The preparation and processing of tussah silk

The preparation and processing of tussah silk

Subrata Das Central Silk Board, BTM hyout, Madivala, Bangalore 560 068, Karnatah, India

Tussah silk is well known for its fast fawn colour, which is often preserved in the manufacture of dress and other fabrics. Hence the processing of tussah for fabrics has not been very common, except for certain ladies’ apparel such as sarees, scarves, etc. Little information on the practical processing of tussah silk has appeared in the technical literature. The present paper will attempt to review the preparation, dyeing, printing and finishing of tussah silk based on laboratory and mill experience.

INTRODUCTION Tussah silk is produced by the larvae of several species of moth, as listed in Table 1. These insects mostly live in the wild on bushes and trees upon which they feed. Their diet is responsible in providing tussah with its characteristic ecru to dark brown colour. It is difficult to remove the natural colouring matters which are held tenaciously by the fibroin [l].

Among the commercially important varieties of wild silk, tussah is the most popular and is produced mainly in the People’s Republic of China, India and Japan. In 1986 China and Japan produced 35 700 and 7905 tonnes of silk respectively, and a considerable proportion, although not the greater portion, of this was tussah. The present annual production of tussah silk in India is about 502 tonnes and exports are valued at about Rs 84.2 million.

COMPOSITION OF TUSSAH SILK Tussah silk is spun in a single-shelled, oval cocoon, with a fine-grained, hard, non-flossy shell. The cocoons are generally yellow or grey. A list of the major tussah cocoon varieties with their characters are given below in Bble 2.

Tussah differs from mulberry silk in the composition of various amino acids in the fibroin structure. It typically contains 24% glycine, 37% alanine and 10% serine, compared with 44% glycine, 29% alanine and 12% serine found in mulberry silk [2]. The tussah filament consists of two separate brins which are held together by sericin. Fibroin constitutes about 75430% by weight of the raw fibre and the remainder is mostly sericin. Under the micro- scope tussah silk is seen to be comparatively broader than mulberry silk and has longitudinal striations representing minute filaments called fibrils [3].

The resistance of tussah to the action of corrosive chemicals is attributed to the higher crystallinity as compared with cultivated silk.

TUSSAH SILK REELING The sericin in the tussah cocoon shell is mostly fixed by tannins, and the presence of inorganic substance makes the cocoon hard and compact. Simple boiling in water is insufficient to soften the shell [4]. Therefore an elaborate series of processes is involved to obtain reeled yarn from the thick, hard tussah cocoons.

After sorting, the reelable cocoons are further sorted into groups according to size, shell compactness and colour. These are then subjected to stifling for 10-15 h at 80- 90°C to kill the pupae and dry the cocoons. Stifled cocoons are stored either loose (in wire mesh racks) or packed in cotton bags. In order to prevent the peduncle end opening during further handling, the peduncles are cut before the cocoons are exposed to the different steps of cooking.

In the thigh reeling system [MI, tussah cocoons are boiled in soda solution in an earthen pot for 3 4 h, followed by reeling in semi-moist conditions on a ‘natwa’. Typically one reeler can produce about 80-90 g of yarn at 6.1-6.6 tex thickness per day. This system is also practised for reeling inferior quality cocoons, including emerged cocoons which cannot be reeled by machine. In t h s method many cocoons remain too hard to be reeled while some open at the peduncle end. In the enzymatic system [MI, the tussah cocoons are boiled in a solution of 1 gA soap and 1 gA soda for 30 min followed by steam treatment for

Table 1 Tussah silk moths

Species Country Food

Antheraea pernyi China Oak leaves Antheraea yarnarnai Japan Oak leaves Antheraea rny/iffa India Asan, arjun and sal leaves Antheraea proy/ei India Oak leaves

Table 2 Properties of main types of tussah silk cocoons

Single Average cocoon size of Raw silk Recovery weight filament recovery of silkll000

Type (9) (”/.) cocoons (9)

Daba 14 1.11 65 1000 Raily 16 1.11 62 1570 Sukinda 12 1.11 65 905 Bogai 10 0.88 63 750 Sarihan 8 0.88 61 440 Bhandara 8.5 0.77 59 450 Oaktussah 5 0.50 60 225

JSDC VOLUME 108 NOVEMBER 1992 481

45 min at 100 Wa, and are finally soaked with 0.5 g/l Biopril-50 ( h i 1 Starch Products, India) solution for about 16-18 h, initially at a temperature of 45°C and later at room temperature. The reeling is done on a pedal-operated machine developed by the Central Tasar Research and Train- ing Institute, Bihar, at a productivity level of 300 g of 6.14.6 tex yarn per reeler per day. The process is illus- trated in Figure 1.

