Phenolic resins are obtained by the reaction of phenols with aldehydes. The simplest representative of these types of compounds, phenol and formaldehyde, are by far most important. Phenolic resins are mainly used in the production of circuit boards. The development of synthetic resins for surface coating applications has usually followed the use of similar material in the plastic industry. One of the first synthetic resins ever used commercially, both in plastics and in surface coatings was the phenolic resin. Phenolic resins result aldehyde with or without modification. Phenol resin bonded wood materials; particle boards (PB), plywood, fiber board (FB) and glued wood construction element are used for outdoor construction and in high humidity areas because of the high water and weathering resistance of the phenolic adhesive bond and high specific strength. The competitiveness and development of the wood working industry are of utmost importance for the development for thermosetting plastics. This industry is the largest consumer of urea melamine and phenol resins. Phenolic laminates are made by impregnating one or more layers of a base material such as paper, fiberglass or cotton with phenolic resin and laminating the resin saturated base material under heat and pressure. The resin fully polymerizes (cures) during this process. The base material choice depends on the intended application of the finished product. Paper phenolics are used in manufacturing electrical components such as punch through boards and household laminates. Glass phenolics are particularly well suited for use in the high speed bearing market. Other applications of phenolic resins are in chemical equipments, fibers, socket putties, photo resists, tannins, brush putties, etc. Good performance at a reasonable cost has long been an important selling point for phenolic resins, especially in applications such as wood bonding and insulation, where discoloring and other drawbacks can be overlooked because of cost savings. Hence demand of phenolic resins is growing rapidly.
This book basically deals with general reaction of phenols with aldehydes, the resoles, curing stages of resoles, kinetics of a stage reaction, chemistry of curing reactions, kinetics of the curing reaction, the novolacs, decomposition products of resites, acid cured resites, composition of technical resites, mechanisms of rubber vulcanization with phenolic resins, thermosetting alloy adhesives, vinyl phenolic structural adhesives, nitrile phenolic structural adhesives, phenolic resins in contact adhesives, chloroprene phenolic contact adhesives, nitrile phenolic contact adhesives, phenolic resins in pressure sensitive adhesives, rubber reinforcing resins, resorcinol formaldehyde latex systems etc.
The present book covers manufacturing processes of phenolic resins. New entrepreneurs, technocrats, research scholars can get good knowledge from this book.
1. HISTORICAL DEVELOPMENT OF PHENOLIC RESINS
2. RAW MATERIALS
Phenols, Physical Properties of Phenol, Cumene Process (Hock Process), Cresols and Xylenols — Synthesis Methods, Alkylphenols, Phenols from Coal and Petroleum, Other Phenolic Compounds, Resorcinol, Bisphenol-A, Formaldehyde, Properties and Processing, Paraformaldehyde, Trioxane and Cyclic Formals, Hexamethylenetetramine, HMTA, Furfural, Other Aldehydes
3. CHEMICAL STRUCTURE
General Reaction of Phenols with Aldehydes, The Resoles, Curing Stages of Resoles, Kinetics of A-Stage Reaction, Chemistry of Curing Reactions, Kinetics of the Curing Reaction, The Novolacs, Decomposition Products of Resites, Acid-Cured Resites, Composition of Technical Resites
4. PHENOLIC RESINS FROM HIGHER
ALDEHYDES
Acetaldehyde, Butyraldehyde, Chloral, Furfural, Acrolein
5. PHENOLIC RESINS FROM POLYHYDRIC
PHENOLS
6. REACTION MECHANISMS
Molecular Structure and Reactivity of Phenols, Formaldehyde-Water and
Formaldehyde-Alcohol Equilibria, Phenol-Formaldehyde Reaction under Alkaline Conditions, Inorganic Catalysts and Tertiary Amines, Ammonia, HMTA and Amine-Catalyzed Reactions, Reaction Kinetics of the Base-Catalyzed Hydroxymethylation, Prepolymer Formation, Resole Cross-Linking Reactions. Quinone Methides, Acid Curing, Heat Curing, Phenol-Formaldehyde Reactions under Acidic Conditions, Reaction Kinetics in Acidic Medium, Reaction under Weak Acidic Conditions. “High-Orthoâ€-Novolak Resins, Novolak Cross-Linking Reaction with HMTA, Reaction with Epoxide Resins, Reactions with Diisocyanates
7. THE PHYSICAL STRUCTURE OF PHENOLIC
RESINS
Introduction, X-Ray Examination, Electron Microscope Examination, The Isogel Theory of Phenoplast Structure, The Spherocolloid Theory of Phenoplast Structure, Further Swelling Experiments, Development of Structure in A-Stage Resin, General Picture of Phenoplast Structure, Structure of Cast Phenoplasts
8. RESIN PRODUCTION
9. FILLERS FOR PHENOLIC RESIN MOULDING
POWDERS
Types of Filler, Effect of Filler on Impact Strength and Damping, Microscopic Structure of Fillers, Ratio of Resin to Filler, Standard Classification of Phenoplast Molding Powder According to Filler, Properties of Individual Fillers, Cellulose Derivatives, Wood Flour, Walnut-Shell Flour, Cottonseed Hulls, Cellulosic Fibers, Textile By-Products, Proteinaceous Fillers, Carbon Fillers, Mineral Fillers
10. FILLERS AND RESINS FOR LAMINATES
Classification of Laminates, Laminated Phenolic Sheets, Laminated Phenolic Tubes (NEMA Classi-fication), High Strength Paper Laminates, Plastic Bonded Cotton Fiber, Glass Fabric Filler, Resins used for Laminates
11 PHYSIOLOGY AND ENVIRONMENTAL
PROTECTION
Toxicology of Phenols, Toxicology of Formaldehyde, Environmental Protection, Waste Water and Exhaust Air Treatment Processes, Microbial Transformation and Degradation, Chemical Oxidation and Resinification Reactions, Thermal and Catalytic Incineration, Extraction Processes and Recovering, Activated Carbon Process, Gas Scrubbing Processes
12. DEGRADATION OF PHENOLIC RESINS BY
HEAT, OXYGEN AND HIGH ENERGY
RADIATION
Thermal Degradation, Oxidation Reactions, Degradation by High Energy Radiation
13. MECHANICAL PROPERTIES OF MOLDED
PHENOLIC RESINS
Introduction, Mechanical Properties Covered, Pheno-plast Properties at Room Temperature, Effect of Degree of Cure on Physical Properties, Tensile Strength, Modulus of Elasticity, Compressive Strength, Flexural Strength, Shear Strength, Bearing Strength, Impact Resistance, Creep and Stress Endurance, Fatigue Resistance, Influence of Temperature on Mechanical Properties, Influence of Temperature on Creep, Theoretical Discussion of Strength Properties of Phenoplasts, Strength-Weight Comparisons with Metals
14. MECHANICAL PROPERTIES OF LAMINATED
PHENOLIC RESINS
Introduction, Mechanical Properties at Ordinary Temperatures, Tensile Strength, Modulus of Elasticity, Compressive Strength, Flexural Strength, Shear Strength, Bearing Strength, Impact Resistance, Creep and Stress Endurance, Fatigue Resistance, Abrasion Resistance, Influence of Temperature on Mechanical Properties, Effect of Resin Content on Mechanical Properties, Effect of Moisture Content of Paper Filler Before Lamination, Effect of Laminating Pressure, Effect of Degree of Cure, Effect of Moisture Content on Physical Properties, Mechanical Properties of Post-Formed Laminates, Tensile Strength, Flexural Strength, Shear Strength, Impact Strength, Water Absorption
15. MODIFIED AND THERMAL-RESISTANT
RESINS
Etherification Reactions, Esterification Reaction, Boron-Modified Resins, Silicon-Modified Resins, Phosphorus-Modified Resins, Heavy Metal-Modified Resins, Nitrogen-Modified Resins, Sulfur-Modified
Resins
16. COMPOSITE WOOD MATERIALS
Wood, Residues of Annual Plants, Adhesives and Wood Gluing, Phenol Resins, Urea and Melamine Resins, Diisocyanates, Lignosulfonates, Bark Extracts, Physical Properties of Composite Wood Materials, Particle Boards, Wood Chips, Resins and Additives, Wood Chips, Resins, Hydrophobic Agents, Fungicides and Insecticides, Flame Retardants, Production of Particle Boards, Chip Blending, Pressing of Particle Boards, Properties of Particle Boards, Plywood, Resins, Additives and Formulations, Production of Plywood, High-Densified Plywood, Fiber Boards, Wood Fibers, Resins and Additives, Production of Fiber Boards, Structural Wood Gluing, Resorcinol Adhesives
17. MOULDING COMPOUNDS
Standardization and Minimum Properties, Composition of Molding Powders, Resins, Fillers, Reinforcements and Additives, Wood Flour and Cellulose Fibers, Asbestos, Mineral Flour, Other Fillers and Fibers, Colorants, Lubricants and Release Agents, Production of Molding Powders, Thermoset Flow, Manufacturing of Molded Parts, Compression Molding, Transfer Molding, Injection Molding, Selected Properties, Thermal Resistance, Shrinkage and Post-Mold Shrinkage, Thermal Expansion
18. HEAT AND SOUND INSULATION
MATERIALS
Inorganic Fiber Insulating Materials, Inorganic Fibers and Fiber Production, Resins and Formulation, Properties of Fiber Mats, Phenolic Resin Foam, Resins and Additives, Blowing Agents, Surfactants, Foaming Equipment, Foam Properties, Sound Insulating Textile Fiber Mats.
19. THERMAL PROPERTIES OF PHENOLIC
RESINS
Introduction, Coefficient of Expansion, Flame Resistance
20. CHEMICAL RESISTANCE OF PHENOLIC
RESINS
Introduction, Water Absorption, Effect of Reagents, Chemical Applications for Phenoplasts, Resistance to Microorganisms
21. OIL SOLUBLE PHENOLIC RESINS
Introduction, Pure Oil-Soluble Phenoplasts, The Modified Phenoplasts, Reactions of the Phenoplasts with Oils
22. FRICTION MATERIALS
Friction and Wear of Thermosets, Formulation of Friction Materials, Fibers, Fillers, Resins, Manufacturing of Brake- and Clutch Linings, Impregnation Process, Wet Mix “Dough†Process, Dry Mix Process
23. PHENOLIC RESINS IN RUBBERS AND
ADHESIVES
Mechanisms of Rubber Vulcanization with Phenolic Resins, Thermosetting Alloy Adhesives, Vinyl-Phenolic Structural Adhesives, Nitrile-Phenolic Structural Adhesives, Phenolic Resins in Contact Adhesives, Chloroprene-Phenolic Contact Adhesives, Nitrile-Phenolic Contact Adhesives, Phenolic Resins in Pressure-Sensitive Adhesives, Rubber-Reinforcing Resins, Resorcinol-Formaldehyde Latex Systems
24. PHENOLIC ANTIOXIDANTS
25. OTHER APPLICATIONS
Carbon and Graphite Materials, Phenolics for Chemical Equipment, Phenolic Resin/Fiber Composites, Phenolic Resin Fibers, Blast Furnace Taphole Mixes, Photo-Resists, Socket Putties, Brush Putties, Tannins, Ion-Exchange-Resins, Casting Resins
26. TECHNICAL MANUFACTURE OF
PHENOLIC RESINS
Resin Manufacture, Cast Resins, Resin Varnishes, Resin Compound, Molding Powder, Phenoplast Molding Laminates
27. MOULDING TECHNIQUE FOR PHENOLIC
RESINS
Introduction, Compression Molding, Transfer Molding, Injection Molding, Molding Practice, Preheating
28. MISCELLANEOUS TECHNICAL
APPLICATIONS OF PHENOLIC RESINS
Wood Adhesives, Bonding of Insulating Mats, Resins for Bonding Grinding Wheels, Wood Impregnation, Miscellaneous Adhesive Applications, Brake-Lining Resins, Cross Linking of Thermoplasts, War Uses of Phenoplasts.
29. FOUNDRY RESINS
Mold- and Core-Making Processes, Inorganic Binders, Organic Binders, Requirements of Foundry Sands, Shell Molding Process, Precoated Resin “Shell†Sand, Shell Sand Properties, Hot-Box Process, No-Bake Process, Cold-Box Process, Ingot Mold Hot Tops
30. INDUSTRIAL LAMINATES AND PAPER
IMPREGNATION
Electrical Laminates, Materials, Paper, Resins, Production of Electrical Laminates, Laminated Tubes and Rods, Cotton Fabric Reinforced Laminates, Decorative Laminates, Filters, Battery Separators
31. COATINGS
Automotive Coatings, Water-Borne Paints and Electrodeposition, Coatings for Metal Containers, Marine Paints, Shop Primers, Wash Primers, Oil-Modified Phenolic Resin Paints, Printing Inks, Rosin-Modified Phenolic Resins, Other Applications
32. ABRASIVE MATERIALS
Grinding Wheels, Composition of Grinding Wheels, Abrasive Materials, Fillers and Reinforcements, Resins, Manufacturing of Grinding Wheels, Cold Molding Procedure for Non-reinforced Wheels, High-Speed, Reinforced Grinding and Separating Wheels, Compression Molding Process, Snagging Wheels, Fibrous Laminated Wheels, Coated Abrasives, Composition of Coated Abrasives, Abrasive Materials, Adhesives and Coatings, Coating Process, Abrasive Papers, Abrasive Tissues, Vulcanized Fiber Abrasives
33. ELECTRICAL PROPERTIES OF PHENOLIC
RESINS -
Introduction, Theoretical Discussion, Numerical Data on Electrical Properties, Effect of Heating on Electrical Properties
34. ANALYTICAL METHODS
Monomers, Nitrogen and Water, Physical Properties, Reactivity, Chromatographic Methods, Spectroscopy
35. PHENOLIC RESINS AS ION-EXCHANGE
RESINS
Introduction, Application of Ion Exchange: Theory, Application of Ion Exchange: Types of Processes
^ Top
Historical
Development of Phenolic Resins
History
On
July 13 1907 Leo H. Baekeland
applied for his famous heat and pressure patent for the processing of
phenol
formaldehyde resins. This technique made possible the worldwide
application of
the first wholly synthetic polymer material (only cellulose derivatives
were
known before). Even from his first patent application of February 18
1907 it
was clear that Baekeland more than these predecessors was fully aware
of the
value of phenolic resins. Before his involvement with phenolic resins
Baekeland
had worked on photographic problems with the same intensity. His
success in
developing a fast copying photographic paper known throughout the world
under
the name Velox gave him the financial independence which allowed him to
build
his own research laboratory in his home in Yonkers New York. There
starting in
1905 he devoted his whole time to the investigation of phenolic resins.
However
the first patent covering phenolic resins (as substitute for hard
rubber) was
granted to A. Smith in 1899. A. Von Bayer found in 1872 while studying
phenolic
dyes that phenol reacting with formaldehyde was converted to a
colorless resin.
He first noticed that a reddish brown resinous mass was produced during
the
reaction of bitter almond oil with pyrogallic acid. However nothing was
done
with this resinous material. Ter Meer A. Claus and E. Trainer continued
the
experiments. Claus and Trainer obtained a resinous material from 2 mol
of
phenol and 1 mol of formaldehyde and hydrochloric acid. After the non
converted
phenol was distilled off a soluble resin was obtained with a MP of
100°C.
