Graft Copolymerization of Lignocellulosic Fibers - ACS Publications

21 UV Degradation and Accelerated Weathering of Chemically Modified Wood

Downloaded by UNIV MASSACHUSETTS AMHERST on August 30, 2013 | http://pubs.acs.org Publication Date: June 18, 1982 | doi: 10.1021/bk-1982-0187.ch021

WILLIAM C. FEIST and ROGER M . ROWELL USDA Forest Service, Forest Products Laboratory, Madison, WI 53705

The roles of c e l l wall chemical modification, polymer lumen-fill treatments, and a combination of these two treatments in reducing the degradative effects of ultraviolet light on wood revealed widely varying degrees of effectiveness. Under conditions simulating accelerated outdoor weathering (ultraviolet light irradiation alterna ting with water spray), butylene oxide- or butyl isocyanate-modified southern pine sapwood performed no better than untreated controls and surface degradation was severe. With ultraviolet light exposure only, surface degradation was much less for both modified wood and untreated wood. Southern pine with a dual treatment of chemical modification with butylene oxide or butyl isocyanate followed by lumen-fill treatment with methyl methacrylate, or southern pine impregnated with methyl methacrylate and polymerized in situ, resulted in modified woods that were resistant to accelerated weathering and to ultraviolet light alone. Physical, chemical, and microscopic changes occurring as a result of ultraviolet light i r r a d i ation are described. Wood and wood-based products are durable m a t e r i a l s t h a t have long been recognized f o r t h e i r v e r s a t i l i t y and t h e i r a t t r a c t i v e engineering and s t r u c t u r a l p r o p e r t i e s ( 1 ) . L i k e other m a t e r i a l s , wood i s s u s c e p t i b l e t o environmental d e t e r i o r a t i o n . Wood exposed outdoors undergoes p h y s i c a l changes and chemical r e a c t i o n s in a process u s u a l l y r e f e r r e d t o as weathering (2, 3 ) . These changes and r e a c t i o n s p l a y an important r o l e when wood products are used for e x t e r i o r s i d i n g or cladding. The weathering o f wood i s caused p r i m a r i l y by the a c t i o n o f u l t r a v i o l e t (UV) l i g h t , water, heat, and a b r a s i o n . The c o n t r i b u t i o n o f these elements t o the wood weathering process has

©

0097-6156/82/0187-0349$6.25/0 1982 American Chemical Society

In Graft Copolymerization of Lignocellulosic Fibers; Hon, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


