Along with its outstandingly high heat of fusion of up to 240 J/g, its toxicological safety, commercial availability and low price, high-density polyethylene (HDPE) proves to be a candidate phase change material (PCM) for latent heat storages [1]. The melting and crystallization process are used to store and release thermal energy. Above HDPE’s melting temperature (i.e. in the charged state of the PCM), however, the combination of (atmospheric) oxygen and the lack of its crystalline morphology in the melt state (which usually hinders oxygen diffusion into the polymer bulk) results in a rather aggressive environment for the polymer. This study investigates the stability of different types of HDPE when exposed in the melted state in air for up to 7200 h by monitoring stability characteristics during the exposure.
A non-additionally stabilized HDPE and four additionally stabilized HDPE types were exposed as bulky specimens at 160 °C and 180 °C in air for up to 7200 h. A Hindered Amine Stabilizer (HAS) in combination with a benzimidazole were chosen for the additional stabilization due to their long-term efficiency[2] and compounded into a commercially available HDPE grade at weight ratios of up to 2 wt%.
The Oxidation Induction Temperature (OIT) was determined in air on a Differential Scanning Calorimeter (DSC) and decreased with increasing exposure time for all exposed HDPE specimens. This was attributed to a combination of stabilizer degradation (as measured via Fourier-transform infrared spectroscopy (FTIR)) and its physical evaporation (detected weight loss). FTIR-microscopy was applied for monitoring thermo-oxidative degradation. It revealed differences in the oxidation penetration depth from the specimen surface into the specimen depending on the applied stabilizer system. The extent of oxidation of the exposed specimens (measured by the characteristic absorption bands in the wavenumber range from 1780 cm-1 to 1700 cm-1 which represent the stretching vibrations of the carbonyl groups) decreased by going further into the specimen bulk which was in good agreement with diffusion limited oxidation (DLO). After an exposure of 7200 h at 160 °C and 7214 h at 180 °C, the specimens of the non-additionally stabilized HDPE exhibited a smaller oxidation penetration depth than the additionally-stabilized ones. Thus, an additional stabilization of the HDPE proved not to be necessary for the HDPE when applied as PCM.