- Email: [email protected]
LDH nanocomposites
Polymer Degradation and Stability 92 (2007) 1715e1720 www.elsevier.com/locate/polydegstab
Synergistic effects of layered double hydroxide with hyperfine magnesium hydroxide in halogen-free flame retardant EVA/HFMH/LDH nanocomposites Guobing Zhang, Peng Ding, Ming Zhang, Baojun Qu* State Key Laboratory of Fire Science and Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China Received 19 April 2007; received in revised form 24 May 2007; accepted 5 June 2007 Available online 27 June 2007
Abstract The synergistic effects of layered double hydroxide (LDH) with hyperfine magnesium hydroxide (HFMH) in halogen-free flame retardant ethylene-vinyl acetate (EVA)/HFMH/LDH nanocomposites have been studied by X-ray diffraction (XRD), transmission electron spectroscopy (TEM), thermogravimetric analysis (TGA), limiting oxygen index (LOI), mechanical properties’ tests, and dynamic mechanical thermal analysis (DMTA). The XRD results show that the exfoliated EVA/HFMH/LDH can be obtained by controlling the LDH loading. The TEM images give the evidence that the organic-modified LDH (OM-LDH) can act as a disperser and help HFMH particles to disperse homogeneously in the EVA matrix. The TGA data demonstrate that the addition of LDH can raise 5e18 C thermal degradation temperatures of EVA/HFMH/LDH nanocomposite samples with 5e15 phr OM-LDH compared with that of the control EVA/HFMH sample when 50% weight loss is selected as a point of comparison. The LOI and mechanical tests show that the LDH can act as flame retardant synergist and compatilizer to apparently increase the LOI and elongation at break values of EVA/HFMH/LDH nanocomposites. The DMTA data verify that the Tg value (10 C) of the EVA/HFMH/ LDH nanocomposite sample with 15 phr LDH is much lower than that (Tg ¼ 2 C) of the control EVA/HFMH sample without LDH and approximates to the Tg value (12 C) of pure EVA, which indicates that the nanocomposites with LDH have more flexibility than that of the EVA/ HFMH composites. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Layered double hydroxide; Hyperfine magnesium hydroxide; Ethylene-vinyl acetate; Synergistic effect; Nanocomposites
1. Introduction Ethylene-vinyl acetate copolymers (EVA) with different vinyl acetate (VA) contents are extensively used in many fields, especially in the cable industry as excellent insulating materials with good physical and chemical properties [1]. However, EVA resins are particularly flammable and emit a large amount of smoke while burning. Magnesium hydroxide (MH) is one kind of toxic-free and smoke-suppressing halogen-free flame retardant additive with high decomposition
* Corresponding author. Fax: þ86 551 3607245. E-mail address: [email protected] (B. Qu). 0141-3910/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2007.06.004
temperature in flame retardant polymeric materials. But its fatal disadvantages are low flame retardant efficiency and thus very large usage amount, which lead the mechanical properties of flame retardant materials to drop down sharply [2]. In order to minimize this effect, the hyperfinization and surface treatment of MH are two important methods to improve its dispersion and compatibility with polymer matrix. Most commonly used surface modifiers include silane coupling agents, stearates, titanate coupling agents, elastomer, etc [3e6]. However, all these surface modifiers of MH are organic reagents or polymer, which could be detrimental to the combustion properties such as limiting oxygen index (LOI). The hyperfine magnesium hydroxide (HFMH) has been verified to be very effective to improve the flame retardant and mechanical properties of
1716
G. Zhang et al. / Polymer Degradation and Stability 92 (2007) 1715e1720
polymeric materials, but its dispersion in polymer matrix is very difficult [7,8]. In recent years, the intercalated or exfoliated polymer/LDH nanocomposites have attracted great interest in the field of material science because of their excellent thermal stability, flame retardance, and physico-chemical properties [9e16]. The organically modified LDH (OM-LDH) inorganic compounds have nanosized dispersion in the exfoliated polymer/LDH nanocomposites [11,13,17]. In our previous work, we have prepared several kinds of polymer/LDH nanocomposites by direct melt intercalation [18e20]. The melt intercalation is a very promising method for extending the practical applications of polymer/LDH nanocomposites. Recently, Ristolainen [21] and Bourbigot [22] reported that the nanosized layered silicates as cationic clay have the ability to improve the dispersion of MH or aluminum trihydroxide in the polymer matrix. However, the dispersion ability of LDH as anionic clay to inorganic additives in polymeric materials has not reported yet. In the present work, we used hyperfine MH and organicmodified LDH layers as halogen-free flame retardant system and studied their synergistic effects on morphological structure, thermal property, flame retardant property, and mechanical properties of the EVA/HFMH/LDH nanocomposites by X-ray diffraction (XRD), transmission electron spectroscopy (TEM), thermogravimetric analysis (TGA), limiting oxygen index (LOI), mechanical properties’ tests and dynamic mechanical thermal analysis (DMTA). The main purpose of this study is to develop a new type of low smoke and halogenfree flame retardant polymeric nanocomposites and to be applied for the halogen-free flame retardant EVA insulated wire and cable. 2. Experimental
The EVA/HFMH/LDH samples were prepared by melt intercalation of two-step blending. Firstly, the OM-LDH powders and the desired amount of EVA were blended for 10 min by a SXX-300 mixer with the rotor speed of 64 rpm at 130 C. Secondly, HFMH particles were dispersed into the EVA/LDH nanocomposites by blending for another 3 min. The compositions, morphological structures and some flame retardant properties of various samples are summarized in Table 1. 2.3. Measurements The XRD patterns were recorded using a Rigaku D/Max-rA rotating anode X-ray diffractometer equipped with a Cu Ka tube and Ni filter (l ¼ 0.1542 nm). The TEM images were obtained on a Hitachi H-800 transmission electron microanalyzer with an accelerating voltage of 200 kV and camera length of 0.8 m. The samples were ultramicrotomed with a diamond knife on an LKB Pyramitome to give 100-nm thick slices. And the slices were transferred from water to Cu grid. The TGA profiles were performed on a Shimadzu TGA-50H thermoanalyzer under an airflow rate of 6 105 m3/min at a scan rate of 10 C/min. The LOI values were measured using a HC-2 type instrument (made in China) on sheets 120 6.5 3 mm according to the standard oxygen index test ASTM D2863-77. The mechanical properties of tensile strength (TS) and elongation at break (EB) were measured by a universal testing machine (DCS500, Shimadzu) with the crosshead speed of 25 mm/min at temperature 25 2 C. The dumbbell-shaped specimens were prepared according to ASTM D412-87. The DMTA data of the loss tangent (tan d) and the storage modulus (E0 ) with the changes of temperature were recorded on a Diamond DMTA analyzer (Perkin Elmer) in a temperature range of 80 to 80 C at a constant frequency of 5 Hz and at a heating rate of 5 C/min.
