An Extension of Lepoutre Mechanism for the Consolidation of the Structure of Latex-Pigment Coatings

摘要

A new mechanism for the consolidation of the structure of latex-pigment coatings, based on Lepoutre's classic coating consolidation mechanism and on Croll's film formation model for polymer latexes, has been proposed by assuming the formation of filter cake at the coating surface, and by introducing the concept of Latex Coalescence Concentration (LCC). The new mechanism considers that, since water is evaporated at the coating-air interfaces during drying, while the solids are 'filtered' out to remain in the suspension, a filter cake is formed at the surface at FCC, the first critical concentration. Drying after FCC causes 'free water' under the cake to migrate to the surface by capillary flow, and the filter cake becomes thicker, until the front of the cake reaches the substrate at LCC, where a loose, 'gelling', three-dimensional network is formed within the whole coating. Further evaporation after LCC causes the coating pores to become smaller and the capillary forces to become greater. Increased capillary forces induce latex particles to deform and to coalesce, causing the latex to adhere to pigment particles, or the binder particles to adhere to each other, until SCC, the second critical concentration, where the coating structure becomes so rigid that both latex and pigments can no longer deform and move. The new model explains a common industry practice that drying intensity at the middle drying stage (after FCC) must be controlled, because boiling of free water under the skin layer may disturb the filter cake, causing surface defects. It also predicts an observation in the literature that surface latex content for thick coatings would be virtually independent of coating thickness, since the migration of particulate latex mainly takes place in the liquid drying phase before filter cake formation, independent of coat weight.