Adhesives from Renewable Resources - American Chemical Society

Downloaded by NANYANG TECH UNIV LIB on November 5, 2014 | http://pubs.acs.org Publication Date: December 31, 1989 | doi: 10.1021/bk-1989-0385.ch034

Chapter 34 Opportunities for Future Development of Adhesives from Renewable Resources R i c h a r d W. Hemingway Southern Forest Experiment Station Forest Service U . S. Department of Agriculture Pineville, L A 71360 A n t h o n y H. C o n n e r Forest Products Laboratory Forest Service U . S. Department of Agriculture Madison, WI 53705

Adhesives derived from natural resources and the possibilites for their industrial utilization, particularly in the forest products industry, have been a focus of considerable attention in recent times. While such adhesives have been used for most of mankind's history, substantial progress has been made in the last 15 years in producing durable adhesives from renewable resources that come close to meeting today's exacting industry standards. A number of important opportunities exist for further refinement of promising adhesive systems and for the development of new ones. Recent advances demonstrate the breadth of available possibilities for using nature's own storehouse for adhesive production and offer an excellent starting point for future research effort needed for its optimum exploitation. A p r i m a r y source m a t e r i a l for n a t u r a l resource-based adhesives i s b y p r o d u c t streams o f the forest p r o d u c t s i n d u s t r y . F o r t h i s reason, a n d the fact t h a t t h e i n d u s t r y is b o t h a producer o f huge tonnages o f residues a n d a m a j o r consumer This chapter not subject to U.S. copyright Published 1989 American Chemical Society

In Adhesives from Renewable Resources; Hemingway, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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o f adhesives, t h i s b o o k ' s m a j o r focus is o n i n d u s t r y u t i l i z a t i o n o f adhesives f r o m renewable resources d e r i v e d f r o m trees. T h e a m o u n t s o f l i g n i n s , t a n n i n s , a n d c a r b o h y d r a t e s a v a i l a b l e as residues f r o m processing o f forest trees d w a r f the c o m m o d i t y adhesive m a r k e t . A t the same t i m e , the forest p r o d u c t s i n d u s t r y is e s p e c i a l l y reliant o n adhesives, since over 7 0 % o f a l l w o o d p r o d u c t s are b o n d e d , a n d t h e i r p r o d u c t i o n consumes a b o u t 4 5 % o f a l l p h e n o l i c a n d 8 5 % o f a l l u r e a - f o r m a l d e h y d e resins p r o d u c e d i n


the U n i t e d S t a t e s . A selection o f chapters d r a w n f r o m other i n d u s t r i e s has been i n c l u d e d because t h e y offer p a r t i c u l a r l y i n t e r e s t i n g a p p l i c a t i o n s of n a t u r a l p r o d u c t s as a d h e sive p o l y m e r s . T h e s e chapters e m p h a s i z e the e x t r a o r d i n a r y s o p h i s t i c a t i o n t h a t c a n be i n v o l v e d i n m a n i p u l a t i n g the s t r u c t u r e o f adhesive p o l y m e r s t h r o u g h genetic engineering as described i n R o b e r t S t r a u s b e r g ' s c h a p t e r ( C h a p t e r 32) a n d the a p p l i c a t i o n o f n a t u r a l p o l y m e r s t o p a r t i c u l a r l y difficult b o n d i n g s i t u a t i o n s as discussed b y C h r i s t i n e B e n e d i c t ( C h a p t e r 33). T h e b r e a d t h o f n a t u r a l p o l y m e r classes t h a t c a n be a p p l i e d i n s u c h a w i d e v a r i e t y o f adhesives testifies t o the great p o t e n t i a l for renewable resources t o r e g a i n t h e i r p r o m i n e n c e as a source o f adhesive p o l y m e r s as p e t r o c h e m i c a l s b e c o m e scarce a n d t h e i r p r i c e increases. T h e chapters presented i n t h i s v o l u m e show t h a t considerable progress has been m a d e i n the l a b o r a t o r y over the past 15 years. W i t h o n l y a few except i o n s , s u c h as the l o n g s t a n d i n g use o f s t a r c h i n the p a p e r i n d u s t r y a n d m o r e recent use o f w a t t l e t a n n i n s as a basis for adhesives i n the S o u t h A f r i c a n w o o d i n d u s t r y , l i t t l e o f t h i s t e c h n o l o g y has been transferred t o the i n d u s t r i a l sector. T h e f r u s t r a t i o n s b e i n g felt b y scientists p u r s u i n g the development o f adhesives based o n n a t u r a l p o l y m e r s surfaced i n the discussions o n the first m o r n i n g of the s y m p o s i u m a n d l a s t e d i n b o t h f o r m a l a n d i n f o r m a l f a s h i o n t h r o u g h o u t the week.

