Industrial Crops and Products 11 2000 31 – 41
Strawboard from vapor phase acetylation of wheat straw
Greggory S. Karr, Xiuzhi S. Sun
Kansas State Uni6ersity, Grain Science and Industry Department, Manhattan, KS
66506
, 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 R
2
= 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 www.elsevier.comlocateindcrop
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
:
xsswheat.ksu.edu X.S. Sun Contribution No. 99-136-J from the Kansas Agricultural
Experiment Station. 0926-669000 - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 6 - 6 6 9 0 9 9 0 0 0 3 1 - X
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 compres-
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.
The objective of this study was to increase the moisture resistance and dimensional stability of
panel boards made from wheat straw. This in- volved two experimental steps: 1 utilizing re-
sponse surface methodology to evaluate the process parameters needed to acetylate ground
wheat straw in a vapor phase reaction with acetic anhydride; and 2 producing panel type boards
from the acetylated wheat straw and determining the effects of acetylation on the strawboards’ me-
chanical properties, water resistance, and dimen- sional stability.
2. Materials and methods
