Mechanically robust multifunctional starch films reinforced by surface-tailored nanofibrillated cellulose

The objective of this study was to develop a transparent, mechanically robust, and fully biodegradable film made of sweet potato starch (SPS) reinforced with nanofibrillated cellulose (NFC) to serve as a potential alternative for packaging. Various surface modifications were applied to NFC using urea/NaOH (UA), oxalic acid (OA), citric acid (CA), and (3-mercaptopropyl) trimethoxysilane (MT). SPS/NFC blend was innovatively treated by ballmilling, incorporating different concentrations (2-10 wt%) of surface -modified NFC and calcium gluconate as a modifier. Interestingly, the solution -cast blend films possessed improved tensile strength, tensile modulus, crystallinity, barrier property and hydrophobicity after NFC additions. More importantly, ball -milling promoted the physical combination of starch and NFC and generated metal -organic supramolecular interaction between -OH and Ca2+. In particular, the SPS-OA/NFC6 composite film exhibited remarkable improvements in tensile strength and tensile modulus, increasing from 1.46 MPa to 13.7 MPa and 6.04 MPato 531.33 MPa, respectively, compared to the neat SPS film. The contact angle of the SPS/CA-NFC6 film was approximately 79% higher than that of the pure SPS film. Generally, the optimal NFC addition was 6 wt%. This study provides guidance for the production of a new type of starch -based material with high mechanical strength and transparency.