Given a choice, many plants would prefer an "environmentally friendly" or "green" hydraulic fluid, especially if it also provides fire resistance. Fire and oil spills are always a potential problem, and plants specify high-priced hydraulic hoses that meet fireproof or fire-resistant regulations, yet continue to pump flammable, toxic, petroleum-based hydraulic fluids through them.
This doesn't make sense when it's possible to use hydraulic fluids that are fire resistant and environmentally friendly, and that perform as well as or surpass conventional petroleum-based hydraulic fluids and out-perform other "food-grade," white mineral oils that claim to be environmentally friendly.
What does biodegradable mean?
Products claiming to have biodegradability are classified as either readily biodegradable or inherently biodegradable. The hydraulic fluid of choice is readily biodegradable. This is defined as degrading by 80 percent within 21 days as measured by a decrease
in a test sample. Most readily biodegradable hydraulic fluids are derived from naturally occurring triglycerides (vegetable oils).
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The least desirable fluids are inherently biodegradable. These are food-grade lubricants and white mineral oils. Most claim "no sheen," but take longer to degrade in the environment and are toxic to marine life over long periods.
Fire-resistant versus flammable
Hydraulic fluids need to be fire resistant. When in contact with an ignition source, a fire-resistant fluid won't propagate a flame and flames self-extinguish upon removal of the ignition source. Fully compounded vegetable oil-based hydraulic fluids are fire resistant.
Established fire-resistant fluids
During the past four decades, two products have competed in the fire-resistant fluid marketplace: water-based and anhydrous fluids.
Water glycol fluids offer maximum fire resistance at a low price, but they have moderate lubricity only. These four-component fluids contain glycol, polyglycol polymer, additives and from 35 percent to 50 percent water. The problem is the water. Because of its inherently low film strength, the quantity of water required for fire resistance reduces lubricity. Water glycols exhibit a viscosity index greater than 100. This means they tend to exhibit a uniform viscosity across a large temperature range, a behavior that's particularly important at low temperatures.
On the other hand, anhydrous products contain no water. They lubricate much like mineral oil. Synthetic polyol esters (formed by the reaction of a fatty acid with an alcohol, diol or other polyol) are recognized as one of the best-performing biodegradable and fire-resistant hydraulic fluids. Their properties are well documented.
With synthetic polyol esters, lubrication is excellent, as is liquidity and aging stability at low and high temperatures. Polyol esters feature a flash point near 540 F, a viscosity index above 150 and a fire point just above 600 F. As anhydrous fire-resistant fluids, they have proven ideal for applications such as high-performance marine equipment, operating as high as 5,000 psi.
Vegetable oils and additives
Vegetable oils or naturally occurring triglycerides are in the same chemical family as polyol esters. Fully compounded vegetable oil fluids are more than 90 percent biodegradable. Their low total-acid numbers contribute to their good chemical stability and acceptable thermal stability. Their corrosion protection is excellent. While they are good lubricants, their tendency toward rapid oxidation at elevated temperatures has raised concerns about their performance.
A high concentration of linoleic and linolenic fatty acids causes oxidation instability. These acids are characterized by two and three double bonds, respectively. Research shows that the oleic-acid ester distribution in a naturally occurring ester-base stock plays a major role in fluid performance. More double bonds make a material more prone to rapid oxidation. In addition, a high degree of unsaturation leads to increases in viscosity and total acid number, as well as aggressiveness toward copper and copper alloys. However, combining specially formulated additive packages with selected vegetable oil base stocks overcomes these deficiencies.