Two-Dimensional Metal Phenylphosphonates as Novel Flame

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Two-dimensional metal phenylphosphonates as novel flame retardants for polystyrene Junling Wang, Bihe Yuan, Xiaowei Mu, Xiaming Feng, Qilong Tai, and Yuan Hu Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 31 May 2017 Downloaded from http://pubs.acs.org on June 1, 2017

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Industrial & Engineering Chemistry Research

Two-dimensional metal phenylphosphonates as novel flame retardants for polystyrene Junling Wang,a Bihe Yuan,b Xiaowei Mu,a Xiaming Feng,a Qilong Tai,a* Yuan Hua* a

State Key Laboratory of Fire Science, University of Science and Technology of

China, Hefei 230026, China b

School of Resources and Environmental Engineering, Wuhan University of

Technology, Wuhan 430070, China

*

Corresponding author.

Tel/fax: +86-551-63601664. E-mail: [email protected] (Qilong Tai); [email protected] (Yuan Hu).

1

ACS Paragon Plus Environment


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Abstract In this work, layered metal phenylphosphonates are synthesized via hydrothermal method and their morphology is observed by scanning electron microscopy and transmission electron microscopy. Then, their polystyrene (PS) composites with 2 wt% phenylphosphonates are fabricated and thermal stability as well as combustion behavior are investigated. Excellent dispersion state of fillers and strong interfacial interaction between PS and phenylphosphonates are achieved. Char residue yield of the PS composites is enhanced in TGA test and the values of peak heat release rate are decreased appreciably, suggesting the improvements in thermal stability and flame retardancy. Moreover, the amount of aromatic compounds released from PS is decreased remarkably after the incorporation of phenylphosphonates. Keywords: Layered compounds; Polymer-matrix composites; Flame retardancy.

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1. Introduction It is well known that polystyrene (PS) is widely used in various fields because of its superb properties, such as high stiffness, excellent transparency and electrical conductivity, with respect to other polymers. However, PS is easily ignited with high heat release rate and lots of smoke release during the combustion, which limits its application. Therefore, it is of great importance to reduce the fire hazard of PS. Phosphorus-based compound is an effective and widely used flame retardant because of its anti-flaming effect both on gaseous and condensed phases.1-3 Many efforts have been made to develop novel and efficient phosphorus-based flame retardant containing organophosphorus and inorganic groups.4-7 The hybridization technique has been noticed by researchers and it has brought a new perspective to the development of organic-inorganic materials including phosphorus-based flame retardant. For example, the flame retardant hybrid containing organophosphorus and graphene oxide (GO) has been prepared to reduce the heat release rate of polymers during the combustion. Actually, it is feasible that introducing metallic element into flame retardant as the inorganic components can achieve the synergistic effect, which have been investigated in the literature.8-10 Metal phosphonate containing quite stable chemical linkages of P-C and P-O-metal, a kind of synthetic organic–inorganic materials, has a wide range of applications, such as ion exchange,11 proton conductors,12 catalysts,13 sensors14 and flame retardants.15 Metal phosphonate is different from metal phosphate in chemical structure. Metal 3



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phosphonate has organic groups, such as alkyl and phenyl, which are beneficial to form regular morphology of particles and excellent dispersion state in polymer matrix. Up to now, several attempts have been made to fabricate polymer composites with metal phosphate, which showed higher thermal stability and flame retardancy than neat polymers.16-18 However, the dispersion of particles is poor, which is detrimental to the further improvements in these properties of polymer. Therefore, it is necessary to synthesize and investigate the flame retardant performance of metal phosphonate. Recently, metal phosphonates have been applied as flame retardant materials for polymers. Liu et al. prepared a cerium N-morpholinomethylphosphonic acid compound (MMPA-Ce) possessing rod-like morphology and investigated flame retardant properties of ethylene-vinyl acetate copolymer/MMPA-Ce composites.19 Cao et al. synthesized a zinc N,N’-piperazinebis(methylenephosphonic acid) compound (PPMPA-Zn), which was blended with high density polyethylene-g-maleic anhydride (HDPE-MA). Flame retardancy and mechanical properties of HDPE-MA were enhanced significantly by PPMPA-Zn.20 Cai et al. reported the synergistic effect between

layered

lanthanum

phenylphosphonate

and

microencapsulated

red

phosphorus and the flame retardancy of polymer was enhanced markedly with regard to pure polymer.21 Ran et al. prepared the multiwalled carbon nanotube bridged cerium

phenylphosphonate

hybrids

(Ce-MWNTs)

and

found

the

obvious

improvements in thermal stability and flame retardancy of conventional flame retardant HDPE after the incorporation of Ce-MWNTs.22 In our work, four transition 4


