Highly Flame Retardant Expanded Polystyrene Foams from

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Highly Fire-Safety Expanded Polystyrene Foams by Phosphorous-Nitrogen-Silicon Synergistic Adhesives Zong-Min Zhu, Ying-Jun Xu, Wang Liao, Shimei Xu, and Yu-Zhong Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b05065 • Publication Date (Web): 05 Apr 2017 Downloaded from http://pubs.acs.org on April 11, 2017

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Industrial & Engineering Chemistry Research

Highly Fire-Safety Expanded Polystyrene Foams by Phosphorous-Nitrogen-Silicon Synergistic Adhesives

Zong-Min Zhu1, Ying-Jun Xu2, Wang Liao*,2 Shimei Xu and Yu-Zhong Wang*,2 1

College of Chemical Engineering, Sichuan University, Chengdu 610064, China.

2

Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, State Key

Laboratory of Polymer Materials Engineering, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), Sichuan University, Chengdu 610064, China.

ABSTRACT: Three novel environment-friendly flame retardant adhesives (FRAs), i.e. (1) poly N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane (P(NTMS)), (2) P(NTMS-phosphoric acid) (P(NTMS-PA)) and (3) P(NTMS-phosphorous acid) (P(NTMS-POA)) are synthesized via a versatile sol-gel method. The chemical structures are characterized by FTIR and 1H NMR. The thermal stabilities of the FRAs are revealed by thermogravimetric analysis (TGA) and they exhibit single, double and triple degradation processes, respectively. Flame retardant expanded polystyrene foams (EPSFs) are prepared with these novel environment-friendly FRAs by simple coating method and their flammability are investigated by limiting oxygen index (LOI), UL-94 vertical burning test and cone calorimeter (CC). With as high as 57 wt% of P(NTMS) coating, the foam gains no rating in UL-94 test. In contrast, EPSF with 57 wt% of P(NTMS-PA) coating passes UL-94 V-0 grade with a LOI of 31% and the foam with only 40 wt% of P(NTMS-POA) also passes UL-94 V-0 grade with a LOI of 26.5%. CC results demonstrate that 40 wt% of P(NTMS-PA) coating reduced the

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peak heat release rate (PHRR) of EPSF with 62.1% and increase the residue from 0 to 36.2%. For P(NTMS-POA), the corresponding values are 68.8% and 34.0%, respectively. The morphology of the residues were revealed by SEM, and corresponding chemical compositions and carbonaceous structure were studied by FTIR, EDX and Raman spectra. These results indicated a synergy in charring between phosphorous, nitrogen and silicon and the resulted protective layers were responsible for the higher fire safety. KEYWORDS: expanded polystyrene; sol-gel reaction; coating; flame retardancy

1. INTRODUCTION Up to ca. 40% total energy on this planet is consumed to maintain a comfortable interior temperature.1 Thermal insulation materials predominate the energy efficiency of numerous buildings, among which, expanded polystyrene foams (EPSFs) overwhelm the other foam-like materials with the largest market share.2 A light and porous EPSF has the volume content of air as high as 98%, therefore, EPSF is with higher flammability than the its corresponding resin. Fire disasters of EPSF have caused significant loss of properties and lives. Flame retarded EPSFs are typically made by suspension polymerization of a mixture of styrene monomer and flame retardant in water to form beads of styrenic polymer. Up to now, the most effective and economic flame retardants (FRs) for EPSF are still halogen-containing FRs, hexabromocyclododecane (HBCD) for example.3-7 For intance, flame retardancy of EPS foam will be improved with a low levels of HBCD (0.5-1 wt%), which will pass German B2 to DIN 4102 with the both HBCD and bis (2, 3-dibromopropyl ether).8 Moreover, many districts, European Union as a representative, have banned this kind of FRs for their toxic products during the thermal ACS Paragon Plus Environment

