Research and application status of epoxy resin at home and abroad

(School of Chemistry and Chemical Engineering, Donghua University, Shanghai 201620, China)

Epoxy resin can effectively improve its shortcomings such as high brittleness, easy cracking and poor wear resistance after curing, and greatly improve its application in aerospace, electronic and electrical and many other fields. This paper mainly discusses the methods of optimizing and improving the toughness, corrosion resistance, thermal stability and other properties of epoxy resins through modification, as well as the application status of epoxy resins.

[Keywords]: epoxy resin, modification, application

1. Introduction

Epoxy resin generally refers to polymer prepolymers containing two or more epoxy groups with aliphatic, aliphatic or aromatic segments as the main chain. Epoxy resin (EP) has the characteristics of high strength, good chemical stability, high mechanical properties, excellent bonding performance, small curing shrinkage, good heat resistance, ultraviolet aging resistance, wear resistance and impact resistance, and is often widely used as a coating and adhesive in composite materials, ships, aerospace, electronic and electrical and other fields [1]. However, the cured epoxy resin has a three-dimensional network structure, high cross-linking density, and high internal stress, resulting in its brittleness, easy cracking, poor wear resistance, and high thermal expansion coefficient of heating, which limits its application in some fields [2]. For the above shortcomings, it can be optimized and improved by modifying the method.

2. Modification of epoxy resin

2.1 Toughening and modification of epoxy resin

In order to increase the toughness of epoxy resin, the initial method used was to add some plasticizers and softeners, but these low molecular substances will greatly reduce the heat resistance, hardness, modulus and electrical properties of the material. Since the 60s of the 20th century, research on the toughening and modification of epoxy resin has been widely carried out at home and abroad, in order to improve the toughness of epoxy resin under the condition that the thermal, modulus and electrical properties are not too large.

2.1.1 Rubber elastomer toughened epoxy resin

The rubber elastomers used for epoxy resin toughening are generally reactive liquid polymers with a relative molecular weight of 1000~10000, and functional groups that can react with epoxy groups on the end or side groups. The main varieties of reactive rubber elastomers used for epoxy resin toughening are: terminal carboxynitrile butadiene rubber, terminal hydroxy nitrile butadiene rubber, polysulfide rubber, liquid random carboxylonitrile butadiene rubber, nitrile-butyrate-isocyanate prepolymer, terminal hydroxyl polybutadiene, polyether elastomer, polyurethane elastomer, etc. The butyl polyacrylate-epoxy resin interpenetrating network polymer synthesized by simultaneous method has achieved satisfactory results in improving the toughness of epoxy resin.

2.1.2 Thermoplastic resin toughened epoxy resin

The thermoplastic resins used for the toughening and modification of epoxy resins mainly include polyalum, polyether alum, polyetherketone, polyimide, polyphenylene ether, polycarbonate and other engineering plastics with good heat resistance and mechanical properties, which are either mixed into epoxy resin by hot melting or solution.

2.1.3 Hyperbranched polymer toughened epoxy resins

The main factors of the ability of hyperbranched polymers to toughen epoxy resins are that the spherical three-dimensional structure of this kind of polymer can reduce the shrinkage rate of epoxy cured products, and its active end groups can directly participate in the curing reaction to form a three-dimensional network structure, and many end functional groups can accelerate the curing speed; in addition, the size and spherical structure of hyperbranched polymers eliminate the harmful particle filtration effect observed in other traditional toughening systems, and play a role in internal toughening.

2.1.4 Core-shell structural polymer toughened epoxy resins

Core-shell structural polymers refer to a class of polymer composite particles obtained by emulsion polymerization of two or more monomers. By controlling the size of the particles and changing the composition of the polymer to modify the epoxy resin, the internal stress can be reduced, the bond strength and impact can be improved, and the significant toughening effect can be obtained.

2.2 Anti-corrosion modification of epoxy resin

At present, the commonly used modification methods for improving the anti-corrosion performance of epoxy resins include polysulfide rubber modification, organosilicon compound modification, and inorganic nanomaterial modification [3].

