Hydraulic fluid composition with improved properties based on boric acid esters, glycol mono-ethers and bis-(glycolether) formals

Hydraulic fluid essentially consisting of

• (A) 20 to 40% by weight, relative to the weight of the total fluid, of a boric acid ester which is obtained when orthoboric acid, diethylene glycol and an ethylene glycol monoalkyl ether of the formula I
• R(OCH.sub.2 CH.sub.2).sub.x OH (I)
• in which R is an alkyl group with 1 to 4 C atoms and x is an integer from 2 to 4, are reacted in a molar ratio of 1:1:1 at a temperature of 50.degree. to 150.degree. C., the water of reaction formed being removed continuously;
• (B) 30 to 60% by weight, relative to the weight of the total fluid, of at least one ethylene glycol monoalkyl ether of the formula I in which R and x have the meaning given;
• (C) 10 to 40% by weight, relative to the weight of the total fluid, of at least one bis-(ethylene glycol monoalkyl ether)-formal of the formula II
• R.sup.1 (OCH.sub.2 CH.sub.2).sub.n.sbsb.1 O--CH.sub.2 --O(CH.sub.2 CH.sub.2 O).sub.n.sbsb.2 R.sup.2 (II)
• wherein R.sup.1 and R.sup.2 denote an alkyl group with 1 to 4 C atoms and n.sub.1 and n.sub.2 denote an integer from 1 to 4;
• (D) 0.1 to 5% by weight, relative to the weight of the total fluid, of at least one alkylamine of the formula III ##STR5## in which R.sup.3 denotes an alkyl or monounsaturated alkenyl group with 1 to 18 C atoms, R.sup.4 denotes hydrogen, --(CH.sub.2 CH.sub.2 O).sub.y H or ##STR6## in which y is an integer from 1 to 5, and R.sup.5 denotes hydrogen, --(CH.sub.2 CH.sub.2 O).sub.y H or ##STR7## in which y is an integer from 1 to 5, or an alkyl or monounsaturated alkenyl group with 1 to 18 C atoms, with the proviso that the sum of the C atoms in R.sup.3 and R.sup.5 in formula III is not greater than 18;
• and
• (E) 0.05 to 5% by weight, relative to the weight of the total fluid, of at least one ingredient selected from the group consisting of a pH stabilizer, a corrosion inhibitor, and an antioxidant.