Paper-to-paper friction -: paper structure and moisture

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 performed using a newly developed apparatus, designed and constructed at STFI in cooperation with the US Forest Products Laboratory, FPL. This device allows for the measurement of the first static, S1, 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 surface energies, roughnesses and sheet structures. Different structures 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 surface 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 intermolecular interactions that take place across the interfaces that determines the paper-to-paper friction.