Can the Cell Wall be Stabilized?

Transcription

1 1988. In: Suchsland, Otto, ed. Wood science seminar 1: Stabilization of the wood cell wall; December 15-16; East Lansing, MI. East Lansing, MI: Michigan State University: Can the Cell Wall be Stabilized? Roger M. Rowell Forest Products Laboratory* Forest Service U.S. Department of Agriculture Madison,WI For many applications, wood is a preferred material because it is economical, available, renewable, low in processing energy, strong, and aesthetically pleasing. Though wood is one of the few natural products used throughout history with almost no modification of its properties, the tendency to change dimensions with changes in moisture content has caused problems with wood in use. Wood is a three-dimensional, polymeric composite made up primarily of cellulose, hemicelluloses, and lignin. These polymers make up the cell wall and are responsible for most of the physical and chemical properties of wood. Wood changes dimensions with changing moisture content because the cell wall polymers contain hydroxyl and other oxygen-containing groups that attract moisture through hydrogen bonding. Moisture, occupying space within the polymers, swells the cell wall and the wood expands in direct proportion to the moisture sorbed until the cell wall is saturated with water (fiber saturation point). Water beyond this saturation point is free water in the void structure and does not contribute to further expansion. This process is reversible, and the wood shrinks as it loses moisture below the fiber saturation point. Swelling of wood in contact with moisture exerts very large forces. Stamm(1) calculated the theoretical swelling pressure for wood to be 1,630 atmospheres or 24,000 lb/in 2. Tarkow and Turner (2) measured the actual swelling pressure of compressed yellow birch and found about half the calculated value. Nevertheless, when wood in use swells, the forces developed can create serious problems. The ancient Egyptians used this swelling force to quarry large stones by drilling holes for the desired size rock, driving dry wooden stakes into these holes, then adding water which caused the wood to swell and crack the rock from the face of the mountain. If the wood cell wall could be stabilized against changing dimensions as the moisture content of its environment changed, it could find many new applications. Can the cell wall be stabilized against sorption and desorption of moisture? Such a straight forward, basic question should have an easy answer, but it does not. Complete stabilization has only been accomplished through the process of petrification, which is rather time consuming and expensive. In an attempt to answer the the question, four things need to be considered: (I) What needs to be stabilized, (II) Methods of stabilization, (111) Extent of stabilization achieved, and (IV) How much stability is needed. * This article was written and prepared by U.S. Government employees on official time, and it is therefore in the public domain and not subject to copyright.

2 54 What needs to be stabilized? There are two areas to be considered in a discussion of what needs to be stabilized in wood: (A) the matrix the cell wall polymers are in and (B) the cell wall polymers themselves. Cell Wall Matrix Cellulose, the hemicelluloses, and lignin are distributed Figure 1: Representation of the microfibril orientation for each cell wall layer of throughout the wood cell wall and Scotch pine with the chemical composition as a percent of total weight each layer of the cell wall varies in content of these three polymers. The middle lamella is mostly lig nin; however, the bulk of the lig nin content in the whole cell wall is in the S 1 and S 2 layers. The S 2 layer is the thickest layer and has the highest carbohydrate content. Cellulose, along with minor amounts of hemicellulose and lig nin, constitute the microfibrils which are oriented in different directions in each cell wall layer (Figure 1). Microfibrils in the S 2 layer are nearly parallel to the cell axis and swell mainly in the transverse direction as moisture increases. Microfibrils in the S 1 and S 3 layers are oriented more perpendicular to the cell axis and tend to restrain transverse swelling of the cell wall in much the same way as cross laminated veneers do in plywood. These restraining forces, which are due to the matrix the cell wall polymers are in, can be altered drastically when wood is chipped, flaked, or fiberized before composite formation and cause the composite to swell more than solid wood where the cell wall matrix is undisturbed. The amount of swelling that occurs due to the hygroscopic expansion of the three major cell wall polymers and the wall matrix is dependent on the density of the cell wall, i.e., the amount of sorbable material. Latewood cell walls have a higher density than early wood walls and therefore swell more (in southern pine, latewood has a density of 0.70 g/cm 3 while ear lywood has a density of 0.33 g/cm 3 ). Latewood cell walls will swell about twice as much as springwood cell walls. Since the tangential cell wall is thicker than the radial wall, more tan gential swelling occurs than radial swelling (in most species radial changes are about 40 to 70 percent of tangential). Cell Wall Polymers Cellulose, the hemicelluloses and lignin sorb moisture to different extents. The hemicelluloses are more hygroscopic than cellulose which is more hygroscopic than lignin (3) (Figure 2). Sorption of moisture is mainly due to hydrogen bonding of water molecules to the hydroxyl groups in the cell wall polymers. The hydroxyl content for each polymer is different.

