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A Brief Review onOrganophosphorus Flame RetardantsDr. G. Chandra Sekhar Reddy

The range of phosphorus-based flame retardant products is extremely wide, including phosphates, phosphonates, phosphinates, phosphine oxides, phosphates and red phosphorus. These phosphorated flame retardant agents can be used as additives or incorporated into the polymer chain during its synthesis, and are known to be active in the condensed and/or vapor phase.In the condensed phase, the phosphorus-based flame retardants are particularly effective with polymers containing oxygen (polyesters, polyamides, cellulose, etc.). With most of them, thermal decomposition leads to the production of phosphoric acid, which condenses readily to produce pyrophosphate structures and liberate water.At high temperature, ortho- and pyrophosphoric acids are turned into metaphosphoric acid (O)P(O)(OH) and their corresponding polymers (PO3H)n. The phosphate anions (pyro- and polyphosphates) then take part, with the carbonized residues, in char formation. This carbonized layer (char) isolates and protects the polymer from the flames and:- limits the volatilization of fuel and prevents the formation of new free-radicals;- limits oxygen diffusion, which reduces combustion;- insulates the polymer underneath from the heat.

Since the implementation of environmental directives banning or limiting the use of halogenated compounds, the development of flame retardants has focused on more environmentally friendly alternatives. Phosphorus compounds have received a growing attention from both industry and academic community.

Phosphorus flame retardants (PFRs)

The polymers prepared showed initial decomposition temperatures above 340 C, excellent thermal stability, high char yields at 850 C and very high limited oxygen index (LOI) values of 5659. The polymers char yields and their (P+Si) contents showed linear relationships.

Scheme 1: Preparation of polyaryloxydiphenylsilanes Polymers were prepared from reacting various diols with DPDCS, as shown in below scheme, with stoichiometric feeding ratios to yield PES-I or with imbalanced stoichiometric feeding ratios, i.e., with a 15% excess of aromatic diols to yield OES-I, OES-II, and OES-III as detailed in Table 1.

Table 1: Preparation and characterization of polyaryloxydiphenylsilane polymers

Figure 1: FTIR spectra of PES-I and OES-II.

Table 2: Thermal properties of polyaryloxydiphenylsilane polymersThe low melting points of the OES polymers implied that they are easily processed with molten processes, especially while the OES polymers are used as flame retardants for thermoplastic materials.IDT = initial decomposed temperature, IPDT = integral procedural decomposition temperature

Figure 2: The plot of polymers char yields (700 C in air) versus polymers (P+Si) contents.Figure 3: TGA thermograms of the polyaryloxydiphenylsilanesThe thermal stability of the synthesized polymers was investigated with TGA. Reliable degradation temperatures and kinetic parameters, such as the initial decomposed temperature (IDT), the temperature of maximum rate of weight loss (Tmax), the integral procedural decomposition temperature (IPDT) and the activation energy for thermal decomposition (Ea), can be calculated from the TGA analysis data and used to assess the polymers thermal stability.

Polymers containing phosphate or phosphonate structures have been widely observed to show relatively poor thermal stability, owing to the low bond energy of the POC structure. However, the thermal stability of the phosphorus-containing polyaryoxydiphenylsilanes (PES-I, OES-I and OES-II) was good enough for applying these polymers as flame retardants to most thermoplastics. The high char yields of the polymers at high temperature region were noteworthy. As the literature has reported, the high efficiency in char formation results from the synergistic function of phosphorus char enrichment and silicon char reservation (reducing weight loss of char from oxidation). The relatively low char yield from heating in air phosphorus-free polymer OES-III also demonstrated that silicon could only serve as a char protector, but not as a promoter for the char formation. 6

