Relationship between the Cohesive Strength and the Tack of Elastomers: Part II, Contact Time Effects

Abstract The self-tack (also called autohesion or simply tack) of an elastomer is its ability to resist separation from another piece of the same elastomer compound with which it has been brought into contact for a short time under a light pressure. Briefly, the events that take place in the interfacial region to form the tack bond are: 1) the attainment of intimate molecular contact of the polymeric chains from each surface; this requires viscous flow of the material near the surfaces and displacement of any impurity that may prevent contact, and, 2) interdiffusion of chains across the (temporary) interface; shorter chains will diffuse more rapidly. If the two contacted elastomeric layers are uncrosslinked, then the interface between them will eventually disappear and the strength of the junction (tack bond) will become identical to the cohesive strength of the material. This process has been referred to as “healing”. It is instructive to define relative tack as the measured value of tack (given, e.g., by the force to peel apart two contacted elastomeric strips) divided by the cohesive strength of the material determined in the same test geometry. Relative tack can have a value between zero and one. When the value is near zero, then it is inferred that the degree of interfacial contact and/or interdiffusion is quite incomplete. On the other hand, when the value approaches unity, the tack bond behaves as though the junction were completely healed. A very important feature of relative tack is that, even for a given time and pressure of contacting, its value is not unique, but depends markedly upon the test conditions selected to pull the bond apart. This interesting behavior was shown in a previous publication. Specifically, relative tack is rate dependent such that it may have a low value at some test rates (indicating only partial healing), but be equal to one at other test speeds—in spite of incomplete interfacial fusion. In this earlier investigation, measurements were carried out on specimens that possessed only a single extent of healing, i.e., the contacting conditions (time and pressure) were held constant. In this paper (Part II), studies of the rate and temperature dependence of bond strength are extended to include tack bonds possessing varying degrees of healing. This was simply achieved by varying the contact time before measuring tack.