Structure of microporous polypropylene sheets containing CaCO3 filler

The microporous polypropylene sheets were prepared by biaxially stretching polypropylene sheets containing CaCO3 filler (particle size, 0.08-3.0 μm), when the CaCO3 filler content was 59% by weight and the stretching ratio was 2.8 × 1.8. The microstructure of the sheets were investigated in relation to the CaCO3 particle size by a N2 gas permeation method. (1) Effective porosity increases with decreasing mean particle size of filler. (2) The tortuosity factor of the pore is in the range of 25-40 and becomes relatively smaller with decreasing mean particle size of filler. (3) The equivalent pore size becomes relatively smaller with decreasing mean particle size of filler.