Ask Joe Powder April 2024 – A question of temperatures

02 April 2024

PPCJ’s regular columnist, Kevin Biller AKA Joe Powder, answers readers questions on all things powder coatings. Readers can submit questions to [email protected]

Hi Joe,

We powder coat welded components with a high-gloss red polyester powder. These parts consist of a box made from 11-gauge metal that has a section of 1/4″ (6.4 mm) wall pipe welded to it, with a 1″ (25 mm) thick flange at the end of the pipe. We are having trouble with a haze developing on the thick flange after cure, as the part begins to cool. The haze can be wiped off, and the powder is glossy underneath.

We also noticed it on some other parts as well. All were high-gloss polyester powders and mostly on heavier surfaces.

Can you explain what is causing the haze and how to prevent it?

The process for these parts is grit blast, 8 stage iron phosphate pre-treat with a DIP (dry in place) sealer, and the polyester powder is cured at 375°F for 23 minutes.

Thanks for any insight you can provide.

Sylvan Martin, Pennsylvania

Hi Sylvan,

Thank you for the query. What you are observing is a textbook phenomenon known as blooming. This occurs with some polyester powder formulas when the coating is exposed to an elevated but not necessarily high temperature for a long period of time.

The parts you describe are relatively thick gauge, which will take a long period to cool from baking temperature to room temperature. It is this long cool down that allows the polyester resin to exude a cyclic oligomer comprised of the main two monomers used to make polyester resins for powder coatings. This phenomenon is very common with virtually all standard-grade polyester powder coatings.

Eliminating the root cause of this defect can take two paths. One would be to eliminate the long cool-down period of the parts exiting your oven. This could involve quenching the parts with water or a high-velocity cooling tunnel. Neither of these options would be very economical.

The best approach would be to contact your powder coating supplier and have them provide a non-blooming polyester powder coating. This technology is well-known by the better formulators. If your powder supplier doesn’t have non-blooming grades of polyester powder coatings, then it may be time to solicit alternate suppliers.

Best regards,

Joe

READ MORE:

Ask Joe Powder February 2024: Kitchens and wrinkles

Dear Joe,

I do not know how to control the Tg (glass transition temperature) of a powder coating. I know that TGIC reduces Tgvalue by 1.5°C. But what about other ingredients like additives (e.g., flow agents, CAB, texture agents, waxes, fillers, pigments, etc.)?

In the summer, my customers have problems with the sintering of powders. The Tg value of resins that I use seem to be high enough.

Do you have any suggestions about this?

Best regards

Ozlem Ekinci, Turkey

Dear Ozlem,

This is a very good question, especially if the powder coating will be exposed to relatively high ambient temperatures. The Tg or glass transition temperature refers to the point at which a material undergoes a change from a rubbery to a glassy stage. It’s measured analytically by differential scanning calorimetry (DSC), which measures the flow of heat from a material as it experiences a carefully controlled increase in temperature (usually at a rate of 10°C per minute). As a material melts, it absorbs heat (endothermic). When a chemical reaction occurs, heat is released (exothermic). You probably know this already.

In layperson’s terms, Tg is best considered a sophisticated method to measure softening or melt point. One other issue – the Tg of a powder coating before the material undergoes cure is a good indication of melt point and relates to physical storage and handling stability. The Tg of a cured powder coating is a good indication of degree of cure, hardness, chemical resistance, and overall coating performance.

Therefore, your question relates to the effect formulation components have on the Tg of a powder coating, in this case a TGIC polyester. The Tg of a polyester powder coating is mostly influenced by the Tg of the polyester resin used. Nevertheless, other components affect the overall Tg of the powder coating, especially TGIC (as you mentioned, it depresses the Tg about 1.5°C). Other components affect the powder Tg as well, including waxes, CAB (cellulose acetate butyrate), and flow agents to a lesser extent.

Pigments and extenders do not affect Tg, as they are non-reactive and relatively inert – they do not melt or affect the melting point of the resinous components. They may affect the melt viscosity, but this will have little (if any) effect on the Tg.

As mentioned, the Tg of the polyester resin has the greatest effect on Tg. Commercially available polyester resins span a range of Tg from about 52 to 73°C. If you are attempting to increase the Tg of a powder coating, you should investigate formulating with a higher Tg polyester. If your formula contains organic additives such as wax, CAB, or plasticiser, you should evaluate formulas with reduced levels or the elimination of these additives.

Another issue to consider is the quality of the polyester resin. Low-quality polyesters may have low-molecular-weight impurities that can reduce Tg and compromise storage stability.

DSC is the best technique to measure Tg. If you do not have access to a DSC instrument, you can use a more empirical method such as tack temperature or storage stability in a temperature-controlled water bath.

Best regards,

Joe

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