While the use of PPEs in lubricating jet engines has somewhat subsided due to their higher cost, they are still used in some aerospace applications. PPEs are also used as base fluids for radiation-resistant greases used in nuclear power plant mechanisms. PPEs and their derivatives have also found use as vapor phase lubricants in gas turbines and custom bearings, and wherever extreme environmental conditions exist. Vapor phase lubrication is achieved by heating the liquid lubricant above its boiling point. The resultant vapors are then transported to the hot bearing surface. If the temperatures of the bearing surface are kept below the lubricant’s boiling point, the vapors re-condense to provide liquid lubrication. Polyphenyl ether technology can also provide superior fire safety and fatigue life, depending on the specific bearing design. In this application, PPEs have the advantage of providing lubrication both as a liquid at low temperatures and as a vapor at temperatures above . Due to the low volatility and excellent high-temperature thermo-oxidative stability, PPEs have also found use as a lubricant for chains used in and around kilns, metal fabrication plants, and glass molding and manufacturing equipment. In these high-temperature applications, PPEs do not form any sludge and hard deposits. The low soft-carbon residue that is left behind is removed easily by wiping. PPEs' low volatility, low flammability, and good thermodynamic properties make them ideally suited for use as heat transfer fluids and in heat sink applications as well. These polymers are made through oxidative coupling of substituted phenol in the presence of oxygen and copper and amine containing catalysts, such as cuprous bromide and pyridine. See Figure 2 for the PPO structure. PPO polymers can be classified as plastic resins. They and their composites with polystyrene, glass, and nylon are used as high-strength, moisture-resistant engineering plastics in a number of industries, including computer, telecommunication, and automotive parts. PPOs are marketed by SABIC Innovative Plastics under the trademarked name of Noryl.