P.-O. Hagstrand

Novel pulp fibre reinforced thermoplastic composites

The reinforcement potential of pulp fibres is presently not fully explored in thermoplastic composites. One of the reasons is that currently used processing methods comprise several severe thermomechanical steps inducing premature degradation of the fibres. Three pre-forming techniques were developed to prepare pulp fibre reinforced cellulose diacetate (CDA) pre-forms, namely filtration-forming, solvent impregnation, and commingling with polymer fibres. These techniques eliminate all thermomechanical steps, prior to final processing. The CDA polymer was nevertheless found to be very sensitive to the specific process histories relevant to each technique, contrary to the pulp fibres, whose size, shape, and mechanical properties were not affected by neither of the pre-forming processes. The tensile properties of composites compression moulded from solvent impregnated pre-forms were compared to those of ground china reed reinforced CDA. Whereas ground china reed particles were found to act merely as fillers increasing composite stiffness, a remarkable reinforcement effect was observed for the pulp fibre reinforced impregnated pre-forms. A combination of a stiffness increase by a factor 5.2 and a strength increase by a factor of 2.3 relative to the pure polymer was achieved, whereas in typical pulp fibre reinforced thermoplastics, the stiffness increase is frequently obtained at the expense of loss in strength. This work highlights the key factors which control the mechanical performance of pulp fibre reinforcements previously neglected in literature, and demonstrates the remarkable reinforcement potential of such renewable material. Furthermore, the properties achieved by optimising the extraction and processing steps indicate that pulp fibre reinforced thermoplastics composites are appropriate materials for load bearing applications.

Robotic tow placement for local reinforcement of glass mat thermoplastics (GMTs)

A process has been developed where multiple yarns of commingled glass and polypropylene are heated and placed to a desired geometry. The placed unidirectional (UD) tow is then encapsulated by over-moulding with glass mat thermoplastics (GMTs). The effects of both force (20x9680 N), with an equivalent pressure range of 8x9646 bar, and the displacement rate (20x96100 mm/s) during tow placement on void content and flexural modulus were investigated for nine placement conditions. Placement rate affected the tow quality measured before overmoulding. Void contents were reduced after over-moulding. The volume fraction of UD tow (vol. O8 composite/vol. total composite) in the manufactured multi-material structure was 21.5%. Corresponding tensile tests showed a factor of 2 increase in modulus and a factor of 3 increase in strength relative to GMT. Tow placement rate had no significant effect on tensile properties after over-moulding. Tensile behaviour as a function of the UD tow volume fraction was modelled and predicted values agreed with the experimental results. Additionally, the predicted tensile performance of UD tow reinforced GMT has been compared with possible competitive materials using merit indices.