Wood Technology: Chemical Aspects

14 Bark Extracts as Bonding Agent for Particleboard

Downloaded by CORNELL UNIV on August 17, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0043.ch014

ARTHURB.ANDERSON Forest Products Laboratory, University of California, Richmond, Calif. 94804

Until recently the particleboard industry has enjoyed a favorable environment for growth through ample supplies of relatively low-cost bonding agents, including urea-formaldehyde and phenolformaldehyde resins. In the manufacture of water-resistant exterior-type particleboard, phenolic and phenol-resorcinol modified resins are employed. The particleboard industry is facing, at present, a shortage and competing market for phenol and high cost polyhydroxy phenol, such as resorcinol (3). It becomes apparent that it would be desirable to find another low-cost source for phenolics, preferably from a renewable resource. One of the ubiquitous by-products of the forest products industry at plant site is bark residues. While some bark is used as fuel and in agriculture, in particular soil applications, considerable amounts of bark remain unused (6,7). A recent survey in Oregon indicated that the lumber and plywood industries generated annually a total of 3.0 million dry tons of bark. Nearly half (46 percent) of the bark producedwas used as fuel, 10 percent for other purposes--mainly for soil application, and 44 percent was not used (6). Bark was utilized least of all types of residues. And bark disposition is becoming more of a problem due to the increased restrictions on the incineration of bark residues. Chemical processing of bark is limited and the principal chemical products produced commercially from barks are based on the barks phenolic content (1,12). Barks generally are richer than wood in quantity and complexity of extractive components, the most important being a) the monomer ic polyphenols or flavonoid compounds, and b) the polymeric phenolics, such as tannin, phlobaphenes and phenolic acids. Use of the phenolic components of bark extracts in preparing adhesive components used in plywood and particleboard manufacture has been proposed from time to time (8,15). Such preparations are based on the reaction of bark phenolic "components with an aldehyde, usually formaldehyde. The chemical reaction of western hemlock bark extract with ?

235

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

WOOD TECHNOLOGY:


236

CHEMICAL

ASPECTS

formaldehyde has been proposed as a bonding agent f o r plywood (13, 18)« Mangrove tannin-formaldehyde r e s i n has been i n v e s t i g a t e d as a s t r o n g water r e s i s t a n t adhesive f o r plywood ( 4 ) . W a t t l e t a n n i n i s being used i n A u s t r a l i a as a waterproof adhesive i n the manuf a c t u r e of plywood and p a r t i c l e b o a r d (_19»_20) · P i n u s r a d i a t a and ponderosa p i n e bark e x t r a c t s have a l s o been i n v e s t i g a t e d as p o s s i b l e bonding agents f o r p a r t i c l e b o a r d (2»_9). T h i s paper i s a r e p o r t on p r e l i m i n a r y s t u d i e s e v a l u a t i n g the s u i t a b i l i t y of bark e x t r a c t s from f o u r West Coast softwood s p e c i e s as bonding agents f o r p a r t i c l e b o a r d . The barks i n v e s t i g a t e d i n clude w h i t e f i r (Abies c o n c o l o r ) , ponderosa p i n e (Pinus ponderosa) Douglas f i r (Pseudotsuga m e n z i e s i i ) and western hemlock (Tsuga heterophylla). M a t e r i a l and P r e p a r a t i o n The bark was a i r d r i e d and then put through a hammermi 11 u s i n g a 1/16-inch screen. The e n t i r e product was used i n the p r e p a r a t i o n of e x t r a c t i v e s . Wood p a r t i c l e s used f o r the outer l a y e r s were comprised of t h a t f r a c t i o n of Pallmann m i l l e d p a r t i c l e s which passed a 10-mesh screen and were r e t a i n e d on a 16-mesh screen w i t h random lengths to 1/2-inch. Wood f l a k e s which remained on a 10-mesh screen and between 0.008 and 0.012-inch t h i c k , w i t h random lengths to 3/4i n c h and random w i d t h to 1/4-inch were used as core i n the t h r e e l a y e r board. The wax used contained 50 percent s o l i d s and the source of formaldehyde was powdered paraformaldehyde. Bark E x t r a c t s Each of the barks used i n the present i n v e s t i g a t i o n was analyzed f o r e x t r a c t i v e content and the r e s u l t s are summarized i n Table I. Table I I shows the t a n n i n and r e a c t i v e polyphenols formaldehyde-hydrochloric a c i d contents ( 5 ) . On the b a s i s of data from p r e v i o u s l y r e p o r t e d experiments r e l a t i n g t o use of aqueous sodium carbonate as an e x t r a c t a n t and the a d d i t i o n of s u l p h i t e s to i n c r e a s e e x t r a c t s t a b i l i t y , p u l v e r i z ed bark (1500 gms oven-dry b a s i s ) was e x t r a c t e d a t from 70° t o 80°C, a f t e r a d d i t i o n of 2 percent sodium carbonate (o.d. bark b a s i s ) (2,_9). The bark s l u r r y was h e l d a t the e x t r a c t i o n temperature f o r 30 minutes, then f i l t e r e d and washed a t the same temperature by s t i r r i n g the f i l t e r cake f o r 15 minutes. The wash l i q u o r , from which bark had been removed by f i l t r a t i o n was then used i n e x t r a c t i n g the second l o t of bark (1500 gms) and so on. A mixture o f 0.25 percent sodium b i s u l p h i t e and 0.25 percent sodium s u l p h i t e (o.d. bark) was added t o the combined f i l t e r e d e x t r a c t s . The e x t r a c t s were concentrated under reduced p r e s s u r e at 35° t o 55°C i n a c i r c u l a t i n g vacuum evaporator. The e x t r a c t


