Plant fibres reinforce plastic car parts
The automotive industry strives to produce light-weight and high-performance car parts that lead to reduced fuel consumption and enhanced safety. Plastics have facilitated many advances compared to conventional metal parts. For example, polypropylene, a thermoplastic polymer capable of being heated and formed repeatedly into different shapes, is currently utilised in bumpers, body panels and trims. Reinforced polymers are, as their name suggests, polymers ‘filled’ with typically fibrous compounds that enhance mechanical properties. More and more attention is turning to natural fibre reinforcement using materials from plants such as hemp, flax (linen), jute and kenaf. Natural fibres have high mechanical strength and low weight. They also have significant advantages over conventional reinforcements including low cost, sustainability in manufacturing and recyclability. One of the limitations to their widespread use has been their inferior performance at the processing temperatures required for extrusion of polymers. Extrusion is a process whereby melted material is pushed through a screw-like piece of equipment leading to a die that moulds it into a specific shape. European researchers sought to facilitate more flexible use of natural fibre reinforcement through development of processing methods suitable for thermoforming (forming with the use of heat). Specifically, the EXTRU CO2 project sought to use supercritical carbon dioxide (scCO2) as a processing aid. The ultimate goal was to enable extrusion of natural fibre-filled polypropylene sheets without typical detrimental effects on mechanical characteristics, colour and odour. Numerous innovations in compound formulations, natural fibre compound manufacture and scCO2 injection equipment enabled a reduction in resistance to flow that successfully facilitated lower extrusion processing temperatures. Mechanical properties of the extruded reinforced sheets were well suited to automotive industry requirements. Products made from the sheets were superior at reasonable cost. Impressively, up to 50 % of recyclate from the products could be used in new sheet production with no detrimental effect on mechanical properties. EXTRU CO2 thus successfully overcame the obstacle of high-temperature processing previously necessary for extrusion. Innovative technology to produce natural fibre-reinforced thermoplastic sheets for the automotive industry should encourage more widespread use of affordable and recyclable materials.