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GRAFT COPOLYMERIZATION OF LIGNOCELLULOSIC FIBERS

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 (2-5). The outdoor weathering of wood causes r a p i d surface c o l o r changes f o l l o w e d by s u r f a c e roughening as the g r a i n r a i s e s and the wood checks and c r a c k s . As w e t t i n g and d r y i n g occur, boards may warp. The wood l o s e s i t s surface coherence, becomes f r i a b l e , and s p l i n t e r s and f r a g ments come o f f . Of the elements c o n t r i b u t i n g t o the weathering process, UV i r r a d i a t i o n from the sun and s t r e s s e s imposed by w e t t i n g and d r y i n g are the most important in the m a j o r i t y o f climates. Wood i s an e x c e l l e n t l i g h t absorber and almost every chemi c a l c o n s t i t u e n t in wood i s s e n s i t i v e , w i t h consequential d e t e r i o r a t i n g e f f e c t (6, 7 ) . Of the v a r i o u s chemical c o n s t i t u e n t s in wood, l i g n i n appears t o be o x i d i z e d and degraded by UV l i g h t most r a p i d l y ( 5 , 8, 9, 10). Because o f l i g n i n ' s strong UV l i g h t absorbing c h a r a c t e r i s t i c s , i t may a l s o f u n c t i o n as a p a r t i a l s h i e l d i n g agent t o p r o t e c t wood c e l l u l o s i c components from photodegradation by UV ( 9 ) . F o r t u n a t e l y , UV l i g h t does not penetrate wood deeper than approximately 75 μπι ( 11). Consequently, the i n t e r a c t i o n o f wood components and UV l i g h t i s e s s e n t i a l l y a surface r e a c t i o n in which the f r e e r a d i c a l intermediates gener ated p l a y a major r o l e in s u r f a c e d e t e r i o r a t i o n and d i s c o l o r a t i o n (5, 6, 8, 12, 13). Many conventional and experimental surface treatments and m o d i f i c a t i o n s f o r wood have been developed t o reduce o r e l i m i n a t e the e f f e c t s o f the v a r i o u s weathering elements ( 2 , 3, 5, 14). The chemical changes o c c u r r i n g and the mechanisms o f degradation and c o l o r formation during weathering have r e c e i v e d considerable a t t e n t i o n and have been summarized in an e a r l i e r paper in t h i s series (3). Chemical m o d i f i c a t i o n of wood c e l l w a l l components has been shown t o be s u c c e s s f u l in imparting r e s i s t a n c e t o v a r i o u s degrading elements (15, 16, Γ7). Reduced h y g r o s c o p i c i t y of chem i c a l l y modified woods has been demonstrated (18, 19, 20). There are no chemical m o d i f i c a t i o n s p u b l i s h e d in which UV s t a b i l i z a t i o n and c o n t r o l of h y g r o s c o p i c i t y (the main weathering elements) were the primary goals o f the m o d i f i c a t i o n (15). In an e a r l i e r paper ( 3 ) , i t was shown t h a t the springwood of southern pine c h e m i c a l l y modified w i t h butylène oxide or methyl isocyanate f o r p r o t e c t i o n from biodégradation was not adequately p r o t e c t e d a g a i n s t the degradative e f f e c t s of a c c e l e r ated weathering (water and UV i r r a d i a t i o n ) . I n c r e a s i n g the dimensional s t a b i l i t y of the wood and b l o c k i n g l i g n i n p h e n o l i c hydroxyl groups apparently was not enough t o stop the e f f e c t s of these weathering elements. F i l l i n g the wood lumens w i t h methyl methacrylate reduced the r a t e o f springwood e r o s i o n by about 50%. The polymer encasing the wood elements probably reduced the r a t e of water uptake and retarded the l e a c h i n g of degradation products. I t acted as a g l u e - l i k e m a t e r i a l h e l p i n g t o h o l d the surface c e l l u l o s i c f i b e r s in p l a c e even though the l i g n i n component may have been degraded by UV l i g h t . A combination of butylène oxide or methyl isocyanate c e l l w a l l modifying treatment f o l l o w e d by



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methyl methacrylate l u m e n - f i l l i n g treatment r e s u l t e d in a system t h a t p r o t e c t e d the wood f o r 90 days of a c c e l e r a t e d weathering. The a d d i t i o n of methyl methacrylate t o the c e l l w a l l - m o d i f y i n g chemical treatment provided a d d i t i o n a l water r e p e l l e n c y and poss i b l e l i g n i n s t a b i l i z a t i o n and had a s i g n i f i c a n t e f f e c t on reducing weathering. The chemical m o d i f i c a t i o n of wood could p l a y a very import a n t r o l e in c o n t r o l l i n g the n a t u r a l weathering process. The primary purpose of t h i s c o n t i n u i n g research was t o determine the e f f e c t s of chemical m o d i f i c a t i o n of wood on w e a t h e r a b i l i t y and to e l u c i d a t e the mechanism(s) o f UV degradation o f modified wood. Chemical m o d i f i c a t i o n of wood c e l l w a l l s w i t h b u t y l isocyanate (Bulso) or butylène oxide (BuO), l u m e n - f i l l i n g m o d i f i c a t i o n w i t h methyl methacrylate (MMA), and combined c e l l w a l l m o d i f i c a t i o n and l u m e n - f i l l i n g m o d i f i c a t i o n s were compared t o unmodified southern p i n e . P h y s i c a l , m i c r o s c o p i c , and chemical changes o c c u r r i n g on the wood surfaces a f t e r UV i r r a d i a t i o n in the cont r o l l e d a c c e l e r a t e d weathering environments were evaluated f o r springwood and summerwood. Both UV l i g h t and UV l i g h t / w a t e r combinations of exposure were i n c l u d e d in the s t u d i e s . Experimental Wood Specimens. Southern pine sapwood b l o c k s , 2.4 χ 2.4 χ O.6 c m ( l o n g i t u d i n a l χ r a d i a l χ t a n g e n t i a l ) , were prepared from a s i n g l e f r e s h l y c u t l o g . A l l specimens were c u t and planed t o provide a smooth v e r t i c a l - g r a i n surface and ovendried f o r 20 h r s at 105°C. The 70 specimens were randomized f o r use in d i f f e r e n t p o r t i o n s of the study. 3