3 55 Figure 2: Sorption isotherms for wood hemicelluloses (HEMI), holocellulose For example, in southern pine, (HOLO), Klasonlignin (LIG), andwood (WOOD). which is 27.9% lignin, 67.0% holocellulose (48.1% cellulose, 9% hemicellulose pentosans, 9.9% hemicellulose hexosans), the lignin contains 10.2% hydroxyl groups (assuming 1.16 hydroxyl group per C 9 unit, molecular weight ). The cellulose contains 22.6% hydroxyl groups (assuming 3 hydroxyl groups per C 6 unit, molecular weight ), and the hemicelluloses contain 8.1% hydroxyl groups (assuming 2 hydroxyl groups per C 5 unit, molecular weight and 3 hydroxyl groups per C 6 unit.) Not all of these hydroxyl groups are accessible to moisture. Sumi et al (4) showed that only 60 percent of the total hydroxyl groups in spruce wood and 53 percent for birch wood were accessible to tritiated water. Stamm (1) estimated that 65 percent of the cellulose in wood was crystalline and therefore probably not accessible to water. Browning (5), using Eucalyptus regnans wood, studied the fractional contribution of each cell wall polymer to moisture sorption. Of the total water sorption, 47 percent was due to cellulose, 37 percent to hemicelluloses, and 16 percent to lignin. This means that lignin (non-crystalline and probably totally accessible), the hemicelluloses (which are all non-crystalline and nearly totally accessible), the non-crystalline portion of cellulose, and the surfaces of the cellulose crystallites are responsible for moisture uptake by the wood cell wall. In summary, the hygroscopicity of the cell wall polymers attract moisture from the wood's environment which results in swelling of both the cell wall polymers and the matrix they are in. Methods of Stabilization Approaches to the stabilization of the cell wall based on the above discussion would include (A) stabilization of the cell wall matrix in such a way as to restrain the cell wall polymers from swelling, (B) reducing the hygroscopicity of the cell wall polymers so they do not attract as much moisture, or (C) bulking the cell wall polymers to maintain the green or wet volume so moisture does not cause additional swelling to occur. Methods of stabilizing the wood cell wall have been recently reviewed (6-9).

4 56 Stabilization of the Cell Wall Matrix If microfibril units are chemically bonded together (crosslinked), the bonds restrain the units from swelling when moisture is present. One of the most widely studied is the two step reaction of formaldehyde with cell wall hydroxyl groups. The reaction is usually catalyzed with a strong acid (10-12). Crosslinking can take place between two hydroxyl groups on a single sugar residue, on two different sugar residues within a single carbohydrate polymer, on hydroxyl groups within the lignin molecule, or between lignin and carbohydrate hydroxyls. The possible crosslinking combinations are large and theoretically, all of them are possible. The desired reaction is between two different cellulose chains which restrains the cell wall matrix from swelling. Reducing Hygroscopicity Since it is the hydroxyl groups in the cell wall polymers which are primarily responsible for attracting moisture, the optimum method of stabilization would be to remove the hydroxyl groups and thus remove the sites for hydrogen bonding. This is theoretically possible through reductive reactions; however, the cell wall would be nearly destroyed by the severity of the reaction conditions. One method that has been widely studied to reduce hygroscopicity is through heat treatments (13-15).The mechanism of stability is based on the thermal degradation of the hemicelluloses (which are the most hygroscopic polymers in the cell wall and the most susceptible to thermal degradation) and the thermal rearrangement of lignin. The hemicelluloses degrade to furan type compounds which polymerize under heating to produce waterinsoluble polymers. The lignin is also partially degraded by heating but mainly undergoes thermal flow. Heating is usually done in the absence of oxygen to avoid as much oxidation as possible. BulkingCellWallPolymers By adding chemicals within the cell wall which occupy space that water would occupy if the wood sorbed moisture, it is possible to keep the cell wall bulked in the green or wet volume. When this bulked cell wall comes in contact with moisture, very little additional swelling can take place because the cell matrix will restrain swelling beyond the green volume. (There are, however, chemicals that can swell beyond the green volume.) In order to occupy all the sites accessible to moisture, the cell wall bulking chemical must be as small as the water molecule and be able to hydrogen-bond as strongly. Three general types of bulking treatments have been studied: non-bonded and leachable; non-bonded and non-leachable; and bonded and non-leachable. Examples of non-bonded and leachable are treatments of the cell wall with soluble chemicals such as sugars(16), various water soluble salts(17),and polyethylene glycol.(18,19). The wood is soaked in these chemcials, usually green, and the chemicals exchange the water in the cell wall and are deposited in the wall. These types of treatments are leachable when the

