Ask Joe Powder August 2024 – Defects and the matter of VOCs

PPCJ presents its latest column from our resident powder coatings expert, Kevin Biller AKA Ask Joe Powder. Any readers with powder coatings queries can submit questions to askjoepowder@yahoo.com

Dear Sir,

We are an automobile aluminium window manufacturer. We use aluminium extrusions for making windows and these frames are black epoxy powder coated. In our three decades of business experience, it is a common phenomenon that many frames are rejected due to bad powder coating. As the users of these frames are some of the big automobile companies, slight mistakes are detected and the entire window is rejected.

Now, the problem is no one in the powder coating industry does re-coating (i.e., removing the old coat and then re-coating again). Hence the entire window has to be scrapped. Is this because removing the powder coat from aluminium is not possible? Or practically not worth it?

We also did many experiments to remove the coat with various acids and alcohols, but it was not successful or practically viable.

Kindly provide the suggestions.

Regards,

Manabendra Pathak

Chennai, India

Dear Mr. Pathak,

This is a very common problem the world over. Powder-coated parts are assembled and sometimes even make it into the field when a serious visual defect is recognised. The challenge is to remedy the finish to the initial specification without having to scrap the part. This problem is multi-faceted, and I will attempt to address each issue.

The first problem has to do with the incidence of coating defects reaching the customer, in this case, big automotive companies. Obviously, your quality programmes are not sufficient enough to eliminate the delivery of out-of-specification parts to your customer. Two measures need to be implemented: 1. An improvement in your process and quality control to eliminate the root cause of defects, and 2. A better inspection process to eliminate the shipment of bad parts.

Eliminating the root cause through better process control involves a multi-step troubleshooting programme. You will be best served to use a brainstorming session with key production, engineering, sales, and quality personnel to:

1. Define and characterise the defect(s)
2. Examine possible causes (man, method, machine, material, Mother Nature)
3. Qualify and rank possible causes
4. Collect data to characterise defects
5. Explore and recommend remediation actions to eliminate root cause(s)
6. Implement remediation measures
7. Continually monitor quality and adjust processes and procedures as necessary

Concurrent with your root cause analysis efforts, it will be wise to improve your inspection process to catch out-of-specification parts before they are packaged for shipment to your customer. Catching parts prior to packaging will allow you to reprocess them much more easily than after they have left your facility – or worse, after they have been assembled by your customer. Your inspection process should employ highly trained individuals who can identify defects. It is very helpful to take photographs of typical defects and have the pictures prominently displayed in your inspection area. The operators should be aware of what constitutes a defect and what quality is acceptable. You should also keep daily records of defects and post the data on a board in the finishing/inspection area. Include definitive goals for quality.

As for fixing unacceptable parts once they have been assembled by your customer, this where I recommend a non-powder coating solution. You should work in conjunction with a liquid paint supplier to provide a touch-up paint to repair defects on assembled parts. It is requisite to work closely with the paint supplier to ensure acceptable adhesion of the touch-up paint to your powder coating. Likewise, it is important to collaborate on the specific process to repair the defects (i.e., identification of defect, sanding, cleaning, film thickness of touch-up, color requirements, etc.).

I do not recommend using a powder coating to repair an already assembled part, nor do I think it is wise to completely strip the powder coating if the defect can be ameliorated by sanding followed by cleaning and the application of a liquid touch-up paint.

The bottom line – focus on not producing defects and therefore, eliminate the need for fixing bad parts that have left your facility. Best of luck.

Regards,

Joe Powder

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Dear Joe,

Do you have any literature about VOCs (volatile organic compounds) in cured powder coatings? I know there is a very negligible amount, if any, during the curing process, but I cannot find any literature online about the matter.

As I am young and new to the coating process, I am told that there is not enough concentration to worry about, but the end customer wants literature about the matter to further interest him in the powder coating process.

Any help would be greatly appreciated!

Chase Lightner

Georgia

Hi Chase,

Regarding powder coating VOCs, it’s somewhat complicated. The traditional ASTM test method for coatings is run at 110°C for 60 minutes and produces essentially zero VOCs with any powder coating. A little moisture is emitted but nothing else. The current ASTM test methods are D2369-20 Test Method for Volatile Content of Coatings and D3960-05 Determining Volatile Organic Compound (VOC) Content of Paints and Related Coatings. D2369 describes the method and D3960 addresses the calculations.

Powder coatings are typically cured between 135°C and 220°C for 60 seconds to upwards of 30 minutes, depending on chemistry and oven design. Some powder coatings emit volatile organic materials either as low-molecular-weight fractions or volatiles of cure through condensation reactions. To further complicate things, these volatiles may or may not be considered VOCs.

Powder coatings can emit a number of volatiles that evolve in the oven environment during the powder bake process. These compounds are unrelated to volatiles generated through cure chemistry and include:

• Benzoin
• Water
• Low-molecular-weight monomers and oligomers
• Some waxes
• Some antioxidants
• Some light stabilisers (HALS) and UV absorbers (UVA)
• Miscellaneous organic additives

Below is a brief rundown of the VOC picture for powder coatings:

Essentially no volatiles of cure:

• Epoxies
• Hybrids
• Polyester TGIC
• Polyester GMA acrylic
• Polyester urethane (uretdione)

Relatively benign volatiles of cure:

• Polyester HAA (water)
• Polyester TMMGU (methanol)
• Polyester urethane (e-caprolactam)

How you handle your customer requires careful research of your product(s), the customer process, local regulations, and the customer’s culture. I recommend that you evaluate your products in question using ASTM D2369 and also the recommended cure conditions. You can simply gravimetrically determine the non-volatile content at these conditions to provide knowledge of amount of material emitted. You also can have an analytical lab perform a head-space analysis of volatile components using FTIR and GC-MS (gas chromatograph-mass spectroscopy). This will give you a fairly precise read-out of what’s evolving from your product. How you report your findings to your customer is a judgment call.

Please let me know your thoughts.

Thanks,

Joe Powder