Regulating Effect of Exfoliated Clay on Intumescent Char Structure

Article pubs.acs.org/IECR

Regulating Effect of Exfoliated Clay on Intumescent Char Structure and Flame Retardancy of Polypropylene Composites Yapeng Hu,†,‡ Xiangmei Wang,‡ and Juan Li*,† †

Ningbo Key Laboratory of Polymer Materials, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences Ningbo, Zhejiang 315201, P. R. China ‡ North University of China, Jiancaoping District, Taiyuan, 030051, P. R. China S Supporting Information *

ABSTRACT: An exfoliated montmorillonite (E-MMT) was prepared by introducing Ni2+ and melamine formaldehyde into layers of MMT. The E-MMT was applied to regulate the flame retardant properties of polypropylene (PP)/intumescent flame retardant (IFR) composites. The IFR used consists of ammonium polyphosphate (APP) and a charring agent, 1,3,5-triazine-2,4,6-trimorpholinyl (TTM). The results revealed that the E-MMT has good synergistic effect on flame retardancy of PP/IFR composites. The PP composites with 16 wt % IFR and 3 wt % E-MMT can pass the UL94 V-0 test. Moreover, the EMMT not only improves the char residues of IFR and PP/IFR blend but also changes the micromorphology of char, turning the compact surface char produced by PP/IFR into microporous char. This microporous char is so good for isolating heat and mass between PP and the outside that a better flame retardant property is obtained.

1. INTRODUCTION Polypropylene (PP) as one of the five general plastics is used widely in many industries because of its outstanding properties. However, the flammability of PP restricts its application in the electric and electronic, building, and transport industries etc.1 Therefore, it is necessary to improve the flame retardant performance of PP. Considering environmental and safety awareness, developing halogen-free flame retardant materials and technology is the inevitable strategy.2 The intumescent flame retardant (IFR) system has received considerable attention because it produces little smoke, has low toxicity and produces no corrosive gas during burning.3,4 A typical IFR system is mainly composed of three components: acid source, charring source, and blowing agent.5,6 Only when the suitability among the three sources and between the IFR and polymer matrix are good enough, can a protective foamed char layer be formed. The layer can hinder the transfer of the heat and fuel between the gas phase and the resin during combustion and result in self-extinguishing of materials. The quality, quantity, and structure of intumescent char layer are three important factors for improving the flame retardant efficiency of IFR. To obtain good char, lots of methods have been tried, for example, developing new structure IFR or synergistic technology, etc.7−18 Compared to synthesizing IFR with a novel structure, synergistic technology is widely adopted in flame retardant materials because it is simple and feasible and can achieve functional superposition. The synergists used in IFR systems include metallic compound,10,11 solid acid,12 clay,13−15 carbon nanomaterials,16 and polyoxometalate-based ionic liquids17,18 etc. The addition of IFR has been decreased from 25−30 wt % to 15−20 wt % to achieve the UL-94 V-0 due © 2016 American Chemical Society

to these synergists. However, it is still difficult to get a perfect balance among performance, price, processing, and color, etc. More efforts need to be made to achieve a good comprehensive cost-effective performance. Montmorillonite (MMT) is a natural clay with layer structure which shows a good synergistic effect on thermal and mechanical properties of polymers.19,20 Moreover, it is inexpensive compared to other synergists. So MMT is regarded as one of the most promising flame retardant additives in the past years.21 To obtain better comprehensive properties, MMT must be modified by organic intercalation agents, for example organic quaternary ammonium salt (C12−C18), to enhance its compatibility with the matrix. However, these intercalation agents endow it with good compatibility as well as flammability because of their structure. Though some metals with a catalyst effect22 or even cations with a flame retardant group were used to modify MMT23−25 and improved the performance greatly, it is still difficult to solve the contradiction between good dispersion and flame retardant properties. To eliminate the flammability of organic intercalation agents, researchers prepared MMT modified by organic matter containing phosphorus and nitrogen, and achieved good progress.26,27 However, combining catalyst and synergist into MMT meanwhile to regulate the performance of polymer materials has not yet been reported in public. It is well-known that metal nickel (Ni) can promote cross-linking and charReceived: Revised: Accepted: Published: 5892

February 2, 2016 April 13, 2016 May 5, 2016 May 13, 2016 DOI: 10.1021/acs.iecr.6b00480 Ind. Eng. Chem. Res. 2016, 55, 5892−5901

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Industrial & Engineering Chemistry Research

temperature to 50 °C. Again 12.9 g (0.1 mol) of DIPEA was added into the mixture and reacted for another 4 h. Then the temperature was raised to the boiling point and 12.9 g of (0.1 mol) DIPEA was added. After 6 h the precipitate was collected and washed for several times with water and ethanol to remove the unreacted reagents. The product was dried under vacuum at 60 °C for 12 h. Finally white powder was obtained with a productivity of 87.5%. The synthetic route of TTM is shown in Scheme 1.

forming reactions of polyolefin,28 and melamine formaldehyde (MF) resin not only has good flame retardant performance but also shows a good synergistic effect with ammonium polyphosphate (APP). Therefore, an exfoliated MMT (EMMT) containing both Ni2+ and MF was proposed to modify flame retardancy of PP/IFR. In this case, the Ni2+ can catalyze the char forming reaction28 and MF resin can regulate the compatibility and flame retardant properties of PP/IFR composites. The double effects can work together to obtain better comprehensive performance. The used IFR system is a compound of APP and a charring agent: 1,3,5-triazine-2,4,6-trimorpholinyl (TTM) which was prepared by using cyanuric chloride and morpholine. PP/IFR/ E-MMT composites were prepared through melt-blending. The flame retardant properties were investigated by using the limiting oxygen index (LOI), UL-94 vertical burning test, thermal gravimetric analyzer, and cone calorimeter, etc. Scanning electron microscopy (SEM) and infrared spectroscopic analysis (FT-IR) were also applied to observe the morphology and structure of char residues of PP composites after combustion.

