The continued drive for lower emissions and higher fuel efficiency places an ever-greater technical burden on lube formulators. Additives such as pour point depressants play increasingly important parts in achieving this aim.Boris Kamchev presents one additive companys findings on the role these substances have.
The trend toward ever-lower-viscosity oils is pushing the technological boundaries for additive formulators and lube manufacturers. Lower-viscosity oils are increasingly demanded by original equipment manufacturers as regulators enforce stringent standards for lower carbon dioxide emissions and better fuel economy.
There are a number of physical and chemical technologies that can achieve better fuel economy and low emissions. Each of them contributes to at least a 1 percent reduction in CO2 emissions, according to one industry insider.
Lubricants and low rolling resistance tires are the most cost efficient means of reducing CO2 emissions, Yulia Aleksanina, technical support manager at German specialty chemicals company Evonik, told the RPI Global Lubricants conference held in Moscow in October.
Aleksanina added that the highest costs are associated with making significant changes to the powertrain and that there are limitations on how far each technology can reduce emissions. These include low-friction design and materials, lightweight components and improvements to aerodynamics.
However, the fuel economy potential for something like lubricants or tires is less than that of other technologies such as hybridization or downsizing, she said. Based in Moscow, Aleksanina coordinates Evoniks Russian operations.
That said, the development of this new generation of motor oils is made possible by the use of high-quality base oils and additives packages that in many cases contain pour point depressants.
All mineral base oils contain some wax that emerges from solution at low temperatures. PPDs prevent waxy structures from forming, which can inhibit lubricant flow. This is especially critical when ensuring lubricants reach all parts of an engine quickly in cold climates. Efficient lubrication reduces wear and fuel consumption on start-up.
PPDs can be either alkylaromatic polymers that adsorb wax before it forms crystalline networks or polymethacrylates that co-crystallize with the wax to prevent crystal growth, according to ExxonMobils Lubrication Fundamentals.
The movement toward lower-viscosity lubricants has resulted in the introduction of new oil specifications, according to Evonik. The SAE J300 standard incorporated SAE 16 in 2013, as well as SAE 12 and SAE 8 in 2015. In 2017, SAE 0W-16 oils were introduced into the API SN standard, allowing this viscosity grade to be marketed with the API service donut to consumers, Aleksanina explained.
Evonik also found that the market shift toward lower-viscosity fluids takes time.
Before the market share of 0Ws can grow, newer vehicle makes and models that are capable of using lower viscosity engine oils must replace older vehicles that specify higher viscosity grades – this shift may take longer in certain world regions, she said, adding that it is expected that 0Ws – some of the lightest viscosity passenger car motor oils currently on the market – will constitute more than 20 percent of the global PCMO market by 2024, including the North American market, which accounts for roughly 25 percent of the worlds total demand for PCMOs of all grades.
The German lube additive maker also found that growth of 0W-XX engine oils will be at the expense of higher viscosity grades and as such, the 5W-XX market is expected to remain relatively flat while the 10W-XX market will shift toward 5W-XX.
Aleksanina said that oil formulators should be ready for formulation impacts when moving from SAE 5W-20 to SAE 0W-16 oils.
They must rebalance the viscosity and volatility properties of their formulation. This can be a difficult task because low-viscosity base oils often have higher Noack volatility. As such, API Group III oils are necessary to balance and meet CCS [cold cranking simulator] and Noack requirements of a 0W-16, she observed.
The Noack volatility test determines the evaporation loss of lubricants in high-temperature service. The CCS test uses a viscometer that measures torque on a rotating spindle, thus measuring the fluids shear stability.
Aleksanina also said that there is a common misconception that Group III base oils do not require PPD since they are highly refined and very uniform in composition.
However, this is not necessarily the case. Group III oils can display a wide range of low-temperature properties depending on the type and manufacturer. Just a small amount of the proper PPD can improve the performance of both Group III and Group III+ base oils, she contended, adding that in traditional Group III oils, just 0.1 percent of PPD eliminated yield stress and substantially reduced viscosity in a mini rotary viscometer test at minus 40 degrees Celsius.
The difference is less pronounced in Group III+, but nevertheless an improvement is observed, she said.
Question of Choice
Formulators often want to know which PPD to use for specific base stocks. However, base stocks are not always effective predictors of the low-temperature performance of a formulation, according to Evonik.
Even though the base stock is often the driving force behind selecting a PPD, choosing the additive solely based on its performance in a base stock may not ensure that the PPD will work in the final formulation. Other formulation components also exhibit crystalline behavior at low temperatures, Aleksanina observed.
Evonik found that additives such as viscosity index improvers or friction modifiers may also exhibit crystalline behavior at low temperatures.
The V.I. improver likely exhibited strong crystalline behavior at low temperatures that PPD could not mitigate, resulting in yield stress, Aleksanina said.
Base stocks do influence the selection of PPD, according to the additive maker. Aleksanina presented an example of a formulated SAE 5W-20 and varied the base stock while keeping the detergent inhibitor and V.I. improvers the same.
Four unnamed PPDs were tested. Surprisingly, Evonik found the Group II-derived SAE 5W-20 exhibited the least performance variation with the different PPDs, and all four PPDs produced passing results. In the Group III-based formulation, only the second and fourth PPD were effective. This may be accounted for by the fact that while Group III oils contain little to no wax, they are poorer solvents for the relatively small amount of wax within a formulation, which may result in stronger physical bonds.
PPD number four was the only PPD to meet the requirements of all three base oil mixtures, Aleksanina revealed.
The additive maker also said that V.I. improvers can have an impact on PPD selection.
In this example, we have a 0W-16 formulation containing three different V.I. improver types with the detergent inhibitor and base stock mixture held constant. All PPDs had no issues meeting the MRV viscosity requirement, but yield stress is another case.
Aging oil can dramatically change the low temperature performance of a lubricant. As the oil ages, it oxidizes and volatilizes, resulting in increased viscosity, generation of polar species and concentration of wax species, she said.
Evonik also found that one PPD can span multiple viscosity grades when selected properly. The right PPD not only treats the wax in a base oil, it also helps mitigate the crystalline behavior of formulation components such as additive packages, friction modifiers and V.I. improvers.
Aleksanina concluded by emphasizing that the selection of PPD remains an important element in the formulation of low-viscosity lubricants.
The low-temperature mechanism of a PPD remains the same regardless of the type of formulation or the viscosity grade. It also still relies on modification through co-crystallization and steric hindrance. Also, the selection of a PPD is heavily influenced by the formulation components and low-temperature requirements, she said. (Steric hindrance is the prevention or stopping of a chemical reaction.)
Base stocks, V.I. improvers and the additive package all influence low-temperature performance, according to Evonik.
Despite formulation differences between a 0W-16 and a 5W-20, one PPD can satisfy the demands of both SAE grades. However, formulators should engage in more considerations to choose the correct PPD, she said.
As base stock improvers, PPDs and new additive technologies are now needed more than ever to meet the evermore stringent environmental regulations of the future.