Combination of Carbon Nanotubes with Ni2O3 for Simultaneously

Jul 6, 2009 - Fax: +86 (0) 431 85262827. .... Round samples 25 (diameter) × 1 mm (thickness) were run at 160 °C. Frequency sweep was performed from ...
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J. Phys. Chem. C 2009, 113, 13092–13097

Combination of Carbon Nanotubes with Ni2O3 for Simultaneously Improving the Flame Retardancy and Mechanical Properties of Polyethylene Haiou Yu,†,‡ Jie Liu,† Zhe Wang,† Zhiwei Jiang,† 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: March 8, 2009; ReVised Manuscript ReceiVed: May 6, 2009

Effects of multiwalled carbon nanotubes (MWCNTs) and Ni2O3 on the flame retardancy of linear low density polyethylene (LLDPE) have been studied. A combination of MWCNTs and Ni2O3 showed a synergistic effect in improving the flame retardancy of LLDPE compared with LLDPE composites containing MWCNTs or Ni2O3 alone. As a result, the peak value of heat release rate measured by cone calorimeter was obviously decreased in the LLDPE/MWCNTs/Ni2O3 composites. According to the results from rheological tests, carbonization experiments, and structural characterization of residual char, the improved flame retardancy was partially attributed to the formation of a networklike structure due to the good dispersion of MWCNTs in LLDPE matrix, and partially to the carbonization of degradation products of LLDPE catalyzed by Ni catalyst originated from Ni2O3. More importantly, both viscoelastic characteristics and catalytic carbonization behavior of LLDPE/MWCNTs/Ni2O3 composites acted in concert to result in a synergistic effect in improving the flame retardancy. Although both MWCNTs and Ni2O3 were not organically modified, LLDPE composites incorporating both the fillers showed higher mechanical properties compared with those of pure LLDPE. 1. Introduction Fabricating polymer nanocomposites, especially ones containing unsymmetric nanoparticles, has been demonstrated as one of the alternative flame retardant approaches to the use of halogenated flame retardants.1-4 Unsymmetric nanoparticles include clay5,6 and carbon nanotubes (CNTs).2,7,8 Previous reports have shown that the formation of a networklike structure of nanoparticles within a polymer matrix can significantly reduce the flammability of nanocomposite.9,10 This implies that there is a close relationship between viscoelastic characteristics and the flammability properties of polymer nanocomposites. The in situ formation of a continuous networklike protective layer from the nanoparticles is critical for significant reduction in heat release rate (HRR), because the protective layer thus acts as a thermal shield of energy feedback from the flame. In order to achieve a significant improvement in the flame retardancy of polymer nanocomposites, the requirement of the formation of the network structure is applicable to the use of both clay and CNTs as fillers.10 Comparatively, well-dispersed CNTs show a stronger trend than clay to form an integral protective layer on the burning surface because of their highly elongated shape (high aspect ratio). However, poorly dispersed CNTs will result in the formation of a discontinuous layer consisting of fragmented islands rather than the continuous network protective layer. Very interestingly, the combination of CNTs with clay particles has presented a synergistic effect on the flame retardancy of polymers.11,12 The synergistic mechanism has been generally explained, in which CNTs act as a sealing agent that can connect clay sheets and reduce surface cracks of chars, leading to the increase of barrier resistance to the evolution of * To whom correspondence should be addressed. Tel.: +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.

flammable volatiles and oxygen ingress to the condensed phase. Recently the synergistic effect between CNTs and clay on improving the flame retardancy of acrylonitrile-butadienestyrene (ABS) resin was attributed to the formation of a networklike structure.13 Rheological measurements have confirmed that the coexistence of clay and CNTs in the ABS nanocomposites could promote the formation of networklike structure, which can hinder the movement of polymer chains and improve flame retardancy. Recently our group has reported a novel nonhalogen method for improving the flame retardancy of polymers, in which the combination between nickel catalysts and solid acids could efficiently catalyze carbonization of the degradation products during combusting polyolefin/nickel catalyst/solid acid composites.14-16 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 results from the degradation products of the polymer itself exposed to fire conditions; thus, the amount of flammable volatiles evolved is reduced, and the flame retardancy of the polymer is also improved. This method is efficient to improve flame retardancy of polyolefins, including polypropylene (PP) and linear low density polyethylene (LLDPE).17 In this paper we aim to report a synergistic effect between multiwalled carbon nanotubes (MWCNTs) and Ni2O3 particles in improving flame retardancy of LLDPE. The results showed that adding two types of fillers improved not only flame retardancy but also mechanical properties compared with those of pure LLDPE. It is very attractive to prepare polymer nanocomposites showing significant improvements in flame retardancy without compromising other performance, such as mechanical properties. Our attention was focused on the influences of the mesoscopic filler network and the carbonization of LLDPE catalyzed by Ni2O3 on the flammability of LLDPE. The mechanism of the synergistic effect between Ni2O3 and MWCNTs in improving the flame retardancy has been studied.

