Causes and Remediation Techniques for Excessive Solvent Residues in Polyurethane Adhesive-Based Lamination Processes

In the production of dry laminaton for flexible packaging, excessive solvent residues are a common issue that directly affects the compliance of food packaging. Even if the polyurethane laminating adhesive itself is qualified, due to multiple factors such as raw materials, equipment, and processes, it is still prone to have high residues, leading to odors, migration, and risks such as customer factory inspections failing.

 

The quality of the solvents is a fundamental influencing factor. The acetic acid ethyl ester used for dilution contains excessive water and alcohol impurities, which not only interfere with the cross-linking reaction of the polyurethane laminating adhesive but also these high-boiling-point impurities are difficult to volatilize in the drying tunnel and remain largely in the adhesive layer and ink layer, thereby increasing the overall residue value. Excessive dilution during the mixing process will also increase the evaporation load of the drying tunnel.

The matching of the drying tunnel system and the production speed is particularly crucial. An unreasonable temperature gradient setting in the drying tunnel will cause the surface of the adhesive liquid to quickly form a crust due to the excessively high temperature in the front section, and the solvents will be trapped inside the adhesive layer and cannot escape; insufficient wind speed and poor exhaust ventilation will prevent the volatilized solvents from being promptly removed. If the production speed is too fast, the substrate will not have sufficient heating time, and the solvents will not have time to fully volatilize before being wound up, which will seal the solvents inside the film roll.

 

The environment of the winding room is also not to be ignored. Poor ventilation in the winding room and high environmental solvent concentration will cause the film to adsorb air solvents in reverse. The residual solvents in the printed film are already high, and the composite process is difficult to completely compensate for it, resulting in a direct accumulation of total residues in the finished product.

 

In actual production, priority should be given to using high-purity solvents and controlling the dilution ratio of the mixing. The drying tunnel should adopt a stepped temperature increase, increase the exhaust air volume, and set the production speed reasonably based on the adhesive liquid and the substrate. Manage the residual solvents of the printed semi-finished products, maintain ventilation in the winding area, and through multiple collaborative measures, the solvent residues can be effectively controlled within the compliant range.