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montmorillonite nanocomposites
Polymer Degradation and Stability 78 (2002) 555–559 www.elsevier.com/locate/polydegstab
Preparation and flammability of ethylene-vinyl acetate copolymer/montmorillonite nanocomposites Yong Tanga, Yuan Hua,*, ShaoFeng Wanga,b, Zhou Guia, Zuyou Chenb, WeiCheng Fana 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 Received 7 June 2002; received in revised form 9 July 2002; accepted 23 July 2002
Abstract Ethylene-vinyl acetate copolymer (EVA)/montmorillonite (MMT) nanocomposites have been prepared using direct melt intercalation by blending EVA and pristine MMT with two different particle sizes: MMTa (average size 38 mm) and MMTb (average size 48 mm). Their structures and flammability properties were characterized by X-ray diffraction (XRD), high resolution electron microscopy (HREM) and Cone Calorimetry. XRD and HREM show that an intercalated structure is formed. The heat release rate (HRR) of the nanocomposite is 40% lower than that of pure EVA and 34% lower than that of the microcomposite. The microcomposite behaves very similarly to the pure EVA. The HRR of nanocomposite loaded with 5% MMTa is lower than that of the nanocomposite loaded with 5% MMTb # 2002 Elsevier Science Ltd. All rights reserved. Keywords: EVA; MMT; Nanocomposites; Intercalated
1. Introduction Polymer/clay nanocomposites have aroused people’s interest since the Toyota group developed PA6/clay nanocomposites with excellent mechanical properties [1]. Nanocomposites are particle-filled polymers for which at least one dimension of the dispersed particles is in the nanometer range. Polymer layered-silicate nanocomposites (PLSNs) are hybrids composed of layered silicates dispersed in a polymer matrix in the form of reticular layers of crystals about 1 nm thick and with a lamellar aspect ratio of between 100 and 1000. The molecular level interaction created in the PLSN is likely to affect not only physical properties, but also its chemical behaviour. PLSNs have demonstrated flameretardant properties namely diminution of the heat release rate (HRR) peak, formation of protective char, good gas barrier properties and decrease in the mass loss rate during combustion [2–5]. * Corresponding author. Tel.: +86-551-360-1664; fax: +86-551360-1664. E-mail address: [email protected] (Y. Hu).
EVA is among the major thermoplastic polymers used in the electrical cable sheathing industry. In this work we have synthesized EVA/clay nanocomposite by a novel approach, which is an original ‘‘one pot’’ reactive process, starting directly from pristine montmorillonite (MMT) and reactive compatibilizer hexadecyl trimethyl ammonium bromide (C16) [6]. We have studied the flammability and thermal stabilities of the nanocomposites compared to those of pure EVA and EVA-microcomposites. Two kinds of pristine MMT particle sizes (MMTa, MMTb) were used and show different behaviour in combustion.
2. Experiments 2.1. Materials Ethylene-vinyl acetate copolymer (EVA) was supplied as pellets by Beijing Petrochemical, China. The pristine montmorillonite (MMT, with a cation exchange capacity of 97 meq/100 g) was kindly provided by Ke Yan Company: MMTa is a fine powder (average size 38 mm);
0141-3910/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(02)00231-8
556
Y. Tang et al. / Polymer Degradation and Stability 78 (2002) 555–559
MMTb is fine powder (average size 48 mm). The compatibilizer is hexadecyl trimethyl ammonium bromide (C16). 2.2. The Preparation of EVA–clay hybrid A twin-roll mill (XK-160, JiangShu, China) was used for the preparation of EVA/clay hybrids. The temperature of the mill was maintained at 140 C and the roll speed was maintained at 50 rpm. For EVA/clay nanocomposites, EVA was added to the mill at the beginning of the blending procedure. After it was molten, MMT and C16 were added to the molten EVA at the same time and melt-mixed for about 10 min to give EVA/clay nanocomposites. The same procedure was used to prepare EVA/clay microcomposite without adding C16. Their different contents are listed in Table 1. 2.3. Evaluation of dispersibility of the clay in EVA matrix The dispersibilities of the silicate layers in the EVA were evaluated using X-ray diffraction (XRD) and high resolution electron microscopy (HREM). Thin films (1 mm) of the nanocomposites were pressed at 150 C for the XRD measurements. X-ray diffraction experiments were performed at room temperature on a Japan Rigaku D/max-rA X-ray diffractometer (30 kV, 10 mA) with Cu (l=1.54178 A˚) irradiation at a scan rate of 2 / min in the range of 1.5–10 . The specimen of EVA (2) for HREM was cut at low temperature using an ultramicrotome (Ultracut-1, UK) with a diamond knife from the EVA (2) film embedded an epoxy block. Thin specimens, 50–80 nm, were collected in a trough filled with a solution of dimethylsulfoxide and glycerin and then placed on 200 mesh copper grids. HREM images were obtained with a JEOL2010 microscope at an acceleration voltage of 200 kV.
