10 Activation of W o o d Surface and Nonconventional Bonding E U G E N E ZAVARIN Forest Products Laboratory, University of California, Berkeley, C A 94804
Nonconventional bonding includes many different methods of bonding wood, all of them radically different from the conventional phenol-formaldehyde and u r e a -formaldehyde and related methods. In many cases the methods rely, at least in principle, on formation of covalent bonds to wood surfaces. Some of the systems involve direct covalent bonds between the wood surfaces, some employ bifunctional monomers for joining the surfaces, and others covalently bridge the surfaces by polymeric chains. The last methods appear to bridge the gaps between wood surfaces with the least difficulty. The methods include gluing by spent sulfite liquor at low pH; gluing by a mixture of spent sulfite liquor, furfuryl alcohol, and maleic anhydride with oxidative surface activation; gluing by water-soluble carbohydrates with a catalyst; and gluing by isocyanates. Some methods are at the pilot plant stage, some are at the laboratory stage, while gluing by isocyanates has been in industrial use for some time. The products often exhibit improved dimensional stability and water resistance, but tend to suffer from abnormally high variability in the mechanical properties. Progress is handicapped by insufficient knowledge of the chemical composition of wood surfaces as well as of the chemical processes involved in bonding. The reacting wood surfaces are commonly richer in lignin than the bulk of the wood and are covered with a layer of polar and nonpolar extractives. This coverage as well as chemical transformations during surface preparation and history can influence the formation of covalent bonds to wood. Acid or oxidant activators can be required for bond formation. Such activators could promote cross-linking of the introduced polymers without formation of covalent bonds to wood surface, could 0065-2393/84/0207-0349/$13.80/0 © 1984 American Chemical Society
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change the polymer and enable it to form covalent bonds with the wood surface, or could change the wood surface and enable it to form covalent bonds with the polymer. Some systems involve the lignin portion of wood for formation of covalent bonds (lignophilic systems), others preferentially form covalent bonds with cellulose or hemicelluloses (cellophilic systems).
JL H E T E R M N O N C O N V E N T I O N A L B O N D I N G h a s b e e n i n u s e f o r s o m e t i m e ,
b u t i t s c h o i c e c a n b e h a r d l y t e r m e d as f o r t u n a t e . I n t h e first p l a c e i t is n e g a t i v e , i . e . , i t is b a s e d o n c o n c e p t s o u t s i d e o f t h e s c o p e o f d e f i n i t i o n . S e c o n d l y , it l a c k s t i m e s t a b i l i t y ; w h a t is n o n c o n v e n t i o n a l t o d a y m i g h t b e c o n v e n t i o n a l t o m o r r o w . T h i r d l y , i t is t o o b r o a d , as i t c a n relate to n o n c o n v e n t i o n a l glues, n o n c o n v e n t i o n a l practices, or e v e n to n o n c o n v e n t i o n a l e q u i p m e n t . P r o b a b l y t h e o n l y r e a s o n for u s i n g t h i s t e r m is t h e l a c k o f a p p r o p r i a t e a l t e r n a t i v e s . In c o m m o n usage the t e r m n o n c o n v e n t i o n a l b o n d i n g of w o o d has b e e n a p p l i e d s o m e w h a t i m p r e c i s e l y to a g r o u p o f b o n d i n g p r o cedures i n v o l v i n g a w i d e variety of c h e m i c a l m o n o m e r i c or p o l y m e r i c reagents. T h e s e reagents are different f r o m the conventionally u s e d a d h e s i v e s , s u c h as 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 . T h e w o r d " d i f f e r e n t " is r a t h e r a m b i g u o u s a n d a l l o w s f o r a n a p p r e c i a b l e gray area. I n this c h a p t e r o n l y those b o n d i n g agents that i n v o l v e c o m p l e t e l y n e w ways of b o n d i n g a n d c r o s s - l i n k i n g are i n c l u d e d , and the agents that b e a r a p p r e c i a b l e s i m i l a r i t y to 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 (e.g., p h e n o l - f o r m a l d e h y d e r e s i n s i n c l u d i n g t a n n i n as a p a r t i a l p h e n o l s u b s t i t u t e ) a r e e x c l u d e d . M o s t n o n c o n v e n t i o n a l b o n d i n g systems share the idea of cova l e n t l y b o n d e d w o o d surfaces. I n c o n v e n t i o n a l b o n d i n g the w o o d s u r f a c e r e p r e s e n t s , o r is t h o u g h t t o r e p r e s e n t , a s e c o n d a r y r e a c t i o n p a r t n e r o n l y , w i t h c o v a l e n t b o n d i n g r e s t r i c t e d m a i n l y to c r o s s - l i n k i n g reactions of the b o n d i n g agents ( 1 - 3 ) . S o m e of the methods i n v o l v i n g c o v a l e n t l y b o n d e d s u r f a c e s r e q u i r e a c t i v a t i o n o f t h e w o o d s u r f a c e s as a necessary p r e r e q u i s i t e for successful b o n d i n g . A c t i v a t i o n of w o o d ' s e x t e r n a l surfaces causes a change i n the c h e m i c a l b e h a v i o r of the w o o d a n d e n a b l e s t h e c o m p o n e n t s o f w o o d e i t h e r to u n d e r g o n e w 1