5 57 treated wood comes into contact with liquid water and, for the most part, would not be of interest in composite products. This is not only due to the leachable nature of the bulking chemicals but also because these treatments usually make the cell wall more hygroscopic and the chemicals interfere with adhesive bonding. Examples of non-bonded and non-leachable treatments include water soluble phenol-formaldehyde resin-forming compounds in which the chemicals are not attached or bonded to the cell wall polymers but form insoluble polymers which are less hygroscopic than the cell wall polymers and not leachable with water (20). One problem with this type of treatment is that, in general, when monomers polymerize, the molecule tends to shrink, which leaves less bulk in the wall after polymerization than before polymerization. Examples of bonded and non-leachable treatments include all of the organic chemicals which can react with cell wall hydroxyl groups reducing hygroscopicity, bulk the cell wall and are leached out with water or organic solvents. Chemicals used to react with cell wall hydroxyl groups can be divided into two classes: those that react with a single hydroxyl group (single site addition) and those that react with a hydroxyl group and then polymerize (polymerization addition). The single-site reaction most studied is acetylation (7,8) It has been done with both acetic anhydride and ketene. Other single-site reactive chemicals include other anhydrides, methylating chemicals, alkyl chlorides, and various aldehydes( 7). Polymerization reactions most studied are epoxides (21) and isocyanates (22). Other chemicals included in this type of reaction include acrylonitrile and ß-propiolactone(7). Extent of Stabilization Achieved A variety of terms is used to describe the degree of dimensional stability given to wood by various treatments: swelling percent, dimensional stabilization efficiency, antiswelling efficiency, and percent reduction in swelling. Stamm (1) introduced the term antishrink efficiency (ASE) which many people use to describe improvements in reducing both shrinking and swelling. Generally, treatments to stabilize the cell wall do not give equal efficiencies to both reducing swelling upon wetting and shrinking upon drying. For this discussion, we will use the term stabilization efficiency (SE) and refer to only reductions in swelling in water vapor or liquid water. SE is calculated as:

6 58 where S = volumetric swelling coefficient, V 2 = wood or cell wall volume after humidity conditioning or wetting with water, and Vi = wood or cell wall volume of ovendried sample before conditioning or wetting. Then where SE = reduction in swelling resulting from a treatment, S 2 = treated volumetric swelling coefficient, Si = untreated volumetric swelling coefficient. One problem, among many, which arises when trying to compare data from different researchers is that there is no standardized test procedure for determining SE. Some use a one hour water boiling, some use a 24 hour or 5 day room temperature water soak, and some use changes in relative humidity at a given temperature. Table 1 gives a comparison of SE values for the Table 1 - Stability of pine wood to dimensional changes due to moisture sorption (5 days in liquid water, 24 C) three major stabilzation methods described in the Stabilization Method Stabilization Weight of Chemical Uptake Stabilization Efficiency previous section. Formal- 4 4 dehyde does react with hydroxyl groups to reduce of cell wall matrix hygroscopicity as well as Formaldehyde Crosslinking some bulking, but since low levels of chemical bonding results in high SE values, Reducing hygroscopicity Heat Treatments the mechanism of stabiliza- Bulking cell wall polymers tion is definitely based on Polyethelene glycol crosslinking structural cell Water soluble phenol- wall units. A maximum of formaldehyde resins 90 percent SE is achieved at Acetylation only 7 percent chemical weight gain and it is interesting to note that higher chemical weight gains do not increase SE. The fact that 100 percent SE is not achieved by crosslinking shows that fixing the cell wall matrix alone is not enough to give complete stabilization.

