Plastic yellowing: Light + heat?

Problems with your machine in general.
User avatar
stephen_usher
Posts: 5580
Joined: Mon Nov 13, 2017 7:19 pm
Location: Oxford, UK.
Contact:

Plastic yellowing: Light + heat?

Post by stephen_usher »

I've just brought my TT back down from the loft and it's very much more yellow than it was when it went up there a few weeks ago. The previously retrobrightened parts have almost returned to their previous colour.

I've seen reports that heat accelerates or "develops" the yellowing of plastic and I think that this proves it as we've had some very high temperatures recently, which would have been even higher in my loft.

The plastic the TT's made of seems particularly light sensitive now, though it wasn't when it was new. It spent its first few years in a brightly light room and didn't yellow at all, it was only during the storage, after 1995, that the colour turned, and that was only in the areas previously exposed to sunlight. Now the plastic is quickly yellowing even in the other areas, suggesting the plastic is now far more photo-sensitive.

It's quite possible that to make the TT look the correct colour I may have to spray paint it with a satin-white paint.

P.S. Some of the areas which have re-yellowed have not had much light exposure since the retro-brightening, suggesting that this is old photo-effects triggering the yellowing. Also, when the machine was in the loft over winter there was no yellowing, suggesting that temperature is a major factor.
P.P.S. The keyboard, which had been retro-brightened again just before going into storage has also gone yellow again.
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.
User avatar
exxos
Site Admin
Site Admin
Posts: 23497
Joined: Wed Aug 16, 2017 11:19 pm
Location: UK
Contact:

Re: Plastic yellowing: Light + heat?

Post by exxos »

In my retrobright thread I half concluded that heat is the issue in yellowing. My Falcon yellowed and its been covered up for years. So only heat can really be a factor.

I do have on test a MEGA ST currently in sunlight, it does of course get warm, but I think the UV lightens up the plastic (albeit very slowly)
https://www.exxosforum.co.uk/atari/ All my hardware guides - mods - games - STOS
https://www.exxosforum.co.uk/atari/store2/ - All my hardware mods for sale - Please help support by making a purchase.
viewtopic.php?f=17&t=1585 Have you done the Mandatory Fixes ?
Just because a lot of people agree on something, doesn't make it a fact. ~exxos ~
People should find solutions to problems, not find problems with solutions.
mattsoft
Posts: 4
Joined: Thu Jul 11, 2019 6:48 am

Re: Plastic yellowing: Light + heat?

Post by mattsoft »

Not entirely ST-related, but definitely yellowing related: I have a PC PS/2 keyboard (from an old Sony VAIO) that was yellow on the outside AND on the inside! Since the inside is mostly protected from light, clearly another force was at play -- obviously time and plastic materials, but probably heat as well. The yellow was easily removed with a gentle peroxide bath in sunlight.
User avatar
stephen_usher
Posts: 5580
Joined: Mon Nov 13, 2017 7:19 pm
Location: Oxford, UK.
Contact:

Re: Plastic yellowing: Light + heat?

Post by stephen_usher »

I've found that you need to be careful retro-brightening the TT case as after a while in the sunlight the brightening stops and it starts to yellow again.
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.
Atarian Computing
Posts: 444
Joined: Tue Aug 22, 2017 4:27 am

Re: Plastic yellowing: Light + heat?

Post by Atarian Computing »

As with any chemical reaction, heat always plays a crucial part.
User avatar
stephen_usher
Posts: 5580
Joined: Mon Nov 13, 2017 7:19 pm
Location: Oxford, UK.
Contact:

Re: Plastic yellowing: Light + heat?

Post by stephen_usher »

At least with the plastic formula Atari used there seems to be a particular temperature above which the reaction progresses far faster, however.

Also, it's highly photo-sensitive. Other plastic formulations bleach in the sun, not Atari's.
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.
User avatar
rubber_jonnie
Site Admin
Site Admin
Posts: 10472
Joined: Thu Aug 17, 2017 7:40 pm
Location: Essex
Contact:

Re: Plastic yellowing: Light + heat?

Post by rubber_jonnie »

I think it's a combination of many factors, but I'm yet to be convinced that it is heat alone.

