Sunday, September 30, 2012

Various embroidery stitches

Various embroidery stitches

Good collection and tutorial for various embroidery stitches.

'via Blog this'

Tuesday, August 7, 2012

Eco standards in textile Industry through eco auditing

The growth in garment industry in India follows the same pattern as that in the Western World, where individual tailoring has slowly faded away and mass production of apparel has penetrated even the rural markets. The readymade garment market has boomed in the recent years. Garment production and quality control has gained increasing importance in the world. Customer/buyer awareness and competition will lead to further growth and strict quality parameters.

Product manufacturing and testing may be looked at under the following broad categories:
♦ Raw material specifications.
♦ Apparel end-use specifications.
♦ Performance specifications.
♦ Disposal of used products.

Today’s ecological requirements
Today in Europe, ecological and toxicity factors are gaining prime importance in the business of fabric and apparel trade. In 1992, Germany banned the use of metallic components in all consumer articles, which contained nickel. Soon thereafter followed restrictions on pentachlorophenol (PCP) and Azo dyes, which liberated banned Amines. Pollutants, Allegan & Carcinogens are now being severely restricted in the manufacturing of consumer goods sold all over Europe. Therefore, a proper selection of processes is essential in confirming to standards demanded by a customer.

Enzyme technology
Enzymes and enzyme technology are widely touted as the way of the future for many processing industries. Enzyme wash is a technique involving the use of enzyme products designed to produce a moderate level of abrasion without the use of pumice stones. Enzymes are organic catalysts highly specific both in the reaction catalysed and their choice of reactants. Enzymes' properties are:
♦ Physically, enzymes are of colloidal nature and chemically they are of the nature of proteins.
♦ Enzymes are complex and have high molecular weights.
♦ Enzymes are destroyed by high temperatures because proteins get denatured.
♦ Enzymatic reactions are reversible.
♦ The activity of enzymes is limited to a narrow range of pH.
♦ Enzymes are inhibited by cyanide, sulphides, azides, etc.
♦ The range of specificity varies in different enzymes.

Water in textile wet processing
The textile industry consumes large amounts of water in its varied processing operations mainly for two purposes: First as a solvent for processing chemicals and secondly, as a washing and rinsing medium. Successful results in the textile wet processing are strongly dependent on a clean and consistent supply of good quality water.

Water consumption
Amount of water consumption in the wet processing depends up on the following factors:
♦ Machine design
♦ Complexity of process
♦ Method of process
♦ Nature of operation

Tolerances for water for textile industry
Quality of water required for textile industry depends on the location, available sources like river water, sea water, bore water, etc. That means different sources of water having different levels of contamination like presence of metallic parts, salt level. Organic, inorganic content which affect the tolerance limit for the textile wet processing. Table 1 shows the tolerances limit for bleaching and dyeing operation in cotton wet processing industry and also mentioned for wool scouring processing industry. The factor which affects the quality of water in the textile wet processing and consciousness to be taken in these areas is shown in Figure 1.

Efficiency of enzyme
The efficiency of enzyme reaction depends up on the following factors, which are shown in Figure 2.

Eco-auditing
Eco-auditing is a systematic, documented and objective view of the facility, operations, practices and products and related to meeting environmental requirements. It is the assessment of the textile industry with regard to their conformance with the norms or criteria of certain eco-parameters that are required for maintaining eco-standard.

Eco-criteria
The eco-criteria for textile products are built around three key areas:
I. Environmental requirements concerning the fibre types used.
II. Environmental requirements concerning the processes and chemicals used in the production of textiles.
III. Requirements concerning the usability of the final textile products.

Keywords to meet eco-criteria:
Due to the various keywords to meet eco-criteria, the following possibilities may arises.
1. To analyse eco-pressures on the industry hence impact possibilities to soil, air and water.
2. Environmental auditing of all inputs, including raw materials used chemicals and dyes.
3. An action plan should be developed to interpret the results of chemical audit.
4. Substitution of chemical and dyes.
5. Laboratory bench testing.
6. Pilot test testing.
7. Production scale testing.
8. Process optimisation.
9. Development of a quality assurance system and quality assurance manual.

EU Eco-audit: This system lays down 9 requirements to be fulfilled for acceptance of a place and a concern:
♦ Carrying out eco-analysis.
♦ Laying down basis for eco-analysis.
♦ Introduction and maintenance of an eco-management system.
♦ Laying down aims of eco-management.
♦ Carrying out necessary eco-analysis tests.
♦ Setting up an ecology programme.
♦ Preparing an ecology situation statement.
♦ Testing of confirmation to above statement.
♦ Handing over such an ecology statement to the relevant authorities and entry in the local Eco-register.

Quality assurance system: The quality assurance manual must provide the following information:
♦ Dyes ad pigments along with their CI number
♦ Chemicals in use
♦ Material safety data sheets for all items
♦ Processing methods used
♦ Quality parameters of the final product
♦ Test methods for each of the items
♦ Frequency at which each test needs to be conducted
♦ Eco-label requirements

Profit of Eco-audit: By introducing the eco-audit in textile wet processing industries, the following benefits are obtained:
♦ Cost reduction due to systematic analysis from material input to delivery at every stage of processing
♦ Legal safety
♦ Improvement of the environmental protection within the industry
♦ Improvement of reliability
♦ Improvement of acceptance within the official and the public

Eco-labels

Buyers in the Europe markets frequently ask for certification of common eco-labels. Eco-labels are basically of two categories, those which are Government administrated and other which are commercially introduced. Some of these labels are given in Table 3.

