Wood Technology: Chemical Aspects

3 Chemical Methods of Improving the Permeability of Wood

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D A R R E L D. N I C H O L A S Institute of Wood Research, Michigan Technological University, Houghton, Mich. 49931

In the past, it has been w e l l documented that the relative permeability of wood has a s i g n i ficant influence on the effectiveness of p r e s e r v a t i v e treatments. Without adequate p e n e t r a t i o n , even the best preservat i v e s will not provide s u f f i c i e n t protection and the wood will fail prematurely. Consequently, any d e v e l opments i n methods which increase the treatability of wood which are difficult to t r e a t could have a s i g n i f icant impact on the wood preserving i n d u s t r y . The o b j e c t i v e of t h i s paper i s to review methods of improving the permeability of wood. Both mechanical and chemical methods are possibilities, but t h i s d i s cussion will be l i m i t e d to the latter. S t r u c t u r a l and Chemical Factors i n Wood Which Affect Flow A complete review of the s t r u c t u r e of wood i s presented i n another paper i n t h i s symposium. Hence, t h i s d i s c u s s i o n will be l i m i t e d to a review of the p r i n c i p a l s t r u c t u r a l and chemical factors which could p o s s i b l y influence the flow of l i q u i d s i n wood. Structural Factors. Wood i s e s s e n t i a l l y a c l o s e d c e l l u l a r s y s t e m , and t h e c e l l w a l l s a r e c h a r a c t e r i z e d by t h e p r e s e n c e o f numerous p i t p a i r s w h i c h s e r v e as flow paths for l i q u i d s i n l i v i n g c e l l s . A f t e r the c e l l s d i e and t h e y a r e t r a n s f o r m e d i n t o h e a r t w o o d , t h e p i t s u n d e r g o a s p i r â t i o n and become o c c l u d e d w i t h wood extract ives. T h i s r e s u l t s i n a reduc t i o n i n the e f f e c t i v e pore s i z e which i n turn r e s t r i c t s flow of m a t e r i als . N e v e r t h e l e s s , e v i d e n c e s u g g e s t s t h a t t h e maj o r f l o w p a t h from c e l l to c e l l ( i n e i t h e r r a y s or t r a c h e i d s ) i s t h r o u g h t h e p i t membrane s i n c e t h i s i s t h e path of l e a s t r e s i s t a n c e . C o n s e q u e n t l y , an u n d e r s t a n d -

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Goldstein; Wood Technology: Chemical Aspects ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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i n g o f t h e s t r u c t u r e and c h e m i c a l c o m p o s i t i o n o f t h e p i t membrane i s p a r a m o u n t i n o u r a t t e m p t t o i m p r o v e t h e p e r m e a b i l i t y o f wood. Pit Structure. B e c a u s e t h e p i t membrane a p p e a r s t o be t h e c o n t r o l l i n g f a c t o r i n f l o w t h r o u g h wood, an e x a m i n a t i o n o f i t s s t r u c t u r e w o u l d be p e r t i n e n t . S i m p l e p i t p a i r s h a v i n g c o n t i n u o u s p i t membranes ( 1 ) are found between parenchyma c e l l s . T h e r e f o r e , the f l o w t h r o u g h t h i s t y p e o f c e l l must p a s s t h r o u g h t h e p i t membrane. C o n v e r s e l y , most s o f t w o o d t r a c h e i d s have b o r d e r e d p i t p a i r s which have a d i f f e r e n t i a t e d membrane c o m p o s e d o f a n e t w o r k - l i k e o p e n margo and a thickened c e n t r a l port i o n c a l l e d the t o r u s ( 1 ) . As l o n g as t h e b o r d e r e d p i t p a i r i s i n t h e u n a s p i r a t e d s t a t e , f l o w can o c c u r r e l a t i v e l y e a s i l y t h r o u g h the porous margo. However, the p i t p a i r s of heartwood a r e f r e q u e n t l y c l o s e d t o f l o w due t o a c o m b i n a t i o n o f a s p i r a t i o n and o c c l u s i o n by e x t r a c t i v e s o r l i g n i n - l i k e substances (2, 3). I n s a p w o o d t h e p i t s may o r may n o t be a s p i r a t e d , b u t do n o t c o n t a i n o c c l u s i o n s so f l o w i s g e n e r a l l y l e s s r e s t r i c t e d than i n the heartwood. I n an a s p i r a t e d p i t , the flow could occur e i t h e r through the t h i c k e n e d t o r u s , o r p o s s i b l y b e t w e e n t h e t o r u s and t h e overhanging border. B a s e d on e v i d e n c e a c c u m u l a t e d t o d a t e , i t i s not p o s s i b l e to determine which of these paths i s the major pathway of f l o w t h r o u g h a s p i r a t e d p i t p a i r s , b u t b o t h a r e p r o b a b l y f u n c t i o n a l and v a r y i n i m p o r t a n c e w i t h i n and among s p e c i e s . E l e c t r o n m i c r o s c o p i c s t u d i e s ( 4 , 5, 6, 7) r e v e a l t h a t t h e b o r d e r e d p i t membrane i s f i b r i l l a r i n n a t u r e . U n t i l r e c e n t l y , t h e margo was c o n s i d e r e d t o be an o p e n network of m i c r o f i b r i l s i n the green s t a t e . However, t h e w o r k by S a c h s and K i n n e y ( 8 ) i n d i c a t e s t h a t t h e margo i s e s s e n t i a l l y a c o n t i n u o u s membrane i n t h e g r e e n s t a t e but becomes q u i t e p o r o u s as a r e s u l t of d r y i n g stresses during seasoning. As l o n g a s wood i s d r i e d p r i o r to treatment, which i s n o r m a l l y the case, t h i s p o i n t i s s u p e r f l u o u s w i t h r e s p e c t to p e r m e a b i l i t y . A l t h o u g h t h e r e a r e no v i s i b l e o p e n i n g s i n t h e t o r us , e v i d e n c e s u g g e s t s t h a t o p e n i n g s do e x i s t ( 9 ) . I t i s e n v i s i o n e d t h a t f l o w through the t o r u s would occur through t o r t u o u s paths between randomly o r i e n t e d m i c r o f i b r i l s s i m i l a r to the openings between the f i b e r s i n f i l t e r paper. Extending t h i s analogy, the s t r u c t u r e and c o m p o s i t i o n o f t h e f i l t e r p a p e r d e t e r m i n e s t h e r a t e of f l o w . The f i n e r and more c o m p a c t t h e e l e m e n t s o f t h e p a p e r a r e , t h e s l o w e r t h e f l o w and t h e more s u s c e p t i b l e i t i s t o p l u g g i n g by p a r t i c u l a t e m a t t e r . This s i t u a t i o n would a l s o a p p l y t o t h e s t r u c t u r e of the mem-


