1 Introduction
Epoxy resin is a commonly used advanced composite resin matrix. Cured epoxy resin has excellent physical and mechanical properties, electrical insulation properties, chemical corrosion resistance, heat resistance, and adhesive properties, so it is widely used in coatings, electronics Electrical, composite materials, civil construction, and other fields. It plays an extremely important role in the development of the national economy and is one of the most important fine polymer materials in the world.
2. Selection of flame retardants in epoxy resin systems
The flame retardant methods of epoxy resin can be divided into filler type and structural type. The former refers to the method of adding various flame retardant additives that do not participate in the curing reaction into the epoxy resin to make it flame retardant; the latter refers to the method of introducing a flame retardant structure into the epoxy resin to achieve the purpose of flame retardancy.
2.1 Halogen flame retardants
Halogen-based (brominated and chlorine-based) flame retardants are a very commonly used flame retardant. The amount added is relatively small and the flame retardant efficiency is high, especially brominated flame retardants [1]. Mainly include tetrabromobisphenol A, decabromodiphenyl ether, tetrabromophthalic anhydride, dibromoneopentyl glycol, etc. However, the serious disadvantages of brominated flame retardants are that they reduce the UV resistance of the flame-retardant substrate and generate more smoke, corrosive gases, and toxic gases that are harmful to the human body and the environment when burned. In 1986, Swiss scientists reported that when polybrominated diphenyl ethers are burned, they produce toxic and carcinogenic polybrominated dibenzodioxins and dibenzofurans, which pose a serious threat to humans and the environment. Therefore, the environmental safety issues caused by halogenated flame retardants have attracted widespread attention. At present, non-toxic and pollution-free inorganic flame retardants and organophosphorus flame retardants with environmental safety and use safety are increasingly used in flame retardant materials.
2.2 Nano aluminum hydroxide
Aluminum hydroxide is the most important type of inorganic flame retardant. In terms of consumption, it ranks first among all flame retardants. Due to its low toxicity, aluminum hydroxide currently accounts for the world's total consumption of flame retardants. 45% of the total, accounting for 70% of the inorganic flame retardant consumption. Because of its smoke suppression, low corrosion, and low price, it is expected that the average annual growth rate of its usage will be higher than that of all other types of flame retardants in the next few years [2]. The aluminum hydroxide flame retardant currently used does not have strong flame retardant ability. Its particles are generally in the micron level and require high filling to be flame retardant. The flame retardant efficiency is not high and will also affect the physical and mechanical properties of the material. To improve the dispersion of the flame retardant in the resin system and increase the flame retardant effect of the flame retardant, it is generally required that the particles of the flame retardant be as fine as possible [3]. The use of nanoscale aluminum hydroxide flame retardant can be more uniformly dispersed in the matrix resin due to enhanced interfacial interaction, thereby more effectively improving the mechanical properties of the blend, and its filling amount will be greatly reduced, making it flame retardant. Efficiency will also be doubled.
The refinement of aluminum hydroxide and the synergistic effect of ATH and flame retardants have transformed it from an inefficient flame retardant type to a highly efficient flame retardant type, and from ordinary products to a series of products with high functionality and high added value, which has greatly It improves the quality of aluminum hydroxide and expands the application scope of ATH.
2.3 Phosphate flame retardant
Organophosphorus flame retardants are one of the most important organic flame retardants, and they are also the focus of developing environmentally friendly flame retardants. Organophosphorus flame retardants include phosphate esters and their derivatives, phosphites, organophosphorus salts, phosphine oxide phosphorus-containing polyols, and phosphorus-nitrogen compounds. As flame retardants, the most widely used are phosphate esters and phosphonic acids. ester.
There are two types of phosphorus flame retardants: reactive type and additive type. The additive type includes low molecular weight compounds, such as alkyl phosphates, and oligomers, such as polyphosphates. At present, additive phosphorus-based flame retardants are mainly used, but the use of reactive flame retardants to chemically modify polymers to impart flame retardancy to materials is also gaining increasing attention. There are two effective ways to chemically modify polymers with phosphorus compounds: the first is to change the structure of the parent polymer by introducing flame-retardant modified functional groups into the polymer; the second is to use reactive retardants. The reaction between flame agent and unsaturated polymer or the copolymerization technology involving flame retardant monomer.
