Preparation And Selective Laser Sintering Of High Performance Polyamide 12 Composite Powder

Selective laser sintering（SLS）is an additive manufacturing technique for fabricating end-use parts layer by layer.It can fabricate complex geometris without the need of mould,therefore has been widely applied in various industries,such as aerospace and automatic.However,powders are subjected to high temperature（～170^oC）for several hours,and this may leads to degradation of powders,therefore influence the re-use of supporting powders and the mechanical properties of sintered parts.Addition commercial antioxidants can improve the thermal degradation of materials.However,many of these antioxidants are small molecules which are easy to lose physically during the processing.As a result,the optical elements in the SLS equipment may be contaminated,or the performance of the sintered parts would be reduced.Moreover,although the research on the SLS process has been investigated for many years,the demand for functional products should be still exploited by incorporation of nanofillers.However,the nanofiller acts as nucleating site in the polymer matrix and leads to issue that the sintered parts has warpage and deformation phenomenon.This work focus on the thermal degradation and recycling of powders,as well as the limitation of the usage of the sintered parts and the warpage of the parts,aims to introduce polydopamine（PDA）and carbon nanotubes（CNT）to regulate the properties of powders and corresponding parts,the main research work and fingdings are as follows:（1）Preparation and selective laser sintering of PA12/PDA powdersPDA coated PA12 composite powders were prepared by physical mixing method and in-situ coating method,and the effects of PDA on the properties and laser sintering process of PA12 powders were studied.PDA can improve the thermal stability of PA12,but the effect of PDA on the crystalline properties of PA12 are dependent on the powder preparation methods.PDA prepared by physical mixing or in-situ coating method with low DA contents can accelerate the crystallization rate of PA12,while PDA formed by in-situ coating with high contents of DA can inhibit the crystallization rate of PA12,and its benefical to widen the sintering window.Although the coalescence rate of powders is affected by PDA contents,the sitered parts still have low defects,good mechanical properties and excellent thermo-oxidation stability.It turned out that the in-situ coating method is a simple and efficient methods to prepare PA12/PDA powders suitable for the laser sintering process.（2）Selective laser sintering of PA12/CNT conductive compositesPA12/CNT composite powders were prepared by ball mill,and the effect of CNT concentration and the processing parameters on the properties of laser sintered PA12/CNT parts were studied.The mixing method did not singnificantly alter the morphology and particle size of PA12 powders.CNT acts as nucleating site and accrlerates the crystallization behavior of PA12.As a result,the sintering window decreases from 24.9 ^oC of pure PA12 to 11.7 ^oC for PA12/2 wt.%CNT,and the corresponding crystallisation half-time dramatically decreases by 69.8%.The microstructure and mechanical properties of CNT/PA12 parts are mainly determined by the strong laser absorption ability of CNT,the retard coalescence behavior,dispersity of CNT and the laser density.For PA12/2 wt.%CNT,the electrical conductivity of SLS parts and compression molding parts are 10^-3 S/cm and 10^-12 S/cm,respectively.Therefore,SLS process can fabricate PA12/CNT parts with better conductive properties.However,the sintered PA12/CNT parts shows distinct warpage phenomenon due to the heterogeneous nucleation of CNT,and the warpage of the sintered parts is highly relyed on the CNT concentration and the processing parameters.（3）The influence of PDA on the warpage of SLS fabricated PA12/CNT partsThe PA12/CNT/PDA powders were prepared by three different methods（one-step method,two-step method and physical mixing method）,and the effect of PDA on the warpage and deformation of PA12/CNT parts was investigated.The powder preparation methods can influence the dispersity of CNT and the fusion behavior of powders.DSC results reveal that the presence of PDA can weaken the heterogeneous nucleation of CNT in PA12 matrix,results the sintering window of PA12/CNT/PDA¹,PA12/CNT/PDA² and PA12/CNT-PDA increase from 14.1 ^oC of PA12/CNT to 22.0^oC,22.4 ^oC and 18.7 ^oC,while the crystallisation half-time increase by 9 min,8.6 min and 4.1 min.The PA12/CNT/PDA powders prepared by one-step and two-step methods show excellent thermal stability,and the fabricated parts have higher tensile strength and modulus,while the elongation at break improves by 27.5%and 9.4%compared to the pure PA12 parts.Ascribed to the presence of PDA,the sintered PA12/CNT/PDA parts exhibit little warpage and deformation compared to the PA12and PA12/CNT parts.This research successfully solved the warpage phenomenon of laser sintered PA12/CNT parts by introducing PDA,therefore provides a new method for preparing functional parts with excellent dimensional accuracy.（4）The re-usability of PA12 and PA12 composite powdersThe aged PA12,PA12/CNT and PA12/PDA powders were prepared by SLS printer to investigate the effect of thermal exposure on the powder recycling and performance of sintered parts.The melting and crystallization behavior of powders are determined by the powder ageing process,thereby the sintering window and crystallization half-time of aged powders are increased compared to the fresh one.The rheology and thermal stability results indicate that the polycondensation is the main ageing process of PA12,therefore results aged powders with higher zero-shear viscosity and thermal stability.The microstructure of PA12 parts is highly related to the powder heating cycles,the quantity of un-melted powder increases and the fusion between particle become worse as increasing the ageing time,while for the PA12/CNT and PA12/PDA parts,the high photothermal conversion efficiency of CNT and PDA can compensate the influence of heating cycles on the microstructure,results parts with similar microstructure as the fresh PA12/CNT and PA12/PDA parts.As the powder ageing time increases,the tensile strength and modulus of fabricated PA12,PA12/CNT and PA12/PDA parts are declined.Compared with parts sintered by fresh powder,the elongation at break of PA12-12 h and PA12/PDA-12 h parts has improved by 139.1%and 216.7%,while decreased 23.4%for PA12/CNT parts.In summary,the effect of CNT and PDA on the PA12 re-usability are difference,the addition of CNT accelerates the polycondensation of PA12 and dramatically influence the sintering process of PA12/CNT aged powders,while PDA reveals nearly no influence on the powder recycling.The study provides new aspects for designing polymer structure and improving the powder re-usability.