Dynamic Schiff Base Chitosan–Lignin Bioplastics with Thermally Assisted Repair and a Reprocessable Network

The development of renewable polymer networks combining mechanical performance with repairability and reprocessability is important for advancing sustainable materials. In this study, chitosan-dialdehyde lignin (Ch-DAL) bioplastic films were fabricated through dynamic Schiff base cross-linking between tartaric acid-protonated chitosan and periodate-oxidized lignin, with glycerol as a plasticizer. Films with Ch:DAL ratios of 1:1, 2:1, and 1:2 were characterized by Fourier-transform infrared (FTIR) spectroscopy, tensile testing, qualitative solvent-resistance assessment, a thermally assisted repair test, and two successive thermo-mechanical reprocessing cycles. FTIR analysis was consistent with the possible formation of imine (C=N) linkages and the contribution of hydrogen-bonding interactions, although aldehyde content, degree of conversion, and cross-link density were not quantitatively determined. The mechanical properties were composition-dependent, with the 1:1 formulation exhibiting the highest tensile strength (8.85 ± 0.09 MPa) and Young’s modulus (118.7 ± 0.7 MPa), whereas the 2:1 formulation exhibited the highest elongation at break (238 ± 0.1%). Thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG) showed distinct thermal-degradation profiles for alkali lignin, DAL, and the 1:1 Ch–DAL film, with residual masses of approximately 17%, 57%, and 33% at 600 °C, respectively. Qualitative assessment showed that the films retained structural integrity in water, 1 M sodium hydroxide (NaOH), and the investigated organic solvents but underwent visible fragmentation in 1 M hydrochloric acid (HCl). Following hot pressing of the cut 1:1 film at 50 °C, macroscopic continuity was re-established and tensile strength increased to 12.9 ± 0.11 MPa; however, elongation at break decreased from 19.0 ± 0.1% to 4.3 ± 0.05%, indicating a mechanically distinct repaired state rather than recovery of the original properties. Fragmented films could also be remolded through two successive thermo-mechanical reprocessing cycles, although tensile strength decreased substantially after the first cycle and recovered only partially during the second cycle. These findings demonstrate composition-dependent mechanical behavior, thermally assisted repair, and remolding capability in Ch-DAL films, while further quantitative characterization, durability testing, and environmental assessment are required before claims regarding long-term performance or sustainability can be established.