Flame Retardant Functional Textiles

EMPA20110483

5 Flame retardant functional textiles

S. GAAN , V. SALIMOVA , P. RUPPER , A. RITTER and H. SCHMID , Swiss Federal Laboratories for Materials Testing and Research (EMPA), Switzerland

Abstract: This chapter is a brief review of research and developments in the fi eld of fl ame retardant textiles. The review focuses mainly on the currently available fl ame retardant solutions for different kinds of textiles. It gives insights into the general mode of action of fl ame retardants, types of fl ame retardant fi bers, fl ame retardant additives for fi bers and surface treatments and standard test methods for fl ame retardant textiles. The existing commercial fl ame retardant treatments for textiles are over 50 years old and some of them have created environmental concerns. Over the past few decades there have been considerable developments in inherently fl ame retardant synthetic fi bers but effective and environmentally friendly fl ame retardant coatings or fi nishing solutions for textiles still elude us.

Key words : fl ame retardant, inherently fl ame retardant fi ber, textile, phosphorus, polymer.

5.1 Introduction Textiles are manufactured from fi ber forming natural polymers such as cellulose and protein and a wide variety of synthetic polymers such as poly- Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231 esters, polyolefi n, polylactide, polyamides, polyaramids, polyetherketones, polyacrylonitrile and cellulose acetate. All of these polymers are suitable for use in textiles because of excellent fi ber-forming properties, but they share a common problem, namely that most of them are combustible under normal environmental conditions and pose serious fi re hazards in case of fi re accidents. These organic polymers are a rich source of hydrocarbons and thus they are an excellent source of fuel during the burning process. The term ‘fl ame retardant textiles’ usually refers to textiles or textile-based materials that inhibit or resist the spread of fi re. The hazards and risks associated with textiles are well documented elsewhere (Horrocks, 2001 ) and will not be discussed in this review. Flame retardant textiles fi nd use in various areas such as (a) manufacturing of uniforms for fi re fi ghters (Prezant et al., 1999 ), military/police personnel (Adanur and Tewari, 1997 ; Duran

et al., 2007 ; Rodie, 2008 ) and industrial workers; (b) high-performance sports applications (Stegmaier et al ., 2005 ); (c) upholstery for home furnishing (Kamath et al ., 2009 ), offi ce/commercial infrastructure and transportation (Flambard et al ., 2005 ); and (d) sleepwear for children and elderly people (Horridge and Timmons, 1979 ; Horrocks et al ., 2004 ). Flame retardant (FR) textile materials help save lives, prevent injuries and property losses and protect the envir onment by helping to prevent fi res from starting and limiting fi re damage. Flame retardant textiles can be constructed from inherently fl ame retardant fi bers such as Nomex, Kevlar, wool, Trevira CS, modacrylics, melamine fi bers and polyvinyl chloride (Marsden, 1991 ; Weil and Levchik, 2008 ). Alternatively, fl ame retardant textiles can also be manufactured either by surface treatments with fl ame retardant compounds with various chemistries or by incorporating fi re retardant additives in polymer bulk before fi ber spinning.

5.2 Factors affecting flammability and thermal behavior of textile fibers and fabrics The fl ammability of a textile material is a very complex phenomenon and depends on many factors such as the polymer itself, construction of the textile (i.e. weave/knit of fabric, yarn construction), weight/unit volume, additives in the fi ber, the type of chemical treatments and the test conditions (Nair, 2000 ; Ozcan et al., 2003 ). All textiles can burn provided they are exposed to the right conditions of fl ame, heat, oxygen concentration and time of exposure to fl ame.

5.2.1 Fabric construction In fl ammability tests, the inherent resistance of polymers to burning was the major factor affecting fabric performance with weight, weave and thickness Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231 being secondary. Fabric structure, weight and thickness were more important when measuring thermal protective capability. A combination of fi ber non-fl ammability and optimum fabric structure should be used to achieve maximum non-fl ammability with the lightest weight for the specifi c fabric application (Barker and Brewster, 1982 ; Ross and Stanton, 1973 ). Availability of oxygen is a critical factor in determining the fl ammability of a fabric. The construction of a fabric can play an important role as it determines the amount of air present, the active surface area and the fl ow of air through the fabric (Hendrix et al ., 1972 ). Fabrics with open constructions may be more combustible and the fl ame propagation will also be faster. A fabric with raised surface (e.g. fl eece-style fabrics, fl annelettes and terry toweling) and containing fi ber protruding out to the surface needs special care during processing and use. The fl ammability hazard with raised fi ber

surface fabrics involves the phenomenon called ‘surface fl ash’ whereby a fl ame can travel rapidly over the fabric surface, singeing the fi ber ends. This fl ash, in itself, may not be dangerous unless the intensity of the fl ame is suffi - cient to ignite the base fabric. In testing, this is known as timed surface fl ash with base burn. The twist of a yarn may also affect the porosity of the fabric. A tightly woven fabric with higher density, constructed from yarns with high twist will provide better fl ame protection. Fabric blends can also play a crucial role in defi ning the fl ammability of fabrics. Blends of fabric made from two different fi brous polymers may show fl ammability characteristics quite different from what is exhibited by each component of blend indepen- dently. For example, fabrics made from polyester are less fl ammable than cotton fabrics, but polyester/cotton blended fabrics burn more rapidly. This is because of the ‘scaffolding’ effect, where the charred cotton in the blend acts as a support for the polyester fi bers. The melting polyester in the blend does not drip away as it may do in 100% polyester fabrics, and continues to burn (wicking effect). The fl ammability and thermal shrinkage are relatively independent of the fabric weight and weave design and primarily dependent on the thermal stability of component fi bers (Minister of Health, 2009 ; Stepniczka and Dipietro, 1971 ).

5.2.2 Type of textile fi ber The fi bers used in textiles may be broadly classifi ed based on natural polymers or synthetic polymers. Cotton, wool, silk and fl ax are some of the major natural textile fi bers while viscose, Tencel™ and cellulose acetate are derived from natural polymers. The synthetic textile fi bers owe their origin to several polymer classes such as polyester, polyamide, polyacrylonitrile, polyolefi ns, polylactide, polyaramid, melamine based, polyurethane, etc. The fl ammability properties of the textile fi bers differ and can be grouped based on their burning characteristics into the following (Minister of Health, Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231 2009 ):

• Readily fl ammable: these fi bers ignite readily and burn rapidly, leaving a light ash residue (e.g. cotton acetate, triacetate, rayon and ramie). • Moderately fl ammable: these fi bers are more diffi cult to ignite. The syn- thetics tend to melt and drip, sometimes self-extinguishing upon removal of the ignition source (e.g. acrylic, nylon, polyester, olefi n and silk). • Relatively non-fl ammable: in general, these fi bers will not support combustion after removal of the ignition source (e.g. wool, modacrylic, vinyon and aramids).

Burning behavior of ﬁ bers is related to the thermal transition temperatures

and thermodynamic parameters such as glass transition temperature ( T g )

and melting point ( T m ). Chemically related transitions such as temperature

for onset of pyrolysis ( T p) and combustion ( T c) are also very important.

