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A Roadworthy Synthetic?

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Oils based on polyalkylene glycol sure would be great at lubricating automotive engines. They could push all the right performance buttons that engine designers say they need: low volatility and good oxidative stability for extended drains and long life, excellent friction and load-bearing characteristics to help boost fuel economy, and no sulfur or ash-producing components that can poison emissions systems. They have the high viscosity indices and low pour points needed for good temperature and flow properties, and they also resist forming deposits and varnish.

The drawback – and its a big one – is that PAGs are not generally compatible with hydrocarbon based oils, pointed out Johan A. Thoen of Dow Chemicals Dow Benelux subsidiary in Terneuzen, Netherlands. Great care must be taken to keep PAGs segregated from mineral oil based lubricants, and systems that are already lubricated with mineral oil cannot be switched over to use PAGs without encountering great difficulty.

Polyalkylene glycols have very good performance, the Dow senior scientist told last months 16th Colloquium Tribology at the Technische Akademie Esslingen in Ostfildern, Germany. They fit certain classes of high-performance applications as well as polyol esters and polyalphaolefins do. But they have one basic shortcoming: They are not very soluble in mineral oil, and mineral oil dominates over 90 percent of the worlds lubricant applications.

About two dozen companies worldwide make PAG base stocks for use in lubricants and although they are rarely used in automotive oils, they are well-known in many industrial applications. Originally they were developed during World War II, in response to the U.S. Navys need for hydraulic fluids that would be both fire resistant and able to endure wide temperature ranges on its ships. In the post-war years, PAGs moved into specialty areas such as textile lubricants, quenchants, compressor and refrigeration oils, metalworking fluids and food-grade lubricants.

Today, the global lubricants market is approaching 40 million metric tons per year, of which just 3 percent are based on synthetic fluids, Thoen estimated. PAGs have grown to be the third largest category of synthetics, after PAO and esters, he added, and account for about one-quarter of synthetic lubricants sold. However, the automotive market is the largest lubricants segment, and PAGs could see explosive growth if they could just get a foot in the door.

If we can develop a PAG that we can blend at 10 or 20 or 30 percent in mineral oil, and therefore deliver the performance benefits of PAG, we could find a place in the market for driveline applications – especially crankcase oil, he said.

A PAG Primer

PAGs usually are made by polymerizing ethylene oxide and propylene oxide in the same molecule, using a mono- or polyhydric initiator (such as butanol), explained Thoen in his presentation, which was coauthored by Dows Martin Greaves. The ratio and order of the oxide, plus the choice of the chemical initiator and its molecular weight, can vary widely, and so will the properties of the resulting PAG. The fluids pour point and lubricity characteristics, for example, can be changed by tweaking the ratio of ethylene oxide to propylene oxide.

The PAG industry today is largely based on ethylene oxide and propylene oxide, Thoen said. Higher oxides are generally not used, but a few years ago we at Dow began asking, Can we? Theres a whole world of catalysis out there, and it lets you play with different molecular weights and distributions. The catalysis side of the PAG equation is still under study – and largely proprietary – but Thoen did offer a look into the other pieces of this chemical puzzle.

In addition to the basic ethylene oxide-propylene oxide combination, he pointed out, PAGs can be synthesized using higher alkylene oxides, such as butylene oxide and octene oxide. Only two companies make butylene oxide, Thoen told the Colloquium attendees. Thats our friends at BASF, and Dow Chemical.

Using homopolymers of butylene oxide resulted in more sophisticated PAGs, and in the 1990s these became the basis for Dows Synalox OA and OD grades. They are somewhat oil-soluble and were developed as alternatives to diesters, polyol esters, polyalphaolefins and poly-isobutenes. These offered some oil solubility, but we werent happy enough with butylene oxide, so we moved to Generation II, using olefin oxides, Thoen said.

Generation II

Octene oxide, the olefin which became the object of Dows latest investigations, is also known as hexene oxylate. Octene oxide seemed promising because it has very low pour points and, unlike ethylene oxide, is fully oil soluble. One disadvantage, though, is that octene oxide doesnt deliver the excellent friction-reduction that is seen when using ethylene oxide to make PAG. Nevertheless, Dows researchers aimed to deliver some moderate extreme pressure performance, similar to that of conventional mineral oil or PAO.

