Excessive Solvent Residue

The amount of residual solvent after curing and drying of the coating exceeds the standard limit (for example, food packaging requires a total solvent residue of ≤5mg/m²). Excessive solvent residue directly endangers food safety and human health, and is a mandatory control indicator for food flexible packaging coated products. Residual solvent also affects the long-term adhesion between the coating and the substrate — the slow volatilization of the solvent causes the coating to continuously shrink and generate stress, leading to later cracking or peeling.

1. Material/formulation factor: insufficient curing temperature or insufficient drying time. 2. Equipment factor: insufficient exhaust air volume of the oven, and high solvent vapor concentration limits further volatilization. 3. Comprehensive factor: the coating amount is too large, exceeding the drying capacity. 4. 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.

1. Process parameter optimization and adjustment: increase the curing temperature and reduce the line speed (increase the effective drying time). 2. Mold/tooling improvement and maintenance: increase the exhaust air volume of the oven to discharge solvent vapor in time. 3. Equipment inspection and system maintenance: appropriately reduce the coating thickness or increase the length of the drying section. 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 excessive solvent residue 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.