Title of Invention

"A METHOD FOR DYEING OF A FABRIC"

Abstract

The present invention relates to a method for dyeing of a fabric with a dye and products thereof, more particularly to a method for dyeing of a fabric with indigo, even more particularly to a method for dyeing of cotton fabric with indigo and to products thereof having better dye uptake. The method for dyeing of cotton fabric with indigo comprises of reduction of indigo with organic alkali in the presence of a reducing agent, step-wise reaction of reduced indigo with the cotton fabric, step-wise squeezing of reacted cotton fabric, step-wise air oxidation of dyed cotton fabric, washing of dyed fabric with water and drying at about 85 degree centigrade for few minutes. The proposed method results in the better dye uptake and uniform colour yield.

Full Text

INVENTORS : The present invention is by Raosaheb Balwantrao Chavan, Indian National and Md. Sarwoar Jahan, Bangladeshi National, both of Department of Textile Technology, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110016, India. FIELD OF THE INVENTION : The present invention relates to a method for dyeing of a fabric with a dye, particularly to a method for dyeing of a fabric with indigo, more particularly to a method for dyeing of a cotton fabric with indigo, and the products thereof having better dye uptake. BACKGROUND OF THE INVENTION : Over the years, the textile industry probably has produced more indigo denim than any other woven fabric, and indigo dyed blue denim have been popular over a longer period of time than any other item of apparel. Probably, the popularity will remain though they are facing competition with other classes of dyes like direct, developed blues, hydron blues, fiber reactive blues, anthraquinone derivative vat blues etc., only because it can maintain its brightness and hue on repeated washing without staining the white filling although gradually looses in depth and it is utilitarian. Further more it is readily destroyed by bleach to give desired effects and can be given different kind of washes like peach finish, enzyme wash, ammonia treatment, telluric treatment which give soft fabric of good feel and good looking garments. The use of natural indigo dates back a very long time and reference is made to this product in Sanskrit, Arabic, Latin, Anciect Greek and Celtic writings under several names like Greek and Roman writings refer to it as Indicon and Indican. The earliest records of use of indigo can be found in the Indus and the Nile areas. In 1870 a small quantity of synthetic indigo was first produced in the laboratory by the two German Chemists named Engler and Emmerling though their method was too complex for the industrial production. In 1883, Adolf Von Bayer obtained the first indigo patent for synthesising indigo. Bayer's early process was still unacceptable and numerous number of German and Swiss chemists were kept working on it. The first indigo pure was launched in 1897, but the process remained very expensive. In India, the natural indigo was produced by way of cultivation. In late 1880, about 1,400,000 acres of land was used for cultivating indigo in India. But, now the commonly used indigo is synthetic indigo. Indigo belongs to class of Vat dyes and contain about 96% indigotin and therefore is a very strong colour, requiring only about 2% of dye to produce a Navy Blue. It is insoluble in water, alkali or dilute acid but soluble in glacial acetic acid while boiling aniline or pyridine are its best solvents. The dyeing method, as known in the art, for dyeing the fabric with indigo comprises of reduction of indigo with alkali, sodium hydroxide, in the presence of a reducing agent followed by treatment of reduced indigo with desired fabric.This techniques has been widely used and is being used for dyeing of fabric, particularly cotton fabric with indigo. The major disadvantage of this method is that the concentration of reactants, that is of dye, that is indigo, reducing agent, that is hydrosulphite, and alkali, that is sodium hydroxide, in the dye bath changes due to strong alkali. Such variation of these constituents gives inferior quality dyed fabric. Further, the change of concentration of the constituents, as stated above, may result change selected colour of choice. For example, the shade turns green and light for excess amount of reducing agent, if an attempt is made to control the amount of the reducing agent, which may also result in the excess amount of alkali and insufficient amount of reducing agent, the shade may turn to reddish colour. Still further, on the contrary, the less amount of alkali results in green and dull shade. Therefore, in order to maintain the correct amount of alkali and the reducing agent, regular tests should be made and recorded every 30 minutes. Further, to overcome this disadvantage, that is to maintain a constant concentration of dye, reducing agent, and alkali in the dye bath, continous chemical replenishment is required to be done frequently at a rate of needed amount. The another disadvantage of such known method is that the extent of affinity is too less to