Role of Particle Size on Latex Deformation During Film Formation

By Charles S. Kan

The ability of latex particles to deform and coalesce to form an integral film upon drying is an important property in many latex coating applications. Many theories have been proposed to account for the origin of the deformation forces. The capillary forces which depend inversely on particle size have been accepted as important for latex deformation and film formation.

The minimum film forming temperature (MFFT) has been found to be a function of the particle size of la-texes and has been used as evidence that the capillary forces are responsible for film formation. In this study, the deforming force at MFFT has been determined from the moduli of water-equilibrated latex polymers. No particle size dependence was observed.

The magnitude of the deforming forces was at least an order of magnitude lower than that predicted by the capillary force theory. Electron microscopy of film formed below the MFFT, a condition that corresponds to early stage film formation, showed significant deformation, indicating that at the beginning of film formation, forces of magnitude predicted by the capillary force theory are present.

However, the magnitude of the forces decreases rapidly as film formation progresses. The MFFT particle size dependency can be explained by the difference in the degree of water plasticization. Evidence that latexes of different particle size were plasticized by water to different extents was determined from the T_gs of the latex emulsions.