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degradation. Halogen-free flame retardants are eco-friendly substitutions. But the loading of FRs is usually at a high level to achieve UL-94 V-0 rating and a high limiting oxygen index (LOI). Yan et al.9 reported a highly effective FR, composing of ammonium polyphosphate (APP) and a charring agent (CA), for PS. LOI value of FR/PS sample reached 32.5% and passed V-0 rating with a loading of 30% of FR. An intumescent char layer formed during combustion, which acted as a strong barrier to avoid the transfer of heat and combustible gases. In addition, a good synergistic effect between APP and the CA was found. The authors also prepared PS resin with 25% aluminum hypophosphite (AHP) composites,10 which showed good fire resistance with significantly higher LOI, lower peak heat release rate (PHRR) and could even pass V-0 rating. Xia et al.11 synthesized a 1-hydroxy ethylidene-1, 1-diphosphonic ammonium salt (HEDPA) and used HEDPA with pentaerythritol (PER) and melamine (MEL) to flame-retard PS. The FRs-PS had a LOI of 26.7 and UL-94 V-0 rating with 25% HEDPA/PER/MEL. However, the effectiveness of halogen-free FRs are still comparatively low in contrast to the halogen-containing ones. Larger amounts of halogen-free FRs are added into EPSFs to obtain good flame retardancy, which, unfortunately, always compromise the foam quality. Coating EPS beads with flame retardant adhesives (FRAs) seems the most profitable solution to impart the matrix a good fire resistance with little influence on the other properties. For instances, Burt in Monsanto Chemicals Ltd. prepared flame-retardant foams with a thermoplastic resin and pentabromophenyl alkenyl ether or tribromophenyl alkenyl ether as FR.12 Murphy and Rauniyar in Shell International Research used a brominated triazine derivative coating to improve the fire safety of the foam.13 Hahn et al. in BASF produced kinds of flame retardant EPSFs coated with organic bromine compounds with self-extinguishing properties.14 However, these FR coatings are still halogenated, requiring greener solutions. The research on halogen-free flame retardant coating for ACS Paragon Plus Environment


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EPSF is very little because of its higher flammability. Hamdani-Devarennes et al.15 prepared flame retardant-EPSF with water-based flame retardant coating based on nano-boehmite and poly(vinyl alcohol). In a certain coating level, flame retardancy of EPSF was improved. However, the foam gains no rating in vertical burning test. To obtain a better flame retardant EPSF, an effective flame retardant should be used for EPSF. Among the ‘greener’ flame retardants, silicon, phosphorous and/or nitrogen are the most popular elements.16-21 For example, natural silicates, polyhedral oligomeric silsesquioxane (POSS) and organic siloxane are widely used for they can form a positive silica layer at high temperature which protects matrix against degradation.22-24 On the other hand, phosphorus-containing FRs can catalyze char formation in the condensed phase and/or capture active radicals in the gas phase.25-29 And nitrogenous FRs always generate inert gases which dilute the combustible materials and heat in the flame.30 However, to the best of our knowledge, no silicon-nitrogen-phosphorus FR coating has been developed and neither their synergism in flame retardancy of EPSF has been studied. In this work, sol-gel reactions were applied to synthesize three novel environment-friendly organic/inorganic hybrids and used as effective FRAs while preparing fire-safety EPSFs. Sol-gel reaction is a versatile technique to produce organic/inorganic hybrids with Si-O-Si frameworks,31-35 which imparts environment-friendly thermal stability and flame retardancy to materials.36,37 In addition, corresponding fire behaviors of the foams and flame retardant mechanisms were revealed by integrated methods.

2. EXPERIMENTAL SECTION 2.1. Materials ACS Paragon Plus Environment

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N-β-(aminoethyl)- γ-aminopropyltrimethoxysilane (NTMS) was provided by Diamond Chemical Co., Ltd. EPS bead (Non-flame-retardant), granule size: 0.7-1.1 mm and expansion ratio: 45-50, was purchased from Dongguan Plastics & Rubbers materials Co., Ltd and used as the control sample. Phosphoric acid (PA) and phosphorous acid (POA) were supplied from Kelong Chemical Co. Deionized (DI) water was used throughout.

