Formation of a Compact Protective Layer by Magnesium Hydroxide

May 9, 2014 - AGT Company, Wujin District, Changzhou 213102, China .... Novel phosphorus–nitrogen–silicon flame retardants and their application i...
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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 Kai Lu,† Xiaojun Cao,† Qiushi Liang,† Hengti Wang,† Xiaowen Cui,‡ and Yongjin Li*,† †

College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, No. 16 Xuelin Road, Hangzhou 310036, China ‡ AGT Company, Wujin District, Changzhou 213102, China S Supporting Information *

ABSTRACT: Magnesium hydroxide (MH) and an intumescent flame retardant (IFR) have been incorporated into a thermoplastic vulcanizate (TPV) for the purpose to fabricate halogen-free flame retardant elastomeric materials. Significant synergistic effects of MH and IFR have been observed for the TPV in terms of both flame retardant properties and mechanical performance. The mechanism of the synergistic effects has been investigated. The results indicate that a small amount of IFR accelerates the degradation of the matrix and induces a shrinkage matrix, leading to a compact and unbroken MgO protective layer. Such compact MgO layer on the surface of the material prevents the bulk material from further degradation, and a high flame retardant performance was achieved. Obviously, this novel flame retardant system paves new possibility for the high performance nonhalogen flame retardant polymeric materials and should also be applied to other polymers. ration of mechanical and processing properties.8,9 Consequently, the combination of MH with other fire retardants or adjuvants has received increasing attention to improve the performance of MH.10−14 Fabien Carpentier et al.10 reported that zinc borate was capable acting as a synergistic agent with MH in ethylene-co-vinyl acetate (EVA) matrix. They found that zinc borate enables one to cushion the degradation of EVA, creating a vitreous protective residual layer that could act as a physical barrier and a glassy cage for polyethylene chains. Laoutid et al.11 found that the partial substitution of both MH and hydromagnesite (HM) by organo-montmorillonite (OMMT) in EVA leads to an improvement in the fire properties with decreased heat release rate peak value and enhanced limiting oxygen index. The more compact char layer was observed by the combination of MH and OMMT after combustion of the composites in comparison with the behavior of EVA/MH or EVA/HM compositions at the same global loading (60 wt %). Ye et al.12 observed the synergistic effects of multiwalled carbon nanotubes (MWNTs) with MH on flame retardant. Although numerous works have reported on the synergistic effects of different flame retardants, no publications appear in the literature concerning the combination of MH and IFR for halogen-free flame retardant materials, to the best of our knowledge. It is generally accepted that the inorganic filler (metal hydroxide) inhibits the expansion of the material, and no synergistic effects can be expected for the combination of metal hydroxide and IFR. However, we considered that the

1. INTRODUCTION Flame retardants (FR) are one of the most important additives for combustible polymers to improve their fire resistance.1−3 The most effective flame retardants are the halogen-containing chemicals, but they usually release harmfully toxic gases as well as heavy smoke during combustion. The substitution of halogen-containing flame retardants attracts great attention, but it is still a significant challenge in both academia and industry. Among the all halogen-free flame retardants, nitrogen−phosphorus-based intumescent flame retardants (IFR) and metal hydroxides are the most widely used additives for achieving flame retardant performance. IFR has been widely applied in industry due to the high flame retardant efficiency, low density, and the good precessability. All intumescent systems generally include three basic ingredients: an acid source, a charring agent, and a blowing agent. Acid source, such as ammonium polyphosphate (APP), can produce acidic species acting as a catalyst at the critical temperature. Charring agent is mainly polyhydric compound, such as pentaerythritol (PER) and its derivatives, which can form carbonaceous materials under acid catalysis. Blowing agent can release gaseous products, which cause the swelling of the char and consequently decreased heat conductivity and enhanced insulation effects during burning.4,5 However, it is reported that IFR releases harmful gases upon the degradation due to containing nitrogen and phosphorus elements. Metal hydroxides, mainly including aluminum hydroxide (ATH) and magnesium hydroxide (MH), are the real environmentalfriendly flame retardant, and they are much preferred in industry due to the very limited ecological impact and relatively low costs.6,7 However, high loading (usually at least 60 wt %) of metal hydroxides is always needed to achieve adequate fire retardance, which inevitably leads to the significant deterio© 2014 American Chemical Society

