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clay nanocomposites prepared by new in situ Ziegler–Natta catalyst
Materials and Design 30 (2009) 2309–2315
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Investigation of properties of polyethylene/clay nanocomposites prepared by new in situ Ziegler–Natta catalyst S. Javan Nikkhah, A. Ramazani S.A. *, H. Baniasadi, F. Tavakolzadeh Polymer Group, Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran
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Article history: Received 9 September 2008 Accepted 13 November 2008 Available online 28 November 2008 Keywords: Nanocomposite (B) In situ polymerization (C) Mechanical property (E)
a b s t r a c t This paper is devoted to investigation of morphological and physical–mechanical properties of polyethylene (PE)/clay nanocomposites prepared via in situ polymerization method using bi-supported Ziegler– Natta catalyst. Bentonite type clay and MgCl2 (ethoxide type) were used as the support of TiCl4. Catalyst support and polymerization process have been done in slurry phase using Triisobutylaluminum as the cocatalyst. The microstructure of the nanocomposites was examined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD and TEM indicated that almost fully exfoliated PE/clay nanocomposites were produced successfully using this method. According to permeability measurements, it was found that oxygen permeability values of the nanocomposite samples prepared with in situ polymerization method were dropped more than 200% introducing only 1 wt% clay to polymeric matrix. Differential scanning calorimetry (DSC) results indicated that the crystallization temperatures of samples are significantly higher than that of virgin PE. Moderate thermal stability enhancement of in situ polymerized nanocomposites was confirmed using thermogravimetric analysis (TGA).The storage modulus, Young’s modulus and tensile strength of prepared samples were increased where the toughness was declined slightly. It seems that good dispersion and exfoliation of clay during polymerization should be responsible to get more effective reinforcing properties for clay in this method comparing to melt blending method for preparation of polyethylene nanocomposites. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Over the last decade, preparation and characterization of organic–inorganic nanocomposites is the subject of numerous research strivings [1–12]. They are of interest; both from fundamental academic research aspect and industrial applications, due to their potentials to advanced properties at very low filler concentrations often a fraction that is so much less than typically required with conventional fillers [1,4]. Major reason of these improvement results from large surface area per unit volume [5]. Among the different nanoparticles used in polymer nanocomposites, clay has been extensively used, because of low cost and good mechanical properties [6–8]. In addition, the ability of the silicate particles to disperse into individual layers in polymeric matrix and ability to fine tune their surface reactions, result in excellent mechanical and physical property enhancements of polymer/clay nanocomposites [6,9]. The efficiency of reinforcement depends on the filler aspect ratio, the filler mechanical properties and adhesion between the matrix and the filler. The typical enhanced properties include tensile/flexural strength, modulus, heat deflection temperature, thermal sta-
* Corresponding author. Tel./fax: +98 21 66165431. E-mail address: [email protected] (A. Ramazani S.A.). 0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.11.019
bility, flame retardancy, barrier property, electrical resistance, and so on [3,5]. It is worth mentioning that the enhancement of all these characteristics occurs only if the clay particles are exfoliated or at least delaminated in to single layers or tactoids of a small number of layers. Delimitation or exfoliation is the result of the incorporation of macromolecules between the layers of the particles [9,10]. PE is a plastic with the world’s highest production volume owing to its excellent processability, high volume production plants, high chemical resistance, low dielectric constant, high safety, and low production cost. PE has found a variety of application such as packaging, consumer good, pipes, durable equipment and industrial machinery. Every year over 60 million tons of PE is produced worldwide [10,11]. However due to limitation of PE such as low stiffness and tensile strength, owning to its inherent chemical nature, this polymer need to be improved to extend its engineering applications [10,12]. In order to improve this weakness, the intercalation–exfoliation method gets applied to PE to prepare PE/clay nanocomposites. In order to prepare PE/clay nanocomposites, melt blending [13–15] and in situ polymerization [16–20] methods have been used. In melt blending method exfoliated or delamination is occurred only if the thermomechanical stress is well transmitted from the melt to the layered particles. Generally good adhesion
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between matrix and particles is then a necessary condition for delamination or exfoliation in the first method. When polymer melt and filler are not incompatible (e.g. a nonpolar matrix and polar particles) the use of an adhesion promoter is necessary [10]. In general, there are two ways to modify the surface of inorganic particulates. The first one is carried out through surface absorption or reaction with small molecules, such as silane coupling agent. The second method is based on grafting polymeric molecules through covalent bonding to the hydroxyl groups existing on the particles [21,22]. For polyolefins, such as PE and PP, which do not include any polar group in their backbone, silicate layers even modified by above two methods are polar and incompatible with polyolefin [23]. Therefore in situ polymerization, because of the clay dispersion during polymerization, is most effective method to prepare exfoliated PE/clay nanocomposites compared with melt blending method [24,25]. The discovery of Ziegler–Natta catalysts in 1953 revolutionized the industry of polyolefins manufacture [26]. A large number of reports have focused on the polymerization of olefins using Ziegler– Natta catalysts [27–29]. A variety of compounds have been used to enhance the performance of TiCl4-based Z–N catalyst. These catalysts comprise a TiCl4 catalyst on a specially prepared MgCl2 support. The most effective support has resulted from the use of MgCl2. Electron donor of various types is incorporated in to the MgCl2 support and also as a ‘‘selectivity control agent”. Electron donors are most frequently aromatic esters [30,31]. Due to the low polarity of PE and difficulty of entering of PE long chains in to the intergalleries of clay layers, it is only possible to prepare intercalated PE/clay nanocomposites with melt blending method. But in in situ polymerization method, small gas monomer molecules can easily introduce in to the intergalleries to achieve to the prepared catalyst active sites and produce polymer chains. The produced polymer between clay layers by this method can increase d-spacing and generate intercalated or even exfoliated structure. Although in recent years, some publications are devoted to report preparation of in situ PE/clay nanocomposites via Ziegler–Natta or Metallocene catalysts, but rarely one can find a publication devoted to investigation of mechanical properties of these nanocomposites. So, this work is devoted to preparation and investigation of physical–mechanical morphological, and barrier properties of these composites. Relation of the mechanical properties of produced nanocomposites with their morphological structure is also considered in this work.
