Flame Retardancy and Thermal Stability of Polypropylene Composite

Article pubs.acs.org/IECR

Flame Retardancy and Thermal Stability of Polypropylene Composite Containing Ammonium Sulfamate Intercalated Kaolinite Wufei Tang, Sheng Zhang, Jun Sun, and Xiaoyu Gu* Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, China ABSTRACT: Ammonium sulfamate (AS) intercalated kaolinite (K) was successfully prepared through a three-step method, and then, it was introduced in association with intumescent flameretardants (IFR) into polypropylene (PP). The structure of intercalated kaolinite (AS-K) was characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and X-ray photoelectron spectrometry (XPS). The flammability evaluation by limit oxygen index (LOI), vertical burning test (UL-94), and cone calorimeters test (CCT) revealed that the LOI value was increased to 35.3, and UL-94 grade reached V-0 rating for the PP/IFR (23.5 wt %)/AS-K (1.5 wt %) sample; the heat release rate (HRR) value was lower than other PP composites. The thermostability analysis by TGA demonstrated that the addition of AS-K could enhance the thermal stability and the formation of char residue. It was proposed that AS-K could react with P, N, and polyaromatic rings to form a ceramic-like compact and continued to char during burning. increasing attention in fire science in the last several years.17,18 Our previous research demonstrated that the combination of raw kaolinite and intumescent flame retardant (IFR) could improve the flame retardancy of polymers.19 Nowadays, polypropylene (PP) has acted as one of the popularly consumed polymer due to its lightweight, low cost, and excellent mechanical properties. However, its further applications have been limited greatly by its ignitability, no residues, and high amount of heat release during combustion.20,21 Consequently, it is urgent to improve the flame retardancy of PP to meet the need of safety standards. It has been reported that, after intercalation with compounds containing N and/or P elements, LDHs and MMT showed improved flame retardancy in PP.10,22,23 Meanwhile, the work of introducing flame retardant containing S into polymer to enhance flame retardancy has been done.24 In this work, ammonium sulfamate (AS) was selected and intercalated into kaolinite. A low amount of K-AS in association with IFR was melt blended with PP. The flammability and possible pyrolysis mechanism of PP/IFR/AS-K composites were evaluated and analyzed.

1. INTRODUCTION In the last two decades, nanocomposites composed of polymer/ natural clay have attracted much attention1−6 for the significant improvement in many properties such as mechanical, barrier, and fire resistant properties.7−10 Layered double hydroxides (LDHs) and montmorillonite (MMT)11,12 have drawn more attention than other inorganic clays for they are relatively easy to be intercalated or exfoliated. Kaolinite (K) is an abundant mineral across the world, and it is widely applied in industrial fields as paper fillers, ceramics, polymer additives, and so on; however, it is much less used in polymer than LDHs and MMT, which is mainly due to the difficulties during intercalation. Kaolinite is a 1:1 nonexpensive layered silicate with a chemical formula of Al2Si2O5(OH)4.13,14 The stacked layers are linked by strong hydrogen bonds. The asymmetrical superposition of the Al−O tetrahedral and Si−O octahedral sheets in the 1:1 layers creates large superposed dipoles. Consequently, the asymmetry makes intercalation of guest organic molecules more difficult than LDHs and MMT. According to the literature, one usually needs three steps to obtain intercalated kaolinite: first, small or dipolar molecules (for example, DMSO) are introduced into the interlayers directly to break the strong interlayer hydrogen bonds; second, the above intercalated substance is replaced by aqueous potassium acetate (KAc);15,16 finally, the target substance is intercalated by replacing KAc. Conventional intercalation paid much attention to improve the dispersion of kaolinite in the substrate, and the intercalated surfactants usually contain aliphatic chains which might decrease the fire resistance of the polymer. However, kaolinite has attracted © 2016 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials. The commercial PP, with a melt flow index of 3 g/(10 min), was kindly provided by Sinopec Maoming Company Received: Revised: Accepted: Published: 7669

May 4, 2016 June 20, 2016 June 24, 2016 June 24, 2016 DOI: 10.1021/acs.iecr.6b01722 Ind. Eng. Chem. Res. 2016, 55, 7669−7678

