21 Finishing agents – flame retarding agents, antimicrobial UV protective, antistatic, ant pilling agents

V. RameshBabu

Flame retarding agents

The field of flame retardancy has witnessed a vigorous development of new technologies and new products and materials to meet the challenge of the needs of new industries-such as computer, electronics and telecommunication industries. Flame retardants are also used in health care settings, Intravenous pumps, hospital beds Hospital curtains. An additional challenge is the growing awareness of environmental issues and the stiffening demands of consumer safety, which has been put forward by governments and public agencies. New flame-retardant systems are needed to meet the new product and market demands.

Fire safety is a significant cause of property damage and of death. Standards are therefore set for electrical appliances, textiles upholstery and many other materials to minimize these losses. To meet fire safety standards, products made of synthetic materials are modified with flame retardant chemicals that inhibit the ignition and spread of flames. Recently, there has been a great deal of interest in providing effective flame retardants for normally flammable substrates. For example, there is great interest in the development of flame retardant finish on synthetic fibers like polyester, nylon, polypropylene etc, without disturbing the desirable physical characteristics of the fibers. Textiles consist of highly ignitable materials and are the primary source of ignition. They contribute to rapid fire spread; however, reduction of ignitability can be obtained by

1:  Use of Inorganic materials {Asbestos, Glass etc}

2:  Through chemical treatment with FR {Flame Retardant chemicals}

3:  Through modification of the polymer.

Function of a Flame Retardant:

Flame retardants are chemicals are applied to fabrics to inhibit or suppress the combustion process. They interfere with combustion at various stages of the process e.g. during heating, decomposition, ignition of flame spread. Fire is gas phase reaction. For a substance to burn, it must become a gas. As with any solid, a textile fabric exposed to a heat source experiences atemperature rise. If the temperature of the source (either radiative or gas flame) is high enough and the net rate of heat transfer to the fabric is great, pyrolytic decomposition of the fibersubstrate will occur. The products of this decomposition include combustible gases, non-combustible gases and carbonaceous char. The combustible gases mix with the ambient air and its oxygen. The mixture ignites, yielding a flame, when its composition and temperature are favourable. Part of the heat generated within the flame is transferred to the fabric to sustain the burning process and part is lost to the surroundings.

Mechanism of Flame Retardancy

• Flame retardant systems for synthetic or natural polymers can act physically and/or chemically by interfering at particular stages of burning
• By cooling Endothermic processes triggered by the flame retardants cool the substrate.
• By forming a protective layer: The heat transfer is impeded, fewer pyrolysis gases are evolved, and the oxygen is excluded.
• By dilution.: Substances, which evolve inert gases on decomposition, dilute the fuel in the solid and gaseous phases. The concentrations of combustible gases fall under the ignition limit.
• Reaction in the gas phase: The free radical mechanism of combustion processes which takes place in the gas phase could be interrupted by flame retardants.
• Reaction in the solid phase: One mechanism is the accelerated breakdown of polymers.

Types of Flame Retardants:

• Brominated flame retardants
• Chlorinated flame retardants
• Phosphorous-containing flame retardants {Phosphate ester such as Tri phenyl phosphate
• Nitrogen-containing flame retardants (i.e. Melamines)
• Inorganic flame retardants.

These can be further classified as: Inorganic, Organo Phosphorous, Halogenated organic and Nitrogen based compounds.

2:   Halogenated organic flame retardants are further classified as containing either Chlorine or Bromine {Brominates Flame Retardants BFR}

There are three types BFRs currently produced. These are Poly Brominated DiPhenyl Ethers {PBDE}, Tetra BromoBisphenol A {TBBPA} and HexaBromoCyclodecane {HBCD} The PBDEs that are commonly used in products are Deca, Octa, and Penta BDE .The concentration of BFRs in products ranges from 5 to 30 % .Compounds containing Iodine are known, but of limited utility as flame retardants, due to their poor thermal stability and dark colour of iodine. Compounds containing Fluorine generally exist as functional polymers rather than materials to be added to other polymeric systems to provide flame retardancy. These polymers are oxidatively stable and only decompose at very high temperature.

