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This document is the accepted manuscript version of the following article: Nazir, R., & Gaan, S. (2019). Recent developments in P(O/S)–N containing flame retardants. Journal of Applied Polymer Science, 47910 (27 pp.). https://doi.org/10.1002/app.47910

Recent developments in P(O/S)-N containing flame retardants Rashid Nazir, Sabyasachi Gaan Additives and Chemistry Group, Advanced Fibers, Empa Swiss Federal Laboratories for Materials Science and Technology, St Gallen, Switzerland

Abstract Synthesis of organophosphorus compounds containing nitrogen as heteroatom and its application as a flame retardant have attracted much attention in the academic and industrial communities over the past decade. Such compounds are relatively easy to synthesize and offer advantages such as high thermal stability, which is useful for high temperature processing, improved char stability, density and yield during the thermal decomposition of polymer, and release phosphorus species active in the flame inhibition process. Though a variety of P-N compounds can be found in the literature, this review mostly summarizes the recent (since 2013) development in phosphorus (O)-nitrogen containing flame-retardants which have been published in peer-reviewed journals. General strategies of synthesizing P(O)-N compounds as flame retardants from various phosphorus-based starting materials are highlighted in this review. Some of the most common classes of researched P(O)-N containing compounds as flame retardants include the phosphinamides, phosphonamides, phosphoramides, phosphoramidates, phosphorodiamidates, phosphonamidatesand their thio counterparts which are usually obtained via a one or two-step synthetic strategy. Incorporation of these compounds as flame retardants in various polymer systems such as polyurethane, epoxy resins, polyamides, cellulose, polylactide, polybutylene terephthalate, polycarbonates, and acrylonitrile-butadiene-styrene are discussed in detail in this review. Special emphasis on the various fire and thermal performances of the new materials are also summarized. The mechanical performance of new materials and the influence of these additives on polymer processing are also briefly discussed.

Corresponding Address: Dr. Sabyasachi Gaan Group Leader (Additives and Chemistry) Advanced Fibers, Empa Swiss Federal Laboratories for Materials Science and Technology Lerchenfeldstrasse 5 9014 St. Gallen Switzerland Tel +41 58 765 611 Fax +41 58 765 862 [email protected] www.empa.ch

1. Introduction In a modern society, polymeric materials are increasingly used in diverse sectors such as construction, transportation, electronics, packaging, industrial machinery, aerospace, etc. Increased use of organic polymers in such materials heighten the risk of fire, as most of them ignite easily. Thus the development of effective flame retardant (FR) strategies, which guarantee the public health and safety, motivates academia and industry. In addition, the development of such technologies helps lower the danger of fire accidents and loss of property. Some flame retardants (FRs) based on halogens which were extensively used in the past but have been proven to be toxic and thus banned from use today.1 New phosphorus based FRs have been considered as an alternative to halogenated FRs, and their usage in various applications have been explored extensively.2-4 In addition, the development of sustainable halogenated FRs has also risen dramatically.1 The versatile nature of phosphorus, i.e., high efficacy (low loadings in polymers and coatings), chemical diversity and ability to exhibit multiple mode of action, makes it a key element for a new FR development. The mode of action of a FR additive is crucial when developing the right formulations of polymer materials to meet specific fire tests. Mode of action of the FR strongly de- pends on its relationship with the polymer matrix. Generally, the mode of action of FRs are classified into two phases, i.e., the condensed phase and the gas phase, 2 with phosphorus based flame retardants (P-FRs) exhibiting both mechanisms.5, 6 In the condensed phase, P-FRs facilitates the formation of char via crosslinking, facili- tating cyclization, aromatization or graphitization by dehydration of the polymeric material. Additionally, some P-FRs act via intumescence, where the residue acts as a protecting sheet, reduce the heat transfer speed to the underlying polymeric material.7-9 It is also claimed that for some P-FRs, gas-phase flame inhibition mechanism and condensed-phase mode of action occur in parallel during the thermal decomposition of the material.10 All compounds discussed in this review are either incorporated into the polymer matrix as complete polymer chains or as a part of the copolymer chain as a co- monomer. Their incorporation in the material can also be done via surface modification or blending or by mixing of unreactive additives. Thermosets contain functional groups such as alcohols, amines, epoxy, halogens, etc. which allow for the incorporation of reactive additives during curing.3 Usually reactive FRs have an advantage since they are covalently bonded to the polymer matrix and therefore have a minor effect on the physical properties of the material. In contrast, addition of nonreactive FR additives results in a decrease in the glass transition temperature and the possibility of leaching is obvious. However, non-reactive additives dominate the market because of their lower price and easy processability. In addition, reactive FRs requires careful formulation of various ingredients, whereas non-reactive FR additives could be used in various polymer matrices without much modification.2, 11 P-FRs containing heteroatoms, for instance, silicon, nitrogen, boron, and sulfur offer a vast choice of specific beneficial interactions compared to the phosphorus compounds made only of pure hydrocarbons.12 Such interactions lead to the reduction of the load of FRs in the material and improved FR efficacy. 13-18 More recently DOPO based phosphonamidate (EDA-DOPO) has been shown to be non-toxic and successfully up-scaled in the industry.19, 20 The use of phosphorus-nitrogen (P-N) compounds as FRs are extensively studied, and their FR effect in many cases is attributed to P-N synergism. It is believed that P-N synergism encourages the cross-linking of polymeric chains during the thermal decomposition process. It boosts the retention of phosphorus in the condensed phase and produces thermally stable char. Additionally, the density of the residue formed at elevated temperatures increases and the matrix is more likely to be retained during the combustion process, consequently contributing to condensed-phase action and giving higher char yields.21 Cone calorimetry and

3 thermal experiments of polyurethane foams containing P(O)-N compounds have indicated their possible gas phase flame inhibition effect.22, 23 Recently, gas phase species such as phosphorus nitride (PN) and other PN species have been detected during the thermal decomposition of dimethyl phosphoramidate. The flame inhibition effect of such species is unknown.24

2. Nomenclature of P-N containing phosphorus compounds A wide variety of P-N containing organophosphorus compounds are known in the literature. The general structure of various P-N bond containing trivalent and pentavalent organophosphorus compounds are summarized in Figure 1.

Trivalent phosphorus

R P NR R N P NR R P NR2 R P NR2 R2N P NR2 R NR2 NR2

phosphine(amin) phosphine(diamin) phosphinetriamin

RO P NR RO P NR2 RO P NR2 R P NR2 OR NR2 OR

phosphoramidite phosphorodiamidite phosphonamidite

Pentavalent phosphorus

O O O O

R P NR2 R P NR2 R2N P NR2 RO P NR2 R NR2 NR2 OR

phosphinamide phosphonamide phosphoramide phosphoramidates

O O S S RO P NR R P NR RO P NR2 R P NR2 RO P NR2 R P NR2 NR2 OR OR OR

phosphorodiamidate phosphonamidate phosphoramidothioates phosphonamidothioic acid

S S S S R N P NR R2N P NR2 R P NR2 R2N P NR2 R2N P NR2 OR R R NR2 phosphorodiamidothioic acid phosphinothioic amide phosphonothioic diamide phosphorothioic triamide

Figure 1: Nomenclature of PN containing organophosphorus compounds

3. Synthesis of P(O)-N compounds 3.1 General Synthesis Methods A straightforward and common way to synthesize P(O)N and P-N compounds is by nucleophilic substitution reactions of P(O)-Cl and P-Cl compounds respectively.25, 26 Thiophosphorylated aminoheterocycles have been synthesized using this methodology.27 The P(O)-N containing compounds are also commonly synthesized from the

P-H (phosphonates and phosphonates) using carbon tetrachloride(CCl4) and an amine via an Atherton-Todd reaction where it is believed that a P-Cl bond is formed insitu as an intermediate.28 However, regardless of the easy handling of this reaction, it has found no commercial successes as CCl4 is a carcinogen and depletes the ozone.29, 30 More recently one pot two-step approach has been employed to convert P-H bonds to P(O)-N bonds. In these cases, P(O)-H bond has been transformed to P-Cl via the use of several chlorinating agents such as tert-butyl hypochlorite (t- 31 32-34 35 BuOCl), chlorine gas (Cl2), Copper(II) chloride (CuCl2), sulfuryl chloride 36 22, 37 23, (SO2Cl2), n-chlorosuccinimide (NCS), and trichloroisocyanuric acid (TCCA). 36, 38-40 and P(O)-Br via N-bromosuccinimide (NBS).41 Furthermore, P(O)-Cl bond formation has also been investigated via Michaelis-Arbuzov rearrangement of trialkyl 42 phosphite with Cl2 . Moreover, synthesis of P (O)-N bonds can also be achieved via 43, 44 45, 46 an oxidative coupling method using iodine (I2) or copper (Cu) as catalysts. However, upscaling of these synthetic procedures are challenging because of their exothermic nature, expensive raw material as well as side products and moderate reactions yields. As a possible alternative, the Staudinger-phosphite reaction has also been proposed to achieve P(O)-N bond.47-50 This synthesis procedure involves the reaction of alkyl azides with trialkyl phosphite to produce phosphorimidates, followed by rearrangement to their corresponding phosphoramidates. Another approach to the synthesize P(O)-N bond is metal-catalyzed C-H bond amidation by using phosphoryl azides51-54, however, the use of hazardous organic azides is a major limitation of this method. Recently, the synthesis of phosphoramidates was carried out via a green synthetic methodology using an organic dye as a photocatalyst. However, this synthetic methodology is only useful for diethyl phosphite derivatives.55

