Performance and viscosity index explained.
Parameter that characterizes how a liquid responds to temperature changes.
Making sure the viscosity modifier is used in the most cost-effective way.
The most common application is for passenger car and heavy-duty trucks.
The PARATONE brand is an industry leader in viscosity modifiers with heritage dating back to the 1930s. The PARATONE product history includes the creation of the first olefin copolymer (OCP) viscosity modifier in 1970 and major innovations ever since. Our products are designed to provide robust, low-temperature performance, high-thickening efficiency and extended shear stability.
In 2018, Oronite’s Gonfreville, France, additive plant will start blending PARATONE viscosity modifier concentrates.
We’ve recently qualified six new third-party blenders around the globe, expanding our reliable international supply chain.
Developing next generation viscosity modifier products to meet ever changing market needs. Our PARATONE 24EX viscosity modifier is an excellent example.
Two primary roles of engine oils are to lubricate moving engine parts and reduce the friction and wear of metal surfaces. To perform these functions well, engine oils must exhibit acceptable viscosity characteristics throughout a broad range of engine operating temperatures and shear environments. Lubricants can achieve this by inclusion of viscosity modifiers.
Viscosity is a measure of a liquid’s resistance to flow. A liquid that has a high viscosity is described as “thicker” or “heavier” while a liquid that has a low viscosity is described as “thinner” or “lighter.”
Viscosity index (VI) is a commonly used method of indicating a fluid's change of viscosity in relation to temperature. Engine oil formulators rely on viscosity modifiers to enhance a formulation’s viscous response to temperature. The terms viscosity index improvers and viscosity modifiers are used interchangeably within the industry.
Generally speaking, the viscosity of any liquid is sensitive to temperature. As temperature increases, viscosity decreases, and vice versa. Viscosity Index (VI) is a parameter that characterizes how a liquid responds to temperature changes. Specifically, it describes the degree to which viscosity changes between the temperatures of 40°C and 100°C. An engine oil formulation that exhibits a large viscosity change between these two temperatures has a low VI number, while another oil having a less dramatic viscosity increase will have a higher VI number.
While the science can be complex, viscosity is important because it impacts engine lubricant properties such as oil film thickness (critical for protecting engine parts in high temperature engine environments) and low-temperature pumpability (needed to protect engines during starting in cold climates). The ability to tailor lubricant viscosity is also important in meeting government-mandated fuel economy targets.
Viscosity modifiers are made from polymers, which are long and flexible molecules used in the production of a diverse range of products, including electrical wire coating/insulation, automobile trim, roofing tiles, coatings, paints, rubbers and lubricant additives. When polymer coils interact with oil and each other, they become increasingly resistant to flow, which means we can add them to oils to increase their viscosity.
To make sure the viscosity modifier is used in the most cost-effective way, polymer thickening efficiency (TE) is important. TE describes the boost in kinematic viscosity at 100°C of an oil following the addition of a specific amount of polymer. Oronite calculates a unit-less TE, which ranges from about 0.5 to 4.0. A polymer having a high value of TE indicates that it is a potent thickener.
TE is primarily a function of polymer chemistry and molecular weight. Large molecules are better thickeners than small ones and, at the same molecular weight, some polymer chemistries are better thickeners than others. There is a trade-off, though. While large molecules are good thickeners, they are also more easily broken, which impacts the shear stability of the oil. A viscosity modifier polymer’s shear stability index (SSI) is defined as its resistance to mechanical degradation under shearing stress.
Viscosity modifiers are used in multi-grade engine oils, automatic transmission fluids, power steering fluids, gear oils, greases and certain hydraulic fluids. By far, the most common application is for passenger car and heavy-duty trucks. Over 80% of all viscosity modifiers sold in the lubricant market globally are used in these applications.
Recognizing that oil viscosity was a critical lubricant performance parameter, in 1911 the Society of Automotive Engineers (SAE) created a graduated system to classify motor oil viscosity in a way that would be easy for the consumer to understand. Today, the SAE International® organization continues to update and maintain the SAE J300 Engine Oil Viscosity Classification system, which identifies – for both hot and cold conditions – specific grades and their associated viscosity limits.
Viscosity modifiers are used by engine oil formulators to ensure their multi-grade lubricant products meet the viscosity requirements of the desired SAE J300 grades (e.g., 5W-20, 15W-40).
All engine oils must deliver “in-grade” viscosity performance throughout the engine’s operating range. To achieve this, engine oil formulators rely on viscosity modifiers to deliver the required viscosity performance in both low-shear and high-shear environments while exposed to a wide range of lubricant temperatures – very cold to very hot. The automotive industry has adopted several key tests specifically to quantify an engine oil’s performance over a broad range of temperature and shear conditions.