Received: Revised: Accepted: Published: 8784

February 25, 2014 April 14, 2014 May 9, 2014 May 9, 2014 dx.doi.org/10.1021/ie5008147 | Ind. Eng. Chem. Res. 2014, 53, 8784−8792

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mm thickness. The obtained sheets were used for further characterization and property measurements. 2.3. Measurements. Limiting oxygen index (LOI) was carried out in an HC-2 oxygen index meter (Jiang ning Analysis Instrument Co., China) with sample measurement of 120 × 10 × 4 mm3, following the procedure described in the ASTM D2863 standard. Vertical burning test was measured according to the American National Standard UL-94. All cone data were taken from a low oxygen standard cone calorimeter (manufactured by fire testing technology) at an incident heat flux of 50 kW/m2 according to ISO 5660-1 standard. All samples (100 × 100 × 1 mm3) were laid on a horizontal sample holder. Thermogravimetric analysis (TGA) was carried out (Q 500, TA) at a heating rate of 10 °C/min from room temperature to 650 °C at continuous high-purged nitrogen or air atmosphere. The scanning electron micrographs of the char layers were analyzed by a Hitachi S-4800 emission scanning electron microscope. The specimens were previously coated with a conductive layer of gold. The phase structure of halogen-free flame retardant thermoplastic elastomer was observed by transmission electron microscopy (TEM) (Hitachi HT-7700). The samples were ultramicrotomed at −80 °C to a section with a thickness of about 70 nm. The sections were then stained using ruthenium tetroxide (RuO4). Tensile tests were carried out according to ASTMD 412 on dumbbell-shaped specimens using a universal tensile testing machine (Instron 5966) at a constant cross-head speed of 20 mm/min. The samples were dumbbell-shape punched out from the molded sheets. Five specimens were tested for each sample.

addition of IFR might benefit the formation of compact metal oxide protective layer. Intumescent materials begin to swell and then to expand when the temperature is higher than a critical value. The process leads to a foamed cellular charred layer on the surface, which protects the underlying material from the action of the heat flux or the flame.15−17 The charred layer acts as a physical barrier, which slows heat and mass transfer between gas and condensed phases.18 Metal hydroxides such as magnesium hydroxide (Mg(OH)2) decompose endothermically, and release water at temperatures similar to those at which polymer decomposition takes place. Their flame retardant effects are based on heat sink mechanisms, gas cooling, fuel dilution, and the final physical protective layer of the metal oxide residue. The shrinkable matrix by the IFR will benefit the formation of the compact protective layer, which in turn improves the flame retardant performance by the combination of these two nonhalogen flame retardants. In this article, the burning behaviors and physical properties of a thermoplastic vulcanizate incorporated with both magnesium hydroxide and IFR have been investigated. It was found that MH and IFR show synergistic effects in both the mechanical performance and the flame retardant properties.

2. EXPERIMENTAL SECTION 2.1. Materials. The EVM sample used in this study was Levapren500 (VA = 50 wt %, Lanxess Deutschland GmbH, Germany). The EVA sample was ELVAX 260 (VA = 28 wt %, DuPont Co., U.S.). The admixture of dicumyl peroxide (DCP) was produced by Shanghai Fangreda Chemical Co., Ltd., 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 um was supplied by Albemarle Corporation Co., Ltd., U.S. The intumescent flame retardant (IFR), NP, was obtained from Shenchuan Chemical Co., Ltd., Japan. Similar to the other IFR, NP includes an acid source, a charring agent, and a blowing agent. The MH and NP were dried in a vacuum oven at 110 °C for at least 12 h. 2.2. Sample Preparation. The EVA/EVM TPVs have been prepared according to the previously reported strategy.19 Simply, EVA/EVM (50/50 by weight) TPVs were first dynamically vulcanized in a Haake batch mixer at 160 °C with the screw rotation speed of 80 rpm. Next, the prepared TPVs were directly melt-mixed with MH and NP using the same mixer at 160 °C with a screw rotation speed of 80 rpm. The formulation of the prepared sample is displayed in Table 1. The total loading of MH and IFR is 50 wt % for all of samples, but different MH to IFR ratios have been used to investigate the synergistic effects of the two flame retardants. The obtained composites were then hot-pressed at 180 °C into a sheet with 4

