Integrated experimental–modelling study of microstructural development and kinematics in a blown film extrusion process: I. Real-time Raman spectroscopy measurements of crystallinity

Abstract

Integrated experimental–modelling studies on microstructural development and kinematics of blown film extrusion of low- and high-density polyethylenes (LDPE and HDPE) films are reported. First, the microstructural evolution along the machine direction of the film line is reported from Raman spectroscopy. The 1418 cm⁻¹ peak associated with wagging of the –CH₂ group in the crystalline phase was measured using mobile Raman probes. The changes in take-up ratio, blowup ratio and freeze line height displayed varied levels of influence on the rate of crystallisation. The two polyethylenes develop significantly different extents of crystallinity in the final product form, but higher strain-rates led to faster kinetics and confirmed flow-enhanced crystallisation phenomenon in both polymers. The effect of processing conditions on the development of crystallinity in LDPE and HDPE was consistent with that reported earlier for linear low-density polyethylene (LLDPE), thus confirming the use of Raman spectroscopy as an efficient and reliable analytical technqiue for monitoring microstructure in real time. These results are compared with predictions from a flow-enhanced crystallisation model in a companion paper.