Causes and Rectification Measures for Excessive Residual Solvents in Flexible Packaging Polyurethane Laminating Adhesives

Excessive residual solvents in flexible packaging composite production are a key compliance issue, especially for food and baby care packaging, which have strict residual limit values. If the coating and drying process of polyurethane laminating adhesives is not thorough, acetate ethyl ester and other solvents will remain inside the adhesive layer. This not only causes odors but also leads to solvent migration and contamination of the contents, resulting in product non-compliance and customer returns and claims.

 

Unsatisfactory drying process is the primary cause. Fast production line speed, low setting of drying oven temperature, and unreasonable temperature control gradient of four stages, all result in the solvents not fully evaporating and being sealed inside the adhesive layer after being covered with film. In high-solid content adhesives and thick coating conditions, the adhesive layer is thick, and the surface solvents quickly form a crust, while the internal solvents are difficult to be released, easily causing hidden residual solvents to exceed the limit.

The quality of solvents and operating habits also have a significant impact. Using inferior acetate ethyl ester containing water and alcohol will increase the boiling point of the solvent and reduce the evaporation rate, significantly increasing the residual risk. Inconsistent dilution ratios of the adhesive mixture, high viscosity of the adhesive solution, uneven coating flow and thickness of the adhesive layer, and severe residual solvents in thick adhesive areas. In addition, insufficient curing temperature and short curing time also cannot assist in evaporating trace residual solvents.

 

The rectification requires optimizing the segmented temperature rise process of the drying oven, matching a reasonable production line speed, and eliminating the rapid drying and sealing of the surface layer. Using high-purity and compliant solvents, standardizing the dilution ratio. Adjusting the coating dry amount according to the film material to avoid thick adhesive accumulation. Cooperating with sufficient constant temperature curing. Systematically optimizing process parameters can stably control the residual solvent indicators and meet the safety and compliance standards for food packaging.