Thermal and mechanical properties of polyolefins/wax PCM blends prepared with and without expanded graphite
Mochane, Mokgaotsa Jonas
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The study deals with the preparation of polyolefin/wax blends as form-stable, solid-liquid phase change materials (PCM) with the aim of enhancing both the thermal conductivity and flame resistance properties of the shape-stabilized PCMs. In such a composite, the wax serves as a latent heat storage material and the polyolefins (EVA and PP) act as supporting materials, preventing leakage of the molten wax and providing structural strength. To improve the thermal conductivity and flammability resistance of these blends, expanded graphite at loadings of 3, 6 and 9 wt% was added into the samples, whereas the polymer/wax blend was kept at a 1:1 weight ratio. To further improve the flammability resistance, combinations of EG with Cloisite 15A clay and diammonium phosphate (DAP) in EVA and an EVA/wax blend were investigated. Both the blends and composites were subjected to different characterization techniques in order to establish their morphology and thermal and mechanical properties. The techniques used were scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermal conductivity, thermogravimetric analysis (TGA), cone calorimetry, dynamic mechanical analysis (DMA), X-ray diffraction (XRD), tensile testing, and impact testing. It was generally observed that the EG particles agglomerated in the absence of wax, but dispersed much better in the blends when wax was present, probably because the wax penetrated in between the EG layers as a result of the better interaction between wax and EG, and separated the layers into smaller and better dispersed EG particles. This gave rise to better thermal conductivity and flame resistance. In the presence of EG+Cloisite 15A the material formed a dense and stable char layer (carbonized ceramic) which significantly improved the flame resistance of the materials. It was observed that the thermal degradation mechanisms of the polymers and blends did not change in the presence of EG, although the EG particles retarded the evolution of the volatile degradation products. There were no significant changes in the melting temperature of EVA in the EVA/EG composites, while the crystallinities of EVA were observably lower in the presence of EG. The EVA composites showed a decrease in impact strength with increasing EG and wax contents. The impact strength of the PP/wax/EG composites increased with increasing EG content in all the samples, but decreased with increasing wax content.