Chain extension study of aqueous polyurethane dispersions

In the several decades, aqueous polyurethane dispersion has been investigat ed by many researchers. However, relatively little systematic work has been reported in detail on chain extension process. This work describes the rea ction of chain extension step (chain extended reaction) as the variation of residual NCO group and subsequent weight average molecular weight of the p olyurethane during the chain extension step. Polyurethane dispersion was pr epared by neutralization emulsification method. The dispersions with prepar ed different average particle sizes were obtained by varying the degree of neutralization and each of them chain extended subsequently by water-solubl e chain extender: 1,6-hexanediamine. Time-dependent change of fret: and res idual NCO group was measured by FT-IR spectroscopy during the chain extensi on process. As the total surface area of polyurethane particle decreased, t he amount of residual NCO group and consequently required amount chain exte nder 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 m olecules. Additionally, it was found that excess amount of chain extender h ad an unfavorable influence on adhesive strength. (C) 2001 Elsevier Science B.V. All rights reserved.