2. Experimental 2.1. Materials Polymerization grade ethylene from Arak Petrochemical Co. after purification by passage through columns of activated 13X and 4A type molecular sieves has been used. Triisobutylaluminum (TIBA) (96%) supplied by Fluca chemika Co. was diluted using nhexane prior to use. Argon with purity of 99 .999% was provided by Arkan Gas Co. and was purified by passage through columns of P205, KOH and activated 13X/4A type molecular sieves. Industrial garde of n-hexane and toluene were distilled with sodium benzophenole complex and then used as diluent. Diisobutyl phthalate (DIBP) as internal donor was produced by Merck. Titanium tetrachloride (TiCl4, 0.8 mol/l solutions in hexane, 99%) was from Leder Co. Magnesium ethoxide (Mg (OEt) 2, 95%) was supplied by Fluca chemika Co. Bentonite clay (Kunimine Industries Co.,Tokyo, Japan), with 200-mesh sieve, cation exchange capacity (CEC) 115 meq/100 g clay, was used as nanofiller.
2.2. Polymerization of PE/clay nanocomposites The catalyst was prepared according to reference [32]. Ethylene was polymerized in hexane by slurry polymerization procedures in the Buchi (1 l) type reactor at the pressure and temperature of 7 bar and 60 °C, respectively. TIBA was used as the co-catalyst. The prepared catalyst was injected into the reactor after hexane and co-catalyst. Hydrogen was used as chain transfer agent in polymerization system. At the end of the polymerization period (1 h), the polymer was washed with ethanol several times. Then filtered and dried in a vacuum oven at 70 °C for 24 h. Standard tensile test samples and notch Izod bars of in situ prepared nanocomposites were formed by injection molding machine from nanocmposite granules prepared by a Coperion Werner and Peleidere twinscrew extruder having co-rotating intermeshing screws with D = 250 mm, L/D = 40 mm. Molding was conducted at a barrel temperature of 180 °C and injection pressure of 80 bar. 2.3. Measurements and characterizations 2.3.1. Morphological analysis The degree of intercalation, exfoliation, and dispersion has been traditionally characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). While both are effective tools, they are limited in that they only probe a small volume of the sample and can be costly for routine characterization of nanocomposites. Furthermore, neither XRD nor TEM alone can accurately describe the levels of clay dispersion and polymer nanocomposite structure. Multiple techniques (usually XRD, TEM combined with another technique) are needed for nanocomposite analysis to properly understand what type of nanocomposite has been made, as no one technique can adequately describe the system [33,34]. The d-spacing of the clay, supported clay and the obtained nanocomposites were examined with X-ray diffractometer (XRD, X’pert PRO MRD, Philips, Netherlands) using Cu Ka radiation (k = 1.5406 Å) at a generator voltage of 40 kV and generator current of 40 mA. Scanning was in 0.02° at a rate of 1°/s. The interlayer spacing (d001) of Bentonite was calculated in accordance with Bragg equation: 2 d sin h = k. TEM analysis was used to confirm the morphological information obtained from the XRD data on the platelet dispersion and distribution. Because the XRD analysis alone does not have the potency to characterize the unconditional morphology. The absence of scattered intensity peaks in the XRD diagrams cannot always surely demonstrate achievement of the disorderedintercalated or the exfoliated structures [33]. Ultra-thin sections measuring approximately 70 nm were ultra-microtomed by Richert OM-U3 (Austria) ultra-microtome using a diamond knife and samples were placed on copper grids to be viewed under Philips CM200 with an accelerating voltage of 200 kV. 2.3.2. Permeability measurements To determine oxygen permeability of nanocomposite films, pure gases were fed from high pressure capsules by a tube to a membrane cell which was composed of two plates with internal diameter of 90 mm. The capsules had control valves which were regulated at 5 105 Pa (5 bar). The gas then entered the cell. The membrane set between the cell plates and was belted by pairs of bolts and nuts. The gas after passage from the cell was connected to a holler of soap water to create soap bubble. A burette was adjoined to the holder from the top. Permeability of gases calculated from
P¼
NL ; DP
ð1Þ
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where, N is the gas volumetric flow rate per unit area of the membrane (cm3 (STP)/(s cm2)), L the thickness of membrane (cm), DP the pressure difference across the membrane (cmHg), P is the permeability (cm3 (STP).cm/(s cm2 cmHg)). 2.3.3. Mechanical properties The tensile and impact bars were prepared using injection molding machine (Eckert & Ziegler GmbH injection molding machine). The tensile test was performed according to ASTM D638-81 with a universal testing instrument (Instron 1185, UK). The tensile test speed was 60 mm/min. The Izod impact test was performed according to GB/T 1043-93 with impact tester XJJ-5, Japan instrument. Both tests were carried out at room temperature. Fig. 1. XRD patterns of clay and supported clay (catalyst).
2.3.4. Dynamic mechanical thermal analysis (DMTA) Dynamic mechanical properties of PE/clay nanocomposites were measured using a dynamic mechanical thermal analyzer (DMA-Triton, Model: Teritec 2000 DMA) according to ASTM E1640-04 with a strain of 0.02%. The temperature range was between 50 and 160 °C with a heating rate of 5 °C/min. The frequency used was 1.0 Hz. The size (length width thickness) of the test samples was 10 7 0.3 mm3. 2.3.5. Thermal analysis The melting and crystallization temperatures, as well as heat of fusion and crystallization of the samples were measured on a Perkin–Elmer, Pyris 1DSC model differential scanning calorimeter (DSC), under a nitrogen atmosphere. The samples were first heated to 250 °C for 2 min to eliminate their thermal history and subsequently cooled to 50 °C at a rate of 10 °C/min. The second endotherm was recorded by heating at 10 °C/min. The degree of crystallinity of the samples was calculated using the equation X % ¼ ðDHf =DHf Þ 100%, where DHf is the heat of fusion determined by DSC, and DHf is the heat of fusion of pure PE, which has the value 293 J/g [31].
