Current Status of Research And Application of Epoxy Resin

Current Status of Research And Application of Epoxy Resin

I. Preamble

Epoxy resin refers to polymer prepolymers containing two or more epoxy groups, with aliphatic, alicyclic or aromatic chain segments as the main chain. Epoxy resin (EP) has high strength, good chemical stability, high mechanical properties, excellent adhesive properties, curing shrinkage is small, good heat resistance, UV aging, wear and impact resistance and other mechanical properties, often as coatings, adhesives are widely used in composites, marine, aerospace and electrical and electronic fields. However, the cured epoxy resin is three-dimensional network structure, high cross-linking density, high internal stress, resulting in its brittleness and easy to crack, poor abrasion resistance, and its temperature coefficient of thermal expansion is high, limiting its application in some fields. For the above shortcomings can be optimized and improved by modification methods

II. Modification of Epoxy Resin

1.Toughening Modification of Epoxy Resin

To enhance the toughness of epoxy resin, the initial approach involved the addition of plasticizers and flexibilizers. However, these low-molecular-weight substances significantly reduced the material's heat resistance, hardness, modulus, and electrical properties. Since the 1960s, research on the toughening modification of epoxy resin has been widely conducted both domestically and internationally, aiming to improve the toughness of epoxy resin with minimal impact on its thermal properties, modulus, and electrical performance.

• Rubber Elastomer Toughening of Epoxy Resin

The rubber elastomers used for toughening epoxy resin are typically reactive liquid polymers with a relative molecular weight of 1000 to 10000, featuring functional groups at the terminal or side positions that can react with epoxy groups. The main types of reactive rubber elastomers used for toughening epoxy resin include: carboxyl-terminated butadiene acrylonitrile rubber, hydroxyl-terminated butadiene acrylonitrile rubber, polysulfide rubber, liquid random carboxyl butadiene acrylonitrile rubber, butadiene acrylonitrile-isocyanate prepolymer, hydroxyl-terminated polybutadiene, polyether elastomer, and polyurethane elastomer. The interpenetrating polymer network of polybutyl acrylate and epoxy resin synthesized by the synchronous method has achieved satisfactory results in improving the toughness of epoxy resin.

• Thermoplastic Resin Toughening of Epoxy Resin

The thermoplastic resins used for the toughening modification of epoxy resin mainly include polysulfone, polyethersulfone, polyetherketone, polyimide, polyphenylene ether, and polycarbonate, which are engineering plastics with good heat resistance and mechanical properties. These resins are either blended into epoxy resin by thermal melting or in solution.

• Core-Shell Structured Polymer Toughening of Epoxy Resin

Core-shell structured polymers refer to a class of polymer composite particles obtained through emulsion polymerization of two or more types of monomers. The interior and exterior of these particles are enriched with different components, exhibiting a special bilayer or multilayer structure. The core and shell have distinct functions. By controlling the particle size and altering the composition of the polymer to modify epoxy resin, internal stress can be reduced, and adhesion strength and impact resistance can be enhanced, achieving significant toughening effects.

2.Corrosion Resistance Modification of Epoxy Resin

Currently, common methods for improving the corrosion resistance of epoxy resin include modification with polysulfide rubber, organosilicon compounds, and inorganic nanomaterials.

• Polysulfide Rubber Modification

Polysulfide rubber is a long-chain flexible compound with thioether bonds that can undergo block copolymerization reactions with epoxy resin, thereby increasing the toughness of the epoxy resin . Researchers often use polysulfide rubber to modify epoxy resin. Researchers in China and others used polysulfide rubber as a modifier to modify phenolic epoxy resin (F-51), effectively improving the toughness of the coating. Adding low-viscosity epoxy resorcinol resin as an active modifier in the coating formula can effectively reduce the viscosity of the epoxy system, allowing for an increased amount of pigments and fillers to be added, and also enhancing the heat resistance and chemical resistance of the coating.

• Modification with Organosilicon Compounds

Organosilicon compounds possess good resistance to oxidation, weathering, and hydrophobicity, as well as excellent cold and heat resistance and high dielectric strength. By modifying epoxy resin with organosilicon compounds, Si−C, Si−O, and Si−H bonds can be introduced into the epoxy resin, thereby improving its toughness and enhancing its corrosion resistance. Researchers in China and others found that the co-modification of lacquer phenol and amino-terminated silicone oil (AS) with epoxy resin (EP) can significantly improve the mechanical properties, heat resistance, hydrophobicity, and corrosion resistance of the coating.

• Modification with Inorganic Nanomaterials

Inorganic nanparticles exhibit numerous excellent characteristics such as small size effect, surface effect, and dielectric effect. Modifying epoxy resin with these nanoparticles not only improves the brittleness of the coating but also strongly inhibits the formation of micropores during the curing process, thereby enhancing the shielding properties of the coating and thus strengthening the corrosion resistance of epoxy resin. Researchers in China modified low molecular weight bisphenol A epoxy resin with nano-silica to prepare a solvent-free epoxy varnish and tested the performance of the paint film. The test results showed that the flexibility, heat resistance, impact resistance, and adhesion of the modified paint film were all improved, and the corrosion resistance was excellent.

