| 3D printing techniques are widely used in various fields due to its rapid prototyping and ability to form complex geometries.Among them,fused deposition modeling technology(FDM)has a wide variety of raw materials and low cost,and the printing equipment is simple and convenient to operate.Poly(lactic acid)(PLA),as a kind of biodegradable materials,is one of the most commonly used raw materials for FDM technology.It can get rid of the dependence on the depleted petroleum-based resources and can completely degrade in the natural environment without causing damage to the environment.PLA has an advantage of high strength and large modulus,and its performance is similar to that of general plastics.However,due to its shortcomings such as brittleness and high cost,its application has been limited.Combining biomass materials with PLA can reduce costs relatively on the basis of improved performance.In this study,the PLA-based 3D printed composites are toughened and modified by polymer blending and polymer grafting.And by incorporation of carbonaceous conductive fillers,the composites may gain a certain electrical and thermal conductivities.That provides a certain theoretical basis and technical guidance for expanding its application fields.The main research contents of this study are as follows:(1)In order to improve the toughness and interfacial properties of poplar wood flour/PLA composites fabricated by FDM,the effect of toughening agents was discussed.The results showed that,the addition of thermoplastic polyurethane(TPU)significantly increased the impact strength of the composites by 51.31%.Comparing to other toughening agents,such as polycaprolactone(PCL)and poly(ethylene-co-octene)(POE),the compatibility between wood flour and PLA was superior when TPU was used.With the addition of TPU,the complex viscosity and the storage modulus of the composites were also increased.The toughening mechanism can be further explained by molecular dynamics simulation from a microscopic view.The results showed that,compared with PCL and POE,TPU,as a connecting phase between PLA and cellulose,can form a more stable combination with both PLA and cellulose,so that the performance of the composite was more excellent.The graft copolymers,synthesized by free-radical melt grafting using glycidyl methacrylate as a graft monomer and dicumyl peroxide as an initiator,were used as compatibilizers in the composites.With the addition of 5wt%wood flour loading,the grafting degree of the graft copolymer was enhanced by 61.54%.At a 2wt%loading of the graft copolymer,the impact strength and tensile strength of composites were increased by 7.75%and 8.39%,respectively,and the interfacial adhesion of the ternary composites were improved.By adjusting the infill patterns during the printing process,smooth and completely infilled models can be printed by FDM.(2)To improve the electrical and thermal conductivity of the poplar wood flor/PLA/TPU composites by FDM,carbonaceous conductive fillers were incorporated into the composites.By optimizing the types and contents of carbonaceous conductive fillers,composites with a certain electrical and thermal conductivity were successfully prepared.The results showed that,compared with the composite without nano-graphite,when the addition amount of nano-graphite reached 25pbw,the conductivity of the composite was improved and the volume electrical resistivity was reduced by 4 orders in magnitude,which decreased from 1012?·m to 10B?·m.It also had good thermal conductivity,mechanical properties and thermal stability.The adding of the redox graphene(rGO)combined with graphite into the composites,compared to the tannic acid-functionalized graphite or the multi-walled carbon nanotubes,can be an effective method to improve the performance of the wood flour/PLA composites,because the volume electrical resistivity reduced by one order in magnitude and the thermal conductivity increased by 25.71%.Models printed by FDM illustrated that the composite filaments have a certain flexibility and can be printed onto paper or flexible baseplates.(3)The incorporation of carbonaceous conductive fillers was aimed at improving the conductivity of PLA-based composites and the effects of carbonaceous conductive fillers types and contents,such as graphene nanoplatelets and multi-walled carbon nanotubes were optimized.The composites with the addition of graphene nanoplatelets or multi-walled carbon nanotubes were fabricated by the masterbatch melting method in order to improve the dispersion of the two kinds of nano-fillers,and thereby improve the electrical conductivity of the composites.The results showed that,compared with the composite without rGO,the conductivity of the composite with the addition of 9wt%rGO was significantly improved,the volume electrical resistivity reached the minimum(decreased from 1011?·m to 103?·m).And the interfacial compatibility,apparent viscosity and the thermal stability of the composite were also good.The rGO functionalized by sodium dodecylbenzene sulfonate(SDBS)was an efficient method to further improve the electrical conductivity of the composite,compared with tannic acid or multi-walled carbon nanotubes.The volume electrical resistivity was reduced by an order in magnitude.Patterns printed onto different baseplates by FDM illustrated that the fuctionalized composite has a certain flexibility and it is suitable for printing complex shapes.(4)In order to further improve the electrical conductivity of rGO/PLA composites,the processing techniques of the composites was optimized to prepare rGO/PLA conductive composites.The method of solution mixing was used to prepare rGO/PLA conductive composites,and the effects of rGO with different properties on the conductive properties of the composites were discussed.The mechanism of the reduction on the mechanical properties of the composites with the addition of rGO was discussed by molecular dynamics simulation.The results showed that the volume electrical resistivity of the rGO/PLA composites prepared by adding SDBS modified rGO is smaller than that of the unmodified rGO/PLA composites regardless of the processing techniques.The method of solution mixing was beneficial to improve the electrical conductivity of the composites,and the volume electrical resistivity of the modified rGO/PLA composite can be reduced from 102?·m to to 10-1?·m.The LED can shine under the normal voltage in a LED circuit where a flake of the modified rGO/PLA composite was inserted.The results of the Raman spectrum and XRD test showed another type of graphene nanoplatelets(N-rGO)has better conductivity and fewer layers compared with the original rGO.The volume electrical resistivity of the N-rGO/PLA composite prepared by adding N-rGO was reduced to 10-3?·m.Due to the barrier effect of microcrystalline cellulose(MCC),the volume electrical resistivity of the N-rGO/PLA composite by incorporation of MCC was increased by an order in magnitude,reaching 10-2?·m.Both the composites can make LED light at normal voltage.The results of molecular dynamics simulation showed that the mechanical properties of the PLA/TPU system decreased with the addition of rGO.On one hand,it can be attributed to a part of TPU in the system was binded to rGO,which led to a reduction in binding sites of hydrogen bond between TPU and PLA,and the binding between TPU and PLA can be weaken accordingly.On the other hand,the weak binding between the molecular chain of PLA and rGO caused a slippage of the PLA chain. |
PDF Full Text Request