Bacterial Cellulose Nanocomposites with Lignin: Fabrication and Characterization of Structure, Mechanical & Thermal Properties

Abstract

From 1950 to 2018, the production of synthetic plastics has grown dramatically from 1.5 million tons per year to 360 million tons per year 1. The unsustainable production of synthetic plastics have not only introduced massive waste in the ocean, released greenhouse gases but also pose potential health threat to all life forms. Therefore, there is a significant need to produce sustainable polymers, from renewable recourses, that are either recyclable or biodegradable. Cellulose, the world’s most abundant biopolymer is synthesized from plants, algae as well as some bacteria, and because of its exceptional properties, such as being bio-degradable, easily functionalizable, and having remarkable mechanical properties which are comparable to Kevlar and carbon fiber 2,3, it has received the significant attention during the past decades. Among many types of cellulose, bacterial cellulose (BC) fibers come in as a matrix-free form, and do not require purification process as complicated as wood-extracted cellulose. BC in the form of pure nanocellulose microfiber has been reported giving rise to materials with remarkable Young’s modulus (~66GPa 2), strength (~1GPa 2), and toughness (16.9 MJ/m 2) due to its high fiber aspect ratio, high crystallinity, and massive hydrogen bonding network 4,5. However, BC-based materials are extremely hydrophilic due to the presence of their surface hydroxyl groups, and due to their high rigidity, they are also brittle. In natural wood, lignin, formed by phenolic monomers, serves as a binder, together with other polymers, for cellulose, providing rigidity to the plant cells wall, supporting water transport, adding hydrophobic characteristic, and protecting wood against microorganisms 3,6. Recent literature shows that introducing lignin in wood-extracted cellulose fibers can create nanocomposite papers with improved mechanical properties, thermal stability and hydrophobicity compared to conventional cellulose paper 6.In this work, we fabricate a series of BC/lignin nanocomposites through a design of experiments method which allows systematic investigation of processing conditions. In particular, we study the effects of hot pressing time (10-30 minutes), temperature (120-160 °C), and pressure (5-15 MPa) in the structure and mechanical properties of the BC/lignin nanocomposites, as well as in the control BC. We also examine the effects of lignin as a binder in single-layer and multi-layered BC/lignin configurations. Our characterization methods include scanning electron microscopy, thermogravimetric analysis, X-ray diffraction, tensile testing, contact angle analysis, and Fourier-transform infrared spectroscopy. We find that processing conditions affect both the single-layer BC and BC/lignin nanocomposites. In the single-layer BC, the introduction of lignin improves the elongation to break, at the expense of Young's modulus and strength. In the multi-layered BC, the Young’s modulus and strength increase in the nanocomposites compared with the pure BC, demonstrating the efficiency of lignin as a binder for multi-layered cellulose composites. Moreover, we find both processing and introduction of lignin can improve the hydrophobicity of BC.