Effect of curing time on physical and me (3)

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Effect of curing time on physical and mechanical properties
of phenolic-treated bamboo strips
U.M.K. Anwar a,∗ , M.T. Paridah b , H. Hamdan a , S.M. Sapuan c , E.S. Bakar b
a
b
c

Forest Product Division, Forest Research Institute Malaysia, 52109 Kuala Lumpur, Malaysia
Faculty of Forestry, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia
Faculty of Engineering, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia

a r t i c l e

i n f o

Article history:

a b s t r a c t
Effect of pressing time on physical and mechanical properties of phenolic-impregnated bam-


Received 14 November 2006

boo strips was evaluated. Bamboo strips (Gigantochloa scortechinii) were impregnated with low

Received in revised form

molecular weight phenol formaldehyde (LMwPF) resin. Samples were submerged in LMwPF

30 April 2008

resin using a vacuum chamber of 750 mmHg for 1 h before it was released within 1.5 h.

Accepted 3 May 2008

Treated strips were dried in an oven with a temperature of 60 ◦ C within 6–9 h. It was hot
pressed at 14 kg m−2 and a temperature of 140 ◦ C for 5, 8, 11, 14 and 17 min. The physical
and mechanical properties of the test indicated that the properties of phenolic-treated strips

Keywords:


have significantly increased as compared to control samples. Dimensional stability (water

Bamboo strips

absorption, thickness swelling and linear expansion) of the phenolic-treated properties were

Impregnation

significantly lower than control after 5-min pressing time. The antishrink efficiency (ASE) of

Phenolic resin

phenolic-treated strips increased when pressing time were extended from 5 to 17 min. The

Pressing time

mean value of modulus of rupture (MOR) for the control samples (177 N mm−2 ) showed a
significant difference with phenolic-treated strips after 17-min pressing time (224 N mm−2 ).
However, there is no significant difference in compression parallel to grain. The MOE of
phenolic-treated strips was 21,777 N mm−2 and for control was 18,249 N mm−2 , whereas the

compression parallel to grain values for phenolic-treated and control samples were 94 and
at 77 N mm−2 , respectively.
© 2008 Elsevier B.V. All rights reserved.

1.

Introduction

Bamboo is one of the fastest growing plants that can be used
for multipurposes. The modern processing techniques of bamboo have further extended its use. Bamboo is used in splits,
strips or round form depending on the application. For being
used as constructional material, bamboo culms must be converted into bamboo strips to make plybamboo, bamboo mat
board and laminated bamboo. The term of strips is defined as
squared splits resulted by removing the outer and inner skin
of the bamboo splits with a planner. However, bamboo strips

Corresponding author. Tel.: +60 3 6279 7390; fax: +60 3 6280 4623.
E-mail address: mkanwar@frim.gov.my (U.M.K. Anwar).
0926-6690/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.indcrop.2008.05.003



have some inherent properties, such as dimension changes
due to varying moisture content, relative humidity, and biotic
and abiotic degradation (Hamdan, 2004; Mansur, 2000; Deka
et al., 2003). Anwar et al. (2005) observed that the shrinkage of
strip was significantly higher when the epidermises and inner
layer of bamboo splits were removed. According to Abd. Latif
and Liese (1995) bamboo started to shrink from the very beginning of the drying process. Shrinkage generally decreases with
age and culms height where the dimensional stability of the
top portion of older bamboo is much greater than that of basal
of young ones.

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215

i n d u s t r i a l c r o p s a n d p r o d u c t s 2 9 ( 2 0 0 9 ) 214–219

In order to enhance the properties of lignocellulose material, a lot of research has been done especially through
resin impregnation. The resin caused significant increase

in strength properties of bamboo and exhibited higher
dimensional stability when compared to untreated bamboo (Deka et al., 2003). Mahlberg et al. (2001) also found a
significant improvement in the mechanical properties and
dimensional stability of wood fiber when impregnated with
polypropylene. Gindl et al. (2003) noticed that impregnation of
melamine–formaldehyde (MF) resin into wood could improve
a number of its wood properties, such as surface hardness and
weathering resistance. As mentioned by Furuno et al. (2004)
the phenolic resin was penetrated into the cell walls, thereby
contributing the enhancement of dimensional stability and
decay resistance in the resin-impregnated wood. It has been
assumed that the deposition of polymer within the wood cell
walls resulted in high decay resistance as well as an improvement in the dimensional stability (Imamura et al., 1998). The
objective of this study is to determine the optimum curing
time of phenolic-treated strips and its effect on physical and
mechanical properties.

2.

