Ultrasonic Assisted Assembly of Lignin Nanoparticles for UV-absorbent Thin Films

Abstract

Lignin is a biproduct made from pulping wood and is the second most available biopolymer in the world. Although it is composed of useful aromatic and phenolic groups, making it a great UV absorber and antioxidant, its macrostructure is extremely complex and difficult to characterize and utilize with today’s technology. In addition to these challenges, its characteristics can change based on the species of wood and type of pulping process used to extract the lignin. In the past lignin has been considered a waste rather than a high value product and is typically burned for energy, rather than utilizing its advantageous properties. Many recent studies have successfully found ways to valorize lignin by synthesizing nanoparticles from them for many applications such as: nanocomposite fillers for mechanical reinforcement, UV absorbing sunscreens and films, drug delivery, antioxidants, and many other applications. Here lignin nanoparticles are synthesized from alkali lignin and used for UV absorption and mechanical reinforcement in a polyvinyl alcohol (PVA) polymer matrix. In chapter 1 ultrasonic forces were utilized for the self-assembly of bimodally distributed sizes of lignin molecules to a monodisperse particle solution. The pH dependence of the reactions was studied and a starting pH of 10.7 was found to create the best monodisperse nanoparticle solution with great colloidal stability, having a particle diameter of 204 nm and a zeta potential of -60 mV. In chapter 2 these particles were investigated for their UV and mechanical reinforcement. With only a 2.5 wt% addition of lignin nanoparticles the SPF value of the films increased from 0 to 26. At the same nanoparticle loading the ultimate tensile strength of the PVA nanocomposite was increased from 41 MPa to 60 MPa, the elastic modulus increased from 1256 MPa to 1713 MPa, while the change in strain in break was not statistically significant. In chapter 3 an acetylation reaction was done on the lignin and resulted in superior UV performance with an increase to 33 SPF, however the mechanical reinforcement suffered greatly. The research done here has shown that with science, technology, and innovation that lignin can be converted into a high value product.