Assessment of lifetime of hot-water polyethylene pipes based on oxidation induction time data

摘要

A method is presented for assessment of lifetime of polyethylene pipes undergoing thermal oxidation (Stage III failure) based on oxidation induction time (OIT) data obtained by DSC. The method requires pressure testing for limited time periods, sampling from unexposed and exposed pipes, the establishment of a linear relationship between the OPT and the antioxidant concentration, and the taking of OIT profiles through the pipe wall. The method is not applicable to pipes with improper dispersion of the antioxidant. A mathematical procedure based on a model for the migration and chemical consumption of the antioxidant system is used to extrapolate short-term OIT profiles to obtain the OPT profiles after extended exposure. The lifetime prediction is also based on the empirical establishment of a criterion, a critical OIT value, for the onset of thermal oxidation of the polymer (Regime C). The total Lifetime is then obtained under the assumption that the time period of Regime C constitutes 10% of the total lifetime. The pressure testing time required to obtain adequate data for modeling is only a fraction (20% to 70%) of the failure times of the pipes studied. The average difference between the actual and predicted times was 15% for the MDPE pipes studied. [References: 14]