Investigation into the mechanical response and microstructure evolution of high density polyethylene, under conditions applicable to the injection stretch blow moulding process

Abstract

High density polyethylene (HDPE) is extensively used for a variety of containers but these containers are currently made by extrusion blow moulding (EBM) and not injection stretch blow moulding (ISBM), on a large-scale production basis. Therefore, there is currently a gap in the market for producing HDPE containers via ISBM. The aim of this study was to conduct a detailed investigation into the response of HDPE, under industrially relevant ISBM processing conditions, characterising both the mechanical response and the microstructure evolution during and post forming. The response of HDPE under ISBM conditions was firstly analysed using the free stretch blow (FSB) process. The results indicated a small temperature processing window, between 126 - 130°C, were the deformation was applied at high rates, between 3 – 16 s-1, under equal-biaxial (EB) and sequential biaxial (SQ) deformation modes. Likewise, the stress-strain response was investigated under displacement control, where a temperature change of 1°C within the temperature processing range was sufficient to significantly shift the stress-strain response. The post deformation results showed that the elastic modulus could be enhanced by a factor of 4.0. However, there was no definitive correlation between any of the microstructure parameters and the elastic modulus enhancement. The evolution of the microstructure during deformation was analysed using time-resolved x-rays analysis techniques carried out at Diamond Light Source. The results indicated that the deformation proceeded via fragmentation and recrystallisation. Furthermore, the initial crystallinity of 56% was reduced to 32% after heating and then, was further reduced to 20% immediately after deformation before finally increasing to 61% during cooling. The results indicated the ability to form HDPE under ISBM conditions and outlined the optimal process conditions required to produce the highest modulus post deformation.

Date of Award	Dec 2018
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	Gary Menary (Supervisor), Peter Martin (Supervisor) & Shiyong Yan (Supervisor)