Facing up to the challenges of natural fibres as an engineering composite reinforcement : Abstract and Presentation

Thomason, James (2013) Facing up to the challenges of natural fibres as an engineering composite reinforcement : Abstract and Presentation. In: Composites Week @ Leuven, 2013-09-16 - 2013-09-20.

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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). 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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