The stress buffering property of flexible packaging polyurethane adhesives

In the process of bag making, dropping, bending, alternating high and low temperatures, etc., the composite film layers of flexible packaging will continuously generate internal stress. The stress buffering capacity of polyurethane adhesives directly determines whether the composite finished product can maintain stability for a long time and reduce defects such as delamination and burst cracks.

 

Polyurethane molecules have flexible segments by themselves. The cured adhesive layer is not a rigid hard film, but an intermediate layer with certain elasticity. When the film is pulled or bent by external force, the adhesive layer can absorb and disperse the interface stress through the expansion and rebound of the molecular chains, avoiding the stress concentrating at the bonding interface and protecting the bonding connection points between the films. If the adhesive layer is too hard, the stress cannot be released, and it is very likely to separate at the stressed position; if the flexibility is too high, it will bring problems such as slippage and wrinkling after compounding. Therefore, the formulation needs to find a balance between the elastic modulus and cohesion.

The stress buffering effect is closely related to the structure of the polyol skeleton. Long-chain flexible polyols can enhance the toughness of the adhesive layer, and the rigid groups are used to maintain the adhesive layer's anti-rheological ability. At the same time, the expansion and contraction differences of the base material will also bring thermal stress. Especially when different material films are compounded, the inconsistent thermal expansion and contraction will cause interface pulling. High-quality polyurethane adhesives can weaken this deformation difference-induced interface tension.

 

In practical applications, steam-sealed bags and frozen packaging bags have higher requirements for stress buffering. By matching an appropriate curing system and controlling the thickness of the adhesive layer, the buffering effect can be further optimized, allowing the flexible packaging to maintain the integrity of the composite structure in complex working conditions during transportation and storage.