The Effect of Lignin on the Mechanical, Thermal, and Morphological Properties of Carboxymethylcellulose/Pectin Bio-composite Films for Food Packaging Applications

Abstract

Biopolymers have been regarded as a promising alternative green material to curb the environmental and ecological concerns caused by the increased use of petroleum-based polymers in flexible packaging films. The current work incorporates kraft lignin and deep eutectic solvent (DES; propionic acid and urea in a 2:1 molar ratio) into a carboxymethylcellulose (CMC)/pectin bio-composite matrix. Three sets of films were prepared and analyzed for tensile strength (TS) and elongation at break (EB), water vapour permeability (WVP), film composition, surface morphology, and thermal properties using a force tester, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). The first set of films was prepared to compare biopolymer films with varying DES loadings. The second set of films was prepared using a mixture design of experiments (DoE) and response surface methodology to explore varying film compositions with CMC, pectin and lignin as variables. The experimental validation of TS and EB confirmed that the prediction model was valid. The third set of films was prepared to evaluate one promising composite film formulation in detail by comparing the original formulation to similar formulations without lignin or lignin-DES. The formulation most suited for packaging applications (within the composition range studied) consisted of CMC/pectin/lignin weight ratio at 1.213 g/0.737 g/0.05 g per 2 g w/v (dissolved in 100 mL milli-Q water) with 1.5 g DES. In general, the DES acted as a green plasticizer, and lignin exhibited a plasticizing effect at low loadings. The as-prepared samples provide a promising low-cost approach for fabricating biodegradable plastic films from waste materials and have potential applications in food packaging.