Agricultural and Synthetic Polymers - American Chemical Society

Chapter 21

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Chemical Modification of Lignoceilulosic Fibers To Produce High-Performance Composites Roger M. Rowell Forest Products Laboratory, Forest Service, U.S. Department of Agriculture, Madison, WI 53705-2398

The performance properties of composites made from wood and other lignocellulosic materials can be greatly improved by changing the basic chemistry of the cell wall polymers. This paper reviews published research on reducing dimensional instability and susceptibility to degradation by biological organisms, heat, and ultraviolet radiation to produce high-performance lignocellulosic composites based on acetylation of the furnish before product formation.

Wood and other l i g n o c e i l u l o s i c materials are three-dimensional, polymeric composites made up primarily of c e l l u l o s e , hemicelluloses, and l i g n i n . These polymers make up the c e l l wall and are responsible f o r most of the physical and chemical properties of these materials. Wood and other l i g n o c e i l u l o s i c materials have been used as engineering materials because they are economical, renewable, and strong and have low processing-energy requirements. They have, however, several undesirable properties, such as dimensional i n s t a b i l i t y due to moisture sorption with varying moisture contents, biodegradability, flammability, and degradability by u l t r a v i o l e t l i g h t , acids, and bases. These properties are a l l the r e s u l t of chemical reactions involving degradative environmental agents. Because these types o f degradation are chemical i n nature, i t should be possible to eliminate them or decrease t h e i r rate by modifying the basic chemistry o f the l i g n o c e i l u l o s i c c e l l wall polymers.

This chapter not subject to U.S. copyright Published 1990 American Chemical Society

In Agricultural and Synthetic Polymers; Glass, J. Edward, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

21. ROWELL

Chemical Modification ofLignoceilulosic Fibers

243

Most research on chemical modification of l i g n o c e i l u l o s i c materials has focused on improving e i t h e r the dimensional s t a b i l i t y or the b i o l o g i c a l resistance o f wood. This paper reviews the research on these properties for wood and other l i g n o c e i l u l o s i c composites and describes opportunities to improve f i r e retardancy and resistance to u l t r a v i o l e t degradation.


Reaction Chemistry Reactive organic chemicals can be bonded to c e l l wall hydroxyl groups on c e l l u l o s e , hemicelluloses, and l i g n i n . Much of our research has involved simple epoxides (1-3) and isocyanates (4), but most o f our recent e f f o r t has focused on acetylation. Acetylation studies have been done using fiberboards (5,6). hardboards (7*"ϋ) · particleboards (12-20), and flakeboards (21-23), using vapor phase acetylation (8,2ΐΓ^26Τ, l i q u i d phase acetylation (1,27), or reaction with ketene (28).

0 Wood-OH + CH -C-0-C-CH 3

Acetic anhydride

3

II Wood-0-C-CH Wood acetate

0 II + CHC -OH Acetic acid

I f t h i s acetylation system does not include a strong c a t a l y s t or cosolvent, only the e a s i l y accessible hydroxyl groups w i l l be acetylated. We developed an acetylation system that uses no strong c a t a l y s t or cosolvent and probably acetylates only e a s i l y accessible hydroxyl groups (27). Several l i g n o c e i l u l o s i c fibers were acetylated using t h i s procedure; reaction times from 15 min to 4 h were used on Southern Pine, aspen, bamboo (29), bagasse (30), jute (31), pennywort, and water hyacinth (32). A l l the l i g n o c e i l u l o s i c materials used were e a s i l y acetylated. Acetyl content r e s u l t i n g from acetylation plotted as a function of time shows a l l data points f i t t i n g a common curve (Fig. 1). A maximum weight percent gain (WPG) of about 20 was reached i n a 2-h reaction time, and an additional 2 h increased the weight gain only by 2 to 3 percent. Without a strong catalyst, acetylation using acetic anhydride alone levels o f f at approximately 20 WPG for the softwoods, hardwoods, grasses, and water plants. Moisture Sorption Sorption o f moisture i s due mainly to hydrogen bonding of water molecules to the hydroxyl groups i n the c e l l wall polymers. By replacing some o f the hydroxyl groups on the c e l l wall polymers with acetyl groups, the hydroscopicity of the l i g n o c e i l u l o s i c material i s reduced. Table I shows the equilibrium moisture content (EMC) o f several l i g n o c e i l u l o s i c materials at 65 percent r e l a t i v e humidity (RH). Reduction i n EMC at 65 percent RH of acetylated f i b e r referenced to unacetylated f i b e r plotted as a function of the bonded acetyl content i s a straight l i n e (Fig. 2). Although the points shown i n Figure 2 come from many d i f f e r e n t l i g n o c e i l u l o s i c materials, they



