Powder Coatings Pretreatment

 PRETREATMENT STEPS

Cleaner
Cleaner Rinse
Phosphate
Rinse
Passivation Rinse
Final Rinse
Dry-off

SELECTION OF PRETREATMENT

Cleaner

The first and most important stage in the spray washer is the cleaner stage. If the part is not completely cleaned while passing through this stage, the quality of the finished part will suffer. The cleaners are alkaline, and contain surfactants to lower surface tension. Spray pressure is important in effective cleaning. The highest pressure, without blowing the parts off the hanger is usually the most desirable. Cleaner solution concentration and temperature must be carefully controlled. Planned dump cycles must be implemented to assure that soils are not redeposited on the surface of the parts. Normal design dwell time in the cleaner stage is 90 seconds. If line speeds have been changed, the chemical supplier should be consulted for recommended changes in solution concentration and temperature. Between each stage is a drain zone. The drain zone should be long enough to keep solutions from being carried from one stage to the next, but short enough that the parts do not dry between stages.

Cleaner Rinse

The rinse after the cleaner is normally an overflowing clean water rinse. It is operated at the same temperature as the cleaner. The alkalinity of this stage is limited to no more than three percent of the cleaner concentration to limit the carry-over into the acid third stage.

Phosphate

Two generic types of phosphates are widely used in powder paint finishing systems, iron phosphate and zinc phosphate. From both the equipment and chemical standpoint, iron phosphates are more economical. They are easier to control and have fewer maintenance requirements.

A detergent iron phosphate product is available that requires only three stages. This system is used in some powder finishing systems, however, it is not recommended where a high quality end product is required. The ultimate performance of the final product, cost, and end use must be balanced to determine the phosphate to use.

Crystalline zinc phosphate and micro-crystalline zinc phosphate, when properly maintained, provide the best substrate, with respect to surface appearance, adhesion, detergent resistance, and corrosion resistance. However, the crystalline coatings exhibit poor flexibility, and generally require higher paint film thickness to achieve equal gloss or iron phosphated parts. No matter what generic type of phosphate is chosen, solution chemistry and temperature must be carefully controlled. Due to the fact that both iron and zinc phosphates form sludges in their normal operation, regular scheduled clean-out of all spray piping and tanks is required to assure adequate spray patterns.

Rinse

The rinse after phosphate is an overflowing fresh water rinse, its concentration must be kept at no more that one percent of the phosphate tank concentration to avoid contamination of the passivation solution in Stage No 5.

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Passivation Rinse

The passivation of the phosphated surface with "chromic acid" solution is a very important step in improving the overall performance of the paint film. Salt spray resistance is strongly affected by this step. PH control in this stage is required.

The chromic rinse serves the following purposes:

It is very important to follow the chemical-suppliers recommendations concerning dumping and recharging of this stage.

Final Rinse Stages

The first rinse after the passivation stage is usually a cold tap-water rinse. This should be followed by an overflowing, recirculating D.I. rinse. D.I. water is water that has been passed through an ion exchange column to remove water soluble salts. A measure of how completely these salts have been removed is the specific resistance of the water. This rinse should be followed by a short drain time and a final fresh D.I. rinse which drawings into the overflowing D.I. rinse. The specific resistance of the overflowing D.I. rinse should be less than 20,000 ohm/cm. The fresh D.I. should have a specific resistance of no less than 50,000 ohm/cm.

Dry-off

The time and temperature of the dry-off oven is dependent upon the pretreatment and part shape. With iron phosphate, the requirement of changing the crystal structure is added. The desired crystal dehydration improves corrosion resistance. Usually, a metal temperature of 350°F for five minutes is sufficient to cause dehydration. With both iron and zinc phosphate, metal temperatures about 400°F and long oven times should be avoided as they can cause deterioration of pretreatment.

Parts exiting the dry-off oven should be completely dry and exhibit a uniform surface appearance. The presence of any streaks, splotches or bare areas is a sure indication of problems in the system. Problems of this type will cause poor durability of the part to be painted. This is true even though the powder coating may completely hide these deposits under paint film. The chemical supplier is the best source of help when these problems are present.

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Selection of Pretreatment

Steel = Cold Rolled Steel
Al = Aluminium Sheet and Extrusion
HDGB = Hot Dipped Galvanised Steel
EZ = Electrogalvanised Steels
IZ  = Iron/Zinc Alloy-coated Steel
Zn/Al = Zinc/Aluminium Alloys and Die Castings

These specification guidelines are given in good faith, but as specific conditions may vary, no responsivbility can be taken for coating system performance. It is the responsibility of the user to ensure that the product and specification meet the particular application.