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metal elements (Co, Ni, Cu, Fe) were selected as the inorganic part of flame retardant because of their possible catalyzing carbonization and smoke suppression effect on PS. Gong et al. have prepared carbon nanospheres by catalytic carbonization of PS, under the existence of organic montmorillonite (OMMT) and cobalt catalyst. They have found that the released light hydrocarbons and aromatics were dehydrogenated and aromatized by cobalt catalysts.23 Yu et al. have investigated effects of multiwalled carbon nanotubes and Ni2O3 on the flame retardancy of linear low density polyethylene. The enhancement in flame retardancy was partially due to the catalyzing carbonization effect of nickel catalyst on degraded products.24 Gong et al. have synthesized carbon-based materials using chlorinated polyvinyl chloride (CPVC) as carbon source. The carbonization of the surface of CPVC microspheres was promoted after the addition of Fe2O3.25 Chen et al. have studied influence of cuprous oxide on enhancing the flame retardancy of epoxy resins. More compact char layer was formed and the catalyzing oxidation of CO to CO2 was observed, suggesting the catalyzing carbonization and smoke suppression function of Cu2O.8 Actually, the formed char has protective effect on polymer matrix and is regarded as an important factor of fire resistance of polymer. However, this work is designed to develop low-cost and effective flame retardant with layered structure. The synthesis of phenylphosphonates containing more oxophilic metal (such as Ti, V) is seemed to more high-cost and complicated according to literature26, 27. Then, the performance of those elements is not investigated in our work. 5



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Herein, several layered metal phenylphosphonates with transition metal elements (CoPP, NiPP, CuPP and FePP) were synthesized via hydrothermal method and crystal structures were investigated by X-ray diffraction patterns. According to the literature,28-32 the metal atoms are dodecahedrally coordinated by eight oxygen atoms of phosphonate groups. Both phosphonate groups chelate the metal atoms while the third oxygen of each group bridges to an adjacent metal atoms. Phenyl groups are bonded to the phosphorus atoms dispersed above and below the metal atom plane alternatively. Therefore, intense conjugate effect between PS and phenylphosphonates was formed, which is beneficial to the dispersion of particles. Apart from the barrier effect of layered structure, metal phenylphosphonates may possess catalyzing carbonization function due to the presence of transition metal elements, which has been illustrated above. Thermal stability and dispersion of the particles were studied by thermogravimetric analysis and scanning electron microscopy, respectively. Moreover, the influences of these metal phenylphosphonates on combustion behavior and thermal stability of PS were explored completely.

2. Experimental 2.1 Materials Nickel

nitrate

hexahydrate

(Ni(NO3)2·6H2O),

cobalt

nitrate

hexahydrate

(Co(NO3)2·6H2O), copper nitrate trihydrate (Cu(NO3)2·3H2O), ferric chloride (FeCl3), tetrahydrofuran (THF) and ethanol were purchased from Sinopharm Chemical 6


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Reagent Co., Ltd. (China). Sodium hydroxide (NaOH) was obtained from Jiangsu Qiang Sheng Chemical Reagent Co., Ltd. (China). Phenylphosphonic acid (PPA) was provided by Wuhan Yuan Cheng Gong Chuang Technology Co., Ltd. (China). PS was obtained from BASF-YPC Co., Ltd. (Germany).

