Developments in environment friendly functional finishes
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The document discusses developments in environmentally friendly functional finishes for cotton fabrics and garments. It provides an overview of finishes that are formaldehyde-free, as well as silicone softeners, bio-finishes, water repellent breathable finishes, and anti-microbial finishes. Emerging technologies discussed include plasma treatment, fiber coating, and genetically modifying cotton through biotechnology. The document then focuses on wrinkle-free finishes for cotton and methods for imparting these finishes through pre-cure, post-cure, dip, and tumble processes.
Developments in environment friendly functional finishes
- 1. Developments in environment friendly functional finishes for cotton fabrics and garments R.B.Chavan Department of Textile Technology Indian Institute of Technology Hauz-Khas, New Delhi 110016 Abstract Global awareness for environment protection combined with stringent legislation on industrial effluents had led to the search for environment friendly chemical processes for textiles. In the present paper an overview of environment friendly functional finishes for cotton fabrics and garments has been given. The developments in formaldehyde free wrinkle free finishing, silicone softeners, bio-finishing, water repellent breathable finishes, anti- microbial finishes, soil release finishes etc are critically discussed. Plasma treatment, finishing through fibre coating, the use of biotechnology for genetic modification of cotton are considered to be some of the emerging technologies for imparting functional finishes to cotton. Introduction "The polluters must pay" is the concept, which has been emerged recently for industrial production. There are stringent legislations particularly in developed countries on eco-toxicological considerations, health and safety during storage, use and safe disposal of chemicals in water, landfills or release in air during processing or during incineration. In order to meet these requirements an integrated pollution control approach is essential. This has necessitated having a re-look towards the entire textile production processes. As a result of this some of the well-established chemicals, dyes, finishing agents and auxiliaries have been replaced by environment friendly substitutes for the production of textiles. Consumers particularly in developed countries prefer eco-friendly textiles. Therefore, the manufacturing of textiles in developing countries for the export market in developed countries is oriented towards achieving this goal, though the cost of production is high. Among the various fibres used for apparels, cotton dominates the market due to its several advantages. It is readily available at affordable price compared to silk and wool. Being natural it is considered to be eco-friendly. In the present article an attempt has been made to review critically the developments in functional finishes for cotton fabrics and garments. Eco-friendly cotton Cotton is cultivated using pesticides, fertilizers and other crop related chemicals. The residues of these chemicals remain on cotton bolls. These residues are removed during the preparatory processes and enter into the wash liquor resulting in water pollution. Therefore in true sense, cotton cultivated by using such chemicals is not considered to be eco-friendly. A trend is started to cultivate cotton without the pesticides, fertilizers and other chemicals. Such 1
- 2. cotton is considered to be eco-friendly and is known as natural cotton, green cotton or organic cotton. The use of biotechnology to introduce disease resistant cotton and organic farming techniques may decrease the use of chemicals, pesticides, herbicides, fungicides, defoliants and harvest aid chemicals used for cotton cultivation. This would help to decrease soil pollution during cultivation and water pollution during preparatory processes. Naturally coloured cotton Naturally coloured cotton of various colours particularly green and brown varieties were cultivated since ancient times in many countries. However, such cotton did not gain commercial popularity due to low yield, short staple length, poor fibre strength, poor spinnability and possible contamination due to pollination. The present environment protection trends have given impetus to the revival of cultivation of naturally coloured cotton. Such cotton is not subjected to dyeing and thus is free from pollution caused by dyeing operations. Finishing Though attempts are being made to produce raw cotton itself as environment friendly as possible. Nevertheless, to make salable consumer products, the fibre, yarn, fabric or readymade garments have to go through various chemical-processing sequences such as preparatory, dyeing, printing and finishing. Among these the finishing assumes considerable significance because the value addition is realized through functional finishing of cotton in fabric or garment form to impart desirable properties. Some of the most important finishes are 1. Easy care/Durable press/Wrinkle free finishes 2. Softening 3. Enzyme/bio finishing 4. Water proof breathable finishes 5. Soil release and stain release finishes 6. Anti microbial finishes Some of the recent developments related to eco-friendliness of the above finishes are critically described in the present paper. Easy care/wrinkle free finishes The primary function of clothing is to prevent the loss of body heat to allow the sweat to pass through it to the outside atmosphere. Cotton is good in each of these functions. These properties together with its hydrophilicity, freedom from static charge generation and pleasant natural feel makes it excellent fibre for tropical wear. However one of the major shortcomings of cotton fabrics/garments is that, it is prone to creasing during wear and washing. A treatment with cross linking agent is given to impart wrinkle free properties to cotton. The process is traditionally known as resin finishing. This finish imparts crease resistant properties to cotton and keeps shrinkage under control. However when the desirable properties are imparted at attractive levels, the strength is reduced to unacceptable level. Of the several technological attempts addressing this issue a few have been able to achieve a 2
