9 Natural Mordants

S. Karpagam Chinnammal

1. Introduction

Natural dyes belong to one of two classes substantive and adjective. Substantive type does not require a mordant, and adjective, which includes majority of the natural dyes, requires the use of a mordant to fix the color to the fiber. For substantive dyes, the use of a mordant, though not a necessity, is still sometimes an advantage, particularly to obtain a variety of shades. So, mordants are an integral part of natural dyeing.

2. Learning objectives

• To understand the need for mordants in natural dyeing
• To learn about the types of mordants and the methods of mordanting
• To get familiar with the procedure for dyeing of different fibers using natural dyes
• To get an insight into the fastness properties of natural dyes

3. Mordants

The term Mordant is derived from the French ‘mordre’, meaning to bite. Mordant is a substance which has an affinity for both fiber and dye stuff and so is used to fix dyes on fibres. Most of the natural colourants are adjective in nature and have no substantivity on cellulose or other textile fibres, they require an intermediate to fix into the fibre, which is called a mordant. Mordants does not serve as a colour source on its own. Mordant is a substance which helps a chemical reaction to take place between a fiber and a dye. The mordants which are usually metallic salts form metal complex with the fiber during mordanting, and during the dyeing process it attracts the dyeand creates a coordinating bond and attaches the dye firmly to the fabric. Thus, for proper fixation of natural dyes on any textile fibre, mordanting is essential in most of the cases.

The scheme of reaction of mordants with dye and fibre is illustrated below:

4. Types of Mordants

Mordants come from primarily two groups – plant based, especially plants high in tannins and mineral based such as alum, iron, tin, copper sulphate and chrome. In Ancient times mordants were found in nature, albumen from egg, ox-blood, urine, cowdung, rotten mud etc was used as natural mordant. Nowadays most dyers use mineral mordant’s .

Different mordants give different colour or shade with the same dyestuff. For example, Cochineal gives

Mordants also improve the dye pick up and fastness of colour to light and washing. Therefore, final colour, their brilliance and colour fastness properties are not only dependant on the dye itself but are also determined by varying mordants.

A good mordant should:

•  Produce good colour yield at low cost
•  Physical properties of fibre or fastness properties of dyes must not be affected
•  It must not cause harmful effect during processing
•  Dyed material should be free from carcinogens

Mordants are classified into three categories:

4.1. Metalic salts or Metallic mordants

Salts of aluminium, chromium, iron, copper and tin fall under this category. The metallic mordants may further be classified as

4.1.1. Brightening mordants

1. Alum: it is the most commonly used mordant. It is a naturally occurring metallic mordant available in two forms aluminium potassium sulphate and aluminium ammonia sulphate. Potassium aluminium sulfate is economical, easily available and safe. It does not affect colour. It improves and brightens the final colour. More alum gives sticky and harsh feel to textiles. It is usually used with cream of tartar which helps evenness and brightens slightly.
2. Chrome (potassium dichromate):.Chrome is good for obtaining yellows. It is also called red chromate. It is expensive. Due to the presence of heavy metal beyond certain limit, its use is limited as per the eco norms. It is light sensitive and so changes colour when exposed to light.

• Tin (stannous chloride): Tin brightens colours than any other mordant. It brightens especially reds, oranges and yellows. It gets oxidized when exposed to air and imparts a rigid hand to the fabric making it weak. If applied above a certain concentration it causes loss of tensile strength.

4.1.2. Dulling mordants

1. Copper (cupric sulphate): Called blue vitriol, is soluble in water and is easily applied. It saddens colours. It gives some special shades, which are otherwise difficult to obtain. It contains heavy metals so beyond a certain limit it is objected by the eco-standard norms.
2. Iron (ferrous sulphate): is one of the oldest mordants and is easily available. It is also called green vitriol and is soluble in water. It is used for darkening /browning and blackening of the colours/ shades. It normally gives grey to black shades Alum and iron are the most environmentally friendly of the mineral mordants while chrome, tin and copper are considered more toxic. Some additional chemicals used with natural dyes, like cream of tartar, acetic acid and vinegar as well as the plant based mordants and tannic acid are also safe to use.

