Textile Gate Questions (from 1989 to 2010):
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Monday, October 1, 2012
Sunday, September 30, 2012
Various embroidery stitches
Various embroidery stitches
Good collection and tutorial for various embroidery stitches.
'via Blog this'
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 enzymeThe 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-criteriaThe 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.
Wednesday, March 21, 2012
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.
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.ShearingPurpose 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.
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.
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.
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:
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.
Chemicals Batch Process Continuous Process
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
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
Thursday, February 23, 2012
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 Tenacity | Normal Tenacity | Staple |
| Dry | 6-7 | 4.5-5.5 | 3.5-4 |
| Wet | 6-7 | 4.5-5.5 | 3.5-4 |
| Elongation (%) | |||
| Dry | 12.5-7.5 | 25-15 | 40-25 |
| Wet | 12.5-7.5 | 25-15 | 40-25 |
| Density | 1.38 | 1.38 | 1.38 |
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
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.
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.
BASIC WEAVES
1. Plain weave
The simplest of all patterns is the plain weave. Each weft yarn goes alternately over and under one warp yarn. Each warp yarn goes alternately over and under each weft yarn. Some examples of plain weave fabrics are crepe, taffeta, organdy and muslin. The plain weave may also have variations including the following:
Rib weave: the filling yarns are larger in diameter than the warp yarns. A rib weave produces fabrics in which fewer yarns per square centimeter are visible on the surface.
Matt Weave or Basket weave: here, two or more yarns are used in both the warp and filling direction. These groups of yarns are woven as one, producing a basket effect.
Interlaced with 1 warp yarn over first filling yarn and 1 warp yarn under next filling yarn forming each repeat. (1/1)
Look for an even repeat of yarns that looks like a checkerboard
Yields fabrics with: highest interlacing most raveling snag resistance most tendency for wrinkling lower tear strength Regular basket weave:
Irregular basket weave:
2. Twill weave:
Twill weave is characterized by diagonal ridges formed by the yarns, which are exposed on the surface. These may vary in angle from a low slope to a very steep slope. Twill weaves are more closely woven, heavier and stronger than weaves of comparable fiber and yarn size. They can be produced in fancy designs.
Interlaced with 2 or 3 warp yarns over and one or 2 warp yarns under respective filling yarns
Diagonal ridge formed left-to-right or right-to-left
fewer interlacing and therefore more yarns per inch
more raveling
more pliable drape and hand
more wrinkle resistance
more resistance to showing soil and soiling
more durability and heavier
tendency to have defined face and back
twill direction defined as left or right hand or variation
angle of twill can vary from 15¤ to 75¤ with 45¤ typicalIdentifying the Weave3. Satin weave
Structuring Process
Interlacing float over 4 or more yarns before a single interlacing (4/1, 7/1 or 11/1)
float in warp direction (satin), floats in filling direction (sateen)
Warp-faced fabric have vertical floats while filling-faced fabrics have horizontal floats
Shiny surface on float side if structured with smooth, shiny yarns
Flat, lustrous, smooth surface
Surface slides easily for linings
Floats result in fewest number of interlacing among plain, rib, twill weaves and therefore yield highest potential yarn count
Long floats (7/1, 11/1) and filament fabrics subject to snagging and poor abrasion resistance
Short floats (4/1, 1/4) and spun fabrics can be tough, compact, durable fabrics with low luster (sateen is formed with spun yarns, usually cotton)
Matt or Basket Weave
Basket weave is the amplification in height and width of plain weave. Two or more yarns have to be lifted or lowered over or under two or more picks for each plain weave point. When the groups of yarns are equal, the basket weave is termed regular, otherwise it is termed irregular.
This is commonly used for edges in drapery, or as a bottom in very small weave repeats, because the texture is too loose-fitting for big weave repeats; moreover, yarns of different groups can slip, group and overlap, spoiling the appearance. This is why only basket weaves 2-2, 3-3 and 4-4 exist
Satin weave
Satin weave is characterised by floating yarns, used to produce a high luster on one side of the fabric. Warp yarns of low twist float or pass over four or more filling yarns. Low twist and floating of warp yarns, together with fiber content, give a high degree of light reflection. Thanks to the distribution of interlacing points, all emphasized diagonal effects are avoided. As with twill, there is only one interlacing point on each thread and pick of the weave repeat. Satins differ from twills by having a step number different from 1.
