COATING SHRINKAGE DURING EVAPORATION: observation, measurement and modelling within a network structure

Coating shrinkage upon drying is a phenomenon well known to the paper coating industry where it often causes changes in the final structure of the coating layer, leading to poor results in terms of gloss, light scattering, surface strength, coverage and printability. The aim of this paper is to improve the understanding of coating shrinkage by measuring the forces related to the capillarity, acting at the free liquid front, in combination with the flocculation, film forming and compressibility characteristics of the coating structure and binders. Shrinkage while the coating layer dries has been successfully measured by observing the deflection of coated strips of a dimensionally stable synthetic substrate. Ground calcium carbonate was used as the coating pigment together with latex binders of low and high Tg latex, respectively, and using starch additionally as an example of a natural film-forming binder. The final dry coatings were analysed with mercury porosimetry and by scanning electron microscopy in order to characterise their porous structure. The mercury intrusion curves were analysed with Pore-Cor, a software package that is able to generate a threedimensional network structure representative of the porous medium. The simulated structures were compared with the microscope images and provided evidence of the drying behaviour of the different components of paper coating and their influence on the final structures of the dry coating layer. The shrinking behaviour of coating colour formulations, which has usually been interpreted as shrinkage of the polymer upon film-forming, has been explained in terms of surface capillary forces acting on the packing structure of the coating at the first critical concentration. The structures undergoing shrinkage are shown to be affected by the degree of flocculation within the coating, which depends on the nature of the binder used.