Smart flame retardant textile coatings and laminates

Smart flame retardant textile coatings and laminates

9

A.R. Horrocks University of Bolton, Bolton, United Kingdom

9.1

Introduction: general requirements and properties of fire retardant coatings

The established and lamination coating technologies for textiles have been reviewed by Woodruff (2003) although those specifically relevant to flame retardant applications were not highlighted. A more recent review by the author has focused on coatings and surface treatments (Horrocks, 2008) while Alongi, Frache, Malucelli, and Camino (2013) have focused on developments in multicomponent coating techniques. However, most types of coating and lamination processes incorporate flame retardancy if required by the application performance requirements. For example, automotive seatings and linings are often laminated structures comprising at least two different textile substrates, a face and backing fabric, in combination with polymer coatings on the reverse face to create moldability and adherence to the underlying automotive component surface. For European and US markets, the whole composite must pass the internationally accepted US Federal Motor Vehicle Safety Standard FMVSS 302 horizontal burning test in which a clamped 356 mm (14 in.) 3 102 mm (4 in.) specimen not exceeding 12.7 mm (0.5 in.) thickness is horizontally mounted and one edge subjected to a burner flame for 15 s. The specimen passes the test if the burning rate is less than 102 mm/s (4 in./min). While this is only a moderate test of flame retardancy, other coated textiles may have to pass far more stringent test conditions. Examples here are simulated leather, coated textile contract furnishing fabrics, which in the UK must pass the ignition criteria defined in BS7176 in which the specimen, mounted around a filling such as polyurethane foam in both seat and back geometries, is subjected to a wooden crib (BS 5852: 1990: Source 5). After the crib has been ignited, it burns with an energy output equivalent to two burning sheets of newspaper (B300 J). The sample must selfextinguish after the crib has burnt out and no evidence of continuing afterflame or smoulder should occur within the total test period of 10 minutes. While these two examples do not represent extremes, since even higher levels of fire resistance may be required in aerospace and military applications, they do represent a fair spread of flame retardancy levels required in many of the nonspecialist or consumer-oriented application areas. In generating respective levels of flame resistance, coated fabrics may comprise a balance of normal nonflame retardant components (e.g., normal polyester yarns and fabrics) in combination with less Smart Textile Coatings and Laminates. DOI: https://doi.org/10.1016/B978-0-08-102428-7.00010-9 Copyright © 2019 Elsevier Ltd. All rights reserved.

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flammable or inherently flame retardant fibers or fabrics and/or flame retardant coatings. These latter may comprise an inherently flame retardant resin such as poly(vinyl chloride) (PVC) or a flammable one such as an acrylic copolymer or a synthetic rubber in which a flame retardant additive chemical is included. In other words, there is a significant armory of flame retardant fibers, polymers, and additives available in order to ensure that a given coated textile or laminate can achieve a desired level of flame resistance. However, none of these flame retardant products may be designated as “smart” since in each case a formulation has been designed and produced to generate a specified level of performance defined by a specific performance standard. A truly smart or intelligent flame retardant or resistant coating should be able to respond to a variety of fire hazards in a proportionate manner by being able to “switch on” or activate varying levels of resistance. At the present time, no such smart coated or laminated textile structures exist but it is true to say that they are becoming “smarter” in their ability to have increased versatility, be processed using more efficient processes and be generally more environmentally acceptable than hitherto. In fact, it may be argued, that the increasing concerns about the environmental sustainability of a number of currently used flame retardant systems (Horrocks, 2013) are driving the need for the next generations of flame retardant coatings and treatments to be “environmentally smarter”. This chapter reviews such recent developments as well as attempt to point the way to the means of developing truly smart and intelligent coatings and laminations.

9.2

Main types of fire retardant/resistant coatings and laminates

Normally flame retardant coatings are expected to confer a defined level of flame retardancy to the overall coated textile or laminate and their effectiveness are often determined by the flammability of the underlying substrate fibers. In many cases these may be conventional unretarded fibers and blends, e.g., pure cotton, cotton/ polyester, 100% polyester, 100% polyamide, or be already flame retarded fibers and blends, e.g., flame retardant cotton and flame retardant cotton-rich/synthetic fiber blends. However, in the latter case, antagonisms may arise between flame retardant species present in the coating and those present in the underlying fibers/textile and so care must be taken when developing such formulations. The simplest situation is probably one where the coating contains the flame retardant species and the underlying substrate has no such presence. In the case of inherently flame resistant fibers, antagonisms may still exist if the flame retardant property is conferred by an additive or comonomer since the underlying chemistries are similar to those present in the flame retardant present in the coating. Thus, for instance, the phosphorus-containing species present in the inherently flame retardant polyester Trevira CS (Trevira GmbH) is similar to phosphorus-containing additives that could be present in a coating formulation. While it is possible that antagonisms might occur in such a situation,

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it is more likely that both flame retardant systems will function with a degree of additivity. However, if a halogen-based flame retardant is present in either fiber or coating, it is possible that some antagonistic interactions might occur. Anecdotal evidence suggests that some flame retarded PVC coating formulations when applied to cotton flame retarded with some phosphorus-containing agencies are inferior to the same coating applied to unretarded cotton. Unfortunately, flame retardancy is not an exact science and predicting antagonisms and indeed synergisms is challenging; empirical formulation is usually the preferred means of achieving a flame retardant, coated textile, or laminate having a defined performance in terms of both flammability and other required properties (Horrocks, 2003). Typical coating polymers include the following (Woodruff, 2003): G

G

G

G

G

G

G

G

G

Natural and synthetic rubbers of which the latter include polyisobutylene (or “butyl”), styrene butadiene (SBR), poly(butadiene-acrylonitrile) (or “nitrile”), poly(chloroprene) (or neoprene), chlorosulphonated polyethylene, poly(fluorocarbon), and silicone elastomers. PVC plastisols and emulsions are widely used to confer the most cost-effective balance of both water resistance and flame retardancy because of the high polymer chlorine content. However, environmental concerns regarding their high chlorine and plasticizer contents have encouraged their replacement by alternatives, although at a higher cost. Poly(vinyl alcohols) or PVAs have varying degrees of water solubility depending on the degree of saponification of the parent poly(vinyl acetate). They find application in end uses where wash durability is not greatly significant and because of this, flame retardant inclusion not only has to render the very flammable base polymer flame retardant but also possess a similar level of durability. Formaldehyde-based resins, including phenol-, urea-, and melamine-formaldehydes offer a relatively cheap range of durable, coating polymers in which flame retardants may be introduced. The phenolics have the advantage of relatively high inherent flame resistance. Acrylic copolymers (or more simply, “acrylics”) offer a combination of high levels of flexibility and softness as well as some degree of moisture permeability and so find preferred applications in many textile areas where aesthetics are important. Typical applications include curtains and linings, roller and pleated blinds, mattress tickings and bedding to confer dust and microbe impermeability, and textile back-coatings as carriers for flame retardants. It is in this last area where significant interest in flame retardancy has lain since the UK furnishing regulations of 1988 (Consumer Protection Act, 1987). Vinyl acetate copolymers (including crosslinkable varieties) with vinyl chloride and/or ethylene. These are flexible coatings ideal for upholstered furnishings and behave in a manner similar to the acrylics; presence of vinyl chloride will add to the overall flame retardant formulation property. Polyurethanes (or PURs) may be applied to textiles by solvents and more recently by hot melt coating and as direct powders. Addition of flame retardants is effective but has to be selected to suit the application method. Silicones offer water repellency and an inherent flame resistance because of their tendency to promote the formation of a silicaceous char and eventually silica. However, when present on synthetic textiles they may prevent melt dripping and so prevent energy being removed from the burning textile as flaming drips. Thus the silicone-coated textile may appear to be quite flammable relative to the textile and resin alone. Fluorocarbons are typified by poly(tetrafluoroethylene) (PTFE) although others exist such as fluorinated ethylene polymers (or FEPs), poly(vinyl fluoride) (PVF), and poly

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(vinylidene difluoride) (PVDF). All have varying levels of inherent flame resistance which when applied to flame retardant substrates will enhance performance although their presence is often insufficient to fully flame retard a flammable substrate. A number of fusible polymers that may be applied in powder or hot melt form such as low and high density polyethylenes, polyamides (e.g., PA6 and PA66), polyesters, copolyesters, and ethylene-vinyl acetate (or EVA) copolymers. While being inherently flammable, they offer the opportunity of incorporation of flame retardant additives during the resin compounding stages.

