How does a liquid coating become a solid film?

Surface coatings transition from a liquid state to a solid, adherent film through three primary mechanisms, often used in combination:1

• Evaporation: In this process, a fully developed polymer is dissolved in a solvent. Once applied to a substrate, the solvent evaporates, leaving behind a solid film. Due to environmental regulations regarding solvent emissions, this method is increasingly limited.21
• Coalescence: This occurs primarily with latex polymers dispersed in water or solvent. As the carrier liquid evaporates, small polymer particles are forced together by surface-mediated forces, fusing into a continuous film. If the polymer is too rigid, coalescing agents are added to temporarily soften the particles until the film forms.34
• Cross-linking: This involves chemical reactions that build a three-dimensional network of polymers. It can occur through oxidative reactions with atmospheric oxygen (as seen in drying oils) or through reactive two-component systems that polymerize upon mixing. Catalysts are often employed to accelerate these reactions to ensure the film cures within a practical timeframe.56