Coating Blushing

After curing, the coating shows blushing or hazing, and the originally transparent or high-gloss coating becomes translucent or milky white. The essence of coating blushing is that when the ambient humidity is high, the rapid volatilization of the solvent absorbs heat, causing the coating surface temperature to drop below the dew point, and water vapor condenses in the coating to form micro water droplets that scatter light. Blushing not only affects the appearance transparency, but also means that moisture has been mixed into the coating, which may affect interlayer adhesion and long-term durability.

1. Comprehensive factor: when the ambient humidity is high, the heat absorption from solvent volatilization causes the coating surface temperature to drop to the dew point, producing water vapor condensation. 2. Process parameter factor: the solvent volatilizes too fast, causing sudden cooling of the coating surface. 3. Substrate surface condition factor: The surface tension, cleanliness and microscopic roughness of the substrate are the basic conditions for coating adhesion and leveling. When the substrate surface tension is lower than the surface tension of the coating liquid, wetting is insufficient and craters occur; surface contamination leads to local adhesion failure, which is the primary direction for troubleshooting coating defects. 4. Curing process factor: the curing temperature and time determine the completion degree of the coating crosslinking reaction — insufficient temperature or time leads to incomplete curing (tacky, low hardness, poor solvent resistance), while excessive temperature or time leads to over-curing (embrittlement, yellowing, cracking), which requires precise matching to the curing window of the coating liquid system.

1. Process parameter optimization and adjustment: control the coating environment humidity below 70%. 2. Mold/tooling improvement and maintenance: select a solvent system that is not prone to causing blushing. 3. Equipment inspection and system maintenance: appropriately preheat in the oven to remove surface water vapor. 4. Systematic troubleshooting method: troubleshooting of coating problems follows the sequence of "substrate → coating liquid → coating head → curing → environment". First check the substrate surface tension (corona treatment effect) and cleanliness, then check the viscosity, solid content and degassing state of the coating liquid, next check the coating head gap and cleanliness, further verify the curing temperature and time, and finally evaluate the influence of ambient temperature, humidity and cleanliness. 5. Preventive management: establish standardized process parameter cards and quality control limits for each mold/product, and regularly conduct process capability index (CPK) evaluations. For recurring problems, use fishbone diagrams or the 5Why analysis method to trace the root cause, and solidify the countermeasures as PFMEA control points. Establish a regular equipment maintenance system, and implement life management for key tooling such as dies/spinnerets/calender rolls/coating heads.

This problem is a typical manifestation of coating blushing in coating forming. The core of the solution lies in ensuring the substrate surface condition, optimizing the rheological properties of the coating liquid, and precisely controlling the curing process conditions. It is recommended to start from the two aspects of substrate and coating liquid, and gradually troubleshoot to the coating equipment and curing system.