and Synergistic Effect with Intumescent Flame ... - ACS Publications

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Facile Preparation of Nickel Phosphide (Ni12P5) and Synergistic Effect with Intumescent Flame Retardants in Ethylene−Vinyl Acetate Copolymer Keqing Zhou,† Bibo Wang,† Saihua Jiang,†,‡ Haixia Yuan,† Lei Song,*,† and Yuan Hu*,†,‡ †

State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, People’s Republic of China ‡ Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute for Advanced Study, University of Science and Technology of China, 166 Ren’ai Road, Suzhou, Jiangsu 215123, People’s Republic of China S Supporting Information *

ABSTRACT: In this paper, nanoporous nickel phosphide (Ni12P5) was synthesized by using a novel hydrothermal method, and its structure was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Then it was used as a synergistic agent with intumescent flame retardant (IFR) in the ethylene−vinyl acetate (EVA) copolymer. With the addition of 2 wt % Ni12P5 and 28 wt % IFR, the LOI value increased from 30.5 to 34.5. The UL-94 test showed that EVA with 25 wt % IFR burned and had no rating, but with the addition of 2 wt % Ni12P5 and 23 wt % IFR, it could reach V-0 rating. The CCT results revealed that the Ni12P5 and IFR system could result in excellent flame retardance. The TGA data indicate that Ni12P5 can increase the thermal degradation temperature and the charred residues after burning. Moreover, the mechanical and electrical properties of EVA composites are also investigated. and gas source, which provide polymer flame retardancy by changing the mode of decomposition into a condensed phase mechanism. A typical and widely studied IFR system is composed of ammonium polyphosphate (APP), pentaerythritol (PER), and melamine (MEL). The IFR technique has emerged as a promising method to afford flame retardancy upon polymers,10−12 its advantages including low smoke and toxic gas production during burning and an excellent antidripping property. However, intumescent flame retardants also have some drawbacks compared with halogen-containing flame retardants13,14 such as low flame retardant efficiency, low thermal stability, inevitable migration to the surface, and water solubility problems. The high loading needed in IFR is still a difficult problem to avoid; the high loading will harm the physical and mechanical properties of polymer composites. To reduce the fraction of IFR in polymer composites, some synergistic agents, such as zeolites,15 fumed silica,16 some transition metal oxides (MO), metal compounds,17−19 and organic boron siloxane,20,21 have been reported. It has been reported that these synergistic agents can effectively improve the mechanical strengths and thermal stability of char residues through catalytic reactions among the components of the IFR system.17,18,22 In our group, we also investigated the synergistic effect of the fumed silica in the PBS/IFR system16 and the synergistic effect of the nickel compounds such as nickel phosphates (VSB-1) and NaNiP in the PP/IFR and TPU/IFR systems.23,24

1. INTRODUCTION Ethylene−vinyl acetate copolymer (EVA) is readily available in the form of rubber, thermoplastic elastomers, and plastic and is widely used in the wire and cable industry due to its desirable physicochemical properties and its easy acceptance of additives.1 However, the limiting oxygen index (LOI) value of EVA is only 17%,2 so EVA is easily flammable and emits a large amount of smoke while burning. The ease of ignition and subsequent flaming combustion with the release of large volumes of toxic smoke restricted the application of EVA-based materials in high-temperature service environments. A feasible solution to this problem is the addition of flame retardant additives, which may improve the fire safety of EVA-based materials. Many researchers have been interested in the flame retardancy of EVA due to its easy flammability and low LOI.2,4,7 In the past decades, halogenated compounds were commonly used as flame retardants to improve the fire resistance of EVA. Unfortunately, their fire retardant action is accompanied by negative effects, such as the generation of corrosive, toxic smoke that is a threat to the environment and human life and health. Therefore, these flame retardants have been prohibited in consideration of environmental and life safety. Metal hydroxides, such as aluminum hydroxide and magnesium hydroxide, are other kinds of flame retardant additives that have a higher frequency of utilization in EVA. However, high loadings are needed to obtain the same flame retardancy when compared to halogen-containing flame retardants, resulting in destruction to the mechanical properties obviously.3 In recent years, intumescent flame retardant (IFR) additives have been widely utilized in the flame retardation of flammable polymers4−9 because of their low toxicity and smoke. IFR systems usually consist of three parts, acid source, char source, © 2013 American Chemical Society