Materials and methods


Strips were obtained from 4-year-old bamboo (Gigantochloa
scortechinii) and the moisture content was about 10%. Specimen size of 400 mm × 20 mm × 4 mm was submerged in low
molecular weight phenol formaldehyde (LMwPF) resin in a
container and put inside a vacuum chamber set for 750 mmHg.
The vacuum pressure was maintained for 1 h before the air
in the vacuum chamber was slowly released within 1.5 h.
The samples were dried in an oven maintained at 60 ◦ C for
6–9 h. After being dried, the impregnated bamboo strips were
pressed using hot press at different pressing durations of 5, 8,
11, 14 and 17 min. The hot press temperature was set at 140 ◦ C
and pressure of 14 kg cm−2 (Anonymous, 1999; Anwar et al.,
2004). All the samples were stacked in a conditioning chamber with a temperature of 20 ± 2 ◦ C and a relative humidity of
65 ± 3% until they reached equilibrium moisture content.

2.1.

Evaluation of physical properties

The water absorption test of the samples was based on weight

of the specimens before and after immersion in water for 24 h.
The weight of soaked samples was measured immediately
after the removal of the excess water with a dry cloth. Same
specimens were used for the determination of dimensional
stability. The properties were evaluated based on thickness
swelling and linear expansion after being soaked in water.
Sixty specimens (25 mm × 20 mm × 5 mm) were prepared and
the width and thickness of all samples were measured before
and after horizontally immersion in water (30 mm below the
water surface) for 24 h. Measurements were taken by using
digital vernier calipers with a precision of 0.01 mm. Due to the
absence of an international standard that can be used as a reference for testing the physical and mechanical properties of
bamboo strips, the testing protocol thus follows different standards and methods used by other researchers on similar study.
The water absorption and dimensional stability (thickness

swelling, perpendicular to the grain) were evaluated based
on European Standard EN 317 (Anonymous, 1993) with some
modification on the size of the specimens based on Hamdan
(2004). The antishrink efficiency tests procedure was prepared
after Kollman et al. (1975). Water absorption and dimensional

stability (linear expansion, thickness swelling and antishrink
efficiency) of the specimens were calculated from the following equations:
Water absorption (%) =

W2 − W1
× 100
W1

where W1 = weight before soaking, g; W2 = weight after soaking, g.
Thickness swelling and linear expansion (%)
=

D1 − D2
× 100
D2

where D1 = initial thickness or linear expansion, mm; D2 = final
thickness or linear expansion, mm.
Antishrink efficiency =


St − Su
× 100
Su

St = treated volumetric swelling coefficient; Su = untreated volumetric swelling coefficient.
where

S=

V2 − V1
× 100
V1

S = volumetric swelling coefficient; V2 = wood volume after
wetting with water; V1 = wood volume before wetting with
water.

2.2.

Evaluation of mechanical properties


The specimens for static bending test were prepared in accordance with the method developed by Ghanaharan et al. (1994)
whereas samples for compression parallel to grain were prepared using the method described by Janssen (1981). A total
of 120 specimens were used for the determination of static
bending and compression parallel to the grain. The tests were
performed using an Instron testing machine with a capacity
of 100 kN in a controlled room temperature of 20 ± 2 ◦ C and
65 ± 3% RH.

3.

Statistical analysis

The changes in dimensional stability of phenolic-treated and
control strips were discussed using column graph. Standard error was used as an indicator for significant difference
between the variables. For mechanical properties, the statistical analysis was carried out using the statistical analysis
software (SAS). A least significant difference (LSD) method
was used to identify the dominant factor and its interaction that affect the means at p ≤ 0.05. This method ranks the
means and calculates the least difference that must occur
between the means was employed to analyse the difference
in mechanical properties of phenolic-treated bamboo strip
and control samples. In order to avoid inherent factors, the
analysis of covariance (ANCOVA) was also used to detect
the differences in mechanical properties among the samples

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i n d u s t r i a l c r o p s a n d p r o d u c t s 2 9 ( 2 0 0 9 ) 214–219

Fig. 1 – Water absorption percentage after 24-h soaking for
control and phenolic-treated bamboo strip.
Note: Means followed with the same letters a,b were not
significantly different (p ≤ 0.05). Vertical bar represent
standard error.

Fig. 3 – Linear expansion perpendicular to the grain
percentage after 24-h soaking for control and
phenolic-treated bamboo strip.
Note: Means followed with the same letters a,b were not
significantly different (p ≤ 0.05). Vertical bar represent
standard error.

when the strength properties were adjusted for moisture content and specific gravity (SG) at the time of test.

4.

Results and discussion

4.1.

Physical properties

The extent and magnitude of water absorption and dimensional stability, i.e., thickness swelling, linear expansion
perpendicular to grain and antishrink efficiency (ASE), of the
samples, are shown in Figs. 1, 2, 3 and 4, respectively. Results
indicate that phenolic-treated bamboo strips exhibited lower
water absorption and improved the dimensional stability,
which lesser extent of thickness swelling and linear expansion
compared to those of control samples. Water absorption after
24-h water soaking for phenolic-treated (5-min pressing) and
control samples was 33.25 and 51.32%, respectively. Samples
pressed for 17-min showed an improvement of 135% water
absorption.