Sunday, February 12, 2012
Fabric defects
The finished fabrics can show various kind of faults
which can be ascribed to the operations which
follow one another till the realization of the finished
fabric. The most common defects which
appear in more or less extended areas of the fabric are:
·
Knot
·
crease mark
·
abrasion or hole
·
tear
·
stain
·
dirt contamination
·
moirè = presence of vawy areas in periodical sequence,
reflecting the light and due to a
different compression of weft or also of warp.
·
grain = presence of
designs with streaked and sinuous lines.
The most common fabric defects due
to warp are:
- Faulty thread = a thread or pieces of thread which are
coarse, fine, irregular owing to higher or
lower twist or to other twist direction, of different
colour, with two or three ends;
- missing thread = a thread or pieces of ground or effect
threads which are missing in the fabric
weave;
- tight/slack thread = a thread or pieces of thread which
are tighter or slacker than the other
pieces/threads;
- incorrectly woven yarn = a thread which in some parts
only of the fabric is not interlaced in the
standard way
- broken warp = small pieces of cut or missing warp
thread
- reversed thread = crossed, exchanged threads or thread
pieces;
- warp stripes = one or more faulty threads giving rise
to zones of different aspect; it can be due
to scraping or rubbing from members of production
machines or to inaccurate reeding;
The most
common fabric defects due to weft are:
· Faulty weft = a weft or pieces of weft
which are coarse, fine, irregular (slubs, etc.), twisted,
reversed, with different twist, of different colour, double weft
· missing weft = weft or pieces of weft
missing in the fabric weave
· tight/slack weft = a weft or pieces of weft
which are tighter or slacker than the other
pieces/wefts
· incorrectly woven weft = a weft which in
some parts only of the fabric is not interlaced in the standard way
· cut wefts = short pieces of cut wefts
· weft bars (starting marks) = visual
light/dark effect in weft direction due to higher or lower weft density caused
by the weaving machine.
The quality control on the fabrics is
carried out on a special inspecting machine, equipped with
special lamps which facilitate the defect
detection by the operator, marks them with labels of
different colours according to the fault
type and importance.
Depending on the number of faults and on their
importance, the fabric pieces can be classified as
standard (in respect to quality specifications) or can be
subjected to a more or less serious
degrading with consequent compensations to the customers
or with the sale of the fabric at a
reduced price.
Various defects can arise during the stages of weaving
preparation (warping, sizing, threading-in
into the heddles and into the reed) as well as during
weaving itself. It is therefore important to
regulate accurately the various devices of the weaving
machine and to understand how to act in
case of anomalous operating situations which create
defects and/or reduce weaving efficiency.
Thursday, February 9, 2012
Thursday, February 2, 2012
NATURAL DYEING OF TEXTILES
Introduction
Dyeing is an ancient art which predates written records. It was practised during the Bronze age in Europe. Primitive dyeing techniques included sticking plants to fabric or rubbing crushed pigments into cloth. The methods became more sophisticated with time and techniques using natural dyes from crushed fruits, berries and other plants, which were boiled into the fabric and gave light and water fastness (resistance), were developed.
Some of the well known ancient dyes include madder, a red dye made from the roots of the Rubia tinctorum, blue indigo from the leaves of Indigofera tinctoria, yellow from the stigmas of the saffron plant, and dogwood, an extract of pulp of the dogwood tree. The first use of the blue dye, woad, beloved by the Ancient Britons, may have originated in Palestine where it was found growing wild. The most famous and highly prized colour through the age was Tyrian purple, noted in the Bible, a dye obtained from the spiny dye-murex shellfish. The Phoenicians prepared it until the seventh century, when Arab conquerors destroyed their dyeing installations in the Levant. A bright red called cochineal was obtained from an insect native to Mexico. All these produced high-quality dark colours. Until the mid-19th century all dyestuffs were made from natural materials, mainly vegetable and animal matter.