The lower the T c values and the higher the fl ame temperature, the higher is the fl ammability of a fi ber. The limiting oxygen index (LOI) of fi bers is also an important property which determines the fl ammability of the fi bers. Fibers with LOI values greater that 21% burn slowly and LOI values of 26–28% are suffi cient to pass small burning tests. The thermal and fl ammability properties of fi bers are discussed in detail elsewhere (Horrocks, 2001 ; Stegmaier et al ., 2005 ). All cellulose-based fi bers have very low LOI values (<19%) and are thus easy to burn. Among the natural fi bers wool has relatively high LOI value (approx. 25%) and is a common component in manufacturing fl ame retardant clothing for fi refi ghters in some countries such as Australia. Another criterion which determines the fl ammability of fi bers is the heat release rates of fi bers (i.e. the speed at which the heat is released during thermal decomposition) (Horrocks, 2001 ; Yang et al ., 2009 ). Peak heat release rates of some common fi bers like cotton, rayon, cellulose acetate, silk, nylon, polyester, polypropylene, acrylic fi bers, Nomex and Kevlar have been recently measured using micro-scale combustion calorimetry. Cellulose-based fi bers, polypropylene, nylon and polyester have relatively high peak heat release rates (>200 W/g) whereas m-aramid (Nomex), a commonly used fi ber in manufacturing fl ame retardant fabrics, has very low values (~80 W/g) (Yang et al ., 2009 ).

5.2.3 Thermal behavior of polymers The polymer composition of the textile materials is the most important factor determining their ﬂ ammability. Understanding the thermal behavior of polymers alone and in the presence of additives is useful in design- ing an effective ﬂ ame retardant system. Subjecting polymers to elevated temperatures may lead to several physical and chemical changes which Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231 are related to the physical and chemical structure of the polymer. Physical

changes like Tg and Tm are quite important in case of thermoplastics and may also inﬂ uence the ﬂ ammability of the polymers. Details of physical changes for polymers commonly used in textiles are discussed elsewhere (Hischler, 2000 ; Horrocks, 2001 ). Thermoplastic polymers like polyesters

have T m values much lower than the decomposition temperature (T d ) values, and thus tend to become fl uids before they start to decompose. This physical property of the polymer greatly infl uences its fi re performance. Under certain fi re tests where the fl ame source is static, such as LOI and vertical fl ammability tests, once the polymer is subjected to fl ame the polymer melts away from the fl ame and stops burning. Thus one has to be careful in determining the real fi re performance of polyester-based textiles.

Thermal decomposition of polymers leads to extensive changes in the chemical structure of the polymer, whereas the thermal degradation of a polymer may lead to the loss of physical, electrical or mechanical properties of the polymer. When a polymer is subjected to a temperature greater

or equal to T p or T c, the polymer chains start to break up into smaller frag- ments, some of which could be volatile fl ammable products. These fl ammable products break down further to fi nally release hydroxyl (OH ˙) and hydrogen (H ˙ ) radicals, which will combine with oxygen to release heat and light. The resultant heat could be released to the environment or could feed back into the burning cycle to cause further decomposition of the polymer. The burning process is a feedback process and will continue until the whole polymer has been consumed. The presence of fl ame retardant additives or absence of oxygen could interrupt this feedback process. The thermal decomposition of polymer is a very complex phenomenon and could lead to formation of several products such as fl ammable volatiles, non-fl ammable volatiles and solid residues. The fl ammable volatiles could be low molecular weight alkanes, alkenes, alkynes, aldehydes, ketones, ethers, etc. Water vapor and carbon dioxide are commonly formed non- combustible gases. The solid residue could be carbonaceous or inorganic in nature or even a combination of both (Horrocks, 2001 ). Some polymers like cellulose (wood), protein (wool) or aramids (Kevlar) tend to form carbonaceous chars upon heating. Some, like olefi ns, polyester and polyamides, do not leave any residues. Presence of additives may infl uence the amount of residue formed during the thermal decomposition process. Some phosphorus-based compounds would react with the substrate to accelerate formation of char. Presence of inorganic additives like silica derivates can form protective glassy layers which could insulate the inner layers from further decomposition. Thermal decomposition of polymers can happen via one or more of the following routes (Hischler, 2000 ): Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231

(a) random-chain scission where chain breakage happens along the chain length at random places; (b) end-scission: the scission of polymer happens at the chain end mainly in the form of monomers; (c) chain stripping: the main polymer chain remains more or less intact with removal of atoms or groups linked to the backbone; (d) cross-linking: new bonds are formed between functional groups of adja- cent chains.

Most of the vinyl polymers break down via routes (a) and (b) whereas route (c) is more common for halogenated oleﬁ ns and acrylonitriles.

5.3 Types, chemistry and mode of action of flame retardant additives Flame retardant additives used in textiles can be classifi ed according to their elemental composition or their mode of action. Flame retardant additives are typically made up from compounds containing elements such as (i) phosphorus (in various organic and inorganic forms); (ii) halogens (brominated, chlorinated organics, fl uorides of zirconium); (iii) silicon (organic or inorganic forms); (iv) boron; (v) metals (hydroxides of Al, Mg, Ca, phosphinates of Al, Zn and Ca and Zinc borates, fl uorides of zirconium); (vi) nitrogen (either alone or together with phosphorus, inorganic or organic); (vii) antimony (oxides together with halogens). Flame retardants can also be grouped by their mode of action, i.e. gas- phase action and condensed phase action. (a) Gas-phase action is characterized by evolution of reactive species during the burning process. This action is mostly exhibited by halogen (Georlette et al., 2000 ) and phosphorus-based fl ame retardants (Brauman, 1977 ; Huang et al., 2008 ). The reactive species could be radicals of halogen (X˙ ) or phosphorus derivatives which act as free radical scavengers in the vapor phase. Halogen-based fl ame retardants may release halogen radical which may form hydrogen halide (HX) by abstracting hydrogen from the fl ame retardant (RX) or the polymer (PH). The hydrogen halide may react further with the hydrogen and hydroxyl radicals (reactions 5.1 and 5.2, respectively) formed from decomposition of polymers and thus interfere with the burning cycle. → HX+ H˙ H 2 +X ˙ [5.1] → O H ˙ + HX H2 O + X˙ [5.2]

Flame retardant activity of halogen compounds can be further enhanced

Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231 (synergistic interaction) by addition of antimony compounds (Sb 2 O3 ). This may be because of the formation of volatile antimony halogen compounds

such as SbX 3 , which act as an effi cient inhibitor of the gas-phase oxidation in the fl ame (Camino et al ., 1991 ). Certain phosphorus species have also been found to have gas-phase action. Using direct mass spectrometric investigations of fl ames, research-

ers have identiﬁ ed phosphorus species like PO, HPO 2 , PO2 and P2 with PO being the major constituent. On the molecular basis, the phosphorus species act by third-body mechanism, catalyzing the recombination of hydrogen atoms according to equations [5.3] and [5.4] (Granzow, 1978 ).

