Background Epoxy resin is a very important thermosetting resin because there are many epoxy groups in pure epoxy resin. Therefore, the chemical cross-linking density of the cured structure is high, the molecular chain flexibility is low, and the internal stress is large, resulting in the epoxy cured material being more brittle and having poor impact resistance and fatigue resistance durability.So the application and development of epoxy resin in high-tech fields with durability and reliability requirements are limited. Therefore, it is necessary to toughen and modify epoxy resin while maintaining its excellent properties.   Toughening modification methods 1. Rubber elastomer toughened epoxy resin Rubber elastomers are the earliest and most widely used tougheners. Rubber elastomers used for toughening epoxy resins are usually reactive liquid polymers (RLP), that is, the end or side groups have active functional groups (such as -COOH, -OH, -NH2, etc.), which can chemically react with epoxy groups.  Factors that determine the toughening effect of rubber elastomer:a.The solubility of rubber molecules in uncured EP. b. Whether rubber molecules can precipitate during the curing process of epoxy gel and be evenly dispersed in the ring with appropriate particle size and ideal form. in oxygen resin. Currently commonly used RLP rubbers and elastomers include amine-terminated nitrile rubber (ATBN), epoxy-terminated nitrile rubber (ETBN), hydroxyl-terminated nitrile rubber (HTBN), carboxyl-terminated nitrile rubber (CTBN), polyester Sulfur rubber (PSR), PUR and silicone rubber (SR), etc. Among them, CTBN contains very polar nitrile groups (-CN) and has good molecular flexibility. Its toughened EP system forms a "sea-island" microscopic phase separation structure that helps improve the toughness of composite materials. 2. Core-shell polymer toughened epoxy resin Core/shell structure polymer (CSP) toughened epoxy resin technology is used. CSP particles are enriched with different material components inside and outside, resulting in their core and shell having different functions. Compared with the traditional EP/RLP system, due to the good flocculation of the CSP shell, it is incompatible with EP after blending and can form a complete "sea-island" phase separation structure after solidification. By controlling the core-shell material components and particle size, which can significantly improve the toughness of EP. 3. Thermoplastic resin toughened epoxy resin Due to the low molecular weight of rubber elastomers, their introduction into EP will reduce the strength, modulus and heat resistance of the cured product. In order to solve these problems, researchers have developed high toughness, high strength and high heat resistance properties. The TP toughening EP approach can significantly improve EP toughness. The commonly used TPs include polysulfone (PSF), polyethersulfone (PES), polyetherketone (PEK), polyetheretherketone (PEEK), polyetherimide (PEI), polyphenylene ether (PPO), etc. 4. Thermotropic liquid crystal polymer (TLCP) toughened epoxy resin Thermotropic liquid crystal polymer (TLCP) is a type of TP with special properties. Its molecular structure contains a certain amount of flexible segments and a large number of mesogenic rigid units (methylstyrenes, esters, biphenyl, etc.), which exhibits high strength and high Excellent mechanical properties such as modulus and self-reinforcement as well as better heat resistance. Liquid crystal epoxy resin (LCEP) has the advantages of both EP and liquid crystal, and has good compatibility with EP and can be used to toughen epoxy resin. 5. Polymer interpenetrating network structure (IPN) toughened epoxy resin IPN not only improves the impact strength and toughness of composites, but also maintains or even improves their tensile strength and heat resistance. This is because unlike mechanical blends, the polymer component materials in IPN are entangled and penetrated at the molecular segment level, thus showing "forced inclusion" and "synergistic effects" 6. Hyperbranched polymer (HBP) toughened epoxy resin The mechanism of HBP tougheningepoxy resin is to assemble functional groups in the outer layer of HBP molecules, which reduces the degree of molecular chain entanglement in the system and reduces the crystallinity, thereby regulating the phase structure of EP and improving the toughness of the resin system. Some scholars have synthesized hyperbranched polyurethane (HBPu) using a quasi-one-step method, and then used it to toughen acid anhydride-cured bisphenol A-type glycidyl ether (DGEBA). Research shows that after the introduction of HBPu, the resin viscosity of the uncured EP system is significantly reduced; the impact properties of cured EP are significantly improved. 7. Nanoparticle toughened epoxy resin Nanoparticles have become one of the hot topics in recent materials research due to their synergistic effect on both strengthening and toughening of polymers, which is attributed to properties such as nanoparticle surface effects and quantum size effects. Among them, inorganic fillers are widely used because of their low cost, low thermal expansion and shrinkage, and high elastic modulus and impact toughness of the composite materials produced. For example: Nano-zirconia (ZrO2), etc. Carbon nanomaterials, including CNT and graphene (GE), have a higher surface area to volume ratio due to their unique one- and two-dimensional structures, making them more conducive to improving the mechanics, electricity, thermal and barrier properties of the polymer matrix. Properties are currently a hot research topic in material modification. Due to the low surface activation energy of carbon nanomaterials, their compatibility with EP is not ideal, so researchers modified the carbon nanomaterials for use. Organic nanoelastomers, such as carboxyl nitrile elastomers, butylbutylene elastomers, etc., in addition to the characteristics of nanomaterials, also have the toughness of elastomers, and have good compatibility with EP. They are a type of elastomer with broad development prospects material. 8. Ionic liquid toughened epoxy resin Ionic liquids are molten salts composed of inorganic anions and organic cations. They are liquid at or near room temperature. They are recognized as "green materials" because of their non-volatility. Ionic liquids have "designability" and are used as plasticizers, lubricants, nucleating agents and antistatic agents for polymers. Some scholars have used butane ionic liquids to dope GE-modified EP composites, and their tensile properties and bending properties have also been significantly improved.  