2.2. Synthesis of FRAs The preparation process was typical Sol-Gel reaction.31 NTMS (0.25 mol) was dissolved in 400 ml DI water at room temperature (RT) and then the temperature was raised to 80~120 oC for 8 h. When the reaction was finished, the mixture was cooled down back to the RT and the solvent was removed. The flame retardant adhesive prepared from only NTMS as referred to as P(NTMS). P(NTMS-PA) and P(NTMS-POA) were prepared by the same process as P(NTMS), in which H3PO4 or H3PO3 (0.5 mol) were added dropwisely before heating.



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Scheme 1. The chemical routes of P(NTMS), P(NTMS-PA) and P(NTMS-POA) and resulted EPSFs. 2.3. Sample preparation Expanded polystyrene beads were heated at 110 oC for 10-15 min to be pre-foamed. The calculated amount of pre-foamed polystyrene beads (aging time: 6-12 h) and FRAs were mixed with a high-speed mixer for 30 min. The mixture was transferred into a suitable mold at 110 oC for 10 min, in which EPS beads expanded, solidified and filled the shape of the mold. The foam board was subsequently quickly taken out and quenched down to the RT. Synthesis of the FR adhesives and the preparation of EPSFs was summarized in Scheme 1.


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2.4. Characterization The fourier transform infrared (FT-IR) spectra were recorded on a Nicolet 6700 spectrophotometer by KBr-pellet. 1

H NMR was collected a Bruker Acance spectrometer (400 MHz) with D2O as the solvent.

The LOI value was performed measured on sheets 150 mm ×10 mm×10 mm according to ASTM D2863-97 as descried below: LOI=([O2]/[O2]+[N2]) ×100%. [O2] and [N2] were the concentration of oxygen and nitrogen in a mixture of the two gases, respectively. The UL-94 vertical burning level was tested on a CZF-2 instrument (Jiangning, China) according to ASTM D3801 and dimension of all samples was 150 mm×20 mm×20 mm. The combustion behaviors were measured with a cone calorimeter (CC) device (Fire Testing Technology). The samples with a size of 100 mm×100 mm×20mm were exposed to a radiant cone at a heat flux of 50 kW/m2. Thermogravimetric analysis (TGA) was carried on a TG 209F1 (NETZSCH, Germany) thermogravimetric analyzer at a heating rate of 10 oC/min of the sample was examined under nitrogen atmosphere. The char residues of the flame-retardant EPSFs after the CC tests were analyzed by scanning electronic microscopy (SEM) of a JEOL JSM-5900LV coupled with energy-dispersive X-ray (EDX) spectroscopy (INCA, Oxford Instruments, Abingdon, Oxfordshire, U.K.). The char residues were recorded after gold coating surface treatment. Raman spectroscopy measurement was carried out at room temperature with LabRAM HR800 laser Raman spectrometer (SPEX Co., USA) by a 532 nm helium-neon laser line.



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Compressive strengths were tested according to ISO 844: 2004, with a relative deformation of 10 % done in a minute. Values of mechanical properties reported here were the averages of at least five individual measurements. The dimensions for all samples were 50 mm×50 mm×20 mm. The densities of EPSFs were measured according to ISO 845: 2006, based on the average values of five samples larger than 100 cm3.

3. RESULTS AND DISCUSSION 3.1. Characterization of the FRAs Figure 1 showed the FT-IR spectra of NTMS and its derivative FRs, i.e. P(NTMS), P(NTMS-PA) and P(NTMS-POA). For NTMS, the absorption peaks at around 1100 cm-1 were assigned to stretching vibration of Si-O-CH3 bond.38 After the reactions, all the flame retardant adhesives had the characteristic absorption peaks at 1133 and 1048 cm-1, which were assigned to stretching vibration of Si-O-Si bonds,39 suggesting that Si-O-CH3 after a process of hydrolysis were changed into Si-O-Si. The peaks at 932 cm-1 were assigned to bending vibration of the Si-OH bonds, indicating that Si-OH groups still existed after hydrolysis. For the spectrum of P(NTMS), the absorption peaks at 3366 cm-1, 3288 cm-1 and 1471 cm-1 were attributed to stretching and bending vibration of N-H bonds. In addition, the absorption peaks of P(NTMS-PA) at 3024 cm-1 and 1504 cm-1 were attributed to NH2+ and NH3+，and the absorption peaks of P(NTMS-POA) at 2381 cm-1 belonged to the stretching vibration of P-H.


