Research on the post-chain extension of aqueous polyurethane dispersions

The post-chain extension efficiency of aqueous polyurethane dispersions (APUDs) was in inverse proportion to the amount of residual amine (NH 2 ) groups, which could be measured by the color reaction with ninhydrin. In this research, the absorbance of chromogenic groups which appeared at 550-570 nm was investigated using ultraviolet-visible spectrophotometry. The results showed that there existed a higher chain extension efficiency at both high and low dispersion temperatures when water was used as the chain extender. The post-chain extension efficiency was affected by the "cage effect" of the inner particles of the APUDs. When ethylene diamine was used as the chain extender, the chain extension efficiency of APUDs increased with the decreasing of the particle size at low temperature, whereas the chain extension efficiency increased with the increase of particle size at the higher temperature. This unexpected phenomenon may be caused by the interaction of the "cage effect" and the interface reaction. The post-chain extension efficiency of APUDs were influenced by the particle size and the "cage effect" whatever the water or polyamine as the chain extender. The post-chain reaction mechanism: (a) small particle size; (b) big particle size.