Isolated Protective Char Layers by Nanoclay Network: Significantly

Article pubs.acs.org/IECR

Isolated Protective Char Layers by Nanoclay Network: Significantly Improved Flame Retardancy and Mechanical Performance of TPV/MH Composites by Small Amount of Nanoclay Xiaojun Cao, Kai Lu, and Yongjin Li* College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, No. 16 Xuelin Road Hangzhou 310036, China

Downloaded by UNIV OF NEW SOUTH WALES on August 31, 2015 | http://pubs.acs.org Publication Date (Web): July 1, 2015 | doi: 10.1021/acs.iecr.5b01478

S Supporting Information *

ABSTRACT: The synergistic effects of organically modified montmorillonite (o-MMT) and magnesium hydroxide (MH) on the flame retardance and mechanical properties of a thermoplastic vulcanizate (TPV) have been investigated. The incorporation of more than 6 phr o-MMT induces a drastically decreased peak heat release rate and smoke production rate while simultaneously enhancing the mechanical properties of the TPV/MH composites. The improvements in both flame retardance and mechanical properties are attributed to the double role of o-MMT in the composites. On the one hand, the increase in viscosity due to o-MMT suppresses the vigorous bubbling caused by polymer pyrolysis. On the other hand, well-dispersed oMMT serves as a rigid network that prohibits massive shrinkage of both the MH and TPV matrix during burning. Therefore, isolation of the protective chars by the clay network together with MgO is achieved, and such a structure prohibits heat and mass transfer during burning.

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INTRODUCTION Over the past decade, thermoplastic elastomers (TPEs) have been widely used as jacketing materials in the wire and cable industry because of their excellent physical and mechanical properties.1−5 However, such applications have also been strictly limited because the TPEs are usually flammable and release massive amounts of toxic smoke during burning. The addition of flame retardants (FRs) is one of the most effective strategies to fabricate fire-resistant polymer materials.6−10 Halogen-containing chemicals show superior flame retardant efficiency for various polymers, but the combustion of such halogen-containing materials usually results in the emission of toxic gases and heavy smoke. Therefore, significant attention has been paid to the substitution of the halogen FRs by environmental friendly FRs in both academia and industry. Among various types of non-halogen flame retardants, aluminum hydroxide (ATH) and magnesium hydroxide (MH) have attracted the most attention because they are real environmentally friendly additives. At the same time, ATH and MH are also compatible with real industrial applications because of their relatively low costs. However, metal hydroxides usually have much lower flame retardant efficiency than halogen FRs. In order to achieve sufficient flame retardance, a high content of metal hydroxides (mainly ATH and MH) has to be loaded into the polymer matrix, but this deteriorates both the physical properties and processability.11−13 The FR mechanism of MH and ATH has been well-investigated.11 It is generally accepted that the metal hydroxides decompose endothermically and release water at high temperatures during combustion. The metal hydroxides simultaneously are transformed into metal oxides, which act as the protective layer for the inner polymer matrix. Therefore, metal hydroxides exhibit multiple functions to prohibit spreading of the flame.14,15 Numerous efforts have been carried out to reduce the loading © 2015 American Chemical Society

of metal hydroxides in the composites to enhance both the mechanical properties and processability by using synergistic additives. Ye et al.16 reduced the minimal effective content of MH in ethylene−vinyl acetate copolymer (EVA) by adding multiwalled carbon nanotubes. Lewin and Weil17 observed the synergistic effects of melamine with MH on flame retardancy, and they found that melamine reduces the afterglow.17,18 On the other hand, it was reported that the combination of zinc or calcium borates with MH promotes the formation of char19−21 and that transition-metal compounds can reduce the overall filler level.22,23 It was also found that silicone-containing compounds and organo(boro)siloxanes act as ceramic precursors to enhance the FR performance of the final composites.24 Finally, zinc stannate and zinc hydroxystannate were also incorporated into MH composites to enhance the fire resistance of the final materials.25,26 In recent years, layered nanofillers, such as layered double hydroxide (LDH)27−29 and layered silicate30,31 have been extensively studied as new flame retardants for polymers. It was found that the addition of such layered nanofillers reduces the heat release rate significantly as a result of the formation of a protective surface barrier/insulation layer. The protective layer consists of accumulated clay platelets with a small amount of carbonaceous char.32,33 However, to date there have been very few literature reports on the polymer flame retardance performance of the combination of metal hydroxides with layered silicates.34,35,36 Marosfoi et al.34 reported the fire retardancy behaviors of polypropylene (PP)/MH/montmorillonite (MMT) ternary composites. They found that fibrous Received: Revised: Accepted: Published: 6912

