Youve heard it before: The most important property of any lubricant is its viscosity. But what exactly is viscosity, and if its so critical to oil performance, how do we control it? To answer that, formulators reach for one of the most indispensable tools in their kit: viscosity index improvers.
When formulators talk viscosity they are referring to the internal friction between the oil molecules – generally speaking, its resistance to flow. Of course, temperature affects oil viscosity; the higher the temperature the lower the viscosity. The rate at which an oils viscosity changes with temperature is called its viscosity index, or V.I.
Get alerts when new Sustainability Blog articles are available.
For decades, lubricant blenders have used polymers to change this natural property. Chosen correctly, the right polymers will make the oil less viscous when its cold, and more viscous when the thermometer rises.
Additive manufacturers and oil marketers call these polymeric materials V.I. improvers or, less frequently viscosity modifiers. The terms are interchangeable, but by any name its a big business. According to a study published last year by Kline & Co., V.I. improvers account for about 23 percent of global lubricant additive sales, by volume.
Additive companies have an assortment of V.I. improver molecules to work with. Historically, the first were polyisobutenes, introduced in the 1940s. The classic of the genre is Paratone N, originally supplied by Exxon Chemicals Paramins division but sold to Chevron Oronite in 1998 when Paramins came together with Shell Chemicals additives business to form Infineum.
In the 1950s, Rohm and Haas introduced the Acryloid brand of polymers to the North American market. Now sold under the Dynavis name by Evonik Oil Additives, these are based on polymethacrylates (PMA), and are widely used for both engine oil and powertrain applications. PMA also can be tailored to provide additional properties such as dispersancy.
Olefin copolymers (OCP) of ethylene and propylene also made their appearance in the late 1950s and early 1960s. OCPs were originally supplied by Paramins and also by Texaco Chemical, which was acquired in the 1990s by Ethyl Corp.; now Afton Chemical, it sells them under the HiTec brand.
OCPs are the most widely used V.I. improvers, and other suppliers include Lubrizol, Infineum, Oronite, Mitsui Chemicals and Lion Copolymer. They offer good performance at reasonable cost, which makes them attractive. In the mid-1980s the processes used to manufacture these workhorse polymers were improved to allow for the formation of so-called block copolymers, which optimize thickening efficiency while allowing for improved low-temperature performance.
Styrene-isoprene polymers were introduced by Shell Chemical in the 1970s. Sold under the Shellvis brand, they brought some advantages in shear stability and low-temperature performance. These polymers also continue to be widely used, especially in engine oil applications.
U.K.-based consultant David Wedlock has described how polymeric V.I. improvers work. Many believe its due to the polymers temperature-dependent solubility. At lower temperatures, the theory goes, the polymers are less soluble (or the molecule is more coiled up), while they become more soluble at higher temperatures (the molecule relaxes).
This is a popular misconception, but in fact, Wedlock says, solubility is largely irrelevant. Writing recently in LubesnGreases Europe-Middle East-Africa, he explains that V.I. is affected by the carbon number (a means of expressing molecular size) of a molecule. The higher the carbon number, the higher the V.I. Simply stated, V.I. improvers are bigger molecules and will raise the V.I. of any base oil having a much lower carbon number.
The net effect, as Wedlock explains, is that viscosity is low enough to enable an oil to flow freely at lower temperatures, and at higher temperatures the viscosity is high enough to provide protection against metal-to-metal wear.
One of the benefits is that automakers can write their lubricant specifications around lower-viscosity base oils, which have lower overall internal friction and less viscous drag. That adds up to better energy efficiency. This property is especially useful for engine oils and powertrain fluids and is accomplished through a phenomenon called temporary shear loss.
When an oil containing a V.I.-improving polymer is subjected to high pressure and/or high shear rates, the polymer molecules tend to become more organized and line up with the direction of flow; this results in a lower apparent viscosity. When the pressure or shear is removed, the polymer once again becomes randomly dispersed in the oil, raising the bulk viscosity.
The downside of V.I. improvers is that repeated shearing tends to break down the polymer into shorter chain lengths which are not as effective at thickening. This is referred to as permanent shear loss. Depending on the polymer structure and size, permanent shear loss can be minimal or quite substantial. The larger the polymer, the larger the shear loss; smaller polymers dont shear as much, but they also are less efficient at thickening. The trick is to balance performance against molecular size.
Todays V.I. improver market is seeing a great deal of activity. In discussions with the major V.I. improver marketers, several common themes developed. While the approach varies from supplier to supplier (who tend to be closemouthed about their proprietary research), all are giving a great deal of thought to this component and how it can be even more useful in the future.
