Key Difference – Polyamide vs Polyimide
Polyamide and polyimide are high-temperature-resistant thermoplastic elastomers with a wide range of applications. The major difference between polyamide and polyimide is in their chemical structures; polyamide has the amide (-CONH-) linkages in their polymer backbone, while polyimide has imide group (-CO-N-OC-) in their polymer backbone.
These two polymers are also known for their excellent electrical and physical properties in addition to high thermal stability.
CONTENTS
1. Overview and Key Difference
2. What is Polyamide
3. What is Polyimide
4. Side by Side Comparison – Polyamide vs Polyimide in Tabular Form
5. Summary
What is Polyamide?
Polyamides are high-performance thermoplastic elastomers characterized by their high service temperature, good heat aging, and solvent resistance. In addition, polyamides have high modulus and impact properties, low coefficient of friction, and high abrasion resistance. Although polyamides are developed to have a wide range of properties, they all consist of amide (-CONH-) linkages in their polymer backbone. Nylon is the most common and widely used type of polyamide; this was one of the earliest polymers developed by Carothers. Today nylon is among one of the most important and widely consumed polymers in the United States.
Figure 1: Polyamide
The amide group is a polar group, which allows polyamides to build hydrogen bonds between chains, thus improving the interchain attraction. This enhances the mechanical properties of polyamide. In nylon, the flexible aliphatic carbon groups in the chain improve the processability of the material by reducing the melt viscosity. The strength and stiffness can be reduced by increasing the number of carbon atoms between the amide linkages. Therefore, the length of the hydrocarbon backbone is a key property that determines the performance of polyamide material. Due to the polarity of the amide group, polar solvents, mainly water, can affect polyamides.
There are two types of polyamides: aliphatic and aromatic polyamides. Nylon can be either an aliphatic or semi-aromatic polyamide. The main applications of polyamides include radiator header tanks in cooling systems, switches, connectors, ignition components, sensors and motor parts in auto electrical systems, wheel trims, throttle valves, engine covers, heat resistant under-bonnet components, airbrake tubing, etc.
What is Polyimide?
Polyimides are high performance polymers, containing imide group (-CO-N-OC-) in their repeating units. The polymer chains are either open chain or closed chain. Polyimides are known for their excellent thermal stability, along with decent electrical and mechanical properties. Usually, polyimide has short exposure temperature stability. Moreover, polyimide shows excellent wear and solvent resistance, and also high abrasion resistance. These properties have led polyimide to be used as a specialty material in certain applications including insulating films, laminates, coatings, molded parts, structural adhesives, high-modulus fibres, permseletive membranes and high-temperature composites.
Figure 2: Polyimide
Soluble copolyimides are used to make adhesives, sealants and molding resins. Good oxidative stability and a high glass transition temperature (tg) can be achieved with polyimide with aromatic structures. Based on the synthesis method, polyimides are classified into three categories, namely; condensation polymers-non melting and thermoplastic, addition polymers and hybrid polymers.
What is the Difference Between Polyamide and Polyimide?
Polyamide vs Polyimide |
|
| Polyamides are made of amide linkages (-CONH-). | Polyimides are made of imide linkages (-CO-N-OC-). |
| Synthesis | |
| Polyamides are synthesized by the polymerization between diamines and dicarboxylic acid. | Polyimides are synthesized by the polymerization between dianhydride and diisocyanate or diamine. |
| Chemical Formula | |
| Common trade names are Nylon and Kelvar. | The common trade name is Kapton. |
| Applications | |
| Polyamides are used as radiator header tanks in cooling systems; switches, connectors, ignition components, sensors and motor parts in auto electrical systems; wheel trims, throttle valves, engine covers, heat resistant under-bonnet components, airbrake tubing, etc | Polyimides are used as insulating films, laminates, coatings, molded parts, structural adhesives, high-modulus fibres, permselective membranes and high-temperature composites |
Summary – Polyamide vs Polyimide
Both polyamide and polyimide are thermoplastic elastomers with high thermal and oxidative stability. Polyamide consists of amide linkages in their backbone and is synthesized by the polymerization between diamine and dicarboxylic acid. Polyimide contains imide linkages in their backbone and is synthesized by the polymerization between dianhydride and diisocyanate or diamine. This is the difference between polyamide and polyimide.
Reference:
1. Kricheldorf, Hans R. Handbook of Polymer Synthesis. Dekker, 1992.
2. Chanda, Manas, and Salil K. Roy. Industrial Polymers, Specialty Polymers, and Their Applications. CRC Press, 2009.
3. Ghosh, Malay K., and K. L. Mittal. Polyimides: Fundamentals and Applications. Dekker, 1996.
4. Mckeen, Laurence W. Fatigue and Tribological Properties of Plastics and Elastomers. Elsevier, 2016.
Image Courtesy:
1. “Polyamide Amidegroup V.1” By Jü – Own work (CC0) via Commons Wikimedia
2. “Polyimide” By Ed (Edgar181) – Own work (Public Domain) via Commons Wikimedia