New star-shaped phosphorus-containing flame retardants were synthesized by phospha-Michael-addition of diphenylphosphine oxide (DPhPO) and dimethyl phosphite (DMP) to oligofunctional acrylates. The products were examined via UL 94 V-rating and thermogravimetric analysis in three different epoxy resin systems: (i) RTM6, (ii) the two-component RTM system bisphenol A diglycidyl ether (DGEBA) and (iii) an epoxy novolak resin (Dow DEN438) cured with dicyandiamide (DICY) as hardening agent and 1,1-dimethyl-3-phenylurea (fenuron) as accelerator, a three-component system predominantly used for prepreg applications, i.e. for printed wiring boards.The influence of a systematically altered chemical environment of the phosphorus atom on the flame retardancy and the char yield was investigated. Furthermore, the glass transition temperatures of the modified resins were studied via differential scanning calorimetry, including the deviation after an additional treatment at elevated temperature. (i) RTM6, a commercial premixed three-component system widely used for resin transfer molding (RTM) applications predominantly in aviation; (ii) the two-component RTM system bisphenol A diglycidyl ether (DGEBA) cured with 4,4-methylenebis(2-methylcyclohexylamine) (DMDC) predominantly used for vehicle manufacturing; and (iii) an epoxy novolak resin (Dow DEN438) cured with dicyandiamide (DICY) as hardening agent and 1,1-dimethyl-3-phenylurea (fenuron) as accelerator, a three-component system predominantly used for prepreg applications, i.e. for printed wiring boards.

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Scheme 2: Synthesis of DPhPP (1), DPhPI (2), DOPP (3), DOPI (4), DMPP (5), and DMPI (6).(A)(B)8

Table 3: Molecular weight, phosphorus content, and chemical environment of the phosphorus atom for DPhPP (1), DPhPI (2), DOPP (3), DOPI (4), DMPP (5), and DMPI (6)The compounds 16 were blended into the epoxy resins with phosphorus contents between 1.0 and 3.0% depending on the system and processed as described above.

Table 5: UL 94-V screening and glass transition temperatures for DGEBA cured with DMDC, blended with 16. Neat resin: n.c.; Tg: 172 CTable 4: UL 94-V screening and glass transition temperatures for RTM6 blended with 16. Neat resin: n.c.; Tg: 229 CThe UL 94-V tests in RTM6 for 1.0, 1.5, and 2.0% phosphorus. The four compounds based on DPhPO and DOPO (14) all exhibited a good flame retardancy, reaching V0 for 2.0% P in all cases and V1 for 1.5 and 1.0% P in most cases, without any evident difference in efficiency between the PETA- and the TAEI-based additives. The derivatives of DMP (56), however, turned out to be even more effective, reaching V0 in case of DMPP (5) and V1 in case of DMPI (6) for blends with 1.0% P, as well as V0 in the cases of all higher loadings. For DGEBA cured with DMDC, the screening was shifted to 2.0, 2.5, and 3.0% phosphorus content (Table 3) due to its inherent higher flammability.

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Table 6: a. UL 94-V screening and glass transition temperatures for epoxy novolak (DEN438) cured with DICY and fenuron, blended with 14. Neat resin: n.c.; Tg: 182 Cb. UL 94-V screening and glass transition temperatures for epoxy novolak (DEN438) cured with DICY and fenuron, blended with 5 and 6. Neat resin: n.c.; Tg: 182 C

Phosphorus-containing Polysulfones A Comparative StudyThorsten Hoffmann, Doris Pospiech, Liane Huler, Karin Sahre, hartmut Komber, Christina Harnisch, Maria Auf Der LandwehrLeibniz-Institut fr Polymerforschung Dresden e.V., Hohe Str. 6, 01069, Dresden, Germany

Alexander Schfer, Manfred DringResearch Center Karlsruhe GmbH, Institute of Technical Chemistry, 76021, Karlsruhe, GermanyHigh Performance Polymers, 00: 000000, 2010 DOI:10.1177/0954008309353951Aromatic phosphorus-containing polysulfones (P-PSU) are reported.The properties of these polymers are compared to bisphenol A-based polysulfones.Both diols have the phosphorus unit as substituent and differ in the number of oxygen atoms within the environment of the phosphorus changing from DOPO-HQ (phosphinate structure to DPPO-HQ (phosphine oxide structure). The use of DPPO-HQ decreased the number of side reactions and enhanced slightly the molecular weight. The thermal stability of DPPO-HQ-based PSU is slightly better than DOPO-HQ-based PSU.