Antimony oxide is another important component flame retardant composition, containing halogen, particularly Chlorine and Bromine. It is totally ineffective if used with out halogen. The Tri oxide is the common material used although the Pentoxide can also use. The pentoxide has a much finer particle size and is more effective per unit weight added than the trioxide. Polyesters are very sensitive to residual acidity in all forms of antimony oxide. Alkaline salts of antimony oxides are used in these critical cases. Antimony oxide acts as synergists with chlorine and bromine.

Antimony tri bromide is a dense white product and is one of the main components of the typical white smoke that is seen from burning polymers containing halogen and antimony oxide. High levels of water from normal combustion cause reversion of SbBr3 to HBR and Sb203.The remaining antimony oxide is then available to react with fresh HBR from decomposing brominated compound. Typically compounds used in flame retardant application contain either 40 to 70 % Chlorine or 45 to 80% Bromine, depending on the flame retardant requirements from 20 to 40 parts of Brominated compound would be used per 100 parts of polymer. Antimony oxide used is typically 1/4th to that of the halogenated material.

Many of the flame retardants do not remain on the fabric, instead they slowly leak from the products in the atmosphere. Brominated flame retardants are a subject of scrutiny. Evidence shows that they are likely to persist in the environment, bio accumulate in the food chain andfinally in to our bodies. A survey of the newer flame retardants suggests a simple theory for their constitution. The molecule should be water-insoluble to achieve durability in laundering. A solvent-soluble organic molecule will give better results. The ortho-phosphate group should be present in the molecule to dehydrate catalytically the cellulose substrate. The molecule should contain polymerizable groups to effect a permanency of finish. The molecule should contain halogen or other groupings to reduce the flammability of the gases of decomposition.

When chemical free alternative materials or designs are not feasible, non halogenated flame retardants can be used to meet fire safety standards. Numerous alternatives are available. It is also confirmed that flame retardants based on Aluminum Trioxide, Ammonium Polyphosphates and Red phosphorous are less problematic in the environment.

Application on Textiles:

One of the most preferred processes of applying FR on cotton is the “Precondensate”/NH3 process. This is an application of one of several phosphoniums “precondensates,” after which the fabric is cured with ammonia, then oxidized with hydrogen peroxide Precondensate is the designation for a Tetrakis-hydroxymethylphosphonium salt pre-reacted with urea or another nitrogenous material. The amount of anhydrous sodium acetate is approximately 4% of the amount of precondensate used. Some precondensates are formulated along with the sodium acetate. Softeners are also added along with precondensates.

The pH of the pad bath should be approximately 5.0.The amount of flame retardant required depends primarily on fabric type, application conditions, and test criteria to be met. Screening experiments should be conducted to determine the minimum application level for a fabric. Application of FR to fabric can be accomplished with conventional padding, padding with multiple dips and nips, followed by 30 to 60 seconds dwell gives good results. A critical factor in the successful application of precondensate/NH3 flame retardant is control of fabric moisture before ammoniation. Generally, moisture levels between 10% and 20% give good results.

Antimicrobial Agents

The inherent properties of the textile fibres provide room for the growth of micro organisms. Besides, the structure of the substrates and the chemical processes may induce the growth of microbes. Humid and warm environment still aggravate the problem. Infestation by microbescause cross infection by pathogens and development odour where the fabric is worn next to skin. In addition, the staining and loss of the performance properties of textile substrates are the results of microbial attack. Basically, with a view to protect the wearer and the textile substrate itself antimicrobial finish is applied to textile materials.

Necessity of antimicrobial finishes

Antimicrobial treatment for textile materials is necessary to fulfill the following objectives:

• To avoid cross infection by pathogenic micro organisms. To control the infestation by microbes.
• To arrest metabolism in microbes in order to reduce the formation odour.
• To safeguard the textile products from staining, discolouration and quality deterioration.

Requirements for antimicrobial finish

Textile materials, in particular the garments are more susceptible to wear and tear. It is important to take into account the impact of stress strain, thermal and mechanical effects on the finished substrates. The following requirements need to be satisfied to obtain maximum benefits out of the finish:

Durability to washing, dry-cleaning and hot pressing. Selective activity to undesirable micro organisms.

Should not produce harmful effects to the manufacturer, user and the environment. Should comply with the statutory requirements of regulating agencies.

1. Compatibility with the chemical processes.
2. Easy method of application. No deterioration of fabric quality.
3. Resistant to body fluids; and resistant to disinfections/sterilisation.

