Cross-Shade Variation (CSV) In Indigo Dyeing : Causes and Solutions

 

CSV is a serious problem that occurs on the great majority of Indigo dyeing equipment in which yarns dyed on one side of the machine are a different shade from the yarns on the opposite side. This problem is so common that it is generally accepted as unavoidable, especially in sheet ranges.

Basically, the problem arises as a result of the uneven distribution of Indigo within the dye box: 

• The Indigo dye is initially mixed in a feed tank at concentrations of from 70-100 g/l. This concentrated mix is then pumped into a recirculation line in which the dye flows in a pipe that passes around the outside of the dyeing section of the machine.
• The recirculation line normally has smaller pipes  that carry fresh dye directly  into the dye boxes on one side of the boxes and pipes on the other side of the boxes that pull the partially exhausted dye back into the recirculation line.
• In the recirculation line, concentrated dye mix from the feed tank is blended with the diluted dye mix removed from the dye boxes which creates an equilibrium between the 70-100g/l Indigo feed mix and the dye box concentrations which range from 1-5 g/l.
• The CSV problem results because as the more concentrated dye and chemical mix enters the dye boxes, there is a sudden increase of dye and chemicals on the entry side of the dye boxes and as the dye/chemical mix spreads across the yarn sheets, the concentrations are gradually reduced as they are pulled toward the exit side of the dye box.

Problem of Solubility:

• Vat dyes, which include Indigos and sulfurs, exist as colloidal particles when subjected to reduction.

• While reduction renders these dyes soluble enough to penetrate cellulose, they do not exist as a true solution as do acid or cationic dyes, which will immediately and uniformly distribute in water.

• The colloidal particles, which exist as clusters involving varying numbers of dye particles are affected by gravity and will exhibit settling behavior. This means that once the initial acceleration of being forced into the dye box losses force, the larger clusters slow down and begin to sink, while lighter particles travel to the other side of the box.
• Mathematically: V = 2r² (s-s’)g/ 9n, where V = rate of settling, r = particle radius, s =density of the particle, s’ = density of fluid, g = 980 (gravity constant) and n = viscosity of fluid.

Improving Solubility of Indigo Dye

• Some additives can improve the solubility of the dye which will reduce the CSV and also  shade variation from beginning to end of the dyeing.
• Dispersing agents are commonly used in Indigo dyeing, however only very few are effective in the dye box. Insist that chemical suppliers demonstrate the dispersing action of reduced Indigo.
• Since potassium hydroxide possesses vastly greater solubility than sodium hydroxide, an effective practice is to replace about 30% of the sodium hydroxide with the potassium form.
• Include methanol or isopropyl alcohol in the dye, about 40 g/l in the feed mix and 10 g/l to start in the dye boxes.
• Reduce concentrations in the feed mix. The typical practice in the U.S. for many years was to keep Indigo concentrations at 80 g/l, sodium hydroxide at 140 g/l (50%) and hydrosulfite at 70 g/l. Reducing the Indigo to 50 g/l is a proven way to improve shade consistency. The objection to this is that more liquor must be pumped into the machine, causing an overflow in the dye boxes, however this can be controlled by having low wet pickup on yarn entering the 1^st dye box and increasing the wet pickup on yarn leaving the last dye box (75-80%).

Machinery Considerations

• CSV occurs on most types of Indigo dye machines, but is particularly a problem on sheet  Ranges.
• In a sheet range the yarn is dyed, then sized immediately and carried to weaving with the yarns arrayed on the loom in the same order as they passed through the dye machine. Therefore, if there is a difference in shade from side-to-side in dyeing, the same variation will be present in the woven fabric.
• Rope ranges have side-to-side variation in most machine designs, however the effects of  CSV can be easily corrected, by blending the ropes of yarn which instead of being arrayed as a single, flat-sheet are separated into bundles of 300-400 yarns each. For example, in a 24-rope range, the 1^st 12 ropes on the left side may be, on average, darker than the 12 ropes on the right side. By utilizing a blending scheme such as combining the odd- numbered ropes (1,3,5…) into one fabric set and the even-number (2,4,6…), the variation will be averaged-out.
• On sheet ranges, the optimal solution is to improve distribution of dye in the box.
• The traditional recommendation for avoiding problems like CSV was to gently keep the dye in the box stirred in order to prevent settling which results in differing dye and chemical concentrations from side-to-side, top-to-bottom and from front-to-rear in the dye box. The practice was to “turn-over” the dye in the box at least 3 times per hour. In other words, with a 2000 liter dye box, the circulation system should pump 6000 liters into the box and pull out 6000 liters per hour. Unfortunately, circulation systems on Indigo machines have been downsized over many years, making this impossible. About 20 years ago, in collaboration with Morrison Textile Machinery, we were able to modify the recirculation system on a U.S. machine to correct this problem. By calculating a mass balance for the specific machine and dyeing objective and explaining it to the machinery engineer, a relatively simple modification solved the problem. An understanding of the complex dye chemistry along with fluid mechanics will solve any dyeing problem. Chemical engineers are the best qualified for significant improvements and collaboration with the machinery supplier can reap great rewards.
• A final possibility is to use suitable submersible pumps inside the dye box for uniform distribution of dye and chemicals. The design of dye boxes on many machines present obstacles to insertion of such units into the box, but there is the possibility of mounting the pumps outside the box and submersing only the tubes, with one pump per dye box.
• The CSV problem in the past has resulted from uneven wet pick up in the dye boxes. Check the wet pickup on each side of the squeeze roller by soaking a 10 gram sample of yarn and passing it through right and left sides and weigh again. If unequal, the roll pressure needs adjusting.
• Another possible source of CSV is uneven yarn tension which can be caused by a guide roll that is not aligned. Higher yarn tension on one side of the dye box results in less dye penetration.

A number of years ago, the U.S. company BJM Pumps developed a submersible pump that is not affected by alkalis or reducing agents and appears to be a good solution to CSV.

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.