Charing polymer wrapped carbon nanotubes for simultaneously improving the flame retardancy and mechanical properties of epoxy resin

Charing polymer wrapped carbon nanotubes for simultaneously improving the flame retardancy and mechanical properties of epoxy resin

Polymer 52 (2011) 4891e4898 Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer Charing polymer wra...

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Polymer 52 (2011) 4891e4898

Contents lists available at ScienceDirect

Polymer journal homepage: www.elsevier.com/locate/polymer

Charing polymer wrapped carbon nanotubes for simultaneously improving the flame retardancy and mechanical properties of epoxy resin Haiou Yu a, b, Jie Liu a, *, Xin Wen a, Zhiwei Jiang a, Yujie Wang a, b, Lu Wang a, b, Jun Zheng a, b, Shaoyun Fu c, Tao Tang a, * a b c

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China Graduate School of the Chinese Academy of Sciences, Beijing 100039, China Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 April 2011 Received in revised form 15 July 2011 Accepted 11 August 2011 Available online 16 August 2011

Molybdenum-phenolic resin (Mo-PR) was grafted onto the surface of multi-walled carbon nanotubes (MWCNTs) to obtain modified MWCNTs (CNT-PR). Compared to epoxy resin, epoxy resin/CNT-PR nanocomposites showed the improvements in flame retardancy and mechanical properties. Structural characterization showed that the grafted Mo-PR improved the dispersion of MWCNTs in epoxy resin and enhanced the interfacial interaction between CNT-PR and epoxy resin. On the other hand, the grafted Mo-PR could show high char yield during the process of combustion. Thus the flame retardancy of nanocomposites was improved, especially for the heat release rate and total smoke production. Furthermore, the combination of CNT-PR with melamine dramatically promoted the LOI value and the level of UL-94 rating. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Carbon nanotubes Epoxy resin Flame retardancy

1. Introduction Epoxy resin has been commercialized for 50 years, due to the attractive characteristics of high adhesion to many substrates, good chemical and corrosion resistance, and superior electrical properties, it was widely used in various industrial fields such as coating, potting, adhesives, laminates, and composites, etc [1e3]. However, one of the main drawbacks of epoxy resin is its inherent flammability, which restricts its application in many fields for safety consideration. Therefore, it is an important issue to improve the flame retardancy of epoxy resin. Epoxy resin can be rendered fireretardant either by incorporating fire-retardant additives [4e7] or by copolymerization with reactive flame retardants [8e11]. The addition of fire-retardant additives is a simple and common way to promote the flame retardancy of epoxy resin. But this method faces the problem of poor dispersion of flame retardants, which deteriorates the physical properties of epoxy resin matrix and decreases the efficiency of flame retardants. Because polymer/CNTs nanocomposites hold the promise of delivering excellent mechanical properties and multi-functional characteristics, they have attracted increasing interest for researchers in the field of polymer and materials from both * Corresponding authors. Tel.: þ86 431 85262004; fax: þ86 431 85262827. E-mail addresses: [email protected] (J. Liu), [email protected] (T. Tang). 0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2011.08.013

academia and industry [12e14]. However, the dispersion property becomes more important when CNTs are blended into polymer matrix. CNTs tend to remain as entangled agglomerates, so it is not easy to disperse CNTs uniformly in a polymer matrix. Furthermore, the lack of interfacial interaction limits load transfer from the matrix to nanotubes because of the atomically smooth non-reactive surface of CNTs. Up to now, the functionalization of CNTs has been explored using different approaches, such as noncovalent [15e18], or covalent [19e22], to improve the dispersion degree of CNTs and load transfer between nanotubes and matrix. Although noncovalent functionalization can confer novel properties on the CNTs without changing their structural features, the dispersion state of CNTs in the matrix is not stable [23]. Covalent functionalization can significantly improve solubility of CNTs in solvents and compatibility of CNTs with polymer matrices. For example, various polymer chains with functional groups have been employed to wrap CNTs to obtain composites with remarkable performance and stable dispersions. Recently, Fang’s group prepared functionalized CNTs grafted with an intumescent flame retardant, which could improve the dispersion of CNTs and the interfacial interaction between polymer and CNTs [24,25]. Consequently, the addition of the modified CNTs would improve the flame retardancy and mechanical properties of nanocomposites. Many methods have been tried to promote the dispersion of CNTs in epoxy resin, such as ultrasonication [26], high shear mixing

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Scheme 1. The molecular structure (a) Epoxy resin (b) Mo-PR.

