Chapter 11 - Film Formation

Abstract

The formation of films from latexes is the most important practical application of polymer colloids because of its relevance to the paint and coatings industries, including graphic arts, adhesives, caulks, varnishes, and floor finishes. To make a film, one spreads liquid latex onto the substrate, allows the medium to evaporate and a clear, continuous film remains. By mixing pigment into the colloid prior to application, opaque, white or colored films are formed. This chapter considers water as the chosen medium but it also emphasizes that an organic medium can also be used. A wet film is spread upon a substrate. Water initially evaporates at the constant rate at which pure water evaporates. There may be some cooling of the film because of the latent heat of vaporization. This usually continues until the volume fraction of solids reaches 0.60 to 0.75. Often the particles form a regular array, fcc or bcc, depending upon the ionic strength, particle size uniformity, and viscosity. The evaporation rate slows down as particles are crowded and occupy surface sites. A continuous film is first formed usually at the surface, so that further evaporation occurs by diffusion of water through this skin, first through interstitial channels and finally through the continuous polymer film. Particles after coming into contact start to deform into polyhedral structures. Water and nonvolatile, water-soluble components are squeezed into interstitial or interfacial regions. Polymer segmental interdiffusion may occur across the former particle boundaries.