bk-2009-1013.ch003

Chapter 3

Downloaded by PENNSYLVANIA STATE UNIV on June 8, 2012 | http://pubs.acs.org Publication Date: April 27, 2009 | doi: 10.1021/bk-2009-1013.ch003

Functionalized-Carbon Multiwall Nanotube as Flame Retardant for Polylactic Acid Serge Bourbigot, Gaëlle Fontaine, Antoine Gallos, Caroline Gérard, and Séverine Bellayer Procédés d'Elaboration des Revêtements Fonctionnels (PERF), LSPES-UMR/CNRS 8008, ENSCL, Avenue Dimitri Mendeleïev - Bât. C7a, B.P. 90108, 59652 Villeneuve d'Ascq Cedex, France

In this work, we have investigated the benefit of combining the grafting of chemical function to improve the solubility and the dispersion in an organic matrix with the opportunity of grafting fireproofing chemicals (melamine-based compound) on multiwall carbon nanotube (MWNT). The functionalized MWNT (f-MWNT) is incorporated in polylactide (PLA) via melt blending. Transmission electron microscopy (TEM) reveals that a high level of nanodispersion is achieved in PLA with f-MWNT while the dispersion is poor with virgin MWNT. The P L A nanocomposite containing f-MWNT exhibit a significant reduction of peak of heat release rate (PkHRR) in cone calorimeter experiment but this effect is probably not optimized because we suspect an antithetic effect of the melamine-based compound grafted on the surface of the nanotube.

© 2009 American Chemical Society

In Fire and Polymers V; Wilkie, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

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Introduction Polymeric materials are commonly used in everyday life increasing fire hazards and so flame retardants are very often incorporated into them to limit their flammability 0. O f particular interest is the developed nanocomposite technology consisting of a polymer and nanoparticles because they often exhibit remarkably improved mechanical properties and various other properties as compared with those of virgin polymer at loading as low as 3-5 wt.-% 0. Numerous nanocomposites containing different nanoparticles, including organomodified clays 0-5), nanoparticles of T i 0 0, nanoparticles of silica 0, layered double hydroxides (LDH) 0-0, carbon nanotubes (CNT) 0 or polyhedral silsesquioxanes (POSS) 0, have been prepared and characterized. A l l those materials exhibit low flammability associated with other properties such as enhanced mechanical or electrical properties. Aliphatic polyesters, and particularly polylactic acid (PLA), currently deserve particular attention in the area of environmentally degradable polymer materials. They are well suited for the preparation of disposable devices because of their biodegradability 0-0. In P L A nanocomposites it was reported that this family of composites exhibits improved properties including a high storage modulus both in the solid and melt states, increased flexural properties, a decrease in gas permeability, increased heat distortion temperature, an increase in the rate of biodegradability of pure P L A , etc... 0. Kashiwagi et al. reported the first study on the flammability of polymer carbon nanotube nanocomposites 0. They showed significant flame retardant effectiveness of polypropylene (PP)/multi-walled carbon nanotubes ( M W N T ) (1 and 2% by mass) nanocomposites. Concurrently, Beyer demonstrated a small improvement in flammability properties of ethylene-vinyl acetate ( E V A ) / M W N T (2.5 and 5% by mass) nanocomposites 0. Based on those results, one of the goals of this paper is to investigate the flame retardancy of P L A in a P L A / M W N T nanocomposite. A n issue is to evenly disperse M W N T in the polymeric matrix to get the best flame retardancy properties. In previous work, we have shown that the nanodispersion (including M W N T ) should be achieved to get the lowest peak of heat release rate (PkHRR) in a cone calorimeter 0. The chemical functionalization of carbon nanotubes is one of the few methods used to enhance the interfacial adhesion between the nanotubes and the matrix 0. The chemical treatment permits to generate functional groups at the surfaces of the nanotubes. These functional groups could react with other chemicals, prepolymers, and polymers, thus enhancing the interfacial bond between the matrix and the tubes for their further application to polymer nanocomposites. Here our basic idea is to combine the benefit of grafting chemical function to improve the solubility and the dispersion in an organic matrix with the opportunity of grafting fireproofing chemicals on the nanotube. 2


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This approach came out very recently in the literature with a reasonable success in acrylonitrile-butadiene-styrene (ABS) polymer using a phosphonate compound 0. Melamine-based compounds are well known to be flame retardants and our strategy is to graft melamine onto the suface of nanotube. The synthesis of the functionalized carbon nanotube will presented in the first part of the paper. Nanodispersion of the nanotubes will be then examined. The third part of the paper will investigate the reaction to fire of the nanocomposites containing virgin nanotubes and functionalized nanotubes.