Table 9.1 lists these generic polymer types and gives approximate measures of their flammability in terms of limiting oxygen index (LOI) values (Horrocks, Price, & Tunc, 1989) Table 9.1 Limiting oxygen index (LOI) values of typical coating and laminating resins Polymer or resin Natural rubber Synthetic rubbers Polyisobutylene Styrene butadiene Poly(butadiene-acrylonitrile) Poly(chloroprene) Chlorosulphonated polyethylene Poly(fluorocarbon) Silicone elastomers Poly(vinyl chloride) Poly(vinyl alcohols) and poly(vinyl acetate) Formaldehyde resins Phenolic Urea Melamine Acrylic copolymers Polyurethanes Silicones Ethylene-vinyl acetate and related copolymers (emulsions); vinyl chloride presence will increase LOI Poly(fluorocarbons) Poly(tetrafluoroethylene) Fluorinated ethylene polymer Poly(vinyl fluoride) Poly(vinylidene fluoride) Fusible/powders Low density poly(ethylene) High density poly(ethylene) Polyamides Polyesters Ethylene-vinyl copolymers

Acronym or trivial name

LOI, vol% oxygen 19
21

Butyl rubber SBR Nitrile rubber Neoprene

PVC PVA

20
21 19
21 20
22 38
41 26
30 .60 26
39 45
47 19
22

EVA; EVA-VC

21
22 B30 B30 17
18 17
18 .26 .19
20

PTFE FEP PVF PVDF

98 B48 23 44

LDPE HDPE PA6, PA66 PES EVA

17
18 17
18 24
26 20
21 19

Acrylics PURs

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noting that for acceptable levels of flame retardancy LOI . 26
27 vol% is typically required. It should be noted that actual LOI values depend on sample dimensions, polymer processing history, presence of fillers, etc. Notwithstanding this, it is evident that certain polymer coating and laminating matrices will have varying levels of inherent flame retardancy (e.g., PVC and chlorine- and fluorine-containing polymers), although the more commonly used polymers and copolymers are quite flammable and so the presence of flame retardants is necessary to flame retard both the coating matrix polymer and the underlying textile substrate.

9.2.1 Use of additives Most flame retardant additives are selected from the range offered across the flame retardant additive field and the reader should consult standard texts and Horrocks (2003), Horrocks and Price (2001), and Lewin (1990
2010). Selecting those for coating and lamination applications is largely dependent upon the following factors apart from flame retardant efficiency: G

G

G

compatibility with matrix polymer; e.g., the flame retardant should mix and disperse well and even dissolve in the polymer if possible; have minimal effect on coating/lamination processing efficiency; e.g., the additive should be stable during processing and have minimal effect on rheological properties; have minimal effect on overall product properties including aesthetics; e.g., presence of large diameter solid particulate flame retardants will reduce surface lustre and cause unacceptable roughness.

Specific examples presented below are flame retardants with particular benefits to coating and laminating applications.

9.2.1.1 Phosphorus-containing agents Ideally, liquid flame retardants are preferred providing they do not unduly plasticize the polymer film although in many cases some level of plasticization is required to achieve a level of acceptable flexibility in the coated fabric. Table 9.2 lists typical examples of acceptable flame retardants, including the long chain alkyl/aryl-substituted phosphate examples where plasticization is also required. While Table 9.2 concentrates on single chemical entities, many commercial proprietary flame retardants are formulated mixtures or blends of more than one species which are especially achievable when component FRs are liquids. Such blends enable balances of flame retardancy to be achieved while offering acceptable processing and end-product performance. Where coating or resin layer thicknesses are significant, then solid, particulate agents based on ammonium polyphosphate (APP) chemistry may be used. These may be part of an overall intumescent system and so are considered in the next section. On a final point, P-containing agents have efficiencies which depend on the polymer type and chemistry. Since they act primarily in the condensed phase by

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Table 9.2 Selected phosphorus-containing flame retardants for use in coating and lamination Chemical formula/ name

Commercial examples

Comments

Triaryl phosphates

Reofos 35
95; Lanxess (formerly Chemtura) Phosflex 71B; ICL-IP Kronitex CDP; Lanxess (formerly Chemtura) Kronitex TCP; Lanxess (formerly Chemtura) Kronitex TXP; Lanxess (formerly Chemtura); Phosflex 179; ICL-IP Fyrol TPE; ICL-IP Phosflex 390; ICL-IP

Proprietary formulations with 7.6
8.0% P

Cresyl diphenyl phosphate Tricresyl phosphate Trixylyl phosphate

Triethyl phosphate Isodecyl diphenyl phosphate Oligomeric phosphatephosphonate Cyclic organophosphates and phosphonates

Nitrogen-containing polyol phosphate

Fyrol 51; ICL-IP Antiblaze CU; Hangzhou Electrochemical Group Co. Ltd (HEGC) (formerly Rhodia) Pekoflam PES; Archroma (formerly Clariant) Aflammit PE Conc and PCO 900; Thor Exolit OP 920; Clariant

9.1% P 8.4% P 7.8% P

17% P Functions as plasticizer in PVC; 7.9% P Textile back-coatings; 20.5% P Substantive to PES fibers but may be incorporated in most coating resins; B22% P

Nonhalogen FR for lattices with plasticizing effects; 16% P, 9% N

promoting the formation of carbonaceous char, this depends on the polymer matrix structure. Polymers rich in hydroxyl or pendant ester groups favor dehydration and carbonization under the Lewis acid-driven activity of phosphorus-containing species when heated (Lyons, 1970; Weil, 1978). Consequently, in poor char-formers like poly(ethylene) and polyesters, P-containing agents are not very efficient unless part of an intumescent system.

9.2.1.2 Halogen-containing flame retardants Unlike the phosphorus-containing flame retardants, halogen-containing flame retardants are not polymer-specific in that they act primarily in the vapor phase by suppressing the flame chemistry. Bromine-containing agents predominate because not only are they more efficient than similar chlorine-containing species, but also the high atomic weight of bromine ensures that it is present in a high mass fraction within most organobromine compounds. Typically for many polymers

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acceptable levels of flame retardancy are achieved if about 5% (w/w) bromine is present in the final formulation. In flame retardants such as the very commonly used, decabromodiphenyl ether (decaBDE; see Table 9.3) where bromine contents are as high as 83% (w/w), flame retardant presence is often less than 10% (w/w), which is quite low compared with most flame retardant polymers containing other flame retardants. Concerns about the environmental sustainability about brominated flame retardants like decaBDE have been mounting in recent years (Horrocks, 2013) and in the US production of this chemical ceased voluntarily from the end of 2013 and a year later it was declared a Persistent Organic Pollutant or POP by the United National Environmental Programme (Anon, 2014). Its continued commercial use is currently under great pressure across Europe. The alternative flame retardant, decabromodiphenyl ethane or 1,2-bis(pentabromophenyl) ethane is being used as a potentially more environmentally acceptable replacement. However, brominated flame retardants by themselves are not very efficient and require the presence of a synergist such as antimony III oxide (ATO) (Hastie, 1973). For greatest effectiveness the molar ratio Br:Sb 5 3 (reflecting the possible formation of SbBr3 as an intermediate; Wang et al., 2000; Weil, 1978) is commonly Table 9.3 Halogen-containing flame retardants for coatings and laminates Chemical formula/name

Commercial examples

Comments

Dibromostyrene and poly (dibromostyrene) Decabromodiphenyl ether (or decaBDE)

DBS and PDBS-8; Lanxess (formerly Chemtura) DE-83R; Lanxess (formerly Chemtura) FR-1210; ICL-IP Saytex 102E; Albemarle Myflam and Performax; Noveon Saytex 8010, Albemarle

59% Br

1,2-Bis(pentabromophenyl) ethane Hexabromocyclododecane (HBCD)

Tetrabromophthalic anhydride and diol

Tetrabromobisphenol A (TBBA)

Dodecachlorododecahydro dimethanodibenzocyclooctene (C18H12Cl12)

CD-75; Lanxess (formerly Chemtura) FR-1206; ICL-IP Saytex HP-900; Albemarle PHT4 and PHT4-DIOL; Lanxess (formerly Chemtura) Saytex RB-49; Albemarle BA-59P; Lanxess (formerly Chemtura) FR-720; ICL-IP Dechlorane Plus; OxyChem

Principal FR for textile backcoatings; 83% Br

As for decaBDE; 82% Br Competes with decaBDE in textile backcoatings; 73% Br 68% Br 46% Br 68% Br

