Epoxy resin curing agent is a chemical reaction with epoxy resin to form a net-shaped three-dimensional polymer, and the composite material is encased in the net-shaped body. Additives that make linear resins tough and solid.
First, the type of curing agent
1. Alkaline type
Alkaline curing agent WTF: including aliphatic diamines and polyamines, aromatic polyamines, other nitrogen-containing compounds and modified fatty amines.
Acidic curing agents: including organic acids, anhydrides, and boron trifluoride and their complexes.
3. Additive molding
Additive curing agent: This type of curing agent and epoxy group undergo an addition reaction to form a part of the cured product segment, and through the stepwise polymerization reaction, linear molecules are cross-linked to form structural molecules. This type of curing agent is also called melon-type curing agent. .
4. Catalytic type
Catalyzed curing agent: This type of curing agent only initiates epoxy resin. After opening epoxy group, it catalyzes the polymerization of epoxy resin itself into a network structure, producing a homopolymer with ether bond as the main structure.
5. Explicit type
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. 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, it can be called a functional latent curing agent.
6. Amine curing agent
The curing effect of primary and secondary amines on epoxy resins is that the active hydrogen on the nitrogen atom opens the epoxy group, which makes it cross-linking and curing. Aliphatic polyamines such as ethylenediamine, hexamethylenediamine, diethylenetriamine, triethylenetetramine, diethylaminopropylamine, etc., have greater activity and can crosslink and cure epoxy resins at room temperature; while aromatic polyamines are more active Low, such as m-phenylenediamine, must be cured at 150 ℃ to complete.
7. Anhydride curing agent
Dibasic acids and their anhydrides such as maleic anhydride and phthalic anhydride can cure epoxy resins, but they must be baked at a higher temperature to cure completely. The acid anhydride first reacts with the hydroxyl group in the epoxy resin to form a monoester, and the carboxyl group and the epoxy group in the monoester undergo addition esterification to form a diester.
8. Synthetic resin curing agent
Low molecular weight polyamide resin is amber viscous resin produced by the reaction of linoleic acid dimer or tungoleic acid dimer with aliphatic polyamine such as ethylenediamine and diethylenetriamine.
9. Latent curing agent
This curing agent is stable under normal conditions, but when heated to a certain temperature, it shows its activity and cures the epoxy resin. Such as dicyandiamide, mixed with epoxy resin, it is stable at room temperature. If it is 145-165°C, the epoxy resin can be cured within 30 minutes. Boron trinitride ethylamine complex is stable at room temperature and can cure epoxy resins at temperatures above 100°C.
Second, the three stages of epoxy resin curing
Operation time (also working time or service life) is part of the curing time. After mixing, the resin/curing agent mixture is still liquid and can work and is suitable for application. In order to ensure reliable bonding, all construction and positioning work should be done within the curing operation time.
The mixture begins to enter the solidified phase (also known as the curing stage), at which point it begins to gel or "mutate." The epoxy at this time has no possibility of long-term work and will also lose its tack. At this stage, there must be no interference. It will become a soft gel like hard rubber, you can press it with your thumb.
Because the mixture is only partially cured at this time, the newly used epoxy resin can still be chemically linked to it, so the untreated surface can still be bonded or reacted. In any case, these capabilities of the near-cured mixture are decreasing.
The epoxy mixture reaches the stage of solidification and solidification, at which time it can be sanded and shaped. At this time, you can't press it with your thumb. At this time, the epoxy resin has about 90% of the final reaction strength, so you can remove the fixing clip and leave it at room temperature to maintain a few angels. It continues to cure.
At this time, the newly used epoxy resin cannot be chemically linked with it, because the epoxy surface must be properly pretreated, such as sanding, to obtain good bonding mechanical strength.
3. Curing temperature of curing agent and heat resistance of cured product
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 increase in the following order: aliphatic polyamine<alicyclic polyamine<aromatic polyamine≈phenolic formaldehyde<anhydride.
The heat resistance of the catalytic addition polymerization type curing agent is generally at the level of aromatic polyamines. 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. However, if the curing temperature is too high, the performance of the cured product is often reduced, so there is an upper limit for 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;
T-31 modified amine and YH-82 modified amine developed and put into production in China in recent years can be cured below 0℃. There are many types of room temperature curing types: aliphatic polyamines, alicyclic polyamines; low molecular polyamides and modified aromatic amines. 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 gelation. After reaching a gel state or a state slightly higher than the gel state, post-cure is performed by heating at high temperature. The previous stage curing is pre-cure.
Fourth, various curing agents with different uses
curing agent can be divided into room temperature curing agent and heating curing agent according to the application. As mentioned above, epoxy resins generally have good performance when cured at high temperature, but the coatings and adhesives used in civil constructions are difficult to heat and need to be cured at room temperature; therefore, most of them use fatty amines, alicyclics, and polyamides. In particular, paints and adhesives used in winter have to be used in combination with polyisocyanate, or use polyalcohols with a foul odor.
As for the medium-temperature curing agent and the high-temperature curing agent, it should be selected based on the heat resistance of the object and the heat resistance, adhesiveness and chemical resistance of the cured product. The choice focuses on polyamines and anhydrides. Since the cured anhydride has excellent electrical properties, it is widely used in electronics and electrical appliances.
Excellent aliphatic polyamine cured product has excellent adhesion, alkali resistance and water resistance. Aromatic polyamines are also excellent in chemical resistance. Since the nitrogen element of the amino group forms a hydrogen bond with the metal, it has an excellent rust prevention effect. The higher the amine mass concentration, the better the anti-rust effect. The anhydride curing agent forms an ester bond with the epoxy resin, shows high resistance to organic acids and inorganic acids, and the electrical properties generally exceed polyamines.