Study On The Thermal Conductivity And Electromagnetic Shielding Performance Of Polyethylene/Graphene Composites Fabricated Through Fused Deposition Modeling 3D Printing

With the continuous miniaturization,integration and high power of modern advanced electronic devices,the demand for multifunctional materials with efficient heat dissipation,high conductivity and excellent electromagnetic shielding performance is increasingly urgent.Polymer composites（CPCs）have been broadly used due to their specific advantages of lightweight,easy functionalization,facile design,excellent moldability and corrosion resistance.In general,the CPCs can be fabricated by introduction of functional fillers into polymer matrix.However,the dispersibility of fillers in polymer matrix and the interfacial compatibility of fillers with matrix would directly affect the properties of composites.On the other hand,the traditional processing technologies are facing great challenges in fabricating architectures with complex geometric structure due to the limitation in process technology and mould fabrication.Recently,the fused deposition modeling（FDM）3D printing technology has been developed rapidly to fabricate parts with complicated shapes and structures due to its easy operation and low costs.With regard to the above challenges,the novel functional low-density polyethylene（LLDPE）/graphene nanoplatelets（GNP）composites parts were successfully fabricated.The LLDPE was employed as polymer matrix due to its low cost and excellent processability.The GNP fillers were carefully selected as both the conductive and thermal conductive filler.The FDM printability and relationship between structures and performances of composites parts were studied.Based on systematically investigating the relationship between FDM printing temperature and the internal stress/crystallization behavior of the LLDPE/GNP composites,the LLDPE/GNP composite 3D printed parts were successfully printed using FDM 3D printing technology.Then,the microwave irradiation post-treatment technology was used to enhance the interlayer mechanical strength of the FDM printed parts.The ball milling technology was also used to realize the selective distribution of GNP,and the corresponding specific layered porous 3D printed parts with high electromagnetic shielding performance were accordingly fabricated.Furthermore,the solid-state shear milling（S³M）technology was used to realize the exfoliation and enhanced dispersion of GNP and also the remarkable improvement in the interfacial compatibility between GNP and LLDPE.Finally,the FDM aligining printing strategy was successfully combined to fabricate the composites parts with exellent anisotropic thermal conductivity and electromagnetic shielding effectiveness（EMI SE）.The detailed investigations and the results were illustrated as follows:（1）Based on the systematical investigation of the relationship between FDM printing temperature and the internal stress/crystallization behavior of the LLDPE material,the pure LLDPE and LLDPE/GNP composite were successfully processed and printed by using FDM 3D printing technology.The temperature exploration showed that,when the printing bed temperature was close to the melting temperature of LLDPE,the crystal nucleus formation become slow and crystal particles growth speed would become fast,thus greatly decreasing the interference in rearrangement of LLDPE molecular chains and hence the corresponding residual stress of the printed parts.In this way,the printed parts would have no obvious shrink and warp.The rheological analysis showed that LLDPE is sensitive to shear and no obvious temperature dependence.When the nozzle temperature is in the range of 165 to 175℃,the printed parts would represent the high interlayer strength and dimensional accuracy.（2）The interlayer strength of LLDPE/GNP printed parts was enhanced by innovatively introducing microwave（MW）irradiation technology.The GNP content of the parts is positively correlated with the power and the time of MW irradiation.With the increase of GNP content,there would be more charge carriers in the printed part which would induce the conversion of microwave energy to heat energy.This would result in decrease in both MW irradiation power and time for achieving the effective enhancement of mechanical performance.Furthermore,the MW enhancement efficiency is correlated with the dispersion of GNP in the parts.Compared with the simple melt-compounding method,the ball milling technology could realize the selective dispersion of GNP in LLDPE matrix,which facilitated the formation of GNP filler network structure and resulted in lower percolation threshold.Therefore,the parts could be rapidly heated and resulted in higher enhancement efficiency of interlayer strength.（3）The layered porous LLDPE/GNP parts with high electromagnetic shielding effectiveness（EMI SE）were fabricated by designing and constructing the FDM 3D printing models with different filling structure patterns.When the filling rate is 100%,the EMI SE of printed parts with filling pattern of"linear"type,"Z"type and"*"type showed the little differences in EMI SE（achieving32.4 d B）.However,as the filling rate decreases,relative to the other filling pattern,the decline in EMI SE of the printed parts with"*"star pattern was slowered due to the intersecting hierachical filament interfaces.The obtained layered porous parts also showed the better impedance match with free volume,thus resulting in more EM wave absorption and electrical loss.It was found that the"*"-type printed part with 40%filling rate showed the EMI SE of 29 d B and specific electromagnetic shielding efficiency（SSE/t）of 318 d B·cm²g^-1.Furthermore,the difference in the thermal conductivity of the printed parts with three different filling patterns was less remarkable.Comparatively,among the three filling patterns,the"liner"-type parts represented the relatively higher thermal conductivity due to the internal regularly aligning structure.The"Z"-type and"*"-type printed parts represented the relatively lower thermal conductivity due to the existence of more printing joints and internal pores.（4）The LLDPE/GNP composite parts with GNP structures aligning along different direction were successfully fabricated by combining solid-state shear milling（S³M）and FDM 3D printing aligning technology.The powerful three-dimensional shearing stress field exerted by S³M process facilitated the exfoliation and dispersion of GNP in LLDPE matrix.Furthermore,compared with the simple melt-compounding method,the S³M process proved to obviously improve the interfacial compatibility and the FDM printability of composites,thus significantly enhancing the comprehensive mechanical properties,including tensile strength,tensile strain and Young’s modulus.For example,relative to unmilled parts,the tensile strength and Young’s modulus of the 30 wt%GNP fillers incorporating parts milled increases by 95%and 330%,respectively.（5）The LLDPE/GNP 3D printed parts with the aligning GNP structure were successfully obtained through innovatively designing and constructing the aligning FDM printing model.These aligned LLDPE/GNP parts proved to have the excellent thermal conductivity and electromagnetic shielding performance,which are also anisotropic.The vertically aligned（VS）parts,where the biggest plane of the parts is perpendicular to the FDM printing direction,i.e.,the direction of heat flow conduction,could maximize the in-plane thermal conductivity of GNP due to no existence of printing pores and filament interfaces along the heat flux direction.Furthermore,combined with the synergistic effect of S³M process,the thermal conductivity of LLDPE/GNP composite parts could be remarkably enhanced due to the significant decrease in the interfacial thermal resistance between LLDPE and GNP filler caused by the effect of pan-milling mechanochemistry.As a result,the printed VS parts showed the excellent through-plane thermal conductivity,as high as 3.43 W·m^-1·K^-1.In addition,the horizontally aligned（FS）parts,where the biggest plane of the parts is parallel to the FDM printing direction and also the involved aligned GNP sheets were perpendicular to the direction of the incident electromagnetic wave,could effectively block and greatly attenuate the inside electromagnetic waves,thus imparting the parts the excellent electromagnetic shielding performance,due to the so many layered interfaces and GNP filler network structures in FS parts.Accordingly,the FS parts demonstrated the excellent EMI SE up to 47.1 d B,where the absorption of the incident EM waves dominates.To summarize,in this paper we have successfully realized the FDM printing of LLDPE/GNP composites,and fabricated the multifunctional 3D printing parts with enhanced interface strength,high thermal conductivity and high electromagnetic shielding performance.The broadly used polyethylene polymer materials with low cost have been sucessfully functionalized and given superior performance/high added value through combining FDM 3D printing technology,solid-state shear milling technology and microwave irradiation strategy,thus finding their wide applications in light weight,diversification and portable devices fields.