Fiber Reactive Dyes
The chemistry of fiber reactive dyes...
So, fiber reactive dyes are slightly different than acid dyes. Acid dyes work by loose associations with fiber. So while they are “fast” (don’t tend to wash out) they’re not actually “bound” in the chemical sense. That is, if you drew a picture, there are no hard dark lines between the chemical structure of the dye and that of the fiber, just dotted ones.
Fiber reactive dyes take those loose associations one step further to make actual chemical (covalent) bonds with the fiber. One reason for this is that plant fibers are slightly basic, while animal fibers are slightly acidic. That means that an acid dye (which requires an acidic environment to stick) will never stick to the basic plant fibers.
Here’s an example of a reactive dye. As usual, you can ignore most of it because the really big parts are just to make it have color. The important part is that SO3 part in the top right. That’s our “reactive” group.
This particular dye has a masking group on the reactive part (The Na – sodium – to the right, plus one of the O’s) which prevents the dye from reacting with water (that SO2 sulfur dioxide group LOVES to react with things) until you either boil it or add a very basic compound like soda ash to knock the sodium off. Then the C-SO2 can bond with an C-O-H on the fiber and make C-S-O-C which is muuuuch more stable.
I suppose I should note here that this is not a typical reactive dye, structurally. Most reactive dyes are unmasked and use a slightly different reactive group.
This is a Procion Yellow dye. In this, the loops behind the chlorine (Cl) are the reactive groups. The chlorine comes off, and the ring reacts with the cellulose to make an “ether” bond, C-O-C (the corners of the rings are C, carbons). The chlorine (unlike the NaO above) is more than happy to fall off as soon as you add water, so it’s called a “leaving group” instead of being a masking group that has to be knocked off.
Dyes like Procion MX don’t have that masking group, so you can’t put water in them until you’re ready to use them, ’cause that reactive group will react with you water! Then it can’t react with your fiber anymore, and much sadness abounds. However, not having the group means that you don’t have to get the masking group off at all, which means almost all your dye will react with something, even if it’s the water. So, if you’re fast, the non-masked dyes are “more reactive” (more of it will react with your fiber) than the masked dyes.
If you actually want to do fiber reactive dyeing, Knitty has a good article.