exercise exhaust dyeing, rather several dipping and nipping technique is followed for gradual build-up of shade. Still another disadvantage of such known method of dyeing of fabric, particularly a cotton fabric with indigo is that the chemical like sodium hydrosulphite, which is being used as reducing agent is very unstable and require repeated addition in the dye bath to retain stability of the reduced dye bath. Furthermore, gradual decomposition of sodium hydrosulphite necessitates over-dosing of alkali to neutralise acidic by-products formed during dyeing process. Still further, such unstability of sodium hydrosulphite is more particular due the selected alkali, that is sodium hydroxide, being more strong. Yet other disadvantages of such known process will be more apparent when the mechanism of dyeing of fabric with indigo is understood. It is known in the art that indigo exist in any one of the following four forms depending on the pH of the dye bath : i) indigotin structure, at lower alkaline pH, ii) reduced but non-ionic form, at moderate alkaline pH, iii) monophenolate form, at relatively higher pH or iv) biphenolate form, at very high pH. The structures i) and ii) of indigo exist below pH 9.0 to 9.5; their relative fraction depends on exact pH value. The increase in pH of dye bath, results in collapse of structure i) and formation of either structure ii) or structure iii) or both. Further increase in pH of the dye bath slowly converts all of these to structure iii) and such pH is above 10.0. A standard pH around 11.5 indicates almost all indigo molecules are in their monophenolate form, that is structure iii). Further addition of alkali increases pH and slowly attacks second C=O group of indigo in the presence of excess of sodium hydrosulphite with a result of conversion of monophenolate [structure (iii)] to biphenolate form [structure (iv)]. The extent of conversion of structure iii) to structure iv) has been observed to be strictly dependent on pH and excess of sodium hydrosulphite in the dye bath. The extent of formation of structure iv) will predominate only when pH of the dye bath is above 12.5, which normally will be the pH of the dye bath, if alkali used is sodium hydroxide. Indigo in the form of structure iv) shows reduced affinity for fabric, particularly cotton fabric and thus dye uptake is reduced at this pH. This can further be explained by analysing charge on the cotton fabric, when the same is dipped in the alkaline bath. It has been observed that cotton gives negative charge, when dipped in alkaline bath and such charge increases with increase in pH, which (pH) is expected to increase with sodium hydroxide being strong alkali. Therefore, the higher the pH, higher the negative charge on the fabric and more is the repulsion between dye and fiber, which in turn results in reduced dye uptake. This is the reason that the known method for dyeing of the fabric, particularly cotton fabric with indigo using sodium hydroxide as alkali has poor dye uptake, as described herein above. Further, it is not commercially viable to attempt to achieve an optimum pH with sodium hydroxide, as the recommended concentration of sodium hydroxide always produces pH greater than 11.5 and at such high pH only structure iv) of indigo can exist. Still further, the alkalinity level of such a system is extermely low that shade depth fluctuation and severe streaking inevitably occur. Still further, drawback of such known method is that the colour yield is poor and not uniform. A possible explaination for this selected pH is that the distribution of the dye, that is indigo in the cross section of the cotton fabric is influenced by dye bath pH. Photomicrographic experiments conducted on the dyed fabric with indigo as dye showed that though each of the yarn contains approximately the same amount of dye, but the depth of the shade for each yarn is not the same, that is the colour yield is not the same for each dyeing. The fabric, particularly the cotton fabric shows no affinity for dye, that is indigo when it is present in structure i), whereas shows negligible affinity for indigo, when it is present in structure ii). The reason for poor affinity of fabric for indigo in these structures, that i) and ii) is that indigo is present in non-ionic form. Therefore, monophenolate form of indigo, that is structure iii) is the only choice of form and this form has been shown to have highest affinity as well as strike rate to give higher dye uptake, but its's existence in the presence of sodium hydroxide is not possible. Therefore, the search is for an alternative alkali which can give pH in between 10.5 to 11.5. The alternative alkali medium offered in the prior art for dyeing of fabric, particularly cotton fabric is the buffered alkali, which can give pH less than 11.5. One of the buffered alkali system as known in the prior art is the 1:1 mixture of sodium hydroxide and sodium carbonate. This method also has disadvantage, particularly being technically difficult due to the formation of sludge. The another known methods comprising of buffered alkali as medium have limitation that the