Chemistry and Technology of Phenolic Resins and Coatings

47 Chemistry and Technology of Phenolic Resins and Coatings J. S. FRY, C. N. MERRIAM, and W. H. BOYD

Downloaded by UNIV OF LEEDS on June 19, 2016 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch047

Union Carbide Corporation, Bound Brook, ΝJ 08805

Early History Chemistry Raw Materials Base-Catalyzed Reactions Acid-Catalyzed Reactions Classification of Phenolic Resins Unsubstituted and Heat Reactive Unsubstituted and Nonheat Reactive Substituted and Heat Reactive Substituted and Nonheat Reactive Applications Waterborne Phenolic Resins Conclusions

Phenolic resins, being the f i r s t truly synthetic resin, already had a highly developed technology by 1924. The phenolic industry had grown to be a large and profitable plastic business closely a l l i e d in supplying materials to the other technological industries of that day. Major uses included automotive applications, the telephone, and the new mass communications medium, the r a d i o . Leo H. Baekeland, founder of the phenolic industry, was recognized as a very capable chemist as well as a businessman. While always interested in the basic science of phenolic resins, the business demands kept intruding on his efforts to elucidate the complex nature of phenolic resin chemistry. Baekeland's attitude toward the commercial world i s i l l u s t r a t e d by the following quote from his presidential address to the September National American Chemical Society (Ithaca) Meeting 51 years ago (1), 1924: "Whether we l i k e to admit it or not, much of the history of science has been shaped by the needs and the outside influences of commerce or industry, brought about by the early manufacture of synthetic dyes.... The very nature of that new industry puts an unprecedented premium on research in organic chemistry. Fame, honors, and monetary rewards were the alluring inducements offered to the best available. I am 0097-6156/85/0285-1141S06.00/0 © 1985 American Chemical Society

Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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APPLIED POLYMER SCIENCE

not one of those the c r e a t i o n of enormous fund of which has helped goes on to say, means of getting

who tend to exaggerate the benefits of chemistry i n thousands of new s y n t h e t i c dyes except f o r the new chemical knowledge we have gathered thereby and immensely i n other more v a l u a b l e directions." He "Anyone who l o o k s f o r n o t h i n g i n c h e m i s t r y but a r i c h has chosen the wrong career."


Early History In o r d e r to b e t t e r understand the r a p i d t e c h n i c a l development of p h e n o l i c r e s i n s , one s h o u l d go back not 51 years but 76 years to 1909, the year of phenolic r e s i n commercialization, and look at the state of s e v e r a l important industries. The e l e c t r i c a l industry was r a p i d l y e x p a n d i n g , though e l e c t r i c a l l i g h t i n g was not yet commonplace. The automotive i n d u s t r y was i n i t s i n f a n c y but f a s t growing. In the communications i n d u s t r y , w i r e l e s s t e l e g r a p h was j u s t s t a r t i n g , t e l e p h o n e s e x i s t e d but were not widespread, and radios were unknown. Phonographs were big business. A new material was needed by these i n d u s t r i e s t h a t was more amenable to mass production methods, had better e l e c t r i c a l i n s u l a t i n g properties, had more heat s t a b i l i t y , and had greater strength. The materials being used and with which phenolics would compete were C e l l u l o i d , hard and s o f t rubber, bituminous binders, and s h e l l a c . The goal of i n i t i a l phenolic r e s i n e f f o r t s was the development of a cheap thermoplastic r e s i n replacement f o r s h e l l a c . The r e s u l t was the phenol/formaldehyde, nonheat-reactive resins that Baekeland referred to as novolaks. These products did not have the toughness of s h e l l a c and, therefore, never became successful substitutes. During these s t u d i e s , Baekeland became more i n t e r e s t e d i n the i n s o l u b l e , i n f u s i b l e masses sometimes obtained. He reasoned that i f he could c o n t r o l t h i s reaction, making i t occur when, where, and i n the form he wanted, he would have a product of commercial v a l u e . This e f f o r t r e s u l t e d i n the development of the heat-reactive resins and the heat and pressure technique of molding them. The r e s u l t i n g cured p h e n o l i c produced hard, s t r o n g , i n f u s i b l e , and i n s o l u b l e o b j e c t s w i t h good e l e c t r i c a l i n s u l a t i n g p r o p e r t i e s . These properties p l u s the r e l a t i v e l y short molding c y c l e s made phenolics j u s t what the aforementioned i n d u s t r i e s needed. W i t h i n 1 year of the announcement of t h i s technology, Bakélite had a dozen customers r e p r e s e n t i n g many of the a p p l i c a t i o n s s t i l l important today. The f i r s t commercial use of t h i s h e a t - r e a c t i v e r e s i n was found i n m o l d i n g m a t e r i a l as a replacement f o r hard rubber and amber i n a p p l i c a t i o n s where resistance to heat and pressure was needed. The f i r s t customer was the Boonton Rubber Co., who purchased h e a t r e a c t i v e r e s i n , blended i t w i t h a s b e s t o s f i b e r , and produced moldings. One of Boonton Rubber's f i r s t customers was the Weston E l e c t r i c a l Instrument Co., who, under the leadership of Dr. Edward Weston, was looking f o r a molding material that could be molded to c l o s e tolerances and not d i s t o r t under heat and pressure. A l l known molding materials had been found inadequate. Being near the end of h i s r e s o u r c e s , Weston chanced to meet Baekeland, who put him i n c o n t a c t w i t h the Boonton Rubber Co. Through t h i s c o o p e r a t i o n , Weston was a b l e to s o l v e h i s problem, r e s u l t i n g i n the f i r s t commercial use of phenolic p l a s t i c i n the e l e c t r i c a l i n d u s t r y — t h e



