Applications of Nonwovens in Technical Textiles || Flame retardant nonwovens

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© Woodhead Publishing Limited, 2010

4Flame retardant nonwovens

S. D U Q U E S N E and S. B O U R B I G O T,

Ecole Nationale Supérieure de Chimie de Lille (ENSCL), France

Abstract: This chapter deals with flame retardant (FR) nonwovens. After abrief introduction, a basic review of the types of flame retardants is reported,the way they work, advantages and drawbacks as well as their potentialapplications in the textile industry is discussed. Then, the use of thesesystems for nonwoven applications is commented upon. Several approachesthat can be used to flame retard nonwovens including surface treatment, theuse of high performance fibers as well as the use of FR fibers are reviewed.Applications of FR nonwovens for filtration, as fire-blockers for seats andupholstery and as protective garments are illustrated.

Key words: flame retardant finishing treatments, high performance fibers,fire-blockers, protective garments.

4.1 Introduction

Nonwoven products are mainly manufactured using synthetic fibers such aspolyolefin, polyester or nylon (for more detail refer to Chapter 2) that representhighly flammable products. Polypropylene, in particular, burns very rapidly witha relatively low amount of smoke and without leaving a char residue because of itswholly aliphatic hydrocarbon structure (Zhang and Horrocks, 2003). Its self-ignition temperature is around 570 °C and it presents a rapid decomposition ratecompared with wood and other cellulosic materials. Einsele et al. (1984) reportsthe heat of combustion for polypropylene to be 40 kJ/g, which is higher than manyother fiber-forming polymers. For comparison, the heat of combustion ofpolyethylene terephthalate is reported to be around 20 kJ/g (Walters et al., 2000).The use of nonwovens manufactured with synthetic fibers can thus lead to anincreased fire risk in many cases. This has to be taken into account even morenowadays since there is a trend to replace high cost materials by lower costmaterials, for example polypropylene.

As an example, dealing with operating room fires, it is reported that theirfrequency has been diminished since the introduction of non-flammable anestheticdrugs but not totally eliminated because of the introduction of new potential fuelssuch as, for example, surgical drapes that are not designed with fire safety as apriority (Wolf et al., 2004). Nonwoven cellulose drapes combined with polyesterrepresent a common class of surgical drape. Such products easily ignite in air after

66 Applications of nonwovens in technical textiles


4.1 Publications regarding flame retardant (FR) nonwovens since 1980.Source: SciFinder.

a laser impact and thus represent an ignition source and potential risk products inoperating rooms (Wolf et al., 2004).

Another example concerns upholstery and in particular mattresses. Fire statis-tics show that in the US, each year, an estimated 20,800 fires are attributed tomattress/bedding fires. These fires cause 2,200 injuries, 380 fatalities, and US$104million in property loss (USFA, 2002). This is mainly due to the fact thatpolyurethane foams are used to manufacture mattresses and easily burn withmelting and dripping (Lefebvre et al., 2004). The molten material can flowdownward with gravity leading to the formation of a pool fire. If the pool fire isclose enough to nearby products, the result can be a self-propagating fire thatfurther promotes heat release (Hirschler, 2008).

The use of barriers, so called fire-blockers, can lead to furnishings complyingwith modern fire tests in some cases (Damant, 1994). As an example, Europeanpatent EP 1780322 describes a fireproof nonwoven cover for spring mattressesmade from cellulose fibers and viscose, stitch bonded with synthetic yarns andimpregnated with a fireproof agent. According to the patent, the use of suchnonwoven mattress covers meet Californian standards (TB 603 regulations)(Barberis, 2006).

The need to develop new flame retardant (FR) nonwovens or barrier nonwovensto protect underlying materials can be seen in the examples above. Although thedevelopment of FR nonwovens is not recent (the first patent dealing with FR

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finishing treatment for nonwoven fabrics was applied for in 1973 (Orito et al.,1973)), the literature has greatly increased in the last few years. It can be also beseen that the literature is mainly composed of patents and that there are only a fewdetailed studies (see Fig. 4.1).

This chapter will first present a basic review of the types of flame retardants, theway they work, advantages and drawbacks as well as their potential applications inthe textile industry. Then, the use of such systems for nonwoven applications willbe detailed. Several approaches that can be used to flame retard nonwovensincluding surface treatment, the use of high performance fibers as well as the useof FR fibers, will be described. Applications of FR nonwovens as fire-blockers forseats and upholstery and as protective garments are illustrated.

4.2 Basics of flame retardancy

The process of ignition and burning can be described in short as a gas phasereaction (Bourbigot et al., 2003). Thus, a substance must become a gas for burning.As with any solid, a textile fabric exposed to a heat source undergoes a temperaturerise. If the temperature of the source (either radiative or a gas flame) is high enoughand the net rate of heat transfer to the fabric is high, pyrolytic decomposition of thefiber substrate will occur. The products of this decomposition include combustiblegases, non-combustible gases and carbonaceous char. The combustible gases mixwith the ambient air and oxygen. The mixture ignites, yielding a flame, when itscomposition and temperature are favorable. Part of the heat generated within theflame is transferred to the fabric to sustain the burning process and part is lost to thesurroundings. The considerable fire hazards posed by textiles both in historicaltimes and to the present day are a consequence of the large surface area of the fibersand the ease of access to atmospheric oxygen (Horrocks et al., 2005). The goal offlame retardancy is then to inhibit or even suppress the combustion process actingchemically and/or physically in the solid, liquid or gas phases (Hirschler andPiansay, 2007). It can interfere with combustion during a particular stage of thisprocess, e.g. during heating, decomposition, ignition or flame spread.

Various methods can be used to protect materials more effectively from fire(Bourbigot, 2007). The first method is to use inherently flame retarded polymersor high performance polymers but it implies the use of specific materials that mightnot have the required properties. The second method is to chemically modify theexisting polymer to synthesize the FR polymer. The third method is to use flameretardants and/or particles (micro- or nanodispersed) directly incorporated in thematerials (e.g. thermoplastics, thermosets or synthetic fibers) or in a coatingcovering their surface (e.g. structural steel or textiles). In this section we only focuson the mechanism of action of FR materials. Our intention is to provide the readerwith the general principles of flame retardancy.

