The chemistry of yellowing - slightly technical

[susiesadeyes]

Very use full info.! Heat I would not have thought about...I keep some dolls on display with UV plexi in front to help keep sunlight away....but a friends' doll yellowed in a box in a basement...Enjoy them Now! Thank you for the great post.

[pbrennan42]

If you have a nice cold basement room out of direct sunlight that you can remodel, this would be the best place to set up a display room for your dolls.

Phil.

[gelfling21]

My bjd, Kit, is an older Type 1 El head on a brand new (made in October) Type 3 body. I noticed that his head was noticeably yellow in comparison to his body. His head was pre-owned. But after having his body these past few months, I notice it has yellowed some, too, though I keep him in a cool dark room all day long, taking him out at night when I return from work.

I don't have anything technical or scientific to add here. But on a sentimental note I think that a yellowed/grayed/worn appearance to those special figures, plushies, etc. that we love is an indication that they ARE loved and not just sitting on a shelf or in protective wrap as a collectible curio to be stared at. I have those, too. All of them put together bring me nowhere near the happiness my little yellow resin-head does!

Embrace the yellow!

[toshirodragon]

I found with my DZ white skinned boy that most of his yellowing disappeared when I washed the crap MSC off of him. Apparently that was what went yellow not his resin. He currently has only the thinnest of coats on his chest.

[s0yuz]

My Soom big boy seems to have weathered Las Vegas summers just fine, despite no A/C during the day. Not something I generally take chances with though, so he and everyone else got moved to a closet under the stairs when I went out to work for the day.

[cutup01]

Good explanation! <:

[Red-wolf-Ink]

Thanks foe the heads up! This just adviod me going into panic mode one day~

[lady_diamond]

Great explanation

[adam]

For those who want more than slightly technical:

Most ball jointed dolls are made from polyurethane, specifically polyurethanes made from reacting 4,4'-Methylenediphenyl Diisocyanate (MDI), an aromatic diisocyanate, with a blend of polyether polyols (alcohols with more than one hydroxyl group). The resulting polymer is a repeating chain of urethane groups, which have -NH-(C=O)-O- format linkages. It happens that aromatic urethanes absorb 400nm and shorter wavelengths of visible and UV light, and the photon energy causes two primary forms of degradation:

1. Photolysis: Photo-Fries Rearrangement, where the -NH-C=O-O- link literally splits and jumps onto the aromatic ring as -C=O-O and -NH2 groups, terminating the polymer chain at that point. Alternately the -C=O-O- group can release as CO2 gas, and you get a free-radical reaction of the -NH+ terminus with whatever it comes into contact with. It will react with catalysts and sensitizers left in the compound. Byproducts of the rearrangements and free radical reactions can be various colors, with yellow one of them. The exact colors you get depends a lot of what catalysts were used, and what other chemicals were in the mix as sensitizers, plasticizers, etc.

2. Oxidation of the -CH2 group between the aromatic rings into -CH-O-O-H and ultimately peroxidation of the adjacent aromatic rings (releasing the O2) to form a diquinone imide, whose rings contain just two double bonds instead of a full resonant aromatic ring. The quinone ring is well known to be a yellow chromophore (quinones are yellow)

The oxidation path requires UV light and exposure to oxygen, but because yellowing is caused by more than one mechanism, it's not enough to keep your doll in an inert gas environment. Interestingly, using white titanium dioxide based pigments to get skin tones actually increases yellowing rates and it seems to force oxidation over photolysis.

It would be nice if BJDs could be made from aliphatic urethanes, as these don't absorb much visible light, instead requiring very energetic short-wave UV to degrade them (which is why they are popular for outdoor coatings like automobile clearcoat). Unfortunately, the aliphatic diisocyanates are not nearly as reactive as the aromatics, making it impractical to cast multiple dolls from the same molds (MDI based polyurethane parts can release from a mold in as little as a couple of minutes, whereas aliphatic polyurethanes generally need at least 4 hours if heated, 12-24 hours at room temp).

[IamGears&Lace]

Thank you for this useful information! Tis sad the yellowing but I love my bjds regardless of the color of their skin...and then there is always dyeing!

[javasoy]

@Adam, as a Chemistry major 20 years ago, I really appreciate your explanation. Thanks!

[TinaB]

Thank you for all the helpful info. :-)

[waytodawn]

I heard somewhere about a tan doll yellowing and turning green in the sun.
Was that just a fluke?
Or a common thing with tan dolls?

[FunnyLori]

Tan dolls do turn green or grey. It is because the red pigments break down before the green do. I've worked on green/tanned dolls before, there's a definite line where the UV hit them.

[waytodawn]

Oh.
Looks like I'll be getting a normal skin doll then.

[pixieweed]

I know this is an old thread now, but I too want to thank you guys for sharing info freely like this - VERY helpful even to an old-hat like me x

[silverbeam]

Huh, very interesting.

[KiraKat]

Originally Posted by adam

For those who want more than slightly technical:

Most ball jointed dolls are made from polyurethane, specifically polyurethanes made from reacting 4,4'-Methylenediphenyl Diisocyanate (MDI), an aromatic diisocyanate, with a blend of polyether polyols (alcohols with more than one hydroxyl group). The resulting polymer is a repeating chain of urethane groups, which have -NH-(C=O)-O- format linkages. It happens that aromatic urethanes absorb 400nm and shorter wavelengths of visible and UV light, and the photon energy causes two primary forms of degradation:

1. Photolysis: Photo-Fries Rearrangement, where the -NH-C=O-O- link literally splits and jumps onto the aromatic ring as -C=O-O and -NH2 groups, terminating the polymer chain at that point. Alternately the -C=O-O- group can release as CO2 gas, and you get a free-radical reaction of the -NH+ terminus with whatever it comes into contact with. It will react with catalysts and sensitizers left in the compound. Byproducts of the rearrangements and free radical reactions can be various colors, with yellow one of them. The exact colors you get depends a lot of what catalysts were used, and what other chemicals were in the mix as sensitizers, plasticizers, etc.

2. Oxidation of the -CH2 group between the aromatic rings into -CH-O-O-H and ultimately peroxidation of the adjacent aromatic rings (releasing the O2) to form a diquinone imide, whose rings contain just two double bonds instead of a full resonant aromatic ring. The quinone ring is well known to be a yellow chromophore (quinones are yellow)

The oxidation path requires UV light and exposure to oxygen, but because yellowing is caused by more than one mechanism, it's not enough to keep your doll in an inert gas environment. Interestingly, using white titanium dioxide based pigments to get skin tones actually increases yellowing rates and it seems to force oxidation over photolysis.

It would be nice if BJDs could be made from aliphatic urethanes, as these don't absorb much visible light, instead requiring very energetic short-wave UV to degrade them (which is why they are popular for outdoor coatings like automobile clearcoat). Unfortunately, the aliphatic diisocyanates are not nearly as reactive as the aromatics, making it impractical to cast multiple dolls from the same molds (MDI based polyurethane parts can release from a mold in as little as a couple of minutes, whereas aliphatic polyurethanes generally need at least 4 hours if heated, 12-24 hours at room temp).

This thread was interesting to start out with, but this post totally completes it! I love knowing EXACTLY what's going on...this clears up a bunch! Also, I'm thankful my basement is like a freezer-- slowing the reaction slows the yellowing so I guess freezing my but off isn't that bad if I'm prolonging my doll's unyellowiness.

[roxythekiller]

This thread is very helpful
Although it's been said before, it can't be said enough. Thanks for giving this info out for free

[Jangki]

SO how long does it usually take for a doll to yellow?
>3< sorry to ask