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15 Degradation of W o o d by Chemicals IRVING S. GOLDSTEIN
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Department of Wood and Paper Science, North Carolina State University, Raleigh, N C 27695-8005
The loss of its identity as wood is an inevitable consequence when wood is pulped or converted into chemicals. But wood also is exposed frequently to the action of chemicals during other types of processing and during ordinary use, yet it still retains its identity as solid wood. These initial processes of chemical attack and the accompanying changes in some important properties of the wood are described. Hydrolysis and oxidation are the most important degradative processes. The sites of initial hydrolytic attack are acetal linkages in carbohydrates and aryl ether linkages in lignin. Mechanisms for these hydrolytic and oxidative reactions are given. The principal consequences of chemical attack on the polymeric components of wood are depolymerization and solubilization.
IN CONSIDERING THE EFFECT OF CHEMICALS ON
WOOD, i t is u s u a l t o f o c u s o n t h e e n d r e s u l t o f t h e c h e m i c a l a c t i o n . S u c h a s p e c t s as t h e f r a c t i o n a t i o n o f w o o d i n t o its separate c o m p o n e n t s , the c o n v e r s i o n o f t h e wood components into various low molecular weight or polymeric products, a n d the d e t e r i o r a t i o n of w o o d b y chemicals are some of the major categories into w h i c h w o o d c h e m i s t r y a n d technology m a y be divided. A l t h o u g h apparently diverse, the categories of solid w o o d p r o cessing, p u l p i n g , a n d c o n v e r s i o n i n t o c h e m i c a l s do possess a c o m m o n f e a t u r e . W h a t e v e r t h e final p r o d u c t o r p r o d u c t s o r t h e b e h a v i o r o f intermediates, all the processes b e g i n w i t h the interaction of a c h e m ical reagent w i t h the bonds a n d functional groups present i n the w o o d . B e c a u s e t h e s t r u c t u r e s o f w o o d a n d its c o m p o n e n t s are q u i t e w e l l d e f i n e d , i t is p o s s i b l e t o r e l a t e t h e s e i n i t i a l p r o c e s s e s t o s p e c i f i c r e a c t i o n s of r e a g e n t s a n d r e a c t i v e sites i n t h e w o o d . A s t h e s e r e a c t i o n s p r o c e e d i t is i n e v i t a b l e t h a t c h a n g e s i n o n e or m o r e i m p o r t a n t properties of the w o o d w i l l occur. T h e s e changes a r e c o m m o n l y c a l l e d degradation, although the value of the resultant 0065-2393/84/0207-0575/$06.00/0 © 1984 American Chemical Society Rowell; The Chemistry of Solid Wood Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
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w o o d p u l p o r w o o d sugar m a y b e greater t h a n that of t h e starting material. Perhaps disorganization w o u l d be a better term, b u t deg r a d a t i o n d o e s a p p l y t o s u c h p r o p e r t i e s as s t r e n g t h a n d c o l o r . In this chapter the emphasis w i l l b e o n the initial processes of c h e m i c a l a t t a c k . D u r i n g t h e e a r l y stages o f a n y t y p e o f c h e m i c a l p r o c e s s i n g t h e w o o d s t i l l r e t a i n s i t s i d e n t i t y as s o l i d w o o d . T h u s , despite any changes i n properties or degradation resulting from c h e m i c a l a c t i o n , these i n i t i a l processes still fall w i t h i n t h e f r a m e w o r k of this book.
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Sites of Initial Attack T h e o n l y c h e m i c a l r e a c t i o n s t h a t c a n affect t h e p r o p e r t i e s o f w o o d are those i n v o l v i n g c h e m i c a l bonds o r functional groups present i n t h e w o o d . It w i l l b e h e l p f u l , therefore, to r e v i e w t h e structure o f the w o o d components w i t h special attention to the most abundant f e a t u r e s . T h e n , s p e c i f i c r e a c t i o n s at t h e s e sites c a n b e c o n s i d e r e d . Structure of W o o d Components. A s d e s c r i b e d i n C h a p t e r 2, w o o d consists o f cellulose, hemicelluloses, l i g n i n , a n d extractives. T h e first t h r e e a r e p o l y m e r i c a n d a r e i n t i m a t e l y a s s o c i a t e d w i t h e a c h o t h e r at t h e m o l e c u l a r l e v e l t o f o r m t h e c e l l w a l l . C a r b o h y d r a t e c o n tent (cellulose a n d hemicelluloses) m a y reach 7 5 % of the w o o d sub s t a n c e , so t h e r e a c t i o n s o f c a r b o h y d r a t e s a r e e s p e c i a l l y i m p o r t a n t . A l t h o u g h the extractives are extraneous materials, their presence can often i n f l u e n c e reactions w i t h t h e c e l l w a l l materials, a n d some w o o d p r o p e r t i e s also m a y b e affected b y reactions i n v o l v i n g extractives.
