A study found that:
1. Comparison of moisture permeability and air permeability of 2-layer composite bags.
LDPE (low density polyethylene) film has more linear macromolecular structure, its crystallinity and density are low, and its gas barrier properties are poor; compared with PE film, PP film side group has non-polar -CH3, and its moisture barrier is stronger than LDPE. , but the air permeability is still poor; after directional stretching treatment (BOPP) the barrier property is stronger than the ordinary film (CPP). PA film is a linear crystalline polymer with a large number of amide groups on the main chain of the molecule; it has good gas barrier properties. Due to the strong polarity of the PA film molecule, it is a typical hydrophilic polymer, which absorbs water. Strong, making the long-term moisture resistance poor. PET film has high strength and toughness because the main chain contains benzene ring, and has good flexibility because of ether bond; its gas barrier and moisture barrier are good.
The order of moisture barrier properties of single-layer film is basically BOPP>LDPE≈CPP>PET>PA
The order of oxygen permeability of single-layer film is: PA>PET>CPP≈BOPP>LDPE.
2. In the two-layer composite film, BOPP/LDPE has the best synergistic effect of moisture and oxygen resistance. When the packaged material needs strong oxygen resistance, PA or PET is generally used as one of the base materials of the composite bag. For better moisture resistance, BOPP is generally selected as one of the substrates of the composite film.