Last updated on September 18th, 2023 at 12:09 am
This is a special treatment for cotton yarns, fabrics which improves the fabric luster and wet ability, ensures a covering effect for dead cotton, improves dimensional stability and dyeing efficiency.
This treatment is done by using caustic soda, which determines the contraction and swelling of the fibres; they become translucent their tensile strength, but reduce their flexural and torsion strength. The section of the fibre which is bean-like becomes first elliptic and then circular, allows the better reflection of light with a consequent increase of luster. The treatment is generally carried out under tension, with caustic soda at 280 to 300 C. If the concentration is lower than 250 C, the treatment is called causticization and aims at enhancing the dyeing liquor penetration into the fabric or yarn.
The liquor temperature usually ranges between15 – 200 C and its uniform absorption is assured by adding mercerizing wetting agents which is stable in alkaline environment. Once the operation has been done, alkalinity must immediately be neutralized by using 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 reactivity is more than natural cellulose.
Points to be Considered
Cotton wetting entails shrinkage of the material, which should be kept under tension, to avoid a fuzzy and woolen appearance. Mercerization is carried out on yarns, fabrics or open or tubular knits. As far as yarns are concerned, before the mercerizing process in special machine, they undergo a singeing treatment to remove the fuzzy and end fibres-which could otherwise prevent the perfect reflection of light after mercerizing.
There are two different types of machines are used for woven fabrics: a chain system and a cylinder system.
With the chain mercerizing process the fibres achieve perfect brightness. This system runs slowly and allows no flexibility when the width of the fabric varies.
This is 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.
Mercerizing process can also be applied 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, neutralized and rinsed.
Another alternative 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 (toxicity, formation of blends that can explode in presence of air and very strict regulations concerning the welding of steel sheets used to build these systems that operate at very high pressures since the boiling point of ammonia is usually .33°C).
Purpose of Mercerizing
The main changes occur in mercerizing of cotton are:
- Solubility in solvents is developed.
- Increase of luster.
- Area of cloth and length of yarn is reduced.
- Removes immature or dead cotton.
- Tensile strength is also increased.
- Dyestuff absorption is increased.
- The physical compactness of cloth and yarn is increased.
- Absorption of water is increased.
- Reactivity with oxygen (air) is increased.
- Even at lower temperature cotton reactivity is increased.
- Sodium hydroxide (caustic soda) is preferentially absorbed during the process.
- Lowers dye cost (up to 40 % on certain colors).
Appearance can be improved by increasing luster, a deepening of the color and the production of a transparent look, the feel of the fabric can also be improved through a resulting soft hand and improved smoothness, and strength and elongation are also improved, along with the addition of good stretching ability. The treatment and handling can be adjusted by fitting different requirements, thus allows the best application of the results of different processing.
You may also like: An Overview of Water Resistant and Wind Resistant Clothing