E x t e r n a l w o o d surfaces, occasionally s i m p l y designated as w o o d surfaces, are c o m m o n l y artificially created a n d c o m p r i s e the interfaces b e t w e e n w o o d a n d the external w o r l d . Internal w o o d surfaces c o m p r i s e the interfaces b e t w e e n c e l l walls a n d c e l l l u m e n a . T h e d e p t h of the surface layer is not restricted to m o n o m o l e c u l a r thickness, but is r e g a r d e d as the d e p t h necessary to p r o d u c e a certain surface effect. T h e definition o f d e p t h is thus relative a n d d e p e n d s o n the type of interaction. C o n s e q u e n t l y , analytical results a r r i v e d at b y different methods o f surface analysis are not strictly comparable. 2
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r e a c t i o n s o r to r e a c t at a n i n c r e a s e d r a t e . S u c h i n c r e a s e d r e a c t i v i t y r e s u l t s i n c e r t a i n e x t e r n a l effects ( e . g . , i m p r o v e d a d h e s i o n ) . B e c a u s e o f t h e v a r i e t y o f t h e s e effects, a c t i v a t i o n is a r e l a t i v e c o n c e p t a n d d e p e n d s u p o n t h e n a t u r e o f t h e effect a n d c a n n o t b e d i s c u s s e d p e r se. T h u s , i n a d d i t i o n t o n o n c o n v e n t i o n a l b o n d i n g , a c t i v a t i o n o f l i g n o c e l l u l o s i c m a t e r i a l s forms t h e basis of grafting of organic m o n o m e r s to l i g n o c e l l u l o s i c s u r f a c e s (4) a n d is r e s p o n s i b l e f o r c e r t a i n i m p r o v e m e n t s i n t h e p e r f o r m a n c e o f t h e s u r f a c e c o a t i n g s o f w o o d (5-8). A d v a n t a g e s of n o n c o n v e n t i o n a l b o n d i n g are associated w i t h co v a l e n t l y b o n d e d w o o d surfaces (external a n d to s o m e e x t e n t i n t e r n a l ) and i n c l u d e d i m e n s i o n a l stability of the products. Occasionally i n c r e a s e d b r i t t l e n e s s a n d a loss i n m e c h a n i c a l p r o p e r t i e s d u e to a c i d i c degradation of carbohydrates are observed. S o m e n o n c o n v e n t i o n a l b o n d i n g m e t h o d s are based o n the use of agricultural by-products, i.e., on nonpetroleum-based materials; this use constitutes another advantage. S o m e n o n c o n v e n t i o n a l l y b o n d e d materials p r o d u c e r e d u c e d a m o u n t s of toxic gaseous m a t e r i a l s , s u c h as f o r m a l d e h y d e , t h a t m a k e t h e m p r e f e r a b l e to p h e n o l formaldehyde products and u r e a - f o r m a l d e h y d e resins. E c o n o m i cally, t h e n o n c o n v e n t i o n a l m e t h o d s do not offer a n y p a r t i c u l a r a d v a n t a g e s , a l t h o u g h t h e y a p p e a r to b e c o m p e t i t i v e w i t h t h e c o n ventional methods. This chapter includes discussions on the chemical composition o f t h e w o o d surface p r i o r to i n t e r a c t i o n s w i t h b o n d i n g a g e n t s — a topic often n e g l e c t e d i n discussions of the surface reactions of w o o d ; nonconventional b o n d i n g methods based on direct, covalent, or wood-to-wood b o n d i n g ; b o n d i n g through intermediacy of bivalent molecules; b o n d i n g through intermediacy of a cross-linked polymer, c o m m o n l y c o v a l e n t l y attached to w o o d surfaces; a n d f u n d a m e n t a l research i n these areas.
Wood Surface Composition Prior to Activation or Bonding T h e c h e m i c a l c o m p o s i t i o n of a w o o d surface does not necessarily c o r r e s p o n d to t h e c h e m i c a l c o m p o s i t i o n o f t h e b u l k o f t h e w o o d a n d is a f u n c t i o n o f t h e c o n d i t i o n s a n d m e t h o d s o f s u r f a c e f o r m a t i o n ; t h e r e d i s t r i b u t i o n of extractives f o l l o w i n g or d u r i n g the surface f o r m a t i o n ; the i n c o r p o r a t i o n of foreign materials d u r i n g surface f o r m a t i o n a n d thereafter; a n d t h e c h e m i c a l c h a n g e s i n t i m e d u e to i n t e r a c t i o n s w i t h a i r - o x y g e n , light, a n d other c h e m i c a l and physical reagents. A l t h o u g h t h e k n o w l e d g e o f t h e c h e m i c a l c o m p o s i t i o n o f t h e s u r f a c e is of great i m p o r t a n c e i n u n d e r s t a n d i n g the surface p e r f o r m a n c e i n n o n conventional b o n d i n g , the a m o u n t of information c u r r e n t l y available o n t h e a b o v e a r e a s is s c a r c e . Conditions and Methods of W o o d Surface F o r m a t i o n . The
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conditions a n d m e t h o d s of w o o d surface formation can strongly i n fluence the percentages of lignin, hemicelluloses, a n d cellulose i n t h e s u r f a c e l a y e r o f w o o d . W i t h t r a n s v e r s e s u r f a c e s (cuts m a d e p e r p e n d i c u l a r to w o o d t r u n k , i . e . , p e r p e n d i c u l a r to t h e l e n g t h o f t h e tracheids), the percentages of above w o o d constituents should not d e v i a t e m u c h f r o m those o f total w o o d . S u c h surfaces are h o w e v e r less p r a c t i c a l l y i m p o r t a n t t h a n o t h e r surfaces. H o w e v e r , t h e s i t u a t i o n is d i f f e r e n t w i t h r a d i a l a n d t a n g e n t i a l s u r f a c e s . M o r p h o l o g i c a l e v i dence from microscopic studies [including scanning electron micros c o p y ( S E M ) ] o f w o o d s u r f a c e s a n d w o o d fibers h a s b e e n p r o v i d e d f o r s p r u c e a n d b i r c h w o o d (9-15) a n d on black spruce. I n t h e w o r k o n b l a c k s p r u c e [Picea mariana ( M i l l . ) B S P ] (14, 15) t h e w o o d s u r f a c e s w e r e p r o d u c e d at t e m p e r a t u r e s r a n g i n g b e t w e e n —190 a n d 2 5 0 °C b y t a n g e n t i a l a n d r a d i a l tensile