Soluble organic additive effects on stress development during drying of calcium carbonate suspensions

The effect of polymer, plasticizer, and surfactant additives on stress development during drying of calcium carbonate particulate coatings was studied using a controlled-environment apparatus that simultaneously monitors drying stress, weight loss, and relative humidity. We found that the calcium carbonate coatings display a drying stress evolution typical of granular films, which is characterized by a sharp capillary-induced stress rise followed by a rapid stress relaxation. The addition of a soluble polymer to the CaCO₃ suspension resulted in a two-stage stress evolution process. The initial stress rise stems from capillary-pressure-induced stresses within the film, while the second, larger stress rise occurs due to solidification and shrinkage of the polymeric species. Measurements on the corresponding pure polymer solutions established a clear correlation between the magnitude of residual stress in both the polymer and CaCO₃-polymer films to the physical properties of the polymer phase, i.e. its glass transition temperature, Tg, and Young's modulus. The addition of small organic molecules can reduce the residual stress observed in the CaCO₃-polymer films; e.g., glycerol, which acts as a plasticizer, reduces the drying stress by lowering Tg, while surfactant additions reduce the surface tension of the liquid phase, and, hence, the magnitude of the capillary pressure within the film.