The loss of sericin on prolonged steaming or enzymatic treatment can cause the intricate crossings of reelable filament to lose their cohesiveness. As a result a mass of reelable loops and slugs come off the cocoons during reeling and form thick slubby patches in the yarn.

In order to overcome the problems that dry-reeled yarn can give in subsequent weaving, a wet-reeling system has been introduced. In this method it has been found that In- dian tussah cocoons are found completely cooked by boiling for 5 min in 1% hydrogen peroxide (30%) with 1% Sunlight soap at pH 10.8. In wet reeling the reeling is done from a water basin (temperature 6040°C) on an improved mulberry charkha (i.e. a hand-reeling device for raw silk) developed by the Central Sericultural Research and Train- ing Institute, Mysore, by which means up to 600 g of 6.1- 6.6 tex thickness of yarn can be produced per working day. The path of the wet reeled yarn is indicated in Figure 2. A Chinese wet-reeling method has been described in the literature [7].

The comparative properties of various reeled yarns are summarised in Table 3, the properties being superior in case of wet-reeled yarn.

The portion of tussah cocoons left over after taking out about 5945% reelable silk is spun into katia yarn. The pierced and inferior quality cocoons are spun into ghicha

Swift -

t Spindle driving mechanism

Figure 1 Dry reeling system for tussah yarns reeling on CTR&TI machine

Reel

Traverse guide v I ,- Croissure angle

Figure 2 Wet reeling system for tussah yarns reeling on CTR&TI machine for mulberry silk

yarn, while the peduncles are utilised for the production of balkal yarn.

TUSSAH SILK WEAVING In tussah weaving the reeled yarn is almost exclusively used in the warp, while various types of yarn can be used for the weft. The warp is stretched out and sized by sprinkling sizing mixtures containing the juice of boiled rice, mustard or coconut oil and water. The entire warp is thoroughly brushed and dried. Weaving is done on both throw and fly shuttle looms.

PREPARATION OF TUSSAH FABRIC The total amount of sericin present in the shell of tussah cocoon ranges from 10 to 18% and this amount is reduced to 7 to 10% in the fibre or yarn state. The gummy matters are largely removed during the alkaline or enzymatic cooking process. To provide adequate protection in weaving, the reeled yarn is sized. After weaving the fabric is desized by using 0.5% hydrochloric acid or sulphuric acid for 2 h at room temperature.

Degumming Degumming is an important treatment of tussah silk fabric processing. The presence of gum makes silk harsh and stiff and masks its natural lustre. Tussah is more difficult to degum than cultivated silk because the fibre contains more mineral matter and the gum is harsher and more resistant to the action of chemicals. The sericin is embedded

482 JSDC VOLUME 108 NOVEMBER 1992

Table 3 Comparative propertes of various reeled tussah yarns

Type of reeled Tenacity Cohesion Cleanness Neatness Evenness yarn (cN/tex) (stroke) (Yo) (W (“A)

Thigh 1.14-1.23 5 63.9 40.0 73.8 Machine (dry) 1.41-1.58 68.1 55.0 70.0 Machine (wet) 2.20-2.64 120 95.6 73.5 97.5

in the strongly fibrillated fibre and is difficult to remove completely.

Alkaline &gumming Tussah silk, being more resistant to alkali, can be degummed in a strong alkaline bath [8]. The first bath is prepared with 5 g/l Marseilles soap, 3 g4 nonionic surfactant (fatty alcohol polyglycol ether), 3 gA polyphos- phate and 2-3 gA soda. The desized fabric is treated for 1 h at 95T, adjusting the pH to 9.5-10. The tussah fabric is again treated in a second alkaline bath of the same composition. If the silk still retains its fawn colour, the bath can be cooled down to 60-70°C and treatment continued for 30- 45 min, adding 1-3 g/l sodium hydrosulphite. Finally the fabric is rinsed with hot and cold water and neutralised with acetic acid.

Alkaline degumming is also carried out effectively by boiling the silk in the first bath with 2 gA soap and 0.3 gl sodium carbonate solution for 2 h followed by washing and subsequent soaking in hydrochloric acid solution for about 12 h [9]. The silk is then washed well in cold water before treating in a second alkaline bath with 1 gA soap and 0.2 gA soda solution for 1 h at the boil. After boiling in the second bath, the silk is washed well in hot water and then in cold water.

