Polypropylene as a potential matrix for the creation of shape stabilized phase change materials

Abstract

Phase change materials, based on isotactic polypropylene (PP) blended with soft and hard Fischer−Tropsch paraffin wax respectively, were studied in this paper. DSC, DMA, TGA and SEM were used to determine the structure and properties of the blends. While paraffin waxes in the blend changed state from solid to liquid, the PP matrix kept the material in a compact shape. Strong phase separation was observed in both cases, which was more pronounced in the case of soft paraffin wax. Despite the fact that both grades of paraffin wax are not miscible with PP due to different crystalline structures, it was shown that the hard Fischer−Tropsch paraffin wax is more compatible with PP than the soft one. Both waxes plasticized the PP matrix. TGA showed that PP blended with the hard Fischer−Tropsch wax degrades in just one step, whereas blends containing soft paraffin wax degrade in two distinguishable steps. SEM exposed a completely different morphology for the two paraffin waxes and confirmed the lower compatibility of PP and soft paraffin wax. The soft and hard characters of the waxes were manifested in the viscoelastic properties, where the blends containing soft paraffin wax exhibited a lower elastic modulus than pure polypropylene, whereas the hard Fisher−Tropsch paraffin wax solidified the matrix. However, both kinds of blends were able to sustain the dynamic forces applied by the DMA within five cycle runs implying good shape stability.