Some labels have special requirements that the processor will need to understand what the customer/buyer needs before deciding processes, process parameters and dyes & chemicals that should be used.

Steps for achieving Eco-label award:

♦ Management commitment.
♦ Selection of eco-label product lines.
♦ Establishment of factory implementation team.
♦ Preparation of process flow diagram.
♦ Eco-audit of chemical & dyestuff.
♦ Process optimisation.
♦ QA system & QA manual.
♦ Award of eco-label certification.

Measures to over come problems associated with waste water quality
♦ Colour of process water can be removed by flocculation and filtration method where it is treated with alum (aluminium sulphate), and lime or soda ash.
♦ Metallic impurities can be removed by aeration, coagulation with alum settling and filtration. However some of the waters require special treatment.
♦ Membrane filtration, ion exchange and electrolysis can remove sulphate and chlorides.
♦ Silica-scale growth in process water can be prevented using scale inhibitors of good dispersants.
♦ Silica can be removed through reverse osmosis (RO) and ion exchange techniques, as well as disilicisers. RO membranes are not immune to silica scale, which forms as a gelatinous mass on the membrane surface. It then can dehydrate, forming a cement like deposit.
♦ Water hardness can be removed by ion exchange method. This is commonly referred as ‘Zeolite Softening’. Hardness can also be removed by treatment with lime and soda ash; This is known as ‘Soda-Softening’.
♦ Turbidity can be removed by clarification using alum and soda ash or lime followed by filtration.

The future in textile requirements
Quality parameters and specifications in the future are expected to investigate the complete “Eco-Cycle” or “Life-Cycle” of a textile product. Bio-degradable products will need to be produced to replace the ones that are to be phased out. Human rights concerns, socio-economic issues such as child labour, work environment and occupational hazards will need to be effectively addressed.


References
1. Lopmundra Nayak, and P S Nayak: A Part of Ensuring Eco-Standard, Clothesline, July 2004.
2. Edward Menzes, and Dr Bharat Desai (Rossari Biotech): Water in Textile Wet Processing -- Quality and Measures, Clothesline, 2004.
3. Eskel Nordell: Water Treatment for Industrial and Other Uses, 1951, Reinhold Publishing Corporation, New York.
4. Paul Roshan and Naik S R: Textile Dyer & Printer, 1997, 13.
5. NCUTE programme on Finishing of Garment & Knits, 2001.
6. Dr G P Nair: Developments in Garment Finishing Machinery.
7. D V Alat: Enzymes in Garment Finishing.
8. Dr R B Chavan: Eco-friendly Specially Chemicals in Garment Finishing.
9. Dr Ulhas Nimkar: Quality Control and Testing in Garment Finishing.
10. Achwal W B: Colorage, 40 (11) (1993), 23.
11. Lantto R, Miettinen and Suominen: American Dyestuff Reporter, 85 (1996).
12. S Perkins: Dyeing and Finishing, Asian Textile Journal, 1997, 72-74.
13. Nalankilli G: Application of Enzymes in Wet Processing of Cotton.
14. Nalankilli G: Indian Textile Journal, C3 (12) Sep 1992, 110.
15. www.rossari.com, www.biocon.com, www.yogeswarchemicals.com.
16. www.texanlab.com.

Monday, February 27, 2012

Pre-treatments of textile fibers


The term ‘‘pre-treatment” includes all operations of preparing the textile materials, such as fibres, Yarn. Woven and knit fabric for the subsequent processes of dyeing, printing and finishing. For all practical purposes pretreatments are carried out incontinuation of dyeing or printing and their equipment is pat of the wet processing plant.The main object of pretreatment is to impart a uniform and high degree of absorptivityfor aqueous liquors with the minimum possible damage to the Fibrous material. Thecotton fabric, for example, after the pretreatment should become free of all natural impurities like pectin, wax, protein and husks and the sizing chemicals comprising of adhesives and softeners. Besides high and uniform absorptivity the textile materialsshould have adequate degree of whiteness so as not to mar colour and brilliance of theapplied colours. Normally achievement of whiteness of about 80% remission (As. c100% reflectance from barium sulphate) and a D.P. of 1,600 to 2,000 are aimed at for thecotton goods.The grey cotton fabric normally undergoes the following pretreatment procedures:

•Inspection and marking
•Shearing or cropping
•Singeing
•Desizing
•Scouring
•Mercerizing
•Bleaching

Shearing and singeing are always carried out in open width but desizing, scouringand bleaching of woven fabric may be done either in rope or in open-width Forms by batch, semi-continuous or hilly continuous systems. The rope processing equipment iscomparatively less expensive but is not quite suitable for the heavier-weight and thewider-width fabrics. Such fabrics tend to develop “rope marks’’ that cause uneven dyeinglater. In the rope form, woven and knit fabrics are scoured and bleached in kiers(autoclaves), winches and jets for batch and in J box or a similar storage system for continuous working. In open width processing, jiggers, pad-batch and pad- roll machinesare used for batch system and J-box, U-box, normal steamer, pressure steamer, roller bedconveyor steamer or perforated belt steamer are employed for the continuous working.Choice of the equipment depends mainly on the volume of cloth to be processed but the present clay trend is to install the open-width continuous pretreatment machines.

