De-sizing Process and its types

De-sizing Process

A process carried out to remove the sizing material (like starch) from the cloth. 

Objective of De-sizing

             1.     To eliminate the water repellent nature of sized cloth.             
             2.     To increase the absorbency.             
             3.     To reduce the consumption of chemicals in subsequent process.

Importance of De-sizing

                The importance of de-sizing is mainly because of water repellent nature of “Sizing materials”. So, it is important to remove the size material before conducting any  coloration or finishing process.

Enzymatic desizing

What are enzymes?

Name

·         Greek, meaning “In Yeast”

Nature

·         Bio-catalyst (proteins)

Structure

·         Usually large & complicated

Properties

·         Specific action

·         Thermo ability

·         Low energy of activation

·         Narrow working pH arrange

Classification

·         According to origin

·         According to action

·         According to structure

·         According to working temperature

Sources of Amylases

·         Malt

·         Bacteria

·         Pancreas

Requirements of desizing:

• Desizing materials
• Desizing methods
• Desizing machine

Desizing processes

Desizing, irrespective of what the desizing agent is, involves impregnation of the fabric with the desizing agent, allowing the desizing agent to degrade or solubilise the size material, and finally to wash out the degradation products. The major desizing processes are:

• Enzymatic desizing of starches on cotton fabrics
• Oxidative desizing
• Acid desizing
• Removal of water-soluble size

Enzymatic desizing

Enzymatic desizing is the most widely used 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..

Amylases is the enzyme that hydrolyses and reduce the molecular weight of amylose and amylopectin molecules in starch, making it water soluble enough to be washed off the fabric.

Effect of PH and Temperature on enzyme desizing

Effective enzymatic desizing require strict control of pH, temperature, water hardness and electrolyte addition Activity of enzymes increase with temperature; however, above a critical temperature, enzymes are deactivated. The effectiveness of enzymes exhibit a maximum at certain temperatures, usually 40 -75 0C. Bacterial enzymes are the most thermally stable and can be used up to 100 0C under special stabilizing conditions. Certain salts increase the activity of specific enzymes. Pancreatic amylase is ineffective without the addition of salt. A combination of sodium chloride

Exhaust Process

                           Enzyme                    4 g/l

                            NaCl                        4 g/l

                           Wetting Agent         1 g/l

            Continuous Process

1. Saturate fabric with a solution containing:

Bacterial Amylase 0.8 - 1.0 %

Wetting agent 0.1 - 0.2 %

Sodium Chloride 10 %

2. Hold:

Open-width Range: Steam 1 to 2 minutes at 200 to 212 ⁰F.

Rope Range: Store in J Box for 20 to 30 minutes at 170 to 180 ⁰F. or

over night at room temperature.

3. Thoroughly rinse with hot water, 175 0F or hotter.

           

Oxidative desizing

In oxidative desizing, the risk of damage to the cellulose fiber is very high, and its use for desizing is very rare. Oxidative desizing uses potassium or sodium persulfate or sodium bromite as an oxidizing agent.

Acid desizing

Cold solutions of dilute sulphuric or hydrochloric acids are used to hydrolyze the starch, however, this has the disadvantage of damaging  cellulose fiber in cotton fabrics.

Removal of water-soluble sizes

Fabrics containing water soluble sizes can be desized by washing using hot water, perhaps containing wetting agents (surfactants) and a mild alkali. The water replaces the size on the outer surface of the fiber, and absorbs within the fiber to remove any fabric residue.

Assessment of size removal:

A drop of iodine solution placed on a test specimen resulting in a characteristic blue color is the universal test for identifying starch. It can be used as a qualitatively test to show whether all the starch was removed. Absence of the blue color signifiesthat all the starch has been removed. The intensity of the color is some what related to how much is left. Usually, if the color is faintly perceptible, the remaining starch will come out in the scouring and bleaching steps that follow.

SINGEING and its types

SINGEING:

Singeing    

The process of removal of protuding fibers from the surface of gray fabric by burning is called singeing.

OBJECTIVES OF SINGEING

Objectives of Singeing of  Woven Fabrics

Ø  Surface hairs helps to entrap air in the fabric when immersed in water it takes longer for water to enter the fabric since it must first displace the air .Singeing therefore helps to increase fabric wet ability.

Ø  It create a smooth surface for printing. It may be possible to print fine details on the Hairy surface but once the hairs moves again after printing and fine details  becomes fuzzy.

Ø  To emphases the woven Structure of the fabric if that is considered desirable.

