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2

Phosphorus-based FRs

Sergei Levchik

ICL-IP America, 430 Saw Mill River Rd., Ardsley, NY 10502

AbstractPhosphorus-based fl ame retardants are on the fast growing track mostly due to

environmental considerations, although sometimes effi ciency, lower density and

good light stability are signifi cant factors. Discontinuation of use of decabromo-

diphenyl oxide in polyolefi ns stimulated development of new intumescent fl ame

retardants and systems. Patents dealing with the fl ame retardancy of polycarbon-

ate and its blends are especially numerous. Well established resorcinol-based and

bisphenol A-based oligomeric aryl phosphates are included in many formulations

but there are also new developments directed to more thermally stable phos-

phates and phosphonates. Th ere are a substantial number of patents and academic

publications dealing with dialkylphosphinic acid salts, and, more recently, with

hypophosphite salts which are useful in thermoplastic polyesters and polyamides.

Largely driven by the waste disposal regulations and “green” marketing strate-

gies by OEMs, interest has increased in non-halogen fl ame retardant systems for

printer wiring boards. Many patents and publications have appeared on epoxy sys-

tems in which 9, 10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide is reacted

into epoxy polymer or used as a curing agent. Fast changing regulations in fur-

niture fi re safety stimulated development of new phosphorus-based reactive and

oligomeric fl ame retardants for fl exible polyurethane foams.

Keywords: Phosphorus fl ame retardant, intumescent, char, plastic, textile, epoxy

resin, polyurethane foams

2.1 Introduction

It is generally accepted that phosphorus fl ame retardants are more eff ec-tive in the oxygen- or nitrogen-containing polymers, which could be either

*Corresponding author: [email protected]

Alexander B. Morgan and Charles A. Wilkie (eds.) Non-Halogenated Flame Retardant Handbook,

(17–74) 2014 © Scrivener Publishing LLC

18 Non-Halogenated Flame Retardant Handbook

heterochain polymers or polymers with oxygen or nitrogen in the pen-dant groups. Phosphorus fl ame retardants are more specifi c to the polymer chemistry than halogen-based fl ame retardants. Th is relates to the con-densed phase mechanism of action where phosphorus fl ame retardants react with the polymer and involve it in the charring. Th e char impedes the heat fl ux to the polymer surface and retards diff usion of the volatile pyrolysis products to the fl ame.

However, if conditions are right, the phosphorus-based moieties can volatilize and be oxidized producing active radicals in the fl ame. Volatile phosphorus compounds are among the most eff ective inhibitors of com-bustion. However, it has been challenging to design phosphorus-based fl ame retardants, which will volatilize into the fl ame at relatively low tem-peratures and at the same time will not be lost during polymer processing. Th erefore, there are not many commercial phosphorus-based fl ame retar-dants which provide mostly gas phase action.

In the past the author of this chapter co-authored two reviews on phosphorus-based fl ame retardants [1–2]. Th is current chapter is an update and extension of the previous reviews. Th is chapter does not cover the large class of chloroalkyl phosphates since they are not halogen-free, but these products were reviewed previously. Although there is large body of aca-demic publications and patent literature on new phosphorus fl ame retar-dants, this chapter focuses only on commercial FRs and products which, to the best of author’s knowledge, are in advanced commercial development. Broader non-selective reviews were published elsewhere [3–4]. Th e eff ect of phosphorus fl ame retardants on human health and environment was recently reviewed by Van der Veen and De Boer [5]

2.2 Main Classes of Phosphorus-based FRs

Th e ammonium phosphate treatment of cellulosic materials (canvas, wood, textiles etc.) has been known for almost three centuries. However, only with commercialization of synthetic polymeric materials in the twentieth century, organophosphorus compounds have become an important class of fl ame retardants.

All phosphorus-based fl ame retardants can be separated into three large classes

• Inorganic represented by red phosphorus, ammonium phos-phates and metal hypophosphates.

Phosphorus-based FRs 19

• Semi-organic represented by amine and melamine salts of phosphoric acids, metal salts of organophosphinic acids and phosphonium salts.

• Phosphate and phosphonate esters.

Phosphate esters is the most diverse class of phosphorus fl ame retardants which can be further separated into

• Aliphatic phosphates• Aliphatic chloro-phosphates• Aromatic phosphates• Phosphonates• Phosphinates • Phosphine oxides (not in commercial use)• Phosphazenes

Water-soluble phosphorus fl ame retardants mostly used for topical treatment of wood, textile and other cellulosic products. Some water sol-uble FRs can be further reacted with cross-linkers (cured) which provides durable water resistant treatment. Water-insoluble phosphorus FRs fi nd a very broad range of applications in thermoplastics, thermosetting resins, synthetic foams, coatings etc.

Phosphorus fl ame retardants have certain advantages over other fl ame retardants (mostly halogen based) but also have some disadvantages which are both listed below:

Advantages:

• Low specifi c gravity which results in light plastic parts• Achieving fl ame retardant effi ciency at lower phosphorus

content compared to the halogen content needed for the same rating

• No need for antimony trioxide synergist• Eff ective in promoting char barrier/formation in charrable

polymers• Better UV stability than most halogen-based FRs• Less tendency to intensify smoke obscuration• High comparative tracking index (CTI) test performance• Less acidic smoke compare to halogen FRs• Most phosphorus FRs are biodegradable and therefore not

persistent


• No halodioxin/furan formation (provided no halogen in phosphorus FR structure) even in poor incineration of the plastics

Disadvantages:

• Very low effi ciency in polyolefi ns, styrenics and elastomers unless charring agent is added.

• Absence of good general synergist.• Many phosphorus FRs are hydrophilic and possibly cause

moisture uptake limiting use in some applications.• May hydrolyze to give acids which decrease molecular

weight of acid-sensitive polymers (polycarbonates, polyes-ters, polyamides etc.)

• Apart from red phosphorus, inorganic phosphates have low thermal stability and therefore their use is limited to low processing temperature polymers

• Recycling of acid sensitive polymers is problematic due to hydrolytic instability of organophosphates.

• Some phosphates are toxic to aquatic organisms.• Apart from a few selected cases, the cost/effi ciency of phos-

phorus FRs is higher than halogen based FRs.

2.3 Polyolefi ns

Upon thermal decomposition, polyolefi ns produce signifi cant amounts of aliphatic hydrocarbons which are highly fl ammable. Furthermore, poly-olefi ns melt, fl ow and drip during combustion because of the relatively low melting point of these polymers. Th ey burn relatively cleanly with very little, if any, char left behind. All of this creates serious challenges in fl ame retardancy of polyethylene, polypropylene and their copolymers. Although polyethylene and polypropylene produce similar aliphatic hydrocarbons, polypropylene is relatively easier to fl ame retard because it decomposes at lower temperature and there is better match with the temperature range of decomposition of common fl ame retardants.

Th e successful fl ame retardants for polyolefi ns have usually been hal-ogen types, most oft en synergized by antimony oxide, or endothermic types used at high loadings, like ATH or magnesium hydroxide. It is also generally accepted that phosphorus based fl ame retardants are ineffi cient in polyolefi ns unless they provide signifi cant gas phase effi ciency or are


combined with an intumescent system. In order to adapt most common phosphorus FRs for the latter, they should be utilized along with a char-ring agent. In the past, a signifi cant eff ort was made by industry and aca-demic laboratories in development of intumescent fl ame retardant systems for polyolefi ns. Th e intumescent fl ame retardant systems require three essential components: (1) a charring agent, typically pentaerythritol, (2) a strong acid which promotes charring, usually originated from decomposi-tion of ammonium phosphates and (3) a foaming agent which is typically melamine or a melamine salt. Th e intumescent systems concept was origi-nally developed for fl ame retardant coatings [6] and later adapted for low temperature processed polymers, like polyolefi ns.

Numerous academic publications on intumescent fl ame retardant sys-tems for polyolefi ns are out of the scope of this chapter. Th e broad subject of intumescent fl ame retardants was discussed in a book edited by Le Bras et al. [7] and also in a more recent review [8] and book chapter [9].

Although intumescent systems based on ammonium phosphates are very effi cient in polyolefi ns the main factors limiting their broad applica-tion are thermal stability and water solubility. Both thermal stability and water solubility can be improved by increasing the chain length (molecular weight) of the polyphosphate. Two crystalline phases of ammonium poly-phosphate (APP, forms I, and II) are commercially sold as fl ame retardants.

It is believed that form I has a linear chain structure and relatively low molecular weight (from 30 to about 150 repeating units). Form I has relatively low thermal stability (onset of weight loss at about 230°C) and relatively high water solubility. Th is form is available from ICL-PP as Phos-Chek® P30. It is mostly used in coatings.

Th e form II is available from Clariant as Exolit® 462 and related prod-ucts, from Budenheim in their FR CROS product group, from ICL-PP as Phos-Chek® P40 and from numerous Asian producers. It is believed that form II has a cross-linked structure [10] and its molecular weight is much higher (700–1000 repeating units) than form I. Form II is more thermally stable (beginning of thermal decomposition at about 270°C) than form I and less water soluble. Many varieties of APP form II with various coat-ings/encapsulations which further decrease water solubility are commer-cially available. For example, Budenheim off ers a range of surface coated APP as FR CROS 486- a silane surface-reacted, FR CROS 487 – melamine formaldehyde resin coated, FR CROS C30/C40 – melamine surface reacted and FR CROS C60/C70, FR CROS 489 – melamine-formaldehyde surface reacted [11]. ICL-PP off ers the coated grade of APP as Phos-Chek® P42. Th ese surface treatments allow decreasing water solubility of form II APP from 0.5 to 0.01–0.1 g/liter. Th ese coatings can also provide a synergistic


eff ect to APP because they can work as charring agents to further enhance the activity of APP.

Recently G. Liu et al. [10, 12] reported commercially viable processes of synthesis of APP, Form V. Th is form has similar thermal stability and water solubility as Form II. It is believed that Form V is also high molecu-lar weight cross-linked polyphosphate, but the branching units instead of ultraphosphate as in Form II, consist of triazine structures. To the author’s best knowledge there is no commercial production of the Form V at the time of writing of this chapter.

Over many years, APP producers and compounders tried to develop fl ame retardant compositions (formulated packages) which included along with APP the charring and foaming agents. Nitrogen-containing low molec-ular weight or polymeric products behave the best because they combined both charring and foaming functions. For example patents suggest that an early Exolit®IFR by Hoechst contained tris(hydroxyethyl)isocyanurate [13, 14]. Another combination of charring agent with APP was developed at Himont (now Basel) as Spinfl am® MF80 and MF82, [15] where an oligomer consisting of triazine rings linked by diamine was used in those products [16]. Oft en urea-formaldehyde or melamine-formaldehyde resins are used in two functions as encapsulating and charring agents. Active research on APP coatings continues in China, where for example this recent publication [17] shows use of poly (p-ethylene terephthalamide) as a charring agent.

