A Systematic Study of Noncross-linking Wet Strength Agents

Cellulosic fibers are inherently hydrophilic, and the modification of them to withstand moisture is important both commercially and scientifically. The usual methods are based on the cross-linking chemistry of reactive groups such as epichlorohydrins. Here, we present that it is possible to attain paper with wet strength from a combination of polymers that lack cross-linking chemistry, namely, carboxymethyl cellulose (CMC) and Polybrene. To accomplish this, we first altered the surface charge of the fibers by adsorption of CMC. Subsequent adsorption of Polybrene, forming the fibers as paper sheets, and drying yielded paper with wet strength properties. The wet strengthening was further investigated by (i) varying the molecular weight of the CMC, (ii) varying the cationic polyelectrolyte, and (iii) synthesizing polymers called ionenes to study the structural properties behind the wet strength of Polybrene. The results showed that (i) drying was necessary to obtain wet strength, (ii) wet strength seemed to be a surface effect, (iii) high Mw CMC played an important role in the development of wet strength, and (iv) only asymmetric ionenes ([3,6] and [6,12]-ionenes) could attain wet strength while symmetric [3,3] and [6,6]-ionenes failed to show wet strength properties.