Article

Industrial & Engineering Chemistry Research Scheme 1. Preparation Process for AS Intercalated Kaolinite

three times before being dried at 60 °C for 12 h. The product was named as KAc-K. Step 3, AS intercalation: 2 g of KAc-K was homogeneously dispersed in 40 mL of AS/H2O (8 mol/L) solution under vigorous stirring at 50 °C for 24 h. The mixture was washed by ethanol for three times before being dried at 60 °C for 12 h. The product was named as AS-K. 2.3. Preparation of PP Composites. The PP composite was prepared by melt mixing with a micro twin-screw extruder (Wuhan Rui Ming Plastics Machinery Co. Ltd.) at a screw speed of 40 rpm. The processing temperature from hopper to die was fixed at 170, 180, and 190 °C, respectively. Table 1 presents the formulations of PP composite (the total weight percentage of IFR/clay was 25 wt %, and the mass ratio of IFR was MCAPP/ PEPA = 2:1). 2.4. Preparation of Muffle Samples. Every cubic sample (about 0.1 g) was placed on the surface of crucible lid and then introduced into the muffle under N2 in order to investigate the char forming process at different temperatures (350, 450, 600, and 800 °C) for 10 min. 2.5. Measurements. The X-ray diffraction (XRD) was performed with a D/max-2500 diffractometometer using Cu Kα

(Maoming, China). Ammonium polyphosphate (APP) was purchased from Jin Ying Tai Chemical Co., Ltd. (Jinan, China). Melamine (MA) was obtained from Jin Tong Le Tai Chemical Product Co., Ltd. (Beijing, China). Pentaerythritol phosphate (PEPA) was a product of Victory Chemistry Co., Ltd. (Zhangjiagang, China). Formaldehyde (POM), dimethyl sulfoxide (DMSO), potassium acetate (KAc), and AS were purchased from Beijing Chemical Factory (Beijing, China). The raw kaolinite with 1−46 μm size range (mean size of 12 μm) and the specific surface area of 17 m2/g (the purity >95%) was kindly supplied by Xing Yi Mineral Processing Plant (Shijiazhuang, China). 2.2. Preparation of AS Intercalated Kaolinite. Scheme 1 shows the procedure of preparing AS intercalated kaolinite, which includes three steps. Step 1, DMSO intercalation: 4 g of kaolinite (K0) was added to a mixture of 40 mL of DMSO and 4.5 mL of deionized water. The solution was reacted under ultrasonic with a power of 200 w for 4 h. The reacted mixture was washed by ethanol for three times before being dried at 60 °C for 12 h. The product was named as DMSO-K. Step 2, KAc intercalation: 2 g of DMSO-K was introduced to 40 mL of saturated KAc water solution under vigorous stirring at 50 °C for 24 h. The reacted mixture was washed by ethanol for 7670

DOI: 10.1021/acs.iecr.6b01722 Ind. Eng. Chem. Res. 2016, 55, 7669−7678

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Industrial & Engineering Chemistry Research Table 1. Formulation of PP Compositesa samples

PP (%)

neat PP PP/K0 PP/IFR PP/IFR/K0

PP/AS-K

PP/IFR/AS-K a

100.0 99.5 98.5 97.0 75.0 75.0 75.0 75.0 99.5 98.5 97.0 75.0 75.0 75.0

IFR (%)

K0 (%)

AS-K (%)

0.5 1.5 3.0 25.0 24.5 23.5 22.0

24.5 23.5 22.0

0.5 1.5 3.0 0.5 1.5 3.0 0.5 1.5 3.0

In weight percentage. Figure 1. FT-IR spectra of K0 and AS-K.

radiation source at 40 kV and 20 mA (λ = 0.154 nm). The interlayer space of clay was calculated according to Bragg’s eq 1. λ = 2d sin θ

In the spectrum of AS-K, special bands of AS such as 3527 (O−H), 3288 and 3196 (N−H), 1401 (Vas, SO), 1258 and 1206 (Vs, SO) and 803 cm−1 (S−O) emerged, which indicated AS had been introduced into K0. 3.1.2. XRD Analysis. Figure 2 shows the XRD patterns of K0 and AS-K. The interplanar spacing change and intercalation rate