2.1. Materials 3. Results and discussion Commercial EVA with 28 wt% VA was supplied by Sumitomo Chemical Co. Ltd. The hyperfine magnesium hydroxide (HFMH) with a particle size of 0.1e1.0 mm used in this work was supplied by Jingjiang Kexing Nanomaterials Co. Ltd., China. Mg(NO3)$6H2O and Al(NO3)$9H2O (analytical pure) were supplied by Shanghai ZhenXing Chemicals No. 1 Plant. Sodium dodecyl sulfate (SDS) and NaOH (analytical pure) were obtained from China Medicine (Group) Shanghai Chemicals Reagent Corporation. 2.2. Preparation of samples The OM-LDH was prepared by co-precipitation method described in our previous work [18]. The pH of 300 ml aqueous solution containing 0.03 mol Mg(NO3)$6H2O, 0.01 mol Al (NO3)$9H2O and 0.05 mol C12H25SO4Na was adjusted to pH w 10 with 1 mol L1 NaOH aqueous solution. The slurry was aged for 3 days at 80 C, then filtered, and washed three times with distilled water. The white powder OM-LDH was obtained by drying at 60 C in an oven.
3.1. Structural characterization Fig. 1 gives the low-angle XRD patterns of the OM-LDH, EVA/HFMH, and EVA/HFMH/LDH samples with different loadings of OM-LDH. The basal spaces of the OM-LDH sample are measured to be 2.75 nm from (001) diffraction peak at 2q ¼ w3.2 and 1.38 nm from (002) diffraction peak at Table 1 Compositions, morphological structures and LOI of various samples Sample code
EVA (phr)
HFMH (phr)
LDH (phr)
Morphological structures
LOI (%)
EM-0 EM-2 EM-5 EM-10 EM-15 Pure EVA E-10 E-15
100 100 100 100 100 100 100 100
100 98 95 90 85 0 0 0
0 2 5 10 15 0 10 15
e Exfoliated Exfoliated Intercalated Intercalated e Intercalated Intercalated
29 31 32 32 34 18 e e
G. Zhang et al. / Polymer Degradation and Stability 92 (2007) 1715e1720
1717
Fig. 1. Low-angle XRD patterns of various samples: (a) OM-LDH, (b) EM-15, (c) EM-10, (d) EM-5, (e) EM-2, (f) EM-0.
Fig. 2. Comparison of low-angle XRD patterns of four samples with or without HFMH: (a) EM-10, (b) E-10, (c) EM-15, (d) E-15.
2q ¼ w6.4 (Fig. 1a), whereas the EVA/HFMH sample has no peaks at corresponding 2q ¼ w3.2 and w6.4 (Fig. 1f). It can be seen from Fig. 1bee that the EVA/HFMH/LDH samples have the exfoliation trend with decreasing LDH loading because the XRD (001) peaks at the low-angle range disappear gradually with increasing the gallery height of the OM-LDH due to the EVA intercalation. The basal spaces of OM-LDH from samples EM-15 and EM-10 can be determined as 4.10 and 5.05 nm from the weak d001 diffraction peaks at 2q ¼ 2.0e3.0 , respectively. These data indicate that the EM-15 and EM-10 samples have partly exfoliated and partly intercalated structures. The (001) diffraction peaks cannot be observed from the samples EM-5 and EM-2 (see Fig. 1d and e), which mean that the exfoliated structures have been formed in the EVA/HFMH/LDH nanocomposites when the contents of LDH decrease to 5 or below 5 phr. The structural features of main samples are listed in Table 1. The small peaks at 7e8 in Fig. 1bef could be formed by the crystals of EVA under the participation of HFMH particles. Fig. 2 compares the low-angle XRD patterns of EM-10, EM-15 and E-10, E-15 samples with or without the HFMH additive in the range of 2q ¼ 1.5e10 . It can be seen that the EM-10 (Fig. 2a) and E-10 (Fig. 2b) samples show the similar intercalated structures, but the space of OM-LDH component in the EM-10 sample with the 90 phr HFMH increases to 5.05 nm from 3.26 nm of E-10 sample without the MH loading. This indicates that more EVA molecular chains in the EM-10 sample were intercalated into the OM-LDH layers in the presence of HFMH than those in the E-10 sample without HFMH. The EM-15 and E-15 samples show the same result that the space of OM-LDH component in the EM-15 sample with 85 phr HFMH increases to 4.10 nm from 3.02 nm of E-15 sample without HFMH, as shown in Fig. 2c and d. The increasing of interlayer spaces in the samples with HFMH give the evidence that the HFMH can enhance the EVA intercalation into the OM-LDH layers in the EVA/MH/LDH nanocomposites.
