Effects of annealing on the hierarchical crystalline structures and mechanical properties of injection‐molded bars of high‐density polyethylene

Abstract

The effect of annealing on the microstructural evolution and mechanical properties of high-density polyethylene parts molded via gas-assisted injection molding was investigated using scanning electron microscopy, differential scanning calorimetry, two-dimensional wide-angle X-ray diffraction and tensile testing. The results indicated that a variety of annealing temperatures could induce considerable variations in the hierarchical structures, crystallinity, lamellar thickness and yield stress of the molded bars. According to these results, the annealing temperatures could be divided into three regions. In the low-temperature region of annealing at 80• C, the spatial variation of the superstructure developed along the thickness direction and mechanical properties of the annealed sample were mainly unchanged and similar to those of the original specimen. At 100 and 120• C, the intermediate temperature region of annealing, the thickness of the crystals, degree of orientation and yield stress of annealed samples were greatly improved. Finally, at 127• C, the degree of orientation decreased and yield stress slightly improved, an indication of the high-temperature annealing region being characterized by increasing melting/recrystallization and causing relaxation of oriented molecular chains. A model is proposed to interpret the mechanism of the annealing treatment of the samples at various temperatures.

Effect of high temperatures on the physical and mechanical properties of HDPE geomembranes in air

“…Thus, the preliminary thermal cycle was omitted in the current study and the DSC thermograms were obtained by heating the specimen to 200°C at a rate of 10°C/min. This approach was also consistent with the procedure used by previous investigators (Scheirs et al 1991;Lu et al 1992aLu et al , 1992bZhou et al 1993;Wunderlich and Czornyj 1997;Wang et al 2014) in exploring the morphological changes in the polymer due to annealing. The basic features studied in Figure 1.…”

“…According to Meille et al (2011), annealing is a 'thermal treatment of a solid polymer material at a fixed or changing temperature, leading to desired changes in its physical structure and properties, without involving complete melting or dissolution'. When the polymer is annealed for a sufficient length of time, some of the polymer crystals melt and then recrystallise to give thicker lamella crystals (Petermann et al 1976;Lu et al 1992aLu et al , 1992bWang et al 2014). Although several studies (Lu et al 1992a(Lu et al , 1992bWang et al 2014) have shown morphological changes in the polymer due to annealing, the testing times were very short (2-24 h as compared to more than 2 years in the present study) and thus the effects of long-term exposure to elevated temperatures were not quantified.…”

“…Different studies (Petermann et al 1976;Lu et al 1992aLu et al , 1992bWang et al 2014) have examined the effect of annealing of polyethylene samples and the subsequent effects on the morphology of the polymer. According to Meille et al (2011), annealing is a 'thermal treatment of a solid polymer material at a fixed or changing temperature, leading to desired changes in its physical structure and properties, without involving complete melting or dissolution'.…”

“…One means of investigating morphological changes in the polymer is to examine the DSC thermograms after incubation at elevated temperatures (Gedde and Ifwarson 1990;Lu et al 1992aLu et al , 1992bZhou et al 1993;Lang 1998a, 1998b;Wang et al 2014). Thus, 5-6 mg GMB specimens were taken from samples of the tested HDPE GMBs that had been incubated at different elevated temperatures for different durations and tested in a TA Instruments Q-100 series DSC in accordance with ASTM D3418 test method.…”

“…Lu et al (1992aLu et al ( , 1992b showed that a change in the morphology of the material, as evidenced from the DSC thermograms, increased with increasing exposure temperature and had a significant effect on the SCG after only 24 h exposure. Wang et al (2014) annealed HDPE for 2 h at temperatures between 80 and 127°C. When annealed at a lower temperature (80°C), some of the crystalline defects were eliminated but the crystals remained largely unchanged and there were no changes in the mechanical properties.…”