A simple method for determining water‐induced gelling times of isocyanate prepolymers

A spectral assay has been developed that permits reliable determination of polymer gelling times. Water‐soluble, isocyanate‐containing polyether prepolymers initiate chain extension on contact with water and after some period of time form a semisolid hydrogel matrix. Techniques based on visual observations used to determine the end point for this process generally produce erratic results. By following scattered transmittance of visible light by an aqueous solution of prepolymer during the gelation process, it is possible to obtain a more objective determination of the gelation time. It was empirically demonstrated that the minimum of the first derivative of the scattered transmittance versus time spectrum correlated with the observed gel time over a wide time scale.