commercial flame retardancy of polyurethanes

http://jfs.sagepub.comJournal of Fire Sciences

DOI: 10.1177/0734904104040259 2004; 22; 183 Journal of Fire Sciences

Edward D. Weil and Sergei V. Levchik Commercial Flame Retardancy of Polyurethanes

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Commercial Flame Retardancyof PolyurethanesEDWARD D. WEIL*

Polytechnic University, Six Metrotech CenterBrooklyn, NY 11201, USA

SERGEI V. LEVCHIK

Akzo Nobel Chemicals, 1 Livingstone Ave.Dobbs Ferry, NY 10522, USA

(Received May 17, 2003)

ABSTRACT: The review covers those flame retardants (combustion modifiers)which are in commercial use or which have had active development leading topotential commercial use in polyurethanes and isocyanurates, with emphasis onfoams but brief coverage of elastomers and coatings. The review also coversfactors such as polyol choice, catalyst, surfactant, and blowing agent whichimpact on performance of flame-retardant polyurethanes and isocyanurates.Performance factors such as scorch and fogging are discussed in relation to thechoice of flame retardant for foam.

KEY WORDS: polyurethanes, flame retardants (additives, reactives), foams,isocyanurates, polyols, blowing agents, scorch, fogging.

THE MAIN APPLICATIONS of polyurethanes are in rigid foams, flexiblefoams, coatings, and elastomers. The closely related isocyanurate foams,which are often partly polyurethanes, are included in the presentchapter. Flame retarding is dealt with only briefly in monographs onpolyurethanes such as by Szycher [1] and Oertel [2]. A comprehensivereview of flame retardants for polyurethanes was published in 1975 byPapa [3]. Another comprehensive and nonselective review covering the

*Author to whom correspondence should be addressed.

JOURNAL OF FIRE SCIENCES, VOL. 22 – MAY 2004 183

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more recent literature and patents has recently been prepared by thepresent authors and published elsewhere [4]. It should be mentionedthat the term ‘‘combustion modifiers’’ is preferred rather than ‘‘flameretardants’’ by some in the industry to avoid giving the impression thatsuch ingredients will prevent ignition and burning under all conditions.Moreover, it must be recognized that the standards are based on small-scale testing, and in general the ‘‘flame retarded’’ materials arecombustible in a large real fire, i.e., they are never ‘‘fireproof ’’ (indeed,a term which should be considered obsolete and misleading).

COMMERCIAL FLAME RETARDANCE OF RIGID FOAMS

Because of the frequent use of rigid polyurethane foams as thermalinsulation in buildings (boardstock and panels for roofing and sheath-ing) and transportation (refrigerated trucks and rail cars), there aremany circumstances where flammability and smoke regulations must bemet. A good summary of the many tests used throughout the world isavailable in the handbook by Troitzsch [5]. Typically, building codestandards are applied; in the U.S. many of these are based on ASTME-84, the 25-foot Steiner tunnel . Class A (Class I) has a flame spreadindex of 0–25. Class B (Class II) a flame spread index of 26–75, andClass C a flame spread index of 76–260 (the flame spread indices arecalculated from observed flame spread rate by a formula). A larger scaletest, required for some insurance purposes, is the Factory Mutual cornertest. Insulated roof decks, a very important market for rigid foams, arerated in the U.S. by the Factory Mutual (FM) roofing test [6] whereinfuel contribution rates in a specified calorimetric test must fall below acertain level to permit Class I to be achieved. Efforts have been made tofind correlations between small-scale tests and large-scale tests, withsome moderate successes [7,8].

Rigid foams, often made into sandwich structures, are used in manytypes of panels, shelves, equipment housings, doors, and other structuralapplications, sometimes requiring flame retardancy depending on theend-use. In sandwich or laminate structures, the surface to which thefoam is laminated plays an important part in its fire resistance.

A recent (as of 2002) US-oriented discussion of fire code requirementsin buildings is presented by Ross and Hagan [9]. Regarding upholsteredfurniture, a thorough review (as of 2002) was written by Hirschler [10].The proposed tests, as of fall 2002, involve the fabric, the filling and thetotal assemblage [11]. A detailed discussion is outside the scope of thischapter, and would be subject to change as regulations evolve.

184 E. D. WEIL AND S. V. LEVCHIK




Additives in Rigid Foams

The leading method for flame retarding rigid foam is to use additives,although reactive diols are occasionally employed where there is somespecial requirement. The leading additives are tris(2-chloroethyl)phosphate (Akzo Nobel FYROL CEF, Rhodia ANTIBLAZE 79, BayerDISFLAMOLL TCEP) and tris(1-chloro-2-propyl) phosphate (alsoknown as tris(chloroisopropyl) phosphate) (Akzo Nobel FYROL PCF,Rhodia ANTIBLAZE 80, Bayer LEVAGARD PP). Both of these arefairly low viscosity liquids, made from phosphorus oxychloride andethylene oxide or propylene oxide. Their moderate volatility is not aproblem in rigid closed-cell foams.

A typical formulation, using the pentane blowing agent, for a 1.8–2 lb/ft3 foam that might be suitable for a roofing laminate is as shown inTable 1.

This foam would be expected to attain Class I by the FM roofing testand Class II by the ASTM E-84 test. If a UL 94 Class I is needed withfoam of this type, a brominated diol (discussed below) might be included.

One distinction between tris(2-chloroethyl) phosphate and tris-(2-chloroisopropyl) phosphate as additives for rigid foams is that thechloroisopropyl phosphate, because of ‘‘steric hindrance,’’ is much morehydrolytically stable and unreactive toward the amine catalysts used infoam-making [12]. Nevertheless, tris(2-chloroethyl) phosphate is ade-quately stable in many formulations and has been used widely inpolyurethanes. Where shelf-life of a premix is important, a substantialadvantage will be found with tris(2-chloroisopropyl) phosphate overtris(2-chloroethyl) phosphate. The stability of a water-containingpremix system is favored by the use of the more hydrolytically stableflame retardants, avoiding strong amine catalysts and by the use of

Table 1. Formulation for a 1.8–2 lb/ft3 pentane-blown foam.

