Quantifying stabilizing additive hydrolysis and kinetics through principal component analysis of infrared spectra of cross-linked polyethylene pipe

Abstract

Peroxide crosslinked high-density polyethylene (PEX-a) is increasingly being used to replace traditional metal and concrete pipes in applications such as water, gas, and sewage transport. Stabilizing additives play important roles in enhancing the long-term stability of PEX-a pipes and understanding changes to these additives under in-service conditions is critical to further improvements in pipe lifetimes. We used infrared (IR) microscopy to measure spectra within the central portion of the walls of PEX-a pipe subjected to two different types of ageing: exposure to high temperature (85∘C) air and exposure to high temperature (85∘C) water. To analyze these data, we used principal component analysis (PCA) to implement an unsupervised multivariate analytical approach. This allowed us to identify distinct ageing pathways for the two types of ageing in the two-dimensional space defined by the first two principal components PC1 and PC2, which together account for 88% of the variance in the data. This representation of the data allowed us to associate each PC with different ageing processes in the pipes: changes in PC1 were due primarily to hydrolysis of stabilizing additive ester linkages, whereas changes in PC2 were due primarily to elevated temperature. The PCA showed that ageing in high temperature water produced spectral changes consistent with those measured for an in-service pipe and that water is the key component driving the changes. The results provide important information for PEX-a pipe ageing and stabilizing additive formulation design.