Zum Schereinfluß auf die Kristallisation und die mechanischen Eigenschaften von isotaktischem Polypropylen

Abstract

The influence of the cooling conditions and the shear on the mechanical properties of isotactic Polypropylene were investigated. Former works about the crystallization and the morphology (1–5) were the basis.

The mechanical properties were discussed by density, Vicat temperature, tensile strength, impact strength and hardness. The differences between the investigated samples were that the tubes crystallized under shear influence and the sheets crystallized without any shear influence. The mass and bath temperatures of tubes als well als sheets and the pressure at crystallization were kept constant. The relations between density, tensile strength, impact strength and hardness, found by other authors (13) with injection moulded Polypropylene, were analogous to ours. The tubes which crystallized under shear influence had rendered higher or equal values of the tensile strength, the tear strain, the impact strength and the hardness than those ones of the sheets which crystallized without any shear influence. These differences cannot be explained by the different cooling media (Silicon oil, water). If the sheets were cooled with water — as the tubes were — both the density and the mechanical properties of the sheets would even be lower than the rendered values because of the higher warmth conductivity of water as that of Silicon oil.

Polymorphism of it-PP (10) may provide one explanation, and other the great differences in textures of tubes and sheets (tubes — strong texture; sheets — low texture) found by P. F. Mayer (9).

The textures and the polymorph structures are enclosed in certain layers of the samples (2, 11, 14–16). In a later work these influences should be discussed by investigating the mechanical properties of separated layers (“differential”) of the samples and then comparing the single values to the integral properties.