f

14.

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was concentrated to 36 to 47 percent s o l i d s and s t o r e d i n a r e f r i g e r a t o r f o r f u t u r e use. The e x t r a c t y i e l d s and percent r e a c t i v e p o l y p h e n o l i c s are summarized i n Table I I I .


Three-Layer

Particleboard

Weighed amounts of prepared wood p a r t i c l e s were tumbled i n a l a b o r a t o r y blender and a wax emulsion (1 percent s o l i d s oven-dry b a s i s ) was added from a spray gun, a f t e r which concentrated bark e x t r a c t (8 percent s o l i d s oven-dry b a s i s ) was sprayed onto the tumbling mixture. Powdered paraformaldehyde (1 percent oven-dry b a s i s ) was added slowly to the s t i r r e d mixture. Boards of 3/8- and 3/4-inch i n thickness were prepared, u s i n g a 10-1/2 square i n c h wooden deckle box. Weighed amounts of bark e x t r a c t t r e a t e d wood p a r t i c l e s were h a n d - f e l t e d i n t o the forming frame t o form the f a c e , and t h i s was followed by h a n d - f e l t i n g the core f u r n i s h and subsequently h a n d - f e l t i n g the p a r t i c l e s f o r the t h i r d l a y e r . The coarse core comprised of about 50 percent of the t o t a l f u r n i s h . The mat was prepressed at 250 p s i f o r 1 minute. A f t e r removing the frame, the compressed mat was put between aluminum cauls and t r a n s f e r r e d to the hot p r e s s . The 3/8-inch board was pressed f o r 3 minutes a t 180°C p l a t e n temperature, i n c l u d i n g 30 seconds f o r c l o s i n g to stops. The 3/4inch boards were pressed f o r 8 minutes at 180°C. The d e n s i t y of the boards was v a r i e d by adding v a r i o u s q u a n t i t i e s of core m a t e r i a l , w h i l e keeping the weight of the outer l a y e r s constant. Testing A f t e r the boards were removed from the hot press they were conditioned a t room temperature f o r s e v e r a l days b e f o r e t e s t i n g . Three 2 χ 10-inch s t r i p s were cut from each board f o r determining breaking l o a d . A 0.24-inch-per-minute l o a d i n g r a t e and a 9-inch span were used. Thickness of each s t r i p was measured at the p o i n t where the l o a d was to be a p p l i e d , and a f t e r the s t r i p had been hroken each h a l f was cut i n t o two 2 χ 2-inch samples. These samples were used to o b t a i n data necessary f o r determination of oven-dry d e n s i t y , water a b s o r p t i o n and thickness s w e l l i n g , i n t e r n a l bond, and a 2-hour b o i l i n g - i n - w a t e r thickness s w e l l i n g t e s t . A 3 χ 6-inch sample was used f o r the l i n e a r expansion t e s t ; i n t h i s i n s t a n c e the sample was conditioned a t 50 percent R.H. measured and then exposed f o r 3 weeks a t 90 percent R.H. and then measured again. R e s u l t s and D i s c u s s i o n As i n d i c a t e d i n Table I, the hot-water s o l u b l e e x t r a c t i v e content of the v a r i o u s barks v a r i e d from 12.9 to 14.7 percent. The hot-water s o l u b l e contains t a n n i n , which i s normally d e t e r -


