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An experimental study of the surface distribution of filler material in paper Serles, Andrew

Abstract

A series of experiments were conducted to observe the effect of different pulp suspension and formation characteristics on the variation in filler concentration on the surface of paper. Hand sheet samples were formed in a laboratory apparatus. 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. The analysis was focused on the variation in surface filler concentrations on the scale of individual strands of the forming fabric. The procedure involved locating areas of interest in the samples, particularly the area of paper formed over knuckles, threads, and openings in the fabric, at which point the sample was analysed with Energy Dispersive X-Ray Spectroscopy and image analysis techniques to determine relative filler concentrations. In samples formed by gravity and vacuum drainage, Kaolin displayed a significantly greater variation in local surface filler concentration than PCC, though the difference was reduced under vacuum drainage conditions. This effect is attributed to the electrostatic attraction between PCC and pulp fibres in contrast with the repulsion felt by Kaolin filler. The attraction resists the distributing forces of the flow at low drainage velocities but the bonds are broken by large shear forces. The distribution on the top side of the paper was comparable between the filler types, due to the more uniform flow field at a distance from the forming fabric. Vacuum drainage increased the spatial variation of both fillers by a similar amount. It was found that 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, it was found that the addition of retention aids produced a significant improvement in the uniformity of the filler material. Finally, it was found that a finer forming fabric improved the uniformity of filler distribution.

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