Deposition of polyelectrolyte complexes as a mechanism for developing paper dry strength

Fibers from recycled copy paper were protreated with two polymers, cationic poly-DADMAC followed by anionic CMC. Results were slightly different from the results of earlier work, which showed that the maximum strength was obtained when the amount of poly-DADMAC was just sufficient to saturate the fibers. In this study, higher tensile strength was obtained when poly-DADMAC addition exceeded the saturation level. It was hypothesized that the strength increase arises from the formation of polyelectrolyte complexes (PECs) in the bulk phase followed by deposition of PECs onto the fibers. Pre-formed complexes were retained efficiently by the fibers, especially when their surfaces had been pretreated with poly-DADMAC to saturation. Surprisingly, such pretreatment increased the retention efficiency of all of the PEC mixtures tested. PEC deposition yielded an additional increase of about 13% in dry strength beyond what could be achieved by treatments not involving complexation.