Polyimide (PI) Properties & Uses

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Basic Information

  • Name: Polyimide

  • Alias/Abbreviation: PI

  • Appearance: Light yellow powder

  • Bending Strength (20°C): ≥170 MPa

  • Density: 1.38–1.43 g/cm³

  • Impact Strength (Notched): ≥28 kJ/m²

  • Tensile Strength: ≥100 MPa

  • Vicat Softening Point: >270°C

  • Water Absorption (25°C, 24h):

  • Elongation: >120%

  • Characteristics: A polymer with imide repeating units

  • Main Categories: Aliphatic, semi-aromatic, and aromatic polyimides


Types and Characteristics

1. Condensation Type

Synthesized from aromatic diamines and aromatic dianhydrides or related compounds. Difficult to produce pore-free composites due to high boiling point solvents. Mostly used for films and coatings.

2. Addition Type

Developed to overcome drawbacks of condensation types. Includes:

  • Bismaleimide (BMI): Easy processing, low cost, but brittle.

  • Norbornene-Terminated Polyimide (PMR-Type): Developed by NASA, suitable for fiber impregnation.

3. Subtypes

Divided into:

  • Homo-aromatic PI

  • Soluble PI

  • Polyamide-imide (PAI)

  • Polyetherimide (PEI)


Performance Features

  • Thermal Decomposition Temperature: Starts around 500°C; some varieties reach 600°C.

  • High-Temperature Resistance: Usable above 400°C, continuous use from -200°C to 300°C.

  • Extreme Low-Temperature Resistance: Withstands temperatures as low as 4K (-269°C).

  • Mechanical Properties: Tensile strength >100 MPa; modulus up to 500 GPa in fibers.

  • Hydrolysis Resistance: Stable in dilute acid; weak in alkali but recyclable.

  • Acid and Solvent Resistance: Wide solubility range; resistant to many organic solvents.

  • Thermal Expansion Coefficient: As low as 10⁻⁷ °C⁻¹, comparable to metals.

  • Radiation Resistance: Maintains strength after high-dose radiation.

  • Dielectric Properties: Dielectric constant ~3.4; dielectric strength 100–300 kV/mm.

  • Flame Retardancy: Self-extinguishing, low smoke, high char residue (>50%).

  • Biological Safety: Non-toxic, sterilizable, good biocompatibility.


Synthesis Methods

  • Low-Temperature Polycondensation: Produces polyamic acid, then heated to ~300°C for imidization.

  • Chemical Dehydration Cyclization: Using acetic anhydride and tertiary amines.

  • One-Step Method: High boiling solvents like phenols.

  • Others: Via esters, intermediates, and special functional group introduction for tailored properties.


Application Areas

Aerospace

  • Uses: Spacecraft structures, thermal protection, cable insulation.

  • Highlight: Used for China's Chang'e-4 moon mission national flag.

Microelectronics

  • Uses: ICs, chip packaging, flexible circuits, photoresists.

Electrical Insulation

  • Uses: Wire insulation, motor slot insulation, transformer wrapping.

New Energy

  • Uses: Fuel cells, solar panels, lithium-ion battery separators.

Fibers and Fabrics

  • Uses: Bulletproof vests, fire-resistant clothing, high-temperature filtration.

  • Note: Only Germany's Evonik and China's Changchun Gaoqi have industrialized production.

Other Fields

  • Uses: Separation membranes, coatings, adhesives, engineering plastics.


Development Trends

  • Global & Domestic Competition: Rapid market expansion, especially in China.

  • Trends:

    • Tech innovation & new properties

    • Market scaling

    • High-end applications & localization

    • Expanding sectors like 5G, OLED, aviation

    • New materials like fluorinated and photosensitive PI

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