In OilChat 97 we discussed rheology in general. In this issue we will address the rheological behaviour of lubricating oil.
The most important rheological parameter of liquid lubricants is viscosity, hence viscosity is reported on all oil specifications and product data sheets. It also affects the tribological properties like friction and wear between moving surfaces in contact with each other.
The smooth and reliable operation of machinery and mechanical components depends on consistent lubrication at the contact area between moving surfaces. To achieve this, the lubricating oil must be of Newtonian nature to maintain a robust film to separate the moving surfaces. The viscosity of Newtonian oils does not change with shear/stress or rate of flow. Most monograde lubricating oils (mineral and synthetic) can be classified as Newtonian.
Having said that, monograde oils may become non-Newtonian in service. A typical example is diesel engine oil that is loaded with soot (partially burned fuel). When the soot concentration in engine oil reaches a level that can no longer be dissolved by the dispersant additive, the soot particles clump together to increase the viscosity of the oil. High viscosity may result in cold-start problems and risk of oil starvation. Once the oil is circulated by the oil pump the soot particles are dispersed in the oil and the viscosity will decrease, but when the engine is shut down the oil will thicken up again. In this instance the rheological behaviour of the oil is thixotropic.
Multigrade oils may also exhibit non-Newtonian behaviour when subjected to stress. Multigrade oils are formulated with Viscosity Index Improvers, also referred to as Viscosity Modifiers. These are normally long chain polymers that expand with increase in temperature to improve the “thickening characteristics” of the oil at elevated temperatures. The rheological behaviour of these polymers is described below:
Fig 1: When the oil temperature is low, the polymers curl up into tight balls that flow readily with the oil molecules and with no effect on the oil viscosity.
Fig 2: As the temperature increases, the polymers expand into large stringy coils that restrict the normal oil flow, which has a thickening effect on the oil. When the oil cools down, the polymers return to their original shape. The result is that when these polymer based additives are blended in the correct proportion with for example SAE 15W base oil, the oil flows like a SAE 15W at low temperatures and similar to a SAE 40 oil at high temperatures. The outcome is a SAE 15W40 multigrade oil that will provide adequate protection over a wide temperature range. It should be noted that there is actually no SAE 40 base oil in a SAE 15W40 formulation.
Fig 3: The viscosity of most multigrade engine oils decreases with increase in the rate of oil flow or shear. This is because the ‘viscous grip’ of the oil on the expanded polymers is reduced with increased stress which causes distortion and rotation of the polymers. The decrease in viscosity of these non-Newtonian multigrade oils is recovered when the shear rate is reduced and the phenomenon is called Temporary Viscosity Loss or TVL.
Fig 4: When the oil is violently sheared, the polymers may be stretched beyond their ability to accommodate the motion of the oil and one or more of the polymers may be broken. Consequently, the viscosity of the oil-polymer solution decreases permanently because the viscous contributions of the broken, shortened polymers are less than that of the original long chain polymers before degradation. In contrast to TVL, this polymer degradation phenomenon is called Permanent Viscosity Loss or PVL since, in this case, the viscosity lost is not recovered and the rheological behaviour of the oil is non-thixotropic. In the early days of multigrade oils, PVL was a factual problem but with modern, shear-stable polymer technology it is unlikely to occur if the oil is used for the intended application. If, however, a modern multigrade oil is used incorrectly, PVL may still occur. A typical example is when a multigrade engine oil is used in a heavily loaded gear system.
Q8Oils offer a comprehensive range of shear stable multigrade oils for a wide variety of automotive, commercial, construction, mining, agricultural and other applications. For more information about the complete range of Q8 lubricants, phone 011 462 1829, email us at info@bcl.co.za or visit www.bcl.q8oils.co.za.