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Offline pcm81

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Question about yellow plastic chemistry
« on: May 06, 2021, 12:09:53 PM »
Online there are multiple how-too's for whitening yellow plastic. The process even got a name "retrobrite" because many dorks like us use these methods to restore old game console and computer cases.

What i am trying to determine in this question is the underlying chemical process that all these methods contribute to.

The methods used are:
1. Hydrogen peroxide + UV light
2. Hydrogen peroxide + elevated temperature
3. Ozone from ozone generator + UV
4. Ozone from ozone generator + elevated temperature.

Elevated temperature (above 60*C) induced bleaching effect of peroxide, is a well known phenomenon. So it makes sense that peroxide+heat works better than just peroxide.
So my first thought would be bleaching effect of peroxide, but then the need for UV light is irrelevant. https://en.wikipedia.org/wiki/Peroxide-based_bleach
well not completely irrelevant, but somewhat. O=O or O-H bond is 5.15eV (https://en.wikipedia.org/wiki/Bond-dissociation_energy) which is equivalent to UV at 238nm.
Ozone has dissociation energy of 3.76eV per bond, so that is 329nm.
My point being that the basic UV lamps that most people buy are 390 or 365 nm wavelength... sooo only a small fraction of accidental high energy UV rays from those lamps actually does any work?


Or is there more to the chemistry of brightening the plastics than just formation of reactive oxygen?
The elevated temperature with peroxide definitely works, but all i can see from elevated temperature is increase of reaction rate, i can't see any mechanism, such as UV that would form reactive oxygen...

What am I missing?
Is there something else going on?

Thanks
« Last Edit: May 06, 2021, 12:38:51 PM by pcm81 »

Offline Corribus

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Re: Question about yellow plastic chemistry
« Reply #1 on: May 06, 2021, 02:33:20 PM »
High energy (UV) radiation can destroy chromophores (colored compounds) through photochemistry.

https://en.wikipedia.org/wiki/Photobleaching
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline pcm81

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Re: Question about yellow plastic chemistry
« Reply #2 on: May 06, 2021, 02:49:16 PM »
High energy (UV) radiation can destroy chromophores (colored compounds) through photochemistry.

https://en.wikipedia.org/wiki/Photobleaching

I agree; but it seems to me the wavelength required is shorter than the UV lamps that people use. So is it just a small fraction of "unusually short wavelength" photons that get "accidentally" generated in 365 and 390 nm UV light that do this work?  I thought one needs UV-C light to do what you are suggesting, not UV-A or UV-B that are readily available to general public.

Offline Corribus

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Re: Question about yellow plastic chemistry
« Reply #3 on: May 06, 2021, 10:13:50 PM »
Sorry I read that previous post very quickly. Hydrogen peroxide will photodissociate in UV light, although higher energy radiation will be more efficient, so the wavelength of your UV lamp will matter. Probably sitting your contraption up in the sun will be best because there are all kinds of wavelengths... plus you don't have to buy a UV lamp.

https://www.h2o2.com/technical-library/physical-chemical-properties/radiation-properties/default.aspx?pid=65&name=Ultraviolet-Absorption-Spectrum

http://www.trusselltech.com/technologies/advanced-oxidation/factors-that-affect-either-uv-h2o2-aop-alone-or-ozone-aops-alone
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline pcm81

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Re: Question about yellow plastic chemistry
« Reply #4 on: May 07, 2021, 06:25:55 PM »
Sorry I read that previous post very quickly. Hydrogen peroxide will photodissociate in UV light, although higher energy radiation will be more efficient, so the wavelength of your UV lamp will matter. Probably sitting your contraption up in the sun will be best because there are all kinds of wavelengths... plus you don't have to buy a UV lamp.

https://www.h2o2.com/technical-library/physical-chemical-properties/radiation-properties/default.aspx?pid=65&name=Ultraviolet-Absorption-Spectrum

http://www.trusselltech.com/technologies/advanced-oxidation/factors-that-affect-either-uv-h2o2-aop-alone-or-ozone-aops-alone