3.2 Synthetic approaches of P(O)-N compounds used as a flame retardants

The general strategy of synthesizing P(O)-N FR compounds from various phosphorus-based starting materials are summarized in scheme1. P(O)-N bond creation using chlorinating agents from various phosphorus-based starting materials are the most commonly used methods in academics for development of FRs. Some methods discussed in this review are also commercially suitable. There are two major synthetic pathway to obtain P(O)-N compounds; the first pathway is the nucleophilic substitution of trivalent phosphorus compound A, and the next step is the oxidation of compound B to achieve the desired product C as shown in scheme 1.25 The second possibility synthesizing P(O)-N compounds is the oxidation of compound A followed by chlorination of compound D using various chlorinating agents followed by nucleophilic substitution to obtain the desired product C. The method to synthesize compound C involves the chlorination of F in boiling thionyl chloride (SOCl2) for 18 hours to produce compound E, which is then reacted with an amine in the presence of a base.26 Another approach to obtained compound 56 C involves the reaction of A with chlorine (Cl2) gas , to form G intermediate, which upon hydrolysis yields intermediate E. The synthesis of thiophosphorus P(S)-N derivative involves a two-step strategy, starting with the reaction of A with hydrogen sulfide (H2S) to generate compound H followed by an Atherton–Todd reaction which yields the desired compound I. Other possibility of synthesizing thiophosphorus compounds involves the nucleophilic substitution of A in the first step to obtain B 25 followed by an oxygen sulfur exchange using S8.

R3 R4 + Et N + CCl4 3N S H S R3 R2 P H R2 P N R R1 R1 4

H I

H2S S8

R R 3 4 CCl + Et N R Cl N 4 3 3 H R P N R P 2 2 R R1 R1 Cl A 2 B Cl Cl R P 2 Cl R1 H O G 2 PCl3 or R-OH [O] H2O CH(OEt)3 R R 3 N 4 O O CA H O R3 R2 P H R2 P Cl R2 P N Base R R1 R1 R1 4 E D C

H2O2

O SOCl2 R2 P OH R1 F R1 = O-R, O-Ar, alkyl, aryl R2 = O-R, O-Ar, alkyl, aryl R3 = H, alkyl, aryl R4 = H, alkyl, aryl Chlorinating agent (CA) = CCl4, Cl2, NCS, TCCA, SO2Cl2

Scheme 1: General pathways for P(O)-N bond synthesis The following sections describe the application of P(O)-N based compounds as FRs for specific polymers.

4.1 Polyurethanes (PUs) Polyurethanes are a broad class of materials used widely in many applications in various forms such as foams (rigid or flexible), coatings, adhesives, sealants, solid components and etc.57 Rigid polyurethane foam (RPUF) is often used as an insulation material , especially in buildings, due to its superior physical and mechanical proper-

7 ties and low thermal conductivity, while flexible foams are used for application in a variety of consumer and commercial products, including bedding, furniture, automotive interiors, carpet underlay, and packaging.58-63 Due to their extensive use in eve- ryday life, it is necessary to treat these materials with FR additives especially for fire safe applications. It is required that these FRs shouldn't have negative influence on the physical properties of the materials and the environment. Phosphoramidates64-66 and phosphonates67 are well studied as FRs for PU foams. These compounds produce active PO species in the gas phase during the thermal decomposition process, which interferes with the combustion process of the flammable volatiles.

O O O O O O O O P P P O P N N N N H H H H

1 2 3 4

O O O O O H O P N N P O N P P O P N O N O H O H O

5 6 7 Figure 2: Chemical structures of DOPO-derivatives (1-7) synthesized by the Ather- ton-Todd reaction.36, 39

The toxic nature of the carbon tetrachloride (CCl4) limits the industrial upscaling of

P(O)-N based DOPO derivatives via Atherton-Todd reaction. To replace CCl4, especially for industrial upscaling, different chlorinating agents were explored. Along with CCl4 two compounds, sulfuryl chloride and trichloroisocyanuric acid (TCCA) were investigated as alternative chlorinating agents. The TCCA reaction is exothermic and it was suggested that it might not be suitable for use in industrial-scale syn- 36 thesis. In contrast, sulfuryl chloride (SO2Cl2) releases toxic gasses (HCl and SO2) as byproducts.36 The fire performance of P(O)-N containing DOPO derivatives 1, 2, 4, 6, and 7 was studied by incorporating them in polyether based flexible PU foams and compared to the commercially available FRs (TCPP, DOPO and Exolit® OP 560). FR evaluation based on UL-94 HB test on the PU foams showed that PU foam with bis-DOPO based phosphonamidates display superior fire performance in con- 8 trast to the commercial FRs. Compound 7 was shown to exhibit the best FR behavior compared to the other DOPO-phopshonamidate compounds (1, 2, 4 and 6). A concentration of only 5 wt.% (based on the weight of polyol) of compound 7 was needed to achieve HF1 rating in the UL 94 fire test. 68

O O O P P N O H O

8 Figure 3: Chemical structure of DOPO-derivative 8 synthesized via Atherton-Todd reaction.

A similar DOPO-phosphonamidate derivative ETA-DOPO (8)69, 70 was incorporated in the RPUFs and evaluated for flame retardancy. Small-scale fire tests show that by addition of 2 wt.% phosphorus in RPUFs a HF1 rating in the UL 94 test and a limiting oxygen index (LOI) of 22.2% can be achieved. Based on the cone calorimetry and thermogravimetric analysis (TGA) tests, RPUF containing compound 8 exhibited a higher char yield and lower smoke production. Compound 8 is active in both phases, i.e. in the gas phase by formation of low active phosphorus species and in the condensed phase by catalyzing tough char, which act as a barrier for fire.

A library of phosphoramidate derivatives (9-16) as shown in figure 4 have been synthesized and investigated for their flame retardancy in FPUFs. Being liquids, these compounds exhibit good compatibility with the PU matrix. Fire test results of the PU foams showed compounds 10 and 13 have superior FR performance compared to their structural analogs. The TGA and pyrolysis combustion flow calorimeter (PCFC) analysis showed that except for compound 16, all other compounds are mostly active in the gas-phase.23

O O O NH NH O NH NH P P P P MeO MeO MeO PhO OMe OMe OMe OPh

9 10 11 12

N NH O O O P OMe O P NH NH NH P P OMe HN PhO PhO OPh OPh

13 14 15 16

Figure 4: Chemical structures of mono-substituted secondary dimethyl/diphenyl and tri-substituted phosphoramidates 9 and 16

Dendrimer DPC–PAMAM (17) containing a phosphorus-nitrogen unit (figure 5) was synthesized and integrated into the PU matrix via in-situ polymerization to form a gel- phase network. Incorporation of the compound 17 in the PU matrix significantly improves the tensile strength of the PU material. TGA and cone calorimetry results showed that the insertion of compound 17 enhances the thermal stability and improves the flame retardancy of the PU matrix. The SEM analysis of residual char obtained from cone calorimeter experiments indicated a compact and uniform char layer for 17/ PU matrix compared to the char residue of the blank PU. This suggests that DPC-PAMAM works primarily in the condensed phase during combustion.71

O O P O NH

HN O H H O N N O P O N P O HN O O O O O O HN O P HN N H O HN O O H N N N N N N N H H O NH O H N NH O P O P HN O O O O O O O O N P N N H H O HN

HN O P O O

17 Figure 5: Chemical structure of synthesized dendrimer 17. A reactive additive diethyl-N, N-bis(2hydroxyethyl) phosphoramide(DEPA) (18) was synthesized from diethyl phosphate and diethanolamine. Compound 18 together with 8 wt. % expandable graphite (EG) was incorporated in RPUFs. LOI value of such RPUF improved by 10 % as compared to the blank foam.

OH O O O P O O P N O O N O O P N N P O H H OH O O

18 19 Figure 6: Chemical structures of phosphoramidate derivatives 18 and 19.

It was proposed that expandable graphite (EG) and compound 18 acted via the two- phase mechanism (gas phase and condensed phase).72 In the gas phase, the mate- * rial produces water (H2O) and carbon dioxide (CO2) along with PO species and sup- press the production of flammable gasses such as ether and benzene. In the condensed phase, continuous and dense char layers are formed with phosphorus-rich residues. Due to this two-phase mechanism, EG/DEPA system exhibit excellent flame retardancy in RPUF. In a similar work, compound 19 was synthesized from diethyl phosphonate and diethylenetriamine via Atherton-Todd reaction. Addition of 19 to RPUF alters its thermal decomposition process and helps increase the char formation in TGA experiments. The LOI values of RPUF containing 8 wt. % EG and 16 wt.% of 19 were around 30%. This study suggested that EG/19 interaction enhances

11 the formation of a stable char layer on RPUFs which act as a fire obstacle during the combustion process. Microscale combustion calorimetry (MCC) experiments on the RPUFs containing EG/19 showed a lower heat release rate for it compared to the neat RPUF. The scanning electron microscope (SEM) and energy dispersive X-Ray analysis (EDX) results on the chars obtained for RPUFs containing EG and 19 show a compact and phosphorus-rich layer which improve the barrier property of the char.73

