Wet Processing of Terry Fabric

Wet processing is one of the most important portions of textile processing. In fact it can be said a vital part of whole processing as its objective is to improve the attractiveness of the gerige product. Terry towels are some bit different from the usual fabrics. Therefore they need some bit different processing techniques.

Chemical Processing of Terry Fabrics:
Grey terry towels contain various unwanted constituents such as contents of size recipe, natural wax and coloring matters, fragmented seed coats, leaves, stems etc which need to be removed from the fabrics with a view to obtain desired functional as well as aesthetic properties in terms of softness, water absorbency, whiteness etc.

As all above-mentioned impurities are embedded in yarn, which is strongly interlaced in fabric, therefore, removal of these impurities from the fabric is a complex job. Treatment of fabrics with a suitable combination of chemicals at required temperature and pressure for the specified duration helps in dissolving the deeply embedded impurities, while mechanical treatment in the form of stirring, rinsing, etc. helps in separating out the dissolved impurities from the fabric/yarn.

Terry towels dyeing
Fig: Terry towels dyeing
As the terry fabric contains piles/loops, therefore, the processing of these fabrics requires gentle treatment so that the configuration of piles is maintained in desired manner.

Kiers, winches, open tubes, etc., are conventionally used for de - sizing and bleaching for most type of fabrics. In these equipments, no specific arrangement was installed for treatment of delicate fabrics like terry towels. But after the invention of ‘soft flow machine’ the effectiveness of chemical processing in terms of both cost and quality is increased a lot.

In addition to soft flow machines, development of continuous dyeing range enhanced significantly the productivity and quality of terry fabrics.

Soft flow terry towel dyeing machine: 

Texlink Engineering Dongguan Co Ltd, China, represented in India by the Delhi-based J K Consultancy & Services Ltd, offers SL-ECOT atmospheric soft flow towel dyeing machine, which is designed specially for towel fabric dyeing combined with the advanced dyeing and environmental friendly technology.

Considering huge lint during the dyeing process for towel fabric, an extra pump filter is equipped to the machine, and both pump filter and heat exchanger filter are fitted with BGF Easy Clean Filter, which can greatly save the time for cleaning filter & assure smoothly circulation of the dyeing liquid.

Due to the heaviness & bulkiness of the towel fabric, SL-ECOT is equipped with specially designed extra large nozzle and fabric delivery tube, combine with more powerful driving motor for the pump, litter reel, plaiting device and layout device which are tailor-made for heavy towel fabric.

Advanced liquid/fabric separating system and rapid circulation technology have been adopted during design of the fabric storage to achieve maximum actual capacity and low liquor ratio of 1:5-6. Other features include high efficiency heat exchanger, level Indicator and controller, service tank with feed pump and pre-pump filter. A disposal filtering cloth bag properly designed and run during dyeing makes hard cleaning job of filter unnecessary. An advanced DTR Controller has been designed and fabricated with latest dyeing technology for future network.

Why Singing is not Done in Case of Terry Towel:
It is really amazing that; Singing that is the first station of wet processing; is not done in case of terry towel. In fact singing is strictly prohibited in case of terry fabrics wet processing. Its reasons are as follows.

  1. Singing is mainly done for plain fabrics; in order to remove hariness from their surface & to smoother them. But terry fabrics contain loops/piles on their surface. No smooth surface effect is required in this case.
  2. If singing is done in case of terry fabric; then the piles would be burnt out & the fabric would become faulty.
We have visited the wet processing floor of Asia Towels (BD) LTD. (Unit-2); CEPZ, Chittagong & Oriontex Industries (BD) LTD, CEPZ, Chittagong for the purpose of our Project Work. Our practical finding about wet processing of terry towels is presented here.

In those industries we found that two types of products are produced. These are:

  1. Full Bleach Product
  2. Dyed Product
The flow chart of wet processing used in Premier Towels (BD) Ltd is given bellow:
 
1. Flow Chart of Full Bleach Product:

Gray Fabric Stitching

Desizing + Scouring + Bleaching (Full Bleaching)

Washing 1

Washing 2

Washing 3

Drying

2. Flow Chart of Dyed Product:

Gray Fabric Stitching

Desizing + Scouring + Bleaching (Half Bleaching)

Washing-1

Washing-2

Washing-3

Finishing

Drying

Dyeing

Washing-1

Washing-2

Washing-3

Washing-4

Finishing

Drying

Procedures of each stage of wet processing department are described bellow:

Procedure for Full Bleach Product:
The basic difference between full bleached products & dyed products is that; the full bleached products are not dyed. Those are exported in optical white color. But the dyed products are half bleached at fist then dyed. In case of full bleaching, the required chemicals are used in superior quantity than that of half bleaching.

