The aging mechanism and long-term stability of flexible packaging polyurethane adhesives

The composite film made with flexible packaging polyurethane adhesives will gradually age in storage, exposure to light, and alternating temperature and humidity environments, directly affecting the shelf life of the packaging. The root cause of its aging lies in the combined effect of chemical structure changes within the adhesive layer and the external environment.

 

The amide bond in the polyurethane molecular chain is the core weak site. It will undergo hydrolysis and chain breakage under long-term heat and water vapor erosion, causing the degradation of the macromolecules and a decrease in the internal cohesion of the adhesive layer. Macroscopically, this is manifested as the attenuation of the composite film's peel strength and local delamination. Ultraviolet light will accelerate the oxidation of the molecular chain, generating polar groups such as carbonyl, and the adhesive layer will gradually turn yellow and become brittle. Especially for light-colored food packaging, the deterioration of appearance will directly affect the product's visual appeal.

The formulation system is the foundation for determining the aging resistance. Selecting a hydrolysis-resistant main structure polyol can enhance the stability of chemical bonds; appropriately introducing antioxidants and hydrolysis stabilizers can capture free radicals and inhibit bond breakage, delaying the aging process. The selection of curing agents is also crucial. The aromatic isocyanate curing system has excellent bonding strength but has relatively poor weather resistance, making it more suitable for light-protected inner packaging; the fatty isocyanate-based adhesive has outstanding yellowing resistance and is mainly used for high appearance requirements outer packaging.

 

In addition to the formulation, the properties of the base material and the composite process cannot be ignored. Some film additives will migrate to the adhesive layer interface, catalyzing the degradation of polyurethane, accelerating interface failure. Imbalanced mixing ratio and insufficient curing conditions will cause incomplete curing, and residual active groups are more likely to react with water vapor, significantly shortening the service life of the composite film. During the product selection stage, it is necessary to combine the packaging storage environment and usage cycle to match the corresponding stability grade of polyurethane adhesives to ensure that the flexible packaging maintains reliable bonding performance throughout its entire life cycle.