Good Viscosity for Coating Applications

A rule of thumb in the coatings industry is that a viscosity of approximately 100 cps (1 P, 0.1 Pa•s) provides acceptable spray, brush or roller application. However, this viscosity is not measured under any conditions, nor with any viscometer. All aspects of paint flow, including agitation, pumping, transfer, sag and application, involve shearing. It is conceivable that the cut puts a small amount of the liquid in the palm of the hand and spreads it by placing the other hand over the first. Apply high shear stress to the paint by spraying and brushing. The same goes for reverse roller coating. Direct roll coating is considered a low shear process (coating rolls usually just kiss the substrate),

Shearing of the paint destroys its structure and the viscosity is reduced to a lower level (usually much lower) than the paint at rest. This is called shear thinning, and the rate at which the paint deforms when sheared is called the shear rate (in reciprocal seconds, s-1). While the key parameter affecting viscosity is shear stress, those in the coatings industry have traditionally viewed viscosity as a function of shear rate. One reason for this is that Rotational Viscometers usually display the shear rate value as a change in velocity, or it is easy to calculate the shear rate from the viscometer speed. Coating application is a high shear stress, high shear rate process. Air spraying has a shear rate of 1,000 to 40,000 s -1 depending on the Spray Gun, settings etc. Brushing involves similar shear rates.

Airless spray has a higher range of 10,000 to about million s -1 . Reverse roll coating can see shear rates in excess of 100,000 s -1 and coatings in direct roll coating may see shear rates of at least several thousand s -1 .

Different viscometers measure viscosity at different ranges of shear rates. The Stormer apparatus (see ASTM D562), which is often used to test architectural and maintenance coatings, operates at low shear rates of about 60 s-1, which is suitable for testing stirrability, but not sprayability or flammability. A conventional spindle Rotational Viscometer cannot provide shear rates much higher than 100 s-1, so it is also not used. I highly recommend a high shear cone/plate viscometer (ASTM D4287) with a shear rate up to 12,000 s -1 (10,000 s -1 where 50 Hz power is used). I tested the ease of application with old ICI cones/plates and recent Brookfield Cap 1000 and 2000 instruments. Special viscometers and sensors can measure up to 100,000 s -1 compared to airless spray and reverse roller coating. However, my experience tells me that they are not necessary, as the viscosity of almost all paints flattens out at fairly low shear rates. The automotive industry has used a No. 4 Ford cup (shear rate around 400-500 s-1, ASTM D1200) to check spray performance for many years, but some companies have switched to modified cone/plate viscometers measuring at 500 s-1 (ASTM D7395).

Let's get back to the nice high shear viscosity application. Earlier, I said 100 cps was a good goal. I have used sprayable paints with viscosities as low as 50 cps and as high as 500 cps. The low end can cause turbulence (bad for the gun and nozzle), overspray, which can cause sagging. The upper end is prone to poor atomization, possibly gun or cobwebs, and an unacceptable appearance. Viscosity affects the ease of brushing, coverage, and ease of application of architectural coatings. I once worked on a hidden problem that was caused by painters applying very thin coats due to the ease with which the paint brushes. The high shear viscosity is only about 70 cps. Raising it solved the problem. If the viscosity is much higher than 100 cps, the resistance of the brush is so great that the painter will have a sore wrist and will not buy the paint again. I don't recall the extent of the roller application, but viscosities much lower than 100 cps may cause fogging, while higher viscosities may create stickiness and other unwanted surface patterns.

What if you are a paint user and don't have a high shear viscometer? I've been at the paint store and they thin a portion of the paint until they like the spray pattern, then use the rest of the roller or tote to reduce. Many spray coaters and roll coaters are reduced to a certain cup viscosity based on experience with spray or coat coats of that product or color. Architectural paint formulation labs often have a designated painter to test for flammability, or call in professional painters to test new products.