However they also could not think of application for this material and
reported
disappointedly it is not possible to crystallize this resinous
material.
Claisen and
Kleeberg continued the experiments after the company Merklin and
Losekam
brought the formaldehyde on to the market in 1889. Kleeberg obtained a
cross
linked insoluble resin using an excess of formaldehyde and hydrochloric
acid in
a vigorous reaction. There was no interest in the product obtained.
After
laboratory investigations performed by Manasse and Lederer the Bayer
Company
applied for a patent for a process for the production of o and p
hydroxybenzylalcohol but without mentioning a formation of the resin.
Speier
obtained an insoluble material from resorcinol formaldehyde and ammonia
as
catalyst which could be used as an antiseptic. Speier Smith and Luft
were the
first ones to draw the attention to technical applications for curable
phenolic
resins. Smith in particular pointed out the valuable properties of the
new
material which did not melt was a good insulating material and could
serve as a
substitute for ebonite and wood. Luft tried to flexibilize the brittle
material
obtained by Smith by addition of solvents glycerin and organic acids.
He
recommended the following applications for his plasticized phenolic
resins water
proof coatings for fabrics fibers which are carbonized to form
filaments for
light bulbs acid and alkali resistant vessels billiard balls buttons
handles imitation
amber and corals when colorants and fillers are added. Almost at the
same time the
Louis Blumer Company applied for a patent for the production of
synthetic
resins as substitutes for shellac. The solid soluble phenolic resins
made with
organic acids as catalysts were the first commercial scale phenolics in
the
world sold under the trade name Laccain. In February 1903 Henschke
continued
the experiments of Manasse and using alkali hydroxide as catalyst for
the P/F
reaction obtained an insoluble resin. Further improvements to the
preparation
of phenolic resins were made by Fayolle and Story. Story worked without
catalysts. Laire tried to find a substitute for copal and damar by
heating
phenol alcohols. He obtained high melting condensation products which
were not
soluble in low boiling alcohols but which could be dissolved in
turpentine or
camphor oil.
So that
when
Baekeland started with his studies of phenolic resins the following
facts were
already known
Phenols
and formaldehyde are converted to resinous products in the presence of
acidic
and alkaline catalysts. These may be permanently fusible and soluble in
organic
solvents or heat curable depending upon the preparation conditions.
Phenolic
resins were already being sold as substitutes for shellac ebonite horn
and
celluloid. These are colorable can be mixed with fillers and under the
influence of heat shaped in molds into solid parts.
However
economic
production of molded parts was not yet possible. The heat and pressure
patent
became the turning point indicating clearly the importance of economic
processing techniques for market acceptance. Phenolic resins mixed with
fillers
could be hardened in a press or an autoclave which was called
bakelisator under
pressure at temperatures above 100 °C in
a considerably short time and without the formation of blisters.
According to
the first Baekeland patent phenol formaldehyde catalysts and fibrous
cellulosic
material were reacted (in the cellulose matrix) at elevated
temperatures. The
impregnation of the fibrous material can be improved by application of
vacuum
and pressure infusible products being obtained only if formaldehyde was
used in
excess. Soon afterwards he reco mmended the impregnation of the
cellulosic
fibers with liquid phenolic resins acid catalyzed resins were being
used at
this stage. According to a patent application by Lebach in February
1907
insoluble and infusible condensation products useful as plastic
materials could
be obtained if phenol is reacted with surplus formaldehyde using
neutral or
basic salts as catalysts. In the same year Baekeland also patented a
process
for the preparation of phenolic resins using alkaline catalysts
preferably
ammonia NaOH and Na2CO3. A patent was granted to him in the USA but not
in
Germany because of the lack of inventive steps considering previous
publications by Henschke. It was in this patent however that resin
manufacture
was described for the first time just as it is carried out today
The
reaction is performed in a closed vessel with a reflux condenser to
prevent
loss of volatile materials.
The
reaction is interrupted when the desired viscosity is obtained.
Distillation
is performed in a vacuum and can be continued until a solid product
which is
still soluble in alcohols is obtained.
In 1908 the
cure
of phenolic resins at ambient temperatures by addition of strong
inorganic
acids was reported by Lebach.
Between
1907 and
1909 Baekeland conducted small scale trials with a few industrial
companies and
as result patented numerous applications for phenolic resins till 1909.
Molding
compounds can be made of pulverized fusible phenolic resins made in
alkaline
environment and fillers and molded to a shaped part of high toughness
strength
and chemical resistance.
Phenolic
resins
are excellent binders for abrasive materials.
Ammonia
catalyzed solid resins in organic solvents can be used for valuable
varnishes
and coatings for food containers.
Temperature
and
steam resistant lining materials can be made of phenolic resin
impregnated
asbestos fibers paper or cloth.
Phenolic
resins
are useful for coating wood yielding a hard and abrasive resistant
surface of
high gloss or can be applied as adhesive for veneer facing.
For
production
of fiberboards an aqueous wood fiber pulp is mixed homogeneously with
phenolic
resin. After a drying process similar to that used in the manufacturing
of
paper the fiber mats obtained are hardened between hot metal plates
under
pressure.
On February
5
1909 at a meeting of the New York Section of the American Chemical
Society Baekeland
reported for the first time the results of his thorough studies of
phenolic
resins which he called Bakelite. His report was received with great
enthusiasm
by a large audience. He stated his theory that the reaction of phenols
with
formaldehyde in the presence of catalysts occurs in three phases
The
formation of
a soluble initial condensation product which could be liquid viscous or
solid
and which he called A the formation of a solid intermediary
condensation
product which could still swell in solvents and which he designated as
B and
the formation of an infusible and insoluble product C.
In 1909
Lebach
suggested calling the liquid curable resin resol the B phase material
resitol and
the hardened phenolic resin resit while Aylsworth recommended the name
condensite.
In the same year Baekeland proposed the designation novolak for the
fusible thermoplastic
resin indicating the suggested substitution of shellac.
The
production
of paper laminates and laminated paper tubes made of liquid or
dissolved resins
the manufacture of noiselessly running cog wheels phenolic resin
putties and
glues to bond various materials and impregnating resin for coils and
similar
electrical devices were also suggested by Baekeland at this early
stage. The
low reactivity of o and p cresol was mentioned and recommended as a
means of
delaying the hardening reaction and increasing plasticity. The high
reactivity
of m cresol had already been mentioned. In another application the use
of
phenyl and cresyl phosphates was recommended for making PF resins more
flexible. Also tung oil was recommended as plasticizing additive for
impregnating and coating resins and the resin preparation method
mentioned. A
further process for manufacturing phenolic resin bonded fiber boards
was
patented by him in 1915. The phenolic resin solution added to the fiber
pulp is
precipitated on the fibers by the addition of acidic salts according to
this
disclosure.
After these
successful preliminary studies the time came to put what had been
learned into
practice. After a visit by Baekeland in June and July 1909 to Germany
the
companies Rutgerswerke AG and Knoll & Co together with
Baekeland founded
the Bakelite Gesellschaft mbH at Erkner near Berlin on May 25 1910.
This was
the first company in the world to produce synthetic resins. On October
10 1910 Baekeland
founded the General Bakelite Company in the USA and later other
companies in
England France Japan and Canada. On March 22 1922 the Bakelite
Corporation was
founded incorporating Redmanol Chemical Products Company and Condensite
Company. This corporation was taken over by the Union Carbide &
Carbon
Corporation in 1939. The first customers of the German Bakelite Corp.
were the
big electrical companies. They mainly used shellac for the
manufacturing of
laminated paper.
In 1910
1911 the
production of cresol molding compounds was also started. At that time
cresol
was preferred to phenol because it was cheaper J. W. Aylsworth a co
founder of
the Condensite Company also contributed a lot to the development of
resins and
the production of molding compounds. In 1910 he found that novolaks
which he
obtained from 3 mol of phenol and 2 mol of formaldehyde could be very
favourably cured by the addition of hexamethylenetetramine or
trioxymethylene.
RAW MATERIALS
Phenolic
resins
are produced by the reaction of phenols with aldehydes. The simplest
representatives of these types of compounds phenol and formaldehyde are
by far
the most important. As an average considering all applications the
production
of 1 ton of phenolic resin requires approximately 440 kg phenol
(containing
about 10% cresols and xylenols) and 220 kg formaldehyde as well as
solvents additives
and water.
Phenols
Phenols are
a
family of aromatic compounds with the hydroxyl group bonded directly to
the
aromatic nucleus. They differ from alcohols in that they behave like
weak acids
and dissolve readily in aqueous sodium hydroxide but are insoluble in
aqueous
sodium carbonate. Phenols are colorless solids with the exception of
some
liquid alkyl phenols. The most important phenols are listed in Table
below.
Data regarding the molecular structure of phenols and cresols are
listed below.
Table 1.Physical properties of some phenols
Physical Properties of Phenol
The melting
point of pure phenol at 40.9 °C is considerably lowered by traces of
water approximately
0.4°C per 0.1% of water. A water content of < 6% renders it
liquid even at
room temperature. To produce phenolic resins a mixture of 90% of
phenol/10% of
water is preferably used. Above 65.3°C phenol can be mixed with water
at any
ratio. During the cooling period of those solutions which may contain
28 92% of
water two phases are being developed phenol/water and water/phenol.
Phenol is
highly
soluble in polar organic solvents but not very soluble in aliphatic
hydrocarbons. Phenol crystallizes in the form of colorless prisms.
Exposed to
air phenol rapidly develops a reddish color especially if it contains
traces of
copper and iron. This happens if phenol is reacted in copper clad or
iron
reactors or if phenolic resins are stored in iron barrels. Additional
safety
and technical data of phenol are listed in Table 2 below
In 1978
only
approximately 3% of the world production of phenol was gained from
coal. Among
the synthetic processes the cumene process in the most frequently used.
The
basic raw materials for the cumene process and thus for the production
of
phenol are benzene and propylene. To help understand the dependence of
the
availability and price for phenolic resins on the crude oil situation
the base
material supply is given in Figure 1.
Fig.
2. Breakdown of benzene and
propylene consumption
Phenol Production Processes
The cumene
process is by far the most important synthetic process for the
production of
phenol and today probably accounts for 90% of the synthetic phenol
capacity in
the Western world.
A two step
oxidation process based on toluene was developed by Dow Chemical.
Kalama
Chemical in USA and DSM in The Netherlands are working according to
this
process 2.1.
The
hydrolysis
of halogenated aromatic compounds in 1930 developed by Raschig was
later
improved to the Raschig Hooker process. The first step the
oxychlorination of
benzene with hydrochloric acid in the presence of a copper on alumina
catalyst
at 275 °C is followed by hydrolysis of the chlorobenzene with water
(steam) on
a copper promoted calcium phosphate catalyst at 400 450 °C.
The
sulfonation
process the oldest one is almost of no importance nowadays.
Cumene
Process (Hock Process)
In Germany
the
phenol synthesis based on cumene was discovered by H. Hock and
published by him
and Sho Lang.
Soon after
World
War II the first pilot plant was constructed jointly by Rutgers werke
and
Bergwerksgesellschaft Hibernia with Hock s assistance. The commercial
production was first developed by the Distillers Co. (GB) and Hercules
Powder
(USA).
The
cumene (isopropylbenzene)
required for the Hock process is produced by alkylation of benzene with
propylene by use of a solid phosphoric acid catalyst (UOP Process Eq.
2.2).
Cumene is
oxidized with oxygen in air in the liquid phase to cumene hydroperoxide
(CHP)
according to reaction (2.3) yielding small amounts of dimethylbenzyl
alcohol
and acetophenone as by products. The mechanism mentioned in parenthesis
for the
acid catalyzed peroxide decomposition (Eq. 2.4) was postulated by
Seubold and
Vaugham.
CHP a
liquid
compound with relatively low vapour pressure is stable at normal
temperature
and conditions but decomposes very rapidly under acidic conditions and
higher
temperatures. During the second stage of the process the concentration
of CHP
and separation of unreacted cumene occur. The concentrated reaction
product is
then converted in a splitting plant by use of sulfuric acid as catalyst
to a
crude mixture of phenol and acetone which also contains a methylstyrene
as a by
product. Then various purification and distillation steps follow. a
Methylstyrene can also be hydrogenated and returned to the process.
CHP is a
potentially hazardous material. Therefore many safety regulations have
to be
observed and safety equipment must be installed in technical plants.
Cresols and Xylenols Synthesis Methods
Cresols
hydroxy
derivatives of toluene commonly designated as methyl phenols exist in
three
isomers depending on the relative position of the methyl towards the
hydroxyl
group. The molecular configuration is described in Section. The main
source of
cresols was originally coal tar. Today however synthetic processes
dominate mainly
based on toluene and phenol. The importance of the petroleum industry
as a
source of cresols and xylenols is relatively insignificant. Starting
with
toluene the cresols are obtained either by sulfonation by alkylation
with
propylene or by chlorination. In the sulfonation process the main
product is
the para derivative together with some ortho derivative. In the
chlorination
process the meta isomer prevails (about 50%) with an approximately
equal o/p
ratio. This route is advantageous for resin grade cresols. The
chemistry of the
toluene alkylation is very similar to the cumene process with
differences in
the oxidation step. Toluene is reacted first with propylene in the
presence of
A1C13 or other catalysts to obtain a mixture of cymenes. In this
process the
m/p ratio of approximately 2 1 by less than 5% o cymene is reported.
Fig.
3. Flow diagram of the
cumene process (Drawing Phenolchemie
More
important
among the synthetic processes is the production of cresols and xylenols
based
on alkylation of phenol with methanol. In the gas phase process the
methanol
and phenol vapours pass an aluminium oxide catalyst at approximately
350 °C
under moderate pressure. Mainly o cresol and 2 6 xylenol are obtained.
If 2 6
xylenol is desired as main product which is used for PPO production
magnesium
oxide is employed as catalyst. The following purification and
separation is
accomplished by vacuum distillation for o cresol (99% purity) and
crystallization for 2 6 xylenol (98%). The further purification of 2 6
xylenol
as needed for PPO is made by counter current extraction with aqueous
sodium
hydroxide solution (Pitt Consol). The yield of the gas phase process is
more
than 90% with regard to the phenol used and more than 85% with regard
to
methanol.
Other
processes
operated by Chemische Werke Lowi and UK Weaseling are performed in the
liquid
phase. The Lowi process is carried out at 300 350 °C at a pressure of
40 70 bar
Al methylate is used as catalyst. Thus mainly o cresol is obtained.
Higher
methanol ratio favors the formation of 2 4 and 2 6 xylenol. By
transalkylation
of xylenols in the presence of phenol the yield of cresols can be
increased.
UK
Wesseling
produces o and p cresol of 99% purity 2 6 xylenol of 98% and 2 4
xylenol of 92%
purity by use of zinc bromide as catalyst. The synthesis of p cresol
used
mainly for BHT or similar antioxidants is also performed by sulfonation
of
toluene.
