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Epoxy resins are thermosetting prepolymers and oligomers that carry two or more epoxide (oxirane) rings per molecule and cure—by nucleophilic addition or cationic ring-opening—into densely cross-linked networks with high adhesion, chemical and electrical resistance, low shrinkage, and tunable glass-transition temperature. In practice the “resin” refers to the base epoxide-functional oligomer (liquid or solid) that will later be combined with a curing agent and modifiers to make an application-ready system. Chemically, the mainstream compositions are glycidyl-type epoxies produced from substrates bearing active hydrogen: diglycidyl ethers of bisphenol A (DGEBA) and bisphenol F (DGEBF) for general-purpose structural and electrical use; epoxy phenol/cresol novolacs (EPN/ECN) that provide higher functionality for heat and chemical resistance; glycidyl amines such as TGMDA/TGDDM for high-temperature aerospace laminates; and specialty grades including hydrogenated bisphenol-based epoxies (HBPA) for improved CTI in electronics and brominated epoxies for flame-retardant laminates. Non-glycidyl epoxies—principally cycloaliphatic epoxies—feature epoxidized alicyclic double bonds and are favored where low viscosity, UV or cationic curing, high dielectric strength, and outdoor weatherability are required. Resin packages may include reactive diluents (mono- or low-functionality glycidyl ethers to reduce viscosity), tougheners (CTBN/ATBN rubbers or thermoplastics), flow/leveling aids, pigments, fillers, and flame-retardant packages, but the base resin is specified by epoxy equivalent weight (EEW), viscosity, functionality, hydrolyzable/total chlorine, color, and purity. For glycidyl ethers, esters, and amines, the industry standard is the epichlorohydrin (ECH) process: the phenolic, carboxylic, or amine substrate is first glycidylated by addition of ECH under Lewis-acid or phase-transfer catalysis to form β-chlorohydrin intermediates, followed by dehydrohalogenation with aqueous alkali to close the oxirane rings. The organic phase is then separated, washed to remove salts and residual chloride, stripped of volatiles (including unreacted ECH), and polished through filtration/ion-exchange as needed. Liquid DGEBA grades are obtained by operating with excess ECH and controlling advancement; solid grades are produced by advancing the epoxy with additional bisphenol under basic catalysis to raise molecular weight to target EEW and softening point, then flaking/pastillating. Novolac epoxies are made analogously by glycidylating phenol or cresol novolac resins to high functionality. Glycidyl amines are obtained by glycidylation of aromatic diamines or multi-amines to give di- to tetra-functional products. Cycloaliphatic epoxies are not made via ECH; they are produced by epoxidizing carbon–carbon double bonds in alicyclic precursors through Prilezhaev-type peracid routes (e.g., peracetic/m-chloroperbenzoic acid) or hydrogen-peroxide catalytic systems, followed by neutralization and purification.
