The flame resistance properties of expandable polystyrene foams coated with a cheap and effective barrier layer

Construction and Building Materials 176 (2018) 403–414

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

The flame resistance properties of expandable polystyrene foams coated with a cheap and effective barrier layer Luyao Wang a,b, Cheng Wang a,b, Pingwei Liu a, Zhijiao Jing a, Xuesong Ge a, Yijun Jiang a,⇑ a b

Key Laboratory of Bio-based Materials, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China University of Chinese Academy of Sciences, Beijing 100049, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 A cheap and effective flame retarded

EPS system was developed.  A synergistic effect of flame

resistance between fly ash and aluminum hydroxide was found.  Fly ash enhanced the compactness of char layer with improved flame resistance and smoke suppression properties.  The main flame resistance mechanism was the effective barrier effect of the char layer.

a r t i c l e

i n f o

Article history: Received 25 January 2018 Received in revised form 16 April 2018 Accepted 4 May 2018

Keywords: Expandable polystyrene Phenolic resin Aluminum hydroxide Fly ash Flame resistance Barrier effect

a b s t r a c t A cheap and effective flame retarded EPS system was developed by using a high silica content based fly ash (FA) synergistic with aluminum hydroxide (ATH) in the thermosetting phenolic resin (PF) coating layer. In this study, scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analyses confirmed the formation of a barrier layer which uniformly encapsulated each EPS microsphere. The flammability of EPS-PF/ATH/FA foam with different compositions were characterized by the limited oxygen index (LOI) and UL-94 vertical burning tests, an evident flame retardant effect has been observed with up to 29.6% LOI value and UL-94 V-0 rating. The cone calorimetric analysis (CONE) indicated that the heat release rate (HRR), total heat release (THR) and smoke production rate (SPR) of the resulting EPS-PF/ATH/ FA foam decreased obviously, and the foam maintained structural integrity after combustion. The thermal stability of EPS-PF/ATH/FA foams has been evaluated by the thermogravimetric analysis (TGA). It is obvious that the presence of PF-ATH/FA flame retarded binder has delayed the pyrolysis process of EPS matrix by about 10 °C. From analysis conducted on the combusted residues, it can be inferred that the dominant flame retardant action of EPS-PF/ATH/FA foam occurred in the compact char layer, which acted as an effective fire-proofing and heat shielding barrier in the condensed phase. Ó 2018 Published by Elsevier Ltd.

1. Introduction

⇑ Corresponding author at: No. 189, Songling Road, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China. E-mail addresses: [email protected] (L. Wang), [email protected] (C. Wang), [email protected] (Y. Jiang). https://doi.org/10.1016/j.conbuildmat.2018.05.023 0950-0618/Ó 2018 Published by Elsevier Ltd.

EPS foam has been extensively used in construction, packing materials, marine and automobile due to its appealing features such as excellent thermal insulation properties, moisture resistance, effective buffering, good chemical resistance, the convenience of processing, light weight, low cost, etc [1–3]. However, EPS foam is extremely flammable and difficult to be flame retarded

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because of their chemical compositions and large specific surface areas [4]. It can be easily ignited with a LOI value of only 18.0%, with no UL-94 rating. The pyrolysis of EPS complies with a radical chain mechanism, and volatile products which can act as the fuels are produced with the toxic black smoke during the combustion [5]. Lots of serious fire disasters caused by the poor flame retardation of EPS foam have posed great threats to people’s properties and lives in recent years [6,7]. Therefore, it is imperative to improve the flame resistance properties of EPS foams. Previous efforts have almost exclusively focused on incorporating halogenated flame retardants into polystyrene matrix before expansion by copolymerizing or blending method [8], while the halogenated flame retardants are environmentally hazardous and toxic to the human body [9]. Thus, the use of halogen-free flame retardants to reduce the flammability of EPS foam and fume products has become a crucial part of the application for EPS materials [10,11]. Recently, the coating method with halogen-free flame retardants has become a promising way to protect materials against fire due to the diversity of flame retardants and the simplicity of processing [12,13]. The flame retarded EPS foam is usually carried out by treatment of EPS microspheres adopting halogen-free flame retardants coating technology followed by foaming and molding. Many thermosetting resins have been used as binders, such as amino resin [4], epoxy resin [12], melamine resin [14] and phenolic resin [15] because of their inherent flame resistance as well as their good compatibility with EPS microspheres and inorganic fillers [16]. The phenolic resin is widely used in flame retardants due to its effective heat resistance, self-extinguishing, low smoke, resistant flame penetration properties. Moreover, phenolic resin materials have good mechanical properties, especially with outstanding instantaneous high temperature burning performance [17]. Hydrated mineral fillers, such as aluminum hydroxide and magnesium hydroxide are widely used flame retardants. In terms of aluminum hydroxide, the dominant flame retardant mechanism is the endothermic dehydration reaction which leads to the dilution of combustible gases and the cooling of flame zone [18,19]. However, to effectively reduce the fire hazards, very high loadings is necessary, which leads to a significant deterioration of the mechanical performance [20]. Efficient barrier effect is another crucial flame retardant mechanism [21]. When EPS foams degrade during combustion, a barrier layer is subsequently formed due to the aggregation of hydrated mineral at the surface, and this layer cuts off the supply of flame with flammable gases and protects the underlying polymer from burning [22–24]. Nonetheless, the EPS foams cannot satisfactorily meet the flame retardant requirement with a single addition of hydrated mineral flame retardant [11]. To enhance the flame retardant properties of hydrated mineral filled thermosetting flame retarded binder, different additional halogen-free flame retardants can be used as synergistic agents, such as SiO2, Fe2O3, clay, etc [9]. These fillers used in combination with flame retardants can facilitate the formation of compact residues and subsequently enhance the fire-proofing and barrier effects [23]. Although these fillers are effective to improve the flame retardant system, they are expensive to be used in the application. It is desirable to explore some cheap and effective agents to improve ATH derived flame retardant system. Fly ash generated in thermal power plants during the combustion of coal as a waste or by-product is mainly composed of SiO2 and Al2O3 [25,26]. Most fly ash disposal methods eventually lead to the dumping of ash on open land, and the environmental hazards of fly ash have been fully recognized [26].The comprehensive utilization of fly ash has already been done in agriculture as soil amelioration agent, geopolymers, high value fly ash concrete, mesoporous materials, etc [27], while it has not been received much attention as a potential fire retardant.

