In addition to the molecular size and physical properties of the gaseous substance, the factors affecting the barrier performance of the plastic composite film also require the internal structure of the plastic film itself, molecular structure and molecular aggregation state, and the compatibility between the plastic and the permeable substance.
1. Molecular polarity
When the crystallinity is fixed, polar macromolecules or strongly polar macromolecules are difficult to diffuse due to the tight bonding between the molecules. The greater the molecular polarity, the lower the resin permeability and the better the gas barrier performance. Among the commonly used plastic resins, PET and PVA are strong polar resins, PA and PVC are polar resins, PS are weak polar resins, and PE and PP are non-polar resins. Their gas barrier properties increase with the increase of molecules. For example, PET and PE have a very different difference in O2 transmission rate.
Water vapor is a polar molecule. According to the principle of similar miscibility, the dissolution and diffusion speed of water vapor in polar molecular plastics are both higher than that of non-polar plastic molecules, and its moisture permeability coefficient value is also large. High-barrier material PET molecule has strong polarity, and its moisture permeability coefficient value is greater than non-polar molecule PE , so PE is an excellent moisture-proof packaging material.
2. Molecular crystallinity
The diffusion energy required for gas and water vapor to pass through the crystalline plastic film is higher than that of the non-crystalline plastic film, and the diffusion coefficient is small, so the crystalline plastic film exhibits good gas barrier properties. Under the same conditions, the higher the crystallinity of the plastic film molecules, the better the barrier performance.
3. Molecular orientation
Due to the stretching during molding, the plastic macromolecules are subjected to different degrees of orientation, arranged regularly and closely arranged, and the barrier properties of the film are improved. The higher the degree of orientation, the better the barrier property. Especially after the film is biaxially stretched, not only the grain size can be greatly reduced, but also the crystallinity can be increased.
It can be explained that on the one hand, stretching causes the original crystalline particles to break and become smaller; on the other hand, stretching increases the orientation of the macromolecules and the arrangement is more regular and orderly, thereby improving the crystallinity and the arrangement density of the macromolecules.
4. Molecular hydrophilicity
The main hydrophilic properties of plastic films are PVA and PA . The hydrophilic resin can swell the resin due to its strong water absorption, and the distance between the molecules increases but the barrier property decreases. Generally, the water vapor diffusion coefficient of a hydrophilic plastic film is not a constant, it increases with the concentration of water vapor, resulting in a change in the moisture permeability coefficient; the moisture permeability of non-hydrophilic plastic films is hardly affected by the ambient humidity influences.
5. Ambient temperature
Temperature has an effect on the molecular structure of the plastic film. The increase in temperature will reduce the crystallinity and orientation of the resin, increase the molecular spacing, and reduce the density, which will reduce the barrier properties of the plastic film.
Generally, the gas permeability of plastic films increases and decreases according to the exponential law with temperature changes. In comparison, the gas barrier properties of PVDC are less affected by temperature, and the hand temperature of non-plastic aluminum foil materials is less affected. Generally, these two films are more suitable for high-temperature cooking bags. In contrast, the barrier properties of ultra-high barrier silicon dioxide coating materials are less affected by temperature.
In practical applications, several materials such as EVOH , PVDC copolymer, PAN copolymer, PA , PEN , PET, etc. are often used as barrier materials, among which EVOH , PVDC , PAN copolymer and aromatic nylon MXD6 are barrier materials, among which EVOH , PVDC , PAN copolymer and aromatic nylon MXD6 are high barrier materials, while PA and PET are medium barrier materials. Although EVOH , PVDC , PEN , PEN , and PAN have excellent barrier properties, they have poor processability, high price, or incomplete performance. Generally, they are not used alone, and are often used in blending and compound coating modification.
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