Double Functions of Chlorinated Carbon Nanotubes in Its

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J. Phys. Chem. C 2010, 114, 13226–13233

Double Functions of Chlorinated Carbon Nanotubes in Its Combination with Ni2O3 for Reducing Flammability of Polypropylene Haiou Yu,†,‡ Zhenjiang Zhang,†,‡ Zhe Wang,†,‡ Zhiwei Jiang,†,‡ Jie Liu,†,‡ Lu Wang,†,‡ Dong Wan,†,‡ and Tao Tang*,† State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China and Graduate School of the Chinese Academy of Sciences, Beijing 100039, China ReceiVed: May 10, 2010; ReVised Manuscript ReceiVed: June 17, 2010

Chlorinated carbon nanotubes (CNT-Cl) and its combination with Ni2O3 were used to prepare polypropylene (PP) composites via melt mixing. The cone calorimetry and TGA results showed that both the flame retardancy and the thermal stability of the composites containing CNT-Cl were better than the samples containing unchlorinated CNTs. NMR measurements confirmed that CNT-Cl could react with the double bond of degradation products at high temperature. Furthermore, the role of CNT-Cl in the combination between CNTCl and Ni2O3 was investigated by carbonization experiments and rheology measurements. The results demonstrated that CNT-Cl could significantly promote the carbonization of degradation products of PP catalyzed by nickel catalyst during combustion. In contrast, the combination between unchlorinated CNTs and Ni2O3 could not promote the carbonization of degradation products of PP compared to the case containing Ni2O3 alone. This was attributed to Cl radicals releasing from CNT-Cl, which worked as a catalyst to accelerate dehydrogenation-aromatization of degradation products of PP and promoted the carbonization of degradation products catalyzed by nickel catalyst. 1. Introduction Carbon nanotubes (CNTs) have been successfully used for many different applications due to their special combination of dimension, structure, and topology.1-5 In recent years, it has been of interest to use CNTs at low-loading content to obtain materials with reduced flammability.6-10 Among these reports, the most dominant mechanism is the in situ formation of a continuous, network structured protective layer from the CNTs, which is critical for significant reduction in heat release rate (HRR), because the layer thus acts as a thermal shield for feedback from the flame. In addition, Peeterbroeck et al. also found that crushed CNTs could substantially delay the time to ignition while maintaining a much reduced HRR.11 This was explained by the chemical reactivity of radical species at the surface/extremities of crushed CNTs during the combustion process of EVA nanocomposites. However, crushed CNTs were obtained through the ball-milling process for 15 h, and the average length was reduced to ca. 300 nm, meaning that the character of CNTs, such as the high aspect ratio, had been destroyed. The combustion process of polymers is complicated, which takes place at least in three interdependent phases, namely, condensed phase, gas phase, and interphase. However, from the viewpoint of reaction, it can be simply thought to involve the breakdown of polymer chains to form macroradicals and H•, which in turn speed up the degradation of the polymers.12,13 Recently, C60 has been found to reduce the flammability of polypropylene (PP) by trapping free radicals and in situ forming * To whom correspondence should be addressed. Phone: +86 (0) 431 85262004. Fax: +86 (0) 431 85262827. E-mail: [email protected]. † Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences.

a gelled ball cross-linking network to improve the flame retardancy in the condensed phase.14 Barrera used fluorinated CNTs to reinforce polymer matrix by in situ direct covalent bonding between the CNTs and the matrix via defluorination during melt compounding.15 If the covalent bonds are formed in the interfacial region between polymers and CNTs during combustion, the amount of volatile components into the flame region will be reduced. As a result, the flammability of the composites will be decreased. Recently, our group reported a novel method for improving the flame retardancy of polymers in which solid acids16-18 or a trace amount of halogenated compounds19 could efficiently improve the carbonization conversion of the degradation products during combusting polyolefin/nickel catalyst composites. As a result, a large amount of char, including CNTs, was in situ formed in the middle stage of combustion. In this case, the char resulted from the degradation products of the polymer itself exposed to fire conditions; thus, the amount of flammable volatiles evolved was reduced, and the flame retardancy of the polymer was also improved. This method is efficient to improve the flame retardancy of polyolefins, including PP and polyethylene.19,20 On the basis of the above analysis, we assume that if the surface of CNTs contains some chemical bonds which will be stable during melt mixing for preparing polymer composites and can break easily at high temperature to form free radicals on the surface of CNTs, the surface of CNTs may trap more degradation products caused by degradation of the polymer during combustion. As a result, the thermal stability and even flame retardancy can be improved. Furthermore, if the chemical bonds on the surface of CNTs are carbon-halogen bonds, the released halogen radicals can efficiently promote catalytic carbonization of the degradation products during combusting polyolefin/nickel catalyst composites.19 To test these hypotheses, in this work, we used chlorinated CNTs (CNT-Cl) as a model

