The Effects of Pre-evaporation Time on the Structure and Performance of Na-Lignosulfonate/PVA Composite Films

Lignin, an abundant renewable biopolymer in nature, contains specific structural limitations affecting its application strategies. So, a favoured strategy is to use it into the manufacturing of composite materials with other natural or synthetic polymers. This study examines the impact of controlled accelerated solvent removal (pre-evaporation), a subject that is still not thoroughly explored, on the structural, morphological, and thermal characteristics of Na-lignosulfonate (LS) and polyvinyl alcohol (PVA) composite films. Aqueous solutions containing LS, PVA, glycerol and urea were formulated for the casting of composite films. A pre-evaporation at 85-90 °C for 0 to 240 min was employed to solution enhancing its concentration prior to being poured into moulds. The 13C-NMR and FTIR used for structural characterisation validated the specific aromatic and aliphatic structure of LS. FTIR analysis identified interactions between LS and PVA in the films, indicating changes in the ether bond peaks (~1039 cm⁻¹). TGA demonstrated that LS enhanced the thermal stability of the composites and facilitated carbonisation above 450°C. Examination by SEM of the composite films indicated that the pre-evaporation duration significantly affected the morphology. A 5–14% decrease in film thickness and an increase in pore size and number, particularly on the exposed surface, were observed, but the change did not occur uniformly. LS/PVA films were effectively produced without internal stress during pre-evaporation durations of up to 150 min. At times exceeding 150 min, excessive solvent loss and increased solution viscosity resulted in significant morphological defects such as internal stresses, large pores, and structural integrity failure, leading film casting impossible at 240 min. The study determined that the 150 min pre-evaporation procedure is an essential limit for the production of composite films from these polymers. The results obtained are crucial for improving the processing parameters of lignin-based composites intended for packaging or carbon precursor applications.