In the field of food packaging engineering, the molecular structure design of high barrier composite materials has become the core technical direction for solving gas permeation problems. This article focuses on the microscopic structure-activity relationship of multilayer co-extruded film materials, analyzing how they achieve long-term barrier of gases such as oxygen and water vapor through molecular chain arrangement and interface synergy.
Material structure and barrier mechanism
High barrier composite materials usually adopt a three-layer or more co-extruded structure, and achieve nano-level interface fusion through melt extrusion process. The inner layer selects polar polymers (such as EVOH), whose hydroxyl groups can form hydrogen bonds with oxygen molecules to slow down the gas diffusion rate; the middle layer uses high crystallinity materials (such as PA) to construct a dense lattice barrier, and uses the regular arrangement of molecular chains to reduce the free volume pores; the outer layer uses polyolefin materials (such as PP) to provide mechanical support and heat sealing properties. Experiments show that when the EVOH layer thickness accounts for 12%, the oxygen permeability can be reduced to below 3 cm³/(m²·24h·0.1MPa), which is 2 orders of magnitude higher than the performance of a single-layer film.
Process Optimization and Interface Control
The key to the multi-layer co-extrusion process lies in the precise control of the temperature field and shear rate. In the die flow channel design, a gradient cooling strategy (from 220°C to 160°C) is adopted to allow each layer of melt to form an interpenetrating molecular chain network at the interface. By introducing plasma pretreatment technology, the surface energy of the material is increased to more than 45 mN/m, promoting the interlayer bonding strength to exceed 8 N/15mm, and avoiding microcracks caused by stress deformation during transportation. In addition, the intercalation dispersion technology of nano-montmorillonite additives can increase the water vapor barrier efficiency by 40%, and its lamellar structure forms a "maze effect" in the polymer matrix, significantly extending the gas diffusion path.
Performance Verification and Application Scenarios
In the accelerated aging experiment, after the packaging samples loaded with freeze-dried strawberries were stored in a 40¡æ/75%RH environment for 180 days, the composite film still maintained an oxygen permeation increment of less than 0.8%. Electron microscopy observations showed that there was no obvious stratification or holes in the cross section of the material, confirming the stability of the interface bonding. This technology has been extended to moisture-proof and anti-oxidation packaging in the fields of puffed food and pharmaceutical intermediates, especially under the condition that the temperature difference between day and night in the shipping container reaches 25¡æ, the packaging integrity can still be maintained.
Technology Evolution Trend
With the maturity of molecular dynamics simulation technology, the research and development of high-barrier materials will turn to the directional modification of functional groups in the future. By grafting fluorinated groups or constructing a metal-organic framework (MOF) composite system, it is expected to construct a selective gas screening channel at the nanoscale to achieve dynamic barrier regulation of oxygen and nitrogen. Such technological breakthroughs will promote the evolution of food packaging from passive protection to intelligent response.
