Epoxy adhesives amine curing agents

The polyaddition reaction is the most commonly used type of reaction for the cure of epoxy resins. The curing agents used in this type of reaction have an active hydrogen compound, and they include amines, amides, and mercaptans. With this reaction mechanism, the most important curing agents for adhesives are primary and secondary amines containing at least three active hydrogen atoms and various di- or polyfunctional carboxylic acids and their anhydrides. 

Amines are one of the most important curing agents for epoxy resins. They provide fast cures with a relatively high crosslink density. Unmodified amine cured epoxy resins are generally too brittle for adhesive applications, and so there are many derivatives that have been developed. More information on amine curing agents can be found in Chap. 5. 

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. 

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. 

Table 12.9 shows a formulation for an accelerated general-purpose one-component, dicyandiamide cured epoxy adhesive compared to one with a modified aliphatic amine curing agent. Notice that the dicyandiamide cured system provides a higher glass transition tem-... 

For maximum heat resistance, pyromellitic dianhydride is often used. The composition and properties of metal-to-metal adhesives based on a combination of epoxy novolac and a bisphenol A epoxy resin have been described in Sec. 12.5.1. Depending on the cure temperature and the choice of amine curing agent, strength as high as 3000 psi at room temperature and over 1000 psi in the range of-55 to + 150°C is possible. 

Some materials involved in the epoxy adhesive formulations are more dangerous than others, and some affect certain parts of the body whereas others do not. Amine curing agents, for example, are generally the most irritating to the skin and eyes. They may also be considered to be strong caustics, and they produce serious local injury on short exposure. If they are worked in a hot condition, their fumes pose additional hazards. 

One possible adhesive formulation contains 20 pbw (parts by weight) of epoxy resin, 70 pbw of filler, and 10 pbw of a reactive diluent and an aromatic amine curing agent such as diamino diphenylsulfone (DADS) whose amine-equivalent weight is 62. If the resin is Epon 825 or DER 332, having an epoxy equivalent of 174, the modified EEW will be ... 

For compounds of epoxy resins based on epoxy-diane ohgomers and resorcinol diglycidyle esters set with amine curing agents, the dependence of Tad oil the content and type of the modifying additive displays distinct maxima. The adhesion strength is enhanced most when the SKD-KTRA rubber is applied at 20% (see Figs. 3.17 and 3.18). With SKN-KTR as modifier the effect is less. 

Adhesives based solely on epoxy resins and amine curing agents give cross-linked materials that have high modulus but are very brittle and have no resistance to peel forces. Epoxy resins became much more useful when it was found that rubber flexibilizers could be added to increase crack resistance. Further, it was found that under appropriate reaction conditions, these rubbers could act to "toughen" the epoxy system. That is, addition of rubber flexibilizers tends to reduce the Tg and shear properties of an epoxy. When an epoxy is "toughened" little or no change occurs in the Tg and shear properties while a substantial increase in... 

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