Pin-holes in Powder Coatings

A common complaint or comment about powder coatings is that they are sometimes pin-holed; a small crater visible, pin-holes in the surface of the finished powder coating.

PIN-HOLES

There are many reasons for pin-holes and not all of them are related with the way the powder coating materials are applied.

De-gassing agent Benzoin if used can prove to be problematic. The substance is a precursor to Benzil, a photoinhibitor and is activated by copper, nitric acid and atmospheric oxygen. So if not fully encapsulated the substance can be the route cause of issues found. Normally this is overcome by the powder coating material manufacturer at the extrusion stage of manufacture.

Other problems at the manufacturing stage are related to contamination by volatile matter; such as high-boiling solvents or low molecular weight resin compounds. Incorrect extruder settings can lead to pin-holing in the finished coating.

Plant conditions at the applicator can be an influence of the eventual finish applied. Cross contamination of the application equipment can have a detrimental effect on the control of pin-holes although not as significant as contamination at the extrusion stage of manufacture. Probably the major contributor from application mistakes is from back-ionization, the effect of particles of powder being thrown back from the surface being coated taking other particles with it and leaving a small crater or pin-hole.

Pin-holes from back-ionization are far more common when coating iron/steel parts than aluminium parts. If this is the case then lowering the Kv may help to overcome this problem.

Other coating plant issues could be related to the quality of the compressed air used, which should be dry and free from oil. Moisture and oil become a contaminant leading to among other problems pin-holes. Emulsions of oil/water; more water give micro-pin-holes and more oil give larger, flatter with fatter borders type pin-holes.

At the pretreatment stage problems can be introduced. Over-pickling and poor de-gassing can be the cause of some pin-holes, this is particularly the case with galvanized steel products. Where substrates have been oxidized and greater then normal pickling is required this can lead to a greater amount of zinc phosphate being attracted. The zinc phosphates applied generally encapsulate molecules of moisture which is then boiled-off during the powder coating cure stage.

Pin-holes are under these conditions the result of gas & steam escaping through the powder coating after the powder has started to flow and skin-over (very similar to the predominately water/oil emulsion effects described above).

In this last case the steel chemistry will heavily influence the pick-up of zinc during galvanizing allowing silicone and phosphorus to migrate to the zinc surface both providing ideal conditions for the incidence of pin-holes. Where the galvanizer has not taken account of condensation or the production of oxidized surface salts from storage and transportation the powder coating applicator is left with a substrate which will have to be over-pickled.

The above is not an exhaustive list of reasons why powder coating pin-holes but gives a range of possible technical reasons why, which suggests that the incidence of pin-holes is far from a simple matter. Control of processes from the powder manufacturer and the galvanizer as well as the steel chemistry have a significant influence of the incidence of pin-holes in the finished powder coating.

Link:  http://powdercc.com/images/pdf/How%20to%20eliminate%20outgassing,%20the%20powder%20coating%20faux%20pas.pdf

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