Experimental Materials. PLA (melt flow index (190 °C, 2.16 kg) = 6.61 g/10 min) was supplied by Nature Works and dried overnight at 110 °C before use. CNT are multiwall carbon nanotube (MWNT) supplied by Nanocyl (Nanocyl-7000 at 90% purity). Unless otherwise noted, all materials were obtained from commercial suppliers and used without further purification. A l l reactions involving air- or moisture-sensitive compounds were performed in a nitrogen atmosphere. Functionanized carbon nanotubes (f-MWNT). Oxidation of MWNT by acid treatment was carried out by sonification for 2h (100W, 50kHz) of a suspension of l g of MWNT in 250mL of sulfuric acid (97%)/nitric acid (65%) (3/1). The oxidized MWNT were washed with water until the pH was neutral, then filtered through 0.45 μπι Millipore membrane. The oxidized MWNT were dried at 80 °C for 20h. A mixture of 1 g of oxidized MWNT and 250 mL of thionyl chloride was refluxed with stirring for 18h then unreacted thionyl chloride was removed by distillation and the activated MWNT (MWNT-COC1) were dried under reduced pressure. To a cold suspension of 2g of MWNT-COC1 in 200 mL of dried dichloromethane were added dropwise 2mL of ethylene diamine; when no more acidic vapor evolved, the suspension was refluxed with stirring for 18h. The ethylene diamine functionalized MWNT were washed, filtered and dried at 80 °C. The sample of 6-chloro-2,4-diamino-l,3,5-triazine was obtained by the reaction of 2.0g of 2,3,6-trichloro-1,3,5-triazine (0.011 mol) with 1.47 mL of aqeous ammonia solution (28%) in a mixture of 25mL H 0 and 25mL acetone at 45 °C overnight (this reaction is well known and can be found in textbooks about triazines). The precipitate was washed with water, filtered and dried under reduced pressure at ambient temperature leading to 1.5g of 6chloro-2,4-diamino-l,3,5-triazine (95%). A suspension of l g of ethylene diamine functionalized MWNT with an exess of 1 g of 6-chloro-2,4-diamino1,3,5-triazine in 200mL of water was refluxed for 18h. The f-MWNT were washed with water, filtered and dried under reduced pressure before use. 2



28 The acidic sites of the nanotubes were evaluated according to method described for single carbon nanotubes 0. The quantity of carboxylic acid function was evaluated to 1 mmol per gram of M W N T . Processing. P L A was melt-mixed with M W N T and f-MWNT (2 wt.-%) at 185 °C (PLA) using a Brabender laboratory E350 mixer measuring head (roller blades, constant shear rate of 50 rpm) for 10 min in nitrogen flow to avoid hydrolysis. Fourier Transform InfraRed. FTIR spectra were performed on Nicolet Impact 400D spectrometer in K B r pellets. Transmission electron microscopy. A l l samples were ultra microtomed with a diamond knife on a Leica ultracut U C T microtome, at room temperature for P L A samples, to give sections with a nominal thickness of 70 nm. Sections were transferred to Cu grids of 400 mesh. Bright-field T E M images of nanocomposites were obtained at 300 k V under low dose conditions with a Philips CM30 electron microscope, using a Gatan C C D camera. Low magnification images were taken at Π,ΟΟΟχ and high-magnification images were taken at 100,000x. Fire testing. A n FTT Mass Loss Calorimeter was used to carry out measurements on samples following the procedure defined in A S T M Ε 906. External heat flux of 35 kW/m was used for running the experiments. The mass loss calorimeter was used to determine heat release rate (HRR). 2

Results and Discussion Synthesis and characterization of the functionalized carbon nanotubes. Functionalized M W N T (f-MWNT) were synthesized in four steps according to the protocol described in Figure 1. In the first step the M W N T are oxidized by treatment with a mixture of sulfuric acid/nitric acid. The carboxylic acid functions formed at the suface of the M W N T are then activate by thionyl chloride, this step was followed by the reaction with ethylene diamine. The 6chloro-2,4-diamino-l,3,5-triazine, obtained by reaction of ammonia with 1,3,5trichlorotriazine, react with the amine function on the M W N T and we obtained the melamine-based f-MWNT. The purpose is twofold: (i) to increase the compatibility of M W N T with an organic polymeric matrix, and (ii) to nanodisperse flame retardant (FR) (here the melamine-based compound) in polymer. Figure 2 shows FTIR spectra of pristine M W N T , oxidized M W N T , M W N T - C O N H - C H - C H - N H , and f-MWNT. The spectrum of pristine M W N T exhibits two broad bands centered at 3430 cm" (-OH) and 1080 cm" (C-O) suggesting some impurities on the M W N T s . The oxidation of M W N T is clearly shown with the appearance of the band at 1635 cm" assigned to carbonyl group 2

