Wet-strength development of cellulose sheets prepared with polyamideamine-epichlorohydrin (PAE) resin by physical interactions

The mechanism of wet-strength development of cellulose sheets prepared with a typical water-soluble polymer, polyamideamine-epichlorohydrin resin (PAE), was studied. Especially, the effect of physical interactions between fibrillated cellulose fibers with no carboxyl groups and PAE molecules in PAE-containing cellulose sheets on wet-strength development was investigated, and the sheets were prepared by soaking once-dried cellulose sheets in PAE solutions. Because either unfibrillated or fibrillated celluloses used in this study had no carboxyl groups, the PAE-containing cellulose sheets should have no chances to form ester or ionic bonds between cellulose fibers and PAE molecules. Since PAE molecules have both carboxyl groups at the ends of polyamideamine chains and the 3-hydroxy-azitidinium groups reactive with carboxylate groups, it is possible to form inter- and intra-molecular cross-linked structures between PAE molecules and within one PAE molecule, respectively, by heating the PAE-containing cellulose sheets. The obtained results showed that wet-tensile strength of the PAE-containing cellulose sheets were clearly improved by heating, when the sheets were prepared from fibrillated celluloses. Thus, it is likely that cellulose fibrils at the inter-fiber bonding areas in the sheets are entangled in the cross-linked PAE molecules by heating the PAE-containing cellulose sheets, resulting in the clear wet-strength development. These physical interactions occurring between fibrillated cellulose fibers and crosslinked PAE molecules by heating the PAE-containing cellulose sheets are significant as well as chemical interactions such as formation of ester bonds in wet-strength development of cellulose sheets by PAE.