Study On Preparation And Properties Of Cellulose/Poly(Lactic Acid)Bio-Composites Based On Fused Deposition Modeling 3D Printing

With the urgent needs of society for environmental protection and resource conservation,it is consistent with the new trend of global economic sustainable development to exploit novel bio-composites by replacing non-renewable resources with biomass materials.On the other hand,three-dimensional?3D?printing,as an innovative technology in the manufacturing field,has unique advantages in the fabrication of personalized and complicated products.Its emergence provides new opportunities for diversification and high-quality utilization of biomass materials.Poly?lactic acid??PLA?is an environment-friendly bio-polymer material with good biocompatibility and processability.Due to its low shrinkage and low warpage during molding process,PLA has become an important filament for fused deposition modeling?FDM?3D printing.This thesis aims at the key issues in the effective use of biomass resources and development of new 3D printed PLA materials.PLA was modified with sugarcane bagasse fiber and cellulose nanocrystals-organic montmorillonite hybrid nanoparticles,respectively,and the relationship between the preparation,structure and performance of PLA bio-composites and FDM 3D printing was systematically studied.Meanwhile,PLA was physically blended with biodegradable polymer material,poly??-caprolactone??PCL?,and functionalized products with thermo-responsive shape memory effect were fabricated by FDM.On this basis,the PLA/PCL blend matrix was appropriately modified and hybrid nanoparticles were incorporated into the blend,and the structure and properties of PLA/PCL nanocomposites were studied in depth.Sugarcane bagasse is both a by-product of the sugar industry and a kind of natural plant fiber.In this thesis,sugarcane bagasse fiber?SCBF?was extracted from raw sugarcane bagasse?RSCB?by continuous chemical treatment with sodium hydroxide?Na OH?solution and sodium chlorite?Na Cl O₂?/glacial acetic acid mixed solution.The physical and chemical properties of sugarcane bagasse before and after chemical treatment were evaluated.The results indicated that the cellulose content of RSCB was only 45.7%,while that of SCBF was as high as 88.5%.Compared with RSCB,SCBF exhibited distinct fibrous morphology and structure and better thermal stability.The microstructure,mechanical properties,crystallization properties and thermal stability of SCBF/PLA composites were systematically studied via FDM 3D printing technology.The results showed that the mechanical properties of SCBF/PLA composites were improved compared to RSCB/PLA composites.Due to the orientation effect of fiber filler,when SCBF content was 6wt%,the tensile strength of composites was better;but the flexural strength continued to decrease as the filler content increased.By modifying the G-code of 3D model of tensile specimen,the influence of different printing methods on the tensile performance of SCBF/PLA composites was further explored.The results indicated that the optimal SCBF content of 3D printed specimens prepared by the other three methods“Default”,“Cross”and“Parallel”was 6wt%except for“Vertical”method.Since the orientation degree of fiber filler and PLA molecular chain inside3D printed specimen was related to its raster angle,the fully oriented tensile specimen printed by“Parallel”method had higher tensile strength.In addition,both RSCB and SCBF possessed a sufficiently high thermal decomposition temperature,which can meet the requirements of melt extrusion processing of PLA composites,and SCBF significantly promoted the crystallization of PLA.Cellulose nanocrystals?CNC?were extracted from bleached softwood pulp via sulfuric acid hydrolysis,and montmorillonite?MMT?was modified by cetyltrimethylammonium bromide?CTAB?to obtain organic montmorillonite?OMMT?.A novel CNC-OMMT hybrid nanomaterial was constructed by electrostatic self-assembly.The microscopic morphology showed that one-dimensional needle-shaped CNC was adsorbed on the surface of two-dimensional sheet-like OMMT,thereby forming multi-dimensional hybrid nanoparticles.In the hybrid nanoparticles,CNC and OMMT still maintained their original chemical structure.The size of hybrid nanoparticles was controlled by the initial dimensions of CNC and OMMT.Infrared characterization analysis and Zeta potential results indicated that there was not only hydrogen bonding between CNC and OMMT,but also strong electrostatic adsorption,and the dispersion behavior of CNC-OMMT hybrid nanoparticles was affected by electrostatic