WOOD TECHNOLOGY:

238

Table I .

CHEMICAL

ASPECTS

Percentage E x t r a c t i v e s (oven-dry b a s i s ) Solvent


Bark White F i r Ponderosa Pine Douglas-fir Western Hemlock

Table I I .

Bark

Ether

Hot Water

Ethanol

8.4 5.5 8.0 4.9

12.9 14.0 13.9 14.7

19.0 20.5 21.6 20.4

Percentage Tannin and Polyphenols (oven-dry b a s i s )

Soluble Solids

White F i r Ponderosa Pine Douglasfir Western Hemlock

Tannin (hide-powder)

NonTannin

Form-HCl Phenolics

% FormHCl Phenolics of SS

12.7

7.7

5.0

10.5

82.6

15.3

7.5

7.8

10.2

66.6

14.1

8.0

6.1

10.0

70.9

14.7

8.1

6.6

10.7

72.7

Table I I I .

Y i e l d of E x t r a c t and Percent R e a c t i v e P h e n o l i c s

Bark

Yield (o.d. bark) %

White f i r Ponderosa P i n e Douglas-fir Western Hemlock

Soluble Solids %

Form-HCl Reactive Phenolics %

17.5 16.1 17.9

42.6 45.0 43.3

79.4 69.2 69.7

17.6

43.0

78.3



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mined by the ALGA hide-powder method (5). Tannin i s a polyphenolic polymer and s i n c e some p h e n o l i c s do not r e a c t w i t h hide-powder, a more r e l e v a n t measurement would be the r e a c t i o n i n which con densed t a n n i n and other p h e n o l i c s undergo the S t i a s n y formaldehydeh y d r o c h l o r i c a c i d condensation ( 5 ) . Table I I shows t a n n i n content according t o the hide-powder and S t i a s n y ' s method. The r e a c t i v e p h e n o l i c s v a r i e d from 66.6 t o 82.6 percent of the t o t a l s o l u b l e s o l i d s . Table I I I g i v e s the average y i e l d s of e x t r a c t s o l i d s obtained from each of the barks on e x t r a c t i o n w i t h d i l u t e sodium carbonate as e x t r a c t a n t . The t o t a l y i e l d s v a r i e d from 320 t o 360 pounds per ton of dry bark. The amount of r e a c t i v e p h e n o l i c s i n each of the bark e x t r a c t s v a r i e d from 69.2 t o 79.4 percent based on t o t a l s o l u b l e s o l i d s . Table IV g i v e s the average p h y s i c a l property values of the t h r e e - l a y e r b a r k e x t r a c t bonded p a r t i c l e b o a r d s , i n v o l v i n g each of the four coniferous bark e x t r a c t s . The r e s u l t s reported are the averages of 9 boards, v a r y i n g i n d e n s i t y from 0.68-0.76 f o r each bark e x t r a c t bonded p a r t i c l e b o a r d . The p r o p e r t i e s of a l l boards meet medium d e n s i t y p a r t i c l e b o a r d s p e c i f i c a t i o n s i n bending ( i . e . , 2500 p s i ) and i n t e r n a l bond ( i . e . , 60 p s i ) . A d d i t i o n a l l y , these boards