Wood-Polymer Materials - ACS Publications - American Chemical

6 Wood-Polymer Materials J O H N A. M E Y E R

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 6, 2017 | http://pubs.acs.org Publication Date: May 5, 1984 | doi: 10.1021/ba-1984-0207.ch006

State University of New York, College of Environmental Science and Forestry, Syracuse, NY 13210 Treatment of solid wood over the years for increased utility included many chemical systems that affected the cell wall and filled the void spaces in the wood. Some of these treatments found commercial applications, while some remain laboratory curiosities. A brief description of the earlier treatments is given for heat-stabilized wood, phenol-formaldehyde-treated veneers, bulking of the cell wall with polyethylene glycol, ozone gas-phase treatment, ammonia liquidand gas-phase treatment, and β- and γ-radiation. Many of these treatments led to commercial products, such as Staybwood, Staypak, Im preg, and Compreg. This chapter is concerned primarily with wood—polymer composites using vinyl monomers. Generally, wood-polymers imply bulk polymerization of a vinyl-type monomer in the void spaces of solid wood. This bulk polymerization takes place in the vessels, cap illaries, ray cells, etc., but not in the cell wall or middle lamellas. The monomer is introduced into the solid wood by a vacuum process. The wood-monomer is then con verted into the solid polymer by γ-radiation or a heat -sensitive polymer Background

and

catalyst dissolved in the monomer. The is fabricated into the final product.

wood

History

W o o d , as a r e n e w a b l e r e s o u r c e , h a s p r o v i d e d p e r s o n s w i t h t o o l s , weapons, a n d shelter since the b e g i n n i n g of our coexistence on this planet. D u r i n g the m i l l e n n i u m of our d e v e l o p m e n t w e learned h o w to m a k e w o o d h a r d e r a n d s t r o n g e r b y d r y i n g a n d h e a t - t e m p e r i n g o u r w o o d e n tools a n d w e a p o n s . A s o u r k n o w l e d g e of t h e w o r l d w e l i v e d i n i n c r e a s e d , w e a t t e m p t e d o t h e r m o d i f i c a t i o n s t o b e t t e r fit o u r i n c r e a s e d r e q u i r e m e n t s . O v e r the years tars, p i t c h e s , creosote, resins, a n d salts h a v e b e e n u s e d t o c o a t w o o d o r t o fill i t s p o r o u s s t r u c t u r e . T r e a t m e n t o f w o o d to m o d i f y its p r o p e r t i e s h a s t w o o b j e c t i v e s : d i m e n s i o n a l stabilization d u e to m o i s t u r e c o n t e n t a n d i m p r o v e m e n t in physical and mechanical characteristics. D u r i n g the period from 1930 to 1960 r e s e a r c h was c a r r i e d out a n d m a n y attempts w e r e m a d e 0065-2393/84/0207-0257/$09.25/0 © 1984 American Chemical Society Rowell; The Chemistry of Solid Wood Advances in Chemistry; American Chemical Society: Washington, DC, 1984.

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to a l t e r w o o d p r o p e r t i e s b y t h e a p p l i c a t i o n of heat, p r e s s u r e for s u r face d e n s i f i c a t i o n , c h e m i c a l a d d i t i o n to b u l k t h e c e l l w a l l s , i m p r e g nation w i t h p o l y m e r s , alteration of the c h e m i c a l composition of w o o d c o m p o n e n t s u s i n g l i q u i d a n d gaseous reagents, irradiation of w o o d w i t h 7- a n d β - r a d i a t i o n , as w e l l as o t h e r t e c h n i q u e s . T w o e x c e l l e n t r e v i e w s o f t h e s e p r o c e s s e s h a v e b e e n p u b l i s h e d ( I , 2). E a c h o f t h e m a j o r s y s t e m s is o u t l i n e d i n t h e f o l l o w i n g p a r a g r a p h s . A s i m i l a r o u t l i n e h a s b e e n p u b l i s h e d e l s e w h e r e (3, 4). Heat-Stabilized Wood (Staybwood). I n this process k i l n - d r i e d l u m b e r is h e l d at 1 5 0 - 3 0 0 ° C f o r v a r i o u s l e n g t h s o f t i m e r a n g i n g f r o m m i n u t e s to h o u r s . T h i s p r o c e s s gives u p to 6 0 % a n t i s h r i n k efficiency ( A S E ) . T h i s t y p e o f d i m e n s i o n a l s t a b i l i z a t i o n is a c c o m p a n i e d b y a s e r i o u s loss i n s t r e n g t h , 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 (5, 6). H e a t i n g g r e e n w o o d i n w a t e r also i m p a r t s s o m e d i m e n s i o n a l s t a b i l i z a t i o n t o w o o d (7). T h e r e is n o c o m m e r c i a l a p p l i c a t i o n for S t a y b wood. H e a t - S t a b i l i z e d C o m p r e s s e d Wood (Staypak). Pressures of 4 0 0 - 4 0 0 0 p s i a r e a p p l i e d t o t h e w o o d a f t e r i t has b e e n h e a t e d . B o t h h e a t a n d p r e s s u r e p l a s t i c i z e w o o d . A t 1 6 0 °C a n d 1 2 % m o i s t u r e c o n t e n t , t h e m a x i m u m p l a s t i c y i e l d p e r i n c r e m e n t o f p r e s s u r e o c c u r s at 1 1 0 0 p s i . P r e s s u r e s o f 1 5 0 0 - 2 5 0 0 p s i a r e r e q u i r e d to y i e l d a s p e c i f i c g r a v i t y o f 1.3. H i g h l y d e n s i f i e d w o o d m u s t b e c o o l e d i n t h e p r e s s . S o m e s t r e n g t h p r o p e r t i e s , s u c h as i m p a c t s t r e n g t h a n d h a r d n e s s , a r e i n c r e a s e d i n d i r e c t p r o p o r t i o n to t h e density. S t a y p a k finds l i m i t e d a p p l i c a t i o n f o r s i l v e r w a r e h a n d l e s a n d d e s k l e g s (6). P h e n o l - F o r m a l d e h y d e Wood Composite (Impreg). In this p r o c e s s , k i l n - d r i e d v e n e e r is i m p r e g n a t e d w i t h a w a t e r s o l u t i o n o f p h e n o l - formaldehyde p r e p o l y m e r (trimers a n d tetramers) w i t h a m o l e c u l a r size s m a l l e n o u g h that it can be c a r r i e d into the c e l l w a l l b y t h e w a t e r . T h i s s y s t e m has t h e a d v a n t a g e c a u s i n g t h e c e l l w a l l t o s w e l l u p t o 2 5 % b e y o n d t h e s w e l l i n g i n w a t e r , so t h a t a f t e r c u r i n g t h e c o m p o s i t e has a f i n a l v o l u m e a l m o s t e q u a l to that o f w a t e r - s w o l l e n w o o d (6, 8). A f t e r i m p r e g n a t i o n u s i n g v a c u u m a n d p r e s s u r e , t h e w a t e r is r e m o v e d b y d r y i n g a n d t h e p r e p o l y m e r is p o l y m e r i z e d a n d c r o s s - l i n k e d to a h i g h m o l e c u l a r w e i g h t b y a p p l y i n g heat i n a press or d r y kiln. G r e e n veneers can be treated directly. A t a loading of 3 5 % b y w e i g h t o f t h e d r y w o o d t h e r e is n o v i s i b l e d e p o s i t i o n o f t h e p o l y m e r i n t h e v o i d s p a c e s o f t h e w o o d , a n d t h e c o m p o s i t e has a n A S E o f 7 0 - 7 5 % . T h e h i g h A S E is a n i n d i c a t i o n t h a t m o s t o f t h e p o l y m e r is i n t h e c e l l w a l l . I m p r e g is i n c o m m e r c i a l u s e , p r i m a r i l y for d i e m o l d s a n d p a t t e r n s i n t h e a u t o m o b i l e i n d u s t r y . T h e d e s i r e d t h i c k n e s s is b u i l t f r o m l a y e r s o f i m p r e g n a t e d v e n e e r s h e e t s ; t h e a s s e m b l y is t h e n h e a t c u r e d . P h e n o l - F o r m a l d e h y d e C o m p r e s s e d Wood C o m p o s i t e (Compreg). T h i s c o m p o s i t e is s i m i l a r t o I m p r e g i n t h a t t h e v e n e e r , g r e e n

