Any industrial part is not isolated but is immersed in an environment more or less complex made of gases, liquids and other solids. A very simple goods such as food packaging film is in contact with air, food, and eventually another plastic constituting the rigid base of the package.
Additives can:
* Desorb toward this environment following kinetics specific to the matter in contact, creating a heterogeneous state inside the part. Monomers, oligomers, plasticizers… desorb the more so as the temperature rises and the pressure decreases.
* Be consumed by aggressive species from the environment, often oxygen consuming protective agents
* Be extracted by chemicals absorbed from the environment. Solvents, oils… swell the polymer and then, some additives are dissolved and extracted.
Apart losses during processing, storage, and service life, polymers can also absorb chemicals of the environment that dissolve additives or modify polymer properties. For example butyl rubber is used for gas mask, notably mustard gas, but becomes permeable if soiled with diesel fuel.