Today, dyeing is a complex, specialised science. Nearly all dyestuffs are now produced from synthetic compounds. This means that costs have been greatly reduced and certain application and wear characteristics have been greatly enhanced. But many practitioners of the craft of natural dying (i.e. using naturally occurring sources of dye) maintain that natural dyes have a far superior aesthetic quality which is much more pleasing to the eye. On the other hand, many commercial practitioners feel that natural dyes are non-viable on grounds of both quality and economics. In the West, natural dyeing is now practised only as a handcraft, synthetic dyes being used in all commercial applications. Some craft spinners, weavers, and knitters use natural dyes as a particular feature of their work.
In many of the world’s developing countries, however, natural dyes can offer not only a rich and varied source of dyestuff, but also the possibility of an income through sustainable harvest and sale of these dye plants. Many dyes are available from tree waste or can be easily grown in market gardens. In areas where synthetic dyes, mordants (fixatives) and other additives are imported and therefore relatively expensive, natural dyes can offer an attractive alternative.
Dyeing of textiles Practical Action
The knowledge required for sourcing and extracting such dyes and mordants is, however, often not available as extensive research work is required to identify suitable plants, minerals, etc. In Zambia for example, there is a wealth of plants available for producing
natural dyes, but due to lack of knowledge of the processes involved in harvesting and processing the plants, little use is made of this natural resource. In some countries, such as India, Nigeria and Liberia, where this research has been carried out, or where there exists a tradition of natural dyeing, natural dyes and mordants are used widely.
Types of textiles suitable for dying
Natural dyes can be used on most types of material or fibre but the level of success in terms of fastness and clarity of colour varies considerably. Users of natural dyes, however, tend to also use natural fibres, and so we will look in more detail at this group. Natural fibres come mainly from two distinct origins, animal origin or vegetable origin. Fibres from an animal origin include wool, silk, mohair and alpaca, as well as some others which are less well known. All animal fibres are based on proteins. Natural dyes have a strong affinity to fibres of animal origin, especially wool, silk and mohair and the results with these fibres are usually good. Fibres of plant origin include cotton, flax or linen, ramie, jute, hemp and many others. Plant fibres have cellulose as their basic ingredient. Natural dyeing of certain plant based textiles can be less successful than their animal equivalent. Different mordanting techniques are called for with each category. When a blend of fibre of both animal and plant origin is being dyed, then a recipe should be chosen which will accentuate the fibre which is required to be dominant.
Equipment needed for home dyeing and very small-scale commercial dyeing
Most equipment needed for dyeing fabrics at home, or at the very small-scale commercial level, can be found in almost any market place throughout the world. The following is a list of the equipment requirements and a brief explanation of their use.
1.
•
Heat source. This can be any type of cooking stove; gas, wood, kerosene,
charcoal, electricity. This is used for heating the liquid used during
mordanting and dyeing.
2.
•
Pestle and mortar. Used for milling the natural dye or minerals, where
this is called for.
3.
•
Mordanting and dyeing pans. Stainless steel or enamel pans are the most
suitable for dyeing. The size of pan depends upon the quantities of fabric that
will be dyed. Do not use pans made from copper, aluminium or iron, unless
absolutely necessary, as these metals have properties which can change the colour
of the dye.
4.
•
tirring rods. Stainless steel or glass rods are best as they can be
cleaned and used for different colour dyes. If wooden stirring rods are used
then there should be a different spoon for each colour.
5.
•
Thermometer. This is used to measure the temperature of the liquid
during mordanting and dyeing. A long thermometer (to reach the liquid at the
bottom of the pan) is preferred, with a range of 0 – 100oC (32 – 210oF).
6.
•
Measuring jugs. These are used to measure the quantities of liquid called
for in the recipe. Sometimes precise quantities are called for.
7.
•
Storage containers. Used for storing the dyestuffs and mordants. Large
glass and plastic jars are ideal. Some mordants and dyes are sensitive to light
and should therefore be stored in sealed light-proof containers.
8.
•
Plastic bowls and buckets. A variety of plastic bowls or buckets of
varying sizes are useful when wetting or rinsing fabrics.
9.
• Strainer.
Used for
straining the liquid off the dyestuff in the dyebath.
10.