Scheme 1. Synthetic Route of TTM

2. EXPERIMENTAL SECTION 2.1. Materials. Cyanuric chloride, morpholine, melamine, and formaldehyde (37 wt % in water) were purchased from Aladdin Industrial Inc. (Shanghai, China). Toluene, NiCl2· 6H2O and N,N-diisopropylethylamine (DIPEA) were bought from Sinopharm Chemical Reagent Company Limited. All solvents were used without further purification. PP (F401) was supplied by Yangzi Oil Co. Ltd. with a melt index of 2.0 g/10 min. APP (n > 1500, Preniphor TM EPFRAPP231) was purchased from Presafer Chemical Co. Ltd. (Guangdong, China) and sodium based MMT (Na-MMT) was supplied by Zhejiang Feng Hong New Material Co. Ltd. 2.2. Preparation of E-MMT. First, 10 g od Na-MMT was dispersed in 500 mL od distilled water for 2 h in a dry threenecked glass flask with mechanical stirring at 60 °C. NiCl2· 6H2O (3.5 g, 0.015 mol) was dissolved in 50 mL of water, and the obtained solution was added dropwise into the flask over a period of 1 h. The reaction was stirred for 4 h after completion of the addition. The mixture was cooled and filtered. The precipitate was washed for three times with distilled water to take away the redundant metal ion. Then, the clear precipitate (Ni-MMT) was dispersed again in 500 mL of water for use in the next step. Second, 10 g of melamine, 19.3 g of formaldehyde (37 wt % in water) and 50 mL of water were added into another glass flask. The pH of the mixture was 8−9. The mixture was heated slowly to 75 °C with stirring. The heat was stopped as a transparent solution appeared; the solution was added dropwise into the Ni-MMT suspension at 60 °C. Stirring continued for 2 h after the solution was dropped completely, followed by adjusting the pH value of the system to 4.5−5.5. The reaction lasted for another 6 h, and the pH value was kept unchanged. The precipitate was collected and washed with distilled water for three times. The product was dried under vacuum at 80 °C for 12 h. Finally, the E-MMT was obtained. 2.3. Synthesis of TTM. An 18.4 g sample of (0.1 mol) cyanuric chloride, 12.9 g of (0.1 mol) DIPEA and 300 mL of toluene were added to a 500 mL three-necked flask. After complete dissolution, 26.1 g (0.3 mol) of morpholine was added dropwise for about 1 h at 0−5 °C with stirring. The reaction was stirred for another 2 h, followed by a raise of

2.4. Preparation of Flame Retardant PP. All materials were dried in a vacuum oven at 60 °C for 12 h before processing. The composites were prepared on a Brabender mixer at 200 °C with a rotation speed of 50 rpm for 8 min. In this work, The IFR consists of APP and TTM with a weight ratio of 2:1. After mixing, the samples were hot-pressed under 10 MPa at 200 °C for 3 min into a sheet in the dimensions of 100.0 × 100.0 × 3.2 mm3, and then cut into bars with suitable size for the LOI and UL-94 testing. 2.5. Characterization. Fourier transform infrared spectrometry (FTIR) was recorded from samples pressed into pellets with KBr powder by using Nicolet 6700 FT-IR. Nuclear magnetic resonance spectrometry (NMR) spectra were tested with a 400 MHz AMX NMR spectrometer using CDCl3 as solvent. XRD tests were performed on a Bruker D8 Advance X-ray diffractometer with Cu Kα1 radiation, at a scanning rate of 2°/ min from 0° to 10°, operated at 40 Kv and 20 mA. The interlayer space of MMT (d001) could be calculated according to the Bragg equation. LOI was carried in a 5801 digital oxygen index analyzer (Kunshan Yang Yi test Instrument Co., Ltd.) with a sample size 100.0 mm × 6.5 mm × 3.2 mm according to ASTM D2863-97. UL-94 vertical tests were performed on an AG5100B vertical burning tester (Zhuhai Angui Testing Equipment Company, China) with a sample measurement of 100.0 mm × 13.0 mm × 3.2 mm according to ASTM D3801. Cone calorimeter testing was performed on FTT0242Standard Cone (Fire Testing Technology Limited., UK) at 35 kW/m2 heat flux. Thermal gravimetric analyzer (TGA) was conducted on a Mettler Toledo TGA/DSC1 analyzer; 3−5 mg specimens were heated from 50 to 800 °C at a heating rate of 10 °C/min under nitrogen (N2) or air atmosphere (50 mL/min). SEM was used to examine the morphology of the residue char obtained from LOI test by using S4800 (Hitachi Corp., Japan). All the samples were sputter-coated with a platinum layer before testing. TEM was conducted to observe the dispersion of E-MMT in matrix by using JEM2100 (JEOL Limited., Japan). 5893

DOI: 10.1021/acs.iecr.6b00480 Ind. Eng. Chem. Res. 2016, 55, 5892−5901

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3. RESULTS AND DISCUSSION 3.1. Characterization of E-MMT and TTM. XRD patterns for MMT before and after modification are shown in Figure 1.