244

AGRICULTURAL AND SYNTHETIC POLYMERS

Figure 1—Rate of acetylation of various lignoceilulosic materials. O, Southern Pine; aspen; Δ , bamboo; Ot bagasse; X, jute; +, pennywort; V , water hyacinth.



21.

ROWELL


10

15

20

25

245

30

Percent acetyl Figure 2--Reduction i n equilibrium moisture content (EMC) as a function o f bonded acetyl content f o r various acetylated l i g n o c e i l u l o s i c materials. O, Southern Pine; • , aspen; Δ , bamboo; Ο» bagasse; X, jute; +, pennywort; Vf water hyacinth.


246

AGRICULTURAL AND SYNTHETIC POLYMERS Table I. Equilibrium Moisture Content (EMC) o f Various Acetylated L i g n o c e i l u l o s i c Materials (65 Percent RH, 27°C)


Material Southern

Pine

Reaction weight gain (percent)

0 6.0

Bamboo

21.1 0

20.1

4.3

3.9

11.1

7.3

10.1

14.2

16.9

7.8 5.9

17.9

19.1

4.8

0 10.8 14.1 0

Water hyacinth

8.9

13.1

5.3

16.6 20.2

4.4

3.4

3.7 8.8

14.4

5.3

15.3

4.4

17.6

19.0

0

15.6 Pennywort

3.2

6.0

12.2

9.4 Jute

12.0

9.2

17.0 Bagasse

1.4

EMC (percent)

7.0 15.1

14.8 Aspen

Acetyl content (percent)

0 10.1 0

8.3 18.6

3.0 16.5

3.4

9.9 4.8

1.3

18.3

14.0 1.2 10.8

8.6

17.8

0.7

1.7 1.2

a l l f i t a common l i n e . A maximum reduction i n EMC i s achieved at about 20 percent bonded a c e t y l . Extrapolation o f the p l o t to 100 percent reduction i n EMC would occur at about 30 percent bonded a c e t y l . Because the acetate group i s larger than the water molecule, not a l l hygroscopic hydrogen-bonding s i t e s are covered. The f a c t that EMC reduction as a function of acetyl content i s the same f o r many d i f f e r e n t l i g n o c e i l u l o s i c materials indicates that reducing moisture sorption and, therefore, achieving c e l l wall s t a b i l i t y are controlled by a common factor. The l i g n i n , hemicellulose, and c e l l u l o s e contents of a l l the materials plotted i n Figure 2 are d i f f e r e n t (Table II). E a r l i e r r e s u l t s showed that the bonded acetate was mainly i n the l i g n i n and hemicelluloses (33) and that i s o l a t e d wood c e l l u l o s e does not react with uncatalyzed acetic anhydride (34). Acetylation may be c o n t r o l l i n g the moisture s e n s i t i v i t y due to the l i g n i n and hemicellulose polymers i n the c e l l wall but not reducing the sorption o f moisture i n the c e l l u l o s e polymer because


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In Agricultural and SyntheticOX. Polymers; Glass, Washington, 2003 6 J. Edward, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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248 1. 2.


3.

AGRICULTURAL AND SYNTHETIC POLYMERS these materials vary widely i n t h e i r l i g n i n , hemicellulose, and c e l l u l o s e content, acetate i s found mainly i n the l i g n i n and hemicellulose polymer, and i s o l a t e d c e l l u l o s e does not acetylate by the procedure used.

