Improvements in or relating to hydraulic fluids

Hydraulic fluids, which are substantially free from esters of ricinoleic acid, e.g. castor oil, comprise as the base fluid one or more polyoxyalkylene glycols or ethers thereof, and a minor proportion of one or more of the following additives: (a) a polyester derived from an aliphatic dicarboxylic acid of formula HOOC.(CH2)n.COOH where n is 2-10 and a polyalkylene glycol of formula HOR(OR)mOH where m is 1-10 and R is an ethylene, propylene or butylene radical, and (b) an ester derived from a dicarboxylic acid as defined in (a) and a C2-18 alkanol, cyclohexanol, or a C1-6 alkyl-substituted cyclohexanol. Typical esters are polyethylene glycol (200) sebacate, di-isooctyl azelate, and di(methyl cyclohexyl)adiptate. Optional additives mentioned include dibutylamine, diamylamine, morpholine, triethanolamine, and benzotriazole.