In-service gedrag van vlasvezelversterkte composieten voor hoogwaardige toepassingen

摘要

The use of flax fibres in the composites industry has been steadily growing, thanks to their good mechanical properties, manufacturing effectiveness, acoustic and thermal insulation, good vibration damping, low density and renewability. Flax fibres are a good alternative to less sustainable fibres, such as glass fibres, and they are good candidates to be used for high performance composite applications. The performance of flax fibre composites is believed to be controlled by the intrinsic properties of the flax fibres and the polymer matrix as well as the textile architecture features (e.g. twist). To achieve a good understanding of these effects on the flax composite properties, the following methodology was are undertaken:1 - Understanding the effect of textile architecture/matrix combinations on the quasi-static mechanical behaviour of flax composites;2- Investigation of the internal geometry of the flax composite and their implementation in modelling tools to predict the quasi-static properties;3- Evaluation of the effect of fibre architecture on the impact and fatigue behaviour of flax composites;4- Environmental impact assessment of flax composites end-of-life technologies and a comparative life cycle assessment of the flax composites in comparison to standard materials for two automotive cases.These studies were carried on using commercially available textile architectures (random mat, plain weave, twill 2x2, quasi-UD and UD) and two types of matrices, a thermoset epoxy and a thermoplastic maleic anhydride polypropylene (MAPP). The internal geometry of textile reinforcements is known for having a significant influence on the mechanical properties of composites. The study of the quasi-static properties (tensile and flexural) showed that the matrix type intrinsic properties have an important effect of the stiffness and strength properties of the composite. No large difference between the different types of woven fabric was seen, although the effect of specific reinforcement geometry parameters (crimp, twist, weave style, wet or dry spun yarns) could be identified. With the quasi-static properties known, the second step of this work aimed to predict these data through modelling. The combination of the textile features with the effective mechanical properties of the fibre/yarn and the matrix properties would make an easy tool to assess the potential of each textile architecture combination, while saving time on the trial-and-error methodology. The internal geometry was analysed via micro-CT imaging. The effective properties of the fibre/yarn used in the textile were assessed using the impregnated fibre bundle test method (IFBT). These parameters were then inputted into the Wisetex-Lamtex-Texcomp trio of software to reconstruct the textile 3D model and predict its quasi-static properties. The results showed that the predicted moduli were comparable to the experimental results with a maximal variation of 10%. The investigation of the impact and fatigue properties of flax-based composites is key in order to understand which material parameters determine the safety and longevity of flax composite products. For impact, the matrix choice was found to greatly influence the absorbed energy as well as the damage area. The absorbed energy at perforation for the flax-MAPP composite was more than 50% higher than for the flax-epoxy one. Furthermore, the use of a MAPP instead of epoxy led to a decreased impact damage area by 38% to 59% with limited delaminations. The decrease of the flexural properties after impact of the flax-MAPP composite was marginal, which is related to the increased absorbed energy per area, while the flax-epoxy composites experienced a stronger decline in properties after impact. The hypothesis that the presence of delaminations has an important influence on the impact performance of the flax composites has been proved wrong, since a limited amount of small delaminations was seen after a non-perforation impact. This was due to the high interlaminar fracture toughness properties of flax composites, which increases by at least 2-3 times over that of the unreinforced brittle epoxy polymer. The tensile toughness was found to be a good indicator of the capacity of a material to sustain perforation or non-perforation impact. The characterization of the tension-tension fatigue properties of flax-epoxy composites showed that the fibre architecture has a strong effect on the fatigue behaviour, where higher quasi-static strength and modulus combinations present the best fatigue characteristics. They have a delayed damage initiation and increased fatigue life as well as a reduced damage propagation rate combined with higher energy dissipation in the early stages of fatigue loading. Furthermore, a comparison with a glass fibre benchmark showed that the behaviour of flax-epoxy composites is comparable to glass fibre reinforced composites, when expressed in terms of specific stress (stress/density) and thus, flax composites are suitable for many new or existing industrial applications. To close the life cycle loop of the flax composite product, three end-of-life (EOL) options were investigated for the flax-MAPP composites: chemical recycling, mechanical recycling and incineration. It was found that the chemical recycling technique is feasible based on the mechanical properties of the recycled composite. However, its processing time, chemicals needed and equipment have negative effects on the environment. The second method, the mechanical recycling, resulted in a recycled, injection moulded composite with discontinuous fibres and hence somewhat lower mechanical properties compared to those of fresh random mat composites. The main advantage of the mechanical recycling technique is the speed of the process. Very large quantities of waste can be shredded and processed into new components while reducing the environmental burden of producing fresh MAPP and flax fibre. The last EOL method studied was incineration with energy recovery and it has been found to be a good alternative as well since all the material can be fully combusted and embodies a relatively high calorific value. Finally, to justify the use of flax composites instead of standard materials, two cradle-to-grave comparative life cycle assessments (LCA) were carried on for two automotive cases, a car roof (flexural design) and a bumper (impact design). A mass factor methodology was used in order to fairly compare equivalent amounts of flax and glass fibre based on the mechanical performance that need to be achieved. For the impact design case, the results of the life cycle analysis showed that the replacement of glass composite by flax composite was beneficial in the 40% and 50% fibre volume fraction case. However, it was not the case for the 30% fibre volume fraction case since the mass factor was higher than 1. This means that, at 30% fibre volume fraction, a higher amount of flax (1.11 kg vs 1 kg of glass fibres) is needed to be used in a composite to fulfil the same impact resistance performance. For the flexural design case, it was found that the use of flax composite has lower environmental impact than their glass composites counterparts. Overall, the LCA results showed that flax fibre composites are an environmentally favourable choice in comparison to glass fibre composites. However, the trade-off between mechanical properties has to be evaluated for each design case specifically.