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How do polyurethane varnishes cure after only the solvent evaporates?

I believe that polyurethane varnishes have as components isocyanates & polyols that react to form the polyurethane portion of the cured varnish. How is it that this doesn't happen spontaneously while the varnish is still in the can & only after it's been left for the solvent (water or organic solvent) to evaporate? What's the chemistry there?

Does the polyol only become a polyol after the solvent have evaporated? If so, again, what's the chemistry there?

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  • $\begingroup$ Don't tell me the moisture is the polyol & it only gets to the isocyanate after the hydrocarbon solvent evaporates. Because that's only true for water repelling, hydrocarbon-based solvent formulas. What about water-based formulas, which is what most polyurethane varnishes are? $\endgroup$
    – Tempus Nomen
    Commented Jun 13, 2023 at 6:54
  • $\begingroup$ A lot of polyurethane resins start with beads of pre-made polymers dispersed in water with very little polymerisation happening on curing (or limited controlled cross linking of existing long chains using other types of cross-link). $\endgroup$
    – matt_black
    Commented Jun 13, 2023 at 13:49
  • $\begingroup$ What stops these pre-made polymers from polymerizing completely? The fact that they need to get settle down & get close to each other as the solvent evaporates? $\endgroup$
    – Tempus Nomen
    Commented Jun 13, 2023 at 19:16
  • $\begingroup$ A good answer might require a whole book to explain. Perhaps this one . But one highly simplified explanation is that the cross linking of dispersed droplets or resin is different to the reactions creating the oligomers and is inhibited in solution by the surfactants creating a stable dispersion. $\endgroup$
    – matt_black
    Commented Jun 14, 2023 at 11:26

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