B Component PHPAromatic polyester polyol 100Tris(chloroisopropyl) phosphate (FYROL PCF) 15Silicone surfactant L6912 2.5Water 0.5Tertiary amine catalyst (C5) 0.32Potassium octoate catalyst (T-45/K15) 2.68Cyclopentane/isopentane blowing agent 23

A ComponentPolyisocyanate (PAPI 27) (NCO index 300) 196.7

Flame Retardancy of Polyurethanes 185




hydrolysis-resistant aromatic polyols [13]. Based on limited test data,there may also be a toxicological advantage of the chloroisopropylphosphate over the chloroethyl compound.

A nonhalogenated phosphorus additive, which has had usage in rigidpolyurethane foam for a long time, is dimethyl methylphosphonate(DMMP). This compound contains 25% phosphorus, the basis of its highflame retardant activity, and only about 8 phr is required in a sucrose-amine-based rigid foam [Stauffer data sheet]. This compound has ‘‘R46’’(mutagen) labeling in Europe so is not used much there. Diethylethylphosphonate or triethyl phosphate are also used for the samepurpose, and have better label status in Europe. Bayer has recentlyintroduced dimethyl propylphosphonate (LEVAGARD DMPP) whichthey advocate as a replacement for the halogen-containing flameretardants. It is a low viscosity liquid, less volatile than the methylphos-phonate.

Triethyl phosphate along with a specific polyester polyol wasrecommended in a R-245fa water co-blown polyurethane or polyisocya-nurate roofing spray foam to meet fire standards [14].

Triaryl phosphates, specifically triphenyl phosphate, isopropylphenyldiphenyl phosphate, tricresyl phosphate, trixylenyl phosphate all findsome use in rigid foam formulations. A blend of triethyl phosphate andtriphenyl phosphate, Bayer’s LEVAGARD TPP, is used in rigidpolyurethane and isocyanurate foams, particularly in Europe. It issometimes combined with the reactive diol phosphonate (Bayer’sLEVAGARD 4090N or Akzo Nobel’s FYROL 6 – see below).

Also, for nonhalogen applications of importance in Europe, highmolecular weight finely-divided ammonium polyphosphate (APP,Clariant EXOLIT AP 422 or Budenheim Iberica’s FR CROS 484) hasbeen found effective in pentane-blown polyurethane or polyisocyanuratefoams. These demand enhanced flame retardant additive levels toovercome the adverse flammability effect of the pentane. The use ofAPP, whichworks by a solid phasemechanism, also provides lower smokecompared to halogen-containing foams of similar density according toClariant workers [15]. Dispersions in polyol are available. APP is alsouseful in rigid integral skin foams.

Ammonium polyphosphate formulated with a char former andblowing agent to make a complete intumescent system is also usefulin both rigid and some flexible foams. These formulations containingEXOLIT AP 750 can meet many requirements of railway and aircraftstandards for integral skin rigid and flexible polyester foams. Thehigh viscosity of the APP dispersions makes them somewhat difficultto use.





Stabilized red phosphorus also has some usage in Europe in rigidpolyurethane foams. It is highly efficient on a weight basis, and can beused at rather low loadings to meet stringent flammability standards.Dispersions in polyol, castor oil or tris(chloroisopropyl) phosphate arevariously available from Clariant or Italmatch.

Fillers such as expanded clay, expanded glass, slate, or the like may beadded for various reasons. Limited data suggests that layered clays likemontmorillonite, particularly after quaternary ammonium treatment tofavor exfoliation, will reduce the rate of heat release. The addition oflong glass fiber reinforcement to foam board is likely to provideimproved fire resistance [16].

Reactive Flame Retardants – General Comments

Over several decades, much effort has been put into developing diolsor polyols containing phosphorus and/or halogen, as a means forbuilding flame retardancy into the polyurethane structure by chemicalbonding [3]. There is often a belief that this approach will lead to greaterpermanence, although experimental evidence for that benefit is hard tofind. Although reactive phosphorus- or halogen-containing flameretardants are on the market, it is believed that the use of additives isdominant in both rigid and flexible polyurethanes.

Reactive Flame Retardants in Rigid Polyurethane Foams

An early reactive diol, VIRCOL 82, was marketed for some years byAlbright & Wilson, then by Rhodia and now by Albemarle. It is thereaction product of dibutyl acid pyrophosphate with propylene oxide andis a substantially neutral diol with about 11.3% P and an OH number ofabout 205. Its main application has been in rigid polyurethane foamprepolymers and premixes, where it facilitates the compatibility of theingredients as well as providing its flame retardant effect.

FYROL 6 was invented in the 1960s at Victor Chemical Works, laterStauffer, now sold by Akzo Nobel (as FYROL 6) and Bayer (asBAYTHERM or LEVAGARD 4090N). It is a reaction product of diethylphosphite with formaldehyde and diethanolamine and has the structure(C2H5O)2P(¼O)CH2N(CH2CH2OH)2. The primary hydroxyl groupsreadily react into a urethane structure. It has primarily been used inspray foams or pour-in-place foams. Because of the reacted-in aspect, ithas sometimes been chosen for applications where the foam will besubjected to severe long term heating, as for example in a stadium dome;other uses are in large refrigerators. Good retention of the flame





retardant can be assured [17]. The amino group in the molecule hassome catalytic contribution to the foaming (a practical advantage inspray foams), and also may have some buffering (stabilizing) effect onthe finished foam. Since the phosphorus ester group is not in thebackbone of the polymer, if it suffers hydrolysis, it does not representcleavage of the polyurethane structure. Formulations of FYROL 6 inpolyol have good shelf life. The mode of action of FYROL 6 may beincreased char yield [18].

Impact of Blowing Agent on Flame Retardancyof Rigid Foams

The chlorofluorocarbon blowing agents are being (or have been)phased out to avoid their upper-atmosphere ozone-depleting action. Theuse of pentane (cyclopentane, isopentane, n-pentane, or mixtures) forblowing of foams in place of chlorofluorocarbon blowing, imposes needsfor a higher degree of flame retardancy to counteract the flammability ofthe blowing agent. The usual flame retardants such as FYROL CEF andPCF can still be used but the level will usually have to be raised. Thehigher the isocyanurate index, the less the required flame retardantlevel to meet a standard.