failures. T h e l o w softening p o i n t o f h e m i c e l l u l o s e s ( 5 0 - 6 0 °C) a n d of l i g n i n ( 9 0 - 1 0 0 °C), t h e t w o materials that b i n d t h e microfibrils o f the c e l l w a l l , strongly i n f l u e n c e d t h e m o r p h o l o g y o f t h e surfaces. T h u s w i t h b o t h radial a n d tangential failures, the percent of tracheids broken b y transwall failure decreased b e t w e e n 0 a n d 200 °C from 4 0 - 5 0 % d o w n t o ~ 0 % . F u r t h e r m o r e , w i t h t a n g e n t i a l s u r f a c e s t h e fiber faces p r o d u c e d at o r b e l o w 1 0 0 ° C r e v e a l e d m a i n l y t h e S s u r f a c e s t r u c t u r e ; above 150 °C a p r e d o m i n a n t l y p r i m a r y w a l l s t r u c t u r e , h e a v i l y e m bedded i n , or covered b y an amorphous matrix of lignin and h e m i celluloses was p r o d u c e d . T h e results suggested that w i t h an increase in temperature the w o o d fibers are more easily separated, then b r o k e n , f r o m p a r e n t w o o d ( F i g u r e 1). B e c a u s e t h e r a t i o o f h e m i c e l luloses to c e l l u l o s e percentages a n d t h e l i g n i n percentage increases f r o m t h e s e c o n d a r y w a l l t o t h e m i d d l e l a m e l l a (16-18) the results also suggest that w o o d surfaces o f different c h e m i c a l c o m p o s i t i o n are p r o d u c e d u n d e r different c o n d i t i o n s . T h i s is i m p o r t a n t i n respect to t h e reactions c o n n e c t e d w i t h t h e activation o f w o o d surfaces; for e x a m p l e , o x i d i z i n g , a c t i v a t i n g a g e n t s s u c h as h y d r o g e n p e r o x i d e ( H 0 ) a n d c e r t a i n n i t r a t e s r e a c t p r e f e r e n t i a l l y w i t h l i g n i n (19, 20). r
2
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M e t h o d o l o g i e s a l l o w i n g direct assessment of the c h e m i c a l c o m position o f t h e w o o d surface are those based o n A u g e r spectroscopy and p a r t i c u l a r l y o n electron spectroscopy for c h e m i c a l analysis ( E S C A ) . E x c e p t for o n e m a r g i n a l l y successful a t t e m p t to d e t e r m i n e t h e s u l f u r d i s t r i b u t i o n o n t h e surface o f a s u l f o n a t e d cross s e c t i o n of a b l a c k s p r u c e w o o d s a m p l e (21), A u g e r s p e c t r o s c o p y h a s n o t b e e n used on wood. A p p r e c i a b l y m o r e w o r k has b e e n d o n e w i t h E S C A , h o w e v e r . F r o m t h e results o f E S C A a ratio o f oxygen to c a r b o n atoms o n the s u r f a c e (NQ/N ) c a n b e calculated. T h e o r e t i c a l l y for p u r e cellulose t h i s r a t i o i s 0 . 8 3 , f o r m i l l e d w o o d l i g n i n f r o m c o n i f e r s i t is a r o u n d C
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TEMPERATURE,°C Figure 1. Percentage of tracheids broken by transwall failure on the tan gential fracture surface as a function of temperature (15). 0.37, a n d from hardwoods a r o u n d 0.43. F o r nonpolar extractives ( m o n o t e r p e n o i d s , r e s i n a c i d s , f a t t y acids) t h i s r a t i o is a r o u n d 0 . 1 0 o r lower. T h e E S C A analyses of lignocellulosic materials available i n c l u d e t h o s e o n l i g n i n a n d l i g n o c e l l u l o s i c f i b e r s (22-24) on lignocel l u l o s i c f i b e r s a n d c h i p s o f p i n e w o o d (25), a n d o n m a p l e w o o d (26). T h e N Q / N v a l u e s for l i g n i n a n d p u r e c e l l u l o s i c fibers (cotton, filter p a p e r ) a g r e e d r e a s o n a b l y w e l l w i t h t h e c a l c u l a t e d v a l u e s , p a r t i c u l a r l y i f t h e fibers w e r e e x t r a c t e d w i t h E t O H , a c e t o n e , o r s i m i l a r solvents. A s s u m i n g that the surface of lignocellulosic materials was c o m p o s e d after e x t r a c t i o n of o n l y cellulosics a n d l i g n i n , D o r r i s a n d G r a y (22, 23) e s t i m a t e d 3 0 - 4 5 % o f l i g n i n o n t h e s u r f a c e o f g r o u n d w o o d p u l p fibers. T h i s is a p p r e c i a b l y h i g h e r t h a n t h e l i g n i n c o n t e n t of wood. I n a related w o r k 7% lignin i n the b u l k and 17% on the s u r f a c e o f e x t r a c t e d s u l f i t e p u l p s w e r e r e p o r t e d (27, 28). T h e a m o u n t o f l i g n i n o n t h e s u r f a c e i n c r e a s e d w i t h t e m p e r a t u r e o f d e f i b r a t i o n (25) w h i c h a g r e e s w i t h t h e w o r k o f K o r a n (15, 16). T h e e x t r a c t e d p i n e c h i p s h a d a n N /N ratio s i m i l a r to p u r e l i g n i n . T h e s e results suggest that l i g n i n was p r o b a b l y the m a i n c h e m i c a l c o m p o n e n t of the w o o d s u r f a c e (Table I). C
Q
C
W i t h o u t e x t r a c t i o n w i t h p o l a r s o l v e n t s t h e Nq/N ratios w e r e regularly m u c h lower. T h e s e ratios w e r e w e l l b e l o w those of cellulose and l i g n i n for m a p l e w o o d a n d p i n e w o o d , a m o u n t i n g to 0.15 a n d 0 . 2 6 , r e s p e c t i v e l y . T h e e x t r a c t i o n effect w a s d u e t o n o n p o l a r w o o d e x t r a c t i v e s ; n o effort h a s b e e n m a d e to i d e n t i f y t h e s e e x t r a c t i v e s . A l t h o u g h the l i k e l i h o o d of extractives c o v e r i n g the surface cannot b e C
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T a b l e I. E S C A NJN Ratios for Various Lignocellulosic Materials Ç
Lignocellulosic
Material
NQ/N