Figure 3 presents the FTIR spectra of different MMT. The Na-MMT shows the characteristic peaks at 3625, 1043, 522,

Figure 1. XRD patterns for (a) Na-MMT, (b) Ni-MMT, (c) E-MMT.

Figure 3. FT-IR spectra of the MMT: (a) Na-MMT, (b) Ni-MMT, and (c) E-MMT.

The characteristic diffraction peak of Na-MMT appears at 2θ = 7.08°, corresponding to the gallery height (d001 spacing) of 1.24 nm. When Na+ is replaced by Ni2+, the peak shifts to 2θ = 6.38° indicating the increase of d001-spacing (1.38 nm) in MMT. After being modified by MF the peak disappears completely in E-MMT suggesting that MMT has been exfoliated. The exfoliated structure of MMT helps it to be dispersed uniformly in polymer. Figure 2 shows the EDX of different MMTs. The main elements of Na-MMT include silicon (Si), aluminum (Al),

and 469 cm−1. The peak at 3625 cm−1 is related to O−H stretching. The peaks at 1043 and 469 cm−1 are corresponding to the Si−O stretching and Si−O−Si blending vibrations, respectively. The peak at 522 cm−1 is attributed to Al−O stretch vibrations. Along with the characteristic peaks of NaMMT, little change is observed for the Ni-MMT. While new peaks appears in the FTIR spectrograph of E-MMT (Figure 3c). The peak at 2972 cm−1 is assigned to C−H stretching of −CH2−, the peaks at 1553 and 1341 cm−1 are attributed to the vibration of the −CN− and N in the triazine ring. These new peaks belong to MF resin. The results further demonstrate the structure of E-MMT. Figure 4 shows the FTIR spectra of cyanuric chloride, morphine, and TTM. The peaks at 2959 and 2858 cm−1 can be

Figure 4. FTIR spectra of (a) cyanuric chloride, (b) morphine, (c) TTM.

assigned to the VC−H in −CH2−CH2−, the peak at 1545 cm−1 is attributed to VCN in the triazine ring, the peaks at 1361, 1251, and 1112 cm−1 are corresponding to Vtr‑N, VC−N, and VC−O−C. The disappearance of characteristic absorption bands of the VN−H at 3316 cm−1 in morphine and 848 cm−1 of the Vtr‑cl in cyanuric chloride indicates TTM is prepared successful. 1 H NMR spectrum of TTM is presented in Figure 5. No H atom exists in the structure of cyanuric chloride. The peaks at chemical shifts of 3.6−3.8 are ascribed to the H atom of the −CH2−CH2− groups in morphine which supports the structure of the product further. 3.2. Morphology of PP/IFR/E-MMT Composites. Figure 6 gives the SEM and TEM micrograph of PP/IFR/E-MMT. Many IFR particles are observed in the PP matrix from SEM, and little evidence of accumulation is found suggesting relative uniform dispersion of IFR in PP. It is difficult to identify MMT because it shows no characteristic morphology in SEM. To

Figure 2. EDX of (a) Na-MMT, (b) Ni-MMT, and (c) E-MMT.

magnesium (Mg), Na, and oxygen (O). The Na disappears in Ni-MMT and E-MMT because it is replaced by Ni2+ through an ion exchange reaction. The elements carbon (C) and nitrogen (N) appear in E-MMT which belong to MF resin. The results show that E-MMT was prepared successfully. 5894


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PP, the LOI value is increased to 28.2 though no UL-94 rating is obtained suggesting good synergistic effect between APP and TTM. When the loading of IFR is increased to 25 wt %, the PP/IFR system achieves a LOI value 30.6 and passes the UL-94 V-0 test. Nevertheless, when 1 wt % E-MMT is combined with 19 wt % IFR, the PP4 achieves the UL-94 V-1 rating without melt dripping and a LOI value 27.4. Keep the total content of IFR/E-MMT unchanged and adjust the addition of E-MMT from 1 wt % to 5 wt %, it is found that the flame retardant properties of PP composites improve first and decrease again. PP6 achieves the best flame retardancy, so the optimum additive amount of E-MMT is about 3 wt %. The addition of too much E-MMT will deteriorate the flame retardant properties. Only suitable content of E-MMT has a good synergistic effect on the flame retardant performance. The PP composites which can pass the UL94 V-0 test need no less than 16 wt % IFR and 3 wt % E-MMT. The photos of PP and its composites after LOI and UL-94 tests are shown in Figure 7. It is evident that PP burns with

Figure 5. 1H NMR spectrum of TTM.

