Thermal and mechanical properties of LPDE/SISAL fibre composites compatibilized with paraffin waxes
Nhlapo, Lefu Piet
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The effects of maleic anhydride grafted Fitscher-Tropsch paraffin wax (MA-g-wax) and oxidised Fischer-Tropsch paraffin wax (OxWax) content on the morphology, thermal, mechanical and thermomechanical properties of LDPE/sisal fibre composites were examined in the current study. The composites and blends were prepared through melt mixing in a Brabender Plastograph internal mixer. Samples of different compositions were characterized for their morphology, thermal properties, mechanical properties and thermomechanical properties using scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), tensile testing and dynamic mechanical analysis (DMA). Compatibility of the wax and the LDPE matrix is necessary as morphology has a significant effect on the composite properties. OxWax was partially miscible with LDPE at 5% wax content and co-crystallized with the LDPE matrix, but only partially miscible at 10% wax content. MA-g-wax, on the other hand, seemed to be more miscible with LDPE. TGA results showed an increase in the thermal stability in the presence of both waxes. In the presence of both 5 and 10% OxWax the tensile strength decreased at low sisal fibre contents (up to 15% fibre), but increased at higher fibre contents. The modulus increased in the presence of 5% OxWax and with increasing sisal fibre contents in the composites. However, the 5% OxWax containing composite samples showed higher moduli than the 10% OxWax containing composite samples as a result of co-crystallization of the wax and polymer at low wax content. The presence of MA-g-wax also increased the tensile strength of the composites. Elongation at break decreased with increasing fibre content in all cases. This indicated that the presence of fibre in the matrix reduced the ability of the sample to deform. The LDPE/MA-gwax showed higher elongation at break values than the respective LDPE/OxWax blends. This is because MA-g-wax was more compatible with LDPE so that there was better interaction between the polymer and the wax and therefore better stress transfer. The OxWax, on the other hand, behaved differently due to the crystallization of the wax in the amorphous phase of the LDPE, and the crystals formed defect centres in the polymer matrix. The storage modulus generally increased with wax and fibre content. The MA-g-wax composite systems showed a shift in the β-transition indicating a better interaction between the LDPE and the fibres at higher fibre contents, which was not the case for the OxWax composite systems.