Effect of different morphologies on the creep behavior of high-density polyethylene

Abstract

With the wide use of polymer materials as pressure parts, people have started paying more attention to the safety and longevity of polymeric materials. Creep is one of the most important factors to evaluate materials. In this study, a self-designed oscillatory shearing injection molding (OSIM) device was utilized to prepare pure HDPE specimens with special morphologies. According to a comparison of the creep behavior of the OSIM specimens with conventional injection molding (CIM) specimens, the distinction between the resistivity to creep due to the special morphologies was observed. Two initial external stress levels (10 MPa and 15 MPa) and three temperatures (ambient temperature 25 °C, 40 °C and 60 °C) were employed in this experiment. Different morphologies resulted in different responses to creep. The deformation and compliance of the CIM specimens were triple or more than those found for the OSIM specimens under the same conditions. The instantaneous deformation of the OSIM specimens was 0.2% compared with 0.6% found for the CIM specimens under 10 MPa at 25 °C. The deformation of the OSIM specimens was 4% after creep for an hour, but the CIM specimens were already necked at less than 50 min under 15 MPa at 40 °C. At 60 °C, too much plastic deformation appears in the creep behavior of the CIM specimens and the creep behavior was nearly not observed under these conditions. In addition, the creep behavior of the OSIM specimens can be observed at 60 °C. According to our tests and analysis, the property of creep resistivity for the OSIM specimens was better than that found for the CIM samples, in both the amorphous phase and crystalline region. In addition, the creep behavior of the OSIM and CIM specimens was satisfactorily described using the generalized Kelvin–Voigt model with one retardation time.