Mechanical properties of composites manufactured from low twist hybrid yarns made of discontinuous carbon and polyamide 6 fibres

Abstract

Composites based on e.g. randomly oriented nonwovens and injection moulded structures from discontinuous carbon fibre (CF), which are processed industrially exhibit low tensile strengths of 200–300 MPa and 400 MPa, respectively. In contrast, composites based on twisted hybrid yarns from discontinuous CF and thermoplastic fibre show higher tensile strengths such as 1150 MPa. Reasons are improved fibre orientation and higher fibre volume content. However, tensile strength is still lower than that of uni-directional carbon fibre reinforced composites (CFRP) based on continuous filament yarn, which is around 1560 MPa. The reason is a low fibre orientation due to yarn twisting and high fibre shortening, which occurs throughout the yarn manufacturing process. As mechanical properties of composites largely depend on fibre orientation and length, there is a high potential to achieve higher mechanical properties in CFRP by ensuring gentle processing of CF and reducing yarn twist. However, due to brittleness and smooth surface of CF, spinning of yarns with low twist (<60 T/m (twist per meter)) is challenging. In order to exhaust the potential of hybrid yarns, a semi-industrial process chain to produce hybrid yarns while ensuring gentle processing of discontinuous CF (fibre length 100 mm) and polyamide 6 fibres (fibre length 80 mm) at significantly low level of twist 20 T/m is reported in this paper. Carbon fibre reinforced composite manufactured from these hybrid yarns show high tensile strength (1453 ± 27 MPa), Young’s modulus (94 ± 6 GPa), flexural strength (1090 ± 41 MPa) and flexural modulus (98 ± 5 GPa).