(1)

where d is the basal spacing and θ is the diffraction angle. The intercalation rates (InR) are calculated according to eq 2. Ii(001) InR = × 100% Ii(001) + Ik(001) (2) where Ik(001) and Ii(001) are the peak intensities of raw kaolinite and intercalated products, respectively. The micrographs were observed by a Hitachi S-4700 SEM under the voltage of 20 kV and a 200 kV JEOL JEM-2100 high resolution TEM. The limit oxygen index (LOI) was tested by JF-3 oxygen index apparatus (Jiangning Nanjing Analytical Instrument Co. Ltd.) according to the standard oxygen index test method of ISO 4589-2. The efficient dimensions of all samples are 50 × 6.5 × 3 mm3. The vertical burning behavior was tested by a CZF-3 apparatus (Jiangning Nanjing Analytical Instrument Co. Ltd.) according to the UL-94 test standard. The specimens used were of efficient dimensions of 130 × 13 × 3 mm3. The cone calorimeter (FTT Co., Ltd.) test was performed according to the standard ISO 5660. The samples with a dimension of 100 × 100 × 3 mm3 were tested at horizontal position with heat radiant flux density of 50 kW/m2. Specimens were wrapped in aluminum foil, leaving the upper surface exposed to the radiator, and then placed on a ceramic backing board at a distance of 25 mm from the cone base. The experiments were repeated three times. Fourier transform infrared spectroscopy (FTIR) spectra were recorded in the 4000−500 cm−1 spectral region using a Nicolet IS5 under the resolution of 1 cm−1 in 32 scans. Thermogravimetric analysis (TGA) analysis was conducted using a synchronous thermal analyzer (STA449C, Netzsch) from 30 to 800 °C at a heating rate of 10 °C/min under nitrogen atmosphere. The mass for each sample was around 10 mg.

Figure 2. XRD patterns of K0 and AS-K.

were calculated according to eqs 1 and 2, respectively. The diffraction peak of d001 left shifted from 12.4° in K0 to 7.45° in AS-K, indicating the interplanar spacing was enlarged from 0.72 to 1.20 nm. It can be concluded that AS had been intercalated into K0, and the intercalation rate of AS-K was up to 90.1%. 3.1.3. Thermal Properties. Figure 3 presents TGA/DTG curves of K0 and AS-K. The major mass loss of K0 during 400−650 °C in the TGA curve corresponded to the peak at 543 °C in the DTG curve, which was attributed to the dehydroxylation.12 The minor mass loss during 220−280 °C was attributed to the elimination of crystal water in the interlayer of K0. The residue of K0 at 700 °C was 86.4%. Comparatively, AS-K undergoes seven decomposition stages. The first and second mass losses (130−260 °C) were assigned

3. RESULTS AND DISCUSSION 3.1. Characterization of AS-K. 3.1.1. FT-IR. Figure 1 shows the FTIR spectra of K0 and AS-K. In the spectrum of K0, the bands at 3694, 3670, and 3654 cm−1 corresponded to interlayer hydroxyl stretching, while the band at 3621 cm−1 was attributed to the stretching vibrations of the internal hydroxyl groups.25,26 7671


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Industrial & Engineering Chemistry Research

Figure 3. TGA and DTG curves of K0 and AS-K.

Figure 4. XPS spectra of K0 and AS-K.

water and ammonia; the fifth and sixth mass losses (330− 450 °C) might be ascribed to release of oxysulfide. The last one in 450−600 °C with the DTG peak of 525 °C was attributed to the dehydroxylation of clay. The residue at 700 °C was 59.9%.

to the dimerization of AS along with the release of ammonia27 and removal of crystal water. The next four steps occurring in the range of 260 and 450 °C were ascribed to the removal of intercalated dimerization of AS: the third and fourth mass losses (270−320 °C) might be attributed to release of 7672


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Industrial & Engineering Chemistry Research Scheme 2. Possible Chemical Structure of AS-K

Table 2. Formulation, LOI, and UL-94 Tests of PP Composites samples PP PP/K0 PP/IFR PP/IFR/K0

PP/AS-K

PP/IFR/AS-K

PP 100.0 99.5 98.5 97.0 75.0 75.0 75.0 75.0 99.5 98.5 97.0 75.0 75.0 75.0

IFR

K0

AS-K

LOI/%

UL-94

0.5 1.5 3.0 0.5 1.5 3.0

18.0 ± 0.1 18.3 ± 0.1 18.3 ± 0.1 18.4 ± 0.1 31.1 ± 0.2 31.5 ± 0.2 32.5 ± 0.2 31.9 ± 0.3 18.4 ± 0.2 18.6 ± 0.2 18.7 ± 0.2 34.0 ± 0.2 35.3 ± 0.2 32.1 ± 0.2

NR NR NR NR V-2 V-0 V-0 V-0 NR NR NR V-0 V-0 V-0

0.5 1.5 3.0 25.0 24.5 23.5 22.0

0.5 1.5 3.0

24.5 23.5 22.0

Figure 5. XRD patterns of PP/IFR/1.5K0 and PP/IFR/1.5AS-K.

