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SPECIAL CHEMICAL FINISHES

20.1 INTRODUCTIONThis chapter deals with finishes of the cellulosic fabrics for certain specific end uses like water-repellency, flame-retardancy and mildew and rot-proofing. Such chemical treatments are mostly applied to the materials required for commercial, industrial and military uses. However there are notable exceptions to this generalised statement where these treatments are used for apparel and household linen such as flame-retardancy of garments and bed-wear for children and waterproofing of raincoats.

Like other finishes, the water-repellent and the flame-retardant finishes are undergoing constant changes not only for improved results but also in consideration of the ecological effects, ease in application and the last but not the least, the ultimate cost. A relatively greater stress for improvement has recently been on the fire-retardant finishes because these involve safety of human lives. The old and the new finishes on these two subjects, both temporary and permanent, are described below.

20.2 WATER- REPELLENCY AND WATER- PROOFING In the past different terms were used to express the state of hydrophobicity of a textile material and degree of effectiveness of a particular product. Multiplicity of such terms was confusing especially when these were not based on any precise and standard testing procedures. However with passage of time all the hydrophobic treatments have been gradually classified into two main groups termed as water-repellent and waterproofing finishes that are expressed with fairly well defined characteristics. The term ‘water-repellent’ expresses a degree of resistance of a fabric to surface wetting, water penetration, water absorption or any combination of these properties but its assessment is dependent upon the test conditions used. In general, the water-repellent finishes are resistant to wetting and wicking by rain drops but are permeable to air and also a little to the water vapours.

Waterproofing normally represents the condition where a textile material can prevent absorption and penetration of water to its structure. Thus the waterproof surface provides a barrier to water under all end-use conditions.

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However in practice a waterproof fabric is generally required to allow no penetration by water below a hydrostatic pressure of 100 cm (10 Kpa). Waterproofing is achieved by coating the fabric with a solid polymer like neoprene (synthetic rubber), polyvinyl chloride or polyurethane. Being non-breathable, the waterproof fabrics are uncomfortable to wear because the perspiration of the user is not permeated and dried due to nonporous nature of the material. The coated materials are therefore mainly used for commercial and industrial purposes.

20.3 CONDITRIONS FOR HIGH WATER-REPELLENCY FOR FABRICS High levels of water-repellency on fabrics depend upon the following factors.

1. Fineness of yarn and close packing of textile structure.

2. A thoroughly prepared fabric surface that is free from impurities like size or surfactants that have rewetting properties. Removal of impurities helps in bringing the repellent chemicals in close contact with the fabric surface. Shearing and singeing of the fabrics also serves the same purpose.

3. Uniform application of the repellent-finish to provide a low-energy surface that has a lower critical surface tension than the surface tension values of water (and other liquids against which the repellent finish is required).

20.4 CHEMICAL NATURE OF WATER-REPELLENT FINISHESThe water-repellent chemicals for the textile materials, as developed over the years, are of a very diverse nature. The more important of these are mentioned below although only a few of these are used for the protein and the synthetic fibres.

20.4.1 Metal salts like Aluminium Acetate (CH3COO)3 Al: This is one of the earliest compound used to impart water-repellency to cotton canvas fabrics. Its application consists in padding or treating the fabric with a solution of 5-90Tw. aluminium acetate or formate, followed by drying. During drying, the aluminium salt is converted into water-insoluble

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aluminium oxide (Al2O3) to make the fabric water-repellent. The main problem with the finish is its tendency to dust off the fabric and so this led to an improved method of depositing another aluminium compound that is usually called, a technically incorrect name, the aluminium soap.

