Polyurethane curing agents typically refer to the components containing isocyanate groups (NCO) in two-component polyurethane products, mainly used in combination with active hydrogen. Common components with active hydrogen include polyether polyols, polyester polyols, epoxy resins, hydroxypropyl acrylic resin, etc. In these products, the addition of curing agents not only increases the crosslinking and cohesion of the products but also enhances the strength, weather resistance, and other properties of the final products. Currently, polyurethane curing agents are widely used in adhesives, coatings, inks, and other fields.
Generally, polyurethane curing agents can be categorized into three main types: solvent-based, water-dispersible, and blocked polyisocyanates.
Solvent-based curing agents are the most commonly used in applications, offering enhanced strength, weather resistance, and mechanical properties to the cured materials. They consist of polyisocyanates and solvents. The isocyanates within solvent-based curing agents react with components containing active hydrogen, such as polyols and resins, to form a durable and crosslinked polyurethane network.
The inclusion of solvents in this type of curing agent facilitates the application and processing of the mixture, ensuring uniform coverage and penetration into substrates. Moreover, solvent-based systems help improve various material properties, including adhesion, flexibility, and resistance to environmental factors.
Solvents used in polyurethane curing agents include acetic ethyl ketone, butyl acetate, propylene glycol monomethyl ether acetate, acetone, toluene, mixed xylene, dichloromethane, and others. The choice of solvent depends on several factors, including chemical compatibility with other components, volatility affecting drying time, and viscosity.
Solvents play a crucial role in curing agents, especially in solvent-based formulations. In general, the main purpose of using solvents is to reduce viscosity for easier application during construction. In addition, solvents help dissolve and disperse isocyanates, promoting their effective reaction with hydroxyl-containing compounds such as polyols and resins. The addition of solvents also enhances the properties of cured polyurethane products, including improved adhesion and flexibility.
With the growing emphasis on environmental awareness, waterborne polyurethane materials have gained increasing attention. In two-component waterborne polyurethane products, the isocyanate component plays a crucial role. A common practice involves the hydrophilic modification of isocyanates to achieve dispersion in water. The primary methods of modification include non-ionic modification and anionic modification.
Non-ionic modification involves using hydrophilic substances such as polyethylene glycol monomethyl ether (MPEG) and polyethylene glycol (PEG) to chemically modify polyisocyanates. The resulting modified polyurethane curing agent not only exhibits a certain degree of hydrophilicity but also encapsulates the residual isocyanate groups (NCO groups), allowing them to remain stable in water for a period of time.
This modification helps in the preparation of waterborne polyurethane products by ensuring the uniform dispersion of polyisocyanates in water and maintaining stability over a specified duration.
Hydrophilic polyisocyanates can also be obtained through ionic modification. Common ionic modification methods include carboxylate modification and sulfonate modification.
Carboxylate modification typically involves the use of dimethylolpropionic acid (DMPA), while sulfonate modification generally utilizes ethylenediamine-based sulfonic acid salts and amino sulfonic acid salts. These compounds introduce carboxyl or sulfonic acid groups onto the molecular chain, enabling the polyurethane curing agent to emulsify in water.
The NCO groups in polyurethane curing agent components are highly reactive and can easily react with atmospheric moisture at room temperature, leading to degradation. To improve storage stability and reduce raw material loss, a blocked curing agent can be produced by reacting a blocking agent with the NCO groups, resulting in a stable curing agent at room temperature.
Upon heating, the blocked curing agent regenerates its original isocyanate structure and reacts with components containing hydroxyl groups. This formulation strategy typically involves introducing a blocking agent into solvent-based or water-dispersible polyurethane curing agents, effectively capping excess NCO groups.
Different blocking agents have varying deblocking (unblocking) temperatures, allowing for customized selection according to specific application requirements. Common blocking agents and their corresponding unblocking temperatures are shown in the table below.
| Blocking Agent | Unblocking Temperature/℃ |
| Caprolactam | 175-200 |
| Ethanol | 180-185 |
| Methyl ethyl ketoxime | 140-150 |
| Phenol | 140-145 |
| Acetoxime | 130-140 |
| Ethyl acetoacetate | 125-150 |
Blocked polyisocyanate curing agents are widely used in can and coil coatings. Upon heating to the activation temperature, curing takes place, forming a durable and stable network structure that exhibits enhanced toughness, wear resistance, and scratch resistance.
