Why are Newtonian Properties of Fluids Important in Industrial Coatings?

This article focuses on several different liquids, analyzing their behavior in viscosity and coating process.

lubricating oil

Lubrication applications are well suited for Newtonian fluids. Recall that these are fluids that do not change viscosity with shear, even though they do change viscosity with temperature. Motor oil is a good example, such as 10W-30. In this case, the "W" stands for "winter," not "weight," and refers to its viscosity as a function of temperature. In short, it stays fluid at lower temperatures (important if we want to be able to start our car in winter), but not too thin when the engine is operating at higher temperatures.       

The key here is that it doesn't change the viscosity as it gets sheared between the parts of the engine. This is very important to separate the parts from each other and reduce friction and wear if it is to do its job. If viscosity drops with shear, displacement can easily occur and metal parts will destroy each other!   

coating

On the other hand, coating applications are well suited for non-Newtonian fluids. These fluids change viscosity with changes in shear and temperature. Obviously, we can heat or cool the coating to change its viscosity. But the shear-thinning properties of modern coatings play other roles during application.     

For example, take the spraying process. In our section "How Newtons Affect Your Coating Process" (click here if you missed it or want to review it) we referred to the gun hole as the "final shear device". Adding shear here in the distribution results in a thinner coating. This is good because when it hits the part it is still at a low viscosity which helps it flow out for a nice even coverage and surface finish. Then, as the fluid recovers from the nozzle's shear, its viscosity increases, helping to hold it in place for a long-lasting set as it goes through the curing process.       

Temperature - Shear - Deposition Rate - Recovery Time...

It is the interplay of a host of chemical and process variables, all of which work together to determine the outcome of the coating process. As we noted in that article, "It can be very confusing!" 

Glue / Adhesive

Glues and adhesives are extreme examples of the paint scenario we just described. They usually start out with a higher viscosity, but are affected by both shear and temperature. Typically, we heat and shear them so they flow through our dispensing system onto our parts. However, we want them to stay in place when the components are assembled, rather than "squeeze out" to create a mess. They also need to provide the required bond strength throughout the life of the product. Here, placement and viscosity are as important as the coating process, and the outflow characteristics of our fluids require even more attention if they are to be successful!         

Temperature and shear are common conspirators

So, as we have seen, there are times and purposes where both Newtonian and non-Newtonian fluids are advantageous. The key is knowing when, where and how to make the most of these properties. 

No doubt it takes teams of chemists and engineers to bring all of this together and make it all work!