Nanocellulose, Cationic Starch and Paper Strength

摘要

The purpose of this work is to help understand the effects of adding nanocellulose - in combination with a dry-strength agent such as cationic starch -on the strength properties of paper. Recently it has been found that combinations of nanofibrillated cellulose (NFC) with high levels of cationic starch can achieve a much greater increases in paper's tensile strength and stiffness compared to either additive on its own. This paper considers how the observed effects can be accounted for based on capillary forces, the forming of hydrogen bonds during paper drying, and forming of polyelectrolyte complexes. Recent research showed that adding pre-mixed cationic starch and NFC to papermaking furnish made it possible to maintain tensile strength and stiffness while decreasing the refining level and achieving much lower levels of apparent density. The resulting paper was bulkier but still strong. It is proposed here that the observed effects involve formation of dense attachments at the nano-scale, but that a "spring-back" effect of the NFC-starch complex avoids the overall densification that occurs when papermakers rely on refining of the main furnish to achieve paper strength objectives. The initial, nano-scale densification is predicted by the capillary force theories of Campbell and Page. Elastic forces appear to facilitate retraction of some meniscuses, allowing the paper to regain some of its bulk after pressing. It appears that NFC can play a synergistic role as a kind of "extender" for cationic starch, allowing the starch to behave as if it were a long, fibrillar material capable of filling the pore spaces within typical paper. Pronounced decreases in air-permeability of the resulting paper, even when the apparent density has been decreased relative to a default paper sheet, provide supporting evidence for such a concept. Because the cationic starch and NFC surfaces have opposite charge, the concepts of polyelectrolyte complexes may apply.