Control of Residual Monomers in Flexible Packaging Polyurethane Adhesives

In the flexible packaging Polyurethane Adhesive system, isocyanate-based monomers are the key substances that affect packaging safety and processing performance. The control of residual monomers runs through the entire process of formulation design, synthesis, and curing.

 

When the content of residual monomers is high, on the one hand, it is prone to react with environmental moisture, forming bubbles and pinholes inside the composite film, which will damage the structure of the composite layer; on the other hand, the unreacted monomers have a migration risk, which directly affects the compliance of food contact materials. In high-temperature boiling and scenarios involving contact with oily contents, the migration risk will further increase.

From the formulation perspective, the ratio design of polyols and isocyanates is the foundation. Reasonable adjustment of functional groups and feeding ratios, combined with appropriate reaction conditions, can increase the monomer conversion rate and reduce the level of free monomers. At the same time, the removal process in the later stage of the reaction and the coordinated regulation of additives can further inhibit the retention of free monomers.

 

In the downstream application stage, the curing process is an important stage for the secondary consumption of residual monomers. Appropriate curing temperature and time can allow the remaining isocyanates in the system to continuously react with hydroxyl groups, gradually reducing the residual amount. However, the curing conditions should not be overly aggressive; excessively high temperatures can easily cause the aging of the adhesive layer and substrate shrinkage, etc., as secondary problems.

 

In terms of industry development trends, low-free-monomer polyurethane adhesives have become the mainstream direction. Considering the compatibility for composite processing and food safety requirements, they are suitable for various flexible packaging scenarios such as boiling packaging, snack packaging, and sauce packaging. This is the direction that adhesive formulation development continues to focus on.