3. RESULTS AND DISCUSSION 3.1. Burning Behaviors. 3.1.1. LOI Tests. To investigate the synergistic effect of the two types of flame retardants, the LOI values of all of the TPV samples with various amounts of NP and MH have been measured and summarized in Table 1. The higher LOI value represents the better flame retardancy.20 Neat TPV is flammable and very easy to burn in air with the LOI of 17.8%. When the TPV was flame retarded by 50 wt % MH or NP separately, LOI values increased to 32.2% and 30.2%, respectively, indicating the improved flame retardant properties by both additives. Surprisingly, considerable flame retardant synergism could be observed when a small amount of NP was combined together with MH. As one can see from Table 1, the LOI values at first increase with increasing amount of NP until a maximum LOI value, but with the further increase in NP-IFR content, the LOI value starts to decrease. The highest LOI value reaches 35.8% for the MH92-NP8 sample. Moreover, the vertical burning test indicates the sample reached UL-94 V-0 rating. Note that further increasing the NP content results in the gradual decreasing of the LOI, indicating that the best synergistic effect between NP and MH occurs at the relatively low NP contents. The fact that MH is dominant in the flame retardant system is very important because MH is the environmental-friendly flame retardant with only the water releasing during the combustion. 3.1.2. Cone Calorimeter Tests. Cone calorimetry is often used to evaluate the fire behaviors of materials, and the measurements provide several useful burning parameters.21−26Figure 1 shows heat release rate (HRR) and total heat release (THR) curves of the neat TPV and typical flame retarded samples with different MH to NP ratios. HRR is a

Table 1. Formulations and LOI Values of TPV/MH/NP Samples samples

TPV (%)

MH (%)

NP (%)

LOI (%)

TPV MH100-NP0 MH96-NP4 MH92-NP8 MH80-NP20 MH60-NP40 MH20-NP80 MH0-NP100

100 50 50 50 50 50 50 50

0 50 48 46 40 30 10 0

0 0 2 4 10 20 40 50

17.8 32.2 35.2 35.8 33 31.2 30.8 30.2 8785

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originates from the residue char (or MgO layer), which forms a physical barrier to avoid or at least inhibit heat and mass transmission. Similar to the LOI results, the lowest PHRR occurs for the MH92-NP8 sample. Two HRR peaks are usually observed for the IFR incorporated polymers. The first peak is assigned to the ignition and the flame spread on the surface of the material.27 In this period, the polymer is protected by the intumescent shield. The second peak is explained by the destruction of the intumescent structure and the formation of a carbonaceous residue.28−31 It is observed from Figure 1a that the TPV/MH/ NP systems exhibit the two HRR peak characteristics. It is considered that the incorporation of NP into MH results in a more effective insulating multicellular protective shield of stacked surface layer, which limits the heat and mass transfer between the heat source and the substrate during the combustion of the material due to the lower degradation temperature of NP (see the TGA analysis in the following section). This insulating layer is not only a barrier to oxygen supply and heat conduction, but also a suppressor to the release of flammable gases.32 The following peak originates from the full heat releasing of the insulating layer on the surface. Figure 1b shows the total heat release (THR) curves of the typical flame retarded samples and the virgin TPV. The MH92NP8 exhibits a THR value of 24.1 MJ/m2, which is only 30% of the virgin TPV. Moreover, the MH92-NP8 sample has the lowest THR value among all flame retardant samples, again indicating the best flame retardant performance. The decreased THR of flame retardant TPV means that a part of the polymer is not completely burnt. The compact protective layers on the surface serve as a thermal insulation layer to inhibit polymer pyrolysis and prevent the evolution of combustible gases to feed the flame, and also separate oxygen from burning materials.33 The main combustion parameters obtained from the cone calorimeter were summarized in Table 2. It should be mentioned that the peak mass loss rate of the MH92-NP8 sample is lowest, which again indicates the best flame retardant performance when 8% NP was incorporated into the 92% MH. The evolution of smoke is considered as another important parameter in the halogen-free flame retardant materials. The formation of CO in fires takes place at low temperatures in the early stages of fire development, primarily attributed to incomplete combustion of the pyrolyzed polymer volatile fuels. When the fire develops, the higher temperature favors the formation of CO2, which is particularly dependent on oxygen availability to the fire.34 Figure 2 show the CO2 and CO releasing rate during combustion. It can be seen that the CO2 and CO productions of the flame retardant TPV samples are much lower than those of the virgin TPV during combustion. The CO and CO2 peak values of the MH92-NP8 sample are the lowest among all flame retardant samples. The flame retardant low smoke polymers have been the long pursued target. It is well-known that metal hydroxide presents