10.1021/jp104216r  2010 American Chemical Society Published on Web 07/16/2010

Double Functions of Chlorinated Carbon Nanotubes to study the influence of CNT-Cl and its combination with Ni2O3 on the thermal stability and flame retardancy of PP. The mechanism of the effect was investigated. 2. Experimental Section 2.1. Materials and Preparation of Samples. Commercially available polypropylene (PP, melt flow index: 0.8 g/min) was obtained from Yanshan Chemical Co. Carbon nanotubes (CNTs) with diameters ranging from 10 to 20 nm were purchased from Chengdu Organic Chemicals Co. Ltd. Before using, chlorinated CNTs (CNT-Cl) was synthesized via reflux in the mixture of nitric and hydrochloric acid (HNO3:HCl ) 3:1; both were analytical grade, HNO3 70 wt %, HCl 37 wt %) for 6 h. The sample was then washed and neutralized with deionized water until a pH value of about 6 was reached. CNT-Cl was finally filtered and dried in a vacuum oven at about 80 °C. For comparison, another modified CNT (HCNT) was prepared by refluxing in HNO3 for 6 h. To enhance the dispersion of CNTs and Ni2O3 (with an average diameter of 250-300 nm, from Lingfeng Chemical Co., Shanghai) in PP matrix, maleated polypropylene (PPMA, with 2.3 wt % maleic anhydride, Mw ) 40 000, purchased from Sanyo Co.) was used as a compatibilizer considering that carboxylic acid or hydroxyl groups were introduced onto the surface of CNTs after reflux in acid.21 To improve the dispersion of fillers, the master batches were fabricated by solution blending. CNT-Cl and/or Ni2O3 were added to xylene in a beaker and then sonicated in an ultrasonator bath (B2500S-DTH, Branson) at room temperature for 1.5 h. After sonication, the xylene suspension was added into the xylene solution of PP and PPMA and vigorously stirred at 130 °C for another 1.5 h. Then the mixture was poured into a large quantity of ethanol for precipitation. The precipitate of PP composite was washed with ethanol three times and then put in a ventilation hood under an infrared lamp for 12 h. The resultant composites were dried at 60 °C for 48 h in a vacuum. PP composites containing HCNT were prepared using the same process. The contents of CNTs and Ni2O3 were 20 and 25 wt % in the master batches, respectively. To prepare PP nanocomposites, the master batches were diluted with PP in a Brabender mixer with a rotating speed of 100 rpm at 180 °C for 10 min. The content of PPMA was 8 wt % in the final PP composites. The resultant samples were designated as xCNT-ClyNi (or xHCNTyNi). Here x and y denote the weight percentage of CNTs (or HCNT) and Ni2O3 in the samples, respectively. For example, 1CNT-Cl5Ni shows that the sample contains 1 wt % CNT-Cl and 5 wt % Ni2O3. 2.2. Characterization. X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Thermo) and inductively coupled plasma-atomic emission spectrometry/mass spectrometry (ICPAES/MS, X Series, Agilent) were used to examine the chlorine on CNTs. The morphologies of composites were observed by transmission electron microscope (TEM, JEL1011, JEOL, Japan) at a 100 kV accelerating voltage. Ultrathin sections were cryogenically cut using a Leica Ultracut and a glass knife at -80 °C. The samples were collected on carbon-coated copper TEM grids. The residues were characterized by X-ray diffraction (XRD) on a Rigaku (Tokyo, Japan) model D/max 2500 and field-emission scanning electron microscope (SEM, XL30, FEI). Flammability tests were performed on a Dual Cone Calorimetry (FTT, U.K.) according to ISO5660 Standard at a heat flux 35 kW/m2. Exhaust flow rate was 24 L/s, and the spark was continuous until the sample ignited. The specimens with sizes of 100 × 100 × 6 mm square plaques for cone calorimetry were prepared by compression molding at 180 °C. Thermal