have low water a b s o r p t i o n and t h i c k n e s s s w e l l i n g v a l u e s , together w i t h r e l a t i v e l y good s t a b i l i t y i n t h i c k n e s s s w e l l i n g a f t e r the 2-hour b o i l i n g - i n - w a t e r t e s t . These p r o p e r t i e s i n d i c a t e t h a t the bark e x t r a c t bonded boards may be c l a s s i f i e d as waterproof comparable t o s y n t h e t i c p h e n o l i c bonded p a r t i c l e - ^ board (Type 2, C l a s s 2 (phenolic-bonded) Density Β (37-50 l b s / f t ) by Commercial Standard CS236-66 Mat-formed Wood P a r t i c l e b o a r d ) . T h i s suggests t h a t the b a r k e x t r a c t responds as a h i g h l y r e a c t i v e polyhydroxy p h e n o l i c compound comparable t o r e s o r c i n o l s i n c e g e l a t i o n time f o r each i s immediate i n the presence of formaldehyde h y d r o c h l o r i c a c i d a t room temperature (14). Catechin i s among the p r i n c i p a l p o l y p h e n o l i c monomers i n white f i r and western hemlock barks ( 1 0 , ^ , ^ 3 ) . Q u e r c i t i n occurs i n ponderosa p i n e b a r k , w h i l e d i h y d r o q u e r c i t i n i s found i n Douglas f i r bark (16*17). The s t r u c t u r e of these compounds are as follows:

OH

0 Dihydro quercitin



3562

2804

0.72

0.72

0.71

Ponderosa P i n e

Western Hemlock

Douglas-fir

2804

3104

0.72

White F i r

15.3 18.7

116

15.5

132 157

14.4

141

3/4-Inch

27.0

138

2584

0.70

Douglas-fir

24.5 25.6

0.69

Western Hemlock

165

20.7

5.0

4.2

4.1

33.7

28.7

28.6

24.7

36.8

3.3

33.4

0.21

0.14

0.21

0.22

0.26

0.19

0.24

35.8

9.8

0.26

(%)

Linear Expansion

30.8

2-hr B o i l Test Thickness Swelling (%)

9.5

8.7

7.6

24-hr Water Immersion A b s o r p t i o n Thickness Swelling (%) (%)

149

2572

0.72

Ponderosa P i n e

179

Internal Bond (psi)

2564

2879

Modulus of Rupture (psi)

0.71

Density O.D. (gm/cc)

White F i r

Bark E x t r a c t s

3/8-Inch

Table IV. Three-Layer Bark E x t r a c t Bonded P a r t i c l e b o a r d


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As i n d i c a t e d by the s t r u c t u r e s of these molecules, the A r i n g con t a i n s r e s o r c i n o l p h e n o l i c h y d r o x y l s , w h i l e the Β r i n g contains the c a t e c h o l o r adjacent p h e n o l i c hydroxy groups, both o f which would be expected t o be h i g h l y r e a c t i v e i n r e s i n formation. T h i s h i g h r e a c t i v i t y would a l s o h o l d f o r the condensed tannins present i n the bark e x t r a c t , s i n c e they are polymeric f l a v o n o i d s (14).