4.2. Tannins and Tannic acid

Tannins are naturally occurring mordants. Tannin is an astringent vegetable product found in a wide variety of plants. Tannins are high molecular weight water soluble polyphenoic compounds having capacity of gelling under certain conditions. Tannic acid give dark shades ie brown and grey shades. Pre treatment with tannic acid followed by metal salt treatment in cotton introduces additional hydroxyl and carboxyyl groups in the fibers. Tannins themselves do not act as mordants but tannin –metal salt combination can act as a mordant for the natural dye. Myrobalan (harda) and galls/sumach are the commonly used mordants in dyeing of textiles. Pomegranate fruit rind is also rich in tannins.

4.3. Oils type mordants

Vegetable oils or Turkey red oil (TRO) fall under this category. TRO as mordant is chiefly used in dyeing of deep red colour from madder. Alum is soluble in water and so has no affinity for cellulose and is easily washed away from the fabric. TRO forms a complex with alum when used as a main mordant. Sulphonated oil posses better binding-capacity than the natural oils. Samples mordanted with oil mordants displays excellent fastness and colour.

5. Methods of Mordanting

There are two processes concerned with the dyeing of vegetable colours. The first is mordanting and second is colouring. Mordanting is a treatment which prepares the material to be dyed to receive the dye.

Mordanting can be accomplished in three ways –

  5.1. Pre mordanting: In this method, the material is entered into a mordanting solution at room temperature and it is gradually raised to the optimized temperature e.g 70 – 100 °C. Mordanting is continued for optimized time say 45 minutes with a ML ratio of 1:30 and then dried. The mordanted textile material is then dyed. After dyeing, the textile material is washed and soaping is carried out.

5.2. Simultaneous mordanting: In simultaneous mordanting, the material is immersed in a bath containing both the mordant and dye and dyeing is started at optimum condition. Dyeing auxiliaries are added as required for the dyeing process. After dyeing, the textile material is washed properly and soaping is carried out. Advantage is it’s a single process .It is not suitable for fibre’s which will be damaged quickly.

5.3. Post mordanting: In post-mordanting method, the dyeing is carried out on bleached material and the dyed fabric is treated in a bath containing the mordanting solution. The material is washed properly and soaping is carried out.

Sufficient time should be allowed for the mordant to thoroughly penetrate the fiber. If the mordant is only superficial, the dye will be uneven: it will fade and will not be bright .

6.    Dyeing process for various fibres using natural colourants

The main factors to be considered in natural dyeing are

i) Nature of material to be dyed: Based on the chemical nature of the fibres, dyeing can be carried out in acidic, alkali or neutral bath. Animal proteins, like wool and silk are dyed best in acidic conditions and are weakened by alkaline. If it is dyed in alkaline condition, it is best to end with a diluted vinegar rinse to restore a slightly acidic pH to the fibers before they dry. Plant material like cotton, flax are best in alkaline conditions and are weakened by acids. If cotton is dyed in acidic condition, it is best to end with a weak washing soda bath to restore the fibers to slightly alkaline before they dry

ii)Measurement of mordants and dyestuff: Ingredients are specified in weight rather than measures. It is expressed as a ratio to fiber weight.

iii)Temperature: Different dyes work better at different temperatures. Most plant dyes benefit from being heated, but some ex madder change colour if allowed to boil. Sappan wood also has a tendency to change colour when heated for a long time. Some dyes work best at lower temperatureex safflower and indigo

iv)Agitation: For getting even dye uptake, one should move the fibers around as much as possible during dyeing. But, when wool is heated and agitated it tends to felt, so we be gentle to avoid felting.

6.1. Cotton

The fiber must first be cleaned before natural dyeing. Desizing, scouring and Hydrogen peroxide bleaching are normally followed. Natural dyeing of cotton is more difficult than silk and wool. Cotton is not very porous and will not hold the dyestuff without complicated preparation, so mordanting is very important for cotton dyeing. The usual method of preparing linen or cotton is to boil it first with an astringent such as tannic acid or a mordant high in tannins such as myrobalan, sumach or gall nuts and then following with a mineral mordant. Cotton has a natural attraction for tannic acid, so once steeped in its solution it is not easily removed by washing. Tannic acid aids the attraction of the coloring matter to the fiber and adds brilliancy to the colors.