Ø  To prevent a frosty appearance after dyeing. The projecting Dyed Hairs give the Fabric Surface The appearance of being poor   then rest the body of the fabric

Ø  To prevent or minimize the tendency of blend of Fabrics to form Pills. Pills  are the little Bolls of the Fibre that arise on the surface of some fabrics as a result of abrasion that occur during Usage.

Machines for singeing:

•   Rotary cylinder singeing machine
•   Plate singeing machine
•  Flame singeing    

PLATE SINGING MACHINES OPERATIONS

1.     In plate singeing the fabric is pass over one or two curved plates of cupper with size 1”-2” thickness.

2.     Guide rollers are used to ensure the enough contact of cloth with the plate surface so that the projecting fibers are removed without any damage.

3.     Two levers are used to move the plates up and down depends upon the life of the plate.

PARAMETRES OF SINGEING MAHINES

1.     The plates are heated up to 150 °C.

2.     Speed of the cloth is 135 to 225 meters per minute.

ROLLER SINGEINGE MACHINE OPERATIONS

1.     In roller singeing machine fabric is passed over a hallow cylinder of cupper or cast iron cylinder which is heated by internal firing.

2.     The cylinder revolves slowly opposite to fabric direction.

3.     The face and back surface of the fabric may be singed in single passage by using two rollers.

Parameters of Flame singeing Machine:

         1.      Composition of flam

      1 part LPG (liquid petroleum gas)

      2 parts Air

         2.      Nature of flame

      Use oxidizing flame because it produces minimum amount of ash

      Temperature of the flame is about 1300°C

Zones of Singeing Machine .

Ø There are five zones of singeing Machine

Ø Entrance Zone

Ø Brushing Zone

Ø Singeing Zone

Ø Washing Zone

         3.      Distance of flame from fabric surface should be 1.5 to 4 mm.

         4.      Speed of the fabric

·         Speed of the fabric depends upon quality of the fabric

·         Speed of low quality fabric should be 120 meter per minute.

·         Speed of High quality fabric should be 60 to 80 meter per minute.

          5.      Angle of the flame

· Perpendicular to fabric structure(for heavy weight fabric)

· Tangential to fabric structure(for light weight fabric)

Inspection of gray fabric

Inspection of gray fabric:

            The process of observing and locating different types of faults in gray fabric is known as inspection of gray fabric.

Types of faults in the gray fabric:

• v  Broken picks
• v  Broken ends
• v  Cut weft (pinhole in fabric due to cut r breakage of pick)
• v  Neps, peels, and cracks
• v  Missing of interlacement of pick and end
• v  Yarn contamination
• v  Floating or protruding fibers
• v  Hang Pick( a pick is out of line for short distance creating hole cavity)
• v  Wrong Drawing (wrong order of drawing ends through heals and reeds

  Four Point inspection system of fabric:

           The 4-Point System assigns 1, 2, 3 and 4 penalty points according to the size and significance of the defect. Points are assigned according to the following criteria:

                           

Penalty Point Defects in Warp Except Holes Torn	1 Up to 3²	2 From1²to 6²	3 From 1²to 9²	4 From1² to 36²
Cutting Faults	Hole and Tear ¼ and over Heavy Crack and Float

Penalty points grading:

The following fabric penalty point grading standards are to be used during inspection.

1. No more than 4 penalty points may be assigned for any single defect.
2. No more than 4 penalty points may be assigned to one linear meter, regardless of the number of defects with in that one meter.
3. A continuous defect shall be assigned 4 points for each meter in which it occurs.
4. Any roll having a running defect through more than three continuous meters shall be rejected regardless of points count.
5. No roll shall be accepted that contains a full width defect in the first or last three meters.
6. A hole or torn is considered to be a major defect and shall be penalized 4 points.

No roll shall be accepted as first quality that exhibits a noticeable degree of looseness of tightness or ripples, puckers, folds or creases in the body of the fabric

Major & minor Penalty Points

           

Average Point: Average point is calculated by following formula.

                                                                                (Total Points) X (3937)

Total Average Points=

Per 100 Meter²                                               (Total length in Meters) X (Width)

                                                                              (Total Points) X 3600

Total Average Points =

Per 100 Yards²                                              (Total length in Meters) X (Width)

• Ø According to SOP (Standard operating Procedure) if there are 15 faults in 100 square meter then the fabric is acceptable for further processes.
• Ø If the faults are in between 15-18 then we correspond with the customer for its requirement. Customer decides whether it is acceptable or not.
• Ø If total faults in 100 sq meter are more than 18 then the fabric is rejected.