A Swiss company MCA technologies recently introduced the condensa-tion product of melamine, morpholine and piperazine (PPM Triazine HF) as a charring agent and synergist with APP [18, 19]. Reportedly, APP com-bined with PPM Triazine HF provides a V-0 rating in PP at only 20 wt.% loading [20]. Interestingly, aliphatic polyamides which are considered as not very charrable can be also used along with APP as a charring agents. For example, the group of French researchers [21] developed a formulated system of APP/polyamide 6/EVA, where EVA is used as a compatibilizer for polyethylenic polymers.

It has been long recognized [22] that addition of a small amount (typi-cally 2–3 wt.%) of multivalent metal salts or oxides provides synergistic eff ect in APP based intumescent systems. Some natural products like talcs, zeolites [23] and clays show similar behavior. Th e synergistic eff ect is observed in a very narrow concentration range and it is believed to be due to formation of cross-links in polyphosphoric acid involving multivalent metals [24]. Increasing the concentration of the synergist results in forma-tion of stochiometric crystalline phosphates which negatively aff ect intu-mescence and the eff ect switches from synergistic to antagonistic. In the recent academic literature there are numerous publications on addition of


organically modifi ed clays to the intumescent systems. Synergistic eff ects are oft en, perhaps erroneously, attributed to the physical eff ect of the clay reinforcing char, whereas it could be the same eff ect of chemical interac-tion with polyphosphoric acid and cross-linking.

More advanced formulated systems are on the market nowadays. For example Clariant off ers two series of Exolit® AP 75x and AP 76x. Exolit® AP 750 is a standard grade of the formulated system suitable for polypro-pylene and polyethylene, AP 751 (TP) is a special grade for reinforced polypropylene and AP 752 is for PP copolymers. All these grades require about 30 wt.% loading to achieve a V-0 rating, which is a lower loading than needed for mineral fl ame retardants (ATH and MDH) and some halogen-based systems. A proprietary improvement, Exolit AP 760, has been introduced and this product appears especially suited for cable ducts and trays. AP 765 grade provides further improvement in the glow wire ignition test (GWIT) which achieves 800°C [25]. Budenheim sells APP based systems containing a charrable component under the trade names BUDIT 3077 and BUDIT 3076 DCD [11]. Th e main advantages of these APP-based systems over halogenated fl ame retardants are lower smoke and excellent UV stability. However, relatively high hydrophilicity still lim-its their application in electrical and electronic products and some outdoor products. Although cable manufacturers are trying to adopt APP or APP formulated systems in the cables jacketing [26] it seems to still have a very limited application due to the water absorption issues.

Th e mechanism of char formation in the pentaerythritol-APP systems was very extensively studied and described in great detail by Camino and Delobel [27]. Since some of the principal intermediates in the char for-mation are bicyclic or spirocyclic pentaerythritol phosphates, signifi cant eff ort has been channeled into development of pentaerythritol phosphate-based intumescent fl ame retardants.

For example, in the past, Great Lakes marketed bis(melamine) salt of pentaerythritol bis(acid phosphate) (Formula 2.1) for use in polypropylene. Originally this product was developed in the Borg-Warner laboratories, [28] later it was studied at Alcan Chemicals [29] in the UK and apparently it still continues to be of interest in China, as shown by recent studies [30, 31]. A decade ago, Budenheim introduced to the market phosphate esters of aliphatic alcohols (probably pentaerythritol) having acid groups neutral-ized with melamine under trade names BUDIT 3118 and 3118F [11, 32]. Th e structure of these products can be similar to that shown in Formula 1. Th ese products can be used as intumescent additives in coatings and poly-propylene sheets and fi bers. Th ey allow smoother surfaces in those sheets and fi bers probably because they melt or partially melt during processing.


(2.1)N

N

N

NH3+O–

PO

O

OP

O –O+H3N

N N

NH2N

NH2

NH2

NH2

O O

Another product of high interest for the intumescent fl ame retar-dants is pentaerythritol bicyclic phosphate (PEPA) (Formula 2.2). Th is phosphorus-containing alcohol was available from Great Lakes, but proba-bly discontinued now. It is available in China. PEPA can be further reacted with phosphorus oxychloride to form phosphate with four phosphorus atoms. Reportedly, this phosphate is commercial in China [33] . Th e salt of di-PEPA acid phosphate and melamine (Formula 2.3) was also in advanced development [28, 29].

(2.2)HO CH2 COP

OO

O

O PO

OCH2 O P

OO

O CH2OP

OO

O

O-+NH3

N

N

N

NH2H2N

(2.3)

Th e blends of PEPA and melamine phosphate were more successful as suggested in patents of Great Lakes’ researchers [34, 35]. It is believed these mixtures were the basis for Great Lakes Reogard® 1000 and 2000 products, recommended for extruded profi les and electrical parts to meet a V-0 rat-ing with good impact strength and heat distortion temperature. For exam-ple, at 19% of the PEPA-melamine phosphate mixture with 0.8% (amount critical) of the montmorillonite, a V-0 rating was obtained in polypropyl-ene [34, 35].

Melamine phosphate also has been originally developed for intumescent coatings but found some use in polyolefi ns. Later additions to this fam-ily of the more thermally stable melamine pyrophosphate and melamine polyphosphate ensured safe processing in most polyolefi ns. Non-coating applications of the melamine phosphates (including the pyrophosphate) were reviewed by Weil et al. [36]. Melamine phosphate and pyrophos-phate are available in the USA from Cytec Industries and Broadview


Technologies. Melamine polyphosphate is sold in the USA and Europe by BASF as Melapur® 200. In an intumescent formulation in a polyolefi n, the melamine phosphates such as Melapur® 200 have been shown to have an advantage over ammonium polyphosphate by causing less mold deposition and having better water resistance [37]. However melamine phosphates are also less effi cient than APP, because they are more thermally stable and have lower phosphorus content.

Th ere are also intumescent fl ame retardants which are based on diff er-ent amine salts other than melamine salts. It is advantageous to use at least a diamine which allows higher thermal stability and lower water solubility compare to monoamines. An example of such successful product is eth-ylenediamine phosphate (EDAP) fi rstly introduced by Albright & Wilson as Amgard® EDAP in the late 80s. [38]. In contrast to APP and melamine salts EDAP shows self-intumescent behavior because it melts at about 250°C, right around where its thermal decomposition starts and because it contains aliphatic carbons which undergo charring. So it quickly melts and activates as an intumescent once it reaches this temperature. EDAP is more soluble in water compared to the form II of APP and less thermally stable which limits its applications in polyolefi ns. In the USA EDAP is available from Broadview Technologies, Unitex Corp and JJI Technologies.

In order to improve thermal stability and decrease water solubility EDAP has oft en been sold as a mixture with melamine or melamine phosphates. Some of these mixtures are also synergistic because the temperature of ther-mal decomposition of EDAP and melamine phosphates is diff erent and the extended temperature interval better matches the thermal decomposition of the host polymer. Some further synergists, such as phase transfer cata-lysts (quaternary ammonium salts) or spirobisamines may further enhance the action of EDAP and melamine pyrophosphate or APP combinations as claimed in Broadview [39] and JJI [40] patents. Recently Th or GmbH introduced a new intumescent fl ame retardant Afl ammit® PPN903 which reportedly does not contain APP but has high thermal stability (>270°C), low solubility and better acid resistance [41]. Various inorganic synergists, like talc or zinc borate, were reported [42] for phosphorus-based intumes-cent systems.

Another interesting development was use of ammonium salt of amide aminomethyl phosphonic acid (Formula 2.4) as a self-intumescent FR [43]. At 25 wt.% it showed LOI=29 and UL-94 V-0 rating in polypropylene. Th is product was developed in Russia and briefl y marketed by Isle Firestop (UK) under a trade name of Bizon. Because of water solubility it was sold as a coated/encapsulated form.


NCH2

PH2N ONH4

O

CH2 P

O

NH2

ONH4CH2PO

NH2

NH4O(2.4)

Intumescent systems based on the mixed salts of melamine and pipera-zine phosphates were fi rst developed in Italy by Montel [44] (Basel now) and marketed as Spinfl am® MF-83 for wire and cable applications [45]. Recently, Asahi Denka [46] developed improved method of synthesis of piperazine pyrophosphate, which allows obtaining a product with superior thermal stability. Another patent [47] shows milling of piperazine pyrophosphate together with melamine pyrophosphate and addition of some polymethylsi-loxane oil probably for decreasing dusting and improving processability. Th is is probably the basis for the new Asahi Denka ADK STAB FP-2200 product [48]. Th is intumescent fl ame retardant is said to be eff ective in polypropylene at about 20%, and in LDPE, HDPE or EVA at about 30%. It is stable enough to permit extrusion and molding at 220–240°C. It appears to be better than the previous intumescent system ADK STAB FP-2100 in regard to water resis-tance, processability, shear stability, heat and mechanical properties.

In spite of the fact that red phosphorus is eff ective in both condensed and gas phase [49] it achieves V-0 rating only in charrable polymers, mostly engineering thermoplastics. In polyolefi ns, red phosphorus was found useful for a V-2 rating and high LOI especially in polyethylene [50]. It is believed there is better match between the temperature of the thermal decomposition of polyethylene and the volatilization of red phosphorus compare to polypropylene [51]. It was shown that a V-2 rating at 1.6 mm could be obtained at as low as 2.5% fi nely-divided red phosphorus (5 μm)[52]. To obviate the risk of handling fi nely-divided red phosphorus, mas-terbatches of encapsulated red phosphorus are available from Italmatch in various polyolefi ns [53] a nd from Clariant in low melting wax and novolacs [54]. Th e masterbatches in polyamide 6.6 and other charrable polymers are also useful as additives in polyolefi ns [53] because they provide charring to enhance the fl ame retardant eff ect of the phosphorus [55].

Some time ago Italmatch introduced to the market aluminum hypo-phosphite (Phoslite® IP-A) and calcium hypophosphite (Phoslite® IP-C) [56]. A surface treated version of the aluminum salt, Phoslite® B85AX combined with a NOR type hindered amine stabilizer shows a V-2 rating in PP copolymers and homolymers at 6 wt.% loading. Phoslite® B361C and B712A are combinations of the hypophosphite salts with brominated FRs, the most effi cient of which seems to be melamine hydrobromide [57].


Th ese synergistic blends allow achieving a V-2 rating in PP copolymers at a level below 3 wt.% especially when combined with a free-radical initiator or NOR type hindered amine [58]. At such a low loading, the content of bromine in the polymer is below 900 ppm, which qualifi es it as halogen-free according to IEC 61249-2-21.

Recently, Th or introduced to the market under the trade name Afl ammit® a series of phosphorus-nitrogen FRs based on proprietary organophospho-rus FRs and their blends. Afl ammit® PCO 700, containing 19.5% phos-phorus and 17.5% nitrogen requires 4–8 wt.% to pass DIN 4102 B2 rating in 50–500 μm LDPE fi lm and only 2–3 wt.% when combined with NOR HALS synergist [59]. Afl ammit® PCO 800 containing 14% phosphorus and 37% nitrogen is less effi cient than PCO 700 but more thermally stable and less water sensitive. Patent literature [60] and a recent presentation [61] indicates that this could be proprietary blends or reaction products of a bicyclic phosphonate and melamine phosphates.