Received: Revised: Accepted: Published: 6303

September 12, 2012 February 28, 2013 April 18, 2013 April 18, 2013 dx.doi.org/10.1021/ie3024559 | Ind. Eng. Chem. Res. 2013, 52, 6303−6310

Industrial & Engineering Chemistry Research

Article

Table 1. Formulations of the EVA and the EVA Composites and the Related UL-94 and LOI Test Results sample

EVA (%)

MCAPP (%)

PER (%)

Ni12P5 (%)

LOI

UL-94

dripping

1 2 3 4 5 6 7 8 9 10

100 70 70 70 70 70 70 70 75 75

0 22.5 20 24 22.125 21.75 21 18.75 18.75 17.25

0 7.5 10 6 7.375 7.25 7 6.25 6.25 5.75

0 0 0 0 0.5 1.0 2.0 5.0 0 2.0

18.5 30.5 29.0 30.0 31.5 32.5 34.5 30.0 26.0 29.5

no rating V-0 V-0 V-2 V-0 V-0 V-0 V-1 V-2 V-0

yes no no yes no no no no yes no

thermal method. In a typical experiment, after 1.425 g of NiCl2·6H2O had been dissolved with deionized water, 0.84 g of urea and 0.3 g of PVP were added to the solution under vigorous stirring until the solution changed to green transparent, and then 1.5 g of Red phosphorus was added to the solution. All of the above steps were carried out under vigorous stirring. The mixture was further stirred for 20 min. The final mixture was sealed in Teflon-lined stainless steel autoclaves at 180 °C for 24 h under autogenous pressure. Then the autoclaves were removed from the oven and cooled at room temperature. The obtained precipitation was then filtered off and washed with abundant deionized water at room temperature and dried at 100 °C for 4 h. 2.3. Preparation of Samples. The samples are listed in Table 1. All of the materials were dried in an oven at 80 °C for 12 h before use. They were melted and mixed in a twin-roller mixer (Kechuang Machinery Co., Ltd., Shanghai, China) for 10 min. The temperature of the mixer was maintained at 140 °C, and the roller speed was 100 rpm. The composites were pressed into sheets in a 1.00 MN semiautomatic molding press (HPC-100D, Xima Weida Machinery Co., Ltd., Shanghai, China). The resulting composites were hot-pressed into sheets of suitable thickness and size for LOI and UL-94 tests. 2.4. Characterization. X-ray Diffraction (XRD) Analysis. XRD patterns were performed with a Japan Rigaku D/Max-Ra rotating anode X-ray diffractometer equipped with a Cu Kα tube and Ni filter (λ = 0.1542 nm). Transmission Electron Microscopy (TEM). TEM images were obtained on a Jeol JEM-100SX transmission electron microscope with an acceleration voltage of 100 kV. Specimens for the TEM measurements were obtained by placing a drop of sample suspension prepared by ultrasonic dispersion on a carbon-coated copper grid and dried at room temperature. Scanning Electron Microscopy (SEM). The scanning electron microscopy (SEM) image of the char residue after the cone calorimetry experiment was taken using a DXS-10 scanning electron microscope produced by Shanghai Electron Optical Technology Institute. The char was adhibitted on the copper plate, and then coated with gold/palladium alloy ready for imaging. Limiting Oxygen Index (LOI). LOI was measured using an HC-2 oxygen index meter (Jiang Ning Analysis Instrument Co., China) on sheets 100 × 6.7 × 3 mm3 according to the standard oxygen index test ASTM D2863-77. UL-94 Vertical Burning Test. The vertical burning test was conducted by a CZF-II horizontal and vertical burning tester (Jiang Ning Analysis Instrument Co., China). The specimens used were 127 × 12.7 × 3 mm3 according to UL-94 test ASTM D3801-1996 standard.

