Epoxy resin (EP) is a very important thermosetting resin, which is formed by the reaction of EP oligomer and curing agent. It is generally divided into 5 categories according to the combination of chemical structure and EP group: glycidylamine, Glycidyl esters, glycidyl ethers, aliphatic epoxy compounds, alicyclic epoxy compounds.

Because there are a large number of EP groups in pure EP, the structure after curing has high chemical cross-linking density, low molecular chain flexibility and high internal stress, resulting in greater brittleness of the EP cured material, poor impact resistance and fatigue resistance, which limits it The application and development of it in some high-tech fields that require high durability and reliability. Therefore, it is necessary to toughen and modify EP while maintaining the excellent performance of EP.

At present, there are mainly three ways to toughen and modify EP.

(1) Add rubber elastomer, core-shell polymer, thermoplastic resin (TP), thermotropic liquid crystal polymer (TLCP) and nanoparticles to the EP matrix to form a microscopic phase separation or homogeneous structure;

(2) TP is continuously penetrated in the EP three-dimensional cross-linked network to form an interpenetrating network structure for toughening;

(3) By adjusting the microstructure of EP, for example, introducing more flexible segments into the three-dimensional cross-linked network to improve its molecular flexibility, or introducing a microscopic phase separation structure to enhance the synergy of deformation of molecular segments to achieve increased tough.

1 Toughened EP preparation method

There are three main methods for preparing toughened EP, namely ontology compound method, solution compound method, mechanical compound method, etc.

1.1 Ontology compound method

The ontology compound method is a common method for preparing toughened EP. The toughening agent, such as nanofiller, is mixed with EP through mechanical shearing to achieve dispersion, sometimes with heating.

1.2 Solution compound method

In the solution compounding method, when the solution is added to the EP system, the viscosity of the mixture is often low, which is conducive to the uniform dispersion of the toughening agent, so it is very conducive to the preparation of membrane materials.

1.3 Mechanical compound method

The mechanical compounding method is to mechanically mix the toughening agent and EP directly through a mill. The method has low cost, simple process flow, no solvent introduction during processing, environmentally friendly, and is widely used in industrial production.

2 Toughening method

2.1 Rubber elastomer toughened EP

Rubber elastomers are the earliest and most widely used toughening agents. The rubber elastomer used for EP toughening is usually a reactive liquid polymer (RLP), that is, the end group or side group has active functional groups (such as -COOH, -OH, -NH2, etc.), which can chemically react with EP. The factors that determine the toughening effect of rubber elastomers: ①The solubility of rubber molecules in uncured EP; ②Whether the rubber molecules can be precipitated during the curing process of EP gel and be uniformly dispersed in EP with suitable particle size and ideal form .

Currently commonly used RLP rubbers and elastomers include amino-terminated nitrile rubber (ATBN), epoxy-terminated nitrile rubber (ETBN), hydroxy-terminated nitrile rubber (HTBN), carboxy-terminated nitrile rubber (CTBN), poly 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 to improve the toughness of composite materials.

2.2 Core-shell polymer toughened EP

Since the 1990s, people began to use core/shell structured polymer (CSP) to toughen EP technology. The inside and outside of CSP particles are enriched with different material components, as a result, the inner and outer shells have different functions. Compared with the traditional EP/RLP system, due to the good flocculation of the CSP shell, it is incompatible with EP after being blended. After curing, it can form a complete “sea-island” phase separation structure. By controlling the core-shell material composition And particle size, can significantly improve the toughness of EP.

2.3　TP toughened EP

Due to its inherent limitations such as low molecular weight of rubber elastomers, its 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 The method of toughening EP with high heat resistance TP can significantly improve the toughness of EP. The most commonly used TPs are polysulfone (PSF), polyethersulfone (PES), polyetherketone (PEK), polyetheretherketone (PEEK), polyetherimide (PEI), polyphenylene ether (PPO), etc. .

2.4 TLCP toughens EP

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 (methyl styrenes, esters, biphenyls, etc.). It 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) combines the advantages of EP and liquid crystal, and has good compatibility with EP, and can be used to toughen EP.

