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layered zinc hydroxide nitrate nanocomposites with enhanced thermal, mechanical and combustion properties
Materials Chemistry and Physics 141 (2013) 582e588
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Fabrication of thermoplastic polyester elastomer/layered zinc hydroxide nitrate nanocomposites with enhanced thermal, mechanical and combustion properties Wei Yang, Liyan Ma, Lei Song, Yuan Hu* State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, PR China
h i g h l i g h t s We We We We
prepare zinc hydroxide nitrate modified by sodium benzoate (SB-ZHN). prepare and characterize thermoplastic polyester elastomer/SB-ZHN nanocomposites. investigate the thermal and combustion properties of the nanocomposites. study the thermodynamic properties of the nanocomposites.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 January 2013 Received in revised form 5 May 2013 Accepted 27 May 2013
The objective of this study is to explore the potential of layered zinc hydroxide nitrate modified with sodium benzoate as nanoparticle in thermoplastic polyester elastomer (TPEE). The organically modified zinc hydroxide nitrate was compounded with TPEE using solution blending method. The nanocomposite structure was characterized by means of X-ray diffraction and transmission electron microscopy. The results showed that the nanoparticle was homogenously dispersed in TPEE matrix, and partially exfoliated structure was formed. The thermal behavior, mechanical and thermal combustion properties of the novel nanocomposite were studied respectively through differential scanning calorimeter (DSC), dynamic mechanical analysis (DMA) and microscale combustion calorimeter (MCC). For the nanocomposite containing 7 wt% nanoparticle, the crystallization temperature evaluated by DSC was increased by 10 C. The storage modulus at 95 C measured by DMA was improved by around 26%. The heat release capacity (an indicator of a material fire hazard) from MCC testing was reduced by about 56% (compared to the results of neat TPEE). Ó 2013 Published by Elsevier B.V.
Keywords: A. Elastomers A. Inorganic compounds A. Nanostructures D. Thermal properties D. Thermodynamic properties
1. Introduction Layered materials, including cationic clay, layered double hydroxide (LDH) and alpha-zirconium phosphate, have attracted considerable attention because of their ability to intercalate neutral guest molecules or to exchange interlayer inorganic or organic ions [1]. One of the most important features is that they can serve as inorganic filler to fabricate polymer based nanocomposites. LDH known as anionic clay is one of the most commonly investigated layered materials, which is related to the mineral hydrotalcite. The most important group of LDHs can be represented by the general n 2þ 3þ and formula ½M2þ ð1xÞ Mx ðOHÞ2 ½ððx þ 2Þ=nÞA $mH2 O, where M 3þ M are divalent and trivalent cations, respectively; x is equal to the
* Corresponding author. Tel./fax: þ86 551 63601664. E-mail address: [email protected] (Y. Hu). 0254-0584/$ e see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.matchemphys.2013.05.074
ratio of M3þ/(M2þþM3þ) and A is an anion of valence n (2, 3) [1]. Up to now, LDHs have been widely used as filler materials to prepare polymer matrix nanocomposites to enhance various properties of polymeric materials, such as thermal, mechanical and fire performances [2e4]. It has recently been found that layered hydroxide salts (LHSs) are closely related to anionic clay and structurally similar to LDHs, which can also undergo anion exchange reactions. In general, the structure of LHSs can be simply denoted as the combination of brucite-like hydroxide layers and interlayer anions. For example, the structure of Zn5(OH)8(NO3)2$2H2O is composed of brucite-like layers in which one-fourth of the octahedral sites are vacant and tetrahedrally coordinated Zn2þ cations locate on either side of the vacant octahedral. Three vertices of the tetrahedron are occupied by hydroxyls, shared with the octahedral sheet, and the apex is occupied by a water molecule. The nitrate anions are present between the hydroxide layers and are surrounded by water molecules, but
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not coordinated to the zinc atoms [5e7]. Although the synthesis, characterization and application of LHSs are not extensively investigated as LDHs, these materials may offer more possibility of varying the identity of both the metal constituents and the exchangeable interlayer anion to enable tuning physic-chemical properties of the materials in order to optimize their performance for selected applications [8]. Polymer/layered material nanocomposites have been of considerable interest in the past decades because of their superior mechanical, physical, thermal and barrier properties. Layered materials are dispersed in polymer matrix to form exfoliated or intercalated structure nanocomposites. However, few reports have focused on the preparation and performance investigation of polymer/LHS nanocomposites. In Hossenlopp’s study [8], a zincbased LHS with a boron-containing exchangeable anion was used as flame retardant and compounded with PMMA using melt intercalation to produce PMMA nanocomposite in order to improve the flame retardant properties. Wypych et al. [7] reported that zinc hydroxide nitrate adsorbed with anions of colored dyes was applied as inorganic filler to prepare PVA nanocomposites and enhance their mechanical performance. Although several improved properties were achieved, well dispersion of LHS and exfoliated structure were not obtained in the reported articles. The commercial introduction of thermoplastic elastomers (TPEs) in recent years has triggered great interest in the investigation and application of these unusual materials, which can cover the boundary between rubbers and plastics. Among TPEs, thermoplastic polyester elastomer (TPEE) exhibits superior oil, chemical resistance and excellent low temperature impact properties, which have been widely used in automobile, electric and electronic application [9e11]. In order to further improve their thermal, mechanical and other properties, layered materials are always used to achieve these purposes. To the best of our knowledge, no work has been published on the use of LHSs layered nanoparticles into TPEE. Therefore, the objective of this study is to explore the potential of layered hydroxide salts modified with sodium benzoate as nanoparticles in TPEE. The thermal behavior, mechanical and thermal combustion properties of the novel nanocomposites will be studied respectively through differential scanning calorimeter, dynamic mechanical analysis and microscale combustion calorimeter. 2. Experimental 2.1. Materials Thermoplastic polyester elastomer (TPEE, H6555) (melt index: 12e15 g/10 min, Density: 1.20 g cm3) was provided by Sichuan Sunshine Plastics Co., Ltd, China. The chemical structure of TPEE was shown in Fig. 1. Sodium hydroxide, zinc nitrate hexahydrate, sodium benzoate, phenol and tetrachloroethane (analytical pure) were obtained from Sinopharm Chemical Reagent Co., Ltd, China. All chemicals were used without further purification. 2.2. Preparation of organo-modified layered zinc hydroxy nitrate The layered zinc hydroxy nitrate (ZHN) precursor was prepared according to reported literature [1]. In a typical experiment, ZHN
Fig. 1. Chemical structure of TPEE.