T h e c o m p a r a t i v e l y large research effort w i t h so few e x a m p l e s d e m o n -

s t r a t i n g the i m p l e m e n t a t i o n o f new adhesive technology b y i n d u s t r y suggests a careful c o n s i d e r a t i o n o f prospects for the f u t u r e . Industry Needs T h e forest p r o d u c t s i n d u s t r y is faced w i t h t w o very significant p r o b l e m s :

1)

w h a t t o do w i t h processing residues other t h a n u s i n g t h e m t o recover l o w - v a l u e heat, a n d 2) how t o meet the d e m a n d for m a t e r i a l s s h o u l d the p e t r o l e u m c r u n c h o f the e a r l y 1970*8 recur. A l a n L a m b u t h , w h o opened the s y m p o s i u m , speaks n o t o n l y f r o m the needs felt b y his c o m p a n y b u t also as the C h a i r m a n o f the N a t i o n a l Forest P r o d u c t s A s s o c i a t i o n ' s W o o d A d h e s i v e C o m m i t t e e o n E v a l u a t i o n of R e s e a r c h . H i s c o m m i t t e e ' s l o n g t e r m s u p p o r t for research o n the d e v e l o p m e n t o f new adhesives based o n renewable resources confirms t h a t the forest p r o d u c t s i n d u s t r y i n the U n i t e d States r a n k s t h i s as h i g h - p r i o r i t y research. T h e excellent p a r t i c i p a t i o n b y i n d u s t r y i n the s y m p o s i u m t h a t l a i d the f o u n d a t i o n for t h i s b o o k is f u r t h e r evidence t h a t m a n y c o m p a n i e s have deemed t h a t i n v e s t m e n t s i n


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research a n d development o f adhesives based o n renewable resources have g o o d potential economic returns.


M o s t o f the chapters i n t h i s v o l u m e d i r e c t l y address the research needs expressed i n A l a n L a m b u t h ' s overview ( C h a p t e r 1). A l t h o u g h t h i s b o o k c a n o n l y be representative o f the m u c h w i d e r a t t a c k o n the p r o b l e m o f o b t a i n i n g adhesives f r o m renewable resources, i t does d e m o n s t r a t e t h a t research is c u r r e n t l y u n d e r w a y i n i n d u s t r y , u n i v e r s i t i e s , a n d government l a b o r a t o r i e s t h a t is focused o n a c r i t i c a l f u t u r e need o f i n d u s t r y . Obstacles to U s e of N e w Adhesive

