Fabric pre-treatment

Wet processingFabric pre-treatmentWet processing We can consider the wet processing the sector in a textile mill to have three distinct sections included within it. These are the Preparatory ( singeing, desizing, scouring, mercerization) Colouartion ( Dyeing, printing)Finishing (Water repellency, Flame repellence, Anti-soiling finish)Grey FabricSingeing

Heat Setting(PET and Nylon)ScouringBleachingDyeing orPrintingFinishing(chemical, mechanical)WashWashWashWashWashDesizing(Woven)Textile Wet ProcessingMercerization(Cotton)Textile Pre-treatment Pretreatment refers to any treatment, which is done before the actual colouration (dyeing and printing) process. It is the series of cleaning operations.All impurities which causes adverse effect during dyeing and printing are removed during the pretreatment processCotton finishing route

Silk finishing root

Synthetic fibre finishing root

Wool finishing root

Objectives of pre-treatment

Good desizing effect Removal of seed husksRemoval of foreign substances from the fibersLowest possible fiber damageHigh degree of WhitenessGood Physical/Technological ratingsHigh color yieldLevelness of the effectsHigh and even Hydrophilicity / RewettabilityConstituents of cotton As discussed earlier pretreatment is the series of cleaning operations. In pretreatment all the impurities present in cotton are removed. Cotton fiber by nature contains:Cellulose 86.8%Natural Impurities:Oil and Waxes 0.7%Pectins 1.0%Carbohydrates 0.5%Proteins 1.2%Salt 1.0%Water 8.5%Colour Pigments:Others 2.0%Except cellulose and water all the impurities are removed in pretreatmentImpurities removed during pre-treatmentShort Fibres SingeingApplied Impurities (Size Material) DesizingArtificial Impurities (Oil ,Dust, Dirt) ScouringNatural Impurities (Oil, Wax, Pectins, Proteins) ScouringColour Pigments (Naturally present in cotton) Bleaching1 Singeing IntroductionSingeing, desizing, mercerizing, scouring and bleaching are the basis for all subsequent processes. Mistakes made in this stage show up in later stages where correction is difficult.

Singeing is the process in which protruding fibres present on the surface of a material (yarn/fabric) are burnt away to give the material a smoother surfaceIt is usually the first stage in the pretreatment process done on grey fabrics. The material is singed on one or both sides by passing either over heated plates or heated cylinder or over a gas flame Process Singeing is usually done by passing the fabric through a burning gas flame at high speed followed by quenching in water or the desizing bath to extinguish the smoldering fibers.

SingeingTypically passed directly from singer into desize bathRollJ-ScraySingerFireFireObjectives of Singeing To get rid of protruding fibresTo create smooth surface for printingTo increase wettability of fabrics/ yarns To improve rubbing fastness and washing fastness of the dyed or printed fabricsDesizing Desizing is the process in which size material is removed.Size is deliberately added on the yarns before the weaving processes so as to increase the strength of yarn, as yarn has to under go great stress and strain on the loomThis process of desizing is necessary otherwise the fabric will give varying and uneven results in dying, printingDesizing can be done either be carried out either by hydrolysis or oxidation Warp Size CharacteristicsStrengthen warp yarnsSize should not penetrate beyond surfaceLubricate warp yarnsAdhere to yarnBe extensible

Be flexibleResistant to abrasionEasily removedInexpensiveNot support bacteriaDesizing involves converting starch to a to a product with a higher degree of water solubilityDesizing process includes: Impregnation the fabric with desizing agent. Allow the time for the agent to the absorbed into the size and degrade or solubilize it.Wash out the degrade size materialTypes of desizing Desizing can be done in a number of ways.