2.2.1 Polysulfide rubber modification

Polysulfide rubber is a long-chain flexible compound with a thioether bond that can undergo block copolymerization with epoxy resins to increase the toughness of epoxy resins [4]. Researchers often use polysulfide rubber to modify epoxy resins. Guo Changqing et al. [5] used polysulfide rubber as a modifier to modify phenolic epoxy resin (F-51), which effectively improved the toughness of the coating, and added a low-viscosity epoxy resorcinol resin active modifier to the coating formulation, which could effectively reduce the viscosity of the epoxy system, improve the addition of pigments and fillers, and improve the heat resistance and chemical resistance of the coating.

2.2.2 Modification of organosilicon compounds

Silicones have good oxidation resistance, weather resistance and hydrophobicity, as well as good cold and heat resistance and high dielectric strength. Silicone-modified epoxy resins can be used to introduce Si−C, Si−O, Si−H and other bonds in silicones into the epoxy resin, improving the toughness of the epoxy resin and improving the anti-corrosion performance of the epoxy resin [6]. Qiao Xingming et al. [7] found that urushiol and terminal amino silicone oil (AS) can significantly improve the mechanical properties, heat resistance, hydrophobicity and anti-corrosion properties of the coating.

2.2.3 Modification of inorganic nanomaterials

Inorganic nanoparticles have many excellent characteristics such as small size effect, surface effect, and dielectric effect. Using its modified epoxy resin can not only improve the brittleness of the coating, but also have a strong barrier to the micropores generated during the curing process, which can improve the shielding property of the coating, and then enhance the corrosion resistance of the epoxy resin. Han Shizhong [8] used nano-silica to modify the low-molecular-weight bisphenol A-type epoxy resin to prepare a solvent-free epoxy varnish and test the properties of the paint film. The test results show that the flexibility, heat resistance, impact resistance and adhesion of the modified paint film are improved, and the corrosion resistance is excellent.

2.3 Other modifications of epoxy resins

2.3.1 Thermal stability modification

Increasing the degree of cross-linking, introducing heat-resistant groups such as imino group, isocyanate group, umazolidinone, etc., and forming an interpenetrating polymer network are the most important means to improve heat resistance; using aniline diphenyl ether resin containing terminal amine group as curing agent to modify epoxy resin, the obtained composite material has high initial decomposition temperature in air atmosphere and good damp heat resistance; polydimethylsiloxane contains aliphatic epoxy group, which can improve its compatibility with epoxy resin matrix, and can improve the thermal stability of the modified cured substance. Moisture resistance and aging resistance [9]: Polyimide molecular chains contain benzene rings and imide groups, which make it have good heat resistance, outstanding mechanical properties, and low dielectric properties, and are widely used in microelectronics, liquid crystals, and electronic communications [10]. It can not only improve the toughness of epoxy resin, but also improve the thermal stability of epoxy resin and reduce the dielectric constant [11]. JIY ET AL. [12][12] SUCCESSFULLY SYNTHESIZED A NOVEL TRIFLUOROMETHYL POLYIMIDE (PIS) AND MODIFIED THE EP BY PHYSICAL BLENDING METHOD. The results show that the PIS-modified EP has good thermal stability and toughness, and its fracture mode changes from brittle fracture to ductile fracture with the increase of PIS content. Essmeister J et al. [13] added highly crystalline poly-terenephthalenetetracarboxylimide (PPPI) particles to EP, and the PPPI particles were uniformly combined with the EP, and covalent bonds were formed between the two, so that the obtained material had a high flexural modulus, storage modulus and low bending strain at break, and the thermal stability of the obtained material was significantly improved with the increase of PPPI content.