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


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o r d r i e d , is s o a k e d i n a w a t e r o r a l c o h o l s o l u t i o n o f l o w m o l e c u l a r weight p h e n o l - f o r m a l d e h y d e prepolymers (trimers a n d tetramers), a n d t h e p r e p o l y m e r is c a r r i e d i n t o t h e c e l l w a l l b y t h e w a t e r . T h e w a t e r is r e m o v e d at a t e m p e r a t u r e l o w e n o u g h t o p r e v e n t c u r i n g o f t h e p r e p o l y m e r . A s t a c k o f t h e t r e a t e d v e n e e r s h e e t s is p l a c e d i n a p r e s s w i t h h e a t e d p l a t e n s a n d , as t h e c o m p o s i t e m a t e r i a l is h e a t e d , p r e s s u r e u p t o 1 0 0 0 p s i is a p p l i e d t o c o m p r e s s t h e w o o d a n d c o l l a p s e t h e c e l l s t r u c t u r e (6, 8 , 9). T h e d e n s i t y o f t h e final, c u r e d ( p o l y m e r i z e d a n d cross-linked) composite approaches that of the c e l l w a l l (solid c e l l w a l l s u b s t a n c e ) w i t h a s p e c i f i c g r a v i t y f r o m 1.3 t o 1.4. I n c o r p o ration of the p o l y m e r into the cell wall prevents springback i n the presence of h i g h relative h u m i d i t i e s and imparts h i g h d i m e n s i o n a l stability. A r e t e n t i o n i n the w o o d of about 3 0 % p o l y m e r gives o p t i m u m s t a b i l i t y a n d a h i g h A S E ( 9 0 - 9 5 % ) . C o m p r e g w i l l a b s o r b less t h a n 1% m o i s t u r e w h e n i m m e r s e d i n w a t e r for 24 h . S t r e n g t h (par ticularly i n compression), hardness, a n d abrasion resistance are a l l i n c r e a s e d , a n d t h e c o m p o s i t e is q u i t e r e s i s t a n t t o d e c a y a n d t e r m i t e s . I m p a c t s t r e n g t h , h o w e v e r , is a d v e r s e l y a f f e c t e d b y t h e p r o c e s s , a n d t h e c o m p o s i t e is m u c h m o r e b r i t t l e . M a n y specialty items, knife and cutlery handles i n particular, are m a d e f r o m C o m p r e g . T h i s composite can be m a c h i n e d w i t h great precision a n d the surface finish can be r e n e w e d b y sanding a n d b u f f i n g b e c a u s e t h e p o l y m e r is t h r o u g h o u t t h e w o o d . C o m p r e g is also u s e d for e l e c t r i c a l i n s u l a t o r s r e q u i r i n g h i g h t e n s i l e s t r e n g t h . Polyethylene Glycol Treatment. T h e polymers of d i h y d r i c a l cohols are polyethers w i t h an oxygen atom separating the h y d r o carbon groups and reactive hydroxyl groups on the ends only; mo l e c u l a r w e i g h t s u p t o 6 0 0 0 a r e h i g h l y s o l u b l e i n w a t e r (6,10). Because o f t h e l o w v a p o r p r e s s u r e o f p o l y e t h y l e n e g l y c o l ( P E G ) 1, i t r e m a i n s i n t h e c e l l w a l l s w h e n t h e w o o d is d r i e d ; t h i s b u l k i n g a c t i o n p r e v e n t s the w o o d from shrinking. A s water evaporates and increases the c o n centration of P E G i n the solution, the rate of diffusion into the c e l l w a l l i n c r e a s e s (6). T h i s is e v i d e n t b y t h e s w e l l i n g o f t h e t r e a t e d w o o d as i t d r i e s . G r e e n c r o s s - s e c t i o n a l d i s k s o f s o u t h e r n p i n e s a p w o o d , 3.18 c m t h i c k t r e a t e d w i t h P E G h a v e b u l k e d s u f f i c i e n t l y to p r e v e n t c h e c k i n g d u r i n g a i r - d r y i n g . T r e a t m e n t consists o f a n o v e r n i g h t , o r l o n g e r , soak i n a 3 0 % s o l u t i o n o f P E G - 1 0 0 0 o r t w o surface coats o f m o l t e n P E G - 1 0 0 0 a d a y a p a r t . F o r t h i c k e r d i s k s , soak t i m e s h o u l d i n c r e a s e i n p r o p o r t i o n to the square of the thickness. H e a r t w o o d requires m o r e soaking time or m o r e coats t h a n s a p w o o d (11). H O - C ^ C H s - O - C H s C H ^ i O - C ^ C H ^ - O - C H ^ ^ - O H 1 G r e e n l o b l o l l y p i n e 1 2 . 7 c m l o n g , 7.6 c m w i d e , a n d 0 . 9 5 c m t h i c k c a n b e d r i e d i n 1 0 - 4 0 m i n b y i m m e r s i o n i n m o l t e n P E G - 1 0 0 0 at 1 3 5