2.5 Polymer Interpenetrating Network (IPN) Toughened EP

IPN toughening EP is a new technology developed in the 1980s. IPN can not only improve the impact strength and toughness of composite materials, but also maintain or even improve its tensile strength and heat resistance. This is because different from mechanical blends, there are entanglements and penetrations at the level of molecular chain segments between the polymer component materials in IPN, thus exhibiting “forced inclusion” and “synergistic effect.”

2.6 Hyperbranched polymer (HBP) toughened EP

The mechanism of HBP toughening EP is to assemble functional groups through the outer layer of HBP molecules, which reduces the degree of molecular chain entanglement in the system and reduces its crystallinity, thereby regulating the phase structure of EP and improving the toughness of the resin system.

Some scholars synthesized hyperbranched polyurethane (HBPu) by a quasi-one-step method, and then used it to toughen the anhydride-cured bisphenol A glycidyl ether (DGEBA). Studies have shown that after the introduction of HBPu, the resin viscosity of the uncured EP system is significantly reduced; the impact performance of EP after curing is significantly improved.

2.7 Nanoparticle toughened EP

Nanoparticles are one of the hotspots of recent material research due to their synergistic effect on polymer reinforcement and toughening. This is attributed to the characteristics of nanoparticle surface effects and quantum size effects. Among them, inorganic fillers are widely used because of their low cost, low thermal expansion and shrinkage rate, high elastic modulus and impact toughness of the composite materials. For example: Nano Zirconia (ZrO2) and so on.

Carbon nanomaterials, including CNT and graphene (GE), have a higher surface area to volume ratio due to their unique one-dimensional and two-dimensional structures, making them more conducive to improving the mechanical, electrical, thermal, and barrier properties of the polymer matrix Sexuality is currently a popular research on material modification. Due to the low surface activation energy of carbon nanomaterials, its compatibility with EP is not ideal, so researchers modified the carbon nanomaterials and used them.

Organic nano-elastomers, such as carboxyl butyronitrile elastomer, styrene butadiene elastomer, etc., in addition to the characteristics of nano-materials, they also have the toughness of elastomers, and their compatibility with EP is good, which is a kind of broad prospects for development. material.

2.8 Ionic liquid toughened EP

Ionic liquids are molten salts composed of inorganic anions and organic cations. They are liquid at or near room temperature. Because of their non-volatility, they are recognized as “green materials.” Ionic liquids have “designability” and are used as plasticizers, lubricants, nucleating agents and antistatic agents for polymers.

Some scholars doped GE modified EP composites with butane ionic liquid, and its tensile properties and bending properties have also been significantly improved.

2.9 Composite toughened EP

With the development of technology, researchers realized that the combined use of two toughening agents at the same time has a better application effect than a single toughening agent. EP/(GE/KH-GE)/MWCNTs-OH composite materials were prepared by adding GE and hydroxylated multi-walled CNT (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.

2.10 Flexible segment curing agent toughens EP

Methods of modifying EP based on physical or chemical principles have disadvantages such as complex and long 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, the flexibility of the molecules is improved and the resin structure is plastically deformed. On the other hand, the flexible segment will also produce a microscopic phase separation structure in the resin system, which can relieve stress concentration. Therefore, the soft segment curing agent can greatly improve the toughness of EP without increasing the complexity of the process.

Compared with traditional rigid aromatic amine curing agents, after curing EP with aromatic amine curing agents (RAn) containing ether bonds (—O—) and saturated alkane chains [—(CH2)n—] and other flexible groups, the resin system will stretch The tensile properties and impact properties have been improved to a certain extent.

Prospective

With the in-depth understanding of the toughening mechanism, and based on the continuous improvement of material genome technology, on the basis of traditional toughening and reinforcement, exploring new toughening methods/processes and developing new multifunctional toughening agents can further improve the EP performance Thermal performance, and endow it with such properties as heat conduction, electrical conductivity, wave absorption, electromagnetic shielding, damping and shock absorption.