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The notion of life cycle for a product or a technical component is a key aspect of any eco-design approach. With this in mind, recyclability needs to be taken into account from as early as the design stage, and it confers a definite advantage on the end product.
CHRISTOPHE BALEY, PROFESSOR, ALAIN BOURMAUD, RESEARCH ENGINEER, UNIVERSITÉ DE BRETAGNE SUD, LABORATOIRE D’INGÉNIERIE DES MATÉRIAUX DE BRETAGNE (LIMATB)(Published on February-March 2010 – JEC Magazine #55)
Reuse by means of a recycling step is possible, within certain limits, for thermoplastic-matrix composites. Generally, this recycling consists in the crushing of the parts followed by a mixing phase in order to blend in these recycled materials with virgin material. This mix is then processed under high pressure and temperature by means of conventional injection or extrusion technologies.
With each new recycling, the properties of the matrix and of the fibres, and those of the fibre-matrix interface, change [1-3]. This is particularly true for the aspect ratio of the fibres. This parameter, which represents the ratio between the length and the diameter of the reinforcement fibres, plays a key role in the reinforcement mechanisms of a composite, since it conditions the load transfer between fibre and matrix. As part of our recent research works, we have been focusing on the recycling of thermoplastic-matrix composite materials made of polypropylene (PP) and polylactic acid (PLA) resins reinforced with plant fibres such as flax, sisal and hemp [1-3]. These materials were processed via injection then crushed and ground using an industrial cutting mill. The size of the pellets obtained after milling was between 2 and 4 mm.
Initially, recyclability studies were carried out on polypropylene reinforced with 30% hemp fibres by weight as manufactured by AFT Plasturgie (France) . Six successive injection and milling cycles were carried out in order to test the recyclability of this material. To assess the recycling behaviour of the composite, we carried out the same operations on the virgin polypropylene matrix and on polypropylene with 30% by weight of glass fibre reinforcement. Figures 1 and 2 present the evolution of tensile modulus and tensile strength as a function of the number of recycling cycles.
Material and process choices
The evolution of the mechanical properties as a function of the number of processing cycles (Figures 1 et 2) demonstrates that the stiffness and tensile strength of the polypropylene-hemp fibre composite remain stable, unlike those of the polypropylene-glass fibre composite.
The stable mechanical properties of composite materials reinforced with hemp fibres could be due to the limited evolution of the aspect ratio over the number of cycles (Figure 3).
Figure 3 illustrates the high initial difference in aspect ratio between the two types of fibres. Glass fibres are produced industrially and calibrated so as to have a constant diameter. Furthermore, they are perfectly individualised.
Within a plant, things are quite different. These fibres, which serve as support tissues, are assembled in bundles, with each bundle containing several dozen fibres. This initial condition means that packets of fibres are to be found within the compound. The average diameter of the bundles measured in the initial compound is 125±23 microns as against 10 microns for a glass fibre.
During the milling and injection steps, the high shear rates cause a reduction in the length of the fibres along with a division of the bundles, naturally reducing their average diameter. The conjunction of these two phenomena explains the low loss of aspect ratio and the relatively stable resistance and stiffness of the composites. Our work has also brought to light an evolution in the properties of the polymer and of the interface over the course of the recycling cycles .
Similar results have been obtained for composites reinforced with sisal fibres .
Micromechanical models were used to estimate the Young’s modulus of composite materials according to the number of recycling cycles, with good correlations . These models take account of the marked anisotropy of plant fibres. The longitudinal Young’s moduli were measured via tensile on the unit fibre and the transverse Young's moduli via nanoindentation.
Another aspect of our work addresses the recycling of polylactic acid-flax fibre biodegradable composites . The composites used in this work were either reinforced with short fibres and shaped via injection, or reinforced with long fibre mats (10 mm) and processed via film stacking (high-temperature compression of a stack of PLA films and flax mats). The Young’s modulus proved to be stable over the course of the cycles. This result can be explained by the low impact of recycling on the aspect ratio, as previously stated. Furthermore, a loss of strength may be noted over the number of cycles, which may essentially be explained by the evolving properties of the PLA matrix.
These various experiments [1, 2, 3] demonstrate that it is possible to recycle thermoplastic-matrix composites reinforced with plant fibres.The aspect ratio of the fibres remains practically constant despite the recycling steps (injection and milling), resulting in fairly stable mechanical properties.
The use of a biodegradable matrix reinforced with plant fibres offers environmental benefits by enabling end-of-life recycling or composting
 Bourmaud, A., Baley C., Investigations on the recycling of hemp and sisal fibre reinforced polypropylene composites. Polymer Degradation and Stability, 2007. 92(6): p. 1034-1045.
 Bourmaud, A., Baley C., Rigidity analysis of polypropylene/vegetal fibre composites after recycling. Polymer Degradation and Stability, 2009. 94(3): p. 297-305.
 Le Duigou A., Pillin I., Bourmaud A., Davies P., Baley C., Effect of recycling on mechanical behaviour of bio-compostable flax/poly(L-lactide) composites. Composites Part A, 2008. 39 (9): p. 1471-1478.