Chain extension study of aqueous polyurethane dispersions

In the several decades, aqueous polyurethane dispersion has been investigated by many researchers. However, relatively little systematic work has been reported in detail on chain extension process. This work describes the reaction of chain extension step (chain extended reaction) as the variation of residual NCO group and subsequent weight average molecular weight of the polyurethane during the chain extension step. Polyurethane dispersion was prepared by neutralization emulsification method. The dispersions with prepared different average particle sizes were obtained by varying the degree of neutralization and each of them chain extended subsequently by water-soluble chain extender; 1, 6-hexanediamine. Time-dependent change of free and residual NCO group was measured by FT-IR spectroscopy during the chain extension process. As the total surface area of polyurethane particle decreased, the amount of residual NCO group and consequently required amount chain extender for optimum chain extension decreased. According to the GPC and FT-IR data, optimum amount of 1, 6-hexanediamine was determined by the location of the NCO group and contact area between the residual NCO groups and water molecules. Additionally, it was found that excess amount of chain extender had an unfavorable influence on adhesive strength.