FORCES OPERATIVE DURING FILM FORMATION FROM LATEX DISPERSIONS

In this paper, the different forces operative on the latex particles d uring film formation are examined and estimates are given of the contr ibution of the forces to the deformation of these particles. The force s examined are gravitational forces, Van der Waals forces, electrostat ic repulsion forces due to the overlap of diffuse double layers, capil lary forces due to the receding water/air interface, and capillary for ces due to liquid bridges between the latex particles. The magnitude o f these forces is compared to the force needed to obtain sufficient de formation, i.e. the closure of the voids between the particles. Calcul ations show that both capillary forces are from the same order of magn itude, 1-3 10(-7) N. The Van der Waals contribution is smaller by a fa ctor of 20 than the contribution due to the capillary forces. However, for deformation the Van der Waals forces may he of considerable impor tance 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, t he electrostatic repulsion forces are approximately a factor of 1000 s maller than the capillary forces. The gravitational forces, 1 x 10(-16 ) N, are negligible. The force needed for successful deformation amoun ts to 10(-7) N, assuming that the Hertz theory is applicable in the de scription of polymer particle deformation. Furthermore, an equation fo r the capillary force due to the receding water/air interface is deriv ed which is applicable for a wider range of degrees of deformation tha n 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 presenc e of Van der Waals forces, and (iii) the Yang-theory for linear visco- elastic spheres. These descriptions are combined with both capillary f orces resulting in criterions determining successful deformation. (C) 1997 Elsevier Science S.A.