An adhesive is a material used for holding two surfaces together. In the service condition that way adhesives can be called as “Social” as they unite individual parts creating a whole. A useful way to classify adhesives is by the way they react chemically after they have been applied to the surfaces to be joined. There is a huge range of adhesives, and one appropriate for the materials being joined must be chosen. Gums and resins are polymeric compounds and manufactured by synthetic routes. Gums and resins largely used in water or other solvent soluble form for providing special properties to some formulations. More than 95% of total adhesive used worldwide are based on synthetic resins. Gums and resins have wide industrial applications. They are used in manufacture of lacquers, printing inks, varnishes, paints, textiles, cosmetics, food and other industries.
Increase in disposable income levels, rising GDP and booming retail markets are propelling growth in packaging and flexible packaging industry. Growth of disposable products is expected to increase, which leads to increase in consumption of adhesives in packaging industry. The global value of adhesive resins market is estimated to be $11,339.66 million and is projected to grow at a CAGR of about 4.88% in coming years. Rapid urbanization coupled with growing infrastructure and real estate construction projects is projected to further fuel demand for adhesives in India.
This handbook covers photographs of plant & machinery with supplier’s contact details and manufacturing aspects of various adhesives, glues & resins. The major contents of the book are glues of animal origin, fish glues, animal glues, casein glues & adhesives, blood albumen glues, amino resin adhesives, cyanoacrylate adhesives, epoxy resin adhesives, phenolic resin adhesives, polychloroprene resin adhesives, polysulfide sealants & adhesives, resorcinolic adhesives, furan resin adhesives, lignin adhesives, polyamide adhesives, rosin adhesive, tannin adhesives, terpene based adhesives, starch adhesives, acrylic adhesives and sealants, pressure sensitive adhesives, hot melt adhesives, alkyd resins, acrylic modified alkyd resins, alkyd –amino combinations based on neem oil, amino resins, carbohydrate modified phenol- formaldehyde resins, epoxy resins etc.
It will be a standard reference book for professionals, entrepreneurs, those studying and researching in this important area and others interested in the field of adhesives, glues & resins technology.
GLUES
OF
ANIMAL ORIGIN
Animal glues
are essentially natural high-polymer proteins.
These
organic
colloids are derived from collagen which is the
protein
constituent of animal hides, connective tissues, and
bones. There
are two principal types of animal glues, hide and
bone,
differing
in the type of raw materials used. In both cases,
animal glue
is
obtained by hydrolysis of the collagen in the raw
material.
Animal glues
find application in a wide range of industrial
uses. They
are
used in woodworking for such applications as
assembly,
edge
gluing, and laminating. In the paper industry,
they are
used
as sizing materials and as binders in paper coating,
and also for
paper creping. Animal glues find wide use during
paper
manufacture for the retention and recovery of paper fibers
and pigments.
The coated
abrasive industry uses animal glues in the
manufacture
of
abrasive paper and cloth. Closely allied with the
coated
abrasives is the use of animal glue in preparation of
compounds
for
coating wheels, discs, belts, etc.
Animal glues
are widely used in the manufacture of gummed
papers and
tapes and in paper and paperboard converting.
Animal glues
and glue-based compounded products are used
in paper
containers—set up and folding boxes, spiral and
convolute
tube
winding, and laminating. Applications in
bookbinding,
magazine and catalogue production, and allied fields
include
binding, casemaking, padding, looseleaf binders, and
various
luggage
and case covering applications.
Animal glues
are employed as warp sizing, throwing, and dyeleveling
agents in
the
textile industry. They are used in the
match
industry for match-head
compositions. Other uses include
paper
gaskets,
cork compositions, rubber compounding,
compositions
for printing, coating and graining rollers, mining,
ore
refining, and metal plating.
Glue
molecules
consist of amino acids connected through
polypeptide
linkages to form long-chain polymers of varying
molecular
weights. In hot aqueous solution the glue molecules
take up
random
configurations of essentially linear form. A wide
range of
molecular weights, varying from 20,000 to 250,000 have
been
reported.
Acidic and basic sites on the amino acid side
chains and
terminal groups affect the interactions among the
protein
molecules and water, and are believed to be responsible
for the
gelation and rheological properties of animal glues.
Because of
the
presence of both acidic and basic functional
groups in
the
protein molecule, the molecules are amphoteric
and can bear
either
a positive or negative charge. Animal glues
can act
either
as acids or bases depending upon the pH in water
solution. In
acidic solution, the protein molecules have an
overall
positive charge and function as cations, in alkaline
solution the
molecules are negatively charged and behave as
anions. The
point where the net charge on the protein is zero
is known as
the
isoelectric point (IEP). The isoelectric point of
animal glues
usually lies in the pH range of 4.5-5.6. Glues in
solution at
pH
values lower than their IEP have cationic
characteristics
while they have anionic characteristics at pH
values above
their IEP. Many properties of glue solutions, such
as
viscosity,
solubility, gel strength, and optical clarity, pass
through a
maximum or minimum at this point.
Commercial
animal glues are dry, hard, odorless materials
available in
granular or pulverized form which vary in color from
light amber
to
brown. They may be stored indefinitely in the dry
form.
The density
of
dry animal glue is approximately 1.27 g/ml.
A moisture
content range of 10-14% is considered normal for
the
commercially dried product. Inorganic ash content, consisting
mainly of
calcium salts, may vary from 2% to 6%. Hide glues
are
generally
neutral in water solution with a usual pH range of
6.5-7.5, and
bone glues are slightly acidic with values in the pH
range of
5.5-6.5.
Animal glues
are soluble only in water. They are insoluble
in oils,
greases, alcohols, and other organic solvents. When
placed in
cold
water, the glue particles absorb water and swell
to form a
spongy gel. When heated the particles dissolve to form
a solution.
When the solution is cooled the glue forms an elastic
gel. This
property is thermally reversible, and upon application
of heat the
gel
liquifies. The gelling or melting point of an animal
glue
solution
will vary from below room temperature to over 120ºF,
depending
upon
glue grade, concentration, and the presence of
modifiers.
Viscosity in
solution and the gel-forming characteristic when
cooled are
important properties of animal glues, especially in
adhesive and
sizing or coating applications. These properties vary
with the
degree
of hydrolysis of the collagen precursor and have
a marked
bearing on working properties. Animal glues are graded
as to
viscosity
(fluidity) and gel strength (stiffness of gel
formation)
under standard conditions and are available in a wide
range of
viscosities and gel strengths.