Phosphorus-containing flame retardants have both flame retardant and plasticizing functions and are widely used. Among them, phosphate ester flame retardants have good gas phase and solid phase flame retardant effects, and their effects are much higher than halogen-containing flame retardants. When burned, the phosphorus compound first forms a non-combustible liquid film of phosphoric acid, which is then further dehydrated to form metaphosphoric acid, which is then polymerized to form polymetaphosphoric acid. Among them, the liquid film generated by phosphoric acid plays a covering role; polymetaphosphoric acid is a strong acid and strong dehydrating agent, which can dehydrate and carbonize polymer materials. This carbon film isolates the air so that the phosphide plays a flame-retardant role.
3. Development trend of flame retardant epoxy resin
Researching and using epoxy resins containing flame retardant elements such as phosphorus, nitrogen, and silicon to replace the widely used tetrabromobisphenol A epoxy resin is the main method for developing non-halogen flame retardant epoxy resins.
3.1 Research on nitrogen-containing epoxy resin
Nitrogen-containing epoxy resins include glycidylamine epoxy resin and polyisocyanurate-oxazolidinone resin. Glycidylamine epoxy resin includes triglycidylamine cyanurate with triazine as the core skeleton, p-aminophenol epoxy resin, and diaminodiphenylmethane epoxy resin. The triglycidylamine cyanurate molecule contains 3 epoxy groups and the nitrogen content is as high as 14%, so it is self-extinguishing. It also has the characteristics of high cross-linking density, high-temperature resistance, and good arc resistance. Polyisocyanurate-oxazolidinone resin is a type of epoxy resin that contains a large number of five- and six-membered nitrogen heterocycles in the structure. Because the structure contains a large number of five- and six-membered rings, this type of material has excellent barrier properties. Flammability, heat resistance, medium resistance, and mechanical strength.
3.2 Research on phosphorus-containing epoxy resin
Introducing phosphorus elements into epoxy resin can make it have good heat resistance and flame retardancy. Phosphorus-containing flame-retardant epoxy resin can be produced by reacting the active nitrogen on the phenanthrene-type phosphorus compound with the epoxy group of the epoxy resin under the catalysis of triphenylphosphorus and stirring at 120~180°C. In this epoxy resin composition, if the phosphorus content reaches 0.8~8%, it can have excellent flame retardancy. When the material is heated, it generates oxygen-containing acids of phosphorus. These acids catalyze the dehydration of the compounds into carbon, reducing the material's mass loss rate and the amount of combustibles produced, while most of the phosphorus remains in the carbon layer. The flame retardant efficiency mainly depends on the generated carbon layer [4].
3.3 Research on silicone-containing epoxy resin
All silicone polymers with epoxy groups can be used as silicone epoxy resins. Different molecular weights of silicone cause great changes in the dispersion state of the silicone phase in the cured resin, resulting in great differences in performance.
Silicone-modified epoxy resin is an effective way developed in recent years that can not only reduce the internal stress of epoxy resin, but also increase the toughness, high-temperature resistance, and other properties of epoxy resin. At present, there are two methods of silicone-modified epoxy resin: blending and copolymerization. The key is to solve the compatibility problem.
4. Conclusion
The development of flame-retardant epoxy resin has tended to be halogen-free, new technology, multi-functional, and systematic. At present, many problems need to be solved in the practical application of flame-retardant epoxy resins. For example, some flame-retardant epoxy resins will produce thick smoke when burned. Although some flame-retardant epoxy resins have excellent smoke suppression functions, their physical and mechanical properties have dropped too much and cannot meet the engineering requirements. Therefore, we should further research and develop new halogen-free structural flame retardant curing agents with high thermal stability, high molecular weight, and good compatibility, and explore the production cost preparation process of epoxy monomers or curing agents with high flame retardant element content, from Fundamentally solve the negative impact on other properties of materials caused by adding flame retardants.
Reprinted in Zhihu: Research Progress of Flame-Retardant Epoxy Resin Systems Composite Materials Gathering | Composite Materials
