PVC Permeability
Shahab Jafarzadeh
General manager of Danpolymer & CEO of Pakan Polymer Jam & Research and Development Deputy of SVI
All polymers are permeable to gases, vapours, and liquids to a greater or lesser extent. At the molecular level even the best-made, porosity-free plastics have intermolecular voids into which gas or vapour molecules can diffuse. As would be expected, anything that inhibits the free passage of gas or vapour will increase a material’s resistance to permeation. Thus, increasing the crosslink density will decrease the permeability.
Plasticisers invariably increase the permeability because of the increased mobility of the polymer molecules and the greater free volume between the chains. Similarly, if there is a significant orientation of the polymer molecules, e.g., by blowing, stretching or calendering, the permeability is reduced.
In principle, the thickness of the material has a proportional effect on the rate of permeation, if chemical interactions between the permeant and the membrane are absent. Increasing the temperature increases the permeability. Permeation takes place in two stages: initially dissolution into the surface of the polymer, then diffusion through the material under the concentration gradient so formed.
Permeability of Water and Liquids
Water vapour permeability is an essential property when determining a material’s resistance to moisture or the ability to protect contents from the transmission of water vapour.
Rigid PVC has low permeability to water and water-based products. However, plasticised PVC has a relatively high permeability and it is this factor that is used in food packaging, allowing foods such as meat and cheese to breathe. It also prevents condensation and the product can be clearly seen.
Flexible PVC is also permeable to steam. The water vapour permeability of PVC film is compared with other plastics in bellow Table.
Permeability of Gases
PVC is valued for its vapour permeability, preventing condensation. Rigid PVC gives very good oxygen impermeability due to its closely packed and semi-crystalline structure. Flexible PVC does not give very good barrier properties, due to the plasticiser increasing the molecular chain mobility and intermolecular distances. The relatively high CO2 permeability of PVC-U in comparison to PET explains the use of PET for carbonated water bottles. The permeation rate of PVC for different gases is compared with other polymers in following Table.