3.2. Morphological structures The TEM images of EM-0, EM-5, and EM-15 samples are shown in Fig. 3. It can be observed from Fig. 3 that the MH particles in the EM-0 sample were agglomerated badly, as the dark platelets pointed by the arrows in Fig. 3a. On the contrary, relatively well-dispersed MH particles instead of agglomeration can be observed from the EM-5 and EM-15 samples (see Fig. 3b and c) although the HFMH particles and LDH layers are difficult to discern from the TEM images. The above results indicate that the compatible interaction of LDHs with the MH particles to form the exfoliated or intercalated structures can help MH to disperse evenly in the EVA matrix. This means that the LDHs act as disperser and compatilizer during the melt blending of HFMH with EVA and thus help HFMH particles to disperse homogenously in the EVA matrix. 3.3. Thermal degradation behavior of EVA/HFMH/LDH nanocomposites Fig. 4 presents the TGA curves of pure EVA, E-10, E-15 and five EVA/HFMH/LDH samples with different amounts of LDH. It can be seen that thermal degradation temperatures of E-10 and E-15 samples are higher than that of the pure EVA. However, the degradation temperatures of these two samples only with LDH are much lower than those of EVA/ HFMH/LDH samples because of the latters’ high loading HFMH and LDH additives. The first weight loss for the four EVA/HFMH/LDH samples with LDH occurs at about 120e 300 C due to the evaporation of physically absorbed water in the layers and the loss of hydroxide of LDH layers. The main thermal degradation processes of all the five samples occur via two steps at the temperature range of 300e500 C. The first degradation step can be ascribed to the dehydration reaction of MH and the loss of acetic acid in the EVA at the range of 300e360 C, whereas the second degradation step in the
1718
G. Zhang et al. / Polymer Degradation and Stability 92 (2007) 1715e1720
Fig. 3. TEM images of samples with different amounts of LDH: (a) EM-0, (b) EM-5, (c) EM-15.
Fig. 4. TGA curves of various samples: (a) pure EVA, (b) E-10, (c) E-15, (d) EM-0, (e) EM-2, (f) EM-5, (g) EM-10, (h) EM-15.
range of 360e500 C can be due to the degradation of ethylene-based chains. The degradation rates of the three EVA/ HFMH/LDH samples (Fig. 4feh) in the temperature range of 360e500 C are apparently slower than that of the EVA/ MH sample without LDH (Fig. 4d). The beneficial effect can be due to the hindered effect of LDH layers for the diffusion of oxygen and volatile products throughout the composite materials. When 50 wt% weight loss was selected as a point of comparison, the thermal decomposition temperatures for the EVA/HFMH and EVA/HFMH/LDH samples with the OM-LDH of 2, 5, 10, and 15 phr were determined as 467, 468, 472, 477, and 485 C, respectively. It can be seen that the thermal decomposition temperatures of EM-5 to EM-15 samples are 5e18 C higher than 467 C of the EVA/HFMH sample. The thermal decomposition temperature of EM-2 sample is almost same as that of the EM-0 because of too low amount of LDH additive. And the thermal decomposition temperature of intercalated EVA/HFMH/LDH sample with 10 phr LDH is 5 C higher than 472 C of sample with 5 phr
G. Zhang et al. / Polymer Degradation and Stability 92 (2007) 1715e1720
1719
LDH although the latter is of the exfoliation structure. Moreover, when the LDH content further increases to 15 phr, the thermal decomposition temperature of the EM-15 sample is about 13 C higher than that of sample with 5 phr. These data indicate that the increase of thermal degradation temperature mainly depends on the increase of LDH loading instead of the exfoliated LDH structures.
3.4. Effect of LDH on the LOI of EVA/HFMH/LDH nanocomposites The LOI values of various samples with and without LDH are listed in Table 1. It can be seen that the addition of LDH instead of the same amount of MH apparently increases the LOI values compared with the EM-0 sample. The LOI value of EM-0 sample with only 100 phr HFMH is 29, while the LOI values of samples with loading 2, 5, 10, and 15 phr LDH increase to 31, 32, 32, and 34, respectively. This further gives the evidence that LDH can act as flame retardant synergistic agent of MH because the LDH layers acted as good barrier and can help the HFMH additive disperse more evenly in the EVA matrix and thus increase the LOI values compared with the sample without LDH.
3.5. Mechanical properties of EVA/HFMH/LDH nanocomposites Table 2 lists the mechanical properties of the EM-0, EM-2, EM-5, EM-10, and EM-15 samples. It can be seen that the elongation at break (EB) values of the samples with LDH increase with increasing the amount of OM-LDH additive to above 480% from 159% of the EM-0 sample without the LDH additive whether the OM-LDH layers in these samples are exfoliated or not. The EB improvement is mainly attributed to the synergistic dispersion effect of LDH layers for the HFMH particles which prevents the HFMH from agglomerating in the EVA matrix. However, the tensile strength (TS) values of the above samples decrease slightly with increasing the LDH content. When the OM-LDH content increases to 15 phr, the TS value decreases to 9.1 MPa from 10.5 MPa of the EM-0 sample without OM-LDH. This is probably due to the high content of low molecular SDS in the OM-LDH layers of the EM-15 sample compared with the same loading of HFMH. The similar observation from the surface modification of coupling agents has been reported in the literature [23e25].
Fig. 5. Loss tangents (tan d) of various samples vs temperature at a constant frequency of 5 Hz and a heating rate of 5 C/min.