Antimicrobial finishing methodologies

The antimicrobial agents can be applied to the textile substrates by exhaust, pad-dry-cure, coating, spray and foam techniques. The substances can also be applied by directly adding into the fibre spinning dope. It is claimed that the commercial agents can be applied online during the dyeing and finishing operations. Various methods for improving the durability of the finish include:

• Insolubilisation of the active substances in/on the fibre.
• Treating the fibre with resin, condensates or cross-linking agents.
• Micro encapsulation of the antimicrobial agents with the fibre matrix. Coating the fibre surface.
• Chemical modification of the fibre by covalent bond formation.
• Use of graft polymers, homo polymers and/or co-polymerisation on to the fibre.

Mechanism of antimicrobial activity

Negative effect on the vitality of the micro organisms is generally referred to as antimicrobial. The degree of activity is differentiated by the term cidal, which indicates significant destruction of microbes and the term, static represents inhibition of microbial growth without much destruction.

The activity, which affects the bacteria, is known as antibacterial and that of fungi is animistic. The antimicrobial substances function in different ways. In the conventional leaching type of finish, the species diffuse and poison the microbes to kill. This type of finish shows poor durability and may cause health problems. The non-leaching type or biostatic finish shows good durability and may not provoke any health problems. A large number of textiles with antimicrobial finish function by diffusion type.

The rate of diffusion has a direct effect on the effectiveness of the finish. For example, in the ion exchange process, the release of the active substances is at a slower rate compared to direct diffusion ad hence, has a weaker effect. Similarly, in the case of antimicrobial modifications where the active substances are not released from the fibre surface and so less effective. They are active only when they come in contact with micro organisms. These so-called new technologies have been developed by considering the medical, toxicological and ecological principles.

The antimicrobial textiles can be classified into two categories, namely, passive and active based on their activity against micro organisms. Passive materials do not contain any active substances but their surface structure (Lotus effect) produces negative effect on the living conditions of micro organisms (Anti-adhesive effect). Materials containing active antimicrobial substances act upon either in or on the cell.

Antimicrobial textiles

Actigard finishes from Clariant are used in carpets to combat action of bacteria, house dust mites and mould fungi. Avecia.sPurista-branded products treated with Reputex 20 which is based on poly (hexamethylene) biguanide hydrochloride (PHMB) claimed to posses a low mammalian toxicity and broad spectrum of antimicrobial activity. PHMB is particularly suitable for cotton and cellulosic textiles and can be applied to blends of cotton with polyester and nylon. In addition to the aforesaid antimicrobial agents, the fibres derived from synthetic with built-in antimicrobial properties are listed in Table.

Antistatic Finish

Antistatic finishes are used for the removal in synthetic fibres of the unwanted effects of electrostatic charge produced during production and wear of fabrics and knits. Electrostatic charge causes an undesirable adhesive power and a resultant shabbiness. It is applied by means of an anti-static chemical treatment, the effect of which may be temporary or permanent.

There are two types of Antistatic finish

1. Non-durable finishes
2. Durable finishes

Non-durable finishes

Non- durable antistatic agents are preferred for fiber and yarn processing finishes since ease of removal is important. Other important requirements of spin finish and fiber lubricants are heat resistance and oil solubility. This group of mostly hygroscopic materials includes surfactants, organic salts, glycols, polyethylene glycols, polyelectrolyte, quaternary ammonium salts with fatty alkyl chains, polyethylene oxide compounds and esters of salts of alkyl phosphonium acids. The general requirements for non durableantistats are:

• Low volatility
• Low flammability
• Non yellowing (heat stable)
• Non corrosive
• Low foaming
• Durable Antistats

Obtaining antistatic properties that are durable to repeated launderings from a single finish application is difficult to achieve.

The basic principle is to form a cross linked polymer network containing hydrophilic groups. Typically, polyamines are reacted with polyglycols to make such structures. These polymers can be formed prior to application to fabrics, or they can be formed in situ on the fiber surface after pad application.

Mechanism of Antistatic Finishes

The principle mechanisms of antistatic finishes are increasing the conductivity of fiber surface (equivalent to lowering the surface resistivity) and reducing frictional forces through lubrication. The surface resistivity is defined as a ‘material property of a substance whose numerical value is equal to the ratio of the voltage gradient to the current density. The resistivity is in effect the resistance of the fiber to electrical flow. Increasing conductivity produces a lower charge buildup and a more rapid dissipation while increased lubricity decreases the initial charge buildup.