P O ˙ + H˙ → HPO [5.3] → HPO +H ˙ H 2 +PO˙ [5.4]

More recently the gas-phase mechanisms of trimethyl phosphate and tri- phenyl phosphate derivatives have been studied using Fourier-transform infrared spectroscopy (FTIR). Acidic intermediates (phosphoric acid and its derivatives) formed during the thermal decomposition could also have gas-phase action (Clive and Ed, 2000 ). (b) Condensed phase action is characterized by the formation of a protective barrier between the pyrolyzing polymer and the heat source. This protective barrier/char is mostly formed by reaction of the fl ame retardant additive with the polymer during the thermal decomposition of the polymer. The reaction of fl ame retardant additive with the polymer is catalytic in nature and causes the decomposition of the polymer at lower temperatures. The action of fl ame retardant is such that it either changes the pyrolytic pathways of thermal decomposition or it could change the rate of formation of combustible materials. In case of hydroxyl-containing polymers such as cellulose, a large amount of water molecules are released by reaction of phosphorus acidic species with the cellulose. In some instances the phosphorus compounds decompose to form a polyphosphoric acid which forms a thermal insulation barrier on the surface of decomposing polymers (Granzow, 1978 ; Horrocks, 1983 , 2006). The condensed phase action of phosphorus fl ame retardants could be further enhanced (P-N Synergism) in presence of external nitrogen additives such as urea, guanidine and melamine. The enhanced action of phosphorus fl ame retardant was attributed to possible chemical reactions between decomposed products of nitrogen additive and phosphorus compound to form a protective coating on the surface of char (Gaan et al ., 2008 ). (c) Other actions like intumescent and heat sink effects are also mechanisms by which some fl ameretardant systems work. Intumescent systems are characterized by formation of a foam-like thermal insulation barrier when exposed to fl ame. A typical phosphorus–nitrogen-based intumescent system consists of a blowing agent (nitrogen-containing compounds Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231 like urea, guanidine and melamine), acid source (phosphorus-based fl ame retardants capable of forming phosphoric acid during thermal decomposition) and char former (generally polyhydroxyl compounds such as penta- erithritol are used). The intumescent effect is due to the combined effect of charring (reaction of phosphorus fl ame retardants with polyhydroxy compounds), and foaming (evolution of gases like ammonia from nitrogen source) (Camino and Lomakin, 2006 ). During the burning process the gases evolving from the thermal decomposition of blowing agent get trapped in the char, which is being formed simultaneously, thus forming foam of char on the surface of the material. A heat sink effect is commonly exhibited by additives such as metal hydroxides (aluminum hydroxide) which decompose to release water at elevated temperatures. The heat sink effect could be due to several factors such as endothermic decomposition, heat absorption by

water formed and dilution of gases formed due to pyrolysis of polymers (Weil and Levchik, 2009 ).

5.4 Flame retardation of textile materials Flame retardant textiles can be manufactured via the following routes:

(a) Using inherently fl ame retardant fi bers (i) Fibers from inherently fl ame retardant polymer. These kinds of fl ame retardant fi bers owe their fl ame retardant property to the polymer. Aramids, polyimide, melamine, glass, basalt, halogen- containing olefi ns, oxidized polyacrylonitrile and polyphenylene sulfi de-based fi bers are some examples of inherently fl ameretardant polymers. Figure 5.1 shows chemical structures of inherently fl ame retardant polymers. (ii) Fibers from polymers containing special fl ame retardant co- monomers. Modifi ed polyester such as Trevira CS and modacrylics are common examples of fi bers containing special monomers with fl ame retardant characteristics. (iii) Fibers containing special non-reactive fl ame retardant additives. Viscose fi bers such as Viscose FR® and Visil® are fl ame retardant cellulosic fi bers containing very high concentration of fl ame retardant additives (~30%). (b) Finishing or coating Application of fl ame retardants at fabric stage by a fi nishing process and subsequent fi xation by various techniques such as thermal and ammonia curing, UV and plasma polymerization, etc.

5.4.1 Fibers from inherently fl ame retardant polymers Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231 Aromatic polyamides or aramids , which were fi rst developed in 1960s, are one of the most commonly used polymers for developing fl ame retardant textiles now. These fi bers fi nd usage in manufacturing fi refi ghters’ uniforms, uniforms for industrial workers, military clothing and accessories. Dupont, USA (Nomex® and Kevlar®), Teijin, Japan (Twaron®, Technora®, Teijinconex®) and SRO Group, China (X-Fiper®), Yantai Spandex Co. Ltd, China (Newstar®), Kermel, France (Kermel®) are some major manufacturers of different kinds of aramid fi bers. Para-aramid fi bers have high tenacity and tensile modulus, heat resistance and dimensional stability and fi nd applications mostly in automotive brake pads and other friction materials, tire cords and protective clothing and are also used as alternatives to asbes- tos. Meta-aramid fi bers are characterized by their long-term heat and fl ame resistance, and are favored for such applications as heat-resistant fi lters and

S NAr O O olyphenylene sulfideolyphenylene Poly(p-phenylene)benzobisoxazole P 2 Copyrighted Material downloaded from Woodhead Publishing Online Publishing Woodhead from downloaded Material Copyrighted by http://www.woodheadpublishingonline.com Delivered (147-04-082) Gaan Sabyasachi 10:56:41 PM 27, 2011 September Tuesday, 77.56.188.231 IP Address: H N C H CNH O -Aramid ame retardant polymers. n para N H NH Polyetherimide Inherently ﬂ Ar =

N HN 5.1 NN C O O O N H N Melamine based polymer

other industrial materials and fi reproof clothing for fi refi ghters. Details of aramid fi ber production, application and properties can be found elsewhere (Rebouillat, 2001 ; Yang, 1993 ). Melamine fi bers are produced by Basofi l Fibers LLC, Enka, NC and mar- keted under the trade name Basofi l® Fiber (TNC Global Inc, 2010 ). Melamine fi bers are manufactured from fi ber-forming synthetic polymer composed of at least 50% by weight of a cross-linked melamine polymer. The cost of melamine fi ber is comparatively low for similar properties. These fi bers can operate at high temperatures; they have high LOI values and typically target hot gas fi ltration and safety and protective apparel markets. Melamine fi bers are weak and generally need blending with stronger fi bers such as aramids to improve their processing. The major application of melamine fi bers is in areas of the bedding industry, upholstery manufacturing of seats for automo- biles and aircrafts, protective clothing for fi refi ghters and industrial workers and in the fi ltration industry involving high temperature applications. While there are different kinds of commercial polyimide polymers, only some are used in manufacturing textile fi bers. P84 is a polyimide fi ber manufactured by Evonik fi bers GmbH, Austria (Weinrotter, 1988 ). The polymer structure of this fi ber is shown in Fig. 5.1 . P84 polyimide fi ber is derived from aromatic dianhydrides and aromatic diisocyanates and exhibits outstanding thermal stability. The polymer is non-melting and has a glass transition temperature of 315°C. This fi ber fi nds application in different areas such as high temperature fi ltration, protective clothing, braided packing and insu- lating material (Evonik Industries, 2010 ). Fibers based on polyetherimide resin (Ultem®) have been recently developed by General Electric (Dris et al ., 2008 ). GE’s Ultem® resin is an amorphous, high-performance polymer that exhibits outstanding high heat resistance, strength, modulus and broad chemical resistance. This fi ber has a high glass transition temperature of 217°C, a relative thermal index (RTI) of 170°C, is inherently fl ame resistant and has a very low smoke evolution. Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231 Polybenzimidazole (PBI) is an organic fi ber with excellent thermal resistant properties and a good hand. The Federal Trade Commission defi nition for PBI fi ber is ‘A manufactured fi ber in which the fi ber-forming substance is a long chain aromatic polymer having recurring imidazole groups as an integral part of the polymer chain’. PBI does not burn in air and does not melt or drip. PBI Performance Products Inc. is the world’s only producer of high-performance PBI fi ber (Performance Products Inc, 2010 ). The high LOI coupled with its good chemical resistance and good moisture regain makes PBI an excellent fi ber for fi re blocking end uses such as safety and protective clothing and fl ame retardant fabrics. Its physical properties are relatively inferior, but PBI can be processed on most types of textile equipment. It blends well with other materials such as carbon and aramid fi bers and is most often done for performance reasons as well as cost. PBI has

