Causes and Rectification Solutions of Air Bubbles in Flexible Packaging Solvent-Free Polyurethane Laminating Adhesive

Bubbles are a common appearance defect in the production of solvent-free Polyurethane Adhesive lamination. They can directly cause problems such as voids in the laminated film, decreased peel strength, and uneven light transmission, seriously affecting the qualification rate of flexible packaging products. Unlike solvent-based adhesives, the solvent-free system has no solvent evaporation channels. Once bubbles form, they are difficult to eliminate on their own, so the early process control is particularly crucial. 

 

The primary cause of bubble formation is the air intake during the mixing process. The viscosity of solvent-free two-component adhesives is relatively high. If the mixing speed is too fast or the mixing method is improper, a large amount of air will be entrained into the adhesive liquid. Changing the adhesive bucket, air entering the supply pipeline, and loose pump body seal can also lead to adhesive bubbles, resulting in dense point-like bubbles after coating. Secondly, high humidity in the workshop is the main cause. The isocyanate groups react with water to produce carbon dioxide, which causes the adhesive layer to bubble and turn white. 

Incompatibility of process parameters can also cause problems. If the coating temperature is too low, the glue will have poor fluidity and weak defoaming ability, and tiny bubbles will not be able to escape. Insufficient lamination pressure and uneven contact of the pressure rollers will prevent the air between the film layers from being completely expelled, resulting in bubble defects. 

 

Actual production rectification requires a multi-pronged approach, including low-speed and uniform stirring, static defoaming followed by coating, and sealing of the supply system. The temperature and humidity in the workshop should be strictly controlled, avoiding production in high-humidity environments. Reasonably increasing the coating temperature and optimizing the compounding pressure can completely solve the problem of bubbles in solvent-free adhesive lamination, ensuring the flatness and transparency of the lamiantion film.