I have a huge lot of yellowed Lego's, I want to try to de-yellow them. I have searched around the internet and this method seems the most promising:


I have a few questions about this : - What is happening at the surface of the ABS plastic ? - My knowledge on this subject is very limited, but would the process go faster when you perform the process in a ultrasonic bath ?

A bit of unrelated question : - Is it possible to combine the de-yellowing with filling any scratches gaps in the bricks ? It seems not to me because you have to have some filler in the solvent or break the bonds in the lattice, i guess ?

any help, information is welcome, I'm like to learn new stuff !! I can only understand it a little bit from a nanotech point of view.


1 Answer 1


According to the retrobright website the process involves the use of activated hydrogen peroxide and possible UV light. This won't work as well in an ultrasonic bath as the bath will keep out most of the light.

Yellowing is thought to be caused by some degradation reaction of some of the additives in the ABS plastic (probably brominated flame retardants, though it could be something else). What is probably happening is that the yellow products of that degradation are being destroyed by activated peroxide. The UV light probably helps this reaction along a little faster, possibly by activating the degradation products (a yellow colour is often a sign that there is a strong absorption of light from the molecules in the near UV).

If you follow one of the recipes on their site (with their safety precautions) it looks as though it should work for Lego.

Filling scratches, though, will probably not be easy as this is not a chemical problem but a physical one. The only effective way would probably involve remelting the ABS and recasting it which seems like overkill (especially since you would need original Lego moulds). Any chemical that will dissolve the ABS is more likely to destroy it than to selectively reshape the surface.

  • $\begingroup$ Also, hydrogen peroxide threatment may lead to significant brittling of the plastic. $\endgroup$
    – permeakra
    Commented Jan 27, 2016 at 20:26
  • $\begingroup$ What does activated in activated hydrogen peroxide mean? $\endgroup$
    – wuschi
    Commented Jan 27, 2016 at 22:27
  • 1
    $\begingroup$ @wuschi hydrogen peroxide doesn't work well when cold. Certain chemicals can enhance the bleaching power (the TARP used here is a common additive used to enhance peroxide bleaching in low temperature laundry). Basically TARP reacts with peroxide to give a more active intermediate. $\endgroup$
    – matt_black
    Commented Jan 27, 2016 at 22:37
  • $\begingroup$ @matt_black thanks for your awesome answer, if i do analysis with the spectroscope i could see whether bromine is the cause ? or i could perform a afm sweep to determine different materials ? I know there is another method to determine the material properties (forgot the name). if i have more questions about selective reshaping the surface, should i open a new question ? $\endgroup$
    – pwghost
    Commented Jan 28, 2016 at 10:09
  • $\begingroup$ @pwghost Optical/UV spectroscopy will certainly tell you whether the treatment is working but it might be hard to verify what the actual coloured contaminants are. Other methods might be better. It would be a very good experiment to do as it would contribute useful results to science if you can identify the degradation products. $\endgroup$
    – matt_black
    Commented Feb 2, 2016 at 11:30

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