had signifi cant success in the fi refi ghters’s apparel market, where, blended in a 60/40 para-aramid/PBI mixture, it has become the standard ‘premium’ material. PBI’s characteristic gold color blends well with other materials for a pleasing appearance. Glass fi bers are made up on a basis of inorganic amorphous polymer and were commercialized in the late 1930s. They fi nd application in insulation (glass batts in home insulation and industrial insulation in mats and fabric form) and are widely used in reinforcing thermoplastic composites in products from circuit boards to boat hulls. They are also used in the manufacturing of barrier fabrics for furniture and military applications and high temperature fi ltration applications. Halogenated fi bers based on modacrylics and polyvinyl chlorides fi nd specialized uses in the development of home furnishings and industrial applications which need fi re protection and excellent chemical resistance. These fi bers owe their fl ame retardancy to the presence of chlorine and may liberate large amount of toxic gases during the burning process. Shrinkage problems in the presence of heat, corrosive monomers and environmental regulations make them less attractive nowadays. Polyphenylene sulfi de fi bers are known for their moderate temperature resistance and excellent chemical resistance. The polymer is produced by several manufacturers such as Chevron Phillips Chemical Company LP (Ryton®) (Chevron Phillips Chemical Company LLC, 2008 ), Toboyo (Procon®), Toray (Toray PPS®) (Toray Industries, 2006 ) and Celanese (Fortron®). The low moisture regain of PPS often prevents its use in protective apparel; the fi ber has an uncomfortable hand, but the good chemical resistance makes it very attractive for industrial applications, especially for fi ltration. Poly- phenylene benzobisoxazole is a new high-performance organic fi ber fi rst introduced by Toyobo in 1999 as Zylon® (Zylon-Department, 2010 ). It possesses outstanding thermal properties and has almost twice the tensile strength of conventional para-aramid fi bers. Its high modulus makes it an Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231 excellent candidate for composites reinforcement. PBO has a very high LOI value, more than twice the LOI of meta-aramid fi bers. Oxidized polyacrylonitrile fi bers are partially carbonized fi bers which can be further converted into carbon or graphite fi ber when subjected to further carbonization in an inert atmosphere at high temperature (Rahaman et al ., 2007 ). These fi bers do not burn, melt or drip; instead, they char and self-extinguish. Oxidized polyacrylonitrile fi ber is relatively weak and has limited abrasion resistance and is thus often blended with other high- performance fi bers such as aramid, polyvinyl halide, PBI, fl ame retardant rayon and polyester fi bers creating a strong durable product with excellent fl ame resistance (Smith, 2007 ). Zoltek corporation (PYRON®) manufac- tures oxidized polyacrylonitrile in different forms such as fi bers, fabrics and felts (Zoltek Corporation, 2010 ).

Inherently fl ame retardant polyesters are commercially available from various manufacturers such as Trevira GmbH (Trevira CS®), Invista (Avora®) and Toyobo (Heim®). These kinds of fi bers contain reactive organophosphorus additives based on phosphonic acid derivatives. Structures of various reactive co-monomers used in manufacturing commercial fl ame retardants are shown in Fig. 5.2. A phosphorus content of 0.7–1% is suffi cient to achieve satisfactory levels of fl ame retardancy. The mechanism of fl ame retardant action of these phosphorus compounds may be due to reduction in melt viscosity caused by acid hydrolysis. The phosphorus atom in these polyesters could be an integral part of the main backbone or it could be present as a pendant group to the main chain. The positioning of phosphorus atom in the polyester does not have any effect on the fl ammability of the fi ber, but it does affect the hydrolysis stability of the polymer. Studies have shown that the main-chain-type hydrolyzes about two times faster than the side-chain type (Maki et al., 2000 ). Further studies on reaction kinetics have shown that main-chain-type phosphorus-containing monomer like 3-(hydroxymethyl

O P OH HO P OH HO O

3-(Hydroxymethyl 3-(Hydroxyphenyl phosphinyl) propanoic acid (HMP) phosphinyl) propanoic acid (HPP)

Main chain type Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231

P O O O

9,10-Dihydro-9-oxa-10-(2,3- dicarbonylpropyl)- OH 10-phosphophenanthrene-10-oxide (DI) O Side chain type 5.2 Phosphorus-containing monomers for polyester.

phosphinyl) propanoic acid (HMP) showed higher chemical reactivity in esterifi cation reactions than the side-chain-type monomer, 9,10-dihydro-9- oxa-10-2,3-dicarbonylpropyl)-10-phosphophenanthrene-10-oxide (DI). In addition, for production of polyester having the same phosphorous content in commercial scale, it is more benefi cial to apply HPP than DI, because of low input of FR due to low molecular weight (Seung-Cheol and Jae Pil, 2007 ). Polyesters with HPP in the backbone had a higher amount of ethylene glycol in the main chain, thus resulting in lower thermal stabilities. Highly sterically hindered DI was more stable against thermal degradation and alkaline attack, and the DI polyester showed similar stability to that of the normal polyester. For higher phosphorous content polymer production, it is profi table to adopt main-chain-type phosphorous FRs rather than pendant types, because of their lower molecular weight and higher reactivity (Yang and Kim, 2007 ). The polymer chain of main-chain-type polyester fi ber is more fl exible than that of pendant type; it adsorbs the dyestuff at lower temperatures and reaches exhaustion more quickly (Seung-Cheol and Jae Pil, 2008 ). More recently Neopentyl glycol and 3-(hydroxyphenylphosphinyl) propionic acid have been used as the third and fourth co-monomers to syn- thesize phosphorus-containing poly[(ethylene terephthalate)- co -(neopentyl terephthalate)] (PENT) with both fl ame retardancy (LOI, 25–34%) and low melting temperature (170°C) properties (Xin-Ke et al ., 2009 ). Fully aromatic polyester fi ber (Fig. 5.3) developed by Celanese Acetate LLC and now manufactured by Kuraray Co., Ltd (Vectran®) (Kuraray America Inc, 2010 ) is one of the newest high-performance polymers used for manufacturing textile materials where high thermal and fl ame protection is important. Vectran HT® and Vectran UM® fi bers exhibit high LOI values of 28% and 30%, respectively. These fi bers are characterized by low smoke generation and don’t drip in vertical fl ammability tests (Beers and Ramirez, 1990 ). Important physical and fl ame retardant properties of the above-men- tioned high-performance synthetic fi bers are given in Table 5.1. A detailed Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231 review of these properties can be found elsewhere (Greer, 2000 –2002).

C O

C O O nm

Structure of Vectran fiber 5.3 Aromatic polyester.

Table 5.1 Properties of ﬁ bers made from inherently ﬂ ame retardant polymers

Limiting Operating Tenacity Elongation oxygen Chemical temperature Fiber (g/den) (%) index (LOI) resistance (°C)

Meta-aramid 3.8–7.2 25.0–40.0 30 Mild–good 200 Para-aramid 22.0–26.0 2.4–4.4 25–28 Mild–good 280 Polyphenylene 3.5–4.5 32.0–49.0 34 Very good 500 sulfi de Melamine 2.0 18.0 32 Mild–good 400 Polyphenylene 42.0 3.5 68 Mild–good 550–600 benzobisoxazole PBI 2.7 29.0 41 Good– 250 excellent Polyphenylene 3.5–7.0 35.0 34 Excellent 190 sulfi de Polyimide 4.2 30.0 38 Good 500 Oxidized PAN Good– fi bers 2.1 25.0 40–52 excellent 570–1000

PAN, polyacrylonitrile.

