Characteristics of adhesives

The JKR model predicts that the contact radius varies with the reciprocal of the cube root of the Young s modulus. As previously discussed, the 2/3 and — 1/3 power-law dependencies of the zero-load contact radius on particle radius and Young s modulus are characteristics of adhesion theories that assume elastic behavior. 

As is evident, there are several distinctive characteristics of adhesion-induced plastic deformations, compared to elastic ones. Perhaps the most obvious distinction is the power-law dependence of the contact radius on particle radius. Specifically, the MP model predicts an exponent of 1/2, compared to the 2/3 predicted by either the JKR or DMT models. 

Adhesive Disadvantages. There are some limitations in using adhesives to form assemblies. The major limitation is that the adhesive joint is formed by means of surface attachment and is, therefore, sensitive to the substrate surface condition. Another limitation of adhesive bonding is the lack of a nondestructive quality control procedure. Finally, adhesive joining is still somewhat limited because most designers of assemblies are simply not familiar with the engineering characteristics of adhesives. 

The following section offers a brief description of how certain additives and formulation parameters are used to control the characteristics of adhesive systems. The roles of specific additives are defined in Chaps. 6 through 10. 

Easily deformable, especially Cu sheets, thus deformation characteristics of adhesive layers important for adhesive selection. 

Mostovoy, S. Ripling, E. J. "Final Report - Fracturing Characteristics of Adhesive Joints" Materials Res. Lab Chicago, 1975. 

This chapter considers a study of two types of composites based on polyhydroxyether and graphite with various amounts of a filler. Using various methods it is possible to estimate the adhesion characteristics and interfacial layer, including its thickness and tensile strength and the interdependence between these values and the adhesion properties. The results were treated on the basis of the theory of irreversible aggregation, cluster theory of the polymer structure and fractal analysis. It was established that all the important characteristics of adhesion, the interfacial layer and mechanical properties are interconnected by the fractal dimensions of the surface of the aggregates of filler particles and of the polymer matrix, whose structure is distorted under the influence of the filler surface. 

Curing In the curing process, the adhesive molecules are cross-linked to create a strong adhesive joint. To solidify the adhesives, several methods are utilized such as heat, pressure, time, catalyst, vulcanizing, reactivation, and radiation based on characteristics of adhesives (Fig. 6) (Cognard 2006). 

In our view, the failure to determine correlation between the thermodynamic characteristics of adhesion and the strength of adhesive-bonded joints lies in the fact that what was being studied was the relationship of the initial compoimd properties (in the hquid state) with the strength of joints in which the adhesive was in the cured state. In addition to ignoring the essential differences of the cured adhesive from the initial liquid, there was no accoimt for irregularity of the process of formation of adhesive-bonded joints itself, which means that the most favorable adhesive structure (from the thermodynamic point of view) is not realized for kinetic reasons. The necessity for accounting for these factors has been established in previous sections. 

The exact cause of premature adhesive failure is very difficult to determine. If the adhesive does not wet the surface of the substrate completely, the bond strength is certain to be less than maximal. Internal stresses occur in adhesive joints because of a natural tendency of the adhesive to shrink during setting, and because of differences in physical properties of adhesive and substrate. The coefficient of thermal expansion of adhesive and adherend should be as close as possible to minimize the stresses that may develop during thermal cycling or after cooling from an elevated temperature cure. Fillers are often used to modify the thermal expansion characteristics of adhesives and limit internal stresses. Another way to accommodate these stresses is to use relatively elastic adhesives. 

Characteristics of Adhesive Materials Table 5.3 Most Important Tape and Film Adhesives ... 

---