2.2 Synthesis of layered metal phenylphosphonate 3.66 g of PPA and 6.74 g of Co(NO3)2·6H2O were dissolved into 60 mL of deionized water with stirring and ultrasonication for 30 min to obtain homogeneous solution. Aqueous NaOH (1M) solution was added dropwise to the above solution until its pH value reached 5-6. Then the mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave, which was sealed and heated at 150 ◦C for 24 h. The precipitate obtained was separated by filtration and washed with water several times until the pH of the filtrate reached neutral. The product was dried at 80 ◦C for 12 h and annealed at 300 ◦C for 2 h in air, to eliminate some impurities completely. The other kinds of layered phenylphosphonates were prepared by using the same way described above. The layered phenylphosphonates containing Co, Ni, Cu and Fe element were denoted as CoPP, NiPP, CuPP and FePP, respectively.

2.3 Fabrication of PS/layered phenylphosphonate composites PS/layered phenylphosphonate composites were prepared by a master batch-based melt mixing method. PS master batch was fabricated by solution blending approach. 4 g of phenylphosphonate was dispersed in 200 mL of THF with ultrasonication and stirring for 2 h. Then, the dispersion was heated to 35 ◦C, and 6 g of PS resin was 7



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dissolved in the dispersion, which was maintained for stirring for 4 h. Then, the mixture was slowly poured into superabundant low temperature ethanol to obtain the master batch, which was dried at 80 ◦C for 24 h. PS composites with 2 wt% layered phenylphosphonate were prepared by melt-mixing of PS and the master batch in a twin roller mill for 10 min. The temperature and roller speed were set at 180 ◦C and 80 rpm, respectively. The PS composites with CoPP, NiPP, CuPP and FePP were denoted as PS/CoPP, PS/NiPP, PS/CuPP and PS/FePP, respectively. The samples were hot-pressed at 185 ◦C and 10 MPa to obtain sheets with suitable sizes.

2.4 Characterization Fourier transform infrared (FTIR) spectra were recorded on a Nicolet 6700 FTIR spectrophotometer (Nicolet Instrument Co., U.S.). X-ray diffraction (XRD) patterns were collected with a Rigaku D = Max-Ra rotating anode X-ray diffractometer (Rigaku Co., Japan), using Cu Kα radiation (λ = 0.1542 nm) as the X-ray source. X-ray photoelectron spectroscopy (XPS) was employed to investigate chemical composition of sample using a Thermo ESCALAB 250 electron spectrometer (Thermo VG Scientific Ltd., UK). Scanning electron microscopy (SEM) images of particles and the section of PS composites were obtained on FEI Sirion 200 scanning electron microscope with an acceleration voltage of 5 kV. Transmission electron microscopy (TEM) was performed on a JEOL JEM-2100F microscope (JEOL Co., Ltd., Japan) to investigate the morphology and dispersion state of particles in polymer matrix. Thermogravimetric analysis (TGA) was recorded on a TA Q5000IR 8


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thermo-analyzer (TA Instruments Inc., U.S.) with a heating rate of 20 ◦C/min under air and N2 atmosphere. Raman spectra of char residues were performed on a LABRAM-HR laser confocal Raman spectrometer (Jobin Yvon Co., Ltd., France) with a 514.5 nm argon laser line. Thermogravimetric analysis/infrared spectrometry (TG-IR) was recorded on a TA Q5000IR thermo-analyzer combined with a Nicolet 6700 FTIR spectrophotometer with a heating rate of 20 ◦C/min. About 5.0 mg of the sample was placed in an alumina crucible and heated from 30 to 800 ◦C under N2 atmosphere. Combustion test was performed on a cone calorimeter (FTT, Co., Ltd., UK). The specimen with the sizes of 100 × 100 × 3 mm3 wrapped in an aluminum foil was exposed horizontally to a heat flux of 35 kW/m2.

