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Sherwin-Williams


Problem Solving Forum

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March 4 - March 10, 2013

What causes blistering/crater formation in freshly applied polyurethane topcoats, and how can this be avoided?

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Selected Answers

From Stephen Bothello of Jotun Paints on March 6, 2013:
     Blistering is the formation of bubbles or raised protrusions, like pimple,s whereas cratering is the formation of small depressions on the surface of the paint film  that do not expose the underlying coat. Blistering is most often caused by moisture contamination in the hose/paint line or during mixing, particularly during high humidity conditions or even during  cold conditions, e.g., night shifts when condensation is caused in the line/metal surface as temperatures cool down.
      The hardener or isocyanate has a general preference to react with moisture/water. Ensure hose/line is cleaned by flushing with fresh thinner. Make sure relative humidity is below 85% and that there is no condensation on steel.The steel temperature should be 3  C over the dew point at the beginning of painting.
      A  frequent, operator-related cause of blistering or bubble formation is solvent entrapment due to application of higher than recommended paint thickness per coat, wherein the surface of the wet paint film dries rapidly, preventing the solvent release from within the paint film. The solvent will, however, rise to the surface within the paint film, as a result of restriction from the already dried surface to form bubbles/blisters at the surface. In certain case, the blisters/ bubbles will burst, forming circular depressions that do not expose the underlying coats, called craters. This can be avoided by eliminating all chances of moisture contamination and applying paint at the recommended thickness by reducing with the recommended thinner at prescribed levels.
     Cratering is also caused by air pockets that are produced in the specially fast drying PU topcoats, by fine particle contamination in the paint or excessive/over mixing just before application. In this case ensure proper filtering and straining and avoid over mixing. Cratering can also be caused by silicone/ oil contamination in the paint line or more than required level of flow and wetting additives added in the paint itself! In this case, it will require a proper investigation by the manufacturers technical representative.

From Jorge Lizarraga of International Paint on March 5, 2013:
    There are several causes for the blistering/crater formation in a freshly applied polyurethane coating, such as a high wet film thickness applied in one coat, high ambient humidity that reacts with the isocyanate leading to CO2 bubbles, high surface temperature, or a very light solvent used in high ambient temperatures.  But the most common cause is the application of high wet film thickness in one coat. This is caused by the evaporation of the solvent, in the form of bubbles, which, when they reach the coating surface, or burst,  a crater results, or if the bubbles are trapped, blisters form. The way to avoid this effect is to apply the coating in several light coats, in a moisture-controlled atmosphere, with a suitable solvent.

From Dipesh Vyas of CARBOLINE on March 8, 2013:
     Besides the formulation part , I would like to add a few points from my expereince which can be helpful in the field when troubleshooting. (1) When applied over epoxy mio-rich mid- coat , one solution is to apply a mist coat and then apply the full coat. (2) Consider contamination from the spray assembley, hose & pump etc., especailly when you have finished applying the epoxy mid-coat. The majority of epoxy thinners are not compatibile with PU, and this can be overcome by pre-cleaning the spray assembly with pu thinner before starting, and do not use this thinner for PU paint dilution.

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Tagged categories: Blistering; Coating failure; Failure analysis; Polyurethane; Topcoats


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