One of the recent developments in composite technology in South Africa is the increasing use of natural fiber materials in the manufacture of plastic products. Most of the previous work on natural fiber-reinforced composites has focused on sisal fiber as it was commercially available. In this study, the mechanical and thermal properties of composites made with locally produced hemp fibers, were compared with composites made with hemp fibers produced in France. New commercial polypropylene random copolymer was used as matrix because it can be processed at lower temperatures when compared with other commercial propylene copolymers. The composites were produced by extrusion compounding and were further processed into tensile test bars by injection molding. Up to 30% fiber loading could be achieved. It was observed in all composites that increasing the amount of fiber resulted in an increase in tensile strength, elastic modulus, and flexural strength and a decrease in impact strength. The thermal properties of the composites were analyzed by the thermogravimetric method. It was found that the fiber/PP composites showed excellent properties when compared to fiber and the matrix separately. The addition of hemp fibers shifted the start of the degradation process towards higher temperatures. The results obtained show that the mechanical and thermal properties of South African long hemp fiber composites compare favorably well with the French bleached and unbleached hemp fibers.

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

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Surface properties of high-strength and high-modulus carbon fibers were studied by inverse gas chromatography. London components of the surface free energy were calculated from the increment per methylene group in the free energy of adsorption of n-alkanes at zero coverage. Values typical of low energy surfaces were obtained for both types of "as received" carbon fibers. Cleaning of the fibers at elevated temperatures under nitrogen caused a significant increase in the London component. This was attributed to the desorption of physically adsorbed species (CO$ sb2$, H$ sb2$O) that occupied the high energy sites on "as received" fibers. Similar results were obtained in the finite coverage region where London components were calculated from the spreading pressures of the hydrocarbons. Type II adsorption isotherms were measured for n-alkanes on carbon fibers. The fibers were pretreated by heating to various temperatures under nitrogen. The BET surface areas of the fibers increased with increasing pretreatment temperature, due to the presence of microporosity. From the spreading pressures of ethanol on carbon fibers, the polar interactions between ethanol and the fibers were estimated.

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