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clay nanocomposites by melt intercalation method from Na+ montmorillonite
Materials Letters 57 (2003) 2675 – 2678 www.elsevier.com/locate/matlet
Preparation and flammability properties of polyethylene/clay nanocomposites by melt intercalation method from Na+ montmorillonite Shaofeng Wang a,b, Yuan Hu a,*, Qu Zhongkai c, Zhengzhou Wang a, Zuyao Chen b, Weicheng Fan a a
State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230026, Anhui, PR China b Department of Chemistry, University of Science and Technology of China, Hefei 230026, Anhui, PR China c National Engineering Research Center for Synthetic Fiber, 48 Jinyi Road, Jinshan District, Shanghai 200540, PR China Received 29 May 2002; accepted 15 October 2002
Abstract Polyethylene (PE)/clay composites have been prepared by melt intercalation technique direct from Na+ montmorillonite (MMT) while using hexadecyl trimethyl ammonium bromide (C16) as reactive compatibilizer. Their degree of dispersion and intercalation spacings, as determined by X-ray diffraction (XRD), were typical of either a microcomposite or an intercalated nanocomposite, depending on the content of C16 in composites. Combustion experiments showed that the microcomposite burns in the same way as pure PE, whereas the heat release rate (HRR) is reduced by 32% when nanocomposite is with low silicate loading (5 wt.%) and C16 loading (4 wt.%). D 2002 Elsevier Science B.V. All rights reserved. Keywords: Nanocomposites; Polymers; X-ray technique
1. Introduction Recently, much attention has been paid to polymer nanocomposites, especially polymer-layered silicate nanocomposites, which represent a rational alternative to conventional filled polymers. Nanocomposite technology has been described as the next great frontier of material science. Because by employing minimal addition levels ( < 10 wt.%), nanoclays enhance
* Corresponding author. Tel./fax: +86-551-3601664. E-mail address: [email protected] (Y. Hu).
mechanical, thermal, dimensional and barrier performance properties significantly [1]. In order to facilitate silicate layers interaction with a polymer, the clay is modified with an alkylammonium salt (surfactant molecule) by a cation exchange reaction, because the alkylammonium make the hydrophilic clay surface organophilic. Although the modified clay is miscible for polar polymers such as nylon [2], epoxy resin [3], polystyrene (PS) [4], etc., the organically modified clay does not disperse well in the nonpolar polymer such as polypropylene (PP) and polyethylene (PE) since such nonpolar polymers are still too hydrophobic [5]. Thus, it is important to
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(02)01354-X
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synthesis nonpolar polymer/clay nanocomposites. Initial attempts to create the nonpolar polypropylene/ clay nanocomposites by simple melt-mixing were based on the introduction of a modified oligomer to mediate the polarity between the clay surface and polymer [6]. However, these techniques all used modified clay. Recently, Alexandre et al. [7] report a novel technique on the preparation of EVA/clay nanocomposites starting directly from natural (Na+-base) clay while using an ammonium salt bearing long alkyl chains as polymer/clay reactive compatibilizer. This paper aims at synthesis of PE/clay nanocomposites starting directly from natural (Na+-base) clay and study on the fire properties of PE/clay nanocomposites and mechanism of this new method.
2. Experiments 2.1. Materials High-density polyethylene (HDPE, PE-LA-50D12) has been used as a model in this study. Purified sodium montmorillonite (MMT, China), with cation exchange capacity (CEC) of 96 meq/100 g and an ˚ , has been used as the interlayer spacing d001 = 15 A nanofiller precursor. Hexadecyl trimethyl ammonium bromide (C16) and tetrabutyl ammonium bromide (C4) have been chosen as clay/matrix reactive compatibilizer. 2.2. The preparation of HDPE/clay composites HDPE was melt-mixed with MMT and different amount of compatibilizer (C16 or C4) at 165 jC using a twin-roll mill (XK-160, made in Jiangshu, China) for 10 min to yield composite. 2.3. Evaluation of dispersibility of the clay in polymer matrix Morphology studies have been carried out on all composites. X-ray diffraction (XRD) experiments were performed at room temperature by a Japan Rigaku D/max-rA X-ray diffractiometer (30 kV, 10 ˚ ) irradiation at the rate of mA) with Cu (k = 1.54178 A 2j/min in the range of 1.5 –10j.
2.4. Cone calorimetry Cone calorimeter experiments were performed at an incident heat of 50 kW/m2. Peak heat release rate (HRR), mass loss rate (MLR), specific extinction area (SEA), ignition time (tign) and specific heat of combustion data are reproducible to within 10% when measured at 50 kW/m2. The cone data reported here are the average of three replicated experiments.