Technology

G i v e n the h i g h p r i o r i t y for new adhesive technology based o n renewable resources, w h y t h e n is there n o t a higher degree of technology transfer f r o m the l a b o r a t o r y b e n c h t o i n d u s t r i a l practice? I n N o r t h A m e r i c a at least, a significant answer t o t h a t question m u s t be t o u t e d i n economics (i.e., the c o m p a r a t i v e l y l o w p r i c e for p h e n o l - f o r m a l d e h y d e a n d u r e a - f o r m a l d e h y d e resins). G r a h a m A l l a n ( C h a p t e r 5) focuses o n the e c o n o m i c considerations t h a t need t o be a d dressed i n the development o f any new adhesives based o n renewable resources. C l e a r l y , the price differential between c u r r e n t l y - u s e d adhesives based o n p e t r o chemicals a n d new renewable resource-based f o r m u l a t i o n s m u s t be s u b s t a n t i a l t o entice i n d u s t r y t o a t t e m p t a s u b s t i t u t i o n . A s p o i n t e d o u t i n A l a n L a m b u t h ' s i n t r o d u c t i o n , i n d u s t r y w o u l d prefer to m a k e t h i s s u b s t i t u t i o n w i t h essentially n o m o d i f i c a t i o n t o current m a n u f a c t u r i n g processes. T h i s is often a difficult r e q u i r e m e n t t o meet. T o n y P i z z i ' s discussion ( C h a p t e r 19) o f the p r o b l e m s faced i n g e t t i n g i n d u s t r y a p p r o v a l for use o f t a n n i n - b a s e d adhesives i n S o u t h A f r i c a focuses o n a n u m b e r of other p r o b l e m s , whether real or o n l y p e r c e i v e d , t h a t i m p e d e d the a d o p t i o n o f t h i s technology b y m a n u f a c t u r e r s o f b o n d e d w o o d p r o d u c t s . M u c h o f the p r o b l e m m u s t b e a s c r i b e d s i m p l y t o m a n ' s resistance t o change n o m a t t e r how s m a l l the r e a l or perceived change m i g h t be. R e s e a r c h scient i s t s dedicate t h e i r lives t o change, b u t a p l y w o o d m a n u f a c t u r e r , for e x a m p l e , wants n o surprises i n m e e t i n g p r o d u c t i o n quotas. P i z z i ' s experience is i m p o r t a n t for a l l of us t o consider. W e m u s t expect questions such as: " W h a t is a c a r b o h y d r a t e or l i g n i n ? " , " W h y is the color o f the glue different f r o m t h a t o f a p h e n o l - f o r m a l d e h y d e r e s i n ? " , " W h y i s n ' t the resin at p H 10.5, since a l l the p r e v i o u s resins have been used at t h i s p H ? " A n d these types o f questions m u s t be answered i n a way t h a t satisfies those i n the m i l l s w h o w i l l a c t u a l l y use the resin. A n o t h e r , a n d perhaps the m o s t i m p o r t a n t , obstacle t o increased use o f a d hesive p o l y m e r s based o n n a t u r a l p r o d u c t s relates t o o u r i n a d e q u a t e knowlege o f the f u n d a m e n t a l c h e m i s t r y of these systems. A large b o d y o f e m p i r i c a l form u l a t i o n w o r k is generally r e q u i r e d i n the development o f new adhesives, a n d success often rests o n how w e l l t h a t w o r k is done. However, knowledge o f the structures a n d reactions of these p o l y m e r s is j u s t as i m p o r t a n t a n d u s u a l l y re-


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q u i r e d t o g u i d e the adhesive f o r m u l a t o r to r a t i o n a l approaches. T h e response t o the p e t r o l e u m shortage o f the e a r l y 1970s was t o invest very h e a v i l y i n a p p l i e d research because of the urgent need t o keep p l a n t s o p e r a t i n g . Investments i n m o r e f u n d a m e n t a l research lagged far b e h i n d , a n d they c o n t i n u e t o be rare i n t o d a y ' s m o r e n o r m a l e n v i r o n m e n t of adequate p e t r o c h e m i c a l supplies.


