Variability of flax fibre morphology and mechanical properties in injection moulded short straw flax fibre-reinforced PP composites

The influence of compounding and injection moulding on the initial variability and morphology of short straw flax fibres is determined and the mechanical properties for the injection moulded fibre reinforced composites are measured. It is found that the composition of the straw flax, flax fibre bundles and woody parts, together with the cutting process strongly affects the initial fibre morphology and its variability. In the initial fibres, small particles as well as long fibres with large width were found. A filter was used to reject the fibres with an aspect ratio below 15 before calculating statistics because these fibres have a negligible contribution to the composite reinforcement. After processing, the initial fibre length and width decrease strongly (−38% to −66% for length and −22% to −72% for width). Also, the variability is affected resulting in a standard deviation shifted towards lower fibre lengths and widths (−55% for length and −71% for width). The improvement of mechanical properties of the flax compound compared to the pure matrix material for the injection-moulded samples is found to be similar to the results for compounds with further processed flax fibres such as scutched and hackled fibres. An increase of tensile strength by 20% was found, for stiffness the increase is in the order of 50–70%. This indicates that despite the very large variability of the initial straw flax fibres and the strong changes of the variability in each processing step, a compound is obtained with improved mechanical properties.