Figure 1. Heat release rate (HRR) (a) and total heat release (THR) (b) curves of TPV/MH/NP composites as a function of time.

measurement of the heat release per unit surface area of a burning specimen, which is considered to have a significant influence on fire hazard, while the peak value of HRR is regarded as an essential parameter with respect to the evaluation of the risk during burning. The HRR of neat TPV showed a significant increase after ignition up to the peak value (PHRR) and decreased sharply toward the end of burning. This kind of heat release characteristic was long-term considered as a typical case for “thin noncharring” material. Both the MH and the NP show significant flame retardant effects for the TPV with a drastically decreased peak heat release rate (PHRR). The combination of MH and NP leads to the improved flame retardant performance of the materials when compared to the separated addition of MH and NP. The PHRR value for the MH92-NP8 sample is as small as 140 kW/m2, which is only 30% of the virgin TPV. On the other hand, it can also be seen from Figure 1 that the PHRR value of the MH 92-NP8 sample is smaller than those of the MH100-NP0 and MH0-NP100 samples, indicating the synergistic effects between MH and NP. It is considered that the drastically decreased PHRR value

Table 2. Combustion Parameters Obtained from Cone Calorimeter

a

samples

PHRR (kW/m2)

THR (MJ/m2)

TTI (s)

TSP (m2)

TSR (m2/m2)

PMLRa (g/s)

MH100-NP0 MH92-NP8 MH20-NP80 MH0-NP100

200 140 147 175

24.5 24.1 27.5 27.6

177 225 282 386

2.49 2.26 4.67 4.89

281 256 528 666

0.128 0.076 0.077 0.094

PMLR stands for peak mass loss rate. 8786

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accumulation of NP intumescent flame retardant together with MH near the regressing sample surface acts as not only a heat insulation layer, but also as a barrier that prevents the decomposed volatile products from migrating to the sample surface. 3.2. Thermogravimetric Analysis. Thermogravimetric analysis (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. Figure 4 presents the TGA curves under N2 and air for the flame retardant TPV samples with different MH to NP ratios. The characteristic degradation parameters from the TGA curves under N2 (Figure 4a and b) are listed in Table 3. Two distinct weight-loss peaks (T = 335, 447 °C) are observed for the MH100-NP0 sample. The first degradation step, in the temperature range of 300−400 °C, can be assigned to the evolution of acetic acid and the decomposition of MH, whereas the second degradation step in the temperature range of 400−500 °C is due to the degradation of polyethylene chains and evaporation of the residues.19,35 The incorporation of NP in the flame retardant system leads to the acceleration for the degradation process, as evidenced by the decreased T1% for MH92-NP8 and MH20-NP80 samples in Table 3. However, it can also be observed that the following weight loss was retarded to higher temperature for the MH92-NP8 sample. The T5% for the MH92-NP8 sample is 341 °C, which is the highest temperature among all of the samples. This means that the combination of MH and NP leads to the acceleration of the decomposition first and then protects the sample from further degradation. Obviously, this behavior is highly consistent with the burning performance of the sample shown in the previous section. It is also seen from Table 3 and Figure 4b that the only MH incorporated sample (MH100-NP0 sample) has a very high residual char originated from the formation of MgO during the degradation. NP is an organic flame retardant, so the final residue of MH0-NP100 sample is only about 1.7 wt %. The combination of MH and NP makes the intermediate residual char between the two samples. Suppose the final residue of the sample follows the simple mixture rule; the residual mass of corresponding samples can be calculated and is shown in Table 3. It is seen that the real residue mass is higher than the calculated one for the MH−NP combined sample. It is considered that the formed MgO layer protects the further degradation of the polymer matrix and therefore enhances the thermal stability of the matrix. The TGA curves under air for the flame retardant TPV samples with different MH to NP ratios are shown in Figure 4c and d. The characteristic degradation parameters from the TGA curves under air are listed in Table 4. The degradation behavior for TPV samples under air is very similar to those under N2 with lower thermal stability. However, it is interesting to find that the final char weight in air is more than that in N2 for the same sample, indicating that the oxygen is favorable to the char formation of the composites. 3.3. Mechanical Properties. It is generally accepted that the high loading of metal hydroxide leads to the severe deterioration of the mechanical properties. Therefore, the balance between the flame retardancy and the mechanical properties is a big issue for this type of halogen-free flame retardant materials. Figure 5 and Table 5 show the mechanical properties of TPV composites with various MH and NP loadings. Virgin TPV is soft with a very high elongation at break

