About the air permeability principle and influencing factors of packaging materials
Food and drug safety has always been the focus of people’s attention, and packaging is the guarantee of food and drug quality. The barrier properties of packaging materials have a major impact on the shelf life, quality stability, moisture resistance and oxidation resistance of food and drugs. At present, the permeability of materials Testing and moisture permeability testing have become more popular. The research on the air permeability of packaging materials is particularly important in the pharmaceutical and food industries, which directly affect the quality of products. A single film cannot meet the overall demａnd for flexible packaging. In recent years, multi-layer composite films have been widely used in various fields with their excellent barrier properties and chemical resistance. Among them, the barrier layer material plays a key role.
The air permeability principle of packaging materials:
Gas transmission rate. Under an atmospheric pressure difference, per square meter of permeable area, the amount of gas permeated in 24 hours (under standard conditions).
Permeability coefficient. The air permeability per unit area and unit thickness of the film in unit time and unit pressure difference (under standard conditions).
The permeation of gas through the film is a single molecule diffusion process, which is a mass transfer process. It consists of three steps. When there is a certain gas with different concentrations on both sides of the film, the gas is first dissolved in the film. Because the solid film has molecular and intramolecular gaps, the gas changes from high concentration to high concentration in these gaps. Diffusion occurs at a low concentration, and finally evaporates on the other side of the membrane. The permeation of gas into the membrane is composed of countless single molecules permeating the membrane.
Different gases (for example: oxygen, nitrogen, carbon dioxide, etc.) have different air permeability coefficients for the same packaging material. In food packaging, the main research is the oxygen permeability of packaging materials, that is, the oxygen permeability of packaging materials.
The factors affecting gas permeability are as follows:
① The polarity of the molecular chain. Among high polymer materials, non-polar materials include polyethylene, polypropylene, polybutadiene, polytetrafluoroethylene, etc.; weakly polar materials include polystyrene, polyisobutylene, natural rubber, etc.; polar polymers include poly Vinyl chloride, nylon, polymethyl methacrylate, etc.; strong polarity includes phenolic resin, polyester, polyvinyl alcohol, etc. Polar molecules have high mutual force, high cohesive energy density, good barrier properties, and low diffusion coefficient.
②The rigidity and side groups of the molecular chain. Materials with high molecular chain rigidity and inflexible main chain materials with high glass transition temperature have lower gas permeability. The side groups of the molecular chain are asymmetric, and the free space of the polymer is large, and the transmittance is relatively high.
③ Crystallinity. The higher the crystallinity, the closer the molecular chain is arranged, and the more diffusion activation energy is required for gas to pass through crystalline substances than through amorphous substances, so the barrier properties are better. On the one hand, the transmittance of crystalline polymers is lower than that of amorphous polymers. On the other hand, the high crystallinity of the same polymer is better than the low crystallinity
④The density of the polymer. Similar to crystallinity, high polymer has high density, good barrier properties and low permeability.
⑤ Degree of orientation. By changing the stretching orientation of the polymer, the gas permeability can be significantly reduced. Especially for crystalline polymers, the orientation can rearrange the crystals in a certain direction and promote crystallization, so that the penetrant molecules need to go through a more tortuous path Only through packaging materials.
⑥ Moisture sensitivity. Some polymers contain hydroxyl -OHI, amide group -CNH-, etc., which are sensitive to water. When water molecules penetrate in, hydrogen bonds are formed, which expands and relaxes the polymer and increases air permeability. In addition, some polymers containing ester groups—C-0—ａnd chlorine groups Pi C=N, although they absorb water, do not affect the barrier properties because they do not depend on hydrogen bonds.
⑦Temperature. In general, the permeability of gas or liquid in high polymers increases with increasing temperature. This is because as the ambient temperature increases, the factors affecting the barrier properties of the plastic film will change accordingly. The rigidity of polymer molecular bonds decreases, the degree of cohesion decreases, and the free volume increases. The increase in ambient temperature will also reduce the orientation energy between polymer molecular chains after stretching. These changes make the transmittance of the film increase as the ambient temperature rises.
