Effect of shear strain in coating on the particle packing of gelled-clay particle dispersions during drying

Particle aggregates formed in a dispersing medium may be destroyed depending on the shear history applied in the coating process. Yielding behavior of gelled-clay particle dispersion can be interpreted as the destruction of particle network structure with the increase in shear strain. In the present study, gelled-clay particle dispersion was coated at different shear strain to control the initial status of particle aggregation of the drying process. The packing behavior of clay particles in the coated film was investigated from the viewpoint of void fraction in the film by the simultaneous measurements of weight loss and thickness decrease. Additionally, the change of surface roughness was researched based on the scattering pattern analysis. As a result, it was found that dispersed particles started to be packed tightly in the constant drying rate period, and that void fraction and surface structure of particle-packed layer scarcely changed in the latter part of the falling drying rate period. In contrast, if clay particle aggregates remained at the beginning of drying, a loosely packed particle layer was formed without changing surface structure in the constant drying rate period. However, the surface roughness was increased continuously in the falling drying rate period in spite of keeping constant film thickness, probably because void in the particle layer was collapsed.