Figure 6. Micrograph of PP/IFR/E-MMT: (a) SEM, (b) TEM.

investigate the dispersion of MMTs in PP, a partial magnified view was obtained by using TEM, and the result is shown in Figure 6b. In the TEM micrograph, the darker lines are exfoliated E-MMT layers since they have higher electron density and white domains represent the PP matrix. Evidently the exfoliated layers of E-MMT have not been broken or accumulated together during processing which demonstrates the good dispersion of E-MMT in PP composites. Combining SEM and TEM, the PP/IFR/E-MMT composites have a good dispersion state that is necessary for achieving good and stable performance. 3.3. Flame Retardant Properties. The flame retardant properties of PP/IFR/E-MMT composites are shown in Table 1. PP exhibits a LOI value of 17.5 and is not classed (NC) in the UL-94 test. The single addition of APP or TTM, LOI of PP composites is only about 18. While adding 20 wt % IFR into

Figure 7. Photos of the char residues after LOI (top) and UL94 (bottom) tests for: (a) PP0, (b) PP12, (c) PP13, (d) PP3, (e) PP2, (f) PP1, (g) PP4, (h) PP5, (i) PP6, (j) PP7, (k) PP11, (l) PP8.

severe melt-dripping, and no char residue is left. Adding 20 wt % APP or TTM individually, only a little char layer is obtained on the surface of the samples. However, some intumescent char could be observed on the surface of PP3 which contains 20 wt % IFR. Figure 7f is the photo of PP1 containing 25 wt % IFR. It shows obvious intumescent char which could act as an effective protective shield for underlying matrix and achieve good results in the LOI and UL-94 tests. This reveals that there exists good

Table 1. Flame Retardant Properties of PP Composites

a

samples

PP wt %

PP0 PP1 PP2 PP3 PP4 PP5 PP6 PP7 PP8 PP9 PP10 PP11 PP12 PP13 PP14 PP15

100 75 78 80 80 80 80 80 75 78 81 82 80 80 80 80

APP wt %

TTM wt %

16.7 14.7 13.3 12.7 12 11.3 10 14.7 12.7 10.7 10 20

8.3 7.3 6.7 6.3 6 5.7 5 7.3 6.3 5.3 5

E-MMT wt %

t1/t2 sa

dripping

UL-94

LOI

1 2 3 5 3 3 3 3

>30/− 0/4 0/17 0/>30 0/12 0/7 0/6 0/20 0/0 0/2 0/9 0/15 >30/− >30/− 0/>30 >30/−

Y N N Y N N N N N N N N Y Y Y Y

NC V-0 V-1 NC V-1 V-0 V-0 V-1 V-0 V-0 V-0 V-1 NC NC NC NC

17.5 30.6 29.8 28.2 27.4 28.8 29.2 26.9 31.2 30.6 28.6 27.2 18.4 17.6 24.8 17.7

20 17

3 3

17

t1/t2 is the average value of five tests in the UL-94 testing. 5895


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Figure 8. TGA curves of the IFR/E-MMT under (a) air and (b) N2 atmosphere.

Table 2. TGA Data of IFR and E-MMT under Air and N2 Atmosphere air char residues (wt %) samples

T1 wt % (°C)

T5 wt % (°C)

T10 wt % (°C)

T50 wt % (°C)

600 °C

700 °C

800 °C

IFR E-MMT IFR/E-MMT−calc IFR/E-MMT−expt

210.1 72.5 138.7 153.5

266.8 165.8 264.3 260.7

280.2 319.8 283.1 277.2

515.6 539.7 519.2 540.3

12.3 39.7 16.4 43.9

9.1 37.1 13.3 38.9

7.7 36.9 12.1 33.2

N2 char residues (wt %) samples

T1 wt % (°C)

T5 wt % (°C)

T10 wt % (°C)

T50 wt % (°C)

IFR E-MMT IFR/E-MMT−cal. IFR/E-MMT−exp.

238.3 88.5 202.3 179.6

270.1 197.7 269.2 246.5

283.5 349.5 284.8 266.8

522.8 585.3 531.3 409.8

600 °C 34.2 48.6 36.4 42.0

700 °C 23.9 42.8 26.7 35.4

800 °C 15.4 39.6 19.1 29.8

Figure 9. TGA curves of the PP/IFR/E-MMT under (a) air and (b) N2 atmosphere.

results are closely related to the flame retardancy of PP composites. All samples that passed UL-94 V0 have a short burning time so their shape remains good. 3.4. Thermal Degradation Behaviors. Figure 8 shows the TGA curves of the IFR/E-MMT in air and N2 atmosphere, and the detailed data are listed in Table 2. In addition, the TGA curves of APP/TTM are shown in Figure S1 in the Supporting Information. The experimental (exp.) curves are obtained from TGA of blend directly. The calculated (cal.) curves are calculated by using the curves of IFR and TTM according to their weight percentage in the blend. The temperatures at 1, 5, 10, and 50 wt % degradation are noted as T1 wt %, T5 wt %, T10 wt % and T50 wt %, respectively. The results reveal that at early degradation, T1 wt % for the IFR/E-MMT-exp. in air atmosphere is 153.5 °C, which is higher than their cal. value. While in N2 atmosphere, an opposite result is obtained. But T5 wt % for IFR/ E-MMT-exp. in air and N2 atmosphere are both lower than

