Keeping environmentally safe hydraulic fluids in their place
By M. F. Wangsgaard
Vegetable oils eliminate some environmental concerns, but water contamination from them (or other fluids) attacks urethane seals. A new urethane material solves that problem.
The Environmental Protection Agency (EPA) continues to advocate the use of environmentally safe hydraulic fluids in place of conventional petroleum-based hydraulic oils &emdash; 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 exposed to the atmosphere over 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). The major benefits of these fluids: small spills are readily biodegradable which reduces the cost of clean-ups; and the fluid is unlikely to harm plant life, fish, animals, and humans who come in contact with it.
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.
From left, green proprietary P4301A90 seals and yellow conventional urethane seals as manufactured; after 1493 hours of exposure to 212° F water; and after 1493 hours of exposure to steam.
Three base oils
Three different base oils have been tried as environmentally safe hydraulic fluids. They are synthetic esters, polyglycols, and vegetable oils. Synthetic esters can be formulated as biodegradable fluids with superior lubrication performance, but their high cost has limited their usage. Polyglycols - attractive because they are 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 rapeseed oil, have excellent natural biodegradability, are in plentiful supply, and are inexpensive. They have become the most commonly used environmentally safe fluids in hydraulic systems.
Urethane and vegetable oils
The properties of urethane have made it a popular material for a broad range of hydraulic-sealing applications. However, one negative factor is its susceptibility to hydrolysis. As urethanes are produced, water is the byproduct of the chemical reaction. If water is re-introduced to urethanes later at a temperature high enough (generally 140° F) to cause a second chemical reaction, polymer bonds are broken and the urethane begins to deteriorate. The material hardens and then flakes apart. This phenomenon is known as hydrolysis. If a urethane seal is exposed to ambient water - and particularly hot water or steam - for extended periods, the seal may disintegrate completely.
Many vegetable oils have an inherent property of water absorption. If such oils are installed in hydraulic systems, their water component introduces a fluid mixture which jeopardizes seal performance. This phenomenon prohibits the use of conventional urethane seals with vegetable oils (as well as water-based or water-mixed fluids) in common hydraulic applications - which typically run at temperatures high enough to precipitate hydrolysis.
A new formulation
In response to this issue, we developed a proprietary high-grade urethane - identified as P4301A90 - which, due to its unique chemical composition, resists hydrolysis while maintaining excellent physical properties at operating temperatures as high as 212° F.
As seen from the test results listed in the table, P4301A90 retains as much as 74% of its original tensile strength in water and is virtually unaffected in pure rapeseed oil. Volume swell is held to a minimum, and there is a negligible change in hardness in both types of media. This indicates that P4301A90 seals may continue to operate very near to their original capability when exposed to both water and rapeseed oil.
We anticipate that the demand for use of environmentally safe hydraulic fluids in a variety of applications will grow as emissions standards become more stringent. In the past, these fluids could only be used successfully in applications where seal longevity was of little consequence. The development of P4301A90 provides a urethane with continuous sealability for more applications using environmentally safe hydraulic fluids - allowing them to perform as they were conceived, without risk of environmental contamination.
Mark Wangsgaard is an applications engineer, Parker Seals Packing Div., Salt Lake City, Utah.