Wet Strength Improvement of Nanofibrillated Cellulose Film Using Polyamideamine-Epichlorohydrin (PAE) Resin: The Role of Carboxyl Contents

An approach to improve water resistance and wet strength of films from nanofibrillated cellulose (NFC) was investigated using polyamideamineepichlorohydrin (PAE) as a cross-linker. To increase the cross-linking reaction, carboxymethylation of pulp fiber was conducted as a chemical pretreatment. NFC was prepared by grinding, and the pass number required in the grinding process differed depending on the carboxyl contents introduced by the carboxymethylation process. First, PAE was added to the NFC suspension, and then NFC films were prepared by casting the suspension followed by heat treatment. The covalent bond formation between the azetidium groups of PAE and carboxyl groups of NFC was confirmed by Fourier transform infrared (FT-IR) spectroscopy analysis. The water contact angle indicated that hydrophobicity of PAE/NFC films remarkably increased by the carboxyl content of NFC indicating that the carboxyl groups interacted with the cationic PAE to give higher water contact angles. While the dry tensile strength of NFC films was not influenced by PAE, wet tensile strength was clearly improved with PAE.