Causes and Solutions of Composite Bubble Defects in Flexible Packaging Polyurethane Laminating Adhesive

Composite bubbles are the most common appearance defect in dry, solvent-free laminating of flexible packaging, manifested as point-like, strip-like, or sheet-like voids. These bubbles cannot be eliminated on their own after curing and directly lead to scrap and rework. The cause of bubble problems is not a single factor; it mainly falls into three categories: mechanical bubble trapping, gas production from the adhesive, and abnormal process environment. It is necessary to precisely identify the source and solve the problem accordingly.

 

Mechanical bubble trapping is the most common. During high-speed laminating, the exhaust is not thorough at the moment of roller contact, resulting in microscopic wrinkles on the film surface, unstable tension, and insufficient flatness of the rollers, which causes air to be trapped between the two layers of substrates, forming regular or random bubbles. Inconsistent tension when unwinding the film, high static electricity on the film surface, and deviation in lamination will exacerbate the bubble phenomenon, especially for light-colored transparent films.

The production of gas from the adhesive itself is a latent cause. When the polyurethane curing agent reacts with water vapor, it will produce carbon dioxide. High humidity in the workshop, solvent containing water, and loose sealing of the adhesive bucket can all lead to slow gas production in the adhesive, resulting in dense and tiny bubbles after curing. In addition, too fast mixing speed of the adhesive and direct operation without waiting for degassing before putting it on the machine can result in micro-bubbles in the adhesive. After coating and curing, it will form needle-like bubbles or random bubbles across the entire sheet.

 

To eliminate bubbles completely, it is necessary to optimize equipment and processes, adjust reasonable composite pressure and gradient tension, ensure smooth and uniform exhaust by flat and uniform roller contact. Strictly control the humidity and purity of the workshop, allow the adhesive to stand and degas after mixing, and avoid coating with bubbles. For high-speed production, match appropriate machine speed and drying parameters to prevent air from being trapped in the surface sealing. Multi-dimensional control can completely eliminate composite bubbles and improve the yield rate and appearance quality.