Thermoplastic polyolefin based polymer – blend-layered double hydroxide nanocomposites

Thermoplastic polyolefin based polymer – blend-layered double hydroxide nanocomposites

Composites Science and Technology 68 (2008) 3234–3239 Contents lists available at ScienceDirect Composites Science and Technology journal homepage: ...

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Composites Science and Technology 68 (2008) 3234–3239

Contents lists available at ScienceDirect

Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech

Thermoplastic polyolefin based polymer – blend-layered double hydroxide nanocomposites T. Kuila a, S.K. Srivastava a,*, A.K. Bhowmick b, A.K. Saxena c a

Inorganic Materials and Nanocomposite Laboratory, Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, India Rubber Technology Centre, Indian Institute of Technology, Kharagpur 721302, India c Defence Materials and Stores Research and Development Establishment, Kanpur 208013, India b

a r t i c l e

i n f o

Article history: Received 23 April 2008 Received in revised form 7 August 2008 Accepted 9 August 2008 Available online 15 August 2008 Keywords: Blend Layered double hydroxide Nanocomposites B. Mechanical properties D. TGA

a b s t r a c t Layered double hydroxides (LDHs) are a new class of promising nanomaterials which improve the physicochemical properties of the polymer matrix. The present work deals with the preparation of nanocomposites of ethylene vinyl acetate (EVA-28)/low density polyethylene (LDPE) blend (1:1 by wt.) with varying amounts of organomodified layered double hydroxide (DS-LDH) by solution blending. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) have been used to find out the dispersion of Mg–Al nanolayers in EVA/LDPE blend. This confirms complete exfoliation of Mg–Al nanolayers in EVA-28/LDPE/DS-LDH nanocomposites. The measurements of mechanical properties of these nanocomposites show maximum tensile strength for 3 wt.% of DS-LDH content. However, elongation at break for the nanocomposites remains lower for entire filler loading with respect to neat EVA/LDPE blend. Thermogravimetric analysis shows successive improvement in the thermal stability behavior of the nanocomposites with increasing the concentration of DS-LDH. The limiting oxygen index values are also improved with increasing DS-LDH concentration. Swelling property measurement shows that solvent resistance properties of the nanocomposites are better at low filler loading. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Over the past decade, polymer/layered silicate nanocomposites have attracted a great deal of interest because of their remarkable improvement in mechanical properties, thermal stability, reduced gas permeability, and other physicochemical properties [1–5] compared to the neat polymer or conventional polymer composites. It is evident that a few percent of layered silicate dispersed in polymer matrix can significantly enhance various properties. Many pristine polymers have been used to prepare nanocomposites with layered silicate, but little work on polymer blend system has been carried out [6]. Polymer blend nanocomposites have combined properties of polymer blend and merit of polymer nanocomposite [7–10]. Recently, thermoplastic nanocomposites containing rigid inorganic layered silicate and soft elastomer blend seems to be the new approach in the engineering nanocomposites [7–15]. Layered double hydroxides (LDH) constitute one of the most promising nanomaterials as filler in the preparation of polymer nanocomposites [16–18]. The lamellar structure and anion exchange properties of LDHs make them attractive for technological application such as ion-exchangers, adsorbents, pharmaceutical

* Corresponding author. Tel.: +91 3222 283334; fax: +91 3222 255303. E-mail address: [email protected] (S.K. Srivastava). 0266-3538/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2008.08.003

stabilizers, and precursors of new catalytic materials [19]. The general composition of LDHs can be represented by the ideal formula [MII(1 x)MIIIx(OH)2]x+Am x/mnH2O, where MII is divalent cation such as Mg2+, Zn2+ etc., MIII is trivalent cations such as Al3+, Cr3+ etc. [19], and A is an anion with valency m (like Cl , CO3 2 , SO4 2 , and NO3 etc.). Structurally, LDH consists of cationic brucite-like layers [M2+(1 x)M3+x(OH)2]x+ in which M2+ is partially substituted by M3+, and anion as well as by water molecules (AxnH2O)x between the brucite-like layers. The strong electrostatic attraction operating between the hydroxide sheets and short inter-layer distance makes LDH materials difficult for the intercalation/exfoliation by the polymer matrix [20]. So, the pivotal prerequisite is to develop methods of preparing a stable uniform dispersion of inorganic phase into the polymer nanocomposite system. For this reason only, organomodification of pristine LDH is necessary. Ethylene vinyl acetate (EVA) copolymer is available as rubbers, thermoplastic elastomers, and plastics. It is used for various purposes, especially for electrical insulation, cable jacketing and repair, fabrication of midsole, which lies between insole and outsole of running shoes [21]. Recently, ethylene vinyl acetate copolymers are blended with different polymers, such as PP [11], styrene butadiene rubber (SBR) [12], ethylene propylene diene terpolymer (EPDM) [13], ethylene-1-butene copolymer (EtBC) [21], polystyrene [14], natural rubber [15], PE [15] etc. to increase the physical properties e.g. tensile strength, thermal stability, and