Opportunities for the F u t u r e O n e of the h i g h l i g h t s of the s y m p o s i u m was the dialogue t h a t o c c u r r e d a m o n g adhesive chemists f r o m diverse b a c k g r o u n d s . U n f o r t u n a t e l y , the discussions t h a t followed the f o r m a l presentations of the i n d i v i d u a l papers c o u l d not be i n c l u d e d i n t h i s b o o k . T h e m a j o r i t y of the p a r t i c i p a n t s expressed considerable o p t i m i s m a b o u t the f u t u r e for adhesives based o n renewable resources. M a n y of the c a r b o h y d r a t e - b a s e d adhesives, like s t a r c h , described i n d e t a i l b y H a r r y K e n n e d y ( C h a p t e r 23), a n d cellulosics, s u m m a r i z e d b y D a v i d H o n ( C h a p t e r 21), a l r e a d y have s u b s t a n t i a l t e c h n i c a l as w e l l as e c o n o m i c advantages over p e t r o c h e m i c a l based p r o d u c t s , a n d these p o l y m e r s h o l d significant m a r k e t s . B i l l Detlefsen ( C h a p t e r 31) showed t h a t other adhesives s u c h as those d e r i v e d f r o m b l o o d a n d casein have a p a r t i c u l a r l y f a s c i n a t i n g h i s t o r y a n d s t i l l find use i n s p e c i a l b o n d i n g a p p l i c a t i o n s . I n d u s t r y ' s need for f u r t h e r development of adhesives based o n renewable resources is b e i n g addressed w i t h some interesting new f o r m u l a t i o n s based o n l i g n i n s , t a n n i n s , c a r b o h y d r a t e s , a n d proteins as s h o w n i n the p r e c e d i n g chapters. L i g n i n s . N o r m a n L e w i s ' s i n t r o d u c t i o n ( C h a p t e r 2) t o the use of l i g n i n s i n adhesives r e m i n d s us t h a t m a j o r research a n d development efforts have been u n d e r t a k e n w i t h no significant a p p l i c a t i o n of l i g n i n s i n adhesives to d a t e . E v e n m o r e m u s t be learned a b o u t the c h e m i s t r y of l i g n i n s , p a r t i c u l a r l y as r e l a t e d t o t h e i r m o d i f i c a t i o n t o p r o v i d e s u i t a b l e properties as adhesive p o l y m e r s a n d t h e i r r e a c t i o n w i t h c r o s s l i n k i n g agents. N e w N M R e x p e r i m e n t s like those des c r i b e d b y L a r r y L a n d u c c i ( C h a p t e r 3) c a n be of considerable help i n s t u d y i n g these p o l y m e r s . W o l f g a n g G l a s s e r ' s review ( C h a p t e r 4) of v a r i o u s approaches t o c r o s s l i n k i n g o f l i g n i n s after different m o d i f i c a t i o n reactions shows the m a n y v a r ied approaches t o t h i s p r o b l e m t h a t c e r t a i n l y need a d d i t i o n a l research. Subsequent t o S h e n a n d C a l v e ' s w o r k o n lignosulfonates for b o n d i n g of p a r t i c l e b o a r d , there has been l i t t l e research o n the use of these l i g n i n s i n N o r t h A m e r i c a , p o s s i b l y because of the c o m p a r a t i v e l y few sulfite m i l l s a n d the w a t e r - s o l u b i l i t y conferred b y the sulfonate f u n c t i o n . G r a h a m A l l a n ' s discussion ( C h a p t e r 5) o f the c o m p a r a t i v e prices between lignosulfonates a n d k r a f t l i g n i n s a n d the large tonnages t h a t are b e i n g p r o d u c e d makes one question whether the focus o n k r a f t l i g n i n s is a d v i s a b l e . H i s phenolysis m o d i f i c a t i o n r e a c t i o n c e r t a i n l y seems t o i m p a r t s u i t a b l e properties a n d at acceptable costs. P i z z i a n d colleagues ( C h a p t e r 7) have cleverly taken advantage of the spec i a l circumstances offered i n the k r a f t l i g n i n s f r o m bagasse t o develop b o t h c o l d - s e t t i n g w o o d l a m i n a t i n g a n d p a r t i c l e b o a r d adhesives. L i g n i n o x i d a t i o n s



