Dyeing 101: Know Your Dyes – What’s the Difference Between an Acid Dye and a Reactive Dye?

I have a weekend update to do, and some fiber to get up, but my brain is in chemistry mode right now, so I’m going to post about that instead.

So, there are a lot of dyes out there in the world, made by a lot of different companies.

The most common kinds you will run into are as follows:

Acid Dyes

Reactive Dyes

Direct Dyes

There are subclasses of most of these, and some crossovers. I’ll also briefly touch on a group called “Disperse Dyes.”

Ok. So, the difference between each of these classes of dye is in the way they interact with the fiber you’re dyeing. That is, they have a different chemistry. No, not scary! Get back here! I’m going to explain things logically and slowly, and if you have questions, feel free to ask them. I’ll try not to overwhelm you with jargon without explanation.

I’m going to go through them from one end of the chemical spectrum to the other, starting with my favorites, acid dyes.

Acid Dyes:

First, what does this group include? Some of the things you’re probably familiar with…

Kool Aid
Most food dyes, especially reds

and some you’re likely less familiar with…

Jacquard Acid Dyes
Prochem Washfast Dyes
Greener Shades Dyes

And more! But they all work via a similar chemical mechanism.

I’ve posted about acid dyes before, but I’ll duplicate some of it here.

First, the “acid” in “acid dyes” refers not to the dyes themselves, but to the environment they bind in. For kool aid, the acid is already present in the mix (in the form of citric acid crystals), for other dyes, it’s added as vinegar (which is acetic acid) or sometimes citric acid.

Safety Sidenote: Both acetic and citric acid are food acids, that is, they’re not terribly toxic and won’t kill you. However, when dealing with concentrated citric acid, or any hot bath with acid in it, be careful. Inhaling large amounts of vinegar steam can irritate your throat and lungs and eyes. Submerging your hands in these acids for any length of time can cause irritation and some skin peeling. Better safe than sorry.

Now, some acid dye chemistry!

Acid dyes work via a combination of interactions with the fiber. I’ll explain each one.

Primary: Ionic bonding
Secondary: van der Waals, Hydrogen bonding

Ionic bonding is a bonding between different charges on two molecules. Imagine acid dyes like little magnets. Remember how if you put two magnets together, you have to put the S and N ends together, or they’re push instead of pull together? Some molecules work the same way.

van der Waals interactions are like mini ionic interactions. Instead of a whole magnetic pole interaction, it’s a tiny fraction of one. But they add up.

Hydrogen bond interactions are weaker than ionic and stronger than van der Waals interactions, but use the same idea but specifically involving hydrogen.

Acid dyes are “anionic” meaning they have one full negative charge, think of it as one “south” magnet end.

Acid dyes can therefor bind to fibers which are “cationic” meaning they have one full positive charge, or one “north” magnet end.

An acid dye binds, like a magnet, when its “south” end binds to a fiber’s “north.”

There are many different ways to build an acid dye to get this “south” magnet effect. Some are strong, and some are weak. Here are a few examples.

You’ll notice that in all of these examples, an SO3 group (or two) is present. This is the “south pole” for these dyes. The rings are what gives it color.

Acid Orange, many reds and oranges are shaped like this.
Acid Orange

Acid Green, many if not most greens are shaped like this. Some purples and blues as well.
Acid Green

Acid Blue, this overall elongated structure is what a lot of the “leveling” dyes use.
Acid Blue

A “leveling” dye, is a dye which naturally makes for a more even color coating. The way this is done is by making a dye molecule with weaker binding potential (a weaker south pole) so that molecules can bind and unbind to move about the fiber evenly. This is convenient, but the trade-off is that the dyes can come off the fiber even when you don’t want them to. This is annoying if you want to wash your fiber in warm or hot water.

Another concern that many have with acid dyes is that to get the deeper and intense colors, especially blues, many of them are made with metal ions…

For example, here’s ProChem’s black dye, acid black. It’s a “true black” or “primary black” meaning that it’s not blended. But the intensity of color comes from that “Cr” in the middle of the molecule, which is a Chromium atom. Chromium is a heavy metal, which makes this a dye you don’t want to pour into your groundwater.

Acid Black

(You’ll notice this dye, as well, uses the “sulfonate” SO3 group as its ion, but also has a “nitro” NO2 group as well)

As a rule, companies try to limit the amounts of these heavy metals they need to use, and some companies like Greener Shades try to eliminate them altogether. However, some colors are hard to get without the use of metals.

So, in short…

Acid dyes are anionic (south pole) dyes, which use heat and an acidic environment to form ionic bonds with fiber.