OH O O O O O O N P NH HN P P NH N HN P N O O O O HO 21 20 21 Figure 7: Chemical structures of 20 and 21. A bisphosphoramidate derivative 20 was synthesized via a two-step reaction, which involves the synthesis of spirocyclic pentaerythritol bisphosphonate disphosphoryl chloride in the first step, followed by its reaction with ethanolamine in the presence of a base. RPUF were prepared with different concentration of compound 20 with the best fire results achieved at a loading of 25 wt.% of 20 (i.e., V-0 rating in the UL 94 HB test and a LOI value of 27.5%).74 Compound 20 was also incorporated in a waterborne polyurethane (WPU) dispersion and its fire performance was evaluated. For the WPU matrix containing 9 wt.% of 20, 1.62 wt% char content in TGA measurements were observed, and a higher LOI value of 26% was achieved . The LOI and cone calorimetry experiments indicated that by increasing the concentration of compound 20 in the matrix, the flame retardancy of waterborne polyurethane (WPU) films was improved. It was proposed that compound 20 is active in two phases, i.e. in the gas phase, it produces non-flammable gases, and in the condensed phase, it produces phosphoric acids. Further, phosphoric acid condenses into polyphosphoric acid to enable char formation via the carbonization process.75 A novel P−N contaning FR, octahydro‐2,7‐di(N,N‐dimethylamino)‐1,6,3,8,2,7‐ dioxadiazadiphosphecine (21) in 3 steps was synthesized. In the first step phosphorus oxychloride (POCl3) reacts with dimethylamine hydrochloride in dichloromethane solution in the presence trimethylamine base. In the second step, ethanolamine was reacted with intermediate dimethylphosphoramidic dichloride in dichloromethane solution using trimethylamine as a base followed by decompressing and distillation of the intermediate to acquire the final product 21. 12

In the next step, compound 21 was used as a chain extender to formulate waterborne P–N synergistic flame‐retardant polyurethane (DPWPU). Polyurethanes were prepared with the different concentration of compound 21(5 to 20 wt.%) and V-0 rating in the UL 94 test and a LOI value of 30.2 % was achieved at a loading of 10 wt.% . For the DPWPU matrix containing 15 wt.% of 21 a LOI value of 30.6% was achieved, on the other hands it was observed that when FR 21 content increased up to 20 wt.%, LOI value drops down to 29.8%. Moreover, it was found that the char residue increased with the increase of FR contents at 480°C.76 A firm and highly cross- linked reside formed during the burning tests was attributed to P-N synergism and keeps the material parts in the condensed phase for a long time. 4.2 Epoxy Resins (Eps) Epoxy resins (Eps) exhibit strong mechanical and adhesive properties, satisfactory electrical insulation, resistance toward chemicals and heat, making them ideal matrix for applications such as adhesives, composites, and coatings.77-80 However, Eps are also highly flammable polymers and therefore requires protection against fire by suitable chemical modificatons or by incorporation of FR additives.81-87 A new multi- element compound 21 containing phosphorus/nitrogen/sulphur elements were designed and manufactured (figure 8) via Atherton-Todd (AT) reaction and reacted with the Eps. The modified Eps showed lower thermal stability as well as lower glass- transition temperature (Tg) as compared to the blank Eps.

O S O S P P N N N N O H O H

22 23 Figure 8: Chemical structure of DOPO-derivatives (22and 23) synthesized by Ather- ton-Todd reaction.88, 89 The tensile strength of the modified Eps were lower compared to the virgin polymer, however, the flame retardancy of these Eps are significantly improved and successfully achieves a V-0 rating in the UL-94 test and a LOI value of 33.5%. Moreover, compound 22 showed activity in reducing heat release and reducing smoke discharge in cone calorimetry experiments. Evolved gas analysis using coupled techniques such as thermogravimetricanalysis–Fourier transform infrared (TGA-FTIR) spectroscopy and pyrolysis-gas chromatography/ mass spectrometry (PY-GC/MS) showed that compound 22 is active in the gaseous phase through N2/S volatiles. 13

Volatiles such as ammonia, hydrogen cyanide, nitric oxide, hydrogen sulfide, sulfur dioxide, and sulfur trioxide along with oxygen and phosphorus-based free radicals were also observed in these experiments. A similar compound 23 was also synthesized (figure 8) via Atherton-Todd reaction from DOPO and 2-aminothiazole and combined with Eps. When 5 wt.% of 23 used, a LOI value of 34.7% was achieved, and the material successfully passed V-0 rating in the UL 94 test. For Eps containing 10 wt.% of 23, a 20% increase in tensile strength was observed compared to the blank epoxy resin. It was hypothesized that its activity in the gas phase plays a vital role in stopping the heat discharge of epoxy resin during combustion.89

O O O H O , EtOAc 2 2 P N N P O

HN NH O O P N N P P O Cl CHCl3, Et3N

DOP-Cl 24

O S S P N N P O S8, Toluene

25 Scheme 2: Synthetic approach of 6 and 25 via DOP-Cl90

6‐chloro‐6H‐dibenz[c,e][1,2]oxaphosphorin (DOP-Cl) is an important precursor to synthesize DOPO and its derivatives. DOP-Cl is an intermediate formed in the DOPO manufacturing process and therefore commercially available. DOP-Cl is highly reactive, and thus it is widely used as a starting material to react with other nucleophiles to synthesize P-heteroatom compounds. For instance, the reaction of DOP-Cl with piperazine in dry chloroform yields compound 24 (Scheme 2).91 Subsequently, by using hydrogen peroxide (H2O2), 24 was oxidized in ethyl acetate solvent to yield compound 6. By reacting 24 with sulphur (S8) in toluene as a solvent, 6,6`-(piperazine-

1,4-diyl)-bis(6H-dibenzo[c,e][1,2]oxophosphinine-6-sulphide) (25) was obtained (Scheme 2).92, 93

S S S8 P P N N P P S S

O O O P P OH P P H N NH P-Cl 2 2 N N P P N N Base P P O O

O S Cl P P O P S P P-Cl = P = O O P = O P = O

26 27 Cl S P P P

O O O O O O P P P S Cl

29 Scheme 3: Synthesis of the tetraphosphorylated compounds 26, 27, 28 and 29.

A series of P(O)-N containing bridged compounds were synthesized from corresponding phosphorus chloride and nitrogen based nucleophiles, followed by oxidation with tert-butyl hydroperoxide (Scheme 3) to the pentavalent P(O)-N 25 or heated with elemental sulfur (S8) in toluene to obtain the di(thiophosphon) amidate (26), di(thiophosphin)amide (27) and di(thiophosphor) amidate (28) respectively. DOPO-Cl shows a different reactivity compared to DOP-Cl. When DOPO-Cl is reacted with a primary amine, it forms phosphonamidate (29), compared to DOP-Cl with the same amine leads to the formation of corresponding diphosphonamidates 25 (i.e. primary amine reacts with two DOP-Cl molecules). The thiophosphonamidate 30 was prepared via two-step methodology, in the first step, nitrogen based nucleophiles (m-

15 phenylenediamine) and DOPAM-3-propyl were transformed into phosphonamidite intermediate via a condensation reaction under reduced pressure. In the final step phosphonamidite was reacted with S8 in toluene to form thiophosphonamidate (30).

O H H O S H H S O N N O O N N O P P P P

30 31

O O P O P S N P N P O O S O

32 33

O O P O P S N O P O N S P O O P O N O P S N O P S P O O

34 35

O O P S O S P O P S O S P O N S N N S N O P S O S P O P S O S P O O

36 37

O P O H O O P O HN N N P H P O H O HN N N O N N NH P N N O P N N O O N N NH P O

38 39 40 Figure 9: Structure of DOPO based P-N containing compounds 30-40. 92-94

The advantages of diphosphonamidates (25 and 26) over phosphonamidates (29 and 30) are their higher thermal stability, higher phosphorus contents and higher molecular weight. Their fire performance was investigated in DEN438/DICY/Fenuron

16 based Eps, and the UL-94 test results were found to be dependent on the FR loading levels. For Eps containing compounds 25, 26 and 30 (1.5 wt. %P) a V0 classification was achieved, as compared to Eps containing 27 and 29 for which only a V1 rating could be achieved at similar phosphorus loading. Compound 29 does not showed adequate FR effect in Eps and is thus not classified in the UL 94 test. The char yields

(TGA experiments in N2) of FR containing Eps improved from 20 to 40 wt.% at 700 ºC as compared to the virgin epoxy resin (30 wt.%). A highest char yield of 40 wt.% was observed for Eps containing 29 which supports the hypothesis that compound 29 is active mainly in the condensed phase.95 For Eps containing compounds 26, 27, 28, 30 and 31, a lower amount of char formation was observed compared to the blank Eps, indicating their gas phase flame inhibition activity.77, 96

Recently, phosphorylated melamine based derivatives were synthesized from DOP-

Cl or DPP-CI as starting materials followed by oxidation in H2O2 and ethyl acetate as a solvent to afford pentavalent P(O)-N derivatives (38,39 and 40) . The fire performance of compound 38 was investigated in Eps (DEN438/DICY/Fenuron) and for comparison commercial available DOPO was chosen. For Eps containing DOPO (1.6 wt. %P), a V0 classification was achieved, whereas Eps with 1.4 wt. %P ( compound 38) V0 rating could be achieved.94

NH2

H N NH2 2 NH O O H N 2 O H 2 NH HN P NH HN P N O P O 2 HN NH NH2 O H2N

H2N NH2

41 42 Scheme 4: Schematic synthesis of 41 and 42. Phosphortriamidates 41 and 42 were synthesized by reaction of triethyl phosphate with an excess of ethylene diamine and benzene-1,3-diamine respectively (Scheme 4). Both compounds were successfully used as crosslinkers for the Eps. Their FR behaviors were investigated, and UL 94 tests showed that for the Eps with 2 wt.% P