synergistic effect on char formation between APP and TTM. After the introduction of E-MMT, the volume of char residues covered on the samples from PP4 to PP7 is bigger than that of PP3. Especially, PP6 obtains the maximum intumescent char which is a good barrier to inhibit the transfer of combustible gas and heat flow; hence, the flame retardant property is improved compared to those samples without E-MMT. Too much of EMMT will harden or destroy the char residue. So an appropriate addition of E-MMT promotes the generation of swelling char layer, and thus results in good flame retardant property. In the UL-94 test, PP0, PP13, PP12, and PP3 burned to the clamp because of their flammability. PP2 is out of shape due to the long burning time before extinguishment. The shape of PP1 changes slightly. PP5 and PP6 keep better shape because the PP composites extinguished quickly, while obvious shapechange and abundant char residue are observed for PP7. The 5896


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Industrial & Engineering Chemistry Research Table 3. TGA Data of PP/IFR/E-MMT Composites air

char residues (wt %) samples

T1 wt % (°C)

T5 wt % (°C)

PP IFR E-MMT PP/IFR/E-MMT−exp. PP/IFR/E-MMT−cal.

249.7 210.1 72.5 241.8 254.0

278.3 266.8 165.8 278.8 276.5

T10 wt % (°C)

T50 wt % (°C)

600 °C

800 °C

326.3 515.6 539.7 358.8 329.3

0 12.3 39.7 13.7 3.9

0 7.7 36.9 6.2 3.0

288.5 280.2 319.8 290.5 287.7 N2

char residues (wt %) T

samples PP IFR E-MMT PP/IFR/E-MMT−exp. PP/IFR/E-MMT−cal.

1 wt %

(°C)

363.8 238.3 88.5 207.0 267.5

T

5 wt %

T

(°C)

10 wt %

409.3 270.1 197.7 294.1 306.6

(°C)

425.5 283.5 349.5 369.2 395.5

T

50 wt %

(°C)

600 °C

800 °C

0 34.2 42.8 6.5 7.2

0 15.4 39.6 5.0 3.7

455.0 522.8 585.3 463.8 456.0

Figure 10. TGA curves of PP composites under (a) air and (b) N2 atmosphere.

Table 4. TGA Data of Different PP Composites air char residues (wt %) samples

T1 wt % (°C)

T5 wt % (°C)

T10 wt % (°C)

T50 wt % (°C)

600 °C

700 °C

800 °C

PP0 PP3 PP4 PP6 PP7

249.7 250.5 245.3 241.8 226.2

278.3 275.7 290.2 278.8 277.1

288.5 286.2 302.5 290.5 289.3

326.3 356.5 347.6 358.8 354.8

0 9.0 11.6 13.7 12.0

0 2.9 5.6 7.9 8.3

0 2.6 4.4 6.2 7.1

N2 char residues (wt %) samples

T1 wt % (°C)

T5 wt % (°C)

T10 wt % (°C)

T50 wt % (°C)

600 °C

700 °C

800 °C

PP0 PP3 PP4 PP6 PP7

363.8 210.2 239.3 207.0 213.5

409.3 296.5 298.5 294.1 286.0

425.5 377.8 372.5 369.2 370.0

455 464.7 464.3 463.8 464.5

0 5.9 5.6 6.5 4.7

0 2.1 4.4 5.7 4.0

0 1.5 3.6 5.0 3.4

their cal. values. Furthermore, the char residues at 800 °C for IFR/E-MMT are 33.2 wt % which is 21.1 wt % higher than the cal. value in air atmosphere. The results suggest that E-MMT causes the IFR to decompose at a lower temperature, but improves the quantity of char residue at high temperature. The gap of char residues between cal. and exp. in air atmosphere is higher than in N2 suggesting air helps IFR/E-MMT to form more char residues. Enough of a char layer of high quality can

act as a good protective barrier, which prevents the underlying matrix from decomposition and combustion and provides a better microenvironment for intumescent char formation. The thermal degradation behaviors of PP/IFR/E-MMT composites under air and N2 atmosphere are shown in Figure 9 and Table 3. The cal. and exp. curves are obtained as the above method and the weight percentage is PP:IFR:E-MMT = 80:17:3. In air atmosphere, the T1 wt % of PP/IFR/E-MMT-exp. 5897


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Figure 11. Relationship between (a)HRR, (b) THR, (c) ML and time for PP composites.

