Viscosity improvers for lubricants: how to select the right VII based on application, stability, and performance
Viscosity is one of the most important properties of a lubricant. It defines the oil’s ability to form a film between moving surfaces, influences friction, affects energy consumption, and determines wear protection. However, in a real formulation it is not enough to achieve a specific viscosity at 40 °C or 100 °C. The challenge lies in maintaining proper performance across the entire operating temperature range of the lubricant.
Viscosity improvers, also known as viscosity index improvers, VI improvers, VII, or viscosity modifiers, are polymeric additives designed to reduce the change in oil viscosity as temperature varies. Their main function is to help the lubricant maintain sufficient fluidity at low temperatures while preserving an adequate lubricating film when temperature increases.
This technology is especially relevant in multigrade lubricants, high-viscosity-index hydraulic oils, gear oils, transmission fluids, engine oils, greases, and other industrial formulations where viscosity stability has a direct impact on performance.
Àlex
Andrés
| Bases | PAO |
|---|---|
| PAG | |
| Esteres | |
| PIB | |
| Esteres Carboxilicos | |
| Siloxanos | |
| PFPE | |
| Esteres Fosfóricos | |
| G-I, G-II, G-III | |
| Aditivos | Antidesgaste/Extrema presión(AW/EP) |
| Antioxidantes | |
| Inhibidores de Corrosión | |
| Pasivadores de metales | |
| Modificadores de fricción | |
| Antiespumantes | |
| Filantes | |
| Anti-niebla | |
| Paquetes | |
| Agentes "coupling" | |
| Polímeros | Mejoradores del índice de Viscosidad (VII) |
| Depresores punto fluidez (PPD) | |
| Espesantes | |
| Grasas | |
| - | |
| Alquilsulfonatos | Sulfonato sódico |
| Sulfonato cálcico | |
| Sulfonato Magnesio | |
| Aminas | ALCANOLAMINAS |
| Aminas | |
| Polieter Aminas | |
| Aminas Multifuncionales | |
| Lubricantes sólidos | Grafito |
| DISULFURO de Molibdeno | |
| Nitruro de Boro (BN) | |
| Cerflon | |
| PTFE | |
| Tensioactivos | Alcoxilatos |
| Co-Polimeros Block | |
| Alcoholes etoxilados | |
| Ácidos grasos etoxilados | |
| Esteres etoxilados | |
| Aminas etoxilados | |
| Esteres de sorbitan | |
| Esteres de sorbitan etoxilados | |
| Esteres de fosfatos | |
| Baja espuma | |
| Poliméricos | |
| Agentes solventes (coupling) | - |
| Alcoholes grasos | Alcoholes grasos |
| Ácidos grasos | Ácidos grasos |
| Ceras | Ceras |
| Agentes de superficie | Agentes de superficie |
Viscosity and viscosity index: two concepts worth distinguishing
Kinematic viscosity indicates a fluid’s resistance to flow under the influence of gravity. In lubricants, it is typically measured at 40 °C and 100 °C using standardized methods such as ASTM D445. These values allow oils to be classified, ISO VG or SAE grades to be assigned, and different formulations to be compared.
The viscosity index describes how viscosity changes with temperature. A high viscosity index indicates that the oil loses less relative viscosity as temperature increases. This is important because the lubricating film largely depends on the fluid maintaining sufficient viscosity under operating conditions.
In industrial applications, a high viscosity index can help stabilize lubricant performance in equipment exposed to thermal fluctuations. In automotive applications, it enables the formulation of oils capable of performing both during cold start and under high temperature, load, and speed conditions. For this reason, VII should not be considered merely as “thickeners,” but as additives that modify the oil’s viscosity–temperature profile.
How viscosity index improvers work
Viscosity improvers are oil-soluble polymers. Their effect depends on the hydrodynamic volume they occupy in the fluid, their molecular weight, their molecular architecture, and their interaction with the base oil. These factors determine how much the polymer contributes to viscosity increase and how it behaves under temperature and shear conditions.
At low temperatures, the polymer tends to adopt a more compact conformation. Under these conditions, its effect on viscosity is limited, which helps maintain lubricant fluidity. This characteristic is important for cold starts, mobile hydraulic systems, transmissions, and equipment operating outdoors.
At high temperatures, the polymer occupies a larger volume within the oil and increases resistance to flow. In this way, it partially compensates for the natural loss of viscosity of the base oil. The result is a lubricant with reduced viscosity variation between low and high temperatures.
The effectiveness of this mechanism does not depend solely on the amount of polymer added. It is also influenced by the modifier’s chemistry, mechanical stability, compatibility with the base oil, and the presence of other additives in the formulation. A VII may improve viscosity index, but if it does not withstand shear well or negatively affects low-temperature behavior, it may not be the best choice for the application.