Summary and Conclusion The y i e l d of bark e x t r a c t s from four West Coast c o n i f e r o u s barks v a r i e d from 320 to 370 l b s of e x t r a c t s o l i d s per ton of oven-dry bark. When a s m a l l amount of paraformaldehyde i s added to wood p a r t i c l e s which have been sprayed w i t h bark e x t r a c t and processed i n t o board, formaldehyde r e l e a s e d during the hot-press c y c l e r e a c t s i n s i t u w i t h p o l y p h e n o l i c compounds present i n the e x t r a c t and forms a b o i l - p r o o f bonding agent. The bark e x t r a c t bonded p a r t i c l e b o a r d s met s p e c i f i c a t i o n s r e q u i r i n g the inherent d u r a b i l i t y provided by p h e n o l i c adhesives. These products are used f o r f l o o r decking f o r modular homes, s p e c i a l i z e d f u r n i t u r e uses, home s i d i n g , garage door panels and more r e c e n t l y , as a w a l l and r o o f sheathing and s i n g l e l a y e r f l o o r decking i n c o n v e n t i o n a l home c o n s t r u c t i o n . Thus, phenol and p h e n o l - r e s o r c i n o l modified r e s i n s can be replaced by a low-cost bark product. This use of bark would be a p r o f i t a b l e o u t l e t f o r bark r e s i d u e s and could l e a d t o v i r t u a l independence o f the wood p a r t i c l e b o a r d i n d u s t r y from the petrochemical i n d u s t r y .

Literature Cited 1. 2.

Anderson, A. B. Econ. Bot. (1967) 21(1):24-27. Anderson, Α. Β., A. Wong and Κ. T. Wu. For. Prod. J . (1974) 24(8):48-53. 3. Anon. For. Prod. J . (1974) 24(1):7. 4. Brandt, T. G. Tectona (1953) XLII p. 137-150. 5. Chang, Y. and R. L. Mitchell. TAPPI (1955) 38(5):315-320. 6. Corder, S. E . , T. C. Scroggins, W. E. Meade and G. D. Everson. Wood and Bark Residues in Oregon (1972) Res. Paper 11, Oregon State University Forest Products Lab., Corvallis, 16 pp. 7. Dost, W. A. For. Prod. J . (1965) XV(10):450-452. 8. Hall, J. A. Utilization of Douglas-fir bark. (1971) Pac. N.W. Forest & Range Exp. Sta. For. Serv. USDA Portland, Oregon, pp. 84-85. 9. Hall, R. Β., J . A. Leonard and G. A. Nicholls. For. Prod. J . (1960) 10(5):263-272. 10. Hergert, H. L. and E. F. Kurth. TAPPI (1953) 36(3):137-144. 11. Hergert, H. L. and E. F. Kurth. Jour. Org. Chem. (1953) 18(5):521-529. 12. Hergert, H. L. "Economic importance of flavonoid compounds


242

13. 14. 15. 16.


17. 18. 19. 20.

WOOD TECHNOLOGY: CHEMICAL ASPECTS in Geisman, T. A. The Chemistry of Flavonoid Compounds." pp. 553-593, Ν. Y. MacMillan. (1962) Herrick, F. W. and L. H. Bock. For. Prod. J . (1958) 8(10): 269-274. Herrick, F. W. and R. J . Conca. For. Prod. J . (1960) 10(7): 361-368. H i l l i s , W. F. "Wood Extractives." pp. 196-198, Academic Press, New York (1962). Kurth, E. F. and J . K. Hubbard. Ind. Eng. Chem. (1951) 43, 896-900. Kurth, E. F. TAPPI (1953) 36(7):119A-122A. Maclean, H. and J . A. F. Gardner. Pulp and Paper Mag. of Canada (1952), pp. 111-114. Plomely, K. F. CSIRO Div. of For. Prod. Tech. Paper No. 46, (1966) Melbourne, Australia, pp. 16-19. Plomely, K. F. and A. Slashevski. CSIRO For. Prod. Newsletter, No. 363 (1969), Melbourne, Australia.


Wood Technology: Chemical Aspects

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