3.1.4. X-ray Photoelectron Spectrometry. Figure 4 gives the X-ray photoelectron spectrometer (XPS) spectra of AS-K and K0. S2P (169.2 eV), S2S (233.6 eV), and N1S (401.3 eV) peaks28,29 can be clearly observed from the survey spectrum of AS-K. From the higher resolution spectra of Al2P and Si2P, it can be found that the peak center of Al2P of K0 was located at 74.9 eV;30

Figure 7. HRR curves of PP and its composites.

however, it was widened and shifted to 74.2 eV in AS-K. It was suggested that chemical situation of Al2P had been changed after

Figure 6. SEM images of PP composites: (A1), (A2), and (A3) PP/IFR/1.5K0; (B1), (B2), and (B3) PP/IFR/1.5AS-K. 7673


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Industrial & Engineering Chemistry Research Table 3. Key Cone Calorimeter Data of PP Composite properties pHRR1 (kW/m2) pHRR2 (kW/m2) tPHRR1 (s) tPHRR2 (s) pSPR1 (m2/s) pSPR2 (m2/s) THR (MJ/m2) TSP (m2/m2) TTI (s) AMLR (g/s) AHRR (kW/m2) residues (%) EHC (MJ/kg)

PP 1474 145 0.110 142 1655 27 0.131 751 0 51.0

PP/IFR

PP/1.5K0

329 438 104 331 0.065 0.082 123 2673 18 0.058 272 13.1 41.6

1346 130 0.104 140 1589 27 0.131 740 1.08 49.4

AS was intercalated into K0. Meanwhile, two peaks for Si2P appeared at 100.4 and 103.5 eV for K0, which were attributed to Si−O−H and Si−O−Si, respectively.19 However, for AS-K, only one Si2P peak at 102.6 eV could be found, and the peak at 100.4 eV (Si−O−H) almost disappeared completely, that means the Si−O−H bonds were disrupted and new chemical bonds between AS and K0 were formed. The possible chemical structure of AS-K was shown in Scheme 2. 3.2. Dispersion of Kaolinite. The XRD patterns of PP/IFR composite containing 1.5 wt % K0 or AS-K are shown in Figure 5. It was noted that the 001 characteristic peak of K0 (2θ = 12.4°) still existed in the PP/IFR/K0, which demonstrated that it was difficult for PP molecular chains to intercalate into the layer space of K0. However, the 001 characteristic peak of AS-K (2θ = 7.45°) almost disappeared in the PP/IFR/AS-K, which indicated AS-K was almost exfoliated in PP/IFR. The distributions of K0 and AS-K in PP/IFR were investigated by SEM (Figure 6). One can see K0 particles with a size of about 15 μm, while the distributed AS-K particle size was less than 5 μm with a thickness of less than 50 nm. This result indicated that AS-K scattered into PP matrix with smaller size than K0. During combustion, after the early decomposition of AS, it was in favor of the immigration of smaller kaolinite particles to the surface to form the shield. It could be concluded that AS intercalation could improve the dispersion of K0 in PP. It was suggested that the inert gas released by AS during combustion and the good distribution of AS-K were in favor of the immigration of kaolinite particles to the matrix surface to form char layers. 3.3. Flammability and Flame Retardant Mechanism. 3.3.1. LOI and UL-94. The LOI values and UL-94 ratings of neat PP and its composites are presented in Table 2. Neat PP had a LOI value of only 18.0, and it could not pass any rate in UL-94 tests. The improvement of both LOI value and UL-94 rate was minimal for PP composite containing a low amount (≤3 wt %) of K0 or AS-K, respectively. Although the LOI value sharply increased to 31.1 after adding 25 wt % IFR into PP, the UL-94 grade only reached V-2. It was demonstrated that the presence of clay alone could not effectively improve the flame retardant of matrix,31−34 and it should be used with IFR to further improve the flame retardancy of the composite. The 0.5−1.5 wt % substitution of IFR with K0 resulted in a slight increase of LOI from 31.5 to 32.5, but a further increase in the amount of K0 decreased the LOI. However, 0.5−1.5 wt % substitution of IFR with AS-K increased LOI to 34.0 or above.