- 3. favourable balance of wrinkle resistance and mechanical properties and have become commercially successful. The development of all cotton wrinkle free apparel in 1990s has raised consumer's expectations on the performance and appearance of cotton garments. The consumers expect the wrinkle free performance to remain almost intact over at least 50 launderings. The rule of thumb is that if the performance stands 10 washes (each of 10 minutes duration at 50-600 C in presence of 1-5g/l detergent) it is likely to stand 50 washes. There are different routes for imparting wrinkle free/easy care properties to cotton fabrics/garments. The most important routes are 1. Pre-cure and post-cure processes 2. Dip and tumbling processes Pre-cure process This is used for fabrics which do not require pleats. The advantages being uniform distribution of chemicals and better process control. The steps involved are pad-dry-cure and are carried out at mill level. Post cure process In this pad-dry application is done at mill level to produce sensitized fabric which is then transported to garment manufacturing unit. Care during storage of sensitized fabric is essential. Many ship the fabric by air and store under refrigeration. The time between garment sensitization and garment making followed by curing is kept minimum (not more than 3 months). The typical steps involved are Pad with resin recipe (60-70% pick up). Dry to 10-12% moisture content. Sanforize without moisture spray. Garment making Garment pressing to introduce creases at desirable places Garment curing at 150-1600C for 8-10 minutes. Finishing in garment form The principal benefits of wrinkle free processing in garment form are 1. Permanent fixing of creases in garment at desirable places 2. No risk of pre-mature cross linking during drying, storage and transportation. 3. Puckering of garment seams is minimized. The following points must be considered. 1. Careful selection of sewing thread buttons zips etc. since these are exposed to corrosive chemicals and heat during wrinkle free processing. 2. Resin finishing offers only one chance. If there is any thing wrong it can not be corrected. This applies both to fabrics and garments. However the control may be more difficult in case of garments than fabrics. Stripping of resin and re-finishing causes more damage. There are two methods of garment finishing 1. Dip process 2. Tumbling process 3
- 4. Dip process 1. Dip the garment inside out in the finishing chemicals keeping MLR 1:5. Suitable washing machine may be used. 2. Rotate the garments for 20 minutes. 3. Hydro-extract to 70-80% pick up. 4. Tumble dry at 700 C to moisture content 10-12%. 5. Turn the garments right side out. 6. Iron/steam press the garments to set the creases at the desired places. 7. Cure at 150-1600C for 8-10 minutes. The solution recovered from the hydro-extractor can be reused. Tumble method In this process the garments are placed (inside out) into a machine with sealed (not perforated) drum and application of resin recipe by pumping or spraying. The drum is turned for 20 minutes. There should not be excessive dripping of chemicals from the garments and also there should not be dry spots on the garments. This method is being used more and more due to the fact that there is no wastage of chemicals. After saturation the garments are hydro extracted, tumble dry at 700C to 10-12% moisture content. Iron/steam press the garment to set the creases. Cure at 150-1600C for 8-10 minutes. Important features MLR should not be less than 1:0.85 for the garments weighing up to 600 gms/garment and 1:1 for those weighing more than 600 gms. Minimum time of tumbling should be 20 minutes. Tumble rotations speed 28-30 rpm. Tumble drying temperature should not be more than 700C Moisture retention after drying 10-12%. If it goes below 10% re-dip and dry. Chemicals used Cross linking agent Low formaldehyde modified glyoxal based (DMDHEU) with external or built in catalyst and buffered. Various brand products are available in the market. Catalyst Facilitates cross-linking reaction. Most commonly used is magnesuim chloride:citric acid system Wetting agent Helps to achieve quick wetting and even distribution of resin recipe. High-density polyethylene emulsion Imparts hand, improve tear strength, abrasion resistance and sewability. Amino silicone or reactive silicone softener Impart soft hand and slick surface feel. Improve wrinkle recovery and sewability. 4
- 5. Silicone elastomer Impart springiness, improve strength. Acrylates Improve soil release. Typical application recipe Cross linking agent 40-120 g/l Magnesium chloride 10-25 g/l Citric acid 0.3 g/l Softeners combination (total) 40 g/l Wetting agent 1 g/l Acetic acid to pH 4-4.5 After resin application and drying turn the garments right side out. Steam press. Hung from non-staining hanger. The assembly is then transferred to moving rails. The rail takes the garments through curing oven and then to packing. Important precautions Pre finishing stage 1. Good water absorbency (<3seconds) 2. Free from size. 3. High tensile and tear strength (sufficiently high to remain acceptable even after 50% loss after resin finishing). 4. Use mercerized cotton. 5. Fabric pH 5. 6. Fabric and garment should be free from softener. Resin finishing stage 1. After drying the residual moisture content should not be <10%. Low moisture gives high dry crease recovery at the cost of heavy loss in tensile and tear strength. For wrinkle free performance both dry and wet crease recovery are important. Wet crease recovery is achieved when moisture content in dry fabric is high. 2. Resin impregnated sulphur dyed fabric/garment deteriorates due to generation of acid from sulphur dye. The generated acid may cause pre mature curing of resin. Sulphur dye should be thoroughly oxidized. 3. Addition of 1-2% urea to the finishing bath is often useful to minimize the release of formaldehyde from resin. Since urea would reduce the efficiency of the resin it may be necessary to increase the resin concentration by 10%. However, it is best to use low formaldehyde etherified DMDHEU. Garment stage 1. Stone washing and other garment processes are done before wrinkle free finishes. 2. Lower curing temperature is required for white to prevent yellowing. 5
