Historical Development of Phenolic Resins
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.
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 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.
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 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 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.
Formaldehyde is the almost exclusively used carbonyl component for the synthesis of technically relevant phenolic resins. Special resins can also be produced with other aldehydes for example acetaldehyde furfural or glyoxal but have not achieved greater technical importance. Ketones are very seldom used.
The physical properties of aldehydes are compiled in Table4.
Formaldehyde Properties and Processing
Formaldehyde is produced by dehydrogenation of methanol over either an iron oxide/molybdenum oxide catalyst or over a silver catalyst. Because of hazards in handling mixtures of pure oxygen and methanol air is used as oxidizing gas. Oxygen is used to burn the developing hydrogen.
When the silver catalyst is used the reaction mixture of methanol and air is prepared so as to be over the upper flammability limit this is reversed when the oxide catalyst is used. Reactor effluent passes to the absorption train where formaldehyde and other condensables are recovered by condensation and absorption in recirculating formalin streams. The raw formaldehyde solution is then purified by stripping out the unconverted methanol. Formaldehyde assay may be adjusted by regulating amounts of water added to the absorber column or by subsequent diluting in storage tanks. Inhibitors are added to retard the formation of paraformaldehyde in storage.
The Formox process works with a mixture of iron oxide and molybdenum oxide as catalyst. The reaction proceeds at relatively low temperatures between 250 400 °C almost up to completion (95 98%). As a side reaction the formaldehyde produced is oxidized to yield carbon monoxide and water (2.12).
Table 4. Physical properties of some aldehydes
The Perstorp/Reichhold Montecatini Nissui Topsoe CdF Lummus and Hiag/ Lurgi processes function in accordance with this method.
In the BASF and Monsanto processes a silver catalyst is used. Here in general methanol is partially oxidized and dehydrogenized at 330 450 °C on silver crystals or silver nets. The BASF process uses a vapour/methanol/air mixture. The conversion is considerably high at approximately 90%. Silver catalyst processes with an incomplete methanol conversion performed at 330 380 °C have been developed by Degussa and ICI.
A further process is based on the direct oxidation of methane with oxygen from air at approximately 450 °C and 10 20 bar on an aluminium phosphate contact. This process however has not yet achieved any technical importance.
Under the influence of acids hemiacetals react to form acetals by eliminating water. A very important reaction is the formation of HMTA from ammonia and formaldehyde. The overall reaction the reaction mechanism is discussed in Section. By the catalytic action of strong bases e.g. sodium hydroxide formaldehyde undergoes a disproportionation reaction known as Cannizzaro reaction yielding methanol and formic acid according to Eq. (2.16).
Formaldehyde solutions always contain minor quantities of formic acid due to the Cannizzaro reaction generally around 0.05%. The formic acid content can easily be determined by titration with sodium hydroxide.
The Kriewitz Prins reaction may have some importance in modification reactions of phenolic resins with unsaturated compounds. Olefins can react with carbonyl compounds under non free radical conditions to result predominantly in unsaturated alcohols (2.17) m dioxanes(2.18) and 1.3 glycols (2.19).
As in the hydroxymethylation of phenols the hydroxy methylene Carboniumion CH2 OH is the alkylating agent.
When storing formaldehyde solutions attention should be given to the fact that at lower temperatures or higher concentrations paraformaldehyde may separate. The stabilization of aqueous formaldehyde solutions can be achieved with alcohols preferably methanol. Urea melamine methylcellulose and guanidine derivatives are also recommended for stabilization among other compounds. Proper storage containers should be of stainless steel iron containers are not suitable. Containers with plastic lining or RP containers may also be used.
The major use of formaldehyde is in the production of thermosetting resins based on phenol urea and melamine.
Table 6. Breakdown of formaldehyde consumption
As shown in Table 6 approximately 55% of the total formaldehyde consumption occurs in the production of thermosetting resins. Since 30 55% aqueous solutions are being used for resin production it is often the case that the consumers install their own small formaldehyde plants which are supplied with methanol by a large central methanol plant. In spite of the high transportation costs for formaldehyde solutions the relatively small capital investment will be soon repayed. For this reason there are about 53 formaldehyde plants in Western Europe at the present time with a total capacity of 4.3 million tons per year in 1976.
Formaldehyde is the more reasonably priced component in phenolic resins. As an average throughout all fields of application about 1.6 mol formaldehyde including HMTA per mol phenol are used.
Instead of formaldehyde solutions of 30 55% higher concentrated aqueous or alcoholic ones may be used. These solutions are produced by dissolving paraformaldehyde in water at 80 100 °C by addition of a small quantity (1%) of NaOH or tertiary amines as depolymerization catalyst. It is also possible to concentrate formaldehyde solutions by adding paraformaldehyde.
Table 7. Specification of a formaldehyde solution
Paraformaldehyde 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.
Despite the rapid commercial growth of the phenoplasts our knowledge of their chemical structure and of the chemical reactions which take place during curing had been meager until recently. The reaction of phenol with formaldehyde leads to the formation of a number of products which are difficult to separate and are readily susceptible to resinification by heat or reagents. Consequently their study tended to be unattractive to organic chemists. The difficulty of the problem was recognized by Baeke land who wrote in 1911
It should be pointed out that we have to deal here with substances which are amorphous noncrystalline nonvolatile and cannot be purified in the usual ways. Furthermore in any of these reactions several substances are liable to be produced at the same time. These substances can form solid solutions one with another or with any excess of the reacting materials employed.
Within the last five years however very significant work has been done in the study of the chemistry of the pheno plasts.
GENERAL REACTION OF PHENOLS WITH ALDEHYDES
In general phenols react with aldehydes to form condensation products if there are free positions on the benzene nucleus ortho or para to the phenolic hydroxy group and if the length of any of the substituents on the nucleus is not so great as to cause steric hindrance. Because of its greater reactivity formaldehyde is by far the most widely used aldehyde. For this reason the discussion in this chapter will be confined to reactions with formaldehyde the use of other aldehydes will be discussed in later chapter. The discussion in this chapter will also be limited to the monohydric phenols that is phenols with only one hydroxy group for each benzene nucleus.
The reaction of phenol with formaldehyde in the absence of any other reagents is very slow and catalysts are always added to accelerate the reaction. These catalysts may be either acids or bases and the nature of the reaction product depends considerably upon the type of catalyst which is used.
The mechanism of the addition of formaldehyde to phenol is not entirely understood. Manasse suggested in 1894 that the formaldehyde may react in alkaline solution as methylene glycol as indicated in Equation (1) or it may undergo an acetal addition with subsequent rearrangement of the hemiformal as shown in Equation (2). The latter suggestion was also made by von Tollens and later by Baekeland and Bender.
Walker favours the idea of the formation of a primary phenolic hemiformal and suggests that the formation of the phenol alcohols may involve tautomeric rearrangements of the type indicated below
The hemiformal is very unstable and rearranges rapidly to the phenol alcohol because of this instability the hemiformal from a phenol has never been isolated.
Whatever the exact mechanism of formaldehyde addition may be the phenolic hydroxy group activates the benzene ring so that the methylol groups always enter the nucleus in ortho and para positions to the phenolic hydroxy group. When some of the ortho and para positions are occupied the reaction of the phenol becomes much slower when all of the ortho and para positions are unavailable no reaction takes place. The presence of substituents in the meta position also has a pronounced effect upon the rate of reaction with formaldehyde. Alkyl groups in the meta position tend to accelerate both the initial condensation and the subsequent resinification. The presence of a hydroxy group in the meta position as in resorcinol greatly increases the reactivity.
The nature of the reaction products depends upon the type of catalyst used. When alkaline catalysts are used the primary reaction products are phenol alcohols and are called resoles. When acid catalysts are used the primary reaction products are apparently also phenol alcohols but these rearrange quickly under the influence of the catalyst to give diphenylmethane derivatives to which the name novolacs was given by Baekeland in 1909. Because of this important difference the chemical structure of these two classes of phenoplasts will be discussed separately.
The resoles are formed when formaldehyde acts upon a phenol in alkaline solution. Almost any alkali may be employed alkali metal hydroxides hydroxides of the earth metals such as barium or calcium ammonia or quaternary ammonium bases. There is some evidence to indicate that the products formed are not identical when different bases are used especially in those cases where the phenol has several reactive positions. It is almost definite that ammonia and amines when employed as catalysts enter into the condensation reaction. The nature of the catalyst may affect to some degree the position in the ring which is occupied by the methylol group. For example Auwers states that strong alkalis favour the production of para methylol derivatives. However, not enough work has been done to present any definite data on this point.
