Room-temperature-curing epoxies

The corrosion resistance and polymer-bonding compatibilities of the lonizable organophosphonates and the neutral organo-silanes are directly related to their inherent chemical properties. Specifically, NTMP inhibits the hydration of AI2O2 and maintains or Improves bond durability with a nitrile-modified epoxy adhesive which is cured at an elevated temperature. The mercaptopropyl silane, in addition to these properties, is compatible with a room temperature-cured epoxy-polyamide primer and also exhibits resistance to localized environmental corrosion. These results, in conjunction with the adsorbed Inhibitor films and the metal substrate surfaces, are subsequently discussed. 

Room-temperature-cured epoxy films on glass (hours in 70°C water to adhesion loss)... 

The specimen used for ITS testing can be any section of the composite, provided that after mounting and polishing, fibers normal to the polished surface can be found. A section of composite approximately 1 cm x 1 cm was cut from the laminates previously prepared. The sections were embedded in 1 in. diameter phenolic ring mounts with room temperature-cured epoxy mounting media. The sections were oriented so that the fiber ends were normal to the face of mount to be polished. The initial steps of the polishing were accomplished on a Struers Abramin polisher and the final step was a on Beuhler Vibromet I vibrating lap polisher. The steps used are listed in Table 2. 

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

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. 

TABLE 11.1 Advantages and Disadvantages of Room Temperature Curing Epoxy Adhesives... 

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 a family of curing agents for epoxy resins, the amidoamines are lower in viscosity than the polyamides. They exhibit very good adhesive properties due to their chemical structure and easy penetration. Amidoamine cured epoxy adhesives have shown very good properties on concrete and other porous substrates. They cure extremely well under humid conditions. In fact amidoamine cured epoxy formulations have been used to cure underwater in certain applications. A typical general-purpose room temperature curing epoxy-amidoamine system is described in Table 11.7. This adhesive is used as a general-purpose metal-to-metal adhesive and body solder in the automotive industry. 

Aliphatic amines typically provide two-component, room temperature curing epoxy formulations with fast reactivity. They are often used as accelerators for polyamide and... 

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. 

TABLE 11.12 Starting Formulations for Rapid-Setting, Room Temperature Curing Epoxy Adhesives17... 

Several methods can be used by the formulator to improve the flexibility of room temperature curing epoxy resin adhesive formulations ... 

The remainder of this section explores room temperature curing epoxy formulations where a great deal more flexibility is required. The most important application for this type of epoxy system is as a sealant or potting compound. 

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. 

---