Evaluation of biochar and polylactic acid composites for sustainable plant containers

Abstract

With the rise in concerns regarding plastic pollution and lack of efficient recycling strategies, research has begun to explore new sustainability routes for materials. Biodegradable polymers present an alternative strategy for sustainability, with their complete decomposition allowing for environmental preservation and diminished concerns of microplastic toxicity. In particular polylactic acid (PLA) has become highly commercialized compared to other biodegradable polymers and is advantageous due to its high mechanical properties and bio-based derivation. However, PLA has significant drawbacks including increased brittleness, expensiveness, and difficult biodegradation process (requiring industrial composting). This work seeks to address these issues by investigating a filler material known as biochar for use in PLA composites. Biochar, the solid byproduct of the biofuel production process, is a renewable and inexpensive material that is highly tailorable. Because of its carbon sequestration capabilities and soil amendment properties, biochar and the compostable polymer PLA, are candidates for replacement of traditional polymer plant containers, of which millions are produced each year. However, biochar further increases polymer brittleness, especially at high concentrations (>10 wt.%). Therefore, the focus of this dissertation is compatibilization of the biochar and PLA composites through biochar tailoring and additive investigations, resulting in flexible plant containers that efficiently degrade in the environment. Additive investigations included two plasticizers (polyethylene glycol and STRUKTOL) as well as the cellulose/lignin-based cork particles. It was determined that though cork may have shown more promise in maintaining tensile properties, polyethylene glycol (PEG) was a promising plasticizer that should be used in greater amounts (>3 wt.%) for effectiveness. For the study on biochar, the biochar was produced and tailored according to production temperature, resulting in varying functional groups, composition, and textural parameters. It was determined that biochar produced at a higher pyrolysis temperature (537°C) provided better surface interactions through its surface area, porosity, and wettability. This resulted in improved mechanical properties to that of the lower temperature (450°C) biochar. It was then determined that higher temperature biochar and PEG addition (10 wt.%) could assist in optimizing properties, and the composites were tested for their viscoelasticity and environmental stability. It was found that composite stiffness decreased due to the presence of the plasticizer, as well as improved material ductility. Furthermore, material biodegradation improved due to the addition of high amount of biochar and PEG, attributed to hydrolysis of the composites through increased water permeability. Overall, up to 40 wt.% of the expensive PLA polymer was replaced with biochar and PEG, resulting in a more flexible and environmentally degradable plant container.