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Figure 1. FT-IR spectra of NTMS, P(NTMS), P(NTMS-PA) and P(NTMS-POA).

1

H NMR spectra of the products were shown in Figure 2, in which P(NTMS): 0.5 ppm (Si-CH2-),

1.6 ppm (Si-C-CH2-), 2.7 ppm (C-C-CH2-), 2.8 ppm (N-CH2-CH2-). After the reaction NTMS and phosphorus-containing acid, compared to P(NTMS), there were an obviously shift of two peaks from 2.7 ppm and 2.8 ppm to 3.1 ppm and 3.4 ppm, respectively. In the spectrum of P(NTMS-POA), the chemical shifts in 6.0 and 7.5 ppm were assigned to P-H bonds. No resonance of NH2+ or NH3+ proton appears in D2O solution resulted from effect on solution exchange for active H of NH2+ or NH3+.40



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Figure 2. 1H NMR spectra of P(NTMS), P(NTMS-PA) and P(NTMS-POA). 3.2. Thermal Degradation Behaviors of the FRAs. Thermogravimetric (TG) analysis is the standard approach to reveal the thermal degradation behaviors of materials. Thermal degradation behaviors can be modeled recently.41,42 TG and derivative thermogravimetric (DTG) curves of the FRAs were presented in Figure 3 and the characteristic parameters were summarized in Table 1. The temperatures at which 5% weight were loss (T5%) were recorded to indicate the thermal stabilities of the FRAs, which are 212 oC for P(NTMS), 216 oC for P(NTMS-PA) and 262 oC for P(NTMS-POA), respectively. P(NTMS-POA) thus shows a remarkably higher thermal stability than the others. The degradation behaviors of the FRAs were rich of diversities. One-step degradation was observed for P(NTMS) with a single maximum weight loss at 431 oC (Tmax) in its DTG curve (Figure 3b). In its FTIR spectrum (Figure 4a), the absorption peaks of N-H (3366 cm-1, 3288 cm-1 and 1468 cm-1) became weaker at the Tmax, indicating breakage of N-H bond and degradation of P(NTMS). At 700 oC, abovementioned N-H absorptions totally disappeared and the peaks of Si-O-Si at 1129 cm-1 and 1038 cm-1 merged into one (1038 cm-1). Therefore, 48.8% of P(NTMS) ACS Paragon Plus Environment

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remained as residue which mainly consist of Si-O-Si and played a role of a protective barrier.43 Based on the DTG curves (Figure 3b), the degradation of P(NTMS-PA) could be divided into two-steps, i.e. Tmax1 at 186 oC and Tmax2 at 384 oC. The first decomposition of P(NTMS-PA) at 186 °C was attributed to the elimination of water which was crosslinking of Si-OH groups in P(NTMS-PA). In addition, the absorption peaks of NH3+ and NH2+ (3020 cm-1 and 1503 cm-1) became weaker at 381 oC, attributing to the breakage of ionic bonds between phosphorous acids and ammoniums and the further formation of polyphosphoric acid derivatives (P-O-P, 1081 cm-1).44 Furthermore, the peaks of Si-O-Si at 1125 cm-1 and 1042 cm-1 shifted to 1000 cm-1 and the peaks of NH3+ and NH2+ disappeared when the temperature was higher than 381 oC. There were 3 steps for the thermal degradation of P(NTMS-POA) (Figure 3b), and the corresponding Tmax were 255 oC, 321 oC and 388 oC, respectively. The FTIR spectrum in Figure 4c again clearly revealed the details of the decomposition process. The first decomposition of P(NTMS-POA) was due to the elimination of water by crosslinking of Si-OH groups. The second decomposition was mainly for the breakage of P-H. Simultaneously, polyphosphoric acid derivatives (P-O-P, 1081 cm-1) formed. The third decomposition with a maximum decomposition rate at 388 oC was attributed to the breakage of the ionic bond between anionic phosphorous acids and the cationic ammoniums. More than half (58.5 wt%) was left as a residue. Based on the discussion, none of the three FRAs were thermally stable at the very initial stage for the easily decomposed polyhydroxy structures. The Si-O-Si groups, which formed at higher temperature, were effective to enhance the amount of the char residues. In addition, the higher residues of P(NTMS-PA) and P(NTMS-POA) comparing with that of P(NTMS) could be attributed to the synergy between phosphorus, nitrogen and silicone.



