What is the raw material for pyrolytic graphite sheets? And the current status of graphite sheet raw materials in the market.

Polyimide (PI) film is known as the "golden film" and is a key insulation material in fields such as electronics, electronics, space, etc. High performance PI film, after high-temperature carbonization and graphitization treatment, can be made into graphite heat sinks with a thermal conductivity several times that of copper, which is the core material for solving heat dissipation problems in electronic products. With the upgrade of communication technology from 4G to 5G, the introduction of high frequency, continuous improvement of hardware integration, and continuous miniaturization of chips, the heat generation of devices has increased dramatically, and heat dissipation has become a bottleneck problem faced by 5G terminals.

At present, PI on the market is divided into two categories: thin PI films below 90um and thick PI films above 90um thickness. The thickness of PI film is only below 90 μm, corresponding to graphite sheets of 17-40μm; the thickness of ultra thick PI film is 90-250 μm, breaking the industry's upper limit of thickness for single-layer graphite sheets of 45-130 μm.

The traditional PI film manufacturing technology for graphite sheets can only produce products with a thickness of less than 90 μ m, and can only prepare graphite sheets with a specification of 17-40 μ m. Single layer heat dissipation is low, and multiple layers must be stacked for use; The stacking of the adhesive layer will form a thermal resistance effect, significantly reducing the thermal diffusion ability, and the process is complex and costly. However, the direct preparation of ultra thick PI films with a thickness of over 90 μm faces multiple obstacles: traditional precursor resins still react slowly with acid anhydrides at both ends of the molecular chain after polymerization, and the reaction endpoint is difficult to control. They need to be stored at ultra-low temperatures around -20 ℃, which not only leads to excessive apparent viscosity and poor leveling of the resin, but also causes defects such as uneven thickness and pinholes in the thick film. The requirements for catalyst activity and dosage are also very strict; More importantly, the expansion of ultra thick PI film in the Z-axis direction during high-temperature graphitization is difficult to control, and it is prone to excessive foaming, resulting in defects such as delamination, powder loss, and cracking after rolling, making it impossible to obtain high-density thick graphite sheets.

The thickness of the ultra thick polyimide film can reach 90-250 μ m, and its preparation method is divided into three steps: first, surface modified carbon nanotubes (treated with carboxylation, amination or fluoridation, with a diameter of 10-200 nm and a length of 0.5-50 μ m) and inorganic fillers (boron nitride, silicon carbide, silicon oxide, calcium phosphate, etc.) are dispersed in an organic solvent; Add the diamine monomer again, wait for it to dissolve, and then add the dianhydride monomer in batches. Finally, introduce a cross-linking end capping agent (such as 4-phenylethynylphthalic anhydride), defoam and prepare the polyamide acid resin, which is stored at 0-20 ℃; Subsequently, a dehydrating agent was added and mixed evenly with the catalyst. The ultra thick polyimide film was prepared by salivation at 100-200 ℃, imidization and crosslinking at 200-450 ℃, and annealing and heat setting at 250-350 ℃. The film is carbonized, graphitized, and rolled to obtain graphite sheets with a single layer thickness of 45-130 μ m.