Fig. 2. XRD patterns of prepared nanocomposites (3 and 5 wt%.).
3. Results and discussion
layer spacing is about 2 nm. It indicates that clay successfully exfoliates in to polymer matrix. On the other hand, in Fig. 3b, one can observe that layers of clay are clearly exfoliated in the polymer matrix. Typical layers are 100–300 nm in length and 1 nm in thickness. These results agree well with those of XRD. It can be concluded from both TEM and XRD measurements that exfoliated PE/clay nanocomposites were prepared by in situ polymerization method. The completely homogeneous dispersion of nanoparticles in the PE matrix is critical for the enhancement of the performance of PE/clay nanocomposites, because if the matrix consists of aggregate of particles, the stress concentration in the vicinity of the aggregate will be high, causing in easier crack initiation and propagation, and consequent premature failure.
3.1. The morphology
3.2. Mechanical properties
Fig. 1 shows the XRD spectra of the clay, supported clay and the prepared nanocomposites. The peak in XRD patterns corresponds to the (0 0 1) reflection peak of layered silicate. In Fig. 1, the d-spacing of the clay calculated using Bragg’s law is about 11.5 nm. For the supported clay, the characteristic (0 0 1) peak of the clay shifted to a lower angle of 2h = 5.872, which corresponds to d-spacing of 15.03 nm. It seems that, because of ionexchanging of chloride ions of catalyst with the clay cations, the parallel stacking of nanofiller is partially disrupted. In Fig. 2, PE/ clay nanocomposites with 3 and 5 wt% of clay do not have the characteristic plane peak, which indicates the exfoliation of the layered silicates. The XRD patterns do not change with clay content. TEM micrographs obtained from prepared PE/clay nanocomposites which contains 3 wt% of clay are shown in Fig. 3. In the low magnification image (Fig. 3a) one can see that reasonably good dispersion of the clay in the polymer has been obtained. The high magnification image indicates the stacked silicate layers. The average size of the stacked tactoid includes two or three layers and the
Fig. 4 presents the stress–strain curves of neat PE and prepared nanocomposite specimens tested at room temperature. Clearly, the brittle-ductile behavior of the prepared nanocomposites is different as compared with neat PE. Neat PE sample fractured in a very ductile behavior where creeping, necking and large cold-drawing can be observed in its strain–stress curve. The stress–strain curves of prepared nanocomposites show proof of major yielding followed by elastic region where necking has not been occurred. The mechanical properties, including the Tensile strength (a), Young modulus (b), elongation at break (c) and impact strength (d) of PE and prepared nanocomposites by in situ polymerization method have been plotted in Figs. 5–8. Young’s modulus of the samples was determined from the initial slope of the elastic region. As shown in Figs. 5 and 6, tensile strength and Young modulus of prepared samples were increased by adding a small amount of filer. According to Fig. 7, elongation at break drops from 700% to 260%, when clay loading increases to 5%. Due to having an increased tendency for better dispersion of clay during polymerization in to the PE matrix, a strong solid-like clay network was
2.3.6. Thermogravimetry (TGA) The thermogravimetries were carried out according to ASTM E1131-03 in nitrogen (50 CC/min) in PL-1500 using 2 mg samples, which were heated under a nitrogen atmosphere from ambient temperature up to 650 °C at a heating rate of 20 °C/min.
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Fig. 3. TEM micrograph of prepared nanocomposite: (a) low magnification and (b) high magnification.
Fig. 4. Stress–strain curves of PE and nanocomposite samples.
Fig. 6. Young modulus as a function of clay loading.
Fig. 5. Tensile strength as a function of clay loading. Fig. 7. Elongation at break as a function of clay loading.
composed and therefore resulted in considerably good tensile properties in prepared nanocomposite. Also, the notched Izod impact strength of prepared nanocomposites was affected by clay content. The impact strength (toughness) is generally demonstrated the energy dissipating incidents that occurs in the vicinity of the notch [9]. According to Fig. 8, the impact strength decreases by raising clay loads. Because of strong polymer/clay network, resulting more brittle failure behavior, the levels of dissipation energy and consequently the impact strength values of nanocomposites are lower than that of virgin PE, depending to clay content and its morphology.
3.3. Dynamic mechanical thermal analysis (DMTA) Dynamic mechanical properties can specify useful information about the viscoelastic behavior of the investigated samples. For this analysis, virgin PE, in situ nanocomposites samples with 1, 3 and 5 wt% clay loadings were investigated. Fig. 9a depicts the storage modulus (E0 ) as a function of temperature for the prepared samples. These samples exhibit similar decreasing E0 procedures with increasing temperature. The increase in segmental polymer
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tion region. This evident E0 reinforcement is ascribed to the nanoscaled dispersion of clay within the PE matrix. This result was caused by the strong solid-like behavior of in situ polymerized PE/clay network. Fig. 9b shows the loss tangent (tan d) versus temperature for the samples. The peak, corresponding to the glass transition temperature of PE, is perceived for each sample. By increasing the clay contents of samples, peak temperatures (i.e., glass transition temperatures) slightly increased. In addition, heights of the peak which is accordance to energy dissipation due to polymer chain motion, decayed by raising the concentration of clay in PE matrix. 3.4. Thermogravimetric analysis (TGA) Fig. 8. Impact strength as a function of clay loading.