59% Br

Used in elastomeric coatings (synthetic and silicone); 65% Cl

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Table 9.4 Selected phosphorus- and chlorine-containing flame retardants Chemical formula/name

Commercial examples

Comments

Tris (1,3-dichloroisopropyl) phosphate (TCDP) Oligomeric chloroalkyl phosphate ester Chlorinated phosphate ester Tris (2-chloroisopropyl) phosphate (TCPP)

Fyrol 38 & FR-2; ICL-IP

7.1% P, 49% CI

Fyrol 99; ICL-IP

14% P; 26% CI

Antiblaze 78; Albemarle Fyrol PCF; ICL-IP

12% P; 34% Cl 9.5% P; 32.5% CI

used and this equates in the case of decaBDE to a mass ratio Sb2O3: decaBDE 5 1:2, thereby ensuring that the total flame retardant concentration present in the polymer may be as high as 15% (w/w) or so. Recently, a number of tin compounds including zinc stannate (ZS) and zinc hydroxystannate (ZHS) have been shown to be synergistic with halogen-containing flame retardants, but unlike ATO, bromine-containing FR/ZS or ZHS combinations have to be selected for maximum efficiency (Cusack & Hornsby, 1999). Furthermore, the zinc stannates are nontoxic and more environmentally sustainable than ATO (Horrocks, Smart, Price, & Kandola, 2009), which has been listed by the US National Institute of Environmental Health as a Category 2B carcinogen. Table 9.3 lists the more commonly used monomolecular halogen-containing flame retardants which have applications in textile coatings and laminates. Phosphorus and halogen together in the same molecule often produce additivity and even synergy in terms of flame performance with respect to the contributions of each element present (Weil, 1975). A number of phosphorus and chlorine-containing flame retardants have been developed and commercially used in applications often requiring a degree of flexibility such as foams, films, coatings, and laminates. Table 9.4 presents a selection of these.

9.2.2 Intumescent systems that form a carbonaceous/vitreous protective layer over the polymer matrix For coated and laminated textiles requiring high levels of flame barrier properties then it is more usual to incorporate an intumescent system (Camino & Lomakin, 2001; Le Bras, Camino, Bourbigot, & Delobel, 1998) within the polymer. Such formulations may be intumescent in their own right and generate carbonaceous chars independently of the surrounding polymer matrix or they may interact with the matrix so that the flame retardant polymer together give rise to an expanded, intumescent char when exposed to heat and flame. The majority of these are based on ammonium phosphate (APP) and melamine chemistries and selected examples are presented in Table 9.5. All are particulate solids, of which one or more components may be water soluble, and so for water durability they may only be used in

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Table 9.5 Selected intumescent and intumescent component flame retardants Chemical formula/name

Commercial examples

Comments

Ammonium polyphosphate

Phase I typesa:

Water solubility B4 g/100 cm3

Antiblaze MC; Albemarle Exolit AP 412; Clariant FR CROS 480
485; Budenheim Phase II types: Exolit AP 422; Clariant FR CR0S 484; Budenheim Coated Phase II typesa: Exolit AP 462 FR CROS 486; Budenheim

Melamine phosphates

Other melamine salts BUDIT 314/315

Intumescent blends

a

FR CROS 487; Budenheim FR CROS C30/C40/ C60/C70/ 489; Budenheim BUDIT 310; Budenheim Dimelamine phosphate; Hummel Croton Inc. BUDIT 311; Budenheim BUDIT 312; Budenheim Melapur MP; BASF Melapur 200; MP BUDIT 3141; Budenheim BUDIT 313; Budenheim Melamine cyanurate Melapur MC 15/25/50/ XL; BASF BUDIT 3077 and related products; Budenheim Antiblaze NW; Albemarle

Water solubility B4 g/100 cm3

Microencapsulated version of AP 422; water solubility ,0.5 g/100 cm3 Silane-coated: melamineformaldehyde (MF) coated: water solubility B0.1 g/100 cm3 MF-coated: water solubility B0.1 g/ 100 cm3 Surface-reacted MF, varying particle sizes D50 5 7
18 μm; water solubility ,0.1 g/100 cm3 Dimelamine orthophosphate Dimelamine orthophosphate Dimelamine pyrophosphate Melamine phosphate Melamine phosphate Melamine polyphosphate Melamine polyphosphate Melamine borate

Melamine cyanurate

Melamine phosphate and dipentaerythritol

Phases I and II refer to different levels of molecular weight, crosslinking and hence crystalline characteristics. Phase I APP variants have much lower degrees of polymerization and crosslinking and greater water solubilities.

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hydrophobic polymer matrices which may create dispersion problems during processing. Hence, many commercial particulate examples are coated or microencapsulated either to reduce water solubility and/or to improve polymer matrix compatibility. Furthermore, manufacturers are attempting to reduce particle sizes as shown for APP in particular. While APP is not an intumescent in its own right, it is a powerful char-former when in the presence of oxygen-containing polymers and copolymers. However, to ensure intumescent action, it used in combination with other agents such as pentaerythritol and melamine (Camino & Lomakin, 2001). The melamine phosphates shown in Table 9.5 do have a greater degree of inherent intumescent activity since the acid-forming component phosphate is chemically combined with the gas-forming melamine. They also have superior water insolubilities often ,1 g/100 cm3 before any subsequent coating or microencapsulation. Particle sizes are often less than normal APP samples and may have particle diameter values of D50 , 8 μm. Of all flame retardant coating innovations of the last few years, it is probably true to say that those incorporating intumescent flame retardant agents have been the most commonly reported (Camino & Lomakin, 2001; Horrocks, 1996; Horrocks, 2003; Le Bras et al., 1998). Indeed the recent demand for open flameresistant barrier fabrics in US markets driven by Californian regulations for furnishings (TB 133) and mattresses (TB 129) and federally by the US Consumer Product Safety Commission (CPSC 16 CFR 1633) for mattresses (Consumer Product Safety Commission, 2006) has encouraged the development of intumescent coatings applied to inherently fire resistant fiber-containing fabrics including glass which are exemplified by the Springs Industries-patented products (now Springs Global) (Tolbert, Jaco, Dugan, & Hendrix, 1992) and fabrics from Sandel International Inc. Increasing interest in intumescent formulations has also been stimulated considerably by the desire to replace halogen-containing formulations based on related environmental concerns (Horrocks, 2013).

9.3

Increasing flame retardant coating smartness

At the present time, most flame retardant formulations used in coatings and laminated textiles are based on the need to achieve a defined flammability performance measured in terms of a standard test performance pass/fail or graduated criterion. This will define a maximum level that any given product will be expected to achieve and the flame retardant present cannot in any way be defined as being “smart.” Introducing “smartness” into flame retarded coatings and materials may be undertaken in three ways: G

G

the use of conventional flame retardants in an unusual or smart way in order to create a novel effect; the introduction of a flame retardant property into a coated textile or laminate in a novel or smart way; and

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G

215

the development of a flame retardant formulation that reacts to a fire and/or heat hazard in a measured and proportionate way. In this case, the flame retardant property may be passive in that the maximum level is fixed, but the product may react in a number of different ways to a fire stress below this maximum. Alternatively, the FR property may be reactive in that it responds to a fire hazard in a proportionate and reversible manner. In this latter case, such a formulation would be truly smart in that, for example, it might offer an acceptable level of ignition resistance to low heat flux ignition and heat sources while changing its overall properties very little. However, at higher heat fluxes, the formulation may offer a barrier property which, unlike intumescent treatments via transformation to a voided char, offers protection for only so long as the fire threat is present. If the hazard is removed, the product should then recover and revert to its initial form. It must be emphasized that this is a goal which at the present time has not been achieved (see Section 9.3.3).

