Epoxy
Epoxy resins are thermosetting polymers formed by the reaction of epoxy monomers, which contain reactive oxirane (epoxide) ring groups, with curing agents such as amines or anhydrides to create a highly cross-linked, three-dimensional network structure.[1] The most common epoxy resin is diglycidyl ether of bisphenol A (DGEBA), synthesized from bisphenol A and epichlorohydrin, providing a versatile base for various formulations.[2] Building on epoxide chemistry pioneered by Nikolai Prileschajew in 1909, epoxy resins were developed in the 1930s through key patents by Paul Schlack in 1934 and Pierre Castan in 1938, and have become essential materials due to their tunable properties achieved through controlled curing processes, often involving step-growth polymerization that leads to gelation and vitrification.[3] These resins exhibit outstanding mechanical strength, thermal stability with glass transition temperatures often exceeding 100°C, and excellent resistance to chemicals, corrosion, and environmental factors, alongside low shrinkage during curing and superior adhesion to diverse substrates like metals, composites, and ceramics.[4] Their physical properties can be further enhanced by incorporating fillers, tougheners, or multifunctional monomers such as N,N,N',N'-tetraglycidyl-4,4'-methylenedianiline (TGMDA), allowing customization for specific performance needs like increased toughness or higher cross-link density.[1] Epoxy resins find widespread applications across industries, including as structural adhesives in aerospace and automotive components, protective coatings for corrosion resistance, matrix materials in fiber-reinforced composites for high-performance parts, and encapsulants in electronics for insulation and packaging.[2] Emerging developments focus on bio-based epoxies and self-healing variants to address sustainability and durability challenges, expanding their role in advanced materials like insulating foams and biomedical devices.[1]History
Early discoveries and synthesis
The development of epoxy resins began in the 1930s through independent efforts by chemists seeking novel thermosetting materials. In Germany, Paul Schlack, working for I.G. Farbenindustrie, patented a process in 1934 for the condensation reaction between epoxides and amines, marking an early breakthrough in epoxy polymerization that laid foundational principles for curing these resins. Independently, in Switzerland, Pierre Castan, a chemist at DeTrey Frères in Zurich, synthesized the first practical epoxy resin in 1936 by reacting epichlorohydrin with bisphenol A in the presence of a base, forming a low-melting glycidyl ether that could be hardened into a durable solid. Castan's work resulted in a Swiss patent application in 1936 (granted in 1940) and a corresponding U.S. patent (US2324483) filed in 1938 and granted in 1943, describing the process as producing resins suitable for casting and varnishes without volatile byproducts.[3][5] These early syntheses focused on the formation of the glycidyl ether group, where the hydroxyl groups of bisphenol A react with epichlorohydrin under alkaline conditions to create epoxide rings, followed by attempts to polymerize the resulting prepolymer through ring-opening reactions with hardeners like anhydrides or amines. Castan's experiments at DeTrey Frères initially targeted dental applications, producing amber-colored resins for dentures and fillings due to their adhesion and hardness, while early polymerization tests involved heating mixtures to 150–170°C to achieve thermoset properties. In the United States, Devoe & Reynolds Company began parallel experiments in the early 1940s, exploring similar glycidyl ether formations for casting resins, though their key patent by Sylvan O. Greenlee came in 1947, building on Castan's methods.[6][5] The 1930s patents represented a pivotal timeline, with Schlack's 1934 innovation and Castan's 1936 synthesis enabling the first viable epoxy formulations, yet initial commercialization faced significant challenges in the 1940s due to World War II. Wartime priorities shifted resources toward military uses, such as adhesives for aircraft and coatings for ships, delaying broader industrial adoption until the late 1940s when companies like Ciba (licensing Castan's patents) and Shell (via Devoe & Reynolds) began scaling production. These early efforts established epoxy's potential for high-strength, chemically resistant materials, setting the stage for post-war expansion.[6][7][8]Commercial development and key innovations