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brane t o r u s . Hence, the r e l a t i v e p a c k i n g d e n s i t y of t h e m i c r o f i b r i l s and t h e amount and t y p e o f e n c r u s t a n c e p r e s e n t w o u l d be e x p e c t e d t o h a v e a s i g n i f i c a n t influe n c e on t h e f l o w o f f l u i d s . C h e m i c a l F a c t o r s . B a s e d on t h e s t r u c t u r e and f l o w p a t h s i n w o o d , t h e p i t membrane a p p e a r s t o be t h e comp o n e n t w h i c h s h o u l d be c h e m i c a l l y a l t e r ed i n o r d e r t o i n c r e a s e the p e r m e a b i l i t y . In order to accomplish t h i s , k n o w l e d g e o f t h e c h e m i c a l c o m p o s i t i o n o f t h e p i t membrane i s d e s i r a b l e . A number o f i n v e s t i g a t o r s h a v e s t u d i e d t h e chemi c a l compos i t i o n o f t h e p i t membrane ( 1 0 , 1 1 , 1 2 , 1 3 , 14) us i n g comb i n a t i o n s o f m i c r o s c o p y , s p e c i f i c enzymes, h i s t o c h e m i c a l m e t h o d s and UV microspectrophotometry. From t h i s w o r k , i t a p p e a r s t h a t t h e p i t membrane u n d e r goes c h e m i c a l m o d i f i c a t i o n d u r i n g heartwood f o r m a t i o n . I t ha s b e e n p o s t u l a t ed t h a t t h e m e c h a n i s m f o r t h i s t r a n s f o r m a t i o n i s t h a t the parenchyma c e l l s produce compound s w h i c h m i g r a t e t o t h e p i t membrane and s e r v e a s p r e c u r s o r s f o r t h e f o r m a t i o n o f p o l y p h e n o l i c compounds ( 1 0 ) . P e r o x i d a s e , w h i c h i s f r e q u e n t l y p r e s e n t i n sapwood p i t s , p r o b a b l y s e r v e s a s a c a t a l y s t f o r these r e a c t i o n s . I n s a p w o o d , t h e p i t membranes a p p e a r t o be p r i n c i p a l l y c o m p o s e d o f c e l l u l o s e and p e c t i n ( 1 3 ) . Howe v e r , i n a number o f g e n e r a , t h e sapwood p i t membranes a l s o c o n t a i n p o l y p h e n o l s i n some c a s e s , b u t t h e d i s t r i b u t i o n i s not u n i f o r m even w i t h i n spec i e s . I n h e a r t w o o d , a l l p i t membranes o f t h e g e n e r a s t u d i e d a p p e a r t o c o n t a i n p o l y p h e n o l s i n add i t i o n t o the o t h e r major c h e m i c a l components. The p r e s e n c e o f l i g n i n i n t h e p i t membrane h a s n o t b e e n p o s i t i v e l y established. However, the f a c t t h a t a monomeric C6-C3 compound has b e e n f o u n d i n t h e c a p i l l a r y l i q u i d o f sapwood t r a c h e i d s , s t r o n g l y s u g g e s t s t h a t l i g n i f i c a t i o n may o c c u r i n t h e p i t m e m b r a n e s . I n any e v e n t , i t appears t h a t the polyphenols present i n the h e a r t wood u n d e r g o p o l y m e r i z a t i o n r e a c t i o n s t o h i g h e r m o l e c u l a r w e i g h t compound s w h i c h r e s i s t e x t r a c t i o n by n e u t r a l s o l v e n t s. Henc e, t h e y e x h i b i t p r o p e r t i e s s i m i l a r to t h a t of l i g n i n . One o f t h e s i g n i f i c a n t f i n d i n g s by B a u c h , e t . a l . ( 1 3 ) i s t h e f a c t t h a t t h e p r e s e n c e o f l i g n i n - l i k e compound s i s n o t u n i f o r m w i t h i n s a m p l e s f r o m a g i v e n s p e cies. T h e r e a l s o a p p e a r s t o be a c o n s i d e r a b l e v a r i a t i o n i n the c h e m i c a l c o m p o s i t i o n of the p i t s w i t h i n a single tracheid. This suggests that treatment w i t h c h e m i c a l s t h a t d e g r a d e l i g n i n may n o t be n e c e s s a r y i n a l l c a s e s , s i n e e m o d i f i c a t i o n o f a few p i t membranes


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p e r t r a c h e i d s h o u l d be s u f f i c i e n t t o s i g n i f i c a n t l y i m p r o v e t h e p e r m e a b i l i t y o f wood. I t s h o u l d be p o i n t e d o u t t h a t most o f t h e r e s e a r c h on c h e m i c a l c o m p o s i t i o n o f t h e p i t membrane h a s b e e n l i m i t e d to the bordered p i t s . The c h e m i c a l composition o f t h e s i m p l e p i t membranes i n t h e r a y p a r e n c h y m a c e l l s may o r may n o t be t h e same. However, s i n c e t h e p a r e n chyma c e l l s p r o d u c e t h e p r e c u r s o r s f o r t h e f o r m a t i o n of p o l y p h e n o l i c compounds, i t i s a n t i c i p a t e d t h a t t h e membrane o c c l u s i o n s w o u l d be s i m i l a r . Methods of I n c r e a s i n g

