A Viscosity Modifier three-part series on the fundamentals of viscosity modifiers, chronicling the benefits of viscosity modifiers for engine oil performance, the demand for engine oils that can help improve fuel economy and reduce emissions, and to take an in-depth look at changing trends in high performance engine oils specific to heavy duty diesel vehicles. This first article provides a detailed understanding of viscosity, how viscosity is controlled through diverse viscosity grades for engine oils, and how viscosity is measured.
What is Viscosity?
Viscosity is the measurement of a fluid's resistance to flow. Whenever one layer of a fluid slides across another layer of the same fluid, there is always a level of resistance. When the magnitude of this resistance is high, the fluid is considered to have high viscosity and is generally a thicker fluid, such as honey. When the resistance of the fluid to flow is low, the fluid is considered to have low viscosity and is generally a thinner fluid, such as olive oil. Because the viscosity of many fluids will change with changes in temperature, it is important to consider the performance attributes that a fluid must exhibit from one temperature extreme to another for specific applications.
Viscosity for Engine Oils
Engine oils must lubricate critical components at all temperatures within an engine's normal operating range. Low temperatures tend to thicken engine oil making it more difficult to pump. If the lubricant is slow to reach key engine parts, oil starvation can cause excessive wear. Also, cold, thick oil can make cold starting difficult due to viscous resistance. Conversely, heat tends to thin an engine oil, and at extremes, can reduce the ability of the oil to provide an adequate protective coating on critical parts. This can lead to premature wear and mechanical failure of piston rings and cylinder walls. The trick is finding the right viscosity balance of thickness and fluidity - and viscosity modifiers are the solution. Viscosity modifiers, which are polymers specifically designed to help control the viscosity of a lubricant over a specified temperature range, help the lubricant provide adequate protection and fluidity.
Adjusting Viscosity with Polymers
Anyone recalling their high school chemistry knows that a polymer is a large molecule, composed of many repeated sub-units known as monomers. Natural polymers such as amber, rubber, shellac, silk and wood are all part of our everyday lives. Artificially-made polymers first came into common usage in the 1930s, for synthetic rubber and nylon stockings. By the 1960s, the benefit of adding carbon-based polymers, often referred to as viscosity modifiers, to engine oil was recognized. Throughout this period, Lubrizol has been a leader in polymer chemistry for passenger car and truck engine oil, proposing, researching, testing and confirming its benefits through the efforts of employees all over the world.
Today, viscosity modifiers (VMs) are key ingredients in most engine oils. Their role is to help lubricant blenders achieve desired viscosity (rheological) properties, mainly reducing the tendency of a lubricant's viscosity to change when subjected to temperature fluctuations. This is effectively achieved through the use of defined viscosity grades.
Simply put, a viscosity grade signifies an oil's thickness, or viscosity. There are two types of viscosity grade: mono-grade and multi-grade. Mono-grade oils, such as SAE 30, are generally designed to provide engine protection at normal operating temperature, but can lack fluidity at colder temperatures. Multi-grade oils commonly use viscosity modifiers to achieve more flexibility and can be identified by a viscosity range, such as SAE 10W-30. The letter "W" designates that an oil has been tested to perform in both cold weather as well as at normal engine operating temperatures.
To further understand viscosity grades, it's helpful to use examples. Because multi-grade oils are the standard engine lubricant for most cars and light and heavy-duty trucks worldwide today, we'll start there. SAE 5W-30 multi-grade viscosity grade engine oil, the most widely used passenger car engine oil viscosity grade in North America, operates as a SAE 5 viscosity grade in the winter, and as a SAE 30 viscosity grade in the summer. The 5W (W stands for winter) designates a thinner oil, and makes cold temperature starting easier. Oil quickly flows to all parts of the engine and fuel economy is improved because there is less viscous drag from the oil on the engine. The 30 part of SAE 5W-30 offers the protection of more viscous (thicker) oil for high temperature protection during summertime driving by keeping the oil from excessive thinning where metal-to-metal contact occurs within the engine.
Heavy-duty diesel engine oils currently are higher SAE viscosity grade than passenger car engine oils. Globally, the most widely used SAE viscosity grade is SAE 15W-40, which is more viscous (thicker) than SAE 5W-30 both in the winter (the 5W and 15W part of the viscosity grade designation) and summer (the 30 and 40 part of the viscosity grade specification). In general the higher the SAE viscosity grade number, the more viscous (thicker) the oil.