Enzymatic degumming Degumming with strong alkali causes weakening of fabric strength through vigorous hydrolysis. For this reason degumming with proteolytic enzyme is becoming increas- ingly common for tussah silk to overcome this disadvan- tage [lo]. First the silk is treated with 0.5 g4 nonionic surfactant at the boil followed by degumming with 1 gl papain and 2 gA sodium hydrosulphite for 90 min at 75°C. The enzyme-treated tussah is then boiled with 0.5 g4 nonionic surfactant and subsequently washed with soft water.

The enzymatically degummed tussah silk fabrics have a relatively harsh handle and dull appearance. Enzymatic degumming is practised in China whereas in Europe it is rarely encountered because of economic disadvantages.

The efficiency of the degumming can be assessed by performing a staining test with Solar Brilliant Red BA (S, CI Direct Red 80), picrocarminic acid or Pauly reagent [11,12]. According to Chu and Provost [13], the degree of degumming can also be checked by dyeing with CI Drect Blue 22, which will dye the sericin but not the fibroin.

Bleaching Tussah silk cannot be freed from the inherent fawn colour through degumming. Even after intensive bleaching treatment, it retains its original colour. This may be due to the attachment of the natural pigments inside the fibroin. Critical studies on the pigment in Antheraeu mylitta have brought to light that larval integument (green and yellow) contains carotenoid- and steroid-like compounds [9]. The green pupal haemolymph does not have pigments in the free state but they are present as a chromoprotein complex which yields 0-carotene on hydrolysis. The pigments ex- tracted from the cocoons of A mylitta are flavonoid-like compounds.

Tussah silk can be bleached by oxidation as well as reduction methods. Reduction bleaching is generally carried out with sodium hydrosulphite. In one of the recommended bleaching processes the material is treated for 1 h at 60°C in a solution of 4-5 gl sodium hydrosulphite followed by a thorough rinse.

Hydrogen peroxide is the preferred oxidising bleaching agent for tussah silk. Several different bleaching treat- ments are available [7].

An addition of a fluorescent brightening agent to the bleach bath is recommended. However, a full white is still difficult to obtain without damaging the fibre. For this reason delicate shades cannot be attained with the same brightness as on cultivated silk. This can also lead to problems with pure white discharge prints.

DYEING The natural colour of tussah silk is popular for garments made from this fibre, but where required fabrics can be dyed. The expected colour yield on tussah silk is on average about one-third of that achievable on wool or nylon fibres, and half of that of cultivated silk (assuming the same depth of colour). Since a high concentration of dye produces only pale dyeings on tussah, the chance of ob- taining uneven results on this fibre is quite high. This un- evenness results not only from the irregular distribution of dye on the fabric but also from variations in the substrate itself [q.

Since tussah fibre has a slightly cationic character, with the isoelectric point at about pH 5.0, it is primarily suitable for coloration by anionic dyes, i.e. acid and metal-complex types. In recent years reactive dyes have also become very important for colouring tussah silk, owing to steadily in- creasing fastness requirements and fashion demands. In


special cases vat, Indigosol, chrome, direct, azoic and natural dyes can also be used for colouring tussah.

The general procedures for applying acid, metal-complex and reactive dyes are shown in Figures 3-6. In order to obtain the best fastness to wet treatment, unfixed reactive dye is washed-off with synthetic detergent. An alkaline method of fixation is preferable as it gives a higher degree of fixation. Tussah silk can also be dyed with reactive dyes in a pad-batch method [7].

Chafe marks and creases can sometimes occur as seri- ous faults in tussah silk processing. They can be avoided by adopting correct preparatory processes and by choos- ing the most suitable types of dyeing machinery. Tradi- tionally jigs, winches and star frame dyeing machines have been tried. Sometimes an addition of a fabric lubri- cant is useful in conquering such defects.

PRINTING Printed silk fabrics are popular due to the exclusiveness of

110 90

9 70 9! 5 50

30 0, 0.

0 20 40 60 80 100 Time, rnin

A B t C

t t

A Acetic acid (80%) 2 4 % , Glauber's salt 10-20% B Dyex% C Dye fixing agent 1%, acetic acid 1%

Figure 3 Profile for dyeing tussah silk with milling acid dyes at pH 5-6

30t 0 20 40 60 80 100 120 t t Time, min A B

A Ammonium sulphate 3 6 % , acetic acid 0-2% B Dyex%

Figure 4 Profile for dyeing tussah silk with metal-complex dyes at pH 5-7

designs and coloristic effects that can be achieved. But this is less the case on tussah silk because of its inherent colour and a lack of available technological knowledge. However, printing on tussah silk be successful, depending to a great extent on proper pretreatment such as desizing, degumming and bleaching.

