Epoxy catalytic curing agents

Curing agents account for much of the potential hazard associated with use of epoxy resins. There are several major types of curing agents aliphatic amines, aromatic amines, cycloaliphatic amines, acid anhydrides, polyamides, and catalytic curing agents. The latter two types are true catalysts, in that they do not participate in the curing process. 

Curing agents lhai give the optimum in elevated temperature properties for epoxy novolaks arc (hose with good high icmperaiure performance such as aromatic amines, catalytic curing agents, phenolics. and some anhydrides... 

The catalytic curing agents commonly used include tertiary amines, Lewis acids and bases, and dicyandiamide. Since their function is truly catalytic, the catalyst is added at relatively low concentrations (0 to 5% by weight) to the epoxy formulation. Homopolymerization generally requires both the presence of catalysts and elevated temperatures for the reaction to proceed. Like the polyaddition reaction, the homopolymerization reaction is accelerated by hydroxyl groups or tertiary amines. 

Tetraglycidyl ether of tetraphenolethane is an epoxy resin that is noted for high-temperature and high-humidity resistance. It has a functionality of 3.5 and thus exhibits a very dense crosslink structure. It is useful in the preparation of high-temperature adhesives. The resin is commercially available as a solid (e.g., EPON Resin 1031, Resolution Performance Polymers). It can be crosslinked with an aromatic amine or a catalytic curing agent to induce epoxy-to-epoxy homopolymerization. High temperatures are required for these reactions to occur. 

Tertiary amines are commonly referred to as catalytic curing agents since they induce the direct linkage of epoxy groups to one another. The reaction mechanism is believed to be as follows ... 

Catalytic curing agents require a temperature of 200°F (93°C) or higher to react. These epoxy formulations exhibit a longer working life than the aliphatic amine cured epoxies. The exothermic reaction may be critically affected by the mass of the resin mixture. Typical materials used include piperidine, boron trifluoride ethylamine complex, and benzyl dimethyl-amine (BDMA). 

Catalytic-curing agents (Lewis bases and Lewis acids). Both Lewis bases and Lewis acids in catalytic quantities (2-4 phr) cure epoxy resins rapidly without requiring heat. Lewis bases are compounds that have a free pair of electrons such as tertiary amines while Lewis acids are compounds that can accept a pair of electrons suchasboron trifluoride. Lewis acids such as boron trifluoride are so reactive that, in addition to their being highly... 

The catalytic curing agents are a group of compoimds that promote epoxy reactions without being consumed in the process. In some of the epoxy literature, catalysts... 

L6WiS BaS6S. Lewis bases contain an unshared pair of electrons in an outer orbital and seek reaction with areas of low electron density. They can function as nucleophilic catalytic curing agents for epoxy homopolymerization as cocuring agents for primary amines, polyamides, and amidoamines and as catalysts for anhydrides. Tertiary amines and imidazoles are the most commonly used nucleophilic catalysts. Several different mechanisms are possible ... 

Imidazoles such as 2-methylimidazole (2-MI) and 2-phenylimidazole (2-PI) contain both a cyclic secondary and a tertiary amine fiinctional groups and are used as catalysts, catalytic curing agents, and accelerators (124,125). They are widely used as catalysts for DICY-cured epoxies in electrical laminates. For powder coatings, 2-MI adducts of LER are often used to facilitate dispersion of the components in powder coating formulations and to enhance shelf-life. Other modified imidazoles are also commercially available. The main advantage of imidazoles is the excellent balance of pot life and fast cure. 2-PI is used to increase Tg and thermal resistance. 

Catalytic curing agents require a temperature of 200°F/93 C or higher to react. They exhibit excellent resistance to acids, alkalies, solvents, inorganic salts, and water. The maximum operating temperature is 325°F/163°C, somewhat higher than that of the amine cured epoxies. 

The most popular catalytic curing agents are the Lewis acids and Lewis bases. A Lewis acid is a species that can accept an electron pair during a reaction and a Lewis base is one that can donate an electron pair. Typical Lewis acids which find use with epoxy resins include zinc chloride, boron trifluoride, titanium tetrachloride and stannic chloride. The major Lewis bases are the tertiary amines although some metal... 

These high performance resins can be cross-linked with either an aromatic amine or a catalytic curing agent to induce epoxy-to-epoxy homopolymerization. High temperatures are required for these reactions to occur. 

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