[27], the aid of solvent [28], and the use of chemical modification through functionalization [29]. Zhu et al. fabricated epoxy resin nanocomposites with 1 wt.% fluoridized CNTs, and an increase in mechanical properties by 14% in stress over those of the unfilled epoxy resin was obtained [29]. Fabricating polymer nanocomposites, especially those containing unsymmetric nanoparticles, has been demonstrated as one of alternative flame retardant approaches to the use of halogenated flame retardants [30e35]. However, nanocomposites are not satisfied in the traditional fire tests, such as LOI and UL-94 [36]. Therefore, it may be a promising way to combine nanosized flame retardants with common flame retardants, such as intumescent flame retardants (IFRs) [36,37], in which the typical carbon sources are pentaerythritol, mannitol, sorbitol, etc. Unfortunately, these carbon sources impair the effects of IFRs due to the poor compatibility with polymers. Some charring polymers such as phenolic resin (PR) [38] or nylon [39] were used as carbon source. Thus the role of IFRs can take much longer, and mechanical properties are also improved. For epoxy resin, PR has been used as curing agent and can also take the role of carbon source to improve the flame retardancy [40,41]. In this paper, molybdenum-phenolic resin (Mo-PR) was grafted onto the surface of multi-walled carbon nanotubes (MWCNTs), and the modified MWCNTs (CNT-PR) were used to prepare epoxy nanocomposites. On one hand, Mo-PR can easily form char with high yield after combustion [42], and the structure of residue char is integrated. So the residues may act as a thermal shield for feedback from the flame. Thus the flame retardancy of nanocomposites is expected to be improved, especially for the heat release rate and total smoke production. On the other hand, the Mo-PR on the surface of CNT-PR can provide bonding sites to the epoxy matrix to prevent macrophase separation between polymer and nanotubes. Thus the modification of CNTs using Mo-PR should improve the dispersion of MWCNTs and the interfacial interaction between CNT-PR and epoxy resin. As a result, the load can be efficiently transferred from polymer matrix to the nanotubes, which lead to

the improvement of mechanical properties. In addition, CNTs may act as a cementing agent, which can promote integrity and strength of the residual char. 2. Experimental section 2.1. Materials The epoxy resin was a DGEBA (diglycidyl ether of biphenol A) with the epoxide value of 0.41e0.47 provided by Sanmu Chemical Co., Ltd. Phthalic anhydride (PA) was used as curing agent. Purified MWCNTs with a diameter of 15  5 nm and a length of about 30 mm were synthesized via CVD method. Melamine was bought from Shanghai Chemical Reagent Corporation. Molybdenum-phenolic resin (Mo-PR) was kindly supplied by 53 Institute of China North Industries Group Corporation. The molecular structure of DGEBA and Mo-PR are shown in Scheme 1. 2.2. Preparation of samples 2.2.1. Acid treatment of MWCNTs The oxidizing acid treatment procedure was used to prepare functionalized MWCNTs (CNT-COOH). In a typical experiment, MWCNTs (5 g) were sonicated in 250 ml of a mixture of concentrated nitric acid and sulfuric acid (1:3 by volume) for 30 min followed by refluxing under magnetic stirring at 120  C for 1.5 h. The sample was then washed with deionized water until a neutral pH value was obtained and the CNT-COOH was dried in vacuum at 80  C overnight. 2.2.2. Preparation of molybdenum-phenolic resin graft to CNTs (Mo-PR) The synthesis route of CNT-PR is shown in Scheme 2. CNT-COOH (1 g) was sonicated in 150 ml thionyl chloride (SOCl2) for 1 h and then refluxed at 70  C for 24 h. The unreacted SOCl2 was removed Table 1 The composition of epoxy resin composites and the results of LOI and UL-94 tests.