Alkylphenols
Phenolic
compounds with a saturated carbon side chain containing a minimum of
three
carbon atoms should be termed alkylphenols. These alkylphenols are
produced
from phenols or cresols by Friedel Craft alkylation with olefins mainly
isobutene diisobutene or propylene. At low temperatures e.g. below 50
°C o
substitution predominates o Alkylphenols can be rearranged to p isomers
by
heating up to 150°C with acid catalysts. High yield of o derivates is
achieved
by use of Ca Mg Zn or Al phenoxides as
catalysts at a
temperature of about 150 °C.
In the
resin
area alkylphenols are used for the production of coating resins because
of
their good compatibility with natural oils and increased flexibility or
as
cross linking agents in the rubber industry. Other uses include
antioxidants surfactants
and phosphoric acid esters.
Phenols from Coal and Petroleum
An average
of
approximately 1.5% crude phenols mainly phenol (~ 0.5%) as well as o m
and p
cresols 2.3 2.4 2.5
2.6 and 3.5
dimethylphenol is found in coal tar. Phenols are further obtained from
condensates of coke oven gases and waste waters of coal gasification
plants.
The extraction is performed either according to the older Pott
Hilgenstock
process using benzene/sodium hydroxide or according to Lurgi s
Phenosolvan
process with diisopropyl ether as solvent. The extraction of phenols
from coal
tar is performed with diluted sodium hydroxide (8 12%) followed by
precipitation of the crude phenols with carbon dioxide. The flow
diagram of an
extraction plant is shown in Figure 5. The crude phenoxide solution
contains
approximately 0.5% non phenolic components neutral
oils (hydrocarbons) and pyridine bases
which have to be
removed prior to
precipitation mostly by steam distillation.
Other
processes
recommend selective solvents to extract phenol e.g. aqueous methanol
(Metasolvan process). However only Lurgi s Phenoraffin process which
uses an
aqueous sodium phenoxide solution as selective solvent has achieved
technical
importance. The selectivity of NaOH is superior to all other
recommended
solvents.
A further
source
of cresols is the petroleum industry particularly in the USA. During
the
catalytic cracking process various phenolic compounds are formed.
Similar to
the recovery of phenolic compounds from coal tar the extraction is
carried out
with diluted sodium hydroxide solution. After the separation of the
phenols by
precipitation with carbon dioxide further treatment follows by
distillation.
Thereby phenol (BP 181.8 °C) and o cresol (BP 191.0 °C) can be
recovered
technically pure immediately.
The
separation
of m and p cresol is only possible with special chemical or
physicochemical
methods due to the similar boiling points. This is also applicable to
xylenols.
In the urea process the mixture of m and p cresol is heated with urea.
Upon
cooling a crystalline addition compound of m cresol and urea is formed.
Further
p cresol when gently heated to 90 °C forms a crystalline addition
compound with
anhydrous oxalic acid. m Cresol with sodium acetate results in adducts
with low
solubility. The chemical processes use the differences in the reaction
rate of
sulfonation (sulfuric acid process) followed by crystallization and
hydrolysis
or the different behaviour in the alkylation with isobutylene
(isobutylene
process) and following dealkylation with sulfuric acid.
Other Phenolic Compounds
Cashew
Nut Shell Liquid (CNSL)
An
important
phenolic compound from natural sources is cashew nutshell liquid
(CNSL). This
liquid from the shells of cashew nuts which grow mainly in Southern
India has
become a useful raw material in the manufacture of special phenolic
resins to
be used for coating laminating and brake lining resin formulations.
Those
resins possess outstanding resistance to the softening action of
mineral oils
and high resistance to acids and alkalies.
The CNSL
when
obtained by a special heat treatment which includes decarboxylation
contains a
mixture of mono and diphenols (2.8) with an unsaturated C15 side chain
in the
meta position thereby exhibiting high reactivity towards formaldehyde.
The reacted
CNSL
resin the hardening reaction includes polymerization and polyaddition
yields a
solid infusible product which in powdered form ( friction dust )
retains high
binding power at raised temperatures and is used in brake lining
formulations.
Resorcinol
Resorcinol
as a
dihydric phenol (1 3 dihydroxybenzene) is a very interesting
intermediate
material for the production of thermosetting resins. However it is only
used
for special applications due to its relatively high price. The reaction
rate
with formaldehyde is considerably higher compared with that of phenol.
This is
of great technical importance for the preparation of cold setting
adhesives.
Resorcinol or resorcinol formaldehyde prepolymers can be used as
accelerating
compounds for curing phenolic resins. The addition of 3 10% of such
compounds
permits shorter cure cycles in particleboard and grinding wheel
production.
Furthermore the adhesion of textile materials e.g. tire cord to rubber
can be
greatly improved by pretreating them with resorcinol formaldehyde resin.
Resorcinol
can
otherwise be used as intermediate material for azo and triphenylmethane
and
other dyes pharmaceuticals cosmetics tanning agents textile treating
agents and
antioxidants.
The only
commercial process for the production of resorcinol is the alkali
fusion of m
benzenedisulfonic acid according to the Eq. (2.9).
As an
intermediate stage the disodium salt of the acid is formed. It is then
fused
with sodium hydroxide in a nickel alloy tank. The melt is dissolved in
water
and the resulting slurry acidified with sulfuric acid. The resorcinol
is then
recovered by counter current extraction and purified by distillation.
Plants
with a capacity of 10 000 tons per year each are operated by Koppers
USA and
Hoechst West Germany.
Another
route not
used commercially at the present time is the Hock process starting with
benzene
by alkylation with propene (2.10).
The
oxidation is carried out with
air at about 90°C the decomposition with diluted sulfuric acid in
acetone or
other solvent.
Bisphenol A
Bisphenol A
is
the common name for 2 2 bis (4 hydroxyphenyl) propane. In 1923 the
commercial
production of bisphenol A was introduced by Chemische Werke Albert in
Germany by
the addition of acetone to phenol using hydrochloric acid as a
catalyst.
Today the
main
use for BPA is the production of epoxide resins (~65%) and
polycarbonate.
Sulfuric acid is used in the newer process because of problems
associated with
the volatility and corrosiveness of hydrochloric acid. Sulfur compounds
like
thioglycolic acid or mercaptans further increase the reaction rate of
the acid
catalyzed addition of carbonyl compounds to phenols.
While the
purity
of BPA made by the sulfuric acid process is satisfactory for the use in
formaldehyde
resins high purity BPA is needed for the production of epoxy resins and
especially for polycarbonate. This is normally accomplished by
recrystallization from toluene or by crystallization as phenol BPA
adduct.
Aldehydes
Formaldehyde
is
the almost exclusively used carbonyl component for the synthesis of
technically
relevant phenolic resins. Special resins can also be produced with
other
aldehydes for example acetaldehyde furfural or glyoxal but have not
achieved
greater technical importance. Ketones are very seldom used.
The
physical
properties of aldehydes are compiled in Table4.
Formaldehyde Properties and Processing
Formaldehyde
is
produced by dehydrogenation of methanol over either an iron
oxide/molybdenum
oxide catalyst or over a silver catalyst. Because of hazards in
handling
mixtures of pure oxygen and methanol air is used as oxidizing gas.
Oxygen is
used to burn the developing hydrogen.
When the
silver
catalyst is used the reaction mixture of methanol and air is prepared
so as to
be over the upper flammability limit this is reversed when the oxide
catalyst
is used. Reactor effluent passes to the absorption train where
formaldehyde and
other condensables are recovered by condensation and absorption in
recirculating formalin streams. The raw formaldehyde solution is then
purified
by stripping out the unconverted methanol. Formaldehyde assay may be
adjusted
by regulating amounts of water added to the absorber column or by
subsequent
diluting in storage tanks. Inhibitors are added to retard the formation
of
paraformaldehyde in storage.
The Formox
process works with a mixture of iron oxide and molybdenum oxide as
catalyst.
The reaction proceeds at relatively low temperatures between 250 400 °C
almost
up to completion (95 98%). As a side reaction the formaldehyde produced
is
oxidized to yield carbon monoxide and water (2.12).
Table
4. Physical properties of
some aldehydes
The
Perstorp/Reichhold Montecatini Nissui Topsoe CdF Lummus and Hiag/ Lurgi
processes function in accordance with this method.
In
the BASF and Monsanto
processes a silver catalyst is used. Here in general methanol is
partially
oxidized and dehydrogenized at 330 450 °C on silver crystals or silver
nets.
The BASF process uses a vapour/methanol/air mixture. The conversion is
considerably high at approximately 90%. Silver catalyst processes with
an
incomplete methanol conversion performed at 330 380 °C have been
developed by
Degussa and ICI.
A further
process is based on the direct oxidation of methane with oxygen from
air at
approximately 450 °C and 10 20 bar on an aluminium phosphate contact.
This
process however has not yet achieved any technical importance.
Under the
influence of acids hemiacetals react to form acetals by eliminating
water. A
very important reaction is the formation of HMTA from ammonia and
formaldehyde.
The overall reaction the reaction mechanism is discussed in Section. By
the
catalytic action of strong bases e.g. sodium hydroxide formaldehyde
undergoes a
disproportionation reaction known as Cannizzaro reaction yielding
methanol and
formic acid according to Eq. (2.16).
Formaldehyde
solutions always contain minor quantities of formic acid due to the
Cannizzaro
reaction generally around 0.05%. The formic acid content can easily be
determined by titration with sodium hydroxide.
The
Kriewitz
Prins reaction may have some importance in modification reactions of
phenolic
resins with unsaturated compounds. Olefins can react with carbonyl
compounds
under non free radical conditions to result predominantly in
unsaturated
alcohols (2.17) m dioxanes(2.18) and 1.3 glycols (2.19).
As in the
hydroxymethylation of phenols the hydroxy methylene Carboniumion CH2 OH
is the
alkylating agent.
When
storing
formaldehyde solutions attention should be given to the fact that at
lower
temperatures or higher concentrations paraformaldehyde may separate.
The
stabilization of aqueous formaldehyde solutions can be achieved with
alcohols preferably
methanol. Urea melamine methylcellulose and guanidine derivatives are
also
recommended for stabilization among other compounds. Proper storage
containers
should be of stainless steel iron containers are not suitable.
Containers with
plastic lining or RP containers may also be used.
The major
use of
formaldehyde is in the production of thermosetting resins based on
phenol urea
and melamine.
Table
6. Breakdown of
formaldehyde consumption
As shown in
Table 6 approximately 55% of the total formaldehyde consumption occurs
in the
production of thermosetting resins. Since 30 55% aqueous solutions are
being
used for resin production it is often the case that the consumers
install their
own small formaldehyde plants which are supplied with methanol by a
large
central methanol plant. In spite of the high transportation costs for
formaldehyde solutions the relatively small capital investment will be
soon
repayed. For this reason there are about 53 formaldehyde plants in
Western
Europe at the present time with a total capacity of 4.3 million tons
per year
in 1976.
Formaldehyde
is
the more reasonably priced component in phenolic resins. As an average
throughout
all fields of application about 1.6 mol formaldehyde including HMTA per
mol
phenol are used.
Instead of
formaldehyde solutions of 30 55% higher concentrated aqueous or
alcoholic ones
may be used. These solutions are produced by dissolving
paraformaldehyde in
water at 80 100 °C by addition of a small quantity (1%) of NaOH or
tertiary
amines as depolymerization catalyst. It is also possible to concentrate
formaldehyde solutions by adding paraformaldehyde.
Table
7. Specification of a
formaldehyde solution
Paraformaldehyde
Paraformaldehyde
is a white solid low molecular polycondensation product of methylene
glycol
with the characteristic odor of formaldehyde. The degree of
polymerisation
ranges between 10 and 100. Types of paraformaldehyde common in the
trade
contain approximately 1 6.5% of water. The preparation of
paraformaldehyde is
performed by distillation of 30 37% aqueous formaldehyde solutions.
According
to the conditions (temperature time pressure) different types of
paraformaldehyde are obtained. The values in Table show that
paraformaldehyde
is not a defined compound.
Table
8. Properties of
paraformaldehyde
Paraformaldehyde
is only very seldom used for resin production because of its high price
compared with aqueous formaldehyde solutions and because of problems
associated
with the exothermal heat evolution. Paraformaldehyde and an acid
catalyst may
be used to cure novolak resins. However the odor and high formaldehyde
loss
make it unattractive. Products obtained are of poorer quality than when
HMTA is
used.
On the
other
hand paraformaldehyde is used almost exclusively to crosslink
resorcinol
prepolymers e.g. in cold setting structural wood adhesives. Lower
curing
temperatures are adequate because of the higher reactivity of
resorcinol thus
formaldehyde evolution is greatly reduced. The reactivity of
paraformaldehyde
depends on the degree of polymerization. A fairly accurate reactivity
test
method is the resorcinol test. This test indicates the period of time
in
minutes in which an alkaline resorcinol/paraformaldehyde mixture heats
up to
60°C due to the condensation reaction.
CHEMICAL STRUCTURE
Despite the
rapid commercial growth of the phenoplasts our knowledge of their
chemical
structure and of the chemical reactions which take place during curing
had been
meager until recently. The reaction of phenol with formaldehyde leads
to the
formation of a number of products which are difficult to separate and
are
readily susceptible to resinification by heat or reagents. Consequently
their
study tended to be unattractive to organic chemists. The difficulty of
the
problem was recognized by Baeke land who wrote in 1911
It should be pointed out
that we have to deal
here with substances which are amorphous noncrystalline nonvolatile and
cannot
be purified in the usual ways. Furthermore in any of these reactions
several
substances are liable to be produced at the same time. These substances
can
form solid solutions one with another or with any excess of the
reacting
materials employed.
Within the
last
five years however very significant work has been done in the study of
the
chemistry of the pheno plasts.
GENERAL REACTION OF PHENOLS WITH ALDEHYDES
In general
phenols
react with aldehydes to form condensation products if there are free
positions
on the benzene nucleus ortho or para to the phenolic hydroxy group and
if the
length of any of the substituents on the nucleus is not so great as to
cause
steric hindrance. Because of its greater reactivity formaldehyde is by
far the
most widely used aldehyde. For this reason the discussion in this
chapter will
be confined to reactions with formaldehyde the use of other aldehydes
will be
discussed in later chapter. The discussion in this chapter will also be
limited
to the monohydric phenols that is phenols with only one hydroxy group
for each benzene
nucleus.
The
reaction of
phenol with formaldehyde in the absence of any other reagents is very
slow and
catalysts are always added to accelerate the reaction. These catalysts
may be
either acids or bases and the nature of the reaction product depends
considerably upon the type of catalyst which is used.
The
mechanism of
the addition of formaldehyde to phenol is not entirely understood.
Manasse
suggested in 1894 that the formaldehyde may react in alkaline solution
as
methylene glycol as indicated in Equation (1) or it may undergo an
acetal
addition with subsequent rearrangement of the hemiformal as shown in
Equation
(2). The latter suggestion was also made by von Tollens and later by
Baekeland
and Bender.