17 a V-0 rating and LOI values of 30% could be achieved.97

O HN P NH O O O HN P NH HN P NH O O

43 44 45

O O O HN P NH HN P NH HN P NH

46 47 48 Figure 10: Chemical structures of 43-48. A series of phosphorus nitrogen containing compounds (43-48) were prepared from phenylphosphonic dichloride and different amines in the presence of triethylamine (TEA) as a base. Among these compounds, two compounds (43 and 46) were se- lected and tested for flame retardancy in epoxy resin diglycidyl ether of bisphenol A (DGEBA-type epoxy resin) along with 4,4-diaminodiphenylsulfone (DDS) (curing agent). At a loading of 30 wt.% of 46 in cured epoxy resin, a V-0 rating in UL 94 test and a LOI value of 31% could be achieved. For the cured epoxy resin with a loading of 30 wt.% of compound 43, a V-1 rating in a UL-94 test at and a LOI value 28% was achieved. In addition, in cone calorimetry experiments lower peak heat release rate (pHRR) was observed for epoxy resin/46 compared to the epoxy resin/43 system. It was proposed that compound 46 is active in two-phase, however, 43 is only active in the condensed phase. In the condensed phase, thermal decomposition of both compounds produce acid compounds (H3PO3 and H3PO4) which acted as a source of char formation with epoxy matrix. In the gas phase, thermal decomposition of 46 produced PO* radicals responsible for flame inhibition. Furthermore, the small loading of compounds 43 and 46 had no significant impact on the Tg, tensile strength and storage modulus of Eps.98

O O O P N P N N N O

49 50 Figure 11: Chemical structures of imidazole derivatives 49 and 50.

Imidazole based phosphorus derivatives (49 and 50) were synthesized and used to cure Eps.99 These compounds ensure not only long pot life during storage of epoxy resin and fast curing during application but also excellent FR effects. For epoxy resins containing 15 wt.% of 49, a LOI value of 31% was achieved compared to 21% for a virgin resin. Moreover, in the UL 94 test, a V-0 rating for 1.6 mm thick samples could be achieved. A similar result was found in the UL 94 test and a higher LOI value of 38% was observed for EPs containing 15 wt.% of 50. Cone calorimetry tests showed that 49/EP and 50/EP formulation had excellent flame retardancy and 49/EP clearly demonstrated improved smoke suppression due to its dominant activity in the condensed phase. By loading 15 wt.% of 49, the total smoke production (TSP) of 49/EP dropped to 13.3 m2·m−2 compared with blank EP (25 m2·m−2), and the char residue yield was higher (37.3%). However, TSP value of 15 wt.% of 50/EP increased (28 m2·m−2) as compared to that for the blank sample, and it was thus suggested that compound 49 is more active in the vapor phase.99

R1 O O O n R1

cyclic polymer

N-dealkylation R N N

O O R1 R1 R1 R1 R N 1 R N O R N N N N O O O R propagation zwitterion R1 n

R1 R1 N R N O O OH R 1 n

R N N β-elimination

R R R1 R1 1 1 R O O 1 O OH O OH tautomerization R1 O R1 n-1 n Scheme 5: Possible mechanism of the curing procedure of P (O)-N/EP. The proposed curing mechanism for P(O)-N/EP components is shown in scheme 5. Firstly, the epoxy group of the pre-polymer reacts with tertiary nitrogen of the imidazole derivative to form an adduct (zwitterion), in the next step, further reactions lead to an epoxy chain propagation. Additionally, there are two options for the following reactions, the first possibility is the N-dealkylation reaction which generates a cyclic epoxy macromolecule and the second possibility is the β-elimination which yields an unsaturated macromolecular structure which is finally converted into a carbonyl group. Consequently, it was proposed for compounds 49 and 50, the electron-deficient imidazole ring detaches from the macromolecule and further takes part in other reactions as a catalyst during the curing process.

O O O O N O O P O O O

O O O P O O N O O O O

51 Figure 12: Chemical structures of 51. A multifunctional P(O)-N containing diluent 51 was synthesized using multistep reactions starting from spirocyclic pentaerythritol bisphosphonate phosphoryl chloride followed by its reaction with diethanolamine, and the final product 51 was achieved by reaction with allyl chloroformate. The synthesized compound 51 was successfully incorporated in epoxy acrylate oligomer in various concentration (5-25 wt.%) together with a photo-initiator (PI), applied on wood via coating and finally cured with UV. In the vertical burning test (UL-94), coated wood (10 cm X 7cm) achieves a V-0 classification along with a significant increase in LOI values. When the treated wood was exposed to 5-10 s of flame, a self-extinguishing behavior of the wood was observed.100 By increasing the concentration of compound 51 (5wt.% to 25 wt.%) in the coating formulation, the LOI of the matrix was also increased from 23% to 35%. The char data obtained from TGA shows that by increasing the phosphorus (P), nitrogen (N), and aromatic content of the polymer matrix, char yield increase from 2.53 wt. % to 17.9 wt. % which acts as a barrier against fire.

O NH O H H 2 H H N P N H2N N P N NH2 H2N

53 52 Figure 13: Synthesis of phosphorodiamidates derivate 52 and 53. Two phosphorodiamidates (52 and 53) as shown in figure 13 were synthesized via a condensation reaction of corresponding diamines with phenylphosphonic dichloride. Due to the bifunctional nature of these compounds, it makes them useful candidates as a crosslinkers of epoxy resin. Flammability and thermal studies on the modified 21

Eps show that the incorporation of compound 53 at 2.3 wt. % phosphorus content produces high char yields along with a LOI of 27 % (around 6% higher than untreated epoxy resin). Furthermore, the compound 52, which contains higher aromatic content display excellent FR activity, a LOI value of 31% was obtained at 2.1 wt. % P in the epoxy resin.101

The phosphorus-nitrogen (P–N) synergism in 52 and 53 play a vital part in forming phosphorus-nitrogen rich char during the thermal decomposition of the epoxy polymer. Eps containing 52 gives higher and more stable char compared to that cured with 53.

O P NH

54 Figure 14: Structure of 54. Compound 54 was synthesized by the reaction of diphenylphosphinic dichloride with allylamine in the presence of trimethylamine as a base. It was then incorporated in the epoxy acrylate along with octamercaptopropyl polyhedraloligomeric silsesquiox- ane (POSS-8SH). Various formulations using different weight ratios of compound 54 and polyhedral oligomeric silsesquioxanes (POSS) in an epoxy acrylate resin was prepared. The incorporation of compound 54 into an epoxy acrylate reduced the double bond conversions; in contrast, the addition of POSS-8SH improved the con- version of unreacted double bonds. Moreover, with the addition of compound 54 flame retardancy of the Eps increased but with the addition of POSS, flammability decreased and failed to pass the UL-94 test. Also as the amount of compound 54 in the polymer matrix increased, the tensile strength of the coatings reduced.102 Eps with 20 wt.% 54 loading achieved a V-1 rating in the UL 94 test.

O H N H H S N O S N P N N P N S N S N H

55 56

Figure 15: Synthesis of phenylphosphonic derivatives 55 and 56. Phenylphosphonic derivative 55 and its tautomer 56 (figure 15) was synthesized by reaction of 2-aminobenzothiazole with phenylphosphonic dichloride using triethylamine (TEA) as a base and cross-linked in Eps.103 For epoxy resin containing 0.69 wt. % of P, a V-0 rating in UL 94 test and 31% LOI value was observed.

A phenylphosphonic derivative 57 was synthesized and combined with a bio-based molecule beta-cyclodextrin (β-CD) to form an inclusion complex (IC). The IC was obtained by dissolving beta-cyclodextrin (β-CD) in water at 65 °C, followed by dropwise addition of compound 57 in water at the same temperature (65 °C) for 2 hours and possible inclusion structures were proposed according to the results of 1H NMR spec- tra.

O HN P NH

b-CD inclusion behavior + epoxy resin + epoxy inclusion complex

O O HN P NH HN P NH

Figure 16: Synthesis of epoxy inclusion complex. As a comparison to IC, the physical mixture (PM) of compound 57 and β-CD prepared by mixing them in dimethylsulfoxide (DMSO) at room temperature and evapo- ration of the solvent. The results showed that the addition of 2 wt.% loading of 57 and β-CD obtained via physical mixing (PM) in the Eps increased LOI values to 24.5%, whereas the blank resin had a LOI value 22%. However, PM/epoxy system

23 was not able to pass the UL-94 test. By using the same concentration (2 wt.%) of IC in epoxy resin, higher LOI values (26.5%) were achieved, however, the IC/EP system also showed no rating in UL-94. The TGA results under nitrogen (N2) showed that the char formation of IC was higher (25.9%) than those of PM (16.8%) at 600 °C. Moreo- ver, the thermal decomposition temperature of IC was much higher than PM. The flammability of epoxy resin (EP) with IC was lower as compared with epoxy resin (EP) and PM/epoxy system. These results indicate that IC acts in both condensed and gas phase. In the condensed phase, compound 57 probably acted as an acid source along with β-CD, which functions as a char precursor. On the other side, a decrease in the heat of combustion (EHC) values of EP/IC and increased values of

CO/CO2 in cone calorimetry experiments compared to the blank Eps showed that IC is active in the gas phase.104

