Mordant Dyeing

The term ‘mordant’ is derived from the Latin ‘mordeo,’ which means to bite or to take hole of. Mordant dye have no affinity to textile fiber, they are attached by a mordant, which can be an organic or inorganic substances. The most commonly used mordant is inorganic chromium, so sometimes this dye is called chrome dyes. Other inorganic mordants are Al, Cu, Fe and organic mordant. Tannic acid is rarely used.

Mordant improves the fastness of the dye on the fibre such as water, light and perspiration fastness. The choice of mordant is very important as different mordants can change the final colour significantly. Most natural dyes are mordant dyes and there is therefore a large literature base describing dyeing techniques. Fiber most readily dyed with mordant dyes are the natural protein fibers, particularly wool and sometime synthetic fibers modacrylic and nylon.

It is often noted that when a mordant dye forms a lake with a metal, there is a strong colour change. This is because metals have low energy atoms. The incorporation of these low energy atoms into the delocalised electron system of the dye causes a bathochromic shift in the absorption. It is this delocalised electron system which is fundamentally responsible for colour in dyes. Since different metal atoms have differing energy levels, the colour of the lakes may also differ.

The most commonly used mordant dye is undoubtedly hematein (natural black 1), whose status as a natural product supercedes its mode of dyeing, apparently. Others are eriochrome cyanine R (mordant blue 3) and celestine blue B (mordant blue 14), both used as substitutes for alum hematoxylin but with a ferric salt as the mordant. Alizarin red S (mordant red 3) is valuable for the demonstration of calcium, particularly in embryo skeletons

Reason for so named

Some natural and synthetic dyes can be applied or fixed on wool and other textile fibers with the help of an auxiliary chemical called a mordant. These dyes are therefore called mordant dyes.

The mordant have affinity both for a fiber can be applied by using a mordant.

In wool dyeing, only chromium salts are of importance and hence mordant dyes for wool are usually called chrome dyes.

Classification

On the basis of origin

1. Natural: Alizarine

2. Synthetic: Acid chrome

1. Natural Mordant dye

Alizarine: Alizarine is an example of natural mordant dye. It is obtained from the root of the madder. Alizarine is known as polygenetic mordant dye because it develops a variety of colors on different mordants.

Mordant Color

Al Red

Sn pink

Fe Brown

Cr puce brown

Cu yellowish brown

Haematin: this is extensively used before and only one still in use found from logwood. It yield navy blue or black colors of good fastness with chromium compounds. This is used in nylon and wool.

Recipe: haematin dye: 8-10%

Acetic acid: 1cc/l [pH 4-6]

temperature: 50-90°C

Time: 2hr

As it is time consuming process, the natural mordant dyes are used in lesser extent.

2. Acid mordant dyes:

Acid color + chromium = acid chrome

Few dihydroxy azo dyes could co-ordinate so easily with chromium and they could be dyed as acid dyes and mordanted by aftertreatment with K or Na dichromate. Such dyestuffs are known as the acid mordant dyes & are used extensively for wool & also for polyamide fibers.

They have good wet fastness and most of them possess satisfactory light fastness.

Methods of dyeing:

There are three general methods of application of mordant dyes as described below:

1. Chrome mordant process: two bath process

First bath: mordanting with insoluble chromium hydrate

2^nd bath: dyeing

2. Afterchrome process: two bath/single bath process

First bath: dyeing

2^nd bath: mordanting with chromium

3. Methachrome (or chromate) process

Dye + mordant (dichromate) in same bath

Mechanism of dyeing

Fig shows an example of mordant dye and shows the formation of dye mordant complex. The chromium cation has a valency of 6 (i.e. 6 bonds) which represented by six lines toward the chromium cation.

The mordant dye is shown to the Cr cation by three of the six bonds. The other three bonds have molecules of water attached to them. It is thought that the three molecules of water are there as an intermediate step only and will gradually be water replaced by another mordant dye anion. Thus two mordant dye molecules form a complex with the Cr cation to form a lake or a dye chromium complex. The formation of these relatively large complexes results in very good wash fastness of dye.

Dyeing procedure of Alizarine dyes

· Boil cotton fabric in solution of 1 part TR oil and

10 part water for 12 hrs

· Dry at 40-60°C

· Treat again with 10°Tw acetate at 60°C for 2hrs

· Dry at 40-60°C

· Again treat with 2 part Sodium phosphate

10 part water at 30-45°C

· Dye 1-1.5% shade with calcium acetate at room temperature for 20 mins

· Wash for 30 min at 70°C

· Soap wash, dry

Dyeing of wool with synthetic mordant:

Dyeing recipe

Dye 1-5%

Acetic acid (80%): 2-5% [pH 4-5]

Glauber salt: 10-25%

H₂SO₄: 1%

L:R= 1:20

Time 45-60 min

Mordanting recipe:

K dichromate: 2%

Temperature: 80-100°C

Time 45 min

M:L = 1:20

Dyeing procedure

· Prepare the dye bath with acetic acid and glauber salt.

· Temperature of the bath is raised to 50-60°C and the goods are entered

· The liquor is brought to boil for 30 min

· H₂SO₄ is now added to complete exhaustion and boiling is continued for another 30 mins

· When dye bath is exhausted and boiling has been continued for long enough for the color to be level, the temperature is allowed to drop and Potassium dichromate is added

· Chroming is continued at boil for 30 min

It is extremely important to make certain that goods are dyed uniformly before the dichromate is added because there will be no further migration afterward. The dye liquor must also be virtually exhausted before mordanting, because of dye remains in solution it will be precipitated on the surface of the fibers in form of its insoluble lake and cause poor rubbing fastness.

Disadvantages:

· Color matching is difficult as the process of mordanting means that the color builds up gradually

· Length periods of applications are both detrimental to protein and polyamide fibers and rather costly.

· Dichromate salts become pollutants once they are discharged into sewerage.

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