Desempenho de nanopartículas de lignina kraft como revestimento em papel para embalagem

Abstract

Lignin is one of the most abundant biopolymers in plant biomass and has stood out due to its diverse properties and applications. It has a rich structure with carboxylic groups, functional binders, and phenolic hydroxyls that give lignin properties that make it an excellent base for the development of new materials. Converting a material into nanoscale can enhance its properties, allowing or facilitating the use of this material in the formulation of new products. Additionally, the presence of lignin makes the colors of lignocellulosic fibers darker after exposure to photo-irradiation, which can result in a positive effect for its application as a barrier to ultraviolet (UV) radiation. Ultraviolet radiation is responsible for the discoloration of dyes and pigments, yellowing of plastics and papers, loss of mechanical properties, degradation, and cracking in various products, which leads to the interest of manufacturers of paints, plastics, papers, wood, and cosmetics in offering products that remain unchanged for long periods under severe light exposure conditions. In the synthesis of nanoparticles, the Bottom-up approach consists of the self-assembly of solubilized lignin macromolecules through a mechanism that is triggered by solvent exchange. In this method, also called solvent-antisolvent, solubilized lignin is precipitated with an antisolvent. The aim of the present study was to prepare Kraft lignin nanoparticles through the Bottom-up approach to obtain colloidal dispersions of nanoparticles. These dispersions were used in the formulation of biopolymeric film-forming blends based on starch and used in paper coating for packaging. Initially, lignin was dissolved in an acetone/water solution 7:3 v/v and then added to water, resulting in the precipitation of lignin into nanoparticles and the formation of a colloidal dispersion that was sonicated for 10 minutes at a power of 50% amplitude. Kraftliner papers were coated with blends containing starch (4% w/w) and different concentrations of lignin nanoparticles. The prepared dispersion showed colloidal characteristics, as lignin particles had their size reduced by almost a thousand times compared to the original material. Stability tests showed no phase separation for the diluted dispersion, even after 4 weeks, while the dispersion with a concentration of 8.2 mg/mL reached 80% stability, with slight precipitation observed at the bottom of the tube and phase separation at the top. The morphology of the coated papers was analyzed, and it was observed that Kraft lignin nanoparticles adhered to the paper microfibrils appear as small spheres on the surface. The barrier properties of the coated papers showed an improvement trend with the increase in nanoparticle concentration in starch-containing films, and lower water absorption in papers coated exclusively with lignin nanoparticle dispersion (without starch) was observed. The coated papers showed grease barrier for all treatments coated with starch-containing dispersions. In summary, the Bottom-up approach used in the preparation of Kraft lignin nanoparticles proved to be a promising method for obtaining stable colloidal dispersions. Lignin nanoparticles have potential for application as paper coating material for packaging, providing UV radiation barrier and hydrophobicity.