Recent Progress in Wood Dissolution and Adhesives from Kraft Lignin

Chapter 38

Recent Progress in Wood Dissolution and Adhesives from Kraft Lignin Nabuo Shiraishi

Downloaded by UNIV OF PITTSBURGH on September 23, 2013 | http://pubs.acs.org Publication Date: July 31, 1989 | doi: 10.1021/bk-1989-0397.ch038

Department of Wood Science and Technology, Kyoto University, Sakyo-ku, Kyoto 606, Japan

It is becoming apparent that wood components, especially lignin, are chemically modified by solvents during wood dissolution, and that the resulting wood tars or pastes become highly reactive. Attempts have therefore been made to prepare effective adhesives, moldable resins and other products from wood after dissolution in phenols or polyhydric alcohols. This review presents recent progress on wood dissolution, and on the preparation of epoxy and phenol resin adhesives from kraft lignin. V a r i o u s a t t e m p t s have been m a d e to prepare adhesives f r o m l i g n i n . T h e p r e p a r a t i o n of resol resin adhesives has been s t u d i e d especially extensively. T h e i n t r o d u c t i o n o f phenols i n t o the a or /^-position o f the s i d e c h a i n o f the p h e n y l p r o p a n e u n i t (phenolation of l i g n i n ) has been considered a key r e a c t i o n for the f o r m u l a t i o n of these types of adhesives w i t h adequate g l u ability. It is k n o w n t h a t the p h e n o l a t i o n of l i g n i n takes place under a c i d i c c o n d i t i o n s at elevated temperatures. F o r e x a m p l e , about 0.36 moles o f p h e n o l were f o u n d to be c o m b i n e d w i t h each p h e n y l p r o p a n e u n i t of a k r a f t l i g n i n w h e n t h i s was heated i n a glass a m p u l e c o n t a i n i n g 5 g o f l i g n i n , 5 g o f p h e n o l a n d 6 m m o l h y d r o c h l o r i c a c i d at 130°C for 6 h (1). A n o t h e r e x p e r i m e n t (2) i n d i c a t e d t h a t more t h a n 0.43 moles of p h e n o l were i n t r o d u c e d i n t o l i g n i n b y r e a c t i o n w i t h b o r o n trifloride ( B F ) at 6 0 ° C for 4 h . A slight m o d i f i c a t i o n of t h i s reaction (80°C for 3 h) was f o u n d to raise the i n c o r p o r a t i o n of p h e n o l i n t o l i g n i n to 0.62 moles (3). O n the other h a n d , i t has recently been d e m o n s t r a t e d t h a t u n t r e a t e d w o o d a n d / o r w o o d modified by, for e x a m p l e , esterification or e t h e r i f i c a t i o n c a n be dissolved i n several organic solvents i n c l u d i n g phenols (3-10). C h a r a c t e r i z a t i o n of the r e s u l t i n g w o o d tars has revealed a h i g h r e a c t i v i t y a n d the p r o d u c t s c a n be converted r e a d i l y i n t o adhesives, m o l d a b l e resins, etc. 3

0097-6156/89A)397-0488$06.00/0 © 1989 American Chemical Society

In Lignin; Glasser, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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(5-7, 9-11). It is therefore v e r y l i k e l y t h a t the l i g n i n i t s e l f is h i g h l y m o d i f i e d b y solvents, especially b y phenols. I n consequence, effective m o d i f i c a t i o n of l i g n i n can be considered t o enhance i t s r e a c t i v i t y . I n t h i s s t u d y , k r a f t l i g n i n was c h e m i c a l l y m o d i f i e d w i t h p h e n o l a n d w i t h b i s p h e n o l - A i n order t o enhance i t s p o t e n t i a l as adhesive.