CELLULOSE. C e l l u l o s e ( C h a p t e r 2 , F i g u r e 1) i s a l o n g c h a i n p o l y m e r o f β-D-glucose i n t h e p y r a n o s e f o r m . A n i m p o r t a n t f e a t u r e o f t h e c e l l u l o s e s t r u c t u r e is t h e t e n d e n c y f o r t h e i n d i v i d u a l c e l l u l o s e chains to f o r m b u n d l e s o f crystalline o r d e r h e l d together b y h y d r o g e n bonds b e t w e e n t h e h y d r o x y l groups o f adjacent chains. T h i s crystall i n i t y i n f l u e n c e s r e a c t i v i t y b y c o n t r o l l i n g t h e access o f reagents o r enzymes to functional groups a n d chemical bonds within the crys talline r e g i o n s , a n d also b y i n t e r f e r i n g w i t h t h e changes i n g e o m e t r y r e q u i r e d f o r t h e t r a n s i t i o n states o f v a r i o u s r e a c t i o n s . T h e a m o r p h o u s o r l e s s - o r d e r e d r e g i o n s o f t h e c e l l u l o s e a r e n o t subject to these r e strictions. T h e h y d r o x y l groups i n cellulose are of t w o types, a single p r i m a r y h y d r o x y l o n e a c h a n h y d r o g l u c o s e u n i t as w e l l as t w o v i c i n a l trans s e c o n d a r y h y d r o x y Is. N e x t i n a b u n d a n c e a f t e r t h e h y d r o x y l groups are t h e acetal linkages that f o r m t h e pyranose rings a n d , b y glycosidic bonds, connect the glucose rings i n the cellulose chain. E n d g r o u p s a r e o f m u c h l e s s i m p o r t a n c e i n a h i g h p o l y m e r s u c h as c e l l u l o s e . A l d e h y d i c f u n c t i o n a l i t y is t o b e e x p e c t e d b u t , d e p e n d i n g
Rowell; The Chemistry of Solid Wood Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
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on the history of the sample, carboxyl groups are not unusual. T h e c a r b o n y l c o n t e n t o f t h e c e l l u l o s e c o m p o n e n t m a y also v a r y w i t h its history; this indicates that s o m e reactions m a y take place along t h e chain. HEMICELLULOSE. H e m i c e l l u l o s e s ( C h a p t e r 2 , F i g u r e 4 ) d i f f e r from cellulose i n that they consist, for t h e most part, o f pentose a n d hexose sugars o t h e r t h a n glucose, a r e u s u a l l y b r a n c h e d , a n d have m u c h l o w e r d e g r e e s o f p o l y m e r i z a t i o n . T h e y a r e n o t c r y s t a l l i n e , so d o n o t p r e s e n t t h e s a m e b a r r i e r s t o a c c e s s i b i l i t y as d o e s c e l l u l o s e . Again hydroxyl groups are most abundant. H o w e v e r , p r i m a r y h y d r o x y l g r o u p s m a y b e a b s e n t as i n t h e x y l a n s . A c e t a l l i n k a g e s a n d b a c k b o n e e n d groups c o r r e s p o n d to those i n cellulose. I n a d d i t i o n the hemicelluloses contain ester bonds i n the form o f acetyl groups. U r o n i c acid groups contribute b o t h an acetal linkage a n d a carboxyl g r o u p . M e t h y l ethers a r e also p r e s e n t , b u t are o f n o c o n c e r n i n this d i s c u s s i o n . L i g n i n — c a r b o h y d r a t e c o v a l e n t b o n d i n g m a y a l s o e x i s t as esters, ethers, o r acetals. LIGNIN. L i g n i n s ( C h a p t e r 2 , F i g u r e 6 ) a r e t h r e e - d i m e n s i o n a l network polymers formed from phenylpropane units. T h e most c o m m o n linkages between the phenylpropane units are shown i n F i g u r e 1 a n d t h e i r p e r c e n t a g e s a r e g i v e n i n T a b l e I ( I ) . I t is a p p a r e n t t h a t e t h e r l i n k a g e s a r e v e r y i m p o r t a n t i n l i g n i n , w i t h β-aryl e t h e r s (A) m o s t a b u n d a n t a n d b e n z y l ( a ) - a r y l e t h e r s ( C ) a n d d i p h e n y l e t h e r s (G) significant. L i g n i n s also c o n t a i n 1 - 1 . 5 m e t h o x y l g r o u p s p e r p h e n y l p r o p a n e u n i t , a n d m u c h s m a l l e r ratios o f free p h e n o l i c h y d r o x y l ( 0 . 0 9 - 0 . 3 0 ) , b e n z y l alcohol ( 0 . 1 5 - 0 . 2 0 ) , a n d carbonyl (0.20) groups. EXTRACTIVES. E x t r a c t i v e s a r e t h e e x t r a n e o u s p l a n t c o m p o n e n t s that c a n b e d i s s o l v e d away f r o m t h e i n s o l u b l e c e l l w a l l material. T h e y include m a n y different kinds of chemicals. T h e reactive functional groups encountered include acids, aldehydes, alcohols, stilbenes, phenols, q u i n o n e s , a n d esters. Hydrolytic Reactions. Probably the most important reactions, w i t h regard to t h e i r i m p a c t o n t h e properties o f w o o d , i n v o l v e some type o f h y d r o l y s i s . B o t h carbohydrates a n d l i g n i n a r e affected, a n d h y d r o l y s i s is e n c o u n t e r e d i n almost e v e r y k i n d o f w o o d processing. CARBOHYDRATES. T h e t w o f u n c t i o n a l g r o u p s s u b j e c t t o h y d r o lysis i n w o o d polysaccharides a r e t h e ester a n d acetal linkages, o f w h i c h t h e acetals a r e m o r e i m p o r t a n t b e c a u s e o f t h e i r greater a b u n dance a n d their role i n connecting the monosaccharides i n the poly mers. H y d r o l y s i s m a y take place u n d e r both acidic a n d alkaline c o n ditions. T h e extent a n d consequences o f acid hydrolysis are greater. E s t e r groups i n w o o d polysaccharides are for the most part acetyl substituents o n the hemicellulose components. I n the presence of
