Identifying Accelerated Ageing Pathways for Cross-Linked Polyethylene Pipes using Principal Component Analysis

Cross-linked polyethylene (PEX-a) pipes are widely used for water transport, but their lifetime can be limited by the formation and propagation of cracks in the pipe wall. To disrupt thermo-oxidative degradation pathways and impart long-term stability, stabilizing additives are included in PEX-a pipe formulations. Nevertheless, PEX-a pipes have exhibited cracking and premature failure. We used infrared (IR) microscopy to build a database of high-spatial resolution spectra of cross-sections of pipes subjected to different types of accelerated ageing. The resulting spectra are complex with different correlations between spectral regions and multiple contributions to functional group absorptions. To analyse this convoluted data, we used principal component analysis (PCA) to implement an unsupervised multivariate analytical approach. This allowed us to identify distinct ageing pathways including the detrimental second-order autocatalytic hydrolysis of a key stabilizing additive. The PCA representation of the data allowed us to quantify the kinetics of the hydrolysis reaction and its penetration depth profile into the pipe wall as a function of time and temperature. The results provide important mechanistic information for PEX-a pipe ageing and stabilizing additive formulations.