Supercritical CO2 processing and annealing of polytetrafluoroethylene (PTFE) and modified PTFE for enhancement of crystallinity and creep resistance

A method is reported to improve creep resistance in tension for polytetrafluoroethylene (PTFE) and modified PTFE (M-PTFE). PTFE and M-PTFE from different sources were annealed in air, N2 or supercritical CO2 (scCO2) at a range of temperatures, pressures and time intervals. Annealing PTFE in scCO2 increases crystallinity from 9 to 53%, depending on the material and annealing conditions. No corresponding increase occurs for samples annealed in air or N2. In comparison to as-received PTFE, significant improvements in tensile creep resistance (18-60%) are observed also dependent upon the material and annealing conditions. For a given temperature and duration, the increase in PTFE tensile creep resistance after annealing in air or N2 is greater than after annealing in scCO2 despite the higher crystallinity for post-scCO2 processed PTFE. Density measurements indicate that the effect of increased crystallinity is counterbalanced by scCO2-generated microvoids, particularly at higher pressures, leading to smaller creep resistance. In contrast, thermal annealing in air or N2, which does not significantly change the density or enhance the crystallinity of PTFE or M-PTFE, yields better tensile creep resistance. The detailed morphological origin of improved resistance to tensile creep is unknown, but stress relief by thermal annealing is evident.