Mercerization of Knitwear | Mercerization for Finishing of Fabrics

Mercerization, the treatment of cotton with a strong caustic alkaline solution in order to improve the luster, hand and other properties, was names after its discoverer, John Mercer.

Mercerising is required most of the time for cotton and its blends, where dimensional stability, lustre/brilliance of shade and economics in dyeing are of prime importance. Certain classes of colours / some colours of the same class do not render themselves suitable for application on un- mercerised cotton substrates, particularly coverage of immature / dead cotton. Therefore the choice of colours for application becomes limited. Still majority of circular knitted hosiery is not mercerised; the reason is not that it is not required, but the attendant difficulties restrict the processors to find other methods to achieve results – a compromise that cannot substitute mercerisation. Technology to mercerize in tubular form and the advanced machineries developed in recent times have made it feasible to extend mercerization to tubular knitwear. 
Mercerized knit fabrics
Fig: Mercerized knit items
Fabric Properties Changes During Mercerizing:
  1. A more circular fibre cross-section
  2. Increasing tensile strength
  3. Increased lustre
  4. Increased apparent colour depth after dyeing
  5. Improved dyeability of immature cotton (greater uniformity of appearance)
  6. Increase in fibre moisture regain
  7. Increase in water absorption
  8. Improved dimensional stability.
Mercerization -Theory and Practice:
Application of Caustic Soda swells cotton. Whereas a higher concentration of caustic soda is required at higher temperatures, same level of swelling is obtainable with lower concentrations at lower temperatures. When stretch forces are applied to counter the swelling and the caustic is washed off in the stretched condition the cotton retains the new dimensions. John Mercer was the first to observe the phenomenon of swelling in strong caustic soda and extensive work on this process by later workers led to what has come to stay as mercerization.

The term stretch here should be understood as the stretch force applied to offset the normal shrinking process during swelling to the extent possible without rupture to the matured fibres.

The optimal profiles of Concentration, Temperature and Time were determined as 50 deg Tw (Sp.Gr. 1.25 – 300 grms/liter), 17deg C and 50 secs, respectively

Swollen fibre tends to become cylindrical in shape and the lumen or the central cavity tend to get reduced in size. Due to stretch de -convolution of the cotton fibre is also facilitated – these two physical changes enable efficient and even reflection of incident light, thus improving luster and brilliance. Also much of the crystalline regions are converted to amorphous state and therefore, cotton becomes more permeable providing easy and freer access for various dyes and chemical solutions to diffuse /penetrate in to the substrate. The strength also improves considerably.

The immature cotton gets ruptured due to swelling and along with the dead cotton gets dissolved in the merceriser strength caustic Soda.

Over the years on the basis of cost benefit considerations the approach to mercerisation concentration/temperature profiles have undergone changes but the principle of swelling and stretch has remained the same

Mercerisation Temperature:
Considering the cost of cooling to 17 deg C (refrigeration) with only marginal advantages in swelling, mercerization nowadays is carried out at ambient temperatures The dwell time has not under gone any change and remains at around one min (50 Secs.) that is the minimum.


There have been a number of views put forward by Processors that the desired swelling and dissolution of immature / dead cotton are achieved even at lower concentrations and for very dark shades the reflected light does not contribute much to the luster and therefore lower concentrations of caustic soda would suffice. Causticizing at 12% to 18% strength is still being practised. Therefore strengths in the merceriser impregnation bath are being maintained at lower levels.

This tendency should be avoided to prevent any complacency and ending up with inadequate mercerisation. With due respects to these views we shall proceed further.

The more important aspect is that the mercerizer bath concentration need to be maintained consistently to avoid variation in dyeing/finishing deficiencies. The impregnation bath concentration has been generally established at 270 g p l (48 ºTw) of Caustic Soda.

The density measurements to control the bath strength with hydrometers in º Tw or º Be.. is widely practiced but such measurements are likely to mislead the correct concentration of the caustic soda, as the bath would progressively bet contaminated with leached out impurities from the fabric undergoing mercerization and that would also contribute to density figures.

Mercerization of Knitwear:
Perfectly mercerized knit goods are characterized by:
  1. Pill-free surfaces
  2. Increased dimensional stability in both length and width
  3. Considerable lower residual shrinkage values than with unmercerized cotton
  4. Permanent lustre
  5. Excellent brilliance and depth of colour
  6. Silken lustre
  7. Improve wearing properties
  8. Lower tensile strength loss during high performance finishing
The knitted fabrics must be fed into the impregnating part of the machine completely uncurled, under controlled lengthways and width ways guiding. This requirement must be met without compromise, but it can be satisfied with a combination of pneumatic and mechanical expanding devices and exactly controllable draw nips. The guiding elements should be arranged so that the last fabric expanding and guiding is applied as possible to the immersion point into the liquor. As soon as the fabric is wetted with liquor, the problem of edge curling disappears.

The circular knitted fabrics can be mercerized in the existing knit-mercerising machines after slit opening of the tubes. However, some major problems of such fabrics cannot be solved with existing machines, namely selvage mark, selvage curling and selvage compacting. Mercerisation may give rise to different warp threads densities between the middle of the fabrics and the edge zones. In chainless machines, these differences are mainly due to loss of fabric width during mercerizing, with the edge zones taking whole of the shrinkage and entailing an inevitable increase in the number of warp threads at the edges. The more the fabric is stretched across the width, higher is the difference in warp density, through in practice, a certain difference can be tolerated. Greatest attention is to be given for the patterned goods, especially with circle effects.

Controlled tension is important factor for optimum tube mercerizing, lustre and uniformity. During the critical phase of stabilizing, when the caustic is washed out- the tubes must be stretched in their natural tubular condition. An even stretching is achieved by a steplessly adjustable circular expander, which is placed inside the fabric tube vertically in the centre of the washing tower. Spray rings, arranged around the expander, are equipped with flat spray nozzles through which hot water is sprayed at about 80 ̊C. the circular expander is held vertical by four concave rollers. The base is designed as floating body. The expander stands in the stabilization bath and its weight is nearly compensated by its buoyancy.

Author of This Article:

Md. Jasimuddin Mandal
Govt. College of Engineering and Textile Technology, Serampore, India. (Under West Bengal University of Technology)
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Mazharul Islam Kiron is a textile consultant and researcher on online business promotion. He is working with one European textile machinery company as a country agent. He is also a contributor of Wikipedia.

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