April 20, 2015 June 7, 2015 June 22, 2015 June 22, 2015 DOI: 10.1021/acs.iecr.5b01478 Ind. Eng. Chem. Res. 2015, 54, 6912−6921

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Industrial & Engineering Chemistry Research and layered clay platelets show synergistic effects with MH microfillers to enhance the flame retardancy of PP composites. Chang et al.35 found that EVA composites containing 3 wt % organically modified MMT (o-MMT) and 47 wt % ATH exhibited the best elongation and nice fire resistance (limiting oxygen index (LOI) = 28). Masoomi et al.36 showed that MH and o-MMT have synergistic flame retardant effects on the EVA/linear low-density polyethylene blends. However, the mechanism of the synergistic effects of MH and o-MMT for polymer matrixes has not been investigated yet. Very recently, we prepared a novel thermoplastic vulcanizate (TPV) by dynamic vulcanization of blends of two EVAs with different vinyl acetate (VA) contents, ethylene vinyl acetate rubber (EVM) with 50 wt % VA content and EVA with 28 wt % VA content.37 EVA and EVM have good compatibility, and both of them are polar polymers. The TPV obtained from EVA/EVM exhibits excellent mechanical properties and very nice chemical resistance.37 Such TPV is expected to be a good candidate for cable jacketing materials. We have applied MH and an intumescent flame retardant to fabricate non-halogen flame retardant TPVs, but the intumescent flame retardant contained nitrogen and phosphorus.15 In this work, we have investigated the effects of o-MMT on the thermal stability, flammability, and mechanical properties of TPV filled with MH to fabricate highperformance halogen-free elastomers. Significant synergistic effects of MH and o-MMT with respect to both mechanical properties and flame retardance performance were observed. A novel flame retardant mechanism is proposed on the basis of the detailed residual char analysis. We consider that such a mechanism provides new route for fabricating full environmentally friendly flame retardant materials.

Table 1. Formulations of TPV/MH/MMT Composite Samples sample

TPV (%)

MH (%)

o-MMT (%)

TPV MH100-MMT0 MH98-MMT2 MH94-MMT6 MH90-MMT10

100 50 50 50 50

0 50 49 47 45

0 0 1 3 5

pressed at 180 °C into a sheet with a thickness of 4 mm for further characterization. Measurements. Cone calorimetry tests were carried out using a low-oxygen standard cone calorimeter (Fire Testing Technology, East Grinstead, U.K.) at an incident heat flux of 35 kW/m2 according to the ISO 5660-1 standard. All of the samples (100 mm × 100 mm × 1 mm) were tested on a horizontal sample holder. LOIs were measured using an HC-2 oxygen index meter (Jiangning Analysis Instrument Company, Nanjing, China). The sample size was 120 mm × 10 mm × 4 mm, according to the procedure described in the ASTM D2863 standard. The UL94 test was also carried out following the ASTM standard. Thermogravimetric analysis (TGA) was done on a Q500 thermogravimetric analyzer (TA Instruments, New Castle, DE, USA). The sample was put into an aluminum pan and heated from room temperature to 650 °C at a heating rate of 10 °C/ min under both air and nitrogen atmospheres. The morphology of the char after burning was characterized by field-emission scanning electron microscopy (FESEM) using a S-4800 scanning electron microscope (Hitachi, Tokyo, Japan) at an accelerating voltage of 10 kV. The specimens were coated with a thin gold layer before observation. Rheological characterization of the composites was carried out using an Physica MCR301 rheometer (Anton Paar Instrument, Graz, Austria) at 180 °C. The samples were set within parallel plates with a diameter of 25 mm. Dynamic frequency sweep tests were carried out at a constant 2.5% strain at frequencies ranging from 0.1 to 600 rad/s. The phase structure of the composites was observed by transmission electron microscopy (TEM) using an HT7700 transmission electron microscope (Hitachi, Tokyo, Japan) operating at an acceleration voltage of 80 kV. The samples were microtomed at −80 °C to sections with a thickness of about 70 nm. The sections were then stained using ruthenium tetroxide (RuO4). The char layers after burning were also observed using the same TEM instruments. The char layers were first embedded in epoxy resin and then microtomed to sections with a thickness of about 70 nm at room temperature. Tensile tests were carried out according to the ASTM D 41280 test method using dumbbell-shaped samples punched out from the molded sheets. The tests were performed using a tensile testing machine (model 5966, Instron, Norwood, MA, USA) at a crosshead speed of 20 mm/min at 20 °C and 50% relative humidity. Five specimens were tested for each sample.