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with the ideal composition Zn5(OH)8(NO3)2$2H2O was prepared by precipitation from a 3.5 M solution of Zn(NO3)2$6H2O with 0.75 M NaOH. 50 ml of the NaOH solution was dropwise added to 20 ml of the Zn(NO3)2$6H2O solution (OH/Zn ¼ 0.5) with vigorous stirring at room temperature. The white product was immediately filtered following complete addition of the NaOH solution, washed with deionized water, and dried at 65 C for 24 h. Exchange of the interlayer nitrate anions in ZHN was prepared by the exchange reaction between nitrate anions and organic anions. 0.2 g ZHN was dispersed in 50 ml of 1 N aqueous solutions of sodium benzoate (SB) at room temperature. The mixtures were frequently stirred for 48 h at room temperature. The exchange product was filtered, washed with deionized water, and dried in air at 65 C for 24 h. The exchange product is referred herein as SB-ZHN.
2.3. Preparation of TPEE/SB-ZHN nanocomposites TPEE nanocomposites with different wt% SB-ZHN were prepared by the solution blending method. A desired amount of SB-ZHN was firstly refluxed in 50 ml blended solution (including 25 ml phenol and 25 ml tetrachloroethane) for 5 h under flowing N2. Subsequently 2 g TPEE was added into the above SB-ZHN suspension with stirring for 12 h. The solution was then dried under vacuum oven at 80 C for 48 h. The preparation conditions were same for each composition of hybrids containing 0 wt%, 1 wt%, 3 wt%, 5 wt%, and 7 wt% of SB-ZHN and were respectively designated as TPEE, TPEE/ SB-ZHN1, TPEE/SB-ZHN3, TPEE/SB-ZHN5, and TPEE/SB-ZHN7.
2.4. Characterizations of ZHN and TPEE/ZHN nanocomposites Fourier transform infrared (FTIR) spectra of the layered materials were obtained on a Nicolet 6700 FT-IR spectrophotometer using thin KBr disc. The transition mode was used and the wavenumber range was set from 4000 to 500 cm1. Thermogravimetric analysis (TGA) of the layered materials was carried out using a Q5000 IR thermogravimetric analyzer (TA Instruments Waters, China) at a linear heating rate of 20 C min1 under nitrogen atmosphere. The weight of all the samples were kept within 5e10 mg. Composites in an open Pt pan were tested under the flow rate of 6 105 m3 per minute at temperature ranging from room temperature to 700 C. X-ray diffraction (XRD) was performed using a Rigaku D/Max-rA rotating anode X-ray diffractometer equipped with a Cu K tube and Ni filter (¼0.1542 nm). Data acquisition was performed using a scanning speed of 3.0 min1. The patterns were recorded in the 2q range of 3.0 e60.0 for the layered materials and 1.0 e10.0 for the nanocomposites. The transmission electron microscopy (TEM) images of the nanocomposites were obtained on a JEOL JEM-2100F with an accelerating voltage of 200 kV. The nanocomposite specimens were cut into ultrathin slices under cryogenic conditions with an ultramicrotome (Ultraacut-1, United Kingdom) equipped with a diamond knife.
2.5. Thermal properties of TPEE/ZHN nanocomposites Thermal behaviors were studied by a NETZSCH DSC (204 F1 phoenix). All operations were carried out in a nitrogen atmosphere. Samples were heated from 50 C to 300 C at 10 C min1 and held at 300 C for 5 min to eliminate the thermal history, then cooled to 50 C at 10 C min1. They were maintained at 50 C for 5 min and heated to 300 C subsequently.
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2.6. Dynamic mechanical properties of TPEE/ZHN nanocomposites Dynamic mechanical analysis (DMA) was performed with the Perkin Elmer Pyris Diamond DMA from 95 C to 40 C at a heating rate of 5 C min1 with a frequency of 10 Hz in the tensile configuration. 2.7. Thermal combustion properties of TPEE/ZHN nanocomposites Thermal combustion properties of plastics were measured using a microscale combustion calorimeter (MCC, Govmark) according to ASTM D 7309-07. 4e8 mg of each sample was heated at 1 C s1 from 90 to 600 C and held there for 30 s. During pyrolysis, the volatilized decomposition products are transferred in the stream of nitrogen to a high-temperature combustion furnace where pure oxygen is added and the decomposition products are completely combusted. The amount of oxygen consumed is measured with an oxygen analyzer and used to calculate a heat release rate (HRR). All MCC data obtained were reproducible to about 3%. Fig. 3. XRD patterns of ZHN (a) and SB-ZHN (b).
