Paper-to-paper friction - paper structure and moisture

Christer Fellers, Marie Backstrom and Myat Htun, Swedish Pulp and Paper Research Institute (STFI), Stockholm, Sweden and Goran Lindholm, Institute for Media Technology (IMT), Stockholm, Sweden. Keywords: Friction, Surface energy, Surface structure, Paper structure, Moisture content, Calendering, Beating, Wet pressing. SUMMARY: The purpose of this investigation was to study the effect of moisture content, paper structure and surface energy on paper-to-paper friction. The friction measurements were perfor- med using a newly developed apparatus, designed and construc- ted at STFI in cooperation with the US Forest Products Laboratory, FPL. This device allows for the measurement of the first static, S l, the third static, S3 and the kinetic, K, coefficients of friction. Experiments were conducted on different types of commercial papers and handsheets covering a wide range of sur- face energies, roughnesses and sheet structures. Different structu- res in the handsheets were created by beating, wet pressing and calendering. Friction increased with increasing moisture ratio. Friction could not however be significantly changed by the surfa- ce roughness or structure of the handsheets. Friction values for the smooth side were always found to be higher or equal to those for the rough side. It is suggested that these results and previous results in the literature can be explained by assuming that the contact area is higher for the smooth side and that it is the micro-roughness of these contact areas together with the inter- molecular interactions that take place across the interfaces that determines the paper-to-paper friction.