47. FRY ET AL.

Chemistry and Technology of Phenolic Resins and Coatings 1143

Weston I n s u l a t i n g C o i l Support. Other e l e c t r i c a l a p p l i c a t i o n s r a p i d l y developed, and by the end of 1911, phenolics were being used i n molded push p l a t e s and commutators. The use of p h e n o l i c s i n the e l e c t r i c a l i n d u s t r y was q u i c k l y followed by i t s introduction i n t o the automotive industry. This was i n i t i a t e d by K e t t e r i n g ' s development of the "Delco" i g n i t i o n and s t a r t i n g system based on a Bakélite molded d i s t r i b u t o r head. K e t t e r i n g found t h a t the Bakélite p h e n o l i c , w i t h i t s d i e l e c t r i c s t r e n g t h and immunity t o a d v e r s e e f f e c t s from temperature, a c i d s , o i l s , and moisture, solved the problems p r e v i o u s l y associated with the use of hard rubber. The use of p h e n o l i c s has grown w i t h t h i s industry, and today finds use throughout the transportation industry i n such diverse a p p l i c a t i o n s as laminated timing gears and bonded brake l i n i n g s . Other e a r l y customers were Westinghouse, producing laminates, and General E l e c t r i c , impregnating c o i l s . Use i n consumer products was a l s o e s t a b l i s h e d e a r l y w i t h the molding of t r a n s p a r e n t p i p e stems and c i g a r e t t e holders. The use of phenolics i n communications developed i n the next few years with such objects as telephone handsets. With the development of radios, phenolics found use i n laminated panels, molded d i a l s , and c o i l forms. Phonograph records became a large market with the development by Aylsworth of the use of hexamethylenetetramine as a curing agent f o r the nonheat-reactive novolak-type phenolics. Although i n 1909 Baekeland obtained patent coverage f o r the use of p h e n o l i c s i n bonding a b r a s i v e s , i t was not u n t i l 1921 t h a t i t became commercially useful. Phenolic bonded grinding wheels were found t o be c a p a b l e of being operated s a f e l y a t h i g h e r speeds and with improved q u a l i t y . More r e c e n t new m a r k e t s i n c l u d e convenience e l e c t r i c a l appliances such as e l e c t r i c f r y i n g pans, toasters, and steam irons t h a t r e q u i r e p h e n o l i c s f o r both t h e i r h a n d l e s and e l e c t r i c a l connections. The tremendous growth of the e l e c t r o n i c s and computer industry has a l s o g i v e n a boost t o p h e n o l i c r e s i n s a l e s . P r i n t e d c i r c u i t s are manufactured by bonding copper f o i l t o a l a m i n a t e d p h e n o l i c base, p r i n t i n g the area representing the c i r c u i t with a c i d - r e s i s t i n g i n k , and then e t c h i n g away the remainder of the f o i l . The dimensional s t a b i l i t y , acid resistance, and e x c e l l e n t e l e c t r i c a l s make phenolics a necessity i n the a p p l i c a t i o n . The f i r s t attempt t o use p h e n o l i c r e s i n s i n c o a t i n g s was the unsuccessful use of nonheat-reactive novolaks d i s s o l v e d i n s o l v e n t as a replacement f o r s h e l l a c . The c o a t i n g s were not good, being very b r i t t l e and darkening i n c o l o r on aging. With the development of t h e h e a t - r e a c t i v e r e s i n s , i t was found t h a t t h e i r a l c o h o l s o l u t i o n s , when applied to objects and baked, gave hard g l a s s l i k e f i l m s w i t h e x c e l l e n t r e s i s t a n c e t o water, s o l v e n t s , and most c h e m i c a l s . These r e s i n s o l u t i o n s were found t o be p a r t i c u l a r l y s u i t a b l e f o r coating brass beds that were so popular at the time. Perhaps the s t o r y of the f i r s t p r o d u c t i o n batch of p h e n o l i c r e s i n at Baekeland's new p l a n t i n 1911 w i l l i l l u s t r a t e the state of the i n d u s t r y some 70 years ago. T h i s s t o r y has some elements i n common with present-day s c a l e ups. While the new Perth Amboy, NJ, plant was being i n s t a l l e d , Dr. Baekeland hired a new chemist, J . J . Frank, trained him at the Yonkers Laboratory, and set him to work i n




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the new p l a n t . Present a t t h i s f i r s t l a r g e - s c a l e batch was Dr. Baekeland. the f a c t o r y manager, the few new f a c t o r y workers, some contractors, and Frank. As the day ran i n t o the evening, l i g h t i n g had to be supplied by kerosene lanterns since the e l e c t r i c l i g h t i n g had not been completed. The batch proceeded w e l l . The r e s i n was r e a c t e d , dehydrated, and f i n i s h e d , and the a l c o h o l s o l v e n t was added. Dr. Baekeland asked whether the r e s i n was a l l d i s s o l v e d or i f t h e r e was a lump of "B" ( g e l l e d ) r e s i n around the a g i t a t o r . Frank q u i c k l y grabbed a l a n t e r n , swung open the manhole door, and h e l d the l a n t e r n t o l o o k i n . There was a m i l d "boom," the hot a l c o h o l vapor i g n i t e d , and f l a m e s f l a s h e d out of the manhole. Everyone q u i c k l y l e f t by the nearest doors and windows, except f o r the factory manager and Frank. The manager calmly slammed the s t i l l manhead, extinguishing the flames. Fortunately, Frank was not hurt b a d l y . He l o s t h i s h a i r , eyebrows, mustache, and h i s job. The batch of varnish was saved. Chemistry Understanding the chemistry of p h e n o l i c r e s i n s i s p a r t i c u l a r l y d i f f i c u l t because of the many complex r e a c t i o n s i n v o l v e d and the i n s o l u b i l i t y of the f i n a l cured products. The i n i t i a l c h e m i c a l e f f o r t s were spent i n i s o l a t i n g and studying pure, c r y s t a l l i z a b l e compounds. I t was not u n t i l 30 years a f t e r Baeyer's f i r s t report i n 1872 of the r e a c t i o n of phenol w i t h aldehydes (2) t h a t i n t e r e s t turned to the n o n c r y s t a l l i n e resinous materials. From 1905 to 1909, Baekeland s t u d i e d phenol/formaldehyde r e a c t i o n s and d e f i n e d the differences between acid and base c a t a l y s t s and the e f f e c t s of the mole r a t i o of these r e a c t a n t s . He l e a r n e d how t o c o n t r o l t h i s r e a c t i o n and i s o l a t e t h r e e stages of products: A stage being s o l u b l e and f u s i b l e , Β stage being i n s o l u b l e but s w e l l a b l e and i n f u s i b l e but softenable, and C stage being completely i n s o l u b l e and i n f u s i b l e . The r e s u l t s of t h i s work were presented to the New York S e c t i o n of the American Chemical S o c i e t y i n 1909 (_3). The u s e f u l products known as Bakélite and the novel heat and pressure technique of c u r i n g them were patented (4^) and became the b a s i s of the phenolic industry. The years a f t e r 1909 were p r i m a r i l y spent on a p p l y i n g t h i s knowledge. The development of an understanding of the c h e m i s t r y i n v o l v e d proceeded s l o w l y . T h i s i s understandable when i t i s c o n s i d e r e d t h a t the b a s i s of modern polymer t h e o r y , f o r example, high molecular weights, f u n c t i o n a l i t y , and c r o s s - l i n k i n g , was not developed u n t i l l a t e r . Neither were the powerful t o o l s of chemical a n a l y s i s a v a i l a b l e t o d e f i n e m o l e c u l a r weights, d i s t r i b u t i o n of p r o d u c t s , f u n c t i o n a l groups, e t c . Added to t h i s was the problem t h a t the f i n a l cured products being i n s o l u b l e , i n f u s i b l e , and n o n r e a c t i v e c o u l d not be a n a l y z e d . S t u d i e s of model compounds d u r i n g the t w e n t i e s (5, 6), l e d t o an understanding of the i n i t i a l r e a c t i o n s . Comprehensive r e v i e w s of the development of t h i s chemistry from the 1920s to the 1950s are a v a i l a b l e (^7, 8), and, f o r the purposes of t h i s paper, o n l y a g e n e r a l d e s c r i p t i o n of these reactions w i l l be given. Raw M a t e r i a l s . Phenolic r e s i n s are formed by the condensation of a phenol with an aldehyde using e i t h e r base or acid c a t a l y s i s . Though


47. FRY ET AL.



there a r e many phenols t h a t c o u l d be used, o n l y a few have gained commercial importance. These can be d i v i d e d i n t o two c l a s s e s : unsubstituted and substituted phenols. Phenol, i t s e l f , i s the most important. Small amounts of other unsubstituted phenols, such as r e s o r c i n o l , f i n d use. Of the s u b s t i t u t e d p h e n o l s , the most important are the f o l l o w i n g a l k y l a t e d phenols: various c r e s o l s , ptert-butylphenol, p-tert-amylphenol, and p-tert-octylphenol. Small amounts of n o n y l - and d o d e c y l p h e n o l are a l s o used. The a r y l s u b s t i t u t e d p h e n o l s , o- and p - p h e n y l p h e n o l , have had a l o n g commercial h i s t o r y , though t h e i r high cost and recent s c a r c i t y has a l l but e l i m i n a t e d t h e i r use today. B i s p h e n o l A i s growing i n importance as a raw material i n phenolic resins. OH

OH

(2,2 b i s ( 4 hydroxyphenyl) propane ( B i s p h e n o l A)

p-phenylphenol

OH

p-tert-butylphenol

OH

p-tertamylphenol

OH

p-tert-octylphenol

p-nonylphenol


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Though many aldehydes can be made t o r e a c t w i t h p h e n o l , o n l y formaldehyde, t h e l o w e s t m o l e c u l a r weight and most r e a c t i v e aldehyde, i s of major commercial importance. A c e t a l d e h y d e and b u t y r a l d e h y d e a r e used o n l y t o a l i m i t e d e x t e n t and o f t e n i n combination w i t h formaldehyde. F u r f u r a l i s a l s o used, forming p h e n o l i c r e s i n s t h a t r e a d i l y o x i d i z e r e s u l t i n g i n dark brown t o black products. Since formaldehyde i n i t s pure state i s a h i g h l y r e a c t i v e gas, i t i s c o m m e r c i a l l y used e i t h e r as a s o l u t i o n i n water, known as f o r m a l i n , or as a s o l i d f l a k e p o l y m e r i c form known as paraform. Formaldehyde i n s o l u t i o n e x i s t s not as the pure aldehyde but rather i n hydrated forms such as methylene g l y c o l and low molecular weight g y l c o l ethers: CH 0 + H 0 2

> HOCH2OH-HOCH2OCH2OH + e t c .

