Manufacturing, Property And Application Of Thermal Conductive Polymer Composites

Geothermal Energy is a kind of important energy with advantages of wide coverage, low pollution and low operating cost, etc. Specially, low-medium temperature geothermal has relatively wide distribution, large quantity and these promise it a broad prospect. However, the development and utilization of the geothermal energy in our country are still in primary stage. The corrosion and scaling problems of heat-exchange equipments restrict geothermal energy to be widely and adequately utilized.It is an effective solution to make compact plate-type heat exchanger with thermal conductive polymer composites. Thermal conductive polymer composites have characteristics of corrosion-resistant, anti-scaling, easy-forming and high cost performance. While the compact plate-type heat exchanger provides advantages in compact structure, high efficiency of heat transfer per unit volume, high flexibility for plates adjusted on actual needs, easy wash and reparation, etc. Therefore, application of compact plate-type heat exchanger with thermal conductive polymer composites can effectively tackle the technical barriers such as high equipment costs, corrosion and poor anti-scaling etc.On the purpose of producing compact plate-type heat exchanger with thermal conductive polymer composites, the study investigated following aspects. Firstly, thermal conductive polymer composites were compounded with different matrix and fillers, including polypropylene, polyethylene and graphite, Zinc Oxide, carbon fiber. The thermal conductivity and mechanical performance of the compound were measured. Secondly, the formulas of the compound were optimized and overall performances of thermal conductive composites were enhanced by the investigation of the inherent macro and micro mechanism. Thirdly, the processing characteristic and long-term service duration of the thermal conductive compounds were investigated through the measurement of the dynamic rheological behavior and aging performance of the composites. Lastly, a complete production procedure from thermal conductive polymeric composites production to heat exchanger production was developed. After appropriate structure of the compact heat exchange was determined, the performance of the heat exchanger was tested.The results show that the thermal conductivity of composites is closely related to the performance of matrix and filler. The matrix conductivity discrepancy caused by several reasons such like the degree of crystallization is amplified by several times or even dozens of times in the compounding system. And the compounding conductivity is increased with the increasing of the filler content. The influence of the heat conducting method of the filler upon the thermal conductivity of composites is significant. With the same volume fraction, the thermal conductivity of composites using graphite as filler, which transfers heat by the co-action of electron and phonon, is remarkably higher than that of other composites (only transferring heat by phonon, fillers such as MgO, SiC, Al2O3 etc.). When the inherent thermal conductivity of the filler is far large than the matrix (such as Kfiller/Kmatrix＞100), further increase of the filler's thermal conductivity has no obvious effect on the improvement of composites' thermal conductivity. The grading of filler particles helps increase the thermal conductivity of composites. The combined filling of carbon fiber and graphite into polymers can effectively enhance the mechanical performance of composites.The interfacial phase between filler and matrix exerts relatively important effect to the transfer of heat carriers in the composites. The research of interfacial phase thickness, interfacial phase thermal conductivity and filler facial treatment show that the existence of interfacial phase prevents the formation of the complete thermal conducting network by direct contact between high thermal conductive fillers, which is against the conduction of heat. The thicker the interfacial phase is, the stronger the dispersion and refraction effects are. As a result, heat loss is severe during transmission. Small molecule coupling agent modified filler surface can improve the combining situation of the interface between filler and matrix and reduce flaws in the interface of them. As a result, the thermal conductivity, tensile properties and bending properties of the polymer composites are improved, but the impact strength is reduced. The macro-molecular coupling agent modified filler surface is covered by a layer of low modulus polymer, which is adverse for the transfer of phonon in the filler-constructed heat conductive network. But after introducing the flexible layer, it is beneficial to releasing or removing the phenomenon of stress concentration, while surface treatment of enhancing the impact strength of the polymer composites is good for decreasing the thermal expansion coefficient of the compound material. Controlling the heat resistance of interfacial phase is an effective solution to improve the overall thermal conductivity of the compound material.It is instructive to study the rheological performance and aging performance of the composites for material process and utilization. With the help of ARES rheometer, the rheological performance of the composites is investigated. The results show the network exists in the compound system and grows with the increase of the graphite content. The thermal conductive percolation threshold and rheological threshold are both about 9vol%. Improved Kerner-Nielson heat conduction model is adopted to relate the thermal conductive performance and rheological behavior. It shows the agglomeration structure is determined by the filling method of the filler and the direct contact between particles is mainly responsible for the formation of network structure. The investigation of the acid/alkali resistance and anti-ultraviolet aging performance show that the aging mechanisms for polypropylene and cross-linked polyethylene based composites are different. The change of mechanical performance for polypropylene based composites is due to the plasticizing and matrix crystallization effect. While for the cross-linked polyethylene based composites, it is mainly resulted from the co-effect of solution damaging material crosslink degree and matrix crystallization. The result of infrared spectrum analysis shows that, with ultraviolet radiation, the change in molecular chain of polypropylene is more distinct than that of cross-linked polyethylene.Preparation of the polyolefin thermal conductive composites and the molding of the heat exchange plate were optimized. By using in-line compounding equipments and plate direct compression molding method, commercial production of the thermal conductive polymer composites and heat exchanger was feasible. A cross-flow compact heat exchanger was designed with characteristics of large unit volume heat exchange area and low resistance loss or so. When the cold water and hot water flow speed reach 1 m/s, turbulence flow occurs. The heat exchange coefficient of heat exchanger can reach over 2500 W/m2.K but the pressure drop value is still lower than 4 KPa.