1 Silicon modification
Silicone modification can not only reduce the internal stress of epoxy resin, but also increase its toughness and high temperature resistance. S Ananda Kumar and others have developed a kind of epoxy resin as the matrix, hydroxyl-terminated polydimethyl phenoxane as modifier, and γ-aminopropyltriethoxysilane (γ-APS) as crosslinking agent. Dibutyl glycerol tin clavulanate (DBTDL) as catalyst, polyamidoamine and aromatic polyamine compound as curing agent, new silicon-containing epoxy coating. Compared with before modification, the weight loss temperature of the modified system is generally increased by about 10℃ by 10% to 50%. A AnandPrabu et al. 2 used polyurethane (PU) prepolymers and hydroxyl-terminated polydimethylsiloxane (HTPDMS) to modify epoxy resins with γ-aminopropyl triethoxysilane (γ- APS) and vinyl triethoxy silane (VTES) are used as crosslinking agent, DBTDL is used as catalyst, aromatic polyamine addition compound and aliphatic amine are used as curing agent. The experiment shows that when PU mass fraction is 2.5%-20%, silicon When the mass fraction is 10%, the viscosity increases. The modified resin has the advantages of good thermal stability, oxidation resistance, weather resistance, good low temperature performance, low surface energy, and high dielectric strength, but its mechanical properties, adhesion, etc. Poor, high cost, and compatibility issues have a great influence on the effect of modified epoxy resin.
2 Phosphorus modification
In view of the fact that a large amount of toxic and corrosive smoke generated by the combustion and decomposition of halogen-containing flame retardants will seriously pollute the environment, the development of epoxy resin non-halogen flame retardant systems has become a research hotspot in recent years. Ru-Jong Jeng et al. synthesized a phosphorus-containing epoxy resin (GDP), which was cured with a phosphorus-free amine curing agent and a phosphorus-containing new aromatic or polyoxyethylene amine curing agent. When it reaches 6.19%, the residual carbon rate at 850°C is nearly 30%, and the oxygen index reaches 32.
3 Synergistic modification of silicon, phosphorus and amine
Studies have shown that silicon and phosphorus have a synergistic enhancement effect in flame retardant behavior. On the other hand, the introduction of nitrogen into the phosphorus flame retardant system can increase the LOI value. The synergistic effect of these substances reduces the cost, not only improves the thermal stability, but also increases the flame retardant efficiency.
used bisphenol A and tetraethoxysilane to synthesize an organic-inorganic mixture through a solution gel process. The sol-gel method can successfully combine silicon and phosphorus with the epoxy cross-linked network, thereby improving its flame retardant performance. The initial weight loss temperature of the modified epoxy resin system is reduced (the phosphorus-containing component is decomposed), but the high-temperature residual carbon rate under a nitrogen atmosphere increases significantly, 36% and 31% at 600℃ and 800℃ respectively (unmodified Only 19% and 14.8%), the LOI value increased from 24 to 32.
The phosphorus/nitrogen and silicon/nitrogen epoxy resin systems were cured by maleimide, and their thermal stability was studied in detail by thermal analysis. Studies have shown that in a nitrogen atmosphere, the initial decomposition temperature of all phosphorus-containing epoxy resins is lower than that of phosphorus-free ones, because phosphorus-containing groups decompose at a lower temperature. Compared with silicon and maleyl The imine group does not change the initial decomposition temperature (IDT) of the epoxy resin. On the other hand, the introduction of avian silicon group greatly increases the complete decomposition temperature (IPDT). For the bisphenol A epoxy resin system (BE188), the IPDT increases with the increase of silicon content. For the cresol-formaldehyde novolac epoxy resin system (CNE200), IPDT decreases with the increase of silicon content, which may be caused by the introduction of volatile triphenylsiloxane groups. In addition, the introduction of phosphorus and maleimide groups increases the organic content and reduces IPDT. The cured product of phenol formaldehyde phenolic resin (PFN) has a residual carbon rate of 50.5% at 800°C and an IPDT of 1769.6°C. It can be used as a high temperature resistant structural material. In the air, IDT changes little and big, but IPDT has a big change. The addition of phosphorus increases IPDT. This result is just the opposite of nitrogen atmosphere, because phosphorus can form a residual carbon layer at high temperatures and delay its continued combustion. process.
4 Maleimide modification
Maleimide can improve the high temperature resistance of the resin. The modification method is to use polybismaleimide and epoxy resin to react and crosslink to form an interpenetrating network (IPN), and use a curing agent containing imide groups Curing epoxy resin, using thermoplastic polyimide or polyimide functional group and epoxy resin blending 3 kinds. The main disadvantage of these methods is the poor compatibility of the imide component and epoxy resin, and it is difficult to process and shape. In the other direction, the introduction of imide groups into the epoxy resin backbone is a hot area of research.
