Main factors affecting film barrier properties

1. Crystallinity
In general polymers, both crystalline and amorphous phases are present. In the crystals of the polymer, the molecular chains are tightly packed, even if the smallest gas molecules cannot penetrate and diffuse. Therefore, the higher the crystallinity of the polymer, the smaller the flexibility of the chain, and the lower the transmittance, the better the barrier property. If the crystal in a certain polymer is 100%, the transmittance is zero. In fact, this polymer does not exist, it always has penetration. The degree of crystallinity is not only related to the physicochemical properties of the polymer, but also depends on the molding processing conditions of the polymer.
2. Forming process
Different molding processing conditions, such as molding temperature, cooling rate, and stretching orientation, can also have a great influence on the barrier properties of the film.
The plastic film has a high molding processing temperature and a low crystallinity of the polymer. When the molding processing temperature is low, the polymer crystallinity is high and the barrier property of the film is improved. During the cooling and setting process, the enamel film has a fast cooling rate and a fast crystallization, and the crystallinity inside and outside is very different. If the medium cooling rate is adopted, the crystal inside the film grows well, the crystallization is complete, and the crystallinity is high. In the process of stretching and orientation, the plastic film is subjected to high pressure, which accelerates the crystallization of the polymer, and allows the crystals to be aligned neatly, so that the penetration of gas molecules is more tortuous and difficult, thereby improving the barrier property of the film. The data in Table 8-2 shows the effect of orientation on the transmittance of the film. The biaxially stretched polypropylene film (BOPP) has a lower oxygen transmission rate and a water vapor transmission rate than the unoriented polypropylene film (CPP).
3. The environment used
The permeability of a gas or liquid in a polymer increases with increasing temperature. Therefore, the film barrier property also changes with temperature. As the temperature increases, the gas permeability of the film increases (see Table 83) and the barrier properties decrease.
4. Physical properties of the polymer
The physical properties of the polymer itself give the film a different barrier. The polymer has a high glass transition temperature, and its molecular chain movement at room temperature is difficult. The barrier property of the film is good: the high crystallinity of the polymer and the molecular chain stacking are good, and the molecular voids sandwiched in the solid are small, and the film is small. The barrier property is good; the polymer with high polarity has large molecular aggregation force, strong interaction between molecular chains, and good barrier property of the film. As the density of the polymer increases, the barrier properties also increase accordingly.