Final answer:
Polyurethane varnishes cure when the solvent evaporates because that increases the concentration of isocyanates and polyols, allowing them to react. This process doesn't happen in the can due to stable formulations that prevent the reactants from curing prematurely.
Step-by-step explanation:
Polyurethane varnishes are composed of isocyanates and polyols, which react to form the durable polyurethane network upon curing. The curing process begins after the application when the solvent evaporates, increasing the concentration of the reactive components and often activating a catalyst in the varnish. The reason this reaction doesn’t happen spontaneously in the can is due to the formulation stability wherein reactants are kept at low enough concentrations or separated by solvents or inhibitors to prevent curing. When the solvent evaporates after application, the concentration of isocyanates and polyols increases, allowing them to react more readily. Additives such as catalysts that promote the reaction between isocyanate and polyol may also be activated upon solvent evaporation or by the exposure to atmospheric moisture, if the system is of a moisture-cured type. Exposure to ultraviolet light can also initiate curing in some formulations.
Isocyanates are sensitive to moisture and can react with water, which is usually minimized or controlled in two-component polyurethane systems. In the presence of water, isocyanates can react to form a polyurea link instead of urethane, which is why precautions are taken to prevent moisture contamination before application. Polyols in polyurethane varnishes do not become polyols after solvent evaporation; rather, they are already present in their functional form and react with isocyanates when conditions allow, leading to the curing of the varnish into a solid film.