Properties of Epoxy Adhesives

Thermal and thermomechanical properties. The thermal conductivities of unfilled epoxies, as with all otherunfilled pol5mers, are quite low, typically 0.1-0.2 W/m-K. When filled with metal or thermally conductive nonmetal fillers up to 80%-85% by weight, the thermal conductivities increase a minimum of tenfold. Some silver-filled epoxies are reported to have thermal conductivities as high as 6.5 to 8 W/m K (AI Technology ESP 8350 and ESP 8680, respectively). 

Thermal expansion properties. The CTEs of most filled epoxy adhesives range from 25 to 50 ppm/°C but, as discussed in Ch. 2, increase three- to fourfold above their glass-transition temperatures. 

For electrically conductive metal-filled adhesives, dielectric constants and dissipation factors are not meaningful parameters, but volume resistivities and contact resistances are significant. The volume resistivities for the best silver-filled epoxies range from 1 x 10 to 8 x lO ohm-cm. 

Physical chemistry is an important factor, which leads to the exceptionally good performance properties of epoxy adhesives. Physical chemistry deals with the physiochemical and surface chemistry aspects of the system. Here system is defined as the adhesive, its constituents, the substrates, and the interface region of the joint. The physical chemistry can determine the success or failure of an epoxy adhesive application. 

These properties have a profound effect on the processing properties of the uncured adhesive and on the end properties of the fully cured product. The properties determined by physical chemistry affect both the cohesive strength of the adhesive film as well as the degree of adhesion to the substrate. They also affect the permanence and durability of the adhesive bond once it is placed into service. 

To understand the overall effect of physical chemistry, one must consider the adhesive system in three stages  

In all these stages the adhesive properties will be determined primarily by the molecular structures of the epoxy resin and curing agent or catalyst employed. Additives and modifiers, of course, will also affect these properties, and they are discussed in later chapters. 

There are processes going on in all three stages that ultimately will affect the adhesion and the performance of the adhesive or sealant. It is important to understand these processes and the effects that they will have on the quality of the joint. They are summarized in Table 3.1 and described in the sections that follow. 

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TABLE 3.1 Important Properties of Epoxy Adhesives during Various Stages of Cure... 

The properties of epoxy adhesives in their uncured condition will determine primarily how easily the adhesive can be processed, applied, and cured. They will also determine, to some extent, the performance characteristics of the cured joint. The properties of the individual components as well as that for the mixed formulation are important. 

The degree of tensile strength improvement is often in the 50 to 100 percent range. The effect of various fillers and loading ratios on the strength properties of epoxy adhesive formulation is indicated in Fig. 9.9. The effect of different fillers loaded at a constant 100 pph is indicated in Table 9.12 for shear strength on phenolic laminate and aluminum substrates. 

McGuiness, E. W., Properties of Epoxy Adhesive at Elevated Temperatures, Epoxy Resin Symposium of SPE, Minneapolis, MN, October 1958. 

The durability of the properties of epoxy adhesives is often limited by aging effects. Depending on the aging conditions, such as humidity and temperature, various structural properties of the adhesives can be strongly influenced. However, the underlying mechanisms in the early stage of aging have not been understood sufficiently. 

The properties of epoxy adhesives can be modified by the incorporation of fillers. 

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