Silk having good affinity for colours, and the natural dyes coming back in demand for various advantages as well as eco-concern, efforts are being made to develop extracts for suitable natural dyes. The article presents details of study conducted to dye mulberry and tasar silks with extracts of Acatia catechu, evaluate impacts of dyeing on certain parameters and the effect of anti-microbial content on the silk fabrics.

Silk is widely known for its unique lustre, tactile properties, durability, and dye ability, supported with unusual mechanical properties. It also displays interesting thermal and electro-magnetic responses, particularly in the Ultra Violet range.' Silk has excellent dyeing affinity and is capable of rendering colours in a unique way. The silk fibre is triangular in section and when several yarns are twisted together, they offer a number of dimensions that reflect light.

Natural dyes have been a part and parcel of mankind's life since time immemorial, and well, it had been a common practice in India. In the past few years, there has been great interest in the textiles and garments that offer enhanced comfort as well as protection to the wearer. After a period of setback, natural dyes have made a comeback, more due to consciousness of the consumers.

Natural dyes are considered to be advantageous for the reasons such as, their being obtainable from renewable resources and harmonization with nature while there are neither any chemical reactions involved in preparation nor these dyes pose any health hazards or disposal problems

As such, a study was conducted to explore  the possibilities of dyeing mulberry and tasar silk with extracts of Acatia catechu with the following objectives

1.   To study the effect of natural dye (Acatia catechu) on m u I berry and tasar silk fabrics;

2.  To evaluate the impact of natural dyeing on mulberry and tasar  silk fabrics  by visual inspection, mechanical, physical, comfort and colour fastness property test, and

3.  To study the anti-microbial content of Acatia catechu dye powder on selected mulberry and tasar silk fabrics.

Procedure:

Selection of the natural dyes: The stick lac was crushed and sieved to remove impurities. The sieved material was then repeatedly washed to remove insect parts and other soluble material. The resulting product known as seed lac was further refined by hot melting, filtration and stretched into thin sheets that are subsequently broken into brittle flakes called shellac, was used as a natural dye.

Acatia catechu is a member of the family leguminosae, mimosacae. The dye is obtained from heartwood of the cutch tree. The flakes of Acatia catechu was ground to powder. It possess effective dye uptake and colour yielding capacity. Although lac exhibited good colour fastness properties, the study was conducted on tannin based dye, Acatia catechu for its anti microbial properties.

Selection of mordant: Silk has excellent affinity for acidic medium. As such, formic acid was selected as mordant  for the study. Formic acid causes hydrolysis of the peptide linkage in silk which increases the dye sites which may lead to high dye uptake.

Pilot Study

Optimisation of dye concentration. To optimize the concentration of the lac dye, 25 and 40 per cent of the dye proportions were selected. The mulberry silk samples of 53 g for dyeing 25 per cent shade and 10.15 g for 40 per cent shade, liquor ratio 1 :50 were taken and dyed with the selected two dye concentrations. The maximum dye absorption was taken as optimum concentration of dye.

Procedure for acidic extraction of lac: 1.3 g of lac dye powder was mixed thoroughly with 250 ml of soft water. After boiling the solution for one hour, it was cooled and filtered. This extracted dye solution was used for dyeing of silk samples.

To optimize the shade in pilot study, 10, 30 and 40 per cent shade was used. For this study, 5" x 5" mulberry and tasar silk fabrics were used to find out the correct shade percentage. After the study, 10 and 40 per cent shade was selected with the acidic extraction of Acatia catechu. 5 gram of dye powder was dissolved in one litre of soft water; after boiling for one hour, it was cooled and filtered. The extracted dye solution was used for this study. The same procedure was used for alkaline extraction also which however, was not  used for the further study because required dye shade could not be obtained.

Optimization of dyeing time: To optimize the dyeing time, the fabric samples were dyed for 15, 30, 45 and 60 minutes in the pilot study. Dyeing time of 45 minutes which gave maximum dye absorption was taken as optimum time taken, and selected for further study.