The present work aims to investigate the synergistic flame resistance effect of a kind of fly ash on flame retarded EPS-PF/ ATH foam. ATH and FA provided their own action towards flame resistance property improvement of EPS foam. A cheap and effective hybrid flame retardant system has been developed in this work. The combustion performance for EPS foam samples was evaluated, the morphology of combusted residues and the flame retardant mechanism were also characterized and analyzed. 2. Experimental details 2.1. Materials EPS beads, with a granule size of 1.0–1.2 mm, were an industrial product of Qingdao Chemical Co., Ltd, China. The phenolic resin was supplied by Shandong Shengquan Chemical Co., Ltd, China. Aluminum hydroxide was generously supplied by Qingdao Hainuozhongtian Chemical Co., Ltd, China. Class F fly ash obtained from the thermal power plant at Qingdao in China was used as silica source in this investigation, consisting a total silica (40.5%) and aluminum oxide (16.7%) composition of 57.2%. 2.2. Preparation of the flame retarded EPS foam The process to produce flame retarded EPS foam is illustrated in Fig. 1. A stable and compatible inorganic–organic hybrid binder was firstly prepared by mixing thermosetting PF and synergistic fillers (ATH/FA) together. The formulation is summarized in Table 1. At the second stage, the calculated amount of EPS microspheres were mixed with flame retarded binder until a homogeneous physical blocking layer was formed during the process of blending. At the last stage, the EPS beads with a thin physical barrier layer were filled into a molding device to expand under 100 °C steam, and then the resulting EPS foam sample with sufficient rigidity for demolding was taken out and post cured at 50–60 °C for at least 4 h. Finally, the well-squeezed flame retarded EPS foam with honeycomb-like barrier structure was obtained. 2.3. Characterization methods 2.3.1. The limited oxygen index test LOI test was conducted on a CF-3 oxygen index apparatus (Jiangning Analysis Instrument Co., China). The specimens used for the test were 100  1010 mm3 according to ISO4589. 2.3.2. The UL-94 vertical burning test The UL-94 vertical burning test was carried out on a CZF-3 horizontal and vertical flame tester (Jiangning Analysis Instrument Co., China) with specimen dimension of 127  12.7  10 mm3 according to UL-94 ASTM D3801. Five specimens for each sample were tested in the burning measurements. 2.3.3. Butane spray combustion test Jet 800 °C flame with a butane spray gun for 3 min at a constant distance of 5 cm, meanwhile, a digital camera was used to record the phenomena generated during the combustion. The specimens used for the test were 50  50  50 mm3. 2.3.4. Cone calorimeter test An FTT standard cone calorimeter (Fire Testing Technology Limited, UK) was used to evaluate the flammability of samples according to ISO5660. The specimen with a dimension of 100  100  10 mm3 was irradiated with an external heat flux of 50 kW/m2 which represented a moderate fire [11]. The measured flammability parameters included HRR, THR, SPR and mass retention (MR).

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Fig. 1. Scheme of the preparation process for the flame retarded EPS foam.

