Wednesday, 27 April 2011

SCOURING PROCESS


Definition: Scouring Process involves in the treatment of textile materials in aqueous or other solutions in order to remove natural fats, waxes, proteins and other constituents, as well as dirt, oil and other impurities.

        The cleaning part of the fabrics are involved in this process. To remove pectic substances and cotton wax contained in cotton woven fabrics,the use of caustic soda (NaOH), strong alkali, is the most effective. The caustic soda has the effects of not only converting water-insoluble pectose contained in pectic substances into water-soluble pectic acid soda, but also of hydrolyzing (saponifying) cotton wax. However, a single use of the caustic soda does not provide enough penetration, emulsion, washing, and scouring powers, and so a surfactant is generally used together.

Objects of Scouring:

Impurities: There are two types of impurities can be found they are given bellow:

Natural impurities:



Added impurities:
Scouring depends on:
●Types of the fibre.
●Types of the fabric.
●Types of the fibre color.
●Types of the fibre cleanliness.
●Types of the twist and count of the yarn.
●Types of the fabric construction.

Scouring depends on Natural Fiber. No need man made fiber for scouring.

Changes of cotton at the time of scouring:
1. Saponifiable oils and free fatty acids are converted into soap.               
2. Pectins and pectose are converted to soluble salt of pectic acid.
3. Proteins are hydrolised into soluble degradation products.
4. Mineral matter is dissolved.
5. Unsaponifiable oils and waxes are emulsified by the soluble soaps formed during the hydrolysis of saponifiable  oils.
  6. Adventitious dirt is removed by the soap.
7. Residual sizing materials break down to soluble product.

 Chemicals used in scouring:

1. Alkali.
  2. Wetting agent.
  3. Detergent.
  4. Sequestering agent.
  5. Solvent.
  1. Alkali:




The function of alkali is to convert the oil into water soluble fatty acid and soap. That is to say alkali carries out the saponification reaction.

 CH2 – OOC (CH2)16CH3                                         CH2 – OH

׀                                                                                 ׀ 

 CH – OOC (CH2)14CH3   +  NaOH  +  H2O →       CH – OH     +    NaOOC(CH2)16CH3   +

 ׀                                                                                    ׀       

CH2 – OOC(CH2)12CH3                                          CH2 – OH          NaOOC(CH2)14CH3   +

                                                                               (Glycerin)

                                                                                                           NaOOC(CH2)12CH3

                                                                                                       (Na salt of different acid)

2. Wetting agent:The wetting or surface active agent reduces the surface tension between the caustic water solution and fabric so that the oil, waxes and caustic are in intimate contact with each other.

3. Detergent:The detergent is also a surface active agent which is used for cleaning the substrate.
4. Sequestering agent: The sequestering agent or chelating agent reacts with metallic ions and thus makes the metallic compounds to be a part of a complex anion. It is used to remove water hardness.

 Na4{Na2(PO3)6}   +   Ca++         Na4{Ca(PO3)6}  +  Na+

  (Calagon-T)                               (Water insoluble)

 5. Solvent:Solvent is used to remove more unsaponifiable oils and waxes.




Optimum temperature for scouring:Scouring is generally carried out at above 1000c temperature (between 900 – 1300c). Above 1000c temperature for every 100c increase in temperature will make the reaction to be finished in half of the previous time.

Scouring process:

There are two types of scouring processes –
1. Conventional scouring process:

      a) Kier boiling (discontinuous) scouring.
    b) Scouring in J or L box (continuous).

2. Special scouring process:

     a) Solvent scouring process.
        b) Vapor loc scouring process.

General recipe for scouring:

Alkali (NaOH)         -    2 to 5 gm per litre.
Soda ash                   -    x gm per litre to adjust PH (PH required for scouring is 10.5).
Wetting agent             -    1 gm per litre.
Sequestering agent      -    1 gm per litre.
Detergent                    -    1 to 2 gm per litre.
Temperature                -    100 to 1250c.
Time                            -     6 hours (close vessel)
8 hours (open vessel)
M : L                           -     1 : 10
N.B. : Water level should keep 6 to 8” above the fabric.