Optimization of mordants and mordanting technique• For the pilot study to optimize the mordanting technique, three mordanting technique (pre, simultaneous and post-mordanting) was carried out on mulberry and tasar silk fabrics with 5 ml of formic acid, boiled for 45 minutes and cooled. The evaluation of dyed samples suggested adoption of post-mordanting technique for further study.

Procedure for Dyeing

Selection of dyeing parameters: The optimized parameters selected for dyeing were, 11  mtr of  mulberry and 9 mtr of tasar silk fabric in the liquor ratio of 1 :50 at boiling temperature The optimum concentration of dye was 10 and 40 per cent, optimum time for dye extraction one hour, and for dyeing 45 minutes with formic acid as the mordant. 45 minutes post-mordanting time was selected to carry out dyeing.

Acidic extraction of  Acatia catechu: The dye  powder in  the ratio of 30, 106 g and 15, 64  g of Acatia catechu (dye extraction) was used to dye mulberry and tasar silk fabrics for 10 and 40 per cent shades, respectively. The dye solution with one litre of water was boiled for one hour at boiling temperature to extract the dye solution.

Dyeing. Acidic extraction method was found effective for dyeing of mulberry and tasar fabrics with 10 and 40 per cent shades.

The selected degummed mulberry silk fabric was taken into two parts. Five and half a metre each was used for dyeing with 500 ml of dye extraction and 11.760, 10520 litres of soft water for dilution of dye bath. The pre-soaked fabric was immersed in dye bath boiling at 40oC for 10 minutes. Then, the fabric was removed from the dye bath and 20 ml formic acid added for postmordanting process. Then, the dyed fabric was placed in the dye bath for dyeing. Gradually, the temperature was raised to 85oC- 90oC for 30-45 minutes. After dyeing, the dyed material was thoroughly washed twice with cold water. This procedure was followed for dyeing mulberry silk with 10 and 40 per cent shades, respectively.

The same procedure was followed for tasar silk fabric which was taken into two parts of 41/2 mtr each. Accordingly, the ratio of dye solution and mordant differed. The recipe for dyeing is at Table 2.

Washing of dyed silk fabrics: After  dyeing,  the mulberry and tasar  silk fabrics  were washed  thoroughly with 6 and 3 g of soft soap (Ezee) in 12 and 6 litres of soft water, respectively. The soap was dissolved thoroughly and the solution boiled at 60oC for 15 minutes with the fabric. Then, the material was taken out and washed with cold water twice thoroughly and then dried in shade. The recipe for washing is at Table 3.

Results

The sample DMS (Dyed mulberry silk) 10 per cent had maximum increase in breaking strength when compared to UDMS (Undyed mulberry silk) and DMS 40 per cent. And there were no significant increase in breaking strength between DTS (Dyed tasar silk) 10 and 40 per cent when compared to UDTS (Undyed tasar silk) along the weft direction (Table 4)

The statistical analysis proved that there were significant difference in breaking strength along warp direction within DMS 10 and 40 per cent as well as within DTS 10 and 40 per cent when compared to its original with '1' test value of 26.868 and 81.983 at 1  per cent level. Similarly, it is proved that there were significant difference in breaking strength along weft direction within DMS 10 and 40 per cent with 't' test value of 16.388 when compared to its original at 5 per cent level whereas there were no significant difference within DTS 10 and 40 per cent when compared to its original.

The sample DMS 40 and 10 per cent and DTS 10 per cent had an increase in elongation along the weft direction. But, the value of DMS 40 and 10 per cent remained same although there was increase in elongation along the weft direction (Fig.1).

The statistical analysis proved that there were significant difference in elongation along warp direction within DMS 10 and 40 per cent as well as within DTS 10 and 40 per cent when compared to its original with '1' test value of 3.732 and 17.399 at 1  per cent level. Similarly, it is proved that there were significant difference in elongation along weft direction within DMS 10 and 40 per cent as well as within DTS 10 and 40 per cent when compared to its original with 't' test value of 35.653 and 9.934 at 1  per cent level, respectively

The sample DMS 10 and 40 per cent, DTS 40 per cent had significant weight loss due to abrasion when compared to respective undyed samples but there were increase in abrasion resistance with DTS 10 per cent when compared to UDTS and DTS 40 per cent (Fig 2).