3. Results and discussions For the sake of comparison, different content of reactive compatibilizers (C16 or C4) have been used to prepare HDPE/clay composites. Compounds 1(a), 1(b), 1(c), 1(d) and 1(e), filled with the same amount of inorganic (5 wt.% MMT) and different amount of compatibilizer (C16 or C4, Table 1), were prepared. From Fig. 1 analysis, it shows that natural sodium montmorillonite (MMT) is immiscible with HDPE ˚ ), while adding only 1 wt.% (Fig. 1a, d001 = 15.1 A C16, we get a notable main diffraction peak at 38.7– ˚ (Fig. 1b). When the C16 is 2 wt.%, the dif43.5 A fraction peak of MMT disappeared and we get a sharp ˚ with orders second and main diffraction peak at 40.6 A third diffraction peaks (Fig. 1c). However, when the content of C16 increases to 5 wt.%, the d001 does not increase but decreases slightly (Fig. 1d). This indicates that the intercalation capability has a certain limit and the result is similar to the report of Wang et al. [8]. The carbonic-chain length of surfactant also affects the morphology compound. When C4 is used as compatibilizer of clay/HDPE, the d001 of compound ˚ , far less that of compound 1(c). This 1(e) is 16.9 A indicates that certain carbonic-chain length is needed to expand the layer of clay.
Table 1 Summary of HDPE/clay composites Compound Polymer (wt.%)
Clay Compatibilizer Main diffraction ˚) (wt.%) (wt.%) peak (d001) (A
1(a) 1(b) 1(c) 1(d) 1(e)
MT MT MT MT MT
HDPE HDPE HDPE HDPE HDPE
(95) (94) (93) (90) (93)
(5) (5) (5) (5) (5)
C16 (1) C16 (2) C16 (5) C4 (2)
15.1 38.7 – 43.5 40.6 38.8 16.9
S. Wang et al. / Materials Letters 57 (2003) 2675–2678
Fig. 1. X-ray diffraction patterns of compounds 1(a) (HDPE + 5 wt.% MMT), 1(b) (HDPE + 5 wt.% MMT + 1 wt.% C16), 1(c) (HDPE + 5 wt.% MMT + 2 wt.% C16), 1(d) (HDPE + 5 wt.% MMT + 5 wt.% C16) and 1(e) (HDPE + 5 wt.% MMT + 2 wt.% C4).
3.1. Flammability properties Characterization of the flammability properties of a variety of polymer/clay nanocomposites, under firelike conditions, using the cone calorimeter, has revealed improved flammability properties for many types of polymer/clay nanocomposites [9,10]. The
Fig. 3. Scheme of polymer/clay compound.
Fig. 2. Heat release rate (HRR) data for pure HDPE, HDPE + 5 wt.% MMT and HDPE + 5 wt.% MMT + 4 wt.% C16.
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cone calorimeter is one of the most effective benchscale methods for studying the flammability properties of materials. Heat release rate in particular peak HRR has been found to be the most important parameter to evaluate fire safety [9]. The heat release rate (HRR) plots for pure HDPE, HDPE/5 wt.% MMT microcomposite and HDPE/5 wt.% MMT/4 wt.% C16 (HDPE/MMT/C16) nanocomposite at 50 kW/m2 are shown in Fig. 2. The peak HRR of HDPE/5 wt.% MMT (immiscible) is very similar to that of pure HDPE. Its peak HRR is lower because it contains 95 wt.% HDPE. However, the peak HRR of HDPE/MMT/C16 nanocomposite decreases 32% compared to that of pure HDPE. Moreover, it should be noted that the time to ignite (tign, 50 s) HDPE/MMT/C16 nanocomposite is relatively short in comparison with pure HDPE (tign, 75 s). The initial HRR for nanocomposite (Fig. 2) is higher for the first minute following ignition (50 –125 s from the beginning of the experiment), probably because of thermal decomposition of reactive compatibilizer (C16) resulting in the formation of volatile combustibles, as shown in Ref. [12]. 3.2. Mechanism of intercalation Ishida et al. [13] have reported the use of epoxy as swelling agent of polymer/organophilic clay nanocomposites. In this paper, C16 serves to modify, as well as to swell, the clay layers. We deduce the mechanism: at the process of melting, where, at first, the surfactant diffuses to the round of clay layers, then, through static adsorption where the surfactant intercalates into the silicate layers and expands them; and at this process, where the C16 can act as a carrier to transport the chains of matrix into the interlayer of clay simultaneously. The scheme of this kind of compounds is shown in Fig. 3 [11].