Dissolution of

Wood

T h e d i s s o l u t i o n of c h e m i c a l l y m o d i f i e d w o o d has been developed recently (3-7). T h e s o l u b i l i z a t i o n of u n t r e a t e d w o o d was f o u n d t o be possible as w e l l (7-10). A t least three m e t h o d s have been f o u n d t o be a p p l i c a b l e t o the s o l u b i l i z a t i o n of c h e m i c a l l y m o d i f i e d w o o d . T h e first e x p e r i m e n t (4) ( D i rect m e t h o d ) e m p l o y e d severe d i s s o l u t i o n c o n d i t i o n s . F o r e x a m p l e , i n 20150 m i n a t 200-250°C, w o o d samples esterified by a series o f a l i p h a t i c acids c o u l d be dissolved i n b e n z y l ether, styrene oxide, p h e n o l , r e s o r c i n o l , b e n z a l d e h y d e , aqueous p h e n o l s o l u t i o n s , etc. F o r c a r b o x y m e t h y l a t e d , a l l y l a t e d a n d h y d r o x y e t h y l a t e d w o o d s , the c o n d i t i o n s p r o v i d e d for d i s s o l u t i o n i n p h e n o l , r e s o r c i n o l or their aqueous s o l u t i o n s , f o r m a l i n , etc., b y s t a n d i n g or s t i r r i n g at 170°C for 30 t o 60 m i n (5). T h e second m e t h o d of d i s s o l u t i o n is based o n solvolysis ( S o l v o l y s i s m e t h o d ) ( 6 , 7 , 1 1 , 1 2 ) . U n d e r m i l d e r c o n d i t i o n s ( 8 0 ° C for 30 t o 150 m i n ) p h e n o l a t i o n was a c c o m p l i s h e d w i t h a n a p p r o p r i a t e c a t a l y s t , a n d the c h e m i c a l l y m o d i f i e d w o o d was dissolved i n p h e n o l (11). U n d e r s i m i l a r c o n d i t i o n s , woods d e r i v a t i z e d b y a l l y l a t i o n , m e t h y l a t i o n , e t h y l a t i o n , h y d r o x y l a t i o n a n d a c e t y l a t i o n have also been f o u n d t o dissolve i n p o l y h y d r i c alcohols, s u c h as 1 , 6 - h e x a n e d i o l , 1 , 4 - b u t a n e d i o l , 1,2-ethanediol, 1 , 2 , 3 - p r o p a n e t r i o l (glycerol), a n d b i s p h e n o l - A (6). T h e t h i r d m e t h o d of d i s s o l u t i o n is based o n the m i l d c h l o r i n a t i o n of c h e m i c a l l y m o d i f i e d w o o d a c c o r d i n g to S a k a t a a n d M o r i t a (13) ( P o s t c h l o r i n a t i o n m e t h o d ) . C h l o r i n a t i o n , i n fact, resulted i n e n h a n c e d s o l u b i l i t y of c h e m i c a l l y m o d i f i e d w o o d i n solvents, i n c l u d i n g p h e n o l . F o r e x a m p l e , c h l o r i n a t e d c y a n o e t h y l a t e d w o o d does not o n l y dissolve i n cresol even at r o o m t e m p e r a t u r e , b u t i t also dissolves (under heating) i n r e s o r c i n o l , p h e n o l and a LiCl-dimethylacetamide solution. U n m o d i f i e d w o o d has been dissolved i n various n e u t r a l o r g a n i c solvents or solvent m i x t u r e s b y a d o p t i n g the " D i r e c t m e t h o d " described above. T h i s o b s e r v a t i o n was m a d e d u r i n g a n i n v e s t i g a t i o n i n t o the r e l a t i o n s h i p between the degree of c h e m i c a l m o d i f i c a t i o n of w o o d a n d i t s s o l u b i l i t y i n o r g a n i c c h e m i c a l s . So f a r , we have f o u n d t h a t under the c o n d i t i o n s of 200-250° C for 30-150 m i n , b o t h u n t r e a t e d w o o d chips a n d g r o u n d w o o d c a n be dissolved i n the f o l l o w i n g solvents: phenols, bisphenols, p o l y h y d r i c alcohols s u c h as 1,6-hexanediols, 1 , 4 - b u t a n e d i o l , oxyethers such as m e t h y l cellosolve, e t h y l cellosolve, diethylene g l y c o l , t r i e t h y l e n e g l y c o l , p o l y e t h y l e n e g l y c o l , a n d others. T h e d i s s o l u t i o n of m o d i f i e d or u n m o d i f i e d w o o d gives rise t o t a r or paste-like solutions w i t h a h i g h w o o d c o n c e n t r a t i o n ( 7 0 % , for e x a m p l e ) . T h e dissolved w o o d components were f o u n d t o be degraded t o a c e r t a i n


