Editing Corrosion (section)

=== Corrosion of polymers ===
[[File:Ozone cracks in tube1.jpg|thumb|[[Ozone cracking]] in [[natural rubber]] tubing]]
[[Polymer degradation]] involves several complex and often poorly understood physiochemical processes. These are strikingly different from the other processes discussed here, and so the term "corrosion" is only applied to them in a loose sense of the word. Because of their large molecular weight, very little [[entropy]] can be gained by mixing a given mass of polymer with another substance, making them generally quite difficult to dissolve. While dissolution is a problem in some polymer applications, it is relatively simple to design against.

A more common and related problem is "swelling", where small molecules infiltrate the structure, reducing strength and stiffness and causing a volume change. Conversely, many polymers (notably flexible [[polyvinyl chloride|vinyl]]) are intentionally swelled with [[plasticizer]]s, which can be leached out of the structure, causing brittleness or other undesirable changes.

The most common form of degradation, however, is a decrease in polymer chain length. Mechanisms which break polymer chains are familiar to biologists because of their effect on [[DNA]]: [[ionizing radiation]] (most commonly [[ultraviolet]] light), [[Radical (chemistry)|free radical]]s, and [[redox|oxidizer]]s such as oxygen, [[ozone]], and [[chlorine]]. [[Ozone cracking]] is a well-known problem affecting [[natural rubber]] for example. [[Plastic additive]]s can slow these process very effectively, and can be as simple as a UV-absorbing [[pigment]] (e.g., [[titanium dioxide]] or [[carbon black]]). [[Plastic shopping bag]]s often do not include these additives so that they break down more easily as ultrafine particles of [[litter]].