Comparing industrial and analytical kraft lignins as biobased UV-protective additives in coatings

Abstract

Lignin as the second most abundant biopolymer on Earth has the potential to become the alternative to petroleum-derived materials. It exhibits excellent UV absorption ability due to its aromatic structure and the presence of numerous phenolic, ketone, and intramolecular hydrogen bonds. Due to its complex nature, it is important to investigate its properties which is a very important step towards the valorization of lignin. Revealing its structural complexity allows for a better examination of its influence on the properties of the final lignin-based materials. In our research, we used two different kraft lignins: commercial analytical kraft lignin (AL) and industrial LignoBoost kraft lignin (KL) as UV-protect additives in BPA(Bisphenol A)-free polymer coatings based on diurethane dimethacrylate (DIUR). The maximum addition of KL and AL was 2 wt%. Both lignin samples were characterized in detail (composition analysis, ash content, molar mass and polydispersity, surface morphology, thermal properties, and quantitative measurement of hydroxyl group content). We investigated the influence of lignins on the textural and thermal properties of the coatings. Finally, we studied the application of lignin as a value-added UV-protective component by UV-Vis electronic absorption spectroscopy. KL with a higher purity and lower number of aliphatic OH had a better dispersion in the polymer matrix than AL lignin which had more agglomeration in the polymer matrix. The better dispersion resulted in producing a smoother surface in the coatings made from KL. Finally, a noticeable and significant impact of KL additive on the photoprotective properties of the coating material was demonstrated. These results showed a potential application and opportunity in the valorization of available industrial lignins toward sustainable and value-added products.