Group III Versus PAO Performance
Historically, PAOs have had superior lubricating performance characteristics such as V.I., pour point, volatility, and oxidation stability that could not be achieved with conventional mineral oils. Now, in modern base oil manufacturing, V.I., pour point, volatility, and oxidation stability can be independently controlled. Modern Group III oils today can be designed and manufactured so that their performance closely matches PAOs in most commercially significant finished lube applications.
As well-designed Group III base oils become abundant in the marketplace, the performance gap between Group III and PAO (Group IV) is closing. Here are some key examples:
Pour Point – Pour point is the one property where Group III oils allegedly fall short of PAO. While it is certainly true that the pour point of the neat Group III base oil is substantially higher than that of a PAO of comparable viscosity, it is important to understand that the pour point of the fully formulated lubricant (base oils plus additives) is the critical property. Base oils manufactured with modern isomerization catalysts respond very well to pour point depressant additives. For example, turbine oils formulated with conventional Group II base oils (-12°C base oil pour point) are available with a formulated pour point of -36°C. Fully formulated Group III based lubricants can be made with pour points of -50°C or below.
Products such as motor oils made with the lighter-grade PAOs, on the other hand, typically have higher pour points than the base fluid, so the gap in final product pour point between PAO-based and UCBO-based lubricants is much smaller than in the base fluids themselves. Moreover, it is entirely possible with modern Group III manufacturing technology to produce base oils of even lower pour point. However, this is not common practice in the industry, because it is more economical to meet finished lube low temperature performance using pour point depressant additives rather than using special Group III oils having exceptionally low pour points.
Cold Crank Simulator – Viscosity in engine journal bearings during cold temperature startup is a key factor in determining the lowest temperature at which an engine will start. Cold Cranking Simulator (CCS) viscosity, as measured by ASTM Method D 5293, is determined under conditions similar to those experienced in engine bearings during starting. For base oils, this viscosity is determined almost entirely by
viscosity and V.I. Since Group III stocks typically have V.I. comparable to that of 4 cSt PAO, one would expect comparable CCS performance. This is demonstrated in Figure 3, where it can be seen that a 4 cSt Group III base oil, with a kinematic viscosity of 4.2 cSt at 100°C and a V.I. of 129, and PAO 4, with a viscosity of 3.9 cSt and V.I. of 123, have similar CCS values, both about half that of a 4 cSt Group II base stock of about 100 V.I. This performance makes the Group III stock very effective for formulating fuel efficient multi-viscosity engine oils in the 0W-20 to 0w50 range, one that has historically been achieved only with PAO-based product
Noack Volatility – Noack volatility of an engine oil, as measured by ASTM D 5800 and similar methods, has been found to correlate with oil consumption in passenger car engines. Strict requirements for low volatility are important aspects of several recent and upcoming engine oil specifications, such as ACEA A-3 and B-3 in Europe and ILSAC GF-3 in North America. Figure 4 shows that from a blender’s perspective, Group III base oils are similarly effective as PAOs for achieving these low volatility requirements in engine oil applications. The V.I. of modern Group III oils typically match or exceed PAO, so they can match the volatility of PAOs at a reasonable distillation cut width.
Oxidation Stability – Oxidation and thermal stability are among the most important advantages that “synthetics” bring to the table. Better base oil stability means better additive stability and longer life. High stability is the key to making the premiumquality finished oils of the future with longer drain intervals. Here Group III oils routinely challenge PAO performance. The stability of modern Group III stocks depends mostly on their V.I., because V.I. is an indication of the fraction of highly stable isoparaffinic structures in the base oil [10]. However, because modern Group III stocks also undergo additional severe hydrofinishing after hydrocracking and hydroisomerization, they achieve an additional boost in stability because only trace amounts of aromatics and other impurities remain in the finished stocks. On the other hand, PAO performance seems to depend largely on residual olefin content. Olefins are an intermediate in PAO production that contribute to instability.
Future Evolution
Looking to the future, the trend is toward lubricants and base oils with even higher purity, lower volatility, and longer life. The molecular structure of base oils will probably look even more like PAO as they become more concentrated in the most favorable molecular species needed for superior lubrication performance. It is likely that recent and ongoing developments in base oil technology will enable lubricants with exceptional performance to be marketed in much greater volumes than was feasible when PAO was the only stock capable of such performance levels. There are many possible routes for improving base oil quality. Continued evolution of the all-hydroprocessing route is one likely possibility. Selectivity toward desired molecular compositions could be improved by improving the catalysts and the
processing technology. Improving the feedstock can also improve the product. Very paraffinic (waxy) feedstocks such as Fischer-Tropsch wax from natural gas-to-liquids plants can potentially be further processed into high quality base oils. Volumes and applications are expected to grow, as ultra-waxy feedstocks become more widely available. Other competing technologies are likely to emerge. New routes for
manufacturing PAOs have been proposed that use cheaper feedstocks such as ethylene and propylene rather than 1-decene Future improvements in base oil technology will assuredly lead to further improvements in the performance of turbine oils and other sensitive applications with low additive treat rates.
Conclusions
Lubrication technology evolved slowly from ancient times until the middle of the 20th century. Then solvent refining technology emerged and displaced naturally occurring petroleum distillates due to its improved refined properties. Starting in the 1960s, hydroprocessing technologies were introduced which improved base oil purity and performance further. In the 1970s and 1980s, Group II base oils were manufactured and recognized as a separate API category in 1993, due to their positive differentiation over conventional stocks. Modern hydroisomerization technologies, such as ISODEWAXING, became widely accepted and grew rapidly since it was first commercialized in 1993. Widespread licensing of this technology has created an abundant supply of Group II oils that have exceptional stability and low temperature performance relative to their Group I and Group II predecessors. This technology is now used to make about one-third of all base oils in North America.
A similar trend appears to be emerging with Group III base oils, especially those made using modern hydroisomerization. They offer most of the performance advantages of traditional PAO-based “synthetic” oils and can be manufactured in volumes unachievable by PAO. Most manufacturers of modern Group II base oils can make modern Group III base oils as well.
Selected top-tier lubricants requiring PAO should continue to coexist with Group III oils as they have for years in Europe. But widespread availability of modern Group II and III mineral oils is accelerating the rate of change in the finished oil markets. New improved base oils are helping the engine and equipment manufacturer meet increasing demands for better, cleaner lubricants.
This is particularly true for turbine oils because turbine oils usually contain more than 99% base oil. Turbine oils made from hydroisomerized Group II base oils and the appropriate additives have demonstrated significantly longer TOST lives than turbine oils made with Group I base oils. In fact, they commonly outperform the traditional “synthetics” made with PAO.
As base oil technology continues to evolve and improve, consumers will enjoy even greater protection of automobiles, trucks, and expensive machinery such as turbines. Lubrication performance that currently can be achieved only in small-volume niche applications, using PAO and other specialty stocks, will be more widely available using the new generation of Group II and Group III oils.
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