Characterisation and degradation study of historic polyolefin gas pipes

Zodros, Naima (2014) Characterisation and degradation study of historic polyolefin gas pipes. Masters by Research thesis (MSc), Manchester Metropolitan University.

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Abstract

Polyethylene (PE) pipes are widely used by the water and gas industries to reline or replace corroded or leaking mains. PE pipe systems are designed on the basis of a 50-year service life. Polyethylene gas pipes were first introduced to the UK in 1969. Changes within the additives package of these new materials were frequent from this date. The additives package would also vary within the same material grade during manufacturing. This would lead to changes in the general properties of the PE pipes such as their pressure bearing capacity. The MSc thesis relates to work carried out on historic medium and high-density polyethylene gas pipes after almost 50 years of service. The work involves identification of the polyolefin grade used for these gas pipes and the study of their remaining lifetime. The project presents and discusses the results of an in-depth investigation into the characterisation and the degradation of the exhumed pipes. The degradation has been studied using Fourier Transform Infra-Red (FTIR) and Differential Scanning Calorimetry (DSC). The identification work was conducted using DSC, Melt Flow Rate (MFR) and density measurements. The exposed outside surfaces of the polyethylene pipes (the remaining faces of the samples were masked with silicone elastomer) were aged in water at various temperatures. The depletion of antioxidants from the exposed face as a function of ageing time in water was monitored by conducting Oxidation Induction Time (OIT) of thin samples cut from the exposed face. Previous work showed that this relationship could be used to predict the residual lifetime of a modern blue PE100 water pipe1. However, this was found to be untrue for the historic gas pipes, due to the high variability of OIT results before exposure masking / confounding any potential changes. Medium and high-density PE were differentiated using well established analytical methods such as MFR and density. The medium-density Polyethylene grades were differentiated using a recently introduced DSC technique known as Stepwise Isothermal Segregation (SIS) which was performed on the DSC instrument. In this study the sample mass normalised SIS data was treated in a novel way in attempt to simplify the data. The method involved taking the first derivative of the sample mass normalised SIS data. This approach led to improved resolution of crystal structures of differing lamellar thickness than possible using the original sample mass normalised SIS data. The SIS technique was proven to be an efficient identification technique. It was possible to obtain reliable fingerprints for each type of PE and material grade using this technique. The degradation study conducted on the unexposed old gas pipe materials using OIT, and FTIR indicated that significant amounts of antioxidants were still remaining in the pipe. However, as assessment of the mechanical performance was beyond the scope of this study, it is not possible to confirm that these old pipes are still fit for service. The OIT data for samples taken around the circumference of the pipe showed a high level of variability as did OIT data for samples taken along the length of the pipe. This variability in stability clearly indicates very poor distribution of stabilisers during manufacture and probably points to early use of a rudimentary masterbatch. Therefore it was not unexpected that the variability in OIT data was even higher after hydrothermal ageing. The scatter in the data was so severe that it was not possible to determine the residual lifetime of the pipes.