Microbubble problems in airless spraying of polyurethanes

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Dear experts, I’ve recently run into a tricky problem related to the microbubbles that appear on the surface during the airless spraying of polyurethane. I’ve tried many solutions without success and really don’t know what to do anymore. Let’s see if anyone has any suggestions. 1. Overview: A two-component solvent-based polyurethane system based on hydroxyacrylic acid and HDI trimer, with a volume solid content of 55%, and a typical dry film thickness of 50–75 microns. The surfaces obtained by brushing, rolling, pouring, and air spraying are all excellent, free from defects such as microbubbles; however, once cured by air spraying, the surface develops numerous microbubbles. 2. Solution 1) has been tried: air-free construction conditions. Adjusting the pressure, nozzle size, and dilution ratio only yields limited improvements ； 2) A new defoaming system was tried; many options were considered, and well-known suppliers were contacted for recommendations, but there was still no improvement ； 3) Setting of the curing rate: increasing it speeds up the process (with the aim of preventing bubbles), while decreasing it does not bring any improvement ； 4) Adjustment of solvent evaporation rate results in limited improvement. 3. Questions for assistance 1) Causes of microbubble formation. Since it cures with isocyanates, we all assume that water vapor is the cause. But under the same conditions, construction with air does not produce microbubbles; how can this be explained? (Although I didn’t think so, I still tried adding substances to remove water, but there was no improvement.) 2) This problem does not occur in the formula of the foreign version. But once this native hydroxyacrylic resin microbubbles are used, that’s when the problem arises. Just looking at the resin, what do you think are the differences in the resin itself that cause this difference? 3) Suggestions for solving this microbubble problem. Thank you all for the active discussions!