1962 Cadillac (1 Viewer)

[bobm]

More reading if anyone is interested in the process.​

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Retrobright: Complicated Chemistry - An Educated Opinion​

It's a lazy Sunday here and I decided to take on the task of reading up on Retrobright chemistry and figure out if it's really worth it. I have a raised eyebrow on the peroxide formulas for the Retrobright process. Since my PhD thesis involves radical chemistry, and I have some experience specifically in peroxide industrial use and production in UV-Protection coatings, I figured I can give an educated opinion on the process.
First, I want to link this PDF to a Hack-a-day post on the process. The author claims to have had a conversation with James Pickett, a PhD in Polymer Chemistry on the finer details of plastic yellowing & Retrobright. The chemistry is correct throughout, albeit surface level. I won't rehash any statements made in the PDF and encourage readers to glance over the linked file.
Now, I will have some definitions to educate readers on some chemical terms that will be used in this post. A Radical is a molecule with a single, unpaired electron. In the attached image is an *OH radical, the main species in hydrogen peroxide (HOOH) that is involved in chemistry. The yellow mesh is the simulated volume that the unpaired electron occupies around the Oxygen atom on a ball-and-stick model. When UV light hits a molecule of peroxide, it Initiates homolysis; 'cutting' a peroxide in half to form two *OH radicals. This chemistry is not unique to peroxides, and can be found in other X-Y single bonds. An Anion is a a molecule with a pair of electrons and has a formal negative charge. In the attached image is -OH (hydroxide) anion. The electrons occupy the same volume as the radical *OH, but this time has a phasing (arbitrarily colored Red and Blue in the image.) From a 1000 ft view, while the atoms in both *OH and -OH are the same, chemically, they are very different and *OH is far more reactive.
The main form of retrobright is an oxidation bleaching. This is the use of some potent oxidizer to break down triple and double bonds into single bonds while adding some functional group. The above linked PDF claims that the conjugated double bonds (responsible for the yellow color of degraded plastic) can be revered by adding OH groups (and halogens such as Chlorine in the PDF's example from Master Organic Chemistry). This is not the only way to oxidativly bleach, but what we're interested in.
The paper on the production of using tetraacetylethylenediamine (TAED) and HOOH to produce a bleaching agent peracetic acid (PAA) is found here and here. They cite an earlier paper from 1974 in use of a paired electron process (read Anionic) to form -OH and -OOH from hydrogen peroxide and peracetic acid (although A. Laurent & N. Pellissier research Cope and Claisen rearrangements & pyrroles - not plastics). On first glance, D. Davis & M. Deary perform a kinetics experiment on the formation of peracetic acid and its reactants; taking frequent aliquots. They report that the maximum transformation of TAED to PAA happens at about 1 hour, and then the concentration becomes linear afterwards. The takeaway being that the additive TAED does not take full effect until 1 hour after mixing, but doesn't get any stronger the longer it sits. The reaction also forms anions and not radicals (remember; very chemically different), but the paper was published in 1991. Radical chemistry is far more understood now three decades later.
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To get into the meat of the chemistry, the retrobright procedure involves these ingredients per the Wikipedia page:

• Hydrogen Peroxide Solution (30% is consumer-grade concentration)
• TAED (1mL per 3L of solution)
• UV light (380 nm is your textbook sweet-spot wavelength)
• Xanthan Gum/ Arrowroot for a thickening paste (this is just cellulose basically, save money by buying corn starch)

UV light is the propagater in the formation of radical species. It is what will break down hydrogen peroxide and PAA to its respective radical species. These radical species will then break down the conjugated plastics; bleaching it. TAED, from the 1991 paper, forms PAA in a paired electron process, forming anionic species. This means that by including TAED, you would have now two competing chemical processes: UV-initiated radical chemistry and peroxide initiated, paired electron chemistry.
If we're comparing the production of an acetic radical from PAA, the enthalpy of homolysis is ~45 kcal/mol. The enthalpy of homolysis of the *OH radical from hydrogen peroxide is ~56 kcal/mol. (enthalpy of homolysis is calculated using Hess's Law.) I propose that the PAA would be forming radical species in concurrence with the paired electron species (argumenative to the 1991 paper) and that the production is 10 kcal/mol more favorable than hydrogen peroxide homolysis. Having said that, the amount of TAED added per volume of solution is minuscule ( 0.03% v/v). When it comes to industrial scale peroxide addition to catalyze reactions, you generally add ~1-2% mol/mol. This means that if you followed the above recipe, even though that the TAED would be a favorable addition, it's literally a drop in the bucket on any meaningful chemistry. Increasing the amount of TAED would be favorable, and decrease the time you would need to have a part sit in the irradiation bucket. Alternatively, if we're only concerned about making radical species and breaking down double bonds as cheaply as possible, then forgo the addition of TAED and let the plastic soak longer. A negative side effect is that radicals are not selective to react to only double bonds. They will react with anything piratically. By increasing TAED in a meaningful amount of mass, you could end up with a case where the radicals produced work too good and end up degrading the plastic further. Someone needs to perform experiments on this and see if this is a possibility.
The addition of a coagulant to make a paste is okay in theory, but from an ease of process standpoint, I'd just use the peroxide bucket and UV light setup.
It should also be stated that this process is not permanent, and will eventually return to yellow over the years as the plastic is exposed to oxygen. You'd reform conjugated plastics or entirely break down the polymer bonds, making the plastic brittle and chip.
To circumvent that, I propose one should add a UV coating on top of a brighted shell. I'd recommend benzophenones as the main ingredient we should be looking for. Benzophenone-6 ( 2,2'-Dihydroxy-4,4'-dimethoxybenzophenone) has a max UV-absorbance at 340 nm, our magic radical initiating wavelength, with high absorbance as well in 280 nm. It's also safe to the touch. Unfortunately, in concentrated layers it will give a slight yellow tinge. This should prevent further UV damage in the plastic.
Other notes:
Sodium Percarbonate is just an anhydrous form of Hydrogen Peroxide. You might be able to make a more concentrated solution of hydrogen peroxide using sodium percarbonate, but I don't know why you'd want to.
Potassium Permaganate (KMnO4) can also be used as a bleaching agent, but might be more expensive than 30% hydrogen peroxide based on your location.
All calculations were performed on Spartan '20 using the w-B97X-D/6-31G* for geometry and energy calculations. (If any other Quantum Chemist reads this, I was too lazy to increase my basis set for a post on r/Gameboy)
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tl;dr: Retrobright doesn't fix plastic, only oxidizes the outside layer. Adding TAED doesn't hurt, but the current recipe is functionally probably insignificant. A benzophenone UV coating might be our best bet for further UV protection.
EDIT: Minor proofreading