Walker has shown that from the condensation of phenol with formaldehyde under alkaline conditions it is possible to isolate a tetramethylol derivative of 4 4 dihydroxy diphenyl methane having the probable constitution of Formula I Seebach isolated the same product melting at 145°C. by the action of more than three moles of formaldehyde on one mole of phenol using a little magnesium oxide as a catalyst. From o cresol the product which is obtained has the constitution shown in Formula II. From these observations it is concluded that the first condensation takes place in the para position. The existence of the tetramethylol derivative implies branching of the chains at an early stage.
Reference has already been made to the fact that the methylol group enters the ring ortho or para to the phenolic hydroxy group. When more than one such position is available polymethylol compounds are formed. Thus from phenols with three reactive positions the series of methylol derivatives shown in Formulas III may be formed. In the case of the reaction of formaldehyde with phenol the presence of all of these alcohols has been confirmed either through isolation of the alcohol itself or of a derivative. The monophenol alcohols saligenin (o hydroxybenzyl alcohol) and homosaligenin (p hydroxybenzyl alcohol) were separated and identified was able to prove the presence of the two dialcohols by methylation of the phenolic hydroxy group followed by oxidation of the methylol groups to carboxyl. The formation of the trimethylol compound was definitely established by Bruson and McMullen. They condensed three moles of formaldehyde with phenol in the presence of a strongly basic nonaromatic secondary amine. With morpholine for example a definite crystalline compound melting at 106 107°C. was formed which had the structure shown in Formula IV. The quantity of dialcohols formed depends upon the ratio of formaldehyde to phenol but even with equimolar ratios some quantities of polyalcohols are formed because of the great velocity of the addition reaction.
It is difficult to obtain the trialcohol but derivatives may be formed such as the addition compound described by Bruson. Stager attempted to isolate the trialcohol in pure form but failed. As Granger states the addition of the third molecule of formaldehyde becomes very slow as the reaction progresses although it is comparatively rapid in the earlier stages.
Curing Stages of Resoles
The mechanism by which the phenol alcohols condense to resins is very complex. Our present knowledge of the curing reactions will be discussed in detail later in this chapter. For convenience the curing mechanism has been divided into three phases. The three phases are
stage resin (resole). This represents the initial condensation product of phenol and formaldehyde. The resin consists mainly of phenol alcohols although it is probable that some condensation has taken place to give methylene ethers methylol derivatives of diphenylmethane and perhaps methylenequinone or its polymers. Lebach termed this stage resole from the Latin resina (a resinous body) and ol which referred to the solubility in alkalis and pointed to the probable presence of hydroxy groups.
stage resin (resitol). This represents the second stage of condensation. The resin is no longer soluble in alkalis because the molecular weight has advanced to such a size that the alkaline salts are no longer soluble. It is partly or even completely soluble in organic solvents such as acetone or alcohol. Cross linkage however has not proceeded very far and the resin is still softened by heat and is plastic while hot although hard and brittle when cold. Lebach called this stage the resitol.
stage resin (resite). This represents the final stage of polymerization with a large amount of cross linkage. The resin is completely insoluble and infusible. Lebach called this the resite.
It is important to note that these stages of resin formation are not clearly defined but pass gradually one into the other.
As has been indicated above even the A stage resin does not consist entirely of phenol alcohols but contains appreciable amounts of higher condensation products. In the B stage phenol alcohols are still present together with much more highly condensed resins and appreciable amounts of partly cross linked resins. As will be explained later it is probable that methylene ethers are present in this stage. There are usually present even in the C stage resin from 3 to 6% of products which can be removed by vigorous extraction with acetone.
Kinetics of A Stage Reaction
Although a great deal is known qualitatively about the nature of the initial reaction of phenol and formaldehyde there have been few quantitative measurements on the kinetics of the reaction. Novak and Cech attempted to follow the progress of resinification by a study of the refractive index viscosity and bromine value. Another empirical approach is represented by the work of Holmes and Megson who studied the behaviour of various phenols with a series of catalysts. In their work 0.4 gram mole of the phenol was mixed with 0.6 gram mole of 40% formalin and the catalyst and immersed in boiling water. An arbitrary time the resinification time was measured from initial heating to the appearance of a permanent turbidity. Table I shows the resinification time in minutes for various phenols when 0.5 g. of trimethylamine was used as a catalyst. More detailed experiments were then carried out with phenol the three cresols and m 5 xylenol the catalysts employed were trimethylamine triethylamine pyridine and ammonia. The reaction curves were roughly hyperbolas of the form
Table 1. Resinification time for Various Phenols
For the purpose of comparing the catalytic activities of the more common bases m cresol was condensed in the presence of 0.75 g. of each base (except ethylenediamine where 0.375 g. was used). Table 2 shows the resinification times obtained. The values for sodium hydroxide potassium hydroxide and lithium hydroxide when reduced to molecular proportions lie on the same curve.
The effect of higher temperatures on the condensation of m cresol was examined by condensing one mole fraction (27 g.) with one mole fraction of paraformaldehyde (7.5 g.) and 2 g. of pyridine in cyclohexanol as a solvent.
Table 2 Relative Activity of Basic Catalysts
Nordlander studied the ammonia catalyzed condensation of phenol with formaldehyde. He classified the two general types of reaction as (a) the primary reaction in which phenol and formaldehyde react to form water soluble intermediates of the oxymethylene phenol type (A stage) and (b) the secondary reaction in which these intermediates react further by condensation to give water insoluble resinous products. The reaction was studied by measuring the rate of disappearance of formaldehyde and the manner in which the bromine value of the water soluble portion of the reaction mixture varied as the reaction proceeded.
The results disclosed that the primary reaction is confined to the interaction of one mole of formaldehyde with one mole of phenol no formaldehyde reacts with any of the intermediates formed. This reaction is apparently of monomolecular order with the rate proportional to the concentration of free phenol. The influence of the catalyst is complex apparently both hydrogen ions and hydroxyl ions and probably other ions derived from the catalyst promote the reactions. At very low ammonia concentrations the reaction order changes to one of apparently bimolecular type which is characterized by a reaction rate proportional to the square of the formaldehyde concentration.
The secondary reaction seemed to be of monomolecular order the catalyst action was complex and similar but not identical to that governing the primary reaction. Weakly alkaline catalysts which are sufficiently active to bring about a primary reaction of the same type as that induced by ammonia are unable to initiate the secondary reaction to any extent. When the catalyst was a weak alkali such as ammonia phenol did not take any part in the secondary reaction which was then limited to the phenol alcohols. Nordlander reported that the temperature coefficient varied for the two reactions the secondary reaction rate increased much more rapidly with the temperature than the primary reaction rate.
Sprung made a very thorough study of the kinetics of the reaction of paraformaldehyde in the presence of a number of phenols using triethanolamine as a catalyst. He determined that the addition phase of the reaction apparently followed a first order rate law. Figure 1 compares the reactivities of the various phenols at 98°C. The apparent first order rate contants as taken from the slopes of the straight lines are listed in Table 3. The introduction of a methyl group in the meta position increased the reaction rate by a factor of 2.8. The introduction of a methylol group as in the case of saligenin depressed the reactivity of phenol to about the same extent as a methyl group similarly placed. This indicates that the rate law as experimentally determined for a di or trifunctional phenol apparently expresses a summation of the rates at which the first second and presumably also the third molecule of formaldehyde react.
Table 3. Effect of Substitution on Reactivity of Phenols
Chemistry of Curing Reactions
As has been previously stated very little definite information on the mechanism of curing has been available until within the last few years. It was generally accepted that the phenol alcohols condensed with the elimination of water to yield three dimensional macromolecules which were cross linked by methylene bridges. However in the case of phenols with three reactive positions the curing reactions were so rapid and so complex that little progress had been made in isolating and identifying compounds from the later stages of the condensation.
In order to overcome these difficulties recent workers have studied the curing reactions of phenols in which one or two of the reactive ortho or para positions are blocked. In this way only mono or di phenol alcohols can be formed and insoluble cross linked products in general cannot be obtained. In most cases considerable yields of crystalline products are obtained and these products can be identified and their further reactions studied. It was through this line of attack that Zinke Hultzsch von Euler and their respective associates have built up our present knowledge of the curing mechanism. As a result of their studies it has been shown that the phenol alcohol which results from the primary condensation of a phenol and formaldehyde in alkaline solution undergoes a complex series of reactions. The extent to which these various reactions take place depends upon the structure of the initial phenol the temperature at which the phenol alcohol is heated and the time of heating. Scheme 1 taken from Adler gives in diagrammatic form the various reactions which a phenol alcohol may undergo on curing. Those compounds which have been isolated in pure form are in heavy type. These various reactions will now be discussed in more detail.