Animal glues
are compatible with and may be modified by
such water
soluble materials as glycerin, sorbitol, glycols, sugars,
syrups, and
sulfonated oils to act as plasticizers and modify the
working
properties of the glue. A degree of moisture resistance
and increase
in
the solution melting point of animal glues may
be imparted
by
the proper use of such materials as aldehyde
donors and
metal salts.
Since they
possess amphoteric properties, animal glues are
highly
effective with suitable modification as colloidal flocculants
or
suspending agents.
Methods
of Manufacture
Both major
types of animal glues are prepared by the
hydrolysis
of
collagen and differ mainly in the type of raw material
used and the
manufacturing processes employed.
Hide glues
are
prepared by initially washing the raw material
with water,
followed by curing in a
calcium hydroxide (lime)
solution
which
conditions the collagen for subsequent glue
extraction
by
hydrolysis. The cured stock is then washed, treated
with dilute
mineral acid, such as sulfuric, sulfurous, or
hydrochloric,
for pH adjustment, followed by a water rinse. The
stock is
then transferred
to extraction kettles or tanks and is
heated with
water to extract the glue. Several hot water
extractions
are
made until the glue is completely removed from
the stock.
Dilute glue
solutions are filtered, concentrated by vacuum
evaporation
and
dried. The dry product is ground to the desired
particle
size.
Bone glues
are
made from the collagen occurring in animal
bones. Green
bone glues are prepared from fresh bones and
extracted
bone
glues from bones which have been degreased
prior to
processing for glue.
Both types
of
bone glues are initially conditioned by cleansing
with water
and/or dilute acid solutions. The glue is extracted
in pressure
tanks with a series of steam and hot water
applications.
The dilute glue solutions are filtered or centrifuged
to remove
suspended particles and free grease, followed by
vacuum
evaporation, drying, and grinding.
Animal glues
contain preservatives added during
manufacture
to
provide adequate protection under conditions of
normal usage
and may contain foam control agents, depending
upon the end
use.
Commercial
Grades and Specifications
Animal glues
are graded according to standard methods
developed
and
adopted by the National Association of Glue
Manufacturers
(NAGM). Grades are based on gel strength and
viscosity
values.
It is common
to
market animal glue under brand names or
grade
designations identified by the midpoint gram values shown
in Table 1
or
by National Association of Glue Manufacturers’
grade number.
Table 2
lists
the typical properties of hide and bone types
of glues.
Viscosity of
animal glue solutions vary over a wide range,
depending
upon
grade, concentration, and temperature. Table 3
lists
typical
viscosity values at 140ºF for a range of dry glue grades
at various
concentrations.
CASEIN
GLUES AND ADHESIVES
Introduction
Casein is
milk
protein, obtained from skimmed milk by
precipitation
with sulphuric, hydrochloric or lactic acid, to a pH
of about
4.5.100 kgs. of milk usually yields about 3 kgs. of casein.
The
precipitated casein is filtered, washed thoroughly, ground
and screened
to
get 20 mesh or finer product for glue
manufacture.
Commercial casein contains about 80-90 per cent
of protein,
1-4
per cent ash, 0-1-3 per cent of butter fat, 7-10
per cent
moisture, 0-4 per cent lactose and 0-3 per cent acids
expressed as
lactic acid. The composition and amounts of
impurities
depend on, among other factors, the method of
manufacture.
Reunet casein is not, as it is, suitable for glue
manufacture
due
to high ash content.
Properties
Casein is an
off white powder. Its molecular weight is about
13000-19,000.
It is insoluble in water at its isolectric point pH
4.6; the
solubility increasing acidity or alkalinity, in the latter
it is more
readily soluble. Fixed alkalies like sodium hydroxide,
remain in
the
glue line as sodium caseinate, a soluble salt while
calcium
hydroxide on which water-resistant wood glues are
based, forms
insoluble calcium caseinate in the glue line.
Similar
insoluble caseinates are formed by zinc, chromium and
aluminium
salts
etc. Casein powder has a shelf life of above 1
year at 20ºC.
Casein
adhesives are unsuitable for outdoor use although
they are
more
resistant to temperature changes and moisture than
other
water-based adhesives. Resistance to dry heat up to 70ºC
is good, but
under damp conditions the adhesives lose strength
and are
subject
to biodeterioration. Their resistance to organic
solvents is
generally good. Casein adhesives are often compounded
with
materials
such as latex and dialdehyde starch to improve
durability.
Strong alkaline nature of mixed casein adhesives often
affects the
bonding of timber with high resin or oil content by virtue
of a
saponification action on poorly wetting surface contamination.
Resultant
bonds
may be stronger than those obtained from
synthetic
resins. Hard woods are subject to staining. Gap-filling
properties
are
good. Alkaline nature of casein glues precludes the
use of
copper
or aluminium mixing vessels.
Like animal
glues, casein glues have fairly good bond strength
and those
containing
sodium hydroxide have even better
resistance
to
water than animal glues; also they recover their
original
strength on redyeing. The fact that casein glues do not
gelatinise
makes them much easier to handle. The most serious
drawback of
casein for use in adhesive is the presence of fatty
matter which
has a very adverse effect on the tensile strength
of joint
made.
Classification
of casein glues and adhesives:
Water-resistant
Casein-lime Glue
Water-resistant
casein glue sets to a gel as the result of a
slow
chemical
reaction, sodium caseinate gradually converted to
calcium
caseinate. Some of the calcium hydroxide in the formula
has produced
sodium hydroxide from a sodium salt also in the
formula,
dissolving the casein. The chemicals are dry mixed with
casein as a
ready mix powder and shipped to the user as a
complete
prepared glue for dispersing in water.
Calcium
hydroxide when present in excess, shortens the
working life
but increases the water resistance of the glue line.
Addition of
sodium silicate (silica: soda ratio 3:1) increases the
working life
of
the glue, at all levels of alkalinity. A simple
formula
using
lime and alkali is given below. It has good water
resistance,
good working life about 7 hours.
Casein
Blend Glues
Casein
blends
with blood are dry powder glues for cold
pressing
plywood. The blood constituent in casein glue
contributes
quick setting, thus reducing clamp time, and both
dry and wet
strengths are improved. These glues are used in
the
construction of flush doors, boxes, furniture and other wood
assembly
work
where dark colour imparted by the blood is
accepted.