Scheme 3: Synthesis route for the preparation of the phosphorus-containing P-PSU (P-PSU_A, P-PSU_B)Polysulfones (PSU), Phosphorus-Polysulfones (P-PSU)

Figure 4: Plots of ATR-FTIR spectra in the spectral region of 1050 to 1350 cm-1 (a) and in the spectral region of 900 to 700 cm-1 (b) taken during the P-PSU_A polycondensation in NMP.ATRFTIR in-line monitoring techniqueDuring the synthesis of P-PSU_A, the growth of the polymer chains is indicated by the appearance of the aromatic COC ether band at 1243 cm-1 (Figure 3a).

Otherwise, the disappearance of characteristic monomer bands of DFDPS at 713 cm-1 and DOPOHQ at 781, 755 and 713 cm-1 is clearly observed in the spectral region below 900 cm-1.

However, the CF bands (DFDPS) at 835 and 818 cm-1 are strongly overlapped by bands of solvent (NMP) and newly formed polymer (Figure 3b).

The SO2 bands of DFDPS did not significantly change during the polymerization reaction.It was clearly shown that the formation of P-PSU_A already started in the first reaction step at 135 C during the azeotropic distillation of toluene/water and finished at 165 C. Moreover, a side reaction, the ring opening of the phenantrene-like phosphorus-ring system (DOPO) to form an OH-group could be proven by the formation of the respective FTIR bands at 1192 cm-1 ([O-P=O]-) and at 1131 cm-1 (P-phenyl). The intensity of these bands was only weak, but nevertheless observable. This ring-opening reaction of DOPO-HQ in K2CO3-containing NMP/toluene mixture was also detected by 1H-NMR spectroscopy. All P-PSU_A syntheses carried out under the reaction conditions given above gave reproducible results. The structure of the phosphorus-containing polysulfones could be proven by means of 1H, 13C, and 31P-NMR spectroscopy. The phosphorus signals found by 31P-NMR were significantly shifted compared to the monomers (P-PSU_A: 26.20 ppm vs. 23.42 ppm in the monomer P-PSU_B: 7.77 ppm vs. 1.71 ppm solvent: DMSOd6). The complex polymer structure of PSU_A does not allow a complete assignment of all signals observed in the 1H- and 13C-NMR spectra of the polymer.14

Table 7: Chemical and thermal characterization of phosphorus-containing P-PSUs incomparison with PSU.

The phosphorus-containing PSUs show higher glass transition temperatures than BPA-PSU. This may be caused by an enhanced chain stiffness of the polymer due to the bulky substituents. However, differences between the phosphorus polyethers exist in the increase of Tg (Tg, P-PSU_A Tg, PPSU_B). This result could be attributed to the more bulky side group of DPPO-HQ (anthracene-like), which inhibits the free rotation more than the phenantrene-like DOPO-HQ15

Figure 5: TGA of P-PSU_A (dotted line) and of P-PSU_B (dashed line) in comparison with the neat PSU (solid line) in nitrogen atmosphere.Thermogravimetric analysis (TGA) indicated a two-step decomposition mechanism of the phosphorus-containing polysulfones compared to PSU with a single decomposition process (Figure 5). Moreover, a higher amount of char at 750 C for both PPSUs (P-PSU_A = 32 wt.%, P-PSU_B = 57 wt.%) in comparison with PSU (29 wt.%) was detected.The decomposition curves of P-PSU_A and PPSU_B differ significantly from each other mainly with respect to the weight loss in the two decomposition steps (see Table 7). The first decomposition step, below the PSU composition, is caused by scission of the polyether main chain and the release of SO2 and phenol. The second weight loss is caused by the decomposition of the substituents and takes place above the PSU decomposition.P-PSU_B yields a higher char amount at 750 C at P-PSU_A which is discussed in terms of a higher thermal stability of P-C bonds in P-PSU_B than of POC bonds in P-PSU_A.16