Loose cotton Fibres, slubbing and yarn in both hank and packages of cone, cheeseor warp beams are scoured and bleached in package processing equipment. If the yarn isto be mercerized later, pretreatment is carried out on hanks because yarn is commonlymercerized in hank form. Hanks are made into a chain by linking together in the formshown in fig.4. 1 and are then packed in kiers for scouring.

A brief description of the above pretreatment processes and the equipment used inthese is given below. Mercerization of cotton fabric and yarn is a special pretreatmentthat can technically he consider a finishing process.

1.Inspection and Marking

 In composite or vertical mills, the grey cloth is thoroughly checked to identify itsdefects and to determine its quality. Inspection is carried out on slanting ground glass plate tables that are illuminated from below by neon tube-lights. The machines are o lienfitted with ultra-violet ray illumination system that spots oil stains and variation indifferent qualities of the cottons that are blended prior to spinning. The cloth is pulledover the glass plate either manually or by a variable-speed motor and different clothdefects are marked and recorded for the quality control purposes. The fabric pieces arenext marked, for identification purposes and flat stitched end to end to make a continuouslength. Marking done with special ink that would withstand subsequent contact withscouring and bleaching chemicals. This indelible ink is usually made with mineral pigments dispersed in a solution of chlorinated rubber binder.

2.Shearing


Purpose of shearing is to remove loose threads and fibre tufts on the surface of thefabric. These are shaved off by carefully running cloth in close contact with razor-sharphelical blades mounted on steel cylinders that are rotating at a high speed in a directionopposite to the movement of the cloth. Shearing is not always necessary for fabrics meantfor bleaching or dyeing but is useful for printing that requires a fabric with a very smoothface. For the same reason, the cloth meant for printing is sometimes sheared after bleaching.

3.Singeing


Fabrics made from staple fibres show protruding fibre ends at the surface of clothand this nap disturbs appearance of the dyed and the printed fabrics. To improve the surface appearance of fabric, the fibre ends are removed by singeing with a flame or a hot plate. Cotton yarn especially that used for sewing thread is also singed to give a smooth appearance.
 Prior to singeing, the fabric is brushed to loosen and raise the fibre ends andthen rapidly moved at a speed of around 100 meters per minute, over a row of gas burnersto burn projecting fibres without scorching the cloth.Singeing can also be done by contact with hot plates but the flame singeing is preferred because flames penetrate into pores of the material, in some machines indirectheat is provided by a red-hot clay plate that is heated by the gas flame.
The best result is achieved by singeing after desizing but it is not practiced because itinvolves an extra drying of the fabric. The burner flame should be free from soot to avoiddiscoloration of the material and so only the more volatile and non-aromatic fuels areused for this purpose. An exhaust fan continuously removes fumes of the burnt fibres.Modern singeing machines have proper safety arrangements against fire hazardsand fuel supply is cut oft immediately on an unforeseen stoppage of the machine. Themachine room has also fireproof doors that automatically close in ease of an accidentalfire to eliminate any chance of spreading fire in the entire department. To quench anylingering flame, the singed fabric is directly fed into a trough full of water that usuallycontains a desizing agent also.

4.Scouring


Cotton goods as yarn and woven and knit fabrics contain about 8 to 10% of natural impurities and the woven fabric carries additional 5 to 7% of the sizing material.While most of the sizing material is removed from the cloth during desizing, the naturalimpurities require treatment with strong alkalis like caustic soda at elevated temperaturefor their breakdown and dispersion in water.For this purpose, a variety of equipment is available to process the materials indifferent forms and by batch, semi-continuous and continuous processes. The ultimateaim of scouring is to make the material tin formally and highly absorbent in a costeffective manner so that there are no difficulties in the later processes of dyeing, printingand finishing. Some of the more common batch and continuous scouring machines aredescribed below.

a.High Pressure Kier


A kier is cylindrical steel autoclave that is capable of withstanding high steam andhas a capacity varying between 1 to 3 tons. To eliminate formation of rust marks. kier iscoated with a mixture of lime and sodium in silicate that is periodically renewed. Kier hasa false bottom at the base on which the cloth pieces, already sewn end to end to loon arope, are piled up to its entire height. A manhole with a pressure lid is provided at the topfor entry of the fabric rope that may be fed manually or mechanically by an overhead.After closing the manhole the scouring liquor is circulated by a centrifugal pumpafter collecting it from the bottom and sprayed at top of the fabric pile through a circular perforated pipe.

On its way up, the liquor is heated with steam indirectly in a multi-tubular heater, an air escape valve at top is left open till the liquor starts boiling and steam starts issuing from it for a few minutes. This is done to remove all the air from the kier otherwise oxygen in the air will react with cellulose to for oxycellulose that causes severe tendering of the material. Composition of the scouring liquor, its temperature and duration of the treatment depend upon density of the fibre to be processed and ultimatedegree of absorption required.A typical recipe, based on the dry weight of the fabric and a liquor ratio of 4:1, is as under:

•Sodium Hydroxide3.0%
•Sodium Silicate0.5%
•Wetting Agent0.1 %

At elevated temperatures sodium hydroxide completely breaks down proteins and pectates that are mainly present in the cuticle layer and converts these into water-soluble products. The oils and fats are converted into soap and this in turn emulsifies waxes thatare removed by washing later. Residues of leaves and husks are degraded but are notcompletely removed by hot caustic soda solution. However, the degraded substances aredestroyed in the subsequent hypochlorite or peroxide bleach. Sodium silicate by virtue of its colloidal nature serves to keep the reacted impurities suspended and reduces their tendency to settle on the scoured fabric.
The wetting agent that may be amixture of anionic and non-ionicsurfactants for having synergisticeffect reduces surface tension of the scouring liquor and so helps inits quick penetration into the fibres.The wetting agents should be ableto withstand high temperature andstrong alkaline solutions. Theliquor is rapidly raised to atemperature of 130°C and is keptcirculating for 6 to 8 hours. Whenscouring is complete the brown colored liquor is discharged, preferably in stages with additionof hot water in to the kier so that the impurities do not settle and stick to the fibres and are removed morecompletely. After washing fabric in the kier, the remaining impurities are removed in arope washing machine (Fig.) preferably with a counter-current flow of water.

5.Desizing

To improve efficiency of weaving, warp yarns are ‘sized’ i.e. coated with anadhesive containing an oily product to prevent their fraying and breaking during weaving.The sizing materials hinder absorption of water or dye-liquor and so their removal isessential prior to attempting any wet process. The cotton warps are usually sized with a preparation based on starch. Starch swells hut does not dissolve in water and dispersible products by either certain enzymes so it is removed by breaking it down into water or oxidizing chemicals. The latter products are comparatively expensive but are faster acting and so are suitable for the continuous processing.

1.Enzyme Desizing

The enzyme preparations are of 3 types. Diastastic, Pancreatic and Bacteria!.These stereospecific enzymes split a-glucoside bonds of starch but not the 3-glucoside bonds of cellulose. The enzymes break down starch (amylose) chains into water-solubleor water- dispersible products. Activity of the enzymes depends on pH temperature and presence of activators (Na+, K +and Ca++ ions) and also the wetting agents.Of all the enzymes the bacterial amylase are the most popular because these arefaster in action and more tolerant to pH and temperature variations. The quenching box of singeing machine contains about 1 % solution of the enzyme with 5g sodium chloride.The optimum pH and temperature of the solution is 6-7 and 60-70 at 18°C respectively.The fabric saturated in the solution is batched on a roller rotating at about 4 rpm. Toimprove penetration, the material is padded twice with an intermediate passage on thetiming-guide rollers before batching.The roll of cloth is wrapped in plastic sheet to avoid drying of fabric and slowlyrotated for a couple of hours. The degraded starch products are then removed by washingthe fabric in boiling water in a washing range. Addition of 5 to 10 g/l of sodiumhydroxide in the first two washing cisterns promotes the washing efficiency. In the earlier practice, the amylase soaked fabric rope was piled in wooden tanks or cement pits andafter about 24 hours the degraded starch was washed off with boiling water in a rope washing machine.

2.Oxidative Desizing

Under controlled conditions, certain oxidizing agents decompose starch into water dispersible particles without damaging cellulose of the fabric. The starch degradingaction is sufficiently rapid to make the process compatible with the continuous fabricscouring and bleaching system. Another advantage of the process is that motes and other non-cellulosic impurities present in fibers are also attacked resulting in partial break down and this helps in their removal in the subsequent scouring and bleaching processes.The more popular oxidizing agents are Sodium Bromite (NaBrO3), Hydrogen Peroxide (H2O2) and Ammonium or potassiumPersulphate (NH4SO4)2Sodium Bromite has provedvery effective.

For rapid oxidation of starch and is applied in a concentration oil 1 g/l of active bromite at a pH of 10 to 10.5(brought with Disodium Hydrogen phosphate-Na2HPO4) at70-90 temperature at 180°C. After a short dwell time of about 20 minutes, thedecomposed starch products are thoroughly washed off with boiling water’ in a washingrange. Alternatively the oxidative desizing agent is added into the scouring liquor toreduce the production time.The water soluble sizes like carboxymethyl cellulose (CMC), Polyvinyl alcohol(PVA) and acrylates need no chemical desizing and only thorough washing with hotwater is enough for their removal, Addition of 1-5 g of a wetting agent and provision of alittle dwell time is helpful in removal of the size especially those containing PVA thattakes comparatively longer time to swell and dissolve in water.

6.Bleaching 

Scouring removes almost all the impurities of cotton fibres except husk amnatural coloring matter that are ultimately removed by oxidizing agents. The oxidationtreatment or bleaching is necessary for producing white goods as finished products or for dyeing paste shades. Even for dark shades, bleaching improves the brilliance andevenness of the shade. Bleaching is normally carried out with hypochlorites, hydrogen peroxide or sodium chlorite.

1.Bleaching with Hypochlorite

The active constituent of the hypochlorites is chlorine o hypochlorous acid that isstabilized as calcium or sodium salts. In earlier times, the calciun compound known as bleaching powder was used exclusively. It is a mixture of calcium hypochlorite Ca (OCI)2, and basic calcium chloride, CaCl2Ca (OH)2.H2O .Bleaching powder being solid is easyto transport and has long shelf life but its use involves excessive and careful handling indissolving and removal of the non-oxidizing solid constituents.Sodium hypochlorite has now replaced bleaching powder because it is moreeconomical at easier to handle. 

