Preparation and characterization of sustainable lignin from oil palm empty fruit bunch (OPEFB) for polylactic acid (PLA) biocomposite material in 3rd printing

Abstract

3D printing is one of the additive manufacturing technologies that has widely been used in the automotive and manufacturing industry. Polylactic acid (PLA) is one of the materials used in 3D printing, made up of linear polymeric structure, that resulted in lower mechanical properties compared to other polymer materials used such as polyamide (PA), acrylonitrile butadiene styrene (ABS), and polycarbonate (PC). The reinforcement of lignin into PLA is not only capable to improve its stiffness but also provides thermal stability and antioxidant properties in PLA/lignin biocomposite. However, the interfacial adhesion between PLA and lignin had reduced the tensile strength and elongation at break of this biocomposite. Hence, this study aimed to utilize the lignin from oil palm empty fruit bunch (OPEFB) by dioxane-based extraction in PLA/lignin biocomposite. A dioxane-based extraction method is one of the solvent extraction processes capable to extract the native structure of lignin from lignocellulosic biomass. OPEFB was used as the source of lignin due to its availability in South East Asia (SEA). The lignin extraction process was optimized by using one-factor-at-time (OFAT) and response surface method (RSM) optimization. The factors that been optimized were temperature (range: 70 to 90ºC), dioxane concentration (range: 90 to 97 %(v/v)), solvent/solid ratio (range: 6 to 10 ml/g), hydrochloric acid concentration (range: 0.1 to 0.5 M) and retention time (range: 40 to 140 min). The optimized factors were further used to extract lignin for PLA/lignin biocomposite. The PLA/lignin biocomposite samples were prepared with a lignin content of 0.5, 1.0, 1.5 and 2.0 wt% in filament and 3D printed form. The highest extraction yield of lignin was 10.64% by using 1,4-dioxane with 0.1M of hydrochloric acid (HCl) as an acid catalyst at 90ºC and 10 ml/g of solvent/solid ratio for 140 minutes. The extracted lignin consisted of 92% of acid-insoluble lignin and 0.1% of acid-soluble lignin. The Fourier-transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) confirmed the release of lignin with low contamination of cellulose and hemicellulose. Apart from that, lignin from OPEFB showed an additional carbonyl group in the chemical structure of lignin. Thermogravimetric analysis (TGA) showed that the extracted lignin started to degrade around 200ºC. The Young’s modulus had increased 27% after the reinforcement of 0.5 wt% of lignin (PLAL0.5) compared to PLA. No reduction in tensile strength and elongation at break was observed during the tensile test. Lignin also acted as a nucleation crystallization agent, which could increase the crystallinity of PLA/lignin biocomposite and provide mechanical strength. The differential scanning calorimetry (DSC) confirmed that the crystallinity of PLA/lignin biocomposite was increased only after 1 wt% of lignin reinforcement (PLAL1.0). The 3D printing that involved the melting and cooling process further improved the degree of crystallinity (Xc) of PLAL1.0. Hence, the PLAL1.0 was selected as the best lignin content into PLA with the highest value of Young’s modulus of 2.14 GPa. Also, no interlayer adhesion was observed in 3D printed PLAL1.0. The lignin from OPEFB by dioxane-based extraction successfully increase the stiffness without any reduction in the ductility of PLA for 3D printing application.