Rowell; The Chemistry of Solid Wood Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
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K>
K>
Figure 1. The most common linkages between phenylpropane units in lignins. (For proportions see Table I.) (Reproduced with permission from Ref 18. Copyright 1981, Academic Press.) alkali these are h y d r o l y z e d to liberate free h y d r o x y l groups o n the h e m i c e l l u l o s e a n d acetate ions i n t h e solution. H y d r o x y l ions i n so l u t i o n are r e p l a c e d b y t h e acetate l e a d i n g to a r e d u c t i o n i n alkalinity. In conjunction w i t h t h e free carboxyl groups present i n the c e l l w a l l c o m p o n e n t s a n d extractives this c o n s u m p t i o n o f alkali can neutralize significant quantities o f an alkaline reagent a n d possibly limit further alkaline hydrolysis. U n d e r acidic conditions, however, the liberation of free acetic a c i d d u r i n g h y d r o l y s i s o f t h e esters c a n increase t h e acidity a n d e n hance further hydrolysis of not only additional ester groups, b u t a c e t a l l i n k a g e s a n d l i g n i n b o n d s as w e l l . A p r i m e e x a m p l e is t h e s o c a l l e d a u t o h y d r o l y s i s r e a c t i o n (2) i n w h i c h a c e t i c a c i d l i b e r a t e d b y steam can cause c o m p l e t e hydrolysis of the hemicelluloses a n d con vert the lignin into soluble fragments. A c i d hydrolysis of t h e acetal linkages i n w o o d polysaccharides f o l l o w s t h e n o r m a l h y d r o l y s i s o f g l y c o p y r a n o s i d e s w i t h fission o f t h e
Rowell; The Chemistry of Solid Wood Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
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glycosyl oxygen b o n d between the rings. T h e mechanism involves protonation of this oxygen atom, followed b y slow b r e a k d o w n of the c o n j u g a t e a c i d to t h e c y c l i c c a r b o n i u m i o n , w h i c h is t h e n a t t a c k e d r a p i d l y b y w a t e r . T h e r e a c t i o n m e c h a n i s m is d e p i c t e d i n F i g u r e 2. B e c a u s e t h e f o r m a t i o n o f t h e c y c l i c c a r b o n i u m i o n is t h e r a t e d e t e r m i n i n g s t e p , t h e h y d r o l y s i s r a t e is a f f e c t e d m a r k e d l y b y t h e c r y s t a l linity of the polysaccharide. T h e ion must assume a partially planar half-chair configuration, but the intermolecular hydrogen bonds i n c r y s t a l l i n e r e g i o n s as i n c e l l u l o s e w o u l d t e n d to k e e p t h e p y r a n o s e r i n g i n its p u c k e r e d c h a i r f o r m a n d thus r e t a r d the f o r m a t i o n of the p l a n a r i n t e r m e d i a t e r e q u i r e d f o r h y d r o l y s i s (3). F o r t h i s a n d o t h e r p o s s i b l e r e a s o n s t h e h e t e r o g e n e o u s r a t e o f c e l l u l o s e h y d r o l y s i s is s e v e r a l o r d e r s o f m a g n i t u d e less t h a n t h a t o f s i m p l e g l y c o s i d e s o r n o n c r y s t a l l i n e p o l y s a c c h a r i d e s (4). T h u s , i n i t i a l a t t a c k b y a c i d s i n v o l v i n g acetal hydrolysis i n cellulose w o u l d involve o v e r w h e l m i n g l y the a m o r p h o u s r e g i o n s to t h e a l m o s t c o m p l e t e e x c l u s i o n o f t h e c r y s t a l l i n e regions. I n t h e n o n c r y s t a l l i n e h e m i c e l l u l o s e s a l l acetals are suscep t i b l e to i n i t i a l attack. U n d e r a l k a l i n e c o n d i t i o n s h y d r o l y s i s o f g l y c o p y r a n o s i d e s is m u c h s l o w e r a n d p r o c e e d s o n l y at h i g h e r t e m p e r a t u r e s . T h e m e c h a n i s m may involve intramolecular displacement of the exocyclic glycosidic oxygen w i t h formation of a cyclic 1,6-anhydroglycopyranose (5).
T a b l e I. Percentages of D i f f e r e n t Types of Bonds i n L i g n i n s of Spruce a n d B i r c h
Bond
Type
0
Spruce (Picea abies)
Birch (Betula verrucosa)
48
60
2
2
A r y l g l y c e r o I - 3 - a r y l e t h e r (A) Glyceraldehyde-2-aryl e t h e r (B) N o n c y c l i c benzyl(ot)aryl e t h e r (C) P h e n y l c o u m a r a n (D) 2- or 6-Position condensed structures (E) Biphenyl(F) D i p h e n y l e t h e r (G) 1,2-Diarylpropane-1, 3 - d i o l (H) β , β - L i n k e d s t r u c t u r e s (I) " Letters A - 1 refer to F i g u r e 1. ( R e p r o d u c e d with p e r m i s s i o n from Réf.
6-8 9-12 2.5-3 9.5-11 3.5-4 7 2 1. C o p y r i g h t 1977,
6-8 6 1.5-2.5 4.5 6.5 7 3 Springer-Verlag.)
