ii) Hot mercerization: Due to the rapid and extensive swelling of cotton fibres during impregnation with caustic soda solution, the structure is compacted at the surface and further penetration of sodium hydroxide is almost impossible, resulting in non uniform mercerization. Mercerizing at elevated temperature lowers the level of swelling and facilitates more even penetration of liquor into the inner parts of the yarn. In the process, the fabric is saturated with caustic soda solution at 100 degree Celsius, followed by controlled hot stretching, controlled cooling and finally traditional tension controlled washing and rinsing. Through better penetration of liquor is achieved, the complete conversion of crystalline structure from cellulose I to Cellulose II is retarded at high temperature. The principle of hot mercerizing is shown in the adjacent diagram.

The process sequence of hot mercerization is given below:

Step-1:Saturation of cotton with sodium hydroxide solution (240 to 280 gpl for full mercerizing effect and 110 to 135 gpl for semi mercerizing effect) preferably under relaxed condition at temperature 80 to 100 degree Celsius for 5 to 10 seconds

Step-2: Controlled hot stretching 

Step-3: Cooling the stretched material to a temperature less than 25 degree Celsius

Step-4: Tension controlled washing up to the caustic concentration in the fabric reaches 60 gpl (Stabilisation)

Step-5: Final washing under normal conditions

Advantages:

1. The degree of mercerizatrion is more uniform

2. Due to the better penetration of alkali solution, the desired mercerising effect is achieved more rapidly

3. The period of contact between material and caustic solution can be reduced by 50-80% (5 to 10 Seconds) resulting in greater productivity.

4. More uniform colouration than cold mercerised cotton

5. Improved lustre, tensile strength, dimensional stability, easy care properties and better receptivity to subsequent scouring and bleaching.

Two important mercerizing processes, these are:

i) Slack Mercerization

ii) Hot Mercerization

i) Slack Mercerization:  The slack mercerizing is a process in which good moisture absorption, good dye uptake and stretchability (ability to extend and recover after extension) is required but strength and lustre is not so important. The slack mercerizing can be carried out for the products like bandages, casual wear, skirts, boat covers etc.  In this process, cotton fabrics are allowed to shrink without tension in a solution of caustic soda (25 to 30%) at around 30 degree Celsius for 60 seconds. The fibres during then, shrinks to a level of 15 to 19% accompanied by considerable lateral swelling of fibres in the yarn. Following three changes take place in the slack mercerizing:

a) An increase in diameter due to permanent swelling

b) A decrease in length i.e shrinkage

c) An increase in the atmospheres volume at the loss of crystalline region.

How do you carry out Slack Mercerizing:

If slack mercerization is to be carried out continously then J-Box can be used. The J-Box consists of caustic soda liquid (23%) within which the fabric is plated and remains for 5 minutes. It then passes through a nip followed by a series of was boxes (Open Soaper) and then cylinder or stenter for drying purpose. Nipping is done very near to the places where the fabric just comes out of the J-Box in order to reduce the tension. Open soaper are used for washing and souring in which the bottom rollers are over driven to reduce the tension.