It is often prepared in the mills by dissolving chlorine gasslowly in a cold solution of caustic soda or sodium carbonate as per following reactions

2 NaOH + Cl2 → NaOCl + NaCI +H2O
Na2CO3+ Cl  →   NaOCI + NaCI + CO2

Chlorine is abundantly available as a by-product in the electrolytic manufacture of sodium hydroxide. A percentage of 15- 18% of available chlorine are generallymaintained in the concentrated hypochlorite solution but it may be noted that theconcentration falls gradually during storage and needs to he checked he lore use.

Hypochlorites are strong oxidizing agents and it is, therefore, necessary to carryout bleaching under the prescribed conditions of pH, temperature and time so as to avoiddamage to the cellulose. Careful control of the pH is very necessary because theoxidizing entity changes with variation in pH. At pH above 100 the oxidizing componentexists as hypochlorite ion (-OC1-) but at lower nil between 5 and 8.5, it is converted intohypochlorous acid. At pH below 5 liberation of chlorine gas starts and at pH 3 whole of the hypochlorous acid is converted into chlorine. It is well known that the rate of bleaching or oxidation of cellulose becomes maximum between pH 5 and 9 and that pHrange corresponds to maximum generation of the hypochlorous ion.For the minimum damage to the cotton goods bleaching should, therefore, bedone at pH 10-11. Sodium hypochlorite solution has a pH of 11.5 but during the bleaching reaction, hydrochloric acid is generated and pH of the solution starts falling. Tocounter this, tile hypochlorite solution is buffered with 5-I 0 g/l of sodium carbonate.Rate of bleaching also depends on temperature and rises with increase in the reactiontemperature. Normally bleaching is carried out with 5 to 9 g/l of active chlorine at room temperature for about 2-4 hours but it is quite sale to process at 40°C to reduce the production time. Contact with copper and iron metals should be avoided as these catalyze the oxidation reaction and may tender the goods. Equipment for bleaching may be same as that used for scouring.

2.Discolorination or anti-color treatment

It is necessary to remove traces of the residual chlorine alter bleaching otherwiseit may damage cotton goods during storage. This is done either by treatment with 5 g of sulphuric or 20 g/l of hydrochloric acids or with 5 to 10 g/1 of sodium bisulphite(NaHSO3) solution at 60°C for 30 minutes. The acid converts hypochlorite into volatilechlorine while the bisulphite reduces it into a harmless sodium or calcium chloride.In an alternative method alkaline hydrogen peroxide (3cc/I H2O2+ 0.3 g/ NaOHat 90°C) is used that besides dechlorination improves quality and stability of thewhiteness. This principle is also a basis for a continuous bleaching process for cottongoods the reaction may be expressed as under:

HOCl + H2O→2HCl + H2O +O2

3.Bleaching with hydrogen Peroxide

The oxidizing agent most commonly used today in bleaching of textiles ishydrogen peroxide. It is used in batch; semi-continuous and continuous processes as wellas for bleaching colored goods and for combined scouring and bleaching of thelightweight cotton goods. Other advantages of peroxide over the hypochlorites are lower loss in weight of goods, reduced oxidative damage and economy in water usage and solesser cost of effluent treatment t. The bleaching equipment is the same as that used for scouring for both the batch and continuous processes.In the latter, two storage chambers can either be installed in tandem for scouringand bleaching or these processes may be done in one chamber alternately. Hydrogen peroxide has a low dissociation constant and is a weak acid. In an alkaline solution the peroxide anion is produced that is the active bleaching agent (Reaction 1 and 2). In asecondary reaction (3) molecular oxygen is formed that has no bleaching action.

H2O2+ OH = H2O + HO2-………………………………….1
HO2-= OH-+ O………………………………………………2
2H2O2+ 2H2O + O2………………………………………….3

A high concentration of the hydroxyl ions has an accelerating effect on the rate of bleaching and so to control the tale of reaction it is necessary to add a stabilizer. The most common stabilizer is sodium silicate and it also gives protection against metalcontaminants. Silicate is more effective in the presence of salts of magnesium and this provides a rare case where hard water is preferred over the soft type. Magnesium salts are sometimes added into the peroxide bath in calculated concentrations.A disadvantage of the silicate is that it deposits hard insoluble incrustations ontile sides on the side of machine, which scuff the fabric and the scuffmarks show in dyeing. For this reason silicate is sometimes replaced partly or completely with tetra sodium pyrophosphate (Na2P2O2) or organic proprietary stabilizers.

A typical recipe for a wet on wet bleaching in a roller bed continuous system is asunder:

Hydrogen Peroxide 35% (135 volume) 50-60 ml/l
Sodium Silicate 30°Be 10 ml/l
Organic Stabilizer 10 g/l
Sodium Hydroxide 3 g
Wetting Agent 1-2 g/1
Liquor Pick-up 100%
Impregnating Temperature 20-30°C
Reaction Time 1-2 hours at about 95°C.

Concentration of the reactants and the time of treatment nay however, varyaccording to the degree of impurities and quality of whiteness required. Concentration of the chemicals in the feeding bath is about 3 times of the above mentioned concentrationsin the padding bath but that of hydrogen peroxide needs to he controlled by checkingwith periodic titration, usually with the standard permanganate solution. In the semi-continuous pad-roll process time of treatment is usually 4 to 5 hours.

Addition of certain peroxide ‘activators’ like tetra acetyl ethylene diamine in the bleaching bath is claimed to achieve good bleaching at comparatively lower temperaturesand pH. It also causes minimum damage to the fibres and is specially recommended for bleaching viscose and spandex blends to protect the fibres from fibrillation.