Subsequently, in a separate work, 57 was added into the epoxy matrix and in a glass fiber reinforced epoxy composite (GRE). On addition of 5 wt.% of compound 57, the epoxy matrix showed a significant increase in the LOI from 22% (blank) to 32% and a V-1 rating in the UL-94 test was obtained. Upon addition of 12 wt.% of 57, the highest LOI values (36%) was achieved and the material passed with a V-0 rating. LOI values of blank-GRE (25%) was higher compared to the blank epoxy matrix (22%). With the addition (5 wt. % and 12w.t% ) of compound 57 in GRE, the LOI increased (29% and 33% respectively), however, the impact of compound 57 on LOI of GRE was lower as compared to the epoxy matrix at the same loading. In the UL-94 test, flame-retardant glass fiber reinforced epoxy composites (FGRE) achieved no ratings, but with the addition of 12 wt. % of 57 in GRE, a V-0 rating could be achieved. In addition, pHRR of flame-retardant 12 wt.% 57/epoxy matrix dropped to 66% in comparison to the blank epoxy matrix. In TGA experiments, the char residue of flame-retarded epoxy resins was lower than that of FGRE. Compound 57 was proposed to be active in both the condensed and gas phases. In the gas phase, the thermal decomposition of 57 released PO* radicals and PO* radicals are able to re- combine H* and OH* radicals. In the condensed phase, thermal decomposition of 57 produced the acidic phosphorus compounds which catalyze char formation. Mechan- ical characterization involving nanoindentation test results showed that the addition of 57 did not disturb the interfacial strength and hardness of epoxy matrix and GRE.105

O CH CN O Cl P Cl 3 H H + H N R NH P N R N 2 2 n refluxing

O O S , DDS: R= R= S , CH2 O O O

58 59 60 DDM: R= CH2

DDA: R= O Scheme 6: Synthesis route of 58-60. Polyphosphonamide (PPDA) derivatives were synthesized via nucleophilic substitution reaction of phenlyphosphinic dichloride and various amines, as shown in scheme 6, combined with the Eps and subsequently investigated for their fire performance. All synthesized PPDAs display low flammability and excellent thermal stability. On addition of PPDAs, the flammability of the epoxy composites was significantly improved. It was found that the thermal properties of the final material are highly dependent on the chemical environment of the PPDAs. The synthesized PPDAs were combined with epoxy resins at 80 ºC for 10 min, poured in a PTFE mold and cured for 2 h at 80 ºC followed by post-curing for 2 h at 120 ºC. All samples were then gradually cooled down to room temperature. All formulations prepared with same concentration (15 wt.% ) of PPDA. Pure EP produced poor char residue and no rating in the UL-94 test. However, the addition of PPDAs in EP, LOI and char residue increased. When 15 wt% of 59 was incorporated into EP, the LOI value of cured EP increased to 28.9% and a V-1 rating was achieved in the UL-94 test. Upon incorporation of 15 wt% of compounds 58 and 60 in EP, the LOI values increased to 29.5% and 29.6% respectively, V-0 rating achieved in the UL-94 test in both cases. These investigations show that the combination of the heteroatom in 58 and 60 in the phosphorus structure results in enhanced flame retardancy compared to 59. The PPDAs are active in both the condensed phase and gas phase. In the condensed phase, the thermal decomposition of PPDAs into acid compounds (H3PO3) which promote the char forming process in EP, provide an additional protective layer. This is evident from the increased char yield (21 wt%) for the EP/PPDA composites compared to the blank EP(4.8 wt% ) as observed from cone calorimetry test. In the gas phase, PPDAs showed the predicted flame inhibition effect during ignition. It is known that phospho- 25 rus-containing polysulfone act as toughness modifier for epoxy resins.106 However, for EP/59 and EP/60 systems, no serious drop in toughness was observed.107

O Cl H P N N N CH3CN n O O P O + O O P Cl O N Et3N O H OO O P O

61 Scheme 7: Synthesis of polymeric crosslinker 61. A polymeric crosslinker poly (pentaerythritol phosphate phosphinic acyl piperazine) (PPAP) (61) was synthesized as shown in scheme 7 and its fire performance tested on Eps. At 20 wt. % PPAP loading in the epoxy resin, a LOI value of 35% was observed compared to the blank epoxy resin which has a LOI value of 19%. Further- more, TGA analysis showed an improvement in the thermal stability of the composite. In cone calorimetry experiments for 20 wt.% loading of 61, Eps improved the char formation (92.7 wt.% ) and a 70% decline in pHRR and 60% decrease in the THR was observed. Dynamic mechanical analysis (DMA) of the Eps showed that crosslinker 61 also improves the mechanical property of EP.108

Polyamide 6 (PA6) and polyamide 66 (PA66). Polyamide 6 (PA6) and polyamide 66 (PA66) engineering plastics exhibit high abra- sion resistance, excellent mechanical strength, good processability, and high chemical resistance. These properties make them suitable substrates for electronics, car- pets and textile clothing, automotive, and electrical parts. Even though polyamides show slow-burning behavior, their flame resistance is not enough for most fire safe applications. Due to these risks, polyamides require to be protected against fire.109, 110 Most common method used to decrease the flammability of polyamides materials is to add a FR additive in the material via melt processing.

O P O N

Figure 17: Chemical structure of DOPO-derivative 62

Commercially available diethyl aluminum phosphinate (Exolit® OP 1230) and compound 7 (figure 2) were incorporated in PA6 via an extrusion process. A V-0 rating in the UL94 test was observed for the PA6 formulation containing both additives. In addition, a LOI of 31.7 % was observed for PA6/ 7 formulation.111 A P(O)-N containing monomer DOPO-DAAM (62) was synthesized, grafted on PA66 fabric via a UV treatment which was shown to improve its flame resistance. Prior to the FR treatment, the PA66 fabric surface was first modified via an HCl treatment to enhance the grafting of the FR molecules. A pass was achieved in a vertical burning test for PA66 fabric treated with 20 wt. % of 62. TGA test of the grafted PA66 fabric samples showed a decrease in the thermal decomposition temperature compared with the blank fabric. Cone calorimetry experiments and TGA analysis of the treated fabric, residual char analysis showed that the 62 decomposed earlier than the virgin PA66 and in such system, a gas phase flame inhibition activity is dominant compared to the condensed phase activity.112

O H H H H HOOC N P C C N COOH H H O

63 Figure 18: Chemical structure of polymeric crosslinker 63. Phosphorus-containing polyamide 66 (PA66) was synthesized via polymerization of the PA66 pre-polymer and monomer 63. A two-step reaction procedure was used to synthesize monomer 63. Phenyl phosphinic dichloride first added dropwise to acrylic acid, followed by an addition of aminobenzoic acid to obtain the desired monomer 63. On addition of 5 wt. % 63 as a co-monomer, the modified PA66 passed the UL-94 test with a V-0 rating and achieved a LOI value of 28%.113

H H H H HOOC N P N COOH H2N N P N NH2 O O 2

64 65

N N O N N P H2N N N N N NH2 H H

66 Figure 19: Synthesis of phenyl phosphine oxide based polymeric crosslinker 64-66. In a subsequent work, several new derivatives of phosphorus and nitrogen- containing compounds (64-66) having carboxyl or amino end groups were synthesized and incorporated via copolymerization in polyamide 66 (PA66).114 Synthesis of monomer 64 was carried out via acylation reaction of phenylphosphonic dichloride with aminobenzoic acid. Compound 65 was prepared by the reaction of 1, 4- benzenediamine with phenylphosphonic dichloride by a dropwise addition and triethylamine was used to trap chlorine fumes. The reaction of benzoguanamine with phenylphosphonic dichloride resulted in 66. By using 5 wt. % of these monomers, all modified PA66 could successfully achieve V-0 rating and PA66 containing monomer 65 afforded the highest LOI value of 29%.

O Cl P Cl TEA O H H H + H N NH 2 2 H2N N P N N NH2 67 5

O H H H CH3CH2OH H2N N P N N NH2 + HOOC(CH2)4COOH 5

O + H H H + - - H3N N P N N NH3 + OOC(CH2)4COO 5

68 Scheme 8: Synthesis of phenyl phosphine oxide based pre-polymer 67 and 68. 28

P(O)-N containing poly-N-aniline-phenyl phosphamide (PDPPD) (67) reactive macromolecule as shown in scheme 8 was synthesized. By using adipic acid, prepolymer 68 was prepared followed by the polymerization of PA66 with prepolymer 68. For PA66 containing 4.5 wt. % prepolymer 68, a V-0 rating in UL 94 test along with LOI value of 28% was achieved. However, compared to the virgin PA66 the TGA results of the modified PA66 showed a decrease in the thermal stability of the modified PA66 by 43 °C.115

4.3 Cellulose fibers. Cellulose is one of the most commonly used biopolymer and widely used in the manufacturing of textiles, paper, and architectural products. Like many organic polymers, cellulose is inherently flammable and mostly flame retarded by incorporation of FR additives in bulk or as coatings. Phosphorus based FRs especially the P(O)-N containing compounds are very popular for flame retardation of cellulose.116 In many cases it is claimed that P(O)-N compounds offer synergism and improved flame retardancy of cellulose.116-118

O O O O N N NH NH P P P P MeO EtO EtO EtO OMe OEt OEt OEt

54 55 56 57

MeO OMe EtO OEt H P H P N O N O O O N O N NH P H P H P EtO MeO OMe EtO OEt OEt

58 59 60

Figure 20: Chemical structures of synthesized phosphoramidites 69-75. A series of alkyl ester phosphoramidates derivative (69-75) were synthesized and incorporated in cellulose fibers obtained from a different source (Peat and Cotton fibers). Phosphoramidates with smaller phosphoester group display significantly higher condensed phase activity compared to the phosphoramidates with larger phosphoester groups. Furthermore, the bis-substituted phosphoramidates derivatives showed higher fire performance as compared to the monosubstituted phosphoramidates (54, 55, 56, 57 and 58).119

