Adhesion and Strength of Viscoelastic Solids. Is There a Relationship between Adhesion and Bulk Properties?

Abstract

Strength of adhesion depends upon the rheology of an adhesive as well as upon its interaction with a substrate. This is shown by studies using model joints prepared with different amounts of interlinking between two rubber sheets. The fracture energy G per unit of interfacial area appears to be a product of two terms: G = Go [1 + f(R,T)], where Go is the intrinsic (chemical) strength of the interface and f(R,T), usually much larger than unity, reflects energy dissipated viscoelastically within the adherends at a crack speed R and temperature T. Values of Go range from virtually zero for nonbonded sheets up to the threshold tear strength of rubber, 50−80 J/m2, in proportion to the density of interfacial bonds. Values of G are as much as 1000× greater at high speeds and low temperatures. Like adhesion, the tear strength of a soft rubbery solid is also increased by internal energy losses and shows the same marked dependence upon rate of tearing. By comparing the rate R at which tear strength increases by a certain factor with the angular frequency ω at which the elastic modulus μ increases by the same factor the length δ of the dissipative zone at the crack tip can be estimated: δ = R/ω. But values obtained in this way are only about 1 Å, too small to represent an actual dissipation zone in rubber. It seems likely instead that fracture takes place intermittently, the crack growing at high speed and then stopping. A possible mechanism of crack stopping is by splitting at the crack tip. Indeed, tip blunting can be inferred by comparing tear strength with resistance to cutting, when blunting is suppressed. Cutting resistance increases much more slowly with increasing rate and decreasing temperature. Thus, much of the observed tear strength of soft adhesives and soft solids is attributed to crack-tip blunting. Further research is needed to establish the exact nature of the blunting process and how it is related (as it clearly is) to viscoelastic and dissipative properties. Attention is drawn to other unresolved issues in adhesion science.