Chemistry of Water-based Polyurethane Dispersions

Q: Chemistry of Water-based Polyurethane Dispersions

A: Water based polyurethane dispersions have urethane polymer particles dispersing in water, and “self-dispersing ionomeric PU prepolymers” is one of the most common techniques. In this process, Dimethylolpropionic acid (DMPA) is a moiety that is often used to react into a PU prepolymer for synthesizing ionomeric prepolymer for water based polyurethane dispersions.

The chemistry of forming water based polyurethane dispersions is reacting stoichiometric excess of isocyanate with hydroxy groups and a moiety. Depending on the final application and requirements of water based polyurethane, structures of polyol and isocyanate in PUD prepolymers can be very varied. For example, typical water based polyurethane dispersions are isocyanate terminated; however, hydroxyl-terminated prepolymers are used to formulate two-component water based polyurethane.

Before the water-dispersing process, the carboxylic acid groups in the prepolymer have to be neutralized with a strong base to convert the acid to the carboxylate, which makes the urethane particles to disperse and stabilize in water. After base neutralization, the aqueous dispersion process must follow immediately. During the aqueous dispersion process, chain extender, such as primary or secondary polyamines, is added to the water based polyurethane dispersions to converse the remaining isocyanate groups to the final polymer within the particle. The reason of using primary or secondary polyamines as chain extender is because they have higher reactivity toward isocyanates than water. After all the reactions are completed, ionomeric prepolymer and residual monomeric isocyanate are formed as the final products of the reaction.

There has been exponential growth in the study of the formulation and application of water based polyurethane dispersions (PUDs). The driven force of innovation and adoption of water based polyurethane technology is the environmental and occupational safety and health regulations to reduce exposure to organic solvents. Today, water based polyurethane coatings and adhesives are widely stably used on products for footwear, leathers, textile fibers, fiber glass, automobiles, floor polishes…etc. Especially in the well developed countries, such as countries in North America and European Union, the use of solvent-based coatings and adhesives are phasing out. Solvent-based products are replaced by water based polyurethane, which are solvent free, harmless to human’ s health and eco-friendly, but keep all the properties of the solvent based polyurethane systems.