Application of tannins
Tannin in leather processing [89]
Leather making is an age old antique craft, which has been in existence since 1500 B.C. Tannins are used to convert animal hide into leather using “tan liquor”, which in term gives greater stability and resistance to water, heat,
bacteria and abrasion. During this tanning process, the collagen chains in hide are cross linked by tannin to give
Tannin in medicine [92]
Most of the Indian systems such Ayurveda, Siddha and Unani formulation have churana, rasa, bhasma, sindura and
khurs, etc. contains many types of tannins as ingredients. Tannins are astringent in nature and used in Ayurveda and
Siddha tooth powder in order to strengthen the gums. They also promote rapid healing and the formation of new tissues on wounds and inflammed mucosa. Tannins are used in the treatment of varicose ulcers, haemorrhoids, minor burns, frostbite as well as inflammation of gums. Internally tannins are administered in cases of diarrhoea,intestinal catarrh and in cases of heavy metal poisoning as an antidote. In recent years, these compounds have demonstrated their antiviral activities for treatment of viral diseases including AIDS.
Tannin in textiles
Tannins are used as mordant in dyeing, manufacture of ink, sizing paper and silk, and for printing fabrics. Tannins are significant in textile dyeing for several reasons. Firstly, they produce indispensable mordants for the dyeing of
vegetable fibres like cotton and linen. Secondly, they are very often associated in the plants with the yellow orange,
red and violet colourants, whose colour is reinforced by them in the dye bath by their own pigment. Materials dyed
with tannins have good fastness for washing and light. Harda powder is used as mordant throughout India by the vegetable dyers and printers since ancient times and so far there is no substitute for harda known to the users which is commercially viable and can replace harda by achieving the same. The study reported by Rafai et.al (1992) [93] states that the fabric to be printed in the Multani style was mordanted with myrobalan after removal of starch and treatment with oil. Divi-divi (Caesalphinia coriaria) fruits are used as a mordanting agent in the textile dyeing
process [94]. Gulrajani et.al (1992) [95] studied dyeing of wool with turmeric using several mordants such as alum (10%), copper sulphate (5%), iron (3%), potassium dichromate (5%), tin (1%), tannic acid (5%) and harda (10%) as
a natural mordant. The mordant application resulted in increased a* and b* values of the dyed samples. These values
were higher for harda mordanted samples than many synthetic mordants. Also, harda mordanted samples showed
better wash fastness than alum, tin mordanted samples and it was equal to iron and copper mordanted samples. Jain
[96] reported that the Weavers Service Centre, Delhi had developed a style of printing wool with vegetable dyes
which involved the treatment to the scoured woollen cloth with a solution containing tannin followed by printing with a synthetic mordant mixed with thickener. Therefore, the fabric is steamed and developed in a bath containing vegetable dye, resulting in the development of different colour on printed portions. Teli et.al (2001) [97] reported the use of eco-friendly mordants namely alum and harda on denim with onion extract. The uses of alum or harda alone or in combination as a substitute were employed. The results produced brilliant shades.
Oil-mordants
Oil mordants are used mainly in dyeing of Turkey Red colour from madder. The main function of the oil mordants is
to form a complex with alum used as the main mordant. Since alum is soluble in water and not it has affinity for
cotton, it is easily washed out from the treated fabric. The natural occurring oil contains fatty acid such as palmetic,
Stearic, oleic etc., and their glycerides. The –COOH group of fatty acid react with metal salts and gets converted in to –COOM, where M denotes the metal. Subsequently, it was found that the treatment of oils with concentrated
sulphuric acid produces sulphonated oils which possess better metal binding capacity than the natural oil due to the introduction of sulphonic acid group, -SO3H. The sulphonic acid can react with metal to produce –SO3M. This bound metal can form complex with mordant dye such as madder to give Turkey Red colour of superior fastness and
hue. Therefore, the sulphonated oils were called Turkey Red Oils (TRO). The term TRO is now used only for the
sulphonated castor oil. TRO is also forerunner of the modern synthetic surfactants.