5.4.2 Flame retardant fi bers made from addition of non-reactive additive to polymer Melt additives Adding fl ame retardants during polymer processing is the most widespread and effi cient method of FR protection of polymeric materials, since this method does not require new equipment and is economically effi cient. However, application of this method is limited by the requirements for an additive: it should be thermally stable up to 300°C under mechanical force for considerable time, should dispense easily and have a suitable Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231 melting point and a high degree of dispersion, without degrading or react- ing detrimentally with the polymer (Lawton and Setzer, 1975 ). Halogen- containing organic compounds and organophosphorous compounds are the most commonly used fl ame retardant additives for poly(ethylene terephthalate) (PET) fi bers. Halogen compounds are inert to polyesters under thermal conditions. All phosphorous organic compounds can pro-

duce phosphorous-containing acids like H 3 PO4 , H 3 PO 3 , H3 PO 2, which can cause degradation of the polyester by hydrolysis, leading to reduction of mechanical stability. If phosphinates or other P-compounds are used alone or in combination with other ﬂ ame retardants in polyesters, there is generally some degree of polymer degradation, and this has an adverse effect on the mechanical properties of the polymer system. The melt viscosity of PET is lowered by addition of these compounds. Halogen compounds, typically

hexabromocyclododecane (HBCD), were mainly used as a fl ame retardant for polyester fi bers. However, the use of these compounds is being regulated, because they are persistent and bio-accumulative in nature. Use of many types of halogenated fl ame retardants, especially brominated compounds (2,2′,4,4′,5,5′-hexabromobiphenyl), are banned due to toxicity and environmental issues. Some cyclophosphazene derivatives have been developed in the past as melt additives for polyester fi bers. Cyclophosphazene phenyl and phenoxy derivatives were incorporated (5–10%) into polyester fi bers before spinning to impart fl ame retardancy without harming their dyeing properties. PET chips were melted at 280°C, combined with 60% hexaphe- nylcyclotriphosphazene and spun into yarn which was self-extinguishing in nature (Clutter, 1975 ). Salts of phosphinic acid have been developed as an additive for thermoplastic polyester (Schlosser et al ., 2008 ; Yao et al ., 2006 ). The FR polyester fi bers have been produced by the incorporation of FRs using blending or copolymerization (Levchik and Weil, 2004 , 2005 ). A dial- kylphosphinate salt has been recently introduced as a melt spinning additive in polyester (Weil and Levchik, 2008 ). The availability and commercial success of most of these phosphorus-based non-reactive melt additives are not known yet. Flame retardant polyester fi bers are manufactured mostly from reactive additives or monomers containing a phosphorus component. Unlike polyester fi bers there are no inherently fl ame retardant fi bers from polyamides that are commercially available. There are different kinds of polyamide in commercial use. They vary in their melting points: PA6 (220°C), PA6.6 (260°C), PA6.10 (240°C), PA6.12 (218°C), PA11 (198°C) and PA12 (178°C). There is a temperature range of about 60°C between the melting points which gives the possibility to use different thermo-stable fl ame retardant additive systems for melt-spun fi bers. Other polyamides like thermoplastic fi bers, even when fl ame retarded using either co-monomeric, modifi cations or additives introduced during polymerization and/or fi ber extrusion stages, melt drip and/or form holes when exposed to fl ame. They Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231 cannot, therefore, be used in applications such as protective clothing and barrier textiles, where sustained thermal protection via char formation is an essential requirement (Horrocks, 1996 ). Some additives like low molecular weight phosphorous compounds, chlorinated polyethylene, brominated pentaerythritol, boric acid and antimony oxide have been used in the spinning dope of polyamides. Calcium zinc molybdate (KemGard 425, Sherwin- Williams Company) and a mixture of brominated pentaerythritol, antimony oxide and boric acid were introduced to nylon 6 melt without change of physical mechanical properties. Melt extrusion of nylon 6 at 230°C with red phosphorous has been reviewed elsewhere (Subbulakshmi et al ., 2000 ). Compounds such as phosphorylated pentaerythritol, lead methylphospho- nate and a complex compound of an alkylphosphonic acid and antimony have been investigated as thermally stable fl ame retardant additives for

fi ber spinning (Butylkina et al ., 1989 ). Ammonium polyphosphate (APP) has been applied on different kinds of nylons and thermal decomposition and fl ame retardant behavior have been studied in detail. During the burning process APP acts as an intumescent agent and also forms a thermally stable polyphosphoric acid layer on the surface of decomposing polymer (Levchik et al., 1994 , 1995 ). Resorcinol bis(diphenyl phosphate) and bisphe- nol A bis(diphenyl phosphate) have been used as fi re-retardant additives on nylon 6 and nylon 6,6. It was shown that aromatic phosphates promoted charring of the nylons, but the char yield was insuffi cient to rapidly extinguish the fl ame (Levchik et al ., 2001 ). Recently phosphinic acid derivative additives have been developed as melt additive for application on nylon fi bers (Yao et al., 2006 ). In spite of much research and development in fl ame retardant melt additives, there exist no successful commercial fl ame retardant additives for nylon melt spinning. Polyolefi n fi bers are traditionally made fl ame retardant by adding brominated additives containing antimony or phosphorus (Zhang and Horrocks, 2003 ). These brominated fl ame retardants may adversely affect the light stability of olefi ns and also have shown to have adverse environmental effects. Tris (tribromoneopentyl) phosphate is a thermally stable, effective fl ame retardant additive for polypropylene fi bers. More recently halogen- and phosphorus-free fl ame retardant has been introduced by Ciba (Flamestab® NOR™ 116). It requires an addition of 1–1.5% to achieve some degree of fl ame retardancy (Weil and Levchik, 2009 ).

Additives for wet spun fi bers Flame retardant viscose fi bers such as manufactured by Lenzing, Austria (Viscose FR®) and Sateri International Group (Visil®) are the only inherently fl ame retardant fi bers developed from cellulose. Both these fi bers use different technology to achieve fl ame retardant property. Viscose FR® is Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231 made by incorporating ~30% organophosphorus-based pigment additive ® whereas Visil fi ber contains up to 35% SiO 2 as a fl ame retardant additive. The development of fl ame retardants for viscose is very challenging as the fl ame retardant has to withstand severe alkaline and acidic conditions during the fi ber manufacturing process. Studies on several kinds of phosphorus-based fl ame retardants have shown that 1,3,2-dioxaphosphorinane, 2,2′-oxybis 5,5-dimethyl-, 2,2′-disulfi de ( Fig. 5.4 ) is the most suitable fl ame retardant additive for viscose fi ber (Wolf, 1981 ). This compound is a white pigment, practically insoluble in water and very stable to acid and alkaline hydrolysis. It is currently manufactured by Clariant and sold under the trade name Exolit 5060. Visil fi bers are manufactured by adding sodium silicate in the alkaline spinning solution and regenerating it as polysilicic acid in the coagulating bath.

O SS O

PP O O O 1,3,2-Dioxaphosphorinane, 2,2Ј-oxybis 5,5-dimethyl-, 2,2Ј-disulfide 5.4 Phosphorous-containing ﬂ ame-retardant additive for viscose manufacturing.