Fig. 2 – Thickness swelling percentage after 24-h soaking
for control and phenolic-treated bamboo strip.
Note: Means followed with the same letters a,b were not
significantly different (p ≤ 0.05). Vertical bar represent
standard error.

Fig. 4 – Antishrink efficiency after 24-h soaking for
phenolic-treated strips.
Note: Means followed with the same letters a,b were not
significantly different (p ≤ 0.05). Vertical bar represent
standard error.

In the thickness swelling study, after 5-min press (3.64%),
phenolic-treated strips showed less swelling than control
samples (7.89%), upon soaking for 24 h. The thickness swelling
after 8, 11, 14 and 17-min were 3.55, 2.52, 2.53, and
2.41%, respectively. Linear expansion of bamboo strips for
phenolic-treated and control samples were evaluated. In general, phenolic-treated strips showed less linear expansion
than control samples. The linear expansion value of 5.34%
was obtained from control samples compared to 1.93% for
phenolic-treated (after 5-min pressing). Water absorption,
thickness swelling and linear expansion were reduced when
the pressing time of phenolic-treated strips was extended.
The standard error and LSD for the physical properties
of phenolic-treated strips and control (4-year-old culms of G.
scortechinii) are also presented in Figs. 1–3. The variability of
physical properties was significant between phenolic-treated
and control strips. A significant difference (p ≤ 0.05) was
observed for tested properties (water absorption, thickness
swelling and linear expansion) for phenolic-treated strips and

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i n d u s t r i a l c r o p s a n d p r o d u c t s 2 9 ( 2 0 0 9 ) 214–219

217

on Furuno et al. (2004) using LMwPF resin, it is assumed that
the fraction of higher molecular weight resin remained on the
inner walls as granular forms. Only the part of the resin which
penetrated into the bamboo strips contributed to the enhancement of dimensional stability and mechanical properties of
the samples.

4.2.

Fig. 5 – LMwPF resins were found in parenchyma (100×).

control samples. No significant difference (p ≤ 0.05) was found
within all treated samples for linear expansion, whereas
significant differences (p ≤ 0.05) in water absorption and thickness swelling were observed after 11-min pressing time.
This explained that the physical properties of bamboo strips
decrease when pressing time extended. However, the properties decreased gradually after 11-min pressing time.
The antishrink efficiency (ASE) of phenolic-treated strips
was increased when pressing time extended (Fig. 4). The highest values (75%) were reached after 17 min of pressing time.
This proved that pressing time influences the curing time of
LMwPF resin. According to Collins (1996) during hot pressing
these methylol groups were converted into more methylene
bridges between phenolic rings resulting in the formation of
a very highly cross-linked thermoset polymer. On the other
hand, high ASE that might be due to the availability of treated
strips reduced the holding capacity of water molecules in
parenchyma cells. According to Abd. Latif and Mohd. Tamizi
(1992), parenchyma cells serve as sites for water storage in
bamboo. The higher content of parenchyma cells in the bamboo increases the water holding capacity of the bamboo (Liese
and Grover, 1961).
Improvement of dimensional stability is due to penetration of LMwPF resin into bamboo strips. After being pressed at
140 ◦ C, the resins are cured and thus they prevent absorption
of phenolic-treated strips and the samples became dimensionally stable. However, pressing time does not affect the
improvement of dimensional stability after 11-min pressing
time. Furuno et al. (2004) in agreement with Deka and Saikia
(2000) concluded that good dimensional stability may be due
to the bulking effect where the resin occupied the lumen
either in granules or in patches when cured. This suggests
that the low molecular weight resins were deposited extensively into the wood cell walls and thus were effective in
reducing the swelling of wood specimens during water immersion. It has been assumed that the deposition of polymer
within the wood cell walls resulted improvement in dimensional stability as well as high decay resistance (Imamura et
al., 1998).
Scanning electron micrograph (SEM) was used to observe
the penetration of LMwPF in bamboo strips. The resins were
recognized in parenchyma (Figs. 5 and 6), such as resin granules and patch which forms various shapes and sizes. Based

