The Chemistry of Solid Wood - American Chemical Society

4 Penetration and Reactivity of Cell Wall Components ROGER M. R O W E L L

Downloaded by UNIV OF MINNESOTA on July 18, 2013 | http://pubs.acs.org Publication Date: May 5, 1984 | doi: 10.1021/ba-1984-0207.ch004

U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI 53705

Chemical modification of wood to increase its resistance to biodegradation and photodegradation, to improve its dimensional stability, and to decrease its flammability d e p e n d s on adequate distribution of reacted chemicals in the water-accessible regions of the cell wall. The chemicals used for modifying wood must be capable of swelling the wood to facilitate penetration and must react with the cell wall polymer hydroxyl groups under neutral or mild alkaline conditions at temperatures at or below 120 ° C . The chemicals should react quickly with the hydroxyl groups to yield stable chemical bonds with no by-products. The modified wood must retain the desired properties of the untreated wood. Chemicals used to modify wood include anhydrides, acid chlorides, carboxylic acids, isocyanates, aldehydes, alkyl chlorides, lactones, nitriles, and epoxides. Reaction of these chemicals with wood yields a modified wood with good biological resistance and greatly improved dimensional stability. The reaction takes place in the cell wall and is evident when the increases in the wood volume approach the volume of chemical added, when the leach resistance of the modified wood is high, and by IR data. Studies on the distribution of the bonded chemical show good penetration into the cell wall structure. The lignin component is highly substituted although the carbohydrate components are less substituted.

Physical Properties and Chemical Modification W o o d is a t h r e e - d i m e n s i o n a l , p o l y m e r i c c o m p o s i t e m a d e u p p r i marily of cellulose, hemicellulose, and lignin. These polymers make u p the c e l l w a l l a n d are r e s p o n s i b l e for most of the p h y s i c a l a n d This chapter not subject to U.S. copyright. Published 1984, American Chemical Society In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.


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T H E CHEMISTRY O F SOLID W O O D

c h e m i c a l p r o p e r t i e s e x h i b i t e d b y w o o d . W o o d is a p r e f e r r e d b u i l d i n g a n d e n g i n e e r i n g m a t e r i a l b e c a u s e i t is e c o n o m i c a l , l o w i n p r o c e s s i n g energy, r e n e w a b l e , s t r o n g , a n d aesthetically p l e a s i n g . It has, h o w e v e r , s e v e r a l u n d e s i r a b l e p r o p e r t i e s , s u c h as b i o d e g r a d a b i l i t y , flammability, dimensional instability w i t h varying moisture contents, a n d d e g r a d a b i l i t y b y U V light, acids, a n d bases. T h e s e p r o p e r t i e s are all the result of c h e m i c a l reactions i n v o l v i n g degradative e n v i r o n m e n t a l a g e n t s . W o o d is d e g r a d e d b i o l o g i c a l l y b e c a u s e o r g a n i s m s r e c ognize the polysaccharide p o l y m e r s i n the c e l l w a l l (e.g., the cellulose a n d hemicelluloses) a n d have v e r y specific e n z y m e systems capable of h y d r o l y z i n g these p o l y m e r s into digestible units. Biodégradation of the h i g h molecular weight cellulose weakens the w o o d because c e l l u l o s e p r i m a r i l y is r e s p o n s i b l e for t h e s t r e n g t h i n w o o d (see C h a p t e r 5). S t r e n g t h is l o s t as t h e c e l l u l o s e p o l y m e r u n d e r g o e s d e g radation t h r o u g h oxidation, hydrolysis, and dehydration reactions. T h e same types of reactions take place i n the presence of acids a n d b a s e s (see C h a p t e r 15). W o o d changes d i m e n s i o n w i t h changing moisture content be cause the c e l l w a l l p o l y m e r s contain h y d r o x y l a n d other oxygen-con t a i n i n g g r o u p s t h a t a t t r a c t m o i s t u r e t h r o u g h h y d r o g e n b o n d i n g (see C h a p t e r 3). T h i s m o i s t u r e s w e l l s t h e c e l l w a l l , a n d t h e w o o d e x p a n d s u n t i l t h e c e l l w a l l is s a t u r a t e d w i t h w a t e r . W a t e r b e y o n d t h i s s a t u r a t i o n p o i n t is f r e e w a t e r i n t h e v o i d s t r u c t u r e a n d d o e s n o t c o n t r i b u t e t o f u r t h e r e x p a n s i o n . T h i s p r o c e s s is r e v e r s i b l e , a n d t h e w o o d s h r i n k s as i t l o s e s m o i s t u r e . W o o d burns because the cell wall polymers undergo hydrolysis, oxidation, d e h y d r a t i o n , a n d pyrolysis reactions w i t h increasing t e m p e r a t u r e t o g i v e o f f v o l a t i l e , f l a m m a b l e gases. T h e l i g n i n c o m p o n e n t c o n t r i b u t e s m o r e to char f o r m a t i o n t h a n do the cellulose c o m p o n e n t s , and the charred layer helps insulate the wood from further thermal d e g r a d a t i o n (see C h a p t e r 13). W o o d e x p o s e d to t h e o u t d o o r s u n d e r g o e s p h o t o c h e m i c a l d e g radation c a u s e d b y U V light. T h i s d e g r a d a t i o n takes place p r i m a r i l y i n the l i g n i n c o m p o n e n t a n d causes characteristic color changes. T h e l i g n i n acts as a n a d h e s i v e i n w o o d , h o l d i n g c e l l u l o s e fibers t o g e t h e r . C o n s e q u e n t l y , the w o o d surface b e c o m e s r i c h e r i n cellulose content as t h e l i g n i n d e g r a d e s . I n c o m p a r i s o n t o l i g n i n , c e l l u l o s e is m u c h l e s s s u s c e p t i b l e t o U V d e g r a d a t i o n . T h e s e p o o r l y b o n d e d fibers a r e w a s h e d off t h e s u r f a c e d u r i n g a r a i n , w h i c h e x p o s e s n e w l i g n i n to t h e degradative reactions. I n t i m e , this " w e a t h e r i n g " process can account f o r a s i g n i f i c a n t l o s s i n s u r f a c e fibers (see C h a p t e r 11). B e c a u s e these types of d e g r a d a t i o n are c h e m i c a l i n n a t u r e , it s h o u l d b e p o s s i b l e to e l i m i n a t e t h e m o r d e c r e a s e t h e i r r a t e b y m o d ifying the basic c h e m i s t r y of the w o o d cell wall polymers. C h e m i c a l m o d i f i c a t i o n o f w o o d is a n y c h e m i c a l r e a c t i o n b e t w e e n s o m e r e a c t i v e

In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984.

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part of a w o o d component a n d a simple single chemical reagent, w i t h or w i t h o u t catalyst, that forms a covalent b o n d b e t w e e n the two c o m p o n e n t s . T h e m o s t a b u n d a n t r e a c t i v e c h e m i c a l sites i n w o o d a r e the h y d r o x y l groups on cellulose, hemicellulose, and lignin.


M o s t of the research done i n the area of chemical modification involves the reaction of h y d r o x y l groups. F o r example, biodégrada tion can be p r e v e n t e d b y reacting chemicals w i t h the hydroxyls on t h e c e l l u l o s e c o m p o n e n t . W h e n t h i s is d o n e , t h e h i g h l y s p e c i f i c b i ological e n z y m a t i c reactions cannot take place because the c h e m i c a l configuration a n d molecular conformation of the substrate have b e e n a l t e r e d . M o r e r e s e a r c h has c e n t e r e d a r o u n d i m p r o v i n g d i m e n s i o n a l stability. C h a n g e s i n d i m e n s i o n can be r e d u c e d b y b u l k i n g the c e l l wall w i t h b o n d i n g chemicals. This m e t h o d works because the treat m e n t p u t s t h e w o o d i n a partially, i f n o t c o m p l e t e l y , s w o l l e n state. T h e s e t e c h n i q u e s d e m o n s t r a t e t h a t i t is p o s s i b l e t o c h a n g e t h e basic c h e m i s t r y a n d , therefore, the properties of w o o d c e l l w a l l poly mers through c h e m i c a l reactions. These chemical modifications can greatly enhance the properties of w o o d products. C h e m i c a l m o d i f i c a t i o n o f w o o d f o r b i o l o g i c a l r e s i s t a n c e is b a s e d o n the t h e o r y that the e n z y m e s (cellulases) m u s t d i r e c t l y contact the substrate (wood cellulose), a n d the substrate must have a specific c o n f i g u r a t i o n . I f t h e s u b s t r a t e is c h e m i c a l l y c h a n g e d , t h i s h i g h l y s e l e c t i v e r e a c t i o n c a n n o t take place. O n e w a y to c h e m i c a l l y m o d i f y the s u b s t r a t e is t o c h a n g e t h e h y d r o p h i l i c n a t u r e o f t h e w o o d . I n s o m e c a s e s w a t e r , a n e c e s s i t y f o r d e c a y o r g a n i s m s , is e x c l u d e d f r o m b i o l o g i c a l sites. T h e c h e m i c a l s u s e d for m o d i f i c a t i o n n e e d not b e toxic to t h e o r g a n i s m b e c a u s e t h e i r a c t i o n r e n d e r s t h e substrate u n r e c o g n i z a b l e as a f o o d s o u r c e to s u p p o r t m i c r o b i a l g r o w t h . I n o t h e r w o r d s , the organisms starve i n the presence of plenty. R e s e a r c h i n v o l v i n g c e l l w a l l b u l k i n g t r e a t m e n t s has s h o w n t h a t t h e i n c r e a s e i n w o o d v o l u m e is d i r e c t l y p r o p o r t i o n a l t o t h e t h e o r e t i c a l v o l u m e o f c h e m i c a l a d d e d (I). T h e w o o d v o l u m e i n c r e a s e s w i t h i n c r e a s i n g c h e m i c a l a d d i t i o n t o a b o u t a 2 5 % g a i n i n w e i g h t , at w h i c h p o i n t t h e t r e a t e d v o l u m e is a p p r o x i m a t e l y e q u a l to t h e g r e e n w o o d v o l u m e (2). W h e n t h i s b u l k e d w o o d c o m e s i n t o c o n t a c t w i t h w a t e r , v e r y l i t t l e a d d i t i o n a l s w e l l i n g c a n t a k e p l a c e . T h i s is h o w b u l k i n g t r e a t m e n t s are effective for d i m e n s i o n a l stability. S e v e r a l t e r m s are u s e d to d e s c r i b e t h e d e g r e e o f d i m e n s i o n a l stability g i v e n to w o o d b y various t r e a t m e n t s : a n t i s h r i n k efficiency ( A S E ) , s w e l l i n g p e r c e n t , d i m e n s i o n a l stabilization efficiency, a n t i s w e l l i n g efficiency, a n d p e r c e n t r e d u c t i o n i n s w e l l i n g . G e n e r a l l y the v o l u m e t r i c s w e l l i n g c o e f f i c i e n t is c a l c u l a t e d b y

y2 - y ι S = —

x

100


178


w h e r e V is t h e w o o d v o l u m e a f t e r h u m i d i t y c o n d i t i o n i n g o r w e t t i n g w i t h w a t e r , a n d V is t h e w o o d v o l u m e o f o v e n - d r i e d s a m p l e b e f o r e c o n d i t i o n i n g o r w e t t i n g . T h e n A S E , w h i c h is t h e r e d u c t i o n i n s w e l l i n g or antishrink efficiency resulting from a treatment, can be calculated from 2

x

ASE where S

2

=

S l

~ Si

x

§ 2

100

is t h e t r e a t e d v o l u m e t r i c s w e l l i n g c o e f f i c i e n t , a n d S is t h e x

u n t r e a t e d v o l u m e t r i c s w e l l i n g coefficient. Downloaded by UNIV OF MINNESOTA on July 18, 2013 | http://pubs.acs.org Publication Date: May 5, 1984 | doi: 10.1021/ba-1984-0207.ch004

Reaction Requirements Penetration. I n whole wood, accessibility of the treating re a g e n t t o t h e r e a c t i v e c h e m i c a l s i t e s is a m a j o r c o n s i d e r a t i o n . To i n crease a c c e s s i b i l i t y to t h e r e a c t i o n site, t h e c h e m i c a l m u s t p e n e t r a t e the w o o d structure. Penetration can be achieved by causing the w o o d s t r u c t u r e to s w e l l . I f a reagent p o t e n t i a l l y capable of m o d i f y i n g w o o d d o e s n o t c a u s e t h e w o o d s u b s t a n c e to s w e l l , t h e n c a t a l y s t m a y b e necessary. I f b o t h t h e r e a g e n t a n d catalyst are u n a b l e to cause the w o o d to s w e l l , a w o r k a b l e c o s o l v e n t c o u l d b e a d d e d to t h e r e a c t i o n system. T h e s w e l l i n g o f w o o d b y v a r i o u s o r g a n i c l i q u i d s has b e e n s t u d i e d (3-9). F o r the most part, these studies consisted of soaking ovend r i e d b l o c k s of w o o d for p r o l o n g e d p e r i o d s i n a n h y d r o u s organic l i q u i d s at r o o m t e m p e r a t u r e . T h e d e g r e e o f s w e l l i n g , o r s w e l l i n g coefficient, represents an u n a d j u s t e d average s w e l l i n g coefficient a n d is u s u a l l y e x p r e s s e d as a t h r e e - d i m e n s i o n a l f u n c t i o n , i . e . , v o l u m e t r i c s w e l l i n g coefficient. F o r comparative purposes, v o l u m e t r i c s w e l l i n g c o e f f i c i e n t s a r e u s u a l l y s t a n d a r d i z e d to a v o l u m e t r i c s w e l l i n g c o e f f i c i e n t c o m p a r e d t o w a t e r , s e t t i n g w a t e r at 10. If, f o r e x a m p l e , t h e u n a d j u s t e d average v o l u m e t r i c s w e l l i n g coefficient for w a t e r was ex p e r i m e n t a l l y d e t e r m i n e d t o b e 1 1 . 7 , t h i s c o u l d b e s t a n d a r d i z e d to 10 b y d i v i d i n g 1.17 i n t o 1 1 . 7 . A l l o t h e r v o l u m e t r i c s w e l l i n g c o e f f i c i e n t v a l u e s o b t a i n e d w o u l d t h e n b e d i v i d e d b y 1.17 t o s t a n d a r d i z e t h e m t o a w a t e r v a l u e o f 10. I n o t h e r w o r d s , a d j u s t m e n t is m a d e b y d i v i d i n g e x p e r i m e n t a l v o l u m e t r i c s w e l l i n g coefficient values b y one-tenth the average v o l u m e t r i c s w e l l i n g coefficient v a l u e for w o o d blocks t r e a t e d w i t h water. Tables I, I I , a n d I I I give v o l u m e t r i c s w e l l i n g coefficients for s o u t h e r n p i n e s a p w o o d for various p o t e n t i a l reagents, catalysts, a n d s o l v e n t s (10). T h e s e c o e f f i c i e n t s w e r e d e t e r m i n e d u n d e r t w o sets o f conditions. Specimens from oven-dried southern pine sapwood blocks were measured and their volumes were determined. Ten spec i m e n s f r o m t h i s set w e r e s u b m e r g e d i n a s o l u t i o n a n d e i t h e r t r e a t e d at 1 2 0 ° C a n d a p r e s s u r e o f 150 l b / i n . f o r 1 h , o r t h e y w e r e s o a k e d at 2 5 ° C f o r 4 8 h . 2