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o f Wood

T h e r e a r e a number o f p o s s i b l e c h e m i c a l m e t h o d s w h i c h c o u l d be e m p l o y e d t o i n c r e a s e t h e p e r m e a b i l i t y o f wood. These a r e : a) c h e m i c a l t r e a t m e n t s , b) m o d i f i c a t i o n o f t r e a t i n g s o l u t i o n p r o p e r t i e s , and c ) b i o l o g i c a l treatments. The p o t e n t i a l o f e a c h o f t h e s e m e t h o d s w i l l now be e x a m i n e d i n d e t a i l . Chemical Treatments. Over t h e y e a r s , a c o n s i d e r a b l e amount o f r e s e a r c h h a s b e e n c o n d u c t e d on t h e p o s s i b i l i t y o f us i n g v a r i o u s c h e m i c a l p r e t r e a t m e n t s t o i m p r o v e t h e p e r m e a b i l i t y o f wood. The b a s i c p r i n c i p l e behind such treatments i s e i t h e r t o e x t r a c t extraneous m a t e r i a l f r o m t h e p i t membrane o r d e g r a d e t h e p i t membrane i n order to enlarge the openings. P r e - e x t r a c t i o n o f Wood. Since the major f a c t o r c a u s i n g a r e d u c t i o n i n t h e p e r m e a b i l i t y o f wood d u r i n g h e a r t w o o d f o r m a t i o n i s o c c l u s i o n o f t h e p i t membranes w i t h e x t r a n e o u s m a t e r i a l , one w o u l d ant i c i p a t e t h a t p r e - e x t r a c t i o n o f t h e wood w i t h a s u i t a b l e s o l v e n t w o u l d be a m e t h o d o f i n c r e a s i n g t h e p e r m e a b i l i t y o f wood. T h i s c o n t e n t i o n h a s b e e n v e r i f i e d b y a number o f s t u d i e s ( 1 5 , 1 6 , 1 7 , 1 8 , 1 9 , 2 0 , 3, 2 1 ) . H o w e v e r , i t a p p e a r s d o u b t f u l t h a t s u c h t r e a t m e n t s w o u l d be comm e r c i a l l y f e a s i b l e since the solvents are expensive and e x c e s s i v e t i m e i s r e q u i r e d f o r t h e a d d i t i o n a l s t e p in the t r e a t i n g process. C h e m i c a l D e g r a d a t i o n o f t h e P i t Membranes. Chemi c a l mod i f i c a t i o n o f t h e p i t membranes i s a p o s s i b l e m e t h o d o f i n c r e a s i n g t h e p e r m e a b i l i t y o f wood a s l o n g as i t c a n be a c c o m p l i s h e d w i t h o u t i n c u r r i n g e x c e s s i v e strength loss. I n t h i s r e g a r d , sodium c h l o r i t e , p u l p i n g l i q u o r s , a c i d s and b a s e s h a v e b e e n u s e d t o i n c r e a s e t h e p e r m e a b i l i t y o f wood ( 1 9 , 2 2 , 3, 2 3 ) . U n f o r t u n a t e l y , e x c e s s i v e s t r e n g t h l o s s o f t h e wood r e s u l t e d from these treatments. N e v e r t h e l e s s , i t may be p o s s i b l e


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t o a c h i e v e t h e d e s i r e d r e s u l t s by s e l e c t i n g t h e p r o p e r c h e m i c a l s so t h i s a p p r o a c h s h o u l d n o t be a b a n d o n e d . F o r e x a m p l e , t h e w o r k by Emery and S c h r o e d e r ( 2 4 ) i n d i c a t e s t h a t wood c a n be c h e m i c a l l y o x i d i z e d w i t h a n i r o n c a t a l y z e d r e a c t i o n under a c i d i c c o n d i t i o n s . It i s c o n c e i v a b l e that t h i s type of treatment could deg r a d e t h e p i t membrane a n d i n c r e a s e t h e p e r m e a b i l i t y . F u r t h e r m o r e , T s c h e r n i t z ( 2 5 ) h a s shown t h a t t r e a t m e n t o f R o c k y M o u n t a i n D o u g l a s f i r sapwood w i t h h o t ammonium o x a l a t e improved the t r e a t a b i l i t y of t h i s m a t e r i a l . I n t h i s l a t t e r c a s e , t h e ammonium o x a l a t e p r o b a b l y s o l u b i l i z e d t h e p e c t i n s i n t h e p i t membrane. A n o t h e r c h e m i c a l method o f i n c r e a s i n g t h e p e r m e a b i l i t y o f sapwood i s t o s t e a m t h e g r e e n wood. This t e c h n i q u e i s f r e q u e n t l y used i n p r o c e s s i n g s o u t h e r n p i n e where i t a i d s i n t h e d r y i n g s t e p as w e l l as i n creasing the permeability. I t h a s b e e n shown t h a t t h e p r o b a b l e mechanism i n v o l v e d i n t h e change i n p e r m e a b i l i t y i s ac i d h y d r o l y s i s w h i c h r e d u c e s t h e e f f e c t i v e n e s s o f p i t aspirât i o n s i g n i f i c a n t l y ( 2 6 ) . A t t e m p t s h a v e b e e n made t o i n c r e a s e t h e p e r m e a b i l i t y of heartwood but without success. Furthermore, s t e a m i n g h a s n o t b e e n e f f e c t i v e on s p e c i e s o t h e r t h a n t h e s o u t h e r n p i n e s (27) . Mod i f i c a t i o n o f L i q u i d P r o p e r t i e s . The r e l a t i v e p e r m e a b i l i t y o f wood v a r i e s w i t h t h e t y p e a n d c o n d i t i o n of t h e i m p r e g n a t i n g l i q u i d . C o n s e q u e n t l y , by s e l e c t i n g the a p p r o p r i a t e parameters, i t i s p o s s i b l e to a l t e r the a p p a r e n t p e r m e a b i l i t y o f wood. T h e s e f a c t o r s w i l l now be d i s c u s s e d i n d e t a i l . Type o f L i q u i d . B o t h p e t r o l e u m h y d r o c a r b o n s and w a t e r a r e u s e d a s c a r r i e r s f o r wood p r e s e r v a t i v e s . A number o f s t u d i e s h a v e c l e a r l y shown t h a t p e t r o l e u m h y d r o c a r b o n s p e n e t r a t e wood much more r a p i d l y t h a n w a t e r ( 2 8 , 2 9 , 2 0 , 2 7 ) . The r e a s o n f o r t h i s d i f f e r e n c e i s n o t ent i r e l y c l e a r , b u t i t has been p r o p o s e d t h a t h y d r o g e n b o n d i n g a b i l i t y o f t h e l i q u i d i s t h e maj o r f a c t o r t h a t i n f l u e n c e s i t s a b i l i t y t o p e n e t r a t e wood (28, 29, 2 7 ) . Water has t h e a b i l i t y t o form s t r o n g hydrogen bonds between m o l e c u l e s w h i c h r e s u l t s i n a s t r u c t u r e d medium. I n a d d i t i o n , water has t h e a b i l i t y t o f o r m h y d r o g e n b o n d s w i t h t h e h y d r o x y l g r o u p s i n wood. H e n c e , t h e s e two f a c t o r s c o u l d p r o d u c e a f r i c t i o n a l d r a g a s w a t e r moves t h r o u g h wood a n d e f f e c t i v e l y r e duce the flow rate. S i n c e i t has never been proven c o n e l u s i v e l y t h a t hydrogen bonding a b i l i t y i s t h e reason f o r d i f f e r e n c e s i n p e n e t r a b i l i t y o f l i q u i d s , o t h e r p o s s i b i l i t i e s should