J. Phys. Chem. C, Vol. 114, No. 31, 2010 13227 gravimetric analyses (TGA) were done using a Thermal Analysis Instrument (SDTQ600, TA Instruments) from room temperature to 500 °C under air with a heating rate of 10 °C/min. The rheological properties of PP and its composites were determined on a controlled strain rate rheometer (MCR300, ANTON PAAR), and the size of the samples measured was 25 mm in diameter, with a gap of 0.8 mm. Frequency sweeping was performed at 200 °C at a frequency from 0.05 to 100 rad s-1 in a nitrogen environment, with a strain of 1% in order to make the materials be in the linear viscosity range. Temperature scanning tests were performed after samples were presheared at 180 °C for 10 min and in the range from 180 to 340 °C, with a strain of 1% and a fixed frequency of 0.1 rad s-1. To verify the reaction between CNTs and macromolecular free radical, 1 H NMR spectra were recorded on a Varian Unity 400 MHz instrument (AVANCE 300, BRUKER, Germany) in o-dichlorobenzene-D at 125 °C. For testing the residual char in an imitative combustion experiment, a sample (∼5 g) was placed in a crucible and burned at 700 °C until the tongue of the flame disappeared. The residues were collected and weighted by an analytical balance. The yield of the carbonaceous residue was calculated by the amount of residue after subtracting the amount of the residual catalysts and added CNTs divided by the amount of the sample. 3. Results and Discussion 3.1. Composition Analyses of the Surfaces of CNT-Cl and HCNT. XPS was used to characterize the surface composition of modified CNTs after reflux in a mixture of HNO3 and HCl acids (or in the HNO3). As is well known, graphite shows an asymmetric peak centered at 284.6 eV with a long tail extended to the higher energy region.22 This peak feature has been explained in terms of many-electron interactions of the metallic conduction electrons induced by low-energy electron-hole excitations resulting from the absorption of X-rays. In Figure 1a, two kinds of the modified CNTs showed a strong peak of C 1s at a binding energy of around 284.6 eV and the peak at around 533.3 eV was assigned to O 1s (CdO). For CNT-Cl, a very weak peak at around 200.2 eV (C-Cl) was assigned to Cl 2p,23 which resulted from treating of mixed acids. The asymmetric peak of C 1s was observed for CNT-Cl (Figure 1b), centered at 284.6 ( 0.1 eV. We performed a deconvolution of the C 1s spectrum of CNT-Cl into three Gaussian peaks centered at 284.6 ( 0.2, 285.3 ( 0.2, and 289.2 ( 0.2 eV. The main peak at 284.6 eV originated from both sp2-hybridized graphitelike carbon and carbon atoms bound to hydrogen atoms. The peak at 285.3 eV could be assigned to sp3-hybridized carbon atoms as in diamond-like carbon.24 On the basis of the literature,25,26 the peak at 289.2 eV was assigned to carbon atoms of the C-Cl bond. This shows that the C-Cl bond has been formed on the surface of CNTs by refluxing in a mixture of HNO3 and HCl. The amount of Cl on the surface of CNTs was 0.2 ( 0.01 wt % according to the results of ICP-AES/MS. 3.2. Dispersion States of Modified CNTs and Ni2O3 in PP Matrix. The dispersion states of modified CNTs and Ni2O3 in PP matrix were observed by means of TEM. PPMA was added as a compatibilizer in order to improve the dispersed states of CNTs and Ni2O3 in PP matrix. Figure 2 shows the morphologies of PP composites. The selected TEM observations were representative of the dispersion states of the fillers in PP matrix. From the TEM images, both 3CNT-Cl and 3HCNT composites (Figure 2d and 2e) showed a good dispersion state of CNTs with few aggregates in PP matrix. In contrast, most of CNTs were dispersed as aggregates in PP composites without PPMA

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Figure 1. (a) XPS spectra of CNT-Cl and HCNT; (b) C1s core-level spectrum of CNT-Cl.