3. Results and discussion FTIR spectra of metal phenylphosphonates are exhibited in Fig. 1 and it is clear that these phenylphosphonates have similar spectra. The bands at around 3400 cm-1 is attributed to the O-H stretching. The bands at around 3050 and 1435 cm-1 are assigned to the C-H and C-C stretching vibrations, respectively, of the phenyl ring. The bands at around 1610 and 740 as well as 690 cm-1 can be ascribed to the adsorption of C-P bonds.30, 33 In addition, PO3 vibrations are observed in the range of 1163-1039 cm-1.34 XRD patterns of phenylphosphonates are shown in Fig. 2. Obviously, the results are similar to the patterns of the reported products in the literature.30, 31, 35, 36 There are four diffraction peaks corresponding to (010), (020), (110) and (030) planes of CoPP 9



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(JCPDS-51-2096) in Fig. 2a and three peaks assigned to (010), (020) and (110) planes of NiPP (JCPDS-49-2391) are observed in Fig. 2b. Three peaks attributed to (002), (004) and (002) planes of CuPP are observed in Fig. 2c. There are two peaks (at 6.0° and 6.2°) observed in pattern of FePP. Compared with the standard pattern of FePP·H2O (JCPDS-46-1890), the additional diffraction peak at 6.2° of FePP may be caused by the loss of interlayer water during the calcination. The loss of interlayer water may result in the stack of layer and the decrease in interlayer space. Then, the position of diffraction peak may be backward. In order to realize the element composition of these phenylphosphonates, XPS test was conducted and the corresponding spectra are shown in Fig. 3. The presence of transition metallic, phosphorous, oxygen and carbon elements is confirmed, according to the full-scan survey spectra of these phenylphosphonates. The main Co 2p3/2 and Co 2p1/2 peaks are located at the binding energy of 782.1 and 797.7 eV, respectively, with the corresponding satellite peaks at 786.3 and 802.6 eV.37 The presence of these satellite peaks suggest that the cobalt ions is in +2 valence state.38 The peaks at 856.6 and 858.3 eV assigned to Ni 2p3/2 and another two peaks at 874.3 and 876.1 eV attributed to Ni 2p1/2 can be found in Ni 2p spectrum, suggesting the existence of Ni2+ ions.39, 40 The satellite peak at binding energy of 880.4 eV also can be attributed to Ni2+ species.41 The Cu 2p1/2 peak found at 934.9 and 933.3 eV and the Cu 2p1/2 peak located at 955.9 and 953.4 eV are shown in Cu 2p spectrum, suggesting that the copper ions is in +2 valence state.42, 43 Moreover, the appearance of peak at 962.7 eV 10


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also proves the presence of Cu2+.44 The main Fe 2p3/2 and Fe 2p1/2 are located at 712.6 and 726.4 eV, respectively, which are the characteristic of Fe3+ ions.45, 46 Besides, the satellite peak at 717.5 eV can be ascribed to Fe3+ species.47 In conclusion, these four kinds of transition metal phenylphosphonates are successfully prepared. Thermal stability of phenylphosphonates is evaluated by TGA and DTG under air and N2 atmosphere. The TGA and DTG curves are given in Fig. 4 and 5. The corresponding data is exhibited in Table 1. The decomposition of CoPP is composed of two steps in air. The DTG peak for the first stage appears at 112 ◦C can be attributed to the evaporation of physically absorbed water while the second stage appears at about 609 ◦C can be assigned to the removal of aromatic ring, suggesting the remarkable stability for organically constituted layers.48 The decomposition of NiPP is one-step reaction in air, with Tmax of 565 ◦C, which can be ascribed to the thermal decomposition of organic component in the material along with the formation of Ni2P2O7.36 In air, two-step decomposition is observed from the DTG curve of CuPP. The first peak at 365 ◦C corresponds to the decomposition and oxidation of the interlayer phenyl groups accompanied by the formation of Cu2P2O7 while the second peak at 628 ◦C may be attributed to the degradation of the remained organic component.30 There are two peaks in the DTG curve of FePP with Tmax1 and Tmax2 of 446 and 462 ◦C in air, respectively. It can be assigned to the rupture and burning of the aromatic ring.49 Moreover, solid residues of CoPP, NiPP, CuPP and FePP in air are 63.5, 69.8, 71.4 and 76.4 wt%, respectively. TGA results under N2 of these 11