3. Results and discussion 3.1. Characterization of the layered materials FTIR spectra of SB, ZHN and SB-ZHN are shown in Fig. 2. For both ZHN and SB-ZHN, the band of the hydroxy group appears in the region from 3100 cm1 to 3700 cm1. For ZHN, the peak at around 3518 cm1 and 3438 cm1 can be respectively assigned to stretching vibrations of free layer hydroxyl groups and layer hydroxyl groups that are hydrogen bonded to the interlayer nitrate anion or water molecules [12]. The assignments can also be used to describe the OH vibrations in SB-ZHN. After the exchange reaction, the characteristic asymmetric stretching vibration of nitrate ion at about 1373 cm1 in ZHN disappears and the new peaks at 1405 cm1, 1559 cm1 and 1601 cm1 appear. The strong peaks at 1405 cm1 and 1559 cm1 are attributed to the asymmetric and symmetric vibrations of C]O group [12,13]. The peak at 1601 cm1 should be assigned to the stretching vibration of CeC bond in benzene ring. These assignments are all characteristic absorption bands of sodium benzoate, as shown in trace (a). It is indicated that the nitrate group is completely exchanged with the benzoate anion. The XRD patterns of ZHN and SB-ZHN are presented in Fig. 3. Both compounds exhibit intense 001 reflection which are equally
spaced indicating that the compounds are layered and possess high range ordering, at least to the third order in the c direction [8]. The XRD pattern of ZHN shows an interlayer distance of 0.98 nm, which is in agreement with the reported values [12]. After exchange, the host layer reflections of ZHN are shifted from 8.98 and 18.07 to 4.65 and 9.26 , respectively, corresponding to an interlayer spacing of 1.90 nm for SB-ZHN (Fig. 4). The intercalation of sodium benzoate not only can enlarge the gallery between the platelets, but also can improve the interfacial properties between the SB-ZHN layers and polyester matrix. The absence of nitrate reflections indicates that the exchange is complete, consistent with the FTIR analysis. The TGA curves of ZHN and SB-ZHN are shown in Fig. 5, obtained at 20 C min1 under nitrogen condition. The decomposition of ZHN is characterized by two steps. The mass loss from 50 to 150 C corresponds to the loss of intercalated water. The other mass loss from 150 to 700 C is attributed to the dehydroxylation of layers. The resulting residue is ZnO. For SB-ZHN, the first mass loss from 50 C to approximately 300 C is the removal of intercalated water and dehydroxylation of layers. The second mass loss from 300 C to about 500 C should be assigned to the loss of impurity benzoate anion [1]. A small loss ranging from 500 C to around 550 C may be attributed to the release of benzene ring. 3.2. Characterization of TPEE/SB-ZHN nanocomposites
Fig. 2. FTIR spectra of SB (a), ZHN (b) and SB-ZHN (c).
The XRD patterns of SB-ZHN and TPEE/SB-ZHN nanocomposites are shown in Fig. 6. When SB-ZHN is compounded with TPEE matrix, the weak peaks of TPEE/SB-ZHN5 and TPEE/SB-ZHN7 are observed at lower 2 theta values. These reflections are broad and not well defined. It is indicated that the ordered layered structure of SB-ZHN is disrupted in TPEE matrix. The shifting of the first Bragg’s reflection to a lower scattering angle corresponds to an increase of the interlayer distance by about 1.07 nm. It reveals that the TPEE chains are aligned within the interlayer space SB-ZHN. XRD alone is insufficient to characterize the structure and dispersion of the TPEE/SB-ZHN nanocomposites. Transmission electron microscopy (TEM) is required to directly observe the dispersion of the additive. Fig. 7 shows the TEM images for TPEE/SB-ZHN7 at both low and high magnifications. The TEM image at low magnification in Fig. 7(a) exhibits the homogenous and uniform dispersion of SB-ZHN in TPEE matrix. At higher magnification, the TEM image clearly shows that many SB-ZHN nanosheets are disorderly dispersed in TPEE. At the
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Fig. 4. Schematic illustration of anion exchange in ZHN.
same time, the intercalated structure composed of multilayer tactoids, which are represented by the dark regions, are also observed in Fig. 7(b). The results from TEM analysis are in good agreement with the XRD patterns. It is thus concluded that the SB-ZHN layers are partially intercalated and partially exfoliated in the TPEE matrix through the solution blending processing (Fig. 8). 3.3. Thermal properties The nonisothermal crystallization and subsequent melt behaviors of neat TPEE and TPEE/SB-ZHN nanocomposites were studied by means of DSC. Fig. 9 shows the DSC curves recorded for the neat TPEE and TPEE/SB-ZHN nanocomposites with various SB-ZHN concentrations in the cooling scan at 10 C min1 and the subsequent heating scan at 10 C min1. The thermal properties obtained from DSC including crystallization temperature (Tc), crystallization enthalpy (DHc), melt temperature (Tm) and melt enthalpy (DHm) are summarized in Table 1. As shown in Fig. 9(a), the crystallization temperature (Tc) in TPEE/SB-ZHN nanocomposites slightly increases with the increasing SB-ZHN concentration. For the nanocomposite containing 7 wt% SB-ZHN, the Tc value evaluated by DSC is increased by 10 C in comparison with neat TPEE. The phenomenon is caused by the heterogeneous nucleation effect of SB-ZHN. In the subsequent heating scan, the multiple melt behaviors are observed in TPEE/SB-ZHN nanocomposites and the melt temperature (Tm) values are reduced compared with neat TPEE. The reason may be that the fusion of a certain amount of original crystals, followed by the recrystallization and final melting of more perfect crystals, partly formed during primary crystallization and partly formed
Fig. 5. TGA curves of ZHN (a) and SB-ZHN (b).
through the recrystallization process occurring during the heating scan [14e18]. On the other hand, both the crystallization enthalpy (DHc) and melt enthalpy (DHm) increase first then decrease with the increasing SB-ZHN concentration. It is indicated that the relative crystallinity of TPEE is reduced by the addition of SB-ZHN with high fraction. The results may be caused by the inability of polymer chains to be fully incorporated into the growing crystalline lamella and the limited space imposed on the polymer chains by a high number of platelets and/or tactoids [19,20]. 3.4. Dynamic mechanical properties Dynamic mechanical analysis (DMA) is one of the techniques commonly used to characterize the time, frequency and temperature dependency of the viscoelastic nature of polymers [21]. As shown in Fig. 10(a), the storage modulus in TPEE/SB-ZHN nanocomposites decreases first and then increases with the increasing SB-ZHN concentration in the low temperature region (below the glass transition temperature). For example, the storage modulus at 95 C for TPEE/SB-ZHN7 is improved by around 26% in comparison with neat TPEE. It is indicated that the TPEE/SB-ZHN nanocomposites show a softening effect at low SB-ZHN concentration, consistent with the reported literature [22]. This may be caused by the large fraction of benzoate anion compared with ZHN. A part of this organic anion adhered on the surface of the intercalated and exfoliated layers may be loosely bound leading to
Fig. 6. XRD patterns: (a) SB-ZHN; (b) neat TPEE; (c) TPEE/SB-ZHN1; (d) TPEE/SB-ZHN3; (e) TPEE/SB-ZHN5; (f) TPEE/SB-ZHN7.
