Polyimide (PI) Properties & Uses

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