Coating Cracking

Fine cracks appear on the coating surface, with the crack directions random or arranged in parallel, and in severe cases forming a network of cracks. Coating cracking is the result of internal stress exceeding the fracture toughness of the coating — the difference in shrinkage between the coating and the substrate generates interfacial stress during curing, an excessively thick coating increases the absolute value of the stress, and an excessively fast curing speed leaves no time for the stress to relax. A cracked coating not only loses its protective and decorative functions, but the cracks also become channels for corrosive media to invade the substrate.

1. Material/formulation factor: large difference in shrinkage between the coating and the substrate. 2. Comprehensive factor: excessively thick coating leads to stress concentration. 3. Process parameter factor: excessively high curing temperature or excessively fast curing speed leads to accumulation of internal stress. 4. Comprehensive factor: insufficient flexibility of the coating itself (excessively high crosslinking degree).

1. Process parameter optimization and adjustment: select a coating system with a shrinkage rate close to that of the substrate. 2. Mold/tooling improvement and maintenance: control the coating thickness within the recommended range. 3. Equipment inspection and system maintenance: adopt milder curing conditions (appropriately reduce temperature or time). 4. Material/formulation optimization: add an appropriate amount of toughening agent or reduce the amount of curing agent. 6. 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.

The essence of coating cracking is that the internal stress exceeds the fracture toughness of the coating. The core of the solution is "stress reduction + toughening": control the coating thickness to avoid stress accumulation from excessive thickness, adopt mild curing conditions to allow stress relaxation, add toughening agents to improve the fracture toughness of the coating, and select a coating system with a shrinkage rate close to that of the substrate.