The Chemistry of Solid Wood - American Chemical Society

7 Bioactive Wood-Polymer

Composites

R. V. S U B R A M A N I A N Department of Materials Science and Engineering, Polymeric Materials Section, Washington State University, Pullman, WA 99164

Downloaded by CORNELL UNIV on June 4, 2017 | http://pubs.acs.org Publication Date: May 5, 1984 | doi: 10.1021/ba-1984-0207.ch007

The preparation and properties of bioactive wood- p o l y m e r composites are discussed. The basic, effective approach to bring about simultaneous improvements in decay resistance, dimensional stability, and mechanical behavior of wood is in situ polymerization and c o p o l y merization of organotin monomers carrying the bioactive tributyltin group. Tri-n-butyltin methacrylatemaleic anhydride and tri-n-butyltin methacrylate—glycidyl methacrylate are examples of suitable monomer combinations for in situ copolymerization. Comonomers that carry anhydride or epoxy functional groups graft to wood through esterification or etherification of wood hydroxyls. Electron microprobe analysis for tin atoms shows that a detectable portion of tin copolymer is located in cell walls. The treated wood is effective in providing resistance against white rot and brown rot fungi, and against marine organisms as determined by laboratory and ocean tests. Notable improvements in flexural and impact strengths, and significant reduction in moisture absorption are also observed.

A

H E POLYMERIZATION O F VINYL M O N O M E R S i n t h e v o i d s p a c e s o f

bulk

w o o d results i n w o o d - p o l y m e r composites of i n c r e a s e d strength p r o p e r t i e s a n d d i m e n s i o n a l s t a b i l i t y (see C h a p t e r 6). B e c a u s e t h e d i f f e r e n t e n v i r o n m e n t a l c o n d i t i o n s e x p o s e i n - s e r v i c e t i m b e r to a t t a c k b y n u m e r o u s w o o d - d e t e r i o r a t i n g m i c r o o r g a n i s m s , i t is d e s i r a b l e t o enhance the biodégradation resistance of w o o d , w i t h simultaneous i m p r o v e m e n t s i n mechanical behavior. This chapter summarizes the f o r m a t i o n o f b i o a c t i v e w o o d - p o l y m e r c o m p o s i t e s (1-4). T h e b a s i c a p p r o a c h is s t i l l i n s i t u 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 i n w o o d , w i t h the appropriate choice of a bioactive, toxic, functional group incorporated in the monomer, and with other modifications based on w o o d - p o l y m e r reactions. 0065-2393/84/0207-0291/$06.00/0 © 1984 American Chemical Society Rowell; The Chemistry of Solid Wood Advances in Chemistry; American Chemical Society: Washington, DC, 1984.

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Biological Activity T h e b i o d é g r a d a t i o n o f w o o d , w h e t h e r it is a b o v e g r o u n d , b y s o i l c o n t a c t , o r i n m a r i n e a p p l i c a t i o n s , is b r o u g h t a b o u t b y f u n g i , b a c t e r i a , insects, a n d m a r i n e borers (5). B o t h toxic a n d nontoxic preservative treatments have been adopted in protecting wood. The chemical m o d i f i c a t i o n o f w o o d h a s b e e n i n v e s t i g a t e d as a n o n t o x i c p r e s e r v a t i v e t r e a t m e n t ( 6 , 7 ) . T h e b a s i s f o r s u c h n o n t o x i c t r e a t m e n t s is t h e as s u m p t i o n that the e n z y m e s e x u d e d b y the microorganisms must c o m e d i r e c t l y i n contact w i t h the w o o d a n d that the substrate m u s t have a specific c o n f i g u r a t i o n i n o r d e r for the h i g h l y selective e n z y m e - i n i t i a t e d w o o d - d e g r a d i n g r e a c t i o n to take p l a c e . T h e r e f o r e , i f t h e w o o d y s u b s t r a t e is c h e m i c a l l y m o d i f i e d , e v e n b y t h e u s e o f n o n t o x i c c h e m icals, these reactions cannot take place, a n d c h e m i c a l l y m o d i f i e d w o o d s h o u l d b e c o m e u n r e c o g n i z a b l e as a f o o d s o u r c e to s u p p o r t m i crobial growth. Toxic preservatives function b y d i s r u p t i n g the cellular organi z a t i o n o f m i c r o o r g a n i s m s so t h a t t h e o r g a n i s m d i e s . T h u s , n u m e r o u s o r g a n i c salts o f c o p p e r , z i n c , a r s e n i c , a n d b o r o n h a v e b e e n u s e d as w o o d p r e s e r v a t i v e s , g e n e r a l l y w i t h a d d e d c h r o m i u m c o m p o u n d s , to reduce rapid leaching of the water-soluble compounds. A m o n g or ganic c o m p o u n d s , coal-tar creosote a n d p e n t a c h l o r o p h e n o l are i m portant toxic preservatives i n w i d e c o m m e r c i a l use. T h e i m p r e g n a tion of w o o d w i t h toxic c o m p o u n d s that are l e a c h e d out d u r i n g actual use of w o o d represents the most w i d e l y used, and frequently, the o n l y p r a c t i c a l m e t h o d o f p r e s e r v i n g w o o d . D e t a i l s of t h e v a r i o u s as pects of w o o d a n d biodégradation protection are discussed i n C h a p t e r s 8 a n d 12.

