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Blisters on horizontal surfaces of Polyurethane painted parts

2003

Dear All,

In our paint shop, blisters are formed on sheet metal components, only on horizontal surface. These blisters appear after two days of painting and some appear in the morning and disappear in afternoon. I don't understand these special behavior of blisters, but if you scratch before evaporation it removes the paint. Blisters appear only for PU paint and not visible on Acrylic paint for the same component. we are having two stage degreasing,rising,zinc phosphating,rinsing,non chrome spray,DM rinsing.

Please suggest what and where the problem is and why blisters appear only on horizontal surface ,Why it appears and disappears after two days.

Murali
automobile - Pune, Maharastra, India

Based on my experience I would say you have a rinsing problem. The rinse following the zinc phosphating being the most likely culprit. This rinse is meant to remove reaction products (salts) after phosphating. As this rinse is normally maintained at ambient temperature a lot of coatings go below their glass transition temperature when they hit this rinse. Brittle overspray on the racking then produces paint chips that plug the spray nozzles. The inadequately unremoved salts then get trapped under the paint film. This film then acts as a semi-permeable membrane allowing water to pass. With high humidity osmotic pressure builds up due to the trapped salts causing the blistering. When the humidity goes down the blisters subside. The acrylic coating being more brittle than the polyurethane resists the pressure build up more effectively.

Ronald Zeeman
- Brampton, Ontario, Canada
2003