10.1021/jp902081e CCC: $40.75  2009 American Chemical Society Published on Web 07/06/2009

Effects of MWCNTs and Ni2O3 on LLDPE

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TABLE 1: Effect of the Combination between MWCNTs and Ni2O3 on the Residual Char after Burning at 700 °Ca

samples

MWCNTs (wt %)

Ni2O3 (wt %)

LLDPE (wt %)

yield of residual char (wt %)

PE 3CN 5Ni 1CN5Ni 3CN5Ni

0 3 0 1 3

0 0 5 5 5

100 97 95 94 92

0 0 8.8 13.2 13.7

a After the flame disappeared, the residue was collected, in which the amounts of the residual MWCNTs and the reduced metallic nickel from Ni2O3 were subtracted.

2. Experimental Section 2.1. Materials and Preparation of Samples. Purified multiwalled carbon nanotubes (MWCNTs) with a diameter of 15 ( 5 nm and a length of about 30 µm were purchased from the Chengdu Institute of Organic Chemistry. MWCNTs were ball milled at 400 rpm for 60 min before being used, and the length of MWCNTs was reduced to about 0.3 µm according to morphology characterization. To prepare composites, LLDPE with a melt flow index of 24 g/min was mixed with the milled MWCNTs and Ni2O3 (with the average diameter of 250s300 nm, from Lingfeng Chemical Company of Shanghai) in a Brabender mixer at 100 rpm and 160 °C for 10 min. The resultant samples were designated as xCNyNi. Here x and y denote the weight percentages of MWCNTs and Ni2O3 in the samples, respectively; for example, 3CN5Ni means that the sample contains 3 wt % MWCNTs and 5 wt % Ni2O3. The detailed information is listed in Table 1. 2.2. Characterization. Morphologies of the composites were observed by means of transmission electron microscope (TEM, JEL1011) at 100 kV accelerating voltage. Ultrathin sections were cryogenically cut using a Leica Ultracut and a glass knife at -90 °C. The samples were collected on carbon-coated copper TEM grids. Combustion behavior of samples was measured with a cone calorimeter performed according to ISO5660 Standard at a heat flux of 35 kW/m2. The exhaust flow rate was 24 L/s, and the spark was continuous until the sample ignited. The specimens with the sizes of 100 × 100 × 6 mm square plaques for cone calorimetry were prepared by compression molding at 160 °C. After a cone calorimeter test, the residues were observed with a digital camera and field-emission scanning electron microscope (FESEM, XL30ESEM-FEG). The rheological properties of LLDPE and its composites were conducted on a controlled strain rate rheometer (ARES rheometer) under a nitrogen atmosphere. Round samples 25 (diameter) × 1 mm (thickness) were run at 160 °C. Frequency sweep was performed from 0.01 to 100 s-1, with a strain of 1% in order to make the materials be in linear viscoelastic response. Mechanical properties were measured on an Instron 1121 at an extension speed of 20 mm/min. All data were the average of five independent measurements; the relative errors committed on each data were reported as well. 3. Results and Discussion 3.1. Dispersion States of CNTs and Ni2O3 in LLDPE Matrix. The dispersion states of fillers in LLDPE matrix are shown in Figure 1. From TEM images of LLDPE composites, it can be seen that both Ni2O3 and MWCNTs are uniformly dispersed in binary LLDPE/Ni2O3 and LLDPE/MWCNTs composites (Figure 1a and 1b). Furthermore, both the fillers can be well dispersed in the ternary LLDPE/Ni2O3/MWCNTs

Figure 1. TEM images of LLDPE composites: (a) 5Ni; (b) 3CN; (c) 3CN5Ni; (d) magnified image of circled area in (c).

