Processes and states during polymer film formation by simultaneous crosslinking and solvent evaporation

Coating film formation with simultaneous crosslinking and solvent evaporation, accompanied by passage of the polymer film through glass transition region, is a complex process by which temporary or permanent anisotropic and gradient network structures can be formed. Evaporation and crosslinking are processes that are interdependent. The changes in structure (growth of branched molecules and network evolution) are a function of reaction kinetics, which gets diffusion controlled when the system passes through the glass transition region. Structural changes are determined by branching, gelation, and network build-up and depend on the architecture of network precursors. Thermodynamic interactions of polymer with solvents affect the solvent activity which determines the vapor pressure of the solvent over the film and thus the evaporation rate. The glass transition temperature increases as a result of both the decreasing solvent content and conversion of functional groups into bonds. By interplay of these two factors more or less solvent can be locked in by vitrification. The roles and intensity of these basic processes and interrelations are discussed. Some older results are reviewed and new experimental evidence is added. The interrelations are illustrated by time dependences of solvent evaporation and conversion of functional groups for solvent-based high-solids polyurethane systems composed of a hydroxyfunctional star oligomer and triisocyanate and by the role of the ratio of evaporation to crosslinking rates. Evidence was obtained of gradient formation in which appearance of a glassy surface layer is an important event in the history of film formation that determines solvent retention and other film characteristics.