Gas transmittance. The amount of gas transmitted in 24 hours per square meter of transmitted area at one atmospheric pressure difference (standard conditions). Air permeability coefficient. The air permeability of the film per unit area and thickness per unit pressure difference per unit time (under standard conditions). The transmission of gas through thin film is a single molecular diffusion process, which belongs to mass transfer process. There are three steps in total. When there is a certain gas with different concentration on both sides of the packaing film, the gas first dissolves in the film. Since there are molecular gaps and intramolecular voids in the solid film, the gas diffuses from high concentration to low concentration in these gaps, and finally evaporates outward on the other side of the film. The permeation of gas to thin film is composed of the permeation of numerous single molecules to thin film. Different gases (such as oxygen, nitrogen, carbon dioxide, etc.) have different permeability coefficients for the same packaging material. In food packaging, the main research is the oxygen penetration of packaging materials, that is, the oxygen permeability of packaging materials.
The factors that affect gas permeability are as follows:
1. Polarity of molecular chain. In the polymer materials, non-polar materials are polyethylene, polypropylene, polybutadiene, polytetrafluoroethylene and so on; Weak polarity of polystyrene, polyisobutylene, natural rubber, etc.; Polar polymers are PVC, nylon, polymethyl methacrylate, etc.; Strong polarity of phenolic resin, polyester, polyvinyl alcohol and so on. Polar molecules have strong mutual force, high cohesive energy density, good barrier and low diffusion coefficient.
2. Rigidity and side groups of molecular chains. Materials with large molecular chain rigidity and inflexible main chain have low gas transmittance and high glass transition temperature. The molecular chain side group is not symmetrical, the free space of high polymer is large, the transmittance is relatively high.
3. Crystallinity. The higher the crystallinity, the closer the molecular chain is arranged, the more diffusion activation energy is needed for the gas to pass through the crystalline material than through the non-qualitative material, so the barrier is better. On the one hand, the transmittance of crystalline polymers is lower than amorphous polymers, and on the other hand, the high crystallinity of the same polymer is better than the low crystallinity
4, the density of the polymer. Similar to crystallinity, the polymer has high density, good barrier and low permeability.
5. Degree of orientation. The gas transmittance can be significantly reduced by changing the tensile orientation of the polymer, especially for the crystalline polymer, the orientation can make the crystal rearrange in a certain direction and promote the crystallization, so that the penetrant molecules need to pass through the packaging material through a more zigzaggy path.
6. Humidity sensitivity. Some polymers contain hydroxyl -OHI, amide -CNH-, etc., sensitive to water, when water molecules infiltrate, the formation of hydrogen bonds, so that the polymer expansion, relaxation, so that the permeability increased. Other polymers containing ester -C-O-, chlorine C=N, although water absorption, but do not affect the barrier, because it does not depend on hydrogen bonding.