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° C . ( D r y i n g t i m e i n m o l t e n P E G is s h o r t e r t h a n i n a i r . ) S u f f i c i e n t P E G diffuses i n t o t h e s a m p l e s d u r i n g t h e i m m e r s i o n to g i v e a b o u t a 3 5 % A S E (12). T h e P E G - b u l k e d w o o d f e e l s m o i s t w h e n r e l a t i v e h u m i d i t y is a b o v e 7 0 % b e c a u s e o f its h y g r o s c o p i c i t y , b u t c e r t a i n p o l y u r e t h a n e finishes t e n d t o r e d u c e t h i s c l a m m y f e e l i n g . T h e t r e a t e d w o o d is h i g h l y s t a b l e t o w a r d c h a n g e s i n h u m i d i t y , b u t i n w a t e r , t h e P E G is l e a c h e d o u t o f t h e w o o d w i t h t i m e . T r e a t m e n t c a u s e s a s l i g h t loss o f a b r a s i o n r e s i s t a n c e a n d b e n d i n g s t r e n g t h . T o u g h n e s s is g e n e r a l l y u n a f f e c t e d at a 4 5 % P E G l o a d i n g w i t h t h e A S E i n t h e r e g i o n o f 8 0 % . P E G is u s e d w h e r e w o o d m u s t h a v e d i m e n s i o n a l s t a b i l i t y to p r e v e n t c r a c k i n g a n d c h e c k i n g . V a l u a b l e art carvings have b e e n p r e s e r v e d i n t h i s m a n n e r , a n d P E G t r e a t m e n t has p e r m i t t e d m a r i n e a r c h e o l o g i s t s to p r e s e r v e w a t e r - l o g g e d w o o d e n s h i p s b r o u g h t u p f r o m l a k e s a n d oceans. Ozone Gas-Phase Treatment. T h i s process holds little p r o m i s e for s o l i d w o o d t r e a t m e n t b e c a u s e b o t h t h e c e l l u l o s e a n d l i g n i n are d e g r a d e d . T h e effect o f o z o n e o n s m a l l w o o d s a m p l e s , g r o u n d w o o d , a n d c h i p s is d e s c r i b e d e l s e w h e r e ( 1 3 , 14). Ammonium Liquid- and Vapor-Phase Treatment. Application o f t h i s a m m o n i u m t r e a t i n g p r o c e s s s h o w s p r o m i s e as a n a l t e r n a t i v e to s t e a m b e n d i n g . S o m e o f t h e d i s a d v a n t a g e s o f s t e a m b e n d i n g i n c l u d e r e c o v e r y o f t h e o r i g i n a l shape i f e x p o s e d to h i g h r e l a t i v e h u m i d i t y , t h e n e c e s s i t y o f h o l d i n g t h e p a r t i n its final s h a p e u n t i l t h e w o o d is s e t b y r e d u c i n g t h e m o i s t u r e c o n t e n t t o a m b i e n t c o n d i tions, a n d h i g h breakage d u r i n g the b e n d i n g process. T h e use of a n hydrous a m m o n i a , i n the l i q u i d or vapor phase, permits the b e n d i n g of m a n y w o o d s i n m u c h m o r e c o m p l i c a t e d shapes than steam b e n d i n g (15-17). C o m p l i c a t e d shapes can be f o r m e d w i t h o u t springback because t h e c r y s t a l l i n e s t r u c t u r e o f t h e c e l l u l o s e is c h a n g e d a n d t h e h y d r o g e n bonds reformed u p o n the evaporation of the a m m o n i a . W h e n a m m o n i a - t r e a t e d b e n t w o o d is e x p o s e d t o l i q u i d w a t e r it w i l l c h a n g e s h a p e to s o m e e x t e n t , b u t u p o n d r y i n g t h e t r e a t e d w o o d w i l l r e t u r n to i t s n e w s h a p e . X - R a y c r y s t a l l o g r a p h y s h o w s t h a t t h e o r i g i n a l c e l l u l o s e I p a t t e r n changes to c e l l u l o s e II u p o n t r e a t m e n t of c e l l u l o s e w i t h a m m o n i a . T h e e v a c u a t e d w o o d is e x p o s e d t o a m m o n i a v a p o r at a p r e s s u r e o f 1 5 0 p s i at r o o m t e m p e r a t u r e . T h e t i m e o f e x p o s u r e is d e t e r m i n e d by the thickness of the w o o d and the moisture content; t h i s t i m e c a n r a n g e f r o m m i n u t e s to h o u r s . B e n d i n g o f 180° h a s b e e n a c c o m p l i s h e d w i t h 3.8 c m t h i c k w o o d . S o f t w o o d s r e q u i r e a l o n g e r e x p o s u r e to a m m o n i a v a p o r t h a n d o h a r d w o o d s for the same d e g r e e of b e n d i n g . A f t e r the a m m o n i a t r e a t e d w o o d is b e n t i n t o its f i n a l s h a p e o n l y m i l d r e s t r a i n t is r e q u i r e d


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f o r a s h o r t p e r i o d o f t i m e u n t i l t h e t r e a t e d w o o d is s e t — t h a t i s , u n t i l s o m e o f t h e a m m o n i a i n t h e w o o d has e v a p o r a t e d . I n m o s t c a s e s t h e c o l o r o f t h e a m m o n i a - t r e a t e d w o o d is d a r k e r a n d a p p r o a c h e s a w a l n u t c o l o r . S o m e c o l o r s t r e a k i n g is a d r a w b a c k with certain wood

species.

I n a d d i t i o n , the a m m o n i a v a p o r reacts

with some of the w o o d components

to p r o d u c e a l i q u i d that d r a i n s

from the w o o d u p o n release of the a m m o n i a pressure. T h i s process


is n o t i n c o m m e r c i a l u s e at t h e p r e s e n t t i m e .

Vinyl Composites Radiation Catalysts.

B e c a u s e w o o d is e s s e n t i a l l y a m i x t u r e o f

h i g h m o l e c u l a r w e i g h t p o l y m e r s , h i g h energy radiation exposure [in m i l l i o n electron volts (MeV)] w i l l d e p o l y m e r i z e the polymers creating free radicals along the C - C backbone.

by

I f t w o free radicals

are c r e a t e d o n separate chains i n close p r o x i m i t y , c r o s s - l i n k i n g w i l l take place. O t h e r types of reactions w i l l take place w h e n the

free

r a d i c a l is c r e a t e d n e a r a r e a c t i v e o r f u n c t i o n a l g r o u p . W h e n t h e free r a d i c a l is o n a t e r t i a r y c a r b o n ,

disproportionation will occur

with

c h a i n s c i s s i o n . S u r p r i s i n g l y l i g n i n is m o r e r e s i s t a n t to 7 - r a d i a t i o n t h a n c e l l u l o s e , b u t l i g n i n is also m o r e s u s c e p t i b l e to n a t u r a l U V rays w h e n e x p o s e d to the a t m o s p h e r e . ( U V rays have about 20 e V p e r q u a n t a c o m p a r e d to a b o u t 1.25 M e V f o r 7 - r a y q u a n t a s f r o m t h e

cobalt-60

source. Some slight increase i n mechanical properties a n d a decrease i n h y g r o s c o p i c i t y w e r e n o t e d w i t h r a d i a t i o n e x p o s u r e u p to 1,000,000 r d (rads) (18).

