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-Ortho”-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.

Table 5. Properties of formaldehyde

Fig. 6. Flow diagram of the iron oxide-molibdenum oxide catalyzed formaldehyde production process.

Formaldehyde, a colorless, pungent, irriating gas, is found in aqueous solution almost exclusively as polymethylene glycol.

The portion of formaldehyde CH2=O in aqueous solution is very low (<0.01%). Further equilibria exist in the presence of methanol, which adds to the stabilization by endcaping forming a hemiformal (2.15).

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.

A. 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.

B. 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:

A-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.

B-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.

C-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.

1. 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.      

2. 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.

3. 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.

4.   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:

1. 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.

2. The resin obtained when an excess of phenol is used is soluble in acetone and alcohol, and is permanently fusible.

3. 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:

1. 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.

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. CNDO/2 and CNDO/S calculations.

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

Peaks for n = 0, 1,2,3 have been identified. In a mixture of 70 parts of 40% formalin and 100 parts of phenol approximately 10% of the phenol has reacted with methylene glycol to form phenol hemiformal.

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. The general expression for the overall reaction rate is:

reaction rate = k[Ph—] [methylene glycol]

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.

K < Na < Li < Ba < Sr < Ca < Mg are used as catalysts (Fig. 1) 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

A. 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

1. Good impact and tensile strength in the molded piece.

2.Low moisture absorption.

3.Low specific gravity in the molded piece.

4.Easy wetting by resins and dyes,

5.No chemical or physical effects on steel dies, and particularly, no abrasive effects.

6.Low cost and adequate supply.

7.Nonflammability or low burning rate.

8.Freedom from odor.

9.Supplies must be readily obtainable and of uniform quality.

SECONDARY REQUIREMENTS

1.Good electrical characteristics in the molded piece.

2.Light color, which is retained at, elevated temperatures and in the presence of chem