Analysis of the causes of white spots after the composite of polyurethane laminating adhesive for flexible packaging films

When the flexible packaging film is laminated with Polyurethane Adhesive, irregular white spots and white dots often appear on the surface of the finished product, seriously affecting the appearance and quality of the packaging. This problem is not caused by a single factor. It mainly lies in four aspects: chemical reaction, process operation, material matching, and equipment environment. It is a common quality issue in the production of flexible packaging laminating. 

 

The production of gas through chemical side reactions is the main cause of the formation of white spots. The isocyanate groups in the polyurethane curing agent are extremely sensitive to water. If the glue and the diluent ethanol contain excessive moisture, or if the humidity in the workshop is too high, the moisture will react with the adhesive to form carbon dioxide micro-bubbles. These tiny bubbles are enclosed within the film composite layer. After the material is cured and shaped, dense white spots will form, which is also the most common defect-causing factor in production. 

Improper process operation will aggravate the problem of white spots. Insufficient glue application and excessively high glue viscosity will result in poor leveling of the glue layer, local glue shortage and uneven coating, which will cause gaps during bonding and form white spots. Insufficient composite pressure and uneven film tension will prevent the composite roller from completely removing the air in the interlayer, resulting in residual air spots. At the same time, insufficient oven temperature and incomplete drying will cause solvent residue to trigger local whitening. 

 

Material matching and equipment issues should also not be overlooked. If the compatibility between the ink and the polyurethane laminating adhesive is poor, or if the covering rate of the light-colored ink is insufficient, local whitening and grayish-white spots will occur. If the corona treatment of the film is not up to standard or surface additives are released, it will reduce the wetting property of the adhesive layer, resulting in inaccurate bonding. In addition, foreign substances adhered to the surface of the gluing roller and the composite roller will cause regular local white spots, affecting the qualification rate of the finished products.