Blend
glues may compromise mixtures of casein with
soyabean
meal.
This type of blend is a way to the reduction of
the cost.
Lime
free Casein Adhesives
Casein
solution
not involving lime may be used as adhesives
for
adherends
other than wood. These are prepared by dissolving
casein with
sodium salts, which provide a sufficiently medium
alkali to
dissolve the casein. Commonly used sodium salts are
borax soda
ash,
trisodium phosphate, and others. Casein in
solution in
strong alkalies, such as sodium hydroxide, and
ammonium
hydroxide, also have the adhesive value. Organic
amines
dissolve
casein and there is some small use of alkyl
amines,
ethanolamines and morpholine as the solvent for casein
adhesives.
To
give limefree casein adhesives some measure of
water
resistance, formaldehyde or formaldehyde donor in the
form of
resin
or hexamethylene tetramine may be added. More
commonly, an
oxide or salt of zinc, aluminium or chromium are
added to
improve water resistance. A formulation is given below
used for
plywood.
AMINO
RESIN ADHESIVES
Introduction
Amino resins
are the condensation products of amino
compound
with
aldehydes. The most common and widely used
amino
compounds
are urea and melamine where as formaldehyde
is almost
always the aldehyde.
In a poly
condensation reaction, a reactant of functionality
greater than
two leads to branching and crosslinking. The
resultant
three
dimensional network can attain greater size
indefinitely,
becoming insoluble and infusible. The over all
reaction of
amino resin can be described in three stages. The
first stage
is
the reaction of amino compound and formaldehyde
to a form a
methylol derivatives
RNH2+HCHO
RNHCH2OH
Urea is
tetrafunctional and melamine is hexafunctional.
Theoritically
therefore, the initial reaction can lead to the
formation of
a
tetramethylol derivatives of urea or a hexamethylol
derivatives
of
melamine, of the ratio of formaldehyde to ammonia
is high
enough
for urea, formation of a methyl group slows
formation of
another. These methylol derivatives are condensed
with the
evolution of water of formaldehyde.
The
properties
of the adhesive intermediates are very much
dependent on
the reaction condition. Molecular weight may vary
from a few
hundred to a few thousand, with a wide distribution
of molecular
size. Characteristic of commercial products are
solubility,
viscosity, pH and concentration. The products are
available
either in dry form or in aqueous solutions. Urea resin
adhesives
are
usually marketed in aqueous solution whereas
melamine
resin
adhesives are available in powder form.
Manufacturing
Technology
Fig. 25.1 is
a
flow chart for the manufacture of amino resin
adhesives.
All
the commercial processes are batch type. The unit
operations
are
reflux and condensation, filtration and spray
drying.
Because
of the corrosiveness of formaldehyde and its
formic acid
content, the reaction is usually carried out in a
stainless
steel
vessel. The reaction vessel is equipped with a
turbine
agitator and reflux condenser and jacketed for heating
or cooling.
The order of
addition to the reactor is formaldehyde, boric
acid and
urea.
When all the components are added to the reactor.
pH is
adjusted
to 7.0-7.8 and the charge is heated to 120ºC.
Disappearance
of urea causes the pH to drop to about 4.0. The
reaction
mixture is refluxed at atmospheric pressure for 2 hr.
Vacuum is
applied and distillation is carried out under vacuum
of 28-29 in.
Of
mercury, until approximately 33 parts by weight
of water is
removed. Then the system is shifted to total reflux
and cooled
to
about 30ºC. The pH is adjusted to 7.2-7.4 with
sodium
hydroxide. The molar ratio: formaldehyde, urea, is
usually
1.75:1
to 2:1 for plywood adhesives.
board
applications, to avoid the smell of formaldehyde in the final
product.
General practice is about 1.3-1.5 moles of formaldehyde
per mole of
urea. The pH is adjusted to 8.5 to 9.0 and the mixture
is heated to
the boiling point under agitation. The solution is
refluxed for
40
min and cooled. The pH is then adjusted to
7.0-8.0 with
a
saturated solution of trisodium phosphate.
Adhesives
for Hardwood Plywood
Plywood is
an
assembly of an odd number of layers of wood
(Veneer)
joined
together by means of an adhesive. The difference
between hard
wood, plywood and soft wood plywood is that the
former has a
ply of wood from the broad-leaf tree, e.g. oak,
walnut,
maple
etc. where as the veneers for softwood plywood
comes from
coniferous trees. Hardwood plywood is generally used
for
decorative
purposes, softwood plywood for structural purposes.
The adhesive
is
applied by means of rubber covered rollers in
the glue
spreaders. The coated veneers are alternated with the
uncoated
veneer
in the final assembly. Then the assembled
veneers are
pressed
in a hot press at approximately 90ºC and
150-300 psi
pressure. Press time is about 5-7 min.
Sand
Core Binder
Cores are
projections of sand in the mould cavity for the
purpose of
marking holes in the casting. After casting, the cores
are
surrounded by
metal and should be removed without
damaging the
casting. Urea resins are capable for forming
mechanically
strong cores.
PHENOLIC
RESIN ADHESIVES
Introduction
Phenolic
resins are the reaction products of phenol or
substituted
phenols with formaldehyde. An unlimited variety of
resins
are possible depending on (1) the choice of phenol (2) the
phenol:
formaldehyde molar ratio (3) the type and amount of
catalyst
used (4) the time and temperature of the reaction.
Resole
resin
The
active positions on the phenol molecule are the two
ortho
and one para positions. When there is more than one mole
of
phenol in the presence of an alkaline catalyst, resole is
formed.
The amount of heat determines the final form of product,
e.g.,
whether the resin is of low viscosity, water soluble liquid
or
a grindable solid. If the reaction is carried too far, the resole
can
gel. Therefore the reaction is always conducted under
carefully
controlled conditions of time, temperature, pH and mole
ratio
of formaldehyde to phenol.
Novolac
Resins
The
reaction of one mole of phenol with less than one mole
of
formaldehyde, under acid conditions, results in a novolac
resin.
Novolac resin contains methylene links and are phenol
terminated.