20.4.2 Soap and Metal Salts: In this method water-repellency is achieved by depositing, inside the fabric interstices, insoluble and hydrophobic aluminium soaps that are produced by reacting soaps with soluble aluminium salts. The double-decomposition reaction may be represented as under:

3 C17H35COONa + (CH3COO)3 Al → (C17H35COO)3 Al + 3 CH3COONa

The treatment may be carried out in a jigger or a winch but use of a padder is preferred because it is more productive. The well-prepared cotton fabric is impregnated with 2-5% soap solution at 600C, dried, and then padded again in 5-90 Tw aluminium acetate solution. The fabric is next rinsed to remove sodium acetate formed during the reaction and then dried at a high temperature for proper penetration of the metal soap (aluminium stearate) into the fibres and the fabric interstices.

To simplify the two-stage procedure, the fabric may be padded, using a sturdy padder, in a viscous solution of pre-prepared aluminium stearate [(C17H35COO)3 Al] in the white oil. Alternatively, the fabric can be padded in the molten aluminium soap. This method is generally confined to preparing heavy waterproof canvas fabric required to cover equipment lying in the open or for tents and awnings. Usually 0.5% of a copper salt, commonly copper nepthenate is added for protection against microorganisms (rot proofing). Sometimes mineral wax is also added to the stearate to make the waterproofing still more effective.

20.4.3 Wax Applications: Different mineral and natural waxes have been used for the water-repellent finishes but these are now being replaced with the chemically more complicated and also the fibre-reactive compounds that yield more lasting finish. The waxes commonly used are, a) mineral wax

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(alkanes-melting point 52-560C), b) beeswax C15H31COOC30H61, (m. pt. 62-650C) and c) Carnauba wax C25H51COOC30H61, (m. pt. 83-860C). The waxes are applied by different methods as mentioned below.

i) Dissolving wax in an organic solvent and applying by a lick roller or by spraying, followed by drying. The method is now ecologically unacceptable.

ii) Rubbing fabric against wax bars followed by hot calendaring.iii) Spraying molten wax with steam jets and drying on hot cylinders.iv) Padding fabric in an aqueous wax emulsion, drying and then melting

the wax for its even redistribution.

20.4.4 Pyridinium-Based Quaternary Ammonium Hydrocarbons: These are represented with the following structure.

It is considered that during heating, thermal decomposition of the pyridinium salt takes place and the liberated chloralkyl ether reacts with the hydroxyl group of cellulose as indicated below.

Heat → C5H5N + C17H35OCH2Cl + CellOH→ CellOCH2C17H35 + HCl

The most well known examples of this type of compounds are Velan PF (ICI) and Zelan AP (du Pont) that are considered to be stearamidomethyl pyridinium chlorides. The oily chains were supposed to form covalent bonds with cellulose molecules but later studies showed that only a part of the compound was covalently linked with cellulose. However it was enough to impart fairly permanent water-repellency along with a soft handle to the fabric. These compounds are applied with sodium acetate that converts the strong mineral hydrochloric acid liberated during the reaction into the non-damaging acetic acid and thus serves to protect the cotton fabric from the possible hydrolytic damage.

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The main problem with such water-repellents is liberation of pyridine, which has an unpleasant odour and is also toxic, and so ecologically undesirable. 20.4.5 Organo-Metallic Complexes: These compounds contain chromium, aluminium or copper atoms complexed with stearic or myristic acids and impart semi-permanent water-repellency to both the natural and the synthetic textiles. These are dissolved in isopropanol, and are applied by pad-dry-cure method. The metal ions are supposed to form bonds with the fibres and the hydrophobic fatty components are oriented towards the surface of the fabric. The more effective copper-complex also imparts the anti-microbial property but gives a green colour to the fabric. The colour restricts its use mainly to tents, awnings, boat covers, surgical gowns etc. The chromium complex is not much used on textiles and its main application is on leather and paper.

20.4.6 N-Methylol Derivatives: These self-cross linking or cyclic reactant compounds like dimethyloldihydroxyethylene urea (DMDHEU) or trimethylol melamine have been used on the cellulosic materials for imparting crease-recovery property since long. By incorporating a hydrophobic fatty chain in the molecules of the reactant C.R. agent, a durable water-repellency is obtained along with the crease-recovery property. Wax is often added to increase the effect.

