How does hydrogen peroxide whiten plastic?

UV radiation causes stabilizing agents in the plastic to form so called free radicals which cause the yellowing. This UV light is present in sunlight but also in fluorescent light. Hydrogen peroxide binds to these free radicals, thus reversing the yellowing of the plastic.

matilo.eu - Bleaching yellowed plastic - Matilo Telephones

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It looks tobacco stained, but unfortunately it is not. Yellowed plastic items are usually not a victim of tobacco smoke. Direct sunlight and more specifically certain frequencies of ultra violet light are the real culprits. This yellowing often diminishes the value of the item and it does not look particularly nice. But we’re in luck. This process can be reversed, in many cases. There are some specialised products available that can be used to bleach plastics, but the same result can be easily achieved with good weather and some house hold products. So here are the results of my first experiments with bleaching plastic. Please keep in mind that I am most certainly not the first one to do this or write about it. I read several articles on line, before I started my experiment. My main inspiration was Sam Hallas’ article:

http://www.samhallas.co.uk/collection/retr0bright.htm

Only yellowed thermoplastics

The process described in this article works only on yellowed plastics. It does not work on faded plastics. In the first instance UV light has broken down some of the components in the plastic and the rest products turn the plastic yellow. Fading occurs when light breaks down the colouring agents in the plastic, which makes the colour lighter. That is not reversible with this process. Secondly, the process works only on most thermoplastics. So it does not work on bakelite and urea-formaldehyde. I do not know if it works on tenite, melamine or diakon.

How it works

Basically the process removes the yellowing effect of the UV damage. UV radiation causes stabilizing agents in the plastic to form so called free radicals which cause the yellowing. This UV light is present in sunlight but also in fluorescent light. Hydrogen peroxide binds to these free radicals, thus reversing the yellowing of the plastic. It does not recolour or enhance the colouring agents present in the plastic. So fading is not reversed in this manner. For this process several ingredients are needed. Of course a hydrogen peroxide containing product is needed, sunlight and in particular UV-radiation (how ironic) and heat.

I used the following items:

An oxy action product from Lidl, containing hydrogen peroxide

A transparent storage box with lid, to keep in the heat and to act as a green house.

A sunny day

Experiment # 1, a yellowed body shell, partially submerged

I used yellowed body shell and only submerged it partially, to see if there was a difference in the result of the submerged and unsubmerged part. I used an 18 ltr box and filled it with hot water and 3 scoops of the oxy action product. I left it out in the sun from about 12 o’clock until sundown at about 20.30 hrs. Please note that the box was no longer in direct sunlight from about 18.30 hrs.

First result

I must say I was very pleasantly surprised by the result. The plastic had turned to its original colour after only one treatment. The difference between the submerged and unsubmerged part was clear. I had written off this body shell as unusable, although it was undamaged. Now it had suddenly turned in a useable part. I was so impressed by it that I gave it a second bath in experiment no 2, to do the upper part so the whole body shell would have undergone treatment. This resulted in an even colour, very close to the original. So far, so good.

Experiment # 2, badly yellowed parts with a sticker shadow

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For this experiment I selected a badly yellowed phone. I had put this aside for exactly this purpose. It was originally grey, but had now turned to a tobacco beige. It was discoloured far worse than the body shell from experiment 1. On top of the discolouring, there was a nasty sticker shadow. Normally this is very hard to reverse or otherwise make invisible. This would also serve as a sort of benchmark/indicator of how well the process worked.

Day 1, fresh batch of mixture

After one day in this case the yellowing was very much reduced, but not totally reversed. There was still an obvious difference between the inside and outside of the phone. Also the sticker shadow was still very visible. The outside of the body shell was still slightly yellow.

So I decided to give it a second bath.

The body shell from the first experiment was now an even colour, with no colour difference between the inside and outside. The de-yellowing has turned this part to almost new! Aside from the surface needing polishing.

Day 2, another fresh mixture

Again improvement, but not as much as from the first day. So I decided to leave it for another day.

Day 3, same mixture

No difference. Apparently the mixture has lost its potency.

Day 4, again a fresh mixture

It was hard to tell, but I think there was a slight improvement. But less apparent as from the first and second mixture.

The sticker shadow was still visible, albeit very very slightly.

After so long in this this mixture the surface of the phone has turned somewhat rough to the touch. Also I had left 2 screws for the handset in place, with had started to corrode. I decided to give it one more go, to try to remove the sticker shadow completely.

Day 5

Last try, although inside and outside are almost the same colour. But this time I could see no difference with the results from the day before. The sticker shadow is still very very slightly visible. Please note that this is not due to fading of the rest of the phone. The shadow is lighter in colour than the rest of the body shell. The surface texture of the phone has gotten a bit rough, though. So it need a very thorough polish. Metal bits, like the screws I left in place and metal parts embedded in the plastic, are somewhat corroded.

Rough surface: wash before polishing!

After being treated for 5 days the plastics of experiment # 2 had gone very rough. I thought it was the plastic itself being affected by the bleaching process. When I started polishing, it turned out that it was not the plastic, but rather a residue of the cleaning compound I used. And when I polished it with a polishing agent, I polished it into the plastic rather than polishing it off. This made the residue almost impossible to remove. I was not able to polish or rub it off. I think it needs to be sanded down, but as this is an experiment I did not resort to that. I did give the parts a wash, to get the crusty residue off before polishing other parts. This worked, but it was not easy to wash it off. I was not able to wash the residue off off the part where I polished it in.

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Polishing the clean parts was a lot easier and worked well. It is, however, recommended to wash the parts between bleaching batches. This will prevent the build-up of a crust and makes polishing afterwards a lot easier.

Miracle cure?

Do we have a magic wand here, reversing this ugly yellowing? Well, I must say that I find the result fantastic. It is very easy to do and the appearance of the plastic items improves dramatically. It certainly makes plastic items a lot more valuable. It enables me to save many parts I would otherwise discard.

Disadvantages

There are some drawbacks and limitations to this process, however. First of all it does not undo all discolouration on all plastics. It only works on plastics with certain stabilizers and flame retardants, which yellow because of the forming of free radicals. And the surface of the plastic becomes rough. This requires good cleaning during and after the process and good polishing afterwards.

Also the process corrodes metal parts.

Reading up on the chemistry behind it all I read several reports of concern that the plastic can turn brittle. Until now this is not something I experienced and this may have something to do with the composition of the plastic. And lastly the de-yellowing may not last. In some case the yellowing returns after a while, even though the bleached item is stored in the dark.

Conclusion

All in all a great process: easy to do, with easily available products and items. I had good results with it so far. I am really impressed with it. However, there are some issues with this process as mentioned above, most importantly that the yellowing may return after a while. I do have several telephones that could really use this treatment. So it will certainly not be the last time I bleach phones with this method. The next challenge will be doing an Ericofon. It has a built in receiver that cannot be removed, so I cannot fully immerse it in water. As I gain more experience I will learn more about the pro’s, cons, what to do and what not to do but from these first experiments I am really enthusiastic!

Other experiences with bleaching techniques? Please share them!

I have already lined up a nice project which, among other things, requires bleaching and I would also like to bleach a couple of Ericofons that I have. Each one of these warrants an article or blog. So more about this subject later. I will post a link under this article, as soon as I publish one. Do you have experience with bleaching plastics? Please share them below, so we can all learn from them.

matilo.eu - Bleaching yellowed plastic - Matilo Telephones

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