composites (Figure 1c), and some of the Ni2O3 particles are in contact with MWCNTs. More importantly, the dispersion degrees of Ni2O3 and MWCNTs in the ternary composites are higher than those in the corresponding binary composites; especially, the size of Ni2O3 is obviously smaller than that in the case of LLDPE/Ni2O3 composite (Figure 1a vs Figure 1c). The size histograms of Ni2O3 particles are shown in Figure 2. The mean sizes of Ni2O3 particles in 5Ni and 3CN5Ni are 230 and 121 nm, respectively. There are two possible reasons for the above phenomena. One of them results from the viscosity increase of the matrix due to the fast dispersion of a filler, which can increase shear force during melt mixing and improve the dispersion degree of the other filler. The other is possible interaction between the two fillers, which can prevent reaggregation of the dispersed fillers. Figure 1d shows that a part of the MWCNTs is in contact with Ni2O3 particles in LLDPE matrix. 3.2. Flame Retardancy. The influences of MWCNTs and Ni2O3 on the flame retardancy of LLDPE matrix were investigated by means of cone calorimetry. Cone calorimetry is one of the most effective medium-sized polymer fire behavior tests.18 The principle of cone calorimeter experiments is based on the measurement of the decreasing oxygen concentration in the combustion gases of a sample subjected to a given heat flux (in general from 10 to 100 kW/m2). The measurements of the gas flow and oxygen concentration are used to calculate the quantity of heat released per unit of time and surface area, i.e., heat release rate (HRR). The evolution of the HRR over time, in particular the value of its peak (PHRR), is usually taken into account in order to evaluate the fire properties. Figure 3 shows

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Figure 2. Size histograms of Ni2O3 particles in (a) 5Ni; (b) 3CN5Ni.

Figure 3. Effects of adding MWCNTs and Ni2O3 on heat release rate of LLDPE at an incident heat flux of 35 kW/m2.

HRR plots for LLDPE, LLDPE/Ni2O3, LLDPE/MWCNTs, and LLDPE/MWCNTs/Ni2O3 composites and the PHRR of these plots at a 35 kW/m2 heat flux. The PHRR in the HRR plots of the LLDPE/MWCNTs composites (856 kW/m2 for 1CN and 667 kW/m2 for 3CN) decreases compared to that of LLDPE (1135 kW/m2), indicating that the flammability can be reduced by adding MWCNTs alone in the LLDPE matrix, especially when the content of MWCNTs is 3 wt % (curve-3CN in Figure 3, PHRR ) 667 kW/m2). This is probably ascribed to the formation of a networklike structure of MWCNTs.9,10 Compared to LLDPE, an approximate 50% reduction in the PHRR of LLDPE/Ni2O3 ) 95/5 (by weight) composite (curve-5Ni in Figure 3, PHRR ) 536 kW/m2) may be attributed to the carbonization reaction of the degradation products from LLDPE catalyzed by Ni2O3.17 Obviously the PHRR of 5Ni is lower than that of 3CN. When the combination between MWCNTs and Ni2O3 is applied, the PHRR of the ternary LLDPE composites shows a further reduction compared with those of LLDPE/ MWCNTs and LLDPE/Ni2O3 composites, such as curve1CN4Ni (PHRR ) 450 kW/m2) vs curve-5Ni in Figure 3, suggesting a synergistic effect of MWCNTs with Ni2O3. It is noteworthy that the HRR of LLDPE/CNTs/Ni2O3 ) 92/3/5 (by weight) composite shows the lowest peak value (curve-3CN5Ni in Figure 3, PHRR ) 312 kW/m2) and reduces further after the peak, and stays at a very low level throughout. 3.3. Mechanism of Flame Retardancy. It is very interesting why the combination between MWCNTs and Ni2O3 particles shows a synergistic effect in improving the flame retardancy of LLDPE. However, combustion is a very complex physical and chemical process that is determined by many factors, such as viscoelasticity and the structure of chars formed during combustion. There are three possible reasons for the improvement of flame retardancy of LLDPE by adding MWCNTs and Ni2O3, that is, physical effect of MWCNTs (such as the formation of

Figure 4. Effects of the filler type and concentration on viscoelastic measurements: (a) storage modulus; (b) complex viscosity.

a networklike structure), chemical effect of Ni2O3 (such as catalytic carbonization17), and the combination of physical effect and chemical effect. Evidence of the Network Structure. For polymer nanocomposites containing nanoparticles, it has been proposed that a so-called three-dimensional filler network structure will be formed when the content of nanoparticles reaches a threshold value,19 which depends on the dispersion degree of nanoparticles.20-23 The presence of a nanoparticle network can be demonstrated by the change of dynamic melt rheological properties. Figure 4 shows storage modulus (G′) and complex viscosity (η) of the samples at 160 °C. In the high frequency (ω) regime corresponding to the movement within a small time scale, not much difference in the G′ and the η is seen for the composites with different composition, which implied that the movements of partial polymer chains were not affected by the addition of

Effects of MWCNTs and Ni2O3 on LLDPE

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Figure 5. FESEM images of the residual char after the combustion in the model experiments: (a) 5Ni; (b) 3CN; (c) 3CN5Ni.

