Acrylate-Based Liquid Rubber as Impact Modifier for Epoxy Resin D. RATNA, 1 A. K. BANTHIA, 2 P. C. DEB 1 1 Naval Materials Research Laboratory, Chickloli, Anandanagar P.O. Addl Ambernath (E), Thane— 421 506 India 2 Materials Science Centre, I.I.T. Kharagpur- 721 302 India Received 10 March 2000; accepted 19 July 2000 Published online 16 March 2001 ABSTRACT: Carboxyl-terminated poly(2-ethyl hexyl acrylate) (CTPEHA) having vari- ous molecular weights were synthesized by bulk polymerization in the form of liquid rubber. The liquid rubbers (LR-1 to LR-4) were characterized by 13 C-NMR spectroscopic analysis, nonaqueous titration, and vapor-pressure osmometry (VPO). The liquid rub- ber having the lowest molecular weight ( M ¯ n = 3600) was prereacted with the epoxy resin and the modified epoxy networks were made by curing with an ambient temper- ature curing agent. The modified epoxy networks containing different concentrations of CTPEHA were evaluated with respect to their thermal and impact properties. The optimum properties were obtained at about 10 –15 phr of CTPEHA concentration (phr stands for parts per hundred parts of epoxy resin). Fracture surface analysis by scanning electron microscopy (SEM) indicated the presence of a two-phase microstruc- ture. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1792–1801, 2001 Key words: epoxy; 2-ethyl hexyl acrylate; impact, modified epoxy INTRODUCTION Epoxy resins are a class of versatile thermoset- ting polymers, which are widely used as struc- tural adhesives, composites, surface coatings, and electrical laminates, 1 because of their high strength, low creep, very low cure shrinkage, and excellent resistance to corrosion, good adhesion to many substrates, and appropriate electrical prop- erties. 2 However, the major drawback of epoxy resins is that in the cured state they are brittle materials having a fracture energy of about two orders of magnitude lower than engineering ther- moplastics and three orders lower than metals. 3 This inherent brittleness causes poor damage tol- erance to impact of the composites made from epoxy resin and poor peeling and shear strength of epoxy-based adhesives. Hence, modification of epoxy resins to impart fracture toughness has been the subject of intense research interest. Following the work reported by Sultan and McGarry 4 that a reactive liquid rubber such as a carboxyl-terminated copolymer of butadiene and acrylonitrile (CTBN) is useful for the fracture toughness of epoxy resins, much work has been reported in this field. 5–7 The theory is that liquid rubber reacts with the epoxy through the carbox- ylic functional groups. During initial stage, the reactive liquid rubber is compatible with the ep- oxy– hardener mixture. As the curing reaction proceeds, the molecular weight increases and phase separation occurs at a some stage, leading to the formation of a two-phase morphology. The improvement in fracture toughness is achieved without a significant reduction of thermal and mechanical properties of the crosslinked epoxy resin. However, since the butadiene component of the elastomers contains unsaturation, it would appear to be a site for premature thermal and/or oxidative instability, and such modified resins are Correspondence to: P. C. Deb. Journal of Applied Polymer Science, Vol. 80, 1792–1801 (2001) © 2001 John Wiley & Sons, Inc. 1792