Key properties in selecting a viscosity improver
The selection of a viscosity modifier should be based on several technical parameters. Focusing only on final viscosity can lead to formulation errors, especially in lubricants subjected to high loads, wide temperature ranges, or continuous mechanical stress.
The first parameter typically evaluated is thickening power. This concept describes the polymer’s ability to increase the kinematic viscosity of the base oil at a given treat rate. It depends on molecular weight, polymer structure, solution volume, and compatibility with the medium.
A polymer with high thickening power may allow the use of lower treat rates to reach the target viscosity. However, this advantage must be assessed together with shear stability. In some cases, a highly efficient polymer may suffer greater permanent viscosity loss during service. For this reason, the real cost should not be calculated only per kilogram price, but in terms of treat cost and final performance in the formulation.
Another critical parameter is temporary viscosity loss. Under high temperature and high shear conditions, polymer chains may align in the direction of flow. This temporarily reduces the polymer’s contribution to viscosity. When shear conditions cease, the polymer returns to its original conformation and the effect is reversible.
In engine oils and other demanding applications, this behavior is related to HTHS viscosity (High Temperature High Shear). HTHS measures lubricant viscosity at high temperature and high shear rate, typically at 150 °C. This value is directly related to the oil’s ability to maintain a lubricating film under severe conditions.
A lower HTHS can help reduce friction and improve fuel efficiency, but it may also reduce film thickness if not properly matched to the application. A higher HTHS improves protection under load, although it can increase frictional losses. The choice depends on the balance between efficiency, protection, technical specifications, and equipment type.
| Oil Type / Target Application | Recommended HTHS (mPa·s @150°C) |
| Maximum protection / High load engines | ≥ 3.5 |
| Balanced protection & fuel efficiency | 2.9 – 3.2 |
| Maximum fuel economy | 2.6 – 2.9 |
| Ultra fuel economy (e.g. 0W-16) | 2.3 – 2.6 |
Permanent viscosity loss is another key aspect. It occurs when polymer chains are mechanically broken due to shear forces. Unlike temporary viscosity loss, this phenomenon is not reversible. The lubricant permanently loses part of its viscosity and may fall outside the intended operating range during service.
Mechanical stability is commonly expressed through the Shear Stability Index (SSI). The lower the SSI, the higher the polymer’s resistance to shear-induced breakdown. This parameter is especially important in high-pressure hydraulic oils, gear oils, transmission fluids, engine oils, and greases subjected to mechanical stress.
Low-temperature performance must also be evaluated from the early stages of formulation. A viscosity modifier may improve viscosity index, but if it negatively affects pumpability, cold start behavior, or pour point, it can create issues in the final application. For this reason, VII selection should be coordinated with base oil choice and, when necessary, with the use of pour point depressants.
Types of viscosity improvers
There are different families of viscosity modifiers. Each one offers a different balance between thickening power, shear stability, low-temperature performance, compatibility, cost, and final application.
| Chemistry | Application | Advantages | Disadvantages |
| Polyisobutylene (PIB) | Greases, tackifier packages / adhesive agents | Excellent tackiness. Good thickening capability. | Temperature sensitive. More complex handling. |
| Olefin copolymer (OCP) | Cost-sensitive formulations, engine oils | Excellent thickening efficiency. Good cost-performance ratio. | Limited shear stability. Poor low-temperature performance. |
| Ethylene-propylene oligomer (EPO) | Gear oils, hydraulic fluids, high-performance engine oils | High thermo-oxidative stability. High shear stability. Low treat rate and good cost-in-use. | Higher cost vs standard OCP. Formulation sensitivity: solubility and compatibility. |
| Polymethacrylates (PMA) | Gear oils, hydraulic fluids, ATF | High viscosity index improvement. High shear stability. Good low-temperature performance. | Requires higher treat rates. |
| Hydrogenated styrene-diene copolymer (SBR/HSD) | Gear oils, premium engine oils, greases | High viscosity index improvement. Excellent thickening efficiency. Very high shear stability. | Premium price positioning. |
- OCP: efficiency and competitive cost
OCPs, or olefin copolymers, are ethylene-propylene copolymers. They are widely used because they offer a good balance between cost and thickening efficiency. In many formulations, they allow the target viscosity grade to be achieved with a competitive treat cost.
This family is common in engine oils and can also be used in industrial lubricants when the formulation requires a balance between performance and cost. Their versatility depends on polymer structure, ethylene content, supply form, and the base oil used.
Their main limitations appear when the application requires high shear stability or excellent low-temperature performance. In these cases, other technologies such as PMA, EPO, or styrenic polymers can provide a more robust response, although at a higher initial cost.
- PMA: viscosity control and good low-temperature performance
PMA, or polymethacrylates, stand out for their ability to improve viscosity index and their good low-temperature performance. They are a particularly interesting family in hydraulic fluids, ATF, gear oils, and formulations where pumpability and viscosity index are priorities.