The commercialization of epoxy resins began in the late 1940s, marking a pivotal shift from laboratory synthesis to industrial production. In 1946, Ciba AG in Switzerland introduced the first commercial epoxy products, including adhesives displayed at the Swiss Industries Fair, based on patents licensed from Pierre Castan.[9] Concurrently, Devoe-Reynolds in the United States released bisphenol-A-based epoxy resins, leveraging Sylvan Greenlee's innovations for coatings and bonding applications.[10] Shell Chemical entered the market shortly thereafter in 1947 with its EPON™ resin line, rapidly scaling production through licensed technologies and establishing itself as a major player.[11] Key innovations in the 1950s focused on curing agents, particularly amines, which enabled room-temperature curing and improved versatility for diverse applications. These aliphatic and aromatic amines, such as diethylenetriamine and meta-phenylenediamine, were developed to accelerate cross-linking reactions, enhancing mechanical properties and allowing expansion into adhesives for aerospace components and protective coatings for marine and industrial surfaces.[12] This period saw epoxy formulations evolve from niche casting resins to robust systems suitable for high-performance uses, driven by patents and collaborative R&D among chemical firms. By the 1960s, epoxy resins experienced significant growth in the electronics sector, where their electrical insulation and encapsulation properties supported the burgeoning semiconductor and circuit board industries. Applications in potting compounds and conformal coatings became standard, fueled by the electronics boom and contributing to annual production increases of over 10% globally. The 1970s introduced challenges from emerging environmental regulations, particularly U.S. Clean Air Act amendments targeting volatile organic compounds (VOCs), prompting innovations in low-solvent and waterborne epoxy formulations to reduce emissions while maintaining performance in coatings. Post-World War II economic factors, including surging demand in aviation for lightweight composites and in construction for durable flooring and structural repairs, propelled epoxy from a specialized material to a cornerstone of industrial growth. Aerospace applications, starting with military aircraft adhesives in the early 1950s, expanded into civil aviation, while construction uses in corrosion-resistant coatings drove market penetration. By the 1980s, these sectors had transformed the global epoxy industry into a billion-dollar enterprise, with U.S. consumption alone exceeding 300 million pounds annually and supporting diverse end-use markets.[6]Chemistry
Epoxy functional groups and basic structure
Epoxy resins are thermosetting polymers defined by the presence of one or more epoxide functional groups per molecule, where the epoxide is a strained three-membered ring consisting of two adjacent carbon atoms bonded to an oxygen atom.[13] This oxirane ring, also known as the epoxy group, imparts high reactivity to the monomer, enabling the formation of cross-linked networks upon curing.[14] The term "epoxy" specifically refers to these polymers derived from epoxide-containing monomers, distinguishing them from other thermosets based on their unique ring structure. The basic architecture of common epoxy resins centers on glycidyl ether linkages, where the epoxy group is attached via an ether bond to a polyfunctional backbone such as a phenol or alcohol. These are typically synthesized by reacting epichlorohydrin—a chlorinated epoxide—with a nucleophilic hydroxy compound in the presence of a base.[14] For instance, phenols like bisphenol A or simple alcohols yield monomers with terminal glycidyl groups, represented generally as R-O-CH2-CH-CH2 with the oxygen bridging the carbons in a ring.