3.Other Modifications of Epoxy Resin

• Thermal Stability Modification

Increasing the degree of crosslinking, introducing heat-resistant groups such as imide, isocyanate, and oxazolidinone groups, and forming interpenetrating polymer networks are the most important means of improving thermal stability. Using aniline diphenyl ether resin containing terminal amine groups as a curing agent to modify epoxy resin results in composite materials with high initial decomposition temperatures in air and good moisture and heat resistance. The lipophilic epoxy groups in polydimethylsiloxane can enhance its compatibility with the epoxy resin matrix, thereby improving the thermal stability, moisture resistance, and aging resistance of the modified cured products. The molecular chain of polyimide contains benzene rings and imide groups, which give it good thermal stability, outstanding mechanical properties, and low dielectric properties, making it widely used in fields such as microelectronics, liquid crystals, and electronic communications. Modifying epoxy resin with it can not only improve the toughness of epoxy resin but also increase its thermal stability and reduce the dielectric constant. Researchers in China et al. successfully synthesized a new type of trifluoromethyl polyimide (PIS) and modified EP by physical blending. The results showed that PIS-modified EP had good thermal stability and toughness, and its fracture mode changed from brittle fracture to ductile fracture with the increase of PIS content. Researchers in China et al. added highly crystalline poly(p-phenylene benzobisoxazole) (PPPI) particles to EP. The PPPI particles were uniformly combined with EP, and covalent bonds were formed between them, resulting in materials with high flexural modulus and storage modulus and low fracture flexural strain. With the increase of PPPI content, the thermal stability of the obtained materials was significantly improved.

• Flame Retardancy Modification

Epoxy resin has poor flame retardancy. To improve its flame retardancy, halogens, nitrogen, phosphorus, boron, and silicon, which are flame-retardant elements, are usually introduced into the epoxy resin. These elements can be introduced by using flame-retardant curing agents, such as those containing halogens, phosphorus, boron, and silicon, to cure epoxy resin, or by structurally modifying the epoxy resin to incorporate flame-retardant elements within its molecular structure. Brominated phenolic epoxy resin can serve as a reactive flame retardant for epoxy resins used in encapsulation materials. Researchers in China and others designed and synthesized two organic phosphorus compounds containing methyl substituents, 4-methylphenyl phenylphosphine oxide (4-MPO) and 2,4-dimethylphenyl phenylphosphine oxide (2,4-DMPO), based on the principle of the relationship between functional group molar polarizability and molar volume and dielectric properties. These compounds were used as flame retardants to prepare bisphenol A flame-retardant epoxy resin, and the thermal stability of the flame-retardant epoxy resin was studied. Mechanistic studies showed that the two flame retardants mainly exerted flame-retardant effects through the quenching and dilution effects of phosphorus-containing free radicals in the gas phase and through the barrier effect of the char layer in the solid phase. While maintaining flame retardancy and water absorption resistance, the dielectric properties of epoxy resin were improved. These advantages confirm the potential of 4-MPO and 2,4-DMPO as flame retardants for manufacturing high-performance EP suitable for advanced electrical materials.

• Chemical Modification

By altering the structure of epoxy resin and introducing certain chemical groups into the epoxy resin molecules, the performance of epoxy resin can be improved and its application range can be broadened. For example, by reacting acrylic or methacrylic acid with some epoxy groups in epoxy resin, carbon-carbon double bonds are introduced while retaining some epoxy groups in the molecule. This modification endows the epoxy resin with both photosensitive characteristics and some of the excellent properties of epoxy resin. Alternatively, by introducing hydrophilic groups into the molecule, epoxy resin can be modified into waterborne epoxy resin, giving the modified epoxy resin water dispersibility.

III. Current Applications of Epoxy Resin

1. Applications in Electronic Devices

Among various polymer matrices, epoxy resin is widely used in semiconductor and electronic packaging materials due to its excellent mechanical, electrical, and thermal properties. However, pure epoxy resin has a low thermal conductivity, and issues such as the high thermal expansion coefficient, inherent brittleness, and propensity to crack of epoxy adhesives are particularly prominent in electronic packaging applications, affecting the structural stability and service reliability of packaged devices. To improve the inherent properties of epoxy adhesives, researchers have conducted extensive studies. Modified epoxy resins can be used in the manufacture of flexible copper-clad laminates. With the rapid development of lightweight and miniaturized microelectronic products (such as mobile phones, laptops, etc.), flexible printed circuit boards have gradually become a research hotspot. Polyimide-modified epoxy resin composites can be used as insulating and dielectric layers in the manufacture of flexible copper-clad laminates, further improving the performance and product quality compared to pure epoxy resin. Epoxy resin is also commonly used in the manufacture of semiconductor packaging materials. Materials developed using polyimide-modified epoxy resin adhesives have excellent comprehensive performance and moderate cost, meeting the above requirements and are one of the hot topics in the field of electronic chemical materials.