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n o t be p r e m a t u r e l y r u l e d o u t . For example, the a b i l i t y o f a l i q u i d t o f o r m b u b b l e s may be s i g n i f i c a n t s i n c e i t i s known t h a t t h e p r e s e n c e o f b u b b l e s r e d u c e s t h e f l o w r a t e b e c a u s e c a p i l l a r y p r e s s u r e m u s t be o v e r c o m e d u r i n g imprégnât i o n ( 3 0 ) . I n t h i s r e g a r d , t h e r e i s a f u n d a m e n t a l d i f f e r e n c e b e t w e e n w a t e r and o r g a n i c l i q u i d s s i n e e i t h a s b e e n shown t h a t s t a b i l i z e d g a s m i c r o n u c l e i e x i s t s i n the former but not i n the l a t t e r ( 3 1 ) . Hence, water would have a tendency t o form b u b b l e s whereas o r g a n i c l i q u i d s would n o t . Another p o s s i b l e e x p l a n a t i o n o f t h e d i f f e r e n c e b e t w e e n t h e two l i q u i d s h a s b e e n a d v a n c e d by B a i l e y and P r e s t o n ( 3 2 ) . T h e y contend t h a t the d i f f e r e n c e i s a t t r i b u t a b l e to the dep o s i t i o n of h y d r o p h o b i c mater i a l i n the pores which ef f e c t i v e l y i n c r e a s e s the c o n t a c t a n g l e f o r w a t e r . As a r e s u l t , more p r e s s u r e i s r e q u i r e d t o i m p r e g n a t e wood i n a c c o r d a n c e w i t h J u r i n s equat i o n ( 3 0 ) . The a b o v e a r e o n l y 3 p o s s i b l e f a c t o r s w h i c h c o u l d i n f l u e n c e t h e p e n e t r a b i l i t y o f l i q u i d s i n t o wood. O t h e r p o s s i b i l i t i e s a r e : a) s u r f a c e t e n s i o n , b) m o l e c u l a r s i z e , c ) c h e m i c a l ac t i v i t y , d) s o l v e n c y , and e) a b i l i t y t o s w e l l wood. Some o f t h e s e f a c t o r s may be o p e r a t i v e i n t r e a t m e n t s w i t h p r o p y l e n e o x i d e w h i c h have b e e n c a r r i e d o u t by R o w e l l ( 3 3 ) . I n t h i s s t u d y , he u s e d a m i x t u r e o f 9 5 % p r o p y l e n e o x i d e and 5% t r i e t h y l amine ( v / v ) . He was a b l e t o c o m p l e t e l y t r e a t s o u t h e r n p i n e and r e d p i n e h e a r t w o o d , b o t h o f w h i c h w e r e c l a s s i f i e d as b e i n g r e f r a c t o r y . T h i s w o r k c l e a r l y shows t h a t w i t h t h e p r o p e r t r e a t i n g m e d i u m , h e a r t w o o d c a n be f u l l y penetrated. I n summary, i t c a n be c o n c l u d e d t h a t t h e t r e a t i n g l i q u i d c h a r a c t e r i s t i c s h a v e a s i g n i f i c a n t i n f l u e n c e on the treatment r e s u l t s . Consequently, a b e t t e r unders t a n d i n g of the f a c t o r s which a f f e c t p e n e t r a b i l i t y could l e a d to improved t r e a t m e n t methods. Liquid Contamination. As t r e a t i n g s o l u t i o n s a r e c o n t i n u o u s l y r e u s e d , t h e y become c o n t a m i n a t e d w i t h p a r t i c u l a t e m a t t e r f r o m c h e m i c a l r e a c t i o n s and e x t r a n e o u s sources. I t h a s b e e n shown t h a t t h i s p a r t i c u l a t e m a t t e r c a n s i g n i f i c a n t l y r e d u c e t h e pénétrât i o n o f p r e s e r v a t i v e s o l u t i o n s i n t o wood. C o n s e q u e n t l y , i t appears t h a t methods f o r c o n t i n u o u s l y r e m o v i n g p a r t i c u l a t e matt e r c o u l d r e s u l t i n i m p r o v e d pénétrât i o n o f p r e s e r v a tive solut ions. Chemical A d d i t i v e s . P r e v i o u s r e s e a r c h h a s shown t h a t t h e a d d i t i o n o f c h e m i c a l s t o w a t e r h a s an e f f e c t on i t s a b i l i t y t o p e n e t r a t e wood ( 7 ) . I n t h i s r e g a r d , s t a n d a r d p r e s e r v a t i v e c h e m i c a l s and wood e x t r a c t i v e s