Generally, the clay has two opponent effects in the thermal stability of the polymer/clay nanocomposites. The barrier effect which should improve the thermal stability and the promoter effect which would encourage the degradation process and decrease the thermal stability. By adding a low fraction to the polymer matrix, the clay layers should be well dispersed, the barrier effect is predominant, but with increasing loading, the promoter effect rapidly rises and becomes impressible, so that the thermal stability of the nanocomposites decreases. TGA data for PE, in situ PE nanocomposite samples with deferent clay contents are collected in Table 1 and shown in Fig. 10. The parameters listed are the onset temperature, at which 10% degradation occurs, T0.1, the mid-point of the degradation, T0.5, another indication of thermal stability. The nanocomposites contain 1% and 3% clay actually show an onset temperature which is higher than that of PE itself and increased by clay loading, which means the barrier effect begins to have an influence on the degradation at this level. When the loading of clay reaches 5%, the onset temperature is decreased, suggesting the promoter effect has an influence on thermal stability. Also, these
Fig. 9. The (a) storage modulus, and (b) tan d of the in situ polymerization prepared nanocomposites.
chain movement with temperature was caused this behavior. The storage modulus curves of the samples have three regions: glassy, glass transition and rubbery. In the 40–70 °C rang, the dramatic E0 drop related the glass transition region of PE. By increasing the clay loading from 1 to 5 wt%, the PE/clay nanocomposite samples showed higher E0 values, both in below and above the glass transi-
Fig. 10. Weight loss curves of the samples as a function of the temperature under air.
Table 1 TGA data of PE and PE/clay nanocomposites. Sample
T0.1 (°C)b
T0.5 (°C)c
PE In situ-PECNa-1% In situ-PECN-3% In situ-PECN-5%
272 292 305 285
312 326 356 342
a b c
Polyethylene/clay nanocomposite. The onset temperature at which 10% degradation occurs. The mid-point of the degradation.
Fig. 11. DSC cooling scan thermo grams of PE and its nanocomposites.
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Table 2 DSC result of virgin PE and prepared nanocomposites. Sample
Crystallization temperature (Tc, °C)
Melting temperature (Tm, °C)
Heat of fusion (J/g)
Crystallinity (%)
PE In situ PECNa-3% In situ PECN-5%
113.3 118.8 122.6
138.6 139.8 140.2
186.06 174.80 155.38
63.5 59.6 53.03
a
PE/clay nanocomposite.
Table 3 The gas permeability results of virgin PE and in situ prepared nanocomposites. Sample
Height of bubble rising (cm)
Time (s)
Na 106
Sample film thickness (cm)
DP (cmHg)
P 109 cm3(STP) cm/(s cm2 cmHg)
PE In situ PENCb-1% In situ PENC-3% In situ PENC-5%
0.5 0.5 0.5 0.3
2700 4800 5700 6230
22.6 12.72 10.71 5.88
0.02 0.028 0.029 0.035
387.6 418 425.6 395.2
1.67 0.852 0.713 0.52
a b
Burette and cell surface area are 1 and 8.1898 cm2, respectively. PE nanocomposite.
effects can be observed on the mid-point of degradation, T0.5, of prepared samples. 3.5. Crystallization and structural characteristics Fig. 11 presents the DSC cooling scan thermograms of virgin PE and PE nanocomposites. Crystallization behavior of virgin PE and PE/clay nanocomposites (melting and crystallization temperature, heat of fusion and crystallinity) obtained from DSC experiments summarized in Table 2. As seen from this table the crystallization temperature of virgin PE is 113.3 °C; while 3 wt% clay addition increases this temperature up to 118.8 °C. This is ascribed to the nucleating effect of nanoclay. The melting points of PE appeared to be unaffected on reinforcement with clay. However, the heat of fusion and the crystallinity of PE/clay nanocomposites decreased by introducing clay to polymeric matrix. It seems that the filler network plays a role to limit the motion of molecular chains. 3.6. The gas permeability The gas permeability results of virgin PE and prepared PE/clay nanocomposites are shown in Fig. 12. These results also are summarized in Table 3. As shown in this table, the barrier properties of nanocomposites films were improved dramatically (e.g. dropped 50% with introducing 1 wt% clay to polymeric matrix) and this improvement was pursued with increasing clay loading. These suggest that most of clay layers were oriented vertically against the direction of oxygen flow.
Fig. 12. The gas permeability results of Virgin PE and in situ prepared nanocomposites.
