Crystallinity of polypropylene-silica ash composites affected by the mixing conditions - DSC studies

For particulate polymeric composites, mixing is a crucial step to be optimised; and in the process-control stage, identification of the factors that influence mixing is important for a deeper understanding of the composite mechanical performance. A mixing study of a hydrophobic polymer matrix (polypropylene) filled with a hydrophilic particulate-filler (silica from rice husk ash) was carried out using a batch mixer at a constant filler fraction of 20% (w/w). The study involved varying the mixing-time, screw speed and mixing-chamber temperature used to prepare the composites. Mechanical analysis of the samples showed that the tensile strength and modulus values were dependent on the mixing conditions. Furthermore, DSC studies of the samples revealed that the degree of crystallinity was also affected by the mixing conditions. The observed increase in the tensile strength was attributed to the increased filler-matrix interactions; however, there was difficulty in analysing how the mixing conditions influenced the tensile strength because of a lack of extensive data on filler dispersion. The increase in crystallinity, as affected by mixing conditions, was thought to improve the filler-matrix interaction leading to an increased tensile modulus. Interestingly, samples showed permanent morphological changes after their previous thermal histories were erased, suggesting that there was significant interaction between the silica ash particles and the polymer matrix, even though they are quite incompatible. A statistical analysis based on the tensile data was carried out to optimise the state of mixing. The results indicate that (i) the optimised state of mixing correlated with higher crystallinity and (ii) changes in the parameters of physical mixing might significantly affect the homogeneity and crystallinity, both of which are related to the mechanical performance of the composites. Furthermore, reducing the particle size of the silica ash was also found to increase the crystallinity, which was in turn related to the improved tensile properties of the composites. The investigation attempts to highlight that for apparent incompatible system, homogeneity is very important but it alone cannot explain the moderate filler-matrix interactions and filler bonding characteristics that are known to contribute to improved tensile properties. Somehow, composites mechanical properties are improved by optimising the physical mix-state, and by modifying the particle size but it seems like that the net effect is due to increased interactions. In any case, it is clear that optimisation of interactions can be achieved by obtaining a homogeneous phase.