The various ways in which a flame retardant can act do not occur singly butshould be considered as complex processes in which many individual stages occur



simultaneously, with one dominating (e.g. using hydroxides causes an endothermicdecomposition, cooling down the substrate and diluting the ignitable gas mixturedue to the formation of inert gases associated with the formation of the oxideprotective barrier).

Physical action. There are several ways in which the combustion process can beretarded by physical action (Lewin, 1998):

• By formation of a protective layer. Under an external heat flux the additives canform a shield with a low thermal conductivity that can reduce the heat transferfrom the heat source to the material. It then reduces the degradation rate of thepolymer and decreases the ‘fuel flow’ (pyrolysis gases issued from the degrada-tion of the material) able to feed the flame. This is the principle of theintumescence phenomenon (Bourbigot et al., 2004). Phosphorus additives (orphosphorus grafted on the backbone of polymeric chains or phosphoruscomonomer) may act in a similar manner. Their pyrolysis leads to pyro- orpolyphosphoric species that are thermally stable and which form a protectivevitreous barrier. The same mechanism can be observed using boric acid-basedadditives, inorganic borates, silicon compounds or low melting glasses.

• By cooling. The degradation reactions of the additive can play a part in theenergy balance of combustion. The additive can degrade endothermally, whichcools down the substrate to a temperature below that required for sustaining thecombustion process. Aluminum trihydroxide (ATH) acts partially under thisprinciple and its efficiency depends on the amount incorporated in the polymer(generally 60 (wt%) in thermoplastics).

• By dilution. The incorporation of inert substances (e.g. fillers such as talc orchalk) and additives that produce inert gases on decomposition dilutes the fuelin the solid and gaseous phases so that the lower ignition limit of the gas mixtureis not exceeded.

Chemical action. The most significant chemical reactions interfering with thecombustion process take place in the condensed and gas phases (Lewin, 1998):

• Reaction in condensed phase. Here two types of reaction can take place. Firstly,breakdown of the polymer can be accelerated by the flame retardant causing apronounced flow of the polymer and, hence, its withdrawal from the sphere ofinfluence of the flame, which breaks away. Secondly, the flame retardant cancause a layer of carbon (charring), a ceramic-like structure and/or a glass to beformed on the polymer surface.

• Reaction in gas phase. The radical mechanism of the combustion process thattakes place in the gas phase is interrupted by the flame retardant or itsdegradation products. The exothermic processes that occur in the flame are thusstopped, the system cools down, and the supply of flammable gases is reducedand eventually completely suppressed. In particular, halogenated compoundscan act as flame inhibitors.



The fire retardant additive systems may be used alone or in association with othersystems in polymeric materials to obtain a synergistic effect, i.e. the protectiveeffect is higher than is assumed from the addition of the separate effects of eachsystem (Bourbigot and Duquesne, 2007). In the other sections of this chapter, wediscuss how these principles can be applied and how they act in the particular caseof nonwovens.

4.3 Different approaches for flame retardant

nonwovens

Most fibers are highly combustible (except high performance fibers) and theflammability of derived fabrics largely depends on the construction and density ofthe fabric. Several approaches can be used to enhance the fire behavior of fiber-based fabrics used either alone or in blends with other fibers (Horrocks et al.,2005):

• Coatings and/or finishing treatments may be applied to shield fabrics from heatsources and prevent volatilization of flammable materials. These may take theform of simple protective coatings or, more commonly, the treatment of fabricswith inorganic salts that melt and form a glassy coating when exposed toignition sources. In more advanced forms, intumescent coatings produce a charthat has sufficient plasticity to expand under the pressure of the gases to yield athick, insulating layer.

• Thermally unstable chemicals, usually inorganic carbonates or hydrates, areincorporated in the material, often as a back-coating so as to preserve the surfacecharacteristics of the carpet or fabric. Upon exposure to an ignition source, thesechemicals release CO

2 and/or H

2O, which, in a first step, dilute and cool the

flame to the point that it is extinguished, and in a second step form a protectiveceramic around the charred fibers.

• Materials that are capable of dissipating significant amounts of heat are layeredwith the fabric or otherwise incorporated in a composite structure. These may beas simple as metal foils or other heat conductors or as complicated as a varietyof phase-change materials that absorb large quantities of heat as they decom-pose or volatilize. If sufficient heat is removed from the point of exposure, theconditions for ignition are not reached.

• Char-promoting chemical treatments that may be fiber-reactive or unreactive toyield launderable or non-durable flame retardancy, respectively.

• Chemicals capable of releasing free radical trapping agents, frequentlyorganobromine or organochlorine compounds, may be incorporated into thefabric. These release species such as Br• and Cl•, which can intervene in theoxidation reaction of the flame and break the chain reaction necessary forcontinued flame propagation.

• In the particular case of synthetic fibers (approaches listed above are valid for



both natural and synthetic fibers), the direct incorporation of additives(microfillers and/or nanoparticles) or the chemical grafting/copolymerizationof specific groups.

4.3.1 Surface treatments

The modification of the surface of textile fabrics or of fibers is one of the easiestways to bring flame retardancy to materials. Several processes can be used tomodify the surface. It is possible to classify these processes into various categories.One can distinguish the ‘chemical processes’ including padding and back-coating,and ‘physical processes’ such as plasma treatment or flaming. In the textileindustry, finishing treatments are also generally classified according to theirdurability (non durable, semi-durable and durable). In this chapter, the surfacetreatments will be classified as (i) wet processes in which an FR textile coating isapplied on the fabric using different deposition processes and (ii) dry processeswhere either FR films are thermally bonded into the nonwoven or when plasmadeposition is used.

