Types and properties of curing agents for epoxy resin

According to the multiple classification, the curing agents of epoxy resin can be divided into two types: apparent curing agent and latent curing agent Apparent curing agent is a common curing agent, which can be divided into addition polymerization type and catalytic type The so-called addition polymerization type is to open the ring of epoxy group for addition polymerization, and the curing agent itself participates in the three-dimensional network structure If the amount of such curing agent is too small, the curing product is connected with the epoxy group of the final reaction Therefore, there is a suitable dosage for this kind of curing agent However, the catalytic curing agent can polymerize epoxy group by cationic or anionic way Finally, the curing agent does not participate in the network structure, so there is no suitable amount of equivalent reaction; however, increasing the amount will accelerate the curing speed Addition polymerization curing agents include polyamines, anhydrides, polyphenols, polysulfides, etc Among them, the most important and widely used are polyamines and anhydrides, which account for 71% of all curing agents and 23% of anhydrides From the application point of view, most of the polyamines have been modified, while the anhydrides are mostly used in the original state, or two or three kinds of low-temperature CO melting The latent curing agent refers to: after mixing with epoxy resin, the curing agent is relatively stable for a long time at room temperature (generally more than 3 months is required, so it has great practical value, and the most ideal is more than half a year or 1 year) However, the curing reaction only needs to be exposed to heat, light, moisture and other conditions This kind of curing agent basically uses physical and chemical methods to block the activity of curing agent Among the apparent curing agents, dicyandiamide and adipic acid diacylhydrazide are insoluble in epoxy resin at room temperature, but start curing reaction after dissolving at high temperature, so they also show a latent state Therefore, in some books, these varieties are also classified as latent curing agents, which can be called functional latent curing agents Because latent curing agent can be mixed with epoxy resin to make one liquid complex, which simplifies the coordination procedures of epoxy resin application Its application range is from single package adhesive to coating, impregnating paint, potting material, powder coating, etc Latent curing agent has been paid more and more attention in foreign countries It can be said that it is a key subject of research and development The curing temperature of various curing agents is different, and the heat resistance of the cured products is also very different Generally speaking, the curing agent with high curing temperature can be used to obtain the cured product with excellent heat resistance For addition polymerization curing agent, the curing temperature and heat resistance are improved in the following order: aliphatic polyamines < alicyclic polyamines < aromatic polyamines ≈ phenolic < anhydrides; the heat resistance of catalytic addition polymerization curing agent is generally at the level of aromatic polyamines The heat resistance of anionic polymerization (tertiary amines and imidazolized antiques) and cationic polymerization (BF3 complex) are basically the same, which is mainly because although the initial reaction mechanism is different, the network structure of ether bond is formed eventually The curing reaction belongs to the chemical reaction, which is greatly influenced by the curing temperature, the temperature is increased, the reaction speed is accelerated, the gel time is shorter, the gelation time value generally decreases linearly with the curing temperature rising, as shown in Figure 3-2 However, if the curing temperature is too high, the performance of the cured product will be reduced, so there is an upper limit of the curing temperature; the temperature that can compromise the curing speed and the performance of the cured product must be selected as the appropriate curing temperature According to the curing temperature, the curing agents can be divided into four categories: the curing temperature of low temperature curing agent is below room temperature; the curing temperature of room temperature curing agent is between room temperature and 50 ℃; the curing temperature of medium temperature curing agent is between 50 ℃ and 100 ℃; the curing temperature of high temperature curing agent is above 100 ℃ In recent years, T-31 modified amines and yh-82 modified amines developed and put into production in China can be cured below 0 ℃ There are many kinds of room temperature curing: aliphatic polyamines, alicyclic polyamines, low molecular polyamides and modified aromatic amines Some aliphatic polyamines, tertiary amines, mimidazoles and boron trifluoride complexes belong to the medium temperature curing type The high temperature curing agents include aromatic polyamines, anhydrides, phenolic resins, amino resins, dicyandiamide and hydrazide For high temperature curing system, the curing temperature is generally divided into two stages The gel is solidified at low temperature before reaching the gel state or slightly higher than the gel state Then the post-cure is solidified at high temperature and solidified as pre curing (pre-cure)