Powder paint cure problem: poor impact resistance

A. Hi Paul,

I always thought that the impact test measured a paint film's ability to adhere to a surface after being severely dented on impact. I didn't think it had much to do with whether the paint had cured completely. Does the paint have a spec for MEK rubs? If so, you can check to see if your painted parts meet the spec. If they do, then you don't have a curing problem. If you have an adhesion problem, maybe it is related to the aluminum pretreatment.

Does the paint supplier's spec for the impact test describe the pretreatment that is used on the metal surface? If your pretreatment is different, then you may not get the same result in the impact test. You should also make sure that whatever pretreatment you are using is within the recommended parameters.

A. I don't believe your rapid cooling down is responsible for your impact resistance failure. You may contact the powder manufacture to see if he has a harder coating available, or if you might try a different pretreatment such as a wash primer.

A. Paul,

I have never really seen anyone perform an impact test upon an aluminum test panel. What grade is the aluminum? The reason impact testing is not done upon aluminum is because aluminum simply cannot take the impact pressure. I assume that you are then testing upon a steel surface. Remember that when we specify temperature specifications, it means that the substrate must see that temperature for 10 minutes, not just be exposed to the temp that long. To test this, I would take a commercial "steel" test panel and coat it with your topcoat. Put it in your oven for at least 15 minutes. Most topcoats will easily survive an over cure. Then test it. It should be cured. If not, have the powder vendor send you a panel painted with the same lot number you currently are painting and see if their panel is cured. If it is, check your pretreatment.

Chipping is not always related to under cure situations. It may be that your aluminum is not getting the etch it needs to secure the topcoat. What exactly is your pretreatment process? Generally a high alkaline or acidic cleaner will give very good topcoat performance and meet your specifications. Also, make sure that you give your parts proper rinses after. Remembering that any Total Dissolved Solids "TDS" that remains in the rinse water, end up on your parts, which could sacrifice your specification/s.

A. Paul,

Always look at your pretreat, then step your way through your process.

1)are you running your process within specification of your supplier recommendations?
2)you are positive that you acquired (metal temperature)to achieve cure schedule,at required cure time?
3)do all shape and configurations of parts fail?

Not knowing your pretreat practices, part configuration(density,racking,etc.)several factors may be involved. Pretreat of aluminum on a production line requires constant vigilance, cleaning,rinses and etch merely set the stage for non chrome seal, if this is what you are using. Final pretreat, prior to topcoat, whether liquid or powder will be the key to consistency knowing you have your cure schedule decisive.

A. Impact Resistance is typically a "formulated" property. It is tested on ACT or Q panels in the powder coat manufacturers lab, and the test usually has no use at the point of application due to varying substrate composition and thickness. The best thing you can do is receive a supplier C of A that it passed impact under their controlled environment, etc. The most pertinent tests at the point of application on production parts are:

1) Cross Hatch Adhesion
2) Solvent (MEK) Rub
3) Pencil Hardness
4) Gloss
5) Film Thickness

A. Paul,
What type of powder are you using? When the company I work for began powder coating our aluminum door line we used a urethane based powder from Sherwin Williams. Of course the engineering dept didn't bother to do any testing prior to ordering a couple thousand pounds. We experienced the same type of symptoms you are describing. And of course Sherwin Williams blamed cure and pre treatment initially, however after switching to a polyester hybrid the problems disappeared. Nothing like throwing away several thousand dollars worth of powder due to listening to a salesman rather than real world testing......

A. Hi Gurpreet. We added your inquiry to a thread which was already addressing this problem. You may also find the "Powder Coating over CED Coating Fails Impact Test" subheading section of letter 23620 helpful. Good luck.

Regards,