Increase of the filler content in papermaking by using a silica-coated PCC filler

The increase of filler content in paper without significantly sacrificing the paper mechanical resistances is of high interest for papermakers. In this work, precipitated calcium carbonate (PCC) modified with silica was used as filler for papermaking. Handsheets based on a eucalyptus kraft pulp furnish with different amounts of the modified filler, ranging from 16 to 40%, were produced. For similar levels of filler content it was found that the strength properties of the handsheets produced with the modified PCC were always significantly better than those obtained with the unmodified PCC. In particular, for handsheets with a filler content of 30%, the tensile index and the Scott bond internal resistance improved by ca. 20% and 39%, respectively. Some decrease of the light scattering and opacity values was noted when using the modified PCC, but the brightness was roughly the same, for each filler level evaluated. The enhanced fibre-to-filler bonding can be due to the hydrogen bonding between the cellulosic fibres and the hydroxyl groups of the silica film coating the calcium carbonate crystals, during the sheet formation. The present results with the new modified PCC provide a possibility to increase the filler amount in the papermaking industry.