Rate-dependent adhesion effect

It has recently become common to use the JKR theory (Johnson, Kendall Roberts, 1971) to extract the surface and inteifacial energies of polymeric materials from adhesion tests with micro-probe instruments such as the Surface Force Apparatus and the Atomic Force Microscope. However the JKR theory strictly applies only to perfectly elastic solids. The paper will review progress in extending the JKR theory to the contact mechanics and adhesion of linear viscoelastic spheres. The observed effects of adhesion hysteresis and rate-dependent adhesion are predicted by the extended eory. 

The behaviour of specimens constructed with either soft or stiff adhesives may be quite different(64). For instance, for the same value of tensile stress. Clearly, creep effects with ductile adhesives will redistribute stress concentrations. There may also be a rate-dependent effect on (7 c (initial) when displacing the adherends with bolts. 

Adhesive test methods and their test results are related to but are not the same as other polymer property tests described in this handbook. In general, adhesive property tests are ultimate properties measured at the failure load of an adhesive joint. Adhesive joint properties are certainly related to adhesion (vide supra) but are primarily due to the physical properties of the adhesive and the physical properties of the adherends. In addition, the design of the adhesive joint has a major effect on the measured strength. Indeed, a properly designed adhesive joint will always lead to failure of the adherend. In addition, adhesive joint properties are as temperature- and rate-dependent as the properties of the polymers used to make the adhesive. Unfortunately, in many cases in the literature, the rate of test is not described. In this chapter, the type of adherend or backing will be described, if at all possible. 

Rate and Temperature Effects. Like adhesive tack, autohesion of elastomers is strongly dependent on test rate and temperature. Furthermore, as shown in Figure 13 for the T-peel autohesion of a cold emulsion SBR, relative autohesion Pr (for a given time and pressure of contact) is not unique, but it too depends markedly on test conditions (104). 

Clearly, the peel strength is not a fundamental property for an adhesive. The value of force per unit width required to initiate or sustain peel is not only a function of the adhesive type, but also depends on the particular test method, rate of loading, thickness and stiffness of the adherend(s) and adhesive as well as other factors. Thus, peel tests generally do not yield results that may be used in quantitative design. This does not imply, however, that the peel test is not a useful test. Peel tests provide quantitative comparisons between different adhesive systems, insight into rate and temperature effects, etc. Additionally, peel tests can be used to provide fracture mechanics information as will be discussed in the next section. In the author s opinion, the latter aspect of peel tests has been perhaps most adroitly exploited by Gent and Hamed [18-20] who used peel tests in conjunction with fracture mechanics to obtain insights into time-temperature effects, the role of plasticity, and many other aspects of adhesive fracture. 

Rate and Temperature Effects for Pressure-Sensitive Adhesives. Pressure-sensitive adhesives consist of soft elastomeric semi-solids. Their peel strength depends strongly upon the rate of peel and the test tem-... 

The energy release rate (G) represents adherence and is attributed to a multiplicative combination of interfacial and bulk effects. The interface contributions to the overall adherence are captured by the adhesion energy (Go), which is assumed to be rate-independent and equal to the thermodynamic work of adhesion (IVa)-Additional dissipation occurring within the elastomer is contained in the bulk viscoelastic loss function 0, which is dependent on the crack growth velocity (v) and on temperature (T). The function 0 is therefore substrate surface independent, but test geometry dependent. 

---