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Experiments on coated films of waterborne coatings (WBC) under controlled conditions (i.e., 25 ° C, 65% RH) were carried out to study the mechanism of leveling. Data in a companion paper imply that the development of the surface tension gradients plays an important role in promoting leveling. The stress due to surface tension gradients is found to be much greater than the stress introduced by hydrostatic pressure gradients in the film caused by surface tension and the free boundary curvature of the film. This reconfirms that the use of a co-solvent, which evaporates faster than water, greatly accelerates the leveling of a WBC., INTRODUCTION It is important to obtain coatings with a smooth surface for appearance and for the inherent performance properties of the coating film. The quality requirements for the leveling in [...]

The influence of film thickness on the leveling of a thermosetting waterborne coating (WBC) was examined. The theory of Patton has been revised to give the proper force balance and modified to include the changes in surface tension and viscosity during leveling by the evaporation of volatile components. However, the predicted leveling process by a numerical method did not coincide with the observed results. Thus, the driving force in the theory, the hydrostatic pressure gradient in the film, was considered to be insufficient to describe the leveling process of the WBC. The hydrostatic pressure gradient can be a component of the driving force to promote leveling, but the existence of an additional component is suggested., INTRODUCTION Coyle (1) showed that it is impossible to control the formation of surface ripples, called ribs, during the application process by the modification of the rheological properties of the [...]

A modified observation system for the leveling process of a coated fluid is proposed, and the observation results are presented and compared with previous results. For the thermosetting waterborne coatings, the theory of Orchard, et al., and ones based on it are not applicable and fail to give even a qualitative description. The theory of Overdiep was found to be reasonable. However, it was insufficient to predict the leveling performances of coatings without experimentation. Qualitatively, the original theory suggests the possibility of the reappearance of surface ripples, in other words, the occurrence of the negative amplitude in the surface ripples during the course of leveling, although little data were presented regarding the phenomenon. It was found here that added co-solvent plays an important role on the leveling rate and also the occurrence and the control of the negative amplitude of the surface ripples. These are thought to relate closely to the volatility of the co-solvent and the composition of the solvent residue in the coated film during the leveling process.