5.4.3 Application of fl ame retardants on fabrics by fi nishing treatments Application on synthetic fi ber fabrics Synthetic fabrics made from nylon, polyester and acrylic fi bers are diffi cult to ignite but once ignited they exhibit severe melting and dripping. Nylon fabrics are commonly used in manufacturing of fabrics for military applications, upholsteries and carpet, because of their excellent mechanical property. Several fl ame retardant treatments based on brominated compounds, thiourea formaldehyde resins (Horrocks, 2003 ) and cyclic phosphonate esters have been discussed in the literature (Weil and Levchik, 2004 ). Back coating formulations containing polyvinyl chloride, chloroparafi n wax emulsions, brominated fl ame retardants and antimony oxides with polymeric binders were also suggested in the literature (Weil and Levchik, 2004 ). Treatments of nylon–cotton blended fabrics with hydroxyfunctional oligom- ers have also been reported. Durability of the treatments (up to ten laundry cycles) was ensured by the use of melamine- and urea-based cross-linkers. The treatments improved the char formation and reduced the decomposition temperature and heat release rates of the fabrics (Yang et al ., 2009 ). Treatments with urea formaldehyde resin and brominated compounds lead to very high LOI values, but the treated fabrics could not pass vertical fl ame Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231 tests due to enhanced dripping effect. Non-durable anti-dripping fl ame retardant treatments of nylon 6,6 fabrics have been obtained by the use of intumescent formulations containing pentaerythritol and melamine (Li et al ., 2009 ). Hydroxymethylation of nylon 6 fabrics with formaldehyde and subsequent treatments with N-methylol-3-(dimethoxy)phosphonopropio- amide resulted in increased LOI values (Kang De et al ., 1992 ). Polyester fabrics owing to unavailability of reactive functional groups are diffi cult to make fl ame retardant by conventional fi nishing treatments. The most common method for making fl ame retardant polyester is by apply- ing either phosphorus or halogen-based fl ame retardants by the thermosol method (Weil and Levchik, 2008 ). The fabrics are generally treated with aqueous solutions of fl ame retardants (2–15%), dried at around 100°C and cured at 185–195°C (knitted fabrics) or 190–205°C (woven fabrics)

for 120–160 s. Durable fl ame retardant treatments can be obtained by this method. The fl ame retardants are believed to diffuse into the soft polymer during the curing operation. Once the fabrics are cooled, the fl ame retardant gets trapped and excess fl ame retardant on the surface is washed off. A common phosphorus fl ame retardant composition for polyester fabric is a mixture of cyclic phosphonate compounds ( Fig. 5.5 ) and is sold as Kappafl am P 31 by Kapp-Chemie GmbH, Germany. Diphosphonate products are also available from Thor (Affl amit) or Zschimmer & Schwarz (Flammex). They are slightly acidic in nature, therefore an application solution needs buffer- ing of the pH to 6.5. Usually a fi nal fi xation of 1.5% of the compound is quite suffi cient to achieve satisfactory fl ame retardancy. Halogen-containing novel organophosphorus compound such as dichlorotribromophenyl phosphate (DCTBPP) has been synthesized and applied to polyester fabrics by thermosol method to obtain self-extinguishing fabrics with durability up to 50 laundry cycles (Yoo-Hun et al ., 2001 ). A novel anti-dripping fl ame retardant, poly(2-hydroxypropylene spirocyclic pentaerythritol bisphos- phonate) (PPPBP) was synthesized and applied by the thermosol method to impart fl ame retardancy and dripping resistance to PET fabrics (Chen et al., 2005 ). Polyester fabric has also been treated with inorganic salts like sodium polymetaphosphate (Mostashari, 2007 ) and ammonium chloride to achieve non-durable treatments which could pass vertical fl ammability tests (Mostashari and Mostashari, 2008 ). Polyacrylonitrile fabrics unlike modacrylics are rarely used for manufacturing fabrics which might be of use in areas that require fl ame protection. Nevertheless, one can fi nd several research articles related to fl ame proofi ng

O Et O O H C P P 3 O Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231 O O

Phosphonic acid, P-methyl-, (5-ethyl-2-methyl-2-oxido- 1,3,2-dioxaphosphorinan-5-yl) methyl ester

CH3

O OOP O O O P Et O Et P O O Phosphonic acid, P-methyl-, bis[(5-ethyl-2-methyl-2-oxido- 1,3,2-dioxaphosphorinan-5-yl)] methyl ester 5.5 Cyclic phosphonate compounds for thermosol application.

of polyacrylonitrile fabrics with various phosphorus- and halogen- containing compounds. Several compounds such as decabromodiphenyl oxide and ammonium bromide, urea and phytic acid, hydrazine hydrate, hydroxylamine sulfate, and metal salts have been used as fl ame retardants (Bajaj et al ., 2000 ). More recent investigations have shown that polyacrylonitrile fabrics can be made fl ame retardant by grafting of acrylic-based organophosphorus compounds using environmentally friendly plasma technologies. In this research argon plasma-induced graft polymerization of four acrylate monomers containing phosphorus, diethyl(acryloyloxyethyl)phosphate (DEAEP), diethyl- 2-(methacryloyloxyethyl)phosphate (DEMEP), diethyl(acryloyloxy methyl) phosphonate (DEAMP) and dimethyl(acryloyloxymethyl)phosphonate (DMAMP) was performed. The treatments resulted in moderate fl ame retardant properties for the fabrics with limited durability to several laundry cycles (Tsafack and Levalois-Grützmacher, 2006 ).

Application on natural fi ber fabrics Attempts to impart fl ame retardant properties to cellulose- and protein- based natural fi bers have been made since the early nineteenth century. Since then considerable efforts have been made to develop newer fl ame retardant solutions for these natural fi bers. Production of fl ame retardant cellulose-based textiles has been the most challenging task. Cellulose is a natural polymer and exists in different fi brous forms such as cotton, jute and linen. The only way to produce fi re-resistant cellulosic textiles from these fi bers is via a chemical fi nishing treatment. The intrinsic properties of textiles like the tensile strength, tactile properties, air permeability and dyeabil- ity should not be affected by the treatment. Moreover, the fl ame retardant should meet health requirements and not produce toxic gases when burned. It is also important that the fl ame retardant treatment should last at least 50 industrial washing cycles. These requirements make the development of

Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231 fl ame retardants for cellulosic fi bers very challenging. Textiles made from natural fi bers like cotton and silk are highly fl ammable whereas wool-based textiles are self-extinguishing in nature. The LOI values of untreated cotton, silk and wool are 18.4, 22.8 and 25, respectively (Horrocks, 2001 ). Finishing treatments for cellulose-based textiles can be grouped into the following: (a) Finishes with inorganic phosphates Examples of inorganic salts that are used as fl ame retardants for cellulose are ammonium phosphates, polyphosphates (Fig. 5.6), bromides and borate- boric acid mixtures. Typically, 1–2% of the phosphorus content relative to the weight of the fabric is enough to obtain fl ame retardant cotton. Ammonium bromide can also be used in combination with ammonium phosphates to provide

O O

H4NO P OH P O

ONH4 ONH4 n

Diammonium Ammonium phosphate polyphosphate 5.6 Ammonium salts of phosphoric and polyphosphoric acid.

NH O NH O NH O

H H H H H2N C N P N C NH2 HO PCPN N OH

OH OH OH Diguanidine hydrogen phosphate Monoguanidine dihydrogen diphosphate 5.7 Guanidine phosphate salts.

some vapor-phase fl ame retardant action, particularly helpful for blends (Chellapa and Shah, 1989 ; Davies et al ., 2002 ). Ammonium sulfamate or sulfate is also included in some ammonium phosphate formulations (Weil and Levchik, 2009 ). The use of APP is reviewed in the literature (Davies et al., 2005 ; Horrocks, 2003 ). Attempts have also been made to achieve durable coatings on cellulose by using these organic salts (Hawkes et al ., 2009 ; Webb et al ., 2007 ). Monoguanidine dihydrogen phosphates ( Fig. 5.7 ) and diguanidine hydrogen phosphate are further examples of phosphates used for non-durable cotton treatment (Vroman et al ., 2004 ). Some fl ame retardants like Flovan® CGN (Ciba), Flammentin® FMB (Thor Specialities) and Pyrovatim® PBS (Weil and Levchik, 2009 ) have been used to produce Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231 semi-durable coatings. Treatments with these fl ame retardant agents will survive water soaking or leaching to various degrees, but have poor stability to laundering. Semi-durable fi nishes with these inorganic phosphates are not durable to alkaline laundering conditions. When phosphoric acid groups are chemically attached and neutralized by ammonium cations, the fi nish can become fairly durable to water washing.

