Modelling the Correlation between DHT and True Field Lifetimes for PET Based Backsheet
Looney, K., Brian Brennan · EU PVSEC · 2014
Excellent mechanical and dielectric properties allied to relatively low cost made PET an ideal material as a mid-layer in TPT and lately TPE laminate backsheet structures. Market conditions from 2006 onwards necessitated the need for replacement weatherable layers in these laminates and many PPE structures emerged. The weatherable layer in such structures primarily needs to withstand combinations of sunlight (UV), temperature and humidity to which the reverse of the module is typically exposed. A multiplicity of PET films have been introduced which show improvements over standard PET for the key area of hydrolysis resistance. The standard for moisture resistance which has been adopted is the damp heat test, DHT, namely survival after 1000 hours exposure to 85° C and 85% RH. Films with hydrolysis lifetimes of at least twice the damp heat test standard appeared on the market. Conversely recent academic work has postulated that the 1000 hour damp heat test actually predicts lifetimes of standard PET of at least hundreds of years in external conditions. We will illustrate in this paper that the foundations of this work has been based on the use of incorrect kinetics to describe PET hydrolysis and that when the correct kinetics are used, a DHT of 1000 hours is inadequate to safely predict a field lifetime in excess of 25 years .