Bio toxicity of Organotin Compounds and Polymers T h e t r i a l k y l t i n g r o u p was c h o s e n for i n c o r p o r a t i o n i n m o n o m e r s u s e d for i n situ p o l y m e r i z a t i o n i n w o o d b e c a u s e t r i a l k y l t i n c o m p o u n d s h a v e e m e r g e d as b r o a d s p e c t r u m t o x i c a n t s h a v i n g h i g h t o x i c i t y t o w a r d m a r i n e f o u l i n g o r g a n i s m s (8), as w e l l as w o o d - d e s t r o y i n g o r g a n i s m s (9—11). I n a d d i t i o n , t h e y p o s s e s s a t o l e r a b l e t o x i c i t y t o w a r d m a m m a l s . T h e y a r e a l s o a p p r o x i m a t e l y 10 t i m e s m o r e t o x i c a g a i n s t w o o d - d e s t r o y i n g f u n g i t h a n p e n t a c h l o r o p h e n o l (9). T r i a l k y l t i n c o m p o u n d s e v e n t u a l l y d e g r a d e to h a r m l e s s i n o r g a n i c oxides of tin b y the action of U V light, microbes, etc., and thus present a m i n i m a l e n v i r o n m e n t a l h a z a r d (12). A f u r t h e r a d v a n t a g e to u s i n g v i n y l m o n o m e r s w i t h t r i a l k y l t i n f u n c t i o n a l g r o u p s f o r i n s i t u p o l y m e r i z a t i o n is t h e p o s s i b i l i t y o f a controlled release of the toxic trialkyltin group from the treated wood. In the conventional t e c h n iq u e of w o o d treatment w i t h organic pes t i c i d e s , t h e p r e s e r v a t i v e is d i s p e r s e d i n w o o d , a n d t h e l o a d i n g l e v e l

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


7.

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Bioactive Wood-Polymer Composites

293

o f t h e b i o c i d e is m a x i m i z e d b y v a r i o u s t i m b e r p r e c o n d i t i o n i n g t e c h n i q u e s . T h i s p r e c o n d i t i o n i n g at m a x i m u m l o a d i n g l e v e l s r e s u l t s i n initial o v e r k i l l t h r o u g h the h i g h l e a c h i n g rate of the d i s p e r s e d toxi c a n t a n d c o n s e q u e n t r e d u c t i o n i n t h e d u r a t i o n o f p r o t e c t i o n af forded. O n the contrary the c h e m i c a l a n c h o r i n g of the toxic t r i a l k y l t i n g r o u p to t h e p o l y m e r i m p r e g n a t e d i n w o o d r e q u i r e s the dissociation of t h e c h e m i c a l b o n d l i n k i n g the toxicant to the p o l y m e r , and the subsequent diffusion of the toxic group t h r o u g h the p o l y m e r m a t r i x . H e n c e , a m e t h o d is a v a i l a b l e f o r i n c r e a s i n g t h e d u r a t i o n o f protection significantly t h r o u g h u n i f o r m a n d o p t i m u m release of tox icant throughout the lifetime of the preserved w o o d . F u r t h e r , the e n v i r o n m e n t a l hazards associated w i t h r a p i d release of toxicants to t h e e n v i r o n m e n t is a l s o m i n i m i z e d . T h e b i o c i d a l effect o f t h e t r i a l k y l t i n g r o u p s d e p e n d s o n the n a ture of the alkyl group, a n d the lower homologues (methyl, p r o p y l , e t c . ) p o s s e s s i n g t h e h i g h e s t a c t i v i t y (8). H o w e v e r , f o r a safe b a l a n c e b e t w e e n tolerable toxicity to m a m m a l s a n d h i g h activity against m i croorganisms, ( n - C H ) S n - , the tri-n-butyltin functional group ( T B T ) , is t h e p r e f e r r e d c h o i c e . T h e s y n t h e s i s a n d p r o p e r t i e s o f p o l y mers w i t h T B T functional groups have been investigated extensively a n d t h e i r b i o c i d a l effect e s t a b l i s h e d a n d r e l a t e d t o t h e i r c h e m i c a l s t r u c t u r e i n b o t h l a b o r a t o r y a n d field tests (13-16). The polymers a r e f o r m e d b y p o l y m e r i z a t i o n o f o r g a n o t i n m o n o m e r s s u c h as t r i - n b u t y l t i n methacrylate ( T B T M A ) or b y esterification of polymers car r y i n g c a r b o x y l a t e f u n c t i o n a l g r o u p s , s u c h as s t y r e n e - m a l e i c a n h y dride copolymers w i t h tri-n-butyltin oxide ( T B T O ) . T h e r e l e a s e o f t h e T B T t o x i c a n t f r o m t h e s e p o l y m e r s is c o n trolled b y the p o l y m e r matrix properties. F o r example, copolymers of T B T M A a n d g l y c i d y l m e t h a c r y l a t e ( G M A ) are effective against Pseudomonas nigrifaciens ( m a r i n e b a c t e r i u m ) , Sarcina lutea ( s o i l f u n g u s ) ; a n d Giomeralla cingulata (soil fungus); the l e a c h i n g rate d e t e r m i n e d b y i n h i b i t i o n z o n e s a g a i n s t t h e s e o r g a n i s m s is m o d i f i e d b y t h e n a t u r e a n d e x t e n t o f c r o s s - l i n k i n g o f t h e p o l y m e r s (17, 18). T h u s i t is p o s s i b l e t o v a r y t h e b i o t o x i c i t y o f t h e s e o r g a n o t i n p o l y m e r s b y modifications of chemical structure. 4

9

3

Organotin Polymers in Wood T h e i m p r e g n a t i o n of w o o d by polymers w i t h T B T functional g r o u p s offers a v i a b l e r o u t e t o t h e p r e p a r a t i o n o f b i o a c t i v e w o o d p o l y m e r composites w i t h a n u m b e r of advantages. P o l y m e r i m p r e g n a t i o n m a y b e e x p e c t e d to i m p r o v e the s t r e n g t h p r o p e r t i e s a n d d i m e n s i o n a l stability of w o o d i n water. T h e controlled release of the toxic T B T m o i e t y , c h e m i c a l l y l i n k e d to the p o l y m e r l o c a t e d w i t h i n the w o o d , can increase the service life of w o o d w h i l e e n s u r i n g m i n -