Another phosphorus-based FR [62] f or polyolefi n fi lm applications (PP and EVA) was recently introduced by ICL-IP under trade name Fyrol® P26. Having small particles of D

50 = 2.5 μm and being properly dispersed in PP

or EVA the fi lm shows a translucent appearance. Fyrol® P26 provides a V-2 rating in EVA at 4 wt.% loading and in PP at 6 wt.% loading. Th e polyole-fi n fi lm also passes the DIN 4102 B2 test. Low water solubility and non-migratory performance in polyolefi ns are other advantages of Fyrol® P26.

Aromatic bisphosphates can be liquid or solid, but independently of the physical form, they have very limited compatibility with polyolefi ns. Interestingly it was found [62] that aromatic bisphosphates can be loaded in PP plus EVA at 5 wt.% without visible exudation aft er heating for 72 hours at 70°C. A solid bisphosphate Fyrolfl ex® Sol-DP showed slightly bet-ter performance than liquid Fyrolfl ex® RDP. Th e fi lms with 5% bisphos-phate showed an HB rating in the UL-94 test. Interestingly, the maximum loading of triphenyl phosphate achievable in PP and EVA without exuda-tion was only 3 wt.%. It is believed that bisphosphates can be used in PP fi bers, fi lms and foams to provide some level of fl ame retardancy.

2.4 Polycarbonate and Its Blends

While they are commonly used in polycarbonate based blends, aromatic phosphates are not much used in plain PC because of reduction in clarity, ten-dency to stress-crack and somewhat reduced hydrolytic stability. In the mid ‘80s Bayer developed oligomeric methylphosphonates (Formula 2.5) which can be further copolymerized with polycarbonate to produce transparent


fl ame retardant polymer. Th is technology was not commercialized by Bayer but recently FRX Polymers developed an improved process and started com-mercialization of the polymeric phosphonate as Nofi a® HM1100 and copo-lymers with PC as Nofi a® CO3000 and CO600 [63]. Th e homopolymer can be used as an additive in PC and some other engineering resins. Th e copo-lymer can be used alone as an inherently fl ame retardant plastic or blended with PC or other polymers. Oligomeric methylphosphonates (Formula 2.5) are also available for special applications mostly in thermosets.

HO CCH3

CH3

O PO

OCH3

CCH3

CH3

OHn

(2.5)

Aromatic phosphates or aromatic phosphate oligomers (mostly diphos-phates) are very widely used in PC/ABS blends. Historically triphenyl phosphate (TPP) was the fi rst phosphorus-based fl ame retardant used in PC/ABS. Although TPP is soluble in PC/ABS and doesn’t bloom out at room temperature, it deposits on the mold surfaces during molding. Because of the low melting point of TPP (48°C), it leads to bridging at extrusion feeding ports. Nevertheless, TPP still fi nds limited use in Asia because it is very inexpensive. Th e next generation of aromatic phosphate FR in PC/ABS was isobutylated-phenyl phenyl phosphates. Th ese are mix-tures of triaryl phosphates of diff erent degree of butylation also containing free TPP. Th ese phosphates are liquid and somewhat less volatile than TPP.

In recent years, oligomeric aromatic phenyl phosphates (mainly diphos-phates) are fi nding broader application than monophosphates because of better thermal stability and lower volatility [64]. For example resorcinol bis(diphenyl phosphate) (Formula 2.6) available from ICL-IP under the trade name Fyrolfl ex® RDP, is a mixture of oligomers with two to fi ve phosphorus atoms, but with the distribution heavily shift ed towards the diphosphate [64]. RDP is also available from Great Lakes Chemicals under trade name Reofos® RDP, Daihachi (Japan) under trade name CR-733S, Yoke (China) under trade name PhireGuard RDP and Wansheng (China) under trade name WSFR-RDP. In commercial PC/ABS blends where ABS content normally does not exceed 25%, RDP gives a V-0 rating at 8–12 wt.% loading [65, 66].

O PO

O O PO

OO O

n(2.6)


Poly(tetrafl uoroethylene) (PTFE) is a necessary ingredient in the for-mulation, which is usually added at <0.5 wt.% to retard dripping. Since the glass transition temperature of PTFE is below room temperature, it is soft and diffi cult to handle. To improve PTFE feeding it can be added during production of ABS so that it is embedded in the polymer [67], or it can be specially treated to become free-fl owing [68], or pre-processed as a masterbatch [69]. RDP is somewhat less hydrolytically stable compared to other bisphosphates, which limits its application in humid environ-ments and may cause a problem in recycling. Th is shortcoming of RDP can be alleviated by adding acid scavengers such as epoxies, oxazolines, or ortho esters [70] or oxetanes and hydrotalcite [71]. Inorganic co- additives such as highly dispersed silica [72] or talc [73] can improve physical properties, notably high temperature dimensional stability. Co-addition of organoclay [74] can result in improved fl ame retardancy by decreas-ing aft erfl aming time. Resorcinol bis(diphenyl phosphate) with very low TPP content (0.1 wt.%) has recently became available from ICL-IP as Fyrolfl ex® RDP-HP [75].

B isphenol A bis(diphenyl phosphate) (BDP) (Formula 2.7) was intro-duced to the market in the late 90s as an alternative to RDP. Since bisphe-nol A is less expensive than resorcinol, it was believed that BDP may be more cost effi cient in spite of lower phosphorus content (8.9% P for BDP vs. 10.7% P for RDP). BDP is signifi cantly more viscous than RDP (12500 cP for BDP vs. 600 cP for RDP at 25°C) and therefore it requires a heated storage tank and heated transfer lines, whereas RDP needs only heated transfer lines. Because of the high viscosity, the oligomers content (n>1, Formula 2.7) in the BDP mixture is usually limited to only 10–15% which creates a problem of potential crystallization of BDP during transportation. In recent years BDP was also undergoing scrutiny from environmentalists because it can potentially hydrolyze and release bisphenol A. However, this doesn’t make much sense since the PC is also made from bisphenol A and it can hydrolyze as well, so the contribution of BDP to the bisphenol A release is probably minimal.

O PO

OO C

CH3

CH3

O PO

OO n

(2.7)

In spite of many disadvantages over RDP, BDP because of its lower cost became the major product used in PC/ABS and the second largest


phosphorus-based fl ame retardant produced. On the positive side BDP has better hydrolytic stability than RDP [76] and can be used in high humid-ity applications especially if it is further stabilized by adding epoxy [77], oxetane [78] or calcium carbonate [79] as acid scavengers. BDP is mostly produced in Japan (Daihachi, CR-741 and Adeka, ADK STAB FP-600) and China (Yoke, PhireGuard BDP and Wansheng, WSFR-BDP). PC/ABS with ABS at ≤ 25 wt.% usually needs ≥ 12 wt.% BDP plus a small co-addition of PTFE in order to assure a V-0 rating [80, 81].

BDP and RDP are also used in PC/PBT and PC/PET but further addi-tion of impact modifi er, for example polyethylene copolymer [82] or core-shell copolymer [83] is needed. Recently, new fl ame retardant blends of PC/PMMA [84] (copolymer of methyl methacrylate and phenyl methacrylate) which produce very high gloss were introduced to the market. New FR blends using as one component a bio-based polymer PC/PLA [85] (poly-lactic acid) are also being explored for the use in electronic equipment. Th e content of the bisphosphate in these blends depends mostly on PC content, the higher PC content less bisphosphate is required to achieve V-0 rating.

Although major compounders of PC based blends are likely to be well equipped with liquid feeding systems, small and medium size compound-ers prefer to use solid bisphosphates even if they cost more than BDP. For over 10 years Daihachi was manufacturing resorcinol bis (2, 6-xylyl phosphate) (RXP) [86] (Formula 2.8) under the trade name PX-200. It is also available from Adeka as ADK STAB FP-500. In contrast to RDP and BDP, RXP is mostly pure bisphosphate with very little oligomers present. Because of specifi c chemical structure and high purity RXP is a solid with a melting point of 95°C. Th e steric hindrance provided by the 2, 6-xylyl groups makes this product more hydrolytically stable than BDP. Because of high purity and low TPP content RXP was a preferred FR for high temper-ature thin wall molding where good resin fl ow is required, however nowa-days low TPP BDP is also used in such applications. RXP has phosphorus content of 9.0% and its fi re retardant effi ciency is similar to that of BDP; it provides a V-0 rating in PC/ABS at 12–16 wt.% loading, [87].

O PO

OO

CH3

CH3

H3C CH3

O PO

O

OH3C CH3

H3C

H3C

(2.8)


Recently a few new specialty bisphosphates were introduced to the mar-ket. For example ICL-IP has introduced a new solid bisphosphate [75], Fyrolfl ex® Sol-DP. Th is product is targeting medium and small compound-ers who are not equipped with liquid feeding systems or large compounders where fl exibility of changing extrusion lines is desirable. Because Sol-DP has a higher melting temperature (105–108°C) than RXP it is easier to feed without extensive cooling of the feeding zone. Sol-DP has a higher phos-phorus content of 10.8% compare to RXP and therefore requires 20–25% lower loading in PC/ABS and other PC based blends.

4,4’- Biphenylbis(diphenyl phosphate) (Formula 2.9) a specialty bispho-sphate for high temperature molding of glass-fi lled PC and PC/ABS was recently introduced to the market by Adeka under the trade name ADK STAB FP-800. It has a melting range of 65–85°C and a phosphorus content of about 9.5%. According to Adeka technical literature it gives a V-0 rating in PC at 3.5 wt.% loading and 0.3 wt.% PTFE. As measured by thermo-gravimetry, FP-800 shows 5 wt.% loss at about 405°C which is signifi -cantly higher than other bisphosphates. Recently Daihachi introduced to the market a new aromatic bisphosphate PX-202 [88] identifi ed in early Daihachi patent [89] as 4, 4’-biphenyl bis (2, 6-xylyl phosphate). PX-202 has a melting point of 185°C and its phosphorus content is 8.1%. It shows exceptional hydrolytic and thermal stability but requires twice higher load-ing for a V-0 rating compared to RDP.

O PO

OO O P

OO

O n(2.9)

Recent studies [90, 91] on the mechanism of fl ame retardant action of aromatic phosphates revealed that BDP shows mostly condensed phase action, RDP mixed condensed phase and gas phase, whereas TPP is almost exclusively gas-phase-active. Th is was attributed to the temperature of decomposition of PC and phosphates, e.g. TPP evaporates at a relatively low temperature and doesn’t have a chance to react with PC, whereas bisphosphates react with PC [92, 93]. RDP or BDP which are mostly con-densed-phase-active additives tend to cause PC to produce more char, decreasing the fuel supply to the fl ame and decreasing fl ame temperature. TPP, which has gas phase activity, becomes more eff ective in the gas phase with decreasing fl ame temperature. Another study showed [94] that the hindered structure of 2, 6-xylyl phosphates slows down the reaction with


PC and therefore PX-200 and PX-202 show less condensed phase action compared to RDP and FP-800 respectively.