Rowell; The Chemistry of Solid Wood Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
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^
CH OH 2
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OH H,OCH
OH
H.OCH3
3
"O
•cHaOH
Di
Figure 2. Acid-catalyzed hydrolysis of glucopuranosides. In addition to glucose (G), small amounts of aisaccharides (Di) are aho formed. (Re produced with permission from Ref 18. Copynght 1981, Academic Press.)
LIGNIN. T h e c h e m i c a l b e h a v i o r o f l i g n i n i n d e g r a d a t i v e r e a c t i o n s has b e e n s t u d i e d e x t e n s i v e l y b y s u b j e c t i n g m o d e l c o m p o u n d s as w e l l as v a r i o u s l i g n i n p r e p a r a t i o n s to t h e a p p r o p r i a t e c o n d i t i o n s . T h e s e studies have b e e n useful i n i n t e r p r e t i n g both the structure of l i g n i n a n d its r e a c t i o n s . A m o n g these i n v e s t i g a t i o n s h y d r o l y s i s of l i g n i n l i n k a g e s u n d e r b o t h a c i d i c a n d a l k a l i n e c o n d i t i o n s has r e c e i v e d m u c h a t t e n t i o n (6). E v e n m i l d h y d r o l y s i s w i t h h o t w a t e r o r d i l u t e a c e t i c a c i d is c a pable of cleaving the easily hydrolyzable benzyl(a)-aryl ether linkages ( C i n F i g u r e 1), w h i l e l e a v i n g t h e β-aryl e t h e r l i n k a g e s ( A i n F i g u r e 1) l a r g e l y i n t a c t (7). A s i m i l a r c l e a v a g e o f a - a r y l e t h e r b o n d s i n a c i d sulfite p u l p i n g p r o v i d e s the p r i n c i p a l f r a g m e n t a t i o n of l i g n i n i n that p r o c e s s . M o r e v i g o r o u s a c i d o l y s i s o f l i g n i n as b y d i o x a n e - w a t e r (9:1) c o n t a i n i n g 0 . 2 M H C I d o e s c l e a v e t h e β-aryl e t h e r l i n k a g e , p r e s u m a b l y v i a a b e n z y l l i u m i o n a n d a n e n o l a r y l e t h e r (8). C l e a v a g e of C - C b o n d s can also o c c u r d u r i n g a c i d h y d r o l y s i s , p r o b a b l y b y r e v e r s e c o n d e n s a t i o n r e a c t i o n s . F o r m a l d e h y d e has b e e n o b t a i n e d f r o m β—y c l e a v a g e i n y i e l d s a p p r o a c h i n g 4 % u p o n h e a t i n g l i g n i n s w i t h 1 2 % H C I o r 2 8 % H S U (9). S m a l l a m o u n t s o f v a n i l l i n and vanillic acid encountered i n acidolysis could only result from a β cleavage. 2
4
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W h e n e x p o s e d to a l k a l i n e reagents at e l e v a t e d t e m p e r a t u r e s lignin undergoes m a i n l y cleavage o f the ether linkages b e t w e e n the phenylpropane units with liberation of phenolic hydroxyl groups. C l e a v a g e o f C — C b o n d s a n d s e c o n d a r y c o n d e n s a t i o n reactions also occur. B e n z y l ( a ) - a r y l e t h e r linkages a r e c l e a v e d b y alkali i f there is a f r e e p h e n o l i c h y d r o x y l g r o u p para
to t h e p r o p y l side chain o r a n
a d j a c e n t h y d r o x y l i n t h e β - p o s i t i o n (10). Q u i n o n e m e t h i d e i n t e r m e diates are i n v o l v e d i n the f o r m e r h y d r o l y s i s a n d epoxides i n the latter. A l k a l i n e cleavage of n o n p h e n o l i c β-ethers occurs through an
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e p o x i d e i f t h e r e is a n adjacent h y d r o x y l o r c a r b o n y l g r o u p i n t h e α o r 7 - p o s i t i o n (11). T h e β - a r y l e t h e r s a l s o u n d e r g o c l e a v a g e t h r o u g h t h i s m e c h a n i s m , b u t t h e c l e a v a g e i s less f a v o r e d t h a n f o r m a t i o n o f a n e n o l e t h e r u n l e s s s u l f i d e i o n s a r e also p r e s e n t (12). D i a r y l
ether
cleavage occurs only to a m i n o r extent. M e t h o x y l groups i n lignin generally are resistant toward alkaline hydrolysis. F o r m a l d e h y d e liberation from 7 - m e t h y l o l groups b y alkaline cleavage of the β - 7 C - C b o n d has b e e n observed, and the action of hot alkali o n lignin to form vanillin b y cleavage of the α - β C - C bonds is w e l l k n o w n . T h e s i m u l t a n e o u s f o r m a t i o n o f a c e t a l d e h y d e i n t h e latter case results f r o m a reverse a l d o l c o n d e n s a t i o n . Traces o f guaiacol f o u n d after alkaline h y d r o l y s i s o f w o o d m a y result f r o m cleavage o f the C - C b o n d b e t w e e n t h e α carbon and the ring. Oxidative Reactions.