Scheme 2. Scheme for the preparation of CNT-PR.

Run

Mo-PR (wt.%)

CNT-PR (wt.%)

Melamine (wt.%)

LOI

UL-94

Epoxy resin 5Ma 8Ma 10Ma 2Mo-PR 1Mo-PR8Ma 2Mo-PR8Ma 1CNT-PR8Ma 3CNT-PR8Ma 5CNT-PR8Ma

e e e e 2 1 2 e e e

e e e e e e e 1 3 5

e 5 8 10 e 8 8 8 8 8

21.5 22 22.4 24.6 24 28 29.5 27.7 28.6 29.5

No rating No rating No rating No rating No rating V-2 V-0 V-2 V-2 V-0

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2.3. Characterization 0.6

CNT-PR

Absorbance

0.5 0.4

CNT-COOH

0.3 0.2

Mo-PR 0.1 4000

3500

3000

2500

2000

1500

1000

500

Wavenumber (cm -1 ) Fig. 1. FTIR spectra for purified MWCNTs, CNT-COOH, Mo-PR and CNT-PR.

by vacuum distillation and the remained black powder (CNT-COCl) was dried for 1 h. After that, 5 g Mo-PR and 200 ml anhydrous tetrahydrofuran (THF) were added and sonicated for 30 min to disperse CNT-COCl. Three drops of pyridine were added as catalyst. Then the reaction mixture was stirred by magnetic stirrer at 80  C for 24 h under dry N2 atmosphere. The product obtained was filtered and washed with THF. The obtained CNT-PR was dried to constant weight at 80  C in vacuum. Based on the results of TGA, the amount of Mo-PR grafted to the surface of CNTs was 44.9 wt.%. 2.2.3. Preparation of epoxy resin nanocomposites In a typical experiment, 7.8 g PA was added into 10 g epoxy resin at 110  C and was stirred until the formation of a uniform mixture. Flame retardants (MWCNTs, Mo-PR, CNT-PR or melamine) were added into another 10 g epoxy resin and vigorously stirred for 1 h. Then the mixture of epoxy resin with flame retardants was poured into the mixture of epoxy resin with PA. After stirred for 10 min, the mixture was poured into a polytetrafluoroethylene mold and cured at 150  C for 8 h. The sample compositions are shown in Table 1. The resultant samples were designed by the abbreviations and contents of the components. For example, 3CNT-PR8Ma means that the sample contains 3 wt.% CNT-PR and 8 wt.% melamine, and 3CNT-PR means that the sample only contains 3 wt.% CNT-PR.

FTIR spectra were recorded on a Bio-Rad FTS 135 spectrophotometer. The structure of CNT-PR and the morphologies of composites were observed by transmission electron microscope (TEM, JEL1011, JEOL, Japan) at 100 kV accelerating voltage. Ultrathin sections were cryogenically cut using a Leica Ultracut and a glass knife at room temperature. The samples were collected on carboncoated copper TEM grids. Flammability tests were performed on a Dual Cone Calorimetry (FTT, UK) according to ISO-5660 Standard at a heat flux 35 kW/m2. Exhaust flow rate was 24 L/s and the spark was continuous until the sample was ignited. The specimens with the sizes of 100  100  2 mm3 square plaques for cone calorimetry. The residues were characterized by field-emission scanning electron microscope (SEM, XL30, FEI, USA). Thermal gravimetric analyses (TGA) were done by Thermal Analysis Instrument (SDTQ600, TA Instruments, USA) with a heating rate of 10  C/min. Differential scanning calorimetry (DSC) tests were recorded on a PerkineElmer DSC-2C at a heating rate of 10  C/min under the flow of 20 ml/min N2, and the glass transition temperature (Tg) was calculated by the method of IEC 61006: 2004. The limited oxygen index (LOI) was measured on a JF-3 oxygen index meter (Jiangning, China) with sheet dimensions of 130  6.5  3.2 mm3, according to ISO45891984. The UL-94 rating was tested according to the UL-94 (ANSI/ ASTMD635-77) with sheet dimensions of 125  12.7  3.2 mm3. Mechanical properties were measured on an Instron 1121 at an extension speed of 2 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. The modification of CNTs via grafting Mo-PR Fig. 1 shows the FTIR spectra of MWCNTs, CNT-COOH, Mo-PR and CNT-PR. As expected, purified MWCNTs were devoid of bands for any identifiable functional group. The band at 1575 cm1 is an in-plane E1u mode of monocrystalline graphite [43,44]. For CNTCOOH, two new bands appeared at 1711 and 1183 cm1, which are attributed to the C]O and CeO stretching vibrations of carboxylic groups (eCOOH), respectively [43,45]. It suggests that carboxylic groups have been brought to the surface of MWCNTs after the oxidation in mixed acids. The band of carboxylic groups of

