Research Progress of Thermal Conductive Plastics on High Thermal Conductivity

ZHANG Zheng-lin

【Abstract】In this paper，the research progress of thermal conductive plastics at present is summarized from the categories of thermal conductive plastics，the advantages of thermal conductive plastics over other thermal conductive materials，several common preparation methods of thermal conductive plastics and the key problems in future research. The problems found in the process of summarizing are discussed and prospected.

【Key words】Thermal conductive plastics；High thermal conductivity；Preparation method

中圖分类号： TQ327；TB33 文献标识码： A 文章编号： 2095-2457（2018）24-0190-003

DOI：10.19694/j.cnki.issn2095-2457.2018.24.092

0 Introduction

In recent years，with the rapid development of the computer industry，the integration technology in the field of micro-electronics has been improved，and the volume of electronic components and logic circuits has been gradually reduced.Compared with the previous，the same volume of circuit power has increased several times，electronic components have become more serious heat dissipation problems.This requires materials with high thermal conductivity to transmit heat to the outside world.

The traditional thermal conductive materials include metals，ceramics and composite materials.The metal has a high thermal conductivity，but its electrical insulation is poor.It is a potential safety hazard and heavy weight when used in electronic components.The ceramic has excellent thermal conductivity and is also a commonly used heat dissipation material，but it is difficult to process microelectronic components with high cost.In the field of electronic components，the commonly used heat dissipation insulation material is engineering plastics，light weight， insulation and easy processing，but its own thermal conductivity is low，this requires auxiliary mechanical heat dissipation.In order to improve its thermal conductivity， the high thermal conductive fillers and plastics are combined to make thermal conductive plastics.

Thermal conductive plastics have inherited the advantages of Engineering plastics，such as good processing performance，low price，easy processing，light weight，uniform heat dissipation，high degree of freedom in product design，and their thermal conductivity has been improved.In recent years，they have been widely used in electronic components，special cables，LED lamps，electronic packaging and other fields[1-2].With the deepening of the research on thermal conductive plastics，the research on thermal conductive plastics with higher thermal conductivity will become the focus of the next stage.

1 The classification of the thermal conductive plastics

Thermal conductive plastic is a kind of plastic material with excellent thermal conductivity，which is made up of two parts：the matrix and the filler.When the filler filling reaches a certain degree，the filler forms a chain-like and network-like shape in the system，it means a heat conduction network chain.When the orientation of these heat conduction network chains is parallel to the direction of heat flow，the thermal conductivity of the system will be greatly improved.

The traditional thermal conductive plastics are engineering plastics such as PPS（polyphenylene sulfide）， PA6/PA66 （polyamide 6/polyamide 66），LCP（liquid crystal polymer），TPE（thermoplastic elastomer），PC（polycarbonate）， PP（polypropylene），PPA（polyphenylene diamide），PEEK （polyether ether ether ketone）.Thermal conductive fillers can be divided into metal materials，metal oxides，metal nitrides and inorganic non-metallic materials according to the types of materials.The commonly used fillers include SiC（silicon carbide），Al2O3（alumina），graphite，graphene，fibrous high thermal conductivity carbon powder，flaky high thermal conductivity carbon powder，etc.

The improvement of thermal conductivity of thermal conductive plastics mainly depends on the construction of thermal conductive network in the matrix，which mainly depends on the interaction between filler and polymer matrix and the mixing mode of materials[4].At the same volume fraction，the enhancement of thermal conductivity of plastics by a single shape of thermal conductive filler depends not only on the thermal conductivity of the filler， but also on the interaction between the filler and the plastic polymer matrix，whether the thermal conductive path can be successfully constructed under the condition of low filler content，rather than the higher thermal conductivity of the filler to the material.The enhancement effect is better[5].Liao et al[6]used flake graphite as filler to prepare polyamide plastics.The thermal conductivity of the plastics stabilized at 0.72W/（m·K）when the flake graphite content was 20%-40%.Compared with pure polyamide，the thermal conductivity of the plastics was doubled and did not change with the filling amount. Wang et al[7]used boron nitride as filler to prepare polyamide-6 thermal conductive plastics.The thermal conductivity of the thermal conductive plastics prepared with flake and spherical boron nitride fillers was similar when the mass fraction of filler was less than 10%.The thermal conductivity of the thermal conductive plastics filled with flake filler was higher than that filled with spherical filler when the proportion of filler increased.The thermal conductivity of sheet filler thermal conductive plastics is 1.112W/（m·K）when the content is 25%，which is much higher than 0.866W/（m·K）of spherical filler thermal conductive plastics.

2 The commonly used preparation methods of thermal conductive plastics

In recent years，domestic and foreign scholars have done a lot of research work on the preparation of thermal conductive plastics.It is found that the interaction between plastic polymer and filler depends on the polarity，molecular weight，hydrophobicity and reaction group of polymer，filler and solvent[8].In the current experimental research，melt blending，solution mixing and in-situ polymerization are the most commonly used methods for the preparation of thermal conductive plastics.

