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4.1 Description
The Quality of a Biobased Lubricant is an indication of the length of time that the lubricant's essential properties will continue to perform as expected.
The quality of a Bio Hydraulic Fluid is the length of time that the fluid's essential properties will continue to perform as expected, i.e., the fluid's resistance to change with time. The following discussion uses Bio Hydraulic Fluids to convey the characteristics of Quality.
The primary properties affecting quality are Oxidation Stability, Rust Prevention, Foam Resistance, Water Separation, and Anti-Wear properties. Many of these properties are achieved through use of chemical additives. However, these additives can enhance one property while adversely affecting another. The selection and compatibility of additives is very important to minimize adverse chemical reactions that may destroy essential properties.
4.2 Oxidation Stability
Oxidation, or the chemical union of oil and oxygen, is one of the primary causes for decreasing the stability of hydraulic fluids. Once the reactions begin, a catalytic effect takes place. The chemical reactions result in formation of acids that can increase the fluid viscosity and can cause corrosion.
Polymerization and condensation produce insoluble gum, sludge, and varnish that cause sluggish operation, increase wear, reduce clearances, and plug lines and valves.
The most significant contributors to oxidation include Temperature, Pressure, Contaminants, Water, Metal Surfaces, and Agitation.
(1) Temperature
The rate of chemical reactions, including oxidation, approximately doubles for every 10 °C (18 °F) increase in temperature. The reaction may start at a local area where the temperature is high. However, once started, the oxidation reaction has a catalytic effect that causes the rate of oxidation to increase.
(2) Pressure
As the pressure increases, the fluid viscosity also increases, causing an increase in friction and heat generation. As the operating temperature increases, the rate of oxidation increases. Furthermore, as the pressure increases, the amount of entrained air and associated oxygen also increases. This condition provides additional oxygen to accelerate the oxidation reaction.
(3) Contaminants
Contaminants that accelerate the rate of oxidation may be dirt, moisture, joint compounds, insoluble oxidation products, or paints. A 1 percent sludge concentration in a hydraulic fluid is sufficient to cause the fluid to oxidize in half the time it would take if no sludge were present. Therefore the contaminated fluid's useful life is reduced by 50 percent.
(4) Water and Metal
Certain metals, such as copper, are known to be catalysts for oxidation reactions, especially in the presence of water. Due to the production of acids during the initial stages of Oxidation, the Viscosity and Neutralization Numbers increase. The neutralization number for a fluid provides a measure of the amount of acid contained in a fluid.
The most commonly accepted oxidation test for hydraulic fluids is the ASTM Method D 943 Oxidation Test. This test measures the neutralization number of oil as it is heated in the presence of pure oxygen, a metal catalyst, and water. Once started the test continues until the neutralization number reaches a value of 2.0.
(5) Agitation
To reduce the potential for oxidation, oxidation inhibitors are added to the base hydraulic fluid. Two types of inhibitors are generally used: Chain Breakers and Metal Deactivators.
Chain Breaker inhibitors interrupt the oxidation reaction immediately after the reaction is initiated.
Metal Deactivators reduce the effects of metal catalysts.
4.3 Rust and Corrosion Prevention
Rust is a chemical reaction between water and ferrous metals.
Corrosion is a chemical reaction between chemicals (usually acids) and metals.
Water condensed from entrained air in a hydraulic system causes rust if the metal surfaces are not properly protected. In some cases water reacts with chemicals in a hydraulic fluid to produce acids that cause corrosion. The acids attack and remove particles from metal surfaces allowing the affected surfaces to leak, and in some cases to seize.
To prevent rust, Bio Hydraulic Fluids use Bio Corrosion Inhibitors (BCI) that deposit a protective film on metal surfaces. The film is virtually impervious to water and completely prevents rust once the film is established throughout the hydraulic system.
Rust Inhibitors are tested according to the ASTM D 665 Rusting Test. This test subjects a steel rod to a mixture of oil and salt water that has been heated to 60 °C (140 °F). If the rod shows no sign of rust after 24 hours the fluid is considered satisfactory with respect to rust inhibiting properties.
In addition to Rust Inhibitors, additives must be used to prevent corrosion. Bio Corrosion Inhibitors (BCI) additives exhibit excellent Hydrolytic Stability in the presence of water to prevent fluid breakdown and the acid formation that causes corrosion.
4.4 Air Containment and Foaming
Air enters a hydraulic system through the reservoir or through air leaks within the hydraulic system. Air entering through the reservoir contributes to surface foaming on the oil. Good reservoir design and use of foam inhibitors usually eliminate surface foaming.
Air Entrainment is a dispersion of very small air bubbles in a hydraulic fluid. Oil under low pressure absorbs approximately 10 percent air by volume. Under high pressure, the percentage is even greater. When the fluid is depressurized, the air produces foam as it is released from solution.
Foam and high air entrainment in a hydraulic fluid cause erratic operation of servos and contribute to pump cavitation. Oil oxidation is another problem caused by air entrainment. As a fluid is pressurized, the entrained air is compressed and increases in temperature. This increased air temperature can be high enough to scorch the surrounding oil and cause oxidation.
The amount of foaming in a fluid depends upon the viscosity of the fluid, the source of the Base Stock, the refinement process, and usage.
Foam Depressants are added to Bio Hydraulic Fluids to expedite foam breakup and release of dissolved air. However, it is important to note that foam depressants do not prevent foaming or inhibit air from dissolving in the fluid. In fact, some antifoamants, when used in high concentrations to break up foam, actually retard the release of dissolved air from the fluid.
4.5 Demulsibility or Water Separation
Water that enters a hydraulic system can emulsify and promote the collection of dust, grit, and dirt, and this can adversely affect the operation of valves, servos, and pumps, increase wear and corrosion, promote fluid oxidation, deplete additives, and plug filters.
United Bio Lube's Bio Hydraulic Fluids made from Stabilized High Oleic Base Stocks permit water to separate or demulsify readily. However, some additives such as Anti-Rust treatments actually promote emulsion formation to prevent separated water from settling and breaking through the Anti-Rust Film.
4.6 Anti-Wear Properties
Conventional hydraulic fluids are satisfactory for low-pressure and low-speed applications. However, hydraulic fluids for high-pressure (over 6900 kPa or 1000.5 lb/sq in) and high-speed (over 1200 rpm) applications that use vane or gear pumps must contain Anti-Wear additives. These applications do not permit the formation of full fluid film lubrication to protect contacting surfaces - a condition known as Boundary Lubrication.
Boundary lubrication occurs when the fluid viscosity is insufficient to prevent surface contact. Anti-Wear additives provide a protective film at the contact surfaces to minimize wear.
At best, use of a hydraulic fluid without the proper Anti-Wear additives will cause premature wear of the pumps and cause inadequate system pressure. Eventually the pumps will be destroyed.
Quality assurance of Anti-Wear properties is determined through standard laboratory testing. Laboratory tests to evaluate Anti-Wear properties of a hydraulic fluid are performed in accordance with ASTM D 2882. This test procedure is generally conducted with a variety of high-speed, high-pressure pump models manufactured by Vickers or Denison.
Throughout the tests, the pumps are operated for a specified period. At the end of each period the pumps are disassembled and specified components are weighed. The weight of each component is compared to its initial weight; the difference reflects the amount of wear experienced by the pumps for the operating period. The components are also inspected for visual signs of wear and stress.
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