Epoxy resin Introduction
Epoxy resin broadly refer to organic polymer compounds containing two or more epoxy groups within their molecular structure. With a few exceptions, their relative molecular masses are generally low. The molecular structure of epoxy resins is characterized by the presence of reactive epoxy groups within the molecular chain; these epoxy groups may be located at the chain ends, in the middle of the chain, or arranged in a cyclic structure. Because their molecular structure contains these reactive epoxy groups, epoxy resins can undergo cross-linking reactions with various types of curing agents to form insoluble, infusible polymers possessing a three-dimensional network structure.
Application Characteristics
Diverse Forms: Systems comprising various resins, curing agents, and modifiers can be formulated to meet the physical form requirements of almost any application, ranging from extremely low-viscosity liquids to high-melting-point solids.
Convenient Curing: By selecting different curing agents, epoxy resin systems can be cured within a temperature range spanning approximately 0°C to 180°C.
Strong Adhesion: The presence of inherent polar hydroxyl groups and ether linkages within the epoxy resin molecular chain endows the material with high adhesion to a wide variety of substrates. Furthermore, epoxy resins exhibit low shrinkage during curing—resulting in minimal internal stress—which further contributes to their enhanced adhesive strength.
Low Shrinkage: The reaction between epoxy resins and their corresponding curing agents proceeds via direct addition reactions or ring-opening polymerization of the epoxy groups within the resin molecules; consequently, no water or other volatile byproducts are released. Compared to unsaturated polyester resins or phenolic resins, epoxy resin systems demonstrate significantly lower shrinkage (typically less than 2%) during the curing process.
Mechanical Properties: Once cured, epoxy resin systems exhibit excellent mechanical properties.
Electrical Properties: Cured epoxy resin systems serve as excellent insulating materials, characterized by high dielectric strength, resistance to surface leakage, and resistance to electrical arcing.
Chemical Stability: Generally, cured epoxy resin systems demonstrate excellent resistance to alkalis, acids, and various solvents. Like other properties of cured epoxy systems, chemical stability depends on the specific resin and curing agent selected. By appropriately choosing the epoxy resin and curing agent, the system can be endowed with specialized chemical stability characteristics.
Dimensional Stability. The combination of many of the aforementioned properties gives epoxy resin systems outstanding dimensional stability and durability.
Mold Resistance. Cured epoxy resin systems are resistant to most types of mold and can be utilized in harsh tropical environments.
Classification by Type
Based on their molecular structure, epoxy resins can be broadly categorized into five main classes:
- Glycidyl Ether Epoxy Resins
- Glycidyl Ester Epoxy Resins
- Glycidyl Amine Epoxy Resins
- Linear Aliphatic Epoxy Resins
- Cycloaliphatic Epoxy Resins
In the composites industry, the most widely used variety of epoxy resin is the aforementioned Glycidyl Ether class, with the Bisphenol A type (specifically, the Bisphenol A diglycidyl ether type) being the predominant variety within this category. The second most widely used class is the Glycidyl Amine type.
Glycidyl Ether Epoxy Resin
Glycidyl ether epoxy resins are synthesized through the polycondensation of phenols or alcohols—compounds containing active hydrogen atoms—with epichlorohydrin (ECH).
(1) Bisphenol A Type Epoxy Resins: Bisphenol A type epoxy resins are produced via the polycondensation of Bisphenol A with epichlorohydrin.
Industrial-grade Bisphenol A epoxy resins are, in reality, mixtures containing molecules with varying degrees of polymerization. The majority of these molecules possess a linear structure terminated by two epoxy groups. A small fraction of the molecules may be branched, while a very small minority may terminate in chlorohydrin groups rather than epoxy groups. Consequently, parameters such as the epoxy group content and hydroxyl group content of the resin exert a significant influence on both the curing process of the resin and the performance characteristics of the resulting cured material. In industrial practice, the key control parameters used to characterize these resins are as follows:
① Epoxy Value. The epoxy value is the primary indicator used to characterize the properties of epoxy resins; industrial epoxy resin grades are distinguished based on their differing epoxy values. The epoxy value refers to the molar amount of epoxy groups contained within every 100 grams of resin. The reciprocal of the epoxy value, multiplied by 100, is termed the epoxy equivalent weight. The significance of the epoxy equivalent weight is defined as: the mass (in grams) of epoxy resin required to contain exactly one mole of epoxy groups.
