The book provide wide coverage of building materials such as stone, bricks, lime, mortars, concrete, asbestos, gray iron, cast iron, steel castings, aluminium, wood, architectural paints and so many others with their applications in building construction. The book is very useful for all professionals related to construction field, technocrats, students and libraries.
STONE
All the engineering structures
are made from some materials. Each material, which is used in the construction,
in one form or the other, is known as engineering material. Engineering
materials are also, sometimes, termed, as building materials or materials of
construction. Every engineer has to come across various materials, in carrying
out various engineering works and projects and as such he is supposed to be
fully conversant with their properties and behaviour.
No material, existing in the universe, is
useless. Every material has its own field of application. An engineer has to be
conversant with the properties of most of them. Stone, bricks, timber, steel,
lime, cement, metals etc. are some commonly used materials by a Civil Engineer.
Even engineers in branches of Mechanical Electrical, Electronics etc. are
required to know the properties of these materials. Selection of building
material, to be used in a particular construction, is done on the basis of
strength, durability, appearance and permeability. In order to carry out safe
constructions, some standards for the materials to be used,
are fixed. These standards are fixed by Indian Standards Institutions (ISI).
These standards are continuously reviewed and modified from time to time to
suit to the changed conditions. All the commonly used Engineering Materials
have been discussed in this book, in regard to their properties, place of
occurrence, manufacture, and uses. In the first chapter stone has been
discussed.
It is likely that our country may face
shortage of common building materials like cement, lime, bricks, aggregates,
plywood, plastics etc. It is therefore an urgent need to handle the situation
by manufacturing cheap building materials and also by developing new building
materials. Shortage of building materials and the high costs are likely to
hamper many projects and developmental programmes. It is therefore imperative
to lay greater emphasis on the growth of such industries which use local raw
material resources for producing less costly building materials.
Rock and Stone
Rock is the term used to name a solid
portion of the earth's crust. It has no definite shape and chemical
composition. It is generally very big in size. The rocks have one or more than
one minerals. Rocks having only one mineral is known
as mono-mineralic rock and those having several minerals as Poly-mineralic
rocks. Quartz, sand, pure gypsum, magnesite are examples of mono-mineralic
rocks and granite, basalt, etc. those of poly-mineralic rocks. The rocks are
named after the predominant mineral present in it.
A rock having calcium carbonate mineral as
predominant mineral, is termed as calcarious rock. Similarly rock predominant
in clay is called argillaceous rock. Quartz, felspar, hornblend, mica, augite,
dolomite are some of the common rock forming minerals.
Stone
The stone is always obtained from rock.
The rock quarried from quarries is called stone. Quarried stone may be in form
of stone blocks, stone aggregate, stone slabs, stone lintels, stone flags, etc.
Stone has to be properly dressed and shaped before it is used at the place of
its use.
Formation of Rocks
Solar system consists of sun as the centre
and all other planets revolve around it. Our earth is one which originally was
in form of mass of incandescent gases. The mass of gases after cooling, first
converted into molten mass and then on further cooling, the surface of the
molten mass converted into solid crust. The process of cooling of earth is
still continuing and thus process of solidification of molten matter is also
continuing. Existence of molten matter under earth's crust is reflected by
eruption of volcanos from time to time. The molten matter, of which the earth
and other planets were originally made up and, existence of which is confirmed
by the volcanic eruptions, is known as Lava or Magma.
Classification of Rocks
The stone which is used in the
construction works, in one form or the other, is
always obtained from the rocks. The rocks may be classified in following four
ways.
1. Geological classification
2. Physical classification
3. Chemical classification and
4. Classification based on hardness of the stone.
Geological classification
According to this classification, rocks may be divided into
following three categories.
(i) Igneous rocks
(ii) Sedimentary rocks and
(iii) Metamorphic rocks.
(i) Igneous rocks. As already explained in article
1.3 "formation of rocks," the in side portion of the earths surface
is very hot and it can cause fusion even at ordinary pressures. The molten lava
or magma, occasionally tries to come out of the earth's surface through cracks
or other weak spots. This magma when gets exposed to the outside cooling effect
solidifies in the form of a rock, known as igneous rock. Hence, igneous rocks
are formed as a result of solidification of molten lava lying below or above
the earth surface due to cooling effect. Depending upon the cooling effect,
following different types of igneous rocks are formed.
(a) Volcanic igneous rocks. This type of igneous
rock is formed when molten lava or magma gets exposed to atmosphere, at the
surface of the earth. In this case, cooling of magma is very rapid and, hence,
structures of these rocks are extremely fine grained. This rock may contain
some quantity of glass which is non-crystalline. Example of Volcanic igneous
rock is Basalt.
(b) Hypa-byssal rocks. This rock is formed when
magma is allowed to cool at, comparatively, slower rate. Such conditions of
cooling, generally, prevail at relatively shallow depth under earth crust.
Since rate of cooling is not as fast as in case of volcanic rocks, the
structure of resulting rocks, is fine grained and crystalline, but not as fine
as in case of volcanic rocks. The best example of hypa-byssal rock is Dolerite.
(c) Plutonic rocks. These rocks are formed when
cooling of magma takes places at a very slow rate. Such conditions of cooling generally, exist at a considerable depth from the surface of
the earth. The structure of these rocks is coarse grained, and crystalline.
Stone, obtained from Plutonic rock is most commonly used in building industry.
The best example of plutonic igneous rock is granite.
All the igneous rocks contain minerals like Augite,
Felspar, Horn blende, mica, quartz etc. Before solidification, all these
minerals are in molten state, along with some gases, forming magma.
(ii) Sedimentary rocks. The rocks are formed by the
deposition of broken up materials like sand, clay, Disintegrated rocks, dead sea organisms etc., with the aid of water, wind, frost
etc. on the pre-existing rocks. Earth's crust, when subjected to weathering
cause disintegration, which results in the formation of clay, sand and pebbles.
The disintegrated mass is carried by rain water, streams, wind etc. and settles
as and when conditions become favourable to it. The process of deposition of
new disintegrated matter continues in regular layers. With age this deposited
mass becomes a rock, known as Sedimentary rock. Since the sediments get
consolidated in horizontal or nearly horizontal layers, these rocks show
different layers distinctly. All the layers of this rock may have same or
different composition, colour and structure, as all the layers have deposited
under varying conditions. The formation of these rocks is shown in Fig 1. These
rocks can be easily split, along the bedding plane. Sand stone, limestone,
slate and shale, are some common Sedimentary rocks.
Fig. 1. Sedimentary Rock formation
(iii) Metamorphic rocks. These rocks are formed,
when igneous as well as sedimentary rocks are subjected to a very large heat
and pressure. The process of change due to heat and pressure is known as
metamorphism. The rocks change their character, due to metamorphism, and the
resulting mass of rock change into hard and durable foliated structure; Marble,
quartzite and slate are common examples of metamorphic rocks.
Metamorphism
All the
rocks of igneous and sedimentary origin, represent a
mass of mineral composition. This mass remains in equilibrium under the general
atmospheric conditions. When either temperature, or pressure, or even both are
increased, the equilibrium of the mass gets disturbed and its minerals realign
themselves to re-establish the equilibrium. Re-alignment of minerals change the texture of the rock. This process is known as
metamorphism. It should be remembered that weathering action and sedimentation
action, are not included in metamorphism.
Heat, pressure, and chemically
active fluids, are the three agents which bring about the changes of metamorphism.
Heat may be supplied by the
general rise of temperature inside the earth or by hot magma and pressure may
be caused due to heavy overlay rocks or due to movement of the earth during
earthquakes. Chemical liquids do not take any active part in the process of
metamorphism. Following four types of metamorphisms occur.
(a) Plutonic metamorphism.
(b) Thermal metamorphisms.
(c) Cataclastic metamorphism.
(d) Dynamo-Thermal metamorphism.
(a) Plutonic metamorphism. The metamorphic change
takes place at large depths under the earth. Uniform pressure and high
temperature are responsible for this change. This is due to the fact that rocks
become plastic mass at certain depths, and plastic mass can be in equilibrium
only under uniform pressure.
(b) Thermal metamorphism. The changes brought
about in this metamorphism are predominantly due to high temperature.
(c) Cataclastic metamorphism. The metamorphism
or change is brought about by directed pressure only and temperature, uniform pressure do not play any role in it. This change takes place
at the surface of the earth.
We have used two terms above-uniform pressure and directed
pressure. Directed pressure can be applied to solids only. Directed pressure
when applied to liquids is converted into uniform pressure. Uniform pressure
can be applied to liquids and solids both.
(d) Dynamo-thermal Metamorphism. Temperature
increases with depth inside the earth. The changes brought about in the rock by
combination of heat and directed pressure are known as
Dynamo-Thermal metamorphism. This change takes place not at very large depths,
but at moderate depths.
As a result of metamorphosis, limestone
and marl become marble, Basalt and trap are converted to schist and laterite
and granite becomes Gneiss.
Physical classification of rocks
According to general structure, the rocks may be
classified into following three categories.
(i) Stratified rocks
(ii) Unstratified rocks and
(iii) Foliated or laminated rocks.
(i) Stratified rocks. These are such rocks which
possess planes of stratification or cleavage. These rocks can be easily split
along these planes. An experiences supervisor at the quarry site,
can easily locate these planes. All the sedimentary rocks have distinct layers
of stratification and thus are stratified rocks.
(ii) Unstratified rocks. The structure of
these rocks is compact granular. They do not show any
layers of stratification or cleavage. All the igneous rocks of volcanic origin, are the examples of unstratified rocks.
(iii) Foliated or laminated rocks. These rocks
comprise of thin laminations. They can be split in definite direction and size.
Metamorphic rocks come under the category of foliated rocks.
Chemical classification
Based upon chemical composition, the rocks can be
classified into following three categories:
(i) Silicious rocks
(ii) Argillaceous or clayey rocks
(iii) Calcareous rocks.
(i) Silicious rocks. These rocks consist of silica,
as their predominant constituent. These rocks are very hard and durable and are
not easily affected by weathering agencies.
Granite, quartzite, trap, basalt,
sandstone, etc. are the examples of silicious rocks. Presence of weaker
materials may cause their disintegration.
(ii) Argillaceous rocks. Predominant constituent of
these rocks is clay. The principle constituent alumina, which is nothing but
clay, remains mixed up in varying proportion with siliceous, calcareous and
carboneous matter. These rocks are hard, durable, dense and brittle, in nature.
Laterite, slate, porphyry, are the best example of argillaceous rocks.
(iii) Calcareous rocks. The predominant constituent
of these rocks is calcium carbonate. The durability of these rocks is greatly
dependent upon the constituents of surrounding atmosphere. Lime stone, marble,
dolomite, kankar etc. are the examples of this type of rocks.
Classification based upon hardness of the stone
According to this classification stone may
be classified as soft, medium, hard and very hard.
Very hard rocks.
Granite, trap taconite, are the very hard varieties of
rocks. Hard rocks. Granite, basalt, trap, gravel,
quartzite are the hard varieties of rocks. Medium rocks. Dolomite and lime
stone are the medium varieties.
Soft rocks.
Talc, gypsum, sand stone, slate etc. are the soft varieties of stones.
Scale of hardness of various minerals,
starting from hardest to softest, have been given as follow.
Diamond
(Hardest)-Corundum-Topaz-quartz-Felspar-Apatite-Flouspar-Calcite-rocks
salt-Talc (softest).
Composition of Stone (Rock-forming Minerals)
Chemically the rocks are composed of mineral
earths, alkalies, oxides or iron and manganese etc. Silica (SiO2), alumina
(Al2O3), lime (CaO), and magnesia (MgO) are the mineral earths, which are
usually found is rocks in one form or the other. Soda
(Na2O) and Potash (K2O) are the usual alkalies present in the rocks. Presence
of alkalies in the rocks is not preferred, as it causes stone to disintegrate
when exposed to weather. Generally stones comprise of more than one mineral
earth.
BRICKS AND OTHER CLAY PRODUCTS
This chapter deals with construction
materials such as bricks, tiles, refractory bricks, earthenwares and
stonewares. All these materials are made from clay and are also known as clay
products. Burning of moulded clay products makes them sufficiently strong for
use as construction materials. Though tiles, refractory bricks, earthenwares
and stonewares serve different construction purposes, brick is the most
commonly used building material. It is light, easily available, uniform in
shape and size, and relatively cheaper except in hilly areas. Bricks are easily
moulded from plastic clays, also known as brick clay or brick earth.
BRICK EARTH AND ITS CONSTITUENTS Sources of Brick Earth
Brick earth is derived by the
disintegration of igneous rocks. Potash feldspars, orthoclase or microcline
(K2O: Al2O36SiO2) is mainly responsible for yielding clay mineral in the earth.
This mineral decomposes to yield kaolinite, a silicate of alumina which on
hydration gives a clay deposit Al2 O3 2H2O known as Kaolin.
Qualities of Brick Earth
A good brick earth should be such a
mixture of pure clay and sand that when prepared with water it can be easily
moulded and dried without cracking or warping. It should contain a small
quantity of lime which causes the grains of sand to melt and helps bind the
particles of brick clay together. It should also contain a small amount of
oxide of iron which acts in the same way as lime and moreover lends the brick
its peculiar red colour.
Functions of the constituents of Brick Earth
Silica or sand in brick earth prevents
shrinkage, cracking and warping of bricks but too much of sand will make the
bricks brittle. Clay or alumina makes brick earth plastic and lends the brick
its hardness; but unless mixed with sand it will shrink, crack and warp in the
process of drying and burning. Lime and oxides of iron both act as fluxes
helping the grains of sand to melt and bind the particles of clay together. Oxides
of iron also impart a red colour to the brick but excess of it makes the brick
dark blue. Magnesia present in clay with oxide of iron make
the brick yellow.
Pebbles of Stones and Gravel
These do not
allow the clay to be mixed uniformly and thoroughly and result in weak and
porous brick. Bricks containing grits are likely to crack and cannot be readily
cut or worked.
Alkaline-Salts
Alkaline salts, if present, act as
hygroscopic substances. They absorb moisture from the atmosphere in due course
of time and create damp conditions. The moisture on drying leaves behind a
greyish white deposit known as efflorescence on account of which the appearance
of the building is spoiled. Common salts generally present in soils are
sulphates of calcium sodium and potassium. The presence of Reh or Kallar
consisting of sodium sulphate, with more or less of sodium carbonate and sodium
chloride, renders the clay utterly unsuitable for brick making. Presence of Reh
or Kallar can be easily detected by the efflorescence on the sides of fresh
excavation, if the soil is moist, but it would be appropriate in all cases to
moisten the soil with water and subject it to evaporation and check for
efflorescence.
Limestone and Kankar
Presence of large quantity of lime and
limestone in lumps is detrimental to brick earth, as lumps of limestone, if
burnt in a brick, slake afterwards and split the brick. Thus limestone should
be present in very finely divided state.
Vegetation and Organic Matter
Organic Matter
if present in brick earth will produce porous bricks. This is due to the evolution
of gas during the burning of the carbonaceous matter, resulting in the
formation of small pores.
MANUFACTURE OF CLAY BRICKS
Bricks are made by treating suitable brick
earth or clay, moulding it to shape and size (allowing for shrinkage), drying
it, and then baking, burning or firing it at high temperatures in order to fuse
the constituents to a hard, homogeneous mass: The process of manufacture can be
described under the following heads.
1. Selection of site
2. Preparation of clay
3. Moulding of bricks
4. Drying of bricks
5. Burning of bricks.
Preparation of Clay
According to IS: 2117-1975, brick clay should be prepared
in two stages:
1. Weathering
2. Tempering
Weathering Process
The soil is left in heaps and exposed to
weather for at least one month in cases where such weathering is considered
necessary for soil. The soil is turned at least twice and it is ensured that
the entire soil is wet throughout the period of weathering. The purpose of
weathering is to disintegrate big boulders of clay under the action of
atmospheric agencies to make it a uniform mass and also to eliminate the
impurities which get oxidized.
Tempering Process
After weathering, the required quantity of
water should be mixed with the soil to obtain the right consistency for
moulding. Addition of sand and other materials, if necessary, may be made at
this stage to modify the composition of the soil.
The quantity of water to be added, may range from ¼ to 1/3 of the weight of soil,
sandy soils requiring less water and clayey soils more water. But the nature
and degree of wetness of the soil at this stage should also be duly considered.
The moistened soil is kneaded with spades
or other manual or mechanical equipment into a plastic mass. After the addition
of water and kneading, the soil may be pugged in a pug mill of suitable size
corresponding to quantity of bricks to be manufactured. The pug mill may be
mechanically operated or hand operated, as shown in fig. 1a and 1b.
Moulding of Bricks
Bricks may be moulded by any one of the following methods:
1. Soft mud process (Hand moulding)
2. Stiff mud process (Machine moulding)
3. Semi dry process (Machine moulding)
Soft Mud Process
The clay prepared by using 25 to 30% of
water is pressed into the mould by hand. Some typical moulds of timber or metal
are shown in fig. 2.
Fig. 1 (a) Manually operated pug
mill (b) Mechanically operated pug mill.
Moulds are made either of wood or of thin
steel plates. Seasoned wood should be used to prevent changes in the dimensions
of the mould. The edges are protected with strips of brass or steel. Steel
moulds, made of plates 6 mm thick, are used if the bricks are to be
manufactured on large scale. Normally shrinkage allowance, varying from 10% to
12% is provided. Thus, the size of the mould is such that it would give the
finished brick its required size.
As per IS:
2117-1975 the mould should be constructed preferably of metal. The thickness of
the sides of the mould shall not be less than 3 mm if made of metal and not
less than 10 mm, if made of wood.
The process of moulding bricks with the
help of moulds is also called hand moulding. According to IS:
2117-1975, handmade bricks may be either ground moulded or table moulded.
In case it is ground moulded, a level firm surface should be used. Typical
specifications for accessories used in table moulding are given below and in
Fig. 3.
Moulding table.
The moulding table is 1.2 to 1.8 m long and 0.6 to 1.0 m wide and is made of
wood or iron. It is smoothly finished at the top and supported horizontally at
a height of 1m to 1.2 m. Also there are holes to accommodate accurately the
bottom pins of the stock-board.
Stock board. It
is a wooden board with iron lining around the upper edge and with such
dimensions as to fit accurately but loosely the interior of the mould (Fig.
3.4.). The stock board is provided with four pins one at each corner of the
bottom side, which when fitted into the corresponding holes on the moulding
table hold the board tightly in position during
moulding. The stock board should also have a projection at the top so as to
form the frog of the brick being made.
Pellet boards. These are rectangular
pieces of wood of size 30 cm x 15 cm x 10 mm thick with a smooth surface on one
side. Pellet boards are used for conveying moulded bricks to the drying yard.
Fig. 3 Stock board.
Procedure
Before moulding, the inside of the mould
is cleaned and then sprinkled with sand or ash. If slop moulding is adopted
then the mould is dipped in water and cleaned. The mould is then set firmly on
the level surface.
A quantity of clay slightly more than the
volume of the mould is taken and rolled in sand. It is then shaped suitably
into a single lump and dashed firmly into the mould with a force that is to be
judged by the moulder by experience, so that the clay
completely occupies the mould without air pockets and with minimum surplus for
removal. The surplus soil is scrapped off with a sharp straight edge, known as
a style, or a stretched wire and the top surface is levelled.
The whole assembly of the mould is then
lifted, given a slight jerk, and, inverted to release the moulded brick on a
pellet board in the case of table moulding or on a dry level surface of the
ground in the case of ground moulding.
The ground may
be advantageously sprinkled with sand before releasing the brick over it, so
that bricks do not stick to the ground. When a frog is not needed a bottomless
mould may be adopted in which case inversion to release the moulded bricks from
the mould will not be necessary. It may be added that each moulder can make on
an average about 500 to 1000 bricks per day.
Stiff Mud Process
The selected clay, after proportioning of
the ingredients is mixed with water up to about 12 to 18% and is thus initially
prepared for being put in the machine. There are two stages in machine
moulding:
Fig. 5 Pug mill and extruding machine.
1. The final mixing when kneading and tempering is done in the
pug mill.
2. In the second stage bricks are formed by extruding stiff
clay through a mould or orifice in the extruding machine.
The pug mill and the extruding machine may
be combined in one unit, as shown in Fig. 5.
The upright cylinder or barrel of the
machine, known as the pug mill, is kept filled with prepared clay which is
mixed well and pressed down against the coarse threads of a horizontal spiral
screw. The pressing screw is fixed near the top of the horizontal axle called
`auger'. At one end of the auger, power is applied for driving the machine and
at the other end is fixed a metal die through which the clay extrudes with the
desired section. The extruding clay is a continuous mass and is received on a
conveyor to be further cut into pieces of the correct sizes of bricks.
The bricks are cut either by a single wire
or by a number of wires fixed on a frame as shown in Fig. 6. The bricks
obtained by this method have a smooth rectangular finish.
