Why are we seeing random flat spots in a textured powder after the metal has been washed?

Hey Joe,

Why are we seeing random flat spots in a textured powder after the metal has been washed?

Aaron Kisler

Decatur, IL

What’s up Aaron,

This is a very interesting problem and one that allows me to dust off my chemistry wizard’s hat to help you understand and solve this caper. Random flat spots can occur when using a rather specific powder cure chemistry. In this case it is technically not a “textured” powder finish but a “wrinkle” finish.

In the powder technical world we can create textures through a variety of means. Typically a formulator will load a product with either a dry, absorptive filler or a non-melting particle (or both) to create a texture. The filler dries up the binder to the point of restricting melt flow and thereby creating a texture. The incorporation of non-melting particles (typically PTFE-based, a Teflon^®-like material) causes protrusions in the powder film and hence a texture reflecting the size of the non-melting particles.

The product you’re using and experiencing trouble with probably uses neither formulating approach to achieve its textured surface. Polyester-based wrinkle finishes rely on a funky cure mechanism to create the wrinkled surface. This phenomenon is based on a very specific catalyst system that keeps the melted powder film fluid until long into the curing phase. Then after a certain amount of time in the oven the coating surface kicks over and tightens against the fluid film, creating the wrinkle.

Now here comes the chemistry part. This catalysis system is based on an organic acid. The influence that your wash system has on the wrinkling phenomenon is due to residual alkali on the surface of the metal. Most common metal wash systems use an aqueous solution of a strong base (aka alkali) such as sodium hydroxide. This residue neutralizes the catalytic acid, which essentially stymies the wrinkling mechanism. (Boy am I glad that I finished my chemistry degree.)

So what can you do to correct this problem? I suggest that you find a way to more thoroughly rinse the cleaning solution off the metal. This will eliminate the localized build-up of alkali residue that causes the interruption of the wrinkle. If you don’t want to follow this course of action you could consider switching your powder chemistry from a wrinkle finish to a more traditional texture based on filler or PTFE particles formulating technology.

Thanks for the question Aaron. It’s always a pleasure to justify all those college tuition payments I made long ago.

Regards,

Joe

Dear Joe,

BITS..the most common complaint of powder coating? What are the sources of “bits” in powder and how to eliminate or control them either during production process or during application? Please discuss.

Thank you,

Shafiullah Khan, Senior Manager Technical at Berger Paints, Pakistan Ltd.

Dear Shafiullah,

Indeed this is 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. Here are the most common sources of bits:

• Unclean raw materials - gel particles in resins, large particle filler pigments, agglomerates in additives (flow agents especially). 
• Free flow (dry-blend) additives - These 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.

Kind regards,

Joe

Dear Joe Powder,

What could be weather effects on epoxy polyester coated iron; sunny and salty environment and sun exposure?

Regards,

Ayoub

Dear Ayoub,

The weather effects would be quite severe. The epoxy-polyester-based powder will resist corrosion if formulated correctly, but the UV exposure would quickly degrade the coating. The degradation would manifest itself as chalking, yellowing and eventual polymeric breakdown. If a very thick coating (epoxy-polyester) were applied this may provide protection for a longer period of time, but the coating would erode incrementally. Regardless of which technology you consider, it is imperative to have excellent cleaning and metal pretreatment.

As for the best system for you to consider - I recommend a high-quality polyester or polyurethane applied to a zinc phosphate or iron phosphate pretreatment. An even better system would be an epoxy primer followed by a polyester or polyurethane topcoat.

Best regards,

Joe Powder

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