1. Evaporation (lacquers)
The binder is already a finished polymer dissolved in solvent. As solvent evaporates, chains entangle into a film. No chemical change occurs, so the film can be re-dissolved by the same solvent. Examples include nitrocellulose and many acrylic lacquers. Drying is fast, but solvent content (VOC) tends to be high.
2. Coalescence (latex / waterborne dispersions)
Polymer particles are dispersed in water. As water leaves, particles pack together, deform and fuse. This only works above the minimum film-forming temperature (MFFT). Coalescing aids like TMPD-MIB temporarily soften particles so films form in cooler weather, then slowly leave. Cold or humid conditions during drying can leave a weak, cracked or porous film.
3. Oxidative cure (alkyds and oils)
Drying oils and alkyds contain unsaturated fatty acid chains that react with oxygen from the air, aided by metal driers, to crosslink. The surface dries first and the film keeps hardening for weeks.
4. Chemical reaction (two-component and moisture-cure)
Reactive components form new covalent bonds and a crosslinked network: epoxy with amine, isocyanate with polyol or amine, and moisture-cure urethanes with water from the air. These films are generally the most chemically resistant and cannot be re-dissolved. Cure speed depends strongly on temperature, and many have a recoat window.
| Mechanism | Reversible? | Sensitive to | Examples |
|---|---|---|---|
| Evaporation | Yes | Solvent balance, humidity (blushing) | Lacquers |
| Coalescence | Partly | Temperature vs MFFT, humidity | Acrylic latex, PUDs |
| Oxidation | No | Temperature, film thickness | Alkyds, oils |
| Chemical reaction | No | Temperature, mix ratio, moisture | Epoxy, PU, polyurea, polyaspartic, MMA |