The effect of storage of poly(propylene) pipes under hydrostatic pressure and elevated temperatures on the morphology, molecular mobility and failure behaviour

One of the important applications of random poly(ethylene propylene) copolymer (PPR) is the production of hot water pipes. The pipes can be used under hydrostatic pressure as well as at elevated temperatures up to 70 °C continuously for 50 years and at short time at 80 °C. If a pipe is used at higher temperatures for longer times it could fail earlier. Knowledge of usage time and temperature is vital for determining the origin of a failure of PPR pipes. Several techniques are used for determining changes in chemical and physical structures upon long-time annealing of PPR pipes at different temperatures. Techniques, which are sensitive to thermo-oxidative degradation of PPR and consumption of stabilizers, are not very sensitive for determining storage time longer than one year. The molar mass of PPR does not change upon long-time annealing. It is shown that crystallinity of the samples, as determined by wide angle X-ray diffraction (WAXD), is not largely affected by storage time at elevated temperatures. It is also shown that onset of melting, as measured by differential scanning calorimetry (DSC), increases with increasing storage temperature, which is apparently caused by the perfection of crystalline structure at higher temperatures. Onset of melting allows determining the maximum storage temperature of PPR pipes. It is shown that proton solid-state NMR transverse magnetization (T2) relaxation analysis is the most sensitive tool for determining changes in PPR samples that are caused by storage time of PPR pipes under hydrostatic pressure. The method provides information on molecular mobility and phase composition of PPR samples. Four different phases are analysed with this method: (1) crystalline phase and rigid fraction of the amorphous phase, (2) semi-rigid crystal-amorphous interface, (3) soft fraction of the amorphous phase and (4) rubbery-like material. The most pronounced changes upon long storage time are observed for the rigid fraction of PPR (fraction 1). This suggests that long time annealing of the samples at temperatures far above Tg (about 0 °C) results in (1) perfection of existing crystals and the formation of new crystals, which act as physical junctions leading to immobilization of the amorphous phase, (2) chain elongation in the amorphous phase due to creep under hydrostatic pressure, and (3) an increase in the gradient of concentration of ethylene-rich chain fragments through the mobile fractions of the amorphous phase. All these changes cause embrittlement of the samples. Thus, the combination of DSC and solid-state NMR measurements is a powerful tool for determining the critical time and temperature conditions causing breakage of PPR pipes and fittings.