Polyimides - What makes them popular?
Polyimides - What makes them popular?
Polyimides (PI) are high-performance polymers of imide monomers. They contain two acyl groups (C=O) bonded to nitrogen (N).
Typical performance profile
Polyimides exhibit an exceptional combination of:
The linearity and stiffness of the cyclic backbone allow molecular ordering. This phenomenon results in lower coefficients of thermal expansion. The CLTE values are lower than thermoplastics having coiled, flexible chains. Additionally, long, linear-ordered chains provide solvent resistance to aromatic polyimides.
The rigid structure of polyimides provides high glass transition temperature (Tg > 300°C). It also imparts good mechanical strength and high modulus.
Classification based on main chain constitution
Polyimides have been in mass production since 1955. They exist in two formats: thermosetting and thermoplastic. Depending upon the constitution of their main chain, PI can be classified as:
- 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
Aromatic Heterocyclic Polyimide (L); Linear Polyimide (R)
Key applications served
Polyimides offer excellent mechanical properties. And thus, they find use in applications that demand rugged organic materials, e.g.,
- High-temperature fuel cells
- Flat panel display
- Aerospace applications
- Chemical and environmental industries, and
- Military applications
Available for use as plastics, films, laminating resins, insulating coatings, and high-temperature structural adhesives. They replace conventional glass, metals, and even steel in many industrial applications.
How to synthesize polyimides?
How to synthesize polyimides?
The synthesis of polyamides takes place by adding heterocyclic rings into the polymer chain. These inert imide rings are highly stable and rigid. The presence of the ring and high interchain interaction results in high cohesion among polymer chains. This further imparts high thermal stability in the polymer.
The classical method of polyimide synthesis is by reaction of a dianhydride and a diamine.
The synthesis of aromatic polyimides was first reported in 1908. But, significant advances in PI synthesis and processing were not realized. This is due to the lack of processability via melt polymerization. In the early 1960s, DuPont was the first company to produce polyimide commercially. It was based on pyromellitic dianhydride and 4,4’diaminodiphenyl ether.
This type of reaction consisted of two steps:
STEP 1 - Solution polycondensation of an aromatic diamine and a dianhydride to form poly(amic acid).
STEP 2 - Poly(amic acid) could be processed into a useful shape. This is followed by cyclodehydration of the amide acid to form polyimide.
Most polyimides are infusible and insoluble. This is due to their planar aromatic and hetero-aromatic structures. Thus, they usually need to be processed from the solvent route. This method provided the first such solvent-based route to process these polyimides.
By adding aromatic rings in the backbone and/or side groups of PI, thermal stability can be further improved. The nature of the chemical structure consisting of rigid imide and aromatic rings always provides:
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Excellent mechanical toughness
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Excellent dielectric properties
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High chemical resistance
Moreover, depending on the application's needs, other functionalities can be added to the backbone and/or side groups of PIs. These include:
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Photo reactivity
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Molecular recognition ability
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Nonlinear optical responsibility
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Light emitting ability, and so on
Why use polyimides over other high heat plastics?
Why use polyimides over other high heat plastics?
Below are the strengths and limitations of polyimides over other high heat polymers.
| Strengths |
Limitations |
- High mechanical performance
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Superior temperature adaptability
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Excellent tensile and compressive strength
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Outstanding chemical resistance
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Transparency in many microwave applications
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Radiation resistance
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Superior bearing and wear properties
|
- Has high manufacturing cost
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High temperature requirement in the processing
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Specified operating processes such as annealing operations at specified temperatures
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Sensitive to alkali and acid attacks
|
How to process polyimides?
How to process polyimides?
Polyimide matrix plays a very important role in the fabrication of composite components. They add value to thermal servicing and mechanical properties. They also enhance processing methods and quality.
However, it is difficult to design and synthesize polyimide matrix for high-temperature composites. This is because the matrix is required to have thermal & mechanical properties, and 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:
- suitability for different processing methods such as autoclave and RTM
- excellent combined properties for applications requiring temperatures of 370°C
Processing conditions of polyimides
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Processing temperature: 380 to 430°C
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Drying before processing is highly recommended: 10 h at 180°C or 5 h at 200°C.
For a crystalline polyimide, the injection molded or extruded products are generally amorphous. The service temperature of PI can be improved from 240°C in the amorphous state to 340°C in the crystalline state. This is possible by annealing after processing.
For injection molding of polyimides, a mold temperature of 170-210°C is recommended.
The conditions for extruded polyimides are:
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Extrusion temperature: ~ 400°C
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L/D ratio of at least 20-25 is recommended
Browse polyimide grades in our database, after reading the guide.