Cyclic phenoxyphosphazenes are thermally and hydrolytically stable phosphorus-nitrogen products with favorable electrical properties. A blend consisting of tri- and tetra-phosphazenes with some larger rings was eff ective in PC/ABS at 12 –15 wt.% [95]. Reportedly, phosphazenes showed higher heat distortion temperature compare to aromatic bisphosphates [96]. Bisphosphates (RDP or BDP) or monophosphates (TPP) are syner-gistic with cyclic phosphazenes [97, 98]. Cyclic phenoxyphosphazenes are available from Otsuka under the trade name SPB-100 and from Fushimi as Rabitle® FB110. Th e phosphazenes are fi nding commercial use mostly in Asia.

Many phosphoramides are high melting solids, and because they don’t plasticize PC/ABS there is some expectation that they would favor high heat distortion temperature (HDT). About a decade ago GE (now SABIC) patented a series of bisphosphoramidates with a piperazine bridging unit [99]. For example piperazine bis (2, 6-xylyl phosphoramidate) can provide a V-0 rating at 13.5 wt.% in PC/ABS and as expected they also provide high HDT, but the impact strength was lower than traditional bisphosphates [100]. In spite of signifi cant eff orts, these phosphoramidates seems never been commercialized. A series of new bisphosphoramidates with one or more phenyl groups replaced with morpholine rings, eff ective in PC/ABS at 12 wt.%, was recently patented by Cheil (Korea)[101].

2.5 Polyphenylene Ether Blends

Polyphenylene ether (PPE) is a polymer synthesized by self-condensation of 2, 6-xylenol to produce a polyether chain. Polyphenylene ether cannot be processed alone because of very high melting temperature, but it is readily compatible with many polymers and can be processed as a blend. Depending on the molecular weight and chain ends PPE can be blended with HIPS, polyamides, styrenic elastomers and even epoxy. Apart from improving physical properties of the host polymer, PPE is an excellent charring polymer due to its specifi c thermal decomposition mechanism (Formula 2.10) where PPE undergoes Fries isomerization [102] and forms a phenolic type of resin with numerous OH groups which are reactive with phosphorus FRs.

O O O

CH3

CH3 CH3

CH3 CH3

CH3

CH2 CH2 CH2

OH OH OH

CH3 CH3 CH3

(2.10)


Commercial PPE/HIPS blends, also known as modifi ed PPE, contain from 35 to 65% PPE. Similar to PC/ABS, the fi rst FR used in PPE/HIPS was TPP [103], which was later replaced with RDP and BDP [104]. Typically between 9 and 15 wt.% of phosphate ester is needed to achieve V-0 rating; the lower the PPE content in the blend, the higher the phosphate load-ing is required. PTFE is required to prevent dripping. A copolymer of polydimethyl- and polydiphenyl siloxane can prevent dripping and is at the same time synergistic with RDP [105]. Interestingly the use of 3 wt.% of syndiotactic polystyrene also prevents dripping [106]. Apart from fl ame retardancy, phosphate esters play an important role of plasticization and resin fl ow improvement. Th erefore some phosphate esters can be added to PPE/HIPS even if the fl ame retardancy is not needed. BDP doesn’t have advantages over RDP in PPE/HIPS because PPE is not sensitive to hydroly-sis, therefore RDP can be used in high humidity applications, as for exam-ple parts of water pumps.

In the literature there are numerous patents on combinations of aro-matic bisphosphates with various co-additives intended to improve fl ame retardant effi ciency or physical properties. For example, the addition of zinc borate to the PPE/HIPS fl ame retarded with a triaryl phosphate resulted in a signifi cant decrease of smoke production [107]. As shown in a GE patent application, [108] co-addition of 0.1–0.5 wt.% of organoclay helps to cut aft erfl aming time in the UL-94 test. Another GE patent shows addition from 1 to 10 wt.% low melting glass (490–550°C) boosts the FR effi ciency of RDP [109]. Th e addition of a branched polyester to PPE/HIPS fl ame retarded with RDP helps improving fl ow and enhances some physi-cal properties [110]. Pentaerythritol [111] or melamine polyphosphate with ferrocene [112] are also synergistic with RDP allowing a decrease of RDP loading from 15 to 12 wt.%.

About a decade ago, SABIC introduced to the market new PPE based blends (Flexible Noryl®) with their main intended use in wire and cable insulation. Th e basis of these blends is PPE and thermoplastic elastomers (TPE) but oft en polyolefi ns and HIPS are also included in the blends. As the patent literature indicates, aromatic bisphosphates RDP [113, 114] and BDP [115, 116] were originally used as fl ame retardants in PPE/TPE blends. Bisphosphates are very compatible and soluble in PPE, but not in TPEs and polyolefi ns and because of this, total loading of bisphosphates is limited because of potential exudation. To overcome this problem in the blends containing less than 50% PPE, solids not soluble in the polymer FR is added along with bisphosphates. Patent literature shows combinations of aromatic phosphates with magnesium hydroxide [117, 118], mel am ine phosphates [117, 118] ammonium polyphosphate [119]. Aluminum diethylphosphinate


(Exolit® OP930 or Exolit® OP 1230, Clariant, formula 2.11) alone [120] or in combination with melamine polyphosphate [121].

PPE/polyamide blends are preferably fl ame retarded with aluminum diethylphosphinate alone or combined with melamine polyphosphate or melamine cyanurate [122, 123]. 10 wt.% red phosphorus provides a V-0 rating in compatibilized PPE/polyamide 6.6 blend [124].

Mechanistic studies of the fl ame retardant action of bisphosphates and TPP showed that they catalyze the Fries rearrangement [125] (Formula 2.10) and promote charring and improve the morphology of the char by making it intumescent-like [126]. One comprehensive study [127] looked at a large number of substituted aromatic phosphates and bisphos-phates and compared them with red phosphorus and aliphatic phosphates. Th is study concluded that aliphatic phosphates are the least effi cient because they decompose at the temperature much lower than the decom-position temperature of PPE. Th e effi ciency of all aromatic phosphates and bisphosphates was similar in the range of experimental error and directly proportional to the phosphorus content. Effi ciency of red phosphorus was similar to that of aromatic phosphates at the same phosphorus concentra-tion. However the strongest factor which controls fl ammability of PPE/HIPS blends was PPE content.

2.6 Polyesters and Polyamides

Th e requirements for phosphorus FRs in polyesters and polyamides are stringent because of high processing temperatures, sensitivity to hydrolytic degradation catalyzed by possible acids or catalytic decomposition assisted by some metals. Since the most common use of fl ame retardant polyes-ters and polyamides is in connectors, there is a requirement for long-term dimensional stability, which means minimal water absorption which espe-cially diffi cult to maintain with polyamides. Because polyesters and poly-amides are semicrystalline and a fl ame retardant can be accommodated only in the amorphous regions, there is an issue of exudation (“bloom-ing”) of low molecular weight fl ame retardants. Th ese requirements have eliminated many phosphorus-based fl ame retardants for consideration in polyesters and polyamides.

For many years, the only phosphorus fl ame retardant useful in polyes-ters and nylons was red phosphorus (polymeric form of elemental phos-phorus). Historically, red phosphorus was mostly used in Europe and Asia and almost not used in North America [128]. Red phosphorus is a contro-versial FR because on one hand it is very effi cient, allowing achievement


of a V-0 rating in glass-fi lled polyamides or polyesters at only 6–12% wt. loading and on the other hand it is very combustible and the powder can easily ignite if heated in open air. Being exposed to moist air, red phos-phorus slowly reacts with water producing phosphine and various phos-phorous acids. Th is doesn’t create problems in molded plastics, but storage and processing of red phosphorus should be monitored very carefully. For safe handling, red phosphorus is usually coated and stabilized with metal oxides [129] or metal salts [130, 131] or hydrotalcite [132] which can react and scavenge the phosphine as it forms. Red phosphorus is also available in the form of masterbatches with a variety of polymers, for example from Italmatch [133] or in the form of low melting concentrates (phenolic resin and wax) from Clariant [134]. Another disadvantage of red phosphorus is its red color which is diffi cult to overcome even with a high concentra-tion of other colorants. Because of this, red phosphorus containing molded parts are typically pigmented black [135].

Red phosphorus is particularly useful in glass-fi lled polyamide 6.6 where a high processing temperature (> 280°C) excludes the use of less stable phosphorus compounds [136]. Many industrial studies have been done to fi nd synergists for red phosphorus. It has been found helpful to combine red phosphorus with phenolic resins [136, 51]. It is believed that under burning conditions, red phosphorus-phenolic combinations form a cross-linked network which eliminates fl aming drips by reducing melt fl ow. A typical polyamide 6.6 formulation which contains 25 wt.% glass fi bers, 7% wt. red phosphorus and 5% wt. phenolic resin gives a V-0 rat-ing. A formulation of 6% red phosphorus, 5% layered clay and 4% poly-olefi n compatibilizer gives V-0 in 15% glass fi ber reinforced polyamide 6.6 [137]. It is interesting that PET is synergistic with red phosphorus when used in glass-fi lled PBT and polyamide 6 [138]. Combinations of red phosphorus and melamine salts also allow boosting the glow wire ignition temperature [139].

Early work on the fl ame retardant mechanism of red phosphorus in poly-amide 6 suggests mostly gas phase mechanism of action due to depolymer-ization into white phosphorus and volatilization in the oxygen depleted atmosphere of the pre-fl ame zone [140]. Later Levchik et al. [141] studie d thermal decomposition of polyamide 6 fl ame-retarded with red phospho-rus in nitrogen and found that phosphate esters are formed even in an inert atmosphere. More recent study also confi rms increased solid residue of polyamide 6.6 in the presence of red phosphorus [142]. Although we could not exclude possible interaction of red phosphorus with traces of O

2 and

absorbed moisture [143] as well as H2O formed during the thermal decom-

position of the polyamide [144], it is also possible that red phosphorus


reacts directly with polyamide via a free radical mechanism as ESR study suggests [141]. Similarly it was found that interaction of red phosphorus with PET results in formation of phosphate esters [145].

In the late 70s and early 80s various metal salts of dialkylphosphinates were prepared and tested in PET by Pennwalt [146] and in polyamide 6 by Hoechst [147]. Later, Ticona and Clariant tested zinc, aluminum and calcium dialkylphosphinate salts in glass fi lled polyamides and PBT. Th e Al or Ca salts of ethylmethylphosphinic acid were found to give V-0 at 15 wt.% in plain PBT, at 20 wt.% in glass-fi lled PBT [148, 149], and 30 wt.% in glass-fi lled polyamides [150]. Later, Clariant developed process for the diethylphosphinate salts [151] and commercialized the aluminum salt (DEPAL, Formula 2.11) as Exolit® OP 935 (fi ne particle grade) and OP 1240 (coarser particles grade).

