Whitening yellowed Leica stereos (and others probably)

[Scarodactyl]

Anyone who's looked at used Leica stereos knows that the plastic they made the shells out of likes to yellow. Their initial color is much like the previous generation of metal-shelled, enamel-painted Wild stereos, essentially white, as seen in this Leica stock photo of an MZ6:

But after a while if they're exposed to UV (+chemicals, fumes, skin oils???) they begin to yellow. First a little (thanks to Microscope Central for the next two images)

But then a lot

And after a while it can get out of control (image from eBay seller hagemmi):

This does not affect function per se, but boy does it affect aesthetics! It's especially annoying because a) some parts like ocular tubes were still made with enamel-painted metal which does not yellow, and b) if you mix and match from different scopes you get obvious mismatches:

This is not a problem unique to Leica stereos, or microscopes in general (though I suspect their formulation is particularly bad for it). Plastics over time yellow fairly inevitably from partial breakdown of the polymers that make them up. This pops up in retro computer and gaming hardware as well, and people in those communities found an effective solution, known as "retr0brighting". Basically you either immerse the yellowed plastic in a solution of hydrogen peroxide or apply a hydrogen peroxide-based gel to the surface (as are used for bleaching hair) and expose them to UV, either from LEDs or sunlight. This can reverse the yellowing and partially or entirely restore the original color.
I've done this a couple times now on Leica stereos and it has worked great. Here's an example with my latest Leica acquisition, my MS5. This one was not badly yellowed but it was particularly noticeable with my dead white 3d printed replacement knobs.

There are more involved approaches using either higher concentration H2O2, the aforementioned gels and also lining the tub with foil and using LED UVs. I've typically found using simple off-the-shelf 3% H2O2 and a few hours of direct summer sunlight to be sufficient.

Plastic shell shucked, cleaned and immersed in H2O2 with weighs to hold it down.

The weights are particularly important since the plastic will generate bubbles, so even if it sinks initially they will get trapped in the hollow underside of the shell and float it over the surface.
Final result:

There's still a hint of color but it's very subtle. It was not super yellowed in the first place but now it's very close the original shade. This is typically where the color has ended up on the Leica plastic I've tested, regardless of how yellow they started.

Any drawbacks? Not that I've seen. There have been concerns that this might weaken the plastic but I haven't observed any tendency to crack etc in the couple pieces I've treated. Maybe with higher concentration H2O2, maybe with more time or UV? Leica shells can get brittle over time on their own anyway. Obviously, I make no guarantees on results and you should only try this at your own risk.

[PeteM]

Interesting approach - and new to me. Thanks.

I've spray painted them in the past. The housings come free pretty quickly -- the time is likely about the same as applying peroxide. That said, the bleaching approach keeps the original markings.

Plastic does get brittle from UV exposure. I'd assume the bleaching doesn't reverse that - and might even accelerate it a bit if the plastic housings are exposed again to the sun?

Here in sunny California, we have to paint most every exposed PVC or ABS pipe for UV protection or watch it get brittle in a decade or two.

[jfiresto]

... Any drawbacks? Not that I've seen. There have been concerns that this might weaken the plastic but I haven't observed any tendency to crack etc in the couple pieces I've treated....

The housing could be made of polycarbonate (PC) or ABS, so I would watch for environmental stress cracking where the plastic is under stress, or for an early production microscope, where the plastic might be relieving "baked in" stress. To get the crazing requires both stress and an attacking chemical agent. PC is resistant to hydrogen peroxide but what else is in a gel?

[Scarodactyl]

I have tested this with cracked plastic (broken knobs) but not crazed.
As for the gels I am not sure what else is in them but it's probably pretty inert. Xantham gum is recommended for home formulations but I haven't tried it.
The Leica plastic is some proprietary blend that is ESD safe--that was their big pitch for the switchover from metal cases. But of course the difference between proprietary and off the shelf is sometimes marketing.

[jfiresto]

A quick duckduckgo search found a PC-incompatible ingredient in hair bleaching products: potassium persulfate. That could just be whitening: two more charts I looked at show PC resists it.

[Scarodactyl]

I have only used the hair gel stuff once (on a focus knob permanently attached to its metal pinion) but it worked the same and didn't cause any issues. I haven't uaed it extensively though, one would need to have a look at other retro hardware communities.

[PeteM]

My guess is that the switch from diecast aluminum to plastic injection molding was mostly a cost-reduction, with a marketing cover story. I've not seen a damaged aluminum housing, but multiple broken plastic ones. And if someone in industry really wanted to control static discharge, simply connecting the scope to ground (or ground through a known resistance for dissipation) should more surely do the trick.

Just looking at the damaged cases I've seen, I'd guess that carbon rather than carbon fiber was added to make a conductive plastic.

In any case (pun almost intended), scopes like the Leica MZ6 remain outstanding.

[Phill Brown]

If not already mentioned,apparently it is the flame retarding ingredient that causes the yellowing.
Thin coat of gel is fine in a ziplock bag to stop it drying out.
Not had any issues with the method so far.
All day in hot sunlight might not be such a good idea.
In the UK the sun isn't often that strong but warping could happen from uneven excessive heating.

[Scarodactyl]

Immersed in liquid excessive heat seems unlikely but who knows. Maybe in Phoenix but not where I am.
The next logical step should probably be designs to print replacement shells, though I don't have one on hand to model one off of.

[Phill Brown]

In a ziplock bag in direct sunlight is still a plastic greenhouse.
Most likely it will be just fine and all the yellow/brown gunk will rinse off leaving it looking freshened up at worst,like new at best.

[Scarodactyl]

That's a fair point, and that would add an extra facror for people to be mindful of.