O x i d i z i n g r e a g e n t s affect b o t h t h e c a r
b o h y d r a t e a n d l i g n i n c o m p o n e n t s o f w o o d . It m i g h t b e i n f e r r e d that l i g n i n is m o r e s u s c e p t i b l e to o x i d a t i o n because selective r e m o v a l o f l i g n i n f r o m c a r b o h y d r a t e b y o x i d a t i o n is t h e basis o f a n analytical p r o c e d u r e f o r h o l o c e l l u l o s e as w e l l as t h e b l e a c h i n g o f p u l p s . H o w ever, carbohydrate oxidation cannot be neglected, especially w h e n c o n s i d e r i n g t h e i n i t i a l processes o f c h e m i c a l attack o n s o l i d w o o d . C e l l u l o s e is q u i t e s e n s i t i v e t o w a r d o x i d i z i n g reagents.
CARBOHYDRATES.
M i l d o x i d a n t s s u c h as c h l o r i n e , b r o m i n e , o r
iodine readily convert the aldehyde e n d groups i n the w o o d polysac charides to aldonic acid e n d groups. N i t r o g e n dioxide selectively c o n verts t h e p r i m a r y h y d r o x y l groups o n C - 6 i n cellulose to c a r b o x y l g r o u p s (13). P e r i o d i c a c i d i s a s p e c i f i c o x i d a n t f o r v i c i n a l d i o l s a n d yields formaldehyde from p r i m a r y hydroxyl groups a n d aldehydes from secondary. A l t h o u g h t h e o x i d a t i o n o f p o l y s a c c h a r i d e s b y halogens is c o n fined
largely to t h e a l d e h y d i c e n d group, a n d oxidation b y periodic
a c i d to glycols a n d n i t r o g e n d i o x i d e is c o n f i n e d to t h e p r i m a r y h y d r o x y l g r o u p , o t h e r o x i d i z i n g a g e n t s a r e l e s s s p e c i f i c a n d m a y affect a l l t h e s e g r o u p s as w e l l as e i t h e r o f t h e s e c o n d a r y h y d r o x y l g r o u p s . S t r o n g e r o x i d a n t s s u c h as n i t r i c a c i d , p o t a s s i u m d i c h r o m a t e , a n d p o -
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t a s s i u m p e r m a n g a n a t e c a n c a u s e e x t e n s i v e o x i d a t i v e d e g r a d a t i o n to a series of d i c a r b o x y l i c acids. A n i m p o r t a n t oxidative degradation of polysaccharides occurs by the action of molecular oxygen i n the presence of alkali. This reaction is u s e f u l i n t h e c o n t r o l l e d d e p o l y m e r i z a t i o n o f a l k a l i c e l l u l o s e b e f o r e etherification, but more frequently w i l l have undesirable conse q u e n c e s . T h e m e c h a n i s m involves attack b y radicals g e n e r a t e d b y d e c o m p o s i t i o n o f h y d r o p e r o x i d e s , a n d is c a t a l y z e d b y t r a n s i t i o n m e t a l s s u c h as c o b a l t , i r o n , o r m a n g a n e s e . P e r o x i d e d e c o m p o s i t i o n m a y b e i n h i b i t e d p a r t l y b y m a g n e s i u m salts o r c o m p l e x i n g a g e n t s f o r t h e t r a n s i t i o n m e t a l s . O x i d a t i v e d e g r a d a t i o n b y f r e e r a d i c a l s is also i n v o l v e d i n e x p o s u r e o f w o o d c a r b o h y d r a t e s to i o n i z i n g r a d i a t i o n . C a r b o n y l groups are i n t r o d u c e d into polysaccharides b y the ac tion of chlorine, hypochlorite, and ozone. U n d e r alkaline conditions glycosidic bonds may be cleaved. H y d r o g e n peroxide and chlorine dioxide react m u c h m o r e slowly w i t h polysaccharides, a n d conse q u e n t l y are less d e g r a d i n g . LIGNIN. S t u d i e s o f t h e o x i d a t i o n o f l i g n i n h a v e r e c e i v e d i m p e t u s f r o m b o t h a t t e m p t s t o e l u c i d a t e t h e s t r u c t u r e o f l i g n i n a n d to u n d e r s t a n d s u c h t e c h n i c a l p r o c e s s e s as b l e a c h i n g o f p u l p . T h e r e a c t i o n s m a y b e c l a s s i f i e d i n t o t h r e e categories: d e g r a d a t i o n of l i g n i n to a r o matic c a r b o n y l c o m p o u n d s a n d carboxylic acids, degradation of aro m a t i c r i n g s , a n d o x i d a t i o n o f spécifie f u n c t i o n a l g r o u p s (14). T h e first category i n c l u d e s oxidations w i t h n i t r o b e n z e n e , m o l e c ular oxygen, or m e t a l oxides u n d e r alkaline conditions. A r o m a t i c ring d e g r a d a t i o n results f r o m e x p o s u r e to p e r a c e t i c a c i d , n i t r i c a c i d , chlorine, c h l o r i n e dioxide, ozone, a n d the anions of hypochlorous a n d chlorous acids. N e u t r a l permanganate can b r i n g about b o t h side chain and ring oxidation. Periodic acid and alkali peroxides oxidize s p e c i f i c f u n c t i o n a l g r o u p s . L i g n i n o x i d a t i o n is a l s o i n v o l v e d i n t h e photodegradation of w o o d a n d the enzymatic degradation of lignin. A l k a l i n e o x i d a t i o n o f l i g n i n is t h e c o m m e r c i a l s o u r c e o f v a n i l l i n . T h i s p r o c e d u r e c o n v e r t s a significant p o r t i o n o f t h e l i g n i n to a r o m a t i c fragments that are, for t h e m o s t p a r t , a r o m a t i c a l d e h y d e s . T h e m e c h a n i s m p r o b a b l y involves two steps, hydrolysis of the aryl e t h e r l i n k ages f o l l o w e d b y s i d e c h a i n o x i d a t i o n . O x i d a n t s s u c h as s i l v e r , c o b a l t , m e r c u r i c , a n d c u p r i c o x i d e s i n alkali y i e l d a mixture of aromatic aldehydes or aromatic carboxylic a c i d s o r b o t h b y a c t i n g o n t h e s i d e c h a i n s . S i l v e r is t h e s t r o n g e s t o x i d a n t a n d y i e l d s c h i e f l y a c i d s ; c u p r i c o x i d e is t h e m i l d e s t o x i d a n t and yields chiefly aldehydes. A l t h o u g h molecular oxygen i n alkali c a n c o m p l e t e l y s o l u b i l i z e l i g n i n b y c o n v e r s i o n to l o w m o l e c u l a r w e i g h t a c i d s at e l e v a t e d t e m p e r a t u r e a n d p r e s s u r e , u n d e r less s t r i n gent conditions the aromatic rings are c o n s e r v e d a n d v a n i l l i n can be obtained in good yield.
