The chemistry of yellowing - slightly technical

[FunnyLori]

This article is my own work in progress, I am open to suggestion and correction. I am not a professional, but this is my take on yellowing.

I know yellowing of resin is a hot topic, and I am sure will continue to be a point of discussion. I thought I would share my understanding of the process, and put it up for discussion.

Many pigments are highly UV sensitive. Most plastics are as well, and they will degrade with light. Since the resin in our dolls is essentially a combination of pigments and plastic, there will be a combination of processes causing our dolls to yellow. Some pigments themselves are oxides, while others are known to change as they oxidize with exposure to the air. Yellowing can occur with all types of resin, regardless of color or type, this is because they are being oxidized both by the air, as well as the UV exposure working as a catalyst degrading the molecular bonds. The dyes and pigments will fade first because they are the most unstable.

The reason that french resin appears to be yellowing faster than other 'standard' resins is because of the inherent qualities of the resin itself. French resin is prized for being partially transparent, which makes it so lifelike and luminous. It is the very translucent quality that is the downfall of the french resin. Because light is able to permeate the resin, there by casting fabulously beautiful natural shadows, it is able to degrade the pigments and the resin deeper than a more opaque resin.

The fact is, all resin will eventually yellow. The yellowing may be permanent. I have heard of very severe (and caustic) as well as simple methods of bleaching the resin back to it's natural color. The problem with bleaching is that it only treats the surface of the resin so that it will appear new again, but it soon re-yellows. Bleaching is not always a permanent solution.

You cannot completely prevent yellowing. Sealing resin helps because the sealant (MSC UV Cut in particular) acts as both a barrier to the environment and partially filters the UV light from reaching the resin. MSC though, will eventually yellow as well. Granted, the new formula is not supposed to yellow as much, but how long has it really been tested? Also, I have heard reports that dolls partially sprayed with MSC seem to have no difference in yellowing, parts that were and parts that were not sprayed seemed to yellow equally.

Keeping resin out of direct sunlight will also reduce the amount resin will yellow. Keeping your doll in it's box will not prevent yellowing. The resin will still be exposed to heat and air, and it will still become partially oxidized as it ages.

I am not saying that dolls should be kept in dark vacuum chambers to prevent yellowing, I am just saying that all things age. It is a fact of life, and we should do what we can, but not stress ourselves knowing that the resin will yellow.

I hope I wasn't overly technical, and somewhat useful to people. I'm not a pro, but this is just my understanding of the chemistry behind the dolls.

--------------------Edits---------------------

Added by Blackbox:
"[Some]...resin is made of tree sap. It is a natural material to some degree, very much like shellac. I'm not saying they are exactly the same only that they are both made from tree sap.

The tree sap is refined and other chemicals are added to give it the qualities for different uses such as dolls, figures and other items.

It is my understanding the "natural tree sap" is what causes the yellowing very much the way shellac yellows with time. It is not because of the pigment added to the resin. In fact the pigmented resins appear LESS yellow over time because the pigment somewhat masks the yellowing of the resin.

French resin simply yellows faster because it has little or no pigment. They use dye rather then solid pigment to color the resin. So you see more of the resin and thus yellowing faster.

I also have read about some very caustic treatments to move the yellowing. I do believe the yellow removal will not be permanent. The resin just naturally wants to yellow.

Keeping resin out of sunlight and away from heat will keep the resin from aging as fast."

"I don't think the basic ingredients of resin are a secret. But the individual companies do "tweak" the formulas to make them unique. The basic chemical composition should be easily found on the internet.

Also I do want to stress that heat is A HUGE factor in the yellowing of the resin. Don't leave your dolls in a hot car! Even if they are covered the heat makes the resin turn. Did you ever notice that when your doll gets very warm the resin has a stronger smell? It is the resin gassing off and the chemical reaction is to turn yellow.

So even if your doll is in the back of the closet in a box covered up, it can still turn if the closet is warm. If there is a heating duct under the floor or inside the wall of the closet yellowing will occur. :/

I do want everyone to know I am NOT a chemist..... this is just the info I have received from the factory."