So well prepared cotton material is mordanted with tannic acid solution. It is rinsed and treated in a solution of alum. Mordanting is carried out with M:L ratio as 1:30.The material is entered at room temperature and gradually raised to 90 °C. Mordanting was continued at this temperature for 45 minutes. After mordanting the fiber is squeezed. Dyeing is carried out as follows using standardized conditions. The wet mordanted material is entered into a dye bath with concentration of 50% natural dye (owm) with a material : liquor ratio of 1:30 at room temperature and the temperature is gradually raised upto 90-100°C and maintained for an hour. The pH is maintained at 10. Occasional stirring is done for even dyeing. It is allowed to cool and the dyed material is rinsed in cold water and soaping s done with 2g/L soap solution at 60 °C for 15 min followed by washing and drying. Cationic dye fixing agent may be used to improve its wash fastness.

6.2. Wool and Silk

Wool is easiest fibre to dye with natural dyes, resulting colours are fast to light and washing when used with proper mordant. Silk can also be dyed but we must be very careful with use of strong alkali as it can eat silk. Wool and silk absorbs mordants well and if chrome and iron are used as mordants there is no need for assistants. Alum is the mordant commonly used for animal fibres as it helps achieve clear, vivid colour from the dye. They are dyed through premordanting or post mordanting process.The mordant and dye concentration are determined based on the depth of shade required.

In premordanting, 10-20 % (owm) mordant is dissolved in water maintaining M:L ratio 1:40 and temperature is raised to 30°C, wet fibre is immersed in mordant solution. Temperature of mordant bath is raised till 90°C and left at this temperature for 1 hour with constant stirring. The mordanted sample is rinsed in water to remove superfluous mordant. The mordanted material is saturated in the dye bath. Dyeing is performed at 90°C for 1 hour, with M:L ratio 1:40. Acidic pH 4.5 is maintained (both the natural protein fibres wool and silk are prone to alkali attack ie at pH greater than 9). The dyed material is washed with 2g/l non ionic detergent at 60°C for 10 minutes , cold washed and dried. In post mordanting process, dyeing is done at optimum condition and the dyed material is dipped in a mordant bath, followed by rinsing , soaping and drying.

For silk the procedure is the same as wool except that the dyeing of silk at high temperature for long periods degrades the quality of silk hence material is not heated in the dye bath as a high temperature will spoil the natural luster of silk.

6.3 . Jute and other lignocellulosic fibres

Like cotton fibres, double premordanting process is best for lignocellulosic fibres. It is normally treated with tannin followed by mineral mordant like alum. Bleached and mordanted lignocellulosic fibres, are dyed with natural dyes in alkaline pH (pH≥10) at temperatures of 85-90°C with ML ratio 1:20 for 60 minutes. After dyeing, the material is washed. For improving the fastness property the samples may be treated with cationic dye fixing agents.

6.4. Synthetic fibres

Synthetic fibres are dyed with natural dyes through high temperature high pressure method at acidic pH with or without mordants. Mordant is used mainly to get a range of colours from the same dye. The dye used must be stable at high temperatures.

In recent times Newer energy efficient dyeing processes like Ultrasonic energized dyeing condition, Ultrasonic and sonicator assisted dyeing are adapted to natural dyes.

7.  Colour fastness properties of natural dyes

Colourfastness of dyed fabric denotes the resistance which the fabric posses to change in its colour characteristics or transfer of its colour to adjacent white material when subjected to the action of various agents such as light, washing, rubbing and perspiration, dry cleaning etc .

Among all the types of fastness lightfastness, wet fastness and rubbing fastness is considered generally and perspiration fastness is important for apparels alone

7.1  Light fastness

Light fastness of many natural dyes especially those extracted from flower petals are found to be poor to medium. Most natural dyes have poor light stability compared to synthetic dyes. Both washfastness and lightfastness improve after mordanting

Most yellow dyes have fastness grading equal to or less than good. Red, blue, brown and black vegetable dyes exhibit good to excellent light fastness. Yellow dyes obtained from plant materials are usually pale i.e the depth of shade is low and so fading is quicker. The brighter yellow shades like turmeric is less fast to light. Mordants greatly influence fading of yellow dyes. Use of tin alum mordants results in significantly more fading than chrome, iron ad copper mordant .Red dyes-are based on anthroqunone and are little affected by light. They are stable to light. Blue colour is usually obtained from indigo, its light fastness on wool is much higher than on cotton and 1-2 points higher than on silk. Brown and black dyes-are obtained from plant sources rich in tannin eq pomegranate skin, myrobalan etc Tannins combine with ferrous sulphate to form complexes, which give grey-brown shades. Since the colours are deep and dark lightfastness is good. Lac dye on wool with metallic mordants had very good fastness to light (Mohmed Ibrahim Khan etal 2012 ). Babool and catechu showed good light fastness, Light fastnes of Lac and Turmeric varied from moderate to good on natural fibres (Teli etal 2014) Light fastness of wool dyed with tea was good and it improved after mordanting ( Varindar Kaur etal , 2012) .Hibiscus flower exhibited fair to good lightfastness ( Subha babel etal., 2014).Beetroot dye exhibited poor fastness to light on bamboo fabric ( Divya ojha and Sangita Deodiya 2014) .Garcina indica bark on cotton showed fair fastness to light ( Mamatha G Hegde 2015). Colourfastness of samples dyed with leaves, bark, bark peel and fruit of jamun using babul bark as mordant showed good lightfastness ( Hemalatha jain 2013)