[15] The key synthetic step for forming the glycidyl ether can be simplified as the nucleophilic substitution where the deprotonated alcohol or phenoxide attacks the less substituted carbon of epichlorohydrin, followed by ring closure with elimination of chloride: \ce{R-O^- + Cl-CH2-CH-CH2} \begin{smallmatrix} \chemfig{**3(---O-)} \end{smallmatrix} \ce{ ->[base] R-O-CH2-CH-CH2} \begin{smallmatrix} \chemfig{**3(---O-)} \end{smallmatrix} \ce{ + HCl} This reaction produces monomers with two or more epoxy groups for network formation, such as diglycidyl ether of bisphenol A (DGEBA).[14] The reactivity of the epoxy group stems primarily from the significant angle strain in the 60° oxirane ring, approximately 13-17 kcal/mol higher than unstrained cycloalkanes, which drives facile nucleophilic attack at one of the carbons, opening the ring and relieving strain.[16] This SN2-like mechanism contrasts with the condensation processes in polyesters, which involve diol-diacid reactions eliminating water, or in polyurethanes, which proceed via nucleophilic addition of alcohols to isocyanates without ring involvement.[17] Epoxy ring-opening thus allows for step-growth polymerization with minimal byproducts, yielding void-free, highly cross-linked structures superior for adhesion and mechanical integrity.[14]Bisphenol-based resins
Bisphenol-based epoxy resins are the predominant type in commercial production, primarily consisting of diglycidyl ether of bisphenol A (DGEBA) derived from the condensation reaction between bisphenol A (BPA) and epichlorohydrin (ECH). This synthesis typically occurs in the presence of a basic catalyst, such as sodium hydroxide, where BPA's phenolic hydroxyl groups react with ECH to form glycidyl ether functionalities.[18] The process yields DGEBA as the monomeric unit, with higher molecular weight variants formed through subsequent advancement reactions involving additional BPA.[19] The generalized condensation for the resin formation can be represented as: n \ \ce{(HO-C6H4)2C(CH3)2} + n \ \ce{Epichlorohydrin} \rightarrow \ce{[resin]} + \ byproducts where the stoichiometry and reaction conditions control the chain length.[18] The degree of polymerization (n) significantly influences the resin's physical form and application suitability: at n ≈ 0, DGEBA exists as a low-molecular-weight, low-viscosity liquid suitable for coatings and adhesives, while n > 1 results in high-viscosity, solid resins ideal for structural composites and laminates.[15] These resins are characterized by their high adhesion to diverse substrates, including metals and composites, and superior chemical resistance to acids, bases, and solvents, making them versatile in demanding environments.[20] Reactivity is quantified by the epoxide number, which measures epoxide equivalents per kilogram and typically ranges from 5.0 to 5.5 eq/kg for liquid DGEBA variants, indicating the number of reactive sites available for curing.[21] Bisphenol-based resins dominate the global epoxy market, accounting for over 80% of production due to their balanced performance and cost-effectiveness across industries like electronics, aerospace, and construction.[22] However, concerns over BPA's endocrine-disrupting potential, supported by extensive research linking it to hormonal imbalances, have spurred development of bio-based and BPA-free alternatives to mitigate health and environmental risks.[23]Novolac and other phenolic resins
Novolac epoxy resins are produced through the epoxidation of novolac, a type of phenol-formaldehyde condensate resin, by reacting its phenolic hydroxyl groups with epichlorohydrin in the presence of a base catalyst.[24] This process typically yields multifunctional resins with an average of 3 to 6 epoxy groups per molecule, depending on the novolac's degree of polymerization and reaction conditions.[25] The simplified reaction can be represented as: \text{Novolac-OH} + \text{epichlorohydrin} \rightarrow \text{novolac epoxy} + \text{HCl} These resins form highly branched structures that enable extensive crosslinking during curing.[26] Due to their multi-functionality, novolac epoxy resins exhibit superior thermal stability compared to difunctional bisphenol-based epoxies, achieving glass transition temperatures (Tg) greater than 150°C and often exceeding 200°C in optimized formulations.[27] This high Tg, combined with excellent chemical resistance and low viscosity in the uncured state, makes them ideal for demanding electronics applications, such as semiconductor encapsulation, printed circuit board laminates, and underfill materials.