Monolayers of saturated fatty acids of different chain lengths and of 9-cis-octadecenoic acid were physisorbed onto regenerated cellulose surfaces to study their effect on friction and adhesion. Lateral and normal force measurements on these surfaces were carried out at different humidities using an atomic force microscope with chemically functionalized silicon nitride cantilevers. Friction and adhesion of clean cellulose increased significantly with humidity. Adhesion forces at high humidity were predicted well by theory that accounted for the effect of the Laplace pressure due to capillary condensation. The magnitude of friction and adhesion and their dependence on humidity decreased with increasing hydrophobicity of the contacting surfaces. Friction of cellulose surfaces impregnated with different saturated fatty acids decreased sharply from a high level to a low level at a carbon chain length above 14 carbon atoms. Contact angle data indicated that only saturated fatty acids with 16 carbon atoms or more in the carbon chain formed vertically oriented monolayers on cellulose. Our results suggest that friction of hydrophilic polymer surfaces under ambient conditions is greatly influenced by capillary condensation. Furthermore, lubrication by fatty acids is achieved through the formation of vertically oriented, hydrophobic monolayers that can withstand the normal and shear stresses during sliding and increase the hydrophobicity of the polymer surface, thereby decreasing the degree of capillary condensation.

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ABSTRACT:The purpose of this investigation was to study the origin of the differences in paper-to-paper friction of linerboards based on old corrugated containers (OCC). The sheets were subjected to two extraction stages and analyzed with respect to, surface roughness, and their content of low-molecular-mass lipophilic compounds (LLC). Friction was measured using a friction tester based on the horizontal plane principle. The surface roughness was measured using a Perthometer profiler and the low molecular mass lipophilic constituent of the paper sheets was determined by gas chromatographymass spectroscopy. The sheets were imaged using environmental scanning electron microscopy (ESEM), and the relative compositions of inorganic ions on the paper surfaces were determined by energy dispersive X-ray spectroscopy (EDS). The results showed that a high amount of LLC in the sheets lead to low friction, due to lubrication. It was also observed that large CaCO 3 particles on the surface had a friction-increasing effect, and that there was no relationship between the surface roughness and the friction.