Express bags are widely used in logistics packaging. Once sealed, they have a very high degree of tightness. It's often seen that even if the bag is torn, the sealed part doesn't burst open. This shows that the bonding strength of hot-melt adhesive on express bags is quite high. However, as we all know, hot-melt adhesive is afraid of water, that is, the adhesive layer is prone to come unsealed after getting wet. But why doesn't this happen to express bags when they get wet?
Hot-melt adhesive for express bags has a very important property - the effect of breaking the material. After an express bag is finished, although the glued part of the seal isn't broken by the boiling glue liquid, it still causes moderate damage to the surface of the plastic material, which is required by the gluing process. And the hot-melt adhesive used on express bags is of the pressure-sensitive type. Even at room temperature, it has a strong initial adhesion on the surface. When the package is sealed, just apply a little pressure to the bag opening, and the bag opening will be sealed tightly, forming a sealed state for the bag and protecting the items inside the bag from being contaminated well. When we tear open the package from the bag opening, we usually find that the plastic beside the glued part is torn, while the sealed part can't be pulled apart no matter how hard we try. It's really firmly bonded! This is the effect of breaking the material of the adhesive. Because the molecules of the plastic used to make express bags are relatively loose, we only need to use a little force to tear off a thin layer of plastic. Moreover, such a loose molecular structure also provides a lot of space for the combination with the molecules of the hot-melt adhesive. During the gluing process of the bag, when the plastic surface is heated, its molecules move vigorously. When the glue liquid cools down, the plastic molecular chains and the hot-melt adhesive molecular chains are intertwined, forming a new and stable structure, which is even more stable than the original plastic molecular structure. It's precisely because of such firm bonding that even if the whole express bag is soaked in water, the sealed part won't come unsealed.
On the other hand, the surface of express bags is very slippery. When water drops on it, the water will be quickly slipped away. Therefore, during the logistics transportation process, there's no need to worry even if it rains. Generally, getting rained on won't affect the tightness of express bags. However, the prerequisite is that the bag itself should be sealed well and not be scratched during transportation. A little rain is even less likely to make the firmly sealed part come unsealed.
The bonding performance of hot-melt adhesive on express bags is quite good, but it can only have such performance on certain materials. Although hot-melt adhesive has a wide range of applications, when facing different materials, we can't expect it to have such a good bonding effect as on express bags. Once the guarantee of the bonding effect is lost, when it gets wet or the humidity is high, the possibility of the hot-melt adhesive coming unsealed will be greatly increased.