Water Dispersions of Polyurethane Elastomers

Due to the absence of dispersants used in their formation and due to their ionomeric character, these systems show extremely good film-forming properties, which in many cases equal the film-forming properties of organic solvent solutions. Even dispersions of especially hard polyurethanes, which are useful for the preparation of materials such as airdrying lacquers, show these properties. 

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Special methods are necessary for the production of polyurethane dispersions because of the thermodynamically unstable nature of these two-phase systems. A simple application of the emulsion polymerization techniques for isocyanate polyaddition reactions is not possible, due to the reactivity of the NCO group with water. The extreme water sensitivity of all polyurethane preparation procedures which calls for the complete absence of water is obviously a major problem to be overcome in their preparation. It is surprising, given the basic hydrolytic degradability of polyurethane elastomers, to find that polyurethane latex has good long-term stability when it is a two-phase system. This is in contrast to the situation that applies when it is present in a one-phase system. 

SPECIAL FEATURES OF WATER-DISPERSED POLYURETHANE ELASTOMERS... 

Fig. 10.2. Summary of steps that occur in the formation of a water-dispersed polyurethane elastomer.

The preparation of polyurethane aqueous dispersions can be accomplished in several ways. The procedure claimed to yield at present the highest quality products involves the reaction of an isocyanate-terminated prepolymer in a water-miscible solvent, e.g., acetone, with a diamine carboxylate or sulfonate. The resultant polyurethane ionomer solution is mixed with water which forms the dispersion and further chain-extends the polymer. Finally, the organic solvent is removed by distillation and can be recycled. As in the case of the thermoplastic polyurethane elastomer adhesives, dispersions can also be prepared with different degrees of crystallinity in the polyurethanes depending on the nature of the prepolymer. 

Heath, R., Rungvichaniwat, A. (2002). The examination of the structure property relationships of some water-dispersed polyurethane elastomers. Progress in Rubber and Recycling Technology, 18, 1, 1-48. 

In general, they consist of a pressure sensitive adhesive layer which is composed of a so-called hydrocolloid dispersed with the aid of a tackifier in an elastomer and secondly a film coating composed of a gas permeable but water impermeable, flexible, elastomeric material. A currently available hydrocolloid dressing is a flexible mass with an adherent inner face and an outer semipermeable polyurethane foam. 

Aqueous dispersions Emulsion polymerization is used to prepare this type of adhesives, mostly made of acrylics, chloroprene, and polyvinyl acetate. High shear dispersion can also be used in the presence of surfactants to obtain water home formulations, as with polyurethanes, epoxies, silicones, and some elastomers. 

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