Wet processes

The use of FR coating or impregnations in the textile industry is common. It wasthe first approach used to develop FR nonwovens (Orito et al., 1973) and is stillintensively used (Weil and Levchik, 2008). FR finishing treatments can includeorganic phosphates (such as tri-alkyl or tri-aryl phosphates, tri-chloroalkyl phos-phates, dialkyl phosphites, tetrakis-(hydroxymethyl)phosphonium chloride andrelated structures), halogenated compounds (such as polybrominated diphenylethers and chlorinated paraffins) or inorganic compounds (such as antimonytrioxide, ammonum bromide, boric acid and aluminum hydrate) (Van Esch, 1997).Since some halogenated compounds have environmental impact concerns, as theyare perceived to be persistent environmental hazards and produce toxic smokewhen burned, their uses are becoming more restricted and halogen-free flameretardants are preferred. At present the consumption of halogenated compounds isstill high in the textile industry (as an example, polybrominated diphenyl etherswas found in over 50% of treated furniture (Hofer, 1999)).

The use of intumescent systems could be an answer to FR challenges in thetextile industry (Horrocks and Kandola, 1997). As an example, Magniez et al.(2003) have reported that the use of textile resin binders in association with a FRadditive of 15% as back-coating enabled a large improvement in the FR propertiesof polypropylene (PP) nonwovens. Table 4.1 shows that when a FR treatment isused as back-coating, the time to ignition increases and the peak of heat release rateobtained in the cone calorimeter test decreases. The improvement of the FRproperties of the nonwoven was attributed to the quick formation of the intumes-cent protective layer, trapping the combustible gases released during the



Table 4.1 Cone calorimeter data of polypropylene nonwoven and treatedpolypropylene nonwoven (external heat flux of 30 kW/m²)

Reference materials Peak of heat release rate Time to ignition(PHRR)(kW/m²) (TTI) (s)

Polypropylene nonwoven 230 30Polypropylene bounded by 165 35treated resin

degradation of the textile and limiting heat and mass transfer between the flameand the material.

Similarly, Duquesne et al. (2006) have compared the FR properties of PPnonwovens padded and back-coated with a polyurethane based intumescentformulation (based on ammonium polyphosphate (APP) and melamine (Mel)).When fire retardant additives are added to the polyurethane (PU), the fire retardantproperties of the nonwoven materials are greatly improved whatever the formula-tion and the fireproofing methods (Fig. 4.2). The higher efficiency of APPobtained in the case of back-coating is explained by the fact that APP has to reactwith PU to develop a protective barrier. The additive in back-coating is concen-trated in the resin and thus the efficiency is higher. Similar results are obtained

4.2 Burned surface of the padded and back-coated PP nonwovenobtained in the vertical hybrid fire test (combination of two normalizedtests: IN ISO 11925-2 and NF G07-184).

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4.3 Heat release rate curve of virgin and treated hemp (a) and flax (b)obtained in the cone calorimeter (ISO 5660 at 35 kW/m²).

when APP and Mel are combined together since a condensed phase mechanism isalso involved. On the other hand, the role of melamine is attributed to anendothermic effect induced by its volatilization and its decomposition in the flame.As a consequence, it is thus more active when dispersed in the system via paddingsince it could lead to flame retard PP as well as PU binder.

In the case of natural fibers, only surface treatment can be used. Cellulosics,such as cotton and rayon, are common and well known natural fibers. They are notinherently ignition resistant and usually must be chemically treated to preventignition by small flames. A well known durable FR chemical for cotton is based onorganophosphorus compounds, such tetrakis (hydroxymethyl) phosphonium chlo-ride (THPC). The THPC system is effective in giving durable flame resistance tocotton fabrics, but it requires the use of special equipment in its application.Coating (or back-coating) on fabric is another way to provide flame retardancy tocotton. Horrocks’ group (Davies, 2005) used intumescent back-coatings based onammonium polyphosphate (APP) as the main flame retardant combined withmetal ions as the synergist. Metal ions promote thermal degradation of APP atlower temperatures, and this enables FR activity to commence at lower tempera-tures in the polymer matrix thereby enhancing FR efficiency. However, thisapproach was applied to woven fabrics and it is known that the structure of thetextile may influence its properties. Recently, the polycondensation products of



Table 4.2 Characteristic data obtained in a horizontal flame spread test forhemp nonwovens (NW)

Material Char length Burn Flame spread Observations(cm) time (s) rate (cm/min)

Hemp NW 13 195 4 Total burn, smokeHemp NW + PLA 13 205 3.8 Total burn, smoke,

film meltedHemp NW + PLA/ – – – Charred, self-

APP/PER film extinguishingHemp NW + PLA/ – – – Charred, self-

APP/LIG film extinguishingHemp NW + PLA/ – – – Charred, self-

APP/starch film extinguishing

urea polyborates and polyphosphates were used to flame retard nonwovens madefrom natural raw materials (hemp and flax) (Kozlowski et al., 2007). The advan-tage of this treatment is its low cost and its high efficiency. Indeed, as presented inFig. 4.3, the treatment leads to a sharp decrease of the heat release rate obtained incone calorimeter experiments. However, it has to be noted that the treatment is notresistant to washing.

Dry processes

Due to environmental concerns, there is a trend to develop new processes to flameretard fabrics, in particular limiting the volatile organic compounds (VOC) andthus the use of dry processes is preferred. As a result, a new process that usesintumescent films made from renewable resources to flame retard nonwovens wasdeveloped (Réti et al., 2009). PLA (polylactic acid)-based film containing APPand a char former (lignin (LIG), starch or pentaerythritol (PER)) was extrudedusing a single screw melt extruder equipped with a film die. Films of about 250 µmthickness were then glued onto hemp nonwovens using a press at a temperaturehigher than the melting temperature of PLA. The characteristic data (Table 4.2)obtained in a horizontal flame spread test (similar to the standard FMVSS 302)show that untreated nonwovens or PLA covered nonwovens easily ignite and burntotally. In the presence of a flame, the nonwovens covered with an intumescentfilm are self-extinguishing. The formation of a protective structure stops thecombustion of the sample. This effect was also observed in vertical configurations.This approach is very efficient, easily applied on various kinds of nonwovens (asan example, it has been validated on wool nonwovens). However, the mechanicalproperties of the fabrics are greatly affected and application where softness isrequired should be avoided.