Table 1 Formulations of EPS foams. Sample codes

Neat EPS EPS-PF EPS-PF/ATH15 EPS-PF/ATH25 EPS-PF/ATH35 EPS-PF/ATH45 EPS-PF/ATH50 EPS-PF/ATH45/FA5 EPS-PF/ATH40/FA10 EPS-PF/ATH35/FA15 EPS-PF/ATH30/FA20 EPS-PF/ATH25/FA25

Components (phr) EPS beads

PF

ATH

FA

50 50 50 50 50 50 50 50 50 50 50 50

0 100 85 75 65 55 50 50 50 50 50 50

0 0 15 25 35 45 50 45 40 35 30 25

0 0 0 0 0 0 0 5 10 15 20 25

The appearance of the combusted char residues after the cone calorimeter tests was observed by the digital camera to assess the compactness and char integrity.

2.3.5. Thermogravimetric analysis The thermal stability of EPS foams with different compositions was revealed with a thermogravimetric analyzer (NETZSCH STA 449 F5 Jupiter, Germany) between 30 and 800 °C at a heating rate of 10 °C/min under 25 mL/min nitrogen (N2) flow rate. The samples mass was about 5 ± 1 mg, and the data including 5% mass loss decomposition temperature (T5%), initial decomposition temperature (TE), the temperature of the maximum mass loss rate (TMAX) and mass of char residue at 800 °C were recorded.

2.3.6. Fourier transform infrared spectroscopy (FTIR) analysis The variation of chemical bonds for EPS foams after exposure to different temperature conditions was identified using a fourier transformation infrared spectrometer Nicolet iN10 under the resolution of 1 cm 1 by using KBr pellets. The combusted residues of EPS foams with different composition in tube furnace (Heating rate = 10 °C/min, nitrogen flowing rate = 25 mL/min) were tested.

2.3.7. Scanning electron microscopy and energy-dispersive X-ray analysis The morphology changes of combusted char residue of EPS foams with different compositions were observed using a scanning electron microscopy instrument (Hitachi S-4800, Japan) with an accelerating voltage of 5 kV at 8.0 mm working distance. Elemental mapping of neat EPS and EPS foams treated with flame retarded binder were carried out using energy-dispersive X-ray spectroscopy (HORIBA 7593-H, Japan). Before observation, the crosssections were achieved by fracturing samples in liquid nitrogen and sputter-coated with Pt. Flat sections and powdered samples were directly sputter-coated with Pt. 2.3.8. Physical property analysis The apparent density of samples was tested by measuring the dimension and mass of the EPS foam samples according to GB/T 6343-2009.The compressive strength of EPS samples was characterized by a CMT6503 microcomputer control electronic universal testing machine (MTS System, Co., Led, China). Samples were cut into rectangular shape with a dimension of 50  50  40 mm3, and tested at a constant strain rate of 4 mm/min until the compressive strain was 15%. The final data reported in the article were based on the average of three replicated experiments at 10% strain

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level. The thermal conductivity coefficient of the foam was measured with DZDR-S thermal conductivity analyzer (Nanjing Dazhan Institute of electromechanical technology, China). The specimen dimensions were 50  50  10 mm3. 3. Results and discussions 3.1. Analysis of the microstructure of EPS microsphere It is known from Fig. 2(a) that after the coating procedure, each EPS microsphere was independently coated with the flame retarded binder, and the barrier layer was relatively uniform. Fig. 2(b) demonstrates the corresponding acquisition area of the elemental mapping for EPS-PF/ATH35/FA15 microsphere surface, the distribution of inorganic fillers on this barrier layer were observed from Fig. 2(c) and (d). The dispersion of FA was measured by pointing out the element Si and the dispersion of ATH and Al2O3 were through element Al, which showed a homogeneous dispersion of both flame retardants on the thermosetting barrier layer surface. This ensured the amelioration in the flame resistance of EPS foam. To further analyze the changes in the micromorphology of neat EPS after the presence of flame retarded binder, the cross sectional morphology of neat EPS and EPS-PF/ATH35/FA15 sample were analyzed by SEM and element mapping methods. According to Fig. 3 (a), there was no filling between neat EPS beads, which was confirmed by the corresponding C element mapping in Fig. 3(b). Therefore, the combustible volatiles beneath the cellular walls of EPS bead could easily escape and burned the flame, then the mass and energy exchange during the combustion were accelerated. After the presence of PF/ATH35/FA15 fame retarded binder, the EPS beads were found to be isolated by a flame retarded barrier layer in Fig. 3(c). Meanwhile, the C and Al element mapping in Fig. 3(d) confirmed the existence of flame retardants between EPS beads, which will serve as an effective barrier layer to hinder the combustible volatiles and energy exchange during burning. This phenomenon could contribute to the enhancement of the flame resistance for EPS foam materials. 3.2. Flammability analysis 3.2.1. LOI and UL-94 vertical burning test Table 2 shows the influence of ATH content on the LOI value and UL-94 rating of EPS foams. We observed that the neat EPS foam

