Measuring accurately matters in woodworking and finishing. There are many things that need to be measured accurately for optimal finishing. Last month’s blog covered measuring catalyst and temperature accurately. This month viscosity is covered.
Viscosity, according to merriam-webster (https://www.merriam-webster.com/dictionary/viscosity), is “the property of resistance to flow in any material with fluid properties”. The higher the viscosity the higher the resistance to flow, or the thicker the fluid.
Viscosity is critical to optimal finishing. The correct viscosity allows the coating to atomize properly through spray equipment, flow out on the substrate, and through evaporation and or thixotropy (viscosity changes due to shear or lack of shear) hang on the part without runs or sags. The viscosity increase through evaporation of the solvents keeps the coating from running or sagging. Resistance to runs and sags can also be increased by the use of thixotropic additives. Some 100% solids UV coatings rely solely on thixotropic additives.
Coatings manufacturers formulate the coatings at specific viscosities and test the coatings produced to ensure they are consistent. Part of testing the viscosity is measuring the temperature the viscosity is tested at. Coatings’ viscosities are dependent on temperature. Last month’s blog covered the importance of measuring temperature accurately. The higher the temperature the lower the viscosity and the lower the temperature the higher the viscosity. This relationship between viscosity and temperature means, to accurately test the viscosity, the temperature must be accurately measured. When setting a specification for viscosity the coating is adjusted to a standard temperature. Standard ambient temperature of 77 degrees F is often used.
Consistent finishing requires consistent viscosity. The viscosity can be affected if the coating you are using is higher or lower than 77 degrees F. The more viscosity is affected the more likely problems will occur. Often in the winter, finish shops are cooler than 77 F. This means the finish is less than 77 F, unless the coating is warmed. When the finish is cool the viscosity is high and many finishers add thinner to reduce the viscosity to compensate for the increased viscosity. Summer brings temperatures above 77F and means the viscosity is lower. While we can’t easily remove, or want to remove, solvent from the finish, retarder might be used to compensate for the coating drying quicker due to the temperature increase. The more the temperature of the coating is controlled the more the viscosity is controlled yielding more consistent results.
Each coating has a viscosity specification. Coating manufacturers test the viscosity in a variety of ways. The only way to know the viscosity is to measure it. One of the most common measurements of viscosity is with a viscosity cup. There are a variety of viscosity cups available. Each has a specific volume of coating it will hold and a specific size hole in the bottom. The viscosity is how many seconds it takes a coating to pour completely through the hole. A reading might be 25 seconds in a Zahn #2 cup also shown as 25” Zahn #2. Remember the temperature matters when taking a reading. Always note the coating temperature when measuring the viscosity. It might look like 25” Zahn #2 at 77F. Be sure to check the coating at the temperature it is being used at and check it at the temperature the coating manufacturer used in setting the viscosity specification to verify that the coating is still the same as when it was produced. Measuring the viscosity and temperature will allow you to be proactive in any adjustments needed to prevent problems from occurring.
Always consult your finish manufacturer for more information on checking the coating’s viscosity.