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5 Powder Storage

Powder storage areas should be reasonably cool, dry and free of airborne contaminates. Some powders are more stable in storage thanothers, so it is important to know which materials are most sensitiveand what conditions are required to keep them in good condition.

Heat

Most powders will withstand a certain amount of exposure to heat instorage. Temperatures of 100120F (3846C) usually will not hurt

the material. If the temperature is greater than this, several physicalchanges may take place. The powder can sinter, pack, or clump in thecontainer. Tall containers or boxes stacked high can add to this prob-lem because of the weight of the powder on itself. Boxes should notbe stacked more than 3 high. Usually this clumping can be eliminatedby passing the powder through a screen or sieve but this is very laborintensive.

Powders with low temperature curing mechanisms may undergo achemical change when exposed to excessive heat. Once this hap-pens, the powder will lose its original flow characteristics and must be

scrapped. Most powders are formulated with blocking agents that pre-vent curing below 200F (93C), so this is not usually a problem.

Humidity

Powder will absorb moisture if the air in the storage area is above60% relative humidity. This will cause it to clump together, resist flu-idization and flow poorly. In most cases, the clumping can be elimi-nated by a combination of stirring and fluidizing for a few minutes.Still, it is better to store powder at a range of 40 to 60% RH.

If powder is stored in a controlled environment within these param-eters, it will normally remain stable for at least one year. Storage in acontrolled environment will provide more consistent behavior of thepowder during application.

To avoid problems with powder materials, storage areas should becontrolled as follows:

Control temperature to 80F (27C) or less.

Control the relative humidity between 40 and 60% RH.

Powder Storage

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Rotate stored powder so that it is not kept in storage for extendedperiods of time. First in, first out.

Avoid leaving containers of powder open on the shop floor wherethe powder can easily pick up moisture and dirt.

Precondition the powder before spraying by fluidizing it for afew minutes prior to application.

Like many of the recommendations in this book, these precautionsare not absolutely necessary to the application of powder. However,following them will produce the best possible results with the least

possible labor. Proper storage and a little patience to achieve correctfluidization will help contribute to consistent results.

6 Powder Particle Size

The size of the powder particle can have an influence on the behaviorof the material in the delivery system, the charging system and thefinal film characteristics. The original grind size is determined by themanufacturer to provide certain deposition and appearance charac-teristics. The powder coater must understand the role of particle sizein the electrostatic process and maintain a consistent blend when re-claimed powder is used.

The consistency of powder flow from the gun begins in the feed hop-per. Steady, uniform fluidization is required to achieve a consistentflow through the pumps and out of the gun. As the concentration offine particles increases the powder coating material will begin to clumptogether and resist fluidization. Finer particles fill the voids betweenthe larger particles and force the operator to use higher air pressure.

The air entering the fluidized bed will follow the path of least resist-ance and higher pressure will often cause geysering. The surface of

the fluidized powder is inconsistent with large bubbles and dead spots.This condition will cause the air to surge and deliver inconsistentamounts of powder to the pick-up tube. If the condition is not toosevere the operator may continue to spray but the parts will haveuneven film thickness and possibly surface blemishes. If the conditionis severe the gun will spit a large volume of powder onto the surfaceand leave a blotchy mound of powder.

Powder Particle Size