Though most global soybean production is funneled into the food industry, soybean oil provides good lubricity, high viscosity index and other desirable characteristics for lubricating oils, properties that its proponents say make it an attractive option as a base fluid.
As demand for environmentally acceptable lubricants has grown, so have market openings for soybean oil. Soybean oil can be found in applications such as metalworking fluids, motor oil and hydraulic fluids, where marketers claim it provides a more sustainable material than its petroleum-based counterparts. It has even lubricated the elevator in the Statue of Liberty for nearly two decades.
But sustainability isnt the only consideration for a company deciding whether to switch from traditional base stocks. Cost and performance matter most, and thats where the biggest obstacles to larger adoption of soybean oil lie.
As a vegetable oil, soybean oil is considered biobased. It is composed of triglycerides, or long, polar fatty acid chains that provide high-strength lubricant films. These triglycerides have strong intermolecular interactions, giving them more stable viscosity when temperature varies. On the downside, triglyceride esters are susceptible to hydrolysis-breakdown in the presence of water-and have poor low-temperature properties, Robert Brentin, commercialization manager at Midland, Michigan-based Omni Tech International, summarized at the Society of Tribologists and Lubrication Engineers annual meeting in May.
However, variations on soybean and other vegetable oils are helping to overcome some of their traditional weaknesses.
Approximately 169 million metric tons of soy is grown in the Northern Hemisphere and 191 million tons in the Southern, according to the United States Department of Agriculture, providing a year-round supply. A majority of that production comes from the United States and Brazil.
Soybean oil products are especially useful in applications with proximity to water: locks and dams, construction equipment used near waterways and brown water marine environments such as brackish rivers and estuaries, or wherever an environmentally acceptable lubricant is needed. Environmentally acceptable lubricants can lessen environmental impact and negative public relations consequences in the event of a spill.
Its biodegradable, and thats creating market openings, Brentin said of soybean oil. Particularly in marine and other environmentally sensitive applications, regulations mandating use of biodegradable materials are creating opportunities for soybean oils, he observed.
Legislation like the U.S. Environmental Protection Agencys Vessel General Permit, which requires environmentally acceptable lubricants be used in all oil-to-sea interfaces, and the Clean Water Act, which determines whether an oil spill may be harmful to public health, make biobased lubricants an attractive option for end-users.
To meet the EPAs definition of biodegradable, at least 60 percent of a product must break down into carbon dioxide and water within 28 days in a laboratory environment. Soybean oil has a biodegradability of 72 to 80 percent by the EPAs test methods, compared to 15 to 35 percent for most mineral oils. EALs must also be minimally toxic and non-bioaccumulative.
Stacking Up to the Competition
Soybean oil performs well with respect to flammability and toxicity, and has good friction and wear characteristics.
Developments in additives can help soybean oil keep up with its more traditional counterparts, but the biggest advances have come from increasing the amount of oleic acid, a fatty acid found in animal and vegetable fats and oils. Its not just the lubricant industry facilitating that advancement; the food industry, in search of oils that are more heart healthy, is spearheading innovation that collaterally benefits lube users.
For example, when the U.S. Food and Drug Administration banned partially hydrogenated oils for food use in June, threatening soybean oils used in frying and baking, the United Soybean Board, a group of soy farmers that researches and promotes soy products, had already invested in studies to produce high-oleic soybean oils that do not require partial hydrogenation.
Higher oleic content can be achieved through modifying the seed stalk or by new ways of processing soy. This can give the oil better lubricating properties, such as oxidative stability.
Doug Adams, senior product development chemist with RSC Bio Solutions, an EAL manufacturer based in Indian Trail, North Carolina, conducted a USB-sponsored study comparing the performance of high- and low-oleic vegetable oils and mineral oil. The vegetable-based fluids included a high-oleic canola oil, a low-oleic soybean oil and two different high-oleic soybean oils with oleic content enhanced to 65 percent. All five fluids were formulated into lubricants with the same additive package, viscosity improver, pour-point depressant and antifoam additive.
As researchers expected, the soybean oils had high viscosity indices. In hydraulic fluid blends, the two formulations with high-oleic soybean oils had viscosity indices of 218 and 213, while the fluid made with low-oleic soybean oil had a V.I. of 237. The mineral oil hydraulic fluid had a V.I. of 140. Viscosity index is a measure of a fluids resistance to change in viscosity with temperature, and is particularly important for hydraulic equipment running outdoors in extreme weather.
Each oil passed an air entrainment test, ASTM D892. While all also passed a water separation test, ASTM D1401, the mineral oil blend separated to a passing level in just 10 minutes, compared to 20 minutes for the low-oleic soybean blend and 20 and 25 minutes for the two high-oleic soybean oils.