composition of buffered alkali medium has not been made known. Therefore, such methods have limited use with limited traders, is the printing of a fabric with. The main disadvantage of the known methods of the prior art for dyeing of a fabric, particularly cotton fabric, more particularly of cotton fabric with indigo is that they require the more number of dipping and nipping steps. Such required number of more steps and other disadvantages and drawbacks of the known methods, as described herein above results in increased consumption of dye, that is indigo, increased dyeing cost and dyeing time. Yet another limitation of the methods, as known in the prior art for dyeing of cotton fabric with indigo dye is that such methods ignore the possibility of exhaust dyeing with indigo, which will reduce the machinery cost of continous long indigo dye range as well as space. NEED OF THE INVENTION : Therefore, there is a need to have a method for dyeing of a fabric with a dye, particularly a method for dyeing of a fabric with indigo, more particularly a method for dyeing of a cotton fabric with indigo, and the products thereof having better dye uptake, which can overcome the disadvantages of the prior art as described herein above. OBJECTS OF THE INVENTION : This is the main object of this invention to make a complete disclosure of a method for dyeing of a fabric with a dye, particularly a method for dyeing of a fabric with indigo, more particularly a method for dyeing of a cotton fabric with indigo, and the products thereof having better dye uptake, which can overcome the disadvantages of the prior art as described herein above. Another main object of the present invention is to propose for an alternative alkalies, which can have advantage of producing pH less than 11.5 to form the said structure iii) of the selected dye, that is indigo and yet have other advantages such as successful reduction of indigo for the method of dyeing of a fabric with a dye, particularly for the method of dyeing of a cotton fabric with indigo, and the products thereof. Further an object of this invention is to propose for a method for dyeing of a cotton fabric with indigo, and the products thereof, wherein the constant pH is maintained during the dyeing process so that the affinity and the dye penetration also remain constant for getting better reproducibility of both shade and yarn wash-down characteristics. Still another an object of the present invention is to make a complete disclosure of the method for dyeing of cotton fabric with indigo dye and products thereof having better dye uptake, and increased and uniform colour yield. This is yet an object of the present invention to propose for a method for dyeing of cotton fabric with indigo dye and products thereof, wherein the proposed method is economical, time saving and consumes less amount of dye, that is indigo. Yet another an object of the present invention is to propose for a method as stated herein above, which can result in better dye uptake even in less number of dips and nips. This is further an object of the present invention to disclose a method as stated herein above, which can be used for continous dyeing of cotton fabric with indigo. Still another an object of this invention is to make a disclosure of the method as stated herein above, wherein the composition of the dyeing solution is such that the same can be used even after 8 hours of preparartion without much effecting the dye uptake. Yet another an object of this invention is to make a disclosure of the method as stated herein above, wherein the composition of the dye bath does not change early and frequently and hence is devoid of disadvantages associated with such drawback, as described herein above. This is yet another an object of the present invention to propose for a method as stated herein above, which totally eliminates the use of sodium hydroxide as alkali and hence, the disadvantages and drawbacks associated with the use of sodium hydroxide as alkaline medium. Still further an object of the present invention is to make a complete disclosure of a method for dyeing of cotton fabric with indigo, which does not result in formation of sludge. STATEMENT OF THE INVENTION According to the present invention there is provided a method for dyeing of a fabric comprising following steps: a) reducing an indigo dye with an organic amine as herein described in the presence of a reducing agent, as herein described, wherein the concentration of said dye in the reduced solution is maintained at 1 to 7 gm/lit; b) reacting said reduced indigo dye obtained from step a) with the cotton fabric; c) squeezing of dyed cotton fabric obtained from step b); d) oxidising dyed cotton fabric obtained from step c) by air; e) repeating the steps b), c) and d); f) washing the dyed fabric obtained from step e); and g) drying of dyed fabric obtained from step f) at a temperature of 70 to 95 degree Celsius to obtain the desired dyed fabric. BREIF DESCRIPTION OF THE INVENTION : Accordingly, this invention provides a complete disclosure of a method for dyeing of a fabric with a dye, particularly a method for