As anybody knows, there can be a lot of incorrect information on the internet. Internet “Just so” stories can spread like wildfire if they are believable and explain something neatly. One of those “just so” stories involves older game consoles and computers; over time, we find that our once-white and gray plastics on old systems like Apple II’s, NES consoles, SNES consoles, and so on change colour; they change from white or gray to yellow, and over time that yellow darkens, sometimes even turning brown.
This phenomena is “explained” here. Or is it? Does what is stated there about the process reflect reality? Does it make chemical sense? To the layman or casual observer- hey, it makes sense. Bromine IS brown, after all, it’s added to the plastic. But is there a chemical basis and support for it? What reactions actually take place?
“RetroBright”- which is basically just Hydrogen peroxide – is commonly recommended to “reverse” the effects. The reason I care about the actual chemical properties is because the yellowing itself goin g away isn’t an indication that everything is back to how it was. Colour changes can be the result of all sorts of things. More importantly, if we learn the actual chemical processes involved, perhaps we can come up with alternative approaches.
Basically, the story put forth in the article is a rather commonly repeated myth- a Chemical “just-so” story of sorts- “Bromine is brown so that must be it” Is the extent of the intellectual discussion regarding chemistry, more or less. Generally though there isn’t much drive to look further into it- it all makes sense to the layman on the surface, or even one with rather standard chemistry knowledge. But when you look deeper than the surface of the concept- you see that the commonly held belief that Brominated Flame Retardants are responsible doesn’t seem to hold up.
First we can start with the first inaccuracy in that link- Bromine is not added as a flame retardant- that is flat out, categorically and completely wrong, and trivially easy to refute. Bromine compounds are added as flame retardants, But as they are compounds, the colour of elemental Bromine (brown) is irrelevant, because elemental Bromine is not added to the plastic. Specifically, chemicals like Tetrabromobisphenol A. (C15H12Br4O2).
The article also says that “The problem is that bromine undergoes a reaction when exposed to ultraviolet (UV) radiation” But Bromine doesn’t photo-oxidize. It doesn’t even react with anything in the air on it’s own; creating Bromine dioxide either involves exposing it to Ozone at very low temperatures alongside trichlorofluoromethane, alternatively, gaseous bromine can be made to react with oxygen by passing a current through it. Neither of these seem like they take place in a Super Nintendo. Not to mention elemental bromine is brown, so if it was in the plastic, oxidization would change it from the brown of elemental bromine to the yellow of bromine dioxide.
Back to what IS in the plastic, though- Tetrabromobisphenol A is not photosensitive; it won’t react with oxygen in the air due to UV light exposure, and the bromine cannot be “freed” from the compound and made elemental through some coincidence in a typical environment. It is simply not the cause of the yellowing; (it will yellow without BFR’s as well, which sort of indicates it’s probably not involved).
The Yellowing is inherent to ABS plastics, because it is the ABS plastic itself that is photo-oxidative. On exposure to UV light (or heat, which is why it can happen with systems stored in attics for example), the butadiene portion of the polymer chain will react with oxygen and form carbonyl-b. That compound is brown. There’s your culprit right there. Retrobright works because thsoe carbonyls react with hydrogen peroxide, and create another compound which is colourless. but the butadiene portion of the polymer remains weak- oxalic acid is thought to be one possible way to reverse the original reaction.
So why does it sometimes not affect certain parts of the plastic or certain systems? here the “just so” story is a bit closer to reality- the “story” is that the plastic formulae has different amounts of brominated flame retardants, This is probably true, but as that compound isn’t photo-reactive or involved in the chemical process, it’s not what matters here. What causes the difference is a variance in a different part of the formulae- the UV stabiliser.
UV Stabilisers are added to pretty much All ABS plastic intentionally to try to offset the butadiene reaction and the yellowing effect the resulting carbonyl has. They absorb UV light and dissipate it as infrared wavelength energy which doesn’t catalyze a reaction in the butadiene. Less UV Stabilizer means more UV gets to the Butadiene and causes a reaction and the plastic yellows more quickly. more UV stabilizer means less UV catalyzes reactions and the plastic takes longer to change colour.
As with anything related to this- the best way is to experiment. I’ve decided to pick up some supplies and test both approaches on a single piece of plastic. some standard “retrobright” mixture using hydrogen peroxide, and a variation using oxalic acid. I can apply both to the same piece of yellowed plastic, and observe the results. Are both effective at removing the yellowing color? what happens longer term? It should be an interesting experiment.