JEC COMPOSITES MAGAZINE - Issue #55 - February/March 2010 - 38

FEATURE … … “Natural Fibres & Environment” Recycling g / Recycling composite materials reinforced with plant fibres 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). 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 [1]. Similar results have been obtained for composites reinforced with sisal fibres [1]. Micromechanical models were used to estimate the Young’s modulus of composite materials according to the number of recycling cycles, with good correlations [2]. 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 [3]. 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. Fig. 3: Evolution of fibre aspect ratio with the number of recycling cycles 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. Conclusion 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. More information: http://web.univ-ubs.fr/limatb/ References [1] 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. [2] Bourmaud, A., Baley C., Rigidity analysis of polypropylene/vegetal fibre composites after recycling. Polymer Degradation and Stability, 2009. 94(3): p. 297-305. [3] 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. Focus Thanks to their genuine technical and environmental advantages, natural-fibre-reinforced composite materials are flourishing and can no longer be considered merely as an example of “greenwashing”. The throwaway era is behind us, and these days, we need to focus on end of service life (EOSL) and recycling issues for these materials. The development of life cycle assessment procedures as defined by the ISO 14040 standard has given a further boost to this approach, with selected scenarios leading to “cradle to gate” and then “cradle to grave” cycles; now, the recyclability of materials allows us to consider a “cradle to cradle” work process. jec composites magazine / No55 February - March 2010

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JEC COMPOSITES MAGAZINE - Issue #55 - February/March 2010

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #55 - February/March 2010