Abstract:
Problem Definitionand and Objectives: Growing concerns about plastic pollution have increased the interest in biopolymers such as polylactic acid (PLA) due to their biodegradability and processability. However, the brittleness of PLA poses limitations in structural applications. Natural reinforcements such as wood flour (WF) and microcrystalline cellulose (MCC) can improve the mechanical properties of PLA, but their uniform dispersion and phase coherence depend on the extrusion method and the type of 3D printing. This study investigates the effect of extrusion repetition and comparison of two 3D printing methods, fused filament (FFF) and fused granulation (FGF), on the morphological and structural properties of PLA-WF-MCC biocomposites to determine the most optimal production conditions for structural applications such as chair printing.
Materials and Methods: In this study, PLA granules were used as the main matrix, MCC as the reinforcing phase, and WF as the filler to produce biocomposites. The PLA-WF-MCC blend was passed through a single-stage and two-stage twin-screw extruder to make the composite. After the extrusion process, the biocomposites were divided into four different methods to investigate the effects of the extrusion process repetition and 3D printing methods on their morphological characteristics. 3D printing methods including fused filament (FFF) and fused granule (FGF) were used to make 3D samples. All the setting parameters in the two 3D printing methods included a temperature of 210°C for filament and granule, 60°C for the printing bed, a printing speed of 40 mm/s, a layer thickness of 0.2 mm, and a filler density of 100%. In order to investigate the interphase adhesion, particle dispersion, microstructure, porosity and structural quality, scanning electron microscope (SEM) images of extruded and printed samples were prepared and analyzed. These images were recorded at different magnifications and used to analyze and compare morphological features and structural density.
Results: The results of SEM images showed that in the single-extruded samples, uniform and homogeneous dispersion of MCC was observed in the PLA matrix and the adhesion between the phases was well established. Also, in these samples, no clear boundary was observed between the polymer matrix and MCC, indicating good and homogeneous mixing of the materials. In contrast, the double-extruded samples suffered from interphase separation and the formation of micropores in the phase boundary areas. These differences were due to the high thermal sensitivity of PLA and its partial degradation in the two-step extrusion process, which reduced the structural quality and created porosity in the samples. These results indicate that in biocomposites, the number of extrusion steps plays an important role in maintaining the balance between proper mixing and structural integrity. In the 3D printing process, the once-extruded samples were selected for printing due to their uniform dispersion and better morphological properties. Among the two printing methods, the samples fabricated by the granular method had better morphological properties and showed less porosity than the filament samples. In this method, due to the reduction of thermal stresses and improved bonding of the layers, a denser structure was obtained, which is more suitable for printing a chair. Finally, the chair was successfully printed by the granular method.
Conclusion: Based on the results of this study, the single-extruded samples had a uniform dispersion of MCC in the PLA matrix, which improved interfacial adhesion and structural integrity. In PLA/WF/MCC biocomposites, the double-extrusion process caused interfacial separation and voids, which is related to the high thermal sensitivity of PLA. These results indicate that choosing the optimal extrusion level is essential to maintain a balance between mixing and structural stability. Also, in the 3D printing process, single-extruded samples with desirable microstructural characteristics were selected for the filament and granular methods. In the granular method, fewer voids were observed compared to the filament method and a denser structure was provided for 3D printing, which improved the structural quality of the parts. The granular method was selected as a more suitable method for chair printing due to the advantages of reducing thermal stresses, improving uniform material distribution, and reducing voids.