2

S o l i d paraform consists of h i g h e r m o l e c u l a r weight p o l y m e t h y l e n e g l y c o l e t h e r s h a v i n g a degree o f p o l y m e r i z a t i o n o f 6-100. On heating, these polymers r e a d i l y unzip forming formaldehyde gas again (I). The i n i t i a l products formed on reacting phenol with formaldehyde are dependent on the c a t a l y s t used and whether i t r a i s e s or lowers the i n i t i a l pH o f 3.0 f o r the m i x t u r e o f phenol and aqueous formaldehyde. Base-Catalyzed Reactions. When a base c a t a l y s t i s used, r a i s i n g the pH above 8, t h e f i r s t p r o d u c t s formed are v a r i o u s hydroxybenzy 1 alcohols. When u n s u b s t i t u t e d phenol i s used, f i v e d i f f e r e n t a l c o h o l s can be formed, s i n c e formaldehyde w i l l r e a c t a t the two o r t h o and the one para p o s i t i o n t o the p h e n o l i c h y d r o x y l . These a l c o h o l s , commonly r e f e r r e d to as m e t h y l o l p h e n o l s , are found i n varying r e l a t i v e amounts depending on the r a t i o of formaldehyde to phenol present and v a r i o u s r e a c t i o n c o n d i t i o n s such as time and temperature. OH

OH

OH +

CH OH 2

OH

HOCH 2

OH

CH OH 2


47. FRY ET AL.


I f the r e a c t i o n i s c a r r i e d f u r t h e r , the m e t h y l o l groups w i l l condense by two p o s s i b l e paths, forming e i t h e r d i h y d r o x y d i a r y 1 methanes (methylene l i n k s ) or dihydroxybenzy1 e t h e r s (methylene ether l i n k s ) . These two reactions can occur at e i t h e r ortho or para p o s i t i o n s , and the remaining o r t h o and para p o s i t i o n s can have varying degrees of methylol s u b s t i t u t i o n . This r e s u l t s i n a very large number of p o s s i b l e isomers. High r a t i o s of formaldehyde and low reaction temperatures favor the formation of l a r g e amounts of metHylol groups. Low temperatures and intermediate pH values (4-7) favor methylene ether l i n k formation. High temperatures and high pH values (above 8) favor methylene l i n k s .


OH

OH

OH

2

Methylene Link OH

OH

Methylene

Ether

Further reaction increases the m o l e c u l a r weight, r e s u l t i n g i n h i g h l y branched polymers. I f an excess of formaldehyde i s present, i t i s p o s s i b l e to form a g e l l e d , c r o s s - l i n k e d s t r u c t u r e . F u r t h e r heating increases the c r o s s - l i n k density. The p h y s i c a l form of these products i s dependent on t h e i r m o l e c u l a r weight and the amount of r e a c t e d formaldehyde. The i n i t i a l r e a c t i o n products, mixed m e t h y l o l s of p h e n o l , are low v i s c o s i t y l i q u i d s t h a t are water s o l u b l e . T h i s m i x t u r e can be separated i n t o pure c r y s t a l l i n e compounds. On further condensation, v i s c o u s r e a c t i v e l i q u i d s are o b t a i n e d , which are d i f f i c u l t t o i s o l a t e i n an anhydrous form. These products are s o l u b l e i n a l c o h o l s and other polar s o l v e n t s . Increasing the molecular weight f u r t h e r w i t h o n l y a s l i g h t excess o f formaldehyde r e s u l t s i n g r i n d a b l e , f u s i b l e , r e s i n o u s s o l i d s t h a t are s o l u b l e i n p o l a r s o l v e n t s . F u r t h e r p o l y m e r i z a t i o n decreases the s o l u b i l i t y and increases the softening point u n t i l an i n f u s i b l e , i n s o l u b l e g e l i s obtained. Acid-Catalyzed Reactions. When an acid c a t a l y s t i s used and the pH of the phenol/formaldehyde mixture i s lowered to 0.5-1.5, somewhat l e s s c o m p l i c a t e d products are formed. The i n i t i a l r e a c t i o n of addition of formaldehyde to the aromatic r i n g r e s u l t s i n an unstable i n t e r m e d i a t e t h a t r a p i d l y condenses t o t h r e e p o s s i b l e dihydroxydiarylmethanes. As more formaldehyde i s reacted, the molecular weight increases w i t h the f o r m a t i o n f o r t r i - and t e t r a n u c l e a r compounds w i t h a methylene l i n k a t the v a r i o u s ortho and para p o s i t i o n s . As the

American Chemical Society Library 1155 Applied 16th St., N.W.Science Tess and Poehlein; Polymer ACS Symposium Series; American Chemical Society: Washington, DC, 1985. Washington, D.C. 20036


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molecular weight increases, the number of isomers a l s o i n c r e a s e s — there being 7 possible t r i n u c l e a r isomer compounds, 27 tetranuclear isomers, and 99 pentanuclear isomers. This r e s u l t s i n a very large number of d i f f e r e n t molecules a l l being present i n a r e s i n having a molecular weight of l e s s than 1000. OH

OH

OH


OH

pjp'-isomer When phenols are used that are substituted ortho or para to the phenolic hydroxyl, the f u n c t i o n a l i t y of the phenol i s decreased to two, decreasing the number of p o t e n t i a l structures and forming only l i n e a r polymers. C l a s s i f i c a t i o n of Phenolic Resins Phenolic resins can be divided between heat-reactive and nonheatr e a c t i v e r e s i n s and between r e s i n s made by using u n s u b s t i t u t e d o r substituted phenols. A review of the four r e s u l t i n g c l a s s i f i c a t i o n s follows. U n s u b s t i t u t e d and Heat R e a c t i v e . The f i r s t c l a s s , the unsubstituted, heat-reactive resins, are made by using phenol, c r e s o l s , and x y l e n o l s . They are m u l t i f u n c t i o n a l and thus can be cross-linked t o form f i l m s . They are s o l u b l e i n a l c o h o l s , ketones, e s t e r s , and g l y c o l ethers and i n s o l u b l e i n aromatic and a l i p h a t i c hydrocarbons. They w i l l t o l e r a t e some water i n t h e i r solvents and, i n some cases, are completely water s o l u b l e . They are compatible with polar resins such as amino resins, epoxies, polyamides, and p o l y ( v i n y l b u t y r a l ) , though c o m p a t i b i l i t y on curing i s dependent on reaction between the two r e s i n s . Less p o l a r r e s i n s such as a l k y d s and d r y i n g o i l s are incompatible. U n s u b s t i t u t e d , h e a t - r e a c t i v e phenolic resins are commercially a v a i l a b l e as 100% viscous l i q u i d s , as water and a l c o h o l s o l u t i o n s , and as s o l i d resins. The viscous l i q u i d s are mixtures of monomers and dimers of varying methylolation. The water s o l u t i o n s contain r e s i n s o f monomers and d i m e r s w i t h t h e h i g h e s t d e g r e e o f methylolation. Alcohol s o l u t i o n s contain resins that are higher i n molecular weight and are too r e a c t i v e to i s o l a t e as s o l i d s . Since these resins are so heat r e a c t i v e , i t i s often necessary to s t o r e them under r e f r i g e r a t e d c o n d i t i o n s . T y p i c a l a l c o h o l Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


