Kinetic friction and dynamic elastic contact behaviour of polymers

Friction and contact between a glass hemisphere and plates of various polymers were studied under elastic contact. The contact at various sliding speeds was observed by means of Newton's rings. Although the frictional force was proportional to the area of contact observed, it was necessary to consider the increase in the shearing strength of the adhesive junctions with increasing load W. The area of dynamic contact of low density polyethylene (LDPE) was proportional to W^0.78 in a wide speed range, while the area of static contact was proportional to W^2/3 as expected from Hertz' equation. The shape of the dynamic contact was affected by the retarded elasticity of the polymer and frictional heating. The area of dynamic contact decreased and the shear strength increased in proportion to log v (where v is the speed) provided that frictional heating was negligible. The time dependence of elastic compliance 1/E of LDPE could be obtained by applying Hertz' equation to the dynamic contact observed at various speeds. However, the value of 1/E decreased with decreasing load in sliding. It was shown that the time dependence of elastic compliance plays an important role in determining the area of dynamic contact of polymers.