Flow and surface defects.

Flow is necessary for good appearance, but it also is responsible for many defects. Both too little and too much flow can cause problems. The difficulty is in formulating and manufacturing paint so that there is just enough flow. We require that spray droplets, brush marks, and roll coat patterns flow out to produce smooth films, then we want flow to stop or decrease substantially so that the smoothness is frozen in. Unfortunately, this is difficult, particularly with high-solids paints. They are made with low molecular weight resins that continue to flow even after all of the solvent is gone, especially on baking. Older medium- and low-solids paints used high molecular weight resins that set up before all the solvent had evaporated.

If there is too much flow, the result will be sagging on vertical surfaces and there may be other defects such as craters, dewetting, pull-back from edges, fat edges, and sinks and bumps. The last-named defect looks a lot like orange peel, but occurs after a coating has flowed out and become smooth. Then, surface tension-driven flow and, possibly, shrinkage as the coating cures can produce poor edge coverage and a distinctly bumpy surface. I recall the first time that I observed such unwanted flow under a microscope as I used a hot stage to simulate a bake. The electrodeposition primer on the coupon flowed out to form a perfectly smooth surface, then continued to flow and became rougher and bumpier as I entreated it to stop. It turned out that we had to reduce the flow of the paint, not increase it, if we wanted a smooth surface after the bake. A similar situation often occurs when surface defects such as craters occur. Less flow means fewer defects, although too little flow gives roughness and orange peel.

How do you stop or reduce unwanted flow? The key viscosity is that at low shear stresses or shear rates the viscosity must be high or increase to a high level soon after application if flow is to be restricted. Pigment and extender levels can be raised, but these changes are likely to increase high shear viscosity to the point of hurting application properties. We usually turn to additives such as thickeners, thixotropes, and flow control agents. Exactly which one is chosen will depend on the type of product, whether it is waterborne or solventborne, the gloss requirement, etc. These materials raise low shear viscosity, usually by forming networks, either by themselves or in conjunction with pigments or resin molecules. Many of them cause the wet paint to be thixotropic; meaning that it takes time to build viscosity, thereby allowing leveling before the coating sets up. A combination of additives almost always works better than a single one. For example, a bentonite clay plus a fumed silica can be quite effective in solventborne coatings if high gloss is not necessary. Two or three molecular weights of a cellulosic thickener or a cellulosic with an associative thickener are useful in waterborne coatings.

Many surface defects are caused by flow that is driven by surface tension variations across the wet paint surface. In addition to raising paint viscosity, we also can reduce surface flow by adding materials that come to the surface and provide a continuous, relatively constant surface layer. Silicone and fluorocarbon surfactants are noted for accomplishing this, but a little bit goes a very long way and an excess can lead to such a low solid surface tension that the coating surface cannot be repaired or repainted. A combination of flow control agents and surface tension additives is more effective than either additive by itself. Oxidized polyethylene waxes tend to come to the surface, but cause less reduction of the solid surface tension than do surfactants. In combination with one or more flow agents, these waxes are noted for giving good appearance and fewer defects. As with all formulating, a range of additive amounts must be tested and properties compared to identify the optimum levels.

"Coatings Clinic" is intended to provide a better understanding of the many defects and failures that affect the appearance and performance of coatings. We invite you to send your questions, comments, experiences and/or photos of coatings defects to Cliff Schoff, c/o "Coatings Clinic," CoatingsTech, 492 Norristown Rd., Blue Bell, PA 19422; or email [e-mail].