Mercerisation - Adds Strength and Lusture to cotton fibre !

What is Mercerisation? What are the benefits of mercerising? What is the Technique of mercerising?

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Chemical treatment applied to cotton fibres or fabrics to make them permanently able to accept dyes and various chemical finishes more easily. The method, patented in 1850 by the English calico printer John Mercer, also gives cotton cloth increased tensile strength and greater absorptive properties. Higher-quality cotton goods are usually mercerized. The treatment consists of dipping the yarn or fibre in a solution of sodium hydroxide and then treating the material with water or acid to neutralize the sodium hydroxide.
One of the most important treatments performed on cellulosic fibres to improve properties such as dimensional stability, tensile strength and lustre, is mercerisation. The aim of this work was to study the crystallinity, accessibility and unit cell structure changes occurring in three types of regenerated cellulose fibres (lyocell, modal and viscose) that were mercerised with caustic soda solutions of different concentrations. Differences were observed between the behavior of the viscose type fibres (viscose and modal) and that of the lyocell fibres. For the viscose type fibres, the proportion of crystalline regions increased at low alkali concentrations, while for lyocell fibres a decrease in crystallinity was observed. In all three fibres there was a transformation from cellulose II to amorphous cellulose. While for lyocell the transformation was partial, the modal and in particular the viscose fibres showed a complete transformation, and the swelling agent caused the fibre to dissolve at high caustic concentrations.

Mercerising
Mercerising is carried out in order to improve tensile strength, dimensional stability and lustre of cotton. Moreover an improvement in dye uptake is obtained (a reduction of 30 - 50 % of dyestuff consumption can be achieved thanks to the increased level of exhaustion).
Mercerising can be carried out on yarn in hanks, woven and knitted fabric through one of the following different treatments:
· mercerising with tension
· caustification (without tension)
· ammonia mercerising.
Mercerising with tension
Caustic soda mercerising is the most commonly applied mercerising technique (it is not applied to flax). Cotton is treated under tension in a solution of concentrated caustic soda (270 - 300g NaOH/l, which means also 170 - 350 g NaOH/kg) for approximately 40 - 50 seconds.
During mercerising the temperature is adjusted at low values (5 - 18 °C) when the lustre is the priority and at slightly higher levels when the improvement of the other characteristics is preferred. Because the reaction between caustic soda and cellulose is exothermic, cooling systems are applied to keep down the temperature of the bath.
Beside the conventional cold treatment, a hot mercerising process is also now increasingly applied (for hanks and fabric). The material is soaked in a solution of caustic soda close to boiling point. After hot stretching, the fabric is cooled down to ambient temperature and washed under tension.
In order to ensure a homogeneous penetration of the liquor, especially when operating at ambient temperature, wetting agents are employed. Sulphonates mixed with non-ionic surfactants and phosphoric esters are the most commonly used.
Caustification
In the caustification process, the material is treated at 20 - 30 °C with caustic soda at lower concentration (145 - 190 g/l) without applying tension. The material is allowed to shrink, thus improving the dye absorption.
Ammonia mercerising
Cotton yarn and fabric can be treated with anhydrous liquid ammonia as an alternative to caustic soda. Effects similar to mercerising are obtained, although the lustre grade is inferior to caustic soda mercerising. Traces of ammonia have to be removed, preferably with dry heat treatment followed by steaming.
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