Process for Dyeing of Wool or Silk and Their Blends with Indigo

Abstract

A process for pretreatment of wool, silk or blends thereof rendering it receptive for the subsequent step of indigo dyeing. The pretreatment steps comprise in the step of cationization for imparting cationic charge and generating other dye combining sites.

Description

FIELD OF INVENTION

This invention relates to a process for dyeing of wool, silk or their blends (fibre/yarn/fabric) in tandem with indigo. In particular, this invention relates to a pretreatment of wool, silk or their blends in order to render it receptive to indigo dyeing.

BACKGROUND OF THE INVENTION

The literature on wool discloses that the molecular grid of wool (Keratin) consists of polypeptide chains bound together by salt linkages and the covalent disulphide (—S—S) linkages. Such a literature further discloses that the disulphide bond is easily and readily degraded in the presence of alkali. Alkali disrupts the disulphide bonds and the fission of disulphide bonds results in the formation of thiol groups, which are very labile.

It is known that as a natural polyamide or polypeptide, silk Fibroin (degummed silk) exhibits free amino, carboxyl and phenolic substitutes with accessible hydroxyl groups. Silk and wool have a slightly cationic character with an isoelectric point at about pH 5.0. Thus, in an acidic solution, silk fibroin or wool takes on a positive charge through absorption of hydrogen ions. The resulting electric charge can be counter balanced with negatively charged anions of the dye, whilst dyeing with anionic dyes such as acid, metal complex, direct and reactive dyes.

It is known that indigo dyeing is carried out under highly alkaline pH values and that leuco indigo has an anionic charge. However, silk and wool have a tendency to lose their cationic property in presence of alkali. Both fibre and the dye being anionic under alkaline conditions, the net result is loss of dye-fibre affinity, lighter dyeing and poor fastness.

Two of the silk properties that determine its colour behaviour are, firstly, very fine fibre fibrils and, secondly high fiber orientation. These fine fibrils produce a large fiber surface. Such a large fiber surface results in the colour yield being very low and about twice as much dye is required for achieving a given dark shade. Further, the fastness is poor. A large fiber surface also leads to a high dye-strike rate even at very low temperature resulting into rapid saturation at the fiber surface and unlevel dyeing. The high orientation of the fibres, which is a barrier to diffusion, causes a very slow rate of further dye absorption, after the first rapid saturation.

Further, the surface of silk is very easily chafed by abrasion particularly when the material is in wet swollen state.

The polypeptide chains of silk fibroin may hydrolyze in boiling water or in steam. This becomes critical in the presence of acids or alkalis. Higher concentrations of alkalis rapidly dissolve silk and wool. Degradation of silk and wool is determined by pH and the type of alkali. The mechanical stress leads to splicing of silk threads, which is severe and irreparable damage.

Thus, it would be apparent that the normal known steps of dyeing of silk cannot be employed for indigo dyeing of silk (and wool) in general. Firstly, indigo dyeing is conventionally carried out under highly alkaline pH values. Both silk and wool would cease to be cationic under alkaline conditions and would assume anionic character losing electrostatic affinity for anionic leuco indigo. The result would be lighter and skittery dyeing and poor fastness.

The rope/slasher indigo dyeing machine is a long battery of several dip-squeeze-air modules with pre-wet, Pre-wet washes and post-dye-wash off boxes. The yarn passage through the said machine would be as long as 400-500 mtrs. At dyeing speed of for example 18-20 mtrs/minute, the yarn runs through hostile environment; for 25 minutes including long exposure to caustic soda, hot washes and mechanical stresses. All these could deteriorate the mechanical properties of silk and wool.

OBJECTS OF THE INVENTION

An object of this invention is to propose a novel process for the dyeing of wool, silk or blends thereof with indigo in conventional dyeing machines.

Yet another object of this invention is to propose a novel pretreatment process for dyeing of wool, silk or blends thereof with indigo.

Still another object of this invention is to propose a novel process for the dyeing of wool, silk or blends thereof with indigo, which reduces the severity of dyeing conditions.