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e x t e n t , a n d t h i s resulted i n m i x t u r e s h a v i n g h i g h r e a c t i v i t y , w i t h the ex c e p t i o n o f those o b t a i n e d b y the " P o s t - c h l o r i n a t i o n m e t h o d . " T h e s o l u t i o n s w i t h h i g h w o o d c o n c e n t r a t i o n c a n be used for the p r e p a r a t i o n of adhesives a n d m o l d i n g s . T h i s has opened a new field for the u t i l i z a t i o n o f w o o d materials. A t t e m p t s ( 6 , 1 1 , 1 2 , 1 4 - 1 7 ) have concentrated o n the p r e p a r a t i o n o f w o o d - b a s e d adhesives b y c u r i n g degraded or c h e m i c a l l y m o d i f i e d w o o d components w i t h reactive solvents. P h e n o l s , bisphenols a n d p o l y h y d r i c alcohols have a l l been s h o w n t o be effective reactive solvents. W i t h these s o l u t i o n s (tars or pastes), p h e n o l - f o r m a l d e h y d e resins (such as resol resins), p o l y u r e t h a n e resins, e p o x y resins, etc., have successfully been p r e p a r e d w i t h h i g h contents o f c h e m i c a l l y m o d i f i e d or u n m o d i f i e d w o o d . C u r e reac t i o n s t a k e place w i t h ease. G l u a b i l i t y studies have d e m o n s t r a t e d excellent performance as w o o d - b a s e d adhesives. A d h e s i v e s were f o u n d t o p e r f o r m s a t i s f a c t o r i l y even i n exterior-grade a p p l i c a t i o n s . A resol-type w o o d - b a s e d adhesive has recently satisfied the requirements o f the Japanese A g r i c u l t u r a l S t a n d a r d for w a t e r p r o o f binders (22). S o l u b i l i z a t i o n c o n d i t i o n s for its p r e p a r a t i o n were 120 m i n at 250° C w i t h e q u a l a m o u n t s o f p h e n o l a n d u n t r e a t e d M a k a m b a (birch) w o o d chips. B y a d d i n g s m a l l a m o u n t s (1.64 p a r t s per 100 p a r t s o f wood-based adhesive) o f a l k y l r e s o r c i n o l as h a r d e n e r , the same adhesive was f o u n d t o b i n d p l y w o o d s even at 120°C. T h i s is a l m o s t 20° C below the t e m p e r a t u r e used for c o m m e r c i a l resol-type adhesives. T h e adhesive satisfied exterior-grade s t a n d a r d s . T h i s means t h a t the r e s o l t y p e w o o d - b a s e d adhesive is s i m i l a r i n i t s r e a c t i v i t y t o u r e a - f o r m a l d e h y d e adhesives, a n d higher t h a n c o m m e r c i a l resol resin adhesives. W o o d s o l u b i l i z a t i o n has a l r e a d y been discussed i n a recent review (18) w i t h s p e c i a l e m p h a s i s o n r e a c t i v i t y . T h e l i g n i n c o m p o n e n t o f w o o d gains r e a c t i v i t y b y m o d i f i c a t i o n w i t h solvent d u r i n g s o l u b i l i z a t i o n . N u c l e o p h i l i c s u b s t i t u t i o n w i t h p h e n o l takes place at the α-position o f l i g n i n sidechains. T h i s d e r i v a t i z a t i o n o f l i g n i n is associated w i t h d e g r a d a t i o n t h a t p l a y s a n i m p o r t a n t role i n m a k i n g the w o o d - p h e n o l s o l u t i o n reactive. T h i s p o i n t was s t u d i e d i n d e t a i l b y r e a c t i n g p h e n o l w i t h isolated l i g n i n , a n d c o n v e r t i n g i t i n t o a n adhesive. T h e f o l l o w i n g s u m m a r i z e s t h i s s t u d y . Epoxy Resin Adhesives from Kraft

Lignin

E p o x y resin adhesives f r o m l i g n i n were r e p o r t e d b y T a i et ai (19,20) i n 1967. S a t i s f a c t o r y g l u a b i l i t y was f o u n d . T h e s o l u b i l i t y i n o r g a n i c solvents, however, was f o u n d t o be p o o r . I n essence, the p h e n o l i c h y d r o x y l groups o f k r a f t l i g n i n were g l y c i d y l a t e d d i r e c t l y . W e have also m a d e a n a t t e m p t to prepare l i g n i n - e p o x y resin adhesives (21). However, i n order t o i m p r o v e i t s r e a c t i v i t y , k r a f t l i g n i n was first p h e n o l a t e d w i t h b i s p h e n o l - A . F o r p h e n o l a t i o n , a s m a l l a m o u n t o f aqueous h y d r o c h l o r i c a c i d or B F - e t h y l etherate was used as c a t a l y s t , a n d t h u s t w o k i n d s o f g l y c i d y l a t i o n m e t h o d s were a d o p t e d (21). T h e p h e n o l a t i o n w i t h b i s p h e n o l - A was f o u n d to enhance the s o l u b i l i t y of the l i g n i n d e r i v a t i v e . I n fact, the l i g n i n - e p o x y resins o b t a i n e d were f o u n d to be c o m p l e t e l y soluble i n c e r t a i n o r g a n i c solvents, i n c l u d i n g acetone. 3


