Abstract
The application of natural dyes on textile materials has been gaining popularity recently, because of increasing awareness of environment, ecology, and pollution control. However, the need for metal mordants limits their use. An attempt was made to optimise the use of metal mordant, so that while fast shades and antibacterial properties were observed, safer limits of metals were not exceeded.
Wool and silk fabrics were treated with copper sulphate of different concentrations ie. 1% to 5%, and dyed with natural dyes by a pre-mordanting and dyeing process. The amount of copper ion (ppm level) on wool and silk fabrics before and after dyeing was determined using atomic absorption spectroscopy. The colour depth, wash and light fastness of wool and silk fabrics were tested. Simultaneously, the antibacterial activity was tested on dyed wool and silk fabric with and without mordanting.
The mordant concentration showed positive influence on the depth of dyeing as well as on lightand wash-fastness properties, as compared to the dyeings obtained without the use of mordant. Mordanted and dyed wool and silk fabrics both exhibited enhanced antibacterial activity. The uniqueness of this process will provide consumers with a hygienic product, offering freshness and safety to their health.
Keywords: Antibacterial Activity, Natural Dyeing, Metallic Mordant, Wool, Silk
1. Introduction
Synthetic dyes involve use of petrochemicalbased raw materials and violent chemical reactions for their synthesis. The manufacture of such dyes is energy-intensive, with adverse impact on the environment adding to pollution. Many of these dyes, especially the azo-based ones, are found to be carcinogenic.
As the awareness of the consumers about environmental problems related to the manufacture and application of synthetic dyes increases, there has been increasing interest in the application of natural dyes, which are obtained from plants, insects or mineral-based sources (Al-Amoundi and Osman, 2009). The present paper focuses on the application of herbal colorants, which are found in abundance and are a renewable source, offering sustainability of operation.
Although they are non-toxic, non-hazardous, biodegradable and thus eco-friendly, their limitations include poor affinity towards textile, lack of brilliancy of shades and very low amounts of colouring compounds in the herbal raw materials, which impairs the economy of the operations. Natural dye, having limited substantivity for the fibre, requires use of a mordant that enhances the fixation of the natural colorant on the fibre by the formation of a complex with the dye.
This complex may be formed by first applying the mordant and then dyeing (pre-mordanting process), by simultaneous application of the dye and the mordant (meta-mordanting process) or by after-treatment of the dyed material with the mordant (post-mordanting process). Some of the important mordants used are alum, potassium dichromate, ferrous sulphate, copper sulphate, zinc sulphate, tannin and tannic acid (Maulik and Pradhan, 2005; Nalankilli, 1997). Although some of these metal mordants contribute in developing a wide gamut of shades after complexing with the natural colouring compounds, most of these metals are toxic in nature and their presence is found to be safe for the wearer only in trace quantities.
In order to promote the concept of ecofriendly textiles, a comprehensive system of ecolabels is advocated by European and other western countries. A comparison of the norms/criteria stipulated for eco parameters in the popular eco-labels operating in Europe and in the Indian Eco Mark Scheme for textiles is given in the literature (Nadiger, 2007).
Today, the market is governed by consumers who are increasingly aware about environmental protection and their rights. Hence they look for garments that can offer comfort, hygiene and freshness. Although most of the natural textile materials made of cotton, wool, silk, etc, are susceptible to microbial attack, wool and silk, being proteinic in nature, act as a food for micro-organisms and are more prone to microbial damage. As both of them are relatively expensive fibres, preference in imparting them bacterial resistance becomes economically important.
It is observed that micro-organisms cause the degradation of the polymer chains, bringing down the strength and the durability of such products. Pathogenic micro-organisms such as bacteria, mould, fungi and their crossinfection not only degrade the fibre substrate or discolour it, but also give rise to foul odour, staining and skin rashes. Hence, textiles finished with antibacterial finishes are prefereed by the modern consumer.