Semi Dry Process
In this method only 7 to 10% of water is
added so that it forms just a damp powder. It is then pressed under a pressure
of 1000 to 1200 kg/cm2 with the aid of a plunger machine to form the bricks. At
the first plunger machine the material is automatically measured off, fed into
a steel mould and pressed by plungers (heated to prevent sticking) on two
opposite sides. It is then expelled from the mould and transferred to another
plunger machine where it is again pressed, one of the beds receiving the frog
at this stage. Pressed bricks do not require drying and could be put into the
kiln directly for burning. They are very strong and compact and on account of
the latter quality they are more durable than bricks moulded by the stiff mud
process.
Machine moulding can also be employed for
the soft mud process to press pugged clay into moulds with the aid of the
plunger. It is possible to mould 4 to 8 bricks at a time as against one by
hand-moulding.
LIME
General
Lime is a very important building
material. It has been in use since ancient times. Egyptians used to use lime
for plastering works and Romans for plastering, mortar, and concrete works. In India,
there are numerous historical constructions, where lime had been used in the
form of cementing material. Even today, lime is a very important material not
only for building purposes, but also in so many other manufacturing processes.
Source of Lime
Lime does not occur in nature in free state.
It is obtained from substance having lot of calcarious content in it. Lime
stone, chalk, Kankar are the usual raw materials from which lime is obtained.
All the materials containing calcarious substance have calcium carbonate
(CaCO3) as the chief constituent. When calcarious materials are heated, carbon
dioxide and moisture are driven out, leaving behind calcium oxide (CaO), which
is called lime. Raw material from which lime is obtained by heating vary in chemical composition as well as physical properties
from place to place and as such lime of uniform quality can not be obtained at
all the places. Besides this, methods of burning, slaking, storing and using
also affect the properties of the lime.
MIX-OF-LIMESTONE & COKE
Fig. 3 Lime burning crude clamps.
(i) More fuel is required for burning as, it is wasteful
method.
(ii) There is no control on temperature during burning.
(iii) Fire cannot be regulated properly.
(iv) Quality of lime is not
uniform. Limestone pebbles near surface generally remain unburnt or under
burnt, while those very near to the fire are over burnt.
(v) Supply of burnt lime is intermittent and small.
2. Intermittent Kilns. There are many types of intermittent
kilns in use. But the most commonly used kilns are following:
(i) Intermittent Flame kiln and
(ii) Intermittent flare kiln.
(i) Intermittent Flame kiln. It consists of a round
encloser, open at the top. The encloser is lined by fire clay bricks from
sides. It has fire places and draw holes suitably located in the walls. The
kiln is loaded by crushed lime stone and fuel, arranged in alternate layers.
Horizontal as well as vertical flues are formed at the time of loading. The
loaded kiln is lastly covered with some unburnt material, which may be clay.
The kiln is fired and allowed to burn for about 3 days. The kiln is then
allowed to cool and lastly unloaded.
MORTARS
Definitions
The mortar is a paste like substance
prepared by adding required amount of water to a dry mixture of sand or fine
aggregate with some binding material like, clay lime or cement.
When clay is used as a binding material,
the resulting mortar is known as mud mortar. If, it is lime, the resulting
mortar is lime mortar. Similarly, if cement is the binding material it is known
as cement mortar.
Before properties and uses of different
types of mortars are explained, let us gain some knowledge about the fine
aggregate which is commonly known as sand. Sand is mostly used as inert
material in mortars and concretes.
Sand
It is a form of silica (SiO2) which may be
siliceous, argillaceous, according to composition. Sand
particles consists of small grains of silica. It is formed by the
decomposition of sand stone due to various weathering effects. It is mostly
obtained from pits, shores, river beds and sea beds. Sand may be classified
into three categories as follows.
1. Pit sand
2. River sand.
3. Sea sand.
A brief description of each type of sand has been given.
1. Pit sand. This sand is obtained by forming pits into the
soil. It is sharp, angular, porous and free from harmful
CONCRETE
Concrete is a construction material
obtained by mixing a binder (such as cement, lime, mud etc.), aggregate (sand
and gravel or shingle or crushed aggregate), and water in certain proportions.
The mix is placed properly in moulds or forms to harden in a suitable
environment. When the various ingredients are mixed, these form a plastic mass
which can be moulded into the desired shape and size. The moulded mass when
allowed to cure in suitable environment, hardens to
become a solid mass capable of maintaining its shape and size and sustaining
certain loads. Hardening in concrete is a result of the chemical/physical
combination of the binding material, water and air in a given environment. The
hardened concrete so obtained serves different purposes depending on the type
of binding material used, quality and grade of concrete, location and size of
such concrete component.
The type
of concrete is basically known by its binding material. It is the binding
material which plays the main role in the behaviour and characteristics of the
resulting concrete. Based on the binding materials, the common concretes can be
classified as:
- Mud
concrete
- Lime
concrete
- Cement
concrete
- Polymer
concrete
These concretes are used to serve certain
requirements of various concrete elements cast in different situations.
Behaviour of these concretes can further be modified by the use of certain admixtures,
special treatments, or combination of binding materials. Properties of some of
these concretes can further be improved through certain special techniques of
construction (such as prestressing, reinforcing, impregnation, etc.).
Mud concrete is made by using suitable
mud as the binding material. Mud is prepared from good quality clay and water
by kneading. The mud is mixed with coarse aggregate or shingle to obtain mud
concrete. The mud concrete is laid in suitable layers and compacted by ramming
or tamping. The mud concrete properties are mainly due to interlocking of
aggregate particles and filling of voids by mud. Mud concrete can easily be
affected by moisture and has poor impermeability, durability and strength
characteristics. This type of concrete is generally used for cheap and
temporary type of constructions in foundation bases, non-load bearing walls
with water proofing treatment on external faces, etc.
Lime, is very popular binding material in
Civil Engineering constructions. Properly slaked lime slurry or putty is used
as binding material in lime mortar and lime concrete. Lime concrete is prepared
by mixing lime mortar with aggregate (shingle or gravel). Lime concrete is laid
in layers and compacted suitably by ramming. Lime concrete is commonly used for
foundation base layers, floor base layers, roof insulation layers over stone
patties (slabs), etc. Lime concrete exhibits fairly good properties of
durability, impermeability, and strength specially
suitable for base courses. It has been used in many important old monumental
buildings which have stood the test of time. The details of preparation,
properties and used shall be dealt in a subsequent section. The advent of
cement as binding material in the 19th century revolutionized construction
activities. Cement concrete obtained by mixing cement, sand, gravel or shingle
or crushed aggregate and water is a versatile and popular construction
material. Cement concrete is used in almost all modern structures due to its
superior qualities and appropriate quality controls possible during and after
construction. Cement concrete is used in various forms and grades for different
purposes in construction works. Certain weak points in cement concretes for
specific purposes can easily be overcome by adopting suitable techniques such
as steel reinforcing, prestressing, fibre reinforcing, ferrocement and polymer
impregnation techniques etc. The properties of cement concrete can also
modified by the use of certain admixtures during its preparation. The details
of preparation, properties and uses shall be discussed later.
Recently certain polymers and epoxy
resins have been developed which exhibit superior binding qualities. These
polymers are now being used for preparation of special polymer concretes.
Polymers concrete is obtained by mixing epoxy resins or polymers with plastic
aggregates. Polymer concrete exhibits very high strength. Cost of polymers and
other epoxy resins used as binding material is quite high and hence the cost of
polymer concrete is also high. Due to the high cost and small production of
epoxy resins, the use of polymer concrete is limited. In India, use of such polymer concrete
is yet to pick up. The future potential for the manufacture and use of polymer
concrete for construction of highly sophisticated structures is very good.
1. Mud concrete. This concrete does not carry any
importance. It is prepared by mixing brick bats in mud mortar. Brick bats may
be made from kuchha or pucca bricks. Sometimes, even crushed stone may be mixed
with mud mortar to form mud concrete.
Mud concrete is used for preparing hard
base. Over which lime concrete may be laid and then permanent flooring may be
spread. During construction of ground floor of buildings, filling of earth is
first of all consolidated by sprinkling sufficient amount of water. During
consolidation process broken brick bats or crushed stone pebbles, lying waste
at the site, are also spread and rammed into the fillings by rammers. Earth
filling added with water and broken brick or stone bats, forms mud concrete
which on setting develops a hard surface, over which permanent flooring of any
form may be laid. The same processes of consolidation with mud concrete may be
carried out during preparation of foundation bases.
The coarse aggregate used in mud concrete
is usually of broken bricks of size 4 cm 100 m3 brick ballast is mixed with 40
m3 of prepared mud mortar.
ASPHALT, BITUMEN AND TAR
The history of these materials can be
traced back to ancient times. Their importance was realized by early
civilizations, who employed them for a variety of
purposes ranging from mummification to building temples, palaces and vast
irrigation systems and enduring highways. A ritual pool-dating back to 3000
B.C. discovered in Mohenjodaro, in the valley of the Indus,
was water-proofed with a layer of bitumen on the walls. After an apparently
extinct phase through the middle ages and renaissance
period, bitumen was re-discovered in the form of deposits of impregnated
limestone, in France, Switzerland and Germany in the eighteenth century.
Put to use for side walls and pavings in different parts of the continent, it
proved to be satisfactory. The utility of the material has grown ever since and
today its versatility as a construction material may be well judged from its
use and application for roofing, road surfacing, insulating varnishes, acid
resistant paints and cold-moulded products.
For purposes of ,
asphalt, bitumen and tar are referred to as “Bituminous
Materials’’, which are essentially hydrocarbon materials frequently
accompanied by their non-metallic derivatives. They may be gaseous, liquid,
semi-solid or solid in nature and are completely soluble in carbon-di-sulphide
(CS2.). They possess some common properties, as follows:
1. Thermo-viscosity,
i.e. variance of viscosity (which is roughly the opposite of fluidity) with
temperature.
2. Adhesion
to solid surfaces.
3. Durability.
4. Water-proofing
characteristics under normal circumstances.
The above desirable properties of ‘bituminous
materials’ render them very useful as a protective agent, an adhesive and
a sealant.
GRAY IRON
Although
“gray iron” denotes a certain type of cast iron, yet the chemical
composition, structure, and properties of gray iron may vary over broad limits.
The range of alloy compositions and properties produced as gray irons may be
better understood by consideration of some of the principles of gray-iron
metallurgy. The metallurgy of cast irons depends in large measure upon the
nature of the iron-carbon equilibrium system.
THE METASTABLE IRON-IRON CARBIDE SYSTEM
In the phase system iron-iron carbide,
carbon in the alloys occurs as the metastable compound iron carbide (Fe3C).
During solidification or melting and in thermal treatments in the solid state,
the iron carbide functions according to normal principles of phase
relationships as expected from the equilibrium diagram. For example, freezing
of a hypoeutectic alloy, less than 4.30 per cent carbon,
will begin with the formation of austenite dendrites and be completed by solidification
of the eutectic austenite-iron carbide. After solidification,
cooling in the solid state results in transformation of the austenite to
pearlite.
CHEMICAL COMPOSITION EFFECTS
All the elements normally present in gray
iron exert some influence on the microstructure of the iron. Carbon and
silicon, of course, are fundamental in their effect on cast irons, and may be
considered first.
Carbon
Carbon in gray iron is present from about
2.5 to 4.5 per cent by weight. Two phases occur, elemental carbon in the form
of graphite and combined carbon as Fe3C. The analysis reported ordinarily is
the total carbon percentage in the iron. Since the two forms may be determined
separately by chemical analysis, the degree of graphitization may be assessed
by the following relationship:
% total carbon= % graphitic carbon + % combined carbon
If graphitization is complete, the percentage of total
carbon and the percentage of graphitic carbon are equal. If no graphitization
has occurred, the percentage of graphitic carbon is zero. If about 0.5 to 0.80
per cent combined carbon exists in a gray iron, it generally indicates that the
microstructure is largely pearlitic since pearlite in gray iron having about 2
per cent silicon forms from the austenite eutectoid containing about 0.60 per
cent carbon. Thus the relationship above offers a chemical criterion of the
degree of graphitization in a gray iron. For sufficient graphitization to
develop during solidification of a true gray iron, a certain
minimum total carbon content is necessary, which is probably about 2.20 per
cent, but this value depends on silicon percentage in the iron.
Silicon
Silicon is present in gray iron from about
1.0 to 3.50 per cent by weight. Of course, the important effect of silicon is
its effect on graphitization. It may be noted that increasing silicon percentage
shifts the eutectic point of the iron-carbon diagram to the left. The eutectic
shift is often described by the following relationship:
Eutectic carbon percentage (in Fe-C-Si alloy)
= 4.30-1/3 x % Si (in iron)
Another term, the carbon equivalent (CE), is
often used to describe the relationship of a particular iron to the eutectic
point:
CE = % C (in the iron) + 1/3 x % Si
If the carbon equivalent of a particular
iron is calculated to be 4.3, then that iron corresponds approximately to a
eutectic alloy (even though it is not a true eutectic in the sense of the
ternary phase diagram). If the carbon equivalent of an iron is less than 4.30,
the alloy is a hypoeutectic alloy. The carbon equivalent is a useful expression
because many properties of gray iron have been found related to it. If the
combination of carbon and silicon exceeds 4.30, according to the
carbon-equivalent equation, the iron is a hypereutectic one. In this case, the
freezing process begins with the formation of graphite. When graphite precipitates
first during solidification, the melt is said to form kish. Because of its
buoyancy, kish
pops out of the melt into the air and can be observed as sparkly graphite
flakes floating on the surface of the iron or in the air above the iron.
Not only is the eutectic point shifted by
silicon in cast irons, but it also shifts the eutectoid point and the
solubility limits of carbon in austenite to the left of equivalent points in
the Fe-C system. For this reason pearlite in a 2.0% Si gray iron may contain only
about 0.60% carbon rather than the 0.76% C value on the Fe-C diagram
Micro structurally, silicon occurs
dissolved in the ferrite of gray iron. As such it hardens and strengthens the
ferrite, as pointed out in Chap. 18. Ferrite in pure iron will measure 80 to 90
Bhn, whereas 2.0 per cent silicon in a ferritic iron raises the hardness to
about 120 to 130 Bhn.
Silicon Content and Graphitization
Silicon promotes graphitization. Low
percentages are not sufficient to cause graphitization during solidification,
but will cause nucleation and graphitization in the solid state at high
temperature, as, for example, during malleableizing heat-treatment. Certain
silicon percentages will cause limited graphitization during solidification,
and a mottled iron, partly white and partly gray, results.
A certain minimum silicon (and carbon)
concentration is necessary for graphitization to proceed sufficiently during
solidification to develop a satisfactory gray iron. More accurate diagrams have
as their purpose a limiting description of the silicon and carbon percentages
which will cause an iron to freeze gray in the section sizes of commercial
castings poured into green sand molds. Although these diagrams are useful as a
guide, successful metallurgical performance in the type of castings made in
particular foundries remains the ultimate criterion for the carbon and silicon
content. Hence foundries producing certain sizes of castings and types of gray
irons will ultimately develop silicon and carbon combinations suitable to their
work.
Sulfur and Manganese
Sulfur, which may be present up to about
0.25 per cent, is one of the important modifying elements present in gray
irons. A low-sulfur iron- silicon-carbon alloy, under 0.010% S, will graphitize
most completely. Boyles has shown that higher sulfur percentages favor the
retention of a completely pearlitic microstructure in a gray iron. The latter
effect causes sulfur to be known as an element restricting graphitization
(carbide stabilizing). Above about 0.25 per cent sulfur is considered to
contribute undesirable hardness and decreased machinability because of its
retardation of graphitization.
The influence of sulfur needs to be
considered relative to its reaction with the manganese in the iron. Alone,
sulfur will form FeS in cast irons. The latter
compound segregates into grain boundaries during freezing and precipitates
during the final stages of freezing. When manganese is present, MnS, or complex
manganese-iron sulfides, are found, depending on the manganese content. The manganese
sulfides begin to precipitate early, and continue to do so during the entire
freezing process, and are therefore usually randomly distributed. As MnS, the
effect of sulfur in causing a pearlitic microstructure to be retained is lost
to a major extent. The effect of Mn alone as an alloying element is to promote
resistance to graphitization. Therefore manganese above that necessary to react
with the sulfur will assist in retaining the pearlitic microstructure. The
following rules are advanced to express the relationship involved:
1. %
S X 1.7 = %Mn; chemically equivalent S and Mn percentages to form MnS.
2. 1.7
X % S + 0.15 =%Mn; the manganese percentage which will promote a maximum of
ferrite and a minimum of pearlite.
3. 3
X % S + 0.35 = %Mn; the manganese percentage which will develop a pearlitic
microstructure.
For commercial gray irons in which a
pearlitic microstructure is desired, rule 3 offers a favourable combination of
manganese and sulfur percentage.
Phosphorus
Segregation of phosphorus may result in
lowering of the temperature of final solidification to about 1800 F. The
percentage of steadite present in the final structure may amount to ten times
the percentage of phosphorus in the iron. Because of segregation, the steadite
usually adopts a cellular pattern characteristic of the eutectic cell size
developed during solidification. In certain conditions of melting and chilling,
iron carbide is associated with the phosphide in a ternary iron-iron
phosphide-iron carbide eutectic. Then an amount of the latter constituent
considerably in excess of ten times the per cent phosphorus may be formed. If
the ternary eutectic is accompanied by graphitization of its carbide during
solidification, expansion of the liquid occurs and beads of eutectic exude from
the iron. These are often found at the surface of sprues and risers.
Because it forms a eutectic as it
segregates, phosphorus is often looked upon as increasing the tendency for a
particular iron composition to be a eutectic-type alloy. For this reason, the
carbon-equivalent equation is sometimes modified to include a factor for
phosphorus as follows:
CE = %C + 1/3 (% Si + % P)
The phosphide of iron is hard and brittle,
as is the carbide. Increasing phosphorus percentage in the iron causes a
proportional increase of the hard constituent, and therefore increasing
hardness and brittleness of the iron, especially above about 0.30% P. To a
limited degree, improved fluidity of the molten iron is a desirable property contributed
by phosphorus through its influence on carbon equivalent.
Gray-iron Specifications
Because gray iron is used in so many
different engineering applications, numerous specifications covering its use in
special fields have been developed.
CAST IRON
Cast
irons are the tonnage product of the foundry industry. Cast-iron foundries
produce over a million tons of castings monthly, and thus supply more than
twice as much casting weight as all other foundries combined. Iron foundries
are found everywhere that manufacturing occurs. Of the 5674 foundries in India
2068 produce gray-iron, 350 nodular-iron, and 116 malleable-iron castings.
These foundries send a steady stream of iron castings into every conceivable
industry. The demand for iron castings is based on the nature of cast irons as
engineering materials and their economic cost advantages. Cast irons offer a
tremendous range of the metallic properties of strength, hardness,
machinability, wear resistance, abrasion resistance, and corrosion resistance
and other properties. Furthermore, the foundry properties of cast irons in
terms of yield, fluidity, shrinkage, casting soundness, ease of production, and
others make the material highly desirable for casting purposes. From all
standpoints, the cast-iron family offers a variety of engineering properties
which ensure its continued and widespread use. Since many cast irons of
different properties are employed, it is desirable that a student engineer
obtain an over all picture of the entire field. This chapter offers such a
picture and presents some of the simpler and more fundamental differences
between members of the cast iron family.
SOLIDIFICATION PROCESS
The differences between gray, mottled, and
chilled irons are largely established during the freezing process. The
fundamentals of the freezing process are related to the nature of the
iron-carbide-silicon ternary equilibrium system (Fig. 2). However, a simplified
schematic diagram presenting the essential ideas is given in Fig. 3. With
reference to the diagram, the freezing and cooling of an iron, composition A,
may be described by the following steps:
A. Liquid
melt cools until freezing begins at point 1. At this point solid austenite
dendrites begin to form and grow until the temperature at point 2 is reached.
This step is omitted when the composition is eutectic, at B, on the diagram.
B. Eutectic
(a liquid saturated with respect to two solids) freezing begins as the area at
point 2 is entered with decreasing temperature. The eutectic solids which form
may be a mixture of austenite and carbide or of austenite and graphite. If the
former occurs, the iron is freezing as white iron. If the latter occurs, the
iron is freezing as a gray or a nodular iron. Graphite will prevail if
graphitizing factors, such as high silicon content and slow cooling rate, are
operative. Low silicon content and rapid cooling will cause the eutectic to
freeze as a mixture of carbide and austenite (white). When the temperature has
dropped to point 3, freezing is completed. Thus an iron freezes, as white,
gray, or nodular iron. Actually, the solidification of nodular cast iron is
somewhat more complex than this. If the iron freezes as gray or nodular, the
nature of the graphite is established during freezing Mottled irons are borderline
cases where both graphite and carbide have formed.
C. At
the end of freezing, the structure consists of the solids developed during
steps A and B. In gray and nodular irons these are austenite and graphite, and
in white irons, austenite and carbide.