Also in response to this major technological shift in blowing agents,some new flame retardants have been introduced as alternatives. Forexample, Clariant has introduced the oligomeric phosphate diol,EXOLIT OP 550, discussed later. A related phosphorus ester reactive,EXOLIT OP 560, with about 12% P content and a higher OH number(400), is available where emissions from flexible foam must beminimized.

Other new nonozone-depleting blowing agents containing halogen canallow for less flame retardant, or with a high enough isocyanuratecontent, no flame retardant at all. An example of a non-ozone-depleting blowing agent in commercial development (in 2002) is1,1,1,3,3-pentafluoropropane, available as Honeywells’ ENOVATE3000 (HFC-245fa) or Solvay’s HFC-365mfc. This compound does havea flash point, but can be made less flammable by blending withtetrafluoroethane [21]. There is also a possible trade-off in the use of apentane with HFC-245fa. The pentane lowers the cost of the blowingagent but may require an increase in the flame retardant. Honeywelldata from Williams [22] (Table 2) shows some comparable boardstockformulations, the last three said to have similar costs. The flame retar-dant tris(chloroisopropyl) phosphate has been elevated to compensatefor the pentane. Mixtures with pentanes are much more economical





than the use of ENOVATE alone. Also, the amount of water blowing canbe adjusted to reach an optimum set of properties and cost.

Other examples of halogenated blowing agents that can lower flameretardant requirements are 1,1-dichloroethane and trans-1,2-dichloro-ethylene [23].

Brominated Diols as Reactive Flame Retardants

Some brominated diols are also used in rigid urethane foams. GreatLakes PHT-4 diol (similar to Albemarle SAYTEX RB-79) made fromtetrabromophthalic anhydride has been available for many years and isused mainly in rigid foams to meet ASTM E-84 Class I or II ratings. Thisproduct has one primary hydroxyl group (faster reacting) and onesecondary hydroxyl group (slower reacting). It is a viscous liquid withabout 46% bromine content. A related tetrabromophthalate diol withboth hydroxyl groups primary, thus more reactive, has been introducedby Great Lakes as FIREMASTER 520.

With 25% SAYTEX RB-79 in the polyol and 25 parts of tris(2-chloroisopropyl) phosphate (FYROL PCF) per hundred parts ofpolyol at an index of 300, a borderline E-84 Class I could bereached. According to recent Albemarle data, with a 350 index, 25%RB-79 and 25 parts PCF, a firm Class I was reached, with smoke densityabout 100.

With selected polyols, low smoke can also be achieved even withbromine in the polymer according to information from Great LakesChemical [24]. It has also been recommended in a blend with a liquidphosphate (Albemarle SAYTEX 7980) as a scorch-resistant flameretardant for flexibles and also for pentane-blown isocyanurate roofing

Table 2. Flame-retardant boardstock formulations using isopentane andHFC-245f ([22]).

QuantitiesandProperties

Enovate3000 Isopentane

Isopentaneand 2%

ENOVATE 3000

Isopentaneand 10%

ENOVATE 3000

Index 250 300 300 300FR (TCPP), php 0 10.00 9.00 7.50Water, php 1.00 1.00 1.00 1.00ENOVATE 3000, php 40.00 0 1.00 4.50Isopentane, pbw 0 20.90 20.40 18.75Density, lb/ft3 1.70 1.90 1.90 1.90k-Factor, relative 0.90 1.00 0.97 0.95





foams. A blend of RB-79 with tris(chloroisopropyl) phosphate was said toperform particularly favorably in this application [25].

Dibromoneopentyl glycol, HOCH2C(CH2Br)2CH2OH, is occasionallyused as a reactive in rigid foams, and its combination with tribromo-neopentyl alcohol, dissolved in a liquid triaryl phosphate, has beenclaimed useful for non-scorching flexibles (see discussion of scorchbelow).

Chlorine and bromine-containing polyols (Solvay’s IXOLs) havebeen in use for some years, particularly in Europe. This family ofpolyols is believed to comprise the reaction products of epichloro-hydrin with 2,3-dibromo-2-butene-1,4-diol. They are effective forpermanent flame retardancy of rigid polyurethane and polyisocyanuratefoams [26,27].

Nonhalogenated Polyols Favorable to FlameRetardancy in Rigid Foams

Some early studies at Uniroyal showed that amine-initiated polyolsfavorably affect flame retardancy [28].

Polyols containing aromatic rings, such as the ‘‘Mannich polyols’’made from diethanolamine, formaldehyde, and phenols, are favorable toflame retardancy not only because of the nitrogen content, which alsoprovides some foaming catalysis, but because the phenolic rings enhancechar formation during burning [29]. Rigid water-blown foams withmuch-improved flame retardancy can be made using combinations of anultra-low viscosity Mannich polyol with another polyol, according toDow [30].

The use of aromatic terephthalate polyester polyols (functionality2.0–2.3) of the KoSa TERATE family can contribute not only someimproved compressive strength and thermal stability but can alsosubstantially enhance flame retardancy such that the phosphorus orhalogen flame retardants can be greatly reduced, according to KoSaproduct information. This flame retardant benefit is because of the char-formation propensity of the aromatic backbone structure. TERATEpolyols are inexpensive and are based on byproducts or coproducts ofdimethyl terephthalate manufacture. They are typically used up toabout one-third of the polyol for 2–2.5 lb/ft3 density foam, the limitingfactor being the high equivalent weight of the TERATE. Higher densityfoams can use even larger amounts of a TERATE. They are alsouseful in isocyanurate foams. TERATE polyols of 2.1–2.3 function-ality are most useful in bunstock, panel, pour-in-place, spray, andfoundry systems. TERATE polyols of 2.0 functionality show improved





processability in laminates as well as the other systems. The optimumTERATE choice also is affected by the blowing agent used.

