Mercerising
This is a typical treatment for cotton yarns and fabrics, which improves the fabric luster and wettability, ensures a covering effect for dead cotton, improves dimensional stability and dyeing efficiency.
This treatment is carried out using caustic soda (28 - 30° Bé), which determines the contraction and swelling of the fibres; they become translucent and increase their tensile strength, but reduce their flexural and torsion strength. The bean-like section of the fibre 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 28°- 30° Bé (approx. 270- 330 g/l).
If the concentration is lower than 24° Bé, the treatment is called causticization and aims at enhancing the dyeing liquor penetration into the fabric. The liquor temperature usually ranges between 15-20° C and its uniform absorption is assured by adding mercerising wetting agents stable in alkaline environment. Once the operation has been carried out, alkalinity must immediately be neutralised by means of a diluted acid solution. From a chemical point of view, alkalicellulose 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 with
Rotor spray on the material, which must be kept under tension, to avoid a fuzzy and woollen appearance.
Mercerising is carried out on yarns, fabrics or open or tubular knits.
As far as yarns are concerned, before the mercerising process in special machines, they undergo a singeing treatment to remove the fuzz and end fibres which could otherwise prevent the perfect reflection of light after mercerising.
There are two different types of machines to be used for woven fabrics:
A- chain system
Chain mercerising: with the chain mercerising process the fibres achieve perfect brightness thanks to optimum tension control. This system runs slowly and allows no flexibility when the width of the fabric varies.
B- Cylinder mercerising: this is a more compact and faster system compared to the previous.
cylinder mercerising 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 mercerising machines are also used for flat knits.
Mercerising 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 4 ° Bé by means of a circular shower. The fabric is then washed, neutralised and rinsed.