[jpg28]

Hello everyone!

I've been sourcing out various literatures about how fabrics turn yellow over time, but I can't seem to get to the culprit behind this phenomenon. As I've read, most fabrics are still made of cellulose (the same material that mostly makes up paper) and may be taken from various sources--whether from wood, bamboo piths, etc.

I'm not entirely sure if I can safely assume that paper and, let's say, cotton fabric are somewhat similar (only that the latter went to lots of post modification processes) in composition, but if it is so, then some sources say that what causes paper to turn yellow-brown after ageing is the oxidation of lignin, one of the by-products in obtaining cellulose from wood or other sources. Could my assumption hold true for cotton fabrics as well? Or are there other causes for these to turn yellow over time?

In an almost-entirely-different case, over bleaching (with sodium hypochlorite) may also cause this yellowing in white fabrics. I was thinking if there are any unmanageable reactions that leads to this, too.

Any inputs about my case would be of great aid! Thank you very much, in advance, guys! Also, I hope I posted on the right board.

[fledarmus]

Which fabrics? Only the plant based fabrics, such as cotton and linen are cellulose based. Animal based fabrics such as wool are keratin protein based, and insect based fibers such as silk are based on a very different protein (called appropriately enough, silk protein) which is very high in glycine, with some alanine and serine. These natural fabrics also contain a wide variety of other chemical entities such as proteins and lipids that are not entirely removed by the cloth making process. Synthetic fabrics such as nylon and polyester have more regular structures with none of the extra proteins and lipids and other natural biologics, but may also contain a number of synthetic dyes, softeners, and other additives. And as far as I know, there is no such thing as a truly white natural fiber. All white cloth is bleached and usually treated with white colorants to appear white. Blue colorants may also be present to counteract the normal yellow appearance of the material and make it appear whiter. The bleaching is a chemical process, and given the right conditions, is reversible. The colorants can be lost due to decomposition, wear, or washing, and the chemical bonds making up the fibers can decompose.

For cellulosic fibers, slow oxidative dehydrolysis appears to be one major cause of yellowing. If you look at the structure of cellulose http://en.wikipedia.org/wiki/Cellulose you will see that it is composed of chains of cyclic sugar molecules, with lots of pendant -OH groups. These can be protonated and eliminated to leave behind double bonds, frequently conjugated double bonds, which have electronic transitions in the visible range - hence, color. Bleach, used appropriately, can reoxidize these double bonds and remove the color. Over bleaching breaks the glycosidic bonds between glucose molecules and oxidizes at C-2 and C-3 to form ketones, which can be further oxidize to break the C-2/C-3 bond and form carboxylates.

Lignins are polyphenolic molecules that are present in plant fibers, but not in animal fibers. Bleaching is used for delignification, breaking down the lignin structure so it can be washed away from the fibers, and oxidizing the chromophores that remain to remove their color. This oxidation, however, is a reversible process, and over time and especially in the presence of light, any remaining lignin will eventually return to its colored state. Wood contains much more lignin than cotton or flax, and wood pulp papers especially are much more sensitive to residual or environmental acids and oxidative decomposition than cotton. I am not aware of wood being used for a source of cloth, except through the process of dissolving and reforming the cellulose to make materials like rayon, which are neither truly natural nor truly synthetic.

Rather non-specific, but I hope some of this information helps.

[jpg28]

Thank you very much, Fledarmus, for all your useful inputs. Now I know that I really have much to read about the matter. My problem really is on how to mitigate this yellowing issue especially when bleach is used when washing white clothes, so I was trying really hard to find out what causes it.

I also found other useful information on yellowing being caused by atmospheric pollutants (various oxides of nitrogen and sulfur) and a bit surprisingly, the container in which these are stored.

As for over bleaching, I do agree that it may hasten the degradation of the fabric. I really like how you explained it in a molecular level: having numerous pi-systems in an almost-rigid structure may lead to absorption of light. If only there was some way to protect the cellulose fibers from degrading too fast when using bleach (or maybe leaving behind minute amounts of it in the fiber matrix), then maybe washing white clothes with bleach won't leave it turning yellow over time.

Again, thank you very much for your response! If you have additional information that could aid my problem, I would appreciate it very much.