Methylol and methylene ester groups that are
present
in resole resins are absent in novolac. Therefore, this
type
of resin is incapable of further reaction without the addition
of
more formaldehyde. This is accomplished by the addition of
hexamethylene
tetramine, which is known as “hexa”. Hexa
makes
the non-heat-reactive thermoplastic novolac capable of
reacting
under heat to a cross linked advantage that novolac
resins
have over resoles is that no water of reaction is evolved
during
cure with hexa. Molecular weight of phenolic novolacs are
in
the 500-900 range.
Manufacture
A
typical phenolic resin is made by a batch process in a
jacketed
stainless steel reaction kettle, equipped with anchor
type
agitator and condenser. Molten phenol and formaldehyde
(37-40%)
are charged into the kettle and agitation begin. For a
novolac,
an acid catalyst is added and steam is introduced into
the
jacket to heat the batch with atmospheric reflux. The reaction
is
continued for 3-6 hrs at 100ºC. The reaction time is dependent
upon
pH and phenol: formaldehyde mole ratio. Following the
reaction
period, the batch is dehydrated under atmospheric
pressure
and than vacuum. If the resin is to be solid in solution,
the
solvent is slowly added to the molten resin in the still,
cooled
by refluxing and discharged into drums. Most of the solid
resins
discharged into pans are pulverized and blended with
hexa
before packaging.
To
make resole resin, an alkaline catalyst such as sodium
hydroxide
is added to the phenol and formaldehyde before
heating
the batch to 80-100ºC. Reaction times are generally 1-
3
hr. Since resole resin is capable of gelling in the still,
dehydration
temperature is kept below 105ºC. By the application
of
vacuum solid resoles are discharged into resin coolers. The
low
molecular weight, water soluble resins are finished at as
low
a temperature as possible, usually about 50ºC, whereas the
less
reactive para-substituted resoles can be finished at
temperature
as high as 120ºC.
Adhesive
Compounding
There
are two methods in general used for compounding
polychloroprene/phenolic
adhesives. The first method involves
masticating
the rubber on a two roll mill to reduce crystallinity
and
improve solubility. The time of milling and degree of shear
are
frequently used to control adhesive viscosity. The magnesium
oxide
and zinc oxide are compounded into the rubber on an
unheated
mill. The magnesium oxide is always added before zinc
oxide
to preclude premature curing of the rubber. The antioxidant
is
also added. The compounded rubber is then dissolved with
the
resin in the solvent blend in a cement tub.
Compounders
who do not have milling equipment use the
slurry
method for adhesive preparation. This method consists
of
simply adding of resin, pigments and antioxidants together
with
the unmilled rubber to the solvent blend in the cement tub.
Adhesives
made from unmilled rubber will be more viscous and
therefore,
are usually produced at lower solid content. They will
also
have higher initial cohesive strength. Adhesives from milled
polymer
are however uniform and retain their uniformity upon
ageing.
Vinyl/Phenolic
Vinyl
formal, vinyl acetal, and vinyl butyral may be combined
with
phenolic resin to produce tough structural adhesives with
good
impact strength, resistance to oil and aromatic fuels and
good
salt spray and weathering resistance. The presence of
hydroxyl
groups on the vinyl chain makes it likely that
crosslinking
occurs between the phenolic resole resin and
hydroxyl
groups during elevated temperature cure.
ACRYLIC
ADHESIVES AND SEALANTS
POLYMERIZATION
All
industrial
polymerization processes are carried out at an
elevated
temperature in the presence of an initiator.
Polymerization
can be carried out in bulk, solution, suspension,
or emulsion.
The most important processes for producing acrylics
for
adhesives-solution and emulsion polymerization are dealt
with here.
SOLUTION
POLYMERIZATION
In solution
polymerization, the monomer or monomer
mixture is
dissolved in a solvent which is relatively inert to free
radicals,
e.g.,
ethyl or butyl acetate, benzene, toluene, petroleum
solvent of
ketones;
then polymerization is effected at elevated
temperatures
in
the presence of an initiator such as an organic
peroxide or
an
azo compound which is soluble in the solvent.
Properties
of the product
The type of
solvent used has a great influence on the reaction
speed and
the
molecular weight of the resulting polymer in
solution,
because of the different chain transfer activities of the
various
solvents. Thus, the viscosity of a polymethyl acrylate
solution,
and
the molecular weight of the polymer, decreases in
the
following
order: benzene, ethyl acetate, ethylene dicholoride,
butyl
acetate,
methyl isobutyl ketone, and toluene. The solvent
with the
highest chain transfer activity gives polymers of lowest
molecular
weight. The molecular weights of solution polymers
are normally
lower than those of emulsion polymers. In selecting
the solvent
to
be used, consideration must also be given to the
economic and
safety aspects.
Emulsion
polymerization
The emulsion
polymerization, process for homo-and
copolymerization
of acrylic compounds is of greater significant
than the
solution polymerization process. It is effected in the
presence of
emulsifiers and initiators (e.g. alkali persulfaces),
normally in
water as the external phase. Suitable emulsifiers
are, for
example,
alkali salts of longchain aliphatic carboxylic or
sulfonic
acids,
of sulfated ethylene oxide adducts.
PROPERTIES
At room
temperature, the homopolymers of methacrylic and
acrylic acid
as
well as those of the lower methacrylic acid esters
are hard,
nontacky products which are suitable only for special
applications
in
the adhesives field. Homopolymers of acrylic acid
esters from
alcohols with at least 2 carbon atoms, which are
elastic,
soft,
and partially highly tacky products, are used for a
much larger
range
of adhesive applications.
Further
physical properties of the most important acrylic
homopolymers
in
comparison to those of polyvinyl acetate are
given in
Table
1 (properties increasing in direction of arrow).
FORMULATIONS
AND APPLICATIONS
Adhesives
for paper converting
The
requirements imposed on adhesives for bonding paperto
paper are
normally not too severe. Because the absorptivity
and surface
condition of paper, animal and vegetable adhesives
can attain
satisfactory wetting and encourage and thus yield
adequate
bonding strength.