dyeing of a fabric with indigo, more particularly a method for dyeing of a cotton fabric with indigo, and the products thereof having better dye uptake, and increased and uniform colour yield. According to the preferred embodiments of the presently disclosed method of this invention, the dyeing of a cotton fabric with indigo totally eliminates the use of sodium hydroxide as alkaline medium and comprises of: a) reduction of indigo/dye with organic alkali in the presence of a reducing agent, b) reaction of reduced indigo from step-a with the cotton fabric, c) squeezing of dyed cotton fabric from step-b, d) air oxidation of dyed cotton fabric from step-c, e) washing of dyed fabric from step-d and f) drying of dyed fabric from step-e. Therefore, the presently disclosed invention has the advantages of eliminating the disadvantages of sodium hydroxide as alkaline medium, as described herein above and also has the advantage of producing dyed products having better dye uptake and uniform colour yield. DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS OF THE INVENTION : In accordance to this invention a complete disclosure of a method for dyeing of a fabric with a dye, particularly a method for dyeing of a fabric with indigo, more particularly a method for dyeing of a cotton fabric with indigo, and the products thereof having better dye uptake and uniform colour yield is provided, which comprises of : a) reduction of indigo/dye with organic alkali in the presence of a reducing agent, b) reaction of reduced indigo from step-a with the cotton fabric, c) squeezing of dyed cotton fabric from step-b, d) air oxidation of dyed cotton fabric from step-c, e) washing of dyed fabric from step-d and f) drying of dyed fabric from step-e. In accordance to one of the preferred embodiments of the present invention the reduction of indigo/dye with organic alkali in the presence of a reducing agent is carried out in one go and the reduced dye is kept ready for dyeing the fabric, particularly the cotton fabric. Further according to the another preferred embodiment of the present invention the reaction of reduced indigo from previous step (step-a) with the cotton fabric, squeezing of dyed cotton fabric from said step-b and air oxidation of dyed cotton fabric from said step-c are carried out in step-wise. In accordance to one of the preferred embodiments of the present invention the reaction of reduced indigo with the cotton fabric is followed by squeezing of dyed cotton fabric, which in-turn is followed by air oxidation of dyed cotton fabric and such sequence steps of reaction of reduced dye with fabric, squeezing of dyed fabric and air oxidation of dyed fabric are carried out number of times, preferably twice or more than twice. According to one of the preferred embodiment of this invention the said step-b and step-c are carried out preferably for 30 sees to 90 sees, more preferably for 30 sees to 60 sees, and the step of air oxidation is carried out for about 30 sees to 120 sees, preferably for about 60 sees. In accordance to the preferred embodiment of the present invention the organic alkali, as stated herein above is either aliphatic amine or alcoholic amine. Further, in accordance to one of the preferred embodiments of this invention the aliphatic amine is methyl amine, dimethyl amine, ethyl amine, or diethyl amine, and alcoholic amine is ethanol amine, diethanol amine or triethanol amine. According to the one of the preferred embodiments of this invention the aliphatic amine is taken between 0.5 to 5 ml for each gram of dye/indigo and the alcoholic amine is taken between 2 to 10 ml for each gram of dye/indigo. According to another preferred embodiment of this invention the reducing agent is sodium hydrosulphite (Na2S2O4). Further, 0.5 to 1.5 gms of such reducing agent is taken for each gram of dye/indigo. In accordance to the present invention, the mixture of the organic alkali, the reducing agent and the dye is allowed to react preferably for 5 to 20 minutes, more preferably for 8 to 15 minutes at room temperature. The reaction is allowed to be carried out till the dye gets reduced to monophenolate form. According to one of the preferred embodiment of the present invention the concentration of indigo/dye in reduced solution is maintained at about 1 to 7 gm/lit, preferably at about 2 to 5 gm/lit of reduced solution. In accordance to one of the preferred embodiment of this invention the step-wise reaction of reduced indigo/dye with the cotton fabric is carried out at an interval and such an interval is preferably of about 1 to 2 minutes. Further, the step-wise squeezing of dyed cotton fabric is also carried out at an interval, which is preferably of about 1 to 2 minutes. Still further, the step-wise air oxidation of dyed cotton fabric is carried out at an interval and such interval is preferably maintained for about 1 to 2 minutes. The duration of interval, as referred herein above, is the preferred duration of each step. The duration of interval, however, will also depend on the length of the fabric, which is beyond the scope of the present invention. In accordance to one of the