Al O PO

C2H5

C2H53

(2.11)

Although salts of dialkylphosphinic acid by themselves are only moder-ately effi cient in polyamides they were found to be synergistic with nitro-gen-containing products such as melamine cyanurate [152], melamine phosphate or melamine polyphosphate [153]. Based on this synergism, Clariant commercialized Exolit® OP 1311 for polyamide 6 and Exolit® OP 1312 (also containing zinc borate as a stabilizer) for polyamide 6.6 [154]. Th ese products provide UL-94 V-0 ratings in glass-fi lled polyamides at 15–20 wt.% loading down to 0.4 mm thickness [155]. It see ms there is no actual synergism between DEPAL and melamine salts in glass-fi lled PBT, but about 1/3 of DEPAL can be replaced with melamine cyanurate or melamine polyphosphate without loss of the V-0 rating [155], which is probably benefi cial for cost saving. Compare to brominated fl ame retar-dants used in the same application, DEPAL allows a high Comparative Tracking Index (CTI) > 500 volt [156, 157]. On the other hand DEPAL and DEPAL-based synergistic combinations show signifi cant wear (cor-rosion) of processing equipment [158], which can be decreased by using acid scavengers. DEPAL-based polyamide 6.6 formulation also shows poor recycling compared to typical brominated polystyrene-based for-mulations [159].

About a decade ago BASF discovered that 15 wt.% calcium hypo-phosphite combined with 10 wt.% melamine cyanurate provides V-0 in glass-fi lled PBT [160]. Later Italmatch found that aluminum hypo-phosphite Al(HPO

2)

3 alone or in combination with melamine is a more

eff ective fl ame retardant because it requires only 15 wt.% total loading


for achieving V-0 in glass fi lled PBT [161]. Now, Italmatch is marketing calcium hypophosphite as Phoslite® IP-C and aluminum hypophosphite as Phoslite® IP-A, but mostly focusing on the aluminum salt [162]. Calciu m and aluminum hypophosphites are also available in India from Anan and from a few sources in China. Because of the risk of evolu-tion of phosphine, aluminum hypophosphite is usually used as a coated or double coated grade. Recently Solvay (ex. Rhodia) developed a pro-cess for producing a purer and reportedly more stable version of calcium hypophosphite [163].

Based on the academic studies there is strong indication that DEPAL mostly operates in the gas phase by a fl ame inhibiting mechanism [164, 165]. It w as believed that DEPAL decomposes with evolution of phosphinic acid which evaporates to the fl ame, however there is other evi-dence showing that aluminum alkylphosphinates can volatilize without decomposition. Th e higher volatility of the salt results in the better fl ame retardant effi ciency [166]. Melamine polyphosphate provides condensed phase action by increasing charring of polyamides and thus providing syn-ergistic action with DEPAL [164]. In contrast, melamine cyanurate mostly volatilizes and provides a cooling eff ect to the fl ame [165], therefore its action is mostly adjunctive but not synergistic. Aluminum hypophosphite decomposes at about 300°C with evolution of 1.5 mole of phosphine [167] which is quickly oxidized in the fl ame providing a gas phase inhibition eff ect. Because calcium hypophosphite decomposes at a higher tempera-ture [162] (about 350°C) and produces less phosphine it is less effi cient than aluminum hypophosphite.

A few years ago, the German company Catena (Floridienne now) intro-duced to the market two new melamine polyphosphate fl ame retardants with part of melamine replaced by aluminum (Safi re® 200) or zinc (Safi re® 400) [168]. Both Safi re® grades are recommended as a replacement of melamine polyphosphate in combination with DEPAL in glass-fi lled poly-amide 6.6, improving the thermal stability of the polymer. Also, 8 wt.% of Safi re® 400 in combination 12 wt.% DEPAL provides a V-0 rating in PBT. Safi re® was also found to be effi cient in combination with 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO, Formula 2.12) in poly-amide 6.6 [169] providing good thermal stability and lower corrosion of processing equipment compare to DEPAL.

POH

O(2.12)


Despite signifi cant eff ort to fl ame-retard thermoplastic polyesters or polyamides with organic phosphates, it has been diffi cult to fi nd a com-mercial solution. Th is is mostly because organic phosphates tend to react with polyesters and nylons with very little char formed and do not volatil-ize to the gas phase. Recently it was shown that a phenolic resin (novolac) charring agent can signifi cantly improve fl ame retardant effi ciency of RDP or BDP in PBT [170, 171]. At 10 wt.% BDP, unfi lled PBT reaches a V-2 rating, however at 15 wt.% BDP or higher, some of the fl ame retardant exuded to the surface. A combination of 10 wt.% novolac and 15 wt.% RDP (formula 2.6) or BDP (formula 2.7) allows achieving V-0 rating, and the novolac suppresses the exudation. To increase the fl ame retardancy, TPP was added to the formulation to provide some gas phase mode of action which allows V-0 in glass-fi lled PBT [172].

An ultimate condensation product of urea or melamine and phosphoric acid is a very high melting solid, phosphorus oxynitride (PON)

x, which

can be directly synthesized from ammonium phosphate. Phosphorus oxynitride has been suggested for use in polyesters in combination with melamine phosphate, melamine cyanurate, ammonium polyphosphate or calcium diethylphosphinate [173]. Interestingly, similar condensation product of P

2S

5 and dicyandiamide, which is apparently a sulfur-containing

analog of (PON)x is even more effi cient giving a V-0 rating in glass-rein-

forced PBT at 20 wt.% without the help of a synergist [174]. A mechanistic study showed that (PON)

x is an effi cient char promoter, however the char is

not a good mass transfer barrier and not a good thermal insulator. Partial substitution (5 wt.%) of (PON)

x with Fe

2O

3 improved the barrier character

of the char and led to a V-0 rating [175, 176]. Another mode of the fl ame retardant action of (PON)

x is postulated [177] to be its ability to form a low

melting non-combustible glass on the polymer surface.

2.7 Th ermoplastic Elastomers (TPE) and Th ermoplastic Polyurethanes (TPU)

Th ermoplastic elastomers (TPE) and urethanes (TPU) have relatively long fl exible aliphatic segments and also functional polymeric linkages of poly-esters, polyamides or polyurethanes. In terms of response to phosphorus based fl ame retardants TPE and TPU are positioned between polyolefi ns and engineering thermoplastics, e.g. phosphorus FR can react with the functional groups and phosphorylate the polymer but the groups are too scarce to produce any signifi cant char. Th erefore, a combination of gas


phase active and condensed phase active FRs is the best strategy for the fl ame retardancy of TPE and TPU.

About 15 years ago Elastogran (BASF now) commercialized halogen-free TPU based on about 25 wt.% melamine cyanurate and 5 wt.% RDP [177]. In the UL-94 test this TPU still drips, but the droplets do not ignite cotton and therefore it is rated V-0. Addition of some free isocyanate dur-ing processing creates additional cross-links and prevents dripping [179]. Later many formulations based on RDP and ATH passing the VW-1 rating in UL-1581 test for wire and cables applications were developed [180, 181] and some probably commercialized. Interestingly, the addition of only 2.5 wt.% novolac type epoxy resin provides robustness in pass-ing the test [182] probably by cross-linking and decreasing resin fl ow and dripping.

It was found that about 30 wt.% DEPAL (OP 1240) is required to achieve a VW-1 rating in TPE wire insulation [183] or the same rating can be achieved if about 1/3 of DEPAL is replaced with melamine polyphosphate (Melapur® 200) [184, 185]. Similarly in TPU about 30 wt.% DEPAL and some melamine salt allow achieving VW-1 rating [186]. In TPE, melamine phosphate can be replaced with melamine cyanurate but it requires further addition of phosphate esters like RDP or BDP which improve fl exibility and scratch resistance [187]. Aluminum hypophosphite (Phoslite® IP-A) preferably in the form of a masterbatch can be used instead of DEPAL, combined with melamine cyanurate, RDP and about 2.5 wt.% phenolic novolac as a charring agent [188].

Recently many patents were fi led on TPU formulations for cable application based on piperazine pyrophosphate (ADK STAB FP2100J or FP2200) combined with melamine cyanurate [182], EDAP [189], RDP or BDP [190]. Silicone surface-treated ammonium polyphosphate in combi-nation with pentaerythritol and methylmethoxysiloxane made by reacting trichlomethylsilane with methanol and water provide high LOI = 34 and UL-94 V-0 ratings in TPU [191]. At the time of writing this review, we could not confi rm that intumescent-based systems are commercially used in TPU in cable applications.

2.8 Epoxy Resins

Th e main fl ame retardant market for epoxy resins is in printed wiring boards (PWB). A phosphorus FR can be added to epoxy as an additive or can be incorporated in epoxy network by pre-reacting with epoxy


resin or by using a phosphorus-based curing agent [192]. Reactive FRs are more preferred in epoxy because they show less negative eff ects on physical properties, mostly glass-transition temperature and hydrolytic stability.

Since the P-H bond can add to the epoxy group, this reaction can be used to attach hydrogen phosphonates or phosphinates to an epoxy resin. Th e only example of a commercially available reactant of this type is 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO, Formula 2.12). Origi-nally DOPO was developed as heat stabilizer and an intermediate for FRs in textiles. In spite of the fact that DOPO is monofunctional, it was adopted by the industry for use in PWB laminates [193] and now it is the highest volume phosphorus FR used in epoxy. Th e common practice is pre-reacting DOPO with multifunctional novolac type epoxy [194] in order to achieve phosphorus content of about 3 wt.%. Th is still leaves in average 2–4 epoxy functionalities unconsumed which allows further curing of the phosphory-lated epoxy resin. Th ere are also examples of DOPO use as a part of curing system [195], but they are less preferred. DOPO is produced in Europe by Schill & Seilacher and Krems Chemie, in Japan by Sanko and by a few small manufacturers in China and Taiwan.

Because DOPO is monofunctional it cannot be used with most com-mon bisphenol A epoxy. In novolac type epoxies DOPO provides V-0 at relatively low phosphorus content of 2.0–2.5%. [196]. Th e high effi ciency of DOPO compared to other phosphorus FRs is partially attributed to its gas phase action [197]. Because of pronounced gas phase action, DOPO can be combined with ATH [198] which is normally not the case with many phosphorus FRs showing mostly condensed phase action. When combined with ATH or fi ne silica, DOPO-based laminates require only 1 wt.% P or less to achieve a V-0 rating. Th e main disadvantage of DOPO is inability to achieve glass transition temperature T

g > 150°C even when

combined with a highly functional epoxy.By reacting DOPO with quinone, a phenolic difunctional product can

be made (DOPO-HQ, Formula 2.13). Th is product is available in Japan from Sanko as HCA-HQ. It can be incorporated in an epoxy resin through a chain-extension process similar to tetrabromobisphenol A with difunc-tional epoxies [199]. Although it provides good physical properties and the required level of fl ame retardancy, it is not fi nding broad application because it is low in phosphorus and probably more expensive than DOPO. Th ere are many patents claiming use of the product of reaction of naphtho-quinone and DOPO, for example HCA-NQ from Sanko [200], but the real commercial status of this product is unknown.


P OO

HO OH

(2.13)

Recently Dow Chemical started marketing a halogen-free experimental epoxy system BF 180D consisting of an epoxy resin part and a phosphorus-based hardener part (both in a solvent). Recent patents to Dow [201, 202] make to believe that the hardener (XZ92741.00, 59% solids) also uses the DOPO moiety as a phosphorus containing group. Probably because this hardener is a multifunctional, it allows producing laminates with T

g > 175°C

and high thermal stability as measured by a delamination test. Similar prod-ucts made by phosphorylation of resol phenolformaldehyde resin [203, 204] or a mixed phenol-melamine novolac [205] are reported in the patent lit-erature, but it is not clear if they are commercial. Recently Albemarle intro-duced a new highly effi cient organophosphorus fl ame retardant for epoxy applications [206] and they also patented [207] use in epoxy of non-reactive di-DOPO phosphinate connected by ethylene bridge.

