Resumen de Development of new lightweight green composites reinforced with nonwoven structures of flax fibres
Composite materials offer good mechanical performance with lower weight than classical materials, what has encouraged their use in many industry applications. Moreover, the rising awareness about environmental problems has prompted the progress towards more sustainable materials, thus encouraging the research in natural fibre reinforcements and biopolymeric matrices. This PhD thesis focuses on the development of green composites made of polyhydroxyalkanoate (PHA) matrix with a reinforcement of flax nonwoven fabrics, while considering materials and processes with low environmental impact. However, in order to achieve lighter materials with even further improved specific properties, the strategy chosen is the enhancement of weight reduction of the matrix by means of foaming processes.
The approach considered in this thesis differs from the classical approaches with these materials in its use of foaming techniques to obtain a cellular structure in the matrix, This, in addition, enhances the competitiveness of these composites, since biopolymers present (nowadays) a high cost. However, the cellular structure obtained after foaming leads to a decrease in the mechanical properties. The use of fibres of high strength is a key factor for counteracting this loss. In this sense, a novel approach is given for the use of flax fibres (of high stiffness and low cost) as reinforcement, since they are used in the form of nonwoven structures.
The use of natural fibres in composites has two main concerns: the fibre/matrix interaction and their sensibility to moisture absorption. This drawbacks can be reduced by using sustainable methods. On the one hand, the compatibility with the matrix can be improved by plasma techniques, which modify the roughness and chemistry of the fibre surface (without affecting the bulk properties) with a minimal use of chemicals and no need of water. On the other hand, wet/dry cycling treatments with water are an interesting option to increase fibre stability against water absorption and dimensional changes without the need of chemicals. The effectiveness of these sustainable treatments applied to the nonwoven is evaluated by means of hydrothermal aging and mechanical characterisation on solid PHA/flax composites. The matrix foamability is evaluated by means of extrusion foaming, considering two main strategies for improving this process: the use of chain extenders and cooling by means of a water quenching. Finally, the most challenging novelty (given the limitations imposed by both the reinforcement structures and the matrix foamability) is the achievement of the cellular composites reinforced with the optimized flax fabrics. A gas dissolution batch foaming from solid precursors is used for such a production. This strategy is evaluated and optimised to this purpose, leading to cellular composite materials with good specific properties and reduced density.
The combination of properties of both components leads to a biodegradable, stiff and lightweight material that present good regularity in the foaming process, which prompts a possible replacement for other less sustainable materials in automotive, construction or other applications. Due to the aforementioned, this thesis presents the development of a challenging new approach for lightweight green composites from a multidisciplinary point of view.
