Process for the production of polyurethane lens

A process for producing of a polyurethane lens comprising the steps of:

• (A) selecting two polythiol compounds which have different reaction rates with a polyisocyanate compound by:
• (i) mixing a first candidate polythiol compound with m-xylylene diisocyanate, as a standard polyisocyanate compound, in an --NCO group/--SH group mixing ratio of 1.0, measuring the viscosity of the resultant mixture at 20 .degree. C. immediately after mixing and again 2 hours after preparation of the mixture, and determining a change in viscosity between the first and second measurements,
• (ii) mixing a second candidate polyisocyanate compound with m-xylylene diisocyanate and measuring viscosity change in the same manner as in the above step (i),
• (iii) repeating step (ii) for additional candidate polythiol compounds, and
• (iv) comparing the amount of viscosity change for any two polythiol compounds so measured, and when a difference between the viscosity change is at least 100 cps, selecting these two candidate polythiol compounds which have different reaction rates with the standard polyisocyanate compound;
• (B) adding two or more of the polythiol compounds selected in step (A) to a mixture of polyisocyanate compound and an alkyltin halide compound of the formula (I),
• (R.sub.1).sub.c --Sn--X.sub.4 --c (I)
• wherein R.sub.1 is methyl, ethyl, propyl or butyl, X is a fluorine atom, a chlorine atom or a bromine atom and c is an integer of 1 to 3, allowing them to react, and thereafter
• (C) obtaining a polyurethane lens.