Figure 2. (a) CO production (COP) as a function of time and (b) CO2 production (CO2P) as a function of time for TPV/MH/NP composites.

low smoke during combustion. Figure 3 shows the smoke production rate as a function of burning time for all of the

Figure 3. Smoke production rate (SPR) as a function of time for TPV/MH/NP composites.

samples. The MH100-NP0 sample has a much lower smoke production rate than the MH0-NP100 sample, similar to the most reported phenomena. However, it is surprising that the MH92-NP8 sample exhibits an even lower smoke production rate than the MH100-NP0 sample, which means that the synergetic effects occur also for the smoke production during the burning of the sample. It is considered that the 8787

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Figure 4. TGA (a) and DTG (b) curves of neat TPV and TPV/MH/NP composites in N2; TGA (c) and DTG (d) curves of neat TPV and TPV/ MH/NP composites in air.

Table 3. Thermogravimetric Data of MH, NP, Neat TPV, and TPV-MH with Different Contents of NP Intumescent Flame Retardant in N2a

samples

T1% (°C)

T5% (°C)

Tmax1 (°C)

Tmax2 (°C)

char residue at 650 °C (wt %)

MH NP MH100-NP0 MH92-NP8 MH20-NP80 MH0-NP100 TPV

332.6 294.8 314 289 295 298 310

362.9 325.7 335.8 340.7 321.8 331.1 321.1

394.5 373.0 351 368.3 368.3 400.8 335.8

438.7 473.1 471.7 471.1 470.4 446.6

70.2 3.38 35.1 33.05 11.42 1.69 0

Table 4. Thermogravimetric Data of Neat TPV and TPVMH with Different Contents of NP Intumescent Flame Retardant in Aira

theory of char residue at 650 °C (wt %)

35.1 32.4 8.44 1.69 0

samples

T1% (°C)

T5% (°C)

Tmax1 (°C)

Tmax2 (°C)

char residue at 650 °C (wt %)

MH100-NP0 MH92-NP8 MH20-NP80 MH0-NP100 TPV

287.8 283.4 277.1 279 265

343 341.7 329.5 327.6 317.5

368 362.3 365.5 407.8 350.9

460.8 470.6 471.5 478.4 450.0

34.21 34.93 13.28 5.82 1.15

theory of char residue at 650 °C (wt %) 35.1 32.4 8.44 1.69 0

a

T1% is the temperature of 1 wt % mass loss; T5% is the temperature of 5 wt % mass loss.

a T1% is the temperature of 1 wt % mass loss; T5% is the temperature of 5 wt % mass loss.

tensile strength can achieve 14 MPa. These are really high values for a halogen-free flame retardant polyolefin composite. To investigate the mechanism for the improvement in the mechanical properties of the TPV/MH composites with a small amount of NP, TEM measurements have been carried out for the MH92-NP8 sample and the MH100-NP0 sample, as shown in Figure 6. It is clear that the MH aggregation particle size in the MH100-NP0 sample is bigger than that in the MH92-NP8. This means that the addition of NP induces the homogeneous dispersion of MH particles in the TPV matrix. Obviously, the improved dispersion of MH for the MH92-NP8 sample accounts for better mechanical performance.

of about 900%. The addition of MH leads to the drastic increase in the modulus because of the strengthening effects of inorganic MH. At the same time, the elongation at break of the binary TPV/MH composite is significantly lower than that of the virgin TPV. On the other hand, as a small organic molecule, high loading of NP induces the decreasing in both elongation at break and tensile strength of the materials. However, the MH92-NP8 sample exhibits much better mechanical performance than both the MH100-NP0 sample and the MH0-NP100 sample. The elongation at break is as high as 580%, and the 8788

