Cationic starch as strengthening agent in nanofibrillated and bacterial cellulose nanopapers

Cellulose nanofibrils (CNF) are obtained by mech. fibrillating cellulose fibers into their elementary nanofibrils. By contrast, bacterial cellulose (BC) is produced by the Komagataeibacter strain of bacteria in the form of a dense, wet membrane. The filtration and subsequent drying of a nanocellulose suspension creates a strong network of hydrogen bonded nanofibrils called nanopaper. Both CNF and BC nanopapers are emerging green materials trialled for various applications such as fuel cell membranes, filtration membranes, electronics and polymer composite reinforcement. A need to optimize nanopaper manufg. both in terms of time and efficiency has arisen. Cationic starch is used systematically in the papermaking industry wet end to both enhance paper strength and reduce dewatering time. Inspired from this, we used cationic starch during the manufg. of BC and CNF nanopapers to verify if similar effects could be obtained on nanopapers. BC and CNF nanopapers of 65-70 g m-2 contg. 0, 1, 2, 5 and 8% cationic starch, resp., were manufd. The filtration time of the corresponding suspensions increased incrementally with CS content for the BC while 2% of CS halved filtration time for CNF suspensions. Addn. of more than 2% CS in the CNF nanopaper, however, increased filtration time exponentially. The zeta potential measurement of the suspensions revealed the stability of the BC suspension was not influenced by CS whereas the addn. of CS in CNF suspensions facilitated flocculation. For both BC and CNF nanopapers, the addn. of CS lead to an incremental decrease in porosity. The tensile modulus and strength of BC nanopapers increased with CS content reaching up to 21.4 GPa and 249 MPa resp. This was also the case for CNF nanopapers up to 2% CS. Further addn. of the cationic additive degraded significantly the nanopaper strength which was accompanied by a decrease in strain at break. This was hypothesised to be due to an overlapping effect of CS with the (~23%) hemicellulose in our CNF nanopapers. The mechanisms leading to the increase in mech. properties of the nanopapers with the addn. of CS will also be discussed in the presentation