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compression set [21]. Among these polymers, polyethylene is used in a wide variety of applications because; it can be produced in many different forms. The most important type of PE, commercially exploited is called low density polyethylene (LDPE). It is soft and pliable and has applications ranging from plastic bags, containers, textiles, and electrical insulation, to coatings for packaging materials [22]. It is well known that EVA has been used for the modification of LDPE for better flexibility, toughness, and resistant to environmental stress cracking [23]. Detailed studies on the relaxation between morphology of EVA/LDPE blends and their mechanical properties, dynamic mechanical properties, and electrical resistance have already been reported [24,25–29]. Therefore, the present work is concerned with the preparation of EVA/LDPE nanocomposites using LDH as nanofiller and is expected to provide the blend with much better properties originating from the nano reinforcement of the filler under investigation.

2. Experimental 2.1. Materials EVA with 28 wt.% of vinyl acetate content was supplied by DuPont, India (Elvax 265; melt flow index 3 g/10 min, density 955 kg/m3 at 23 °C). LDPE was obtained from IPCL Baroda, India (24FSO40; MFI 4 g/10 min, density 922 kg/m3). Dicumyl peroxide (DCP, 98%, from Hercules, Inc., United States) was used to prepare the neat EVA/LDPE blend and the EVA/LDPE/DS-LDH nanocomposites. Mg(NO3)26H2O (E. Merck Ltd., India), Al(NO3)39H2O (E. Merck Ltd., India), NaOH (Quest Chemicals, Kolkata, India), and Na2CO3 (E. Merck Ltd., India) were used for the synthesis of pristine LDH. Sodium dodecyl sulfate (SDS) (SRL Pvt. Ltd., Mumbai, India) was used for the modification of pristine LDH. Toluene and Xylene were used as solvent and purchased from SRL, Mumbai, India. All the chemicals were used without further purification.

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2.4. Characterization and measurements X-ray diffraction (XRD) patterns were recorded on a PANalytical (PW 3040/60), ‘X’ Pert Pro using Cu Ka radiation (k = 0.1541 nm). Transmission electron microscopy (TEM) images were taken using JEM-2100 transmission electron microscope (JEOL, Japan) with acceleration voltage: 200 kV and bright field illumination. The tensile properties were measured on a Zwick/Roell Z010 at a strain rate of 100 mm/min at 25 ± 2 °C. The tensile fractured surface of the samples was gold coated under argon atmosphere and the fractured morphology was recorded at 25 ± 2 °C using scanning electron microscope (SEM), JEOL (JSM-5800) with an acceleration voltage of 20 kV. Thermal stability of neat EVA/LDPE blend and its nanocomposites with DS-LDH were measured on Redcroft 870 thermal analyzer, Perkin Elmer with a heating rate of 10 °C/min over a temperature range of 60–600 °C in air. The flame retardancy test of all the sample were carried out by the measurement of limiting oxygen index (LOI) value by the flammability tester (S.C. Dey Co., Kolkata) as per the standard ASTM D 2863-77. The solvent uptake capacity of the nanocomposites was determined using toluene at 25 °C. The gravimetric method (ASTM D 2765-95, Method C) was followed for the swelling measurements. 3. Results and discussion 3.1. Morphology of nanocomposites X-ray diffraction patterns of DS-LDH, neat EVA/LDPE blend and EVA/LDPE/DS-LDH composites with varying amount of DS-LDH loading are displayed in Fig. 1. The diffraction peak (0 0 1) at 2h = 3.2° corresponds to 2.75 nm basal spacing of DS-LDH layers. However, the d001 diffraction peaks disappeared completely for EVA/LDPE blend nanocomposites with 1, 3, and 5 wt.% DS-LDH content. This infers that the Mg–Al layers have been completely exfoliated in the EVA/LDPE matrix. However, XRD results cannot be the absolute evidence for the nanolayer exfoliation, because at