4. Conclusion This study has demonstrated the ability to prepare nonpolar HDPE-base nanocomposites by melt
intercalation starting from Na+ montmorillonite by adding cation surfactant as reactive compatibilizer. Combustion experiments showed that the heat release rate (HRR) is reduced by 32% when nanocomposite with low silicate loading (5 wt.%) and C16 loading (4 wt.%). The supposed mechanism of this technique has been studied.
Acknowledgements The work was financially supported by the Natural Science Foundation of China (No. 50003008), the China NKBRSF project (No. 2001CB409600), Anhui ‘‘the tenth five years’’ tackle key problems project, open project of the Structure Research Laboratory of the University of Science and Technology of China and Hefei Unit Center of Analysis and Test of CAS.
References [1] M. Alexandre, P. Dubois, Mater. Sci. Eng. 28 (2000) 1. [2] J.W. Cho, D.R. Paul, Polymer 42 (2001) 1083. [3] X. Kornmann, H. Lindberg, L.A. Berglund, Polymer 42 (2001) 4493. [4] Y. Hu, L. Song, International Fire Safety Conference (Fire Retardant Chemicals Association), March 11 – 14, 2001. [5] E. Manias, A. Touny, L. Wu, K. Strawhecker, B. Lu, T.C. Chung, Chem. Mater. 13 (10) (2001) 3516. [6] M. Kato, A. Usuki, A. Okada, J. Appl. Polym. Sci. 66 (1997) 1781. [7] M. Alexandre, G. Beyer, C. Henrist, et al., Chem. Mater. 13 (11) (2001) 3830. [8] K.H. Wang, M.H. Choi, C.M. Koo, Y.S. Choi, I.J. Chung, Polymer 42 (2001) 9819. [9] J.W. Gilman, Appl. Clay Sci. 15 (1999) 31. [10] M. Zanetti, T. Kashiwagi, L. Falqui, G. Camino, Chem. Mater. 14 (2002) 881. [11] M. Ogawa, Langmuir 13 (1997) 1853. [12] W. Xie, Z. Gao, K. Liu, et al., Thermal characterization of organically modified montmorillonite, Thermochim. Acta 367 – 368 (2001) 339. [13] H. Ishida, S. Campbell, J. Blackwell, Chem. Mater. 12 (2000) 1260.
Preparation and flammability properties of polyethylene/clay nanocomposites by melt intercalation method from Na+ montmorillonite Shaofeng Wang a,b, Yuan Hu a,*, Qu Zhongkai c, Zhengzhou Wang a, Zuyao Chen b, Weicheng Fan a a
State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230026, Anhui, PR China b Department of Chemistry, University of Science and Technology of China, Hefei 230026, Anhui, PR China c National Engineering Research Center for Synthetic Fiber, 48 Jinyi Road, Jinshan District, Shanghai 200540, PR China Received 29 May 2002; accepted 15 October 2002
Abstract Polyethylene (PE)/clay composites have been prepared by melt intercalation technique direct from Na+ montmorillonite (MMT) while using hexadecyl trimethyl ammonium bromide (C16) as reactive compatibilizer. Their degree of dispersion and intercalation spacings, as determined by X-ray diffraction (XRD), were typical of either a microcomposite or an intercalated nanocomposite, depending on the content of C16 in composites. Combustion experiments showed that the microcomposite burns in the same way as pure PE, whereas the heat release rate (HRR) is reduced by 32% when nanocomposite is with low silicate loading (5 wt.%) and C16 loading (4 wt.%). D 2002 Elsevier Science B.V. All rights reserved. Keywords: Nanocomposites; Polymers; X-ray technique
1. Introduction Recently, much attention has been paid to polymer nanocomposites, especially polymer-layered silicate nanocomposites, which represent a rational alternative to conventional filled polymers. Nanocomposite technology has been described as the next great frontier of material science. Because by employing minimal addition levels ( < 10 wt.%), nanoclays enhance
* Corresponding author. Tel./fax: +86-551-3601664. E-mail address: [email protected] (Y. Hu).