Ni2O3 and MWCNTs. However, the G′ and the η in the low ω regime are significantly dependent on the addition of Ni2O3 and MWCNTs. The rheological properties in the low ω regime reflect the relaxation and the motion of the whole polymer chains. From the results of Figure 4a, it is observed that the terminal slope of G′-ω curves decreases with the addition of the fillers (especially in the cases containing MWCNTs) compared with that of pure LLDPE, which is similar to the phenomena observed in poly(methyl methacrylate)/CNTs,9,23 PP/ CNTs,24 polystyrene (PS)/CNTs, and PS/clay nanocomposites,10 indicating a transition from liquidlike state to solidlike state due to the formation of filler network. In this case, the movement of LLDPE chains is restricted by the spatially confined geometry. The minimal value of the terminal slope of G′-ω curves is found in LLDPE/Ni2O3/MWCNTs ) 92/5/3 (by weight) composite (curve-3CN5Ni in Figure 4a), which indicates a more intact percolated network structure formed in LLDPE/ Ni2O3/MWCNTs composites than in LLDPE/Ni2O3 and LLDPE/ MWCNTs composites. These results are in accordance with those of TEM observation (Figure 1). MWCNTs are onedimensional nanotubes which have a highly elongated shape, and Ni2O3 are three-dimensional particles with a size of 250-300 nm. The coexistence of Ni2O3 and MWCNTs in the composites can form a more effective confined space and enhance the network structure, just like the phenomenon observed in ABS nanocomposites,13 in which the coexistence of clay and CNTs could promote the formation of networklike structure. Considering the above results of rheological measurements and the flammability properties of the samples in this work, it is clear that the formation of the network structure is an important factor strongly affecting the flame retardancy of LLDPE composites, which can improve the barrier resistance to the evolution of flammable volatiles and the oxygen ingress to the condensed phase. However, comparing the rheological result of 3CN with those of 5Ni and 1CN5Ni, although the trend for forming the networklike structure is more obvious in the 3CN sample, the PHRR of 3CN is higher than those of 5Ni and 1CN5Ni. This means that there should be another factor influencing the flame retardancy of LLDPE, such as the catalysis of Ni catalyst on the carbonization of degradation products of LLDPE.17 Catalysis of Ni2O3 on Carbonization of LLDPE. Table 1 presents the yields of the charring residue after burning the samples with various compositions in a crucible at 700 °C until the tongue of flame disappeared. It should be noted that the residual char could continue to burn slowly just like coke under the external heat flux; therefore, the yield of the residue only represents the char generated in the intermediate stage during combustion. It can be seen from Table 1 that the yield of the residual char is zero in the case of pure LLDPE. The addition of Ni2O3 into LLDPE matrix promotes the formation of residual

char in the intermediate stage of combustion albeit with low yield (Table 1), which is similar to our previous results.17 The previous results showed that Ni2O3 could catalyze the carbonization of LLDPE during combustion, which results from the catalysis of metallic Ni particles in situ formed via the reduction of Ni2O3 on the carbonization of degradation products of polyolefin through dehydrogenation and aromatization.17,25 In contrast, the presence of MWCNTs in LLDPE matrix does not lead to the formation of additional residual char except the residual MWCNTs in the intermediate stage of combustion (Table 1). However, in this case, the higher the content of MWCNTs is, the smaller the PHRR (Figure 3). Very interestingly, the coexistence of Ni2O3 and MWCNTs in LLDPE matrix results in a further increase in the yield of residual char (Table 1). Figure 5 shows FESEM images of the residual chars. In the residue of 5Ni (Figure 5a), only amorphous carbon particles, which are originated from the carbonization of degradation products of LLDPE catalyzed by in situ formed Ni catalyst, are observed. In contrast, the residue of 3CN is mainly composed of MWCNTs (Figure 5b). Interestingly, the residue of 3CN5Ni is composed of a mixture of MWCNTs and amorphous carbon particles (Figure 5c). Thus, the increment in the residue of 3CN5Ni should result from the amorphous carbon particles formed from the degradation products of LLDPE. Owing to the uniform dispersion of MWCNTs and Ni2O3 in ternary LLDPE composites, the resultant amorphous carbon particles are well mixed with MWCNTs in the residual char. In our previous work, the presence of solid acids can significantly promote the carbonization of degradation products catalyzed by Ni catalyst,17,25 which results in the formation of a lot of CNTs. In this work, although there are no solid acid sites on the surface of MWCNTs, the yield of the residual char from the ternary LLDPE/MWCNTs/Ni2O3 composites is also increased compared with that of LLDPE/Ni2O3 composite (Table 1). This probably results from extended contacting time of degradation products with Ni catalyst due to physical barrier of the networklike structure of MWCNTs. As we know, the formation of char from the degradation products in the intermediate process of combustion will reduce the release of flammable degradation products; thus, the HRR should decrease during combustion. This is a reason for the lower PHRR in the cases of 1CN5Ni and 3CN5Ni compared with those of the cases only containing Ni2O3 (5Ni) or MWCNTs (3CN). However, the PHRR is different in the 1CN5Ni and the 3CN5Ni although both the samples form the same amount of residual char after combustion (Table 1). Especially in the case of 3CN5Ni, the HRR further reduces after the peak in the HRR plot. The difference in the flame retardancy between the 1CN5Ni and the 3CN5Ni should be originated from different microstructures of the two samples. In the latter case, an obvious trend for forming a networklike structure consisting