PP/IFR/1.5K0 326 373 165 439 0.058 0.079 123 2612 17 0.047 219 14.8 41.1

PP/1.5AS-K 1169 131 0.102 133 1534 28 0.129 698 1.49 45.9

PP/IFR/1.5AS-K 316 309 150 447 0.040 0.058 125 2208 18 0.044 200 15.7 40.1

Figure 8. Digital photographs of residues of (A) PP, (B) PP/IFR, (C) PP/IFR/1.5K0, and (D) PP/IFR/1.5AS-K.

A further increase of AS-K content to 3.0 wt % resulted in a decrease of LOI to 32. In this situation, this phenomenon also showed that the value of LOI could be decreased with a further increase in other inorganic particles.35,36 Moreover, PP composites containing either K0 or AS-K reached V-0 rating. The results indicated that there was an optimal concentration (1.5 wt %) of AS-K where an obvious synergistic effect was observed between AS-K and IFR. 3.3.2. Cone Calorimetry Test. The cone calorimetry test (CCT) is a powerful tool to evaluate the flame retardant and is used to predict the combustion behavior of materials in a real fire.34,37 The heat release rate (HRR) curves of PP composites are shown in Figure 7, and some key data is summarized in Table 3. It can be seen that neat PP (Figure 7) burned very quickly after ignition and appeared as a sharp HRR peak at 1473 kW/m2. The pHRR value of PP/1.5K0 composite was decreased to 1346 kW/m2. Moreover, the addition of 1.5 wt % AS-K can significantly reduce the pHRR value to 1169 kW/m2. These results indicated that AS-K was more effective at restraining the heat release of PP. For PP/25 wt % IFR, the HRR curve showed two peaks with a significantly reduced peak value of 329 and 438 kW/m2, respectively. The first peak (pHRR1) was a little earlier than that of neat PP which was caused 7674


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Figure 9. SEM micrographs of the residual char after CCT: (A) PP/IFR, (B) PP/IFR/1.5K0, and (C and D), PP/IFR/1.5AS-K.

by the earlier decomposition of IFR. The same trend was also found by Bourbigot and coworkers.34 The first peak was attributed to the formation of carbonaceous char. With the absorption of heat flux, fine cracks will be gradually formed on the surface of the protective char. The heat accumulation will finally result in the collapse of char. Consequently, heat, smoke, and other volatile gases would be released simultaneously, and the second peak (pHRR2) emerged.38 After the introduction of K0 or AS-K, the value of pHRR was further reduced. Meanwhile, the whole combustion time was prolonged from 250 s for neat PP to 425 s for PP/IFR and further increased to 525 and 580 s for IFR/1.5K0 and IFR/1.5AS-K composites, respectively. One reason may come from the gas phase flame retardant contribution of AS; the other reason was attributed to the fact of exfoliation of AS-K in PP composite. This structure can promote the formation of more compact char which acted as a barrier to heat, oxygen, and other volatile gases during combustion.39 As a result, pHRR2 of PP composite containing IFR/AS-K was much lower than that of other samples. 3.3.3. Char Morphology. Figure 8 shows digital photos of residues char of PP/IFR, PP/IFR/1.5K0, and PP/IFR/1.5AS-K after CCT. Neat PP completely burns out without leaving any residue. For PP/IFR, a discontinuous char with some cracks was left. However, a uniform and intumescent char layer was formed from PP/IFR/K0, and the char layer became denser in PP/IFR/AS-K. The char was further observed by SEM, and the images are shown in Figure 9. The char of PP/IFR showed a coralloid network structure with discontinuous residual char including lots of cracks and cavities. Heat and volatiles can easily penetrate the char layer during combustion, so it cannot provide good flame shield for the underlying material. However, the char of PP/IFR/1.5K0 and PP/IFR/1.5AS-K was tight, dense, and compact, especially for the sample containing AS-K. 3.4. Thermal Behavior. The curves of PP and its composites are shown in Figure 10. The decomposition temperature corresponding to 5 and 50 wt % weight loss (T5% and T50%), and the amount of residual char are collected in Table 4.

Figure 10. TGA curves of PP and its composites under N2.

