SYNOPSIS MOISTURE SORPTION PROPERTIES OF ACETYLATED LIGNOCELLULOSIC FIBERS

Transcription

1 MOISTURE SORPTION PROPERTIES OF ACETYLATED LIGNOCELLULOSIC FIBERS ROGER M. ROWELL and JEFFREY S. ROWELL U.S. Department of Agriculture Forest Service Forest Products Laboratory 1 One Gifford Pinchot Drive Madison, Wisconsin SYNOPSIS Acetylation of spruce chips followed by fiberization shows that new moisture sorption sites are created during fiberization. Direct acetylation of wood fiber is the most efficient means of reducing equilibrium moisture content. 1The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. 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. The Publisher does not claim copyright in this article, which was written by employees of the U.S. Government as part of their official duties. 343 In: Schuerch, Conrad, ed. Cellulose and wood-chemistryand technology: Proceedings of the 10th cellulose conference; 1988 May 29-June2; Syracuse, NY. New York: John Wiley & Sons, Inc.; 1989:

2 344 ROWELL AND ROWELL Many different types of lignocellulosic materials, including hardwoods, softwoods, grasses, and water plants, acetylate at approximately the same rate using acetic anhydride alone. All types of acetylated lignocellulosic materials show a similar pattern in reducing moisture sorption as a function of acetyl content. These materials vary widely in their lignin, hemicellulose, and cellulose content. Acetylation may control moisture sorption in the accessible lignin and hemicellulose polymers of the cell wall but may not greatly affect cellulose sorption. INTRODUCTION Wood and other lignocellulosic materials are three-dimensional, polymeric composites 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 the lignocellulosic material. The lignocellulosic material 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 material expands in direct proportion to the moisture sorbed until the cell wall is saturated with water (fiber saturation point). Beyond this saturation point, water is present as free water in the void structure and does not contribute to further expansion. This process is reversible, and the lignocellulosic material shrinks as it loses moisture below the fiber saturation point. MOISTURE SORPTION Cellulose, the hemicelluloses, and lignin sorb moisture to different extents. The hemicelluloses are more hygroscopic than cellulose, which is more hygroscopic than lignin [1] (Fig. 1). Sorption of moisture is mainly due to hydrogen bonding of water molecules to the hydroxyl groups in the cell wall polymers.

3 ACETYLATED LIGNOCELLULOSICFIBERS 345 FIG. 1. Sorption isotherms for wood hemicelluloses (HEMI), holocellulose (HOLO), Klason lignin (LIG), and wood (WOOD) [1]. Not all of these hydroxyl groups are accessible to moisture. Sui et al. [2] showed that only 60% of the total hydroxyl groups in spruce wood and 53% for birch wood were accessible to tritiated water. Stamm [3] estimated that 65% of the cellulose in wood was crystalline and therefore probably not accessible to water. Browning [4], using Eucalyptus regnans wood, studied the fractional contribution of each cell wall polymer to moisture sorption. Of the total water sorption, 47% was attributed to cellulose, 37% to hemicelluloses, and 16% to lignin. This means that lignin (noncrystalline and probably totally accessible), the hemicelluloses (which are all noncrystalline and nearly totally accessible), the noncrystalline portion of cellulose, and the surfaces of the cellulose crystallites are responsible for moisture uptake by the woad cell wall. In summary, the hygroscopicity of the cell wall polymers attracts moisture from the lignocellulosic material's environment, which results in swelling of both the cell wall polymers and the matrix they are in. The lignocellulosic cell wall tendency to sorb moisture can be reduced by changing the hydrophilic nature of the cell wall polymers.

4 346 ROWELL AND ROWELL This can be done by a variety of methods, including the reaction of hydrophobic chemicals with the hydroxyl groups on the cell wall polymers [5,6]. One of the most studied chemical reactions is acetylation using either acetic anhydride or ketene. If the acetylation system does not include a strong catalyst or cosolvent, only the easily accessible hydroxyl groups will be acetylated. We developed an acetylation system that uses no strong catalyst or cosolvent and probably acetylates only easily accessible hydroxyl groups [7]. Acetylation of solid wood is limited by penetration of reacting chemicals to accessible cell wall hydroxyl groups. For this reason, the acetylation technology has been applied to lignocellulosic composites where the particle size is small enough for complete penetration. How small (i.e., chips. flakes, particles, or fiber) the particles need to be to achieve acetylation in the moisture-sensitive site is not known. Fiberboard has been targeted as the desired composite for acetylation [8], and whether chips can be acetylated and then fiberized or fiber must first be produced and then acetylated is not known. Also of interest is the effectiveness of acetylation in reducing moisture sorption in a variety of lignocellulosic fibers. The purpose of this research was twofold: (1) to compare the accessibility of hydroxyl groups to acetylation in wood chips and wood fiber and (2) to acetylate several types of lignocellulosic materials and determine equilibrium moisture content. EXPERIMENTAL Acetylation of Chips and Fiber Scandinavia spruce (Picea abies (Karst.)) was chipped into chips of approximately 20 by 15 by 5 mm. Some chips were fiberized using a 12-in laboratory disk refiner. Chips and fiber were ovendried for 12 h before reaction. Ovendry chips and fiber were placed in separate stainless steel mesh containers. Each container was dipped into a tank containing room-temperature acetic

