Effects of fillers on the wet web strength of paper

The wet web strength of handsheets containing PCC (precipitated calcium carbonate) fillers with different particle sizes was evaluated for various concentrations of PCC. Also the friction between wet sheets containing PCC was evaluated in the range 30-55% solids content. It was found that incorporating PCC in the sheet using cPAM (cationic polyacry-lamide) as a retention aid leads to the formation of large PCC aggregates, which have a negative impact on wet web strength. On the other hand, incorporating PCC in sheets using PEI (polyethylene imine) as a retention aid, leads to a decrease in PCC size and the wet web strength is found to be very close to that of sheets without PCC. It is suggested that PCC particles or small aggregates deposited on fibers can increase the friction between fibers, thus increasing the entanglement friction and the wet web strength, whereas large PCC aggregates, which due to their large size can easily detach from fibers, interfere with fiber entanglements during the consolidation of the sheet, resulting in lower strength. The increase in friction was confirmed by measurements of the friction force between two wet sheets. In the regime where capillary forces are absent, depositing PCC particles or aggregates on top of a wet sheet increases the friction between sheets. It was also found that the addition of microfibrils to the PCC-filled handsheets increases the wet web strength, likely because of mechanical entanglements caused by the microfibrils.