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Figure 3. TG (a) and DTG (b) curves of P(NTMS), P(NTMS-PA) and P(NTMS-POA) under N2. Table 1. Characteristic parameters of TGA and DTG curves for FRAs Mass loss rate at Sample

T5%(oC)

Tmax1(oC)

Tmax(%/min)

Mass loss rate

Mass loss rate Tmax2(oC)

atTmax2(%/min)

Tmax3(oC) at Tmax3(%/min)

P(NTMS)

212

431

-4.96

-

P(NTMS-PA)

216

186

-1.05

381

P(NTMS-POA)

262

255

-0.83

321

Residue at 700 oC (%)

-

-

48.8

-4.80

-

-

58.9

-1.84

388

-4.41

58.5


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Figure 4. FT-IR spectra of decomposition products of P(NTMS) (a), P(NTMS-PA) (b) and P(NTMS-POA) (c) at characteristic temperatures of thermal degredation.

3.3. Combustion tests of EPSF-FRAs Limiting oxygen index (LOI) and UL-94 rating are basic and critical parameters to evaluate the flammability of a polymeric material. LOI test is used to judge materials difficulty of ignition in the air. A higher value indicates a better flame retardancy. The results of LOI values and UL-94 ratings of the FRAs-EPSFs were summarized in Table 2. EPSF prepared with non-flame-retardant (abbr. EPSF) was highly flammable with a LOI of only 17% and got no rating in UL-94 tests. The LOI values significantly increased with higher usage of the FR adhesives. For instance, when 12 g preformed PS beads prepared with 8 g of a FRA, i.e. the FRA took 40 % of the total weight, the corresponding LOI values were 19 % for EPSF/8P(NTMS), 25.5 % for EPSF/8P(NTMS-PA) and 26.5 % for EPS/8P(NTMS-POA). After combustion, as shown in Figure 5, no residue could be recognized for the EPSF except melting dripping. The melting dripping would transferred heat and benefit to the flame spread. In contrast, residue of EPSF/8P(NTMS) appeared but was less and loose, while the residues of ACS Paragon Plus Environment


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EPSF/8P(NTMS-PA) and EPSF/8P(NTMS-POA) were comparatively more and denser. These results indicated that adhesive of P(NTMS) could suppress the combustion of EPSF to some extent. Rich Si-O-SiWith modification by the phosphorous-containing acids, the condensation products of P(NTMS-PA) and P(NTMS-POA) could provide better flame retardant efficiency to EPSF than that of P(NTMS). More char residues could be left under catalysis of phosphorous-containing acid. Table 2 also showed that EPSF/8P(NTMS) and EPSF/8P(NTMS-PA) failed to pass UL-94 V-0 rating, comparing with the different situation for EPSF/8P(NTMS-POA), which also indicates the best fire resistant efficiency of the P(NTMS-POA) adhesive. This difference might be related to different phosphorus valances and their chemical structures. Chen et al.45 found lower valance of phosphorus additive showed better fire resistance for flexible polyurethane foams. The highest phosphorus valance (+5) took effect mainly in the condensed phase but the phosphorous with an intermediate valance (+3) might played its roles both in gas phase and condensed phase. Table 2. LOI and UL-94 results of EPSF and FRAs-EPSFs. Samples

EPS

P(NTMS)

P(NTMS-PA)

P(NTMS-POA)

FR

(g)

(g)

(g)

(g)

(wt%)

UL-94

LOI (%)

