Porous structure and functional properties of highly-permeable polypropylene films

Abstract

The porous polypropylene films, containing through channels and permeable for liquids, have been prepared by the method based on extrusion of polymer melt with subsequent annealing, uniaxial extension and thermofixation. We studied the effect of film preparation conditions on the characteristics of porous structure and functional properties of the films (porosity and permeability) at the stages of extension at room temperature ("cold" drawing) and the additional stage ("hot" drawing). Permeability, size and the number of through pores were measured by filtration porosimetry. Overall porosity is determined as a portion of the sample volume occupied by pores by measuring the density ratio of porous and nonporous films. The film surface images, which demonstrate the characteristic features of the porous structure, have been obtained by scanning electron microscopy. The effect of melt orientation degree during extrusion (spin draw ratio) on the films porous structure is determined. It was found that the increasing of spin draw ratio leads to the growth in through permeability, overall porosity and pore sizes of the films. It was shown that the growth of orientation degree at the additional "hot" drawing stage gives the possibility to increase the functional characteristics of porous samples. In the films preparation process, which includes two stages of extension ("cold" and "hot" drawing), the values of overall porosity and permeability of 60 % and 400 l/(m2×h×atm), respectively, were reached. They are ones of the highest values as compared with values of these characteristics presented in the scientific and patent literature for polymer porous films and they determine the efficiency of their practical application as filtration materials