4.Cold Pad-Batch Process

This process is very simple and consists of padding fabric with 40- 50 ml/lhydrogen peroxide (35%), 15-20 ml/l sodium silicate 38° Be, 10-15 g/l caustic soda and3-5 g/l of a wetting agent to a pick-up of 100%. The batch roil is covered with a plasticsheet and slowly rotated (4 rpm) for 15 to 24 hours and then washed in an open width washing range with boiling water. The process has the advantages that it requires inexpensive equipment and is free of danger of catalytic damage to the fibres. However,the degree of whiteness is inferior to that obtained by the steaming process.
If deep shades are to be dyed, bleaching of the grey fabric is occasionally done inone step by the cold-hatch or the pad-roll process. However, the process is not always satisfactory because impurities like traces of the heavy metals in cotton have decomposing effect on the peroxide and may also cause tendering on the fabric.


5.Bleaching with Sodium Chlorite: 

Sodium chlorite (NaClO2) was originally introduced for bleaching the synthetic fibres but is now finding increasing use for cottongoods because of all the bleaching agents it is the least damaging to cellulose. It is sometimes used for bleaching grey cotton goods without prior boil-out but absorbency obtained is just tolerable. In the grey bleaching the impurities of cotton are not removed but are oxidized to colorless products and so there is very little loss in weight of the goods after the treatment.Sodium chlorite is commonly marketed in the form of a crystalline powder in80% strength but it is available in liquid state also. The aqueous solution of the chlorite isslightly alkaline and has a pH of about 8.5. However, it must be acidified to a pH valuewithin a range of 3.5 to 4.5 to liberate the oxidizing agent. Doubts exist about the realoxidizing entity and different workers have suggested this to be chlorine dioxide (ClO2)chlorous acid (HClO2) or even atomic oxygen.

Chlorine dioxide gas is poisonous and explosive and is also very corrosive tometals in the aqueous medium. Chlorite bleaching is therefore often carried out in anexclusive room in the dye house that is very well ventilated. The machines are fabricatedfrom special stainless steels that have a high proportion of molybdenum or titanium.

Alternatively bleaching is done in equipment made of stone, PTFE coated steel or wood.For protection of the stainless steel metal of the machines, it is not uncommon to addsodium nitrate in a quantity equal to the chlorite that moderates decomposition of thechlorite and inhibits corrosion of metals.The pH of around 4 ± 0.2 required for bleaching is maintained with buffers or astermed in industry activators, like sodium acetate or sodium dihydrogen phosphate (NaH2PO4) Latter is usually preferred because it improves whiteness o goods. Neutral or slightly acid chemicals that liberate acid on heating are also used occasionally. Organicesters like ethyl lactate or titrate and their ammonium salts are also suitable for this purpose. Common recipes for the batch and the continuous bleaching are given below:

Chemicals                                        Batch Process           Continuous Process

Sodium chlorite (80%)                      5-7                              20-25g/l
Sodium nitrate                                  2-3                               2-3g/l
Sodium dihydrogenphosphatce         2-4                                0.5-1.0g/l
Wetting agent                                   1-2                                1-2g/l
Formic acid to adjust pH to              3.8-4.                            26-6.5
Temperature                                    85-90°                           85-90°C
Reaction time                                  1-3                                  2-4 Hours

After the treatment, washing with boiling water and dechlorination with 0.5-I %cold sodium bisulphate solution is carried out.

6.Bleaching of Knit Fabrics

Cotton tubular knit goods are commonly scoured and bleached by a one step process in winch or jet machines using 4-5 ml/l caustic soda solution (38° Be), 5-8-ml/lhydrogen peroxide (35%) with a peroxide stabilizer and a wetting agent. Semi-continuouscold pad-batch method has also been developed in which the fabric is first padded withreactants under a minimum of tension, wrapped on a perforated stainless steel tube and covered with a plastic sheet. After the due reaction time, water is forced through the tube
into the batch of the fabric and the oxidation-degraded products are washed off. M/sDornier has modified their “floating cigar’ mercerizing machine. For continuous scouringand bleaching of the knit fabric. The fabric may be mercerized first leaving about 2%alkali in the material during washing off The wet fabric is next padded with hydrogen peroxide its stabilizer and other auxiliaries and stored in a roller-bed steaming unit for 1-2 hours. The bleached fabric is finally washed again on the floating cigars thus making the bleaching process a semi-continuous one.It may not be quite out of place to mention that certain direct dyes are applied tothe unbleached (grey) knit fabrics in the presence of hydrogen peroxide and caustic soda,which is a very economical process as scouring, bleaching and dyeing are combined.Recently it has been suggested that combined scouring and bleaching at a temperature of 130°C in a high-pressure jet machine can save time, steam, water and electric power without sacrificing whiteness and absorbency of the finished material. The maindifference of the process from a conventional one is that rinsing of the fabric is started atabout 120°C by simultaneous feeding of fresh water and draining of reactant liquor whilethe system is still tinder pressure.This technique reduces a considerable quantity of water that is normally requiredfor washing off Moreover treatment with hydrogen peroxide and washing off attemperatures above the boiling point of water reduces the processing time from 170minutes required for the conventional process to only 70 minutes. The method essentiallyconsists in raising temperature of the liquor to 130°C in 40 minutes, treating for 5minutes at that temperature and then adding a hydrogen peroxide killer. To save time rinsing is started during the liquor cooling stage and continuous addition of cold water and discharge of hot liquors is carried out simultaneously. The rinsing stage is completedin 25 minutes. It has been calculated that the high temperature process saves about 25%water, 60% electric power, 6% steam and 59% time as compared with the conventionalsystem.

