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FAQ'S
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
100% 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.
Coating Density
Q: I am filling out an Air pollution control form for our county. On the form it is asking me for the coating density (lbs/gal). How do I know what the coating density is? The reports we print monthly give me the monthly VOC content used, however, there is no place that shows the density. Hope you can help!
A: The coating density is nothing more than the weight of one gallon of the paint. Every MSDS provides this information, but it doesn't always refer to it as "density". Often, it simply gives you WPG in lbs/gal. WPG = "weight per gallon", which is the same as its density. Some MSDS provide the S. G. (specific gravity) of the coating. S. G. is the ratio between the density of the coating and the density of water. In other words, how much lighter or heavier is one gallon of the coating than one gallon water? We know that the density of water at room temperature is 8.33 lbs/gal (some reference books give it as 8.34 lbs/gal). Therefore, if the S. G. is 1.4 (no units), it is 1.4 times heaver than water, and the density must be 1.4 * 8.33 lbs/gal = 11.66 lbs/gal. Similarly, if the S. G. is 0.97, the coating is 0.7 as heavy as water (it is lighter) and its density is 0.7 * 8.33 lbs/gal = 5.83 lbs/gal.
Referring to the MSDS of all your paints, you should now be able to locate or calculate the density. Q: What will be the checkpoints to ensure a good quality powder coating (including chemical pretreatment)? What should an auditor check during powder coating process audit?
A: In my opinion I would do a thorough check of the surface pretreatment process and confirm that you get a good quality phosphate or conversion coating. Your chemical supplier should be able to assist you in setting up an appropriate quality control program.
Adhesion is one of the most important parameters, and I would perform both a dry and wet adhesion test per ASTM 3359 Method B. In addition, you will need to measure dry film thickness of the powder coating. If necessary, look for pin holes and voids, especially if the coated part will be shipped to a corrosive environment. If customers require a Class A finish, I would look for contamination and other defects in the finish. You might need to perform the MEK rub test to insure that you have achieved proper cure and if the coated part eventually will be exposed to sunlight, you might send some samples to a laboratory that can subject the samples to a weather-ometer where UV light (and perhaps also humidity) will accelerate coating degradation. Measurements of gloss and texture might also be necessary. Calculating the Weight of Zinc
Q: What would the weight of one mill of zinc be on 1 square inch of surface area and how is it calculated?
A: Is this pure zinc, or a zinc rich paint? My calculation is for the average density of zinc metal.
One solid gallon of any compound covers 1,604 ft2 at a film thickness of 1 mil. The density of zinc is approximately 59.0 lbs/gal; therefore 1,604 ft2 of zinc, at 1 mil weighs 59.0 lbs. 1 ft2 = 144 in2 Therefore 1604 ft2 = 1604 ft2x 144 in2/ft2 = 230,976 in2 The weight of zinc covering 1 in2 at 1 mil = 59.0 lbs/230,976 = 0.000255 lbs 1 lb = 453.6 grams Wt. of zinc = 0.000255 lbs x 453.6 grams/lb = 0.116 grams = 116 mg. Therefore the approximate weight of zinc covering 1 in2 at a thickness of 1 mil = 116 mg. Orifice Size in Viscosity Cup
Q: What are the orifice sizes in the Ford viscosity cups?
A: According to ASTM D1200 the diameters are as follows:
Ford #1 = 1.90 mm
Ford #2 = 2.53 mm Ford #3 = 3.40 mm Ford #4 = 4.12 mm Ford #5 = 5.20 mm Hope this helps.
Calculating Paint Coverage Rates
Q: 1. I'm interested in knowing if you have an industry standard or a parameter range that would indicate standard painting speeds and feeds, assuming an automtic (robotic) paint line? I fully underestand there's alot of variables here: gun type, paint type, tip aperatures, ect. However, I'm really looking for a general rule of thumb for how much paint coverage (lets say using PU paint) can be achieved/mil at a standard feed rate in/min (or mm/min). It would be nice to have some kind of correlation or time association between the specified thickness and coverage per sq in. For example, at a robot speed of 20ipm the paint coverage is 20um per sq in or whatever it happens to be. Any input is greatly appreciated.