9. Composite toughened epoxy resin With the development of technology, researchers have realized that using two toughening agents in combination has better application effects than a single toughening agent. EP/(GE/KH–GE)/MWCNTs-OH composites were prepared by adding GE and hydroxylated multi-walled CNTs (MWCNTs-OH) to EP. The results show that GE/KH–GE and MWCNTs-OH have a synergistic toughening effect on EP without affecting the mechanical properties of EP. 10. Flexible segment curing agent toughens epoxy resin Methods for modifying EP based on physical or chemical principles have shortcomings such as complex and lengthy process routes. By using macromolecular curing agents containing flexible segments, after the EP is cured, the flexible segments are naturally bonded to the resin system. In the three-dimensional cross-linked network, on the one hand, it improves the flexibility of the molecules and promotes plastic deformation of the resin structure. On the other hand, the flexible segments also produce microscopic phase separation structures in the resin system, which can alleviate stress concentration. Therefore, flexible segment curing agents can greatly improve the toughness of EP without increasing process complexity. Compared with traditional rigid aromatic amine curing agents, after curing EP with aromatic amine curing agents (RAn) containing flexible groups such as ether bonds (—O—) and saturated alkane chains [—(CH2)n—], the resin system has a better The tensile properties and impact properties have been improved to a certain extent.   Outlook With an in-depth understanding of the toughening mechanism and based on the continuously improved material genome technology, on the basis of traditional toughening and reinforcement, new toughening methods/processes and the development of new multi-functional toughening agents can be further improved. Thermal properties and endowed with properties such as thermal conductivity, electrical conductivity, wave absorption, electromagnetic shielding, damping and shock absorption.  

1. Indicators of epoxy groups This is the most important characteristic index of epoxy resin, which is used to indicate the content of epoxy group in the resin molecule, and there are three main ways of expression, including  epoxy value, epoxy index and epoxy equivalent. Epoxy value is defined as the amount of epoxy groups(mol) per 100g of epoxy resin，unit is mol/100g. The definition of the epoxy value is mainly for the purpose of calculating the amount of curing agent to be added to the epoxy resin for curing. The amount of curing agent is the mass of curing agent to be added per 100g of epoxy resin cured. Epoxy index is the amount of epoxy groups(mol) per 1kg of epoxy resin, the unit is mol/kg. In terms of the International System of Measurement (SI units), the epoxy index is more appropriate than the epoxy value, which is 10 times larger than the epoxy value. Epoxy equivalent is the mass (g) of an epoxy resin containing 1 mol of epoxy groups ,the unit is g/mol. The chain segments between the epoxy groups become longer and longer as the molecular weight of the epoxy resin increases, so the epoxy equivalent of epoxy resins with a high relative molecular mass is also elevated. The physical quantity of epoxy equivalent is usually used to describe the epoxy group of epoxy resin in the United States, Japan and Europe.   2. Hydroxyl content Bisphenol A-type epoxy resin molecular chain contains a large number of secondary hydroxyl structure, the larger the polymerization degree n value, its molecular weight is also larger, the higher the hydroxyl content. It can cross-link with phenolic resins, amino resins or Poly isocyanates, and it can promote the curing reaction. Therefore, when controlling the curing process of epoxy resin paint, the hydroxyl content of epoxy resin must be determined. There are two most used methods to express the hydroxyl content. The hydroxyl value F is the amount of hydroxyl contained in 100g of epoxy resin, the unit is mol/100g. And the hydroxyl equivalent H is the mass(g)of epoxy resin containing 1mol of hydroxyl, the unit is g/mol.   3. Softening point Epoxy resin is a mixture of homologous prepolymers with different degrees of polymerization and has no fixed melting point or melting process. The softening point generally refers to the temperature at which the epoxy resin turns from hard to soft and shows a certain fluidity in the process of heating. The softening point of epoxy resin can characterize the average molecular weight size and distribution of the resin, the molecular weight of the high softening point is large, and the molecular weight of the low softening point is small. Epoxy resins can be broadly classified according to the softening point into three types. Type Softening point Degree of polymerisation Low molecular weight epoxy resin <50°C <2 Medium molecular weight epoxy resin 50~95°C 2~5 High molecular weight epoxy resin >100°C >5     4. Viscosity The viscosity of epoxy resins affects the fluidity and workability of resins and coatings. Viscosity increases as the average molecular weight of the epoxy resin increases and decreases as the molecular weight distribution decreases. The viscosity of epoxy resins is extremely sensitive to temperature and decreases rapidly with increasing temperature, so it is generally expressed as the viscosity at a specific temperature.   5. Chlorine value The amount of chlorine contained in an epoxy resin (including organic chlorine and inorganic chlorine) is called the chlorine value. The chlorine in the epoxy resin is divided into organic chlorine and inorganic chlorine by the form of its existence. Organic chlorine comes from the residue of insufficient ring closure in the manufacture of the epoxy resin, which is called easily hydrolysable chlorine. Inorganic chlorine comes from the residual sodium chloride that is not washed sufficiently when manufacturing epoxy resins. Organic chlorine measures the resin reaction, and inorganic chlorine measures the level of post-production treatment processes for epoxy resins. Both are detrimental to the electrical properties of the cured substance and to corrosion resistance.