Usually an add-on of 3-4% is needed to get effective results, which is applied by padding, followed by drying and curing at 1500C for 1-2 minutes. The finish however is not completely fast to dry-cleaning.

20.4.7 Silicone Finishes: Siloxanes are extensively used as softeners in the textile industry and are known to be mildly water-repellent. On curing, the dimethyl derivative, provides good hydrophobicity along with soft handle to the fabric but its complete polymerisation requires a long time at elevated temperatures (200-2500C). To lower the curing temperature, methylhydrogen

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silicone is copolymerised with the dimethyl derivative and the resultant product can be cured at 120-1500C for only a few minutes. The copolymer is therefore commercially acceptable but the handle becomes slightly harsh. The structural formulae of dimethyl and methyl hydrogen polysiloxanes and their copolymer are shown below as (a), (b) and (c) respectively.

In addition to a balanced mixture of the two siloxanes, the water-repellent formulation contains catalysts of zirconium, titanium or other compounds. These are not catalysts in the sense as to enhance rate of polymerisation but these tend to orientate or direct oxygen atoms in the silicones towards the fibre surface for chemical bonding and the methyl groups away from the surface for water-repellency. The optimum results are obtained by ‘ageing’ for 24 hours after curing when the crosslinking process is completed.

Polysiloxanes are soluble in many organic solvents like xylene and also the more economical tetrachloroetylene, and so can be applied in the special solvent-applicant machines with arrangement for recovery of the used solvent. This method gives enhanced water-repellent effect than that obtained with the aqueous emulsion process.

The durability of silicone finishes is fairly high to the normal laundering and dry- cleaning but its quality deteriorates on repeated laundering because the

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fibres swell on wetting and the hydrophobic surface-film develops cracks, through which water can seep.

Polysiloxane finishes are popular because of ease in their application, a relatively low add-on, soft handle, compatibility with other chemical finishes, applicability to different textile materials and above all lower cost as compared with the other permanent finishes. The silicones are often replaced with the fluorochemicals, which though expensive, are more acceptable on account of the combined water and oil-repellency properties.

20.4.8 Fluorochemical finishes: The fluorochemicals or fluorocarbons are best known for their stain and oil-resistant characteristics but these also have outstanding water-repellent properties. These extraordinary chemicals have been successfully exploited for the combined effects of stain and water-repellency.

The fluorochemicals are often mixed with the common water-repellent hydrophobic compounds for a synergistic effect. The complex formulations are marketed as aqueous emulsions for ease in their application by the padding techniques. The chemical add-on is around 0.15-0.3% on the weight of fabric. After impregnation, the fabric is dried at 110-1300C and then cured at 160-1800C for 45-30 seconds.

20.5 WATERPROOF BREATHABLE FABRICSAs mentioned earlier in section 20.2, the waterproof fabrics completely prevent penetration of liquid water whereas the water-repellent fabrics only delay the penetration of water. However water repellent garments are more comfortable to wear because these allow the perspiration of the wearer to gradually permeate out giving a feeling of dryness to skin.

The breathable fabrics allow passive diffusion of water vapours but the fabric can still prevent penetration of liquid water. These have been developed on the consideration that diameter of the smallest raindrop is 100 m and cannot pass through holes of about 10 m while molecules of water vapour having a diameter of 0.0004 m can permeate.

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Waterproof breathable fabrics may be divided into the following three types;i) densely woven fabrics with maximum pore size of 10-4 m.ii) laminated membranes with maximum pore size of less than

0.001m.iii) coatings with maximum pore size of 3 0.1 m.