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EXPERIMENTAL SECTION Materials. The EVM sample used in this study was Levapren 500 (VA = 50 wt %, Lanxess Deutschland GmbH, Langenfeld, Germany). The EVA sample was ELVAX 260 (VA = 28 wt %, DuPont Co., Wilmington, DE, USA). The admixture of dicumyl peroxide (DCP) was produced by Shanghai Fangreda Chemical Co., Ltd. (Shanghai, China). All of the polymers and DCP were dried in a vacuum oven at 40 °C for at least 12 h before use. Fine precipitated MH with a particle size of 0.3−0.7 μm was supplied by Albemarle Corporation Co., Ltd. (Baton Rouge, LA, USA). The clay Cloisite 30B was purchased from Southern Clay Products (Gonzales, TX, USA). The MH and clay were dried at 110 °C for 12 h in a vacuum oven. Sample Preparation. The EVA/EVM TPVs were prepared according to the previously reported strategy.37 Simply, TPVs were prepared by dynamic vulcanization of an EVA/EVM (50/ 50 w/w) blend in a Haake batch mixer. The dynamic vulcanization was carried out at 160 °C with a screw rotation speed of 80 rpm. MH and o-MMT were then melt-mixed with the prepared TPVs using the same mixer under the same processing conditions. The detailed formulations of the TPV/ MH/o-MMT composites are listed in Table 1. The composites are denoted as MHx-MMTy, where x and y denote the percentages of MH and o-MMT, respectively, in the composite. We fixed the total MH/o-MMT loading at 50 wt % for all of the samples while varying the MH to o-MMT ratio. The loading content was set to be 50 wt % because such loading is usually not enough for flame retardant polymer composites using MH only. The obtained composites were then hot-

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RESULTS Burning Behavior. Cone calorimetry testing (CCT) is a useful tool to evaluate the fire behavior of polymer materials.38−43 The heat release rate (HRR) curves of neat TPV, the binary TPV/MH composite, and ternary TPV/MH/ o-MMT composites during CCT at a heat flux of 35 kW/m2 are shown in Figure 1. Neat TPV exhibits a drastically 6913

DOI: 10.1021/acs.iecr.5b01478 Ind. Eng. Chem. Res. 2015, 54, 6912−6921

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Industrial & Engineering Chemistry Research


Figure 1. Heat release rate (HRR) curves of neat TPV and TPV/MH/ MMT composites during burning.

increasing HRR, and the peak HRR (PHRR) appears shortly after ignition, followed by a sharp decrease in the HRR at the end of the burning. This is typical “thin noncharring” burning behavior, similar to that of most flammable polymers. Compared with neat TPV, the MH100-MMT0 sample shows significantly decreased PHRR, indicating the significant flame retardant effect of MH on TPV. The PHRR of the MH100MMT0 sample is 324.6 kW/m2, but the value is 708 kW/m2 for the neat TPV. At the same time, it was also found that the ignition time increases with the addition of MH. Substitution of a small amount of MH with o-MMT in the composites leads to further improvement in the flame retardant performance, as evidenced by the decreased PHRR value. The PHRR value of the MH94-MMT6 sample is 76% lower than that of pure TPV and 48% lower than that of the MH100-MMT0 sample. The results indicate that o-MMT shows perfect synergistic effects with MH for flame retardance of the TPV matrix. However, it can also be seen from Figure 1 that the combustion curve of the MH90-MMT10 sample is very similar to that of the MH94MMT6 sample, indicating that an o-MMT loading of 6 parts per hundred rubber (phr) is saturated for the flame retardant properties in the sample. We considered that the o-MMT forms a perfect network in the composites at an o-MMT loading of 6 phr, as discussed in the next section. The detailed burning parameters for all of the samples are listed in Table S1 in the Supporting Information. It is clear that the combination of a small amount of o-MMT with MH induces suppression in the PHRR and prolongs the ignition time, indicating the synergistic effects of o-MMT and MH. However, it should be noted that the total heat release (THR) of the samples remains almost constant with the substitution of MH by o-MMT. This is rational because the total filler loadings are same and the THR is mainly dependent on the organic content exposed for burning. For an excellent non-halogen flame retardant material, the CO2 and CO emissions should be as low as possible during burning. CO2 forms at high temperature when sufficient oxygen is supplied to the fire.44 In contrast, CO is generally generated as a result of incomplete combustion at lower temperatures with insufficient oxygen, and this usually occurs at the early stages of fire development. Figure 2 shows the CO and CO2 releasing rates during combustion of neat TPV and the composite samples. Neat TPV shows drastically increased CO2 and CO emission upon burning, indicating the heavy toxic smoke release from the virgin TPVs. However, the addition of MH leads to significantly decreased CO2 and CO emissions.

