Two-Component Coatings, Part II

Question:

These remarks are in reference to your October 2001 clinic question on two-component coatings. My company supplies a lot of 2K systems in low/satin gloss. You are correct that changing the catalyst/hardener ratio can cause a difference in the performance properties. The coating will likely cure as the leftover isocyanate will react with the moisture in the air.

Most likely the gloss variation is caused by dry film thickness, which you cite as painter technique number five. Other factors, which might be more important, can be the flash time between when the parts are sprayed and baked and the humidity. You are correct in that a lot of effort has gone into formulating low/satin gloss urethanes. Formulations act differently depending on the inert flatting pigment that is used and the particular resin system. All suppliers are not created equally when it comes to low gloss urethane paints and gloss control. C.S.

Answer:

Thank you for your comments on the importance of following supplier’s instructions for using two-component coating materials and pointing out additional factors that could cause problems. I remember a case where I recommended a two-component primer for power circuit breaker tanks. The paint came off “in sheets.” In the final analysis, it was discovered that the two-component epoxy primer was applied to the steel without the catalyst. It was the first time the painters had used two-component paints. They thought the other can (the catalyst) was a reducing solvent. In those days, the position of painter was the first promotion from the entry-level position of sweeper. Since the vehicle in the paint was a solid resin, it appeared to be dry after the solvent evaporated. The solvent in the subsequently applied topcoat redissolved the primer causing it to lose adhesion, and the entire paint film came off “in sheets.”