9.3.1 Use of conventional flame retardants in a smarter way One of the major issues that has faced the flame retardant industry for the past 20 years or so, has been the desire to remove flame retardant chemicals from use that have been shown to have an unacceptable level of environmental risk (Horrocks, 2013). In summary and as mentioned in Section 9.2.1, all halogen- and, more specifically, bromine-containing flame retardants have come under scrutiny, and while some like penta- and octabromodiphenyl ether have been banned, others like decabromodiphenyl ether (decaBDE) and tetrabromobisphenol A (see Table 9.3) have been subjected to risk assessments and in spite of their being found to be safe (European Union Risk Assessment Draft Report, 2005; European Union Risk Assessment Report For Bis, 2003) remain under considerable pressure to ban or curtail their use. This is especially a challenge for the UK furnishing industry where decaBDE has been a principal flame retardant for many years. In February 2017, the EU Commission proposed a restriction in its use and it will not be manufactured or marketed after 2nd March 2019 (Commission Regulation EU 2017/227, 2017) and this will increase pressure within the UK for its replacement in domestic furnishing fabrics in particular (Consumer Protection Act, 1987). This author has reviewed the challenges posed by the desire to reduce the use of brominated flame retardants in textile back-coatings (Horrocks, Davies, Alderson, & Kandola, 2007; Horrocks, Eivazi, & Kandola, 2016; Wang et al., 2000), which includes previously reported results of a number of initial attempts to replace brominated flame retardants in formulations with varying degrees of success. For instance, gradual replacement of the decaBDE-ATO content in a conventional formulation with a number of bromine-free alternatives including APP, a cyclic oligomeric phosphonate (Amgard CU; Rhodia), alumina trihydrate, and ZHS, shows that when applied as back-coatings to cotton, fabrics may pass a simulated version (Wang et al., 2000) of the small flame ignition source (Source 1) method defined in BS 5852: 1979: Part 1 when total add-ons are 30% (w/w) or less with respect to substrate. Within this test, fabric samples were tested over nonflame retarded polyurethane (PU) foam in order to demonstrate their flame barrier properties. However, while all nonbromine-containing formulations examined pass if present

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as 100% replacements for the decaBDE-based component, in formulations containing both brominated and nonbrominated retardants, only those containing either APP or the cyclic oligomeric agent passed at commercially acceptable add-on levels. In the latter case, however, because it is a liquid, the back-coating formulation was plasticized to the extent of yielding a tacky and unacceptable handle when present at greater than 50% (w/w) with respect to the original decaBDE-ATO component concentration. The UK regulations (Consumer Protection Act, 1987) require that flame retarded upholstered fabrics pass the small flame, simulated match test after a water durability test as defined in BS 5651. After application of the 30 minutes 40 C water soak, however, both formulations failed to pass the small flame ignition source requirement. It is worth noting that the revised BS 5852: 2006 includes the automatic requirement to submit all samples to be tested to Source 1 after this prior 40 C water soak. In a later paper (Horrocks, Wang, Hall, Sunmonu, & Pearson, 2000), the performance of selected, less soluble (including some) intumescent phosphorus-based flame retardants was studied in which a number were applied as standard back-coating formulations to 100% cotton and 35% cotton
65% polyester blend fabrics having typical area densities for furnishing fabrics. Only APP-based formulations could yield passes to the simulated Source 1, BS 5852 test carried out over nonflame retardant PU foam on both cotton and cotton-polyester substrates. This appeared to be associated with their relatively low temperatures of thermal decomposition behavior as determined by thermogravimetric analysis. However, the associated poor water durability caused all APP-containing samples to fail after the required 40 C water soak treatment. A major conclusion from these results was that any phosphorus-based candidate for replacing conventional bromine-containing, back-coating formulations would have to decompose and preferably transform to a liquefied state at temperatures well below the ignition temperature of the most flammable fibers present in the supporting fabric. This would enable the now-fluid flame retardant to wet substrate fiber surfaces and diffuse from the back-coating through the fabric and to the front face and prevent ignition by the igniting flame. In the case of cotton fibers which ignite at about 350 C, this would require the flame retardant component to decompose at about 300 C or less, a condition shown only by APP. To the authors’ knowledge, no single phosphorus-based flame retardant fulfils both the low decomposition temperature and high water insolubility criteria at the present time. Thus in developing a phosphorus flame retardant strategy for the replacement of decaBDE and similar bromine-based formulations, it is evident that the vapor-phase activity of the latter is a key factor in determining their efficiency apart from their excellent insolubility and general intractability. Notwithstanding these prime issues, the outcomes of our previous research (Horrocks et al., 2000; Wang et al., 2000) led to three further strategies that have been proposed to achieve these requirements: 1. the sensitization of decomposition or flame retarding efficiency of phosphorus-based systems (Davies, Horrocks, & Alderson, 2005); 2. the reduction in solubility of successful but soluble systems; and 3. the introduction of a volatile and possible vapor phase-active, phosphorus-based flame retardant component (Horrocks et al., 2007).

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With regard to the first, we have demonstrated that the inclusion of small amounts of certain transition metal salts, notably those of zinc II and manganese II can reduce the onset of decomposition of APP from 304 C to as low as 283 C in the case of 2% (w/w) manganese II sulfate addition (Davies et al., 2005). When applied in a back-coating formulation with APP, the presence of metal ions increases LOI values slightly (of the order of 1
1.5 LOI units for manganese and zinc salts) from 25.1 for APP-only coated cotton, to 26.6 vol% in the presence of 2% (w/w) manganese acetate. However, all coated fabrics still failed the simulated small flame ignition Source 1 version of BS 5852, which is not perhaps surprising since our earlier research indicated that an LOI value for a coated cotton fabric above 26 and closer to 29 vol% was required for a pass. It should be pointed out, however, that even if passes had been obtained, the problem of durability to water soaking would still remain. With this in mind, Bourbigot and coworkers have shown that microencapsulation of otherwise soluble flame retardants like APP with polyurethane shells can improve the durability of coatings containing them (Giraud et al., 2005). However, the preparation of these microencapsulated agents is not an easy process with different techniques being developed in order to improve yields (Saihi, Vroman, Giraud, & Bourbigot, 2005; Saihi, Vroman, Giraud, & Bourbigot, 2006).

9.3.1.1 Role of nanoparticles An extension of the first strategy above is, perhaps, the possible sensitization using nanoparticulate additives. The inclusion of nanoparticles in coating formulations has been investigated by Bourbigot, Devaux, Rochery, and Flambard (2002) and Devaux, Rochery, and Bourbigot (2002). Both nanoclay and polyhedral oligomeric silsesquioxanes (POSS) when present alone in polyurethane coatings applied to polyester and cotton fabrics were found to reduce peak heat release values of backcoated fabrics determined by cone calorimetry, although neither increase ignition times not reduce extinction times. In fact the converse tends to be the case. Subsequent work by Horrocks, Davies, Alderson, and Kandola (2007) has shown that the introduction of nanoparticulate clays by themselves has no beneficial effect to a back-coating polymeric film in terms of enhancing its thermal resistance and the introduction of fumed silica to a flame retarded back-coating formulation reduced its effectiveness when a conventional flame retardant such as APP is present. In other polymers, the incorporation of nanoparticles along with conventional flame retardants indicates that possible synergies may occur, enabling considerably lower amounts of the latter to be used to provide the same level of flame retardant performance. This has been demonstrated by Bourbigot and Duquesne for selected intumescent-montmorillonite clay and intumescent-nanosilica polypropylene/polyamide blend formulations such as EVA-APP/PA6-clay and EVA-APP/PA6-nanosilica (Bourbigot & Duquesne, 2007). We have also demonstrated that the inclusion of nanoclays and APP and similar flame retardants in polyamide 6 and 6.6 films promote synergistic interactions and the opportunity to reduce APP levels by up to

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50% while maintaining an acceptable performance (measured as LOI) (Horrocks, Kandola, & Padbury, 2003). However, to the author’s knowledge there appears to be no published evidence of this potential synergy having been introduced into commercial flame retardant coating formulations for textiles and laminates.