However,
since
the requirements imposed on the bonding
speed-have
increased steadily, they can no longer be met with
adhesives
based
on natural products. Thus, it was not possible
to design
and
use modern automatic paper converting machines
until it was
discovered that polymer dispersions based on
polyvinyl
acetate are suitable for this application. By modifying
these
homopolymeric polyvinyl acetate dispersions by adding, for
instance,
plasticizers, solvents and resins, it was even possible
to render
these
adhesives suitable for bonding coated and
lacquered
paper
and to a certain extent also for bonding paper
to polymer
films. The range of application for dispersions
modified in
this way is, however, limited by the fact that
plasticizer
migration may adversely affect the adhesion and/or
bonded
materials. The demand for raw materials and adhesives
with
improved
specific adhesion therefore increased with the
improvements
of
the materials used in the paper converting
industry,
such
as printing, lacquering, application of water vapor
impermeable
coatings on paper and board, and the use of polymer
films.
Because of
their high specific adhesion to a great variety of
surfaces,
polyacrylic acid esters in the form of aqueous
dispersions
and
organic solutions were found suitable for the
production
of
adhesives for surfaces which are difficult to bond.
The products
in
question are either copolymers of acrylic acid
esters with
one
another, or, especially in the case of packaging
adhesives,
copolymers of acrylic acid esters with vinyl propionate
or vinyl
acetate i.e. copolymers of vinyl acetate with acrylic acid
esters.
Terpolymers produced from acrylic acid, acrylic acid ester,
and vinyl
acetate have increased adhesion to metal foils and
various
plastics and are therefore used for producing adhesives
for this
field
of application. Terpolymers of this type are also
very
resistant
to plasticizer migration, e.g., from plasticized PVC
film.
Flame
Resistant & Pressure Sensitive Adhesive
For some
applications it may be necessary to use a flame
resistant
pressure-sensitive adhesive. Acrylics can be rendered
flame
resistant.
Pressure-sensitive
acrylic adhesives can normally be applied
by the
conventional methods, e.g., direct or reverse roll coating
or by
air-knife
coating. The adhesive compound is applied either
directly to
the
final substrate or a release paper. In the latter
case the
adhesive on the carrier is dried or crosslinked before
it is
transferred to the final substrate. This transfer method is
necessary in
those case where the backing material would
deteriorate
during the drying or crosslinking process. The transfer
method is,
for
instance, commonly used for producing decorative
films.
For
protecting
the adhesive coat of pressure sensitive
adhesive
materials during transport and storage, the adhesive
coat except
in
the case of adhesive tapes in rolls, is usually
covered by a
release paper or a release foil, Silicone treated
paper,
polyethylene or PVC films are, for instance, suitable for
this
purpose.
Long-chained acrylates exhibit good release effects.
The release
material must not have any adverse effect on the
pressure-sensitive
adhesive coating. Undesirable effects can be
obtained
when
unsuitable silicones are used or when the
silicones
are
not properly processed.
Acrylic
Sealants
Linseed oil
and
bitumen were for a long time the commonest
base
materials
for building sealants. Further developments in
building
construction and the steadily increasing demands on
quality
resulted in the development of a number of synthetic
polymers for
sealants. The first work on acrylics for building
sealants was
carried out in response to the technical and
economic
success which was achieved by this product class in
the last
10-15
years in the field of surface coatings, such as
paints. The
first serviceable acrylic compounds were placed on
the market
in
about 1960. In the meantime the acrylics,
particularly
the aqueous acrylic dispersions, gained considerable
significance
in
the production of sealants because of their
outstanding
aging resistance and adhesion properties as well
as their
favourable price.
It is
expected
that the increase of the consumption of
aqueous
acrylic
sealants will be above average in the next few
years. The
total consumption of sealants will increase only be
approximately
20% during the same period.
The acrylics
used nowadays for sealants are tailor-made
copolymers
of
acrylic and/or methacrylic acid esters and other
monomers.
Usually
several
monomers are present in order to achieve
the desired
properties, such as elasticity, adhesion, resistance
to UV
radiation, resistance to chemicals, and hardness, suitable
polymers are
linear polymers, most of which are thermoplastic,
as well as
polymers which can be rendered adequately elastic
by cold
vulcanization, oxidation, or by the effect of alkaline
substance,
such
as caustic soda solution, cement, or lime.
Acrylics
which
can be crosslinked with the aid of oxidative
catalysts or
epoxy resins are also well known. The dominating
raw
materials
for acrylic sealants are based on aqueous acrylic
dispersions.
Solvent-containing
products and solvent-free products are
also
available
on the market. The solvent-free products have
been on the
market only for a short time and the experience
gained with
them is still inadequate. The solvent-containing
products
have
been on the market for a long time but they gained
considerable
less significance than the acrylic dispersions.
Solvent-containing
products are usually 80-90% solutions in
xylene. In
the
initial stage aqueous dispersions were available
with a
solids
content of only 50-55%. Dispersions with a higher
solids
content,
were obtained by improving the polymerization
technique.
All
acrylics must be modified with fillers and other
aids in
order
to achieve optimum properties.
Aqueous
Acrylic Sealants
Aqueous
acrylic
sealants are employed mainly for those
applications
for which compounds bases on linseed oil, butyl
rubber, or
polyisobutylene have been used hitherto, i.e., the
sealing of
joints which are subject to little elongation. Viz joints
between
curtain
walls and door and window frame joints.
In view of
the
experience gained hitherto, it appears that
the aqueous
acrylic sealants are also suitable for joints between
prefabricated
concrete building components with a practical
elongation
of
approximately 10-15%.
Soft acrylic
sealants with a high degree of elongation have
already been
successfully used for many years for joints between
small
building
components and special applications, e.g., aerated
concrete.
Even the
results obtained hitherto in the trials for expansion
joints,
which
were commenced some time ago, have been positive
until now.
Practice will show whether aqueous acrylic sealants
are in fact
suitable for this application.
Harder
compounds
are mainly used for do-it-yourself
application
and
for sanitary equipment, e.g., bathtub and
washbasins.
AMINO
RESINS
Introduction
Amino resin
are
manufactured throughout the industrialised
world to
provide a wide variety of useful products. Adhesives (qv),
representing
the largest single market, are used to make plywood,
chipboard,
and
sawdust board. Other types are used to make
laminated
wood
beams, parquet flooring, and for furniture
assembly.
Some
amino resins are used as additives to modify
the
properties
of other materials. For example, a small amount
of amino
resin
added to textile fabric imparts the familiar washand-
wear
qualities
to shirts and dresses. Automobile tires are
strengthened
by
amino resins which improve the adhesion of
rubber to
tire
cord. A racing sailboat may have a better change
to win
because
the sails of polyester have been treated with an
amino resin.
