Dissolution of cellulose and casein in ionic liquids and further fabrication of films and composites

Abstract

This research explored the dissolution of cellulose from unbleached and bleached birch DES pulp and microcrystalline cellulose, as well as casein from waste milk, in the ionic liquid EmimAc. 5 cellulose-casein ratios (5:1, 3:1, 1:1, 1:3, and 1:5) were prepared to create a bio-composite film. To enable an equal comparison between the ionic liquid systems, 4 additional ionic liquid systems at the 1:1 ratio were tested to determine the effect of the solvent on the characteristics of the bio-composite. All 10 combinations of the materials dissolved completely, though the time and temperature required to dissolve decreased as more casein was included in the mix, with microcrystalline cellulose dissolving the fastest. Fourier Transform Infrared Spectroscopy showed that at the molecular level, casein and cellulose were combined in all films formed from these fabrications by noticing the appearance and the systematic amplification of the amide I and II bands. Also, FT-IR analysis showed that progressive hydrogen bonding was occurring at the interface between the two biopolymers. X-ray diffraction confirmed full conversion of Cellulose I to Cellulose II for each of the samples, demonstrating that molecular dissolution occurred among each of the ionic liquid systems. The 3:1 bleached cellulose-casein film exhibited the highest tensile strength, outperforming unbleached films affected by residual lignin. The study highlighted the unique properties of an innovative film based on an ionic liquid system, which achieved an elongation at break of approximately 43% and exhibited rubber-like mechanical behaviour. A strong linear relationship was observed between surface wettability and casein content in each film. All films were shown to block UV light effectively; however, the unbleached films exhibited near-complete absorption across the entire UV spectrum due to residual lignin chromophores in the composite. All composite films showed complete structural disintegration under soil burial conditions within four weeks, thereby demonstrating complete end-of-life biodegradability. Cellulose-casein/ionic liquid composite films can serve as effective substrates for sustainable packaging, UV barriers, and flexible materials, turning milk by-products into valuable functional raw materials, with further modifications in the future.