Dyeing using plant dyes and mordants
Dyeing of cotton
Cotton is a cellulosic fibre. The most commonly used dyes for cotton are reactive dyes, direct dyes, vat dyes and
sulphur dyes. The reactive dyes form covalent bond with hydroxyl groups of cotton fibre. Acid dyes and cationic
dyes lack affinity for the cotton fabric and only surface deposition occurs due to the ionic attraction. Natural dyes
can be fixed on cotton with the help of natural or metallic mordant. The natural mordant also acts as primary
mordant for metallic salts. For instance, cotton on treatment with tannic acid can absorb all types of metallic
mordants. The metallic mordants form complex with the carboxylic groups of tannic acid. Generally, cotton lacks
affinity for the natural dyes. Hence, the use of mordant is necessary for the application of natural dyes, acid and basic dyes on cotton.
Dyeing of wool and silk
Wool is more receptive towards mordants. This is due to its amphoteric nature wool can absorb acids and bases
equally and effectively. When wool is treated with a metallic salts it hydrolyses the salt into an acidic and a basic
component. The basic component is absorbed at the –COOH groups and the acidic component is removed during
washing. Like wool, silk is also amphoteric and can absorb both acids as well as bases.
Fastness properties of natural dyes
Fastness is the fundamental requirement that coloured textiles should exhibit, to the conditions encountered during
the processes following colouration and during their subsequent usage. The fastness requirements, however, are
largely determined by the end-use of dyed fabrics. Some of the basic fastness properties required are as follows:
Light fastness
Most of the natural dyes have poor light stability as compared to that of the best synthetic dyes, and hence the
colours in historical old textile are often different from their original colours. Fading of colour on the textile occurs
on exposure to light. This degradation occurs when light breaks chemical bonds in dyes. Sunlight is made up of ultra
violet light, visible light and infrared radiation. While short wave UV causes most of the physical property damage
to fibres, it is generally the longer wave UV and visible light that causes dyed textile fade. Poor light fastness of
some of the natural dyes can be attributed to propensity of the dye chromophore to the photochemical oxidation. The
chromophore in some classes can be protected from photochemical oxidation by forming complex with transition
metals, where by a six member ring is formed. The photons sorbed by the chromophoric group dissipate their energy
by resonating within the ring and hence dye is protected. The post mordanting process with metal increases the light
fastness of natural dyed samples. However, the post mordanting with metal salts also results in change in hue of the
dyed fabric, hence the post treatment should be selected rightly. Attempt by Prabhu et.al (2011a,b) [98, 99] has been
made to improve the light fastness properties of popular natural colourants namely turmeric, henna, madder and
pomegranate rind using natural mordants namely tamarind seed coat and emblica officinal’s tannins and also
metallic mordant (safer limit) combinations. The rate of fading has been greatly reduced due to the combined effect
of natural tannins and metal mordant bonding. Additional, the dyed fabric resulted in good antimicrobial protective
fabric using this eco-friendly process.
Washing fastness
Some of the natural dyes undergo a little change in their hue on washing, this may be due the alkaline nature of the
washing mixture mainly the pH. In general, natural dyes (on wool) have only moderate wash fastness as assessed by
the ISO 2 test. However, logwood and indigo dyes exhibit better fastness when applied to different textiles. The
nature of detergent solution suitable for conservation of natural coloured art work has been examined [100]. A liquor
containing 1g/l of sodium polyphosphate is found to be best resulting marginal changes in hue with natural dyes
applied on wool or silk [101]. The small increase in cleaning efficiency attributable to the alkali must be balanced
against possible colour change in the natural dyes, apart from possible damage to the protein fibre under alkaline
conditions. In general, the fastness of a colour can vary with the type of dye, the particular shade used, the depth of
shade and the dyeing process and mainly the nature of the washing mixture.
Some of the shades obtained using natural colourants and modrants are shown in Table 5.