In order to improve the water resistance, solvent resistance, coating hardness, substrate adhesion, aging resistance and other properties of waterborne polyurethane, waterborne polyurethane can be crosslinked modification, so in waterborne polyurethane, what groups can participate in the crosslinking reaction?
At present, most waterborne polyurethane on the market is carboxylic waterborne polyurethane. Taking carboxylic waterborne polyurethane as an example, theoretically, conventional carboxylic waterborne polyurethane dispersions contain only the following groups: carboxyl, carbamate, urethane, ether bond or ester group.

Among these groups, carboxyl group, carbamate group and urea group have crosslinking activities. However, only carboxyl group is the best choice for normal temperature crosslinking. Therefore, most normal temperature crosslinking systems are developed for carboxyl group.
In addition, the end group in polyurethane dispersion is an important group that is often ignored. Although the end group content is small, it can also play a role in the crosslinking system.The end groups of aqueous polyurethane dispersions are complex, usually including amino-nh2, amino-urea-nh-co-nh-nh2 and hydroxy-oh.
Amino group is the main end group form of aqueous polyurethane dispersion. It can be formed by the reaction of the end isocyanate with water or the residue of the reaction of the end isocyanate with a diamine.
When hydrazine is used as chain extender, the end group of isocyanate reacts with one hydrazine to form carbamide.
When the polyurethane prepolymer is synthesized with excessive hydroxyl group, the end group is hydroxyl group.Amino, carbamide and hydroxyl groups are excellent crosslinking functional groups at room temperature. They can react with most developed crosslinking agents at room temperature. Therefore, the crosslinking of end groups is an important factor in the system of crosslinking at room temperature.

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