20.5.1 Densely Woven Fabrics: The earliest fabrics of this type were developed in 1940s for military purposes and were termed as “ventile fabrics”. These are made from combed yarns spun from fine cottons that are then plied to make yarn as smooth and regular as possible. The yarn is converted into fabric with Oxford weave (plain weave with two threads woven as one in the warp) to give the weft yarn minimum crimp and to make fibres parallel to the fabric surface. On wetting, the fibres swell to the maximum extent that results in decreasing the size of the pores in the fabric from 10 m to 3-4 m across. With a fabric density of about 100/cm, these ventile fabrics effectively resist penetration of water from the medium intensity rain showers.

Microfilament yarns of polyester or polyamide with diameter of less than 10 m are also used to make densely woven fabrics that are waterproof breathable. To further improve their hydrophobicity, these may be treated with silicone or fluorochemical finishes, but the treated fabric still remains permeable to moisture vapour.

20.5.2 Laminated Membranes: Membranes for water repellent purposes are manufactured as thin polymeric films of 10 m thickness and are laminated to the conventional textile fabric to provide a suitable barrier to liquid water. Such microporous membranes may be made from a film of polytetrafluoroethylene polymer that is claimed to have 1.4 billion holes per square centimetre. These membranes are usually supported with a layer of hydrophilic polyurethane or modified polyester film. The hydrophilic membrane though non-porous absorbs water (perspiration) and acts as a porous conduit for water vapours and also decreases effect of the oily contaminations of the human body on the water repellent membrane.

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20.5.3 Coatings: These are both microporous and hydrophilic types and may be produced by wet coagulation, thermocoagulation or foam coating methods. These coatings form a large number of interconnected channels that allow the water vapours to permeate.

A special coating (Stomatex) has been introduced by Akzo Noble, which is a closed foam insulating neoprene film incorporating a series of small convex domes that are vented by small apertures at the dome apex. These apertures open and close like plants (leaf stomata). This film works in conjunction with a waterproof breathable membrane and thus provides a strong breathable structure with a high comfort level.

20.6 Waterproofing: The waterproof fabrics are mainly used as covers for military hardware and other commercial materials to protect these from rain showers. The earlier methods of making textile fabrics impervious to water were crude and consisted in application of natural vegetable products like rubber latex exudations and linseed oil. By the beginning of the 19 th century, chemically modified materials were introduced and aluminium soaps either alone or in admixture with wax and fat emulsions were extensively used. As the latex emulsions did not give very satisfactory coatings, these were mixed with mineral waxes and animal fats like stearin. In an alternative process that is still popular, the latex solution is mixed with a vulcanizing agent, fillers and accelerator to form an adhesive rubber film on the fabric. This technique is used for backing of rugs and carpets and also on the canvas transmission belts, hoses, gum boots etc. The heavy tarpaulins for the military hardware used to be coated with a solution of tar, pitches and wax dissolved in naptha. These were often mixed with lithopone (a mixture of zinc sulphide and barium sulphate) and other pigments and the total add-on would be in the range of 30 to 35%. These have now been replaced with the lighter-weight and less messy products, as mentioned before.

20.7 FLAME-RETARDANT FINISHESAll textile fibres are destroyed by fire and produce heat and suffocating fumes that cause injuries and death to the all forms of life- human, animal

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and others. A great deal of work has therefore been done to learn about the mode of destruction of fibres by fire and to devise methods to minimise the damage to life and property originating from accidental fires. Methods used for protection of flammable materials from fire dates back to about 400 B.C. The earliest recorded attempts for prevention of fire to textiles were made in 1640 when clay and plaster of Paris were applied to canvas used in theatres. To these formulations, alum and ammonium phosphate were added about a 100 years later. The first scientific investigation was perhaps carried out by Gay Lussac in 1820 and he concluded that the most effective salts for ‘fireproofing’ were either those that covered the fibres with a glassy layer or those which produced non-flammable vapours on heating. In the later case the vapours extinguish the fire by excluding oxygen near the flame. However these explanations for fireproofing do not explain all the observations. The term ‘fireproofing’ tends to imply that the fireproofed fabric when brought in contact with flame will behave like asbestos. However main aim of ‘fireproofing’ is to suppress the tendency of the textile materials to spread fire and not to protect it from the fire damage. Actually such treatments tend to increase the rate of destruction of the fibres. The more accepted term now for avoiding the fire hazards of textiles is flame-retardancy.

