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Effect of mercerization and its importance:
John Mercer discovered in the year 1844 the mercerization process. He found that tensionless treatment of cotton fabrics with a strong caustic soda solution improves the below properties of the cotton fabric:
* The dye uptake of the cotton gets increased.
* The tensile strength of the cotton is also increased.
* The lustre of the fabric gets improved in mercerizing.
* The outer wall of the cotton fibre gets dissolved and fibre swelling takes place.
* The internal reorientation of the cellulosic structure also takes place.
* The immature fibres present in the cotton fabric get dissolved in mercerizing.
* The durability of the fabric gets improved.
* The dimensional stability of fabric gets improved in the mercerizing process.
The changes in the cotton properties after mercerizing depend upon the alkali solution concentration, dwell time in alkali solution and process temperature.
The concentration of sodium hydroxide necessary to achieve the fully mercerised effect is 25% by wt solution.
Types of fabric mercerization:
Mercerising may be carried out at various stages of the preparation sequence on one of two main types of mercerising machine given as below:
1. Chain mercerizing machine.
2. Chainless mercerizing machine.
Chain Mercerizing machine:
Chain mercerizing is done on a range equipped with stenter chains for tension control. The range consists of a padding mangle followed by a set of timing cylinders and then a clip stenter frame. The chain mercerizer applies tension directly to warp and weft. The clips of the stenter chain pull the fabric in the weft direction. The reduced width during mercerization is recovered up to some extent.
Freshwater is sprayed onto the fabric to remove the caustic soda as it is held tensioned in the stenter frame.
The stenter frame is followed by a series of open width wash boxes which further reduces the caustic level.
Acetic acid is in one of the last boxes to complete the neutralization of caustic.
Chainless Mercerizing machine:
Chainless mercerizing is practised on a range where the cloth is maintained in contact with rotating drums virtually throughout the entire process. The chainless design only applies indirect tension across the weft. Consequently, the fabric construction must allow for this width loss or the fabric must be stentered to a greater width prior to mercerising, which in itself is difficult and sometimes impracticable.
Steps involved in mercerization machine:
* The caustic soda solution of 200 to 220 GPL is applied at the padding mangle at 100% wet pickup.
* The fabric passes over the timing cylinders.
* The number of the timing cylinders must correspond to the range speed and provide at least one minute dwell time.
* The fabric is clipped onto stenter chains and stretch weft wise.
* The fabric is run under the spray washers to remove caustic.
* The tension is released and washing continues in open-width wash boxes to further reduce the caustic.
* The fabric is neutralized with acetic acid and dried.
Fabric flow in Mercerization machine:
Common structure and working principle of mercerizing machine:
This unit consists of a tensioner roller, pressure roller and compensator. The main function of this unit is to collect the fabric during batch changing to avoid machine stoppage.
It contains two compartments; both are filled with caustic soda solution with the required concentration. The fabric is dipped into the caustic soda solution and finally squeezes out the excess caustic at the end of this unit.
Impregnating unit Timing Cylinders:
It consists of a series of steel roller, the fabric passes under tension in rollers and thus reaction between fabric and caustic soda take place.
It consists of a chain drive and spraying unit. To get the required width of the fabric by using the stenter chain track and to remove the excess caustic soda by spraying water.
Washing and Neutralizing Compartment:
In this compartment, it washes the excess caustic soda present in the fabric by using hot water. The fabric pH in the neutralizing compartment is corrected by dosing of mild acetic acid and then washing the excess acid.
Vertical Drying Range:
It consists of Teflon and stainless steel cylinders. The steam is passed inside the cylinder. When the fabric is passed over the cylinder the actual drying happens on the fabric.
To collect the fabric during the batch change to avoid machine stoppage and to provide a perfect out a passage to a batch of fabric without any crease.
Operating sequence of mercerizing machine:
* The Main Power supply switch is switched in ON and the steam, air and water valve are opened.
* The machine control panel is programmed for drying range, Impregnating unit, Washer control, caustic filter on/off caustic level caustic GPL, caustic temp Caustic pump.
* The setting of caustic soda tank, acid dosing system, tight & slack control, emergency offsetting, empty batch are checked properly.
* The main power is switched On and the compressed air, water and steam valves are opened.
* The quality and lot number of the fabric are checked before putting it on the machine by checking the label.
* The fabric to be run is transported to the inlet J-scray of mercerization machine using a hydraulic hand puller.
* The two ends are stitched without a crease. One end of the fabric is and the other leader fabric in the machine are stitched together. The straightness of fabric without crease is ensured.
* The water is filled in all the washers and the temperature of all washing units is set.
* The concentration of the caustic soda and other parameters like the speed of the machine, width etc are set.
* The fabric is observed for any defect before and during the process and is reported to the supervisor if any irregularities are observed.
* The speed of the machine from start to end of the process is kept the same. ( A speed of 50-80 m/min (meters per min) is required for normal mercerization operation requires. it varies depending upon the quality. Lightweight GSM fabric requires more speed and vice versa.)
* The actual flow rate of each chemical from the flow meter as well as the operating monitor is checked.
* The chemicals are kept ready for the entire process.
* The circulating pump is switched on while starting the machine.
* The exit width of the fabric is checked at every 500 meters.
* The fabric is checked for any defects in the mercerised fabric like stains – dust, chemicals, rust, handling stains, crease, water dropping, oil, grease, etc.
* If the machine stops for a long time, it is covered by leader fabric and commence water spraying the impregnating unit immediately without delay
Cleaning in the mercerising machine:
* The accumulated dust and dirt are removed regularly from the machine.
* All the rollers are cleaned properly with dry fabric at starting and end of every programme.
* The entire washer and its filter are cleaned once a day.
* The Padding mangles are cleaned properly and are washed thoroughly.
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