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Fig. 7. TEM images of TPEE/SB-ZHN7: (a) low magnification; (b) high magnification.
the increasing of the mobility of the polymer matrix in the polymer-filler interfacial region. Due to the weak interaction at the interface, the plasticizing effect dominates at low SB-ZHN concentration in the low temperature region. Therefore, the storage modulus is reduced. In the high temperature region (above the glass transition temperature), the storage modulus increases with the increasing SB-ZHN concentration. This means that the Fig. 9. DSC curves of (a) nonisothermal crystallization at a cooling rate of 10 C min1 and (b) subsequent heating scan at 10 C min1 for neat TPEE and TPEE/SB-ZHN nanocomposites.
reinforcing effect dominates in this temperature region. The Tg, which represents one of the major viscoelastic transitions of a material, is often obtained from the maximum of the tan d curve [21], which is shown in Fig. 10(b). No fundamental change in Tg is observed with the increasing SB-ZHN concentration. However, the tan d maximum values of the three nanocomposites are lower than that of neat TPEE, indicating the strong interaction at the interface between SB-ZHN and TPEE.
Table 1 Calorimetric data of the nonisothermal crystallization and second melting process for the neat TPEE and TPEE/SB-ZHN nanocomposites.
Fig. 8. Conceptual illustration of TPEE/SB-ZHN interaction to form the intercalated/ exfoliated nanocomposite.
Sample
Tc ( C)
DHc (J g1)
Tm ( C)
DHm (J g1)
TPEE TPEE/SB-ZHN1 TPEE/SB-ZHN3 TPEE/SB-ZHN5 TPEE/SB-ZHN7
167 170 172 175 177
26.3 28.1 24.6 20.0 18.7
206 196 196 194 194
20.8 24.3 21.6 17.9 16.5
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Table 2 MCC data of neat TPEE and TPEE/SB-ZHN nanocomposites (HRC: heat release capacity, 5 J g1 K1; PHRR: peak of heat release rate, 5 W g1; THR: total heat release, 0.1 kJ g1; TPHRR: temperature at PHRR, 2 C). Sample
HRC (J g1 K1)
PHRR (W g1)
THR (kJ g1)
TPHRR ( C)
TPEE TPEE/SB-ZHN1 TPEE/SB-ZHN3 TPEE/SB-ZHN5 TPEE/SB-ZHN7
552 325 272 235 245
553 327 270 236 244
20.9 19.8 18.6 17.5 17.6
420 416 415 414 414
3.5. Thermal combustion properties
Fig. 10. DMA measurements of neat TPEE and TPEE/SB-ZHN nanocomposites: (a) storage modulus and (b) tangent delta.
Thermal combustion properties of neat TPEE and TPEE/SB-ZHN nanocomposites were characterized by microscale combustion calorimeter (MCC). MCC is a thermal combustion analysis instrument that directly evaluates the fire hazards by measuring the heat of combustion of the gases evolved during controlled heating of milligram-sized samples [23]. The heat release rate curves from MCC testing are shown in Fig. 11 and the data are summarized in Table 2. TPEE is a flammable polymeric material which has high peak of heat release rate (PHRR), heat release capacity (HRC) and total heat release (THR). The HRR curves of TPEE nanocomposites become broad at low SB-ZHN concentration and two peaks appear at high SB-ZHN concentration. The temperature at PHRR slightly decreases with increasing SB-ZHN concentration. The results should be attributed to the decomposition of SB-ZHN. The PHRR value remarkably decreases with the increasing SB-ZHN concentration. For example, the PHRR values decrease from 553 W g1 for TPEE to 236 W g1 for TPEE/SB-ZHN5 with a reduction of around 57%. HRC is a molecular level flammability parameter that is a good predictor of flame resistance and fire behavior [24]. The trend in the variation of the HRC value is similar with that of PHRR. The HRC value for TPEE/SB-ZHN5 is also reduced by about 57% compared to the result of neat TPEE. Furthermore, the THR value steadily decreases with the increasing SB-ZHN concentration. These results show that the fire hazards of TPEE are noticeably reduced by the incorporation of SB-ZHN. This reduction may be attributed to decomposition of SB-ZHN, in which plenty of water vapor is released, cooling the pyrolysis area at the combustion surface.
4. Conclusions This work examined the use of layered zinc hydroxide nitrate (ZHN) modified with sodium benzoate as nanoparticle in thermoplastic polyester elastomer (TPEE). SB-ZHN was added into TPEE using solution blending method. The novel nanocomposites were characterized by XRD and TEM. The results showed that SB-ZHN was uniformly and disorderly dispersed in TPEE matrix, and partially exfoliated structure was formed. The thermal behavior, thermodynamic and thermal combustion properties of the novel nanocomposites were studied respectively through DSC, DMA and MCC. For the nanocomposite containing 7 wt% SB-ZHN, the crystallization temperature evaluated by DSC was increased by 10 C. The storage modulus at 95 C measured by DMA was improved by around 26%. The heat release capacity from MCC testing which is an indicator of a material fire hazard was reduced by about 56% (compared to the results of neat TPEE).
Acknowledgments
Fig. 11. HRR curves for neat TPEE and TPEE/SB-ZHN nanocomposites.