Calculated Cellulose M i l l e d wood lignin Softwood Hardwood N o n p o l a r extractives Found Filter paper M i l l e d w o o d l i g n i n (spruce) Pine chips Acetone extracted Maple wood H N 0 treated
C
Ratio
0.83 0.37 0.43 0.10 0.79-0.83 0.39 0.26 0.42 0.15 0.43
3
d i s m i s s e d , i t is a l s o p o s s i b l e t h a t p a r t o f t h e o b s e r v e d effect r e l a t e s to c h e m i c a l changes d u r i n g surface p r e p a r a t i o n . S u c h changes c o u l d i n v o l v e d e h y d r a t i o n , c o u l d l o w e r t h e N /N ratio, and could render some of the products of decomposition more soluble i n polar solvents. Q
C
A l t h o u g h e l e c t r o n i c a b s o r p t i o n s p e c t r o s c o p y i n its r e f l e c t a n c e v a r i a t i o n h a s b e e n u s e d t o assess t h e d i s t r i b u t i o n o f l i g n i n a c r o s s t h e c e l l w a l l s o f t r a c h e i d s (16, 17), n o m e t h o d b a s e d o n e l e c t r o n i c o r I R s p e c t r o s c o p y has b e e n d e v e l o p e d to e n a b l e q u a n t i t a t i v e d e t e r m i n a tion of the l i g n i n c o n t e n t of w o o d surfaces. R e d i s t r i b u t i o n of E x t r a c t i v e s . R e d i s t r i b u t i o n of the extractives d u r i n g o r after surface p r e p a r a t i o n c o u l d result i n t h e i r d e p o s i t i o n o n t h e surface i n l a r g e r a m o u n t s . I f a surface was c r e a t e d p r i o r to the r e m o v a l of m o i s t u r e f r o m w o o d , or i f the w o o d s u b s e q u e n t l y was w e t t e d (e.g., aqueous solution of a reagent used i n w o o d activation), the w a t e r - s o l u b l e , p o l a r extractives are l i k e l y to m i g r a t e a n d b e c o m e d e p o s i t e d on the w o o d surface d u r i n g the process of d r y i n g . A n u m b e r of studies discussed discolorations d u r i n g kiln or air d r y i n g of w o o d (29-32). S o m e nonpolar, water-insoluble extractives, such as f a t t y a n d r e s i n a c i d s , a l s o m i g r a t e d u r i n g d r y i n g a n d b e c o m e d i s t r i b u t e d o n the w o o d surface. Several studies o n adhesion inactivat i o n of w o o d surfaces are available. T h e vapor-phase translocation of s t e a r i c a c i d as a m o d e l c o m p o u n d w a s s t u d i e d (34) at t e m p e r a t u r e s b e t w e e n 2 5 a n d 105 ° C b y u s i n g w o o d p u l p h a n d s h e e t s . A l t h o u g h t h e r a t e o f t r a n s l o c a t i o n is s t r o n g l y d e p e n d e n t u p o n t e m p e r a t u r e , i t c a n t a k e p l a c e e v e n at t h e a m b i e n t t e m p e r a t u r e . T h e r e d i s t r i b u t i o n o f e x t r a c t i v e s d u r i n g w o o d d r y i n g is s t r o n g l y i n f l u e n c e d b y t h e d r y i n g
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m e t h o d (34). T h e n o n p o l a r t o l u e n e - s o l u b l e e x t r a c t i v e s o f Pinus taeda L . (resin acids, fatty m a t e r i a l s , t u r p e n t i n e c o m p o n e n t s , steroids, a n d o t h e r unsaponifîables) w e r e f o u n d to b e c o m e strongly e n r i c h e d i n the outer layer of w o o d d u r i n g kiln d r y i n g , although practically no e n r i c h m e n t took place i n a i r - d r y i n g of w o o d . H o w e v e r , o n the basis o f w o o d a n a t o m i c a l c o n s i d e r a t i o n s (Betula alleghaniensis Britton— y e l l o w b i r c h ) i t w a s c o n c l u d e d (35) t h a t v a p o r t r a n s p o r t o f f a t t y a c i d s f r o m i n s i d e o f w o o d to t h e surface was c o n c l u d e d to b e h i g h l y u n likely, e x c e p t for the r e g i o n close to the surface. I n H e m i n g w a y ' s o p i n i o n (35), t h e a m o u n t o f f r e e s a t u r a t e d f a t t y a c i d s ( i . e . , n o t o c c u r r i n g as g l y c e r i d e s ) w a s t o o s m a l l i n t h i s r e g i o n t o i n t e r f e r e w i t h g l u i n g t h r o u g h surface deposition. H e favored the deposition of the p r o d u c t s o f a i r - o x i d a t i o n o f l i n o l e i c a c i d (free a n d b o u n d as g l y c e r i d e ) o n the surface. A d d i t i o n a l w o r k d e a l i n g w i t h surface d e p o s i t i o n of n o n p o l a r ex t r a c t i v e s is t h a t o f S u c h l a n d a n d S t e v e n s (36), H a n c o c k (37), C h e n (38), C h o w (39), a n d T r o u g h t o n a n d C h o w (40). I n t h e last w o r k , t h e s u r f a c e o f Picea glauca ( M o e n c h ) Voss v e n e e r was c o v e r e d w i t h a l a y e r o f s i l i c i c a c i d p o w d e r , t h e v e n e e r w a s h e a t e d at 1 5 0 ° C f o r various a m o u n t s of t i m e , a n d the m i g r a n t acetone extractives that b e c a m e a d s o r b e d to s i l i c i c a c i d w e r e i s o l a t e d . T h e t i m e of h e a t i n g ( 2 0 - 8 0 min) d i d not i n f l u e n c e the percent of total acetone solubles t h a t m i g r a t e d , w h i c h a m o u n t e d to a n a v e r a g e o f 0 . 1 1 % f o r h e a r t w o o d a n d 0 . 