RELATIONSHIP BETWEEN STRESS AND DRAW RATIO OF POLYETHYLENE DURING ULTRA DRAW

Abstract

The relationship between stress and draw ratio of crystalline polymer during the ultra draw were investigated in terms of phase equilibrium under stress between the crystalline phase and amorphous phase which is induced by the fracture of crystallite during the deformation. The two types of deformation were considered; one is that caused when the folded chain crystals are fractured by stretching (F type deformation), the other when the folded chain crystals as well as tie links which are induced during the ultra draw were fractured (FT type deformation). From the thermodynamical consideration on the phase equilibrium the stress at an extremely low draw rate, the critical stress σc, was obtained aswhere x: crystallinity, ΔGm: Gibbs' free energy of fusion, αn: neck draw ratio, αm: maximum draw ratio at perfectly extended state, and α: draw ratio during ultra draw.The stress-draw ratio relation of polyethylene was studied at the temperatures from 30°C to 120°C and at the draw rate from 1 to 1000mm/min. It was found for linear polyethylene that F type deformation occurs by the stretching at low draw rate and at high temperature, whereas FT type deformation occurs by stretching at high draw rate and at low temperature. As for the crosslinked polyethylene which was obtained by γ-ray irradiation at the dosage of 10 and 20Mrad, FT type deformation occurred in all stretching conditions.The maximum draw ratio could not be measured by experiment because the rupture took place before the maximum extention. The ratio was obtained only for crosslinked polyethylene by fitting the master curve of stress-draw ratio which was obtained theoretically for FT type deformation to the logarithmic plots of stress-draw ratio relation obtained by experiment. The maximum draw ratios obtained by this method were in good agreement with that calculated from the chain length between crosslinks. It was proved from these results that the adopted models are suitable for explaining the deformation mechanisms.