A further object of this invention is to propose a navel process for the dyeing of wool, silk or blends thereof with indigo, which avoids any damage being caused to wool or silk.

A still further object of this invention is to propose a novel process for the dyeing of wool, silk or blends thereof with indigo, which provides a dark and level indigo shade.

Yet further object of this invention is to propose a novel process for the dyeing of wool, silk or blends thereof with indigo, which provides adequate fastness.

Still a further object of this invention is to propose a novel process for the dyeing of wool or silk or blends thereof with indigo, which is efficient.

DESCRIPTION OF THE INVENTION

It is known that alkali disrupts the disulphide bonds in wool and results in the formation of thiol groups. Such a reaction is utilized in the present invention in that before the labile thiol groups recombine or form fresh cross links, the thiol groups are, in situ, brought in contact with (leuco) indigo, some hydrogen bonds/salt linkages between the dye and the thiol groups in the substrate are effected. Thus, the total dye-combining capacity of the substrate is increased by generating additional dye-combining groups or sites by the action of alkali on wool. The rate and extent of dye absorption is enhanced by virtue of increased accessibility of the fibre to the dye.

In accordance with this invention, a mordanting of wool and silk in the form of cationization on line enhances dye-fibre affinity by virtue of imparted cationic dye-combining sites for indigo which is anionic in alkaline media. Such mordanting also reduces the heterogeneity in the substrate and offer thereby level dyeing.

Wool and silk are sensitive to alkali and might degrade significantly when in contact with alkali particularly caustic soda for any appreciable length of time. In case of wool, such a normally negative point is instead taken advantage of by using generated thiol groups as additional dye combining sites.

Thus, to overcome the disadvantages of the prior art, cationization of wool and silk or deposition of insitu generated cationic polymer/oligomer within fibre provides the required result in that the cationic charge in the substrate could attract the anionic leuco indigo dye

According to this invention there is provided a process for the pretreatment of wool, silk or blends thereof comprising the step of online cationization of silk, wool or blends thereof to render it receptive to indigo dyeing by imparting a cationic charge and generating dye-combining sites.

The treatment comprises in the step of cationization by passing wool, silk or blends thereof through pretreatment baths and then subjecting to the step of a passage in air, which is preferably online. The online pretreatment could be effected either at ambient temperature or at sub-ambient temperature, for example, at 12-15 Decree C. The pretreatment bath comprises 3- to 120 gpl, and preferably 3 to 80 gpl of cationizing compound and 3 to 25 gpl of caustic soda either separately or in admixture. Wool, silk or their blends is subjected to a treatment in the pretreatment baths for 5 to 30 seconds each and to the step of aeration for 30 to 180 seconds.

Reference is now made to the step of dyeing of silk or wool with indigo subsequent to the pretreatment step in a manner known in the art, but the step of dyeing comprises in passing in tandem the pretreated silk, wool or blend through a plurality of indigo dye baths following the dip-squeeze-air (oxidation) steps.

In accordance with this invention, the dye bath contains (leuco) indigo, soda ash sodium hydroxide, sodium hydrosulphite and known chemical auxiliaries. As described hereinabove, wool and silk have a tendency to lose its cationic charge in the presence of alkali. Simultaneously, the addition of alkali to the dye bath is necessary in order to render the dye soluble. However, the pretreatment step renders silk or wool receptive to anionic leuco dye by virtue of imparted cationization and generated dye combining sites in the substrate. Thus, each dye bath contains 1.0 to 3.5 gpl indigo and upto 8 dye baths may be provided. The passage of wool or silk through each dye bath is for a period of 5 to 30 seconds each followed by the step of squeez and oxidation for a period of 30 to 180 seconds for each step of oxidation. The dye-bath recipe contains soda ash mostly and between 15-20 gpl, in place of caustic soda. The latter is used for fine tuning of dye bath pH. The caustic soda and the cationizing chemical in the pre-treatment are kept at subambient treatment in accordance with on embodiment of this invention. This is to reduce the severity of the process and help substrate retain its original strength.

