Post-curing Dark Reaction of UV-cured Coatings Causing Products to Become Sticky After a Long Time

The surface of calendered products is sticky and prone to blocking, with an obvious sticky feeling when touched by fingers, and the film layers are difficult to separate after being pressed together. Product stickiness directly affects winding and storage—film layer blocking makes unwinding difficult and, in severe cases, damages the film surface. Causes of stickiness include plasticizer surface migration, insufficient cooling, and improper formulation design, and it is most common in soft PVC calendered products.

1. Comprehensive factors: incomplete oxygen inhibition in the system or slow deep cationic curing propagation reaction causes residual monomers to be generated in the post-curing process. 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.

1. Process parameter optimization and adjustment: Use nitrogen protection during UV curing to completely eliminate oxygen inhibition. 2. Mold/tooling improvement and maintenance: Increase photoinitiator concentration and exposure dose. 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.

The essence of incomplete curing is that the crosslinking reaction has not reached the designed conversion rate. The core of the solution is the precise matching of the three elements of "temperature + time + formulation": ensure that the curing temperature and time are within the recommended window of the coating fluid system, verify the curing agent ratio and validity period, and for UV curing, regularly test the lamp light intensity and replace it in time.