Hydrolytic desizing Acid de-sizingA weak solution of an acid (0.5-1%) (hydrochloric or sulphuric) is sufficient to solubilize the starch at room temperature in a few hours.If the concentration of the acid, temperature and duration of the acid de-sizing treatment are increased, the de-sizing will takes place very rapidly, but the acid may attacks the cotton resulting in the weakening of the fabric. 23Enzymatic desizing Uses enzymes to degrade starch It is one of the most commonly used methods in the industry as it is absolutely safe process. The desize bath contains bacterial enzymes, common salt, wetting agent and acetic acidThe fabric is passed through the desize bath maintained at a specific temperature and pH range, After passing fabric in the desizing solution it is batched for 8-12hours. During this time starch is converted from insoluble starch to soluble sugars and then simple washing is doneWhile carrying out this process temperature limit is to be maintained, as enzymes are active within a narrow limit of temperature.They work best between 60oC and 70oCEnzymes need a slightly alkaline solution to work out their best; hence a pH range of 5.5-6.5 is needed. The pH is adjusted using acetic acidThe enzymes hydrolyses the starch only (not the cellulose) On the other hand acid de-sizing attacks cotton cellulose under suitable conditions of time, temperature and acid concentration.Enzyme de-sizing is therefore safer than acid de-sizing. (enzymes are however more expensive than acids).

Oxidative desizing

Oxidative Desizing

In this process the desizing is carried out by the oxidation of the oxidizing agents like: -1) Sodium Hypo chlorite2) Hydrogen Peroxide3) Sodium BromiteThe oxidizing agents under optimum chemical and temperature conditions liberate oxygen, which help in the oxidation of the starch into water-soluble form.Scouring Cotton is based on 90% of cellulose and the remaining portion consists of impurities such as fats, waxes & organic compounds. The main impurities which must be removed are fats and waxes which are insoluble in water thus decreasing the water absorbency of fabric raw cotton containsThis process of removing off the hydrophobic impurities from a desized fabric is known as scouringThis is done using an alkali like sodium hydroxideA series of reaction takes place in the fabric. These are: -1) NaOH converts all the saponifiable oils into soluble soap and soluble glycerine.2) Proteins are all degraded into soluble amino acid3) All types of mineral matter is dissolved4) All dust ,dirt particles are removed by the detergents5) Waxes are emulsified by the soluble soapsObjectives of scouringTo remove natural fat, wax, and oil materials contained in the fabrics without damaging the fibresTo accelerate dye and chemical absorption of the fabricsTo improve the handle of the goods (softer)

31The main actions, which take place during scouring, are(i) Saponification(ii) Emulsification(iii) Solvent Extraction (Detergency)Saponification: - is the process in which fats are treated with an alkali leading to the formation of hydrophilic soaps (sodium salts of fatty acids).Emulsification: removal of mineral oils/waxes by use of an emulsifying agent, which keeps the emulsion formed for a longer time. When the mineral oil or waxes are emulsified they can be easily removed by washing.Detergency: The dust and dirt particles are removed by using a good detergent. The detergent keeps the dirt and dust particles in a stable suspension in water and does not allow them to settle again. Soap is a good detergentThe scouring can be done in in a number of ways: -1) Kier and J-Box when scouring is to be done in rope form2) Open width pad roll system when open width treatment is requiredKier scouring is the most commonly used method of scouring Kier is the most often used machine for the scouring process in the industry. Kiers are horizontal and vertical cylindrical vessels in which the fabric is piled up in rope form and then hot alkaline scouring liquor is circulated through the fabric. They are generally made up of iron and are available in various capacities; generally 2-ton kier is used.The process of getting the fabric piled up, the liquor circulating inside the kier is known as kieringBleaching Natural fibers, i.e. cotton, wool, linen etc. are off-white in color due to color bodies present in the fiber. The coloring matter gives a yellowish brown color to the fibers, which is unwanted as far as aesthetic appearance and smooth subsequent finishing process is considered. Bleaching should also decolorize or remove any residual impurities left by scouringCotton fibers usually require bleaching unless the material will be dyed very dark or dull shades.