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T h e g l u a b i l i t y of the l i g n i n - e p o x y resin was also f o u n d t o be i m p r o v e d b y p h e n o l a t i o n , a n d w a t e r p r o o f adhesives resulted (21). T h e t e m p e r a t u r e dependence o f the d y n a m i c viscoelastic response was s t u d i e d o n cured l i g n i n - e p o x i d e films prepared w i t h v a r y i n g degrees o f p h e n o l a t i o n (21). T h e results revealed t h a t well-defined differences were f o u n d i n the glass t r a n s i t i o n t e m p e r a t u r e ( T ^ ) , the storage m o d u l u s ( G ' ) i n the r u b b e r y p l a t e a u r e g i o n , a n d the peak height o f the l o g a r i t h m i c decrement (αχ) curve o f the cured films. T ^ a n d G i n the r u b b e r y p l a t e a u region were f o u n d t o increase w i t h a n increase i n degree of p h e n o l a t i o n . F u r t h e r m o r e , the peak height o f the αχ curve was f o u n d t o be higher for samples f r o m less p h e n o l a t e d l i g n i n - e p o x i d e f i l m s . T h i s suggests t h a t the m o r e b i s p h e n o l a t e d l i g n i n forms, the m o r e crosslinks d u r i n g cure, thereby p r o d u c i n g a b e t t e r developed t h r e e - d i m e n s i o n a l network s t r u c t u r e (21).


;

T h e g l u a b i l i t y of two types of epoxy-resins, prepared f r o m l i g n i n s w i t h different degrees o f p u r i t y , was e x a m i n e d . T o k a i P u l p k r a f t l i g n i n F ( < 4 . 5 % sugars) a n d O j i k r a f t l i g n i n ( 1 1 . 7 % sugars) were used. T h e difference i n l i g n i n p u r i t y d i d n o t reveal a n y i n f e r i o r i t y i n either d r y or wet b o n d s t r e n g t h s , b u t enhanced g l u a b i l i t y was n o t e d for the less p u r i fied l i g n i n , suggesting active p a r t i c i p a t i o n o f the sugar c o m p o n e n t s . B o t h e p o x y resins gave satisfactory d r y - a n d w e t - b o n d strengths after 5 m i n of hot-pressing at 140°C. A l l requirements for " F i r s t - C l a s s P l y w o o d " a c c o r d i n g t o the Japanese A g r i c u l t u r e S t a n d a r d ( J A S ) were satisfied. T h e g l u a b i l i t y o f the l i g n i n - e p o x y resin adhesives was f o u n d t o be i m p r o v e d by the a d d i t i o n of c a l c i u m c a r b o n a t e ( 5 0 % b y weight) t o the l i q u i d resin. T h i s m u s t be a t t r i b u t e d t o the n a t u r e o f the weak a l k a l i i n c a l c i u m c a r b o n a t e as a cure accelerator, a n d t o the reinforcement effect o f fillers. Since w o o d surfaces are a c i d i c , the a d d i t i o n o f alkaline fillers effectively alters the p H o f the glue l i n e . T h e s e results were o b t a i n e d by u s i n g h y d r o c h l o r i c a c i d as the c a t a l y s t for the p h e n o l a t i o n r e a c t i o n w i t h l i g n i n . Besides h y d r o c h l o r i c a c i d , BF3 is k n o w n t o be a n effective c a t a l y s t for the i n t r o d u c t i o n of p h e n o l groups i n t o l i g n i n . BF3 is also k n o w n as a c a t a l y s t w h i c h c a n p r o m o t e g l y c i d y l a t i o n not o n l y of phenolic b u t also of alcoholic h y d r o x y l groups. T h u s , the p r e p a r a t i o n of l i g n i n e p o x y resins w i t h B F as c a t a l y s t was s t u d i e d also. T h e e p o x y value f o u n d for the s t a n d a r d l i g n i n - e p o x i d e prepared i n the presence of BF3 was 0.48, whereas t h a t of the epoxide prepared w i t h H C 1 as c a t a l y s t was 0.38. 3