[28] The higher epoxy functionality promotes denser crosslinked networks upon curing, resulting in enhanced mechanical strength and heat deflection temperatures relative to bisphenol A epoxies.[29] Novolac epoxies represent approximately 10% of the global epoxy resin market, valued at around USD 1.2 billion in 2023 within a total market of over USD 14 billion.[30] In recent developments, 2024 research has advanced low-halogen novolac formulations by incorporating phosphorus-based additives, achieving UL-94 V-0 flame retardancy ratings while maintaining high Tg and reducing environmental impact from traditional brominated systems.[31]Aliphatic and cycloaliphatic resins
Aliphatic and cycloaliphatic epoxy resins represent a class of non-aromatic epoxy compounds distinguished by their enhanced ultraviolet (UV) stability and flexibility relative to traditional bisphenol-based aromatic resins, making them suitable for applications exposed to environmental stressors.[32] These resins feature backbone structures derived from linear or cyclic hydrocarbons, which reduce the susceptibility to photo-oxidation and yellowing observed in aromatic variants.[33] Synthesis of these resins typically involves the glycidylation of aliphatic diols or cycloaliphatic polyols with epichlorohydrin under basic conditions, where the hydroxyl groups react to form glycidyl ethers. For instance, polypropylene glycol diglycidyl ether (PPGDGE) is produced by reacting polypropylene glycol, a flexible aliphatic diol, with epichlorohydrin in the presence of a catalyst like sodium hydroxide, yielding a low-molecular-weight resin with terminal epoxy groups.[34] Similarly, hydrogenated diglycidyl ether of bisphenol A (hydrogenated DGEBA), a cycloaliphatic analog, is synthesized from hydrogenated bisphenol A (cyclohexane-based) and epichlorohydrin, preserving the difunctional epoxy structure while eliminating aromatic rings.[35] A representative reaction for cycloaliphatic variants can be depicted as: \ce{(C6H10)(CH2OH)2 + 2 ClCH2CH(O)CH2 -> (C6H10)(CH2OCH2CH(O)CH2)2 + 2 HCl} where a cyclohexane derivative such as cyclohexanedimethanol serves as the diol precursor.[15] These resins exhibit lower viscosity than aromatic epoxies, facilitating easier processing and higher filler loading, alongside superior weather resistance due to the absence of UV-absorbing chromophores.[32] Aliphatic variants like PPGDGE provide greater flexibility and impact resistance in cured networks, while cycloaliphatic resins offer exceptional optical clarity and minimal yellowing, ideal for transparent applications such as lenses.[33] Both types demonstrate improved hydrolytic stability and reduced water absorption compared to aromatic counterparts.[36] In applications, aliphatic and cycloaliphatic epoxies are particularly valued in UV-curable coatings for outdoor surfaces, where their stability prevents degradation under sunlight exposure.[37] They constitute approximately 5-10% of the global epoxy resin market, a segment experiencing growth driven by demand in weather-resistant composites and electronics encapsulation.[38][39]Specialty resins (halogenated, glycidylamine, and diluents)
Specialty epoxy resins are modified variants designed to enhance specific performance characteristics, such as flame retardancy, higher cross-linking density, or improved processability, building on the foundational structures of bisphenol-based or aliphatic epoxies discussed earlier.[15] Halogenated epoxy resins, particularly those incorporating bromine, are widely used to impart flame retardancy. Brominated diglycidyl ether of bisphenol A (DGEBA), derived from tetrabromobisphenol A (TBBPA), serves as a reactive flame retardant that integrates into the polymer network during curing, releasing halogen radicals to inhibit combustion.[40] These resins are essential in electronic applications, such as printed circuit boards, where they enable materials to achieve the UL 94 V-0 flammability rating by minimizing burning and dripping under fire exposure.[41] TBBPA-based epoxies also contribute to higher glass transition temperatures (Tg), enhancing thermal stability without significantly compromising mechanical properties.[42] Glycidylamine epoxy resins are synthesized from aromatic amines reacting with epichlorohydrin, yielding high-functionality epoxies with multiple glycidyl groups per molecule for increased cross-linking.