[kathywmn]

Hi,
Ran across this old post and may be able to lend some information if you're still interested.  I work for the Mrs. Stewart's Bluing company (www.mrsstewart.com).  We have manufactured laundry bluing since 1883 ... your grandmother most likely had it in her laundry room.  White fabric begins as "grey goods".  To make it the whitest white, it is put through a bleach and a bluing process.  Bluing is to add just a bit of blue pigment to the fabric ... just enough to make it a "blue-white" color, which, to our human eye, appears as the brightest white.  Bluing is not permanent.  It rinses out with several washings and rinsings.  At the turn of the century (1900), marketing bluing to consumers gave them the opportunity to add the bluing back into their clothing ... better yet to use it in every whites load to prevent the loss of bluing altogether.  Usually when bluing rinses out, you are left with "the dingies" ... that off white, grey, dirty look.  But sometimes, depending on the fabric, you are left with a "yellowing".  There is more information about this at our website and by search, "laundry bluing".  Different bluings had different blue pigments in them.  Mrs. Stewart's Bluing is ferric hexacyanoferrate ... a naturally occurring mineral ... blue iron.

Another take on yellowing is this.  Bleach (even color safe oxygen bleach ... any kind of bleach actually) works by "burning away" the top fiber of fabric ... the fibers with the dirt on them, revealing the clean bright fabric beneath.  If used as directed and sparingly, bleach makes a good stain remover.  However, used in every load to "keep whites white" doesn't work forever.  Slowly the fabric becomes more and more yellow.  Under a microscope, this appears to be a chemical burn.  This cannot be reversed if it is too far damaged.

For what it's worth.  It's not a "chemical" explanation.  But it's what we've learned and have been explaining to people for over 100 years.

[marquis]

I think you are correct.

I worked for a pharmaceutical company that reduced the coefficient of
friction for rubber stoppers by chlorinating them.  Basically, it was just
mixing bleach with a small amount of weak acid.

While there are papers with a good number of possible explanations, it
was the electron microscope pictures that were most revealing.  An un-
treated surface was smooth.  The chlorinated stoppers were very irregular
with a large amount of the surface removed. This resulted in reduced
contact area and reduced friction.

One other theory was that the chlorination added across the double bonds
of the polymer and the increased size of the chorine atom decreased the
friction.  While there is no doubt that this happens, it didn't seem to
be the major factor.

Thanks for posting.

[meerkat]

This is related to the initial question re yellowing of fabrics, which I am desperate to learn more about. I recently bought two vintage club chairs I adore (circa 1970s/80s) and had them reupholstered in an oyster white synthetic outdoor fabric (100% polyolefine) that our cats won’t butcher.

Within a month of us bringing home the newly upholstered chairs in this gorgeous fabric, they started yellowing in different parts. We immediately sent the chairs back to the upholsterer who wasn't sure what the deal was but agreed to redo the chairs: they sealed the wood with a water-based sealant again, lined it with cotton this time, and then put the fabric on top. They were perfect upon arrival yet once again, the chairs have yellowed, this time after 6 weeks.

We can’t figure out if it’s a chemical reaction b/c of the synthetic fabric, if something is coming up from the wood, or what the deal is. No one seems to have any clue. What we can tell is that the yellowing is coming through and radiating on the fabric where the wood seems to be on the other side, so perhaps something wasn't sealed properly with the water-based sealant.

We are sending the chairs to a woodworker who is going to sand them down and put a strong oil-based sealant on them. We may try a new fabric this time, but wondering if anyone would know why any fabric would turn yellow in such a short period of time. Would the synthetic fabric be the reason? Otherwise, even if these chairs sat in a damp basement for years, wouldn't that be mitigated by a wood sealant? Or was the sealer not strong enough? Why would something oxidize SO quickly? I can find nothing on this anywhere. Hoping three times will be a charm for us in this endeavor. Thank you so much for any insight

[Corribus]

Hard to speculate without more information. The pattern of yellowing can tell you a lot about where it's coloring, as can where the fabric is being stored. It could be something leeching from the wood, as you suggested. It could also be something that the fabric is treated with - say, upon exposure to sunlight. A good "control" experiment would be to put a sample of the fabric next to the chair for a while and see if it yellows - this will tell you whether it is something in the fabric itself or something in proximity to it (wood from the chair).