Desizing + Scouring + Bleaching:
Desizing is intended to remove size from the fabric to ensure even bleaching, level dyeing and soft handle. Desizing processes differentiate according to the sizing agent used. The classical desizing process consists of removing the starch from towel fabric using enzymes. This desizing process simply involves liquefying the film of size on the product. Bacterial, malt and pancreas amylases are used as desizing agents. Enzymatic desizing is the classical desizing process of degrading starch size on cotton fabrics using enzymes. Enzymes are complex organic, soluble bio-catalysts, formed by living organisms that catalyze chemical reaction in biological processes. Enzymes are quite specific in their action on a particular substance. A small quantity of enzyme is able to decompose a large quantity of the substance it acts upon. Enzymes are usually named by the kind of substance degraded in the reaction it catalyzes.

Scouring is done in order to remove natural impurities & spin finish from the fabrics.

Bleaching is done to obtain optical white fabric, that ensures level dyeing.

The wet processing procedure starts with gray fabric stitching. Gray fabric pieces are stitched together in order to make a continuous form of fabric. Then the fabric is feed to gray feed by beam player in rope form. From there the fabric is feed to the saturator of combined bleaching & scouring unit. In fact in this saturator bleaching & scouring is done together.

The chemicals used for bleaching & their functions are given bellow:

chemicals used for bleaching & their functions

The enlisted chemicals are mixed in an over head mixing tank. There are two tanks in this unit each having capacity of 2000 liters. One is for mixing & the other is reserve tank. The chemicals are sent to the mixing tank by using dozing pump according to the recipe. There is arrangement of agitation for mixing the chemicals uniformly.

From tank mixed up chemicals (liquor) is sent to the saturator. Fabric is feed to the saturator in rope form from the gray feed. The saturator is provided with immersion roller & padding rollers. After impregnation & squeezing, material is send to the J-Box in rope form. Between saturator & J-Box a sensor controlled feed control tray called denser is used. Its purpose is to synchronize feed to the J-Box & delivery from the saturator automatically.

Steaming in J-Box:
In J-Box, the feed & delivery of material is controlled in such way that the material remain inside the J-Box for a certain period. Depending on the size of the towel this dwell time varies between 1.5 hrs to 2.0 hrs. During this time steaming is done to the material inside of the J-Box. Steaming is done in order to remove the natural color of cotton, which is the basic objective of bleaching. Steam temperature is kept within 90-1100C. It depends on the fiber composition & volume of material inside the J-Box.

Washing:
After steaming the fabric is sent for washing via a denser. For half bleach product, three washing m/c is used one after another. In each washing m/c fabric is passed in rope form. Water At room temperature is feed to the m/c & steam is also feed to heat the water at desired temperature. As the fabric passes through hot water so it is washed out. After washing the fabric is squeezed & the used water is drained out. Fabric passes from one washing m/c to another in rope form via denser. Same process is used in each washing m/c but the temperature remains different. The temperatures used in different washing m/c are given bellow:

  1. Washing m/c 1: 90-95 0C
  2. Washing m/c 2: 85-90 0C
  3. Washing m/c 3: 80-85 0C
Finishing:
After third washing the bleached fabric is delivered to the wet feed via denser. From there fabric is feed to the finishing pad. The objective of finishing is to soften the fabric as well as to remove dirt materials if any.

In the finishing pad two mangles are used. Each mangle is provided with an immersion roller & a pair of squeezing rollers. In the first mangle water at room temperature is supplied. In the second mangle finishing liquor is supplied by a perforated pipe from an over head tank. This m/c is provided with two over head tank; One for mixing & other for reserve. Capacity of each tank is 1000 liters.

The chemicals used for finishing & their functions are given bellow:

chemicals used for finishing & their functions

At first the fabric passes in open width form through the first mangle containing water & then through the second mangles containing liquor. After squeezing in the second mangle the fabric is delivered in to a trolley via a spreader.

Drying:
The next step is drying. Loop Dryer is used in this mill. Loop dryer is provided with eight cylinders (at the beginning) & five chambers. There is arrangement of heating the cylinders & chambers by steam. The fabric is feed over the cylinder surface to the m/c. As the fabric enters into the m/c; fabric loop is formed. This m/c is provided with chain that carries many rollers. Fabric passes through the m/c in loop form over these rollers.

Temperatures of cylinders & chambers are enlisted bellow:

  1. Temperatures of cylinders : 105-110 0C
  2. Temperatures of chamber-1: 85-90 0C
  3. Temperatures of chamber-2: 80-85 0C
  4. Temperatures of chamber-3: 40-45 0C
  5. Temperatures of chamber-4: 60-65 0C
  6. Temperatures of chamber-5: 80-90 0C
Finally, after drying fabric is delivered to the quality department situated in the second floor via some guide rollers.

Procedure for Dyed Product:

Desizing + Scouring + Bleaching (Half Bleaching):
Similar procedure as full bleached product is used in case of dyed product. But in this case Half Bleaching is done. That means here the chemicals for bleaching are used in minor quantity. The processing technique in saturator, steaming parameters & methods are all the same as described before.