Plasma treatments are another approach to flame retard textile fabrics and inparticular nonwovens. There are two approaches:



4.4 Acrylate monomers containing phosphorus that can be grafted andpolymerized on cotton fabric. DEAEP, diethyl(acryloyloxyethyl)phos-phate; DEMEP, diethyl-2-(methacryloyloxyethyl)phosphate; DEAMP,diethyl(acryloyloxymethyl)phosphonate; DMAMP, dimethyl(acryl-oyloxymethyl)phosphonate; DEAEPN, diethyl(acryloyloxyethyl)phosphoramidate (DEAEPN); BisDEAEPN, acryloyloxy-1,3-bis(diethylphosphoramidate)propan.

• the plasma induced graft polymerization (PIGP), which consists of the simulta-neous grafting and polymerization of functionalized monomers, such as acrylatemonomers, on the surface of a material;

• and the plasma enhanced chemical vapor deposition (PECVD), consisting ofthe injection in the plasma of non-functionalized monomers that becomeexcited and partially decomposed in the plasma with the subsequent formationof a highly cross-linked polymers thin film.

The first approach was used to give permanent fire proofing properties to cottontextiles (Tsafack and Levalois-Grützmacher, 2006). In this study, the simultaneousgrafting and polymerization of fire retardant acrylate monomers (Fig. 4.4) contain-ing phosphorus on cotton fabric induced by argon plasma were investigated. Thelimiting oxygen index (LOI/ISO4589 is the minimum concentration of oxygenthat will just support flaming combustion in a flowing mixture of oxygen andnitrogen) of the cotton increases from 19 vol.% for untreated fabric to 28–29 vol.%for phosphoramidate monomers. This approach was validated on PET/cottonfabrics (Vannier, 2007) but was never reported for nonwoven fabrics. Tsafack andLevalois-Grützmacher (2006) report that, for cotton fabrics, the efficiency of thetreatment differs when the surface density of the fabrics vary from 120 g/m² to210 g/m². This difference is attributed to the fact that heavier fabrics are morecompact than lighter fabrics. Because of this compactness, the surface contact

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4.5 Nonwovens based on high performance fibers untreated (a) andplasma treated (b) after exposure to a heat radiator (around 35 kW/m²).

between the fabric and the solution of the monomer is weaker for heavier fabricsthan that of lighter fabrics and thus grafting and polymerization are less efficient.

The second approach, plasma enhanced chemical vapor deposition (PECVD),deposes an organosilicon thin film obtained from the polymerization of 1,1,3,3-tetramethyldisiloxane (H(CH3)2Si-O-Si(CH3)2H) monomer using the cold remotenitrogen plasma (CRNP) process on bulk polymer (Polyamide 6, PA6) (Bourbigotet al., 1999). A similar approach was developed in our laboratory at that time inorder to improve the thermal stability of high performance fibers. First results areencouraging (see Fig. 4.5). (Jimenez et al., 2009): after exposure to the heatradiator test the untreated nonwoven is greatly degraded, the treated nonwovenonly presents some shrinkage.

4.3.2 High performance fibers

High performance fibers are driven by special technical functions that requirespecific physical properties unique to these fibers (Sikkema, 2002). They usuallyhave very high levels of at least one of the following properties: tensile strength,operating temperature, heat resistance, flame retardancy or chemical resistance.Applications include uses in the aerospace, biomedical, civil engineering, con-struction, protective apparel, geotextiles and electronic areas. The resistance toheat and flame is one of the main properties of interest for determining the workingconditions of these fibers. In this section we will not consider the inorganic, man-made fibers because very few of them are found in nonwoven applications.

The principal classes of high performance fibers are derived from rigid-rodpolymers (lyotropic liquid crystalline polymers and heterocyclic rigid-rod poly-mers), modified carbon fibers, synthetic vitreous fibers, phenolic fibers,

(a) (b)



poly(phenylene sulphide) fibers and others. Typical high performance fibersare poly(p-phenylene-2,6-benzobisoxazole) (PBO or Zylon from Toyobo), poly-p-phenylenediamine-terephtalamide (PPTA or Kevlar, DuPont), co-poly(p-phenylene-3,4-oxidiphenylene-terephthalamide) (TECH or Technora, Teijin),poly(2,6-diimidazo[4,5-b-4',5'-e]pyridinylene-1,4(2,5-dihydroxy)phenylene)(PIPD or M5, Magellan), phenolic fibers (Kynol, Kynol), melamine fibers(Basofil, Basofil Fibers LLC), oxidized polyacrylonitrile (PAN) and polyamide-imide fibers (Kermel, Kermel).

Three categories of fibers can be determined, rated according to their perform-ance (Bourbigot and Flambard, 2002). The first group is PBO and PIPD. Thesefibers have a very low rate of heat release rate (RHR) and in the conditions of post-flashover (external heat flux > 50 kW/m² in the cone calorimeter) would not beexpected to fire spread; they also have a high limiting oxygen index (LOI) (> 50vol.%), and they do not emit smoke. Kynol and recycled oxidized PAN fibers arein the second group because they have a moderate RHR (< 150 kW/m²). Therecycled oxidized PAN fibers and Kynol exhibit high LOI values (> 30 vol.%), andthey emit little smoke. The third group is the p-aramid fibers, which, while havingcomparatively high RHR values (~ 300 kW/m²), contribute to fire growth and emitsmoke; they also have a high LOI (> 27 vol.%) values. According to the aboveratings, the heterocyclic rigid-rod polymers (PBO and PIPD) exhibit the bestperformance. The two structures of the polymers are highly conjugated and areheteroaromatic. Moreover, they do not have flexible mid-chain groups that maylead to the reduction of their thermal stability. These factors provide high levels ofstability to the polymer and promote high flame resistance. In contrast to PBO andPIPD, p-aramid fibers have phenylene groups linked by amide bridges. Thesebridges (–CONH–) lead to a reduction in the thermal stability of the fibers, whichconsequently yield flammable molecules upon heating. Finally, Kynol and oxi-dized PAN fibers are cross-linked networks with methylene (Kynol) or ether(oxidized PAN) bridges. These flexible groups lead to the reduction in thermalstability as for the p-aramids, but the stabilizing character of the cross-linkednetwork enhances char yields. It follows that the better fire performance of Kynoland oxidized PAN fibers in comparison with p-aramids can be attributed to theformation of higher yields of char. This enhanced char can act as an insulativeshield when burning and can protect the substrate.