(not shown). The dispersion state of CNTs in the sample of 3HCNT was similar to that in the 3CNT-Cl. Thus, there was no obvious influence of C-Cl bonds on the surface of CNTs on the dispersion states of CNTs in PP matrix. Ni2O3 particles were uniformly dispersed in the 5Ni, 3HCNT5Ni, and 3CNTCl5Ni composites (Figure 2a, 2b, and 2c). The well-dispersed states of CNTs and Ni2O3 were attributed to the compatibilization of PPMA due to strong interaction between the modified CNTs (or Ni2O3) and the maleic anhydride group of PPMA.21 3.3. Comparing the Efficiency for Capturing Macroradicals Using CNT-Cl and HCNT in PP Nanocomposites. Many polymers will degrade at high temperature via a radical reaction in spite of the inert or air environment. Addition of CNTs can improve the thermal stability of polymers due to trapping radicals of CNTs.27 Figure 3 presents TGA curves of PP/PPMA blend and its nanocomposites containing the modified CNTs under air atmosphere. Detailed data are listed in Table 1. From these results it is evident that the thermooxidative stabilities of all the nanocomposites were increased compared with that of PP/PPMA blend. In particular, for the sample of 3CNT-Cl, its thermooxidative stability was the best among the samples. T5wt%, T10wt%, and Tmax of 3CNT-Cl were around 284, 311, and 397 °C, respectively, which were higher than those of PP/MAPP blend (the increments were 19, 32, and 29 °C, respectively). As for the 3HCNT nanocomposite, its T5wt%, T10wt%, and Tmax were 10, 18, and 20 °C higher than those of PP/PPMA blend, indicating that the presence of CNT-Cl obviously improved the thermooxidative stability of PP. The main reason is probably ascribed to more efficiently trap macroradicals or other degradation products by CNT-Cl compared to HCNT. To confirm the difference in the reaction between degradation products and two kinds of modified CNTs, the following

Figure 2. TEM images of PP/MAPP nanocomposites: (a) 5Ni, (b) 3HCNT5Ni, (c) 3CNT-Cl5Ni, (d) 3HCNT, and (e) 3CNT-Cl.

Figure 3. TGA and DTG curves of PP/PPMA blend and its nanocomposites.

comparative experiments were done. PP/CNT-Cl (or HCNT) ) 94/6 (by weight) mixture was fabricated by solution mixing in xylene. After the samples were precipitated and dried, they were annealed in an oven at 320 °C for 10 min under air atmosphere. From the TGA curves shown in Figure 3 this temperature is located in the range between T10wt% and Tmax for all samples. For comparison, PP was also annealed under the same conditions (labeled as HPP10). It is well known that the degradation of PP is a free radical reaction through β-scission of PP chains, and one double bond (CdC) will be formed after

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TABLE 1: Summary of the TGA Results (Figure 3) for PP/PPMA and Its Nanocomposites run

T5wt% (°C)a

T10wt% (°C)a

Tmax (°C)b

PP/PPMA 1CNT-Cl 1HCNT 3CNT-Cl H3CNT

265 267 266 284 275

279 284 282 311 297

368 380 357 397 388

a

T5wt% and T10wt% are the temperature where 5 and 10 wt % weight loss occurred. b Tmax ) temperature at which the maximum weight loss rate occurred.

Figure 5. 1H NMR spectra of HPP10, HPP20, heat-treated HPP10/ CNT-Cl, and HPP10/HCNT mixtures in o-dichlorobenzene-D at 120 °C. Conditions for heating treatments: 320 °C for 10 min under air atmosphere.

Figure 4. 1H NMR spectra of PP, heat-treated PP (HPP10), heat-treated PP/CNT-Cl, and PP/HCNT mixtures in o-dichlorobenzene-D at 120 °C. Conditions for heating treatments: 320 °C for 10 min under air atmosphere.

