Room Temperature Curing Epoxy Adhesives

One of the distinct advantages of epoxy adhesives is that they can be cured at room temperature or even at lower temperatures. Epoxy adhesives are often divided into room temperature curing types and elevated-temperature curing types. This chapter discusses room temperature epoxy formulations. The major advantages and disadvantages of room temperature curing epoxy adhesives are shown in Table 11.1. 

Room temperature curing epoxy adhesives are two-component adhesives, indicating that the curing agent portion and the epoxy resin portion are packaged separately. These are often referred to as 2K adhesives, with the K derived from the German spelling of komponent. 

The two parts are mixed just prior to application. Metering occurs before the mixing operation at the time of assembly, or the two components are packaged in preweighted containers of various types and sizes. 

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TABLE 11.1 Advantages and Disadvantages of Room Temperature Curing Epoxy Adhesives 

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FIGURE 1.1 Characteristics of a particular room temperature curing epoxy adhesive under different cure time and temperature conditions.1... 

Another characteristic difference between a two-component, room temperature curing epoxy adhesive system and a one-component, heat curing system is the shelf life. The shelf life of a one-component epoxy can be 3 to 6 months when stored at room temperature and may even require refrigeration or freezing for a practical shelf life. The shelf life of each component in a two-component system is generally 6 months to 1 year. 

With most room temperature curing epoxy adhesives, a significant drop in tensile shear strength occurs between 80 and 120°C. The exact temperature where the drop in strength occurs will depend very much on the specific curing agent used, the fillers or modifiers... 

Room temperature curing epoxy adhesives provide widely varying application and performance properties depending on the formulation employed. The following sections highlight certain formulations and commercial products that fall under this popular classification of epoxy adhesives. 

As with amidoamine and polyamide cured adhesives, epoxy resins cured with aliphatic amines exhibit tensile shear strength that is dependent on the type of filler and concentration. Table 11.10 shows the effect of filler loading on strength of a simple general-purpose, room temperature curing epoxy adhesive composed of liquid DGEBA epoxy mixed with 10 pph of a tertiary amine. 

Table 11.12 shows two rapid-setting, room temperature cure epoxy adhesives based on epoxy acrylate resins with aliphatic amine curing agents. These adhesives have gel times of less than 5 min for a 100-g mass. The bond strength development is rapid with handling strength occurring in about 1 h at room temperature. 

The room temperature cured epoxy adhesives discussed thus far exhibit a general lack of flexibility, especially when considered next to elastomeric sealants. Flexibility is generally desired when the performance requirements include high peel strength, impact strength, and resistance to thermal shock or thermal cycling. 

Two-component, room temperature curing epoxy adhesives have evolved over three distinct generations, from brittle to flexible to toughened. The quest for toughening is due to the inherent lack of peel and impact strength properties in brittle epoxy adhesives and... 

There are several ways by which the formulator can moderately improve the heat or chemical resistance of room temperature curing epoxy adhesives. Using an elevated-temperature cure or a postcure will, of course, improve the temperature resistance by virtue of improved crosslink density. However, this section describes formulations that have been developed for moderately improved heat resistance after only a cure at room temperature. Optimal (heat-curing) high-temperature and chemically resistant epoxy adhesives are discussed in Chap. 15. 

Although room temperature curing epoxy adhesive can also be cured at elevated temperatures, there are important differences in morphology and internal stresses between a room temperature cured adhesive and a heat cured epoxy adhesive (see Chap. 3). One should not assume that the properties would be equivalent or better if the adhesive were cured at elevated temperature rather than room temperature. A number of factors need to be considered in such a deliberation. 

Whereas most room temperature curing epoxy adhesives are cured with aliphatic amines, polyamides, or amidoamines, most elevated-temperature curing epoxy adhesives are cured with aromatic amines, modified aliphatic amines, alcoholic and phenolic hydroxyls, acid anhydrides, Lewis acids, and a host of other curatives. Latent curing agents, such as dicyan-diamide and imidazoles, are typically used in one-component epoxy adhesives systems. 

One approach to solve this problem has been the development of fast-reacting room temperature curing epoxy adhesive systems, as discussed in Chap. 11. However, they often result in brittle joints, poor adhesion due to reduced time to achieve substrate wetting, and... 

Quick-curing epoxy adhesive cured with polymercaptan Starting formulations for rapid-setting, room temperature curing epoxy adhesives... 

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