A great deal of work has been done in recent years to understand the mode of combustibility of different textile fibres and the factors responsible for propagation of fire. Burning behaviour of the different textile fibres may be summed up briefly as follows. The cellulosic fibres are charred and produce heat and volatile products; the latter catch fire and further spread the fire. The charred matter has the property of ‘afterglow’ (flameless or smouldering combustion) that rekindles the fire after extinguishing it. The synthetic fibres like polyester and nylon melt and catch fire producing a smoky flame. However the fibres shrink away from the flame and the molten mass drips down thus removing heat from the flame front and reduces chances of propagation of fire. This is in contrast to burning of cotton as discussed above. However the flaming-drippings can spread fires separately if these fall on carpets, or other combustible materials. Besides these drips cause severe burns to living beings that may come in contact with the melt-

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drippings. In composite textiles like Polyester/cotton, a blend of thermoplastic and non-thermoplastic materials, the former melt on ignition but spreads flame on to the cotton component that propagates it by the so-called scaffolding effect.

The protein fibres like wool and silk catch fire but do not propagate the flame and are self-extinguishing when moved away from the flame. Fabrics of protein fibres are therefore considered to be safer than those of other fibres and are specifically recommended for children’s night-ware. The inherent good flame-retardancy of wool is due to its high contents of moisture (16%), nitrogen (15%) and sulphur (3-4%) and low content of hydrogen (6-7%). Nevertheless wool materials are given the flame-retardant treatments when used for the aeroplane seat covers to meet the stringent safety requirements.

From the above it is clear that each group of chemically different fibres require some specific flame-retardant finishes that are compatible with their chemical natures. A brief review of the existing flame retardant treatments is given below for different types of fibres.

20.7.1 Cellulosic Fibres: In principle any Lewis acid forming substance will promote flame retardancy by inducing char formation and many common acid salts have been used since long for this purpose. However the ideal salt should not generate acid below 1500C i.e. during the usual drying and curing stages. A large number of proprietary products, both organic and inorganic, have been marketed that have varying degrees of durability. These include boron, antimony and ammonium salts and organic compounds containing phosphorous, nitrogen and bromine. The marketed formulations usually contain both inorganic and organic compounds in different concentrations. Some of the more common flame-retardants are briefly discussed below.

One of the earlier non-durable inorganic flame-retardant is a mixture of borax (Na2B4O7) and boric acid (H3BO3) in a 7:3 ratio with add-ons of 10-15% but this suffers from poor after-glow retardancy. Another common non-durable finish is diammonium phosphate {(NH4)2HPO4)} but it has now been almost completely replaced with a more durable ammonium polyphosphate

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(APP) whose durability increases with increase in the molecular weight of the polymer. The phosphorous in these two compounds reacts with cellulose (phosphorylation) that improves durability of the finish but an add-on of 1.5-4.5% phosphorous is required for adequate protection against spreading of flame. Fire-retardancy of APP is further improved by addition of urea that has a two-fold function. Firstly urea enables APP to be cured at a relatively high temperature of 1500C and this increases degree of phosphorylation of cellulose and consequently durability of the finish. Secondly urea, by virtue of its swelling action on cellulose fibres, improves penetration of APP in the fine structure of cellulose and provides more uniform and better effect. Some organo-phosphorous compounds were also developed that would react with cellulose at about 1500C to impart a wash-resistant flame-retardant finish. Chemical structures of three phosphorous compounds are indicated below.

