Q: I am looking for insight on some samples that we cured for a potential customer. We are trying to cure a clearcoat epoxy, and we used three processes:
- Infrared (IR) using quartz tubes at 70% power for 4 minutes
- IR for two minutes and then 10 minutes in a convection oven at 375°F
- Curing fully in the convection oven for 18 minutes at 375°F
Here’s the problem – all IR-cured samples cracked, and the convection samples did not crack at all. We are thinking that we just cured too fast with the IR and “burned” the powder. We would greatly appreciate your thoughts and input.
A: Using IR for a clearcoat is tricky business. IR works best and most completely when the coating contains a pigment. With clearcoats, the dry powder absorbs heat from the IR until it melts. It then becomes transparent and essentially stops absorbing much more radiant energy. I suspect that the IR-cured sample is partially cured and hence the cracking.
A good way to check is testing solvent resistance. Rub each sample with acetone, MEK, or lacquer thinner. This will give you an idea of degree of cure.