Reactions 1 and 2 when a phenol alcohol is heated some formaldehyde is split off with a regeneration of the original phenol. The phenol then combines with the unchanged phenol alcohol with the splitting out of water and a dihydroxy diphenylmethane derivative is formed. A typical reaction of this type is shown for 4 hydroxy 3 5 dimethylbenzyl alcohol (Eqs. 4a and 4b). The extent to which this reaction takes place depends very much upon the structure of the initial phenol.
Reaction 3 Reaction 3 as shown in Equation (5) indicates the formation of dihydroxydibenzyl ether. When a dialcohol is used as for example the dialcohol from p cresol long chain ethers are formed as shown in Equation (6). It is apparent that this reaction is in general the most important primary reaction in the curing of highly substituted phenol alcohols. In many cases the dibenzyl ethers form the largest single product which can be isolated from the cured reaction mass.
In the case of 3 (2 hydroxy 5 methylbenzyl) 2 hydroxy 5 methylbenzyl alcohol Adler showed that only the reaction indicated in Equation (7) took place. An analogous reaction took place when the dialcohol was used but in this case in addition to resinous linear ether about 6 10% of cyclic ether was formed with the structure shown in Formula V.
The ether formation appears to take place more slowly in the case of p methylol derivatives than in the case of the ortho derivatives. Ether formation is retarded by an increase in the curing temperature or by the presence of alkali. When alkalis are present the formation of methylene bridges is favoured. Thus when a resole from p tert butylphenol contains alkali it yields on heating a fairly large quantity of a crystalline product which was identified by Ziegler as the cyclic compound VI. It is interesting to note that this compound is similar in structure to the cyclic ether shown in Formula V.
Apparently the action of the alkali is to split off formaldehyde from the ether producing the cyclic methylene compound.
Reaction 4 on further heating particularly at temperatures higher than are needed to form the ether the latter may split off formaldehyde and give a dihydroxydiphenylmethane which is identical with that obtained through reactions 1 and 2. Reaction 4 may be written as in Equation (8).
Hanus have been able to show that when phenol alcohols are heated to a certain temperature only water is split off when the temperature is then raised to another definite point formaldehyde is split off indicating the beginning of reaction 4. The temperature increase required to initiate reaction 4 over that required for reaction 3 is definite and depends upon the size and nature of the substituent group. Table 4 summarizes the effect for dialcohols of various para substituted phenols. The amount of water which is split off is very nearly one mole for every two moles of phenol alcohol (or one mole for every mole of phenol dialcohol) which reacts. Even under the most favourable conditions however much less than one mole of formaldehyde is split off for one mole of the ether. In their best experiment Zinke and Hanus were not able to get more than 0.6 mole of formaldehyde split off. This is attributed either to side reactions or to the tendency of formaldehyde to combine further with the dihydroxydiphenylmethane derivative.
It is interesting to note that the presence of a free phenolic hydroxy group is required for reaction 4 to take place although a free phenolic hydroxy group is not needed for the ether formation (reaction 3). This was shown by Ziegler who treated the dialcohol from p cresol with p toluenesulfonyl chloride to form the tosyl derivative. The latter split off water readily to form chain ether but this ether was stable and on further heating did not split off formaldehyde. Adler state that neither reaction 3 nor 4 takes place when the phenolic hydroxy group is etherified. Thus the monomethyl ether of p cresol dialcohol was almost unchanged after heating one hour at 160°C. although the free p cresol dialcohol resinified quickly at 130°C.
Reaction 5 Small quantities of phenol aldehydes are usually found in the cured products. Zinke has suggested that these may be formed by the thermal cracking of the dihydroxydibenzyl ethers as shown in Equation (9). The equation also shows that a nuclear methylated body is formed simultaneously. The quantities of aldehydes formed are usually quite small (4%) even from the highly substituted phenols employed in these experiments and the yield depends upon the nature of the substituent groups on the original phenol. Ziegler have shown that very considerable amounts of aldehyde are formed upon heating the dibenzyl ether of 3 5 dichloro 2 hydroxybenzyl alcohol (obtained by the action of formaldehyde on 2 4 dichlorophenol).
Hultzsch does not favour the theory that phenol aldehydes are formed as indicated above. He points out that the reaction as written would require equimolar quantities of phenol aldehyde and nuclear methylated phenol to be formed experiment shows that this is not always the case. Hultzsch therefore prefers to consider the phenol aldehyde formation as an oxidation reduction action of methylenequinone. The presence of both dialdehydes and monoaldehydes in the curing products of cyclohexylphenol dialcohol was confirmed by Mayer through ultraviolet light absorption studies. In any case it is important to note that aldehyde formation plays only a minor part in the curing reaction this is particularly true in the case of commercial phenoplasts where phenols with two or three reactive positions are used.
Reactions 6 and 7 the formation of methylenequinone or quinonemethide is particularly interesting because it indicates a mechanism by which phenoplasts or at least a part of the resin may be formed by polymerization rather than condensation. Baekeland had suggested that polymerization played a part in the formation of phenoplasts although he offered no definite mechanism. Wohl and Mylo suggested in 1912 that the polymerization might proceed through the methylene derivative of the tautomeric form of phenol although definite proof was lacking. Novak also felt that polymerization played a part in the curing reaction it is interesting to note that these investigators showed that the curing reaction was catalyzed by treatment of the formaldehyde with ozone whereby peroxides are presumably formed. The latter would evidently be catalysts for a polymerization reaction and would be expected to have no effect upon a condensation. Later investigators have definitely identified dimers and trimers of methylenequinone in the reaction products from the curing of highly substituted phenols. The monomer itself has never been identified as it is very unstable and polymerizes rapidly. The methylenequinone may be formed directly from the phenol alcohol as shown in Equation (10a) or it may be formed by loss of water from the dihydroxy benzyl ether as shown in Equation (10b).
The equations above indicate only the formation of para methylenequinone but the ortho compound is also formed when the methylol group is in the ortho position as is shown in Equation (11). Generally speaking the course of the reaction is the same whether the methylol group is in the ortho or para position relative to the phenolic hydroxy group.
In the curing of phenol alcohols it is probable that both reactions shown in Equation (10) take place simultaneously. However the formation of the dihydroxydibenzyl ether is usually the predominating reaction and this compound is usually formed at low temperatures (below 150°C.) it is rather stable at these temperatures and only breaks up into methylenequinone at higher temperatures (about 200°C.). The monomeric form of methylenequinone is unstable and polymerizes rapidly to form dimers or trimers. The dimer is colored yellow and Pummerer and Cherbuliz have attributed to this compound the general formula of cyclic quinone ether as shown in Formula VII. The presence of the quinone nucleus accounts for the
PHENOLIC RESINS FROM HIGHER ALDEHYDES
The discussion in previous chapter on the chemical structure of phenoplasts has covered only condensations with formaldehyde. Higher aldehydes are occasionally used in the manufacture of phenoplasts although by far the greatest percentage of these resins is made from formaldehyde. There are two reasons for this (1) Condensation with formaldehyde gives in general resins which have shorter curing times than those of higher aldehydes and (2) formaldehyde is subject to few if any side reactions in the presence of the condensation catalysts. The latter consideration is important from the chemical viewpoint. The Cannizzaro reaction is the only side reaction which takes place to any extent in the case of formaldehyde under Ordinary conditions of phenoplast manufacture. This reaction involves the reduction of one molecule of formaldehyde accompanied by the oxidation of a second and is normally catalyzed by alkalis
With sodium hydroxide and formaldehyde alone the reaction takes place slowly at room temperature but the velocity approximately triples for every rise of 10°C. in temperature and is very rapid at 100ºC. The presence of appreciable quantities of phenols greatly retards the Cannizzaro reaction with formaldehyde and virtually no Cannizzaro reaction takes place even at refluxing temperatures when the molar ratio of phenol to alkali is greater than six to one.
Acetaldehyde and most of the higher aldehydes also undergo self resinification when treated with strong acids or bases. The facility with which such side reactions take place limits the usefulness of the higher aldehydes in the manufacture of phenoplasts.
