Desizing

Desizing is done in order to remove the size from the warp yarns of the woven fabrics. Warp yarns are coated with sizing agents prior to weaving in order to reduce their frictional properties, decrease yarn breakages on the loom and improve weaving productivity by increasing weft insertion speeds. The sizing material present on the warp yarns can act as a resist towards dyes and chemicals in textile wet processing. It must, therefore, be removed before any subsequent wet processing of the fabric.

  

Objects of  Desizing :

1. To remove the starch material from the fabric.
2. To increase the absorbency power of the fabric.
3. To increase the affinity of the fabric to the dry chemicals.
4. To make the fabric suitable for the next process.
5. To increase the luster of the fabric increase of dyeing and printing. 

  

Factors of Size removal Efficiency :

• Type and amount of size applied
• Viscosity of the size in solution
• Ease of dissolution of the size film on the yarn
• Nature and the amount of the plasticizers
• Fabric construction
• Method of desizing, and
• Method of washing-off

Some Sizing Materials :

v  Starch: Corn,

                          Potato,

                          Sago,

                          CMC (Carbon Methyl Cellulose),

                          CMS (Carbon Methyl Starch).

v  Natural gums:  Locust bean,

                                       Tragacanth .

v  Synthetic sizes:  PVA,

                                        PAN,

                                        PVAC, etc.

Theory of Sizing :

The major portion of the size consists of starch, wax and tallow. All these remain on the warp  yarns even after weaving the cloth. Usually an emulsifying agent is added to the sizing mixture to facilitate its subsequent removal from the cloth.

It is necessary to remove the size material from the cloth; otherwise the hydrophobicity of the wax and the tallow hinder the subsequent dyeing and printing processes.  Wax  and  tallow  are  removed  in  the  later  process

(scouring or kier boiling), while the starch is removed during desizing. Thus desizing is the term usually restricted to the process of removal of starch (of the size) from the cloth. Desizing is a chemical process and the rate of this process, i.e. the rate of hydrolysis of starch can be controlled. For example, if the desizing liquor is stronger, the process can be completed in a short time so much so that the process can be made a continuous one. If weaker solutions are used, the process is bound to be batch one.

A process  to  remove  the  sizing  material  from  the  fabric  is  called Desizing. Starch is widely  used as a sizing agent, being readily available, relatively cheap and based on natural, sustainable raw materials. 75% of the sizing agents used worldwide is starch and its derivatives.

We use Enzymatic Desizing. Enzyme are complex organic soluble by catalyst formed by living organisms that catalyze chemical reaction in biological process.

Amylases are enzymes that hydrolyse and reduce the moleculer weight of amylase and amylopectin molecule in starch. The traditional approach is enzymatic Desizing in which α -amylase or diastase enzyme is used to attack the 1:4 glycosidic links in the starch breaking down the macromolecules into small soluble saccharides such as maltose and glucose. Enzymatic Desizing is safer than acid Desizing where cellulose may also get hydrolysed if the concentration of acid is higher than the optimum value.

1. The object of desizing is to remove from the grey fabric the size that has been applied during weaving and thus to make the fabric ready for further processes.
2. The main ingredient in size that is not water-soluble is usually starch.
3. Chemically starch is poly-glucopyranose  in w hich straight chain and branched chain polymers are present.
4. Both the constituents of starch are insoluble in water but they can be made soluble by hydrolysis of these long chain compounds to shorter ones.
5. Grey cotton fabric contains both natural impurities as well as ‘added matter’.
6. The added matter is called ‘size’. It is added by man in a process called ‘sizing’, as it facilitates weaving.
7. The size contains substances  such as starch, thin boiling starch, CMC, PVA, vegetable oil, mutton tallow, etc.

Classification of Desizing Process : 

  

Hydrolytic Method :

Enzymatic Desizing :

Enzymatic desizing is the most widely used method for the removal of starch, amylases being  particularly suitable. The advantage in the use of enzymes is that starches are decomposed without damaging cellulose fibre. These  are  fairly  sensitive  to  temperature  changes  from  the  optimum. Bacterial desizing agents like Rapidase are active over a wider temperature range and have certain other advantages, like tolerance of variation in pH.

