Thursday, January 12, 2017

Mercerizing Process in Textile Industry

What is mercerizing:
Mercerization is a typical treatment for cotton yarns and fabrics, which improves the fabric silky luster and wet ability, ensures a covering effect for dead cotton, and improves dimensional stability and dyeing efficiency. Mercerization may be carried out on grey fibers or after scouring/bleaching. Sewing and embroidery cotton yarns are mercerized with tension in the form of hank. Stretched yarn can be made by mercerizing without tension. The stretched materials are used for bandages, casual wear skirts, boat covers etc.

Types of mercerizing:
Mercerization are two types:

1. Tension Mercerization: The purpose of mercerization is to increase luster of Cotton fibers. In this mercerizing process fiber become untwists and swells, lumen becomes rounder in cross-section and it gains luster. Dye affinity and chemical reactivity increase. Fabric becomes stronger and smoother.

2. Slack Mercerization: It is not as lustrous like as tension mercerization. Elongation and recovery properties improve and thus have been used to produce comfort stretch garments and fabric bandages, which need to conform to body shapes.

Mechanism of mercerization:
Mercerization is commonly done after scouring in textile industry, but it is preferable to mercerize after bleaching if continuous bleaching plant is available. This treatment is carried out using caustic soda, which determines the contraction and swelling of the fibers; they become translucent and increase their tensile strength, but reduce their flexural and torsion strength. The bean-like section of the fiber becomes first elliptic and then circular, allowing a better reflection of light with a consequent increase of luster. The treatment is usually carried out under tension, with caustic soda at 280 -300 Be(bom)’ (approx. 270-330g/1). If the concentration is lower than 240 Be'(bom), the treatment is called causticization and aims at enhancing the dyeing liquor penetration into the fabric.
Mercerizing machine
Fig: Mercerizing machine
The liquor temperature usually ranges between 15-200C and its uniform absorption is assured by adding mercerizing wetting agents stable in alkaline environment. Once the operation has been carried out, alkalinity must immediately be neutralized by means of a diluted acid solution. From a chemical point of view, alkali cellulose is the first material to form; the next material, which forms after repeatedly water washing is hydrocellulose, which is more reactive than natural cellulose.

Cotton wetting entails shrinkage of the material, which must be kept under tension, to avoid a fuzzy and woolen appearance. Mercerizing is carried out on yarns, fabrics or open or tubular knits. As far as yarns are concerned, before the mercerizing process in special machines, they undergo a singeing treatment to remove the fuzz and end fibers-which could otherwise prevent the perfect reflection of light after mercerizing.

Mercerizing process can also be carried out on tubular knitted goods: after the wetting process, the fabric is left reacting in a padding mangle. The withdrawal of the fabric width is controlled by means of an adjustable ring spreader while the withdrawal of the fabric length is controlled by “slowing down” the fabric before the final squeezing. The sodium hydroxide concentration is brought down to approximately 40 Be’ by means of a circular shower. The fabric is then washed, neutralized and rinsed.

A typical mercerizing process for tubular knitted fabric as follows:
The mercerizing machine has a de-twisting unit, a water trough, a sodium hydroxide liquor trough, swelling unit, four washing units and a neutralizing unit. The de-twisting unit removes twist from fabric during feeding. In water trough required water is supplied for caustic. In sodium hydroxide trough required amount of caustic is come from a central reservoir. The central reservoir store caustic at particular concentration. There is also a chiller which maintains the caustic temperature. The swelling unit has some dancing rollers which control the fabric tension. After swelling unit the fabric enters into first washing unit and then second washing unit. Each washing unit has cigger which extends the fabric circumference as washing can be done very well. The cigger widths are kept same for first and fourth and same for second and third. But the temperatures are kept different for the all four washing units. Before entering third and fourth washing units the fabric is passed through neutralization padder. Here acetic acid is used and impregnation temperature is kept 700C

Another well-proven mercerizing agent is liquid ammonia, which has to be applied for very short times (about half a second). There are very few systems based on liquid ammonia due to the difficulties connected to the use of liquid NH3.

Engineers have recently developed continuous mercerizing cycles and machines for combined mercerizing and bleaching process.

Types of machines used in mercerizing:

There are two different types of machines to be used for woven fabrics: a chain system and a cylinder system.

Chain mercerizing: With the chain mercerizing process the fibers achieve perfect brightness thanks to optimum tension control. This system runs slowly and allows no flexibility when the width of the fabric varies.

Cylinder mercerizing: This is a more compact and faster system compared to the previous one; cylinder mercerizing does not allow the contraction of the warp because the fabric is drawn in on the cylinders. The contraction of the filling yarns is also prevented thanks to the tension produced by the simultaneous action of the cylinders and of the fabric wetting. Cylinder mercerizing machines are also used for flat knits.