Fig. 2. TEM images of CNT-COOH (a) and CNT-PR (b).

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To compare the solubility of CNT-COOH and CNT-PR in organic solvent, 4 mg of both samples were added into 10 ml THF followed by ultrasonication for 5 min and sedimentation for 10 min, respectively. A remarkable difference in suspension stability was observed (Fig. 3). CNT-COOH could not be suspended stably in THF, meaning that the CNT-COOH was not well dispersed in THF even after ultrasonication (Fig. 3a). In contrast, a stable suspension of CNT-PR was formed due to the good solubility of grafted Mo-PR (Fig. 3b). The improvement in the suspension stability of CNT-PR further confirmed that Mo-PR was covalently linked onto the sidewalls of MWCNTs. Most importantly, the solubility of CNTs in the organic solvent was improved. 3.2. Dispersion states of CNT-COOH and CNT-PR in epoxy resin matrix

Fig. 3. Photographs for the suspension stability of CNT-COOH (a) and CNT-PR (b) in THF by ultrasonication for 5 min and sedimentation for 10 min.

CNT-COOH shifted from 1711 to 1727 cm1 after acylation and reacting with Mo-PR, which is ascribed to C]O of ester groups in the resultant CNT-PR [45]. Meanwhile, the characteristic bands of Mo-PR appeared, such as the band at 1450 cm1, which is attributed to C]C of benzene ring. The broad peak at 3300 cm1 in the FTIR spectrum of CNT-PR, which is ascribed to hydroxyl group of phenol, became weaker than the homologous peak of Mo-PR. Furthermore, the band of in-plane deformation of hydroxyl groups of phenol at 1362 cm1 almost disappeared in the CNT-PR [46]. It suggests that Mo-PR has grafted to the surface of MWCNTs. In order to get a visual proof, the comparison for the microstructures of CNT-COOH and CNT-PR was shown in Fig. 2. In Fig. 2a, CNT-COOH was obtained by centrifugation from the THF solution of CNT-COOH and Mo-PR. It was shown that most tops of CNT-COOH were open due to the oxidation of mixed acids, and the cannular structure was clear. For CNT-PR, there was a thin layer of Mo-PR on the surface of CNTs, and the cannular structure became obscure. It is obvious that Mo-PR has been successfully grafted to the surface of CNTs.

The dispersion states of CNT-COOH and CNT-PR in epoxy resin matrix were observed by means of TEM, as shown in Fig. 4. The present TEM images were representative of the dispersion states of the fillers in epoxy resin matrix. It could be seen that CNT-COOH aggregated in the epoxy resin matrix due to the van der Waals force between CNTs (Fig. 4a). In contrast, CNT-PR showed a good dispersion in the epoxy resin matrix and no aggregate was observed (Fig. 4b). According to the FTIR results (Fig. 1), the grafted phenolic resin contains an amount of hydroxyl group, which can take part in the curing reaction of epoxy resin [41,47]. Therefore Mo-PR grafted to CNTs can improve the dispersion of CNTs and enhance the interfacial interaction. 3.3. Thermal properties The glass transition temperatures (Tg) of epoxy resin and its composites were measured by means of DSC. The results are shown in Table 2. Compared to pure epoxy resin, the samples containing melamine (Run 8Ma), Mo-PR (Run 2Mo-PR) or CNT-PR (Run 5CNTPR) showed a high Tg, which demonstrates that the addition of melamine and CNT-PR can promote the degree of cure. Meanwhile, the sample containing both CNT-PR and melamine showed a higher Tg than that of the sample containing CNT-PR alone. The reason is that the amine group of melamine and the hydroxy group of Mo-PR may take part in the cure reaction of epoxy resin. It can promote the degree of cure, thus the Tgs of composites are increased. Fig. 5 presents TGA curves of epoxy resin and its composites under air atmosphere. Detailed data are listed in Table 2. The onset temperature (T5 wt.%) of all composites was lower than that of epoxy