[43] A prominent example is triglycidyl-p-aminophenol (TGPAP), a trifunctional resin with low viscosity at room temperature, which promotes dense network formation and elevated Tg values suitable for demanding structural applications.[15] The synthesis typically involves the nucleophilic attack of the amine on epichlorohydrin, followed by cyclization: \text{Ar-NH}_2 + \text{Epichlorohydrin} \rightarrow \text{Ar-N(CH}_2\text{CH(OH)CH}_2\text{Cl)}_n \rightarrow \text{Glycidylamine (Ar-N(CH}_2\text{CH-CH}_2\text{O})_n} where Ar represents the aromatic moiety and n denotes functionality.[44] TGPAP is particularly valued in aerospace composites due to its ability to deliver superior mechanical strength and toughness when reinforced with fibers.[45] Reactive diluents, such as monofunctional glycidyl ethers, are incorporated to lower the viscosity of high-molecular-weight epoxy formulations, facilitating better wetting and impregnation during processing.[46] Butyl glycidyl ether (BGE), a common aliphatic monofunctional example, reduces viscosity effectively at low addition levels (typically 5-10 wt%) while participating in the curing reaction to maintain substantial cross-link density, though excessive use can slightly diminish it.[47] In 2025, industry trends emphasize phthalate-free diluents, driven by regulatory pressures and sustainability goals, with bio-based alternatives like those derived from plant oils gaining traction to replace traditional petroleum-derived options without compromising performance.[48]Production
Raw materials and synthesis processes
Epoxy resins are primarily synthesized from epichlorohydrin (ECH), bisphenol A (BPA), and sodium hydroxide (caustic soda). ECH is derived from propylene through a chlorohydrin process involving the reaction of propylene with hypochlorous acid to form chlorohydrins, followed by dehydrochlorination.[49] BPA is produced via the acid-catalyzed condensation of phenol and acetone, typically using a strong acid catalyst like hydrochloric acid or a sulfonic acid resin, yielding 2,2-bis(4-hydroxyphenyl)propane.[50][51] Caustic soda serves as the base for dehydrohalogenation in the epoxy formation step, facilitating the closure of epoxy rings.[52] The standard synthesis of bisphenol A-based epoxy resins, such as diglycidyl ether of bisphenol A (DGEBA), proceeds in a two-stage reaction. In the first stage, glycidylation occurs where BPA reacts with excess ECH in the presence of a catalytic amount of caustic soda to form the chlorohydrin intermediate. This is followed by the second stage of washing and neutralization, where additional caustic soda is added to dehydrohalogenate the intermediate, yielding the epoxy resin with removal of salt byproducts.[53][54] The process typically achieves yields of approximately 90% or higher when excess ECH is used to favor monomeric product formation.[54] The overall reaction for DGEBA synthesis can be represented as: \text{BPA} + 2 \text{ ECH} + 2 \text{ NaOH} \rightarrow \text{DGEBA} + 2 \text{ NaCl} + 2 \text{ H}_2\text{O} [53] For novolac-based epoxy resins, the process begins with the acid-catalyzed condensation of phenol and formaldehyde to form the novolac phenolic resin, which is then reacted with ECH under similar glycidylation and dehydrohalogenation conditions as for BPA-based resins.[24] This variation produces multifunctional epoxy resins with higher cross-linking potential compared to bisphenol-based types.[54]Industrial manufacturing and scale-up
Industrial manufacturing of epoxy resins primarily relies on large-scale chemical processes that build upon the synthesis of key intermediates like epichlorohydrin (ECH) and bisphenol A, scaled to meet global demand of approximately 4.6 million metric tons annually as of 2025.[55] Major producers include Dow Inc., Hexion Inc., Huntsman Corporation, Olin Corporation, and Kukdo Chemical Co., Ltd., which collectively dominate capacity through integrated facilities focused on high-purity resin output.[56] In 2025, expansions such as DIC Corporation's new facility at its Chiba Plant in Japan, supported by a ¥3 billion government subsidy, aim to boost production for semiconductor applications, adding specialized capacity starting in 2029.