Coating Yellowing/Discoloration

The coating turns yellow in color after curing or during use. When this defect appears in coating-formed products, it is usually closely related to the coating head precision, the rheological properties of the coating liquid, the substrate surface condition, or the curing process conditions. The distribution characteristics of coating defects (transverse/longitudinal/random) provide important clues for diagnosing coating head and curing system problems. Continuous occurrence will cause the functional performance of the coating to fall below standard, and coordinated troubleshooting is required from three aspects: coating liquid formulation, coating equipment, and curing process.

1. Material/formulation factor: excessively high curing temperature causes thermal oxidative discoloration of the coating liquid components. 2. Material/formulation factor: certain components in the coating liquid formulation (such as amine curing agents, aromatic compounds) are naturally prone to yellowing. 3. Material/formulation factor: additives in the substrate (such as plasticizers, antioxidants) migrate into the coating and participate in reactions causing discoloration.

1. Process parameter optimization and adjustment: lower the curing temperature while extending the curing time. 2. Mold/tooling improvement and maintenance: select yellowing-resistant raw materials (such as aliphatic curing agents, UV stabilizers). 3. Equipment inspection and system maintenance: evaluate the chemical compatibility between the substrate and the coating. 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 yellowing/discoloration 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.