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The Tensile Strength of Petung Bamboo Fiber Reinforced Epoxy
Composites: The Effects of Alkali Treatment, Composites Manufacturing,
and Water Absorption
To cite this article: Gunawan Refiadi et al 2019 IOP Conf. Ser.: Mater. Sci. Eng. 547 012043
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IOP Publishing
IOP Conf. Series: Materials Science and Engineering 547 (2019) 012043 doi:10.1088/1757-899X/547/1/012043
The Tensile Strength of Petung Bamboo Fiber Reinforced
Epoxy Composites: The Effects of Alkali Treatment, Composites
Manufacturing, and Water Absorption
Gunawan Refiadi1* Yusi Syamsiar2 Hermawan Judawisastra3
1
Mechanical Engineering Vocational Education Study Program, College of Teacher
Training and Education (STKIP) Sebelas April, Jl. Anggrek Situ 10 Sumedang 45323,
Indonesia
2
Textile Chemistry Department, Polytechnic of Textile Technology, Jl. Jakarta 31
Bandung 40272, Indonesia
3
Material Engineering Department, Faculty of Mechanical and Aerospace
Engineering, Bandung Institute of Technology (ITB), Jl.Ganesha 10 Bandung 40132,
Indonesia
*gunawan_refiadi@stkip11april.ac.id
Abstract. Natural fiber application due to its competitiveness attracted many research in green
composites. However, the tensile properties of natural fiber composites might be influenced by
several factors such as treatment of the fibers, manufacturing processes, and water absorption
levels. In this research, we use petung bamboo fibers as natural fibers candidate for a good
reinforcement in green composites. The study focused on the tensile property of bamboo fiber
reinforced epoxy composites due to alkali treatment, manufacturing process, and water
absorption. The composites were made with three variations: the fibers were varied without
and with alkaline treatment (5% NaOH); manual lay-up method and hot press were applied as
manufacturing variation; moisture content in the composites was varied in dry condition and
after water absorption treatment. Tensile testing and Scanning Electron Microscope (SEM),
were performed to improve tensile strength of composite and fiber-matrix interface quality.
The alkali treatment has less effect to the tensile strength than of the volume fraction gain and
the void content. Composite manufacturing by hot press has a significant effect to the
improvement of tensile strength up to 37% compare to the hand lay-up method. Water
absorption up to 8.8% had decreased the tensile strength of composites up to 29%. The alkali
treatment on the bamboo fibers had effectively reduced water absorption into the composite as
well as the tensile strength reduction.
Keywords: petung bamboo, alkali treatment, composites manufacturing, water absorption
1. Introduction
Polymeric composites such as CFRP (Carbon Fiber-Reinforced Polymer) and GFRP (Glass
Fiber-Reinforced Polymer) had abundantly produced due to their advantages as well as high
mechanical specific performances. Unfortunately, the drawback of those materials is lack of
sustainable environment due to pollutant expel both to the air and the land. However, this
limitation in turn opens up other material research opportunities such as green composites.
The composites use lignocellulosic structure natural resources as reinforcement such as ramie,
coconut, hemp, jute, and bamboo. In tropical country such Indonesia, bamboo has a potential
advantage as an abundance resource of natural fibers. In addition, bamboo have low-cost, fast
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IOP Conf. Series: Materials Science and Engineering 547 (2019) 012043 doi:10.1088/1757-899X/547/1/012043
growth, renewable, CO2 free, flexibility, as well high specific strength and stiffness compared
to glass fiber [1][2]. Several considerable research dealing with bamboo were reviewed [3]
and in Indonesia petung bamboo (Dendrocalamus asper) in the form of stems, slats, and fibers
was studied also [4]–[6]. However, petung bamboo fibers as well as the other lignocellulose
materials still have several weaknesses and problems. High variability in properties and high
moisture absorption [7] affected the interfacial bonding quality between fibers and matrix.
Meanwhile, composite materials in its application could be affected by hygroscopic
conditions. To face the problem, several chemical methods such as alkalization, graph
copolymerization, and coupling agents have been proposed to improve both interfacial quality
and hydrophobing of the fibers [8], [9]. Studies of water absorption effects on bamboo fiberreinforced polyester through fibers surface modifications through alkali treatment [6] have
been reported. In terms of composite manufacture methods, hand layup and hot press were
commonly used to fabricate natural fiber composites considering time and cost [11].