Inspired by the effectiveness of nickel and phosphorus, a novel compound containing nickel and phosphorus (Ni12P5) was prepared and used as synergistic agent to improve the flame retardancy of an EVA/IFR composite. As far as we know, nickel phosphide had been synthesized for the environmental treatments and photocatalytic degradation organic dyes.25−27 Ni et al. have reported the synthesis of porous Ni12P5 superstructures through an improved hydrothermal synthesis strategy, and the as-prepared porous Ni12P5 superstructures showed good adsorption capacities for Pb2+ and Cd2+ ions in water and also could strongly degrade some organic dyes under irradiation of UV light.25,26 Li et al. reported the successful synthesis of Ni12P5 hollow spheres via a facile hydrothermal route, and the as-prepared Ni12P5 hollow spheres could photocatalytically degrade some organic dyes such as Safranine T and Pyronine B under irradiation of UV light.27 To the best of our knowledge, there are no reports about using nickel phosphide to improve the flame retardancy of the polymer. In the present work, porous nickel phosphide (Ni12P5) was prepared by using a novel hydrothermal method, and then its synergistic flame retardancy in an EVA/IFR matrix was studied. The preparation method of Ni12P5 is simple with high yield and friendly to the environment. In the present study, we mainly use Ni12P5 as a synergistic agent to flame-retard EVA, along with an IFR, which is composed of PER and silica-gel microencapsulated ammonium polyphosphate (MCAPP). In our previous work, MCAPP with silica gel resin with an in situ polymerization method was investigated and reported.28 The results suggested that MCAPP can significantly increase the water resistance of flame retardant composites and the interfacial adhesion between APP particles and the polymer matrix. The flame retardant properties, thermal stabilities, mechanical properties, and electrical properties of EVA composites are systematically investigated. Through the research we can provide a more promising way to design new efficient intumescent materials that can further reduce the addition of flame retardants.

2. EXPERIMENTAL SECTION 2.1. Material. Nickel chloride hexahydrate (≥98 wt %), urea, Red phosphorus, and polyethylene pyrrole (PVP) were bought from Sinopharm Chemical Reagent Co., Ltd., China. APP and PER were kindly provided by KeYan Co. (Hefei, China). Silica gel MCAPP was synthesized in our laboratory according to the reported method.28 EVA copolymer containing 28 wt % vinyl acetate was supplied by Samsung Total Petrochemical (Korea). 2.2. Preparation of Nickel Phosphide (Ni12P5). Nanocrystalline nickel phosphide was prepared by using a hydro6304

dx.doi.org/10.1021/ie3024559 | Ind. Eng. Chem. Res. 2013, 52, 6303−6310


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Thermogravimetric Analysis (TGA). TGA was carried out using a Q5000IR thermoanalyzer instrument from 50 to 800 °C at a linear heating rate of 10 °C/min under an air flow of 60 mL/min. The mass of sample used was 5−10 mg. Cone Calorimeter Test (CCT). The CCTs were carried out on the cone calorimeter, following the procedures in ISO5660. Square specimens (100 × 100 × 3 mm3) were irradiated at a heat flux of 35 kW/m2, corresponding to a mild fire scenario. Mechanical Properties. The mechanical properties were measured with a universal testing machine (Instron 1185) at temperatures of 25 ± 2 °C. The crosshead speed was 20 mm/ min. Dumbbell-shaped specimens were prepared according to ASTM D412. The tensile strength and elongation at break were recorded. Dynamic Mechanical Analysis (DMA). Dynamic mechanical properties were measured with DMA Q800 (TA, USA). The dynamic storage modulus was determined at a frequency of 10 Hz and a heating rate of 5 °C/min over the range of −80 to 80 °C. The dimensions of the samples were approximately 1 mm thickness, 20 mm length, and 5 mm width. Electric Properties. Volume resistivity of the composites was measured at room temperature by a high-insulation resistance meter (Shanghai Precision and Scientific Instrument Co., China). Square samples with an area of 100 × 100 mm2 were used after they were cut from the molded sheets.

