Polyethylene/paraffin binary composites for phase change material energy storage in building: A morphology, thermal properties, and paraffin leakage study

Phase change materials (PCMs) are able to melt and solidify at a certain temperature with a high heat of fusion. These promising functional materials for acting as energy as latent heat storage units have one major problem, leakage of the PCMs when molten. Polymer/phase change material (PCM) blends were investigated as a possible solution of PCM leakage. In this research, paraffin was used as the PCM and three types polyethylene (PE) were used as the structural matrix. To investigate the morphology of PE/ paraffin blends and evaluate the influence on blend behavior, paraffin was blended with either high density polyethylene (HDPE), low density polyethylene (LDPE), or linear low density polyethylene (LLDPE) using a parallel co-rotating twin screw extruder. Chloroform extraction was utilized to estimate the maximum amount of paraffin able to be released by the PE network at equilibrium. Scanning electron microscope (SEM) images of chloroform-extracted samples were collected to investigate the blend structure. Thermal transition temperatures and crystallinity of components in the blends were characterized by differential scanning calorimetry (DSC) and polarized optical microscopy (POM) was used to investigate crystal perfection changes of the polyethylenes before and after blending with paraffin. Paraffin leakage of the blends was investigated by accelerated degradation test in an oven at 60 °C. The leakage behavior was analyzed by Korsmeyer-Peppas equation to determine the underlying mechanisms. Chloroform extraction indicated that almost all the paraffin would leak out from the blends in the long run. A co-continuous structure of the blends was evident from the chloroform extraction and subsequent SEM images. While this co-continuous structure controlled leakage behavior, it had little influence on paraffin thermal transition temperatures and crystallinity based on DSC study. In contrast, PE crystal perfection decreased with paraffin level as evident in POM images. This structure resulted in leakage of paraffin in all formulations.