4.3.3 Flame retardant fibers for nonwovens

To make flame retarded fibers, several approaches can be considered: (i) theincorporation of FR additive(s) in the polymer melt or in the solution prior toextrusion, (ii) the copolymerization or the grafting of FR molecules to the mainpolymeric chain, and (iii) the use of semi-durable or durable finishing. The thirdapproach will not be considered here as it has been covered in Section 4.3.1. Theonly candidates for applying the first two approaches are synthetic fibers and our



discussion will focus on these. Synthetic fibers are numerous and all of themrequire flame retardancy, appropriate to their chemical formulation. We will thenfocus on the most established and used fibers. They include polylactic acid (PLA),polyester, polyamide and polypropylene. All these fibers are used for makingnonwovens.

PLA fibers were the first man-made (synthetic) fibers made from 100% annu-ally renewable resources, and were publicly launched by Cargill Dow in early2003 (Vink et al., 2004; Vink et al., 2007). Upon heating, PLA melts, drips andreaches its temperature of ignition very quickly. Very little work has been carriedout on the flame retardancy of PLA textiles, but in our laboratories (Solarski et al.,2007), we have developed PLA-clay nanocomposites able to be melt spun to makemultifilament yarn (and further nonwovens). Various quantities of organomodified(OM)- montmorillonite (MMT) (from 1 to 4 percent in weight (4 wt.%) have beenadded to PLA by melt blending via the usual procedures to produce PLAnanocomposites and then into yarn by melt spinning. It has been found that it isnecessary to use a plasticizer to melt-spin a blend with 4 wt.% of OM–MMT, andthe dispersion of the clay (dispersion at the nano-scale) in the yarns is quite good.The multifilaments were knitted and the flammability studied using cone calorimetryat an external heat flux of 35 kW/m². Depending on the clay loading, the peak valueof RHR is decreased by up to 38 % demonstrating the improved fire performanceof these PLA fibers. Formation of char is observed in the case of the nanocompositessuggesting a mechanism of condensed-phase.

Polyester fibers are the main synthetic fibers used in the industrial manufactur-ing sector and can be found in several areas of application. As polyester fibers areeasily flammable, flame retardancy is a significant issue. One of the commonsolutions to flame retard polyester is to incorporate a comonomeric phosphinicacid unit into the PET polymeric chain (trade name Trevira CS) (Horrocks et al.,2005). Trevira CS polyester does not promote any char formation and there isevidence that the phosphorus compound acts in the gas phase. Several flameretardants have also been designed for polyester extrusion (bisphenol-S-oligomerderivatives from Toyobo, cyclic phosphonates (Antiblaze CU and 1010) fromRhodia or phosphinate salts from Clariant). Note that the cyclic phosphonate mayalso be applied as textile finish as well as a melt additive. All these flame retardantswere developed in the 1980s (except the phosphinate salt which was developed inthe 1990s) and their modes of action have been described in the literature(Horrocks et al., 2005) but very little on this topic has been published recently.Nevertheless, a new halogen-free FR master batch for polyester has been devel-oped in our laboratories, which at only 5 wt.% incorporation in PET allows fabrics(including nonwovens) to meet several standards such as the NF P 92 501 or NF P92 503 (M classification), FMVSS 302 or BS 5852 (Crib 5) (Almeras et al., 2008).

Like polyester, polyamides are synthetic fibers made from semicrystallinepolymers and are used in a variety of applications in textiles similar to polyester.Polyamides, however, have proved difficult to render durably flame retardant by



incorporation of additives because of their melt reactivities. Semi-durable finishesbased on thiourea derivatives are used but usually only on industrial polyamidetextile where launderability is not an issue (Horrocks et al., 2005). The most recentdevelopments for the flame retardancy of polyamides concern mainly the inclu-sion of nanoparticles. Nylon 6 or PA-6/clay hybrid fibers have been made by meltblending and by melt spinning (Bourbigot et al., 2002). RHR curves recorded bycone calorimetry of knitted PA-6 and PA-6/clay nanocomposite fabrics at anexternal heat flux of 35 kW/m² showed that the peak of RHR of the nanocompositedecreased by 40% compared to that of the pure PA-6. Visually, while a char layercan be seen on each of the two textiles, that in PA-6 seems to be crumbly andcontains holes, whereas the char produced by PA-6nano is uniform and withoutholes. This structure could explain the better performance of PA-6nano againstPA-6. Recent work by Horrocks et al. (2005) has been the investigation of theeffect of adding selected flame retardants based on ammonium polyphosphate,melamine phosphate, pentaerythritol phosphate, cyclic phosphonate and similarformulations into nylon 6 and 6.6 in the presence and absence of nanoclay. Theyfound that in nylon 6.6 all of the effective systems comprising the nanoclaydemonstrated significant synergistic behavior except for melamine phosphatebecause of the agglomeration of the clay. They then report in the case of nylon 6that the presence of nanoclay acts in an antagonistic manner (in terms of LOI). Toexplain why the nanoclay lowered the LOI value of the FR-free nylon 6 film butnot that of the nylon 6.6, they proposed that the nanoclay reinforces the fiberstructure both in solid and molten phases, thereby reducing its dripping capacity.Such an effect would be likely to reduce the LOI value as the melting polymer hasgreater difficulty in receding from the igniting flame.