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phenylphosphonates are exhibited in Fig. 5. Similar decomposition process can be observed. Due to the easily-moisture-adsorbed performance of phenylphophonates, obvious evaporation of physically absorbed is found before the appearance of Tmax. This may lead to decrease in Tinitial, compared with results in air. Moreover, these phenylphosphonates show higher Tmax and lower maximum mass loss rate (MMLR) under N2. In short, these prepared phenylphosphonates have excellent thermal stability under air and N2 in terms of its high decomposition temperature and solid residue. SEM and TEM images of these phenylphosphonates are exhibited in Fig. 6 and 7. It is clear that all phenylphosphonates exhibit layered structure. Particles with lamellar structure may show barrier effect on the transfer of heat and degraded products, leading to the inhibition in pyrolysis or combustion of polymer. Moreover, these phenylphosphonates can be regarded as microscale materials in terms of length and thickness. These phenylphosphonates sheets are overlapped and the large surface area of particles can be observed from TEM micrographs. Large surface area of sheets is beneficial to the reinforcement of barrier effect and then more volatiles attached on the layer of particles can be catalyzed into char residue or non-toxic gases. The dispersion state and interfacial interaction between fillers and polymer matrix are confirmed as important factors in affecting the properties of polymeric composites. To evaluate the dispersion state of metal phenylphosphonates in the PS matrix, the morphologies of fractured surfaces are observed by SEM and the images are shown in 12


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Fig. 8. Compared with neat PS, the fractured surface of PS composites show considerably different fractographic features, as exhibited in Fig. 8b-e. After the incorporation of phenylphosphonates, the surfaces of PS composites become much rougher than PS. Moreover, red circles displayed in Fig. 8b-e represent the phenylphosphonates. Clearly, phenylphosphonates are well dispersed and embedded in the matrix without apparent aggregation, which is probably attributed to conjugate action between PS chains and phenyl groups of fillers. Fig. 8f is the enlarged view of one red circle in Fig. 8e and the substances in blue rectangle are FePP. Obviously, FePP particles are embedded tightly in PS matrix, indicating the strong interfacial interaction between phenylphosphonates and PS matrix. Thermal stability of PS composites under air and N2 is measured and the corresponding curves are shown in Fig. 9 and 10, respectively. The related thermal data are exhibited in Table 2. From Fig. 9a and b, PS shows one step decomposition under air with Tinitial and Tmax of 332 and 396 ◦C, respectively. Compared with neat PS, PS composites show higher Tinitial and Tmax, except for PS/NiPP, which exhibited similar Tinitial and slightly decreased Tmax of 332 and 394 ◦C, respectively. There is no char residue left for neat PS at 700 oC while its composites show obvious improvement in char yield with the presence of phenylphosphonates, suggesting the enhanced thermal stability of PS. Furthermore, obvious reduction in MMLR is observed, suggesting the barrier function of phenylphosphonates. Thermal decomposition of PS and its composites under N2 is also investigated. One step 13



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decomposition of PS with with Tinitial and Tmax of 372 and 409 ◦C can be found. However, the improvement in Tinitial and Tmax isn’t obvious. Inversely, slight decrease in these data can be found. Moreover, char residue yield in N2 is improved. However, there is a modest enhancement in char residue. Then, the protective effect of char may be marginal here. Moreover, the values of MMLR of PS composites are decreased appreciably, which can be ascribed to the barrier function of phenylphosphonates. Fig. 11 presents heat release rate (HRR) and total heat release (THR) curves of polymer materials and the corresponding combustion data are shown in Table 3. It can be noted that a high HRR peak appears in the curve of pure PS with a peak heat release rate (PHRR) value of 1642 kW/m2 and its THR is 108.8 MJ/m2. Obviously, the composites containing phenylphosphonates show lower PHRR and THR than pure PS, suggesting the improvement in flame retardancy. PS/CuPP shows the most remarkable decrease in PHRR and THR for 34.8% and 13.7%, respectively. This may be attributed to the best barrier performance of CuPP in the release of decomposed fragments, which is reflected by the maximum decline in MMLR both in N2 and air. Moreover, the decrease in PHRR for PS/FePP, PS/CoPP and PS/NiPP are 30.2, 25.0 and 22.5%, respectively. Compared with neat PS, the transfer of heat and decomposed fragments may be inhibited by these layered fillers due to its barrier action, resulting in less fuel for combustion. Then, the declined value of PHRR and THR are observed. However, the composites are ignited earlier than neat sample, which may be due to the thermal degradation of instable phosphonate groups in fillers under this high 14