Added by Hobbywhelmed:
"'ll testify that some resin doesn't need light to change. I've had a brand new body of a particular brand sitting in its box since I got it, since I can't find a head I like to match the color of the resin, which is very pink-white. When I took it out of its box the other day, after it being in the back of the closet in its box for about 2-3 months, I'll swear the pink is shifting a little to the peach side. <edit> So even in darkness, it appears to be changing. "


Added by Quixote:
Of interest to this thread might be the wikipedia entry for resin:
http://en.wikipedia.org/wiki/Resin

One thing I would note there is that there are synthetic resins as well as natural tree resins. At least some dolls are made from polyurethane resin. Polyurethane is synthesized, not from tree sap. (Although there certainly could be other dolls made from natural tree sap resins).

However, polyurethane is UV sensitive.

For a very technical view of polyurethane, see the wikipedia entry:
http://en.wikipedia.org/wiki/Polyurethane

A useful excerpt from that entry:
"Exterior use of polyurethane varnish may be problematic due to its susceptibility to deterioration through ultra-violet light exposure. It must be noted, however, that all clear or translucent varnishes, and indeed all film-polymer coatings (i.e.paint, stain, epoxy, synthetic plastic, etc.) are susceptible to this damage in varying degrees. Pigments in paints and stains protect against UV damage, while UV-absorbers are added to polyurethane and other varnishes (in particular "spar" varnish) to work against UV damage. Polyurethanes are typically the most resistant to water exposure, high humidity, temperature extremes, and fungus or mildew, which also adversely affect varnish and paint performance."

[april]

It didn't seem overly technical to me! :)

[Shizen]

Thank you. Explanations like this would be best for most of the participants of our hobby, actually.

[VinylFaerie]

Heat also contributes to yellowing; otherwise, Bravo! :)

[harlowe]

Thanks for posting this! It wasn't at all overly technical and makes perfect sense.

What is hard to hear is that there is no way of really "preventing" it, more like delaying the inevitable. Which is a shame because I really do LOVE french resin.

[Taco]

Made sense to me and is very interesting.

[alanna214]

Is there a place for this in the wiki? I think that would be a great place for it. :) Thank you for your work.

[blackbox]

On a resent trip to Hong Kong during one of the meetings with my factory, I was a bit shocked to find out resin is made of tree sap. It is a natural material to some degree, very much like shellac. I'm not saying they are exactly the same only that they are both made from tree sap.

The tree sap is refined and other chemicals are added to give it the qualities for different uses such as dolls, figures and other items.

It is my understanding the "natural tree sap" is what causes the yellowing very much the way shellac yellows with time. It is not because of the pigment added to the resin. In fact the pigmented resins appear LESS yellow over time because the pigment somewhat masks the yellowing of the resin.

French resin simply yellows faster because it has little or no pigment. They use dye rather then solid pigment to color the resin. So you see more of the resin and thus yellowing faster.

I also have read about some very caustic treatments to move the yellowing. I do believe the yellow removal will not be permanent. The resin just naturally wants to yellow.

Keeping resin out of sunlight and away from heat will keep the resin from aging as fast.

I am not posting this as to debate anyone merely just passing along the information I have received from, what I would consider, a very reliable source.

[FunnyLori]

Thank you very much for your contribution! I did not know that the resin being used was based with refined tree sap. Most companies keep their formulas fairly secret, so finding this information is a real treasure!

On a resent trip to Hong Kong during one of the meetings with my factory, I was a bit shocked to find out resin is made of tree sap. It is a natural material to some degree, very much like shellac. I'm not saying they are exactly the same only that they are both made from tree sap.

The tree sap is refined and other chemicals are added to give it the qualities for different uses such as dolls, figures and other items.

It is my understanding the "natural tree sap" is what causes the yellowing very much the way shellac yellows with time. It is not because of the pigment added to the resin. In fact the pigmented resins appear LESS yellow over time because the pigment somewhat masks the yellowing of the resin.

