Research On Preparation,Structure And Performance Of Polylactic Acid/Lemongrass Composite Based On Fused Deposition 3D Printing

In recent years,due to restrictions on the use of disposable plastic products,environmentally friendly biodegradable polymers and their composites have ushered in new development opportunities and challenges.On the other hand,fused deposition modeling(FDM)3D printing is a rapid additive manufacturing technology based on the accumulation of layers of molten thermoplastic polymer,which can form products with complex shapes without molds.Polylactic acid(PLA)is a widely used biodegradable polyester and the most commonly used printing consumable for FDM technology.However,PLA also has shortcomings such as brittleness and slow crystallization rate.Therefore,it is of great significance to study the blending of green fiber and PLA,the toughening modification of PLA,and and the structure and performance of composite materials under the change of FDM parameters.In this thesis,lemongrass fiber(LF)is used as a nature fiber,and the influence of compatibilizers on the structure and performance of PLA/LF composites is deeply studied.At the same time,PLA was blended with the biodegradable polyester polyadipate/butylene terephthalate(PBAT)and the interface was modified to achieve the effect of toughening PLA.On this basis,a new type of PLA/PBAT/LF green-friendly composite was prepared and the influence of FDM parameters on the structure and performance of the composite was investigated in depth.Lemongrass is a plant that grows extensively in the south.It has antiseptic,antibacterial,and anti-oxidant characteristics and has a special lemon scent.In this paper,LF was prepared by mechanical crushing method and the content of cellulose,hemicellulose and lignin in LF was determined.FTIR and X-ray diffraction tests show that LF has a glycoside chemical structure and cellulose I crystal structure.The particle size analysis test shows that the average particle size of LF is about 65 ?m,and the median diameter is about 44.7 ?m.Thermal weight loss analysis shows that the thermal decomposition temperature of LF is about 174.2°C,and the fastest decomposition temperature is about 333°C.Based on the analysis of the properties of LF,polylactic acid grafted maleic anhydride(PLA-g-MAH)was prepared by melt grafting method and used as the interface compatibilizer of PLA/LF composites.And the influence of the addition of LF and PLA-g-MAH on the mechanical properties,crystallization properties and microstructure of PLA/LF composites was systematically studied after using the FDM 3D printing molding process.The acid-base titration results showed that the grafting rate of PLA-g-MAH increased firstly and then decreased with the increase of initiator DCP or monomer MAH content.The addition of LF will increase the flexural modulus and crystallinity of PLA,accelerate the crystallization speed of PLA and reduce the cold crystallization temperature.After adding the PLA-g-MAH,the crystallization rate of PLA is further improved,but the crystallinity does not change much.The mechanical properties of PLA/LF composites increase first and then decrease with the increase of PLA-g-MAH content.The tensile strength and flexural strength of the composites reach the highest when 5 wt% PLA-g-MAH is added.It can be seen from the scanning electron microscope that the addition of PLA-g-MAH can significantly reduce the cavities and fiber peeling of the composites,there is no obvious gap between the two phases,and the interface compatibility of PLA and LF has been improved.From the perspective of high toughness and biodegradability,PBAT is considered the material for PLA toughening.In this thesis,specimens of PLA/PBAT blends were prepared by melt extrusion-FDM 3D printing,and styrene-glycidyl methacrylate copolymer(PSG)was used to compatibilize the PLA/PBAT blends.The rheological properties,mechanical properties,thermal properties and micro morphology of PLA/PBAT and PLA/PBAT/PSG blends were systematically studied.Impact strength of PLA/PBAT blends gradually increased with the increase of PBAT content,but the tensile and bending properties decreased significantly.The addition of PSG significantly improves the impact strength of the samples of the PLA/PBAT30 component.Compared with PLA/PBAT30 and pure PLA,the impact strength is increased by 2.6 times and 7.7 times,respectively.At the same time,the tensile properties and bending properties are higher maintained rate compared to PLA/PBAT30 blends.Rheological studies have shown that the addition of PSG can significantly increase the viscosity of PLA/PBAT blends,which may be related to the compatibilizing effect of epoxy groups.The addition of PSG can increase the entanglement of the PLA and PBAT molecular chain.The crystallization of PLA is limited,and the thermal stability and vicat softening temperature have been improved to a certain extent,and the glass transition temperature of PBAT is also close to the glass transition temperature of PLA.SEM test show that PBAT and PLA are thermodynamically immiscible systems,but the addition of PSG can emulsify the interface between PLA and PBAT and improve the compatibility of the interface.The increase in impact toughness can be attributed to the shear deformation after debonding of the PBAT phase and the formation of molecular bridges.Furthermore,PLA/PBAT/LF composite wires were prepared by melt extrusion technology and used in FDM 3D printing research.On the basis of comparing the effects of injection molding and FDM printing on the structure and mechanical properties of the sample,the effect of printing parameters on the structure and properties of the composite is analyzed.The research results show that in the layer-by-layer FDM technology,the adhesion between the filaments and the layers is not strong,which leads to the existence of internal pores,and the mechanical properties are not as good as the injection molded samples.The thicker the grating layer,the larger the internal pores of the product,the lower the tensile and bending properties,and the improvement of the impact strength is related to the energy dissipation of the micropores.Too fast the printing speed will affect the dimensional stability of the sample,and too slow will affect the adhesion between the layers,which cause the mechanical properties of the sample to decrease.The difference of the grating angle will cause the difference in the orientation and failure mode of the filaments in the grating layer,thereby affecting the mechanical properties.The nozzle temperature affects the thermal diffusion of molecules between the grating layer interfaces,thereby affecting the interface bonding,and there are also differences in mechanical properties.The temperature of the printing platform affects other properties of the composite by changing the crystallization of PLA.The higher the platform temperature,the higher the crystallinity,the more brittle the sample,the lower the tensile strength,the impact strength and the bending strength,while the rigidity and resistance to thermal deformation of the material are enhanced.When the printing speed is 40mm/s,the grating layer thickness is 0.1 mm,nozzle temperature is 210°C,the bed temperature is 60°C and the grating angle is 90°/90°,it can satisfy the best mechanical properties of the product while also improve printing efficiency and maintain dimensional stability.