preferred embodiment of this invention the step-wise air oxidation of dyed cotton fabric is preferably carried out after each step-wise squeezing of dyed cotton fabric and before next step-wise reaction of reduced indigo with the cotton fabric. Further, in accordance to the present invention the step-wise reaction of reduced indigo with the cotton fabric, that is the dipping and the step-wise squeezing of dyed cotton fabric, that is nipping, are preferably carried out for 2 to 10 times preferably for 4 to 8 times, more preferably by 5 or 6 times for better dye uptake. Similarly, the step-wise air oxidation of dyed cotton fabric are also preferably carried out for 2 to 10 times preferably for 4 to 8 times, more preferably by 5 or 6 times for better dye uptake. The step-wise washing of dyed fabric, in accordance to the present invention is carried out after selected number of steps of step-wise reaction of reduced indigo with the cotton fabric, step-wise squeezing of dyed cotton fabric and step-wise air oxidation of dyed cotton fabric, as described herein above. The step-wise washing of dyed fabric is preferably carried out with cold water preferably for three to six times. The washed fabric is subjected to drying and said drying of dyed fabric is carried out at about 70 to 95 degree centigrade, preferably at about 80 to 90 degree centigrade for about 2 to 10 minutes. The dyed products obtained in accordance to the preferred embodiments of the presently disclosed invention have better dye uptake, and higher and uniform colour yield of dye/indigo on said fabric. The present invention has been explained with the help of the following working examples, which are not intended to limit the scope of the present invention and are given for the better understanding of the working and the manner in which it is to be performed. The commercially bleached unfinished cotton fabric with following specification was used : a) Weave -Plain weave poplin b) Count -Warp 93 -Weft 84 c) Threads per inch -Warp 110 -Weft 85 d) Fabric weight (gm/sq mts) -113.5 The dye used was Indian origin indigo dye and reducing agent used was sodium hydrosulphite powder. The organic alkalies used were as stated herein above. The reduction of indigo dye was carried out by taking 2 gm of dye and 1.5 gm of sodium hydrosulphite powdre in 100 ml of water. One of the organic alkali was added to this in the following composition, as given in table-l. As described herein above that the composition of the organic alkali will depend on the nature of the alkali used, that is whether it is aliphatic or alcoholic in nature. Table-l (Table Removed) The above mixtures are allowed to react for about 10 min at room temperature to reduce indigo to monophenolate form of structure. The dye baths, containing dye, reducing agent and one of the organic alkali (each such dye bath constitutes one set of experiments) then were diluted to a volume of 657 ml to obtain 3 gm/lit of reduced indigo in the reduced solution of each dye bath. The cotton fabric was reacted with the reduced indigo solution and oxidised in air to get back the original colour. The dwell time in dye bath was 30 sees followed by a squeeze through the padder (82% pick up) for another 30 sees followed by air oxidation for about 60 sees. The 6 dip and 6 nip were allowed. The pH of each dye bath pH was measured before and after dyeing and was observed to be almost constant. The dye uptake was observed to be higher and colour depth was observed to higher and uniform. It is known in the art that during of cotton fabric with indigo, both reducing agent and alkali are continously consumed in the presence of air. Further, a certain minimum concentration of alkali and of reducing agent is required to be maintained in order to keep indigo in the reduced form, as described herein above. The stable dye bath means alkali and reducing agent is sufficient to keep the whole amount of indigo dye in the dye bath in reduced form to get uniform dyeing with good reproducibility. This requirement necessitates the determination of stability of reduced indigo solution with respect to time and the colour build up. Therefore, in the present experiments also it is expected that both sodium hydrosulphite and organic alkali will be consumed over the period of time. In order to check the stability of reduced indigo solution and the colour build up, the following experiments were carried out. The 3 g/l solution of indigo containing adequate quantity of sodium hydrosulphite and different organic alkalies were prepared and the results were compared with the 3 g/l solution of indigo containing adequate quantity of sodium hydrosulphite and sodium hydroxide. Sodium hydroxide was used merely for comparision of merits of the presently disclosed method with the known method of the prior art. For each alkali (organic alkali and sodium hydroxide), the total volume of indigo solution was divided into 6 parts. These solutions were stored for different intervals of time from zero, 2, 4, 6, 8 and 24 hours. The dyeing of cotton fabric with these solutions after storing for selected length of time (0 hrs, 2 hrs, 