9.3.1.2 Introduction of volatile phosphorus-containing species In addressing the need for such vapor phase activity (strategy (3) above), it must be remembered that the generally poor performance of all APP-containing coated fabrics is associated with our earlier observation that enhanced char formation alone at the rear of a fabric is insufficient to prevent burning of the front face (Horrocks et al., 2000). The obvious way of overcoming this situation is to introduce phosphorus-containing species that may be rendered volatile and so enter the flame chemical reactions in a manner similar to bromine-containing retardants. However, if this were to be effected, there remains the question of the flame retarding efficiency of such volatile species and here the literature is not very helpful, with usually only indirect evidence of vapor phase activity being cited. For instance Rohringer, Stensby, and Adler (1975) proposed that the relatively superior flame retarding efficiency of flame retardants based on tetrakis(hydroxymethyl)phosphonium chloride (THPC) applied to polyester-cotton blends may be associated with the evolution of volatile phosphine oxides, which then act in the vapor phase and retard the burning polyester component. Day, Ho, Suprunchuk, and Wiles (1982) have also provided evidence that the flame retarding efficiency of now-banned tris (2,3-dibromopropyl) phosphate or “tris,” when applied to polyester, is also a consequence of vapor phase activity of phosphorus species. Hastie and Bonnell (1980) used spectroscopic and high pressure sampling mass spectrometry to study possible flame inhibition effects of a number of phosphorus-containing compounds including trimethyl phosphate, phosphoryl chloride, and triphenylphosphine oxide. When mixed with methane and propane fuels, flame inhibition was noted in diffusion flames burning in air, although in premixed flames (with air), some P-containing additives could increase flame strength. These same experiments suggested that previous considerations that the PO radical was the predominant species in flames would have to be revisited since now it appeared that the HPO2 radical was more significant. It was then suggested that this then interacts with H and OH radicals in manner similar to halogen radicals, thus interfering with the main flame propagation reactions as follows: G

G

G

G

HPO2 1 H ! PO 1 H2 O HPO2 1 H ! PO2 1 H2 HPO2 1 OH ! PO2 1 H2 O Work by Babushok and Tsang (2000) concerning the inhibition of alkane combustion in premixed flames suggests that in the vapor phase, phosphorus may be

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more effective than halogen. In accordance with these findings, our recent work (Horrocks et al., 2007) initially considered four potentially volatile phosphorus flame retardants selected from their reported boiling or decomposition data and these were the monomeric cyclic phosphate Antiblaze CU (mass loss occurs above 197 C), tributyl phosphate (TBP) (melting point (MP) 5 280 C, boiling point (BP) 5 289 C with decomposition), triphenyl phosphate (TPP) (MP 5 48
52 C, BP 5 244 C at 10 mm Hg, 5% weight loss at 208 C), and triphenylphosphine oxide (TPPO) (MP 5 156
158 C), this last being one studied by Hastie and Bonnell (1980). Because interest lay in generating new back-coatings for both polypropylene and cotton fabrics, thermogravimetric studies suggested that TBP would be most suitable because it starts to lose mass (i.e., produces volatiles) at about 150 C, well below the melting temperature of polypropylene (B165 C) and the ignition temperature of cotton (B350 C), although tackiness was anticipated to be a problem. TPP was also selected as the next most volatile agent with volatilization starting at about 200 C. Each was combined with an intumescent char-forming agent, the former, now obsolete, Great Lakes NH 1197 (Chemtura) comprising phosphorylated pentaerythritol (Horrocks et al., 2000) (see also Table 9.5) in formulations that maintained constant overall flame retardant contents, although in varying volatile:nonvolatile phosphorus ratios. These formulations when applied to cotton, generated passes to the simulated Source 1, BS 5852 test (Wang et al., 2000), although poor water soak durability was a problem. Further evidence of volatile phosphorus activity was gained by determining the retention of phosphorus in charred samples comprising the following back-coating flame retardants: G

G

G

G

ammonium polyphosphate (Antiblaze MCM, Albemarle), melamine phosphate (Antiblaze NH, Albemarle), cyclic phosphonate (Amgard CU, Rhodia), oligomeric phosphate-phosphonate (Fyrol 51, ICL),

where the liquid Amgard CU and Fyrol CU species (see Table 9.2) were selected as potentially vapor-phase active flame retardants. In order to produce chars having different thermal histories, back-coated samples of known weight were then placed in a furnace at 300, 400, and 500 and 600 C for 5 minutes in an air atmosphere, which were then analyzed for residual phosphorus. Table 9.6 summarizes the results Table 9.6 Back-coated fabric LOI values and loss of phosphorus (ΔP) from chars (Horrocks et al., 2007) Flame retardant/dry parts by weight

Initial add-on (%)

LOI (vol%)

ΔP (%) 300 C

400 C

500 C

600 C

Antiblaze MCM/250 Antiblaze NH/250 Amgard CU/250 Fyrol 51/250

13.9 11.0 11.9 16.6

23.2 20.8 26.3 26.1

0.41
0.16 1.91 1.62

1.35
0.24 4.77 2.78

4.93 0.9 10.51 7.64

3.07 1.87 23.95 7.59

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of char phosphorus content analyses expressed as ΔP%, the respective phosphorus loss from each char, where ΔP equals the theoretical phosphorus content assuming 100% retention in the char minus the experimental value (Horrocks et al., 2007). It is evident that phosphorus loss is lowest for the fabrics containing the charpromoting Antiblaze MCM (APP) and NH (melamine phosphate) retardants and highest for the Amgard CU and Fyrol 51 liquid components. These two also exhibit the highest coated fabric LOI values suggesting that not only is the phosphorus present volatile but, when released into the flame, it reduces flammability.

9.3.2 Novel or smart ways of introducing flame retardant coatings to textiles and laminates 9.3.2.1 Coating levels versus nanotechnological challenges Before considering potential for and present evidence of novel means of introducing flame retardant coatings, it is pertinent to address a number of factors which will influence their success or failure. Unlike some surface treatments such as those that confer extreme hydrophobicity and/or soiling resistance and which may be effective if only a single molecular layer of an appropriate chemical species is deposited on fiber surfaces, there needs to be a critical minimal presence of a flame retardant treatment if it is to be effective. For a typical retardant to prevent ignition of a typical fiber-containing textile, concentrations of between 5% and 20% (w/w) with respect to the textile are usual. This is because for a phosphorus-containing flame retardant, phosphorus levels typically between 1% and 3% (w/w) are required. Most commercial flame retardants contain only 8%
20% (w/w) P (see Table 9.2) which results in actual chemical levels present on the fabric ranging from as low as 5% up to as high as 25% (w/w). With brominated flame retardants in which bromine contents are much higher (see Table 9.3), the levels of flame retardant may be less but once the additional ATO synergist is taken into account, total flame retardant concentrations approach similarly high levels as discussed in Section 9.2.1. In the particular case of coated textiles, the flame retardant present must not only act on the textile fibers present, but also on the coating resin, which, unless it has an inherent flame retardant property (see Table 9.1), will be similarly flammable. Thus levels of flame retardant present in many coatings are often higher than is necessary to be effective on the fabric alone. Once the level of effective flame retardant has been established to achieve the required flame retardancy, the application requirement may also influence the final concentration and physical form required. In free-standing textiles such as curtains, linings, and drapes, ignition resistance and self-extinction time are the major flame retardant requirements. However, if the flame retardant textile must act as a barrier to an underlying surface, such as the filling in upholstered furnishings or as an underlying clothing layer in protective clothing, then the flame retardant treatment should maintain or even enhance the insulative property of the outer fabric layer, usually by char promotion. For char-forming fibers such as the cellulosics and wool, this is quite easily achieved and in back-coated furnishing fabrics made from

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these fibers, total dry coating levels of 20%
30% (w/w) of a formulation containing about two-thirds by weight of flame retardant and one-third by weight of resin are typical (Horrocks, 2003). However, if the fibers are thermoplastic and possibly fusible, that is, polyester, polyamide, and polypropylene, then the coating formulation must be char-promoting in its own right and be able to support the melting/ shrinking substrate fibers in order to maintain an effective flame barrier. This is why back-coatings for polyester and polypropylene furnishing fabrics, for example, are applied at levels typically in the 50%
100% (w/w) range. Thus any novel or smart means of applying flame retardant coatings must be able to achieve high enough levels of application to confer acceptable levels of flame retardancy. A number of reviews (Alongi, Carosio, & Malucelli, 2013; Horrocks, 2017; Hyde & Hinestroza, 2007; Stegmaier et al., 2007) highlight the possibilities of conferring films and coatings at nanodimensions on to fiber and textile surfaces in order to achieve high level novel effects such as hydrophobicity, soil release, self-cleaning, bioactivity, etc. Methods cited include: G

G

G

G

G

nanoparticle adsorption/deposition (Alongi, Carosio, et al., 2013; Stegmaier et al., 2007); self-assembly of nanolayer films (Alongi, Frache, et al., 2013; Alongi, Carosio, et al., 2013; Horrocks, 2017; Hyde & Hinestroza, 2007; Stegmaier et al., 2007); sol
gel, silica-based coatings (Alongi, Carosio, et al., 2013; Horrocks, 2017); surface grafting of polymer nanofilms (Alongi, Frache, et al., 2013; Horrocks, 2017; Luzinov, 2007); and synthesis of smart switchable hybrid polymer nanolayers (Minko & Motornov, 2007).

