Polyimide, typically abbreviated as PI or referred to tradenames such as Vespel, Plavis and Duratron-PI, is the second most powerful high temperature engineering polymer on the market today, right behind Celazole. In this blog post, we are going to discuss the characteristics and uses of this powerful high-performance polymer.
Where Polyimides Are Used
Vepsel has been proven ideal for the most rugged conditions involving high temperatures (up to 570°F), chemical attack, vacuum pressures, and even cryogenic applications.
It can be found in just about any industry that requires heat-resistant polymers, including oil & gas, semiconductor, transportation, and aerospace applications. Polyimide can be found in centrifugal pumps, material handling equipment, aircraft engines, boiler pump, cooling towers, and automotive parts.
More specifically, graphite, PTFE, and molybdenum disulfide filled grades of polyimide have been found ideal for various types of high performance bearings, bushings, and seals because of its thermal performance, low friction, dry running, and excellent wear capabilities.
Characteristics of Polyimides
Polyimide provides outstanding heat resistance and good chemical resistance. It has very good strength and impact characteristics, and very good wear resistanceat temperatures where most thermoplastics lose their strength and usefulness. It offers excellent creep resistance and good dimensional stability, which can be further improved through the addition of graphite as a filler.
This lightweight but strong polymer works well in vacuum applications and is considered a viable replacement for many metal applications.
Polyimide is also known for very low friction, which can be lowered through additives such as graphite, PTFE, of molybdenum disulfide. Friction ranges from 0.29 for unfilled polyimide down to 0.12, depending on the fillers used. It provides excellent wear characteristics, too.
This lightweight but strong polymer works well in vacuum applications and is considered a viable replacement for many metal applications. Its primary drawback is a tendency to absorb moisture, with a maximum water absorption rate of 24 hours of .24% for unfilled but down to .14% for some filled grades.
Polyimide also offers low outgassing and resistance to radiation. As an added bonus, it has a reputation for being very easy to machine.
Types of Polyimides Available
Virgin polyimide can operate in environments within a range of temperatures from cryogenic to 570°F. It has good strength and excellent thermal and electrical insulation. Unfilled polyimide offers the greatest tensile strength and compressive strength of available grades at 12.5 ksi and 19.3 ksi, respectively.
Polyimide can be filled with graphite, usually either 15% or 40% by weight. The addition of graphite reduces friction, improves dimensional stability, and increases wear resistance. Another common grade of polyimide involves a combination of 10% PTFE and 15% graphite, resulting in a blend that has an extremely low coefficient of friction and outstanding wear resistance in temperatures up to 300°F.
There are also blends of polyimide with molybdenum disulfide which work well where graphite would be considered too abrasive. These blends are well adapted to vacuum and moisture-free environments and provide improved wear and friction characteristics.
Conclusion
Polymide (Vepsel) is one of the best high performance polymers, falling right behind Celazole. It can handle a wide range of temperatures from cryogenic to 570°F without losing its key characteristics, and is available in a variety of filled options. Polymide is a popular choice for bearings, bushings, and seals that will be facing hostile environments.
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