Walker
favours
the idea of the formation of a primary phenolic hemiformal and suggests
that
the formation of the phenol alcohols may involve tautomeric
rearrangements of
the type indicated below
The
hemiformal
is very unstable and rearranges rapidly to the phenol alcohol because
of this
instability the hemiformal from a phenol has never been isolated.
Whatever
the
exact mechanism of formaldehyde addition may be the phenolic hydroxy
group
activates the benzene ring so that the methylol groups always enter the
nucleus
in ortho and para positions to the phenolic hydroxy group. When some of
the
ortho and para positions are occupied the reaction of the phenol
becomes much
slower when all of the ortho and para positions are unavailable no
reaction
takes place. The presence of substituents in the meta position also has
a
pronounced effect upon the rate of reaction with formaldehyde. Alkyl
groups in
the meta position tend to accelerate both the initial condensation and
the
subsequent resinification. The presence of a hydroxy group in the meta
position
as in resorcinol greatly increases the reactivity.
The nature
of
the reaction products depends upon the type of catalyst used. When
alkaline
catalysts are used the primary reaction products are phenol alcohols
and are
called resoles. When acid catalysts are used the primary reaction
products are
apparently also phenol alcohols but these rearrange quickly under the
influence
of the catalyst to give diphenylmethane derivatives to which the name
novolacs was
given by Baekeland in 1909. Because of this important difference the
chemical
structure of these two classes of phenoplasts will be discussed
separately.
THE RESOLES
The resoles
are
formed when formaldehyde acts upon a phenol in alkaline solution.
Almost any
alkali may be employed alkali metal hydroxides hydroxides of the earth
metals such
as barium or calcium ammonia or quaternary ammonium bases. There is
some
evidence to indicate that the products formed are not identical when
different
bases are used especially in those cases where the phenol has several
reactive
positions. It is almost definite that ammonia and amines when employed
as
catalysts enter into the condensation reaction. The nature of the
catalyst may
affect to some degree the position in the ring which is occupied by the
methylol group. For example Auwers states that strong alkalis favour
the
production of para methylol derivatives. However, not enough work has
been done
to present any definite data on this point.
Walker
has shown that from the
condensation of phenol with formaldehyde under alkaline conditions it
is
possible to isolate a tetramethylol derivative of 4 4 dihydroxy
diphenyl
methane having the probable constitution of Formula I Seebach isolated
the same
product melting at 145°C. by the action of more than three moles of
formaldehyde on one mole of phenol using a little magnesium oxide as a
catalyst. From o cresol the product which is obtained has the
constitution
shown in Formula II. From these observations it is concluded that the
first
condensation takes place in the para position. The existence of the
tetramethylol derivative implies branching of the chains at an early
stage.
Reference
has
already been made to the fact that the methylol group enters the ring
ortho or
para to the phenolic hydroxy group. When more than one such position is
available
polymethylol compounds are formed. Thus from phenols with three
reactive
positions the series of methylol derivatives shown in Formulas III may
be
formed. In the case of the reaction of formaldehyde with phenol the
presence of
all of these alcohols has been confirmed either through isolation of
the
alcohol itself or of a derivative. The monophenol alcohols saligenin (o
hydroxybenzyl alcohol) and homosaligenin (p hydroxybenzyl alcohol) were
separated and identified was able to prove the presence of the two
dialcohols
by methylation of the phenolic hydroxy group followed by oxidation of
the
methylol groups to carboxyl. The formation of the trimethylol compound
was
definitely established by Bruson and McMullen. They condensed three
moles of
formaldehyde with phenol in the presence of a strongly basic
nonaromatic
secondary amine. With morpholine for example a definite crystalline
compound melting
at 106 107°C. was formed which had the structure shown in Formula IV.
The
quantity of dialcohols formed depends upon the ratio of formaldehyde to
phenol but
even with equimolar ratios some quantities of polyalcohols are formed
because
of the great velocity of the addition reaction.
It is
difficult
to obtain the trialcohol but derivatives may be formed such as the
addition
compound described by Bruson. Stager attempted to isolate the
trialcohol in
pure form but failed. As Granger states the addition of the third
molecule of
formaldehyde becomes very slow as the reaction progresses although it
is
comparatively rapid in the earlier stages.
Curing Stages of Resoles
The
mechanism by
which the phenol alcohols condense to resins is very complex. Our
present
knowledge of the curing reactions will be discussed in detail later in
this
chapter. For convenience the curing mechanism has been divided into
three
phases. The three phases are
stage
resin (resole). This
represents the initial condensation product of phenol and formaldehyde.
The
resin consists mainly of phenol alcohols although it is probable that
some
condensation has taken place to give methylene ethers methylol
derivatives of
diphenylmethane and perhaps methylenequinone or its polymers. Lebach
termed
this stage resole from the Latin resina (a resinous body) and ol which
referred
to the solubility in alkalis and pointed to the probable presence of
hydroxy
groups.
stage
resin (resitol). This
represents the second stage of condensation. The resin is no longer
soluble in
alkalis because the molecular weight has advanced to such a size that
the
alkaline salts are no longer soluble. It is partly or even completely
soluble
in organic solvents such as acetone or alcohol. Cross linkage however
has not
proceeded very far and the resin is still softened by heat and is
plastic while
hot although hard and brittle when cold. Lebach called this stage the
resitol.
stage
resin (resite). This
represents the final stage of polymerization with a large amount of
cross
linkage. The resin is completely insoluble and infusible. Lebach called
this
the resite.
It is important to note that
these stages of resin
formation are not clearly defined but pass gradually one into the
other.
As
has been indicated above even
the A stage resin does not consist entirely of phenol alcohols but
contains
appreciable amounts of higher condensation products. In the B stage
phenol
alcohols are still present together with much more highly condensed
resins and
appreciable amounts of partly cross linked resins. As will be explained
later it
is probable that methylene ethers are present in this stage. There are
usually
present even in the C stage resin from 3 to 6% of products which can be
removed
by vigorous extraction with acetone.
Kinetics of A Stage Reaction
Although
a great deal is known
qualitatively about the nature of the initial reaction of phenol and
formaldehyde
there have been few quantitative measurements on the kinetics of the
reaction.
Novak and Cech attempted to follow the progress of resinification by a
study of
the refractive index viscosity and bromine value. Another empirical
approach is
represented by the work of Holmes and Megson who studied the behaviour
of
various phenols with a series of catalysts. In their work 0.4 gram mole
of the
phenol was mixed with 0.6 gram mole of 40% formalin and the catalyst
and
immersed in boiling water. An arbitrary time the resinification time
was
measured from initial heating to the appearance of a permanent
turbidity. Table
I shows the resinification time in minutes for various phenols when 0.5
g. of
trimethylamine was used as a catalyst. More detailed experiments were
then
carried out with phenol the three cresols and m 5 xylenol the catalysts
employed were trimethylamine triethylamine pyridine and ammonia. The
reaction
curves were roughly hyperbolas of the form
Table
1. Resinification time for
Various Phenols
For the
purpose
of comparing the catalytic activities of the more common bases m cresol
was
condensed in the presence of 0.75 g. of each base (except
ethylenediamine where
0.375 g. was used). Table 2 shows the resinification times obtained.
The values
for sodium hydroxide potassium hydroxide and lithium hydroxide when
reduced to
molecular proportions lie on the same curve.
The effect
of
higher temperatures on the condensation of m cresol was examined by
condensing
one mole fraction (27 g.) with one mole fraction of paraformaldehyde
(7.5 g.)
and 2 g. of pyridine in cyclohexanol as a solvent.
Table
2 Relative Activity of
Basic Catalysts
Nordlander
studied the ammonia catalyzed condensation of phenol with formaldehyde.
He
classified the two general types of reaction as (a) the primary
reaction in
which phenol and formaldehyde react to form water soluble intermediates
of the
oxymethylene phenol type (A stage) and (b) the secondary reaction in
which
these intermediates react further by condensation to give water
insoluble
resinous products. The reaction was studied by measuring the rate of
disappearance of formaldehyde and the manner in which the bromine value
of the
water soluble portion of the reaction mixture varied as the reaction
proceeded.
The results
disclosed that the primary reaction is confined to the interaction of
one mole
of formaldehyde with one mole of phenol no formaldehyde reacts with any
of the
intermediates formed. This reaction is apparently of monomolecular
order with
the rate proportional to the concentration of free phenol. The
influence of the
catalyst is complex apparently both hydrogen ions and hydroxyl ions and
probably other ions derived from the catalyst promote the reactions. At
very
low ammonia concentrations the reaction order changes to one of
apparently
bimolecular type which is characterized by a reaction rate proportional
to the
square of the formaldehyde concentration.
The
secondary
reaction seemed to be of monomolecular order the catalyst action was
complex
and similar but not identical to that governing the primary reaction.
Weakly
alkaline catalysts which are sufficiently active to bring about a
primary
reaction of the same type as that induced by ammonia are unable to
initiate the
secondary reaction to any extent. When the catalyst was a weak alkali
such as
ammonia phenol did not take any part in the secondary reaction which
was then
limited to the phenol alcohols. Nordlander reported that the
temperature
coefficient varied for the two reactions the secondary reaction rate
increased
much more rapidly with the temperature than the primary reaction rate.
Sprung made
a
very thorough study of the kinetics of the reaction of paraformaldehyde
in the
presence of a number of phenols using triethanolamine as a catalyst. He
determined that the addition phase of the reaction apparently followed
a first
order rate law. Figure 1 compares the reactivities of the various
phenols at
98°C. The apparent first order rate contants as taken from the slopes
of the
straight lines are listed in Table 3. The introduction of a methyl
group in the
meta position increased the reaction rate by a factor of 2.8. The
introduction
of a methylol group as in the case of saligenin depressed the
reactivity of
phenol to about the same extent as a methyl group similarly placed.
This
indicates that the rate law as experimentally determined for a di or
trifunctional phenol apparently expresses a summation of the rates at
which the
first second and presumably also the third molecule of formaldehyde
react.
Table
3. Effect of Substitution
on Reactivity of Phenols
Chemistry of Curing Reactions
As has been
previously stated very little definite information on the mechanism of
curing
has been available until within the last few years. It was generally
accepted
that the phenol alcohols condensed with the elimination of water to
yield three
dimensional macromolecules which were cross linked by methylene
bridges.
However in the case of phenols with three reactive positions the curing
reactions were so rapid and so complex that little progress had been
made in
isolating and identifying compounds from the later stages of the
condensation.
In order to
overcome these difficulties recent workers have studied the curing
reactions of
phenols in which one or two of the reactive ortho or para positions are
blocked. In this way only mono or di phenol alcohols can be formed and
insoluble cross linked products in general cannot be obtained. In most
cases considerable
yields of crystalline products are obtained and these products can be
identified and their further reactions studied. It was through this
line of
attack that Zinke Hultzsch von Euler and their respective associates
have built
up our present knowledge of the curing mechanism. As a result of their
studies it
has been shown that the phenol alcohol which results from the primary
condensation of a phenol and formaldehyde in alkaline solution
undergoes a
complex series of reactions. The extent to which these various
reactions take
place depends upon the structure of the initial phenol the temperature
at which
the phenol alcohol is heated and the time of heating. Scheme 1 taken
from Adler
gives in diagrammatic form the various reactions which a phenol alcohol
may
undergo on curing. Those compounds which have been isolated in pure
form are in
heavy type. These various reactions will now be discussed in more
detail.
Reactions 1
and
2 when a phenol alcohol is heated some formaldehyde is split off with a
regeneration of the original phenol. The phenol then combines with the
unchanged phenol alcohol with the splitting out of water and a
dihydroxy
diphenylmethane derivative is formed. A typical reaction of this type
is shown
for 4 hydroxy 3 5 dimethylbenzyl alcohol (Eqs. 4a and 4b). The extent
to which
this reaction takes place depends very much upon the structure of the
initial
phenol.
Reaction 3
Reaction
3 as shown in Equation (5) indicates the formation of dihydroxydibenzyl
ether.
When a dialcohol is used as for example the dialcohol from p cresol
long chain
ethers are formed as shown in Equation (6). It is apparent that this
reaction
is in general the most important primary reaction in the curing of
highly
substituted phenol alcohols. In many cases the dibenzyl ethers form the
largest
single product which can be isolated from the cured reaction mass.
In the case
of 3
(2 hydroxy 5 methylbenzyl) 2 hydroxy 5 methylbenzyl alcohol Adler
showed that
only the reaction indicated in Equation (7) took place. An analogous
reaction
took place when the dialcohol was used but in this case in addition to
resinous
linear ether about 6 10% of cyclic ether was formed with the structure
shown in
Formula V.
The ether
formation appears to take place more slowly in the case of p methylol
derivatives than in the case of the ortho derivatives. Ether formation
is
retarded by an increase in the curing temperature or by the presence of
alkali.
When alkalis are present the formation of methylene bridges is
favoured. Thus when
a resole from p tert butylphenol contains alkali it yields on heating a
fairly
large quantity of a crystalline product which was identified by Ziegler
as the
cyclic compound VI. It is interesting to note that this compound is
similar in
structure to the cyclic ether shown in Formula V.
Apparently
the
action of the alkali is to split off formaldehyde from the ether
producing the
cyclic methylene compound.
Reaction 4
on
further heating particularly at temperatures higher than are needed to
form the
ether the latter may split off formaldehyde and give a
dihydroxydiphenylmethane
which is identical with that obtained through reactions 1 and 2.
Reaction 4 may
be written as in Equation (8).
Hanus have
been
able to show that when phenol alcohols are heated to a certain
temperature only
water is split off when the temperature is then raised to another
definite
point formaldehyde is split off indicating the beginning of reaction 4.
The
temperature increase required to initiate reaction 4 over that required
for
reaction 3 is definite and depends upon the size and nature of the
substituent
group. Table 4 summarizes the effect for dialcohols of various para
substituted
phenols. The amount of water which is split off is very nearly one mole
for
every two moles of phenol alcohol (or one mole for every mole of phenol
dialcohol) which reacts. Even under the most favourable conditions
however much
less than one mole of formaldehyde is split off for one mole of the
ether. In
their best experiment Zinke and Hanus were not able to get more than
0.6 mole
of formaldehyde split off. This is attributed either to side reactions
or to
the tendency of formaldehyde to combine further with the
dihydroxydiphenylmethane derivative.
It is
interesting to note that the presence of a free phenolic hydroxy group
is
required for reaction 4 to take place although a free phenolic hydroxy
group is
not needed for the ether formation (reaction 3). This was shown by
Ziegler who
treated the dialcohol from p cresol with p toluenesulfonyl chloride to
form the
tosyl derivative. The latter split off water readily to form chain
ether but
this ether was stable and on further heating did not split off
formaldehyde.
Adler state that neither reaction 3 nor 4 takes place when the phenolic
hydroxy
group is etherified. Thus the monomethyl ether of p cresol dialcohol
was almost
unchanged after heating one hour at 160°C. although the free p cresol
dialcohol
resinified quickly at 130°C.
Reaction 5
Small
quantities of phenol aldehydes are usually found in the cured products.