Ammonium Polyphosphate Tris(1-aziridinyl) Phosphine Oxide Tetra kis hydroxymethyl

Phosphoniumchloride APP A.P.O (T.H.P.C.) A large number of proprietary products, both organic and inorganic as well as their mixtures of varying durability, are available in market and most of these formulations contain nitrogen, phosphorous and halogens (mainly bromine) in different proportions. An interesting development is the use of crosslinking silicone combined with compounds containing nitrogen and phosphorous. This finish is fairly durable and is popular for back-coating outdoor fabrics like tents and curtains in theatre and cinema houses. The two most popular products that are durable for at least 50 launderings are marketed under the names of Pyrovatex CP (Ciba), Pekoflam DPM (Clariant) or Amgard TFR 1 (Rhodia) and Proban CC (Rhodia, formerly Albright and Wilson).

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Pyrovatex CP is a cellulose reactive methylolated phosphonamide of the following chemical structure.

The fire retardant Pyrovatex CP is bound with cellulose molecules via reactive resins like methylolated melamine or dihydroxydimethlolethylene (DHDMEU) in the presence of an acidic catalyst. A typical recipe consists of 280 g/I Pyrovatex CP, 35g/I methylolated melamine, 25g/I Orthophosphoric acid (catalyst) with a softener and a wetting agent. The chemistry of reaction may be represented as under.

The fabric is carefully padded or foam-applied with the liquor, dried at 1300C and cured at 1500C for 4-5 minutes or at 1700C for 1 minute. Generation of formaldehyde and other volatile compounds during the reaction is a serious drawback but is contained by scrubbing the volatile products with water. To neutralise any acid, the fabric is treated with an alkaline solution at 500C and then washed and dried. The treated fabric binds 1.5-2% phosphorous to get a fairly permanent flame retardancy and also acquires some crease recovery property with only about 20-25% loss in tensile strength. The fabric remains compatible with dyes and is superior to Proban CC in this respect.

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Proban CC is a THP salt-urea complex {(CH2OH)3P+(Cl-)CH2NHCONH2} that requires an ammonia cure and a final oxidative treatment with hydrogen peroxide. The complex is applied along with wetting and softening agents by a pad or foam application method, dried with a moisture content of 8-10% and then exposed to ammonia vapours to create crosslinks between THPC-urea complex chains. The reaction yields an insoluble polymeric phosphine having a molar P:N ratio of 1:2. The resulting phosphine polymer has a highly reducing nature and is oxidised with dilute hydrogen peroxide to stabilise it as an oxide. The treated fabric acquires some stiffness but becomes softer after laundering that removes the surface polymer. The chemistry of the possible reaction is given below.

The above two proprietary products are fairly permanent and are so far known to have no toxic effects but are expensive. The high cost of these products has given rise to use of relatively less durable but inexpesive

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expensive halogenated hydrocarbons especially for the back-coated outdoor fabrics like tents and tarpaulins. A common and economical product is chlorinated paraffin waxes {CnH(2n-m+2)Clm}. More recently back-coating formulations comprising antimony-bromine compounds present in acrylic resins have become popular. A typical recipe consists of 3% decabromodiphenyl oxide (DBPO), 17% antimony oxide (Sb2O3) and 50% acrylic binding resin to get a mole ratio for Sb:Br of 1:3. The commercial product may also contain antifoaming agent, viscosity modifiers etc. The fabric is back-coated with the mix and dried. An add-on of 20-30% is maintained in the finished product to get the optimum result.

20.8 ENVIRONMENTAL IMPACT OF FLAME-RETARDANT FINISHES: The above mentioned flame-retardant chemicals are not environment and friendly especially antimony, halogen and even phosphorous compounds. A great amount of work is going on to reduce the impact of these compounds on the environment and more efficient methods have been developed to minimise waste and also to recycle the by-products. Improvements have also been made in the scrubbing systems for containing formaldehyde and other toxic vapours and to eliminate their escape to the atmosphere. Special efforts are being made to reduce antimony and bromine (Sb-Br) concentrations in the back-coatings or to entirely replace these with ammonium polyphosphate formulations. Another interesting development is the introduction of intumescent products that is briefly discussed below because much information of the chemicals is not available as yet.

