Scouring process in textile processing

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  • Grey fabric contains mainly three type of impurities:

    Waxes, proteins, pectic substances and mineral matters

    Mechanically held impurities e.g. motes, leaves

    Spin oils, waxes, grease

    Machine oils, tars, greases, dirt, dust

  • Cellulose 85-96%

    Oil and Wax 0.5 -1%

    Proteins, Pectoses 3-5%

    and colouring Matter

    Mineral Matter 0.5-1%

    Moisture 7-8%

  • Natural fibers contain oils, fats, waxes,minerals, leafy matter and motes asimpurities that interfere with dyeing andfinishing.

    Synthetic fibers contain producer spinfinishes, coning oils and/or knitting oils.

    Mill grease used to lubricate processingequipment mill dirt, temporary fabricmarkings and the like may contaminatefabrics as they are being produced.

  • Scouring is a purifying treatment of textiles.

    Also known as kiering, kier boiling or boilingout.

    The objective of scouring is to reduce theamount of impurities sufficiently to obtainlevel and reproducible results in dyeing andfinishing operations.

  • Selection of Scouring Agents depends on: Kind of fiber

    Wool and silk are dissolved by alkali

    Acetate and triacetate are converted to cellulose

    Cotton absorbs it

    Fabric type i.e. woven or knitted

    Fabric density i.e. thick or thin

    Texturised or non-texturised

    Extent of impurities present in the fiber

  • Scouring mechanism involves the following processes:

    (for vegetable oils, animal fats, mineral oils)

    (for pectins, minerals, heavy metals, amino acid, proteins)

    (for mineral oils, waxes)

    (for dirt, dust, china clay)

  • Sponification

    The name given to the chemical reaction thatoccurs when a vegetable oil, animal fat and mineraloils (not soluble in water) are mixed with a strongalkali. The products of the reaction are two: soapand glycerin

    Fats, oils and waxes (glycerides of fatty acids)

    Soap formed enhances scouring

    Quantity of residual waxes in particular decide theabsorbency of fabric

  • Oil + Caustic Soda Glycerine + Soap

  • Solublisation

    NaOH swells the impurities & makes them water soluble

    For Pectins, protiens and motes

    Minerals and heavy metals are converted to soluble salts by use of acids

    Amino acids and protiens are removed by formation of their sodium salts

  • Emulsification

    An emulsion is a mixture of two ormore liquids that are normally immiscible

    Non-sponifiable mineral oils and waxes which areesters of higher fatty alcohols and fatty acids(natural and added) can be removed by emulsifyingthem with a surfactant (emulsifying agent)

    Ordinary soap (washing soap) is a good emulsifyingagent

  • Emulsification

    An emulsion consists of a base liquid, with the other liquid contained and spread within the base liquid in tiny droplets.

    The base liquid is called the continuous phase and the contained liquid is called the dispersed phase.

  • In order to keep the liquids uniformly distributed andstable, an agent used is called emulsifying agent.

    These agents have both a hydrophilic and a lipophilicpart in their chemical structure.

    They concentrate at and are adsorbed onto theoil:water interface to provide a protective barrieraround the dispersed droplets.

    They also reduce the interfacial tension of the system.

    Some agents enhance stability by imparting a chargeon the droplet surface.

    In scouring it is a soap formed already.

  • The remaining constituent of size and otherdust and dirt particles, stains, are removed bydetergency.

    Detergency is closely bound up with surfacetension.

  • When a piece of greige fabric is placed on the surfaceof water in a beaker, the piece does not sink but floatson its surface.

    This is because every molecule of water is attracted byevery other molecule of water.

    The molecules inside the water counterbalance theforces from all sides and cancel out the overall effecton them.

    While the surface molecules are only pulled sidewaysand downward, due to which they form and interfacebetween liquid and solid.

  • Generally, surface tension between textile fibres, and wateris high, and hence wetting of the fibre surface does nottake place thoroughly and quickly.

    A surfactant is a substance which, when applied in lowconcentration, markedly reduces the surface tension of asolution.

    A good surfactant, for example, will reduce the surfacetension of water from 72 dynes/cm to 30 dynes/ cm at aconcentration of less than 0.1%.

    The concentration at which no further reduction in surfacetension occurs is known as 'critical micelle concentration'.

  • When a surfactant (soap) is dissolvedin water the hydrocarbon chain(hydrophobic part tail, ==) tries toget away from the water medium dueto its hydrophobicity and concentrateat the surface.

    The hydrophilic grouping (carboxylhead, O) just dip in the water at thesurface and the sodium cation is inthe vicinity of negatively chargedcarboxyl head.

    Thus this compound, sodiumstearate (soap), distort the structureof water and decrease the freeenergy of the system.

  • The reason for reddish shade after bleaching with peroxide is the high content of manganese.

    The contents of Fe, Mn, Ca and Mg varies depending upon the origin of cotton

    Can be done by complexion and consequent solublizationusing an acid or alkali

    Usually done by sequestering action.

    They form complexes with metal ions and keep them awayfrom fabric.

    Generally good pre-treated materials should have 100-300 p.p.m, of Ca and Mg and 4-8 p.p.m, iron.

  • Advantages

    Better Levelness and more Brilliance in textile dyeing

    lower peroxide consumption in bleaching

    Increase in degree of Whiteness

    regular decomposition of peroxide and No catalytic damage

  • Absorbency: Drop Test Min 3 Sec and max 10 sec

    Weight loss is also checked

    Residual Wax Content Wax is dissolved in a solvent

    Solvent is distilled off

    The sample is reweighed

  • Dyeing and Chemical Technology of Textile Fibers, By: E.R. Trotman

    Chemical Technology in the pretreatment processes of Textiles, By: S.R. Karmakar.

  • Absorbency: Drop Test

    Less than 5 sec

    Other tests

    Wicking Height

    Sinking Test

    Weight loss is also checked

    Residual Wax Content Wax is dissolved in a solvent

    Solvent is distilled off

    The sample is reweighed

    Removal of Motes Observed visually and given a rating of 1-5