Benzimidazole Derivatives and their Complexes as Accelerators for Curing of Epoxy Resin P. V. Reddy, 1 Sakuntala Samuelson, 2 N. M. Nanje Gowda, 2 D. N. Sathyanarayana 3 1 Polymer Laboratory, Central Power Research Institute, Bangalore 560 080, India 2 Department of Chemistry, Central College, Bangalore University, Bangalore 560001, India 3 Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India Received 7 June 2006; accepted 12 November 2007 DOI 10.1002/app.27845 Published online 4 February 2008 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Bisphenol-A-based epoxy resin was cured with an anhydride hardener in the presence of benzimida- zoles and their complexes as accelerators. Investigations have revealed an appreciable reduction in the cure and gel times. The kinetic studies based on DSC showed that the rate of cur- ing increases with enhanced concentration of accelerators. The coefficient of thermal expansion has been reduced con- siderably in the presence of the benzimidazole. The electrical properties of the cured epoxy resin system were unaffected by lower concentrations of benzimidazoles. A mechanism for curing has been proposed with 1,3-bis(2-benzimidazolyl)ben- zene as accelerator based on IR studies, which indicate the involvement of the secondary and tertiary nitrogen of the benzimidazole in the crosslinking of the resin system. Ó 2008 Wiley Periodicals, Inc. J Appl Polym Sci 108: 2101–2106, 2008 Key words: curing; epoxy resins; benzimidazole deriva- tives INTRODUCTION Epoxy resins owing to their remarkable chemical re- sistance, good adhesion, extraordinary toughness, high mechanical strength, and good heat resistance are extensively used as electrical insulators. When properly cured, they yield tough materials, which find application in industrial flooring, foam, and electrical insulation. The applications of epoxy resins depend on the nature of curing. Generally curing takes place in the presence of a hardener or a curing agent. Anhydrides are used as curing agents for ep- oxy resins, and the latter are used in casting and lamination. Bases such as tertiary amines are com- monly used to accelerate the curing process. Chemi- cal reactions that take place during the period of cur- ing determine the properties of the cured thermoset. Organometallic compounds are known to act as cur- ing agents for various epoxy resin systems. 1–3 Sev- eral acetyl acetonates act as latent accelerators for anhydride cure of bisphenol-A-based resins. 4–8 In this work, an attempt has been made to use benzimidazoles and their complexes as accelerators for curing epoxy–anhydride systems. 2-Phenyl benz- imidazole, 1,3-bis(2-benzimidazolyl)benzene, dichloro- aquo[1,3-bis(2-benzimidazolyl)benzene]copper(II), dichloroaquo[1,3-bis(2-benzimidazoly1)benzene]zinc- (II), dichloroaquo[1,3-bis(2-benzimidazoly1)benzene]- cobalt(II), dichloroaquo[1,3-bis(2-benzimidazoly1)- benzene]nickel(II), 1,2,4-tri(2-benzimidazolyl)benzene, and 1,2,4,5-tetra(2-benzimidazolyl) benzene were pre- pared 9–16 and used as accelerators. The cure and ki- netic aspects of the resin and the anhydride in the presence of these compounds have been studied. Gel time and storage life studies have also been carried out. Thermomechanical properties for 1,3-bis(2-ben- zimidazolyl)benzene has been carried out and com- pared with that of the blank and the commercially available accelerator. Electrical properties such as loss factor and capacitance of the resin–anhydride system have been investigated. A cure mechanism has been suggested for the epoxy resin with carboxy- lic anhydride in the presence of 1,3-bis(2-benzimida- zolyl)benzene involving tertiary and secondary nitro- gen atoms. 17–25 EXPERIMENTAL Materials and methods Bisphenol-A-based epoxy resin, Lapox C-17 with an epoxy content of 5.2–5.5 equiv/kg, K-12 hardener (anhydride content of about 94%), and commercial accelerator K-112 were procured from Cibatul, India, and were used as such. Silica flour (w-12 grade) of 300-mesh was also procured from Cibatul, and it was dried at 1108C for 3 h and then used as filler. 2-Phenyl benzimidazole (2phBz), 1,3-bis(2-benzi- midazolyl)benzene (1,3Bz), dichloroaquo[1,3-bis(2- Correspondence to: P. V. Reddy (pvreddy43@hotmail. com). Journal of Applied Polymer Science, Vol. 108, 2101–2106 (2008) V V C 2008 Wiley Periodicals, Inc.