exposed in a Stanton Redcroft cone calorimeter according to ASTM 1356–90 under a heat flux of 50 kw/ m2. The experiments were repeated three times. When measured at 50 kw/m2 flux, the results from the Cone Calorimeter are considered reproducible to within 10% .The cone data reported in this paper are the average of three replicated experiments.
3. Results and Discussion 3.1. Dispersibility of EVA-clay hybrids Fig. 1 shows the XRD patterns of the mixtures of EVA and pristine clay. The d001 peak of pristine clay at 2=5.8 corresponds to 1.4 nm (Fig. 1a) interlayer spacing. The d001 peak of EVA-pristine clay composite without C16 (Fig. 1b) is almost same as that of pristine clay (Fig. 1a). This indicates that EVA could not intercalate into the silicate layers. However, there are different results when reactive compatibilizer C16 is used in hybrids, the d001 peaks of the mixture (Fig. 1c, d) are observed at lower angle (2=2.3 ) than that of pristine clay, these indicate an average basal spacing increase from 1.4 to 3.78 nm. These results indicate the EVA with C16 could intercalate into the silicate layers and expand the basal spacing. The dispersibility of the silicate layers in the EVA is also confirmed by HREM shown in Fig. 2. The image confirmed that EVA-layered silicate nanocomposites are formed, as we can see the intercalated structure in Fig. 2. 3.2. Flammability properties Characterization of the flammability properties of a variety of polymer/clay nanocomposites, under fire-like conditions, using the Cone Calorimeter has revealed improved flammability properties for many types of
2.4. Combustion and cone calorimeter studies The combustion properties of EVA–clay hybrids were evaluated using a Cone Calorimeter. The signals from the Cone Calorimeter were recorded and analyzed by a computer system. All the samples (10103cm3) were Table 1 Different contents of EVA and EVA/MMT hybrids Samples
EVA (wt.%)
Silicate (wt.%)
EVA0 EVA1 EVA2 EVA3 EVA4
100 95 92.5 89.5 92.5
MMTa MMTa MMTa MMTb
Compatibilizer (wt.%) (5) (5) (7) (5)
C16 (2.5) C16 (3.5) C16 (2.5)
Fig. 1. XRD patterns of MMT and EVA–MMT hybrids (a) MMT; (b) EVA+MMTa; (c) EVA+MMTa (5%)+C16; (d) EVA+MMTb (5%)+C16.
Y. Tang et al. / Polymer Degradation and Stability 78 (2002) 555–559
Fig. 2. The HREM image of EVA (2).
