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RESEARCH PAPER
Effects of alkali treatment and elevated temperature on the mechanical
properties of bamboo fibre-polyester composites
(Title contains 15 words)
Running headline: Effects of alkali treatment and elevated temperature on the mechanical properties of bamboo fibre-polyester composites
(104 characters)
by
Allan C. Manalo
1
, Evans Wani
1
, Noor Azwa Zukarnain
1
, Warna Karunasena
1
,
Kin-tak Lau
2
1
Centre of Excellence in Engineered Fibre Composites (CEEFC),
Faculty of Health, Engineering and Sciences, University of Southern Queensland,
Toowoomba, Queensland 4350, Australia
2
Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung
Hom, Kowloon Hong Kong SAR
Submitted to
Composites Part B: Engineering
Corresponding Author:
Allan Manalo
Senior Lecturer in Civil Engineering
Centre of Excellence in Engineered Fibre Composites (CEEFC),
Faculty of Health, Engineering and Sciences,
University of Southern Queensland,
Toowoomba, Queensland 4350, Australia
Tel: +61 7 4631 2547 Fax: +61 7 4631 2110
E-mail: manalo@usq.edu.au
Manuscript summary:
Total pages 33 (including 1-page cover)
Number of figures 14
Number of tables 9
1
Effects of alkali treatment and elevated temperature on the mechanical
properties of bamboo fibre-polyester composites
Allan C. Manalo
1
, Evans Wani
1
, Noor Azwa Zukarnain
1
, Warna Karunasena
1
, Kin-tak Lau
2
1
Centre of Excellence in Engineered Fibre Composites (CEEFC),
University of Southern Queensland, Toowoomba, Queensland 4350, Australia
2
Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon Hong
Kong SAR
Email: manalo@usq.edu.au; w0063044@umail.usq.edu.au; NoorAzwa.Zulkarnain@usq.edu.au;
Karu.Karunasena@usq.edu.au; mmktlau@polyu.edu.hk
ABSTRACT
Bamboo fibre reinforced composites are not fully utilised due to the limited understanding on
their mechanical characteristics. In this paper, the effects of alkali treatment and elevated
temperature on the mechanical properties of bamboo fibre reinforced polyester composites
were investigated. Laminates were fabricated using untreated and sodium hydroxide (NaOH)
treated (4 to 8% by weight) randomly oriented bamboo fibres and tested at room and elevated
temperature (40, 80 and 120
o
C). An improvement in the mechanical properties of the
composites was achieved with treatment of the bamboo fibres. An NaOH concentration of
6% was found optimum and resulted in the best mechanical properties. The bending, tensile
and compressive strength of this composite is 44.2, 21.0, 111.2 MPa, respectively while the
stiffness is 4.0 GPa which are 7, 10, 81, and 25%, respectively higher than the untreated
composites. When tested up to 80
o
C, the flexural and tensile strength are slightly enhanced
but the bending stiffness and compressive strength decreased as these latter properties are
governed by the behaviour of resin. At 40 and 80
o
C, the bond between the untreated fibres
and polyester is comparable to that of untreated composites and resulted in almost same
mechanical properties. However, a significant decrease in all mechanical properties was
observed for composites tested at 120
o
C.
3
1. Introduction
Synthetic fibres such as glass, carbon and aramid have been used for several years in many
applications varying from aerospace components to civil infrastructures. However, the high
production and material costs of these fibres limit their wider use for the development of
composite materials. As a result, there is an increasing interest in utilizing the less expensive
natural fibres as reinforcement in composites because of their added advantages such as
lightweight, renewability and biodegradability. Joshi et al. [1] also revealed that natural fibres
are environmentally superior to glass fibres making them an emerging and realistic
alternative to synthetic fibres in some engineering applications. It is anticipated therefore that
the use of sustainable natural fibres in the development of new generation composites will be
a necessity and will play a crucial role in the near future.
Among the well-known natural fibres, bamboo has one of the most favourable
combinations of low-density and high stiffness and strength [2]. Nugroho and Ando [3]
indicated that these properties of bamboo make them a promising material for the
manufacture of various engineered composite products. However, it is only in recent years
that the interest in utilising bamboo as reinforcing materials for composites is increasing
because of limited availability of the fibres [2]. This is because it is technically difficult and
expensive to extract long and straight bamboo fibres [4-5]. Another significant challenge in
using bamboo is the inherent flaws within fibres which reduce their compatibility with
polymer matrices resulting in poor mechanical properties of the composites [6]. This is due to
the hydrophilic nature of natural fibres and the hydrophobic nature of the polymer matrix but
their poor compatibility can be improved by fibre surface modifications using chemical
treatments.