Zinke
has suggested that these may be formed by the thermal cracking of the
dihydroxydibenzyl ethers as shown in Equation (9). The equation also
shows that
a nuclear methylated body is formed simultaneously. The quantities of
aldehydes
formed are usually quite small (4%) even from the highly substituted
phenols
employed in these experiments and the yield depends upon the nature of
the
substituent groups on the original phenol. Ziegler have shown that very
considerable amounts of aldehyde are formed upon heating the dibenzyl
ether of
3 5 dichloro 2 hydroxybenzyl alcohol (obtained by the action of
formaldehyde on
2 4 dichlorophenol).
Hultzsch
does
not favour the theory that phenol aldehydes are formed as indicated
above. He
points out that the reaction as written would require equimolar
quantities of
phenol aldehyde and nuclear methylated phenol to be formed experiment
shows
that this is not always the case. Hultzsch therefore prefers to
consider the
phenol aldehyde formation as an oxidation reduction action of
methylenequinone.
The presence of both dialdehydes and monoaldehydes in the curing
products of cyclohexylphenol
dialcohol was confirmed by Mayer through ultraviolet light absorption
studies.
In any case it is important to note that aldehyde formation plays only
a minor
part in the curing reaction this is particularly true in the case of
commercial
phenoplasts where phenols with two or three reactive positions are used.
Reactions 6
and
7 the formation of methylenequinone or quinonemethide is particularly
interesting because it indicates a mechanism by which phenoplasts or at
least a
part of the resin may be formed by polymerization rather than
condensation.
Baekeland had suggested that polymerization played a part in the
formation of
phenoplasts although he offered no definite mechanism. Wohl and Mylo
suggested
in 1912 that the polymerization might proceed through the methylene
derivative
of the tautomeric form of phenol although definite proof was lacking.
Novak
also felt that polymerization played a part in the curing reaction it
is
interesting to note that these investigators showed that the curing
reaction
was catalyzed by treatment of the formaldehyde with ozone whereby
peroxides are
presumably formed. The latter would evidently be catalysts for a
polymerization
reaction and would be expected to have no effect upon a condensation.
Later
investigators have definitely identified dimers and trimers of
methylenequinone
in the reaction products from the curing of highly substituted phenols.
The
monomer itself has never been identified as it is very unstable and
polymerizes
rapidly. The methylenequinone may be formed directly from the phenol
alcohol as
shown in Equation (10a) or it may be formed by loss of water from the
dihydroxy
benzyl ether as shown in Equation (10b).
The
equations
above indicate only the formation of para methylenequinone but the
ortho
compound is also formed when the methylol group is in the ortho
position as is
shown in Equation (11). Generally speaking the course of the reaction
is the
same whether the methylol group is in the ortho or para position
relative to
the phenolic hydroxy group.
In the
curing of
phenol alcohols it is probable that both reactions shown in Equation
(10) take
place simultaneously. However the formation of the dihydroxydibenzyl
ether is
usually the predominating reaction and this compound is usually formed
at low
temperatures (below 150°C.) it is rather stable at these temperatures
and only
breaks up into methylenequinone at higher temperatures (about 200°C.).
The
monomeric form of methylenequinone is unstable and polymerizes rapidly
to form
dimers or trimers. The dimer is colored yellow and Pummerer and
Cherbuliz have
attributed to this compound the general formula of cyclic quinone ether
as
shown in Formula VII. The presence of the quinone nucleus accounts for
the
yellow
color.
PHENOLIC RESINS
FROM HIGHER ALDEHYDES
The discussion in previous
chapter on the chemical
structure of phenoplasts has covered only condensations with
formaldehyde.
Higher aldehydes are occasionally used in the manufacture of
phenoplasts although
by far the greatest percentage of these resins is made from
formaldehyde. There
are two reasons for this (1) Condensation with formaldehyde gives in
general resins
which have shorter curing times than those of higher aldehydes and (2)
formaldehyde is subject to few if any side reactions in the presence of
the
condensation catalysts. The latter consideration is important from the
chemical
viewpoint. The Cannizzaro reaction is the only side reaction which
takes place
to any extent in the case of formaldehyde under Ordinary conditions of
phenoplast
manufacture. This reaction involves the reduction of one molecule of
formaldehyde accompanied by the oxidation of a second and is normally
catalyzed
by alkalis
With sodium
hydroxide and formaldehyde alone the reaction takes place slowly at
room
temperature but the velocity approximately triples for every rise of
10°C. in
temperature and is very rapid at 100ºC. The presence of appreciable
quantities
of phenols greatly retards the Cannizzaro reaction with formaldehyde
and
virtually no Cannizzaro reaction takes place even at refluxing
temperatures when
the molar ratio of phenol to alkali is greater than six to one.
Acetaldehyde
and
most of the higher aldehydes also undergo self resinification when
treated with
strong acids or bases. The facility with which such side reactions take
place
limits the usefulness of the higher aldehydes in the manufacture of
phenoplasts.
Acetaldehyde
Acetaldehyde
was
condensed with phenol to form a resin by Baeyer as early as 1872. Fabinyi mixed an excess of
phenol with
paraldehyde added stannic chloride slowly and formed a dark brown resin
which
distilled over in part under a pressure of less than 10 mm. The distillate was
crystallized from benzene
and yielded dihydroxydiphenylethane. Lunjak obtained a similar result
using
hydrochloric acid as catalyst. Claus dissolved two moles of phenol and
one mole
of acetaldehyde in ether and passed in hydrochloric acid gas. After removal of the ether a
dark brown resin
was left which could not be crystallized. The ultimate analysis
corresponded to
dihydroxydiphenylethane. Baekeland obtained a similar result. It will be noted that these
were all rather
vigorous chemical treatments.
Under mild
conditions however initial condensation products may be isolated. Thus
Adler von
Euler and Gie have shown that in the presence of dilute aqueous
hydrochloric
acid at room temperature the primary condensation of acetaldehyde with
phenol
produces a carbinol (Eq. 3 a) which then adds another mole of
acetaldehyde to
give the cyclic acetal benzodioxin (Eq. 3 b). When the phenol has two
or more
reactive positions polycondensation can take place with the formation
of
resins. When the phenol has only one reactive position as in the case
of 2 4
dimethyl phenol treatment with warm hydrochloric acid decomposes the
benzodioxin
to yield first an ortho vinyl phenol which rapidly dimerizes to yield
the
chroman derivative (Eq. 3 c).
In view of
our
present knowledge on novolacs it seems probable that the acid
condensation of
phenol with acetaldehyde under technical conditions yields a series of
linear
polymers in which the phenol groups are linked together by ethane
bridges as
shown in Formula 1. The bridges may occur in a random manner either
ortho
or para to the phenolic
hydroxy group. The arrangement in Formula 1 is of course highly
idealized.
The resins
from
the acid condensation of phenols with acetaldehyde are soluble and
permanently
fusible just as are the novolacs. Like the latter the acetaldehyde
phenol
resins may be converted to the insoluble resites by alkaline
condensation with
formaldehyde or by heating with a source of methylene groups such as
hexamethylenetetramine. It is much more difficult to condense
acetaldehyde with
phenol in the presence of an alkaline catalyst because the acetaldehyde
tends
to undergo aldol condensations and self resinification.
Butyraldehyde
Baekeland
and
Bender studied the condensation of phenol with normal butyraldehyde in
the
presence of hydrochloric acid. They obtained a resin which on heating
and
vacuum distillation gave a fairly good yield of
dihydroxydiphenylbutane. The
authors concluded that the primary reaction was the formation of 1
phenoxy 1
p
hydroxyphenyl n butane which
rearranged on heating according to Equation (4). When heated with
paraformaldehyde an insoluble and infusible resin was obtained from the
initial
resinous condensation product of phenol and butyraldehyde.
Chloral
Chattaway
studied the condensation of chloral with various phenols in the
presence of
sulfuric acid. When phenol itself was added to chloral suspended in
concentrated sulfuric acid an immediate reaction occurred and an oily
liquid
separated which rapidly changed to an opaque white solid. The latter
readily
dissolved in alcohol and gave a colorless solution from which no
crystalline
matter could be obtained. On evaporating the solvent a viscous liquid
was left
which solidified to a colorless transparent resin. The constitution of
the
latter was not determined.
When a para
substituent was present on the phenol nucleus no resin was obtained.
Instead a
good yield was obtained of a crystalline compound. For example p
nitrophenol
yielded anhydro 5 nitro 2 (b b b trichloro a hydroxyethoxy) b b b
trichloro a
hydroxyethylbenzene. Chattaway postulated that the reaction proceeded
according
to the scheme of Equation (5). Harden and Reid also condensed a number
of
phenols with chloral in order to study the bactericidal efficiency of
the
products.
Furfural
Of all the
higher aldehydes which have so far been discussed furfural probably has
the
most commercial importance in the manufacture of phenoplasts. It had
been known
since 1860 that furfural could be condensed with phenols to give
resinous
bodies. In 1921 Novotny covered practical details of the condensation
in U. S.
Pat 1 398 146. Trickey Miner and Brownlee studied the condensation in
1923 and
came to the following conclusions
In the case
of
acid condensed resins
In
order to obtain an infusible insoluble
resin the molecular proportions preferably should be slightly in excess
of 1
mole of furfural to 1 mole of phenol.
The
resin obtained when an excess
of phenol is used is soluble in acetone and alcohol and is permanently
fusible.
The
resins obtained by the use of
varying amounts of acid as a condensing agent were similar but the time
necessary to complete the reaction varied from two weeks when 0.2% of
hydrochloric acid was used (based on the weight of the total reaction
mass) to
ten hours in the case of 0.6% acid.
In the case
of
alkaline condensed resins
In
order to obtain an insoluble
resin the proportions are preferably about 1.25 moles of furfural to 1
mole of
phenol.
2.
The resins formed by an excess
of phenol are solid and brittle when cold melt easily and are readily
soluble
in acetone alcohol and furfural. When heated with enough furfural to
bring the
molecular proportions up to 1.25 of furfural to 1 of phenol the resins
pass
over to the infusible state.
By careful
condensation of furfural with phenol in dilute acid solution it is
possible to
prepare the primary product hydroxyphenyl furyl carbinol as shown by
the
reaction in Equation (6). The position of the carbinol group has not
been
established. It appears then that the behaviour of furfural in
phenoplast
condensations is very similar to that of formaldehyde. There are
however certain
differences between the formaldehyde and furfural condensation
products. The
phenoplasts from furfural have a dark purplish black color which
differs from
the yellow or brown color of the phenoplasts made from formaldehyde.
The
condensation of the phenols with furfural is initially more sluggish
than with
formaldehyde and furfural itself has a marked tendency to polymerize
under acid
conditions.
phenolic resins
from polyhydric phenols
The discussion so far has
concentrated upon the
monohydric phenols that is phenols which have only one hydroxy group
upon the
nucleus. From the commercial point of view the monohydric phenols are
by far
the most important since they are more readily available and less
expensive.
The theoretical studies leading to our knowledge of structure have
dealt mostly
with the monohydric phenols. monohydric phenols which have been highly
substituted in order to decrease their reactivity. The polyhydric
phenols are in
general much more reactive than the monohydric phenols since the effect
of the
second hydroxy group is to activate the benzene ring further. This is
particularly true in the case of resorcinol where the hydroxy groups
are meta
to one another. The resonance produced by this arrangement activates
the ortho
and para positions of the nucleus as a result the resorcinol resins are
very
rapidly cured.
Baeyer in
1872
condensed resorcinol and pyrogallol with various aldehydes. When the
aldehyde
was relatively inactive crystalline compounds could be obtained. For
example benzaldehyde
and pyrogallol gave a crystalline product. With resorcinol and
acetaldehyde or
formaldehyde resinous products were obtained. Caro in 1892 condensed an
excess
of resorcinol with formaldehyde in the presence of dilute hydrochloric
acid the
product obtained was identified as tetrahydroxydiphenylmethane (Formula
1). The
product obtained in a similar manner with pyrogallol was
hexahydroxydiphenylmethane.
Boehm
prepared
methylol derivatives of resorcinol by reduction of the corresponding
dicarbomethoxy aldehydes. The compound shown in Formula 2a was stable
and not
sensitive to acids while compound b could not be prepared in a pure
form but
always occurred as a resin. From this time on no studies were made on
the
structure of the resins from polyhydric phenols until von Euler and
associates
included this type of condensation product in their work.
In the case
of
hydroquinone it was shown that alkaline condensation with two moles of
formaldehyde gave the dialcohol illustrated in Formula 3a while four
moles of
formaldehyde gave the tetraalcohol illustrated in Formula 3b. Both the
di and
tetraalcohols resinified on heating.
On
treatment
with a weak acid the dialcohol quickly resinified to an insoluble
amorphous
product to which von Euler Adler and Gie ascribe a methylene bridge
structure.
The
alkaline
condensation of catechol with formaldehyde gave only catechol dialcohol
in
which the position of the methylol groups was established by von Euler
Adler Kispeczy
and Fagerlund according to Formula 4. The catechol dialcohol also
resinified on
heating. Rosenmund and Boehm prepared the monomethylol derivative of
catechol
by reduction of the corresponding aldehyde.
Kyrning
studied
the mechanism or curing of the di and tetraalcohols from hydroquinone
and
catechol. The dialcohol of hydroquinone apparently went through a
number of
reactions involving ether formation methylenequinone formation and the
formation of methylene bridges. The tetraalcohol of hydroquinone was
particularly interesting. It has no free nuclear positions and hence
diphenylmethane formation cannot take place. When cured at 210°C. it
gives off
about three moles of water per mole of tetraalcohol. This indicates
methylene
quinone formation in addition to the development of ether linkages as
shown in
Equation (I).
Kyrning
also
studied the tetraalcohol from p quinone and showed that at 180°C. it
lost
slightly less than two moles of water indicating the formation of ether
linkages only. Methylenequinone formation could not take place because
no
phenolic hydroxy group was present. This experiment showed that the
quinone
groups conferred a reactivity similar to that obtained when the hydroxy
groups
were present. However methylation of the phenolic hydroxy groups in
hydroquinone tetraalcohol gave a product which was completely stable at
180°C.
Dubusay
studied
the rate of gel formation in alkaline catalyzed resorcinol formaldehyde
resins.
They concluded that the ratio of aldehyde to resorcinol had a marked
effect
upon the curing time as the ratio increased the gel time passed through
a
minimum and then increased. The position of the minimum gel time was
affected
by the concentration of sodium hydroxide present. Von Euler and co
workers
point out that the methylol derivatives of the polyhydric phenols will
produce
infusible and insoluble resins when reacted with phenols which have
only two
reactive positions though the latter normally give only soluble and
fusible
resins. This is because the dihydroxyphenols behave as tetrafunctional
compounds and when combined with the difunctional phenols the
dihydroxyphenols
supply a sufficient number of reactive positions to cause the formation
of
three dimensional cross linked molecules.
Because of
its
great reactivity resorcinol and its methylol derivatives are used
commercially
today either as such or in combination with other resins to increase
the rate
of curing of phenoplasts. The addition of from 3 to 20% of resorcinol
will
either decrease the time of cure required at high temperatures or will
permit
curing at relatively low temperatures even at room temperatures.