O O O O P Cl HN NH O P N N P O 76 O O O

O O O O P Cl H N N N P 77 2 O O HN N N

Scheme 9: Synthesis of phosphoramidites 76 and 77. Two piperazine derivatives 76 and 77 as shown in scheme 9 were synthesized, and their flame retardancy was investigated on a cellulose substrate (cotton fabric). Vari- ous thermal and chemical analytical techniques such as TGA–FTIR, Py-GC/MSFTIR and thermogravimetricanalysis–Fourier transform infrared (TGA–FTIR) were used to understand the mechanism of the thermal decomposition for the cellulose treated with 76 and 77. The analysis showed that the phosphorus additives were stable to the treatment conditions. The untreated cellulose generates more flammable gasses compared to the cellulose containing compounds 76 and 77. 120 Cotton fabrics treated with 7 to 13 wt. % of 76 and 77 passed the vertical flammability. For the corresponding treatments, the LOI values of the treated cotton fabrics with compound 77 were 26.7–33.1% and for 76 were 23.7–31.0.121 O O O O O O O O N N N P P N N P P N O O O O

78 79

Figure 21: Chemical structures of phosphoramidates derivatives 78 and 79. Siprocyclic phosphoramidate derivate 78 was synthesized via one-step reaction of imidazole with spiralphosphodicholoride (SPDPC) 122 and coated on cotton fabrics with various aqueous formulations of 78, cyanuric acid and phosphoric acid. Before treatment, the cotton fabric was boiled in water for 30 min and dried at room temperature. Then blank cotton fabric was immersed in an aqueous solution of different formulations of 78. In these formulations, compound 78 was used as an intumescent

FR, cyanuric acid as a cross-linking agent and phosphoric acid (H3PO4) as a catalyst. Five different formulations were used from 10-30 wt.% loading of 78. In fire tests, the treated cotton fabrics with 30 wt. % of compound 78 exhibited self-extinguishing be- 30 havior and achieved a LOI value of 36.6%. The cured cotton fabric exhibited improved thermal stability and 13.3 % decrease in the tensile strength compared to the untreated cotton fabric. Cone calorimetry experiments on the treated cotton showed a decrease in heat release rate by 57 % and total heat formation decreased by 60 % compared to the blank cotton fabric. Furthermore, a similar precursor of phosphoramidate derivate 79 was synthesized by reacting pyrrole with di(phosphate mono- chloride) (SPDPC) in the presence of triethylamine as a base in acetonitrile. The TGA data of 79 showed the initial decomposition temperature was around 270 ºC, along with 5% char yield at 600 ºC and may be useful as FRs for some polymer during melt processing.

OEt O EtO Si N P O O H P OEt O N HO OH

80 81 Figure 22: Chemical structures of 80 and 81. Hydroxyl containing spirocyclic phosphoramidate 80 (figure 22) was prepared by reacting neopentyl glycol with phosphorus oxychloride in acetone, followed by dropwise addition of diethanolamine (DEA) and triethylamine in acetone at 0–5 °C.123 Cotton fabrics were treated with various concentration (10%, 15%, 20%, 25%, and 30%) of compound 80 in acetone/water (1:4 vol/ vol) solutions. The treated fabrics passed the vertical burning tests and achieved higher LOI values (27.0–29.3%), respectively compared with neat cotton (19%). The cone calorimetry results showed that improved flame retardancy of treated cotton fabrics with compound 80 was due to the phosphorylation reaction, which promotes phosphorus-rich char formation layer in the condensed phase and restricts the release of combustible gases in the gaseous phase. A silicone containing phosphoramidate 81 (figure 22) was synthesized by reacting diphenylphosphinic chloride with (3-aminopropyl)trimethoxysilane in the presence of a base. The cotton fabric was treated with various concentrations of compound 81 in ethanol/ water solutions. Subsequently, the treated fabrics were dried at 140 °C and evaluated for their thermal stability and flame retardancy. TGA measurements showed increased char formation for treated cotton fabrics above 500 °C. Treated cotton fabrics exhibited self- extinction as soon the flame source was removed. The treatment was shown to be durable to several washing cycles.124

O O O O P P N OH N OH O H O H

82 83 Figure 23: Chemical structures of phosphoramidate 82 and 83.

Two hydroxyl functional phosphoramidate derivatives, dimethyl 3- hydroxypropylphosphoramidate (MHP) 82 and hydroxypropylphosphoramidate (EHP) 83 were synthesized and applied on cotton twill fabric at 5 to 20 wt. % loadings.125 Cotton twill fabric with 5 wt. % of both compounds showed self-extinction behavior. Furthermore, all fabrics treated with 82 exhibited higher LOI values (27.0 − 37.2%) compared to the 83 treated fabrics (25.8 − 33.4%). In the MCC test, lower values for THR and pHRR were observed for all fabrics treated with compound 82 as compared to the fabrics treated with compound 83. Furthermore, the THR (3.9−3.8 kJ/g) of 82 treated samples decreased with increasing add-on, while the THR (4.6−6.3 kJ/g) of 83 treated cotton increased with increasing add-on. TGA results showed that the char yield increased for 82 (33−36%) and 83 (28−31%) treated fabrics with the increasing add-on. Evolved gas analysis using TGA-FTIR showed that compound 82 decomposes to yield a terminal amine. Further, this terminal amine could react with decomposing cellulose to enhance its char formation or volatilize into an active species in the gas phase. Additionally, the FR efficacy of 82 was related to the ability of O-alkyl group to form a covalent bond with cellulose. H O O H O N O N O P O P P N HN P NH O N O P O HN H HN H HN HN HN CF3 NO NO2 2 Cl

84 85 86 87 88 Figure 24: Chemical structures of phosphoramides and phosphoramidates 84-88.

Various phosphoramides and phosphoramidates (Figure 24) were synthesized, applied together with a thermal initiator on cotton fabrics and cured at high tempera-

32 ture.126 Compounds 85 and 87 showed a lower degree of grafting and char formation in TGA experiments compared to compounds 84, 86 and 88. The degree of grafting (DG) for each monomer was estimated at three different concentrations of monomers. It was observed that the degree of grafting increased by increasing the monomer concentration. Monomers (84, 86 and 88) containing unsaturated (C=C) bonds have the ability to graft to the cotton fabrics. Thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) results of the treated cotton showed an enhancement in the char formation and confirmed the action of monomers in the condensed phase.

O EtO P OEt Si N N Si EtO H H OEt OEt OEt

89 Figure 25: Chemical structures of phenylphosphonic derivative 89 Triethoxysilane derivative 89 (figure 25) was synthesized and applied on cotton fabric. The cotton fabrics were treated with a 10 wt. % solution (water and ethanol, v/v, 1:1 of 89 for different treatment times. The treated cotton fabrics instantly self- extinguished once the flame source was detached.127 Hybrid materials prepared by combining compound 89 and thiol-ene photocurable curable resin. The sol-gel precursor was synthesized by reaction of methacryloxypropyltrimethoxysilane (MAPTMS) with an alkoxide in ethanol. Water was used for hydrolysis in the presence of a catalyst (p-toluene sulfonic acid). The combination of sol-gel precursor and compound 89 not only increases the thermal stability of the materials but also improved the flame retardancy of the hybrid material (LOI of the hybrid material was 26.5%).128 Similar synthetic methodology was used to synthesize 90 and subsequently used to coat the cotton fabrics. However, lower thermal stability for treated fibers were observed compared to the fibers treated with the previously synthesized FR hybrid system. An increase in char residue was observed for treated fibers with increasing concentration of compound 90. Fire performance of the treated and untreated fabrics was investigated via LOI and vertical test. When add-on of 80 in fabrics was increased from 5.4% to 18.7%, the LOI value improved from 20.5% to 24.5% respectively.

O O Si O O O Si O

O HNHN O O Si P Si O O N N O HN P P NH N HN NH

O O Si Si O O O O

Figure 26: Chemical structures of cyclotriphosphazene derivative 90

Thus, hybrid compound 90 improved the flame retardancy of the cotton fibers by im- proving char formation, which stops the decomposition of the underlying substrate and restricts the formation of flammable gases throughout the thermal degradation process. The fabric’s treatment durability was also confirmed as there was no change observed in phosphorus content of the fabrics after the laundry experiments.129

H N P N O O n Cl H N O H P 2 NH O 2 Cl

91 Scheme 10: Synthesis of poly(phosphorodiamidate) 91. Poly(phosphorodiamidate) 91 was synthesized by the reaction of ethylenediamine with phenyl dichlorophosphate and was used to coat cotton fabrics. The cotton fabrics were immersed treated with various concentrations of 91 along with tetraethyl ortho-silicate (TEOS) which acts as a cross-linker to increase the networking of 91 on the fabric. The treated cotton fabrics were dried at 160 °C and were laundered before further fire tests. Vertical flame tests were used to investigate the flammability of treated cotton fabrics. Based on phosphorus–nitrogen synergism, oligomer 91 exhibited excellent flame retardancy on treated cotton fabrics compared to the untreated cotton fabrics. The fire test showed, for a loading of 3.5 wt.% of 91 and 1 wt. % tetraethyl-ortho-silicate (TEOS), a LOI value of 27.2% achieved. TGA results showed that treated cotton fabrics with 91 were more thermally stable as compared with blank cotton fabrics.130

2 N H O P N N Cl O Cl Cl n P O Cl O O O

N O N O P P N O N O O OH O 92 Scheme 11: Synthesis of polymer 92. Polymer 92 was synthesized from the polymerization of methacryloyloxyethylortho- phosphortetraethyl diamidate via a free radical polymerization with benzoyl peroxide as an initiator. The monomer was synthesized via a three-step reaction shown in scheme 11. Different concentration of aqueous solutions of polymer (92) was used to treat the cotton fibers in the presence of a cross-linker and a binder (vinyl acetate). The cotton fabrics treated with polymer 91 (3-4 wt.%) and vinyl acetate as a binder, their LOI values range 18.6%–19% respectively as compared with blank cotton fabric (17%). Some improvement in LOI values (2% to 3%) was observed but it is was considered not enough to act as flame barriers.131

O O O O HO P O P OH N N H O O H

Figure 27: Chemical structures of dioxaphosphinane derivatives 93.