Acetaldehyde 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.
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.
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.
Of all the higher aldehydes which have so far been discussed furfural probably has the most commercial importance in the manufacture of phenoplasts. It had been known since 1860 that furfural could be condensed with phenols to give resinous bodies. In 1921 Novotny covered practical details of the condensation in U. S. Pat 1 398 146. Trickey Miner and Brownlee studied the condensation in 1923 and came to the following conclusions
In the case of acid condensed resins
In order to obtain an infusible insoluble resin the molecular proportions preferably should be slightly in excess of 1 mole of furfural to 1 mole of phenol.
The resin obtained when an excess of phenol is used is soluble in acetone and alcohol and is permanently fusible.
The resins obtained by the use of varying amounts of acid as a condensing agent were similar but the time necessary to complete the reaction varied from two weeks when 0.2% of hydrochloric acid was used (based on the weight of the total reaction mass) to ten hours in the case of 0.6% acid.
In the case of alkaline condensed resins
In order to obtain an insoluble resin the proportions are preferably about 1.25 moles of furfural to 1 mole of phenol.
2. The resins formed by an excess of phenol are solid and brittle when cold melt easily and are readily soluble in acetone alcohol and furfural. When heated with enough furfural to bring the molecular proportions up to 1.25 of furfural to 1 of phenol the resins pass over to the infusible state.
By careful condensation of furfural with phenol in dilute acid solution it is possible to prepare the primary product hydroxyphenyl furyl carbinol as shown by the reaction in Equation (6). The position of the carbinol group has not been established. It appears then that the behaviour of furfural in phenoplast condensations is very similar to that of formaldehyde. There are however certain differences between the formaldehyde and furfural condensation products. The phenoplasts from furfural have a dark purplish black color which differs from the yellow or brown color of the phenoplasts made from formaldehyde. The condensation of the phenols with furfural is initially more sluggish than with formaldehyde and furfural itself has a marked tendency to polymerize under acid conditions.
phenolic resins from polyhydric phenols
The discussion so far has concentrated upon the monohydric phenols that is phenols which have only one hydroxy group upon the nucleus. From the commercial point of view the monohydric phenols are by far the most important since they are more readily available and less expensive. The theoretical studies leading to our knowledge of structure have dealt mostly with the monohydric phenols. monohydric phenols which have been highly substituted in order to decrease their reactivity. The polyhydric phenols are in general much more reactive than the monohydric phenols since the effect of the second hydroxy group is to activate the benzene ring further. This is particularly true in the case of resorcinol where the hydroxy groups are meta to one another. The resonance produced by this arrangement activates the ortho and para positions of the nucleus as a result the resorcinol resins are very rapidly cured.
Baeyer in 1872 condensed resorcinol and pyrogallol with various aldehydes. When the aldehyde was relatively inactive crystalline compounds could be obtained. For example benzaldehyde and pyrogallol gave a crystalline product. With resorcinol and acetaldehyde or formaldehyde resinous products were obtained. Caro in 1892 condensed an excess of resorcinol with formaldehyde in the presence of dilute hydrochloric acid the product obtained was identified as tetrahydroxydiphenylmethane (Formula 1). The product obtained in a similar manner with pyrogallol was hexahydroxydiphenylmethane.
Boehm prepared methylol derivatives of resorcinol by reduction of the corresponding dicarbomethoxy aldehydes. The compound shown in Formula 2a was stable and not sensitive to acids while compound b could not be prepared in a pure form but always occurred as a resin. From this time on no studies were made on the structure of the resins from polyhydric phenols until von Euler and associates included this type of condensation product in their work.
In the case of hydroquinone it was shown that alkaline condensation with two moles of formaldehyde gave the dialcohol illustrated in Formula 3a while four moles of formaldehyde gave the tetraalcohol illustrated in Formula 3b. Both the di and tetraalcohols resinified on heating.
On treatment with a weak acid the dialcohol quickly resinified to an insoluble amorphous product to which von Euler Adler and Gie ascribe a methylene bridge structure.
The alkaline condensation of catechol with formaldehyde gave only catechol dialcohol in which the position of the methylol groups was established by von Euler Adler Kispeczy and Fagerlund according to Formula 4. The catechol dialcohol also resinified on heating. Rosenmund and Boehm prepared the monomethylol derivative of catechol by reduction of the corresponding aldehyde.
Kyrning studied the mechanism or curing of the di and tetraalcohols from hydroquinone and catechol. The dialcohol of hydroquinone apparently went through a number of reactions involving ether formation methylenequinone formation and the formation of methylene bridges. The tetraalcohol of hydroquinone was particularly interesting. It has no free nuclear positions and hence diphenylmethane formation cannot take place. When cured at 210°C. it gives off about three moles of water per mole of tetraalcohol. This indicates methylene quinone formation in addition to the development of ether linkages as shown in Equation (I).
Kyrning also studied the tetraalcohol from p quinone and showed that at 180°C. it lost slightly less than two moles of water indicating the formation of ether linkages only. Methylenequinone formation could not take place because no phenolic hydroxy group was present. This experiment showed that the quinone groups conferred a reactivity similar to that obtained when the hydroxy groups were present. However methylation of the phenolic hydroxy groups in hydroquinone tetraalcohol gave a product which was completely stable at 180°C.
Dubusay studied the rate of gel formation in alkaline catalyzed resorcinol formaldehyde resins. They concluded that the ratio of aldehyde to resorcinol had a marked effect upon the curing time as the ratio increased the gel time passed through a minimum and then increased. The position of the minimum gel time was affected by the concentration of sodium hydroxide present. Von Euler and co workers point out that the methylol derivatives of the polyhydric phenols will produce infusible and insoluble resins when reacted with phenols which have only two reactive positions though the latter normally give only soluble and fusible resins. This is because the dihydroxyphenols behave as tetrafunctional compounds and when combined with the difunctional phenols the dihydroxyphenols supply a sufficient number of reactive positions to cause the formation of three dimensional cross linked molecules.
Because of its great reactivity resorcinol and its methylol derivatives are used commercially today either as such or in combination with other resins to increase the rate of curing of phenoplasts. The addition of from 3 to 20% of resorcinol will either decrease the time of cure required at high temperatures or will permit curing at relatively low temperatures even at room temperatures. Methods for accomplishing this have been described in British Pat. Shiskov has shown that chemically resistant resins for coatings may be prepared from 15 parts of phenol to 12 parts of resorcinol. In this proportion the resin hardens rapidly and has physical properties which are only slightly inferior to those obtained when pure resorcinol is used.
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.
Formaldehyde Water and Formaldehyde Alcohol Equilibria
Formaldehyde is by far the most reactive carbonyl compound. In aqueous medium a very fast acid and base catalyzed hydration reaction of formaldehyde to methylene glycol occurs. The equilibrium indicated in Eq. (3) far on the side of methylene glycol can be estimated by UV spectroscopy (n p) transition of the carbonyl group) by NMR or by polarographic methods.
Methylene glycol is found in aqueous solutions as a low molecular condensation polymer. It is also obtained by dissolving parafor maldehyde. The concentration of monomeric non hydrated formaldehyde is very low generally less than 0.01%. The MWD of poly methylene glycol in a 40% aqueous solution is indicated in Table 6.
The methylene glycol is present as monomer only in very dilute aqueous formaldehyde solutions (1 2%). The depolymerization of aqueous polyoxymethylene glycol (4) in the presence of acidic and basic catalysts is of importance for the overall reaction rates for resol and novolak formation.
Alcohols are often present in the P/F reaction. Methanol is present at least in small amounts (~1%) because the formaldehyde production process starts from methanol. In addition it can be formed from formaldehyde during storage by disproportionation (Cannizzaro reaction). Furthermore methanol may be added because it is very efficient in stabilization of concentrated aqueous formalde hyde solutions. The chain termination prevents the formation of low soluble polymers so that precipitation or turbidity will be omitted. Alcohols can react with aqueous formaldehyde in a neutral pH to form hemiformals (5). Diformals are not formed under these conditions.
Also the reaction between hydroxymethylphenols and methylene glycol must be considered. The extent of this reaction with the hydroxymethyl group (6) as well as with the phenolic hydroxyl group (7) has been studied by high resolution NMR
Formaldehyde is therefore consumed in the P/F reaction also to form hemiformals which constitute a potential source of formaldehyde and may be detected by usual titrimetric methods but is otherwise not more directly involved in the hydroxy methylation reaction. This should be a sufficient reason for the apparent reduction of the reaction rate at rising conversion.
