Epoxy adhesives latent curatives

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. 

Adhesives which are meant to cure at temperatures of 120 or 171°C require curatives which are latent at room temperature, but react quickly at the cure temperatures. Dicyanodiamide [461-58-5], (TH INI is one such latent curative for epoxy resins. It is insoluble in the epoxy at room temperature but rapidly solubilizes at elevated temperatures. Other latent curatives for 171°C are complexes of imidazoles with transition metals, complexes of Lewis acids (eg, boron trifluoride and amines), and diaminodiphenylsulfone, which is also used as a curing agent in high performance composites. For materials which cure at lower temperatures (120°C), these curing agents can be made more soluble by alkylation of dicyanodiamide. Other materials providing latency at room temperature but rapid cure at 120°C are the blocked isocyanates, such as the reaction products of toluene diisocyanate and amines. At 120°C the blocked isocyanate decomposes to regenerate the isocyanate and liberate an amine which can initiate polymerization of the epoxy resin. Materials such as Monuron can also be used to accelerate the cure of dicyanodiamide so that it takes place at 120°C. 

As mentioned above, many structural adhesives require latent curatives which react in the system at an elevated temperature. Latent epoxy curing agents are usually materials which are insoluble in the resin at room temperature but become soluble at some high temperature. One of the most widely used latent curing agents for epoxy resins is dicyandiamide. The chemical structure of this material is... 

Room temperature latency of the nitrogen-containing curatives is typically achieved in one of two ways. One method is to employ a curative which is insoluble, or only marginally soluble, in the adhesive mixture at room temperature, but which becomes soluble at the cure temperature. The classic example of this is dicyandiamide (10). The second method is to employ a curative precursor which is inactive at room temperature, but which converts to the active curative at the cure temperature. Examples include Monuron (11) and the di-urea adduct (12) of toluene diisocyanate and dimethylamine. Full cure in one hour at about 250 F is achieved. These solid curatives are incorporated into the epoxy under high-shear conditions. Efforts to replace the water-soluble dicyandiamide with adipic dihydrazide and solid adducts of ethylenediamine such as HY-940, sold by Ciba-Geigy, have been successful. Most latent curatives typically have some activity at room temperature, which often requires the storage of the uncured adhesive at subambient temperatures. 

Adhesives for use at 350°F to 450°F contain polyfunctional epoxies and usually a latent curative with an aromatic diamine structure such as methylene dianiline, mcta-phenylenediamine, or diaminodiphenylsulfone. Heat distortion temperatures of up to 325°F can be obtained with these aromatic amine hardeners. Toughness has now been incorporated into these high-temperature adhesives (Table I). 

Primary amine curatives are most commonly used for room temperature curing of epoxy adhesives. However, aromatic primary amines can be used as latent catalysts for one-part heat curing products (Table VI). Mixtures of bis(aminopropyl)tetraoxaspiroundecane and m-phenylenediamine or bis(aminophenyl)ether, amine, sulfoxide, or thioether have been used to cure DGEBA-type resins at two temperatures (40-100°C and 120-160°C) to give cured specimens with good fracture toughness. Aromatic primary diamines can offer improved adhesion of epoxy adhesives to metals and... 

One-part epoxy systems in the paste (liquid) form and in film form share much the same formulation ingredients. However, in comparison to the two-part adhesives, the one-part adhesives use very different curatives. Because these systems are meant to have all of the components of the adhesive in one part, the curatives used for two-part adhesives cannot be used as these would cure the adhesive in storage. Hence, latent curatives for these adhesives have been developed. Latenc/ in this case means that the curative is dormant in the formulation until a certain temperature is reached, at which point the curative begins to cure the formulation. Therefore, such latent catalysts have to have some means to stay inactive. For epoxy systems, this means that the curative remains insoluble in the epoxy resin imtil the cure temperature is reached or the curative is blocked by some chemical means which then unblocks at the... 

There are three main polymers presently used for structural adhesive bonding and they are phenolics, epoxies and urethanes. We can also include in our classification, the initial physical state of the uncured adhesive since it governs the type of application and curing conditions. Structural adhesives are manufactured in the form of films or pastes. The films are one-part adhesives, i.e., they contain a latent catalyst which requires heat for activation. Structural adhesive films are made in a number of different thicknesses and can either be supported (containing a scrim) or unsupported. In general, film adhesives require the application of pressure during cure in order to obtain ultimate properties. Paste adhesives are either one- or two-part materials. That is, pastes can either contain a latent heat activatable catalyst, or they can be a system which is separated into two parts, one of which contains the cur a t i ve/ca talys t. Two-part pastes cure at room temperature. In this section the chemistry of one-... 

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