Melt drawing as a route to high performance polyethylene

Abstract

Well established routes for obtaining stiff and strong polyethylene (PE) involve solid state drawing either of solution crystallized gel films or melt crystallized spherulitic PE. The aim of this work is to show the potential of melt deformation as an alternative route for obtaining highly oriented products. Our previous work on the melt deformation route showed that oriented PE fibers could be directly extruded under appropriately controlled conditions [8,9]. Here, we show that PE films (or filaments) can also be "melt drawn" in the temperature window 130-160 ~ thus yielding oriented products. The advantage of melt drawing over direct melt extrusion is that it allows a wider operational latitude and thus does not require such carefully controlled conditions. The morphology produced by melt deformation is different from solid state deformation and consists of extended chain fibrils with platelet overgrowths. The relative amount of fibrils and platelets depends on operating parameters. The temperature window of PE melt drawing is identified with the regime where some flow induced crystallization takes place. The conditions for melt drawability are of wider generality for crystallizable flexible chain polymers. They are: (i) adequate strain rate to overcome entropic resistance to chain extension, (ii) but not high enough to activate the elastic response of the transient networks in the entangled system, (iii) sufficient strain to fully extend the chain, (iv) appropriate temperature for flow-induced crystallization and strain hardening, and (v) cooling to freeze the oriented structure.Ultra high molecular weight PEs were not the most suitable for melt drawing due to their high recoverable elongation in the melt (melt elasticity) in addition to added limitations imposed by their nascent grain systeme. Our work suggests that an optimum molecular weight for melt drawing is M~ -(400-900) x 103 with further possibilities for improvement through multimodal distributions.

Melt drawing of ultra-high molecular weight polyethylene: Comparison of Ziegler- and metallocene-catalyzed reactor powders

“…As per Bashir and Keller, 9 the longer chains crystallize earlier than the shorter chains during drawing from melt. Thus, a combination of chains having different length may have a dissolution effect where the shorter chains play a plasticizing role for longer chains, like solution-crystallization of PE with less entanglements.…”

“…Such advantages of HMW on ductility for drawing from PE melts have been examined by Kaito et al, 7,10 van Aerle et al, 8 and Bashir and Keller. 9 However, it has been also reported that much higher MW over 10 6 never gives the high ductility. 9,10 Thus, there may be an optimum MW for drawing from melt, as well as the solid-state drawing, depending on the drawing techniques, initial morphologies, and MW distribution (MWD).…”

“…9 However, it has been also reported that much higher MW over 10 6 never gives the high ductility. 9,10 Thus, there may be an optimum MW for drawing from melt, as well as the solid-state drawing, depending on the drawing techniques, initial morphologies, and MW distribution (MWD).…”

Novel in situ NMR Measurement System for Evaluating Molecular Mobility during Drawing from Highly Entangled Polyethylene Melts

“…[1][2][3][4][5] Transient crystallization into a hexagonal form occurs during oriented crystallization into a final orthorhombic form during melt-drawing of UHMW-PE, as determined by in situ wide-angle X-ray diffraction (WAXD) measurements using synchrotron radiation. [6] Furthermore, we examined the correlation between this transient crystallization and molecular entanglement characteristics of the sample films, i.e., MW or MW distribution (MWD).…”

Morphology of chemically crosslinked ultrahigh molecular weight polyethylene

“…[29][30][31] The temperature at which UHMWPE flows is 215°-220°C. 23,32 Complete fusion occurs at temperatures exceeding 220°C, the higher temperatures necessary for sufficient disentanglement and chain translation.…”