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Aluminium profiles painting, paint curing problem

October 14, 2009

Dear Sirs
My name is Miriam Grosu, I am a retired chemical engineer. I worked 40 years in the aluminium sector as specialist for surface treatment . Now I continue my activity as Technical Adviser, and am facing a problem.
One of my customers, an aluminium shutter manufacturer who has also a coil coating line, wants to replace his usual liquid polyester paint with a powder paint used in aluminium profiles' painting. The required working parameters for this powder are curing 180-200 degree C 12-15 min. The coil coating line's working parameters are 240-250 degree C 2-3 min. Despise my advise that the paint will be overcured , the customer wants to do it with a dark grey paint, so it cannot be seen if the pigment is burned. How can I check that the paint is overcured and has lost his weathering proprieties, the coil has 0.3 mm thickness so I cannot perform the impact test.
Thanks & best regards
Miriam Grosu

Miriam Grosu
Technical Consulter , Specialist in Metallic Surface Treatment - Kiriath Motzkin, Israel

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First of two simultaneous responses --

There are two tests you could carry out ¨ the first would evaluate the adhesion of the powder to the aluminium and the second the effectiveness of the cure.
a)Adhesion can be monitored by carrying out a cross hatch test, check internet for procedure.

b)The cure of a coating can be determined by gouging a groove into the cured powder film. Using a silver coin, press this hard down into the powder film, then pull the coin over the powder surface trying to destroy the integrity of the film surface. A smooth grove indicates an adequate cure a jagged grove indicates under cure.

Depending upon the mass of the item, as this will determine the time for the metal to reach the cure point, and unfortunately the temperature you mention may be inadequate and time factor could possibly require increasing up to 4 or 5 minutes - (requires evaluating before production run).

Terry Hickling
Birmingham, United Kingdom
October 19, 2009

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Second of two simultaneous responses --

Check the gloss, low gloss will be the first indicator that the powder is over cured.
You should be able to do all of the destructive testing that you want to do on a sample panel. Make a 6"x6" panel of the same material as the product and put it through the system, then beat the crap out of it.

Sheldon Taylor
supply chain electronics
Wake Forest, North Carolina
October 20, 2009

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November 12, 2009

Miriam, this is a difficult question without seeing the operation/process in action or knowing the specific chemistries that make up your powder. I wonder what type of system your running to cure the substrate, I.E. I.R., conventional etcetera.

Generally, one would never make a change in the operation line until "all" testing had been completed and you understand and approve of the outcome of your product.

Even though the temperature is very high, the dwell time is very short for your powder to achieve proper cure. Generally speaking, most powders can assume a 100% overcure in an oven. This means that you can run it twice in the oven and most powders will not be affected. Remember I said "most".

In your case, I don't believe the powder will react fast enough in it's filiform flow rate to lay a smooth finish on the substrate. You'll have to try it at your powder suppliers building first.

You have discussed this with them haven't you?

Bob Utech
Benson, Minnesota