Strawboard from vapor phase acetylation of wheat straw
Greggory S. Karr, Xiuzhi S. Sun *
Kansas State Uni6ersity,Grain Science and Industry Department,Manhattan,KS66506,USA
Received 8 April 1999; accepted 25 June 1999
Abstract
Commercial ground wheat straw was used in a central composite response surface experimental design to examine four acetylating process variables: reaction temperature, reaction time, initial moisture content of straw, and the vapor flow rate of chemical reagent. The response variable was acetyl content determined as a function of straw weight gain. Diphenylmethyane diisocyante was used as a binder to prepare board samples with a hot press. Equilibrium moisture content (EMC) was determined at 65 and 90% RH at 27°C, and dimensional stability was determined using a humidity cycle of 30 – 90% RH at 27°C. ASTM D1037-93 standard method for a 3-point flex test was used to measure mechanical properties. The microstructures of both treated and untreated wheat straw and boards were observed with a scanning electron microscope. The vapor phase acetylation system used acetylated ground wheat straw to a 24% weight gain (dry weight basis). A mathematical model (R2=0.97) was developed to
predict the weight gain as a function of the four acetylation processing variables. The maximum reduction in all strawboard properties occurred at the highest weight gain (24%). The strawboard EMC decreased (30% maximum reduction) as weight gain increased at both 65 and 90% RH. The strawboard dimensional stability increased as the weight gain increased (maximum reductions of 80% in thickness swell and 50% in linear expansion). The initial mechanical properties of the strawboards decreased as the weight gain increased (maximum reductions of 64% in strength and 48% in stiffness). The density of the strawboards decreased as the weight gain increased (23% maximum reduction). SEM micrographs showed no physical evidence of structural damage to cell walls from the acetylation. © 2000 Elsevier Science B.V. All rights reserved.
Keywords:Wheat straw; Vapor phase; Acetylation; Dimensional stability; Mechanical properties
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1. Introduction
The commercial strawboard industry is rela-tively new in the United States. The cost of wood fiber is on the rise, and the demand is surpassing supply (Erwin, 1997). This has been the main driving force behind the search for alternative fiber sources in the panel board industry. Straw-board is a reconstituted lignocellulosic composite * Corresponding author. Tel.: +1-785-532-4077; fax: +
1-785-532-7010.
E-mail address:[email protected] (X.S. Sun)
Contribution No. 99-136-J from the Kansas Agricultural Experiment Station.
that uses ground wheat straw as a fiber source. Wheat straw has the same basic components as wood: cellulose, lignin, and pentosan (Rowell, 1992). Strawboards are now competing against reconstituted wood products, such as particle and fiber boards, in markets for floor underlays, furni-ture and cabinet construction.
Reconstituted lignocellulosic products have a well documented problem of water sorption and lack of dimensional stability. Youngquist et al. (1986a) stated that when a reconstituted product is made, a mat of lignocellulosic material is re-strained in a hot press. The heat, pressure, and binder ‘set’ the material in place but also impart compressive stresses in the product. When the reconstituted product absorbs moisture, two types of swelling occurs: reversible and irreversible swelling. Reversible swelling will occur in two directions; thickness swelling (parallel to sion) and linear expansion (normal to compres-sion). Irreversible swelling, which occurs mainly as thickness swelling, is the greater problem in reconstituted products. Irreversible swelling is caused by the release of compressive stresses that are in the board from the compression process.
One strategy to improve the water absorption and dimensional stability of these products is to chemically modify the cell wall polymers, which will modify the physical properties of the lignocel-lulosic composite. Rowell (1982) defined the chemical modification of wood as the formation of a covalent bond between a cell wall component and a single chemical reagent. Hydroxyl groups are the most abundant reactive sites on the cell wall polymers of a lignocellulosic material (Row-ell, 1982). Many reagents have been used to mod-ify the cell wall polymers with varying degrees of success, including anhydrides, acid chlorides, iso-cyanates, aldehydes, alkyl halides, lactones, ni-triles, and epoxides (Rowell, 1982). Acetylation has been the most widely used and successful chemical modification and is a single site reaction that replaces a hydroxyl group with an acetyl group. Acetyl groups are more hydrophobic than hydroxyl groups, therefore, replacing some of the hydroxyl groups with acetyl groups reduces the hydrophilic property of the cell wall polymers (Rowell, 1992). The acetyl group is also larger
than the hydroxyl group; therefore, the material undergoes permanent expansion. This increases the dimensional stability of the modified material because when moisture is sorbed, the swelling caused by water is only slightly higher than the permanent expansion caused by acetylation (Westin and Simonson, 1992). Rowell (1992) stated that the reduction in equilibrium moisture content (EMC) as a function of acetyl content is the same for a variety of lignocellulosic materials, and therefore, acetylation could be used to im-prove the dimensional stability of products made with a wide variety of lignocellulosic materials.