2.3.2 Flame retardant modification

Epoxy resins are less flame retardant, and in order to improve their flame retardancy, flame retardant elements such as halogens, nitrogen, phosphorus, boron, and silicon are usually introduced into epoxy resins. The method introduced can be to cure the epoxy resin using flame-retardant curing agents, such as those containing halogen, phosphorus, boron, and silicon, to modify the structure of the epoxy resin, and to introduce flame-retardant elements into the epoxy resin molecule. Brominated phenolic epoxy resins can be used as reactive flame retardants for epoxy resins for encapsulation materials. Wu Mei et al. [1] designed and synthesized two organophosphorus compounds containing methyl substituents, 4-toluene phenylphosphorus oxide (4-MPO) and 2,4-xylphenylphenylphosphorus oxide (2,4-DMPO), based on the relationship between functional group molar polarizability and molar volume and dielectric properties, and prepared bisphenol A-type flame retardant epoxy resin with these two compounds as flame retardants, and studied the thermal stability of flame retardant epoxy resin. Mechanism studies have shown that the two flame retardants exert flame retardant effect mainly through the quenching and dilution of phosphorus-containing free radicals in the gas phase, and exert the flame retardant effect through the barrier effect of the carbon layer in the solid phase. The dielectric properties of epoxy resins are improved while maintaining flame retardancy and water absorption resistance. These advantages confirm the potential of 4-MPO and 2,4-DMPO as flame retardants to manufacture high-performance EPs suitable for advanced electrical materials.

2.3.3 Chemical modification

By changing the structure of epoxy resin and introducing some chemical groups into the epoxy resin molecule, the performance of epoxy resin can be improved and the scope of its application can be broadened. For example, acrylic acid or methacrylic acid is used to react with part of the epoxy group in the epoxy resin, and the carbon-carbon double bond is introduced while retaining part of the epoxy group in the molecule, so that the modified epoxy resin not only has photosensitive characteristics, but also retains some excellent characteristics of the epoxy resin;

3 Application status of epoxy resin

3.1 Application in electronic devices

Among the various polymer matrices available, epoxy resins are widely used in semiconductor and electronic packaging materials due to their excellent mechanical, electrical, and thermal properties [14]. However, the low thermal conductivity of pure epoxy resins [15], the high coefficient of thermal expansion of epoxy adhesives, inherent brittleness, and easy cracking are particularly prominent in electronic packaging applications, which affect the structural stability and service reliability of packaged devices. In order to improve the intrinsic properties of epoxy adhesives, researchers have conducted a lot of research. Modified epoxy resins can be used in the manufacture of flexible copper clad laminates. With the rapid development of light and small microelectronic products (such as mobile phones, laptops, etc.), flexible printed circuit boards have gradually become a research hotspot. Among them, polyimide modified epoxy resin composites can be used as insulating and dielectric layers for the manufacture of flexible copper clad laminates, which can further improve the performance and product quality of flexible copper clad laminates compared with pure epoxy resins. Epoxy resins are also commonly used in the manufacture of semiconductor packaging materials. The materials developed by polyimide modified epoxy resin adhesives have the characteristics of excellent comprehensive performance and moderate cost, taking into account the above requirements, and are one of the hot spots in the field of electronic chemical materials.

3.2 Applications in the aerospace field

Epoxy resins are widely used in the field of thermal protection of projectiles and arrows, such as solid rocket engine nozzles, aerodynamic heat protection of missile bodies, and heat protection of re-entry spacecraft surfaces [16]. With the rapid development of science and technology, the aerospace field has put forward higher requirements for the comprehensive performance of EP. It is of great theoretical value and practical significance to further improve the thermal protection efficiency of resin-based thermal protection materials. SiB6 powder is expected to play a multiple modification role when added to resin-based thermal protection materials as a filler, and significantly improve the thermal protection performance of resin-based thermal protection materials. Therefore, Wang Bin et al. [16] used the multiple modification mechanism of SiB6 to modify epoxy resin, and explored the effect of its addition on the ablation and thermophysical properties of epoxy resin matrix composites. The results show that the addition of SiB6 powder increases the density and hardness of epoxy resin composites, increases the pyrolysis residual weight, and significantly improves the ablation resistance of composites. In the ablation process, the addition of SiB6 powder can form a molten liquid phase on the surface of the composite, which can strengthen the adhesion of the surface carbonization layer and improve the anti-ablation performance of the composite.