Mechanical properties

Table 1 shows the mean values of mechanical properties of
G. scortechinii after being impregnated and pressed under various pressing times. The modulus of rupture (MOR), modulus
of elasticity (MOE) and compressive stress were studied. The
treatment was significantly reduced moisture content at about
4.8% in average compared to control strips. A slight increment
in the SG of phenolic-treated strips was observed at 0.8 as
compared to 0.75 for control samples. This result is considered good since there was only a small increase in weight. The
finding (SG) was in good agreement with Zaidon et al. (1990)
and Shams et al. (2004) but differed to that found by Deka et al.
(2003), who treated Bambusa tulda Roxb. with a thermosetting
resin and found that the SG remained unchanged.
The strength properties of control samples were slightly
lower than phenolic-treated after 5-min pressing time. The
MOR values were 177 N mm−2 and 205 N mm−2 , respectively.
The highest values of MOR, MOE and compression parallel
to grain (224 N mm−2 ; 21,719 N mm−2 and 94 N mm−2 ) were
achieved after 17-min pressing. The strength properties were
significantly improved of 26, 21 and 22% for MOR, MOE and
compression parallel to gain, respectively.
The results of statistical analysis (LSD and ANCOVA) are
shown in Table 1. The mean values were analysed with LSD
and found that the MOR values were significantly different at
p ≤ 0.05 when comparing control and phenolic-treated strips.
However, in MOE and compression to the grain values, a significant difference (p ≤ 0.05) was found after 17-min pressing
time.
When the mechanical properties values were adjusted
which the MC and SG used as factor. The results indicate
that pressing time had a significant effect on MOR and MOE.
However, in compression parallel to grain no significant difference at p ≤ 0.05 (which is in contrast to the case of MOR
and MOE) was found between control and phenolic-treated

Fig. 6 – Resins were found at parenchyma with granules
and patch filled in parenchyma (350×).

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218

Note: Values are average of 120 specimens. Values in parentheses are standard deviations. Means with the same letter a,b in the same column are not significantly different (p < 0.05).

75a
77a
84a
86a
90a
93a
14,896a
18,771b
20,329b
20,158b
20,455b
22,369b
110a
186b
225b
236b
242b
250b
(13.2)
(8.7)
(7.9)
(11.4)
(11.5)
(10.9)
77a
81a
82a
85ab
86ab
94b
17,813a (3,492)
19,683ab (3,012)
19,884ab (2,462)
20,050ab (3,591)
20,362ab (3,204)
21,719b (3,691)
(27.6)
(27.7)
(23.4)
(20.2)
(33.3)
(26.8)
177a
205b
210b
215b
218b
224b
(0.04)
(0.07)
(0.06)
(0.06)
(0.07)
(0.06)
0.75a
0.80a
0.80a
0.80a
0.82a
0.82a
8.5a
5.6b
4.9b
4.6b
4.6b
4.4b
0 (control)
5
8
11
14
17

(0.16)
(0.17)
(0.18)
(0.18)
(0.04)
(0.13)

MOE
MOR
Compress
parallel to grain
MOE
MOR

Mean values
(N mm−2 )
SG
MC (%)
Pressing
time (min)

Table 1 – Mechanical properties of impregnated G. scortechinii strips with low molecular weight phenol formaldehyde

Strength adjusted
(N mm−2 )

Compress
parallel to grain

i n d u s t r i a l c r o p s a n d p r o d u c t s 2 9 ( 2 0 0 9 ) 214–219

bamboo strips. The results suggest that the pressing time only
affects the MOR and MOE not compression parallel to grain of
bamboo strips. However, generally there is no significant effect
within the pressing time observed.
In general, the results were also consistent with the fact
that impregnation with LMwPF resin improves the strength
properties of lignocelluloses material. The MOR, MOE and
compression values of treated samples were higher than
control strips, which may be attributed to the present of
LMwPF resin in bamboo strips. It also could be explained
that, during pressing, the resins were polymerized and cured
(see Figs. 5 and 6). Therefore, when pressure and heat were
applied, the strips became densified while the resin polymerized. Rowell (1991) noted that the resin usually starts as a
low molecular weight pre-polymer and builds the degree of
polymerization in the curing process. According to Deka and
Saikia (2000), MOR and MOE values were increased due to the
fact that the bulked volumes of the treated samples remained
the same after curing. They also found that MOR and MOE of
wood which treated with PF increased the values at about 21
and 12%, respectively. In other study by Shams et al. (2004),
they note that the mechanical properties of treated samples
increased when the pressure increased.

5.

Conclusion

The treatment of bamboo strips with LMwPF resin followed
by pressing at 140 ◦ C improved the dimensional stability and
strength properties of the strips. The treatment improved
water absorption, thickness swelling and linear expansion
perpendicular to grain after 24 h of cold water soaking. The
values for MOR, MOE and compression parallel to grain of
phenolic-treated strips were increased when the pressing time
increases. The highest strength properties were attained at
17 min of pressing time.

Acknowledgements
The author acknowledges the Forest Research Institute
Malaysia (FRIM), Ministry of Science Technology and Innovation (MOSTI) and Universiti Putra Malaysia (UPM) for funding
this project. The resins provided by Malaysian Adhesive
Chemical (MAC) are gratefully acknowledged. The results are
part of the PhD project of U.M.K. Anwar.

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