Assessment of Fillers for Opacity Improvement of Printing Papers - A Combination of Theory and Laboratory Work

摘要

Opacity is an important property of fine papers, and the opacity of pulp can vary widely. At an Australian mill using imported pulp it was desired to increase the opacity of all grades by up to 4%. Two options were available: to use a finer ground calcium carbonate (GCC) filler or to mix some calcined clay with the current GCC filler. The laboratory work to evaluate the proposed options involved a series of experiments in which the absorption and scattering coefficients were determined for a particular pulp and four fillers. The factors affecting opacity are described, and the relevant equations are used to show the importance of using the optimum filler type or combination to increase sheet opacity. The fillers investigated were the coarser GCC currently in use at the mill, a finer GCC and two calcined clays supplied by Engelhard: Ansilex 93 and Excaliber. From the results for each individual filler, it is possible to predict the opacity if a mixture of fillers is used. These results showed that the most suitable option for achieving the desired opacity increase was to include some Excaliber calcined clay with the current GCC filler. On a total solids basis less than 6% is required, dependent on the grammage and desired opacity increase. The method was validated by a subsequent machine trial, which gave a very similar result to the prediction. A small amount of Excaliber is now used in lightweight products to achieve the desired opacity.