![]() Many sprayers and roll coaters reduce to a certain cup viscosity based on experience of what sprays or roll coats well for that product or color. What if you are a paint user and do not have a high shear viscometer? I have been in paint shops where they diluted a portion of paint until they liked the spray pattern and used that reduction with the rest of the drum or tote. I do not recall the ranges for roll coating, but viscosity much below 100 cps is likely to cause misting, and high viscosity probably will give ropiness and other unwanted surface patterns. If the viscosity is much above 100 cps, the brush drag will be so high that the painter will end up with a sore wrist and will not buy your paint again. The high shear viscosity was only about 70 cps. I once worked on a hiding problem that turned out to be due to the paint being so easy to brush that painters applied very thin coats. Viscosity affects ease of brushing, coverage, and tends to spatter of architectural paints. The upper end tends to give poor atomization, possibly gun spits or cobwebs, and unacceptable appearance. The low end is likely to cause turbulence (bad for spray guns and nozzles), fine overspray and, possibly, contribute to sagging. I have worked with paints that were sprayable at viscosities as low as 50 cps and as high as 500 cps. Earlier, I said 100 cps was a good target. Let’s get back to optimum high shear viscosities for application. 4 Ford cups (shear rate around 400-500 s -1, ASTM D1200) to check sprayability for many years, but some companies have changed to modified cone/plate viscometers that measure at 500 s -1 (ASTM D7395). However, my experience tells me that they are not necessary because the viscosities of nearly all paints flatten out at considerably lower shear rates. ![]() Special viscometers and sensors are available that enable measurements up to about 100,000 s -1 comparable to airless spray and reverse roll coating. I have had many useful results with the old ICI cone/plate and the more recent Brookfield Cap 10 instruments to test ease of application. I highly recommend high shear cone/plate viscometers (ASTM D4287) that can apply a shear rate of 12,000 s -1 (10,000 s -1 where 50 Hz power is used). A conventional spindle-type rotational viscometer does not give shear rates much above 100 s -1, so it is not of use either. The Stormer instrument (see ASTM D562) often used to test architectural and maintenance paints operates at a low shear rate of about 60 s -1, which works well to test stirrability, but not sprayability or brushability. Reverse roll coating can give shear rates over 100,000 s -1, and the paint in direct roll coating probably sees shear rates of at least a few thousand s -1.ĭifferent viscometers measure viscosity at different shear rate ranges. Brushing involves similar shear rates.Īirless spray has a higher range of 10,000 to perhaps a million s -1. air spray gives shear rates of 1,000 to 40,000 s -1. Paint application is a high shear stress, high shear rate process. One reason for this is that rotational viscometers often display shear rate values as speed changes or it is easy to calculate shear rate from the viscometer speed. ![]() Although the key parameter affecting viscosity is the shear stress, people in the paint industry historically have looked at viscosity as a function of shear rate. This is called shear thinning, and the rate of deformation of the paint as it is sheared is called the shear rate (units are reciprocal seconds, s -1). Shearing of paint breaks up its structure and the viscosity drops to a lower level (often much lower) than the paint had at rest. Direct roll coating is thought of as a low shear process (the application roller often just kisses the substrate), but the doctor blade and pre-application rollers may apply high shearing stresses. This also is true of reverse roll coating. ![]() With spraying and brushing, a high shearing stress is applied to the paint. Shearing can be envisaged in terms of placing a small amount of liquid in the palm of your hand and smearing it by passing your other hand over the first one. All aspects of paint flow, including stirring, pumping, transferring, sagging, and application involve shearing actions. However, that viscosity is not measured under just any set of conditions or done with just any viscometer. s) provides acceptable spraying, brushing, or roll coating.There is a rule of thumb in the paint industry that a viscosity of approximately 100 cps (1 P, 0.1 Pa I thought that I would extend my answer to other application methods and share the information. I recently was asked about the optimum viscosity for spraying paint.
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