T h e l i g n i n - e p o x i d e resins f r o m the r e a c t i o n w i t h BF3 were also tested as adhesives for p l y w o o d , u s i n g t r i e t h y l e n e t e t r a m i n e as c u r i n g agent w i t h hot-pressing at 140°C. T h e results of adhesion tests showed t h a t the w a t e r p r o o f adhesive strengths were i m p r o v e d b y use o f BF3 as c a t a l y s t . T h e use of B F as c a t a l y s t p e r m i t t e d the a d o p t i o n of hot-pressing t i m e s as short as 3 m i n for p r e p a r i n g t h r e e - p l y p l y w o o d panels w i t h 6 m m thickness. T h i s resulted i n satisfactory w a t e r p r o o f adhesive performance. T h e a d d i t i o n o f c a l c i u m carbonate ( 5 0 % b y weight) t o the l i q u i d adhesive was a g a i n f o u n d t o enhance the w a t e r p r o o f g l u a b i l i t y . 3


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Resol-Type Phenol Resin Adhesives from Kraft

Lignin

T h e r e have been m a n y a t t e m p t s t o prepare resol resin adhesives f r o m k r a f t l i g n i n (23). P h e n o l a t i o n o f l i g n i n c a n , i n these cases, also b e considered a k e y r e a c t i o n . T h a t i s , effective m o d i f i c a t i o n of l i g n i n w i t h p h e n o l c a n enhance i t s r e a c t i v i t y , r e s u l t i n g i n g o o d g l u a b i l i t y . L i g n i n was c h e m i c a l l y m o d i f i e d p r i o r t o r e s i n i f i c a t i o n , a n d the effect o f p h e n o l a t i o n was e x a m i n e d o n the g l u a b i l i t y of the resol r e s i n adhesives (23). T h e p h e n o l a t i o n was p e r f o r m e d either w i t h H C 1 ( 8 0 ° C , 60 m i n ) or w i t h o u t c a t a l y s t ( 2 0 0 ° C , 60 m i n ) . P r i o r t o adhesive t e s t i n g w i t h t h r e e - p l y p l y w o o d , 5 p a r t s of coconut husk powder were m i x e d w i t h 100 p a r t s of the resin. R e s u l t s are s h o w n i n F i g u r e 1. W e t - b o n d tensile-shear adhesion strengths are c o m p a r e d for two k i n d s of adhesives prepared u s i n g different p h e n o l a t i o n c o n d i t i o n s . T h e l i g n i n - r e s o l resin adhesive prepared w i t h o u t c a t a l y s t revealed sufficient w a t e r p r o o f adhesion s t r e n g t h t o meet or exceed the Japanese I n d u s t r i a l S t a n d a r d ( J I S ) requirement (0.98 M P a ) w i t h o n l y 6 m i n h o t - p r e s s i n g at 120°C. B y c o n t r a s t , the adhesive p r e p a r e d w i t h H C 1 as c a t a l y s t required 9 m i n of hot-pressing at the same t e m p e r a t u r e before i t reached the same level of s t r e n g t h . T h i s result emphasizes t h a t the p h e n o l a t i o n w i t h o u t c a t a l y s t results i n better m o d i f i c a t i o n of l i g n i n c o m p a r e d t o t h a t w i t h h y d r o c h l o r i c a c i d as c a t a l y s t . F u r t h e r m o r e , the l i g n i n - r e s o l resin adhesives prepared w i t h o u t c a t a l y s t c a n be c u r e d under c o n d i t i o n s u n d e r w h i c h a m i n o resin adhesives are generally used (i.e., 120°C w i t h a h o t - p r e s s i n g rate o f 1 m i n per 1 m m p l y w o o d t h i c k n e s s ) . I n order t o prepare better adhesives, the effect of p h e n o l a t i o n t i m e , p h e n o l a t i o n t e m p e r a t u r e , r e a c t i o n t i m e for resol r e s i n i f i c a t i o n , a n d degree o f l i g n i n p u r i t y were e x a m i n e d o n the adhesive p r o p e r t i e s . T h e results i n d i c a t e d t h a t o p t i m u m c o n d i t i o n s for p h e n o l a t i o n involve 2 0 0 ° C for 60 m i n a n d 60 m i n resol resinification at 9 0 ° C a n d at p H 9. T h e c o n d i t i o n s for resol resinification correspond w e l l t o those o f the c o n v e n t i o n a l m a n u f a c t u r i n g m e t h o d (24). Tests for the effect of l i g n i n p u r i t y o n the g l u a b i l i t y i n v o l v e d two k i n d s of resol resin adhesives p r e p a r e d f r o m O j i k r a f t l i g n i n ( 8 7 . 8 % p u r i t y ) a n d Tokai P u l p kraft lignin F (> 9 5 % purity). B o t h lignins produced satisf a c t o r y w e t - b o n d adhesion s t r e n g t h t h a t met J I S requirements b y a h o t pressing rate of 1 m i n per 1 m m p l y w o o d thickness. T h e difference i n l i g n i n p u r i t y h a d no effect o n g l u a b i l i t y . T h e same conclusions were d r a w n for e p o x y resin adhesives f r o m k r a f t l i g n i n . T h i s suggests t h a t p o l y s a c charide c o m p o n e n t s m a y be converted i n t o reactive m a t e r i a l s , such as 5h y d r o x y m e t h y l - 2 - f u r f u r a l , f u r f u r a l , etc., t h r o u g h h y d r o l y s i s a n d d e h y d r a t i o n d u r i n g p h e n o l a t i o n (18). F o r c o m p a r i s o n , a c o n v e n t i o n a l resol resin adhesive w i t h o u t l i g n i n was p r e p a r e d (24), a n d i t s g l u a b i l i t y was e x a m i n e d . T h i s resin was f o u n d t o require a hot-pressing rate of at least 1.5 m i n per 1 m m p l y w o o d thickness before a satisfactory w e t - b o n d adhesion s t r e n g t h was achieved at the low h o t - p r e s s i n g t e m p e r a t u r e of 120°C. T h i s indicates t h a t r e p l a c i n g a p a r t of the p h e n o l w i t h l i g n i n does not i m p l y a mere extender a d d i t i o n , b u t t h a t a p o s i t i v e role is achieved w h i c h enhances the r e a c t i v i t y of the adhesive.