E x p o s u r e s above this l e v e l d e g r a d e d the cellulose a n d

i m p a i r e d the m e c h a n i c a l p r o p e r t i e s ; the w o o d was c o m p l e t e l y soluble a b o v e 3 x 1 0 r d (20, 21). T h e u l t r a s t r u c t u r e o f 7 - i r r a d i a t e d D o u g l a s 8

fir a n d y e l l o w p o p l a r w a s s t u d i e d b y d e L h o n e u x (22).

T h e results

w e r e s i m i l a r for b o t h w o o d s o v e r the range of cobalt-60 7 - i r r a d i a t i o n (1.25 M e V ) o f 3 2 - 1 5 0 0 M r d . T h e s c a n n i n g e l e c t r o n m i c r o s c o p e

re

vealed that the m i d d l e lamella a n d the p r i m a r y w a l l w e r e the most resistant to t h e 7 - r a d i a t i o n ; t h e s e c o n d a r y w a l l d e p o l y m e r i z e d r a p i d l y w i t h l o w exposure. A t 1500 M r d the w o o d was c o m p l e t e l y soluble. D u r i n g t h e e a r l y 1960s a n e w class o f c h e m i c a l s c o n t a i n i n g o n e o r m o r e d o u b l e b o n d s was u s e d to treat w o o d — v i n y l - t y p e m o n o m e r s that c o u l d be p o l y m e r i z e d i n t o the s o l i d p o l y m e r b y means of free r a d i c a l s (23).

T h i s v i n y l m o n o m e r p o l y m e r i z a t i o n was an i m p r o v e

m e n t o v e r the c o n d e n s a t i o n p o l y m e r i z a t i o n reaction because the free radical catalyst was n e i t h e r acidic n o r basic, n o r does the reaction l e a v e b e h i n d a r e a c t i o n p r o d u c t , s u c h as w a t e r , t h a t m u s t b e r e m o v e d f r o m t h e final c o m p o s i t e . T h e a c i d a n d b a s e c a t a l y s t s u s e d w i t h t h e other treatments degrade the cellulose c h a i n a n d cause a brittleness in the composite.

V i n y l p o l y m e r s have a large range of properties

f r o m soft r u b b e r to h a r d b r i t t l e s o l i d s d e p e n d i n g u p o n t h e


groups

262

T H E

a t t a c h e d to t h e C - C b a c k b o n e .

C H E M I S T R Y

O F

A few examples of v i n y l

S O L I D

W O O D

monomers

are s h o w n here: CH

= C H

2

2

C H = C H

C H = C H

2

2

I CH


ethylene

I

C H = C H

C H = C - C H

C N

0 = C - C H

2

I

propylene

vinyl

acrylonitrile

= C H

2

3

methyl

chloride

CH

3

I

Cl

3

2

methacrylate

CH, = C H

CH, = C H

Û styrene

CH

(CH ) 3

3

2

/ \

vinyltoluene

CH3 CH3 fert-butylstyrene

M o s t v i n y l m o n o m e r s a r e n o n p o l a r ; c o n s e q u e n t l y , t h e r e is l i t t l e i f a n y i n t e r a c t i o n w i t h t h e h y d r o x y l groups a t t a c h e d to the cellulose molecule. In general, v i n y l polymers simply bulk the w o o d structure by

filling

the capillaries, vessels, a n d other voids i n the w o o d struc

ture. T h e U . S . A t o m i c E n e r g y C o m m i s s i o n , i n t h e e a r l y 1960s, s o r e d r e s e a r c h that u s e d 7 - r a d i a t i o n to d e v e l o p w o o d - p o l y m e r s .

spon The

7-radiation generated free radicals i n the w o o d - m o n o m e r , w h i c h i n turn initiated the polymerization of the v i n y l m o n o m e r support was e x p a n d e d

t o o t h e r o r g a n i z a t i o n s , s u c h as

Georgia who supplied wood-polymer

This

samples for i n d u s t r i a l e v a l u a

t i o n , N o r t h C a r o l i n a S t a t e U n i v e r s i t y (25, the w o o d - p o l y m e r

(24).

Lockheed-

26) f o r t h e e v a l u a t i o n o f

physical properties, and Arthur D . Little C o m

p a n y for e c o n o m i c e v a l u a t i o n o f the w o o d - p o l y m e r s .

T h e first p a p e r

o n the catalyst-heat process for m a k i n g w o o d - p o l y m e r s

was

pre

s e n t e d at t h e 1 9 6 5 a n n u a l m e e t i n g o f t h e F o r e s t P r o d u c t s R e s e a r c h S o c i e t y i n N e w Y o r k C i t y (27).

D u r i n g the past 20 years the i n d u s t r i a l

d e v e l o p m e n t has b e e n s l o w b u t steady for b o t h 7 - r a d i a t i o n a n d cat alyst-heat processes. I n g e n e r a l , the free radicals u s e d for the p o l y m e r i z a t i o n reaction c o m e f r o m t w o sources: t e m p e r a t u r e - s e n s i t i v e catalysts a n d cobalt-


6.

M E Y E R

263


6 0 7 - r a d i a t i o n . I n e a c h p r o c e s s a f r e e r a d i c a l is g e n e r a t e d f r o m

one

of these sources. After free-radical generation the v i n y l p o l y m e r i z a t i o n m e c h a n i s m is t h e s a m e . E a c h p r o c e s s f o r g e n e r a t i n g f r e e r a d i c a l s has its o w n p e c u l i a r i t i e s . W h e n 7 - r a d i a t i o n is u s e d as a s o u r c e o f f r e e r a d i c a l s m a n y c o m p l i c a t i o n s a r i s e i m m e d i a t e l y , t h e l e a s t o f w h i c h is t h e c h e m i s t r y

of

the process. T h e use of radiation mandates compliance w i t h govern


m e n t regulations a n d arouses the c o n c e r n of environmentalists Safety r e q u i r e m e n t s m u s t be satisfied before a cobalt-60

(28).

source

can be installed a n d licensed. Radiation-trained personnel must

be

o n t h e staff b e f o r e a l i c e n s e c a n b e i s s u e d . A t l e a s t 5 0 0 , 0 0 0 - 1 , 0 0 0 , 0 0 0 C i (curies) of cobalt-60 are r e q u i r e d i n a p r o d u c t i o n source making wood-polymers,

for

a n d at $ 1 . 0 0 o r m o r e p e r c u r i e , a c o n s i d

erable investment must be made before production can begin.

Be

sides cost considerations, the cobalt-60 r a d i a t i o n process does have some distinct advantages i n m a k i n g w o o d - p o l y m e r

composites.

