Influence of structure and surface chemistry on the cohesion of paper coatings

Abstract

The effects of particle size, particle shape and surface chemistry on the cohesion of pigmented coatings based on clay, calcium carbonate and polystyrene (PS) latex, using carboxymethylcellulose (CMC) as a binder, has been determined. The critical (minimum) binder content (CBC) required to form a coherent film was assessed by an abrasion test. Z-direction strength was characterized by a delamination (90° peel) test. The results indicate that coating cohesion, assessed by CBC or delamination, increases with increasing pigment particle size, for the three pigments examined. Differences between clay and CaCO3 are ascribed mainly to a shape effect. Coatings based on plate-like clay are more resistant to in-plane stresses, and coatings based on isometric CaCO3, to out-of-plane stresses. The significant difference in both CBC and peel strength between isometric CaCO3 and PS (at similar equivalent spherical diameter) is ascribed to much weaker acid-base interactions between pigment and the CMC binder in the case of the PS latex. Size distribution (CaCO3 is polydisperse, PS is monodisperse) might play a role in stress distribution.