20.9 INTUMESCENT APPLICATIONSIntumescence is a phenomenon where heat or some other agent promotes formation of an expanded or a swollen condition; in case of the flame-retardant this is an expanded char. Intumescent coatings applied to the textile fabrics are activated with heat and generate a flame-resistant foamed layer on the fabric surface that stops or retards propagation of flame.

Intumescent coatings contain four major components-a char former (carbonific), a catalyst, a ‘spumific’ or gas former and a binder. On heating, the binder melts and spreads to form a skin over the fabric surface and thus

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protects the inner layers from oxygen in atmosphere and so combustion is avoided. The heat then activates the catalyst, which decomposes to yield an inorganic acid and chars the carbonific compound by a dehydration mechanism. At the same time heat makes the intumescent compound generate non-flammable gases, which are trapped within the viscous carbonaceous char, blowing up the layer to form a thick thermally-insulating foam. This foam acts as a char-barrier and mechanically resists spread of flame and also reduces emission of smoke and other toxic emissions.

In a more recent work, a mixture of ammonium and melamine phosphates and intumescents applied in a resin binder to cellulosic fibres imparts such a high flame-retardancy that is comparable with the performance of aramid fibres. These finishes are equally effective on wool and polyamide fibres. Presently a lot of research and development work is going on intumescent flame-retardant-complex finishes as substitutes to ecologically unacceptable formulations like Sb-Br compounds. It is expected that in the next few years there will be increased use of the intumescent-based compounds.

20.10 PROTECTION AGAINST MICROBIOLOGICAL DEGREDATIONSpores of fungi and bacteria exist everywhere and under hot and humid conditions these rapidly multiply feeding on their hosts. In case of textiles, these microorganisms attack the natural fibres as well as the finishes applied to these and cause mildew and rotting damages. The enzymes that degrade cotton are cellulase and cellobiase and these reduce strength, produce coloured and foul smelling spots and even holes on the exposed fabrics. It appears that accessibility of the hydroxyl groups of cellulose to the microorganism has something to do with the damage because the viscose rayon with greater amorphous content than cotton is more susceptible to damage while the acetate is immune to their attack.

Among all textiles, cotton goods are most exposed to the mildew damage because these being less expensive and more durable, are extensively used for the outdoor requirements like covers, tents, awnings, screens, sail cloth etc. Even otherwise, these are liable to damage during storage and transportation over high seas especially when starch is present in their

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finishes. A lot of work has, therefore, been done to develop effective yet economical antiseptics for cotton and now a variety of inorganic, organic and organo-metallic compounds are available for this purpose. Ideally an antiseptic should be colourless, odourless, nonvolatile, non-toxic to human beings, efficient in low concentrations and above all have a low cost. It is understandable that such a perfect product has not become available as yet but a few of the important antiseptics are discussed below in a more or less chronological order of their development.

20.10.1 Inorganic Compounds: It is well known that certain metal cations like copper, zinc and cadmium are toxic to microorganisms and fabrics are protected by a simple treatment of padding in solutions of salts of these metals followed by drying at a low temperature. However these salts are not wash-resistant and also reduce strength of the material during drying. The most successful compound in this category is cuprammonium hydroxide that is prepared by dissolving cupric hydroxide in concentrated ammonia liquor. The solution applied by the padding method dissolves the surface layers of cellulose fabric and the copper compound is deposited there. The material acquires a greenish colour but otherwise has a very good antimildew property. On squeezing the softened fabric between rollers, the fabric-interstices are closed forming a water-resistant film and so a combined water and mildew-proof finish is obtained. This treatment, known as the Willesden finish, had been extensively applied to heavy canvas fabrics meant as covers for military hardware but has now been replaced with other chemicals.

