For the past century, availability and cost positioned paraffinic oils as the dominant base oils for lubricants. The performance properties of what came to be defined as API Group I oils-and, after the 1970s and 1990s, respectively, Group II and III oils-helped define lubricant specifications for many transportation and industrial applications. Though they are not formal definitions, convention has further segmented the range of paraffinic grades to include Group II+ and Group III+ oils.
Yet composition, viscosity index and waxy molecules that thicken paraffinics at low temperatures place limits on lubricant performance. How much longer will paraffinic base oils prevail in a world of evolving technologies and priorities such as energy efficiency?
Polyalphaolefins-hydrocarbons with side chains and comb-like form-are synthesized by reacting specific raw materials under carefully controlled conditions. PAOs have a well-defined chemistry and are free of sulfur, unsaturated carbon-carbon bonds and waxes present in paraffinic oils. Their stability at high temperatures and in the presence of water, plus fluidity at low temperatures, sometimes outweigh their high cost and limited solubility of additives. Will PAOs continue to be confined to low volume, technically demanding niches?
And what about naphthenic mineral oils? Unlike paraffinics, naphthenics are obtained by refining naphthenic crudes, which contain more of the cyclic, saturated ring structures that are the main components of naphthenic base oils. Despite their low-temperature fluidity and ability to dissolve additives, their low V.I. and available viscosity grades can present limitations for lubricant formulations. Will this continue to be case, or are there new opportunities waiting in the wings for naphthenics?
Advances in lubricant formulation and testing of PAOs, paraffinic and naphthenic base oils were presented at the Society of Tribologists and Lubrication Engineers annual meeting in Nashville, Tennessee, in May.
What do these developments augur for lubricant formulators and end users?
High Performance at Lower Cost
Sipho R. Masilela and Philip L. de Vaal, of the University of Pretoria, South Africa, performed a head-to-head comparison of Group I, Group III+ and PAO oils. Group III+ is a marketing term generally accepted in the industry to refer to API Group III base oils with exceptionally high viscosity index, typically 130 or greater.
The most commercialized base oils are Group I solvent-refined mineral oils, Masilela told his audience. Group I paraffinics are the least expensive base stocks that provide satisfactory volatility, oxidation stability, low-temperature lubricity performance and good solvency for additives. However, he pointed out that the oils low V.I. can result in increased wear as oil evaporates and thickens at higher temperatures.
In contrast, Masilela said, Group IV PAOs are known for their excellent lubricity over a wide range of temperatures due to their high V.I., high thermal and oxidative stability and low volatility compared to Group I oils. However, they have limited solvency for polar additives and are very costly compared to paraffinics.
Group III+ base oils have physical characteristics between those of Group III paraffinics and PAOs, he continued. These oils have long and complex molecular structures with 20 to 30 or more carbon atoms that are more than 90 percent saturated with few cyclic paraffins. They have very low sulfur content (at or below 0.03 percent by weight), pour points around minus 30 degrees Celsius depending on viscosity grade, high V.I., satisfactory solvency to polar additives and increased oxidation stability.
Can Group III+ oils deliver the lubricity performance that formulators seek from PAOs at prices closer to those of Group I? In a study jointly funded by the South African National Research Foundation and Sasol Research and Technology, Masilela and de Vaal tested oils with nearly the same viscosity (between 4 centistokes and 5 cSt at 100 C). However, the oils differed significantly in their response to temperature due to their different chemical compositions and viscosity indices, which ranged from 103 (Group I) to 125 (PAO) and 131 (Group III+).
Masilela also pointed out that pressure, just like temperature, has an effect on the viscosity behavior of base oils. When a base oil is compressed by the application of stress at a constant temperature, viscosity increases. This is because the space between molecules decreases, resulting in resistance to further compression and shear. This contributes to increased friction, especially in the elastohydrodynamic and mixed lubrication regimes. To select the most efficient base oil for low friction, formulators must know the pressure-viscosity behavior of a fluid at the temperatures of interest, he explained.
Masilela and de Vaal measured the coefficient of friction under oscillatory sliding conditions using an SRV test machine at temperatures between 40 and 120 C. The mixed lubrication regime-where film thickness is intermediate between full film lubrication and boundary lubrication with significant contact between asperities-was predominant.
The types of lubricant that can be formulated from the base oils in this study include bearing, gear and motor oils, which are commonly of the grades SAE 5W-30, SAE 10W-30 and SAE 10W-40, de Vaal told LubesnGreases. This study focused on ball bearing lubrication where point contacts occur. Typical applications of lubricated ball bearings would be in automobile starter motor and transmission shafts.
Results showed that the Group III+ base oil demonstrated increased resistance to shear stress at all experimental temperatures while maintaining higher friction stability, temperature stability and wear reduction compared to the Group I and PAO, said Masilela. (See Page 39.) It also demonstrated similar wear mechanisms to the PAO base oil with increasing temperature.