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NH

2

Figure 1. Synthetic steps for the preparation of f-MWNT

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in carboxylic function and of the broad and intense band centered at 3430 cm" assigned to hydroxyl group. The reaction of oxidized M W N T with the ethylene diamine leads to the formation of amide group. Amide and carboxylic groups exhibits characteristic bands at very close wavenumbers (intense band of carbonyl in - C O N H and - C O O H at 1635 cm" and a broad band o f - O H / - N H groups centered at 3430 cm" ). Evidence of the formation of an amide bond is given by the band at 1550 cm" characteristic of a monosubstitued amide. The last step of the functionalization of the M W N T s is evidenced by the shift of the broad band centered at 3430 cm" to 3330 cm" corresponding to the transformation of - N H to - N H . Additional bands observed on the spectrum are assigned to melamine-based M W N T compound. Characterization of the nanodispersion. Figure 3 shows T E M images at low and high magnification of P L A / M W N T . Large bundles of nanotubes can be distinguished on the picture at low magnification (Figure 3-a) and numerous agglomerates are observed at high magnification (Figure 3-b). Using nonfunctionalized nanotubes the dispersion is poor and must be enhanced to expect good flame retardancy properties. f-MWNTs are much better dispersed in P L A than M W N T as revealed by T E M at low and high magnification (Figure 4). Tiny agglomerates can be distinguished on the picture at low magnification (Figure 4-a) but almost all nanotubes are single and relatively well separated from each other (Figure 4-b). So, it is demonstrated that the functionalized nanotubes with a melamine-based compound permit one to achieve a high level of nanodispersion. 1

1

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Figure 2. FTIR spectra of (a) pristine MWNT, (b) MWNT-COOH, (c) MWNT-CONH-CH CHrNH , and (d)fMWNT r

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Figure 3. TEM images of PLA/MWNT at low magnification (a) and at high magnification (b).



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Figure 4. TEM images of PLA/fMWNT at low magnification (a) and at high magnification (b).

Reaction to fire. PLA/f-MWNT exhibits lower HRR values than pure PLA and PLA/MWNT (reduction by 28% of PkHRR compared to virgin PLA) while PLA/MWNT does not exhibit any flame retardancy (Figure 5). This might be assigned at least partially, to the excellent dispersion of f-MWNT in PLA 0. In the final residues, the char of PLA/f-MWNT is relatively compact and cohesive while that with MWNT exhibits islands of char without any cohesion. It is also noteworthy that the times to ignition of PLA/MWNT and of PLA/f-MWNT are not enhanced compared to virgin PLA. According to the previous work of Kashiwagi et al. 0, we may assume this is because of an increase in the radiation in-depth absorption coefficient by the addition of carbon nanotubes. We also know that the incorporation of nanotubes in a polymer increases its viscosity in the melt compared to virgin polymer 0 upon heating. In the same conditions, virgin PLA melts and 'boils' creating strong convection due to mass transfer. At this time, heat conductivity of the nanocomposite becomes lower than that of the virgin polymer while it is the reverse before heating. Radiation absorption and lower heat conductivity causes accumulation of heat at the surface of the material upon heating and the nanocomposite reaches its ignition temperature more rapidly. We have also examined if melamine is a flame retardant for PLA. The incorporation of 30 wt.-% (the typical amount for conventional FR) of melamine in PLA provides a high LOI value (35 vol.-%) but it increases PkHRR and shortens the time to ignition, which means that melamine is not an efficient flame retardant for PLA in terms of cone calorimetry. Visual observations suggests that melamine decreases the viscosity of the materials upon heating


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PLA/Melamine(30%)

Time (s) Figure 5. HRR curves as a function of time of PLA/MWNT and PLA/f-MWNT compared to virgin PLA and PLA/Melamine.

(LOI test conditions). During an LOI test, the burning materials flows away along the bar and the combustion is stopped, which might explain in such conditions why LOI value is quite high while cone performance is poor. This discussion might also explain why the reduction of PkHRR of P L A / f - M W N T is not high as expected while the nanodispersion level is excellent. Further work is in progress with other F R compounds to be grafted onto the surface of nanotube to investigate this.

Conclusion In this work, we have investigated the synthesis of functionalized carbon nanotubes containing flame retardant groups (melamine-based compounds) and the reaction to fire of P L A nanocomposites containing M W N T and f-MWNT. High level of nanodispersion is achieved with f-MWNT while the dispersion is poor with virgin M W N T . The nanocomposite containing f-MWNT exhibits significant reduction of PkHRR but this effect is probably not optimized because it is suspected that the melamine-based compound grafted on the surface of the nanotube creates an antithetic effect.


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Acknowledgment


This work was partially supported by the European project Interreg IV ' N A N O L A C . The authors are indebted to Mr. Pierre Bachelet from our group for skilful technical assistance and for helpful discussion. Mr. Michael Claes from Nanocyl (Sambreville, Belgium) is gratefully acknowledged for supplying carbon nanotubes, for helpful collaboration and discussion.

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