The statistical analysis proved that there were no significant difference in abrasion resistance within DMS 10 and 40 per cent as well as within DTS 10 and 40 per cent when compared to its original.

There was a maximum gain in drape co-efficient of DMS 10 per cent when compared to UDMS and DMS 40 per cent as well as there was maximum gain in drape co-efficient DTS 10 per cent when compared to UDTS and DTS 40 per cent (Table 5).

The statistical analysis proved that there were• significant difference in drape ability within DMS 10 and 40 per cent when compared to its original with 't' test value of 3.717 at 5 per cent level whereas there were no significant difference in drapability within DTS 10 and 40 per cent when compared to its original.

From anti-microbial test, it was proved that dye powder and mulberry and tasar silk fabric showed positive results.

The anti-microbial test was conducted by disc diffusion method for dye powder and parallel streak method for mulberry and tasar silk fabrics. The Staphylococcus aureus and Klebsiella pneumonia did not activate on Acatia catechu 1 and 4 per cent dye powder which was dyed on mulberry and tasar silk fabrics. Hence, this natural dye functioned as natural' antimicrobial agent on the selected fabric (Table 6)

The wicking of silk samples proved that OMS 10 per cent and OTS 40 per cent had the best wickability compared to its respective undyed and dyed samples (Fig 3)

The statistical analysis proved that there were significant difference in wickability within OMS 10 and 40 per cent as well as OTS 10 and 40 per cent when compared to its original with 't' test value of 8.931 and 3.881 at 1 and 5  per cent level, respectively.

The sample OMS 10 per cent and OTS 10 per cent had the best sinking value compared to rest of the sample.

The statistical analysis proved that there were no significant difference in sinking values within OMS 10 per cent and OMS 40 per cent as well as OTS 10 and 40 per cent when compared to its original fabric, respectively.

Evaluation of colour fastness depict that all the samples had excellent colour fastness properties. Results of colour fastness tests conducted on the dyed silk samples are illustrated in Table 7. It reveals that dyed samples of mulberry silk with 10 and 40 per cent shades and tasar silk with 10 and 40 per cent shades showed good colour fastness property. In the wash test, M4 and T1 sample showed excellent results, whereas M3 and T2 samples exhibited noticeable colour change. Regarding stain on silk and stain on cotton all the 4 samples exhibited excellent results.

The colour fastness pertaining to dry rubbing, the samples T1 had excellent fastness, the rest of the samples (T2, M3 and M) scored very good, whereas the colour fastness with respect to wet staining, T1, M3 and M4 scored excellent, whereas sample T2 scored very good. The samples T1, T2 and M3 and M4 have exhibited very good colour fastness to sunlight

In relation to acid perspiration T1 sample scored from good to very good, pertaining to colour change whereas T2, M3 and M4 scored very good.

Regarding stain on cotton, all the 4 samples (T1, T2, f\:13 and M4) exhibited colour fastness of in the range of excellent whereas for stain on silk, T2 and M4 samples exhibited score of very good, for M3 it is from good to very good and for T1 it is very good to excellent.

In relation to alkali perspiration, pertaining to colour change, sample T, scored from good to very good and T2, M3 and M4 scored upto very good whereas for stain on cotton, all the samples scored excellent and for stain on silk all the samples scored very good.

In the pressing test in relation to dry and wet state all the samples exhibited excellent results. Regarding stain on cotton all the 4 samples (T" T2, M3 and M4) exhibited excellent results and for stain on silk they scored maximum in the range of excellent. Hence, it could be concluded that M3, M4, T, and  T2 had superior colour fastness properties.

About the Authors

The authors are with the Department of Textiles and Clothing, Avinashilingam University for Women, and PSG College of Arts and Science College, Peelamedu, Coimbatore, respectively

Originally published in Indian Silk: September 2009