Fig. 2. SEM image (a) elemental mapping area (b) silicon (Si) element mapping (c) and aluminum (Al) element mapping (d) for EPS-PF/ATH35/FA15 microsphere surface.

was extremely flammable, with a low LOI value of 17.3% and no UL-94 rating, meanwhile produced a large amount of smoky and sooty flame in the vertical burning test. After thermosetting PF was introduced, the LOI value of EPS-PF foam increased rapidly to 26.0%. When 25 wt% of PF was substituted by ATH in EPS-PF sample, the LOI reached a value of 27.0%. The endothermic dehydration reaction of ATH which diluted the flammable gas and reduced temperature around the flame zone can be responsible for the improvements in flame resistance [19]. In the UL-94 test, a V-1 rating can be easily obtained by the presence of 15 wt% ATH. The conclusion was drawn that the flammability of the EPS foam can be improved by the formation of a thermosetting PF/ ATH barrier layer. However, V-0 rating could not be achieved with ATH as the single filler. Table 3 shows the synergistic effects of FA with ATH ingredient on the LOI value and UL-94 rating for EPS-PF/ATH/FA foam. As listed in Table 3, the presence of high amount of ATH (EPS-PF/ ATH50) did not highly improve the LOI value (LOI = 27.4%). While by substituting 15 wt% of ATH with FA in EPS-PF/ATH50 sample, the LOI value could increase up to 29.6%. Furthermore, V-0 rating can be easily obtained by the presence of 5 wt% FA, and the vertical flame self-extinguished immediately after the second ignition in the UL-94 test. These results reflected that the flame retardant performance of EPS-PF/ATH foam could act better with the partial substitution of ATH by FA. Namely, the synergistic effect of ATH and FA on flame retarded EPS-PF/ATH/FA foam was confirmed. It is proposed that the remarkable improvements in the flame retardant may due to the effective barrier layer formed during combustion, which prevented the entire decomposition of EPS foam. This aspect will be discussed in the SEM and CONE analysis part. Contrary to the expectations, the LOI value reduced to 27.6% for EPS-PF/ ATH25/FA25 sample, which was slightly higher than that of EPSPF/ATH50 sample. This implied a less efficient barrier effect caused by the high FA substitution level. 3.2.2. Butane spray combustion phenomena The flame retarded EPS foam after coating and molding treatment was composed of many independent honeycomb-like structure units, and each unit was covered by a flame retarded barrier layer. Fig. 4 recorded the combustion phenomena and char residue photos of neat EPS, EPS-PF and EPS-PF/ATH35/FA15, which suffering from the jet of the butane spray gun and LOI test, respectively. From Fig. 4(a) and (d), the neat EPS sample melt strongly, produced a large amount of smoky and sooty flame and almost had no char residue left after combustion. However, from Fig. 4(b–d), the EPSPF and EPS-PF/ATH35/FA15 samples would not melt or shrink during the combustion, only carbonized on the surface, and lots of char residues were generated to encapsulate the sample which can inhibit the production of melt-dripping, reduce smoke and prevent the propagation of flame. The formation of charring layer was due to the compact cross-linked curing structure generated by carbon source-thermosetting PF at the EPS foaming and combustion procedure. More significantly, a type of honeycomb-like charring layer structure was highlighted to exert a great support effect on the EPS foam. Compared to Fig. 4(c), this unique structure would collapse to some extent in the absence of fillers in Fig. 4(b). Therefore, the synergistic fillers could improve the rigidity of the charring layer. The integrated honeycomb-like structure made the EPS foam well protected, inhibiting the spread of oxygen and heat to some extent, thus enhancing the flame resistance properties of the EPS foam materials effectively. 3.2.3. The morphology of combusted char residue To further assess the effects of two fillers on the morphology of char layer structure, the SEM images of char residue surface of EPSPF, EPS-PF/ATH35 and EPS-PF/ATH35/FA15 foams are shown in

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Fig. 3. SEM image (a) and C element mapping (b) for neat EPS sample. SEM image (c) C and Al element mapping (d) for EPS-PF/ATH35/FA15 sample.

Table 2 Flammability test results for the flame retarded EPS-PF/ATH foam. Sample codes

EPS EPS-PF EPS-PF/ATH15 EPS-PF/ATH25 EPS-PF/ATH35 EPS-PF/ATH45 EPS-PF/ATH50 a b

LOI (%)

17.3 26.0 26.8 27.0 27.1 27.3 27.4

Vertical burning Drop

Cotton ignited

t1/t2(s)a

UL-94b

YES NO NO NO NO NO NO

YES NO NO NO NO NO NO

No self-extinction/No self-extinction/13.1/3.2 12.7/2.1 11.8/3.4 12.0/2.0 11.5/2.5

NR NR V-1 V-1 V-1 V-1 V-1

t1: average after-flame time of the first ignition for five individual specimens; t2: average after-flame time of the second ignition for five individual specimens. The category of ratings follows the UL-94 standard.