4. Conclusions PE/clay nanocomposites were prepared by in situ polymerization of ethylene with TiCl4/MgCl2 (ethoxid type)/clay bi-supported catalyst. TEM and XRD results reveal the complete exfoliation dispersion of clay during polymerization. Loading the polymer with nanoclays dramatically improved the gas barrier properties. With 1 wt% clay, the oxygen permeability decreased about 50%. The improvement of tensile strength, Yong’s and storage modulus and the decreased in impact strength for in situ nanocomosite samples in low concentration of clay indicates that clay should have exfoliated structure in these nanocomposites. Considerable enhancement in thermal stability of polyethylene by introducing very low clay loading also indicate that in addition to exfoliated structure for clay, dispersion of clay should be also very good in prepared nanocomposites. References [1] Chiu FC, Lai SM, Chen YL, Lee TH. Investigation on the polyamide6/organoclay nanocmposites with or without a maleated polyolefin elastomer as a tougher. Polymer 2005;46:11600–9. [2] Wagener R, Reisinger TJG. A rheological method to compare the degree of exfoliation of nanocomposites. Polymer 2003;44:7513–8. [3] Sheng N, Boyce MC, Parks DM, Rutledge GC, Abes JI, Cohen RE. Multiscale micromechanical modeling of polymer/clay nanocomposites and the effective clay particle. Polymer 2004;45:487–506. [4] Fornnes TD, Paul DR. Modeling properties of nylon 6/clay nanocomposites using composite theories. Polymer 2003;44:4993–5013. [5] Luo JJ, Daniel IM. Characterization and modeling of mechanical behavior of polymer/clay nancomposites. Compos Sci Technol 2003;63:1607–16. [6] Deshmane C, Yuan Q, Misra RDK. High strength-toughness combination of melt intercalated nanoclay-reinforced thermoplastic olefins. Mater Sci Eng A 2007;460–461:277–87. [7] Yang IK, Hu CC. Preparation and rheological characterization of poly(nbutyl methacrylate)/montmorillonite composites. Eur Polym J 2006;42:402–9. [8] Yuan Q, Misra RDK. Impact fracture behavior of clay-reinforced polypropylene nanocomposites. Polymer 2006;47:4421–33. [9] Dasari A, Yu ZZ, Mai YW. Effect of blending sequence on microstructure of ternary nanocomposites. Polymer 2005;46:5986–91. [10] Incarnato L, Scarfato P, Scatteia L, Acierno D. Rheological behavior of new melt compounded copolyamide nanocomposites. Polymer 2004;45:3487–96. [11] Wang M. Master of science dissertation. Morphological studies of polyethylene. University of Alberta, 2006. [12] Kurauchi T, Okada A, Nomura T, Nishio T, Saegusa S, Deguchi R. Nylon 6-clay hybrid-synthesis, properties and application to automotive timing belt cover SAE Technical Paper Ser 1991, 910584. [13] Kawasumi M, Hasegawa N, Kato M. Preparation and mechanical properties of polypropylene–clay hybrids. Macromolecules 1997;30:6333–8. [14] Hasegawa N, Kawasumi M, Kato M. Preparation and mechanical properties of polypropylene–clay hybrids using a maleic anhydride-modified polypropylene oligomer. J Appl Polym Sci 1998;67:87–92.
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Investigation of properties of polyethylene/clay nanocomposites prepared by new in situ Ziegler–Natta catalyst S. Javan Nikkhah, A. Ramazani S.A. *, H. Baniasadi, F. Tavakolzadeh Polymer Group, Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 9 September 2008 Accepted 13 November 2008 Available online 28 November 2008 Keywords: Nanocomposite (B) In situ polymerization (C) Mechanical property (E)
a b s t r a c t This paper is devoted to investigation of morphological and physical–mechanical properties of polyethylene (PE)/clay nanocomposites prepared via in situ polymerization method using bi-supported Ziegler– Natta catalyst. Bentonite type clay and MgCl2 (ethoxide type) were used as the support of TiCl4. Catalyst support and polymerization process have been done in slurry phase using Triisobutylaluminum as the cocatalyst. The microstructure of the nanocomposites was examined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD and TEM indicated that almost fully exfoliated PE/clay nanocomposites were produced successfully using this method. According to permeability measurements, it was found that oxygen permeability values of the nanocomposite samples prepared with in situ polymerization method were dropped more than 200% introducing only 1 wt% clay to polymeric matrix. Differential scanning calorimetry (DSC) results indicated that the crystallization temperatures of samples are significantly higher than that of virgin PE. Moderate thermal stability enhancement of in situ polymerized nanocomposites was confirmed using thermogravimetric analysis (TGA).The storage modulus, Young’s modulus and tensile strength of prepared samples were increased where the toughness was declined slightly. It seems that good dispersion and exfoliation of clay during polymerization should be responsible to get more effective reinforcing properties for clay in this method comparing to melt blending method for preparation of polyethylene nanocomposites. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Over the last decade, preparation and characterization of organic–inorganic nanocomposites is the subject of numerous research strivings [1–12]. They are of interest; both from fundamental academic research aspect and industrial applications, due to their potentials to advanced properties at very low filler concentrations often a fraction that is so much less than typically required with conventional fillers [1,4]. Major reason of these improvement results from large surface area per unit volume [5]. Among the different nanoparticles used in polymer nanocomposites, clay has been extensively used, because of low cost and good mechanical properties [6–8]. In addition, the ability of the silicate particles to disperse into individual layers in polymeric matrix and ability to fine tune their surface reactions, result in excellent mechanical and physical property enhancements of polymer/clay nanocomposites [6,9]. The efficiency of reinforcement depends on the filler aspect ratio, the filler mechanical properties and adhesion between the matrix and the filler. The typical enhanced properties include tensile/flexural strength, modulus, heat deflection temperature, thermal sta-
* Corresponding author. Tel./fax: +98 21 66165431. E-mail address: [email protected] (A. Ramazani S.A.). 0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.11.019
bility, flame retardancy, barrier property, electrical resistance, and so on [3,5]. It is worth mentioning that the enhancement of all these characteristics occurs only if the clay particles are exfoliated or at least delaminated in to single layers or tactoids of a small number of layers. Delimitation or exfoliation is the result of the incorporation of macromolecules between the layers of the particles [9,10]. PE is a plastic with the world’s highest production volume owing to its excellent processability, high volume production plants, high chemical resistance, low dielectric constant, high safety, and low production cost. PE has found a variety of application such as packaging, consumer good, pipes, durable equipment and industrial machinery. Every year over 60 million tons of PE is produced worldwide [10,11]. However due to limitation of PE such as low stiffness and tensile strength, owning to its inherent chemical nature, this polymer need to be improved to extend its engineering applications [10,12]. In order to improve this weakness, the intercalation–exfoliation method gets applied to PE to prepare PE/clay nanocomposites. In order to prepare PE/clay nanocomposites, melt blending [13–15] and in situ polymerization [16–20] methods have been used. In melt blending method exfoliated or delamination is occurred only if the thermomechanical stress is well transmitted from the melt to the layered particles. Generally good adhesion
