micro cracks in polymer coating 2

[josefel]

Hello, we use a polymer coating on steel and although it comes out of the salt/fog chamber fine, after about 24 hours it shows micro cracks. Any ideas? Thanks in advance.

 

[cloa]

What is the ambient conditions and what is the polymer?

 

[josefel]

My first day on the job is today. I will find out for myself and then pass it along. I am unsure of the polymer, but the ambient conditions are just regular RT temperature and humidity as far as I know. Thanks for your response, cloa.

 

[josefel]

The polymer is a spray on powder coat polyester. It is fine when it comes out of the salt/fog chamber, but after aging for ~24 hours at RT, micro cracks/crazing is observed. According to the Datapaq cure cycle profile it is being cured properly. Thoughts? Thanks.

 

[Compositepro]

Micro-cracks indicate shinkage and perhaps degradation. Polyester resins can hydrolize (react with water) which degrades the polymer. They will also physically absorb water which will cause some swelling and then shrinkage when drying.

 

[TVP]

josefel,

Based on your most recent description, one of the things I would look at is the ratio of resin to hardener used for the powder coating. If the paint is being cured properly (time and temperature), then it is possible that the ratio is not correct, making the part more susceptible to water absorption and subsequent shrinkage/cracking. I suggest discussing this issue with your paint supplier or a knowledgeable independent test lab.

 

[josefel]

Thanks for the responses. They were very helpful.

 

[josefel]

compositepro, wouldn't the polyester resin have to be under-cured to hydrolyze? If it were fully cured would it still be able to react with the water?

 

[Compositepro]

Polyester is a polymer formed in a "condensation" reaction. This means that water is a byproduct of the reaction. Most chemical reactions can go either way to reach an equilibrium between the forward an reverse direction. The actual equilibrium depends on concentration of the reactants and products, and temperature and pressure. In high temperature and pressure water polyester resin will disintegrate quickly. At lower temperature and pressure and water concentration it can last a long time. Thin coatings, however, have the least longevity.

 

[josefel]

Compositepro, we are not synthesizing the polyester. It is a prepared powder coat from a reputable vendor. We are applying the polyester clear coat onto a metal substrate, curing it and then subjecting it to salt/fog testing. Then after ~24 hrs at rest at RT, the micro-cracking/crazing is observed. From what I can tell, this is a problem with improper curing. Why the problem takes ~24 hrs at RT to manifest itself is what is flummoxing me. Your point regarding water uptake, then drying leading to shrinking/cracking seems on the right track. I apologize if I am being redundant.

 

[swall]

Is your coating thickness within the limits recommended by the manufacturer for this particular powder formulation?