N.Ra

17

40

N.R

19

12

50

N.R

20.1

16

57

N.R

20.5

8

40

V-2

25.5

12

12

50

V-1

28

EPS/16P(NTMS-PA)

12

16

57

V-0

31

EPS/8P(NTMS-POA)

12

8

40

V-0

26.5

EPS/12P(NTMS-POA)

12

12

50

V-0

28

Neat EPS

12

EPS/8P(NTMS)

12

8

EPS/12P(NTMS)

12

EPS/16P(NTMS)

12

EPS/8P(NTMS-PA)

12

EPS/12P(NTMS-PA)


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EPS/16P(NTMS-POA) a

12

16

57

V-0

30

N.R: no rating.

Figure 5. Digital photographs of the samples after LOI tests: EPSF (a), EPS/8P(NTMS) (b), EPS/8P(NTMS-PA) (c) and EPS/8P(NTMS-POA) (d).

3.4. Cone calorimeter analysis Cone calorimetry (CC) based on the oxygen consumption principle is at present effective method to assess flammability behavior of a polymer material in real fire, because its result correlates well with those from large-scale fire tests. 46 CC test will give out the some parameters such as the time to ignition (TTI), heat release rate (HRR), total heat release (THR), time to peak of heat release (TTPHRR), peak of heat release (PHRR), fire growth rate (FIGRA = PHRR/TTPHRR) and the residual char content to estimate the material’s fire safety. Figure 6 and Table 3 showed the HRR curves and THR curves of EPSF-FRAs and the corresponding combustion data under a heat flux of 50 kW/m2. All of the foams samples were ignited easily under that high heat flux value. In spite of this, TTI values slightly increased when FRAs were used. PHRR value must be the most vital one to evaluate the fire risky of a material.47 As Figure 6 shown, these values remarkably decreased



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once FRAs were used. For instance, PHRR of the neat EPSF was 689 kW/m2 and the value decreased to 423 kW/m2 (-38.6%) for EPSF/8P(NTMS). Even so, as mentioned before, the EPSF/8P(NTMS) still failed to pass V-0 rating with the mass fraction of the adhesive as high as 57%. EPSF with phosphorous-containing adhesives have higher fire safety: PHRR of EPSF/8P(NTMS-PA) decreased to 261 kW/m2 (-62.1 %), and the corresponding value further decreased to 215 kW/m2 (-68.8 %) for EPSF/8P(NTMS-POA). Moreover, THR of FRs-EPSF decreased from 29 MJ/m2 for EPS to 22 MJ/m2 (-24.1 %) of EPS/8P(NTMS-PA), 20 MJ/m2 (-31.0 %) of EPS/8P(NTMS) and 19 MJ/m2 (-34.5 %) of EPS/8P(NTMS-POA), respectively. FIGRA is composite parameter to assess the fire risk, and the lower the value, the higher fire safety of a material.48 About a half cut of FIGRA was achieved by NTMS-PA and NTMS-POA adhesives. These results implied that a protection char formed by the Si-O-Si was not competent enough, which was indicated by the residue of 13.5 % for EPSF/8P(NTMS). Under the assistant of phosphorous-containing acid, more stable and more compact char residues could form by crosslinking, and were reflected by growth of the residues.

Figure 6. HRR (a) and THR (b) curves of EPSF, EPSF/8P(NTMS), EPSF/8P(NTMS-PA) and EPSF/8P(NTMS-POA) under a heat flux of 50 kW/m2. ACS Paragon Plus Environment

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Table 3. Characteristic parameters of cone calorimetry results of EPSF, EPSF/8P(NTMS), EPSF/8P(NTMS-PA) and EPSF/8P(NTMS-POA). TTI

PHRR

Time to PHRR

(s)

(kW/m2)

(s)

EPSF

4

689

45

29

15.3

0

EPSF/8P(NTMS)

5

423

40

22

10.6

13.5

EPSF/8P(NTMS-PA)

6

261

40

20

6.5

36.2

EPSF/8P(NTMS-POA)

6

215

20

19

10.8

34.0

Sample

THR

FIGRA

(MJ/m2) (kW/s·m2)

Residue (%)

Figure 7 showed the digital photos of the residues of the samples after the CC tests. For the nonflame-retardant EPSF, no residue could be recognized (Figure 7a). For the EPSF/8P(NTMS), a discontinuous

and

broken

char

left

(Figure

7b).

