Presumably everyone is familiar with epoxy resin curing agents. What are the classifications of epoxy resin curing agents? And its curing process, let's take a look at it together.
Anhydride curing agent: dibasic acid and its anhydride such as maleic anhydride and phthalic anhydride can cure epoxy resin, but it needs to 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.
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.
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.
Types of curing agent
Alkaline type: alkaline curing agent WTF: including aliphatic diamines and polyamines, aromatic polyamines, other nitrogen-containing compounds and modified fatty amines.
Acidic: acidic curing agents: including organic acids, anhydrides, and boron trifluoride and their complexes.
Addition molding: Addition curing agent: This kind 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 into body structure molecules. This type of curing agent is also called melon Type curing agent.
Catalytic type: Catalytic curing agent: This type of curing agent only initiates the epoxy resin. After the epoxy group is opened, the epoxy resin itself is catalyzed to polymerize into a network structure to form a homopolymer with an ether bond as the main structure.
Explicit type: Explicit type curing agent is 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. Catalytic curing agents use ring-opening addition polymerization of cationic or anionic methods. The curing agent does not participate in the network structure, so there is no suitable amount of equivalent reaction; however, increasing the amount will cause curing Speed up. 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.
Latent type: Latent curing agent refers to relatively long-term stability at room temperature after being mixed with epoxy resin (epoxy resin generally requires more than 3 months to have greater practical value, ideally requires half a year or 1 Years or more), and only need to be exposed to heat, light, moisture and other conditions, the curing reaction will begin. This type of curing agent basically uses physical and chemical methods to block the curing agent activity. 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. Latent curing agents are attracting more and more attention abroad. It can be said that they are the subject of research and development. Various varieties of curing agent modification and new technologies are emerging endlessly and are very active.
Amine curing agent: The curing effect of primary amine and secondary amine on epoxy resin 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 cross-link and cure epoxy resins at room temperature; aromatic polyamines are more active Low, such as m-phenylenediamine, must be cured at 150 ℃ to complete.
Three stages of epoxy resin curing
The operating time (also working time or service life) is part of the curing time. After mixing, the resin/curing agent mixture is still liquid and works and is suitable for use. 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 can not be chemically linked with it, because the epoxy surface needs to be properly pretreated, such as sanding, to achieve good bonding mechanical strength.
Various curing agents with different uses
Curing agents can be divided into room temperature curing agents and heating curing agents according to their uses. As mentioned above, epoxy resins generally have good performance when cured at high temperatures, but the coatings and adhesives used in civil constructions require heating at room temperature due to the difficulty of heating; 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.
The medium-temperature curing agent and high-temperature curing agent 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 is polyamine and anhydride. Since the cured anhydride has excellent electrical properties, it is widely used in electronics and electrical appliances.
The 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.