According to the multiple classification method, the epoxy resin curing agent can be divided into: obvious curing agent and latent curing agent.
Explicit curing agent is a commonly used curing agent, and can be divided into addition polymerization type and catalytic type. The so-called addition polymerization type means that the ring of the epoxy group is opened for the addition polymerization reaction, and the curing agent itself participates in the three-dimensional network structure. If this kind of curing agent is added in too small amount, the cured product is connected to the unreacted epoxy group. Therefore, for this type of curing agent, there is a suitable amount. The catalytic curing agent uses cationic or anionic methods to ring-open addition polymerization of epoxy groups. 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 Make the curing speed faster.
Additional polymerization curing agents include polyamines, acid anhydrides, polyphenols, and polythiols. Among them, polyamines and anhydrides are important and widely used. Polyamines account for 71% of all curing agents, and anhydrides account for 23%. From the perspective of application, most of the polyamines have been modified, and the anhydrides are mostly used in their original state, or two or three low-temperature eutectic blends.
The latent curing agent refers to this kind of curing agent mixed with epoxy resin, which is relatively long-term stable at room temperature (generally requires more than 3 months to have greater practical value, ideally requires half a year or 1 year or more), and only need to be exposed to heat, light, moisture and other conditions, the curing reaction will start. This type of curing agent basically uses physical and chemical methods to block the curing agent activity. Among the obvious curing agents, varieties such as dicyandiamide and adipic acid dihydrazide are insoluble in epoxy resins at room temperature, but start to cure after dissolving at high temperatures, so they also show a latent state. Therefore, in some books, these varieties are also classified as latent curing agents, which can actually be called functional latent curing agents. Because the latent curing agent can be mixed with epoxy resin to make a one-component complex, simplifying the application procedures of epoxy resin, its application range from single packaging adhesive to coating, dipping paint, potting material, powder coating, etc. Development.
The curing temperature of each curing agent is different, and the heat resistance of the cured product is also very different. Generally speaking, using a curing agent with a high curing temperature can obtain a cured product excellent in heat resistance. For the addition polymerization type curing agent, the curing temperature and heat resistance are improved in the following order: aliphatic polyamine <alicyclic polyamine <aromatic polyamine ≈phenolic <acid anhydride<catalyzed addition polymerization type curing agent The heat resistance is generally in Aromatic polyamine levels. The heat resistance of anionic polymerized (tertiary amine and imidazolium antiquities) and cationic polymerized (BF3 complex) is basically the same. This is mainly because although the initial reaction mechanism is different, they eventually form a network of ether bonds. structure.
The curing reaction is a chemical reaction. It is greatly affected by the curing temperature. The temperature increases, the reaction speed is accelerated, and the gel time becomes shorter. The logarithm of the gel time generally decreases linearly with the increase of the curing temperature, but the curing temperature is too high. The performance of the cured product decreases, so there is an upper limit of the curing temperature; a temperature that compromises 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 agent can be divided into four categories: the curing temperature of the low-temperature curing agent is below room temperature; the curing temperature of the room temperature curing agent is room temperature ~ 50 ℃; the curing temperature of the intermediate temperature curing agent is 50 ~ 100 ℃; the curing temperature of the high temperature curing agent is above 100 ℃ . There are few types of curing agents that belong to low-temperature curing types, such as poly-alcohol type and polyisocyanate type; there are many types of room-temperature curing types: aliphatic polyamines, alicyclic polyamines; low molecular weight polyamides and modified aromatic amines Wait. Some of the middle-temperature curing types include cycloaliphatic polyamines, tertiary amines, azoles, and boron trifluoride complexes. High-temperature curing agents include aromatic polyamines, acid anhydrides, resol resins, amino resins, dicyandiamide, and hydrazides.
For high-temperature curing systems, the curing temperature is generally divided into two stages. Low-temperature curing is used before the gel. After reaching the gel state or a state slightly higher than the gel state, the high-temperature heating is used for post-curing. Curing.