(187g) Comparing the Miscibility of Paraffin In Different Polyethylenes for Form-Stable Phase Change Materials

Form-stable phase change materials (PCM) are utilized for thermal energy storage and their structure and properties are highly correlated to blend miscibility which will affect their efficiency on energy saving application dramatically. The miscibility evaluation of paraffin in three types of polyethylene as form-stable PCM was carried out by differential scanning calorimetry and atomic force microscopy. Paraffin with a chain length of 18 carbons was compounded with HDPE, LDPE and LLDPE separately by employing a parallel co-rotating twin screw extruder. A depression of the equilibrium melting temperature was observed for both of polyethylene and paraffin in their blends. Interaction parameter values were obtained from analysis of the melting point depression via the Flory-Huggins approximation of the thermodynamic mixing of two polymers and some extended theories. The crystallinity of each component is formulation dependent and drops with increasing of the other component. Two paraffin crystallization peaks for PE/Paraffin blends were displayed and the enthalpy of one peak increased at the expense of the other. AFM characterized the dispersion of paraffin in PE and two distinct phases with an additional intermediate phase were demonstrated. This structure verified partial miscibility of paraffin in polyethylenes. Miscibilities of paraffin in polyethylenes were compared based on degree of existence of intermediate phase, crystallization temperature shifting, equilibrium melting point depression and changes on crystallinity according to the investigation above. A conclusion that paraffin has a miscibility with the polyethylene identified in this study in an order of HDPE<LDPE<LLDPE was made.