Chemical Methods of Improving the Permeability of Wood

pit membrane is paramount in our attempt to improve the permeability of wood. Pit Structure. Because the pit membrane appears to be the controlling fa...
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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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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.

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

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.

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

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.

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