Analysis of Cohesive Strength and Bonding Strength of Flexible Packaging Polyurethane Adhesives

In the application of flexible packaging composites, polyurethane adhesives have two key indicators that are easily confused: cohesive strength and bonding strength. Both jointly determine the reliability of the composite film's use, but they represent different performance dimensions.

 

Bonding strength is the bonding force between the adhesive layer and the film substrate interface, reflecting the adhesive's ability to wet the substrate surface and form an interface bond. When bonding is insufficient, interface separation occurs during the peel test, with the adhesive layer remaining intact on one side of the film. Common causes include substrate corona attenuation, surface oil stains, and poor adhesive wetting properties.

Cohesive strength refers to the bonding force between the molecular chains of the adhesive layer after curing, representing the adhesive layer's own tear resistance. If the cohesive strength is too weak, the adhesive layer will tear during the peel test, and the adhesive will transfer bidirectionally to both film surfaces. The formulation's polyol structure, curing agent ratio, and curing conditions all directly affect the cohesive level.

 

High-quality polyurethane composite adhesives require a balance between these two properties. Focusing solely on high bonding strength but with insufficient cohesive strength is prone to adhesive layer fracture during steaming, stretching, and kneading processes; while excessive cohesive strength will make the adhesive layer too hard, reducing its flexibility, and causing the composite film to crack and have reduced bending resistance. The formulation design usually regulates the ratio of soft and hard segments to ensure both a reliable interface attachment and appropriate elasticity of the adhesive layer.

 

During actual production and testing, the failure type can be quickly distinguished by the failure mode of the peel test, and the adhesive liquid selection, mixing process, and curing parameters can be adjusted specifically to avoid later delamination and damage of the composite product.