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F i g u r e 1. C o m p a r i s o n of w e t - b o n d adhesion strengths o f l i g n i n - r e s o l resin adhesives p h e n o l a t e d w i t h a n d w i t h o u t a c i d c a t a l y s t . L e g e n d : φ : p h e n o l a t i o n w i t h a c i d c a t a l y s t at 80°C for 60 m i n ; Ο p h e n o l a t i o n w i t h o u t c a t a l y s t at 2 0 0 ° C for 60 m i n . Note: N u m e r i c a l values i n parentheses are percentages of w o o d failure; hot-press t e m p e r a t u r e : 120°C. :

F o r the purpose of further e n h a n c i n g the r e a c t i v i t y a n d g l u a b i l i t y of kraft l i g n i n - r e s o l resin adhesives, the a d d i t i o n o f a l k y l r e s o r c i n o l was ex a m i n e d . T h e results of a d d i n g 2-10 p a r t s of a l k y l r e s o r c i n o l t o 100 p a r t s o f l i g n i n - r e s o l resin adhesive p r i o r to a p p l i c a t i o n i n d i c a t e a significant i m p r o v e m e n t i n g l u a b i l i t y , especially i n w a t e r p r o o f g l u a b i l i t y . T h e w a t e r p r o o f g l u a b i l i t y as described b y w e t - b o n d adhesion s t r e n g t h tesing of two types of adhesives prepared w i t h a n d w i t h o u t a l k y l r e s o r c i n o l (10 parts) are c o m p a r e d i n F i g u r e 2. T h e a d d i t i o n of a l k y l r e s o r c i n o l resulted i n a n i m p r o v e m e n t o f w e t - b o n d adhesion s t r e n g t h . T h e adhesive met the requirements for " F i r s t - C l a s s P l y w o o d " a c c o r d i n g to J A S even after o n l y 3 m i n of h o t pressing (a h o t - p r e s s i n g rate of 0.5 m i n per 1 m m p l y w o o d thickness) at 120°C. It was also f o u n d t h a t the l i g n i n - r e s o l resin adhesives satisfied the J I S requirements for n o n - v o l a t i l e content, p H v a l u e , viscosity, gel t i m e , a n d d r y a n d wet adhesion s t r e n g t h . F u r t h e r m o r e , the low t e m p e r a t u r e c u r a b i l i t y t y p i c a l l y f o u n d i n a m i n o resin adhesives c o u l d also be achieved. T h u s , i t c a n be concluded t h a t a n effective u t i l i z a t i o n of l i g n i n is possible w i t h s i m u l t a n e o u s i m p r o v e m e n t of the properties o f resol resin adhesives.