D. Further
cooling between points 3 and 4 results in the precipitation of carbon from the
austenite present since the austenite may contain as much as 2.0% C at the end
of freezing, but only about 0.60 to 0.80% as the temperature decreases to point
4. The excess of carbon in the austenite is precipitated as carbide in white
irons and as graphite in gray and nodular irons.
E. Between
points 4 and 5, the final change occurs in the solid state during cooling.
Austenite-transforms over the temperature range of points 4 to 5. Because this change is quite complex, only a few generalizations
are offered. With the most favourable of graphitizing conditions, only
ferrite is formed in gray and nodular irons. With less severe graphitizing
conditions, ferrite and pearlite or only pearlite is formed. In nodular cast
iron, mixed structures of ferrite and pearlite form as “bull’s
eyes” of ferrite around the graphite spheroid (Fig. 1). In white irons
only pearlite is formed. The final microstructure of white iron such as is used
to produce malleable castings.
F. Cooling
below point 5 to room temperature produces little change in the iron.
From the foregoing it can be seen that the
type of iron, whether white, mottled, chilled, or gray, is largely established
during the freezing process. Furthermore, the room-temperature microstructure
reflects the entire freezing and cooling process of the iron. Thus the
properties of cast irons are greatly influenced by the thermal and chemical
changes occurring during its entire history from liquid melt to cooled casting.
STEEL CASTINGS
MOLDING PROCESSES AND SANDS
Molding for steel castings is no different
from that for other casting alloys. However, because of certain characteristics
of steel, certain methods cannot be used and others are not used to the extent that
they are employed in other metals.
Steel can be cast into molds made by any
of the sand-molding processes. Dry-sand molds, core-sand molds, skin-dried
molds, and cement-bonded molds are used to a greater extent in steel foundries
than for most of the other casting alloys. The reason for this is the severe
conditions imposed by steel. The problems associated with various molding
methods should become more apparent as these methods are discussed.
With reference to molding methods other
than those using sand, the high pouring temperature required for steel prevents
its being made by the permanent-mold process, except in certain special cases,
or by die casting, or plaster molding. Steel can be poured in investment molds
because the investment materials are sufficiently refractory. Graphite molds
can be used for steel if precautions are taken to avoid carbon pickup. Ceramic
molds can be and are being used.
Green-sand Molding
Many steel castings are made using
green-sand molds. The general practice is no different from that for other
alloys. However, steel-foundry sands differ from others chiefly in the
following characteristics.
Refractoriness
Because sand in contact with steel may be
heated to an excessively high temperature, the molding sand must be of sufficient
purity so that it will not fuse together or deteriorate. Figure 15.6
illustrates that the sand at a metal-mold interface may reach high
temperatures, but a short distance away the sand does not get so hot nor does
it heat up so rapidly as at the interface. As a consequence of the demand for
high thermal stability, most green-sand molding for steel is done with compounded
sand mixtures; the bond is usually bentonite. Associated with refractoriness of
the sand is the problem of durability. The high-temperature exposure to which
the sand is subjected alters the sand and its bond both physically and
chemically, leading to a gradual change in its properties unless it is amply
replenished with new sand. Unfortunately, there is no simple test to indicate the
occurrence of these gradual changes. In one investigation it was observed that
the rate of deterioration of the sand could be linked with the development of
relatively high hot strength and sensitivity to thermal shock, with progressive
build-up of “cokey” coatings on the sand grains.
High Permeability and Low Moisture Content
These two requirements are linked together
because they are inter, related. When sand is heated, part of the moisture in
the sand is changed to steam. The air in the mold is heated and increases in
volume, and organic additions may decompose to gaseous products. These gases
must be vented away from the mold cavity. Steel heats the mold to higher
temperatures than do other alloys; hence a greater gas volume may develop and
more venting is needed. The necessary conditions can be achieved for steel by
increasing the permeability above that required for other alloys and
restricting the moisture content to a relatively low value (around 3 per cent).
Much of the gas can escape through risers and other openings in the mold.
Organic and Other Additions
The use of synthetic sands with a relatively low binder content for steel is accompanied by
a tendency toward certain casting defects such as scabs, buckling, and rattails
that result from the expansion of the sand as it is heated. The addition of
certain materials to the sand may reduce the tendency to form these defects.
The net effect of these special conditions
imposed by steel on green sand properties results in establishing a range of
properties that differ rather markedly from those for molding sand mixtures
used for other alloys. These differences are demonstrated by the data in Table
5.9, which lists typical sand compositions and properties for various alloys,
including steel.
Much green-sand work is done with a facing
sand which is especially compounded to produce the desired properties, and a
backing sand which, being essentially reused facing sand, is also controlled as
to properties and grain size. This practice, although it adds to the complexity
of molding since it involves delivery of both facing and backing sand to the
molder, has the advantage of cutting down the quantity of sand that must be
treated with additives and ensures sand properties at the metal-mold interface
that are always under close control.
Green-sand-molding Casing Defects
In addition to such defects as rattails,
buckles, scabs, hot tears, etc. which are discussed elsewhere in this book, and
also treated thoroughly in reference material, another defect that can develop
is pinhole porosity.
It is characterized by small smooth-walled
holes, elongated in a direction perpendicular to the mold wall and occurring
immediately below the casting skin. The exact cause of the defect is still a
matter of debate, but it is generally agreed that the formation of either CO or
H2O or both by a reaction at the metal surface or slightly below is
responsible. The fact that the defect occurs more frequently in green-sand
molds suggests that it is at least aggravated by certain conditions existing at
the metal-sand interface; and since the only major difference between
green-sand and dry-sand molds would be in moisture content, the formation of
H2O by reaction between hydrogen and oxygen in the steel is strongly suspected
as at least a contributing factor. Moisture in the sand could aggravate the
condition by being dissociated to hydrogen, which could then diffuse into the
steel and react with dissolved oxygen. This would explain why pinhole porosity can
be prevented by deoxidizing the steel with aluminum before pouring, since the
oxygen would react with the aluminum instead of the hydrogen.
Dry-sand Molds and Skin-dried Molds
Green-sand molding is preferable to other
methods of molding because it is more economical and gives maximum production
rates. There are times, however, when, because of the need to increase the
strength of the mold or to avoid pinholes, or for other reasons, drying of the
mold before pouring is desirable. Superficial drying can be accomplished by
heating the surface with torches, infrared lamps, or hot air, or the molds can
be dried in large car-type ovens at temperatures up to 500F.
The moisture content of the green sand
used for skin-dried or dry-sand molds may be somewhat higher than for ordinary
green-sand work for greater moldability, and also because a higher moisture
content leads to greater dry strength.
Other Types of Molds
A few foundries have used cement as a sand
binder, but the practice has not been very popular in this country.
One field where investment molding has
proved effective is in the castings of the special alloys and shapes used for
gas-turbine blades and other parts subject to high-temperature service that
cannot be readily formed by other methods.
Shell molds have been used with some
success, but there is a tendency to form surface defects. These can be
eliminated by use of hill-type shell molds. Ceramic molds are also feasible.
These permit pouring thinner sections than with conventional sand molds. A special
process combining graphite molds and air-pressure pouring has been used to
produce steel car wheels and other shapes.
Molding Methods
The usual methods of molding, such as hand
ramming, jolt ramming, squeezing, and sand-slinger ramming, are used on steel
sands; no difference exists in the ramming methods used for steel in comparison
with other casting alloys.
ALUMINIUM AND MAGNISIUM ALLOYS
Pure aluminium and magnesium being
relatively poor casting materials, aluminium and magnesium castings are actually
produced from alloys. The casting alloys used are
those having properties peculiarly suited to casting purposes. Since a large
number of aluminum-and magnesium-base casting alloys are available, it is
evident that quite widely different properties may be obtained from the various
alloys. For all these alloys two types of properties should be considered: the
casting properties, those characteristics of the alloy which determine the ease
or difficulty of producing acceptable castings, and the engineering properties,
those properties which are of interest to the designer or user of the castings.
These two sets of properties can be used as a basis for studying the
similarities and differences of the large number of aluminium and magnesium
casting alloys.
ALUMINUM ALLOYING PRINCIPLES
The aluminium-base alloys may in general
be characterized as eutectic systems, containing intermetallic compounds or
elements as the excess phases. Because of the relatively low solubilities of
most of the alloying elements in aluminium and the complexity of the alloys
that are produced, any one aluminium-base alloy may contain several metallic
phases, which sometimes are quite complex in composition. These phases usually
are appreciably more soluble near the eutectic temperatures than at room
temperature, making it possible to heat-treat some of the alloys by solution
and aging heat-treatments. Specific instances of the application of these
heat-treatments are given in subsequent paragraphs.
All the properties of interest are, of course,
influenced by the effects of the various elements with which aluminium is
alloyed. The principal alloying elements in aluminium-base casting alloys are
copper, silicon, magnesium, zinc, chromium, manganese, tin, and titanium. Iron
is an element normally present and usually considered as an impurity. Some of
the simpler effects of alloying can be considered.
Copper
The diagram shows solubility of copper in
aluminium increasing in the solid state from less than 0.50 per cent at room
temperature to 5.65 per cent at 1018 F. Copper above the solubility limit at
any temperature appears micro structurally as the phase. The latter phase has a
composition approximating the formula CuAl2 (46.5% A1-053.5% Cu) and is a hard
brittle constituent. By comparison the solid-solution phase is relatively soft
and ductile. Structurally, then, increasing copper content in Cu-Al-base alloys
result in an increasing percentage of the hard phase. The mechanical properties
of hardness and strength can then be expected to increase as copper content
increases while the ductility decreases. A limited percentage of copper thus
has a beneficial effect of strengthening and hardening in Cu-Al-base alloys.
Furthermore, ductility is reduced to a very low level and brittleness results in
alloys of high copper content. Therefore copper percentages do not exceed 12
per cent in most aluminium casting alloys. Actually, the copper percentages in
aluminium casting alloys are adjusted so that the lower contents, 2 to 5 per
cent, are used in alloys required to have optimum ductility (or toughness),
whereas the higher percentages are used when greater hardness and strength are
desired.
Heat-treatment of Cu-Al Alloys
The mechanical-property curves of Cu-Al
alloys are shown to be markedly shifted by solution heat-treatment and age
hardening. In fact, the degree of strengthening obtainable by heat-treatment is
greater than that gained by alloying alone. A few elements, namely, Cu, Mg, Zn,
and combinations of Mg and Si confer heat-treating potentialities to Al-base
alloys in which they are present. These are referred to as
“heat-treatable” grades of aluminium alloys, and they greatly
extend the range of properties available in aluminium castings.
Solution heat-treatment.
Solution heat-treatment of aluminium casting alloys consists of a thermal cycle
of heating, a suitable period of holding the metal at some elevated
temperature, and then rapid cooling of the castings, usually by quenching in
water. The temperature and time of holding are exceedingly important factors in
the treatment. The temperature must be high enough to cause a substantially
large amount of the alloying elements (usually present as intermetallic
compound phases) to dissolve in the aluminium-rich solid-solution phase.
After sand casting and slow cooling to
room temperature, this alloy consists micro structurally of the aluminium-rich
phase k and the hard
phase, copper being concentrated mainly in the latter phase.
Reheating the alloy to a temperature of about 900 to 950 F causes the phase to disappear from the
microstructure, since, the higher temperature permits all the copper in the
alloy to be dissolved by the aluminium; hence the name “solution”
heat-treating. Of course, adequate time for dissolving of the phase into the K phase must be
allowed. Thus emphasis is placed on the “time at temperature” of
the solution heat-treatment. A sufficient holding period at the solution
heat-treating temperature is one which results in the aluminium-rich phase
having reached a uniformly high percentage of dissolved alloying elements. When
this condition exists, rapid cooling from the elevated temperature will retain
the enriched solid-solution phase, 4% Cu-96% Al in the present case, down to
room temperature. The end microstructure after solution heat-treating then is a
supersaturated Al-rich solid-solution phase. In this case, the k phase contains
4 per cent dissolved copper rather than the normal amount of less than 0.50 per
cent for the slow or equilibrium-cooled condition. Since solution heat-treating
results in a more uniform distribution of soluble alloying elements, it also
assists in minimizing the harmful effects of segregation developed during
solidification.
Accompanying the microstructural effects
of solution heat-treatment are improvements in mechanical properties. A marked
increase in tensile and yield strengths and an improvement in ductility are
revealed in Fig. 3 as a consequence of this treatment. Most important is the
fact that solution heat-treatment is the necessary step in preparing the alloys
for age or precipitation hardening from which further benefits may be obtained.
Solution heat-treatment
by chill casting. Rapid cooling from any elevated temperature,
particularly above 700 to 800 F, will cause retention of a supersaturated
A1-rich phase down to room temperature. Hence casting processes such as
permanent-mold or die casting which are inherently rapid in their cooling
effect have this possibility. Sand casting, by contrast, is a slow cooling
process. Therefore, if a given alloy, Cu-A1, for example, is cast in a metal mold, it will usually show higher hardness, strength, and
ductility than if the same alloy is cast in a sand mold. This point will be
considered again later.
Age hardening or precipitation hardening.
Natural age hardening is a gradual increase in hardness (and strength) which
occurs with the lapse of time at atmospheric temperatures. The increased
hardness may reach a maximum value in a few days but may require several years
in some alloys. More rapid aging can be caused to occur at elevated
temperatures, 300 to 400 F. Heat-treating to cause aging is called artificial
age hardening, or “precipitation” hardening. Aging effects by
either method are obtained only from alloys which have been previously solution
heat-treated. Or the alloy can be aged, if it has been processed so that
effects similar to solution heat-treatment are retained, as, for example, by
chill casting. The metallurgical changes associated with aging are exceedingly
complex, so that only the more simple details are considered here.
Aging or precipitation-hardening
temperatures are such as to promote precipitation from the supersaturated solid
solution remaining from solution heat-treatment. In the case of the 4% Cu-96%
A1 alloy considered earlier, the direction of microstructural changes during
aging is toward reprecipitation of the phase from the supersaturated k
phase developed by solution heat-treatment. However, the most beneficial aging
effects are obtained before microstructural evidence of precipitation is
revealed. In fact, when the precipitating phase is metallographically visible,
overaging has occurred. Overaging results in a substantial decrease in
hardness, strength, and other properties.
Temperature and time of aging are exceedingly
important factors, determining the end effect of aging. High temperatures are
to cause rapid aging or overaging at extended times. Low temperatures can
prevent aging. Thus it is evident that a proper temperature and time interval
will produce the most desirable properties. Aging treatments for specific
alloys will be considered later.
MALLEABLE IRON
American malleable iron occupies the
unusual position of being truly a product born of the American
foundryman’s inventiveness. The first “blackheart”
malleable-iron castings were developed by Seth Boyden at Newark, N.J.,
starting in 1826. Boyden’s work eventually resulted in the growth of the
American, or blackheart, malleable-iron industry, until it has become the third
largest tonnage producer in the castings field.
Malleable iron is an important engineering
material, largely because its properties offer certain special advantages among
the family of cast irons. Desirable properties include case of machinability,
toughness and ductility, corrosion resistance in certain applications, strength
adequate for wide usage, magnetic properties, and uniformity resulting from 100
per cent heat-treatment of all castings produced. Applications of malleable
castings usually reflect a need for one or more of the foregoing properties.
Principal users of the castings are the automotive and truck industries,
construction-machinery producers, and agricultural-equipment makers.
Examples of truck malleable-iron castings are shown in
Fig.1.
The properties of malleable iron are
mainly related to its metallographic structure. Malleable iron may be defined
micro structurally as a ferrous alloy composed of temper carbon in a matrix of
ferrite containing dissolved silicon. The structure is the result of
heat-treatment applied to white-iron castings. The chemical composition of the
common grades of white iron which may be heat-treated to malleable iron is
given in Table 1.
Heat-treatment converts the massive
carbides and pearlite of the white iron to ferrite and temper carbon.
Chemically, heat-treatment causes a change from combined carbon to graphite or
temper carbon, the combined carbon generally being less than 0.15 per cent by
weight after heat-treatment. The ferrite structure with interspersed graphite
gives malleable-iron mechanical properties in the range of those specified in
Table 2, under standard malleable iron. The tensile properties and Bhn are
characteristic of ferrite alloyed with 1 per cent silicon.
Except for annealing or malleableizing,
the manufacture of malleable-iron castings involves the same basic foundry
processes used with other alloys. Molding, coremaking, cleaning, melting,
pouring, etc., are adapted to the special casting properties of malleable iron,
which are primarily related to its metallurgical nature. This area will
therefore be considered first.
MELTING
Melting iron for malleable casting is
generally performed in the air-furnace, the cupola, induction, or direct are
electric furnaces, or a combination of these furnaces when duplexing is
employed.
Batch-melting Process
The cold-melt air furnace shown in Fig. 3
is used for batch melting. The air furnace is a reverberatory-type furnace
fired with pulverized coal or oil. Common furnace capacities range from 15 to
40 tons. The furnace hearth is rectangular and provides a molten-bath depth of
generally less than 12.0 in. Tapholes are-provided on the
side of the furnace. The side walls are made of firebrick supported by
steel, and the bottom is either silica sand or firebrick. The furnace top
consists of a series of removable firebrick arches known as
“bungs.” By removing some of the bungs, the furnace may be charged
with cold metal through the top. A typical furnace charge is given below:
Smaller-size charge materials are usually
placed on the bottom of the furnace. Both charges listed above contain about 50
per cent sprue because this is the usual percentage of remelt in a malleable
foundry. The balance of the air-furnace charge is selected so that the iron
will melt down at about 2.65 to 2.85 C, phosphorus and sulfur percentages below
the maximum permitted, and silicon and manganese within or slightly below the
desired analysis range. Less than 0.07 per cent, and preferably less than 0.03
per cent, chromium should be in the charge, since this element interferes with
annealing. Melting down is performed with a fuel-air mixture which will produce
flame temperatures of about 3080 to 3150 F and hold oxidation of the metal to a
minimum. A slag forms during melting down from metal-oxidation products and
refractory attrition. During melting down and as the bath reaches a temperature
of about 2600 F, the slag is skimmed. The bath temperature is then raised to
the desired pouring temperature, usually 2800 to 2900 F. Losses of silicon and
manganese occur during melting down and until the metal has reached a
temperature of about 2700 F. At higher temperatures, carbon losses can occur
rapidly under oxidizing atmospheres, but there may be a silicon pickup from the
refractories and slag. The iron gains about 0.05 to 0.15 per cent silicon per
hour at 2800 to 2900 F from reduction of silica by carbon in the iron. Typical
composition changes during a heat are given in Table 3.
Carbon losses are counteracted by melting
with a higher fuel-to-air ratio (reducing), by adding graphite, petroleum coke,
or proprietary recarburizer, or by dropping powdered coal on the metal surface
from the burners.
The analysis changes occurring in the
course of an air-furnace heat are accompanied by structural changes in the
solidified iron. Early in the heat, iron cast into a bar about 1¾ to 2
in. in diameter and 8 to 10 in. long will freeze gray or mottled. Mottling results from the formation of flake graphite during
freezing, the iron then not being a completely white iron. As the
temperature increases above 2600 F and the carbon percentage in the iron drops,
mottling gradually disappears. Finally, before tapping, the test bar will cast
white and will have a completely white fracture as illustrated in Fig. 5.
Generally, the objective of quality malleable-iron melting is to produce a
completely white iron with no free flake graphite in the castings since-flake
graphite lowers the properties of malleable iron. Melting may be conducted to
favor white iron by using high temperatures, oxidizing conditions, low carbon
and silicon percentage in the iron, additional steel in the charge, moisture in
the air, and a number of other practices. When the iron has reached the necessary
composition limits and is known to freeze white, it is tapped from the furnace.
Furnace addition of ferrosilicon and ferro-manganese may be employed if it is
necessary to adjust the analysis of the iron. Tapping is usually done at 2800
to 2900 F, and pouring occurs at 2600 to 2800 F, depending on casting-section
thickness. Tapping in air-furnace heat may require from 30 min to over an hour,
depending on the furnace size and pouring facilities.
ENGINEERING PROPERTIES
The tensile properties of malleable iron and other
mechanical properties are tabulated below:
|
Bhn
|
110-145 (115-135 usual range)
|
|
Endurance ratio
|
0.40-0.575
|
|
Notch endurance ratio
|
0.35
|
|
Modulus of elasticity in tension
|
25 x 104 psi
|
|
Shear strength
|
0.80 UTS
|
|
Compressive strength
|
Greater than UTS
|
|
Impact resistance
|
6.5-16.5 ft-lb, depending on test conditions
|
|
Machinability rating
|
120%
|
The strength of malleable iron combined
with its ductility makes it suitable for many applications. Probably its
greatest engineering value rests in the combination of its mechanical
properties, service life, cost, and suitability to many fabricating and
processing operations. Among these advantages are:
1. Machinability.
Malleable iron is among the most machinable of ferrous alloys. Especially
desirable is the fact that a high degree of uniformity of machinability in
large numbers of castings can be maintained because every casting has been
heat-treated.
