Non-conventional Base Stocks

Gear Oils Gain Efficiency


Although the volume of the overall lubricant market is not expected to grow, the share of synthetic lubricants (not including API Group III oils)-currently at 1.3 million metric tons globally-is expected to increase along with performance demands on lubricants.

The benchmark for high-viscosity synthetic lubricants used in gear oils is API Group IV base stocks, or polyalpha­olefins. API Group V base oils, which encompass all other base stocks except paraffinic mineral oils or PAOs, represent approximately 50 percent of the synthetic lubricant market. The most prominent representatives of Group V synthetics are esters and polyalkylene glycols, with approximately 600,000 tons of combined demand.

PAGs are known for outstanding properties like low friction coefficients, cleanliness, antiwear properties and long drain intervals. Additionally, PAGs offer high viscosity indices, typically above 200, which can provide improved fuel economy and energy efficiency as well as more robust oil films for better wear protection.

On the other hand, it is generally known that PAGs show no or only limited compatibility with mineral oils and PAOs. Some oil-soluble PAGs were introduced to the market about five to 10 years ago. They addressed the compatibility with mineral oils by reacting a fatty alcohol with higher alkylene oxides, such as butylene oxide. However, oil-soluble PAGs show significantly higher friction than PAO and four to five times higher friction than a typical water-soluble PAG. Also, the viscosity index is approximately 25 percent lower than a typical water-soluble PAG.

To fill the gap between compatibility and performance, BASF developed a base stock with a unique polyether based hybrid structure. Dubbed an energy-efficient base stock, parts of the structure contribute to excellent frictional behavior while other parts help to achieve compatibility with hydrocarbon oils. The base stock is most suitable for applications such as gear oils, axle fluids, compressor lubricants and greases.

Polyether based fluids are known for thermal stability, oxidation stability, low vapor pressure and radiation resistance. While traditional polyether fluids have been limited by a relatively high pour point-around 4 degrees Celsius-the polyether based hybrid fluid has a pour point below minus 37 C.

Two different grades of BASFs new hybrid polyether fluid have been introduced with kinematic viscosity of 45 centistokes and 133 cSt at 100 degrees Celsius. (See Table 1.)

The frictional behavior of both neat base stocks was studied using a Mini Traction Machine in the hydrodynamic regime. The MTM is laboratory testing equipment that allows fast screening of different oils in all stages of the Stribeck curve: the boundary, hydrodynamic and mixed lubrication regimes. It also allows adjustment of the slide-to-roll ratio, which is important to simulate different gears.

Test conditions were set at speed of 4 meters per second, load of 1 gigapascal and temperature of 70 degrees C. Compared to PAOs of similar viscosity, the coefficient of friction was reduced by 30 percent with the polyether hybrid base stocks. Test temperature was chosen based on the temperature-viscosity curve to reduce any differences due to viscosity.

Performance as an Additive

High-viscosity base stocks such as EEB 130 or a 150-viscosity metallocene PAO are often used in combination with low-viscosity base stocks to achieve the desired finished lubricant viscosity. Although the viscosity of the neat EEB 130 is approximately 35 percent lower than the viscosity of mPAO 150, in solution with low-viscosity API Group I to IV oils, the viscosity of the blend is about 10 percent higher if EEB 130 is used. This demonstrates a greater thickening efficiency.

The friction of two blends of Group II base oil, one with 10 percent EEB 130 and one with 10 percent mPAO 150, was investigated using a high-frequency, linear-oscillation test machine (SRV). The experiments were performed in a 100Cr6 steel ball-on-disc setup (ball diameter 10 mm; disc 24 by 7.9 mm), applying a load of 100 newtons/451 megapascals at 50 C. The sweep frequency was at 50 Hertz and 1 millimeter. Since no additives were added to the oil blend, a run-in time of 30 seconds at 20 N was applied, as well. The experiment provides data on frictional behavior at 100 percent sliding and allows wear analysis of the ball after the experiment. Results are shown in Figure 1.