3.3 Application in the field of ships

Epoxy resins have excellent mechanical properties such as abrasion resistance and impact resistance, good adhesion properties to metal substrates, and lower cost than silicones, and are commonly used in marine anti-corrosion coatings [17]. Hydrophobic modification of epoxy resin as a matrix can be used to develop anti-corrosion and anti-fouling coatings, which can improve anti-fouling performance by reducing surface energy, and using drag-reducing substances to delay the turbulence of boundary layer fluids and improve drag-reducing performance. Silicone oil is incompatible with epoxy resin, and after curing, silicone oil will slowly precipitate on the surface of the coating, and the precipitated silicone oil is conducive to improving the antifouling and drag reduction performance of the coating. The addition of dimethylsilicone oil can significantly improve the hydrophobicity of epoxy resin coatings, inhibit the adhesion of diatoms and improve the drag reduction performance, which has the potential to be applied in the anti-fouling and drag reduction of ships. Xu Xiaoyuan et al. [17] used the method of physical blending and modification to use silicone oil to modify epoxy resin to prepare a coating with multi-functional functions of anti-corrosion, anti-fouling and drag reduction, and carried out performance tests. The results show that the silicone oil modified epoxy coating adheres well to the surface of the aluminum alloy substrate, which makes the surface hydrophobic and inhibits the adhesion of diatoms, and the inhibition rate reaches 70%. The addition of dimethyl silicone oil to physically blend and modify the epoxy resin can effectively improve the hydrophobicity, antifouling and drag reduction of the epoxy coating while maintaining the good adhesion of the coating to the substrate.

3.4 Application in the field of construction

With its excellent impermeability durability, bonding and other properties, epoxy resin is increasingly used in engineering construction, mainly as a building structure adhesive for concrete structure secondary structure reinforcement, concrete hole honeycomb exposed ribs and other damage repair, crack repair, as well as steel structure bonding and underground pipeline dam foundation and other interface repair reinforcement and sealing anti-corrosion, pool, building interior and exterior wall anti-seepage, waterproof, anti-corrosion, moisture-proof and other repair work. However, epoxy resin building structural adhesive has defects such as low strength, high brittleness, low elastic modulus, easy cracking and low tensile strength, so epoxy resin structural adhesive needs to be modified. Li Wenhui [18] added nano calcium carbonate and silicon powder to the epoxy resin for modification, and tested the tensile strength, compressive resistance and flow properties of the modified epoxy resin, the results showed that the modified epoxy resin had good mechanical properties, the low amount of nano calcium carbonate could significantly improve its tensile properties, and the silicon powder could improve its compressive strength.

4 Conclusion

Due to its excellent mechanical, electrical, heat-resistant, wear-resistant and good adhesion properties, epoxy resin is widely used in many fields in China's industry, and has great application prospects and broad market potential. With the rapid development of science and technology, aerospace, shipbuilding, electronic and electrical and other fields have put forward higher requirements for the comprehensive performance of epoxy resin. Modification of epoxy resin can optimize and improve its toughness, corrosion resistance, thermal stability, flame retardancy and wear resistance, etc., and better meet the requirements of social development.

References

(1) Wu Mei, Xu Xiaolei, Li Xiao et al.Study on methyl substituted diaryl phosphine oxide flame retardant modified epoxy resin[J/OL].Materials Reports:1-29[2023-10-26].

(2) Li Yang, Niu Yongping, Yang Kang, et al.Research on tribological properties of modified epoxy resin of molybdenum disulfide/copper sulfide nano-hybrid materials[J/OL].New Chemical Materials:1-7[2023-10-26].

Chemical Technology and Development,2023,52(10):67-70+93.

(4) Pan Wei, Li Yu, Xin

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