47. FRY ET AL.


s o l u t i o n s of the most r e a c t i v e r e s i n s w i l l g e l i n 3-6 months a t ambient temperatures. Even the s o l i d r e s i n s , based on p h e n o l , stored at ambient temperatures can polymerize to the point of being i n s o l u b l e and i n f u s i b l e . A new, s o l i d , h e a t - r e a c t i v e r e s i n based on b i s p h e n o l A was introduced recently (10) that i s usable a f t e r even a year i n storage at ambient (70-80 °F) temperatures. This r e s i n polymerizes w e l l at common curing temperatures of 300-400 °F and may be formulated i n t o both s o l u t i o n and powder coatings (11) as w i l l be discussed. The importance of these resins i n high-performance coatings i s large. They are u s u a l l y applied from a l c o h o l s o l u t i o n s by spray, d i p , brush, or r o l l e r c o a t i n g techniques. Some s o l i d r e s i n s f i n d a p p l i c a t i o n as powder coatings. Once the a l c o h o l s o l u t i o n c o a t i n g i s a p p l i e d , the c o a t i n g i s baked, f i r s t removing the s o l v e n t s and then heat polymerizing the resin. During t h i s polymerization, water i s released by condensing m e t h y l o l groups, and formaldehyde i s r e l e a s e d by decomposing methylene ether groups. Some of t h i s formaldehyde reacts, and some i s l o s t w i t h the s o l v e n t and water. T h i s r e l e a s e of v o l a t i l e s during curing l i m i t s the f i l m thickness to l e s s than 1 m i l . Thicker f i l m s w i l l d e v e l o p b l i s t e r s and p i n h o l e s on baking. For many a p p l i c a t i o n s , one-coat, c l e a r f i l m s of 0.2-0.7 m i l s provide a l l the protection necessary. These f i l m s are baked a t temperatures r a n g i n g from 135 t o 300+ °C. Times may vary from s e v e r a l minutes to s e v e r a l hours. The time/temperature c y c l e used w i l l have a large e f f e c t on the degree and type of cure r e s u l t i n g i n d i f f e r e n t f i l m properties. Increased cure caused by longer times and higher temperatures w i l l r e s u l t i n increased corrosion, chemical and s o l v e n t resistance, and decreased f l e x i b i l i t y . Short, high-temperature bakes product a more r e s i s t a n t f i l m than l o n g , low-temperature bakes. The c o l o r of the c o a t i n g darkens on baking, going from very l i g h t greenish y e l l o w s to golden to deep red browns. T h i s c o l o r i s caused by the f o r m a t i o n of v a r i o u s o x i d a t i v e and u n s a t u r a t e d s t r u c t u r e s such as quinones, s t i l b e n e s , and methides. F i l m s t h i c k e r than 1 m i l can be o b t a i n e d by a p p l y i n g s e v e r a l t h i n c o a t s . I n t e r m e d i a t e c o a t s are g i v e n a p a r t i a l bake of 10-20 rain at 135 °C t o remove the s o l v e n t s and most of the r e a c t i o n v o l a t i l e s . The s o l v e n t s i n subsequent c o a t s w i l l not l i f t t h i s p a r t i a l l y cured f i l m . After the f i n a l coat the f i l m i s given a f u l l bake s c h e d u l e , d u r i n g which enough f l o w and c u r i n g o c c u r s t o f u s e the s e v e r a l coats i n t o one f i l m . These thick, g l a s s l i k e coatings must be cooled s l o w l y i n order to minimize stresses and s t r a i n s . The r e s u l t i n g cured coatings are l e s s affected by s o l v e n t s than any o t h e r type of o r g a n i c c o a t i n g s . They remain u n a f f e c t e d a f t e r years of exposure to a l c o h o l s , ethers, esters, ketones, aromatic and a l i p h a t i c hydrocarbons, and c h l o r i n a t e d s o l v e n t s . They have e x c e l l e n t r e s i s t a n c e to b o i l i n g water, aqueous s o l u t i o n s of m i l d acids, and a c i d i c and neutral s a l t s but poor a l k a l i resistance. A report by the National Association of Corrosion Engineers (12) made recommendations f o r the use of t h i s type of c o a t i n g , f i n d i n g i t recommended f o r over 90% of some 500 chemicals tested. These c o a t i n g s are very hard w i t h smooth, dense s u r f a c e s and poor f l e x i b i l i t y . Their low conductance and low moisture absorption r e s u l t i n good e l e c t r i c a l i n s u l a t i n g properties, r e s i s t i n g up to 500 V/mil. Resistance to a broad range of temperatures i s good, being Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.



capable of withstanding dry heat temperatures as high as 370 °C f o r short periods and having l i t t l e change i n properties on c o o l i n g to very low temperatures. Extended e x t e r i o r exposure of c l e a r and pigmented coatings r e s u l t s i n only darkening of c o l o r and l o s s of gloss. These f i l m s are completely odorless, t a s t e l e s s , and nontoxic and f i n d wide use as i n t e r i o r coatings f o r food cans. Unsubstituted, heat-reactive coatings f i n d use i n a v a r i e t y of a p p l i c a t i o n s on r i g i d metal substrates where maximum resistance i s required. A few examples w i l l s u f f i c e to i l l u s t r a t e t h e i r use. D r i l l Pipe. Iron oxide pigmented coatings are used to protect o i l w e l l d r i l l pipe. In t h i s a p p l i c a t i o n , phenolics are required f o r t h e i r resistance to abrasion, acids, hydrocarbons, and water at high temperatures and pressures, as encountered i n d r i l l i n g operations. P r i n t i n g P l a t e Backing. Baked p h e n o l i c s are used to p r o t e c t the back of copper p r i n t i n g p l a t e s t h a t are processed by e t c h i n g i n strong acids. These p l a t e s are reetched many times without degrading the backing. Any f a i l u r e of the backing would cause p a r t i a l etching of the back and uneven pressure during high-speed p r i n t i n g , r e s u l t i n g i n f a i n t l y printed areas. L i n i n g f o r S o l v e n t Drums. These c o a t i n g s have been used f o r many years as l i n i n g s f o r drums used to ship solvents. Their e x c e l l e n t resistance to s o l v e n t s makes them i d e a l f o r t h i s a p p l i c a t i o n . Other coating a p p l i c a t i o n s are i n hardware, such as door handles and hinges; food p r o c e s s i n g , such as food, f r u i t , and m i l k c o n t a i n e r s ; the l i q u o r i n d u s t r y , such as beer and wine tanks and r a i l r o a d cars; ships, such as p r o p e l l e r s and o i l tanker i n t e r i o r s ; and such miscellaneous areas as b e l t buckles, munitions c a r t r i d g e cases, and razor blades. In the t o t a l phenolic market t h i s c l a s s i s by f a r the l a r g e s t , h a v i n g more than 50% of the t o t a l p h e n o l i c r e s i n volume. T h e i r noncoating uses i n c l u d e v a r i o u s bonding, l a m i n a t i n g , and m o l d i n g a p p l i c a t i o n s . The l a r g e s t volume usages are the bonding of wood veneer i n making plywood and bonding g l a s s and rock wool f i b e r i n making thermal and a c o u s t i c a l i n s u l a t i o n s . In these a p p l i c a t i o n s the low molecular weight r e s i n s are used e i t h e r as 100% l i q u i d s or as water s o l u t i o n s . Other l a r g e volume bonding a p p l i c a t i o n s include the bonding of abrasives f o r making brake l i n i n g s , grinding wheels, and sandpaper, the bonding of sand f o r making foundry molds, and the bonding of wood c h i p s and wafers f o r making c o n s t r u c t i o n board. Molding materials are made by compounding with wood f l o u r , f i b e r s and other f i l l e r s . Paper and c l o t h with l i q u i d or a l c o h o l s o l u t i o n s are used to form many laminated products. A l l of these a p p l i c a t i o n s are based on the o r i g i n a l - t y p e heatr e a c t i v e Bakélite r e s i n s and the heat and pressure technique f o r curing them. The phenolic r e s i n brings to these various composites the p r o p e r t i e s of good adhesion, s t r u c t u r a l s t a b i l i t y , h i g h r e s i s t a n c e to most environments, and good e l e c t r i c a l p r o p e r t i e s . The other phase of the composite, be i t c e l l u l o s i c s , g l a s s f i b e r , or chopped chicken feathers, adds mechanical toughness and lowers the cost.