Figure 2. (a) CO production (COP) and (b) CO2 production (CO2P) curves for neat TPV and TPV/MH/o-MMT composites during burning.

The lowest CO2 and CO peak values and lowest release rate were achieved for the MH94-MMT6 and MH90-MMT10 samples. The CO2 and CO release behaviors of the MH94MMT6 sample are almost same with those of the MH90MMT10 sample. Figure 3 displays the smoke production rate as a function of burning time for neat TPV and the composite samples. Neat

Figure 3. Smoke production rate (SPR) curves for neat TPV and TPV/MH/0-MMT composites during burning.

TPV shows a very sharp smoke release curve with a peak of 0.15 m2/s. The incorporation of MH decreases the peak value of the smoke production rate significantly. The MH100-MMT0 sample has a peak smoke release rate of 0.08 m2/s, which is only about half of that for neat TPV. The binary incorporation of MH and o-MMT leads to a drastic synergistic effect on the smoke release behavior, as evidenced by the peak smoke release rate of 0.027 m2/s. Obviously, o-MMT and MH play a good role as a barrier to prohibit mass transfer from the inside of the samples to the air during burning, so the decomposed volatile products cannot immigrate to the sample surface and the smoke emission is greatly prohibited. 6914


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Figure 4. Thermal analysis curves for neat TPV and TPV/MH/o-MMT composites: (a) TGA and (b) DTG in N2 atmosphere; (c) TGA and (d) DTG curves in air atmosphere.

(especially in air atmosphere). These results indicate that the ternary TPV/MH/o-MMT composites have better thermal stability than the binary TPV/MH composite at high temperature. It is clear that the presence of o-MMT enhances the char formation and prevents the diffusion (emission) of volatile products during decomposition in the ternary composites. Mechanical Properties. The stress−strain curves of TPV composites are shown in Figure 5 and summarized in Table S4

It should again be noted that both the CO2 and CO emissions and the smoke production reach a limitation when the o-MMT loading in the samples is higher than 6 phr, similar to the heat release rate behavior during burning. Once more, this indicates the saturation of o-MMT in the samples at 6 phr, at which point the o-MMT forms a network, as discussed in the next section. On the other hand, the incorporation of a small amount of o-MMT into the TPV/MH composite leads to slightly increased LOI. The samples with more than 6 phr oMMT can pass the UL-94 V-0 rating, as shown in Table S1 in the Supporting Information. Thermogravimetric Analysis. Figure 4 shows the TGA curves of neat TPV and the TPV/MH/o-MMT composites in nitrogen and air atmospheres. Similar to pure TPV, the MH100-MMT0 sample exhibits a two-stage degradation pattern.15 The first weight loss is attributed to the evolution of acetic acid and the decomposition of MH, while the second stage corresponds to the degradation of polyethylene chains in the TPV matrix.15,45 The two peaks in the derivative thermograms (Figure 4b,d) correspond to the two maximum weight loss rates during degradation. The temperatures of 5 wt % mass loss (T5%), 10 wt % mass loss (T10%), and maximum mass loss (Tmax) along with the residue yields from Figure 4 are summarized in Tables S2 and S3 in the Supporting Information. The incorporation of o-MMT results in acceleration of the degradation process, as evidenced by the decreased T5% for the MH94-MMT6 and MH90-MMT10 samples. This loss may be attributed to thermal degradation of the organics on the clay for the surface modification. However, the maximum decomposition temperatures increase greatly, especially for the first stage of MH94-MMT6 and MH90MMT10 samples in N2 atmosphere and the second stage of TPV/MH/o-MMT samples in air atmosphere. In addition, the residues of the TPV/MH/o-MMT samples are also higher than that of the TPV/MH sample, revealing that the incorporation of o-MMT improves the thermal stability and enhances the formation of char, i.e., the carbonization of the polymer