French resin simply yellows faster because it has little or no pigment. They use dye rather then solid pigment to color the resin. So you see more of the resin and thus yellowing faster.

I also have read about some very caustic treatments to move the yellowing. I do believe the yellow removal will not be permanent. The resin just naturally wants to yellow.

Keeping resin out of sunlight and away from heat will keep the resin from aging as fast.

I am not posting this as to debate anyone merely just passing along the information I have received from, what I would consider, a very reliable source.

[kitteh]

Thanks to everyone contributing! This is seriously the most informative thread I've read in a while. I agree it should go in the wiki.

[hobbywhelmed]

I'll testify that some resin doesn't need light to change. I've had a brand new body of a particular brand sitting in its box since I got it, since I can't find a head I like to match the color of the resin, which is very pink-white. When I took it out of its box the other day, after it being in the back of the closet in its box for about 2-3 months, I'll swear the pink is shifting a little to the peach side. (In this case, I HOPE it does! Will be easier to match it to something!)

So even in darkness, it appears to be changing. I will keep "checking in" to see if that continues or if it stops short at some point.

[ShaDrouet]

This is a very informative thread.. thanks so much for all this valuble information..

: )

[blackbox]

I don't think the basic ingredients of resin are a secret. But the individual companies do "tweak" the formulas to make them unique. The basic chemical composition should be easily found on the internet.

Also I do want to stress that heat is A HUGE factor in the yellowing of the resin. Don't leave your dolls in a hot car! Even if they are covered the heat makes the resin turn. Did you ever notice that when your doll gets very warm the resin has a stronger smell? It is the resin gassing off and the chemical reaction is to turn yellow.

So even if your doll is in the back of the closet in a box covered up, it can still turn if the closet is warm. If there is a heating duct under the floor or inside the wall of the closet yellowing will occur. :/

I do want everyone to know I am NOT a chemist..... this is just the info I have received from the factory.



Thank you very much for your contribution! I did not know that the resin being used was based with refined tree sap. Most companies keep their formulas fairly secret, so finding this information is a real treasure!

[alanna214]

Isn't it ironic that we, warm-blooded creatures, who love our resin kids so much, are slowly causing them damage simply by loving them. It's sad, too. :cry

[gayle]

Outstanding explanation!!

[Cookiekitty]

How soon does yellowing start on a doll that is kept from the sunlight?

[Faireyangel]

This is very awesome information to have, thank you for putting it together for us. My Roda is definitely yellowing more rapidly than my other guys. :|

Do you know if process levels out at some point?

[junoboy]

I feel like I have a complete education... thanks so much!

[FunnyLori]

Thank you everybody. I am glad that it has been useful.

I especially like the contributions and testimony some of you have added. If this gets made into the wiki, I hope that all of the information is included.

I do not as of yet know if the yellowing process levels out. The yellowing process likely begins as soon as the resin cures. This is because it is now being exposed to the environment. However, severe yellowing does not usually occur at this stage unless the resin is highly unstable. The rate of yellowing varies from doll to doll, resin type to resin type, company to company, and so on. Some dolls will yellow faster than others. Some dolls yellow very slowly, there is no standard measure of time that a doll will become "yellowed".

I also updated the first post. Please let me know if there are any objections.

[Sassie_L]

Thank you for this. I found it really useful as I am really new to this hobby and had read quite a few references to "yellowing" and was unsure of what causes it.

[Chops]

Very good points, and it makes sense. Thank you!

[Vega]

That was very well written and very enlightening. It was technical, but you made it read like it wasn't.
*feels edumacated now*
:heart -V

[blackbox]

Wow Thanks for including my information in your first post.

I'm honored :)
Rin

[Orrery]

Thanks for the information. I've never been quite sure what caused yellowing besides light. ^^

[quixote]

Just thought I'd add to the conversation.

Of interest to this thread might be the wikipedia entry for resin:
http://en.wikipedia.org/wiki/Resin

One thing I would note there is that there are synthetic resins as well as natural tree resins. At least some dolls are made from polyurethane resin. Polyurethane is synthesized, not from tree sap. (Although there certainly could be other dolls made from natural tree sap resins).