For

EPSF/8P(NTMS-PA)

and

EPSF/8P(NTMS-POA) foams, continuous and compact char residues could be seen (Figure 7c and d). These results could be attributed to the polyphosphoric acid and pyrophosphatic acid which formed during combustion and played a role of charring crosslinkers.



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Figure 7. Digital photographs of char residues after cone calorimetry tests: EPSF (a), EPSF/8P(NTMS) (b), EPSF/8P(NTMS-PA) (c) and EPSF/8P(NTMS-POA) (d).

3.5. Analysis of the protective char residues. An effective protection offered by the char layers will improve the fire retardancy of materials during combustion. Thus, it is necessary to analyze char layers for further understanding the mechanism of flame retardant.49,50 To better understand the microstructure and function of the protective chars, the char residues after cone calorimeter tests were systematically investigated by combination of SEM photographs (Figure 8), EDX spectra (Figure 9), FT-IR spectra (Figure 10), and Raman spectra (Figure 11). In Figure 8, SEM photographs of the char residues of EPSF/8P(NTMS-PA) and EPSF/8P(NTMS-POA) after cone calorimeter tests were shown. Figure 8 a1 and a2 showed the char layer of EPSF/8P(NTMS-PA) were not compact, and there were some big hole in the inter surface of the chars. Thus, char layers of EPSF/8P(NTMS-PA) could not provide an effective barrier for underlying material during combustion. Comparing with the char ACS Paragon Plus Environment

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morphologies of EPSF/8P(NTMS-PA), more compact microstructure at the outer and inner surfaces formed for EPSF/8P(NTMS-POA), and also less and smaller holes (Figure 8 b1 and b2). This more compact char layer impeded both mass and heat transfer and were responsible for the better flame retardancy of P(NTMS-POA).

Figure 8. SEM images of the char residues after cone calorimetry tests for EPSF/8P(NTMS-PA) (a1, outer surface; a2, inter surface), EPSF/8P(NTMS-POA) (b1, outer surface; b2, inter surface).

EDX spectroscopy is used to determine the elements on the surface of char residues, and the detailed results of EPS/8P(NTMS), EPS/8P(NTMS-PA) and EPS/8P(NTMS-POA) were showed in Figure 9. For EPS/8P(NTMS), only silicon and oxygen elements were found and no carbon element, suggesting P(NTMS) adhesive did not promote a protecting char layer in burning and the EPS matrix was burned out. Carbon, oxygen, silicon and phosphorus elements could be distinguished on the residues of EPS/8P(NTMS-PA) and EPS/8P(NTMS-POA), indicating complex ACS Paragon Plus Environment


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char residues formed.

Figure 9. EDX spectra of EPS/8P(NTMS), EPS/8P(NTMS-PA) and EPS/8P(NTMS-POA) char residues after the cone test.

The chemical structures in the residues were further investigated by FTIR. In Figure 10, the absorption peaks at 1070 cm-1 were assigned to the stretching vibration of Si-O-Si (Figure 10a), and absorption peaks at 1631 cm-1 were assigned to the stretching vibration of C=C bond (Figure 10a). The weak absorption for C=C indicates that the EPS matrix in the EPSF/8P(NTMS) decomposed completely and did not involve in the formation of char residues, what left are decomposition products of P(NTMS). For the foams with phosphorous-containing adhesives, the peaks at 1007 cm-1 and 1100 cm-1 were assigned to Si-O-Si bond and P-O-P bond (Figure 10b). In addition, the new peaks at ca. 1200 cm-1 were assigned to P-O-C structure,51 suggesting more carbon were brought into char residues with such as bonds. This result indicated that phosphorus compounds generated polyphosphoric acid which catalyzed the char formation and protected the


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foam matrix from the both oxygen and heat transfer. This protective layer acted as a barrier to the release of fuel gases from the surface of EPS matrix.52

Figure 10. FT-IR spectra of char residues after cone calorimeter tests: EPSF/8P(NTMS) (a), EPSF/8P(NTMS-PA) (b) and EPSF/8P(NTMS-POA) (c).