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temperature. Char residue can be regarded as efficient shield for polymer and block the transfer of mass and heat. Moreover, the char residue can be strengthened by the metal phosphates formed, resulting in persistent protection for polymer matrix under high temperature. Fig. 12 shows the digital photos of char residue and there is a modest enhancement in char yield, suggesting the marginal protective function of char. Then, it is inferred that barrier function of these phenylphosphonates may be the primary effect in this work. To investigate the effect of phenylphosphonates on the gaseous volatiles during pyrolysis, the volatiles of PS and PS composites are studied by TG-IR technique. Fig. 13 shows the 3D gas phase spectra of neat PS and PS composites with a heating rate of 20 ◦C/min under N2 atmosphere. It is obvious that the typical thermal degradation process of the composites is similar to that of pure PS. Then, to further investigate the component evolved in the gas phase of PS and PS composites, the TG-IR spectra at maximum weight loss rate are plotted in Fig. 14. It also shows that the typical thermal decomposition process of PS composites is similar to that of neat PS. Furthermore, the decomposed products of PS under nitrogen are mainly monomer, dimer, and trimer of phenyl alkenyl and the absorption bands at 3073, 1496, 773, and 698 cm-1 are assigned to aromatic compounds, and the absorption band observed at 1597 cm-1 can be ascribed to alkenyl units.50 To further study the decomposition difference between PS and PS composites, the Gram-Schmidt (GS) curves is exhibited in Fig. 15a. The absorbance values are normalized by the original mass of samples for 15



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comparison purpose. Obviously, the absorbance intensity of decomposed products are decreased after the incorporation of phenylphosphonates. Two explanation are presented for this phenomenon. Firstly, the layered phenylphosphonates can act as barrier, which can decrease the thermal decomposition rate of polymer and limit gas diffusion. Secondly, the volatile aromatic compounds can be catalyzed to form char by in the presence of transition metal element. As mentioned above, aromatic compounds may be the main decomposed volatiles of PS. Then, the decreased amount of the decomposed volatiles may further result in the suppression of smoke, since the condensed aromatic compounds may be aggregated to form smoke.51 Here, PS/CuPP shows the lowest absorbance intensity of pyrolysis products, suggesting that the formation of volatile aromatic compounds are restrained more efficiently after the addition of CuPP. This is in accord with the superior function of copper element on smoke suppression.9, 10 Then, the quantity of combustible fuels is reduced, further resulting in the evident declines in PHRR and THR. For the purpose of investigation on the change of decomposed products, the FTIR absorbance of pyrolysis product (773 cm-1) for PS and PS composites vs. time is revealed in Fig. 15b and the absorbance values are normalized by the pristine mass of samples. It is clear that the absorbance intensity of pyrolysis product is decreased after the incorporation of phenylphosphonates, which implies the amount of volatile aromatic compounds released from PS composites is less than those from pure PS. In short, the release of decomposed products is inhibited by layered phenylphosphonates. 16


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In order to investigate the effect of metal phenylphosphonates on the char formation, the quality of char residues can be evaluated by Raman spectra, as shown in Fig. 16. Higher quality char residue means that polymer matrix underlying char can be protected efficiently due to better thermal stability of the char. The spectra in Fig. 16 exhibit two strong peaks at approximately 1355 and 1583 cm-1, which are attributed to the vibration of carbon atoms with dangling bonds in the plane terminations of disordered graphite or glassy carbons (D band) and vibration of sp2-hybridized carbon atoms in a graphite layer (G band), respectively.52,

53

Graphitization degree of char residue can be evaluated by the ratio of integral intensity of D and G bands (ID/IG). Lower value of ID/IG indicates that higher graphitization degree is obtained for char residue. Obviously, char residue of PS/CuPP shows the highest graphitization degree, indicating that polymer can be efficiently protected from burning by this char. Crystal structure of char residues can be explored by XRD and Fig. 17 shows corresponding patterns of char residues after combustion. It is verified that the main products in the char residue of CoPP, NiPP, CuPP and FePP, can be assigned to Co2P2O7, Ni2P2O7, Cu2P2O7 and Fe2P2O7, respectively. Six diffraction peaks shown in Fig. 17a can be attributed to (-102), (021), (012), (220), (032) and (-424) plane of Co2P2O7 (JCPDS-49-1091).54 Four peaks exhibited in Fig. 17b can be assigned to (210), (002), (212), (230) and (-513) plane of Ni2P2O7 (JCPDS-74-1604).55 In addition, seven peaks observed in Fig. 17c can be ascribed to (002), (-202), (022), (220), (132), 17