Of these, the first four methods will be considered in greater detail below. However, there does remain the challenge for a successful, smart method to achieve the loadings typically required of flame and heat resistant coatings present on textiles. The possibility does exist of reducing coating thickness while retaining overall constant levels, if the coating, instead of being applied on textile surfaces, is applied to all component fiber surfaces within the upper surface fabric layers. In the case of the application of fluorocarbons at about 0.6% (w/w) to a typical polyester fiber of 10 dtex (B30 μm diameter), the surface layer thickness is calculated to be above 50 nm (Stegmaier et al., 2007). At microfiber dimensions (,10 μm diameter), the surface layer thickness on the increased fiber surface area reduces to about 10 nm and at submicrofiber dimensions, even thinner films are theoretically possible. However, flame retardant coatings will be required to be present at 10
20 times these fluorocarbon concentrations yielding much thicker theoretical film thicknesses as well as problems associated with interfiber adhesion and occlusion of fiber interstices.

9.3.2.2 Nanoparticle adsorption/deposition Notwithstanding the above discussion, there are the possibilities of gaining some degree of heat and fire protection using coatings or films applied at the nanolevel if they are not seen to be simple replacements for conventional flame retardant coatings. Coated textiles and laminates are physically quite thin materials when compared with more conventional ones such as bulk polymers. In fire science terms, they are also

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more often to be defined as thermally thin materials (Drysdale, 1999) in which the temperature of the surface is assumed to equal the temperature of the interior during heat exposure. In thermally thick materials, such as bulk polymer extrusions and moldings, a thermal gradient exists in a fire-exposed sample with the surface temperature being close to that of the igniting source and the interior much cooler. This enables the material surface to offer insulative properties to underlying structures and, if the heated surface forms an intumescent char, the overall insulative character of the whole sample may actually increase. In normal flame retardant textiles and coated fabrics and laminates, unless they are quite thick ( . 3
5 mm), the ability to form a thick, surface insulating char is limited and the underlying fibers soon reach temperatures approaching that of the igniting source ( . 500 C) when they degrade and may ignite. Even the most inherently flame resistant fibers such as the poly(meta- and para-aramids), poly(benzimidazole), semicarbons, etc. (Horrocks, Eichhorn, Schwaenke, Saville, & Thomas, 2001) are only able to offer a thermal barrier during sustained high heat exposures for limited periods. If, however, we are able to convert a thermally thin textile into one showing socalled thermally thick behavior, its overall fire protective character will increase and many conventional coatings, especially those comprising intumescent additives, attempt to do this. It is highly unlikely that nanocoatings could promote a similar effect unless they could offer a heat shield property of unusual efficiency. However, it has been proposed by a number of workers including ourselves that nanocoatings of inorganic species such as montmorillonite clays may form nanoceramic thermal barriers when deposited on the surfaces of textile fibers (Alongi, Carosio, et al., 2013; Horrocks, Kandola, Nazare´, & Price, 2011; Horrocks, 2005). These will be discussed in further detail below because such studies have been associated with concurrent plasma treatments in order to maximize fiber-particulate physicochemical interaction. In the area of heat protective textiles (Horrocks, 2005), use is made of the deposition of reflective metal films on to fabric surfaces to reduce the effects of heat radiation from a fire source and it is in this area that nanofilm and nanocoating deposition may have opportunities.

9.3.2.3 Self-assembly of nanolayer films: layer-by-layer treatments Layer-by-layer (LbL) surface treatments have a history stretching back about 15 years or so which has been reviewed by Alongi, Carosio, et al. (2013) and more recently by the author (Horrocks, 2017). This is a nanoparticle deposition process, which can be repeated multiple times using different reagents at each adsorption step to enable large numbers of bilayers (BLs) to be deposited on surfaces including textile fabrics. Multilayer film formation requires the alternate immersion of the substrate into an oppositely charged polyelectrolyte solution (or nanoparticle dispersion). This leads to a total surface charge reversal after each immersion step to create a structure of alternatively positively and negatively charged layers piled up on the substrate surface as represented schematically in Fig. 9.1.

Wash (e.g., in deionized water)

Immerse in negatively charged particulate dispersion

Single bilayer (BL) cycle

Fabric

Wash (e.g., in deionized water)

Figure 9.1 Schematic diagram of the cyclical layer-by-layer assembly process.

Immerse in positively charged particulate dispersion

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Alongi, Carosio, et al. (2013) have defined two types of LbL treatments that may give rise to significantly thick coatings following the previously discussed requirement for the relatively high concentrations for effective flame retardation. 1. LbL inorganic nanocoatings and 2. LbL hybrid organic
inorganic or intumescent nanocoatings.

Both Grunlan and co-workers (Schulz et al., 2010) and other researchers cited by Alongi, Carosio, et al. (2013) reported the deposition on cotton fabric surface nanoparticulates such as a lamellar clay (e.g., laponite as the negatively charged counterpart) coupled with a branched polyethylenimine (the positively charged counterpart), colloidal silica and a POSS. Vertical fabric flame testing (ASTM D6413) showed that 10 BLs did not significantly improve the flame-retardancy properties of cotton. When applied to polyester fabrics, LbL deposition of nanoparticles such as alumina-coated, silica nanoparticles or α-zirconium phosphate nanoplatelets with different counterparts (i.e., poly(diallyldimethylammonium chloride)) showed that melt dripping could be reduced significantly (Carosio, Alongi, & Malucelli, 2011). However, to date LbL deposition of inorganic nanoparticles has not produced self-extinction of fabrics subjected to vertical strip testing. Char-forming or even intumescent-like features have been introduced on cottonrich polyester blends (70/30) comprising 5 BLs and 10 BLs of APP and chitosan and of APP and silica (Carosio & Alongi, 2015). The main challenges posed are those of achieving vertical strip self-extinguishment after an acceptable durability treatment and while Carosio and Alongi (2015) have achieved some level of wash durability in water at 65 C for 1 hour following deposition of three bilayers of APP (doped with an acrylic polyurethane latex) and chitosan on polyester-cotton followed by UV exposure, no evidence of self-extinction was presented. Greater success in this respect has been achieved with LbL hybrid organic
inorganic depositions (type 2 above). Of all the recently published work, Mateos, Cain, and Grunlan (2014) have been particularly successful in achieving selfextinguishability after 30 BL of a chitosan (CH)/poly(sodium phosphate) (PSP), polycationic/polyanionic system applied in a semiautomated process, although the problem of lack of durability remained. However, development of a “one pot” branched poly (ethylenimine) (PEI)/PSP polyelectrolyte system, which after coating cotton at pH 7 and then reducing it to pH 2, resulted in a finished surface deposit that yielded selfextinguishability in a single deposition step (Cain, Murray, Holder, Nolen, & Grunlan, 2014). Subsequent work with a poly(allylamine)/PSP polyelectrolyte system has compared a 30 BL application with a “one pot” method on polyester-cotton to achieve both self-extinguishability and five domestic wash (at 30 C) durability (Haile et al., 2016). In this work the “one pot” treatment generated self-extinguishability at 17.9% add-on and retained this level of flame retardancy after five washes to AATCC 135 at 30 C and also after 8 hours in boiling water. The authors claim this to be the first flame retardant polyelectrolyte coating to show such impressive wash-durability on fabric, a consequence of the insolubility of the polyelectrolyte complex, even at high solution pH. This process is currently being assessed for its commercial potential. No indication of the actual thickness of the surface coating was reported, however.

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Hydrolysis: (RO)4 Si

+ H 2O

(RO) 3 Si OH

(RO)3 Si OH + H 2O

(RO) 2 Si (OH)2

+

ROH +

ROH

Aqueous Condensation: (RO)3 Si OH

+

OH Si (RO) 3

(RO)3 Si O Si (RO)3

+ H 2O

Alcoholic Condensation: (RO)3 Si OH

+

(RO) 4 Si

(RO)3 Si O Si (RO)3

+

ROH

Polycondensation: x ((RO) 3 Si OH) + y ((RO) 4 Si) + cross-links

z ((RO) 2 Si (OH)2)

linear chains of - Si (RO)2 O – with - Si (RO) O O

where R = CH 3, C2H5, etc.

Figure 9.2 A schematic representation of the basic sol
gel chemistry (Alongi, Carosio, et al., 2013).