Figure 2. TEM image of Ni12P5.

a significant effect on the flame retardant efficiency of the EVA composites (samples 2−4). Compared with samples 3 and 4, sample 2 has the highest LOI value and can obtain a V-0 rating, which indicated that sample 2 has the best flame retardancy. Therefore, we chose 3/1 as the weight ratio of MCAPP/PER in the following EVA/IFR/Ni12P5 composition. From Table 1, it can be observed that the LOI values increase with the dosage of Ni12P5 increasing at first. The highest LOI value is 34.5% with 2 wt % Ni12P5 and 28 wt % IFR and also passing the V-0 rating. Afterward, when the weight ratio of Ni12P5 increases to 5%, the LOI value decreases to 30.0%, and the UL-94 rating decreases from V-0 to V-1. Therefore, the addition of Ni12P5 can enhance the flame retardancy of EVA/IFR composites. However, the optimal synergistic effect is based on a just-right formulation between IFR and Ni12P5 in EVA. In this paper, 2 wt % Ni12P5 is the best proportion in the EVA/IFR composite; the sample has the best flame retardancy effect compared with the other samples. To further demonstrate the synergistic effect between Ni12P5 and IFR, the total amount of additives was decreased to 25 wt %. It can be found that EVA with 25 wt % IFR has only a V-2 rating, but after the addition of 2 wt % of Ni12P5 and 23 wt % of IFR, it can pass the UL-94 test. At the same time, the LOI value increased from 26.0 to 29.5%. Because only the addition of Ni12P5 could not improve the LOI values and UL-94 rating of EVA, as discussed above, some important conclusions could be drawn: The addition of Ni12P5 could improve the flame retardancy of the EVA/IFR system. With a suitable content of Ni12P5, there exists a synergistic effect between Ni12P5 and the IFR system, and there is an optimal content in this system for the best flame retardancy of EVA. 3.3. CCT of EVA Composites. The cone calorimeter based on the oxygen consumption principle has widely been used to evaluate the flammability characteristics of materials. Although the CCT is on a small scale, the obtained results have been found to correlate well with those obtained from a large-scale fire test and can be used to predict the combustion behavior of materials in a real fire. Several key parameters such as heat release rate (HRR), total heat release (THR), peak heat release rate (PHRR), time to ignition (TTI), time to peak HRR, average mass loss rate (AMLR), and average smoke extinction area (ASEA), which could be employed to evaluate the development, spread, and intensity of fires, can be obtained from cone calorimetry. To identify the role of Ni12P5 in the

3. RESULTS AND DISCUSSION 3.1. Characterization of Ni12P5. The XRD pattern (Figure 1) of Ni12P5 is in agreement with a previous report.27 All peaks

Figure 1. XRD pattern of Ni12P5.

can be indexed as the tetragonal Ni12P5 phase by comparison with JCPDS Card Files No. 74−1381. The strong diffraction peaks indicate that the product has good crystallinity. No other impurity peaks are detected, indicating that the product is pure. The morphology of the as-synthesized product was characterized by TEM. Figure 2 shows a representative TEM image of the product. Many porous structures with a mean diameter of 50 nm can be easily found. 3.2. LOI and UL-94 Tests of EVA Composites. Table 1 lists the composition of the EVA mixture and the results of UL94 and LOI tests. Apparently, the pure EVA composite has no rating along with serious dripping. After the addition of the IFR flame retardants, the EVA/IFR composites demonstrate higher flame retardancy. The weight ratio of the MCAPP and PER has 6305



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that of sample 2. At the same time, it also can be seen that the second peak of the HRR curve of sample 2 is ahead of time compared with sample 7, indicating that the char formed in sample 2 cannot bear the high temperature for a longer time as compared to sample 7. It is possible that the presence of Ni12P5 can promote char formation in the EVA/IFR composites and form a char layer of better quality, which can endure the higher temperatures and protect EVA from decomposing during a fire. The result is in accordance with the TGA results (discussed under Thermal Stability of the EVA Composites), so it can be concluded that there exists a synergistic effect between Ni12P5 and the IFR system. Total Heat Release (THR). Figure 4 presents the THR for all samples. The slope of the THR curve can be assumed as

formation of the intumescent coating during the combustion process of the EVA and EVA/IFR samples, the cone calorimeter has been used. HRR. The HRR curves of EVA and flame retardant EVA composites are shown in Figure 3, and the related THR, PHRR,

Figure 3. Heat release rates versus burning time for EVA and EVA composites.