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T a b l e I . V o l u m e t r i c S w e l l i n g C o e f f i c i e n t s (S) f o r S o u t h e r n P i n e Sapwood in Various Reagents


Reagent M e t h y l isocyanate Acetic anhydride Formaldehyde solution Water Epichlorohydrin Acrolein Propylene oxide Acrylonitrile M e t h y l isothiocyanate Butylène oxide

120 °C; 150 lb/in. ; 1 h 2

25 °C;

52.6 12.3 12.3 10.0 6.9 6.7 5.2 4.6 4.5 4.1

Soaking 5.1 1.5 12.3 10.0 5.9 7.0 5.0 4.5 4.1 0.7

Table I shows that the v o l u m e t r i c s w e l l i n g coefficients for the potential reagents u n d e r the two conditions are n e a r l y the same ex c e p t i n t h e cases of m e t h y l isocyanate a n d acetic a n h y d r i d e . T h e a m o u n t o f s w e l l i n g f o r t h e s e t w o r e a g e n t s is m u c h g r e a t e r at 1 2 0 ° C t h a n at 2 5 ° C b e c a u s e at 1 2 0 ° C a r e a c t i o n w i t h w o o d h a s o c c u r r e d using b o t h m e t h y l isocyanate a n d acetic a n h y d r i d e . T h e large i n c r e a s e i n v o l u m e at 1 2 0 ° C is c a u s e d b y r e a c t e d c h e m i c a l s b u l k i n g t h e c e l l w a l l . M u c h less c o n s i s t e n c y b e t w e e n t h e t w o t r e a t i n g c o n d i t i o n s is s e e n i n T a b l e I I w i t h c a t a l y s t s . P i p e r i d i n e a n d a n i l i n e h a v e h i g h s w e l l i n g c o e f f i c i e n t s at 1 2 0 ° C b u t v e r y l o w s w e l l i n g c o e f f i c i e n t s T a b l e I I . V o l u m e t r i c S w e l l i n g C o e f f i c i e n t s (S) f o r S o u t h e r n P i n e Sapwood in Various Catalysts

Reagent n-Butylamine Piperidine Dimethylformamide Pyridine Acetic acid Aniline Water Diethylamine N-Methylaniline N-Methylpiperidine N.N-Dimethylaniline Triethylamine

120 ° C , 150 lb/in. ; 1 h 2

15.5 13.3 12.8 11.3 11.1 11.0 10.0 5.0 2.6 2.2 0.3 -0.1

2 5 °C;

Soaking 15.2 0.0 12.5 13.1 8.8 0.5 10.0 11.0 0.8 1.6 0.5 2.1


180



at 2 5 ° C . I f t h e s o a k i n g at 2 5 ° C is c o n t i n u e d , p i p e r i d i n e r e a c h e s a n e q u i l i b r i u m s w e l l i n g c o e f f i c i e n t o f 1 3 . 1 a f t e r 9 0 - 1 0 0 d (3, 4) a n d a n i l i n e r e a c h e s a n e q u i l i b r i u m s w e l l i n g c o e f f i c i e n t o f 10 a f t e r 9 0 d (4). T r i e t h y l a m i n e a c t u a l l y c a u s e s t h e w o o d s t r u c t u r e t o s h r i n k s l i g h t l y at 1 2 0 ° C . T h i s is a l s o o b s e r v e d w i t h h e x a n e (Table I I I ) . T h e r e are no striking differences b e t w e e n the high a n d l o w t e m p e r a t u r e s o a k i n g v a l u e s for t h e s o l v e n t s l i s t e d i n T a b l e I I I . I n fact, t h e r e s e e m s to b e a c o r r e l a t i o n b e t w e e n h y d r o p h i l i c n a t u r e a n d d e g r e e o f s w e l l i n g . S t a m m (7) h a s s u g g e s t e d t h a t s o m e s o l v e n t s a c t u a l l y swell the carbohydrate-type polymers i n the cell wall and others swell l i g n i n . A t t e m p t s h a v e b e e n m a d e (4-7) t o c o r r e l a t e o b s e r v e d s w e l l i n g behavior w i t h certain p h y s i o c h e m i c a l properties of the liquids. Trends were noted between the degree of swelling and the dielectric constant, or b e t w e e n the surface tension, d i p o l e m o m e n t , m o l e c u l a r size, or the t e n d e n c y to h y d r o g e n b o n d w i t h m e t h a n o l . E v e r y t r e n d , h o w e v e r , h a d its e x c e p t i o n s . I t is k n o w n t h a t s w e l l i n g is d i r e c t l y r e l a t e d t o t h e d e n s i t y o f t h e w o o d (7, 11, 12). B e c a u s e l a t e w o o d o f m o s t s p e c i e s h a s a d e n s i t y m o r e t h a n t w i c e t h a t o f e a r l y w o o d , l a t e w o o d is a m a j o r c o n t r i b u t o r to s w e l l i n g . M a n y p h y s i c a l differences exist b e t w e e n latewood a n d earlyw o o d (see C h a p t e r 1). I n s o f t w o o d s , e a r l y w o o d t r a c h e i d s h a v e t h i n T a b l e III.

V o l u m e t r i c Swelling Coefficients (S) for S o u t h e r n P i n e Sapwood i n V a r i o u s Solvents

Reagent D i m e t h y l sulfoxide Dimethylformamide Cellosolve M e t h y l cellosolve Water Methanol 1,4-Dioxane Tetrahydrofuran Acetone Dichloromethane M e t h y l ethyl ketone E t h y l acetate Cyclohexanone 4-Methyl-2-pentanone Xylenes Cyclohexane Hexanes

120 °C; 150 lb/in. ; 1 h 2

13.3 12.8 10.6 10.3 10.0 9.0 6.5 5.4 5.1 3.8 3.6 2.4 2.3 0.4 0.1 0.1 -0.2

25 °C;

Soaking 11.7 12.5 10.2 10.0 10.0 9.3 0.6 7.2 5.6 3.3 5.0 4.2 0.5 1.5 0.2 0.1 0.2


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cell walls, large l u m e n s w i t h ends o v e r l a p p i n g those of other t r a cheids, a n d large a n d n u m e r o u s pit-pairs distributed along the radial face. H o w e v e r , p i t - p a i r s a r e m o s t a b u n d a n t o n t h e o t h e r e n d s w h e r e tracheids overlap each other. L a t e w o o d tracheids have thick cell walls, narrow l u m e n s , and fewer and smaller pit-pairs o n the radial wall. I n a d d i t i o n , the latewood tracheids are p r e d o m i n a n t l y p i t t e d o n t h e i r t a n g e n t i a l w a l l s ( 1 3 , 14). T h e thick cell walls of latewood [mainly caused by a thicker S l a y e r i n t h e c e l l w a l l (15)] r e s u l t i n less p i t a s p i r a t i o n o n d r y i n g (1618). T h e m a i n f l o w o f l i q u i d s i n s o f t w o o d s is t h r o u g h t h e l u m e n s o f tracheids by way of bordered pit-pairs.


2

Several studies have b e e n concerned w i t h the penetration of l i q u i d s i n t o l a t e w o o d a n d e a r l y w o o d ( J J , 16-23). U n d e r atmospheric pressure, the penetration of nonpolar liquids into softwood latewood may be caused, i n part, b y capillary action i n the very small lumens a n d passage t h r o u g h u n a s p i r a t e d p i t m e m b r a n e s . I n a s p i r a t e d ear l y w o o d this penetration w o u l d not occur. Penetration of nonpolar l i q u i d s m a y also b e t h r o u g h d r y i n g c h e c k s i n the t h i c k l a t e w o o d c e l l walls. A s the t e m p e r a t u r e a n d pressure of the l i q u i d are raised, p e n e t r a t i o n of p o l a r l i q u i d s i n e a r l y w o o d w o u l d b e e x p e c t e d to increase because of softening of the pit structure a n d displacement of the pit m e m b r a n e . B e c a u s e t h e c e l l w a l l o f e a r l y w o o d is t h i n n e r t h a n t h a t of latewood, penetration into earlywood walls w o u l d be quicker and facilitated b y swelling. Incrustation occurs i n the pit membranes of s o u t h e r n p i n e l a t e w o o d (24); t h i s w o u l d r e t a r d l i q u i d p e n e t r a t i o n . Reactants. C e l l u l o s e , h e m i c e l l u l o s e s , a n d l i g n i n are d i s t r i b uted throughout the w o o d cell wall. These three hydroxyl-containing p o l y m e r s m a k e u p the solid phase of w o o d . T h e v o i d structure or l u m e n s i n w o o d c a n b e v i e w e d as a b u l k s t o r a g e r e s e r v o i r f o r c h e m i c a l reactants, w h i c h c o u l d b e u s e d to m o d i f y the c e l l w a l l p o l y m e r s . F o r example, the v o i d v o l u m e of southern pine earlywood w i t h a density o f 0 . 3 3 g / c m is 0 . 7 7 c m v o i d s / c m w o o d o r 2 . 3 c m / g . F o r l a t e w o o d w i t h a d e n s i t y o f 0 . 7 0 g / c m , t h e v o i d v o l u m e is 0 . 5 2 c m / c m o r 0 . 7 4 c m / g . T h e c e l l w a l l c a n a l s o s w e l l a n d act as a c h e m i c a l s t o r a g e reservoir. F o r s o u t h e r n p i n e , the change i n c e l l w a l l storage v o l u m e from o v e n - d r y t o w a t e r - s w o l l e n is 0 . 0 7 7 c m / c m . T h e s e d a t a s h o w t h a t t h e r e is m o r e t h a n e n o u g h v o l u m e i n t h e v o i d s i n w o o d t o h o u s e 3

3

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3

3

3

3

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sufficient c h e m i c a l reactants for a r e a c t i o n to take p l a c e w i t h t h e c e l l wall polymers. P o t e n t i a l reactants m u s t c o n t a i n f u n c t i o n a l groups that w i l l react w i t h h y d r o x y l groups of the w o o d components. T h e r e are m a n y l i t e r a t u r e reports o f c h e m i c a l s that f a i l e d to react w i t h w o o d c o m p o n e n t s w h e n , i n fact, the c h e m i c a l s d i d n o t c o n t a i n f u n c t i o n a l g r o u p s that c o u l d react.



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T H E CHEMISTRY O F SOLID WOOD

T h e c h e m i c a l b o n d desired b e t w e e n the reagent a n d the w o o d c o m p o n e n t is o f m a j o r c o n s i d e r a t i o n . F o r p e r m a n e n c e , t h i s b o n d s h o u l d h a v e e n o u g h s t a b i l i t y to w i t h s t a n d e n v i r o n m e n t a l s t r e s s e s . I n s u c h cases, t h e e t h e r l i n k a g e m a y b e the m o s t d e s i r a b l e co v a l e n t C O b o n d . E t h e r linkages are m o r e stable than the glycosidic acetal bonds b e t w e e n sugar units i n the w o o d polysaccharides; therefore, the polysaccharides w o u l d degrade before the b o n d e d ethers. Less stable b o n d s c a n also b e f o r m e d w h i c h w o u l d b e u s e f u l for t h e release of a b o n d e d c h e m i c a l u n d e r e n v i r o n m e n t a l stresses. A c e t a l s a n d es ters are less stable t h a n e t h e r b o n d s a n d c o u l d b e u s e d to b o n d b i o l o g i c a l agents o r fire retardants to the w o o d i n s u c h a w a y that they w o u l d be released u n d e r certain conditions. Unless all of the r e a g e n t s k e l e t o n b e c o m e s b o n d e d to t h e w o o d , i . e . , n o b y - p r o d u c t s are generated, e c o n o m i c s m a y dictate that a recovery system be i m plemented. G a s r e a c t a n t s a r e d i f f i c u l t to h a n d l e b e c a u s e t h e y r e q u i r e h i g h p r e s s u r e e q u i p m e n t . A l s o , t h e l e v e l o f c h e m i c a l s u b s t i t u t i o n is u s u a l l y l o w e r i n gas t h a n i n l i q u i d s y s t e m s , a n d gas p e n e t r a t i o n c a n b e v e r y d i f f i c u l t . T h e b e s t s u c c e s s , t o d a t e , o f c h e m i c a l s y s t e m s is w i t h l o w b o i l i n g l i q u i d s t h a t s w e l l w o o d e a s i l y . I f t h e b o i l i n g p o i n t is t o o h i g h , i t is d i f f i c u l t t o r e m o v e e x c e s s r e a g e n t a f t e r t r e a t m e n t . G e n e r a l l y , t h e l o w e s t m e m b e r o f a h o m o l o g o u s s e r i e s is t h e m o s t r e a c t i v e a n d has t h e l o w e s t b o i l i n g p o i n t . S o m e chemicals react c o m p l e t e l y w i t h a single h y d r o x y l group. S u c h is t h e c a s e , f o r e x a m p l e , w i t h m e t h y l a t i o n u s i n g m e t h y l i o d i d e . O t h e r c h e m i c a l s , s u c h as e p o x i d e s , i n t h e c o u r s e o f r e a c t i n g f o r m a n e w h y d r o x y l g r o u p that reacts f u r t h e r . I n o t h e r w o r d s , cases s u c h as m e t h y l a t i o n i n v o l v e s i n g l e - s i t e s u b s t i t u t i o n , w h e r e a s cases s u c h as e p o x i d a t i o n i n v o l v e p o l y m e r f o r m a t i o n f r o m a s i n g l e graft p o i n t . T h i s w i l l be discussed i n detail later in this chapter. F r o m the s t a n d p o i n t of i n d u s t r i a l a p p l i c a t i o n of reagents for w o o d , toxicity, c o r r o s i v i t y , a n d cost are i m p o r t a n t factors i n s e l e c t i n g a c h e m i c a l . T h e reacted chemicals s h o u l d not be toxic or carcinogenic i n t h e finished p r o d u c t , a n d t h e r e a c t a n t i t s e l f s h o u l d b e as n o n t o x i c as p o s s i b l e i n t h e t r e a t i n g s t a g e . T h i s is s o m e w h a t d i f f i c u l t t o a c h i e v e because c h e m i c a l s that react easily w i t h w o o d h y d r o x y l groups w i l l also react easily w i t h b l o o d a n d tissue h y d r o x y l - c o n t a i n i n g p o l y m e r s . T h e r e a c t a n t s s h o u l d b e as n o n c o r r o s i v e as p o s s i b l e t o e l i m i n a t e t h e n e e d for s p e c i a l t r e a t m e n t of e q u i p m e n t . I n the laboratory e x p e r i m e n t a l s t a g e , t h e h i g h c o s t o f c h e m i c a l s is n o t a m a j o r c o n s i d e r a t i o n . C h e m i c a l c o s t is i m p o r t a n t , h o w e v e r , i n c o m m e r c i a l i z a t i o n o f a p r o cess. Conditions. T h e r e are certain e x p e r i m e n t a l conditions that m u s t b e c o n s i d e r e d b e f o r e a r e a c t i o n s y s t e m is s e l e c t e d . T h e t e r n -