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3.4. Char Residue Analysis. It is very interesting to find the drastically improved flame retardancy with enhanced LOI value, and decreased PHRR and SPR by incorporation of a small amount of NP. This strategy paves new routes to the high performance nonhalogen flame retardant materials with very low N−P contents, excellent mechanical properties, and good processability. To understand the mechanism for the synergistic effects between the MH and NP, the char residue of the sample has been investigated carefully. Figure 7 shows the digital photographs of the flame retardant samples after cone calorimetric analyses. It is clear that the residual char was crisp for the MH100-NP0 sample. The detailed observation of the char surface indicates the surface layer was broken into small pieces. The residual char for the MH0-NP100 sample was thin, and the char was broken into several big pieces. In contrast, the char of the MH92-NP8 sample was compact and continuous, which was an effective barrier for heat and mass transmission. To elucidate the relationship between the microstructure of protective char and flame retardancy, the surface morphology of the char residue after LOI tests was observed by SEM, as shown in Figure 8. For the MH100-NP0 sample, many big cracks can be observed on the surface after burning, as shown in Figure 8a. The detailed observation indicates that the surface for the MH100-NP0 sample is in fact the stacking of the MgO flakes (Figure 8a). We considered that the MH transforms into MgO during burning and the MgO layer protects the inside from further degradation (as shown in Supporting Information Figure S1). However, the transformation process leads to the drastic shrinking of the burning part due to the releasing of water. Obviously, such a process occurs first on the surface during burning where the temperature is highest, while the inside of the sample is still rigid. Therefore, many cracks were observed on the sample surface because of the shrinkage. At the same time, the formed MgO flakes were only simply physically stacked together with almost no adhesions because the polymer was fully burned with almost no residue. Therefore, heat and mass can be easily transferred from the cracks and the gaps between the MgO flakes. The burning can extend into the inside of the sample, so relatively low flame retardant properties were observed from the sample with only 50 wt % MH addition (the MH100-NP0 sample). In fact, this is also the reason that high loadings of MH are necessary for the adequate flame retardant properties using MH as the flame retardant additive. Different from the sample with only MH, the surface for the MH0-NP100 sample was porous after combustion (Figure 8d). The porous char surface can be attributed to both the expansion of the NP during the burning and the organic nature of NP. Note should be paid that NP induces significant carbonaceous residue on the surface due to the charring agent in NP. The carbonaceous materials can also take the role to

Figure 5. Strain−stress curves for TPV, MH100-NP0, MH92-NP8, and MH0-NP100.

Table 5. Mechanical Properties of TPV Composites

samples MH100-NP0 MH92-TPV8 MH0-NP100 TPV

tensile strength (MPa)

tensile set at 100% deformation (MPa)

tensile set at 200% deformation (MPa)

± ± ± ±

7.60 7.61 5.44 1.95

10.22 9.77 6.85 3.12

14.5 15 8.1 12.0

1.1 2.0 2.1 1.2

elongation at break (%) 418 590 320 890

± ± ± ±

40 30 10 25

Figure 6. TEM of TPV composites with different loadings of MH and NP: (a,b) the MH100-NP0 sample, (c,d) the MH92-NP8 sample.

Figure 7. Photos of the charred residue samples collected after cone calorimeter tests: (a) MH100-NP0; (b) MH92-NP8; and (c) MH0-NP100. 8789

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Figure 9. EDX of the MH92-NP8 sample after combustion.

has been made for the MH100-NP0 and MH92-NP8 samples. From the TGA curves in Figure 4, the degradation of NP starts at about 300 °C, at which the degradation of MH has not started, so we have carried out the experiments for the samples at 300 and 400 °C in a muffle for 20 min. A comparison was made in morphologies of samples after their burning at certain temperatures, as shown in Figure 10. It was found that the

Figure 8. SEM micrographs of the char residue of TPV composites: (a) the MH100-NP0 sample, 100×, 5000×; (b) the MH92-NP8 sample, 100×, 5000×; (c) the MH20-NP80 sample, 100×, 5000×; and (d) the MH0-NP100 sample, 100×, 5000×.