3.6. Effects of LDH on dynamic mechanical properties Dynamic mechanical thermal analysis (DMTA) can obtain accurate Tg values of polymer materials. The effects of LDH on the Tg of polymer composites are shown in Fig. 5. It can be seen that the Tg values decrease with increasing the LDH contents towards the Tg value of pure EVA. The Tg value of EM-0 sample without LDH is 2 C. However, the Tg values of samples with 2, 5, 10 and 15 phr OM-LDH decrease rapidly to 3, 5, 7, and 10 C, respectively. The latter value approaches to 12 C of the pure EVA sample. The Tg values of above various samples are listed in Table 2. It seems that the addition of OM-LDH actually enhances the mobility of the polymeric chain segments compared with the EM-0 sample. This is because the OM-LDH layers act as a compatilizer to reduce the agglomerations of the HFMH particles and help them to disperse evenly in the EVA matrix. The homogenously dispersed HFMH particles make the nanocomposites having flexibility with the lower Tg.
Table 2 Mechanical properties and Tg values of EVA and various samples Sample code
Elongation at break (%)
Tensile strength (MPa)
Tg ( C)
EM-0 EM-2 EM-5 EM-10 EM-15 Pure EVA
159 15 223 20 280 10 480 20 483 20 e
10.5 0.15 10.2 0.10 9.7 0.12 9.3 0.10 9.1 0.14 e
2 3 5 7 10 12
Fig. 6. Storage modulus (E0 ) of various samples vs temperature at a constant frequency of 5 Hz and a heating rate of 5 C/min.
1720
G. Zhang et al. / Polymer Degradation and Stability 92 (2007) 1715e1720
The storage modulus of samples vs the temperature is shown in Fig. 6. The storage modulus of EM-0 sample is much larger than that of the pure EVA sample due to the higher stiffness of the filler HFMH. It has been reported that the poly(propylene) storage modulus increased with increasing the MH content [26]. However, the storage modulus of the EVA/HFMH/LDH samples decreases with increasing the loading of OM-LDH, as shown in Fig. 6. This is because the OM-LDH can act as a dispersion agent and help the HFMH disperse evenly in the EVA matrix, which makes the EVA/ HFMH/LDH samples more flexible than that of the EM-0 sample without OM-LDH. These results are in good agreement with those of EB in Table 2. 4. Conclusions The synergistic effects of organic-modified LDH with HFMH in the halogen-free flame retardant EVA/HFMH/ LDH nanocomposites have been studied by XRD, TEM, TGA, LOI, TS, EB, and DMTA. The XRD data show that the exfoliated and/or intercalated LDH structures of EVA/ HFMH/LDH nanocomposites prepared by melt intercalation mainly depend on the LDH loading, whereas the addition of HFMH is benefit to the exfoliation of LDH layers in the EVA matrix. The TEM images give the evidence that the LDHs can act as dispersers and help the HFMH particles disperse evenly in the EVA matrix. The TGA results demonstrate that when 50% weight loss is selected as a point of comparison, the thermal degradation temperatures of EVA/HFMH/ LDH nanocomposites with 5e15 phr OMgAl-LDH are 5e 18 C higher than that of the EVA/HFMH composite without OM-LDH. The LDHs can act as flame retardant synergist and compatilizer and apparently increase the LOI values and improve the elongation at break of EVA/HFMH/LDH nanocomposites. The results from the DMTA test show that the
Tg values of the nanocomposites decrease with increasing the LDH content and approach to the Tg of pure EVA, which means that the LDH can improve the flexibility of the EVA/ HFMH flame retardant system.
References [1] Dutta SK, Bhownick AK, Mukunda PG, Chaki TK. Polym Degrad Stab 1995;50:75. [2] Wang ZZ, Qu BJ, Fan WC, Huang P. J Appl Polym Sci 2001;81:206. [3] Wang J, Tung JF, Fuad MYA, Hornsby PR. J Appl Polym Sci 1996;60:1425. [4] Hornsby PR, Mthupha A. Plast Rubber Compos Process Appl 1996;25:347. [5] Cool M, Harper JF. Plast Rubber Compos Process Appl 1996;25:99. [6] Monte SJ, Sugerman G. Polym Eng Sci 1984;24:1369. [7] Jianping Lv, Longzhen Qiu, Baojun Qu. Nanotechnology 2004;15:1576. [8] Jianping Lv, Longzhen Qiu, Baojun Qu. J Crystal Growth 2004;267:676. [9] Leroux F, Besse JP. Chem Mater 2001;13:3507. [10] Decker LC, Zahouily K, Keller L, Benfarhi S, Bendaikha T, Baron J. J Mater Sci 2002;37:4831. [11] Chen W, Feng L, Qu BJ. Chem Mater 2004;16:368. [12] Chen W, Qu BJ. Solid State Commun 2004;130:259. [13] Qiu LZ, Chen W, Qu BJ. Colloid Polym Sci 2005;283:1241. [14] Gang Hu, Dermot O’Hare. J Am Chem Soc 2005;127:17808. [15] Ding P, Qu BJ. J Colloid Interface Sci 2005;291:13. [16] Du LC, Qu BJ, Xu ZJ. Polym Degrad Stab 2006;91:995. [17] Ding P, Zhang M, Gai J, Qu BJ. J Mater Chem 2007;11:1117. [18] Du LC, Qu BJ. J Mater Chem 2006;16:1549. [19] Ding P, Qu BJ. Polym Eng Sci 2006;46:1153. [20] Ding P, Qu BJ. J Polym Sci B 2006;44:3165. [21] Ristolainen N, Hippi U, Seppala J. Polym Eng Sci 2005;45:1568. [22] Bourbigot S, Bras ML, Dabrowski F. Proceeding of annual conference on fire research, NISTIR 6242; 1998. p. 43. [23] Demjen Z, Pukanszky B. Polym Compos 1997;18:741. [24] Chuah AW, Leong YC, Gan SN. Eur Polym J 2000;36:789. [25] Wang ZZ, Qu BJ, Fan WC, Li Z. J Funct Polym 2001;14:47. [26] Velasco JI, Morhain C, Martinez AB, Rodriguez-Perez MA, Saja JA. Macromol Mater Eng 2001;286:719.