Additives in Isocyanurate Foams

The isocyanurate structure is much more thermally stable than aurethane structure (which can undergo the reverse reaction of itsformation, back to isocyanate and alcohol). Thus, a substantial degree ofinherent flame retardancy can be built into a rigid foam by using anexcess of isocyanate relative to coreactant hydroxyl groups andcatalyzing the isocyanate to trimerize to the very stable isocyanuratering. A totally isocyanurate foam can be made, but in practice, most ofthese foams are partly urethane, partly isocyanurate. The higher theisocyanurate content, the greater the char yield on fire exposure andthus the more flame resistant, but also the more brittle (friable).Moreover, the isocyanate is more costly than the polyol. An additivephosphate, usually tris(2-chloroethyl) phosphate or tris(chloroisopropyl)phosphate, is often added, mainly to reduce friability, but flameretardancy is also enhanced. Isocyanurate foams with no added flameretardant can be used to make, for example, composite panels, and ifproperly covered, will pass most building code standards.

For demanding applications in roofing, a detailed study shows goodfire performance in the revised large-scale tests from pentane-blownpolyisocyanurate foams containing tris(2-chloroethyl) phosphate, or amixture of DMMP and RB-79 (tetrabromophthalic ester diol), oralternatively, SAYTEX XP-4020 [31].

It was shown by experimental work at Albemarle that a flame spreadindex of 23 (Class I) and a smoke density of 100 (E-84 tunnel) could beattained by using an isocyanate index of 350 with 25% SAYTEX RB-79and 25 parts of tris(chloroisopropyl) phosphate.

The Effect of Catalyst Choice on Flame Retardancyof Rigid Lamination Foams

Trimer modification (isocyanurate formation) of rigid polyurethanefoams has been gaining importance because this method not only allowsthe use of lower cost polyester polyols (like TERATE) but also allowssignificant lowering of the level of the additive flame retardant. Pentaneblowing agents can also be used as a partial alternative to waterblowing, allowing a somewhat lower index and thus saving on the cost ofthe relatively more expensive isocyanate. The catalyst used to producethe foam controls the amount of trimer, and thus affects the flame





retardancy at a given level of flame retardant, since trimer structure isfavorable for flame retardancy. In a careful multivariable study of a widerange of catalysts, using diethyl ethylphosphonate as the additive flameretardant, potassium octoate was found advantageous over potassiumacetate as trimerization catalyst in regard to foam flammability asmeasured by flame height (German DIN4102 test B2). Strong blowingcatalysts such as pentamethyldiethylenetriamine also appeared slightlybetter than strong gelling catalysts in regard to flame height. The use ofa special pentane emulsifier allowed for less, or even zero, water inblowing the foam [32]. With more pentane, more flame retardantadditive or higher isocyanurate content would presumably be used tocompensate for the unfavorable flammability effect of the pentane.

FLAME RETARDANCE OF FLEXIBLE FOAMS

The three largest markets are transportation, furniture and carpetbacking. Carpet backing uses mostly alumina trihydrate (ATH) as theflame-retardant additive, the other markets use an ever-increasingvariety of approaches.

Additives in Flexible Foams

A major fraction of the flexible polyurethane foams used in furnitureis flame retarded.

Additives are the dominant means, although much research hasbeen expended on reactives. In general, the flame-retardant additivesincrease the foam ignition temperature and reduce the rate of flamespread [2].

To pass the automotive standard MVSS 302, typical additive levels forseating foam are about 16 parts per hundred of a chloroalkyl phosphatein 1.0 lb/ft3, and about 7 phr in 1.8 lb/ft3 foam. For simple horizontaltesting of 1/200 thick seating foam, even lower levels may pass MVSS 302.On the other hand, when the foam is laminated to a fabric (oftenuntreated and with less than 1/200 foam), and the test is run with thefabric facing the flame, it is more challenging to pass MVSS 302.

Furniture manufacturers in the U.S. usually try to have theirfoam cushions comply with the CAL 117 tests (currently underrevision). This is usually done with liquid plasticizer-like additives,containing phosphorus or halogen or both. Typically, about 16 parts perhundred of a chloroalkyl phosphate is sufficient for 1.0 lb/ft3 foam and





12 parts for 1.8 lb/ft3 foam and in some cases even lower levels suffice.The addition of a small amount of melamine can prevent failure to passthe test after repeated flexing. In the UK, and perhaps eventuallythroughout the EU, a stringent fire safety standard must be passed byseating cushions. Thus, for UK use (BS 5852), flame retardant levels aregenerally higher and melamine is generally used at more substantiallevels.

Melamine is often used in combination with these additive flameretardants, and is discussed below at greater length.

Both tris(2-chloroisopropyl) phosphate and tris(1,3-dichloro-2-propyl)phosphate are used extensively as additives for flexible polyurethanefoams. Of these, the most frequently used is tris(1,3-dichloro-2-propyl)phosphate (TDCPP, Akzo Nobel FYROL FR-2, Rhodia’s ANTIBLAZE195), a product of reacting epichlorohydrin with phosphorus oxychlor-ide. Ring opening of the epoxy group gives mostly the branchedstructure, with a few percent of the straight chain (the 2,3-dichloro-propyl) group. Many older literature references mistakenly call thisproduct tris(2,3-dichloro-1-propyl) phosphate, mostly before the truestructure was established. Actually, the fact is that the product is mostlybranched but a small straight chain content keeps it from crystallizing,except on long storage under cold conditions. Tris(1,3-dichloro-2-propyl)phosphate tends not to interfere with catalysis and allows considerablelatitude in the tin component. It allows early development of resistanceto compression set.

A typical present-day formulation in slabstock or automotive seatingfoam is shown in Table 3 (in parts by weight). The MVSS 302 results are

Table 3. Formulation for a flame-retardant slabstock orautomotive seating foam.

Component; Flame Test Parts

Polyol, 3000mol. wt. 100Silicone L-5750 1.20Water 3.85Dabco 33-LV amine 0.30Niax A-1 tertiary amine 0.20Stannous octoate 50% 0.42Methylene chloride 1.5080 : 20 TDI (index 111) 51.2FYROL FR-2 12.0Flammability by MVSS-302 1.40 0 Dist., 0 sec ext.





given as distance burned and time to extinguishment in accordance withthe test protocol.

Cold-cure molded foam can meet the standards with lower levels ofTDCPP. Hot-cure molded flexible foam tends to need a somewhat higherpercentage of flame retardant. Here, TDCPP is used, but also the lesscostly and higher phosphorus-content tris(2-chloroisopropyl) phosphate(TCPP, Akzo Nobel’s FYROL PCF). In fact, the latter can be foundsomewhat more active, weight for weight, in some melamine-freecombustion-modified high resilience (CMHR) formulations which arebased on ethylene-oxide terminated polyols. If melamine is also added,then TDCPP is more active than TCPP.