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HOT-PRESS TIME (min.) F i g u r e 2. R e l a t i o n s h i p between hot-press t i m e a n d w e t - b o n d adhesion s t r e n g t h for lignin-resol resin adhesives w i t h a n d w i t h o u t a l k y l r e s o r c i n o l . L e g e n d : Ο w i t h o u t a l k y l r e s o r c i n o l ; φ : w i t h 10 parts a l k y l r e s o r c i n o l . Note: N u m e r i c a l values i n parentheses are percentages o f w o o d failure; p h e n o l a t i o n : 2 0 0 ° C , 60 m i n , w i t h o u t c a t a l y s t ; hot-press: 120°C, 6 m i n . :

Literature C i t e d 1. Kobayashi, Α . ; Haga, T . ; Sato, K . Mokuzai Gakkaishi 1966, 12, 305; Ibid. 1967, 13, 60. 2. Abe, I. Bull. Hokkaido For. Prod. Res. Inst 1970, 55, 1. 3. Shiraishi, N . Kobunshi Kako 1982, 31(11), 500. 4. Shiraishi, N . Japan Patent 1988-1992 (Appl. June 6, 1980). 5. Shiraishi, N . ; Goda, Κ. Mokuzai Kogyo 1984, 39(7), 329. 6. Shiraishi, N.; Onodera, S.; Ohtani, M . ; Masumoto, T . Mokuzai Gak kaishi 1985, 31(5), 418. 7. Shiraishi, N . Tappi Proc. 1987 Inter. Dissolving Pulps Conf., 95-102. 8. Shiraishi, N . ; Tsujimoto, N.; Pu, S. Japan Patent (Open) 1986-261358 (Appl. May 14, 1985). 9. Pu, S.; Shiraishi, Ν . ; Yokota, T . Abst. Papers Presented at 36th Natl. Mtg., Japan Wood Res. Soc. 1986, 179-180. 10. Shiraishi, N . Mokuzai Kogyo 1987, 42(1), 42. 11. Shiraishi, N . ; Kishi, H . J. Appl. Polym. Sci. 1986, 32, 3189. 12. Shiraishi, N.; Ito, H . ; Lonikar, S. V . ; Tsujimoto, N . J. Wood Chem. Technol. 1987, 7(3), 405. 13. Morita, M . ; Shigematsu, K.; Sakata, I. Abst. Papers Presented at 35th Natl. Mtg., Japan Wood Res. Soc. 1985, 215-216. 14. Kishi, H . ; Shiraishi, N . Mokuzai Gakkaishi 1986, 32(7), 520.


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15. Shiraishi, N . Mokuzai Gakkaishi 1986, 32(10), 755. 16. Ono, H.; Sudo, K.; Karasawa, J . Proc. 31st Lignin Forum 1986, 133. 17. Ono, H.; Sudo, K.; Karasawa, J. Abst. Papers Presented at 37th Natl. Mtg., Japan Wood Res. Soc. 1987, 288. 18. Shiraishi, N.; Tamura, Y . ; Tsujimoto, N . Mokuzai Kogyo 1987, 42(11), 492. 19. Tai, S.; Nagata, M . ; Nakano, J . ; Migita, N . Mokuzai Gakkaishi 1967, 13(3), 102. 20. Tai, S.; Nakano, J . ; Migita, N . Mokuzai Gakkaishi 1967, 13(6), 257. 21. Ito, H.; Shiraishi, N . Mokuzai Gakkaishi 1987, 33(5), 393. 22. Pu, S.; Shiraishi, Ν . Abst. Papers Presented at 37th Natl. Mtg., Japan Wood Res. Soc. 1987, 239. 23. Kato, K.; Shiraishi, N . Holzforschung, submitted. 24. Nakarai, Y . ; Watanabe, T. Mokuzai Gakkaishi 1965, 11, 137. RECEIVED February 27,1989


Recent Progress in Wood Dissolution and Adhesives from Kraft Lignin

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