4 hrs, 6 hrs, 8 hrs or 24 hrs) was carried out by 6 dip and 6 nip with the intermediate air oxidation, as described herein above. The samples were then thoroughly rinsed with ample quantity of cold water several times (10 times), dried at room temperature and then hot pressed. The amount of dye present on each of the sample was determined by measurement of K/S and also by measuring the optical density (OD) of the sulphuric acid solution in which a known weight of indigo dyed sample was dissolved. The amount of dye present on sample dyed with sodium hydroxide as an alkali was considered as 100% colour yield. The precentage colour yield on cotton fabric dyed with indigo using organic alkalies of the present invention was expressed in comparision with sodium hydroxide as alkali. The results are shown in table-ll. Table-ll Effect of different alkalies on indigo dye bath stability and colour build up (Table Removed) It is observed from the table-ll, that when all the dye solutions are stored upto 6 hrs, all alkalies other than sodium hydroxide showed colour yield more than 1000% in comparision to that of sodium hydroxide (colour yield taken as 100%), whereas after 8 hrs some of the alkalies such as methyl amine, dimethyl amine, and ethyl amine showed colour yield less than 100% on the basis of OD measurement. All the alkalies, including sodium hydroxide, showed negligible dye uptake when the reduced dye solution was stored upto 24 hrs (data not shown in the table) before use. In order to understand why the organic alkalies have shown higher colour yield than that of sodium hydroxide, the pH of each solution was measured before padding.The results are shown in table-Ill. From this table, it can be seen that sodium hydroxide being strong alkali showed highest pH (12.3), whereas the organic alkalies being week alkalies have shown lower pH in the range of 9.62 to 10.8. Further, the pH remains almost stable with organic alkalies. Table-Ill Change of pH with time before and after 6 dips and 6 nips (Table Removed) As described herein above, that at pH 10.5 to 11.5, maximum amount of indigo is present as monophenolate form which has higher affinity for cotton. With the increase in pH, the monophenolate form of indigo decreases with the corresponding increase in biphenolate form as well as increased ionisation on cellulose and the resultant effect leads to less dye affinity for cotton. Therefore, it is clear from the table-Ill, that with organic alkalies of the present invention the monophenolate form of indigo predominates, which results in increased dye uptake and higher colour yield as compared to sodium hydroxide. Further, in order to check the colour build-up of indigo on the used cotton fabric by above methods and to compare the results of the experiments conducted by the preferred embodiments of the present invention, solutions of indigo containing 5 g/l, 10g/l, 15 g/l, 20 g/l and 25 g/l were prepared using methyl amine and sodium hydroxide (sodium hydroxide was used merely for comparision of merits of the presently disclosed method with the known method of the prior art). The dyeing was carried out by 2 dip and 2 nip without intermediate air oxidation. This procedure was adopted to simulate the 3-bowl padding mangle operation. After 2 dip and 2 nip, air oxidation of fabric carried out. The samples were then rinsed in cold water several times, and dried at room temperature. The amount of dye on each fabric sample was determined by measurement of K/S and also by measuring the OD of sulphuric acid solution in which a known weight of indigo dyed sample was dissolved. The results are given table-IV. From this table it can be seen that the colour yield in terms of K/S or OD measurements was increased with increase in dye bath concentration and at each dye bath concentration, organic alkali produced deeper shades compared to sodium hydroxide. Table-IV Effect of dye concentration on colour build-up (Table Removed) The inventors also propose buffered alkali having composition : sodium hydroxide, trisodium phosphate and sodium silicate in the ratio of 1:1:2 respectively, as an alternative alkali. But this bufferred alkali also suffer from the drawback of sludge formation, as known in the prior art, therefore, the results of buffered solution having the said composition are not presented in this description. In the commercial practice, after dyeing of cotton with indigo dye, the residual dyebath solution is not discarded, but it is used for subsequent dyeing after replenshing with required concentration of reduced indigo. Such a procedure is adopted on regular basis. This means that residual dyebath is not discarded but used again again after replenishment. In order, to check, that whether the reduced indigo solution of the presently disclosed invention can be use again again after replinshment or not, the inventors conducted following experiment. Solutions of indigo containing sodium hydroxide and methyl amine were prepared and devided into two batches (sodium hydroxide was used merely for comparision of merits of the presently disclosed method with the known method of the prior