Upgrading biodegradable lubricant performance

Dr. Neil Canter, Contributing Editor | TLT Tech Beat June 2009

Benefits of PAGs combined with the natural lubricity of biobased oils.

KEY CONCEPTS
• Current biodegradable lubricant basestocks display inferior performance compared to synthetic lubricants from the standpoint of oxidation stability and low temperature characteristics.
• A blend of polyalkylene glycol and high oleic canola oil raises the level of performance in minimizing deposits, sludge and varnish as determined in lab testing while maintaining biodegradability and performance.
• Initial applications for the PAG, high oleic canola oil blend include chain oils, greases, stationary hydraulic fluids and metalworking fluids.

Interest in the use of biodegradable lubricants continues as more emphasis is placed on using fluids that are derived from sustainable raw materials. One area of concern with current biodegradable lubricants is inferior performance compared to synthetic lubricants, particularly from the standpoint of oxidation stability and low-temperature characteristics.

Readily available feedstocks such as soybean oil often do not provide sufficient oxidation stability because of very high levels of polyunsaturated fatty acid chains such as linoleic in the triglyceride structure. These fatty acids are particularly vulnerable to oxidation and, as a consequence, the generation of insoluble varnishes and residues can occur.

Regulatory drivers both in the U.S. and in the EU are attempting to promote the use of biodegradable lubricants in the marketplace. The Farm Bill of 2002 enabled the U.S. Department of Agriculture to designate minimum biobased contents for lubricants and requires federal agencies to purchase these products if they display comparable cost performance to traditional petroleum-based fluids. As an example, mobile and stationary hydraulic fluids qualify if they both have minimum biobased contents of 44%.

In May 2005 the European Union published the criteria for an eco-label for lubricants. This regulation also designated minimum biobased contents for specific lubricant fluids. For hydraulic fluids, the minimum carbon content derived from renewable sources needs to be 50%.

Development of new technology to improve biodegradable lubricant performance is needed to expand the presence of these fluids in the marketplace, particularly in environmentally sensitive applications such as hydraulic fluids. Such a technology has now become available.

PAG/HIGH OLEIC CANOLA OIL BLEND 
Dow Chemical has just developed a product platform called Dow SYMBIO™ that is prepared from a blend of polyalkylene glycol (PAG) and high oleic canola oil. STLE-member Jennifer Butcher, new business development leader for The Dow Chemical Co., says, “This technology is our first new chemistry based on renewable raw materials. It raises the level of performance of biodegradable lubricants beyond that of currently available fluids.” 

STLE-member Dr. Govind Khemchandani, senior technical specialist for Dow Chemical, adds, “Our product is the first ever combination of PAG with high oleic canola oil. We have a lot of experience with PAGs in lubricant applications and found they generate no deposits, sludge or varnish. This led us to blend them with high oleic canola oil to produce this biodegradable base. We believe this product represents the first time that the benefits of PAGs have been combined with the natural lubricity found in vegetable oil esters.” 

The PAG/high oleic canola oil blend has a biodegradability greater than 60% and a renewable content greater than 50%, according to Khemchandani. Biodegradability testing was conducted using the modified Sturm Test (OECD Method 301B) which determines the quantity of carbon dioxide generated in 28 days.

This blend has a viscosity index of 202, flash point (COC) of 295 C and a pour point of -26 C. A formulation prepared with this blend has been evaluated for oxidation and sludge generation using a modified version of ASTM D2893. 

Khemchandani explains, “The test procedure involves blowing air through a sample at 95 C for 13 days. To pass our test, a fluid must not display more than a 50% increase in viscosity. The formulation containing our PAG/high oleic canola oil blend exhibited strong performance and did not display a 50% increase in viscosity until after 25 days.” In contrast, commercially available products based on vegetable oils such as canola oil, high oleic canola oil and rapeseed oil all failed in less than 13 days.

Sludge formation was determined by heating fluids for three weeks at 120 C following the guidelines described in ASTM D2893. Figure 1 shows the contrast between a blend of a synthetic ester & canola oil on the left and the blend of PAG with high oleic canola oil on the right. Fluid darkening and sludge is readily seen in the former while the latter remains translucent and contains no sludge.


Figure 1. In tests using a modified version of ASTM D2893, the PAG/high oleic canola oil blend remains translucent and displays no sludge as seen in the figure on the right. In contrast, a blend of a synthetic ester and canola oil evaluated in the same manner exhibits fluid darkening and sludge as seen on the left. (Courtesy of The Dow Chemical Co.)

The PAG/high oleic canola oil blend was used as the basestock for an ISO 46 antiwear hydraulic fluid. Wear testing was conducted using a Vane Pump Test and following ASTM D7043. Testing was conducted at a pressure of 2000 psi for 100 hours at 65 C. 

The PAG/high oleic canola oil-based hydraulic fluid displayed a total weight loss of cam and vanes of 1.9 mg (milligrams) and a viscosity change of 2.1%. This data are well below the recognized industry values of <5 mg wear and < 5% viscosity change that are both considered to be excellent results. Several other biodegradable hydraulic fluids formulated with other basestocks displayed either comparable or inferior results and many of them showed a severe viscosity change (>10%) after the test period.

Khemchandani says, “The synergism we found between the PAG/high oleic canola oil blend and the antiwear additive used enabled us to generate this excellent result. We are confident that this fluid should outperform mineral oil-based hydraulic fluids in this test.”

Switching to a PAG/high oleic canola oil-based lubricant will not be a problem for end-users. Khemchandani says, “Our product is compatible with mineral oil-based fluids and biodegradable lubricants derived from vegetable oils and esters. Preferred seal materials for use with the PAG/high oleic canola oil blend are the common elastomers hydrogenated nitrile butadiene rubber (HNBR) and Viton.” 

The hydraulic fluid based on the PAG/high oleic canola oil blend is being targeted for stationary equipment use in environmentally sensitive applications. Khemchandani adds, “We believe that the hydraulic fluid can be used in such applications as stationary marine equipment, locks and dams and hydraulics in the entertainment arena (amusement park rides and large fountains). The PAG/high oleic canola oil blend also can be used as a base to formulate other biodegradable lubricants, including chain oils, greases and metalworking fluids.” 

Trials are underway to evaluate the PAG/high oleic canola oil blend in hydraulic fluid and metalworking fluid applications. Khemchandani says, “We are evaluating the hydraulic fluid in a conversion forklift at one of our facilities. We also have evaluated a metalworking fluid based on the PAG/high oleic canola oil blend in a CNC lathe application. Both trials are running smoothly.” 

Khemchandani indicates that the PAG/high oleic canola oil blend will be registered as a biopreferred product under the USDA program. Future work will involve the development of a second generation renewable fluid for mobile equipment for the construction, agriculture and marine industries.

Further information can be found at www.ucon.com/symbio or by contacting Khemchandani at GVKhemchandani@dow.com.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.