Light transmittance and gas permeability of alkaline lignin-poly vinyl alcohol film with formaldehyde crosslinker

Abstract: In order to improve the utilization value of an industrial alkaline lignin and expand the application range of alkali lignin, with alkaline lignin and poly (vinyl alcohol) as the primary materials, and formaldehyde as a crosslinker, alkaline lignin/poly (vinyl alcohol) cross-linked film was prepared by casting. The effects of the dosage of alkaline lignin, formaldehyde, and the pH values on the light transmittance and gas permeability of the films were investigated one at a time for different variables. The light transmittance and gas permeability properties were measured using a double-beam UV-Vis spectrophotometer and a VAC-VBS pressure difference gas permeameter respectively. The surface morphology and chemical structure of the films were characterized by SEM and FT-IR. The results were as follows. (1) Influence of different conditions on the optical properties of the films: Alkaline lignin had a great influence on the absorbance and transmittance of films. The UV absorbance of the reaction films was nearly 100% among 200-400nm and the transmittance decreased during 400-800nm in the visible area. When the mass ratio of the alkaline lignin and PVA was 1:4, the transmittance was 16.12% at 600nm. The more formaldehyde, the higher the transmittance of the reaction films in the visible area. Besides, when the pH value was 9, the film transmittance was better. (2) Influence of different conditions on the gas permeability properties of the films: Both the carbon dioxide and oxygen transmittances were reduced after adding alkaline lignin, but increased with the formaldehyde added and first increased then decreased with the pH value changing from 5 to 11. (3) FT-IR showed that the ether linkage was formed and the cross-linking reaction was occurring between the alkaline lignin and PVA. The surface of the reaction films was smooth. The water contact angle of the reaction film was bigger than the blend, although the water contact angle of the composites both increased with the alkaline lignin added, which showed that the water resistance of the film increased. When formaldehyde as cross-linking agent was compared with glutaraldehyde, the crosslinking reaction between alkali lignin and PVA was greater and the transmittance in the visible area was better. Alkaline lignin-PVA reaction film may be applied as a good UV absorption material in the mulching film.