Table 3 Flammability test results for the flame retarded EPS-PF/ATH/FA foam. Sample codes

LOI (%)

EPS EPS-PF/ATH50 EPS-PF/ATH45/FA5 EPS-PF/ATH40/FA10 EPS-PF/ATH35/FA15 EPS-PF/ATH30/FA20 EPS-PF/ATH25/FA25

17.3 27.4 27.5 28.4 29.6 27.9 27.6

Vertical burning Drop

Cotton ignited

t1/t2(s)

UL-94

YES NO NO NO NO NO NO

YES NO NO NO NO NO NO

No self-extinction/11.5/2.5 9.6/3.0 7.0/0 2.1/0 8.0/0 14.0/0

NR V-1 V-0 V-0 V-0 V-0 V-1

Fig. 5. Obviously, the char layer was successfully generated for EPSPF foam, but this char barrier was not firm enough to effectively inhibit the spread of fire, as the char residue surface of the EPSPF foam was full of irregular voids which could provide exchange channels for combustible volatiles. Consequently, the combustible volatiles beneath the char residue can easily go out of the barrier and fuel the flame. For the EPS-PF/ATH35 foam, the compactness of the char layer was slightly improved by the incorporation of ATH, while the existing voids still attributed to the mediocre performance of the fire resistance. This was consistent with the results

of LOI test, with just 4% improvement in LOI value compared to EPS-PF foam. Meanwhile, the dispersion of ATH seemed to be less homogeneous, as agglomerates were clearly presented in Fig. 5(b). On the contrary, in Fig. 5(c) for EPS-PF/ATH35/FA15 sample, substituting 15% PF by FA in PF/ATH35 flame retarded binder, the char residue surface became compact and smooth, and almost no voids were present. Moreover, a homogeneous distribution of both fillers in the residue was observed, with the absence of any agglomerate of ATH. Therefore, it could be concluded that the addition of FA could have facilitated the thermosetting PF/ATH flame retarded

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Fig. 4. Combustion phenomenon photos of neat EPS (a) EPS-PF (b) EPS-PF/ATH35/FA15 (c) and photos of EPS samples before and after LOI test (d).

Fig. 5. SEM images of the char residue after LOI test: EPS-PF (a) EPS-PF/ATH35 (b) EPS-PF/ATH35/FA15 (c).

binder to form more compact char layer with higher toughness. It is well known that the compact char layer can be functioned as a barrier layer to the materials, as it delays the volatilization of flammable gasses, as well as, it could cut off the mass change and heat transfer between the gaseous flame area and the underlying poly-

mer composites[28]. Namely, the flame resistance of EPS foam materials was enhanced by the formation of a compact and tough char layer, which has already been confirmed by up to 71% improvements in LOI value for EPS-PF/ATH35/FA15 sample compared to neat EPS.

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3.3. Thermal stability analysis TGA was employed to characterize the thermal stability of neat EPS and flame retarded EPS foams with different compositions under N2 atmosphere. The detailed results are recorded in Table 4. According to Fig. 6(a), the neat EPS showed a rapid one-stage decomposition, ranged from 388 to 427.6 °C, and almost had no char residue left after thermal degradation. Compared with T5% of neat EPS (370 °C), the value of EPS-PF sample decreased by a certain extent (222 °C), this was mainly ascribed to the deepening crosslinking reaction of thermosetting phenolic along with water release in this period. The EPS matrix decomposed during the second degradation stage between 392 and 445 °C, and the yield of residue char significantly increased by 32.7%. The presence of flame retarded synergists (ATH/FA) affected the thermal decomposition behavior of PF and EPS. For one hand, inorganic fillers increased the thermal stability of PF in the initial decomposition stage with up to 30% increase of T5% for EPS-PF/ATH25/FA25 sample in comparison with EPS-PF sample. The endothermic dehydration reaction of ATH and high thermal stability of FA (98.3 wt% left at 800 °C) accounted for the improved thermal stability in this period. ATH released a large amount of water (34.4 wt%) between 265 and 289 °C, resulting in the dilution of flammable gas, reduction of flame zone temperature and the major mass loss. On the other hand, the thermal stability of EPS matrix improved slightly by the presence of flame retarded coating layer, TE of EPS-PF/ATH35/ FA15 was around 10 °C above that of neat EPS. From Fig. 6(b) and Table 4, TMAX of EPS-PF, EPS-PF/ATH50 and EPS-PF/ATH35/ FA15 increased by 7.4, 7.2 and 7.7 °C, respectively, compared to that of neat EPS. The shifts of TMAX were due to the thermalinsulation properties of the barrier layer, and the reduction of mass loss rate would probably slow down the degradation of EPS matrix, which had a positive influence on enhancing the flame resistance of EPS matrix. Meanwhile, the amount of char residue at 800 °C was greatly improved by the increasing content of FA, which achieved a maximum of 42.8% for EPS-PF/ATH25/FA25 sample. It should be figured out that the residual mass of flame retarded EPS foam was close to the carbonization degree of PF and original loading level of ATH/FA (except the loss on ignition). This indicated that PF/ATH/FA flame retarded binder may not promote the charring of EPS matrix but generate a stable and intact char layer structure, which acted as an effective barrier layer to reduce the heat and mass transfer during the severe combustion procedure. 3.4. FTIR analysis of combusted char residues FTIR is an effective method to obtain the chemical bond formation of the char residue. Fig. 7 demonstrates the FTIR spectra for the combusted residues of different EPS foams after tube furnace combustion. Combined with Fig. 6, it was believed that the maximum mass loss rate of neat EPS foam occurred at around 410 °C