In hydraulic oils, PMAs can be a preferred option when a high viscosity index and good cold-start performance are required. This combination is useful in mobile machinery, equipment exposed to temperature changes, or systems requiring precise hydraulic response.
Their main drawback is usually the higher treat cost compared to more economical options such as OCP. However, in applications where low temperature performance, stability, and precision are critical, cost must be evaluated in relation to final fluid behavior.
- PIB: viscosity, tackiness, and film persistence
PIB, or polyisobutylene, provides thickening, oil compatibility, and a strong tackifying effect that is highly valued in certain formulations. It is not used only for its contribution to viscosity, but also for its ability to improve lubricant persistence on surfaces.
This family is common in greases, gear oils, tackifier packages, process oils, and formulations where higher adhesion or reduced runoff is required. In applications involving open gears, chains, cables, or exposed surfaces, this property can be particularly useful.
Its use as a primary viscosity improver in high-performance multigrade engine oils is less common. In these cases, technologies with a better balance between viscosity index, shear stability, HTHS, and low-temperature behavior are typically preferred.
- EPO: stability and high-performance formulation
EPOs, or ethylene propylene oligomers, are liquid viscosity modifiers based on ethylene-propylene structures. They can offer a good combination of viscosity control, mechanical stability, thermo-oxidative stability, and ease of handling.
This family is used in gear oils, hydraulic fluids, ATF, greases, and high-performance lubricants. In certain formulations, EPOs allow low shear viscosity loss while maintaining a good balance between efficiency and stability.
- Styrenic polymers: efficiency and stability under severe conditions
Hydrogenated styrene-diene, styrene-isoprene, and other styrenic architectures are used when high thickening efficiency and strong shear stability are required. They may be presented in diblock or star structures, each providing a different performance profile.
Their main limitation is typically cost. However, when the lubricant must maintain viscosity over long periods or under demanding mechanical conditions, the technical value can justify a more specialized solution.
Common mistakes when selecting a viscosity modifier
Uno de los errores más frecuentes es seleccionar el modificador de viscosidad únicamente por su poder espesante. Alcanzar la viscosidad objetivo con una dosis baja puede parecer una ventaja clara, pero no siempre garantiza estabilidad durante el servicio. Si el polímero tiene baja resistencia a la cizalla, la viscosidad final puede caer después de pocas horas de trabajo.
Otro error común es comparar productos solo por precio por kilo. En formulación, el dato más útil suele ser el coste por tratamiento efectivo y por rendimiento conseguido. Un producto de mayor precio puede ser más competitivo si permite mejorar la estabilidad, reducir dosis, evitar reformulaciones o cumplir especificaciones más exigentes.
También es importante no seleccionar el VII de forma aislada. El modificador debe ser compatible con el aceite base y con el paquete de aditivos. Antioxidantes, aditivos antidesgaste, extrema presión, inhibidores de corrosión, detergentes, dispersantes y antiespumantes pueden influir en el comportamiento final del sistema.
Por último, conviene validar siempre la formulación en condiciones representativas. Los datos de viscosidad inicial son necesarios, pero no suficientes. Ensayos de cizalla, baja temperatura, estabilidad oxidativa, compatibilidad y rendimiento específico permiten confirmar si la selección del VII es adecuada para la aplicación final.
Practical criteria for selecting the right VII
Selection should start from the application and not from the chemistry. Before choosing between OCP, PMA, PIB, EPO, or styrenic polymers, it is essential to define what the lubricant must do in service.
The first step is to establish the target viscosity at 40 °C and 100 °C, the desired viscosity index, and the low-temperature requirements. After that, shear conditions, operating temperature, load, type of equipment, and expected service life must be evaluated.
The base oil must also be considered. A Group I, Group II, Group III, PAO, ester, or other synthetic base can significantly affect polymer solubility, treat response, and low-temperature behavior. The polymer architecture and the diluent used in the concentrate can also influence compatibility.
As a general guideline, OCPs are typically suitable when thickening efficiency and cost competitiveness are priorities. PMAs perform well when low-temperature behavior and high viscosity index are critical. PIBs are useful when tackiness or film persistence is required. EPOs offer a balanced combination of stability and performance. Styrenic polymers are usually reserved for formulations where shear stability and efficiency justify a premium solution.
The role of technical support in formulation
The selection of a viscosity improver should not be based solely on a property table. The same chemistry can behave differently depending on the base oil, treat rate, additive package, and final application. For this reason, technical support is an important part of the selection process.
At Lumar Química, we help formulators of industrial and automotive lubricants select raw materials according to the technical objective of each formulation. In the case of viscosity index improvers, this involves evaluating target viscosity, viscosity index, SSI, HTHS, low-temperature performance, compatibility, and real operating conditions.
A well-selected viscosity improver enables the formulation of more stable, efficient lubricants that are better adapted to the application. It is not only about increasing viscosity. It is about controlling how viscosity evolves during service and ensuring that the lubricant maintains its protective function under the conditions for which it was designed.