Amino resins can improve the strength of paper
even when it
is
wet. Molding compounds based on amino resins
are used for
parts of electrical devices, bottle and jar caps,
molded
plastic
dinnerware, and buttons.
Amino resins
are also often used for the cure of other
resins such
as
alkyds and reactive acrylic polymers. These
polymer
systems
may contain 5-50% of the amino resin and are
commonly
used
in the flexible backings found on carpets and
draperies,
as
well as in protective surface coatings, particularly
the durable
baked enamels of appliances, automobiles, etc. The
term amino
resin is usually applied to the board class of
materials
regardless of application, whereas the term aminoplast
or sometimes
amino plastic is more commonly applied to
thermosetting
molding compounds based on amino resins. Amino
plastics and
resins have been in use for the past fifty years.
Compared to
other segments of the plastics industry, they are
mature
products, and their growth rate is now only about half
of that of
the
plastics industry as a whole.
Most amino
resins are based on the reaction of formaldehyde
with urea or
melamine. Although formadehyde combines with
many other
amines, amides, or amino triazines to give useful
products,
only
a few have found commercial utility, and they are
of minor
importance compared to the major products based on
urea and
melamine. Benzoyuanamine, e.g., is used is amino
resins for
coatings because it provides excellent resistance to
laundry
detergent, a definite advantage in coatings for automatic
washing
machines, dihydroxyethyleneurea is used for making
amino resins
that provide wash-and-wear properties to clothing.
Aniline-formaldehyde
resins were formerly important because of
their
excellent
electrical properties, but have been supplanted
by newer
thermoplastics. Nevertheless, some aniline resins are
still used
as
modifiers for other resins. Acrylamide occupies a
unique
position
in the amino resin field since it not only contains
a
formaldehyde-reactive site but also a polymerisable double
bond. Thus
it
forms a bridge between the formaldehyde
condensation
polymers and the versatile vinyl polymers and
copolymers.
Formaldehyde
links two molecules together and is hence
diffunctional.
Each amino group has two replaceable hydrogens
that can
react
with formaldehyde and thus is also difunctional.
Since urea
and
melamine, the amino compounds commonly used
for making
amino resins, contain two and three amino groups,
they react
polyfunctionally with formaldehyde to form threedimensional,
cross-linked
polymers. Compounds with a single
amino group,
such as aniline or toluenesulfonamide, can react
with
formaldehyde to form only linear polymer chains.
This is true
under mild conditions, but in the presence of
an acid
catalyst a higher temperatures, the aromatic ring of
aniline,
e.g.,
may react with formaldehyde to produce a crosslinked
polymer. The
use of thiourea improved gloss and water
resistance,
but
stained the steel molds. As amino resins
technology
progressed the amount of thiourea the formulation
could be
reduced and finally eliminated altogether.
Melamine
resins
were introduced about ten years after
molding
compound. They were very similar to those based on
urea but had
superior qualities. Melamine resins rapidly
supplanted
urea
resins and were soon used in molding,
laminating,
and
bonding formulations, as well as for textile and
paper
treatments. The remarkable stability of the symmetrical
triazine
ring
made these products resistant to chemical change
once the
resin
had been cured to the insoluble, crosslinked
state.
Future
markets for amino resins and plastics appear to
be secure
because they provide unusual qualities. New
developments
will probably occur in the areas of more highly
specialine
materials for treating textiles, paper, etc, and for use
with other
resins in the formulation of surface coatings where
a small
amount
of an amino resin can significantly increase the
value of the
basic material. Looking further into the future, the
fact that
amino
resins are largely based on nitrogen may put
them into a
position to compete with other plastics as raw
materials
based
on fossil fuels become more costly.
Raw
materials
Urea
Urea
(carbamide) is the most important building block for
amino resins
because urea-formaldehyde is the largest selling
amino
resins,
and urea is the raw material for melamine, the
amino
compound
used in the next largest selling type of amino
resin. Urea
is
also used to make a variety of other amino
compounds,
such
as ethyleneureas, and other cyclic derivatives
used for
amino
resins for treating textiles. They are discussed
below:
Urea is
soluble
in water, and the crystalline solid is some
what
hygroscopic,
tending to cake when exposed to a humid
atmosphere.
For
this reason , urea is frequently pelletised or
prilled
(formed
into little beads) to avoid caking and making it
easy to
handle.
Only about 10% of the total ureas production
is used for
amino resins, which thus appear to have a secure
source of
low-cost raw materials. Urea is made by the reaction
of carbon
dioxide and ammonia at high temperature and
pressure to
yield a mixture of urea and ammonium carbamate;
the latter
is
recycled.
CO2 + 2HN3 → NH2CONH2
+ H2O = H2NCOONH4
Melamine
Melamine
(cyanurrotriamide, 2,4,6-triamino-s-triazine) is a
white
crystalline solid, melting at approximately 350ºC with
vaporisation,
only slightly soluble in water, commercial product,
recrystallised
grade, is at least 99% pure. Melamine was
systhesised
early in the development of organic chemistry, but
it remained
of
theoretical interest until it was found to be a
useful
constituent of amino resins. Melamine was first made
commercially
from dicyandiamine but is now made from urea, a
much cheaper
starting material. The urea is dehydrated to
cyanamide
which
trimerises to melamine in an atmosphere of
ammonia to
suppress the formation of deamination products.
The ammonium
carbamate also formed in recycled and converted
urea. For
this
reason the manufacture of melamine is usually
integrated
with
much larger facilities with much larger facilities
making
ammonia
and urea. Since melamine resins are derived
from urea,
they
are more costly and are therefore restricted to
applications
requiring superior performance. Essentially all of
the melamine
produced is used for making amino resins and
plastics.
Formaldehyde
Pure
formaldehyde is a colorless, pungent smelling reactive
gas. The
commercial product is handled either as solid polymer
paraformaldehyde,
or in aqueous or alcoholic solutions. Marketed
under the
trade
name Formcel, solution is methanol, n-butanol,
and
isobutanol,
are widely used for making alcohol-modified urea
and melamine
resins for surface coatings and treating textiles.
Aqueous
formaldehyde, known as formalin, is usually 37 wt %
formaldehyde,
though more concentrated solutions are available.