Dimensional S t a b i l i t y Dimensional i n s t a b i l i t y , e s p e c i a l l y i n the thickness d i r e c t i o n , i s a greater problem i n l i g n o c e i l u l o s i c composites than i n s o l i d wood because composites undergo not only normal swelling (reversible swelling) but also swelling caused by the release of residual compressive stresses imparted to the board during the composite pressing process ( i r r e v e r s i b l e swelling). Water sorption causes both reversible and i r r e v e r s i b l e swelling, with some of the r e v e r s i b l e shrinkage occurring when the board d r i e s . Dimensional i n s t a b i l i t y of l i g n o c e i l u l o s i c composites has been the major reason for t h e i r r e s t r i c t e d use. We are i n the process of producing fiberboards from various types o f acetylated l i g n o c e i l u l o s i c f i b e r s . Most o f our research has been on pine or aspen particleboards or flakeboards, so the data presented here on dimensional s t a b i l i t y and b i o l o g i c a l resistance come mainly from these types of boards. The rate of swelling i n l i q u i d water of an aspen flakeboard made from acetylated flakes and phenolic r e s i n (27) i s shown i n Figure 3· During the f i r s t 60 min, control boards swelled 55 percent i n thickness, while the board made from flakes acetylated to 17.9 WPG swelled less than 2 percent. During 5 days of water soaking, the control boards swelled more than 66 percent, while the 17.9-WPG board swelled about 6 percent. Control boards made from bamboo p a r t i c l e s using a phenolic adhesive swelled about 10 percent a f t e r 1 h, 15 percent a f t e r 6 h, and 20 percent a f t e r 5 days. Particleboards made from acetylated bamboo p a r t i c l e s swelled about 2 percent a f t e r 1 h and only 3 percent a f t e r 5 days (35). Thickness changes i n a s i x - c y c l e water-soaking/ovendrying test for an acetylated aspen flakeboard are shown i n Figure k (27). Control boards swelled more than 70 percent i n thickness during the s i x cycles, compared with less than 15 percent f o r a board made from acetylated flakes. Acetylation greatly reduced both i r r e v e r s i b l e and r e v e r s i b l e swelling. In a s i m i l a r f i v e - c y c l e water-soaking/ovendrying test on bamboo particleboards, control boards swelled more than 30 percent, while boards made from acetylated p a r t i c l e s swelled about 10 percent. Figure 5 (27) shows changes i n thickness of aspen flakeboards made from control and acetylated flakes using a phenolic adhesive at d i f f e r e n t r e l a t i v e humidities. After four cycles of 30 to 90 percent RH, control boards swelled 30 percent i n thickness, while acetylated boards at 17·9 WPG swelled about 5 percent. The r e s u l t s of both l i q u i d water and water vapor tests show that acetylation of l i g n o c e i l u l o s i c materials greatly improves dimensional s t a b i l i t y of composites made from these materials.



21. ROWELL


min

hrs

days

Figure 3—Rate o f swelling i n l i q u i d water o f aspen flakeboard made from acetylated flakes. 0 = control; + = 7-3 WPG; X = 11.5 WPG; Δ = 14.2 WPG; • = 17-9 WPG.


249


250


Ovendry/water soaking no. Figure 4—Changes i n ovendry (0D) thickness i n repeated water-soaking test of aspen flakeboard made from acetylated flakes. Ο = control; + = 7-3 WPG; X = 11.5 WPG;

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21. ROWELL


OD 30% 90% 30% 90% 30% 90% 30% 90% 30% OD

1 2

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Humidity cycle Figure 5—Changes in ovendry (OD) thickness at 30 percent and 90 percent relative humidity of aspen flakeboard made from acetylated flakes (27°C). 0= control; + = 7-3 WPG; X = 11.5 WPG; Δ = 14.2 WPG; • = 17-9 WPG.



