Strawboard from vapor phase acetylation of wheat straw

Industrial Crops and Products 11 (2000) 31 – 41
www.elsevier.com/locate/indcrop
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
1. Introduction
* Corresponding author. Tel.: +1-785-532-4077; fax: + 1785-532-7010.
E-mail address: [email protected] (X.S. Sun)
Contribution No. 99-136-J from the Kansas Agricultural
Experiment Station.
The commercial strawboard industry is relatively 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. Strawboard is a reconstituted lignocellulosic composite
0926-6690/00/$ - 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
32
G.S. Karr, X.S. Sun / Industrial Crops and Products 11 (2000) 31–41
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, furniture 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 restrained 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 compression) and linear expansion (normal to compression). 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 lignocellulosic 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 (Rowell, 1982). Many reagents have been used to modify the cell wall polymers with varying degrees of
success, including anhydrides, acid chlorides, isocyanates, aldehydes, alkyl halides, lactones, nitriles, 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 improve 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, hemicellulose, and lignin (Rowell et al., 1994). Other similar 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 involved two experimental steps: (1) utilizing response 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’ mechanical properties, water resistance, and dimensional stability.
G.S. Karr, X.S. Sun / Industrial Crops and Products 11 (2000) 31–41
33
2. Materials and methods
2.2. Experimental design
Ground wheat straw was obtained from Natural Fiber Board Inc. (Minneapolis, KS). The commercial 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 anhydride was obtained from Aldrich Chemical
Company Inc. (Milwaukee, WI). A diphenylmethyane diisocyanate resin binder, (Rubinate
1840), was obtained from ICI Polyurethanes
(Geismar, LA).
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 reported as the SD for all data analysis.
2.1. Acetylation process
A process flow diagram of the acetylation system used in this research is presented in Fig. 1.
Air was passed through drierite to remove moisture, 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 containing an aqueous sodium hydroxide solution.
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-adjusted 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,
Fig. 1. Process flow diagram of the straw acetylation system.
34
G.S. Karr, X.S. Sun / Industrial Crops and Products 11 (2000) 31–41
Table 1
Process variables levels of the response surface central composite design
Process variables
Moisture content (%)
Oven temperature (°C)
Reaction time (h)
Air flow rate (cc/min)
Levels
−1.5
−1
0
+1
+1.5
2.5
72.5
0.5
250
5
80
1
500
10
95
2
1000
15
110
3
1500
17.5
117.5
3.5
1750
and the air flow passed directly through the reactor 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 byproduct 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 control 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 0.499 0.02 cm in thickness and
then cut into two 15.2× 4.4 cm samples for testing 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 significantly 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
The equilibrium moisture content (EMC) of
each sample was determined at 65 and 90% RH.
The samples were stored for 3 weeks at 65% RH
or 90% RH at 27°C, then were oven-dried for 2 h
at 130°C, and EMC was calculated. Each point
was the average of two 4.4× 4.4 cm samples.
G.S. Karr, X.S. Sun / Industrial Crops and Products 11 (2000) 31–41
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).
35
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 samples were viewed with an E-Tech Auto Scan scanning electron microscope and micrographs were
taken.
4. Results and discussion
4.1. Response surface experiment
3.5. Mechanical properties
Method (D1037-93) (ASTM, 1995) was followed 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 equations given in this method. Each value of MOR
and MOE were the average of two board specimens (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
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 a F-test. This indicates that each of
the four variables was significant in the acetylation 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 experiment 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
The EMC values of the strawboards at 65 and
90% RH decreased with increasing acetyl content
G.S. Karr, X.S. Sun / Industrial Crops and Products 11 (2000) 31–41
36
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 compared to the control boards. These results indicate
that the straw became more hydrophobic (due to
fewer hydrogen bonding sites) as it was acetylated, which also agrees with trends reported previously. 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 expansion during the humidity cycle test are illustrated in Fig. 2. The lines at 6.1, 12.9, and 18.1%
weight gains are the averages of five samples
within a 9 1.2% weight gain level that were
Table 2
Results of the response surface experiments
Run number
Reaction time
(h)
Reaction temperature (°C) Moisture content
(%)
Air flow rate (cc/min)
Weight gaina
(%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
3
1
3
1
3
1
3
1
3
1
3
1
3
1
3
1
3.5
0.5
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
110
110
80
80
110
110
80
80
110
110
80
80
110
110
80
80
95
95
118
73
95
95
95
95
95
95
95
95
95
95
95
95
95
1500
1500
1500
1500
1500
1500
1500
1500
500
500
500
500
500
500
500
500
1000
1000
1000
1000
1000
1000
1750
250
1000
1000
1000
1000
1000
1000
1000
1000
1000
24.2
10.6
19.4
8.1
16.9
7.3
7.1
5.3
19.2
7.8
12.9
5.0
16.5
5.4
7.9
4.1
18.8
3.8
–b
9.5
12.8
5.6
13.9
7.1
14.6
11.9
13.6
13.3
14.1
13.0
13.8
13.1
12.3
a
b
15
15
15
15
5
5
5
5
15
15
15
15
5
5
5
5
10
10
10
10
17.5
2.5
10
10
10
10
10
10
10
10
10
10
10
SD of centerpoint of CCD= 1.26%.
Not obtained for this point because the straw was burned during the reaction.