2.2. Preparation of Mg–Al LDH and DS-LDH The pristine LDH and DS intercalated LDH (DS-LDH) were prepared following our previous reported methods [20]. In this method 19.65 g Mg(NO3)2 and 9.25 g Al(NO3)3 were dissolved in 100 ml water. This metal nitrate solution was added to 100 ml water containing 2.65 g Na2CO3 under stirring condition at constant pH of 8–9 by adding 1 mole/liter aqueous NaOH solution. The resultant slurry obtained, aged at 70–75 °C for 12 h. Finally, the precipitate was filtered, washed by hot distilled water, and dried at room temperature for 24 h followed by vacuum drying at 80 °C for 24 h. The organomodified LDH (DS-LDH) was obtained from the rehydration of calcined LDH. In this process, 2 g of LDH sample was calcined at 500 °C for 6 h, and then suspended in 100 ml water containing 2 g SDS. The suspended solution was stirred for 12 h at 70 °C and then refluxed for another 6 h to yield a white powder of organophilic LDH (DS-LDH). 2.3. Preparation of EVA/LDH nanocomposites The nanocomposites of EVA/LDPE blend with different amount of DS-LDH (1, 3, 5, and 8 wt.%) were prepared by solution intercalation process. The method involved refluxing of desired amount of DS-LDH in 30 ml xylene at 100 °C for 6 h. Subsequently the dispersion so obtained was added to the solution of 15 g of EVA/LDPE already dissolved in 100 ml toluene and stirred vigorously for 6 h at 100 °C. Finally, DCP was added as curing agent to this solution and subsequently the solvent was removed under reduced pressure and the resultant composites were roll milled at room temperature followed by compression molding at 150 °C for 45 min.

Fig. 1. XRD spectra of (a) DS-LDH (b) pure EVA/LDPE blend and the nanocomposites of EVA/LDPE blend with (c) 1, (d) 3, (e) 5, and (f) 8 wt.% DS-LDH content.

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lower concentration of nanofiller the diffraction patterns may be largely affected. Therefore TEM observation has to be accompanied. When the DS-LDH content increases to 8 wt.%, a broad and weak diffraction peak appears at 2h = 2.6°, corresponding to an inter-layer distance of 3.4 nm. This observation suggests that an intercalated/exfoliated structure has been formed for higher amount of DS-LDH loading in the EVA/LDPE blend. Hsueh et al. [30] also observed similar type of intercalated/exfoliated polyimide/LDH nanocomposites at higher filler loading prepared from solution intercalation of polyamic acid (polyimide precursor) and LDH–amino benzoate using N,N-dimethylacetamide as a solvent. Morphological structures of exfoliated EVA/LDPE/DS-LDH nanocomposites are further studied by the TEM analysis. Fig. 2a–c displays the typical TEM images of the EVA/LDPE/DS-LDH nanocomposites with 3 and 8 wt.% DS-LDH content. The grayish white areas represent the EVA/LDPE matrix and black areas represent DS-LDH layers. It shows that the DS-LDH layers are mostly dispersed throughout the polymer matrix. Apparently, an inhomogeneous distribution of DS-LDH layers is evident in the EVA/LDPE matrix. However, TEM of this sample at higher magnification in Fig. 2b confirms that the LDH layers are disorderly oriented providing an ample evidence of crystal layer delamination/exfoliation from their surfaces. The thickness and lateral sizes of the exfoliated LDH layers are 8–10 nm and 30–40 nm, respectively. In addition,

