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The kinematic viscosities in the undiluted solutions of propylene glycol and the polypropylene glycols were proportional to their molecular weights, but the friction coefficients and compressibilities in polypropylene glycols hardly depended on their molecular weights except for those in propylene glycol. In the friction coefficient measurement, the sliding metal surface was oxidized by strong mechanical shearing force and heat. The boundary lubricant film formation between the polypropylene glycols and the metal oxide layer on the sliding surface was investigated by the contact angle and infrared absorption measurements. The contact angle of the polypropylene glycol at 40°C decreased with an increase in the heat-treated temperature of the metal plate. The infrared absorbance of the polypropylene glycol at 3496 cm was proportional to the lubricant film thickness, but the absorbance intercept in the plot of the absorbance vs. the film thickness was negative at zero film thickness. The negative intercept indicates that the hydroxyl group of the polypropylene glycol disappeared owing to the adsorption of the lubricant molecules to the metal oxide surface. The molecular weight dependence of the friction coefficient was explained by the boundary lubricant film formation of the polypropylene glycol on the metal oxide surface.

Relationship between Friction Coefficient, Contact Angle and Infrared Absorbance of Polypropylene Glycol on Metal Surface