The work was financially supported by National Natural Science Foundation of China (No. 51036007), the Joint Fund of NSFC and
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Guangdong Province (No. U1074001) and Specialized Research Fund for the Doctoral Program of Higher Education (20103402110006). References [1] S.P. Newman, W. Jones, J. Solid State Chem. 148 (1999) 26. [2] S.P. Lonkar, S. Morlat-Therias, N. Caperaa, F. Leroux, J.L. Gardette, R.P. Singh, Polymer 50 (2009) 1505. [3] C.H. Tseng, H.B. Hsueh, C.Y. Chen, Compos. Sci. Technol. 67 (2007) 2350. [4] C. Nyambo, P. Songtipya, E. Manias, M.M. Jimenez-Gasco, C.A. Wilkie, J. Mater. Chem. 18 (2008) 4827. [5] W. Stählin, H.R. Oswald, Acta Crystallogr., Sect. B: Struct. Sci. 26 (1970) 860. [6] M. Louër, D. Louër, D. Grandjean, Acta Crystallogr., Sect. B: Struct. Sci. 29 (1973) 1696. [7] R. Marangoni, A. Mikowski, F. Wypych, J. Colloid Interface Sci. 351 (2010) 384. [8] S. Majoni, S.P. Su, J.M. Hossenlopp, Polym. Degrad. Stab. 95 (2010) 1593. [9] S. Cho, Y. Jang, D. Kim, T. Lee, D. Lee, Y. Lee, Polym. Eng. Sci. 49 (2009) 1456. [10] Y. Nagai, T. Ogawa, L.Y. Zhen, Y. Nishimoto, F. Ohishi, Polym. Degrad. Stab. 56 (1997) 115.
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Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Fabrication of thermoplastic polyester elastomer/layered zinc hydroxide nitrate nanocomposites with enhanced thermal, mechanical and combustion properties Wei Yang, Liyan Ma, Lei Song, Yuan Hu* State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, PR China
h i g h l i g h t s We We We We
prepare zinc hydroxide nitrate modified by sodium benzoate (SB-ZHN). prepare and characterize thermoplastic polyester elastomer/SB-ZHN nanocomposites. investigate the thermal and combustion properties of the nanocomposites. study the thermodynamic properties of the nanocomposites.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 January 2013 Received in revised form 5 May 2013 Accepted 27 May 2013
The objective of this study is to explore the potential of layered zinc hydroxide nitrate modified with sodium benzoate as nanoparticle in thermoplastic polyester elastomer (TPEE). The organically modified zinc hydroxide nitrate was compounded with TPEE using solution blending method. The nanocomposite structure was characterized by means of X-ray diffraction and transmission electron microscopy. The results showed that the nanoparticle was homogenously dispersed in TPEE matrix, and partially exfoliated structure was formed. The thermal behavior, mechanical and thermal combustion properties of the novel nanocomposite were studied respectively through differential scanning calorimeter (DSC), dynamic mechanical analysis (DMA) and microscale combustion calorimeter (MCC). For the nanocomposite containing 7 wt% nanoparticle, the crystallization temperature evaluated by DSC was increased by 10 C. The storage modulus at 95 C measured by DMA was improved by around 26%. The heat release capacity (an indicator of a material fire hazard) from MCC testing was reduced by about 56% (compared to the results of neat TPEE). Ó 2013 Published by Elsevier B.V.
Keywords: A. Elastomers A. Inorganic compounds A. Nanostructures D. Thermal properties D. Thermodynamic properties
1. Introduction Layered materials, including cationic clay, layered double hydroxide (LDH) and alpha-zirconium phosphate, have attracted considerable attention because of their ability to intercalate neutral guest molecules or to exchange interlayer inorganic or organic ions [1]. One of the most important features is that they can serve as inorganic filler to fabricate polymer based nanocomposites. LDH known as anionic clay is one of the most commonly investigated layered materials, which is related to the mineral hydrotalcite. The most important group of LDHs can be represented by the general n 2þ 3þ and formula ½M2þ ð1xÞ Mx ðOHÞ2 ½ððx þ 2Þ=nÞA $mH2 O, where M 3þ M are divalent and trivalent cations, respectively; x is equal to the
* Corresponding author. Tel./fax: þ86 551 63601664. E-mail address: [email protected] (Y. Hu). 0254-0584/$ e see front matter Ó 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.matchemphys.2013.05.074
ratio of M3þ/(M2þþM3þ) and A is an anion of valence n (2, 3) [1]. Up to now, LDHs have been widely used as filler materials to prepare polymer matrix nanocomposites to enhance various properties of polymeric materials, such as thermal, mechanical and fire performances [2e4]. It has recently been found that layered hydroxide salts (LHSs) are closely related to anionic clay and structurally similar to LDHs, which can also undergo anion exchange reactions. In general, the structure of LHSs can be simply denoted as the combination of brucite-like hydroxide layers and interlayer anions. For example, the structure of Zn5(OH)8(NO3)2$2H2O is composed of brucite-like layers in which one-fourth of the octahedral sites are vacant and tetrahedrally coordinated Zn2þ cations locate on either side of the vacant octahedral. Three vertices of the tetrahedron are occupied by hydroxyls, shared with the octahedral sheet, and the apex is occupied by a water molecule. The nitrate anions are present between the hydroxide layers and are surrounded by water molecules, but
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not coordinated to the zinc atoms [5e7]. Although the synthesis, characterization and application of LHSs are not extensively investigated as LDHs, these materials may offer more possibility of varying the identity of both the metal constituents and the exchangeable interlayer anion to enable tuning physic-chemical properties of the materials in order to optimize their performance for selected applications [8]. Polymer/layered material nanocomposites have been of considerable interest in the past decades because of their superior mechanical, physical, thermal and barrier properties. Layered materials are dispersed in polymer matrix to form exfoliated or intercalated structure nanocomposites. However, few reports have focused on the preparation and performance investigation of polymer/LHS nanocomposites. In Hossenlopp’s study [8], a zincbased LHS with a boron-containing exchangeable anion was used as flame retardant and compounded with PMMA using melt intercalation to produce PMMA nanocomposite in order to improve the flame retardant properties. Wypych et al. [7] reported that zinc hydroxide nitrate adsorbed with anions of colored dyes was applied as inorganic filler to prepare PVA nanocomposites and enhance their mechanical performance. Although several improved properties were achieved, well dispersion of LHS and exfoliated structure were not obtained in the reported articles. The commercial introduction of thermoplastic elastomers (TPEs) in recent years has triggered great interest in the investigation and application of these unusual materials, which can cover the boundary between rubbers and plastics. Among TPEs, thermoplastic polyester elastomer (TPEE) exhibits superior oil, chemical resistance and excellent low temperature impact properties, which have been widely used in automobile, electric and electronic application [9e11]. In order to further improve their thermal, mechanical and other properties, layered materials are always used to achieve these purposes. To the best of our knowledge, no work has been published on the use of LHSs layered nanoparticles into TPEE. Therefore, the objective of this study is to explore the potential of layered hydroxide salts modified with sodium benzoate as nanoparticles in TPEE. The thermal behavior, mechanical and thermal combustion properties of the novel nanocomposites will be studied respectively through differential scanning calorimeter, dynamic mechanical analysis and microscale combustion calorimeter. 2. Experimental 2.1. Materials Thermoplastic polyester elastomer (TPEE, H6555) (melt index: 12e15 g/10 min, Density: 1.20 g cm3) was provided by Sichuan Sunshine Plastics Co., Ltd, China. The chemical structure of TPEE was shown in Fig. 1. Sodium hydroxide, zinc nitrate hexahydrate, sodium benzoate, phenol and tetrachloroethane (analytical pure) were obtained from Sinopharm Chemical Reagent Co., Ltd, China. All chemicals were used without further purification. 2.2. Preparation of organo-modified layered zinc hydroxy nitrate The layered zinc hydroxy nitrate (ZHN) precursor was prepared according to reported literature [1]. In a typical experiment, ZHN
Fig. 1. Chemical structure of TPEE.
