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.