Direct printing Selected acid, metal-complex, direct and reactive dyes can be used in tussah silk printing. The following recipe and operating procedures may be followed for non-reactive dyes: Acidmetal-compleddirect dyes x g Thiodiethylene glycol 50g Thickener, e.g. bean flour ( p a r ) gum 500 g Urea 50-100 g Acid donor, e.g. tartaric acid 60-100 g Water to 1000 g

110 I

0 20 40 60 80 100 1 Time, min

A B C t t

A Dye x % , formic acid (85%) 0.5% B Formic acid (85%) 3.5% C Synthetic detergent 0.1%

!O

Figure 5 Profile for dyeing tussah silk with cold-dyeing reactive dyes (acid fixation method)

60 80 100 120 0 20 40

B A t 1 Time, min

C D t t

A Dye x%, sodium sulphate (anhydrous) 1% B Sodium sulphate (anhydrous) 2% C Sodium carbonate 0.25% D Synthetic detergent 0.1%

Figure 6 Profile for dyeing tussah silk with cold-dyeing reactive dyes (alkaline fixation method)


The printed fabric is dried under mild conditions to retain a good printed mark and prevent the goods from mark- ing-off during subsequent processes. The prints are fixed by steaming in a star steamer at atmospheric pressure at 102104°C. The steaming time is set according to the depth of the colour and degree of pattern coverage required. The printed fabric is rinsed thoroughly in cold water.

With reactive dyes the following recipe can be used: Reactive dye x g Urea "I g

Thickener, e.g. sodium alginatr Yg Water to 1000 g

Anionic oxidation agent, e.g. Revatol S (S) Sodium bicarbonate 8-15 g

10 g

The dried prints are protected from the influence of acid before they are steamed for 10-15 min at atmospheric pressure at 102-104°C. To attain good fastness properties, reactive dyes must be washed-off at higher temperature than acid or metal-complex dyes.

Discharge printing For discharge printing of tussah silk only reducing agents are used, especially Rongalit C (BASE sodium formalde- hyde sulphoxylate). The use of tin(@ chloride is of limited value due to the release of acid during steaming, which leads to corrosion.

The ingredients shown in Table 4 are normally used in typical discharge recipes. The prints should be steamed for 10-15 min at 102°C for white discharges while steaming for 10-20 min is required at 102°C for coloured discharges. Festoon steamers are suitable for this purpose to control steaming conditions effectively. Because of the limitation in fastness properties of various coloured discharges, it is advisable not to exceed 30°C in washing-off the prints.

The appropriate selection of ground colours is of great importance in successful discharge printing. Dyes that are suitable for the ground normally contain no azo groups that can be split by reduction. The colour of amines produced by reductive cleavage is also important. If they are not completely removed during subsequent washing, their residues will slowly darken due to oxidation, and a white discharge that was initially satisfactory will become

Table 4 Typical recipe for discharge printing of tussah fabric

Coloured

(parts by wt) White (illurninant) (parts by wt)

Urea Thiodiglycol Rongalite C Zinc oxide Thickener (gaur gum) Fluorescent brightener Discharge-resistant dye Water to

0-50 20-40 30-40 30-1 50 50-1 50 50-75 500600 500-600 9-1 0

1000 1000 X

unacceptable. Ground shades for discharge printing are normally chosen from acid, metal-complex or reactive ranges, e.g. Sandosilk Turquoise AS (S), Navimill Red B (IDI), Lanasyn Brown GRL (S, CI Acid Brown 369) and Levafix Brilliant Red E-4BA (BAY, CI Reactive Red 158).

Since discharge printing of tussah silk is carried out with reducing agents the illumination colours have to withstand the action of dischargmg agents under severe fixation conditions. The range of colours available is thus very limited; examples of dyes that can withstand the action of Rongalit C include Solar Yellow BG (S, CI Direct Yellow 28), Acidol Bordeaux M-B (BASE CI Acid Violet 90), Sandosilk Rhodamine B (S) and kocion Blue H-EGN (ICI, CI Reactive Blue 198).

Printing techniques Printing of tussah silk is normally carried out on hand- screen tables with either steam or electrical heating [14]. The normal screen printing parameters, screen mesh, squeegee pressure, paste viscosity and type and number of squeegee strokes, have to be adjusted for a particular design and a specific fabric.

FINISHING Finishing of tussah silk is not often carried out. However, depending upon the specific requirements, chemical or mechanical finishing processes can be followed.

Chemical finishing To improve the handle of dyed or printed tussah fabrics the following recipe can be used [lo]: Starch or arrowroot 500 g Softnol NOF (Ahura) 100 g Glycerine 100 g Urea 200 g Water to 1000 g

The treated goods are sun dried. In order to impart a scroopy finish, tussah silk can be

treated with 40 ml/l Dextrin (2:l) and 4 ml/l tartaric acid (10%). Various other chemical finishing processes reported in the literature are more of theoretical than practical inter- est [7].