Purpose The goal of bleaching is to decolorize the impurities which mask the natural whiteness of fibers. A good bleaching agent should have following properties: - It should ensure a pure and permanent whiteness to the fabric. It should give level dyeing properties.It should not cause any tendering of the fabric, which causes the loss in tensile strength of the fabricTypes Of BleachingDifferent chemicals are available to carry out the process of bleaching. These include: By using dilute Sodium Hypo chlorite solution at room temperatureBy using Hydrogen Peroxide solution at 80- 85oC.By using Sodium Chlorite solution at boil.By using peroxy compounds e.g. peracetic acidFull Bleach-Full white / Snow whiteHalf Bleach-to dye light / pale / medium shade.No Bleach-to dye dark shades.H2O2 Bleaching Hydrogen Peroxide (H2O2) bleaching is the most common bleaching method, and is environmentally friendly (degradable into water and oxygen). The best bleaching whiteness can be obtained at a pH 11 adjusted using caustic soda.

Since hydrogen peroxide contains an atom of loosely combined oxygen, it has powerful oxidising properties. Due to this it has a bleaching action on textile substrates. In a neutral aqueous solution, hydrogen peroxide is ionised in to perhydroxyl and hydrogen ions H2O2 HOO- + H+HOO- is the active bleaching agent. But the bleaching effect is dependent greatly on the pH of the solution. In acidic or neutral pH, perhydroxyl and hydrogen ions are found in solution. These hydrogen ions have a tendering effect on cellulose due to its acidic nature. In acidic or neutral pH tendering effect of H+ ions is more than the bleaching effect of the HOO- ions, which causes yellowing of the cotton substrateIn alkaline condition following equilibrium exists: H2O2 + OH- HOO- + H2OThis gives a more bleaching effect and less tendering of cottonIn an alkaline medium although the damage to cotton fibre is less, the stability of peroxide itself is also very less. In absence of stabiliser, in alkaline medium at temperatures as high as 100- 1100C, the entire peroxide will decompose HOO- (unstable) OH + O*The presence of water hardness and the iron or metal contamination in commercial processes further enhances the action of peroxide decompositionTo prevent this use of a stabilising agent in bleaching baths to get good bleaching performance is advised.Sodium Silicate is the widely used stabiliser in peroxide bleaching

Sodium Chlorite (NaClO2) bleachingSodium Chlorite (NaClO2) bleaching is performed after pH adjustment to 3.5 with an acid such as a formic acid. It provides a soft hand with less embrittlement of cotton. Compared with the H2O2 bleaching, some insist that even bluish whiteness can be achieved with the NaClO2 bleaching.NaClO2 bleaching is free from worries about pin-hole generation, which often occurs in the H2O2 bleaching. However, a harmful chlorine dioxide gas is generated during the bleaching process, and therefore, suitable measures including the equipment need to be taken for metallic corrosion problems and dechlorination purposesAfter bleaching, a dechlorination process needs to be provided. A trace of chlorine which exists in fabrics reacts with nitrogen compound to form chloroamine, or cause yellowing of the fabrics when exposed to light and heat.Note: a special reducing agent, sodium thiosulfate (hypo), or hydrogen peroxide (so-called, `cosmetic bleaching`) is used for the dechlorination purposes.

Sodium Hypochlorite (NaClO) bleaching

Sodium Hypochlorite(NaClO) bleaching is characterized in that the bleaching is performed at room temperature without requiring a heating apparatus such as a steamer, and that the chemical cost is low. However, the disadvantages of the NaClO bleaching include relatively low bleaching whiteness and embrittlement of fabrics.No particular pH adjustment is required in the NaClO bleaching.

The NaClO bleaching is usually employed for bleaching fabrics after Kier-scouring, or for bleaching towels. For towels, the two-step bleaching comprising the NaClO bleaching and the H2O2 bleaching is preformed to obtain full whiteness. The NaClO bleaching is also employed for jeans fade adjustment, etc.Note: that a dechlorination process needs to be provided as in the NaClO2 bleaching.

Mercerization Mercerization is an optional process in the sequence of the wet processingIt involves the treatment of the fabric with the highly concentrated alkaline solution. Generally the material is treated with 25% of NaOH for relatively short time under tension This is because the longer duration may even cause the degradation of the fabric. After the treatment the fabric is thoroughly washed to remove the traces of the alkaliCotton is generally mercerized in the under given forms: -Yarn formFabric formEffect of mercerization Swelling Increased lustreIncreased tensile strengthChange in cellulose structure