Method for preparing a photochromic polyurethane latex and application to ophthalmic optics

A method of preparing a photochromic polyurethane latex comprising:

• forming a hydrophilic polyurethane prepolymer with isocyanate end groups, by polyaddition of at least one polyol and at least one polyisocyanate in the presence of at least one internal emulsifying agent, wherein the amount of the internal emulsifying agent is from 3 to 5% by weight with respect to the total weight of polyol, polyisocyanate, and internal emulsifying agent;
• mixing the polyurethane prepolymer with a photochromic agent;
• adding water or a mixture of water and solvent, under agitation, to the mixture of the polyurethane prepolymer and the photochromic agent, to form an emulsion;
• and
• adding at least one chain extension agent to the emulsion and performing a chain extension reaction to give a final photochromic polyurethane latex.