Advances on the pyroresistive behaviour of conductive polymer composite
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The positive temperature coefficient (PTC) effect in conductive polymer composites (CPC) are still poorly understood with the thermal expansion of the polymer matrix accepted as the main cause. This thesis aims to study a model system able to explain the effect of the filler size and shape on the PTC behaviour of CPCs. Silver coated glass spheres and flakes are used as conductive fillers due to the ease in controlling uniform size and shape. In a controlled system it was demonstrated that the PTC intensity increases with increasing filler size and with decreasing filler content, both for conductive fillers. Combinations of different conductive fillers were investigated to explore the possibility to obtain both low percolation thresholds and high PTC intensities. Model systems in which at least one of the two conductive fillers is of relatively homogenous size and shape were used to facilitate unravelling some of the complicated relationships between (mixed) conductive fillers and the PTC effect. The PTC intensity of mixed fillers composites were dominated by the filler with the lowest PTC intensity, even at very low volume fractions. The PTC intensity was not only influenced by the conductive particle size but also by its size distribution. The effect of difference in linear coefficient of thermal expansion (CTE) of conductive fillers and polymer matrix based on a change in filler core on PTC behaviour was investigated. Damage to the particles due to the poor adhesion between the silver coating and the PMMA bead lead to the composite behaving like mixed filler composite. Hybrid polymers filled with silver coated glass flakes was also examined in order to enhance the PTC intensity. The PTC intensity of the composite increased with increasing PPE content but the negative temperature coefficient (NTC) effect was observed in all the composites.
AuthorsAsare, Eric Kwame Anokye
- Theses