Synergistic effects of layered double hydroxide with hyperfine magnesium hydroxide in halogen-free flame retardant EVA/HFMH/LDH nanocomposites Guobing Zhang, Peng Ding, Ming Zhang, Baojun Qu* State Key Laboratory of Fire Science and Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China Received 19 April 2007; received in revised form 24 May 2007; accepted 5 June 2007 Available online 27 June 2007
Abstract The synergistic effects of layered double hydroxide (LDH) with hyperfine magnesium hydroxide (HFMH) in halogen-free flame retardant ethylene-vinyl acetate (EVA)/HFMH/LDH nanocomposites have been studied by X-ray diffraction (XRD), transmission electron spectroscopy (TEM), thermogravimetric analysis (TGA), limiting oxygen index (LOI), mechanical properties’ tests, and dynamic mechanical thermal analysis (DMTA). The XRD results show that the exfoliated EVA/HFMH/LDH can be obtained by controlling the LDH loading. The TEM images give the evidence that the organic-modified LDH (OM-LDH) can act as a disperser and help HFMH particles to disperse homogeneously in the EVA matrix. The TGA data demonstrate that the addition of LDH can raise 5e18 C thermal degradation temperatures of EVA/HFMH/LDH nanocomposite samples with 5e15 phr OM-LDH compared with that of the control EVA/HFMH sample when 50% weight loss is selected as a point of comparison. The LOI and mechanical tests show that the LDH can act as flame retardant synergist and compatilizer to apparently increase the LOI and elongation at break values of EVA/HFMH/LDH nanocomposites. The DMTA data verify that the Tg value (10 C) of the EVA/HFMH/ LDH nanocomposite sample with 15 phr LDH is much lower than that (Tg ¼ 2 C) of the control EVA/HFMH sample without LDH and approximates to the Tg value (12 C) of pure EVA, which indicates that the nanocomposites with LDH have more flexibility than that of the EVA/ HFMH composites. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Layered double hydroxide; Hyperfine magnesium hydroxide; Ethylene-vinyl acetate; Synergistic effect; Nanocomposites
1. Introduction Ethylene-vinyl acetate copolymers (EVA) with different vinyl acetate (VA) contents are extensively used in many fields, especially in the cable industry as excellent insulating materials with good physical and chemical properties [1]. However, EVA resins are particularly flammable and emit a large amount of smoke while burning. Magnesium hydroxide (MH) is one kind of toxic-free and smoke-suppressing halogen-free flame retardant additive with high decomposition
* Corresponding author. Fax: þ86 551 3607245. E-mail address: [email protected] (B. Qu). 0141-3910/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2007.06.004
temperature in flame retardant polymeric materials. But its fatal disadvantages are low flame retardant efficiency and thus very large usage amount, which lead the mechanical properties of flame retardant materials to drop down sharply [2]. In order to minimize this effect, the hyperfinization and surface treatment of MH are two important methods to improve its dispersion and compatibility with polymer matrix. Most commonly used surface modifiers include silane coupling agents, stearates, titanate coupling agents, elastomer, etc [3e6]. However, all these surface modifiers of MH are organic reagents or polymer, which could be detrimental to the combustion properties such as limiting oxygen index (LOI). The hyperfine magnesium hydroxide (HFMH) has been verified to be very effective to improve the flame retardant and mechanical properties of
1716
G. Zhang et al. / Polymer Degradation and Stability 92 (2007) 1715e1720
polymeric materials, but its dispersion in polymer matrix is very difficult [7,8]. In recent years, the intercalated or exfoliated polymer/LDH nanocomposites have attracted great interest in the field of material science because of their excellent thermal stability, flame retardance, and physico-chemical properties [9e16]. The organically modified LDH (OM-LDH) inorganic compounds have nanosized dispersion in the exfoliated polymer/LDH nanocomposites [11,13,17]. In our previous work, we have prepared several kinds of polymer/LDH nanocomposites by direct melt intercalation [18e20]. The melt intercalation is a very promising method for extending the practical applications of polymer/LDH nanocomposites. Recently, Ristolainen [21] and Bourbigot [22] reported that the nanosized layered silicates as cationic clay have the ability to improve the dispersion of MH or aluminum trihydroxide in the polymer matrix. However, the dispersion ability of LDH as anionic clay to inorganic additives in polymeric materials has not reported yet. In the present work, we used hyperfine MH and organicmodified LDH layers as halogen-free flame retardant system and studied their synergistic effects on morphological structure, thermal property, flame retardant property, and mechanical properties of the EVA/HFMH/LDH nanocomposites by X-ray diffraction (XRD), transmission electron spectroscopy (TEM), thermogravimetric analysis (TGA), limiting oxygen index (LOI), mechanical properties’ tests and dynamic mechanical thermal analysis (DMTA). The main purpose of this study is to develop a new type of low smoke and halogenfree flame retardant polymeric nanocomposites and to be applied for the halogen-free flame retardant EVA insulated wire and cable. 2. Experimental
The EVA/HFMH/LDH samples were prepared by melt intercalation of two-step blending. Firstly, the OM-LDH powders and the desired amount of EVA were blended for 10 min by a SXX-300 mixer with the rotor speed of 64 rpm at 130 C. Secondly, HFMH particles were dispersed into the EVA/LDH nanocomposites by blending for another 3 min. The compositions, morphological structures and some flame retardant properties of various samples are summarized in Table 1. 2.3. Measurements The XRD patterns were recorded using a Rigaku D/Max-rA rotating anode X-ray diffractometer equipped with a Cu Ka tube and Ni filter (l ¼ 0.1542 nm). The TEM images were obtained on a Hitachi H-800 transmission electron microanalyzer with an accelerating voltage of 200 kV and camera length of 0.8 m. The samples were ultramicrotomed with a diamond knife on an LKB Pyramitome to give 100-nm thick slices. And the slices were transferred from water to Cu grid. The TGA profiles were performed on a Shimadzu TGA-50H thermoanalyzer under an airflow rate of 6 105 m3/min at a scan rate of 10 C/min. The LOI values were measured using a HC-2 type instrument (made in China) on sheets 120 6.5 3 mm according to the standard oxygen index test ASTM D2863-77. The mechanical properties of tensile strength (TS) and elongation at break (EB) were measured by a universal testing machine (DCS500, Shimadzu) with the crosshead speed of 25 mm/min at temperature 25 2 C. The dumbbell-shaped specimens were prepared according to ASTM D412-87. The DMTA data of the loss tangent (tan d) and the storage modulus (E0 ) with the changes of temperature were recorded on a Diamond DMTA analyzer (Perkin Elmer) in a temperature range of 80 to 80 C at a constant frequency of 5 Hz and at a heating rate of 5 C/min.