Figure 6: Decomposition of PSU (solid line), P-PSU_A (dotted line), and P-PSU_B (dashed line): heat release rate as function of temperature obtained by pyrolysis combustion flow calorimeter.The pyrolysis combustion flow calorimeter (PCFC) characterizes the thermal combustion of a sample in a non-flaming test by rapid, controlled pyrolysis in an inert gas stream (N2) followed by complete combustion of the pyrolysis gases in an excess of oxygen at 900 C for 10 s. PCFC determines the heat release rate (W g-1) of the sample, which is calculated from the flow rate of the gas stream and the oxygen consumed by combustion of the pyrolysis gases.Both phosphorus-containing PSUs possess a two-step thermal combustion behavior that is comparable with the DTG mass loss curve in Figure 5b. The maximum of heat release rate for P-PSU_A was achieved at 452 C (only visibly as shoulder) and at 484 C for P-PSU_B and occurred in the temperature region of PSU. In contrast, PSU has only a single peak of heat release rate at 557 C. The second maximum of the heat release rate is detected at 563 C (P-PSU_A) and 589 C (P-PSU_B), respectively. The combustion behavior of both P-PSUs are comparably and was characterized by a clearly decrease of the heat release rate compared to PSU. The non-combustible residues after the PCFC measurements were higher than those obtained after TGA. 17

The pyrolysis and fire behavior of halogen-free flame-retarded DGEBA/DMC, RTM6 and their corresponding 60 vol.-% carbon fibers (CF) composites were investigated. A novel phosphorous compound (DOPI) was used. Its action is dependent on the epoxy matrix. Only flame inhibition occurs in the gas phase. RTM6 shows flame inhibition and a condensed phase interaction increasing charring. Both mechanisms decrease with increasing irradiance, whereas in RTM6-CF charring is suppressed at low ones. RTM6+DOPI shows a higher LOI (34.2%) than DGEBA/ DMC+DOPI and a V-0 classification in UL 94. Adding CF only enhances the LOI, DOPI+CF leads to a superposition in LOI for DGEBA/DMC-CF+DOPI (31.8%, V-0) and a synergism for RTM6-CF+DOPI (47.7%, V-0).

Scheme 4: a) Synthesis of DOPI. b) Components of DGEBA/DMC. c) Main components of RTM6.Dopac-3-isocyanurate [DOPI]

Table 8: Investigated materials. The amount of CF corresponds to 60 vol.-%. And glass transition temperatures, Tg (3 Hz), of the investigated materials with an error of 2K.

All thermosets containing 2 wt.-% of phosphorus in the final product were preparedThe decrease in Tg is attributed to a reduction in the effective number of crosslinking points per volume, mainly due to the replacement of epoxy resin by the flame retardant as modification

Figure 7: Mass and mass loss rate of (a) DGEBA/DMC (filled squares) and DGEBA/DMC+DOPI (open squares) and (b) RTM6 (filled circles) and RTM6+DOPI (open circles) under nitrogen at a heating rate of 10 K min-1.Table 9: Thermal analysis of DGEBA/DMC, DGEBA/DMC+DOPI, RTM6 and RTM6+DOPI (ML= mass loss).

The reduction in thermal decomposition temperature is attributed to the lower crosslinking density of the systems or the ester group within the flame retardant, respectively.The reduction in thermal decomposition temperature is attributed to the lower crosslinking density of the systems or the ester group within the flame retardant, respectively.

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Table 10: Time to ignition (tig) of the investigated materials, LOI results and UL 94 results for a sample thickness of 3.0mm.In agreement with results received in pyrolysis it is concluded that both flammability tests are influenced by gas phase and condensed phase mechanisms.

While a gas phase mechanism betters the LOI in both materials, the additional action of DOPI in the condensed phase of RTM6+DOPI changed melt flow properties in the burning zone and therefore flaming dripping in the UL 94 was suppressed.

Further, an additional char-protecting layer was formed, which protects the material underneath from heat and hinders pyrolysis gas transport.

Figure 8: (a) HRR and (b) THR of DGEBA/DMC (filled squares), DGEBA/DMC+DOPI (open squares), RTM6 (filled circles) and RTM6+DOPI (open circles) at an external heat flux of 50 kW.m-2.

