Long-term behaviour of welded polypropylene liners for pit thermal energy storages

Abstract

This paper deals with the global ageing behaviour of a polypropylene random copolymer (PP-R) liner material with tailor-made primary structure, morphology and stabilization package. Specific focus was given to the long-term behaviour of the liner material in the welding zones. Conventional weld line testing was not feasible due to plastic deformation of the peel arms. Hence, a novel testing approach based on micro-sized specimen was implemented and used. The PP-R liner was welded varying the hot wedge temperatures ranging from 320 to 410 °C. For accelerated ageing, welded micro-specimens with a thickness of 100 μm were manufactured and exposed to hot air at 95, 115 and 135 °C. The investigated ageing indicators included phenol index and oxidation temperature which are highly sensitive to stabilizer degradation. Moreover, the carbonyl index and strain-at-break values were monitored reflecting primarily polymer degradation. To evaluate potential effects of welding, welded and unwelded micro-specimens were compared. By assessment of phenol index and oxidation temperature a critical temperature dependent consumption of stabilizer was ascertained. Nevertheless, the rate of stabilizer degradation was comparable for unwelded and welded PP-R. The investigated hot wedge temperatures had no significant impact on further stabilizer depletion or polymer degradation. So far, failure of the PP-R material was obtained for an exposure temperature of 135 °C after 3750 h. The long-term behaviour and the embrittlement time of the polymer material was not affected by welding.