Impact modifiers: (3) their incorporation in epoxy resins

Abstract

As thermosetting resins, epoxies possess many desirable properties such as high tensile strength/modulus, excellent adhesion, chemical and solvent resistance, dimensional and thermal stability, good creep resistance, and fatigue properties. Two major categories of epoxy resins are generally used: difunctional and tetrafunctional types. The tetrafunctional type is typified by tetraglycidyl-4,4′-diaminodiphenylmethane (TGDDM) (Ciba Geigy MY-720), and the difunctional type by the diglycidylether of bisphenol-A (DGEBA) family, typified by Epon-828 (Shell), Epi-Rez 510 (Celanese), DER-331 (Dow Chem.), and Araldite-6010 (Ciba-Geigy) etc. Novolac-type epoxy resins are produced by reacting epichlorohydrin with a phenolic novolac resin. The novolac epoxy resins contain epoxide groups and a phenolic backbone with an average of three or more epoxide groups per molecule. Some specialty epoxy resins have also been synthesized to achieve higher T_g, or lower viscosities, or to impart some special functions. Epoxy cure can be achieved over a wide temperature range at various rates by selection of special curing agents. Epoxy can be cured with various aliphatic primary and secondary amines; at elevated temperatures, aromatic amines are used. Anhydrides are also often utilized for curing. For more detailed discussion on the types of epoxy resin and curing agents, readers are advised to consult textbooks or handbooks on epoxies. Major characteristics of epoxy resins includes: (1) excellent adhesion to almost any surfaces, (2) no volatiles upon cure, (3) thermal and mechanical stability over wide temperature ranges, (4) extremely low shrinkage, and (5) easy modification to suit various purposes. These versatile characteristics have helped to gain acceptance in widespread applications, such as adhesives in joining and fastening technology and as encapsulating agents for electronics or microelectronics parts. Over the past few years, increasing applications of epoxy resins have also been found in composites for aircraft, transportation vehicles, sports goods etc. However, epoxy resins are generally brittle due to high crosslink densities. Improvement of impact properties of epoxy matrix composites is generally needed for applications in structural parts. Furthermore, applications in coatings, adhesives, or microelectronics encapsulation etc., also require that the applied epoxy materials exhibit long-term mechanical stability under thermal/stress cycling environments. In such cases, impact improvement of the epoxy resins can usually resist microcracking.