            Machine capacity may be from 1ton to 5 ton.


Description and working principle of scouring process:
Kier boiler is a long mild steel or cast iron cylindrical vessel provided with two perforated tube sheets (disc with a number of holes). One is placed at the bottom and another is top. These discs are connected by a number of tunes which carry the liquor from the bottom compartment to the upper one. In the middle compartment steam is passed. Thus the tubes carrying the liquor are surrounded by steam which heats them.The hot liquor from the multitublar heater is sprayed over the cloth, packed in the kier, through a hollow perforated ring. The liquid passes slowly over the packed cloth, collects below the false bottom, from where it is pumped into the auxiliary heater by a centrifugal pump and the cycle repeats. 

Precaution:
a. Kier boiler should be cleaned.
b. Material should be packed evenly.
c. Complete immersion of the fabric need.
d. After boiling the liquor should be removed in absence of water.
e. Before starting all the joining parts should be checked.
f. Fabric should always keep under scouring solution.

Scouring in the J Box:
In continuous process J-box is used for cotton scouring. It looks like English letter j. so it is called J-box. In J-box cotton is scoured open width and rope form.

General recipe of scouring in J-box
    Alkali                                      :           4 to 6 g/l
    Wetting agent                          :           4 to 5 g/l
    Impregnation temperature       :           70to 800c.
 Impregnation time                   :           40 to 90 second.
    Pick up                                    :           90 to 100%
 Storing time in J box               :           1 to 2 hours.
   Temperature in J box               :           98to 1020c.

Rinsing with water and finally hot wash (not less than 800c) and cold wash is done.

Description and working principle:
The scouring process in J-box can be divided into four units. They are
 a. impregnation box.
    b. pre-heater.
 c. J-box.
   d. Washing unit.

a. Impregnation box:
Fabric is passed into impregnation box in open width form and through the guide rollers and padded by caustic soda and wetting agent. The temperature for impregnation should be maintained at 70 to 800c for about 40 to 90 seconds. Then the fabric is squeezed and passed on to the next unit.

b. Pre heater:
In this unit the material is passed to the thermostatic controlling system at temperature of about 90 to 1000c for about 30 seconds. Then the material is passed to the J-box
c. J-box:







The capacity of normal J-box is about 12000 to 15000 pounds. The material from the J-box is passed to the washing unit passing through squeeze roller.

d. Washing unit:The water soluble impurities or products that were left on the material are removed by washing. First of all the materials are washed in hot water at the temperature above 800c, then it is cold washed and finally dried.

Advantage:

1. The process is a continuous process. So consumes less time.
2. The process is economical.

Disadvantage:

1. The result of scouring is not good as compared with kier boiler.
   2. The process is not hydrophilic as kier boiler.

Estimation or scouring effect:
The scouring effect can be estimated by carrying out one of the following tests-
    1. Measurement of weight loss.
  2. Test of absorbenca) Immersion test.
 b) Drop test.
 c)  Wicking or column test.

Scouring of Cotton
In the scouring process the cotton cellulose material is treated with a solution containing alkali ( soda ash and caustic soda), an anionic and/or nonionic detergent, a wetting agent, a complexing and sequestering for the removal of metal ions and polyacrylates or polyphosphonates as special surfactant free dispersing agents, at high temperature.
The scouring operation was conventionally done in kier machines and hence the process was called kier boiling , but now a days the scouring is done mostly in the dyeing machines itself as a part of combined or single operation such as scouring and bleaching.

