Crystallization Behavior and Mechanical Properties of Microinjection Molded High Density Polyethylene Parts

Abstract

Ultra-thin high density polyethylene (HDPE) parts with two different molecular weights were prepared by microinjection molding (MIM). The dependence of crystalline morphology and orientation, as well as the resulting mechanical properties of the samples, on molecular weight is described. The toughness of the high-molecular-weight (HMW) sample was over 2 times that of the low-molecular-weight (LMW) one, in parallel with a significant increase of tensile strength. Microstructure characterizations, including differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS), were performed to investigate the variations of the microstructure. It is suggested that the increased crystallinity and higher degree of both molecular and lamellar orientation were beneficial to the enhancement of strength of the HMW sample. SAXS results showed that a highly oriented crystalline structure, i.e. shish-kebabs, were formed in parts of both of the two HDPE. Furthermore, a larger number of shish and kebab structure or lamellae was formed in the HMW sample due to the fact that the crystallinity was increased and the lamellar thickness and lateral crystallite size was reduced. Therefore, a stronger physical cross-linking network was formed in the HMW sample because of the increased connection points, which was in favor of the notable improvement of toughness. We suggest this issue is of great significance for achieving materials with high performance by tailoring the microstructure.

Keywords:

• Microinjection molding
• microstructure
• molecular weight
• HDPE
• shish-kebab structure
• mechanical property

Acknowledgments

We express our great thanks to the National Natural Science Foundation of China (Grants number: 51173171, 11172271, 11172272), the Program for Science & Technology Innovation Talents in Universities of Henan Province (Grants number: 2012HASTIT002), State Key Laboratory of Materials Processing and Die & Mould Technology as well as The Key Laboratory of Polymer Processing Engineering, Ministry of Education, China. The authors would also like to thank Prof. Liangbin Li and Prof. Guoqiang Pan of the University of Science and Technology of China for WAXD measurements at the National Synchrotron Radiation Laboratory.