The chemistry of yellowing -- slightly technical
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:
"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. [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."
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:
"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. [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."