Polyphenyls radiation resistance

One hquid in this class intended for aircraft engine use is described in military specification MIL-L-87100 for operation from +15 to 300°C. Limitations of this class of synthetics are pour points of +5°C and higher, relatively poor lubricity, and high cost of 265/L ( 1000 + /gal) (44). Polyphenyl ether greases are available with good radiation resistance for appHcations in the temperature range of +5 to 288°C. 

Perfluoroalkyl ether greases thickened with polytetrafluoroethylene (MIL-G-38220 and MIL-G-27617) are used from —40 to 200°C in missiles, aircraft, and appHcations where fuel, oil, and Hquid oxygen resistance is needed (55). Polyphenyl ether greases find special use from 10 to 315°C in high vacuum diffusion pumps and for radiation resistance. 

Polypheny I Ethers. Both alkyl-substituted and iinsubxtiinlcd polypheny ethers are included in this class of synthetic lubricants. General preparation involves the (.llhiian ether synthesis. The unsubstituted polyphenyl ethers have outstanding thermal, oxidative and radiation resistance, however, poor low-temperature characteristics arc a major drawback. Alkyl substitution improves low-temperature viscosity, but detracts from stability. Most lubricant uses are developmental in nature and involve aircrali and aerospace applications. 

Others Phosphate esters Silicone oils Halogenated fluids Polyphenyl ethers Fire resistant hydraulic fluids, gas turbine oils High temperature hydraulic fluids, brake fluids, compressor oils Extremely fire-resistant hydraulic oils Radiation-resistant, heat transfer fluids... 

In some instances synthetic fluids are proposed for lubricating service on the basis of their resistance to degradation in a radioactive environment. These are likely to be unsubstituted or short-chain substituted polyphenyls or polyphenyl ethers. Their radiation stability outweights their disadvantages in other respects, such as high pour point and poor temperature coefficient of viscosity. 

The ordinates represent the viscosity increase relative to that of the material before irradiation (mostly at 100°F), so that they give a general indication of the extent of decomposition due to radiation exposure. This figure illustrates that aromatic hydrocarbons (n-butyl benzene) are more resistant to radiation damage than are aliphatic compounds (hexadecane). The most resistant of all are the polyphenyls, of which diphenyl is the simplest example. 

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