OIL-SOLUBLE PHENOLIC RESINS K. NORRIS SHREVE
AND BRAGE GOLDING
Purdue University, Lafayette, I n d .
E
V E X though
o f the many known oil-soluble condensation products of coniture, between 0.25 and I mole pletely oil-soluble phenols and formaldehyde, the para-substituted phenols, of catalyst per mole of phenol phenolic resins have been as a class, are most suitable for use in oleoresinous varwas desirable. made since 1930 ( 2 7 ) and it nishes. As there was no danger of The preparation of the resins most likely to be used romhas long been known that the too complete a condensation mercially and their influence on the physical and cherriia d d i t i o n of even a small with acid-catalyzed resins, cal properties of varnishes made therefrom were investiquantity of a phenolic resin they were made at reflux gated. temperature, using only a to a natural resin greatly imProperties such as oil solubility, oil reactivity, color stasmall amount of catalyst. proves the physical characbility, drying rate, and resistance to moisture and alkali hIany of the basic-condensed teristics of t h e v a r n i s h e s are listed tabularly for comparison. resins, on the other hand, made from them, very little has been published on tended to react too rapidly and completely a t r c f l u x , methods of manufacture and necessitating careful control of catalyst amount and lengt,h physical properties of the resin? \Thiit has been published is of reaction. If the reaction went too far, the mass became mainly patent literature, and m m v preparative techniques and too viscous, and it was difficult), if not impossible, to neutralize, results claimed are questionable. wash, and dehydrate it properly. If bhe reaction had not In 1941 Turkington and Allen published the first comprehensive proceeded far enough, the possibility of unreacted phenols review (78) of the properties and general preparation of oil-solremaining in the resin mass was great. The most satisfactory uhle phenolic resins, including a rather general evaluation of the procedure for alkaline-catalyzed resins was to allow the reaction varnishes made from these resins. to take place a t temperatures between 25 O and 50' C. for 48 to 72 Many patents ( 1 , 4 4 , 8 , 9, 12-16, 19-21, 2%%26,28, S I , 53, 36, hours. At t'hese low temperatures more careful control was 38, 39, 42, 43, 46, 51, 65, 57, 68,63, 67-71, 13, 14,16, 81, 83) have possible, as the rmction rate, though fairly rapid a t first, tapered claimed advantages for an alkylated or arylated phenol or its off after the first hour or two. For most resins, the maintenance inethod of manufacture, but little has been published concerning of the reaction mass a t 50" C. in a thermostatically cont,rolled a methodical evaluation of these phenols and the resins made from oven for 48 hours gave a resinous mass, upon neutralization, of them. [At least two excellent patents (12, 44) are available, satisfactory consistency. however, on general theory and preparation.] The present This low temperature necessitated the use of larger amounts authors have attempted to evaluate what they believe to be of catalyst. An amount slightly in excess of that sufficient to among the best of the phenolics in order to ascertain differences, dissolve the phenol was considered best. This produced a oneif any, among them. Only para-substituted phenols were used, phase system, eliminating slowness of reaction due to slow diffubecause they were, in general, superior t o others in color, color sion rates, yet kept the catalyst concentration low enough to rtahility, oil eolubility, and alkali rePistance (18). permit,fairly easy washing out of the salt formed upon neutralizaPREPARATION O F THE RESINS tion. The amount of alkaline catalyst necessary to accomplish solution of the phenol varied with the particular phenol, but The most satisfactory apparatus for preparing the resins were usually was in the neighborhood of 0.25 to 1 mole of catalyst to I special "resin flasks" made of glass and heated with made-to-order glass-fiber electric mantles (Glas-Col) (Figure 1). These flasks mole of phenol. (of 2000-ml. nominal capacity) handled from 2 to 5 moles of phenol The p H a t which final condemation and polymerization took plus other chemicals needed. The flasks were strong, came apart place TTas found to exert a profound influence on the rapidity of easily a t the horizontal flange for easy cleaning, permitted obresinification and the final color of the resin. If too much acid servation of the