The viscosity of a lubricant is determined by its base oils and its additives. One of these additives is the viscosity modifier. The secret to stabilizing the viscosity of the oil lies in the structure of the viscosity modifier, which is capable of increasing the viscosity index of the engine oil. This unique property is obtained at the molecular level: the molecules contained in the viscosity modifiers expand and contract their protuberances when the temperature changes. When the temperature rises, the molecules begin to expand and stick to each other to stabilize the viscosity of the liquid. The molecules contract when the temperature drops, allowing other molecules to move more freely.
A new generation of viscosity modifiers has arrived on the market. These advanced additives offer many advantages over previous generations of viscosity modifiers. They derive these benefits from their revolutionary form. The ends of the molecules force the molecules to expand in several directions, thus optimizing the space occupied. New generation viscosity modifier molecules contract more efficiently, allowing other molecules to flow more freely than ever. By combining compact contraction and extended expansion, these products widen the operating temperature range of engine oils.
This structure improves shear stability because the increased amount of ends reduces the impact of high pressures. When an older generation viscosity modifier succumbs to pressure, its effectiveness is seriously affected. The molecule’s capacity for contraction and expansion is reduced and the engine oil loses its viscosity. New generation viscosity modifiers remain fully effective under high pressure. When one of the ends breaks, the others are put back in place to compensate for the lost protrusion. This improved shear stability provides extended lubrication intervals and ensures optimal lubrication for longer under difficult conditions.
These two improvements allow mix designers to seek new opportunities. First of all, it is now possible to improve the fluidity of engine oils at low temperatures without compromising the protection of the engine at operating temperature. Customers who live or work in extremely cold regions will no longer have to sacrifice the protection of their engine at operating temperature to benefit from a quick cold start.
Second, the different combinations possible between the new viscosity modifiers and the base oil groups open up a new window of opportunity. It is now possible to use lower grade synthetic base oils mineral base oil in combination with an innovative viscosity modifier, instead of having to use a very expensive higher-grade synthetic base oil.
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.
Lubricants are essentially needed to minimize metal-to-metal contact or friction within the engine. Lubricants with additives and friction modifiers play an important role, as they reduce the friction among the moving components contained in the engine. The appropriate application of friction modifiers can also make a vital contribution to the sturdiness of vehicles. The molecules of these modifiers provide a cushion, as one of the coated surfaces comes in contact with another coated surface.
While the manufacturers are trying hard to enhance the fuel economy, innovative varieties of engine oils having lower viscosity are appearing. Viscosity modifiers play an important role in protecting the engine. Fuel economy and low viscosity are closely connected. But, there is not one specific grade of oil for all engines.
Achieving optimum fuel efficiency involves more than just using friction modifiers and lower viscosity oils. The ultimate formulation needs to appropriately balance a number of components that work together to retain durability, and simultaneously reduce the overall friction.
Generally speaking, the viscosity of any liquid is sensitive to temperature. As temperature increases, viscosity decreases, and vice versa. Viscosity Index 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.
Thickening efficiemcy 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 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 cars and heavy-duty trucks. Over 80% of all viscosity modifiers sold in the lubricant market globally are used in these applications.
TEMPERATURE AND SHEAR OPERATING REGIMES
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.
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 the 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 temperatures 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.