Epoxy curing agent categories and performance

According to multiple classifications, epoxy curing agents can be classified into: visible curing agents and latent curing agents.

The apparent curing agent is a commonly used curing agent, which can be further divided into an addition polymerization type and a catalytic type. The so-called addition polymerization type, that is, the epoxy-opened ring undergoes an addition polymerization reaction, and the curing agent itself participates in the three-dimensional network structure. If such a curing agent is added in too small an amount, the cured product is linked to the unreacted epoxy group. Therefore, there is a suitable amount for such curing agents. Catalytic curing agents, on the other hand, cationically, or anionically, epoxy ring-opening addition polymerization. Ultimately, the curing agent does not participate in the network structure, so there is no appropriate amount of equivalent reaction; however, increasing the amount will Make curing faster.

Examples of addition polymerization type curing agents include polyamines, acid anhydrides, polyhydric phenols, and polythiols. The most important and most widely used are polyamines and anhydrides, polyamines account for 71% of all curing agents, and anhydrides account for 23%. From the application point of view, most of the polyamines are modified, and the anhydrides are mostly used in the original state, or mixed in two or three kinds of low-temperature comonions.

The latent curing agent means that the curing agent is mixed with an epoxy resin and is relatively stable at room temperature for a long period of time (generally required for more than 3 months, it has a greater practical value, and ideally requires six months Or more than one year), and only need to be exposed to heat, light, moisture, etc., to start the curing reaction. Such curing agents essentially block the curing agent activity physically and chemically. Among the typical curing agents, dicyandiamide, adipic acid dihydrazide and the like, which are insoluble in epoxy resin at room temperature, start to cure after dissolution at a high temperature, and thus exhibit a latent state. Therefore, in some books, these varieties are also classified as latent curing agents, which may actually be called functional latent curing agents. Because the latent curing agent can be mixed with epoxy resin to form a liquid type complex, simplifying the application procedures of epoxy resin, its application range from single packaging adhesive to paint, dipping paint, potting material, powder coating, etc. Aspect development. China's epoxy resin industry online () reporter learned from the international synthetic resin organizations, latent curing agent has attracted increasing attention in foreign countries, can be said that research and development is a key topic, a variety of new curing agent variety and new technology with endless Is very active.

The curing temperatures of various curing agents are different, and the heat resistance of cured products is also very different. In general, a cured product having excellent heat resistance can be obtained using a curing agent having a high curing temperature. For the addition polymerization type curing agent, the curing temperature and the heat resistance improve in the following order: Aliphatic polyamines <Cyclic polyamines <Aromatic polyamines phenolic < anhydride

The heat resistance of the catalytic polyaddition curing agent is generally at the aromatic polyamine level. The heat resistance of anionic polymerization (tertiary amines and imidazole antiquities) and cationic polymerization (BF3 complexes) is basically the same. This is mainly because although the initial reaction mechanism is different, they eventually form an ether-bonded network. structure.

The curing reaction is a chemical reaction, which is greatly influenced by the curing temperature. The temperature increases, the reaction speed increases, and the gelation time becomes shorter. The logarithm of the gelation time tends to decrease linearly with the increase of the curing temperature, as shown in Figure 3-2. However, if the curing temperature is too high, the properties of the cured product are often degraded, so there is an upper limit of the curing temperature; the temperature at which the curing rate and the properties of the cured product are compromised must be selected as an appropriate curing temperature. The curing agent can be divided into four categories according to the curing temperature: the curing temperature of the low-temperature curing agent is lower than room temperature; the curing temperature of the room-temperature curing agent is room temperature to 50°C; the curing temperature of the medium-temperature curing agent is 50 to 100°C; and the curing temperature of the high-temperature curing agent is 100°C or higher . There are few types of curing agents that are low temperature curing type, such as polyfurfuryl alcohol type, polyisocyanate type, etc.; in recent years, T has been developed and put into production in China.
-31 modified amine, YH-82 modified amine can be cured below 0 °C. There are many types of room-temperature curing type: aliphatic polyamines, alicyclic polyamines, low-molecular polyamides, and modified aromatic amines. There are alicyclic polyamines, tertiary amines, carbazoles, boron trifluoride complexes, and the like, which are intermediate temperature curable types. Examples of high-temperature curing agents include aromatic polyamines, acid anhydrides, resole resins, amino resins, dicyandiamide, and hydrazide.

For the high-temperature curing system, the curing temperature is generally divided into two stages. The low-temperature curing is performed before the gel. After reaching a gel state or slightly higher than the gel state, the high-temperature heating is used for post-cure. The previous stage cures to pre-cure.

Source: 21st Century Fine Chemicals Network