Ionized Carboxyl Protonation Strategy to Improve Water Resistance of Carboxymethyl Cellulose-Based Ultraviolet-Shielding Films

The extensive and inflexible molecular structure, coupled with the significant presence of ionized carboxyl groups (−COO–) and hydroxyl groups (−OH) in carboxymethyl cellulose (CMC), leads to inadequate water resistance and mechanical properties of films made from CMC. Consequently, their application in the packaging industry is limited. Herein, CMC/sodium alginate (SA)/carbon quantum dots (CQDs)/H+ (CSCH) films are prepared by ionized carboxyl protonation (acid impregnation) using CMC as a substrate and SA as a reinforcement filler. The protonation of the −COO– groups of CMC and SA by the H+ from the acid reduces the electrostatic repulsion between the molecular chains of CMC and SA. Meanwhile, this also strengthens the hydrogen-bonding network within the CMC composite film and collaborates with SA to enhance the strength of the CMC composite film and improve its water resistance. Additionally, the introduction of CQDs provides excellent ultraviolet (UV)-shielding properties of CSCH films. As a result, the water-resistant CSC2H150 film exhibits outstanding mechanical properties (tensile strength: 60.00 MPa), water resistance (solubility rate: 20.32%), and an average UV transmittance of 5.18%. The study proposes a simple approach for making water-resistant UV-shielding packaging materials with good mechanical properties, promising many applications.