Epoxide adhesives: curatives

Epoxide adhesives , then, comprise a very wide and important range of materials. They may be cured (cross-linked) in a variety of ways and their mechanical properties may be markedly changed by toughening . This article is concerned principally with the epoxy polymers as such Epoxide adhesives curatives is concerned with cross-linking reactions. Toughened epoxide adhesives is the subject of further articles. 

The cross-linking of these polymers is further discussed in Epoxide adhesives curatives. 

Epoxide adhesives curatives J A BISHOPP Cross-linking agents... 

Epoxide adhesives comprise epoxy resin, many of which are prepared from phenols and epichlorohydrin, for example, the diglycidyl ether of bis-phenol A or bis-phenol F usually, these resins are a mixtnre of molecular weights blended to fit the applications. The most-common cnratives for epoxy resins are polyanfines (used in stoichiometric amounts), usually a chain-extended primary aliphatic amine, for example, diethylene triamine or triethylene tetraamine or chain-extended equivalents, which react rapidly with the epoxy resin at room temperature. Aromatic amines react slowly at room temperature but rapidly at higher temperatures. Most epoxide adhesives also contain catalysts, typically, tertiary amines. Dicyanimide is the most-common curative for one-component high-temperature-cured epoxide adhesives. Mercaptans or anhydrides are used as curatives for epoxide adhesives for specialist applications, for example, for high-speed room-temperature cures or for electronic applications. A smaller number of epoxide adhesive are cured by cationic polymerization catalysed by Lewis acids photogenerated at the point of application. Lewis acid photoinitiators include diaryliodonium and triarly sulphonium salts. See Radiation-cured adhesives. 

Curatives have been prepared via condensation of (meth)acrylonitrile with various hydrazides such as carbohydrazide, oxalyl dihydrazide, and succinyl dihydrazide (Eq. 16). When these cyanoethylated and cyanopropylated hydrazides are mixed with DGEBA-type resins, viscosity increases are noted in 4-5 days at room temperature. However, these partially reacted mixtures are still uncrosslinked and flowable at temperatures near their final cure temperature of around 120°C. At this temperature reaction occurs very rapidly for a variety of epoxide-to-amine ratios. Adhesive compositions of DGEBA-type epoxy resins and the cyanoalkylated hydrazides give extremely strong bonds on substrates such as sheet moulding compound (SMC) and steel. 

Epoxy adhesives get their name from the ,2-epoxy, epoxide, or oxiranc ring, which provides the reactive sites on the polymers. This three-member ring consists of two carbon atoms joined to an oxygen atom. The strained geometry contributes to the reactivity with nearly all nucleophilic compounds, not only curatives but also adherend surfaces. 

The alternative approach involves the determination of the water uptake by the adhesive, checking at the same time that the difihision leading to saturation obeys Pick s two laws, deducing the diffusion constant from the rate of uptake of water and thence calculating the distribution of water in the joint. These steps are outlined by Comyn (1981) together with data on a number of epoxide-curative combinations. The epoxide resins were aU based on the commonly used bis phenol A with various amine curing agents. 

---