Be

c a u s e t h e m o n o m e r is n o t c a t a l y z e d i t c a n b e s t o r e d at a m b i e n t c o n d i t i o n s as l o n g as t h e p r o p e r a m o u n t o f i n h i b i t o r is m a i n t a i n e d . T h e r a t e o f f r e e r a d i c a l g e n e r a t i o n is c o n s t a n t f o r a g i v e n a m o u n t o f c o balt-60 and does not increase w i t h temperature. W h e n 7 - r a d i a t i o n p a s s e s t h r o u g h a m a t e r i a l s u c h as w o o d o r a v i n y l m o n o m e r i t l e a v e s b e h i n d a s e r i e s o f i o n s a n d e x c i t e d states as t h e e n e r g y o f t h e 7 - r a y is a b s o r b e d t h r o u g h p h o t o e l e c t r i c ,

Compton,

a n d p a i r p r o d u c t i o n c o l l i s i o n s (see F i g u r e 1). ( C o b a l t - 6 0 p r o d u c e s 7 - r a y s o f 1.17

a n d 1.33

two

M e V . A p p r o x i m a t e l y 3 0 e V is r e q u i r e d to

r u p t u r e a c o v a l e n t b o n d a n d to cause ionization.) T h e ions a n d e x c i t e d states g e n e r a t e d i n t h e a b s o r b i n g m a t e r i a l i m m e d i a t e l y r e a r r a n g e t o f o r m free radicals, w h i c h i n t u r n initiate the p o l y m e r i z a t i o n process. (Monomer)* -» R + E x c i t e d State

H'

F r e e Radicals

T h e free radicals u s u a l l y consist of H* a n d the radical m o n o m e r .

Once

t h e f r e e r a d i c a l is g e n e r a t e d , t h e p o l y m e r i z a t i o n r e a c t i o n is t h e s a m e as t h a t o f a n o r m a l f r e e - r a d i c a l - c a t a l y z e d , v i n y l m o n o m e r b u l k p o l y merization

rf

(27).

· " r f r f

e" M*

M* e" f f j f r f

* \ ^ \ Μ * f f r f β"

Figure 1. Ionized and excited molecules along the path of a η-ray.


264

T H E

C H E M I S T R Y

O F

S O L I D

W O O D

T h e initiation step can be represented i n general by R + M ( m o n o m e r ) —» R - M a n d the propagation step b y R-(M) -M' + M -» n

R-(M)

n + 1

-M'

T h e t e r m i n a t i o n step r e c o m b i n e s the g r o w i n g radicals R-(M) -M- + R-(M) -M'-> R - ( M ) - M - M - ( M ) - R Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 6, 2017 | http://pubs.acs.org Publication Date: May 5, 1984 | doi: 10.1021/ba-1984-0207.ch006

n

n

n

n

B e c a u s e t h e w o o d ' s c e l l w a l l s t r u c t u r e is n o t s w o l l e n b y t h e v i n y l m o n o m e r , t h e r e is l i t t l e o p p o r t u n i t y f o r t h e m o n o m e r t o r e a c h t h e free-radical sites, g e n e r a t e d b y t h e 7 - r a d i a t i o n o n t h e c e l l u l o s e , to f o r m a v i n y l p o l y m e r b r a n c h . F r o m t h i s s h o r t d i s c u s s i o n , i t is r e a sonable to c o n j e c t u r e that t h e r e s h o u l d b e little i f any difference i n the physical properties of catalyst-heat initiated or 7-initiated i n situ polymerization of vinyl monomers in wood. C h e m i c a l Catalysts. Vazo or 2,2'-azobisisobutyronitrile cata l y s t (2) is p r e f e r r e d o v e r p e r o x i d e c a t a l y s t s b e c a u s e o f its l o w d e c o m p o s i t i o n t e m p e r a t u r e a n d its n o n o x i d i z i n g n a t u r e . Vazo w i l l not bleach dyes dissolved i n the m o n o m e r d u r i n g polymerization.

C H

3

CH3

CH3

I

I

CH3

I

- C - N = N - C - C H

I

3

2CH3-C

I

+

N

I

2

CN C N C N T h i s first-order r e a c t i o n is i n d e p e n d e n t o f t h e c o n c e n t r a t i o n o f v a z o a n d i n d e p e n d e n t o f t h e t y p e o f m o n o m e r (29). T h e r a p i d d e c o m p o s i t i o n of vazo catalyst w i t h an increase i n t e m p e r a t u r e (Table I) c a n b e u s e d to a d v a n t a g e i n t h e b u l k v i n y l p o l y m e r i z a t i o n s i n w o o d . A m o d e r a t e t e m p e r a t u r e of 60 °C c a n be Table I. H a l f - L i f e of Vazo Catalyst vs. T e m p e r a t u r e Temperature 0 7 18 30 46 70 100

(°C)

T,„

(min)

4 x 10 1 x 10 1 x 10 1 x 10 1 x 10 270 5.5

( R e p r o d u c e d from Ref. 30. C o p y r i g h t 1977, ical Society. )

7

7

6

5

4

American C h e m -



6.

M E Y E R

265


u s e d t o i n i t i a t e t h e r e a c t i o n , a n d , b e c a u s e t h e h a l f - l i f e is m o r e t h a n 4 0 , 0 0 0 , 0 0 0 m i n o r a b o u t 2 0 y e a r s at 0 ° C , t h e c a t a l y z e d m o n o m e r can b e s t o r e d safely for m o n t h s . C a t a l y z e d m o n o m e r s have b e e n s t o r e d f o r o v e r a y e a r at 5 ° C . T h e n i t r o g e n r e l e a s e d d u r i n g t h e v a z o c a t a l y s t d e c o m p o s i t i o n is n o r m a l l y s o l u b l e i n t h e m o n o m e r - p o l y m e r s o l u t i o n . A t h i g h t e m p e r a t u r e s t h e n i t r o g e n f o r m s gas b u b b l e s i n t h e highly viscous m o n o m e r - p o l y m e r solution and the final w o o d p o l y m e r w i l l c o n t a i n v o i d s p a c e s . I n t h e w o o d - p o l y m e r t h i s is o f little consequence because the m e t h y l methacrylate monomer ( M M A ) s h r i n k s a b o u t 2 5 % d u r i n g t h e p o l y m e r i z a t i o n to create a d d i t i o n a l v o i d s p a c e s i n t h e s o l i d p o l y m e r . T h e c o s t o f v a z o c a t a l y s t is in the range of $ 1 - $ 1 0 a p o u n d d e p e n d i n g u p o n the amount ordered. T h e o r e t i c a l l y , 1 g w i l l p r o d u c e 7.4 x 1 0 f r e e r a d i c a l s a n d at $ 1 0 / l b ; t h i s is 3 . 3 x 1 0 f r e e r a d i c a l s p e r d o l l a r , o r a b o u t $ 0 . 0 2 / g . 2 1

