Viscosity modifiers are a component of most modern engine oils. These are often times thought to be a required component of multigrade viscosity oils, but that is not necessarily the case. Multigrade oils achieve their performance due to their viscosities at low and high temperature, and although a viscosity modifier may contribute to that property, it is neither necessary nor automatic that multigrade oil has one in the formula. You can read more about that in Viscosity & Viscosity Grades. But just what is a viscosity modifier?
Let’s start by going over some of the common attributes most of them share:
Viscosity modifiers attain their abilities through some basic functions of their molecular structures and how that structure is arranged. Put into basic terms, the molecules become smaller at low temperatures making their viscosity increase at a lower rate than most oils, and conversely they expand at higher temperatures making their viscosity decrease at a lower rate than most oils. This property gives them an extremely high viscosity index (VI) compared to base oils and so they are sometimes also referred to as VI improvers.
There is a subset of viscosity modifiers known as pour point depressants (PPD) which are often put into a class of their own. However I personally group them with viscosity modifiers since they do modify the viscosity of oil at extremely low temperatures. However, they rarely affect an oil at high temperature because they are used at very low treat rates in comparison to VI improvers. Their function is to reduce the point at which an oil solidifies or its pour point. They work by disrupting the formation of wax crystals and molecular matrices from forming solid structures within the oil.
Viscosity modifiers come in a number of different chemistries and each one has its benefits and detriments.
The benefits that a viscosity modifier imparts on a complete oil formula are typically lower low temperature viscosity and higher high temperature viscosity (ie. better viscosity index) than would be possible with only petroleum or synthetic base oils. Depending on the rest of the formulation, they may also assist in additive solubility and may contribute to load carrying ability due to their potential contribution to film strength.
One fairly common detriment to most viscosity modifiers is their cost. They are significantly more expensive than most base oils including most synthetics, so their use is somewhat limited due to cost restraints.
An often touted, but sometimes misunderstood, property of viscosity modifiers is their shear stability. Some polymer types are able to withstand shearing forces and do not shear any significant amount. Other types are quite vulnerable and will lose a significant amount of their original viscosity due to shear. This shear resistance is completely dependent on the chemical structure of the polymer including its shape and the strength of the chemical bonds.
Viscosity shear is a property exhibited by polymer additives and viscosity modifiers in particular are very prone to it. The most commonly misunderstood concepts are what is capable of shearing, and how shear actually occurs. I talk to many people that are under the impression that base oils are capable of shearing. However, except for some extremely high viscosity PAOs that are fairly uncommon, base oils do not shear any measurable amount. The other thing about shear and how it occurs is that it is a mechanical/physical process. When a molecule is sheared it is literally sliced in two by two surfaces that come in contact with one another. This often occurs in gearboxes and transmissions because gears will typically enter the boundary lubrication regime compared to the hydrodynamic or elastohydrodynamic regimes associated with most engine lubrication. When the gear teeth make physical contact, the clearances are small enough to physically cut a large molecule into smaller ones.
For simplicity’s sake, if you were to start out with an 18 carbon chain polymer:
Then it was to go through a transmission and be sheared one time right down the middle.
Now there are two 9 carbon polymers.
These resulting molecules now exhibit a lower viscosity and less overall flexibility than the original, larger molecule had. A consolation with regards to shearing is that once it is done and the molecules reach a certain size, they are small enough that that are unlikely to shear any further. So there is at least a limit to how much a polymer will shear and you are unlikely to lose all of the beneficial properties it bestows. The shear potential or the maximum shearing a polymer can go through is known as the shear stability index (SSI). The SSI is given as a number between 0 and 100 and refers to the percentage of viscosity lost after a polymer shearing test. There are multiple shearing tests that can be used to determine the SSI and different ones may have a better correlation to specific applications. Commonly used tests to determine are the KRL tapered roller bearing test, diesel injector shear test and a sonic shear test. The sonic shear test is one that may report a temporary shear loss rather than a permanent result. Be wary of those numbers because temporary shear loss is not as important as permanent shear loss.
