Biobased Hydraulic Fluids: Selection, Use and Maintenance
By: Dr. David Sundin
Since their introduction in the early 1990s, biobased hydraulic fluids have become accepted by industry and recognized as having significant advantages in certain applications. The market for vegetable oil based hydraulic fluids has grown because of relative advantages that these fluids have, when compared with other fluid types. This article will discuss the history, characteristics, applications and advantages of vegetable-based hydraulic fluids, to aid the user in selecting and applying them.
Description and Characteristics:
Vegetable seed oils belong to a chemical family called “natural esters”. These are simply oils that are extracted from vegetable seeds. Nearly all types of seeds contain oil, however only some type are present in sufficient quantities or posses characteristics needed to be commercially viable lubricant oils.
Chemically, natural esters are called “triglycerides”. These are made up of a glycerol molecule plus three molecules of “fatty acids”. There are many naturally occurring fatty acids, which are simply chains of hydrogen and carbon atoms, attached by either single chemical bonds or double chemical bonds. Most naturally occurring fatty acids have chains with four to 28 carbon atoms.
The molecular structure of a triglyceride can be pictured as shown in Figure 1.
The types of fatty acids in the natural ester molecule will determine the ester’s physical and chemical characteristics, such as viscosity, pour point, and especially, its resistance to oxidation. If the fatty acid portion of the molecule has many double bonds between the different carbon atoms, then the resulting natural ester will have poor oxidation stability, as the oxidation reaction occurs at the double bond.
Oleic acid is a fatty acid that is particularly stable with respect to oxidation.
For this reason, natural esters that are high in oleic acid content are generally more stable against oxidation or polymerization than natural esters with lower concentration of oleic acid.
Oleic acid has only one double bond (Figure 2), which gives oleic acid higher resistance to oxidation, crosslinking or polymerization.
Fig. 2. Oleic acid (note that there is only one double bond, highlighted above)
As mentioned before, there are only a few vegetable seed oils that have the characteristics necessary to be commercially viable hydraulic fluids. Oils extracted from soy, sunflower and rape (canola) seeds have been the most widely used.
A short history of vegetable-based lubricant use:
Vegetable oils have been used for thousands of years as lubricants. The ancient Romans used olive oil as a lubricant. Most of the incentive for using vegetable oils in the last hundred years was as an alternative to expensive imported petroleum oils. In the late 1980s and early 1990s, there was renewed interest in the use of vegetable seed oils as lubricants, driven by users’ desire to use more environmentally friendly alternatives to petroleum oils. By this time, oxidation stability of vegetable oils had improved, through the use of modern breeding and oil refining techniques.
How are natural esters processed for use?
Natural esters go through a series of purification steps before they can be used as hydraulic fluids or as lubricants. These steps differ a little from manufacturer to manufacturer, but in general can be described as follows:
Step 1: Extraction: The seeds (soybean, sunflower, canola) are crushed and the oil is extracted from them either by mechanical presses or with solvents.
Step 2: Degum & Alkali Refining: The crude vegetable seed oils can be used as lubricants in this form, but they contain many impurities. Purified and refined forms of natural esters have longer life and better lubrication characteristics. The first purification step is to remove chemicals that would form gums or varnishes. This is done by contacting the oil with phosphoric acid, which combines with these undesirable constituents. Then excess acids in the oil are removed by contacting the degummed oil with sodium hydroxide (NaOH) and water.
Step 3: Bleaching: The oil is now contacted with activated attapulgite clay to remove ionic contaminants. This step improves the oxidation characteristics of the oil, and lightens its color.
Step 4: Deodorizing: The oil is heated to about 250 oC. and distilled under vacuum to remove any aldehyde or ketones, which occur naturally in the vegetable oils. This step improves the oxidation stability of the oil as well as its odor.
Step 5: Winterizing: This step of the oil’s refining process removes waxes in the oil. This step is done by chilling the oil (which solidifies the waxes present) and then passing the oil through a filter. This improves the low temperature flow characteristics.
Step 6: Additives: The last steps of the manufacturing process are similar to the process for making standard lubricant or hydraulic oils. The refined vegetable oil is combined with additives such as lubricity improvers, antioxidants and low temperature flow improvers.
Current Use of Vegetable-Based Hydraulic Oils:
Today, vegetable seed based hydraulic oils are being manufactured in North and South America, Europe, India and China, from a variety of manufacturers. Seed-based fluids have been successfully applied in a wide variety of hydraulic systems, both mobile and stationary. There does not seem to be an inherent limit to the pressures with which natural esters can be used, however their use is often discouraged at high temperatures.
Applicable Industry Standards
The physical and performance characteristics of vegetable-based hydraulic oils vary with the type of base oil chosen and the additives that are used. Vegetable-based hydraulic oils from several different bases have been manufactured that meet key industry standardized tests, such as the Dennison Vane Pump test, the Eaton-Vickers 35VQ Vane Pump test, the Sundstrand Piston Pump Test.
Advantages of Vegetable Seed Oils:
There are many advantages of using vegetable seed oils as hydraulic fluids, when compared to standard mineral oil:
- Higher Flash and fire point: The ASTM D92 Open Cup Flash point of vegetable oils is above 320 oC. This adds a significant margin of safety against fires in hydraulic systems, compared to comparable mineral oil based fluid, with a flash point of ~130 oC.
