Amazing polyphenyl ethers used in the SR 71 for lubrication set new standards.

Polyphenyl ethers used in the SR 71 for lubrication are a class of lubricants having unusual properties, most significantly high thermal and radiation resistance, chemical stability, a high refractive index, and positional stability. They are most useful as robust lubricants in a variety of extreme environments.

In 1906, during the exciting early days of synthetic organic chemistry, a German researcher named F. Ullman set out to make diphenyl ether. By
substituting an oxygen atom between two benzene rings, he reasoned that he would produce a substance that would remain liquid at lower temperatures.

What are polyphenyl ethers used in the SR 71 for lubrication?

Ullman was successful in creating the first polyphenyl ether - a class of compounds having from 2 to 10 benzene rings with ether linkages between them. Initially the compound that Ullman made had no immediate application. It was simply a chemical oddity. He would never have imagined that polyphenyl ethers used in the SR 71 for lubrication would make history.

Each polyphenyl ether has numerous isomers. The positions of the four oxygen atoms on the benzene rings are a very critical feature: they permit molecular rotation of the various parts of the molecule. This flexibility in turn means that a polyphenyl ether can remain liquid at fairly low temperatures - an important characteristic for many lubricants. The pour point of a 5-ring polyphenyl ether, for example, is 4° C. A sequence of 5 benzene rings without the oxygen atoms would be extremely rigid, would pack tightly, and would consequently become solid at higher temperatures.

Polyphenyl ethers used in the SR 71 for lubrication are unique.

The thermal stability of polyphenyl ethers led to their first major use as a lubricant. In the 1960s, the SR-71 spy plane was under development by the Air Force. No lubricant based on petroleum distillates could be used for the turbine engines, because the plane was designed to fly too high (>60,000 feet) and, as a result, the temperature of the bearing surfaces was too high 316° C. A 5-ring polyphenyl ether was found to be stable at these temperatures, making polyphenyl ethers used in the SR 71 for lubrication ideal.


Two engine driven oil systems are used to lubricate the J-58s, and are located on each engine. This oil is also clear in color, and was specifically designed for the high temperatures encountered. This oil must be heated to seventy degrees Fahrenheit prior to each engine start. Below this temperature, it is as thick as molasses.

Polyphenyl ethers used in the SR 71 for lubrication under extreme conditions.

It was not only the 316° C. temperature that made the SR-71 turbine engine an extreme environment. The lubricant must not evaporate at the low atmospheric pressures of high altitude, and in this atmosphere the lubricant could oxidize to unstable molecules. Polyphenyl ethers have an extremely low vapor pressure, and hardly evaporate at all under standard temperature and pressure. At 260° C. , the vapor pressure of a 5-ring polyphenyl ether is only 0.0102 mm Hg. At 343° C.– a temperature well above the SR-71’s requirements - vapor pressure increases, but is still only 11.8 mm Hg.

The oxidative stability of polyphenyl ethers used in the SR 71 for lubrication is high because the resonance stability of the electron cloud of the benzene rings. This reduces the possibility that oxygen can attack the molecule. Mixing a very small quantity of an oxygen-grabbing additive to the polyphenyl ether made the lubricant even more stable at the high temperatures found in the SR-71. The effect of the additive is dramatic: in a standard oxidation-corrosion test at 650° C., the additive lowers the 100° F. viscosity increase from 18% to 22%. The result was a lubricant that permitted continuous operation at temperatures beyond those that could be tolerated by petroleum/ester-based lubricants.

This is an extreme example of the benefits of using synthetic lubricants in extreme applications. You might not have a SR71, but youe surely do have a car or piece of industrial or mining equipment that could benefit from using a high quality fully synthetic lubricant such as produced by Habot Oil. So even if you don’t need polyphenyl ethers used in the SR 71 for lubrication but would still like to find out how we can assist you save money, give the professionals at Habot oil a call.