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are considered f r o m two quite different v i e w p o i n t s . T o r S c h u l t z a n d his coworkers ( C h a p t e r 6) are t a k i n g a new look at a n o l d r e a c t i o n used t o p r o d u c e v a n i l l i n f r o m l i g n i n i n a n a t t e m p t t o f o r m u l a t e l o w - m o l e c u l a r - w e i g h t phenolics for a d hesive synthesis. A n n e g r e t H a a r s a n d her colleagues ( C h a p t e r 10) t a k e the o p p o s i t e a p p r o a c h i n u s i n g phenoloxidase enzymes to p o l y m e r i z e l i g n i n s i n the b o n d l i n e as a p a r t i c l e b o a r d adhesive. B o t h Hse ( C h a p t e r 8) a n d G i l l e s p i e ( C h a p t e r 9) have developed adhesive f o r m u l a t i o n s t h a t i n c o r p o r a t e s u b s t a n t i a l p r o p o r t i o n s of k r a f t l i g n i n s after h y d r o x y m e t h y l a t i o n . T h e r a p i d l y e x p a n d i n g oriented s t r a n d b o a r d i n d u s t r y i n the U n i t e d States c a n be expected t o s u b s t a n t i a l l y increase d e m a n d s for phenolic resins, a n d the studies j u s t referred t o suggest t h a t k r a f t l i g n i n s c a n be used t o s u b s t i t u t e for significant p r o p o r t i o n s of t h e p h e n o l usage i n these resins. A l l of the studies m e n t i o n e d are directed t o l i g n i n s d e r i v e d f r o m p u l p a n d p a p e r m a n u f a c t u r e . T h i s requires t h a t a t r a d i t i o n a l l y conservative p u l p a n d p a p e r i n d u s t r y e m b a r k o n f u r t h e r development as a c h e m i c a l p r o d u c e r . H o w ever, H e l e n a C h u m a n d her colleagues ( C h a p t e r 11) derive t h e i r phenolics f r o m l i g n i n b y fast p y r o l y s i s of w o o d r a t h e r t h a n as b y p r o d u c t s f r o m the p u l p a n d p a per i n d u s t r y . O p p o r t u n i t i e s m i g h t be better for development of adhesives f r o m a c h e m i c a l m a n u f a c t u r e r d r a w i n g o n waste w o o d as a r a w m a t e r i a l m u c h the same as has developed i n the p r o d u c t i o n of f u r a n resins derived f r o m a g r i c u l t u r a l residues described b y B i l l M c K i l l i p ( C h a p t e r 29). A n area not t r e a t e d i n d e t a i l a n d covered o n l y i n N o r m a n L e w i s ' s review ( a n d one t h a t deserves a t t e n t i o n i n the future) is the use of h y d r o l y s i s a n d organosolve l i g n i n s i n w o o d adhesives. S h o u l d w o o d p u l p p r o d u c t i o n by s t e a m e x p l o s i o n or organosolve p u l p i n g become m o r e significant c o m m e r c i a l processes, m o r e a t t e n t i o n w i l l c e r t a i n l y b e directed t o these l i g n i n s . I n s u m m a r y , progress o n the use of l i g n i n s i n w o o d adhesives has been m a d e , b u t the level of effort directed t o l i g n i n u t i l i z a t i o n i n general has been s m a l l i n c o m p a r i s o n w i t h the e n o r m i t y of the p r o b l e m , p o s s i b l y because of the l i m i t e d advances t h a t have been m a d e i n t h i s field i n the past. U n l e s s h i g h p e t r o l e u m prices r e t u r n , we c a n expect t o see slow c o n t i n u a l i m p r o v e m e n t i n o p p o r t u n i t i e s t o use l i g n i n s as a basis for adhesives. T a n n i n s . H e r b H e r g e r t ' s i n t r o d u c t i o n ( C h a p t e r 12) t o t h e use o f condensed t a n n i n s i n adhesives is especially interesting because he provides some reasons w h y c o m m e r c i a l success is l a c k i n g i n the use o f condensed t a n n i n s f r o m conifer b a r k s despite s u b s t a n t i a l effort w o r l d w i d e to p a r a l l e l the S o u t h A f r i c a n success i n the use of w a t t l e t a n n i n s . M u c h of the p r o b l e m i n the use of conifer b a r k t a n n i n s r e m a i n s centered o n o u r i n a d e q u a t e u n d e r s t a n d i n g o f the f u n d a m e n t a l c h e m i s t r y o f these p o l y m e r s . F o r e x a m p l e , L a w r e n c e P o r t e r ( C h a p t e r 13) p r o v i d e d the first measurements of the viscosities of s o l u t i o n s of p u r i f i e d c o n densed t a n n i n isolates of k n o w n m o l e c u l a r weight a n d the reactions of these p o l y m e r s w i t h f o r m a l d e h y d e . It is i n c r e d i b l e t h a t t h i s has not been done p r e v i o u s l y c o n s i d e r i n g the hundreds of papers t h a t have been p u b l i s h e d o n t a n n i n use i n w o o d adhesives. F u r t h e r evidence for the c o m p a r a t i v e l y l i m i t e d knowledge