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T H E CHEMISTRY OF SOLID WOOD

imal adverse impact on the surrounding environment. B y decreasing the amount of water absorbed by the wood, polymer impregnation m i n i m i z e s the s w e l l i n g a n d s h r i n k i n g of w o o d i n cyclic moisture ex p o s u r e , a n d t h e r e b y s l o w s t h e loss o f t o x i c a n t f r o m w o o d . T h u s , i m p r o v e m e n t i n biodégradation resistance and mechanical behavior may be achieved. A l t h o u g h i t is p o s s i b l e to i n c o r p o r a t e o r g a n o t i n p o l y m e r s i n w o o d b y v a c u u m or pressure i m p r e g n a t i o n w i t h solutions of p r e f o r m e d p o l y m e r , i t w o u l d b e p r e f e r a b l e to u t i l i z e m o n o m e r i m p r e g nation followed b y i n situ p o l y m e r i z a t i o n because the smaller molec u l a r s i z e , as w e l l as t h e l o w v i s c o s i t y o f m o n o m e r s , is c o n d u c i v e t o efficient p e n e t r a t i o n of w o o d . Therefore, v i n y l m o n o m e r s i n w h i c h t h e T B T g r o u p is c h e m i c a l l y b o n d e d , s u c h as T B T M A , a r e u s e d f o r i n s i t u p o l y m e r i z a t i o n i n w o o d (2). I n the selection of m o n o m e r s for i n situ p o l y m e r i z a t i o n , the possibility of chemical reaction w i t h hydroxyl groups i n w o o d must b e c o n s i d e r e d . T h i s r e a c t i o n p o s s i b i l i t y is a c h i e v e d b y s e l e c t i n g c o m o n o m e r s s u c h as m a l e i c a n h y d r i d e ( M A n h ) o r G M A f o r c o p o l y m e r ization w i t h the m o n o m e r containing T B T In reaction with T B T M A M A n h c a n r e a c t w i t h w o o d h y d r o x y l s b y e s t e r i f i c a t i o n as s h o w n i n R e a c t i o n 1, as w e l l as c o p o l y m e r i z i n g t h r o u g h t h e d o u b l e b o n d s . S i m i l a r l y , G M A c a n r e a c t b y e t h e r i f i c a t i o n o f h y d r o x y l s , t h r o u g h its epoxide group. A s a result of these reactions, the copolymer does not j u s t fill i n t h e v o i d s o f t h e w o o d m a t r i x , b u t i t is c h e m i c a l l y b o n d e d , i . e . , grafted to the w o o d s t r u c t u r e . T h e r e f o r e , i n situ c o p o l y m e r ization in these systems involves copolymerization through the CH Wood-OH

+

— v ^ C H - C H - C H z - C ^ C.=

TBTMA-MAnh

3

0-Sn(Bu)

3

Copolymer

j, W o o d - 0 - oC - C H o- =Cc H - C H 11

X

2

- C

c=o X

O H

0-Sn(Bu)

3

T B T M A - M A n h C o p o l y m e r grafted to w o o d Reaction 1 double bonds of the comonomers, a n d grafting through the additional f u n c t i o n a l g r o u p s p r e s e n t (the a n h y d r i d e o r e p o x i d e ) . H o m o p o l y m e r i z a t i o n o f e p o x i d e g r o u p s m a y also occur. T h u s , the m o n o m e r



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p a i r s T B T M A - M A n h o r T B T M A - G M A a r e i n t e r e s t i n g s y s t e m s to s t u d y (1, 2). C o n v e r s i o n o f w o o d h y d r o x y l s b y a c e t y l a t i o n or e t h e r i fication can i m p r o v e d i m e n s i o n a l stability a n d biodégradation resis t a n c e i n w o o d (7, 9, 20). C o p o l y m e r i z a t i o n of T B T M A - M A n h a n d T B T M A - G M A was c o n d u c t e d i n situ b y free r a d i c a l initiators b e n z o y l p e r o x i d e or azob i s i s o b u t y r o n i t r i l e a f t e r v a c u u m i m p r e g n a t i o n o f g r a n d f i r (Abies grandis) w o o d s p e c i m e n s (0.64 x 2 . 5 4 x 1 1 . 4 c m ) c o n t a i n i n g w o o d f i b e r s e i t h e r p a r a l l e l t o l e n g t h o r p a r a l l e l to w i d t h . W o r k i n g w i t h 1 0 4 0 % (by w e i g h t ) o f m o n o m e r s i n s o l u t i o n i n b e n z e n e o r a c e t o n e , i t w a s f o u n d t h a t 1 5 - 6 0 % (by w e i g h t ) l o a d i n g o f c o p o l y m e r i n w o o d can be obtained. T h e amount of p o l y m e r incorporated into treated w o o d was m u c h h i g h e r w h e n acetone, a moderate w o o d s w e l l i n g s o l v e n t , w a s u s e d t h a n f r o m b e n z e n e s o l u t i o n s . ( B e n z e n e is a n o n s w e l l i n g solvent for w o o d . ) T h e viscosity of b e n z e n e solutions of the m o n o m e r s is s i g n i f i c a n t l y h i g h e r t h a n t h e v i s c o s i t y o f a c e t o n e s o l u tions. T h e results of solvent extraction of p o l y m e r - i m p r e g n a t e d w o o d s a m p l e s p o i n t t o a h i g h d e g r e e o f g r a f t i n g o f t h e p o l y m e r to w o o d (Table I) as r e v e a l e d b y t h e u n e x t r a c t a b l e f r a c t i o n o f t h e i m p r e g n a t e d polymer. C o m p a r i s o n of these data w i t h those obtained w h e n only the i n d i v i d u a l m o n o m e r s w e r e h o m o p o l y m e r i z e d showed that M A n h , or G M A , still gives a h i g h degree of grafting but T B T M A d i d not. T h e r e f o r e , u n d e r the p o l y m e r i z a t i o n conditions used, the a c y l ation a n d etherification of w o o d hydroxyls by M A n h or G M A , re s p e c t i v e l y , r e a d i l y c a n o c c u r i n a d d i t i o n to h o m o p o l y m e r i z a t i o n r e a c tions. Polymer Distribution. T h e m a c r o d i s t r i b u t i o n of p o l y m e r i n treated w o o d specimens can be examined by scanning electron m i croscopy ( S E M ) of transverse a n d l o n g i t u d i n a l sections of the speci-

T a b l e I. F r a c t i o n s of V a r i o u s P o l y m e r s a n d C o p o l y m e r s to W o o d

Polymer TBTMA-MAnh

TBTMA-GMA

Polymer in Wood (wt %) 16.2 37.9 57.6 17.6 32.5 51.5

N O T E : T h e solvent u s e d for extraction was

Grafted

Grafting

(%) 87.3 93.5 91.5 86.0 88.3 83.4

benzene.