47. FRY ET AL.


U n s u b s t i t u t e d and Non Heat R e a c t i v e . The second c l a s s , the u n s u b s t i t u t e d , n o n h e a t - r e a c t i v e r e s i n s , are the n o v o l a k r e s i n s developed by Baekeland and Thurlow by the acid-catalyzed reaction of formaldehyde with an excess of phenol and mixed c r e s o l s . They are n o n f i l m formers, being b r i t t l e , permanently f u s i b l e , and s o l u b l e s o l i d s . They are s o l u b l e i n a l c o h o l s , ketones, and esters, though not as p o l a r as t h e i r h e a t - r e a c t i v e analogues and, t h e r e f o r e , i n s o l u b l e i n water. They a r e a l s o i n s o l u b l e i n aromatic and a l i p h a t i c hydrocarbons. Some uses depend on the f a c t that heating a novolak r e s i n under pressure with a source of excess formaldehyde w i l l r e s u l t i n a c r o s s - l i n k e d , i n s o l u b l e , i n f u s i b l e , "C -staged m a t e r i a l (13). One way of doing t h i s i s to compound w i t h a h e a t r e a c t i v e r e s i n having a large excess of methylol groups that can be used to react with the novolak resin. A second way, practiced f i r s t by A y l s w o r t h i n 1911, uses hexamethylenetetramine, "hexa," as a source of both formaldehyde and base c a t a l y s t . Hexa i s a s o l i d product formed by combining formaldehyde and ammonia. This r e s u l t s i n the terminology of two-step resins. The f i r s t reaction or step ( a c i d c a t a l y s t w i t h a d e f i c i e n c y of formaldehyde) i s brought to completion, r e s u l t i n g i n a permanently f u s i b l e and s o l u b l e r e s i n t h a t , on compounding w i t h hexa, can become r e a c t i v e a g a i n . P u l v e r i z e d s o l i d resins containing hexa f i n d a p p l i c a t i o n i n molding materials and i n bonding a p p l i c a t i o n s where a minimum of penetration i s desired. In volume they are second of the four types but coming way behind t h e i r heat-reactive counterparts. P h e n o l i c n o v o l a k r e s i n s have been a l s o used as c o r e a c t a n t s (hardeners) w i t h epoxy r e s i n s to produce thermoset systems w i t h high-quality "engineering p l a s t i c " properties.


ff

ff

ff

The base-catalyzed reaction of an epoxy r e s i n with the phenolic r e s i n produces a c r o s s - l i n k e d polyether structure that i s r e s i s t a n t to chemicals and heat and i s a good moisture vapor barrier. Since the curing mechanism does not produce byproducts, t h i c k sections may be o b t a i n e d w i t h o u t v o i d s and low s h r i n k a g e . A p p l i c a t i o n s t h a t employ the advantages of epoxy-phenolic formulations include molding materials, laminates, coatings, and adhesives. Single-package epoxy-phenolic molding m a t e r i a l s (14) u s u a l l y u t i l i z e a s o l i d epoxidized novolak and a phenolic novolak r e s i n i n a formulation such as that shown below: Epoxy r e s i n Phenolic r e s i n Catalyst Lubricant (stéarate) Filler—silica

20% 10% 2% 1% 67%

The c a t a l y s t s used are u s u a l l y base c a t a l y s t s such as an a l k y 1i m i d a z o l e or t e r t i a r y amines. The c a t a l y s t s may be b l o c k e d or Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


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coated i n m o l e c u l a r s i e v e s t o produce l a t e n c y . The molding m a t e r i a l s a r e used i n t r a n s f e r molding processes t o produce encapsulated semiconductors, d i s c r e t e devices such as t r a n s i s t o r s , c a p a c i t o r s , diodes, r e c t i f i e r s and r e s i s t o r s , and general-purpose components such as c o i l s , bushings, armatures, and potentiometers. Since a l l the above devices require e x c e l l e n t long-terra e l e c t r i c a l properties, phenolic resins work w e l l because they can be made low i n i n o r g a n i c i o n content. W h i l e the o l d e r standard r e s i n s have given r e l i a b l e performance, newer, lower free phenol resins w i l l lower the mold s t a i n i n g and c l e a n i n g problems and i n c r e a s e production e f f i c i e n c y . These p h e n o l i c r e s i n s are a l s o combined w i t h epoxy r e s i n s f o r prepreg l a m i n a t i n g . Prepregs c o n s i s t o f a r e i n f o r c i n g f a b r i c , u s u a l l y g l a s s c l o t h , impregnated with a p a r t i a l l y cured (B-stage) r e s i n system. These materials a r e used t o manufacture e l e c t r i c a l c i r c u i t boards, c o r r o s i o n r e s i s t a n t f i t t i n g s , and r e c r e a t i o n a l equipment such as water s k i s , tennis rackets, and f i s h i n g rods. In a d d i t i o n t o the above a p p l i c a t i o n s , t h i s c l a s s of p h e n o l i c r e s i n s may be used w i t h epoxy r e s i n s t o form e i t h e r s o l u t i o n or powder c o a t i n g s f o r p i p e s , e l e c t r i c a l p a r t s , or metal items t h a t require excellent corrosion resistance. A representative formulation i s shown: Parts by wt Phenolic novolak (equiv wt 117) Epoxidized novolak (equiv wt 225) Bisphenol A epoxy r e s i n (equiv wt 525) Leveling agent Base c a t a l y s t Pigment f i l l e r Solvent ( i f not powder coating)

33 50.8 29.5 0.4 0.2 24.0 80-100

Phenolic-epoxy adhesive systems f o r s t r u c t u r a l bonding use s i m i l a r f o r m u l a t i o n s but a r e f r e q u e n t l y m o d i f i e d w i t h a l i n e a r polymer h a v i n g r e a c t i v e end groups ( c a r b o x y l , amino) f o r toughness and v i b r a t i o n resistance. S u b s t i t u t e d and Heat R e a c t i v e . The t h i r d c l a s s , substituted and h e a t - r e a c t i v e r e s i n s , are made by using p a r a - s u b s t i t u t e d phenols where the substituent i s a four-carbon or higher group such as t e r t b u t y l , t e r t - o c t y l , and phenyl. Small amounts of ortho-substituted phenols and unsubstituted phenols are sometimes coreacted; but, i n g e n e r a l , the f u n c t i o n a l i t y i s 2, and o n l y l i n e a r m o l e c u l e s a r e formed. They a r e b r i t t l e s o l i d s t h a t do not form f i l m s . The substituent makes the resins l e s s polar and hence they are s o l u b l e i n ketones, e s t e r s , and aromatic hydrocarbons, w i t h l i m i t e d s o l u b i l i t y i n a l c o h o l s and a l i p h a t i c hydrocarbons. The phenolic resins based on longer chain a l i p h a t i c phenols are more compatible with drying o i l s , alkyds, and rubbers. These resins are l e s s important i n coatings than t h e i r nonheatr e a c t i v e counterparts. They do f i n d use i n combination with drying o i l s i n making e l e c t r i c a l c o i l impregnation v a r n i s h e s . The o i l s used i n t h i s type a p p l i c a t i o n a r e blown w i t h a i r so t h a t they c o n t a i n enough oxygen t o c r o s s - l i n k but a r e s t i l l f l u i d . The