Figure 5. Stress−strain curves for neat TPV and the TPV/MH/oMMT composites.

in the Supporting Information. Pure TPV has a tensile strength of 12.0 MPa, modulus at 100% of 1.95 MPa, and modulus at 200% of 3.12 MPa. The incorporation of only MH leads to an increased modulus, while the elongation at break decreases drastically. The substitution of a small amount of MH by oMMT leads to obvious enhancements in both the tensile strength and the elongation at break. As shown in Figure 5, when only 2 phr o-MMT is added, the ternary MH98-MMT2 sample exhibits 526% elongation at break and a tensile strength of 15.2 MPa, compared with 460% and 14.5 MPa, respectively, 6915


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for the binary MH100-MMT0 sample. The maximum mechanical performance of the ternary composites was achieved with the incorporation of 6 phr o-MMT. The tensile strength was 16 MPa and the elongation at break was 590%, which are 10% and 28.3% higher, respectively, than those of the MH100-MMT0 sample. Morphology Analysis of Ternary Nanocomposites. We have synthesized halogen-free flame retardant ternary composites with excellent mechanical performance. The morphologies of the ternary composites were characterized to elucidate the structure−property relationships of the composites. Figure 6

Figure 7. Digital photographs of the flame retardant samples after the cone calorimetry test.

testing. It is clear that the aluminum foil was exposed and residual char broke into several pieces after CCT of the MH100-MMT0 sample, indicating that there was not enough flame retardance. In contrast, the char became more compact and continuous with incorporation of even 2 phr o-MMT, and fully compact char was achieved with more than 6 phr o-MMT. Such a compact char structure indicates the excellent flame retardant properties of the samples. In order to elucidate the relationship between the microstructure of charred layer and flame retardance of the sample, we characterized the char morphology after burning by SEM. For the MH100-MMT0 sample, it can be observed that there were many big cracks on the char surface after burning, as shown in Figure 8A,B. Obviously, the MH near the surface layer is transformed into MgO by dehydration during burning. The MgO layer plays a role to protect the polymer substrate from further degradation. However, the transformation process from microsized Mg(OH)2 into nanosized MgO (Figure S1 in the Supporting Information) results in drastic shrinkage of the burning part due to the release of water. The massive shrinkage leads to the formation of a lot of cracks. Obviously, such big cracks provide routes for heat and mass transfer between the polymer under the burning surface and the air. Therefore, the burning can easily extend into the inside of the sample through these cracks, and no high flame retardant performance can be achieved for the sample with only 50 wt % MH addition (the MH100-MMT0 sample). In other words, to achieve adequate flame retardance with the only addition of MH, a higher loading is necessary, but this will inevitablely induce the deterioration of both the mechanical properties and processability. Only a large MH loading leads to full coverage of MgO on the surface to protect the inside from further burning. In contrast to the sample with only MH, few big cracks can be observed on the surface of the MH-MMT samples with increasing o-MMT content after combustion (Figure 8C−H). For the MH90-MMT10 sample, a perfect compact char without cracks was observed after combustion (Figure 8G,H). To make a detailed analysis of the flame retardant mechanism for all of the composites, the residue char layer

Figure 6. TEM images of TPV composites with different loadings of MH and MMT: (A, B) the MH100-MMT0 sample; (C, D) the MH94-MMT6 sample.

shows TEM images of the MH100-MMT0 and MH94-MMT6 samples. It can be observed that the MH particles form many agglomerates in the MH100-MMT0 sample (Figure 6A,B), while the microsized MH particles are well-dispersed and isolated by the exfoliated clay layers in the MH94-MMT6 sample (Figure 6C,D). The distance between the neighboring exfoliated layers is very small, and the clay layers form a kind of network structure embedded in the polymer matrix. Obviously, the formation of the clay network and the well-dispersed MH particles play a key role in the improvements in both the mechanical properties (Figure 5) and flame retardant performance of the ternary TPV/MH/o-MMT composites. Char Residue Analysis. To understand the flame retardant mechanism of TPV/MH/o-MMT ternary composites, the structure of the residue char after burning was investigated. Figure 7 presents photographs of the flame retardant samples with the indicated amounts of o-MMT after cone calorimetric 6916


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Figure 8. SEM images of the outer surface for different samples after burning: (A, B) the MH100-MMT0 sample; (C, D) the MH98-MMT2 sample; (E, F) the MH94-MMT6 sample; (G, H) the MH90-MMT10 sample.