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of the f u n d a m e n t a l c h e m i s t r y o f reactions of these p o l y m e r s is p r o v i d e d b y the need for research o n the reactions o f m e t h y l o l p h e n o l s w i t h t a n n i n s . C r o s s l i n k i n g o f condensed t a n n i n s w i t h p o l y m e t h y l o l phenols has been a d v o c a t e d since M c L e a n a n d G a r d n e r ' s p a p e r i n 1952, a n d t h i s a p p r o a c h has been used i n m a n y t a n n i n - b a s e d resin f o r m u l a t i o n s . However, l i t t l e is k n o w n a b o u t these react i o n s . W a y n e M c G r a w ( C h a p t e r 14) describes c o m p e t i t i v e reactions o f orthoa n d p a r a - h y d r o x y b e n z y l alcohols w i t h m o d e l c o m p o u n d s for condensed t a n n i n s . I n c o m p a r i s o n w i t h t h e i r e x t r e m e l y r a p i d r e a c t i o n w i t h f o r m a l d e h y d e , the c o n densed t a n n i n s react o n l y s l u g g i s h l y w i t h oWAo-methylols. R o l a n d K r e i b i c h ( C h a p t e r 15) has t a k e n advantage o f the r a p i d c o n d e n s a t i o n o f conifer b a r k t a n n i n s w i t h f o r m a l d e h y d e i n f u r t h e r development o f e n d - j o i n t i n g adhesives u s i n g his h o n e y m o o n p r i n c i p l e . A n a l o g o u s w a t t l e t a n n i n - b a s e d e n d j o i n t adhesives are used c o m m e r c i a l l y i n S o u t h A f r i c a , b u t K r e i b i c h has s h o w n t h a t i n e x p e n s i v e sulfonate extracts f r o m s o u t h e r n pine b a r k c a n p r o v i d e e n d j o i n t b o n d s m e e t i n g the essential requirements of the A m e r i c a n I n s t i t u t e o f T i m ber C o n s t r u c t i o n . T h e development o f these adhesives was based o n f u n d a m e n t a l studies o f the s u l f o n a t i o n a n d d e s u l f o n a t i o n o f p o l y m e r i c p r o c y a n i d i n s a n d the rearrangement of these p o l y m e r s i n alkaline s o l u t i o n done b y H e m i n g w a y a n d his c o l l a b o r a t o r s . D a v i d R o u x ( C h a p t e r 16) describes his g r o u p ' s studies of the b a s e - c a t a l y z e d rearrangement o f the p r o f i s e t i n i d i n s a n d p r o r o b i n e t i n i d i n s i n quebracho a n d w a t t l e t a n n i n s . H i s chapter emphasizes the i m p o r t a n c e o f a n d difficulty i n v o l v e d i n u n d e r s t a n d i n g the b e h a v i o r o f condensed t a n n i n s even i n such s i m p l e s i t u a t i o n s as t h e i r s o l u t i o n i n m i l d a l k a l i n e c o n d i t i o n s . T h e c o m m e r c i a l i z a t i o n of conifer b a r k extracts i n adhesive systems requires larger m a r k e t s t h a n are afforded by the t i m b e r l a m i n a t i n g i n d u s t r y . I n one a p p r o a c h , B r i d g e t te D i x a n d R a i n e r M a r u t z k y ( C h a p t e r 17) have e x p l o r e d the use o f b a r k e x t r a c t s as w e l l as c a r b o h y d r a t e s a n d proteins i n c o m b i n a t i o n w i t h d i isocyanates for the p l y w o o d a n d p a r t i c l e b o a r d i n d u s t r i e s . I n another a p p r o a c h , G a r y H a m e d a n d colleagues ( C h a p t e r 18) have e x a m i n e d the p o t e n t i a l for use o f condensed t a n n i n s as s u b s t i t u t e s for resorcinol-based adhesives i n the c o m p a r a t i v e l y large r u b b e r i n d u s t r y . B o t h appear t o have p r o m i s e . P i z z i ' s discussion ( C h a p t e r 19) o f the t r i a l s faced i n o b t a i n i n g c o m m e r c i a l acceptance o f w a t t l e based adhesives i n S o u t h A f r i c a is p a r t i c u l a r l y apropos to H a m e d ' s efforts t o g a i n acceptance o f condensed t a n n i n s i n the h i g h - v a l u e , l a r g e - v o l u m e r u b b e r market. Progress i n the use o f condensed t a n n i n s i n adhesive f o r m u l a t i o n s m i g h t be expected t o be m o r e r a p i d t h a n is the case for l i g n i n s because o f the i m p e t u s p r o v i d e d b y the c o m m e r c i a l i z a t i o n o f w a t t l e t a n n i n - b a s e d adhesives a n d because o f the e x t r a o r d i n a r i l y h i g h r e a c t i v i t y o f t a n n i n s i n reactions w i t h f o r m a l d e h y d e . T h i s r e a c t i v i t y offers a n o p p o r t u n i t y t o s u b s t i t u t e t a n n i n for resorcinol (curr e n t l y p r i c e d at a b o u t $ 1 . 8 0 / l b ) i n s t e a d o f p h e n o l ( a b o u t $ 0 . 4 0 / l b ) . N o w t h a t w a t t l e t a n n i n s have been successfully i n t r o d u c e d , t h e i r a p p l i c a t i o n can be expected t o continue t o e x p a n d . T h e s i t u a t i o n r e m a i n s d i f f i c u l t , o n the other h a n d , for use o f conifer b a r k t a n n i n s i n adhesives. H e r b H e r g e r t is c e r t a i n l y


