Discoloration due to sun exposure

[tibbles]

I've got questions about the weathering damage on my Lego set for the white Saturn rocket.  It's not the fact that sitting in sunlight for years has started discoloring it, I'm curious why the discoloration happened as it did.  Instead of discoloring in bands matching where sun exposure most made it through slits in my blinds, some pieces are a discolored yellow while others are still plain white.  Either a whole brick yellowed evenly at once or not at all and the ship is now a checkerboard of white and yellowed bricks.  Why?  

And, once discolored, is there any way to rewhiten?  I'm guessing not, but might as well ask.

Thanks.

[SpacePolice89]

My 10021 USS Constellation has been partially exposed to sunlight for over 20 years. I recently moved it to my basement. Some of the gray pieces are somewhat yellowed and others are not yellowed at all. It must be some difference in the material because the pieces are next to each other. 

[Mylenium]

The distribution of the pigments is different across different production batches and even within the same production runs since the colors are added directly in the injection molding machine. They all appear the same initially, but technically the amount of actual white can vary and this comes to light as they age. The rest is just how the underlying chemistry works on a broader level. Once the aging process starts it doesn't require direct exposure to the initial factors anymore. The process is self-propagating because there's an imbalance in the material that is trying to even out itself. You can read long-winded explanations on how the polymer strands shorten and re-order or how molecules migrate within the medium on respective sites. It's perfectly normal. In combination of both factors you get those typical checkerboard patterns.

Mylenium

[SpacePolice89]

53 minutes ago, Mylenium said:

the colors are added directly in the injection molding machine.

This is a more recent method. Before the plastic granules came pre colored and then the colors were more consistent. Even bricks from that era yellow very individually, some very much and others almost not at all. When there were fewer colors this was the preferred solution and the quality was higher but it was also more expensive.

"LEGO used to produce bricks from pre-coloured raw material, however they've switched base+color pellets nowadays. I'm not entirely sure why (probably due to cost saving), but this introduced problems with color consistency, which is still hasn't been resolved until today"

https://bricks.stackexchange.com/questions/12940/why-was-lego-reluctant-to-use-additional-colours-for-regular-bricks-in-former-ti

[ChrisXY]

LEGO elements contain a brominated flame retardant (“BFR”). Basically LEGO wants to prevent that their parts are highly flammable. It´s a safety concern. When exposed to ultraviolet (UV) light the compound decomposes, the bromine migrates to the surface of the part and bonds with atmospheric oxygen, which exposes the inherent brown color of bromine. The effect is particularly visible on white, trans-clear, old grey and blue. As the BFR wasn´t used in the exact proportion in every brick, some bricks yellow, some don´t. Even if they came from the same set and have same sunlight exposure.

There is a developed procedure using hydrogen peroxide (H2O2) and sunlight to reverse the effect. Chemically, the oxygen is displaced by hydrogen.

This at least a reasonable explanation. Some might say now: That means that sets that are stored away, should not have yellowed. That could be true, but only when you are 100% certain that at no time (beginning in the production) the part was NEVER exposed to UV light. That might be untrue, that´s why parts also yellow in dark places in sealed sets.

Edited May 26 by ChrisXY

[tibbles]

Interesting, thanks, all.  So, too late for the set, but upside, I guess I don't need to feel guilty about continuing to leave it in the sun longer.

[Toastie]

5 hours ago, ChrisXY said:

LEGO elements contain a brominated flame retardant (“BFR”)

Hmmm - don't think so . 

Since decades now, “BFRs” have been of environmental concern. Many of these compounds (there are myriads) have been phased out since long. I don't believe that TLG takes the risk of using remaining halogen/bromine containing flame retardants - if that becomes public, all hell would break loose. Furthermore, since many years now, non-halogen containing self extinguishing ABS formulations have become available, just type into Google "abs flame retardant halogen free" and you'll get access to numerous data sheets, sources, particularly filaments for 3D printers and so on and so forth. 

