In this article, we’ll go further in depth on how to understand the term viscosity when it comes to grease, particularly associated with PFPE grease. Before reading this article, it’s highly recommended that you read our article; Introduction to Grease, if you are new to grease.
Units for Viscosity
When it comes to industrial chemicals, the kinematic viscosity or sometimes called thickness, is often expressed in two ways; Centistokes (cSt) or mm²/s. 1 cSt = 1mm²/s.
When looking at grease spec sheets, you will most likely see a section called Viscosity Index. This is overall viscosity of the particular grease. Take one of our best-selling PFPE grease for example, and look for the viscosity index, you’ll quickly realize it’s measured in cSt.
Moreover, the viscosity index is directly determined by how viscous or thick the grease is, and that is due to the thickener or soap used to form the actual grease. Every grease, to begin with is actually base oil, which is later mixed with a thickener or soap and the amount or formula used is what will determine the overall viscosity index of the final product.
Something to take note of is that every type of PFPE grease, regardless of model of viscosity, it is always thickened by PTFE (Polytetrafluoroethylene), which is a fluoropolymer and mostly comes in the form of nano-sized powder. The reason why PTFE is used for PFPE grease, is due to the fact that PFPE base oil, won’t bond with any other thickener, so PTFE is the only substance compatible with PFPE oil.
Base Oil Viscosity
This can sometimes be a bit confusing for some, as base oil viscosity and overall viscosity index has nothing to do with each other. As covered in the previous section, the overall viscosity is for the finish product, but the base oil/fluid viscosity is the viscosity of the actual base oil used to make the grease.
In most PFPE greases, the base oil viscosity is somewhere between 15 and 500 cSt or mm²/s. This range might seem quite broad, but that is to take into consideration the wide range of applications PFPE grease can be used for. As different applications require different base oil viscosities, imagine a car door as supposed a fast spinning bearing in a machine, they both require PFPE grease, but the applications and performances are completely different. The main reason why different base oil viscosities are needed, is that PFPE base oil can sometimes start to separate, sometimes called bleed, if the viscosity is not right for the particular application.
The higher viscosity base oil does not equate to higher overall viscosity index, as that is entirely up to the amount of thickener and different formulas used to create the PFPE grease. Usually higher viscosity base oil is used for PFPE greases that are used for more slow moving applications, and lower viscosity base oils for more extreme and fast moving applications. Another huge factor is the temperature range in which the application operates in, but all that will be covered by the specification sheets of any PFPE grease.