Effect of Silica Gel in Causticized Calcium Carbonate from Nonwood Pulping on the AKD Sizing Efficiency of Paper

The application of causticized calcium carbonate (CCC) from nonwood pulping as a filler in papermaking is restricted by a marked decrease in alkyl ketene dimer (AKD) sizing efficiency. But the reason for this adverse effect has not been clear. In this paper, effect of carbonization on the AKD sizing efficiency was investigated systematically. The results showed that the hydrated calcium silicate (C-S-H) in the original CCC had been converted into silica gel after carbonization, and this caused the carbonized CCC to have a higher specific surface area and larger pore volume. This could explain the reason why the CCC had a worse AKD sizing efficiency than the conventional CaCO3 fillers. In addition, the effect of pH value of carbonization on the AKD sizing efficiency was also studied. It was discovered that the silica gel generated and the AKD sizing efficiency started to decline dramatically at the pH value of 8.5. Based on the research above, it can be included that pH value of 8.5 is an appropriate pH of carbonization end to maintain the AKD sizing efficiency at a good level.