Continuously Extruded Micro-Textured Polypropylene Films Byron S. Villacorta, 1 Sarah Hulseman, 2 Andrew H. Cannon, 2 Ralph Hulseman, 2 Amod A. Ogale 1 1 Chemical Engineering and Center for Advanced Engineering Fibers and Films (CAEFF), Clemson University, Clemson, South Carolina 29634 2 Hoowaki LLC, Pendleton, South Carolina 29670 The continuous extrusion of micro-textured isotactic polypropylene (i-PP) film and its tribological properties are reported. As analyzed by scanning electron micros- copy, a rectangular-semicircular micro-patterned die successfully produced extruded cast-films with hill-like micro-texture. Such films displayed static and kinetic coefficients of friction (COF s and COF k ) values of 0.247 6 0.028 and 0.245 6 0.003, respectively, for a steel fixture sliding along the micro-texture of the film. The equivalent COF s and COF k values for non-textured (NT) films were 0.369 6 0.036 and 0.340 6 0.024, respectively, confirming that textured films displayed a reduction in COF of about 30%. For textured and NT films sliding on textured films, the COF k values were even lower at 0.161 6 0.013 and 0.113 6 0.004, which represents about 20–40% reduction as compared with that for the NT-NT counterparts. Films coated with a silicone lubricant dis- played COF s and COF k values of 0.334 6 0.042 and 0.099 6 0.012, respectively, for NT films sliding over lubricated NT films, and 0.426 6 0.031 and 0.063 6 0.006, respectively, for textured films sliding over lubricated textured films. The COF k values for textured/non-lubri- cated films approach those of NT/lubricated films, indi- cating that micro-textured polypropylene films may be used in environmentally sensitive applications where lubricants and fluorinated additives may not be used for reducing friction. POLYM. ENG. SCI., 00:000–000, 2013. V C 2013 Society of Plastics Engineers INTRODUCTION Numerous applications of polymeric films require low sliding friction surfaces, such as high-speed packaging equipment and low-friction tape wraps [1–4]. Conventional methods of reducing the sliding friction of polymeric surfa- ces include: (a) external coating of liquid lubricants such as silicone oil or powdered solid lubricants such starch or talc, and (b) internal fluoropolymer additives [5–8]. Fric- tion reducing chemicals that are applied to film surface or ones that bloom to the surface can possibly contaminate products and add processing complexity. Prior literature studies have established that the fric- tional force, F, developed between two sliding surfaces has a dual, molecular-mechanical nature [9]. Hence, fric- tional force depends on the contact bonding shear stress, s 0 , arising from the adhesive forces (molecular interac- tions), and on the contact area, A t , between the two surfa- ces [2, 9]: F 5 s 0 A t . Thus, the fabrication of regular micro-textures that reduce contact area (A t ) of the film surface can lead to a reduction in COF. Previous studies have reported that by creating micro-sized features in metallic and polymeric surfaces, the tribological charac- teristics and hydrophobicity can be modified [10–17]. This can help avoid the use of external lubricants, such as silicone oils and fluorinated compounds, that must be avoided in environmentally sensitive applications. For various materials, micro-textural features have been typically created by means of batch techniques such as mold casting using lithographically made micro-molds [14, 18], plasma processes [19], and chemical/ion beam etching or laser texturing [11]. These processes can be expensive and slow for bulk production of polyolefin films. However, the processing of micro-textured films by continuous melt- extrusion of polymers, a scalable technique, has not been systematically reported in the literature studies. Therefore, the objectives of this study were to (i) assess the microstruc- ture of isotactic polypropylene (i-PP) films continuously extruded through a micro-patterned die, and (ii) measure the effect of the surface micro-texture on the sliding coefficient of friction (COF) characteristics of the films. EXPERIMENTAL Materials The polymer used throughout this study was PP Dow INSPiRE TM 114, a film grade i-PP. It has a melting point Correspondence to: Prof. Amod Ogale (e-mail: ogale@clemson.edu) Contract grant sponsor: National Science Foundation; contract grant numbers: EEC-1128481, EEC-9731680. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. DOI 10.1002/pen.23762 Published online in Wiley Online Library (wileyonlinelibrary.com). V C 2013 Society of Plastics Engineers POLYMER ENGINEERING AND SCIENCE—2013