Chemical M o d i f i c a t i o n . Ovendried b l o c k s were reacted in a s t a i n l e s s s t e e l v e s s e l a t 120°C., 150 p s i n i t r o g e n p r e s s u r e , 10 b l o c k s w i t h Bulso and 10 w i t h BuO monomer u s i n g a technique d e s c r i b e d e a r l i e r (18, 19). Bulso was reacted w i t h wood in the presence of dimethylformamide (65/35, v/v) and BuO w i t h t r i e t h y l amine (95/5, v / v ) . Another s e t of 10 b l o c k s was t r e a t e d in a g l a s s chamber w i t h MMA monomer c o n t a i n i n g 5% t r i m e t h y l o l propane t r i m e t h a c r y l a t e as a c r o s s - l i n k i n g agent and O.25% a z o b i s i s o b u t y r o n i t r i l e as a c a t a l y s t ( 3 ) . Separate b l o c k s in a f o u r t h group were f i r s , t modified w i t h e i t h e r the Bulso or BuO system (10 each) and then t r e a t e d w i t h MMA. The Bulso o r BuO systems r e s u l t in c h e m i c a l l y modified ( c e l l wall-bound) m a t e r i a l and MMA in lumenf i l l i n g m o d i f i c a t i o n ( 3 ) . A l l specimens were ovendried a f t e r treatment and weighed t o determine chemical add-on expressed as weight percent g a i n (WPG) (19) (Table I ) . The UV l i g h t - o n l y specimens were exposed in an a c c e l e r a t e d weathering chamber w i t h UV i r r a d i a t i o n o n l y ; the l i g h t / w a t e r specimens were exposed t o a l t e r n a t i n g c y c l e s o f l i g h t and water. A c c e l e r a t e d Weathering. The r a d i a l faces of all t e s t s p e c i mens were exposed t o a 6500-W xenon a r c l i g h t source (which




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Table I.--Specimen p r e p a r a t i o n f o r a c c e l e r a t e d weathering, v e r t i c a l - g r a i n e d southern pine sapwood WPG 1/ Specimen

Treatment

Light only

Light/ water

Control

None

B u t y l isocyanate (Bulso)

Bulso + dimethylformamide, 120°C

27.2

26.3

Butylène oxide (BuO)

BuO + t r i e t h y l a m i n e

28.4

30.3

c a t a l y s t , 120°C Methyl methacrylate (MMA)

MMA, c a t a l y s t , 70°C

49.6

48.1

Bulso + MMA

Bulso treatment f o l l o w e d by MMA

25.2 43.9

27.3 39.0

BuO + MMA

BuO treatment f o l l o w e d by MMA

30.6 51.1

30.4 57.3

1/ Weight percent gain as a r e s u l t of treatment, determined from ovendry weight before and a f t e r treatment; average of 2 replicates.