Table 2. Related Cone Data of EVA and Flame0Retardant EVA Composites time to sample

TTI (s)

PHRR (s)

PHRR (kW/m2)

THR (MJ/m2)

FPI (m2 s/kW)

FGI (kW/sm2)

1 2 7

30 46 51

125 250 240

1333.14 321.16 238.45

94.43 70.33 73.77

0.02 0.14 0.21

10.67 1.28 0.99

Figure 4. Total heat release versus burning time for EVA and EVA composites.

representative of fire spread.29 From Figure 4 it can be seen that the THR is decreased by the IFR system and Ni12P5. It is very clear that the flame spread of samples (samples 2 and 7) has decreased. It also suggests there is a synergistic effect of flame retardancy between Ni12P5 and the IFR system. CO and CO2 Production Rates. Figure 5 shows the CO and CO2 produced from EVA and flame retardant EVA composites under a heat flux of 35 kW/m2. The incomplete combustion of flame retardant composite can be seen in the CO production rate. Compared to pristine EVA, the CO production rate of flame retardant systems is greatly decreased throughout the whole range of fire in the experiments. Furthermore, with the addition of Ni12P5, the CO production rate decreases especially at the high temperature range. It is very interesting that the carbon monoxide production rate is greatly decreased with the addition of Ni12P5. The above phenomena can be illustrated in the following. The presence of Ni12P5 can promote char formation in the EVA/IFR composites. This implies there is more compact char residue formed on the surface of the sample with Ni12P5. The compact char residue can restrain combustible gases, so the released flammable gases can be completely combusted, which leads to the low carbon monoxide production rate. Another reason may be that the carbon monoxide can be oxidized to carbon dioxide under the catalyzing of Ni12P5, which can migrate onto the surface of the sample.29 The CO2 production rate data essentially mirror the HRR data. The CO2 production rates of the flame retardant systems

TTI, and time to peak HRR are recoded in Table 2. It can be seen that the pure EVA resin begins to catch fire when exposed to fire after 30 s and burns out within 170 s after ignition. A very sharp HRR peak appears at the range of 50−170 s with a PHRR of 1333.14 kW/m2. In the case of samples 2 and 7, the time to ignition extended to 46 and 51 s, respectively. The PHRR values are greatly reduced, and the combustion times of the composites are prolonged in comparison with that of neat EVA. For samples 2 and 7, the PHRR values of these two composites are 321.16, 238.45 kW/m2, respectively. The HRR curve of flame retardant EVA composites exhibits two peaks. The first peak is assigned to the development of the intumescent protective char. After the first peak, the HRR curve forms a plateau in some cases, in which the increase in HRR is suppressed because of the presence of the efficient protective char can protect the composites from both the mass and the heat transfer. The second peak is due to the gradual degradation of the protective layer as the sample is continuously exposed to the heat and the formation of a new protective char in some formulations (samples 2 and 7). As can be seen in Figure 3, the addition of 2 wt % Ni12P5 decreases the PHRR value of sample 7 from 321.16 to 238.45 kW/m2 as compared to that of sample 2. Meanwhile, the results of PHRR values are consistent with the results of the LOI test, and this indicates that the flame retardancy of sample 7 is better than 6306



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Figure 5. Carbon monoxide and carbon dioxide versus burning time for EVA and EVA composites.

