The flow and leveling to remove scratches and small defects has been a problem for paint shops since they first applied coatings. Generally, flow and leveling can be improved through rheological modification and the use of surface tension modifiers. If the viscosity is low enough, flow is good; but with increasing viscosity, the flow decreases. However, if the viscosity is too low, sag will be the result.

The use of thixotropes can help allow better flow with less sag. This allows the use of shear to decrease viscosity initially but allows viscosity to build when shearing ends. However, more solvent will be required.

The use of fluoro-surfactants, silicone-based surfactants or other surface-active agents can result in lower surface tension and less cavitation in defects and scratches, but this comes with a price. Most surfactants also reduce adhesion and can affect several other properties. Therefore, the best solution is proper surface preparation, sanding and buffing to minimize the defects. Multiple coats, usually with sanding between coats, will produce a relatively defect-free surface. Sorry, no short cuts.

Coatings for High Temperatures

What coatings types are appropriate for high temperatures?

High temperature is a relative term. Silicone-modified alkyds will perform well at modest temperatures--about 100°C. Pure silicone resin systems will perform up to 200°C. Above this temperature, ceramic-type coatings, such as glass, aluminum oxide, and many other inorganic materials must be used.

Many of these can be used on structures that are ridged with constant or slowly changing temperatures. Large temperature variations or a substrate that must flex will generally result in coatings failure. Usually there is a difference in the thermal expansion coefficient between the ceramic layer and the substrate. This difference can set up a stress zone, which can result in delamination. Since the ceramic layer is usually relatively brittle, flexing of the substrate can, with the aid of the thermal stress, cause delamination. Impact properties are also compromised with these coatings. For example, a classic ceramic coating was porcelain for pots and pans, which has been found to be very prone to chipping.

Judging Foam Stability

Generally, foam is classified into macro and micro foam. Macro foam comprises large bubbles that are relatively easy to break. Micro foam requires magnification to see with the eye. After application, macro foam will result in large surface defects that are obvious to the eye. Micro foam can be seen with a 20X magnification as small surface craters, which look like pores in a person's skin.

To quantify foam, many workers have used a graduated cylinder and agitation to form foam. The stability of the foam is indicated by following the volume of the paint as a function of time. Pigmented systems can be difficult to analyze in this manner since it coats the cylinder, making quantitative measurements difficult or impossible. Since this is a volume measurement, accuracy is limited.

We have developed a method that uses a pint can filled to 75% with paint. It is shaken in a paint shaker for three minutes and then rolled slowly, 1/3rpm for 30 minutes, after which the weight-per-gallon (density) is checked at a set temperature. Knowing the true density of the paint without air can yield the air content of the paint. The time can be varied by application.

This method gives a reproducible measure of the entrained air, even micro foam. It allows easy comparison of defoamers and anti-foaming aids, as well as the effect of the various surfactants. Surfactants are one of the major sources of foam stability. Therefore, it is important to evaluate foam stability as well as the surfactant's ability to wet the pigments or the other functions for which it is being used.

Impact of Air Quality

Usually, a dry, oil-free air source is recommended with a good particle filter. The lubricating oils in the pump can cause adhesion and cratering problems on the substrate. If the air is not dry, water or rusty water can deposit on the surface. The use of a particulate filter obviously is needed to remove particles from the air to avoid the deposition of new particles. Remember, when air at high velocity impacts the part, other surfaces nearby can have dust disturbed. Removing the airborne particulates from the work area is as important as the impact of the air quality; therefore, appropriate exhaust is required.