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p e r a t u r e r e q u i r e d for c o m p l e t e r e a c t i o n m u s t be l o w e n o u g h that it causes little or no w o o d degradation. H o w e v e r , the rate of reaction m u s t b e r e l a t i v e l y fast. A safe u p p e r l i m i t is a b o u t 1 2 0 ° C , b e c a u s e l i t t l e w o o d d e g r a d a t i o n o c c u r s at t h i s t e m p e r a t u r e o v e r a s h o r t p e r i o d of t i m e . It is i m p r a c t i c a l to d r y w o o d to less t h a n 1 % m o i s t u r e , b u t t h e w a t e r c o n t e n t of the w o o d d u r i n g reaction is, i n most cases, critical. T h e h y d r o x y l i n w a t e r is m o r e r e a c t i v e t h a n t h e h y d r o x y l g r o u p s a v a i l a b l e i n w o o d c o m p o n e n t s , i . e . , h y d r o l y s i s is f a s t e r t h a n s u b s t i t u t i o n . T h e m o s t f a v o r a b l e c o n d i t i o n is a r e a c t i o n s y s t e m i n w h i c h t h e r a t e o f r e a g e n t h y d r o l y s i s is r e l a t i v e l y s l o w . It is a l s o i m p o r t a n t to k e e p t h e r e a c t i o n s y s t e m s i m p l e . I t is b e s t to a v o i d m u l t i c o m p o n e n t s y s t e m s t h a t r e q u i r e c o m p l e x s e p a r a t i o n p r o c e d u r e s to r e c o v e r t h e c h e m i c a l s a f t e r t h e r e a c t i o n . T h e o p t i m u m system w o u l d be w h e n the reacting c h e m i c a l swells the w o o d struc t u r e a n d acts as t h e s o l v e n t . A l m o s t a l l c h e m i c a l reactions r e q u i r e a catalyst. S t r o n g a c i d cat alysts c a n n o t b e u s e d w i t h w o o d b e c a u s e t h e y cause extensive d e g radation. T h e most favorable catalyst f r o m the standpoint of w o o d d e g r a d a t i o n is a w e a k l y a l k a l i n e o n e . A l k a l i n e c a t a l y s t s a r e a l s o f a vored because m a n y of t h e m swell the w o o d structure a n d give better p e n e t r a t i o n (see T a b l e II). T h e c a t a l y s t u s e d s h o u l d b e e f f e c t i v e at l o w r e a c t i o n t e m p e r a t u r e s , easily r e m o v e d after r e a c t i o n , n o n t o x i c , and n o n c o r r o s i v e . I n most cases, the organic tertiary a m i n e s are best s u i t e d for this p u r p o s e . T h e reaction conditions m u s t be m i l d enough that the reacted w o o d still possesses the desirable properties of w o o d . T h e w o o d s h o u l d r e m a i n s t r o n g , r e t a i n its n a t u r a l c o l o r ( u n l e s s a c o l o r c h a n g e is d e s i r a b l e ) , s t i l l b e a g o o d e l e c t r i c a l i n s u l a t o r , n o t b e c o m e d a n gerous to h a n d l e , a n d b e g l u a b l e a n d p a i n table.

Reactions with Wood Esters. ACETYLATION. T h e most studied of all the c h e m i c a l m o d i f i c a t i o n t r e a t m e n t s f o r w o o d has b e e n a c e t y l a t i o n . T h e e a r l y w o r k was d o n e w i t h acetic a n h y d r i d e catalyzed w i t h p y r i d i n e ( 2 5 ) or z i n c c h l o r i d e (26). I n t h e r e a c t i o n w i t h a c e t i c a n h y d r i d e , a c e t y l a t i o n o c c u r s , a n d a c e t i c a c i d is s p l i t o u t as a b y - p r o d u c t : Ο

Il

Wood-OH

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T h e r e a c t i o n is a c i d o r b a s e c a t a l y z e d . M a n y c a t a l y s t s h a v e b e e n t r i e d , i n c l u d i n g p o t a s s i u m a c e t a t e a n d s o d i u m a c e t a t e (27), d i m e t h y l f o r m a m i d e ( D M F ) (28-30), u r e a a m m o n i u m s u l f a t e (29), m a g n e s i u m p e r c h l o r a t e (31-33), t r i f l u o r o a c e t i c a c i d (32), b o r o n t r i f l u o r i d e (30), s o d i u m a c e t a t e (31), p o t a s s i u m h y d r o g e n p h o s p h a t e (34), a n d 7 - r a y s (35). T h e b e s t a c e t y l a t i o n c o n d i t i o n , h o w e v e r , is u n c a t a l y z e d a c e t i c a n h y d r i d e i n x y l e n e at 1 0 0 - 1 3 0 ° C (36). A c e t y l a t i o n is a s i n g l e - s i t e r e a c t i o n , t h a t i s , o n e a c e t y l p e r r e acted h y d r o x y l group w i t h n o p o l y m e r i z a t i o n . T h i s means that a l l the weight gain i n acetyl can be converted directly into units of hydroxyl groups blocked. W h e n p o l y m e r chains are f o r m e d the weight gain cannot be converted into units of blocked hydroxyl groups. A t w e i g h t gains above 1 7 % , acetylated w o o d was found i n soilb l o c k tests (90 d ) t o r e s i s t a t t a c k b y t h e f u n g i Coniophora puterana (36), Lentinus lepideus (36), Porta incrassata (36, 37), Polyporus ver sicolor (36-38), Gloeophyllum trabeum (36, 38, 39), Porta monticola (36), Porta microsporia (37), a n d Coniophora cerebella (40-43). Acet y l a t e d , l a m i n a t e d v e n e e r s o f y e l l o w b i r c h i n g r o u n d contact stake tests at 1 9 . 2 % w e i g h t g a i n h a d a n a v e r a g e l i f e o f 1 7 . 5 y e a r s c o m p a r e d t o 2 . 7 y e a r s f o r u n t r e a t e d c o n t r o l s (44). A c e t y l a t i o n to a w e i g h t gain of 2 0 - 2 5 % s h o w e d a 7 0 % r e d u c t i o n i n s w e l l i n g o r A S E (37, 38, 45, 46). S o u t h e r n y e l l o w p i n e w e a t h e r e d for 12 m o n t h s d e c r e a s e d s l i g h t l y i n a c e t y l c o n t e n t . Its A S E d r o p p e d from 7 8 t o 6 4 % (38). DENSITY. A c e t y l a t e d w o o d is m o r e d e n s e t h a n u n t r e a t e d w o o d a n d h a s f e w e r fibers o f l i g n o c e l l u l o s e p e r u n i t v o l u m e (47). T h i s effect is c a u s e d b y t h e b u l k i n g o f t h e a c e t a t e , w h i c h i s m o r e d e n s e t h a n water. W o o d u s u a l l y gets s l i g h t l y d a r k e r after a c e t y l a t i o n w i t h u n c a t a l y z e d acetic a n h y d r i d e ; i t also loses m u c h o f its n a t u r a l b r i l l i a n c e (48) . T h e c h a n g e i n c o l o r w i t h c a t a l y z e d a c e t y l a t i o n v a r i e s d e p e n d i n g o n t h e reaction conditions a n d catalyst. C o l o r changes from a slight d a r k e n i n g to almost b l a c k w i t h p y r i d i n e a n d D M F have b e e n f o u n d . A c e t y l a t e d w o o d i s l e s s p e r m e a b l e t o gases t h a n u n t r e a t e d w o o d (49) . T h i s m a y b e c a u s e d b y t h e b u l k i n g c h e m i c a l s w h i c h r e s t r i c t t h e p o r e space. M o i s t u r e a b s o r p t i o n decreases b y a factor o f t w o to three (50) as d o e s o v e r a l l w a t e r r e s i s t a n c e ( 5 1 , 52). A c e t y l a t i o n i n a N 0 Ν , Ν - D M F - p y r i d i n e s y s t e m causes a p e r m a n e n t loss o f c e l l u l o s e c r y s t a l l i n i t y ( 5 3 , 54). T h e loss o f c r y s t a l l i n i t y y i e l d s a u n i f o r m d i s t r i bution of acetyl groups i n cellulose. 2

4

T h e m e c h a n i c a l p r o p e r t i e s of acetylated w o o d are generally e q u a l to those o f u n t r e a t e d w o o d . H o w e v e r , shear strength parallel t o t h e g r a i n d e c r e a s e s i n t r e a t e d w o o d (47), a n d t h e m o d u l u s o f e l a s t i c i t y d e c r e a s e s s l i g h t l y (54). I m p a c t s t r e n g t h (38) o r m o d u l u s o f e l a s t i c i t y (or stiffness) a r e u n c h a n g e d (47). W e t a n d d r y c o m p r e s s i v e


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s t r e n g t h (38, 4 7 ) , h a r d n e s s , fiber s t r e s s at p r o p o r t i o n a l l i m i t (47), a n d w o r k t o p r o p o r t i o n a l l i m i t (47) a r e i n c r e a s e d . M o d u l u s o f r u p t u r e is i n c r e a s e d f o r s o f t w o o d s b u t d e c r e a s e d f o r h a r d w o o d s (47). R e s u l t s o f a 2 - y e a r p a i n t s t u d y i n d i c a t e a c e t y l a t e d w o o d is a b e t t e r p a i n t i n g s u r f a c e (37) t h a n u n t r e a t e d w o o d . U V r a d i a t i o n d a r k e n s u n a c e t y l a t e d w o o d , b u t t h e r e is n o c h a n g e o r a s l i g h t b l e a c h e d effect w i t h a c e t y l a t e d w o o d (37). I n g e n e r a l , a c e t y l a t i o n r e d u c e s t h e a d h e s i v e s t r e n g t h o f w o o d (48). A d h e s i v e s t r e n g t h is r e d u c e d w i t h u r e a - f o r m a l d e h y d e r e s i n s (54, 55) a n d c a s e i n g l u e s (55), b u t t h e r e i s v e r y l i t t l e e f f e c t w i t h r e s o r c i n o l - f o r m a l d e h y d e r e s i n s (55). M a n y of the properties of acetylated w o o d d e p e n d on the m e t h o d of acetylation. T h e temperature of treatment, time of reac tion, a n d t y p e a n d a m o u n t of catalysts p l a y a significant role i n the e x t e n t t h a t fibers d e g r a d e d u r i n g t r e a t m e n t . T h e a m o u n t o f m o i s t u r e p r e s e n t i n t h e w o o d a l s o is i m p o r t a n t . S o m e m o i s t u r e ( 2 - 5 % ) s e e m s to b e n e e d e d to o b t a i n t h e b e s t r e a c t i o n , b u t a b o v e this l e v e l t h e w a t e r h y d r o l y z e s t h e a c e t i c a n h y d r i d e to a c e t i c a c i d . T h i s loss b y h y d r o l y s i s a c c o u n t s f o r a 5 . 7 % loss o f a n h y d r i d e w i t h e a c h 1 % o f w a t e r i n t h e w o o d (36). T h e r a t e o f a c e t y l a t i o n d e c r e a s e s as m o i s t u r e c o n t e n t i n c r e a s e s (37). T h e a n h y d r i d e m e t h o d of acetylation gives an acid b y - p r o d u c t t h a t r e s u l t s i n a n a c i d i c c o n d i t i o n i n t h e w o o d a n d a loss o f 5 0 % o f t h e r e a c t i o n c h e m i c a l . T h e s e b y - p r o d u c t s m u s t b e r e m o v e d to p r e vent degradation. A c e t i c acid, the by-product of acetylation w i t h a c e t i c a n h y d r i d e , is v i r t u a l l y i m p o s s i b l e t o r e m o v e c o m p l e t e l y f r o m w o o d . T h i s results i n a p r o d u c t that smells of acetic acid, acid c o n ditions that catalyze the r e m o v a l of m o r e acetyl groups, acid h y d r o l y s i s o f c e l l u l o s e fibers w h i c h r e s u l t s i n s t r e n g t h losses o v e r a l o n g t e r m , a n d a c i d corrosion of m e t a l fasteners u s e d i n the w o o d p r o d u c t . A c e t y l a t i o n can also b e d o n e b y v a p o r - p h a s e t r e a t m e n t s , b u t t h e d i f f u s i o n r a t e v a r i e s i n v e r s e l y as t h e s q u a r e o f t h e t h i c k n e s s (37, 56). B e c a u s e o f t h i s e f f e c t , v a p o r - p h a s e t r e a t m e n t has b e e n a p p l i e d o n l y to t h i n v e n e e r s . A n o t h e r m e t h o d for the acetylation of w o o d involves reaction w i t h k e t e n e gas d i s s o l v e d i n a c e t o n e o r t o l u e n e (57-61): Ο Wood-OH

+ C H = C = 0-> 2

W o o d - 0 - C - C H

3

R e a c t i o n s c a r r i e d o u t at 5 5 - 6 0 ° C f o r 6 - 8 h p r o d u c e w e i g h t g a i n s o f 2 2 % (59). M u c h o f t h e w o r k w i t h k e t e n e , h o w e v e r , has r e s u l t e d i n m u c h l o w e r w e i g h t gains. A t the h i g h e r l e v e l of treatment, the acet-


186

T H E CHEMISTRY O F SOLID WOOD

y l a t e d w o o d s h o w s a r e d u c t i o n i n w a t e r a b s o r p t i o n b y 352, t a n g e n t i a l s w e l l i n g b y 71%, a n d r a d i a l s w e l l i n g b y 69$

(58).

Vapor-phase acetylation w i t h ethanethioic acid produces a m o d ified w o o d w i t h s l i g h t l y l o w e r w e i g h t gains t h a n acetylation

with

a c e t i c a n h y d r i d e (62). A t w e i g h t g a i n s o f a b o u t 1 7 % , t h e t r e a t e d w o o d has a n A S E o f 4 8 % . E t h a n e t h i o i c a c i d is l e s s c o r r o s i v e t h a n a c e t i c a n h y d r i d e , b u t t h e t r e a t e d w o o d c o n t i n u e s to e m i t h y d r o g e n

sulfide

because of the e n t r a p m e n t of small amounts of ethanethioic acid. In spite of the vast a m o u n t of research i n the acetylation of w o o d , t h e p r o c e s s has n o t b e e n a p p l i e d c o m m e r c i a l l y . T w o a t t e m p t s ,


i n t h e U n i t e d S t a t e s (38)

one

a n d o n e i n R u s s i a (63, 64), c a m e c l o s e t o

commercialization but were discontinued, presumably because they w e r e not cost

effective.

PHTHALYLATION.