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(221) and (-222) plane of Cu2P2O7 (JCPDS-74-1614).56 There are three peaks shown in Fig. 17d corresponding to (021) (201) and (220) plane of Fe2P2O7 (JCPDS-76-1762).57 These results indicates the formation of phosphorous-O-metal bridge, which can strengthen char residue to resist high temperature. Moreover, valence transition from Fe3+ to Fe2+ can be observed, suggesting the redox reactions between Fe3+ species and the degraded products of polymer. Chemical composition of char residues is confirmed by XPS and the results are given in Fig. 18. Clearly, the peaks of P 2p located at 134.0 eV can be assigned to pyrophosphate or polyphosphate compounds, which indicates the formation of a phosphor carbonaceous char.58, 59 Two peaks at 782.5 and 798.6 eV are assigned to Co 2p3/2 and Co 2p1/2, respectively, and the existence of Co2+ can be confirmed according to the literature.60 Furthermore, two peaks shown in the spectrum of NiPP are ascribed to Ni 2p3/2 and Ni 2p1/2 and the bivalent Ni ion can be observed.61 For the char residue of CuPP, the peaks at 935.3 and 955.7 eV are attributed to Cu 2p3/2 and Cu 2p1/2, respectively, indicating the presence of Cu2+.62 In addition, two strong peaks at 712.6 and 726.1 eV correspond to Fe 2p3/2 and Fe 2p1/2, respectively, which are confirmed as the characteristic of Fe3+ species.63 It is deduced that partial Fe3+ species are transformed into Fe2+ accompanied with the result of XRD, indicating the occurrence of redox reactions between Fe3+ species and degraded products of PS, such as aromatic compounds. Moreover, the existence of pyrophosphates is confirmed by XPS results. Here, the char residues containing pyrophosphates and carbon materials 18


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are suggested to possess high thermal stability and perform efficient barrier role in hindering the transfer of heat and mass.

4. Conclusions In this work, four kinds of transition metal phenylphosphonates were designed to enhance the fire safety of polymer. The enhancement in fire safety of polymer can be achieved by the inhibition of heat release and suppression of smoke or toxic gases. The values of PHRR and THR are decreased with the presence of these phenylphosphonates, indicating the heat release inhibition. Moreover, the absorbance intensity of aromatic compounds is greatly reduced by phenylphosphonates, suggesting the possible smoke suppression, since the condensed aromatic compounds may be aggregated to form smoke. These layered phenylphosphonates may have barrier and catalyzing carbonization effect on the degraded products of polymer. The maximum declines in MMLR and absorbance intensity of pyrolysis products are obtained owing to the barrier effect. More char residue with higher graphitization degree are formed because of the catalyzing effect. Moreover, the protective effect of char may be marginal in this work. Then, transfer of degraded products are mainly blocked by the barrier function of fillers. This work may provide a novel strategy for the preparation of low-cost and effective flame additives with layered structure for polymer. 19



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Acknowledgements The work was financially supported by the National Natural Science Foundation of China (51403196), the Natural Science Foundation of Jiangsu Province (BK20130369) and the Fundamental Research Funds for the Central Universities (WK2320000032).

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29



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Table captions Table 1. Related TGA data of layered metal phenylphosphonates under air and N2. Table 2. Corresponding thermal data of PS and its composites under air and N2. Table 3. Related results of cone calorimeter measurements.