Table 4. Key Data of TGA Curves of PP Composites samples

T5%/°C

T50%/°C

residues (wt %)/W700 °C

PP PP/IFR PP/IFR/1.5K0 PP/IFR/1.5AS-K

420.2 361.7 388.2 388.0

455.6 464.7 468.6 468.2

0 7.9 12.5 14.7

It could be seen that PP was a one-step decomposition during 400−500 °C with no residue left (above 500 °C). The initial decomposition temperature was decreased after the introduction of 25 wt % IFR. Two-step decomposition could be observed for the sample containing IFR: the onset temperature (361.7 °C) of the first step was earlier than that of the main decomposition of PP (420.2 °C), which may be due to the release of water and ammonia from IFR; the second step was prolonged to a higher temperature (above 450 °C), assigned to the main decomposition of polymer matrix33 and yielded a final residue of around 7.9 wt % at 700 °C. TGA curves in Figure 10 and data in Table 4 indicated the partial substitution of IFR with K0 or AS-K (1.5 wt %) had minimal effect on the thermal 7675


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Figure 11. Digital photographs and FTIR spectra of PP/IFR/1.5AS-K heated at different temperatures.

degradation temperatures; however, the amount of residual char at 700 °C was increased to 12.5 and 14.7 wt %, respectively. 3.5. Condensed-Phase Analysis. In order to further investigate the char forming process, the PP/IFR/1.5AS-K sample was calcined in the muffle at different temperatures (350, 450, 600, and 800 °C) for 10 min. The digital photographs and FTIR spectra of residual char of condensed phases at different temperatures are shown in Figure 11. When the temperature increased to 350 °C, the sample still maintained the initial shape. This reason was the earlier degradation of IFR produces a char layer on the surface of PP, which could protect PP from further decomposition. From the FTIR spectra, one can still see the typical chain skeleton peaks of PP at around 2800−3000 cm−1, 1456 cm−1 (−CH2), and 1372 cm−1 (−CH3). A little emerged peak at 1719 cm−1 (CO) indicating the beginning oxidization of PP. Up to 450 °C, the sample was oxidized, but the matrix was not carbonized completely. From the FTIR spectra, characteristic bands of PP could still be found but several bands changed, which illustrated the degradation of PP. For example, the increase of the intensity of the band around 1719 cm−1 (CO) documents the oxidization of PP was aggravated; the new band around 1640 and 1170−1290 cm−1 (CC) demonstrated −CH3 fell off from backbone of PP. Two broad bands found above 3000 cm−1 were attributed to O−H and/or N−H,28 which was contributed by the decomposition of AS and IFR. A swelling and compact char layer was formed on the surface of residues at 600 °C. The dehydrogenation of PP reduced the PP skeleton vibration bands around 2800−3000 cm−1. Special bands attributed to the combination of P, O, and C were jumbled together, including 2360 (OP−OH), 1401 (OP), 1003 (P−O−P) cm−1 and so on. It was suggested that the cross-linking structure may be formed from PP, IFR, and AS-K during heating. Only a series of bands corresponding to the combination of P, O, and C (1000−1700 cm−1) were observed at 800 °C. It was suggested that olefin was transformed to a cross-linking network structure. It could be assumed that AS-K reacted with IFR to form a ceramic-like structure during heating and polyaromatic rings were formed when AS-K migrated to the surface of composite.40,41 The compact and continuous char layer was formed with improved mechanical performance.37,42 3.6. Flame Retardant Mechanism. The burning process was described as Scheme 3. During combustion, IFR

Scheme 3. Possible Burning Process of PP/IFR/1.5AS-K

decomposed and divided into smaller molecules containing P and N, as well as noncombustible gases, and then formed an intumescent and viscous layer. The gases blow the kaolinite particles to immigrate to the surface of the composite.43,44 With the cooperation of the stiff kaolinite particle, the char layer was strengthened. AS-K was more effective in improving the flame retardancy of PP than K0, which maybe due the following two possible reasons: the first one was the contribution from AS to release inert gases such as NH3 and SO2 during heating; another one was the distribution of AS-K in PP/IFR was better than that of K0. The possible decomposition and char formation scheme of PP/IFR/AS-K is presented in Scheme 4. MCAPP decomposed into polyphosphoric acid derivatives and released NH3 and H2O simultaneously during heating. Meanwhile, AS-K also releases some inert gases, and then, PP was dehydrogenated to form −OH groups on the backbone. −OH groups could further react with polyphosphoric acid derivatives to form phosphoruscontaining compounds and olefin polymer. The olefin polymer then will transform to polyaromatic rings which connected with P, N, and AS-K to form a ceramic-like structure at high temperature. The formed compact and continuous char layer can isolate flame, heat, oxygen, and other volatile gases during burning. 7676


