Forces operative during film formation from latex dispersions

Abstract

In this paper, the different forces operative on the latex particles during film formation are examined and estimates are given of the contribution of the forces to the deformation of these particles. The forces examined are gravitational forces, Van der Waals forces, electrostatic repulsion forces due to the overlap of diffuse double layers, capillary forces due to the receding water/air interface, and capillary forces due to liquid bridges between the latex particles. The magnitude of these forces is compared to the force needed to obtain sufficient deformation, i.e. the closure of the voids between the particles. Calculations show that both capillary forces are from the same order of magnitude, 1–3 10 −7 N. The Van der Waals contribution is smaller by a factor of 20 than the contribution due to the capillary forces. However, for deformation the Van der Waals forces may be of considerable importance since the Van der Waals forces diverge for very small distances. A sound incorporation of the Van der Waals forces can be achieved by using the JKR equations. Under the assumption of constant potential, the electrostatic repulsion forces are approximately a factor of 1000 smaller than the capillary forces. The gravitational forces, 1×10 −16 N, are negligible. The force needed for successful deformation amounts to 10 −7 N, assuming that the Hertz theory is applicable in the description of polymer particle deformation. Furthermore, an equation for the capillary force due to the receding water/air interface is derived which is applicable for a wider range of degrees of deformation than is the Mason-equation. Three descriptions of the particle's response to deformation are examined: (i) the Hertz theory for purely elastic spheres, (ii) the JKR-theory for purely elastic spheres in the presence of Van der Waals forces, and (iii) the Yang-theory for linear visco-elastic spheres. These descriptions are combined with both capillary forces resulting in criterions determining successful deformation.