[57] Production processes typically employ batch reactors for flexibility in handling variable resin formulations, allowing precise control over reaction conditions during the condensation of ECH with bisphenol A, though continuous reactors are increasingly adopted for high-volume commodity grades to enhance throughput and reduce labor costs.[58] ECH purification, critical for minimizing impurities in the final resin, involves multi-stage distillation to separate it from dichlorohydrins and water, often integrated with pervaporation membranes in hybrid systems to recover up to 98% purity and lower energy use.[59] Scale-up from pilot to industrial levels presents significant engineering challenges, particularly in managing the exothermic heat released during the synthesis reactions, which risks thermal runaway without advanced cooling systems or staged addition of reactants in reactors up to 50,000 liters.[60] Impurity control is equally vital to maintain epoxy value—the measure of reactive epoxy groups per unit mass—at levels above 5.2 eq/kg for standard diglycidyl ether of bisphenol A (DGEBA), achieved through rigorous filtration and hydrolysis steps to limit chloride ions below 0.1% that could degrade resin performance.[61] Energy efficiency and waste management in epoxy production focus on handling chlorine-based byproducts from ECH synthesis, such as chlorohydrins and hydrochloric acid, which are neutralized and recycled via effluent treatment processes to comply with environmental regulations and recover up to 90% of salts for reuse.[62] Industry efforts are shifting toward propylene-derived routes for ECH, including hydrogen peroxide (H2O2)-based epoxidation of allyl chloride (derived from propylene) to ECH, reducing chlorine dependency and wastewater by 50% compared to traditional chlorohydrin methods.[60]Curing Mechanisms
Homopolymerization and catalytic curing
Homopolymerization of epoxy resins involves the self-polymerization of epoxide groups through chain-growth mechanisms, typically initiated by catalysts without the need for co-reactant hardeners. This process proceeds via anionic or cationic ring-opening polymerization, where the strained three-membered epoxy ring opens to form linear or branched polyether chains.[63] In anionic homopolymerization, Lewis bases such as tertiary amines (e.g., 1-methylimidazole or benzyldimethylamine) act as initiators by nucleophilic attack on the epoxy oxygen, generating an alkoxide species that propagates the chain through successive ring openings. Cationic homopolymerization, conversely, employs Lewis acids like boron trifluoride (BF₃) complexes or onium salts (e.g., diaryliodonium salts), which coordinate to the epoxy oxygen to form an oxonium ion intermediate, facilitating electrophilic ring opening and chain extension. These mechanisms enable curing in 100% solids formulations, avoiding solvents and supporting applications like coatings.[63][64][65] The generalized reaction scheme for catalytic homopolymerization can be represented as:More specifically, the ring-opening propagation yields repeating polyether units such as −CH₂−CH(R)−O−, where R is the substituent from the epoxy monomer, forming a network through branching, particularly in multifunctional epoxies. This process is often accelerated by heat or UV light in cationic systems, achieving gel times as short as 30–45 minutes at 120–130°C.[63][65] Catalytically cured epoxies exhibit rapid curing kinetics, enabling short processing cycles, but the resulting networks are often brittle due to high crosslink density and linear chain dominance in homopolymerization, with heat distortion temperatures reaching 150–170°C yet limited toughness. These properties make them suitable for prepreg applications in carbon fiber reinforced composites, where fast cure supports efficient molding and electrical insulation.[64][66][67] A key limitation of cationic systems is their sensitivity to moisture, as water can quench active cationic species (e.g., oxonium ions), reducing polymerization efficiency and cure completeness, particularly in humid environments. Anionic systems are generally less affected, though overall brittleness may require flexibilizers for broader use.[63][Epoxy monomer](/page/Monomer) + [catalyst](/page/The_Catalyst) → polyether chain[Epoxy monomer](/page/Monomer) + [catalyst](/page/The_Catalyst) → polyether chain