Q: “Bits” seem to be a common complaint in powder coating. What are the sources of bits in powder? How can I eliminate or control them, either during the production process or during application?
A: Indeed, powder coating bits are the bane of any powder producer or applicator. These defects arise seemingly out of nowhere and drive technologists crazy until the issue either disappears or is resolved. Simply defined, “bits” are objectionable protrusions in a powder coating film.
Bits can emanate from a variety of sources during powder manufacture, application, and curing. The most common sources of bits include:
- Unclean raw materials – gel particles in resins, large particle filler pigments, agglomerates in additives (flow agents especially)
- Free flow (dry-blend) additives – will cause bits if not deagglomerated during incorporation into a powder
- Gel particles created in the extruder
- Purge compound left in the extruder
- Lint from wiping rags
- Environmental dirt encountered in powder production due to poor housekeeping, dirty containers, torn boxes/bags
- Dirty compressed air lines – steel compressed air lines can corrode and send rust particles into the application air stream
- Environmental dirt from ovens – baked-on residues can come loose; these bits will reside on top of the coating surface
- Dirty substrates – weld spatter, environmental dirt; these are represented as “buried” particles
- Dirty racks and hooks
A simple magnifier of 6X to 10X can tell you volumes about the nature of a “bit.” The qualities to observe are size, color, and shape. Lint is easiest to discern, as it is typically observed as a coiled filament. Irregular brown or yellowish lumps are usually charred resin or powder coating binder. Suspected ferrous particles can be extracted with a fine probe and tested for magnetism. Light-colored nodular defects are usually free-flow additive agglomerates or large-particle-size fillers.
If optical analysis is unsuccessful, the defects can be analyzed with electron microscopy coupled with emission spectroscopy. This can determine elemental composition, which usually is successful with identifying the defect causing culprit.