(b) Finishes with organophosphorus compounds The two most successful products with more than 60 years’ history for ﬂ ame retardation of cellulose are Pyrovatex® CP (Ciba) and Proban® CC (Rhodia). The basic molecule in Pyrovatex formulation is 3-(dimethylphosphono)- N-methylolpropionamide (DMPMP) ( Fig. 5.8 ), and in Proban – tetrakis (hydroxymethyl) phosphonium chloride (THPC) (Fig. 5.8). Durable

O HO OH O – PCl+ H3COPC (CH2)2

HN CH2OH HO OH OCH3 3-(Dimethylphosphono)-N-methylolpropionamide Tetrakis (hydroxymethyl) phosphonium chloride 5.8 Organophosphorus ﬂ ame retardants for durable treatment on cellulose.

treatments of DMPMP on cellulose-based fabrics are generally obtained by the use of a formaldehyde-based cross-linker such as melamine formaldehyde. Presence of formaldehyde in DMPMP and the cross-linker creates environmental problems during the application process due to the release of formaldehyde fumes. Formaldehyde may also be released during the subsequent use of the treated fabric. Fixation of THPC on cellulosic textiles requires the use of patented ammonia curing process which limits its widespread usage. These compounds are presently the best solutions for fl ame retardant fi nishing of cellulose-based textiles. Recent studies on the fl ame retardant action of DMPMP have shown that it has superior action to some phosphates and phosphoramidate compounds. The superior action of DMPMP has been attributed to the formation of acidic intermediates during the thermal decomposition process, which could further catalyze the char formation of cellulose (Gaan and Sun, 2007a ). DMPMP treated cotton has higher activation energy of decomposition, higher char content, lower heat of combustion and forms a protective coating on the char surface (Gaan and Sun, 2007b ). Application of THPC as a fl ame retardant on cellulose-based textiles has been extensively investigated by many researchers. THPC was applied together with urea and melamine on cotton, cotton–polyester blends and viscose to obtain fi nishes Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231 which could withstand 20 laundering cycles (Donaldson et al ., 1973 , 1975 ; Guthrie et al ., 1955 ; Hamalainen et al ., 1956 ; Reeves and Guthrie, 1953 , 1954 ; Reeves et al ., 1957 ). Recently the use of oligomeric hydroxyl functional organophosphorus compound has been investigated in durable fl ame retardant fi nishing of cellulose (Fig. 5.9). HFPO has been used to make fl ame retardant cotton and cotton/nylon blends (Yang and Wu, 2006 ). In this study HFPO was shown to signifi cantly lower the thermal decomposition temperature of cotton and increases the amount of char formed during the burning process. HFPO was used in combination with various cross-linkers such as trimethylolmelamine and dimethyloldihydroxylethyleneurea. The cotton fabric treated with this system maintained satisfactory fl ame retarding performance (LOI more than 29%) after 40 home laundering cycles (Stowell and Yang, 2003 ; Yang

O O O O P OH HO P O O

2x x

Hydroxy functional organophosphorus oligomer (HFPO) 5.9 Hydroxy functional oligomeric organophosphorus compound.

and Wu, 2004 ). Numerous studies have been performed in order to improve the durability of the chemical fi nishing to washing. Eco-friendly cross- linkers such as multifunctional carboxylic acids (phosphorus-containing maleic acid, polycarboxylic acids) have also been used as fi nishing agents for cotton since the late 1980s (Blanchard and Graves, 2002 , 2005 ; Welch, 2001 ; Wu and Yang, 2008 ). Developments in organophosphorus fl ame retardants for cellulose have focused on novel phosphoramidate compounds (Gaan et al ., 2009b ). Phosphoramidates have been shown to have superior fl ame retardant action to the analogous phosphates (Gaan et al ., 2009a ). Phosphoramidates with hydroxyl terminating alkyl groups (Gaan et al ., 2009a ; Rupper et al ., 2010 ) and methyl ester phosphoramidates have been shown to have superior fl ame retardant action (Gaan, 2008 ).

(c) Flame retardancy with sulfur derivatives The sulfur derivatives are used mainly for treating cellulosics with ammonium, aluminium or other metallic sulfates and produce limited non- durable ﬂ ame retardant coatings. Sulfation treatments with urea sulfamates yielded fabrics that withstood 50 hard and soft water launderings. However, the treatments suffered from drawbacks like reduced tensile strengths and Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231 after-glow effect (Lewin, 2005 ).

(d) Grafting fl ame retardant monomers to cellulose Some attempts have been made to develop fl ame retardant cellulose by chemical graft polymerization techniques. Methacryloyloxyethyl orthophos- phorotetra ethyl diamidate was used as fl ame retardant for graft polymerization on cellulose. Durability to about 50 washing cycles has been achieved by varying the parameters of graft polymerization (Abdel-Mohdy, 2003 ). Other researchers have used radiation graft polymerization technique to fl ame retard cotton fabrics. Orthophosphoric acid was used as a fl ame retardant, where the treatments had limited durability (Reddy et al ., 2005 ). 4-Vinyl pyridine has been used as fl ame retardant with benzophe- none as photoinitiator for grafting onto cellulose fabrics using UV radiation

technique. The thermogravimetric analysis (TGA) of the grafted fabrics showed lower decomposition temperatures with slight increase in char yield (Kaur et al., 2007a ). Methacrylamide has also been grafted onto cotton with the use of low energy UV irradiation technique to introduce func- tionalities that can react with phosphorus-containing compounds to impart fl ame retardancy. Subsequent phosphorylation of grafted fabric showed improved fl ame retardant behavior in comparison to grafted fabric (Kaur et al ., 2007b ). More recently plasma technology has been used as an environmentally benign technique to produce fl ame-resistant textiles. Grafting and polymerization of fi re-retardant monomers on the cotton fabrics have been carried out using microwave argon plasma to produce durable fl ame retardant coatings (Tsafack and Levalois-Grützmacher, 2006 , 2007 ). It is postu- lated that the fl ame retardant properties of treated cotton depend primarily on the structure of the FR monomer and not on the phosphorus content of the fabrics. The durability of the coatings obtained has been attributed to the covalent bond formed between fl ame retardant molecule and cellulose during plasma-induced polymerization. Sodium silicate has been applied onto cellulose textiles using radio frequency low pressure non-equilibrium oxygen plasma to produce phosphorus-free fi re-resistant fabrics. It was observed that samples treated in plasma withstood 50 laundering cycles due to the cross-linked structure of the coatings (Totolin et al ., 2009 ). Wool textiles have the highest inherent non-fl ammability and pass the required fl ame-retardancy tests untreated, because of its highly cross-linked chemical structure, high amino- and amido-nitrogen content (16%) and moisture content (10–14%). Hexafl uorozirconate (Zirpro® treatment) has been the basis for the widely used and commercial fl ame retardant system for wool textiles, which was developed by Benisek in the early 1970s. The Zirpro® treatments act in the solid phase by producing char around the fi bers. A detailed review of fl ame retardant treatments for wool is discussed elsewhere (Horrocks, 1986 ). In 2005 a new fl ame retardant for wool Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231 was introduced (Nofl an®). It is based on amidoalkylophosphonic acid and contains about 14% phosphorus. It is claimed, that Nofl an® enhances char formation in the solid phase, thus minimizing smoke and toxic gas emis- sions (Benisek and Andrews, 2004 , 2005 ). In addition, Nofl an® is an environmentally benign fl ame retardant, and can be applied by continuous pad-dry technique. Silk is widely used in apparel and home furnishing for its luxurious appearance, soft handle, high wearing comfort and other desirable aesthetic appearances (Currie, 2001 ; Robson, 1998 ). Treatment of silk with a mixture of phosphoric acid resulted in high level fl ame retardancy (LOI more than 28%) with limited wash durability (Achwal et al ., 1987 ). Pyrovatex CP has also been used to render silk fl ame retardant. Treated silk is claimed to have LOI values greater than 30%. The fl ame retardant properties were retained

