In -Situ Precipitation of CaCO3 within Pulp Fibers Enhances Optical and Printing Properties of Paper

Writing and printing papers are usually loaded with white pigment particles to improve many paper properties such as light scattering, smoothness, pore-size distribution, and ink-paper interactions. In the conventional method of loading, the filler particles are added directly into the papermaking stock before forming the paper sheet. In an alternative approach, the filler particles are placed within the lumen or the cell wall of the pulp fibers either by mechanical diffusion or by in-situ precipitation. Many researchers have found that the fiber-loading method may offer several advantageous over the conventional filler loading such as lower requirements of retention aids and improved formation of the sheet, and higher sheet strength and cleaner white water system at equal level of filler addition.

We have studied the effect of fiber-loading with CaCO₃ on paper sheets made from bagasse pulp. The bagasse pulp fibers were first soaked in a dilute aqueous suspension/solution of calcium hydroxide, and then, CaCO₃ was precipitated in-situ within the fiber cell wall and/or lumen by passing CO₂ gas through the pulp suspension. Standard laboratory sheets were prepared from the fiber-loaded pulp. Strength and optical properties of the sheets were determined using standard test methods. The sheets were printed in the laboratory using an IGT printability tester. It was observed that the fiber loading by in-situ precipitation of CaCO₃ very significantly improved the light scattering coefficient of the bagasse pulp. As a consequence, the print through of the paper was decreased, which is particularly desirable in low gramage papers.