However, the effects of combination between manufacturing methods, alkali treatment and
water absorption to tensile properties of petung bamboo fiber reinforced composite has not
been investigated yet. This study is aimed to investigate tensile strength of petung bamboo
fiber-reinforced epoxy composites resulting from alkali treatment, composites manufacturing,
and water absorption.
2. Materials and Methods
2.1 Materials
Supplied from Hutan Penelitian Bambu in Arcamanik, Bandung (West Java, Indonesia),
petung bamboo stems then extracted into 100~300 mm fiber bundles by chemo-mechanical
methods. The resulting fibers then variably prepared both by non-alkali (NA) and alkali
method using 5% v/v NaOH solution (A). Resulted before [12], both treated fibers (NA and
A) has density of 1.05 and 1.11 g/mm3, respectively. The 5% alkali was found as the optimum
solution resulted maximum bamboo fiber strength [12] and least water uptake in bamboo
composites [10]. The matrix used in this study was Lycal GLR 1011 Part A epoxy resin with
curing agent GLR 1011 Part B with density of 1.13 g/mm3.
2.2 Composites Manufacturing and Testing
Prior to composites manufacturing by hand lay-up (LU) and hot press (HP) methods, bamboo
fibers were oven dried 24 hours – 110oC to achieve dry condition. After petung bamboo fiber
aligned unidirectionally on the molding surface, we poured epoxy mixing. For HP method,
pressure, temperature, and holding time were set on 10 MPa 60 oC, and 1 hour, respectively.
With the LU process, impregnation were apllied manually using specific roll at 25oC.
Demolding then performed after the composite 24 h cured.
Composites characterization were evaluated physically by volume fraction and density
measurements. Fiber volume fraction (Vf) and void volume fraction (Vv) calculated using eq.
(1) and eq. (2) as per ASTM D-3171 based on bamboo petung fibers and epoxy densities. We
measured composite density by pycnometry method.
(𝑤𝑏 ⁄𝜌𝑏 )
𝑉𝑓 (%) = [
] 𝑥 100%
(𝑤𝑏 ⁄𝜌𝑏 ) + (𝑤𝑚 ⁄𝜌𝑚 )
.......... (1)
𝑉𝑣 (%) = 1 − 𝜌𝑐 [(𝑤𝑏 ⁄𝜌𝑏 ) + (𝑤𝑚 ⁄𝜌𝑚 )]𝑥 100%
.......... (2)
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where Wb, Wm, are weight fractions of bamboo fibers, epoxy matrix and b, m and c are
densities of bamboo fibers, epoxy matrix, composites, respectively.
Water absorption tests were performed to the tensile specimen as per ASTM D570-98 but
modified as per previous research [6]. in this study, we use water absorption by two intervals
of immersion. Firstly, weighing the samples in boiling water 60 minutes for every 10 minute.
Secondly, with 30 minute intervals from the minutes of 60 up to 240. The water uptake in the
composites was weighted by electronic balance with accuracy 10-4. Moisture content in the
composite samples then calculating by weight difference using Eq.3.
𝑊𝑡 (%) =
𝑊1 − 𝑊0
𝑥 100%
𝑊0
.......... (3)
where Wt total water-absorbed in composites, W0 and W1 are composites weighs on H0 and
H1 respectively. Tensile testing of composites were conducted on both H0 and H1 samples
per ASTM D3039. We use Tensilon machine equipped by a 5 kN load cell to get the tensile
property. Besides, SEM (Scanning Electron Microscope) analysis using JEOL GSM-636OLA
type were also conducted after tensile test to evaluate fracture surface of the composites.
3. Results and discussion
3.1 Physical Properties of Petung Bamboo-Reinforced Epoxy
Table 1 and Figure 1(a) and (b) show the fiber volume fraction and the void volume fraction
of petung bamboo-reinforced epoxy under varying processing conditions. The fiber volume
fraction was 22% up to 28% and the void volume fraction ranges from 0% to 6%. The higher
pressure with uniform distribution in hot press processing method resulted in higher fiber
content as shown in Figure 1(a). Alternatively, the composite fabricated by manual lay-up had
a higher void content as shown in Figure 1(b).