The blend with EEB 130 showed about 20 percent lower coefficient of friction compared to mPAO 150 at the same concentration. Because the viscosity of the EEB blend is about 10 percent higher, one would expect a further reduction in the coefficient of friction if the viscosity were the same as the mPAO blend.

The wear scar of the ball used for the EEB 130 based oil was approximately 20 percent less in comparison to the mPAO 150 based oil.

Results indicate that the addition of the EEB 130 at relatively low concentrations leads to a more robust lubricating film and a significant reduction of wear scar.

Performance in Gear Oil

Past studies have found a significant reduction of friction in ISO Viscosity Grade 320 formulations containing the polyether hybrid fluid. (See Table 2.) These experiments were performed using an MTM; however, a twin disc bench test is closer to a real-life situation.

In a twin disc test rig, the discs are loaded with a normal force and rotate at different speeds. The resulting frictional force is determined by a force transducer. The coefficient of friction is calculated as a ratio of the frictional and normal force. The slide-to-roll ratio is given by the different speeds of both discs and can be varied.

Test parameters were set with velocity of 5 to 10 meters per second, Hertzian pressure at 1,100 to 2,100 MPa, slide-to-roll ratio from 0 to 50 percent, and injection temperature of 70 C.

The relative friction coefficients of both the EEB based and PAO based ISO VG 320 formulations (Table 2) showed a reduction of up to 40 percent with the energy efficient base stock.

The experiment is not isothermal. Due to the increased friction, heat is generated and the temperature of the oil rises depending on pressure and velocity. Applying velocity of 10 m/s, pressure of 2,500 MPa and slide to role ratio of 50 percent, the temperature increases to approximately 110 C in the case of the EEB based formulation and to 135 C with the PAO based formulation.

The increase in temperature indicates significant reduction of friction with formulations containing the polyether hybrid. It results from a combination of reduced internal friction within the lubricant as well as reduced friction between metal parts due to greater lubricating film thickness.

Finally, the performance of the energy efficient base stock was investigated using a worm gear, a type of gear known to be less efficient than others. The gear had a wheel base of 100 mm and a transmission with a 20:1 gear ratio. The test rig consisted of an engine (21.5 kilowatts) and a generator (44 kW). Output torque was 600 newton meters, and the efficiency was determined as a function of speed. In this setup, a more efficient oil would lead to lower input power. The power loss can be determined, and the efficiency calculated as a ratio of input and output power.

A 50-hour run-in of both flanks was applied. Three different ISO VG 320 oils were investigated, including two commercial oils-a PAO based oil and a water soluble PAG oil-and one experimental EEB based oil. (See Table 3)

The worm gear exhibited efficiency of 81 to 87 percent, depending on speed and oil, and efficiency increased with speed (Figure 2). The commercial PAO oil showed the lowest efficiency, while the commercial PAG and the EEB oil exhibited the highest efficiency. Efficiency gains of up to 5 percent over the PAO were shown.

The EEB oil, consisting mainly of EEB 45, showed similar performance to PAG. This means that EEB 45 could be used to formulate lower-viscosity formulations in combination with mineral oils.

It should be noted that both EEB oils exhibit 20 to 25 percent higher viscosity indices compared to the commercial PAO based oil. The higher V.I. provides additional efficiency gains during start-up of gears because the oil can be formulated to have a lower viscosity at 40 C, while the viscosity at working conditions is the same as conventional PAO oils.

The lower friction properties of the polyether hybrid base stock can translate into lower operating costs because less power input is needed. This can also result in less emissions, which may vary with the individual application and type of gear.

The fluid is expected to be available in the market next year.

Edith Tuzyna, Kian Molawi, Henrik Heinemann and Philip Ma also contributed to this article.

Frank Rittig is technical marketing manager for base stocks and metalworking fluids with BASF, where he has worked for 17 years. He holds a Ph.D. from the University of Leipzig and a chemistry degree from the Freiberg University of Mining and Technology.