2. Ductility
in processing. Many processing operations such as coining, crimping, press
fits, punching, and straightening can utilize or require ductility.
3. Ductility
or toughness in service. Many applications are best served when the casting is
capable of deforming rather than fracturing when overstressed. Clamps,
pipe-fitting threads, chain links, tractor bolster posts, and many other cases
may be cited.
4. Surface
coatings. Corrosion resistance of malleable iron may be greatly increased by
coatings of zinc, cadmium, aluminium, and lead. Hot dip galvanizing may be
applied to clean malleable castings to provide good corrosion resistance to
exposure in a wide variety of outdoor conditions which may be encountered by
electrical conduct boxes and fittings fence fixtures, playground-equipment
castings, and numerous other applications.
5. Wear
resistance. Malleable iron with a ferritic structure does not have inherent
wear resistance other than that normal to soft-ferrous alloys. It may be
hardened, however. If the metal is heated to the austenitic temperature range,
carbon goes back into solution and permits a hard martensitic structure to be
obtained by quenching. Caster wheels, cams, rollers, and other items may be
flame-or induction-hardened to give wear resistance.
6. Magnetic
properties.
RESIN CHARACTERIZATION
MECHANICAL PROPERTIES OF A PARTIALLY CURED RESIN -- DMA
CHARACTERIZATION
A Du Pont 983 DMA was used for all
mechanical measurements of resin samples. In tests using this DMA, the sample,
a special 0.2 by 12.5 by 35 mm nonwoven glass filter cloth impregnated with
resin, is clamped horizontally between the ends of two parallel arms (Figure
2). One arm and the sample are driven to oscillation at a
prescribed amplitude by an electromagnetic drive. Energy dissipation by
the sample causes the actual sample strain to be out of phase with the driver
signal. The instrument detects this time shift as a phase angle and
calculations are made to determine the storage modulus E', a measure of the
material's stiffness, and the loss modulus E", a measure of the material's
viscosity. The ratio of these two properties, E"/E', is designated tan
delta, which is useful in determining the extent of resin mechanical cure.
In practice, the resin-impregnated glass
cloth sample is first conditioned at room temperature in a humidity chamber
(for the rest of the paper called precure conditioning). The sample is then
"precured" for a period of time in an atmosphere of controlled
temperature and humidity. After reconditioning (for the rest of the paper called
pre-DMA conditioning) at 91% relative humidity (RH) over a barium chloride
solution, the sample is tested in the DMA to determine initial stiffness and
its reaction to further heating while in the DMA.
Fig. 1 Du Pont 983 dynamic mechanical
analyzer.
Fig. 2 Close-up of the dynamic mechanical
analyzer showing the sample-holding arms.
There were numerous difficulties
encountered in developing the testing technique. Substrate selection, resin
application, resin shrinkage, clamping mode, clamping torque, oscillation amplitude,
fixed and resonant frequency modes of DMA operation, and isothermal and
isochronal heating have been discussed in detail elsewhere. The precure
conditioning is necessary to prevent sudden resin expansion or shrinkage with
resultant crazing when the sample is exposed for a short time to the desired
environmental conditions. For a dry exposure, the sample is conditioned over
phosphorous pentoxide to bring it close to 0% moisture content. The pre-DMA
conditioning at 91% RH plasticizes the sample, enhancing
the response of the partially cured resin to DMA conditions and accentuating
differences caused by precure treatments.
Storage modulus E' curves for two samples
precured for two minutes at two temperatures are shown in Figure 3. Typically,
a sample precured at the higher temperature (160°C) showed a higher initial
modulus. However, after exposure to further heat in the DMA, in this case at a
constant temperature of 150°C, the sample cured at the lower temperature
(130°C) attained a higher ultimate modulus. It was hypothesized that this
phenomenon was caused partially by the lengthening of molecular chains in the
precure exposure which reduced their mobility and their subsequent ability to
cross-link during the final curing process in the DMA.
Heat-induced softening, counteracted by a
loss of moisture, determined the shape of the storage modulus curve prior to
the point where the glass transition and storage modulus rise due to rapid
molecular cross-linking. Previously, the counteracting effects of heat softening
and moisture loss have hindered the expansion of DMA techniques to
solvent-based adhesives. Considerable progress in following resin cure has been
made due to the refinement of specific procedural techniques, such as substrate
selection and pre-DMA conditioning at high RH. Additionally, in the pursuit of
data interpretation, the focus has been shifted from the storage modulus E' to
tan delta.
The area under the tan delta curve is
related to the development of mechanical stiffness. Tan delta curves for a
resin exposed to 115°C for three precure times are shown in Figure 4. The
areas under the curves indicate the extent to Fig. 3 Storage modulus, Resin A
precured at two temperatures; temperature in the dynamic mechanical analyzer
was 150°C.
Fig. 4 Tan delta, resin A precured
at 115°C for three time periods.
Fig. 5 Mechanical cure from tan delta
area, precure at 115°C.
Fig. 6 Mechanical cure from tan delta
area, precure at 140°C.
Which further development of mechanical
stiffness is possible and , as such, decrease with
longer precure exposures.
In Figure. 5 the
tan delta area has been plotted as a function of precure time at 115°C for
two different phenolic resins. Compared with resin A, resin B had a higher
molecular weight and contained less free formaldehyde and more NaOH at
115°C resin B reacted at a much lower rate than did resin A. However, after
precuring at 140°C, both resins developed mechanical properties at
relatively fast rates, indicated by a rapid decrease in the tan delta areas (Figure
6).
Chemical Advancement of a Partially Cured
Resin—Differential Scanning Calorimeter CharacterizationTo determine how
resins differ in their chemical response to environmental conditions, a
Perkin-Elmer DSC-2 differential scanning calorimeter (DSC) was used to test a
partially cured sample trimmed from the DMA specimen immediately following
precure exposure and reconditioned to 91% RH. A 10 mg sample was placed in a
stainless steel capsule that had been fitted with an O-ring to prevent release
of volatile components. The partially cured sample was compared to an empty
control capsule. The difference in energy supplied to maintain a constant
temperature rise of 10°C/min in each capsule was a measure of the
exothermic reaction remaining in the partially cured resin and thus, the
chemical response of the resin to heat. The weight-corrected area under the
exothermic heat curve decreased with precure exposure time.
The chemical responses of two resins
precured at 115°C and 140°C are shown, respectively, in Figures 7 and
8. Unlike their mechanical responses (Figure 5 and 6), the chemical responses
of these two resins maintained the same general relationship at both
temperatures.
Chemical-Mechanical Relations
The extent of mechanical cure of a
partially cured resin sample can be expressed as the difference between the
area under the DMA tan delta curve for the partially cured sample (Ac) and that
for an uncured sample (Au), expressed as a percentage of the area under the tan
delta curve of the uncured sample:
Percentage cure = [(Au - Ac)/(Au)]
100 Likewise, percentage chemical cure can be obtained from the areas under the
exothermic curves for partially cured and uncured DSC test samples. The
relationship between percentage mechanical cure and percentage chemical cure at
115°C for resins A and B is shown in Figure 9. This type of plot is
referred to as a chemical-mechanical response curve and it has been found to be
very useful in comparing the responses of different resins to various
environmental exposures.
The chemical-mechanical relations of the
two resins at 115°C showed relatively slight differences. That is, a small
increase in chemical cure (12%) caused a large increase in mechanical cure
(60%) for both resins. However, the rate of increase, as denoted by the time
(in minutes) associated with each data point of both chemical and mechanical
characteristics, was much less in resin B. Where resin A
attained 85% of its full mechanical cure in five minutes, the mechanical cure
of resin B did not exceed 50% even after 20 minutes of exposure.
Fig. 7 Chemical cure from residual DSC
heat retention, precure at 115°C.
Fig. 8 Chemical cure from residual DSC
heat retention, precure at 140°C.
The relative position of the chemical
mechanical curves for these two resins changed quite drastically at 140°C
(Figure 10). At this higher temperature, the mechanical cure of resin B
preceded its chemical cure to an even greater extent then was observed at
115°C. Maximum mechanical cure was developed within five minutes in both
resins. Chemical reactions for resin B, although generally faster and more
extensive at 140°C than at 115°C, lagged considerably behind those
observed for resin A. A precure period of 5-8.5 minutes was necessary to attain
20% chemical cure for resin B, where the same degree of chemical cure was
attained after only one minute of precure for resin A.
Moisture Content as a Variable
During initial developmental work, precure
exposures were limited to oven dry conditions. Later experiments incorporated
moisture content as a variable in the precure environment. The samples were
exposed to controlled conditions of temperature and humidity in a specially
designed treatment chamber (Figure 11). The desired conditions below 100°C
were obtained by the additional heating of an airstream that had been saturated
with moisture at a controlled temperature (Figure 12A). At these operating
conditions, the chamber was assumed to be at atmospheric pressure. Conditions
above 100°C were obtained by superheating saturated steam and controlling the
final temperature and pressure in the treatment chamber (Figure 12B). The
superheater was capable of reaching temperatures of 200°C, and the chamber
was designed to withstand a maximum pressure of 150 lb/in2
(1.034 Mpa). This allowed the research to obtain a RH of 62% at
200°C. and a RH of 100% at any temperature below
180°C. Because the chamber had to be decompressed and opened to insert the
specimen, a lag of up to one minute occurred before the equipment stabilized at
exposure temperatures above 100ºC. An air lock will be constructed to
minimize this time lag in future studies.
Fig. 9 Chemical-mechanical curves, precured at 115°C,
dry.
Fig. 10 Chemical-mechanical curves, precured at 140°C,
dry.
Test procedures were modified to condition the test sample
to the same RH at which it was exposed during precure. All precured test
samples were then reconditioned to 91% RH prior to testing in the DMA. Figure
13 compares Fig. 11 Environmental chamber.
Chemical-mechanical curves for resin A,
precured at 115ºC and at RH levels of 41 and 91%, and those obtained at
dry conditions. Increasing the humidity to 41% promoted faster chemical
response, but had little effect on the development of mechanical properties.
Further increasing RH to 91% did not alter appreciably the rates of either
chemical or mechanical cure beyond those at 41% RH.
In contrast to observations for resin A,
increasing the humidity to 41% dramatically increased the mechanical cure of
resin B (Figure 14) with only slight increases in chemical cure. Increasing the
RH to 91% hastened the early development of mechanical properties and
furthermore, increased the extent of chemical cure at all periods of time that
were investigated.
Chemical-mechanical curves clearly provide
a good measurement of a resin's sensitivity to moisture in the curing
environment. Chemical changes measured by the DSC are a summation of all the
reactions that occur during testing; the data do not indicate the type of
chemical reactions that are taking place. Addition of moisture may affect the
type of chemical-reactions and, therefore, the change in rate of mechanical
strength development. The study information indicates that molecular
immobility, whether caused by precure exposure or by pre-DMA conditioning at
low RH, reduces the formation of cross-linking reactions that occur in a curing
resin. The molecular properties of the uncured resins were analyzed using GPC,
NMR, and FTIR techniques. It is planned that the FTIR data will be expanded to
include information on the structure of partially cured resins.
Flake Bonding
The curing mechanisms characteristic of a
resin are important only in that they determine the formation of an adequate
bond. Those environments that enhance resin curing do not necessarily provide
the best situation for bonding. A technique to follow the strength development
of an adhesive-bonded joint is needed. Humphrey measured the effects of time,
temperature, and moisture variations on the bonding of two wood wafers that
were still hot. This research was extended using two flakes bonded in a
lap-shear configuration. These tests are useful in determining the ability of a
composite board to resist delamination forces when the press is opened. A
procedure will be investigated that will indicate the shear strength of a bond
that is in a cooled condition.
The technique utilizes two flakes bonded
and tested in lap-shear configuration. The bond can be formed easily in the
environmental chamber, which is equipped with two opposing air-operated
cylinders capable of applying 792 kg. This is sufficient to obtain a pressure
of 40.2 Mpa on the 200 mm2 area being bonded. Steam or conditioned air is
introduced to the chamber through the faces of the pressure platens. The 0.8 by
15 by 75 mm flakes are cut with a microtome from water-saturated quarter-sawn
blocks of aspen lumber. The knife is positioned at a slight angle to the grain
and moves in a direction perpendicular to the grain. The flakes are dried under
glass plates to prevent any warp or curling. Resin is applied in a 15 mm-wide
strip across the end of one flake. Following a short open assembly time the
second flake is positioned on the adhesive-coated flake and the assembly is
bonded in the environmental chamber under exposure to the chosen conditions.
Also, the flake pair can be bonded easily within, and retrieved from, a pressed
board. Testing is accomplished in a standard tension tester. The flake-bonding
work to date has been exploratory in nature with emphasis on the development of
a technique.
Initial results indicate that the method
will be useful in determining the influence of environment on the development
of bonds by specific resins.
Measurement of Pressing Environments Defined many of the basic relationships between board
pressure, temperature, and vapor pressure in conventionally pressed boards.
Temperature and vapor pressure in these boards changed relatively slowly. Vapor
pressure differentials throughout the thickness and width of the conventionally
pressed board were slight and lasted for relatively short periods of time.
Temperatures and vapor pressures in steam-injected boards, on the other hand,
changed rapidly and drastically. Vapor pressure differentials throughout the
board may be large and remain for a relatively long time.
Resin Penetration
The lapped flakes used in testing adhesive
bonds can be sectioned readily to determine resin penetration. Work begun at
the Forest Products Laboratory in this area is now being continued at VPI. A
technique has been developed to dye a microtomed section of the flake pair and
observe the bond line under a fluorescent light microscope. The technique works
well with phenolics and isocyanate resins. Photographs of the bond line are
digitised in an image analyzer, and the data are used to quantitatively
determine the degree of penetration.
Practical Application
Each of the seven discussion subjects
described so far in this paper are, by themselves, interesting from an academic
standpoint. Combined, their value increases. In conjunction with other modern
chemical and molecular analysis such as FTIR and NMR techniques, the
information provides extremely useful guidelines in synthesizing a resin for a
specific application or in modifying or controlling a process within the parameters
defined by the resin's characteristics. As with any characterization process,
the information describing a specific resin is more meaningful as the data base
expands. It is hoped that the knowledge of resin behavior will be broadened by
investigating other phenolics, ureas, and isocyanates. Incorporation of new
techniques such as dielectric or sonic cure measurements may also prove
beneficial. Finally it may be useful, especially from the standpoint of
improved dimensional stabilization, to combine a resin's curing an bonding characteristics with information describing the
viscoelastic reaction of the wood substrate under the same environmental
conditions.
WETTABILITY AND WATER REPELLENCY OF
WOOD
Water repellency of wood is important in
outdoor uses of wood and wood-based products. Use of effective water repellents
on wood has been linked to improved performance of wood outdoors. Conversely,
good wetting is required in operations involving coatings, adhesives, and some
types of protective treatments that require penetration of wood by liquids.
Efficient wetting of wood by adhesives has been correlated with strength of
adhesive bond.
Wettability is usually defined as the
contact angle that a liquid makes when in contact with a solid surface. From
contact angle data, surface free energy of a solid can be estimated. Good water
repellency is the special case of poor wettability (large contact angle). Wood
is both hygroscopic and briefly water repellent. Direct contact angle
measurements, tilted plate, and capillary rise techniques have been used to
study forces acting on liquids in contact with wood surfaces, but all involve
experimental complications, and are relatively slow. A rapid, reliable
procedure for measuring wettability and related properties on wood and on wood
products was clearly needed, and we devised one. A modified wilhelmy method had
been used to determine the forces acting on immersed fiber samples. Casilla, used a larger, conical wood sample to obtain a
"wettability index". We used that procedure as a starting point, and
combined an automatic surface tension apparatus with a microcomputer for
control, data collection, and processing, for the study of wood surfaces.
EXPERIMENTAL
Wood materials
Green logs of various species were obtained
immediately after harvesting and were stored at 2°C prior to sawing into
edge-grain boards about 1.3 centimeters (cm) thick. The wood was kiln dried
below 70°C. The boards were planed, and sawn into 3x 0.64 cm sticks.
Automated surface tension analyzer
Surface tension measurements of liquids
and determinations of forces on wood samples during immersion in liquids were
done with a Fisher Model 215 Autotensiomat Surface Tension Analyzer that was
interfaced with an IBM-PC-XT microcomputer using Metrabyte Corporation Dash 16
multifunction high-speed analog to digital expansion board.
The analyser consists of a strain
sensitive wire affixed at one end to the balance beam and the other to a
transducer. The transducer's signal is proportional to the load. The output of
the analyzer is 0 to 1 mv which is calibrated to equal 0 to 100 dynes/cm. This
signal is amplified and filtered; noise above 2 HZ is attenuated before
entering the Metrabyte Dash-16 Analog to Digital interface board installed in
the IBM-PC-XT.
The elevator in the surface tension
analyzer is controlled by the digital output port of the Dash-16.
Computer program: wood wettability study
The BASIC program automates experimental
procedures, and is divided into three major modules: Surface Tension, Run New
Sample, and Analyze Data, and one major submodule, Graph.
The Surface Tension module has two main
features: electronic calibration and automatic apparatus control. Surface
tension of a liquid is determined, and the value is filed. Run New Sample
routine calibrates and controls the apparatus. The collected data is filed
under a name based on sample statistics. Analyze Sample Data module retrieves
file to be analyzed and reads it into computer memory. The main module has
sub-modules: Graph, Calculation of Contact Angle from Attractive Force, from
Work of Adhesion, Estimate of Surface Free Energy, Calculation of Interaction
Factor, and Data Summary.
GRAPH
The Graph
submodule features variable axis scaling, and rescaling that enlarges sections
of the graph. Figure 1 is the graph of a typical wood sample run.
Fig. 1 Force (dynes) _ vs _ time (seconds).
Contact angle from attractive force
The contact angle is calculated by use of
the Wilhelmy equation:
Attractive force = gL P cos q
were
gL = surface tension of the
liquid,
P = the perimeter of the wood sample, cm, and
q = the contact angle.
The point of initial contact of the wood
with the immersion liquid is located, and correction for buoyancy is made. This
gives the attractive force at zero immersion from which the contact angle, q,
is determined.
Contact angle from work of adhesion
The contact angle, q, is calculated with the Young-Dupre
equation:
Work of adhesion=gL (1+ cos q)
The work of adhesion is obtained from the area under
immersion curve.
Surface free energy estimation
The well-known Zisman critical surface
tension gC, was determined by plotting the cosine of
the contact angle on a wood surface versus the surface tension of a series of
liquids. The point at which cos q =1 (zero contact angle) line is intercepted,
determines gC. Mixtures of water with ethylene glycol
and with glycerol provided a range of surface tensions. End grain of wood
samples was sealed with a small amount of beeswax to prevent rapid capillary
flow of liquids into wood samples.
Interaction parameter calculation
The interaction parameter, was
calculated according to the procedure used by Good:
gS = surface tension of solid
Aging effect
Wood samples, cut at the same time, were
stored under constant 27°C, 30% relative humidity conditions for several
weeks; elapsed time shown denotes length of exposure of exposure to laboratory
illumination and atmosphere. Each point represents a wood sample cut from the
same board.
FLAME RETARDANT TREATMENT OF WOOD
Wood has many inherently good properties,
which make it a preferred building material for many applications. However, the
flammability of wood can be a problem when wood is used to build permanent
structures. The tendency to burn can be greatly reduced by adding
fire-retardant chemicals to wood. However, in many instances, these chemicals
introduce other problems, including increased hygroscopicity and corrosivity,
and reduced adhesive bonding. These problems occur especially when inorganic
salts are used as fire retardants.
A great deal of research has been done to
develop treatments that make wood flame retardant but free of the problems
associated with leachable inorganic salts. One such treatment is the amino
resin system, which uses chemicals such as dicyandiamide, melamine, urea,
formaldehyde, and phosphoric acid. The amino resin fire retardants greatly
reduce or eliminate hygroscopicity, corrosivity, chemical blooming, and
leaching. Leach resistance is attributable to polymerization of the components
within the wood and possibly to some reaction with the cellulose in wood.
Despite the reported leach resistance of the amino resin treatment, Juneja
reports that water leaching can remove as much as 91% of the phosphorus from
shingles treated with dicyandiamide-phosphoric acid-formaldehyde and 71% of the
phosphorus from shingles treated with melamine-dicyandiamide-phosphoric
acid-formaldehyde.
Urethane foams
are commonly made flame retardant by reaction of a phosphorus polyol with an
isocyanate. Malz prepared flame-retardant compounds by reacting chemicals
having free hydroxyl groups with cross-linking agents such as isocyanates. Von
Bonin propose flame-proofing absorbent substrates used in roofing and packaging
with a mixture of a polyisocyanate and a condensate that contains phosphorus
and has two or more hydroxyl groups. Isocyanates have been reacted with wood to
increase its dimensional stability and decay resistance.