2. It appears that a general rule is 1 solid gal. of paint covers 1604 sq ft at .001" thick. Can I apply this constant with a paint mixture (base coat+thinner+ hardner)?
A: Off-hand I don't have coverage rates that relate to the speed of the robot; however, the general algorithm for calculating coverage is:
Coverage (ft2) = 1604 ft2/gal/mil x Gals x % Volume Solids x % Transfer Efficiency
Dry film thickness (mils)
This algorithm applies to all paints, regardless of their mixture with or without thinners. To use the calculation you do need to know the % volume solids of the coating and the transfer efficiency (TE). With regard to the TE you can conduct online tests to determine this value, or if you don’t have the luxury of conducting tests you must guess.
I have not calculated the coverage rate as a function of time, but it is not too difficult to add a time dimension to the algorithm. First, you need to experimentally determine the fluid flow rate (g/min) or (mL/min) and then you need to know the % Wt. solids of the coating and possibly also its density (g/L) or (lbs/gal). The required input data depends on how you measure your fluid flow rate; by weight or volume.
Calculating Thickness
Q: If I know the dry film amount in grams on a known sq. ft. substrate and I know the volume solids and density, how do I calculate the mil thickness?
A: You can calculate the film thickness from the algorithm:
Coverage (ft2) = 1604 x Gals x Fraction Volume Solids
Dry film thickness (mils) Where 1604 is the coverage of 1 gallon of solids applied to a film thickness of 1 mil (= 0.001 inches)
Notice that density of the paint is not required for this calculation.
Therefore:
Dry film thickness (mils) = 1604 x Gals x Fraction Volume Solids Coverage (ft2)
Gals = Weight (lbs)
Density (lbs/gal) Therefore, substituting for Gals:
Dry film thickness (mils) = 1604 x Weight (lbs) x Fraction Volume Solids Density (lbs/gal) x Coverage (ft2)
Since 1 lb = 453.6 grams
Dry film thickness (mils) = 1604 x 453.6 x Weight (grams) x Fraction Volume Solids Density (lbs/gal) x Coverage (ft2)
Paint Making Formulations and Chemistry
Q. I want to set up a laboratory for testing pigments and other components of paints, and the idea is to compare the properties related to the components in paints we will prepare and then test (for as many uses and applications as it is possible), but my problem is that we have few formulas. Can you suggest me some pages on Internet, books and/or newspapers where I will find this information? A. The Raw Material Suppliers of resins and pigments often publish starting formulations. If I were you, I would start by looking up the names of resin and pigment suppliers and then going to their web sites or writing to their lab directors. Also, you might like to go to Specialty Chem www.paintandcoatings.com which is an excellent rerseource for the type of information you require. Finally. go to the bookstore at http://www.coatingstech.org/ which is the web site for the Federation of Societies of Coatings Technologies.
Why are Paints Glossy?
Q. What is the ingredient in semi-gloss paint that makes it glossy that flat paint doesn't have? A. The ingredients of glossy and flat paints are essentially the same. The difference is that in glossy (high gloss) paints the ratio of pigments to resin (pigment volume concentration or PVC) is lower than in flat paints where you have considerably more pigments for the same amount of resin. When the paint dries some of the pigments at the surface break through the upper layer and diffuse the incident light. In some low gloss paints a very small concentration of light weight flatting agent is added so that it quickly rises to the top to break up the incident light.
