Test to determine if powder coating is fully cured

• Topic 15333 "MEK/Solvent Rub Test? Cure Tests"

A. Hi Matt

The other way of determining the curing effect is "Solvent rubbing resistance"

Solvent used for this test is normally Methyl Ethyl Ketone.

Test Method:
Dip a plain cloth in the described solvent and make 10 double rub on the part.
Evaluation:
If your part is not cured properly the cloth will have some paint on it.

Hope this test will helps to clear your doubts.

Let me know if you have further assistance on this.

Vijendran

A. Are you using a convection oven or IR?
If IR then the problem is easily explained.
Regards,
Bill

A. Hi Again,
Have you, yourself conducted an MEK 32 rub cure test on the offending components?
If not, please do so.
Cure testing with polyesters has become a bit of a black art.
The long established and proven 32 rub test with MEK has been somewhat hi-jacked by some, most notably Orika/Dulux who tend to substitute the solvent resistance test as an alternative.
This they do by providing the customer with "cure test solvents" which are not straight MEK.
In general to your last post I would say never, ever mix powders from different supply.
There are very good reasons why every box is marked with both batch & box numbers.
This is largely to provide traceability in the event of problems such as you are encountering.
Hope this helps,
Regards,
Bill

Hi Anna,
I am a little confused by your posts.
I originally understood that you had conducted a 32 rub MEK cure test with satisfactory result and then were disturbed to find that a droplet of Acetone completely dissolved the powder film.
Your subsequent post seems to contradict that position?
I am assuming that you are using Polyester Powder?
If so, at the end of the day your product is failing the standard 32 rub MEK cure test?
If that is the case then you only have two variables:
1 Faulty batch of Powder(unusual but not unheard of)
2 Faulty cure schedule (time at metal temperature)

The most common error with cure schedule is to assume that oven air temperature translates directly to metal temperature. Have you run a thermal data logger through your system attached to a piece of the same material that is creating the problem?
If not it is essential that you do so to create a true time/metal temperature graph (cure schedule)
I have some difficulty with your references to glass transition temperature.
I may be wrong but I can conceive that initial Gelling of the powdercoat could be regarded as Tg.
Initial gelling does not ensure that full cross polymerisation of the powder has occurred, rather that it has merely initiated.
Your Powder Supplier must have a thermal data logger if you do not. They should provide a check against your own methods on a regular basis in any instance.
Please keep me updated with your progress.
Regards,
Bill

Hi again Anna,
I am intrigued.
Your Powder supplier is probably correct about consistency within the same box.

It seems the most likely way to achieve inconsistency in the powder applied would be to do so within the process.
If you are using a vibratory feed system rather than an aeration system for example (this usually only causes problems with metallic powders).

I have not asked if your process is a batch system or a fully conveyorised system?

I recall training a group in Australia who had a batch system. Their oven was set up with a system which kicked in to time once the object had reached cure temperature and then send an alarm to unload.
They always had perfect Time/ Temperature graphs.
Unfortunately, they also had consistently undercured products.

Sound familiar?

Their problem was that no-one understood the significance of where to place the thermal data logger clamps.
For convenience they left it clamped to the inner surface of the oven rather than the workpiece.
The only cured pieces out of that oven were the last ones.

I hope that I have assisted you and would very much like to hear how your problem is resolved.
Best Regards,
Bill

A. Hi Anna,

Just check whether you are using right temperature grade of grease or lubrication oil in your conveyor.

A relatively low temperature grease (very close to the curing temperature, say 250 °C) will burn and form fumes. These fumes can interfere your coating randomly; however more temperature you cure the substrate. Watch for smoke from oven entry/exit doors.

Use lube oil with a working temperature of 350 °C and avoid greasing, if used.

Please update your feedback.

A. We in Australia, refer to this Epoxy/Polyester blend as Hybrid. Normal cure test procedures are used: 32 rubs with MEK resulting in negligible discolouration on the white rub rag signifying acceptable cure. We also have a solvent resistance test (quite separate) which uses Acetone.
It appears that your product is significantly under-cured.
The usual error for "young players" is to assume that oven air temperature directly translates to metal cure temperature.
This is obviously quite wrong.
I would suggest you acquire a thermal data logger and check the actual "cure schedule" : time at metal temperature you are actually working with.
In the first instance you could impose on your powder supplier to conduct a test with their equipment.
The logger should cost <US$2000.00.
It is essential equipment for any quality coater.
Hope this helps.
Regards,
Bill