This is a guest post by Harry Mercer. This is a very technical description – read on if you are technically oriented..
Here are given some important dyeing processes related to indigo dyeing – specially on Rope Dyeing .
Pre-treatment is conducted in the 1st tank. The most common pre-treatments are :
1. Sulfur bottoming
2. Scouring with sodium hydroxide
3. Causticizing or Mercerizing
Pre- Treatment : Sulphur Bottoming
•The purpose of sulfur bottoming was to:
Originally to produce a dark shade on denim using less Indigo for lower costs
In the U.S. sulfur bottoms were dyed using a combination of blue and black dyes
In denim operations outside the U.S. the bottom is normally dyed with sulfur black
Pre – Treatment : Cotton Scouring
Cotton fibers contain impurities like waxes, pectins and minerals that will interfere with Indigo dyeing and result in streaks.-Sodium hydroxide at low concentrations (<5%) are applied at high temperatures (>85 C) in order to remove impurities and melt natural cotton waxes.
Pre – Treatment : Causticizing
Causticizing generally refers to using sodium hydroxide at below Mercerizing concentrations (<18%).
Cold causticizing of cotton yarn results infaster Indigo dye fading from laundryabrasion, darker Indigo color with the same % of Indigo and unique washdowns.
Hot causticizing improves colorfastness
Pre – Treatment : Mercerizing
Mercerizing is the use of strong sodium hydroxide (18-30%) to swell surface fibers.
When using strong sodium hydroxide it is important to remove all of it.
If sodium hydroxide is on the yarn as it enters the Indigo tanks, the yarn color will change.
Concentrations of sodium hydroxide more than 18% are not a solution, but a gel and are difficult to remove.
Mercerized yarns are more ring-dyed and dye more darkly than non-Mercerized.
Mercerizing is normally conducted at low temperatures, but hot Mercerizing can be employed for a more abraded appearance after garment laundering.
Indigo dyeing is unique and because of the complex chemical reactions should be correctly viewed a a form of chemical engineering.
Only Indigo dyeing requires multiple dye applications for a dark shade.
Color consistency of Indigo in recent decades has been unsatisfactory as a result of machine designs that do not apply basic principles of fluid mechanics properly and unstable dye mixes.
Commonly, a single dye lot will have between 8 and 15 visually different shades from beginning to end and also have shade differences from one side to the other.
Indigo Dyeing Methods
Indigo dyeing follows the same basic steps regardless of machine design.
Scour or dye bottoming in a heated tank,
washing tanks, dyeing(1-20),a heated tank for topping (optional) and wash tanks.
In different areas of the world,the same color is produced using 1.8, 2.0 or 4% Indigo depending on dyeing method.
The Indigo dyeing process begins with a concentrated mixture of Indigo, sodium hydroxide and reducing agent. This concentrated mixture (70-90 g/L Indigo) is delivered by pipes to the Indigo dye tanks where the dye concentration is reduced to 1-4 g/L for dyeing the cotton.
Dye Mixing Procedures
Many denim companies find it difficult to control original and washed Indigo shades.
The primary source of color differences is the instability and inconsistency of Indigo mixtures.
As the concentration of reducing agent going to the dye machine changes, the color changes.
Uniform Indigo Mixtures
For consistent Indigo dyeing, the mixture must have consistent concentrations of Indigo, sodium hydroxide and reducer from the top of the mixture to the bottom.
The main cause of inconsistent Indigo mixtures relates to concentration levels.
Instability of Indigo mixtures results from the decomposition of sodium hydrosulfite.
Consistency of Concentration
There is a limit to the amount of any chemical that can be dissolved in water.
When the limit of solubility of any chemical •In water is exceeded, precipitation occurs.
Indigo mixes should not have more than 20% solids. At higher levels, chemicals and dye sink to the bottom of the tank.
Improving Dyeing Consistency
When reducing agent sinks to the bottom of the tank, there is a higher concentration than in the top of the tank. As the dye enters the machine, the higher concentration results in a lighter, greenercolor and as the dye from the top of the tank enters the machine, the color is darker and redder.
Dye Control In Feeding Tank
Stirring the tank for 2 minutes will improve dye uniformity between top and bottom.
To avoid settling of dye and chemicals the total solids should not exceed 20%.
The “glass plate” test can be used to test concentrations of hydrosulfite in the top and bottom. If dye requires 50 seconds to oxidize, there is about 50 g/L of reducer.
Buffers In Indigo Dyeing
Alkaline buffers have been used to make very dark shades of Indigo with as little as 1% dye, more ring-dyed, faster fading.
Reductive buffers can eliminate color differences in Indigo-dyed denims and can reduce hydrosulfite use by 30-50%.
Cold Dyeing Methods
Sulfur colors can be dyed at low temperatures with specific buffers which produce more colorfast dyeings with no color variation.
Cold dyeing methods have been used to blend Indigo and sulfurs, eliminating the need for separate bottoming and topping, while eliminating shade changes in both.
Special Dyeing Techniques
On rope ranges, space dyeing techniques can be simple and produce a wide range of special effects in denim.
By dyeing part of the yarns with a sulfur top and leaving the rest un-dyed, many companies produce a slub appearance with regular yarns.
Blending ring yarns of different sizes also produces a slub appearance.
This is a guest post by Harry Mercer. Mr. Mercer has 30 years experience in the denim business including 3 prominent U.S. denim companies. He is an expert colorist for measurement and color matching as well as textile testing.