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with the ideal composition Zn5(OH)8(NO3)2$2H2O was prepared by precipitation from a 3.5 M solution of Zn(NO3)2$6H2O with 0.75 M NaOH. 50 ml of the NaOH solution was dropwise added to 20 ml of the Zn(NO3)2$6H2O solution (OH/Zn ¼ 0.5) with vigorous stirring at room temperature. The white product was immediately filtered following complete addition of the NaOH solution, washed with deionized water, and dried at 65 C for 24 h. Exchange of the interlayer nitrate anions in ZHN was prepared by the exchange reaction between nitrate anions and organic anions. 0.2 g ZHN was dispersed in 50 ml of 1 N aqueous solutions of sodium benzoate (SB) at room temperature. The mixtures were frequently stirred for 48 h at room temperature. The exchange product was filtered, washed with deionized water, and dried in air at 65 C for 24 h. The exchange product is referred herein as SB-ZHN.
2.3. Preparation of TPEE/SB-ZHN nanocomposites TPEE nanocomposites with different wt% SB-ZHN were prepared by the solution blending method. A desired amount of SB-ZHN was firstly refluxed in 50 ml blended solution (including 25 ml phenol and 25 ml tetrachloroethane) for 5 h under flowing N2. Subsequently 2 g TPEE was added into the above SB-ZHN suspension with stirring for 12 h. The solution was then dried under vacuum oven at 80 C for 48 h. The preparation conditions were same for each composition of hybrids containing 0 wt%, 1 wt%, 3 wt%, 5 wt%, and 7 wt% of SB-ZHN and were respectively designated as TPEE, TPEE/ SB-ZHN1, TPEE/SB-ZHN3, TPEE/SB-ZHN5, and TPEE/SB-ZHN7.
2.4. Characterizations of ZHN and TPEE/ZHN nanocomposites Fourier transform infrared (FTIR) spectra of the layered materials were obtained on a Nicolet 6700 FT-IR spectrophotometer using thin KBr disc. The transition mode was used and the wavenumber range was set from 4000 to 500 cm1. Thermogravimetric analysis (TGA) of the layered materials was carried out using a Q5000 IR thermogravimetric analyzer (TA Instruments Waters, China) at a linear heating rate of 20 C min1 under nitrogen atmosphere. The weight of all the samples were kept within 5e10 mg. Composites in an open Pt pan were tested under the flow rate of 6 105 m3 per minute at temperature ranging from room temperature to 700 C. X-ray diffraction (XRD) was performed using a Rigaku D/Max-rA rotating anode X-ray diffractometer equipped with a Cu K tube and Ni filter (¼0.1542 nm). Data acquisition was performed using a scanning speed of 3.0 min1. The patterns were recorded in the 2q range of 3.0 e60.0 for the layered materials and 1.0 e10.0 for the nanocomposites. The transmission electron microscopy (TEM) images of the nanocomposites were obtained on a JEOL JEM-2100F with an accelerating voltage of 200 kV. The nanocomposite specimens were cut into ultrathin slices under cryogenic conditions with an ultramicrotome (Ultraacut-1, United Kingdom) equipped with a diamond knife.
2.5. Thermal properties of TPEE/ZHN nanocomposites Thermal behaviors were studied by a NETZSCH DSC (204 F1 phoenix). All operations were carried out in a nitrogen atmosphere. Samples were heated from 50 C to 300 C at 10 C min1 and held at 300 C for 5 min to eliminate the thermal history, then cooled to 50 C at 10 C min1. They were maintained at 50 C for 5 min and heated to 300 C subsequently.
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2.6. Dynamic mechanical properties of TPEE/ZHN nanocomposites Dynamic mechanical analysis (DMA) was performed with the Perkin Elmer Pyris Diamond DMA from 95 C to 40 C at a heating rate of 5 C min1 with a frequency of 10 Hz in the tensile configuration. 2.7. Thermal combustion properties of TPEE/ZHN nanocomposites Thermal combustion properties of plastics were measured using a microscale combustion calorimeter (MCC, Govmark) according to ASTM D 7309-07. 4e8 mg of each sample was heated at 1 C s1 from 90 to 600 C and held there for 30 s. During pyrolysis, the volatilized decomposition products are transferred in the stream of nitrogen to a high-temperature combustion furnace where pure oxygen is added and the decomposition products are completely combusted. The amount of oxygen consumed is measured with an oxygen analyzer and used to calculate a heat release rate (HRR). All MCC data obtained were reproducible to about 3%. Fig. 3. XRD patterns of ZHN (a) and SB-ZHN (b).