Several different methods of acetylation have been developed. One of the more commonly used procedures involved dipping the lignocellulosic material into acetic anhydride for 2 min, draining off excess reagent, and then placing the material in an oven at 120°C for a given reaction time (Rowell et al., 1986a). This procedure has been used to acetylate southern pine and aspen flakes (Rowell et al., 1986a); pine chips and jute cloth (Tillman, 1987); sugarcane bagasse fiber (Rowell and Keany, 1991); solid aspen wood and aspen fibers (Feist et al., 1991a,b); and solid southern yellow pine, Monterey pine and the isolated cell wall polymers, holocelluloses, cellulose, hemicellu-lose, and lignin (Rowell et al., 1994). Other simi-lar procedures have been used to acetylate aspen flakeboard (Youngquist et al., 1986a,b); oil palm stem and rubberwood blocks (Ibrahim and Mohd Ali, 1991); spruce veneers and Sugi sapwood (Imamura, 1993); and rubberwood flakes (Hadi et al., 1995). Vapor phase acetylation procedures also have been tested (Klinga and Tarkow, 1966; Rowell et al., 1986b,c; Tillman, 1987).
2. Materials and methods
Ground wheat straw was obtained from Natu-ral Fiber Board Inc. (Minneapolis, KS). The com-mercial wheat straw had been ground to a particle size range of 1.9 cm to dust with the nodes and residue grain removed. Reagent grade acetic an-hydride was obtained from Aldrich Chemical Company Inc. (Milwaukee, WI). A diphenyl-methyane diisocyanate resin binder, (Rubinate 1840), was obtained from ICI Polyurethanes (Geismar, LA).
2.1. Acetylation process
A process flow diagram of the acetylation sys-tem used in this research is presented in Fig. 1. Air was passed through drierite to remove mois-ture, the dried air then flowed through a flow meter, which regulated and measured the flow rate. The dry air then was bubbled through acetic anhydride (AA) in two saturation bottles in series that were housed in a constant temperature oven. The AA saturated air then flowed into the top of a 2-l glass reactor vessel (also in the oven) that contained ground wheat straw. Finally, the air stream with chemical residues exited the bottom of the reactor vessel, and then the oven and was neutralized by passing through a scrubber con-taining an aqueous sodium hydroxide solution.
2.2. Experimental design
Response surface methodology with a central composite design (CCD) was used in this study. The four variables were reaction time, reaction temperature, initial moisture content of the straw, and air flow rate, and the response was the extent of acetylation as determined by add-on weight calculated on a dry basis. The levels of each variable entered into the central composite design matrix are listed in Table 1. The straw weight gain response was analyzed with Statistical Analysis System software (SAS, 1992), and the RS-reg function was used to develop a model equation. The standard deviations (SD) of each dependent variable at the centrepoint of the CCD was re-ported as the SD for all data analysis.
3. Procedure
The moisture content (MC) of the straw was adjusted to the desired level two days before acetylation, and the straw was sealed in a plastic bag. For the experiment, the reactor vessel was filled with 150 g (dry weight basis) of MC-ad-justed straw. The vessel and the saturation bottles containing the AA were placed in the preheated oven. The air flow was set at a given rate and continued for the reaction time and then the saturation bottles were removed from the oven,
Table 1
Process variables levels of the response surface central composite design
Process variables Levels
−1.5 −1 0 +1 +1.5
5 10
Moisture content (%) 2.5 15 17.5
Oven temperature (°C) 72.5 80 95 110 117.5
1 2
0.5 3
Reaction time (h) 3.5
250
Air flow rate (cc/min) 500 1000 1500 1750
and the air flow passed directly through the reac-tor vessel. The oven temperature, then was dropped to 50°C, and the air flow rate was set to 500 cc/min to remove excess reagent and by-product and air flow was continued for 16 – 19 h. Sheen (1992) did preliminary studies on a 100 kg/day pilot-scale acetylation process and found that a flow of air at 50°C through the fiber was the most efficient method to remove the excess reagents.