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c l o s e l y approximates n a t u r a l s u n l i g h t spectrum in the v i s i b l e and UV regions) in an enclosed chamber a t 45°-50°C and 50% r e l a t i v e humidity ( 4 ) . One s e t of specimens was exposed t o t h i s r a d i a t i o n f o r 24 hrs/day. The second s e t of specimens was exposed to l i g h t a l t e r n a t e d w i t h a spray of d i s t i l l e d water a t ambient temperatures w i t h the l i g h t o f f ; each c y c l e c o n s i s t e d of 4 h r s of d i s t i l l e d water spray f o l l o w e d by 20 hours of l i g h t . Exposure time i s always expressed as hours of exposure to l i g h t . E r o s i o n of springwood and summerwood was measured using a technique described e a r l i e r f o r v e r t i c a l - g r a i n e d specimens ( 4 ) . E r o s i o n was determined a f t e r 600, 1,200, and 1,800 h r s of l i g h t . The specimens f o r e r o s i o n measurement had the upper one-half of the exposed face p r o t e c t e d w i t h a s t a i n l e s s s t e e l cover. Weight Loss. Specimens f o r weight l o s s determinations were placed in both a c c e l e r a t e d weathering chambers ( l i g h t c y c l e and l i g h t / w a t e r c y c l e ) w i t h f u l l y exposed faces. The specimens were ovendried and weighed before and a f t e r exposure (1,800 h r s of l i g h t ) and t h e i r weight l o s s c a l c u l a t e d as a f u n c t i o n of ovend r i e d weight a f t e r chemical m o d i f i c a t i o n (Table I I ) . Chemical A n a l y s i s . Three separate specimens were used f o r chemical analyses f o r each m o d i f i c a t i o n . The f i r s t was the unexposed wood. The second was the outer O.5 mm of wood (removed by s l i c i n g w i t h a r a z o r ) exposed in the a c c e l e r a t e d weathering chamber ( r e f e r r e d t o as "outer specimen"). The t h i r d specimen was the remainder of the exposed specimen a f t e r removal of the O.5 mm of exposed wood surface ( r e f e r r e d t o as " i n n e r specimen"). A l l specimens were ground t o pass a 40-mesh screen and ovendried f o r 16 hrs a t 105°C before chemical a n a l y s i s . L i g n i n determinations were by a method s i m i l a r t o T e c h n i c a l A s s o c i a t i o n of the Pulp and Paper I n d u s t r y Standard T13 (21). Samples were t r e a t e d w i t h 72% s u l f u r i c a c i d f o r 1 hr a t 30°C and 3% s u l f u r i c a c i d f o r 4 hrs a t r e f l u x temperature t o hydrolyze* and s o l u b i l i z e the wood carbohydrate. The i n s o l u b l e residue was measured g r a v i m e t r i c a l l y as l i g n i n . The h y d r o l y s a t e from the l i g n i n determination was used f o r the reducing sugar analyses (22). A l l values shown are uncorrected f o r e x t r a c t i v e s , chemical add-on from m o d i f i c a t i o n , and f o r the s m a l l amount of degradation during h y d r o l y s i s (Table I I I ) . Scanning E l e c t r o n Microscopy (SEM). S e l e c t e d specimens, both unweathered and a r t i f i c i a l l y weathered, were mounted on 9-mm-diameter c i r c u l a r holders w i t h a mixture of s i l v e r p a i n t and c e l l u l o s e acetate cement. The stub holders were then t r a n s f e r r e d t o a high-vacuum evaporating u n i t and coated w i t h 10-20 nm of gold. The specimens were examined w i t h a scanning e l e c t r o n microscope (Cambridge Stereoscan) a t 20 kV. Both r a d i a l surfaces ( o r i g i n a l , exposed, and unexposed) and t a n g e n t i a l surfaces ( s p l i t through springwood w i t h a r a z o r , exposed and unexposed) were examined.




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Table I I . — R a t e of a c c e l e r a t e d weathering and o v e r a l l weight l o s s of southern p i n e sapwood a f t e r 1,800 hr l i g h t exposure

Ovendry weight loss, % Specimen Light only

Light/ water

1/ E r o s i o n r a t e , μιη/hrSpringwood

Summerwood

Light only

Light/ water

Light only

Light/ water

Control

O..5

5..7

O..008

O.,150

O..008

O..042

Bulso

2..0

5..2

O..017

O.,150

0,.008

O..033

BuO

1..3

6..4

O..033

O.183

O..017

O..067

MMA

0,.4

3..5

O..008

O..033

0,.008

O..017

Bulso + MMA

1,.6

2..7

0,.042

O..017

O..025

O..008

BuO + MMA

1,.0

2..5

0,.017

O..017

0,.008

0,.008

1/ E r o s i o n r a t e determined between 1,200 and 1,800 h r , expressed in μπι/hr of l i g h t exposure.


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S e l e c t e d specimens from both l i g h t and l i g h t / w a t e r exposure s t u d i e s were photographed u s i n g c o n v e n t i o n a l camera systems f o r comparative purposes.