- 6. 3. Pockets, belt loops, labels, sewing threads, buttons, zippers etc. should be resistant to chemicals and heat. The resin recipe liberates acid during garment curing. Quality control Physical properties Efficiency of wrinkle free finishing is tested by wet and dry crease recovery angle, smooth appearance rating, resin add on, tear strength, tensile strength, abrasion resistance, dimensional stability etc. Cuen (1 N cupriethylene diamine hydroxide) test Pull the yarn from the backside of the finished garment. Fray the yarn. Put it on microscope slide. Wet out the yarn with 1-2 drops of Cuen. Put the cover slide and observe the swelling after15 minutes under the microscope. The Cuen rating is given by comparing the standard photographs Swelling Rating Remarks None 5 Fully cross linked Slight or none 4 Good cross-linked Moderate 3 Moderately cross linked Moderate to heavy 2 Partially cross-linked Heavy, rapid 1 Low cross-linked Rapid and dissolves 0 No cross linking Rating 3-4 is normally taken as acceptable. Anything below 3 fails due to lack of resin cure and anything above 4 fails due excessive loss of physical properties. Eco-friendly cross linking agents Majority of cross-linking agents used today is formaldehyde based including DMDHEU and etherified DMDHEU, which has low formaldehyde level. Formaldehyde based cross-linking agents are cost effective and efficient. However, the release of formaldehyde vapours during finishing processes as well as during subsequent storage and consumer use of finished products has caused world wide concern on its impact on human health and environment because of the fears that it is carcinogenic and its well known dermatitis effects. The release of formaldehyde is restricted to 20-ppm level. Some of the approaches to limit the release of formaldehyde are 1. After wash of cured fabric/garment 2. Addition of formaldehyde acceptors or scavengers such as urea, carbohydrizide to the finishing bath. 3. Application of 10-30% urea solution with 5-10% add on by atomized spray technique using BASF fog chamber. 4. Modification of DMDHEU with alcohol to produce etherified DMDHEU to decrease formaldehyde release e.g. treatment of DMDHEU with diethylene glycol or 2- propanediol. 6
- 7. Zero formaldehyde derivatives of DMDHEU are more expensive and less effective than DMDHEU at the same add on levels. More active curing systems are also often required. Polycarboxylic acids (PCA) as cross linking agents An alternative approach has been based on the use of PCAs. In 1998 Welch reported that cotton fabric treated with 1,2,3,4 butane tetra carboxylic acid (BTCA) in presence of sodium hypo phosphite showed high level of wrinkle resistance and strength retention as well as good durability to home launderings. However, exceedingly high cost has prevented the use of BTCA on commercial scale. Citric acid (CA), a low priced tri-functional carboxylic acid is less effective to home launderings than BTCA. It also causes yellowing of fabric under curing conditions. Mechanism of PCA cross linking The crease resistance performance of cotton fabrics treated with traditional N- methylol compounds is dependent on ether bond cross-linking with cellulose while BTCA and other PCAs are based on ester-links. In general the esterification reaction takes place in two steps. Step 1 Two adjacent -COOH groups dehydrate and transfer in to cyclic acid and anhydride under high temperature curing conditions. Step 2 The acid and hydride undergoes esterification reaction with -OH groups of cellulose. Requirement for step 1 is that two adjacent -COOH groups should be located at the same side of the main chain which is responsible for easy dehydration and creating cyclic anhydride. If O O the two -COOH groups are located at the two sides of the main chain, cyclic anhydride will not be generated and esterification of cellulose will not take place. Weakly basic alkali metal salts of phosphorous containing mineral acids accelerate the formation of cyclic anhydride. C HC C OH HC Among various phosphates sodium hypophosphite monohydrate (NaH2PO2.H2O) is found to V V - H2O be excellent. The catalyst increases the rate of. anhydride formation as well as the rate of O cross linking through esterification. HC C OH H C V C V O O Polycarboxylic aci Cyclic anhydride O O HC C O R HC C cellulose HO – R V V cellulose O Cellulose HC C HC C O H V V O O 7 Cyclic anhydride Polycarboxylic acid bonded to cellulose through an ester linkage
- 8. Fig. 1. Reaction of polycarboxylic acid with cellulose Other carboxylic acids Performance of various PCAs has been compared with conventional methylol derivatives as wrinkle free finishing agents using sodium hypophosphite as catalyst. Most of these acids imparted DP rating of 4.3-4.7, and crease recovery angle 285-300°. However, the resultant finishes differed considerably in durability in alkaline laundering. The acids arranged in order of decreasing durability in terms of maximum number of washings and tumble drying cycles withstood were as follows: BTCA>CA>Maleic acid.>Succinic acid. Principle of PCA selection The principles on which PCAs can be used as non-formaldehyde finishes are as follows: 1. In terms of number of COOH groups, saturated Carboxylic acids should possess at least three and unsaturated at least two COOH groups. 2. Regarding the position of COOH, the two COOH in aliphatic PCA should be in cis configuration and two in aromatic PCA should be in adjacent position. According to above principles it may be possible to develop new low cost PCA finishes whose DP rating is as good as or greater than BTCA. It has been thought that polymaleic anhydride (PMA) can be used as PCA finish but the reaction conditions should be strictly controlled. The DP of polymerization of PMA should be within the range of 4-6. CH CH n C C O O O Fig 2. Polymaleic acid anhydride Demerits of PCA finish Eco-friendliness of catalysts For cross linking of cotton with BTCA or CA the catalysts used are weakly basic alkali metal salts of phosphorous containing mineral acids. Some of the salts which act as catalysts are 8
