The chain extension of anionic prepolymers in the preparation of aqueous poly(urethane-urea) dispersions

Anionic polyurethane prepolymers end-capped with isocyanate groups were dispersed and chain-extended in aqueous media using three different extension agents: hydrazine, 1,2-ethylene diamine (EDA) and 1,2-propylene diamine (PDA). Two types of prepolymer were used. The first was prepared from isophorone diisocyanate (IPDI), alpha,alpha-dimethylol propionic acid (DMPA) and poly(propylene oxide) diol (PPO) and the second from alpha,alpha,alpha',alpha'-tetramethyl-1,3-xylylene diisocyanate (m-TMXDI), poly(caprolactone) diol (PCL) and DMPA. The colloidal particles which formed in the dispersion process and the constituent poly(urethane-urea) chains were characterised by a combination of dynamic and static light scattering, gel permeation chromatography and FTIR spectroscopy. Using EDA as the extender, a study was made of how the degree of extension depended on the molar ratio of amine to isocyanate groups, [NH2]/[NCO] (=R-A,R-I). It was found that using a stoichiometric balance of isocyanate and amine groups did not lead to high degree of extension, and better chain extension was obtained at lower R-A,R-I values. In a comparative study using stoichiometric balances of isocyanate and amine groups, the degrees of extension obtained using PDA and EDA were approximately the same, while hydrazine was the least effective. Force-extension studies were carried out on samples prepared from films cast from the aqueous poly(urethane-urea) dispersions in order to assess the influence of chain-extender type and stoichiometry on bulk properties; values of Young's modulus, tensile strength and maximum extension are reported. (C) 2003 Society of Chemical Industry.