1 8 % f o r s a p w o o d ( d r y w o o d p e r c e n t basis). C o n v e r s e l y , t h e a m o u n t of free fatty acids o n the surface i n c r e a s e d f r o m 65 p p m for 2 0 m i n to 74 p p m for 80 m i n o f d r y i n g t i m e . This supports the previously mentioned work based on E S C A that n o n p o l a r extractives c a n b e d e p o s i t e d o n w o o d surface d u r i n g its p r e p a r a t i o n a n d s u b s e q u e n t h i s t o r y . D e p o s i t i o n of F o r e i g n M a t e r i a l s . F o r e i g n materials are d e p o s i t e d o n t h e surface o f w o o d d u r i n g a n d after surface f o r m a t i o n . S o m e d e p o s i t s , s u c h as d u s t a n d w a t e r o f c o n d e n s a t i o n , a r e r e l a t e d to w o o d storage, a n d t h e i r a m o u n t c a n b e c o n t r o l l e d i n p r i n c i p l e . T h e others are, h o w e v e r , c o n n e c t e d w i t h the m e t h o d s of surface formation. T h u s i n various m a c h i n i n g operations small amounts of m e t a l , p r i m a r i l y i r o n , f r o m c u t t i n g p a r t s a r e l i k e l y to b e t r a n s f e r r e d to w o o d surfaces. A l t h o u g h , d u e to t h e i r m i n u t e a m o u n t s , s u c h m a terials are n o t g e n e r a l l y l i k e l y to i n f l u e n c e t h e c h e m i c a l b e h a v i o r o f the w o o d surfaces greatly, i n s o m e instances t h e i r p r e s e n c e can b e felt. T h u s , i n interactions of l i g n o c e l l u l o s i c materials w i t h H 0 , t r a c e s o f i r o n c a n e x e r t a n a p p r e c i a b l e c a t a l y t i c effect o n t h e r a t e o f H 0 d e c o m p o s i t i o n . I n laboratory e x p e r i m e n t s , traces of i r o n can b e r e m o v e d e a s i l y b y t r e a t m e n t w i t h c h e l a t i n g a g e n t s , s u c h as s o d i u m salts o f e t h y l e n e d i a m i n e t e t r a a c e t i c a c i d ( E D T A ) a n d i r o n r e i n t r o 2
2
2
2
356
T H E CHEMISTRY OF SOLID WOOD
d i i c e d i n c o n t r o l l e d q u a n t i t i e s as d e s i r e d . I n i n d u s t r i a l p r a c t i c e , h o w ever, the q u a n t i t a t i v e c o n t r o l of the traces of i r o n or other catalytically acting materials w o u l d represent a m o r e difficult p r o b l e m . Chemical Changes. T h e c h e m i s t r y of the w o o d surface can be c h a n g e d d u r i n g a n d a f t e r p r e p a r a t i o n o f t h e s u r f a c e d u e to i n t e r a c t i o n w i t h various physical a n d c h e m i c a l natural reagents. Some sawing conditions, c o m m o n l y w i t h excessive saw vibrations, cause the w o o d s u r f a c e t o b u r n . A l t h o u g h n o t m u c h is k n o w n a b o u t t h e t e m p e r a t u r e s of the w o o d surface d u r i n g sawing, the temperatures of plane c i r c u l a r saws w e r e f o u n d to b e a b o u t 4 0 - 6 0 ° C , occasionally 100 °C, a n d e v e n 160 ° C , a b o v e t h e a m b i e n t t e m p e r a t u r e , d e p e n d i n g u p o n t h e d i s tance f r o m the t e e t h . T h e t e m p e r a t u r e s of the saw teeth are g e n e r a l l y a p p r e c i a b l y h i g h e r , r e a c h i n g as h i g h as 7 7 4 ° C (41, 42). T h u s , t h e possibilities are g i v e n for p y r o l y t i c a n d oxidative changes o n w o o d s u r f a c e , a l t h o u g h t h e t i m e s o f e x p o s u r e a r e v e r y s h o r t a n d t h e effects c o r r e s p o n d i n g l y less. A d d i t i o n a l cases of s o l i d - w o o d e x p o s u r e to e l e vated temperatures are m e t i n d r y i n g w o o d particles a n d veneer. T h e d e g r a d a t i o n o f w o o d at m o d e r a t e l y e l e v a t e d t e m p e r a t u r e s o r at s h o r t e x p o s u r e s t o h i g h e r t e m p e r a t u r e s i n t h e p r e s e n c e o f a i r is c o m p o s e d o f p y r o l y t i c a n d o x i d a t i v e c h a n g e s . A t l o n g e r e x p o s u r e s t o h i g h e r t e m p e r a t u r e s t h e c o m b u s t i o n p r o c e s s sets i n — a u t o c a t a l y t i c pyrolytic decomposition coupled w i t h oxidation of the p r o d u c e d v o l atiles a n d char. P y r o l y t i c a n d oxidative changes of cellulose, h e m i c e l l u l o s e s , a n d l i g n i n at m o d e r a t e t e m p e r a t u r e s p r o c e e d i n d e p e n dently of each other, i.e., w o o d behaves like a m i x t u r e of these m a t e r i a l s (43). P y r o l y s i s of cellulose was a subject of n u m e r o u s i n v e s t i g a t i o n s a n d h a s b e e n r e v i e w e d s e v e r a l t i m e s (44—48). T h e p r o cess b e g i n s w i t h d e p o l y m e r i z a t i o n o f t h e p o l y s a c c h a r i d e s b y t r a n s g l y c o s y l a t i o n to y i e l d g l u c o s a n a n d o t h e r m o n o s a c c h a r i d e a n d o l i g o s a c c h a r i d e d e r i v a t i v e s . C o n c u r r e n t l y , d e h y d r a t i o n to u n s a t u r a t e d c o m p o u n d s t a k e s p l a c e (46). W i t h l i g n i n t h e l o w t e m p e r a t u r e d e c o m p o s i t i o n is d o m i n a t e d b y c o n d e n s a t i o n s a n d f o r m a t i o n o f e t h e r l i n k ages b e t w e e n t h e η - p r o p y l s i d e c h a i n s , a n d b y g e n e r a t i o n o f a l k y l a r y l b o n d s , w h i c h is p a r a l l e l e d b y d e h y d r a t i o n r e a c t i o n s t h a t f o r m d o u b l e b o n d s i n t h e s i d e c h a i n s (49, 50). O x i d a t i o n o f c e l l u l o s e a p p a r e n t l y t a k e s p l a c e at o r a b o v e 1 4 0 ° C a n d is a c c o m p a n i e d b y d e polymerization a n d formation of carbonyl and carboxyl groups, fol l o w e d b y s o m e decarboxylation. M o i s t u r e strongly catalyzes the p r o c e s s (51, 52, 53). T h e i n f o r m a t i o n on the p y r o l y t i c a n d oxidative changes that o c c u r o n w o o d surfaces r e s u l t i n g f r o m the h i s t o r y of t h e i r f o r m a t i o n is u n s a t i s f a c t o r y . To a l a r g e e x t e n t s u c h i n f o r m a t i o n is c o n n e c t e d w i t h i n v e s t i g a t i o n s o f s u r f a c e i n a c t i v a t i o n t o w a r d c o n v e n t i o n a l g l u i n g (54),
10.
