Polyimide (PI) resin is a kind of engineering plastics with high modulus, high strength, low water absorption, hydrolysis resistance, radiation resistance, excellent insulation and heat-resistant oxidation stability. The graphitization of polyimide at high temperature can make it not only have the characteristics of low density, high temperature resistance, corrosion resistance, high strength and high modulus of traditional carbon materials, but also have excellent conductivity, and become the preferred material to solve the heat dissipation problems in the advanced industrial fields such as electronic devices, aerospace vehicles, etc. Therefore, the graphitization of polyimide is popular in material enterprises.
What is graphitization? Why graphitization can improve the performance of polyimide materials? According to the insiders, the graphitization process of polyimide refers to the high-temperature decomposition reaction of materials at a certain carbonization temperature, resulting in a large volume of gas. The polymer chain breaks and rearranges into a nitrogen-containing carbon hexagonal carbon layer structure. With the increase of temperature, nitrogen is released, and carbon atoms change from disordered carbon to regular carbon The whole graphite crystallized hexagonal carbon net structure grows and finally forms graphite material. Polyimide can be carbonized at a certain temperature to form a disordered carbon structure material. If the material is graphitized at high temperature, it can form regular graphitization crystallization, thus effectively improving the comprehensive properties of the material.
Domestic researchers have studied the polyimide graphite films prepared by the same heating up procedure with different polyimide films as raw materials. From the appearance, it can be seen that the shrinkage of domestic polyimide films is uneven after heating, and the surface wrinkle is serious. However, the shrinkage of foreign polyimide films is even after high temperature, and the surface is still flat and the structure is dense.
It is said that some researchers in China have prepared high orientation graphite films with resistivity less than 1.1 μ? · m and thermal conductivity up to 1100 w / (m · K) by carbonization and high temperature graphitization. However, there is still much room to improve the theoretical thermal conductivity of graphene. At present, the conductivity of carbon film, graphite film and polyimide film is more than 6.5 × 103s / m, 2 × 105s / M and 1500W / m · K. However, the domestic polyimide film is difficult to meet the industrial demand because of uneven heat shrinkage, poor quality of the graphite film and low conductivity. It can be seen that the performance and development of polyimide graphite film are directly determined by the limitations of the preparation process conditions of polyimide film.
In terms of polyimide film preparation, China began to explore the technology and industry of polyimide film in 1966, and now there are more than 50 domestic manufacturers. In recent years, with the development of industry, many kinds of polyimide films have been developed in China, but generally speaking, there is a big gap between domestic and foreign films. Most domestic enterprises adopt the casting process, which is small in scale, large in pollution and poor in quality. The polyimide film produced can only be used for electrical insulation. When the requirements for product performance stability are high or used in other special environments, it can only rely on imports. Therefore, in order to develop the graphitization technology of polyimide in China, the first thing to be improved is the preparation technology of polyimide film, such as using biaxial oriented equipment for production.
In view of the current research status and existing problems of polyimide graphite film, experts in the industry suggest that two aspects of work should be carried out in the future: first, continue to develop the technology of carbonization and high temperature graphitization to prepare flexible graphite film devices with high heat conductivity, adjust the process parameters such as heating rate and holding time, and observe the dynamic process of orientation state formation The formation mechanism of graphite lamellae was revealed; Secondly, based on the characteristics of graphene and modern testing technology, the factors influencing the thickness and size of graphene flakes which control the composition of graphene film are analyzed, the transformation mechanism of high orientation polyimide precursor polymer to large-size graphene microcrystalline with high orientation along the tensile direction of graphene mesh is clarified, and the correlation between structure and performance is clarified, so as to solve the key scientific problems.
In short, the development and utilization of polyimide graphite film, a new functional film device, will further alleviate the urgent demand for high thermal conductivity materials in many fields such as national defense industry and electronic packaging devices, and promote the progress and development of science and technology in the above fields.