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between matrix and particles is then a necessary condition for delamination or exfoliation in the first method. When polymer melt and filler are not incompatible (e.g. a nonpolar matrix and polar particles) the use of an adhesion promoter is necessary [10]. In general, there are two ways to modify the surface of inorganic particulates. The first one is carried out through surface absorption or reaction with small molecules, such as silane coupling agent. The second method is based on grafting polymeric molecules through covalent bonding to the hydroxyl groups existing on the particles [21,22]. For polyolefins, such as PE and PP, which do not include any polar group in their backbone, silicate layers even modified by above two methods are polar and incompatible with polyolefin [23]. Therefore in situ polymerization, because of the clay dispersion during polymerization, is most effective method to prepare exfoliated PE/clay nanocomposites compared with melt blending method [24,25]. The discovery of Ziegler–Natta catalysts in 1953 revolutionized the industry of polyolefins manufacture [26]. A large number of reports have focused on the polymerization of olefins using Ziegler– Natta catalysts [27–29]. A variety of compounds have been used to enhance the performance of TiCl4-based Z–N catalyst. These catalysts comprise a TiCl4 catalyst on a specially prepared MgCl2 support. The most effective support has resulted from the use of MgCl2. Electron donor of various types is incorporated in to the MgCl2 support and also as a ‘‘selectivity control agent”. Electron donors are most frequently aromatic esters [30,31]. Due to the low polarity of PE and difficulty of entering of PE long chains in to the intergalleries of clay layers, it is only possible to prepare intercalated PE/clay nanocomposites with melt blending method. But in in situ polymerization method, small gas monomer molecules can easily introduce in to the intergalleries to achieve to the prepared catalyst active sites and produce polymer chains. The produced polymer between clay layers by this method can increase d-spacing and generate intercalated or even exfoliated structure. Although in recent years, some publications are devoted to report preparation of in situ PE/clay nanocomposites via Ziegler–Natta or Metallocene catalysts, but rarely one can find a publication devoted to investigation of mechanical properties of these nanocomposites. So, this work is devoted to preparation and investigation of physical–mechanical morphological, and barrier properties of these composites. Relation of the mechanical properties of produced nanocomposites with their morphological structure is also considered in this work.
2. Experimental 2.1. Materials Polymerization grade ethylene from Arak Petrochemical Co. after purification by passage through columns of activated 13X and 4A type molecular sieves has been used. Triisobutylaluminum (TIBA) (96%) supplied by Fluca chemika Co. was diluted using nhexane prior to use. Argon with purity of 99 .999% was provided by Arkan Gas Co. and was purified by passage through columns of P205, KOH and activated 13X/4A type molecular sieves. Industrial garde of n-hexane and toluene were distilled with sodium benzophenole complex and then used as diluent. Diisobutyl phthalate (DIBP) as internal donor was produced by Merck. Titanium tetrachloride (TiCl4, 0.8 mol/l solutions in hexane, 99%) was from Leder Co. Magnesium ethoxide (Mg (OEt) 2, 95%) was supplied by Fluca chemika Co. Bentonite clay (Kunimine Industries Co.,Tokyo, Japan), with 200-mesh sieve, cation exchange capacity (CEC) 115 meq/100 g clay, was used as nanofiller.
2.2. Polymerization of PE/clay nanocomposites The catalyst was prepared according to reference [32]. Ethylene was polymerized in hexane by slurry polymerization procedures in the Buchi (1 l) type reactor at the pressure and temperature of 7 bar and 60 °C, respectively. TIBA was used as the co-catalyst. The prepared catalyst was injected into the reactor after hexane and co-catalyst. Hydrogen was used as chain transfer agent in polymerization system. At the end of the polymerization period (1 h), the polymer was washed with ethanol several times. Then filtered and dried in a vacuum oven at 70 °C for 24 h. Standard tensile test samples and notch Izod bars of in situ prepared nanocomposites were formed by injection molding machine from nanocmposite granules prepared by a Coperion Werner and Peleidere twinscrew extruder having co-rotating intermeshing screws with D = 250 mm, L/D = 40 mm. Molding was conducted at a barrel temperature of 180 °C and injection pressure of 80 bar. 2.3. Measurements and characterizations 2.3.1. Morphological analysis The degree of intercalation, exfoliation, and dispersion has been traditionally characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). While both are effective tools, they are limited in that they only probe a small volume of the sample and can be costly for routine characterization of nanocomposites. Furthermore, neither XRD nor TEM alone can accurately describe the levels of clay dispersion and polymer nanocomposite structure. Multiple techniques (usually XRD, TEM combined with another technique) are needed for nanocomposite analysis to properly understand what type of nanocomposite has been made, as no one technique can adequately describe the system [33,34]. The d-spacing of the clay, supported clay and the obtained nanocomposites were examined with X-ray diffractometer (XRD, X’pert PRO MRD, Philips, Netherlands) using Cu Ka radiation (k = 1.5406 Å) at a generator voltage of 40 kV and generator current of 40 mA. Scanning was in 0.02° at a rate of 1°/s. The interlayer spacing (d001) of Bentonite was calculated in accordance with Bragg equation: 2 d sin h = k. TEM analysis was used to confirm the morphological information obtained from the XRD data on the platelet dispersion and distribution. Because the XRD analysis alone does not have the potency to characterize the unconditional morphology. The absence of scattered intensity peaks in the XRD diagrams cannot always surely demonstrate achievement of the disorderedintercalated or the exfoliated structures [33]. Ultra-thin sections measuring approximately 70 nm were ultra-microtomed by Richert OM-U3 (Austria) ultra-microtome using a diamond knife and samples were placed on copper grids to be viewed under Philips CM200 with an accelerating voltage of 200 kV. 2.3.2. Permeability measurements To determine oxygen permeability of nanocomposite films, pure gases were fed from high pressure capsules by a tube to a membrane cell which was composed of two plates with internal diameter of 90 mm. The capsules had control valves which were regulated at 5 105 Pa (5 bar). The gas then entered the cell. The membrane set between the cell plates and was belted by pairs of bolts and nuts. The gas after passage from the cell was connected to a holler of soap water to create soap bubble. A burette was adjoined to the holder from the top. Permeability of gases calculated from
P¼
NL ; DP
ð1Þ
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where, N is the gas volumetric flow rate per unit area of the membrane (cm3 (STP)/(s cm2)), L the thickness of membrane (cm), DP the pressure difference across the membrane (cmHg), P is the permeability (cm3 (STP).cm/(s cm2 cmHg)). 2.3.3. Mechanical properties The tensile and impact bars were prepared using injection molding machine (Eckert & Ziegler GmbH injection molding machine). The tensile test was performed according to ASTM D638-81 with a universal testing instrument (Instron 1185, UK). The tensile test speed was 60 mm/min. The Izod impact test was performed according to GB/T 1043-93 with impact tester XJJ-5, Japan instrument. Both tests were carried out at room temperature. Fig. 1. XRD patterns of clay and supported clay (catalyst).