reacting materials, and were equipped with an adjustable packing gland seal for the glass, multiple-bladed stirrer, was present during final dehydration and condensation, the reacso that a vacuum could be applied if necessary. tion proceeded very quickly in most cases, causing the resinous All equipment in contact with the reactants was made of glass mass to gain rapidly in viscosity, which interfered with complete to prevent coloring of the resins by contamination, particularly dehydration. The use of a vacuum helped somewhat; neverthedue to iron, as ferric ion in the presence of phenols forms deeply colored complexes. less, the high viscosity caused the mass to foam and prevented the use of sufficient vacuum t'o dehydrate the mixture properly. 4fter considerable experimentation with para-substituted On the other hand, if the pH was kept' too high, the rate of phenols, the following general conclusions about resin preparation condensation was too slow. This necessit'ated a long period of were drawn. dehydration and condensation which was uneconomical of time, In either acid- or base-catalyzed reactions, the quantity of and, in general, caused darkening of the resin. catalyst-i.e., the pH-had no appreciable effect on the type of The final p H also directly influenced the color of the resin. I t product formed. The only difference observed was in the rate was found that a p H too high invariably produced yellow resins, and extent of reaction (in a given period of time), especially in the some light, some dark. Too low a pH also gave off-colored rescase of base-catalyzed resins. ins, most of them darker in color than would otherwise be obThe smaller the amount of catalj st used, the more easily it was tained. removed from the reaction mass after initial condensation; hence, Using aliquot portions of the same initial condensation product as small a quantity as waR consistent with time economically and varying only the final pH, the following was found: available was desirable. Higher trinperature of reaction gave a pH > 7 Yellow product more rapid reaction rate. Thus, only a small amount of catalyst p H 6 t o 7 Pale product, b u t very long time of condensation was needed if the reaction was carried on a t reflux temperature. pH 5 to 6 Pale product, optimum time of condensation pH < ?I T o o rapid a condensation, often a dark product the best quantity was on the order of 1%,based on the phenol. If a nonvolatile acid or alkali was used, a further disadvantage If the reaction was carried out approximately a t room tempera-
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January 1951
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I N D U S T R I A L A N D E N G l: N E E R I N G C H E M I S T R Y
in using either t o excess was the danger of producing a resin in which considerable quantities of free acid or alkali were retained. Although phenols and formaldehyde will react without the presence of any catalyst, the reaction rate can be greatly increased by its use, without affecting the equilibrium. A large number of compounds have been tried as catalysts (3, 7 , 10, 1 1 , 17, 82, 29, 34, 55, S7, 40, 49, 56-54, 56, 59-62, 66, 76, 75). Although many claims have been made for particular catalysts, relatively few fulfill all or most of the conditions deemed desirable. Several catalysts were tried, in an effort t o determine any significant differences among them. Among the basic catalysts tried were sodium hydroxide, sodium carbonate, aqueous ammonia, and calcium oxide. Many organic catalysts have been recommended, such as amines and quaternary ammonium compounds. However, preliminary work with other catalysts indicated that the added expense would outweigh any claimed advantages. Sodium hydroxide appeared to be preferable to the others, and hence was used for all base-catalyzed condensations. Several acidic substances were tried, among them sulfuric acid, hydrochloric acid, phosphoric acid, acetic acid, and oxalic acid. All gave somewhat satisfactory results, but some had certain disadvantages that caused them t o be discarded. Oxalic acid was, perhaps, one of the best catalysts. It produced very pale resins and was volatile enough (owing principally to sublimation) to permit removal of excess acid: It was, however, too weak a n acid to accelerate the reaction rate appreciably in some cases. As a compromise, hydrochloric acid was usually used to effect the initial aqueous condensation. It was then washed out of the reaction mass, and the desired