Until now, environmentally acceptable lubricants have largely been used in sensitive industrial applications, Nigel Battersby of Shell Global Solutions in Chester, U.K., pointed out to last Septembers World Tribology Congress. These include hydraulic equipment used in ports and water-ways, chains saws for forestry, ski slope machinery, wire ropes used to control dams, and other areas where there is high risk of leaks and contamination. Hydraulic oils are the leading application for environmentally acceptable lubricants by far, with slightly over half the volume according to an E.U. Study.

Northern Europe led the way with biodegradable lubricants, prodding consumption via legislative means, such as rules covering operators working in conservation areas (Germany), biodegradable two-cycle marine outboard engine oils (Austria), and forestry (Sweden). Europe also has plied users with incentives, such as Dutch and German programs that offer tax breaks or compensate users for added costs of putting biodegradable and non-toxic oil in equipment. Even so, after nearly 20 years of availability, only about 4 percent of the lubricants market in Germany is now biodegradable, out of a total 1.1 million metric tons, says Fuchs Petrolub AG. Contrast that with a recent University of Amsterdam (IVAM) study that said as much as 90 percent of all lubricants could be replaced by biological alternatives.

Obviously theres a long way to go, but things are edging in that direction, says oleochemicals maker Cognis GmbH. It points to a study by international market analysts Frost & Sullivan, which pegged the value of the European bio-lubricants market at $670 million in 2006, nearly triple the $230 million seen in 1999. This is a year-on-year growth of 16.3 percent – versus the essentially flat growth seen for lubricants as a whole. Its even better than the 2.2 percent growth SRI Consulting pinned on the regions market for synthetic lubricants.

The biggest challenge for vegetable-based products, says Cogniss Markus Scherer in Dusseldorf, continues to be a perception of poor performance. Although they are environmentally friendly, vegetable oil esters such as triglycerides have had limited temperature range and poor oxidative stability. As long as mineral oil, polyalphaolefins and synthetic esters are stronger in these areas, the drive to natural products could remain stalled.

However, we do not believe that sustainability and performance are incompatible, Scherer told the 15th International Colloquium Tribology in January. Our company is committed to a sustainability focus, which means including more environmentally compatible and ecological products. It means using renewable raw materials meeting international standards. It means having products with longer life and improved disposal efficiency.

And it also means developing hydraulic fluids with superior oxidative stability, he acknowledged to the meeting, which was hosted by the Technische Akademie Esslingen. Of no less importance are antiwear performance, low-temperature viscosity, cleanliness and compatibility with seal materials. Hydrolytic stability is another hurdle, since so much hydraulic equipment operates under wet conditions.

Cognis set its sights on creating a natural ester based ISO 46 hydraulic fluid with good oxidative and hydrolytic stability, capable of performing for 2,000 hours. It needed to have good seal compatibility too, especially with NBR [nitrile buna rubber], which can be a problem, Scherer said, and good viscosity at low temperatures. This all pointed to using esters, but some ester types can have drawbacks such as seal compatibility, and poor hydrolytic stability especially compared to PAO. So we looked at our ester library, realized wed need to tweak their chemistry, and how they are made to improve them.

First ruled out were low-tier simple esters based on rapeseed (canola) oil. Although such fluids have good seal compatibility and lubricity, they are high in unsaturates which means they have double bonds, so there are always those places where the oxygen atoms can attack the molecule and lead it to decompose under high temperatures. By contrast, saturated fats have better oxidative stability, but poor NBR seal compatibility.

Cognis tested a range of ester-based hydraulic fluids, all using the same additive package. One fluid was based on a saturated branched ester (SBE), another on saturated linear ester (SLE), and third on unsaturated linear ester (ULE). A fourth formulation added some Group III mineral oil to the saturated linear ester, to nudge its seal compatibility.

Oxidative and hydrolytic stability tests pointed to the winners: the SBE and SLE formulations. The SLE-plus-Group III combination also performed well, but due to its mineral oil content was a less attractive candidate when biodegradability and toxicity needs were added, Scherer commented.

Both saturated ester fluids – SBE and SLE – offered good oxidative stability, with greater than 4,000 hours in the ASTM D943 thermal oxidative stability test. But the linear version fell by the wayside when measured for seal compatibility, which excluded such an ester as a top-performing hydraulic fluid.

In the end, the SBE fluid showed the strongest package of performance attributes overall. It had good oxidative stability and seal compatibility, plus showed good antiwear properties in FZG, Vickers vane pump and 4-ball studies, Scherer showed. In summary, the saturated branched ester allows for the design of a bio-hydraulic fluid that fulfills the highest performance criteria. It also eliminated concern about hydrolytic stability.

While conceding that cost is still high compared to its mineral oil based competitors, he added that the robust SBE fluid can enable end users to employ longer drain intervals while seing less varnish or sludge in valves, pipes and hydraulic fluid reservoirs. Disposal costs also are lower.

He also pointed to a key attribute that may sweeten the appeal of saturated branched esters: their high renewables content, which is over 90 percent. This would make them downright overqualified for the new Eco-label approved by the European Commission in May 2005 for environmentally acceptable lubricants. The approval covers hydraulic fluids, greases, chain saw oils, concrete release agents, two-stroke oils and other total-loss lubricants. At least 50 percent of such products by weight must be based on renewable raw materials, to wear the Eco-label, and other requirements for biodegradability and nontoxicity must be met. Makers of products meeting the Eco-labels requirements will be able to mark their packages with a flower logo – which may give considerable marketing pull with consumers.

SBEs high renewables content also would easily meet rules designed to encourage U.S. sales of biobased products, as required by the 2002 Farm Bill. The guidelines for these emerged recently from the Department of Agriculture, and the first group of items designated by USDA to be accorded preference in all federal purchases includes mobile equipment hydraulic fluids and penetrating lubricants. The final rule, published in the March 16 Federal Register, also set thresholds for the biobased content of these products. To qualify for the preferred procurement status, the minimum biobased content for mobile equipment hydraulic fluids must be 44 percent; clearly the 90 percent renewables content of the SBE products would surpass this.