significantly decrease because the compact char residue prevents combustible gases from being diffused into air, resulting in a complete combustion of the released flammable gases.29 To judge the fire hazard more clearly, the fire performance index (FPI) and the fire growth index (FGI) were selected. The former is defined as the proportion of TTI and the peak HRR. It is reported that there is a certain correlation between the value of FPI of material and the time to flashover. When the value of FPI reduces, the time to flashover will be advanced. Thus, it is generally accepted that the FPI value of a material is smaller and its fire risk is higher. The latter is defined as the proportion of peak HRR and the time to peak HRR. According to a previous paper, the larger the value of FGI, the shorter time it takes to arrive at a high peak HRR and the greater fire hazard the materials have. The comparison between the values of FPI and FGI of the EVA specimens has also been shown in Table 2. Apparently, the fire risk of sample 7 is much smaller than that of samples 1 and 2. The dense degree of the char layer has an intimate connection with the HRR, THR, and smoke produced during a fire. As is well-known to all, the dense char layer can reduce the HRR, THR, and smoke production. To further testify to the effect of the char layer during combustion, the morphologies of the chars of the samples after cone calorimeter testing were studied by SEM. From Figure 6a, many porous structures can be seen on the surface of the char residue of sample 2 due to insufficient char formation or less condensed char during the burning process. This poor char quality cannot effectively prevent the underlying EVA from degradation during combustion, especially at higher temperatures, which may be the primary reason for the poor flame retardancy. However, the

char surface of sample 7 illustrated in Figure 6b is compact and tight. The impact char serving as a barrier can prevent the penetration of oxygen and combustible gases. The char layer can also resist mass and heat transfer and effectively retard the degradation of underlying material, so sample 7 has the lower HRR and CO and CO2 production rates. 3.4. Thermal Stability of the EVA Composites. TGA is one of the commonly utilized techniques for rapid evaluation of the thermal oxidative stability of different materials and also reveals the decomposition of polymers at various temperatures.30 To understand the effects of Ni12P5 on EVA/IFR composites, the thermal oxidative degradation behavior and the amount of residual char obtained from different EVA composites are compared by TGA. TGA and DTG curves of EVA and flame retardant EVA composites under air atmosphere are shown in Figure 7. The initial degradation temperature (the initial degradation temperature is defined as T−5 wt %, where 5 wt % mass loss takes place in our laboratory), the 50 wt % loss temperature (T−50 wt %), the maximum decomposition temperature (Tmax), and the char residue at 800 °C are listed in Table S1 of the Supporting Information. It can be observed that the thermal degradation of pure EVA is composed of two main steps and leaves almost no residue. The maximum weight loss temperatures (Tmax) for the two decomposition steps are 348 and 440 °C, respectively. Compared with pure EVA, the initial degradation temperature (T−5 wt %) of flame retardant EVA composites decreases owing to the formation of ester mixtures (T < 280 °C), decomposition of APP (280 ≤ T ≤ 350 °C), and the formation of char layer.31 The flame retardant EVA composites exhibit an enhanced thermal behavior at temperatures ranging from 350 to 800 °C, mainly because ITR forms an intumescent char layer, which protects the EVA matrix from heat and combustion. Moreover, in the presence of Ni12P5, the thermal stability of the EVA/IFR composite is enhanced significantly (Supporting Information, Table S1); the initial degradation temperature (T−5 wt %), T−50 wt %, and Tmax2 shift to higher temperatures. Compared to the EVA/IFR composite, the initial decomposition temperature of EVA/IFR/Ni12P5 composite increases from 300 to 314 °C, the T−50 wt % and the Tmax2 increase 16 and 27 °C, respectively. Meanwhile, the amount of residue of EVA/IFR/Ni12P5 composite at 800 °C increases from 7.12 to 17.06%, and also the addition of Ni12P5 in IFR obviously reduces the maximum decomposition rate of EVA/ IFR composites, which are obtained easily from the DTG curves. It is possible that the presence of Ni12P5 can promote

Figure 6. SEM images of the char residue after cone calorimeter testing: (a) sample 2; (b) sample 7. 6307



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Figure 7. Thermal stability of EVA and EVA composites measured by TGA under air atmosphere: (a) TGA curves; (b) DTG curves.