polymer/clay nanocomposites [5,7–10]. The Cone Calorimeter is one of the most effective bench-scale 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 [5]. The HRR for nanocomposites, the microcomposite and pure EVA at 50 kw/m2 heat flux are shown in Fig. 3. The peak HRR of the nanocomposite is 40% lower than that of pure EVA and 34% lower than that of the microcomposite. The reduction in the peak HRR of the nanocomposite is typical of all the nanocomposites
557
reported in the literature [11,16]. From Fig. 3, we can see that the microcomposite behaves very similarly to the pure EVA, but nanocomposites display remarkably different combustion behaviour. At the end of combustion pure EVA leaves no residue, and the microcomposite leaves only a little powder. The nanocomposites leave a solid, consistent char-like residue. Fig. 4 shows the residue weight increasing in the order of nanocomposites > microcomposite > pure EVA. At the same time, the effect of MMTa+C16 loading on the HRR is also shown in Fig. 3. The peak HRR reduces as the mass fraction of MMTa+C16 increases. It is reasonable that as the fraction of clay increases, the amount of char that can be formed increases and the rate of heat release decreases. Fig. 6 shows that the peak HRR of EVA-nanocomposite with MMTa is lower than that of EVA-nanocomposite with MMTb, this may be because the two different particle size layered silicates disperse differently in the EVA matrix. The particle size of MMTa is smaller than that of MMTb, which shows that the particle sizes have influence on the peak HRR of the nanocomposites. The primary parameter responsible for the lower HRR of the nanocomposite is the mass loss rate (MLR) during combustion, which is significantly reduced from those values observed for the pure EVA and microcomposite (Fig. 5). Fig. 5 shows that the MLR decreased in the order of EVA3 > EVA2> EVA1 > EVA0, this trend is the same as those of the HRR in the Cone Calorimeter (Fig. 3). These similarities indicate that the mechanism of the observed reduction in HRR and also in MLR by addition of the layered silicate depends mainly on the condensed phase process instead of the gas phase process [12,14]. From Figs. 3 and 5, it is interesting to note that the HRR and the MLR of EVA-layered silicate nanocomposite are higher than those of pure EVA and
Fig. 3. Heat release rate (HRR) for pure EVA and EVA/MMT hybrids.
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Y. Tang et al. / Polymer Degradation and Stability 78 (2002) 555–559
Fig. 4. Mass loss for pure EVA and EVA/MMT hybrids.
Fig. 5. Mass loss rate (MLR) for pure EVA and EVA/MMT hybrids.
Fig. 6. Heat release rate (HRR) for pure EVA and different size MMT nanocomposites.
Y. Tang et al. / Polymer Degradation and Stability 78 (2002) 555–559
EVA-clay microcomposite at the beginning of combustion. This may be explained by two reasons [13,14]. On the one hand, in the early stages of nanocomposite combustion the decomposition of organic alkylammonium cation (C16), which resulted in the formation of volatile combustibles, contributes to the combustion. On the other hand, the thermodegradation of EVA takes place in two stages, the first step involves the deacylation with the elimination of acetic acid. However in EVA nanocomposite, the elimination of the deacylation reaction is accelerated, which is probably due to a catalytic effect of acidic sites of the layered silicates deriving from Hoffman elimination reaction of the organic alkylammonium cation (C16). In nanocomposites those acidic sites are active owing to intimate contact between the polymer and the silicate. Accelerated evolution of acetic acid might contribute to greater heat release in the early stages of nanocomposite combustion compared to pure EVA and EVA microcomposite. So in Figs. 3 and 5, we see similar phenomena. Thermal volatilization of the deacetylated polymer is slowed by the ‘‘labyrinth’’ effect of the silicate layers in the polymer matrix, which lowers the rate of diffusion of the degradation products into the gas phase [13,15]. In air, nanocomposites protect and stabilize against thermooxidation. This property might derive from the barrier produced by the diffusion of both the volatile thermo-oxidation products to the gas phase and oxygen from the gas to polymer. What is more, this barrier effect increases because of ablative reassembly of the reticular layers of the silicate on the surface of the polymer in the process of volatilization, a sort of ceramic char-layered silicate nanocomposite can be made [14,16], so at last the nanocomposite shows a significant delay of weight loss and HRR.
4. Conclusions In this work, EVA/clay nanocomposites have been synthesized by melt intercalation from pristine MMT by adding compatibilizer C16 in a ‘‘one-pot’’ reactive process. The combustion in the Cone Colorimeter indicates that combustion behaviours of the nanocomposites and the microcomposite are remarkably different. The HRR peaks of nanocomposites are much lower than those of pure EVA and microcomposite. The behaviours of microcomposite are very similar to the pure polymer. Meanwhile the dispersers of clay have influence on the HRR. The initial HRR for the nanocomposite is higher at
559
the beginning of combustion probably because of decomposition of the organic alkylammonium cation (C16) and accelerated evolution of acetic acid owing to catalytic effect of acidic sites of the layered silicates deriving from Hoffman elimination reaction of C16. However, the peak HRR is reduced and the weight loss is delayed owing to good gas barrier properties and ablative reassembly of the reticular layers of the silicate in nanocomposites.
