Micro-bubble Defects of Barrier Coatings (Such as PVDC) Under Humid and Hot Conditions

After PVDC coated products are stored for several months, dense tiny bubble spots are visible to the naked eye. When this defect occurs in coated products, it is usually closely related to coating head precision, coating fluid rheological properties, substrate surface condition, or curing process conditions. The distribution characteristics of coating defects (transverse/longitudinal/random) provide important clues for diagnosing coating head and curing system problems. If it persists, it will cause the coating's functional performance to fail standards, requiring coordinated investigation from the three aspects of coating fluid formulation, coating equipment, and curing process.

1. Comprehensive factors: the PVDC coating slowly absorbs water under humid heat and coalesces into bubbles at weak interfacial bonding points. 2. Substrate surface condition factors: 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 coating fluid surface tension, wetting is insufficient and cratering occurs; surface contamination causes local adhesion failure; this is the primary direction for troubleshooting coating defects. 3. Curing process factors: Curing temperature and time determine the completion degree of the coating crosslinking reaction—insufficient temperature or insufficient time leads to incomplete curing (tacky, low hardness, poor solvent resistance), excessive temperature or excessive time leads to over-curing (embrittlement, yellowing, cracking); the curing window of the coating fluid system must be precisely matched.

1. Process parameter optimization and adjustment: Strengthen the anchoring force design between the substrate and the PVDC coating. 2. Mold/tooling improvement and maintenance: Add a primer layer to make it firmly bonded to the substrate. 3. Systematic troubleshooting method: Troubleshooting of coating problems follows the order of "substrate → coating fluid → coating head → curing → environment". First check the substrate surface tension (corona treatment effect) and cleanliness, then verify the coating fluid viscosity, solid content, and degassing status, then 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. 4. 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 5Why analysis to trace the root cause, and solidify 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 micro-bubble defects of barrier coatings (such as PVDC) under humid and hot conditions in coating. The core of the solution lies in ensuring the substrate surface condition, optimizing the rheological properties of the coating fluid, and precisely controlling the curing process conditions. It is recommended to start with the substrate and coating fluid and gradually investigate to the coating equipment and curing system.