In a pin and vee test, ASTM D3233B, where higher jaw load and torque indicate better extreme pressure properties, the mineral oil was only able to withstand 1,015 pounds of load and 41 pound-inches of torque. One high-oleic soybean oil withstood 1,623 lbs. of load and 41 lbs.-in. of torque, and the other high-oleic soybean oil handled 1,531 lbs. and 68 lbs.-in. The low-oleic soybean blend withstood 1,450 lbs. and 68 lbs.-in.
Pour point, which Adams pointed out can be a problem with vegetable based oils, had mixed results for the various blends. In the ASTM D97 test method, the mineral oil and one of the high-oleic soybean oils had the worst results at minus 21 degrees Celsius. The other high-oleic soybean oil had a pour point of minus 27 C, and the low-oleic soybean blend came in at minus 26 C.
Tests for protection from steel corrosion (ASTM D665A) and copper corrosion (ASTM D130) showed good results across the board.
The most notable differences between the high-oleic blends and the low-oleic blend were oxidation stability and four-ball wear test results. Oxidation stability was measured using the rotating pressure vessel oxidation test (ASTM D2272), in which a sample is oxidized in a pressure vessel and rotated until a specified drop in pressure occurs. The longer a sample lasts, the better. The two high-oleic soybean oil samples lasted 194 minutes and 153 minutes, while the low-oleic sample maxed out at 45 minutes. The mineral oil blend lasted 600 minutes.
What was really enlightening was how higher oleic content allowed for the oxidation performance to improve dramatically, Adams told LubesnGreases. Theres a chance we can keep it in equipment longer than a season, which is what most of the current product is recommended for.
Tyler Housel, head of the Milledgeville, Georgia-based Lexolube division of Zschimmer and Schwarz, said that in lubricants, better oxidation stability is the most important difference between high-oleic soybean oil and low-oleic soybean oil. When a base oil oxidizes, it thickens, making it harder to pump and thus harder to reach the point of application. The fact that high-oleic soybean oil has three times the oxidative stability of low-oleic soybean oil is a significant improvement.
In four-ball wear testing, where less wear scar indicates better antiwear properties, the two high-oleic soybean oils had scars of 0.38 millimeters and 0.48 mm, results Adams described as very good and acceptable, respectively. The low-oleic soybean oil had 0.50 mm of scarring, and the mineral oil formulation had the largest wear scar of 0.55 mm.
A lubricant with a higher oleic content has better film strength, which typically leads to better four-ball wear results, so those are a real plus, said Adams. That would typically mean you could back off on the antiwear additives, or [less friction] means you have a lower operating temperature in the application, leading to a longer-life product.
Adams noted his study was introductory, and no field tests have yet been conducted. But the results are promising, and the base fluids used in Adams study were already a year old by the time they were used. Presumably, soy crops grown in the past year have continued to progress.
The Cost of Business
One hurdle for soybean oils is price. Refined conventional soybean oils are around 38 cents per pound, and the better performing high-oleic soybean oil will run about 10 cents higher. Motiva Enterprises API Group II base oil, marketed as Star 6 with viscosity of 220 to 230, costs 38 cents per pound as of early October.
Both Adams and Brentin are hopeful that cost can be reduced over time. The price volatility for soybean oil is less than petroleum … Soybean oil has fluctuation but not as much, Brentin said. There is a premium [on high-oleic soybean oil] relative to conventional soybean oil, but as volumes, competition and efficiencies of scale increase over time, this will decrease.
Housel said cost is a factor, but its not the only one. I dont see it as a choice of soybean oil versus mineral oil. You cant take a formula with mineral oil and just substitute soybean oil. You have to make some modifications, he said. A company with an established mineral oil formula likely wont make the change to a soybean oil-based formula without a specific initiative, and the cost of creating a new formula must be factored in as well.
Soybean oil is the most commonly used vegetable-based oil in North America because of ready local supply. According to Qualisoy, an independent soybean distributor, supply of high-oleic soybean oil is projected to grow from less than 500 million pounds in 2018 to almost 9 billion pounds in 2028. By 2025, the high-oleic oil producing plants will be the fourth-largest grain and oilseed crop in the U.S. due to growing acreage in the region.
The North American soybean based lubricants market was estimated at $191.5 million in 2016, according to Grand View Research. While global lubricant growth sat at 0.8 percent last year, global biobased lubricant growth reached 5.3 percent, according to CEO of Biosynthetic Technologies Mark Miller.
Uptake of soybean oil for lubricants continues to be slow. Government regulation helps the biobased lubricants industry; for example, the Federal Acquisition Regulation mandates that federal agencies purchasebiobased productsin 139 categories, including lubricants.
However, the general lubricants industry is resistant to change; it remembers past failures of applications using biobased oils. And no one has ever been fired for maintaining the status quo, said Miller. Theres a lot of pushback in the industry; its tough to sell the concept, he told an audience at the ICIS North American Industrial Lubricants Congress in Chicago in September.
Cost is the driver. It doesnt matter if you have the best product, you still have to sell it, said Adams.