Fire Behavior

Figure 9: (a) PHRR, (b) THE, (c) THE/ML and (d) residue of DGEBA/DMC (filled squares), DGEBA/DMC+DOPI (open squares), RTM6 (filled circles) and RTM6+DOPI (open circles) at different external heat fluxes.Cone calorimeter measurements at different external heat fluxesThe HRR curves for DGEBA/DMC, DGEBA/DMC+DOPI and RTM6 were typical for non charring materials with an increase in HRR up to a small shoulder and a subsequent intense peak at the end of burning. This is in good agreement with thermal analysis results, where all of these materials left only a small amount of residue.

The shape of the curve of RTM6+DOPI shows a more distinctive shoulder, which almost exhibits a low plateau followed by a strong peak heat release rate (PHRR) and a final decrease in HRR at the end of the burning. The more distinctive shoulder is caused by the formation of a char layer at the surface of the burning sample. This corresponds to the enhanced amount of residue found in thermogravimetry, but the strong PHRR at the end of burning also reveals that the potentially formed char layer is not a very efficient protection layer. The PHRR and THR for neat DGEBA/DMC were higher than for neat RTM6. DOPI strongly decreased the PHRR and the THR in both epoxy resins. This is in good agreement with both the difference in flammability (reaction to small flame, LOI and UL 94) between DGEBA/DMC and RTM6 and the amelioration of flammability by adding DOPI.23

Figure 10: (a) HRR and (b) THR of DGEBA/DMC-CF (filled triangles) and DGEBA/DMC-CF+DOPI (open triangles) and RTM6-CF (filled inverse triangles) and RTM6-CF+DOPI (open inverse triangles) at an external heat flux of 50 kW.m-2.Fire Behavior

Figure 11: (a) PHRR, (b) THE, (c) THE/ML and (d) residue of DGEBA/DMC-CF (filled triangles), DGEBA/DMC CF+DOPI (open triangles), RTM6-CF(filled inverse triangles) and RTM6-CF+DOPI (open inverse triangles) at different external heat fluxes.Cone calorimeter measurements at different external heat fluxes24

Table 11: Results of cone calorimeter measurements at different external heat fluxes for all investigated materials

(PHRR = peak of heat release rate, THE = total heat evolved, THE/ML = effective heat of combustion of the volatiles multiplied by the combustion efficiency, tig = time to ignition).

The values are averaged from two measurements.

With the introduction of phosphorus containing segments, polyester onset weight-loss temperatures in both nitrogen and air are enhanced. Thermal degradation kinetics were analyzed and which showed a decreased activation energy for the co-polyester compared with that for neat Poly(trimethylene terephthalate) (PTT), strongly suggesting that the increased onset weight-loss temperature in nitrogen is a result of enhanced carbonization. Improved flame-retardant properties of co-polyester were observed with incorporated phosphorus components.

Scheme 5: Synthesis of PTTEP via solid-state polymerization.

Table 12: PTT and PTTEPs polymers.The actual phosphoruscontent of the purified co-polyester was determined using oxygen flask combustion-inductively coupled plasma atomic emission spectroscopy (ICP-AES) method.27

Figure 12: TG curves of PTT and PTTEPs at the heating rate of 10 C/min in N2 (a) and air atmosphere (b)

Table 13: TG and DTG results of PTT and PTTEPs.Under a nitrogen atmosphere, neat PTT and PTTEP show single decomposition processes. It has been reported in similar systems that the introduction of phosphorus-containing compounds produces a decreased onset decomposition temperature (T5%), reflecting the weaker P-C bond in the polymer backbone. The onset weight-loss temperature of PTTEPs in the present study increased with increasing phosphorus incorporation, notably different from the prior literature. To explain this phenomenon, wenote that the char residue increases from 6.7% to 16.0% with the introduction of DOPO-HQ-HE. The incorporation of DOPO-HQ-HE obviously increases the carbonization. Such a significant quantity of char might well block the volatilization of decomposition products, thus giving the illusion of enhanced thermal stability with introduction of phosphorus. The copolyesters, thus, would begin to degrade but without losing weight.

