The aging yellowing of polyurethane laminating adhesives is caused by quality control of anti-yellowing properties.

Transparent packaging and light-colored printed packaging have extremely high requirements for the yellowing resistance of the adhesive layer. After the composite film is stored for a period of time, it may exhibit problems such as yellowing, darkening, and yellowing through transmission, collectively referred to as adhesive aging yellowing. Yellowing not only damages the transparent texture and appearance grade of the packaging, but also accompanies aging of the adhesive layer, strength reduction, and decreased weather resistance. It is a common complaint issue for high-end packaging shipments.

 

The yellowing of polyurethane laminating adhesives mainly stems from the molecular structure and oxidative aging. Ordinary aromatic polyurethane raw materials contain benzene ring structures. Over time, they are affected by ultraviolet rays, light, and high-temperature storage environments, and the molecular chains are prone to oxidation and breakage, generating chromogenic groups, causing the adhesive layer to change from transparent to slightly yellow or dark yellow, and the aging process is irreversible. Excessive high-temperature curing and long-term exposure to light stacking will significantly accelerate the yellowing process.

Defects in auxiliary materials and processes can exacerbate aging and color change. Excessive curing agent dosage and residual free monomers in the system are prone to oxidation and yellowing in the later stage. Insufficient purity of solvents and the presence of impure additives make the adhesive layer less dense after curing, and it is more susceptible to erosion by air and light. At the same time, excessive internal stress in the composite film, long-term stress fatigue will accelerate the aging of the adhesive, simultaneously inducing appearance yellowing and fogging.

 

The first choice to solve the yellowing problem is aliphatic anti-yellowing polyurethane laminating adhesives, which can eliminate the root cause of oxidative color change from the molecular structure. Production strictly controls the accuracy of dosage ratios, reduces free residues, uses high-purity solvents, and standardizes the curing process to avoid excessive high-temperature baking. The finished product is stored in a light-protected and constant-temperature environment, which can maintain the high transparency and low yellowing state of the composite film for a long time, meeting the aging-resistant appearance standards for high-end packaging.