Is acrylic Waterborne coating being over cured?

A. An indication of overbake is that the color swings yellow. Have you put a sensor pack into your oven to check the temperature? What about lowering the temperature at different intervals on some test pieces to see if you can obtain the proper color, and then test the paint to see if it has retained it's mechanical properties. Your supplier isn't going to be too interested until you prove to him that it's his problem; if the paint is overbaking at his recommended cure cycle, then it's his problem.

A. The manufacturer usually specifies a temperature for a time period. Is your product in a heat tunnel? If so, this may be your problem. The product may be heating for longer periods at different and inconsistent temperatures. Also, how well are you mixing your received product? Things to think about.

A. Dear Micael,

You do not inform us whether the aluminium powder coated items, once baked, meet the demands of your customers. If they do - then why worry? You could test for the adhesion of the powder to the aluminium by applying a cross hatch followed by tape. If it sticks, then it is okay. As a supplier of powder coating materials we, like all other mfgrs., test the material on 6 x 4 panels under ideal conditions. Our technical data sheets specify certain parameters at which we would like our powders to be cured/baked. I would suggest that not all users will or can adhere to that. Example: The alloy wheel mfgr., 40 mins minimum at 210 °C or keg mfgr. 165 °C for 14 mins. There are many more examples but, if at the inspection point the powder does not crack, has good adhesion with the correct gloss and colour -- forget what the mfgr. tells you. If what comes off the line has no defects continue as you are.

A. Dear Micael,

I will attempt my best for the limited information you provided. This could be a pretreatment issue but I will assume for the sake of this discussion it is an oven or powder issue. I do not know if this oven is convection or not.

First, you need to provide the temperature that the powder manufacturer recommends. You have not explained if every color yellows or if it is whites or clears. You said the Datapaq indicates you have a lower temperature than the other two probes. Does your color change from top to bottom also? I do not know if your oven is bottom, side or topflow, but you should have even air distribution within your oven to achieve even coatings. Remember that the sole purpose of your oven is to cause a chemical reaction of the powder. The proper amount of heat at a given time insures that the desired decorative, chemical, and mechanical properties are realized.

A powder coat convection oven should be able to exhaust the entire oven air volume several times per hour (typically exhausting 10 times minimally per hour).

Your oven should have some means to keep the heat contained within the oven via an air-curtain, etc. Once you dial in your oven to achieve the proper temperatures and can maintain them, you should try different powder manufacturers to see if you're having the same problems with yellowing with their products. You can also ask your powder supplier to provide their own in-house testing to see if they see the same reactions you are seeing.

A. Micael,

Your oven temperatures are average and are okay. Generally speaking, you should bring your topcoat temperature up rapidly because powder tends to flow better and quicker which helps to give a nice cosmetic finish. I have seen a few ovens that have their burners flaming directly into the oven, which could affect the outcome of the finish though I doubt you have this issue. If you had this issue, it would definitely show up when you ran an oven profile. I would like to know if you have ran any tests such as a cross-hatch test, mandrel bend, and M.E.K. test? Do you have only one powder supplier and if not, do your other powder suppliers have this same issue?

A. Micael,

If your oven has a direct heat fired doghouse, you may or may not see a discernible observation on a recorder. If the conveyor runs directly in front of the flame and is close enough, you will see a noticeable rise on the data recorder. If the flame is far enough away, you will not.

My guess is the powder is your culprit. You need to get powder from a different supplier and run it through your system. I would bet you will not see the same yellowing. Sometimes powder supply companies use a poorer grade of filler in their powder in order to save them and you money and you end up with this result. If you don't complain, they keep sending you the same powder. You owe it to them to check this out.

I am concerned because you stated you were going to ask the powder supplier to cure some panels in their laboratory and it sounds as though this has not happened. When you don't initiate corrective practices, it sends messages to the entire industry that there are problems associated with powder coating, when really, these are easily solved issues.

A. The powder consumable you are using will have a manufacturer recommended cure schedule.
That will be a time at METAL TEMPERATURE not a time at oven air temperature.
To accurately assess your compliance with manufacturer recommended cure schedule you will need to have a thermal data logger to regularly check your compliance with manufacturer specs. Without such a checking mechanism you are "flying blind".
It certainly sounds like overbake or burn but why not just get a bit professional and ensure that you do it right first time every time?
Hope this helps.
Regards,
Bill

A. I agree with Bill, it sounds like over cure -- the most common cause of yellowing.
Start with powder maker's recommendation, then check that you get this. It's a time and temp thing, and metal temp, not air temp inside the oven. For many applications under 200 °C is desirable to adjust time to suit. You'll need a recording set of thermocouples that you place in the oven to see the temp/time curve for different thicknesses of steel in different parts of the oven.