Current understanding of the deformation of latex particles during film formation

This article reviews models of polymer particle deformation in film formation. Dillon et al. modelled polymer particles as viscous bodies whose surface tension provides the main driving force for particle deformation. In Brown's alternative model, deformation is driven by capillary pressure due to water evaporation and opposed by the elastic force of the polymer itself. Brown, however, assumes the area on which capillary and deformation pressures act to be equal and the particles to be elastic rather than visco-elastic. The first assumption is addressed by Mason and the second by Lamprecht. Brown, Mason and Lamprecht modelled the polymer particles response as the response of two non-attracting spheres pressed together. Kendall and Padget followed an alternative approach using the JKR theory thus incorporating the van der Waals attraction. The existence of a sharp transition in the film forming qualities near Tg is predicted by these theories through a sharp change in the elastic modulus. The predicted inverse proportionality for the particle radius is not observed experimentally. Experiments performed by Sperry et al. cast further doubt on the applicability of these theories. Keddie et al. propose a new approach to particle deformation. According to them transparent films can either be obtained along an ‘easy’ route, by particle deformation due to capillary forces, or along a “hard” route, by particle deformation due to other surface forces; e.g. the polymer/water interfacial tension, the polymer/air surface tension or the forces due to residual water left between the particles.