interaction.Based on the dual function of hydrogen bonding and electrostatic adsorption,the nanosheet structure of OMMT effectively shielded the sulfate groups on the surface of CNC and delayed the thermal decomposition of CNC,thereby significantly improving its thermal stability.Further,the effects of three nanofillers?CNC,OMMT and CNC-OMMT?on the microstructure,mechanical properties,crystallization properties and thermal stability of PLA nanocomposites were systematically analyzed via FDM 3D printing technology.Moreover,the heat resistance of 3D printed specimens prepared at different printing platform temperatures was explored combined with the isothermal melting crystallization kinetics of PLA nanocomposites.The results showed that CNC-OMMT played a synergistic modification effect on enhancing the mechanical properties of PLA nanocomposites.The dispersion of hybrid nanoparticles in the PLA matrix was not only significantly improved,but also did not cause hole defects in the polymer matrix.In terms of crystallization performance,multi-dimensional hybrid nanoparticles acted as nucleation aids to greatly improve the crystallization rate of PLA in the crystallization temperature range.According to the isothermal melting crystallization kinetics,PLA nanocomposites had different crystallization rates at different printing platform temperatures,thus exhibiting different heat resistance.Due to the faster crystallization rate at 120°C and the formation of?crystals with a more ordered crystal structure,3D printed specimens prepared at this temperature had higher vicat softening temperature?VST?.By introducing the time dimension on the basis of 3D printing,the shape of 3D printed products can be changed over time to achieve 4D printing,which meets the requirements of functional 3D printing.The thermo-responsive shape memory performance of 3D printed PLA/PLA blends with different mass ratios was systematically studied using a stress-controlled dynamic mechanics analyzer in tension mode,and the shape memory effects?SME?mechanism was elucidated in combination with the crystallization-melting behavior,dynamic mechanical performance and morphology of PLA/PCL blends.The results showed that the shape fixity performance of 3D printed PLA/PCL blends were excellent.This is due to the rapid crystallization of the reversible phase PCL,so that the temporary shape was fixed in time.The shape recovery performance was mainly affected by the physical crosslinking between the two-phase interfaces.As the PCL content increased,the ability of physical crosslinking limiting the irreversible slippage of PCL phase was gradually weakened,resulting in a decrease in shape recovery performance.In addition,since the glass transition of PLA adversely affected the shape recovery,the deformation temperature?T_d?used in shape memory cycle should be lower than the glass transition temperature of PLA.The orthogonal experiment results of 3D printing parameters showed that layer thickness and raster angle had great impacts on the shape recovery performance of PLA/PCL blend,while infill density had little influence.The SME of 3D printed product was tuned by optimizing the 3D printing parameters used in FDM process.As a result,3D printed product fabricated with raster angle of 45°/-45°,infill density of 80%and layer thickness of 0.05mm exhibited favorable SME at T_d of 65?.In order to further improve the thermo-mechanical cycle performance,strength and modulus of PLA/PCL blend,the blend matrix was first modified appropriately.Poly??-caprolactone?diol?PCL-diol?was modified with isocyanate group?-NCO?using hexamethylene diisocyanate?HDI?to obtain PCL-NCO.The effect of PCL-NCO on the cyclic shape memory properties of PLA/PCL blends was studied via FDM 3D printing.The results indicated that PCL-NCO improved the interface interaction of PLA/PCL blend.When4%PCL-NCO was added,PLA/PCL blend showed better cyclic shape memory performance in three thermo-mechanical cycles.On this basis,the effect of hybrid nanoparticles on the structure and properties of PLA/PCL nanocomposites was investigated using FDM technology.The results showed that the addition of less than 1%of hybrid nanoparticles did not have evident influence on the cyclic shape memory properties of PLA/PCL nanocomposites,which was mainly affected by the thermal response of the blend matrix itself.In addition,hybrid nanoparticles had an enhanced effect on the storage modulus of PLA/PCL nanocomposites,and hybrid nanoparticles also effectively improved the mechanical properties of nanocomposites at a low content of less than 1%.