3. Results and discussion 3.1. Characterization of the layered materials FTIR spectra of SB, ZHN and SB-ZHN are shown in Fig. 2. For both ZHN and SB-ZHN, the band of the hydroxy group appears in the region from 3100 cm1 to 3700 cm1. For ZHN, the peak at around 3518 cm1 and 3438 cm1 can be respectively assigned to stretching vibrations of free layer hydroxyl groups and layer hydroxyl groups that are hydrogen bonded to the interlayer nitrate anion or water molecules [12]. The assignments can also be used to describe the OH vibrations in SB-ZHN. After the exchange reaction, the characteristic asymmetric stretching vibration of nitrate ion at about 1373 cm1 in ZHN disappears and the new peaks at 1405 cm1, 1559 cm1 and 1601 cm1 appear. The strong peaks at 1405 cm1 and 1559 cm1 are attributed to the asymmetric and symmetric vibrations of C]O group [12,13]. The peak at 1601 cm1 should be assigned to the stretching vibration of CeC bond in benzene ring. These assignments are all characteristic absorption bands of sodium benzoate, as shown in trace (a). It is indicated that the nitrate group is completely exchanged with the benzoate anion. The XRD patterns of ZHN and SB-ZHN are presented in Fig. 3. Both compounds exhibit intense 001 reflection which are equally
spaced indicating that the compounds are layered and possess high range ordering, at least to the third order in the c direction [8]. The XRD pattern of ZHN shows an interlayer distance of 0.98 nm, which is in agreement with the reported values [12]. After exchange, the host layer reflections of ZHN are shifted from 8.98 and 18.07 to 4.65 and 9.26 , respectively, corresponding to an interlayer spacing of 1.90 nm for SB-ZHN (Fig. 4). The intercalation of sodium benzoate not only can enlarge the gallery between the platelets, but also can improve the interfacial properties between the SB-ZHN layers and polyester matrix. The absence of nitrate reflections indicates that the exchange is complete, consistent with the FTIR analysis. The TGA curves of ZHN and SB-ZHN are shown in Fig. 5, obtained at 20 C min1 under nitrogen condition. The decomposition of ZHN is characterized by two steps. The mass loss from 50 to 150 C corresponds to the loss of intercalated water. The other mass loss from 150 to 700 C is attributed to the dehydroxylation of layers. The resulting residue is ZnO. For SB-ZHN, the first mass loss from 50 C to approximately 300 C is the removal of intercalated water and dehydroxylation of layers. The second mass loss from 300 C to about 500 C should be assigned to the loss of impurity benzoate anion [1]. A small loss ranging from 500 C to around 550 C may be attributed to the release of benzene ring. 3.2. Characterization of TPEE/SB-ZHN nanocomposites
Fig. 2. FTIR spectra of SB (a), ZHN (b) and SB-ZHN (c).
The XRD patterns of SB-ZHN and TPEE/SB-ZHN nanocomposites are shown in Fig. 6. When SB-ZHN is compounded with TPEE matrix, the weak peaks of TPEE/SB-ZHN5 and TPEE/SB-ZHN7 are observed at lower 2 theta values. These reflections are broad and not well defined. It is indicated that the ordered layered structure of SB-ZHN is disrupted in TPEE matrix. The shifting of the first Bragg’s reflection to a lower scattering angle corresponds to an increase of the interlayer distance by about 1.07 nm. It reveals that the TPEE chains are aligned within the interlayer space SB-ZHN. XRD alone is insufficient to characterize the structure and dispersion of the TPEE/SB-ZHN nanocomposites. Transmission electron microscopy (TEM) is required to directly observe the dispersion of the additive. Fig. 7 shows the TEM images for TPEE/SB-ZHN7 at both low and high magnifications. The TEM image at low magnification in Fig. 7(a) exhibits the homogenous and uniform dispersion of SB-ZHN in TPEE matrix. At higher magnification, the TEM image clearly shows that many SB-ZHN nanosheets are disorderly dispersed in TPEE. At the
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Fig. 4. Schematic illustration of anion exchange in ZHN.
same time, the intercalated structure composed of multilayer tactoids, which are represented by the dark regions, are also observed in Fig. 7(b). The results from TEM analysis are in good agreement with the XRD patterns. It is thus concluded that the SB-ZHN layers are partially intercalated and partially exfoliated in the TPEE matrix through the solution blending processing (Fig. 8). 3.3. Thermal properties The nonisothermal crystallization and subsequent melt behaviors of neat TPEE and TPEE/SB-ZHN nanocomposites were studied by means of DSC. Fig. 9 shows the DSC curves recorded for the neat TPEE and TPEE/SB-ZHN nanocomposites with various SB-ZHN concentrations in the cooling scan at 10 C min1 and the subsequent heating scan at 10 C min1. The thermal properties obtained from DSC including crystallization temperature (Tc), crystallization enthalpy (DHc), melt temperature (Tm) and melt enthalpy (DHm) are summarized in Table 1. As shown in Fig. 9(a), the crystallization temperature (Tc) in TPEE/SB-ZHN nanocomposites slightly increases with the increasing SB-ZHN concentration. For the nanocomposite containing 7 wt% SB-ZHN, the Tc value evaluated by DSC is increased by 10 C in comparison with neat TPEE. The phenomenon is caused by the heterogeneous nucleation effect of SB-ZHN. In the subsequent heating scan, the multiple melt behaviors are observed in TPEE/SB-ZHN nanocomposites and the melt temperature (Tm) values are reduced compared with neat TPEE. The reason may be that the fusion of a certain amount of original crystals, followed by the recrystallization and final melting of more perfect crystals, partly formed during primary crystallization and partly formed
Fig. 5. TGA curves of ZHN (a) and SB-ZHN (b).
through the recrystallization process occurring during the heating scan [14e18]. On the other hand, both the crystallization enthalpy (DHc) and melt enthalpy (DHm) increase first then decrease with the increasing SB-ZHN concentration. It is indicated that the relative crystallinity of TPEE is reduced by the addition of SB-ZHN with high fraction. The results may be caused by the inability of polymer chains to be fully incorporated into the growing crystalline lamella and the limited space imposed on the polymer chains by a high number of platelets and/or tactoids [19,20]. 3.4. Dynamic mechanical properties Dynamic mechanical analysis (DMA) is one of the techniques commonly used to characterize the time, frequency and temperature dependency of the viscoelastic nature of polymers [21]. As shown in Fig. 10(a), the storage modulus in TPEE/SB-ZHN nanocomposites decreases first and then increases with the increasing SB-ZHN concentration in the low temperature region (below the glass transition temperature). For example, the storage modulus at 95 C for TPEE/SB-ZHN7 is improved by around 26% in comparison with neat TPEE. It is indicated that the TPEE/SB-ZHN nanocomposites show a softening effect at low SB-ZHN concentration, consistent with the reported literature [22]. This may be caused by the large fraction of benzoate anion compared with ZHN. A part of this organic anion adhered on the surface of the intercalated and exfoliated layers may be loosely bound leading to
Fig. 6. XRD patterns: (a) SB-ZHN; (b) neat TPEE; (c) TPEE/SB-ZHN1; (d) TPEE/SB-ZHN3; (e) TPEE/SB-ZHN5; (f) TPEE/SB-ZHN7.