47. FRY ET AL.


combination of the heat-reactive r e s i n with the blown o i l makes i t p o s s i b l e to heat cure very t h i c k coatings i n the absence of a i r . Combination w i t h neoprene r e s u l t s i n rubber c o a t i n g s used f o r waterproofing concrete and f o r general-purpose maintenance (15). In these coatings the phenolic r e s i n i s complexed with a t e t r a v a l e n t m e t a l oxide such as magnesium or z i n c oxide to form a s t a b l e i n f u s i b l e compound. T h i s treatment improves the adhesion and hardness of the neoprene. The l a r g e s t use f o r these r e s i n s i s i n neoprene c o n t a c t a d h e s i v e s . A f o r m u l a t i o n s i m i l a r to t h a t used i n neoprene rubber c o a t i n g s i s used. The p h e n o l i c i s used t o improve adhesion to metals and g l a s s , cohesive strength, elevated temperature strength, and tack of the rubbers. T h i s a d h e s i v e f i n d s use i n shoe making, automotive u p h o l s t e r y and w e a t h e r s t r i p p i n g adherence, l a m i n a t e j o i n i n g to wood f o r t a b l e s and counter tops, and t r a d e s a l e s as adhesives f o r wood, c l o t h , p l a s t i c , rubber, and metal. S u b s t i t u t e d and Non Heat R e a c t i v e . The fourth c l a s s , substituted and nonheat-reactive r e s i n s , are produced with the same substituted phenols as t h e i r h e a t - r e a c t i v e c o u n t e r p a r t s . They are b r i t t l e , permanently f u s i b l e , and s o l u b l e s o l i d s and thus non f i l m formers. S i m i l a r to t h e i r n o n h e a t - r e a c t i v e analogues they have broad s o l u b i l i t y and c o m p a t i b i l i t y . T h e i r importance i n c o a t i n g s i s l a r g e . Being non f i l m formers, they are always used to modify f i l m i n g r e s i n s , such as drying o i l s and alkyds. Their l a r g e s t use, from which they r e c e i v e the o i l - s o l u b l e p h e n o l i c name, i s i n the preparation of oleoresinous varnishes. Varnishes existed many years before the development of p h e n o l i c s , being made from n a t u r a l l y occurring o i l - s o l u b l e r e s i n s and drying o i l s . The f i r s t successful a d d i t i o n of a synthetic r e s i n to a drying o i l i s r e p o r t e d by A y l s w o r t h i n 1914 u s i n g an o - c r e s o l p h e n o l i c r e s i n (16). This r e s i n and i t s varnish were not very d e s i r a b l e due to t h e i r dark c o l o r and poor c o l o r s t a b i l i t y . Attempts to improve the c o l o r and o i l s o l u b i l i t y of these r e s i n s r e s u l t e d i n r o s i n m o d i f i e d p h e n o l i c s . T h i s type r e s i n i s made by r e a c t i n g a heatr e a c t i v e phenolic with r o s i n and then e s t e r i f y i n g w i t h g l y c e r i n e . They were f i r s t produced by Kurt A l b e r t of Germany i n 1917 (17). They were referred to as A l b e r t o l s or A l b e r t o l acids, depending on whether they were e s t e r i f i e d or not (18). Commercial A l b e r t o l s and Amberols were i n t r o d u c e d i n t h i s country by the Resinous Products Co. i n 1924 and are s o l d today by the Reichold Corp. E f f o r t s to further improve the hard r e s i n s were f r u i t l e s s u n t i l s u b s t i t u t e d phenols became a v a i l a b l e . In the l a t e 1920s, pp h e n y l p h e n o l became a v a i l a b l e . I t was i s o l a t e d from the t a r r y residue of commercial phenol plants. With t h i s new phenol came the development of the f i r s t high-performance 100% pure phenolic o i l s o l u b l e r e s i n (19), marketed as BR-254 by the Bakélite Corp. i n 1929. The para s u b s t i t u e n t r e s u l t e d i n a l i g h t c o l o r r e s i n w i t h good l i g h t s t a b i l i t y . I t s combination with drying o i l s r e s u l t e d i n a more r a p i d d r y i n g v a r n i s h f i l m w i t h b e t t e r e x t e r i o r d u r a b i l i t y than had p r e v i o u s l y been obtainable with any other hard resin. The e x c e l l e n t performance of varnishes containing t h i s r e s i n r e s u l t e d i n i t s i n c l u s i o n i n many i n d u s t r i a l and governmental s p e c i f i c a t i o n s written during the 1930s. This r e s i n remains today the standard of the industry, but because of diminishing a v a i l a b i l i t y and high costs


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of the monomer, i t s use i n the l a s t s e v e r a l years has decreased to almost nothing. During the t h i r t i e s many substituted phenols were screened f o r a p p l i c a b i l i t y i n v a r n i s h e s . Patents on the p r e p a r a t i o n of o i l s o l u b l e resins were issued to Honel (20) using p-tert-butylphenol and p - t e r t - a m y l p h e n o l and t o T u r k i n g t o n and B u t l e r (21) u s i n g pt e r t - b u t y l p h e n o l , o c t y l p h e n o l , and others. Turkington and A l l e n (22) reported the e f f e c t of the a l k y l substituent on the phenol and the influence on r e s i n and varnish properties. Of some 40 tested, o n l y the a c i d - c a t a l y z e d , n o n h e a t - r e a c t i v e r e s i n s made w i t h pp h e n y l p h e n o l and a few para t e r t i a r y a l k y l a t e d p h e n o l s gave good performance. These premium v a r n i s h e s can best be d e s c r i b e d as o l i g o m e r s o l u t i o n s of phenolic resins and drying o i l s . The phenolic r e s i n , i n a d d i t i o n to p r e v i o u s l y d i s c u s s e d p r o p e r t i e s , has a high g l a s s t r a n s i t i o n temperature, 80-100 °C, and a m o l e c u l a r weight of 6001200. The d r y i n g o i l s a r e any of t h e n a t u r a l l y o c c u r r i n g t r i g l y c e r i d e s of mixed C\q to C 2 f a t t y acids having some degree of unsaturation, low v i s c o s i t y , low g l a s s t r a n s i t i o n , -30 °C and lower, and m o l e c u l a r weights of about 900 ( f o r most common C\q fatty a c i d s ) . They are c a l l e d d r y i n g o i l s because, on s t a n d i n g a t room temperature, they w i l l absorb oxygen from the a i r and c r o s s - l i n k by v a r i o u s r e a c t i o n s , thus being transformed from a l o w - v i s c o s i t y l i q u i d to dry s o l i d f i l m s . This c r o s s - l i n k i n g occurs through the unsaturation of the o i l s . O i l s with higher degrees of unsaturation are more r e a c t i v e and c r o s s - l i n k more t i g h t l y . This polymerization i s accelerated by heat and t r a n s i t i o n metal s a l t s commonly c a l l e d driers. A more complete d e s c r i p t i o n can be found i n other r e f e r e n c e s (23, 24). Varnishes have h i s t o r i c a l l y been prepared by "cooking" the raw o i l s and r e s i n s at high temperatures, 230-310 °C, u n t i l the r e s i n was a l l d i s s o l v e d and the o i l s had p o l y m e r i z e d t o the d e s i r e d v i s c o s i t y . S o l v e n t was then added, the v a r n i s h was c o o l e d , and various a d d i t i v e s such as d r i e r s , UV absorbers, antiskinning agents, and mildewcides were included. With the c o n t i n u i n g t e c h n o l o g i c a l development of p h e n o l i c s came r e s i n s having better s o l u b i l i t y i n drying o i l s and higher molecular weights. This made i t possible to decrease the amount of cooking to o b t a i n r e s i n s o l u b i l i t y and desired varnish v i s c o s i t y . This work r e s u l t e d i n the development by S. H. Richardson (25) of the c o l d mix v a r n i s h technique, which was i n t r o d u c e d at the P a i n t I n d u s t r i e s Show i n 1954. Using a new a l k y l a t e d p h e n o l r e s i n h a v i n g improved o i l s o l u b i l i t y and c o m p a t i b i l i t y and higher s o l u t i o n v i s c o s i t i e s , i t was possible to d i s s o l v e r e s i n and o i l s i n v a r n i s h - t y p e s o l v e n t s a t room temperature, thus o b t a i n i n g a s t a b l e premium-quality v a r n i s h . Hence, i t became p o s s i b l e f o r a p a i n t company h a v i n g o n l y roomtemperature mixing equipment to manufacture phenolic varnishes that, f o r the m a j o r i t y of a p p l i c a t i o n s , performed as w e l l as the best cooked v a r n i s h e s . The m a t u r i t y of the v a r n i s h i n d u s t r y and i t s reluctance to change from time-proven procedures r e s u l t e d i n a slow but steady acceptance of t h i s procedure. The advent of a i r p o l l u t i o n r e g u l a t i o n s i n the l a t e 1960s g r e a t l y increased the use of t h i s technique since no v o l a t i l e s were g i v e n o f f d u r i n g manufacture. Producers of cooked varnishes have 2