A w o o d p r o d u c t that has a v e r y h i g h i n i t i a l A S E

can be obtained b y using phthalic anhydride. T h e initial A S E de c r e a s e s i f t h e w o o d is s o a k e d r e p e a t e d l y i n w a t e r (65). S t a r t i n g at a n ASE

o f 1 0 0 % o n t h e first s o a k c y c l e , t h e A S E v a l u e d r o p s t o a b o u t

7 0 % o n t h e s e c o n d c y c l e , 6 0 % o n the t h i r d c y c l e , a n d d o w n to 5 0 % o n t h e s i x t h c y c l e . T h e r e is a c o r r e s p o n d i n g loss o f b o n d e d

chemical

after e a c h s o a k i n g , w h i c h s h o w s t h e s u s c e p t i b i l i t y to h y d r o l y s i s t h e p h t h a l y l g r o u p (66).

of

P h t h a l y l groups have a greater affinity for

w a t e r t h a n d o t h e h y d r o x y l g r o u p s i n w o o d , so p h t h a l y l a t e d w o o d is m o r e h y g r o s c o p i c t h a n u n t r e a t e d w o o d (66, 67). W h e r e a s t h e m e c h a n i s m of A S E effectiveness the h y d r o x y l groups,

b y a c e t y l a t i o n is b y c h e m i c a l b l o c k i n g o f

phthalylation operates

mainly by

mechanical

b u l k i n g o f t h e s u b m i c r o s c o p i c p o r e s i n t h e w o o d c e l l w a l l (68). P h t h a l ylation produces

v e r y h i g h w e i g h t g a i n s (65, 69). M o s t

researchers

h a v e f o u n d t h a t a c e t y l a t i o n w e i g h t g a i n s r a n g e f r o m 15 to 2 1 % , w h e r e a s p h t h a l y l a t i o n w e i g h t g a i n s r a n g e f r o m 4 0 t o 1 3 0 % (65,

69).

T h e s e h i g h w e i g h t gains m a y result f r o m a p o l y m e r i z a t i o n reaction. OTHER ANHYDRIDES.

O t h e r anhydrides have been reacted w i t h

wood, including propionic and butyric anhydrides in xylene without catalyst. T h e s e c o m p o u n d s react slower than acetic a n h y d r i d e

(36).

A f t e r a 1 0 - h r e a c t i o n t i m e ( i n x y l e n e at 1 2 5 ° C w i t h p o n d e r o s a

pine)

a c e t y l a t i o n p r o d u c e s w e i g h t g a i n s o f 1 7 % , c o m p a r e d t o less t h a n 4 % for p r o p i o n y l a t i o n a n d n o w e i g h t gain for b u t y r y l a t i o n . A f t e r 30 h of reaction, p r o p i o n i c a n h y d r i d e produces a weight gain of 10%.

Reac

t i o n w i t h b u t y r i c a n h y d r i d e p r o d u c e d l i t t l e o r n o w e i g h t g a i n (36). ACID CHLORIDES. t i o n r e a c t i o n s (70).

A c i d c h l o r i d e s c a n also b e u s e d i n e s t e r i f i c a -

T h e p r o d u c t is t h e e s t e r o f t h e r e a c t e d a c i d c h l o

r i d e , w i t h h y d r o c h l o r i c a c i d as a b y - p r o d u c t : Ο Wood-OH

Ο

+ R-C-C1-H> W o o d - O - C - R

+

HC1



4.

ROWELL

187


U s i n g l e a d a c e t a t e as a c a t a l y s t w i t h a c e t y l c h l o r i d e , S i n g h e t a l . (71) f o u n d a l o w e r a c e t y l c o n t e n t t h a n w i t h t h e a c e t i c a n h y d r i d e method. T h e y obtained m u c h higher A S E values, however, with a c e t y l c h l o r i d e ( 6 0 - 8 4 % for a c e t y l c h l o r i d e vs. 4 7 % for acetic a n h y dride). B y u s i n g a 2 0 % l e a d acetate s o l u t i o n , the a m o u n t of free H C 1 r e l e a s e d i n t h e r e a c t i o n is r e d u c e d . T h i s v e r y s t r o n g a c i d c a u s e s extensive degradation of the w o o d , a n d because of this v e r y little w o r k has b e e n d o n e i n this area. CARBOXYLIC ACIDS. C a r b o x y l i c acids h a v e b e e n e s t e r i f i e d to w o o d c a t a l y z e d w i t h t r i f l u o r o a c e t i c a n h y d r i d e (72, 73). S e v e r a l u n saturated carboxylic acids react w i t h w o o d b y the trifluoroacetic a n h y d r i d e impelling m e t h o d to g i v e a n increase i n o v e n - d r y v o l u m e and A S E , a n d a decrease i n w o o d crystallinity a n d m o i s t u r e content (74). R e a c t i o n s o f w o o d w i t h β - m e t h y l c r o t o n i c a c i d ( R e a c t i o n 1) g i v e a d e g r e e o f s u b s t i t u t i o n h i g h e n o u g h to m a k e t h e e s t e r i f i e d w o o d s o l u b l e i n a c e t o n e a n d C H C 1 t o t h e e x t e n t o f 3 0 % (75). 3

H - C - C O O H

Ο

I

H

3

C - C - C H

3

II

+ W o o d - O H - * W o o d - O - C - C - H

II

H C - c - CH3 F u r t h e r e s t e r i f i c a t i o n i n c r e a s e s t h e s o l u b i l i t y b u t is a c c o m p a n i e d

(1)

3

considerable degradation of w o o d components.

by

Solubilization seems

t o b e h i n d e r e d b y b o t h l i g n i n a n d h e m i c e l l u l o s e (76, 77). Isocyanates.

A n i t r o g e n - c o n t a i n i n g e s t e r is f o r m e d i n t h e r e a c

tion of w o o d hydroxyls w i t h isocyanates: Ο

il

Ο

,

il

Wood-OH + R - N = C-» W o o d - O - C - N H R W o o d v e n e e r s w o l l e n i n D M F was e x p o s e d to v a p o r s o f p h e n y l i s o c y a n a t e at 1 0 0 - 1 2 5 ° C (29). T h e w o o d g a i n e d n o w e i g h t , b u t t h e A S E w a s as h i g h as 7 7 % . T h e m o d i f i e d v e n e e r s s h o w e d i n c r e a s e d m e c h a n i c a l s t r e n g t h w i t h l i t t l e o r n o c h a n g e i n c o l o r . B a i r d (28) r e a c t e d D M F - s o a k e d cross sections of w h i t e p i n e a n d E n g e l m a n n spruce w i t h e t h y l , a l l y l , b u t y l , tert-butyl, a n d p h e n y l isocyanates. Vapor-phase reactions of b u t y l isocyanate i n D M F gave the best results. T h e reac tion p r o d u c e d A S E values of 4 7 % w i t h a 14% gain i n weight a n d 6 7 % w i t h a 3 1 % g a i n i n w e i g h t . W e i g h t g a i n s w e r e as h i g h as 5 0 % w i t h an A S E o f 7 5 - 8 0 % . T h e s a m p l e s t r e a t e d to 6 7 % A S E h a d a b o u t a 2 5 % r e d u c t i o n i n toughness a n d abrasion resistance. W h i t e c e d a r w a s r e a c t e d w i t h 2 , 4 - t o l y l e n e d i i s o c y a n a t e (78) w i t h and w i t h o u t a p y r i d i n e catalyst to a m a x i m u m n i t r o g e n c o n t e n t o f 3.5 and 1.2%, r e s p e c t i v e l y . T h i s c o r r e s p o n d s to w e i g h t gains of 2 1 . 8 a n d 7.5%. T h i s h i g h w e i g h t gain was a c c o m p a n i e d b y an A S E of 5 0 % .


188

THE

CHEMISTRY O F SOLID W O O D


Compressive strength and bending modulus increased with i n creasing nitrogen content. B e e c h w o o d modified w i t h a diisocyanate (79) u p t o 5 0 % w e i g h t g a i n l o s t 4 . 5 - 8 . 1 % w e i g h t after 6 w e e k s o f a t t a c k b y t h e f u n g i Coniophora cerehella a n d Polystictus versicolor. A f t e r f u n g a l attack, t h e m o d i f i e d w o o d lost a l m o s t 2 0 % o f its static b e n d i n g s t r e n g t h as c o m p a r e d t o t h e m o d i f i e d w o o d b e f o r e f u n g a l attack. A t c h e m i c a l add-ons o v e r 1 8 % , w o o d m o d i f i e d w i t h m e t h y l , e t h y l , η-propyl, a n d η-butyl isocyanates was resistant to attack b y Gloeophyllum trabeum (80). M e t h y l isocyanate reacts v e r y q u i c k l y w i t h o u t catalyst to g i v e w e i g h t g a i n s u p t o a p p r o x i m a t e l y 7 5 % (81). M a x i m u m A S E v a l u e s o f 6 0 % a r e o b t a i n e d at w e i g h t g a i n s o f 2 5 - 3 0 % . A b o v e t h i s l e v e l o f b o n d e d w e i g h t g a i n , t h e A S E values start to decrease. S c a n n i n g e l e c t r o n m i c r o g r a p h s s h o w t h a t h i g h l e v e l s o f c h e m i c a l a d d - o n s to t h e cell w a l l p o l y m e r s cause splitting i n the tracheid wall a n d not i n the i n t e r c e l l u l a r s p a c e s (80). I n s o m e c a s e s t h e s p l i t s g o t h r o u g h t h e b o r d e r e d p i t s . W h e n t h e t r a c h e i d w a l l s p l i t s , t h e A S E starts to d r o p a n d c o n t i n u e s t o d r o p as c h e m i c a l w e i g h t g a i n i n c r e a s e s . S p l i t t i n g e x p o s e s n e w fiber s u r f a c e s w h e r e w a t e r c a n c a u s e s w e l l i n g . S w e l l i n g b e y o n d t h e g r e e n w o o d v o l u m e t a k e s p l a c e b e c a u s e t h e c e l l w a l l is r u p t u r e d a n d n o l o n g e r acts as a r e s t r a i n t t o s w e l l i n g . E t h y l , n - p r o p y l , η - b u t y l , a n d p h e n y l i s o c y a n a t e s also r e a c t w i t h w o o d w i t h o u t t h e n e e d f o r a c a t a l y s t ; b u t p - t o l y l i s o c y a n a t e , 1,6diisocyanatohexane, and tolylene 2,4-diisocyanate require either DMF o r t r i e t h y l a m i n e as a c a t a l y s t (80). H i g h w e i g h t g a i n s a r e o b s e r v e d w i t h t h e s e last t h r e e i s o c y a n a t e s , b u t l i t t l e o r n o d i m e n s i o n a l stability results from the reaction. Therefore, polymerization must be taking place i n the void structure. Isocyanates are s e n s i t i v e to m o i s t u r e ; t h e r e f o r e , t h e r e a c t i o n n e e d s t o b e d o n e o n d r y w o o d (82). A s w o o d m o i s t u r e c o n t e n t i n creases b e f o r e r e a c t i o n , m o r e n o n b o n d e d p o l y m e r s are f o r m e d after reaction. R e a c t e d moist w o o d shows v e r y h i g h A S E values on the first w a t e r - s o a k t e s t , b u t l e a c h i n g c a u s e s a s i g n i f i c a n t loss i n A S E . T h i s s h o w s t h a t t h e b u l k i n g c h e m i c a l is n o t b o n d e d t o t h e c e l l w a l l but comes out u p o n water leaching. Acetals. FORMALDEHYDE. W o o d hydroxyls and formaldehyde r e a c t i n t w o s t e p s ( R e a c t i o n 2). B e c a u s e t h e b o n d i n g is b e t w e e n t w o h y d r o x y l g r o u p s , t h e r e a c t i o n is c a l l e d c r o s s - l i n k i n g . Ο Wood-OH

I

+ H - C - H

OH W o o d - Ο - in

2

Wood-OH

>

(hemiacetal) Wood - O - C H 2 - O - Wood (acetal)


φ

4.

ROWELL


189


T h e t w o h y d r o x y l g r o u p s m a y c o m e f r o m (1) h y d r o x y l s w i t h i n a s i n g l e s u g a r r e s i d u e ; (2) h y d r o x y l s o n d i f f e r e n t s u g a r r e s i d u e s w i t h i n a s i n g l e c e l l u l o s e c h a i n ; (3) h y d r o x y l s b e t w e e n t w o d i f f e r e n t c e l l u l o s e c h a i n s ; (4) s a m e as i n (1), (2), a n d (3) e x c e p t r e a c t i o n o c c u r s o n t h e h e m i c e l l u l o s e s ; (5) h y d r o x y l s o n d i f f e r e n t l i g n i n r e s i d u e s ; a n d (6) i n t e r a c t i o n between cellulose, hemicelluloses, and lignin hydroxyls. T h e possible cross-linking combinations are many, a n d theoretically a l l o f t h e m are p o s s i b l e . B e c a u s e t h e r e a c t i o n is a t w o - s t e p m e c h a n i s m , s o m e o f t h e added formaldehyde will be i n the noncross-linked form of hemiacetals. T h e s e b o n d s are v e r y u n s t a b l e a n d w o u l d not s u r v i v e l o n g after treatment. T h e r e a c t i o n i s b e s t c a t a l y z e d b y s t r o n g a c i d s , s u c h as H C l ( 8 3 86), H N 0 (85), S 0 (87, 8 8 ) , p - t o l u e n e s u l f o n i c a c i d , a n d z i n c c h l o r i d e (84, 89). W e a k e r a c i d s , s u c h as s u l f u r o u s a n d f o r m i c a c i d , d o n o t w o r k (85). B a s e s , s u c h as l i m e w a t e r o r t e r t i a r y a m i n e s , c a n i n i t i a t e t h e r e a c t i o n (90), b u t a t t e m p t s w i t h t r i e t h y l a m i n e w e r e u n s u c c e s s f u l (91). W h e n its w e i g h t i s i n c r e a s e d b y 2 % , f o r m a l d e h y d e - t r e a t e d w o o d is n o t a t t a c k e d b y f u n g i (92). T h i s is far s h o r t o f t h e q u a n t i t y o f c r o s s l i n k i n g n e e d e d to p r e v e n t attack o n t h e basis o f h y d r o x y l b l o c k i n g for e n z y m e i n h i b i t i o n . C r o s s - l i n k i n g , w h i c h is effective at these l o w l e v e l s , m u s t b e t y i n g t o g e t h e r s t r u c t u r a l u n i t s (92). A n A S E o f 4 7 % is a c h i e v e d at a w e i g h t g a i n o f 3 . 1 % , a n A S E o f 5 5 % at 4 . 1 % , a n A S E o f 6 0 % a t 5 . 5 % , a n d a n A S E o f 9 0 % at 7 % ( 8 5 , 8 9 ) . T h u s , a w e i g h t g a i n o f 4 % r e s u l t s i n 4 t i m e s t h e A S E as w o u l d b e f o u n d b y b u l k i n g t r e a t m e n t s s u c h as a c e t y l a t i o n . The mechanical properties observed i n formaldehyde-treated w o o d are r e d u c e d c o m p a r e d to those observed i n untreated w o o d . T o u g h n e s s a n d a b r a s i o n r e s i s t a n c e d e c r e a s e g r e a t l y (85, 89), c r u s h i n g s t r e n g t h a n d b e n d i n g s t r e n g t h s d e c r e a s e a b o u t 2 0 % (93), a n d i m p a c t b e n d i n g s t r e n g t h d e c r e a s e s u p t o 5 0 % (93). T h e m e a s u r e m e n t s d o n e t h u s far o n t h e last t w o p r o p e r t i e s h a v e b e e n d o n e o n 7-ray-treated w o o d ; consequently part o f the strength reduction may b e due to the 7 - r a y t r e a t m e n t . T h e loss i n t o u g h n e s s i s d i r e c t l y p r o p o r t i o n a l t o t h e A S E ; i . e . , a 6 0 % A S E i s e q u a l t o a 6 0 % loss i n t o u g h n e s s (85). F o r m a l d e h y d e treatment causes w o o d to b e c o m e brittle. T h i s e m b r i t t l e m e n t m a y b e caused b y t h e short inflexible cross-linking unit o f the - O - C - O - type. I f the i n n e r carbon unit were longer, there w o u l d b e m o r e flexibility i n this unit, a n d t h e e m b r i t t l e m e n t s h o u l d b e r e d u c e d . M o s t o f t h e loss i n w o o d s t r e n g t h p r o p e r t i e s i s probably caused b y the hydrolysis structural cellulose units w i t h a strong acid catalyst. 3

2

OTHER ALDEHYDES.

