Development of the Porous Structure of Clay Coatings Based on Protein or Latex, 1996 Coating Conference Proceedings

Previous investigators have examined the consolidation process of paper coatings with latex as a binder. This work reports on the development of the porous structure of clay coatings containing a soluble binder, protein from the moment of application to the dry state, using the technique of quenching the structure in liquid Nitrogen and freeze drying. Binder content covered a range from 0 to 20 pph. It was found that, as with latex-based coatings, the consolidation of protein-based coatings proceeds through a first and a second critical concentrations (FCC & SCC). However, the solids content at those two critical concentrations was lower for protein-based coatings resulting presumably from the strong adsorption and interactions of the protein molecules with clay. Shrinkage after the FCC increased with binder content for both systems. However, shrinkage was found to be greater with protein-based coatings, indicating that the very bulky structure produced at the FCC is lost after drying. Pore size, determined from void volume and surface area, at all consolidation stages examined in this work were larger with protein coatings than with latex coatings, but the scattering efficiency per unit volume of voids was similar for both systems. It is possible that the pore size of both systems lies on opposite sides of the optimum for light-scattering. With latex coatings, the surface area of the freeze dried coatings decreased mostly between the FCC and the SCC as a result of the coalescence of the latex. With protein the decrease in surface area occurred mostly after the SCC, as the soluble binder molecules collapsed together and on the pigment surfaces. During the SEM investigation of protein binder coatings evidence of a rather unique network structure was observed. This structure has not been previously reported. A description of the structure and a discussion of its potential effect on the consolidation behavior of the coating is reported.