Government regulations aimed at protecting the environment are a fact of life that everyone who designs, builds, or uses hydraulic systems is either dealing with now, or soon will be. At least 700 million gallons of petroleum-based fluids enter the environment annually, more than half through illegal disposal channels and somewhere between 70 and 80% of all hydraulic fluids leave the system through leaks, line ruptures, and spills.

Environmental officials around the world have launched a broad range of green initiatives backed up by significant fines and clean-up cost allocations aimed at the whole user community. Fluid suppliers have responded by introducing new, eco-friendly biodegradable and non-toxic fluids based on a variety of different chemistries.

Unfortunately, none of these fluids are a direct one-for-one replacement for the petroleum-based fluids the hydraulics industry has grown up with over the last century, so previous experience is no guarantee of future success. The situation is complicated by economic pressures that are moving the industry toward higher operating pressures, temperatures, and working cycles — all of which are more efficient but also increase the stress on components and lubricants.

Defining biodegradable and non-toxic
Biodegradation is the process of chemical breakdown or transformation of a substance caused by organisms or enzymes. Two common measures of degradation are complete (technically, ultimate degradation), in which the only end products are carbon dioxide, water, and any indigestible inorganic elements. Incomplete degradation (technically, primary degradation) is measured by the reduction of hydrocarbons in the original solution. The relevant test standards are OECD 301 or ASTM D5864 for ultimate degradation and CEC-L-33-A-93 for primary degradation.

Two other terms in common use are readily and inherently biodegradable. A readily biodegradable fluid will undergo primary degradation greater than 80% within 21 days or ultimate degradation greater than 60% within 28 days. Inherently biodegradable fluids break down slowly over time, usually measured in years, and cause substantial damage to the environment.

Eco-toxicity is a measurement of the concentration required to kill various organisms over a short period of time — ranging from 24 to 96 hr. The toxicity of a fluid is described by a loading rate in parts per million (ppm) of fluid that has a 50% effect or causes 50% mortality of the organisms after the stated time.

The important thing to remember is that both biodegradability and non-toxicity are relative terms. Environmental friendliness absolutely does not mean the fluid is totally harmless to the environment and/or living things, only that it is less harmful than something else. Environmentally friendly or green fluids absolutely must be disposed of carefully and responsibly.

Finding a green alternative to petroleum
Four basic types of environmentally-friendly hydraulic fluids are in common use:
• HETG — hydraulic environmental triglyceride (water insoluble triglycerides),
• HEES — hydraulic environmental ester synthetic (water insoluble synthetic ester),
• HEPG — hydraulic environmental poly glycol (water soluble poly alkylene glycol [PAG]), and
• HEPR — hydraulic environmental polyalphaolefin and related fluids (water insoluble poly alpha olefins [PAO] and related hydrocarbon-based fluids).

Each of these chemistries is best applied to a specific range of application categories and operating conditions. They also have different interactions with seal materials and other system components. These interactions need to be taken into account when weighing considerations about whether or not they will be a suitable replacement for petroleum based fluids in any particular hydraulic system.