A c e t a l d e h y d e (85) a n d b e n z a l d e h y d e (85,

94)

react w i t h w o o d b y u s i n g e i t h e r H N O 3 o r z i n c c h l o r i d e catalysts. Acetaldehyde modification produces a high A S E , but benzaldehyde


190


modification yields an A S E of only 4 0 % . M e c h a n i c a l properties of t h e s e t r e a t e d w o o d s a r e t h e s a m e as t h o s e o f f o r m a l d e h y d e - t r e a t e d wood.


D i f u n c t i o n a l aldehyde (dialdehydes) reactions have b e e n cata lyzed with zinc chloride, magnesium chloride, phenyldimethylamm o n i u m c h l o r i d e , a n d p y r i d i n i u m c h l o r i d e (94). G l y o x a l , g l u t a r a l dehyde, a n d α-hydroxyadipaldehyde all show A S E values of 4 0 % w i t h w e i g h t g a i n s o f 1 5 % a n d t h e h i g h e s t A S E (50%) at 2 0 % w e i g h t g a i n . W i t h t h e s e t h r e e c o m p o u n d s , c r o s s - l i n k i n g is p o s s i b l e ; h o w e v e r , w i t h t h e l o w A S E at h i g h w e i g h t p e r c e n t g a i n , i t is c l e a r t h a t b u l k i n g is the m e c h a n i s m for the A S E a c h i e v e d . C h l o r a l (trichloroacetaldehyde) w i t h no catalyst gives a 6 0 % A S E at 3 0 % w e i g h t g a i n (94). A f t e r 15 w e e k s at 7 0 % r e l a t i v e h u m i d i t y , h o w e v e r , a l l w e i g h t g a i n w a s l o s t as w e l l as t h e A S E . T h i s s h o w s a very unstable, perhaps reversible, b o n d formation. Phthaldehydic acid i n acetone catalyzed w i t h p-toluenesulfonic a c i d g i v e s a n A S E o f 4 0 % at a w e i g h t g a i n o f 3 4 % (94). T h e A S E reaches 5 0 - 7 0 % w h e n p h t h a l d e h y d i c a c i d or its d e r i v a t i v e s are c u r e d at 1 0 0 ° C u n c a t a l y z e d f o r 1 6 - 2 4 h (95). O t h e r aldehydes and related compounds have been reacted ei ther alone or catalyzed w i t h sulfuric acid, zinc chloride, magnesium c h l o r i d e , a m m o n i u m c h l o r i d e , o r d i a m m o n i u m p h o s p h a t e (94). C o m p o u n d s s u c h as l , 3 - b i s ( h y d r o x y m e t h y l ) - 2 - i m i d a z o l i d o n e , g l y c o l a c e t a t e , a c r o l e i n , c h l o r o a c e t a l d e h y d e , h e p t a l d e h y d e , o- a n d p - c h l o r o benzaldehydes, furfural, p-hydroxybenzaldehyde, and ra-nitrobenzaldehyde all achieve the A S E by a b u l k i n g mechanism and not by l o w - l e v e l c r o s s - l i n k i n g . A t w e i g h t gains of 1 5 - 2 5 % , the highest A S E r e p o r t e d is 4 0 % . Ethers. METHYLATION. T h e s i m p l e s t e t h e r is t h e m e t h y l e t h e r . R e a c t i o n o f w o o d w i t h d i m e t h y l s u l f a t e a n d N a O H (54, 55), o r m e t h y l i o d i d e a n d s i l v e r o x i d e (54) a r e t w o s y s t e m s t h a t h a v e b e e n r e p o r t e d . M e t h y l a t i o n u p t o 1 5 % w e i g h t g a i n d i d n o t affect t h e a d hesive properties of casein glues. T h e mechanical properties of m e t h y l a t e d w o o d are greatly r e d u c e d because of the severe reaction c o n ditions required. ALKYL CHLORIDES. In the reaction of alkyl chlorides w o o d , H C 1 is f o r m e d as a b y - p r o d u c t ( R e a c t i o n 3). Wood-OH

+ R - C l -> W o o d - O - R

+ HC1

with

(3)

B e c a u s e o f t h i s , t h e t r e a t e d w o o d is n o t v e r y s t r o n g . R e a c t i o n o f w o o d w i t h a l l y l c h l o r i d e i n p y r i d i n e (96, 97) o r a l u m i n u m c h l o r i d e s g i v e s h i g h i n i t i a l A S E ; b u t w h e n t h e w o o d is d r i e d a n d r e s o a k e d , t h e effects o f a l l y l a t i o n a r e l o s t (97). I n t h e a l l y l c h l o r i d e - p y r i d i n e c a s e ,


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t h e A S E is n o t c a u s e d b y t h e f o r m a t i o n o f a l l y l e t h e r s i n c e l l u l o s e o r lignin, but b y the b u l k i n g caused b y the formation of allyl p y r i d i n i u m c h l o r i d e p o l y m e r s , w h i c h are w a t e r soluble a n d easily l e a c h e d o u t (98). O t h e r a l k y l c h l o r i d e s t e s t e d a r e c r o t y l c h l o r i d e (99) a n d n - a n d terf-butyl c h l o r i d e s (J00) c a t a l y z e d w i t h p y r i d i n e . A g a i n , t h e A S E is o n l y t e m p o r a r y a n d the l i b e r a t e d H C 1 causes severe degradation. β-PROPioLACTONE. T h e reaction of β-propiolactone w i t h w o o d produces different products d e p e n d i n g on the p H of the reaction. A c i d c o n d i t i o n s ( R e a c t i o n 4) r e s u l t i n a n e t h e r b o n d to t h e h y d r o x y l group, along w i t h a free-acid e n d group.

(4)

W o o d - 0 - C - C H

2

- C H

2

- O H

(5)

U n d e r b a s i c c o n d i t i o n s ( R e a c t i o n 5), a n e s t e r b o n d is f o r m e d w i t h a primary alcohol end group. U n c a t a l y z e d β-propiolactone reactions i n southern yellow pine (pH = 5) g i v e a c a r b o x y e t h y l d e r i v a t i v e (101). H i g h c o n c e n t r a t i o n s of β-propiolactone cause d e l a m i n a t i o n a n d splitting because of the v e r y h i g h d e g r e e o f s w e l l i n g (91). A t a 2 5 % w e i g h t gain, treated w o o d strongly resists (2% w e i g h t l o s s o r l e s s ) r o t (101, 102) c a u s e d b y Lentinus lepideus, Lenzites trabea, Porta monticola, a n d Coniophora puteana i n s o i l - b l o c k tests. I n c r e a s i n g t h e w e i g h t g a i n to 4 5 % d o e s not c h a n g e t h e rot resistance in either w e a t h e r e d or u n w e a t h e r e d samples. A t 3 0 % weight gain, t h e t r e a t e d w o o d has a n A S E of 6 0 % . T h e m a j o r p r o b l e m i n β - p r o p i o l a c t o n e r e a c t i o n s is t h a t β - p r o p i o l a c t o n e has b e e n l a b e l e d a v e r y active c a r c i n o g e n . F o r this reason, little f u t u r e r e s e a r c h c a n b e e x p e c t e d o n this c h e m i c a l . It w o u l d b e i n t e r e s t i n g , h o w e v e r , to l o o k at t h i s c h e m i c a l r e a c t i o n u n d e r t h e b a s i c conditions that p r o d u c e ester formation. ACRYLONITRILE. W h e n a c r y l o n i t r i l e is r e a c t e d w i t h w o o d i n t h e p r e s e n c e o f a n a l k a l i n e c a t a l y s t , c y a n o e t h y l a t i o n o c c u r s ( R e a c t i o n 6). Wood-OH

+ C H

2

= C H - C N -> W o o d - 0 - C H C H C N 2

2

(6)

W i t h N a O H , a w e i g h t g a i n u p to 3 0 % has b e e n a c h i e v e d . A t t h i s l e v e l , t h e w o o d has an A S E of 6 0 % . A t a w e i g h t gain of 2 5 % , t h e r e


192



w a s n o loss i n s a m p l e w e i g h t i n s o i l - b l o c k tests w i t h Poria monticola, Coniophora puteana, Lenzites trabea, o r Lentinus lepideus (101). W i t h a n i t r o g e n c o n t e n t o f 8 . 5 % , t h e t r e a t e d w o o d is resistant to Poria vaporaria (103). W i t h o n l y 1 % f i x e d n i t r o g e n , t h e w o o d r e s i s t s a t t a c k b y Lentinus lepideus, Poria monticola, Lenzites trabea, and Polyporus versicolor (104, 1 0 5 ) . C y a n o e t h y l a t e d s t a k e s i n g r o u n d c o n t a c t at 1 5 % w e i g h t g a i n h a v e a n a v e r a g e l i f e o f 7 . 8 y e a r s , c o m p a r e d t o 3 . 9 y e a r s f o r u n t r e a t e d s t a k e s (44). To s h o w t h a t t h e d e c a y r e s i s t a n c e o b s e r v e d is c a u s e d b y a b u l k i n g m e c h a n i s m a n d n o t t h e toxicity o f acrylonitrile o r its reaction p r o d u c t s , c y a n o e t h y l a t e d w o o d w a s e x t r a c t e d w i t h h o t w a t e r (104) t o s h o w t h a t t h e l e a c h a t e h a d n o t o x i c effects o n Lenzites trabea. The l e a c h e d blocks lost t h e i r decay resistance, w h i c h m a y b e caused b y the r e a c t i o n o f a c r y l o n i t r i l e w i t h t h e a m m o n i a catalyst that was u s e d to f o r m w a t e r - s o l u b l e p o l y m e r s i n t h e c e l l w a l l . C y a n o e t h y l a t e d w o o d ( w h i c h w a s p r e p a r e d w i t h N a O H as c a t alyst) h a d a l o w e r i m p a c t s t r e n g t h t h a n u n t r e a t e d w o o d (101). E x p o sure o f 2 5 % a c r y l o n i t r i l e i n M e O H to 1 0 rads o f i o n i z i n g r a d i a t i o n g a v e a n A S E o f o n l y 4 0 % at a w e i g h t g a i n o f 2 9 % (106). T h i s l o w A S E may be caused b y the acrylonitrile reacting with the M e O H and forming polymers i n the l u m e n rather than i n the cell wall. 7

Epoxides. T h e reaction between epoxides a n d hydroxyl groups is a n a c i d - o r b a s e - c a t a l y z e d r e a c t i o n ; h o w e v e r , a l l w o r k i n t h e w o o d field has b e e n w i t h base-catalyzed reactions:

Wood-OH

+ R - C H - C H

2

- » W o o d - 0 - C H

2

C H - R

T h e simplest epoxide, ethylene oxide, catalyzed with trimethyla m i n e , h a s b e e n u s e d as a v a p o r - p h a s e t r e a t m e n t . A t a w e i g h t g a i n o f 2 0 % , t h e r e is a 6 0 % A S E (107). A n A S E o f 8 2 % w i t h a w e i g h t g a i n of 1 0 % for t h e same process o r w i t h p r o p y l e n e oxide has b e e n c l a i m e d a l s o (108). U n d e r s i m i l a r c o n d i t i o n s , a w e i g h t g a i n o f 2 2 % g i v e s l e s s t h a n 1 % t a n g e n t i a l a n d r a d i a l s h r i n k a g e (J09). B y u s i n g a n o s c i l l a t i n g pressure rather than a constant pressure system with ethylene oxide a n d t r i m e t h y l a m i n e , a n A S E o f 4 2 % is f o u n d for a w e i g h t g a i n o f 1 1 % (110). M o r e w o r k (111) w i t h p r o p y l e n e o x i d e , b u t y l è n e o x i d e , a n d e p i c h l o r o h y d r i n s h o w s a n A S E o f 7 0 % at w e i g h t gains o f 2 2 - 2 5 % . I f N a O H is u s e d w i t h e t h y l e n e o x i d e i n a v a p o r t r e a t m e n t , e x t e n s i v e s w e l l i n g r e s u l t s , w h i c h c a u s e s b u r s t i n g o f t h e w o o d s t r u c t u r e (112). A s w i t h t h e m e t h y l isocyanate s y s t e m , h i g h w e i g h t gains w i t h p r o p y l e n e a n d b u t y l è n e o x i d e s c a u s e t h e A S E t o f a l l ( F i g u r e 1) (2). F o r p r o p y l e n e o x i d e , t h e m a x i m u m A S E ( 6 0 - 7 0 % ) i s a t t a i n e d at a weight gain between 25 a n d 3 3 % . F o r butylène oxide, a w i d e r range


4.