Phenol Formaldehyde Reaction under Alkaline Conditions
The reaction between formaldehyde and phenol in the alkaline pH range was first mentioned in 1894 by L. Lederer and O. Manasse. It is therefore occasionally also designated as the Lederer Manasse reaction. At a pH above 5 bis and tris alcohols are formed as well as mono alcohols and other compounds. The simplest product of this reaction 2 hydroxybenzylalcohol (saligenin) was already isolated from the glucoside salicyn by hydrolysis with diluted acid.
Inorganic Catalysts and Tertiary Amines
Sodium hydroxide ammonia and HMTA sodium carbonate calcium magnesium and barium hydroxide and tertiary amines are used as catalysts in the alkaline hydroxy methylation reaction. In aqueous solutions as used in all technical processes formaldehyde is present as methylene glycol. Phenol reacts quickly with alkali hydroxide to form the phenoxide ion which is stabilized by resonance according to Eq.(10)
In the following reaction catalyzed by alkalis C alkylation in ortho and para position occurs almost exclusively. Meta substitution is hardly not evident.
The quinoide transition state is stabilized by proton shifting as indicated in the Eqs. (11) and (12). This reaction mechanism was recommended for dilute solutions. The monomethylol derivative continues to react with formaldehyde forming two dimethylol and one trimethylol compounds.
The kinetics of the base catalyzed phenol formaldehyde reaction has been thoroughly researched and is relatively well understood. In general a second order reaction type was found with the exception of the ammonia catalyzed reaction which rather corresponds to one of the first order.
It must be pointed out however that the actual constitution of the hydroxyalkylating agent in the alkaline catalyzed reaction is not yet fully understood. It is not clear how methylene glycol would react with the phenoxide ion. The concentration of non hydrated formaldehyde is too low to explain the reaction rates.
A deviating reaction mechanism was proposed very early by Claisen later by Walker and others. The presence of hemiformals 3.7 in aqueous phenol formaldehyde solutions has been proven by the means of NMR.
As evidence for the formation of hemiformals as intermediates followed by a shift of the hydroxymethyl group according to Eq.(15) the absence of the reaction if the phenolic group is etherified was pointed out. This statement however is not correct since the nucleophily of the phenoxide anion is the decisive parameter in the alkaline hydroxymethylation.
A series of experimental results indicates that the constitution of the transition state is considerably more complex than indicated in the Eq. (11). The strongest evidence is the dependence of the ortho/para substitution ratio on the type of catalyst. The ortho/para ratio decreases from 1.1 at pH 8.7 to 0.38 at pH 13.0. It has been recognized that the ortho substitution is considerably enhanced if metal hydroxides of the first and second main group along the series.
Even more distinct is the effect of the hydroxides of transition metals. The ortho substitution is the more favoured the higher the chelating strength of the cation. The directing effect of the Fe Cu Cr Ni Co Mn and Zn ions is explained by Peer as formation of chelates as transient compounds according to formula (16). Boric acid also has a strong ortho directing effect (17).)
The formation of high ortho novolaks if magnesium oxide or zinc oxide are used as catalysts. The different activity of some of the more frequently used catalysts and their effect on MWD examined by GPC is shown in Figure 1. Identical reaction conditions have been used for the production of the resols. The catalyst was neutralized with hydrochloric acid and the resin analyzed without previous distillation. The highest degree of ortho orientation is observed when zinc acetate is used (Fig. 5) followed by magnesium oxide and tri ethylamine as catalysts (Fig. 1).
The interpretation of the orientation and directing effects depending upon the type of catalyst must be performed with care. The concentration of any particular alcohol in the reaction mixture does not only depend upon the rate of formation but also upon the rate of disappearance due to further reaction i.e. it also depends upon the mol ratio of phenol to formaldehyde and the reaction time. The order of appearance of the individual methylol phenols in the GPC chromatogram depends despite of molecular size also on the number of hydroxyl groups because of solvent interaction.
fillers for phenolic resin moulding powders
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
Good impact and tensile strength in the molded piece.
Low moisture absorption.
Low specific gravity in the molded piece.
Easy wetting by resins and dyes
No chemical or physical effects on steel dies and particularly no abrasive effects.
Low cost and adequate supply.
Nonflammability or low burning rate.
Freedom from odor.
Supplies must be readily obtainable and of uniform quality.
Good electrical characteristics in the molded piece.
Light color which is retained at elevated temperatures and in the presence of chemical materials used in the preparation of the molding powder.
Inertness to acids alkalis and solvents.
Availability in controlled mesh size and bulk factor.
TYPES OF FILLER
Fillers are usually classified primarily according to their general chemical nature organic or mineral and then are further divided into subdivisions according to their chemical composition and physical structure.
EFFECT OF FILLER ON IMPACT STRENGTH AND DAMPING
The principal mechanical effect which is produced by different fillers is a change in the impact resistance of the molded plastic. The change in impact resistance results from an increase in the capacity of the molded specimen to absorb mechanical shock waves and thus prevent their amplification at certain points through resonance. The ability to absorb mechanical shock waves and transform them into heat is known as damping. In order to obtain a high degree of damping it is necessary to have a high degree of nonhomogeneity in the molded plastic this is usually best accomplished by the use of fibrous fillers. The filler itself must have very high tensile strength so that it may resist the disruptive tensile forces which are created by the shock wave.
De Bruyne in 1936 studied the effect of damping and pointed out that the increase in shock resistance of fiber filled molding compounds was connected with their greater damping capacity. De Bruyne has also emphasized the importance of high damping capacity in assuring freedom from vibration. On the other hand a high damping capacity involves a high energy absorption in the part which is subject to vibration and the energy absorption is necessarily accompanied by the development of heat which may of itself be destructive to the plastic. Leaderman has studied the damping capacity by the decrement of free oscillations using solid cylindrical specimens tested in torsion. Four grades of phenoplast molding materials were employed (a) phenol formaldehyde resin white transparent and unfilled (b) commercial phenoplast molding material with wood flour filler (c) shock resisting material with a filler consisting of fabric snippings impregnated with phenol formaldehyde resin and (d) shock resisting material with a filler consisting of closely packed cords running parallel to the axis of the specimen.
Figure 1 taken from Leader man shows the rate at which free torsional oscillations decreased when these four fillers were used. From these data the mean specific damping capacity in torsion was computed (in per cent) and is given in Table 1. To illustrate the relation between damping capacity and impact resistance Table 1 also contains the approximate impact resistance for these various materials as measured by the standard A.S.T.M. test.
MICROSCOPIC STRUCTURE OF FILLERS
The microscopic structure of both cellulosic and inorganic fillers has been examined. It was demonstrated that the resilient fibrous fillers vary considerably in particle size. In the cellulosic fillers the fibers are frequently split in many cases the microscopic examination showed that only 20 to 30% of the fibers were intact.
RATIO OF RESIN TO FILLER
The ratio of resin to filler has a profound effect upon the molding qualities of the powder and upon the physical and chemical properties of the molded plastic. (In all cases in this chapter weight ratios are meant.) When cellulose fillers are employed the molding composition usually contains approximately equal parts by weight of resin and filler. With this proportion the best general combination of molding qualities strength properties and water resistance is obtained. When less resin is employed molding becomes more difficult because the combination has less flow. Some strength properties such as tensile and flexural strength tend to drop off rather rapidly. Impact strength values usually rise to a maximum at about 30% resin content and then decrease rapidly. However whatever virtue there may be to this gain in impact strength is offset by the loss in tensile and flexural strength. Water resistance and resistance to other chemical agents decrease as the resin content decreases because the particles of filler are not thoroughly covered and protected by a film of resin. For the same reason the appearance of the piece is poor because the filler particles are exposed.
If the resin content is increased much above 50% when cellulosic filler is used the molding tends to become more difficult because of excessive flow of the combination. There is not much change in tensile or flexural strength but the impact strength decreases because the molded piece tends to become more homogeneous in composition and consequently has less damping capacity.
When fillers are used which contain much natural resin such as lignin extended wood flour or redwood flour the ratio of phenoplast resin to filler may be decreased. The natural resin present compensates for the lower quantity of phenoplast and the total quantity of resin present in the combination is still approximately 50%. When mineral fillers such as mica or asbestos are used a higher weight ratio of filler is usually employed about 70% filler to 30% resin. These mineral fillers have a higher specific gravity than the cellulosic fillers and consequently the volume ratio of filler to resin is about the same as in the case of the cellulosic fillers.
