This paper deals with the global aging behaviour of a polypropylene random copolymer liner material for seasonal thermal energy storages. Hot air aging experiments at elevated temperatures of 95 degrees C, 115 degrees C and 135 degrees C were carried out using micro-sized specimens with thicknesses ranging from 50 to 2000 Am. The aging indicator strain-at-break was monitored for an exposure of up to 1500 days. Potential and Arrhenius equations were used to fit the experimental hot air aging data referring to thickness and temperature dependency. A semi-empirical model was established and used to assess the lifetimes of polypropylene based thermal energy storage liners. The high quality of the thickness/temperature-model was approved by low divergence of the lifetime values between experimental and modelled failure data. Depending on the temperature loading profile of the seasonal thermal energy storage lifetime values ranging from 20 to 47 years were deduced for a PP liner with a thickness of 2 mm.