As @Mylenium has said, there are so many sources regarding the "break-down" mechanisms out there. As far as I am concerned, it all begins with the initial breaking of remaining C=C double bonds. Sunlight is certainly a secure route for initiating degradation, but also all other means of chemical activation may come into play. ABS tends to absorb H2O to some extent - and thus certainly other chemicals as well. These may also be involved in starting the processes, even in the dark. After all, cosmic radiation is everywhere. Once initiated, the degradation process is indeed of self propelling (chain) nature. As ABS formulations are so diverse (clutch power is one aspect, there are many, many more), the extent of remaining double bonds as well as the 3D-structure of the final polymer may intentionally or unintentionally change during production. How the 3D structures of the polymers actually look like after production in country X on machine Y in an environment Z is really a good question. Yes, there will be very strict process and quality control measures but ...

It simply happens. And as far as I am concerned: I like it. It gives a very special touch to MOCs and models, particularly when they are decades old. As I am.

All the best
Thorsten

 

4 minutes ago, tibbles said:

but upside, I guess I don't need to feel guilty about continuing to leave it in the sun longer

Absolutely true! Just enjoy the special weathering touch to it. It makes it unique!

 

[tibbles]

I'd enjoy the special weathering touch a bit more if it was further from being a bit uriney yellow.  Weathered white isn't so great.

I don't know the chemistry, but I'm pretty sure the sunlight must be a major catalyst though.  That or brick material change.  I've got old pieces from 35 years ago that spent their life mostly in the dark closest when not played with and pretty much all the white pieces are still white.  Maybe not bright white, but definitely not yellowed.

[RichardGoring]

If the yellowing is self-propelling, is there any explanation for the many pieces I have that are yellowed on one side, or even only half of a side, with the other half still nicely white? 

That implies to me that it's sunlight that causes it, but only to the area exposed.

But at the same time, I am not a chemist or materials scientist. Is this a case of multiple potential mechanisms that all lead to a similar outcome?

[Toastie]

11 hours ago, RichardGoring said:

Is this a case of multiple potential mechanisms that all lead to a similar outcome?

As far as I am concerned, exactly this is the case. (I am a chemist, but according to my colleagues not a real chemist - I am a physical chemist , and even worse, work at a university). The starting or activation step though needs to be bond breaking. So sunlight is certainly a very good candidate. And sometimes it is just some “residue” in the ABS formulation that may be responsible. Metal atoms are always good candidates for starting or promoting catalytic processes.  

Being self-propelled does not mean it grows all over the place. With a microscopic region, the activation begins and then will slowly lead to yellowing. Light may very well initiate this process, but as you all know, a white brick does not yellow within minutes of sunlight exposure - it is rather a process that takes months or years and is spatially limited to the “activated” area. And which is really slow. But when activated on a surface area, that area may eventually yellow - even in the dark. All speculation, however, literature suggests that this may be the case. We had some discussions here on EB with ample of literature citations and references.

As said: I like it.

All the best
Thorsten

[RichardGoring]

7 hours ago, Toastie said:

...Being self-propelled does not mean it grows all over the place. With a microscopic region, the activation begins and then will slowly lead to yellowing. Light may very well initiate this process, but as you all know, a white brick does not yellow within minutes of sunlight exposure - it is rather a process that takes months or years and is spatially limited to the “activated” area. And which is really slow...

Ah, that makes sense, yes! Thanks.

FWIW, you are clearly an excellent chemist who is good at teaching others. Pretty perfect for what you do, regardless of what those in the analytic gang say! 

[AmperZand]

On 5/26/2025 at 1:05 AM, tibbles said:

And, once discolored, is there any way to rewhiten?  I'm guessing not, but might as well ask.

You certainly can re-whiten parts as long as they don’t have stickers on them that you’re not willing to sacrifice. Here’s how to re-whiten yellowed parts: https://www.eurobricks.com/forum/forums/topic/7167-brick-de-yellowing-techniques/#findComment-1977178

 

[Toastie]

15 hours ago, RichardGoring said:

regardless of what those in the analytic gang say

Thank you for the flowers, I really appreciate that! 

Heehee - it wasn't the analytic gang - my colleague in Analytical Chemistry is equally regarded as would-be chemist - the really tough guys are (in this order ) synthetic organic chemists followed by hardcore inorganic chemists. However, we all get along quite nicely. And it's OK, as anyone knows, there is nothing physical chemists cannot accomplish . 

All the best
Thorsten