34.

H E M I N G W A Y A N D C O N N E R

Future

Developments

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correct i n his assessment t h a t one o f the m o r e i m p o r t a n t advances t h a t needs t o be m a d e is the i m p r o v e m e n t o f e x t r a c t y i e l d s . T h i s m u s t be a c c o m p l i s h e d w i t h the r e t e n t i o n o f h i g h r e a c t i v i t y , a r e q u i r e m e n t t h a t w o u l d seem t o exclude the use o f a l k a l i n e e x t r a c t i o n at elevated t e m p e r a t u r e . W e m u s t also continue t o i m p r o v e o n o u r u n d e r s t a n d i n g o f the c h e m i s t r y o f these p o l y m e r s , p a r t i c u l a r l y i n t e r m s o f t h e i r reactions. C a r b o h y d r a t e s . C a r b o h y d r a t e s are a v a i l a b l e i n huge q u a n t i t i e s f r o m p l a n t a n d a n i m a l sources. T h e y represent the single m o s t i m p o r t a n t class of c o m p o nents o b t a i n a b l e f r o m renewable resources. T o n y C o n n e r demonstrates i n his overview ( C h a p t e r 20) t h a t c a r b o h y d r a t e p o l y m e r s , o l i g o m e r s , a n d m o n o m e r s , as w e l l as p r o d u c t s f o r m e d f r o m t h e i r reactions, have been h i s t o r i c a l l y u t i l i z e d i n adhesives. I n a d d i t i o n , he shows t h a t each of these classes of c a r b o h y d r a t e m a t e r i a l s offers excellent o p p o r t u n i t i e s for f u r t h e r u t i l i z a t i o n i n adhesives. T h e use of c a r b o h y d r a t e p o l y m e r s i n adhesive systems is addressed f r o m five very i n t e r e s t i n g , a n d i n some respects very divergent, v i e w p o i n t s . D a v i d H o n ( C h a p t e r 21) a n d H a r r y K e n n e d y ( C h a p t e r 23) t h r o u g h l y explore the use o f two v e r y i m p o r t a n t c a r b o h y d r a t e p o l y m e r s i n adhesives-cellulose a n d s t a r c h . B o t h p o l y m e r s have l o n g histories associated w i t h t h e i r use as i n d u s t r i a l r a w m a t e r i a l s i n general a n d as adhesives i n p a r t i c u l a r . B o t h continue t o be i m p o r t a n t i n present adhesive a p p l i c a t i o n s . G e r r i t v a n der K l a s h o r s t ( C h a p t e r 22) investigates the use of hemicellulose (an i m p o r t a n t class o f heterogeneous c a r b o h y d r a t e p o l y m e r s ) f r o m s o d a bagasse p u l p i n g l i q u o r s as a n adhesive i n corr u g a t e d b o a r d adhesives. S t u d i e s o f adhesives systems such as t h i s offer m u c h insight i n t o the c h e m i s t r y o f d u r a b l e adhesives based o n c a r b o h y d r a t e p o l y mers. R a m a n i N a r a y a n a n d colleagues ( C h a p t e r 24) explore a n area t h a t w i l l become i n c r e a s i n g l y i m p o r t a n t i n the very near f u t u r e , the b o n d i n g o f h y d r o p h o b i c p l a s t i c m a t e r i a l s t o n a t u r a l p o l y m e r s , for e x a m p l e , d u r i n g the f o r m a t i o n of p l a s t i c / w o o d composites. T h e techniques used i n t h e i r s t u d y w i l l be of interest t o researchers i n v e s t i g a t i n g other areas o f adhesion a n d adhesives. Several chapters also d e m o n s t r a t e the use o f s m a l l e r m o l e c u l a r - w e i g h t c a r b o h y d r a t e s (i.e., m o n o m e r s ) i n adhesives. T o n y C o n n e r a n d his colleagues ( C h a p ter 25) explore the p a r t i a l replacement of p h e n o l - f o r m a l d e h y d e adhesives used to b o n d w o o d w i t h various w o o d - d e r i v e d c a r b o h y d r a t e s . A l C h r i s t i a n s e n ( C h a p ter 26) a n d J o e K a r c h e s y a n d his coworkers ( C h a p t e r 27) investigate the very c o m p l i c a t e d c h e m i s t r y a n d the p r a c t i c a l a p p l i c a t i o n o f adhesives based o n the reaction of a carbohydrate w i t h urea and phenol. T i t o V i s w a n a t h a n (Chapter 28) describes his a t t e m p t s t o u t i l i z e a very large c a r b o h y d r a t e waste s t r e a m , w h e y permeates f r o m the processing of cheese, for the p r o d u c t i o n of w o o d a d hesives. A n u m b e r of v e r y reactive c o m p o u n d s c a n be o b t a i n e d f r o m c a r b o h y d r a t e s . F u r f u r a l , a f u r a n , is one such c o m p o u n d t h a t is p r o d u c e d by r e a c t i n g a p e n tose c a r b o h y d r a t e u n d e r a c i d i c c o n d i t i o n s . B i l l M c K i l l i p ( C h a p t e r 29) gives a n excellent overview o f f u r a n resin c h e m i s t r y a n d the use o f furans for the p r o d u c t i o n o f p o l y m e r i c m a t e r i a l s such as adhesives. F u r a n s , i n p a r t i c u l a r f u r f u r a l ,


