Effect of multilayer extrusion technology on the transparency of polypropylene sheets

摘要

To improve the transparency of isotactic polypropylene sheets, multilayer extrusion was conducted using a resin with lower melt viscosity for the surface layers than one for the core layer. By using multilayer extrusion, the number of spherulites near the surface decreased dramatically and the internal haze of the sheet was improved. The number of spherulites of the sheet cross-section and the internal haze were closely correlated to each other. Each distribution on the flow velocity, shear rate and shear stress in a die lip section was calculated using a finite element method. The shear stress in the sheet thickness direction was reduced when the resin with lower melt viscosity was laminated on both surfaces, and then it was surmised that the stress induced crystallization was considerably restrained in these layers. Furthermore, the cooling simulation in a belt quenching process was conducted, and the prediction formula was derived by multiple regression analysis, using the number density of spherulites as a response variable, and both the plateau area time of the temperature in each thickness direction and shear stress as independent variables, respectively. As a result, the number density of spherulites in various surface layer ratios was predicted using obtained regression formula, and the optimization of the multilayer composition to obtain higher transparency was proposed.