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Flow-induced crystallisation of Polyethylene and Polypropylene |
Introduction
Lino's research concerned structure development during flow-induced crystallisation (FIC). This determines to a high degree the properties of semi-crystalline polymers. Processing parameters strongly affect the crystal nucleation and therefore the final morphology of polymeric products. The aim of his work was to carry out systematic experiments in order to understand the physical processes that govern the flow-induced crystallisation of polyethylene (PE) and polypropylene (PP).
Method
Isothermal experiments were carried out in the Cambridge MultiPass Rheometer (MPR) on PE and PP. The polymer melt was cooled to the desired temperature and then crystallisation induced by moving the pistons and forcing the material to flow within a narrow-section geometry. The structure development inside the material was followed optically or by X-ray diffraction using specially designed test-sections.
Key Observations
HDPE. Fibrous crystallisation occurs during the pistons movement: the fibrous crystals form locally in the high stress zones of the flow ( i.e. the walls of the slit and the downstream "fang" region) and modify the flow-field and the stress distribution, which in turn affects crystallisation. The deep coupling between crystallisation and flow presents a challenge in modelling FIC of polyethylene.
PP. This polymer has a broader temperature window for FIC MPR experiments, corresponding to a wider range of observation. An interesting aspect is that the line nuclei which dictate the crystal morphology are generated during flow, but most of the crystallisation process can occur in the form of lamellar and spherulitic growth after the flow has stopped. So, crystallisation and flow can be uncoupled, and the understanding of FIC be considerably simplified. However, other complications arise, related to the fact that both the crystal morphology and the influence of flow on FIC are more complex. For HDPE only oriented fibrous crystals are generated by flow and their formation is thought to be essentially related to a local critical stress threshold, whereas for PP spherulites generally develop in parallel with the oriented structures, and the crystallisation development is a function of the whole thermo-mechanical history of the sample.
Numerical Simulations
Observations on a metallocene-catalysed HDPE melt at 125°C revealed the formation of very localised crystal filaments. Experimental melt flow birefringence and pressure drop measurements at 155°C were satisfactorily matched with numerical simulations using a multi-mode Pom Pom model and the solver FlowSolve. Subsequent simulations at 125°C revealed a relation between the regions of visible localised FIC during the flow experiment and the regions of high Specific Work undergone by local fluid elements. A method was proposed to estimate the value of critical specific work for the onset of FIC.
References
Saquet O, Mackley MR, and Moggridge G (2000). Direct experimental evidence for flow induced fibrous polymer crystallisation occurring at a solid/melt interface. Journal of Materials Science 35 (2000), 5247-5253.
Scelsi L and Mackley MR (2008) Rheo-optic flow-induced crystallisation of polypropylene and polyethylene within confined entry-exit flow geometries. Rheol Acta, Nov 2007 DOI: 10.1007/s00397-008-0278-z
Hassell DG and Mackley MR (2008) Localised flow-induced crystallisation of a polyethylene melt. Rheol Acta, 47(9), 435-446
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