Stress Relaxation in High Density Polyethylene. Effects of Orientation and Gamma Radiation

Abstract

ABSTRACT:The stress relaxation of unoriented (A= I) and oriented().= 9) high density polyethylene (HDPE) was studied for different levels of radiation (0, 100, 300, and 500 kGy). The relaxation behavior is analyzed in terms of a formerly established two-process model. In that model, one process is related to the crystal and the other to the amorphous fraction of a polyethylene sample. The modified viscoelastic properties of HDPE, due to effects of orientation and irradiation, are reflected in changes in viscosity coefficients and elasticity moduli of these two fractions. This made possible better comprehension of the nature of changes in the relaxation behavior of HDPE induced by irradiation and orientation.KEY WORDS Stress Relaxation I Crystal Fraction I Amorphous Fraction I Polyethylene IThe effects of ionizing radiation and orientation on the mechanical relaxation of polyethylene have been the subject of recent publications. 1 -6 Some of these studies have shown an increase in intensity of the stress relaxation with orientation. 1 -3 This behavior is a consequence of the reduced mobility of highly stretched tie molecules in oriented samples. It has also been shown that radiation crosslinking can improve stability to applied stress up to 30%. 2 -6 Formation of network suppresses the cold flow and increases internal resistance of the sample.For better understanding of the influence of irradiation and orientation on the relaxation behavior, in our previous paper 4 we proposed a two-process model for the stress relaxation in linear low density polyethylene (LLDPE). The model is based on the assumption that the stress relaxation can be represented with two thermally activated processes acting in parallel, each related with one fraction of polyethylene sample: crystal or amorphous. The process which takes place in a crystal domain is, in fact, a propagation of defects through the lattice, as it has been established earlier by Wilding and Ward. 7 This approach is explored further in the present paper by examining the effects of orientation and different adsorbed doses of radiation on the relaxation behavior of high density polyethylene (HDPE). Since the relaxation process is very sensitive to prior structure of the samples, we used differential scanning calorimetry (DSC) and gel measurements to find out the changes in the crystal fraction and the degree of network formation respectively. EXPERIMENTALIn these investigations HDPE HIPLEX EHM 6003 p=0.96gcm-3 and Mw=76700 was used. Preparation and irradiation of the samples are described in previous studies. 3 -5 There are two types of samples, unoriented (A= 1) and oriented (A=9), which were exposed to 6°C o gamma-source up to absorbed doses of 0, 100, 300, and 500 kGy. Initial elasticity moduli of unirradiated samples were E=0.6GPa for A= 1 and E=2.9GPa for A= 9. These values were obtained from stress-strain 1194 measurements using Zwick tensile testing machine at a load rate of 20 em min-1 .The stress relaxation measurements are also performed on a Zwick tensile...

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