Additives in Flexible Foams – VolatilityConsiderations (‘‘Fogging’’)

When slightly volatile additives such as tris(1,3-dichloro-2-propyl)phosphate are used as flame retardants in automobile seating foams,there is often a detectable fogging of the inside of the windshield if thepassenger compartment of the vehicle is warm. Other components of thefoam, such as the catalysts and surfactants can also contribute tothe fogging. There are various industry tests for windshield fogging [33].Where the monophosphate fails this test, it is usual to use diphosphatesor oligomeric phosphates or phosphonates.

Several of these have been commercialized. These are larger moleculeswith less vapor pressure than that of the monophosphates. The availableones are: Akzo Nobel’s FYROL 99, a mixture (ClCH2CH2O)2P(¼O)O[CH2CH2OP(¼O)(OCH2CH2Cl)O]nCH2CH2Cl (more suitable for rigidsand thermosets) and Great Lakes’ FIREMASTER 100 or Rhodia’s V6,a compound having the structure (ClCH2CH2O)2P(¼O)OCH2C(CH2Cl)2CH2OP(¼O)(OCH2CH2Cl)2.

FIREMASTER 100 has some advantages besides low volatility. Thebranchy dichloroneopentyl group in the middle of the molecule tends tostabilize it toward hydrolysis and gives it good stability in a premix witha polyol.

Halogen-free Additives for Flexible Foams

An oligomeric ethyl phosphate additive containing 19% phosphorushas been introduced by Akzo Nobel as FYROL PNX. Because of the high% phosphorus, it is quite efficient and as little as 5 php is effective inpassing the California 117 test and MVSS 302 in a 1.5–1.8 lb/ft3 foam.





For automotive applications, this low-volatility additive has theadvantage of low fogging propensity.

Triaryl phosphates, such as isopropylphenyl diphenyl phosphate, findsome use in flexible foam formulations sometimes in combination with abromine-containing additive (see Scorch discussion). A recent introduc-tion, Great Lakes’ RHEOFOS NHP, a low viscosity liquid, probably amember of the aryl phosphate family, is a formulation designed to meetMVSS 302 for hot-molded automotive seating, said to be cost effectiveand nonfogging.

In polyester-polyol-based flexible foams, which have a greatercharring propensity than polyether-polyol-based foams, insoluble highmolecular weight APP, crystal type II, is especially useful as a low-smoke non-migrating flame retardant additive. It is available as a finelydivided powder (several suppliers) or as a thixotropic dispersion(Clariant EXOLIT AP 452). To minimize interactions with other foamcomponents, a more expensive resin-coated variety may be used such asEXOLIT AP 462.

Overcoming the ‘‘Scorch’’ Problem in Flexible Foams

In the manufacture of flexible polyurethane foams, allowing the foamto reach an excessively high temperature during the latter stages offoaming, after the addition of the water, can lead to ‘‘scorch.’’ This is,minimally, a discoloration of the interior of the foam slab or bun, moreseriously a loss of mechanical properties indicative of structuraldegradation, and in extreme cases, a fire can result. Scorch is aggravatedby excessively fast foaming, by warm ambient conditions as might occuron a hot summer day, by drafts, by a formulation imbalance or meteringmalfunctions, by traces of soluble iron or copper compounds [34], and bythe presence of some flame retardants such as the chloroalkylphosphates. Aminophenyl oxidation products are blamed for most ofthe discoloration.

Those flame retardants which are either most reactive towards thehypothesized arylamino groups, or simply more hydrolyzable, tend to bethose which aggravate scorch. Therefore, as a class, the aliphaticphosphates tend to be more prone to aggravating scorch than thearomatic phosphates. The more hydrolyzable, the more aggravating.Chloroethyl phosphates are scorchier than tris(chloroisopropyl) phos-phate [35]. Tris(1,3-dichloro-2-propyl) phosphate is still less scorchy butrequires avoidance of excessively high exotherm temperature. It can begiven more latitude by various antioxidant ‘‘packages’’ [36–39]. Thecommon antioxidants of the 2,6-di-tert-butylphenol type, which are





often used in the polyols, are not too effective by themselves as scorchinhibitors, and may themselves generate yellowish quinoidal chromo-phores.

The polybromoaromatic compounds have unreactive bromineatoms and act as inert additives under the conditions of scorch, thusfoams containing them tend to be scorch resistant. Consequently,a widely used low-scorch flame-retardant additive has been a liquidblend of pentabromodiphenyl ether and isopropylphenyl diphenylphosphate/triphenyl phosphate, both quite stable liquids [40] whichperform well together as Akzo Nobel’s FYROL PBR or Great Lakes’DE-61. Either component alone is effective only in very undemandingapplications such as MVSS 302 seating. In the early 2000 era,environmental concern regarding pentabromodiphenyl ether [41] hasled to a sharp decline in its usage, and regulatory actions in Europehave the effect of a ban. Manufacturers in the U. S. are also seekingalternatives.

Alternatives introduced into the market include a tetrabromobenzo-ate ester, Great Lakes FIREMASTER BZ-54 [42] which can be usedalone or blended with an alkylphenyl diphenyl phosphate. When usedalone, this 54% bromine content additive has a favorable effect on flamelamination, and also unlike the pentabromodiphenyl oxide formulations,it does not cause center softening in high resilience foam formulations.Its blend with an isopropylphenyl phosphate may be Great Lakes newFIREMASTER 550, used in both high-resilience and conventionalfoams.

A hydrolytically stable and low scorching aromatic phosphate EAC003developed by Akzo Nobel can be used without halogen [43,44].

Reactive Flame Retardants for Flexible Foams

The diols used in rigid foams, such as FYROL 6, have too high an OHnumber for use in flexible foams, except in small amounts. A largeamount of research has been done on reactive phosphorus-containingdiols and polyols for flexible foams without much commercial success [3].The reasons for the lack of success are mostly cost, adverse propertiesrelated to excessive hydrophilicity, and the need for more reformulationto accomodate reactive flame retardants in each foam grade for eachapplication.

