Epoxy resin curing agents Dicyandiamide

Similar information is presented in Table 14.5 for starting adhesive formulations made from an epoxy resin emulsion and dicyandiamide latent curing agent. This adhesive has exceptionally good water resistance when cured. The adhesive was applied to the indicated substrates in a manner similar to that described above, and it was cured for 3 min at 65°C followed by 10 min at 175°C. 

Cyanamide and its aromatic derivative, such as 4,4 -methylene bis(phenyl cyanamide), were reported to cure an epoxy resin at elevated temperatures.(4) It is also well known that the dimer of cyanamide (dicyandiamide) is the most important epoxy curing agent in one-package epoxy compounding.(5) Unfortunately, this dimer precipitates from the dispersion causing uneven mixing upon standing. 

Benzyldimethylamine (BDMA) is another tertiary amine that can be used as either a sole catalyst or an accelerator with other curing agents. It is used with DGEBA epoxy resins at 6 to 10 pph. The pot life is generally 1 to 4 h, and the cure will be complete in about 6 days at room temperature. When used by itself, BDMA can provide epoxy adhesive formulations with high-temperature resistance (Chap. 15). However, BDMA is mostly used as an accelerator for anhydride and dicyandiamide cured epoxy resins. 

The BF3-MEA complex offers a slightly faster rate of cure and a reduced shelf live in liquid epoxy systems when compared to unaccelerated dicyandiamide cured epoxy formulations. However, when used as a sole curing agent, BF3-MEA has not had the commercial success of dicyandiamide because of their lower bond strength and brittleness. The BF3-MEA complex compounds also hydrolyze in the presence of moisture, so that mixtures with epoxy resins must be stored in tightly closed containers to maintain shelf life. 

Latent curing agents such as dicyandiamide are dissolved into solvent solutions of solid epoxy resins. This is then followed by evaporation of the solvent. 

Resin I contains roughly equal amounts of diglycidyl ether of bisphenol A (DGEBA) and an epoxy cresol novolac. Sufficient dicyandiamide (DICY) as a curing agent is present such that the amine/epoxy ratio is 0.85. Monuron is present as an accelerator. The supplier s recommend standard cure is two hours at 127°C. Previous work (1 ) has shown that this cycle produces a fully cured system, as indicated by the disappearance of the epoxide absorbance band in the infrared spectrum. 

Typically, a low molecular weight solid epoxy resin and latent curing agent, such as dicyandiamide dissolved in an appropriate solvent, are widely used in dry lay-up formulations for electrical laminates for computers, aerospace applications, and communications equipment. Wet lay-up systems are almost exclusively based on low-viscosity resins cured with aliphatic amines. These ambient cure resin systems are used primarily for manufacture of large chemical-resistant tanks, ducting, and scrubbers. Filament winding is used... 

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... 

Dicyandiamide n. (cyanoguanidine) H2NC (=NH)NHCN. The widely used, but incorrect name for the dimmer of cyana-mide. Cyanoguanidine is used mainly in the production of melamine, but also as a stabilizer for vinyl resins and curing agent for epoxy resins. 

The rate of cure of epoxy resins with tertiary amines depends primarily upon the extent to which the nitrogen is sterically blocked. The homopolymerization reaction depends on the temperature as well as the concentration and type of tertiary amine. Benzyldimethylamine (BDMA) and TDMAMP are mainly used as accelerators for other curing agents, in the cining of anhydride-and dicyandiamide-based systems. Other tertiary amine catalysts include 1,4-diazabicyclo(2,2,2)octane (DABCO) and diazabicycloundecene (DBU). 

Historically, the most common curing agent used in epoxy resin systems for printed circuit base materials has been dicyandiamide, or dicy. Figure 7.10 illustrates the polymerization of... 

The model adhesive used was Dow Chemical epoxy resin D.E.R. 331 mixed with an M-5 silica filler, a dicyandiamide ( DICY ) curing agent, a tertiary amine accelerator (PDMU), and various amounts of rubber toughener (Kelpoxy G272-100), details of which have been listed elsewhere [33], The final products, according to the rubber concentration level, were designated as adhesives A (0% rubber), B (4.1 %), C (8.1 %), and E (15%). Adherends were cut from 25-mm-wide 6061-T6 aluminum alloy bar stock. 

Figure 6 Chemical formulae of the catalysts and co-reactants used as curing agents of epoxy resins electrophilic 33 and nucleophilic 34-36 catalysts, dicyandiamide 37, ureas 38, 39, imidazoles 40-47, aliphatic amines 48, 49, polyamide 50, and cycloaliphatic...

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