Influence of supercritical carbon dioxide on structure and mechanical properties of porous polypropylene membranes

The influence of supercritical carbon dioxide (scCO2) on structure and mechanical properties of porous polypropylene tubular membranes subjected to different process conditions was investigated. The membranes were treated with scCO2 at pressure of 18 MPa and at three different temperatures (40 oC, 70 oC, and 100 oC) for 2 h in a batch reactor. The obtained samples were analyzed using a scanning electron microscope (SEM) to determine the impact on the membrane structure, tensile testing for ultimate strength assessment, and bubble point test for determination of the pore size distribution and the filtration coefficient (UFC). A membrane not treated with scCO2 was used as reference sample for comparison. SEM pictures of side surfaces and cross-sections of treated tubular membranes did not reveal any changes in membrane structure. Tensile testing of treated and non-treated samples showed that after scCO2 treatment the ultimate strength slightly decreased (less than 10%), while the Young’s modulus was reduced by almost 50%. The bubble point test showed that scCO2 causes an increase in the number of pores and an increase in the UFC value. In the range 40-100 oC no significant temperature dependence was observed. The results confirm that supercritical carbon dioxide can be used as a medium in porous polypropylene membrane production, maintenance and modification.