Oil as a Lubricant: Viscosity and other Properties
When people think of oil, motor oil is one of the first things to come to mind. The light brown, transparent quality of motor oil when it is fresh offers a stark contrast to its dark black, opaque quality when it has been used and needs to be changed. Why do we use oil as a lubricant and why does it degrade over time such that it must periodically be replaced? As we will see, lubrication is an essential part of any moving machine and certain petroleum fractions have special properties that make them valuable lubricants.
What is a lubricant?
Most people are probably aware that a lubricant is used to reduce friction between two moving surfaces. It also functions in transporting contaminants and other foreign particles away from moving parts so that they do not due damage. These particular properties are true of any lubricant whether it is used in an engine, on a door, or even on the human body. As it turns out, there are a few other properties that make for a good lubricant.
- Reduces Friction
- High Boiling Point and Low Freezing Point
- High Viscosity
- Thermal Stability
- Corrosion Retardation
- Resistance to Oxidation
We’ll treat each of these properties in turn, but will take them slightly out of order, leaving viscosity for the end.
Lubricity – Reducing Friction
The term lubricity means that a substance reduces friction. The customary way to measure lubricity is to determine how much wear occurs when two moving surfaces are separated by a thin film of the substance being tested as a lubricant. Traditional tests include “Ball-on-three-discs” and “Ball-on-cylinder.” In both of these cases, the wear that occurs between two moving parts under varying conditions in the presence of a given substance is compared to the wear when either no lubricant is present or when a known lubricant is present. The ability of a substance to prevent wear (scuffing, grooving, etc.) is then quantified and used to rate lubricity.
What is essentially happening in all of these cases is that the lubricant is providing a layer of material between the moving parts that prevents them from having too much contact. Because the lubricant is more “slippery” than the components it is applied to, it reduces friction. A reduction in friction leads to less wear on components, easier movement, and reduced energy needs of the system. In fact, some well-designed engine oils can actually increase a car’s fuel economy.
High Boiling Point and Low Freezing Point
All this says is that for a lubricant to be effective it cannot boil away or freeze solid. In either case, the lubricant would cease to reduce friction. In the first case, it would simply disappear and leave the moving parts to grind upon one another. In the second case, it would actually increase friction, perhaps to the point of preventing movement altogether. In motor oils, these two properties are explained through viscosity numbers like 10w – 40, which will be discussed more in the last section of the article.
Friction generates heat, so for a lubricant to be successful, it needs to retain its lubricity even when it gets hot. If a lubricant lacks thermal stability, then it begins to break down when it gets hot, which leads to increased friction. This problem is sometimes seen in car engines where it is known as “sludging.” Sludging is a process whereby oil becomes sticky as a result of repeated exposure to hot engine conditions. In extreme cases, sludge can “gum-up” an engine leading to failure and even “seizing” in which the pistons are unable to move against the walls of the cylinders and thus the engine cannot function.
Lubricants help to stop or prevent corrosion by coating components in a thin layer that protects them from exposure to oxygen and other “oxidizers” that can cause chemical reactions to occur that lead to damage to the surface of a material. The best known example of corrosion is rust. Rust occurs when iron is exposed to oxygen. Rusting can be increased by exposure to water and salt (as anyone who lives in a snowy region where salt is applied to the roads will know).
Water is present in the atmosphere and salts are often contaminants of fuels. To protect engine components from these substances, motor oil, coats the sides of cylinders, pistons, and other moving engine parts. This coating prevents water, oxygen, salt, and abrasive substances from coming into contact with the surface of the steel (which contains iron).
Resistance to Oxidation
This is similar to thermal stability except that it applies to chemical reactions rather than to the breakdown of the lubricant due to heat. Like any chemical, lubricants can undergo chemical reactions that change the structure of molecules. For the most part, such reactions are undesirable because they lead to a change in the properties of the lubricant such that it is less effective at reducing friction and preventing corrosion.
Structure of Petroleum that Creates a Lubricant
Before getting to viscosity, it is worth taking a look at what fractions of petroleum best fit the above requirements and why. To begin, we need a substance that is slippery enough to reduce friction. If you have ever felt mineral oil or motor oil between your fingers, you know that it is very slippery. Why is that?
Why Oil is Slippery
Explaining why oil is slippery requires a look at its chemical properties. First, oil is non-polar, which means it does not have a positive or negative charge. Some molecules, like water, have a “charge distribution,” which means the molecule acts almost like a battery, part of it has a positive charge and part of it has a negative charge. The result, because positive is attracted to negative and vice versa, is that water and other “polar” molecules stick to each other. Oil doesn’t have this problem, so one oil molecule can slide past another more easily than one water molecule can slide past another.
