Kinetic study of the crystallization of LLDPE and wax in LLDPE/wax phase change blends used for thermal energy storage

Abstract

The main purpose of this research was to kinetically study the influence of each component in an LLDPE/wax blend on the crystallization behaviour of the other component, and also to evaluate the effectiveness of wax as a phase change material when blended with LLDPE. Phase change materials are used to store and release energy through phase changes, be it melting and solidification processes or solid state phase transitions. Paraffin wax is one of a large number of phase change materials that store and release large amounts of thermal energy through melting and solidification. Since molten wax has a low viscosity, it is important to contain the wax in some medium. A lot of research has gone into the preparation and characterization of immiscible polymer/wax blends, in which the wax crystallizes separately in the amorphous phase of the polymer. These wax crystals can then melt and solidify without affecting the polymer, which should have a significantly higher melting temperature than the wax. It is, however, possible for some of the wax to be trapped in the amorphous part of the polymer, in which case this wax fraction will not be available for thermal energy storage, making the system less effective as a phase-change blend. The crystallization kinetics results described in this thesis showed that the overall crystallization rate of LLDPE decreased with an increase in wax content, due to the dilution effect of the wax. Although the wax crystallized faster when blended with LLDPE, it showed lower melting enthalpies indicating fewer wax crystals, which directly impacts on its effectiveness as a phase-change material for thermal energy storage. The results obtained by successive self-nucleation and annealing (SSA) indicated that LLDPE can be thermally fractionated, whereas the medium-soft paraffin wax was not susceptible to thermal fractionation because of its linear short chain hydrocarbons. It was also shown that the wax acts as a solvent for LLDPE inducing a 'dilution effect' without co-crystallization.