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t e n d t o r e d u c e t h e f l o w o f l i q u i d s i n wood. Hence, p r o p e r s e l e c t i o n o f c h e m i c a l s and t h e r e m o v a l o f e x t r a c t i v e s could i n s u r e that the treatment i s maximized. The f a c t t h a t t h e r e i s a s i g n i f i c a n t d i f f e r e n c e i n t h e p e n e t r a b i l i t y o f p o l a r and n o n - p o l a r l i q u i d s s u g g e s t s t h a t altérât i o n s o f t r e a t i n g s o l u t i o n s w i t h add i t i v e s may be a m e t h o d o f i m p r o v i n g t r e a t m e n t . In t h i s r e g a r d , B u c k m a n , e t . a l . ( 2 8 ) and H a r t m a n n - D i c k ( 2 0 ) h a v e shown t h a t t h e f l o w o f w a t e r i n wood c a n be s i g n i f i c a n t l y i n c r e a s e d by t h e add i t i o n o f t h e z i n c chloride. The r e a s o n f o r t h i s i m p r o v e m e n t i s n o t k n o w n , b u t i t s u g g e s t s t h a t t h e r e may be some p o t e n t i a l i n t h i s a r e a and o t h e r add i t i v e s s h o u l d be i n v e s t i gated . B i o l o g i c a l Treatments. The p o s s i b i l i t y o f u s i n g b i o l o g i c a l treatments to i n c r e a s e the p e r m e a b i l i t y of wood has r e c e i v e d c o n s i d e r a b l e a t t e n t i o n by r e s e a r c h e r s . I n g e n e r a l , t h i s t y p e o f t r e a t m e n t c a n be separated i n t o 3 c a t e g o r i e s ; n a m e l y , a) t r e a t m e n t w i t h f u n g i , b) t r e a t m e n t w i t h b a c t e r i a , and c ) enzyme t r e a t m e n t s . Treatment w i t h Fungi. The u s e o f f u n g i t o i n c r e a s e t h e p e r m e a b i l i t y o f wood was i n i t i a t e d by L i n d g r e n and H a r v e y ( 3 4 ) and B l e w ( 3 5 ) . In these s t u d i e s , T r i c h o d e r m a m o l d was u s e d t o i n c r e a s e t h e p e r m e a b i l i t y of s o u t h e r n p i n e sapwood. F o l l o w i n g t h i s work, o t h e r s t u d i e s w e r e c o n d u c t ed w i t h D o u g l a s f i r ( 3 6 , 3 7 ) , j a c k ρine ( 3 8 ) , and s p r u e e and a s p e n (3 9 ) . A l l of t h e s e s t u d i e s i n v o l v e d the use of T r i c h o d e r m a mold. H o w e v e r , i n a l a t e r s t u d y by E r i c k s o n and Depreitas ( 4 0 ) , G l i o c l a d i u m f u s a r i u m , and C h a e t o m i u m w e r e u s e d along with Trichoderma. Improved p e n e t r a t i o n , h i g h e r p r e s e r v a t i v e r e t e n t i o n , and more u n i f o r m t r e a t m e n t s w e r e t h e g e n e r a l r u l e i n the above stud i e s . Fur t h e r m o r e , the r e s u l t s w e r e s i m i l a r among t h e s p e c i e s o f f u n g i t e s t e d . The m e c h a n i s m f o r i n c r e a s i n g p e r m e a b i l i t y i s i l l u s t r a t e d i n F i g u r e 1. T h i s shows a f u n g a l h y p h a e g r o w i n g i n s i d e a c e l l l u m e n and a b r a n c h h a s f o r m e d a t t h e p i t a p e r a t u r e so t h a t i t c a n p e n e t r a t e i n t o t h e next c e l l . I f t h i s o c c u r s f r e q u e n t l y enough, i t would e f f e c t i v e l y i n c r e a s e the p e r m e a b i l i t y . T h e r e a r e two maj o r l i m i t a t i o n s i n t h e processes d e s c r i b e d above. F i r s t of a l l , t h e f a c t t h a t t h e s e f u n g i a r e e f f e c t i v e o n l y i n t h e sapwood, w h i c h i n most instances i s r e a d i l y t r e a t a b l e , reduces i t s usefulness. S e c o n d l y , i t has b e e n shown by J o h n s o n and Gj o v i k ( 4 1 ) t h a t e x t r a n e o u s b a c t e r i a , r a t h e r t h a n f u n g i , may a c t u a l l y be r e s p o n s i b l e f o r t h e p i t membrane d e g r a d a t i o n .