Under an air atmosphere, both neat PTT and PTTEPs show twostep decomposition processes, including major (with the peak temperature in the range of 399.6403.2 C) and minor (with peak temperature in the range of 488.2587.0 C) weight loss stages. The onset weight-loss temperature shows the same trend as was observed under nitrogen testing.28

Figure 14: PHRR and THR value of PTT and PTTEPs at 1 C/s heating rate in N2.Figure 13: HRR curves of PTT and PTTEPs at 1 C/s heating rate in N2.Microscale combustion calorimetry was carried out to test the flammability behavior of the co-polyester.Thermal combustion properties of PTT and PTTEPs were characterized by microscale combustion calorimetry (MCC). Thermal combustion properties of PTT and PTTEPs were characterized by microscale combustion calorimetry (MCC). During MCC tests, volatile products of the samples are burned and corresponding heat release data recorded (which should correspond to the flammability properties of the decomposition products generated during TGA testing). The heat release rate (HRR) curves, peak of heat release (PHRR) and total heat release (THR) from MCC testing are shown in Figures 13 and 14, respectively. A reduction of both HRR and THR is observed for phosphorus-containing PTT. 29

Figure 16: Ozawa plots of PTT and PTTEP30 at following fractional weight losses (): 0.05, 0.10, 0.15, 0.20 and 0.35.

Table 14: Activation energy of PTT and PTTEPs using the FlynnWallOzawa method.Thermal degradation kinetics were analyzed using the FlynnWallOzawa method.

Figure 15: TG curves of PTT and PTTEP30 at different heating rates in N2.Showing correlation coefficient above 0.99; thus indicating the applicability of this method to the present system. 30

Figure 17: GC traces for the evolved products of PTT and PTTEP30 during pyrolysisPyrolysis GCMSDOPOHQ-HE does not significantly change the pyrolysis products of the samples, with benzoic acid, methyl vinyl terephthalate and diallylterephthalate accounting for 65% of all the pyrolysis products.

PTTEP30, however, exhibited a greater number of chromatographic peaks than was observed with the degradation of neat PTT. This difference can be attributed to the introduction of phosphorus containing flame retardant. DOPO-HQ-HE has a different chemical structure than PTT, thus can yield different compounds.

ConclusionsPhosphorus flame retardants generally act in both gas and condensed phase. They can also be chemically tailored to favor one mechanism or the other.

Unlike other halogen-free alternatives, phosphorus compounds have demonstrated flame retardancy with relatively low loadings, regardless of their mode of addition.

The altered bond nature between phosphorus and adjacent atoms (oxygen and carbon) provoked different thermal stability and consequently changed the decomposition mechanism.

High phosphorus contents led to polymers exhibiting high activation energy of decomposition and leading as best flame retardants in literature.

DOPO derivatives interest because they lead to excellent flame resistance which is caused by the rapid decomposition of the DOPO unit, accompanied by the release of dibenzofuran and a PO radical at increased temperatures in the case of fire.

Hence, new multifunctional reactive DOPO derivatives (like, DOPO-HQ, DPPO-HQ, DOPO-HQ-HE, etc.), phosphorus containing oligomers and phosphorus-based curing agents that would not have a detrimental effect on the Tg of the cured resin are subject to extensive research.

ConclusionsIn the DOPO-HQ as well as DPPO-HQ the phosphorus is located in the side chain and has differed only in the number of oxygen atoms within the environment of the phosphorus. While the oxygen O1 bound to the P atom in the DOPO unit can interact with the OH-group (O4) to form an intramolecular hydrogen bond able to disturb the poly-condensation, the phosphine oxide structure in DPPO-HQ seems more suitable for poly-condensation.

Such reactive DOPO derivatives or phosphorus containing oligomers will also answer the problem of leaching that occurs with the use of additive flame retardant.

In addition, efforts are currently invested towards developing new halogen-free flame retardant systems investigating both new phosphorus containing structures as well as synergism with other existing inexpensive inorganic additives.