Formalin is
the
general-purpose formaldehyde of commerce
supplied
unstabilised or methonol-stabilised. The latter may be
stored at
room
temperature without precipitation of solid
formaldehyde
polymers because it contains 5-10% of methyl
alcohol. The
uninhibited type must be maintained at a
temperature
of
at least 32ºC to prevent the separation of solid
formaldehyde
polymers.
Large quantities are often supplied in
more
concentrated solutions. Formalin at 44, 50, or even 56%
may be used
to
reduce shipping costs and improve manufacturing
efficiency.
Heated storage tanks must be used. For example,
formalin
containing 50% formaldehyde must be kept at a
temperature
of
55ºC to avoid precipitaton. Formaldehyde
solutions
stabilised with urea are used and various other
stabilisers
have been proposed. With urea-stabilised
formaldehyde
the user only adjust the U/F (urea/formaldehyde)
ratio by
adding
more urea to produce a urea resin solution ready
for use.
Paraformaldehyde
is a mixture of polyoxymethylene glycols,
HO (CH2O)n
H,
with n from 8 to as much as 100. It is
commercially
available as a powder (95%) and a flake (91%). The
remainder is
a
mixture of water and methanol. Paraformaldehyde
is an
unstable
polymer that easily regenerates form-aldehyde
in solution.
Under alkaline conditions, the chains depolymerize
from the
ends,
whereas in acid solution the chains are randomly
cleaved.
Paraformaldehyde is often used when the presence of
a large
amount
of water should be avoided as in the preparation
of alkylated
amino resins for coatings. Formaldehyde may also
exist in the
form of the cyclic trimer trioxane. This is a fairly
stable
compound
that does not easily release formaldehyde,
hence it is
not
used as a source of formaldehyde for making
amino
resins.
Approximately 25% of the formaldehyde produced
in India is
used in the manufacture of amino resins and plastics.
Other
materials
Benzoguanamine
and acetoguanamine may be used in place
of melamine
to
achieve greater solubility inorganic solvents and
greater
chemical resistance. Aniline and toluenesulfonamide
react with
formaldehyde to form thermoplastic resins. They are
not used
alone,
but rather as plasticizers for other resins
including
melamine and urea-formaldehyde. The plasticizer may
be made
separately or formed in situ during preparation of the
primary
resins.
Water
borne epoxy resins and derivatives
Electrodeposition
is an important new technique for coating
metals, and
water-borne epoxy ester-based vehicles are among
the leading
coating systems thus applied. The coatings are used
as
corrosion-resistant primers for automobiles, appliances, and
electrical
parts. An early approach involved maleinising epoxylinsed
fatty acid
ester (effecting a Diels-Alder condensation,
between
maleic
anhydride and a fourcarbon conjugated
unsaturated
segment of the fatty acid), then dissolving in butyl
cellosolve
and
subsequently neutralising 80% of the composition
by ammonia
solution or tertiary amine.
R-COOH + NH3
→
R-COO-
+ NH4
+
Maleated
resin
Resin anion
The resin
anion
is deposited on an anode under an applied
dc voltage
of
100 volts or more. The coating is then cured by
baking at
elevated temperatures. The epoxy ester can be made
water-soluble
by esterifying unreacted hydroxyl groups with
phthalic
acid
and thus preparing a phthalic acid half ester of
specific
acid
number. Other approaches include the use of dimer
acid and
versatic acid. This is still an expanding field, and further
advances are
expected.
Emulsions of
epoxy resins themselves are generating
interest
because of their ecological advantages. Coatings applied
from
water-based epoxies reduce hazards from fire, toxicity,
pollution
etc.
A bisphenol resin modified with a reactive diluent,
and
containing
emulsifier, is easily emulsified just before use
in a
high-speed
agitator with gradual addition of water. The
coatings
based
on this resin are cured with polyamide emulsions.
Among
suitable
emulsifying agents for a bis-epi resins are the
derivatives
of
nonylphenol and ethylene oxide.
Diluents
and modifiers
Many
applications have requirements for viscosity, flexibility,
impact
resistance, adhesion, pot-life, cost etc., which can be met
by the use
of
diluents and various modifiers.
Diluents
These
liquids
are used primarily to reduce the viscosity of
the epoxy
resin
system. They may be nonreactive. The
nonreactive
diluents may be volatile organic solvents or
nonvolatile
plasticizers. Solvents are used to obtain deep
penetration
in
such applications as prepreg laminating and
filament
winding. Ketones, esters, and glycol ethers are true
solvents for
epoxy resins; but aromatic hydrocarbons and
alcohols are
sufficiently compatible to function as diluents. Some
of the
commercial medium-viscosity (2000-4000 cps) resins
contain
dibutyl
phthalates as a nonreactive diluent. Viscosity
reduction of
70-80% is obtained by the use of about 15 phr of
dibutyl
phthalate. The products obtained from such resins are
generally
softer, less brittle and have less solvent resistant than
products
based
on unmodified or 100% reactive resins. Pine oil
was
suggested
as a nonreactive diluent.
Reactive
diluents are those that may take part in the curing
reaction and
become an integral part of the crosslinked system.
The reactive
sites on these may be either epoxides or other
functional
groups. Monoepoxide diluents include butyl glycidyl
ether,
cresyl
glycidyl ether, phenyl glycidyl ether, styrene oxide.
The first
two
are highly efficient diluents providing very great
reduction of
viscosity in small amounts. About 12 p butyl glycidyl
ether per
100
standard liquid resin brings the viscosity down
from more
than
10,000 cps to about 500-700 cps at 25°C. Since
monoepoxides
reduce crosslinking, some of the properties of the
cured resin,
such as water resistance, flexural strength, and
heat
distortion
temperature, are somewhat lowered. To overcome
this
disadvantage, diepoxides may be used as diluents, e.g., 1,4-
butanediol
diglycidyl ether, bis (2,3-epoxy cyclopentyl) ether. The
nonepoxy
type
reactive diluents include triphenyl phosphite and
butyrolactone.
Details of reactive diluents are shown in Table
6.
Flexibilisers
The bis-epi
type resins, epoxy nonvolacs, and other epoxy
resins
containing aromatic ring structures, when cured with the
usual
amines,
or anhydrides, give products that are hard and
brittle,
with
rather low impact resistance and poor elongation.