Methods for
accomplishing this have been described in British Pat. Shiskov has
shown that
chemically resistant resins for coatings may be prepared from 15 parts
of
phenol to 12 parts of resorcinol. In this proportion the resin hardens
rapidly
and has physical properties which are only slightly inferior to those
obtained
when pure resorcinol is used.
reaction mechanisms
The
conditions mainly
pH and temperature under which reactions of phenols with formaldehyde
are
carried out have a profound influence on the character of the products
obtained. Three reaction phases have to be considered formaldehyde
addition to
phenol chain growth or prepolymer formation at temperatures <
100 °C and
finally the cross linking or hardening reaction at temperatures above
100 °C.
The rate of the phenol formaldehyde reaction at pH 1 to 4 is
proportional to
the hydrogen ion concentration above pH 5 it is proportional to the
hydroxyl
ion concentration indicating the change in reaction mechanism. Two
prepolymer
types are obtained depending on pH.
Novolaks
are
obtained by the reaction of phenol and formaldehyde in the acidic pH
region. In
general the reaction is carried out at a molar ratio of 1 mol phenol to
0.75
0.85 mol of formaldehyde. Novolaks are mostly linear condensation
products
linked with methylene bridges of a relatively low MW up to
approximately 2 000.
These resins are soluble and permanently fusible i.e. thermoplastic and
are
cured only by addition of a hardener almost exclusively formaldehyde
applied as
HMTA to insoluble and infusible products.
Resols are
obtained by alkaline reaction of phenols and aldehydes whereby the
aldehyde is
used in excess. P/F molar ratios between 1 1.0 to 1 3.0 are customary
in
technical resols. These are mono or polynuclear hydroxymethylphenols
(HMP) which
are stable at room temperature and by application of heat seldom of
acids are
transformed into three dimensionally cross linked insoluble and
infusible
polymers (resits) over different intermediate stages (resitols).
However the
limited storage stability of resols at ambient temperature must be
taken into
consideration.
The
methylene
bridge is thermodynamically the most stable cross linkage. It is
prevalent in
completely cured phenolic resins. Theoretically 1.5 mol of formaldehyde
is needed
for the complete three dimensional cross linking of 1 mol of phenol
(1). A
higher proportion of formaldehyde is used in technical resins. On an
average considering
all fields of application approximately 1.6 mol formaldehyde is used.
This
excess is necessary to meet distinct technical requirements for
instance resin
efficiency or free phenol content.
Phenols as
condensation monomers have a functionality of 1 to 3 according to the
substitution. The functionality of the aldehydes always has the value 2.
Table
1.
Molecular Structure and Reactivity of Phenols
The
molecular
configuration in solution and crystal structure of phenol is determined
by a
strong inclination to form hydrogen bonds. In the solid state phenol
forms H
bonded chains in the form of a threefold spiral. In solution e.g. in
benzene
containing small amounts of water trimolecular species Ph3 Ph2 H2O
Ph
2 H2O are formed.
For Ph3
a cyclic structure was proposed. 2 Hydroxy methylphenol forms a strong
intramolecular hydrogen bond. The tendency and extent of H bonding of
phenols
can be easily detected by NMR chemical shift or by infrared frequency
change. A
linear relation between the thermodynamic functions for H bond
formation and
pKa values exists. In Table 2 it is shown that alkyl substituted
phenols have
only a little lower acidity compared to phenol. This is also confirmed
by
calculated electron densities listed in Table 5. The differences
between phenol
and cresols are very small. More pronounced is the effect of bulky
substituents
in ortho position because of steric factors. Hydroxymethylphenols are
stronger
acids than phenol. Phenols in their electronically excited states are
more
acidic than the ground state molecules as deduced from spectroscopic
data.
The
hydroxyl
group is in the benzene ring plane even for 2 6 di tert butylphenol
Ortho and
meta substituted phenols exist as cis (a) and trans (b) isomers with
reference
of the Table 2. Acidity of phenols at 25 °C.
The
hydroxyl
group is an inductive electron withdrawing (–1) and conjugatively
electron
releasing (+R) group. Both effects favour para substitution. Steric
reasons
also decrease the accessability of the ortho positions. In comparison
with
other activating groups the following order with decreasing activating
power.
The oxide
ion group
in the phenoxide ion is a very strong activating substituent stronger
than NR2 and
more ortho directing than the hydroxyl group. The calculations (Table
4)
indicate clearly the higher electron density of the ortho position
compared to
the para position in the phenoxide ion. This differentiation is not so
marked
in the neutral molecules. The direct experimental comparison of phenol
substitution rates with benzene to indicate the relative activating
power of
the hydroxyl group on the benzene nucleus is impracticable because of
the
extremely large difference in the reaction rates in the order of 10.
The
formaldehyde
phenol reaction corresponds to an electrophilic aromatic substitution
in acidic
as well as in alkaline environments. It is generally assumed that this
reaction
type involves the rate determining formation of a p complex followed by
rapid
loss of a proton. The actual pathway of reaction however is much more
complicated with phenols because of solvent interactions and inter and
intramolecular hydrogen bond formation the abnormal and wide variation
in ortho
/para ratio supports this.
The attack
on
the para position is favoured by polar solvents and acidic conditions
while the
attack on the ortho position is favoured by nonpolar solvents alkaline
conditions
and group II metal oxide hydroxide or
acetate catalysts.
In the last
few
years because of the fast growth of the electronic computers it has
been
possible to employ more sophisticated quantum theoretical calculations
which
not only include the p electrons but all valence electrons of large
molecules.
These
theoretical calculations have proved to be a powerful tool in
structural
chemistry. One of the most applied semi empirical calculations which
includes
the 1s electron of hydrogen the 2 s 2 px 2py 2pz electrons of carbon is
the
CNDO/2. This method has been used by Knop for the calculation of the
electronic
structure for the ground and the excited singlet and triplet states of
the
following molecules (Table 3).
The
electron
density distribution of the neutral molecules is not a sufficient basis
for the
interpretation of the reported kinetic data. This applies especially to
the
excited states (1st singlet and triplet state). An increased
differentiation of
the electron density distribution is found for the corresponding ions
(Table
5). The electron density of the para position in the phenoxide ion is
remarkably higher than that of the ortho position and is therefore a
Table 5.
Formaldehyde Water and Formaldehyde Alcohol Equilibria
Formaldehyde
is
by far the most reactive carbonyl compound. In aqueous medium a very
fast acid
and base catalyzed hydration reaction of formaldehyde to methylene
glycol
occurs. The equilibrium indicated in Eq. (3) far on the side of
methylene
glycol can be estimated by UV spectroscopy (n p) transition of the
carbonyl
group) by NMR or by polarographic methods.
Methylene
glycol
is found in aqueous solutions as a low molecular condensation polymer.
It is
also obtained by dissolving parafor maldehyde. The concentration of
monomeric
non hydrated formaldehyde is very low generally less than 0.01%. The
MWD of
poly methylene glycol in a 40% aqueous solution is indicated in Table 6.
The
methylene
glycol is present as monomer only in very dilute aqueous formaldehyde
solutions
(1 2%). The depolymerization of aqueous polyoxymethylene glycol (4) in
the
presence of acidic and basic catalysts is of importance for the overall
reaction rates for resol and novolak formation.
Alcohols
are
often present in the P/F reaction. Methanol is present at least in
small
amounts (~1%) because the formaldehyde production process starts from
methanol.
In addition it can be formed from formaldehyde during storage by
disproportionation (Cannizzaro reaction). Furthermore methanol may be
added
because it is very efficient in stabilization of concentrated aqueous
formalde
hyde solutions. The chain termination prevents the formation of low
soluble
polymers so that precipitation or turbidity will be omitted. Alcohols
can react
with aqueous formaldehyde in a neutral pH to form hemiformals (5).
Diformals
are not formed under these conditions.
Also the
reaction between hydroxymethylphenols and methylene glycol must be
considered.
The extent of this reaction with the hydroxymethyl group (6) as well as
with
the phenolic hydroxyl group (7) has been studied by high resolution NMR
Formaldehyde
is
therefore consumed in the P/F reaction also to form hemiformals which
constitute a potential source of formaldehyde and may be detected by
usual
titrimetric methods but is otherwise not more directly involved in the
hydroxy
methylation reaction. This should be a sufficient reason for the
apparent
reduction of the reaction rate at rising conversion.
Phenol Formaldehyde Reaction under Alkaline Conditions
The
reaction
between formaldehyde and phenol in the alkaline pH range was first
mentioned in
1894 by L. Lederer and O. Manasse. It is therefore occasionally also
designated
as the Lederer Manasse reaction. At a pH above 5 bis
and tris alcohols are formed as well as
mono alcohols and other compounds. The simplest product of this
reaction 2
hydroxybenzylalcohol (saligenin) was already isolated from the
glucoside
salicyn by hydrolysis with diluted acid.
Inorganic Catalysts and Tertiary Amines
Sodium
hydroxide
ammonia and HMTA sodium carbonate calcium magnesium
and barium hydroxide and tertiary
amines are used as catalysts in the alkaline hydroxy methylation
reaction. In
aqueous solutions as used in all technical processes formaldehyde is
present as
methylene glycol. Phenol reacts quickly with alkali hydroxide to form
the
phenoxide ion which is stabilized by resonance according to Eq.(10)
In the
following
reaction catalyzed by alkalis C alkylation in ortho and para position
occurs
almost exclusively. Meta substitution is hardly not evident.
The
quinoide
transition state is stabilized by proton shifting as indicated in the
Eqs. (11)
and (12). This reaction mechanism was recommended for dilute solutions.
The
monomethylol derivative continues to react with formaldehyde forming
two
dimethylol and one trimethylol compounds.
The
kinetics of
the base catalyzed phenol formaldehyde reaction has been thoroughly
researched
and is relatively well understood. In general a second order reaction
type was
found with the exception of the ammonia catalyzed reaction which rather
corresponds
to one of the first order.
It must be
pointed out however that the actual constitution of the
hydroxyalkylating agent
in the alkaline catalyzed reaction is not yet fully understood. It is
not clear
how methylene glycol would react with the phenoxide ion. The
concentration of
non hydrated formaldehyde is too low to explain the reaction rates.
A deviating
reaction mechanism was proposed very early by Claisen later by Walker
and
others. The presence of hemiformals 3.7 in aqueous phenol formaldehyde
solutions has been proven by the means of NMR.
As evidence
for
the formation of hemiformals as intermediates followed by a shift of
the
hydroxymethyl group according to Eq.(15) the absence of the reaction if
the
phenolic group is etherified was pointed out. This statement however is
not
correct since the nucleophily of the phenoxide anion is the decisive
parameter
in the alkaline hydroxymethylation.
A series of
experimental
results indicates that the constitution of the transition state is
considerably
more complex than indicated in the Eq. (11). The strongest evidence is
the
dependence of the ortho/para substitution ratio on the type of
catalyst. The
ortho/para ratio decreases from 1.1 at pH 8.7 to 0.38 at pH 13.0. It
has been
recognized that the ortho substitution is considerably enhanced if
metal
hydroxides of the first and second main group along the series.
Even more
distinct is the effect of the hydroxides of transition metals. The
ortho
substitution is the more favoured the higher the chelating strength of
the
cation. The directing effect of the Fe Cu Cr Ni Co Mn and Zn ions is
explained
by Peer as formation of chelates as transient compounds according to
formula
(16). Boric acid also has a strong ortho directing effect (17).)
The
formation of
high ortho novolaks if magnesium oxide or zinc oxide are used as
catalysts. The
different activity of some of the more frequently used catalysts and
their
effect on MWD examined by GPC is shown in Figure 1. Identical reaction
conditions have been used for the production of the resols. The
catalyst was
neutralized with hydrochloric acid and the resin analyzed without
previous
distillation. The highest degree of ortho orientation is observed when
zinc
acetate is used (Fig. 5) followed by magnesium oxide and tri ethylamine
as
catalysts (Fig. 1).
The
interpretation of the orientation and directing effects depending upon
the type
of catalyst must be performed with care. The concentration of any
particular
alcohol in the reaction mixture does not only depend upon the rate of
formation
but also upon the rate of disappearance due to further reaction i.e. it
also
depends upon the mol ratio of phenol to formaldehyde and the reaction
time. The
order of appearance of the individual methylol phenols in the GPC
chromatogram
depends despite of molecular size also on the number of hydroxyl groups
because
of solvent interaction.
fillers for
phenolic resin moulding powders
INTRODUCTION
The
idea of adding filler to a
phenoplast resin in order to produce a combination which could be
molded
satisfactorily originated with Baekeland and was one of his pioneer
contributions to the art. Filler is any substance either organic or
inorganic which
is mixed with a resin to produce a nonhomogeneous mixture which can
subsequently be molded. The filler facilitates the molding process
which is
usually very difficult with pure resin and also improves the physical
properties of the molded article.
The
selection of
the proper filler for a molding powder has an importance which is
secondary
only to the selection of the phenoplast resin. The filler is of most
importance
in controlling mechanical and strength properties of the finished
molded
product to a lesser extent it affects the electrical qualities and heat
resistance. The resin is important because it must give the proper flow
and the
proper bond and must also permit curing to a well finished piece in a
reasonable length of time. The exact nature of the resin is important
in
securing the most desirable electrical properties. This point will be
discussed
further in a later chapter.
The general
requirements for a satisfactory filler have been classified into two
groups primary
and secondary with the idea that the primary requirements are essential
for
satisfactory molding while some compromise is possible for certain uses
in the
case of the secondary requirements.
The
classification is as follows
PRIMARY REQUIREMENTS
Good
impact and tensile strength
in the molded piece.
Low
moisture absorption.
Low
specific gravity in the
molded piece.
Easy
wetting by resins and dyes
No
chemical or physical effects
on steel dies and particularly no abrasive effects.
Low
cost and adequate supply.
Nonflammability
or low burning
rate.
Freedom
from odor.
Supplies
must be readily
obtainable and of uniform quality.
SECONDARY REQUIREMENTS
Good
electrical characteristics
in the molded piece.
Light
color which is retained at elevated
temperatures and in the presence of chemical materials used in the
preparation
of the molding powder.
Inertness
to acids alkalis and
solvents.
Easy
machinability.
Heat
resistance.
Availability
in controlled mesh
size and bulk factor.
TYPES OF FILLER
Fillers are
usually classified primarily according to their general chemical nature
organic
or mineral and then are further divided into subdivisions according to
their
chemical composition and physical structure.
EFFECT OF FILLER ON IMPACT STRENGTH AND DAMPING
The
principal
mechanical effect which is produced by different fillers is a change in
the
impact resistance of the molded plastic. The change in impact
resistance
results from an increase in the capacity of the molded specimen to
absorb
mechanical shock waves and thus prevent their amplification at certain
points
through resonance. The ability to absorb mechanical shock waves and
transform
them into heat is known as damping. In order to obtain a high degree of
damping
it is necessary to have a high degree of nonhomogeneity in the molded
plastic this
is usually best accomplished by the use of fibrous fillers. The filler
itself
must have very high tensile strength so that it may resist the
disruptive
tensile forces which are created by the shock wave.
