Mechanical properties of uniaxially and biaxially rolled polymer sheets

Thick sheets of polypropylene and polyethylene were rolled uniaxially and also biaxially at from 100 to 140°C. Moreover, the effects of rolling elongation on the tensile strength, Young's modulus, molecular orientation and degree of crystallinity were investigated in order to obtain thick sheets of polypropylene and polyethylene which have large tensile strength, high Young's modulus and small anisotropy of these properties. The tensile strength of the rolled polypropylene sheet, initially elongated by 5.0 in L-direction and then elongated by 1.5 in C-direction, reached almost 100 N/mm², and the value was three times as large as that of the stock in all directions in the plane. This combination of the first rolling elongation λ₁=5.0 and the second rolling elongation λ₂=1.5 is the condition to get substantially the same rolling elongation in both L- and C-directions because the uniaxially rolled sheets become thicker by shrinking when reheated for biaxial rolling and the substantial second rolling elongation is larger than 1.5. The tendency of molecular orientation corresponds with the tensile strength characteristics of uniaxially and biaxially rolled sheets. Degree of crystallinity of the rolled sheets decreases with increasing rolling elongation.