In the solvent-based polyurethane dry lamination production process, coating orange peel, wave texture, and uneven leveling are very common appearance defects. These are manifested as uneven surface of the adhesive layer, obvious light and dark patterns on the film surface, and a wavy feeling when transparent, and the cured layer cannot self-level and repair, which not only affects the appearance of the composite but also causes local uneven thickness of the adhesive layer, resulting in deviations in peel strength, high residual solvent content, local delamination and other potential hazards, which are the most typical process defects of dry lamination.
The inaccurate control of adhesive viscosity is the primary cause of poor leveling. Insufficient dilution ratio of polyurethane laminating adhesive, high working viscosity, large molecular resistance of the adhesive liquid, cannot be quickly spread and wet the substrate after machine coating, and the mesh roller dots cannot be completely closed, directly forming point-like and strip-like orange peel patterns. Conversely, excessive dilution and low viscosity of the adhesive liquid, too strong fluidity, easily cause edge run-off and middle thin coating, resulting in uneven thickness of the wave defects.
Inappropriate temperature gradient in the drying tunnel will cause curing and leveling defects. Dry lamination relies on solvent evaporation to achieve film formation of the adhesive layer. If the inlet temperature of the drying tunnel is too high, the solvent on the surface of the adhesive layer will rapidly evaporate and form a thin layer on the surface in an instant, while the internal adhesive liquid has no time to flow and level, resulting in hard orange peel patterns. Abnormal temperature and humidity in the workshop, dry air, and excessive wind speed will also accelerate the formation of a thin layer on the surface, completely locking in the leveling defects.
Inappropriate matching of mesh roller status and machine speed also affects film formation. Mesh roller blockage, wear, uneven dot depth, uneven adhesive application amount. If the equipment speed is too fast, the leveling time of the adhesive liquid is greatly compressed, the dots cannot close, and regular coating patterns are formed. In addition, uneven mixing of the adhesive liquid and mixing of new and old adhesives can lead to unstable rheology, and intermittent leveling differences and local orange peel patterns will also occur.
Improvement requires precise control of working viscosity, matching the optimal dilution ratio according to the mesh roller model. Adopting a gradient temperature drying tunnel process can avoid rapid drying at the inlet. Regular cleaning and maintenance of mesh rollers to ensure uniform adhesive application, matching a reasonable machine speed to reserve sufficient leveling time. Stabilizing the adhesive state and coating conditions can completely solve the problems of orange peel and poor leveling of
Polyurethane Adhesive, ensuring the adhesive layer is flat and transparent, and the composite quality is stable.