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m e n s (2). S m a l l s e c t i o n s f r o m e d g e a n d e n d s u r f a c e s c o u l d t h u s b e c o m p a r e d w i t h sections f r o m t h e b o d y center of the specimens. I n t h e c a s e o f G M A p o l y m e r i z e d f r o m a c e t o n e s o l u t i o n s , w o o d c e l l s fill to t h e same e x t e n t i n a l l parts o f t h e s p e c i m e n ; therefore, t h e p o l y m e r is d i s t r i b u t e d u n i f o r m l y t h r o u g h o u t t h e t r e a t e d s p e c i m e n . H o w e v e r , w h e n T B T M A - G M A or T B T M A - M A n h are copolymerized from benzene solutions, p o l y m e r distribution i n treated wood was not u n i -

10 μ Figure 1. Concentration of tin atoms in the cell walls of TBTMA-GMAtreated wood. Key: top, S EM using secondary emission electrons of treated wood; and bottom, concentration of tin determined by electron microprobe moving along line cc' (top). (Reproduced

with permission

from

Ref. 2. Copyright

1981,

Springer-Verlag.)


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suBRAMANiAN


f o r m . Samples taken from the b o d y center of the treated w o o d s h o w e d the highest fraction of e m p t y w o o d cells, a n d those taken from t h e s h o r t e d g e o f t r e a t e d w o o d s h o w e d t h e h i g h e s t f r a c t i o n o f f i l l e d c e l l s . T h u s , t h e p o l y m e r d i s t r i b u t i o n is u n i f o r m at t h e s u r f a c e but declines toward the center of the sample. B o t h the nonswelling nature of benzene a n d the higher viscosity of m o n o m e r solutions i n b e n z e n e c o n t r i b u t e to t h e o b s e r v e d n o n u n i f o r m i t y i n p o l y m e r d i s tribution. P o l y m e r migration into the cell walls of treated w o o d can be e x p e c t e d to h a v e i m p o r t a n t c o n s e q u e n c e s o n i m p r o v e m e n t i n m e c h a n i c a l p r o p e r t i e s , d i m e n s i o n a l s t a b i l i t y , a n d r e s i s t a n c e to w o o d decay. B e c a u s e the c o p o l y m e r i n the T B T M A systems contains t i n , the m i c r o d i s t r i b u t i o n of the c o p o l y m e r i n treated w o o d can be ex a m i n e d b y d e t e r m i n i n g the location of t i n atoms w i t h the a i d of e l e c t r o n m i c r o p r o b e analysis. T h e results for T B T M A - G M A co p o l y m e r a r e s h o w n i n F i g u r e 1. A s u b s t a n t i a l a m o u n t o f t i n a t o m s a n d , p r e s u m a b l y , the T B T M A - G M A c o p o l y m e r are d i s t r i b u t e d w i t h i n t h e c e l l w a l l s o f t r e a t e d w o o d . A l t h o u g h b e n z e n e is a n o n s w e l l i n g s o l v e n t , t h e c o p o l y m e r is f o r m e d i n w o o d c e l l w a l l s i n d e tectable amounts. O r g a n o t i n p o l y m e r s are incorporated u n i f o r m l y into w o o d b y i n situ p o l y m e r i z a t i o n of organotin m o n o m e r s . T h e use of c o m o n o m e r s containing functional groups capable of reacting w i t h w o o d hydroxyls l e a d s t o t h e g r a f t i n g o f t h e c o p o l y m e r t o w o o d . T h e effects o f o r ganotin p o l y m e r i m p r e g n a t i o n o n the properties of w o o d may be examined now. D i m e n s i o n a l Stability. A s expected, the antishrink efficiency ( A S E ) is i n c r e a s e d , a n d t h e a m o u n t o f w a t e r a b s o r b e d is d e c r e a s e d because of p o l y m e r i m p r e g n a t i o n . Table II summarizes the results of T a b l e II.

A n t i s h r i n k E f f i c i e n c y of W o o d T r e a t e d with Various Polymers Antishrink

Soaking Time (h) 1.5 6 18 48

0

TBTMA -MAnh (wt %)

TBTMA-GMA (wt %)

25.4

59.6

16.2

57.6

17.6

51.5

67.0 62.0 61.5 61.0

73.0 66.0 62.5 62.0

68.0 60.0 57.5 52.0

82.0 76.0 71.5 67.0

66.0 60.5 56.0 52.0

77.5 74.0 72.5 71.0

N O T E : Samples h a d w o o d fibers parallel to sample length. A n t i s h r i n k efficiency = (1 — % swelling o f treated w o o d / % 100. ( R e p r o d u c e d , w i t h p e r m i s s i o n from Ref. 3. C o p y r i g h t 1981, a

X

GMA (wt %)

Efficiency

swelling of control) S p r i n g e r - Verlag. )


298

THE CHEMISTRY OF SOLID WOOD

T a b l e III.

