Text and Science Care

Textile Science and Care 3(2+1)

Lesson 29 : Chemical Finishes

Finishes that are imparted with the help of chemicals are termed as chemical finishes.

• Bleaching: Bleaching is an optional finish for fabrics intended for white fabrics or dyed in pastel colors. Bleaching is a process of producing good white color by removing the natural brown/grey color of the woven goods. Any discoloration or stains are removed from the fabrics and are made ready for dyeing.

  Bleaching can be done at yarn stage or fabric stage. Bleaching agents are oxidizing or reducing. The oxidizing bleaches supply nascent oxygen to the colored compound and make it colorless. The reducing bleaches remove oxygen or supply hydrogen and make it colorless.

  Cellulosic fibres are bleached by the use of oxidizing bleaches such as sodium, hydrogen peroxide, sodium chlorite etc. Hypochlorite bleach is cheaper than hydrogen peroxide bleach and often employed for bleaching cellulosics especially cotton. Due to the toxic effluents obtained from this process, its use in recent years is limited.

  The industries are considering hydrogen peroxide bleaches, as it does not produce any toxic substances and the effluent does not damage the environment. The H2O2 bleaching is carried out at higher temperature with 2 to 4 gram of H2O2 in a litre solution at 10 to 11 pH. H2O2 bleach can also be sued on protein fibres. Careful handling of fabric is required in this process. The bleaching process consists of a J-shaped box where in the fabric after impregnating with the bleach liquor is piled up and left for one hour at a higher temperatures of 1000C. Later it is washed and thoroughly dried.

  Often scouring is combined with bleaching. In this case the fabric is impregnated with scour liquor and is pulled into first J box and piled up for 1 hour at 1000C and removed and washed. Then it is impregnated again with bleach liquid and made to enter into second J box and the process continues.

  Manmade fibres are easier to bleach. As there is very little foreign material, it does not require much bleaching. Peroxide bleaches are safely used. Nylon fibre generally does not react to any of the commonly used bleaches. Strong oxidizing bleach such as peracetic acid is used for bleaching nylon.
  It is to be noted that all bleaches damage the fabric to a certain extent. Selection of appropriate bleaches reduces the damage.

• Mercerizing: Mercerizing is important finish that alters the appearance of cotton fabric. It produces lustrous cotton after finishing.

  The fabric is treated with 20-23 percent w/w of caustic soda for around half an hour and then washed. The mercerizing machine consists of roller and a tenter frame.

  1. The fabric is first wetted out in water and impregnated with caustic soda solution. The saturated fabric is fed between the rollers to ensure uniform penetration of the solution throughout.
  2. Later the fabric is fed into the tenter frame in order to keep the fabric under tension so that it can control the shrinkage.
  3. After the holding the fabric tight in caustic solution for a predetermined time, the fabric is thoroughly washed, neutralized by treating with weak solution of acid and washed on rolled and dried.

  Cotton fibre swells in caustic solution and becomes round. The surface of the fibre then reflects the light and thus the fabric becomes lustrous. The convolutions are removed from the fibre due to swelling and the fibre is slightly decrystallised. The cotton polymer improves orientation and thus becomes stronger. The dye sites are improved on the surface and thus it will become more absorbent and improves dyeability.

  If the mercerization is done under slack condition, the fibre becomes lustrous but fabric decreases in strength due to decrystallisation. However it will have increased elasticity.

• Parchmentization: It is an aesthetic finish that affects the drape of a fabric. Mainly done to cottons by using sulfuric acid. Care is taken while handling fabric as sulfuric acid can dissolve and damage the fabric on long exposure. There are two types of parchmentization finishes
  • All over: A crisp and transparent fabric called organdy is produced with this finish. Sheer lawn fabric is singed, desized, bleached, mercerized and dyed with colors that resist acid damage. Then it is immersed in acid solution for a split second and dried. Cotton dissolves on exposure to acid and hardens as a cellulosic film upon drying. This finish imparts crispness and makes the fabric transparent. The cloth is later neutralized in an alkali, washed and calendered.
  • Localised: The fabric is printed in the form of a design with an acid resist material. Later it is immersed in an acid solution. This produces both opaque and transparent areas. The acid resist area remains opaque while the rest of the fabric becomes transparent on acid exposure.


• Plisse: Interesting textures on fabrics can be obtained by printing the cotton fabric with caustic soda (caustic soda) or thermoplastic fabrics with phenol. The printed areas shrink causing the untreated areas to pucker. Texture of the fabric changes permanently. Lawn when treated with caustic soda produces plisse fabric. Plisse effects can also be produced through weaving and they are called seersucker.


• Burnt out and etched effects: Blended fabrics like rayon and polyester are more suitable for producing burnt out effects. A chemical usually an acid is printed in the form a design, which then dissolves one fibre leaving the area transparent and other areas opaque. They are also known as etched fabrics. This is an example of subtractive finish as a part of the fabric is dissolved.