Monday, December 20, 2021

Fabric desizing process, types of desizing method, structure and working principle of continuous desizing machine

Requirement of fabric desizing:

When the fabric weaving with the single ply spun yarn is done, the sizing of the warp yarn gets mandatory because the single ply spun yarn pose very poor weavability. 

A very thin film of the starch, binders and softeners is applied to the warp yarn surface. This thin film of starch makes single ply spun yarn weavable. The typical sizing agents used are natural starch (maize, potato, wheat, tapioca, rice, arrowroot, sago) and modified starch derivatives, polyvinyl alcohol (PVA), carboxymethylcellulose (CMC), guar gum etc.

When the dyeing of this fabric is carried out, many types of difficulties come due to the size material present in the fabric. This size material causes poor moisture absorbency. If we do not remove this size material from the fabric, improper fabric appearance results after dyeing. However, during the subsequent process, this applied size (surface coating on yarn) has to be removed from woven fabric for further wet processing comprising bleaching, dyeing, printing and finishing.

All these problems necessitate removing the starch and other size material from the fabric before dyeing.  

Desizing process: 

Desizing is a process of removing the size material from the fabric, which is applied in warp yarn before weaving. The sized material is removed to facilitate the penetration of dyes and chemicals in the subsequent wet processing operations. 

Methods of desizing: 

Desizing of cotton fabric can be done by physical, chemical or combination of physical and chemical mechanisms. The popular methods of the desizing process are given below:

1- Rot steeping 

2- Acid steeping 
3- Enzymatic desizing. 

1- Rot steeping:

In this method, grey cotton fabric is steeped in water in the suitable box at a temperature of about 30-40° c for about 24 hours. During the storage, micro-organisms develop and the starch gets a water-soluble compound. This is a very slow and time-consuming method. This desizing method requires large space 

2- Acid desizing: 

In this desizing method, cotton fabric is treated with dilute sulfuric acid or hydrochloric acid. 0.5 - 1.0 % solution of sulphuric acid or hydrochloric acid is used for fabric desizing. The quantity of the chemical is calculated as per the weight of the fabric lot. The process temperature is kept at about 40°c. This method of desizing process takes 3-4 hours. The starch and other sizing materials become water-soluble and get eliminated during the desizing process from the fabric.
3- Enzymatic desizing: 

In enzymatic desizing, the grey fabric is first padded with the desizing mixture containing enzyme - 0.5-2% and wetting agent at 60-70°. During this period the Enzymes reacts with starches and make them soluble. 

There three main steps are involved in the enzymatic desizing method: 

1- Enzyme application, 

2- Starch digestion  

3- Digestion products removal. 

Components of enzymatic desizing bath:

* Amylase enzyme

* pH stabiliser

* Chelating agent



* The enzymes being active within a specific pH range, need to be maintained by a suitable pH stabiliser. 

* Chelating agents used to sequester calcium or combine heavy metals may be injurious to the enzymes and must be tested before use. 

* Certain salts are used to enhance the temperature stability of enzymes. 

* Surfactants help improve the wettability of the fabric and improve the size removal. Generally, non-ionic surfactants are suitable but it is always recommended to test the compatibility of surfactants before use.

Machines used for desizing:

Desizing on a jigger machine:

This is a simple method where the fabric from one roll is processed in a bath and rewound on another roll. 

First, the sized fabric is washed in hot water (80 - 95°C) to gelatinize the starch. 

The desizing liquor is then adjusted to pH 5.5 - 7.5 and a temperature of 60 - 80°C depending on the enzyme. 

The amylase is added in an impregnation stage and degraded starch in the form of dextrins is then removed by washing at 90 - 95°C for two minutes. 

The jigger process is a batch process. 

In contrast, in modern continuous high-speed processes, the reaction time for the enzyme may be as little as 15 seconds.

Desizing on continuous desizing machine:

Desizing on pad rolls is continuous in terms of the passage of the fabric. 

However, a holding time of 2 - 16 hours at 20 - 60°C is required using low-temperature alpha-amylases before the size is removed in washing chambers. 

With high-temperature amylases, desizing reactions can be performed in steam chambers at 95 - 100°C or even higher temperatures to allow a fully continuous process.

Disadvantages of enzymatic desizing:

The disadvantages of enzymatic desizing are given below:

1- Lower additional cleaning effect towards other impurities

2- No effect on certain starches (eg, tapioca starch) 

3- Possible loss of effectiveness through chemicals, which act as enzyme deactivators or poisons.

 Advantages of enzymatic desizing:

Since the enzyme only targets the starch, it does not negatively affect the cellulose as is the case with acidic or oxidative methods of desizing. The specific advantages of enzymatic desizing are given below:

  • Effective solubilisation and removal of starches.
  • Excellent biodegradability.
  • No aggressive chemicals are needed, thus maintaining the tensile strength of the substrate.
  • Safe handling and operation.
  • Improved wettability.
  • Improved fabric quality.
  • Reproducible performance and ease of use.

