Chemical processing of cotton fabrics

(Pre treatment)

BY:OLIYAD EBBA

ETHIOPIAN TEXTILE INDUSTRY DEVELOPMENT INSTITUTE

Cotton textiles

• Fibers - Loose Stock

• Yarn - Hank, Package (Cone / Cheese)

• Fabric - Open width, Rope form.

Chemical Processing of Textile Materials

• Preparatory/pre-treatment Process

• Dyeing / Printing

• Finishing

Preparatory/pre-treatment process

• What is the objective of fabric preparation in textile processing?

Preparation:• To remove the natural and added impurities

from fibers.

• To achieve high and uniform dye uptake

• To get required brightness of shade in the fabric after dyeing

• To impart good hydrophilic properties combined with high absorbency

• To produce acceptable degree of whiteness

• What will happen if the impurities are not removed properly ?

Preparatory Process

• If the impurities are not removed, the material will not be receptive to the subsequent processes – dyeing / printing / finishing.

• Impurities are also to be removed to improve the appearance

What are these impurities?• In cotton

Impurities in cotton

• In Fibers – natural impurities such as wax, oils, pectin's, pentose's, vegetable matter and COLOURING MATTER (Yellowish / Brownish)

• In Yarns – All natural impurities plus traces of lubricants - like oils & greases, rust, dirt, etc.,

Cont…..

• In Fabrics Woven: all impurities that are present in fibers

and yarns plus sizing materials (if yarn is sized) such as starches, PVA etc.,

Knits: for knitting yarn is not sized. Therefore knits are likely to have all impurities that

woven fabric have except the sizing materials.

Natural constituents of raw cotton

% by wt. of raw cotton

Cellulose 88.0 – 96.0

Nitrogenous matter 1.0 – 1.9

Waxes 0.3 – 1.0

Pectins 0.7 – 1.2

Ash 0.7 – 1.6

Malic, citric and other organic matter

0.5 – 1.0

Total sugars 0.1 – 0.3

Others 0.8 – 0.9

Moisture regain ̃~ 8%

what are the steps to be taken before going to wet processing?

Following are the steps to be taken:

Grey inspectionStitching Shearing and cropping

GREY FABRIC INSPECTION

• Fabric is inspected for defects before it is dyed i.e. grey stage.

• Longer yarns can be cut

• A record of the defects in a fabric length is kept to determine its quality .

• The process of giving points and assigning a quality level is called fabric grading.

Fabric Stitching• Most of the machines in preparatory section

are huge capacity machines. Therefore to utilize them to its full capacity it is necessary to feed them with suitable lots prepared by stitching together of fabrics from different job orders but of similar quality and same preparatory sequence requirement, thus enabling us to process them in a single batch

Shearing and cropping

• This is used for removing loose yarns of length up to 8cm from the surface of the fabric

What are the various operations in preparatory

process?

The various operations in preparatory process are:

Singeing DesizingScouringBio polishingBleachingMercerization

• Typically a woven cotton fabric would be prepared by sequence of process as shown above

• In case of knitting sizing step is not involved

Singeing

• Burning of protruding fibers from the surface of fabric or yarn - A mechanical process

What it do?• Makes fabrics smooth

• Prevents pilling

• Improves luster

What should control during singeing operation?

• Intensity and uniformity of flame

• Working speed of the fabric to be singed

• Effectiveness of singeing : should not add harshness in the fabric

• Should not damage the synthetic filament yarn in the fabric

• More care should be taken while singeing blended fabrics,

What is Bio polishing?

It is an enzyme treatment designed to improve fabric quality and provide following advantages:

• Improve pilling resistance

• A clear, lint and fuzz-free surface structure

• Improved drape and softness

• The effect are durable

When it should be carried out?

• It can be carried out at any time, but after bleaching is most popular

• After dyeing shade may be affected

Process

• Cellulase enzyme is used

• Enzyme dosage 1-2% owf

• pH 4.5-5.5

• Temperature 40-55oC

• Time 30-60 min.

It is similar to singeing as it also provide similar effect on the fabric

DESIZING

• To remove size material from the warp yarn• Size material: Starch, PVA, Synthetic ingredients• If not desized? Problems in dyeing like patchy or uneven dyeingWhat is sizing? This is a process of coating warp yarn with sizing

material to make it strong so it can withstand during weaving operation . In other words it will not break during weaving operation.

Desizing methods• Rot steeping

• Acid steeping

• Enzymatic Desizing

• Oxidative Desizing

Mechanism of Desizing

• Starch Amylopectin &Amylose(insoluble)

Dextrin(soluble) Dextrin(insoluble)

Maltose(soluble) Alpha glucose(soluble)

Enzymatic Desizing:Enzyme Desizing:• Enzyme solution in water with 0.5% to 2.0% on

weight of fabric enzyme• Require quantity of Common salt is also required • Neutral pH• Overnight Padding fabric with solution• Other method fabric is run continuously in

machine having enzyme solution.• Enzyme: Amino acid based catalyst.