Usually polyimide or imide compound is added to epoxy matrix, or used as curing agent to improve the thermal stability and flame retardancy of epoxy resin. However, Chuan-Shao Wu et al. used triphenylphosphine and methyl ethyl ketone as catalysts and solvents for the first time to perform a simple addition reaction between maleimide with hydroxyl groups and epoxy groups to obtain an interpenetrating network structure. The glass transition turbulence of the cured epoxy resin modified by maleimide increased from 369℃ to 381～386℃, in N2 atmosphere
The maximum residual carbon rate at 800°C is 27.3%, and the LOI value reaches 29.5.
The synthesized 4-(N-maleimidephenyl) glycidyl ether (MPGE) epoxy resin was cured with DDM and DICY and DEP (diethyl phosphite) to obtain a cross-linked network. In N2 atmosphere, 5% weight loss temperature can reach 355℃, complete decomposition temperature (IPDT) reaches 2287℃, 800℃ residual carbon rate reaches 60.38%; in air, 5% weight loss turbulence reaches 348℃, complete decomposition temperature reaches 669℃, 800℃ residual carbon rate reaches 11.01%. The highest LOI value can reach 48.0.
5Rigid rod epoxy modification
Because rigid rod-shaped epoxy resin has good thermal, mechanical and electrical properties, it is usually used in electrical and space technology fields. For example, the study of Mi Ja et al. showed that epoxy resins containing azomethine groups have higher thermal stability than ordinary bisphenol A epoxy resins. Research by WFA Su et al. showed that epoxy resin cured by sulfonamide or methylcyclohexylamine has higher thermal stability because it has azomethine group or bisphenol rigid rod-like group. They also used trimellitic anhydride and diaminodiphenyl sulfone to cure the diphenol epoxy resin, respectively, and the cured resin also showed good thermal and electrical properties. Lu et al. 2 also found that diphenylaminomethane or 4,4'-diaminophenoxyhexane cured bisphenol epoxy resin has good high temperature resistance. Wei-Fang Su et al. used rigid tetramethyl biphenyl (TMBP) and flexible bisphenol A epoxy resin (DGEBA) to cure with phthalic anhydride (PA) and phenol formaldehyde phenol resin (PF5110) respectively. PF5110 cured product has better thermal performance than PA cured product, and has a higher glass transition temperature and thermal decomposition temperature. The 5% weight loss temperature of the DGEBA/PF5110 system is 384C, and the carbon residue rate at 450℃ is 37.57%; the 5% weight loss temperature of the TFMBP/PF5110 system is 363℃, and the carbon residue rate at 450℃ is 37.84%.
6 Aromatic heterocyclic and alicyclic modification
By introducing groups such as naphthalene ring and alicyclic ring into the main chain of epoxy resin, the resulting "structural hybrid" (structural hybrid) epoxy resin has good heat resistance and humidity resistance, and can be used for laminated circuit boards and large-scale integration Circuit potting material. Table 1 shows the thermal properties of dicyandiamide (DICY) and bisphenol A formaldehyde phenolic resin (PN) cured with naphthalene ring and cycloaliphatic functional group modified epoxy resin. Sample 3 only introduces 1-naphthol, sample 4 introduces 1-naphthol and 2-naphthol in a certain proportion (the mass fraction of 1-naphthol is 80% and 60% respectively). Thermal analysis shows that the Tg of epoxy resin cured with DICY increases linearly with the increase of the mass fraction of 2-theophyl. For example, sample 4 has the smallest molecular mass, but the Tg is the highest. This may be related to the steric hindrance of the alicyclic or methylene groups on 2-naphthol and 1-naphthol. The PN curing system of sample 3 has a higher coefficient of thermal expansion (CTE) than the DICY system, which is probably due to the higher crosslinking density of the former, and the DICY system has much lower water absorption than the PN system. Compared with thermosetting phenyl compounds, this system shows better water resistance. Due to the small polarity of the naphthalene ring and alicyclic ring in the main chain, the electrical insulation performance of the NL epoxy resin system is significantly better than that of the DGE-BA epoxy resin system. 2-Naphthol has a slight effect on the thermal stability of epoxy cured products. The residual carbon rate of NL resin at 600°C is much higher than that of DGEBA, and the residual carbon rate in air towels is higher than that in nitrogen. This result shows that the thermosetting substances obtained from aromatic epoxy resins have different decomposition mechanisms under different conditions. It may be that the oxides generated in the presence of oxygen act as physical barriers and delay the decomposition of polymers.