It is clear that the exfoliated clay platelets play important roles in the flame retardant properties. We performed a careful analysis of the residual char of the samples with o-MMT after burning. Liquid epoxy resin with a curing agent was carefully dropped onto the surface of the residual char and then crosslinked by being heated to 150 °C. The structure of the residue was then fixed by the epoxy. The sample was finally microtomed into thin slices and subjected to TEM observation. As we can see from Figure 10, o-MMT platelets form a perfect network in the char residue. On the other hand, MgO particles that originated from the dehydration of MH during burning can also be observed in the char residue. More important, the MgO particles are mainly located in the area of the clay networks. By comparison with the TEM images in Figure 6 before burning, it is easily understood that the clay network stays intact and undergoes almost no change during burning while MH

was carefully trimmed to expose the inner surface of the char layer. The inner surface was characterized by SEM, and the results are displayed in Figure 9. Many air holes from the inner surface of the char residue can be observed for MH100-MMT0 (Figure 9A). The sizes of the holes range from 1 to 20 um. In combination with Figure 8A,B, it should be mentioned that these air holes play the role of transferring the heat sources and the flammable compound between the substrate and the burning surface, together with the broken MgO layer. However, with increasing o-MMT content, no air holes were observed in the inner surface of the char residue for the ternary TPV/MH/ o-MMT samples after burning, as shown in Figure 9B−D. This means that the gallery for transferring the heat, combustible volatiles, and oxygen does not exist. Therefore, higher flame retardant performance is achieved. 6917


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surface. In contrast, a drastic expansion was observed for the MH90-MMT10 sample at 400 °C. Moreover, the burning surface was curved with no cracks, which means that the surface was covered by the full char residue, as evidenced by the SEM image after burning at 500 °C. When we combine these observations with the results in Figure 8, it is rational to attribute the different burning behaviors of the two samples to the significantly higher melt viscosity of the samples with oMMT. Degradation of the polymer matrix and dehydration of MH during burning generate many gas bubbles inside the sample. These bubbles rise and expand to the sample surface when the surrounding polymer melt has a low viscosity, as is the case for the MH100-MMT0 sample. Therefore, we observed many holes in the char inner surface of the sample (Figure 8). Obviously, these holes will transfer the mass and the heat during burning. In contrast, with the addition of o-MMT, the melt viscosity of the sample increases, as shown in Figure 12. The high melt viscosity suppresses the vigorous bubbling and the rising of the formed bubbles. Therefore, no holes were observed, and a significant swelling of the sample was observed. Moreover, the burning surface stays compact, continuous, and dense with no cracks after burning at 500 °C. We also considered that the formation of a clay platelet network in the polymer matrix contributes greatly to the enhancement of the flame retardance of the materials. One can see from Figure 6 and Figure 10 that the exfoliated clay forms a network structure at more than 6 phr loading. Such a network divides the sample into small pieces, so the dimensional shrinkage during burning is confined within the small area surrounded by the clay platelets. Therefore, no big cracks form by the massive shrinkage of the MH-MMT sample. Obviously, the network can form only when the o-MMT loading is high enough (at about 6 phr), as evidenced by the almost constant flame behaviors of MH94-MMT6 and MH90-MMT10 samples. It should be mentioned that the clay platelet network also contributes to the enhancement in the melt viscosity of the sample. On the basis of the above analysis, we propose a schematic burning process for the TPV/MH composites with and without o-MMT, as shown in Figure 13. The temperature of the sample surface increases rapidly upon burning, and the viscosity of the molten sample near the surface decreases accordingly. The sample starts to generate pyrolysis products by degradation when the temperature of the TPV sample becomes sufficiently high. The majority of the degradation products are monomers, cyclic oligomers, and small quantities of gaseous volatiles. These degradation products are superheated and readily nucleate to form bubbles in the molten layer. The bubbles rise quickly and break through the sample surface if the viscosity of the surrounding polymer layer is low, as shown in Figure 13A,B, for the samples without o-MMT. Therefore, many channels (holes) form, and they act as the route to transfer the heat and mass that forms. On the other hand, the polymer layers show much higher melt viscosity with the incorporation of o-MMT, and the bubbles hardly rise and expand to the sample surface, as shown in Figure 13C,D. Additionally, the o-MMT forms a rigid network in the composites, which limits the massive shrinkage of the samples during combustion, leading to a compact and full protective layer covering the whole surface without any cracks.

