Chemical thermoplasticization of lignocellulosic fibers by reactive extrusion

Abstract

Cellulosic thermoplastics are anticipated as promising replacements to petroleum-based thermoplastics, but their high manufacturing costs have limited wide-spread application. The primary objectives of this thesis were to use low-cost lignocellulose, practically forestry waste, as the raw material rather than more expensive purified cellulose in the preparation of new plastics and, consequently, to develop an economical reactive process focused on diminishing the use of expensive solvents in the thermoplasticization of lignocellulose. The thermoplasticization of lignocellulosic fibers started by developing a high solids content (60 wt%) twin-screw extrusion technique to defibrillate the raw material for the subsequent chemical modification. By this approach, the received lignocellulosic fibers showed improving handling as a feedstock for extrusion as well as chemical accessibility. To effectively wet the lignocellulosic fibers for chemical modification and avoid using expensive and largely ineffective solvents, a low-cost additive was derived by mimicking aspects of an ionic liquid using benzethonium chloride (hyamine) and sulfuric acid. The effectiveness of the hyamine/sulfuric acid wetting agent was demonstrated initially in a bench-top method where the additive also became chemically bonded to the lignocellulose and strongly contributed to its thermoplasticity. During acetylation, this new and low-cost wetting/functionalizing agent converted the lignocellulosic fibers into a compression-moldable thermoplastic. The molar ratio of benzethonium chloride to sulfuric acid was found to be the most significant variable to determine grafting behaviour as well as degradation of the polymer chains. Subsequently, this new modification chemistry was translated over to the environment of a twin-screw extruder to devise a continuous, greener method of thermoplasticization for lignocellulose. The new reactive extrusion process had a short reaction time of 45-90 s and yet showed a good tendency for producing a flowable thermoplastic suitable for melt molding without plasticizers. A notable benefit to the method was the moldable lignocellulosic bioplastic maintained the excellent stiffness inherent to cellulose. Moreover, by the reactive extrusion method, the properties of the lignocellulosic thermoplastics were found to be tunable with the selected esterifying agents (butyric anhydride versus acetyl anhydride) and the molar ratio of benzethonium chloride to sulfuric acid. A statistical analysis based on a Design of Experiment method revealed details on desirable extrusion conditions. The project concluded with improvements to the high solids-content process was exploring the novel concept of a recycle stream for reactive extrusion. The excessive esterifying agent content used in the initial studies was necessary to lubricate the fibrous mass inside the extruder else it would jam the process. This meant that the extrudate left the extruder with an unnecessary amount of reactant and required costly cleaning. The idea of recycling a portion of the newly made cellulosic thermoplastic was to add a natural lubricant and thereby lower the content of the esterifying agent in the extruder. Under optimal recycling conditions, a significant 50% decline in reactant was possible without decreasing the degree of modification or harming the thermoplasticity of the modified lignocellulose.