[Resin Materials] Summary of epoxy resin curing agent types, curing process, structure and properties, types and uses, and curing mechanism

[Resin Materials] Summary of epoxy resin curing agent types, curing process, structure and properties, types and uses, and curing mechanism

Alkaline curing agents: WTF — includes aliphatic diamines and polyamines, aromatic polyamines, other nitrogen-containing compounds, and modified aliphatic amines.

Acidic curing agents: include organic acids, acid anhydrides, and boron trifluoride and its complexes.

Addition-type curing agents: these react with epoxy groups in an addition reaction and become part of the chain segments of the cured product, crosslinking linear molecules into a three-dimensional network through step-growth polymerization. They're also called polyaddition curing agents.

Catalytic curing agents: these only initiate the epoxy resin. After opening the epoxy groups, they catalyze the resin itself to polymerize into a network structure, forming a homopolymer mainly based on ether bonds.

Latent vs. non-latent: Non-latent curing agents are the ordinary ones, and they split into addition-polymerization and catalytic types. With addition-polymerization types, the curing agent opens the epoxy ring and joins the three-dimensional network itself. If you add too little, the cured product ends up with unreacted epoxy groups hanging off it. So there's a proper dosage for these. Catalytic types open the epoxy ring and polymerize it via a cationic or anionic mechanism, and the curing agent doesn't end up in the network, so there's no exact equivalent dosage — but more of it speeds up curing. Among non-latent curing agents, dicyandiamide and adipic acid dihydrazide don't dissolve in epoxy resin at room temperature; they dissolve and start curing at high temperature, so they behave in a latent way. You could call them functional latent curing agents.

Latent curing agents: once mixed with epoxy resin, they stay relatively stable at room temperature for a long time (usually 3 months or more for practical value; ideally half a year to a year or more), and only start curing when exposed to heat, light, moisture, etc. These basically block the curing agent's activity by physical or chemical means. Some books classify these as latent curing agents, but really they're functional latent curing agents. Because latent curing agents can be mixed with epoxy resin into a one-component system, they simplify the mixing steps, and their use has spread from single-pack adhesives to coatings, impregnating varnishes, potting compounds, powder coatings, and more. They're getting more attention abroad and are a key R&D topic — new modified types and new mixing techniques keep coming out, very active.

Amine curing agents: Primary and secondary amines cure epoxy resin when the active hydrogen on the nitrogen opens the epoxy group and crosslinks it. Aliphatic polyamines like ethylenediamine, hexamethylenediamine, diethylenetriamine, triethylenetetramine, and diethylaminopropylamine are quite active and can cure epoxy at room temperature. Aromatic polyamines are less active — m-phenylenediamine, for example, needs 150°C to cure fully.

Anhydride curing agents: Dibasic acids and their anhydrides, like maleic anhydride and phthalic anhydride, can cure epoxy resin, but they need baking at higher temperature to cure fully. The anhydride first reacts with hydroxyl groups in the epoxy resin to form a monoester, and the carboxyl group in the monoester adds to the epoxy group to form a diester.