High density polyethylene as a thermal storage material studied by MAS C-13 NMR spectroscopy

In order to improve the stability of high density polyethylene (HDPE) as a thermal energy storage medium, HDPE was cross-linked by electron beam irrad iation. Solid state NMR spectroscopic methods were used to analyze the cros s-linked HDPE and its thermal degradation. The influence of ethylene glycol (EG) as a heat transfer fluid on the cross-linked HDPE was also investigat ed. Neither oxidation nor phase transition was observed in HDPE kept in vac uum, even after 1000 h at 150 degrees C. However, both phenomena were obser ved in HDPE either kept at 150 degrees C or thermally cycled in air. HDPE c ompletely immersed in EG was not oxidized, but phase transition from crysta lline to amorphous, probably caused by the penetration of EG into the HDPE, was detected. The observed increase of amorphous phase was attributed to a result of crystalline to amorphous phase transition of perturbed regions i n the crystalline phase close to defect sites occupied by oxidation product s or EG molecules. (C) 1999 Elsevier Science Ltd. All rights reserved.