Potential synthetic lubricants: Diol ester of C₁₈‐saturated cyclic acids

Abstract

Monoalkyl esters of linseed‐oil‐derived C18‐saturated cyclic acids (HCal), described in earlier work, showed promise as lubricant base stocks for turbine aircraft as set forth in the military specification MIL‐L‐7808E. These esters, however, did not exhibit the increased oxidative stability and higher viscosity required by the more recent specification MIL‐L‐23699. Six diol esters of HCal have now been prepared. Both hindered and unhindered dihydric alcohols were used, including ethylene glycol, 1,4‐cyclohexanedimethanol, 1,4‐benzenedimethanol, 2,3‐dimethyl‐1,3‐propanediol and 2,2,4,4‐tetramethyl‐1,3‐cyclobutanediol (I). The viscosities of these esters at 210F ranged from 10.1 to 19.6 centistokes and the pour points, from ‐35 to ‐62F. Oxidative stabilities at 400F were determined with 0.5% each of phenyl‐a‐napthylamine (PANA) andp,p'‐dioctyldiphenylamine as inhibitors. The esters of the unhindered diols had poor stability, whereas esters of the hindered diols, in particular I, exhibited excellent resistance to the formation of acidic decomposition products and sludge. Although the HCal ester of I by itself is too viscous to meet specification MIL‐L‐23699, its ASTM slope (0.650) is excellent. Blends of this material with less viscous hindered esters, commercially available, may find application as lubricants for high‐performance turbine engines used in various aircraft.