Retrobright, also known as Retr0bright is a process of bleaching sun damaged ABS plastic back to the original colour using a solution of Hydrogen Peroxide and laundry oxidixer containing TAED. It was originally developed for restoring retro computers which left the factory an off-white / light beige colour and often went yellow or even brown with age.
As you may know I've started collecting / restoring old telephones and the ABS plastic the GPO used to make phones in the 60s onwards also suffered from the same yellowing issue. It's most obvious on the Ivory models (Frequently sold incorrectly as "Yellow") but also affects the appearance of the coloured models, most notably the green and grey versions.
I've retrobrighted a couple using UV light, but in the UK the sun is variable and almost non-existant in winter, so I thought I'd try using heat instead of UV, and do some testing with different concentrations, temperatures, and amounts of oxidizer.
The most accurate colour match is probably for the 3% solution at 60 degrees. The Oxidizer appeared to make little difference.
As you can see, with green plastic the results are dramtically different to the Ivory plastic, with significant bleaching and discolouration to the point where some of the parts are almost turquoise.
It's not obvious from the image but the parts that were in solutions containing the oxidizer have developed an uneven cloudy finish, most noticably in the 12% concentrations.
The most accurate colour match is 3% H2O2 at 40C.
It's said that the best way to cook meat is "Low and Slow" - and perhaps unsurpisingly the same applies to bleaching plastics.
The green plastic in particular was very sensitive to bleaching and it was very easy to over do it and dramatically alter the colour. It seems you can get away with stronger solutions and temperatures with Ivory plastics.
The original recipe calls for Oxidizer, but for my experiements it either had little effect or made things worse - especially for the coloured plastics. I probably won't be using it in future as its just another thing to buy and measure out.
I also had an issue at one stage with the 12% Hydrogen Peroxide and Oxidizer solution going exothermic and appearing to be close to boiling. I measured it as being over 80°C whilst the surrounding waterbath was at 60°C. I'm not sure what happened there - it would appear a contamination to the jar, solution, plastic, hot glue or stainless steel nut triggered some kind of reaction. After the jar was emptied and re-filled it didn't occur again.
With the above in mind, and given the outcome of the experiement, I won't be using 12% again, but aiming for 3% with coloured plastics and possibly as high as 6% for ivory and whites. (I'll still buy 12% and dilute it to reduce shipping costs.) I'll also be sticking to 40°C because at 60°C I had issues with the hot melt glue going soft and the weights falling off.
Keep in mind that results may vary between different colours and batches of ABS, so it's best to keep an eye on it and remove once its done. I've also observed that the colour can continue to change after being removed from the H2O2 and rinsed, so you're better to take it out early, wait, and put it back in if required rather than over doing it.