STANDARD CLASSIFICATION OF PHENOPLAST MOLDING POWDER ACCORDING TO FILLER
There are several standard classifications of phenoplast molding materials in which the method of classification is based upon the nature of the filler. Where cellulosic fillers are used the impact strength of the molding also varies with the nature of the filler. Such classifications therefore also give an indication of the strength of the composition. It is important to note that the word strength as affected by the nature of the filler applies principally to impact strength and not to other strength properties such as tensile or flexural strength. As a matter of fact in so far as phenolic filled molding plastics are concerned the tensile strength is scarcely altered in changing from wood flour (the weakest of the cellulosic fillers in impact resistance) to tire cord (the strongest in impact resistance) while the flexural strength is increased only slightly.
fillers and resins for laminates
The fillers for laminated phenoplasts differ from those used in molding powders principally in that the fillers for laminates are continuous webs rather than discrete particles. The chemical nature of fillers is quite similar they may be paper cotton or linen fabric sisal mat or woven asbestos. More recently woven glass fiber has become available for applications in which exceptionally high strength is required. The filler greatly increases the strength properties of the laminate over those of the pure resin the increase is greater than is obtained with molding powders because of the continuous web which is present. The filler has the same type of damping effect upon impact waves as has been described in the case of molding powders.
The resin usually in the form of an alcoholic impregnating solution is applied to the filler. Aqueous solutions however are frequently used particularly in the case of paper base laminates.. The impregnated sheet which may contain from 25 to 65% resin (usually about 40%) is then dried and pressed between metal plates at a high temperature. In cases in which a highly polished finish is required on the surface of the laminate the surface sheets may contain a higher resin content approximately 50%. For standard flat laminates the usual pressing temperature ranges from 140° to 180°C. and the molding pressure is from 1000 to 2500 p.s.i. Laminated phenoplast tubing may be formed by rolling the impregnated sheet material upon mandrels between heated pressure rolls and then either oven baking or pressing in a heated mold to complete the curing of the resin.
In recent years there have been important developments in the molding of laminates into more intricate forms such as seats for airplane pilots air ducts ammunition boxes and similar articles. There are two general methods by which such contoured laminates may be formed (1) Layers of laminate cut to the proper size are formed in a mold under heat and pressure. The amount of draw or the degree of contour is usually limited since the base of the laminate will tear if subjected to too much strain in molding. The molding pressures may vary from a high range (1200 to 2000 p.s.i.) down to a low range (100 to 250 p.s.i.). Somewhat better strength properties are obtained at the higher pressures but the lower pressures permit the use of cheaper dies and less expensive presses. (2) By selection of the proper resin and careful control of the curing cycle during lamination it is possible to produce a flat laminate which can be heated and reshaped under relatively low pressures. Such a process is known as post forming. It was originally thought that the resin used in bonding the laminate should be under cured later work has indicated that a fully cured resin can be formed just as well and the use of a well cured resin is in fact desirable. The filler is a cotton fabric with a weave chosen especially so that some stretch is possible in two directions. As in the case of low pressure laminating post forming permits the production of relatively large articles with cheap dies and light presses. A certain amount of draw is possible when molding by this process. The maximum draw may be determined from an index (r/R) which is calculated by dividing the cup radius (r) by the blank radius (R) of the piece to be drawn. This index should lie between 0.67 and 0.77 for satisfactory hot drawing of laminated phenoplast sheets in thicknesses of from 1/32 to 1/16 in.
CLASSIFICATION OF LAMINATES
Standard laminated sheets and tubes are classified according to properties and functional use which in turn are dependent upon the type of filler. Paper has been the most important single filler used according to Hanson the laminating industry produces about 70 000 000 Ib. of finished stock per year. Prior to the war 80% of this material had paper used as a base this represented an annual consumption of 28 000 000 Ib. of paper.
In the United States the National Electrical Manufacturers Association (NEMA) has set up standard classifications for laminates made under high pressure (i.e. 1000 to 2500 p.s.i.). The standard NEMA grades are listed below. These grades do not include recent developments in high strength paper glass fabric and low pressure laminates. These new materials will be described in separate paragraphs.
Laminated Phenolic Sheets
Grade X A strong paper base laminated material primarily intended for mechanical applications where electrical requirements are of secondary importance. Should be used with discretion when high humidity conditions are encountered. Not equal to fabric base grades in impact strength.
Grade P A paper base laminated material primarily intended for punching. More flexible and not quite as strong as Grade X. Moisture resistance and electrical properties intermediate between Grades X and XX
Grade XX A paper base laminated material suitable for usual electrical applications.
Grade XXP A paper base laminated material similar to Grade XX in electrical and moisture resisting properties but more suitable for hot punching. Intermediate between Grades P and XX in punching and cold flow characteristics.
Grade XXX A paper base laminated material suitable for radio frequency work for high humidity applications and with minimum cold flow characteristics.
Grade XXXP A paper base laminated material similar to Grade XXX but having lower dielectric losses and being more suitable for hot punching. This grade has greater cold flow than Grade XXX and is intermediate between Grades XXP and XXX in punching characteristics.
Grade C A fabric base laminated material made throughout from cotton fabric weighing over 4 oz. per sq. yd. and having a count as determined from inspection of the laminated plate of not more than 72 threads per in. in the filler direction nor more than 140 threads per in. total in both warp and filler directions. A strong tough material suitable for gears and other structural forms exposed to high impact. The heavier the fabric base used the higher will be the impact strength but the rougher the machined edge consequently there may be several subgrades in this class adapted for various sizes of gears and types of mechanical service. Should not be used for electrical applications except for low voltages.
Grade CE A fabric base laminated material of the same fabric weight and thread count as Grade C. For electrical applications requiring greater toughness than Grade XX or mechanical applications requiring greater resistance to moisture than Grade C. Exceptionally good in moisture resistance.
Grade L A fine weave fabric base laminated material made throughout from cotton fabric weighing 4 oz. or less per sq. yd. As determined by inspection of the laminated plate the minimum thread count per inch in any ply shall be 72 in the filler direction and 140 total in both warp and filler directions. For purpose of identification the surface sheets shall have a minimum thread count of 80 threads per in. in each of the warp and filler directions. Not quite as tough as Grade C. Should not be used for electrical application except for low voltage.
Grade LE A fine weave fabric base laminated material of the same fabric weight and thread count as Grade L. For electrical applications requiring greater toughness than Grade XX. Better machining properties and finer appearance than Grade CE also available in thinner sizes. Exceptionally good in moisture resistance.
Grade A An asbestos paper base laminated material. More resistant to flame and slightly more resistant to heat than other laminated grades because of high inorganic content. Suitable for only low voltage applications. Minimum dimensional changes when exposed to moisture.
Grade AA An asbestos fabric base laminated material. Similar to Grade A but stronger and tougher. Minimum dimensional changes when exposed to moisture.
Laminated Phenolic Tubes (NEMA Classification)
There are two types of tubes rolled and molded. The rolled are oven baked after rolling on mandrels while the molded are cured in molds under pressure. The rolled tubes are less dense and generally less resistant to moisture than molded tubes but are of uniform strength around the circumference whereas molded tubes have mold seams which are sources of weakness both mechanically and electrically particularly in thin walled tubes. Each type has its own particular applications and characteristics.
X Rolled A high mechanical strength paper base tubing with good punching and fair machining qualities. Low power factor and high dielectric strength under dry conditions.
XX Rolled A paper base tubing with good machining punching and threading qualities. Not as strong mechanically as X Rotted but better moisture resistance. Best grade for low dielectric losses particularly on exposure to high humidity.
X Molded Better in moisture resistance and machining qualities than X Rolled. Strongest paper base except in thin walls. Dielectric strength may be low at molded seams.
XX Molded Best paper base grade from moisture resisting standpoint. Good machining and good electrical properties except in very thin wall forms.
C Rolled A fabric base tubing made from a cotton fabric weighing more than 4 oz. per sq. yd. As determined by inspection of the laminated tube the thread count shall not be more than 72 threads per in. in the filler direction. The total thread count per inch in both warp and filler direction shall not exceed 140. This tubing is intended primarily for mechanical purposes. Dielectric strength is relatively low and moisture absorption greater than for other fabric base grades.
CE Molded A fabric base tubing made of same fabric weight and thread count as Grade C Rolled. For use when a tough dense fabric base material is required having fair electrical properties along with excellent mechanical properties and good resistance to moisture. Dielectric strength may be low at molded seams especially in thin walls.