mechanical, thermal, dimensional and barrier performance properties significantly [1]. In order to facilitate silicate layers interaction with a polymer, the clay is modified with an alkylammonium salt (surfactant molecule) by a cation exchange reaction, because the alkylammonium make the hydrophilic clay surface organophilic. Although the modified clay is miscible for polar polymers such as nylon [2], epoxy resin [3], polystyrene (PS) [4], etc., the organically modified clay does not disperse well in the nonpolar polymer such as polypropylene (PP) and polyethylene (PE) since such nonpolar polymers are still too hydrophobic [5]. Thus, it is important to
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(02)01354-X
2676
S. Wang et al. / Materials Letters 57 (2003) 2675–2678
synthesis nonpolar polymer/clay nanocomposites. Initial attempts to create the nonpolar polypropylene/ clay nanocomposites by simple melt-mixing were based on the introduction of a modified oligomer to mediate the polarity between the clay surface and polymer [6]. However, these techniques all used modified clay. Recently, Alexandre et al. [7] report a novel technique on the preparation of EVA/clay nanocomposites starting directly from natural (Na+-base) clay while using an ammonium salt bearing long alkyl chains as polymer/clay reactive compatibilizer. This paper aims at synthesis of PE/clay nanocomposites starting directly from natural (Na+-base) clay and study on the fire properties of PE/clay nanocomposites and mechanism of this new method.
2. Experiments 2.1. Materials High-density polyethylene (HDPE, PE-LA-50D12) has been used as a model in this study. Purified sodium montmorillonite (MMT, China), with cation exchange capacity (CEC) of 96 meq/100 g and an ˚ , has been used as the interlayer spacing d001 = 15 A nanofiller precursor. Hexadecyl trimethyl ammonium bromide (C16) and tetrabutyl ammonium bromide (C4) have been chosen as clay/matrix reactive compatibilizer. 2.2. The preparation of HDPE/clay composites HDPE was melt-mixed with MMT and different amount of compatibilizer (C16 or C4) at 165 jC using a twin-roll mill (XK-160, made in Jiangshu, China) for 10 min to yield composite. 2.3. Evaluation of dispersibility of the clay in polymer matrix Morphology studies have been carried out on all composites. X-ray diffraction (XRD) experiments were performed at room temperature by a Japan Rigaku D/max-rA X-ray diffractiometer (30 kV, 10 ˚ ) irradiation at the rate of mA) with Cu (k = 1.54178 A 2j/min in the range of 1.5 –10j.
2.4. Cone calorimetry Cone calorimeter experiments were performed at an incident heat of 50 kW/m2. Peak heat release rate (HRR), mass loss rate (MLR), specific extinction area (SEA), ignition time (tign) and specific heat of combustion data are reproducible to within 10% when measured at 50 kW/m2. The cone data reported here are the average of three replicated experiments.
3. Results and discussions For the sake of comparison, different content of reactive compatibilizers (C16 or C4) have been used to prepare HDPE/clay composites. Compounds 1(a), 1(b), 1(c), 1(d) and 1(e), filled with the same amount of inorganic (5 wt.% MMT) and different amount of compatibilizer (C16 or C4, Table 1), were prepared. From Fig. 1 analysis, it shows that natural sodium montmorillonite (MMT) is immiscible with HDPE ˚ ), while adding only 1 wt.% (Fig. 1a, d001 = 15.1 A C16, we get a notable main diffraction peak at 38.7– ˚ (Fig. 1b). When the C16 is 2 wt.%, the dif43.5 A fraction peak of MMT disappeared and we get a sharp ˚ with orders second and main diffraction peak at 40.6 A third diffraction peaks (Fig. 1c). However, when the content of C16 increases to 5 wt.%, the d001 does not increase but decreases slightly (Fig. 1d). This indicates that the intercalation capability has a certain limit and the result is similar to the report of Wang et al. [8]. The carbonic-chain length of surfactant also affects the morphology compound. When C4 is used as compatibilizer of clay/HDPE, the d001 of compound ˚ , far less that of compound 1(c). This 1(e) is 16.9 A indicates that certain carbonic-chain length is needed to expand the layer of clay.
Table 1 Summary of HDPE/clay composites Compound Polymer (wt.%)
Clay Compatibilizer Main diffraction ˚) (wt.%) (wt.%) peak (d001) (A
1(a) 1(b) 1(c) 1(d) 1(e)
MT MT MT MT MT
HDPE HDPE HDPE HDPE HDPE
(95) (94) (93) (90) (93)
(5) (5) (5) (5) (5)
C16 (1) C16 (2) C16 (5) C4 (2)
15.1 38.7 – 43.5 40.6 38.8 16.9
S. Wang et al. / Materials Letters 57 (2003) 2675–2678
Fig. 1. X-ray diffraction patterns of compounds 1(a) (HDPE + 5 wt.% MMT), 1(b) (HDPE + 5 wt.% MMT + 1 wt.% C16), 1(c) (HDPE + 5 wt.% MMT + 2 wt.% C16), 1(d) (HDPE + 5 wt.% MMT + 5 wt.% C16) and 1(e) (HDPE + 5 wt.% MMT + 2 wt.% C4).