Finishing:
In case of dyed product finishing is done twice. The first finishing is done after half bleaching & the second finishing is done after dyeing & washing. The first finishing is done only with water, no chemical is used. Water at room temperature is taken in both mangles of finishing pad. After the first finishing the material is dried is sent for drying.

Drying:
After the finishing the material is dried in loop dryer. Drying is done in similar way as described before. Then the material is sent for Dyeing.

Dyeing:
Reactive Dyes is mostly used for dyeing of terry towels. The reasons of selecting reactive dies are given bellow:

1. Bright Shade: The reactive dyes are the brightest dyes available for the cellulose fibres and have a full range of shade

2. Good Fastness Properties: Color Fastness is the resistance of a material to change in any of its color characteristics, the resistance to transfer its colorant to adjacent materials, or both, as a result of the exposure of the material to any environment that might be encountered during the processing, testing, storage, or use of the material by the consumer.

3. Better Wash Fastness: Towels dyed with reactive dyes have very good wash fastness properties. The wash fastness rating is about 4-5 per AATTCC standards.

4. Better Light Fastness: Towels dyed with reactive dyes have very good light fastness. The light fastness rating being about 6 per AATTCC standards. These dyes have a very stable electron arrangement and provide very good resistance to the degrading effect of the UV component of sunlight. There are, however, some reactive dyes with only fair light fastness.

5. Better Bleaching Fastness: The reactive dyes are stable to peroxide bleaching and so are suitable for towels which might be used in the Bathroom around cosmetics. Strong reducing agents and chlorine, however, destroy the chromogens.

6. Easy Application: Reactive dyes offer a great flexibility in application methods with a wide choice of equipment and process sequences and so they have become very popular with most all modern mills. These are applied through exhaust and continuous systems both very comfortably.

7. Moderate Cost: Reactive dyes as compared to vats are of lesser costs considering the fastness properties of both. The dyeing process involved in vat dyeing is also costly which involves certain steps like reduction and oxidation. On the other hand reactive dyeing is free from these steps.

Dyeing Procedure:
Tarry towels are dyed in continuous & open width form. At first the dye liquor is prepared according to the recipe. In this mill Cold Brand Reactive Dyes are user. Dyes of Hungsman Co. are mostly used. The dyes are mixed with required amount of cold water according to the recipe. The dyeing m/c is provided with two over head tanks, one for dye reserve & another for chemical reserve. Capacity of each tank is 2000lt.

Name & purpose of dyes & chemicals used are given bellow:

Name & purpose of dyes & chemicals

Dye liquor & chemicals are supplied to the dye bath separately by separate dozing pump. Dye liquor & chemicals are supplied to a funnel via separate supply pipes. The bottom end of the funnel is joined with a perforated pipe, set horizontally in the dye bath.

The half bleached fabric is feed to the m/c in open width form via some guide rollers & a feed roller. Then the fabric passes through the dye liquor & chemicals by an impregnation roller. Then the fabric passes through a pair of squeezing roller that extract out excess amount of liquor from the fabric. Then the fabric pass over some guide roller & finally delivered to a trolley via a fabric spreader. There is arrangement of draining out the already used liquor.

After dyeing the dyed fabric is covered with a sheet of polythene & kept covered for 12hrs. Then the material is sent for washing.

Washing:
Four washing is used for dyed product; among them in the first, second & third washing 5 gm/lit soaping agent or detergent is used (e.g.: Eripon-R). The fourth washing is done at room temperature, only with water.

Temperature used in different washing m/c is given bellow:

  1. Washing m/c 1: 85-90 0C
  2. Washing m/c 2: 70-85 0C
  3. Washing m/c 3: 60-65 0C
  4. Washing m/c 4: Room Temperature.
Finishing:
The next operation is finishing. From the fourth washing m/c the material is sent to the finishing pad via denser.

The chemicals used for finishing & their functions are given bellow:

chemicals used for finishing & their functions

Drying:
After the finishing the material is dried in loop dryer. Drying is done in similar way as described before.

Stripping:
Stripping is done in order to remove the color from any dyed material. It is done if the material is dyed with faulty color. In this mill stripping is done in Winch m/c.

The chemicals used for stripping & their functions are enlisted bellow:

chemicals used for stripping & their functions

The winch m/c is provided with a guide roller, a winch roller & a perforated cage. The fabric that to be stripped is passed over the guide roller & the winch roller. At first the fabric is run in hot water at rope form for 45-60mins. Then the liquor is given in to the m/c & the fabric is run until the color is removed & the fabric become white. The temperature inside the m/c is kept within 90-1000C.

There is arrangement of supplying water & steam into the m/c & discharging used up chemicals via separate pipes.

After stripping the material is sent for drying & then re-dyed.

After completion of wet processing the material is inspected & then sent to the stitching department.

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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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