W e i g h t P e r c e n t of W a t e r A b s o r b e d b y U n t r e a t e d W o o d and Various Treated Woods Water

™ ë Time (d)

GMA (wt%)

kin


0.50 1.50 4.50 8.50

Untreated 51 60 76 88

Absorption

(%)

TBTMA-MAnh (wt%)

TBTMA-GMA (wt%)

25.4

59.6

16.2

57.6

17.6

51.5

25 39 58 64

13 24 35 38

12 20 34 45

4 13 23 28

14 26 37 42

10 21 32 38

( R e p r o d u c e d w i t h p e r m i s s i o n from Ref. 3. C o p y r i g h t 1981,

S p r i n g e r - Verlag. )

A S E d e t e r m i n e d b y thickness (tangential) s w e l l i n g of t r e a t e d a n d u n t r e a t e d g r a n d fir w o o d s p e c i m e n s i m m e r s e d i n d i s t i l l e d w a t e r . T h e effectiveness of the c o p o l y m e r s T B T M A - M A n h a n d T B T M A - G M A is s l i g h t l y b e t t e r t h a n t h a t o f t h e G M A h o m o p o l y m e r i n i m p r o v i n g ASE. Similarly, water absorption d e t e r m i n e d b y weight gain following i m m e r s i o n i n w a t e r is d e c r e a s e d c o n s i d e r a b l y b y p o l y m e r i n c o r p o r a t i o n (Table I I I ) . F o r a l l t h r e e p o l y m e r t y p e s , w a t e r a b s o r p t i o n d e creases w i t h i n c r e a s i n g p o l y m e r c o n t e n t initially, b u t l e v e l s off to a c o n s t a n t v a l u e at h i g h e r p o l y m e r c o n t e n t s . T h u s , a l l a c c e s s i b l e sites f o r w a t e r a b s o r p t i o n a r e n o t c o m p l e t e l y s e a l e d o f f e v e n at t h e h i g h e s t levels of p o l y m e r loading. F u r t h e r m o r e , the polymers themselves w i l l have s o m e levels of w a t e r absorption. T h e effectiveness i n d e creasing water absorption decreases i n the order T B T M A - M A n h > T B T M A - G M A > G M A . T h i s is t h e o r d e r o f d e c r e a s i n g h y d r o p h o bicity of the three polymers. T h e degree of reaction between epoxy g r o u p s o f G M A a n d w o o d h y d r o x y l s is e n h a n c e d b y T B T g r o u p s i n t h e T B T M A c o m o n o m e r c o m p a r e d to r e a c t i o n of G M A alone i n w o o d (3). W i t h l a r g e r n u m b e r s o f w o o d h y d r o x y l s l e f t u n r e a c t e d , t h e G M A - w o o d system shows more water absorption. Mechanical Properties. Significant improvements i n strength p r o p e r t i e s a r e o b s e r v e d to v a r y i n g e x t e n t s i n g r a n d fir s p e c i m e n s treated w i t h the different p o l y m e r systems. In specimens w i t h fibers p a r a l l e l to t h e l e n g t h , m o d e r a t e increases i n f l e x u r a l s t r e n g t h , b e t w e e n 50 a n d 65%, a r e o b s e r v e d . T h e f l e x u r a l s t r e n g t h is i n c r e a s e d m u c h more drastically i n the transverse direction, by 317%, 6 1 % , a n d 2 4 3 % for G M A - , T B T M A - M A n h - , a n d T B T M A - G M A - t r e a t e d w o o d s p e c i m e n s , r e s p e c t i v e l y (see F i g u r e 2). S u c h s i g n i f i c a n t i m p r o v e m e n t i n t h e w e a k t r a n s v e r s e d i r e c t i o n is h i g h l y d e s i r a b l e . Similarly, the i m p r o v e m e n t s i n flexural modulus of elasticity


7.

299


suBRAMANiAN


10.0 ρ

2D L

ι

ι

I

I

0

20

40

60

80

POLYMER

IN W O O D

(wt

I 100

%)

Figure 2. Dependence of flexural strength on polymer content of wood treated with GMA (Π), TBTMA-MAnh (O), and TBTMA-GMA (A). The specimens had fibers parallel to their width. (Reproduced

with permission

from

Ref

3. Copyright

1981,

Springer-Verlag.

)

w e r e a l s o m o d e r a t e i n t h e l o n g i t u d i n a l d i r e c t i o n . F o r e x a m p l e , at 4 0 % (by w e i g h t ) p o l y m e r c o n t e n t , the f l e x u r a l m o d u l i w e r e : 1460 M P a f o r T B T M A - M A n h , c o m p a r e d to 1 2 6 5 M P a f o r T B T M A - G M A , 1110 M P a for G M A , a n d 1000 M P a for u n t r e a t e d w o o d . A s s e e n i n F i g u r e 3, G M A , T B T M A - M A n h , a n d T B T M A - G M A b r i n g a b o u t i m p r o v e m e n t s to the extent o f 5 3 5 % , 8 0 % , a n d 4 5 6 % , respectively, in the transverse flexural modulus. T h e T B T M A - M A n h copolymer is h a r d a n d b r i t t l e c o m p a r e d t o t h e o t h e r t w o c o p o l y m e r s . C o n s i d e r i n g that the T B T M A - M A n h c o p o l y m e r produces the highest i n crease i n m o d u l u s i n the l o n g i t u d i n a l d i r e c t i o n , the o b s e r v e d l o w efficiency for this p o l y m e r i n the transverse d i r e c t i o n c o u l d b e d u e to t h e f o r m a t i o n o f l o n g i t u d i n a l m i c r o c r a c k s i n w o o d t r a c h e i d s d u r i n g t h e t r e a t m e n t p r o c e s s . T h i s h y p o t h e s i s is c o n s i s t e n t w i t h t h e o b s e r v a t i o n t h a t t h e t e n s i l e s t r e n g t h o f s p e c i m e n s w i t h f i b e r s p a r a l l e l to the w i d t h a c t u a l l y decreases w i t h p o l y m e r c o n t e n t for T B T M A M A n h , a l t h o u g h i n c r e a s i n g m e a s u r a b l y for the o t h e r two systems ( F i g u r e 4). T h e i m p a c t s t r e n g t h s o f s p e c i m e n s w i t h f i b e r s p a r a l l e l to the length a n d treated w i t h T B T M A - M A n h increase u n i f o r m l y w i t h p o l y m e r c o n t e n t , w h e r e a s w e l l - d e f i n e d m a x i m a are o b s e r v e d for t h e o t h e r t w o p o l y m e r s ( F i g u r e 5). T h e c o m p l e x m i c r o s t r u c t u r e a n d s u b microscopic ultrastructure of w o o d c o m b i n e very effectively i n c o n t r i b u t i n g to t h e f r a c t u r e t o u g h n e s s o f w o o d (21). C u r r e n t l y , o n l y s o m e g e n e r a l features of m e c h a n i c a l p r o p e r t y


