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Ask the Expert Question-and-Answer Archive

by Ron Joseph

October, 2004

Relationship Between Wet Film Thickness and Dry Film Thickness

Q. Do you have an equation that predicts the thickness at which a coating will run based on viscosity, density, surface wettability, etc?

The relationship is very simple. Dry film thickness is directly proportional to the volume solids of the coating. Therefore, if you apply a wet film of 2.0 mils and the coating has a volume solids of 50%, you can expect the dry film thickness to be 1.0 mils. The equation is given below:

Dry Film Thickness (DFT) = Wet Film Thickness (mils) x %Volume Solids

Although the equation is perfectly simple you need to be aware that in the real world the relationship will not necessarily yield accurate results, possibly because of the substrate or because not all of the solvent will be released from the coating during the drying and curing process.

Here is the procedure I would follow: Apply the coating to your substrate at a range of wet film thicknesses making a note of each film thickness and the corresponding dry film thickness. Plot a graph of the relationship between the two for that particular coating on that particular substrate. Bear in mind that the relationship will only hold true if you always use the coating at the same volume solids. If you thin the coating to adjust its viscosity, the volume solids will very from one day to the next and the calibration curve will no longer be valid. If you have several colors and/or different coatings, I would perform the calibration for each one.

You can calculate volume solids from an MSDS, but if you intend to add thinners to adjust the viscosity you will need to know exactly how much was added so that you can perform the calculation. Alternatively, if this is really important to you, it is possible to experimentally determine the volume solids by following the procedures outlined in ASTM 2369. In this method you simply need to purchase some disposable plastic syringes, small disposable aluminum dishes, you will need an electronic balance and a laboratory oven that allows solvents to be directed out of the oven to prevent a fire or explosion. Perhaps you already have some of the equipment, but if not the investment is very small; perhaps a few hundred dollars. The methodology is quite simple and you can easily train your operators to perform the test.

If you do not like this suggestion then perhaps you can try to calibrate each coating by comparing its viscosity with its corresponding dry film thickness. Bear in mind that you will need to measure viscosity more accurately than you would if you were using a simple Zahn cup. In fact, I would probably use Ford 4 cup which is more accurate. In addition, you will always need to measure the viscosity at one specific temperature, because viscosity can change dramatically with changing temperatures.

I hope that this answer is of help. If you need assistance to calibrate your coating by either of the methods discussed above we will be happy to do so for you but will need to charge a consulting fee.

Best wishes,

Ron Joseph

I must have stated the question incorrectly because the answer doesn't fit. My question relates to how thick can a wet coating be (with no drying) before it will start to run on a vertical surface. I'm sure viscosity, density and surface tension are iteracting and I don't know the equation.

I have never seen a mathematical relationship between these parameters. I wouldn't be surprised if an empirical relationship can be derived, but it would need to take many other factors into account. For instance, the method of application will determine how quickly the solvents are released during the application. Suppose you use a spray gun to apply the coating, the atomizing air pressure, fluid flow rate, perhaps also the ambient humidity (in the case of waterborne coatings), gun-target distance, fan size, painter's technique, etc., will play a role in establishing how quickly the coating sets up. If the coating formulation contains a thixotropic agent, this too will affect the result. Air movement in the spray booth can play a role, and I'm sure I can think of other factors that will influence the results.

Experimental data will probably be more reliable than a mathematical approach.

I hope this better fits your question.

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