When you are spraying a coating material with a spray gun, do you ever have days when the coating comes out more coarsely (larger droplets) or just won't flow out to a smooth and even covering? How about an aerosol that spits this morning when it didn't yesterday afternoon?
How about a coating that runs or curtains on a vertical surface even if you aren't doing anything different today than you were the last time you used this product and didn't have this problem? Could there be a difference in the temperature of the coating, the equipment or the workpiece? Temperature is one of the most important factors to consider when applying a coating regardless of the method of application. But when spraying a finish, it is vitally important. Temperature affects the viscosity of the material. In an aerosol, it also affects the pressure in the can.
Typically, a manufacturer will specify the viscosity of the material on a product data sheet or perhaps on the label. It might publish a "package viscosity," indicating the viscosity specified for the material in the package prior to use. The recommended spraying viscosity will be different unless the material is sold as Ready to Spray (RTS), Ready to Use (RFU) or with similar wording indicating that reducers will not be needed under ordinary conditions.
Still, you might want to know a little more about it.
What is viscosity?
Stated simply, viscosity is the resistance to flow exhibited by a liquid. It is an extremely important factor in spraying liquid coatings because it determines how easily the liquid is atomized and thus determines the proper orifice size, air pressure and fluid pressure for optimum spray application. It also affects flow and leveling of the applied material after the material is on the workpiece.
While there are a variety of instruments to measure viscosity, for most laboratory and field use in the coatings industry, simple viscosity cups are used. These cups will momentarily contain a specified amount of liquid that is allowed to drain from the cup. The time it takes to drain (efflux time) is measured with a stopwatch.
The viscosity is usually expressed in seconds, as measured in a specified cup, at a specified temperature. While the most common cup for laboratory use is the Ford cup, the Zahn cup is more often seen in the field, as it is less expensive, easier to use and it provides adequate accuracy. Both the Ford- and Zahn-type cups come with a variety of orifice sizes to cover different viscosity ranges. Conversion charts are available to convert results from one type of cup to another. For higher and lower viscosity materials, different cups, or even different measuring methods, may be necessary. For most of the coatings you will be using, testing is typically done at 25 C (77 F), and deviations from this temperature might induce errors in the results. Viscosity is inversely proportional to temperature. That is, when the temperature goes up, the viscosity goes down and vice versa. Liquids tested may vary in viscosity from 3 to 8 percent per degree of change (Centigrade) or 1.5 to 4 percent (Fahrenheit).
The variation is greater in higher viscosity materials. The variation is generally close to linear between 23 and 27 C (73.4 and 80.6 F), but the user should be aware that the change may not be truly linear even within a narrow temperature range. Because even slight variations in the temperature of the fluid may affect viscosity - and thus efficiency in spraying - the air temperature, fluid temperature and the temperature of the test equipment should be very close to 25 C (77 F). This is especially important when measuring fluids containing fast-evaporating solvents, where evaporative cooling can be a factor.
Applying the results
Viscosity testing may be used to determine that a coating is within specifications, and to adjust the viscosity to suit the intended use in the intended application equipment. Attempting to spray too high a viscosity material could result in insufficient volume delivered, poor atomization, and orange peel or rough finish. In the attempt to spray a too viscous material, the operator might turn up the fluid and/or air pressure too high, thus increasing energy costs. If viscosity is too low, more coats will be required to achieve the desired film thickness, waste will increase, and runs, sags and curtaining will more likely occur. Too thin a finish could also cause more dry overspray in low humidity/high temperature conditions.
It is generally better to adjust the temperature of a coating to achieve the proper viscosity than to add reducers. Typically this is more economical and has fewer side effects than using reducers, retarders and flow-enhancing additives.
Factors that affect viscosity in a liquid coating include:
- The type and quantity of resin used
- The type(s) and quantity of solvent used
- Temperature of the coating
- Type and quantity of additives
- Type, size and quantity of pigments or other undissolved solids
The finisher should frequently test the viscosity of the materials he is spraying, as directed above. Over time, he will find the conditions that, with his equipment, yield the best results. Only when it is determined that the equipment is appropriate for the material, and that the temperatures at application are correct, should reducer, retarder or flow enhancers be added to the material to improve performance.
Accurate viscosity cups are precision instruments and need to be properly maintained. For very critical uses (generally in a lab environment) periodic recalibration of the instrument is recommended. Cheap stamped metal or plastic cups don't provide the consistency you need for reliable readings and even when you have one that is providing adequate consistency for your use, they are easy to damage in imperceptible ways. For instance, a plastic cup dragged across a rough surface may change readings because of a roughening of the edge of the orifice.
There are many factors determining how your spray equipment - whether it is a simple aerosol can or a sophisticated a multithousand-dollar air-assisted airless system - applies a particular coating. Viscosity, regulated by chemical additives or temperature, is one of the most important, and least understood, factors. Get the coating right first and everything else gets easier.
Greg Williams, formerly senior touch-up and finishing instructor for Mohawk Finishing Products, is now a freelance instructor and consultant for finishing and touch-up. He can be reached at gregalwil@ yahoo.com.
This article originally appeared in the December 2009 issue.