FACING UP TO THE CHALLENGES OF NATURAL FIBRES AS A
REINFORCEMENT FOR ENGINEERING COMPOSITES
J.L.Thomason
Department of Mechanical and Aerospace Engineering, University of Strathclyde, 75 Montrose Street, Glasgow
G1 1XJ, United Kingdom.
Email: james.thomason@strath.ac.uk
ABSTRACT
The use of glass fibre reinforced polymer materials has increased dramatically during the last
half century, becoming the standards of high performance in automotive, aeronautical and
innumerable other high performance applications. Glass fibres currently represent more than
95% of the reinforcement fibres used globally in the engineering composites industry.
However, the increasing pressure on natural resources and the large amounts of energy
required in glass fibre production has led to an upsurge in the research of fibres derived from
natural sustainable sources as potential reinforcements for high performance composite
materials. It has been claimed that natural fibres show significant potential as environmentally
friendly alternatives to conventional reinforcements such as glass fibres (Bledzki 1999,
Mohanty 2002, Wambua 2003). Nevertheless, a certain level of reinforcement performance is
required from such fibres in order to succeed in engineering applications. In this context many
researchers refer to the respectable level of axial modulus of some natural fibres, which can
be made to appear even more attractive by comparing modulus/density ratios (see Table 1).
Flax
Hemp
Sisal
Jute
Glass
Table 1 Typical range of properties of natural and glass fibres
Modulus
Strength
Density
(GPa)
(GPa)
(10-3kg/m3)
27-80
0.35-1.5
1.4-1.5
60-80
0.55-0.9
1.4-1.5
9-38
0.4-0.7
1.33-1.5
10-42
0.4-0.8
1.3-1.46
75
1.5-2.5
2.6
“Diameter”
(m)
45-60
20-200
50-300
25-50
10-17
However, such claims implicitly depend on the use of simple micromechanical models
suitable only for isotropic materials and often ignore the well documented low levels of shear
and transverse modulus of these highly anisotropic natural fibres (Cichocki 2002, Baley 2006,
Ntenga 2008, Thomason 2009, Gentles 2010, Thomason 2010, Shah 2012). Consequently, an
overwhelming number of the published results based on such justifications have failed to
fulfil the expectation of matching glass fibre reinforced composite performance. Despite the
very large amount of research resource expended in this area, natural fibres continue to
exhibit only moderate reinforcement in stiffness and very little positive (and often negative)
effects on the strength and impact resistance of composites, often resulting in materials with
many characteristics lower than the original base polymer.
The majority of the widely available and commercially attractive, low cost, “technical”
natural fibres present a number of challenges in terms of their characterization, processing and
performance. A list of these challenges would include the very high levels of natural fibre
variability, the very high levels of natural fibre anisotropy leading to very poor transverse and
shear reinforcement performance, the non-circular natural fibre cross section with “diameters”
often orders of magnitude larger than man-made fibres, the introduction of voids in the
composites through the natural fibre lumen, the very poor (often negative) reinforcement
performance of natural fibres composites, the high moisture content of natural fibres both
before and after processing into composites, the health and performance issues related to the
bioactive nature of these fibres, the temperature sensitivity during composite processing of
natural fibres – in particular odour issues, the very low levels of stress transfer capability at
the natural fibre – matrix interface, the unsuitable delivery form of many natural fibres for use
in standard composite industry processes, and the need for many high cost treatments and
processing steps involved in attempting to overcome these, and many other, challenges in
order to produce a reinforcement that has anywhere near the potential to replace glass fibres
in any high volume engineering application.
This paper will review a number of these challenges and focus in detail on two areas, natural
fibre anisotropy and their non-circular cross section, which although almost universally
acknowledged, have received little attention in respect of their consequences on the
characterization and performance of natural fibres as a composite reinforcement.
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