② Inorganic Chlorine Content. Chloride ions present in the resin can undergo complexation reactions with amine-based curing agents, thereby interfering with the resin’s curing process. Furthermore, they adversely affect the electrical properties of the cured resin; consequently, chlorine content constitutes a critical parameter for epoxy resins.
③ Organic Chlorine Content. The organic chlorine content within the resin serves as an indicator of the proportion of chlorohydrin groups in the molecule that failed to undergo the ring-closure reaction. This content should be minimized as much as possible; otherwise, it will compromise both the curing of the resin and the performance characteristics of the cured product.
④ Volatile Content.
⑥ Viscosity or Softening Point.
(2) Phenolic Polyepoxy Resins. This category encompasses phenol-formaldehyde and o-cresol-formaldehyde polyepoxy resins. Compared to bisphenol A-type epoxy resins, these linear-chain molecules contain more than two epoxy groups. Consequently, the cured products exhibit a high crosslink density and possess exceptional thermal stability, mechanical strength, electrical insulation properties, water resistance, and corrosion resistance. These resins are synthesized through the polycondensation of linear phenolic resins with epichlorohydrin.
(3) Other Polyhydroxyphenol Glycidyl Ether Epoxy Resins. Representative examples of practical utility within this class include resorcinol-type, resorcinol-formaldehyde-type, tetraphenol-ethane-type, and triphenylmethane-type epoxy resins. Upon curing, these multifunctional glycidyl ether resins exhibit high heat distortion temperatures and rigidity. They may be utilized independently or blended with general-purpose “Type E” resins to serve as matrix materials for high-performance advanced composite materials (ACMs), printed circuit boards, and similar applications.
(4) Aliphatic Polyol Glycidyl Ether-Type Epoxy Resins: The molecules of aliphatic polyol glycidyl ethers contain two or more epoxy groups. The vast majority of these resins exhibit very low viscosity; most consist of long-chain linear molecules, and are therefore highly flexible.
Other Types of Epoxy Resins
(1) Glycidyl Ester-Type Epoxy Resins: Compared to bisphenol A-type epoxy resins, glycidyl ester-type epoxy resins offer several advantages: low viscosity and excellent processing characteristics; high reactivity; superior adhesion strength relative to general-purpose epoxy resins, resulting in cured products with excellent mechanical properties; good electrical insulation properties; and excellent weather resistance, including exceptional performance at ultra-low temperatures—under such extreme conditions, they still maintain higher bonding strength than other types of epoxy resins. Additionally, they possess good surface gloss, light transmittance, and weather resistance.
(2) Glycidyl Amine-Type Epoxy Resins: The advantages of this class of resins include high functionality, a high epoxy equivalent weight, and high crosslink density, resulting in significantly improved heat resistance. Currently, both domestically and internationally, the superior adhesion and heat resistance of glycidyl amine epoxy resins are being utilized to manufacture carbon fiber-reinforced polymers (CFRP) for use as secondary structural materials in aircraft.
(3) Alicyclic Epoxy Resins: These epoxy resins are synthesized by the epoxidation of double bonds within alicyclic olefins. Their molecular structures differ significantly from those of bisphenol A-type epoxy resins and other epoxy types; in the former, the epoxy groups are directly attached to the alicyclic ring, whereas in the latter, the epoxy groups are linked to benzene rings or aliphatic hydrocarbon chains via glycidyl ether linkages. The cured products of alicyclic epoxy resins exhibit the following characteristics:
① High compressive and tensile strengths; ② The ability to retain excellent mechanical properties even after prolonged exposure to high-temperature conditions; ③ Superior arc resistance, resistance to UV-induced aging, and weather resistance.
(4) Aliphatic Epoxy Resins: The molecular structures of this class of epoxy resins contain neither benzene rings nor alicyclic ring structures. They consist solely of aliphatic chains, with the epoxy groups directly bonded to these aliphatic chains. Epoxidized polybutadiene resin exhibits excellent strength, toughness, adhesion, and resistance to both positive and negative temperatures after curing.
