The aging mechanism and control points of flexible packaging polyurethane adhesives

After the lamination process of flexible packaging polyurethane adhesives is completed, aging is a crucial step that determines the final bonding performance. Essentially, it is a process where the main component polyol and the curing agent isocyanate continuously undergo cross-linking reactions, gradually forming a dense three-dimensional network structure. The initial viscous state of the adhesive layer gradually transforms into a stable, cohesive, medium-resistant, and flexible film. The degree of reaction during this process directly affects the overall performance of the packaging product.

 

The aging process is subject to multiple constraints. Environmental temperature is the core influencing factor. An appropriate temperature can increase the molecular movement rate, promoting the full progress of the cross-linking reaction; low temperatures will significantly slow down the reaction rate, resulting in incomplete aging and insufficient bonding strength; high temperatures are more likely to cause rapid reactions, increasing internal stress in the adhesive layer, and leading to warping and delamination risks in the later stage. At the same time, environmental humidity cannot be ignored. Water vapor will preferentially consume the curing agent, disrupting the balance of the ratio, and causing insufficient cross-linking of the adhesive layer.

In addition to temperature and humidity, the thickness of the adhesive layer and the type of substrate also affect the aging rhythm. Thin adhesive layers age faster, while the internal reactions in thick adhesive layers are lagging. For adhesives used in steam-treatment packaging, the cross-linking system design is more complex, and the required aging period is relatively longer. In production, it is not sufficient to determine the aging endpoint based solely on a fixed duration; it is necessary to combine strength testing and peel-off status for comprehensive judgment.

 

Reasonable control of the aging process ensures that the cross-linking fully utilizes the adhesives' ability to withstand steam treatment and resist oil, while reducing residual active groups and lowering the risk of food contact migration. In the context of the green trend, new polyurethane adhesives also optimize the cross-linking activity in their formulations, balancing performance stability while shortening the aging period and helping to improve the efficiency of the flexible packaging production line.