one time of β-scission of PP chains during the heating process.12,13 Thus, the content of the double bond can reflect the reaction degree of β-scission of PP chains. 1H NMR spectra were recorded to analyze the change of the double bond after the heating treatments (Figure 4). The peaks at about 7-7.5 ppm are from o-dichlorobenzene-D (as solvent during NMR measurements). Compared to original PP, there were two new chemical shifts at 4.72 and 4.77 ppm (hydrogen of double bond) in the 1H NMR spectrum of HPP10. The content of the double bond in HPP10 was 1.78 mol %. Comparatively, the PP/HCNT mixture contained 1.03 mol % of the double bond after the same heating treatments. Very surprisingly, the heat-treated PP/CNTCl mixture under the same conditions did not contain the double bond. These results imply that the presence of the modified CNTs could restrain the β-scission of PP chains due to trapping macroradicals of PP by the modified CNTs. Meanwhile the modified CNTs probably reacted with the double bond of the degradation products, especially in the presence of CNT-Cl, which was confirmed in the following model experiments. First, the above HPP10 was used to prepare HPP10/CNT-Cl and HPP10/HCNT mixtures. Then these mixtures were annealed again at 320 °C for 10 min under air atmosphere. As a control, HPP10 was also annealed by the same procedure (labeled as HPP20). Figure 5 presents 1H NMR spectra of these annealed samples. Further annealing led to the increase of the double bond in HPP20. Compared to HPP10, the content of the double bond slightly decreased in the HPP10/HCNT mixture but dramatically decreased in the HPP10/CNT-Cl mixture. This further confirms that the modified CNTs could trap macroradicals to restrain β-scission of PP chains. More importantly, the

Figure 6. TEM images of (a) CNT-Cl and (b) the residue CNTs of heat-treated PP/CNT-Cl after extraction in xylene.

TABLE 2: Char Yield of PP Mixtures Burned in Crucible at 700°C run

char yield (wt %)

PP/PPMA 5Ni 1CNT-Cl 3CNT-Cl 3HCNT 1CNT-Cl5Ni 3CNT-Cl5Ni 3HCNT5Ni 1NH4Cl5Ni

0 8.9 ( 0.5 0.5 ( 0.1 0.7 ( 0.1 0 31.4 ( 0.5 54.3 ( 2.7 10.5 ( 0.4 53.7 ( 2.0

free radicals on the surface of CNT-Cl formed by rupturing C-Cl bonds could react with the double bond of the degradation products of PP. Figure 6 shows TEM images of CNT-Cl and the residue of heat-treated PP/CNT-Cl composite after extraction in xylene for 48 h. Compared to CNT-Cl (Figure 6a), there was a thin layer of polymer on the surface of CNT-Cl from the residue (Figure 6b). It provides ocular proof for the reaction between CNT-Cl and the degradation products of PP. 3.4. Carbonization Behavior of PP Composites Containing the Modified CNTs and Their Combination with Ni2O3 during Combustion. Table 2 presents the yields of the charring residue after burning the samples with various compositions in a crucible at 700 °C until the tongue of the flame disappeared. The yield of the residual char was zero in the case of PP/PPMA blend. Addition of Ni2O3 into PP matrix promoted the formation of residual char in the intermediate stage of combustion albeit with low yield (8.9 wt %, Table 2). However, the char yields for the samples of 1CNT-Cl and 3CNT-Cl were 0.5 and 0.7 wt %, respectively (Table 2). As for the 3HCNT sample, there was

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Figure 8. XRD profiles of PP/PPMA, 5Ni, CNT-Cl, HCNT, and the residue chars of 5Ni, 3HCNT5Ni, and 3CNT-Cl5Ni.

Figure 7. SEM images of the residues of composites (a) 5Ni, (b) 3CNT-Cl, (c) 3HCNT, (d) 3HCNT5Ni, and (e) 3CNT-Cl5Ni, and (f) TEM image of the residue of 3CNT-Cl5Ni.

