By: Dr. S.N. Chattopadhyay, Dr. N.C. Pan and Dr. A.K. Roy


Golden fibre, jute, is now being utilized for a number of valueadded and diversified products as it has not lost its glitter. Traditional useof jute fabric i.e., sacking and hessian will continue for its use as packagingmaterial, carpet backing, etc. Non traditional use in curtains, upholstery, furnishingtextiles and apparel textiles will increase gradually due to its eco-friendlyand biodegradable character. So, modification of feel and look of the fabrichas become essential depending on the end use requirement resulting inincreasing the demand for bright, wash fast and light fast jute fabric.


Chemical composition of jute is complex. It containscellulose, hemicellulose and lignin as the major constituent. So, chemical processing of jute is delicate as severe treatment may cause damage to the fabric. This fibre isvery sensitive to alkali due to the presence of hemicellulose and the presence of cellulose makes it sensitive to strong acid. At every stage of chemical processing, a com promising recipe is to be formulated which produces acceptable resultretaining sufficient physical properties.


First step of dyeing of jute fabric is known as preparatory process where fibre is made clean by removing its natural and added impurities whichis known as scouring. Proper scouring is the basic requirement for even dyeing.


The next step is bleaching. By this process, the fabric ismade white by removing or modifying the colouring matter present in the fibre.The main colouring matter in jute fibre being lignin, which also acts ascementing material and contributes to the strength of the fibre.


Table 1: Preparatory processing of jute fabric

Process

Chemicals used

Results

Scouring

Sodium carbonate / sodium hydroxide and non-ionic detergent

Removes natural and added impurities like sizing materials, fats, oil and waxes to make the fabric more absorbent.

Bleaching

Hydrogen peroxide, sodium silicate, sodium hydroxide and non-ionic detergent

It modifies the colouring matter present in the fabric and produce white fabric.

  • Full bleach - Full white, sold as bleached fabric
  • Half bleach - For pale and medium shade dyeing
  • Quarter bleach - For deep shade dyeing


For deep shade mild bleaching is sufficient but for producing light and pale shade full bleach is needed. Depending on the shades to be produced, the method of bleaching as well as bleaching agent can be selected. Differentoxidizing and reducing agents can be utilized for bleaching of jute fabric andamong them hydrogen peroxide, a non-chlorine oxidative bleaching agent, is themost popular.


The bleached fabric thus produced is ready for dyeing. Allthe dyes that are suitable for dyeing of cellulosic fibre can be applied fordyeing of jute fabric as cellulose is the main constituents which take part inthe dyeing process.


 

Direct, reactive, vat, sulphur dyes can be applied on jute fabric to produce variety of shade depending on the end use requirement. Presence of noncellulosic constituents in jute has made it unique with respect to dyeing as jute fibre can also be dyed with dyes like acid, basic, metal complex etc. without any modification.


So, a big range of dyes can be applied on jute fabric to produce wash fast, light fa9t and bright coloured fabric. Selection of dyes and their performance are tabulated in table-2


Table 2 : Performance of dyed jute fabric

Dyes Applied

Performance

Direct

Process is very simple and cost of dyeing is low. It produces moderate to low wash fastness and moderate to high light fastness

Reactive

Process is simple and cost of dyeing is moderate. It produces high wash fastness and moderate to high light fastness. It produces bright colours.

Sulphur

Process is slightly complex and cost of dyeing is low. It produces very high wash fastness and moderate light fastness. Generally deep shades are produced and shades are comparatively dull. All sorts of shades are not available in this dye.

Vat

Process is complex and cost of dyeing is high. It produces very high wash and light fastness. The dyed fabric is very bright and suitable for pale to medium depth of shade.


The basic requirements for dyeing of jute fabric are as follows:


Dyestuff


Colored compound must contain certain unsaturated groups (nitro, azo, keto etc) which are known as chromophores and the compound containing chromophore is called a chromogen. Chromophores are also known as colour bearing ingredient.


Certain groups are called auxochrome (colour intensifying groups) are present in the chromogen. Auxochromes includes - OH, -NH2, -COOH, -OSO3H groups. These groups are responsible for the affinity to the fibres and solubility in water.


A dye applicable to jute or other cellulosic fibre must have the properties like particular colour, substantivity or affinity to the fibre and solubility in water.


Dyestuff auxiliaries


Different chemicals like salts, acids, alkalis, surface active agents, sequestering agents, buffering agents, oxidizing agents, reducing agents, etc., are used during dyeing of jute fabric are popularly known as dyestuff auxiliaries.


Jute fabric


A well prepared jute fabric is needed for good dyeing. The fabric should be well scoured and bleached so that the dyes are applied evenly on the fabric.


 

Water


Water is the main medium for transportation of dye to the fibre surface and, from the fibre surface to the inside of the fibre through the fibre pores. Water is also responsible for swelling of the fibre so that the dye molecules can enter inside the fibre through the enlarged pore size on the surface of the fibre. Water used for dyeing of jute fabric must be soft so that the dyestuffs and other auxiliaries can get easily solubilised. Moreover, water is also needed during washing and soaping of the dyes for removing the unfixed dye from the fabric surface to improve its wash fastness.


