Way to whiten old bricks that have yellowed slightly that doesn't quickly get worse?

[CSEverett1759]

I'm trying to use some white bricks that have been out of production for decades, and naturally they've yellowed slightly, and a bit too much for use with a white rocket. Is there any way to whiten them that doesn't get far worse then before after a year? I've whitened bricks with hydrogen peroxide in the past, and now they're far worse then when I started.

 

Thanks.

[Murdoch17]

In short: No, there is nothing you can do. Retrobrighting is just a temporary fix and parts will get worse over time.

[MAB]

You could try hydrogen peroxide again. I found when I did it, some yellowed again whereas others have remained white for years.

Or buy replacement bricks from bricklink.

 

Edited October 29, 2025 by MAB

[Toastie]

4 hours ago, MAB said:

You could try hydrogen peroxide again. I found when I did it, some yellowed again whereas others have remained white for years.

I was waiting for such a reply, before beginning bragging ... that result indeed is very interesting (!) and contradictory to what I wanted to ague. On the other hand, I really believe all that yellowing depends on so many (not controllable by us) parameters, that any global prediction is rendered useless. However ;) statistically (with some sort of narrowed error of margin), I'd argue:

Colored ABS yellows (white) over decades (years), because some unsaturated bonds in the ABS polymer break, shifting the absorption spectrum deeper into the visible region. White brick = close to 100% reflection, yellowed brick = loss of refection within the visible (VIS) spectrum. It is not necessarily the presence of free electrons in the polymer, but much more so "saturation" of the initial radical sites by reaction with another ubiquitous biradical always present, the oxygen molecule O2, forming oxidized sites in the polymer. Which in turn shifts the absorption spectrum into the VIS region.

Upon H2O2 treatment, these sites are oxidized again, and a "temporary" effect is observed. However, I believe H2O2 is not selectively attacking the culprit sites, it also bites into the saturated ABS polymer sites, generating free radicals where there should be no free radicals generated upon exposure to light and such.

Which in turn accelerates the yellowing processes, as now, formerly passive saturated bonds are activated.

All total and utter speculation, but I am still favoring the radical mechanism (found in the literature, along with many others) as the main cause/driving force for yellowing. 

This proposed "mechanism" depends heavily on the parameters a) degree of polymerization (= yield/creation of single bonds) in the original ABS polymer, b) exposure time to actinic light, c) change of surface layer composition (grease, water etc.), d) change of surface constitution (scratches, dents, etc.), and so many more.

And H2O2 may lead to temporary shifts, but in the end, induces sites accelerating the yellowing process.

As said, total speculation.

Do I know any other means of “whitening”? Yeah: White (spray) paint. But that is generally not accepted at all. They for sure do it in the LEGO Lands, as I and others have literally observed it.

All the best
Thorsten    

 

[CSEverett1759]

An article about "cleaning" mentioned a method involving baking soda and "brushing it off" with a toothbrush - does that method help whiten? Any experience as to if it might have better long-term results? It almost seems that removing the tiniest fraction of a millimeter of the surface would expose fresh plastic.

[JopieK]

See this ten year old topic: 

 

[MAB]

13 hours ago, CSEverett1759 said:

An article about "cleaning" mentioned a method involving baking soda and "brushing it off" with a toothbrush - does that method help whiten? Any experience as to if it might have better long-term results? It almost seems that removing the tiniest fraction of a millimeter of the surface would expose fresh plastic.

Yes it can help, but in the same way as using fine grit sandpaper. It dulls the surface but can also get rid of scratches too.

[Toastie]

13 hours ago, JopieK said:

See this ten year old topic: 

As well as these ...

https://www.eurobricks.com/forum/forums/topic/7167-brick-de-yellowing-techniques/
https://www.eurobricks.com/forum/forums/topic/57827-yellowing-in-misb-sets/
https://www.eurobricks.com/forum/forums/topic/97365-lego-and-sunlight/
https://www.eurobricks.com/forum/forums/topic/171633-the-old-yellowed-lego-returns-to-shine/
https://www.eurobricks.com/forum/forums/topic/199644-mindstorms-nxt-retrobright-easy-technique-fix-your-yellowed-nxt/

... and even more. There is one thing, though, I'd rather remove from any "why" discussion: Bromine. That is certainly not the case, as has been discussed elsewhere. It would never work; if bromine as brown molecule would be present in the outer layers of ABS accessible to light/chemistry, it would react away within no time.

Everything else (light, heat, other chemicals, surface damage, polymer formulation etc.) is up to speculation. It for sure is a "close to the surface effect" as you can sand off the yellow @MAB has said. It does work, but removes material ...

Best
Thorsten