to some extent even after 50 washing cycles (Guan and Chen, 2006 ). Graft copolymerization of a vinyl phosphorus-based monomer such as DEMEP has also been carried out on silk fabrics. Treated silk fabrics were found to be stable to 30 hand washing cycles (Guan and Chen, 2008 ). Hydroxyfunctional organophosphorus polymer in combination with 1,2,3,4-butanetetracarbox- ylic acid has been used as a formaldehyde-free fl ame retardant fi nishing system for silk. Treated fabrics were claimed to be stable to 15 washing cycles (Guan et al., 2009 ). Metal oxides and salts such as antimony oxide, lead monoxide, manganese dioxide and ammonium chloride have also been used in fl ame retardant treatments on silk fabrics. Antimony oxide has been shown to have better fl ame retardant effect than lead monoxide (Tarafder and Singh, 2007 ).

5.5 Environmental issues related to flame retardants During the last few decades the knowledge about toxicity and environmental impact of chemicals has rapidly grown and people have become more aware of potential dangers associated with them. A large number of fl ame retardant compounds are on special lists of national or international environmental committees because of their harmful properties. Many of these substances are either banned or their use has been restricted. Currently tetrabromobis- phenol A (TBBPA), decabromodiphenyl ether (DecaBDE) and HBCD are the most widely used brominated fl ame retardants worldwide. DecaBDE is mainly used as fl ame retardant for plastic casing of electrical and electronic equipments, for furniture and textiles. TBBPA is mainly used in manufacturing circuit boards and plastics. HBCD is mainly used in expanded and extruded polystyrene insulations, textiles and to a lesser extent for plastic casings of electrical and electronic equipments (Ittershagen, 2008 ). Scientifi c and political discussions have focused on these products because of their growing worldwide presence in sediments, dust in buildings, micro- Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231 organisms, fi sh and animals such as polar bears, seals, raptors and their eggs and even in human blood, tissues and breast milk (D’Silva et al ., 2004 ; Hardy et al ., 2009 ; Kang et al., 2009 ; La Guardia et al ., 2007 ; Lilienthal et al ., 2009 ; Schecter et al., 2009 ; Smolarz and Berger, 2009 ; Stapleton et al ., 2009 ; Uddin, 2003 ). Environmental risks associated with pentabromodiphenyl ether (PentaBDE), octabromodiphenyl ether (OctaBDE) and DecaBDE, have been assessed in the recent past and resulted in an EU Directive (Offi cial Journal of the European Union, 2003 ). This Directive prohibits the application and use of PentaBDE and OctaBDE. More recently (2008) the use of DecaBDE in electrical and electronic devices has been banned in Europe. In Norway and Sweden there are national prohibitions for the use of DecaBDE. The US EPA has recently (2009) reached an agreement

with three manufacturers of DecaBDE to voluntarily phase out its production and use in all consumer products by the end of 2013. Studies have shown that DecaBDE persists in the environment, potentially causes can- cer and may impact brain functions. DecaBDE can also degrade to form more toxic by-products which are frequently found in the environment (Environmental Protection Agency, 2009 ). According to the new European REACH-Regulation, all persistent, bio-accumulating and toxic compounds will be assessed and their substitution by less hazardous chemicals encour- aged. The basic goal of REACH (Registration, Evaluation, Authorization and Restriction of Chemicals) is the protection of human health and the environment from the risks posed by chemicals (Kemmlein et al ., 2009 ). The following halogenated fl ame retardants for fi reproofi ng garments are restricted (European Commission, 2008 ):

• tris(2,3-dibromo-1-propyl)-phosphate • tris-(aziridinyl)-phosphinoxide • polybrominated biphenyls.

The use of antimony trioxide as synergist together with halogenated-based ﬂ ame retardants (e.g. in textile back coatings, ﬂ ame retarded paints, rubber, textiles) is also under review within the European Union Commission. The International Antimony Association opposes the ban of antimony oxides under the European Union’s Restriction of Hazardous Substances (RoHS) regulation as there is no conclusive evidence regarding the toxicity or environmental impact of antimony oxides (International Antimony Association, 2009 ). Local legislations in some countries may not be able to induce a broad sustainable protection of mankind and the environment as many chemicals remain unchanged in nature for a long period of time and may also spread over large regions of the planet. The Stockholm Convention with more than 160 member states will decide on the possible ban of POPs (persis-

Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231 tent organic pollutants) which have shown toxicity and environmental risks (Secretariat of the Stockholm Convention, 2009 ). Thus stronger coopera- tion between countries and international organizations is required to tackle the problems associated with the use of harmful chemicals.

5.6 Test standards for flame retardant textiles There are several kinds of standard test methods to determine the fi re risks of textile materials. The use of any particular standard test method is based on the end use of the textile material and the country where the material is to be used. The test method generally outlines various specifi cations such as the type and size of a sample, source of the fl ame/heat, and duration of the application of the fl ame/heat to the substrate. Table 5.2 outlines several important test methods for various kinds of textiles.

Table 5.2 List of standard test methods for ﬂ ame retardant textiles

Textile type Standard test methods Comments

Furnishing fabricsBS 5852: Pts 1 and Cigarette and simulated 2: 2006 match fl ame (20 s ignition) BS 5852: 1990(1998) Small fl ames and wooden ISO 8191: Pts 1 and 2 cribs applied to small (same as BS 5852: 1990) and full scale tests BS EN 1021–1: 2006 Cigarette BS EN 1021–2: 2006Simulated match fl ame (15 s ignition) BS 6807: 2006Ignitability of mattresses or divans BS 5867: Part Small fl ame test for 2: 1980 (1990) curtains and drapes NightwearBS 5722: 1991 Small fl ame AS1249 Australian Standard for children’s nightwear Bedding BS 7175: 1989 (1994)Ignitability of bed covers and pillows BS ISO 12952–1/4: 1999 Ignitability of bedding BS ISO 12952–2/3: 2001 items by cigarette and small fl ame sources France Decree 2000–164 French Standard for bedding (cigarette resistance) Protective clothing EN 533, NF P92 503 (M1), BS 7175 Crib 5 Carpet DIN 4102(B1), FAR25–853 Non-woven NF P92 503

Copyrighted Material downloaded from Woodhead Publishing Online Delivered by http://www.woodheadpublishingonline.com Sabyasachi Gaan (147-04-082) Tuesday, September 27, 2011 10:56:41 PM IP Address: 77.56.188.231 Textiles in aircraftsASTM E 906 1983, uses Ohio Irradiate under 35 kW/m2 State University heat with small ﬂ ame igniter release calorimeter NF P 92501: 1995 French Irradiate with small burner ‘M test’

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