With the growing quality consciousness among the buyers and exporters they are insisting on the penetration and fastness properties of the shades applied on coir for the manufacture of various coir products. Coir being a lignocelluloses material. Its natural colour changes from light brown to deep brown during exposure to light. So whatever fast shades are applied on coir, the shades are likely to change because of variation in the natural colour of the coir on which the dyeing has been done.

In the conventional method, the dyeing is carried out by treating the coir yarn/fiber in a hot solution containing the dyestuffs and the chemical, for a specific duration of time with occasional stirring of the material. In this method usually the factors controlling the dyeing namely the temperature, effective contact of the dye liquor with the material etc. are not maintained in a uniform way. When the material is dipped in the dye bath manually, the material coming in contact with the hot liquid takes up more dyestuffs from the bath and the stirring is started effectively, only after putting all the material in the dye bath. This causes unevenness in the shades especially when the light shades are taken on coir. The use of certain dye bath assistants for even pick up of the dyestuffs can improve this draw back to a certain extent. The dye bath assistants inhibit the on rushing of the dyestuffs to the material to provide, comparatively, even dyeing.

The heating of the dye bath, in the conventional method, is done by burning the firewood from below the dye vat placed on a hearth. As the firewood is fed manually it is not possible to attain a uniform temperature in the dyeing bath. Moreover the heat value of the firewood itself is not uniform, as the firewood contains unevenly distributed natural moisture, which inhabits the evolution of uniform heat. With a view to heat the dye liquor uniformly, the heating system was switched over to steam generating boilers operated by burning firewood, coal or furnace oil.

Usually the coir material is dyed with an optimum liquor ratio (for 1 kg. coir yarn - 10 liters of water and for 1 kg. of coir fibre - 15 liters of water in the mechanized dyeing system) at adequate temperature, for a specific duration of time and with forced circulation of the liquor throughout the period of dyeing for effective contact of the liquor with the material.

To get the best desired results, the coir yarn is dyed in the mechanized dyeing units, comprising of Dyeing Vats with mechanized circulation system to force the hot dye liquor to pass through the uniformly stacked yarn in two directions, inside out and outside in. After the dyeing operation is over, hydro extractors are used to drive out major part of the mechanically held up water and finally these material is dried on a drier with endless conveyors for efficient drying. The drier is designed to drive away the moisture in the coir yarn uniformly with the hot air emerging from the steam heated coils with the help of powerful blowers.

The sequence of operations in the mechanized dyeing unit is as follows:-

The knows in the export hanks of coir yarn are loosened and stacked in the carrier placed in the cylindrical pit. The main dye vat fitted with the circulation system is filled with the required quantity of water, heated with open steam and the dyestuffs and chemicals are adds as per the shade requirement. When the dye bath is ready, the carrier containing the coir yarn is placed in the dye solution by means of the hoist and the liquor circulation is started. The forced circulation of the dye liquor, in two directions alternatively, is continued throughout the period of dyeing. The dyeing is continued for one hour. After the dyeing period is over, the hot spent liquor is pumped up to the storage tank for using as standing bath a the dyed yarn is washed once in cold water. The drained yarn, usually containing an equal weight of mechanically held up water, is then subjected to hydro extraction for a period of 5-7 minutes, to drive out about 40% of water, and is fed to the continuous steam drier for efficient drying without exposing the material to the direct sunlight.

Due to the introduction of a more advanced technology, the heating by open steam is replaced by the heating of the dye liquor and air in the drier by hot thermic fluid passed through coils of pipes placed at the bottom of the dye vat and by the side of the drier the dye liquor in the dye vat and the air from the blower in the drier, the cold thermic fluid is being reheated in the thermic boiler and circulated to attain the required temperature. The thermic fluid can be heated up to 300oC and the dye liquor is brought to required temperature very quickly. As soon as the required temperature is attained by the liquid the circulation of the thermic fluid is cut off automatically by means of effective thermo control. This system is sufficient to obtain efficient and steady temperature without dilution of the dye liquor due to live steam, through out the dyeing operation. Besides that, it is compact, easy to operate and provides neatness of working atmosphere.

By K.G. Mohanan, Rt. Dyeing Master, CCRI, Kalavoor, Alappuzha, Kerala, India
© Copy Right 1999 Shyam Net, Cochin-16 India. All rights reserved. Revised: March, 2006 .