is 241.8 °C which is lower than that of pure PP and the cal. value, while the degradation temperatures are higher than the cal. value above T5 wt %. Additionally, the char residue at 800 °C reaches 6.2 wt % which is 3.2 wt % higher than the cal. value. As for N2 atmosphere, the T1 wt % of PP6-exp. is 207.0 °C, which is nearly 60 °C lower than the cal. value. However, the degradation temperatures are not improved evidently with increasing temperature. There is 5.0 wt % char residue at 800 °C for PP/IFR/E-MMT and it is only 1.3 wt % higher than the cal. value. The results suggest that the decomposition temperature of PP/IFR/E-MMT is advanced at low temperature and the char formation is promoted by E-MMT at high temperature. Thermal degradation behaviors of different PP composites are shown in Figure 10 and Table 4. Under air atmosphere, after the addition of IFRs, the degradation temperature form T1 wt % to T10 wt % changes slightly. While T50 wt % of PP3 is 30.3 °C higher than PP0. The char residue of PP3 is 2.6 wt % at 800 °C. With the addition of E-MMT, T1 wt % of PP composites is reduced evidently. Moreover, this decrease is improved with the E-MMT content improving. It is caused by the early degradation of E-MMT itself. In addition, the char residues have a significant increas at 800 °C, for example the char residues for PP6 is 6.2 wt %. Under N2 atmosphere, T1 wt % for all PP composites are reduced dramatically. E-MMT does not affect the temperature greatly as in air. All samples show similar T50 wt %, which is about 14 °C higher than that of PP0. However, the incorporation of E-MMT increases the char residues of PP composites. PP6 has the biggest char residue at the temperature range from 600 to 800 °C. While PP7 containing more E-MMT than PP6 only obtains 3.4 wt % char residue at 800 °C. Therefore, only at a suitable ratio can good synergistic effect be obtained, which is in accordance with the flame retardant properties. 3.5. Cone Calorimeter Analysis. Cone calorimeter is usually used to evaluate the heat release rate during

combustion. Some useful parameters can be obtained to simulate the real fire conditions such as heat release rate (HRR), total heat release (THR), and mass loss. Figure 11 shows the relationship between HRR, THR, ML, and time for PP and its composites at a heat flux of 35 kW/m2, the related data are presented in Table 5. Table 5. Cone Calorimeter Test Data of PP Composites samples

TTI (s)

PHRR (kW/m2)

THR (kJ/m2)

mean MLR (g/s)

char residues (wt %)

PP0 PP3 PP4 PP6 PP7

48 38 36 31 30

1007 360 363 328 305

120 86 80 77 107

0.104 0.038 0.026 0.025 0.054

0 29 36 34 21

PP0 is flammable and it shapes a sharp HRR peak of 1007 kW/m2. Double HRR peaks are observed for other PP composites which is typical behavior of PP/IFR systems. The first HRR peak is attributed to the formation of intumescent char layer and the second HRR peak is ascribed to collapse of unstable char layer. With incorporation of 20 wt % IFR into PP, the HRR peak of PP3 is reduced dramatically to 360 kW/m2. When combining E-MMT with IFR, the HRR peaks of PP6 and PP7 are decreased further to 328 and 305 kW/m2, respectively, that for PP4 does not change greatly because of too little E-MMT in the PP composites. Although PP7 gets the lowest HRR peaks among them, but the HRR values between 100 to 200 s are obviously higher than that of other PP composites. The reason can be attributed that the quality of char layer formed by PP7 is poor as shown in Figure 12e. THR result is closely related to HRR. The THR decreases from 120 kJ/m2 of PP0 to 87 kJ/m2 of PP3. PP6 has the lowest THR value 77 kJ/m2. While the THR value of PP7 increases to 107 kJ/m2. 5898


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3.6. Morphology Analysis of Char Residues. Taking into consideration that the microstructure of charring layer plays a significant role in flame retardant performance, the morphology of the char layers were examined by SEM which are shown in Figure 13. Some holes and cracks are found in the

Figure 12. Photos of PP composites after cone calorimeter tests: (a) PP0, (b) PP3, (c) PP4, (d) PP6, (e) PP7.

Figure 13. SEM photographs of char residues of (a) PP3, (b) PP4, (c) PP6, (d) PP7.

char layers of PP3. With the introduction of E-MMT, the inside structure of char is still composed of large poles as shown in Figure 13b. However, the surface structure of the char layer is regulated by E-MMT obviously. Lots of micropores are observed for PP4, PP6, and PP7. When the content of EMMT is not enough, part of the char layer keeps the same morphology as PP3 as noted by arrows 5 and 6, and part char layer becomes a microporous structure. With the content of EMMT improving, the whole char layer looks microporous as shown in PP6 and PP7. Too much E-MMT makes the char brittle, so there is a large crack in the char layer of PP7. The char layer with an intumescent inside structure and a microporous outside structure helps to block gas and heat exchange between the flame zone and resin. Therefore, a high efficiency of flame retardancy for PP composites is achieved. 3.7. FT-IR of the Char Residues. The FT-IR spectra of char layers are shown in Figure 14. All samples have similar FTIR curves. The peaks above 3000 cm−1 are assigned to OH. The peak at 1642 cm−1 belongs to CC, 1391 cm−1 is attributed to βO−H and 1008 cm−1 belongs to P−O. The result indicates that the E-MMT does not change the chemical structure of the char. So the char-forming speed, micropores structure, and quantity

