A Brief Analysis of the Initial Adhesion of Flexible Packaging Polyurethane Laminating Adhesives

The initial adhesion force is the core performance indicator of polyurethane laminating adhesives for flexible packaging. It refers to the initial adhesive ability of the adhesive to quickly adhere and fix the film substrate after being coated and laminated, without the need for continuous external pressure for a short period of time. It directly determines the stability of the flexible packaging laminating process and the qualification rate of the finished products, and is a key parameter that cannot be ignored in the lamination production process. 

 

In the dry laminating and solvent-free laminating processes of flexible packaging, insufficient initial adhesion is a common problem in the industry. The solvent-free polyurethane laminating adhesive has a low initial molecular weight and weak internal cohesion at room temperature, resulting in a slow curing reaction rate. This makes it prone to defects such as creasing, shifting, and tunneling of the laminated film, and it is very likely to cause interlayer misalignment after the rolled material is wound up, significantly increasing the waste rate. In contrast, achieving the required initial adhesion enables the film to be instantly set after lamination, ensuring smooth operation of subsequent processes such as winding and cutting, and laying a foundation for the steady improvement of the final bonding strength during the subsequent curing process. 

The molecular structure of adhesives, production conditions, and the addition of additives are the three core factors influencing the initial adhesion force. The ratio of the soft and hard segments in polyurethane determines the cohesion and wettability within the adhesive. The faster the soft segment crystallizes, the better the initial adhesion. The coating temperature and environmental humidity affect the flow of the adhesive and the wetting effect on the substrate. Imbalance in temperature and humidity will directly weaken the initial bonding effect. In addition, adding appropriate tackifying resins such as rosin and terpene resins can enhance the surface activity of the adhesive, strengthen the interface bonding force with non-polar films like PE and PP, and quickly improve the initial adhesion performance. 

 

In actual production, it is necessary to balance the initial adhesion force and the final bonding strength: blindly increasing the initial adhesion force may lead to poor fluidity of the glue, resulting in uneven coating, bubbles and other problems. By optimizing the molecular formula, adjusting the process parameters of the composite, and compounding special adhesion enhancers, the initial adhesion and the final bonding strength of the adhesive can be balanced in both directions, meeting the production requirements of different flexible packaging scenarios such as food packaging and steaming packaging.