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C o n s e q u e n t l y , i t does not appear t h a t t h i s p a r t i c u l a r a p p r o a c h w i l l p r o v e t o be p r a c t i c a l i n c o m m e r c i a l a p plications . S i n c e i t h a s b e e n shown t h a t t h e d e g r a d a t i o n of t h e p i t membranes i n wood i s t h e most l o g i c a l m e t h o d o f i n c r e a s i n g p e r m e a b i l i t y , f u n g i must be c a p a b l e o f a t t a c k i n g t h i s s t r u c t u r e i n o r d e r t o be s u c c e s s f u l . As i s shown i n F i g u r e 1, t h e f u n g a l h y p h a e a r e a b l e t o l o c a t e and p e n e t r a t e t h e p i t membrane so t h i s i s a v i a b l e method. Because the heartwood p i t s c o n t a i n p o l y p h e n o l i c s u b s t a n c e s , t h e f u n g i must d e g r a d e t h e s e compounds. I n t h i s r e g a r d , t h e w o r k by K i r k (42) i s significant. K i r k has f o u n d t h a t t h e f u n g u s P h a n e r o chaete chrysosporium r e a d i l y degrades l i g n i n . Hence, i t m i g h t be p o s s i b l e t o u s e t h i s o r a s i m i l a r f u n g u s t o i m p r o v e t h e p e r m e a b i l i t y o f wood. S i n c e i t may be d e s i r a b l e t o d e g r a d e t h e c e l l u l o s e as w e l l as p o l y p h e n o l i c c o m p o u n d s , i t m i g h t be n e c e s s a r y t o u s e a m i x e d c u l t u r e ( 4 3 ) . T h a t s u c h a f u n g u s e x i s t s was v e r i f i e d by G r e a v e s and B a r n i c l e ( 4 4 ) when t h e y d i s c o v e r e d t h a t a f u n g u s was r e s p o n s i b l e f o r t h e i n c r e a s e d p e r m e a b i l i t y o f k a r r i h e a r t w o o d by s e l e c t i v e a t t a c k on t h e p i t membrane and wood r a y s . I f t h e r i g h t f u n g i c o u l d be f o u n d , t h e n i t may be p o s s i b l e t o p r e - t r e a t wood w i t h a s p o r e s u s p e n s i o n w h i c h w o u l d be a l l o w e d t o r e a c t s u f f i c i e n t l y t o o p e n up t h e s t r u c t u r e b e f o r e t r e a t m e n t . T h i s , of course, assumes t h a t the f u n g i w i l l s u f f i c i e n t l y d e g r a d e the p i t membranes b e f o r e s i g n i f i c a n t l y d a m a g i n g t h e c e l l w a l l s t r u c t u r e and c a u s e e x c e s s i v e s t r e n g t h l o s s . This may be p o s s i b l e b e c a u s e o f t h e a c c e s s i b i l i t y o f t h e p i t membranes. F u r t h e r m o r e , t h e i n c u b a t i o n p e r i o d m u s t be r e l a t i v e l y s h o r t i n o r d e r f o r s u c h a p r o c e s s t o be feasible. K i r k (42) i n d i c a t e s t h a t t h e s e r e a c t i o n s p r o c e e d r a p i d l y u n d e r t h e p r o p e r c o n d i t i o n s so t h i s may n o t be a s e r i o u s p r o b l e m . I n o r d e r t o make i t f e a s i b l e t o t r e a t h e a r t w o o d t o s a t i s f a c t o r y d e p t h s , i t may be n e c e s s a r y t o i n c i s e w i t h an o p e n p a t t e r n p r i o r t o a p p l y i n g t h i s s p o r e suspension. S i n c e t h i s c o u l d be d o n e i n a s i n g l e o p e r a t i o n , i t w o u l d be e c o n o m i c a l l y a t t r a c t i v e . Such a s y s t e m might p r o v i d e a means o f a c h i e v i n g a u n i f o r m p r e s e r v a t i v e d i s t r i b u t i o n , which i s c u r r e n t l y not p o s s i b l e w i t h i n c i s i n g a l o n e w i t h o u t i n c u r r i n g e x c e s s i v e damage t o t h e wood. Treatment w i t h B a c t e r i a . S e v e r a l y e a r s ago, i t was d i s c o v e r e d t h a t t h e p e r m e a b i l i t y o f wood was i n c r e a s e d by b a c t e r i a l a t t a c k when l o g s w e r e s o a k e d i n water f o r a p e r i o d of time (45). Following this,


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add i t i o n a l s t u d i e s w e r e c o n d u c t e d t o d e t e r m i n e w h i c h b a c t e r i a w e r e i n v o l v e d , t h e i r mode o f a c t i o n and o p t i mum o p e r a t i n g c o n d i t i o n s ( 1 1 , 12, 46, 4 7 , 4 8 , 4 5 , 49, 5 0 , 5 1 , 5 2 , 5 3 , 54, 5 5 , 56, 57, 5 8 , 5 9 , 60, 6 1 ) . I n p i n e , i t was f o u n d t h a t B a c i l l u s p o l y m y x a was the major s p e c i e s i n v o l v e d (45, 55), whereas, i n s p r u c e t h e m a j o r s p e c i e s w e r e B a c i l l u s s u b t i l l s and Flavobacterium pectinovorum (49). In another study ( 5 3 ) , C l o s t r i d ium o m e l i a n s k i i was i d e n t i f i e d as t h e species at t a c k i n g softwoods. I n a l l s t u d i e s , i t was f o u n d t h a t t h e b a c t e r i a l a t t a c k on t h e p i t membranes was t h e r e a s o n f o r i n c r e a s e d p e r m e a b i l i t y o f t h e wood. F u r t h e r m o r e , i t was shown by F o g a r t y and Ward ( 4 9 ) t h a t b a c t e r i a d e g r a d e d t h e p i t membranes by e x c r e t i n g t h e enzymes a m y l a s e x y l a n a s e , and p e c t i n a s e . A t y p i c a l sapwood p i t membrane t h a t h a s b e e n a t t a c k e d by b a c t e r i a i s shown i n F i g u r e 2. As c a n be s e e n , t h e t o r u s i s s e v e r e l y d e g r a d e d and h a s w e l l d e f i n e d o p e n ings. The i m p o r t a n t t h i n g t o remember i n m o s t o f t h e s e e x p e r i m e n t s i s t h a t the b a c t e r i a have been used to imp r o v e the p e r m e a b i l i t y of sapwood. For s p e c i e s l i k e S i t k a s p r u c e w h i c h h a s sapwood t h a t i s i m p e r m e a b l e t o c r e o s o t e , t h i s t y p e o f t r e a t m e n t may p r o v e t o be commercially feasible. However, i n o r d e r to f u l l y exp l o i t t h i s method f o r i n c r e a s i n g p e r m e a b i l i t y , i t w i l l be n e c e s s a r y t o f i n d b a c t e r i a w h i c h h a v e t h e a b i l i t y to d e g r a d e h e a r t w o o d p i t membranes. I n d e e d , t h i s may be p o s s i b l e s i n c e G r e a v e s (50) h a s shown t h a t some b a c t e r i a can i n c r e a s e the p e r m e a b i l i t y of heartwood. In a d d i t i o n t o t h i s , i t w o u l d be d e s i r a b l e t o a c c e l e r a t e t h e r e a c t i o n as much a s p o s s i b l e i n o r d e r t o make i t compatible with a commercial operation. P o s s i b l e s t r e n g t h l o s s i s i m p o r t a n t w h e n e v e r an a t t e m p t i s made t o i n c r e a s e t h e p e r m e a b i l i t y o f wood. V a r i a b l e e f f e c t s h a v e b e e n r e p o r t e d by r e s e a r c h e r s . F o r e x a m p l e , B a u c h , e t . a l . ( 1 1 , 12) and U n l i g i l (58) observed a s i g n i f i c a n t r e d u c t i o n i n the impact s t r e n g t h o f wood t h a t had b e e n p o n d e d t o i m p r o v e p e r m e a b i l i t y . On t h e o t h e r h a n d , D u n l e a v y and F o g a r t y ( 4 8 ) r e p o r t e d no s i g n i f i c a n t s t r e n g t h l o s s i n s p r u c e p o l e s t h a t w e r e exposed to b a c t e r i a l a t t a c k . The r e a s o n f o r t h i s d i f f e r e n c e i n r e s u l t s may be due t o t h e b a c t e r i a l s p e c i e s involved. G r e a v e s ( 5 1 ) s t u d i e d t h i s i n d e t a i l and f o u n d t h a t some b a c t e r i a w e r e a b l e t o i n c r e a s e t h e p e r m e a b i l i t y o f wood w i t h o u t a f f e c t i n g t h e s t r e n g t h . Conv e r s e l y , o t h e r b a c t e r i a were a b l e to i n c r e a s e the p e r m e a b i l i t y b u t a t t h e same t i m e t h e y d e c r e a s e d t h e strength signif icantly. T h i s d i f f e r e n c e may a l s o be due t o t h e p r e s e n c e o f o t h e r m i c r o o r g a n i s m s s i n e e one