R O W E L L

193



80

20

25

% WT ADD

30

ON

Figure 1. Relationship between antishrink efficiency (ASE) and chemical add-on caused by epoxide modification. Key: •, butylène oxide; and O , propylene oxide. o f m a x i m u m A S E v a l u e s is o b s e r v e d : 6 0 - 7 3 % A S E f o r w e i g h t g a i n s b e t w e e n 21 a n d 3 3 % . T h e difference b e t w e e n these two examples may be caused by the greater hydrophobicity of butylène oxide an d the difference i n molecular weight. B o t h treatments show a d o w n w a r d trend i n A S E above 3 3 % weight gain. S c a n n i n g e l e c t r o n m i c r o g r a p h s s h o w t h e effects o f a d d i n g l a r g e amounts of chemicals. F i g u r e 2 A shows a radial-split sample of u n treated southern pine. F i g u r e 2 Β shows a radial-split sample of s o u t h e r n p i n e t r e a t e d w i t h p r o p y l e n e o x i d e to a 2 9 . 5 % w e i g h t g a i n . T h e t r a c h e i d w a l l s a r e i n t a c t , a n d t h e r e a r e n o v i s i b l e effects o f t h e c h e m i c a l a d d e d . F i g u r e 2 C s h o w s t h e s a m e t y p e o f s a m p l e at a 3 2 . 6 % w e i g h t g a i n . N o t e t h a t c h e c k s a r e s t a r t i n g to f o r m i n t h e t r a c h e i d w a l l s . I n F i g u r e 2 D , at a w e i g h t g a i n o f 4 5 . 3 % , t h e c h e c k s i n t h e t r a c h e i d w a l l a r e v e r y l a r g e . T h e s p l i t t i n g is a l w a y s i n t h e t r a c h e i d w a l l , n o t i n t h e i n t e r c e l l u l a r spaces; i n s o m e cases, the splits go t h r o u g h the b o r d e r pits. M o s t of the checks are i n the latewood p o r t i o n o f t h e t r e a t e d w o o d . T h e less d e n s e e a r l y w o o d m a y b e a b l e to a c c o m m o d a t e m o r e c h e m i c a l a d d - o n b e f o r e t h e c e l l w a l l w o u l d r u p t u r e . It is a l s o p o s s i b l e t h a t t h e r e is less c h e m i c a l a d d - o n i n t h e



194


Figure 2. Scanning electron micrographs of radial-split southern pine, showing the swelling of wood when treated with propylene oxide-triethylamine. No swelling is shown in A, the untreated control (990 x ). In B, wood is treated to the green volume (990 x ). In C, wood is superswollen above the green volume at 32.6% weight gain and cell wall rupture is apparent (540 x ). In D, rupture is pronounced at 45.3% weight gain (495 x ). e a r l y w o o d . I f so, a n d b e c a u s e t h e w e i g h t p e r c e n t g a i n is a n a v e r a g e for the w h o l e s a m p l e , t h e w e i g h t gain i n the l a t e w o o d w o u l d be higher than 3 3 % w h e n the cell walls rupture. O n l y the epoxide a n d isocyanate treatments have been reported to a d d to w o o d c e l l w a l l c o m p o n e n t s to s u c h a d e g r e e that t h e y cause t h e w o o d s t r u c t u r e i t s e l f to b r e a k a p a r t (2, 80). O t h e r c h e m i c a l s u b s t i t u t i o n t r e a t m e n t s o f w o o d c o m p o n e n t s a d d to w o o d u p to a b o u t 3 5 % weight gain w i t h no cell wall rupture. T h e epoxide and iso cyanate systems s e e m to s w e l l the c e l l w a l l , react w i t h it, a n d c o n t i n u e to s w e l l a n d react to t h e p o i n t o f c e l l w a l l r u p t u r e a n d b e y o n d . I n t h e case o f t h e e p o x y s y s t e m , after the i n i t i a l r e a c t i o n w i t h a cell wall h y d r o x y l group, a n e w hydroxyl group originating from the e p o x i d e is f o r m e d . F r o m t h i s n e w h y d r o x y l , a p o l y m e r b e g i n s t o


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form. T h e ionic nature of the reaction a n d the availability of alkoxyl ions i n the w o o d components probably produce chain transfer, thereby y i e l d i n g a short chain length. T h e formation of a p o l y m e r i n t h e c e l l w a l l m a y b e t h e c a u s e o f c e l l w a l l r u p t u r e at h i g h c h e m i c a l w e i g h t gains. A t a w e i g h t gain o f a p p r o x ima t el y 2 0 % t h e v o l u m e of the t r e a t e d w o o d is e q u a l to t h e o r i g i n a l u n t r e a t e d g r e e n w o o d v o l u m e (49). W h e r e t h e w e i g h t g a i n i s m o r e t h a n a b o u t 3 0 % , t h e v o l u m e o f t h e t r e a t e d w o o d is g r e a t e r t h a n t h a t o f g r e e n w o o d . T h i s is t h e l e v e l w h e r e t h e A S E s t a r t s t o d r o p , w h i c h m a y m e a n t h e p o l y m e r l o a d i n g s a r e n o w so h i g h t h e y h a v e b r o k e n t h e c e l l w a l l a n d a l l o w e d the w o o d to supers w e l l above the green w o o d v o l u m e . T h e s i m p l e epoxides are sensitive to moisture levels i n the w o o d d u r i n g r e a c t i o n (82). T h e p r o p y l e n e o x i d e r e a c t i o n s y s t e m s e e m s t o b e t h e m o s t a f f e c t e d b y m o i s t u r e , as i s s h o w n b y h i g h w e i g h t l o s s e s b y e x t r a c t i o n o f n o n b o n d e d c h e m i c a l a n d b y losses i n A S E . T h e b u t y l è n e o x i d e s y s t e m is l e s s s e n s i t i v e t o m o i s t u r e , b u t s t i l l r e s u l t s in formation of large amounts of n o n b o n d e d glycols. S o i l b u r i a l tests w i t h e p i c h l o r o h y d r i n - o r d i c h l o r o h y d r i n - t r e a t e d s p e c i m e n s s h o w n o d e c a y a f t e r 2 m o n t h s (113). L o n g e r field tests show that butylène oxide stakes, treated above 2 0 % w e i g h t gain, resist attack b y g r o u n d o r g a n i s m s after 7 years i n n o r t h e r n U . S . e x p o s u r e s b u t s h o w s o m e d e c a y i n s o u t h e r n e x p o s u r e s (44, 114). L a b o r a t o r y s o i l - b l o c k t e s t s w i t h t h e b r o w n - r o t f u n g i Gloeophyllum trabeum (111, 114) a n d Lentinus lepideus (111) a n d w i t h t h e w h i t e - r o t f u n g u s Coriolus (Polyporus) versicolor (114) s h o w b u t y l è n e o x i d e m o d i f i e d w o o d to b e resistant to attack above about 1 7 % w e i g h t gain. B u t y l è n e o x i d e - m o d i f i e d b l o c k s w e r e r e s i s t a n t i n l a b o r a t o r y tests t o a t t a c k b y s u b t e r r a n e a n t e r m i t e s (Reticulitermes flavipes) (114, 115). R e s i s t a n c e s e e m s a t t r i b u t a b l e p r i m a r i l y t o t h e w o o d ' s u n p a l a t ability. W h e r e a s t h e w o o d t r e a t e d to h i g h e r w e i g h t p e r c e n t gains lost little w e i g h t u n d e r attack, t e r m i t e m o r t a l i t y p a r a l l e l e d that for a star v a t i o n set. M o r t a l i t y m a y b e a t t r i b u t a b l e to e i t h e r a n e n h a n c e d star v a t i o n effect o r a s l o w - a c t i n g t o x i c effect. T h e s e t w o o p t i o n s a r e d i f f i c u l t t o assess b e c a u s e p a t h o g e n i c m i c r o b e s i n g r o u p s o f s t a r v a t i o n ally w e a k e n e d termites confound data interpretation. F i g u r e 3 shows that after o n l y 2 w e e k s o f t e r m i t e attack t h e c o n t r o l s p e c i m e n is almost c o m p l e t e l y d e s t r o y e d . T h e e p o x i d e - m o d ified block suffered only m i n o r damage, because the termites d i d s o m e surface g r a z i n g b u t d i d n o t attack. U n m o d i f i e d control specimens are destroyed b y marine borer a t t a c k i n less t h a n 1 y e a r i n a m a r i n e e n v i r o n m e n t . E p o x i d e - m o d i f i e d specimens have b e e n tested for over 5 years w i t h v e r y little m a r i n e b o r e r a t t a c k (114). T h e m e c h a n i s m o f e f f e c t i v e n e s s o f m o d i f i e d w o o d i n r e s i s t i n g attack b y m a r i n e o r g a n i s m s is u n k n o w n . A s w i t h t h e



196


Figure 3. Butylène oxide-modified block (left) and control (right) after 2 weeks of termite attack. laboratory t e r m i t e tests, u n p a l a t a b i l i t y m a y be the largest single factor. M o s t of the mechanical properties of propylene oxide-modified w o o d a r e r e d u c e d (116). T h e m o d u l u s o f e l a s t i c i t y i s r e d u c e d 1 4 % , m o d u l u s o f r u p t u r e is r e d u c e d 1 7 % , fiber s t r e s s at p r o p o r t i o n a l l i m i t is r e d u c e d 9 % , a n d m a x i m u m c r u s h i n g s t r e n g t h is r e d u c e d 1 0 % (116). E t h y l e n e o x i d e - m o d i f i e d w o o d s h o w e d n o r e d u c t i o n i n static b e n d i n g tests (107). Radial, tangential, a n d longitudinal hardness indexes of pro p y l e n e o x i d e - m o d i f i e d w o o d w e r e t h e s a m e as f o r u n t r e a t e d c o n t r o l s (116). T h e d i f f u s i o n c o e f f i c i e n t t o w a t e r v a p o r w a s i n c r e a s e d 2 9 % . T h e t h e r m a l s t a b i l i t y o f m o d i f i e d w o o d as s h o w n b y b o t h t h e r m o g r a v i m e t r i c a n a l y s i s a n d e v o l v e d gas a n a l y s i s d e c o m p o s i t i o n t e m peratures was slightly increased b y epoxide bonding, the same w i t h a c e t y l b o n d i n g , a n d s l i g h t l y l o w e r e d b y i s o c y a n a t e b o n d i n g , as c o m p a r e d t o c o n t r o l s (117). T h e a m o u n t o f c h a r g e n e r a t e d d u r i n g p y r o l ysis w a s n e a r l y t h e same for u n t r e a t e d w o o d , a c e t y l - a n d isocyanateb o n d e d w o o d , a n d less for e p o x i d e - b o n d e d w o o d . T h e e p o x i d e b o n d s e e m s t o s t a b i l i z e t h e c o m p o n e n t s t h a t d e g r a d e at 3 2 5 ° C — h e m i c e l l u l o s e s — w h i c h a p p a r e n t l y gasifies w i t h t h e c e l l u l o s e c o m p o n e n t at 3 8 5 ° C . T h e e t h e r l i n k a g e is c h e m i c a l l y m o r e s t a b l e a n d a p p a r e n t l y t h e r m a l l y m o r e stable than t h e acetyl linkage that bonds t h e p o l y saccharides. T h u s , t h e e p o x i d e m a y still have b e e n b o n d e d to t h e c a r b o h y d r a t e at t h e t e m p e r a t u r e at w h i c h c a r b o h y d r a t e p y r o l y s i s o c curred. A c e t y l - a n d isocyanate-bonded chemicals d i d not stabilize the c o m p o n e n t s d e g r a d i n g at 3 2 5 ° C , b u t s h o w e d t h e s a m e t h e r m o g r a v i m e t r i c a n d e v o l v e d gas a n a l y s i s p r o f i l e s as d i d t h e c o n t r o l s . B e c a u s e e s t e r a n d u r e t h a n e b o n d s a r e n o t as s t a b l e t o w a r d p y r o l y s i s as e t h e r l i n k a g e s at h i g h t e m p e r a t u r e s , t h e r e w a s a p a r t i a l r e l e a s e o f b o n d e d c h e m i c a l at l o w t e m p e r a t u r e s (117). E v o l v e d gas a n a l y s i s s h o w e d t h a t t h e e p o x i d e - b o n d e d w o o d h a d


4.

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197


a higher heat o f combustion of volatiles than d i d the control. T h e heat o f c o m b u s t i o n o f volatile products from acetyl isocyanate- a n d m e t h y l i s o c y a n a t e - b o n d e d w o o d w a s a l m o s t t h e s a m e as t h a t o f t h e control. T h e h i g h heat of c o m b u s t i o n of volatiles observed for e p o x i d e - b o n d e d w o o d is p r i m a r i l y d u e to t h e h y d r o c a r b o n c o n t e n t attached to t h e b o n d i n g group. T h e epoxide b o n d i n g g r o u p , - C H 2 C H - , accounts for part of the heat of combustion, b u t the - C H a n d - C H C H a d d e d b y p r o p y l e n e oxides a n d butylène oxides, respec t i v e l y , a l s o c o n t r i b u t e (117). Types of Wood. F o r the most part, c h e m i c a l modification has b e e n d o n e w i t h relatively f e w species of w o o d . A m o n g the softwoods, Douglas-fir, ponderosa pine, a n d southern pine have been used; a m o n g t h e h a r d w o o d s , h a r d m a p l e a n d b i r c h h a v e b e e n u s e d . It is easy to g e n e r a l i z e o n t h e t y p e o f w o o d u s e d a n d extrapolate infor m a t i o n to a n u n t r i e d species w i t h t h e rationale that i f it w o r k e d o n o n e i t w i l l w o r k o n t h e other. T h i s is a d a n g e r o u s a s s u m p t i o n a n d m o r e often t h a n n o t , i t is i n c o r r e c t . 3


2

3

In a recent study, 13 species o f w o o d w e r e treated w i t h e i t h e r propylene oxide or butylène oxide a n d catalyzed w i t h triethylamine (118). W e i g h t g a i n w a s d e t e r m i n e d a n d , i n m o s t c a s e s , so w e r e A S E v a l u e s (Table I V ) . S p e c i e s s u c h as r a d i a t a , s o u t h e r n a n d p o n d e r o s a pines, h a r d maple, walnut, e l m , cativo, a n d eucalyptus a l l h a d ac ceptable w e i g h t gains a n d m e d i u m to h i g h A S E values. R e d oak a n d teak gave g o o d w e i g h t gains w i t h little o r n o d i m e n s i o n a l stability. T h e r e a s o n for this is n o t clear, b u t t h e extractives i n teak s e e m to interfere w i t h both c h e m i c a l penetration a n d reactivity. I f additional species w e r e u s e d i n further research, e v e n greater variability w o u l d be expected.

Proof of Bonding T h r e e c r i t e r i a h a v e b e e n u s e d as e v i d e n c e t h a t a c h e m i c a l h a s r e a c t e d i n t h e c e l l w a l l a n d that it has b o n d e d w i t h t h e c e l l w a l l p o l y m e r s : (1) i n c r e a s e s i n w o o d v o l u m e as a r e s u l t o f r e a c t i o n , (2) r e s i s t a n c e t o l e a c h i n g o f a d d e d c h e m i c a l a f t e r r e a c t i o n , a n d (3) I R data. Increases i n W o o d V o l u m e . Oven-drying green southern pine causes a s h r i n k a g e o f 6 - 1 0 % f r o m t h e o r i g i n a l g r e e n w o o d v o l u m e (Table V ) . W h e n w o o d i s t r e a t e d t o a w e i g h t g a i n o f a b o u t 2 0 % , t h e o v e n - d r y v o l u m e o f t h e t r e a t e d w o o d is e q u a l t o t h e o r i g i n a l u n treated g r e e n w o o d v o l u m e . Table V I shows that for p r o p y l e n e oxide, m e t h y l isocyanate, a n d acetic a n h y d r i d e , v o l u m e expansion i n the w o o d i s n e a r l y e q u a l t o t h e v o l u m e o f c h e m i c a l a d d e d (I). A l t h o u g h this is s t r o n g e v i d e n c e that t h e b u l k i n g c h e m i c a l s a r e i n t h e c e l l w a l l , these results d o n o t i n d i c a t e w h e t h e r o r n o t t h e c h e m i c a l is b o n d e d .


