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Key Features & Applications of Polyimide (PI)

Comprehensive Guide on Polyimide (PI)

As an attractive dielectric material, polyimide has been widely used in the field of electronics, aerospace, and automobiles fulfilling increasing need for materials that can perform well under harsh conditions, such as elevated temperatures. Polyimides are an important class of step-growth polymers due to their high temperature stability, mechanical properties, and superior chemical resistance.

Explore Polyimides (PI) in detail along with its key properties like mechanical, thermal, electrical, etc. and understand what makes it an ideal choice in high-end engineering applications.

Overview

Polyimides - What makes them popular?

Polyimides - What makes them popular?

Polyimides (PI) are high-performance polymers of imide monomers which contain two acyl groups (C=O) bonded to nitrogen (N). These polymers are known for their high temperature performance in the 400-500°C range as well as chemical resistance.

They are used to replace the conventional use of glass, metals and even steel in many industrial applications.

Polyimides offer excellent mechanical properties and thus find use in applications that demand rugged organic materials, e.g.

  • High temperature fuel cells
  • Flat panel display
  • Aerospace applications
  • Chemical and environmental industries
  • As well as various military applications

They are available for use as plastics, films, laminating resins, insulating coatings and high temperature structural adhesives.

Polyimides exist in two formats: thermosetting and thermoplastic.

Depending upon the constitution of their main chain, Polyimide can be classified as aliphatic, aromatics, semi-aromatics thermoplastics and thermosets.

  • Aromatic polyimides are derived from an aromatic dianhydride and diamine.
  • Semi-aromatic ones contain any one of the monomer aromatics: i.e., either the dianhydride or diamine is aromatic, and the other part is aliphatic.
  • Aliphatic polyimides consist of the polymers formed as a result of the combination of aliphatic dianhydride and diamine.

Molecular Structure of Aromatic Polyimide
Molecular Structure of Aromatic Polyimide


Aromatic Heterocyclic Polyimide (L); Linear Polyimide (R)
Aromatic Heterocyclic Polyimide (L); Linear Polyimide (R)

Polyimides have been in mass production since 1955.

Synthesis of Polyimides

Synthesis of Polyimides

Polyimides are prepared by incorporating highly stable and rigid heterocyclic ring systems into the polymer chain. Hence, the presence of an inert imide ring and high interchain interaction, i.e., high cohesion among the polymer chains imparts high thermal stability in the polymer.

The classical method of polyimide synthesis is by reaction of a dianhydride and a diamine.

Synthesis of aromatic polyimides was first reported in 1908. However, due to the lack of processability via melt polymerization, significant advances in polyimide synthesis and processing were not realized. In early 1960s, DuPont was the first company to produce polyimide commercially. It was based on pyromellitic dianhydride and 4,4’diaminodiphenyl ether.

Classical method of polyimide synthesis is by reaction of a dianhydride and a diamine

This type of reaction consisted of two steps

  • The solution polycondensation of an aromatic diamine and a dianhydride to form poly(amic acid)
  • Poly(amic acid) could be processed into a useful shape, followed by cyclodehydration of the amide-acid to form polyimide

Most polyimides are infusible and insoluble due to their planar aromatic and hetero-aromatic structures and thus usually need to be processed from the solvent route. This method provided the first such solvent based route to process these polyimides.

The thermal stability of polymer can further be improved by incorporating aromatic rings on the backbone and/or side groups. In addition to such high thermal stability, the nature of the chemical structure consisting of rigid imide and aromatic rings always provides:

  • Excellent mechanical toughness
  • Excellent dielectric properties
  • High chemical resistance

Moreover, depending on the application needs, other functionalities can be added into the backbone and/or side groups of PIs. These include:

  • Photo reactivity
  • Molecular recognition ability
  • Nonlinear optical responsibility
  • Light emitting ability, and so on


Well Known Properties of Polyimides

Well Known Properties of Polyimides

Polyimides exhibit an exceptional combination of thermal stability (>500 °C), mechanical toughness, and chemical resistance. They have excellent dielectric properties.

The linearity and stiffness of the cyclic backbone allow for molecular ordering. This phenomenon results in lower coefficients of thermal expansion (CTE) than those found for thermoplastic polymers having coiled, flexible chains.

Additionally, the morphology of long, linear ordered chains provides solvent resistance to the aromatic polyimides.

The comparatively rigid structure of polyimides provides high glass transition temperature (Tg > 300°C) and imparts good mechanical strength and high modulus.

Strengths and Limitations of Polyimides


Strengths Limitations
  • High mechanical performance
  • Superior temperature adaptability
  • Excellent tensile and compressive strength
  • Outstanding chemical resistance
  • Transparency in many microwave applications
  • Radiation resistance
  • Superior bearing and wear properties
  • Has high manufacturing cost
  • High temperature requirement in the processing
  • Specified operating processes such as annealing operations at specified temperatures
  • Sensitive to alkali and acid attacks
Further, PI matrix pays a very important role in the fabrication of composites components. PI composites not only add value in thermal servicing and mechanical properties, but also enhance processing methods and quality.

However, it is difficult to design and synthesize polyimide matrix for high-temperature composite components, because the matrix resins are required to have not only enough thermal and mechanical properties, but also appropriate melt processability...learn how you can accurately measure melt temperature for your high heat plastics

Some carbon fiber-reinforced thermoset polyimides-based composites have demonstrated the suitability for different processing methods such as autoclave and RTM and excellent combined properties for 370°C applications.


Polymer Matrix Selection for Very High Temperature Composites


How to Process PI?

How to Process PI?

  • Processing temperature: 380 to 430°C
  • Drying before processing is highly recommended: 10 h at 180°C or 5 h at 200°C.

Even if Polyimide is a crystalline polymer, the extruded or injection molded products are generally amorphous. In order to improve service temperature (from 240°C in the amorphous state to 340°C in the crystalline state), annealing after processing is possible.

For injection molding of polyimides, a mold temperature of 170-210°C is recommended.

And, extrusion conditions are:

  • Extrusion temperature: ~ 400°C
  • L/D ratio of at least 20-25 is recommended.


PES can be processed by conventional thermoplastics methods such as: injection molding, extrusion, blow molding or thermoforming. It exhibits low shrinkage. For injection molding, barrel temperatures of 340-380°C with melt temperatures of 360°C are recommended.

Mold temperatures should be in the range of 140-180°C. For thin-walled molding, higher temperatures may be required. Unfilled PES can be extruded into sheets, rods, films and profiles.

  • Processing temperature : 340 to 390°C
  • Drying is recommended, in order to obtain % of water < 0.04 %: 4 h at 150°C or 2h at 180°C.

Injection Molding


  • A mold temperature of 120-160°C is recommended. 
  • Screws of L/D around 25 are recommended. 
  • Polyethersulfone is suitable for injection of very small parts with tight dimensional tolerances due to its low lineic shrinkage.

Extrusion (film/sheet/tube)


  • Extrusion temperature : 340-390°C 
  • L/D around 20 is recommended 
  • Elasticity is generally high, so the orientation is weak.


Find Suitable Polyimide (PI) Grade

View a wide range of polyimide (PI) grades available in the market today, analyze technical data of each product, get technical assistance or request samples.

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1 Comments on "Comprehensive Guide on Polyimide (PI)"
Ilhem A Aug 31, 2021
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