- 9. sodium hypophosphite, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, tetrasodium pyrophosphate etc. mono di and tri sodium citrate are also used in BTCA finishing. The most effective catalyst is sodium hypophosphite. But it has serious disadvantages. For example: 1 High cost 2 Being reducing agent it brings about shade changes in most sulphur dyes and reactive dyes. 3 Environment impact is also a major concern Compounds containing phosphorous can consume large amount of oxygen in water, deteriorate the quality of water and influence the reproduction of fishes. Recently attempts have been made to replace sodium hypophosphite by trisodium citrate and a combination of sodiun oxalate and sodium formate as catalysts. These catalysts do not have any effect on sulphur or reactive-dyed fabrics. Cost of BTCA Due to extremely high price of BTCA, there are limitations in its industrial application. Two alternative aspects may be taken in to consideration for cost reduction. 1. Cost of synthesis The synthesis process of BTCA is extremely sophisticated, therefore, one should strive to seek an optimum production to decrease its cost. Industrially used BTCA may be manufactured without purification. The solution of BTCA can be decolorized and concentrated in the paste form for the application. 2. Cost of Finishing One can incorporate other PCAs or catalysts in to finishing formulation to reduce the amount of BTCA in finish bath. Inexpensive citric acid may be used. Some auxiliary catalysts such as triethanol amine (TEA) may be incorporated to lessen the finishing cost and to enhance the strength of finished fabric. Discoloration/yellowing effect of CA finished fabrics When CA is applied, as cross-linking agent discoloration of finished fabric is observe The reason for discolouration is that unsaturated poly carboxylic acids are produced during curing, generating conjugated double bond system as a chromophore. Investigations showed that major products formed during the curing process of CA treated fabrics are aconitic acid, citraconic acid and etaconic acid. H2C COOH H2C COOH HC COOH HO C COOH HC COOH H2C COOH H2C COOH Citric acid BTCA 9
- 10. HC COOH CH2 CH3 HOOC C C COOH C COOH H 2C COOH H2C COOH HC COOH trans – Aconitic Itaconic acid Citraconic Fig. 3. Formulae of PCAS used as crosslinking agents and of the unsaturated PCAS formed during curing process of CA-treated fabrics In order to minimize the yellowing, special additives such as triethanolamine (TEA), N,N-bisHydroxy ethyl) glycine, boric acid and polyethylene glycol may be incorporated in the finish bath. It has been observed that TEA can suppress the yellowing effect efficiently. The mechanism of the yellowing suppression is suggested to be that, the OH in TEA can react with OH in CA to form ester bonds which can block the OH in CA and prevents CA from hydrating and decomposing to unsaturated polycarboxylic acid when it is exposed to elevated temperature. As a result the formation chromophores responsible for discoloration is prevented. Poor wash fastness of finished fabrics The difference in structure of CA and BTCA is that one OH group exist in structure of CA. Accordingly, the difference in easy care effects between CA and BTCA is attributable to the disturbance impact of the OH group in CA on esterification reaction between the COOH in CA and OH in cellulose molecule. Due to this reason the hydrolysis rate of the ester bonds formed by CA under alkaline conditions is greater, than those formed by BTCA. Thus the OH group in CA is the origin of the low wash fastness of CA finished fabrics. In order to improve the durability of the finish, OH group in CA must be modified. Investigations indicate that when PMA is incorporated in to CA finish bath, the OH in CA can undergo esterification reaction with the COOH in PMA to form extremely complicated molecules under elevated temperature curing conditions. As a result the OH in CA will he blocked and enhance the wash fastness of finished fabrics. From the above-mentioned drawbacks it can be seen that there are many problems associated with PCA based non-formaldehyde finishing. These include cost of the PCA, the environmental problem of catalyst containing phosphorous, the yellowing effect and the low wash fastness of CA finished fabrics. All these problems are under investigation and some of which have been overcome satisfactorily. It is believed that it will not be too long before the traditional N methylol amide finishes are substituted completely by PCA non-formaldehyde finishes. A new approach to the production, of formaldehyde free DP finishes has been the 10
- 11. recent work on the ester crosslinking of cotton by two polymers of maleic acid, namely homopolymer (PMA) and a terpolymer (TPMA) based on maleic acid, acrylic acid and vinyl alcohol. The terpolymer approach showed a significant improvement in DP performance of cotton fabric and retention of physical properties compared to the DMDHEU. Softeners Fabrics and garments are usually comfortable to wear if they are soft to touch. Chemical pretreatments remove natural cotton waxes rendering cotton harsh to handle. This is usually made worst after wrinkle free finishing. To compensate this; softeners are widely used. They also act as fibre lubricants decreasing both fibre-fibre and fibre-metal friction. The draping and, cutting properties are also enhanced. The modem requirements are for softeners of good durability to machine washing, and retain the wettability and absorbency as well as properties against static charge under adverse conditions. The trend is towards the use of silicone softener that provide a soft luxurious handle, thereby imparting a higher quality and added value to the material. Silicone softeners Silicone is a general term that refers to class of man made polymers based on the framework of alternate silicone and oxygen (Siloxane) bonds with organic substituents attached to silicone. Methyl group (Me) is the most important of organic sustituents used in commercial silicones. The most dramatic changes in the field of softening have been the introduction of novel Polysiloxane softeners in both macro and micro emulsion forms. Softeners and elastomers based on Epoxy and amino functional polysiloxanes provide an ultra soft handle to cotton. Such softeners generally increase