STUDIES ON PREPARING OF BIAXIALLY STRETCHED MICROPOROUS POLYPROPYENE SHEET FILLED WITH INORGANIC FILLERS .1. BIAXIALLY STRETCHED MICROPOROUS POLYPROPYLENE SHEET FILLED WITH INORGANIC FILLERS

Microporous sheets were prepared by biaxial stretching of polypropylene (PP) sheets filled with calcium carbonate (CaCO3) and silica (SiO2). Pore volume and water vapor transmission (WVTR) increase for higher degrees of biaxial stretching. Maximum pore size (D(max)), WVTR and air permeability increase with increasing of the content of filler. Maximum pore size is positively correlated with average pore size, and negatively correlated with water resisting pressure. Average pore sizes of sheet increase with increasing of particle size of filler. Air permeability, D(max) and water resisting pressure are different for different kinds of fillers. Between one surface and the other surface of the sheet, there are many large voids and micropores. Micropores are located in the stretched PP layer, they connect with large voids.