Th e aluminum diethylphosphinate, Exolit® OP 930 or OP 935 (DEPAL, Formula 2.11), which Clariant has developed for polyesters and nylons (see above) apparently fi nds some application in epoxy. It is used as a fi ne dispersion in combination with ATH [208]. Combinations with melamine polyphosphate [209] or zinc borate also have been found benefi cial [210]. Because DEPAL doesn’t dissolve in epoxy resin and behaves as a fl ame retardant fi ller it is useful in low dielectric loss compositions [211]. Recently Clariant introduced to the market two new products Exolit® EP 150 mono-functional liquid with phosphorus content of 25 wt.% and Exolit® EP 200 difunctional solid with phosphorus content of 29 wt.% [212]. Reportedly these are very high in phosphorus FR require 4.0–4.5 wt.% P loading and 3.0–3.4 wt.% nitrogen-based synergist to achieve V-0 loading in epoxy laminates. Th is technology may be related to one of Clariant’s early patents on use of phosphinic acids in epoxy [213].

Because phosphine oxides are thermally and hydrolytically very stable, phosphine oxide structures have been proposed to impart fl ame retardancy to epoxy resins. Many studies have been reported in the literature concern-ing curing of epoxy resins with bis(aminophenyl)methylphosphine oxide (Formula 2.14) and combustion performance of the fi re retardant struc-tural composites [214–216]. Because of strong nucleophilic character of


the amino group it cures epoxy at the relatively low temperature of 150°C. In spite of good performance this curing agent was not commercialized because of the expensive multistep synthesis.

H2N

PO

CH3

NH2

(2.14)

In the patent literature there many recent publications on triarylphos-phine oxides with phenolic OH functionalities [217, 218] which can be used as curing agents or can be further functionalized with epichlohydrin to produce phosphorus-containing epoxies [219]. Th ese OH functional tri-phenyl phosphine oxides can provide only V-1 in epoxy laminates, but com-bined with ATH the laminates can be rated V-0 [220]. Recently Great Lakes introduced to the market a new phosphorus based curing agent under the trade name Emerald™ 2000 [221] which could be related to these patents.

Some time ago ICL-IP (ex. Akzo Nobel) introduced a curing agent poly(1,3-phenylene methylphosphonate), (Fyrol® PMP, Formula 2.15) specially designed for epoxy resins in electrical and electronic applica-tions [222, 223]. It is a semi-solid at room temperature, but it melts at 45–55°C. Th e product is very rich in phosphorus (17.5%) and is ther-mally stable with weight loss starting only above 300°C. Fyrol® PMP shows a unique mechanism of curing by opening the epoxy group and insertion into the phosphonate ester linkage [224]. From 20 to 30 wt.% of PMP provides a V-0 fl ammability rating in epoxy laminates. Because PMP has better processability and higher functionality it shows higher glass transition temperature and better thermal stability compared to DOPO based laminates [225].

(HO)n O PO

CH3

O O PO

O

CH3

(OH)m

p m, n = 0 or 1

(2.15)

Th ere is extensive patent literature on the use of cyclic phosphazenes in epoxy laminates [226, 227], because cyclic phosphazenes are hydrolyti-cally stable and show low polarity and therefore good electrical properties. However, it seems only the mixture of tri- and tetracyclic phenoxyphosp-hazenes produced by Otsuka in Japan (SPB-100) fi nds some commercial use [96, 228].


2.9 Unsaturated Polyesters

Unsaturated polyesters are very fl ammable and they produce copious smoke upon combustion [229]. Typical use of unsaturated polyesters is in composite elements for construction or transportation (trains and ships). It is very diffi cult to produce halogen-free fl ame retardant composites because these industries require large scale rigorous fl ammability tests. Typically passing these tests is achieved with high loading of aluminum hydroxide with addition of phosphorus based fl ame retardants as a sec-ondary FR.

For years low molecular weight phosphate esters like triethyl phos-phate and dimethyl methylphosphonate (DMMP) were used in highly fi lled ATH systems or in glass-fi ber composites with the main purpose of viscosity reduction [230]. For example 55–60 wt.% ATH and 1–2 wt.% DMMP allows passing the UL 723 test with class I for ventilation stacks [231]. Recently the use of DMMP was signifi cantly restricted because of suspected mutagenicity but it is still apparently used in Asia.

ATH suppresses intumescent performance of APP in polyolefi ns and therefore these two FRs almost never used together in polyolefi ns, but it seems not to be the case in unsaturated polyesters [232]. In order to decrease loading of ATH it can be partially or completely replaced with more effi cient APP. Clariant technical literature recommends use of 15–25 parts (per 100 parts resin) APP (Exolit® AP 422) and 50 parts ATH to obtain a V-0 rating. To achieve M2 class in NFP 95-901 test, 10 parts of APP and 90 parts of ATH is needed whereas without APP about 225 parts ATH which is very diffi cult to process would be required. Clariant also off ers Exolit® AP 740 and AP 742 as a formulated APP blends for light weight UP composites and gel coats. It requires 40 parts of AP 740 to pass German DIN 5510, Part II for railway transport [233]. AP 740 c an be also combined with ATH. Recent academic publication [234] shows tha t silane treated APP at 35 wt.% loading allows decreasing of heat release rate by 70%, but more important total smoke released decreases by 50%. It was also shown that combination of APP with expandable graphite is benefi -cial and probably shows a synergistic eff ect [235]. Red phosphorus (Exolit® RP6540) can be used instead of APP in many applications and typically requires lower loading [233].

Melamine salts, e.g. melamine pyrophosphate [234] seems to be less effi cient in unsaturated polyesters compared to APP. Weil and Kim [236] reported interesting research where a dispersion of melamine in uncured polyester resin was prepared by using a high-shear mixer and then it


was reacted with fertilizer grade “superphosphoric acid” forming in situ melamine phosphate dispersion. Th e cured low-styrene formulation showed a V-0 rating whereas addition of triethyl phosphate or dimethyl methylphosphonate was needed for a high-styrene formulation. Although not commercial yet this inexpensive approach seems to have commercial potential.

Lanxess recently introduced a new additive, dimethyl propylphospho-nate (Levagard® DMPP) replacing DMMP. It was suggested to use 5–10 wt.% DMPP as viscosity reducer and synergist with APP and ATH [237]. Surprisingly only 10 wt.% ATH, 4 wt.% EDAP and 1 wt.% DMPP provide V-0 rating in a glass-fi lled polyester composite [238].

Recent developments at Clariant showed high effi ciency of DEPAL alone [239] or in combination with ATH, APP or melamine [240] or melamine polyphosphate [241]. For example, a combination of 10 wt.% aluminum diethylphosphinate and 10 wt.% melamine polyphosphate provides a V-0 rating in a 30% glass-fi lled composite and shows an LOI of 42. DEPAL can also be pre-dispersed in polyester/styrene prepolymer [242] which results in higher LOI values compare to freshly added DEPAL.

Th ere is a substantial literature on reacted-in phosphorus co-mono-mers in unsaturated polyester resins [243] but almos t all of these prod-ucts were not commercialized. Th e water evolved in esterifi cation reaction leads to hydrolysis of phosphate esters, side reactions and high acidity. However, this problem seems to be less severe if phosphorus is in a pendant group. For example the product of condensation of DOPO and itaconic acid (Formula 2.16) can be further copolymerized with diols and maleic anhydride to form a prepolymer [244, 245] which can be cured with styrene. Similarly DOPO-HQ (Formula 13) can be co-polym-erized into the polyester chain [246], but this copolyester seems not to have been commercialized.

P OOCH2

CHCO

O CH2 C

O

O CH3CH3

(2.16)

Researchers at the Industrial Technology Research Institute (Taiwan) showed transesterifi cation of simple phosphorus compounds such as dimethyl methylphosphonate into unsaturated polyester resins [247]. Similar work was recently performed in China [248], where it was found


that addition of about 15 wt.% DMMP to the reactive mixture in esterifi ca-tion synthesis results in UPE composites with V-0 rating. Recently Clariant applied for US patents [249, 250] showing difunctional phosphinates being incorporated in the polyester chain without signifi cant hydrolysis probably because phosphinates are more hydrolytically stable than phosphates and phosphonates.

2.10 PU Foams

Rigid polyurethane foams are the best heat-insulating materials, and are widely used in the building industry. Th ere is no offi cial small-scale or bench-scale test for the rigid PU foams, but they need to be tested using one of large scale construction material test, the most common of which in the USA is the “25-foot tunnel”, ASTM E-84.

Tris(chloroisopropyl) phosphate (TCPP) is the principal FR used in rigid PU foams, but it is out of the scope of this chapter because it contains chlorine. TCPP is also the largest commercial phosphorus based fl ame retardant.

Dimethyl methylphosphonate (DMMP) for many years was used in rigid PU foams [251], but now it is eff ectively removed from the market in the USA and Europe because it was categorized as a suspected muta-gen. It is still used in China for passing stringent fi re tests requirements for high rise buildings. For many years Rhodia (ex. Albright & Wilson then Albemarle) was marketing diethyl ethylphosphonate (DEEP) as a DMMP alternative, but the current commercial status of this product is not clear. Some time ago Lanxess (ex. Bayer) introduced dimethyl propylphospho-nate (Levagard® DMPP) as a replacement to DMMP [252, 253].

Triethyl phosphate (TEP), now produced only in Asia, is used in rigid PU foam as a co-additive with TCPP or brominated FRs as a viscosity cutter. TEP also helps with decreasing smoke, although, in fact, it doesn’t reduce smoke but just doesn’t increase it as much as halogen-containing FRs tend to do. For example 9 parts TEP provides a B-2 rating in DIN 4102 in high density rigid PU foam and shows lower smoke [254] compared to chloroalkyl phosphates.

In a recent publication, Tebbe and Sawaya [255] compared DMMP, DEEP, DMPP and TEP with TCPP and tris(chloroethyl phosphate) (TCEP, removed from the market a few years ago). It was surprisingly found that the halogen-free phosphates and phosphonates show higher LOI, 25–26.5 compared to chloroalkyl phosphates. It seems that high volatility


of halogen-free FRs compensated for lack of chlorine. TEP, DEEP and DMPP showed good compatibility with blowing agents n-pentane and water, which resulted in overall better shelf life of the mixed composi-tion. On the negative side, halogen-free FRs showed lower compression strength and elastic modulus, probably due to stronger plasticization of the PU polymer. Another recent study [256] found similar FR effi ciency of TEP (phosphate) and TCPP (phosphonate) confi rming that volatility of the FR plays a more important role compared to the oxidative state of the phosphorus atom.