7.2 Wash fastness

Most natural dyes have only moderate washfastness. The alkali present in washing detergent may also affect the dye. Wash properties of yellow dyes range from fair to excellent. Red dye is stable to washing. Blue dye usually indigo is applied in soluble form but once it is inside the fiber it gets oxidized to insoluble form and so has excellent washing fastness. Tannins-black being complex high molecular weight have good affinity for cellulosic and protein and has good fastness to washing. Lac dye on wool with metallic mordants have moderate to very Good fastness to washing (Mohmed Ibrahim Khan etal 2012). Babool showed very good wash fastness while Catechus wash fastness varied from good to very good, Wash fastness of Lac and Turmeric varied from fair to moderate on natural fibres (Teli etal 2014). Hibiscus flower exhibited fair to good washfastness ( Subha babel etal., 2014).Beetroot dye exhibited poor fastness to staining during washing and very poor fastness to washing on bamboo fabric ( Divya ojha and Sangita Deodiya 2014). Garcina indica bark showed fair fastness to washing on cotton( Mamatha G Hegde 2015). Colourfastness of samples dyed with leaves, bark, bark peel and fruit of jamun using babul bark as mordant showed good washfastnes ( Hemalatha jain 2013). Washfastness of wool dyed with tea improved after mordanting ( Varindar Kaur etal , 2012)

7.3.Rubfastness

Rub fastness of most natural dyes have been found to be moderate to good. Lac dye on wool with metallic mordants showed fair to Good fastness to wet rubbing and good to very good fastness for dry rubbing (Mohmed Ibrahim Khan etal 2012). Beetroot dye exhibited good fastness to dry rubbing and fair fastness to wet rubbing on bamboo fabric ( Divya ojha and Sangita Deodiya 2014). Garcina indica bark had excellent fastness to rubbing on cotton ( Mamatha G Hegde 2015)

8.Conclusion

This module has highlighted on the need for mordants, types of mordants and its effects on colour, mordanting techniques compatible with natural dyes, dyeing processes for various fibres and fastness properties of natural colourants.

Natural dyeing is a necessity for a sustainable future and mordants play an important role in natural colouring. Continuous research efforts are being carried out to improve the draw backs like non reproducibility and poor colourfastness in natural dyeing.

REFRENCE

1. Sankar Roy Maulik and Kunal Bhandari, Dyeing of Silk and Wool with Madhuca indica, Asian dyer, oct-nov-2012, p-42
2. M.D.Teli, Sanket P.Valia and Shruti Venkatram, Eco friendly colouration of natural fibre Pandanus Utilis, Asian Dyer, Feb-March-2014, p.24-26
3. Varinder Kaur etal., Dyeing of wool with Tea as a natural dye, Colourage Feb 2012, P.42.
4. Mohamed Ibrahim Khan etal., Studies on application of lac natural dye on wool using ecofriendly metal mordants
5. Dr. Subha babel et.al., Natural dye from Hibiscus Rosa (Gundhal) flower on cotton, Textile trends, Dec 2014 , p.5
6. Divya Ojha and Sangita Deodiya, Development of different dye shades of beetroot dye with natural mordant (myrobalan) on bamboo fabric, Textile trends, June 2014 p.43
7. Rakesh kumar and RameshwarDayal, Utilization of tageltes minnta stem for extraction of natural dye, Asian dyer, Feb-march 2012, p.37
8. Mamatha G.Hegde, Application of Garina Indica bark and fruit dye on organic cotton fabric, Asian dyer, Sep 2015, P.28
9. Hemalatha Jain, Dyeing of cotton fabrics with extract of jamun tree and its by products using natural mordants, Colourage, November 2013, P.42-43