2.3.4. Dynamic mechanical thermal analysis (DMTA) Dynamic mechanical properties of PE/clay nanocomposites were measured using a dynamic mechanical thermal analyzer (DMA-Triton, Model: Teritec 2000 DMA) according to ASTM E1640-04 with a strain of 0.02%. The temperature range was between 50 and 160 °C with a heating rate of 5 °C/min. The frequency used was 1.0 Hz. The size (length width thickness) of the test samples was 10 7 0.3 mm3. 2.3.5. Thermal analysis The melting and crystallization temperatures, as well as heat of fusion and crystallization of the samples were measured on a Perkin–Elmer, Pyris 1DSC model differential scanning calorimeter (DSC), under a nitrogen atmosphere. The samples were first heated to 250 °C for 2 min to eliminate their thermal history and subsequently cooled to 50 °C at a rate of 10 °C/min. The second endotherm was recorded by heating at 10 °C/min. The degree of crystallinity of the samples was calculated using the equation X % ¼ ðDHf =DHf Þ 100%, where DHf is the heat of fusion determined by DSC, and DHf is the heat of fusion of pure PE, which has the value 293 J/g [31].
Fig. 2. XRD patterns of prepared nanocomposites (3 and 5 wt%.).
3. Results and discussion
layer spacing is about 2 nm. It indicates that clay successfully exfoliates in to polymer matrix. On the other hand, in Fig. 3b, one can observe that layers of clay are clearly exfoliated in the polymer matrix. Typical layers are 100–300 nm in length and 1 nm in thickness. These results agree well with those of XRD. It can be concluded from both TEM and XRD measurements that exfoliated PE/clay nanocomposites were prepared by in situ polymerization method. The completely homogeneous dispersion of nanoparticles in the PE matrix is critical for the enhancement of the performance of PE/clay nanocomposites, because if the matrix consists of aggregate of particles, the stress concentration in the vicinity of the aggregate will be high, causing in easier crack initiation and propagation, and consequent premature failure.
3.1. The morphology
3.2. Mechanical properties
Fig. 1 shows the XRD spectra of the clay, supported clay and the prepared nanocomposites. The peak in XRD patterns corresponds to the (0 0 1) reflection peak of layered silicate. In Fig. 1, the d-spacing of the clay calculated using Bragg’s law is about 11.5 nm. For the supported clay, the characteristic (0 0 1) peak of the clay shifted to a lower angle of 2h = 5.872, which corresponds to d-spacing of 15.03 nm. It seems that, because of ionexchanging of chloride ions of catalyst with the clay cations, the parallel stacking of nanofiller is partially disrupted. In Fig. 2, PE/ clay nanocomposites with 3 and 5 wt% of clay do not have the characteristic plane peak, which indicates the exfoliation of the layered silicates. The XRD patterns do not change with clay content. TEM micrographs obtained from prepared PE/clay nanocomposites which contains 3 wt% of clay are shown in Fig. 3. In the low magnification image (Fig. 3a) one can see that reasonably good dispersion of the clay in the polymer has been obtained. The high magnification image indicates the stacked silicate layers. The average size of the stacked tactoid includes two or three layers and the
Fig. 4 presents the stress–strain curves of neat PE and prepared nanocomposite specimens tested at room temperature. Clearly, the brittle-ductile behavior of the prepared nanocomposites is different as compared with neat PE. Neat PE sample fractured in a very ductile behavior where creeping, necking and large cold-drawing can be observed in its strain–stress curve. The stress–strain curves of prepared nanocomposites show proof of major yielding followed by elastic region where necking has not been occurred. The mechanical properties, including the Tensile strength (a), Young modulus (b), elongation at break (c) and impact strength (d) of PE and prepared nanocomposites by in situ polymerization method have been plotted in Figs. 5–8. Young’s modulus of the samples was determined from the initial slope of the elastic region. As shown in Figs. 5 and 6, tensile strength and Young modulus of prepared samples were increased by adding a small amount of filer. According to Fig. 7, elongation at break drops from 700% to 260%, when clay loading increases to 5%. Due to having an increased tendency for better dispersion of clay during polymerization in to the PE matrix, a strong solid-like clay network was
2.3.6. Thermogravimetry (TGA) The thermogravimetries were carried out according to ASTM E1131-03 in nitrogen (50 CC/min) in PL-1500 using 2 mg samples, which were heated under a nitrogen atmosphere from ambient temperature up to 650 °C at a heating rate of 20 °C/min.
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Fig. 3. TEM micrograph of prepared nanocomposite: (a) low magnification and (b) high magnification.
Fig. 4. Stress–strain curves of PE and nanocomposite samples.
Fig. 6. Young modulus as a function of clay loading.
Fig. 5. Tensile strength as a function of clay loading. Fig. 7. Elongation at break as a function of clay loading.
composed and therefore resulted in considerably good tensile properties in prepared nanocomposite. Also, the notched Izod impact strength of prepared nanocomposites was affected by clay content. The impact strength (toughness) is generally demonstrated the energy dissipating incidents that occurs in the vicinity of the notch [9]. According to Fig. 8, the impact strength decreases by raising clay loads. Because of strong polymer/clay network, resulting more brittle failure behavior, the levels of dissipation energy and consequently the impact strength values of nanocomposites are lower than that of virgin PE, depending to clay content and its morphology.
