The causes and improvement measures for the warping of lamination films caused by flexible packaging polyurethane laminating adhesives

Warping and curling of lamination films occur during the winding and aging of the films, which is a very common problem in flexible packaging processing. In many cases, it is closely related to the internal stress generated during the curing of polyurethane laminating adhesives, which can affect subsequent bag-making and cutting processes, and in severe cases, directly result in the destruction of the finished products.

 

Polyurethane adhesives will undergo volume contraction during the crosslinking and curing process, and internal stress will form within the adhesive layer. If the thermal shrinkage coefficients of the two film sides are significantly different, combined with the shrinkage stress of the adhesive layer, the film will curl towards one side. Imbalance in coating amount is an important cause. If the adhesive layer on one side is too thick, the curing shrinkage stress is greater, and it is very likely to cause unilateral warping. The viscosity of the solvent-free system is too high, and when the coating is uneven, the stress distribution becomes disordered, and the warping phenomenon becomes more prominent.

The selection of substrate combinations should also not be ignored. The combination of rigid materials such as nylon and PET with soft PE films has a large inherent property difference, and improper combination can easily cause curling. Excessive temperature during the aging process in the workshop and too rapid cooling can quickly amplify the stress difference between the adhesive layer and the film, exacerbating the warping defect.

 

To address these issues, low-shrinkage polyurethane laminating adhesives can be selected to reduce the internal stress during curing. During the production stage, stable coating dry amount should be maintained to ensure uniform coating thickness. After aging, a slow cooling method should be adopted to reduce the impact of temperature differences. At the same time, the membrane material structure should be reasonably matched, the winding tension should be adjusted, and multiple measures should be coordinated to release the internal stress, effectively improving the warping phenomenon of the lamination film and ensuring the smooth progress of subsequent processing.