there also exist some aggregates or stacked (marked by ‘‘A”) in this nanocomposites. This may be explained similar to the observation of Costa et al. [31] of the LDPE/Mg–Al layered double hydroxide nanocomposites. According to this, initially the polymer chains penetrate within the inter-layer region of the LDH particles and then push the metal hydroxide sheets apart from each other. However, with the course of intercalation, some LDH particles remain unaffected and appear as aggregates. In our case, this has been marked by ‘‘A” in the TEM images of the EVA/LDPE/DS-LDH nanocomposites. The TEM image (Fig. 2c) for 8 wt.% DS-LDH content clearly shows that the DS-LDH layers are aggregated in the EVA/ LDPE matrix. 3.2. Mechanical properties Fig. 3 shows the effect of DS-LDH content on the tensile properties, including tensile strength (TS) and elongation at break (EB) to all of the EVA/LDPE/DS-LDH nanocomposites. It is seen that the tensile strength for the nanocomposites is much higher with respect to neat EVA/LDPE blend and it is maximum for the nanocomposites with 3 wt.% of DS-LDH content. The strong chemical bonding between the hydroxyl group of DS-LDH and polar acetate group of EVA molecules results in good compatibility between these two phases [32]. As the loading of the DS-LDH is increased

Fig. 2. TEM images of (a) EVA/LDPE/DS-LDH (3 wt.%) nanocomposites at low magnification (b) EVA/LDPE/DS-LDH nanocomposites (3 wt.%) at high magnification, and (c) EVA/LDPE/DS-LDH (8 wt.%) nanocomposites.

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Fig. 3. Variation of tensile strength (TS) and Elongation at break (EB) with 1, 3, 5, and 8 wt.% DS-LDH contents in EVA/LDPE matrix.

Fig. 4. Stress vs. strain plot of (a) pure EVA/LDPE blend and the nanocomposites with (b) 1, (c) 3, (d) 5, and (e) 8 wt.% DS-LDH.

beyond 3 wt.%, the system shows a decrease in TS and is augmented for the nanocomposites with 8 wt.% of DS-LDH contents. This is due to the extended aggregation of DS-LDH particles in the EVA/LDPE matrix. It is also evident from Fig. 3 that the EB for the nanocomposites is much lower compared to neat EVA/LDPE blend. Such observations are exactly similar to the PE/Mg–Al LDH [18] and PE/organophilic-MMT [33–35] nanocomposites system. The typical stress–strain properties of neat EVA/LDPE blend and their nanocomposites are presented in Fig. 4. Interestingly, it shows that the modulus at different elongation percents increases systematically with increasing DS-LDH content from 1 to 5 wt.%. Such an improvement in the mechanical properties can also be accounted on the basis of static adhesion strength as well as interfacial stiffness because of the efficient stress transfer at the interface and elastic deformation originating from the large aspect ratio of the nanofiller [13]. However, on further addition of DS-LDH (8 wt.%), the tensile modulus is reduced due to the aggregation of DS-LDH nanolayers [36].

towards the crack propagation is different. Such type of rough surface morphology can be related with the improvement in mechanical properties of the nanocomposites [37].

3.3. Fracture surface morphology Fig. 5a and b displays SEM micrographs obtained from the tensile fracture surfaces of neat EVA-28/LDPE blend and its nanocomposites containing 3 wt.% of DS-LDH. The image of the fractured surface of neat blend shows relatively smooth surface. On the contrary, a cloud like rough fracture surface is obtained for the nanocomposites with 3 wt.% of DS-LDH content and the tendency

3.4. Thermogravimetric analysis Fig. 6 shows TG curves for the oxidative thermal degradation of EVA/LDPE/DS-LDH nanocomposites with varying amount of DSLDH content. Similar to neat EVA/LDPE blend, nanocomposites also undergo two-step decomposition. The first step corresponds to the emission of acetic acid, while the second step is associated with the polyethylenic main chain scission [3,38]. It is seen that the initial weight loss for the nanocomposites is accelerated, mainly due to the early degradation of DS molecules [39]. On the contrary, when 50% weight loss is selected as a point of comparison, the thermal decomposition temperatures for pure EVA/LDPE blend and their nanocomposites with 1, 3, 5, and 8 wt.% DS-LDH contents are 431, 437, 442, 442, and 445 °C, respectively. The increase in thermal stability is mainly attributed to the homogeneous dispersion of DS-LDH in the EVA/LDPE matrix, as depicted in the TEM images. During the course of thermal decomposition, the homogeneously dispersed DS-LDH forms a protective oxide layers on the nanocomposites surface which impedes the burning process by reducing the oxygen supply to the bulk and preventing the emission of degraded small gaseous molecules [40]. As a result, thermal stability of the nanocomposites increases with the progress of the thermal degradation process. This fact is supported well when 70% weight loss is

Fig. 5. SEM images of the tensile fracture surface of (a) pure EVA/LDPE blend, and (b) EVA/LDPE/ (3 wt.%) DS-LDH nanocomposites.

