Physical properties of poly(ether ether ketone) exposed to simulated severe oilfield service conditions

The property changes occurring when poly(ether ether ketone) (PEEK) is subject to methane and carbon dioxide at high pressures (108 Pa) and high temperatures (175-200 °C) are reported. Differential scanning calorimetry, density gradient techniques, positron annihilation lifetime spectroscopy, dynamic mechanical thermal analysis and tensile tests measurements were used to monitor the changes which occur during the ageing process. Over the period of the study an overall increase in the tensile strength was noted, with little or no change in 0.2% and 2% proof stresses, whilst there was a decrease in bending modulus and glass transition temperature due to the effects of plasticization. The Young's modulus generally increases for samples exposed to a temperature of 175 °C in the presence of a mixture of 90% methane and 10% carbon dioxide, or carbon dioxide alone, but it decreases at 200 °C in the presence of carbon dioxide alone. The observed effects are consistent with the polymer undergoing initially a densification of the matrix associated with annealing-induced crystallisation, followed by plasticization as the gases permeate into the disordered regions of the matrix. When de-pressurised, the gas dissolved in the matrix attempts to leave the matrix and morphological changes are observed. The complexity of the changes in crystallinity and plasticization in the disordered phase are reflected in changes in the positron annihilation data.