The reaction vessel was removed from the oven, and the straw was collected and sealed in a plastic bag. Preliminary tests had shown that all the moisture was removed during acetylation. The straw moisture content then was adjusted to 7%. The bag then was sealed, shaken, and allowed to equilibrate overnight.
3.1. Strawboard preparation
Rubinate 1840 binder (5%) was mixed into the acetylated straw with a paddle mixer (Hobart model N50). The resinated straw was pressed into boards using a 15.2×15.2 cm mold and a hot press (Carver model 3889 auto C). A 15.2×15.2 cm×:0.64 cm board was produced. The press conditions were 2.68 MPa (389 psi) and 176.7°C (350°F) for 3 min. One treatment procedure was performed per day. A total of six untreated con-trol strawboards were made with the same press conditions and reported as control samples having 0.0% weight gain.
Because the compressibility of the acetylated straw was influenced by the degree of acetylation, the board samples had a range of thicknesses (0.6 – 0.9 cm). Samples were sanded (both sides) to a uniform thickness with a table belt sander. The
15.2×15.2 cm boards were sanded to 0.5490.05 cm in thickness then a 15.2×4.4 cm sample was cut off the board. This sample was used in the humidity cycle test. The remainder of the sample was sanded to 0.4990.02 cm in thickness and then cut into two 15.2×4.4 cm samples for test-ing of mechanical properties. It was assumed that boards with the initial thickness of 0.6 – 0.9 cm would have a relatively uniform cross-sectional density profile, and that sanding would not sig-nificantly influence board properties.
3.2. Acetyl weight gain
The amount of acetyl groups added to the straw during reaction was estimated by the weight gain during treatment, on a dry weight basis using Eq. (1),
AC=(AT−(BT−(WS×MC)))
/(WS−(WS−MC)) (1) where AC=weight gain, AT=weight of straw plus reactor vessel after treatment, BT=weight of straw plus reactor vessel before treatment, WS= weight of straw in reactor, and MC=moisture content of straw. The MC of the treated straw was assumed to be zero; therefore, all the weight gain during the treatment, calculated on a dry straw basis, was caused by acetylation.
3.3. Equilibrium moisture content
3.4. Dimensional stability
A humidity cycle test was used to determine the dimensional stability of the board samples. The relative humidity was cycled between 1 week at 90% and 1 week at 30% RH at 27°C. The sample length and thickness were measured by calipers, and the linear expansion and thickness swell were determined at each humidity transition for six cycles. Each value of percent linear expansion and thickness swell were the average of two measured points on one board specimen (15.2×4.4×0.54 cm).
3.5. Mechanical properties
Method (D1037-93) (ASTM, 1995) was fol-lowed for a 3-point flex test using an Instron universal testing machine with a crosshead speed of 5 mm/min and a 101.6 mm span. Modulus of rupture (MOR) and modulus of elasticity (MOE) then were calculated for each sample using equa-tions given in this method. Each value of MOR and MOE were the average of two board speci-mens (15.2×4.4×0.49 cm).
3.6. Board density
The samples were preconditioned at 65% RH and 25°C for 1 week prior to measurement. The board density of each sample was obtained by measuring the average thickness, width, and length with calipers to calculate board volume, and then dividing the mass of the sample board by the volume. The reported density is the average of the two board specimens from the mechanical properties test prior to testing
3.7. Scanning electron microscopy
The cross section of an individual piece of a straw was obtained by putting a wheat straw sample in a plastic drinking straw and filling it with ethanol. The ends of the drinking straw were clamped, and then the entire straw was dropped into liquid nitrogen. The frozen drinking straw then was cut with a razor blade, which produced a sharp cut normal to the direction of the straw
fibers. This procedure was done with samples of untreated wheat straw and acetylated wheat straw sample treated to a 19% weight gain. Strawboard samples about one cm3 made from the same two straw treatments were cleaned with distilled water in an ultrasonic cleaner, and then dried. All sam-ples were viewed with an E-Tech Auto Scan scan-ning electron microscope and micrographs were taken.
4. Results and discussion
4.1. Response surface experiment
The results of the response surface experiments are shown in Table 2. A mathematical model expressing weight gain of wheat straw with four variables gave an R-square of 0.966. All four variables were found to be significant at P\0.05 a-value from aF-test. This indicates that each of the four variables was significant in the acetyla-tion process. An F-test showed that all of the regression terms in the model were significant at a 0.05 a-value. This indicates that each of the three types of terms (linear, quadratic, and crossproduct) was significant to the model. The modeled surface had a saddle stationary point but no maximum or minimum point.