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of char layer during LOI test measurements for the nanocomposites, whereas no char formation is observed in case of pure EVA/ LDPE blend. Presence of char layer in the EVA/LDPE/DS-LDH nanocomposites acts as a barrier and prevents flame. According to Costa et al. [31], at low filler loading, the char layer can not provide efficient barrier effect as its thickness is very small. However, on increasing DS-LDH content in EVA/LDPE, the thickness of char layer is sufficient enough to prevent burning. In addition, formation of water vapors due to the endothermic decomposition of LDH is also likely to produce sufficient smoke impeding thereby the burning of the nanocomposites. 3.6. Swelling properties

Fig. 6. TGA curves of (a) pure EVA/LDPE blend and the nanocomposites of EVA/LDPE blend with (b) 1, (c) 3, (d) 5, and (e) 8 wt.% DS-LDH content.

selected as a point of comparison. At this stage, the decomposition temperature of pure EVA/LDPE blend and their above mentioned nanocomposites are 442, 446, 453, 456, and 460 °C, respectively. It is also noted that above 500 °C, pure EVA/LDPE blend shows almost no residue left. But the nanocomposites with 1, 3, 5, and 8 wt.% DS-LDH content shows about 0.16, 1.1, 2.5, and 3.5 wt.% of char residue respectively. Formation of char residue is mainly attributed to the decomposition of LDH materials. Interestingly, the presence of such char residues on the surface of the nanocomposites enhances directly the thermal stability of the nanocomposites and may find applications as fire safety materials [40].

Fig. 8 shows the influence of DS-LDH in the solvent (toluene) uptake properties on EVA-28/LDPE/DS-LDH nanocomposites. It is observed that the solvent uptake capacity decreases for EVA-28/ LDPE blend nanocomposites containing 1 and 3 wt.% of DS-LDH. This is possibly due to the nano level dispersion of LDH particles offering whole surface area available for the interaction between the polymer and DS-LDH. Indeed, this strong interfacial interaction between the polymer chains and DS-LDH layers inhibits the penetration of toluene molecules from surface to bulk region of the nanocomposites leading to reduction of toluene uptake [41]. Additionally, the high aspect ratio of LDH layers possessing the excellent barrier properties by the formation of polymerbridging flocculation in solvent also cannot be ruled out. Meanwhile, the increase in solvent uptake at higher DS-LDH contents may be due to the aggregation of DS-LDH layers in the polymer matrix. 4. Conclusion

Limiting oxygen index (LOI) values provides the first hand information regarding the effectiveness of fire-retardant materials [31]. Fig. 7 shows the effects of different contents of DS-LDH on the LOI values of the EVA/LDPE/DS-LDH nanocomposites. It is evident that the LOI values of the nanocomposites are relatively much higher with respect to neat EVA/LDPE blend and the value increases with increasing DS-LDH content. A maximum value of LOI is recorded for the nanocomposites with 8 wt.% of DS-LDH content and it is 23.3 compared to 16.3 for pure blend. This is due to the formation

The EVA/LDPE/DS-LDH nanocomposites have been prepared using solution intercalation technique. XRD analysis shows that fully delaminated structure has formed at lower DS-LDH content in EVA/LDPE matrix, whereas partially exfoliated structure is observed for higher filler loading. Molecular level dispersion of DS-LDH in the polymer matrix is observed by TEM analysis. Mechanical and thermal analysis show that the as prepared nanocomposites possess improved mechanical and thermal properties. Swelling property analysis shows that the solvent resistance property of the nanocomposites is improved with respect to neat EVA/LDPE blend.

Fig. 7. Effect of DS-LDH content on limiting oxygen index of EVA/LDPE/DS-LDH nanocomposites.

Fig. 8. Effect of DS-LDH content on toluene uptake of EVA/LDPE/DS-LDH nanocomposites.

3.5. Effects of DS-LDH on the LOI values

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Acknowledgement Authors are thankful to CSIR and DRDO, India for the financial support.

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