I. Saponification  of fats into water soluble soap and water miscible glycerin  under alkaline conditions.
II. Hydrolysis of proteins into water soluble degradation products.
III. Dissolution of amino compounds.
IV. Solubilising of pectose and pectins  by converting into soluble salts .
V. dissolution and extraction of mineral matter.
VI. Emulsification and solubilisation of natural oils and waxes.
VII. removal and dispersion of dirt particles and kitty by the action of alkali and detergent.
The material after scouring is more absorbent , free from natural impurities and coloring matter.This treatment can be carried out on filaments, yarns and fabrics.
    Saponification:
When a grey cotton fabric is dipped in water the oil present in the size will not allow the water to spread on the fibre, i.e, the oil resists the wetting of the fabric by water. For further processes such as bleaching, dyeing, printing etc. to be effective, it is essential for the fabric to wet easily when treated with solutions of chemicals and dyestuffs. In other words, the cloth should be highly absorbent towards water. To achieve this objective, the oil has to be removed from the fabric. Also China clay (of the size) is firmly fixed on the grey cloth with the help of oils and waxes which act as binding agent for the clay particles. Thus it is necessary to remove the oils and waxes for removing the China clay from the fabric.  

Scouring of Silk
The scouring of pure silk is a degumming process used to remove sericin (silk gum) from
fibroin floss. Sericin is the gummy element which keeps together the fibroin floss and gives the
silk a hard hand and dull appearance. It is carried out on yarn, on dyed yarn, piece-dyed fabric or on products ready for printing. The treatment, which causes a loss of weight ranging between 24 and 28%, gives the degummed silk a lustrous appearance and a soft hand; the treatment is
carried out with soapy solutions or with buffer dissolving agents. It is also possible to use enzymes (protease), which hydrolyses sericin.


Scouring of Wool
On wool, the scouring process removes oils and contaminants accumulated during upstream
processing steps and can be carried out on slivers, yarns and fabrics with solutions containing
sodium carbonate with soap or ammonia, or anionic and non-ionic surfactants, which carry out a
softer washing to avoid any damage to the fibres.

Scouring of Synthetics
The scouring process applied to synthetic fibres removes oils, lubricants and anti-static substances, dust, contaminants and can be carried out on yarns and fabrics (when warp yarns
have been bonded, the treatment is called debonding). It is carried out by means of surfactants,
detergents and emulsifying agents.

Scouring of colored woven goods:
In certain type of fabrics, dyed yarns are used for warp and weft along with white yarns to produce colored woven designs. Thus colored borders of saris and dhotis, colored check effects, shirtings etc. are woven using white and dyed yarns. These are known as colored woven goods and the dyed yarns are known as colored effect threads.
Since the woven cloth has to undergo rigorous conditions of scouring and bleaching, a restriction is imposed on the selection of dyes for dyeing these yarns. For example, if direct dues are used for the purpose, bleeding of the dye from the yarns takes place during scouring with the consequent staining of the white yarns present in the fabric being scoured. When the scoured fabric is subsequently bleached with sodium hypochlorite solutions, most of the direct dyes are destroyed by the bleaching agent. If sulphur dyes or reactive dyes are dyed for producing such colored threads, sodium hypochlorite destroys most of these dyes. Therefore, these classes of byes are not suitable for the purpose. Generally, vat and azoic colors are dyed on such yarns. Vat dyes re superior to azoics in this respect. These liquor, neither are they destroyed by the subsequent bleaching.


Bio-Scouring:
Bio-scouting with pectinases(enzyme) have shown promise in replacing the traditional alkakine scouring treatment. Some auxiliaries’ ksuppliers have introduced an enzymatic process to remove hydrophobic and other non-cellulosic components forn cotton. The new bio-scouring process operates at mils pH conditions over a broad temperature range and can be applied using equipment such as jet machines.
It is claimed that, due to a better bleachability of enzyme-scoured textiles, bleaching con be carried out with reduced amounts of bleaching chemicals and auxiliaries. Bio-scouring enzymes actually make the substrate more hydrophilic (which could explain better bleachability), but they ae not able to destroy wax and seedsm which are therefore removed in the subsequent bleaching process.
 There is no need for the use of caustic soda in enzymatic scouring. So this process reduced pollution loads, high TDS, BOD and COD in the effluent.

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