Chevron Oronites Bill Paschal, global V.I. improver product manager in Bellaire, Texas, says his company is actively evaluating alternative chemistry. Oronite remains committed to our core technology but is also looking outside of our immediate toolbox to expand our portfolio, he says, adding, There are new molecules we are looking into, but arent able to share any information at this stage. Broadly speaking they are unique polymer structures, such as block copolymers, tailored to specific application requirements.
Olefin copolymers will continue to represent the bulk of the market, says Bill Dimitrakis, VM commercial manager at Lubrizol in Wickliffe, Ohio. However, in 2008 Lubrizol introduced a bristling new PMA structure. This star-like polymer, called Asteric, offers a very high V.I. which results in fuel economy gains, according to Dan Visger, Lubrizols VM technology manager. He says Asteric polymers are favored in markets like Japan, where OEMs recommend very low viscosity grades. Visger notes that Asteric is also well suited for automatic transmission fluids and rear-axle lubes where lower-viscosity products can provide added fuel economy.
Darmstadt, Germany-based Evonik also has a new patented PMA technology, which Thomas Schimmel, product manager, hydraulic fluids, describes as having a comb structure. He says it too provides excellent fuel economy performance in lower viscosity fluids such as Japanese engine oils and ATF.
Of great interest these days is how future engine oil specifications will impact V.I. improver design. Carlo Rovea, Infineum Internationals segment manager for VM and pour point depressants, points out that the coming upgrades of gasoline-fueled engine oil (GF-6) and heavy-duty diesel oil (PC-11) both are heavily emphasizing fuel economy.
The lubricant itself provides for some level of fuel economy, but just as important, it must protect the engine against wear, reminds Rovea, who is based in Savona, Italy. Viscosity modifiers play a key role by allowing formulators to meet viscometrics in order to maximize fuel economy and deliver engine protection in all climates. Different viscosity modifiers can deliver best-in-class shear stability and low-temperature pumpability and wear protection. The ideal viscosity modifier would also allow for significant formulating flexibility, in order to optimize base stock selection.
However, Lubrizols Dan Visger cautions that the new oils need for both better shear stability and lower viscosity may provide less blending cushion, resulting in a narrower formulating window. As well, tougher stay-in-grade requirements for heavy-duty engine oils are being investigated for PC-11, he says. This could lead to use of more shear-stable VMs in those oils compared to todays market.
Infineums Rovea agrees that the formulating window is getting tighter, especially compared to todays most common SAE viscosity grades. Viscosity modifiers may be very critical in meeting some very tight formulation windows, and in order to deliver maximum fuel economy while still protecting the engine.
Alex Boffa, Oronites technical team leader for V.I. improvers, doesnt antici pate that drastic changes will be needed for the bulk of the market. We think any new V.I. improver designs will not impact the core GF-6 or PC-11 products but rather be targeted to specialty products, like the fuel-conserving version of PC-11.
In addition, Boffa continues, we believe the pressure on fuel economy will drive the need for more polymer options, especially for PC-11. Formulators will want more choices – another reason we believe Paratone to be positioned well. The need for cost-effective solutions may impact V.I. improver designs, so we also believe that economics may play a part in driving improved performance.
Evoniks Thomas Schimmel points to other applications, beyond the automotive crankcase, where V.I. improvers are being challenged. While the objective of fuel economy is shared by driveline, engine oil and hydraulic fluid applications, the concepts and technology addressing fuel economy are fundamentally different among them. It is low viscosity and high V.I. for driveline and engine oils; it is high viscosity and high V.I. for hydraulic fluids.
Transmission hardware is also becoming more diverse, leading Lubrizol to observe that ATFs may become more specific to transmission type. Result: Even more splintering of the transmission fluid market than seen today.
Drivers for automatic transmissions include longer drain intervals, higher power density, smaller sumps, better efficiency, and greater durability. All of these have an impact on the V.I. improvers, especially durability and power density.
Lubrizol cites gear lubricants as another application where more shear-stable V.I. improvers could result in better fuel efficiency. They might also allow for the use of API Group III base stocks, as opposed to polyalphaolefins, for such grades as SAE 75W-90 or possibly SAE 75W-85.
Both Lubrizol and Evonik say they are delving into the possibility of using lower molecular weight, highly branched V.I. improvers at higher dosages for areas such as gear lubricants for stationary engines and industrial applications. These products promise much improved shear stability as well as greater efficiency and reduced operating temperatures in critical equipment such as wind turbines.
Lube Report Asia occasionally includes articles originally published in sister publications of LNG Publishing Co. This article was originally published in September 2012 issue of LubesnGreases – Volume 18, Issue 9 – under the headline The Never Ending Puzzle of Flow.