the tensile resiliency and decrease the bending rigidity. Another aspect of elastomeric finishes is that they can be used to replace some of the cross Linking agents used in typical easy care formulations. When ethylene diamino functional siloxanes are cured for easy care finishing, yellowing can occur through the formation of azo and azoxy compounds as a result of oxidation of amine groups. Secondary amines such as cyclo hexyl amine and tertiary amines such as piperazine have been studied for use in polysiloxane softeners. Yellowing is greatly decreased but the finish is less hydrophilic and somewhat dryer than that obtained using primary amino functional siloxanes. Carboxy funtional, epoxy functional and amido funtional siloxanes have also been developed. The Later is prone to yellowing and more hydrophilic than amino functional siloxanes. Particularly, interesting is the research involving carboxy functional siloxanes which may improve resistance to oily soiling. The use of polysiloxane softeners modified to incorporate various organo functional groups will increase in the next decade as cotton finishers strive to improve the strength retention of easy care fabrics. Silicones have been used as textile softeners since 1960. Initially, polydimethyle siloxane (PDMS) were used but in late 70s the introduction of amino functional PDMS opened new dimensions. Currently available silicone softeners can be classified in to following classes: 1. non-reactive 2. Reactive 11
- 12. 3. Organo reactive. Non-reactive silicones These are based on polydimethyle siloxanes. They impart desirable properties to fabrics due to their flexible polymer backbone, stable bonds and low intermolecular forces. However, the softening effect is not very durable to washing due to absence of reactive groups. - 2. Reactive silicones: these are PDMS polymers modified with silane hydrogen or silanol functional groups. Conventional reactive silicones form a cross-linked siloxane rework on the fabric surface in presence of water and Organo metallic catalyst. The durability is better than non-reactive PDMS. Silanol functional polymers are also the basis for silicone elastomers and textile finishes to improve hand and DP performance. 3. Organo functional silicones: In this amino functional groups, which are bound to PDMS backbone improve the orientation and substantivity on fibre. The improved orientation of amino functional silicone leads to extremely soft hand which if frequently described as "super soft". In this Organo reactive groups such as amines epoxides and alcohols are introduced in PDMS. Me Me Me Me Me3 SiO (SiO) Si Me3 Me3 SiO (-SiO) (-SiO) Si Me3 XMe3 SiO (-SiO) (-SiO)n Si Me2X n a b Me Me Me Me Nonreactive silicon polymar Reactive silicone polymer Me Me Me3 SiO (-SiO) (-SiO) Si Me3 -HC CH2 - NR +Cl- , -COOH , NHCOR` , -O(EO) (PO) X a b 3 a b Me R O Y X = -OH or RO- Y = - NH2 Organo functional reactive silicone polymar Fig. 4. Examples of non-reactive, reactive and Organo functional reactive silicone polymers Eco-friendliness of silicone softeners The textile processing operations produce an aqueous effluent, which is discharged to waste water treatment plant for purification. Silicones are a minor part of that discharge. PDMS becomes part of the plant wastewater stream in the form of tiny dispersed droplets and attach to suspended solids. Because non-volatile PDMS fluid is essentially insoluble in water, these materials become a minor component of the sludge in the treatment plant. The specific route 12
- 13. of non-volatile silicones in the environment depends on how the individual wastewater treatment plants handle the sludge. If the sludge is incinerated the silicone gets converted to amorphous silica, water and CO2. Sludge, if used as a fertilizer it may introduce PDMS to soil, where it is subjected to natural degradation processes. Similarly soil catalyzed degradation is also expected to occur if sludge bound PDMS is land filled. Though PDMS materials are highly resistant to biodegradation they break down in to lower molecular weight silaxonols and silanols during soil contact. The degradation products are susceptible to biological decomposition eventually oxidizing to natural silica. Me Me Soil HO (-SiO) H Me3 SiO H y + Me3 SiO (SiO) Si Me3 Water x Me Me y = mainly 1 Fig. 5 Degradation of PDMS Effects PDMS is ecologically inert and has no effect on aerobic or anaerobic bacteria. It does not inhibit the biological process by which the waste water is treated. No adverse effects of PDMS were seen on seed germination or survival percentage of plants. Studies found no evidence that PDMS may inhibit the bacteria responsible for nitrogen fixation in some crops. If PDMS enters the aquatic environment they do not bio concentrate. Their molecular size prevents them from passing through the biological membrane of fish or other animals. PDMS fluids attached to particulate matter and are effectively removed by the natural cleansing process of sedimentation. They do not partition back in to water column. PDMS fluid exhibit insignificant BOD. Testing on aquatic plants and animal life revealed no measurable adverse effects even under highly exaggerated conditions of exposure. No significant change in the growth of algae or other marine organisms has been found. PDMS has not been found to pose threat to insect population and birds. While manufacturers across the world continue to seek environment friendly alternatives for many performance additives and finish chemicals in the textile industry, the inherent eco-friendly silicones and their low concentration usage indicate the likelihood of there sustained presence in textile manufacturing. Water resistant breathable finish Present trend is to impart water repellency without affecting water vapour permeability of fabrics and garments for use in out door activities, foul weather-clothing etc. Improving the transpiration rate of perspiration through the textile materials is particularly important in sports activities, where the relative metabolic rate is high. Water-resistant breathable finishes are impermeable to water droplets but allow the escape of water vapour. It is possible to achieve this property because of enormous size difference between liquid water 13
