Development and characterisation of bast and basalt fibre hybrid polymer composites for automotive applications

Natural fibres such as kenaf, hemp, and flax, also known as bast fibres, offer several benefits such as low density, low cost, carbon dioxide neutrality, sustainability and renewability. In Europe, their composites are used intensively by almost all car manufacturers, mostly in interior applications. Compared to glass fibres, they are safe in processing, renewable, and recyclable. Other than that, their specific mechanical properties are either close to or at times higher than glass fibres. They have great potential to replace a segment of the glass fibre reinforced composites in automotive applications. The shortcomings of bast fibres are their poor mechanical strength, varying fibre characteristics (because of climate, cultivation, soil), and low processing temperature (approximately 200°C). Therefore, they cannot meet the structural and durability demands of automobile parts as do glass fibres. This research work aimed to improve the mechanical performance of bast fibre reinforced polymer com-posites by hybridisation with high performance natural basalt fibres. They are natural inorganic fibres with mechanical and thermal properties higher than bast fibres and close to or higher than common glass fibres (E-glass). As bast fibres, flax, hemp, and kenaf were selected for reinforcement. Polypropylene and acrylic based polyester resin were used as matrix. The target was to prepare the composites by established processing methods for the applications of natural fibre reinforced composites in the automotive sector, such as carding, resin impregnation, and compression moulding. The first step in the production of composites was the preparation of nonwovens by carding process and needle punching. It was challenging to card fine and brittle basalt fibres. The carding process was therefore intensively optimised so that the nonwovens could be produced without fibre damage and with homogeneously distributed fibres. For thermoplastic composites, nonwovens were prepared with poly-propylene fibres. The reference composition was a mixture of polypropylene and bast fibres (50:50). For the hybrid compositions, the bast content was gradually replaced by basalt fibres. Nonwovens were ready for compression moulding immediately after their production. Nonwovens for thermoset resin impregnation were prepared with only natural fibres. The reference composition was a mixture of flax and kenaf (50:50). For the hybrid compositions, the bast content in the reference composition