JEC COMPOSITES MAGAZINE - Issue #37 - December 2007 - 34

FEATURE “Biocomposites” High-performance natural fibres? Using natural fibres to reinforce polymers is a topical issue. Thanks to the state of development and the availability of a range of biopolymers, it is now possible to produce “biocomposites”. Some of the advantages of these materials are that they constitute a renewable resource, leave a smaller environmental footprint, and can be composted at their end of life; another is that parts made with them have good carbon storage potential. Similarly in a plant fibre, the physical properties are determined mainly by the chemical and physical composition, structure, cellulose content, microfibrillar angle, cross section, and degree of polymerization [2]. To simplify: for a given cellulose content, the lower the microfibrillar angle, the stiffer and stronger will be the fibre; the greater the microfibrillar angle, the higher will be elongation at break. Cellulose fibrils have a Young’s modulus in the macromolecular chain direction of 138 GPa (cellulose I) and an estimated tensile strength of 17.8 GPa [3]. The highest Young’s modulus for a plant fibre with high cellulose content, low microfibrillar angle, and absence of defects is estimated at 128 GPa [3]. For example, the microfibrillar angle of flax fibres is 11° [4] and of ramie fibre (a member of the nettle family), 3° [5]. A closer look at a plant’s cell wall shows that the fibre structure consists of a primary wall and a secondary one that has three layers: S1, S2 and S3. The S2 layer is the thickest, and determines the behaviour of the fibre as a whole. Due to this structure, the material is highly anisotropic [6]. CHRISTOPHE BALEY, ASSISTANT PROFESSOR, UNIVERSITÉ DE BRETAGNE SUD, L2PIC P otential users of natural fibres in the composites sector often wonder about their mechanical properties: how welldistributed and reproducible these properties are, how available the fibres are, and how much they cost. It is not unreasonable to search for the “ideal” plant, i.e. the one that will supply the fibres for the best compromise. Note that, out of the many existing natural fibre varieties, those with the most advantageous properties are the ones which play a structural role in nature.The object of this article is not to present an inventory, but to give examples of “high-performance” natural fibres. Plant-fibre structure A plant fibre can be compared to a composite material that is reinforced with cellulose fibrils (Figure 1) [1]. These are oriented helically at an angle called the microfibrillar angle. In a composite material, the elastic and break properties are usually a function of the reinforcement content and fibre orientation. Comparing tensile properties Table 1 shows a comparison of the tensile properties (measured by tests on single fibres) of various plant fibres (jute, sisal, hemp and flax), of regenerated cellulose fibres (viscose and lyocell) and of glass fibres (the most commonly used reinforcement for composite materials). The mechanical properties for jute, sisal, hemp and flax fibres from [8] are consistent with average values found in the literature. While the order of magnitude is favourable, these properties are not directly exploitable; the mechanical properties of the fibres from any one given plant are a function of many things. These include variety (there are, for example, about 300 different varieties of flax), the growth conditions (soil, temperature, how much water and sunlight the plant has received, farming practices, etc.), plant maturity at harvest, fibre-extraction methods used (retting or not, scutching, combing, etc.), the percentage of water absorbed and the mechanical characterization protocol. Therefore, we must avoid reaching hasty conclusions about whether or not the extracted fibres from a given plant are of interest. In [9,10,11,12], all of Fig.1: Plant-fibre structure [1] JEC Composites Magazine / No37 December 2007

JEC COMPOSITES MAGAZINE - Issue #37 - December 2007

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #37 - December 2007