• pH: Indicates acid, neutral and base character of the material.

QUALITY CONTROL CHECKS

• Iodine test

SCOURING

• It is process to remove all undesirable impurities except COLOURING MATTER. After scouring, oils and waxes are removed and hence the material becomes receptive to water, dyes and chemicals. In other words, the absorption capacity of the material is improves.

• Natural Impurities: Based on the composition of natural material like cotton, wool, silk etc.

Mechanism of Scouring

• Saponifiable oils & free fatty acids are converted into soap and glycerol

• Pectic substances(pectin, pectose and pectates)converted into soluble salts of pectic or meta pectic acids

• Proteins are degraded to simple soluble amino acids

Cond…

• Mineral matter is dissolved • Unsaponifiable oils & waxes are emulsified by

soaps• Adventitious dirt is removed and retained in

suspension by the soap • Other added impurities are broken down into

soluble products

Scouring of Cotton

• Sodium hydroxide and Soda ash are used for scouring of cotton at boiling temperature: called alkali scouring

• Now enzymatic scouring is also used

• Solvent scouring was also used few year back by using solvent by using trichloroethylene, which dissolve wax-But now it is not used

BLEACHING

• It is a process of whitening-fibers, yarns, or fabrics having natural color.

It is carried out by using oxidizing agents like:

Sodium hypochlorite (NaOCl)

Hydrogen peroxide (H2O2)

Sodium Chlorite (NaClO2)

• Among these hydrogen peroxide is widely used.

Styles of bleaching

• Full Bleach - Full white / Snow white

• Half Bleach - to dye light / pale / medium shade.

• No Bleach - to dye dark shades.

Hydrogen Peroxide for cotton• Most popular and environmentally accepted

process• But require higher temperature near boiling• Hydrogen peroxide is used along with NaOH (to

maintain pH 10-11)Drawback: Poor stability for small amount of Iron

and copper present in the textiles or the processing water. Cause catalytically decomposition of the peroxide, This loss of H2O2 mean less available bleaching agent. To prevent it Sodium silicate is used as stabilizer along EDTA.

Mechanism of peroxide bleaching

• H2O2 is a weak acid and ionizes in water to form:

HOO- (unstable) OH + O* (active or nascent oxygen)

O* + X X-O, X- oxidizable substance

• H2O2 decomposition is catalyzed by metal ions e.g. Cu++, Fe+

+ ...undesired rxn: no bleaching effect and causes fiber damage.

(active bleaching agent)

calculation of active oxygencontent in H2O2-solutions

• For the determination of the active oxygen content the bimolecular decomposition reaction of hydrogen peroxide is taken as basis:

34.0146 g/mol 18.0152 g/mol + ½ · 31.9988 g/mol

• Out of 34.0146g H2O2 100 % result from ½ · 31.9988g =15.9994g of so-called active oxygen.

• The conversion factor of % by weight of H2O2-solution in active oxygen is the following:

active oxygen [g/kg] = % in weight H2O2

solution * 4.704

Examples:

1.How much active oxygen in g/kg does a H2O2-solution of 50% contain ?

2. How much active oxygen does a bleaching bath with 10ml/l H2O2 50% (density ρ20 = 1.132g/cm3) contain?

solutions

#1.Active oxygen = 50 · 4.704

Active oxygen = 235 g/kg

Active oxygen = 23.5 %

•#2. 10 ml H2O2 50 % contain

10 1.195 = 11.95 g H2O2 50 %⋅or 11.95 g H2O2 50% /1000 ml

x=11.95*50*4.7 1000 =2.8 g/l

i. Effect of pH

H2O2 is an extremely weak acid. Caustic neutralizes the proton and shifts the

reaction to the right.• pH <10, H2O2 is the major species so no

bleaching.• pH = 10 -11, moderate conc. of per hydroxyl ions.• pH = 10.2-10.7 ( with NaOH) is optimum.• pH >11, rapid generation of per hydroxyl ion.• pH 11.8 all H2O2 is converted to HOO- and rxn is

out of control.

ii. Effect of Time and Temperature

• Stabilized H2O2 does not decompose at high temperature therefore faster and better bleaching occurs at 95 to 100 0C… ideal for continuous operations.

• Temp - rate of bleaching … but solution becomes unstable and degradation of cotton increases.

• Below 80 0C the evolution of per hydroxyl ion is very slow so also the rate of bleaching.

Effect of concentration of liquor and time

iii. Effect of concentration of liquor• Batch process = 2-4% (o.w.f.) H2O2 MLR of

10:1 to 20:1. • In the continuous process =1-2% (o.w.f.)

H2O2 Very high concentration may damage the fiber.

iv. Effect of time

• The time depends on temp, class of fiber and equipment used for bleaching.

• temp. of bleaching bath time of bleaching

Auxiliaries for Bleaching With H2O2

1. Stabilizers• To control the decomposition of H2O2 . Provide buffering action to control the pH

and to complex with trace metals which catalyze the degradation of the fibers.