The time to ignition (TTI) of PP/IFR composites is reduced due to the existence of IFR. TTI for PP composites containing E-MMT are decreased further. Moreover the more the E-MMT is, the shorter the TTI is. This may be caused by the early degradation of IFR and E-MMT, because their degradation will release some flammable gas which helps the ignition of samples. Of course a char layer will be generated after ignition and it will cover the surface of the substrate and prevent the heat and gas from transferring to the underlying resin, so the HRR of the PP composites grows slowly. On the other hand, with the increase of E-MMT, the content of IFRs is decreased. Lack of IFR will also shorten TTI. Figure 11c shows the change of the residual mass of the samples with combustion time. Pure PP produces little char residue. When combined with IFR or IFR/E-MMT, the composites present an increase in the residual mass. PP4 and PP6 have a larger char residue than PP3, while PP7 achieves the lowest char residue among all PP composites indicating that excessive addition of E-MMT has some negative effects on flame retardant performance. Combining all these data, it can be concluded that a suitable addition of E-MMT has a positive synergistic effect on the PP/IFR system. It promotes the formation of a char layer which serves as a good barrier to slow down the heat and gas release, resulting in the reduction of the THR of the composites and increase of char residues. However, too much E-MMT will make the PP composites more flammable and reduce char residue. Figure 12 shows the photos of PP composites after cone calorimeter testing. PP0 leaves nothing after combustion. With the addition of 20 wt % IFR, an intumescent char layer is formed. When IFR is combined with E-MMT, the intumescent volume of char residues becomes greater. Moreover, the volume of char layers for PP6 and PP7 is larger than that of PP4. However, a lot of large holes are observed in the char layer of PP7, which cannot prevent combustible gases and heat from transferring into the inside. The char layer for PP6 and PP4 looks better than PP7, few macroscopic holes are observed.

Figure 14. FT-IR spectra of char layers for (a) PP3, (b) PP4, (c) PP6, and (d) PP7. 5899



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should be the key factors for improving flame retardancy of PP composites. In general, appropriate addition of E-MMT improves the quantity, modifies the quality, and regulates the porous structure of char, resulting in good flame retardant of PP/IFR composites.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b00480. The exp. and cal. TGA curves of APP/TTM system (PDF)

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REFERENCES

(1) Green, J. The flame retardation of polyolefins. Flame-Retardant Polymeric Materials; Springer: New York. 1982, 1. (2) Van der Veen, I.; de Boer, J. Phosphorus flame retardants: properties, production, environmental occurrence, toxicity and analysis. Chemosphere 2012, 88, 1119. (3) Camino, G.; Costa, L.; Trossarelli, L.; Costanzi, F.; Pagliari, A. Study of the mechanism of intumescence in fire retardant polymers: Part VI-Mechanism of ester formation in ammonium polyphosphatepentaerythritol mixtures. Polym. Degrad. Stab. 1985, 12, 213. (4) Alongi, J.; Han, Z. D.; Bourbigot, S. Intumescence: Tradition versus novelty. A comprehensive review. Prog. Polym. Sci. 2015, 51, 28. (5) Enescu, D.; Frache, A.; Lavaselli, M.; Monticelli, O.; Marino, F. Novel phosphorous-nitrogen intumescent flame retardant system. Its effects on flame retardancy and thermal properties of polypropylene. Polym. Degrad. Stab. 2013, 98, 297. (6) Su, X.; Yi, Y.; Tao, J.; Qi, H.; Li, D. Synergistic effect between a novel triazine charring agent and ammonium polyphosphate on flame retardancy and thermal behavior of polypropylene. Polym. Degrad. Stab. 2014, 105, 12. (7) Nie, S.; Hu, Y.; Song, L.; He, Q.; Yang, D.; Chen, H. Synergistic effect between a char forming agent (CFA) and microencapsulated ammonium polyphosphate on the thermal and flame retardant properties of polypropylene. Polym. Adv. Technol. 2008, 19, 1077. (8) Li, B.; Xu, M. Effect of a novel charring−foaming agent on flame retardancy and thermal degradation of intumescent flame retardant polypropylene. Polym. Degrad. Stab. 2006, 91, 1380. (9) Liu, Y.; Zhao, J.; Deng, C. L.; Chen, L.; Wang, D. Y.; Wang, Y. Z. Flame-retardant effect of sepiolite on an intumescent flame-retardant polypropylene system. Ind. Eng. Chem. Res. 2011, 50, 2047. (10) Zhou, K.; Jiang, S.; Wang, B.; et al. Combined effect of transition metal phosphide (MxPy, M= Ni, Co, and Cu) and intumescent flame retardant system on polypropylene. Polym. Adv. Technol. 2014, 25, 701. (11) Wang, D. Y.; Liu, Y.; Ge, X. G.; et al. Effect of metal chelates on the ignition and early flaming behaviour of intumescent fire-retarded polyethylene systems. Polym. Degrad. Stab. 2008, 93, 1024. (12) Liu, Y.; Wang, Q. Catalytic action of phospho-tungstic acid in the synthesis of melamine salts of pentaerythritol phosphate and their synergistic effects in flame retarded polypropylene. Polym. Degrad. Stab. 2006, 91, 2513. (13) Stretz, H. A.; Wootan, M. W.; Cassidy, P. E.; Koo, J. H. Effect of exfoliation on poly(styrene-co-acrylonitrile)/montmorillonite nanocomposite flammability. Polym. Adv. Technol. 2005, 16, 239. (14) Kiliaris, P.; Papaspyrides, C. D. Polymer/layered silicate (clay) nanocomposites: an overview of flame retardancy. Prog. Polym. Sci. 2010, 35, 902. (15) Beyer, G. Filler blend of carbon nanotubes and organoclays with improved char as a new flame retardant system for polymers and cable applications. Fire Mater. 2005, 29, 61. (16) Peeterbroeck, S.; Laoutid, F.; Swoboda, B.; et al. How carbon nanotube crushing can improve flame retardant behaviour in polymer nanocomposites? Macromol. Rapid Commun. 2007, 28, 260. (17) Chen, S.; Li, J.; Zhu, Y.; Guo, Z.; Su, S. Increasing the efficiency of intumescent flame retardant polypropylene catalyzed by polyoxometalate based ionic liquid. J. Mater. Chem. A 2013, 1, 15242. (18) Chen, S.; Li, J.; Zhu, Y.; Su, S. Roles of anion of polyoxometalate-based ionic liquids in properties of intumescent flame retardant polypropylene. RSC Adv. 2014, 4, 32902. (19) Kozlowski, R.; Wesolek, D.; Wladyka-Przybylak, M.; Duquesne, S.; Vannier, A.; Bourbigot, S.; Delobel, R. Intumescent flame-retardant treatments for flexible barriers. Multifunctional Barriers for Flexible Structure; Springer: Berlin, Heidelberg, 2007, 39. (20) Xie, W.; Gao, Z.; Pan, W. P.; Hunter, D.; Singh, A.; Vaia, R. Thermal degradation chemistry of alkyl quaternary ammonium montmorillonite. Chem. Mater. 2001, 13, 2979. (21) Tang, Y.; Hu, Y.; Wang, S.; Gui, Z.; Chen, Z.; Fan, W. Intumescent flame retardant−montmorillonite synergism in polypropylene-layered silicate nanocomposites. Polym. Int. 2003, 52, 1396.

