What are the factors that affect the barrier property of plastic film?
Plastic film due to stretching during molding, plastic macromolecules are subject to varying degrees of directional role, respectively, and the rules are closely arranged, the barrier to improve the film. The higher the degree of orientation, the better barrier property. Especially after biaxial stretching treatment, the grain size can be greatly reduced, and the crystallinity can be increased. It can be explained that on the one hand, stretching causes the crystalline particles to become smaller and smaller; on the other hand, the stretching increases the orientation of macromolecules and makes the arrangement more orderly, thus increasing the crystallinity and the arrangement density of macromolecules.
The temperature has an influence on the molecular structure of the plastic film. The increase of temperature will reduce the crystallinity and orientation of the resin, widen the molecular spacing and decrease the density, which will reduce the barrier property of the plastic film.General plastic film transmittance of all other exponentially with the change of temperature change, in contrast, gas barrier PVDC was little affected by temperature, non plastic foil material affected by temperature is smaller, the general choice of these two kinds of films do more suitable high-temperature cooking bags. In contrast, the barrier properties of ultra high barrier silicon dioxide coated plastic films are less affected by temperature.In practical application, EVOH, PVDC copolymer, PAN copolymer, PA, PEN, PET and other materials are often used as barrier materials, including EVOH, PVDC, PAN copolymer and aromatic nylon MXD6 high barrier materials, and PA, PET as the medium barrier material. EVOH, PVDC, PEN, PAN, although the barrier is very good, but or processing is not good, or high prices, or performance is not comprehensive, generally not used alone, often used in blending, composite and coating modified.
The hydrophilic properties of plastic film mainly include PVA, PA and so on. The hydrophilic resin can swell the resin because of its strong water absorption, and the molecular spacing increases, and the barrier property decreases. Usually, please water plastic film water vapor diffusion coefficient is not a constant, it is with the water vapor solubility increases, resulting in the change of permeability coefficient, permeability and influence of non hydrophilic plastic film is almost not affected by environmental humidity.
The diffusion energy required by the gas and water vapor through the crystalline plastic film is smaller than that of the amorphous plastic film, so the crystalline plastic film exhibits better resistivity. Under the same conditions, the higher the crystallinity of the plastic film, the better the barrier properties.
When the crystallinity is constant, the polarity of macromolecules or strong polar molecules is difficult to diffuse because of the tight combination between molecules. The greater the molecular polarity is, the smaller the permeability of the resin is, the better the gas barrier is. In common plastic resins, PET and PVA are strong polar resins, PA, PVC are polar resins, PS is weak polarity resin, PE and PP are non-polar resins. Their gas resistance increases with the increase of molecular polarity, such as PET and PE, the transmittance of O2 is very different.Water vapor is a polar molecule. According to the principle of similar dissolution, the water vapor in the polar molecular plastics is integrated and diffused faster than non-polar plastic molecules, and its moisture permeability is also larger. High barrier materials and PET molecules have strong polarity, and their moisture permeability is greater than non-polar molecular PE. Therefore, PE is an excellent moisture resistant packaging material.