no residue left except for the added CNTs. It is known that the only difference between CNT-Cl and HCNT is the C-Cl bonds on the surface of the modified CNTs; thus, it is reasonable that the formed radicals on the surface of CNT-Cl after Cl radical leaving can react with the degradation products bringing double bonds during combustion. In the cases of combined CNT-Cl with Ni2O3, the char yield was greatly increased. For the sample 3CNT-Cl5Ni, the char yield was 54.3 wt % (Table 2). However, the char yield of 3HCNT5Ni composite was similar to that of the sample of 5Ni, meaning that C-Cl bonds on the surface of CNT-Cl can promote the efficiency of catalytic carbonization by nickel catalyst, which is similar to our previous results.19 Previous results showed that halide radical released from halogen-containing additives acted as a catalyst to accelerate dehydrogenation-aromatization of degradation products of PP, which promote the degradation products to form the residual char catalyzed by nickel catalyst. Comparatively, the char yield of the sample 1NH4Cl5Ni (containing 1 wt % NH4Cl and 5 wt % Ni2O3) was close to that of 3CNT-Cl5Ni composite (Table 2). However, the contents of chlorine in the composites of 1NH4Cl5Ni and 3CNT-Cl5Ni were 1.87 × 10-2 and 6 × 10-3 wt %, respectively. This shows that the catalytic efficiency of chlorine in 3CNT-Cl5Ni composite is three times higher than in the sample of 1NH4Cl5Ni. Figure 7 presents the morphologies of the residual chars. There was a lot of amorphous char in the residual char from 5Ni composite (Figure 7a). However, the residual char was mainly composed of fiber-like structure in the case of 3CNTCl and 3HCNT nanocomposites (Figure 7b and 7c). Interestingly, the residue from 3HCNT5Ni composite consisted of a mixture of fiber-like structure and amorphous char (Figure 7d). In contrast, a lot of fiber-like structure was formed in the residual char from 3CNT-Cl5Ni composite (Figure 7e). TEM observation showed that most of the fiber-like char was multiwalled carbon nanotubes (MWCNTs) (Figure 7f). Figure 8 shows XRD profiles of the samples before and after combustion. Compared to the XRD profiles of the original

Figure 9. Heat release rate curves for PP/PPMA and its nanocomposites at an incident heat flux of 35 kW m-2.

samples, the diffraction peaks of both graphite and metallic nickel appeared in the residual chars; meanwhile, the diffraction peaks of PP disappeared completely, and the diffraction peaks of Ni2O3 became weak, especially in the case containing both CNT-Cl and Ni2O3. It can be clearly seen that the intensity ratio of diffraction peaks of the graphite to those of metallic nickel in the residual char from 3CNT-Cl5Ni composite was the largest among the three residual chars. Raman spectra showed that the ratio of the D-band to G-band was almost the same in all residual chars. This implies that more residual char with graphitic structure has been formed in the residual char from 3CNT-Cl5Ni composite. 3.5. Influence of Modified CNTs and Their Combination with Ni2O3 on the Flammability of PP Composites. The influence of the modified CNTs and their combination with Ni2O3 on the flammability was investigated by means of cone calorimetry. The results are shown in Figure 9 and Table 3. For the composites containing the modified CNTs only, the heat release rate (HRR) was lowered with an increase of the content of the modified CNTs (Figure 9). Interestingly, the peak heat release rates (PHRR) of the samples containing the same amount of HCNT were higher than those of the CNT-Cl containing samples. For example, the PHRR of 3HCNT and 3CNT-Cl were 799 and 755 kW/m2, respectively (Table 3). More importantly, the time for the appearance of PHRR (tPHRR) was delayed in the 3CNT-Cl sample (370 s) compared to the HCNT sample (325 s). For the samples containing the modified CNTs only, the total heat release (THR) was almost the same as that of

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TABLE 3: Summary of the Cone Calorimetric Results (Figure 9) for PP/PPMA and Its Composites run

tPHRR (s)

PHRR (kW/m2)

THR (MJ/m2)

PP/PPMA 5Ni 1CNT-Cl 1HCNT 3CNT-Cl 3HCNT 1CNT-Cl5Ni 1HCNT5Ni 3CNT-Cl5Ni 3HCNT5Ni

350 ( 5 315 ( 4 330 ( 4 315 ( 5 370 ( 5 325 ( 4 105 ( 3 305 ( 3 80 ( 2 320 ( 3

1486 ( 60 597 ( 21 888 ( 27 1124 ( 55 755 ( 28 799 ( 35 436 ( 15 584 ( 23 400 ( 10 500 ( 22