This model was used to predict weight gain for two sets of process variables. These two sets of variables then were tested experimentally using the same process and procedures. The predicted values and two experimental values are listed in Table 3. The model produced from this experi-ment was able to predict the weight gain very well. Statistically, the model fit the data, and the two predicted points fell close to the experimental values. This indicates not only that the level of acetylation can be predicted for this system but also that the vapor phase acetylation of wheat straw is a predictable and reproducible chemical reaction.
4.2. Equilibrium moisture content
at both RH’s (Table 4). The standard deviations (SD) reported in Table 4 are the SDs for the centrepoint of the CCD. The boards with the highest level of acetylation (24%) had about a 30% reduction in EMC at both humidities com-pared to the control boards. These results indicate that the straw became more hydrophobic (due to fewer hydrogen bonding sites) as it was acety-lated, which also agrees with trends reported pre-viously. Acetylation could reduce the EMC of pine chips by more than 60% at several different levels of relative humidity (Tillman, 1987). Similar
reductions in EMC of fiberboards, made from acetylated aspen fibers and bagasse fibers, were observed by Clemons et al. (1992) and Rowell and Keany (1991), respectively.
4.3. Dimensional stability
The changes in thickness swell and linear ex-pansion during the humidity cycle test are illus-trated in Fig. 2. The lines at 6.1, 12.9, and 18.1% weight gains are the averages of five samples within a 91.2% weight gain level that were
Table 2
Results of the response surface experiments
Reaction time Reaction temperature (°C)
Run number Moisture content Air flow rate (cc/min) Weight gaina
(h) (%) (%)
1 3 110 15 1500 24.2
110 15 1500 10.6
1
21 17.5 1000 12.8
2
aSD of centerpoint of CCD=1.26%.
Table 3
Comparison of weight gain predicted from the mathematical model and the experimental values
Experimental conditions
crease in dimensional stability of acetylated strawboards is the result of two effects. The acety-lated straw is more hydrophobic and sorbs less moisture, as indicated in the EMC results, and the acetyl group causes prebulking or permanent ex-pansion of the wheat straw’s cell wall, which will limit the swelling caused by water. Other re-searchers have reported similar results. Acetyla-tion was found to reduce the thickness swelling and linear expansion caused by water absorption of reconstituted boards made from southern pine and aspen flakes (Rowell et al., 1986a); southern pine, douglas fir, and aspen flakes (Rowell et al., 1986c); pine chips (Rowell et al., 1986b; Tillman, 1987); oil palm stem and rubberwood (Ibrahim and Mohd Ali, 1991); aspen fibers (Clemons et al., 1992); sugarcane bagasse fiber (Rowell and Keany, 1991); and rubberwood (Hadi et al., 1995).
4.4. Mechanical properties
The mechanical properties, MOR and MOE of the control and acetylated strawboards are listed in Table 5. The SDs reported in Table 5 are the SDs for the centrepoint of the CCD. Both the MOR and MOE decreased as the straw weight gain increased, with overall reductions of about 64 and 48%, respectively, compared to untreated control boards. These results indicate that the strawboards lost initial strength and stiffness as the straw was acetylated. The cause of this reduc-tion in mechanical properties is not understood completely. It could be due to some chemical change in the lignocellulose cell walls which affect the straw’s structural properties and then the strawboard’s strength and stiffness. Another pos-sible cause is a physical effect such as the adhe-sion of the binder to the acetylated straw’s surface or the loss of compaction of the straw during compression, which is shown by the board density data. Results of previous studies show similar reductions in mechanical properties. Youngquist et al. (1986b) measured the MOR and MOE of aspen flakeboard made with untreated and acety-lated flakes using ASTM method (D 1037). Re-sults showed that boards acetylated to a 20% acetyl content had a 37% reduction in MOR and grouped together. The line at 0.0% represents the
untreated control samples. The line at 24.2% weight gain was the result from one sample that had the highest weight gain produced by this experiment. The data show that as the level of acetylation increased the dimensional stability in-creased. The thickness swell of the strawboard with the highest level of acetylation, 24%, was less than a fifth of the swell of the untreated straw-board. The linear expansion of the strawboards followed the same trend; strawboard with 24% acetylation showed about one-half the linear ex-pansion of the untreated strawboard. This
in-Table 4
Fig. 2. Dimensional stability of control and acetylated strawboards during RH cycles. (a) Thickness swell; and (b) linear expansion.
