Structure-property relationships for rolled polypropylene

The relationship between certain structural parameters and mechanical properties have been studied in rolled polypropylene. Sheets of isotactic polypropylene of initial sherulitic (unoriented) structure were rolled both uniaxially and biaxially (cross rolled) at room temperature. Samples were also rolled in the region of the α-transition at 135°c. In addition effects of annealing the rolled samples were studied.

Wide angle x-ray diffraction measurement of pole figures for uniaxially rolled polypropylene show a gradual rotation of the chain axes into the rolling direction. For cross rolled samples, pole figures show that the c-axis tend to align parallel to the surface of the sheet with no preferred orientation about the normal direction. Hot rolled samples have nearly the same texture as the cold rolled samples.

SAXs patterns for uniaxially rolled polypropylene show that the lamellae have tilt of 45° from the rolling direction toward the normal direction with little spread toward the transverse direction. Whereas, for cross rolled samples these lamellae have a tilt of 45° from both rolling directions.

The effect of rolling on mechanical properties seems to be general, i.e., the yield strength and tensile strength are increased in the rolling direction and are decreased in the transverse direction in conventional (uniaxial) rolling. Young's modules increases in all three directions (0° , 45° , 90° to the rolling direction), but most in the rolling direction. Cross rolled samples exhibit practically no difference in properties with the test direction in the sheet for samples cut in three directions (RD 1, RD 2, 45° to either RD). Flongation to break at low deformations in the uniaxial rolled samples increases by more than a factor of two over the unrolled samples. Similar but somewhat smaller increases are also observed in the cross rolled samples.

Annealing of rolled samples causes little change in the unit cell orientation, but results in stacks of lamellae parallel to the rolling direction in uniaxial rolling but with no preferred orientation about the normal direction in cross rolling. Free annealing results in large decreases in ductility, yield strength, tensile strength and modulus. Fixed annealing increases yield strength, tensile strength and modulus while decreasing the ductility (especially in the TD). At high deformation, hot rolling causes structural changes which may be considered equivalent to the combination of cold rolling and annealing. The mechanical properties of hot rolled samples are comparable with properties of the cold rolled with subsequent fixed annealing but not with free annealing.