What is ABS plastic?

ABS is a plastic/polymer made up of acrylonitrile, butadiene, and styrene as seen by the simple reaction below:

According to the Wikipedia page, ABS is a general plastic that is useful for its tough properties and ability to be shiny. The multiple monomers that make up ABS are useful at providing different properties to enhance the polymer. The important part we will be looking at for how ABS yellows is that butadiene part. Quick organic chemistry: butadiene is incredibly reactive because the 4 carbons are all “conjugated”, which essentially means that the electrons have higher reactivity from being so close together. This component is the cause of ABS yellowing.

How ABS yellows

ABS yellowing is a well known radical based mechanism. A radical is any atom that only has an unpaired electron outside of its base state (electrons really love being in pairs). A mechanism is simply the process of a reaction. You don’t need to be knowledgeable in organic chemistry to understand this as its a fairly simple process

Source: Polymer Photodegradation: Mechanisms and experimental methods by JF Rabek

As you can see, only 2 things are involed, the carbon that is affected and simple oxygen. This process can be initiated by both heat and UV light, though UV increases the reaction rate by a much higher degree. And because this is radical based, there is no way to 100% stop it. It also ruins the mechanical strength of the plastic, making it much more brittle than fresh ABS.

Bromine and yellowing

A very common misconception, probably caused by Nintendo, is that the bromine flame retardants (BFRs) used in ABS plastics is the cause of the yellowing process. I can safely say that this is not the case. If the BFRs degraded into a bromide or bromate salt, then they would simply dissolve in water and the plastic would not stay yellowed. Yet that doesn’t happen. Another suggestion is that it degrades into bromine, which would also not keep the plastic yellow. Bromine is a volatile liquid and evaporates very quickly. It is also one of the most pungent smells that you could experience and even little amounts linger (have worked with bromine before). It just doesn’t cause the plastic to yellow. It is entirely caused by the degradation of ABS.

How deyellowing works

There have been no real studies into how the deyellowing process works unfortunately, so what I say here is only from my knowledge in chemistry and what results I could find. The deyellowing process only requires hydrogen peroxide and a source of energy like UV light or heat. My guess is that it is simply the reverse reaction, but it does remove the pesky oxygen from the affected carbon. This entire process is enhanced by a peroxide bleaching agent like Tetra-acetylethylenediamine (TEAD). What is interesting is why this occurs, and in my research the plastic does not become deyellowed because of the hydrogen peroxide, but because of a bleaching agent. Yes, everything you’ve heard about the Retr0bright process is not entirely true. You can see the reasons why in this video and this post on Quora. This entire process effectively bleaches the plastic, which is why it also further damages the plastic. If you want to do the process in the video, don’t. It is very dangerous even for skilled chemists and I don’t recommend doing it all.

How should you deyellow then?

Ignore Retr0bright entirely. Buy the strongest peroxide solution you possibly can (products for hair are typically decently high), use a UV lamp or careful heating, and add a bit of TEAD to enhance the bleaching process of peroxide. Submersion is the easiest method and the hydrogen peroxide will last a fair bit, but not over time. If you are skilled and knowledgeable enough, another bleaching agent like sodium hypochlorite (sold as household bleach and pretty low concentration) would also work to some degree, but may not be as strong of a bleach as hydrogen peroxide. Calcium hypochlorite is also fairly cheap and sold as pool chlorine. Creating a strong solution of that is fairly trivial. The advantage of not using peroxide is that you don’t need a source of energy at all. The bleach is already activated by being dissolved. However, little research of any variety has been done about other bleaching agents outside of chlorine gas, so you will need to experiment to see if it actually does work well enough.