We were surprised that this Group III+ base oil competed closely with the PAO under these harsh experimental conditions in the mixed regime and showed even better wear reduction at 100 and 120 C, Masilela stated. Based on the laboratory results obtained, the Group III+ base oil is a strong competitor to the PAO in the mixed lubrication regime.
Paul Norris, senior R&D scientist at Afton Chemical Ltd. UK, compared gear oils in a presentation on efficiency of industrial gearboxes. Norris told his audience that gear boxes cover a wide range of efficiencies depending on their type, design and manufacturer, as well as operating conditions and applications. Even small improvements in energy efficiency can have a substantial cumulative effect during their long service life.
Norris and his colleagues used Mini-Traction Machine (ball on disk) and FZG gear pitting tests to evaluate experimental designs of formulations for lubricating hypoid gears. They compared formulations based on API Group I and PAO oils with different polymer thickeners and various additives. All formulations were prepared at 35 cSt (100 C).
Overall, whatever the test [MTM or FZG], we see that the PAO gives much lower friction in the hydrodynamic lubrication regime than Group I oil, Norris explained. Hydrodynamic lubrication is typical in gear operation. We believe this is due to the lower pressure-viscosity coefficient of the synthetic oil.
We also noted that at high speed in the FZG test, the differentiation [between base oils] was lost. This indicates that churning losses become dominant as the speed increases, he observed. Churning loss is energy lost when gears move through the lubricant, which is affected by oil level, viscosity and rotational speed.
Concerning the impact of friction modifiers and thickeners on PAO formulations, Norris added, Like the base oil differences, the choice of thickener can influence the hydrodynamic friction coefficients due to differences in the pressure-viscosity coefficients. This is the same impact for both mineral and synthetic base oils.
For industrial gear oils, the OEM landscape is evolving, Norris said. Some specifications include a clutch test requirement, which requires high friction and thus could impact our ability to make more efficient lubricant formulations.
However, given that the higher efficiency gains may be obtained in the hydrodynamic lubrication regime, while the clutch friction is governed by additive effects in the boundary layer regime, the two dont have to be mutually exclusive. The PAO could still deliver desired efficiency with an additive system that meets the clutch test requirement, he concluded.
New Roles for Naphthenics
Thomas Norrby, technical manager at Nynas AB, presented new formulation guidelines and experimental results for gear oils based on naphthenics.
Norrby noted that the total industrial gear oil market is estimated to be above 1 million metric tons per year, or approximately 3 percent of the total global lubricants market. For industrial gear oils, ISO VG 220 is the most widely utilized grade, followed by VG 150 and VG 320. Even heavier VG 1,000 and 1,500 grades are used for draglines, haul trucks and other heavy-duty applications.
A traditional industrial gear oil in the 150 to 480 VG range is typically formulated from blends of API Group I paraffinic bright stock and SN 500/600 oil, he continued. For higher-viscosity grades, viscosity index improvers or viscosity modifiers like polyisobutylenes are employed to create viscosities such as VG 680, 1,000 and 1,500-above what can be reached using typical bright stock, which tops out around 500 cSt at 40 C.
Industrial gear oil standards such as ISO 12925-1 and DIN 51517-3 require a minimum V.I. of 85 or 90. Naphthenics offer benefits with regards to solvency and low temperature properties, but do not have the V.I. required by these standards.
Therefore, Norrby blended commercial naphthenics with V.I. improvers, viscosity modifiers and high V.I. base fluids to formulate model gear oils that met both kinematic viscosity at 40 C and V.I. requirements for ISO VG 150, 220, 320, 460, 680, 1,000 and 1,500 gear oils.
The key challenge was to simultaneously meet the viscosity and V.I. targets for the fully formulated oils. Norrby found that some of the viscosity index improvers and viscosity modifiers, including two olefin copolymers and two PIBs, increased the viscosity faster than the V.I., or did not raise the V.I. enough before the oil slipped out of the viscosity grade specification limits.
However, he did find five of the viscosity index improvers to be useful in various combinations at different treat rates to produce the targeted sets of viscosity and V.I. These included an oil soluble polyalkylene glycol, a polyalkyl methacrylate, a biobased complex ester, one ethylene-propylene copolymer, and an energy-efficient polyether base stock.
Especially for the higher viscosity grades (680, 1,000 and 1,500), these new fluid candidates demonstrate novel routes to high-viscosity base fluids for industrial lubricants, Norrby concluded.
Mary Moon, Ph.D., is a professional chemist, consultant and technical writer and is technical editor of The NLGI Spokesman. Contact her at firstname.lastname@example.org or (+1) 267-567-7234.