Acknowledgements The work was financially supported by the National Natural Science Foundation of China (No. 50003008), the China NKBRSF project (No. 2001CB409600), 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] Fukushima Y, Okada A, Kawasumi M, Kurauchi T, Kamigaito O. Clay Miner 1988;23:27. [2] Alexander M, Dubois P. Mater Sci Eng 2000;28:1–63. [3] Hu Y, Song L, JXu L, Chen Zuyao, Fan Weichemg. Colloid Polym Sci 2001;279:819–22. [4] Wang S, Hu Y, Song L, Wang, Z, Chen Z, Fan, W. Polym Degrad Stab 2002;77:423–60. [5] Gilman JW. Appl Clay Sci 1999;15:31–49. [6] Alexandre M, Beyer G, Henrist C, Cloots R, Rulmont A, Jerome R, Dubois P. Chem Mater 2001;13:3830. [7] Bourbigot S, Le Bras M, Dabrowski F, Gilman JW, Kashiwagi T. Fire Mater 2000;24:201. [8] Zanetti M, Kashiwagi T, Falqui L, Camino G. Chem Mater 2002:881–7. [9] Zhu J, Morgan AB, Lamelas FJ, Wilkie CA. Chem Mater 2001; 13:3174–80. [10] Gilman JW, Jackson CL, Morgan AB, Harris Jr R. Chem Mater 2000;12:1866–73. [11] Zhu J, Morga AB, Lamelas FJ, Wilkie CA. Chem Mater 2000; 13:3774. [12] Kashiwang T, Gillma JW, Butler KM, Harris RH. Fire Mater 2000;24:277–89. [13] Zanetti M, Camino G, Thomann R, Mu¨lhaupt R. Polymer 2001; 42:4501–7. [14] Zanteei M, Kashiwagi T, Falqui L, Camino G. Chem Mater 2002;14:881–7. [15] Gilman JW, Kashivagi T, Lomakin S, Giannelis E, Manias E, Lichtenahan J, Jones P. Fire retardancy of polymer: the use of intumescence. Cambridge: The Royal Chemical Society Pub; 1998. [16] Gilman JW, Jackson CL, Morgan AB, Harris R, Manias E, Giannelis EP, Wuthemow M, Hilton D, Phillips SH. Chem Mater 2000;12:1866.
Preparation and flammability of ethylene-vinyl acetate copolymer/montmorillonite nanocomposites Yong Tanga, Yuan Hua,*, ShaoFeng Wanga,b, Zhou Guia, Zuyou Chenb, WeiCheng Fana 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 Received 7 June 2002; received in revised form 9 July 2002; accepted 23 July 2002
Abstract Ethylene-vinyl acetate copolymer (EVA)/montmorillonite (MMT) nanocomposites have been prepared using direct melt intercalation by blending EVA and pristine MMT with two different particle sizes: MMTa (average size 38 mm) and MMTb (average size 48 mm). Their structures and flammability properties were characterized by X-ray diffraction (XRD), high resolution electron microscopy (HREM) and Cone Calorimetry. XRD and HREM show that an intercalated structure is formed. The heat release rate (HRR) of the nanocomposite is 40% lower than that of pure EVA and 34% lower than that of the microcomposite. The microcomposite behaves very similarly to the pure EVA. The HRR of nanocomposite loaded with 5% MMTa is lower than that of the nanocomposite loaded with 5% MMTb # 2002 Elsevier Science Ltd. All rights reserved. Keywords: EVA; MMT; Nanocomposites; Intercalated
1. Introduction Polymer/clay nanocomposites have aroused people’s interest since the Toyota group developed PA6/clay nanocomposites with excellent mechanical properties [1]. Nanocomposites are particle-filled polymers for which at least one dimension of the dispersed particles is in the nanometer range. Polymer layered-silicate nanocomposites (PLSNs) are hybrids composed of layered silicates dispersed in a polymer matrix in the form of reticular layers of crystals about 1 nm thick and with a lamellar aspect ratio of between 100 and 1000. The molecular level interaction created in the PLSN is likely to affect not only physical properties, but also its chemical behaviour. PLSNs have demonstrated flameretardant properties namely diminution of the heat release rate (HRR) peak, formation of protective char, good gas barrier properties and decrease in the mass loss rate during combustion [2–5]. * Corresponding author. Tel.: +86-551-360-1664; fax: +86-551360-1664. E-mail address: [email protected] (Y. Hu).
