Dyeing of Cotton with Indigo and Products Thereof

Introduction

Indigo dye blue denim is popular than any items of apparel because it maintains its brightness and hue on repeated washings. Indigo belongs to a class of vat dyes and contain about 96% indigotin, requiring only about 2% dye to produce the navy blue. Dyeing method comprises of reduction of the dye (indigo) with alkali (sodium hydroxide) in the presence of a reducing agent (hydrosulphite) followed by treatment of the fabric in the dye bath. Major disadvantages of this method are, variation in concentration of reactants in the dye bath affecting the quality of dyed fabric; frequent replenishment of reactants and lesser affinity which require repeated dipping and nipping techniques. In the case of cotton fabric it has other disadvantages such as unstability of reducing agent, poor dye uptake, shade depth fluctuation, poor and non uniform colour yield, no affinity for dye, etc. From this point of view, particularly for dyeing cotton fabric with indigo, a dyeing process with the use of an alternative alkali has been developed. This process is economical, time saving and consumes less amount of dye (Indigo).

The dyeing process developed is related to the replacement of Sodium Hydroxide with organic alkalies during the dyeing of cotton warp sheet with Indigo for the manufacture of denim fabric. The method is based on the principle that the Indigo yield is considerably higher if the dyeing is carriedout in the pH range of 10.5 to 11.5. The use of Sodium Hydroxide gives the pH range 12 - 13 at which Indigo yield is much lower, whereas the organic alkalies give the desired pH during dyeing. The process comprises of the following steps : a) Reduction of dye (indigo) with organic alkali in the presence of a reducing agent; b) Reaction of reduced indigo with the cotton fabric; c) Squeezing of dyed cotton fabric; d) Air oxidation of dyed cotton fabric; d) Air oxidation of dyed cotton fabric; f) Drying of dyed fabric. The dye, organic alkali and the reducing agent taken in the specified ratio is mixed and allowed to react till the dye gets reduced to monophenolate form. Reaction of the cotton fabric is carried out at prescribed duration of interval. This is followed by squeezing, air oxidation washing and drying of the fabric, which are carried for a specified time and, at a specified temperature.

Special Features

The method of dyeing developed particularly for cotton has the following advantages: Replacement of sodium hydroxide alkali medium by organic alkali medium; Reduction of dye in one go; Better dye uptake; Higher colour yield of dye (Indigo); Higher colour yield of dye (Indigo)

Prospective Users

All fabric manufacturers particularly cotton denim

Keywords

Fabric, Cotton, Dye

Type of Technology

Process

Status of IPR Protection

Indian Patent Application No. 2931/DEL/98 dated 5.10.98 with title "Process for dyeing of cotton with Indigo"

Major Equipment & Facilities required

Existing facilities for the production of blue denim cotton could be used. No additional equipment/facilities are required

Requirement of Raw Materials/Components

Organic alkali replaces the use of Sodium Hydroxide with the other raw materials remaining the same for the preparation of blue indigo dye

For further information please contact

Managing Director
Foundation for Innovation and Technology Transfer (FITT)
Indian Institute of Technology, Delhi
Hauz Khas, New Delhi-110016, INDIA
Tel : 91-011-26597167, 26857762, 26581013, 26597153
Fax : 91-011-26851169
E-mail : drkdpn@gmail.com