A
fluid composition suitable for use as an aircraft
hydraulic fluid is disclosed. The
fluid composition comprises a fire resistant
phosphate ester base stock comprising between about 10% and about 100% by weight of a trialkyl
phosphate, between about 0% and about 70% by weight of a dialkyl
aryl phosphate, and from about 0% to about 25% by weight of an
alkyl diaryl phosphate, with the proviso that the sum of the proportionate amount of each base stock component must equal 100%. The
alkyl substituents of the trialkyl phosphate, the dialkyl
aryl phosphate, and the
alkyl diaryl phosphate contain between 3 and 8 carbon atoms, preferably between 4 and 8 carbon atoms, more preferably between 4 and 5 carbon atoms, and are bonded to the phosphate
moiety via a primary carbon. It is still further preferred that the alkyl substituents of the trialkyl phosphate, the dialkyl
aryl phosphate, and the alkyl diaryl phosphate are isoalkyl groups. The
fluid composition further comprises an acid
scavenger, an anti-
erosion additive, a
viscosity index improver, and an
antioxidant. A novel additive combination comprises a high molecular weight butyl / hexyl
methacrylate viscos ity index improver, a perfluoroalkylsulfonate anti-
erosion additive, a 3,4-epoxycyclohexanecarboxylate or a diepoxide acid
scavenger, a di(alkylphenyl)amine, and a
phenolic antioxidant comprising a mixture of a 2,4,6-trialkylphenol and a hindered
polyphenol compound selected from the group consisting of bis(3,5-dialkyl-4-hydroxyaryl)
methane, 1,3,5-trialkyl-2,4,6-
tris(3,5-di-tert-butyl-4-hydroxyaryl)
benzene and mixtures thereof. Preferably, the fluid composition further comprises a
benzotriazole derivative as a
copper corrosion inhibitor, and a 4,5-dihydroimidazole derivative, as an iron
corrosion inhibitor and to enhance the stability of the fluid. <INS-S DATE="20010320" ID="INS-S-00001">The issued reissue patent is the reexamination
certificate. <INS-E ID="INS-S-00001">