Polymer structure effects on crystallization and properties in polypropylene film casting

One possible way to modify the properties of isotactic polypropylene (iPP) is to use the material’s polymorphism which in case of film casting means to balance the content of α-crystalline and mesomorphic phase in the product. A cast film study was performed with several iPP homopolymers of different isotacticity and an ethylene-propylene (EP) random copolymer at two different film thickness levels (50 and 200 μm), varying the cooling rate in solidification by adapting the processing conditions. The results from film characterization can be correlated to findings from a parallel investigation with a combination of DSC and FSC techniques in a cooling rate range of 0,01 to 1.000°C/s. A similar effect of crystallinity on tensile modulus was found for both iPP grades studied and at both levels of film thickness. The dominant processing parameter is the chill roll temperature. Output and line speed have a limited effect on crystallinity, but a stronger one on transparency (haze & clarity), where microscopy revealed surface roughness modifications as significant contributing factor.