9.3.2.4 Sol
gel, silica-based coatings Research within this area has been recently reviewed (Alongi, Carosio, et al., 2013) with regard to two-step reactions (hydrolysis and condensation) starting from alkyl silicates to yield silica-based architectures on the surfaces of viscose, cotton, polyester, and cotton/polyester blends. This work showed that the additional presence of phosphorus-containing species is required if moderate levels of flame retardancy are to be achieved, although self-extinguishability during vertical strip testing has rarely been achieved and durability to water soaking is poor (Horrocks, 2017; Salmeia, Gaan, & Malucelli, 2016). The main issue with regard to the poor durability is that the formation of sol
gel, silica-based coatings is the generation of hydrolytically sensitive 3dimensional silica networks deposited on the fiber/fabric surfaces. The relevant chemical reactions are shown schematically in Fig. 9.2 (Alongi, Carosio, et al., 2013).

9.3.2.5 Surface grafting of polymer nanofilms: applications of plasma technology Plasma technology offers a means of developing novel nanocoatings having the desired thermal and flame shielding effects, although the literature is sparse with regard to reported examples. Prior to about 2005, most plasma research studies were undertaken with vacuum plasma which would be wholly unsuitable for scaling up to workable, textile processing system. Examples of such vacuum systems which have developed flame retardant properties on underlying textile substrates include the following significant research results.

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1. Shi has demonstrated that low pressure, radio frequency discharge plasma treatment of a number of polymer surfaces including poly(ethylene terephthalate) in the presence of gaseous (CF4/CH4) leads to flame retardation (Shi, 1999). Later studies in which EVA copolymers were plasma-exposed for times up to 15 minutes followed by immersion into acrylamide, gave very high yields of surface grafted poly(acrylamide) and LOI values approaching 24 vol% at 47% (w/w) grafting levels (Shi, 2000). 2. Tsafack and coworkers (Tsafack and Levalois-Gru¨tzmacher, 2006; Tsafack, Hochart, & Levalois-Gru¨tzmacher, 2004) reported the successful grafting of phosphorus-containing acrylate monomers (diethyl(acryloyloxyethyl)phosphate (DEAEP), diethyl-2-(methacryloyloxyethyl)phosphate (DEMEP), diethyl(acryloyloxymethyl) phosphonate (DEAMP), and dimethyl(acryloyloxymethyl)phosphonate (DMAMP)) to polyacrylonitrile (PAN) fabrics (290
300 g/m2). In the presence of a grafting agent, ethyleneglycoldiacrylate (EGDA), graft yields were optimized (as high as 28% (w/w)) resulting in LOI values as high as 26.5 vol%, although after accelerated laundering this reduced to about 21 vol%. Fabric samples were first immersed in a solution of monomer in ethanol followed by plasma exposure. This and Shi’s techniques would not be expected to provide nanofilms since this type of grafting may be perhaps best considered as a variation of established polymer surface and textile-grafting procedures (Bhattacharya and Misra, 2004) and the high yields (28% (w/w) in the case of grafted DMAMP, CH2 5 CH  CO  O  CH2  P(CH3)2) would explain both the level of flame retardancy and poor launderability achieved. When extended to cotton (120 and 210 g/m2), low pressure argon plasma graft polymerization of these same acrylate monomers (Tsafack and Levalois-Gru¨tzmacher, 2006), again yielded grafted fabrics having elevated LOI values as high as 26.0 vol% in the case of DMAMP. However, even higher and more acceptable levels of flame retardancy were achieved only if synergistic nitrogen was also present in grafts which they demonstrated following the grafting of the phosphoramidate monomers, diethyl(acryloyloxyethyl)phosphoramidate (DEAEPN), and acryloyloxy1,3-bis(diethylphosphoramidate)propan (Bis-DEAEPN). These yielded LOI values of 28.5 and 29.5 vol% respectively at grafting levels of 38.6 (53.36%P) and 29.7 (53.29%P) % (w/w). Launderability was improved when the crosslinking agent, EGDA, was present at high concentration and, in the case of BisDEAEPN, after a simulated laundering, graft level reduced to 26.7% (w/w) and LOI reduced to 25.0 vol%. The improved durability achieved here is probably associated with the greater reactivity of the plasma-activated cellulose chains compared with those generated on PAN fiber surfaces. In this work, while the challenge of achieving the high flame retardant agent levels has been reached, no grafted film thicknesses were reported, although they are probably within the micron range and not the nanometer range. 3. Marosi and coworkers (Ravadits, To´th, Marosi, Ma´rton, & Sze´p, 2001) have used plasma treatment of a polyethylene substrate surface-treated by vinyltriethoxysilane and by organoboroxo-siloxane (OBSi), and an OBSi-containing intumescent flame-retarded compound (IFR-OBSi) based on polypropylene, AAP, and pentaerythritol in attempts to improve the oxygen barrier properties of the intumescent coating. Plasma treatment did in fact reduce the oxygen permeability of the coating by one order of magnitude, although the effects that this has on the fire barrier properties are not reported. 4. Jama et al. (2001) reported the possibility that plasma deposition of silicon-based films might improve the flame retardancy of underlying polymer surfaces. Here normal and nanocomposite polyamide 6 films were activated by a cold nitrogen plasma and then transferred to a reactor containing 1,1,3,3-tetramethyldisiloxane (TMDS) vapor in an oxygen carrier gas for 20 minutes. This remote plasma-assisted polymerization is similar to that used by Tsafack et al. above except that the monomer is in the vapor phase prior

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to polymeric deposition. The oxygenated polysiloxane coating transformed to a silicabased structure at about 800 C creating a thermal barrier effect which gave a moderate increase in flame retardancy of a coated polyamide 6 film and a surprising increase in the flame resistance of nanocomposite polyamide 6 films, as determined by LOI with LOI values exceeding 45 vol% for the latter. A subsequent paper (Que´de´ et al., 2005) demonstrated that on scaling up the experiments using a larger low pressure plasma source and reactor, thereby enabling larger and more consistently coated samples to be produced. Of particular interest to the present discussion is that the film thicknesses obtained in the earlier and smaller reactor were about 48 μm, whereas those from the larger reactor reduced to only 1.5 μm and yet coated nanocomposite polyamide 6 films continued to yield LOI values as high as 48 vol%. Furthermore if film thickness was increased above 1.5 μm, LOI reduced to a constant value of about 42 vol%. The residues after cone calorimetric exposure showed that the coated nanocomposite film transformed to a silica-like structure and it is this that creates the thermal shielding effect.

However, of greater relevance to this chapter is the potential for atmospheric plasma treatments which have been available commercially since about 2000 and which lends itself to the continuous processing requirements of the textile industry (Herbert, 2007). Very recent work in our own laboratories has led to a patented process (Horrocks, Kandola, Nazare´, & Price, 2009; Horrocks et al., 2011) in which using atmospheric plasma we demonstrated that the flash fire resistance of a conventionally flame retarded fabric was improved by surface treatment in the presence of a nanoparticulate, functionalized montmorillonite clay (Cloisite Na1 functionalized with vinyl triphenyl phosphonium bromide) and a silicon-containing monomer, hexamethyldisiloxane (HMDSO). Table 9.7 shows the changes in cone calorimetric behavior of a 200 g/m2 woven meta-aramid fabric subjected to an argon plasma alone and the plasma in the presence of a silicon-containing monomer, a nanoclay alone and a combination of silicon-containing monomer and nanoclay. The fabric alone failed to ignite when exposed at the more typical heat flux of 50 kW/m2 but did ignite when exposed at 60 kW/m2. Flash fire testing is usually associated with heat fluxes of 80 kW/m2 (NFPA 2112, 2007) or more and this level was not achievable by our equipment. The results show that even after argon plasma treatment alone, slight increases in both the time-to-ignite (TTI) and time-to-peak (TTP) heat release were observed with a similarly slight reduction in PHRR. The loss of mass is associated with the effect of surface ablation following plasma treatment. When any of the combinations of silicon-containing monomer and nanoclay were introduced, the fabric became nonignitable under a 60 kW/m2 heat flux. The effect of additional components gave rise to either reduced mass loss or an actual increase in sample mass as would be expected if a surface layer were being deposited. Clearly the already high heat flux ignition resistance of the metaaramid fabric was significantly improved following plasma treatment. Increasing the incident heat flux to 70 kW/m2 caused untreated and treated fabrics to ignite but the presence of the plasma treatment reduced PHRR values both before and after a simulated laundering (see Fig. 9.3). The PHRR values reduced from 119 to 113 kW/m2 for HMDSO only, 109 kW/m2 for clay only, and 99 kW/m2 for HMDSO/clay samples immediately following plasma treatment. After simulated

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Table 9.7 The cone calorimetric behavior of m-aramid-containing fibers exposed to 60 kW/m2 heat flux after subjecting them to various atmospheric plasma treatments (Horrocks et al., 2011) Sample and treatment

Mass change (%)

Time-toignition, TTI (s)

Time-topeak heat release, TTP (s)

Peak heat release rate, PHRR (kW/m2)

Meta-aramid alone Argon plasma only Argon plasma with siliconcontaining monomer Argon plasma with nanoclay Argon plasma with siliconcontaining monomer and nanoclay



2.8
0.6

13 16 NIa

16 20


83 73


1.6

NIa







3.5

NIa







a

Sample did not ignite.