11% reduction in MOE compared to the control boards. Two other groups used the same ASTM method to measure changes in mechanical proper-ties. Rowell and Keany (1991) reported that fiberboards made from acetylated sugarcane bagasse fibers had lower MOR and MOE than control boards. Westin and Simonson (1992) found that over a range of board densities the acetylated boards had lower MOR and MOE than the control boards. These authors believed that the initial loss of MOR was caused by the increased mass of the fibers from the addition of the acetate group. Because the acetylated fibers are heavier, a board with the same volume and density will contain fewer fibers than an untreated board. Boards made with fibers that are acety-lated to a 20% acetyl content will have only about 80% of the number of fibers of a control board with equal density and volume.
4.5. Board density
The board densities for the control and acety-lated strawboards are listed in Table 5. All the samples were made with the same press condi-tions. They were not pressed to a uniform thick-ness, but to a constant pressure. The density of the treated boards decreased with increasing weight gain. The samples around 7% weight gain appeared to have higher densities than the un-treated samples, but then density decreased to the
lowest value of 0.65 g/cm3
at a weight gain of 24%. The change in density could be explained by the straw’s permanent swelling caused by the bulky acetyl groups and/or the loss of compress-ibility with acetylation. This reduction in density with acetylation agrees with a study by Youngquist et al. (1986b), where acetylated and control flakes were pressed into boards with a density of 0.6418 g/cm3
. To achieve the same densities, the acetylated flakes required 8% more pressure than the control flakes under the same press temperature and time. The acetylated flakes were less compressible and had a much larger ‘spring back’ than the control flakes. Measure-ments of density profiles through the thickness of the board showed no large differences from the
Table 5
Mechanical properties of strawboards made from control and acetylated strawa
Weight gain MOR MOE (MPa)c Density
(%) (MPa)b (g/cm3)d
19.6 0.84
0.0 1930
15.9
7.1 1900 0.89
12.6
12.3 1890 0.81
9.2
18.8 1480 0.77
24.2 7.2 1020 0.65
Fig. 3. SEM micrographs of untreated and acetylated straw and strawboard. (a) Cross-section of untreated wheat straw at 200×
magnification; (b) cross-section of acetylated wheat straw (19%) at 200× magnification; (c) cross-section of untreated strawboard at 500×magnification; and (d) cross-section of acetylated strawboard (19%) at 500× magnification.
control boards. However, the control boards ap-peared to have a more compact structure with fewer voids than the acetylated boards.
4.6. Scanning electron microscope
Cross sections of the untreated and acetylated straw culm are shown in Fig. 3a and b, respec-tively. No damage is visible in the structure of the hypodermal cells (small circles) or parenchyma cells (large circles) from acetylation. Therefore, we concluded that the loss in mechanical properties of the acetylated strawboards was not due to
com-pacting to the same degree as the untreated straw, the resultant strawboard would have fewer fibers per unit volume and more void space. Fewer fibers per unit volume would reduce the mechani-cal properties, because the load per fiber would remain constant. With an increase in void space, the resin binder that is coated on the straw would have less contact area to bind to other straw segments. Fewer of these contacts, or cross-links, would produce lower mechanical properties in the resultant strawboard. These explanations were not tested in this study, and further research would be required to confirm them.
5. Conclusions
The results obtained from this research agree with previous published results for the acetylation of other lignocellulosic materials.
Wheat straw can be acetylated with acetic
an-hydride using a vapor phase process with no added catalyst. Four major acetylation process parameters (reaction temperature, reaction time, initial moisture content of straw, and vapor flow rate of the reagent) significantly affect the level of acetylation. The weight gain of the treated straw using this acetylation sys-tem and procedure can be predicted by the mathematical model developed from this research.
A strong trend was found for decreasing
equi-librium moisture content of the strawboard with increasing weight gain of the wheat straw. This indicates that wheat straw becomes more hydrophobic as it is acetylated.
The acetylated strawboards were found to be
more dimensionally stable than untreated strawboards. This was due to the straw being more hydrophobic and the prebulking effect caused by acetylation.
The initial strength and stiffness of the
straw-boards as measured by a 3-point flex test de-creased as the weight gain of the straw increased.
