Viscosity Index Modifiers
The viscosity index (VI) is a characteristic commonly used to indicate the viscosity of a fluid at different temperatures. The smaller the viscosity variation, the higher the VI. Viscosity index modifiers are able to modify this property, helping to maintain adequate lubricity at different temperatures, which is a fundamental characteristic of lubricants.
Viscosity is the property that determines the shear strength of a fluid.
It is one of the most important properties in lubricants, as it is responsible for determining the oil film thickness, improving the system’s lubricity and reducing friction and wear, which avoids unscheduled maintenance.
Furthermore, viscosity influences heat generation in some systems; determines the sealing effect of an oil and consequently its consumption rate; and it also defines how smooth machines will start working in low temperature conditions.
Viscosity can be determined by measuring the force required to overcome fluid friction in a film of known dimensions. When measured this way, it is called the dynamic or absolute viscosity, which is usually reported with the unit Poise (P).
The kinematic viscosity can be obtained by dividing the fluid dynamic viscosity value by its density, when measured at the same temperature. The most used unit for this property is centistokes (cSt).
Kinematic viscosity can be obtained with the aid of calibrated glass capillary viscometers, by measuring the time required for a specific volume of fluid subjected to gravity to flow through the capillary. One of the most recognized reference methodologies for this analysis is ASTM D445.
The measurement of Saybolt viscosity is also made with the aid of a viscometer, which has a cylindrical section and an orifice at the bottom. Viscosity is obtained by measuring the time it takes for 60ml of fluid to flow into another container. The method that describes this analysis is ASTM D88 – 07 and the result is reported with the unit according to Universal Saybolt (SSU).
Viscosity Index (VI) is the property of a fluid that quantifies the relationship of viscosity with temperature. A low VI indicates a greater difference in this relationship, while a high VI indicates a relatively small change, which is important so that the lubricity of the equipment is not affected at different temperatures.
The VI of mineral oils produced through the conventional refining route varies depending on the refinery technology, and may be below 0 and even slightly above 100. Hydrotreated mineral oils and some synthetic oils may have a VI above 120. The VI is considered to be very low when it is less than 0, low between 0 and 40, medium between 40 and 80, high between 80 and 120, and very high above 120.
The VI is calculated based on an empirical scale created by Dean and Davis, which has standards with two series of oils: one arbitrarily considered as VI equal to 100 and another designated as VI equal to 0.
For the VI of an oil to be calculated, it is necessary to measure its viscosities at 40°C and at 100°C. Then, the calculation is made from its viscosity at 40°C and the viscosities at 40°C of the standards, having a viscosity at 100°C equal to that of the oil whose VI is sought to be determined, according to the formula below:
L = viscosity at 40°C of an oil of the VI=0 standard, having the same viscosity at 100°C as the oil under study;
H = viscosity at 40°C of an oil of the VI = 100 standard, having the same viscosity at 100°C as the oil under study;
U = Viscosity at 40°C of the oil under study.
In the ASTM D2270 method it is possible to consult the tables for determining the VI of Dean and Davis based on kinematic viscosity or Saybolt viscosity at 40°C and 100°C.
Viscosity Index Modifiers
Viscosity index modifiers are long-chain, high molecular weight polymers that work by providing a higher relative viscosity increase of an oil at high temperatures than at low temperatures. This is usually due to the way the polymer changes its physical configuration as the temperature of the mixture increases.
It was observed that the polymer molecule takes on a contracted shape at low temperatures, as the oil’s solvent power is lower, causing the polymer molecules to mutually attract each other.
At high temperatures, however, the oil’s solvent power increases, causing the polymer molecules to unwind and increase in volume. The interaction of oil with these long molecules causes an increase in the viscosity of the mixture.
The main compounds used as viscosity index modifiers are:
- Polyisobutene (PIB);
- Vinyl-acetate copolymers;
- Olefin copolymers – ethylene-propylene, styrene-butadiene, etc;
- Alcoholic polystyrenes
All of these polymers have a linear structure and the choice of which to use will depend on the final application of the finished lubricant.
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