Raman spectroscopy provides a suitable method to characterize the different carbonaceous types after combustion.53,54 The corresponding spectra of the residue chars of EPSF/8P(NTMS-PA) and EPSF/8P(NTMS-POA) after CC tests were shown in Figure 11. The spectra exhibited two broad and strongly overlapped peaks with intensity maxima at ca. 1580 cm-1 and 1360 cm-1. The former band (G band) corresponded to the stretching vibration mode with E2g symmetry in the aromatic layers of the graphite crystalline, whereas the latter one (the D band) represented disorder graphite or glassy carbon.55 The relative ratio of the integrated intensities of D band and G band (ID/IG) was inversely proportional to an in-plane microcrystalline size.56 As the figure showed, each spectrum was fitted into 2 Gaussian bands. The bigger the ratio of ID/IG was, the smaller size of carbonaceous microstructures was, which meant better flame retardancy.57,58 In Figure 11, ID/IG ratio of ACS Paragon Plus Environment


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EPSF/8P(NTMS-POA) (2.48) was slightly bigger than that of EPSF/8P(NTMS-PA) (2.39), indicating P(NTMS-POA) promoted more integrate chars during the combustion of the EPSF. This result supported the data listed in Table 3.

Figure 11. Raman spectra of the char residues of EPSF/8P (NTMS-PA) (a) and EPSF/8P(NTMS-POA) (b) after cone calorimeter tests.

3.6. Mechanical properties of FRAs-EPSF Compressive strength of a polymeric foam closely relates to its application ranges. These data of the foams studied in this work were recorded at a 10% compressive deformation and the resultant strengths were listed in Table 4. The compressive strength of the non-flame-retardant EPSF was 95 kPa. This value increased to 135 kPa (+42 %) for EPSF/8P(NTMS) and 166 kPa +74 %) for EPSF/8P(NTMS-PA) and 156 kPa (+64 %) for EPSF/8P(NTMS-POA), respectively. Due to the closed-cell structure of EPS, FRAs only coated in the surface of EPS beads.15 These FRAs became harder with the volatilization of solution and contributed to higher compressive strengths of the


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flame retarded EPSFs. Table 4. Mechanical properties of EPSFs. Sample

Compressive strength (kPa)

Density (g/cm3)

EPSF

95 ± 25

0.044 ± 0.001

EPSF/8P(NTMS)

135 ± 17

0.044 ± 0.001

EPSF/8P(NTMS-PA)

166 ± 13

0.044 ± 0.001

EPSF/8P(NTMS-POA)

156 ± 24

0.044 ± 0.001

4. CONCLUSION In this work, three novel flame retardant adhesives were synthesized by sol-gel method. TGA results showed that all flame retardant adhesives could form more than 49% residue over 700 oC. The results of LOI, UL-94 tests and cone calorimetry tests indicated that the flame retardant adhesives improve the fire safety of EPSF, especially the phosphorous-containing adhesives. The SEM, EDX, FTIR and Raman results indicated that the three FRAs mainly played their flame retardant roles in the condensed phase. A Si-O-Si network of P(NTMS) could suppress the combustion of EPSF to some extent, and the synergistic effect of silicon, nitrogen and phosphorus provided by P(NTMS-PA) and P(NTMS-POA) could further improve the foams charring capability and hence endow EPSF better flame retardancy.

AUTHOR INFORMATION Corresponding Authors * Tel. & Fax: +86-28-85410755; E-mail: [email protected] (Y. Wang); [email protected] ACS Paragon Plus Environment


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(W. Liao)

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Grants 51320105011 and 51121001) and Program for Changjiang Scholars, Innovative Research Team in University (IRT. 1026) and Sichuan Province Youth Science and Technology Innovation Team (No. 2017TD0006).

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82x44mm (300 x 300 DPI)


Highly Flame Retardant Expanded Polystyrene Foams from

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