CHEMICAL ASPECTS CONCERNING THE FRICTION AND ABRASION OF RUBBER

The frictional force (f(F)) is proportional to the normal force; for r ubber it is corrected by a factor involving the pressure and elasticit y due to a change of the contact surface. During friction induced by a slider, f(F) fluctuates, which is conventionally explained by a stick -slip mechanics. However, the effect of the temperature and velocity o n f(F) as well as the frequencies of vibration and pattern abrasion pr oducing worn products of various size, are not studied using mechanica l model. On the contrary, the author discusses here the chemical mecha nism of stick-peeling based on a model involving pseudo crosslinks of multi-sizes. The frictional force (f(F)) is taken to be equal to the p eeling force (f(p)), For adhesives, f(p) is proportional to the adhesi on force (f(A)) affected by such rheological factors as the peeling ve locity (v), the thickness (h), and the relaxation time (tau); f(p)=f(A )(tau vh)(0.5). Here, f(A) is proportional to the fraction of the pseu do link (v) and the wetting energy (W) divided by the peeling distance , which is almost equal to the bond length (l) and f(A) is expressed a s f(A)=(v/N)(2/3)(W/l). v is the number of links formed on the surface of the rubber and is varied with the sizes of the links 4 (link A) an d 16 (link B) and their relaxation times of 10(-3.4) s (tau(A)) and 10 (-1) s (tau(B)). Also, for the frictional force (f(F)) the same equati on is obtained when v and h are taken to be the velocity of the slider and the thickness of the rubber layer deformed by the slider, respect ively. Friction produces two kinds of vibrations by the dissociation o f links A and B; their frequencies are given by the reciprocal of tau( A) and tau(B), respectively. Abrasion is caused by the scission of rub ber chains connected with links A and B in the peeling process. The fo rmer abrasion yields powdery wear products by crazing, whereas in the latter the scission of chemical bonds of the energy (D) develops to gi ve crest-shape tearing i.e. so-called pattern abrasion. The sizes of w ear products are proportional to the relaxation distances correspondin g to tau(A) and tau(B). The resistance to abrasion of rubber is expres sed by a ratio of the force at a break (f(B)) to f(F),f(B)/f(F)=(v/N)( 1/3)(D/Wl(2))(v tau h)(-0.5). Carbon black improves the abrasion resis tance due to its reinforcing ability, and the resistance is enhanced b y increasing the content, specific surface area and adhesion ability o f carbon black. Their optimum values were also estimated theoretically .