EVA is among the major thermoplastic polymers used in the electrical cable sheathing industry. In this work we have synthesized EVA/clay nanocomposite by a novel approach, which is an original ‘‘one pot’’ reactive process, starting directly from pristine montmorillonite (MMT) and reactive compatibilizer hexadecyl trimethyl ammonium bromide (C16) [6]. We have studied the flammability and thermal stabilities of the nanocomposites compared to those of pure EVA and EVA-microcomposites. Two kinds of pristine MMT particle sizes (MMTa, MMTb) were used and show different behaviour in combustion.
2. Experiments 2.1. Materials Ethylene-vinyl acetate copolymer (EVA) was supplied as pellets by Beijing Petrochemical, China. The pristine montmorillonite (MMT, with a cation exchange capacity of 97 meq/100 g) was kindly provided by Ke Yan Company: MMTa is a fine powder (average size 38 mm);
0141-3910/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(02)00231-8
556
Y. Tang et al. / Polymer Degradation and Stability 78 (2002) 555–559
MMTb is fine powder (average size 48 mm). The compatibilizer is hexadecyl trimethyl ammonium bromide (C16). 2.2. The Preparation of EVA–clay hybrid A twin-roll mill (XK-160, JiangShu, China) was used for the preparation of EVA/clay hybrids. The temperature of the mill was maintained at 140 C and the roll speed was maintained at 50 rpm. For EVA/clay nanocomposites, EVA was added to the mill at the beginning of the blending procedure. After it was molten, MMT and C16 were added to the molten EVA at the same time and melt-mixed for about 10 min to give EVA/clay nanocomposites. The same procedure was used to prepare EVA/clay microcomposite without adding C16. Their different contents are listed in Table 1. 2.3. Evaluation of dispersibility of the clay in EVA matrix The dispersibilities of the silicate layers in the EVA were evaluated using X-ray diffraction (XRD) and high resolution electron microscopy (HREM). Thin films (1 mm) of the nanocomposites were pressed at 150 C for the XRD measurements. X-ray diffraction experiments were performed at room temperature on a Japan Rigaku D/max-rA X-ray diffractometer (30 kV, 10 mA) with Cu (l=1.54178 A˚) irradiation at a scan rate of 2 / min in the range of 1.5–10 . The specimen of EVA (2) for HREM was cut at low temperature using an ultramicrotome (Ultracut-1, UK) with a diamond knife from the EVA (2) film embedded an epoxy block. Thin specimens, 50–80 nm, were collected in a trough filled with a solution of dimethylsulfoxide and glycerin and then placed on 200 mesh copper grids. HREM images were obtained with a JEOL2010 microscope at an acceleration voltage of 200 kV.
exposed in a Stanton Redcroft cone calorimeter according to ASTM 1356–90 under a heat flux of 50 kw/ m2. The experiments were repeated three times. When measured at 50 kw/m2 flux, the results from the Cone Calorimeter are considered reproducible to within 10% .The cone data reported in this paper are the average of three replicated experiments.
3. Results and Discussion 3.1. Dispersibility of EVA-clay hybrids Fig. 1 shows the XRD patterns of the mixtures of EVA and pristine clay. The d001 peak of pristine clay at 2=5.8 corresponds to 1.4 nm (Fig. 1a) interlayer spacing. The d001 peak of EVA-pristine clay composite without C16 (Fig. 1b) is almost same as that of pristine clay (Fig. 1a). This indicates that EVA could not intercalate into the silicate layers. However, there are different results when reactive compatibilizer C16 is used in hybrids, the d001 peaks of the mixture (Fig. 1c, d) are observed at lower angle (2=2.3 ) than that of pristine clay, these indicate an average basal spacing increase from 1.4 to 3.78 nm. These results indicate the EVA with C16 could intercalate into the silicate layers and expand the basal spacing. The dispersibility of the silicate layers in the EVA is also confirmed by HREM shown in Fig. 2. The image confirmed that EVA-layered silicate nanocomposites are formed, as we can see the intercalated structure in Fig. 2. 3.2. Flammability properties Characterization of the flammability properties of a variety of polymer/clay nanocomposites, under fire-like conditions, using the Cone Calorimeter has revealed improved flammability properties for many types of
2.4. Combustion and cone calorimeter studies The combustion properties of EVA–clay hybrids were evaluated using a Cone Calorimeter. The signals from the Cone Calorimeter were recorded and analyzed by a computer system. All the samples (10103cm3) were Table 1 Different contents of EVA and EVA/MMT hybrids Samples
EVA (wt.%)
Silicate (wt.%)
EVA0 EVA1 EVA2 EVA3 EVA4
100 95 92.5 89.5 92.5
MMTa MMTa MMTa MMTb
Compatibilizer (wt.%) (5) (5) (7) (5)
C16 (2.5) C16 (3.5) C16 (2.5)
Fig. 1. XRD patterns of MMT and EVA–MMT hybrids (a) MMT; (b) EVA+MMTa; (c) EVA+MMTa (5%)+C16; (d) EVA+MMTb (5%)+C16.