252


B i o l o g i c a l Resistance Chemical modification of wood composite furnish f o r b i o l o g i c a l resistance i s based on the theory that the p o t e n t i a l l y degrading enzymes must d i r e c t l y contact the substrate and that the substrate must have a s p e c i f i c chemical configuration and molecular conformation. Reacting chemicals with the hydroxyl groups on c e l l wall polymers chemically changes the substrate so that the highly s e l e c t i v e enzymatic reactions cannot take place. Chemical modification also reduces the moisture content of the c e l l wall polymers to a point where b i o l o g i c a l degradation cannot take place. Particleboards and flakeboards made from acetylated flakes have been tested f o r resistance to several d i f f e r e n t types of organisms. In a 4-week termite test using Reticulitermes flavipes (subterranean termites), boards acetylated at 16 to 17 WPG were very r e s i s t a n t to attack, but not completely so (17,36,37). This may be attributed to the severity of the test. However, since termites can l i v e on acetic acid and decompose c e l l u l o s e to mainly acetic acid, perhaps i t i s not s u r p r i s i n g that acetylated wood i s not completely resistant to termite attack. Chemically modified wood composites have been experimentally exposed to decay fungi i n several ways. Untreated aspen and pine particleboards and flakeboards exposed to white-, s o f t - , and brown-rot fungi and tunneling b a c t e r i a i n a fungal c e l l a r were destroyed i n less than 6 months, while particleboards and flakeboards made from furnish acetylated to greater than 16 percent acetyl weight gain showed no attack a f t e r 1 year (Table III) (36-38). In a standard 12-week single-culture s o i l - b l o c k test, control aspen flakeboards exposed to the white-rot fungus Trametes v e r s i c o l o r l o s t 34 percent weight, while acetylated flakeboards (17 percent acetyl weight gain) l o s t no weight (36-38). In exposure to the brown-rot fungus Tyromyces p a l u s t r i s , control aspen flakeboards l o s t only 2 percent weight when a phenol-formaldehyde adhesive was used but l o s t 30 percent weight when an isocyanate adhesive was used. I f the flakeboards were water leached before the s o i l - b l o c k test was run, control boards made with phenol-formaldehyde adhesive l o s t 44 percent weight with the brown-rot fungus Gloeophyllum trabeum (34-36). This shows that the adhesive can influence results of fungal t o x i c i t y , e s p e c i a l l y i n a small, closed test container. Weight loss r e s u l t i n g from fungal attack i s the method most used to determine the effectiveness of a preservative treatment to protect wood composites from decaying. In some cases, e s p e c i a l l y for brown-rot fungal attack, strength loss may be a more important measure of attack since large strength losses are known to occur i n s o l i d wood at very low wood weight loss (39). A dynamic bending-creep test has been developed to determine strength losses when wood composites are exposed to a brown- or white-rot fungus (40). Using t h i s bending-creep test on aspen flakeboards, control boards made with phenol-formaldehyde adhesive f a i l e d i n an average of 71 days with T. p a l u s t r i s and 212 days with T. v e r s i c o l o r (4l). At f a i l u r e , weight losses averaged 7.8 percent f o r T. p a l u s t r i s and 31.6 percent f o r T. v e r s i c o l o r . Isocyanate-bonded control


21. ROWELL


253

Table I I I .

Fungal C e l l a r Tests on Aspen Flakeboards a b Made from Control and Acetylated Flakes '

Acetyl


Rating at intervals**

WPG

2 mo

3 mo

4 mo

5 mo

6 mo

0 7.3 11.5 13.6 16.3 17.9

S/2 S/0

S/3

S/3

—

s/i

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S/4

S/l

s/4

0 0 0 0

0 0 0 0

S/0

s/i

0 0 0

0 0 0

S/3 S/2 S/0

0 0

0 0

C

12 mo

S/3 s/i

^ o n s t e r i l e s o i l containing brown-, white-, and s o f t - r o t fungi and tunneling b a c t e r i a . Flakeboards bonded with 5 percent phenol-formaldehyde adhesive.