respectively. After addition of the flame retardants, both the tensile strength and elongation at break obviously decrease. Whereas the flame retardant loading is 30 wt %, the tensile strength and the elongation at break of sample 2 decreased to 13.2 MPa and 559%, respectively. However, after the addition of a small amount of Ni12P5, the tensile strength and elongation at break are show almost no change in comparison with EVA/ IFR system; it is interesting to find that the tensile strength and elongation at break for sample 5 even increase a little compared with the EVA/IFR system (sample 2), although the mechanical properties are lower than those of pure EVA. Dynamic Mechanical Thermal Analysis. The tan δ and storage modulus of EVA and flame retardant EVA composites are shown in Figure 8. The temperature at maximum of tan δ is usually taken as the glass transition temperature (Tg). In contrast to the pure EVA (−6.5 °C), the Tg of the flame retardant EVA composites shifts to high temperature (Figure 8a). It can be observed that the Tg values of samples 2 and 7 are −2.0 and 3.6 °C, respectively, mainly because the rigid filler (IFR system) limits the mobility of the polymer chains.2 The storage modulus of EVA and flame retardant EVA composites, as a function of temperature, are shown in Figure 8b. Above 0 °C, there are no obvious differences in modulus for the three materials, whereas below 0 °C, the flame retardant EVA composites have higher storage modulus than that of pure EVA. This is because the rigid filler has imparted stiffness behavior to

char formation in the EVA/IFR system, and the dense char layer will provide a good barrier to prevent the transfer of heat and volatiles, resulting in significant improvement of the thermal stability EVA composites during a fire. Therefore, the EVA/IFR/Ni12P5 composite demonstrates better thermal stability than the EVA/IFR composite. Combined with the analysis of LOI, UL-94, cone tests, SEM images, and TGA, a conclusion can be made that there exists a synergistic effect between the IFR and the Ni12P5 at low amount. The appropriate addition of Ni12P5 can improve the flame retardancy and thermal stability of EVA/IFR systems. 3.5. Mechanical Properties. Table 3 shows the tensile strength and elongation at break of EVA and flame retardant Table 3. Tensile Strength, Elongation at Break, and Volume Resistivity of EVA and Flame Retardant EVA Composites sample

tensile strength (MPa)

elongation at break (%)

1 2 5 6 7 8

28.2 13.2 14.6 13.7 13.0 13.1

587 559 569 539 506 487

volume resistivity (Ω·cm) 3.58 3.92 4.60 4.59 6.05 1.22

× × × × × ×

1014 1013 1013 1013 1013 1014

EVA composites. The tensile strength and the elongation at break of the pure EVA (sample 1) are 28.2 MPa and 587%,

Figure 8. Temperature dependence of tan δ (a) and storage modulus (b) of EVA and flame retardant EVA composites. 6308



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the filled EVA composites. Moreover, the storage modulus of sample 7 has a higher value than sample 2. 3.6. Electrical Properties. The most widely used application of the EVA is in the electrical wire and cable industry, and resistivity studies are very important for insulating materials, which is because the most desirable character of an insulator is its ability to resist the leakage of electrical current. The electrical properties of pure EVA and flame retardant EVA composites are listed in Table 3. It can be noted that the volume resistivity of sample 2 decreases significantly compared to the pure EVA. After the addition of 2% Ni12P5, sample 7 shows higher volume resistivity than sample 2. From the results, it can be concluded that the addition of Ni12P5 not only improves the flame retardancy of the EVA composites but also retains the mechanical and electrical properties, even showing some improvements compared with the EVA/IFR system.

ASSOCIATED CONTENT

S Supporting Information *

TGA data of the pure EVA and EVA composites. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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4. CONCLUSION In this study, Ni12P5 was successfully prepared and then added to an EVA/IFR system through a melt blending process as a synergistic agent. TGA results indicate that the addition of this synergist into the EVA/IFR composite can improve the thermal stability and char yields. Moreover, a series of tests were employed to characterize the flame retardancy of the EVA composites. With the addition of 2 wt % Ni12P5, the LOI value of the EVA/IFR system was increased apparently from 30.5 to 34.5, and the system can reach the V-0 rating. The CCT data reveal that the values of HRR, THR, CO, and CO2 of EVA/ IFR/Ni12P5 blends apparently decrease. The incorporation of this synergist into IFR leads to a remarkable influence on charring of EVA formulations as revealed by TGA and cone data. The morphological structure of char residue proves that the addition of Ni12P5 is capable of initiating a compact and homogeneous char on the surface, which turns out to be of most importance for the flame retardant performance. Furthermore, the mechanical, dynamic, and electrical properties of the EVA/IFR/Ni12P5 system were not damaged; even some improvements were obtained in comparison with the EVA/IFR system. In summary, the EVA/IFR/Ni12P5 system developed in this study may be a promising formulation for ITR EVA with excellent properties.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-551-3601664. Fax: +86-551-3601664. E-mail: [email protected] (Y. H.) or [email protected] (L.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financially supported by the National Basic Research Program of China (973 Program) (2012CB719701), the joint fund of NSFC and Guangdong Province (No. U1074001), the Program for the joint fund of Guangdong province and CAS (No.2010A090100017), and the youth innovation fund of USTC. 6309



