Polyurethane curing agents are widely used in adhesives, coatings, inks and other fields

Polyurethane hardeners generally refer to isocyanate-based (NCO)-containing components in two-component polyurethane products, mainly used in combination with components containing active hydrogen. Common components containing active hydrogen are polyether polyol, Polyester Polyol, epoxy resin, hydroxyacrylic resin, etc., in these products, the addition of curing agent can not only improve the cross-linking degree and cohesion energy of the product, but also enhance the strength and weather resistance of the final product. Polyurethane curing agents are currently widely used in adhesives, coatings, inks and other fields.

Classification of polyurethane hardeners

For polyurethane curing agents, they can be classified into three categories: solvent-based, water-dispersible and closed.

Solvent-based curing agent

For most traditional products, solvent-based curing agents are more commonly used, which can give products better strength, weather resistance, mechanical properties, etc.

Solvent-based curing agent is composed of common substances

Some commonly used structural formulas of polyisocyanates

Water-dispersible curing agent

With the enhancement of people's awareness of environmental protection, waterborne polyurethane materials have gradually received attention. In two-component waterborne polyurethane products, the isocyanate component plays a crucial role. At present, the more common practice is to achieve dispersion in water by hydrophilic modification of isocyanate, and the main methods of modification are non-ionic modification, anionic modification and anion-nonionic modification.

Nonionic modification

Nonionic modification is mainly the use of polyethylene glycol monomethyl ether (MPEG), polyethylene glycol (PEG) and other polyisocyanate hydrophilic modification, the modified polyurethane curing agent not only has a certain hydrophilicity, but also the remaining NCO group is wrapped, so that it can stably survive in water for a period of time.

Nonionic modified composition of common substances

Anionic modification

Hydrophilic polyisocyanates can also be obtained by ion modification. The usual ion modification mainly includes carboxylate modification, sulfonate modification, etc., carboxylate modification generally uses dihydroxymethylpropionic acid (DMPA), sulfonate modification generally uses ethylenediamine ethanesulfonate and sulfamate, etc., they introduce carboxyl or sulfonic acid groups on the molecular chain to make polyurethane curing agent emulsified in water.

Anionic modification of common substance composition

Anonionic modification

At present, non-ionic modification is commonly used for polyethylene glycol monomethyl ether for modification, but the introduction of monomethyl ether will bring water resistance problems and crystallinity tendency, although ion modification can solve the problem of water resistance, but its pH requirements for the solution are high. The anion-non-ionic hybrid modification can better overcome the above problems.

Anionic and nonionic modified composition of common substances

Closed polyurethane hardener

The NCO group in the curing agent component has high activity, and it can react with atmospheric water at room temperature to deteriorate. In order to facilitate storage and reduce raw material loss, the blocking agent can be used to react with NCO to prepare a stable blocking curing agent at room temperature.

Blocking hardeners can be heated to restore their original isocyanate structure and react with hydroxyl-containing components. The formulation design idea is generally to introduce a sealing agent on the basis of the above common solvent-based and water-dispersible polyurethane curing agents, so as to end the excess NCO groups. The unsealing temperature of different sealing agents is different, we can also choose different sealing agents according to specific use scenarios, common sealing agents and their unsealing temperature are shown in the following table.

Commonly used sealing agents and their unsealing temperature
Sealing agent unsealing temperature/°C

Caprolactam 175-200

Ethanol 180-185

Methylethyl ketoxime 140-150

Phenol 140-145

3,5-Dimethylpyrazole 130-150

Acetone oxime 130-140

Ethyl acetoacetate 125-150

Diethyl malonate 130-140

Imidazoles 130-140

Sodium bisulfite 50-70

Application of polyurethane hardeners

With the rapid development of polyurethane, its curing agent is also produced and develops rapidly.

Polyurethane curing agent is used in adhesives, printing pastes and inks and other fields, which can improve adhesion performance and improve washing fastness; It is used as an external crosslinking agent in wood paint, coating, leather finishing agent, etc., which can maintain high gloss, improve hardness, improve water resistance, solvent resistance and other properties.

In some baking paint systems, such as coil steel paint, most of the time the closed polyurethane curing agent is selected, which is cured after heating and unsealing, and the coating has better toughness, weather resistance and scratch resistance after film formation.

In some high-quality furniture industries, such as matte wood paints, the coating of varnishes with matting agents is not transparent and has poor media resistance. The matting effect can be achieved from the molecular structure through the curing agent, and the same kind of paint, with different structures of matte curing agent, can get the gloss we need.

In some food flexible packaging fields, most of them use some solvent-based aromatic polyurethane curing agents, such as: TDI-TMP adducts. In recent years, for food safety considerations, researchers are working on the development of low free NCO curing agents and alternative aliphatic polyurethane curing agents.