Chemicals that contain phosphorus change
the thermal degradation processes in wood. They are effective as flame
retardants because they reduce the temperature at which pyrolysis occurs and
increase the amount of residual char. Phosphorus compounds are acid precursors
during combustion or pyrolysis, and the acids formed cause selective decomposition
of the carbohydrate materials. Dehydration and char formation are enhanced, and
combustible volatile formation is suppressed.
Objectives of our research were to develop
new fire-retardant treatments for wood that are leach-resistant and, at the same
time, not hygroscopic or corrosive. To accomplish these objectives, an
aliphatic diisocyanate and an oligomer phosphonate were mixed in an appropriate
solvent, impregnated into the wood, and cured with heat. Reaction of the
diisocyanate with the oligomer phosphonate and the wood should enhance leach
resistance.
Preparation of specimens
Specimens of ponderosa pine and southern
pine were cut into 25 by 25 by 6 mm (radial by tangential by longitudinal)
pieces for dimensional stability tests. Specimens 140 by 7 by 3 mm were cut for
leaching, thermogravimetric analysis, and elemental analysis. All specimens
were oven-dried at 105 C before weighings and reactions.
Treatment of specimens
Specimens were placed in a container
inside a desiccator, and a vacuum was drawn for 1 h using a water aspirator.
Specimens were then covered with solution and the pressure returned to
atmospheric. After a 1-h soak, the solution was drained and a vacuum drawn for
10 min to remove excess chemicals and solvent. The specimens were then cured
overnight at 105 C, reweighed, and the weight percent gain (WPG) calculated
based on the original oven-dry weight. Ten specimens of each size were treated
at each level of treatment.
Dimensional stability tests
Dimensional
stability was determined using a flat bed micrometer by measuring the increase
in volume (swelling) of treated and untreated specimens. Specimen volume was determined oven-dry and after specimens were
soaked in water. The volume was measured after each hour for the first 5 h,
then after 24 h. The percentage of swelling was calculated from the wet volume
of the specimen compared to its treated oven-dry volume.
Thermogravimetric analysis
Thermogravimetric analysis
(TGAs) were done using a Perkin Elmer TGS-2 system. Specimens were
pyrolyzed in a flow of nitrogen (40 ml/min). Pyrolysis temperatures were
programmed from 30 to 600 C at 20 C per minute. The specimen weight remaining
at 600 C was used to calculate the percentage of residual char. The temperature
at maximum rate of pyrolysis was recorded.
FUNGAL AND TERMITE RESISTANCE OF WOOD
Protecting wood against attack by fungi
and termites by methods based on modification of the cell-wall polymers has
been investigated at the Forest Products laboratory for several years. Most of
this research has dealt with the bonding of reactive organic monomers to the
hydroxyl groups on lignin, hemicelluloses, and cellulose. More recently we have
investigated chemicals that form stable complexes with cell-wall hydroxyl
groups and are resistant to water-leaching.
Two chemicals that form stable complexes
with wood are periodic acid and sodium periodate. Periodic acid is known to
react with diols in carbohydrates, with cleavage and oxidation taking place
between the two diols. This method has been used for many years to determine
carbohydrate structure. Even though these chemicals have been reacted with
isolated carbohydrate polymers and monomers, their reactions with whole wood
have not been reported.
In preliminary investigations, we found
that both periodic acid and sodium periodate reacted with wood to form
leach-resistant complexes. Little oxidation of the cell-wall polymers took
place, as evidenced by infrared spectroscopy.
The purpose of this research was: (1) to
determine the resistance of reacted wood to brown-and white-rot fungi and
subterranean termites in standard laboratory tests, and (2) to study the
reactions of periodic acid and sodium periodate with wood.
MATERIALS AND METHODS
Fungal evaluations
Loblolly pine or sweetgum sapwood blocks
1.9 X 1.9 X 1.9 cm (radial X tangential X longitudinal) in size were selected
according to the American Society for Testing and Materials (ASTM 1976)
standards, with three to four annual rings per centimeter. Blocks were reacted
with aqueous solutions of either periodic acid or sodium periodate at six
concentration levels: 1% 0.5%, 0.25%, 0.1% 0.05%, and 0.01.%
(w/w).
For each test, wood blocks were placed in
a vacuum chamber for 1 h at 16 to 22 mm Hg. They were then impregnated with one
of the six aqueous solutions. Blocks treated with periodic acid were soaked for
2 h, those treated with sodium periodate for 4 h. After soaking, seven blocks
per treatment were dried under a hood for 1 day and then conditioned at 27 C
and 30% relative humidity (RH) for 3 weeks. Another seven blocks per treatment
were leached in 350 ml of distilled water daily for 2 weeks. After leaching,
the leached blocks were also conditioned at 27 C and 30% RH for 3 weeks.
Soil-block fungal decay tests were run
according to the ASTM standard. Gloeophyllum trabeum, a brown-rot fungus, was
used with loblolly pine blocks, and Coriolus versicolor, a white-rot fungus,
was used with sweetgum blocks. Five replicate blocks from each treatment and
five control blocks were tested for decay resistance over a period of 12 weeks.
The extent of fungal attack was determined by weight loss. Solution retention
concentration that resulted in weight loss by decay of less than 2% was
generally considered as the threshold retention.
Termite evaluations
Loblolly pine sapwood blocks 0.4 x 2.5 x
2.5 cm (tangential x radial x longitudinal) in size were selected according to
the ASTM standards with four to six annual rings. Blocks were reacted with
aqueous solutions of either periodic acid or sodium periodate at five
concentration levels: 5%, 1%, 0.5%, 0.25%, and 0.1% (w/w); in addition, 10%
concentration was included for the periodic acid solution.
For each test, eight blocks were placed in
a vacuum chamber for 20 min at 16 to 20 mm Hg. They were then impregnated with
one of the six aqueous periodic acid or five sodium
periodate solutions and soaked in the treating solution for 24 h. After
soaking, four blocks per treatment were dried under a hood for 1 day and then
conditioned at 27 C and 30% RH for 3 weeks. Another four blocks per treatment
were leached in 200 ml of distilled water daily for 2 weeks. After leaching,
the leached blocks were also conditioned at 27 C and 30% RH for 3 weeks.
Reticulitermes flavipes for this study
were freshly collected at Janesville,
WI. Each treated and control
block was exposed to 1.0 g of termites (natural caste mixture averaging 270
undifferentiated functional workers, 1 soldier, and 0.3 nymph).
Termite resistance of the treated wood was evaluated for a period of 4 weeks.
The extent of termite attack and mortality were determined by weight loss of
the blocks and final live weight of the termites. Threshold retentions based on
weight loss and termite mortality were determined.
Reaction time and chemical analysis
Fourteen loblolly pine blocks (1.9 X 1.9 X
19 cm) were placed in a vacuum chamber for 1 h at 16 to 22 mm Hg. They were
then impregnated with 0.1% periodic acid solution and soaked at room
temperature for 2,8, and 24 h. After soaking, seven
blocks per treatment were dried under a hood for 1 day and then conditioned at
27 C and 30% RH for 3 weeks. Another seven blocks per treatment were leached in
350 ml of distilled water daily for 2 weeks. After leaching, the leached blocks
were also conditioned at 27 C and 30% RH for 3 weeks. Five replicate blocks
from each treatment and five control blocks were tested for fungal decay
resistance by G. trabeum over a period of 12 weeks. The extent of fungal attack
was determined by weight loss.
RESULTS AND DISCUSSION
Decay resistance
For wood reacted with periodic acid,
threshold retention with G. trabeum was 0.26% in both unleached and leached
blocks; with C. versicolor, threshold retention was 0.05% in unleached and
0.11% in leached blocks (Table 1). Leaching of periodic acid-reacted blocks did
not decrease G. trabeum decay resistance but did decrease C. versicolor decay
resistance.
For wood reacted with sodium periodate,
threshold retention with G. trabeum was 0.26% for unleached blocks and 1.05%
for leached blocks; with C. versicolor, threshold retention was 0.11% for both
unleached and leached blocks (Table 1). Leaching of sodium periodate-reacted
wood caused a fourfold decrease in G. trabeum decay resistance but did not
decrease C. versicolor decay resistance.
For termite tests, threshold retention for
periodic acid-reacted wood with R. flavipes was 1.4% in unleached blocks and
14% in leached blocks (Table 2). Leaching of periodic acid-treated blocks
caused a tenfold decrease in termite resistance. Threshold retention for sodium
periodate-reacted wood with R. flavipes was 6.8% for unleached blocks and was
not determined for leached blocks.
The effect on weight loss of soaking
loblolly pine blocks in 0.1% periodic acid solution for various times was
investigated for G. trabeum. For control blocks, weight loss with G. trabeum
was 61.8%. By soaking wood blocks in 0.1% periodic acid solution for 2 h,
weight loss with G. trabeum was reduced to 32.1% (Table 3).
Weight loss was further reduced from 14.6%
to 5.4% as length of soaking increased from 8 to 24 h, respectively. This means
that the fungal decay resistance of periodic acid-reacted wood can be improved
substantially by lengthening the reaction time from 2 to 24 h.
Chemical analysis
Chemical analysis of periodic acid-reacted
wood showed that periodic acid reacted with wood in the blocks within 2 h and
remained in the wood even after 2 weeks of water-leaching. For example, after
treatment with 0.1% periodic acid solution, chemical analysis showed 90 mole percent of periodic acid in the wood (Table 4). This
means that 90 mole percent of periodic acid reacted with wood to form
leach-resistant products. At 1% periodic acid solution, 114 mole percent of
periodic acid was found in wood (Table 4). This more than 100% value may
indicate that periodic acid is able to react with wood quickly (within 2 h) to
form stable complexes with cell-wall polymers of wood. Therefore, the reaction
of periodic acid with wood is not merely a deposition of periodic acid in wood.
Rather, in involves bond formation between periodic acid and polymers of wood,
particularly the more accessible and configurationally more favourable
hemicelluloses and diols in lignin polymers. The C-2 and C-3 vicinal diols of
mannopyranose-containing hemicelluloses in wood are in cis configuration, which
is favourable for the bonding of periodic acid to these polysaccharides. At low
concentration of periodic acid, there was no change in the infrared spectrum in
the carbonyl region. At the 10% concentration level, a slight increase in the
carbonyl region was evident. The high percentage of periodic acid found in wood
even after leaching could explain why leached periodic acid-reacted blocks were
as effective as unleached blocks in preventing attack by brown-and white-rot
fungi. The high iodine content (0.021%) in wood being treated with 0.01%
periodic acid (Table 4) may be due to a greater error in chemical analysis when
the samples have a very low concentration of periodic acid.
WEATHERING OF WOOD
Wood is an extremely durable material even
under adverse conditions, but the durability depends upon the environment (Fig.
1). Buried deep under ground, fully exposed to the weather, submerged under
water, or hidden in an ancient tomb, it can last for tens of centuries. There
are many examples of structures, ships, and other wooden objects which have
survived centuries of use. The same type of wood exposed to an unfavourable
environment, however, may vanish almost without a trace within a year or two.
In the tropics, a wooden house may under certain conditions disintegrate in a
few years.
Like other biological materials, wood
consists of organic substances, primarily polysaccharides and polyphenolics:
cellulose, hemicelluloses, and lignin. Extractives are also present in
relatively small quantities and their concentration determines color, odor, and
other properties of a wood species.
The degradation of wood by any biological
or physical agent modifies some of its organic molecules. The cause of the
change may, for example, be an enzyme, a chemical, or
electromagnetic radiation, but invariably the net result is a change in
molecular structure through some chemical reaction. Stalker conveniently
divided the environmental agencies that bring about wood degradation into
categories. "physical" forms of energy were
used to describe all factors other than fungi, insects, or animals. In Table 1,
the importance of the various destructive agents on wood can best be considered
by comparing two situations, inside and outside wood structures. The most
serious risk to wood indoors comes from the intense heat of an accidental fire.
Outdoors, the factor most deserving of attention is
weathering, a complex combination of chemical, mechanical, and light energies.
Fig. 1 Old Fairbanks house at Dedham, Massachusetts.
Built in 1637, most of the white pine clapboard siding was replaced in 1903 and
has stood 75 years without paint.
Weathering is not to be confused with
decay, which results from the presence of excess moisture for an extended
period of time. The condition of decay can lead to rapid deterioration of the
wood and result in a phenomenon far different than that observed for natural
outdoor weathering.
Weathering Factors
Action of water.
The principal cause of weathering is frequent exposure of the wood surface to
rapid changes in moisture content. The action of water on wood has been
thoroughly described. Rain or dew falling upon unprotected wood is quickly
absorbed by capillary action in the surface layer of the wood, then adsorbed within wood cell walls. Water vapor is taken
up directly by adsorption under increased relative humidities. Adsorbed water
has been shown to virtually add its volume to that of the cell walls, resulting
in swelling. Stresses are set up in the wood as it swells and shrinks as a
result of moisture gradients between the surface and the interior. These
induced stresses are greater the steeper the moisture gradient, and are usually
concentrated near the surface of the wood. When unbalanced, they may result in
warping, cupping, and face checking. Grain raising results
from differential swelling and shrinking of summerwood and springwood.
Fig. 2 White oak log cabin near Middleton, Wisconsin,
constructed about 1845 and never painted or finished.
Fig. 3 Close-up view of weathered white oak logs in Fig. 2.
Action of light. The
photochemical degradation of wood or wood-related materials has been reviewed
in several publications. It was recognized quite early that the initial color
change of wood exposed to sunlight was a yellowing or browning. The graying of
wood occurs after browning and was thought to be related to iron salts.
Sunlight, particularly the ultraviolet (UV) end of the spectrum, degrades the
organic materials in wood; lignin decomposes preferentially to a relatively
shallow depth of 0.05-0.5 mm. Photo degradation by UV light induces changes in
chemical composition, particularly in the lignin.
Browne found that infrared light
penetrated deeper than visible light, while the penetration of UV light was
negligible. Stout using reflectance curves showed that absorption of UV light
is primarily due to lignin and lignin like substances. Pine cellulose exhibited
a high reflectance, whereas the reflectance curve for lignin substances closely
approximated that for wood. Sandermann and Schlumbom made a comprehensive study
of color changes of numerous wood species. In another study, decomposition by
UV light, as indicated by the coloring process of wood during the first several
hours of exposure, appeared to be independent of ambient atmosphere; exposed
wood samples darkened with or without oxygen in the environment. Desai
described the photo degradation process for cellulose.
It is important to note here that the two
most significant elements of weathering-light irradiation and water-tend to
operate at different times. Exposed wood can be irradiated after having been
wet by rain or when surface moisture content is high from overnight high
humidity or from dew. Time of wetness, therefore, is an important parameter in
relating climatic conditions to exterior degradation. The action of the
combined elements can follow different degradation paths, with irradiation
accelerating the effect of water or the converse.
Action of heat.
The role of temperature in the natural weathering process is generally felt to
be of less importance than those of light and water.
ARCHITECTURAL PAINTS
Architectural paints are used as protective
and decorative coatings on structures such as homes, offices, and factories.
The many types of architectural paints may be grouped in the following four
general classifications:
1. Exterior paints for wood.
2. Interior paints for plaster and wallboard.
3. Exterior paints for cement blocks, stucco, and masonry.
4. Floor paints for wood and concrete.
In some cases architectural paints are
sold by the manufacturer directly to the consumer, and in other cases they are
marketed through trade outlets such as paint dealers and hardware stores. In
the latter case the paints are referred to as "trade sales items."
The two principal groups of consumers for architectural paints are painting
contractors and the general public. The contractors must figure costs
carefully; therefore, they are interested in actual price per gallon, square
feet of surface covered per gallon, minimum number of coats to obtain
satisfactory appearance, and case of application. The general public, or the
"do-it-yourself" painters, primarily want ease of application with
results that look like professional work. The price per gallon is of secondary
importance because they save the large item of application cost. The resin
emulsion or so-called "latex" paints have made a strong appeal to the
general public because of their case of application and the ease of removing
paint spots or cleaning brushes and rollers with soapy water. In most cases the
actual cost per square foot of surface painted is higher with the latex paints
than with the oil or resin paints.
It has long been recognized by the paint
industry that durability and performance of architectural paints are determined
not only by the composition of the paint but also by factors such as condition
of the surface on which the paint is applied, structural characteristics and
conditions of occupancy in the building, weather conditions during application,
and proper application of the paint according to the instructions of the
manufacturer. Premature failure of paint frequently is due to one or more of
these factors, but it may be difficult to prove this when a complaint is made a
long time after the paint was applied. For example, some types of oleoresinous
paints do a remarkably good job of binding the loose chalk on masonry surfaces,
but other types of paint have poor adhesion to such surfaces and may fail
quickly. Structural characteristics which permit water to accumulate behind
paint coatings may be the cause of blistering, cracking, and peeling of the
coating. This type of failure also may be caused by relatively large amounts of
water vapor passing through the walls from the inside of the house. The vapor
will condense and accumulate behind the coating in cold weather, and the water
tries to escape through the coating. The coating may swell and lose adhesion,
and a slight pressure may be developed which blisters the coating and finally
causes it to crack and flake off. On wooden structures this condition may be
remedied by proper ventilation in the house and use of sheet materials as moisture
barriers or by application of barrier coatings to the interior walls.
Ventilation behind the siding on exterior walls may be used in extreme cases,
but this interferes with good thermal insulation.
Painting contractors with many years of
experience are apt to think that modern ready-mixed paints can be
"improved" by addition of oil or other material. This practice stems
from the time when the contractor mixed his own paint, but it can produce
disastrous results with the carefully balanced composition of good ready-mixed
paints. It is also a common fallacy to believe that second grade paints is
"just about as good" as first grade paint. At present, competition is
too keen to permit putting different labels on cans of the same paint; therefore
the lower price second grade paint is probably an inferior product. Since cost
of application is by far the largest item in the painting contract, it is false
economy to use second grade paint. It is also false economy to allow the paint
on a structure to deteriorate severely before repainting. However, it is
equally bad to repaint too often, because excessively thick coatings crack
badly and must be removed entirely, which is an expensive operation.
EXTERIOR PAINTS FOR WOOD
Despite several centuries of experience,
the subject of exterior paint for wood is highly controversial. There is ample
evidence that good paint jobs are being produced on various types of wood
siding. But there is also the disturbing evidence that bad failures from
blistering, cracking, and peeling of paint are not uncommon. As noted
previously, it is becoming more widely recognized that premature failure of
paint on wooden houses is not necessarily the fault of the paint; frequently
such failures can be traced to construction characteristics which allow
moisture to accumulate behind the coating, or premature failure may occur from
improper application of the paint. As a result, the paint maker can blame the
painter while the painter claims it was bad paint. And, both can attribute the
failure to construction characteristics; whereas the contractor is sure the
painter did not put the paint on right, and the painter insists that it was
poor paint in the first place. In a more enlightened period of civilization we
may expect paint makers to apply the paints and maintain them in satisfactory
condition under a contract for fifteen to twenty-five years. Under such
conditions there would be guaranteed satisfaction for the consumer, paint would
enjoy a better reputation than it does at present, and there would be greater
incentive for the paint maker to develop a paint system which would need little
or no maintenance over the period of the contract.
There are several types of wood and grades
of siding used on the exterior of houses, and the durability of exterior paint
varies considerably with these different types and grades of lumber. Color also
is an important factor in the durability of exterior paints. Extremely durable
paints can be made with pigments such as red iron oxide and carbon black, but
most people want white or light tints. Unfortunately, white and the popular
colors are not as durable as black or iron oxide red.
In addition, climatic conditions markedly
affect the performance of exterior paints. A simple comparison of the paint on
the north and on the south sides of a house will illustrate the effect of
climate. Paint usually lasts longer on the north side because it receives less
sunlight with its destructive ultraviolet radiation. In some sections of the
country this advantage may be offset somewhat by excessive mold growth on the
north side. In addition, there may be more condensation due to the colder
conditions on the north side. Over a long period of time the reduced wear on
the north side or under eaves may result in excessive thickness and failure by
cracking.
CHARACTERISTICS OF WOOD SIDING
Siding for the exterior of houses is
produced from the so-called "softwoods" which are obtained from
coniferous trees. Browne classified the various softwoods into four groups in
an attempt to indicate the relative durability of paint over these woods. His
classification was based on tests made with two types of paint: Type A that
fails by checking and crumbling, such as straight white lead paint, and Type B
that fails by cracking and flaking, such as paints which contain a high
percentage of zinc oxide. Browne also showed a general relationship between the
specific gravity of the wood and the durability of paint over it. The
classification and specific gravity data are shown in Table 1.
The classification in Table 1 is a very
useful guide to the durability which may be expected from paints on the various
softwoods. However, it should be remembered that there is considerable
variation in any given species of wood, and siding may be obtained in several
grades. It is quite possible that the best grade of siding of a wood in a
slightly lower classification may have better "paintability" than a
poor grade of a better wood.
It will also be noted that there is a
general relationship between specific gravity of the wood and the durability of
paint on it. Woods on which paint has the poorest durability have the highest
specific gravity, but a few exceptions to this rule will be seen. The specific
gravity of soft woods depends largely on the ratio of springwood to summerwood.