Fig. 4. TEM images of epoxy nanocomposites with 1 wt.% CNTs. (a) CNT-COOH/epoxy resin; (b) CNT-PR/epoxy resin.

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Table 2 Summary of the TGA results for epoxy resin and its composites under air atmosphere. Run

Tga ( C)

T5

Epoxy resin 8Ma 2Mo-PR 2Mo-PR8Ma 1CNT-PR 3CNT-PR 5CNT-PR 1CNT-PR8Ma 3CNT-PR8Ma 5CNT-PR8Ma

104.3 132.4 123.4 132.0 e e 120.0 e e 131.4

335 303 273 261 324 314 303 261 285 293

a b c d

wt.%

b 

( C)

Tmaxc( C)

Char yieldd (wt.%)

517 540 523 517 509 490 497 498 522 529

7.3 7.2 21.4 12.2 18.6 20.5 23.1 10.3 15.7 17.6

Tg was caculated by the method of IEC 61006: 2004. T5 wt.%, at which 5 wt.% weight loss rate occurred. Tmax, at which maximum weight loss rate occurred. Char yield at 800  C.

resin. This should result from the decomposition of melamine and Mo-PR, which is good for protecting the polymer matrix from heat flux. A similar phenomenon can be observed in the system containing intumescent flame retardants. Tmaxs of the composites containing Mo-PR and melamine were higher than that of epoxy resin, and the highest Tmax was 540  C for the sample 8Ma. For the samples containing CNT-PR only, Tmax were lower than that of epoxy resin. It might be caused by the increasing thermal conductivity of CNTs [48]. When combining CNT-PR with melamine, Tmax increased with the content of CNT-PR. Compared to epoxy resin, the addition of melamine did not change the char yield

Fig. 6. Heat release rate plots of epoxy resin and its composites (measured by cone calorimeter at 35 kW/m2).

at 800  C. Surprisingly, the char yield of the sample 2Mo-PR was increased to 21.4 wt.%. There are two possible reasons for the formation of high char residue. One of them is that the char layer from molybdenum-phenolic resin or CNT-PR prevents the release of the degradation products of epoxy resin. The other is that CNT-PR and Mo-PR can promote the degree of cure of epoxy resin. However, the char yield was decreased again in the case containing combined Mo-PR (or CNT-PR) with melamine. It is a reason that the gas released from melamine destroyed the integration of residual char. 3.4. Flame retardancy The LOI, representing the lowest oxygen volume content for sustaining the flame in an environment, was used for quantifying the flame retardancy of epoxy resin. The oxygen volume content in ambient atmosphere is about 21%. Therefore, a material exhibiting its LOI above 21 might show flame-retardant property. Generally, materials with LOI values higher than 26 might show selfextinguishing behavior and were considered to be high flame retardancy. The results of LOI and UL-94 for epoxy resin and its composites are listed in Table 1. The LOI value of epoxy resin increased slightly after adding melamine or Mo-PR, and the dripping phenomenon could be still observed during UL-94 tests. When the combination of melamine and Mo-PR (or CNT-PR) was added, the values of LOI were further increased compared to the corresponding samples containing only one kind of additives. Furthermore, UL-94 test could reach V-0 for the sample 2Mo-PR8Ma and 5CNT-PR8Ma. These results demonstrate that a synergistic effect between Mo-PR (or CNT-PR) and melamine on the flame retardancy of epoxy resin occurs.