Table 1. Fiber and void volume fractions of petung bamboo fiber-reinforced epoxy
Manufacture
Processes
LU
HP
Vf (%)
NA
A
24
22
28
28
Vv (%)
NA
A
1
6
1
0
Vf – fiber volume fraction, Vv – void volume fraction; NA – Non-Alkalized; A – 5% Alkalized
(a)
(b)
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IOP Conf. Series: Materials Science and Engineering 547 (2019) 012043 doi:10.1088/1757-899X/547/1/012043
Figure 1. Fiber volume fraction, (a) and void volume fraction, (b) of composites as per
manufacturing methods and alkali treatments. [LU: Hand Layup, HP: Hot Press]
The high void content in composite primarily due to several factors such as air bubbles
entrapped during matrix mixing and composite fabrication. The air from the process of
manual lay-up, and the volatile component from the heat process due to chemical reactions
[13]–[15]. The void content resulted in both of hot press and lay up methods are the same
which is only 1.0%. After the alkali treatment the void content become 6% and 0% in both of
the hand lay-up and hot press methods, respectively. Tightly controlled parameters in the hot
press method through closed-loop system resulting in the lower void content whereas the
open-loop controlled system in the hand lay-up method showed an anomaly result by
increasing void content.
3.2. The Effects of Alkali Treatment on Tensile Strength and Water Uptake
Table 2 shows the influence of alkali treatments (0% and 5% NaOH) on tensile strength and
water absorption of petung bamboo fiber reinforced epoxy composites were varied from 79
MPa up to 126 MPa while the water absorption gain was ranged from 3 up to 8.8%.
Table 2. The tensile properties and water absorption of petung bamboo fiber reinforced epoxy
composites before and after alkali treatment.
Manufacture
Process
LU
HP
Tensile Strength (MPa)
NA
A
83
79
126
126
Water absorption (%)
NA
A
5,15
3,79
8,82
6,57
NA – Non-Alkalized; A – 5% Alkalized
Figure 2 shows that the alkali treatment effect in the hot press (HP) method did not
increase the composite’s strength as stated in the literature [16]. Moreover, using lay up (LU)
method, the effect of alkali treatment showed slight reduction of tensile strength due to high
void content, 6% (see Table 1). The void causes stress concentrations and tend to weaken the
fiber-matrix bonding [17]. The tensile strength of hot-pressed composites seem to be more
dominated by high fiber volume fraction, 28%Vf, and lower void content than that of the
alkali effect.
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IOP Conf. Series: Materials Science and Engineering 547 (2019) 012043 doi:10.1088/1757-899X/547/1/012043
Figure 2. Tensile properties of petung bamboo fiber reinforced epoxy before and after alkali
treatment as per hand layup (LU) and hot press (HP) methods. [NA: Non-Alkalized, A: 5% Alkalized]
Figure 3 shows water absorption reduction after fiber alkali treatment on lay-up (LU) and hot
press (HP) composites. The higher fiber volume fraction in the hot press composites lead to
higher water uptake than that of the lay-up composites. This uptake was due to the inherent
hygroscopic properties of bamboo fibers as lignocellulosic materials[16] which contain polar
hydroxyl groups produced by both cellulose and hemicellulose content.
Figure 3. Water absorption characteristics of composites based on alkali treatment.
[NA: Non-Alkalized, A: 5% Alkalized]
Both composites have 26% reduction of water uptake after the alkali treatment. This was due
to the removal of hemicellulose and lignin content during the treatment [16]. In the previous
report [10] hemicellulose was responsible for water absorption of the fibers. Alkali treatment
makes the fibers less hydrophilic as the number of hydrophilic hydroxyl groups reduces by
react which 5% NaOH [17], [18]. Therefore, reduction of water absorption enhanced the
water resistance of composites. Besides, alkali treatment increases fiber fibrillation and
therefore decreases fiber diameter, which in turn improve the effect of surface area for contact
with the matrix [16]. This might be contributing to decrease water absorption due to bamboo
fibers more encapsulated by the epoxy matrix.
3.3. Effect of Manufacture Process on Tensile Strength
Figure 4 shows the comparison of tensile strength of composites resulting from manual lay-up
(LU) and hot press (HP) processing. The highest tensile strength of composites was obtained
by using hot press method. It can be seen that hot press processing has improved the tensile
strength of composites up to 37%. The combination of higher fiber volume fraction and lower
void content improved tensile strength, see Table 1. The higher fiber volume fraction in the
hot press (HP) was achieved by homogenous temperature and steadier pressure distributions.
In this method temperature and pressure are tightly controlled in closed-loop system.
