How to de-yellow Lego

Background

When you buy an old Lego set on Bricklink, the chances are big that you’ll receive a bunch of  yellowed bricks. Mostly the colours white, grey and blue are affected by this. This has to reasons:
  • Bromine contents in the plastic: bromine has been used as a flame retardant. If plastic that contains bromine is exposed to UV light, the bromine molecules can destabilize and work their way to the surface;
  • Polymer oxidation: due to the exposure to UV light, the polymers of the plastic start to oxidize, giving the yellow colour to the plastic.

This yellowing can be restored by submerging the yellowed Lego parts into a hydrogen peroxide solution and exposing the solution to UV-A light.

 
Important parameters of the process

The following parameters of the process are important:
  • Percentage of hydrogen peroxide in solution;
  • Water temperature;
  • Amount of UV-A light.

Percentage of hydrogen peroxide solution and amount of UV-A light
Hydrogen peroxide solutions are for sale in different solution percentages. I worked the whole summer with a 3% solution in combination with the sunlight. Which solution works the best for you depends on your location on Earth. I live in the Netherlands, so if you live on approx. the same longitude (like southern Canada), 3% will do in summer time. You could also use a 6% solution, which makes the process a bit faster but also the risk to overexpose the Lego parts, which results in a white film on the parts.

If you live on a more southern longitude (in case of the Northern Hemisphere), 3% will always do the job, there’s no need to use higher percentages.

Water temperature
I found out that the process works the best with water temperatures of 25 degrees Celsius or above. The process works also below this temperature but it’s less efficient and it takes longer to get the same result. I don’t know if there’s an upper limit, as far as I know there’s not. But keep in mind that the warmer your water becomes, the faster it evaporates.

De-yellowing Lego without the sun

In case you live in a place where you don’t get enough sunlight/radiation for the de-yellowing process, you could also build your own sun. What’s important to know is that you have the right frequency/wavelength of UV light. The sweet spot for de-yellowing Lego is UV-A light with a wavelength of 325 nm. UV-B lights won’t work, see image below.
 
 
Blacklight is also UV-A light, but blacklight tubes and LED’s emit around 365-370 nm with a very steep curve. This means that there won’t be a lot of intensity left at 325 nm. See also the image below, which is a curve of the emission of a blacklight tube (blacklight LED’s are comparable).
 

That being said, maybe the process will work with blacklight as there may be some overlap. It is not clear how wide the bandwith of the sweet spot at 325 nm is: there may be some overlap with the blacklight emission curve above. I don’t know this: you could perform a test. What certain is, is that the process will run less effeciently so it will take more time to get the same results (if these results could be obtained in the first place).
 
I use OSRAM ULTRA VITALUX 300W lightbulbs. One lightbulb could do the trick, but two is better, as most of the light doesn’t diverge enough to expose a baseplate of 32×32 evenly. Please note that these lightbulbs become extremely hot: if you use them in a confined space, add a cooling fan. WARNING: don’t look into the lights without the proper eye protection!
 
If you want to know more about de-yellowing Lego in this manner, check out this video below.