When low shear is encountered at low temperature in the oil sump and lines that carry oil from the sump to the engine, viscosity modifiers must deliver needed viscosity control. Oil that is too thick at these conditions can cause oil starvation.
At low-temperature/high-shear conditions, high shear is encountered in the engine bearings – high viscosities here can result in too much resistance to engine cranking and failure to start the car.
The traditional high-temperature/lowshear measurement is kinematic viscosity at 100°C (kV100C). This defines the oil’s SAE hightemperature grade. Hightemperature, low-shear conditions are seen in leak paths (oil seals, behind piston rings), and a viscosity that is too low can affect oil consumption.
The high-temperature/high-shear (HT/HS) viscosity test which is run using oil heated to 150°C, measures viscosity and indicates the oil film thickness that might be encountered in bearings, cams, etc., under severe high-speed operations. An oil that is too thin under these conditions may not provide the needed lubricant protection, which could result in significant wear in these critical engine parts.
The use of viscosity modifiers in lubricant oil compensates for the poor temperature response of base oil alone, which tends to get thinner at high temperatures and thicker at low temperatures. A flexible polymer molecule dissolved in the lubricating oil improves its temperature response by attenuating changes in viscosity through changes in size of the polymer itself.
At low temperatures, the polymer coil energy is reduced and it becomes small. Its impact on the flowing oil is therefore less and its contribution to the oil’s viscosity at low temperature is small. When the oil is heated, the polymer molecule expands. A larger coil volume impedes the free movement of the oil more than a small coil, which helps to prevent a decrease in viscosity. The thickening impact of the polymer on the oil’s viscosity at high temperatures is therefore greater than the impact at low temperatures, leading to the “viscosity index improver” effect.
During routine engine operation and through continued use, engine oils are exposed to more extreme shearing mechanisms that break down the polymer molecules, reducing the oils molecular weight. This can lead to viscosity loss and a subsequent decrease in oil film thickness. If too severe, this can cause undesired friction and engine wear with oils that are not formulated with the proper viscosity characteristics.
Under no shear/no flow conditions, the polymer coil is roughly spherical in shape. As the oil begins to flow, the flexible polymer coil reacts to the velocity gradient within the oil. The coil deforms (becomes elongated) and becomes aligned to the direction of flow. The distorted coil impedes the oil’s flow less than the original spherical coil did, and the oil’s observed viscosity falls. This is known as “shear-thinning” behavior. When the shear stress is removed, the distorted coil resumes its original spherical shape and the oil’s viscosity returns to its original value. This shear thinning is therefore termed “temporary viscosity loss.”
In a permanent viscosity loss scenario, the polymer undergoes physical breakage that cannot be reversed when the shear is removed. Consequently, the oil’s viscosity is permanently reduced. The Kurt Orbahn Diesel Injector Test is a frequently used test to quantify permanent shear stability. It measures the permanent reduction in an oil’s viscosity after (commonly) 30 cycles through the test apparatus.
The oil’s viscosity falls during the test due to polymer coil breakage. In other words, only that part of the oil’s viscosity, which is contributed by the viscosity modifier polymer, is susceptible to breakage. Neither the base oil nor the additive performance package suffers permanent viscosity loss. Moreover, different viscosity modifier polymers have different shear stability characteristics, depending on the molecular weight and chemical nature of each. Those viscosity modifiers having higher molecular weight have a greater propensity for polymer coil breakage.
A viscosity modifier polymer’s Shear Stability Index (SSI) is defined as its resistance to mechanical degradation (polymer coil breakage) under shearing stress. Example: An oil is formulated with base oil of viscosity 5 centistokes (cSt) and a viscosity modifier is used to increase its viscosity to 15 cSt. The viscosity modifier’s viscosity contribution is therefore 10 cSt. During the shear test, the oil’s viscosity falls to 12 cSt. It has permanently lost 3 cSt of viscosity. The viscosity modifier polymer’s shear stability index (SSI) is therefore 3 cSt (loss) divided by 10 cSt (viscosity modifier contribution), or 30% SSI.
Viscosity modifiers are available across a range of the SSI, and oil formulators choose the appropriate viscosity modifier product that allows them to meet their finished oil performance and marketing needs.
Different lubricant shear rates are commonly encountered in different parts of the engine. The graph below shows how the shear rate can affect the polymer’s viscosity contribution for a typical oil. Both high operating temperatures and high shear rates cause viscosity reduction resulting in reduced oil film thicknesses. Viscosity retention of multi-grade oils in field service is an important performance characteristic since adequate oil film thickness is required for engine wear protection.