7.BLUEING

After bleaching, the cellulosic materials retain a slight yellowish tone to which the human eye is very sensitive to detect. The principle of blueing is to neutralize this tone with a blue or violet light and this is achieved by the following two procedures.

1.Blueing by Subtractive Effect

In this cotton is shaded with a blue or violet disperse pigment dispersion that arenot substantive to cellulose. This application suppresses the yellow tone but slightlyreduces brightness of the white. Disperse dyes; vat pigments and Ultramarine Blue arecommonly used for this effect.
2.Blueing by Additive Effect or Optical Brightening

Certain organic compounds have the property of fluorescence i.e. they can absorblight of short wavelengths and re-emit it at the longer ones. Many of such compoundsabsorb ultra rays and re-emit as visible light in the range of 4,000 to 7,000 A0. Textilefibres containing a fluorescent compound reflect more light than an untreated one andthus increase its brightness and whiteness.
During the last half century a large number of fluorescent chemicals, also termed as optical brighteners or FBA have been developed for both natural and man-made fibres that have affinity to the fibres and most of these can also he applied during the bleaching process.Chemically these can be classified as derivatives of stilbene and behave like thedirect dyes for the cellulosic fibres. These can be applied by both exhaustion and paddingtechniques in concentrations that would fix about 0.2-0.4%. of it on the fibres. For cottonfibres, these are generally applied along with the peroxide during bleaching but only aselected few can withstand the hypochlorite. It may be kept in view that the degree of brightness obtained with the FBA depends on proportion of the ultraviolet light in theincident light and is less apparent in the artificial light than the sunlight

Textile - Reference Book for Weaving

Textile - Reference Book for Weaving

textile reference book for fininshing

Textile Reference Book for Finishing

Monday, February 13, 2012

Polyester fiber

 

The British scientists John Whinfield and James Dickson first invented polyester cloth in 1941 in England. After World War II was over, in 1945, the United States company DuPont bought the right to make polyester and by 1950 a factory in Delaware was beginning to actually make it.
People make polyester out of oil. You take the oil, which is a kind of very big hydrocarbon molecule, and break it down into two smaller molecules, ethylene glycol and dimethyl naphthalate, both still made entirely of oxygen, carbon, andhydrogen atoms. Dimethyl terephthalate is an ester, and ethylene glycol is a kind of alcohol. When you mix the ester and the alcohol together, they form molecules with both positive and negative charges, and the charges make the molecules line up in chains of crystals that hold together as long fibers. 
The polymerized material comes out of the machine in long ribbons, and you cut the ribbons into little chips and let them harden. Then you melt the chips again and push the goo out through little holes to make thinner ribbons, and wind the thinner ribbons around spools. Then you heat the thinner ribbons and stretch them out to about five times their original length, to make them thin enough to use as thread to weave cloth. 
Since the 1960s, polyester has been the cheapest kind of cloth, and almost half of all the world's clothing is made of polyester. You are probably familiar with polyester mainly from team shirts like for soccer or basketball.
Polyester fiber is a " manufactured fiber in which the fiber forming substance is any long chain synthetic polymer composed at least 85% by weight of an ester of a dihydric alcohol (HOROH) and terephthalic acid (p-HOOC-C6H4COOH)" [3]. The most widely used polyester fiber is made from the linear polymer poly (ethylene terephtalate), and this polyester class is generally referred to simply as PET. High strength, high modulus, low _shrinkage, heat set stability, light fastness and chemical resistance account for the great versatility of PET.




These fibres are also known as Terylene, Terene, Dacron etc.

These fibres are synthetic textile fibres of high polymers which are obtained by esterification of dicarboxylic acids,

with glycols or by ester exchange reactions between dicarboxylic acid esters and glycols.

Thus Terylene is made by polymerising using ester exchange reation between dimethyl teraphthlate and ethylene glycol.


Raw Materials

The main raw materials required for the manufacture of Terylene polyester fibres are p-xylene ethylene glycol and methanol.or Dacron ( Du Pont ) is produced by polycondensation reaction using Teraphthaleic Acid (TPA) and Ethylene Glocol

Manufacture of TPA

P-xylene-- Air, nitric Acid-->P-Toluic Acid--> Teraphthaleic Acid

Manufacture of DMT

p-xylene--Air 200 degC, co-toluate--> Toluic Acid--Ch3OH--> Monomethyl toluate--oxidation--> Monomethyl teraphthalate--CH3OH--> DMT

The use of Dimethyl Teraphthalate is preferred instead of Teraphthalic acid as the purity of the reacting chemicals is essential and it is easier to purify DMT than teraphthalic acid.

Manufacture of Ethylene Glycol



Ethylene--Oxidation with air-->Ethylene Oxide--Hydrolysis-->Ethylene Glycol
or
Ethylene--Hypochlorous Acid HOCl--> Ethylene Chlorohydrin--Alkaline Hydrolysis--> Ethylene Glycol

Production
The polymer is made by heating teraphthalic acid with excess of ethylene glycol ( Both of high priority) in an atmosphere of nitrogen initially at atmospheric pressure. A catalyst like Hydrochloric acid speeds up the reaction.

