Union Dyeing Techniques, Factors and Limitations

Last Updated on 25/10/2021

Union Dyeing Techniques, Factors and Limitations

Shareful Islam Palash
Lecturer, Institute of Textile Engineering & Information Technology
Email: palash313@gmail.com

Union Dyeing:
The advent of regenerated and synthetic man made fibers has greatly increased the scope for making blends of two or more components. The oldest mixture of all is wool and cotton, which is used to make cloths known as unions. For this reason dyeing of mixtures of these two fibers is often referred to as union dyeing. Union dyeing is a method of dyeing a fabric containing two or more types of fibers or yarns to the same shade.

Union dyeing often involves coloring each fiber component separately using appropriate dyes in two different dyebaths. This will be the longest type of union dyeing process. Ideally, the dyeing of each type of fiber is so selective that the dyes for coloring one fiber completely reserve the other. This is rarely the case. Some of the dyes selected for coloring one component of the blend often stain the other type of fiber. Such cross-staining must be distinguished from cross-dyeing. Staining of a fiber implies that the dye is not well fixed and the color will have poor fastness properties. In most of these dyeing, therefore, minimizing the degree of cross-staining is essential.

In union dyeing, there are several possibilities for coloring the different fibers:

  1. Solid shade dyeing: All the fibers are identically colored;
  2. Reserve dyeing: At least one fiber is undyed and remains white;
  3. Cross-dyeing: The different fibers are dyed different hues;
  4. Tone-in-tone dyeing: The different fibers have the same hue but with different depths.

Ways of Union Dyeing:
There are three possible ways of union dyeing of protein and cellulosic fibers. Those are:

  1. Dyeing the protein or cellulosic fiber and leaving the other white.
  2. Dyeing both fibers the same color.
  3. Dyeing the two fibers different color

Union Dyeing Techniques of Protein and Cellulosic Fibers:

Dyeing the wool/silk fibers, leaving the cotton fibers white:
In this purpose generally acid dye is used to complete union dyeing process. Because, acid dye have no affinity of cotton fibers. So it is very easy to dyeing the wool/silk in union without staining the cellulose. For this process acetic or formic acid is use to instead of sulphuric acid to protect the cellulose. After dyeing the materials need to be washing well to remove the acid.

Union Dyeing
Fig: Union Dyeing

Dyeing the cotton fibers, leaving the wool/silk fibers white:
Direct dye is used for this dyeing process. Because at low temperature direct dyes will exhaust on the cellulose and at low temperature direct dye have no affinity of wool/silk. So it is best to dyeing cotton fibers, to leaving wool/silk white. In this process soda must be used, because it works as a protector for wool/silk fibers dyeing.

Dyeing the cotton and wool/silk fibers by different color:
There are different process to dyeing the wool/silk and cotton fibers by using different dyes. In this process, firstly cotton fibers are dyeing by direct dye at low temperature and then wool/silk fibers dyeing by acid dye. Finally we get different color union dyed materials.

Dyeing the cotton and wool/silk fibers by using same color:
In this purpose direct dye is used to complete unions dyeing same color. For this direct dye apply at high temperature and acidic medium. This process is very easy and cheap.

Union Dyeing Mixtures of Silk and Wool:
Silk and wool union dyeing process is partial difficult, because only without of temperature question both fibers have same affinity for dye. Silk is dyeing with acid and basic dye at low or medium temperature and wool is dyeing with acid and basic dye at high temperature.

Dyeing wool fibers, leaving the silk white:
In this process acid dye is used and it apply on wool fiber at boiling temperature to produced color. In this process some color may be adjust on silk surface. But it can remove by treatment with ammonia acetate.

Dyeing silk fibers, leaving the wool white:
In this purpose, basic dye is used to complete union dyeing. Because basic color have no affinity of wool at low temperature. So this process can be complete easily to apply basic color on silk at low temperature.

Dyeing silk and wool fibers by using different color:
Firstly wool fiber is dyeing by acid dye like as before process. Then, remove color from silk by using ammonium acetate and dyeing silk fibers by basic dye at low temperature.

Dyeing silk and wool fibers by using same color:
It is very difficult process, because wool need to dyeing at boiling temperature and silk need to dyeing at low temperature to complete this union dyeing process. Generally acid dyes are used for this process.

Factors of Union Dyeing:
It is obviously desirable that union dyeing takes less time than that required for dyeing each fiber in separate steps. Direct dyeing processes, in which both fibers are dyed simultaneously with a mixture of different or similar types of dyes in a single bath, are more economical. A number of factors, however, must be considered.

These include:

  1. The color effects and fastness properties required;
  2. The compatibility of different dyes in the dyebath;
  3. The degree of anticipated cross-staining of each fiber;
  4. The influence of the dyeing conditions on the different fibers present;
  5. The stability of the dyeings to finishing processes;
  6. The type and availability of dyeing equipment;
  7. The process costs.

Limitations of Union Dyeing:
One-bath union dyeing to give a solid shade is difficult to achieve and there are few union dyeing processes of this type. For example, there are several important limitations in the dyeing of unions containing wool or cotton because of their different sensitivity to alkalis and acids. Thus, the cotton in cotton/wool blends cannot be dyed with vat dyes because the high alkalinity required for the leuco dye solution causes extensive wool degradation. Large amounts of cotton/polyester blends are still dyed in two separate processes, one for application of disperse dyes to the polyester, followed by a second dyeing of the cotton with reactive or direct dyes.

Different fibers can be used to union dyeing. I only give some basic knowledge about it. This dyeing process need to be done very carefully because, mistake of something can damaged the materials.

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