The environmental effects of hydraulic leaks can be large. Leaked oil permeates down through the ground, eventually reaching the water table where it can spread beyond the boundaries of the offending plant. The growing interest in the environment has therefore led to the introduction of hydraulic fluids with minimal environmental impact. There are two considerations: toxicity, which is the harm that fluid can cause to animals and plants, and biodegradability, which describes the speed at which the fluid changes to innocuous products.
Three different base oils are used as environmentally safe hydraulic fluids. The commonest, and cheapest, is vegetable oil (such as rape seed oil). This has excellent natural biodegradability. In most environmentally sensitive applications vegetable oil is the main choice.
Other oil types are polyglycols and synthetic esters. These are more expensive than vegetable oil, markedly so for synthetic esters, and are not as environ- mentally friendly. They are used where the characteristics of vegetable oil, such as a limited temperature range and poor water tolerance, preclude its use. They can also be used as a blend with vegetable oil.
Whichever base oil is used additives will still be required to provide lubrica- tion, reduce foaming, etc. These additives must also have low toxicity and be biodegradable.