3.3. Dynamic mechanical thermal analysis (DMTA) Dynamic mechanical properties can specify useful information about the viscoelastic behavior of the investigated samples. For this analysis, virgin PE, in situ nanocomposites samples with 1, 3 and 5 wt% clay loadings were investigated. Fig. 9a depicts the storage modulus (E0 ) as a function of temperature for the prepared samples. These samples exhibit similar decreasing E0 procedures with increasing temperature. The increase in segmental polymer
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tion region. This evident E0 reinforcement is ascribed to the nanoscaled dispersion of clay within the PE matrix. This result was caused by the strong solid-like behavior of in situ polymerized PE/clay network. Fig. 9b shows the loss tangent (tan d) versus temperature for the samples. The peak, corresponding to the glass transition temperature of PE, is perceived for each sample. By increasing the clay contents of samples, peak temperatures (i.e., glass transition temperatures) slightly increased. In addition, heights of the peak which is accordance to energy dissipation due to polymer chain motion, decayed by raising the concentration of clay in PE matrix. 3.4. Thermogravimetric analysis (TGA) Fig. 8. Impact strength as a function of clay loading.
Generally, the clay has two opponent effects in the thermal stability of the polymer/clay nanocomposites. The barrier effect which should improve the thermal stability and the promoter effect which would encourage the degradation process and decrease the thermal stability. By adding a low fraction to the polymer matrix, the clay layers should be well dispersed, the barrier effect is predominant, but with increasing loading, the promoter effect rapidly rises and becomes impressible, so that the thermal stability of the nanocomposites decreases. TGA data for PE, in situ PE nanocomposite samples with deferent clay contents are collected in Table 1 and shown in Fig. 10. The parameters listed are the onset temperature, at which 10% degradation occurs, T0.1, the mid-point of the degradation, T0.5, another indication of thermal stability. The nanocomposites contain 1% and 3% clay actually show an onset temperature which is higher than that of PE itself and increased by clay loading, which means the barrier effect begins to have an influence on the degradation at this level. When the loading of clay reaches 5%, the onset temperature is decreased, suggesting the promoter effect has an influence on thermal stability. Also, these
Fig. 9. The (a) storage modulus, and (b) tan d of the in situ polymerization prepared nanocomposites.
chain movement with temperature was caused this behavior. The storage modulus curves of the samples have three regions: glassy, glass transition and rubbery. In the 40–70 °C rang, the dramatic E0 drop related the glass transition region of PE. By increasing the clay loading from 1 to 5 wt%, the PE/clay nanocomposite samples showed higher E0 values, both in below and above the glass transi-
Fig. 10. Weight loss curves of the samples as a function of the temperature under air.
Table 1 TGA data of PE and PE/clay nanocomposites. Sample
T0.1 (°C)b
T0.5 (°C)c
PE In situ-PECNa-1% In situ-PECN-3% In situ-PECN-5%
272 292 305 285
312 326 356 342
a b c
Polyethylene/clay nanocomposite. The onset temperature at which 10% degradation occurs. The mid-point of the degradation.
Fig. 11. DSC cooling scan thermo grams of PE and its nanocomposites.
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Table 2 DSC result of virgin PE and prepared nanocomposites. Sample
Crystallization temperature (Tc, °C)
Melting temperature (Tm, °C)
Heat of fusion (J/g)
Crystallinity (%)
PE In situ PECNa-3% In situ PECN-5%
113.3 118.8 122.6
138.6 139.8 140.2
186.06 174.80 155.38
63.5 59.6 53.03
a
PE/clay nanocomposite.
Table 3 The gas permeability results of virgin PE and in situ prepared nanocomposites. Sample
Height of bubble rising (cm)
Time (s)
Na 106
Sample film thickness (cm)
DP (cmHg)
P 109 cm3(STP) cm/(s cm2 cmHg)
PE In situ PENCb-1% In situ PENC-3% In situ PENC-5%
0.5 0.5 0.5 0.3
2700 4800 5700 6230
22.6 12.72 10.71 5.88
0.02 0.028 0.029 0.035
387.6 418 425.6 395.2
1.67 0.852 0.713 0.52
a b
Burette and cell surface area are 1 and 8.1898 cm2, respectively. PE nanocomposite.
effects can be observed on the mid-point of degradation, T0.5, of prepared samples. 3.5. Crystallization and structural characteristics Fig. 11 presents the DSC cooling scan thermograms of virgin PE and PE nanocomposites. Crystallization behavior of virgin PE and PE/clay nanocomposites (melting and crystallization temperature, heat of fusion and crystallinity) obtained from DSC experiments summarized in Table 2. As seen from this table the crystallization temperature of virgin PE is 113.3 °C; while 3 wt% clay addition increases this temperature up to 118.8 °C. This is ascribed to the nucleating effect of nanoclay. The melting points of PE appeared to be unaffected on reinforcement with clay. However, the heat of fusion and the crystallinity of PE/clay nanocomposites decreased by introducing clay to polymeric matrix. It seems that the filler network plays a role to limit the motion of molecular chains. 3.6. The gas permeability The gas permeability results of virgin PE and prepared PE/clay nanocomposites are shown in Fig. 12. These results also are summarized in Table 3. As shown in this table, the barrier properties of nanocomposites films were improved dramatically (e.g. dropped 50% with introducing 1 wt% clay to polymeric matrix) and this improvement was pursued with increasing clay loading. These suggest that most of clay layers were oriented vertically against the direction of oxygen flow.
Fig. 12. The gas permeability results of Virgin PE and in situ prepared nanocomposites.
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