Q Weight percent gain. ^Rating system: 0, no attack; 1, s l i g h t attack; 2, moderate attack; 3f heavy attack; 4, destroyed; S, swollen. flakeboards f a i l e d i n an average of 20 days with T. p a l u s t r i s and 118 days with T. v e r s i c o l o r , with average weight loss at f a i l u r e o f 5·5 percent and 34.4 percent, respectively (4l). Very l i t t l e or no weight loss occurred with e i t h e r fungi i n flakeboards made using phenol-formaldehyde or isocyanate adhesive with acetylated flakes. None o f these specimens f a i l e d during the test period. Deflection-time curves f o r flakeboards are shown i n Figure 6. The curves show an i n i t i a l increase o f d e f l e c t i o n f o r both c o n t r o l and acetylated flakeboards, then a stable zone, and f i n a l l y , f o r control boards, a steep slope to f a i l u r e . The study showed that less than 5 mm of d e f l e c t i o n was caused by creep due to moisture alone (4l). Mycelium f u l l y covered the surfaces of isocyanate-bonded control flakeboards within 1 week, but mycelial development was s i g n i f i c a n t l y slower i n phenol-formaldehyde-bonded control flakeboards. Both isocyanate- and phenol-formaldehyde-bonded acetylated flakeboards showed surface mycelium c o l o n i z a t i o n during the test time, but the fungus did not attack the acetylated flakes, so l i t t l e strength was l o s t . In s i m i l a r bending-creep tests, both control and acetylated pine particleboards made using melamine-urea-formaldehyde adhesive f a i l e d because T. p a l u s t r i s attacked the adhesive i n the g l u e l i n e (42). Mycelium invaded the inner part of a l l boards, c o l o n i z i n g i n both wood and g l u e l i n e i n control boards but only i n the g l u e l i n e i n acetylated boards. A f t e r a 16-week exposure to T. p a l u s t r i s , the i n t e r n a l bond strength o f control aspen flakeboards made with phenol-formaldehyde


254



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21. ROWELL

255

Chemical Modification ofUgnoceUu^

adhesive was reduced more than 90 percent and that of flakeboards made with isocyanate adhesive was reduced 85 percent (43). After 6 months of exposure i n moist u n s t e r i l e s o i l , the same control flakeboards made with phenol-formaldehyde adhesive l o s t 65 percent of t h e i r i n t e r n a l bond strength and those made with isocyanate adhesive l o s t 64 percent i n t e r n a l bond strength. F a i l u r e was due mainly to great strength reductions i n the wood caused by fungal attack. Acetylated aspen flakeboards l o s t much l e s s i n t e r n a l bond strength during the 16-week exposure to T. p a l u s t r i s or 6-month s o i l b u r i a l . The isocyanate adhesive was somewhat more r e s i s t a n t to fungal attack than the phenol-formaldehyde adhesive. In the case of acetylated composites, l o s s i n i n t e r n a l bond strength was mainly due to fungal attack i n the adhesive and moisture, which caused a small amount of swelling i n the boards. Acetylated pine flakeboards have also been shown to be r e s i s t a n t to attack i n a marine environment (44). Control flakeboards were destroyed i n 6 months to 1 year, mainly because of attack by Limnoria tripunctata, while acetylated boards showed no attack a f t e r 2 years. A l l laboratory tests f o r b i o l o g i c a l resistance conducted to t h i s point show that acetylation i s an e f f e c t i v e means of reducing or eliminating attack by s o f t - , white-, and brown-rot fungi, tunneling bacteria, and subterranean termites. Tests are presently underway on several l i g n o c e i l u l o s i c composites i n outdoor environments. Other Properties Other properties of l i g n o c e i l u l o s i c composites can be improved by changing the basic chemistry of the furnish (45). Acetylation has been shown to improve u l t r a v i o l e t resistance of flakeboards. Table IV shows that acetylation greatly reduced weight l o s s and errosion rate due to l o s s o f surface f i b e r from aspen flakeboards. The rate o f errosion f o r boards made from acetylated flakes i s h a l f that o f control boards. In outdoor tests, flakeboards made from acetylated pine flakes were s t i l l l i g h t yellow i n color when control boards had turned dark orange to l i g h t gray.

Table IV. Weight Loss and Rate of Erosion f o r Acetylated Aspen Flakeboard A f t e r Accelerated Weathering Weight loss (percent) a f t e r weathering period 500 h

800 h

Control

0.9

2.0

2.8

3-7

0.12

Acetylated

0.0

0.5

1.2

2.4

0.06

1,700

h

Erosion rate at 2,400 h Wh)

