Influence of Fiber Swelling on Coating Penetration, 2002 Coating Conference Proceedings

S. Akinli-Kocak, A. Van Heiningen and D. W. Bousfield

The water uptake behavior of fibers, especially those at the surface of paper, may determine the initial coating consolidation and the penetration of coating into the paper. However, quantitative studies on this topic have not been reported in the literature. The goal of this work is to characterize the degree of coating penetration onto paper with fibers of different water uptake behavior.

Three batches of fibers with different water retention values (WRVs) were produced by carboxymethylation of softwood kraft pulp. Handsheets were made from these batches and the original pulp. These four handsheets and three commercial sheets, a light weight coated (LWC), a woodfree, and a thermo-mechanical pulp (TMP), were characterized in a number of ways. Kaolin clay coatings with or without a polymer thickener were applied to these sheets by rod draw down and high-speed laboratory coaters. The coating penetration is characterized by the roughness of the backside of the coating.

The fluid absorption rates of the handsheets made from carboxymethylated fibers suggest that swelling of the surface fibers can close the surface pores and slow down absorption. The coating dive-in is reduced by carboxymethylation of the fibers. Polymer addition to the coating formulation also reduces penetration as expected. For commercial sheets, the coating dive-in is small for all coatings studied. A new test method, to characterize coating penetration, is described.