Polypropylene (PP) is presently one of the fastest growing fibers for technicalend-uses where high tensile strength coupled with low cost are essential features.Because of its wholly aliphatic hydrocarbon structure, polypropylene by itselfburns very rapidly with a relatively smoke-free flame and without leaving a charresidue. While polypropylene fibers may be treated with FR finishes and back-coatings in textile form with varying and limited success (Zhang and Horrocks,2003), the ideal FR solution for achieving fibers with good overall performancedemands that the property is inherent within the fiber. An acceptable flameretardant for polypropylene, especially fiber-forming grades, should have (i) athermal stability up to the normal PP processing temperature (< 260 °C), (ii) agood compatibility with PP and no migration of the additives, (iii) flame retardancyproperties when present in the fiber and (iv) efficiency at a relatively low level(typically less than 10 wt.%) to minimize its effect on fiber/textile properties aswell as cost. With such an approach, Zhang and Horrocks identified five principaltypes of generic FR systems for inclusion in polypropylene fibers as phosphorus-containing, halogen-containing, silicon-containing, metal hydrate and oxide, andthe more recently developed nanocomposite FR formulations. They concludedthat apart from antimony–halogen or in some cases tin–halogen formulations, only



one single FR system, tris(tribromoneopentyl) phosphate, is presently effective inpolypropylene when required for fiber end-uses. Presently, the use of phosphorus-based, halogen-free flame retardants in PP fibers is prevented by the need to haveat least 15–20 wt.% additive presence. Since the latter are char-promoting while allhalogen-based systems are essentially non-char-forming in polypropylene, theway forward for a halogen-free, char-forming flame retardant conferring accept-able levels of retardancy at additive levels of 10 wt.% will require either completelynew FR chemistry or the development of a suitably synergistic combination basedon the understanding reviewed above. The use of nanoparticles might offer thisopportunity.

The ‘nanocomposite approach’ (incorporation and nanodispersion of particles)alone does not provide enough flame retardancy for PP fabric. That is whyHorrocks and his group combined nanoclays with conventional flame retardants asthey did in the case of polyamides (Zhang et al., 2006). Flame retardants usedincluded ammonium polyphosphate and a hindered amine stabilizer known tohave flame retarding characteristics in polypropylene (Zhang and Horrocks,2003). They reported that the flammability of polypropylene is reduced by theaddition of small amounts of clay in conjunction with a conventional phosphorus-containing and a hindered amine flame retardant. The authors also suspect a P–Nsynergism to exist and the LOI value for the best formulation is 22 vol.% comparedto 19 vol.% for neat PP, with only 6 wt.% total loading of additives.

4.4 Applications of flame retardant nonwovens

In order to ensure the safety of the public with regard to fire, standards, regulationsand requirements in this field are continually discussed and modified. It is not easyto navigate the maze of testing methods and standards. In Europe, harmonizationwas initiated in the 1990s and is still progressing. The new regulations present newchallenges to the flame retardancy industry. Examples of applications of FRnonwovens are reported below. The application of nonwoven materials to manu-facture mats for composites also require FR properties (Frechette and Bootle,2003) but will not be considered in this chapter.

4.4.1 Protective garments

The field of protective garments is relatively large with differing requirementssince it incorporates the protection of men at work and military applications, aswell as clothing for firefighters. This problem is also relatively complex since anumber of properties are required for a material to be used in this field of applica-tion. Indeed, heat protective performance is needed, but also heat-moisture transferproperties and comfort performance including lightness, for example, have to betaken into account, and usually a balance between the heat and moisture barrier hasto be found. Usually, protective fabrics are multilayer clothing containing



4.6 Typical structure of protective garments for firefighters. Source:Rawas, 2008.

up to five or six layers. Fire protective clothing for firefighters consists of at leastfour layers: outer and inner shell, moisture barrier, and thermal liner (see Fig. 4.6).These layers are expected to provide adequate heat, flame, liquid, chemical andmechanical protection.

Nonwovens composed of high performance fibers are typically used as thermalliners in protective garments. Various specialized companies have been involvedin the process of developing fire protection fabrics (Anon, 2004). Among them,Consoltex introduced CRYONTM technology combining the comfort of naturalcotton with the protection of modacrylic fibers and proposed some solutions forfirefighters and workwear using Nomex®, Kevlar, PBI and/or Kermel® technolo-gies. Montreal-based company Securitex has developed firefighter products thatameliorate the interaction between the firefighter and his clothing. Meanwhile,Difco Performance Fabrics have also introduced Breezeway® and Genesis® thatact as safe alternatives for flame resistant fabrics. Duflot Industries, a companydeveloping technical nonwovens, launched the first nonwoven thermal barrier(Duflot®) in 1986 using Inidex fibers from Courtaulds. This barrier was immedi-ately adopted by the London fire brigade for their structural firefighting kit. Morerecently, Duflot have developed the Isomex® IsoAir® barrier for heat protectionto deal with heat and moisture transfer properties. These barriers are used by theHelsinki, Berlin and London fire brigades (Rawas, 2008).

4.4.2 Fire-blockers for seat and upholstery

Fire-blockers are usually highly fire resistant materials that are placed beneath theexterior cover fabric of furnishing and the first layer of cushioning materials in seats,mattresses and upholsteries. The bulky cushioning materials represent the majorfuel source and therefore the greatest hazard potential. The fire-blocker acts as a

Outer shell

Moisture barrier

Thermal liner

Inner shell

Heat transfer

Moisture transfer



barrier between the heat source (flame, cigarette, etc.) and the cushioning materialslimiting fire growth and development (Damant, 1996). Fabric-like fire-blockersinclude woven and needle punched fabrics made from highly resistant textile fiberssuch as glass, Nomex, Kevlar, PBI, etc. Some of the fabric-like fire-blockersavailable are engineered textile products that use a combination of different fibersand fabric treatments. The first modern generation of fire-blockers was introducedby Dupont under the trade name VonarTM during the 1970s. In the 1990s, HoechstCelanese and Dupont developed products based on PBI blended with other fiberssuch as aramids, cotton and rayon, and based on Kevlar® and Nomex®, respec-tively. Kemira in Finland also introduced the use of Visil® fibers, a viscose rayonfiber with a silicic acid backbone, in the field of fire-blockers. More recently, it hasbeen reported that Lenzing FR, an inherently flame resistant cellulose fiber, can beused for seat covers in aircraft, railway vehicles, ships and for technical applicationssuch as fire-blockers and insulation felts (Gstettner, 2005).