The resulting low molecular weight ethylene glycol teraphthalate is then heated at 280 deg C for 30 minutes at atmospheric pressure and then for 10 hours under vacuum. The excess of ethylene glycol is distilled off. the ester can polymerise now to form a product of high molecular weight. The resulting polymer is hard and almost white substance, melting at 256 deg C and has a molecular weight of 8000-10000. Filaments are prepared from this.

Spinning of Polyester Fibres

The polymer is extruded in the form of a ribbon. This ribbon is then converted into chips.The wet chips are dried and fed through a hopper, ready for melting. This molten polymer is then extruded under high pressure through spinnerettes down to cylinder.

Each spinnerette contains 24 or so holes. A spinning finish is applied at this stage as a lubricant and an antistatic agent. The undrawn yarn is then wound onto cylinders.This yarn goes to the drawing zone, where draw twist machines draw it to about four times their original length. This is hot drawn in contrast to cold drawing of nylon filaments.
For the production of staple fibres, the filaments are first brought together to from a thick tow. These are distributed in large cans. The tow is drawn to get correct strength. Then it is passed through a crimping machines, the crimps being stabilized by heating in ovens. It is then cut into specified lengths and baled ready for dispatch.


Properties of Polyester


Tenacity (gpd)High TenacityNormal TenacityStaple
Dry6-74.5-5.53.5-4
Wet6-74.5-5.53.5-4
Elongation (%)
Dry12.5-7.525-1540-25
Wet12.5-7.525-1540-25
Density1.381.381.38

Moisture RegainAt 65% RH and 70 deg F--> 0.4%Because of low moisture regain, it develops static charge. Garments of polyester fibres get soiled easily during wear.
Thermal Properties

Polyester fibres are most thermally stable of all synthetic fibres. As with all thermoplastic fibres, its tenacity decreases and elongation increases with rise in temperature. When ignited, polyester fibre burns with difficulty.

Shrinkage

Polyester shrinks approx 7% when immersed in an unrestrained state in boiling water. Like other textile fibres, polyester fibres undergo degradation when exposed to sunlight.

Its biological resistance is good as it is not a nutrient for microorganisms.

Swelling and Dissolving

The fibre swells in 2% solution of benzoic acid salycylic acid and phenol.

Alcohols, Ketones, soaps, detergents and drycleaning solvents have no chemical action on polyester fibres.

Chemical Resistance

Polyester fibres have a high resistance to organic and mineral acids. Weak acids do not harm even at boil. Similarly strong acids including hydrofluoric acids do not attack the fibres appreciably in the cold.

CHEMICAL PROPERTIES
Polyester fibers have good resistance to weak mineral acids, even at boiling temperature, and to most strong acids at room temperature, but are dissolved with partial decomposition by concentrated sulfuric acid. Hydrolysis is highly dependent on temperature. Thus conventional PET fibers soaked in water at 70oC for several weeks do not show a measurable loss in strength, but after one week at 100oC, the strength is reduced by approximately 20%.


Polyesters are highly sensitive to bases such as sodium hydroxide and methylamine, which serve as catalysts in the hydrolysis reaction. Methylamine penetrates the structure initially through noncrystalline regions, causing the degradation of the ester linkages and, thereby, loss in physical properties. This susceptibility to alkaline attack is sometimes used to modify the fabric aesthetics during the finishing process. The porous structures produced on the fiber surface by this technique contribute to higher wettability and better wear properties [7].

Polyester displays excellent resistance to oxidizing agents, such as conventional textile bleaches, and is resistant to cleaning solvents and surfactants. Also, PET is insoluble in most solvents except for some polyhalogenated acetic acids and phenols. Concentrated solutions of benzoic acid and o-phenylphenol have a swelling effect.


PET is both hydrophobic and oleophilic. The hydrophobic nature imparts water repellency and rapid drying. But because of the oleophilic property, removal of oil stains is difficult. Under normal conditions, polyester fibers have a low moisture regain of around 0.4%, which contributes to good electrical insulating properties even at high temperatures. The tensile properties of the wet fiber are similar to those of dry fiber. The low moisture content, however, can lead to static problems that affect fabric processing and soiling.


 OPTICAL PROPERTIES
PET has optical characteristics of many thermoplastics, providing bright, shiny effects desirable for some end uses, such as silk-like apparel. Recently developed polyester microfiber with a linear density of less than 1.0 denier per filament (dpf), achieves the feel and luster of natural silk [23].
THERMAL PROPERTIES
The thermal properties of PET fibers depend on the method of manufacture. The DTA (Fig. 5.) and TMA (Fig. 6) data for fibers spun at different speeds show peaks corresponding to glass transition, crystallization, and melting regions. Their contours depend on the amorphous and crystalline content. The curves shown for 600 m/min and above are characteristic of drawn fiber. The glass transition range is usually in the range of 75oC; crystallization and melting ranges are around 130oC and 260oC, respectively.
Uses of Polyester

1. Woven and Knitted Fabrics, especially blends.
2. Conveyor belts, tyre cords, tarpaulines etc.
3. For filling pillows
4. For paper making machine
5. Insulating tapes
6. Hose pipe with rubber or PVC
7. Ropes, fish netting and sail cloth.