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The application of polyimide

The application of polyimide

Polyimide refers to a class of polymers with an imide ring in the main chain and is one of the best overall organic polymer materials. It has a high-temperature resistance of over 400°C, a long-term use temperature range of -200 to 300°C, no significant melting point in parts, high insulation properties, a dielectric constant of 4.0 at 103 Hz, a dielectric loss of only 0.004 to 0.007, and is a class F to H insulation.

What are the properties of polyimide?

  • For the fully aromatic polyimide, according to the thermogravimetric analysis, the beginning of its decomposition temperature is generally at about 500 ℃. Polyimides synthesised from biphenyl dianhydride and p-phenylenediamine have a thermal decomposition temperature of 600 °C and are by far one of the most thermally stable varieties of polymers.
  • Polyimides can withstand very low temperatures, e.g. at 4 K (-269 °C) in liquid helium, without brittle cracking.
  • Polyimide also has very good mechanical properties. The tensile strength of unfilled plastics is mostly above 100 MPa, while homopolymer polyimide films (Kapton) have a tensile strength of 250 MPa and biphenyl polyimide films (Upilex S) have a tensile strength of 530 MPa. As for engineering plastics, the modulus of elasticity is usually 3 to 4 GPa. According to theoretical calculations, polyimide fibres made from homophthalic anhydride and p-phenylenediamine can reach a modulus of 500 GPa, second only to carbon fibres.
  • Polyimides are more stable to dilute acids, but in general, the varieties are not very resistant to hydrolysis, especially alkaline hydrolysis. This seemingly disadvantageous property gives polyimides a great advantage over other high-performance polymers, namely the possibility of recovering the raw materials dianhydride and diamine by alkaline hydrolysis, for example, up to 90% for Kapton films. Structural changes can also lead to varieties that are quite resistant to hydrolysis, e.g. withstanding 120 °C and 500 h of cooking. However, polyimides, like other aromatic polymers, are not resistant to concentrated sulphuric acid, nitric acid and halogens.
  • Polyimides have a wide solubility spectrum and, depending on their structure, some species are insoluble in almost all organic solvents, while others are soluble in common solvents such as tetrahydrofuran, acetone, chloroform and even toluene and methanol.
  • The coefficient of thermal expansion of polyimides ranges from 2 × 10 - 5 to 5 × 10 - 5 K - 1, with biphenyl-type polyimides reaching 0 - 6 K - 1, on the same level as metals, and some varieties even reaching 10 - 7 K - 1. 10 - 7 K - 1.
  • Polyimide has a high resistance to irradiation, its film can still retain 86% strength at an absorbed dose of 5 × 107 Gy, and a polyimide fiber after 1 × 108 Gy fast electron irradiation, the strength retention rate is still 90%.
  • Polyimides have very good dielectric properties, with ordinary aromatic polyimides having a dielectric constant of around 3.4. The introduction of fluorine reduces the dielectric constant to around 2.5 by dispersing the air in the polyimide in nano-sized groups. The dielectric loss is 10-3, the dielectric strength is 100-300 kV/mm and the volume resistance is 1017 W-cm. These properties are maintained at a high level over a wide temperature and frequency range.
  • Polyimide is a self-extinguishing polymer with a low smoke emission rate.
  • Polyimide gives off very little gas at a very high vacuum.
  • Polyimides are non-toxic and can be used to make tableware and medical instruments and can withstand thousands of sterilizations. Some polyimides are also very biocompatible, for example, they are non-haemolytic in blood compatibility tests and non-toxic in vitro cytotoxicity tests.

Applications of polyimides

Polyimides are widely used due to the above-mentioned properties, synthetic chemistry and the fact that they can be processed in a variety of ways.
It is difficult to find a material with such a wide range of applications as polyimide, and each of its applications shows outstanding properties.
  1. Films: One of the earliest commercial products of polyimide was used for slot insulation in electric motors and cable winding materials. In recent years, there has been a huge industry for polyimide films in flexible printed circuit boards. The main products are Kapton from DuPont, the Upilex series from UBE and Apical from Jong-un.
  2. Coatings: Used as insulating varnishes for electromagnetic wires or as high-temperature-resistant coatings.
  3. Advanced Composites: Used for structural components and engine parts for aerospace, aircraft and rockets. It can be used for hundreds of hours at 380 °C or higher and can withstand temperatures of 400 to 500 °C for a short period of time, making it the most heat-resistant resin-based composite material.
  4. Fibres: Common heat-resistant fibres can be used as filtration materials for high-temperature media and radioactive materials, and as flame-retardant fabrics. High-strength fibres are used as reinforcement for advanced composites and as protective, cut-proof fabrics. Nanofibres obtained by electrospinning can be used as separators for lithium batteries to improve their safety and charging speed. In addition, they can be used in fine filtration materials.
  5. Foam: used as thermal and acoustic insulation for high and ultra-low temperature resistance.
  6. Engineering plastics: thermosets and thermoplastics, which can be moulded, injection moulded or transfer moulded. Mainly used for self-lubrication, sealing, insulation and structural materials.
  7. Adhesives: Used as high-temperature structural adhesives.
  8. Separation films: For the separation of various gases such as hydrogen/nitrogen, nitrogen/oxygen, carbon dioxide/nitrogen or methane, for the removal of water from the air, hydrocarbon raw gases and alcohols. They are also used for permeation vaporization membranes and ultrafiltration membranes. Polyimide is of particular importance in the separation of organic liquids and gases due to its resistance to heat and organic solvents.
  9. Photoresists: There are negative and positive resists, which can be used with aqueous developer solutions with a resolution of up to submicron level. They can be used in combination with pigments or dyes for colour filter films, which greatly simplifies the process.
  10. Applications in microelectronic devices: as a dielectric layer for interlayer insulation, and as a buffer layer to reduce stress and improve yield. As a protective layer, it can reduce the influence of the environment on the device, and can also play a shielding role against alpha particles, reducing or eliminating the soft error of the device (soft error).
  11. Orientation aligners for liquid crystal displays: Polyimide plays a very important role in the orientation of TN-LCD, STN-LCD, TFT-LCD and ferroelectric liquid crystal displays.
  12. Electro-optical materials: Used as passive or active waveguide materials, optical switch materials, etc. Polyimide containing fluorine is transparent in the communication wavelength range; using polyimide as a base for chromophores can improve the stability of the material.
  13. Proton transport membranes: Used as diaphragms for fuel cells, especially methanol fuel cells, where the methanol transmission rate is much lower than that of conventional perfluorosulfonic acid membranes (Nafion).
  14. Medical materials: Polyimide is low in toxicity and can be used as a medical material in many ways.

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