300

T H E C H E M I S T R Y O F S O L I D WOOI


90 r

10

fc

0

1

1

1

20

1

40

1

60

POLYMER IN WOOD

1

80

100

(wt %)

Figure 3. Dependence of flexural modulus on polymer content of wood treated with GMA (D), TBTMA-MAnh (O), and TBTMA-GMA (A). The specimens had fibers parallel to their width. (Reproduced

with permission

from

Ref 3. Copyright

1981, Springer-

Verlag. )

5.0

4.0

3.0

2.0

40

60

80

100

POLYMER IN WOOD (wt %) Figure 4. Dependence of tensile strength on polymer content of wood treated with GMA (Ώ), TBTMA-MAnh (O), and TBTMA-GMA (A). The specimens had fibers parallel to their width. (Reproduced

with permission

from

Ref 3. Copyright 1981, Springer-

Verlag. )


7.


3.0

301


suBRAMANiAN r

1,0

I

»

1

"

1

0

20

40

60

80

POLYMER

IN W O O D

(wt

1

100

%)

Figure 5. Dependence of impact strength on polymer content of wood treated with GMA (Π), TBTMA-MAnh (O), and TBTMA-GMA (A). The specimens had fibers parallel to their length. (Reproduced

with permission

from

Ref

3. Copyright

1981,

Springer-

Verlag.

)

i m p r o v e m e n t by p o l y m e r impregnation can be inferred. I n addition to filling u p v o i d s a n d m i c r o c r a c k s , a n d t h u s r e d u c i n g d e f e c t s i n t h e s t r u c t u r e , t h e i m p r e g n a t e d p o l y m e r , b e c a u s e o f its h i g h c o h e s i v e s t r e n g t h , acts as a s t r o n g b i n d e r m a t r i x i n t h e w o o d s t r u c t u r e . B e t t e r stress transfer w i t h i n the s t r u c t u r a l e l e m e n t s o f w o o d m a y also b e f a c i l i t a t e d b y p o l y m e r i m p r e g n a t i o n . A l t h o u g h i t is n o t e a s y t o i n t e r p r e t t h e d e t a i l s o f t h e o b s e r v e d effects, i t s h o u l d b e r e w a r d i n g t o study fracture a n d the details of crack initiation and propagation i n these systems. T h e s e studies s h o u l d be c o u p l e d w i t h an examination o f f a i l u r e m o d e s , as r e v e a l e d i n i n s t r u m e n t e d i m p a c t tests a n d s c a n n i n g e l e c t r o n m i c r o g r a p h s o f f r a c t u r e s u r f a c e s , i n o r d e r to l e a d t o a better u n d e r s t a n d i n g of the changes caused b y p o l y m e r impregnation of wood. Biodégradation Resistance. Organotin polymers increase the d e c a y r e s i s t a n c e o f g r a n d fir w o o d to a t t a c k o f s e v e r a l m i c r o o r g a n i s m s e x a m p l e , Coniophora puteana, a b r o w n - r o t fungus, a n d Polyporus versicolor, a white-rot fungus, completely covered untreated and versicolor, a w h i t e rot fungus, completely covered untreated a n d G M A - t r e a t e d w o o d blocks i n only 4 weeks, w h i l e the organotin p o l y m e r - t r e a t e d s p e c i m e n s w e r e , to v i s u a l e x a m i n a t i o n , c o m p l e t e l y


302

T H E CHEMISTRY O F SOLID WOOD


free of fungal attack e v e n after 8 w e e k s of exposure. T h e u n t r e a t e d c o n t r o l s a m p l e s e x p o s e d t o Coniophora puteana for 8 w e e k s suffered a w e i g h t loss o f 5 . 2 % ; t h e G M A - t r e a t e d s a m p l e s s h o w e d a 1 % w e i g h t loss. B u t t h e T B T M A - M A n h - a n d T B T M A - G M A - t r e a t e d samples s h o w e d n o w e i g h t loss at a l l . T h u s , b a s e d o n v i s u a l a p p e a r a n c e a n d w e i g h t loss, the T B T M A - M A n h a n d T B T M A - G M A t r e a t m e n t s p r o v i d e s u p e r i o r p r o t e c t i o n a g a i n s t d e c a y as c o m p a r e d t o e i t h e r u n treated or G M A - t r e a t e d w o o d . T h i s confirms the expectations based on the presence of the bioactive tri-n-butyltin group i n T B T M A M A n h and T B T M A - G M A copolymers and the slow leaching of the toxicant from the p o l y m e r - t r e a t e d wood. T h e d e c a y r e s i s t a n c e p r o v i d e d b y o r g a n o t i n p o l y m e r s has b e e n c o n f i r m e d b y f i e l d t e s t s c o n d u c t e d i n t h e o c e a n (4, 22). T h e T B T M A G M A polymer treatment and other similar polymers carrying T B T f u n c t i o n a l g r o u p s w e r e a p p l i e d t o s o u t h e r n y e l l o w p i n e (Pinus sp) s p e c i m e n s d e s i g n e d f o r e x p o s u r e at K e y W e s t N a v a l S t a t i o n , K e y W e s t , F l o r i d a . A t each semiannual inspection, i n June a n d again i n D e c e m b e r , the racks of s p e c i m e n s w e r e p u l l e d u p , the surface f o u l i n g o n t h e c o u p o n s w a s s c r a p e d off, a n d t h e e x p o s e d s u r f a c e w a s i n s p e c t e d c a r e f u l l y a n d r a t e d b y d e g r e e o f a t t a c k o n a scale f r o m 0 to 10. A r a t i n g o f 10 s i g n i f i e s n o a t t a c k ; 9, t r a c e a t t a c k ; 7, m o d e r a t e a t t a c k ; 4, h e a v y a t t a c k ; a n d 0, n o s t r u c t u r a l i n t e g r i t y left. H e a v i e s t attack takes place b e t w e e n J u n e a n d D e c e m b e r w h e n the w a t e r t e m p e r a t u r e is h i g h e r . T h i s t e s t m e t h o d c o r r e s p o n d s to A S T M D 2 3 8 1 ( " A c c e l e r a t e d E v a l u a t i o n of W o o d P r e s e r v a t i v e s for M a r i n e S e r v i c e s by M e a n s of Small-Size Specimens"). T h e f i e l d test data are g i v e n i n Table I V , a n d c o n f i r m the e v i d e n c e o b t a i n e d i n l a b o r a t o r y tests o f w o o d d e c a y w i t h g r a n d f i r s p e c i m e n s d i s c u s s e d e a r l i e r . A f t e r t h e first 2 years of e x p o s u r e , n o n e of the s p e c i m e n s w i t h o r g a n o t i n t r e a t m e n t s s h o w e d any attack. T h e total failure of u n t r e a t e d samples i n this p e r i o d demonstrates the p r e s e n c e of s a m p l e i n o c u l u m for attack. A f t e r 24 m o n t h s , t h e m a r i n e creosote-treated controls h a d a n average r a t i n g of 8.3 a n d the special m a r i n e creosote t r e a t m e n t r e s u l t e d i n a r a t i n g of 9.7. I n 36 m o n t h s , however, they have b e e n attacked a n d damaged severely, whereas t h e o r g a n o t i n p o l y m e r - i m p r e g n a t e d systems d o not suffer a n y d e t e rioration. T h e l o n g - t e r m e f f e c t i v e n e s s o f T B T M A as a b i o c i d e h a s b e e n c o n f i r m e d (23). T h e d e t e r i o r a t i o n o f s o u t h e r n p i n e w o o d s p e c i m e n s i m p r e g n a t e d w i t h T B T M A has b e e n c o m p a r e d w i t h s p e c i m e n s that were treated w i t h pentachlorophenol methacrylate ( P C P M A ) or p e n t a b r o m o p h e n o l m e t h a c r y l a t e ( P C B M A ) . T h e t h r e e esters w e r e i n c o r p o r a t e d as t h e c o p o l y m e r b y i n s i t u c o p o l y m e r i z a t i o n w i t h m e t h y l m e t h a c r y l a t e ( M M A ) at d i f f e r e n t m o l a r r a t i o s o f t h e c o m o n o m e r s .


