In our last few posts, we’ve been exploring the differences between Newtonian and non-Newtonian fluids and how they behave both when measuring viscosity and when we use them in our coating processes. In this segment, we’ll take a look at why we might choose between these different behaviors for a few common processes.
Lubrication applications are well-suited for Newtonian fluids. Recall that these are fluids that do not change viscosity as a function of shear, even though they do change viscosity as a function of temperature. Motor oil is a great example, say 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 the winter), but doesn’t get too thin at higher engine operating temperatures.
The key here is that it doesn’t change viscosity as it is sheared between the parts in the engine. This is important if it is going to do its job, separating the parts from one another and reducing friction and wear. If the viscosity fell as a function of shear, it would be easily displaced and the metal parts would destroy one another!
Coating applications, on the other hand, are well-suited for non-Newtonian fluids. These fluids change viscosity both as a function of shear and as a function of temperature. Obviously, we can heat or cool the coating to change its viscosity. But the shear-thinning property of modern coatings serves other purposes in the application process.
Take a spray process, for example. In our segment “How Newton Affects Your Coating Process” (click here if you missed it or want to review) we described the spray gun orifice as “the ultimate shear device”. Adding shear at this point in the dispense causes thinning of the coating. This is good as when it hits the part, it is still at a low viscosity, which helps it to flow out for good even coverage and surface finish. Then, as the fluid recovers from the shear of the nozzle, its viscosity increases, helping to hold it in position as it goes through the curing process to be set forever.
Temperature – Shear – Deposition Rates – Recovery Time…
This is an interplay of a host of chemical and process variables, all working together to determine the outcome of the coating process. As we noted in that article, “It can be very confusing!”
Glues and adhesives are an extreme example of the coatings scenario we just described. They generally start at a much higher viscosity, but are subject to both shear and temperature. Often, we both heat and shear them to get them to flow through our dispensing system and onto our parts. But then, we want them to stay in position while we assemble the components and not “squish out” to create a mess. They also have to provide the desired bond strength over the life of the product. Here, placement and viscosity are just as important as in the coating process, and the flow out properties of our fluid requires as much or even greater care if we are to be successful!
Temperature and Shear are Co-Conspirators
So, as we can see, there are times and purposes where both Newtonian, and non-Newtonian fluids are advantageous. The key is to know when, and where, and how to make the best use of these properties.
It’s no wonder it takes a team of chemists and engineers to put it all together and make it all work right!