Machineries


Machineries are needed for even dye application, penetration and fixation of dyes on the jute fabric. Jute fabric can be dyed by either exhaustion method or padding method. Exhaustion method is more popular due to the simplicity of the application. Mainly a machine known as "JIGGER" is used for dyeing of jute fabric by exhaustion method. In this machine, liquor is kept stationary and the jute fabric is kept in circulation. Jute fabric is passed from one roll to the other roll through the dye bath in forward and reverse direction several times.


Heating system


Different temperatures are needed during different steps of dyeing i.e., dye solubilisation, dye application, dye fixation, washing of dyed fabric, soaping, stripping, after treatment etc., to achieve optimum colour yield and fastness properties of the dyed fabric.


Moreover, the temperature of application varies from one class of dyes to the other. Reproducible shades are produced only when all the factors are followed properly.


Mechanism of dyeing of jute


The principles of application of different dyestuffs to jute fibre are given in brief:


  • Physical adsorption of water soluble dyestuffs from an aqueous medium to jute fabric by reversible attachment to active sites present in the fibre. Direct dyes are held in jute fibre by hydrogen bonding.
  • Mechanical retention of water insoluble dyestuffs in the jute fibre as in the case of vat and sulphur dyes. The dyestuffs are applied by temporary solubilisation before application to jute fibre and then reconverted to the insoluble form inside the fibre after the application.
  • In case of reactive dyes, the dyes are held on the fibre by forming a covalent bond.


Technology of application


a) Direct dyeing


Jute fibre has affinity towards direct dye. Direct dyeing offers better exhaustion and depth of shade in shorter dyeing time. Bleached jute fabric is dyed with direct dyes at 80-90C for one hour. The dye bath contains dye, trisodium phosphate and sodium sulphate.


After dyeing the fabric is washed with cold water, soaped with non-ionic detergent, again washed and finally dried in air.


b) Reactive dyeing


Dyeing of jute fabric with reactive dyes involves two steps i) exhaustion and ii) Fixation. Exhaustion is carried out in the dye bath containing reactive dye and glaubers' salt. Bleached jute fabric is dipped into the dye bath and kept for one hour with stirring at 30C. After the exhaustion is over, fixation of dye is carried out by adding sodium carbonate to the dye bath. The fixation process continues for 45 minutes. Thereafter, dyed jute fabric is washed with cold water, soaped with non-ionic detergent at boil followed by usual cold washing and drying.


 

c) Vat dyeing


Vat dyeing of bleached jute fabric involves different steps like vatting-dyeing-washing-oxidation-further washing-souring-washing-soaping-washing and finally drying. Application method of vat dye is given below:


Vat dye is pasted with TRO (Turkey Red Oil) and then hot water is added to the bath. Sodium hydroxide and sodium hydrosulphite are added to the vatting bath and vatting is continued for 20 min at 60C.The colour of the dye solution changes after vatting. After the vatting of the dye, required amount of water is added.


Further sodium hydroxide, sodium hydrosulphite, glaubers' salt are added to the dye bath and dyeing is continued for 60 min at 60C. Fabrics are then washed thoroughly in cold water and oxidized by air oxidation method or chemical oxidation method. Fabrics are washed thoroughly and soured in a bath of mild acetic acid solution for 15 min at ambient temperature.


After oxidation of the dyeing, the original colour of the dye is produced. Then soaping of the fabric is done at boil for 30 min in a bath containing sodium carbonate and non-ionic detergent. Ultimately the fabrics are dried in air after thorough washing with running water.


d) Sulphur dyeing


Sulphur dyes are widely used for fast colours, mostly black, brown, deep blue and are applied to the fibre as substantive leuco compounds which are subsequently converted into insoluble polydisulphides inside the fibre by exposing the dyeing to air or by treating them with chemical oxidizing agents.


Sulphur dyes are reduced by using sodium sulphide, the most common reducing agent for converting the sulphur dye to the alkali soluble leuco form. Then bleached jute fabric is entered in the reduced sulphur dye bath and dyeing continued at high temperature for one hour.


As the leuco sulphur dyes are of low substantivity for jute fibre, glaubers' salt is added to improve its dye uptake.


The fabric is then oxidized either in air or in chemical oxidizing bath for regeneration to the original insoluble sulphur dyes. The fabric is finally soaped at boil followed by through washing.


Conclusion


  • Pretreatment of jute fabric is the most important for the production of reproducible shade as final shade is mainly dependent on the substrate.


  • Jute can be dyed with direct, reactive, vat, sulphur class of dyes to produce variety of shade in different fastness characteristics depending on the value of the end product and end use requirement.


  • After treatment of dyed jute fabric produces optimum fastness characteristics. So, proper care must be taken during after -treatment to maintain optimum treatment condition.


  • Hence, production of good quality dyed jute product is now a reality.


About the Authors


The authors are associated with Chemical & Biochemical Processing Division in National Institute of Research on Jute & Allied Fibre Technology, Indian Council of Agricultural Research, Kolkata, India