Construction Of Heat Conduction Networks In Insulating UHMWPE Composite System Under The Extensional Flow Field

Ultra-high molecular weight polyethylene（UHMWPE）has simple structural units and excellent overall performance.It has broad development prospects in the fields of artificial joints,wear-resistant pipes,sports equipment and thermal management of electronic products.However,due to its low thermal deformation temperature（80 ^oC）and poor thermal conductivity(thermal conductivity<0.5 Wm^-1K^-1),UHMWPE products are prone to accumulate heat leading to the local overheating and deformation,which limits the application in the field of thermal management.Therefore,it is of great significance to develop UHMWPE materials with high thermal conductivity and insulation,and broaden their applications in the field of thermal management of electronic products.In this paper,carbon nanotubes（CNT,one-dimensional）and boron nitride（BN,two-dimensional）were used as thermally conductive materials.Using extensional rheological extrusion technology（ERE）,the dispersion and orientation of one-dimensional materials and two-dimensional materials in the UHMWPE matrix could be effectively controlled by adjusting the processing conditions to construct a network structure with thermal conduction but non-conducting electricity,and obtain an insulating UHMWPE composite with high thermal conductivity.The research has broken through its key technology of continuous preparation,summarized the influence of elongational rheological processing methods on the structure of UHMWPE composites,as well as revealed the root cause of the external field induced forming process to improve the thermal conductivity.More importantly,this work solved the technical bottleneck problem that conventional processing methods cannot take into account the high thermal conductivity and high insulation of polymer composites,excellent mechanical properties and recycling.The details are as follows:Firstly,the typical one-dimensional material carbon nanotubes（CNT）were used as thermally conductive fillers,and compression molding（CM）and ERE technologies were used,respectively.UHMWPE composites with isolated thermal network structure and oriented thermal network structure were constructed.The influence of different processing techniques on the microstructure and macroscopic properties of composite materials was studied.The results show that,compared with CM,the thermal conduction network constructed by ERE is more continuous and dense,and the thermal conduction performance is significantly improved,and presents anisotropic characteristics.When CNT is 7 wt%,the in-plane thermal conductivity（λ）of CM-UHMWPE/7CNT composite is only 0.64 Wm^-1K^-1,while the in-planeλof ERE-UHMWPE/7CNT reaches 0.98Wm^-1K^-1,by increasing in 53.13%.The microstructure analysis results of the composites show that the ERE molding process has a significant orientation effect on both UHMWPE and CNT.The results of the Agari model analysis show that CNT forms a effective thermal conduction path under the action of the extensional flow field.The effective medium theoretical model also proves that the extensional flow field is beneficial to reduce the interface thermal resistance of the composite.In addition,the thermal stability of ERE-UHMWPE/7CNT composite is also greatly improved,while its volume resistivity is still maintained at 2.01×10¹⁰Ωcm,which is higher than the lower limit of 10⁹Ωcm for the above insulating materials.It can be seen that the use of extensional rheological extrusion technology can prepare insulating UHMWPE/CNT composites with good thermal conductivity.Secondly,the two-dimensional material BN was employed as a thermally conductive filler,the ERE technology was used to construct a thermally conductive and insulating UHMWPE/BN composites with a layered stacked structure oriented along the extrusion direction.The effect and mechanism of the extensional flow field on the dispersion and orientation of sheet-like BN were studied.The relevant parameters of the Nan model were optimized,and a thermal conduction model describing the UHMWPE/BN system based on the extensional flow field was established.The results of the Agari model analysis show that the sheet-shaped BN can effectively disperse and undergo in-plane orientation under the action of the extensional flow field,thereby forming a thermal conduction path in the UHMWPE matrix,and effectively improving the thermal conduction performance of the UHMWPE composites.When the addition amount of BN is 40 wt%,the in-planeλof the composite reaches 3.77 Wm^-1K^-1,which is5.85 times higher than that of pure UHMWPE.At the same time,the tensile strength of ERE-UHMWPE/40BN composite reaches 25.02 MPa,which is 85.61%higher than that of CM-UHMWPE/40BN composite,which shows that the effect of extensional flow field is also benefical to improve the mechanical properties of composites.The electrical conductivity test results show that the volume resistivity of the ERE-UHMWPE/40BN composite is 7.42×10¹²Ω·cm,showing a good insulation property.This research explores a new way for the preparation of insulating UHMWPE composites with high mechanical properties and high thermal conductivity.Thirdly,the blend of CNT（one-dimensional）and BN（two-dimensional）was used as a composite thermal conductive agent,and ERE technology was used to prepare insulating UHMWPE/BN/CNT composites with high thermal conductivity.The effect of the extensional flow field on the microstructure and macroscopic properties of composites was studied.The mechanism and model of the“line-surface”structure synergistic construction of an efficient hybrid filler thermal network were discussed.It is clear that when the amounts of BN and CNT are 40 wt%and 7 wt%,respectively,the in-planeλof ERE-UHMWPE/BN/CNT composite is 4.16 Wm^-1K^-1,which is 6.56 times higher than that of pure UHMWPE,and is higher than ERE-UHMWPE/7CNT composite(0.98 Wm^-1K^-1)and ERE-UHMWPE/40BN composite(3.77 Wm^-1K^-1).The analysis results of Nan model show that the introduction of CNT can effectively reduce the thermal resistance of the interface between BN and UHMWPE matrix,and improve the thermal conductivity of the composite.The volume resistivity of ERE-UHMWPE/40BN/7CNT composite is 8.5×10¹¹Ω·cm,revealing their good insulation properties.Finally,based on the extremely mismatched viscosity of r-UHMWPE and HDPE,BN was used as a thermal conductivity agent,and ERE technology was used to construct a selective distribution system of BN.Two composites of HDPE/r-UHMWPE/BN and HDPE/r-UBN were prepared respectively.The relationship between the distribution characteristics of BN and the thermal conductivity was studied,and the thermal network structure model of composites was proposed.It results show that in the HDPE/r-UHMWPE/BN system,HDPE is the continuous phase,r-UHMWPE is the dispersed phase,and BN are selectively distributed in the HDPE matrix.As for the HDPE/r-UBN system,HDPE is the continuous phase and r-UBN is the dispersed phase.Part of the BN dispersed in UHMWPE is transferred to the HDPE matrix during the extrusion process,constituting a thermal conduction path.Compared with HDPE/r-UHMWPE/BN composites,HDPE/r-UBN exhibits more efficient thermal conductivity.When the BN content is 20 wt%,itsλreaches 0.83 Wm^-1K^-1,which is 7.8%higher than the HDPE/r-UHMWPE/20BN(λ=0.77 Wm^-1K^-1).The volume resistivities of HDPE/r-UBN and HDPE/r-UHMWPE/BN composites are 2.32×10¹⁰Ω·cm and3.35×10¹⁰Ω·cm,respectively,demonstrating good insulation performances.Moreover,the two composites subjected to the tensile flow field have good mechanical properties,wear resistance and thermal properties.These results bring about new ideas for the construction of UHMWPE high-efficiency thermal conduction network and the high-quality recycling of waste UHMWPE.