LE Rolled Made from a fine weave cotton fabric weighing 4 oz. or less per sq. yd. As determined by inspection of the laminated tube the minimum thread count per inch shall be 72 in the filler direction and 140 total in both warp and filler directions. Best concentricity and dielectric strength of any fabric base grade. For use when the seams from a molded tube may be objectionable and when the application requires good machining qualities together with good electrical and mechanical properties.
L Molded Made from a fine weave cotton fabric of the same weight and thread count as Grade LE Rolled. Has high density and good moisture resistance. For mechanical applications primarily when finer machined appearance than with CE Molded is desired or when tougher material than LE Molded is required. Should not be used for electrical applications except for low voltage.
LE Molded Made from a fine weave cotton fabric of the same weight and thread count as Grade L Molded. Has excellent machining and moisture resisting characteristics. For use in electrical applications even under humid conditions when a tougher material than Grade XX tubing is required at some sacrifice of electrical properties. Dielectric strength may be low at molded seams especially in thin wall forms. Grade LE Molded is better electrically than Grade CE Molded but not quite as tough.
High Strength Paper Laminates
In recent years a new type of paper base for phenoplast laminates has been developed which gives higher tensile strength properties than the usual paper bases. This type of filler is not included in the NEMA specifications. This new base was principally developed because of the need for paper laminates of improved strength characteristics in the construction of component parts for aircraft particularly when molded at low pressure ranges (i.e. 50 to 200 p.s.i.). The paper is made from Mitscherlich spruce sulfite pulp because with a minimum of mechanical treatment it gives a sheet of paper with exceptionally high tensile strength. The fibers are laid on the paper machine so that they are largely oriented in the direction of the machine. A thin sheet gives the best results and the weight of the paper is generally held to 35 lb. per ream. For best results the gauge of the paper must be uniform in all directions. Although spruce is ideal for the purpose a number of other softwoods yield pulps of about the same characteristics. In place of Mitscherlich pulp that from the Kraft process can be used satisfactorily provided the papermaking conditions are altered to suit the pulp.
When such a paper is used as a laminate and the plies are all oriented in the same direction an ultimate tensile strength as high as 35 000 p.s.i. in the direction of the fibers may be obtained. The strength in the opposite direction however is rather low. The paper is usually used by laying the plies in alternate directions so that the strength properties are substantially the same in any direction.
Plastic Bonded Cotton Fiber
It is possible to prepare a laminate by using as filler cotton fibers which have all been laid in a parallel direction. The laminate then develops very high tensile strength (up to 35 000 to 40 000 p.s.i.) in the direction of the fiber although the strength properties across the fiber are low. Such laminates have been described by Goldman.
Glass Fabric Filler
Woven glass fabric has recently become available as filler for laminates and this type of filler is also not included in the NEMA specifications. The grade most commonly used has the glass fibers running in one direction and is woven together by a fine cotton fiber. To obtain equal strength properties in both directions the laminates are cross banded that is the alternate layers of fabric are oriented at an angle of 90° to each other. With this filler it is possible to obtain an ultimate tensile strength in the neighborhood of 30 000 p.s.i. at room temperature. The high strength is to some extent offset by the relatively high specific gravity of the laminate 1.67 as compared with 1.34 for a paper laminate with comparable resin content. The impact strength of the laminated glass fabric is particularly high the notched Izod value at room temperature is 22 ft. lb. per inch of notch as compared to 1 ft. lb. per inch of notch for the usual paper laminate.
RESINS USED FOR LAMINATES
In general the resins used in laminates are very similar to those used for molding powders. Where exceptionally good electrical properties are required cresylic acid is frequently used as the base instead of phenol or some special type of extender is added to the phenolic resin. The proportion of resin to filler is usually lower than in the case of molding powders it may be as low as 25% based on the weight of the finished laminate and usually averages about 40%. Less resin can be used because less flow is required in the molding operation there are no sharp corners in the mold to be filled out by flow of the resin and filler. The effect of the quantity of resin on physical properties is further discussed in the chapter on the physical properties of laminated phenoplasts.
The degree of impregnation obtained in paper filler varies markedly with the molecular complexity of the resin as the molecualr size is reduced an improvement in the impregnation is observed. There is considerable difference between the inpregnation achieved by a low molecular weight resin dissolved in water and a more highly condensed resin which must be dissolved in a solvent such as alcohol. The more highly condensed resin does not penetrate the fibers to the same extent as the water soluble resin.
physiology and environmental protection
In order to judge the risks connected with the handling of phenolic resins a clear distinction must be made between phenols phenolic resin prepolymers and cured phenolic resins. Apart from constitutional characteristics the MW is of great importance regarding the physiological effects. The physiological activity of phenolic
prepolymers depends upon the content of free phenol and formaldehyde. Cured phenolic resins are entirely harmless. The FDA permits articles molded from phenolic resins to come into contact with food.
Toxicology of Phenols
Mononuclear phenols are protein degenerating and highly toxic. The oral LD50 value (rats) is 530 mg/kg. Human skin which has come in contact with phenol first becomes white subsequently red and wrinkled a strong burning sensation is quickly perceived. Longer contact destroys the skin tissue. Solid and liquid phenols are absorbed by the skin very quickly and cause very severe damage. Contact with large amounts leads to death through paralysis of the central nervous system. Minor intoxications lead mostly to damage of the kidneys liver and pancreas. If phenol is inhaled or swallowed local cauterizing occurs and headaches dizziness vomiting irregular breathing respiratory arrest and heart failure are the results.
The destructive effect of phenol on the human skin is reduced by introduction of lypophilic groups (methyl higher alkyl or chloro groups). The neutral molecules are far more active than the corresponding ions. The biological activity of phenols is the result of their ability to alter biological structures i.e. cracking of bacterial cell walls. The disruptive effect on cytoplasmic membranes and cell walls develops it is believed by the creation of pores large enough to permit cytochromes to diffuse out. Cresols are similar to phenol in their action but less severe in their effects. Chlorophenols are not used for resin production.
The surface activity of alkyl phenols leads to their concentration on the cell surface but does not explain the destructive effect to the cells. The bacterial and anthelmintic action of phenols is also influenced by soaps which are usually used to solubilize phenols in water for use as disinfectants.
Toxicology of Formaldehyde
Formaldehyde in an aqueous solution is a protoplasm poison with a cauterizing and protein degenerating effect. The use of aqueous formaldehyde to preserve medical or biological preparations is well known. Formaldehyde is believed to injure bacteria by reacting with the amino groups of proteins which are thereby changed in nature and action. Formaldehyde in the organism is quickly oxidized to formic acid which is partly separated by the urine.
Formaldehyde in the form of gas or aerosol the effect of both is comparable is very irritating to the mucous membranes. The pungent smell is noticeable even at concentrations below 1 ppm. The MAK value is 1 ppm. Formaldehyde is a dangerous material to work with and has received the same rating as phenol.
Concentrations of formaldehyde up to 10 ppm cause conjunctivitis within a few minutes as well as rhenitis with anosmia and pharyngitis. It can be observed that one can get used to formaldehyde to a certain extent. At 10 15 ppm dispnoe cough and pneumonia develop.
The environmental policies of progressive industrial nations require not only the return of used media to the environment in a treated condition but aim to work with these media without causing damages or injuries i.e. new production processes must be developed which prevent contamination of the environment in the first place. An example is the legislation against environmental contamination in West Germany. Aims and instruments of environmental policies are set forth in the Bundesimmission sschutzgesetz . The Technische Anleitung zur Reinerhaltung der Luft (Technical Instructions on Clean Air Maintenance) 1974 describes the minimum requirements for plants and their operation and a series of limiting values for emissions and immissions. According to these requirements organic compounds of Class I phenol and formaldehyde amongst others belong to this class must not exceed a mass concentration of 20 mg/m3 at a mass flow of 0.1 kg/h and more. In order to protect the waterways from contamination the Wasserhaushaltungsgesetz 1976 was passed. The law called Wasserabgabengesetz 1976 includes a scale of fees determined by the quantity of waste water emitted and the amount of injurious substances (according to the COD and BOD) in the water and deposits as well.
Permissible levels of phenols in waste water have been established in USA by the Environmental Protection Agency (EPA) in the Federal Register. These guidelines generally establish phenol levels of 0.1 mg/1 for the Best Practical Control Technology Currently Available (BPCTCA) for 1977 and 0.02 mg/1 for the Best Available Control Technology Economically Achievable (BACTEA) for 1983.