A newer halogen-free phosphorus-containing diol was developed byHoechst for rigid or flexible foams and is now marketed as EXOLIT OP550 by Clariant [45]. The product is a hydroxyethyl-terminated ethylphosphate oligomer diol with about 17% P for rigid or flexible foams,





especially for molded and high density slabstock flexible foams. It causesless plasticization than small molecular weight additives such as triethylphosphate. Clariant also has a related diol, EXOLIT OP 514, with somechlorine content as well as phosphorus. Typical loadings of EXOLITOP 514 or 550 are 2–10 php to pass MVSS 302 and 6–12 php to passCAL 117. A related OH-functional methylphosphonate methyl phos-phate oligomer, Akzo Nobel’s FYROL 51 is available but used mainly inpaper and textile applications. It is not available in Europe at thepresent time.

Polyols Favorable to Flame Retardancyin Flexible Foams

Many cold-cured flexible molded foams can pass lenient flammabilityrequirements, such as MVSS 302, with no flame retardant or verylittle. The CMHR foams are substantially more flame retardant witha given amount of flame-retardant additive, and indeed some highresilience foams can pass some of the more lenient flammabilityrequirements with no flame retardant added, probably because of theirmelt-flow characteristics. This mode of retardancy can be defeated bylaminating or even being adjacent to a fabric which can retard the meltflow. CMHR or HR foams with a degree of inherent flame resistancewere made by Mobay from grafted high molecular weight polyols withpolyacrylonitrile grafts [46,47] and/or dispersed styrene-acrylonitrilepolymer [48], or from polyols with added polyurea dispersion (Bayer’sPHD Polyols) [49]. In these systems, the more the surface meltis coherent and protective, the more flame retardant. To pass thestringent British Standard 5852, Part 2, Source 5, 20–30 parts byweight of melamine is usually added, usually along with a chloroalkylphosphate.

It should in general be noted that foams with retardancy relying onmelt-flow can be rendered flammable with even a small amount of aninfusible solid, such as ATH.

Dow has introduced particular SPECFLEX polyols for use withparticular MDI-type isocyanates which can provide improved flamm-ability by the CAL TB 117 criteria in water-blown flexible foams overthe entire isocyanate index range without the need for additive flameretardants [50].

A family of ‘‘PIPA’’ (Poly Isocyanate Poly Addition) foams requirelower amounts of fire retardants to pass tests such as BS5852 (Source 5).The choice of polyol seems to influence the melting properties. Although





the softness of the foam can be controlled by the ethylene oxide contentof the polyol, this seems to have little correlation to the fire testperformance [51].

Polyols developed by Arco were found to yield slabstock foams whichrequire little or nomelamine to pass the Cal TB 133 or BS 5852 (Source 5)[52]. Avoidance of melamine avoids viscosity and cell uniformityproblems.

Polyols with polymers dispersed in them so as to cause a lowertemperature of decomposition were developed by Asahi Glass to favorpassing the automotive standard MVSS 302 with no added flameretardant. The mode of action seems to be based on the endothermicityof the decomposition and also on the formation by the decomposedpolymer of a viscous molten surface layer which shields the foam [53].These polyols are said to function in hot molded, slab, and low densityHR foams.

Melamine in Flexible Foams

Especially in Europe, melamine is frequently used as part of the flameretardant system in flexible foams. Melamine, dispersed in selectedpolyols, is effective as a flame retardant additive in flexible foams forupholstered furniture or bedding, and affords advantageous comfort,foam strength, and cost properties [54,55]. Melamine is used also in theU.S. but patents impose limitations on its use with some dispersedpolymer-modified polyols [56,56a]. The patented formulations allow forreduced levels of melamine, resulting in more stable dispersions and lessimpairment of the physical properties of the foam by the solid additive.

Certain combinations of melamine with specific amine-based polyol-isocyanate condensation products provide enhanced flame retardancy[57]. The choice of specific BASF polyols, alternative to the CMHRtypes, with melamine has provided the basis for a successful series offlame-retardant furniture foams, some even passing stringent FAAaircraft seating requirements [58].

Melamine is often used in combination with a haloalkyl phosphate,such as tris(1,3-dichloro-2-propyl) phosphate. A representative CMHRformulation to pass the British Standard 5852 (Source 5) is shown inTable 4.

A synergistic interaction has been noted with melamine and TDCPP(but not with the more volatile TCPP) in CMHR foam, and explained onthe basis of a chemical interaction producing char or a difficultlyignitable low-melting semisolid [59].





Melamine can cause problems of viscosity increase, settling, pluggingof the equipment, and nonuniform cell size problems. Attention to thegrade of melamine (use of uniform fine particle size) can minimize theproblems. Small particles such as DSM’s type 003, with average particlesize of 50�, may be preferred and have been shown to have a physicalproperties advantage; the best properties at a given melamine loadingare said to be obtained if the melamine particles are smaller than thestrut thickness of the foam [60].

The California standard 133 governing furniture in public placesrequires that the entire piece of furniture pass an open flame test. Thismore stringent requirement can be met if the foams also containmelamine, as well as a chloroalkyl phosphate flame retardant. In someformulations, ATH is also added but it should be noted that in someother formulations the addition of a solid such as ATH works against theflame retardant melt flow action. Combustion modified high resiliencyfoam used in high risk places such as hotels, hospitals and prisons mayneed a combination of several flame retardants.

Other Solid Additives in Flexible Foam

Where polyester polyols are used in slabstock instead of polyetherpolyols, some problems with hydrolytic stability are sometimesencountered. In Europe, finely divided APP is sometimes used as aflame retardant in polyester polyol based foams. It can be metered as apaste predispersed in polyol [2]. Such applications are mostly forapplications where UL -HF1 flame requirements must be met.

Table 4. Representative CMHR formulation to passBritish Standard 5852, source 5.

