Forming fabric weave-scale variations in paper filler content

Abstract

A series of experiments were conducted on handsheets to investigate small-scale variations in filler concentration on the surface of paper. The surface distribution of two types of filler material was investigated: precipitated calcium carbonate (CaCO3) and kaolin clay (Al2Si2O5(OH)4). The effect of retention aids, dewatering rate, and forming fabric geometry on filler distribution was tested. Local filler concentration was found to be strongly correlated with the relative flow velocity during formation. In samples formed by gravity and vacuum drainage, kaolin displayed a significantly greater variation in local concentration than precipitated calcium carbonate, though the difference was reduced under vacuum drainage conditions. The distribution on the top side of the paper was comparable between the filler types, independent of drainage velocity. Under vacuum drainage, retention aids did not improve filler uniformity on the wire side. However, on the top side of the paper, a moderate reduction in spatial variation was observed. Additionally, on the wire side of samples made with gravity drainage, the addition of retention aids produced a significant improvement in the uniformity of the filler material. These filler distribution trends are believed to be related to variable filler retention as a function of filler type, drainage velocity, and chemical retention aids. Additionally, samples made with an industry forming fabric showed the same distribution trends and an improvement in the uniformity of the filler material.