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S t r o n g o x i d a n t s s u c h as p e r m a n g a n a t e a n d d i c h r o m a t e i n a c i d i c solution degrade l i g n i n c o m p l e t e l y to carbon dioxide a n d dibasic acids. I n b e t w e e n t h e side c h a i n oxidants d e s c r i b e d above a n d t h e s t r o n g o x i d a n t s l i e s a g r o u p o f less d r a s t i c o x i d a n t s i n c l u d i n g c h l o r i n e , nitric acid, chlorine dioxide, sodium hypochlorite, peracetic acid, h y d r o g e n p e r o x i d e , a n d o z o n e w h i c h attack p r i m a r i l y t h e aromatic nuclei of the lignin. Because they exhibit a relative selectivity i n attacking lignin more rapidly than carbohydrate, they have found utility i n b l e a c h i n g p u l p . A n important m e c h a n i s m for these lignin oxidations involves formation of quinones with subsequent ring o p e n i n g t o p r o v i d e d e r i v a t i v e s o f d i c a r b o x y l i c a c i d s s u c h as m u c o n i c , maleic, a n d fumaric acids. S o d i u m periodate specifically oxidizes guaiacyl groups to q u i nones. H y d r o g e n a n d s o d i u m p e r o x i d e s i n alkali a r e also s o m e w h a t selective i n oxidizing lignin a n d destroy chromophoric groups such as q u i n o n e s a n d c a r b o n y l f u n c t i o n s w h i l e a l s o d e g r a d i n g o n l y a r o matic units w i t h free p h e n o l i c hydroxyls to dibasic acids. Other Reactions. T h e h y d r o l y t i c a n d oxidative reactions d e scribed i n t h e p r e c e d i n g sections account for most o f the degradation of w o o d b y chemicals. H o w e v e r , there are several other processes significant e n o u g h to warrant special m e n t i o n . T w o o f these m i g h t perhaps have been i n c l u d e d under hydrolysis because decrystallizat i o n c o u l d b e c o n s i d e r e d as r e p r e s e n t i n g h y d r o l y s i s o f i n t e r m o l e c u l a r h y d r o g e n b o n d s a n d p e e l i n g as a s p e c i a l c a s e o f a l k a l i n e h y d r o l y s i s . T h e examples to b e c i t e d u n d e r discoloration demonstrate t h e i n f l u ence of extractives o n thec h e m i c a l behavior o f wood. A n o t h e r reac t i o n t h a t i n v o l v e s e x t r a c t i v e s a n d affects t h e u t i l i z a t i o n o f w o o d i s t h e inhibition o f t h e sulfite p u l p i n g o f p i n e heartwood b y t h e stilbene p i n o s y l v i n a n d i t s m e t h y l e t h e r s (15).
