The increasingly important issues of environmental acceptance and biodegradability are the drivers behind the quest for alternatives to mineral oil as a base stock for hydraulic fluids. This search for replacements has led in several directions. Currently there are three commercially available biodegradable base stocks. They are polyglycols, polyol esters and naturally occurring triglycerides (known more widely as vegetable oils).
Polyglycols were the first base stocks to be formulated into biodegradable fluids. They offer excellent oxidation stability and are so non-toxic that they are widely used in the food-supply industry. But they are the highest priced biodegradable fluids. Synthetic polyol esters are the reaction product of a fatty acid — derived either from animal fat or vegetable oil —; and a synthetic polyol. Synthetic polyol esters represent the industry standard, especially for heavy-duty fluid applications. Naturally occurring triglycerides, or vegetable oils, while favored for their inherent biodegradability and low environmental impact, traditionally have exhibited low oxidative stability. This is a critical shortcoming that has limited their widespread acceptance and use. Improved base stocks and additive technology are now helping to overcome this deficiency.
A Third Performance Factor
Wherever fire is a possibility from a spark or heat source, a third performance factor enters the picture. The hydraulic fluid also should be fire-resistant. Note that the term fire-resistant means
* sprayed fluid that contacts an ignition source will not propagate flame, and
* if the source of ignition is removed from a fluid spray, any flames will self extinguish.
Where fire resistance is a primary concern, synthetic polyol esters have performed well in extreme operations for over 25 years. As a result, polyol esters have emerged as the fluid of choice, despite costs ranging up to six times more than mineral-based oils. An industry goal has been to develop a vegetable-oil-based (or naturally occurring-ester) fire-resistant fluid that can equal the performance properties found in synthetic esters while reducing fluid costs in the process. This goal can be achieved. By optimizing additive packages and using selected base stocks, naturally occurring ester fluids can have oxidation stability and lubricity performance on a par with synthetic polyol ester fluids.
Established Fire-Resistant Fluids
Over the past four decades, two general types of products have competed in the fire-resistant fluids marketplace: water-based and anhydrous fluids.
Water glycol fluids offer maximum fire resistance and low price, but have only moderate lubricity. These 4-component fluids consist of glycol, polyglycol polymer, additives, and 35 to 50% water. Because of its inherently low film strength, the water required for fire resistance reduces lubricating film strength. Water glycols exhibit a viscosity index above 200 and have excellent low-temperature properties.
Anhydrous or non-water containing products, on the other hand, provide lubrication much like mineral oil. Synthetic polyol esters (formed by the reaction of a fatty acid with an alcohol, diol, or other polyol) have been regarded as one of the best performing of the biodegradable and fire-resistant hydraulic fluids. Their properties have been well-documented.
For synthetic polyol esters, biodegradability can be as high as 90%. Lubrication is excellent, as is liquidity and aging stability at low and high temperatures. Polyol esters feature a viscosity index above 150; a flash point between 530° and 550° F; and a fire point between 600° and 615° F. As an anhydrous fire-resistant fluid, synthetic polyol esters have proven ideal for applications such as high-performance mill equipment operating at pressures above 3000 psi (as high as 5000 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 over 90% biodegradable. Their low total-acid numbers contribute to their good chemical and acceptable thermal stability. Corrosion protection is excellent. While these oils are good lubricants, their tendency toward rapid oxidation at elevated temperatures raises concerns about their performance.
The oxidation instabilities are due to a high content of linoleic and linolenic fatty acids. Research shows that the oleic-acid ester distribution in a naturally occurring ester-base stock plays a major role in fluid performance. These acids are characterized by two and three double bonds, respectively. More double bonds makes a material more prone to rapid oxidation. The high degree of unsaturation, in fact, also leads to increases in viscosity and total acid number, and fluid aggressiveness toward copper and copper alloys. However, by combining specially formulated additive packages with selected vegetable-oil base stocks, these known deficiencies can be overcome.
Additive technology is used to provide fire resistance consistent with Factory Mutual guidelines. Tests have now demonstrated that additive technology can further be applied to enable vegetable oils to perform like polyol esters. This is because, with the exception of the ester base stock, the formulations for finished synthetic polyol ester and naturally occurring triglyceride fluids are identical. Both contain viscosity index modifiers, rust and oxidation inhibitors, extreme-pressure additives, copper passivators, and defoamers.
To determine whether additive packages could be used to fortify vegetable oils to greatly improve their oxidation stability and thus equal the desired performance characteristics of synthetic polyol esters, we conducted several laboratory comparison tests.
The process began with canola oil as the base stock. This was then formulated with additives for benchmark comparison against a commercial, premium synthetic polyol-ester fluid. When testing was completed, the canola oil with optimized additive package was found to provide overall performance comparable to the synthetic fluid, including these areas:
Lubrication - When properly formulated, both fluids passed FZG stage 12; Shell 4-Ball results (1,800 rpm, 40 kg, 1 hr, RT) were 0.30 mm.
Pump testing - The Vickers 35VQ Vane Pump Test is a severe, mobile-equipment wear test using a Vickers Model 35VQ25 vane pump. The standard test procedure requires that the same charge of fluid be tested with satisfactory results in three successive 50-hour test runs. (Total ring and vane wear must be below 90 mg in each.) A commercial vegetable oil-based fluid passed the Vickers 35VQ pump test. Results from ASTM's D2882 pump-wear test also were exceptional. Ring and vane loss was less than 12 mg for both the synthetic polyol ester and properly formulated canola oil. These results equate with high-performance hydraulic oil.
Oxidation stability - The Rotating Bomb Oxidation Test was applied with these results: bomb life ranged from 55 to 180 minutes for several synthetic polyol esters that were evaluated. The result for canola oil with an optimized additive package in a oleic-acid ester base stock was 165 minutes.
The conclusion from these (and other tests) is that a fluid based on naturally occurring triglycerides can be formulated to perform as an equal with synthetic polyol ester fluids in terms of lubricity, oxidation stability, and fire resistance.
New Product Developed
Based on the test results and proven optimized formulation, Houghton International has developed Cosmolubric B-230, a vegetable oil-derived fire-resistant hydraulic fluid with additives to enhance corrosion protection, metal passivation, and oxidative inhibition. This product is approved by Factory Mutual as a less-hazardous fluid. Among its physical properties are a viscosity index of 214, ASTM flash point of 495° F, and ASTM fire point of 610° F. This fluid passed ASTM corrosion test D-665A.
The combination of a growing desire in the fluid market for biodegradability as well as renewable, less-costly alternatives suggests that this canola oil-based product can serve as a practical replacement for synthetic polyol esters, especially in applications where fire resistance is a paramount concern.
Rich Adams is manager, fluid power research and development; Joseph P. Kromdyk, is general manager, fluid power products; and Tony Noblit is technical specialist; all at Houghton International Inc., Valley Forge, Pa.