198


Table I V . W e i g h t Percent C a i n s for Various W o o d A p p l i e d to S e v e r a l W o o d Species

Species R e d oak


Hard maple

Teak Walnut Elm Cativo Persimmon Eucalyptus obligva Radiata p i n e (sapwood) (heartwood) S o u t h e r n p i n e (sapwood) (heartwood) Ponderosa pine Douglas-fir Spruce

Treatment" PO PO PO BO BO PO PO PO BO PO PO BO BO BO BO PO PO PO PO PO BO BO PO BO

Treatments

Time (min)

Weight Percent Gain

30 40 35 60 180 30 60 3 240 40 40 240 180 240 240 40 40 40 40 40 300 360 40 360

21.8 25.6 27.3 18 32 20.5 20.7 26.2 28.3 28.2 29.7 22.8 22 33 22 34.2 32.1 35.5 24.6 26.9 20.7 24.6 32.6 30.4

ASE

b

0 2.1 41.1 52.2 61.0 0 0 46 53 46.3 42.2 64.2

— — 46.4 67.3 52.3 68.3 59.7 36.5

— — —

—

Treatments: P O , p r o p y l e n e oxide; a n d B O , butylène oxide. A n t i s h r i n k efficiency after one water soak. C o n d i t i o n s : t e m p e r a t u r e , 120 °C; solvent, e p o x i d e / t r i e t h y l a m i n e , 95/5, v/v; a n d p r e s sure, 150 l b / i n . . a

b

2

F o r a c r y l o n i t r i l e , t h e r e is a g r e a t e r v o l u m e o f c h e m i c a l a d d e d t h a n t h e r e is a n i n c r e a s e i n w o o d v o l u m e . T h i s m e a n s t h a t n o t a l l t h e c h e m i c a l i n t h e w o o d is i n t h e c e l l w a l l . T h i s is v e r y e v i d e n t w h e n using m e t h y l methacrylate, w h i c h shows a very large addition of chemical v o l u m e w i t h very little increase i n w o o d volume. T h e m e t h a c r y l a t e p o l y m e r is m a i n l y i n w o o d l u m e n s . R e s i s t a n c e to L e a c h i n g . If the c h e m i c a l that caused the c e l l w a l l t o s w e l l is b o n d e d t o t h e c e l l w a l l p o l y m e r s , t h e n s o l v e n t e x traction cannot leach it out. N o n b o n d e d chemicals w i l l leach out r e s u l t i n g i n w e i g h t loss. Table V I I shows that m e t h y l isocyanate-, butylène oxide-, a n d acetic a n h y d r i d e - m o d i f i e d w o o d are v e r y resis-


In The Chemistry of Solid Wood; Rowell, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1984. 6.9 10.0 6.6 8.3 6.7 9.4 10.0 9.9 10.1

3.24 3.24 3.42 3.30 3.36 2.11 2.15 2.17 2.13

3

3.48 3.60 3.66 3.60 3.60 2.33 2.39 2.41 2.37

3

AV (%)

Oven-Dry Volume (in. )

Green Volume (in. ) Propylene oxide Propylene oxide Propylene oxide Propylene oxide Propylene oxide Acetic anhydride Acetic anhydride Acetic anhydride Acetic anhydride

Treatment

15.9 21.1 26.1 34.1 41.0 13.9 17.5 19.5 22.8 3.42 3.60 3.66 3.66 3.72 2.30 2.33 2.39 2.37

3

Weight Percent Gain

Oven-Dry Volume After Treatment (in. )

Table V . Changes i n V o l u m e of Southern Pine upon D r y i n g and C h e m i c a l Treatment



200

T a b l e V I . V o l u m e C h a n g e s i n Southern P i n e u p o n Chemical Treatment

Treatment


Propylene oxide

M e t h y l isocyanate

Acetic anhy dride

Acrylonitrile

Methyl methacrylate

Weight Percent Gain

Increase in Wood Volume with Treatment (cm )

0

3

Calculated Volume of Chemical Added (cm ) b

3

26.5 28.8 34.3 36.2

7.1 6.4 8.4 8.9

7.5 7.2 8.0 9.0

12.4 25.7 47.7 51.2

0.16 0.21 0.46 0.54

0.14 0.27 0.54 0.58

17.5 19.5 22.8 25.7 28.7 36.0

3.0 3.6 3.9 0.46 0.26 0.74

2.9 3.3 4.0 0.77 0.39 1.2

58.0 91.4

0.6 0.9

7.6 10.1

D i f f e r e n c e i n v o l u m e b e t w e e n treatments is d u e to different sample size. D e n s i t y u s e d i n v o l u m e calculations: p r o p y l e n e oxide, 1.01; m e t h y l isocyanate, 0.967; acetic a n h y d r i d e , 1.049; acrylonitrile, 0.806; a n d m e t h y l methacrylate, 0.94. a

b

tant to t h e l e a c h i n g o f a d d e d chemical(s). T h e starting chemicals a n d n o n b o n d e d by-products w o u l d b e very soluble i n benzene or water. S o x h l e t e x t r a c t i o n o f g r o u n d - m o d i f i e d w o o d ( 2 0 - 4 0 m e s h ) is a severe e n v i r o n m e n t that exposes a v e r y large i n t e r n a l surface area to t h e extracting solvent. P r o p y l e n e oxide-modified w o o d shows more w e i g h t loss t h a n t h e t h r e e a f o r e m e n t i o n e d c h e m i c a l s y s t e m s . P r o p y l e n e o x i d e is m o r e m o i s t u r e s e n s i t i v e t h a n b u t y l è n e o x i d e , a n d thus forms more n o n b o n d e d polymers d u r i n g reaction. Acrylonitrile m o d i f i c a t i o n u s i n g a m m o n i u m h y d r o x i d e as c a t a l y s t r e s u l t s i n a l m o s t n o p e r m a n e n t l y b o n d e d c h e m i c a l e v e n i n a m i l d w a t e r - s o a k i n g test. W o o d t r e a t m e n t s u s i n g N a O H as c a t a l y s t s h o w a l o w e r w e i g h t loss i n w a t e r t h a n d o w o o d t r e a t m e n t s u s i n g a m m o n i u m h y d r o x i d e as a catalyst. H o w e v e r , w e i g h t loss is still s i g n i f i c a n t l y h i g h e r t h a n a n y other chemically bonded system. A n o t h e r test for resistance to l e a c h i n g o f b o n d e d c h e m i c a l c a n



oxide

4

Acrylonitrile + NH OH Acrylonitrile + NaOH

Butylène oxide Acetic anhydride

Propylene

Control M e t h y l isocyanate

Reagent

13.5

—

1.6 1.0 1.2

0.6 1.0 1.0 9.7 4.0

25.7

12.5 11.7 9.7 12.2

— — — —

11.2

(%)

Water 7 d Soaking Blocks (%)

21.7

— — —

10.8

— — — —

4.7

(%)

Water 24 h Soxhlet 40 Mesh

—

2.3 2.9 6.5 9.6 5.2 6.8 3.8 2.3 2.8

(%)

Benzene 24 h Soxhlet 40 Mesh

26.1

0 10.0 23.5 47.2 29.2 38.0 27 16.3 22.5

Weight Percent Gain

Benzene/Ethanol 4 h Soxhlet 20 Mesh

Table V I I . O v e n - D r y W e i g h t Loss of C h e m i c a l l y M o d i f i e d W o o d L e a c h e d w i t h Various Solvents


202



b e s e e n i n d a t a g e n e r a t e d for r e p e a t e d w a t e r - l e a c h i n g A S E tests (I). Table V I I I shows that w o o d m o d i f i e d w i t h p r o p y l e n e or butylène oxide, m e t h y l isocyanate, a n d acetic a n h y d r i d e maintains a 5 0 - 6 0 % A S E v a l u e e v e n after f o u r s o a k i n g - d r y i n g cycles. T h i s v a l u e shows that t h e b u l k i n g c h e m i c a l is s t a y i n g i n t h e c e l l w a l l . A c r y l o n i t r i l e modified wood catalyzed with both a m m o n i u m hydroxide a n d sodium h y d r o x i d e loses b u l k i n g c h e m i c a l e v e n after o n e s o a k i n g c y c l e . T h e A S E v a l u e o n t h e s e c o n d s o a k i n g c y c l e is n e g a t i v e , w h i c h m e a n s t h e m o d i f i e d w o o d i s l e s s d i m e n s i o n a l l y s t a b l e t h a n t h e c o n t r o l . T h i s loss i n stability m a y b e d u e to h e m i c e l l u l o s e extraction d u r i n g reaction u n d e r strong alkaline catalyst conditions. IR Data. E v i d e n c e that a c h e m i c a l reaction has taken place w i t h t h e w o o d c e l l w a l l h y d r o x y l g r o u p s is s e e n i n t h e I R s p e c t r a o f m e t h y l i s o c y a n a t e - m o d i f i e d s o u t h e r n p i n e ( F i g u r e 4). S a m p l e s w e r e first m i l l e d t o p a s s a 4 0 - m e s h s c r e e n a n d e x t r a c t e d w i t h b e n z e n e / e t h a n o l (2/1, v/v) f o l l o w e d b y w a t e r i n a Soxhlet extractor. A n y u n reacted reagent a n d isocyanate h o m o p o l y m e r formed d u r i n g the reac tion w o u l d b e r e m o v e d b y this procedure. T h e spectrum for u n reacted w o o d i n the region of 1730 c m shows some carbonyl s t r e t c h i n g v i b r a t i o n s ( F i g u r e 4 A ) . A f t e r t h e w o o d is m o d i f i e d to 1 7 . 7 % w e i g h t g a i n , t h e c a r b o n y l b a n d is s t r o n g e r ( F i g u r e 4 B ) . A t 4 7 . 2 % weight gain (Figure 4 C ) this b a n d becomes one of the major b a n d s i n t h e I R s p e c t r a . T h e i n c r e a s e i n c a r b o n y l is d u e to t h e for m a t i o n o f R - O - C - N - R i n t h e u r e t h a n e b o n d . T h e r e is also a n i n c r e a s e i n t h e a b s o r p t i o n b a n d s as t h e w e i g h t p e r c e n t g a i n i n c r e a s e s : at 1 5 5 0 c m " , Ν — H d e f o r m a t i o n f r e q u e n c i e s o f s e c o n d a r y a m i n e s ; at 1 2 7 0 c m " , C - N v i b r a t i o n o f d i s u b s t i t u t e d a m i n e s ; a n d at 7 7 0 - 7 8 0 c m , N - H deformation of b o n d e d secondary amines. N o unreacted r e a g e n t r e m a i n s i n t h e s a m p l e s , as s h o w n b y t h e a b s e n c e o f i s o c y a n a t e a b s o r p t i o n at 2 2 7 5 - 2 2 4 0 c m (Figures 4 Β a n d 4 C ) . -

1

1

1

- 1

-

1

T h e s t r o n g a b s o r p t i o n at 3 4 0 0 c m " a n d 2 9 5 0 c m " i n a l l t h e I R s p e c t r a is c a u s e d b y h y d r o x y l a b s o r p t i o n . B e c a u s e s u b s t i t u t i o n is n o t h i g h e n o u g h to e l i m i n a t e all h y d r o x y l groups, these bands are always present. 1

1

T h e holocellulose (cellulose a n d hemicellulose) from a sample m o d i f i e d b y m e t h y l isocyanate to a w e i g h t gain o f 1 7 . 7 % was isolated b y t h e s o d i u m c h l o r i t e p r o c e d u r e (119). T h e I R s p e c t r u m o f t h e holocellulose ( F i g u r e 4 D ) shows that urethane b o n d i n g has taken place i n the carbohydrate component of wood. T h e I R spectrum of l i g n i n i s o l a t e d f r o m a m e t h y l i s o c y a n a t e - m o d i f i e d s a m p l e at 4 7 . 2 % w e i g h t gain b y t h e H S 0 p r o c e d u r e (J20) shows that u r e t h a n e b o n d i n g has o c c u r r e d i n t h e l i g n i n c o m p o n e n t of w o o d ( F i g u r e 4 E ) . T h e lignin s p e c t r u m shows the characteristic aromatic skeletal v i b r a t i o n a t 1 5 1 5 c m " (121). T h i s b a n d i s m i s s i n g from t h e m o d i f i e d 2

4

1



0 29.2 0 27.0 0 16.3 22.5 0 26.1 0 25.7 0 21.6 29.9

15.8 6.0 13.6 3.6 13.8 5.1 4.1 14.1 2.7 20.3 10.5 14.0 5.5 4.7 60.4 66.4

—

48.3

—

80.9

—

63.0 70.3

—

73.5

—

62.0

ASEf 15.8 9.0 12.4 5.7 13.3 5.1 3.8 13.9 15.3 16.8 18.8 13.8 6.6 6.0

c d

—

—

52.0 56.8

neg.

neg.

—

61.7 71.4

54.0

—

43.8

ASE/ 15.9 7.8 12.4 5.2 13.6 5.3 4.0 14.0 14.4 16.7 17.5 13.7 6.5 4.8

s/

52.6 65.0

neg.

neg.

—

—

61.0 70.6

58.1

—

50.9

ASEf

1

h

g

e

d

0

b

a

4

x

Samples r e c o r d e d at 0 % are controls. V o l u m e t r i c s we l l i ng coefficient d e t e r m i n e d from initial o v e n - d r y v o l u m e a n d first water-swollen v o l u m e . A n t i s h r i n k efficiency based o n S . D e t e r m i n e d from first w a t e r - s w o l l e n v o l u m e a n d r e o v e n - d r y i n g . B a s e d o n So. f D e t e r m i n e d f r o m r e o v e n - d r y v o l u m e a n d s e c o n d water-swollen v o l u m e . B a s e d o n So. D e t e r m i n e d from s e c o n d water-swollen v o l u m e a n d second r e o v e n - d r y i n g . Based on S .

4

Acrylonitrile + NH OH Acrylonitrile + NaOH M e t h y l isocyanate

Acetic anhydride

Butylène oxide

Propylene oxide

Treatment

Weight Percent Gain"

13.5 6.3 5.3

—

— —

—

h

15.9 7.9 12.9 5.6 13.3 5.3 4.1

S

—

53.3 60.7

— —

— — —

60.2 69.2

—

56.6

50.3

ASEJ

T a b l e V I I I . V o l u m e t r i c S w e l l i n g C o e f f i c i e n t s (S) a n d A S E as D e t e r m i n e d b y t h e W a t e r - S o a k i n g M e t h o d



204


WÛVENUMBER CM"

1

Figure 4. IR spectra of methyl isocyanate-modified southern pine. Key: A , southern pine control; B, methyl isocyanate-modified southern pine to 17.7% weight gain; C, methyl isocyanate-modified southern pine to 47.2% weight gain; D, holocellulose fraction from methyl isocyanate-modified southern pine to 17.7% weight gain (0.07% lignin); and E lignin fraction from methyl isocyanate-modified southern pine to 47.2% weight gain. y

h o l o c e l l u l o s e c u r v e ( F i g u r e 4 D ) , s h o w i n g that the c h l o r i t e p r o c e d u r e does r e m o v e substituted lignins. S i m i l a r results are o b s e r v e d on I R spectra of acetylated w o o d (122). A s t h e w e i g h t p e r c e n t g a i n i n c r e a s e s u p o n a c e t y l a t i o n , t h e a b s o r p t i o n b a n d at 1 7 3 0 c m " i n c r e a s e s b e c a u s e o f t h e c a r b o n y l g r o u p in the acetyl bond. 1

Distribution of Bonded Chemical C h e m i c a l m o d i f i c a t i o n o f w o o d to i m p a r t d e c a y resistance a n d to p r o v i d e d i m e n s i o n a l s t a b i l i t y d e p e n d s o n a d e q u a t e d i s t r i b u t i o n of r e a c t e d c h e m i c a l s i n w a t e r - a c c e s s i b l e r e g i o n s o f t h e c e l l w a l l . It is i m p o r t a n t , therefore, to d e t e r m i n e the d i s t r i b u t i o n of b o n d e d c h e m icals. T h i s i n f o r m a t i o n m a y also l e a d to a b e t t e r u n d e r s t a n d i n g o f h o w c h e m i c a l modification of w o o d changes the c h e m i c a l properties of cell wall polymers. T h e d i s t r i b u t i o n o f b o n d e d c h e m i c a l as a f u n c t i o n o f d e p t h o f


4.