2.1 The melt-compounding method

In melt blending process，the high thermal conductivity filler is directly mixed with the plastic polymer melt without adding solvent.The traditional extrusion or injection method is usually used to mix the plastic polymer and filler mechanically at high temperature，and the plastic polymer chain is intercalated between the filler sheets to form the modified thermal conductivity plastics.This is a common method for preparing thermoplastic polymer composites，and some polymer systems without active functional groups or unsuitable for in-situ polymerization can also use this method[9-10].Hu et al[11]prepared thermal conductive plastics by melt blending of magnesium oxide and alumina into polypropylene.It was found that the stronger the thermal conductivity and the higher the viscosity of the matrix，the better the thermal conductivity of the plastics.The thermal conductivity of pure polypropylene is 0.202W/（m·K），when filled with 60% alumina，0.38W/（m·K），when filled with 60% magnesium oxide，the thermal conductivity of thermal conductive plastics is increased to 0.866 W/（m·K），and when filled with 60% magnesium oxide and graphite，the thermal conductivity of thermal conductive plastics is increased to 1.267W/（m·K）.Zhang et al[12]filled with graphite and modified by Polyamide46（PA46） by melt blending.With the increase of graphite filling，the thermal conductivity of thermal conductive plastics increased significantly，and the mechanical properties decreased slightly.The thermal conductivity of thermal conductive plastics reached 3.743W/（m·K），13.91 times that of pure PA46 matrix.And the thermal diffusivity reached 2.344mm2/s，15.95 times that of pure PA46 matrix.

2.2 The solution-mixing method

Solution blending is to use solvent to insert polymer molecules into the filler with lamellar structure to form modified thermal conductive plastics.The solution blending method is based on solvent system of plastic polymer and filler can be dissolved or dispersed in it.Fillers need to be dispersed in suitable solvents such as water，acetone， chloroform，tetrahydrofuran（THF），dimethylformamide （DMF） or toluene.The mixed plastic polymer is adsorbed on the peeled filler sheet，and when the solvent evaporates，the sheets are re-stacked and the plastic polymer is sandwiched between the layers to form a filler-plastic polymer composite.The driving force between polymer intercalation and filler sheet is the increase of entropy produced by desorption of solvent molecules，which compensates for the decrease of conformational entropy of polymer chain during intercalation.Therefore，a large number of solvent molecules need to be adsorbed from the packing to adsorb the polymer chain[13].The main advantage of this method is the preparation of modified thermal conductive plastics based on low polarity or non-polar.

2.3 The in-situ polymerization

The in-situ polymerization is to pre-mix the solution of the plastic polymer monomer with the colloidal dispersion of the filler，so that the polymer monomer molecules are inserted into the filler，and then put into the initiator to initiate the polymerization，so that the polymerization takes place between the layers of the filler， and the resulting reaction solution can be processed to obtain the modified thermal conductive plastics[14].In addition，the expansion effect caused by exothermic interlaminar polymerization makes the space between the filler sheets expand，which is conducive to further peeling and makes the filler disperse more evenly in the matrix.Many plastic polymers can be prepared by this method to obtain a variety of modified thermal conductive plastics，such as polypropylene，polystyrene，polymethyl methacrylate，polyamide and polyethylene terephthalate.

Chen[15]used polyamide 6 as matrix material to prepare graphene modified thermal conductive plastics by in-situ polymerization.The thermal conductivity of pure polyamide 6 was 0.32W/（m·K）.When the content of graphene in thermal conductive plastics reached 10%，the thermal conductivity of thermal conductive plastics increased to 1.37W/（m·K），and the filler was changed into graphene oxide.When the content of graphene in thermal conductive plastics reached 10%，the thermal conductivity changed to 0.41W/（m·K）. When graphene filler was added with graphene oxide solution（5% of the total mass of filler），and then filled with thermal conductive plastics to 10%，the thermal conductivity of thermal conductive plastics increased to 2.14W/（m·K）.

3 The urgent problems to be solved in thermal plastics

At present，there are three urgent problems to be solved for thermal conductive plastics，including preparation methods，selection of high thermal conductive fillers and improvement of thermal conductivity at the interface.The solution mixing method and in-situ polymerization method are often used in the laboratory research stage，which are difficult to be applied in practical engineering.It is urgent to put the laboratory research results into practical engineering applications.At present，the choice of thermal conductive fillers is very important in the preparation of high thermal conductive plastics.The effect of high thermal fillers on the thermal conductivity of different matrix materials is different.Selecting the most suitable thermal conductive fillers and filling ratio becomes the focus of the next research.At present，the key to restrict the thermal conductivity of thermal conductive plastics is whether the high thermal conductive fillers and matrix materials can achieve efficient heat transfer at the interface.At present，more research needs to be carried out at the interface of thermal conductive plastics to enhance the heat transfer at the interface，which is also one of the focuses of the next study.

4 Conclusion

Thermal conductive plastics have a wide range of application prospects and important research significance， which is worthy of further research and discussion.With the development of research，the thermal conductivity of thermal conductive plastics will be further improved，which will bring more help to the current industrial production and people's lives.

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