Flexibilisers
are employed to improve the impact resistance and
increase the
elongation of the cured products. Some improvement
in these
properties may be achieved with plasticizers such as
dibutyl
phthalate, but only at the expense of gross reduction in
other
properties such as solvent resistance. In epoxy technology
the term
flexibiliser generally refers to those compounds that
undergo
reaction and impart flexibility to the system by
increasing
the
distance between the crosslinks, interposing
segments
with
greater free rotation. Among favoured categories
of reactive
flexibilisers are the aliphatic diepoxides, the
polysulfide
telomers, and the amido-amine crosslinking agents
discussed
later. The flexible aliphatic epoxy resins, when used
alone give
soft-cured compositions having low physical strength.
They are
best
utilised in blends with the bisphenol-A based
epoxy
resins.
Incorporation of about 10-30% of flexible resins
retains most
of
the desirable properties of unmodified system
while
improving
the impact resistance and elongation.
Improvement
in
the flexibility of the modified system will also
depend on
the
type and chain length of the flexibilising resin,
the ether
type
and long-chain resins being more effective than
the ester
type
and shorter chain resins. Some epoxy are based
on dimerised
C18 fatty acid, polyurethane diglycidyl ethers and
cycloaliphatic
diglycidyl ethers. The diglycidyl ethers with
urethane
linkages provide tough products with excellent impact
resistance
at
temperatures as low as 6-55°C. The glycidyl groups
of the
cycloaliphatic resins react readily with polyamines at
ambient
temperatures.
The
polysulfides of commercial significance in epoxy resin
technology
are
Thiokol’s LP liquid polymers, which are essentially
mercaptan
terminated poly (ethyl formaldisulfide):
The various
grades of polysulfide polymers differ in amount
of branching
and in molecular weight, which may range from 600
to 7500. The
liquid polymers most often used with epoxy resins
are low
molecular weight polymers with approximate molecular
weight of
1000.
These polymers flexibilise epoxy resins by
extending
chain
length.
This
reaction
proceeds very slowly at room temperature, and
no useful
products are obtained unless curing agents such as
amines or
anhydrides are used. The polysulfide-epoxy resin
compounds
are
usually formulated as two component systems
to give good
shelf life and permit easy handling. In most
applications,
for every 100 parts of epoxy resin, about 75 parts
of
polysulfide
liquid polymer is used with about 10 parts of
amine curing
agent, commonly a tertiary amine such as 2,4,6-
tris
(dimethylaminomethyl) phenol. Aliphatic amines are not
compatible
with
polysulfides and tend to settle out.
Bituminous
modifiers
Coal
tar-modified epoxy coatings are used for pipes, tanks,
machinery
foundations, and boats, because of their outstanding
resistance
to
acids, alkalies, and brine. A 50/50 mixture of a
low
molecular
weight epoxy resin and coal tar pitch, incorporating
7.5 parts of
diethylene triamine per hundred parts of blends,
cured to a
corrosion-resistant, rubbery product within 24 hours.
Such
mixtures
also can be cured with amine adducts and
polyamides.
When added to flexible epoxy resins, coal tar
provides
better
elongation without reduction of tensile strength.
These resins
can be compounded with asphalt; addition of 30
phr aromatic
distillate results in elongation in excess of 300%
even at
-18°C.
Synthetic
polymers as modifiers
Various
thermoplastic and thermosetting polymers, including
elastomers,
have been incorporated to modify the properties of
the cured
epoxy
resin products. A nylon soluble in ethanol-water
mixture, is
used in epoxy-nylon film adhesives to obtain high
peel
strength
as well as good heat resistance. The nylon can be
a major or
minor component in the blend. Room temperature
peel
strength
usually increases with increasing amount of
polyamide,
but
with the sacrifice of high temperature resistance.
Excessive
deformation under high temperature curing can be
reduced by
blending with high-temperature-melting nylon
particles of
uniform fine size. A thermoplastic polyurethanemodified
epoxy resin
has
been developed which is reported to
give better
peel strength at cryogenic temperatures than that
obtained
with
epoxy-nylon.
Polyvinyl
formal and polyvinyl acetals show good compatibility
with epoxy
resins and improve peel strength of adhesives
Polyvinyl
formal improves impact resistance of powder coatings
remarkably
when
used at 35-100 phr level with silica and BF3-
amine
complex.
Tough powder coatings are claimed by blending
with
irradiated
polyethylene. Among the thermosetting resins,
phenolics
have
long been blended to obtain heat resistance in
adhesives
and
chemical resistance in coatings. Xyleneformaldehyde
resins are
useful in formulating epoxy casting
systems.
Butylated amino resins are used to crosslink high
molecular
weight epoxy resins or epoxy esters to obtain light
colored,
chemical-resistant coatings. Solid epoxy resins modified
with
hydroxy-functional silicone intermediates yield reaction
products
with
free terminal epoxy groups available for further
reaction
with
fatty acids or common curing agents. The siliconeepoxy-
based
products
have good heat stability, chemical and
moisture
resistance as well as good electrical properties making
them
suitable
for protective coatings, laminates, and molding
materials.
Elastomers
provide greater elongation and impact strength.
Polysulfides,
the most commonly used elastomer to flexibilise
epoxy
resins,
have been discussed already. Epoxy-chloroprene
(neoprene)
rubber blends have been cured with polyphenols or
aromatic
amines
to give tough, chemically resistant products.
Epoxy-nitrile
rubber blends yield high-peel-strength adhesives.
Carboxyl-terminated
nitrile rubbers, introduced by Goodrich,
were shown
to
toughen the cured epoxy resin at only 5 phr
loading.
Improvements in impact have been obtained by the
addition of
5-35% by weight of carboxyl terminated nitrile rubber
to
cycloaliphatic resins.
Fillers,
reinforcements, and other additives
Incorporation
of fillers and reinforcements into epoxy
formulations
can result in higher viscosity, longer pot life, lower
exotherm and
lower shrinkage. Properties of the cured polymers
may be
improved. Above all, use of fibrous fillers may lower the
cost of the
formulations. For mechanical strength, asbestos,
glass,
graphite
and boron fibers are used. Glass fibers the most
common
reinforcement, not only increases tensile, flexural, and
impact
strength, but also raised heat resistance.and reduces
shrinkage
and
thermal expansion. Graphite and boron fibers, very
high in
modulus
and thermal tensile strength, are used for high
performance
aerospace application where strength-weight
characteristics
are critical. Coated graphitised carbon fibers of
391,000 psi
have been incorporated in 60 volume % in epoxy
resins.
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