Direct Dyes:

Dyes in this category are…

Cushings Direct Dyes
Jacquard iDye Direct Dyes
ProChem’s Diazol Direct Dyes, now sold by Aljo Mfg.
Any “universal” dye like RIT will have a Direct Dye as one of the two components

Direct dyes also use a combination of forces to bind to fiber…

Primary: van der Waals
Secondary: Hydrogen bonding

You can see already, that in some ways they’re similar to acid dyes, but their primary binding mode utilizes a much weaker interaction. Still, they bind similarly to similar fibers.

Direct dyes tend to be very large in order to have more “subtantivity.” This means having more surface area to contact the fiber and engage in more of these small van der Waals interactions.

Many if not most of the direct dyes will function as acid dyes as well. Look at this Direct Red dye and you’ll see why…

Direct Red

See that SO3 group? Yep, acid dyeing group. The problem with these dyes is that being as long as they are, and generally more balanced along their length, they again are easier to dislodge. You’ll notice it looks almost identical to the “leveling” category of dyes, that’s because… the classes overlap! More on what this means when I get to fiber chemistry in the next Dyeing 101 post.

You’ll also notice there’s more H’s hanging off, hydrogens that can be used in hydrogen bonds to stick this dye to fiber that doesn’t have that “north pole” that the acid binding group (the SO3) needs.

Since these dyes rely generally on weaker interactions, they are less washfast and lightfast than most acid and reactive dyes. Being big and floppy, they also tend to be duller in color (though this trait is not uniform). The advantage of them is that they don’t require fiber with positive “north pole” groups.

So, direct dyes in brief…

Use some ionic, but mostly hydrogen bonding and van der Waals forces to bind to fiber. Less wash and lightfast than reactive or acid dyes, but better leveling and don’t require cationic “north pole” groups to bind.

Reactive dyes:

Dyes in this category…

Procion MX
Dylon Cold
Cibacron F
Prochem Sabracron F
Vinyl Sulfone

Reactive dyes us primarily a singular mode of binding, though most can use another as well.

Primary: Covalent bonds
Secondary: As acid dyes

Covalent bonds are what you think of when you think of molecules. “Actual” bonds. The kind that hold the H-O-H of water together. Those little lines in all these diagrams are covalent bonds.

Covalent bonds are the strongest bonds, as such reactive dyes are the toughest when it comes to being washfast.

I wish there was a better picture of this, but here’s a common reactive dye molecule…

Reactive Red

The first thing you might notice is those SO3 groups. That’s what makes this able to function as an acid dye if you add vinegar.

But here’s the important part of the molecule, over there on the left…


Funny looking bit, isn’t it? Chlorines (note that when on the dye, one chlorine is replaces with the dye)? Negative charges? Those chlorines would really like to rip a hydrogen off of your fiber and float off happily as HCl (Hydrochloric acid), but that would leave that ring behind it bare! But wait, whatever is behind the H the Cl ripped off is ALSO bare, usually an O or an N (Oxygen or Nitrogen) so the two bare things can interact and form a new covalent bond, yay!

That means when the process is done, you have a dye molecule that is actually PART of your fiber molecule, pretty cool? And depending how those interaction groups are designed, they can react with all SORTS of things. Neat. The highly basic environment most reactive dyeing is performed in is to make this process easier, similar in reverse to why acid dyes are done in an acidic environment.

So, an overview of reactive dyes…

Covalently bonded to oxygens and nitrogens on the fiber, making them ultimately washfast. Lightfasteness varies with the structure of the dye molecule.

Disperse Dyes:

Disperse dyes are horrible and gross. They’re what you have to use to dye polyester and acrylic and similar plastics. Ok, that’s not all true. There are ways to dye polyester without having to use the noxious chemicals and super heat, Jacquard has dyes for it. But they’re still not pleasant dyes. I’m not going to talk about the chemistry of these.

Brief overview for those who want the differences quick:

Acid Dyes: Anionic (negative), bind to cationic (positive) fibers. Can’t bind nonionic fibers. Fairly washfast, pretty lightfast.

Direct Dyes: Often anionic (negative) binds to nonionic fibers, and if anionic then it can bind cationic fibers as an acid dye. Not very washfast or lightfast. Many can be used as weak acid dyes. A component in “universal” dyes like RIT

Reactive Dyes: Covalent interactors, often with anionic (negative) bits as well. Given the right environment can bind covalently to nonionic fiber, and possibly to cationic fiber. Many can also function as acid dyes on cationic fiber.

Now, I’m sure by now you’re waiting for me to tell you WHAT fibers are cationic, and what fibers are nonionic. You might be wondering if there’s anionic fibers too. We’ll get to that, but not in this post. That’s in the next post. Cellulose, protein, animal, plant, extruded?