7.

suBRAMANiAN

303


T a b l e I V . F i e l d Test D a t a for M a r i n e D e c a y of

Wood

Rating Sample Number Al A2

12 Months

Treatment P-13 M a r i n e G r a d e C r e o s o t e (368 k g / m )

10 10

3

A3 A4

10 10 9 10

A5 A6 A13 A14


18 Months

10 10

T B T M A - M M A Copolymer (223 k g / m ) 3

A15

10

A16 A17

10 10

24

30

Months

Months

36 Months 10

10 4 10 9 7

—4 — —4

10 10 10 10

10

10

10 10

10 10

10

10 10

A31 A32 A33 A34 A35 A36

T r i b u t y l T i n E s t e r of Methyl Vinyl Ether/Maleic A n h y d r i d e (52.5 k g / m )

10 10 10 10 10 10

10 10 10 10 10 10

10 10 10 10 10 10

A37 A38

P-13 M a r i n e G r a d e

7

A39 A40 A41

9 10 10 10 10

— — 10

A42

10

A18

A43 A44 A45

3

C r e o s o t e (254

kg/m ) 3

10

Special M a r i n e C r e o s o t e , 40% Naphthalene (435 k g / m )

10 9 10 10 10

3

A46 A47 A48 B44 B45 B46 B77

T B T M A - M M A , i n situ Polymerization (365

kg/m ) 3

B78 B79 B57 B58 B59 B60

T B T M A - G M A i n situ P o l y m e r i z a t i o n , 10% Polymer in W o o d

10 10

10 10

10 10 10 10

10 10 10 10

10 10 10

10 10 10 10

10

10

10 10 10 10 10

10

—0 7 4 4 10

9 9 10 10 10 10

10 9 10 10 10 10 10 10 10 10 10 10

(Reproduced from Ref. 4. Copyright 1982, American Chemical Society.)

B o t h soil block samples a n d m a r i n e exposure samples of southern pine w e r e i m p r e g n a t e d thus b y M M A , or copolymers of P C P M A , P C B M A , and T B T M A with M M A . S o i l b l o c k s w e r e e x p o s e d to a b r o w n rot fungus Gloeophyllum trabeum f o r 12 w e e k s t o d e t e r m i n e w e i g h t loss b y b i o d e c a y . T h e


304

THE

CHEMISTRY

OF

SOLID

WOOD


u n t r e a t e d c o n t r o l s h o w e d 4 2 % w e i g h t loss, a n d the M M A - t r e a t e d s p e c i m e n s s u f f e r e d 1 7 % w e i g h t l o s s . T h e w e i g h t loss o f b l o c k s t r e a t e d w i t h P C P M A - M M A a n d P C B M A - M M A was close to that o b s e r v e d w i t h M M A t r e a t m e n t alone. T h e inference was that the r e d u c t i o n i n w e i g h t loss i n t h e s e c a s e s w a s d u e t o t h e effect o f t h e i m p r e g n a t e d p o l y m e r as a m o i s t u r e b a r r i e r , a n d n o t d u e t o r e l e a s e o f t h e i n c o r p o r a t e d toxic c h e m i c a l s , P C P M A o r P C B M A . I n m a r k e d contrast to these r e s u l t s , t h e w e i g h t loss i n t h e p r e s e n c e o f T B T M A - M M A was n e g l i g i b l e , a n d shows that T B T M A was i n d e e d effective i n p r e v e n t i n g attack b y the b r o w n rot fungus. T h e f a i l u r e o f esters, P C P M A a n d P C B M A , i n these tests to a f f o r d a n y l e v e l o f p r o t e c t i o n is p r o b a b l y d u e t o t h e s t a b i l i t y o f t h e s e esters, w h i c h p r e c l u d e s release of the toxic p e n t a c h l o r o p h e n o l a n d p e n t a b r o m o p h e n o l (23). T h e s e o b s e r v a t i o n s s e r v e t o e m p h a s i z e t h e i m p o r t a n c e o f a c h i e v i n g c o n t r o l l e d release of toxicants for p r o t e c t i o n against b i o l o g i c a l degradation. I n m a r i n e t e s t s c o n d u c t e d w i t h s p e c i m e n s t r e a t e d as d e s c r i b e d p r e v i o u s l y , T B T M A - M M A was f o u n d effective i n p r e v e n t i n g attack b y m a r i n e o r g a n i s m s (23) f o r o v e r 12 m o n t h s . T h e s e r e s u l t s c o n f i r m o b s e r v a t i o n s r e p o r t e d e a r l i e r (3, 4). T h e u n t r e a t e d c o n t r o l s p e c i m e n s w e r e largely destroyed b y l i m n o r i a and teredine borers, w h i l e spec imens treated with P C P M A - M M A and P C B M A - M M A already s h o w e d s i g n s o f i n c i p i e n t a t t a c k . ( T h e s e tests a r e b e i n g c o n t i n u e d . )

