Extraction of Lignin Using scCO2 and Preparation of High-Performance Composite Films with Poly(vinyl alcohol)

Abstract

Lignin (SCEL) was separated from steam-exploded eucalyptus wood fibers using a supercritical carbon dioxide (scCO2)–ethanol/water composite medium, which was compared with traditional industrial alkaline extraction of lignin (AL) and high-temperature ethanol extraction of lignin (EL). All three kinds of lignin were spray-dried to form microparticles. The obtained SCEL, AL, and EL particles were mixed with poly(vinyl alcohol) (PVOH) to prepare a composite film. Compared with the pure PVOH film, the transmittance of the composite film to ultraviolet light was reduced from more than 70% to less than 20%, and the tensile strengths of the SCEL/PVOH, AL/PVOH, and EL/PVOH films were increased by 79.2%, 39.1%, and 66.0%, respectively. The water contact angle was increased from 35.5° to 95.3°, 59.5°, and 66.7°, respectively. In terms of performance improvement, SCEL had a better effect. The SCEL particle size was small and uniform, and SCEL contained more S-type structural units and β-O-4 bonds, which is conducive to its uniform dispersion in the PVOH matrix and increases the contact area between lignin and ultraviolet light, thereby improving its anti-ultraviolet performance. The hydrophilic groups in lignin form a large number of hydrogen bonds with PVOH, which promotes the combination of lignin and PVOH, and at the same time exposes a large number of hydrophobic groups of lignin, which improves the mechanical properties and the hydrophobicity of the film.