4. CONCLUSIONS In this paper, an E-MMT was prepared by introducing Ni2+ and MF resin into layers of MMT. The E-MMT was used as a synergist to regulate the flame retardant properties of polypropylene PP/IFR composites. The IFR is a mixture of APP and a charring agent, TTM, at weight ratio of 2:1 which achieves good flame retardant performance in PP. The results are shown as follows. (1) The E-MMT shows exfoliated structure because of modification by Ni2+ and MF. It can be dispersed in PP resin uniformly as shown in SEM and TEM photos. (2) The E-MMT improves the flame retardant efficiency of PP/IFRs at a content no more than 3 wt %. The PP composites containing 16 wt % IFRs and 3 wt % E-MMT can be classified at the UL94 V-0 level. In addition, the cone calorimeter test suggested that the HRR, THR, and ML are also decreased. (3) The interaction between E-MMT, IFR, and PP were studied by TGA. The results indicated that E-MMT promotes the degradation of IFR at lower temperature and induces the char formation of IFR and PP/IFR at higher temperature in both air and nitrogen atmosphere. (4) The volume of intumescent char layers becomes larger than that without E-MMT. The E-MMT regulates the superficial structure of the char layer which changes from compact to microporous. It is the char layer with an intumescent inside and a microporous outside that helps to block gas and heat exchange between the flame zone and resin. Therefore, better flame retardant performances for PP/IFR are obtained based on the physical (char structure) and chemical (char formation) effects of E-MMT. Of course, too much or too little E-MMT does not help to form a perfect char layer, only a suitable addition of E-MMT improves both quantity and quality of char.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 0086-574-86685256. Fax: 0086-574-86685186. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported financially by the National Natural Science Foundation of China (Nos.21274159 and 51473178) and the Ningbo Science and Technology Innovation Team (No. 2015B11005). 5900


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Industrial & Engineering Chemistry Research (22) Kong, Q.; Lv, R.; Zhang, S. Flame retardant and the degradation mechanism of high impact polystyrene/Fe-montmorillonite nanocomposites. J. Polym. Res. 2008, 15, 453. (23) Huang, G.; Yang, J.; Wang, X.; Gao, J. Nanoclay, intumescent flame retardants and their combination with chemical modification for the improvement of the flame retardant properties of polymer nanocomposites. Macromol. Res. 2013, 21, 27. (24) Tai, Q.; Yuen, R. K.; Song, L.; Hu, Y. A novel polymeric flame retardant and exfoliated clay nanocomposites: Preparation and properties. Chem. Eng. J. 2012, 183, 542. (25) Fang, K. Y.; Li, J.; Ke, C. H.; Zhu, Q. L.; Tao, K.; Zhu, J.; Yan, Q. Intumescent flame retardation of melamine-modified montmorillonite on polyamide 6: enhancement of condense phase and flame retardance. Polym. Eng. Sci. 2011, 51, 377. (26) Huang, G.; Gao, J.; Li, Y.; Han, L.; Wang, X. Functionalizing nano-montmorillonites by modified with intumescent flame retardant: Preparation and application in polyurethane. Polym. Degrad. Stab. 2010, 95, 245. (27) Huang, G.; Li, Y.; Han, L.; Gao, J.; Wang, X. A novel intumescent flame retardant-functionalized montmorillonite: Preparation, characterization, and flammability properties. Appl. Clay Sci. 2011, 51, 360. (28) Tang, T.; Chen, X.; Meng, X.; Chen, H.; Ding, Y. Synthesis of multiwalled carbon nanotubes by catalytic combustion of polypropylene. Angew. Chem., Int. Ed. 2005, 44, 1517.

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Regulating Effect of Exfoliated Clay on Intumescent Char Structure

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