protect the inside sample. It is surprising that the char structure of the MH92-NP8 sample is homogeneous and very compact with no flaws, which is different from the char structures of the MH100-NP0 and MH0-NP100 samples. The char layer is thicker and more solid than the only MH and only NP incorporated samples. Especially, the high magnification displayed that there are no defects on the surface. Therefore, such a compact layer is very effective to insulate the emission of heat and the mass during the burning, and better flame retardant performance can be achieved. To make the detailed analysis of the char layer structure, the EDX has been carried out for the residue of the MH92-NP8 sample. The Mg, O, and C mapping images are shown in Figure 9. It is clear that the residual was dominantly MgO for the MH92-NP8 sample. It should also be noted that C was observed and well distributed on the char surface. The C originates mainly from the carbonaceous residue by NP. Therefore, it can be depicted that the MgO particles from MH were bonded together by the carbonaceous residue from NP to form a thick and compact MgO layer during burning. Such a compact layer takes a critical role for the flame retardancy of the materials. 3.5. Formation Mechanism of the Compact Protective Layer. It is obvious that the compact MgO layer by the combination of dominant MH and minor NP during combustion protects the inside of the material from further burning upon the fire. It is important to elucidate the mechanism for the formation of the protective layer. We tried to simulate the burning process by putting the samples into a muffle oven at different temperatures. The comparison

Figure 10. SEM of char residue of MH100-NP0 and MH92-NP8 after calcination: (A) MH100-NP0 after calcination at 300 °C; (B) MH100NP0 after calcination at 400 °C; (C) MH92-NP8 after calcination at 300 °C; and (D) MH92-NP8 after calcination at 400 °C (inserts are the appearance of the corresponding sample after calcinations).

sample with only MH has no any expansion at 300 °C and the surface is rather flat with many cracks. In contrast, the drastic expansion was observed for the MH92-NP8 sample at 300 °C, and the dilatation value is up to 800%, as shown in Figure 10C. It is therefore believed that a small amount of NP enables the significant swelling. After continuing to burn at 400 °C, the cracks propagate and become more severe due to the 8790

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Figure 11. Schematic diagram of the formation of compact protective layers in the process of combustion ((A)−(D) indicates the burning process for the MH100-NP0 sample without NP; (a)−(d) indicates the burning process for the MH92-NP8 sample with a small amount of NP).

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dehydration of MH and the shrinkage by the dehydration, as shown in Figure 10B. However, for the MH92-NP8 sample, the previous expansion becomes smaller when further exposed at 400 °C, and meanwhile a smooth surface emerges, which means the formation of a compact protective layer of the sample. On the basis of the above analysis, a schematic process during burning for the combination of MH and NP has been proposed in Figure 11. For the sample flame retarded with only MH, MH decomposes to produce MgO, and the matrix TPV decomposes with the elevation of the temperature during burning. Of course, the releasing of the water during the decomposing of MH reduces the surface temperature and dilutes flammable gases content, resulting in the flame retardant effects. However, many cracks occur because of the shrinkage by both the decomposing of MH and the TPV matrix. These cracks serve as the effective pathway of materials and energies to deteriorate the retarded efficiency severely, as shown in Figure 11D. In contrast, for the MH92-NP8 sample with the presence of NP, NP expands before the decomposing of MH, and the expansion provides a shrinkable layer for the following decomposing of MH. Therefore, almost no cracks were formed during the process. At the same time, the carbonaceous residue from NP during the combustion bonds the MgO particles, and a compact thick protective layer was created. Therefore, a better flame retardant performance was achieved. We considered that the synergistic mechanism should also be applicable to other polymer matrix and other MH-IFR combinations in which IFR expands earlier than the decomposing of MH. Related works have been undergone and will be reported in the near future.

ASSOCIATED CONTENT

S Supporting Information *

XRD spectra for the MH92-NP8 sample after burning. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 86-571-2886-7206. Fax: 86-571-2886-7899. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Science Foundation of China (51173036, 21374027) and the Ministry of education program for New Century Excellent Talents, and PCSIRT (IRT 1231).

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REFERENCES

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4. CONCLUSION In this article, MH and NP were utilized together as a novel combined flame retardant system to improve the flame retardance of TPV for the first time. A small amount of NP enhanced both the flame retardance and the mechanical properties of the MH filled TPV drastically. The addition of NP induces the improved MH particle dispersion in the TPV matrix; thus the final flame retardant TPV exhibited enhanced mechanical properties. On the other hand, a small amount of NP accelerates the degradation and induces a shrinkage matrix, leading to a compact and unbroken MgO protective layer. Therefore, enhanced LOI and the drastically depressed peak heat release rate were achieved. This novel flame retardant system paves a new possibility for the high performance nonhalogen flame retardant polymeric materials, and it should also be applied to other polymers. 8791

dx.doi.org/10.1021/ie5008147 | Ind. Eng. Chem. Res. 2014, 53, 8784−8792

Industrial & Engineering Chemistry Research

Article

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