Although PU industry prefers dealing with liquid FRs similar to other PU foam components, sometimes the use of solid FRs is a more eco-nomical way of achieving high fl ammability standards even if it requires installation of special equipment. For example, use of fi nely divided APP combined with ATH and a cyclic phosphonate allows achieving class I in E-84 tunnel test [257] and a combination of APP, ATH and zinc borate allows passing the UL 790 roof assembly [258] test of spray foam roofi ng. Red phosphorus alone or combined with APP is another very effi cient FR allowing passing stringent tests like DIN 374 or GB 822 [259]. Other very effi cient combinations are APP with expandable graphite [260] and red phosphorus with expandable graphite [261]. Usually TEP or some other low viscosity liquid FR is used in combination with solids to improve processability.

Th ere is some market interest in reactive fl ame retardants for rigid PU and polyisocyanurate (PIR) foams. Th e advantage of a reactive FR is its permanence in the foam which is especially important in roofi ng appli-cations in hot desert and tropical climates where the temperature of the roof can be very high and non reactive FRs can be lost. For many years ICL-IP has been manufacturing and selling a product of reaction of diethyl phosphite, formaldehyde and diethanolamine (Fyrol® 6, Formula 2.17). It is also produced by Lanxess as Levagard®4090N. Th e main application of Fyrol®6 is in roofi ng spray foam and in the insulation foam for large refrig-erators. A mechanistic study on analogs of Fyrol®6 showed [262] that even though most of the phosphorus splits off and volatilizes from the foam, it still helps with signifi cant char increase which indicates that this reactive FR provides both condensed phase and gas phase mode of action.

HO CH2 CH2 N CH2 CH2 OHCH2

PO

OC2H5C2H5O(2.17)


Interesting research towards reactive FRs for rigid PU foams was recently reported from Korea [263]. Large amount of TEP or trimethyl phosphate or TCPP was added to waste PU foam and the mixture was heated to 190°C for 6 hours. At this temperature PU decomposes and polyol fragments transesterify phosphate ester thus producing phos-phorylated polyol. Rigid foam produced with the addition of this recy-cled polyol showed a decrease in peak heat release rate as measured by cone calorimeter.

Flexible PU foams have mostly open cell structure. Because of this, fl exible foams are very combustible with LOI in the range of 16–18 [264], fast fl ame spread and high heat release rate [265]. Th e fl ammabil-ity of PU foams strongly depends on the foam density and openness of the cells (air fl ow). Light foam with open cells burns very fast. Flexible PU foam is the main and most combustible component of upholstered furniture, mattresses and car seats. Fires involving PU foams are the most deadly.

Because PU foam has high thermal inertia (dissipates heat slowly) it is easy to ignite. However, paradoxically it is also easy to extinguish when the fl ame is still small (match, cigarette igniter or similar) because the preheated layer of decomposing polymer is shallow and combustion is unstable. Th is means that the most effi cient way of preventing large furniture fi res is preventing (extinguishing) small fi res before they can grow. For about 25 years California’s Technical Bulletin 117 was man-datory for that state and it was voluntarily accepted by many furniture manufacturers in other states for testing PU foams. Th e requirement of resistance to small fi re sources (lighters, matches, and candles) has been recently removed from the standard which is an obvious step backward in public fi re safety.

In manufacturing of fl exible polyurethane foams, if the foam reaches an excessively high temperature, “scorch” can occur. Scorch is, at the least, a discoloration of the interior of the slab or bun, and more seriously the loss of mechanical properties because of polymer degradation. Some of the commonly used fl ame retardants can aggravate scorch. Mechanistic studies showed [266, 267] that scorch is largely the result of oxidation of aromatic amino groups arising from the hydrolysis of isocyanate groups which became isolated in the PU network. Th e formation of the chro-mophoric groups is aggravated by the presence of fl ame retardants with alkylating capabilities such as the alkyl phosphates because alkylated amin-ophenyl structures are more easily oxidized to quinoneimines. Conversely, phosphorus compounds that cannot alkylate amino groups are those


which do not aggravate scorch; examples are aryl phosphates or hindered alkylphosphates.

For more than 15 years ICL-IP (ex. Akzo Nobel) was marketing an oligomeric ethyl ethylene glycol phosphate additive (Formula 2.18) con-taining 19% phosphorus (Fyrol® PNX). Because of the high phosphorus content, it is quite effi cient and as little as 4–8 php is eff ective in passing the automotive MVSS 302 test in a 1.5–1.8 lb/cu.ft . foam [268]. PNX is halogen-free and it has been especially of interest in Europe, particu-larly with respect to low-fogging low-volatiles-emission requirements of the automotive industry which is achieved because of low additive level in the foam. PNX has been recommended for use in combination with alkylphenyl phosphates, which improve the fl ame retardant perfor-mance and also decrease the additive viscosity [269]. PNX causes some scorch, especially in low density water blown foam, therefore the foam needs to be stabilized [270]. Aliphatic butylphosphonate oligomers with butylene bridging group were developed at Solvay (ex. Rhodia) [271], but apparently they are not commercial at the time of writing of this chapter.

C2H5O PO

O (CH2)2 O PO

OC2H5

OC2H5 OC2H5n (2.18)

Triaryl phosphates, such as isopropylated or isobutylated tripheny phos-phates [272], fi nd some use in fl exible foam formulations sometimes in combination with a bromine-containing additives [273]. Newly introduced by Great Lakes, Emerald® NH-1 [274] (replacing Rheofos® NHP [275]) is a low viscosity liquid, probably a member of the aryl phosphate family. It is designed to meet MVSS 302 for hot-molded automotive seating and also, in combination with melamine, to meet British BS 5852 for upholstered furniture in the UK. Lanxess’s Disfl amol® DPK, also a member of the tria-ryl phosphate family, is recommended for both furniture and automotive low fogging application [276].

In recent years ICL-IP (ex. Supresta) has introduced a series of new halogen-free phosphate esters Fyrol® HF-4, HF-5 [277, 278] and HF-9 [279]. HF-4 is de signed for low-scorch application by providing high oxidative stability, which is needed for production of PU foams in hot and humid summer weather. HF-5 is an oligomeric highly effi cient phosphate ester fl ame retardant for upholstered furniture specially designed to satisfy new IKEA requirement related to very low emis-sions and absence of triphenyl phosphate and chloroalkyl phosphates.


HF-9 is a highly effi cient low scorch FR for broad upholstered furniture market.

Similar to rigid foams there is a market desire to have a reactive phos-phorus based fl ame retardant for fl exible foams. However, technical development of such a product is more diffi cult because the cell structure of fl exible foams is more sensitive to the variations in the composition compared to rigid foams. For example, Fyrol® 6, broadly used in rigid PU foams, can be used in fl exible foams only as a co-additive at the levels of 1–2 phr because OH functionality is very high compared to typical fl ex-ible foam polyols.

In recent decades some attempts were made by Clariant (ex. Hoechst) to commercialize halogen-free phosphorus-containing diols for fl exible foams, Exolit® OP 550 and OP 560 [280]. OP 550 is a hydroxyethyl termi-nated ethyl phosphate oligomer [281, 282] with abo ut 17% P. It is primarily recommended for use in molded and high density slabstock fl exible foams, where it passes the MVSS302 test at 7.5 parts [281]. Th e main advantage of this product is permanency in the fl exible foam which allows achieving low VOC. However because of diffi culties of formulating foam with OP 550, the market penetration of this product is unclear. Another phospho-rus ester, Exolit® OP 560, with about 12% P content and a higher OH func-tionality, is a reactive phosphonate [234, 283]. It is mostly recommended for automotive fl exible PU foams where it reacts in and becomes part of the PU network. It is highly effi cient especially in high density foam where pass of MVSS302 test is achieved at < 5 parts [234].

In order to meet low fogging requirements in automotive foam Lanxess started marketing a monofunctional reactive phosphorus-based fl ame retardant Levagard® TP LXS 51053 [284, 237] with high OH func-tionality which seems to be easier to mitigate in the monofunctional FR. Its effi ciency in MVSS302 test is comparable with commercial chloroal-kyl phosphates and brominated FRs, but it has much lower viscosity and this helps with the processing. Based on the recent patent literature, it could be dimethyl 2-hydroxyethylphosphonate [285].

Recently, Daihachi introduced [286] a new reactive monofunctional FR for fl exible foam, Daiguard ®610 with high OH funfctionality. Because of potentially low VOC it is mostly intended for automotive application. It has low phosphorus content of 11% and it requires 18–22 parts for passing MVSS302 test. Recent patent literature shows that Daiguard® 610 can be one of the products made by reacting cyclic neopentyl acid phosphates with propylene oxide or ethylene oxide [287] to produce monohydric alcohols.


2.11 Textiles

Th ere are many ways of applying fl ame retardants to textiles.

• For example, a water-soluble FR can be applied to textile by soaking, padding or spraying and then drying which will result in a FR fi nish non-durable for laundering.

• If FR can be reacted with textile surface (typically cotton) either by itself or by using a binding agent, this leads to semi-durable or durable fi nishing.

• Wide range of solid or liquid FRs formulated with acrylic or latex binder can be applied to the textile as a surface coating (typically backcoating). Th ere are no specifi c requirements to the chemistry of the fl ame retardants in backcoating apart from being effi cient and compatible with the coating formulation.

• For synthetic polyester and polyamide fi bers there is a com-mon practice of applying FRs by the “thermosol” process which is similar to disperse dyeing and oft en can be com-bined with the dyeing. In this process fi ber at the tempera-ture close to the polymer melting process passing through a water solution or suspension of the fl ame retardant. On heating, the fl ame retardant migrates into the soft ened fi ber and then stays locked close to the surface when the fi ber cools down. Exhaust is another dyeing process which is run at lower temperature on knitted textiles and it can be also applied to incorporate fl ame retardants.

• In some cases fl ame retardants can be added to the molten synthetic polymer in the spinning process.

• And fi nally reactive fl ame retardants can be copolymerized with the main polymer during polymer synthesis process.

Non-durable fi nishes are most oft en used for disposable goods, for example medical gowns, party costumes, and sometimes wall covering. Th ey can be used on work clothing and curtains, but the laundry then must reapply them aft er each wash. Typically mono- or diammonium phos-phates or water-soluble short chain ammonium polyphosphate are used for non-durable treatment of cotton or cotton-based blends. Diguanidine hydrogen phosphate or monoguanidine dihydrogen phosphates are also used for non-durable cotton treatment [288]. Depending on fabric weight and density 1–2% of phosphorus provides self-extinguishing performance.


Some organic co-additives can be added to the solution [289] to improve textile wetting and inhibit crystallization upon drying in order to avoid formation of visible crystals of ammonium phosphates.

For some time the UK based company Isle Firestop Ltd. was marketing an adduct of ammonium salt of methylphosphonoamidic acid and ammo-nium chloride (Formula 2.19) [290] under the trade name Nofl an®. Aft er application to the textile the product is cured with melamine-formalde-hyde resin and urea, so it can survive multiple launderings. Th is technol-ogy was fi rst developed and probably commercially used in Russia [291]. It was recommended for use in plain cotton or polyester/cotton blends [292]. Reportedly Nofl an® was used to treat upholstery seats of small airplanes in Europe, but it was removed from the market due to corrosion of metal parts in contact with the textile.