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Fig. 7. TEM images of TPEE/SB-ZHN7: (a) low magnification; (b) high magnification.
the increasing of the mobility of the polymer matrix in the polymer-filler interfacial region. Due to the weak interaction at the interface, the plasticizing effect dominates at low SB-ZHN concentration in the low temperature region. Therefore, the storage modulus is reduced. In the high temperature region (above the glass transition temperature), the storage modulus increases with the increasing SB-ZHN concentration. This means that the Fig. 9. DSC curves of (a) nonisothermal crystallization at a cooling rate of 10 C min1 and (b) subsequent heating scan at 10 C min1 for neat TPEE and TPEE/SB-ZHN nanocomposites.
reinforcing effect dominates in this temperature region. The Tg, which represents one of the major viscoelastic transitions of a material, is often obtained from the maximum of the tan d curve [21], which is shown in Fig. 10(b). No fundamental change in Tg is observed with the increasing SB-ZHN concentration. However, the tan d maximum values of the three nanocomposites are lower than that of neat TPEE, indicating the strong interaction at the interface between SB-ZHN and TPEE.
Table 1 Calorimetric data of the nonisothermal crystallization and second melting process for the neat TPEE and TPEE/SB-ZHN nanocomposites.
Fig. 8. Conceptual illustration of TPEE/SB-ZHN interaction to form the intercalated/ exfoliated nanocomposite.
Sample
Tc ( C)
DHc (J g1)
Tm ( C)
DHm (J g1)
TPEE TPEE/SB-ZHN1 TPEE/SB-ZHN3 TPEE/SB-ZHN5 TPEE/SB-ZHN7
167 170 172 175 177
26.3 28.1 24.6 20.0 18.7
206 196 196 194 194
20.8 24.3 21.6 17.9 16.5
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Table 2 MCC data of neat TPEE and TPEE/SB-ZHN nanocomposites (HRC: heat release capacity, 5 J g1 K1; PHRR: peak of heat release rate, 5 W g1; THR: total heat release, 0.1 kJ g1; TPHRR: temperature at PHRR, 2 C). Sample
HRC (J g1 K1)
PHRR (W g1)
THR (kJ g1)
TPHRR ( C)
TPEE TPEE/SB-ZHN1 TPEE/SB-ZHN3 TPEE/SB-ZHN5 TPEE/SB-ZHN7
552 325 272 235 245
553 327 270 236 244
20.9 19.8 18.6 17.5 17.6
420 416 415 414 414
3.5. Thermal combustion properties
Fig. 10. DMA measurements of neat TPEE and TPEE/SB-ZHN nanocomposites: (a) storage modulus and (b) tangent delta.
Thermal combustion properties of neat TPEE and TPEE/SB-ZHN nanocomposites were characterized by microscale combustion calorimeter (MCC). MCC is a thermal combustion analysis instrument that directly evaluates the fire hazards by measuring the heat of combustion of the gases evolved during controlled heating of milligram-sized samples [23]. The heat release rate curves from MCC testing are shown in Fig. 11 and the data are summarized in Table 2. TPEE is a flammable polymeric material which has high peak of heat release rate (PHRR), heat release capacity (HRC) and total heat release (THR). The HRR curves of TPEE nanocomposites become broad at low SB-ZHN concentration and two peaks appear at high SB-ZHN concentration. The temperature at PHRR slightly decreases with increasing SB-ZHN concentration. The results should be attributed to the decomposition of SB-ZHN. The PHRR value remarkably decreases with the increasing SB-ZHN concentration. For example, the PHRR values decrease from 553 W g1 for TPEE to 236 W g1 for TPEE/SB-ZHN5 with a reduction of around 57%. HRC is a molecular level flammability parameter that is a good predictor of flame resistance and fire behavior [24]. The trend in the variation of the HRC value is similar with that of PHRR. The HRC value for TPEE/SB-ZHN5 is also reduced by about 57% compared to the result of neat TPEE. Furthermore, the THR value steadily decreases with the increasing SB-ZHN concentration. These results show that the fire hazards of TPEE are noticeably reduced by the incorporation of SB-ZHN. This reduction may be attributed to decomposition of SB-ZHN, in which plenty of water vapor is released, cooling the pyrolysis area at the combustion surface.
4. Conclusions This work examined the use of layered zinc hydroxide nitrate (ZHN) modified with sodium benzoate as nanoparticle in thermoplastic polyester elastomer (TPEE). SB-ZHN was added into TPEE using solution blending method. The novel nanocomposites were characterized by XRD and TEM. The results showed that SB-ZHN was uniformly and disorderly dispersed in TPEE matrix, and partially exfoliated structure was formed. The thermal behavior, thermodynamic and thermal combustion properties of the novel nanocomposites were studied respectively through DSC, DMA and MCC. For the nanocomposite containing 7 wt% SB-ZHN, the crystallization temperature evaluated by DSC was increased by 10 C. The storage modulus at 95 C measured by DMA was improved by around 26%. The heat release capacity from MCC testing which is an indicator of a material fire hazard was reduced by about 56% (compared to the results of neat TPEE).
Acknowledgments
Fig. 11. HRR curves for neat TPEE and TPEE/SB-ZHN nanocomposites.
The work was financially supported by National Natural Science Foundation of China (No. 51036007), the Joint Fund of NSFC and
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