Conclusion T h e feasibility of simultaneously i m p r o v i n g decay resistance, d i m e n s i o n a l stability, strength, a n d toughness seems w e l l established b y the results of i n situ polymerization a n d copolymerization of organotin p o l y m e r s i n w o o d . P r o v e n techniques of w o o d impregnation b y m o n o m e r s c a n b e a d o p t e d i n these e x p e r i m e n t s to p r e p a r e w o o d polymer composites stronger than untreated wood. Tributyltin func t i o n a l g r o u p s , i n c o r p o r a t e d t h r o u g h c h e m i c a l a t t a c h m e n t to t h e p o l y m e r i z i n g m o n o m e r s , have p r o v e d effective i n r e n d e r i n g the w o o d p o l y m e r c o m p o s i t e s b i o a c t i v e , w i t h d e c a y resistance s u p e r i o r to that obtained b y c o n v e n t i o n a l treatments of w o o d b y creosote. Scientific a n d t e c h n o l o g i c a l a d v a n c e s i n t h i s field c a n b e e x p e c t e d to p r o c e e d rapidly b y e x p l o r i n g a n d exploiting the concepts u n d e r l y i n g these early bioactive wood—polymer composites.

Literature Cited 1. Mendoza, J. A. "Wood Preservation by In Situ Polymerization of Organotin Monomers"; Masters thesis, Materials Science and Engineering, Wash ington State University; Pullman, WA, 1977. 2. Subramanian, R. V.; Mendoza, J. Α.; Garg, Β. K. Holzforschung 1981, 35, 253.



7.

SUBRAMANIAN


305

3. Subramanian, R. V.; Mendoza, J. Α.; Garg, Β. K. Holzforschung 1981, 35, 263. 4. Andersen, D . M . ; Mendoza, J. Α.; Garg, Β. K.; Subramanian, R. V. in "Bi ological Activities of Polymers"; Carraher, C. E., Jr.; Gebelein, C. G . , Eds.; A C S SYMPOSIUM SERIES No. 186, ACS: Washington, D . C . , 1982; p. 27. 5. Becker, G. Wood Sci. Technol. 1974. 8 (3), 163. 6. Rowell, R. M . ; Gutzner, D . I. Wood Sci. 1975, 7 (3), 240. 7. Rowell, R. M . ; Proc.—Annu. Meet. Am. Wood-Preserv. Assoc. 1975, 1, 1. 8. Phillip, A. T. Prog. Org. Coat. 1973/1974, 2, 159. 9. Hof, T. J. Inst. Wood Sci. 1969, 4 (5), 19. 10. Levi, M . P. J. Inst. Wood Sci. 1969, 4 (19), 45. 11. Cockroft, R. J. Inst. Wood Sci. 1974, 6 (6), 2. 12. Sheldon, A. W. J. Paint Technol. 1975, 47 (600), 54. 13. Dyckman, E . J . ; Montemarano, J. A. "Antifouling Organometallic Poly mers: Environmentally Compatible Materials"; N S R D C Report 4136, David W. Taylor Naval Ship R & D Center: Annapolis, M D , February 1974. 14. Montermoso, J. C.; Andrews, J. M . ; Marinelli, L . P. J. Polym. Sci. 1958, 32, 523. 15. Subramanian, R. V.; Somasekharan, Κ. N. J. Macromol. Sci., Chem. 1981, A16, 73. 16. Subramanian, R. V.; Garg, Β. K. Polym.-Plast. Technol. Eng. 1978, 11, 81. 17. Subramanian, R. V.; Garg, Β. K.; Corredor, J. "Organometallic Polymers"; Carraher, C. E., Jr.; Sheats, J. E.; Pittman, C. U . , Eds.; Academic Press: New York, 1978; p. 181. 18. Somasekharan, Κ. N . ; Subramanian, R. V. in "Controlled Release of Bioac tive Materials"; Baker, R., E d . ; Academic Press: New York, 1980; p. 415. 19. Stamm, A. J . ; Tarkow, H . J. Phys. Colloid Chem. 1947, 51, 493. 20. Goldstein, I. S.; Dreher, W. Α.; Jeroski, Ε. B . ; Nielson, J. F.; Oberley, W. J.; Weaver, J. W. Ind. Eng. Chem. 1959, 51, 1315. 21. Borgin, K. New Sci. 1974, November 21, 556. 22. Anderson, D . M . Report # D T N S R D C / S M E - 7 8 / 4 1 , David W. Taylor Naval Ship R & D Center, Annapolis, M D , 1979. 23. Rowell, R. M . "Controlled Release Delivery Systems"; Roseman, T. J . ; Mansdorf, S. Z., Eds.; Marcel Dekker, Inc.: New York, 1983; Chap. 23. RECEIVED

for review May 17, 1983.

ACCEPTED

July 15, 1983.


The Chemistry of Solid Wood - American Chemical Society

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