quantity of oxalic acid was added for final condensation. Because all the resins made were finally condensed and dehydrated a t p H 5 to 6, the alkaline-condensed resins were always neutralized. For the alkaline-condensed resins, acetic acid was the most Patisfactory. It was relatively weak, so that a small excess was not harmful; i t was volatile, so that even this excess could be removed; the sodium acetate formed is one of the most watersoluble sodium salts; and the sodium acetate and residual acid formed a buffer solution having the p H desired for final condensation. The temperature a t which the resin is poured determines, to a great extent, the melting point of the resin. This is of importance, as the melting point has a marked effect on the physical properties of the varnishes made from the resin. I n general, the higher the melting point, the harder and more durable is the varnish film (15, 7 9 ) ; hence it is desirable to heat the reRin t o aR high a temperature as possible without causing discoloration, infusibility, or insolubility. The acid-condensed resins, being thermoplastic, could be heated t o a considerably higher temperature than the others. Complete dehydration, even with vacuum, is not accomplished much below 120" t o 130" C., because of the viscosity of the melt. It was found t h a t a maximum temperature of 190" t o 210" C. was permissible for the acid-condensed resins, guaranteeing complete dehydration and producing a higher melting resin without appreciable darkening. Because alkali-condensed resins generally discolored and/or condensed too completely at such high temperatures, a maximum of 110' t o 150' C. was allowable. This rather large temperature range is ascribed to the different characteristics of the particular phenol, cresol, for example, requiring a lower temperature than a butylphenol resin. The literature revealed a wide variation of phenol-formaldehyde ratios ( 1 , 4-6, 8, 9, 18-16, 19-61, 83-26, 68, 31, 33, 36, 38, 39, 41-46, 48, 51, 65, 57, 68, 63, 67-71, 73, 74, 76, 81, 83) which affected the properties of the resin and subsequent varnish. The
Figure 1.
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Resin Flasks and Auxiliary Equipment
effect of variations in this ratio is much more pronounced with the basic-condensed resins. In an attempt t o cover adequately the ranges involved, the following ratios were selerted as representative: Acid-Condensed HCH0:phenol = 0.8:l.O
Basic-Condensed HCH0:phenol = 1 . O : l . O and 2.0:l.O
The reason for the low ratio for acid-condensed resins is that certain phenols tend to form oil-insoluble resins if the ratio is 1.0 to 1.0 or greater (SO, 50, 76). By keeping slightly below this ratio, oil solubility for all resins was assured, yet all resins would have the same phenol-formaldehyde ratio. The small excess of phenol causes no great harm, inasmuch as most of the excess is sublimed from the melt a t the final high temperature and vacuum. Satisfactory alkaline-catalyzed resins are claimed from formaldehyde-phenol ratios from 0.5:l.O t o 2.5:l.O and even higher. The two ratios chosen were believed t o cover the range satisfactorily, and were far enough apart t o show any differences. The only conclusion obtained with the basic-catalyzed resins made with the two different ratios was that, generally, the higher the formaldehyde content, the less viscous the initial condensation product and the paler the resin. Differences in the varnishes were more marked. The rate of reaction of formaldehyde with the various phenols varied widely. Although no quantitative experiments were carried out, it was observed during the course of experimentation that the phenols fell roughly into the following classes: Rapid Reaction Rate Medium Reaction Rate Slow Reaotion Rate Phenol p-tert-Butylphenol p-Phenylphenol Cresol p-tert-Amylphenol p-Cyclohexylphenol Bis-phenol-Aa a Trade name for 2,2-bis(p-hydroxyphenyl)propane.
Thus it appeared that the rate of reactivity of various phenols might be predicted approximately by the length and type of substituent group attached-i.e., a short, aliphatic chain or none in-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE I. PHYSICAL PROPERTIES OF R E S I K S ~ Resin S o . 103 108 93 96 68
Type of Phenol
Melting Range, C.
Color
SoIubility
p-Cresol 1: 1 Crystalline 9 p-Cresol 2: 1 Crystalline 6 p-tert-Amyl 2:l 45-50 p-tert-Amyl 1: 1 45-52 Bis-Phenol-Ab €I 47-56 LO2 Bis-Phenol-A 1:1 52-57 11 97 p-Cyclohexyl 2 : l 53-58 6 94 p-tert-Butyl 2:l 55-61
