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

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

[发布机构] University of the Free State