Figure 9. SEM images of the inner surface of the char layer for different samples after the cone calorimetry test: (A) the MH100MMT0 sample; (B) the MH98-MMT2 sample; (C) the MH94MMT6 sample; (D) the MH90-MMT10 sample.

Figure 10. TEM images of the carbonaceous residue collected at the surface of the MH90-MMT10 sample after the cone calorimetry test.

transformed into MgO. The MMT network divides the matrix into numerous small pieces. Therefore, the shrinkage of each small piece during burning is very tiny, and such shrinkage is confined within the small piece surrounded by the rigid MMT platelets. Therefore, we observed a compact char surface after burning.

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DISCUSSION It is very interesting to find that a small amount of nanoclay improves both the mechanical properties and the flame retardance performance of TPV significantly. Moreover, the results indicate that adequate clay loading content is important for the improvement of the flame retardance. These results are extremely informative for the development of new non-halogen flame retardant polymer materials because both nanoclay and MH are environmentally friendly FR additives. In order to elucidate the synergistic mechanism of nanosized o-MMT and microsized MH, the samples were calcinated in a muffle oven at different temperatures to simulate the burning process. Figure 11 shows the morphologies of samples with and without oMMT after calcination at the indicated temperatures. No expansion was observed for the sample with only MH after calcination at 400 °C. At the same time, many cracks were clearly found on the surface of the calcinated sample. The complete burning at 500 °C leads to big cracks on the sample 6918


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Figure 11. SEM images of the char residues of MH100-MMT0 and MH90-MMT10 after calcination at the indicated temperatures.

Figure 12. Complex viscosity as a function of frequency for neat TPV and the TPV/MH/o-MMT composites.

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CONCLUSION Flame retardant thermoplastic vulcanizate (TPV) composites with magnesium hydroxide (MH) and organically modified montmorillonite (o-MMT) were prepared. The incorporation of a small amount of o-MMT into the TPV/MH composites enhances both the flame retardance and the mechanical properties. A compact and continuous char layer is generated in the ternary TPV/MH/o-MMT composites during burning. Such compact and continuous char residue acts as an effective barrier layer to prohibit transfer of heat, oxygen, and combustible volatiles. Specifically, the incorporation of more than 6 phr o-MMT leads to the lowest pHRR and lowest SPR among all of the samples. TGA showed that the thermal

Figure 13. Schematic diagram of the burning process for the samples with and without o-MMT: (A, B) the burning process for the MH100MMT0 sample without o-MMT; (C, D) the burning process for the MH90-MMT10 sample with a small amount of o-MMT.

stabilities of TPV/MH systems are enhanced by adding the layer-structured nanoparticles (o-MMT). TEM images of the char residue after cone calorimetry tests confirmed that initially well-dispersed clay platelets accumulate on the burning surface and form a continuous netlike structure together with MgO. The simultaneous improvements in both flame retardance and mechanical performance have been attributed to the double 6919