Enzymes  suffer  from  one  disadvantage  that  if  the  conditions  of temperature and pH are not favourable, their desizing activity is destroyed. For example, their activity is destroyed they are deactivated above 75°C. An outstanding feature of enzyme desizing is the specific nature of the enzyme action.  Thus  diastase  hydrolyses  starch  but  does  not  tender  cellulose. Therefore enzyme desizing is safer than acid desizing, where cellulose may also  get  hydrolysed  if  the  concentration  of  the  acid  is  higher  than  the optimum value.

Classification of Enzyme :

Mainly two types of enzymes. Such as:

1. Animal enzymes: Example: Viveral, Novofermosol, Degomma, Waste pancreas, Clotted blood, Liver, etc.

2. Vegetable enzymes:

There are two types vegetable enzymes.

a)      Malt extract enzymes: Example: Diastafor, Diastase, Gabahit, Maltoferment, Maltostase etc.

b)     Bacterial enzymes:  Example: Rapidase, Biolase, Arcy etc.

Condition for Enzymatic Desizing :

Enzyme	Conc. (g/l)	Tempt. (°C)	PH value
Malt extract	3-20	50-60	6-7.5
Pancreatic	1-3	50-60	6.5-7.5
Bacterial	0.5-1	60-70	5.5-7.5

Enzymatic Desizing Process :

Four faces must be considered for a successful enzymatic desizing process.

1. Preparation of the desizing mixture: Agents should be added:

1. Water
2. Wetting agent
3. Salt
4. Acid/Alkali
5. Enzyme.

First, salt and wetting agent are added than enzyme.

2. Saturation: Fabrics containing starch as sizing materials are difficult to wet out. So, it is mandatory that the mass of fiber and size be saturated to approx. 100% wet pick up.

3. Digestion: It means the process of converting starch to soluble materials. In a continuous process, fabrics are run through a steamer and conversion is accomplished  during  the  steaming  time  available. In  case  of  J-box, temperature range is 60 C to 90 C and time is 15 to 20 mins.

4. Washing: When desizing has been completed, it should be relatively easy to remove the short chain sugar as they are water soluble.

Main controlling points:

1. Temperature
2. PH
3. Fabric speed
4. Concentration

Continuous Desizing :

Advantages of enzyme desizing

1. — Time required for the desizing process is less.
2. — It is continuous process, so greater production can be achieved.
3. — Closely constructed fabric can be easily desized, due to the effective enzyme action.
4. — There  is  no  chance  for  the  cellulose  to  get  hydrolysed,  as  in  acid desizing.

Disadvantages

—          If  the  conditions   of  temperature,   pH  and   time  are  not  properly maintained, the desizing activity of the enzymes is destroyed.

Rot Steeping :

1. This is the oldest and cheapest method of desizing.
2. Here no special chemical is used.
3. The cloth is first passed through warm water at 40C in a padding mangle where the cloth is squeezed to about 100% expression.
4. The cloth is then allowed to stand for 24 hours.
5.  The  microorganisms,  naturally  present  in  w ater,  multiply  and  secrete starch-liquefying (hydrolyzing) enzymes, which break down the starch present in the size to w ater-soluble products.
6. The cloth is then washed to remove these products.

  

Advantages

— 1. Rot steeping is the cheapest of all the desizing methods.

— 2. No chemicals are required.

Disadvantages

A large floor space is required for this process.

— The process is slow, so desizing time is long.

— Mildew may attack the cloth during steeping and cause stains on the fabric.

Acid Desizing :

• Dilute sulphuric acid or hydrochloric acid may be  used  to hydrolyse  the starch from the sized fabric.

—

• A 0.25% - 0.5 % solution of the acid at room temperature (30o C) is suitable for this process.

— 

• The cloth is impregnated with the dilute acid solution in a two-bowl or three- bowl padding mangle and then stored for 8-12 hours in a closed concrete pit.