A variety of compounds, both natural and synthetic chemicals, have been used for antimi- crobial finishing. Some of the important natural products used are neem oil, clove oil, cashewshell oil, etc. Certain metal-salts complexes, such as copper, zinc, zirconium and silver, on textiles have also exhibited durable antimicrobial activities towards S.aureus, and also gave an 80% increase in rot resistance (Teli, 2005; Teli and Prabhu, 2006).
Although synthetic-chemical formulations are used as antibacterial finishing agents, it is always desirable to investigate the possibility of such properties being imparted to textile material when it is treated with natural colorants, using metal mordants. The aim of this work is to optimise the concentration of copper-suphate mordant in the application of natural dyes on wool and silk, within the safe limit (ie. up to 100ppm), so that on one hand natuarally dyed wool and silk textiles are obtained with all-round performance and, on the other, barrier properties against bacteria are imparted to these textile substrates.
2. Experimental
2.1 Materials
Ready for dyeing wool (164 gsm) and silk (42 gsm) fabrics were purchased from Kiran Threads, Vapi, India.
For microbiological testing, Gram-positive bacteria Staphylococcus aureus (S.aureus) (NCTC 3570) and Gram-negative bacteria Escherichia coli (E.coli) (ATCC 10148) were procured from Haffkine’s Institute, Parel, Mumbai, India.
The natural dyes: Curcuma longa L. (Turmeric), Punica granatum L. (Pomegranate rind), Lawsonia inermis L. (Henna) and Rubia cordifolia L. (Indian madder) were obtained from a local ayurveda market in powder form, Mumbai, India.
Metal mordant: Copper sulphate of analytical grade was obtained from S.D. Fine Chemicals Ltd, Mumbai, India.
2.2 Methods
2.2.1 Extraction of Natural Colorants
All the colorants were extracted using water as the medium. The colorants in the form of fine powder were mixed with 100ml of water and allowed to soak for half an hour, and the mixture was then boiled for one hour. The contents were cooled and filtered. The filtrate was used for dyeing.
2.2.2 Mordanting
Pre-Mordanting: Both wool and silk fabrics were treated with copper-sulphate solution, using 1% to 5% concentrations on weight of the fabric (owf), and this treatment was carried out at 75°C for 45 min, keeping the liquor to material ratio at 20:1. After mordanting, fabrics were subjected to dyeing.
2.2.3. Dyeing
The shade used for dyeing wool and silk fabric was 10 % owf. Dyeing was carried out in Rota Dyer machine, Rossari Labtech, Mumbai, keeping the liquor to material ratio at 20:1. Fabrics were introduced into the dyeing solution at room temperature and slowly the temperature was raised to 85°C. The dyeing was continued at this temperature for 60 min. After dyeing, the fabrics were rinsed and air-dried.
2.2.4 Determination of Metal Ion by Atomic Absorption Spectroscopy
The amount of copper ion on the mordanted and dyed fabric was determined by atomic absorption spectroscopy. The fabric (1cm x 1cm) was placed in a test tube and 2 ml of 60% nitric acid was added. The test tubes were heated in a water bath at 90°C, and after 1 h, sample fabric had been dissolved completely. The copper concentration of the solution was then measured with Atomic Absorption Spectroscopy (Kobayashi, 2002).
2.2.5 Evaluation of Colour Strength
Dyed samples were evaluated for their colour strength by determining K/S values using a Spectra Flash SF 300 computer colour-matching system supplied by Datacolor International, USA. An average of four readings, taken at four different sample areas, was used to get the reflectance values and Kubelka Munk function (K/S) (Etters & Hurwitz, 1986).
K/S = (1-R)2/2R
where R is the reflectance at complete opacity; K is absorption coefficient; S is the scattering coefficient. Dyed fabrics were also evaluated in terms of CIELAB colour space.
2.2.6 Antimicrobial Activities of Mordanted and Dyed Fabrics
Quantitative assessment of antimicrobial activity exhibited on mordanted and dyed cotton and silk fabrics against both micro-organisms were carried out by AATCC Test 100-200414. To evaluate the antimicrobial activities of the treated fabrics, the reduction in number of bacterial colonies formed with respect to the untreated control after incubation (37±1°C, 24 h) was determined. The percentage reduction was calculated with the following equations:
Reduction rate (%) R= (B-A)/B x 100 where R = % reduction in bacterial count; A = the number of bacterial colonies recovered from the inoculated, treated test-specimen swatches in the jar, incubated for a 24 hr contact period; B = the number of bacterial colonies recovered from the inoculated, untreated control test-specimen swatches in the jar, immediately after inoculation (at ‘0’ contact time).