Component Parts by Weight

PHD polyol, OH No. 33 100Toluene diisocyanate (80:20) 46.2–57.7Water 3.0–4.5Melamine 20–30Silicone surfactant 0.3–0.4Triethylenediamine catalyst 0.15Alkanolamine 1.5–2.0Tin catalyst 0.13–0.15Haloalkyl phosphate 2.0–10.0Index �105





Hydrated alumina (ATH) can also be used in flexible slabstock foamsas an additive [2]. High loadings are needed, thus ATH tends to haveproblems of density, serious effects on mechanical properties, andsettling prior to and during the foaming process.

A Systems Approach to Flame Retardant FlexibleFoams without Flame-retardant Additives

Dow has developed a combination of particular isocyanates includingMDI and particular polyols to meet flammability requirements for CAL117 furniture foams using water blowing, over a wide range of densitiesand high-resilience load bearing characteristics. The technology is basedon high molecular weight base polyols in combination with copolymerpolyols, and all-MDI prepolymers with optimized isomer/oligomer ratioand hydroxyl-terminated components [50].

Silicone Surfactants Favorable to FlameRetardancy in Flexible Foams

Polyether polysiloxanes are effective as foaming stabilizers andcommonly (although not universally) used for this purpose. Thesurfactants have a complex action on flammability by virtue of theireffect in controlling cell opening and cell size [61,62]. Besides theireffects during foaming, the surfactants can affect melt rheology underflame exposure conditions. The silicone surfactants, if not optimum forthis application, can actually increase flammability, or in other words,may require that more flame retardant be added to obtain a desiredlevel of flame retardancy. One of the ways a surfactant can affectflammability is by its effect on cell opening and air flow, a complex andconcentration-dependent effect [63]. A substantial minimizing of therequisite level of flame retardant in a flexible foam is made possible bythe use of a silicone surfactant of a type designed for this application,such as Union Carbide’s (now OSI’s) L-5740 and L-5750. Favorablefactors appear to be the presence of a sufficient number of relatively lowmolecular weight polar (polyoxyalkylene) grafts and the optimum totalratio of these grafts on the dimethylsiloxane backbone [64].

While there are now ‘‘universal silicones’’ such as OSI’s L620 whichare effective in both flame-retardant and nonflame-retardant foams,some manufacturers make available silicone surfactants which havebeen optimized for specific foam applications. For example, in the Byk





Chemie SILBYK family of silicone surfactants, recommendations aremade for grade 9110 for flexible polyester slabstock, 9700 for CMHRslabstock, TP 3796 for rigid laminated boards and slabstock, TP 3706 or3801 for laminated boards and metal-faced sandwiches, and newlydeveloped TP 3846 for flexible ether slabstock. The higher thesurfactant activity, the lower the required surfactant loading which isgenerally better for flame retardancy.

Foamers who have to make foams passing the horizontal burnclassification UL 94 HF1 often prefer to avoid silicone surfactants. Twoexplanations bear upon the question of the deleterious effect onflammability seen with less optimum silicones: one is that degradationproducts of the silicone reduce melt viscosity, make more drips ofsmaller diameter, with more surface to have air access and to burn[Buescher J (Byk Chemie), private communication]. Another explana-tion, rather different, is that the silicone burns to silica which acts as awick, stabilizing the flame; this effect can be demonstrated by applyingpowdered silica (or almost any other infusible powder) to a test specimenof flexible foam flame retarded with a chloroalkyl phosphate [Weil, E. D.,unpublished].

From a basic study, it appears that silicones can also improve flameretardancy by forming a silicaceous barrier layer when burnt [65,66].This beneficial effect is most evident in reduction of peak heat releaserate in the cone calorimeter.

Technology utilizing CO2 as auxiliary blowing agent in flexibleslabstock foam was introduced around 1993. Where flame retardantperformance is required, some re-optimized silicone surfactants arepreferred, for example OSI’s NIAX L-631, to obtain desired cellstructure and adequate flame retardancy without requiring an excessiveamount of flame-retardant additive. Air Products DABCO DC5980 is arelatively new high efficiency nonhydrolyzable silicone glycol copolymerwhich is also flame retardant compatible.

Effect of Foaming Catalysts on Air Flow,Flame Retardancy and Smoldering Combustion

While it is desirable to have a good air flow (typically 3 cfm) in aconventional flexible foam in the 1.5–1.8 lb/ft3 range, it is detrimentalto the flame retardant property to have a very high air flow (such as>5 cfm). However, some minimum air flow is necessary to allowproper cooling. Smoldering is also enhanced. Air flow is in fact one ofthe main properties of a foam affecting its fire behavior. Open cellversus closed cell content is obviously related to air flow and thus to





flammability. Foams with high air flow are desirable for good furniturecushioning. Moreover, the air flow will tend to increase with repeatedflexing. Excessive initial air flow can generally be remedied by using asomewhat higher level of the tin catalyst, and this can be expected toimprove flame retardancy and reduce smoldering tendency. Foamsnear the minimum acceptable air flow tend to be erratic inflammability. Experimental foams with poor cell opening (high closedcell content, very low air flow) can be deceptively flame retardant.Foams with high but desirable air flow may require additional flameretardant such as supplemental amounts of melamine to pass theignition test requirement.

Interaction of Upholstery Fabric on the Flammability ofFlexible Foam in Furniture

This is a complex topic beyond the scope of our present review.The fact that there is a strong interaction is in general well known,but specific combinations are unpredictable. An excellent summarywas presented in 1998 to the Consumer Products Safety Commissionto assist them in regulation and test development [67]. It was pointedout that bad combinations of fabric and foam can still occur assurprises, and in some instances even an interliner can cause aproblem.

Effect of Fillers on Flame Retardancy of Flexible Foams

The use of nonflame retardant fillers such as calcium carbonate orbarium sulfate may make a flexible foam more difficult to flame retard,unless very large loadings are present. Any infusible solid powder orfiber can provide a wick-like action and enhance burning. As mentio-ned, this effect can sometimes be seen in the deleterious effect ofnonoptimized silicone surfactants. Infusible pigments can sometimeshave an adverse flammability effect, particularly on smoldering, so it isimportant to check flame retardancy if pigmentation is added orchanged. Pigments containing iron, magnesium, or calcium are reputedto be particularly suspect.

Rebonded Foam

This type of foam can use up to about 75% scrap foam, and is usefulfor applications such as carpet underlay. Flame retardancy can be





usually met by about 10% of FYROL FR-2, which can assist as a fluxingaid in melt bonding.