Nomex samples (unwashed)

Nomex samples (washed)

Peak heat release rate (kW/m2)

140 120 100 80 60 40 20 0 Nomex

Ar-Nomex

Ar HMDSONomex

Ar ClayArNomex

Ar-Clay-Ar HMDSONomex

Figure 9.3 Peak heat release values for Nomex fabrics surface-treated with combinations of HMDSO and clay and subjected to a cold argon flame atmospheric plasma before and after a simulated washing (Horrocks et al., 2011).

washing, PHRR values surprisingly show further slight reductions. Thus, the plasma treatment and its effects achieved simulated wash durability, indicating robust physicochemical bonding between activated fiber surfaces and the clay/polysiloxane coating.

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The advantage of this method is that in principle it may be applied to any textile substrate retrospectively and so offers great opportunity for enhancing the heat and fire resistance of a range of textile substrates. More recent work from Tata, Alongi, and Frache (2012) showed that polyester fabrics could be etched initially by cold oxygen plasma and then finished with hydrotalcite, nanometric titania, and silica aqueous suspensions. Immersion time was a fundamental variable in order to yield the best fire performance and only hydrotalcite-containing treatments promoted consistent increases in TTI, and hence, showed improved fire performance levels, even after washing in demineralized water at 30 C for 30 minutes. A subsequent study (Carosio, Alongi, & Frache, 2011) used plasma surface activation combined with nanomontmorillonite deposition to influence the thermal stability of fabrics in air. Cone calorimetry revealed the best sample had a remarkable improvement in terms of TTI (up to 104%) and a slight reduction in the PHRR (ca. 10%) compared to untreated polyester (PET) fabric. No data were presented regarding wash durability, however. Contemporary research by Totolin, Sarmadi, Manolache, and Denes (2012) reported grafting/crosslinking of sodium silicate layers onto viscose and cotton flannel substrates by using atmospheric pressure plasma which increased fabric burning times during 45 testing. The presence of the silicate on the surface of the fabrics was detected even after ultrasound washes as confirmed by XPS and SEM. A mechanism was suggested in which inter- and intramolecular (OSiO) crosslinks formed between OH groups present in cellulose chains together with pendant OSiO2H groups. However, as observed for sol
gel treatments above, the presence of silicon-containing moieties alone within the cellulose is insufficient to produce high levels of flame retardancy. Contemporary research by Edwards, El-Shafaei, Hauser, and Malshe (2012) studied the introduction of phosphorus as two novel phosphoramidate monomers into the dielectric barrier discharge plasma under a helium atmosphere. Chosen variables with respect to graft yield were exposure time, monomer concentration, crosslinker chemistry, crosslinker concentration, and photoinitiator concentration. Unfortunately, while char levels were increased, flame self-extinguishability during vertical fabric testing was not achieved and correlated with the inability to apply the appropriate phosphorus levels (#0.5%) into the grafted fabric. More recently, a patented technique referred to as multiplexed laser surface enhancement (MLSE) is currently being trialed within the North of England, including our own laboratories (Mistry, 2017). This enables preimpregnated or prepadded fabrics to be simultaneously subjected to a laser UV (308 nm)/atmospheric plasma process in a variety of atmospheres (e.g., argon, nitrogen, CO2, oxygen, and mixtures) in order to graft flame species present into the component fiber structures. Fig. 9.4 shows the system schematically for a fabric being processed by a single plasma/laser head unit, although in practice such heads may be positioned to allow one or more sequential processes to be carried out on one side and a similar number on the reverse fabric face. As an alternative to prepadding, volatile flame retardant agent vapor may be directly introduced into the UV/plasma zone to enable direct

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Figure 9.4 Schematic representation of the MLSE combined atmospheric plasma/excimer laser system. Reproduced with permission from MTIX Ltd., Huddersfield, UK.

interaction with activated fiber species. This technique is proving to hold the possibility of generating truly novel and durable FR treatments for a range of the common fabrics (e.g., cotton, wool, polyester, and blends) with zero waste and minimal effluent. Preliminary work has shown that cotton twill furnishing fabrics pad-dried with a proprietary P- and N-containing flame retardant, after MLSE treatment achieve satisfactory flame resistance after a simulated (BS 5852) water soak test at 40 C for 30 minutes (Wang et al., 2000; Horrocks, Eivazi, Ayesh & Kandola, 2018). This is the same level of durability achieved if the same FR finish is heat cured at 150 C after a similar pad-dry treatment. In conclusion, it should be noted that plasma technological modification of fiber and textile surface has a history spanning about 40 years and although it has gained commercial significance within industrial sectors such as microelectronics and more recently in improving paint/coating adhesion to plastics for automotive and other applications, its adoption by the textile industry has been slow (Shishoo, 2007). One of the main reasons for this is that the majority of successful plasma applications occurred using low pressure plasma and it is only recently that atmospheric pressure plasma technologies have been developed which are considered to be more appropriate to continuous processing of textile fabrics (Herbert, 2007). The eventual success of atmospheric pressure plasma for introducing durable flame retardancy increases further the challenge of achieving high levels of surface deposition since plasma polymerization can be best controlled in low pressure plasmas, which have more well-defined plasma zones (Hegemann, 2006). Hegemann and Balazs (2007) also state the preference for low pressure plasma systems, at least at the research level, because the greater mean free paths of ions within such plasma

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enable greater penetration depths within textile materials, hence the potential for more cohesive nanocoatings. Furthermore, plasma metallization sputtering techniques, currently used to confer conductive nanolayers on textile surfaces, but with the potential for thermally reflective coating deposition, favor the use of low pressure plasmas. However and notwithstanding these arguments, it is most likely that any commercial plasma process acceptable for the textile industry will have to be based on atmospheric pressure technologies and so future research efforts should be cognizant of this requirement, especially given that the established nonthermal plasma processes previously feasible at low pressures have been successfully transferred to atmospheric pressure conditions, as evidenced by the current range of Dielectric Barrier Discharge, arc-jet, microwave and hybrid sources available (Shishoo, 2007; Stegmaier, Dinklemann, Von Arnim, & Rau, 2007). In this latter respect, the combination of plasma and UV in the MLSE process recognizes that increased surface modification may actually require more than one energy source in order to introduce durable functionality into textile fabric and fiber surfaces.

9.4

Truly smart flame retardant coatings and laminates and future trends

At the beginning of Section 9.3, three ways of introducing smartness were defined and discussions in Sections 10.3.1 and 9.3.2 focused on the first two. The third, namely the development of a flame retardant formulation that reacts to and responds to a fire and/or heat hazard in a measured and proportionate way, is an ideal yet to be achieved especially if a degree of reversibility is also required. Given the technological economic challenges of replacing many of the currently useful coating and surface treatments available, successful development of such smart coatings would be in a niche area where the price of the effect and product can justify the cost. Alternatively, the new process will partly replace or be a component within a more conventional process. Potential areas of opportunity may lie within novel hot melt laminating/coating (all dry) where absence of the need for solvents is a prime advantage as well as ease of addition of modifying additives. Furthermore, use of multimelting ceramics which offer a spectrum of thermal protection and hybrid polymer brushes and which not only act as durable surface grafts to substrate polymers and fibers, but also may offer novel functionalities have yet-to-be-realized potential. Finally, development of nanofilm deposition using a range of technologies including chemical vapor deposition and atmospheric plasma alone or in combination with the previously mentioned technologies offers fertile ground for research. In conclusion, it might be commented that while the research literature abounds with interesting novel surface-modifying processes and effects, scaling these up to fully commercial processes within a correct economic model and which meet repeatable performance demands commensurate with today’s requirements of high performance textiles and materials will often pose greater challenges than the original proof-of-concept research.

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