STATIC SHEAR-STRENGTH AND ADHESION FRICTION OF SOME THERMOPLASTICS

The static shear strength and static friction of three thermoplastics have been investigated for the case of contact between a bulk plastic sample and a very smooth metallic plate. The minimum tangential force required to shear the interfacial adhesion bonds between the two surfaces was measured and defined as the adhesion component of friction F(alpha). Experimental results showed that F(alpha) is large when the surface energy is high or the interfacial energy is small and that a correlation may exist between this force and the work of adhesion evaluated from Dupre equation. The real area of contact A(r) was also measured using an optical device designed to handle samples and experimental conditions similar to the static friction tests. It was found that A(r) is proportional to P(alpha)0.9 where P(alpha) is the apparent pressure of contact and an increase of F(alpha) with A(r) was observed. The static shear strength tau has been evaluated as the ratio F(alpha)/A(r), and its relationships with the real contact pressure P approximated by linear functions. It was concluded that tau increases at high P, although it has not been verified for ultra-high-molecular-weight polyethylene. An attempt has been made to explain the observed break in the data. The results indicated that (tau/tau(b)) = k'(tau(alpha)/P(alpha)) where b refers to bulk, alpha to apparent values, and k' is a proportionality constant that depends on the plastic material.