Fig. 6. TGA (a) and DTG (b) curves of neat EPS and flame retarded EPS foams under N2 atmosphere.

during the whole degradation. The peak at 3025 cm 1 in the corresponding FTIR spectra of this temperature in Fig. 7(a) was attributed to the stretching vibration of @CAH [10], and the peaks occurred at 1600, 1491 and 1452 cm 1 were ascribed to the AC@CA stretching vibrations of benzene ring backbone[29]. In comparison with the FTIR spectra of 230 °C, the peak intensity decreased obviously, implying that the neat EPS was mainly degraded into aromatic monomers, dimers, aromatic heterocyclic groups, and so on. From Fig. 7(b) for the EPS-PF sample, the FTIR spectra of 200 °C and 230 °C are almost the same. The peak at 1028 cm 1 was attributed to the CAH stretching vibration of ACH2OH, and the peak at 754 cm 1 belonged to the adjacent CAH external bending vibration of phenol ring. When it came to 400 °C, the peak intensity of 1028 cm 1 and 754 cm 1 almost dis-

Table 4 TGA data for different EPS foams. Sample codes

Neat EPS EPS-PF ATH EPS-PF/ATH50 FA EPS-PF/ATH45/FA5 EPS-PF/ATH35/FA15 EPS-PF/ATH25/FA25 T

(E)(°C)

a

TON (°C) T5% (°C)

TE (°C)a

370.0 222.0 261.4 255.3 None 272.5 276.5 286.3

388.0 392.4 265.0 396.2 None 395.3 397.7 392.7

: Initial decomposition temperature obtained from extension method.

TMAX (°C)

Char Residue (%) 800 (°C)

410.3 417.7 289.0 417.5 None 414.5 418.0 413.5

None 32.7 65.6 36.7 98.3 39.9 40.9 42.8

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Fig. 7. FTIR spectra for the combusted residues of neat EPS (a) EPS-PF (b) EPS-PF/ATH50 (c) EPS-PF/ATH35/FA15 (d) after tube furnace combustion.

appeared, confirming the –CH2OH group experienced a condensation reaction with the adjacent active hydrogen during further cross-link curing process of thermosetting phenolic resin [30]. Concerning 480 °C for EPS-PF sample, the peaks at 2922 and 2851 cm 1 belonged to the asymmetric and symmetric stretching vibrations of –CH2 disappeared basically, meanwhile, the peak at 1200 cm 1 which was attributed to the stretching vibration of C-O for phenolic hydroxyl group was highlighted. The obtained results illustrated that EPS-PF degraded fully at 480 °C, and no chemical reactions have occurred between PF and EPS matrix. From Fig. 7 (c) and (d), the emerging absorption peaks at 3620, 3526, 3463 and 1026 cm 1 were due to the presence of ATH [31], which conducted the dehydration reaction during 260 to 290 °C. Meanwhile, the absorption peaks at 456 and 1095 cm 1, just as those from commercial silica [32,33], suggesting the presence of silica in FA. When the temperature rose to 410 °C, the FTIR spectra were almost the same with that of EPS-PF, and no new chemical bonds were formed, which indicated that the PF/ATH/FA coating layer had degraded completely before 410 °C, and no chemical reactions have occurred among the components of EPS-PF/ATH/FA system during combustion procedure. Namely, the main decomposition stage of EPS-PF/ATH/FA system was the degradation of EPS matrix. 3.5. CONE analysis Cone calorimetric is a scientific and effective method that can be used to investigate the combustion behavior of EPS foam material and predict the fire intensity in real fire conditions [10]. Fig. 8 illustrates the HRR, THR, MR and the SPR of neat EPS, EPS-PF and EPS-PF/ATH35/FA15 samples, and the detailed parameters are showed in Table 5. It can be seen from Fig. 8(a) that neat EPS