Here's a photo of my 65XE case stood up against my STs:

IMG_20170731_190054.jpg
IMG_20170731_190054.jpg (125.89 KiB) Viewed 4319 times

The 65XE has not had any chemical treatments to bring the colour back to original, it's simply been washed with soapy water. I've owned this 65XE from new and I can safely say this is the colour it came out of the box. The XC12 that came with it is the same colour too.

I've never made any special effort to keep it cool or out of the light. It's always lived inside though, so little exposure to UV.

The STs on the other hand are all of various ages, and are all different colours. I can't say how warm they got, or how much light they are exposed to, since I've only had them the last 4 or 5 years.

I think we have to consider that the plastic ingredients play a big part in this.

Also consider my SNES. That went brown IN THE BOX, so no exposure to light. My unboxed Dreamcast that I've also had since new, has a browning case. Inside the cd tray it's original colour though. The outside case has browned in areas exposed to light, but the modem module, exposed to all the same conditions as the rest of the case, is still the original colour.

BUT the weird thing is, if heat were a factor, why does the inside of the case stay the original colour?

My STs and Dreamcast (I haven't yet opened up the SNES, but I expect it to be the same) are all original colour on the inside. The inside will have been warmer then the outside, so why does it not change colour?

So I guess my theory is this:

A plastic moulding that contains the ingredients that are susceptible to browning, is exposed to the small amounts of UV and IR light that are present around us. Being covered up long term does not matter, as at some point they have been exposed to light.

The addition of heat (Central heating, warmth from the sun, warmth from the electronics) and a surface based chemical reaction takes place which causes the browning to occur (As I understand it, bromine used as a fire retardant begins to leech out).

This explains why the insides of cases are not affected and remain the original colour, because despite exposure to heat, they are not generally exposed to light (UV or IR).

If the plastic does not contain the ingredients that will cause browning (I don't believe my 65XE does, nor does the Dreamcast modem module), then you can have as much heat or light exposure as you like, browning will never occur.

My two pennorth!
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
User avatar
stephen_usher
Posts: 5580
Joined: Mon Nov 13, 2017 7:19 pm
Location: Oxford, UK.
Contact:

Re: Plastic yellowing: Light + heat?

Post by stephen_usher »

You're probably correct that it's any UV exposure and it depends very much on the chemical composition of the additives to the plastic.

"Leech out", however, isn't the correct idea though. Basically the bromine is part of a molecule (colourless) which gets broken down releasing molecular bromine (brown). The UV probably knocks an electron off something which then makes it reactive. Then over time and accelerated by heat, possibly requiring a minimum temperature, then breaks the bonds in the fire retardant. It could well be the plasticiser chemical which is being broken and that goes on to react with the flame retardant.

So, in conclusion, it's probably the combination of colourant, plasticiser and flame retardant which determine the plastic's final photo and temperature sensitivity.
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.
User avatar
rubber_jonnie
Site Admin
Site Admin
Posts: 10472
Joined: Thu Aug 17, 2017 7:40 pm
Location: Essex
Contact:

Re: Plastic yellowing: Light + heat?

Post by rubber_jonnie »

stephen_usher wrote: Fri Jul 12, 2019 9:59 am You're probably correct that it's any UV exposure and it depends very much on the chemical composition of the additives to the plastic.

"Leech out", however, isn't the correct idea though. Basically the bromine is part of a molecule (colourless) which gets broken down releasing molecular bromine (brown). The UV probably knocks an electron off something which then makes it reactive. Then over time and accelerated by heat, possibly requiring a minimum temperature, then breaks the bonds in the fire retardant. It could well be the plasticiser chemical which is being broken and that goes on to react with the flame retardant.

So, in conclusion, it's probably the combination of colourant, plasticiser and flame retardant which determine the plastic's final photo and temperature sensitivity.
Yes, 'leech out' isn't the correct term, but it was the best way I could think of describing it.

I expect the combination of colourant, plasticiser and flame retardant also affects the rate and depth of browning too.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
User avatar
stephen_usher
Posts: 5580
Joined: Mon Nov 13, 2017 7:19 pm
Location: Oxford, UK.
Contact:

Re: Plastic yellowing: Light + heat?

Post by stephen_usher »

rubber_jonnie wrote: Fri Jul 12, 2019 10:55 am I expect the combination of colourant, plasticiser and flame retardant also affects the rate and depth of browning too.
Indeed. The more opaque the colour the less penetration.
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.
Post Reply

Return to “HARDWARE ISSUES”