Frequently, this gives rise to heterogeneous drying in which a wet coagulated skin forms at the free surface due to the evaporation-induced flow. Depending on latex properties, solvent evaporation or particle deformation may rate-limit the process of wet film formation. Contrary to dry film formation, wet film formation is aided by the hydroplasticized state of the polymer. Another advantage of wet film formation is the appearance of capillary forces directed towards particle deformation. As a result hydrophilic latices may film-form at temperatures significantly lower than T g of the dry film, which is of practical significance.
Cracks originate from stress built up during densification of coatings and limit their application.
Superficial cracks result from stresses due to the deformation of particles within the skin. Cracks may also form at later stages and then proceed throughout the film. The spatial frequency and direction of the cracks depend on drying conditions. Immediate release of the stress through plastic shrinkage normal to the film plane yields perfect, crack-free films. In this case, water evaporation is the rate-limiting process for film formation.