|dc.description.abstract||Increasing worldwide environmental awareness is encouraging scientific research into the development of cheaper, more environmentally friendly and more sustainable construction and packaging materials. Natural fibre reinforced thermoplastic composites are strong, stiff, lightweight and recyclable, and have the potential to meet this need. Industrial hemp fibre is amongst the strongest of the natural fibres available, and possesses a similar specific stiffness to E-glass, but with additional benefits such as low cost and low production energy requirements.
The favourable mechanical properties of hemp, however, have yet to be transferred successfully to thermoplastic-matrix composite materials. The aim of this thesis was to achieve a greater understanding of the various parameters that contribute to composite strength and stiffness, and to manipulate these parameters in order to produce an improved hemp fibre reinforced polypropylene composite material.
Hemp fibre was alkali treated at elevated temperatures in a small pressure vessel with either a solution of 10wt% NaOH or 5wt% NaOH / 2wt% Na2SO3. Single fibre tensile tests were performed on treated and untreated fibres, and it was found that the NaOH/Na2SO3 treatment produced the strongest and stiffest fibres with a good level of fibre separation. Lignin tests revealed that both alkali treatments were effective in the removal of lignin from hemp fibre, and XRD analysis showed that both alkali treatments resulted in increases in the hemp fibre crystallinity index. TGA and DTA analysis showed that the alkali fibre treatments improved the thermal stability of the treated hemp fibre when compared to the untreated fibre.
Alkali treated hemp fibre, polypropylene and a maleic anhydride modified polypropylene (MAPP) coupling agent were compounded in a twin-screw extruder, and injection moulded into composite tensile test specimens. A range of composites with different fibre and MAPP contents were produced and tested. Tensile tests revealed that the optimum composite consisted of polypropylene with 40wt% NaOH/Na2SO3 treated hemp fibre and 4wt% MAPP, and had a tensile strength of 50.5 MPa and a Young's modulus of 5.31 GPa, respectively.
The effect of MAPP on the fibre/matrix interface of NaOH/Na2SO3 treated hemp fibre/polypropylene composites was assessed by means of the single fibre fragmentation test. A composite consisting of NaOH/Na2SO3 treated fibres in a matrix of 4wt% MAPP and polypropylene was found to have a critical fibre length of 0.83mm and an interfacial shear strength of 16.1 MPa. The effects of MAPP on the composite fracture mechanisms were evaluated by means of SEM microscopy. TGA and DTA analysis showed that untreated hemp fibre composites and NaOH/Na2SO3 treated hemp fibre composites, each with a matrix of 4% MAPP and polypropylene, were less thermally stable than the polypropylene matrix alone.
The Bowyer-Bader model was used to model the strength of an injection moulded composite with a normal fibre length distribution, consisting of 40wt% NaOH/Na2SO3 treated fibre, 4% MAPP and polypropylene. A theoretical composite tensile strength of 149 MPa was obtained from the model, based on the assumption that all the fibres were axially aligned in the composite.
Composites with long, axially aligned fibres were produced using a novel solution mixing technique, where the polymer matrix and MAPP coupling agent were dissolved in a solvent and then precipitated inside an aligned fibre mat. Significant improvements in tensile strength and Young's modulus were achieved for solution mixed composites compared to composites produced by means of extrusion and injection moulding. The strongest solution mixed composite had a tensile strength of 84.7 MPa, and consisted of 56wt% NaOH/Na2SO3 treated fibre, 4% MAPP and polypropylene; and the stiffest injection moulded composite had a Young's modulus of 16.0 GPa, and consisted of 63wt% NaOH/Na2SO3 treated fibre, 4% MAPP and polypropylene.||en_NZ