Mechanical finishing Dried tussah fabric can be moistened and wrapped with cottodsilk fabric and hammered manually by two wooden hammers alternately to impart a soft handle and lustre. This process is known as kundi finishmg and can be compared with a button or knife breaking machine finish, where fabric is passed several times rapidly back and forth over small rollers studded with brass buttons or slanted knives.

Tussah silk fabrics can be calendered on a two-bowl calendering machines for improving handle and appearance [ 101.


CONCLUSION While a perceptible improvement has been achieved in the wet processing of cultivated silk, the natural fawn colour of tussah silk makes it difficult to process to achieve results of similar quality. However, proper understanding of the raw material and of the appropriate techniques at each stage of processing improve the coloration and other effects that can be achieved on this noble fibre in future.

REFERENCES 1. Subrata Das, Indian Silk, 27 (9) (1989) 27.

2.

3.

4. 5. 6. 7. 8. 9.

10. 11. 12. 13. 14.

F Lucas et al., Advances in protein chemistry, Ed. C B Anfinsen Jr et al. (New York Academic Press, 1958). M S Jolly et al., Non mulberry silks, FA0 Agricultural Services Bulle- tin (Rome, 1979) 98,99. Subrata Das, Indian Silk, 28 (11) (1990) 13. Subrata Das and S K Chowdhury, lnd. Text. I., 100 (10) (1990) 98. S K Chowdhury, Indian Silk, 27 (1) (1988) 33. AGLiddiard,J.S.D.C.,98(1982)396. J Hilden, Internat. Text. Bull., (1) (1985) 42. M S Jolly et al., Tusar culture, Central Tasar Research Station (Bombay: Arnbika Publishers, 1974) 220,243. Subrata Das, lnd. Text. J., 101 (10) (1991) 348. K Mahall, Text A s h , (Oct 1985) 95. C Rimington, J. Textile Inst., 21 (1930) T237. K Y Chu and J R Provost, Rev. h o g . Coloration, 17 (1987) 24. Subrata Das, Unpublished work.

Dyeability variation in cotton - a study from fibre bale to finished fabric

P Cooper" and J M Taylor t *Cuurtaulds Textiles, Haydn Ad, Nuttingharn, UK t Courtaulds Research, Spundon, Derby, UK

An extensive study has been conducted to quantify the dyeability variations existing in cotton, sampling from the bale, and throughout processing in the spinning mill and the dyehouse. A dyeing test has been derived to assist dyehouses to rapidly assess the dyeability characteristics of incoming yarns.

INTRODUCTION Developments in microelectronics during the past decade have facilitated the implementation of the technology nec- essary for effective measurement and control systems for the textile coloration industry. The full benefits of automa- tion in the dyehouse have not, however, been fully realised due to the variability of fabric received for processing. For example, the actual improvements in shade reproduc- ibility and reduction in the number of lots for reprocessing achievable have not been fully realised because of lack of control over the quality and consistency of incoming substrate.

A cooperative project between Courtaulds Textiles and Courtaulds Research was therefore initiated to study: (a) Dyeability variations existing in cotton, from raw fibre

through to finished fabric (b) The consistency of dyeability of a single-count

combed yarn delivered to a dyehouse from one particular spinner

(c) Dyeability differences in cotton yarns obtained from many spinners

(d) The effect of physical differences on apparent dyeability.

The aim was to evaluate the relative importance of these

factors and to formulate recommendations that would en- able a dyehouse to rapidly assess dyeability variations in incoming yarns. In the present work dyeability was assessed in terms of the ability to acheve the same colour on a given substrate using a given recipe under a standard set of conditions. A large sampling programme was agreed with spinners and dyers within Courtaulds Textiles to al- low these objectives to be met. In the present paper variations in dyeability are quoted as percentages based on a total integrated colour value obtained using an ICS colour measurement sytem.

SPINNING Cotton fibre samples were taken from individual bales, ribbon lap, sliver and ring tube. The dyeability was assessed by dyeing using a fixed concentration of CI Direct Green 27. The colour of the resultant dyeings was meas- ured spectrophotometrically and the results expressed as percentages summarised in Table 1; shade variations of less than 5% cannot be perceived by the eye. Cross checks with reactive dye recipes showed differences of a similar magrutude.

The figures quoted in Table 1 (repeated over three weeks of full sample study) demonstrate the effectiveness of the spinner's blending process. Fibre from an identical


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The preparation and processing of tussah silk