30


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Table 1. Related TGA data of layered metal phenylphosphonates under air and N2. Sample Tinitiala (◦C)

Tmax1b (◦C)

Tmax2b (◦C)

Tmax3b (◦C)

Solid residue at 700 ◦

C (wt%)

air

N2

air

N2

air

N2

air

N2

air

N2

CoPP

140

150

112

146

609

648

-

-

63.5

60.2

NiPP

547

568

565

651

－

-

-

-

69.8

76.1

CuPP

368

365

365

365

628

418

-

500

71.4

71.7

FePP

411

208

446

494

462

-

-

-

76.4

73.5

a

Temperature at 5 wt% weight loss.

b

Temperature at maximum weight loss rate.

31



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Table 2. Corresponding thermal data of PS and its composites under air and N2. Sample

Tinitial (oC)

Tmax (oC)

Char residue at 700 oC (wt%)

air

N2

air

N2

air

N2

PS

332

372

396

409

0.0

0.7

PS/CoPP

337

368

401

402

1.9

1.8

PS/NiPP

332

366

394

407

2.1

2.1

PS/CuPP

341

365

415

408

2.4

1.3

PS/FePP

335

371

402

407

2.0

2.6

32


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Table 3. Related results of cone calorimeter measurements. Sample

PHRR (kW/m2)

THR (MJ/m2)

PS

1642

108.8

PS/CoPP

1231

103.2

PS/NiPP

1272

101.0

PS/CuPP

1070

93.9

PS/FePP

1146

103.5

33



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Figure captions Fig. 1. FTIR spectra of (a) CoPP, (b) NiPP, (c) CuPP and (d) FePP. Fig. 2. XRD patterns of (a) CoPP, (b) NiPP, (c) CuPP and (d) FePP. Fig. 3. XPS spectra of (a, b) CoPP, (c,d) NiPP, (e,f) CuPP and (g, h) FePP. Fig. 4. TGA and DTG curves of (a) CoPP, (b) NiPP, (c) CuPP and (d) FePP under air. Fig. 5. TGA and DTG curves of (a) CoPP, (b) NiPP, (c) CuPP and (d) FePP under N2. Fig. 6. SEM images of (a) CoPP, (b) NiPP, (c) CuPP and (d) FePP. Fig. 7. TEM images (a) CoPP, (b) NiPP, (c) CuPP and (d) FePP. Fig. 8. Morphologies of fractured surface of (a) PS, (b) PS/CoPP, (c) PS/NiPP, (d) PS/CuPP, (e) PS/FePP and (f) enlarged view of one red circle in PS/FePP. Fig. 9. (a) TGA and (b) DTG curves of PS and its composites under air. Fig. 10. (a) TGA and (b) DTG curves of PS and its composites under N2. Fig. 11. (a) HRR and (b) THR curves of PS and its composites. Fig. 12. Digital photos of char residue after combustion: (a) PS, (b) PS/CoPP, (c) PS/NiPP, (d) PS/CuPP and (e) PS/FePP Fig. 13. The 3D diagrams of the gaseous volatiles during combustion process of (a) pure PS, (b) PS/CoPP, (c) PS/NiPP, (d) PS/CuPP and (e) PS/FePP. Fig. 14. TG-IR spectra at maximum weight loss rate of (a) PS, (b) PS/CoPP, (c) PS/NiPP, (d) PS/CuPP and (e) PS/FePP. Fig. 15. (a) Gram-Schmidt (GS) curves and (b) the absorbance of pyrolysis product of pure PS and its composites. 34


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Fig. 16. Raman spectra of char residues of (a) PS/CoPP, (b) PS/NiPP, (c) PS/CuPP and (d) PS/FePP. Fig. 17. XRD patterns of char residues of (a) CoPP, (b) NiPP, (c) CuPP and (d) FePP. Fig. 18. XPS spectra of char residues of (a, b) CoPP, (c, d) NiPP, (e, f) CuPP and (g, h) FePP.

35



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Fig. 1

36


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Fig. 2

37



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Fig. 3 38


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Fig. 4

Fig. 5 39



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Fig. 6

Fig. 7 40


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Fig. 8

41



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Fig. 9

42


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Fig. 10

43



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Fig. 11

Fig. 12

44


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Fig. 13

Fig. 14

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Fig. 15

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Fig. 16

Fig. 17 47



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Fig. 18

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49


Two-Dimensional Metal Phenylphosphonates as Novel Flame

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