Antistatic agents that increase fiber surface conductivity form an intermediate layer on the surface. This layer is typically hygroscopic. The increased moisture content leads to higher conductivity. The presence of mobile ions on the surface is very important for increased conductivity. The effectiveness of hygroscopic antistatic finishes depends greatly on the humidity of the surrounding air during actual use; lower humidity leads to lower conductivity (higher resistance) and greater problems with static electricity.

Most non-polymeric antistatic finishes are also surfactants that can orient themselves in specific ways at fiber surfaces. The hydrophobic structure parts of the molecule acts as lubricants to reduce charge buildup. This is particularly true with cationic antistatic surfactants that align with the hydrophobic group away from the fiber surface, similar to cationic softeners. The main antistatic effect from anionic and non ionic surfactants is increased conductivity from mobile ions and the hydration layer that surrounds the hydrophilic portion of the molecule since the surface orientation for these materials places the hydrated layer at the air interface.

Application of Antistatic Finishes

Although antistatic finishes applied after dyeing or printing is more common with hydrophobic fibres, fabrics made from cotton, rayon and wool may also be antistatic treated depending on the intended use. The textile products that are treated with antistatic finishes includes –

a) Carpets for computer room.

b)Upholstery fabrics and airbags for automobiles.

c)Conveyor belts.

d)Filtration fabrics.

e)Airmail bags, parachutes.

f)Fabrics for hospital operating rooms and

g) Protective clothing for work with flammable gases, liquids and powdered solids.

Anti-Pilling Agents

PILLING is one of the. major quality problems in spun yarn fabrics particularly made from synthetic fibres and their blends. Pilling is a fabric defect which appears as small balls attached to the fabric surface by fibres. These balls are quite different from the balls observed frequently with the textured filament yarnfabric. The pills are formed only during wear andwashing due to abrasion affecting appearance, touch and handle of the fabrics. Disposal of the garments long before they reach their normal wear life results in customer dissatisfaction.

Chemical finishes:

Different chemical finishing approaches have been made to prevent pill from accumulating on fabric surface which include the following:

Application of polymers by padding and coating techniques.

Reduction in the strength of fibre to reduce pilling to cause the pills to fall off from the material as soon as they get formed and

Application of enzymes (bio finish) to 100% cotton textiles to cause removal of loose fibres in the yarn to reduce pilling tendency.

The polymeric formulations bring about the binding of the loose fibres into the fabric. The products normally used in this type of finish are friction reducing lubricants to minimize damage due to abrasion. The acrylic co-polymers which can be modified to suit the requirements are normally used. In the second approach, polymer structures of synthetic fibres are altered to give fibres of lower tenacity. The fabric from the yarns spun from fibres of lower strength show significant improvement in pill rating as compared to the fabric made from the normal polyester fibre. In the third approach, bio polishing with cellulase enzyme is carried on 100% cellulosic or cellulose rich blend fabrics to eliminate loose fibres which results inclean and smooth surface with improved pill rating.

Ultra Violet Protective Agents

Longterm exposure to UV light can result inAcceleration of skin ageing, Photodermatosis (acne), Phototoxic reactions to drugs, Erythema (skin reddening), sunburn, Increased risk of melanoma (skin cancer), Eye damage (opacification of the cornea) DNA damage.

UV protection ability of textile materials are influenced by the type of dye or pigment, the absorptive groups present in the dyestuff, depth after dyeing, the uniformity and additives present in the finishes. A protective effect can be obtained by dyeing, printing or finishing, which is better than using heavyweight fabrics which are not suitable for summer conditions.

Colour and Dyes: Generally Dark colours (black, navy, dark red) of the same fabric type provide better UV protection than the light pastel colours (white, sky blue, light green) for identical weave due to increased UV absorption. For instance, the UPF of a green colour T-shirt is 10 versus 7 for white.