The most common types of viscosity modifiers used in motor and gear oils are polyisobutylene (PIB), polymethacrylate (PMA) and various copolymers. However there are different variations of those chemistries that will give them different properties even within the same basic chemical family.
PIB offers a high viscosity index and probably has the widest range of viscosities to choose from out of all the viscosity modifiers. There are PIBs almost as thin as water all the way to being basically a solid block of plastic. The way this is possible is simply due to the length of the carbon chain they are made up of. The shorter the chain, the lower the viscosity will be, and vice versa. PIB is a relatively simple compound that does not offer much variety when it comes to chemistries or molecular shape. Due to this configuration of a simple carbon chain, PIB is also highly prone to viscosity shear. One of the benefits to having such a wide viscosity range is that the thinner PIB grades are less prone to shear so some are more suitable than others in motorcycle applications.
PMA is similar in chemistry to acrylic glass commonly known as plexiglass. Some other variations of the PMA chemistry are polyalkyl methacrylate (PAMA), poly(methyl)methacrylate (PMMA). This chemistry can come in liquid or solid forms soluble in oil and can come in many different chemical structures. They can be branched molecules, comb-like, asteric or star polymers or linear chains and can be functionalized to give specific properties.
This structural flexibility
makes PMAs very versatile and different combinations of functional groups and main structures result in a wide range of shear stability. Functional groups may help to contribute to detergency or dispersant performance in a finished lube, so they may even contribute beyond viscometric performance. Several PMA configurations may be used primarily as PPDs or VI improvers or they may be multifunctional and serve both purposes.
Copolymers are a broad categorization of polymers that include chemistries like olefin copolymers (OCP), styrene based copolymers. These types of viscosity modifier incorporate multiple polymer types bonded to one another to form a single unique polymer.
This combination allows copolymers to build a broad range of performance capabilities and have versatile applications. Similar to PMA, copolymers can be used as PPDs or as VI improvers and incorporate properties such as dispersancy, deposit reduction, improved fuel efficiency and many others depending on the polymer combinations. Copolymers are often the most universally compatible viscosity modifiers and are usable in most base fluids with no solubility concerns. Just like PMA, copolymers come in all sorts of molecular structures so the shear stabilities range from very high to very low for different types.
There are other types of viscosity modifiers used, but those three kinds are probably the most widely used as VI improvers and PPDs at this time. Regardless of whether that fact changes, I think it gives you the basic idea of how these additives work and what their purpose is.
Typically the more complex a molecule’s structure is, the more shear stable that viscosity modifier is. This is because the more chemical bonds and the stronger those bonds are, the harder they are to break. With enough pressure any bond will break, but there is a limit to the pressure exerted by the components in an engine and transmission.
In general, straight chain molecules are the weakest when it comes to shear strength. As functional groups and branches are added to chains their shear stability increases. Asymmetric molecules or chemical structures such as the star structure with a central atom are more difficult to shear because they tend to fold onto themselves and prevent a single branch or arm of the star to be singled out under pressure.
Even beyond those basic structures, there can also been combined structures; like a branched star polymer or a combed branched structure and more.
The importance of viscosity modifiers in modern engine and gear oils cannot be understated. Although I said they are not required to formulate multi-grade oils in the beginning of this article, this does not mean they aren’t a crucial part of many high performance products. A good viscosity modifier gives a substantial boost to the inherent performance of the oil alone and offers very little downside. Since viscosity shear is the biggest concern with most viscosity modifiers, selecting a shear stable polymer for the correct application minimizes any detriments the viscosity modifier could have contributed.
Shear can happen in different ways, and different polymers handle those stresses differently. So one that maintains its viscosity in a bearing application may not be the best in a meshing gear system and vice versa. So choosing the right product for the right application is important because the formulators should be taking these considerations into account when putting their products together.