Fig. 5. TGA curves of epoxy resin and its composites under air atmosphere.

Fig. 7. Total smoke production plots of epoxy resin and its composites (measured by cone calorimeter at 35 kW/m2).

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Table 3 Summary of cone calorimetric results for epoxy resin and its composites. Run

pHRR (kW/m2)

TSP (m2/kg)

Residual char (wt.%)

Epoxy resin 8Ma 2Mo-PR 2Mo-PR8Ma 1CNT-PR 3CNT-PR 5CNT-PR 1CNT-PR8Ma 3CNT-PR8Ma 5CNT-PR8Ma

900 750 543 579 580 604 503 527 535 468

26.7 23.4 22.7 21.8 25.4 22.5 22.8 25.2 22.5 20.8

2.9 6.0 15.7 11.4 13.1 15.1 17.1 11.6 12.0 15.0

The cone calorimeter is a fire testing apparatus based on bench scale and provides a wealth of information on combustion behavior of a material. Some cone calorimeter results have been found to correlate well with those obtained from large scale fire tests, thus it can be used to predict the behavior of materials in a real fire. Furthermore, it provides comprehensive insight into fire risk via parameters such as heat release rate (HRR), peak of heat release rate (pHRR) and total smoke production (TSP). The results of cone calorimeter for epoxy resin and its composites are shown in Fig. 6, Fig. 7 and Table 3. Fig. 6 shows the HRR plots of epoxy resin and its composites measured by cone calorimeter at 35 kW/m2. Compared to pure epoxy resin, the HRR of all the composites was decreased. For the sample 8Ma, the HRR decreased slightly and the pHRR decreased 17% compared to epoxy resin. Interestingly, the pHRR for the 5CNT-PR8Ma was the lowest among all the samples, which decreased 48% compared to epoxy resin. In addition, comparing 2Mo-PR8Ma with 5CNT-PR8Ma, the content of Mo-PR in these two samples was the same, but the pHRR of the 5CNT-PR8Ma was lower than that of the 2Mo-PR8Ma. This demonstrates that CNTs can promote the flame retardancy of the composites. Compared to 2Mo-PR, both the HRR and pHRR of 2Mo-PR8Ma were higher. Fig. 7 and Table 3 present the results of TSP for epoxy resin and its composites. Except for the sample 8Ma, the time of initiating smoke for all the other composites was earlier than epoxy resin, it was resulted from Mo-PR decomposed firstly. For the composites containing CNT-PR, the TSP decreased with increasing the content of CNT-PR. Furthermore, the TSP further decreased when both CNTPR and melamine were added simultaneously to epoxy resin. The TSP for 5CNT-PR8Ma was the lowest among all the samples, and it decreased 22% compared to epoxy resin. The char yield of the samples after the cone calorimeter tests is also listed in Table 3. It could be found that the char yields increased after adding Mo-PR or