Since the cellular structure of springwood has thin cell walls and large
cavities filled with air, it has lower density than summerwood, which has thick
cell walls, and the cavities are smaller and contain only about half the volume
of air. In general, a high proportion of summerwood means high specific gravity
and reduced durability of paint applied to such wood.
BINDERS FOR EXTERIOR HOUSE PAINTS
Linseed oil has been the principal binder
for exterior paints for centuries. It maintains this position despite the fact
that it is subject to autoxidative decomposition which causes the aged coating
to become either hard and brittle and fail by cracking
or weak and fragile and fail by chalking and erosion. The type of failure is
determined largely by the kind and amount of pigment used, but the basic cause
is radical change in the binder. In view of the many developments in binders
for other coatings it seems strange that linseed still is used so widely for
house paint. Of course, it has been replaced almost entirely by solutions or
emulsions of high polymer resins for exterior masonry, and these binders are
being tested extensively in exterior finishes for wood. At present the
polyvinylacetate and polyacrylic resins offer most promise, and reference will
be made later in this chapter to their use in actual production of exterior
paints for wood. Usually an oil-type primer is required on bare wood with the
emulsion paints as finish coats.
PIGMENTS FOR COLORED PAINTS
The majority of exterior house paints are
white, but about 25-30% are colored, and most of these
are pastel shades rather than deep colors. White paints can be made to stay
white longer by the self-cleaning mechanism of fairly free chalking. In most
cases free chalking of colored paints would make them appear to-fade rapidly,
because the chalk is much lighter in color than the coating. However, it will
be apparent that a tinted paint coating will appear to fade if it chalks badly
despite the use of fade-resistant colors.
The PVC of the paint and the type of
extender have marked effect on rate and degree of
chalking. McCleary have shown that the special type of calcium carbonate
referred to previously gives much less chalking than magnesium silicate;
therefore it also gives less fading in colored paints. However, calcium
carbonate usually fails by cracking at normal PVC of 30-35%, although cracking
is reduced or eliminated at PVC of 40-45%.
The extender is an important component of
colored paints because the high hiding power of color pigments permits the use
of a large proportion of extender. The ratio of extender also is influenced by
the white pigment used. For example, pink paints may be made with combinations
such as 10% iron oxide, 40-45% leaded zinc oxide, and 45-50% magnesium
silicate; or 10% iron oxide, 15-25% TiO2-ZnO mix, and 65-75% calcium carbonate.
The latter type of pigment combination has from 40-45% PVC, but it gives better
gloss and less apparent fading than the former type which would be used 30-35%
PVC. Obviously, the type containing calcium carbonate would also be lower in
material cost. Magnesium silicate has higher oil absorption than calcium
carbonate, and if these extenders were used at equal volume the paint
containing calcium carbonate would be too thin because it would have more free
binder. When the PVC of the calcium carbonate paint is increased it will
contain about the same amount of free binder which will adjust the consistency
and also give better durability. This comparison shows that two paints having
the same PVC can differ markedly in free binder content.
The iron oxide yellows, reds and browns
are extremely light fast both in mass tone and tints, and the reds are widely
used in barn paints, but the colors are rather dull. Tuscan reds are used where
a somewhat brighter color is required.
For very bright pinks, tinting colors such
as the Monastral reds and parachlor reds, are used as
described in Chapters 19 and 20. It should the emphasized that all color
pigments used for tints should be checked carefully for lightfastness in the
actual formulation in which they will be used. The type of binder, other
pigments, and exposure conditions all affect the lightfastness or degree of
fading of exterior paints.
Chrome yellows tend to darken in exterior
paints, and although they discolor in atmospheres that contain sulfur
compounds, they are widely used because of their other good properties. Zinc
yellow changes to a darker greenish color when exposed to sunlight, but it is
not affected by sulfur fumes. In general, it is not as durable in oil paints as
light chrome yellow of comparable shade. Cadmium yellows are more expensive and
do not have as good hiding power as chrome yellows, but they are not affected
by sulfur fumes. They have good lightfastness in mass tone but they fade in
tints exposed to moisture and sunlight, and they show poor durability. Nickel
titanate yellow has excellent resistance to light and sulfur fumes and good
durability in exterior exposures.
The organic
yellow as Green-Gold YT-562-D has good lightfastness in tints and excellent
chemical resistance, but it may bleed slightly in oil paints. The Hansa and
benzidene yellows do not bleed in oil but are sensitive to solvents, and
although they have good tinting strength, they have low hiding power. In
general, these yellows are not satisfactory in exterior oil paints.
The two principal blue pigments used in
exterior paints are iron blue and phthalocyanine blue, and the latter is slowly
replacing iron blue despite its higher cost per pound because of better light
fastness, brighter tints, and greater tinting strength. The indanthrone blues
have good durability and light fastness even in very light tints, but they are
used more in specialty finishes than in house paints.
Iron blue has good light fastness in
medium and deep tones but may fade slowly in light tints. This pigment is
subject to reduction to the white ferro-ferrocyanide when exposed to strong
light, which may be a factor in its fading in light tints. Reduction to the
white compound also may occur in closed containers with oxidizing oils, but the
blue usually is restored after the paint is applied and oxidized in the drying
process. Since iron blue is very sensitive to alkali, combinations with calcium
carbonate should be checked carefully before using.
Phthalocyanine blue is much more expensive
per pound than iron blue, but it is about twice as strong in tinting strength,
and the tints are very lightfast, are cleaner in color, and are very durable in
exterior paints. Blends of phthalocyanine and iron blue are used in tints to
reduce cost, but they are not as permanent as phthalocyanine alone.
Ultramarine blue is not satisfactory for
tinting exterior paints because it appears to fade badly. The pigment itself is
very lightfast, but it is sensitive to free acids which develop in oxidizing
coatings and may change to a white or colorless compound. In addition, the dry
pigment is much lighter in color than when wetted by oils in the paint;
therefore when the coating chalks it becomes very much paler in color from
liberation of dry blue pigment.
The best green pigment for tints is
phthalocyanine green. It is definitely superior to chrome green or combinations
of phthalocyanine blue and chrome or zinc yellow. Chromium oxide green is
extremely lightfast, but it is a relatively pale green with low hiding and
tinting strength. When used with a small proportion of white it produces a very
pleasing shade of green of excellent permanence, but its low hiding, low
bulking, and low oil absorption make it an expensive pigment to use.
MICROORGANISMS IN PAINTS AND COATINGS
The microorganisms found in paints and
organic coatings are primitive plants such as bacteria, algae, and fungi.
Bacteria are the smallest living things, if viruses are not classified among
living organisms. There are hundreds of different types of bacteria which exist
in three general shapes; the little sticks or rods, called bacilli; the
spherical or grain-like, called cocci; and the spiral or corkscrew shape,
called spirilla. Bacteria, which may be found almost everywhere, fulfill an
essential role in the phenomenon which man calls life. Some bacteria are useful
to man, many have no effect, and others are deadly if allowed to accumulate in
his body beyond the minimum tolerance. One of their very useful functions in
the natural world is to convert organic matter back to the inorganic form; but
householders object when the organic matter is a paint coating.
Bacteria do not contain chlorophyll and
are not capable of photosynthesizing their nutritive requirements from carbon
dioxide, water, and sunlight. However, many bacteria can chemosynthesize these
requirements and are therefore independent of other organisms. Those that
cannot chemosynthesize exist at the expense of other organic matter, either the
dead or living forms. For example, the disease-producing bacteria that exist as
parasites on living organisms may destroy the tissues of the host or produce
poisons which interfere with normal physiological functions. The nitrifying
bacteria, by manufacturing nitrogeneous compounds from carbohydrates and
atmospheric nitrogen, thus help the plants that produced the carbohydrates.
Vannoy investigated twenty-five
preservatives in exterior oil paints for wood and selected the following five
as being most effective after two years exposure:
1. Phenyl mercury salicylate
2. Phenyl mercury oleate
3. Salicylanilide
4. Cuprous oxide
5. Copper-8-quinolinolate
After five years exposure their
effectiveness in mold control had decreased considerably except for cuprous
oxide and copper-8-quinolinolate. However, since these two preservatives and
salicylanidide produced discoloration when used in amounts required for mold
control, they would not be satisfactory for white paints.
Shapiro in the Materials Laboratory, Fort Belvoir, Virginia
investigated a large number of preservatives by both laboratory and field
methods for use in paints on wood. From laboratory results he arranged nine
preservatives in four groups in decreasing order of effectiveness as follows:
1. Copper-8-quinolinoleate
2. Phenyl mercurials
Pyridyl mercury chloride
1-(4-chlorophenyl)-2-4-demethyl-3-nitropyrazole
3. Tetrachlorophenol
Salicylanilide
Phenanthroquinone
4. Mercury chloride
Pentachlorophenol
FORMULATING EXTERIOR PAINTS FOR WOOD
Now that we have considered the principal
components of exterior paints for wood, we are confronted with the problem of
combining them in the best possible proportions for maximum performance of the
paint. This problem was quite simple when basic carbonate white lead was the
sole pigment and raw linseed oil with driers the entire vehicle. This type of
paint contained 28-30% PVC and a 3-coat system was used. The directions
required addition of ½ pint of linseed oil to the gallon for the first
coat, ½ pint of turpentine of the gallon for the second coat, and the
third coat was to be applied as received. This system has ample free oil in the
primer for good penetration of oil into the wood. Also, the application of three
coats practically assured the absence of "holidays" or thin places in
the over all coating.
Following World War I 2-coat systems based
on white lead were used with some success because heavy coats were required to
obtain adequate hiding. With the advent of high-hiding- titanium dioxide it
became easier to formulate 2-coat paint systems that would have equal or better
hiding than 3-coat systems of white lead or combinations of white lead and zinc
oxide. However, a penetrating primer is
not satisfactory in a 2-coat system, because it would not provide sufficient
hold-out for the finish coat. In the early 2-coat systems high pigmentation
with white lead prevented significant penetration of the raw oil. Decreased
penetration also can be obtained by replacing raw linseed oil with bodied
linseed. A mixture of equal parts of raw and bodied linseed is a good
compromise between limited penetration for good adhesion and sufficient
hold-out for uniform appearance of the finish coat. This mixture is
satisfactory for the primer coat but is not as good for finish coat durability
as a combination of 75% raw and 25% bodied linseed. This indicates that a
2-coat system should use different paints for the primer and finish coats.
It is apparent that any marked
incompatibility in physical properties between the various coats of a paint
system will produce stresses which decrease the ultimate durability of the
system. Realization of this situation stimulated the investigation of zinc-free
systems and also systems that eliminate both lead and zinc pigments.
Unfortunately, such systems merely substitute the problems associated with lead
and zinc pigments for those that develop when titanium dioxide is the only
prime pigment. These problems include lack of hardness with conventional oil vehicles,
excessive chalking and erosion, and control of microorganisms. It would appear
that the answer to this dilemma may be with combinations of titanium dioxide
and barium metaborate. Such combinations have been under test for about five
years, with results that are extremely promising; suggested formulas using
barium metaborate (Busan11) will be included in this chapter.
BUILDING CONSTRUCTION ADHESIVES
For most construction applications, the
bonds of construction adhesives must retain this structural integrity
throughout the life of the structure. Hence, long term durability is an
important requirement, but limits the choice of adhesives. While many of these
adhesives possess the necessary strength, their working properties are often
such that they cannot be used directly. For this reason, specially formulated adhesives, are required.
Advantage of Using Adhesives in Construction
Adhesives have the ability to distribute
stress uniformly throughout the bonded area. Adhesives permit assembly of
different materials that could not be joined together with mechanical
fasteners. Adhesives can also significantly reduce the number of mechanical
fastenings required, hence reduce cost of fastenings. Often mechanical
fastenings cause nail popping problems in wall board and underlayment.
Adhesives reduce the problem because members are held together firmly and fewer
fasteners are required. Flexible adhesives permit bonding of dissimilar
materials that have widely differing coefficients of expansion and mechanical
properties. Adhesives also have the ability to yield and absorb internal
stresses.
Elastomeric Adhesives
Elastomeric construction adhesives are
used extensively in the construction of modular housing because they help to
ensure that units can be transported from the factory, then lifted and erected
at the building site, without incurring significant, structural or finish
damage. A polychloroprene construction adhesive might consist of the following
basic ingredients.
An important class of additives in
polychloroprene adhesives compounding is the resin. Since polychloroprene
itself does not adhere well to wood, metal, glass etc., resin is included for
specific adhesion of these substrates. The level of resin added greatly affects
open time or tack range, as well as heat resistance. Metal oxides serve several
purposes in adhesives. Magnesium oxide is a reactant with the resins, and the resinate imparts heat resistance. Magnesium oxide in
conjunction with zinc oxide promotes solvent release, thus aiding early
strength development. MgO and ZnO also act as acid acceptors for possible
damaging HCI given off during cure and ageing. Magnesium oxide is a processing
stabilizer and curing agent. ZnO acts as a cure accelerator and curing agent.
Antioxidants are included to promote long term ageing. Fillers and extenders
are added primarily to reduce costs, but they may improve cohesive strength and
viscosity. Solvents are an important compounding
ingredients because their choice can significantly affect rate of strength
development, open time, viscosity and cost.
Rubber base mastic may be made of reclaim
rubber, having high water and alkali resistance and flexibility and low cost. A
floor tile adhesive is given below:
Formulation
|
Ingredients
|
Parts by Weight
|
|
Smoked sheets
|
45.0
|
|
Calcium carbonate
|
15.0
|
|
Soft clays
|
75.0
|
|
Coumarone-indene resin
|
(110ºC) 50.0
|
|
Antioxidant
|
0.5
|
|
Mineral spirits
|
as required
|
Metal to
wood bonding is troublesome as under dry heat metal expands and wood shrinks.
Due to these reasons rubber based adhesives have been found to be especially
good. Filler should not be present but preservatives to guard against mould
growth.
Formulation
|
Ingredients
|
Parts by Weight
|
|
Natural rubber (60%)
|
100.0
|
|
Ammonium caseinate (25%)
|
11.0
|
|
Sodium hydroxide (25%)
|
0.4
|
|
Formaline (40%)
|
5.1
|
|
Ester gum
|
3.7
|
|
Mineral oil
|
0.4
|
|
Water
|
6.2
|
|
Fungicide
|
As Required
|
Gap-Filling Phenol Resorcinol Adhesives
The gap-filling phenol-resorcinol
adhesives are more than adequate in plywood than lumber, joints containing
asbestos, walnut shell flour and have sufficient strength and serviceability.
They may be used in nail spacings, gaps of bridges and other cantilevered
units.
Polyurethane Adhesives
With the ever present demand to speed
production output in the wood using industries, emphasis has been placed on
Building Construction Adhesives speeding adhesive bonding operation with
shorter assembly line, pressure periods and pot lines. Polyurethane adhesives
suit best.
Resorcinol Resin Adhesives
Resorcinol formaldehyde resin adhesives have filled a great need in the
building product for its great rigidity and strength and they are capable of
curing at normal ambient temperature. Typically one mole or resorcinol is
reacted with 0.60 to 0.65 mole of formaldehyde in the presence of a catalyst at
100-150ºC followed by cooling to control the reaction. After
refluxing and cooling paraformaldehyde is added as a hardener. It should
be noted that the paraformaldehyde is actually a chemical reactant in the
adhesive system and not a catalyst in the reaction. Its function is to provide
additional methylol groups on the resorcinol polymer at the time of use to
provide final cross-linking in curing. Wallnut shell flour is mixed with
paraformaldehyde to impart the desired consistency to the adhesive for
spreading and controlling penetration of wood. Sometimes phenolic resin is
added to reduce the cost.
Casein Adhesives
Casein adhesives are still used
extensively for laminating large structural beams, columns, and arches for
interior installation. They is used also for interior
stressed skin panels, box beams, and assembly bonding of plywood and lumber
floors and walls of the factory housing.
Casein is the main protein of milk. A
formulation for a wet-mix casein adhesive with working life 6 to 7 hr, good dry
strength and water resistance is as follows.:
Formulation
|
Ingredients
|
Parts by Weight
|
|
Casein
|
100
|
|
Water
|
150
|
|
Sodium hydroxide
|
11
|
|
Water
|
50
|
|
Calcium hydroxide
|
20
|
|
Water
|
50
|
Silicate of soda may be used to replace
sodium hydroxide to give a longer working life, when copper salts are added,
the water resistance is improved. Copper acts as a preservative and provides
some protection to the joints when exposed to warm, damp atmosphere where mold
and other micro organism are active. Here is an example.
Formulation
|
Ingredients
|
Parts by Weight
|
|
Casein
|
100
|
|
Water
|
150 to 250
|
|
Calcium hydroxide
|
20 to 30
|
|
Water
|
100
|
|
Silicate of soda
|
70
|
|
Cupric chloride
|
2 to 3
|
|
Water
|
30 to 50
|
Polyvinyl Acetate Resin Emulsion
Polyvinyl acetate resin emulsion came in
to use as wood working adhesives as a substitute for hide glues. Primarily because they are supplied as ready to use liquids and
cure rapidly at room temperature. General purpose ceramic tile adhesives
may be prepared as follows:
Formulation
|
Ingredients
|
Parts by Weight
|
|
50% Polyvinyl acetate emulsion
|
120
|
|
60% natural rubber latex
|
100
|
|
Surfactant, 10% solution
|
10
|
|
Gypsum
|
200-500
|
|
Bentonite clay
|
20
|
|
Tackifier
|
20
|
|
Water
|
as required
|
Another formulation of adhesives used in
vinyl to metal adhesion that eliminates expensive metal pretreatment is given
below:
Formulation
|
Ingredients
|
Parts by Weight
|
|
Polyvinyl acetate emulsion
|
14.30
|
|
Nitrile rubber
|
3.81
|
|
Phenolic resin
|
0.99
|
|
Methyl ethyl ketone
|
37.61
|
|
Isobutyl ether
|
18.51
|
|
Toluene
|
20.00
|
|
MBT
|
0.66
|
|
Carbon disulfide
|
4.75
|
Phenolic Resin Adhesives
Conventional phenol formaldehyde resin adhesives are
used primarily in the manufacture of exterior type soft wood plywood. There are
two basic classes of phenolic resin-the resoles and novalacs. One typical
adhesive mixture in common use in plywood industry is illustrated only as an
example.
Formulation
|
Ingredients
|
Parts by Weight
|
|
Water
|
750
|
|
Co-cob
|
500
|
|
Wheat flour
|
100
|
|
Phenolic resin
|
400
|
|
50% caustic solution
|
110
|
|
Phenolic resin
|
2200
|
In the actual plywood manufacturing
process, softwood veneer is normally dried to a very
low moisture content before bonding often as low as 2%. In a typical five ply
panel production process, the two cross plies are run through high speed double
roll spreaders with special grooving on rubber rolls. This spreader applies
adhesive on both sides of the core-ply pieces and the centre. The assembled
panel is then subjected to hot presses. A press cycle for a typical adhesive
for a ½ in panel, at the temperature of 80ºC is approximately 5 min
near 175 psi.
FLOORING
Epoxy resin flooring is used as the
covering or topping that is frequently placed on top of a sub-floor. This
covering, which is, of course, the wearing surface, can be classified in a
number of ways. The composition of the floor, its method of laying and the
nature of the wear to which the floor will be subjected have all been used as
ways of grouping different floor types. It is sufficient when considering epoxy
resin floors merely to divide the floors into three types:
(1) Domestic floors, as in dwelling houses and flats.
Examples of such floor materials are carpet, PVC, wood, linoleum and cork, each
in tile or strip form.
(2) Institutional floors in schools, offices, banks,
hospitals, showrooms and certain types of factories. Many of the materials used
are the same as those for domestic floors, such as PVC, wood and linoleum, and
in addition other materials, such as concrete, magnesite, ceramic tiles and
terrazzo are employed.
(3) Industrial floors, as in certain types of factory where
severe corrosion and/or heavy mechanical wear occurs, e.g. sweet stuff and food
factories, tanneries, breweries, battery and plating shops and chemical plants.
Examples of the flooring used include acid-resistant quarry tiles, metal
paving, granolithic materials, asphalt and synthetic polymers.
All of these types of floors have similar
basic requirements, but the relative importance of each requirement is
different for each type of floor. This may be shown in the form of ratings as
in Table 1. Naturally, there will be exceptional situations where the
assessment given in Table.1 is not appropriate for that particular example, but
in general terms it is believed that this overall assessment is essentially
accurate.
The data show that decorative appearance
and good acoustic insulation properties are of prime importance for domestic
floors. For institutional floors, non-slip characteristics and the ability to
clean the floor thoroughly are the most important properties, followed by good
sound insulation. This last factor is becoming increasingly important as the
number of industrialised buildings increases, because sound transmission is
usually high in this type of building. In general, there is increasing
awareness of the nuisance and distraction caused by high noise levels, which
result in reduced efficiency. The most direct way of reducing noise is to use a
sound-deadening covering on the floor and the trend has therefore been towards
the use of soft flooring. Industrial floors, as would be expected, should
primarily be resistant to mechanical and chemical attack, be capable of
withstanding thorough, vigorous cleaning, and have non-slip characteristics.