had a high peak HRR (P-HRR) of 299 kW/m2, while the EPS-PF sample had a decreased P-HRR value of 156 kW/m2. Concerning the sample with 35 wt% ATH and 15 wt% FA (EPS-PF/ATH35/FA15), PHRR was further reduced to 120 kW/m2. More importantly, the single shark peak characteristic of neat EPS was evolved into a plateau for EPS-PF/ATH35/FA15 sample, which was generally ascribed to the formation of an effective barrier layer and consequent inhibition of volatile spread and heat transfer during burning [33,34]. It is interesting to note that the ignition time (TTI) of EPS-PF/ ATH35/FA15 sample was 8s, which was slightly ahead of neat EPS (10s). This may correspond to the higher thermal conductivity of EPS-PF/ATH/FA sample than that of neat EPS, which led to a rapid increase of the surface temperature for the sample and decrease of the time to the pyrolysis temperature [35]. Thus, TTI of EPS-PF/ATH35/FA15 sample was reduced. As shown in Fig. 8 (b), the THR value of EPS-PF reached 7.3 MJ/m2, which was higher than the sample with 35 wt% ATH and 15 wt% FA. This reflected a less efficient barrier effect for EPS-PF sample, confirmed by the presence of an earlier second P-HRR (75 s) than that of EPS-PF/ ATH35/FA15 (100 s). The appearance of second P-HRR was relevant to the cracking of the charring layer formed during fire [23]. The burning time of EPS-PF (110 s) and EPS-PF/ATH35/FA15 (120 s) were prolonged compared to neat EPS (85 s). Based on the delayed combustion time and lower THR value for EPS-PF/ATH35/FA15 than EPS-PF, it can propose that the incorporation of synergistic flame retardants (ATH/FA) in PF resin is beneficial to generate more efficient barrier effect, which delays the thermal degradation and prevents the complete decomposition of the EPS matrix. As shown in Fig. 8(c), the mass loss rate of EPS-PF and EPS-PF/ ATH35/FA15 were much lower than that of neat EPS. Neat EPS had a peak mass loss rate (P-MLR) of 0.24 g/s, while after the pres-

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Fig. 8. Cone calorimetric curves of neat EPS, EPS-PF and EPS-PF/ATH35/FA15: HRR (a) THR (b) MR(c) and SPR (d).

Table 5 Cone calorimeter test results of different EPS foams. Sample code

TTI (s)

P-HRR (kW/m2)

FPI (m2s/kW)

THR (MJ/m2)

MR (wt%)

P-MLR (g/s)

P-SPR (m2/s)

TSR (m2/m2)

EPS EPS-PF EPA-PF/ATH35/FA15

10 6 8

299 156 120

0.033 0.038 0.067

8.0 7.3 6.1

0 38 46

0.24 0.15 0.09

0.05 0.025 0.017

95 85 61

ence of PF resin, the value of P-MLR decreased to 0.15 g/s. The PMLR was further reduced to 0.09 g/s after the incorporation of synergistic flame retardants (ATH/FA), which was 62.5% lower than that of neat EPS. The MR of EPS-PF and EPS-PF/ATH35/FA15 remarkably increased to 38% and 46%, respectively, compared to neat EPS with no residues left. Namely, the presence of organic– inorganic hybrid flame retarded barrier layer can effectively increase the char residue for EPS foam. The production of toxic smoke is the most major threat to life during the fire disaster, which can directly cause the death by asphyxiation or inhalation [10]. It is conceivable that smoke suppression is as important as flame resistance improvement. As shown in Fig. 8(d) and Table 5, the peak smoke production rate (P-SPR) and total smoke rate (TSR) of EPS-PF/ATH35/FA15 were 0.017 g/s and 61 m2/m2, which reduced by 66% and 36% in comparison with neat EPS. The reduction of smoke indicated that PF/ATH/FA flame retarded binder can act as the smoke suppression agent, and is helpful for people to escape from the fire. The fire performance index (FPI) used as a flame resistance parameter is defined as the ratio of TTI to PHRR, which can provide an estimation of the spread rate and size of a fire [36]. The higher the FPI value, the smaller the risk of fire. From Table 5, it can be seen that the FPI increases from 0.033 m2s/ kW to 0.038 m2s/kW of EPS-PF sample, and further increases to 0.067 m2s/kW for EPS-PF/ATH35/FA15 sample, demonstrating the

