Hydraulic fluids, also called hydraulic liquids, are the medium by which power is transferred in hydraulic machinery. Most hydraulic fluids have a mineral oil base. Beyond this, other chemical compounds can be used according to the functions and properties required. Considerations need to be made for:
To obtain the right performance properties, hydraulic fluids can contain a wide range of chemical compounds, including: oils, butanol, esters (e.g. phthalates, like DEHP, and adipates, like bis(2-ethylhexyl) adipate), polyalkylene glycols (PAG), phosphate esters (e.g. tributylphosphate), silicones, alkylated aromatic hydrocarbons, polyalphaolefins (PAO) (e.g. polyisobutenes), corrosion inhibitors, etc.
Where biodegradability and renewable sources are considered important (e.g. farming, or marine operations) then natural oils such as rapeseed, or canola oil, are used as the base for the hydraulic fluids.
Typically these oils are available as ISO 32, ISO 46, and ISO 68 specification oils. ASTM standards ASTM-D-6006, Guide for Assessing Biodegradability of Hydraulic Fluids and ASTM-D-6046, Standard Classification of Hydraulic Fluids for Environmental Impact are relevant.