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 mmmin 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 cm
3
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 a F-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 The EMC values of the strawboards at 65 and
90 RH decreased with increasing acetyl 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 9 1.2 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 ccmin Weight gain
a
h 1
1500 3
24.2 110
15 110
15 1500
10.6 1
2 80
15 1500
19.4 3
3 8.1
1500 15
4 80
1 110
5 1500
16.9 3
5 1
110 6
5 1500
7.3 7.1
1500 5
7 80
3 1
80 8
5 1500
5.3 9
500 3
19.2 110
15 1
110 10
15 500
7.8 3
80 11
15 500
12.9 1
80 12
15 5.0
500 3
13 16.5
500 110
5 500
14 5
110 5.4
1 80
3 5
15 500
7.9 500
4.1 16
1 80
5 10
3.5 1000
18.8 95
17 95
10 1000
3.8 0.5
18 2
19 –
b
118 1000
10 20
73 2
10 1000
9.5 2
95 21
17.5 12.8
1000 2
22 5.6
1000 95
2.5 1750
23 10
95 13.9
2 95
2 10
24 250
7.1 1000
14.6 25
2 95
10 10
2 1000
11.9 95
26 1000
13.6 27
2 95
10 28
13.3 1000
10 95
2 1000
10 14.1
95 2
29 10
2 1000
13.0 95
30 2
95 31
10 1000
13.8 95
32 13.1
1000 10
2 33
95 2
10 1000
12.3
a
SD of centerpoint of CCD = 1.26.
b
Not obtained for this point because the straw was burned during the reaction.
Table 3 Comparison of weight gain predicted from the mathematical
model and the experimental values Experimental conditions
Set 2 Set 1
Time 1.5 h
Time 3.0 h
110°C Temp
Temp 110°C
6.7 M.C.
M.C. 6.7
Rate 1500 ccmin
Rate 150ccmin
11.5 19.0
Model pre- dicted
Experiment 12.0
19.1 1
11.2 Experiment
19.2 2
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 Equilibrium moisture content EMC of strawboards at 65 and
90 RH
a
Weight gain EMC
65 RH
b
90 RH
c
7.5 14.4
0.0 6.8
7.1 12.9
12.3 6.4
12.6 18.8
6.2 10.7
24.2 5.1
10.3
a
27°C.
b
SD of centrepoint of CCD = 0.2.
c
SD of centrepoint of CCD = 0.3.
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 gcm
3
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 andor 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 gcm
3
. 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 straw
a
Weight gain MOR
Density MOE MPa
c
MPa
b
gcm
3 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 0.65
7.2 1020
a
5 diphenylmethyane diisocyanate binder.
b
SD of centrepoint of CCD = 1.1 MPa.
c
SD of centrepoint of CCD = 130 MPa.
d
SD of centrepoint of CCD = 0.01 gcm
3
.
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
physical damage to the straw. However, acetyla- tion 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 differ- ent Fig. 3c and d. Differences between the sam-
ples are noticeable. The acetylated strawboard looks less compact and has more void space than
the untreated strawboard. This supports the find- ing 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-
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