Adding to the slipperiness of oil is its tendency to form distinct layers through forces called Van der Waals forces, or more specifically London Dispersion forces (a type of Van der Waals force). These forces, which are the weakest known in science, can help old things together, which would increase friction. However, oils have the unique property of forming forces only within layers because the molecules are essentially planar. Planar just means that molecules are flat as the diagram below emphasizes and only take up space in two dimensions rather than three. Without projections to attach to, forces can only be distributed within the plane and so there are no forces to bond one layer to the next. Thus, two layers of oil don’t bond to one another to any great degree.
In point of fact, oxidation of oil produces carboxylic acid derivatives, which ARE polar. Thus, oxidation damages the viscosity of motor oil and is one of the reasons why an oil that contaminants are unwanted and eventually degrade the quality of oil.
Why Oil Doesn’t Freeze
Next, we need a substance that is resistant to both boiling and freezing. To achieve both of these properties, a substance will need to remain a liquid over a wide range of temperatures. Lubricating oils, as it turns out, have an average chain length of 36 carbon atoms, this gives them a very high boiling point (somewhere around 300 degrees Celsius or 572 degrees Fahrenheit). The lack of polarity in the molecules (they don’t carry a positive or negative charge anywhere) combined with the variation in size of hydrocarbon molecules (the average is 36, but the range is from roughly 18 to 44 carbon atoms) prevents the molecules from forming any kind of repeating, regular pattern needed for freezing. Of course, oil does flow easier or harder depending on temperature, which we will discuss shortly in the section on viscosity.
Viscosity is a complicated term that refers to how well a fluid resists being deformed. In other words, how well a fluid resists shear and tensile stresses determines how viscous it is. A highly viscous fluid is difficult to deform and changes shape only slowly. Examples of such fluids would be honey. On the other hand, water has a low viscosity and flows easily.
Viscosity is affected by temperature. The hotter a fluid is, the easier is flows and the cooler it is, the more difficult it is to deform. A good lubricant needs to be fluid enough at low temperatures to be able to move through an engine (or other system) to provide protection. A good lubricant must also retain its viscosity at high temperatures so that it does not flow out of the engine or become so thin so as to be no more effective than water as a lubricant. The best lubricant is the one that can provide separation between moving parts without being so “sticky” so as to require a great deal of energy to move the parts. A scale referred to as the viscosity index provides a measure of the change in viscosity of a substance with temperature.
The viscosity index (VI) ranges from 0 to above 110, with 0 to 35 representing low viscosity and anything over 110 representing high viscosity. The VI was created by the Society of Automotive Engineers (SAE) and oils are tested between 38 and 99 degrees Celsius (or 100 and 210 degrees Fahrenheit). These values, however, are not what are reported on motor oil. Rather, these numbers are converted into a rating system that uses one or two numbers and often takes the form of 10W-40 or 0W-40 or something similar. So what do the numbers in this system mean?
The number that includes a W, which stands for “winter,” means that the oil was tested at a lower temperature than above. Here is how it works.
The number without the W represents the viscosity of the oil at 99 degrees Celsius (210 Fahrenheit). An SAE of 20 translates into a VI value of 5.6 to 9.3. An SAE of 30 would translate to a VI value of 9.3 to 12.5. So, the higher the number, the more viscous the oil will be at 99 degrees Celcius. This is the “hot number” for oil.
The number with the W is tested at a lower temperature and this is not always consistent across the board. The temperature is reported somewhere on the bottle, but is usually around 30 degrees Celsius. The VI value obtained is then translated into a number followed by a W. The value 0W, for instance, indicates that at 30 degrees Celsius, the oil behaves as a 30 oil would behave at 100 degrees Celsius. In other words, it is as thin at 30 degrees as a 30 oil is at 100 degrees.
Obviously, a lower number for the W is better because it means the oil flows well at low temperatures. Because oil drains out of the higher parts of an engine as it sits, running a cold engine is when the most metal to metal contact occurs and thus the most wear. By similar reasoning, a higher “non-W” number is desirable because it indicates that the oil retains its ability to protect an engine at higher temperatures by retaining its ability to coat parts and not get “too thin.” Most new cars use 5W-30. Some synthetic oils are able to attain ranges like 0W – 40, which offers outstanding protection and improved fuel economy.