Trouble Spraying Inorganics

Question:

We have a problem spraying an inorganic coating. Our company uses a coating, which consists of 45.0% high purity barium sulphate, 0.5% polyvinyl alcohol, 34.25% ethyl alcohol and 34.25% water by weight. In sum, this is a coating of the pigment, barium sulphate, weakly tethered to a polyvinyl alcohol-water-ethyl alcohol base. When we attempted to air spray this coating, the gun clogged.

We recently tried applying this coating using an airless spray gun and the coating clogged the gun. The only gun that works is the rotary type sold at hardware stores. However, these wear out in three to four weeks even though we spray only occasionally during this time. Viscosity readings of 11 to 15 seconds on a Number 3 Zahn cup are average.

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What recommendation in equipment would you suggest? We have more or less exhausted our resources. Please Help! B. M.

Answer:

I can appreciate your frustration with the problem of spraying a finish which has such a high pigment volume concentration. Since you have so little vehicle in the formulation, the coating may not be “wet” enough. This is aggravated by the rapid evaporation rate of the ethyl alcohol component of the solvent blend, making the coating even dryer. Other possible causes of the spray gun orifice clogging are large pigment particle size and pigment agglomeration. I am not sure what you are trying to do with this coating, but I suggest you try the following:

1. Increase the amount of polyvinyl alcohol vehicle to achieve a wetter coating because of a lower pigment volume concentration.
2. Try another co-solvent that evaporates at a slower rate than ethyl alcohol.
3. Try using other water soluble vehicles. There is a host of these materials available in the market place.
4. Use a grade of barium sulphate having a smaller particle size.
5. Modify your compounding process to more completely grind the pigment into the vehicle and at the same time break up any pigment agglomerates.
6. Another approach is to coat the surface with dry barium sulphate or a dry blend of barium sulphate and polyvinyl alcohol using the electrostatic powder coating technique. Inorganic and as well as organic powders have been applied successfully on conductive and non-conductive surfaces using this technique. For more information on electrostatic powder coating, contact suppliers of Powder Coating Equipment, electrostatic spray, in the suppliers database.