ZAVARIN
Nonconventional
Bonding
357
or w i t h the d i m e n s i o n a l s t a b i l i z a t i o n of w o o d b y exposure to m o d erately elevated temperatures. A loss o f h y g r o s c o p i c i t y b y p r o l o n g e d h e a t i n g o f s o l i d c e l l u l o s i c m a t e r i a l s t o 1 0 0 ° C o r h i g h e r w a s e x p l a i n e d b y g r a d u a l loss o f h y d r o x y l s (55). A q u a n t i t a t i v e c o r r e l a t i o n w a s o b t a i n e d b e t w e e n t h e loss o f h y g r o s c o p i c i t y a n d loss o f w e i g h t b y u s i n g y e l l o w p o p l a r a n d l o b l o l l y p i n e w o o d s a m p l e s h e a t e d to 200 °C for 5 m i n . T h i s c o r r e l a t i o n was e x p l a i n e d b y f o r m a t i o n of i n t r a m o l e c u l a r epoxy groups b e t w e e n h y d r o x y l s 2 a n d 3 o f a n h y d r o u n i t s o f c e l l u l o s e (56); i n t e r m o l e c u l a r e t h e r l i n k a g e s a p p a r e n t l y d o n o t f o r m (57). T h e i n c r e a s e d d i m e n sional s t a b i l i z a t i o n o f w o o d after h e a t i n g it to 300 °C was e x p l a i n e d by the decomposition of hygroscopic hemicelluloses and other car bohydrates, followed b y condensation or p o l y m e r i z a t i o n of the r e s u l t i n g f u r a n - t y p e c o m p o u n d s (58). C h a n g e s i n c h e m i s t r y o f m i c r o s e c t i o n s o f w o o d u s e d as m o d e l s f o r s u r f a c e l a y e r s o f w o o d o f Picea glauca ( M o e n c h ) V o s s w e r e s t u d i e d b y C h o w a n d M u k a i (39, 59) b e t w e e n 100 a n d 2 4 0 °C i n a i r a n d i n n i t r o g e n . B e l o w 180 °C t h e changes w e r e associated m a i n l y w i t h o x i d a t i o n , a n d a b o v e 180 °C they w e r e of m i x e d pyrolytic and oxidative nature. T h e h y d r o x y l a b s o r p t i o n i n I R s p e c t r a d e c r e a s e d w i t h t i m e at 180 ° C , t h e c o l o r o f w o o d darkened, a n d both crystallinity and degree of polymerization ( D P ) of cellulose decreased. T h e I R C = 0 absorption of ester a n d c a r b o x y l groups first d e c r e a s e d a n d t h e n i n c r e a s e d w i t h t e m p e r a t u r e . E x t r a c t i v e s w e r e f o u n d to c a t a l y z e t h e r a t e o f o x i d a t i o n . T h i s c a t a l y s i s is p r o b a b l y w h y r e f i n e d f i b e r s g i v e b e t t e r m e d i u m - d e n s i t y f i b e r b o a r d t h r o u g h a n i n c r e a s e i n o x i d a t i o n m o i e t i e s at t h e s u r f a c e . I n c r e a s e d t e m p e r a t u r e s are also l i k e l y to l e a d to changes i n ex tractive m a k e u p o n the w o o d surface. P o l y m e r i z a t i o n of tannins a n d m o n o m e r i c p h e n o l i c materials to s y n t h e t i c p h l o b a p h e n e s a n d s i m i l a r m a t e r i a l s is l i k e l y t o t a k e p l a c e . U n s a t u r a t e d f a t t y a c i d s s u c h as l i n oleic acid apparently can cleave oxidatively, and the resulting smaller m o l e c u l a r w e i g h t f r a g m e n t s a t t a c h t h e m s e l v e s to t h e surface of w o o d (35). A s i d e from special circumstances the changes i n the chemistry of the w o o d surface d u e to e x p o s u r e to l i g h t d u r i n g surface f o r m a t i o n a n d t h e r e a f t e r ( d r y i n g , s t o r a g e ) a r e o f l i t t l e i m p o r t a n c e to n o n c o n v e n t i o n a l b o n d i n g ( e x c e p t i n g l i g h t as a p o t e n t i a l a c t i v a t o r ) . T h e r e a c tions are rather c o m p l i c a t e d a n d d e p e n d u p o n the w a v e l e n g t h a n d intensity of light, temperature, time of exposure, moisture content of wood, atmospheric composition, and presence of light-absorbing s u b s t a n c e s ( a c t i v a t o r s ) (51, 60-64). T h e surface changes i n c l u d e for m a t i o n of free radicals, c h a i n scission, d e h y d r o g e n a t i o n a n d d e h y droxymethylation of cellulose and splitting of double bonds, forma-
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tion of phenoxy radicals a n d q u i n o n e structures, and polymerization of lignin. I n the presence of oxygen and water, H 0 and peroxy g r o u p s also f o r m . W i t h s o l i d w o o d ( 4 5 - 5 0 °C, 5 0 % r e l a t i v e h u m i d i t y , 7 5 d o f e x p o s u r e , a n d X e a r c as l i g h t s o u r c e ) t h e r e is a loss o f l i g n i n a n d h e m i c e l l u l o s e s from t h e s u r f a c e (61); w a t e r s t r o n g l y i n c r e a s e s t h e rates o f loss. T h e c o l o r o f w o o d surface can l i g h t e n or d a r k e n d e p e n d i n g u p o n t h e w a v e l e n g t h o f l i g h t to w h i c h t h e surface was e x p o s e d t o (65). T h e c h a n g e s i n c o l o r d e p e n d s t r o n g l y u p o n t h e a m o u n t a n d k i n d o f e x t r a c t i v e s p r e s e n t . A p h o t o o x i d a t i v e r e a c t i o n o p e r a t e s to t r a n s f o r m f l a v o n o i d s t a x i f o l i n (I) a n d a r o m a d e n d r i n (III) i n t o q u e r c e t i n (II) a n d k a e m p f e r o l ( I V ) , r e s p e c t i v e l y , a n d r e s u l t s i n a g e n e r a l d e c r e a s e i n f l a v o n o i d s a n d i n c r e a s e i n v a n i l l i n - r e l a t e d c o m p o u n d s (65). 2
