by Dr. Tanveer Hussain, Department of Textile Chemistry, National Textile University, Faisalabad.

Textiles are normally singed in order to improve their surface appearance and wearing properties.  The burning-off of protruding fibre-ends, which are not firmly bound in the yarn, results in a clean surface which allows the structure of the fabric to be clearly seen.  Un-singed fabrics soil more easily than singed fabrics.  Similarly, the risk of cloudy dyeings with singed articles dyed in dark shades is considerably reduced than un-singed articles.

Although textile materials can be singed in yarn, knitted or woven forms, singeing of woven fabrics is much more common as compared to other forms. Two main methods of singeing are direct flame singeing and indirect flame singeing. The important direct flame singeing parameters are:

• Singeing position
• Flame intensity
• Fabric speed
• Distance between the fabric and the burner
• Moisture in the fabric coming for singeing

If any one or more of the above parameters are not optimal, the result may be faulty singeing. There may be singeing faults which are optically demonstrable and are quite easily remedied during the actual working process.  On the other hand there may be some singeing faults which are not visible until after dyeing and which can, once occurred, no longer be repaired. A summary of most common problems in the singing of woven fabrics is given in Table 1.

Desizing is done in order to remove the size from the warp yarns of the woven fabrics. Warp yarns are coated with sizing agents prior to weaving in order to reduce their frictional properties, decrease yarn breakages on the loom and improve weaving productivity by increasing weft insertion speeds.  The sizing material present on the warp yarns can act as a resist towards dyes and chemicals in textile wet processing. It must, therefore, be removed before any subsequent wet processing of the fabric. The factors, on which the efficiency of size removal depends, are as follows:

• Type and amount of size applied
• Viscosity of the size in solution
• Ease of dissolution of the size film on the yarn
• Nature and the amount of the plasticizers
• Fabric construction
• Method of desizing, and
• Method of washing-off
  Different methods of desizing are:
• Enzymatic desizing
• Oxidative desizing
• Acid steeping
• Rot steeping
• Desizing with hot caustic soda treatment, and
• Hot washing with detergents

The most commonly used methods for cotton are enzymatic desizing and oxidative desizing. Acid steeping is a risky process and may result in the degradation of cotton cellulose while rot steeping, hot caustic soda treatment and hot washing with detergents are less efficient for the removal of the starch sizes.

Enzymatic desizing consists of three main steps: application of the enzyme, digestion of the starch and removal of the digestion products. The common components of an enzymatic desizing bath are as follows:

• Amylase enzyme
• pH stabilizer
• Chelating agent
• Salt
• Surfactant, and
• Optical brightener

The enzymes are only active within a specific range of pH, which must be maintained by a suitable pH stabiliser. Chelating agents used to sequester calcium or combine heavy metals may be injurious to the enzymes and must be tested before use. Certain salts may be used to enhance the temperature stability of enzymes. Surfactants may be used to improve the wettability of the fabric and to improve the size removal. Generally, non-ionic surfactants are suitable but it is always recommended to test the compatibility of surfactants before use. Some brighteners may also be incorporated in the desizing bath which may be carried through the end of the pre-treatment, resulting in improved brightness but again, their compatibility must be ascertained before use.

Enzymatic desizing offers the following advantages:

• No damage to the fibre
• No usage of aggressive chemicals
• Wide variety of application processes, and
• High biodegradability

Some disadvantages of enzymatic desizing include lower additional cleaning effect towards other impurities, no effect on certain starches (e.g. tapioca starch) and possible loss of effectiveness through enzyme poisons.

Oxidative desizing can be affected by hydrogen peroxide, chlorites, hypochlorites, bromites, perborates or persulphates. Two important oxidative desizing processes are:  the cold pad-batch process based on hydrogen peroxide with or without the addition of persulphate; and the oxidative pad-steam alkaline cracking process with hydrogen peroxide or persulphate. The advantages offered by oxidative desizing are:

• Supplementary cleaning effect
• Effectiveness for tapioca starches
• No loss in effectiveness due to enzyme poisons.

Some disadvantages of oxidative desizing include possibility of fibre attack, use of aggressive chemicals and less variety of application methods.

After desizing, the fabric is systematically analyzed to determine the uniformity and thoroughness of the treatment. It is first weighed to determine the percent size removed. The results are compared with a sample known to have been desized well in the lab. If the size is not adequately removed then either the treatment or washing have not been thorough. Iodine spot tests are then conducted on the fabric. The fabric is not spotted randomly but from side-centre-side at different points along the length of the fabric. The results of this evaluation give some idea of the causes of any inadequate treatment.

Some of the most common problems in enzymatic desizing are given in Table 2.