ROWELL


205


p e n e t r a t i o n w a s d e t e r m i n e d b y t r a c i n g t h e fate o f c h l o r i n e i n e p i c h l o r o h y d r i n - r e a c t e d w o o d (123) a n d a c e t y l g r o u p s i n a c e t y l a t e d w o o d (122). O u t s i d e , m i d d l e , a n d i n n e r s a m p l e s w e r e t a k e n f r o m s p e c i m e n s 1.27 x 1.27 c m t o 5 . 0 8 x 5 . 0 8 c m p r e p a r e d f r o m e p i c h l o r o h y d r i n - m o d i f i e d s o u t h e r n p i n e . I n samples u p to 3.81 x 3.81 c m , no significant differences w e r e observed i n chlorine content from the three sections analyzed. B e y o n d 3.81 c m a concentration differ ence occurred between the outside a n d the inner part of the wood t r e a t e d . R e s u l t s w e r e s i m i l a r f o r a c e t y l a t e d w o o d (122). T h e e p i c h l o r o h y d r i n r e a c t i o n s y s t e m w a s also u s e d to d e t e r m i n e the distribution of chlorine i n earlywood, latewood, sapwood, a n d h e a r t w o o d o f s o u t h e r n p i n e (124). T h e v e r y p o l a r e p o x y s y s t e m r e a c t s m o r e quickly, a n d w i t h greater w e i g h t gains, w i t h the e a r l y w o o d — as o p p o s e d t o l a t e w o o d — c e l l w a l l c o m p o n e n t s . A l t h o u g h t h e a m o u n t of the epoxides was larger i n h e a r t w o o d , b e n z e n e extraction o f the r e a c t e d w o o d e f f e c t e d a g r e a t e r w e i g h t loss f r o m t h e h e a r t w o o d t h a n f r o m t h e s a p w o o d . T h i s w e i g h t loss m a y h a v e b e e n c a u s e d b y r e a c t i o n of the epoxides w i t h heartwood extractives, w h i c h w e r e then r e m o v e d o n benzene extraction. A s t u d y o f soft-rot d e c a y patterns s h o w e d that t h e t an g e n t ial c e l l w a l l is r e a c t e d t o a h i g h e r d e g r e e t h a n r a d i a l c e l l w a l l s i n p o n d e r o s a p i n e r e a c t e d w i t h b u t y l è n e o x i d e at 8 % w e i g h t g a i n (125). T h e r a d i a l w a l l i n l a t e w o o d i s n e a r l y t w i c e as t h i c k as t h e t a n g e n t i a l w a l l , so t h e radial w a l l m a y not be totally penetrated b y the epoxide system. E n e r g y X - r a y analysis of b r o m i n e i n w o o d acetylated w i t h t r i b r o m o a c e t y l b r o m i d e s h o w e d that b r o m i n e was d i s t r i b u t e d throughout the entire secondary wall, suggesting chemical reaction w i t h l i g n i n (126). U s i n g a s i m i l a r t e c h n i q u e , t h e g r e a t e s t p e r c e n t a g e of chlorine i n e p i c h l o r o h y d r i n - m o d i f i e d w o o d reference was found i n t h e S l a y e r o f t h e c e l l w a l l . T h i s is t h e t h i c k e s t c e l l w a l l l a y e r a n d contains the most cellulose. 2

B y taking apart the cell wall of a modified w o o d specimen a n d s e p a r a t i n g t h e c e l l w a l l c o m p o n e n t s f r o m o n e a n o t h e r , i t is p o s s i b l e to d e t e r m i n e t h e d i s t r i b u t i o n o f b o n d e d c h e m i c a l s i n t h e c e l l w a l l p o l y m e r . It is m o r e d i f f i c u l t to d e l i g n i f y m o d i f i e d w o o d t h a n u n m o d i f i e d w o o d , w h i c h m e a n s t h a t t h e l i g n i n h a s b e e n s u b s t i t u t e d (122, 127, 128). T h i s is t r u e f o r w o o d r e a c t e d w i t h b o t h a c e t i c a n h y d r i d e a n d m e t h y l isocyanate. Table I X shows that t h e l i g n i n c o m p o n e n t is always m o r e substituted than the holocellulose components (128). T h i s w o u l d i n d i c a t e t h a t t h e l i g n i n is e i t h e r m o r e a c c e s s i b l e f o r r e a c t i o n t h a n h o l o c e l l u l o s e o r t h a t i t is m o r e r e a c t i v e t h a n h o l o c e l l u l o s e . L i g n i n was found to b e m o r e reactive than cellulose toward acetyl a t i o n (129).


206



T a b l e I X . D e g r e e o f Substitution o f H y d r o x y l G r o u p s i n M e t h y l Isocyanate-Modified Southern P i n e Weight Percent Gain

Lignin

Holocellulose

5.5 10.0 17.7 23.5 47.2

0.17 0.28 0.41 0.59 0.89

0.025 0.047 0.084 0.117 0.209

Lignin: Holocellulose 7.4 6.0 4.9 5.1 4.3

F o r a c e t y l a t e d w o o d at a p p r o x i m a t e l y 2 5 % c h e m i c a l w e i g h t g a i n , a l l o f t h e l i g n i n h y d r o x y l s w e r e f o u n d t o b e s u b s t i t u t e d ( 1 2 2 , 130). M o r e b o n d e d a c e t y l is f o u n d o n t h e c e l l u l o s e t h a n t h e h e m i c e l l u l o s e s at l o w ( 1 3 . 5 % ) c h e m i c a l w e i g h t g a i n , b u t t h i s w a s r e v e r s e d at h i g h e r (24.5%) w e i g h t g a i n (J30). H y d r o x y l substitution calculations are based o n the assumption that a l l h y d r o x y l groups a r e accessible a n d that reaction w i t h acetic a n h y d r i d e o r m e t h y l isocyanate is a single-site s u b s t i t u t i o n r e a c t i o n — i.e., only one reagent reacting w i t h one hydroxyl a n d no p o l y m e r i zation. O n l y 6 0 % of the total hydroxyl groups i n spruce w o o d are a c c e s s i b l e t o t r i t i a t e d w a t e r (131). A b o u t 6 5 % o f t h e c e l l u l o s e i n w o o d is c r y s t a l l i n e a n d , t h e r e f o r e , p r o b a b l y n o t a c c e s s i b l e f o r r e a c t i o n s i n v o l v i n g t h e s e h y d r o x y l g r o u p s (7). B a s e d o n t h e s e e s t i m a t e s a n d a s s u m i n g that o n l y 3 5 % o f the cellulose hydroxyls are accessible for substitution, the degree of substitution i n the holocellulose c o m p o n e n t is m u c h h i g h e r i n t h e accessible regions t h a n s h o w n i n Table I X . T h e data o n t h e d i s t r i b u t i o n o f b o n d e d c h e m i c a l s suggest that a h i g h rate o f l i g n i n s u b s t i t u t i o n does n o t c o n t r i b u t e significantly to the overall protection mechanism of w o o d from decay or dimensional stabilization. T h e d e g r e e o f substitution i n l i g n i n was h i g h i n samples at l o w e r w e i g h t p e r c e n t g a i n o f b o n d e d c h e m i c a l w h e r e l i t t l e o r n o protection from decay or dimensional stabilization was observed. If t h e d e g r e e o f s u b s t i t u t i o n i n l i g n i n d o e s h a v e a n effect o n t h e s e m e c h a n i s m s , i t i s o n l y o b s e r v e d at v e r y h i g h l e v e l s . T h e d e g r e e o f substitution i n t h e holocellulose components seems to b e t h e most i m p o r t a n t factor i n decay resistance a n d d i m e n s i o n a l stability.

Conclusion C h e m i c a l modification of w o o d w i l l be important i n the future b e c a u s e o f its a b i l i t y to e n h a n c e t h e p r o p e r t i e s o f t h e e n d p r o d u c t s i n u s e (132). If, f o r e x a m p l e , fire r e t a r d a n c y is i m p o r t a n t i n a w o o d



4.

ROWELL


207

m a t e r i a l , t h e fire r e t a r d a n t c h e m i c a l c o u l d b e b o n d e d p e r m a n e n t l y to t h e c e l l w a l l o f t h e w o o d . I f t h e l e v e l o f c h e m i c a l a d d i t i o n w e r e h i g h e n o u g h , d i m e n s i o n a l stability a n d s o m e degree of resistance to b i o l o g i c a l a t t a c k w o u l d a l s o b e a c h i e v e d at n o a d d i t i o n a l c o s t . T h e greatest single application of c u r r e n t research m a y b e i n r e constituted products i n w h i c h standard operating procedures call for d r y w o o d m a t e r i a l s , spray c h e m i c a l a d d i t i o n for m a x i m u m d i s t r i b u tion, small sample size for g o o d penetration, a n d h i g h temperature a n d pressure i n product formation. These are exactly the procedures r e q u i r e d for successful c h e m i c a l modification. P e r m a n e n t l y b o n d e d chemicals that p r o v i d e fire retardancy, U V stabilization, color changes, d i m e n s i o n a l stability, a n d resistance to biological attack m a y be possible through chemical modification.

Literature Cited 1. Rowell, R. M . ; Ellis, W. D . Wood Fiber 1978, 20(2), 104-11. 2. Rowell, R. M . ; Gutzmer, D . I.; Sachs, I. B.; Kinney, R. E. Wood Sci. 1976, 9(1),51-54. 3. Stamm, A. J. "Colloid Chemistry of Cellulosic Materials"; U S D A Misc. Publ. No. 240, Madison, WI, 1936. 4. Nayer, A. N . , P h . D . thesis, University of Minnesota, Minneapolis, 1948. 5. Kumar, V. B. Nor. Skogind. 1957, 11(7), 259. 6. Kumar, V. B. Nor. Skogind. 1958, 12(9), 337. 7. Stamm, A. J. "Wood Science"; Ronald: New York, 1964. 8. Ashton, H . E. Wood Sci. 1973, 6, 159-66. 9. Ashton, H . E. Wood Sci. 1974, 6(4), 368-74. 10. Rowell, R. M . ; Hart, S. V. Final Report Project 3212-5-76-3, U.S., For. Serv., For. Prod. Lab. Rep., 1981. 11. Erickson, H . D . For. Prod. J. 1955, 5(4), 241-50. 12. Haslam, J. H.; Werthan, S. Ind. Eng. Chem. 1931, 23(2), 226-33. 13. Phillips, E. W. J. Forestry 1933, 7(1), 109-20. 14. Thomas, R. J . ; Scheld, J. L . North Carolina State College Tech. Rep. No. 19, Raleigh, N C , 1964. 15. Meier, H . 'The Formation of Wood in Forest Trees"; Zimmerman, M . H . , E d . ; Academic: New York, 1964; p. 137-51. 16. Griffin, G. J. J. For. 1919, 17(7), 813-22. 17. Huber, B.; Merz, W. Planta 1958, 51, 660-72. 18. Nicholas, D . D . ; Siau, J. F. in "Wood Deterioration and Its Prevention by Preservative Treatments"; Nicholas, D . D . , E d . ; Syracuse Univ.: Syra cuse, NY, 1973. 19. Erickson, H . D . Minn., Agric. Exp. Stn., Tech. Bull. 1937, 122. 20. Gjovik, L . R.; Roth, H . G . ; Lorenz, L . F. Ρroc.—Annu. Meet. Am. WoodPreserv. Assoc. 1970, 66, 260-62. 21. MacLean, J. D . Proc.—Annu. Meet. Am. Wood-Preserv. Assoc. 1924, 20, 44-73. 22. Nicholas, D . D . For. Prod. J. 1972, 22(5), 31-36. 23. Verrall, A. F. U.S. For. Serv., South For. Exp. Stn., For. Surv. Release No. 157, 1957. 24. Thomas, R. J. Wood Fiber 1969, 1(2), 110-23. 25. Stamm, A. J.; Tarkow, H . U.S. Patent 2 417 995, 1947. 26. Ridgway, W. B.; Wallington, H . T. British Patent 579 255, 1946. 27. Tarkow, H . U.S. For. Prod. Lab., Rep. 1959. 28. Baird, B. R. Wood Fiber 1969, 1, 54-63. 29. Clermont, L . P.; Bender, F. For. Prod. J. 1957, 7, 167-70.


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119. Green, J. W. in "Methods in Carbohydrate Chemistry," Vol. III; Whistler, L.; Wolfrom, M . O., Eds.; Academic: New York, 1963; p. 9-21. 120. Moore, W. E.; Johnson, D . B. U S D A Forest Serv. unnumbered report, Forest Prod. Lab., Madison, WI, 1967. 121. Sarkanen, Κ. V.; Chang, H.-m.; Ericsson, B. Tappi 1967, 50(11), 572-75. 122. Rowell, R. M . Wood Sci. in press. 123. Rowell, R. M . Wood Sci. 1977, 9(3), 144-48. 124. Rowell, R. M . Wood Sci. 1978, 10(4), 193-97. 125. Nilsson, T.; Rowell, R. M . International Res. Group on Wood Preserva tion; Document No. IRG/WP/3211, 1982. 126. Peterson, M . D . ; Thomas, R. J. Wood Fiber 1978, 10(3), 149-63. 127. Callow, H. J. J. Indian Chem. Soc. 1951, 28, 605-10. 128. Rowell, R. M . Wood Sci. 1980, 13(2), 102-10. 129. Callow, H. J. J. Text. Inst. 1952, 43, T247-49. 130. Truksne, D . LLA Raksti 1977, 130, 32-39. 131. Sumi, Y.; Yale, R. D . ; Meyer, J. Α.; Leopold, B.; Ranby, B. G. Tappi 1964, 47(10), 621-24. 132. Rowell, R. M . Proc.—Annu. Meet. Am. Wood-Preserv. Assoc. 1975, 71, 41-51. RECEIVED

for review May 9, 1983.

ACCEPTED

July 7, 1983.


The Chemistry of Solid Wood - American Chemical Society

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