2.1. Materials 3. Results and discussion Commercial EVA with 28 wt% VA was supplied by Sumitomo Chemical Co. Ltd. The hyperfine magnesium hydroxide (HFMH) with a particle size of 0.1e1.0 mm used in this work was supplied by Jingjiang Kexing Nanomaterials Co. Ltd., China. Mg(NO3)$6H2O and Al(NO3)$9H2O (analytical pure) were supplied by Shanghai ZhenXing Chemicals No. 1 Plant. Sodium dodecyl sulfate (SDS) and NaOH (analytical pure) were obtained from China Medicine (Group) Shanghai Chemicals Reagent Corporation. 2.2. Preparation of samples The OM-LDH was prepared by co-precipitation method described in our previous work [18]. The pH of 300 ml aqueous solution containing 0.03 mol Mg(NO3)$6H2O, 0.01 mol Al (NO3)$9H2O and 0.05 mol C12H25SO4Na was adjusted to pH w 10 with 1 mol L1 NaOH aqueous solution. The slurry was aged for 3 days at 80 C, then filtered, and washed three times with distilled water. The white powder OM-LDH was obtained by drying at 60 C in an oven.
3.1. Structural characterization Fig. 1 gives the low-angle XRD patterns of the OM-LDH, EVA/HFMH, and EVA/HFMH/LDH samples with different loadings of OM-LDH. The basal spaces of the OM-LDH sample are measured to be 2.75 nm from (001) diffraction peak at 2q ¼ w3.2 and 1.38 nm from (002) diffraction peak at Table 1 Compositions, morphological structures and LOI of various samples Sample code
EVA (phr)
HFMH (phr)
LDH (phr)
Morphological structures
LOI (%)
EM-0 EM-2 EM-5 EM-10 EM-15 Pure EVA E-10 E-15
100 100 100 100 100 100 100 100
100 98 95 90 85 0 0 0
0 2 5 10 15 0 10 15
e Exfoliated Exfoliated Intercalated Intercalated e Intercalated Intercalated
29 31 32 32 34 18 e e
G. Zhang et al. / Polymer Degradation and Stability 92 (2007) 1715e1720
1717
Fig. 1. Low-angle XRD patterns of various samples: (a) OM-LDH, (b) EM-15, (c) EM-10, (d) EM-5, (e) EM-2, (f) EM-0.
Fig. 2. Comparison of low-angle XRD patterns of four samples with or without HFMH: (a) EM-10, (b) E-10, (c) EM-15, (d) E-15.
2q ¼ w6.4 (Fig. 1a), whereas the EVA/HFMH sample has no peaks at corresponding 2q ¼ w3.2 and w6.4 (Fig. 1f). It can be seen from Fig. 1bee that the EVA/HFMH/LDH samples have the exfoliation trend with decreasing LDH loading because the XRD (001) peaks at the low-angle range disappear gradually with increasing the gallery height of the OM-LDH due to the EVA intercalation. The basal spaces of OM-LDH from samples EM-15 and EM-10 can be determined as 4.10 and 5.05 nm from the weak d001 diffraction peaks at 2q ¼ 2.0e3.0 , respectively. These data indicate that the EM-15 and EM-10 samples have partly exfoliated and partly intercalated structures. The (001) diffraction peaks cannot be observed from the samples EM-5 and EM-2 (see Fig. 1d and e), which mean that the exfoliated structures have been formed in the EVA/HFMH/LDH nanocomposites when the contents of LDH decrease to 5 or below 5 phr. The structural features of main samples are listed in Table 1. The small peaks at 7e8 in Fig. 1bef could be formed by the crystals of EVA under the participation of HFMH particles. Fig. 2 compares the low-angle XRD patterns of EM-10, EM-15 and E-10, E-15 samples with or without the HFMH additive in the range of 2q ¼ 1.5e10 . It can be seen that the EM-10 (Fig. 2a) and E-10 (Fig. 2b) samples show the similar intercalated structures, but the space of OM-LDH component in the EM-10 sample with the 90 phr HFMH increases to 5.05 nm from 3.26 nm of E-10 sample without the MH loading. This indicates that more EVA molecular chains in the EM-10 sample were intercalated into the OM-LDH layers in the presence of HFMH than those in the E-10 sample without HFMH. The EM-15 and E-15 samples show the same result that the space of OM-LDH component in the EM-15 sample with 85 phr HFMH increases to 4.10 nm from 3.02 nm of E-15 sample without HFMH, as shown in Fig. 2c and d. The increasing of interlayer spaces in the samples with HFMH give the evidence that the HFMH can enhance the EVA intercalation into the OM-LDH layers in the EVA/MH/LDH nanocomposites.
