The mechanism of wet-strength development of cellulose sheets prepared with polyamideamine-epichlorohydrin (PAE) resin

Abstract

The mechanism of wet-strength development of cellulose sheets prepared with typical wet-strength additive, polyamideamine-epichlorohydrin resin (PAE), was studied by some analytical techniques. PAE-containing sheets were prepared from cellulose fibers with different carboxyl contents by either internal addition of PAE to slurries of cellulose fibers or soaking treatment of once-dried cellulose sheets with aqueous PAE solutions. The results showed that carboxyl groups in cellulose fibers behaved as anionic retention sites of cationic PAE molecules for both internal addition of PAE in the sheet-making process and soaking treatment of cellulose sheets with PAE solutions. Moreover, wet-tensile strength of the PAE-containing sheets was clearly improved by heating treatment of the sheets at 110 °C for 10 min, when the cellulose fibers of the sheets clearly had carboxyl groups. FT-IR analysis of PAE films showed that ester bond formation between carboxyl groups at the end of polyamideamine chains in PAE and azetidinium groups of PAE, i.e. within PAE molecules, are possible to occur at least in heated PAE films. PAE-containing cellulose sheets were then subjected to cellulase treatment to obtain PAE-rich fractions in the sheets by removing most of cellulose and hemicellulose fractions without any interactions with PAE. The cellulase-treated residues obtained in the yields of about 1% were analyzed by FT-IR. The result revealed that significant amounts of ester bonds formed between azetidinium groups of PAE and carboxyl groups of cellulose fibers were present in the PAE-containing cellulose sheets, and that these ester bonds increased by heating treatment of the sheets. Thus, it was concluded that wet-strength development of PAE-containing cellulose sheets is primarily ascribed to the ester bond formation between azetidinium groups of PAE and carboxyl groups of cellulose fibers, i.e. the covalent bond formation.