The hazards of excessive curing of polyurethane laminating adhesives and precise process control

In the production of flexible packaging, most enterprises focus on the issue of insufficient curing but tend to overlook the quality risks brought about by excessive curing. Excessive curing refers to the situation where the composite film is stored for too long at high temperatures or with excessively high temperatures, causing the cross-linking reaction of the polyurethane laminating adhesive to exceed the optimal balance point, resulting in the reverse attenuation of the material properties of the adhesive layer, leading to a decrease in strength, brittleness of the film body, and poor weather resistance, among other latent defects.

 

The most obvious hazard of excessive curing is the loss of the adhesive layer's toughness. After the polyurethane laminating adhesive has completed effective cross-linking, continuous high-temperature baking will cause the molecular chains to become overly dense and the rigidity to significantly increase, transforming the adhesive layer from flexible to relatively hard and brittle. During subsequent processes such as cutting, bag making, and bending, problems such as crease cracking, micro delamination, and edge bulging are prone to occur, and the resistance to steaming and freezing will also significantly decrease.

At the same time, excessive curing will exacerbate the thermal contraction of the film material and the accumulation of internal stress, causing the composite film to warp, loose edges, and curl deformation, seriously affecting the flatness of bag making and the matching degree of the seal. High-temperature storage for a long time will also accelerate the slight oxidation of the adhesive layer, causing the composite film to turn yellow, decrease in transparency, and affect the appearance texture of high-end color printing packaging.

 

Production control requires the establishment of a standardized curing system, setting corresponding temperatures and durations based on the adhesive type and film structure. After reaching the curing standard, the product should be promptly placed in a room for normal temperature standing. Prohibit over-temperature and over-time stacking, and regularly check the temperature control accuracy of the curing room. Precise control of the curing stage can not only ensure full cross-linking but also avoid the deterioration of material properties caused by excessive curing, stabilizing the comprehensive quality of the finished product.