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Figure 1. A fungal hyphae growing inside the lumen of a tracheid. Note how the hyphae is branched to penetrate the pit membrane. (Courtesy of I. B. Sachs)

Figure 2.

A sapwood pit membrane that has been degraded by bacteria. (Courtesy of I. B. Sachs)


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A Douglas-fir sapwood pit membrane which has been degraded by pectinase. (Courtesy of I. B. Sachs)

s t u d y ( 5 9 ) r e v e a l e d t.hat t h e i n c r e a s e d p e r m e a b i l i t y was due t o f u n g i r a t h e r t h a n b a c t e r i a . Enzymes. S i n c e b o t h b a c t e r i a and f u n g i u t i l i z e e n z y m e s t o d e g r a d e t h e p i t membrane i n w o o d , i t i s n o t s u r p r i s i n g t h a t t r e a t m e n t w i t h i s o l a t e d enzymes p r o duces s i m i l a r e f f e c t s . T h i s was shown t o be t h e c a s e by N i c h o l a s a n d Thomas ( 2 6 ) u s i n g c e l l u l a s e , h e m i c e l l u l a s e and p e c t i n a s e . S i m i l a r r e s u l t s were sub s e q u e n t l y o b t a i n e d by o t h e r r e s e a r c h e r s ( 6 2 , 63, 64, 65) . I n a r e c e n t s t u d y by Tshernitζ ( 2 5 ) , i t was v e r i f i e d t h a t e n z y m e s c o u l d be u s e d t o i n c r e a s e t h e p e r m e a b i l i t y o f Rocky M o u n t a i n Douglas f i r . By p r e t r e a t i n g t h e wood w i t h pec t i n a s e, a c o m p l e t e l y uniform t r e a t m e n t o f t h e sapwood z o n e was p o s s i b l e w i t h c r e o sote. This i s i n contrast to e r r a t i c treatment nor m a l l y o b t a i n e d when t h i s m a t e r i a l i s t r e a t e d . A t y p i c a l p i t membrane w h i c h h a s b e e n d e g r a d e d b y p e c t i n a s e i s shown i n F i g u r e 3. W i t h r e g a r d t o t h e e f f e c t o f e n z y m e s on t h e s t r e n g t h o f t r e a t e d wood, t h e work by Meyer (65) i n d i cated that i t i s p o s s i b l e to s i g n i f i c a n t l y increase t h e p e r m e a b i 1 i t y o f wood w i t h o u t a n y a p p r e c i a b l e


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s t r e n g t h l o s s . Hence, i t does not appear t h a t t h i s would be a problem w i t h the use of enzymes. To d a t e , i t has not been p o s s i b l e to degrade heartwood p i t membranes w i t h i s o l a t e d enzymes. Since c o - f a c t o r s are p r o b a b l y r e q u i r e d ( 4 2 ) , i t appears t h a t use of a whole o r g a n i s m r a t h e r than i s o l a t e d enzymes may be r e q u i r e d i f the p e r m e a b i l i t y of heartwood i s to be i n c r e a s e d .

LITERATURE CITED 1. 2. 3. 4. 5. 6.

Panshin, A. J., DeZeeuw, C., Brown, H. P., "Text book of Wood Technology," Vol. 1, McGraw-Hill Book Company, Inc., New York, Ν. Y., 1964. Cote, W. A., J. Polymer Sci. C., (1963), (2):231-42. Krahmer, R. L., Cote, W. A., Tappi, (1963), 46 (1):42-49. Erickson, H. D., Wood Science, (1970), 2(3):149-158. Cote, W. A., Forest Prod. J., (1958), 8(10):296301. Cote, W. A., Krahmer, R. L., Tappi, (1962), 45(2):119-22.

7. 8.

Nicholas, D. D., Forest Prod. J., (1972), 22(5):31. Sachs, I. Β., Kinney, R. E., Wood Science, (1974), 6(3):200-205. 9. Sachs, I. Β., Personal communication, (1975). 10. Bauch, J., Berndt, H., Wood Science and Technol ogy, (1973), 7(1):6-19. 11. Bauch, J. Adolf, P., Liese W., Holz als Roh-und Werkstoff, (1973), 31(3):115-120. 12. Bauch, J., Liese, W., Berndt, H., Holzforschung, (1970), 24(6):199-205. 13. Bauch, J., Schweers, W., Berndt, H., Holzforschung, (1974), 28(3):86-91. 14. Nicholas, D. D., Thomas, R. J., Forest Prod. J., (1968), 18(1):57-59). 15. Benvenuti, R. R., Unpublished Masters thesis, North Carolina State Univ., Raleigh, N. C. (1963). 16. Buro, Α., Buro, Ε. A., Holz als Roh-und Werkstoff, (1959), 17(12):461-74. 17. Charuk, Ε. V., Razumova, A. F., Holztechnologie, (1974), 15(1):3-6. 18. Comstock, G. L., Forest Prod. J., (1965), 15(10): 441-49. 19. Erdtman, H., Svensk Papperstidning, (1958), 61625-32. 20. Hartmann-Dick, V., Forstwiss. CBI., (1955), 74(5/6):163-83. 21. Scarth, G. W., Spier, J. D., Royal Soc. of Can. Proc. and Trans., (1929), 23:281-88.