Embracing all of the factors considered in
Table 1 is cost. In most instances, the material which satisfies the
performance requirements at the lowest cost will be used. In general, flooring
material that has a lower performance rating that average in a given area
coupled with a high cost can be disregarded.
DOMESTIC FLOORING
Considering the five groups of properties
in Table. 1, one study which compared typical domestic floor finishes
with an epoxy finish gave the following ratings (the lowest figure represents
the flooring with best performance properties):
|
Carpet
|
5
|
|
Vinyl (foam backed)
|
9
|
|
Wood parquet
|
15
|
|
Linoleum
|
15
|
|
Thermoplastic tiles
|
17
|
|
Epoxy
|
20
|
Epoxy flooring on this basis is the least desirable for domestic purposes, chiefly
because of its low level of sound insulation and its lack of pleasing
appearance. Efforts have been made to improve epoxy resins in these respects,
for example by using mixed coloured aggregates or spattering colour on to the
floor surface to improve the appearance, and by incorporating rubber crumb as
an extender to improve the flexibility and `feel' of the floor and to help to
reduce impact noise. However, these modifications have not been successful and
really give a poor imitation of vinyl flooring at higher cost.
In general, carpet and wood parquet are
more expensive than epoxy flooring, but they clearly have much more appeal in
the domestic market. For epoxy resins to enter the domestic market, therefore,
they would need to compete with the cheaper vinyl and linoleum flooring, whose
performance characteristics are nearer to that of epoxy flooring than any other
domestic floor. Unfortunately, these materials have such a low price compared
with epoxy resins that even the development of new, faster ways of laying epoxy
flooring will not overcome the difference.
Clearly, on both technical performance and
cost, epoxy floors are not likely to succeed in the domestic market, which is
moving strongly towards the use of soft flooring and carpets.
INSTITUTIONAL FLOORING
Institutional floors include a wide range
of flooring situations, from `light duty' floors for offices, lecture rooms and
corridors to floors for industrial situations such as laboratories, kitchens in
hospitals, schools, hotels, showers and bathrooms, etc.
The institutional floors that remain
from the above list do not offer a very promising market for epoxy resins,
despite earlier optimism that the self-levelling epoxy or epoxy-terrazzo would
be used widely in office foyers, internal concourses of large office buildings,
etc. The order of preference for the different types of floors is probably as
follows:
|
Carpet
|
7
|
|
Vinyl
|
12
|
|
Wood parquet
|
14
|
|
Epoxy
|
|
|
Linoleum
|
15
|
|
Thermoplastic tiles
|
16
|
At present, low-cost linoleum and vinyl
and thermoplastic tiles account for about 70 per cent of the total area
treated, but on a performance basis, carpet is clearly the preferred flooring
material. Compared with the harder flooring, carpet has improved comfort, sound
insulation and, it is claimed, a 10 per cent advantage in cleaning costs. It
therefore seems unlikely that epoxy flooring will be preferred to carpet or
vinyl tiles.
EPOXY BONDING IN NEW STRUCTURES
There are a number of examples of the
inclusion of epoxy adhesives in the design stage and in the erection of new
massive structures, the adhesive mostly being used to join pre-cast concrete
structural elements during erection. Small-section mullions up to massive
bridge sections weighing perhaps 100 tons each have been bonded in this way.
The merits of building structural beams
and arches from pre-cast units rather than casting in place are well known.
Dry, non-bonded joints can be and are used, but unless they are specially keyed
into each other and post-tensioned, they cannot adequately resist tension and
shear stresses. Careful preparation and casting of adjacent elements is
essential in order for accurate mating of the units to be possible. Upon this
accuracy depend the proper uniform transmission of
stress and the size of the gap between the elements, which, if open, will allow
water and corrosive substances to penetrate the joint. There is therefore a
need for an adhesive between the sections that will act as a load distributor
and gap filler. Cement mortar joints at least 1 cm wide or an epoxy
resin-filled joint can be used, but the resin adhesive have many advantages
over the cement mortar and allow the maximum advantages of the pre-cast
technique to be obtained. These advantages are:
(1) Faster cure and therefore props and supports can be
removed earlier and post-tensioning begun sooner.
(2) Greater tensile and shear strengths are obtained, which
can, in some instances, allow joined, untensioned units to be assembled before
lifting into place as the joined sections are as strong as monolithic concrete.
(3) Shrinkage of the adhesive during cure is negligible and
hence the joints remain watertight.
(4) Thinner joints can be used and are aesthetically more
pleasing than the thick bands of cement mortar. Accurate precision casting of
the concrete section is clearly necessary if thin joints are to be used.
Alternatively, more time must be spent in grinding the mating surfaces to a
close fit.
These advantages of using epoxy adhesives, have been fully realised in the construction of a
number of concrete bridges. Where a bridge is spanning a river or estuary or is
in a situation where casting in situ is extremely difficult and therefore
costly; a segmental construction technique is often used. In this method,
pre-cast box sections are lowered successively into place as cantilevers from
the bridge piers, high-strength epoxy adhesive being used as the jointing
material.
Many large and complex bridges have been
built in this way, including several notable bridges in France. The Pont D'Oleron links the
island of Oleron,
in the Bay of Biscay near the mouth of the Gironde,
with the mainland. It is the longest bridge in France, stretching for 9928 ft
(2900 m), and was erected in only 14 months as a continuous pre-stressed box
beam from pre-cast concrete units bonded with an epoxy formulation. The Paris ring-road system involved the building of two
bridges across the Seine. One of these
bridges, the Pont Aval, carries two carriageways, each of four lanes 3 ½
m wide, a central reservation 3 m wide, and has a total width of 34.6 m. Box
beams of pre-stressed concrete form the deck of each half of the bridge and
their weight varies from 40 to 70 tons each. The pre-fabricated box sections
were cast against one another so as to ensure a perfect fit, the faces of each
section being separated from each other solely by the epoxy composition. It was
found that the use of the epoxy formulation enabled work to proceed
independently of external conditions and ensured that all sections and the
channels for the stressing cables were watertight.
Other bridges erected using this technique
include the Pont Pierre Benite near Lyon, a bridge on highway NI 86 at
Choisy-Le-Roi near Paris, Casteljon bridge over the river Ebro, Pamplona,
Spain, which required about 2 tons of adhesive, and the Rawcliffe bridge over
the Dutch River in Yorkshire. The segmental construction technique was also
used in The Netherlands on three large motorway bridges, all associated with
the development known as the Delta Project, which will lead to a huge extension
of the industrial and residential area of Rotterdam and Europoort. The
Kleinpolderplein fly-over is Holland's
first three-level interchange and cost £7M to complete. It was
constructed from 45-ton prefabricated box beam sections cast off-site and epoxy
resin adhesives were used for the joints between the sections of the higher
levels. The Brielse Maas bridge and the Hartel Kanaal
bridge were also constructed by this technique and were epoxy jointed. But
perhaps the success of the technique can best be demonstrated by the
construction of the 5022 m long Ooster Schelde bridge, which was opened in 2
½ years from driving the first pile and which required very little
temporary scaffolding. Holland
is in an advantageous position to use the pre-cast technique as the massive
concrete section can be transported to almost any part of the country by water.
Probably the most impressive road
structure in which an epoxy jointing composition was used is the 1 ½
mile long elevated section of the N9 highway which follows an extremely sinuous
route along the north-east shore of Lake Geneva between Chillon and Villeneuve
in Switzerland (Plate 10). The road viaduct is mounted on slender piers, some
of which are 135 ft (45 m) high, and pre-cast concrete sections, which weighed
between 45 and 80 tons each, were erected by a giant travelling gantry crane
that moved on rails attached to the bridge deck. The surfaces of all the
pre-cast sections were coated with an epoxy composition before the elements
were placed in position. When a pair of elements was provisionally placed, four
145 ton post-tensioning cables were fixed between the two elements. At a normal
rate of fixing four pairs of elements per day, the viaduct was extended at over
300 m per month. Over 50000 yd3 of concrete and 5000 tons of steel were used in
the 1 ½ mile viaduct, which cost £3M.
Epoxy adhesives have also been used to
bond together large prefabricated concrete sections of buildings. In Durban, South
Africa, a large sugar storage terminal was
built using pre-cast sections, whereas most silos constructed previously were
cast in situ. This latter method would have required the use of very expensive
shuttering, which it would have been necessary to dismantle and move many times
during the construction of the whole of the 800 ft (250 m) long structure. The
building itself consisted of 80 principal arches, each leg of which was
prepared from 18 cast units weighing 5 tons each. When erecting the units, the
epoxy formulation was coated on to the joining surfacing and took up any lack
of uniformity. After four arches had been completed, they were pre-stressed and
the supports moved on to the next four to be erected.
One of the most exciting buildings
architecturally to be built in recent years is the Sydney Opera House, designed
by Jorn Utzon together with structural engineers Ove Arup and Partners. This
cluster of shell forms was built from curved hollow concrete ribs, which
themselves were made from pre-cast segments with matching faces. The whole
string of segments was made continuous by post-tensioning across the transverse
joints of the string. Each segment weighed about 10 tons and varied in width
from 1 to 12 ft and in depth from 4 to 7 ft. An epoxy adhesive was used to bond
the segments together and also served to maintain a uniform load distribution.
O `Brien' reported that the minimum and maximum compressive stresses that might
be applied to the adhesive under extreme conditions would be 300-2500 p.s.i.
(2-17.5 MN/m2) with a maximum shear stress of 500 p.s.i. (3.5
MN/ m2). Under normal conditions the maximum compressive stress applied
would be 1500 p.s.i. (10.5 MN/m2) and the shear stress 100
p.s.i. (0.7 MN/m2).
A further impressive use of an epoxy
adhesive was in the construction of the superstructure of the new Parc des
Princes Stadium in Paris, regarded as one of the world's most ambitious and
successful structures in reinforced concrete (Plate 8). This structure, which
has a capacity for 50000 people, is composed of 50 huge arches of segmental
concrete construction with internal steel stressing cables. An epoxy-based
mortar was used for jointing between the hundreds of concrete segments, ensuring
continuity of the structure and playing a fundamental part in load distribution
both during construction and in the finished building. It also ensures that the
stressing cables and their terminals are fully protected from the environment.
O `Brien' has also recorded many examples
where slender columns and mullions have been joined with epoxy adhesives in
buildings, such as Coventry Cathedral, the University of Exeter Science
buildings, Somerville College, Oxford, and Abbotsinch Airport, Glasgow, and
listed five types of joints that have been formed.
(1) Thin joints between pre-cast concrete units, which are
post tensioned after cure.
(2) Thin horizontal compression joints between sections of
pre-cast concrete columns or mullions with no dowel connector bar.
(3) The same situation as in (2) but with dowels grouted-in
using the same adhesive.
(4) Pre-cast concrete shear connections using bolts to keep
the adhesive in compression.
(5) Various joints used in assembling pre-cast concrete
staircases.
Nearly all of the joints considered so far
have been of the type in which the adhesive is held in compression. There are
not many applications known where the adhesives are used under conditions of
shear stress, no doubt owing to the lack of long-term strength data on these
adhesive, especially when used in thick sections and subjected to long-term
stresses in various environments. However, studies have shown that epoxy resin
adhesives can serve as reliable and safe shear connectors for composite T-beams
under long-term static or dynamic loading. In these tests, carried out on a
number of composite T-beam designs, it was shown that the shear stress between
the metal beam and the concrete slab seldom exceeded 200 p.s.i. (1.4 MN/m2) in
normal applications, whereas the ultimate stresses determined experimentally
were between 600 and 1160 p.s.i. (4.2 and 8 MN/m2), i.e. three to six times as
great. None failed in dynamic loading where test cycling frequencies of 200-250
cycles per minute were being used. The shear stress in the joint during tests
varied between 150 and 300 p.s.i. (1 and 2 MN/m2).
Epoxy resin shear connectors would have an
advantage over mechanical connections such as bolts by providing an improved
distribution of the stress that may result from loading or shrinkage, impact or
thermal cycling. Both techniques would cost about the same initially but in the
long term the resin connectors would show savings on maintenance and repair.
There are a number of limitations to the
use of epoxy adhesives that need to be overcome before they can be more widely
used in construction, viz. fire resistance, bulk mechanical properties and
creep.
GROUTS FOR LEVELLING: MISC.
APPLICATIONS
The precise and accurate location of heavy
equipment and machinery during installation on concrete can be a very difficult
operation, as some shrinkage, which is often uneven during the hardening of the
concrete, is inevitable. But the use of epoxy-based grouting media has greatly
simplified the operation and led to considerable cost savings. Hitherto, the
spaces that remained between foundations and the undersurfaces of machinery
were filled with a cementitious grout, but this procedure has a number of
disadvantages. The grout:
(1) is relatively low in strength;
(2) has a high and unpredictable
shrinkage;
(3) deteriorates in the presence
of chemicals and oil; and
(4) is relatively slow to cure.
For these reasons, epoxy-based grouting
compounds are now used throughout the world and they offer the following
advantages:
(1) Very low shrinkage.
(2) Excellent adhesion to concrete and metal.
(3) Adequate strength in the grout is developed.
(4) Very good dimensional stability and mechanical strength
and vibration damping. The epoxy grouts can be said, broadly, to have at least
three times the compressive strength and four times the tensile strength of
concrete.
(5) Good chemical resistance.
In practice, the preferred method is to
cast the basic concrete structure and to leave a small gap of about ½ in
(13 mm) between the concrete and the machinery that is to be precisely located.
This gap is then filled with a free-flowing epoxy grout, the machinery having
been positioned accurately before the epoxy was cured. Alternatively, the epoxy
can be used to produce the desired accurately levelled surface on the concrete first
and the machinery is placed in position afterwards. This technique has been
used in the siting of heavy compressors and other machinery. Other examples are
the installation of heavy milling, boring and drilling machines, which are
used, for example, in the manufacture of marine diesel engines. These very
heavy machines are normally mounted on a massive reinforced concrete foundation
that is several feet thick. In certain cases, the machine is placed directly on
to the prepared concrete base, accurately positioned and levelled, and the
epoxy grout poured into a mould formed from, for example, foamed plastic strips
around the base.
With very heavy machines, it has been
necessary to support the levelled equipment temporarily with monolevelling
jacks, form a mould as before, and pour the epoxy grout underneath each
supporting jack. A similar method is used to support the rails on which the
boring machine travels.
The method can also be employed in the
regrouting of machinery, but the most impressive uses of self-levelling epoxy
grouts have been concerned with tracks carrying heavy machinery. Two examples
vividly illustrate this use.
The Harland and Woolf Shipyard in Northern
Ireland has a dry-dock 150 m wide and 750 m long, which was built for the
assembly of oil tankers from prefabricated sections, each of which weighs
approximately 800 tons. These sections are carried from the workshops to their
assembly position in the dry-dock by a giant crane 200 ft (60 m) high mounted
on 60 bogies on each side and running on steel rails that extend down the
length of the dock. The high dynamic mechanical stresses which occur when the
crane is working would have caused the breakup of the concrete if the rails
were put directly on to a concrete base. After considerable testing, it was
decided instead to use a filled epoxy resin bedding composition, which
possessed good tensile and compressive properties. The rails were carried on a
steel sole plate 20 in (50 cm) wide, which was laid in 60-ft (18-m) sections.
The bottom face of the plates was shot-blasted and primed with an epoxy primer
under factory conditions. On site, the sole plates were aligned and levelled in
place so as to allow a gap of 1-1 ½ in (2.5-3.75 cm) between the base of
the plate and the concrete foundation. Shuttering was placed so as to allow a
gap of 1-1 ½ in on either side of the plate and the resin grout was then
deaerated and pumped into place so that the grout was level with the top face
of the sole plate. About 2000ft3 (60 m3) of epoxy grout were used in this one
application.
The Jodrell Bank radio-telescope is used
extensively for research in radioastronomy and also for tracking space
vehicles. In this work, it is essential that the telescope is positioned
accurately and maintained steady when aligned on its target. Additions and
modifications to the moving parts of the installation have imposed heavier
loads on the circular rail tracks on which the telescope rotates and it was
found that the existing concrete under the sole plates was not strong enough to
take these additional loads. It was therefore necessary to fit new sole plates
and to grout between these plates and the foundation concrete with a
self-levelling epoxy grout, formulated to have high compressive and tensile
strength. The telescope was fully operational within 7 days of placing the
final section of the grout and the increased load is being supported
satisfactorily. The work was completed without having to dismantle the
telescope or its running gear and with a minimum disruption of the station's
programme.
MISCELLANEOUS APPLICATIONS
In addition to the main uses already
discussed, there are a number of other important applications for epoxy resins
in the construction field, which are discussed below.
Soil consolidation
Oil wells drilled in loose sand can become
blocked if the sand enters the well pipe, thus reducing the output of oil. To
overcome this problem, a solvent containing epoxy composition has been
developed which can consolidate the sand in the immediate vicinity of the well
bore. The consolidated sand then becomes a filter which prevents further
movement of sand into the well itself. The technique of using this system first
requires the water in a small area around the bottom of the well to be removed
by means of an alcoholic solvent. The liquid resin system is then pumped into
the pore space of the loose sand. Cure proceeds and the polymer spreads over
the surfaces of the sand grains, concentrating at contact points between the
grains. When the polymer is fully cured, the grains are firmly held together,
forming a solid but permeable structure. Using this technique, oil fields can
be developed that were previously regarded as unworkable.
In a similar application, part of a Ruhr coalmine shaft has been sealed with an epoxy
composition in order to prevent the ingress of water containing a high
proportion of mineral salts. Epoxy grouts were also injected into the rock
walls of the underground NORAD Command Centre at Colorado Springs, U.S.A.,
in order to prevent rock movement.
Tile grouts
A point of weakness in ceramic tiling that
is susceptible to chemical attack is the joint between the tiles. Normal grouts
are often porous, have poor chemical resistance and adhesion and can allow
bacteria to be harboured in their cracks. Epoxy tile grouts for walls and
floors ensure that the chemical resistance of the tile is matched by that of
the joints and also offer very much better adhesion and the absence of porosity
and shrinkage. The Sidney Opera House contains about 80 miles (130 km) of
ceramic roof tiles jointed with an epoxy system which completely excludes
moisture and water vapour.
Epoxy laminates for concrete moulds
Wooden moulds and shuttering have been
used for many years in casting and decorating concrete. However, there are a
number of drawbacks to this use of timber:
(1) Much work is required to form a mould of complicated
shape, which may only be used a few times before it becomes disfigured at the
surface and be beyond repair. The construction of these moulds in timber also
requires skilled manpower, which is expensive and difficult to obtain.
(2) Wood absorbs moisture from the wet concrete and there
is therefore the risk of porosity developing in the concrete surface and the
mould becoming warped.
Steel has also been used to make shuttering for concrete,
but suffers from the disadvantages that it is very heavy, difficult and
expensive to fabricate into complicated shapes and can easily corrode if it is
not carefully maintained. Epoxy resin-based glass-fibre laminates are ideal
materials for shuttering. The laminate is actually used as a surface or lining
to timber moulds and shuttering. These epoxy-lined moulds are light, strong and
extremely hard wearing and can be used many times. Smooth, flawless castings
are obtained which reproduce perfectly the contours of the moulds.
Resin concrete
The term `resin
concrete' is used to describe materials in which resin, rather than Portland
cement is used as the binder for aggregate particles. The resin serves the same
function as the cement, initially providing a fluid matrix around the aggregate
particles so that the mix can be compacted, and finally, when cured, it
determines for the most part the properties of the material.
Various resins have been used to prepare
resin concretes, including epoxies, polyesters, phenol-formaldehyde and
furfurol-acetone types, and work on these resin mortars in Czechoslovakia has been described
by Bares in a number of papers (reference 8 lists the earlier publications).
Williams mentioned work on resin-sand mixtures in which they incorporated
fibres of materials such as carbon, as a means of increasing the modulus of
elasticity and improving the deformation characteristics of resin binders under
sustained loads.
A further, different approach has been the
addition of small amounts of resin into a normal cement concrete mix as a means
of improving the tensile strength and other mechanical properties of the
material.
Most of this work is still at the
development stage and it is not at all clear whether a resin based concrete
will ever become a primary construction material, Nevertheless, the
improvements shown by resin concretes over Portland cement concrete in terms of
compressive and tensile strengths, elastic modulus, low porosity and chemical
resistance will ensure that development work on the material will continue.
However, resin concretes are expensive, long-term data
on their mechanical behaviour are lacking and they are affected by increased
temperatures and could not withstand a fire in the way in which cement concrete
can.
