Epoxy resin curing agents Imidazoles

Another group of strong bases used as epoxy resin curing agents are the imidazoles ... 

In Chapter 2 the DSC technique is discussed in terms of instruments, experimental methods, and ways of analysing the kinetic data. Chapter 3 provides a brief summary of epoxy resin curing reactions. Results of studies on the application of DSC to the cure of epoxy resins are reviewed and discussed in Chapter 4. These results are concerned with the use of carboxylic acid anhydrides, primary and secondary amines, dicyanodiamide, and imidazoles as curing agents. 

Diglycidyl ether of bisphenol-A (DGEBA), epoxy resin (YD 128, Kuk Do Chem., Mn = 378), and bisphenol-A dicyanate (BPACY, Arocy B-10, Ciba-Geigy) were used as the thermoset resin. 4,4 -diaminodiphenyl sulfone (DDS, Aldrich Chem. Co.) was used as a curing agent for epoxy. Polyetherimide (PEI, Ultem 1000, General Electric Co., M = 18,000) and 2-methyl imidazole (2MZ, Aldrich Chem. Co.) were used as the thermoplastic modifier and catalyst. 

Heat resistant resin compositions based on BMI/aminophenol-Epoxy blends are achieved by reacting a BMI/p-aminophenol 1 1 adduct with epoxy resin (62). Both the secondary amine and phenol functionality may react with the epoxy resin and subsequently cure through an imidazole catalyst. Imidazole catalysts promote both the epoxy/phenol reaction and the anionic maleimide crosslinking. The formation of a 1 2 BMI/aminophenol adduct, as in Fig. 20, is claimed in a patent (63). The hydroxy terminated BMI/aminophenol adduct is an advantageous curing agent for epoxy resins when high temperature performance is desired. 

Technical Bulletin, The Curezol Series Imidazole Group Curing Agents for Epoxy Resins , Shikoku Fine Chemical Corp., Okura and Co. Inc., New York, NY, U.S.A. 

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. 

EINECS 213-234-5 EMl-24 2-Ethyl-4-methylimidazole Imidazole, 2-ethyl-4-methyl- IH-lmidazoie, 2-ethyi-4-methyl- 4-Methyi-2-ethylimidazole NSC 82315. A curing agent for epoxy resins used in low proportions thus improving chemical resistance. Crystals mp = 36-42° bp = 292-295° d 0,9750 soluble in H2O (18,0 g/100 ml), organic solvents. Lancaster Synthesis Co,... 

Imidazoles are very effective catalysts for the esterification of carboxyl group. The reaction between carboxyl group and epoxy group can be effected before gelation of the system by incorporation of 2 phr EMI when epoxy resin is cured by mPDA, or by incorporation of 1 phr EMI when 4,4 -diaminodiphenylsulfone was used as curing agent. 

The inks are prepared by mixing an epoxy resin (Epon 826 epoxy resin) with dimethyl methyl phosphonate, nanoclay platelets (Cloisite 30b), silicon carbide whiskers (SI-TUFF SC-050), and milled carbon fibers (Dialead K223HM). Further, an imidazole-based ionic liquid is employed as a latent curing agent (Basionics VS03) (31). 

Polyamine and organic acid anhydride hardeners serve as co-reactive hardeners, which become incorporated into the epoxy resin, whereas tertiary amines, such as 2,4,6-tris-(dimethylaminomethyl)phenol (tris-DMP) and N,N-dimethylbenzylamine, imidazoles, and boron trihalide amine complexes are catalyst-type curing agents, which may not be chemicaUy bound to the resin molecules during epoxy curing reactions (Guin and Work 1995 Muskopf and McCoUister 1987). 

The reactive groups attached to the molecules of an epoxy resin can react with several curing agents, such as amines, anhydrides, acids, mercaptans, imidazoles, phenols and isocyanates, to create covalent intermolecular bonds and thus to form a three-dimensional network. Among these compounds, due to the enhanced environmental stability of amine-cured epoxy resin (Dyakonov et al., 1996), primary and secondary amines are the curing agents most commonly used in particular aliphatic or cycloaliphatic amines for low-temperature epoxy systems as adhesives or coatings and aromatic amines to produce matrices for liber-reinforced composites (Pascault and Williams, 2010). In Fig. 5.14 the structures of both an aliphatic and an aromatic amine are shown. 

Epoxy resins may be converted to the thermoset state through the epoxy groups by reaction with well over 50 different classes of chemical compounds. Amines, polyamides, anhydrides, Lewis acids, ureas, melamine, imidazoles, BFj amine complexes, imides, and so forth, have been commonly used as cure agents for epoxy resins [53]. 

Azole compounds as imidazole, benzimidazole, benzotiazole, and 2-mercapto-benzimidazole are of interest as curing agents for epoxy resins [95-97]. 

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...

Compounds such as 2-methylimidazole 41, 2-undecylimidazole, and 2-ethyl-4-methylimidazole 42 are highly reactive materials. It has been stated that soluble imidazoles are too reactive to permit their use in one-part adhesive systems stable at ambient temperature. Among the other compounds, the less reactive 4-phenylimidazole 43 would be a convenient curing agent at elevated temperature, although the reaction still occurs over a few days of storage at 25 °C. 4-Methyl-2-phenylimidazole is cited in different patents allowing a 3-day pot life at room temperature for silver-filled epoxy resins based on bisphenol-A, bisphenol-F, and phenol novolac hardener. Imidazole 44 mixed with bisphenol-F... 

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