::waves:: As I said above, feel free to ask questions, clarify, or even argue with my chemistry.

Answers to Questions:

To Velma: By wandering around the web a lot. Here’s what I can tell you about ProChem’s stuff…

Sun Yellow, Basic Red, Bright Blue Red, Turqoise, Bright Blue, and National Blue are all non-metallic. Carbon Black (Acid Black 52) is the chromium containing one. Here’s a link which has some minimal information. As a rule, if you Google either the systematic name or the Color Index name, you can find the structures of things. So for example… Acid Yellow 17 Sigma can be a good place to find these.

To Krissy: Usually, that’s how those fibers are dyed, from what I understand. They’re colored in the vat, before extrusion.

Kellie: Feel free to link away. And anyone who comes here from there can also feel free to ask questions or make comments.

Diane: An ionic bond is like a magnet. It can only occur between two things which are magnetizable (or in the case of ionic bonds, things which have charge). So, like you can’t stick a magnet to plastic, you can’t stick an ionic compound to a non-ionic one.

David: The next big post in this series will cover the basic difference between fibers, animal, plant, and others.

~The Gnome

16 thoughts on “Dyeing 101: Know Your Dyes – What’s the Difference Between an Acid Dye and a Reactive Dye?

  1. Looking forward to follow-up posts to this informative start.

    Where might one look to find the chemical structure of various dyes? I am interested in which of the ProChem dyes (besides black) uses heavy metals, but have been unable to locate this info.

    Also, if, for example, PC black contains chromium, why does its MSDS not read differently than for Greener Shades’? (I cannot find an MSDS for 172, only for their 672). Is chromium not considered hazardous by OSHA / TSCA standards?

    Many thanks for your input! Cheers, Velma

  2. Maybe this is a silly question to ask someone who dyes already-created (-harvested, etc) fiber, but can the plasticky fibers such as acrylic, polyester, etc just be colored as liquid and then extruded as the desired color? Is that even how acrylic and polyester fibers are made? Or shall I await the exciting fiber chem post to find out?

  3. Dan, thank you so much for this. It’s wonderful and really explains the chemistry to me, which until now had eluded me. Plus I was forced to recall all those damn terms from my high school chemistry classes. That was a long time ago.

    I think I’m going to link this on my site if you don’t mind. It’s an excellent source of information and reference and I know I’ll be coming back to it. I’m sure my customers would like to know what it is I’m using to dye the fiber and yarns they’re buying and this will definitely help. Thanks so much for putting it together and up here for us all to benefit from, much appreciated!

  4. Fascinating, as usual. I didn’t take chemistry even in HS – at least, not more than what might’ve been included in biology *g* – so I’ll have to read it a couple times more to get it firmly in da head. But it’s a great, clear explanation of the reasons behind what I know about what dyes work on which fibers.

    And when I see questions on Rav about dyeing, I’m gonna send ’em here!

  5. Ouch, that’s give me brain freeze, much like eating ice cream too fast. Only warmer.
    Really, this is fascinating information.
    Will you get into the whole fiber chemistry topic soon? Like, how wool and silk and nylon can all use the same dyes, but those dyes will only stain cotton?
    And, do ice cubes really get gum off a shag carpet?
    (Ok, no more coffee for me.)

  6. my.brain.is.failing.
    That’s a long post that I’ll have to take considerably more time reading to fully grasp all that you’ve said. Thanks for putting so much effort into explaining it all so well and posting it here for free for the dyers of the world to benefit from.

  7. These are great! Thanks for taking the time to put this all down. I do miss Chem lab – sigh. Looking forward to more info!

  8. Like some information on the dyeability of cotton and soy fibres.
    Why is it that soy performs as a protein and yet it is of a cellulosic nature?
    Wool and soy will then dye both together or only with certain dyestuff ?

  9. Thanks so much for these sorted information, i’v been away from chemistry past several years yet your explanation is awsome , i work on coupling various dye’s to proteins as in a biologist research but you have clearly explained me the chemistry behind it ..thank you so much

  10. Question for the Gnome: This is a great site which explains things in simple terms. Is there any way to determine which dyes are best for which fabrics ? Can this be done by looking at the shape and types of bonding on the dye molecule ? For example Polyester, Cotton and Nylon using dyes such as Direct Red 81, Direct Red 23 or Disperse Yellow 7 ? It is also interresting that when you do a search for these dyes you get a large number of different names for each ? Is there a universal naming system or is it because the dyes are produced by different companies and name them differently ? If this is the case are they identical ?

  11. Hi, I use yarns that are either chemical free or dyed with low impact dyes as much as possible. Would you mind explaining to me where reactive dyes fit into the safety scale for babies and people who are chemically sensitive?
    Thank you

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