Paints & Coatings - Converting from Solids by Weight to Solids by Volume
Q. I'd like to convert the Solids by Weight (SW) data provided by coating manufacturers to Solids by Volume (SV) so that I can then determine dry film thickness (DFT). Is there a formula to convert SW to SV? A. To convert from percent solids by weight to percent solids by volume you need to calculate the percent volatiles by weight and then convert those to percent by volume. From an MSDS you can calculate the percent volatile by volume for each volatile ingredient, and then using the density of each ingredient, you can calculate their respective volumes. The solvent densities are available online. Once you know the volumes of each volatile component you can calculate the total volume of volatiles. By subtracting the total from a 12 gallon quantity, you will have the volume solids.
Humidity and Paint Performance
Q. Our company has recently purhased an automated paint system and have been debating whether or not to purchase a humidification system for the building it resides in. Next to the paint cell we have a wood manufacturing cell where we have been seeing warp issues due to parts drying out. The RH has varied from 22% to 65% this year (Minneapolis, MN). I was wondering how humidity effects spray painting and if there would be a benefit in regulating the humidity in the building. A. Humidity definitely plays a role during the curing of waterborne coatings. You do not want the relative humidity to be too high because it affects the speed at which the water and coalescing solvents evaporate relative to each other.
I have never seen data on the effect of RH on the evaporation of solvents from solvent based coatings, but you must avoid the condensation of moisture vapor on the uncured film. Therefore, you must always insure that while the solvents are evaporating (which is a cooling process), you do not allow the surface temperature at the air/substrate interface to fall below the dew point.
Unfortunately, I don't have actual data in terms of temperature vs. humidity to serve as a guide, but in my opinion 22% RH is within the acceptable range. 65% RH might be somewhat high and you might want to bring that down to 50%. Don't forget that ambient temperature is also important and you should be considering both temperature and humidity. Without performing experiments I can't help you further.
If you would like me to research this further by going to the literature and the major coating research institutes I will be happy to do so, but I will need to do so on a consulting basis. Please get back to me if you are interested.
Calculating Air Flow Rate (Velocity) in a Spray Booth
Q. What should the airflow be in a semi down draught booth with heated air replacement, that is 6.6m long x 4.4m wide x 2.2m high? Can the size of the air inlet filter be reduced? What are the calculations? A. Acording to OSHA Table G10 of 29 CFR 1910.94(c)6)(i) and NFPA-33 you should have a design velocity of 100 ft/min of air passing across the areas were the painters are working in a cross draft booth using manual air spray guns. In a large paint spray booth you are permitted to lower this to 50 ft/min. Many spray equipnment companies regard a downdraft booth to be a large booth and they design for the 50 ft/min target.
From your question is it not clear what you mean by "semi-downdraft" booth. Some booths are designed so that air entering at the back of the booth but from the ceiling moves to the far end of the booth where exhaust filters are located. This is a modification of a conventional cross draft booth.
Other booths are designed with air entering from the ceiling and ehausting through filters that are located immediately above the floor but along two side walls. In other words, the air entering at the ceiling splits with half moving towards the filters on the one side, and the other half moving toward the filters on the other side of the booth. This is a modification of a downdraft booth.
Scenario #1 - Air moving from ceiling at back of booth to the exhaust filters at far end of booth.
Based on these assumptions:
Surface area (ft2) = 4.4m (wide) x 2.2 m (high) x 10.76 (m2/ft2)
= 102.4 ft2
Air velocity = 102.4 ft2 x 100 ft/min
= 10,240 ft3/min Scenario #2: Air entering at ceiling and moving downward toward the filters immediately above the floor along the sides of the booth.
Based on these assumptions:
Surface area (ft2) = 6.6m (long) x 4.4m (wide) x 10.76 (m2/ft2)
= 312.5 ft2
Air velocity = 312.5 ft2 x 50 ft/min
= 15,625 ft3/min If you want uniform air distribution throughout the booth you should size the air intake filters accordingly. If you make the filter section too small, you can expect turbulence in the booth and this will cause paint overspray to settle on the walls, ceiling, lamps, etc. In addition, turbulence lowers transfer efficiency.
While these calculations might be useful to you, I recommend that you consult a spray booth company, such as Global Finishing Systems for the final drawings and the air flow requirements
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