- 14. molecule (100 microns in diameter) and water vapour molecule (0.0004 microns) i.e. there is factor of 2,50,000) between two sizes. Such type of garments is designed for sports wear, skiwear, tracksuits, ram wear, clothing for mountaineering etc. Lightweight fabrics coated with poly (vinyl chloride) and polyurethane have become increasingly popular for foul weather clothing. Such fabrics provide exceptional protection against rain but considerable build up of moisture vapour inside the clothing may cause discomfort. This problem arises because of water vapour impermeability of the polymer coating. To overcome this problem breathable polymer coatings are now available which have greatly enhanced comfort properties of these clothing. Classification of breathable fabrics Breathable fabrics may be classified into three categories High density woven fabrics These are obtained by densely weaving fine smooth micro fibre yarns, this type of weaving results in wind proof fabric with excellent water vapour permeability. However it is not waterproof even after treating for water repellent finish. These fabrics are used for fashionable ski clothing, where water vapour permeability and wind proofing is more important than water impermeability. Laminated fabrics In this approach the fabric is coated with breathable adhesive with the help of rotary screen- printing or spray coating. It is than laminated with micro porous breathable barrier film like PTFE. The breathable films are made through biaxial stretching or by mechanical fibrillation to produce microscopic tears through out the film. Manufacturers claim that such PTFE membrane contains 9 billion pores per square inch with a maximum pore size of 0.2 microns. Such PTFE membranes are used in Goretex two layer and three layer laminates of polyester fabrics. Other commercial micro porous polymeric membranes include 1. Sympatex(hydrphilic polyester): Akzo 2. Bion II (polyurethane): Toyo Cloth 3. Excepor U (Poly amino acid/PU): Mitsubishi-Kasei 4. Thintech (Poly olefin): 3M company Coated fabrics Micro porous coating The most important techniques used to produce micro porous coatings for breathable textiles are 1. Solvent exchange 2. Phase separation 3. Phase inversion Solvent exchange In this process the polymer is dissolved in water miscible solvent and then thinly coated onto 14
- 15. the fabric. Passing through a coagulation bath develops the porous structure where the solvent is displaced by water. Phase separation The coating polymer is applied from a mixture of relatively volatile solvent with a proportion of higher boiling non-solvent. Precipitation of polymer as the micro porous layer occurs as the true solvent evaporates faster during the subsequent drying process. e.g. Ucecoat 2000, a polyurethane based coating Here a low temperature boiling solvent (methyl ethyl ketone) evaporates preferentially as the fabric passes through oven, thereby increasing the concentration of non solvent in the coating. When the concentration of the non solvent reaches a critical level, the polyurethane precipitates out in a highly porous form and remains in this form on complete drying Phase inversion A solution of polyurethane containing a non-solvent is coated on the fabric. In a selective evaporation step, the solvent is eliminated first. The polyurethane then precipitates in a micro porous way, the example is Ucecoat 2000 (S) of UCB specialty chemicals. Bio finishing Processing of cellulose fabrics and garments with cellulase enzymes is generally referred as bio finishing or bio polishing. The concept was started in 1980's. Presently major industrial application of cellulase enzymes is to produce wash down or worn out look (stone wash) on indigo dyed denim. This is known as biostoning. Cellulases are multi component enzyme system that is commonly produced by soil dwelling fungi and bacteria. The most important cellulase producing organisms are fungi of Trichoderma, penicillium and Fusarium. Cellulase consists of at least three enzyme systems working together synergistically. i. Endoglucanases or Endo cellulases hydrolyze cellulose randomly along with the cellulose chains, preferentially the amorphous region. ii. Cellobiohydrolase or Exo cellulase attack the chain ends and primarily produce cellobiose iii. The cellobiose and the small chain oligomers produced by exocellulase are then hydrolyzed by the third enzyme β-( 1,4 )-glucosidase into glucose Depending on the pH of activity, there are three types of cellulase enzymes viz: Acid stable (pH 4.5-5.5) Neutral (pH 6.5-7) Alkali stable (pH 9-10) These enzymes can be used in the temperature range of 40-60° C. Biopolishing consists of cellulase treatment so that there is partial hydrolysis of cellulose at the surface with a weight loss of around 3-5% and loss of strength of 2-7%. Short fibre ends are hydrolyzed and are removed by additional mechanical treatment to prevent pilling particularly in case of PET/cotton blends. Mercerization enhances the rate of enzymatic hydrolysis. Biostoning has achieved considerable importance for treating casual wear garments to give them a wash down or worn out look. Environmentally this is more acceptable because it replaces or decreases the quantity of pumice or synthetic stones that cause damage to machine. It also avoids 15
- 16. occurrence of pumice dust in the environment and in the garments creating a harsher handle. Neutral cellulases are preferred for biostoning of indigo dyed denim trousers and other garments. The bio finishing of cotton or PET/cotton blends reduces pilling, provides durable soft handle, increased gloss or lustre. The cellulase treatment of cotton is now well established. Further refinements in process techniques would emerge. Developments such as the use of one or more specific enzyme types rather than three types working synergistically will depend on commercial separation of the enzymes. Anti microbial finish Clothing and textile materials carriers of microorganisms such as pathogenic bacteria, odour generating bacteria and fungi. Because of the growth of micro organisms on the fabric surface, anti microbial / anti bacterial / anti fungal fabrics gained significant importance in the recent years due to its wide acceptance as