What are the factors affecting the barrier properties of packaging materials?
2018/7/21 16:14:53
The barrier property of packaging materials refers to the property of materials to prevent the transmission of small molecules. The small molecules include water molecules, O2, N2, CO2 and organic gas and other gas molecules, including blocking the outside air from entering the package, and also preventing packaging. The internal moisture, aroma, etc. permeate. Therefore, the barrier properties of the packaging material can be measured by the gas permeation rate (rate) or the gas permeability coefficient. In GB/T 1038-2000, the gas permeation amount refers to the volume of gas per unit area per unit time per unit time at a constant temperature and a unit pressure difference. It is expressed in terms of volume values at standard temperature and pressure in cm3 / m2·d·Pa. The gas permeability coefficient refers to the volume of gas per unit thickness and unit area per unit time per unit time under stable temperature and unit pressure difference. It is expressed in terms of volume values at standard temperature and pressure, and the unit is cm3·cm / cm2·s·Pa.
For the explanation of the permeation mechanism of small molecules in non-porous films, a solution-diffusion model is widely accepted. The model considers that small molecules on one side of the membrane are first dissolved in the membrane, and then diffused from one side of the membrane to the other side under the pressure difference or concentration difference, and then desorbed on the other side surface of the membrane. The decisive factor in this process is dissolution and diffusion. Therefore, the rate of transmission of small molecules in a certain membrane material depends not only on its solubility on the membrane surface, but also on its diffusion rate in the membrane. .
The solubility of small molecules on the surface of the membrane follows the principle of "similarity of compatibility", that is, the polarities of both small molecules and membrane materials are similar, so that small molecules are easily dissolved on the surface of the membrane and then diffused in the membrane, so the transmittance is high. The barrier property is poor; on the contrary, the greater the difference in polarity between the two, the better the barrier property of the membrane material to the small molecule.
The rate of diffusion of small molecules in the membrane after dissolution on the membrane surface is related to the free volume in the membrane, the molecular size and chemical structure of the small molecules, and environmental factors. Free volume refers to the volume that is not occupied by the molecules of the membrane material itself, ie, the molecules between the molecules. Therefore, the larger the free volume of the membrane material, the higher the diffusion rate of the small molecules. The free volume of the material is affected by environmental factors such as temperature and humidity. The higher the temperature, the more intense the movement of the molecule, the larger the free volume, the lower the barrier property of the material, and the greater the influence of the humidity on the hydrophilic material, such as EVOH, PVA and other polymer materials, although with good gas Barrier property, but in a high humidity environment, it will swell due to moisture absorption, and the free volume will increase remarkably, resulting in a sharp decrease in barrier properties.