The influence of different coating latices on printing and surface properties of paper calendered at high temperatures

This report focuses on the possibility of enhancing the effects of temperature-gradient calendering of coated paper by using different types of coating latices. Four latices with different thermal softening behaviours were used; two styrene-butadiene latices, a poly(vinyl acetate) latex and a styrene-acrylate latex. The two styrene-butadiene latices had markedly different softening behaviours. One, S/B1, exhibited a sudden drop in the modulus when the temperature exceeded 40 degreesC. The modulus of the other S/B2, was only slightly reduced in the temperature range from 20-200 degreesC. A woodfree paper was blade coated with coating colours prepared with the four latex binders. The papers were then calendered with different temperatures of the heated roil (740-210 degreesC) in a conventional or an extended soft nip. Printing trials were performed in a 4-colour sheet-fed offset press. It was found that the paper coated with the coating colour containing the S/B1-latex had the highest gloss level and a rather low small-scale roughness, both before and after calendering. The S/B2-latex resulted in a rather low paper gloss and a poor bending stiffness. The poly(vinyl acetate) latex produced a comparatively coarse and porous coating structure that had a high covering ability but rather low levels of paper and print gloss. The styrene-acrylate latex gave high levels of paper and print gloss. In this case, the paper gloss was also somewhat more stable against irreversible roughening induced by the fountain solution and, moreover, the bending stiffness was also more preserved after calendering. An increase in the roil temperature from 140 to 210 degreesC led to a higher paper gloss and higher print gloss. Calendering with the extended soft nip gave a higher print gloss than calendering with the conventional soft nip.