Hydraulic fluids can be categorized as petroleum-based, water glycol or synthetic based (i.e., phosphate ester). All types provide specific properties that need to be considered to meet the needs of a given hydraulic application.
In the past, hydraulic fluids were petroleum based fluids that could potentially cause problems by leaking into the ground and contaminating the area and water supply. Today, the industry is moving toward more environmentally friendly fluids where advances have led to many new generations of “green” fluids.
Green fluids are typically synthetic- or vegetable-based. Green fluid’s base compound may be biodegradable and non-toxic, however, additives in the fluid may not be.
Synthetic fluids are primarily ester-based.
Vegetable oils are gaining popularity since they cost less than synthetic and are more biodegradable. They also have excellent lubricity and a high viscosity index. However, they have a limited temperature range with rapid oxidation at elevated temperatures.