Miscibility studies of paraffin/polyethylene blends as form-stable phase change materials

Thermal energy storage is useful to promote energy conservation in buildings or machinery. One means of achieving a form stable phase change materials (PCMs) with polymers is to utilize immiscible blend pairs, governed by blend miscibility. The degree of miscibility between the polymer pairs may also influence energy efficiency in applications. Binary polyethylene-paraffin blends were melt compounded at different ratios of high-density polyethylene (HDPE), low-density polyethylene (LDPE), and linear low-density polyethylene (LLDPE) using a parallel co-rotating twin screw extruder. The miscibility of the paraffin in the three types of polyethylene was evaluated using differential scanning calorimetry (DSC) and atomic force microscopy (AFM). The DSC data demonstrated two melting temperatures with a depression of the equilibrium melting temperature for the polyethylene in the mixture. Two distinct and an intermediate phase were evident in the atomic force microscopy images. This structure verified the partial miscibility of paraffin in polyethylene. Interaction parameters between paraffin and polyethylene were obtained through melting point depression analysis via the Flory-Huggins approximation for thermodynamic mixing of two components. The crystallinity of each component depends upon the blend concentration. Two paraffin crystallization peaks for the PE/paraffin blends were observed, with the enthalpy of one peak increasing at the expense of the other. The lowest paraffin miscibility in polyethylene is found in paraffin/HDPE blend.