De Bruyne
in
1936 studied the effect of damping and pointed out that the increase in
shock
resistance of fiber filled molding compounds was connected with their
greater
damping capacity. De Bruyne has also emphasized the importance of high
damping
capacity in assuring freedom from vibration. On the other hand a high
damping
capacity involves a high energy absorption in the part which is subject
to
vibration and the energy absorption is necessarily accompanied by the
development of heat which may of itself be destructive to the plastic.
Leaderman has studied the damping capacity by the decrement of free
oscillations using solid cylindrical specimens tested in torsion. Four
grades
of phenoplast molding materials were employed (a) phenol formaldehyde
resin white
transparent and unfilled (b) commercial phenoplast molding material
with wood
flour filler (c) shock resisting material with a filler consisting of
fabric
snippings impregnated with phenol formaldehyde resin and (d) shock
resisting
material with a filler consisting of closely packed cords running
parallel to
the axis of the specimen.
Figure 1
taken
from Leader man shows the rate at which free torsional oscillations
decreased
when these four fillers were used. From these data the mean specific
damping
capacity in torsion was computed (in per cent) and is given in Table 1.
To
illustrate the relation between damping capacity and impact resistance
Table 1
also contains the approximate impact resistance for these various
materials as
measured by the standard A.S.T.M. test.
MICROSCOPIC STRUCTURE OF FILLERS
The
microscopic
structure of both cellulosic and inorganic fillers has been examined.
It was
demonstrated that the resilient fibrous fillers vary considerably in
particle
size. In the cellulosic fillers the fibers are frequently split in many
cases the
microscopic examination showed that only 20 to 30% of the fibers were
intact.
RATIO OF RESIN TO FILLER
The ratio
of
resin to filler has a profound effect upon the molding qualities of the
powder
and upon the physical and chemical properties of the molded plastic.
(In all
cases in this chapter weight ratios are meant.) When cellulose fillers
are
employed the molding composition usually contains approximately equal
parts by
weight of resin and filler. With this proportion the best general
combination
of molding qualities strength properties and water resistance is
obtained. When
less resin is employed molding becomes more difficult because the
combination
has less flow. Some strength properties such as tensile and flexural
strength tend
to drop off rather rapidly. Impact strength values usually rise to a
maximum at
about 30% resin content and then decrease rapidly. However whatever
virtue
there may be to this gain in impact strength is offset by the loss in
tensile
and flexural strength. Water resistance and resistance to other
chemical agents
decrease as the resin content decreases because the particles of filler
are not
thoroughly covered and protected by a film of resin. For the same
reason the
appearance of the piece is poor because the filler particles are
exposed.
If the
resin
content is increased much above 50% when cellulosic filler is used the
molding
tends to become more difficult because of excessive flow of the
combination.
There is not much change in tensile or flexural strength but the impact
strength decreases because the molded piece tends to become more
homogeneous in
composition and consequently has less damping capacity.
When
fillers are
used which contain much natural resin such as lignin extended wood
flour or
redwood flour the ratio of phenoplast resin to filler may be decreased.
The
natural resin present compensates for the lower quantity of phenoplast
and the
total quantity of resin present in the combination is still
approximately 50%.
When mineral fillers such as mica or asbestos are used a higher weight
ratio of
filler is usually employed about 70% filler to 30% resin. These mineral
fillers
have a higher specific gravity than the cellulosic fillers and
consequently the
volume ratio of filler to resin is about the same as in the case of the
cellulosic fillers.
STANDARD CLASSIFICATION OF PHENOPLAST MOLDING POWDER
ACCORDING TO FILLER
There are
several standard classifications of phenoplast molding materials in
which the
method of classification is based upon the nature of the filler. Where
cellulosic fillers are used the impact strength of the molding also
varies with
the nature of the filler. Such classifications therefore also give an
indication of the strength of the composition. It is important to note
that the
word strength as affected by the nature of the filler applies
principally to
impact strength and not to other strength properties such as tensile or
flexural strength. As a matter of fact in so far as phenolic filled
molding
plastics are concerned the tensile strength is scarcely altered in
changing
from wood flour (the weakest of the cellulosic fillers in impact
resistance) to
tire cord (the strongest in impact resistance) while the flexural
strength is
increased only slightly.
fillers and resins for laminates
The fillers
for
laminated phenoplasts differ from those used in molding powders
principally in
that the fillers for laminates are continuous webs rather than discrete
particles. The chemical nature of fillers is quite similar they may be
paper cotton
or linen fabric sisal mat or woven asbestos. More recently woven glass
fiber
has become available for applications in which exceptionally high
strength is
required. The filler greatly increases the strength properties of the
laminate
over those of the pure resin the increase is greater than is obtained
with
molding powders because of the continuous web which is present. The
filler has
the same type of damping effect upon impact waves as has been described
in the
case of molding powders.
The resin
usually
in the form of an alcoholic impregnating solution is applied to the
filler.
Aqueous solutions however are frequently used particularly in the case
of paper
base laminates.. The impregnated sheet which may contain from 25 to 65%
resin
(usually about 40%) is then dried and pressed between metal plates at a
high
temperature. In cases in which a highly polished finish is required on
the
surface of the laminate the surface sheets may contain a higher resin
content
approximately 50%. For standard flat laminates the usual pressing
temperature
ranges from 140° to 180°C. and the molding pressure is from 1000 to
2500 p.s.i.
Laminated phenoplast tubing may be formed by rolling the impregnated
sheet
material upon mandrels between heated pressure rolls and then either
oven
baking or pressing in a heated mold to complete the curing of the resin.
In recent
years
there have been important developments in the molding of laminates into
more
intricate forms such as seats for airplane pilots air ducts ammunition
boxes and
similar articles. There are two general methods by which such contoured
laminates may be formed (1) Layers of laminate cut to the proper size
are
formed in a mold under heat and pressure. The amount of draw or the
degree of
contour is usually limited since the base of the laminate will tear if
subjected to too much strain in molding. The molding pressures may vary
from a
high range (1200 to 2000 p.s.i.) down to a low range (100 to 250
p.s.i.).
Somewhat better strength properties are obtained at the higher
pressures but
the lower pressures permit the use of cheaper dies and less expensive
presses.
(2) By selection of the proper resin and careful control of the curing
cycle
during lamination it is possible to produce a flat laminate which can
be heated
and reshaped under relatively low pressures. Such a process is known as
post
forming. It was originally thought that the resin used in bonding the
laminate
should be under cured later work has indicated that a fully cured resin
can be
formed just as well and the use of a well cured resin is in fact
desirable. The
filler is a cotton fabric with a weave chosen especially so that some
stretch
is possible in two directions. As in the case of low pressure
laminating post
forming permits the production of relatively large articles with cheap
dies and
light presses. A certain amount of draw is possible when molding by
this
process. The maximum draw may be determined from an index (r/R) which
is
calculated by dividing the cup radius (r) by the blank radius (R) of
the piece
to be drawn. This index should lie between 0.67 and 0.77 for
satisfactory hot
drawing of laminated phenoplast sheets in thicknesses of from 1/32 to
1/16 in.
CLASSIFICATION OF LAMINATES
Standard
laminated sheets and tubes are classified according to properties and
functional use which in turn are dependent upon the type of filler.
Paper has
been the most important single filler used according to Hanson the
laminating
industry produces about 70 000 000 Ib. of finished stock per year.
Prior to the
war 80% of this material had paper used as a base this represented an
annual
consumption of 28 000 000 Ib. of paper.
In the
United
States the National Electrical Manufacturers Association (NEMA) has set
up
standard classifications for laminates made under high pressure (i.e.
1000 to
2500 p.s.i.). The standard NEMA grades are listed below. These grades
do not
include recent developments in high strength paper glass fabric and low
pressure laminates. These new materials will be described in separate
paragraphs.
Laminated
Phenolic Sheets
Grade X A
strong
paper base laminated material primarily intended for mechanical
applications
where electrical requirements are of secondary importance. Should be
used with
discretion when high humidity conditions are encountered. Not equal to
fabric
base grades in impact strength.
Grade P A
paper
base laminated material primarily intended for punching. More flexible
and not
quite as strong as Grade X. Moisture resistance and electrical
properties
intermediate between Grades X and XX
Grade XX A
paper
base laminated material suitable for usual electrical applications.
Grade XXP A
paper base laminated material similar to Grade XX in electrical and
moisture
resisting properties but more suitable for hot punching. Intermediate
between
Grades P and XX in punching and cold flow characteristics.
Grade XXX A
paper base laminated material suitable for radio frequency work for
high
humidity applications and with minimum cold flow characteristics.
Grade XXXP
A
paper base laminated material similar to Grade XXX but having lower
dielectric
losses and being more suitable for hot punching. This grade has greater
cold
flow than Grade XXX and is intermediate between Grades XXP and XXX in
punching
characteristics.
Grade C A
fabric
base laminated material made throughout from cotton fabric weighing
over 4 oz.
per sq. yd. and having a count as determined from inspection of the
laminated
plate of not more than 72 threads per in. in the filler direction nor
more than
140 threads per in. total in both warp and filler directions. A strong
tough
material suitable for gears and other structural forms exposed to high
impact.
The heavier the fabric base used the higher will be the impact strength
but the
rougher the machined edge consequently there may be several subgrades
in this
class adapted for various sizes of gears and types of mechanical
service.
Should not be used for electrical applications except for low voltages.
Grade CE A
fabric base laminated material of the same fabric weight and thread
count as
Grade C. For electrical applications requiring greater toughness than
Grade XX or
mechanical applications requiring greater resistance to moisture than
Grade C.
Exceptionally good in moisture resistance.
Grade L A
fine
weave fabric base laminated material made throughout from cotton fabric
weighing 4 oz. or less per sq. yd. As determined by inspection of the
laminated
plate the minimum thread count per inch in any ply shall be 72 in the
filler
direction and 140 total in both warp and filler directions. For purpose
of
identification the surface sheets shall have a minimum thread count of
80
threads per in. in each of the warp and filler directions. Not quite as
tough
as Grade C. Should not be used for electrical application except for
low
voltage.
Grade LE A
fine
weave fabric base laminated material of the same fabric weight and
thread count
as Grade L. For electrical applications requiring greater toughness
than Grade
XX. Better machining properties and finer appearance than Grade CE also
available
in thinner sizes. Exceptionally good in moisture resistance.
Grade A An
asbestos paper base laminated material. More resistant to flame and
slightly
more resistant to heat than other laminated grades because of high
inorganic
content. Suitable for only low voltage applications. Minimum
dimensional
changes when exposed to moisture.
Grade AA An
asbestos fabric base laminated material. Similar
to Grade A but stronger and tougher.
Minimum dimensional changes when exposed to moisture.
Laminated
Phenolic Tubes (NEMA
Classification)
There are
two
types of tubes rolled and molded. The rolled are oven baked after
rolling on
mandrels while the molded are cured in molds under pressure. The rolled
tubes
are less dense and generally less resistant to moisture than molded
tubes but
are of uniform strength around the circumference whereas molded tubes
have mold
seams which are sources of weakness both mechanically and electrically
particularly
in thin walled tubes. Each type has its own particular applications and
characteristics.
X Rolled A
high
mechanical strength paper base tubing with good punching and fair
machining
qualities. Low power factor and high dielectric strength under dry
conditions.
XX Rolled A
paper base tubing with good machining punching and threading qualities.
Not as
strong mechanically as X Rotted but better moisture resistance. Best
grade for
low dielectric losses particularly on exposure to high humidity.
X Molded
Better
in moisture resistance and machining qualities than X Rolled. Strongest
paper
base except in thin walls. Dielectric strength may be low at molded
seams.
XX Molded
Best
paper base grade from moisture resisting standpoint. Good machining and
good
electrical properties except in very thin wall forms.
C Rolled A
fabric base tubing made from a cotton fabric weighing more than 4 oz.
per sq.
yd. As determined by inspection of the laminated tube the thread count
shall
not be more than 72 threads per in. in the filler direction. The total
thread
count per inch in both warp and filler direction shall not exceed 140.
This
tubing is intended primarily for mechanical purposes. Dielectric
strength is
relatively low and moisture absorption greater than for other fabric
base
grades.
CE Molded A
fabric base tubing made of same fabric weight and thread count as Grade
C
Rolled. For use when a tough dense fabric base material is required
having fair
electrical properties along with excellent mechanical properties and
good
resistance to moisture. Dielectric strength may be low at molded seams
especially
in thin walls.
LE Rolled
Made
from a fine weave cotton fabric weighing 4 oz. or less per sq. yd. As
determined by inspection of the laminated tube the minimum thread count
per
inch shall be 72 in the filler direction and 140 total in both warp and
filler
directions. Best concentricity and dielectric strength of any fabric
base
grade. For use when the seams from a molded tube may be objectionable
and when
the application requires good machining qualities together with good
electrical
and mechanical properties.
L Molded
Made
from a fine weave cotton fabric of the same weight and thread count as
Grade LE
Rolled. Has high density and good moisture resistance. For mechanical
applications primarily when finer machined appearance than with CE
Molded is
desired or when tougher material than LE Molded is required. Should not
be used
for electrical applications except for low voltage.
LE Molded
Made
from a fine weave cotton fabric of the same weight and thread count as
Grade L
Molded. Has excellent machining and moisture resisting characteristics.
For use
in electrical applications even under humid conditions when a tougher
material
than Grade XX tubing is required at some sacrifice of electrical
properties.
Dielectric strength may be low at molded seams especially in thin wall
forms.
Grade LE Molded is better electrically than Grade CE Molded but not
quite as
tough.
High
Strength Paper Laminates
In recent
years
a new type of paper base for phenoplast laminates has been developed
which gives
higher tensile strength properties than the usual paper bases. This
type of
filler is not included in the NEMA specifications. This new base was
principally developed because of the need for paper laminates of
improved
strength characteristics in the construction of component parts for
aircraft particularly
when molded at low pressure ranges (i.e. 50 to 200 p.s.i.). The paper
is made
from Mitscherlich spruce sulfite pulp because with a minimum of
mechanical
treatment it gives a sheet of paper with exceptionally high tensile
strength.
The fibers are laid on the paper machine so that they are largely
oriented in
the direction of the machine. A thin sheet gives the best results and
the
weight of the paper is generally held to 35 lb. per ream. For best
results the
gauge of the paper must be uniform in all directions. Although spruce
is ideal
for the purpose a number of other softwoods yield pulps of about the
same
characteristics. In place of Mitscherlich pulp that from the Kraft
process can
be used satisfactorily provided the papermaking conditions are altered
to suit
the pulp.
When such a
paper is used as a laminate and the plies are all oriented in the same
direction an ultimate tensile strength as high as 35 000 p.s.i. in the
direction of the fibers may be obtained. The strength in the opposite
direction
however is rather low. The paper is usually used by laying the plies in
alternate directions so that the strength properties are substantially
the same
in any direction.
Plastic
Bonded Cotton Fiber
It is
possible
to prepare a laminate by using as filler cotton fibers which have all
been laid
in a parallel direction. The laminate then develops very high tensile
strength
(up to 35 000 to 40 000 p.s.i.) in the direction of the fiber although
the
strength properties across the fiber are low. Such laminates have been
described by Goldman.