•
Weighing scales. Used for obtaining the correct quantities as specified
in the recipe. A scales with metric and imperial measurement is useful as
conversions from one system to the other are not then needed.
11.
• Protective equipment. Gloves for holding
hot pans will prevent burns. An apron will protect your clothing. Rubber gloves
will prevent skin irritation caused by mordants, and
2 Dyeing of textiles
Practical Action
1.
will
also prevent you from dyeing your hands. A face mask can cut down the amount of
fumes or powder inhaled during the dyeing process.
Mordants Few natural dyes are colour-fast with fibres. Mordants are substances which are used to fix a dye to the fibres. They also improve the take-up quality of the fabric and help improve colour and light-fastness. The term is derived from the Latin mordere, to bite. Some natural dyes, indigo for example, will fix without the aid of a mordant; these dyes are known as ‘substantive dyes’. Others dyes, such as madder and weld, have a limited fastness and the colour will fade with washing and exposure to light.
Traditionally, mordants were found in nature. Wood ash or stale urine may have been used as an alkali mordant, and acids could be found in acidic fruits or rhubarb leaves (which contain oxalic acid), for example. Nowadays most natural dyers use chemical mordants such as alum, copper sulphate, iron or chrome (there are concerns, however about the toxic nature of chrome and some practitioners recommend that it is not used).
Mordants are prepared in solution, often with the addition of an ‘assistant’ which improves the fixing of the mordant to the yarn or fibre. The most commonly used mordant is alum, which is usually used with cream of tartar as an additive or assistant. Other mordants are:
1.
•
Iron (ferrous sulphate)
2.
•
Tin (stannous chloride)
3.
•
Chrome (bichromate of potash)
4.
•
Copper sulphate
5.
•
Tannic acid
6.
•
Oxalic acid
Using a
different mordant with the same dyestuff can produce different shades, for
example;
1.
•
Iron is used as a ‘saddener’ and is used to darken colours.
2.
•
Copper sulphate also darkens but can give shades which are otherwise
very difficult to obtain.
3.
•
Tin brightens colours.
4.
•
Tannic acid, used traditionally with other mordants, will add
brilliancy.
5.
•
Chrome is good for obtaining yellows.
6.
•
Oxalic acid is good for extracting blues from berries.
7.
•
Cream of Tartar is not really a mordant but is used to give a lustre to
wool.
Mordants are
often poisonous, and in the dye-house they should be kept on a high shelf out
of the reach of children. Always use protective clothing when working with
mordants and avoid breathing the fumes. The mordant can be added before, during or after the dyeing stage, although most recipes call for mordanting to take place prior to dyeing. It is best to follow the instructions given in the recipe being used or experiment on a sample before carrying out the final dyeing. Later in this brief we will explain how the mordant is mixed and used as part of the dyeing process.
These chemical mordants are usually obtained from specialist suppliers or from chemists. Where this is prohibitive, due to location or cost, natural mordants can be used. There are
3 Dyeing of textiles Practical Action
a number of plants and minerals which will yield a suitable mordant, but their availability will be dependent upon your surroundings. Some common substitutes for a selection of mordants are listed below.
1.
•
Some plants, such as mosses and tea, contain a small amount of aluminium. This
can be used as a substitute to alum. It is difficult to know, however, how much
aluminium will be present and experimentation may be necessary.
2.
•
Iron water can be used as a substitute to ferrous sulphate. This can be made
simply by adding some rusty nails and a cupful of vinegar to a bucket-full of
water and allowing the mixture to sit for a couple of weeks.
3.
•
Oak galls or sumach leaves can be used a substitute to tannic acid.
4.
•
Rhubarb leaves contain oxalic acid.
Natural dyestuffs Dyestuffs and dyeing are as old as textiles themselves. Nature provides a wealth of plants which will yield their colour for the purpose of dyeing, many having been used since antiquity. In this section we will look at some of these naturally occurring dyes, their source and the colours they produce. Later in the brief we will look at the application of the dyes to textiles.
Almost any organic material will produce a colour when boiled in a dye-bath, but only certain plants will yield a colour that will act as a dye. The plants given in Table 1 are a selection of plants that have stood the test of time, and are used widely and traditionally by natural dyers. Natural dyes fall into the following categories:
1.
