Development of thermoplastic 3D printing feedstock utilising biomass

Development of thermoplastic 3D printing feedstock utilising biomass

3D printing feedstock constituted by bio-based thermoplastic and biomass filler is increasingly gaining prominence for fused deposition modelling (FDM). Biomass fillers are abundantly available and sustainable resources, which are widely applied in biocomposites for producing cost-effective and sustainable materials. The incorporation of biomass filler in bio-based and biodegradable polylactide (PLA) has obtained considerable attention for FDM 3D printing application.

In this thesis, hemp hurd (HH), the waste product of hemp fibre industry, and bamboo powder (BP), the waste product of bamboo-pole slicing and bamboo-plank sanding, were used as biomass fillers for the preparation of FDM feedstock by meltcompounding and extrusion. Due to the toughness decrease with the inclusion of biomass powder, the toughening modification of PLA biocomposites was investigated using poly(butylene adipate-co terephthalate, PBAT) combined with ethylene-methyl acrylate-glycidyl methacrylate random terpolymer (EGMA), a commercially-available core-shell acrylic impact modifier (BPM520), and polycaprolactone (PCL). The toughening efficacy was compared in PLA/BP biocomposite, and the processability and printability of the toughened biocomposites were examined. PDLA-PCL-PDLA (PCDL) tri block copolymers were investigated as compatibilizers for addressing the phase-separation of PLA/PCL blend and enhancing the toughness of PLA/BP/PCL biocomposite. The optimal toughening agent PBAT/EGMA was used for studying the effects of the biomass powder loading levels and particle sizes on PLA/HH and PLA/BP biocomposites as FDM feedstock. The melt flow, rheological, thermo-mechanical, and mechanical properties of biocomposite pellets, filament quality, and finish quality of FDM-printed parts were systematically investigated.

The key findings of this research include understanding the relationship between toughness enhancement, loading levels and particle size distributions of HH and BP biomass species, and a major range of properties of PLA biocomposites, including melt flow, rheological, thermo mechanical, and mechanical properties of biocomposite pellets, filament quality, and finish quality of FDM printed parts, and the associated mechanisms. Also, PLA/HH and PLA/BP feedstocks were developed and appropriately applied in FDM 3D printing.

Experimental results showed that PLA/BP biocomposite toughened by PBAT/EGMA exhibited higher toughness, superior filament quality, improved processability, and lower surface roughness than BPM520-toughened feedstock, and enhanced toughness than commercial PLA feedstock for both FDM-printed and injection-moulded (IM) specimens. PCDL efficiently improved the compatibility between PLA and PCL, leading to improved toughness. The increment in the toughness of PLA/BP biocomposite using PCL as toughening agent and PCDL as compatibilizer was insignificant. Among the three toughening agents, PBAT/EGMA was optimum with respect to toughness enhancement and processability, together with printability. Increasing biomass loading levels resulted in increased complex viscosity and decreased melt flow, while the FDM filament retained diameter tolerance (within ±0.03 mm) and roundness (0.04 mm) meeting the requirement in GB/T 37643-2019 standard. IM specimens filled with 40 phr HH exhibited 10.8% increase in tensile strength, 12% increase in flexural strength, 62.5% increase in flexural modulus, whereas 38.5% decrease in impact strength, compared to the base polymer matrix. FDM-printed parts with up to 30 phr HH or BP incorporation showed higher impact toughness than the parts fabricated from the commercial PLA filament control (46±2.5 J/m). Also, the FDM-printed parts exhibited greater dimensional accuracy (decreased shrinkage) than the samples from PLA control. The shrinkage of all PLA/HH samples was lower than that of PLA (0.33±0.04 %) and decreased from 0.30±0.06 % (PLA-HH-0) to 0.03±0.01 % (PLA-HH-40), indicating the dimensional accuracy improved with increasing HH loading. The porosity increased from 5.8% for PLA-HH-0 to 17.9% for PLA-HH-40, and 16.9% for PLABP-40. The increase in biomass loading levels and particle sizes did not change the average surface roughness (Ra and Rq) when the particle size of biomass was smaller than the printing layer thickness, while increased the peak-to-valley height (Rz and Rmax) of FDM-printed parts. HH and BP particle sizes exhibited opposite effects on the melt flow and complex viscosity (|η*|) at low frequency, increased particle size
led to increased MFR and decreased |η*| for PLA/HH while decreased MFR and increased |η* for PLA/BP biocomposites. Larger particle size was advantageous for obtaining higher impact strength for both IM and FDM printed specimens for both PLA/HH and PLA/BP. The impact strength was improved from 41.3±3.0 J/m to 54.4±4.3 J/m for PLA/HH biocomposites. Impact strength was retained at around 55 J/m for FDM parts although the porosity increased from 4.86% to 9.85%, with concomitant particle size increase from 35 to 160 µm. PLA-BP-3 exhibited an impact strength of 13% and 38% higher than PLA-BP-1 for IM and FDM parts, respectively.

This thesis contributes to the utilization of biomass filler in 3D printing for obtaining renewable and sustainable feedstock. This research reinforces the understanding of the influence of toughening modification, biomass filler contents, and particle size distributions on the melt flow, mechanical properties, processability, and printability of PLA biocomposites, and the underlying mechanisms. The potential of producing PLA biocomposites and application in FDM are investigated, the future work is discussed.