2 3

Impregnation Process. I n b o t h p r o c e s s e s t h e first s t e p i n t h e i m p r e g n a t i o n o f w o o d is c a r r i e d o u t b y e v a c u a t i n g t h e a i r f r o m t h e w o o d v e s s e l s a n d c e l l l u m e n s (27). A n y t y p e o f m e c h a n i c a l v a c u u m p u m p is a d e q u a t e i f i t c a n r e d u c e t h e p r e s s u r e i n t h e a p p a r a t u s t o 750 m m of m e r c u r y o r less. S o m e i n d u s t r i a l p r o d u c e r s o n l y r e d u c e t h e p r e s s u r e to a b o u t 711 m m o f m e r c u r y . E x p e r i e n c e has s h o w n t h a t t h e a i r i n t h e c e l l u l a r s t r u c t u r e o f m o s t w o o d s is r e m o v e d as fast as t h e p r e s s u r e i n t h e e v a c u a t i o n v e s s e l is r e d u c e d . P u m p i n g f o r 3 0 m i n at 1 m m p r e s s u r e is s u f f i c i e n t t o r e m o v e t h e a i r . T h e v a c u u m p u m p is i s o l a t e d f r o m t h e s y s t e m at t h i s p o i n t . T h e catalyzed m o n o m e r containing cross-linkers, and on occa s i o n d y e s , is i n t r o d u c e d i n t o t h e e v a c u a t e d c h a m b e r t h r o u g h a r e s e r v o i r at a t m o s p h e r i c p r e s s u r e . (See F i g u r e 2, w h i c h i l l u s t r a t e s t h e c o m p o n e n t s necessary for m a k i n g w o o d — p o l y m e r s o n b o t h a l a b o r a t o r y a n d i n d u s t r i a l s c a l e . ) T h e w o o d m u s t b e w e i g h t e d so t h a t i t does not float i n the m o n o m e r solution. I n the radiation process, the c a t a l y s t is o m i t t e d f r o m t h e m o n o m e r . A s u r g e t a n k 10 t i m e s t h e v o l u m e o f t h e t r e a t i n g v e s s e l is i n c l u d e d i n t h e s y s t e m t o a l l o w t h e air d i s s o l v e d i n t h e m o n o m e r to e x p a n d w i t h o u t g r e a t l y c h a n g i n g t h e pressure i n the impregnation vessel. Alternatively, the system can b e p u m p e d as t h e m o n o m e r is a d m i t t e d i n t o t h e e v a c u a t e d v e s s e l . W i t h t h i s p r o c e d u r e m u c h m o n o m e r is l o s t d u e t o t h e h i g h v a p o r p r e s s u r e o f M M A (40 m m at r o o m t e m p e r a t u r e ) . A f t e r t h e w o o d is c o v e r e d w i t h m o n o m e r s o l u t i o n , a t m o s p h e r i c p r e s s u r e is r e g a i n e d , o r , i n t h e c a s e o f r a d i a t i o n p r o c e s s , d r y n i t r o g e n is a d m i t t e d i n t o t h e e v a c u a t e d v e s s e l . I m m e d i a t e l y t h e m o n o m e r s o l u t i o n b e g i n s to f l o w i n t o t h e e v a c u a t e d w o o d s t r u c t u r e t o fill t h e v o i d s p a c e s . C a r e m u s t b e t a k e n t o m a i n t a i n e n o u g h m o n o m e r s o l u t i o n a b o v e t h e w o o d so t h a t a i r is n o t r e a d m i t t e d to t h e c e l l s t r u c t u r e . T h e soaking p e r i o d like the evacuation period depends u p o n the



266

T H E

C H E M I S T R Y

O F

S O L I D

W O O D

MANOMETER

Figure 2. Apparatus used to impregnate wood. structure of the w o o d : m a p l e , b i r c h , a n d other open-celled woods fill in about 30 m i n , and other woods require longer periods of time. A b l o c k of 7.6 x 7.6 x 17.8 c m m a p l e a b s o r b e d 3 0 0 m L of m o n o m e r i n l e s s t h a n 10 m i n . A f t e r t h e m o n o m e r i m p r e g n a t i o n is c o m p l e t e t h e w o o d m o n o m e r is r e m o v e d a n d p l a c e d i n a n e x p l o s i o n - p r o o f o v e n , o r t h e c o b a l t - 6 0 s o u r c e for c u r i n g . O n a l a b o r a t o r y scale o r s m a l l p r o d u c t i o n u n i t t h e w o o d - m o n o m e r is w r a p p e d i n a l u m i n u m f o i l b e f o r e p l a c i n g i n t h e c u r i n g o v e n at 6 0 ° C . I n l a r g e r p r o d u c t i o n o p e r a t i o n s t h e w o o d - m o n o m e r is p l a c e d d i r e c t l y i n t o t h e c u r i n g o v e n , u s u a l l y i n the basket that h e l d the w o o d d u r i n g i m p r e g n a t i o n . I n the radiationc u r e d p r o c e d u r e the t h i n m e t a l can, i n w h i c h the w o o d was i m p r e g n a t e d , is f l u s h e d w i t h n i t r o g e n a n d is l o w e r e d i n t o a w a t e r p o o l n e x t to t h e c o b a l t - 6 0 s o u r c e . W i t h h i g h v a p o r p r e s s u r e m o n o m e r s , t h e w o o d s u r f a c e is d e p l e t e d t o s o m e e x t e n t b y e v a p o r a t i o n , b u t t h i s d e p l e t e d a r e a is u s u a l l y r e m o v e d b y m a c h i n i n g to e x p o s e t h e w o o d p o l y m e r s u r f a c e . A s m e n t i o n e d p r e v i o u s l y , M M A has a v a p o r p r e s s u r e o f 4 0 m m at r o o m t e m p e r a t u r e ; tert-butylstyrene has a v a p o r p r e s s u r e o f o n l y 0 . 8 m m at r o o m t e m p e r a t u r e . Monomers.

M a n y d i f f e r e n t v i n y l m o n o m e r s (32) h a v e


been

6.

Wood-Polymer

M E Y E R

267

Materiab

u s e d to m a k e w o o d - p o l y m e r s

d u r i n g t h e p a s t 10 y e a r s , b u t M M A

appears to b e t h e p r e f e r r e d m o n o m e r for b o t h the c a t a l y s t - h e a t a n d radiation processes.

I n fact, M M A is t h e o n l y m o n o m e r t h a t c a n b e

economically p o l y m e r i z e d w i t h 7-radiation. A l l types of l i q u i d v i n y l monomers

can b e p o l y m e r i z e d w i t h vazo or p e r o x i d e catalysts. I n

m a n y countries styrene a n d s t y r e n e - M M A mixtures are used w i t h vazo or p e r o x i d e catalysts.