Recently compound 93 was synthesized using ditrimethylolpropane with phosphoryl chloride as starting materials (Figure 27) and was applied to polyester, cotton, nylon fabrics via a dipping method. The fabrics were dried first and then cured at 160 °C. The fire performance of the cured fabrics was tested using vertical flame tests and LOI measurements, and results show that treated nylon fabrics possess better flame resistance compared to other treated fabrics.132

4.4 Poly(lactic acid) (PLA). Poly(lactic acid) (PLA) is known for its good biodegradability and biocompatibility as well as having a relatively high melting point compared to other biopolymers. Be- cause of these properties, PLA and its copolymers are used in nonwoven and woven 35 textiles, packaging, electronics, and agriculture.133-136 Due to these useful properties, it is expected that in the future PLA will be increasingly used in sectors such as electronic equipment and transportation were a high level of flame retardancy is required. The flammability of PLA is reduced by mixing it with phosphorus-containing compounds, antimony trioxide, halogenated and nitrogen compounds.137-140

NH2 H2N

K2CO3

Cl Cl O O i) HO NHCOCH3 P P N N N N H N P P NH Cl P P Cl 2 O O 2 N ii) NaOH/H2O N Cl Cl O O

H2N NH2

Intermediate

DOPO Et3N/CCl4

O P O O P NH HN

O O O P P O N N O P O HN O P P O NH N O O

HN O HN O O P P O

94 Scheme 12: Schematic synthesis of 94 using the Atherton−Todd reaction. A series of phosphazene and phosphaphenanthrene derivatives (94-97) as shown in scheme 11 and 12 have been synthesized as FR additives for PLA. The synthesis of 94 was performed via a 3-step strategy. In the final step, DOPO reacted with amino containing triazine intermediate using carbon tetrachloride in the presence of triethylamine via Atherton-Todd reaction to form 94 as shown in scheme 12. Furthermore, condensations of this intermediate by three different types of aldehydes along with DOPO via Kabachnik-Fields reaction with an imine linkage yields compounds 95, 96 and 97 respectively as shown in scheme 13.141

O O P O R P H NH N O R

Carbonyl O O P R O O compounds N N P Intermediate DOPO O HN O P P O NH N P O O O R

HN HN R R= H 95 R O O P O P O OH R= 96

R= OH 97

Scheme 13: Schematic diagram for the synthesis of 95–97 via Kabachnik–Fields. The fire performances of compound 96 were investigated by mixing with PLA via melt blending. A limiting oxygen index (LOI) value of 30% was achieved at a 5% wt. loading of compound 96. However, for PLA composite with 5 wt.% 94, a LOI value of 34.7% and a V-0 rating in UL 94 test was achieved.142

4.5 Polybutylene terephthalate (PBT) Polybutylene terephthalate (PBT) is widely used as an insulator in the electronics and electrical industries. PBT is the semi-crystalline polymer that is resistant to shrinkage and solvents as well as being mechanically and thermally stable up to 150-200 °C. Flame retardancy of this material can be achieved by incorporating FR additives. Phosphorylated ethylamine derivatives (98 and 100) were synthesized and investigated for their FR properties in PBT. Initially, an intermediate was synthesized via an addition reaction between DOPO and N-vinylformamide as a catalyst. Further, an acidic workup gives a stable intermediate, which was used further to synthesize stor- able, stable amines derivatives (98 and 100). The synthesis of 98 was performed via

Atherton Todd reaction using carbon tetrachloride (CCl4) as chlorinating agent. Com- pound 100 was synthesized via multi step synthesis procedure, which involves the reaction of trivalent DOP-Cl with the intermediate in the presence of a base to form 99. Afterwards, oxidation of 99 produces the desired product in high yield. PBT com- 37 posite produced via injection molding containing 20 wt.% of 100 achieves V-2 classification in a UL-94 fire test. The TGA results show that the decomposition temperature of 99 is relatively low to be suitable for engineering plastics and thus it was not further tested. 143

O O O NEt P H 3 P NH HCl P NH + N O 3 O H O O Cl Toluene O MeCN

Intermedite

O DOPO,CCl4, O P P N . CHCl H O NEt3 3 O

O Cl P NH3 O

O O O O O P tBuOOH P N-methylimidazole P N (aq) P N O O P O P DOP-Cl O CHCl3 O

99 100

Scheme 14: Synthesis of bridged DOPO-functionalized molecules 98 and 100

4.6 Polycarbonates (PCs). Polycarbonate (PC) is extensively applied in the electronics, construction and automotive industries because of its outstanding properties such as stiffness, good hardness, transparency, impact strength, thermal stability, and dimensional stability. In the UL-94 test, even if the PC displays a V-2 rating, a lower FR ratings is required for

38 electronic and electric applications. Therefore, many FR additives have been devel- oped to improve the fire performance of PC.144-146

O O O O O N P P N N P P N O O O

6 101

Figure 28: Chemical structures of Phosphonamidates 6 and 101.

The fire performance for the polycarbonate (PCs) containing compound 101 has been investigated and compared to PC containing 6. Only 3% of compound 101 in PC is required to achieve V-0 classification in the UL-94 test. In comparison, a 10% loading of 6 is required to achieve the same (V-0) classification in the UL-94 test. Re- gardless of the similarities in the structures, compound 6 is far less active .147 Even though both FRs show some FR activity in the gas phase, 101 is believed to offer excellent flame retardancy because of the catalyzed degradation of polycarbonates (PCs) through the active tertiary amine group.

O H H O O O O O O N N O HN P P NH P P O O O O

102 103 Figure 29: Chemical structures of 102 and 103. Two intumescent flame retardants (IFRS) 102 and 103 were synthesized as shown in figure 29 and incorporated in polycarbonate (PC). 148 The synthesis of IFR 102 was achieved via reaction of pentaerythritol with phosphorus oxychloride in acetonitrile followed by a second reaction with aniline. The IFR 103 was obtained via a two-step reaction involving the synthesis of 2-chloro-2-oxo-5, 5-dimethyl-1, 3, 2- dioxaphosphorinane(DOPC) via reaction of neopentyl glycol with phosphorus oxychloride in chloroform followed by DOPC with m-phenylenediamine in the presence of a base as a catalyst. PC containing 5 wt. % IFRs passed the UL-94 burning test with V-0 classification with a LOI value of approx. 35 %. TGA experiments proved

39 that both IFRS showed increased char forming capability when tested alone or together with PC.

4.7 Acrylonitrile-butadiene-styrene (ABS). Acrylonitrile-butadiene-styrene (ABS) composites are well known for their chemical resistance, excellent mechanical properties, and easy processing. ABS resins are widely used in electrical appliances, office supplies, and in the automobile industry. However, flammability of acrylonitrile-butadiene-styrene resins is a critical issue. Therefore, it is vital to improve the flammability of ABS resins, which is mostly obtained by incorporating suitable FR additives.

O O H H O P N N N P O O O N N

104 Figure 30: Chemical structures of 104. 4-(Diphenoxyphosphorylamino)-6-phenyl-[l,3,5] triazin-2-y1]-phosphoramidic acid diphenyl ester (104) was synthesized via a reaction of benzoguanamine with diphenyl phosphorochloridate in THF in the presence of a base. To study the flame retardancy of this additive various formulations of ABS resins were studied. These formulations consisted of several concertation of LDHs (Mg-Al–Co–layered double hydroxides) and additive 104. However, in the UL-94 test, only a V-2 classification could be achieved for these formulations. LOI values of ABS/ 104 and ABS/ 104/ LDHs composites was 23.9 % and 24.7 % respectively, which is higher compared to the blank control ABS (18.1%).149

4.8 Pololefins and related polymers Polypropylenes (PP), polyethylene (PE) are relatively inexpensive materials that offer many benefits such as physical flexibility, useful electrical, mechanical, and thermal properties as well as corrosion resistance, natural processability and chemical stability. These properties make them attractive candidates for application in the electronics, automotive, and electrical industries. PP, PE are highly flammable polymers be-

40 cause they contain high amount of carbon content, which burns straightforwardly with a fast fire spread. During combustion, they produce toxic gases; which restrict them to be used in high-risk applications. Therefore, it is essential to improve the flammability of PP, PE.150-154

O2N NH HN O O O HN P P NH O O O NH HN NO2

105

Figure 31: Chemical structures of spirocyclic pentaerythritol diphosphonate di- nitroguanidine 105. P(O)-N containing intumescent flame retardant (IFR) 105 was synthesized and incorporated into linear low-density polyethylene (LLDPE). At a 30 wt. % of IFR 105 in LLDPE composite the LOI values increased to 31% compared to 17% for the blank. The TGA results of the additive showed improved thermal stability and the char formation for LLDPE.155

