Mechanical anisotropy of a monoaxially drawn polypropylene film

The tensile strength values, tensile moduli (measured with an Instron dynamometer) and sonic moduli of a monoaxially oriented polypropylene film were determined. Anisotropy was defined by the ratio between the tensile strength or modulus values in the direction of drawing and in the perpendicular direction. The difference amounting to an order of magnitude between the anisotropy of tensile strength and of the sonic modulus is explained by the existence of cracks between bundles of microfibrils. Anisotropy of the modulus determined with the instron dynamometer is lower than that of tensile strength, but higher than that of the sonic modulus. This is a consequence of the failure of the system which occurs in the transverse direction already at low deformations.