What mercerizing is
Mercerizing is a treatment of cotton yarn or fabric with concentrated caustic soda, usually 28 to 32 °Bé (about 270 to 330 g/L of NaOH), while the material is held under tension. The caustic is then washed out, still under tension, and the fabric is neutralized. The name comes from John Mercer, who described the effect of caustic soda on cotton in 1844.
It is applied to fabric for shirting, bed linen, knitwear and yarn-dyed goods, and to yarn for sewing thread and for the mercerized cotton thread sold for crochet and knitting.
What happens to the fiber
A raw cotton fiber is a flattened, twisted tube with a kidney-shaped cross section and a hollow channel in the center (the lumen). In strong caustic soda the fiber swells: the cross section becomes almost round, the lumen nearly closes and the twists open out. Inside the fiber, part of the crystalline cellulose changes to a less ordered form. These changes are permanent and remain after the caustic is washed out.
What the plant sees:
- Luster. A round, smooth fiber reflects light more evenly. Luster only develops if the fiber is held under tension while it swells; without tension it shrinks instead.
- Higher dye uptake. The less ordered cellulose absorbs more dye. A mercerized fabric reaches the same depth with less dye, and in dark shades with reactive dyes the saving can be 20 to 30%.
- Coverage of dead cotton. Immature fibers swell too, so the white specks that show up in dark shades on unmercerized cotton are largely covered.
- Strength. The tensile strength of the yarn goes up.
- Dimensional stability. Fabric mercerized at the right width has lower residual shrinkage.
Mercerizing, causticizing and sanforizing
These three terms are often confused.
- Mercerizing: concentrated caustic, 28 to 32 °Bé, under tension. It gives luster, dye uptake, strength and stability.
- Causticizing: caustic at a lower concentration, roughly 15 to 24 °Bé, with little or no tension. It does not give luster. It is used to increase dye uptake, cover dead cotton and stabilize the fabric at a lower cost.
- Sanforizing: a mechanical process with no chemicals. The fabric is compressed lengthwise on a rubber belt so it does not shrink later in washing. Many shirting fabrics are mercerized and then sanforized.
Where it goes in the process
Fabric can be mercerized at different points of the preparation:
- Grey, after singeing. It saves steps, but the natural wax and the size slow down wetting, so it needs a strong wetting agent and the luster tends to be less even.
- After scouring, or after scouring and bleaching. The fabric wets out easily and the result is the most even. It is the usual choice when luster matters.
The fabric can enter the range in two ways:
- Dry. Wetting is even and the caustic concentration in the impregnation is easy to keep constant.
- Wet on wet. The fabric comes straight from a previous wet step. It saves a drying pass, but the water the fabric carries dilutes the caustic, so the concentration has to be measured and corrected continuously.
Knitted fabric is also mercerized, on chainless machines that control tension in length and width. Yarn is mercerized in hanks or as a warp, mainly for sewing thread and yarn-dyed fabrics.
Process variables
- Caustic concentration. 28 to 32 °Bé for full mercerizing. Measure it with a Baumé hydrometer at the temperature the hydrometer is calibrated for, and check it several times per shift. A lower concentration gives partial mercerizing, which shows up later as lighter dyeing.
- Temperature. Cold mercerizing, at 10 to 15 °C, gives the most luster but needs cooling and a longer contact time. Hot mercerizing, at 50 to 60 °C, penetrates to the core of the yarn faster and more evenly but gives less luster.
- Contact time. On continuous ranges the reaction is complete after about 40 to 50 seconds cold and 25 to 30 seconds hot. Heavy fabrics or poor wetting need more.
- Tension. The fabric has to be held at width (clips on chain machines, rollers on chainless machines) and in length while it is in the caustic and while the caustic is washed out. If tension is released while the fabric still carries strong caustic, it shrinks and the luster is lost.
- Washing and stabilization. The caustic is washed out with hot water in countercurrent while the fabric is still under tension, until the concentration in the fabric drops to about 6 to 8 °Bé. The weak caustic from this section is normally concentrated again in an evaporator and reused.
- Neutralization. The rest of the alkali is washed out and the fabric is neutralized with acid in the last compartments. The fabric should leave at a pH between 6 and 7. Residual alkali yellows the fabric during drying and changes the pH of the next dye bath.
The wetting agent
Caustic soda at 30 °Bé wets cotton slowly, and grey cotton with its natural wax even more slowly. Most common wetting agents do not dissolve at that concentration: they separate out and stop working. A wetting agent for mercerizing has to:
- stay dissolved and active in caustic up to 30 °Bé or more
- wet the fabric within a few seconds, so the reaction is complete within the contact time of the range
- produce little foam, both in the impregnation and in the caustic recovery evaporator
- be free of cresol, which the older mercerizing wetting agents used and which plants now avoid because it is toxic
The dose depends on the fabric: grey fabric needs more than scoured or bleached fabric. A simple check is the sinking test: a piece of the fabric dropped into the caustic with the wetting agent at the planned dose should sink within a few seconds.
How to check the result
- Barium activity number (BAN), AATCC 89 method. It compares how much barium hydroxide the mercerized cotton absorbs against the same cotton unmercerized. Unmercerized cotton gives about 100, a value above 150 means complete mercerizing, and values in between mean partial mercerizing or causticizing.
- Comparative dyeing. Dye a strip of the mercerized fabric and a strip of the same fabric unmercerized in one bath with a direct dye. The mercerized strip should come out clearly darker, and strips taken from the left edge, the center and the right edge should match each other.
- Width and residual shrinkage against the specification.
- Fabric pH after neutralization.
Common problems and what they usually mean
- Edges and center dye differently: uneven caustic pickup or uneven tension across the width. Check the squeeze pressure and the width setting.
- Bands or streaks along the length: the caustic concentration or the temperature changed during the run, often from dilution in wet on wet processing, or the fabric stood in the caustic during a machine stop.
- Light dyeing or white specks in dark shades: incomplete mercerizing. Check the concentration, the wetting and the contact time, and run a BAN test.
- Little luster: low tension, high temperature, or poor wetting of grey fabric.
- Fabric narrower than specified or high residual shrinkage: tension released before the caustic was washed out.
- Yellowing or harsh hand after drying: residual alkali. Check the fabric pH and the neutralization.
Products for mercerizing and causticizing
CASDITOL FSE is a low-foam anionic wetting agent for mercerizing and causticizing cotton yarn and fabric and their blends. It wets in caustic soda from 12 to 32 °Bé, is free of cresol, and is compatible with anionic and non-ionic auxiliaries. Typical dose: 5 to 10 g/L in continuous processes and 2 to 5% in exhaust, depending on the process and the wetting required.
CASDITOL 54-P is a sequestrant-dispersant for iron, copper, calcium and magnesium, stable at high temperature and alkalinity. It is used in causticizing, scouring and bleaching to keep hardness and metals from depositing on the fabric. More on this in our article on water hardness in textile dyeing.
CASDITOL ACID is an acid neutralizer and sequestrant-dispersant free of acetic acid and other volatile weak acids. It neutralizes the alkalinity down to the inside of the fiber without forming the sodium acetate that yellows the fabric.
To set up a mercerizing or causticizing process or to request a sample, contact our technical team.