The most likely applications for these
mixes would appear to be as a speciality product for pipes, tanks, chemically
resistant floors and perhaps as cladding panels for walls. Certainly in Czechoslovakia,
large diameter pipes for the discharge of highly aggressive effluent from
chemical plants have been made from a resin concrete, although the resin used
was not epoxy but a furfural-furfurol type. In fact, at resin to aggregate
ratios of 1:10 or greater, there is no clear case, on mechanical strength
grounds, for using an epoxy resin rather than a polyester or a furfural resin
in the concrete.
GLASS
Glass was formed naturally from common
elements in the earth's crust long before anyone ever thought of experimenting
with its composition, molding its shape, or putting it to the myriad of uses
that it enjoys in the world today. Obsidian, for instance, is a naturally
occurring combination of oxides fused by intense volcanic heat and vitrified
(made into a glass) by rapid air-cooling. Its opaque, black color comes from
the relatively high amounts of iron oxide. Its chemical durability and hardness
compares favorably with many commercial glasses Pumice, a naturally occurring
foam glass, is replete with tiny pockets of the gaseous products of the
decomposition of many compounds. These gases were trapped by the viscous glass
while it was cooling.
The origin of the first synthetic glasses
is lost in antiquity and legend. Faience was made by the Egyptians who molded
figurines from sand (SiO2 ), the most popular
glass-forming oxide. They coated them with natron, the residue left by the
flooding Nile river,
which was composed principally of calcium carbonate (CaCO3), soda ash (Na2CO3),
salt (NaCI), and copper oxide (CuO). Heating below 1000°C produced a glassy
coating by the diffusion of the fluxes, CaO and Na2O, into the sand and their
subsequent solid-state reaction with the sand. The copper oxide gave the
article an appealing blue color. Glass technology has evolved for six thousand
years, and some of today's principles date back to early times. This includes
what is today known about the structure of glass, its composition, properties,
method of manufacture, and uses.
Common usage of the term glass follows the
definition of Morey (2): "Glass is an inorganic substance in a condition
which is continuous with, and analogous to, the liquid state of that substance,
but which, as the result of a reversible change in viscosity during cooling,
has attained so high a degree of viscosity as to be, for all practical
purposes, rigid." Similarly, ASTM defines glass as "an inorganic
product of fusion that has cooled to rigid condition without
crystallizing." Both organic and inorganic materials may form glasses if
their structure is non-crystalline, i.e., if they lack long-range order. This
includes some plastics, metals, and organic liquids. In principle, rapid
cooling could prevent crystallization of any substance if the final temperature
is sufficiently low to prevent structural rearrangement. Thus, glasses are
formed primarily for kinetic reasons.
Glass is not
merely a super cooled liquid. This distinction is illustrated by the
volume-temperature diagram shown in Figure 1. When a liquid that normally does
not form a glass is cooled, it crystallizes at or slightly below the melting
point (path A). If there are insufficient crystal nuclei or if the viscosity is
too high to allow sufficient crystallization rates, under-cooling of the liquid
can occur. However, the viscosity of the liquid rapidly increases with
decreasing temperatures, and atomic rearrangement slows down more than would be
typical for the super-cooled liquid. This results in the deviation from the
metastable equilibrium curve which is depicted by paths B and C in Figure 1.
This change in slope with temperature is characteristic of a glass. Structural
rearrangement is too slow to be detected experimentally, and additional volume
changes are virtually linear with continued cooling, the same as for any other
single-phase solid. The cooling rate determines when the deviation begins to
occur. Slower cooling (path B), for instance, results in less of a deviation
from the extrapolated liquid curve. Figure 1 shows that the point of intersection
of the two slopes defines a transformation point (glass-transition temperature)
Tg, for a given cooling rate. Practical limitations on
cooling rate define the transformation range Tg ®Tg
as the temperature range in which the cooling rate can affect the
structure-sensitive properties such as density, refractive index, and volume
resistivity. The structure, which is frozen in during the glass transformation,
persists at all temperatures. Thus, a glass has a configurational or fictive
temperature which may differ from its actual temperature. The fictive
temperature is the temperature at which the glass structure would have been the
equilibrium structure. It describes the structure of a glass as it relates to
the cooling rate. A fast-quenched glass would have a higher fictive temperature
than a slowly cooled glass.
Fig. 1 Volume-temperature relationships
for glasses, liquids, supercooled liquids, and crystals.
Glasses can be prepared by methods other
than cooling from a liquid state, including solution evaporation, reactive
sputtering, vapor deposition, neutron bombardment, and shock-wave
nitrification. These techniques suggest that the purely kinetic explanation of
the glassy state is subject to question and that we need to modify the previous
definitions. It has been shown that extrapolation of the thermodynamic
properties of the super-cooled liquid gives the paradoxical results that
entropies, heat contents, and volumes become less than those of the perfect
crystal at the same temperature. Considering the transformation as a
second-order transition has yielded a satisfactory explanation for the
properties of some organic systems, but this theory is still subject to
confirmation for inorganic glasses. The dependence of transformation
temperature on relaxation time has been considered and a new definition based
on structural factors proposed. Isotropic materials with long
structural-relaxation times, e.g., >103 s, would be defined as glasses. The
determination of either the isotropy of the material or structural relaxation
times distinguishes whether or not the material is a glass. This definition
requires information regarding structure and does not consider previous thermal
history as a distinguishing characteristic of a vitreous
materials.
Structure
The basic structural unit of silicate
glasses is the silicon-oxygen tetrahedron in which a silicon atom is
tetrahedrally coordinated to four surrounding oxygen atoms. Oxygens shared
between two tetrahedrons are called bridging oxygens. In pure vitreous silica, virtually
all oxygens are bridging. Those that are not shared, for one reason or another,
can be referred to as non-bridging oxygens. The relationship between these
tetrahedral is controversial and has yet to be completely resolved. The earlier
crystallite theory has been modified by proponents of the random-network
theory. Modern structural methods point to a compromise theory.
In 1921, Lebedev (17) noted a
discontinuous index of refraction of SiO2 near the a-b transition of quartz.
His data and subsequent x-ray investigations of vitreous silica led to the
suggestion (18) that crystallites on the order of 1.5 nm were present. It was
demonstrated, however, that the crystal size would be less than 0.8 nm, and it
was suggested that the term crystal loses meaning for these dimensions.
Zachariasen formulated the random-network
theory of glass in 1932. It proposes that atoms present in glass form a
three-dimensional connected structure without periodic order and with energy
content comparable to that of the corresponding crystalline material. According
to this theory, the coordination number of an atom determines its role in a
glass structure, and the following four rules should be fulfilled for an oxide
to form glass: (1) each oxygen atom must be linked to no more than two cations;
(2) the number of oxygen atoms around any one cation must be small, i.e., three
or four; (3) the oxygen polyhedra must share corners, not edges or faces, to
form a three-dimensional network; and (4) at least three corners must be shared.
For one-component glasses, each polyhedron shares corners with at least three
other polyhedra in such a way that the network is continuous in three
dimensions. In multi-component glasses, additional cations are distributed
throughout holes in the network.
X-ray structural work strongly supported
the random network theory. The x-ray scattering pattern of glass after Fourier
analysis gives radial distribution curves that indicate the distribution of
neighboring atoms about a central atom. No evidence of discrete particles or
voids supporting an ordered structure was observed based upon data for the
first coordination shell. There were both theoretical experimental limitations
to this early work. Later, more complete data were obtained using fluorescence
excitation to eliminate Compton
scattering. Not only was a silicon-oxygen distance of 0.162 nm observed, but
also peaks at 0.265 nm for the oxygen-oxygen distance, 0.312 nm for the
silicon-silicon distance, 0.415 nm silicon-second silicon, and 0.64 nm for the
silicon-third oxygen peak. X-ray scattering investigation of silica suggested a
structural ordering beyond the distances first reported. Data analysis resulted
in a shorter silicon-oxygen bond distance (0.1595 nm) than reported at first.
However, a similarity in bonding topology between tridymite and silica glass
does not imply microcrystallinity of vitreous silica in a crystallographic
sense. The similarity between crystalline and vitreous structure on the basis
of silicon-oxygen-silicon bond angles and, hence, the relative orientation of
the silicon tetrahedrons is pointed out in Figures 2 and 3.
In addition to the question of long-range
ordering (1-2 nm), there still are aspects of the random-network theory that
are often criticized. It is possible, for example, to form glasses when no
three-dimensional network is possible. Glassy orthosilicates (SiO44-) of lead
or sodium and calcium Fig. 2 The distribution of silicon-oxygen-silicon bond
angles in vitreous silica. The function V(a) is the
fraction of bonds with angles normalized to the most probable angle, 144°.
This distribution gives quite a regular structure on the short range, with
gradual distorting over a distance of 3 or 4 rings (2-3nm). Crystalline silica
such as quartz or cristobalite would have a narrower distribution around
specific bond angles.
Fig. 3 Schematic representation of (a) an
ideal crystalline structure (Si—O—Si bond angles = 180°) and
(b) a simple glass (Si—O—Si bond angles = 144° + according to
Fig. 2). The tetrahedra in the schematics represent four oxygens clustered
around a silicon as shown (c). have
been prepared. Furthermore, modifying cations have been shown to occur at
regular interatomic distances ranging over several coordination shells.
Dark-field transmission electron microscopy has been used to infer density
fluctuations of silica which suggest ordered regions of approximately 1.0 nm in
size. However, these results have been criticized.
Glass-forming systems other than silica
have been examined. The fraction of three-and four-coordinated boron in borate
glasses can be determined by nmr (see Analytical
methods). Both nmr and x-ray diffraction (30) results
led to the suggestion that the boroxyl ring is the structural unit of vitreous
B2O3. The intermediate-size boroxyl ring represents a compromise between the
crystallite and the random-network theory.
Composition
Conditions favorable for glass formation
may be deduced from either geometric or bond strength considerations. On the
basis of the rules (21) discussed above, the following oxides should be glass
formers: B2O3 SiO2, GeO2 P2O5 As2 O5, P2O3, As2O3, Sb2 O3, V2O5, Sb2O5, Nb2O5
and Ta2O5. In fact, they are all so used. The only fluoride that fulfills the
rules of glass formation is BeF2 which readily forms a glass.
Glass formers generally have cation-oxygen
bond strengths greater than 335 kJ/mol (80 kcal/mol).
In multiple-component systems, oxides with lower bond strengths do not become
part of the network and are called modifiers. Oxides with energies of ca 335
kJ/mol may or may not become part of the network and are referred to as
intermediates. The dissociation energies used to predict glass formation are
calculated, taking into account the coordination number of the cation (see
Table 1). In multiple-component glasses, the terms formers, modifiers, and
intermediates are frequently used to define the role of the individual oxides.
However, an element such as lead may be either a modifier or intermediate,
depending upon its coordination and the glass system considered.
Glass formation of individual oxides can
be predicted from the melting point, and individual bond energies can be
normalized by dividing by the melting point of the oxide. This ratio is
relevant because the melting point is related to the amount of thermal energy available
to rupture bonds. If the bond energy is large and the melting point low, glass
formation is favored. This explains the ease of glass formation of B2O3 and
from low-melting eutectics in which neither oxide forms a glass separately,
e.g., CaO-AI2O3.
Other correlations of glass formation and
properties have been offered. For example: (1) cation valence should be either
three or greater, (2) glass formation should increase with decreasing cation
size, (3) the Pauling electrpnegativity should be between 1.5 and 2.1 Using
these criteria, four types of oxides are described: (1) strong glass formers
such as Si, B, Ge, As, and P, (2) intermediate formers that require rapid
cooling, such as Sb, V, W, Mo, and Te, (3) oxides that form glasses in binary
mixtures with non-glass formers, such as AI, Ga, Ti, Ta, Nb, and Bi, and (4)
oxides that do not form glasses.
Glass composition work starts with the
application of structural and bonding rules of glass formation. Numerous
ternary systems and their glass-forming regions have been investigated. There
are three types of ternaries: (A) single former and two modifiers; (B) two
formers and one modifier; and (C) three glass formers. Type A is shown in
Figure 4. The structural rules suggested by Zachariasen can also define likely
regions for glass formation. Additions of several percent of other oxides for
property adjustments are usually made to each system to give commercially
useful glasses.
Single-Phase Glasses
Vitreous silica. Vitreous
silica is the most important single-component glass. Highly cross-linked
vitreous silica is viscous and has a thermal expansion coefficient within the
0-300°C range of about 5.5X107/°C. It is an excellent dielectric and
resists attack by most chemicals, except fluorides or strong alkali. Fused
silica has a high spectral transmission, and in addition, is not subject to
radiation damage, which results in browning of other glasses. It is the ideal
glass for space-vehicle windows, wind-tunnel windows, ultrasonic delay lines,
crucibles for growing ultra-pure silicon or germanium crystals, and for optical
systems in spectrophotometric devices.
Fig. 4 Glass-forming region in Type A
system. The shaded area represents the predicted glass-forming region
based upon Zachariasen's rules.
The same
properties that make transparent fused silica useful also cause it to be
difficult to produce and expensive. Vitreous silica may be made by several
processes. Fused quartz made by electrically fusing quartz crystal gives a
product containing very little moisture and having good ir transmission.
However, mineral impurities of natural quartz, including alumina, iron, and
some chlorides, reduce uv transmission. Flame fusion
of quartz or flame hydrolysis of SiCl4, on the other hand, gives glasses
containing larger amounts of water which decreases the ir transmission. Long
heat treatments of vitreous silica below 1723°C may cause crystallization.
Stable crystalline forms of silica at atmospheric pressure are cristobalite
(1723-1470°C), tridymite (1470-867°C), and quartz (below 867°C)
(see Silica, vitreous silica; Silica, synthetic quartz).
Multicomponent Silicate Systems.
Most glasses fall into the category of silicates containing modifiers and
intermediates. Addition of a modifier such as sodium oxide, Na2O, to the silica
network alters the structure by cleaving the Si—O—Si bonds to form
Si—O·Na linkages (see Fig.5).
Fig. 5 The addition of a modifier, in this case one
molecule of Na2O, causes the breaking of one Si—O—Si bond to form
two Si—O-Na linkages.
Separating the silica tetrahedra from each
other makes the glass more fluid and therefore more amenable to conventional
melting and forming methods. Modifiers (or fluxes) also cause a decrease in
resistivity, an increase in thermal expansion, and generally lower chemical
durability. Glasses with a SiO2: Na2O molecular ratio less than one have so
many nonbridging oxygens that they lack a continuous, three-dimensional
structure (Zachariasen's rule 4). Such glasses, referred to as invert glasses, have been made containing Li2O, Na2O and K2O
oxides. Alkali silicates that have a silica : alkali
ratio of 0.5-3.4 are the basis of the soluble silicate glass industry.
The effectiveness of an alkali oxide
(e.g., Li2O, Cs2O) as a flux increases with the size of the cation and
therefore with its polarizability. Large ions such as cesium are easily
polarized and thus more likely to give up their oxygen to break the Si-O-Si
bonds as discussed above. Lithium, on the other hand, is more likely to keep
its oxygen and, therefore, its fluxing power is less. This is consistent with
the ease of glass formation as the size : charge ratio
of the modifier is increased. Phase separation occurs often when less
polarizable oxides are present. Lithium or magnesium silicates have a tendency
to phase-separate during heat treatment. Ionic mobility is related both to
charge and to size. Large alkalies are expected to be more mobile because of
their greater polarizability. Increased size, however, tends to reduce
mobility. Alkaline earth silicates behave similarly to alkali silicates, but
the fluxing power of alkaline earths is less than that of the alkalies.
Mobility of divalent ions is less than that of monovalent ones; hence,
resistivities of alkaline-earth glasses are usually higher. Divalent oxides
increase the resistivity of alkali-containing glasses.
Alumina is frequently used in silicate
glasses. It often adopts a four-coordinated structure with alkalies giving a
NaAIO2 tetrahedral unit which substitutes into the SiO2 network. The extra
negative charge associated with the four bridging oxygens surrounding AI3+ is
offset by the Na+ ion. A maximum in viscosity occurs when the AI2O3:Na2O ratio
equals one.
Boron oxide often behaves as a flux. Boron
softens glass for easier melting but, unlike alkalies, boron oxide increases
expansion only slightly. This is the basis of the easily melted but
low-expansion commercial glasses known as borosilicates.
Soda-Lime Glasses. Mixtures
of alkali and alkaline earths give glasses of higher durability than the alkali
silicates. The actual compositions are usually more complex than the term soda
lime suggests. In addition to Na2O, CaO, and SiO2, these glasses may contain
MgO, Al2 O3, BaO, or K2O and various colorants. Alumina increases durability,
whereas MgO prevents devitrification. Soda-lime glass accounts for nearly 90%
of all the glass produced. The batch materials are inexpensive and relatively
easy to melt. Soda-lime glass is used for containers, flat glass, pressed and
blown ware, and lighting products where exceptional chemical durability and
heat resistance are not required.
Borosilicate Glasses.
Replacement of alkali by boric oxide in a glass network gives a lower-expansion
glass. The fluxing action of the boron facilitates melting by weakening the
network. This has been attributed to the presence of planer three-coordinate
borons that weaken the silicate network at high temperature. Phase
separation of borosilicate glasses often occurs during heat treatment which may
be useful for certain applications. However, most commercial borosilicate
glasses have compositions that are miscible and homogeneous. Borosilicate glass
is applied as ovenware, laboratory equipment, piping, and sealed-beam
headlights.
Aluminosilicate Glasses.
Structural rules suggest that if the R2O: Al2O3 or the RO: AI2O3 molar ratio is
unity, an aluminosilicate glass has a silica structure in which all oxygens are
bridging oxygens. This is true of other silicate minerals and appears to be the
case with glasses. Alumina is expected to be four-coordinated when the alkali
to alumina molar ratio is greater than one, but if the ratio is less than one,
six-fold coordination of alumina has been suggested.
Aluminosilicate glasses are used
commercially because they can be chemically strengthened and withstand high
temperatures. Thus, applications include airplane windows, frangible
containers, lamp envelopes, and top-of-stove uses.
Lead Glasses. Lead oxide is
usually a modifier although at times it may act as a network former. Lead glasses
may be easily melted and have a long working range and a high refractive index
which makes them useful for lead crystal, optical glass, and hand-formed art
ware. Lead-containing glasses effectively shield high-energy radiation and are
therefore used commercially for radiation windows, fluorescent-lamp envelopes,
and television bulbs. Low-melting solder glasses and frit or decorative enamels
are usually based upon low-melting lead compositions.
Borate Glasses. Borates,
including vitreous B2O3, have been studied more than any other glass-forming
system with the exception of silicates (38). Vitreous boric oxide has a
three-coordinate structure consisting of six-membered rings of alternating
boron and oxygen atoms. Many physical properties of alkali borate glasses show
a minimum or maximum at 15-30mol% modifier (boron anomaly). Coordination
changes of boron are detected by nmr, ir, Raman, and
esr techniques. Broad quadripolar coupling typical of triangular boron
coordination is readily distinguished from the sharp coupling of
four-coordinate boron. The fraction of tetrahedral borons present appears
directly proportional to the alkali-to-boron ratio as long as this ratio is
less than 0.5. The very low durability of borate glasses precludes their use in
all except the most special applications. Low molecular weight Lindemann
glasses (Li2O, BeO. B2O3) were developed as x-ray transmitting glasses.
Rare-earth borate glasses have optical uses because of their high refractive
indexes and low dispersion. Additions of Gd2O3 to the latter increase the index
but not the dispersion.
Phosphate Glasses. The
structure of phosphate glass appears to be based on the phosphorous-oxygen
tetrahedron. Like the borates, they tend to have low durability. Important
commercial applications of phosphate glasses do exist, however. Because the
absorption bands of iron oxide in phosphate glasses are sharper in the uv and ir than in silicate glasses, iron-containing
phosphate glasses are nearly transparent to visible light. Almost clear heat-absorbing
glasses with several percent iron oxide are possible.
Phosphate-based glasses also are
more resistant to fluoride than silicate glasses. Some of the optical produced
by Schott, Hoya, Owens-IIIinois, and Corning-Sovirel use phosphate as the primary
glass former. Flourophosphate glasses, designated EK-5 or FK-50 by Schott have
very low optical dispersion with Abbe-numbers of 70.4 and 81.5, respectively.
Other Oxide Glasses.
Germanium, arsenic, and antimony oxides all form stable glasses and their structures
have been predicted. The germania glass structure is
quite similar to silica. Infrared transmission of germania
glasses is higher than that of silica. Tellurium-containing lead glasses with a
very high refractive index (>2.0) are also used commercially.
Chalcogenide Glasses.
Glasses based upon sulfur, selenium, or tellurium rather than oxygen are well
known. These glasses, although often opaque to visible light, transmit ir
radiation of a much longer wavelength than oxide systems, and many are also
semiconductors (qv). Conductivity usually increases with increasing atomic
number. The most-studied chalcogenide glasses contain the Group V elements,
arsenic and antimony.
Halide Glasses. Although
examples of zinc chloride glasses are known, BeF2-containing glasses are more
common. Vitreous beryllium fluoride has a tetrahedral structure analogous to
silica. Its unique spectral properties including transmission from
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