While mono-grade oils, such as SAE 30 and 40 grades, are still common in some markets, they do not contain polymers to modify the viscosity with temperature. The use of multi-grade engine oil, containing viscosity modifiers, allows the consumer to enjoy the dual benefits of ease of oil pumping and starting while maintaining high temperature protection from excessive engine oil thinning. Also, unlike mono-grade engine oils, the consumer does not need to worry about changing from a summer grade to winter grade oil with seasonal temperature fluctuations.
Polymer Performance Beyond Just Thickening
For decades, Lubrizol testing and research has demonstrated the effectiveness of VMs to improve efficiency, cleanliness and low temperature performance of lubricating oils, all the while providing durability and protecting equipment from severe wear. Dating back to the 1970s, Lubrizol holds hundreds of patents on VMs, and its scientists have contributed many hundreds of papers to the SAE and other professional organizations on the benefits of VMs. Lubrizol is the acknowledged global leader in this field.
Types and typical usages of VMs include:
- Polyisobutylene (PIB) was the predominant VM for engine oil 40 to 50 years ago. PIBs are still used in gear oils due to their outstanding load carrying characteristics. PIBs have been replaced by Olefin Copolymers (OCP) in engine oils due to their superior cost effectiveness and performance.
- Polymethacrylate (PMA) polymers contain alkyl side chains that interfere with the formation of wax crystals in the oil, providing excellent low-temperature properties. PMAs are used in super fuel economy engine oils, gear oil and transmissions fluid formulations. Generally, they have a higher cost than OCPs.
- Olefin Copolymers (OCP) see extensive use in engine oils due to their low cost and satisfactory engine performance. Many OCPs are on the market, differing in molecular weight and the ratio of ethylene to propylene content. OCPs are the dominate polymer used for viscosity modifiers in engine oils.
- Hydrogenated Styrene-Diene Copolymers (SBR) are characterized by fuel economy benefits, good low-temperature properties, and superior deposit control performance compared to most other polymers.
- Hydrogenated Radial Polyisoprene polymers have good shear stability at relatively low treatment rates compared to some other types of VMs. Their low-temperature properties are similar to OCP.
The lubricant industry has established and perfected laboratory tests that can measure viscosity parameters and predict how viscosity-modified engine oils will perform. Tests conform to ASTM standards and Lubrizol has in-house capabilities to run all the tests necessary to develop and evaluate viscosity-modified engine oils. For the complete listing of engine oil viscosity specifications refer to Table 1 in SAE J300, revised April 2013.
Kinematic viscosity is the most common viscosity measurement used for engine oils and is the measure of resistive flow of a fluid under the influence of gravity. Kinematic viscosity has been traditionally used as a guide in selecting oil viscosity for use under normal operating temperatures.
Capillary viscometers measure the flow rate of a fixed volume of fluid through a small opening at a controlled temperature. One common test is a high-pressure capillary viscometer, which is used to simulate the viscosity of engine oils in operating crankshaft bearings to measure high temperature high shear (HTHS) viscosity. HTHS can be related to engine durability in high-load severe service applications and to engine oil contribution to fuel efficiency.
Rotary viscometers measure a fluid's resistance to flow by using the torque on a rotating shaft at a constant rotational speed. One type is the Cold Cranking Simulator (CCS). This test measures viscosity at low temperatures to simulate engine start-up at low-temperature. Oils with high CCS viscosity can make it difficult for the starter motor in a vehicle to turn the engine over.
Another common rotary viscometer test is the Mini-Rotary Viscometer (MRV). This test studies the ability of an oil to pump after a specified thermal history that includes warming, slow cooling, and cold soaking cycles. MRV are useful in predicting engine oils prone to field failures under conditions of slow cooling (overnight) in cold climates.
Engine oil is sometimes evaluated by measuring the pour point (ASTM D97) and cloud point (ASTM D2500). The pour point is the lowest temperature where movement is observed in the oil when the sample in a glass tube is tilted. Cloud point is the temperature where a cloud from wax crystal formation is first observed. These methods are no longer used and have been replaced with specifications for low-temperature pumping and cranking and gelation index.
There are consequences to changes in viscosity that can be harmful to engine operation such as excessive wear, low oil pressure, high oil consumption, difficult starting, and low or no flow during cold starts. The impact of viscosity on engine performance and how viscosity modifiers enhance performance will be further discussed in detail in part 2 of this series.