7 59 The SE resulting from heat treatments depends on the conditions of heating. Heating wood, in the absence of oxygen, at 280 to 350 C for 10 to 20 minutes, results in an SE of about 40 percent (Table 1). The SE value can be increased to 65 percent when wood is heated at 320 C for 1 hour. This shows that the hygroscopicity of the cell wall polymers has been reduced. Chemical analysis shows a reduction of the hemicellulose content, and electron micrographs show smooth spots on the surface of the cell wall which is probably lignin. Electron micrographs also show significant cell wall damage. As with formaldehyde crosslinking, SE for heat treatment is not 100 percent. Removing most of the hemicellulose from the cell wall and moving the lignin by thermal flow does not completely stabilize the cell wall. Bulking the cell wall with polyethylene glycol, without lowering hygroscopicity, does result in about 80 percent SE (Table 1). An SE of 100 percent is not achieved because the polyethylene glycol molecule is much larger than the water molecule and cannot penetrate into all the spaces that water can. At an SE of 80 percent, the cell wall is in its fully swollen, set state. The additional swelling is due to expansion of the cell wall beyond its green volume. Bulking the cell wall with a water soluble phenol-formaldehyde solution followed by polymerization does reduce hygroscopicity and swell the cell wall to its green volume. Brown et al (23) impregnated ponderosa pine flakes with water soluble phenol-formaldehyde resin before making the flakes into boards. At 20 to 25% resin weight gain (based on the ovendry weight of flakes), a 3.17 mm impregnated flakeboard swelled in thickness only 9 to 18% as much as a control specimen in a water soaking test. The amount of swelling increased as the amount of impregnated resin was reduced. As with polyethylene glycol, water soluble phenolic resins do not give an SE of 180 percent even though the the cell wall is in its completely swollen state. Bulking to green volume alone, even with some reduction in hygroscopicity, does not give complete cell wall stabilization. Reacting the cell wall Table 2. Equilibrium moisture content (EMC, 27 C), fiber saturation point (FSP) and stability hydroxyl groups with efficiency (SE) of control and acetylated acetic anhydride (24 southern pine 27) or ketene (28) bulks Acetyl the cell wall and greatly Weight EMC at FSP SE Gain reduces hygroscopicity (%) 30% RH 65% RH 90% RH (%) (%) of the cell wall Control polymers. Acetylation Acetylated is a single site reaction, i.e., one hydroxyl group reacted per acetyl group added with no polymerization. At an approximate acetyl weight gain of 17 to 19 percent, the acetylated cell wall is expanded to its green, wet state.