Reference is made to the online pretreatment step in distinction to a batch process. One of the aspects of the present invention resides in the online pretreatment step which provides a short period of contact of wool, silk of its blends with caustic soda and cationizing chemicals. It is known that wool, silk or its blend is sensitive to caustic soda. Thus, one of the aspects of the present invention is to ensure that the wool, silk or its blends is not deteriorated or damaged by the pretreatment step, which is ensured by a reduced contact period with the cationizing chemicals by such an online step.

EXAMPLE

The wool yarn was subjected to online cationization. The original sample as well as the cationized sample were then analyzed for their nitrogen contents by Kjeldahl method. The sample showed increase in nitrogen content over the control by 0.35%.

The nitrogen content of the cationizing chemical viz., 3-chloro-2-hydroxy propyl trimethyl ammonium chloride is 9.18%.

The 0.35% increase in nitrogen content in cationized sample would therefore mean 5.8 gram of cationizing chemical was added up to 100 grams of wool.

The spectral study indicated no major change in the structural pattern of wool.

The nitrogen increase in case of silk was 0.2% which means 3.3 gms of cationizing chemical was deposited onto 100 gms of silk.

DESCRIPTION WITH REFERENCE TO ACCOMPANYING DRAWING

Reference is made to FIG. 1 of the accompanying drawing which illustrates a schematic line diagram of a known rope dyeing apparatus but modified to contain the pretreatment steps.

As shown in FIG. 1, the pretreatment apparatus contains Box 1 and Box 2 with squeeze rollers SR1 therebetween.

First box Box 1 may contain cationizing chemical solution followed by aeration thereafter which allows time for cationizing chemical to penetrate well into wool/silk substrate.

The second box Box 2 may contain caustic soda solution also followed by aeration.

Aeration allows time for reaction between wool/silk and cationizing chemical in the presence of caustic soda. Alternatively, Box 1 may contain both cationizing chemicals and caustic soda.

After impregnation of wool/silk with the said cationizing chemical, an aeration is effected which allows for the time for the reaction between wool/silk and cationizing chemical in the presence of caustic soda.

The second box Box 2 in such an instance could be empty-or could be used for a water-rinse

Indigo dyeing then follow in tandem, with conventional and known steps of indigo dyeing, viz., dip-squeeze-air.

As shown in FIG. 1, the pretreated substrate then passes through a plurality of dye boxes, for example 1 to 8 dye boxes, followed each time by aeration. The substrate is then subjected to post dyeing wash off in wash boxes and then passed through drying cans.

It is to be noted that the present invention is susceptible to modifications, adaptations and changes by those skilled in the art. Such variant embodiments employing the concepts and features of this invention are intended to be within the scope the present invention, which is further set forth under the following claims:

Claims

1-8. (canceled)

10. A process for on line pretreatment of a substrate comprising:

providing a substrate, wherein the substrate is selected from the group consisting of wool, silk, and blends thereof; and

pretreating the substrate, wherein pretreatment of the substrate comprises generating dye combining sites by treatment with at least caustic soda and for a period sufficient to generate said sites and without damaging said substrate.

11. The process of claim 10, wherein the pretreatment step further includes the step of cationizing by passing the substrate through a pretreatment bath followed by aeration to provide a dwell period for the pretreated substrate.

12. The process of claim 11, wherein the pretreatment step comprises treatment with caustic soda and a cationizing agent.

13. The process of claim 10, wherein the step of pretreatment in the bath is carried out for 5 to 30 seconds and aeration for a period of 30 to 180 seconds.

14. The process of claim 11, wherein the pretreatment bath comprises 3 to 80 gallons per liter of a cationizing compound and 3 to 25 gallons per liter of caustic soda.

15. The process of claim 11, wherein the pretreatment bath comprises 3 to 25 gallons per liter of caustic soda.

16. The process of claim 10, wherein pretreatment of the substrate is carried out at ambient temperature.

17. The process of claim 10, wherein pretreatment of the substrate is carried out at 12-15° C.