Processing and Properties of Natural Fibre Reinforced Gluten Composites

Wheat gluten, a co-product from the starch and bioethanol industry, is used as an ingredient in many food applications. Besides, wheat gluten has also been considered for use as raw material for non-food applications, such as for biopolymers and biocomposites. Due to increasing environmental awareness, biodegradable materials such as wheat gluten, have received attention because they are low-cost raw materials, annually renewable, and readily available. In the absence or at low concentrations of plasticizer, high temperature compression moulding of wheat gluten results in rigid, glassy polymer materials with stiffness and strength, approaching those of synthetic resins such as epoxy. Moreover, gluten composites reinforced with natural fibre can obtain higher stiffness and strength compared to pure polymers. Natural fibres have an advantage over synthetic fibres since they are intrinsically biodegradable, annually renewable, and have low energy utilization. However, developing a fabrication process for rigid gluten composites with natural fibres is quite challenging due to the powder form of gluten, the high protein viscosity and the thermoset character of gluten. Indeed, the gluten cross-links formed at high temperatures increase the protein viscosity during compression moulding. Therefore, in this research, some methodologies are proposed to process natural fibre reinforced gluten composites with high mechanical properties. Some possibilities are proposed to process unidirectional flax fibre reinforced rigid gluten composites such as a dry and a wet method. In the dry method, a dry gluten powder is used to make composites. In case of the wet method, an ethanol solution is used to partially dissolve gluten to process gluten composites. The results show that using a wet method gives much better mechanical properties (modulus and strength) than using a dry method. This is expected since gliadin can be dissolved in the alcohol solvent and can easily penetrate in between the fibres, hence the impregnation can be improved. This can also be proved by much higher modulus and strength of gliadin composites (prepared by solution impregnation), compared to glutenin composites (prepared by suspension impregnation). By utilizing a suspension-solution process the entire gluten material can be impregnated into a long fibre composite, with good mechanical properties. However, the interface strength of gluten based composites is relatively modest which needs to be improved. To develop further the gliadin composites (giving the highest modulus and strength among other gluten composites at the same fibre content), the interlaminar fracture toughness was investigated to determine fracture mechanisms at the initiation and propagation region of damage. Moreover, in order to obtain optimal fracture toughness, three different parameters were studied for flax/gliadin composites namely the processing conditions (affect the degree of cross-linking), matrix plasticization (improves the matrix toughness), and fibre treatment (increases the surface roughness to improve the fibre-matrix interface). The results show that interface quality is more dominant than the effect of a low amount of plasticizer for the initiation fracture toughness value. Moreover, crack initiation is closely related to weak links such as the fibre-matrix interface while crack propagation appears to be mainly in the polymer matrix. The initiation and propagation fracture toughness significantly increased with the presence of three combined parameters: cooling down at medium rate, addition of glycerol, and alkali treatment of the fibres. It is hypothesized that this is due to the combination of plasticization of the matrix by glycerol, improved fibre-matrix adhesion by alkali treatment and increased level of non-disulfide cross-linking at medium cooling rate, visible in a more plastic failure mode at the fracture surface. One of the greatest challenges in working with biomaterials is their sensitivity to water. Therefore, in this research, gluten and gliadin polymers and their natural fibre composites were subjected to moisture absorption. Their diffusivities were determined, showing that moisture in gliadin polymers diffuses faster than in gluten polymers. The increase is even more pronounced for the composites. Moreover, the preliminary results show that their mechanical properties are strongly affected by the moisture absorbed, namely showing significant decreasing modulus and dramatic increasing failure strain. In conclusion, this doctoral work has developed some methodologies to make unidirectional natural fibre (flax fibre in particular) rigid gluten based composites with high mechanical properties. A good impregnation is obtained while the interface strength between fibre and matrix is still relatively modest and would be candidate for improvement. The materials are moisture sensitive and good fracture toughness could be obtained in optimised conditions.status: publishe