Thermoplastic matrix composites based on engineering polymers and renewable fibres

This dissertation deals with the development of thermoplastic matrix composites (TMC) based on engineering polymers and renewable fibres. TMCs reinforced with renewable fibres are usually based on lignocellulosic or cellulosic fibres and low flow/melting temperature thermoplastics due to the poor thermal stability of the fibres, which can lead to degradation during processing. Such TMCs are limited to applications requiring a modest level of mechanical and thermal performance. Significantly better properties of TMCs can be achieved by using engineering polymers as matrices, however, their development presents many challenges, several of which were explored in the dissertation.
The first part deals with the investigation of the reinforcing effect of renewable fibres processed at elevated temperatures. Two natural fibres (flax and hemp) and one man-made fibre (lyocell) were compounded into polypropylene (PP) by twin-screw extrusion and injection moulded at various temperatures (180 °C – 260 °C, with 20 K steps). The decomposition behaviour of the fibres was characterised by thermogravimetric analysis (TGA). The microstructure of the TMCs was characterised by optical microscopy (OM). The optical, mechanical, thermal, and rheological properties were characterised by tensile tests, Charpy impact tests, dynamic mechanical analysis (DMA), melt flow rate (MFR) and colorimetry. The mechanical properties of the TMCs decreased at 200 °C, with a steep decrease at 240 °C. Increasing processing temperature led to decreased mechanical properties and darker colours. The TMCs reinforced with lyocell fibres had the highest tensile strength and the best retainment of mechanical properties at high temperatures, followed by flax and hemp fibres. The reinforcement effect of fibres in TMCs processed at elevated temperatures can be predicted by TGA measurements.
The second part deals with the investigation of the geometry influence of renewable fibres on the structure-property relationships of TMCs. Seven different viscose fibres that vary in geometry were compounded by twin-screw extrusion and injection moulded. The microstructure of the TMCs and the length of the extracted fibres were evaluated by OM. Mechanical and thermal properties were evaluated by tensile tests, Charpy impact tests, DMA, and differential scanning calorimetry (DSC). TMCs containing cylindrical fibres with a linear density of 1.7 dtex and length of 5 mm had the best mechanical properties due to the longest and best dispersed and oriented fibres, with lower linear densities and higher initial lengths leading to shorter fibres and worst mechanical properties. Non-cylindrical fibres were well dispersed and distributed fibres, but with low aspect ratios and large fibre-fibre contact areas due to their shape, which was reflected in the poor mechanical properties of the TMCs.
In the last part, a novel one-step approach for the preparation of TMCs based on engineering polymers and renewable fibres is presented. Fibre direct compounding (FDC) technique was modified, combined with a low-temperature processing approach and used for the preparation of TMCs based on PA6 and lyocell fibres. In parallel, analogous composites were compounded by twin-screw extrusion and injection moulded. The microstructure of the TMCs was characterised by OM. The optical, thermal, and mechanical properties were investigated by colorimetry, DSC, DMA, and tensile tests. TMCs prepared by a one-step process exhibited significantly lower fibre thermal degradation, characterised by their lighter colour. Fibre degradation was further decreased by combining a low-temperature processing approach. However, TMCs produced in two steps had fibres well dispersed, while those produced in one step had visible large fibre agglomerates. This was reflected in the composites’ mechanical properties, as the former TMCs had tensile strengths comparable to the neat matrix, while the latter had decreased tensile strengths by approximately 20 %. Both TMCs had significantly increased storage and tensile modulus compared to the neat PA6, albeit the increase was higher for composites prepared in two steps.