Even though most of the fire-blockers have been designed using high perform-ance fibers, there has been an attempt to develop these barriers using fire protectivecoating, such as intumescent coating, applied to certain fabrics. F.R. SystemsInternational, Canada, have applied this technology to introduce fire barriers formattresses for example. The use of FR nonwovens based on natural fibers as firebarriers for upholstery has been reported (Kozlowski et al., 2004), and also thatnatural materials modified with safe fire retardants may be applied as valuablecomposites in sleeping and seating furniture with the advantages of naturalmaterials regarding their permeability and anti-electrostatic properties.

4.4.3 Other applications

Another application of FR nonwovens is flexible insulation panels for buildingconstruction. Although traditional thermal insulation materials such as mineralwool or polystyrene are widely used, the return to ecology and nature noticed inseveral application fields is also observed in the building industry. Natural wool,coconut or even duck feathers are used to design thermal insulation panels.However, all these materials burn easily and thus FR treatments are required. Thedevelopment of needle punched nonwovens and air-laid nonwovens based on fireretardant modified natural fibers has been reported (Kozlowski et al. 2007) and hasbeen demonstrated that such materials meet the requirements for use in buildingapplications.

Finally, it is noteworthy that there are applications where FR properties arerequired in disposable nonwovens, for example for surgical drapes used inoperating rooms as well as air filters used in the automotive industry. However, asonly few studies have been reported these applications will not be fully developedin this chapter.



4.5 Conclusion and future trends

The development of FR nonwovens is an important field of investigation sincein a number of applications fire retardant properties or fire protection is requiredfor the safety of people. Although this topic has been investigated for wovenfabrics over a long period of time, it is relatively new for nonwovens and only afew detailed and comprehensive studies have been reported in the literature.However, we know that the structure of the materials (in particular its surfacearea) plays a significant role in the properties of the materials. At present, themain applications of FR nonwovens concern fire protective garments and fire-blockers for seats, mattresses and upholstery applications. These barriers aremainly prepared using high performance fibers and thus the costs are relativelyhigh. Future developments will concern the development of low cost FRnonwovens. One interesting approach consists of the development of intumes-cent coatings used as back-coatings that could develop a heat protective barrierin case of fire. All these developments could take into account environmentalconcerns and thus developments of bio-based materials and of dry processtechnologies will be preferred.

4.6 References

Almeras X, Vandendaele P, Vannier A, Duquesne S, Bourbigot S, Delobel R, Ortiz M, GuptaG and Pivotto E (2008), ‘New halogen free masterbatch for PET fibers’, Chemical FibersInternational, 58, 178–181.

Anon. (2004), ‘Firefighter and fire-resistant clothing and fabric’, Textile Journal, 121(3),36–42.

Barberis CP (2006), ‘Fireproof non-woven fabric, method of manufacturing thereof andmattress cover obtained thereby’, EP1780322.

Bourbigot S, Jama C, Le Bras M, Delobel R, Dessaux O and Goudmand P (1999), ‘Newapproach to flame retardancy using plasma assisted surface polymerisation techniques’,Polymer Degradation and Stability, 66(1), 153–155

Bourbigot S and Flambard X (2002), ‘Heat resistance and flammability of high performancefibres: A review’, Fire and Materials, 26, 155–168.

Bourbigot S, Devaux E and Flambard X (2002b), ‘Flammability of polyamide-6/clay hybridnanocomposite textiles’, Polymer Degradation and Stability, 75, 397–402.

Bourbigot S, LeBras M and Troitzsch J (2003), ‘Fundamentals – Introduction’, in TroitzschJ, Flammability Handbook, Munich, Hanser Verlag.

Bourbigot S, LeBras M, Duquesne S and Rochery M (2004), ‘Recent advances for intumes-cent polymers’, Macromolecular Materials and Engineering, 289, 499–511.

Bourbigot S and Duquesne S (2007), ‘Fire retardant polymers: Recent developments andopportunities’, Journal of Materials Chemistry, 17, 2283–2300.

Damant GH (1994), ‘Recent United States developments in tests and materials for theflammability of furnishings’, Journal of the Textile Institute, 85(4), 505–525.

Damant GH (1996), ‘Use of barriers and fire blocking layers to comply with full-scale firetests for furnishings’, Journal of Fire Sciences, 14(1), 3–25.

Davies PJ, Horrocks AR and Alderson A (2005), ‘The sensitisation of thermal decomposition



of ammonium polyphosphate by selected metal ions and their potential for improvedcotton fabric flame retardancy’, Polymer Degradation and Stability, 88, 114–122.

Duquesne S, Drevelle C, Bourbigot S and Delobel R (2006), ‘Influence of the fireproofingmethod on the fire retardant performance of intumescent polypropylene non-woven’,Paper presented at the 17th Annual Conference Recent Advances in Flame Retardancy ofPolymeric Materials, May 21–24, 2006, Stamford, CT.

Einsele U, Koch W and Herlinger H (1984), ‘Investigations into the development of heatwhen textiles burn in air’, Melliand Textilberichte, 65(3), 200–206.

Frechette DR and Bootle J (2003), ‘Intumescent mats in composites: Process, testing andperformance criteria’, International SAMPE Symposium and Exhibition (Proceedings),48I, 937–943.

Gstettner A (2005), ‘Flame resistant and functional seating fabrics’, Textiles à UsagesTechniques, 1(55), 24–25.