8 60 Table 2 shows that both the equilibrium moisture content and fiber saturation point are reduced as the level of bonded acetyl increases. FOP pine, both are reduced by more than 50 percent at about 20 percent acetyl weight gain. SE is over 85 percent at this level of acetyl weight gain. Figure 3: Reduction in equilibrium moisture content (EMC) as a function of bonded acetyl contant for various acetylated lignocellulosic materials. If a plot is made of the reduction in equilibrium moisture content at 65 percent relative humidity of acetylated fiber compared to unacetylated fiber as a function of the bonded acetyl content (Figure 3), a straight line plot results. Even though the points shown in Figure 3 come from many different lignocellulosic materials (pine, aspen, bamboo, bagasse, pennywort, jute and water hyacinth), they all fit a common line. A maximum reduction in equilibrium moisture content is achieved at about 20 percent bonded acetyl. Extrapolation of the plot to 100 percent reduction in equilibrium moisture content would occur at about 30 percent bonded acetyl. Since the acetate group is larger than the water molecule, not all hygroscopic hydrogen bonding sites are covered. The fact that many different lignocellulosic materials fit a common graph (Figure 3) indicates that reducing moisture sorption and, therefore, achieving cell wall stability is controlled by a common factor. The lignin and hemicellulose content of all of the materials plotted in Figure 3 are different, but their cellulose content is similar. Earlier results showed that the bonded acetate was mainly in the lignin and hemicelluloses (29) and that isolated wood cellulose does not react with uncatalyzed acetic anhydride. It is possible the acetylation with uncatalyzed acetic anhydride controls the hygroscopicity of accessible lignin and hemicellulose hydroxyls but cannot get to the bulk of the hydroxyl groups in cellulose. It can be concluded that neither cell wall matrix fixation, reducing hygroscopicity of cell wail polymers, nor bulking the cell wall alone results in complete stabilization of the cell wall to changes in moisture content. How Much Stability is Needed? Up until now, the dimensional instability of wood composites has, for the most part, been tolerated by the wood industry, in some ways, with a rather fatalistic attitude, i.e., there is nothing that can be done about it anyway. This has forced many wood composites into second class markets where both cost and performance are low.

9 61 The question of dimensional stability of lignocellulosic materials is a key factor for the inclusion of whole wood fiber in high performance composites. High performance implies higher costs which will allow physical and chemical modifications to be done economically. The environment in which the composite must perform is a critical consideration in the approach to stabilization. A composite which must be stable to long seasonal changes in relative humidity may be stabilized in a completely different way than a composite made to perform in short rain cycles. For wood composites used in automobile parts, linear expansion is a critical factor. For exterior doors and windows, both linear and thickness swelling must be controlled to a high degree. In considering treatments to stabilize the wood cell wall, it is important to maintain the other positive properties of wood: high strength, pleasing color, good electrical insulation, non-toxicity, gluability, and paintability. A treatment that stabilizes wood to a very high degree may result in a furnish which is much more toxic to handle, more flammable, less compressible, causes more irreversible swelling (swelling due to the release of compressive stresses imparted to the composite during pressing) and has greatly reduced strength. For example, crosslinking with formaldehyde and heat treatments give very good levels of stabilization, but cause large reductions in abrasive resistance and toughness properties.(10,13) All treatments described in this review to stabilize the wood cell wall also result in a medium to high degree of resistance to biological, attack. Limiting the moisture content of a lignocellulosic cell wall is an effective mechanism to limit attack by termites, fungi, and bacteria. The problems of cell wall stability and biological resistance are usually studied separately by different research disciplines. Logically they should be studied as one problem. Another problem of diverse research disciplines is the consideration of materials for application in high performance composites. Metals, glass, plastics, natural polymers, and synthetic fibers are, for the most part, studied separately. For many applications, the optimum composite may be a combination of materials to achieve the desired properties and performance. Ultimately, all considerations come down to a single matter of economics: Can the added costs to improve properties be recovered in the selling price? Conclusions Can the cell wall be stabilized against sorption and desorption of moisture? Yes, to some degree, but not completely. We still do not know all of the factors which effect swelling. This means we have critical gaps in our understanding of mechanisms to control stabilization. The problem of lignocellulosic materials stabilization is being researched by many different people in many different countries and has been for many years. Perhaps in an age of decreasing minerals, depleting oil supplies, increasing energy costs and a desire to expand the utilization of renewable materials, the problem will now be solved.