art). One batch of each solution was used immediately for dyeing and the other batch of each solution was used after storage fro seven days with replinshment of required amount of reducing agent and of alkali (organic alkali or sodium hydroxide). The dyeings were carried out by 6 dip and 6 nip technique taking 3 g/l of indigo concentration in to consideration by the preferred embodiments of the present invention for organic alkali and by known method for sodium hydroxide. The colour yield was determined by K/S measurement. The results are shown in table-V. From this table, it can be seen that although the reults interms of K/S were not indentical for these two batches of reduced indigo solutions, but it was clearly established that the residual dye bath can be reused by proper replinshment even after storage for long time. In this experiment also, the inventors observed that organic alkali showed better colour yield as compared to sodium hydroxide. Table-V Simulation of Commercial Dyeing (Table Removed) It is obvious from the foregoing description that various alternatives are possible in view of the examples elaborated herein above, which are not intended to limit the scope of the present invention but are given merely for the better understanding of the present invention and for understanding of the manner in which it is to be performed, whithin the scope of the present invention. We claim: 1 . A method for dyeing of a fabric comprising following steps: a) reducing an indigo dye with an organic amine as herein described in the presence of a reducing agent, as herein described, wherein the concentration of said dye in the reduced solution is maintained at 1 to 7 gm/lit; b) reacting said reduced indigo dye obtained from step a) with the cotton fabric; c) squeezing of dyed cotton fabric obtained from step b); d) oxidising dyed cotton fabric obtained from step c) by air; e) repeating the steps b), c) and d); t) washing the dyed fabric obtained from step e); and g) drying of dyed fabric obtained from step f) at a temperature of 70 to 95 degree celsius to obtain the desired dyed fabric. 2. A method for dyeing of a fabric as claimed in claim 1, wherein said organic amene is aliphatic amine or alcoholic amine. 3. A method for dyeing of a fabric as claimed in claims 1 and 2, wherein said aliphatic amine is methyl amine, dimethyl amine, ethyl amine, or diethyl amine. 4. A method for dyeing of a fabric as claimed in claims 1 and 2, wherein said alcoholic amine is ethanol amine, diethanol amine or triethanol amine. 5. A method for dyeing of a fabric as claimed in claims 1, 2 and 3, wherein said aliphatic amine is taken between 0.5 to 5 ml for each gram of said dye. 6. A method for dyeing of a fabric as claimed in claims 1, 2 and 4, wherein said alcoholic amine is taken between 2 to 10 ml for each gram of said dye. 7. A method for dyeing of a fabric as claimed in claim 1 wherein said reducing agent is sodium hydrosulphite (Na2S2O4). 8. A method for dyeing of a fabric as claimed in claims 1 and 7, wherein 0.5 to 1.5 gms of said reducing agent is taken for each gram of said dye. 9. A method for dyeing of a fabric as claimed in preceeding claims, wherein the concentration of said dye in reduced solution is maintained, preferably at 2 to 5 gm/lit of reduced solution. 10. A method for dyeing of a fabric as claimed in preceeding claims, wherein said reaction of reduced dye with the cotton fabric is carried out at an interval of preferably 1 to 2 minutes. 11. A method for dyeing of a fabric as claimed in preceeding claims, wherein said squeezing of dyed cotton fabric is carried out at an interval of preferably 1 to 2 minutes. 12. A method for dyeing of a fabric as claimed in preceeding claims, wherein said air oxidation of dyed cotton fabric is carried out at an interval of preferably 1 to 2 minutes. 13. A method for dyeing of a fabric as claimed in preceeding claims, wherein said air oxidation of dyed cotton fabric is preferably carried out after said each squeezing of dyed cotton fabric and before said next reaction of reduced indigo with the cotton fabric. 14. A method for dyeing of a fabric as claimed in preceeding claims, wherein said reaction of reduced indigo dye with the cotton fabric, said squeezing of dyed cotton fabric and said air oxidation of dyed cotton fabric are carried out for 2 to 10 times preferably for 4 to 8 times. 15. A method for dyeing of a fabric as claimed in claim 1, wherein said washing of dyed fabric is carried out with water for three to six times. 16. A method for dyeing of a fabric as claimed in preceeding claims, wherein said drying of dyed fabric is carried out preferably at about 80 to 90 degree Celsius. 17. A method for dyeing of a fabric substantially as hereinbefore described.

Documents:

2931-del-1998-abstract.pdf

2931-del-1998-claims.pdf

2931-del-1998-correspondence-others.pdf

2931-del-1998-correspondence-po.pdf

2931-del-1998-description (complete).pdf

2931-del-1998-form-1.pdf

2931-del-1998-form-13.pdf

2931-del-1998-form-19.pdf

2931-del-1998-form-2.pdf

2931-del-1998-form-3.pdf

2931-del-1998-gpa.pdf

2931-del-1998-petition-138.pdf