However, particular dyes can vary considerably in the degree of UV protectiveness because of individual transmission and absorption characteristics. Some direct, reactive and vat dyes are capable of giving a UPF of 50+. Some of the direct dyes substantially increase the UPF of bleached cloth, which depends on the relative transmittance of the dyes in the UV-B region. In many cases, a UPF calculated using a direct dye solution appears to be higher than that of the fabric after dyeing, mainly because the actual concentrations are mostly less than the theoretical concentration. Dyes extracted from various natural resources also show the UPF within the range of 15 – 45 depending on the mordant used.

UV Stabilisers: Several types of UV stabilisers are available, the most common being benzophenones and phenyl benzotriazoles. These molecules are able to absorb the damaging UV rays of sunlight.Classification of UV stabilisers: Ultraviolet stabilisers can be classified into three different categories depending on their mode of action.

UV Absorbers: The amount of UV radiations absorbed by a polymer upon natural weathering can be reduced substantially by using additives, which compete with the photosensitive chromophores of the polymer substrate for the absorption of the incident photons. The UV absorbers provide long-term stabilisation against the UV radiations without getting destroyed.

UV absorbers are organic or inorganic colourless compounds with very strong absorption in the UV range of 290 – 360nm. UV absorbers incorporated in to the fibres convert electronic excitation energy in to thermal energy. They function as radical scavengers and oxygen scavengers. The high-energy short wave UVR excites the UV absorber to a high energy absorbed may then be dissipated as longer wave radiation. Alternatively, isomerisation can occur and the UV absorber may then fragment in to non- absorbing isomers.

Organic UV absorbers are derivatives of o-hydroxyl benzophenones, o-hydroxyphenyltriazes, and o-hydroxy phenyl hydrazines. Titanium dioxide and other ceramic materials have an absorption capacity in the UV region of 280- 400nm and reflects visible and IR rays. UV absorbers for synthetic fibres are Phenyl salicylates, benzophenones, benzotriazoles, cyanoacrylates, phenyltriazines. UV absorbers for natural fibres are benzotriazole derivatives, Oxalic acid dianilide derivatives.

UV absorbers incorporated in dyeing decreases the dye uptake, except in post treatment application. They are compatible with dyes and are applied by normal padding, exhaust, pad thermosol, pad dry cure methods. UV absorbers are applied between 30-40g/l depending on the type of fibre and its construction. The main limitations of UV absorbers are that they cannot be applied in a single bath along with other finishing agents; anything in excess will have a detrimental effect on the fabric.

With the advent of nano science and technology, a new area has developed in the area of textile finishing called Nano finishing. Coating the surface of textiles and clothing with nano particles is an approach to the production of highly active surfaces to have UV blocking properties. Metal oxides like ZnO as UV blocker are more stable as compared to organic UV blocking agents. Zinc oxide (ZnO) nano particles embedded in polymer matrices like soluble starch will enhance the UV blocking property due to their increase surface area and intense absorption in the UV region.

Quenchers: The excited chromophores (C*) responsible for photo oxidation can transfer their energy to an adequate accepter or quencher (Q), before chemical bonds are broken and radical initiated reaction proceed. For effective stabilisation, it is important that further deactivation of the excited quencher to the ground state molecule occur without the production of any reactive species. The commonly used light stabilisers of this kind are organic complexes of transition metals like Ni, Fe, Cr etc. The inherent colour of these metal complexes is a distinct problem, which limits their use as stabilisers for white and clear polymer materials.

Hindered Amine Light Stabilisers (HALs): Derivatives of 2, 2, 6, 6-tetramethylpiperidine are called HALs. HALs protects polymer chemically not physically and their effectiveness depends on the optimum dispersal in the binding agents. HALs are extremely efficient at preventing light induced degradation of most polymers. They do not absorb UV radiation, but instead act to inhibit degradation of the polymer. The basic principle followed here is:

They form nitroxyl radicals in the reaction with hydroperoxides.

•  These nitroxyl radicals react with free radicals in the polymer backbone to form amine ethers.
•  The amine ethers terminate peroxy radicals to cause reversion to nitroxyl radicals.

REFERENCES and URLs

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2. Karmakar S.R., “Chemical Technology in the pretreatment processing of textiles”, Textile Science & Technology, Elsevier Publication, 1999.
3. Bhagwat R.S “Handbook of Textile Processing”, Colour Publication, Mumbai, 1999.
4. T.L.Vigo, “Textile Processing and Properties”, Elsevier, New York, 1994.