Latent acid curing agents

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. 

The novolac resins are prepared by using acidic catalysts and a deficiency of formaldehyde. Because this type of resin is less reactive, cross-linking is accomplished by the addition of a curing agent or catalyst. The most common is hexamethylenetetramine or "hexa." The curing agent serves as a latent source of formaldehyde. As in the case of the resoles, volatiles are emitted during the cure. The chemistry of the phenolic resin is old but complex and well documented in the literature (10). 

One-component products contain curing agents which are inactive at storage temperatures, but which initiate a curing process when heated. Typical latent curing agents include organic acid anhydrides (Table 4, Fig. 5), which are used mainly in the electri-... 

Although there are literally hundreds of curing agents to choose from, the conunonly used ones can be conveniently divided into the major categories aliphatic amines, aromatic amines, acid anhydrides and catalytic curing agents or latent hardeners. 

Galego N, Vazquez A, Riccardi CC, William RJJ (1992) Citric acid-diethylenetriamine salts as latent curing agents for epoxy resins. J Appl Polym Sci 45 607-610... 

FIXAPRET PCL can be applied to cellulosic textiles by all processes and for all finishes in which latent or potent acids are used as catalyst, e.g. classical finishing, moist cross-linking and wet crosslinking. It is the most versatile of all FIXAPRET types. FIXAPRET PCL is one of the less reactive cross-linking agents and must, therefore, be cured under energetic conditions. 

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