While many researchers have developed composites with natural fibres, the work on
bamboo fibre reinforced composites is still very limited. Most of the studies focused mostly
on tensile strength characterisation, thus Khalil el at. [5] suggested that more analysis and
testing are warranted to comprehensively characterise the mechanical properties of bamboo
fibre based composites. Similarly, investigation on the behaviour of such composites under
elevated temperature is scarce. There is a need therefore to comprehensively investigate the
mechanical properties of composites utilising bamboo fibres in order to increase its
acceptance and wider use in the composites industry.
In this study, bamboo fibres were treated with different levels of alkali solution and
infused with polyester resin to produce the composites. Experimental investigation using
coupon specimens following ISO and ASTM test standards were performed to characterise
the effect of alkali treatment on the flexural, tensile and compressive properties of the
bamboo fibre-polyester composites at room and elevated temperature. It is expected that the
results of this study will provide information to support the development and application of
cost-effective and eco-friendly bio-composites utilizing bamboo fibre through an evaluation
and understanding of their mechanical characteristics.
2. Background
Bamboo is a widely available material in many parts of the world and has been used
extensively as a substitute to wood in making furniture and low-cost housing. It is estimated
4
that more than 2.5 billion people depend on the use of bamboo [7]. Bamboo is found in
abundance in Asia and South America [5]. In Australia, bamboo has been in small-scale
cultivation in several areas of Queensland for more than 20 years [8] wherein the current
market is for shoots and culm production. However, the Australian market has long been
importing bamboo products for housing and construction such as flooring, laminates,
composite board, and chipboard. Thus, a study that supports the development and adaptation
of more renewable materials while providing the construction and building industry with an
alternative and environmentally sustainable materials such as bamboo is warranted.
Despite its high mechanical properties, biodegradability and low cost, bamboo is not
fully utilized in modern construction due to its cylindrical shape requiring special joints and
connections. Consequently, bamboo fibres are extracted and used as reinforcement for
polymer composites to practically apply the benefit of bamboo in various engineering
systems. Takagi and Ichihara [9] indicated that bamboo fibre is one of the most attractive
candidates as a strengthening natural fibre. It has several advantages, such as the
environmental load is small, because it is renewable yearly and it grows rapidly, and the
bamboo fibre has relatively higher strength compared with jute and cotton fibres. Rao and
Rao [4] indicated that bamboo fibres are stiffer but weaker than banana and sisal fibres,
which are two of the mostly utilised natural fibres. Amada et al. [10] estimated the tensile
strength, modulus and bulk density of bamboo culm is around 50 MPa, 2 GPa and 670 kg/m
3
and that of fibre is 610 MPa, 46 GPa and 1160 kg/m
3
, respectively. Added to these
advantages, bamboo is typical of unutilised natural bio-resources.
The favourable properties of bamboo fibres have led researchers to investigate its
potential use as reinforcement in composites. Mohanty and Nayak [11] developed short
bamboo fibre reinforced HDPE polyester composites with varying fibre content. They found
that the bamboo composites possess good tensile and flexural strength with fibre loading
from 10 to 30%, beyond which there was a decline in the mechanical strength. Takagi and
Ichihara [9] fabricated green composites from starch-based resin and short bamboo fibres
wherein they found that the strength of bamboo fibre reinforced composites were strongly
affected by fibre aspect ratio. Wong et al. [12] conducted investigation into the effect of
bamboo fibre length to tensile properties. Their research concluded that very short fibres
detracted from the strength of the unreinforced composites. Manalo et al. [13] compared the
mechanical properties of bamboo fibre composites made from non-woven textile, foam core,
and randomly oriented fibres. Their results indicated that the good mechanical properties of
bamboo composites can certainly have an edge over conventional panel products used in the
housing and construction industry. However, these researchers have highlighted the need to
modify the surface of bamboo fibres to enhance its bond with the polymer matrix to
effectively utilise the high strength of bamboo fibres in the developed composites.
Different chemical treatments are now possible to modify the structure and surface of
the natural fibres to improve the bonding with matrix and to enhance composite properties.
Mohanty et al. [14] indicated that alkaline treatment is one of the mostly used and least
expensive chemical treatments given to natural fibres. This treatment removes a certain
amount of lignin, wax and oil covering the external surface of the fibre cell wall resulting in
an increase in the amount of cellulose exposed on the fibre surface. The alkaline treatment