Link:

EXACTLY What I was thinking

 

[WarDamnEagle]

Trivia (delete this post from your thread is desired):

It's really easy to precipitate nice orthorhombic Potassium Permanganate crystals out of a saturated solution; a good school exercise for almost any grade as an introduction to crystallography/mineralogy.

Another interesting property of it as a strong oxidizing agent, is that it will spontaneously ignite when combined with almost any glycerol compound, including car anti-freeze and glycerin used in soap and skin-care products. Growing up, a friend and I used to make our own flare bombs that way and terrorize passing motorists by mixing the components in a small container in the middle of the road as a car was approaching, and using the several seconds of delay before combustion to hide nearby and observe. Fortunately, we never caught anyone's car on fire.

Nice. All I ever did was make my own gunpowder as a young teenager. I did do a lot of reading on Gun Cotton (nitrocellulose) at the time and toyed with making it. Thankfully I didn't pursue that effort very far.

 

[J Mack]

Ok @bobm I have moved on to test #2. I turned up the volume from 40 to 50 and it seems to be working better.

Got some stuff to bump the volume up to 100 tomorrow so we'll see what happens in test #3

 

[bobm]

If anyone figures it out ,it will be you

 

[J Mack]

If anyone figures it out ,it will be you

Thanks buddy, I shifted gears today because I'm back working on that fuel injection on my wife's project so I'm cleaning seats while I wait on other stuffs.

 

[bobm]

Leather came out awesome, looks brand new . Color goes good with the Blue to .

 

[J Mack]

Leather came out awesome, looks brand new . Color goes good with the Blue to .

Thanks Bob, getting closer on the back window too. This is were we are now.

 

[Ol Yeller]

Thanks Bob, getting closer on the back window too. This is were we are now.
View attachment 3581267

That’s an impressive improvement

 

[bobm]

Imagine how many would just toss that rear window without putting in some elbow grease . Looking good Buddy!

 

[J Mack]

Getting serious now..

 

[bobm]

Yup, that's Some serious . Hey i can 👁 garage door all the way through both windshields persistance .

 

[bobm]

?? Is what's going in thar

 

[J Mack]

?? Is what's going in thar

When I bought this car I bought it knowing the engine had issues, I was prepared for a major overhaul if needed but when I was dissembling it yesterday I found a bent pushrod that I believe was the ticking “rod knock” he heard. I will hopefully get it torn down the rest of the way in the next day or two and have a solid plan but for now everything looks great in this engine.

 

[J Mack]

Pistons all measure in spec.

All the bearing look new.

Crank is perfect..

All the cylinders measure within the range.

Other than sitting for 52 years and the oil getting kind of like a coat of varnish this engine looks like it just need to be hot tanked and some seals and gaskets.
Going to take the heads to get looked at tomorrow or the next day when I drop the block off to get cleaned.

 

[Michael B]

Looks like it might have had Quaker State motor oil run in it for awhile.

 

[J Mack]

Looks like it might have had Quaker State motor oil run in it for awhile.

I know he did it a lot, the door looks like it's had a few stickers stuck in there over the years plus I have quite a few I pulled out of the glove box.

 

[Michael B]

True story, my dad swore by QS for most of his adult life. He never listened to anyone trying to make him wiser. He was a pretty good shade tree mechanic. He kept all of our vehicles running and dabbled in hobby class drag racing. When I was 15, he started helping fellow shade tree with his automotive side gig. This guy had warned my dad repeatedly about QS. About the 4th engine they tore down for a rebuild, that had exclusively run QS, and you couldn’t even see the rocker arms on for the paraffin build up. He took the hint and stopped using QS.

 

[bobm]

Awesome! Good spot to be in. Is that the Flathead in background ?? Thought you sold that?? Me confused!!!!

 

[J Mack]

I sold the green car with the Hemi... The little A with the flathead is my daily...

 

[bobm]

Can't beat the sound of a Flathead . Was a place down south Carolina's I that put them in motorcycles . Had 1 in my 47 Firetruck.