3.2. Morphological structures The TEM images of EM-0, EM-5, and EM-15 samples are shown in Fig. 3. It can be observed from Fig. 3 that the MH particles in the EM-0 sample were agglomerated badly, as the dark platelets pointed by the arrows in Fig. 3a. On the contrary, relatively well-dispersed MH particles instead of agglomeration can be observed from the EM-5 and EM-15 samples (see Fig. 3b and c) although the HFMH particles and LDH layers are difficult to discern from the TEM images. The above results indicate that the compatible interaction of LDHs with the MH particles to form the exfoliated or intercalated structures can help MH to disperse evenly in the EVA matrix. This means that the LDHs act as disperser and compatilizer during the melt blending of HFMH with EVA and thus help HFMH particles to disperse homogenously in the EVA matrix. 3.3. Thermal degradation behavior of EVA/HFMH/LDH nanocomposites Fig. 4 presents the TGA curves of pure EVA, E-10, E-15 and five EVA/HFMH/LDH samples with different amounts of LDH. It can be seen that thermal degradation temperatures of E-10 and E-15 samples are higher than that of the pure EVA. However, the degradation temperatures of these two samples only with LDH are much lower than those of EVA/ HFMH/LDH samples because of the latters’ high loading HFMH and LDH additives. The first weight loss for the four EVA/HFMH/LDH samples with LDH occurs at about 120e 300 C due to the evaporation of physically absorbed water in the layers and the loss of hydroxide of LDH layers. The main thermal degradation processes of all the five samples occur via two steps at the temperature range of 300e500 C. The first degradation step can be ascribed to the dehydration reaction of MH and the loss of acetic acid in the EVA at the range of 300e360 C, whereas the second degradation step in the
1718
G. Zhang et al. / Polymer Degradation and Stability 92 (2007) 1715e1720
Fig. 3. TEM images of samples with different amounts of LDH: (a) EM-0, (b) EM-5, (c) EM-15.
Fig. 4. TGA curves of various samples: (a) pure EVA, (b) E-10, (c) E-15, (d) EM-0, (e) EM-2, (f) EM-5, (g) EM-10, (h) EM-15.
range of 360e500 C can be due to the degradation of ethylene-based chains. The degradation rates of the three EVA/ HFMH/LDH samples (Fig. 4feh) in the temperature range of 360e500 C are apparently slower than that of the EVA/ MH sample without LDH (Fig. 4d). The beneficial effect can be due to the hindered effect of LDH layers for the diffusion of oxygen and volatile products throughout the composite materials. When 50 wt% weight loss was selected as a point of comparison, the thermal decomposition temperatures for the EVA/HFMH and EVA/HFMH/LDH samples with the OM-LDH of 2, 5, 10, and 15 phr were determined as 467, 468, 472, 477, and 485 C, respectively. It can be seen that the thermal decomposition temperatures of EM-5 to EM-15 samples are 5e18 C higher than 467 C of the EVA/HFMH sample. The thermal decomposition temperature of EM-2 sample is almost same as that of the EM-0 because of too low amount of LDH additive. And the thermal decomposition temperature of intercalated EVA/HFMH/LDH sample with 10 phr LDH is 5 C higher than 472 C of sample with 5 phr
G. Zhang et al. / Polymer Degradation and Stability 92 (2007) 1715e1720
1719
LDH although the latter is of the exfoliation structure. Moreover, when the LDH content further increases to 15 phr, the thermal decomposition temperature of the EM-15 sample is about 13 C higher than that of sample with 5 phr. These data indicate that the increase of thermal degradation temperature mainly depends on the increase of LDH loading instead of the exfoliated LDH structures.
3.4. Effect of LDH on the LOI of EVA/HFMH/LDH nanocomposites The LOI values of various samples with and without LDH are listed in Table 1. It can be seen that the addition of LDH instead of the same amount of MH apparently increases the LOI values compared with the EM-0 sample. The LOI value of EM-0 sample with only 100 phr HFMH is 29, while the LOI values of samples with loading 2, 5, 10, and 15 phr LDH increase to 31, 32, 32, and 34, respectively. This further gives the evidence that LDH can act as flame retardant synergistic agent of MH because the LDH layers acted as good barrier and can help the HFMH additive disperse more evenly in the EVA matrix and thus increase the LOI values compared with the sample without LDH.
3.5. Mechanical properties of EVA/HFMH/LDH nanocomposites Table 2 lists the mechanical properties of the EM-0, EM-2, EM-5, EM-10, and EM-15 samples. It can be seen that the elongation at break (EB) values of the samples with LDH increase with increasing the amount of OM-LDH additive to above 480% from 159% of the EM-0 sample without the LDH additive whether the OM-LDH layers in these samples are exfoliated or not. The EB improvement is mainly attributed to the synergistic dispersion effect of LDH layers for the HFMH particles which prevents the HFMH from agglomerating in the EVA matrix. However, the tensile strength (TS) values of the above samples decrease slightly with increasing the LDH content. When the OM-LDH content increases to 15 phr, the TS value decreases to 9.1 MPa from 10.5 MPa of the EM-0 sample without OM-LDH. This is probably due to the high content of low molecular SDS in the OM-LDH layers of the EM-15 sample compared with the same loading of HFMH. The similar observation from the surface modification of coupling agents has been reported in the literature [23e25].
Fig. 5. Loss tangents (tan d) of various samples vs temperature at a constant frequency of 5 Hz and a heating rate of 5 C/min.
