Polyimide (PI) is a class of polymeric compounds containing an imide ring (-CO-N-CO-) in its main chain. It possesses excellent properties such as high temperature resistance, radiation resistance, chemical corrosion resistance, high strength, and high modulus, and is widely used in aerospace, electronics, automotive, and machinery industries. The production process of polyimide mainly includes raw material preparation, polymerization reaction, imidization reaction, post-processing, and molding.
In the raw material preparation stage, the main raw materials include dianhydrides (such as pyromellitic dianhydride PMDA) and diamines (such as diaminodiphenyl ether ODA), as well as solvents (such as N,N-dimethylacetamide DMAc, N-methylpyrrolidone NMP, etc.). Raw materials must undergo strict quality inspection to ensure that their purity and impurity content meet production requirements. The choice of solvent has a significant impact on the polymerization rate and the performance of polyimide; therefore, it must be selected based on specific product requirements.
The polymerization reaction is the core step in polyimide production and is typically carried out using a two-step method. The first step is prepolymerization, where dianhydride and diamine are mixed in a solvent in a specific ratio and subjected to a polycondensation reaction at low temperature to generate a polyamic acid (PAA) solution. This step requires strict control of the reaction temperature, time, and raw material ratio to ensure the molecular weight and distribution of the polyamic acid meet requirements. The second step is polymerization, where the prepolymerized polyamic acid solution is further reacted at a higher temperature to allow the molecular chains to grow further, forming high-molecular-weight polyamic acid.
Imidization is the key step in converting polyamic acid into polyimide. Imidization can be carried out through either thermal imidization or chemical imidization. Thermal imidization involves coating the polyamic acid solution onto a substrate and baking it at high temperature to dehydrate and cyclize the polyamic acid, forming a polyimide film. Chemical imidization involves adding a dehydrating agent (such as acetic anhydride) and a catalyst (such as pyridine) to the polyamic acid solution and reacting at a lower temperature to generate polyimide. Chemical imidization has a faster reaction rate, but the product performance may be slightly inferior to that of thermally imidized products.
Post-processing steps include washing, drying, and pulverizing, aiming to remove solvents, unreacted raw materials, and impurities from polyimide, improving product purity and performance. Washing is typically done with water or organic solvents to remove residual solvents and impurities. Drying uses methods such as hot air drying or vacuum drying to remove moisture and volatile substances from the polyimide, ensuring product stability. Pulverizing breaks down the dried polyimide blocks into powder, facilitating subsequent molding processes.
Molding processes involve shaping polyimide powder or film into products of the desired shape and size. Molding methods include injection molding, extrusion, compression molding, and stretching. Injection molding is suitable for producing polyimide products with complex shapes and high dimensional accuracy requirements, such as gears and bearings. Extrusion molding is suitable for producing continuous length polyimide profiles, such as pipes and rods. Compression molding is suitable for producing large-size, thick-walled polyimide products, such as sheets and structural components. Stretch molding is mainly used for producing polyimide films, improving the film's orientation and mechanical properties through stretching.
During the production of polyimide, attention must be paid to safety and environmental protection. The polymerization and imidization reactions may generate harmful gases and waste liquids, requiring effective ventilation and waste gas treatment measures to ensure a safe production environment. Simultaneously, waste liquids and residues must be properly treated to prevent environmental pollution.