•
Leaves and stems
2.
•
Twigs and prunings
3.
•
Flower heads
4.
•
Barks
5.
•
Roots
6.
•
Outer skins, hulls and husks
7.
•
Heartwoods and wood shavings
8.
•
Berries and seeds
9.
•
Lichens
10.
•
Insect dyes
Figure 2: Marigold
| Common Name | Latin Name | Parts Used | General Colour Guide | Suggested Mordant |
| Alder | Alnus spp | Bark | Yellow/ brown/ black | Alum, iron. Copper sulphate |
| Alkanet | Anchusa tinctoria | Root | Grey | Alum, cream of tartar |
| Apple | Malus spp | Bark | Yellow | Alum |
| Blackberry | Rubus spp | Berries, young | Pink, | Alum, tin |
| shoots | Purple | ||||||
| Betel nut | Areca catechu | Nut | Deep pink | ||||
| Blackwillow | Salix negra | Bark | Red, brown | Iron | |||
| Bloodroot | Sanguinaria canadensis | Roots | Red | Alum, tin | |||
| Buckthorn | Rhammus cathartica | Twigs, berries, bark | Yellow, brown | Alum, cream of tartar, tin, iron | |||
| Cherry (wild) | Prunus spp | Bark | Pink, yellow, brown | Alum | |||
| Dahlia | Dahlia spp | Petals | Yellow bronze | Alum | |||
| Dog’s mercury | Mercurialis perennis | Whole plant | Yellow | Alum | |||
| Dyer’s broom | Genista tinctoria | Flowering tops | Yellow | Alum | |||
| Elder | Sambucus negra | Leaves, berreis, bark | Yellow, grey | Iron, alum | |||
| Eucalyptus | Eucalyptus | Leaves | Deep gold, grey | ||||
| Fustic | Chloropho-ria tinctoria | Wood shavings | Yellow | ||||
| Groundnut | Arachis hypogea | Kernel skins | Purple, brown, pink | Copper sulphate, alum | |||
| Henna | Lawsonia inermis | Leaves | Gold | ||||
| Hypogymnia lichen | Hypogymnia psychodes | Whole lichen | Gold, brown | ||||
| Indigo | Indigofera | Leaves | Blue | Not required | |||
| Ivy | Hedera helix | Berries | Yellow, green | Alum, tin | |||
| Madder | Rubia tinctora | Whole plant | Orange, red | Alum, tin | |||
| Maple | Acer spp | Bark | Tan | Copper sulphate | |||
| Marigold | Calendual spp | Whole plant, flower heads | Yellow | Alum | |||
| Nettles | Urtica dioica | Leaves | Beige, yellowy greens | Alum, copper | |||
| Onion | Allium cepa | Skins | Yellow, orange | Alum | |||
| Oak | Quercus spp | Inner bark | Gold, brown | Alum | |||
| Ochrolech-ina lichen | Ochrolech-ina parella | Whole lichen | Orange, red (when fermanted in urine then boiled) | Alum | |||
| Privet | Ligustrum vulgare | Leaves, berries | Yellow, green, red, purple | Alum, tin | |||
| Ragwort | Senecio | Flowers | Deep yellow | ||||
| Safflower | Carthamus tinctoria | Petals | Yellow, red | Alum | |||
| Sloe- Blackthorn |
Prunus spinosa | Sloe berries, bark | Red, pink, brown | Alum | |||
| Tea | Camelia sinensis | Leaves | Beige | ||||
| Turmeric | Circuma longa | Root | Yellow | ||||
| Wild mangosteen | Diospyros peregrina | Fruit | Grey, pink | ||||
| Weld (wild mignonette) | Reseda luteula | Whole plant | Olive green | Alum, cream of tartar | |||
| Woad | Isatis tinctoria | Whole plant | Blue | Lime | |||
The choice of mordant for a particular plant is dependant upon the material with which it will be used. It is necessary to check a recipe before using a plant, or one can experiment to see what effect a mordant has for a particular application.
It is recommended that plants be grown specifically for the purpose of dyeing. Harvesting plants from the wild on a non-sustainable basis can endanger the survival of the plant. Many lichens are registered as protected organisms and it is illegal to gather them from the wild.
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