CNT-PR, but the combination between Mo-PR (or CNT-PR) and melamine reduced the char yield. These results were similar to those presented in Table 2. 3.5. The mechanism of flame retardancy The LOI of char is 65. If a char layer can be formed during the combustion of polymer, it can act as a heat insulator, limiting the heat transfer from the source to the polymer and the mass transfer from the polymer to the flame and resulting in improved fireretardant performances. Therefore, the amount and structure of char layer are important to promote the flame retardancy of polymer. Owing to the super properties of ablative resistance, heat insulation, smoke abatement and flame suppression, Mo-PR was widely used as high temperature ablation-resistant materials and insulation lining materials of rocket. From the results of TGA and cone calorimeter test, it can be found that the char yield of epoxy resin composites increased with the content of Mo-PR. In the case of melamine combined with MoPR (2Mo-PR8Ma), the char yield during cone calorimeter test decreased from 15.7 wt.% (2Mo-PR) to 11.4 wt.% (2Mo-PR8Ma). Thus the HRR of 2Mo-PR8Ma was higher than 2Mo-PR. Although the char yield of 2Mo-PR was similar to that of 5CNT-PR8Ma, and the char yield of 5CNT-PR was higher than that of 5CNT-PR8Ma, the HRR of 5CNT-PR8Ma was the lowest among three samples. It demonstrates that the char yield is not the only factor of determining the flame retardancy of epoxy resin composites. So it is necessary to investigate the morphology of residua after cone calorimeter experiments. Fig. 8 shows the photographs of residua after combustion. For the residue of epoxy resin (Fig. 8a), it could be seen that there was little char left after the cone calorimeter test. In contrast, there was a large amount of char left for the sample of 2Mo-PR (Fig. 8f) after combustion. The char layer of 2Mo-PR was compact, but it was not continuous. In the case of combined Mo-PR with melamine (2Mo-PR8Ma, Fig. 8g), the char layer trended to be relatively continuous, but a lot of holes were formed by releasing gas from the decomposition of melamine. For the binary epoxy resin/CNT-PR (Fig. 8cee) composites, the residual chars trended to be more dense and continuous with increasing the content of CNTPR, especially for the sample of 5CNT-PR. Interestingly, for the ternary epoxy resin/CNT-PR/melamine composites (Fig. 8hej) the residual chars were more continuous than the corresponding binary epoxy resin/CNT-PR composites. Especially for the 5CNTPR8Ma composites, the char layer was more smooth and integrated than 5CNT-PR. This phenomenon was different from the contrast between 2Mo-PR and 2Mo-PR8Ma. It may result from the push of

Fig. 8. Photographs of the residua after the cone calorimeter tests. (a) Epoxy resin; (b) 8Ma; (c) 1CNT-PR; (d) 3CNT-PR; (e) 5CNT-PR; (f) 2Mo-PR; (g) 2Mo-PR8Ma; (h) 1CNT-PR8Ma; (i) 3CNT-PR8Ma; (j) 5CNT-PR8Ma.

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Fig. 9. SEM images of the residua of composites. (a) 2Mo-PR; (b) and (c) 5CNT-PR; (d) 2Mo-PR8Ma; (e) and (f) 5CNT-PR8Ma.

gas released from melamine decomposition. Comparing the binary epoxy resin/CNT-PR with the ternary epoxy resin/CNT-PR/ melamine composites, the char yields of binary composites were higher than those of ternary composites, and the apparent morphologies were similar, but the HRR of the ternary composites were lower, especially for the composites of 5CNT-PR8Ma and 5CNT-PR. So it is necessary to investigate the microstructure of residual chars. Fig. 9 shows SEM images of the residual chars. A lot of holes can be seen in the residue of 2Mo-PR (Fig. 9a). In contrast, the residue from 2Mo-PR8Ma composite became porous structure (Fig. 9d). It might result from the gas release from melamine, which breaks through the char. This is another reason for the higher HRR of 2MoPR8Ma than 2Mo-PR. Compared to 2Mo-PR, the number of holes in the char layer was decreased in the residue of 5CNT-PR (the content of Mo-PR in the composites is the same as that of 2Mo-PR) (Fig. 9b). It indicates that CNTs act as a cementing agent, which can promote integrity of the residual char. Extraordinarily, the residual char of 5CNT-PR8Ma (Fig. 9e) was very dense and continuous, no holes could be found. This was determined by the dispersion states of CNTs in the residues. In the char of 5CNT-PR composite (Fig. 9c), a lot of CNT aggregates could be found. This implies that CNTs reaggregated during combustion. However, CNTs were well dispersed in the char of 5CNT-PR8Ma (Fig. 9f). Thus CNTs can play the role of adhesive to promote the integration of char. As a result, the HRR of 5CNT-PR8Ma was lower than that of 5CNT-PR. 3.6. Mechanical properties In the above results, we have demonstrated the synergistic effect in improving the flame retardancy of epoxy resin using the combination of CNT-PR (or Mo-PR) and melamine. 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 from polymer matrix to fillers, then the strength of the composites will be improved compared to those of polymer matrices. 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. 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.