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(13) Li, Z. Z.; Qu, B. J. Flammability characterization and synergistic effects of expandable graphite with magnesium hydroxide in halogenfree flame-retardant EVA blends. Polym. Degrad. Stab. 2003, 81, 401. (14) Rothon, R. N.; Hornsby, P. R. Flame retardant effects of magnesium hydroxide. Polym. Degrad. Stab. 1996, 54, 383. (15) Lu, K.; Cao, X. J.; Liang, Q. S.; Wang, H. T.; Cui, X. W.; Li, Y. J. Formation of a compact protective layer by magnesium hydroxide incorporated with a small amount of intumescent flame retardant: New route to high performance nonhalogen flame retardant TPV. Ind. Eng. Chem. Res. 2014, 53, 8784. (16) Ye, L.; Wu, Q.; Qu, B. Synergistic effects and mechanism of multiwalled carbon nanotubes with magnesium hydroxide in halogenfree flame retardant EVA/MH/MWNT nanocomposites. Polym. Degrad. Stab. 2009, 94, 751. (17) Weil, E. D.; Lewin, M.; Lin, H. S. Enhanced flame retardancy of polypropylene with magnesium hydroxide, melamine and novolac. J. Fire Sci. 1998, 16, 383. (18) Baillet, C.; Delfosse, L. The combustion of polyolefins filled with metallic hydroxides and antimony trioxide. Polym. Degrad. Stab. 1990, 30, 89. (19) Leeuwendal, R.; Shen, K.; Ferm, D. New application developments in halogen-free flame retardant polyolefin and polyamide engineering plastics using fire brake zinc borates. Intersci. Commun. London 2002, 95. (20) Bourbigot, S.; Le-Bras, M.; Leeuwendal, R.; Shen, K. K.; Schubert, D. Recent advances in the use of zinc borates in flame retardancy of EVA. Polym. Degrad. Stab. 1999, 64, 419. (21) Carpentier, F.; Bourbigot, S.; Le Bras, M.; Delobel, R.; Foulon, M. Charring of fire retarded ethylene vinyl acetate copolymer− magnesium hydroxide/zinc borate formulations. Polym. Degrad. Stab. 2000, 69, 83. (22) Lewin, M.; Makoto, E. Catalysis of intumescent flame retardancy of polypropylene by metallic compounds. Polym. Adv. Technol. 2003, 14, 3. (23) Costa, L.; Luda, M. P.; Trossarelli, L. Mechanism of condensed phase action in flame retardants. Synergistic systems based on halogen−metal compounds. Polym. Degrad. Stab. 2000, 68, 67. (24) Mohai, M.; Tóth, A.; Hornsby, P. R.; Cusack, P. A.; Cross, M.; Marosi, G. XPS analysis of zinc hydroxystannate-coated hydrated fillers. Surf. Interface Anal. 2002, 34, 735. (25) Marosi, G.; Márton, A.; Anna, P.; Bertalan, G.; Marosfoi, B.; Szep, A. Ceramic precursor in flame retardant systems. Polym. Degrad. Stab. 2002, 77, 259. (26) Laoutid, F.; Gaudon, P.; Taulemesse, J. M.; Cuesta, J. M.; Velasco, J. I.; Piechaczyk, A. Study of hydromagnesite and magnesium hydroxide based fire retardant systems for ethylene−vinyl acetate containing organo-modified montmorillonite. Polym. Degrad. Stab. 2006, 91, 3074. (27) Wang, D. Y.; Costa, F. R.; Anastasia, V.; Leuteritz, A.; Scheler, U.; Jehnichen, D.; Wagenknecht, U.; Haussler, L.; Heinrich, G. OneStep Synthesis of Organic LDH and Its Comparison with Regeneration and Anion Exchange Method. Chem. Mater. 2009, 21, 4490. (28) Costa, F. R.; Saphiannikova, M.; Wagenknecht, U.; Heinrich, G. Layered double hydroxide based polymer nanocomposites. Adv. Polym. Sci. 2008, 210, 101. (29) Calistor, N.; Everson, K.; Wang, D. Y.; Wilkie, C. A. Flameretarded polystyrene: Investigating chemical interactions between ammonium polyphosphate and Mg/Al layered double hydroxide. Polym. Degrad. Stab. 2008, 93, 1656. (30) Liu, Y.; Wang, J. S.; Deng, C. L.; Wang, D. Y.; Song, Y. P.; Wang, Y. Z. The synergistic flame-retardant effect of MMT on the intumescent flame-retardant PP/CA/APP systems. Polym. Adv. Technol. 2010, 21, 789. (31) Szep, A.; Szabo, A.; Toth, N.; Anna, P.; Marosi, G. Role of montmorillonite in flame retardancy of ethylene−vinyl acetate copolymer. Polym. Degrad. Stab. 2006, 91, 593.

roles of o-MMT. On the one hand, o-MMT increases the viscosity of the composite and suppresses the vigorous bubbling caused by polymer pyrolysis during burning. On the other hand, well-dispersed o-MMT serves as a rigid network in the sample that prohibits the massive shrinkage of both MH and the TPV matrix during burning.

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ASSOCIATED CONTENT

S Supporting Information *

Burning parameters, physical property data, and SEM images showing the change in MH particle size after burning. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b01478.

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AUTHOR INFORMATION


Corresponding Author

*E-mail: [email protected]. Fax: +86 571 28867899. Telephone: +86 571 28867026. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51173036, 21374027, and 51203134) and the Program for New Century Excellent Talents in University (NCET-13-0762).

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REFERENCES

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Isolated Protective Char Layers by Nanoclay Network: Significantly

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