All vinyl monomers

c o n t a i n i n h i b i t o r s to p r e v e n t

premature

p o l y m e r i z a t i o n d u r i n g transportation a n d storage. If these i n h i b i t o r s , s u c h as 1 , 4 - b e n z e n e d i o l ,

m o n o m e t h y l ether of 1,4-benzenediol,

tert-

b u t y l - 1 , 2 - b e n z e n e d i o l , a n d 2,4-dimethyl-6-ter£-butylphenol, are not r e m o v e d before use the catalyst or radiation m u s t generate free

enough

radicals to use u p the i n h i b i t o r before p o l y m e r i z a t i o n w i l l b e g i n .

This i n d u c t i o n p e r i o d depends u p o n the amount and type of inhibitor present.

I n t h e case o f r a d i a t i o n , t h e i n h i b i t o r m u s t b e k e p t to a

m i n i m u m f o r e f f i c i e n t u s e o f t h e 7 - r a y s. T h e p r o d u c t i o n o f c o m m e r c i a l p o l y m e t h y l m e t h a c r y l a t e r o d o r s h e e t s t o c k , s o l d as L u c i t e o r Plexiglas, requires 0 . 0 1 % vazo catalyst w i t h the i n h i b i t o r r e m o v e d . W i t h 50 p p m of 2,4-dimethyl-6-feri-butylphenol in M M A i n wood, 0 . 2 5 % v a z o c a t a l y s t is r e q u i r e d t o o b t a i n c o m p l e t e p o l y m e r i z a t i o n . W o o d c o n t a i n s n a t u r a l i n h i b i t o r s , w h i c h is t h e r e a s o n f o r t h e h i g h vazo content.

A g a i n , the amount of natural inhibition w i l l

u p o n the species of w o o d . from

depend

M o n o m e r s extract the soluble fractions

the w o o d structure, a n d , w i t h repeated use, the extractive c o n

t e n t b u i l d s u p i n t h e m o n o m e r . T h e r e f o r e , e x c e s s i v e f o a m i n g is p r o d u c e d u n d e r v a c u u m a n d t h e p o l y m e r i z a t i o n r e a c t i o n is c o m p l e t e l y i n h i b i t e d , a n d a d d i t i o n a l catalyst m u s t be

added.

T h e p o l y m e r i z a t i o n o f v i n y l m o n o m e r s is a n e x o t h e r m i c r e a c t i o n i n w h i c h a c o n s i d e r a b l e a m o u n t o f h e a t is r e l e a s e d , a b o u t 7 5 . 3 k j / mol.

I n both the c a t a l y s t - h e a t a n d 7-radiation processes the

heat

r e l e a s e d d u r i n g p o l y m e r i z a t i o n is t h e s a m e f o r a g i v e n a m o u n t

of

m o n o m e r . T h e r a t e at w h i c h t h e h e a t is r e l e a s e d is c o n t r o l l e d b y t h e r a t e at w h i c h t h e f r e e - r a d i c a l i n i t i a t i n g s p e c i e s

is s u p p l i e d a n d at

w h i c h the chains are g r o w i n g . T h e vazo a n d p e r o x i d e catalysts are temperature dependent;

consequently,

the rate of

decomposition,

a n d thus the s u p p l y of free radicals, increases r a p i d l y w i t h an increase i n t e m p e r a t u r e . B e c a u s e w o o d is a n i n s u l a t o r d u e t o i t s c e l l u l a r s t r u c ture, heat f l o w into a n d out of the w o o d - m o n o m e r - p o l y m e r rial

is r e s t r i c t e d . I n t h e c a t a l y s t - h e a t p r o c e s s ,

mate

heat m u s t be i n t r o

d u c e d i n t o t h e wood— m o n o m e r to start the p o l y m e r i z a t i o n , b u t o n c e t h e e x o t h e r m i c r e a c t i o n b e g i n s t h e h e a t f l o w is r e v e r s e d . T h e t e m perature of the w o o d - m o n o m e r - p o l y m e r

composite

increases rap

i d l y , b e c a u s e t h e h e a t f l o w o u t o f t h e w o o d is m u c h s l o w e r t h a n t h e heat generation. F i g u r e 3 illustrates the heat-transfer process


(31).


268

T H E

C H E M I S T R Y

O F

S O L I D

W O O D

Figure 3. Idealized temperature-time exothermic curve. T h e t i m e t o t is t h e t i m e f o r t h e w o o d - m o n o m e r m a s s t o r e a c h 0

o v e n o r c u r i n g t e m p e r a t u r e at 7 V

D u r i n g the period of

constant

t e m p e r a t u r e (the i n d u c t i o n p e r i o d ) , t h e i n h i b i t o r is b e i n g

removed

b y the reaction w i t h the free radicals. W h e n the i n h i b i t o r s are e l i m i n a t e d f r o m the m o n o m e r a n d w o o d , the t e m p e r a t u r e rises to a m a x i m u m that c o r r e s p o n d s to t h e peak o f t h e e x o t h e r m i c p o l y m e r i z a t i o n r e a c t i o n . P o l y m e r i z a t i o n c o n t i n u e s t o c o m p l e t i o n a l t h o u g h at a d e c r e a s e d rate, a n d the t e m p e r a t u r e r e t u r n s to that of the c u r i n g chamber.

T h e t i m e to t h e p e a k t e m p e r a t u r e d e p e n d s u p o n

a m o u n t of catalyst present, the type of m o n o m e r ,

the

the type of cross-

l i n k e r , a n d t h e r a t i o o f t h e mass o f m o n o m e r to that of t h e w o o d . T h e w o o d m a s s acts as a h e a t s i n k . The G e l Effect.

T h e g e l o r T r o m m s d o r f f e f f e c t (33)

is t h e

s t r i k i n g a u t o a c c e l e r a t i o n o f t h e v i n y l p o l y m e r i z a t i o n r e a c t i o n as t h e viscosity of the m o n o m e r — p o l y m e r

solution increases. C h a i n t e r m i

n a t i o n i n v o l v i n g t h e r e c o m b i n a t i o n o f t w o free radicals b e c o m e s dif fusion c o n t r o l l e d ; this results i n a decrease i n the rate of t e r m i n a t i o n . T h e c o n c e n t r a t i o n of active free radicals therefore increases

propor

t i o n a l l y . To s u m u p t h e g e l e f f e c t : t h e r a t e o f v a z o c a t a l y s t i n i t i a t i o n increases w i t h t e m p e r a t u r e , the rate of propagation or p o l y m e r i z a t i o n increases w i t h the viscosity, a n d the rate of t e r m i n a t i o n of

the

g r o w i n g p o l y m e r c h a i n s d e c r e a s e s w i t h t h e v i s c o s i t y . T h e g e l effect also results i n an increase i n t h e m o l e c u l a r w e i g h t of l i n e a r p o l y m e r s , b u t t h i s h a s n o p r a c t i c a l s i g n i f i c a n c e w h e n c r o s s - l i n k i n g is p a r t o f t h e reaction. As m e n t i o n e d previously, a given 7-radiation-source

geometry

w i l l s u p p l y f r e e r a d i c a l s at a c o n s t a n t r a t e f o r v i n y l m o n o m e r


poly-

6.