Glass
Fabric Filler
Woven glass
fabric has recently become available as filler for laminates and this
type of
filler is also not included in the NEMA specifications. The grade most
commonly
used has the glass fibers running in one direction and is woven
together by a
fine cotton fiber. To obtain equal strength properties in both
directions the
laminates are cross banded that
is the
alternate layers of fabric are oriented at an angle of 90° to each
other. With
this filler it is possible to obtain an ultimate tensile strength in
the
neighborhood of 30 000 p.s.i. at room temperature. The high strength is
to some
extent offset by the relatively high specific gravity of the laminate
1.67 as
compared with 1.34 for a paper laminate with comparable resin content.
The
impact strength of the laminated glass fabric is particularly high the
notched
Izod value at room temperature is 22 ft. lb. per inch of notch as
compared to 1
ft. lb. per inch of notch for the usual paper laminate.
RESINS USED FOR LAMINATES
In general
the
resins used in laminates are very similar to those used for molding
powders.
Where exceptionally good electrical properties are required cresylic
acid is
frequently used as the base instead of phenol or some special type of
extender
is added to the phenolic resin. The proportion of resin to filler is
usually
lower than in the case of molding powders it may be as low as 25% based
on the
weight of the finished laminate and usually averages about 40%. Less
resin can
be used because less flow is required in the molding operation there are no sharp corners
in the mold to be
filled out by flow of the resin and filler. The effect of the quantity
of resin
on physical properties is further discussed in the chapter on the
physical
properties of laminated phenoplasts.
The degree
of
impregnation obtained in paper filler varies markedly with the
molecular
complexity of the resin as the molecualr size is reduced an improvement
in the
impregnation is observed. There is considerable difference between the
inpregnation achieved by a low molecular weight resin dissolved in
water and a
more highly condensed resin which must be dissolved in a solvent such
as
alcohol. The more highly condensed resin does not penetrate the fibers
to the
same extent as the water soluble resin.
physiology and
environmental protection
In order to judge the risks
connected with the
handling of phenolic resins a clear distinction must be made between
phenols phenolic
resin prepolymers and cured phenolic resins. Apart from constitutional
characteristics the MW is of great importance regarding the
physiological
effects. The physiological activity of phenolic
prepolymers
depends upon the
content of free phenol and formaldehyde. Cured phenolic resins are
entirely
harmless. The FDA permits articles molded from phenolic resins to come
into
contact with food.
Toxicology of Phenols
Mononuclear
phenols are protein degenerating and highly toxic. The oral LD50 value
(rats)
is 530 mg/kg. Human skin which has come in contact with phenol first
becomes
white subsequently red and wrinkled a strong burning sensation is
quickly
perceived. Longer contact destroys the skin tissue. Solid and liquid
phenols
are absorbed by the skin very quickly and cause very severe damage.
Contact
with large amounts leads to death through paralysis of the central
nervous
system. Minor intoxications lead mostly to damage of the kidneys liver
and
pancreas. If phenol is inhaled or swallowed local cauterizing occurs
and
headaches dizziness vomiting irregular breathing respiratory arrest and
heart
failure are the results.
The
destructive
effect of phenol on the human skin is reduced by introduction of
lypophilic
groups (methyl higher alkyl or chloro groups). The neutral molecules
are far
more active than the corresponding ions. The biological activity of
phenols is
the result of their ability to alter biological structures i.e.
cracking of
bacterial cell walls. The disruptive effect on cytoplasmic membranes
and cell walls
develops it is believed by the creation of pores large enough to permit
cytochromes to diffuse out. Cresols are similar to phenol in their
action but
less severe in their effects. Chlorophenols are not used for resin
production.
The surface
activity of alkyl phenols leads to their concentration on the cell
surface but
does not explain the destructive effect to the cells. The bacterial and
anthelmintic action of phenols is also influenced by soaps which are
usually
used to solubilize phenols in water for use as disinfectants.
Toxicology of Formaldehyde
Formaldehyde
in
an aqueous solution is a protoplasm poison with a cauterizing and
protein
degenerating effect. The use of aqueous formaldehyde to preserve
medical or
biological preparations is well known. Formaldehyde is believed to
injure
bacteria by reacting with the amino groups of proteins which are
thereby
changed in nature and action. Formaldehyde in the organism is quickly
oxidized
to formic acid which is partly separated by the urine.
Formaldehyde
in
the form of gas or aerosol the
effect of
both is comparable is very irritating to the mucous membranes. The
pungent
smell is noticeable even at concentrations below 1 ppm. The MAK value
is 1 ppm.
Formaldehyde is a dangerous material to work with and has received the
same
rating as phenol.
Concentrations
of formaldehyde up to 10 ppm cause conjunctivitis within a few minutes
as well
as rhenitis with anosmia and pharyngitis. It can be observed that one
can get
used to formaldehyde to a certain extent. At 10 15 ppm dispnoe cough
and
pneumonia develop.
Environmental Protection
The
environmental policies of progressive industrial nations require not
only the
return of used media to the environment in a treated condition but aim
to work
with these media without causing damages or injuries i.e. new
production
processes must be developed which prevent contamination of the
environment in
the first place. An example is the legislation against environmental
contamination in West Germany. Aims and instruments of environmental
policies
are set forth in the Bundesimmission sschutzgesetz . The Technische
Anleitung
zur Reinerhaltung der Luft (Technical Instructions on Clean Air
Maintenance) 1974
describes the minimum requirements for plants and their operation and a
series of
limiting values for emissions and immissions. According to these
requirements
organic compounds of Class I phenol and formaldehyde amongst others
belong to
this class must not exceed a mass concentration of 20 mg/m3 at a mass
flow of
0.1 kg/h and more. In order to protect the waterways from contamination
the Wasserhaushaltungsgesetz
1976 was passed. The
law called Wasserabgabengesetz
1976 includes a scale of fees determined by the quantity of waste water
emitted
and the amount of injurious substances (according to the COD and BOD)
in the
water and deposits as well.
Permissible
levels of phenols in waste water have been established in USA by the
Environmental Protection Agency (EPA) in the Federal Register. These
guidelines
generally establish phenol levels of 0.1 mg/1 for the Best Practical
Control
Technology Currently Available (BPCTCA) for 1977 and 0.02 mg/1 for the
Best
Available Control Technology Economically Achievable (BACTEA) for 1983.
Even
relatively
low concentrations of phenols below 10.000 mg/1 water are fatal for
several
kinds of fish after 1 3 days. Lower concentrations are at least
injurious and
deteriorate the taste of the fish flesh considerably so that it is
unfit for
human consumption. Phenols in chlorinated water lead to the formation
of
chlorophenols which will impart objectionable taste and odor to water
even in
quantities below 0.01 mg/1.
For example
in
West Germany the following requirements must be met when waste water is
allowed
to flow into the local waters content of free phenols maximum 0.5 mg/1
temperature
maximum 30 °C pH 6.5 8.0. Furthermore the total quantity of waste water
allowed
to flow in within 24 hours is limited to 75 m.
Table
1 Phenols in Water Threshold
Odour and Taste Concentration and Acute Fish Toxicity
Under
certain
circumstances waste water may be allowed to flow into the municipal
biological
water treatment plants together with the household sewage. In any case
the
municipal requirements have to be adhered to. In general the waste
water must
be as follows temperature maximum 30 35 °C pH 6.0 9.0 content of
phenols or
phenol prepolymers maximum 100 mg/1. In addition the composition of the
waste
water must be such that neither the biological processes nor the plant
operation are affected.
The
disposal of
solid phenolic waste is also regulated by the federal legislation.
Waste
materials containing injurious substances are to be disposed off only
at
official disposal places. Flammable waste materials are preferably
destroyed
within the plant in appropriate incinerators. Used phenolic resin
coated sand
can be disposed off at official disposal places without problems.
Published
results of the behaviour of foundry waste sand at disposal places show
that the
phenol quantities which may be eluted of cured resin bonded sands are
even
lower than amounts found for household waste under similar
circumstances.
The
regulation Verordnung
uber gefahrliche Arbeitsstoffe covering the handling of injurious
working
materials 1975 contains requirements for the rating packing marking and
preparation
of dangerous working materials. Dangerous or injurious materials in the
sense
of this regulation are basic and auxiliary materials and their
formulations
(blends mixtures and solutions) if they are explosive flammable toxic
detrimental
to the health caustic or irritating. To indicate these properties
official
warning symbols are to be put on packaging and containers. The scope of
this
list is in accordance with the requirements of the European Community
of 1967
including their alterations and supplements of May 21 1973 as well as
the EEC
rules and regulations for solvents. Phenols and phenolic resins
formaldehyde the
solvents methanol propanol toluene and some others which are used to
produce
resinous solutions are also governed by these regulations.
All
phenolic
resins containing more than 5% free phenol must be designated poisonous
by a
skull. Phenolic resins containing 1 5% free phenol are considered
detrimental
to the health and are to be marked with a St. Andrew s cross
(Andreaskreuz).
Formulations are not considered toxic if the amount of free phenol is
below
0.2%. The label on the packaging or containers must also show the name
of the
producer and the kind of toxic components and must include warnings of
the
special dangers involved and safety measurements to be taken.
During the
handling of phenolic resins sensitive persons may succumb to
dermatological
diseases. To prevent such reaction it is advisable to treat the hands
arms and
other parts of the body which might be exposed to phenolic resins with
an
appropriate protective cream and to wear rubber or plastic gloves
during work.
After work hands and arms are to be washed with a special soap and
again
treated with protective cream.
Furthermore
special
attention should be given to clean working conditions and effective
ventilation
in the working rooms. The MAK value (maximum concentration at place of
work) is
5 ppm for phenol and 1 ppm for formaldehyde.
Waste Water and Exhaust Air Treatment Processes
There are
no
universal solutions for waste problems for plants working with phenol
and
phenolic resins. The choice of the optimum process requires an
individual
analysis of the kind and amount of injurious substances as well as the
structure of the plant and laboratory performance tests. Occasionally a
combination of different processes may be feasible. Such processes are
microbial degradation thermal combustion physical and physico chemical
scrubbing chemical oxidation or resinification reactions and adsorption
methods.
Microbial Transformation and Degradation
The
breakdown of
aromatic compounds is an important step in the natural carbon cycle and
many
microorganisms eubacteriales pseudomonas actinomyceatas endomicetas
higher
funghi are capable of breaking down aromatic substrates. The essential
step
required for biological degradation is the conversion of the aromatic
compound
to an ortho or para dihydroxybenzene structure. The enzymes responsible
for
this hydroxylation have the character of mixed function oxidases or
dioxygenases. The first steps of the three possible oxidative cleavage
reactions of o and p dihydroxy compounds are shown in the formulae
(1/5). In
the case of 1 2 dihydroxybenzene ortho or meta cleavage (1 2) may
occur. Ortho and
para hydroxybenzoic acids (3 5) may be formed as intermediates during
the
degradation of phenolic resin prepolymers.
The
aliphatic
mono and dicarboxylic acids formed are further converted to 3
oxohexanedioic
acid which is taken up in the Krebs cycle or to fumarate pyruvate
acetaldehyde
and acetoacetate. After this the degradation to CO2
and H2O
follows.
Certain kinds of
microorganisms are able to live and
cause degradation in water containing up to 1 000 mg/1 of phenol. They
are most
active at temperatures between 25 35 °C. Further essential
prerequisites are a
sufficient content of nutritive substances (N P) and oxygen pH between
7.5 8.5
and the absence of heavy metal ions (5 mg/1). In order to provide the
nutritive
substances it is advantageous to treat the waste water together with
household
sewage. Ammonium phosphate is most frequently used as a nutritive
compound. The
effectiveness of the biomass increases with time up to a limiting value
because
biological selection processes take place and the resistance and
degradation
ability of some kinds of micro organisms increase. The basin must have
an
effective aerating and circulating system so that dissolved oxygen is
always
available in excess. The Unox process uses oxygen instead of air in
order to
reach a higher oxygen level.
Biochemical
degradation is the most used and most effective process for treating
waste
waters containing phenol. Final effluents in the range of 0.1 mg/1 are
reported. In order to ensure that feed and environmental conditions for
the
biomass are constant properly designed equalization systems are
required for
optimum efficiency. Particular problems arise if plants are operated
discontinuously or 5 days a week.
Chemical Oxidation and Resinification Reactions
In chemical
oxidation processes phenols are normally destroyed to form intermediate
non
toxic compounds (not CO2 and H2O)
and so only a certain
decrease in COD will result. The removal of phenol may reach final
levels of
less than 1 mg/1 or > 99% according to the ratio of chemicals
used.
Hydrogen
peroxide in the presence
of small amounts of iron manganese chromium
and copper salts is an effective
oxidizer of phenols (and other organics). The temperature has little
effect on
reaction rate and conversion a pH in the range of 3 5 is most
effective.
Hydrogen peroxide may be used to treat concentrated wastes high in
phenols or
for pretreating of high phenol waste before biological treatment to
obtain
uniform phenol levels.
Ozone is a
more
effective oxidant than hydrogen peroxide. Lower amounts are normally
applied as
necessary for complete destruction to carbon dioxide and water. The
selectivity
of ozone is low operating at pH values of 11.5 11.8 appears to result
in
preferential oxidation of phenol. Ozone is often used in the final
treatment
step leading to very low phenol levels (lower than 0.1 ppm).
Sodium
hypochlorite or chlorine dioxide which is the oxidizing agent will
oxidize
phenols to benzoquinones (pH 7 8). At pH above 10 further oxidation to
maleic
acid and oxalic acid will occur chlorophenols are not formed. Chlorine
is not
used because of the formation of chlorophenols which are more toxic and
have
more objectionable taste and odor than the original phenols. Potassium
permanganate or potassium dichromates are also effective oxidants
however the
handling of the precipitated sludge can be a serious problem.
Resinification
reactions followed by precipitation of the polymeric material can be
used for
waste waters which contain phenol phenol prepolymers and formaldehyde
by adding
sulfuric acid or ammonia and reacting at higher temperature. Ferric
chloride or
aluminium sulfate are recommended as precipitants. The deposits are
burned in
most cases. It is customary in the plywood particle board and fiber
board
industries to acidify the waste waters with aluminium sulfate up to pH
4. By
this method the resinous components precipitate almost completely as a
deposit which
settles well and is filterable especially if the precipitation occurs
at
elevated temperatures. Afterwards the water must be neutralized with
caustic
lime (pH 6.5 8.0) and the calcium sulfate which is formed filtered.
Thermal and Catalytic Incineration
The
treatment of
exhaust air by oxidation thermal or catalytic incineration is taken
into
consideration if the recovery of the solvents is not feasible or
uneconomical.
The organic components are oxidized to CO2 CO
and water. The
catalytic incineration occurs at temperatures between 350 400 °C. Metal
oxides preferably
however elements of the platinum group on different supports are used
as
catalysts. Catalysts are very sensitive. Sulfur phosphorus
halogen silicon arsenic
compounds and many others lead to
catalyst poisoning.
In
principle
then catalytic incineration is only preferred if the exhaust air
contains only
minor concentrations of organic substances (
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