What are the four packaging materials?
2018/7/31 17:31:42
Paper, metal, glass (and ceramics), and plastic are called four packaging materials. The paper currently used is mainly composed of plant fibers, the main chemical component of which is hydrophilic cellulose molecules, and the pores between fibers and fibers can be up to the order of micrometers, so the paper material has almost no moisture barrier and gas barrier properties. Conventionally, hydrophobic modification, sizing, etc. are generally used to improve the moisture resistance of the paper, and the barrier property of the paper can be improved by polymer coating to obtain a paper material having a moisture-proof and gas barrier function.
The metal materials commonly used in packaging are iron and aluminum. As an atomic crystal, the metal atoms are arranged very closely, which is not conducive to the transmission of small molecules, and therefore has excellent barrier properties against moisture and gas.
Although the glass is amorphous, its main structure is a network structure composed of Si-O tetrahedrons. The Si and O atoms are closely connected, so there is almost no large pores in the material, which makes it have excellent moisture resistance and gas barrier ability. . At the same time, since the heat resistance of metal and glass is excellent and the structure is stable, their barrier properties are hardly affected by temperature and humidity at normal temperature. However, the high cost and opacity of metallic materials, the high specific gravity of glass and the fragility limit their many applications in packaging.
Plastic is composed of polymer and filler. It has many advantages such as light weight, transparency, low price, easy processing and non-breaking. It is widely used in packaging and the dosage is increased sharply. Because of its diverse structure and many influencing factors, the barrier properties of different plastics are also very different. As mentioned earlier, polymers of different chemical compositions have different polarities and thus have significant differences in solubility for different small molecules. At the same time, the polymer mostly has a structure in which a crystalline state and an amorphous state coexist. In the crystalline structure, the molecules are closely arranged in order and thus have little permeability, so the gas permeability of the entire material depends on the proportion of the crystalline structure. That is, crystallinity. The degree of crystallinity is affected by the molecular structure, molecular weight and processing technology of the polymer. For example, amorphous PET has an oxygen diffusion coefficient of 5 × 10 13 m 2 /s and an oxygen transmission rate of 0.424 cm / m 2 · day · atm, while the PET bottle produced by the cold blowing process has a crystallinity of 22% and an oxygen diffusion coefficient. To 2.5 × 10 13 m 2 / s, the oxygen transmission rate also decreased to 0.171 cm / m 2 · day · atm. At the same time, as the molecular motion of the polymer macromolecules increases with increasing temperature, resulting in an increase in free volume, the barrier properties are reduced or even lost. In addition, the barrier property of a polymer having a hydrophilic group is also greatly affected by humidity. Therefore, when selecting plastic packaging, materials with appropriate barrier properties should be selected according to the characteristics of the packaged food and the storage and transportation environment.