DECRYSTALLIZATION
. T h e c r y s t a l l i n i t y o f c e l l u l o s e is a n i n h e r e n t p r o p e r t y t h a t is a n i m p o r t a n t d e t e r m i n a n t o f its m e c h a n i c a l p r o p e r ties, affinity for water, a n d accessibility to c h e m i c a l reagents. Because cellulose c o m p r i s e s almost 5 0 % o f t h e w o o d , its crystallinity is a d e t e r m i n a n t o f t h e b e h a v i o r o f t h e w o o d as w e l l . A n y d i s r u p t i o n o r change i n t h e crystallinity of the cellulose w i l l cause significant changes i n properties and, thus b y o u r definition, degradation. D e c r y s t a l l i z a t i o n o f c e l l u l o s e b y s w e l l i n g agents o r solvents c a n be brought about b y concentrated s o d i u m hydroxide; amines; m e tallo-organic complexes of copper, c a d m i u m , a n d iron; quaternary a m m o n i u m bases; c o n c e n t r a t e d m i n e r a l acids (sulfuric, h y d r o c h l o r i c , p h o s p h o r i c ) ; c o n c e n t r a t e d salt s o l u t i o n s ( b e r y l l i u m , c a l c i u m , l i t h i u m , z i n c ) ; a n d a n u m b e r o f r e c e n t l y i n v e s t i g a t e d m i x e d s o l v e n t s (16). A n increase i n the crystallinity of cellulose from chemical treat m e n t is u n u s u a l , b u t i t does o c c u r after a c i d h y d r o l y s i s o f the a m o r phous regions. T h e initial hydrolysis of amorphous cellulose actually
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increases crystallinity b y p e r m i t t i n g the b r o k e n chains to have greater f r e e d o m t o b e c o m e o r g a n i z e d i n m o r e h i g h l y o r d e r e d s t r u c t u r e s (17). PEELING. P e e l i n g i s t h e t e r m a p p l i e d t o t h e s t e p w i s e d e p o l y merization of polysaccharides from t h e reducing e n d groups u n d e r alkaline conditions. I n conjunction w i t h the formation of n e w e n d groups b y alkaline hydrolysis of glycosidic bonds, the degradation of the polysaccharides c a n b e extensive. T h e mechanism involves alkali-catalyzed rearrangement of the aldose e n d g r o u p to a 2-ketose. E l i m i n a t i o n o f t h e β-alkoxy g r o u p from the C - 4 position generates a n e w aldohexose e n d group a n d the process continues d o w n the chain. U n d e r alkaline p u l p i n g conditions as m a n y as 5 0 g l u c o s e u n i t s m a y b e p e e l e d f r o m a s i n g l e c e l l u l o s e m o l e c u l e b e f o r e t h e r e a c t i o n is s t o p p e d b y a d i r e c t β-alkoxy e l i m i nation from t h e C - 3 position (IS).
DISCOLORATION. S t a i n i n g o r d i s c o l o r a t i o n o f w o o d b y c h e m i c a l p r o c e s s e s i s a f r e q u e n t l y e n c o u n t e r e d f o r m o f d e g r a d a t i o n . I t is o f t e n confused w i t h discoloration caused b y fungi, but results instead from the conversion o f originally colorless o r light-colored, naturally oc c u r r i n g extractives into i n t e n s e l y c o l o r e d products that m a y i m p a r t an objectionable appearance to the wood. Two mechanisms have been identified. M o s t of the so-called c h e m i c a l stains r e s u l t f r o m o x i d a t i o n o f c e r t a i n w o o d extractives b y air d u r i n g a i r seasoning o r kiln d r y i n g . Colors observed i n c l u d e shades o f b r o w n , b l u e , g r e e n , y e l l o w , a n d r e d . Species i n c l u d e b o t h h a r d w o o d s (oak, b i r c h , m a p l e , a l d e r , b a s s w o o d , g u m , e t c . ) a n d soft w o o d s (eastern a n d w e s t e r n pines, hemlock). W e t w o o d c a n also d i s c o l o r b y contact w i t h i r o n o r c o p p e r w h e n tannins are present to form black iron tannate o r reddish c o p p e r tannate. I n contrast to the c h e m i c a l stains c a u s e d b y o x i d a t i o n , w h i c h do n o t significantly alter the w o o d other than i n color, the p r o l o n g e d action o f iron o r copper may catalyze further chemical b r e a k d o w n of the w o o d s t r u c t u r e b y free radical oxidative m e c h a n i s m s .
Consequences of Chemical Attack T h e properties of w o o d ultimately depend o n the interaction of the three polymeric components cellulose, hemicelluloses, and lignin at t h e m o l e c u l a r l e v e l . T h e y a r e i n t i m a t e l y a s s o c i a t e d , w i t h t h e l i n e a r crystalline cellulose microfibrils e m b e d d e d i n the matrix o f the amor phous hemicelluloses a n d lignin. Intramolecular covalent b o n d i n g w i t h i n t h e i n d i v i d u a l p o l y m e r s is i m p o r t a n t to t h e w o o d structure. But intermolecular bonding between similar molecules a n d among the t h r e e m a j o r c o m p o n e n t s is also c r i t i c a l . H y d r o g e n b o n d i n g , d i p o l e - d i p o l e forces, a n d L o n d o n forces b e c o m e v e r y large i n t h e
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a g g r e g a t e as t h e s i z e o f p o l y m e r s i n c r e a s e s . C o v a l e n t b o n d i n g , as b e t w e e n l i g n i n a n d h e m i c e l l u l o s e , is p r o b a b l y a l s o o f s i g n i f i c a n c e .