The strawboards’ density decreased with
in-creasing weight gain. This was caused by the prebulking of the straw and the reduction in compressibility with acetylation.
References
American Society for Testing and Materials, 1995. Standard Methods of Evaluating the Properties of Wood-base Fiber and Particle Panel Materials, ASTM (D1037-93), ASTM, Philadelphia, PA, pp. 137 – 155.
Clemons, C., Young, R.A., Rowell, R.M., 1992. Moisture sorption properties of composite boards from esterified aspen fiber. Wood Fiber Sci. 24 (3), 353 – 363.
Erwin, L.H., 1997. Strawboard, Biocomposites, and Fields of Dreams, Evergreen, Newsletter for New Uses Council, April 1997.
Feist, W.C., Rowell, R.M., Ellis, W.D., 1991a. Moisture sorp-tion and accelerated weathering of acetylated and methacrylated aspen. Wood Fiber Sci. 23 (1), 128 – 136. Feist, W.C., Rowell, R.M., Youngquist, J.A., 1991b.
Weather-ing and finish performance of acetylated aspen fiberboard. Wood Fiber Sci. 23 (2), 260 – 272.
Hadi, Y.S., Darma, I.G.K.T., Febrianto, F., Herliyana, E.N., 1995. Acetylated rubberwood flakeboard resistance to bio-deterioration. Compos. Manu. Prod. 45 (10), 64 – 66. Ibrahim, W.A., Mohd Ali, A.R., 1991. The effect of chemical
treatments on the dimensional stability of oil palm stem and rubberwood. J. Trop. Forest Sci. 3 (3), 291 – 298. Imamura, Y., 1993. Morphological changes in acetylated
wood exposed to weathering. Wood Res. 79, 54 – 61. Klinga, L.O., Tarkow, H., 1966. Dimensional stabilization of
hardboard by acetylation. Tappi 49 (1), 23 – 27.
Rowell, R.M., 1982. Distribution of acetyl groups in southern pine reacted with acetic anhydride. Wood Sci. 15 (2), 172 – 182.
Rowell, R.M., 1992. Opportunities for Lignocellulosic Materi-als and Composites, ACS Symposium Series 476, ACS, Washington, DC, pp. 12 – 27.
Rowell, R.M., Keany, F.M., 1991. Fiberboards made from acetylated bagasse fiber. Wood Fiber Sci. 23 (1), 15 – 22. Rowell, R., Simonson, R., Hess, S., Plackett, D., Cronshaw,
D., Dunningham, E., 1994. Acetyl distribution in acety-lated whole wood and reactivity of isoacety-lated wood cell-wall components to acetic anhydride. Wood Fiber Sci. 26 (1), 11 – 18.
Rowell, R.M., Simonson, R., Tillman, A.M., 1986a. A sim-plified procedure for the acetylation of hardwood and softwood flakes for flakeboard production. J. Wood Chem. Technol. 6 (3), 427 – 448.
Rowell, R.M., Simonson, R., Tillman, A.M., 1986b. Dimen-sional stability of particleboard made from vapor phase acetylated pine wood chips. Nordic Pulp Paper Res. J. 2, 11 – 17.
Rowell, R.M., Simonson, R., Tillman, A.M., 1986c. Vapor phase acetylation of southern pine, douglas-fir, and aspen wood flakes. J. Wood Chem. Technol. 6 (2), 293 – 309. SAS, 1992. SAS User’s Guide: Statistics, SAS Institute, Cary,
NC.
Youngquist, J.A., Krzysik, A., Rowell, R.M., 1986a. Dimen-sional stability of acetylated aspen flakeboard. Wood Fiber Sci. 18 (1), 90 – 98.
Youngquist, J.A., Rowell, R.M., Krzysik, A., 1986b. Mechan-ical properties and dimensional stability of acetylated as-pen flakeboard. Holz als Roh-und Werkstoff 44, 453 – 457. Tillman, A.M., 1987. Chemical Modification of
Lignocellu-losic Materials, unpub, PhD Dissertation, Dept. Engineer-ing Chemistry II, Chalmers University of Technology, Goteborg, Sweden.
Westin, M., Simonson, R., 1992. High Performance Com-posites from Acetylated Wood Fibers, Pacific Rim Bio-Based Composites Symposium, Nov 9 – 13, Rotorua, New Zealand, pp. 235 – 242.
