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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.
(Published on December 2007 – JEC Magazine #37)
CHRISTOPHE BALEY, ASSISTANT PROFESSOR, UNIVERSITÉ DE BRETAGNE SUD, L2PIC
Potential 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.
A plant fibre can be compared to a composite material that is reinforced with cellulose fibrils (Figure 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.
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 . 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 . 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 . For example, the microfibrillar angle of flax fibres is 11°  and of ramie fibre (a member of the nettle family), 3° . 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 .
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  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 the parameters were controlled. As far as we know, nettle fibres are no longer available for industry. Studying them is still of interest, however: the plant is well adapted to the European climate, the fibres have good mechanical properties, and there is potential for upgrading a whole group of co-products.
We can see from Table 1 that:
The values for the properties of natural fibres are widely distributed – a common reproach. This should not be overlooked, but inspires a few remarks:
Obviously the mechanical properties of an anisotropic fibre are not limited to three tensile properties, and the thermomechanical behaviour under different stress/strain conditions should also be studied. In addition, research needs to be carried out on the fibre/matrix bond and ply properties of a composite material. However, all this is beyond the purview of this paper.
In the near future, we will need to set up channels that are specific to each area of plastics processing. There is also a need to draw up reference documents establishing specifications for natural fibres that are suitable for use in composite materials. Setting up production channels for natural fibres involves taking unforeseen weather factors into account so as to be able to select and standardize batches of fibres from different origins. Crops should be environment-friendly. The utilization of natural fibres often reflects an eco-design approach and a desire to reduce environmental impacts. This requires data. Research is now being published on the environmental impacts of the different methods for producing flax and hemp fibres .
The quest for the “ideal” and available natural fibre is not all that simple. The choice of plant is made not only based on the mechanical properties of the fibres, but also as a function of the potential to upgrade all the co-products (fibres, wood, leaves, seeds). More generally, using natural fibres to reinforce composite materials is part of a strategy to create structures that consume less energy and leave a smaller environmental footprint. By virtue of this, some natural fibres are truly high-performance fibres.