215 ( 5 204 ( 5 216 ( 4 224 ( 6 212 ( 4 217 ( 5 186 ( 6 202 ( 2 164 ( 3 185 ( 4

PP/PPMA blend (Table 3), implying that all polymer materials were burned out. Compared to PP/PPMA blend, an approximate 60% reduction in the PHRR of 5Ni composite (Table 3) was attributed to the carbonization reaction of the degradation products catalyzed by Ni2O3.19 When the combination of the modified CNT-Cl and Ni2O3 was applied, the PHRR of the composites showed a further reduction compared to those of the 5Ni and the corresponding CNT-Cl composites (Figure 9 and Table 3), suggesting a synergistic effect of CNT-Cl with Ni2O3. It is noteworthy that the HRR of 3CNT-Cl5Ni composite shows the lowest peak value, is reduced further after the peak, and stays at a low level throughout. However, when the combination of HCNT and Ni2O3 was applied, the HRR was higher than that of the corresponding CNT-Cl5Ni composites, and the shape of the HRR curve was different. There were two PHRR in the HRR curves of both 1HCNT5Ni and 3HCNT5Ni composites, and the second PHRR was higher than the first one. In contrast, there was no second PHRR for 3CNT-Cl5Ni, and the second PHRR for 1CNT-Cl5Ni was lower than the first one (Figure 9). This is the reason why the composites containing both HCNT and Ni2O3 show a higher tPHRR compared to the corresponding composites containing both CNT-Cl and Ni2O3. More importantly, the reduction of THR for the composites containing both CNT-Cl and Ni2O3 was more obvious than those of the corresponding composites containing both HCNT and Ni2O3 compared to PP/PPMA blend and 5Ni composite. 3.6. Discussion. The above results based on cone calorimetry demonstrated that addition of the modified CNTs, Ni2O3, and their combination can improve the flame retardancy of PP. However, there are obvious differences in the improvements of flame retardancy of PP by adding CNT-Cl alone or combined CNT-Cl/Ni2O3 compared to the cases containing HCNT alone or combined HCNT/Ni2O3. There are four possible reasons for the improvement of flame retardancy of PP by adding CNTs and Ni2O3, that is, the physical effect of CNTs (such as a high melt viscosity and formation of a protective layer), catalytic carbonization by nickel catalyst, effect of chlorine on promoting the efficiency of catalyzing carbonization by nickel catalyst, and capture of macroradicals by CNTs. It is well known that the melting process and degradation will take place before a material starts to combust, and combustible gas products (degradation products) will traverse the decomposition zone to the flame zone to maintain combustion. If the melt viscosity is high, the gas products need more time to reach the flame zone, and thus, the combustion process will be slowed down. The complex viscosity profiles of the composites versus the dynamic frequency sweep at 200 °C are shown in Figure 10. As expected, incorporating the modified CNTs or Ni2O3 increased the complex viscosity of the system over the whole test region. The complex viscosity of 3CNT-Cl was the same as that of 3HCNT, and the complex viscosity of

Figure 10. Effects of the type and content of added fillers on complex viscosity of PP composites.

Figure 11. Dependence of complex viscosity on temperature for PP and its composites.

the composites increased with the content of the modified CNTs. This is because a large amount of CNTs trends to form a threedimensional network in polymer matrix, leading to a higher melt viscosity. Figure 11 presents the curves of temperature dependence of complex viscosity for PP and its composites. Clearly, all samples first showed a decrease in the complex viscosity with the increase of temperature, followed by a subsequent sharp increase for the composites and a continuous decrease throughout to nearly zero for the PP/PPMA blend. This behavior could be explained as follows. Upon heating, the easier movement of polymer chains and the degradation of the polymer at high temperature resulted in a decrease in complex viscosity of the samples. The increase of complex viscosity in PP composites was due to fillers acting as oxidation cross-linking sites and the oxidation cross-linking reaction overwhelming the polymer decomposition. In the case of 1CNT-Cl and 3CNT-Cl composites, the temperature at which the complex viscosity began to increase was ∼296 °C, and those for other composites were all around 310 °C. Moreover, complex viscosities of CNT-Clcontaining composites were much higher than those of the corresponding samples containing HCNT at high temperature (>300 °C). This phenomenon should result from the reaction between the radicals on the surface of CNT-Cl and the double bond of the degradation products of PP. For PP/CNT-Cl composites, this reaction results in the formation of more grafted chains on the surface of CNT-Cl; as a result, the melt viscosity increases. These results account for the enhancement of flame

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Figure 12. Mass loss curves of PP/PPMA and its composites measured by a cone calorimeter at an external radiant flux of 35 kW/m2.