Conversely, processing parameters in hand lay-up (LU) controlled manually through the
operator skills level which is the open-loop system. Dealing with the lower void content in the
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IOP Conf. Series: Materials Science and Engineering 547 (2019) 012043 doi:10.1088/1757-899X/547/1/012043
hot pressed composite, this could be achieved due to uniformly high pressure – forced out the
entrapped void – supported by the molding mechanism.
Tensile Strength, MPa
Layup
Hot Press
140
120
100
80
60
40
20
0
Non Alkalized
5% Alkalized
Alkali Treatment
Figure 4. The comparison of the tensile strength by using manual lay-up (LU) and hot press
(HP) processing
Figure 5 shows the composites fracture surfaces resulting from both the layup and hot
press processes. Lower extent of fiber pullout in HP composites (Fig. 5b) compared to LU
composites confirms the improvement of fiber-matrix interface, resulting in higher tensile
strength. This improvement might be resulted from the combined temperature and pressure
energy supply in HP method.
(b)
(a)
Figure 5. Comparison of the fracture surface of composites manufactured by (a) Hand Lay
Up and (b) Hot Press
3.4. Effect of Water Absorption on Tensile Strength
Table 3 and Figure 6 depicted the effect of water absorption on tensile strength of petung
bamboo fiber-reinforced epoxy composites based on alkali treatment and manufacturing
processes. The absorption of water from 3.79% up to 8.8% leads to the degradation of fibermatrix interface [18]. The result was a reduction of composites tensile strength from 17% up
to 29% (see Table 3). Besides, water composites immersion swelled the hydrophilic bamboo
fibers resulting in micro cracking of the brittle matrix[19]. As water, penetrating into the
interface through the micro cracks fiber-matrix debonding occurs and leads to composite
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failure [20]. High strength reduction on both non-alkalized composites (LU-NA and HP-NA)
as high as 25% and 24% were due to high water absorption 5.15% and 8.8%, respectively.
This water absorption made hydrogen bonds to the fibers and decreased fiber-matrix
interconnections. Large strength reduction on the alkalized lay-up (LU-A) composites was
dominated by high void content (see Fig. 1) than the water uptake in the composites. From
Fig. 6 the best composite with the lowest strength reduction was resulting from alkalized hot
press composites (HP-A). The composite shows only 17% tensile strength reduction, showing
the effectiveness of alkalization to reduce water absorption and minimize the strength
reduction.
Table 3. Tensile properties decrement of Petung bamboo fiber reinforced epoxy composites
due to water absorption
Composite - Fiber Tensile decrements Water uptake
treatments
(%)
(%)
LU-NA
25
5.15
LU-A
29
3.79
HP-NA
24
8.8
HP-A
17
6.57
NA – Non-Alkalized; A – 5% Alkalized
Figure 6. The effect of water absorption on tensile strength petung bamboo fiber reinforced
epoxy composites.
Supporting tensile test results, SEM qualitative evaluation were added in Fig. 7. Fracture
surfaces were based on fiber treatments, manufacturing processes, and water absorption
conditions. Fracture surfaces of composites after water immersion (Fig. 7b, d, f, h) show
larger extent of fibers pullout that related to poor fiber-matrix interface. These results
corroborate with the tensile strength reduction of composites due to water absorption.
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Composites
(Manufacture – Treatment)
Before immersed in water
(H0)
After immersed in water
(H1)
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
Layup-Non Alkali
[LU-NA]
Layup-Alkali
[LU-A]
Hot Press-Non Alkali
[HP-NA]
Hot Press-Alkali
[HP-A]
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Figure 7. Comparison of composites fracture surfaces due to alkali treatment, manufacturing
technique, and water absorption
4. Conclusion
The effects of alkali treatment, composite manufacturing, and water absorption on the tensile
strength of petung bamboo fiber-reinforced epoxy composites have been studied. In this
research, the alkali treatment has less effect to the tensile strength than of the volume fraction
gain and the void content. Hot press composites manufacturing method has significant effect
to the improvement of tensile strength up to 37% than that of lay-up method. Water
absorption up to 8.8% decreased the tensile strength of composites up to 29%. The alkali
treatment had effectively reduced water absorption and consequently minimized the tensile
strength decrement.
Acknowledgements
The authors would like to express their gratitude and appreciation to Directorate General
Research Strengthening and Development, Ministry of Research, Technology and Higher
Education of the Republic of Indonesia (DRPM-Kemristekdikti) that have been founded the
Inter-Universities Collaboration of Research (PEKERTI)
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