Aqueous polyurethane dispersions: A comparative study of polymerization processes

Restrictions on volatile organic compounds have forced the producers of solvent‐based polymers to develop safer products. Water dispersions of polyurethanes are successfully replacing solvent‐borne products in the coating industry, but relatively little systematic work has been reported that compares polyurethanes obtained by different processes. In this study, polyurethanes were produced with three polymerization processes: in a solution, in a dispersion with the acetone process, and in a dispersion with the prepolymer mixing process. The same prepolymer was synthesized in all cases, and the reaction was concluded under the conditions required by the three different methods. In the first phase, nonextended polyurethanes were prepared with different functional group ratios (NCO/OH), and an evaluation of the quality of the dispersions and solutions and of the morphology of the dry polymers was made by a comparison of the products from the three preparation routes. Second, the growth of the molecular weight in water was studied for the prepolymer mixing process. NCO‐terminated prepolymers were dispersed in water and then extended by aliphatic diamines. The extender was added in different percentages that referred to the NCO equivalents of the prepolymer. This led to an evaluation of the effect of the dispersing medium on chain extension.