5 ACETYLATED LIGNOCELLULOSIC FIBERS 347 anhydride (1 h for chips, 1 min for fiber), removed from the treating tank, and drained at room temperature for 5 min. The containers with the wetted chips or fiber were placed in a preheated (120 C) stainless steel reactor for 2 h, after which vacuum gas applied in the cylinder for 1 h at 120 C. A condenser on the bottom of the reactor collected excess acetic anhydride and byproduct acetic acid. The acetylated chips and fiber were then reovendried at 105 C for 12 h. The weight percent gain (WPG) due to acetylation was calculated based on the weight of ovendried unreacted chips or fiber. Following acetylation, some of the chips were refined in the laboratory disk refiner to produce fiber. A portion of this acetylated fiber was reacetylated as previously described. Several other lignocellulosic fibers were acetylated using this procedure; reaction times from 15 min to 4 h were used on Southern Pine, aspen, bamboo, bagasse, jute, pennywort, and water hyacinth. Acetyl content of all control and acetylated samples was determined by gas chromatography. Equilibrium Moisture Content Weighed, ovendried control and acetylated chips and fibers were placed in constant-humidity rooms at 30%, 85%. and 90% relative humidity (RH) levels and 27 C. After 21 days the lignocellulosic materials were reweighed and equilibrium moisture content (EMC) levels determined. Reduction in EMC for acetylated materials was compared with that of controls. RESULTS AND DISCUSSION Table I shows that spruce chips acetylated for 2 h had a WPG of 14.2 with an acetyl content of 15.8%. Acetylation of spruce fiber, under the same reaction conditions, resulted in a WPG of 22.5 with an acetyl content of 19.2%. The acetylated fiber produced by fiberizing acetylated chips had an acetyl. content of 15.4%, indicating that the fiberizing process did not result in losses of acetyl groups. Reacetylating the fiber from the acetylated chips resulted in

6 348 ROWELL AND ROWELL an additional 6.2 WPG with an acetyl content of 20.5%. TABLE I Acetyl Content and Equilibrium Moisture Content (EMC) of Control and Acetylated Spruce at Various Relative Humidity (RH) Levels

7 ACETYLATED LIGNOCELLULOSIC FIBERS 349 The EMC of the acetylated chips was higher at all RH conditions tested than the EMC of fiber acetylated after fiberizing control chips. Many new hydroxyl groups apparently became available for acetylation as a result of fiberization. When the acetylated chips were fiberized and reacetylated, their EMC values were the same as when fiber was acetylated directly. These results are somewhat surprising since acetic anhydride is an excellent wood-swelling and penetrating reagent. Because chip size was such that the entire chip was saturated with reacting chemical, acetylation should have occurred on all accessible hydroxyl groups in the hemicelluloses, lignin, and cellulose. All the lignocellulosic materials used in this study were easily acetylated. Plotting acetyl content resulting from the acetylation of aspen, Southern Pine, bagasse, bamboo, jute, pennywort, and water hyacinth as a function of time shows all. data points fitting a common curve (Fig. 2). A maximum WPG of about 20 was reached FIG. 2. Rate of acetylation of various lignocellulosic materials. Southern Pine; aspen; bamboo; bagasse; X, jute; +, pennywort; water hyacinth.

8 350 ROWELL AND ROWELL in a 2-h reaction time, and an additional 2 h increased the weight gain only by about 2% to 3%. Without a strong catalyst, acetylation using acetic anhydride alone levels off at approximately 20 WPG for the softwoods, hardwoods, grasses, and water plants. Table II shows the EMC of these lignocellulosic materials at 65% RH. A plot of the reductions in EMC at 65% RH of acetylated fiber referenced to unacetylated fiber as a function of the bonded acetyl content is a straight line plot (Fig. 3). Even though the points shown in Fig. 3 come from many different lignocellulosic materials, they all fit a common line. A maximum reduction in EMC is achieved at about 20% bonded acetyl. Extrapolation of the plot to 100% reduction in EMC would occur at about 30% bonded acetyl. Because the acetate group is larger than the water molecule, not all hygroscopic hydrogen-bonding sites are covered. The fact that EMC reduction as a function of acetyl content is the same for many different lignocellulosic materials (Fig. 3) indicates that reducing moisture sorption and, therefore, achieving cell wall stability are controlled by a common factor. The lignin, hemicellulose, and cellulose contents of all the materials plotted in Fig. 3 are different (Table III). Earlier results showed that the bonded acetate was mainly in the lignin and hemicelluloses [9] and that isolated wood cellulose does not react with uncatalyzed acetic anhydride [10]. The acetylation with uncatalyzed acetic anhydride may control the hygroscopicity of accessible lignin and hemicellulose hydroxyls but may not get to the bulk of the hydroxyl groups in cellulose that are accessible to water. CONCLUSIONS New accessible moisture-sensitive sites are exposed when small acetylated spruce chips are fiberized. This means that in the potential production of acetylated fiberboard. fiber must be acetylated to achieve maximum reduction in moisture uptake.

9 ACETYLATED LIGNOCELLULOSIC FIBERS 351 TABLE II Equilibrium Moisture Content (EMC) of Various Acetylated Lignocellulosic Materials (65% RH, 27 C)

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11 TABLE III Chemical Composition of Some Lignocellulosic Materials

12 354 ROWELL AND ROWELL Many different types of lignocellulosic materials, including hardwoods, softwoods, grasses, and water plants, follow a similar pattern of acetylation. Following acetylation, all types of fiber follow a similar pattern in the reduction of moisture sorption as a function of acetyl content. Because these materials vary widely in their lignin, hemicellulose, and cellulose content, because acetate is found mainly in the lignin and hemicellulose polymer, and because isolated cellulose does not acetylate by the procedure used, acetylation may be controlling the moisture sensitivity due to the lignin and hemicellulose polymers in the cell wall but not reducing the sorption of moisture in the cellulose polymer. REFERENCES

13 ACETYLATED LIGNOCELLULOSIC FIBERS 355