Loop Dyeing Process:
Many dyeing methods such as rope dyeing, slasher dyeing and loop dyeing are applied in the indigo dyeing process. Among those methods, loop dyeing was the first method used to dye denim. This dyeing produces more consistent indigo shades and requires less space to operate than other dyeing process. In the loop dyeing process, the yarn is dyed in a single bath instead of several. The desired depth of color is attained by passing the yarn through the vat several times. It involves running ropes of yarn through a vat of indigo dye before releasing them onto the factory’s roof to oxidize before returning them to the dye bath. It’s almost the same as slasher dyeing; the difference is that loop dyeing has only one dye bath, which means it requires less space and, to get deeper shade depths, the sheet is dipped 3 to 4 times in the same dye bath.

Loop dyeing is the original method of dyeing denim. This dyeing technique, known as ‘Loopdye one for six’, was developed in 1980 by Eckhardt Godau and his company Looptex in order to solve many problems associated with both slasher and rope dyeing. In comparison with the other indigo dyeing methods with six or eight dye baths, in the loop dyeing process, the yarns are dyed in only a single bath with one squeezing unit, after passing through the pre-treatment boxes. When the yarns exit the dyeing bath, instead of moving forward, the yarns are carried to the rear of the machine, around the top and rear of the yarn creel from where they started, to pass under the yarn creel, where they return to the indigo dye bath for another dye passage. This continuous passage of yarns that occurs between the yarn creel and the dye box represents a ‘loop’ form. According to the desired shade, the yarns can make multiple loops through the indigo dye bath. Next, they are conducted through the wash boxes and then onto the drying cylinders. After drying, the indigo dyed yarns pass directly to the sizing unit. It can be seen that loop dyeing has various advantages in terms of handling, rapidity and flexibility of process, floor space and investment cost.

List of Machineries are Used in Loop Dyeing:
The loop dyeing technique was developed by Echardt Godau and his company Looptex in 1980. Although the process is also under license by other companies such as Kusters Zima Corporation (USA) and Texima SA (Brazil), there is only Looptex, which now markets the loop dyeing machine under the Loopdye brand name. With a compact layout, this machine is equipped with the famous Looptex technology Nitrogen Indigo Dyeing Reactor (Pat.). It can produce various color shades requested by the market such as sulphur bottoming and topping, blue or 100% black sulphur shades. According to the company, thanks to the reduced length and to the nitrogen atmosphere, Loopdye can dye in half of the space of a classic machine and reaches the darker shades with very good color fastness. The short layout of the machine allows reducing hydrosulphite, water and energy consumption. The company also claims that this machine is the cheapest in the category and the simplest, which enables it to be operated by minimally skilled personnel.

Advantages of Loop Dyeing:

1. This technique may be characterized by a one-stage production process from the back beam to the weaving beam.
2. The loop dyeing machine is smaller and requires less floor space than slasher dyeing and rope dyeing machines.
3. The loop dyeing machine presents the lowest investment costs for continuous indigo dyeing machinery.
4. The dipping and skying times are shorter.
5. Maintenance and energy costs are approximately 20% lower with loop dyeing compared to slasher dyeing and rope dyeing.
6. The short slots are possible with this technique.
7. Dyeing can be performed with climatic control thanks to temperature controlled air oxidation.
8. This technique can also be applied for fine yarns.
9. This dyeing method is able to produce lightweight denims.
10. This technique requires small volumes of dyeing bath (1500 L).
11. Less chemical consumption.

Disadvantages of Loop Dyeing:

1. The productivity is essentially equivalent to a multi-boxed slasher machine.
2. This dyeing method produces more warp yarn breakage in weaving, which decreases productivity as well as fabric and garment quality.
3. Loss of elasticity of yarns.
4. There is a problem of center to selvedge shade variation.
5. During the change of lots, machine stoppage is time consuming.
6. Limits in the production of very dark shades.

References:

1. Denim: Manufacture, Finishing and Applications Edited by Roshan Paul
2. Sustainability in Denim Edited by Subramanian Senthilkannan Muthu
3. Textile Dyes By Dr. N. N. Mahapatra
4. Handbook of Weaving by Sabit Adanur

You may also like:

1. Rope Dyeing Process: Flowchart, Advantages and Disadvantages
2. Slasher / Sheet Dyeing Process with Advantages and Disadvantages
3. Denim Dyeing Process with Indigo Dyes
4. Different Types of Dyeing Methods
5. Process Flow Chart of Yarn Dyeing | Methods of Yarn Dyeing

Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.

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