O O HN NH Cl O O O O O P P O Cl N N P P N N 106 O O O O n

O O O O P P NH H2N N NH2 N H O O N 107 n Scheme 15: Synthesis of PPSPB 106 and PDSPB 107. Phosphorous-nitrogen containing polymeric IFR poly(piperazine spirocyclic pentaerythritol bisphosphonate) PPSPB (106) was synthesized and applied to polyethylene)/ethylene(vinyl acetate) copolymer (LDPE/EVA) mixture. Nanocomposite containing compound 106 and montmorillonite (OMMT) LDPE/EVA was also studied. The combined effects of PPSPB and montmorillonite on thermal stability and flame retardancy was investigated by TGA and cone calorimetry.156 The mixture of 106 and montmorillonite enhanced the thermal stability and significantly decreased the flammability of the composites. The SEM results showed that upon burning, a dense and compact char was obtained for the LDPE/EVA/IFR/OMMT nanocomposites. Com- pound 106 157 was also combined with a triazine-based polymer as a char forming 41 agent (CFA) and ammonium polyphosphate (APP) and melt blended in polypropylene (PP) to reduce its flammability. For formulation containing a total of 30 wt. % of FR [106:APP:CFA (3:6:2 weight %)] loading, a V-0 rating in UL 94 test along with a LOI value 39.8% was achieved. Moreover, cone calorimeter analyses showed that pHRR and the THR of the 106/PP formulations decreased significantly compared to the pure PP. P(O)-N containing oligomeric compound 107 was synthesized and grafted on multi- walled carbon nanotube (MWNT), and ethylene vinyl acetate (EVA) used as a copolymer.158 The EVA/MWNT-g-107 nanocomposites containing 0.2, 0.5, 1.0, 2.0 wt.% of MWNT–107 were prepared through melt compounding at 100 ºC. UL94 tests showed that grafted EVA/MWNT-g-107 nanocomposites failed to pass the flame retardancy tests. O HN P HN Cl

N N NH O O H N P NH Cl N Cl N N NH 2 N N 2 H HN Cl P Cl H N 2 N 2 H H HN N N P H O

NH P HN O 108 Scheme 16: Synthesis route of 108. A novel polymeric FR 108 containing phosphorus oxynitride, phenyl, and triazine groups was synthesized from phenyl phosphonic dichloride, cyanuric chloride and ethylenediamine starting materials and incorporated in polypropylene (PP) along with ammonium polyphosphate (APP).159 By using 25 wt.% of APP: PTPA (2:1) in PP a LOI value of 34.0% and V-0 rating in UL-94 was obtained. In the cone calorimetry tests, the pHRR values decreased by about half on the addition of either APP or 108 separately, compared to the pure PP. However, combining 108 with APP, the PHRR was reduced to almost 3 times (121 kW/m2) compared to the blank PP. The TGA results show that 108 undergoes two stage thermal decomposition process, the first step of the decomposition process occured at 269-359 ºC and the second step at 359- 700 ºC. Cone calorimeter test and SEM of char residue indicates that decomposed products of 108 and APP promote the formation of the firm and compact char

42 layer. In a different work, compound 108 was combined with APP and was used to prepare a FRcomposite of 10 wt.% 108/60 wt.% wood fiber /APP (20 wt.%).160 The LOI values of 31.5% and V-0 rating was achieved for these composites.

O O NH2CH2CH2NH2 Cl P Cl 2HN2HC2HC2HN P NH2CH2CH2NH2

CHCl3

O Dioxane N NHCH2CH2NH P NHCH2CH2NH n N N

NH2 Cl N Cl NH

N N Cl N Cl N NH N N N H H

Cl Dioxane 109 N H Scheme 17: Synthesis of HAPN 109 A macromolecular additive containing phosphorous-nitrogen group (109) with the capability of free-radical quenching was synthesized and used as a FR additive together with APP for polypropylene. The thermal stability and flammability of PP/APP /109 composites were studied by LOI, TGA, UL-94, and cone calorimetry experiments. For 25 wt.% content of 109/APP (mass ratio of 109 to APP was 1:1.) in PP a UL-94, a V- 0 rating along LOI of 29.5% was achieved. In cone calorimeter test, for blank PP, a pHRR of 916.5 kW/m2 was observed at 210 s. However, when 12.5 wt.% of 109 and 12.5 wt.% APP was incorporated in PP, the matrix showed a decrease(72.1%) in the pHRR.161

O O Cl O P P O Cl O O DMF O O O O + P P N B N n O O

HN B NH2 110 Scheme 18: Synthesis of poly(pentaerythritol spirocyclic phosphorusoxy spirocyclic diethanolamine borate) (PPSPSDB) 110. Polymeric FR 110 containing boron was synthesized by the reaction of spirocyclic pentaerythritol bisphosphorate disphosphoryl chloride (SPDPC) with diethanolamine borate (DEAB) in DMF solvent. Compound 110 was combined with ammonium polyphosphate (APP) and incorporated in low-density polyethylene (LLDPE). Improved

43 thermal stability was observed for the modified LLDPE. For a 30 wt.% APP/ PPSPSDB loading in LLDPE, a LOI of 29.6% and V-0 rating in UL 94 test was observed. The addition of 30 wt% polymeric FR 110 into LLDPE increased the thermal stability and char residue up to 15.06 % at 800 °C. The reason for higher flame resistance of 110 is the synergic effect of the P, N and B elements. In the condensed phase LLDPE/110/APP systems thermally decompose, the phosphorus oxide (P=O) bonds provide the acid resource, the nitrogen (N) atoms turn into incombustible gas

(N2), and ammonia (NH3) and finally, the boron (B) atoms get converted into boric anhydride (B2O3) or boric acid (H3BO3). In the condensed phase char layer consist of P=O, B-O, P(O)-N, and P-O-P, etc. substructures. Meanwhile, APP decomposes to produce phosphoric acid and promote the formation of a compact and stable char layer.162

N N N N O O N HN NH Cl P Cl HN N P N NH Cl N Cl N N N N O N N P P O N

111 Scheme 19: Synthesis of a novel hyper-branched IFR 111 A P(O)-N containing hyperbranched macromolecule (HPCFA )111 was synthesized and used with the combination with ammonium polyphosphate (APP) as fire retardant polypropylene (PP). PP composite along with IFR and APP passed UL- 94 V‐0 rating and achieved a LOI value 31% at 20 wt% IFR/APP (1/2, w/w). Besides, 111 was compared with another hyperbranched char forming agent (HCFA) .163 HPCFA (111) and HCFA have similar chemical structure, differing only in the main chain phosphorus containing unit of HPCFA. Cone calorimetery showed that the incorporation of 20 wt.% 111/APP (ratio 1:2) reduces pHRR of PP by 86.24%. Analysis of char residue showed that 111/APP system forms a dense, compact char layer compared to the HCFA/APP system.164 A series of P-N containing compounds were designed and synthesized from corresponding phosphorus chloride and nitrogen based nucleophiles in the presence of lithium source (diisopropylamide or n-butyllithium ) followed by oxidation with elemental sulfur (S8) to obtain the FRs and were used as a stabilizer with polypropylene.

The results indicate that it is possible to process polypropylene (PP) with these additives in the melt (extrusion, injection molding, etc.) without any degradation of PP.165

S S

S N S N N O P P P S O O O

112 113 114

N N S P S P O

115 116 Figure 31: Chemical structures of P-N heterocyclic derivatives 112-116.

5. Conclusion Phosphorus is a central focus for the development of new FRs due to its chemical versatility, activity in both phases (condensed and gas phase)and ability to function as a reactive component or additive. The different oxidization states of phosphorous and synergistic propensity with various elements make phosphorous an ideal candi- date for the advancement of FR materials. P(O)-N bond formation using chlorinating agents such as CCl4, t-BuOCl, Cl2 gas, CuCl2, SO2Cl2, NCS and TCCA from various phosphorus-based starting materials are the most commonly used methods in academia for the development of FRs. Some methods which involves the use of TCCA, NCS discussed in this review are commercially feasible if the FR properties outweigh its cost of production. Furthermore, the use of other synthetic strategies such as Michaelis-Arbuzov rearrangement, oxidative coupling methods using iodine or copper, the Staudinger-phosphite reaction, metal-catalyzed amidation by using phosphoryl azides, and green synthetic methodology, are seldom used by research- ers to synthesize P(O)-N containing organophosphorus FR derivatives. One can find numerous literature which focuses on the development of P(O)-N compounds such as phosphinamide, phosphonamide, phosphoramide, phosphoramidates, phosphorodiamidate, phosphonamidate, pentaerythritol bisphosphonate and P(S)-N containing phosphorus compounds such as phosphoramidathioates and thiophosphina- 45 mide as FR additives on various polymers, however, their commercialization is still limited. They have been shown to be very effective in flame retarding diverse class of polymers such as on polyurethane, epoxy resins, polyamides, polyolefins, cellulose, polylactic acid, polybutylene terephthalate, polycarbonates, and acrylonitrile- butadiene-styrene. With increased demands towards recyclablility, non-toxicity, and sustainable materials, phosphorus-containing compounds have the potential to fulfill all the requirements of future FR additives. Future FR research will focus on sustainable synthetic methodologies, development of polymer based FR additives and the use of bio-based resources. Furthermore, investigations into synergism of phosphorus (P) with various moieties (P-Si, P-B, P-P, etc.) and multicomponent arrangements will keep evolving with the central goal to decrease the FR loading and increase FR performance.

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