APPLICATIONS, TESTS, AND MEASURES OF VISCOSITY LOSS IN SHEAR STABILITY OF MULTI-GRADE, POLYMER-THICKENED LUBRICANTS


Lubricants are substances or fl uids that are used to reduce the friction between mechanical components that come in contact [1]. Lubricants serve many different purposes, a leading one being their usage in vehicles. Lubricants are vital in automobiles due to the importance of properly lubricating engines as they operate at high speeds and temperatures. When mechanical components of an engine rub against each other, a great amount of friction and heat can be generated. With the use of lubricants, an engine can function for a prolonged period of time. A well-lubricated engine experiences a reduction in friction from wear and tear due to contact stresses between the moving surfaces [2]. This reduction increases effi ciency and fuel economy. Therefore, the automobile user saves money and gas as the engine is properly lubricated. Conversely, devoid of the lubricant, the engine parts can be damaged and fused due to high contact temperatures generated by the frictional heating of the moving components.


In order to lubricate and lubricate effectively, the lubricant needs to have an acceptable and preferably optimized viscosity, which is a measure of the fl uid’s ability to fl ow. Viscosity is a function of temperature. In this regard lubricants can be classifi ed into two types: single-grade and multi-grade [3]. Both types contain packages of performance additives such as detergents, dispersants, antioxidants and anti-wear agents depending on the application. Single-grade lubricants have a viscosity optimized for a particular temperature range, usually summer and winter grades. They have to be changed with each season in temperate climates. Multi- grade lubricants work well at both high and low temperatures since they are thinner at low and thicker at high temperatures, relative to single-grade lubricants [4]. Multi-grade lubricants are made with either special, and usually expensive, synthetic base fl uids, or more commonly with mineral oils thickened with oil- soluble polymers. Multi-grade, polymer-thickened lubricants are composed of polymer thickeners and a light base stock lubricating oil. The polymer in the lubricant thickens the fl uid, increasing the viscosity. An important property to consider when manufacturing multi-grade, polymer-thickened lubricants is their shear stability since different kinds of formulated lubricants are used depending on the vehicle component being lubricated and the shears encountered in it. The type of polymer, its molecular weight, and its treat rate is selected to meet initial, fresh oil viscosity grade requirements and also to account for any degradation of the polymer chains so that the oil “stays in-grade” during service or testing. This degradation of the polymer results in the loss of viscosity at the operating conditions and temperatures at which the engine functions and reduces the strength of the oil fi lms formed between the components of the engine. In addition, the


PIN April / May 2022


Figure 1. Graphical defi nition of viscosity.


loss of viscosity affects the ability of the lubricant to remain in high load areas and reduces the strength of the oil fi lm. Minimizing shear degradation is critical to minimizing the viscosity loss of the lubricants. It is also essential to provide the oil’s contribution to fuel economy while still maintaining an adequate oil fi lm strength as those are critical parameters of a lubricant. Manufacturers of multi- grade, polymer-thickened lubricants test their engine protecting performance by subjecting them to extensive performance tests including tests to determine their viscosity loss and shear stability.


Shear stability is a measure of how resistant an oil or lubricant is to change in its viscosity under applied mechanical stresses and shears such as those generated during the operation of a running


engine [5]. Viscosity is defi ned as the ratio between applied shear stress and rate of shear (Figure 1). It is also called the coeffi cient of viscosity or dynamic viscosity. The shear rate is defi ned as the velocity gradient perpendicular to the direction of fl ow. In the example of a lubricant in a mechanical system, such as piston rings and piston walls, or journal bearings, the shear rate is the difference in velocity at the two surfaces divided by the distance between the two surfaces. The unit of shear rate is inverse time, typically s-1. Shear stress is the force per unit area in the direction for the fl ow. When a mechanical stress is induced, for example by the operation of a running engine, thinning of the lubricant may occur. This thinning results from the reduction of viscosity. As


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