G.S. Karr, X.S. Sun / Industrial Crops and Products 11 (2000) 31–41
Table 3
Comparison of weight gain predicted from the mathematical
model and the experimental values
Experimental conditions
Set 1
Time
Temp
M.C.
Rate
Model predicted
Experiment
1
Experiment
2
Set 2
1.5 h
110°C
6.7%
1500 cc/min
Time
Temp
M.C.
Rate
(%)
11.5
(%)
19.0
12.0
19.1
11.2
19.2
3.0 h
110°C
6.7%
150cc/min
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 increased. The thickness swell of the strawboard
with the highest level of acetylation, 24%, was less
than a fifth of the swell of the untreated strawboard. The linear expansion of the strawboards
followed the same trend; strawboard with 24%
acetylation showed about one-half the linear expansion of the untreated strawboard. This inTable 4
Equilibrium moisture content (EMC) of strawboards at 65 and
90% RHa
Weight gain (%)
0.0
7.1
12.3
18.8
24.2
a
EMC (%)
65% RHb
90% RHc
7.5
6.8
6.4
6.2
5.1
14.4
12.9
12.6
10.7
10.3
27°C.
SD of centrepoint of CCD= 0.2%.
c
SD of centrepoint of CCD= 0.3%.
b
37
crease in dimensional stability of acetylated
strawboards is the result of two effects. The acetylated straw is more hydrophobic and sorbs less
moisture, as indicated in the EMC results, and the
acetyl group causes prebulking or permanent expansion of the wheat straw’s cell wall, which will
limit the swelling caused by water. Other researchers have reported similar results. Acetylation 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 reduction 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 possible cause is a physical effect such as the adhesion 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 acetylated flakes using ASTM method (D 1037). Results showed that boards acetylated to a 20%
acetyl content had a 37% reduction in MOR and
38
G.S. Karr, X.S. Sun / Industrial Crops and Products 11 (2000) 31–41
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 properties. 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 acetylated to a 20% acetyl content will have only about
80% of the number of fibers of a control board
with equal density and volume.
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 compressibility 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. Measurements of density profiles through the thickness of
the board showed no large differences from the
4.5. Board density
Weight gain
(%)
MOR
(MPa)b
MOE (MPa)c Density
(g/cm3)d
0.0
7.1
12.3
18.8
24.2
19.6
15.9
12.6
9.2
7.2
1930
1900
1890
1480
1020
The board densities for the control and acetylated strawboards are listed in Table 5. All the
samples were made with the same press conditions. They were not pressed to a uniform thickness, 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 untreated samples, but then density decreased to the
Table 5
Mechanical properties of strawboards made from control and
acetylated strawa
a
5%
SD
c
SD
d
SD
b
0.84
0.89
0.81
0.77
0.65
diphenylmethyane diisocyanate binder.
of centrepoint of CCD=1.1 MPa.
of centrepoint of CCD= 130 MPa.
of centrepoint of CCD=0.01 g/cm3.
G.S. Karr, X.S. Sun / Industrial Crops and Products 11 (2000) 31–41
39
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 appeared 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, respectively. 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
physical damage to the straw. However, acetylation might have caused some physical or chemical
change to weaken the straw fibers that was not
visible. Micrographs of cross sections of untreated
and acetylated strawboards, are noticeably different (Fig. 3c and d). Differences between the samples are noticeable. The acetylated strawboard
looks less compact and has more void space than
the untreated strawboard. This supports the finding that the acetylated strawboards have a lower
density and offers possible explanations for the
loss in mechanical properties with increasing
acetylation. If the acetylated straw is not com-
40
G.S. Karr, X.S. Sun / Industrial Crops and Products 11 (2000) 31–41
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 mechanical 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 anhydride 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 system and procedure can be predicted by the
mathematical model developed from this
research.
“ A strong trend was found for decreasing equilibrium 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 strawboards as measured by a 3-point flex test decreased as the weight gain of the straw
increased.
“ The strawboards’ density decreased with increasing 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 sorption 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. Weathering 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 biodeterioration. 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 Materials 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 acetylated whole wood and reactivity of isolated wood cell-wall
components to acetic anhydride. Wood Fiber Sci. 26 (1),
11 – 18.
Rowell, R.M., Simonson, R., Tillman, A.M., 1986a. A simplified 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. Dimensional 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.
Sheen, A.D., 1992. The Preparation of Acetylated Wood Fibre
on a Commercial Scale, Pacific Rim Bio-Based Composites
Symposium, Nov 9 – 13, Rotorua, New Zealand, pp. 235 –
242.
G.S. Karr, X.S. Sun / Industrial Crops and Products 11 (2000) 31–41
Tillman, A.M., 1987. Chemical Modification of Lignocellulosic Materials, unpub, PhD Dissertation, Dept. Engineering Chemistry II, Chalmers University of Technology,
Goteborg, Sweden.
Westin, M., Simonson, R., 1992. High Performance Composites from Acetylated Wood Fibers, Pacific Rim BioBased Composites Symposium, Nov 9–13, Rotorua, New
Zealand, pp. 235 – 242.
41
Youngquist, J.A., Krzysik, A., Rowell, R.M., 1986a. Dimensional stability of acetylated aspen flakeboard. Wood Fiber
Sci. 18 (1), 90 – 98.
Youngquist, J.A., Rowell, R.M., Krzysik, A., 1986b. Mechanical properties and dimensional stability of acetylated aspen flakeboard. Holz als Roh-und Werkstoff 44, 453 – 457.
.