Flexibility is a core indicator of flexible packaging polyurethane adhesives, directly determining whether the composite film will experience cracking or delamination and discoloration during bending, rubbing, and stretching. During packaging processes such as bag making, drop impact, and low-temperature storage, the film layer continuously undergoes deformation, and if the adhesive layer is too rigid, it cannot deform synchronously with the substrate, making the bonding interface prone to tearing.
The flexibility of polyurethane adhesives is mainly determined by the structure of the molecular chain segments. The soft segment polyols provide elasticity and elongation capabilities, while the hard segment is formed by the reaction of isocyanates, giving it cohesive strength and heat resistance. The key to formulation design is to balance the proportion of soft and hard segments: if the proportion of the soft segment is too high, the adhesive layer will be soft and have good flexibility for bending, but its heat resistance and resistance to media will decrease; if the proportion of the hard segment increases, the adhesive layer will become more rigid, providing more stable heat resistance during boiling, but it will tend to become brittle and have poorer low-temperature bending performance.
In addition to the basic formulation, the selection of substrate materials is also crucial. When combining rigid substrates such as PET and PA with flexible substrates such as PE and CPP, the requirements for the flexibility of the adhesive layer are completely different. For products that undergo boiling, not only must they be gentle and flexible, but they must also not soften at high temperatures and not crack after cooling. At the production end, the curing conditions will also affect the flexibility of the adhesive layer. If the curing is incomplete, the crosslinking density is insufficient, and the adhesive layer will be soft; if the curing is excessive, the crosslinking density is too high, and the adhesive layer will gradually become hard and brittle, and its low-temperature bending performance will deteriorate.
When selecting adhesives in practice, it is not enough to solely pursue high flexibility. One needs to make a choice based on the end-use scenarios, finding an appropriate balance point between flexibility, bonding strength, and heat resistance and media resistance to meet the entire packaging usage requirements.