However, polyurethane is UV sensitive.

For a very technical view of polyurethane, see the wikipedia entry:
http://en.wikipedia.org/wiki/Polyurethane

A useful excerpt from that entry:
Exterior use of polyurethane varnish may be problematic due to its susceptibility to deterioration through ultra-violet light exposure. It must be noted, however, that all clear or transluscent varnishes, and indeed all film-polymer coatings (i.e.paint, stain, epoxy, synthetic plastic, etc.) are susceptible to this damage in varying degrees. Pigments in paints and stains protect against UV damage, while UV-absorbers are added to polyurethane and other varnishes (in particular "spar" varnish) to work against UV damage. Polyurethanes are typically the most resistant to water exposure, high humidity, temperature extremes, and fungus or mildew, which also adversely affect varnish and paint performance.

[wildefae]

Thanks for this! I'm pretty much resigned to the fact that my doll's going to yellow no matter what I do...might as well enjoy her without worrying too much about it.

[MieAga]

This should be stickied! Yellowing it a huge topic and there are a lot of myths and misconceptions surrounding it. Valueable info should be easy for the newer members to find.

[javasoy]

This should be stickied! Yellowing it a huge topic and there are a lot of myths and misconceptions surrounding it. Valueable info should be easy for the newer members to find.

Actually, someone should re-edit the content from this thread and put it on this site's wiki. If there is enough material, I will update the wikipedia article as well.

[FunnyLori]

I just added some more contributions to the first post.

[Mara]

Yay, materials science! Yay, chemistry! This was really enjoyable to read and IMO was presented in a very easily-digested format. Thanks!!

[Nineivre]

wow, that was really a useful bunch of info to know! thanks for your efforts, FunnyLori ^^

[animemadness]

i was thinking of dying my dollfie head a warm light peach colour to kind of mask the yellowing in her face,she is white skin and it shows up awful and it really annoys me,im a bit nervous about dying a doll,but her face is so pretty,but she just looks ill to me yellow and pale,

[chrissa5girl]

Wow! Great info! I have heard (can't remember where) that soaking a doll in isopropyl alcohol may help with yellowing?

[lookame]

I also want to add, resin doesn't always yellow, sometimes it turns green instead, this normally occurs when the doll is put in a box and kept away, whereas yellowing occurs with excessive exposure to light. Also, different types of resin yellow or green differently.

[GambrillGroup]

Can you / should you anti-UV seal or reseal a doll? If SPF products work on our skin, would they work on a doll? (this might be blasphemy or just a plain d u m b question - but this forum has been great for a rookie like me...) Thanks!

[Arkady]

SPF products work to protect human skin by coating it with a light-protective barrier, most commonly titanium oxide; it's most effective if you can still see a thin film of white on your skin. To get the same effect on a doll, you'd basically have to spray-paint it with a coloured paint that acts as a complete barrier - which sort of defeats the purpose of using a coloured resin in the first place.

[r.littlewolf]

There are UV sprays made for sealing dolls such as Mr Super Clear UV Cut that will supposedly help protect your dolls from yellowing. There are also companies (Volks and Iplehouse come to mind) that add some kind of UV protect product to the newer batches of resin to keep it from yellowing to begin with. I'm not any kind of expert, though, so I don't know the details of how any of it works or how successful it is.

[TheSacrificialMaiden]

Good article with useful info!

I would be curious as to what the composition of the actual pigments are, and if there are any that help the resin stay truer, longer.

You're very right about all plastic discolouring when I was a kid my brother threw my She-ra figure on the roof and she was stuck up there all winter! When she came down she had lost her tan! :XD (Even now I don't let him near my BJDs! :p)

[kakei]

Thank you very much for doing this
It's lot of essential things to know

[xblackravenx]

Thankyou for putting this together for us to look through.
I found it really interesting to learn why they yellowed and I didn't think it was too 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. :XD

[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]

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 :)