Combinations of Polyurethane and Other Foaming Polymers

Particularly low density polyurethane-polychloroprene foams madeusing Chestnut Ridge’s reduced pressure foaming process show afavorable combination of fire resistance and physical properties, suchthat they are useful for aircraft seating, mattresses and other fire-regulated applications [68].

Basic Studies on Flammability and Flame RetardantAction in Flexible Foams

Foam density and type of foam were found to influence the ignitionand burn rate – generally burn rate increases as density decreases – butthere is also found a complicated interplay between char forming andmelting [69].

In a study at Polytechnic University, the mode of action of tris(1,3-dichloro-2-propyl) phosphate in a flexible polyurethane foam wasexamined. Two modes of action could be discerned. In upward burning,as in the California 117 test, the main process seemed to be physicalvapor phase action, since the FYROL FR-2 mostly volatilized from fire-exposed foam undecomposed, produced no char, and acted as aretardant whether it was uniformly distributed in the foam or merelyapplied on the outside. When injected into the flame, it did not seem toact as an inhibitor, surprisingly in view of the prevalent idea of halogencompounds as radical scavenging flame inhibitors. The foam itself alsoappeared to release toluenediisocyanate, particularly in the nonflamingpreignition stage, leaving essentially difficultly-ignitable polyol behind.However, in downward burning, FYROL FR-2 did help form a semi-charred ‘‘skin’’ so there was evidence of a condensed phase chemicalaction [70,71].

Polyurethane Elastomers and Cast Resins

A wide variety of flame retardants are used here. In general, itappears that flame retardants are more effective in nonfoamedelastomers; probably, this is because of less air access and lower





surface/volume ratio in the elastomer. This relationship was shown by acone calorimetric study comparing the effects of a variety of commercialflame retardants in foam and nonfoamed urethanes [72]. One surprisingresult in this study was the efficacy of zinc stearate in lengthening timeto ignition.

Insoluble high molecular weight APP, crystal type II, is useful inelastomers and cast urethane resins. It is effective on a weight basis andaffords generally low smoke. It is available as a finely divided powder(multiple sources) or as a thixotropic dispersion (Clariant EXOLIT AP452).

In Europe especially, there has been some use of red phosphorusas an additive for elastomers including polyurethane types, andpolyol masterbatches or dispersions are available (from Italmatch orClariant) which avoid the handling of the flammable red phosphoruspowder.

Hydrated alumina can be used in quite high loadings as a sole flameretardant or in combination with phosphorus additives, in polyurethaneelastomers. ATH not only retards flame but also suppresses smoke.Mineral fillers such as ATH but also clays and mica also providedimensional stability and reduction of thermal expansion, but with ATHalone, processing can be difficult because of viscosity. The level of ATHneeded to reach a flammability rating of V-0 can be reduced by usingAPP, red phosphorus, or a liquid organic phosphonate such as Rhodia’sANTIBLAZE 1045. For example, Clariant shows that V-0 can be reachedby about 140 phr ATH and about 10 phr EXOLIT 422 (APP) or EXOLITRP 652 (red phosphorus), where 350 phr of ATH alone would be neededin a particular cast polyurethane. Flame-retarded cast polyurethanescan have good electrical properties, suitable for insulator applications,but care is taken to select a particularly moisture-resistant grade of APPor red phosphorus.

Melamine alone, at rather high loadings, is effective to pass moderateflame resistance requirements in polyurethane elastomers [73]. Bothtris(chloroisopropyl) phosphate and tris(1,3-dichloro-2-propyl) phos-phate are effective in polyurethane elastomers. Where the halogen isundesired, it has been found possible to use tris(butoxyethyl) phosphate.Diphenyl octyl phosphate and other diphenyl alkyl phosphates can beused in polyurethane and other elastomers to obtain flame retardancywith relatively low smoke.

Expandable graphite is useful as a flame retardant in polyurethaneelastomer as well as in flexible foams [74] and has been used in someEuropean mattress foams (Dunlop). It is made from natural graphite byintercalating an oxidizing acid between the carbon layers. On heating, it





typically expands mostly in the range of 235–300�C [75]. This additiveis used in gaskets and seals. It loses its effectiveness if finely divided,so some granularity must be tolerated.

Reaction-injection-molded Products

This technique is used to make various items such as machineenclosures and furniture parts. The same flame-retardant additivesuseful for rigid foams can be used here.

An effective nonhalogen reactive system, with good heat distortionproperties and thus suitable for reaction injection molding, can beachieved by ‘‘fine tuning’’ the OH number of a phosphorus polyolmixture comprising the high OH number diethyl N,N-bis(2-hydroxy-ethyl)aminomethyl phosphonate (Akzo Nobel’s FYROL 6 or Bayer’sBAYTHERM) admixed with the low OH number oligomeric phosphatephosphonate (FYROL 51) [76].

Urethane Coatings and Sealants

Polyurethanes are one of the several binder types used in intumescentcoatings, which will be reviewed elsewhere as a separate subject. Thetypical formulation is a combination of a charring catalyst, (usuallyinsoluble high molecular weight APP), a char-former (usually mono-,di- or tripentaerythritol) and a blowing agent (melamine can performthis function in coatings). Typically these three ingredients are inabout a 3:1:1 ratio [77]. Polyurethane sealants, such as caulks andelastomeric strips, can be similarly formulated.

Nonintumescent flame retardant polyurethane coatings may be madeusing the brominated diols such as the diester diol from tetrabro-mophthalic anhydride (Albemarle’s SAYTEX RB-79 or Great LakesPHT-4 diol). As usual, antimony oxide may be added to boost the flameretardancy.

Firestops, which are flexible heat-expandable solids for pluggingopenings and gaps where pipes, conduits, and cables penetrate throughfire-rated walls, may be made from urethanes similarly formulatedwith intumescent ingredients. In some cases, hydrated alkali silicatesor expandable graphite can be used as the intumescent agent. APPand combinations thereof with intumescent char forming agents areoften used. Other soft polymers are used besides polyurethanes.Polyolefins, various rubbers, plasticized PVC, and silicones are foundin this field of application. A detailed discussion is beyond the scope ofthis review.





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