Advantages of acid desizing

1. — Acid desizing is an economical process.
2. —The process is effective and gives fairly uniform desizing, as it is a chemical- based process. It does not require specific conditions of pH and can be done at room temperature.
3. — It is a much quicker process than rot steep desizing.

Disadvantage of acid desizing

1. —The main disadvantage of the process is that mineral acid is harmful to cellulose fibres if proper care is not taken.
2. —Especially during the storage stage, the acid-wet fabric must not be allowed to dry.
3. —This would cause the formation of hydrocellulose, which will weaken the fibre.

 Oxidative Desizing :

The “oxidative desizing” technique mentioned above, is applicable not only for water insoluble sizing agents, but also for water-soluble ones. This technique  is  particularly  useful  for  textile  finishers,  but  also  for  water soluble  ones.  This  technique  is  particularly  useful  for  textile  finishers dealing with many different types of fabrics and therefore sizing agents. In Oxidative desizing the starch is oxidized and this produces is also known as grey chemicking.

Desizing with Oxidizing agents

1. —Though the use of oxidants for desizing of cotton fabric is widely accepted but their large scale industrial application is yet to be exploited.
2. —The most important aspects of oxidizing agents are that they can be applicable to wide range of fabrics, the size content of which is often not known.

Table summarizes the necessary conditions for desizing starch in presence of some important oxidizing agents.

Bromite Desizing :

Sodium bromite, the salt of bromous acid, HBrO2  (like sodium chlorite, the salt of cholorous acid, HClO2), has powerful oxidizing action on starch. This is due to the combined effect of bromous acid,  HBrO2  and hypobromous acid, HOBr. This is accompanied by the conversion of bromine dioxide into oxygen and bromine. Hydrolysis of bromine thus formed produces more hypobromous  acid.  Of  the  different  modes  of  oxidation  of  starch,  the following one is the most likely one, involving breaking  up of relatively stable either linkage of the glucose ring by sodium bromite.

If  as  shown  above  ethers  are  vulnerable  to  oxidation  by  sodium bromite, one would expect depolymerisation of the ether linkages (Oxygen Bridge joining two  glucose units). Any significant  degree of depolymerisation would then convert starch into water soluble products.

1. — The main ingredient in size that is not water-soluble is usually starch.
2. —Chemically starch is poly-glucopyranose in which straight chain and branched chain  polymers are present.
3. —Both the constituents of starch are insoluble in water but they can be made soluble by hydrolysis of these long chain compounds to shorter ones.
4. —Thus, under suitable conditions, the following steps show the progressive hydrolysis of starch.
5. —However, in desizing, the hydrolysis of starch is carried out only up to the soluble dextrin stage, as this can be removed off the desized fabric by means of an aqueous wash.

Desizing Efficiency Test :

Desizing efficiency is found in two ways conventional and TEGEWA method.

Conventional Method:

In this method we first take the weight of the sized fabric, let it be W1. Then desize the fabric, dry & take the weight, let it be W2. After that the fabric is treated with 3gpl (35%) HCl at 700 C for 30 min. dry & take the weight of the fabric. Let it be W3.

Total size = W1-W3.

Residual size = W2-W3.

Desizing Efficiency = (Total size – Residual size)/Total size X 100.

Tegewa Rating :

Reagent:  potassium iodide (10 gm. Of KI (100%) in 100 ml water, add 0.6358 gm of iodine (100%) stir and shake; iodine is completely dissolved. Fill up to 800 ml with water then complete to 1000 ml with ethanol. (Shelf life approx 6 months only).

Method: 

Ø  Spot drop wise solution onto fabric.

Ø  Rub in gently.

Ø  Assess change of color.

Note: the test must be carried on fabric cooled down to room temperature; residual alkalinity has to be neutralities prior to the test.

Assessment:  Grey fabric:
No change of color = no starch size present.
Pale blue to bluish = presence of starch size or blend
Violet =of starch size with synthetic size

Desized fabric:
Pale blue to bluish violet = refer to violet scale TEGEWA This indicates residual Starch content.