Y. Tang et al. / Polymer Degradation and Stability 78 (2002) 555–559
Fig. 2. The HREM image of EVA (2).
polymer/clay nanocomposites [5,7–10]. The Cone Calorimeter is one of the most effective bench-scale 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 [5]. The HRR for nanocomposites, the microcomposite and pure EVA at 50 kw/m2 heat flux are shown in Fig. 3. The peak HRR of the nanocomposite is 40% lower than that of pure EVA and 34% lower than that of the microcomposite. The reduction in the peak HRR of the nanocomposite is typical of all the nanocomposites
557
reported in the literature [11,16]. From Fig. 3, we can see that the microcomposite behaves very similarly to the pure EVA, but nanocomposites display remarkably different combustion behaviour. At the end of combustion pure EVA leaves no residue, and the microcomposite leaves only a little powder. The nanocomposites leave a solid, consistent char-like residue. Fig. 4 shows the residue weight increasing in the order of nanocomposites > microcomposite > pure EVA. At the same time, the effect of MMTa+C16 loading on the HRR is also shown in Fig. 3. The peak HRR reduces as the mass fraction of MMTa+C16 increases. It is reasonable that as the fraction of clay increases, the amount of char that can be formed increases and the rate of heat release decreases. Fig. 6 shows that the peak HRR of EVA-nanocomposite with MMTa is lower than that of EVA-nanocomposite with MMTb, this may be because the two different particle size layered silicates disperse differently in the EVA matrix. The particle size of MMTa is smaller than that of MMTb, which shows that the particle sizes have influence on the peak HRR of the nanocomposites. The primary parameter responsible for the lower HRR of the nanocomposite is the mass loss rate (MLR) during combustion, which is significantly reduced from those values observed for the pure EVA and microcomposite (Fig. 5). Fig. 5 shows that the MLR decreased in the order of EVA3 > EVA2> EVA1 > EVA0, this trend is the same as those of the HRR in the Cone Calorimeter (Fig. 3). These similarities indicate that the mechanism of the observed reduction in HRR and also in MLR by addition of the layered silicate depends mainly on the condensed phase process instead of the gas phase process [12,14]. From Figs. 3 and 5, it is interesting to note that the HRR and the MLR of EVA-layered silicate nanocomposite are higher than those of pure EVA and
Fig. 3. Heat release rate (HRR) for pure EVA and EVA/MMT hybrids.
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Y. Tang et al. / Polymer Degradation and Stability 78 (2002) 555–559
Fig. 4. Mass loss for pure EVA and EVA/MMT hybrids.
Fig. 5. Mass loss rate (MLR) for pure EVA and EVA/MMT hybrids.
Fig. 6. Heat release rate (HRR) for pure EVA and different size MMT nanocomposites.
