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Plastic yellowing: Light + heat?

Posted: Wed Jul 10, 2019 8:10 pm
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.

Re: Plastic yellowing: Light + heat?

Posted: Wed Jul 10, 2019 9:18 pm
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)

Re: Plastic yellowing: Light + heat?

Posted: Thu Jul 11, 2019 4:50 pm
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.

Re: Plastic yellowing: Light + heat?

Posted: Thu Jul 11, 2019 5:56 pm
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.

Re: Plastic yellowing: Light + heat?

Posted: Thu Jul 11, 2019 6:38 pm
by Atarian Computing
As with any chemical reaction, heat always plays a crucial part.

Re: Plastic yellowing: Light + heat?

Posted: Thu Jul 11, 2019 10:02 pm
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.

Re: Plastic yellowing: Light + heat?

Posted: Fri Jul 12, 2019 9:21 am
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 522 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!

Re: Plastic yellowing: Light + heat?

Posted: Fri Jul 12, 2019 9:59 am
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.

Re: Plastic yellowing: Light + heat?

Posted: Fri Jul 12, 2019 10:55 am
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.

Re: Plastic yellowing: Light + heat?

Posted: Fri Jul 12, 2019 11:42 am
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.