retardancy (especially for the increase of tPHRR) for the composites of 1CNT-Cl and 3CNT-Cl compared to the 1HCNT and 3HCNT composites. A similar phenomenon was found by Fang et al. in PP/C60 nanocomposites, in which C60 reduced the flammability of PP by trapping free radicals and in situ forming a gelled ball cross-link network.14 Although the complex viscosity of 3CNT-Cl composite was similar to that of 3CNT-Cl5Ni in the mode of dynamic frequency sweep and higher than that of 3CNT-Cl5Ni in the mode of dynamic temperature sweep in the high-temperature region, the HRR value of 3CNT-Cl5Ni composite was much lower than that of the sample 3CNT-Cl. Therefore, the melt viscosity is not a key factor influencing the fire retardancy of PP composites containing both the modified CNTs and Ni2O3. From the results of imitative combustion experiments, the high yield of the residual char from 3CNT-Cl5Ni should be the reason for the low HRR. Furthermore, the amount of the residua was different (Figure 12). Adding Ni2O3 or the combination of the modified CNTs/Ni2O3 resulted in formation of a lot of residual char, especially in cases containing both CNT-Cl and Ni2O3. The microstructures of the residue from the cone calorimeter tests were similar to those of the residue from the model experiments (Figure 7). For example, the residual char from 3CNT-Cl5Ni consisted of a lot of CNTs. Clearly, catalysis of nickel catalyst on the carbonization of degradation products of PP is an important factor affecting the flame retardancy in cases containing both the modified CNTs and Ni2O3. Figure 13 shows a schematic drawing for the mechanism of the combined effect between Ni2O3 and CNT-Cl on improving the flame retardancy of PP. The improved flame retardancy is attributed to the carbonization of degradation products of PP catalyzed by nickel catalyst and the capture of macroradicals by CNTs. However, the C-Cl bond on the surface of CNT-Cl is important to improve the thermal stability and flame retardancy of PP during combustion. On one hand, Cl radicals will be formed when heating samples at high temperature, and PP will degrade to form fragments including macromolecular species and small organic molecules. Generally, breaking of the C-C bond and C-H bond will form carbon radicals and hydrogen radical. In the presence of Cl radicals, it will attack the hydrogen atom attached to carbons of degradation products of PP at high temperature, which easily leads to formation of HCl. Therefore, dehydrogenation occurs after HCl departs from the degradation products. Meanwhile, Cl* will be regenerated via reaction of HX with H*, which results from breaking the C-H bond. The above reactions take place repeatedly. Finally,

Figure 13. Schematic drawing for a possible mechanism between Ni2O3 and CNT-Cl for improving the flame retardancy of PP.

free radical condensates (see ring structure in Figure 13) are formed in the reaction system. In fact, Cl* acts as a catalyst for dehydrogenation of degradation products. In this case, dehydrogenation will be accelerated. Owing to the strong catalysis of Cl*, the dehydrogenation of degradation products, hydrogen transfer, isomerization, cyclization, and aromatization would take place during the degradation of PP, which leads to facile formation of more aromatic hydrocarbons in the degradation products. Simultaneously, Ni2O3 is in situ reduced to Ni, probably by H2 and/or other degradation products of PP. The free radical condensates are catalyzed by Ni to form CNTs.19 On the other hand, free radicals formed on the surface of CNTCl during heating can anchor part of the degradation products bringing the double bond compared to HCNT. In addition, formation of a network-like structure in PP composites containing CNTs also has a contribution to reduce the flammability of PP. 4. Conclusions Double functions of CNT-Cl in PP matrix were demonstrated in this work. On one hand, the formed radicals on the surface of CNT-Cl via releasing Cl radicals could react with the double bond of degradation products, which resulted in formation of more grafted chains on the surface of CNTs. As a result, the presence of CNT-Cl obviously increased the thermooxidative stability and flame retardancy of PP compared to unchlorinated CNTs. On the other hand, the combination between CNT-Cl and Ni2O3 showed a synergistic effect in improving the flame retardancy of PP. It is attributed to chlorine radical releasing from CNT-Cl, which worked as a catalyst to accelerate dehydrogenation-aromatization of the degradation products of PP, promoting the degradation products to form the residual char catalyzed by nickel catalyst. Acknowledgment. We are thankful for financial support from the National Natural Science Foundation of China (nos. 50525311, 20734006, 50921062) and Jilin Bureau of Science and Technology (no. 20060319). References and Notes (1) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. ReV. 2006, 106, 1105.

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