47. FRY ET AL.


had to e i t h e r discontinue making varnishes, make c o l d mixtures, or i n v e s t i n the necessary new equipment, such as caustic scrubbers, or remove the 3-5% o i l y , obnoxious v o l a t i l e s g i v e n o f f d u r i n g h i g h temperature cooking. In order to better s e r v i c e the c o l d mixing varnish industry, a convenient 50% s o l i d s s o l u t i o n was i n t r o d u c e d (26). The s m a l l coatings company can produce a complete l i n e of cold-cut varnishes by simply mixing a few l i q u i d raw materials together. Large-volume companies can o b t a i n a l l raw m a t e r i a l s i n b u l k and c o n t i n u o u s l y prepare c o l d - c u t v a r n i s h e s w i t h the a i d of metering and mixing pumps, thus further reducing manufacturing costs. In the 1980s, h i g h - s o l i d s c o a t i n g s have become p o p u l a r due to EPA a i r p o l l u t i o n requirements and the economics of s o l v e n t s . Phenolic varnishes were already f a i r l y h i g h - s o l i d coatings (50-70% n o n v o l a t i l e s ) , but improvements up to the 75-80% n o n v o l a t i l e l e v e l were made p o s s i b l e by the i n t r o d u c t i o n of a new c o l d - c u t v a r n i s h r e s i n (27) that has shown equivalent performance to the t r a d i t i o n a l ones, p a r t i c u l a r l y i n aluminum pigmented maintenance paints. This same r e s i n may a l s o be used to upgrade the corrosion resistance of h i g h - s o l i d s alkyd v e h i c l e s as noted l a t e r under A p p l i c a t i o n s . P h e n o l i c v a r n i s h e s , e i t h e r cooked or c o l d mixed, o f f e r many e x c e l l e n t c o a t i n g p r o p e r t i e s . They brush w e l l , h a v i n g good f l o w c h a r a c t e r i s t i c s , and dry to a high gloss. The drying o i l s g i v e them good flow and wetting c h a r a c t e r i s t i c s . They have good adhesion to metal and wood surfaces due to the p o l a r i t y of the phenolic hydroxyl groups. I n t e r c o a t adhesion on r e c o a t i n g i s e x c e l l e n t s i n c e the v a r n i s h s o l v e n t s s o f t e n the s u r f a c e of the cured f i l m but do not dissolve i t . Varnishes can be formulated to a i r - d r y to tack-free f i l m s i n 4 h. The c o l o r of these v a r n i s h e s i s l i g h t golden. T h i s c o l o r adds a r i c h n e s s to wood, though i n pigmented c o a t i n g s , pure white cannot be made because of t h i s y e l l o w n e s s . On aging t h i s color i s stable. The cured f i l m s have f a i r resistance to d i l u t e acids and a l k a l i s and a l c o h o l s and a l i p h a t i c hydrocarbon s o l v e n t s . Though these f i l m s w i l l r e s i s t s h o r t c o n t a c t w i t h the c h e m i c a l s , they are not recommended f o r continuous contact. Strong a l k a l i w i l l d i s s o l v e the coatings by hydrolyzing the ester group i n the o i l and making waters e n s i t i v e s a l t s of the p h e n o l i c h y d r o x y l s . S o l v e n t s such as ketones, esters, and aromatics w i l l d i s s o l v e the phenolic r e s i n out of the f i l m s and l i f t the polymerized o i l f i l m s o f f the surface as a s w o l l e n g e l . R e s i s t a n c e to hot and c o l d water i s e x c e l l e n t . The e x c e l l e n t f l e x i b i l i t y of these f i l m s i s w e l l demonstrated by t h e i r use i n can c o a t i n g s where m e t a l sheets are f i r s t coated and then formed i n t o cans. There i s no r u p t u r e of the f i l m i n the v a r i o u s crimping operations. A b r a s i o n r e s i s t a n c e i s a l s o good. The e x c e l l e n t e x t e r i o r d u r a b i l i t y of these coatings, f i r s t recognized i n marine spar v a r n i s h e s , today s t i l l makes them one of the most durable c l e a r coatings f o r the protection of wood. Applications. Clear phenolic varnishes f i n d use today i n i n d u s t r i a l c o a t i n g s as food can l i n i n g s and maintenance primers and i n t r a d e s a l e s c o a t i n g s as g e n e r a l - p u r p o s e c l e a r s , f l o o r v a r n i s h e s , and premium e x t e r i o r c l e a r s . Many of the cooked phenolic varnish can c o a t i n g s f o r m u l a t i o n s have not changed i n the l a s t 40 years, but some cold-cut systems have come i n t o use. The e x c e l l e n t wetting and