20.10.2 Organic Compounds: Simple organic compounds like phenol, formaldehyde and salicylic acid were known to have antiseptic properties since long and initial trials were made with these for the light-weight fabrics. However these are deficient in one respect or the other. Formaldehyde has an unacceptable pungent odour besides being toxic in high concentrations. Phenol is coloured and salicylic acid becomes ineffective on coming in contact with alkalis. However more complex derivatives of these compounds have been found effective even in low concentrations (0.02 to 0.05%) and have comparatively better wash resistance. More common among these are salicylanilide (Shirlan), pentachlorophenol and paranitrophenol, 2,2'-dihydroxy 5,5' dichlorodip-henylmethane (G4).

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Quaternary ammonium compounds having halogen or sulphur atoms in their molecules posses good antiseptic properties and are popular for sanitization of diapers to prevent baby rash. In a process similar to acetylation of surface layers of fabrics, protection from mildew can also be obtained by cyanoethylation reaction that unlike acetylation is uniform and does not lower strength of the material. The reaction is carried out on cotton fabric treated with caustic soda solution of mercerizing strength as shown below.

C6H9O4OHa + CH2=CHCN C6H9O4OCH2CH2CN

It was learnt that a fixation of at least 3.5% of nitrogen gives the required resistance to the microbiological attack. Understanding the role of nitrogen led to use of simpler and consequently cheaper reagents like trimethylol melamine by the pad-cure method.

20.10.3 Organometallic compounds: A large number of organometallic compounds are mentioned in literature but perhaps the more successful ones are copper nepthenate, copper soaps (copper oleate and stearate), cadmium soaps, copper 8-quinolinolate (copper ions chelated with 2 molecules of hydroxyquinoline) and mercury salicylate. A two-bath process using 1% soap solution and copper or cadmium salt solution produces the metal soaps. Copper napthenate is extensively used for heavy canvas fabrics along with wax for a combined water and mildew proofing. Reacting copper salts with napthenic acid, a cycloparaffin derivative obtained from petroleum, produces copper napthenate. Use of tributyl tin, another organometallic compound to be used in concentration of about 0.02%, is also mentioned in literature as a bactericidal for the baby diapers.

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20.11 FRESHNESS TREATMENTSM/s Huntsman have introduced a new and interesting concept in textile finishing in which they claim to covalently link ‘open’ micro-capsules to the textile fibres. These capsules, termed as ‘empty pockets’ by the manufacturers, can be filled with various freshening products like deodorants, perfumes, antimicrobial chemicals etc that may withstand a number of launderings. Chemically the open-capsules are prepared from a cyclo-dextrin that is biodegradable and so is eco-friendly. These can be attached to fabrics both by a batch or a continuous process. The product is of considerable interest and may find application in certain fashion garment trades.

ADDITIONAL READING MATERIAL

1. Textile Finishing, Ed. Heywood, D, Society of Dyers and Colourists, Bradford, 2003.2. Marsh, J.T., Introduction to Textile Finishing. 1966, Chapman and Hall, London.3. Whewell, C.S., The Finishing of Textile Fabrics, Textile Progress, 2, No.3. !971-72.4. Idem., Advances in the Finishing of Textiles, Review of Tex. Progress, 14, 1984.5. Colorants and Auxiliaries, Volume 2, Ed., Shore, J., Society of Dyers and Colourists,

Bradford, 2002.6. BASF, Textile Finishing Manual. 1973.7. Smith, The Chemical Finishing of Textiles, Rev. Prog. Coloration, 6, 1975.8. Shenai, V.A. and Saraf, N.M., Technology of Finishing, 1987. Sevak Publications,

Bombay, (India).9. Harrison (Ed.), Textile Finishing, 1978. The Textile Institute, Manchester.10. Hall, A.J., Textile Finishing, 1966. Tingling & Co, London. 11. Textile Series Reports. U.S. Quarter Master Corps.12. Technical Literature of Textile Auxiliaries Manufacturers.13. Little, R. W., (Ed.) Flame Proofing Textile Fabrics. Reinhold, New York, 1947.

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