140 h

Sample



256


Acetylation does not change the fire properties of lignoceilulosic materials. In thermogravametric analysis, acetylated and control pine sawdust pyrolyzed at the same temperature and rate (46). The heat of combustion and rate of oxygen consumption were also the same for control and acetylated specimens, showing that the acetyl group added to the c e l l wall has approximately the same carbon, hydrogen, and oxygen content as the cell wall polymers. Reactive fire retardants can be bonded to the cell wall hydroxyl groups. The effect would be an improvement in dimensional stability, biological resistance, and fire retardancy. Conclusions Dimensional instability and susceptibility to degradation by biological organisms, heat, and ultraviolet radiation can be greatly reduced by modification of lignoceilulosic cell wall polymers. These modifications result in a furnish that can be converted into composites of any desired shape, density, and size and provide an opportunity for a manufacturer to distinguish a product line based on quality, uniformity, and performance. Of the various reaction systems studied to date, a simple noncatalyzed acetylation process appears to be closest to implementation on a commercial basis. Composites made from lignoceilulosic materials have been restricted from many markets because of their moisture sorption, dimensional instability, and to a lesser extent, biological degradation. These negative properties can be overcome, allowing flakes, particles, and fiber from wood and agricultural residues to find markets related to high-performance composites. For some applications, a combination of materials may be required to achieve a composite with the desired properties and performance. Property-improved lignoceilulosic fibers can be combined with materials such as metal, glass, plastic, natural polymers, and synthetic fiber to yield a new generation of composite materials. New composites will be developed that utilize the unique properties obtainable by combining many different materials. This trend will increase significantly in the future. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8.

Rowell, R. M.; Tillman, A-M.; Zhengtian, L. Wood Sci. Technol. 1986, 20, 83-95. Rowell, R. M.; Ellis, W. D. Reaction of Epoxides with Wood. Res. Pap. FPL 451. U.S. Department of Agriculture, Forest Serv., Forest Prod. Lab., 1984. 41 p. Tillman, A-M. J . Wood Chem. Technol. (In press). Rowell, R. M.; Ellis, W. D. Am. Chem. Soc. Symp. Ser. 1981, 172, 263-284. Bekere, M.; Shvalbe, K.; Ozolinya, I. Latvijas Lauksaimniecibas Akademija. Raksti 1978, 163, 31-35. Bristow, J . Α.; Back, E. L. Svensk Papperstid. 1969, 72, 367-374. Sudo, K. Mokuzai Gakkaishi 1979, 25, 203-208. Klinga, L. O.; Tarkow, H. Tappi 1966, 49, 23-27.


21. ROWELL 9. 10. 11.


12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 3334. 3536.


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37. Rowell, R. M.; Esenther, G. R.; Youngquist, J . Α.; Nicholas, D. D.; Nilsson, T.; Imamura, Y.; Kerner-Gang, W.; Trong, L . ; Deon, G.; Proc. Symp. Protection of Wood-Based Composite Products, 1988, 238-266. 38. Nilsson, T.; Rowell, R. M.; Simonson, R.; Tillman, A.-M. Holzforschung 1988, 42, 123-126. 39. Cowling, Ε. B. Comparative Biochemistry of the Decay of Sweetgum Sapwood by White-Rot and Brown-Rot Fungus, Technol. Bull. 1258, U.S. Department of Agriculture, Forest Serv., 1961, Ρ 50. 40. Imamura, Y.; Nishimoto, K. J . Soc. Materials Sci. 1985, 34, 985-989. 41. Rowell, R. M.; Youngquist, J . Α.; Imamura, Y. Wood Fiber Sci. 1988, 20, 266-271. 42. Imamura, Y.; Rowell, R. M.; Simonson, R.; Tillman, A.-M. Paperi ja Puu 1988, 9, 816-829, 43. Imamura, Y.; Nishimoto, K.; Rowell, R. M. Mokuzai Gakkaishi 1987, 33, 986-991. 44. Johnson, B. R.; Rowell, R. M. Mater. und Org. 1988, 23, 147-156. 45. Rowell, R. M. In Chemistry of Solid Wood; Rowell, R. M., Ed.; Advances in Chemistry Series 207; American Chemical Society: Washington, DC, 1984; Chapter 4, pp 175-210. 46. Rowell, R. H.; Susott, R. Α.; De Groot, W. G; Shafizadeh, F. Wood and Fiber Sci. 1984, 16, 214-223. RECEIVED

January 22, 1990


Agricultural and Synthetic Polymers - American Chemical Society

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