High bio-content fibre composites

Fibre reinforced plastic composites (FRPs) are heavily used in engineering applications such as vehicle structures. In most cases these composites comprise thermosetting epoxy polymers derived from oil and glass or carbon reinforcing fibres. Although the weight saving due to FRPs can offer a more sustainable option, e.g. by reducing fuel consumption, there is room for significant improvement in their environmental impact. This project aims to produce more sustainable composites by using materials with a renewable source, i.e. bio-based epoxy polymers and regenerated cellulose fibres, and to determine the feasibility for these to replace glass fibre reinforced plastic composites. In engineering applications, good mechanical strength and stiffness, accompanied by a high fracture toughness, are essential to provide a resilient material with a long service life. Epoxy polymers are thermosets so are inherently brittle materials, but it is possible to improve their fracture toughness by adding toughening fillers, e.g. microcellulose or nanocellulose particles.

Cellulose is an attractive bio-based material as it exhibits good mechanical properties, and it is a cheap and abundant material. However, some issues arise when processing cellulosic materials with epoxy polymers. Cellulose is hydrophilic, attracted to water, and readily absorbs moisture (undesirable in composites as it degrades performance); whereas the epoxy matrix is hydrophobic. This difference in surface properties leads to poor interfacial strength and air voids. Also, cellulose experiences particle-particle interactions which can lead to agglomeration and poor dispersion. Air voids and agglomerates act as stress concentrators in the material, reducing its strength and toughness. Therefore, it is important to understand how these defects can be minimised and the dispersion of the cellulose particles can be optimized.

During the manufacture of composites using low-cost infusion methods, large particle agglomerates cannot pass through the fibre stack, yielding a poor distribution of filler along the length and through the thickness of the composite. Surface treatments applied to cellulosic fibres and particles can simultaneously reduce the cellulose-cellulose interaction and improve cellulose-epoxy compatibility, hence improving dispersion and reducing water absorption. However, these treatments also have a negative effect on the properties of the cellulose itself. Mechanical methods, such as filtration and agitation will be implemented to reduce the size and frequency of agglomerates in the polymer matrix to improve the overall composite performance.

Silane surface treatments and particle dispersion methods will be used to improve the quality and uniformity of cellulose particle dispersions in bulk epoxy and as a composite matrix. Surface treatments will also be applied to improve the adhesion between epoxy and regenerated cellulose fibres. The dispersion of cellulose particles throughout bulk epoxy samples and fibre composite panels will be quantified. The mechanical and fracture properties of the epoxy-cellulose composites will be measured and related to changes in microstructure to determine the optimum processing conditions. The properties and environmental impact of the resulting cellulose particle toughened composites, with a bio-based epoxy matrix and cellulose reinforcing fibres, will be compared to conventional glass fibre reinforced plastic composites.