R e s u l t s and D i s c u s s i o n The e f f e c t i v e n e s s of c e l l w a l l chemical m o d i f i c a t i o n and l u m e n - f i l l i n g m o d i f i c a t i o n in p r o t e c t i n g wood a g a i n s t the weathering elements of UV l i g h t or UV l i g h t in combination w i t h a l t e r n a t i n g water spray was determined on southern pine sapwood modified w i t h B u l s o , BuO, or MMA or combinations (Table I ) . C e l l w a l l m o d i f i c a t i o n was in the range of 25 t o 31 WPG, the most e f f e c t i v e ranges f o r these treatments (L8, 19); l u m e n - f i l l modif i c a t i o n was in the range of 39 to 57 WPG. Combined treatment specimens were prepared by chemical m o d i f i c a t i o n w i t h Bulso or BuO f o l l o w e d by MMA impregnation and r e a c t i o n . Weight Loss. Determination of weight l o s s by weighing specimens before and a f t e r a c c e l e r a t e d exposure, and measurement of the amount of m a t e r i a l eroded from the exposed wood s u r f a c e are r e l i a b l e methods of a s s e s s i n g degradation due t o UV l i g h t . Ovendry weight l o s s values (Table I I ) show t h a t some m o d i f i c a t i o n s are e f f e c t i v e in reducing weight l o s s when compared to unmodified c o n t r o l s a f t e r exposure to UV l i g h t and water c y c l e s . Weight l o s s i s low in the UV l i g h t - o n l y exposure because there i s no l e a c h i n g of degraded products by water. Weight l o s s values f o r c e l l w a l l - m o d i f i e d specimens were higher than those observed f o r unmodified c o n t r o l specimens. This l o s s may be the r e s u l t of r e s i d u a l bound monomer being v a p o r i z e d or of g r a f t e d polymer being degraded. Weight l o s s values f o r specimens in the UV l i g h t / w a t e r exposure study were from 2 to 11 times g r e a t e r than f o r specimens in the UV l i g h t - o n l y exposure. MMA m o d i f i c a t i o n , e i t h e r alone or in combination w i t h c e l l w a l l m o d i f i c a t i o n , reduced weight l o s s by 40-60% as compared to unmodified c o n t r o l s . A l l these weight l o s s values (Table I I ) i n d i c a t e l o s s of wood substance o n l y from the surface of the exposed specimen because UV l i g h t does not penetrate wood deeply (11) and the weathering process i s a surface phenomenon (2, 4 ) . E r o s i o n of Wood Substance. Wood elements are l o s t d u r i n g weathering or a c c e l e r a t e d weathering as l i g n i n i s degraded by UV l i g h t and the adhesive c h a r a c t e r of t h i s wood component i s reduced ( 2 ) . Degraded l i g n i n and c e l l u l o s e - r i c h f i b e r s are washed away by the a c t i o n of water and the wood g r a d u a l l y erodes away. I n softwoods, springwood erodes much f a s t e r than does summerwood ( 1 , 2 ) . The summerwood of specimens c h e m i c a l l y m o d i f i e d w i t h Bulso or BuO eroded f a s t e r than unmodified c o n t r o l s when exposed to UV l i g h t / w a t e r c y c l e s ( F i g u r e 1). A l l specimens c o n t a i n i n g MMA, e i t h e r alone or in combination w i t h Bulso or BuO, were more



-- -- -- -

28,.0 27..2 27..2 26..3 26..3

31..0 28..4 28..4 30..3 30..3

Bulso Unexposed Light-outer -inner Light/water- •outer •inner

BuO Unexposed Light-outer -inner Light/water- -outer •inner

-•-

Bonded

Control Unexposed Light-outer -inner Light/water- •outer -inner

.

1/ Specimen-

c

Filled

29..6 30,.1 32..5 27..9 31..6

33..1 32..2 33..8 30..7 34..9

34..1 35..1 34..9 36..7 33..8

45..8 45..8 46..4 49..6 45..8

66,.3 65..7 66..3 70..6 67..5

%

%

28,.6 26,.9 29,.1 24,.8 28,.3

Total sug4/ ars,—

Lig. 3/ nin—

63..6 65..2 67..4 64..6 65..4

78..9 78..0 80..2 80..3 80..7

94..9 92..6 95..4 95..4 95..8

%

Lignin + sugars,

87..9 83..9 83..9 87..5 87..7

72..8 72..1 72..6 74..7 71..3

65,.0 67..0 64..3 69..3 65..9

cose

G l u

6..1 6..7 6..9 5..9 6..3

9..4 9..1 8..2 8..7 9..6

10..2 9..8 11..1 8..7 10..0

Xylose

4..5 6..5 6..4 4..7 4..6

13..8 13..0 13..2 12..9 14..0

16..1 15..9 15..0 14..3 16..9

Mannose

O..8 1..5 1..5 1..1 O..8

2..5 4.,3 4..2 2..6 3..5

5..9 4..7 6..7 5..9 5..0

Galactose

Page 1 of 2

O..7 1..4 1..3 O..8 O..6

1..5 1..5 . 1..8 1..1 1..6

2,.8 2,.6 2,.9 1,.8 2,.2

Arabinose

Sugar r a t i o s

Table III.--Chemical analyses of c h e m i c a l l y modified southern pine a f t e r a c c e l e r a t e d weathering--continued


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Graft Copolymerization of Lignocellulosic Fibers - ACS Publications

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