Name: Sakina
Message: I would be greatful if you could kindly answer the following questions for me. I would like to know more about the fastness of direct dyes and reactive dyes on cotton fabric in hot water; which type of dye is better to use that will not easily wash away; and lastly, why are all the mordants transition metals, and why do we then use sodium chloride as a mordant normally. Thank you.

This sounds like an assignment you were given for a class!

First, nobody uses sodium chloride as a mordant. That is not what it's for. Instead, sodium chloride reduces the electronegativity of the fiber and the dye, reducing their tendency to repel each other. Obviously, having the fiber repel the dye would inhibit the formation of a bond between the fiber and the dye. For more information about the use of sodium chloride in dyeing, look in my FAQ for the question "Do I need to use salt?", For more information on the difference between mordants and other auxiliary chemicals for dyeing, see "What's the difference between mordants and other chemical assistants used in dyeing?".

There are many references on my web site to the differences in fastness between direct dye and fiber reactive dyes. See "About Fiber Reactive Dyes", and "About Direct Dyes", which are included in the "About the Dyes" section of my website. To learn about the differences between the chemical bonds which hold fiber reactive dyes to the fiber and the weaker bonds which hold direct dyes to the fiber, see "What kinds of chemical bonds attach dyes to fibers?". The serious deficiencies in washfastness apparent with direct dyes can be ameliorated by the use of cationic dye fixatives, which include Retayne as one example; see "Commercial Dye Fixatives"; unfortunately, they sometimes change the hue of the dye slightly, and they also can significantly reduce lightfastness of some dyes; see "Lightfastness of Different Types of Dyes". However, the cost of direct dyes is lower than that of fiber reactive dyes, and the amount of waste water produced industrially is less, so these trade-offs are often commercially acceptable.

Mordants are not used for fiber reactive dyes, direct dyes, disperse dyes, acid milling dyes, or strong acid dyes . They are not, in fact, used for most types of dye. The only auxiliaries used for most classes of dyes are salts and pH-adjusting chemicals, plus the cationic aftertreatments used for non-washfast dyes. The two classes of dyes for which mordants are used are most natural dyes (with the exception of natural direct dyes such as turmeric or walnut, and natural vat dyes such as indigo or the historic Tyrian purple), and the class of dyes which is known collectively as the mordant dyes. Premetalized or metal complex dyes are not mordant dyes, because they have the metal ions precomplexed to the dye. Chrome dyes are a group of synthetic mordant dyes which require the use of dangerous hexavalent chromium to form a complex between the dye and wool fiber; they are used only because they have a better combination of leveling and fastness properties than any other wool dye. (See David M Lewis's book Wool Dyeing.)

Why are mordants almost always transition metals? That's an interesting question. In order for an ion to form a complex between the negatively charged fiber and the negatively charged dye, the metal must be in the form of a positive ion, something all transition metals are prone to form. In transition metals, the d levels of the electrons are partially filled with electrons. They can form different sorts of ions at different times, with different positive charges. Since the d level electrons are in the inner shell, the valence electrons in the outer shell are more shielded from the nucleus, so they are more easily released in order to make ions.

There are two types of mordant that I can think of that are not transition metal ions. (These do NOT include salt, soda ash, vinegar, citric acid, or other non-mordant dyeing auxiliaries.) One is the complex molecule known as tannin, which is often used in combination with alum to mordant cotton fabrics, which have a simple chemical composition that is relatively difficult to bond to nonreactive dyes. Another group of chemicals that can be used as mordants, for the basic (cationic) dyes, is direct dyes themselves. Cotton that has been dyed with direct dyes can then be dyed with basic dyes, which otherwise would not stick. This is not difficult to picture. Alone of all the dye classes, basic dyes are cationic, that is, they have a positive charge. They can cling to the negatively charged large molecules of direct dyes. This is not a very useful observation, since basic dyes are poorly washfast and inferior to other currently available synthetic dyes for cotton, but it is certainly interesting, if you are interested in how dyes bond to fibers.

It is worth noting how extremely toxic most of the metals used as mordants are, making in many cases the use of natural dyes to be far more dangerous than the use of synthetic fiber reactive dyes. The most popular mordant for dyeing is alum, which is usually either aluminum potassium sulfate or aluminum acetate, simply because it is not dangerous like all of the other metal mordants. It is the only metal mordant whose use I can enthusiastically recommend for home dyers. It is not safe to ingest in large quantities and can be irritating to the skin and eyes. A fatal dose of alum for an adult is 30 grams. (See the alum MSDS provided by JT Baker.)

Chromium is popular both for the brightness of the yellows and other colors it can produce, and the washfastness of the dye-metal-fiber complexes, but it is very dangerous. It is obtained as potassium dichromate, which is in the carcinogenic and toxic hexavalent form. (In contrast, premetalized dyes which contain chromium, such as the Lanaset dyes, contain only small amounts of the less dangerous trivalent for of chromium.) A fatal dose for an adult is about 5 grams, but chronic exposures to lower levels produce many serious or fatal effects. Because chromium is excreted from the body very slowly, its effects may be cummulative. (See potassium dichromate MSDS .)

Tin mordant (stannous chloride) is not as dangerous as chromium, but it is not a safe substance to use carelessly. It is an irritant, and ingestion of a toxic dose "may cause burning in mouth and throat, abdominal pain, reduced blood pressure, stomach bleeding, collapse and convulsions" and "may cause liver and kidney damage". (See stannous chloride MSDS.) Copper mordant, which is cupric sulfate, also known as blue vitriol, can be quite dangerous. A fatal dose of copper sulfate for an adult is about 10 grams. (See copper sulfate MSDS.) Iron mordant, which darkens or saddens colors, can also be extremely dangerous if swallowed: a fatal dose in humans is about 12 grams for an adult, or as little as 0.6 grams for a toddler. (See ferrous sulfate MSDS.)

In contrast to the dangers of these mordants, synthetic fiber reactive dyes are relatively very safe. Although they should be kept out of reach of children, and their use by children very closely supervised, no harm has been reported in cases in which children have accidentally ingested Procion MX dyes. Some direct dyes are known carcinogens, based on their containing and releasing benzidine or o-dianisidine, but they are unlikely to produce immediate death from accidental ingestion, as mordants may.

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