Article

(19) Wu, N.; Yang, R. J. Effects of metal oxides on intumescent flame-retardant polypropylene. Polym. Adv. Technol. 2011, 22, 495− 501. (20) Ravadits, I.; Tóth, A.; Marosi, G.; Márton, A.; Szép, A. Organosilicon surface layer on polyolefins to achieve improved flame retardancy through an oxygen barrier effect. Polym. Degrad. Stab. 2001, 74, 419−422. (21) Marosi, G.; Márton, A.; Anna, P.; Bertalan, G.; Marosföi, B.; Szép, A. Ceramic precursor in flame retardant systems. Polym. Degrad. Stab. 2002, 77, 259−265. (22) Li, J.; Wilkie, C. A. Improving the thermal stability of polystyrene by Friedel-Crafts chemistry. Polym. Degrad. Stab. 1997, 57, 293−299. (23) Nie, S. B.; Hu, Y.; Song, L.; He, S. Q.; Yang, D. D. Study on a novel and efficient flame retardant synergist−nanoporous nickel phosphates VSB-1 with intumescent flame retardants in polypropylene. Polym. Adv. Technol. 2008, 19, 489−495. (24) Shan, X. Y.; Zhang, P.; Song, L.; Hu, Y.; Lo, S. Compound of nickel phosphate with Ni(OH)(PO4)2− layers and synergistic application with intumescent flame retardants in thermoplastic polyurethane elastomer. Ind. Eng. Chem. Res. 2011, 50, 7201−7209. (25) Ni, Y. H.; Liao, K. M.; Li, J. In situ template route for synthesis of porous Ni12P5 superstructures and their applications in environmental treatments. Cryst. Eng. Comm. 2010, 12, 1568−1575. (26) Ni, Y. H.; Jin, L. N.; Hong, J. M. Phase-controllable synthesis of nanosized nickel phosphides and comparison of photocatalytic degradation ability. Nanoscale 2011, 3, 196−200. (27) Li, J.; Ni, Y. H.; Liao, K. M.; Hong, J. M. Hydrothermal synthesis of Ni12P5 hollow microspheres, characterization and photocatalytic degradation property. J. Colloid Interface Sci. 2009, 332, 231−236. (28) Wang, B. B.; Wang, X. F.; Tang, G.; Shi, Y. Q.; Hu, W. Z.; Lu, H. D.; Song, L.; Hu, Y. Preparation of silane precursor microencapsulated intumescent flame retardant and its enhancement on the properties of ethylene-vinyl acetate copolymer cable. Compos. Sci. Technol. 2012, 72, 1042−1048. (29) Jiao, C. M.; Chen, X. L. Synergistic effects of zinc oxide with layered double hydroxides in EVA/LDH composites. J. Therm. Anal. Calorim. 2009, 98, 813−818. (30) Liu, Y.; Zhao, J.; Deng, C. L.; Chen, L.; Wang, D. Y.; Wang, Y. Z. Flame-retardant effect of sepiolite on an intumescent flameretardant polypropylene system. Ind. Eng. Chem. Res. 2011, 50, 2047− 2054. (31) Camino, G.; Costa, L.; Trossarelli, L.; Costanzi, F.; Pagliari, A. Study of the mechanism of intumescence in fire retardant polymers: Part VIMechanism of ester formation in ammonium polyphosphatepentaerythritol mixtures. Polym. Degrad. Stab. 1985, 12, 213−218.

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