In some cases, environmental considerations necessitate the selection of a zinc-free ashless petroleum or a biodegradable hydraulic fluid.

The Environmental Protection Agency (EPA) continues to advocate the use of environmentally safe hydraulic fluids in place of conventional petroleum-based hydraulic oils - particularly in applications where fluid leakage could have a negative impact on the environment. Spills of standard, petroleum-based hydraulic fluids are known to kill marine life and contaminate soil. Environmentally safe hydraulic fluids are formulated to avoid those undesirable results.

To be classified as environmentally safe, a fluid must be readily biodegradable (more than 60% of the fluid must break down into innocuous products when tested in standardized laboratory tests a 28-day period) and virtually non-toxic (more than half the rainbow trout fingerlings in a population must survive after four days in an aquatic solution with concentrations of the fluid greater than 1000 ppm). A major benefit of these highly biodegradeable fluids is that spills may have lower cleanup costs, depending on local regulations. Also, they are less likely to harm plant and animal life that comes in contact with a spill.

Hydraulic applications that could be considered environmentally sensitive include mobile equipment in general, with emphasis on forestry and construction machinery, and marine equipment used on fishing boats, off-shore drilling operations, and hydraulically operated bridges, locks, and dams. Other locations are commercial elevators and equipment in amusement parks.

Three base oils

Three different base oils have been tried as environmentally safe hydraulic fluids. They are synthetic esters, polyglycols, and vegetable oils (which are sometimes called "natural esters"). Synthetic esters can be formulated as biodegradable fluids with superior lubrication performance, but their high cost has limited their usage. Polyglycols — attractive because they have excellent lubricity characteristics and are usually less expensive than synthetic esters — have been used more commonly. However, polyglycols lack required biodegradability and are potentially toxic in water when mixed with lubricating additives. Vegetable oils, such as sunflower, soy, or canola oils, have excellent natural biodegradability, are in plentiful supply, and are inexpensive. They have become the most commonly used environmentally safe fluids in hydraulic systems.

The base fluids of biodegradable hydraulic fluids are usually vegetable oils, selected synthetic esters, or a blend of the two. Biodegradable hydraulic fluids typically contain ashless inhibitors with low toxicity and additives to enhance performance. Properly formulated biodegradable hydraulic fluids can provide effective wear resistance similar to petroleum anti-wear hydraulic fluids. However, some biodegradable base oils, especially vegetable oils, may exhibit poor oxidation stability. The use of a synthetic-ester base usually improves the and oxidation resistance of the fluids.

The tradeoff between environmental advantages and potential performance deficiencies of biodegradable hydraulic fluids suggests that these fluids are most suitable for applications in environmentally sensitive areas. Their use should be considered wherever contamination of ground or water by petroleum lubricants could be a problem.

Additives

Like petroleum oils, vegetable oils or synthetic esters rely on specially selected additives to improve their performance as lubricants. The additives contained in biodegradable hydraulic fluids typically exhibit very low toxicity. Unlike petroleum oils, vegetable oils contain unsaturated hydrocarbons and are natural occurring esters. The unsaturation leads to rapid oxidation at elevated temperatures and poor low temperature flow properties. This low-temperature fluidity can be improved by additives, but their oxidation stability remains a performance concern.

International guidelines

Throughout Europe, the development of guidelines for biodegradable lubricants is typically left to local authorities or non-government organizations. In Germany, Blue Angel labels will be awarded to biodegradable hydraulic fluids. The Blue Angel for biodegradable hydraulic fluids will likely require that the base fluids must be readily biodegradable - greater than 80% biodegradation in 21 days by CEC L-33-A93 Test, or greater than 70% biodegradation in 28 days by the Modified Sturm Test. In addition, all components must be Water Hazard Class 0 or 1, which means the components are not water pollutants. Environmental Choice Program of Canada is currently in the process of reviewing a guideline on biodegradable, non-toxic hydraulic fluids. It will likely include a requirement that base fluids exhibit greater than 90% biodegradation in 21 days by CEC L-33-A93.

In the United States, ASTM D-2.N.3 on eco-evaluated hydraulic fluids has drafted an information guide that addresses the means of assessing the biodegradability of hydraulic fluids. D-2.N.3 is currently developing environmental classifications for hydraulic fluids. A In December 1995, ASTM D-2.12 on Environmental Standards for Lubricants completed a standard test especially designed to determine the aerobic aquatic biodegradability of all lubricants and their components. The test is similar to the Modified Sturm Test, which measures the evolution of carbon dioxide in 28 days. This standard is being published as ASTM D 5864. ASTM D-2.12 is currently developing other environmental standard tests for lubricants, which include an aquatic toxicity test for fish and large invertebrates; a manometric respirometry biodegradation test method; and a Gladhill Shake Flask biodegradation test.

Initially designed to measure the biodegradability of 2-cycle engine oils, CEC L-33-A93 has been the most widely applied biodegradation test for lubricants in Europe since the early 1980s. The test uses infrared spectroscopy to measure the disappearance of certain hydrocarbons over a 21-day period when the lubricant is mixed with an inoculum containing micro-organisms. Thus, the CEC test is a only a measure of primary biodegradation.

Unlike the CEC test, the Modified Sturm Test is a measure of ultimate biodegradation. By measuring the production of CO2 over 28 days, the test estimates the extent to which the carbon in a lubricant is converted by micro-organisms to the elements found in nature - namely: CO2, water, inorganic compounds, and biotic mass. Because this test was designed originally for water-soluble, pure compounds, it is difficult to use for testing lubricants, most of which are water-insoluble, complex mixtures.

The new ASTM D 5864 test is similar to the Modified Sturm Test. It is specially designed for testing water insoluble complex lubricants.