surgical apparels, baby clothing, under garments etc. There is growing concern all over the world on the dangers of microbial contamination. Recent outbreak in diseases such as AIDS, Hepatitis B has increased the awareness of these health hazards and the need for protection against them. Anti microbial finished fabrics for surgical apparel play an important role in reducing diseases transmitted in operation theatre, as the bacterial and viral diseases are spread through air and blood. Anti microbial agents in these fabrics prevent the proliferation of disease causing microbes or reduce their number by the action upon them by destroying their cell membrane. Anti microbial surgical apparels act as physical barrier against the transfer of micro organisms. A surgical patient is at greater risk because the wound created during the operation is open and easily accessible to microorganisms and hence may become septic. For this reason the surgical gowns used by patients should be free from microbes. The doctors and nurses are also to be protected from the blood and other body fluids of the patients, which may contain microorganisms spreading the disease to doctors and nurses. Anti microbial under garments is also useful in preventing skin related diseases, urinary track infection etc. Baby's clothing are advised to be anti microbial and their thin skin may be permeable to microbes. Since the internal intake of anti biotic for the infants is not desirable so protective action against microorganisms should be taken through clothing. Taking all these factors into consideration development of anti microbial fabrics/clothing is important. Requirements in anti microbial finshing 1. Durability to multiple launderings and dry cleaning. 2. No toxic effects on wearer. 3. Acceptable moisture transfer properties. 4. Compatibility with dyes, auxiliaries and other finishing agents. 5. Ready availability at reasonable price. 6. Should not affect the fastness properties of dyes. Mechanism of protection There are three mechanisms by which treated fabrics are made resistant to disease causing bacteria Viz. Controlled release, regeneration model and barrier or blocking action. Controlled release 16
- 17. This finish is considered effective when in the presence of sufficient moisture, the anti bacterial agent is released from fabrics at a rate sufficient to kill or inhibit the growth of bacteria. Regeneration model This model is based on subjecting the fabric to agency that would destroy the bacteria e.g. addition of bleaching agent during laundering or exposure to UV light Barrier or blocking mechanism This include a) insertion of physical barrier films or coatings that are simply impervious to transmission of micro organisms through fabric .. b) Films or coatings that have direct surface contact activity against bacterial growth. Finishing techniques Following processes can be used for imparting anti bacterial finish to cotton or PET/cotton blends Pad-dry-cure process for fixation of anti bacterial additive onto fabric with the help of cross linking agent Grafting Incorporation of anti bacterial agent during fibre finish Pad-dry-cure method Various chemicals, which can impart antimicrobial property to cotton, are shown in Table Table 2. Anitibacterial chemicals Name Chemical Nature Effective Concentration (g/l) Afrotin LC Na salt of heterocyclic compound 20 Mysotox ELN Pentachlorophenyl laurate and Non-ionic emulsifying agent 2.5 Mystox WFE Aqueous solution of Sodium Ortho phenylphenate 1 Antibac MF An organic compound containing Nitrogen with no halogen 40 Antibac MFB Combination of hetrocyclic Organic compound 80 Fungitex OP Derivatives of bezimidazole 100 Above agents can be applied along with cross linking agent by pad-dry-cure technique. However the wash fastness of the finish is not satisfactory. Chitosan and citric acid 17
- 18. Chitosan and citric acid are used as durable press anti microbial finish cotton when applied by pad-dry-cure process CA reacts with OH group of cellulose and NH2 of chitosan to form ester cross links and inter ionic attraction Poly(hexamethylene-biguanide hydrochloride, PHMB) A new anti microbial finish called Ruptex 20 has been found to give durable finish to cotton and cotton blends. It is based on the active agent PHMB. It is applied by padding or exhaustion technique. The Finish can withstand 25-50 wash cycles depending on wash conditions. Tulsi leaves (Ocium santum) One bath process for dyeing and simultaneous anti microbial finish by methanolic extract (1-5%) of Tulsi leaves on cotton has been investigated. It is believed that Urosolic acid (C30H48O3) is the active ingredient, which inhibits the growth of bacteria. Grafting Poly-(2-methyl-5-vinyl pyridine) was grafted onto cellulose followed by immersion in aqueous Kl solution. Such a technique produces idophor, a material that slowly releases iodine to impart anti bacterial and anti fungal activity. Barrier coating Organo silicone The anti bacterial finish based on the concept of barrier coating (direct surface contact activity) is Dow Corning 5700 (3-trimethoxy silyl propyl dimethyl octa decyl ammonium chloride) Organo silicone polymer. Containing pendant quaternary ammonium group that forms a bio barrier onto fabric. This finish is effective in inhibiting the growth of odour causing bacteria and is durable to atleast 40 laundry washes. Soil release and stain release finishes Soil release and stain release finishes are particularly important where leisure or sports wear activities may lead to greater incidence of soiling or staining. This type of finish would become more important as standards of easy care increase and consumer expectations become greater Another factor is the trend towards lower temperature washing of garments where removal of persistent soil and stain removal becomes more problematic. In general, the approach in textile finishing has been to ensure that any soil that is deposited is less strongly bound to facilitate the removal of soil during domestic washing. Because most persistent soils are 18