2
.OH
ΊΚ Direct
Covalent
Wood-to-Wood
Bonding
A t t e m p t s to i n d u c e b o n d i n g b e t w e e n l i g n o c e l l u l o s i c surfaces b y f o r m a t i o n o f d i r e c t s u r f a c e - t o - s u r f a c e c o v a l e n t b o n d s w e r e m a d e as e a r l y as 1 9 4 5 w h e n L i n z e l l p a t e n t e d a p r o c e s s f o r m a k i n g fiber p r o d u c t s b y c o m p r e s s i n g a n d h e a t i n g a m i x t u r e o f l i g n o c e l l u l o s i c fibers a n d a f e r r i c c o m p o u n d as o x i d a n t (66). S c h u r a n d L e v y n o t e d a n i m p r o v e m e n t i n the wet strength of paper upon oxidation of the p u l p w i t h s o d i u m p e r i o d a t e o r s o d i u m h y p o c h l o r i t e (67). A d d i t i o n a l p a t ents i n v o l v e d interaction of lignocellulosic materials w i t h acids i n the a b s e n c e o f o x i d a n t s (68-72). M o r e r e c e n t l y t h e a r e a has d r a w n a t t e n t i o n f o l l o w i n g t h e e x p e r i m e n t s o f S t o f k o (73). H i s i n i t i a l w o r k w a s connected w i t h the manufacture of particle board a n d p l y w o o d from w h i t e fir [Abies concolor (Gord. and Glend.) L i n d l . ] and incensec e d a r [Calocedrus decurrens (Torr.) F l o r i n ] w o o d i n c o m b i n a t i o n w i t h various oxidants i n c l u d i n g aqueous H 0 / f e r r o u s sulfate, aqueous H N 0 , and ethanolic ferric chloride. 2
2
3
It was o r i g i n a l l y h o p e d that free radicals f o r m e d b y oxidative a c t i v a t i o n o f t h e w o o d s u r f a c e w o u l d j o i n v i a o x i d a t i v e c o u p l i n g to f o r m c o v a l e n t b o n d s ( F i g u r e 2). T h e p a r t i c l e b o a r d w a s p r e p a r e d b y
10.
Nonconventional Bonding
zAVARiN
359
Figure 2. Hypothetical mechanism for the direct wood-to-wood bonding through oxidative phenolic coupling.
s p r a y i n g t h e p a r t i c l e s w i t h a c t i v a t o r s , followed b y p r e s s i n g at 1 3 0 ° C for 2 m i n . T h e r e s u l t i n g p a r t i c l e b o a r d h a d a specific g r a v i t y of 0 . 7 0 0.72 g / c m , i n t e r n a l b o n d (IB) of 6 3 . 5 - 7 4 . 0 p s i , 2 4 - h w a t e r a b s o r p t i o n of 4 4 . 0 - 5 5 . 5 % , a n d 24-h thickness s w e l l i n g of 1 0 . 4 - 2 6 % . A d d i t i o n o f h a m m e r - m i l l e d b a r k e x e r t e d a b e n e f i c i a l effect ( w o o d - t o - b a r k r a t i o , 50:50; specific gravity, 0.725 g/cm ; I B , 81.9 psi; 24-h water absorp tion, 5 9 . 9 % ; 24-h thickness s w e l l i n g , 5.6%). Incense-cedar p l y w o o d had a shear strength of 405 or 385 psi (measured on a G l o b e p l y w o o d t e s t e r ) (73). Later experiments concentrated on plywood. Douglas-fir veneer [Pseudotsuga menziessi ( M i r b . ) F r a n c o ] was u s e d u n d e r acidic or a l kaline c o n d i t i o n s , i n c o m b i n a t i o n w i t h an e x p a n d e d list of activators i n c l u d i n g H 0 w i t h f e r r i c c h l o r i d e o r z i r c o n i u m t e t r a c h l o r i d e as c a t alysts; s o d i u m chlorate; s o d i u m h y p o c h l o r i t e ; potassium persulfate; a m m o n i u m nitrate; potassium permanganate; and potassium ferricyanide; or combinations of the above; a n d occasionally i n the pres ence of cobaltous c h l o r i d e a n d sulfate, manganese dioxide, a n d c u p r i c nitrate. S o d i u m chlorate u n d e r alkaline conditions produced p l y w o o d w i t h d r y shear strength of 246 psi, but the bonds w e r e not waterresistant. A c c e p t a b l e water resistance was attained, however, u n d e r acidic conditions. D r y shear was acceptable although an increase i n a c i d i t y b e y o n d a c e r t a i n p o i n t t e n d e d to d e c r e a s e d r y shear, p r o b a b l y b y surface h y d r o l y s i s . P e r c e n t of w o o d failure r e a c h e d values b e t w e e n 9 0 a n d 1 0 0 % b u t not c o n s i s t e n t l y ; it t e n d e d to b e h i g h e r w i t h m o r e a c i d i c m i x t u r e s a n d i n s o m e cases was i n v e r s e l y p r o p o r t i o n a l to s h e a r s t r e n g t h . A d d i t i o n o f w h e a t f l o u r o r o f a m m o n i u m l i g n o s u l fonate to r e d u c e p e n e t r a t i o n of the b o n d i n g reagents i n t o the i n t e r i o r of w o o d resulted i n only small i m p r o v e m e n t s and small r e d u c e d vari ability. T h e m a i n p r o b l e m s w e r e connected w i t h r e p r o d u c i b i l i t y a n d v a r i a b i l i t y o f t h e p l y w o o d c h a r a c t e r i s t i c s as w e l l as w i t h t h e a n t i c i p a t e d t i m e effect o f a c i d o n t h e s t r e n g t h o f t h e p r o d u c t (74, 75). T h e p r o c e s s w a s f i n a l l y p a t e n t e d (76) a n d i n c l u d e d e x a m p l e s o f m a k i n g plywood and particle board by using incense-cedar and Douglas-fir 3