Even relatively low concentrations of phenols below 10.000 mg/1 water are fatal for several kinds of fish after 1 3 days. Lower concentrations are at least injurious and deteriorate the taste of the fish flesh considerably so that it is unfit for human consumption. Phenols in chlorinated water lead to the formation of chlorophenols which will impart objectionable taste and odor to water even in quantities below 0.01 mg/1.
For example in West Germany the following requirements must be met when waste water is allowed to flow into the local waters content of free phenols maximum 0.5 mg/1 temperature maximum 30 °C pH 6.5 8.0. Furthermore the total quantity of waste water allowed to flow in within 24 hours is limited to 75 m.
Table 1 Phenols in Water Threshold Odour and Taste Concentration and Acute Fish Toxicity
Under certain circumstances waste water may be allowed to flow into the municipal biological water treatment plants together with the household sewage. In any case the municipal requirements have to be adhered to. In general the waste water must be as follows temperature maximum 30 35 °C pH 6.0 9.0 content of phenols or phenol prepolymers maximum 100 mg/1. In addition the composition of the waste water must be such that neither the biological processes nor the plant operation are affected.
The disposal of solid phenolic waste is also regulated by the federal legislation. Waste materials containing injurious substances are to be disposed off only at official disposal places. Flammable waste materials are preferably destroyed within the plant in appropriate incinerators. Used phenolic resin coated sand can be disposed off at official disposal places without problems. Published results of the behaviour of foundry waste sand at disposal places show that the phenol quantities which may be eluted of cured resin bonded sands are even lower than amounts found for household waste under similar circumstances.
The regulation Verordnung uber gefahrliche Arbeitsstoffe covering the handling of injurious working materials 1975 contains requirements for the rating packing marking and preparation of dangerous working materials. Dangerous or injurious materials in the sense of this regulation are basic and auxiliary materials and their formulations (blends mixtures and solutions) if they are explosive flammable toxic detrimental to the health caustic or irritating. To indicate these properties official warning symbols are to be put on packaging and containers. The scope of this list is in accordance with the requirements of the European Community of 1967 including their alterations and supplements of May 21 1973 as well as the EEC rules and regulations for solvents. Phenols and phenolic resins formaldehyde the solvents methanol propanol toluene and some others which are used to produce resinous solutions are also governed by these regulations.
All phenolic resins containing more than 5% free phenol must be designated poisonous by a skull. Phenolic resins containing 1 5% free phenol are considered detrimental to the health and are to be marked with a St. Andrew s cross (Andreaskreuz). Formulations are not considered toxic if the amount of free phenol is below 0.2%. The label on the packaging or containers must also show the name of the producer and the kind of toxic components and must include warnings of the special dangers involved and safety measurements to be taken.
During the handling of phenolic resins sensitive persons may succumb to dermatological diseases. To prevent such reaction it is advisable to treat the hands arms and other parts of the body which might be exposed to phenolic resins with an appropriate protective cream and to wear rubber or plastic gloves during work. After work hands and arms are to be washed with a special soap and again treated with protective cream.
Furthermore special attention should be given to clean working conditions and effective ventilation in the working rooms. The MAK value (maximum concentration at place of work) is 5 ppm for phenol and 1 ppm for formaldehyde.
Waste Water and Exhaust Air Treatment Processes
There are no universal solutions for waste problems for plants working with phenol and phenolic resins. The choice of the optimum process requires an individual analysis of the kind and amount of injurious substances as well as the structure of the plant and laboratory performance tests. Occasionally a combination of different processes may be feasible. Such processes are microbial degradation thermal combustion physical and physico chemical scrubbing chemical oxidation or resinification reactions and adsorption methods.
Microbial Transformation and Degradation
The breakdown of aromatic compounds is an important step in the natural carbon cycle and many microorganisms eubacteriales pseudomonas actinomyceatas endomicetas higher funghi are capable of breaking down aromatic substrates. The essential step required for biological degradation is the conversion of the aromatic compound to an ortho or para dihydroxybenzene structure. The enzymes responsible for this hydroxylation have the character of mixed function oxidases or dioxygenases. The first steps of the three possible oxidative cleavage reactions of o and p dihydroxy compounds are shown in the formulae (1/5). In the case of 1 2 dihydroxybenzene ortho or meta cleavage (1 2) may occur. Ortho and para hydroxybenzoic acids (3 5) may be formed as intermediates during the degradation of phenolic resin prepolymers.
The aliphatic mono and dicarboxylic acids formed are further converted to 3 oxohexanedioic acid which is taken up in the Krebs cycle or to fumarate pyruvate acetaldehyde and acetoacetate. After this the degradation to CO2 and H2O follows.
Certain kinds of microorganisms are able to live and cause degradation in water containing up to 1 000 mg/1 of phenol. They are most active at temperatures between 25 35 °C. Further essential prerequisites are a sufficient content of nutritive substances (N P) and oxygen pH between 7.5 8.5 and the absence of heavy metal ions (5 mg/1). In order to provide the nutritive substances it is advantageous to treat the waste water together with household sewage. Ammonium phosphate is most frequently used as a nutritive compound. The effectiveness of the biomass increases with time up to a limiting value because biological selection processes take place and the resistance and degradation ability of some kinds of micro organisms increase. The basin must have an effective aerating and circulating system so that dissolved oxygen is always available in excess. The Unox process uses oxygen instead of air in order to reach a higher oxygen level.
Biochemical degradation is the most used and most effective process for treating waste waters containing phenol. Final effluents in the range of 0.1 mg/1 are reported. In order to ensure that feed and environmental conditions for the biomass are constant properly designed equalization systems are required for optimum efficiency. Particular problems arise if plants are operated discontinuously or 5 days a week.
Chemical Oxidation and Resinification Reactions
In chemical oxidation processes phenols are normally destroyed to form intermediate non toxic compounds (not CO2 and H2O) and so only a certain decrease in COD will result. The removal of phenol may reach final levels of less than 1 mg/1 or > 99% according to the ratio of chemicals used.
Hydrogen peroxide in the presence of small amounts of iron manganese chromium and copper salts is an effective oxidizer of phenols (and other organics). The temperature has little effect on reaction rate and conversion a pH in the range of 3 5 is most effective. Hydrogen peroxide may be used to treat concentrated wastes high in phenols or for pretreating of high phenol waste before biological treatment to obtain uniform phenol levels.
Ozone is a more effective oxidant than hydrogen peroxide. Lower amounts are normally applied as necessary for complete destruction to carbon dioxide and water. The selectivity of ozone is low operating at pH values of 11.5 11.8 appears to result in preferential oxidation of phenol. Ozone is often used in the final treatment step leading to very low phenol levels (lower than 0.1 ppm).
Sodium hypochlorite or chlorine dioxide which is the oxidizing agent will oxidize phenols to benzoquinones (pH 7 8). At pH above 10 further oxidation to maleic acid and oxalic acid will occur chlorophenols are not formed. Chlorine is not used because of the formation of chlorophenols which are more toxic and have more objectionable taste and odor than the original phenols. Potassium permanganate or potassium dichromates are also effective oxidants however the handling of the precipitated sludge can be a serious problem.
Resinification reactions followed by precipitation of the polymeric material can be used for waste waters which contain phenol phenol prepolymers and formaldehyde by adding sulfuric acid or ammonia and reacting at higher temperature. Ferric chloride or aluminium sulfate are recommended as precipitants. The deposits are burned in most cases. It is customary in the plywood particle board and fiber board industries to acidify the waste waters with aluminium sulfate up to pH 4. By this method the resinous components precipitate almost completely as a deposit which settles well and is filterable especially if the precipitation occurs at elevated temperatures. Afterwards the water must be neutralized with caustic lime (pH 6.5 8.0) and the calcium sulfate which is formed filtered.
Thermal and Catalytic Incineration
The treatment of exhaust air by oxidation thermal or catalytic incineration is taken into consideration if the recovery of the solvents is not feasible or uneconomical. The organic components are oxidized to CO2 CO and water. The catalytic incineration occurs at temperatures between 350 400 °C. Metal oxides preferably however elements of the platinum group on different supports are used as catalysts. Catalysts are very sensitive. Sulfur phosphorus halogen silicon arsenic compounds and many others lead to catalyst poisoning.
In principle then catalytic incineration is only preferred if the exhaust air contains only minor concentrations of organic substances (