CCH3 PO

ONH4

NH2 NH4 l (2.19)

For many years the product of addition of dimethyl phosphite to acryl-amide followed by methylolation was marketed by Ciba and now by Huntsman in Europe [293] as Pyrovatex® CP (Formula 2.20). Recently also Th or started marketing it as Afl ammit® KWB. Th is product is fi xed on the cellulose using an amino resin and an acid curing catalyst. Pyrovatex® CP has a mild formaldehyde odor, because it contains some components with less well bound formaldehyde [294, 295]. Th is product is not used in the USA and has limited use in Europe because of potential formaldehyde exposure Th ere are methods of decreasing of formaldehyde release [296] and it is believed that they are used commercially.

CH3O PO

OCH3

CH2 CH2 CO

NH CH2OH (2.20)

For a number of years Akzo Nobel (now ICL-IP) was promoting oligomeric OH-terminated methylphosphonate-phosphate (Fyroltex® HP, Formula 2.21). It has been shown that this oligomeric product can be curable on cotton or blends using dimethyloldihydroxyethyleneurea (DMDHEU) and trimethylolmelamine [297] or melamine-formaldehyde [298] to obtain a durable fi nish with low formaldehyde odor. Fyroltex HP is also effi cient on cotton-nylon blends [299]. It can also be used in non-formaldehyde fi nishes where the bonding to cellulose is achieved by using a polycarboxylic acid such as butanetetracarboxylic acid or citric acid


[300]. Akzo Nobel discontinued production of Fyroltex® HP, but a similar product is available in China or from Allison Associates Intl. in the USA as ALC HP/51.

H O CH2 CH2 O PO

OCH3

O CH2 CH2 O PO

O CH2 CH2 OHCH3

2x x(2.21)

In terms of durability and effi ciency the leading commercial product is tetrakis(hydroxymethyl)phosphonium chloride (THPC) or sulfate (THPS), originally developed by Albright & Wilson and now marketed by Solvay (ex. Rhodia) for the Proban® process. THPC and THPS are water-soluble, but non-hydrolysable phosphonium salts which ensure exceptional dura-bility. In the Proban® process THPC or THPS is reacted with urea fi rst and the product is used to impregnate the textile which is then dried and cross-linked with gaseous ammonia. Finally the textile is treated with aqueous hydrogen peroxide which oxidizes phosphine structures to more thermally stable phosphine oxide. Th e idealized structure [301] which doesn’t have hydrolyzable bonds is shown in Formula 2.22. Th e need for using gaseous ammonia is the major disadvantage of the Proban® process and it requires special equipment.

NH CH2 P

O

CH2 NH C

O

NH CH2 P

O

CH2CH2NH

CH2P

O

CH2NH CH2 NH C

O

NH CH2 P

O

CH2

CH2NH

CH2

(2.22)

Backcoating is a very common and cost effi cient method of fl ame retard-ing cotton or synthetic textiles or their blends. Th e phosphorus-based backcoatings are more limited to cellulosics because their effi ciency relies mostly on charring. Th e durability of backcoating in laundering depends on the binder and the hydrolytic stability of the fl ame retardant. Horrocks et al. [302] studied a wide range of phosphate salts and some phosphate esters and concluded that ammonium polyphosphate is the most effi cient FR for cotton and cotton polyester blends because APP decomposes to polyphosphoric acid and involves cotton in charring [303, 304]. Coated ammonium polyphosphate grades such as Clariant’s Exolit® AP 462 or Budenheim’s FR CROS® 487 or ICL-PP Phos-Chek® 42 are more preferred over untreated APP because of better durability.


Th ermosol fi nishes with phosphorus-based fl ame retardants have been used for many years in PET textiles [305] and probably in polyamides. Th e major product used in the thermosol treatment of polyesters is a liquid cyclic phosphonate (Formula 2.23) marketed by Solvay (ex. Rhodia) as Amgard® CU. It is a mixture of diphosphonate and triphosphonate with the ratio mostly shift ed towards diphosphonate x=1. Th ere are few analogs of Amgard® CU available in China, but reportedly they have higher acid-ity. Usually, a small concentration of phosphorus 0.3–0.5 wt.%, in PET is needed to pass the textile fl ammability test NFPA 701. Aft er the phospho-nate is trapped under the fi bers surface it is resistant to laundering and doesn’t leak out even being highly soluble in water. Daihachi [306] tested a large number of cyclic phosphate-phosphonates similar to the cyclic phos-phonate (formula 2.23) in the thermosol process, however none of them seems to be commercial.

PO

O C2H5

CH2 O P

O

CH3

(OCH3)x

H3C

O2-x

(2.23)

Recently, Huntsman [307] patented use of various aromatic bisphos-phates, more specifi cally RDP. (Formula 2.6) in PET textiles in the presence of polycaprolactone as a dispersing agent and polyethylene diamine as an auxiliary FR helping retaining RDP in the fi ber. RDP and co-additives are applied by an exhaust process. Th e add-on level > 10 wt.% was achieved and the textile passed stringent DIN 54336 test with immediate extinguishment.

Various aromatic bisphosphates (Formulas 2.6–2.9) have shown limited success as melt additives in PET. Even required level of fl ame retardancy was achieved in freshly spun fi bers the bisphosphates tended to leach out with time. Phosphonates seems to be more successful in PET fi bers. For example Antiblaze® 1045 (Formula 2.23, x is mostly 0) has shown more promise in polyamide fi bers [308] where it was applied to a silica carrier prior to com-pounding which aided dispersion in the polymer melt and probably helped to slow down exudation. Antiblaze® 1045 was also useful as a melt addi-tive in PET, poly(trimethylene terephthalate) (PTT) and polyamide used to make nonwoven fi bers [309].

One of the fi rst phosphonates used in PET fi bers was poly (sulfonyldi-phenylene phenylphosphonate) produced by Toyobo in Japan. Th is oligo-mer is easy miscible with PET [310] up to 15 wt.% but for fi ber applications typically less than 5 wt.% loading is needed. Th is product was discontinued in Japan in favor of a reactive type FR (see below), but it is reportedly pro-duced now in China [33]. Recently, FRX Polymers introduced polymeric


bisphenol A methylphosphonate (Formula 2.5) Nofi a® HM1100 for PET fi bers for carpet (ASTM E648) and other FR textile applications [63]. Only 2.5 wt.% of the polyphosphonate added to PET fi bers during spinning allows passing ASTM D6413 textile fl ammability test [311].

A Clariant patent application [312] suggests that zinc diethylphosph-inate is particularly suitable for the fi ber additive application because it melts at the processing temperature. Recent patent application to DuPont [313] suggests that zinc diethylphosphinate, marketed by Clariant as Exolit® OP950 is useful in PTT fi bers for carpeting. It can also be melt processed with semiaromatic polyamide which is blended with rayon fi bers to pass ASTM D6413 [314]. Some time ago Apexical started marketing Pyrapex®, a melt-blendable additive for polyester fi bers and polyamide textiles and nonwovens. According to the company information it is an organic phos-phinate salt, high in %P and melting at 220°C, which indicates that it is probably similar to the OP950 product.

Although use of phosphorus-containing co-monomers was very exten-sively explored [243] only a few products became commercial mostly in PET fi bers because less than 0.5% of phosphorus content is required for good fl ame retardancy [315] and therefore mechanical properties of the fi bers are not aff ected. For many years cyclic 2-methyl-2, 5-dioxa-1, 2- phospholane [316] (Exolit® EP 110, Formula 2.24) developed by Hoechst was copoly-merized with ethylene glycol and dimethyl terephthalate to produce fl ame retardant PET fi bers sold as Trevira® CS (Hoechst) and later Avora® FR (KoSa). Th is product was recently discontinued and is being replaced with an adduct of benzenephosphinic acid and acrylic acid (Formula 2.25) pro-duced in Korea and China. Th is phosphinate can be similarly co-polymer-ized in the PET chain at 0.3–0.9 wt.% which leads to a signifi cant increase in the LOI of PET fi bers [317]. Reportedly it can be also copolymerized in polyamide 6.6 fi bers [318] to produce fl ame retardant carpets.

OP

CH3

OO

(2.24)

HO CO

CH2 CH2 PO

OH(2.25)

Th e adduct of 9, 10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) and dimethyl itaconate [319] (Formula 2.16) is another reactive


fl ame retardant commercially used as a co-monomer in polyester fi bers [320, 321]. Th is fabric is commercial in Japan from Toyobo as HEIM® and the bis(hydroxyethyl) ester reagent is available from Schill & Seilacher (Ukanol® FR 50/1) and from Kolon in Korea. Similarly to the copolymers with the phosphorus group in the main chain, this product is effi cient in PET fi bers at low loading of 0.3–0.65% phosphorus and allows keep-ing good fi ber properties [322]. It was found that placing the phosphorus ester linkage in the side chain, instead of the main chain, aff orded superior hydrolysis resistance [323] and thermal stability [324, 325].

In general, the fl ame retardancy of phosphorus-containing PET and polyamides is mostly achieved by enhanced melt fl ow and melt drip, pre-sumably catalyzed by polyphosphoric acid produced in the process of oxi-dative degradation during combustion.

2.12 Conclusions and Further Trends

Phosphorus-based fl ame retardants are on the fast growth path due to good performance and lesser environmental and/or regulatory problems compared to some halogenated fl ame retardants. Th e most active develop-ment is happening in polycarbonate and polyphenylene ether blends with exploration of new oligomeric aromatic phosphates and improved use of existing products. Another very active area is new thermally and hydrolyti-cally stable intumescent systems for polyolefi ns and elastomers. Signifi cant industrial and academic R & D eff ort is being channeled to new effi cient phosphorus-based fl ame retardants for printed wiring boards and more specifi cally to reactive FRs for epoxies. Th e requirements to new phospho-rus FRs for this market are very stringent due to increasing processing and service temperatures of the boards and need for excellent dielectric prop-erties for high frequency and mobile devices. Another area of fast changes is phosphorus fl ame retardants for fl exible PU foams where regulations are changing very rapidly due to high potential exposure to fl ame retardant chemicals.

Since phosphorus fl ame retardants possess gas phase and condensed phase modes of fl ame retardant action with the gas phase being mostly underutilized there is good prospective for development of new FRs with mostly gas phase activity or discovery of new synergistic combinations. Due to the fact that phosphorus FRs are selectively active only in a hand-ful of highly charrable and heteratomic polymers there is need for devel-opment of more universal products. Th is research can progress either by developing of new highly effi cient and hydrolytically stable intumescent


systems or highly effi cient gas phase active FRs or by a combination of both. Plastics containing phosphorus FRs are poorly recyclable, and so there is interest in more hydrolytically and thermally stable phosphorus fl ame retardants to favor recycling. Following the general trend in the fl ame retardant industry towards polymeric and reactive products which show lesser negative eff ects to the fi nal products and minimal exposure to humans and the environment, phosphorus fl ame retardants will follow this general strategy.

References

1. S.V. Levchik and E.D. Weil, J. Fire Sci., Vol. 24, p. 345, 2006.

2. S.V. Levchik and E.D. Weil, “Developments in phosphorus fl ame retardants”,

in A.R. Horrocks and D. Price eds., Advances in Fire Retardant Materials,

Woodhead, pp. 41–66, 2008

3. P. Joseph and J.R. Ebdon, “Phosphorus-based fl ame retardants”, in C.A.

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