Which kind of brings us back to my original question about destruction of chromophores.
What is the final key reaction that is doing most of the work? Is it O reacting with chromophores and oxidizing them? I am basically trying to come-up with most efficient method.
1. So far it seems hot peroxide is easiest and is not effected by line of site.
2. Arguably, if UV is just disassociating H2O2 then it may not need to shine on the surface, so line of site for complex geometry is not as much of an issue an hot peroxide can be replaced with cool peroxide +UV and agitation.
3. If O3 that disassociates into O and O2 is good enough; then a ozone generator connected to a bag is good enough, without heat or UV and should work for complex geometries too.

My peroxide source is Sodium percarbonate. I wonder if playing with pH rather than temperature of the solution would have bigger effect.
I have a 50 watt 365nm UV lamp and am trying to decide if it is good enough or if i need to go UV-C if that is better than raised temp peroxide...
Another advantage of UV or Ozone generator is that you do not need to store a solution, you can just turn it on once per year. To be honest sodium percarbonate is pretty cheap to just mix up low concentration hydroxide solution and i have even cleaned a pool filter in bath tub with warm peroxide before... but I'd be hesitant to leave a temperature controlled heater running with low concentration sodium percarbonate solution say overnight; which is how long it might take if concentration is low.  just trying to come-up with best method i can use "on the fly" when i need it later.

Offline pcm81

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Re: Question about yellow plastic chemistry
« Reply #5 on: May 07, 2021, 10:58:32 PM »
Update:
Found this page: https://medium.com/@pueojit/a-look-into-the-yellowing-and-deyellowing-of-abs-plastics-db14b646e0ad
The summary is that UV light is just an energy source, just like heating. OH does not need ionizing effect of UV light, just the energy for reaction to take place.

EDIT:
Just got some TAED on Amazon. Will see if room temp solution of sodium percarbonate and TAED will work as well... in theory it should. Any suggestions for the ratio of Sodium percarb to TEAD?
« Last Edit: May 07, 2021, 11:51:45 PM by pcm81 »

Offline Borek

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Re: Question about yellow plastic chemistry
« Reply #6 on: May 08, 2021, 07:48:11 AM »
This can be a bit tricky, as often a lot depends on some unpredictable factors - so while I do like the systematic approach and I do agree trying to understand what is really the mechanism of the retrobrite can help in optimizing the process, in the end it can turn out practice/experiment are the only way of finding the best procedure, no matter what the theory says.

Any suggestions for the ratio of Sodium percarb to TEAD?

Stoichiometry would be a reasonable starting point, wouldn't it?
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Offline pcm81

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Re: Question about yellow plastic chemistry
« Reply #7 on: May 11, 2021, 08:23:09 PM »
This can be a bit tricky, as often a lot depends on some unpredictable factors - so while I do like the systematic approach and I do agree trying to understand what is really the mechanism of the retrobrite can help in optimizing the process, in the end it can turn out practice/experiment are the only way of finding the best procedure, no matter what the theory says.

Any suggestions for the ratio of Sodium percarb to TEAD?

Stoichiometry would be a reasonable starting point, wouldn't it?

Sorry for taking a while to reply, work and RL got in the way...
Anyhow, I thought about doing stoich between sodium percarb and TAED but that would basically create a bunch or peroxyacid most of which would go to waste. Or can it be stored as bleaching solution long term? Would stochiometry vs amount of yellow plastic be a better choice for amount of TAED? Since we would be dealing with peroxyacid after addition of TAED, does pH have a bigger effect than concentration of TAED and therefore peoxyacid on speed of the reaction?
This quickly evolves intro chemistry considerations well beyond my very limited knowledge to know which of these factors can be ignored...

I just got 1lb of TEAD and it ran me $35 vs about $2 i pay for sodium percarb... just trying to avoid wasting TEAD.

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