Hirschler MM (2008), ‘Polyurethane foam and fire safety’, Polymers for Advanced Tech-nologies, 19(6), 521–552.

Hirschler MM and Piansay T (2007), ‘Survey of small-scale flame spread test results ofmodern fabrics’, Fire and Materials, 31, 373–386.

Hofer H (1999), ‘Environmental Aspects of Flame Retardants in Textiles’, GeschäftsfeldToxikologie – Bereich Lebenswissenschaften report for Austrian Standards InstituteConsumer Council, Report N° OEFZS–L-0057.

Horrocks R and Kandola BK (1997), ‘Novel intumescent applications to textiles’, Journalof Coated Fabrics, 27(7), 17–26.

Horrocks AR, Kandola BK, Davies PJ, Zhang S and Padbury SA (2005), ‘Developments inflame retardant textiles – A review’, Polymer Degradation and Stability, 88, 3–12.

Jimenez M, Duquesne S and Bourbigot S (2009), unpublished results.Kozlowski R, Muzyczek M and Mieleniak B (2004), ‘Upholstery fire barriers based on

natural fibres’, Journal of the Balkan Tribological Association, 10(3), 422–428.Kozlowski R, Mieleniak B, Muzyczek M, Mankowski J, Magnies C, Mesnage P (2007),

‘Flammability of lightweight, flexible insulating nonwoven lade from natural fibrous rawmaterials’, Paper presented at the 18th Annual Conference Recent Advances in FlameRetardancy of Polymeric Materials, May 21–23, 2007, Stamford, CT.

Lefebvre J, Bastin B, Le Bras M, Duquesne S, Ritter C, Paleja R and Poutch F (2004), ‘Flamespread of flexible polyurethane foam: Comprehensive study’, Polymer Testing, 23(3),281–290.

Lewin M (1998), ‘Physical and chemical mechanisms of flame retarding polymers’, inLeBras M, Camino G, Bourbigot S and Delobel R, Fire Retardancy of Polymers: The Useof Intumescence, Cambridge, Royal Society of Chemistry, 3–34.

Magniez C, Dubois A, Vouters M, Delobel R and Poutch F (2003), ‘Behavior of anintumescent system for flame retardant materials coated on polypropylene textiles’,Journal of Industrial Textiles, 32(4), 253–266.

Orito Z, Nakagawa O, Uzawa K, Tokuyama S and Machida M (1973), JP 4800060019730109.

Rawas C (2008), Presentation made at Club de Veille Textil’Aisne, December 17, 2008,France.

Réti C, Casetta M, Duquesne S and Bourbigot S (2009), ‘Intumescent biobased-polylactidefilms to flame retard nonwovens’, Journal of Engineered Fibers and Fabrics, 4(2), 33–39.

Sikkema DJ (2002), ‘Manmade fibers one hundred years: Polymers and polymer design’,Journal of Applied Polymer Science, 83, 484–488.

Solarski S, Mahjoubi F, Ferreira M, Devaux E, Bachelet P, Bourbigot S, Delobel R, Coszach



P, Murariu M, Da Silva Ferreira A, Elexandre M, Degeee P and Dubois P (2007),‘(Plasticized) Polylactide/clay nanocomposite textile: Thermal, mechanical, shrinkageand fire properties’, Journal of Materials Science, 42, 5105–5117.

Tsafack MJ and Levalois-Grützmacher J (2006), ‘Flame retardancy of cotton textiles byplasma-induced graft polymerization (PIGP)’, Surface and Coatings Technology, 201(6),2599–2610.

Tsafack MJ and Levalois-Grützmacher J (2007), ‘Towards multifunctional surfaces usingthe plasma-induced graft-polymerization (PIGP) process: Flame and waterproof cottontextiles’, Surface and Coatings Technology, 201(12), 5789–5795.

USFA (2002), Mattress and Bedding Fires in Residential Structures, US Fire Administra-tion, Topical Fire Research Series, 2(17).

Van Esch GJ (1997), ‘Flame retardants: A general introduction’, Environmental HealthCriteria (192), 1–56.

Vannier A, Duquesne S, Bourbigot S, Delobel R, Magniez C and Vouters M (2006), ‘The useof the plasma induced polymerization technology to develop fire retardant textiles’,International Conference on Textile Coating and Laminating, November 8–129, Barce-lona, Spain.

Vink ETH, Rábago KR, Glassner DA, Springs B, O’Connor RP, Kolstad J and Gruber PR(2004) ‘The sustainability of NatureWorks™ polylactide polymers and Ingeo™ polylactidefibers: An update of the future’, Initiated by the 1st International Conference on Bio-basedPolymers (ICBP 2003), November 12–14 2003, Saitama, Japan, Macromolecular Bio-science, 4, 551–564.

Vink ETH, Glassner DA, Kolstad JJ, Wooley RJ and O’Connor RP (2007), ‘The eco-profilesfor current and near-future NatureWorks® polylactide (PLA) production’, IndustrialBiotechnology, 3, 58–81.

Walters RN, Hackett SM and Lyon RE (2000), ‘Heats of combustion of high temperaturepolymers’, Fire and Materials, 24(5), 245–252.

Weil ED and Levchik SV (2008), ‘Flame retardants in commercial use or development fortextiles’, Journal of Fire Sciences, 26(3), 243–281.

Wolf GL, Sidebotham GW, Lazard JLP and Charchaflieh JG (2004), ‘Laser ignition ofsurgical drape materials in air, 50% oxygen, and 95% oxygen’, Anesthesiology, 100(5),1167–1171.

Zhang S and Horrocks AR (2003), ‘A review of flame retardant polypropylene fibres’,Progress in Polymer Science, 28(11), 1517–1538.

Zhang S, Horrocks AR, Hull R and Kandola BK (2006), ‘Flammability, degradation andstructural characterization of fibre-forming polypropylene containing nanoclay-flameretardant combinations’, Polymer Degradation and Stability, 91, 719–725.

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Applications of Nonwovens in Technical Textiles || Flame retardant nonwovens