Consolidation of Pigmented Coatings: Development of Porous Structure, 1995 Coating Conference Proceedings

Anna Stanislawska and Pierre Lepoutre

DepartmentUniversity of Maine Orono

The consolidation of pigmented coatings based on clay or ground calcium carbonate and latex was examined by quenching the wet coatings at various stages of drying in liquid nitrogen and freeze-drying (F-D). From SEM examination of the F-D coatings and measurements of gloss, void fraction, surface area and light-scattering, the process of development of the porous structure could be followed. The overall process is basically the same, independent of pigment shape and latex content: Formation of a loose structure with large voids, at a first critical concentration where particle alignment at the surface is maximum and void size is smallest; coalescence of the latex, in the presence of water, accompanied by a decrease in surface area and a shrinkage that depends on the latex content and its rheological characteristics and by an increase in pore size; emptying of pores after a second critical concentration. Pigment shape was found to affect the consolidation process. With plate-like clay, two consolidation regimes were observed : pigmentcontrolled at low binder content and bindercontrolled at high latex content. Gloss of the all clay structures was always much higher than for CaCO3 . Pore sizes at all stages of the consolidation were smaller with clay than with rhombic CaC03 . Intense drying increased the shrinkage of all coatings but thermal post-treatment of room temperature-dried coatings had no effect on dimensions. Calculation of a void light-scattering efficiency led to the conclusion that, at a given binder level, pores in clay coatings are smaller and more numerous than in CaCO3 coatings. Consolidation of coatings applied at 39 m/min. on paper was examined with the aid of a special coating apparatus. The same behavior was observed (increase in the gloss of F-D coatings up to the FCC followed by a decline); however, initial gloss values on paper and on plastic film were quite lower than that of plastic coatings applied at low speed. This observation was unexpected and is significant. At the later stages of consolidation, the structure of the paper and plastic coatings differed increasingly, according to expectations.