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Figure 6. Photographs of the residua after the cone calorimeter test: (a) 5Ni; (b) 1CN; (c) 3CN; (d) 1CN5Ni; (e) 3CN5Ni; (f) 1CN4Ni.

of the fillers may be profitable to the formation of a perfect protective layer on the burning surface, which improves the flame retardancy of LLDPE composites. Structure of the Residue. Figure 6 shows macromorphologies of the final residue after cone calorimeter tests. Visual observations of the combustion experiments in the cone calorimeter revealed different behavior for pure LLDPE and its composites. Pure LLDPE was completely melted, accompanied by severe bubbling and bursting at the sample surface. At the end of the test, no residue was left behind. In the presence of Ni2O3 or MWCNTs, the combustion process was different. Many discrete islandlike structures were observed after initial formation of numerous small bubbles and bursting at the surface. Even though severe bubbling was not observed, bubbling between islandlike structures was clearly seen. The residue of LLDPE composites containing 5 wt % Ni2O3 or 1-3 wt % MWCNTs are composed of many small discrete islandlike structures (Figure 6a-6c). In contrast, there are more residual chars in the residue of ternary LLDPE composites after combustion (Figure 6d-6f). In addition, the trend for forming a relative continuous carbonaceous layer increases, especially in the case of 3CN5Ni (Figure 6e). This shows that the combination between MWCNTs and Ni2O3 promotes the formation of a relative continuous carbonaceous layer, which is profitable to the reduction of the HRR. The microstructures of the residue from the cone calorimeter tests are similar to those of the residue from the model experiments (Figure 5). In the case of 3CN5Ni, the char from the carbonization of degradation products of LLDPE, acting as an adhesive, makes the MWCNTs network more compacted and consequently prevents the transfer of degradation products, heat, and oxygen, thus further reducing the flammability of LLDPE to some extent. Based on the above results, the improved flame retardancy is partially attributed to the formation of a networklike structure due to good dispersion of MWCNTs in the matrix, and partially to the carbonization of degradation products of LLDPE catalyzed by Ni catalyst originated from Ni2O3. A schematic drawing for the mechanism of the synergistic effect between Ni2O3 and MWCNTs in improving the flame retardancy of LLDPE is shown in Figure 7. The addition of MWCNTs provides a precondition for the formation of a networklike structure in LLDPE matrix. The addition of Ni2O3 affords the catalysis for the carbonization of degradation products of LLDPE. In the case containing both Ni2O3 and MWCNTs, on one hand, the melt viscosity of the material is further increased due to the formation of a networklike structure compared with those of LLDPE/MWCNTs nanocomposites; on the other

Figure 7. Schematic drawing for the mechanism of the synergistic effect between Ni2O3 and MWCNTs on improving the flame retardancy of LLDPE.