Among the olefins tested were 1,2-expoxyoctane and 1,2-expoxydodecane. The latter did not prove fruitful – the pour point was unacceptable – but with the former Dow struck paydirt. On a laboratory scale in a stainless steel reactor, the researchers were able to produce clear and colorless viscous liquids in two molecular weights, with viscosity indices averaging 147 and 159, and pour points of -25 and -22 degrees C.

The next step was to evaluate the new fluids lubricity and oil solubility. First, an Optimol SRV test apparatus was used to measure their friction and extreme pressure properties. This test uses an oscillating steel ball against a steel disk to show lubrication performance; a value of 0.10 in the test is considered excellent and a value higher than 0.14 is poor. Friction coefficients for the PAGs based on 1,2-epoxyoctane are good, with values between 0.125 and 0.13, Thoen reported. By contrast, an ISO 32 visocity grade paraffinic base oil was rated 0.14 in this test, and a 6 cSt PAO was 0.16.

The SRV tests also showed that the PAGs based on 1,2-epoxyoctane had moderate extreme-pressure capabilities, similar to the PAO but not nearly as strong as conventional PAG, as had been predicted.

Finally, to see just how soluble the new PAG fluid was in mineral oil, blends were prepared as 50/50 mixtures with paraffinic base oil, PAO and naphthenic base oils. Each mixed easily and remained clear, and all were then stored for one month at ambient temperatures. At the end of the month, the mixtures were found to be stable and homogeneous, and still optically clear. There was no haziness or separation, as would have been seen with conventional PAG. (As a control, a PAG reference fluid was also blended with the same mineral oils and PAO; it resulted in a hazy mixture right away, which split into phases upon standing.)

Dows goal is to bring the fluids into the market as a blendstock that can upgrade mineral oil performance, and these tests show that it can be done, Thoen told the meeting in Germany. At this point, a pilot-scale process is in place at Terneuzen, Netherlands, that shows the molecules are attainable. The next steps are to scale up the process and commercialize the new generation of fluids, perhaps in just two or three years from now. The initial viscosity grades are likely to be ISO 46 and 68 base oils, and by controlling the polymer molecular weight the range could be expanded to lower and higher viscosity grades. Eventually, the new PAGs probably will be marketed under the Synalox brand name in Europe, and as Ucon fluids in North America.

A Question of Cost

After scale-up, the next hurdle for Dows octene oxide based PAG is likely to be price. PAGs typically cost four to six times as much as mineral oil base stocks, and the automotive lubricant market is notoriously price-sensitive. In response to follow-up questions after his presentation though, Thoen was optimistic that in the right formulation, the new PAG chemistry would prove its worth.

PAG has very good inherent friction and wear properties, and you may be able to replace some of the additives needed now in crankcase lubricants, he suggested. PAG also leaves no combustion residue – no harmful ash or metals that can damage after treatment devices such as catalytic converters – and it facilitates the addition of additives in the blend because it has some polar functionality. This means it will be a good choice as a blendstock when using less-polar base stocks such as PAO and API Group III base oils, which have more difficulty in solubilizing additives. Plus, he continued, theres no soot formation, and theres overall cleaner lubrication that helps avoid sticking valves and deposits.

In its oil solubility tests, Dow achieved blends of 50 percent PAG in mineral oil, but its more likely to be used at rates of 10 to 30 percent, Thoen said. We did blendings of up to 50 percent mineral oil and PAG without problem, but because PAGs are quite a bit more expensive than mineral base oils, the goal is to optimize the blend and to reap substantial performance increases with the minimum amount of PAG, to keep the cost manageable.

The companys target is the high-performance space now occupied in engine oils by additives, polyol esters and polyalphaolefins, he went on. Because they cost more, synthetic engine oils must be very sensitive to showing they can deliver higher performance, higher fuel economy and cleaner operation, he said. We want to increase performance in economical ways. In the United States, GF-5 oils are just around the corner, and we believe this fluid can help meet that specification.

Thoen was asked whether Europe will be first in line to get the new PAGs, since synthetics already have a larger share of the high-end engine oil market there. But he declined to speculate on where the fluids will make their debut.

Because we are a global company, we will have to ensure it is globally available and meets global needs, he said. Its first use is likely to depend on where the prime markets are, but right now, were just beginning to expose this to a broad array of lubricant marketers and to the OEMs. We want to see where their interest lies.

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