494


c a n be r e a d i l y m a d e f r o m a n u m b e r o f renewable resources (e.g., corncobs a n d w o o d ) a n d are a l r e a d y used as a source of a n u m b e r of i m p o r t a n t p o l y m e r i c a n d adhesives systems. H o w e v e r , the c h e m i s t r y o f these types o f c o m p o u n d s is v e r y r i c h a n d offers a n u m b e r of e x c i t i n g a n d i n n o v a t i v e p o s s i b i l i t i e s . A n e x a m p l e is presented i n J o h n S t a n f o r d ' s excellent research ( C h a p t e r 30) i n t o p r o d u c i n g


p o l y u r e t h a n e s f r o m renewable resources. P r o t e i n s . T h e chapters presented i n the section o n p r o t e i n s best d r a m a t i z e the i n t e r e s t i n g h i s t o r y o f a n d f u t u r e for adhesives f r o m renewable resources. B i l l Detlefsen's account ( C h a p t e r 31) o f the use of b l o o d a n d casein adhesives t h r o u g h h i s t o r y is a n i m p o r t a n t r e m i n d e r t h a t the age of adhesives f r o m p e t r o chemicals has been a b r i e f p e r i o d i n t i m e . H i s chapter also r e m i n d s us t h a t the peak p r o d u c t i o n of these p r o t e i n glues o c c u r r e d i n c o m p a r a t i v e l y recent t i m e s (1960 for b l o o d a n d 1973 for casein). T h e p e s s i m i s t i c o u t l o o k for t h e i r f u t u r e s t a n d s i n s t a r k contrast t o the e x c i t e m e n t s u r r o u n d i n g the development o f m u s sel adhesive p r o t e i n for b o n d i n g u n d e r e x t r e m e l y difficult c o n d i t i o n s . R o b e r t S t r a u s b e r g ' s s u m m a r y ( C h a p t e r 32) of efforts to p r o d u c e analogues of M A P p o l y m e r s b y genetic engineering processes was a n excellent e x a m p l e of the f u t u r e i n n o v a t i v e a p p l i c a t i o n s for n a t u r a l p o l y m e r s designed w i t h very specific p r o p e r t i e s . L i k e w i s e , C h r i s B e n e d i c t ' s discussion ( C h a p t e r 33) o n the use o f mussel adhesive p r o t e i n i n b o n d i n g o f cells t o i n e r t substrates or b o n d i n g o f a n i m a l tissues s u c h as i n eye surgery d r a m a t i z e s the e n o r m o u s , as yet u n t a p p e d , p o t e n t i a l for f u t u r e development o f adhesives f r o m renewable resources. Concluding Remarks A vast b o d y o f research lies ahead o f us i f we are t o effectively u t i l i z e renewable resources as adhesives. T h e content o f t h i s b o o k emphasizes t h a t a l l classes o f n a t u r a l p o l y m e r s h o l d p r o m i s e i f o n l y we can l e a r n enough a b o u t t h e i r c h e m i s t r y t o m a k e use o f t h e i r s p e c i a l properties. W e must not w a i t for another p e t r o l e u m shortage for research t o be f u n d e d for the development o f new a d hesives f r o m renewable resources. M a n y of the f o r m u l a t i o n s described i n the chapters i n c l u d e d i n t h i s v o l u m e are e c o n o m i c a l l y a n d t e c h n i c a l l y c o m p e t i t i v e w i t h t o d a y ' s w i d e l y used adhesives, a n d there are enhanced o p p o r t u n i t i e s t o enlarge o n t h e i r advantages. T h e i n c r e a s i n g use o f composites as m a t e r i a l s i n most aspects o f our lives places great d e m a n d s o n adhesives, b u t t h i s f u r t h e r e x p a n d s the p o t e n t i a l for new adhesive s y s t e m technology. T h e research h i g h lighted i n t h i s b o o k a m p l y demonstrates the b r e a d t h of p o s s i b i l i t i e s t h a t exists for the creative researcher t o explore the f o r m u l a t i o n of adhesives f r o m n a t u r e ' s o w n storehouse. I n t h i s sense, t h i s b o o k ends o n a note o f e x p e c t a n t o p t i m i s m . RECEIVED

September 6, 1988


Adhesives from Renewable Resources - American Chemical Society

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