Causes and Control of Bubbles in Flexible Packaging Polyurethane Adhesives

Bubble formation is a common defect in the application of polyurethane adhesives for flexible packaging. The presence of bubbles directly damages the continuous structure of the adhesive layer, reduces the bonding strength, and also affects the appearance and barrier properties of the packaging. In severe cases, it may even cause local delamination.

 

The sources of bubbles can be classified into three categories: the raw materials themselves, the mixing operation, and the composite process. In terms of raw materials, the trace solvents dissolved in the adhesive liquid, the water vapor absorbed from the air by the system, and the reaction with the isocyanate component to form carbon dioxide will remain in the adhesive layer during the curing process and form bubbles. During the mixing stage, if the stirring speed is too fast or the stirring time is too long, a large number of mechanical bubbles will be introduced. If the sufficient defoaming time is not reserved before starting the coating process, the bubbles will directly remain in the adhesive layer. The water vapor adsorbed by the substrate surface and the low-molecular substances released will also become the triggering points for bubbles.

In terms of control measures, it is necessary to ensure proper storage protection of raw materials to prevent the adhesive liquid and curing agent from being exposed to humid air for a long time. During the mixing stage, control the stirring rhythm, and after the mixing is completed, allow it to stand to defoam. If necessary, appropriate defoaming additives can be added. Do not add too much blindly to avoid affecting the bonding and migration indicators. During the coating process, maintain a reasonable temperature gradient in the oven to ensure the full evaporation of volatile substances; match the pressure and gap of the composite rollers to fully squeeze out the air encapsulated between the adhesive layers.

 

Different types of polyurethane adhesives have different sensitivity to bubbles. The solvent-free system is more sensitive to water vapor, while the solvent-based system pays more attention to the balance of solvent evaporation. In daily production, it is necessary to combine the characteristics of the adhesive type and control the environmental humidity simultaneously to reduce the generation of bubble defects from the source.