2. Applications in Aerospace Field

Epoxy resin is widely used in the thermal protection of missiles and projectiles, such as the nozzle of solid rocket engines, aerodynamic thermal protection of missile bodies, and surface thermal protection of re-entry spacecraft. With the rapid development of science and technology, the aerospace field has put forward higher requirements for the comprehensive performance of EP. Further improving the thermal protection efficiency of resin-based thermal protection materials has important theoretical and practical significance. S_iB₆ powder, as a filler, added to resin-based thermal protection materials is expected to play multiple modification roles and significantly enhance the thermal protection performance of resin-based thermal protection materials. Therefore, Researchers in China and others used the multiple modification mechanisms of S_iB₆ to modify epoxy resin and explored the effects of its addition on the ablation and thermophysical properties of epoxy resin-based composite materials. The results showed that the addition of SiB6 powder increased the density and hardness of epoxy resin composite materials, increased the pyrolysis residue weight, and significantly improved the ablation resistance of the composite materials. During the ablation process, the appropriate addition of SiB6 powder can form a molten liquid phase on the surface of the composite material, which plays a bonding and enhancing role on the surface carbonized layer, improving the ablation resistance of the composite material.

3. Applications in Marine Field

Epoxy resin has excellent mechanical properties such as wear resistance and impact resistance, good adhesion to metal substrates, and is relatively cheaper than organic silicon, and is usually used for anti-corrosion coatings of ships. Taking epoxy resin as the matrix and modifying it with hydrophobic properties can develop coatings with anti-corrosion and anti-fouling functions. By reducing the surface energy, the anti-fouling performance can be improved while using drag-reducing substances to delay the turbulence degree of the boundary layer fluid and enhance the drag-reducing performance. Silicone oil is incompatible with epoxy resin. When silicone oil is added to epoxy resin coatings, it will slowly exude on the surface of the coating after curing, and the exuded silicone oil is conducive to improving the anti-fouling and drag-reducing performance of the coating. The addition of dimethyl silicone oil can significantly improve the hydrophobicity of epoxy resin coatings, inhibit the attachment of diatoms, and enhance the drag-reducing performance, showing application potential in ship anti-fouling and drag reduction. Researchers in China and others used the method of physical blending modification to modify epoxy resin with silicone oil and prepared coatings with multiple functions of anti-corrosion, anti-fouling, and drag reduction, and tested their performance. The results showed that the silicone oil-modified epoxy coating adhered well to the surface of aluminum alloy substrates, making the surface hydrophobic and inhibiting the attachment of diatoms, with an inhibition rate of 70%. The physical blending modification of epoxy resin with dimethyl silicone oil effectively improved the hydrophobicity, anti-fouling, and drag-reducing properties of the epoxy coating while maintaining good adhesion to the substrate.

4. Applications in Construction Field

Epoxy resin, with its excellent impermeability, durability, and dense adhesion, is increasingly used in construction projects, mainly as a structural adhesive for concrete structure secondary structure planting bars, repairing damaged areas such as concrete holes, honeycombs, and exposed bars, repairing cracks, as well as bonding steel structures and repairing and reinforcing interfaces such as underground pipelines and dam foundations, and sealing and anti-corrosion. It is also used for waterproofing, anti-corrosion, and moisture-proofing of swimming pools, interior and exterior walls of buildings, and other repair work. However, epoxy resin structural adhesives have defects such as low strength, high brittleness, low elastic modulus, easy cracking, and low tensile strength, which require modification of the epoxy resin structural adhesive. Researchers in China modified epoxy resin by adding nano-calcium carbonate and silicon micropowder, and tested the tensile, compressive, and flow properties of the modified epoxy resin. The results showed that the modified epoxy resin had good mechanical properties, and a low amount of nano-calcium carbonate could significantly improve its tensile performance, while silicon micropowder could enhance its compressive strength.

IV. Conclusion

Epoxy resin, due to its excellent mechanical, electrical, thermal, and wear-resistant properties, as well as good adhesion, is widely used in various fields of China's industry, with great application prospects and broad market potential. With the rapid development of science and technology, fields such as aerospace, marine, and electronics have put forward higher requirements for the comprehensive performance of epoxy resin. Modifying epoxy resin can optimize and improve its toughness, corrosion resistance, thermal stability, flame retardancy, and wear resistance, better meeting the requirements of social development.

The innovative research and development achievements of Yinsu Flame Retardant in the field of epoxy resin include a variety of solutions such as brominated epoxy, epoxy resin red phosphorus paste and bromine antimony flame retardant. These highly efficient flame retardants not only provide excellent flame retardant properties, but also effectively improve the processability and high temperature resistance of epoxy resin, and are widely used in electronics, electrical and other high-demand fields. We are committed to providing customers with customized flame retardant solutions to help improve product safety and market competitiveness.