Table 4 shows mechanical properties of samples. In comparison with epoxy resin and the sample 2Mo-PR, the tensile strength at break (TSB) and elongation at break (EB) of all composites increased due to the good dispersion of CNTs and the strong interfacial interaction between Mo-PR and the matrix. The TSB of 2Mo-PR8Ma was the highest, which is two times as that of epoxy resin. For the binary epoxy resin/CNT-PR composites, both TSB and EB were increased with the content of CNT-PR. When CNT-PR was combined with melamine, TSB was increased with the content of CNT-PR, which was higher than the corresponding binary epoxy resin/ CNT-PR composites. It is a possible reason that the combination between CNT-PR and melamine can increase corsslinking density of epoxy matrix compared to the binary epoxy resin/CNT-PR composites. Compared to epoxy resin, Young’s modulus (YM) of all the composites showed a similar change. For example, for the binary epoxy resin/CNT-PR composites, the YM was increased with the content of CNT-PR. The reason may be CNTs reduce apparent volume fraction of polymer matrix. SEM images for the fracture surface of the 3CNT-PR sample after tensile testing are presented in Fig. 10. It can be seen that CNTs are uniformly distributed in the epoxy matrix and no obvious CNTs pullout can be observed. Namely, CNTs pullout length is very small and CNTs are covered by the epoxy matrix. This is an indication of strong CNT-matrix interfacial adhesion since the interfacial adhesion strength is inversely proportional to critical fiber (here CNTs) length while critical CNTs length is proportional to CNT pullout length [49,50]. Otherwise, there should be great CNT pullout length since original CNTs length is large. It indicates that Mo-PR grafted onto the sidewall of CNTs can strengthen the interfacial interaction

Table 4 Mechanical properties of epoxy resin and its composites. Samples

TSB (MPa)

Epoxy resin 8Ma 2Mo-PR 2Mo-PR8Ma 1CNT-PR 3CNT-PR 5CNT-PR 1CNT-PR8Ma 3CNT-PR8Ma 5CNT-PR8Ma

27.4 43.4 23.8 55.0 32.9 41.4 44.9 33.5 44.8 50.0

         

0.9 0.6 0.8 0.4 0.5 0.3 0.7 1.0 1.1 1.5

YM (MPa) 1020 1383 948 1735 1056 1185 1435 1160 1445 1660

         

35 18 16 39 12 19 22 18 27 42

EB (%) 2.0 3.7 2.5 3.8 3.8 3.6 5.8 3.6 6.6 4.9

         

0.6 0.8 0.2 0.7 0.6 0.3 1.1 0.5 1.2 0.8

TSB: tensile strength at break; YM: young’s modulus; EB: elongation at break.

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Fig. 10. SEM images with high magnification (a) and low magnification (b) for fractured surfaces of 3CNT-PR.

between CNTs and epoxy resin, besides promoting the dispersion of CNTs in the matrix. Thus the improvement in mechanical properties can be reasonably attributed to the good CNT-epoxy interfacial adhesion. 4. Conclusion

[12] [13] [14] [15] [16] [17] [18]

Mo-PR has successfully been grafted onto the sidewall of CNTs, which improved the dispersion of CNTs in epoxy resin matrix. The combination of melamine and CNT-PR (or Mo-PR) was profitable for regular flame tests. For instance, UL-94 tests reached V-0 rate for the composites of 2Mo-PR8Ma and 5CNT-PR8Ma. Simultaneously, the mechanical properties of composites were improved. Furthermore, the results of cone calorimetric experiments indicated that the efficiency of flame retardancy for the composites containing CNT-PR was better than the corresponding composites containing 2Mo-PR or CNTs. This resulted from the different structure of char layers. The char layer was more dense and integrated in the residual char of the composites containing CNT-PR, especially containing the combination of CNT-PR and melamine. The microstructure observation showed that the decomposition of melamine could prevent the aggregation of CNTs during combustion. As a result, the strength of char layer was strengthened, and the structure of the char layer was closely knitted.

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Acknowledgments [35]

This work is financially supported by the National Natural Science Foundation of China for the Projects (no. 50873099, 51073149, 50921062 and 10972216). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

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