M E Y E R

269


m e r i z a t i o n . A n i n c r e a s e i n t e m p e r a t u r e w o u l d o n l y affect t h e p r o p agation a n d t e r m i n a t i o n rates. T h e e x o t h e r m i c heat from the v i n y l monomer composite,

p o l y m e r i z a t i o n is s t i l l r e l e a s e d i n t h e

rate of initiation. C o m p l e t e radiation c u r i n g of composites

wood—monomer

b u t t h e t e m p e r a t u r e is m u c h l o w e r b e c a u s e o f t h e s l o w wood-monomer

u s u a l l y r e q u i r e s 8 to 10 h d e p e n d i n g u p o n t h e

radia

t i o n s o u r c e g e o m e t r y ; t h e v a z o i n i t i a t e d r e a c t i o n is o v e r i n 3 0 t o 4 0


minutes. T h u s , all the catalytic heat of p o l y m e r i z a t i o n of a given m o n o m e r m a s s is r e l e a s e d i n o n e - s i x t e e n t h o f t h e t i m e i t is i n t h e radiation process.

Because the wood - m o n o m e r

material in a thin

m e t a l c a n is i m m e r s e d i n t o a w a t e r p o o l f o r i r r a d i a t i o n , t h e c o o l i n g b y t h e w a t e r r a d i a t i o n s h i e l d a l s o assists i n l o w e r i n g t h e t e m p e r a t u r e . A d d i t i o n a l h e a t is a d d e d t o t h e w o o d - m o n o m e r - p o l y m e r b y the absorption of the 7-rays b y the w o o d ,

composite

a l t h o u g h t h i s h e a t is

s m a l l c o m p a r e d to the e x o t h e r m i c heat f r o m the p o l y m e r i z a t i o n . W h e n t h e h e a t o f p o l y m e r i z a t i o n is r e l e a s e d q u i c k l y i n a w o o d m o n o m e r composite the h i g h temperature increases the vapor pres sure of the moisture i n the cell walls a n d distills the moisture out of the w o o d . T h e c h a n g e i n v o l u m e of the c e l l w a l l causes changes i n dimensions w h i c h are manifested b y shrinkage a n d distortion of the original w o o d shape. W o o d - p o l y m e r composites c u r e d by the cata l y s t - h e a t process

m u s t b e m a c h i n e d to the

final

s h a p e after t r e a t

m e n t . C o n v e r s e l y , b e c a u s e t h e h e a t o f p o l y m e r i z a t i o n b y 7 - r a y s is released over a longer period of time, the temperature of the

wood-

p o l y m e r r e m a i n s l o w a n d n o t as m u c h c e l l w a l l m o i s t u r e is d r i v e n off. T h e r e f o r e , t h e a m o u n t o f d i s t o r t i o n a n d d i m e n s i o n a l c h a n g e is s o m e w h a t l e s s (30,

31).

Vazo Catalyst Effect on Polymerization.

F i g u r e 4 illustrates

the t e m p e r a t u r e - t i m e c u r v e s for the c u r i n g of basswood

samples

i m p r e g n a t e d w i t h M M A containing various concentrations of vazo catalyst. T h e data for each p o l y m e r i z a t i o n have b e e n c o m p i l e d , are p r e s e n t e d i n T a b l e I I , a n d a r e p l o t t e d i n F i g u r e 5 as t h e p e r c e n t v a z o catalyst p e r w e i g h t o f m o n o m e r vs. the t i m e to the e x o t h e r m i c p e a k (t ). p

T h e d a t a s h o w that v a r y i n g the a m o u n t o f catalyst has a d e f i n i t e

effect o n t h e t i m e t o t h e e x o t h e r m i c p e a k , a n d o n t h e p e a k t e m p e r a t u r e (T ). p

exothermic

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

v e r s i o n at h i g h c a t a l y s t c o n c e n t r a t i o n s ( 3 1 ,

con

34).

Increasing the c o n c e n t r a t i o n of catalyst brings about a r e d u c t i o n i n t h e t i m e to t h e e x o t h e r m i c p e a k . T h e e x o t h e r m i c p e a k t e m p e r a t u r e i n c r e a s e s as t h e p e r c e n t a g e o f c a t a l y s t is i n c r e a s e d . T h i s i n c r e a s e i n t e m p e r a t u r e is d u e t o t h e a u t o a c c e l e r a t i o n effect t h a t o c c u r s w h e n the viscosity of the m o n o m e r - p o l y m e r solution increases very rap idly w i t h polymer formation. T h e percentage

o f c o n v e r s i o n is a p

p r o x i m a t e l y c o n s t a n t , e x c e p t f o r a d r o p at t h e 1 . 2 % a n d 1 . 5 %


vazo

270

T H E CHEMISTRY O F SOLID WOOD

J3l C e

0r%VAZ0 0.2%VAZD


0.3%\AZO

CURING TIME (mire.) Figure 4. Temperature- time curves showing the effects of various con centrations of vazo catalyst on the polymerization exotherm of basswoodMMA composite. (Reproduced with permission from Ref. 31. Copyright 1972 Springer- Verlag?) y

Table II. Effect of Vazo Catalyst on Polymerization of B a s s w o o d - M M A Composite BasswoodSample

M (%)

MMA

(g) 48.5 46.7 49.3 49.4 50.5 46.1 49.1 47.3

(e) 56.7

48.4

116.9

59.2 56.4 57.5 56.6 58.2 56.8 58.1

50.8 49.5 49.8

126.7 114.4 116.4

48.6 48.6 44.7 45.3

112.1 126.2 115.7 122.8

Vazo (%)

Ρ

(%) 99.8

85.4

108.7 100.4 100.8 96.2

85.8 87.8 86.6 85.9 83.5

105.4 91.0 95.8

78.7 78.0

0.1 0.2 0.3 0.5 0.8 1.0 1.2 1.5

t (min) 0

102.1 90.1 76.1 71.3 70.0 59.4 53.3 65.5

t (min)

Τ (°é)

208.4 154.1 130.8 101.9 98.8 83.4 76.6 84.1

131.0 139.5 145.0 156.0 152.5 163.0 157.0 163.0

Key: W , weight of oven-dry wood; W , weight of monomer; W , weight of polymer; M, % of monomer; P, % of polymer; and C, % conversion of monomer to polymer. (Reproduced with permission from Ref. 31. Copyright 1972, Springer - Verlag. ) S

m

p


6.


M E Y E R

h

200h

271

~85.4%

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150

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