Sodium silicate, organic compounds and phosphates

Cont.

2.Activator: provide alkalinity

NaOH, Na2CO3, Na3PO4,etc.

3. Sequestering agent: organic stabilizers or separate

EDTA, Sodium hexa meta phosphate.

4. Wetting agent – to provide wetting and detergency

Usability of bleaching agents for various types of fibers

Hydrogenperoxide

Sodiumchlorite

Sodiumhypochlorite

Persulphate Sodiumdithionite

Cotton CO ++ ++ ++ ++ +

Wool WO ++ -- -- -- ++

Silk SE ++ -- -- -- ++

Viscose CV ++ ++ + + +

Acetate AC Δ ++ 0 -- ++

Polyester PES

+/0 ++ +/0 0 +/0

Polyacryl (EL + / 0 -- -- Δ + / 0

++ important process,+ suitable,0 ineffective,-- harmfulΔ suitable under special conditions

scour/ bleach

Scour bleaching recipe on jet.Material to liquor ration:1:6Wetting agent : 1 gplSequestering agent:1 gplStabilizer :0.2 gplLubricant : 1 gplCaustic soda : 2gplHydrogen peroxide:2 gplAcetic acid: 0.5 gplPeroxide killer:0.5 gpl

Process diagram of scour bleaching

Common tests should be done after pretreatment

PH : for reactive dyeing the dye start ph should be in the range of 5.5-6.5,electronic ph meter of litmus paper can be used.

Peroxide residue: there shouldn’t be any residual peroxide for it result in unlevel dyeing. Peroxide stick can be used.

Hydrophility : water or methyl blue reagent can be used to under go water drop test. The water should be absorbed in three seconds time to assure the fabric for dyeing.

Cont.…

The liquor amount is very important. The pick up, water used for dye stuff , soda dissolution should be subtracted from the total liquor volume (fabric weight*M:L ratio).

Whiteness /refractive index: to check for the standard whiteness to be achieved before dyeing.

If brilliant white finish is required in textile, then bleaching is not sufficient. For this purpose Fluorescent brightening agents are used

Fluorescent brightening agent(FBA)

• Fluorescence is the ability of a substance to absorb radiation of light and re-radiate it at a longer wavelength.

Mechanism

• OBAs absorb in the UV range (300–400 nm) and re-

emit into longer visible (400–500 nm) wavelengths.

• Optical brighteners increase the apparent reflectance

of the textile in the visible region by converting UV-

radiation in the visible light and so increase the

whiteness or brightness.

Recipe of full bleachWetting agent : 1 gplStabilizer : 1gplLubricant : 1 gplHydrogen peroxide( 50 % concentration):4gplCaustic soda( flake ) :4 gplOptical brighter : 0.3%Acetic acid :0.5 gplMaterial to liquor ratio: 1:6

Process diagram for Full white

Change in properties after bleaching

• Whiteness Increases

• Absorption Increases

• Drop in strength

Relation between Whiteness and Strength

• Increase in whiteness is directly proportional to loss in strength. That is higher the degree of whiteness achieved, higher will be the loss in strength.

Why loss in Strength after Bleaching?

• The oxidizing agent not only oxidizes the natural coloring matter present in cotton but also oxidizes the cotton cellulose. Thus cellulose is chemically damaged.

• The chemical damage is seen in terms of drop in molecular chain length (DP) and conversion of cellulose into oxy cellulose

What are the adverse effects of Oxy Cellulose formation?

• Drop in reactive dye up take

• Drop in moisture absorption

• Drop in reactivity

These changes are due to conversion of hydroxyl groups into carboxyl and aldehyde groups.

Acceptable quality after bleaching

• Minimum loss in strength – 5 to 10 % depending upon – desired degree of whiteness & – fabric construction (coarse, medium & fine)

• Permanent whiteness – (no yellowness on exposure to sun light)

• High degree of absorption• Fluidity less than 4 or 4.5 depending upon

whiteness required

Conversion of g/l into % or % into g/l

• On discontinuous bleaching proceses (and of course on dyeing and finishing processes), the chemicals, auxiliaries, optical brighteners, dyestuffs etc. are either stated in percent (%) related to the fabric weight or in g/l resp. ml/l.

Cont...

• Conversion of g/l into % or from % into g/l with the given liquor ratio is as follows:

% =g/l * liquor ratio 10

g/l= %*10 liquor ratio

examples

• 3 g/l H2O2 50 % are in the bleaching liquor. What is the concentration in % with a liquor ratio of 1:8 ?

MERCERISATION

• This is the process applicable only on cellulosic fibers especially cotton

• The main purpose of mercerization is converting the inter fiber structure from alpha-cellulose to a thermodynamically more favorable beta-cellulose polymorph. Mercerizing results in the swelling of the cell wall of the cotton fiber. This causes increase in the surface area and reflectance, and gives the fiber a softer feel

Microscopical analysis of mercerized cotton

Fiber cross-section of a non-mercerized cotton

Fibre cross-section of a

mercerised cotton