- Biodegradable and non-toxic: Mineral-based hydraulic fluid is about 25 – 35% biodegradable in standard 28-day tests. Vegetable based fluids are 90 – 98% biodegradable in these same tests. Vegetable oils have been determined to be non-toxic to fish. Depending on local environmental codes, this may allow hydraulic systems filled with vegetable-based fluids to be installed in environmentally sensitive areas where mineral oil would be perceived as a hazard. Also, spill cleanup regulations may give preference to natural ester fluids over petroleum-based fluids. (Again, local environmental codes should be consulted)
- Higher lubricity of vegetable-based fluids. Many natural ester base oils have very high natural lubricity, which protects hydraulic pumps from wear and premature failure. With proper additives, the lubricity of these fluids often surpasses that of mineral or even synthetic fluids.
Disadvantages of Vegetable-based Hydraulic Fluids:
There are several disadvantages to using vegetable-based fluids in hydraulic systems. Most of these, fortunately, can be minimized through design changes.
- Poorer oxidation resistance, compared to mineral oils. Natural esters react with oxygen in a different way than mineral oils do. Instead of forming acids and then sludge, like mineral oils, natural esters will polymerize, or form a gel when they are at the end stage of their oxidation reaction. There is considerable debate in the industry regarding the suitability of applying standard mineral oil oxidation tests to natural esters, and there are task forces working in industry standards groups to answer this question and to find the most appropriate oxidation test standards for natural ester fluids.
- Inferior low temperature behavior, compared to standard mineral oil. Vegetable oils have a pour point (the lowest temperature at which they will freely flow, under prescribed conditions) of about -21 oC. For comparison, mineral hydraulic fluids have a pour point of -30 to 45 oC., depending on the viscosity grade being tested. The higher pour point of natural esters will limit their application in hydraulic systems in low temperature environments
- Higher cost, when compared to mineral oil: Depending on the location and number of suppliers, natural esters cost 2.5 – 3.5 times the price of standard hydraulic oils.
The Use and Handling of Vegetable-Based Hydraulic Fluids:
By and large, vegetable-based hydraulic fluids are used in the same way as standard mineral oil fluids.
Filling Equipment: Vegetable-based hydraulic fluid is normally shipped from its manufacturer in 5-gallon pails, 55 gallon drums, or “tote” containers of 275 or 330 gallons. Vegetable-based fluids should be segregated from other fluids and its identity clearly marked. Although natural ester fluids are miscible and physically compatible with some other fluids, it is not advisable to mix natural esters with mineral oil or synthetic fluids. Dedicated handling equipment should be used.
Most manufacturers of vegetable-based hydraulic fluids recommend minimizing contact between the fluid and air. This means keeping fluid containers sealed and maintaining a nitrogen atmosphere in opened containers (drums, totes, and bulk storage tanks) as well as in hydraulic systems filled with the fluids. Consult the fluid manufacturer for specific handling instructions.
Post-installation Maintenance: Vegetable-based hydraulic fluids are maintained in much the same way as with mineral oils. The characteristics of used natural ester fluids are somewhat different than those of standard mineral oils because of the different oxidation reactions and ageing processes that they undergo. For example, acceptable acid values and moisture content for used natural ester fluids will be much higher than those that are considered normal for mineral oils. Fluid manufacturers are the best source of information on characteristics of used oils that are considered acceptable for continued use.
The Future of Natural Ester Use:
Natural esters are expected to grow in popularity for several reasons:
Lower Fluid Prices: As their use becomes more popular, the price of natural esters will fall.
Maturing Technology: Many hydraulic system owners or manufacturers have adopted a “wait and see” approach to natural ester fluids. As more guidance becomes available from industry standards groups, a greater user base will be developed and natural esters will be seen as a more “mainstream” choice of hydraulic fluids.
Greater Environmental and Fire Safety Awareness: In time, government regulating bodies will incorporate the fire and environmental safety advantages of vegetable seed fluids into fire and environmental codes. Industry experts predict that preference will be given in oil spill situations to fluids that are highly biodegradable, resulting in lower spill cleanup requirements and costs for users.
Although mineral insulating oils will continue to hold the majority share of the hydraulic fluid market for the foreseeable future, most experts believe that vegetable seed oil fluids will gain and hold an appreciable part of the market for themselves. The exact size of this share of the market is difficult to predict, and will depend on such things as the number of manufacturers of these fluids, continued availability of inexpensive mineral oil and adoption of industry standard guidelines and specifications. At this time, the future of natural ester hydraulic oils looks very good.
Vegetable-based, natural ester fluids offer several advantages to the hydraulic system owner, compared to other types of hydraulic fluids. Natural ester fluids have been successfully used in a wide variety of power transfer hardware, at both low and high pressures. Handling and maintenance procedures for natural esters are not significantly different from practices that are used for mineral hydraulic oils.
Natural esters are expected to grow in popularity and market share in coming years as environmental and fire safety regulations become stricter. Basic knowledge of the characteristics and application of natural esters will be a requirement of hydraulic power engineers and asset managers worldwide.Tags: biodegradable hydraulic fluid