1156


adhesion to metal, resistance to steam s t e r i l i z a t i o n , f l e x i b i l i t y , odorless and t a s t e l e s s nature, and a b i l i t y to cure r a p i d l y on baking have kept a place f o r these coatings i n the can industry. The ease of brushing, 4-h a i r - d r y , and r e c o a t a b i l i t y make these e x c e l l e n t trade s a l e s c o a t i n g s . The f a c t t h a t t h e i r water and a l k a l i resistance i s better than that of most other o i l - c o n t a i n i n g v e h i c l e s makes them more d u r a b l e to c l e a n i n g w i t h household detergents. As e x t e r i o r c l e a r s f o r the protection of wood, phenolic varnishes o f f e r premium performance. Their combination of e x t e r i o r d u r a b i l i t y and a b i l i t y to p r o t e c t wood from degradation by u l t r a v i o l e t r a d i a t i o n i s unsurpassed by any other coating. In the mid-1960s, the Baker Castor O i l Co. (now Cas Chem) introduced a new drying o i l polymer, Copolymer 186 (28), f o r use i n c o l d mixing of coatings with improved d u r a b i l i t y over other drying o i l s . The use of t h i s product, c o l d mixed w i t h a p h e n o l i c r e s i n , has shown improved e x t e r i o r d u r a b i l i t y , being i n good c o n d i t i o n a f t e r 4 years of e x t e r i o r exposure 45° f a c i n g south i n New J e r s e y and 18 months i n F l o r i d a . In pigmented coatings, phenolic varnishes f i n d use as v e h i c l e s f o r porch and deck paints where moisture and abrasion resistance are of utmost importance. T h e i r moisture r e s i s t a n c e and good metal adhesion make them useful v e h i c l e s for metal primers. In concrete p a i n t s they outperform most other o i l - c o n t a i n i n g v e h i c l e s due to t h e i r better a l k a l i resistance. As a v e h i c l e f o r aluminum-based maintenance p a i n t used to p r o t e c t m e t a l , the c o l d m i x t u r e v a r n i s h o f f e r s maximum e x t e r i o r d u r a b i l i t y at a minimum of c o s t and e f f o r t . T h i s e a s y - t o - a p p l y , single-coat f i n i s h w i l l maintain f i l m i n t e g r i t y , retard corrosion, and r e t a i n good appearance f o r upward of 4 years, outperforming many other v e h i c l e s (29). The d u r a b i l i t y of t h i s paint i s p a r t l y due to the " l e a f i n g " or f l o a t i n g of the aluminum p l a t e l e t s p a r a l l e l to the s u r f a c e , r e s u l t i n g i n a good m o i s t u r e b a r r i e r . I f the s u r f a c e treatment of the aluminum f l a k e s i s affected by e i t h e r the v e h i c l e , s o l v e n t s , or d r i e r s on aging, the p a i n t w i l l no l o n g e r " l e a f . " C o l d mixture p h e n o l i c v a r n i s h aluminum p a i n t s have been shown to r e t a i n e x c e l l e n t l e a f i n g a f t e r 10 years of aging. Phenolic o i l - s o l u b l e resins a l s o f i n d use as a d d i t i v e s to other v e h i c l e s to i n c r e a s e adhesion, hardness, and a l k a l i and moisture resistance. The addition of 5-20% to an alkyd w i l l s i g n i f i c a n t l y improve t h i s type performance. Another form of p h e n o l i c r e s i n / d r y i n g o i l combination i s the p h e n o l i c d i s p e r s i o n r e s i n , which i s a h i g h l y bodied v a r n i s h d i s p e r s e d i n f a s t e v a p o r a t i n g a l i p h a t i c hydrocarbon solvents. On evaporation of s o l v e n t s , these dispersions form a dry f i l m . Further o x i d a t i v e p o l y m e r i z a t i o n i s not needed. These d i s p e r s i o n s are rather b r i t t l e when used alone and are, therefore, u s u a l l y added to varnishes or alkyds to increase the dry rate and s o l v e n t resistance. T r a f f i c paints formulated with these dispersions w i l l dry i n 3 min. Fast-drying shop primers f o r metal can be sprayed, a i r - d r i e d f o r 5 min, and topcoated with strong solvent-containing lacquers without l i f t i n g the primer. The p h e n o l i c dispersion/alkyd combination i s recommended for fast-drying, c o r r o s i o n - i n h i b i t i n g a i r c r a f t primer (30). Waterborne Phenolic Resins. In addition to the unsubstituted, heatr e a c t i v e , water-soluble resins mentioned e a r l i e r , recent years have Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


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Chemistry and Technology of Phenolic Resins and Coatings1157

seen the introduction of s e v e r a l phenolic dispersions i n water. The d i s p e r s e d r e s i n s a r e h i g h e r i n m o l e c u l a r weight than the w a t e r s o l u b l e resins and have l i t t l e or even no free phenol content. The f i r s t materials of t h i s c l a s s were heat-reactive phenolic r e s i n s d i s p e r s e d i n s i t u d u r i n g the p r e p a r a t i o n r e a c t i o n and s t a b i l i z e d with a mixture of water-soluble gums (polysaccharides). P a t e n t e d by J . H a r d i n g ( 3 1 ) , t h i s t e c h n o l o g y r e s u l t e d i n commercially a v a i l a b l e products with an average p a r t i c l e s i z e of 2-5 ym (32). These d i s p e r s i o n s , s u p p l i e d a t 40-50% t o t a l s o l i d s , a r e f u l l y d i l u t a b l e w i t h water and may a l s o be blended w i t h v a r i o u s l a t e x e s to produce c o m p o s i t i o n s w i t h m o d i f i e d p r o p e r t i e s . A p p l i c a t i o n s i n v o l v i n g the use of these dispersions i n c l u d e f i b e r bonding, p u l p m o l d i n g , paper and f a b r i c i m p r e g n a t i o n , f r i c t i o n element bonding, coated a b r a s i v e s , l a m i n a t e s , wood bonding, and adhesives. A second g e n e r a t i o n of p h e n o l i c d i s p e r s i o n s , patented by J . S. Fry (33), i n v o l v e d the post d i s p e r s i o n of p h e n o l i c r e s i n s i n a mixture of water and water-miscible s o l v e n t s . To conform with a i r p o l l u t i o n regulations, the s o l v e n t was held to 20 volume %, or l e s s , of the v o l a t i l e s . A h e a t - r e a c t i v e phenolic r e s i n dispersion (34) and a phenolic-epoxy codispersion have become commercially a v a i l a b l e based on the above t e c h n o l o g y . S u p p l i e d a t 40-45% s o l i d s , these products, which have a s m a l l p a r t i c l e s i z e (0.75-1.0 ]im), are better f i l m formers than the e a r l i e r dispersions. Used alone or i n blends with other waterborne m a t e r i a l s , corrosion-resistant baking coatings may be f o r m u l a t e d f o r c o i l c o a t i n g p r i m e r s , d i p p r i m e r s , spray p r i m e r - s u r f a c e r s , and c h e m i c a l l y r e s i s t a n t one-coat systems. Products of t h i s type are a l s o t a c k i f i e r s for a c r y l i c latexes, and such systems have been employed as c o n t a c t , heat s e a l , and laminating adhesives for diverse substrates. Conclusions Phenolics have found many uses over the l a s t 50 years, r e s u l t i n g i n a continuing growth. Sales have grown from 100 thousand pounds i n 1911 and 1 m i l l i o n pounds i n 1914 t o 1 b i l l i o n pounds i n 1966. Today's volume i s about 1.5 b i l l i o n pounds and s t i l l growing. S c i e n t i f i c knowledge i s a l s o i n c r e a s i n g w i t h new i n f o r m a t i o n c o n c e r n i n g k i n e t i c s and m o l e c u l a r s t r u c t u r e . The a p p l i c a t i o n of various new a n a l y t i c a l t o o l s w i l l further increase our knowledge and may a s s i s t i n i m p r o v i n g such p r o p e r t i e s as c o l o r , internal p l a s t i c i z a t i o n , molecular weight, and heat s t a b i l i t y . Further growth i s expected to occur i n screw i n j e c t i o n molding, flame r e t a r d a n t a p p l i c a t i o n s , p h e n o l i c foams, microballoons, and wafer board. Usage i n coatings i s expected to continue to expand i n high-performance a p p l i c a t i o n s . Waterborne coatings and adhesives are expanding i n t o new a p p l i c a t i o n s a l s o . Literature Cited 1. 2. 3. 4. 5.

Baekeland, L. H. Ind. Eng. Chem. 1924, 16, 10. Baeyer, A. Ber. 1872, 5, 1095. Baekeland, L. H. Ind. Eng. Chem. 1909, 1, 3. Baekeland, L. H. U.S. Patents 939 966, 1909, and 942 852, 1909. Baekeland, L. H.; Bender, H. L. Ind. Eng. Chem. 1925, 17, 3. Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

1158 6. 7. 8. 9. 10.


11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34.


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Chemistry and Technology of Phenolic Resins and Coatings

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