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Some c h e m i c a l reactions of the w o o d components involve func tional groups that do not f o r m part of the p o l y m e r c h a i n a n d m a y h a v e o n l y a s l i g h t effect o n s o m e w o o d p r o p e r t i e s a n d m a y e n h a n c e some. F o r e x a m p l e , esterification or etherification of free h y d r o x y l groups i n carbohydrates or lignin may reduce hygroscopicity, i n crease d i m e n s i o n a l stability, a n d actually increase w o o d strength b y reducing the e q u i l i b r i u m moisture content. H o w e v e r , any of the m u l t i t u d e of c h e m i c a l reactions that d i s r u p t the intramolecular a n d intermolecular bonds w i t h i n and a m o n g the w o o d c o m p o n e n t s w i l l h a v e d e l e t e r i o u s effects o n w o o d p r o p e r t i e s . T h e obvious extreme w o u l d be the complete depolymerization and dissolution of the w o o d components w i t h complete destruction of the wood. These processes of depolymerization or solubilization of the w o o d c o m p o n e n t s d o n o t h a v e to g o t o c o m p l e t i o n to b r i n g a b o u t m a r k e d changes i n w o o d properties or degradation. Initial attack of the w o o d b y any of the h y d r o l y t i c pathways described above reduces the degree of polymerization of the w o o d p o l y m e r s i n v o l v e d a n d m a y b e r e f l e c t e d i n s u c h m a n i f e s t a t i o n s as s e r i o u s loss i n i m p a c t s t r e n g t h b e f o r e a n y loss i n w e i g h t c a u s e d b y s o l u b i l i z a t i o n c a n b e d e t e c t e d . I n s o f a r as m a n y o f t h e r e a c t i o n s e x hibit selectivity a m o n g the w o o d components, one property or a n o t h e r m a y suffer the i n i t i a l changes. As a broad generalization the cellulose provides impact resis tance a n d t e n s i l e s t r e n g t h , t h e l i g n i n p r o v i d e s stiffness, a n d b o t h m a t r i x p o l y m e r s c o n t r i b u t e to h a r d n e s s a n d c o m p r e s s i v e s t r e n g t h . H y d r o l y t i c a n d oxidative processes that d e p o l y m e r i z e a n d solubilize t h e w o o d c o m p o n e n t s w i l l affect t h e w o o d p r o p e r t i e s , o f t e n t o a n extent far g r e a t e r t h a n m i g h t b e e x p e c t e d f r o m t h e l i m i t e d i n i t i a l reaction. Summary O n e x p o s u r e to c h e m i c a l s , f u n c t i o n a l g r o u p s a n d b o n d s i n t h e c o m p o n e n t w o o d p o l y m e r s u n d e r g o r e a c t i o n s t h a t l e a d to c h a n g e s i n t h e w o o d p r o p e r t i e s . T h e r e a c t i o n s m a y affect b o t h c a r b o h y d r a t e s a n d l i g n i n , a n d are c h i e f l y h y d r o l y t i c or oxidative i n nature. H y d r o l y s i s o f p o l y m e r l i n k a g e s l e a d s to d e p o l y m e r i z a t i o n a n d s t r e n g t h l o s s . V u l n e r a b l e to h y d r o l y s i s a r e a c e t a l l i n k a g e s i n c a r b o h y d r a t e s a n d a r y l e t h e r l i n k a g e s i n l i g n i n . O x i d a t i v e processes also c o n t r i b u t e to frag m e n t a t i o n of the w o o d c o m p o n e n t s . T h e loss of w o o d substance through subsequent solubilization of hydrolysis and oxidation prod u c t s is a n i m p o r t a n t p a r t o f t h e d e g r a d a t i o n p r o c e s s .
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Literature Cited 1. Adler, E . Wood Sci. Technol. 1977, 11, 169-218. 2. Lora, J. H.; Wayman, M . Tappi 1978, 61(6), 47-50. 3. Goldstein, I. S. in "Wood and Agricultural Residues: Research on Use for Feed, Fuels and Chemicals"; Sokes, E . J., E d . ; Academic Press: New York, 1983; 315-28. 4. Harris, J. F. Appl. Polym. Symp. 1975, 28, 131-44. 5. Ferrier, R. J . ; Collins, P. M . "Monosaccharide Chemistry"; Penguin Books: Baltimore, 1972; pp. 61-62. 6. Wallis, A. F. A. in "Lignins"; Sarkanen, Κ. V.; Ludwig, C. H . , Eds.; Wiley -Interscience: New York, 1971; 345-69. 7. Nimz, H . Holzforschung 1966. 20, 105. 8. Adler, E.; Lundquist, K.; Mikshe, G. Ε. in "Lignin Structure and Reac tions," Marton, J . , Ed.; A C S A D V A N C E S IN C H E M I S T R Y SERIES No. 59; ACS: Washington, D . C . , 1966; 22-35. 9. Brauns, F. E. "The Chemistry of Lignin"; Academic Press: New York, 1952; p. 440. 10. Gierer, J.; Noren, I. Acta Chem. Scand. 1962, 16, 1713. 11. Gierer, J.; Lenz, B.; Noren, I.; Soderberg, S. Tappi 1964, 47, 233. 12. Adler, E.; Falkehag, I.; Marton, J.; Halvarson, H . Acta Chem. Scand. 1964, 18, 1313. 13. Yackel, E . C . ; Kenyon, W. O. J. Am. Chem. Soc. 1942, 64, 121. 14. Chang, H - M . ; Allan, G. G. in "Lignins"; Sarkanen, Κ. V.; Ludwig, C. H., Eds.; Wiley-Interscience: New York, 1971; pp. 433-85. 15. Lindstedt, G . ; Misiorny, A. Acta Chem. Scand. 1951, 5, 121. 16. Turbak, A. F. "Solvent Spun Rayon, Modified Cellulose Fibers and Deriv atives"; A C S SYMPOSIUM SERIES No. 58; ACS: Washington, D . C . , 1977. 17. Hermans, P. H.; Weidinger, A. J. Polym. Sci. 1949, 4, 317. 18. Sjostrom, E . "Wood Chemistry"; Academic Press: New York, 1981; pp. 43-46.
RECEIVED
ACCEPTEDJuly 27, 1983.
for review May 19, 1983.
Rowell; The Chemistry of Solid Wood Advances in Chemistry; American Chemical Society: Washington, DC, 1984.