3. NICHOLAS 22. 23. 24. 25. 26. 27. D. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47.

Methods of Improving Permeability

J o h n s t o n , H. W., Maass, O., Can. J. Res., ( 1 9 3 0 ) , 3:140-73. Y o s h i m o t o , T., H a y a s h i , S., K i s h i m a , T., Wood Research, (1970), (52):90-105. Emery, J. A., S c h r o e d e r , Η. A., Wood Sci. and Technology, (1974), 8(2):123-137. T s c h e r n i t z , J. L., F o r e s t P r o d . J., ( 1 9 7 3 ) , 23(3):30-38. N i c h o l a s , D. D., Thomas, R. J., Proc. Am. Wood– Preservers' Assoc., (1968), 64:1-7. N i c h o l a s , D. D., S i o u , J. F., "Wood Deterioration and Its P r e v e n t i o n by Preservative Treatments," D. N i c h o l a s , Ed., Vol. II, S y r a c u s e University P r e s s , S y r a c u s e , Ν. Y., 1973. Buckman, S. J., S c h m i t z , H., G o r t n e r , R. Α., Chem., (1935), 39:103-19. E r i c k s o n , H. D., S c h m i t z , Η., G o r t n e r , R. A., M i n n e s o t a Ag. E x p . S t a . T e c h . Bull. No. 2 2 , ( 1 9 3 7 ) . S i a u , J. F., "Flow in Wood," S y r a c u s e U n i v . P r e s s , S y r a c u s e , Ν. Υ., 1971. Hayward, A. T. J., Am. Scientist, (1971), 59:434443. B a i l e y , P. J., P r e s t o n , R. D., H o l z f o r s c h u n g , (1969), 23(4):113-120. R o w e l l , R. Μ., P e r s o n a l c o m m u n i c a t i o n , ( 1 9 7 5 ) . L i n d g r e n , R. A., H a r v e y , G. M., F o r e s t P r o d . J., (1952), 2(5):250-56. B l e w , J. O., Jr., F o r e s t P r o d . J., ( 1 9 5 2 ) , 2(3): 85-86. Graham, R. D., F o r e s t P r o d . J., ( 1 9 5 4 ) , 4 ( 4 ) : 1 6 4 166. L i n d g r e n , R. Μ., W r i g h t , Ε., F o r e s t P r o d . J., (1954), 4(4):162-164. Panek, Ε., F o r e s t P r o d . J., ( 1 9 5 7 ) , 7 ( 4 ) : 1 2 4 - 1 2 7 . S c h u l z , G., F o r e s t P r o d . J., ( 1 9 5 6 ) , 6 ( 2 ) : 7 7 - 8 0 . E r i c k s o n , H. D., Defreitas, A. R., F o r e s t P r o d . J., (1971), 21(4):53-58. J o h n s o n , B. R., G j o v i k , L. R., P r o c . Amer. WoodPreservers' A s s o c . , (1970), 66:234-240. K i r k , Τ. Κ., P e r s o n a l c o m m u n i c a t i o n , ( 1 9 7 5 ) . S k i n n e r , K. J., Chem. and E n g r . News, ( 1 9 7 5 ) , 53(Aug. 1 8 ) : 2 2 - 4 1 . G r e a v e s , Η., B a r n a c l e , J. Ε., F o r e s t P r o d . J., (1970), 20(8):47-51. E l l w o o d , Ε. L., E c k l u n d , Β. A., F o r e s t P r o d . J., (1959), 9(9):283-92. D e G r o o t , R. C., S c h e l d , H. W., F o r e s t P r o d . J., (1973), 23(4):43-46. D u n l e a v y , J. A., M c Q u i r e , A. J., J. I n s t . Wood Sci., (1970), 5(2):20-28.


45

46 48. 49. 50. 51. 52.

53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65.

WOOD TECHNOLOGY: CHEMICAL ASPECTS

Dunleavy, J. Α., Fogarty, W. M., Proc. British Wood Preserving A s s o c . , (1971), 1-28. Fogarty, W. M., Ward, O. P., Wood Sci. and T e c h . , (1973), 7(4):261-270. Greaves, Η . , Holzforschung, (1970), 24 (1):6-14. Greaves, Η., Wood Sci. and T e c h . , (1971), 5(1): 6-16. Greaves, Η., Levy, J. F., "Proc. of the F i r s t I n t e r n a t i o n a l B i o d e t e r i o r a t i o n Symposium," p. 429441, E l s e v i e r P u b l i s h i n g Company, Ltd., Amsterdam, 1968. Karnop, G., M a t e r i a l and Organism, (1972), 7(3):189-203. Knuth, D. T., Forest Prod. J., (1964), 12(9):437442. Knuth, D. T., Unpublished Ph.D. d i s s e r t a t i o n , Univ. of Wisconsin, Madison. Univ. M i c r o f i l m , Ann Arbor, Michigan, (1964). S u o l a h t i , O., Wallen, Α., Holz als Roh-und Werks t o f f , (1958), 16:8-20. Unligil, Η. Η., J. I n s t . Wood Sci., (1971), 5(6):30-35. Unligil, Η. Η., Forest Prod. J., (1972), 22(9): 92-100. Unligil, Η. Η., Krzyzewski, J., Newfoundland bi monthly Research Notes, (1972), 28 (2/3):11-12. Ward, O. P., Fogarty, W. Μ., J. of the Inst. of Wood Sci., (1973), 6(2):8-12. Banks, W. Β., D e a r l i n g , Τ. Β., Mater. Organismen, (1973), 8(1):39-49. A d o l f , F. P., Holzforschung, (1975), 29(5):181-186. Imamura, Y., Harada, Η., Saiki, Η., Wood Sci. and Tech., (1974), 8(4):243-254. J u t t e , S. Μ., Levy, J. F., Acta Bot. Neerl., (1971), 20(5):453-466. Meyer, R. W., Wood Sci., (1974), 6(3):220-230.


Wood Technology: Chemical Aspects

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