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Industrial & Engineering Chemistry Research Scheme 4. Possible Pyrolysis Mechanism of PP/IFR/1.5AS-K

(4) Batistella, M.; Otazaghine, B.; Sonnier, R.; Caro-Bretelle, A. S.; Petter, C.; Lopez-Cuesta, J. M. Fire retardancy of ethylene vinyl acetate/ ultrafine kaolinite composites. Polym. Degrad. Stab. 2014, 100, 54−62. (5) Nie, X. D.; Adalati, A.; Du, J.; Liu, H. H.; Xu, S. M.; Wang, J. D. Preparation of amphoteric nanocomposite hydrogels based on exfoliation of montmorillonite via in-situ intercalative polymerization of hydrophilic cationic and anionic monomers. Appl. Clay Sci. 2014, 9798, 132−137. (6) Gamelas, J. A. F.; Ferraz, E.; Rocha, F. An insight into the surface properties of calcined kaolinite clays: The grinding effect. Colloids Surf., A 2014, 455, 49−57. (7) Shen, S. Z. Q.; Bateman, S.; McMahon, P.; Dell’Olio, M.; Gotama, J.; Nguyen, T.; Yuan, Q. The effects of Clay on fire performance and thermal mechanical properties of woven glass fibre reinforced polyamide 6 nanocomposites. Compos. Sci. Technol. 2010, 70, 2063−2067. (8) Tzeng, P.; Maupin, C. R.; Grunlan, J. C. Influence of polymer inter diffusion and clay concentration on gas barrier of polyelectrolyte/clay nanobrick wall quad layer assemblies. J. Membr. Sci. 2014, 452, 46−53. (9) Landry, V.; Blanchet, P.; Riedl, B. Mechanical and optical properties of clay-based nanocomposites coatings for wood flooring. Prog. Org. Coat. 2010, 67, 381−388. (10) Hao, A.; Wong, I.; Wu, H.; Lisco, B.; Ong, B.; Sallean, A.; Butler, S.; Londa, M.; Koo, J. H. Mechanical, thermal, and flame-retardant performance of polyamide 11−halloysite nanotube nanocomposites. J. Mater. Sci. 2015, 50, 157−167. (11) Zhu, Y. X.; Zhou, Y. M.; Zhang, T.; He, M.; Wang, Y. J.; Yang, X. M.; Yang, Y. Preparation and characterization of lactate-intercalated Co−Fe layered double hydroxides and exfoliated nanosheet film with low infrared emissivity. Appl. Surf. Sci. 2012, 263, 132−138. (12) Huang, C. B.; Li, Y. J.; Han, L.; Gao, J. R.; Wang, X. A novel intumescent flame retardant-functionalized montmorillonite: Preparation, characterization, and flammability properties. Appl. Clay Sci. 2011, 51, 360−365. (13) Hou, H. J.; Li, H. Q.; Liu, Q. F.; Cheng, H. F.; He, P.; Li, S. P. Theoretical study for the interlamellar aminoalcohol functionalization of kaolinite. Appl. Surf. Sci. 2015, 347, 439−447. (14) Cheng, H. F.; Liu, Q. F.; Yang, J.; Ma, S. J.; Frost, R. L. The thermal behavior of kaolinite intercalation complexes-A review. Thermochim. Acta 2012, 545, 1−13.

4. CONCLUSIONS AS-K was successfully prepared by a three-step ion exchange reaction. The PP composite containing AS-K and IFR (MCAPP/ PEPA= 2:1 in weight) was then fabricated by melt blending. A small replacement of IFR with AS-K (1.5 wt %) could enhance the flame retardancy of PP. The LOI could reach up to 35.3 and pass the V-0 rating; additionally, the value of pHRR was significantly reduced for the sample containing IFR and AS-K. It was suggested that AS could release inert gases which dilute the concentration of combustible gas during combustion, and the improved dispersion was beneficial to form a more compact and continued barrier layer which could restrain heat release and hence enhance the flame retardancy.

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Corresponding Author

*Tel: +86 (10) 64434862. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the National Natural Science Foundation of China (No. 21374004 and No. 51373018) and Fundamental Research Funds for the Central Universities (YS201402) for their financial support of this research.

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Flame Retardancy and Thermal Stability of Polypropylene Composite

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