2.2.7 Wash and Light Fastness Properties
Dyed fabrics were tested for colourfastness to washing according to test method ISO 105- C10-2006. Dyed fabrics were tested for colourfastness to light according to AATCC Test Method 16-2004.
3. Results and Discussion
3.1 Colour Strength
Results in Table 1 indicate that the K/S values of wool and silk increased with the increase in the concentration of mordant, even though no dye was used. The greenish colour imparted was due to increased fixation of copper-sulphate mordant. When these fabrics were dyed, with all the four dyes without mordant, there was a distinct increase in K/S values, indicating dyeing of these fibres. However, it is well established that the fastness properties of such dyeings are inferior and there is always a limitation on the shade build-up if mordants are not used. Hence mordanting becomes essential for dyeing with natural dyes.
The copper-sulphate mordanted wool and silk fabrics were then dyed with pomegranate rind, turmeric, henna and madder (10% shade owf). K/S values of the mordanted wool and silk dyed with all these natural dyes further increased as the mordant concentration increased. Since the concentration of dye in the solution was kept constant, this increase in the depth of dyeing was due to the increase in the concentration of copper sulphate acting as a mordant. Cu (II) ions forms a complex with the natural colouring compounds, resulting in their fixation on the fibre. The higher the concentration of mordant on the fibre, the greater will be the extent of complex formation, which in turn increases colour depth on the fibre.
3.2 Estimation of Metal Ion Absorption
Results in respect of estimation of Cu (II) ions (ppm level) absorbed by wool and silk fabric are given in Table 2. The results indicate that pure and silk fabric did not contain any traces of Cu (II) ions. However, when wool was only mordanted, with copper sulphate of concentrations 1% to 5%, it showed a relatively higher ppm level of Cu (II) ion absorption than allowed by the Oeko-Tex standard, ie. 100 ppm. It was 109.05 ppm of Cu (II) for 1% concentration and this increased to 228.30 ppm as the concentration increased to 5%. However, in case of mordanted wool dyed with pomegranate rind, the presence of Cu (II) ions was found to be lower when compared to samples that were mordanted only (Table 2). This clearly indicates that, during dyeing with pomegranate rind, a certain amount of Cu (II) ions get released in the dyebath.
In the case of turmeric, henna and madder, similar trends were observed. In general, all the wool samples, mordanted and dyed with natural dyes, contained Cu (II) ions in concentrations less than on wool materials that were mordanted only.
Copper-sulphate and natural-dye molecules are individually smaller in size than when Cu (II)- dye complexes are formed. Hence, during dyeing Cu (II) ions that are not complexed with the dye molecules get released in the aqeuous dyebath and, after fixation of the dye, the dyed material contains a Cu (II) ion concentration less than that on the mordanted-only sample.
According to the Oeko-Tex standard, the permissible limit of copper (II) ions on fabric is 100 ppm. Pomegranate rind and henna-dyed wool samples, which were pre-mordanted with copper sulphate with 1%, 2% and 3 % concentrations of mordant solution, were found to exhibit Cu (II) ion concentrations on the dyed fabric well within the permissible limit. However, turmeric and madder-dyed wool showed a critical concentration at 1% of mordant concentration in pretreatment.
In the case of silk, as the mordant concentration increased, the absorption of Cu (II) ions on the fabric also increased to slightly above the permissible limit of 100 ppm. However, the amount of Cu (II) ions present on the silk dyed after mordanting was less than that when silk was only mordanted. For all the silk fabrics dyed after mordanting, the ppm level of Cu (II) ions was well below the permissible limit of the Oeko-Tex standard, ie. 100 ppm.
