In addition to the molecular size and physical properties of the gaseous substance, factors influencing the barrier performance of plastic film, a packaging material, also include the plastic film's internal structure, including its composition, molecular structure, and molecular aggregation, as well as the compatibility between the plastic and the permeable substance.
1. Molecular Polarity
At a certain crystallinity level, large polar or strongly polar molecules, due to their tight intermolecular bonds, hinder gas diffusion. The greater the molecular polarity, the lower the resin's gas permeability and the better its gas barrier performance. Among commonly used plastic resins, PET and PVA are strongly polar, PA and PVC are polar, PS is weakly polar, and PE and PP are non-polar. Their gas barrier properties increase with increasing molecular weight. For example, the O2 permeability of PET and PE differs significantly.
Water vapor is a polar molecule. Based on the principle of like dissolves like, water vapor dissolves and diffuses faster in polar plastics than in non-polar plastics, resulting in a higher water vapor permeability coefficient. PET, a high-barrier material, has highly polar molecules and a higher water vapor permeability coefficient than non-polar PE, making it an excellent moisture-proof packaging material.
2. Molecular Crystallinity
The diffusion energy required for gases and water vapor to pass through crystalline plastic films is higher than that of amorphous plastic films, resulting in a lower diffusion coefficient. Therefore, crystalline plastic films exhibit superior gas barrier properties. All other conditions being equal, the higher the molecular crystallinity of the plastic film, the better the barrier properties.
3. Molecular Orientation
Due to stretching during molding, plastic films experience varying degrees of orientation, resulting in a regular and dense distribution and improved barrier properties. The higher the degree of orientation, the better the barrier properties. In particular, biaxial stretching can significantly reduce crystallite size and increase crystallinity. This can be explained by the fact that, on the one hand, stretching breaks up the original crystalline particles and reduces them in size; on the other hand, stretching increases the orientation of the macromolecules, making their arrangement more regular and orderly, thereby increasing crystallinity and the density of their arrangement.
4. Molecular Hydrophilicity
Plastic films primarily exhibiting hydrophilic properties include PVA and PA. Hydrophilic resins, due to their strong water absorption, can swell, increasing the intermolecular distance but decreasing their barrier properties. Generally, the water vapor diffusion coefficient of hydrophilic plastic films is not constant; it increases with increasing water vapor concentration, resulting in a change in the water vapor permeability coefficient. In contrast, the water vapor permeability of non-hydrophilic plastic films is virtually unaffected by ambient humidity.
5. Ambient Temperature
Temperature affects the molecular structure of plastic films. Increased temperature reduces the crystallinity and orientation of the resin, increases the intermolecular distance, and reduces density, all of which degrades the barrier properties of the film.
The gas permeability of plastic films generally increases and decreases exponentially with temperature. In comparison, PVDC's gas barrier properties are less affected by temperature, and non-plastic aluminum foil is even less affected by temperature. Therefore, these two films are generally more suitable for high-temperature retort bags. In contrast, the barrier properties of ultra-high-barrier silica-coated films are even less affected by temperature. In practical applications, EVOH, PVDC copolymers, PAN copolymers, PA, PEN, and PET are commonly used as barrier materials. EVOH, PVDC, PAN copolymers, and aromatic nylon MXD6 are barrier materials, with high barrier properties, while PA and PET are medium barrier materials. While EVOH, PVDC, PEN, and PAN offer excellent barrier properties, they suffer from poor processability, high prices, or limited performance. They are generally not used alone and are often used in blends, composites, or coating modifications.
