Surface treatment of the substrate and bonding principle of flexible packaging polyurethane adhesives

For the successful bonding of flexible packaging polyurethane adhesives, in addition to the formulation performance of the adhesive itself, the surface condition of the substrate is a crucial factor that cannot be ignored. Plastic films are low surface energy materials with strong molecular structure inertia. If no surface treatment is performed, the adhesive is difficult to fully spread and penetrate on the surface, which directly affects the final bonding strength.

 

The most commonly used treatment method for films is corona treatment. Corona discharge introduces polar oxygen groups on the surface layer of the film, increasing the surface tension of the substrate and allowing the Polyurethane Adhesive to evenly wet and form molecular-level mutual adsorption. However, the effect of corona treatment has a time limit. If the film is placed for too long, the surface polar groups will gradually decay and the surface tension will drop, resulting in insufficient bonding and local delamination in subsequent bonding.

The bonding of polyurethane adhesives relies on the formation of intermolecular forces such as hydrogen bonds between polar groups of the adhesive and the active groups of the substrate surface; at the same time, after curing, the adhesive layer forms a continuous polyurethane polymer network, relying on the internal cohesion of the adhesive layer to lock the two substrates. It is necessary to distinguish between the problems of poor wetting and poor curing of the adhesive itself. Surface oil stains and the released lubricants of the film will also block the contact between the adhesive layer and the film, even if the adhesive liquid quality is qualified, it will still result in adhesive failure.

 

In the selection of materials and production control, in addition to testing the performance of the adhesive liquid, it is also necessary to regularly monitor the surface tension of the film, control the storage period of the film, and avoid the migration of additives to contaminate the surface. Only when the substrate surface is clean and has sufficient surface energy can the polyurethane adhesive fully exert its bonding advantages and ensure the stability and reliability of the flexible packaging composite product.