3.1. Flammability properties Characterization of the flammability properties of a variety of polymer/clay nanocomposites, under firelike conditions, using the cone calorimeter, has revealed improved flammability properties for many types of polymer/clay nanocomposites [9,10]. The
Fig. 3. Scheme of polymer/clay compound.
Fig. 2. Heat release rate (HRR) data for pure HDPE, HDPE + 5 wt.% MMT and HDPE + 5 wt.% MMT + 4 wt.% C16.
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cone calorimeter is one of the most effective benchscale methods for studying the flammability properties of materials. Heat release rate in particular peak HRR has been found to be the most important parameter to evaluate fire safety [9]. The heat release rate (HRR) plots for pure HDPE, HDPE/5 wt.% MMT microcomposite and HDPE/5 wt.% MMT/4 wt.% C16 (HDPE/MMT/C16) nanocomposite at 50 kW/m2 are shown in Fig. 2. The peak HRR of HDPE/5 wt.% MMT (immiscible) is very similar to that of pure HDPE. Its peak HRR is lower because it contains 95 wt.% HDPE. However, the peak HRR of HDPE/MMT/C16 nanocomposite decreases 32% compared to that of pure HDPE. Moreover, it should be noted that the time to ignite (tign, 50 s) HDPE/MMT/C16 nanocomposite is relatively short in comparison with pure HDPE (tign, 75 s). The initial HRR for nanocomposite (Fig. 2) is higher for the first minute following ignition (50 –125 s from the beginning of the experiment), probably because of thermal decomposition of reactive compatibilizer (C16) resulting in the formation of volatile combustibles, as shown in Ref. [12]. 3.2. Mechanism of intercalation Ishida et al. [13] have reported the use of epoxy as swelling agent of polymer/organophilic clay nanocomposites. In this paper, C16 serves to modify, as well as to swell, the clay layers. We deduce the mechanism: at the process of melting, where, at first, the surfactant diffuses to the round of clay layers, then, through static adsorption where the surfactant intercalates into the silicate layers and expands them; and at this process, where the C16 can act as a carrier to transport the chains of matrix into the interlayer of clay simultaneously. The scheme of this kind of compounds is shown in Fig. 3 [11].
4. Conclusion This study has demonstrated the ability to prepare nonpolar HDPE-base nanocomposites by melt
intercalation starting from Na+ montmorillonite by adding cation surfactant as reactive compatibilizer. Combustion experiments showed that the heat release rate (HRR) is reduced by 32% when nanocomposite with low silicate loading (5 wt.%) and C16 loading (4 wt.%). The supposed mechanism of this technique has been studied.
Acknowledgements The work was financially supported by the Natural Science Foundation of China (No. 50003008), the China NKBRSF project (No. 2001CB409600), Anhui ‘‘the tenth five years’’ tackle key problems project, open project of the Structure Research Laboratory of the University of Science and Technology of China and Hefei Unit Center of Analysis and Test of CAS.
References [1] M. Alexandre, P. Dubois, Mater. Sci. Eng. 28 (2000) 1. [2] J.W. Cho, D.R. Paul, Polymer 42 (2001) 1083. [3] X. Kornmann, H. Lindberg, L.A. Berglund, Polymer 42 (2001) 4493. [4] Y. Hu, L. Song, International Fire Safety Conference (Fire Retardant Chemicals Association), March 11 – 14, 2001. [5] E. Manias, A. Touny, L. Wu, K. Strawhecker, B. Lu, T.C. Chung, Chem. Mater. 13 (10) (2001) 3516. [6] M. Kato, A. Usuki, A. Okada, J. Appl. Polym. Sci. 66 (1997) 1781. [7] M. Alexandre, G. Beyer, C. Henrist, et al., Chem. Mater. 13 (11) (2001) 3830. [8] K.H. Wang, M.H. Choi, C.M. Koo, Y.S. Choi, I.J. Chung, Polymer 42 (2001) 9819. [9] J.W. Gilman, Appl. Clay Sci. 15 (1999) 31. [10] M. Zanetti, T. Kashiwagi, L. Falqui, G. Camino, Chem. Mater. 14 (2002) 881. [11] M. Ogawa, Langmuir 13 (1997) 1853. [12] W. Xie, Z. Gao, K. Liu, et al., Thermal characterization of organically modified montmorillonite, Thermochim. Acta 367 – 368 (2001) 339. [13] H. Ishida, S. Campbell, J. Blackwell, Chem. Mater. 12 (2000) 1260.