3
2
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T H E CHEMISTRY O F SOLID
W O O D
v e n e e r a n d w h i t e fir s h a v i n g s . A c t i v a t o r s u s e d f o r p l y w o o d w e r e f e r r i c chloride i n E t O H , H 0 w i t h ferric chloride or w i t h z i r c o n i u m tet r a c h l o r i d e as c a t a l y s t s i n w a t e r , a n d s o d i u m c h l o r a t e i n w a t e r . I n s o m e cases H S 0 or H C 1 was added. T h e m e a n p l y w o o d d r y shear s t r e n g t h for t h e e x a m p l e s c i t e d r a n g e d f r o m 210 to 385 p s i . T h e b o n d was resistant to 4 h o f b o i l i n g . P a r t i c l e b o a r d was m a d e w i t h H 0 / f e r r o u s s u l f a t e i n t h e p r e s e n c e o f H C 1 a n d g a v e a n I B o f 6 5 p s i at a density of 0.70 g/cm . 2
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T h e w o r k o f S t o f k o e t a l . w a s c o n t i n u e d b y P o h l m a n et a l . (77), mainly i n the area of particle board and using primarily H 0 / f e r r o u s s u l f a t e as o x i d a n t . I t w a s t h o u g h t t h a t i m p r o v e m e n t i n m e c h a n i c a l properties a n d decreased variability of particle board properties could be achieved b y stabilizing the H 0 reagent. U s e of phosphoric acid o r o f p y r o p h o s p h o r i c a c i d as s t a b i l i z e r s o r p r e o x i d a t i o n o f w o o d w i t h s o d i u m h y p o c h l o r i t e f a i l e d to p r o d u c e any p o s i t i v e results, h o w e v e r . A s e x p e c t e d , t h e s t a b i l i t y o f H 0 i n c r e a s e d as t h e a m o u n t o f a d d e d ferrous sulfate d e c r e a s e d . D e s p i t e a l l these approaches the most i m p o r t a n t p a r a m e t e r d e t e r m i n i n g t h e I B w a s s t i l l t h e d e n s i t y ; t h u s , at densities of 0.88 g / c m , I B values above 70 p s i w e r e reached, a l t h o u g h at 0 . 8 1 g / c m , I B v a l u e s d r o p p e d to 3 8 p s i (150 ° C , 5 m i n presstime, 0.2% F e S 0 , 4.0% H 0 , 0.5% H P 0 , 0.5% HC1). This is e x e m p l i f i e d i n F i g u r e 3. A d d i t i o n o f b a r k e x e r t e d a f a v o r a b l e effect o n I B , a l t h o u g h at l e a s t 5 0 % b a r k h a d t o b e a d d e d to o b t a i n a n I B o f a b o u t 7 5 p s i at 0 . 7 5 g / c m d e n s i t y . I m p r o v e m e n t i n I B a n d w a t e r resistance w e r e d i r e c t l y r e l a t e d to t h e a m o u n t of H 0 reagent u s e d . 2
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A r e m a r k a b l e i m p r o v e m e n t i n particle b o a r d properties was a c h i e v e d u s i n g b r a n c h w o o d . T h u s at d e n s i t i e s o f 0 . 7 8 - 0 . 8 0 g / c m t h e I B o f p o n d e r o s a p i n e B a u e r - r e f i n e d m a t e r i a l (Pinus ponderosa Laws.) was 76 p s i for n o r m a l h e a r t w o o d , 47 p s i for n o r m a l s a p w o o d , 163 p s i for n o n d e b a r k e d b r a n c h e s , a n d 228 p s i for d e b a r k e d branches w i t h satisfactory w a t e r resistance ( 0 . 0 1 % F e S 0 , 6 . 0 % H 0 , 0 . 5 % H C 1 ) . U n f o r t u n a t e l y , n o t e n o u g h a t t e n t i o n h a s b e e n g i v e n to t h i s r e s u l t . T h e h e a t n e c e s s a r y f o r b o n d i n g is p r o v i d e d b y t h e e x o t h e r m i c r e a c tions t a k i n g p l a c e w i t h t h e h e a t f r o m t h e press p l a t e n s u s e d o n l y for i n i t i a t i o n o f t h e s e r e a c t i o n s ; i n c o n v e n t i o n a l b o n d i n g h e a t i n g is p r o v i d e d m a i n l y b y t h e p l a t e n s (77). 3
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A t t e m p t s to n o n c o n v e n t i o n a l l y b o n d v e n e e r of w h i t e fir, sugar p i n e (Pinus lambertiana D o u g l . ) a n d a s p e n (Populus tremuloides M i c h x . ) were made using peroxyacetic acid under acidic conditions (78). T h e m e a n d r y s h e a r v a l u e s w e r e d e t e r m i n e d a c c o r d i n g t o A S T M D 9 0 5 o n a B a l d w i n U n i v e r s a l t e s t i n g m a c h i n e a n d r a n g e d f r o m 107 t o 9 7 2 , f r o m 2 0 4 t o 9 1 6 , a n d f r o m 3 6 3 to 9 1 8 p s i , w i t h p h e n o l formaldehyde boards g i v i n g d r y shear values of 1 3 6 7 - 1 6 3 8 psi. I n crease i n peroxyacetic a c i d concentration or i n acid strength h a d a p o s i t i v e effect o n s h e a r s t r e n g t h . S h e a r s t r e n g t h v a r i e d w i t h i n a w i d e
10.
ZAVARiN
Nonconventional Bonding
361
100
80H ο o m
60 ο.'
