Orientation of polyoxymethylene by rolling with side constraints J. Mohanraj a,1 , J. Morawiec b , A. Pawlak b , D.C. Barton a , A. Galeski b, * , I.M. Ward c a School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, UK b Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, 90-363 Lodz, Poland c School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK Received 4 July 2007; received in revised form 30 October 2007; accepted 6 November 2007 Available online 7 November 2007 Abstract In this paper, we describe the application of the constrained rolling process to produce highly oriented polyacetal bars with enhanced mechanical properties. In this process, the heated polymer billet is deformed in a channel formed in the circumference of the bottom roll that provides lateral constraint to the material as it deforms. It is a process that has attracted interest due to its capability to produce thick cross-sectional oriented products continuously and at moderate production speeds. Here the focus is on two commercial grades of polyoxy- methylene (a) Delrin Ò 100 and (b) Tarnoform Ò 300. Tarnoform Ò , unlike Delrin Ò , is a copolymer. The compression behaviour of these grades has been investigated in a plane strain channel die to determine the optimum constrained rolling conditions. Samples were then rolled to different reduction ratios close to but below the crystalline melting temperature of the two grades. The modulus and strength increased almost linearly with reduction ratio. Rolled Delrin Ò exhibited higher modulus and strength than Tarno- form Ò . Under impact loading, with the initial notch perpendicular to the rolling direction, the fracture process was incomplete for both resins with the specimens exhibiting a hinge type break. Structural investigations of the rolled samples were carried out by wide and small angle X-ray diffraction. The structures produced were very similar to those produced in plane strain compression test. The pole figures from the (100) reflection suggest that the c axes of the POM crystals are oriented along the rolling direction while ab planes showed clustering of orientation of (100) normals in six directions. SAXS patterns from the rolled samples with the X-ray beam parallel to the force direction showed two-point patterns that suggest the transformation of the spherulitic morphology to a fibrillar structure in this direction. However, perpendicular to the rolling direction, four-point patterns were obtained that suggest cooperative kinking of the lamellae during deformation to produce a chevron- like structure. The enhancement in properties as a result of molecular orientation suggests that these materials can have major commercial applications. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Polyoxymethylene; Rolling; Plane strain compression 1. Introduction Molecular orientation in polymers greatly improves their mechanical and physical properties compared to the isotropic material [1e5]. Oriented polymers have found extensive appli- cations in the production of fibres, films, pipes, sheets, etc. and recently more complex profiles have been manufactured. Molecular orientation in polymers can be achieved by pro- cessing either in the melt or in the solid state. The enhance- ment of properties in the melt state is low due to the high temperature of the melt and the shaping device that increases the mobility of the molecular chains. However, in the solid- state processes, the molecular chains are essentially ‘‘frozen- in’’ leading to significant enhancements in the properties. In these processes, the polymers are processed below the crystal- line melting temperature in the case of semi-crystalline poly- mer or near the glass transition temperature for amorphous polymer. Examples of solid-state orientation processes include tensile free drawing, die-drawing, hydrostatic extrusion, rolling, roll-drawing, constrained rolling process and equal * Corresponding author. Tel.: þ48 42 6803250; fax: þ48 42 6803261. E-mail address: andgal@bilbo.cbmm.lodz.pl (A. Galeski). 1 Present address: Bridon, Carr Hill, Doncaster DN4 8DG, UK. 0032-3861/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2007.11.008 Available online at www.sciencedirect.com Polymer 49 (2008) 303e316 www.elsevier.com/locate/polymer