Chemical processing of knitted fabrics

BY:OLIYAD EBBA

ETHIOPIAN TEXTILE INDUSTRY DEVELOPMENT INSTITUTE

Features of knit fabric • Knitted fabrics are more elastic and unstable mainly

because of the basic element of knitted fabric i.e. loop.

• This loop structure made knitted fabric to have the following main properties:

• too stretchable and cannot bear any undue tension.

• Knitted fabrics are highly elastic but if they are stretched beyond the elastic limits, it is impossible for them to return to stable state and consequently elongation remains.

• Knitted fabrics suffer from inherent spirality problem due to the factors such as yarn structure, twist, knit pattern, loose structure and extent of stress applied

• knitted fabric is inherently easy to distort such , bow, excessive shrinkage, etc.

Chemical Processing of knitted fabrics

• Preparatory/pre-treatment Process

• Dyeing

• Printing

• Finishing

Preparatory/pre-treatment process

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

Objectives of fabric 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• It is a process of making the fabric ready for

dyeing or printing by mechanical or chemical action.

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

….

• 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 knitted cotton fabric

Impurities in knitted cotton fabric

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

• Processing impurities–traces of lubricants - like oils & greases, rust, dirt, etc.,

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 F̃~ 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 knitted fabric

preparatory process?

The various operations in knitted preparatory process are:

SingeingHeat setting ScouringBio polishingBleachingMercerization

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

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.

Objectives of Scouring:

• To make the fabric highly hydrophilic.

• To remove impurities such as oils, waxes, gum, husks as nearly as possible.

• To increase absorbency of fabric or textile materials without physical and chemical damage.

• To produce a clean material by adding alkali.

• To make the fabric ready for next process.• To remove non-cellulosic substance in case of

cotton.

Mechanism:

Saponification:The vegetable oil, which is immiscible with

water, is glyceride of fatty acids. When such oils are heated with a solution of sodium hydroxide in water, the oil splits up into its constituents-fatty acid and glycerine. Glycerine is miscible with water easily and the fatty acids reacts with sodium hydroxide present in the solution forming its sodium salt i.e.soap which is also soluble in water. Thus oil is removed.

…cont

Emulsification:• Wax and non saponifiable oils are

removed by emulsification as they are immiscible in water. Normal washing soap is used as a emulsifying agent which makes emulsion of them.

What are the changes occurring during scouring?

Changes occurs during 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

…Cont

• 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 process depends on:

• The type of cotton.

• The color of cotton.

• The cleanliness of cotton.

• The twist and count of the yarn.

• The construction of the fabric.

• The shade % and type of shade of finished product

Chemicals used in scouring processMain chemicals Use

Caustic soda Neutralize acidic materials, saponify glycerides (waxes and oil),solubilise silicates

Surfactants Reduce surface tension & minimize interfacial tension.

Detergents Emulsify oil, fats, waxes and remove oil-borne stains.

Chelating agent Deactivate metal ions.

Sodium silicate Penetrate & break drown lignins,buffer

Sodium sulphite Reduce time of scouring

Soda ash Maintain pH

SolventThe chlorinated hydrc,trichloro ethyl,perchloro ethyl

Assist emulsification by dissolving oily materials.

Souring:

• The treatment/the process by which the fabric, after processing with alkali or scouring, is treated with Acetic Acid, Hydrochloric acid or dilute H2SO4 for removing alkali or neutralization of alkali is souring.

Difference between scouring and souring

Scouring SouringTo remove oil, waxes

gum soluble impurities.

Not to remove any impurities, only for alkali neutralization.

Scouring is done in alkali solution.

Souring is done dilute HCl or H2SO4

Required heat to boiling.

No need of heat.

Need of definite time. No need of definite time.

Bleaching• Natural fibers are off-white in color due to

color bodies present in the fiber.

• This yellowish and brown discoloration is related to:

flavone pigments of the cotton flower.The climate, soil, drought and frost can

also cause various degrees of yellowness.may also come from dirt, dust, insects or

from harvesting or processing equipment in the form of oils and greases.

Objective of bleaching

• The object of bleaching is to produce white fabrics by destroying the coloring matter with the help of bleaching agents with minimum degradation of the fiber.

Mechanism of Bleaching• The mechanism of bleaching is very complicated

and not completely understood. • One opinion is that the color producing agents in

natural fibers are often organic compounds containing conjugated double bonds. .

• The bleaching agents either oxidize or reduce the coloring matter … thus whiteness obtained is of permanent nature.

• Oxidative bleaches oxidize color bodies into colorless cpds.

Discoloration can occur by breaking up the chromosphere, most likely

destroying one or more of the double bonds within the conjugated system

Styles of bleaching

• Full Bleach - Full white / Snow white

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

• No Bleach - to dye dark shades.

Bleaching agents

Oxidative bleaches are also known to degrade cellulose so the objective in bleaching is to optimize whitening and minimize fiber damage.

Bleaching agents…• Oxidative bleaching – the bleaching agent

is a chemical reagent which decomposes in alkali solution and produce active oxygen.

• The active oxygen is in fact the intrinsic bleaching agent as it will further destroy partly or completely the coloring matter present in the textile material.

• Reductive bleaching – the bleaching agent will destroy the coloring matter by reductive reaction of SO2.

Bleaching with Hypochlorites• Hypochlorite bleaching (OCl-) is the oldest industrial

method of bleaching cotton.

• Originally, calcium hypochlorite, Ca(OCl)2 (bleaching powder) was used.

• Until 1940 most cotton fabrics were bleached with NaOCl … today only 10 % of the cotton.

• It is however the main stay of home laundry bleaching products.

• Their use is declining because of anti-chlorine lobby and environmental pressures.

• Hypochlorites are excellent cidal agents for mildew and other bacteria and are used as disinfectants and to control bacteria in swimming pools.

Bleaching with Hydrogen Peroxide (H2O2)

• By 1940, about 65% and to-day about 90 - 95 % of all cotton and cotton/synthetic blends are bleached with H2O2.

• It is available commercially as 35, 50 and 70 % solutions.

• It is a corrosive, oxidizing agent which may cause combustion when allowed to dry out on oxidizable organic matter.

• H2O2 is an irritant to the skin and mucous membranes and dangerous to the eyes.

H2O2 - Ecologically acceptable

- Economically feasible

Cont…

• 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 ?

Sol/n

#1.Active oxygen = 50 · 4.704

Active oxygen = 235 g/kg

Active oxygen = 23.5 %

Effect of pH on H2O2

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.

Effect of Temperature on H2O2

• 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.

• 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

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 bleach

Wetting 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

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