Why Plastics Yellow - Part 3: Mechanisms of Degradation

April 23, 2026

In Part 2 of Why Plastics Yellow , we looked at chromophores and how they are responsible for color of an object. In this post, we are going to look into the mechanisms of plastic yellowing, or said another way, physically what mechanisms initiate chromophores to change colors and degrade the plastic?

There are several possible ways for polymer degradation to occur which include the following:

Thermolysis (heat-thermal degradation))
Thermooxidation (heat in the presence of oxygen)
Photodegradation (light)
Photooxidation (light in the presence of oxygen)
Ionizing radiation (degradation due to ions)
Mechanical action (stress and shear)
Biodegradation (fungi, bacteria, yeasts, algae, and their enzymes)
Chemical degradation (acids, bases, solvents reactive gases, etc.))
Or combinations of all of these (for example, light and elevated temperatures, in oxygen)

Those examples are taken from this source. For most people interested in Retro-Britening, photooxidation is the likely culprits of yellowing. It is interesting to note that the degradation of polymers usually starts on the outer surface and penetrates gradually into the material (Blaga 1980). So if those are the inputs, what happens in the plastic? That's the subject of our next post, Part 4!