- 19. hydrophobic, the major trend has been to use finishes that render the fibre surface more hydrophilic. On this principle three major groups of finishes are developed. Finishes containing carboxyl groups Finishes containing oxy ethylene and /or hydroxyl groups Fluorocarbon finishes Finishes that contain carboxyl, oxy ethylene or hydroxyl groups may be incorporated within zero formaldehyde based chemical cross-linking systems for cotton. Easy care finishes make cotton more hydrophobic and can increase oily soiling. Whereas the introduction of hydrophilic carboxyl groups can improve the release of oily soil dramatically. It is known that hydrophilicity, anti soiling and soil release properties are not usually associated with silicone softeners, and silicone based water repellent finishes do not provide oil repellency. Fluorochemicals Fluoro chemicals or fluorocarbons are increasing in use because of greater ability to engineer the chemical structure to provide good water repellency and soil and stain release properties. Hybrid flouro chemical finishes have been introduced that contain per-fluoro alkyl containing polymer segments and also hydrophilic segments. In air the fibre-air interface is dominated by closely packed per fluoro alkyl moieties which promote soil repellency. In aqueous wash liquor, molecular reorientation occurs, leading to. the hydrophilic segments being oriented towards fibre-liquid interface, thereby promoting soil release. The use of perfluoralkyl acrylate and methacrylate based copolymers modified to suit particular end uses is well established. Emerging technologies Traditionally chemical finishes are applied by padding or low wet pick up techniques such as lick roll, porous bowls, vacuum extraction or foam. Alternative methods include coating and lamination technologies. New application technologies are emerging. Some of these are briefly mentioned. Nextec Process It is patented by Nextec Applications Inc. USA. It is a novel process by means of which individual fibres within the fabric are encapsulated or wrapped with an ultra thin film of polymer. This can be utilized to provide a breathable barrier film within the fabric, preventing liquid water from penetrating while allowing moisture vapour to pass through at as close to the same as original fabric. The handle of the fabric is relatively unaffected. It is also possible 19
- 20. to introduce chemicals for imparting wrinkle resistance, soil release, stain resistance. Chemical and biochemical additives may be positioned on the fabric in order to provide water repellency, UV protection, flame resistance or viral barriers. So far this unique fibre encapsulation process has been applied using silicone-based polymers, polyurethane, polyacrylate and other polymers. This process would open up many opportunities for providing multi functional finishes on cotton fabrics. Plasma treatment The plasma treatment is based on ionized gases produced by electrical discharges. The plasma treatment is highly surface specific and does not affect the bulk properties. The surface properties enhanced include wettability, adhesion, cross-linking, bio- compatibility, chemical affinity or inertness. Relative free radical intensity of the plasma treated fibres measured by ESR, increases in the following order Cotton > wool > silk > Nylon = PET Plasma treatment may enable to avoid pre-coating and pre-lamination preparation of the substrate, replacing some costly and environmentally polluting processes. The dry treatment and environment friendliness for improved wetting, adhesion and bondability offer scope for improvement in functional finishes for cotton. Use of biotechnology The use of biotechnology for the modification of cotton is at an exciting stage and the greater applicability of enzyme treatments as chemical pre treatments and finishing treatments is worth exploiting. The next decade should bring forward some interesting developments in this field and the genetic manipulation of cotton gene may usher in new cotton varieties with improved properties and performance that can be further exploited in chemical finishing to produce innovative effects. UV protection The problems of ozone depletion in the upper atmosphere have led to increased problems of exposure of skin to UV radiations. The size and depth of Antarctic ozone hole has caused concerned, particularly in Australia. It has been suggested that a decrease of 1% ozone would lead to increase UV radiation at the earth's surface and may eventually lead to a 2-3% increase in skin cancer. Hats, sun screens, sunglasses and clothing can be used to decrease the exposure to UV radiation by a factor of 10 or more. The term UV protection factor (UPF) has been introduced such that a garment of UPF 15 will provide the same amount of protection against solar UV radiation as a sun screen of SPF 15 (Sun protection factor). Garments with UPF value of 20-29 offer high protection (UVR transmission 5-3.3%). A UPF value of 30-40 (UVR transmission 3.3-2.5%) offers very high protection and UPF value of >40 offers maximum protection with UVR transmission of <2.5% This opens up another area of opportunity for cotton fabric and garment producers to manufacture comfortable ranges of clothing for leisure and for out door work wear that offer significant enhanced protection against UV radiation. The appropriate selection of dyes, fluorescent brightening agents and particularly UV absorbers should enable cotton finisher to provide high level of UV protection. 20
- 21. Conclusions Desirable properties can be imparted to cotton by the application of functional finishes. This not only would improve the quality of cotton but also give value addition. Innovations through R&D in chemical finishing will be necessary to stimulate the more demanding consumer and to develop specialty and niche markets for cotton fabrics and garments. The development of finishes with multi-functionality will be particularly interesting field especially with regard to easy care finishing. Novel methods that minimize water and energy costs may be based upon plasma treatments, but such treatments for cotton are still in the development stage. Enzyme treatments also offer many promises. In general many innovative ideas are being explored in the field of functional finishes for cotton and in the next decade quite a few new concepts may be introduced for imparting desirable properties to cotton fabrics and garments. 21