3.6. Effects of LDH on dynamic mechanical properties Dynamic mechanical thermal analysis (DMTA) can obtain accurate Tg values of polymer materials. The effects of LDH on the Tg of polymer composites are shown in Fig. 5. It can be seen that the Tg values decrease with increasing the LDH contents towards the Tg value of pure EVA. The Tg value of EM-0 sample without LDH is 2 C. However, the Tg values of samples with 2, 5, 10 and 15 phr OM-LDH decrease rapidly to 3, 5, 7, and 10 C, respectively. The latter value approaches to 12 C of the pure EVA sample. The Tg values of above various samples are listed in Table 2. It seems that the addition of OM-LDH actually enhances the mobility of the polymeric chain segments compared with the EM-0 sample. This is because the OM-LDH layers act as a compatilizer to reduce the agglomerations of the HFMH particles and help them to disperse evenly in the EVA matrix. The homogenously dispersed HFMH particles make the nanocomposites having flexibility with the lower Tg.
Table 2 Mechanical properties and Tg values of EVA and various samples Sample code
Elongation at break (%)
Tensile strength (MPa)
Tg ( C)
EM-0 EM-2 EM-5 EM-10 EM-15 Pure EVA
159 15 223 20 280 10 480 20 483 20 e
10.5 0.15 10.2 0.10 9.7 0.12 9.3 0.10 9.1 0.14 e
2 3 5 7 10 12
Fig. 6. Storage modulus (E0 ) of various samples vs temperature at a constant frequency of 5 Hz and a heating rate of 5 C/min.
1720
G. Zhang et al. / Polymer Degradation and Stability 92 (2007) 1715e1720
The storage modulus of samples vs the temperature is shown in Fig. 6. The storage modulus of EM-0 sample is much larger than that of the pure EVA sample due to the higher stiffness of the filler HFMH. It has been reported that the poly(propylene) storage modulus increased with increasing the MH content [26]. However, the storage modulus of the EVA/HFMH/LDH samples decreases with increasing the loading of OM-LDH, as shown in Fig. 6. This is because the OM-LDH can act as a dispersion agent and help the HFMH disperse evenly in the EVA matrix, which makes the EVA/ HFMH/LDH samples more flexible than that of the EM-0 sample without OM-LDH. These results are in good agreement with those of EB in Table 2. 4. Conclusions The synergistic effects of organic-modified LDH with HFMH in the halogen-free flame retardant EVA/HFMH/ LDH nanocomposites have been studied by XRD, TEM, TGA, LOI, TS, EB, and DMTA. The XRD data show that the exfoliated and/or intercalated LDH structures of EVA/ HFMH/LDH nanocomposites prepared by melt intercalation mainly depend on the LDH loading, whereas the addition of HFMH is benefit to the exfoliation of LDH layers in the EVA matrix. The TEM images give the evidence that the LDHs can act as dispersers and help the HFMH particles disperse evenly in the EVA matrix. The TGA results demonstrate that when 50% weight loss is selected as a point of comparison, the thermal degradation temperatures of EVA/HFMH/ LDH nanocomposites with 5e15 phr OMgAl-LDH are 5e 18 C higher than that of the EVA/HFMH composite without OM-LDH. The LDHs can act as flame retardant synergist and compatilizer and apparently increase the LOI values and improve the elongation at break of EVA/HFMH/LDH nanocomposites. The results from the DMTA test show that the
Tg values of the nanocomposites decrease with increasing the LDH content and approach to the Tg of pure EVA, which means that the LDH can improve the flexibility of the EVA/ HFMH flame retardant system.
References [1] Dutta SK, Bhownick AK, Mukunda PG, Chaki TK. Polym Degrad Stab 1995;50:75. [2] Wang ZZ, Qu BJ, Fan WC, Huang P. J Appl Polym Sci 2001;81:206. [3] Wang J, Tung JF, Fuad MYA, Hornsby PR. J Appl Polym Sci 1996;60:1425. [4] Hornsby PR, Mthupha A. Plast Rubber Compos Process Appl 1996;25:347. [5] Cool M, Harper JF. Plast Rubber Compos Process Appl 1996;25:99. [6] Monte SJ, Sugerman G. Polym Eng Sci 1984;24:1369. [7] Jianping Lv, Longzhen Qiu, Baojun Qu. Nanotechnology 2004;15:1576. [8] Jianping Lv, Longzhen Qiu, Baojun Qu. J Crystal Growth 2004;267:676. [9] Leroux F, Besse JP. Chem Mater 2001;13:3507. [10] Decker LC, Zahouily K, Keller L, Benfarhi S, Bendaikha T, Baron J. J Mater Sci 2002;37:4831. [11] Chen W, Feng L, Qu BJ. Chem Mater 2004;16:368. [12] Chen W, Qu BJ. Solid State Commun 2004;130:259. [13] Qiu LZ, Chen W, Qu BJ. Colloid Polym Sci 2005;283:1241. [14] Gang Hu, Dermot O’Hare. J Am Chem Soc 2005;127:17808. [15] Ding P, Qu BJ. J Colloid Interface Sci 2005;291:13. [16] Du LC, Qu BJ, Xu ZJ. Polym Degrad Stab 2006;91:995. [17] Ding P, Zhang M, Gai J, Qu BJ. J Mater Chem 2007;11:1117. [18] Du LC, Qu BJ. J Mater Chem 2006;16:1549. [19] Ding P, Qu BJ. Polym Eng Sci 2006;46:1153. [20] Ding P, Qu BJ. J Polym Sci B 2006;44:3165. [21] Ristolainen N, Hippi U, Seppala J. Polym Eng Sci 2005;45:1568. [22] Bourbigot S, Bras ML, Dabrowski F. Proceeding of annual conference on fire research, NISTIR 6242; 1998. p. 43. [23] Demjen Z, Pukanszky B. Polym Compos 1997;18:741. [24] Chuah AW, Leong YC, Gan SN. Eur Polym J 2000;36:789. [25] Wang ZZ, Qu BJ, Fan WC, Li Z. J Funct Polym 2001;14:47. [26] Velasco JI, Morhain C, Martinez AB, Rodriguez-Perez MA, Saja JA. Macromol Mater Eng 2001;286:719.