PVC Powder

Ask an Expert From: Products Finishing, , from Powder Coating Consultants, Div. of Ninan, Inc.

Posted on: 3/1/2004

Question: Could you please tell me if there is a PVC powder coating that can be used to coat a piece of metal using the same equipment (guns, ovens, etc.) as used to do regular industrial powder coating on metal parts?

Question:

Could you please tell me if there is a PVC powder coating that can be used to coat a piece of metal using the same equipment (guns, ovens, etc.) as used to do regular industrial powder coating on metal parts? Are the procedures the same? M. S.

Answer:

There are PVC powder coatings available from several powder coating formulators. Look in PF Online for a list of these suppliers (http://www.pfonline.com/suppliers.html). Tell your prospective suppliers that you want electrostatic spray-grade material to ensure that you can apply it using guns. These PVC powders are classified as thermoplastic type powder coatings, which means that they are not cured but are melted and flowed onto a surface and will soften if heat is re-applied. They are often applied over a liquid primer to promote adhesion to the metal substrate but you should verify this with the powder formulator.

As to your equipment question, here are some issues you should be concerned about. First, the electrostatic spray guns used for most thermoplastic powder coatings (like PVC) are usually positive polarity instead of negative polarity used for all other thermoset powder coatings (typically used in coating industrial metal parts). The importance of polarity for the gun relates to the tribo-charge imparted on the powder as it is fluidized, pumped and atomized in the gun. In most thermoset powders this polarity is negative, and therefore, by using negative polarity guns the charging doesn’t have to “fight” this tribo-charge but rather adds to it. Just the opposite is true for thermoplastic powders, where the tribo-charge effects are of positive polarity. The good news is that changing gun polarity is just a matter of changing the gun. The control box and cables remain the same.

Secondly, as I stated previously, PVC is a thermoplastic powder coating and is therefore not cured but rather melted and flowed onto the substrate. This means a conveyorized cure oven normally used for thermoset powder coatings may have too much time and temperature to just melt and flow a thermoplastic powder coating. Adjusting both time (line speed) and temperature should fix this problem. It is even simpler when using a batch cure oven. You just adjust the batch cycle time and oven temperature, and you will be fine.

Lastly, you may have to apply a liquid primer to the metal substrate for proper adhesion. This will require a liquid spray gun and feeding device (pressure pot, or pump, etc.). The primer will need to flash-off and dry before applying the powder coating. Implementing these steps on an existing automated conveyorized powder coating system can be a problem. However, in a batch process, you will need only a liquid spray gun and booth.

 



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