3.3 L*, a* and b* Values
The L*, a* and b* values for wool and silk fabric mordanted with copper-sulphate concentrations of 1% to 5% and dyed with natural dyes are shown in Figures 1-4. The results indicate that the shades with pomegranate rind, henna and madder lie in the first quadrant of an a*, b* plot, indicating differential tones as the natural dyecolouring component varied. Pomegranate rind gave highest b* values, followed by turmeric and henna, indicating yellowish tones. However, in the case of turmeric, it is a purer yellow, since its a* value is very small and negative, indicating a slight greenish tone. Similarly, madder gave the highest a* value, indicating its reddish shade. Pomegranate rind and henna also had significant redness and thus a* values were also significant.
In Figures 2 and 4 the a*, b* plots clearly indicate the influence of pre-mordanting on dyeing. Respective dyed samples thus showed shifts in their tones, giving beautiful shades.
3.4 Antibacterial Properties
The results for percent reduction of colonies, as shown in Tables 3 and 4, indicate that the percentage reduction in the number of colonies of bacteria increased as the concentration of copper-sulphate mordant increased from 1% to 5% (owf), when compared to untreated or control wool fabric. Similar results were also obtained in the case of silk treated with only copper-sulphate solution. Non-mordanted wool and silk samples dyed with natural dyes – namely pomegranate rind and turmeric – showed more colony reduction than the wool and silk fabric dyed with henna and madder. This suggests that the pomegranate rind and turmeric possess more active antibacterial compounds compared to those present in henna and madder, regardless of whether the dye was applied on wool or silk.
Results from Table 5 indicate that the percent reduction in the number of colonies for wool and silk dyed samples after pre-mordanting with copper sulphate showed a good antimicrobial property ie. 80%. This suggests the existance of great potential in imparting antibacterial properties to wool and silk by mordanting with copper sulphate within a safe limit and dyeing with the above-mentioned natural dyes.
3.5 Wash and Light Fastness Properties
The wash-fastness properties (Table 6) were of the order of 3-4 (midway between good to very good) in the case of wool mordanted only, and 4-5 ( midway between very good to excellent) for samples dyed with pomegranate rind and turmeric dye, and 4 (very good) for henna and madder dyed samples. Similar results were also obtained in the case of silk. This wash-fastness property was measured using the ISO II method in aqueous solution (water), and thus it is possible that in dry-cleaning solvents fastness would be much better, since these dyes are insoluble in dry-cleaning solvents. The light fastness was in the range of 4 and 5-6, ie. good and midway between better and very good, which is quite acceptable.
4. Conclusions
Taking into consideration the permissible limits of Cu (II) ion on the fabric, the concentrations of copper sulphate mordant for wool and silk dyeing with four natural dyes were optimised. The results show that in the case of pomegranate rind and henna-dyed wool, pre-mordanted with copper sulphate of 1%, 2% and 3 % concentration of mordant solution, metal content on the fabric is within the permissible limit (ie. 100 ppm). However, in the case of turmeric and madder dyeing, premordanting with copper sulphate requires a concentration limited to only 1%, in order to achieve a dyeing well within safe limits.
For turmeric and madder, lighter to medium shades can be obtained, with an ability to exhibit antibacterial property. These dyeings will also be eco-friendly and safe to the wearer as Cu (II) ion concentration on them falls well within the permissible limit.
In the case of silk, however, up to 5% mordant concentration can be safely used to obtain varying shades with all four natural dyes, which in turn can exhibit antibacterial property while containing Cu (II) ion concentration within the safe limits, and thus posing no danger to the health of the wearer.
The mordanted samples on dyeing with all the four natural dyes exhibited enhancement in antibacaterial activity. However, such an enhancement of antibacterial activity was more in case of pomegranate rind and turmeric as compared to henna and madder. Further work by varying the percent shade of the individual dye may result in obtaining different depths on the protein fibre. Also, mixtures of the dyes can be tried to obtain a wide shade gamut on the fabric.
Acknowledgement
The author K.H. Prabhu expresses his thanks to the Technical Education Quality Improvement Programme (TEQIP) New Delhi, India for SRF, which enabled this work to be carried out.
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