Figure 8. Tensile stress-strain curves of LLDPE and its composites (inset: magnified stress-strain curve in the region of low strain).

hand, the formation of the residual char is promoted due to the presence of networklike structure compared with LLDPE/Ni2O3 composites, which will reduce the releasable amount of flammable degradation products. The above factors result in the formation of a more compacted protective layer. Therefore the synergistic effect between Ni2O3 and MWCNTs is displayed in improving the flame retardancy of LLDPE. 3.4. Mechanical Properties of LLDPE Composites. In the above results, we have demonstrated the synergistic effect in improving the flame retardancy of LLDPE using the combination of MWCNTs and Ni2O3. Clearly, a nonhalogen method for improving flame retardancy of polymeric materials without sacrificing other properties (such as mechanical properties) simultaneously is very attractive to academic and industrial communities. In polymer/filler composites, if load can be effectively transferred to from polymer matrix to fillers, then the strength and the modulus of the composites will be improved compared with those of polymer matrices. This is because the fillers show higher modulus and higher strength, such as CNTs. However, the well-dispersed states of the fillers in polymer matrices and the good interfacial interaction between two components are the key factors in order to prepare polymer composites with high mechanical properties.

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TABLE 2: Mechanical Properties of LLDPE Compositesa samples

YS (MPa)

PE 5Ni 1CN 3CN 1CN5Ni 3CN5Ni

10.3 11.4 11.7 11.7 12.4 12.8

( ( ( ( ( (

0.1 0.1 0.1 0.1 0.1 0.1

TSB (MPa) 23.6 23.4 25.2 27.1 24.8 25.5

( ( ( ( ( (

1.1 0.4 1.1 1.1 1.3 1.2

EB (%) 1406 1258 1417 1473 1388 1369

( ( ( ( ( (

64 63 59 36 60 31

a YS, yield strength; TSB, tensile strength at break; EB: elongation at break.

catalyzed by Ni catalyst originated from Ni2O3. Compared to LLDPE/MWCNTs and LLDPE/Ni2O3 composites, the higher melt viscosity caused by the formation of percolated network structure of MWCNTs and catalytic carbonization of degradation products of LLDPE by Ni catalyst show a synergistic effect in improving the flame retardancy of LLDPE. This is ascribed to the formation of a compacted carbonaceous protective layer. Although both the fillers (MWCNTs and Ni2O3) were not organically modified, LLDPE/MWCNTs/Ni2O3 ternary composites show higher mechanical properties compared with pure LLDPE. It is believed that optimizing the combination between CNTs with various structural parameters and carbonization catalyst particles with different composition and size will further improve the efficiency. As an example of the combination between nanotechnology and catalytic technology, combining CNTs with carbonization catalyst is a promising general strategy to simultaneously improve the flame retardancy and mechanical properties of polymeric materials. Acknowledgment. This work is financially supported by the National Natural Science Foundation of China for the Outstanding Youth Fund (no. 50525311) and the Fund for Creative Research Groups (no. 50621302).

Figure 9. Comparison of Young’s moduli of LLDPE and its composites.

Otherwise, the addition of the fillers leads to the formation of a defect; as a result, the mechanical properties of the composite will be deteriorated. The representative stress-strain curves are shown in Figure 8. Table 2 summarizes mechanical properties of the samples. In comparison with LLDPE, the yield strength (YS) of all LLDPE composites increases slightly due to the good dispersion of the two fillers in LLDPE matrix as shown in Figure 1. The tensile strength at break (TSB) of LLDPE/MWCNTs and LLDPE/Ni2O3/ MWCNTs composites also shows a little increase, but that of LLDPE/Ni2O3 does not change. At the same time, the addition of Ni2O3 decreases the elongation at break (EB) somewhat, from 1406% for LLDPE to 1258% for LLDPE/Ni2O3 composite. However, the incorporation of MWCNTs slightly improves the TSB and EB of LLDPE/MWCNTs simultaneously when compared with those of LLDPE and LLDPE/Ni2O3 composite. The above results indicate that the interfacial stress can be well transferred from LLDPE matrix to MWCNTs although the MWCNTs were not functionalized. The Young’s moduli of LLDPE and its composites are shown in Figure 9. On the whole, the Young’s moduli of LLDPE composites are much higher than that of pure LLDPE. The incorporation of CNTs or/and Ni2O3 fillers could enhance the Young’s modulus of LLDPE. Comparatively, the ternary composites show a higher modulus compared with the parental binary composites, which might be attributed to the improvement of the dispersed degree of both the fillers in LLDPE matrix (Figures 1 and 2). 4. Conclusions The above results have demonstrated that the combination between MWCNTs and Ni2O3 can further decrease the PHRR and slow down the combustion process of LLDPE compared with the addition of MWCNTs or Ni2O3 alone in LLDPE matrix. The dramatically improved flame retardancy is partially attributed to the good dispersion of MWCNTs in the matrix and partially to the carbonization of degradation products of LLDPE

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