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The low temperature tolerance of the food packaging vacuum shrink bag depends on the dynamic balance between the material phase change behavior and the interface stress. The multilayer co-extruded film structure forms an anisotropic mechanical network through the controllable orientation crystallization technology. The core of the low temperature performance of the food packaging vacuum shrink bag lies in the characteristics of linear low-density polyethylene. When the temperature is lower than the glass transition point, the molecular chain segment movement of the amorphous region freezes and triggers a brittle transition. At this time, the integrity of the package depends on the plastic deformation ability of the interface bonding layer.
The modern process uses the reaction extrusion grafting method to introduce a long-chain branched structure into the ethylene-vinyl acetate copolymer matrix. This molecular design allows the food packaging vacuum shrink bag to remain stretchable in a sub-zero environment while maintaining the tensile strength required for vacuum sealing. The problem of frozen shrinkage stress is solved by the biaxial stretching process. The orientation stress stored in the prefabricated film during molding is limited to below the critical value, avoiding spontaneous shrinkage caused by a sudden drop in temperature and causing packaging rupture.
The surface of food packaging vacuum shrink bag is functionalized to form nano-scale roughness. This structure helps to enhance the adhesion of ice crystals and prevent local stress concentration caused by phase change volume expansion. Its technical innovation lies in the construction of a gradient modulus system, with the high modulus of the outer layer ensuring morphological stability, the middle viscoelastic layer dissipating thermal stress, and the low modulus of the inner layer maintaining the flexible wrapping of the food contact surface.