Effects of improper desizing process:

Inadequate desizing leads to problems in subsequent wet processing. The main problems due to improper desizing are given below:

1- The absorbency of the fabric gets reduced due to improper desizing.

2- Inadequate whiteness of the fabric results due to improper desizing.

3-  Yellowishness in the colour shade results after dyeing. 

4- Spots and patches appear after the dyeing process. 

5- the creasing tendency gets increased due to stiffness. 

6- the tearing strength and the tensile strength get decreased due to improper desizing.

Assessment of desizing quality:

After desizing, the fabric should be systematically analysed to determine the uniformity, effectiveness and thoroughness of the treatment. The commonly used test methods are:

1- Assessment of weight loss 
2- TEGEWA scale rating
3- Water and colour solution absorbency
4- Hand feel
5- Bending length
6- Effect on tensile strength and tearing strength

Assessment of weight loss:

The desized fabric sample is weighed to determine the per cent size removed. The results are compared with a sample known to have been desized well in the lab. If the size is not adequately removed then either the treatment or washing have not been thorough. Ideally, after desizing, the residual size should not be more than 1% fabric weight.

TEGEWA scale rating:

A drop of dilute iodine solution (I2 in KI) is placed on the fabric and the colour change is observed and assessed by the TEGEWA scale. The fabric is not spotted randomly but from the side-centre side at different points along the length of the fabric. A dark bluish violet colour indicates excessive starch and if so, the fabric or garments must be reprocessed.

Passage of fabric in continuous desizing machine:

Details of desizing machine: 

Inlet feeding unit:

To feed the fabric into the machine It has guide bars ensuring smooth passage of fabric and one compensate (fabric tensioner) to set necessary fabric tension. 

 Selvedge guide: 

To align fabric centrally and helps in passing the fabric without folds into the machine. 

Desizing chamber: 

The main function of this chamber is to wet the fabric by using a series of rollers, the fabric is immersed into the desizing chemicals and thus desizes the fabric.

Chemical dosing system: 

It doses the appropriate quantity of required chemicals and water into the chemical mixing tank. The chemicals get mixed in the mixing tank and then dosed into the desizing chamber. 

Squeezing mangle: 

To squeeze out an extra chemical solution from fabric, it has two rubber-coated rollers which are pressed together to squeeze out a chemical solution and pass it back into the desizing chamber. 
Batching unit: 

An important function of the batching unit is to wind the fabric on the batch. The fabric is guided over the driven roller and is then wound onto batch. 

Rotating station: 

To have a uniform distribution of desizing chemicals throughout the batch the fabric after batching is covered completely with a polythene sheet and is then kept in a rotating station for rotation for uniform application of desizing chemicals on the batch. 

Operating sequence of continuous desizing machine: 

1- Stitching of fabric to be desized Feeding of fabric without a crease.
2- Dosing of the chemical into desize tank. 
3- Padding  Indicator of chemical/ temperature 
4-Ensuring proper batching (No defects)  
5- Switch ON main power and open steam, compressed air and water valve. 
6-Transport the grey fabric to be run, to the inlet feeding unit of the desizing machine using a hydraulic hand puller or electric truck from the grey batching section.
7- Prepare a desizing recipe with the guidance of the supervisor. 
8- Adjust machine parts according to the following parameters as per supervisor’s instructions.
9- Temperature of Desize chamber: 
It is measured in degrees Celsius. Set the temperature as instructed to get the correct desizing.
10- Chemical dosing: 
The dosing of chemicals should be optimum. Less quantity of chemicals leads to poor size removal while excess quantity leads to wastage of chemicals. 
11- Determine the pH value of the desize mix in the chamber using a pH meter or pH paper. Since all the enzymes work at a particular pH, this value should be in a specified range. 
12- Pick-up:
It is the amount of chemicals taken by fabric and is expressed in percentage (%) Low pick-up leads to less chemicals on fabric thus causing improper size removal and High pick-up lead to more consumption of chemicals than required i.e. wastage of chemicals. 
13- All the above parameters are to be continuously monitored by the operator while the machine is running. 
14- Verify all the above set parameters in the panel board While the machine is working. 
15- Verify that the chemical dosing is taking place properly.  16- Ensure that the fabric is evenly & uniformly dried before desizing. 
17- Continuously inspect the feed fabric and outlet fabric with the help of another operator. 

Cleaning in desizing machine: 

1-Remove regularly accumulated dust and dirt from the machine. 
2- Clean all the rollers dry fabric at periodic intervals 
3- Clean properly Main chemical tank, preparation tank and other pipelines. 
4- Clean the Padding mangle with water in every shift.
5-  Collect all the waste and store them in a designated place.

Please click on the below video link to watch the full lecture in Hindi:

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