A Process Model for Latex Film Formation: Limiting Regimes for Individual Driving Forces

The deformation of particles, to produce a structure without voids, has been an issue of contention in the film formation community for many years. Four different mechanisms have been proposed. Three involve homogeneous deformation throughout the film, although all are built on the deformation of two isolated particles, described in the viscous limit by Frenkel and in the elastic limit by Hertz and Johnson, Kendall, and Roberts. We derive a linear viscoelastic generalization of Frenkel's model that predicts the deformation of two spheres compressed by a force, F, and surface tension, γ. The resulting equation is then embedded in field equations governing the collapse of macroscopic films. Assuming a uniaxial compression allows derivation of limits for the proposed modes of homogeneous deformation. These limits are shown as surfaces in parameter space. Since temperature alters most profoundly the rheological response of viscoelastic polymers, the controlling deformation mechanism is defined as a function of temperature. Wet sintering requires slow evaporation or a low modulus polymer and is seen at high temperatures. Capillary deformation requires the strain in the film to follow evaporation and appears at intermediate temperatures. Dry or moist sintering is then seen at the lowest temperatures, when the modulus is high and deformation is slow compared to evaporation.