TransForum Vol. 7, No. 2

A New Nanolubrication Technology May Solve Problems Caused by Low-Sulfur Diesel Fuels and Oils

This boric acid "rose" shows the intricately layered microscopic structure of the compound.

As often happens when you try to solve one problem — in science, society, etc. — you end up creating another. When the U.S. Environmental Protection Agency (EPA) mandated steep reductions in the amount of sulfur allowed in diesel fuels and lubricating oils, it took a big step in the much-publicized effort to significantly reduce particulate emissions from diesel-fueled vehicles. (The allowable sulfur content in fuels decreased from more than 3,000 ppm over the years to less than 15 ppm today.)

In lubricating oils, the amount of sulfur- and phosphorous-bearing additives and all the heavy elements are also under increasing scrutiny and demand to be drastically reduced or replaced by environmentally benign alternatives. Besides contributing to particulate emissions, these additives poison and/or adversely affect the performance of aftertreatment devices in diesel engines. In particular, sulfur combines with moisture in engine combustion chambers, forming acids that can lead to corrosion and severe wear problems.

So reducing sulfur and other additives in fuels and lubricants is a good thing, right? Yes and no. Trucking fleets throughout the nation reported fuel component failures after switching to low-sulfur diesel fuels.

Turns out that removing the sulfur reduces the lubricating capacity of the diesel fuel, accelerating wear in fuel system components. Reduction of sulfur and phosphorous in lubricating oils has also been shown to result in higher friction and wear in engines.

To address these issues, Argonne may have a solution, with a recently patented nanoscale lubrication technology — U.S. Patent No. 6,783,561: Method to Improve Lubricity of Low Sulfur Diesel and Gasoline Fuels, and U.S. Patent Applied for No. US2005/0009712: Methods to Improve Lubricity of Fuels and Lubricants. Specifically, Argonne's boron-based additives are environmentally safe, inexpensive, and have an unusual capacity to enhance the antifriction and antiwear properties of sliding surfaces in diesel- and gasoline-powered engines. Fig. 1 shows its ability to restore the lubricity of sulfur-free (Fischer-Tropsch) diesel fuel, even at concentrations down to 250 ppm.

Figure 1. Fuel lubricity performance of a boron-based additive in sulfur-free (Fischer-Tropsch) diesel fuel. For comparison, the fuel lubricity performance of 500 ppm-sulfur containing number 2 diesel fuel is also provided. These tests were run in a ball-on-three disk fuel-lubricity test machine.

Who Can Benefit from Using Boron-Based Lubricants?

Complaints related to low-sulfur diesel fuels without lubricating additives include premature fuel system component wear or failure, unstable engine speed, plugging of injector/injection nozzles, seal failures, hard starting, decreased power, and engine smoke. Reduction of sulfur and phosphorous-bearing additives in engine oils has also resulted in increased friction and wear in critical engine parts and components. These outcomes are bad news for trucking companies and engine and truck manufacturers. In addition, there are claims that, because sulfur is an antioxidant, fuel quality could degrade faster without it — posing potential problems for the makers and distributors of diesel fuel. Whether all of these problems can actually be blamed on low-sulfur diesel is debatable, but a lubricant additive that could economically solve them without causing separation or other harmful refining effects would be a welcome answer.

What Do They Do?

Argonne researcher Ali Erdemir performs a friction test on a metal disc coated with a solution of motor oil with nano-boric acid particles.

Several years ago, Argonne scientists discovered that boric acid, used as a lubricant, is one of the most slippery solids in existence. Earlier tests showed that thin films or bulk powders of boric acid can provide friction coefficients as low as 0.02 to 0.05 — one-fourth to one-sixth the value of other, more expensive solid and liquid lubricants. Recently, patented nanoboric acid powders and liquid media additives containing special boron compounds proved to work even better. Specifically, when used as solid dispersion or liquid media additives in lubricating mineral and synthetic base oils, they were able to reduce friction by 50% to 90% (depending on the concentration) under a wide range of boundary-lubricated sliding conditions. These novel additives were also able to enhance the lubricity of sulfur-free and ultra-low sulfur diesel fuels. They reduced wear-scar diameters by as much as 50% in high-frequency reciprocating and ball-on-three disk fuel lubricity tests; see Fig. 1 for a typical result. The ball-on-three disk is a fuel lubricity test machine, in which a rotating ceramic ball is pressed against three flat disks at an angle. The ball and disks are submerged in liquid fuel; the average size of wear scar diameter that forms on disk surfaces is recorded as an indicator of that fuel's lubricity. The smaller the diameter, the higher the lubricity.

The unusual lubricating mechanism of boric acid in these oils and fuels is controlled by its very special chemical structure and its ability to form a strongly bonded protective boundary film on rubbing surfaces. The compound is crystallized in layers in which the atoms are tightly bonded to each other. The layers themselves are weakly bonded; when stressed, they shear and slide over one another easily, so friction is low. The strong bonding between the layers prevents direct contact between sliding parts, minimizing wear. The boron-based fuel and oil additives developed by Argonne take advantage of this special structure to provide enhanced lubrication in diesel fuels and lubricating engine oils.

The frosting on the sands of Boron, California, wasn’t caused by a snowstorm – rather, it consists of spontaneously forming boron deposits. The natural abundance of boric acid makes it a temptingly cheap and environmentally friendly lubricant.

Although discussions are underway with several companies, the innovative technology is available for licensing through Argonne's Office of Technology Transfer.

This work was funded internally by Laboratory-Directed Research and Development funds.

For information on licensing agreements and working with Argonne, contact Stephen Ban, phone: 630/252-8111 or Stephen Lake, phone: 630/252-5685.

August 27, 2007