Manufacturing and Characterisation of Reactive Polylactic Acid (PLA)-Unidirectional Flax Fibre Composites

Abstract

The main objective in this research was to trial two matrix preparation methods, powder-coating and film extrusion, to manufacture a unidirectional tape prepreg with flax fibres. The preparation was done manually as a precursor to developing a continuous prepreg process. Three processing variables namely time, temperature and pressure were optimised in the prepreg production. Composites were comprised of 50 vol.% Polylactic acid (PLA), with and without fibre treatment of 5% Sodium hydroxide (NaOH) and 1% 3-Glycidoxypropylmethoxysilane (GPTMS), and 50 vol.% unidirectional flax fabric. The secondary objective was to review the changes in interfacial properties between PLA and treated and untreated flax fibres. The fibre treatment was only trialled with powder-coating tape prepreg due to limited availability in raw materials. Three PLA blends were evaluated but only two were selected. Natureworks-2002D and University of Auckland developed Reactive Extrusion-Grade were chosen based on their physical and mechanical properties to continue onto the next stage of manufacturing a composite panel. The unidirectional flax fibre reinforced PLA composites were manufactured by film stacking followed by compression moulding. The manufacturability, interfacial and mechanical properties of the final composites were compared. The composites were evaluated based on their tensile, flexural and impact properties. Overall, the Natureworks-2002D film extrusion prepreg composites displayed the highest stiffness and strengths in both tensile and flexural properties. Reactive Extrusion-Grade film extrusion prepreg displayed high stiffness and strengths in tensile properties. In addition, it was able to absorb more energy in plastic-elastic form during impact. With no fibre treatment, poor adhesion between the fibre-matrix interfaces was observed with the use of Scanning Electron Microscopy (SEM) due to surface incompatibility between the hydrophilic fibres and hydrophobic polymers. With NaOH treatment, there was no sign of improvements in interfacial properties; however with the addition of GPTMS, fibre-matrix adhesion improved dramatically. In addition, tensile and flexural strength increased after fibre treatments. This research clearly shows that the performance of the final composite was dependent on the appropriate selection of manufacturing process and materials.