Micro-patterned Polypropylene Films: Reduced Sliding Friction

Abstract

There are many applications of polymeric films where low sliding friction surfaces are required, e.g., low-friction medical tubing and low-friction electrical wires. Typical methods of reducing the apparent sliding friction of polymeric surfaces are: (a) the use of liquid lubricants such as silicone oil, (b) powdered solid lubricants such micro-sized starch and (c) fluoropolymers [1, 2, 3]. Studies on modifying the coefficient of friction (COF) by adding different chemical entities have been previously reported [1, 2]. Such chemical moieties are partially transferred from the film to other surfaces upon contact, and may contaminate those surfaces [3]. It is known that friction is greatly influenced by many factors such as tribology, sliding speed, normal load, surface roughness, material stiffness and contact geometry [2]. Thus, in this study, the sliding friction of micro-textured isotactic polypropylene films, extruded from micro-patterned dies was assessed. We investigated two micro-patterned extrusion dies, one with rectangular texture and the other with a trapezoidal one. The effect of the texture on the apparent film-on-film COF and film-on-metal COF of was evaluated. The films were extruded at 220°C in costumed-built cast-film line consisting of a 25 mm single screw extruder. The ASTM D-1894 Standard Test Method for Static and Kinetic Coefficients of Friction of Plastic Film and Sheeting (method of assembly C) was used for all COF measurements on 25 mm wide films with the following exceptions: (a) a stainless steel (SS) sled (standard roughness of 400 grit) of 19 mm width was used and (b) a cross-head speed of 50 mm/min was used to pull the sleds. For the film-on-metal tests, the kinetic COF of non-textured films was measured to be 0.348±0.023. The textured films displayed a COF of 0.234±0.013 and 0.218±0.011 for the rectangular and trapezoidal textures respectively. Thus, a reduction in COF of about 35% was observed in textured films as compared to the non-textured films. For film-on-film experiments, the non-textured films possessed a kinetic COF of 0.186±0.013. The films obtained from the rectangular textured die showed a reduction in COF of about 12% for the machine direction configuration at 0.163±.012, whereas the films obtained from the trapezoidal textured die displayed a greater reduction of 28% because the COF was of only 0.134±.012. On the other hand, the COF of the combination of non-textured films on rectangular textured films (COF: 0.113±0.004) in the machine direction configuration had a more significant reduction in friction of about 40%. These results indicate that, micro-textured polypropylene films might be effectively used in potential applications where low-sliding and clean surfaces are required.