Squeaks and rattles from interior components in automobiles are annoying and manufacturers are working to minimise or eliminate them. Many of the squeaks and rattles emanate from sliding contacts of interior plastic components. In this paper the friction characteristics, and mechanical and chemical properties of like-pairs of plastics are reported. The plastics are polypropylene (PP), polycarbonate (PC), acrylonitrile-butadiene-styrene (ABS), polyamide 6 (PA6), polyoxymethylene (POM), PC/ABS blend and ABS/PA blend.
A tensile tester was used to measure the friction force of the plastics sliding on themselves. When stick-slip occurred during sliding the amplitude of the stick-slip was used to characterise the tendency for the sliding pairs to make noise. The effect of the normal load, stiffness of the connection between the force transducer and the sliding polymer, the temperature, surface roughness and sliding speed on the amplitude of the stick-slip were determined.
PP had the lowest stick-slip amplitude mean and standard deviation of all the polymers tested. It also had a surface energy which was 60 percent of those of all other polymers and the highest internal damping. The low surface energy is caused by the absence of polar groups on the surface of the PP. When PP was etched with chromic acid, oxygen containing groups were introduced on the surface. As a result both the surface energy and the stick-slip amplitude was increased. PP also had the highest internal damping of the polymers tested and was one of the three polymers which was semicrystalline.
It is proposed that stick-slip amplitude can be reduced by using polymers which have low adhesion at the contact points. The low adhesion results when the surfaces have low surface energy. In addition, crystallinity will inhibit the mobility of the polymeric chains and minimise diffusion across the interface which would promote better adhesion. Finally, internal energy dissipation will damp out vibrations that are excited by sliding motion.
For the polymers which had large stick-slip amplitudes, increases in the normal load and the temperature caused the amplitude to increase. A decrease in system stiffness and an increase in surface roughness caused half of the polymers to have significant increases in stick-slip amplitude.