DEVELOPMENT OF HIGHLY LOADED POLYLACTIC ACID/LIGNIN BIOCOMPOSITES FOR MATERIAL EXTRUSION ADDITIVE MANUFACTURING

Abstract

There is a constant need to improve existing polymer composites, enhance their physical and mechanical characteristics and reduce costs in order to meet the growing requirements that are important for practical applications, in particular 3D printing. Promising in this regard are composites based on polylactic acid (PLA) and lignin, which are bio based and biodegradable and thus an attractive alternative to petroleum-based polymers. This work focuses on the preparation of cost-effective PLA-based biocomposites with high lignin loadings with improved mechanical properties compared to existing ones, as well as studying their suitability as a feedstock for material extrusion additive manufacturing . First, the effect of a new type of melt-flowable organosolv lignin called Bioleum (BL) on the properties of PLA/BL biocomposites with a BL loading up to 40 wt% was investigated. In addition, the changes in properties after the incorporation of polyethylene glycol (PEG) and triethyl citrate (TEC) plasticizers were also studied. Characterization of microstructure, mechanical, thermal, and spectral properties of the obtained biocomposites was conducted. The obtained results were explained and compared with those reported in the literature. It was found that the BL domain size increased as the BL content was increased, causing a drop in the strength and ductility. However, PLA_BL biocomposites exhibited higher tensile strength than those previously reported. The addition of 1 wt% PEG in PLA composite with 20 wt% BL significantly reduced the BL domain size and enhanced the elongation at break by 221%. Additionally, with 5 wt% PEG, the PLA composite with 20 wt% resulted in an increase of about 9 times the elongation at break compared to pure PLA. Secondly, grafting of glycidyl methacrylate (GMA) onto BL using a melt mixing process was carried out. The obtained BL-g-GMA was used for blending with PLA up to 40 wt%. The performance of PLA-based biocomposites using neat BL and BL-g-GMA was compared, based on which the ability of the latter to improve their mechanical properties was confirmed. The results revealed that PLA and BL compatibility increased significantly with the grafting and resulted in decreased domain size of BL-g-GMA and thus enhanced all the tensile properties (strength, modulus, and elongation at break) at BL loadings as high as 40 wt%. This study revealed that grafting/melt mixing method can be adopted to enhance the performance of lignin based biocomposites. Next, PLA/BL-g-GMA biocomposites with lignin content of 40 wt% in the form of filaments were fabricated through a single-screw extruder. The printability of the optimum filament and the performance of resultant prints were assessed. One factor at a time (OFAT) method was conducted to optimize the process condition during the manufacturing of the composite filaments as well as their 3D printing process. The influence of four printing process parameters viz. nozzle temperature, layer height, print speed and print direction on the overall print quality, mesostructure, and tensile properties of the printed biocomposites was investigated. Overall, the biocomposites exhibited good processability and printability. The 3D printing experiments revealed that the print quality and mechanical properties are strongly dependent on the print temperature, residence time, and GMA content. Lower print temperatures and residence times resulted in higher tensile strengths. The biocomposite with BL-g-5GMA showed the highest tensile strength while the greatest elongation at break was associated with BL-g-10GMA. This suggests that

lignin-based biocomposites are a feasible and scalable alternative for 3D printing feedstock.