thermal and oxidative stability and stability in the presence of ionizing radiation. PPEs have the disadvantage of having somewhat high pour points. For example, PPEs that contain two and three benzene rings are actually solids at room temperatures. The melting points of the ordinarily solid PPEs are lowered if they contain more m-phenylene rings, alkyl groups, or are mixtures of isomers. PPEs that contain only o- and p-substituted rings have the highest melting points. Thermo-oxidative stability PPEs have excellent high temperature properties and good oxidation stability. With respect to volatilities, p-derivatives have the lowest volatilities, and the o-derivatives have the highest volatilities. The opposite is true for flash points and fire points. Spontaneous ignition temperatures of polyphenyl ethers lie between , alkyl substitution reduces this value by ~. PPEs are compatible with most metals and elastomers that are commonly used in high-temperature applications. They typically swell common seal materials. Oxidation stability of un-substituted PPEs is quite good, partly because they lack easily oxidizable carbon-hydrogen bonds. Thermal decomposition temperature, as measured by the isoteniscope procedure, is between . Radiation stability Ionizing radiation affects all organic compounds, causing a change in their properties because radiation disrupts covalent bonds that are most prevalent in organic compounds. One result of ionization is that the organic molecules disproportionate to form smaller hydrocarbon molecules as well as larger hydrocarbons molecules. This is reflected by increased evaporation loss, lowering of the flash and fire points, and increased viscosity. Other chemical reactions caused by radiation include oxidation and isomerization. The former leads to increased acidity, corrosivity, and coke formation; the latter causes a change in viscosity and volatility. PPEs have extremely high radiation resistance. Of all classes of synthetic lubricants (with the possible exception of perfluoropolyethers) the polyphenyl ethers are the most radiation resistant. Excellent radiation stability of PPEs can be ascribed to the limited number of ionizable carbon-carbon and carbon-hydrogen bonds. In one study, the performance of PPE under the influence of 1 ergs/gram of radiation at was compared with synthetic ester, synthetic hydrocarbon, and silicone fluids. PPE showed a viscosity increase of only 35%, while all other fluids showed a viscosity increase of 1700% and gelled. Further tests have shown PPEs to be resistant to gamma and associated neutron radiation dosages of 1 erg/g at temperatures up to . Surface tension PPEs have high surface tension; hence these fluids have a lower tendency to wet metal