Y. Tang et al. / Polymer Degradation and Stability 78 (2002) 555–559
EVA-clay microcomposite at the beginning of combustion. This may be explained by two reasons [13,14]. On the one hand, in the early stages of nanocomposite combustion the decomposition of organic alkylammonium cation (C16), which resulted in the formation of volatile combustibles, contributes to the combustion. On the other hand, the thermodegradation of EVA takes place in two stages, the first step involves the deacylation with the elimination of acetic acid. However in EVA nanocomposite, the elimination of the deacylation reaction is accelerated, which is probably due to a catalytic effect of acidic sites of the layered silicates deriving from Hoffman elimination reaction of the organic alkylammonium cation (C16). In nanocomposites those acidic sites are active owing to intimate contact between the polymer and the silicate. Accelerated evolution of acetic acid might contribute to greater heat release in the early stages of nanocomposite combustion compared to pure EVA and EVA microcomposite. So in Figs. 3 and 5, we see similar phenomena. Thermal volatilization of the deacetylated polymer is slowed by the ‘‘labyrinth’’ effect of the silicate layers in the polymer matrix, which lowers the rate of diffusion of the degradation products into the gas phase [13,15]. In air, nanocomposites protect and stabilize against thermooxidation. This property might derive from the barrier produced by the diffusion of both the volatile thermo-oxidation products to the gas phase and oxygen from the gas to polymer. What is more, this barrier effect increases because of ablative reassembly of the reticular layers of the silicate on the surface of the polymer in the process of volatilization, a sort of ceramic char-layered silicate nanocomposite can be made [14,16], so at last the nanocomposite shows a significant delay of weight loss and HRR.
4. Conclusions In this work, EVA/clay nanocomposites have been synthesized by melt intercalation from pristine MMT by adding compatibilizer C16 in a ‘‘one-pot’’ reactive process. The combustion in the Cone Colorimeter indicates that combustion behaviours of the nanocomposites and the microcomposite are remarkably different. The HRR peaks of nanocomposites are much lower than those of pure EVA and microcomposite. The behaviours of microcomposite are very similar to the pure polymer. Meanwhile the dispersers of clay have influence on the HRR. The initial HRR for the nanocomposite is higher at
559
the beginning of combustion probably because of decomposition of the organic alkylammonium cation (C16) and accelerated evolution of acetic acid owing to catalytic effect of acidic sites of the layered silicates deriving from Hoffman elimination reaction of C16. However, the peak HRR is reduced and the weight loss is delayed owing to good gas barrier properties and ablative reassembly of the reticular layers of the silicate in nanocomposites.
Acknowledgements The work was financially supported by the National Natural Science Foundation of China (No. 50003008), the China NKBRSF project (No. 2001CB409600), 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] Fukushima Y, Okada A, Kawasumi M, Kurauchi T, Kamigaito O. Clay Miner 1988;23:27. [2] Alexander M, Dubois P. Mater Sci Eng 2000;28:1–63. [3] Hu Y, Song L, JXu L, Chen Zuyao, Fan Weichemg. Colloid Polym Sci 2001;279:819–22. [4] Wang S, Hu Y, Song L, Wang, Z, Chen Z, Fan, W. Polym Degrad Stab 2002;77:423–60. [5] Gilman JW. Appl Clay Sci 1999;15:31–49. [6] Alexandre M, Beyer G, Henrist C, Cloots R, Rulmont A, Jerome R, Dubois P. Chem Mater 2001;13:3830. [7] Bourbigot S, Le Bras M, Dabrowski F, Gilman JW, Kashiwagi T. Fire Mater 2000;24:201. [8] Zanetti M, Kashiwagi T, Falqui L, Camino G. Chem Mater 2002:881–7. [9] Zhu J, Morgan AB, Lamelas FJ, Wilkie CA. Chem Mater 2001; 13:3174–80. [10] Gilman JW, Jackson CL, Morgan AB, Harris Jr R. Chem Mater 2000;12:1866–73. [11] Zhu J, Morga AB, Lamelas FJ, Wilkie CA. Chem Mater 2000; 13:3774. [12] Kashiwang T, Gillma JW, Butler KM, Harris RH. Fire Mater 2000;24:277–89. [13] Zanetti M, Camino G, Thomann R, Mu¨lhaupt R. Polymer 2001; 42:4501–7. [14] Zanteei M, Kashiwagi T, Falqui L, Camino G. Chem Mater 2002;14:881–7. [15] Gilman JW, Kashivagi T, Lomakin S, Giannelis E, Manias E, Lichtenahan J, Jones P. Fire retardancy of polymer: the use of intumescence. Cambridge: The Royal Chemical Society Pub; 1998. [16] Gilman JW, Jackson CL, Morgan AB, Harris R, Manias E, Giannelis EP, Wuthemow M, Hilton D, Phillips SH. Chem Mater 2000;12:1866.