presence of PF/ATH/FA flame retarded binder has positive effect in improving the flame resistance properties of EPS foam, which is consistent with LOI and UL-94 test results. Fig. 9 presents the combusted residues of neat EPS, EPS-PF and EPS-PF/ATH35/FA15 samples after CONE test. It is obvious that neat EPS burned completely with no residues left after combustion, while the EPS-PF sample left a fragmentary and cracked char residue which was defective to effectively protect the internal EPS foam materials, and attributed to the higher HRR and THR values than EPS-PF/ATH35/FA15 sample. Optimal performance was found for EPS-PF/ATH35/FA15 sample with an intact honeycomb-like carbonaceous mass structure, which uniformly covered on the surface of aluminum foil after burning. This well-structured char layer can show an effective barrier property in insulating the heat transfer and permeability of oxygen and other pyrolysis products, thus be responsible for the decrease of the HRR, THR and SPR. 3.6. Compressive strength and thermal conductivity analyses Fig. 10 presents the main physical properties which are important parameters for building insulation wall, including compressive strength and thermal conductivity. It should be figured out that the density of flame retarded EPS foams remained stable at 30 kg/m3. Therefore, the compressive strength and thermal con-

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Fig. 9. Photographs of combusted residue after cone calorimeter test.

Fig. 10. Physical properties of different EPS foams: Compressive strength (a) thermal conductivity coefficient (b) SEM image of ATH (c) SEM image of FA (d).

ductivity for flame retarded EPS with different compositions are comparable. The density, compressive strength and thermal conductivity of the neat EPS were 15 kg/m3, 55.6 kPa and 0.0334 Wm 1K 1, respectively. Compared to that of neat EPS, the compressive strength and thermal conductivity of EPS-PF increased by 40% and 6% as a result of the cross-linked network structure of cured phenolic resin on EPS foam. With respect to EPS-PF/ ATH15 sample, the compressive strength increased up to 109 kPa, and the thermal conductivity reached the minimum value at 0.0340 Wm 1K 1. However, when the ATH content was higher than 25 wt%, deterioration of compressive strength and thermal insulation property were observed, and this was attributed to the poor compatibility of hydrophilic groups covered on the surface of ATH with PF binder hindering the formation of homogeneous coating layer and thus the mechanical and thermal insulation properties were vulnerable [37,38]. Conversely, the compressive strength increased up to 120.1 kPa and the thermal conductivity reduced to 0.0336 Wm 1K 1 with the increased FA substitution level. The reason for this was explained by the morphology difference between irregular aggregation of ATH and FA containing solid spherical-shaped SiO2 with reference to Fig. 10 (c) and (d), and the very small SiO2 beads can be beneficial to

improve the compactness and homogeneity of the matrix material [39]. It should be figured out that the thermal conductivities of different EPS foams were all less than 0.036 Wm 1K 1, which is better than that of ordinary EPS foams (0.039 Wm 1K 1) [40]. It is concluded that the flame resistance of EPS foam can be effectively improved with acceptable mechanical characteristic and thermal insulation property. 3.7. Flame retardant mechanism Based on the above analysis, the flame resistance mechanism of EPS-PF/ATH/FA foam system has been proposed as presented in Fig. 11.The neat EPS after coating and molding treatments was composed of many independent honeycomb-like structure units, and each unit was covered by a flame retarded barrier layer. For EPS-PF/ATH35/FA15 sample, the phenolic resin began to carbonize in the initial char layer, and the endothermic dehydration of ATH released a large amount of water which led to the dilution of combustible gases and reduce the temperature of burning area during combustion. At the same time, the presence of FA increased the propensity towards the formation of compact and tough char layer. Since the honeycomb-like charring structure can remain a certain

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Fig. 11. Illustrative scheme of the flame retarded mechanism for EPS-PF/ATH/FA foam.

extent of rigidity during burning, the foam structure could be kept integrated. Meanwhile, the compact char layer could be functioned as a barrier to delay the volatilization of combustible gases, suppress the release of toxic smoke and isolate the penetration of oxygen in the condensed phase. Consequently, the EPS matrix beneath the compact char layer can be fully protected and hence improve the flame resistance of the EPS foams.

4. Conclusions A cheap and effective flame retardant system was developed by introducing FA to the PF/ATH binder and this flame retardant system was successfully coated onto EPS foam to obtain a good heat insulating material. Importantly, the LOI value of this heat insulating material was increased to 29.6% and V-0 rating can be obtained, with 60% and 66% decrease of P-HRR and P-SPR in cone calorimetric test compared to neat EPS, respectively, which is more excellent than other EPS derived heat insulating materials considering the cost and performance.

Conflict of interest We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. Acknowledgments This work was supported by the laboratory of Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, K.C. Wong Education Foundation, Youth Innovation Promotion Association of CAS, the Young Taishan Scholars Program of Shandong Province [tsqn20161052], the Key Basic Research Pro-

ject of Shandong Natural Science Foundation [ZR201708240147] and the National Natural Science Foundation of China [21433001].

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