10 62 References 1. Stamm, A.J. Wood and Cellulose Science. New York, USA Ronald Press Co. 549 pp. (1964).. 2. Tarkow, H. and HD Turner, The swelling pressure of wood, For. Prod. J. 8(7): (1958). 3. Christensen, G.N. and K.E. Kelsey, The sorption of water vapour by the constituents of wood, Holz als Roh-und Werkstoff. 17(5): (1959). 4. Sumi, Y., R.D. Yale, J.A. Meyer, B. Leopold, and B.G. Ranby, Accessibility of wood and wood carbohydrates measured with tritiated water, Tappi 47(10): (1964). 5. Browning, B.L., ed. The wood-water relationshp in The Chemistry of Wood,New York: Wiley (1963). 6. Rowell, R.M., and R.L. Youngs, Dimensional stabilization of wood in use, USDA For. Sew. Res. Note. FPL-0243, 8 pp. (1981). 7. Rowell, R.M. Penetration and reactivity of cell wall components, in Rowell, R.M., ed, Chemistry of Solid Wood. Advance. Chem Ser Washington, D.C.: American Chemical Society. Chapter 4, (1984). 8. Rowell, R.M., and W.B. Banks, Water repellency and dimensional stability of wood, USDA For. Sew. Gen. Tech. Rep. FPL-50, 24 pp. (1985). 9. Rowell, R.M., and P. Konkol, Treatments that enhance physical properties of wood. USDA. FS. For. Prod. Lab. Gen. Tech. Rep. FPL-GTR-55, 12 pp. (1987). 10. Tarkow, H., and A.J. Stamm, Effect of formaldehyde treatments upon the dimensional stabilization of wood, J. For. Prod. Res. Soc. 3(3): (1953). 11. Stamm, A.J. Dimensional stabilization of wood by thermal reactions and formaldehyde crosslinking, Tappi. 42(1):39-44 (1959). 12. Burmester, A., Tests for wood treatment with monomeric gas of formaldehyde using gamma rays, Holzforschung 21: (1967). 13. Stamm, A.J., H.K. Burr, and A.A. Kline, Heat stabilized wood, Ind. Eng. Chem. 38(6): (1946). 14. Seborg, R.M., H. Tarkow, H.J. Stamm, Effect of heat upon the dimensional stabilization of wood, J. For. Prod. Res. Soc. 3(3):59-67 (1953). 15. Stamm, A.J. Thermal degradation of wood and cellulose, Ind. Eng. Chem. 48(3): (1956).

11 Stamm, A.J., Treatment with sucrose and invert sugar, Ind. Eng. Chem. 29: (1937). 17. Stamm, A.J., Effect of inorganic salts upon the swelling and the shrinking of wood, Journal of Am. Chem. Soc. 56: (1934). 18. Stamm, A.J., Dimensional stabilization of wood with carbowaxes, For. Prod. J. 6(5): (1956). 19. Stamm, A.J., Effect of polyethylene glycol on the dimensional stability of wood, For. Prod. J. 9(10): (1959). 20. Stamm, A.J., and R.M. Seborg, Resin-treated wood, (Impreg). Rep Madison, WI: USDA For. Serv., For. Prod. Lab. (1943). 21. Rowell, R.M., and W.D. Ellis, Reaction of Epoxides with Wood, USDA For. Serv. Res. Pap. FPL 451, 41 pp. (1981). 22. Rowell, R.M., and W.D. Ellis, Bonding of isocyanates to wood, Am. Chem. Soc. Symp. Series 172: (1981). 23. Brown, F.L., D.L. Kenaga, and R.M. Gooch, Impregnation to control dimensional stability of particleboard and fiberboard, For. Prod. J. 16:45-53 (1966). 24. Klinga, L.O., and H. Tarkow, Dimensional Stabilization of hardboard by acetylation, Tappi 49:23-27 (1966). 25. Bekere, M, K. Shvalbe, and I. Ozolinya, Some factors affecting the quality of boards made from acetylated wood fibers, Latvijas Lauksaimniecibas Akademija. Raksti. 163:31 35 (1978). 26. Arora, M., M.S. Rajawat, and R.C. Gupta, Effect of acetylation on properties of particleboards prepared from acetylated and normal particles of wood, Holzforsch. Holzverwert. 33:8-10 (1981). 27. Rowell, R.M., A.M. Tillman, and R. Simonson, A simplified procedure for the acetylation of hardwood and softwood flakes for flakeboard production, J. of Wood Chem. and Tech. 6(3): (1986). 28. Rowell, R.M. R.H.S. Wang, and J.A. Hyatt, Flakeboards made from aspen and southern pine wood flakes reacted with gaseous ketene, J. of Wood Chem and Tech. 6(3): (1986). 29. Rowell, R.M., Distribution of reacted chemicals in southern pine modified with acetic anhydride, Wood Sci. 15(2): (1982).