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TiCl4 for in situ synthesis of polypropylene nanocomposites
Applied Clay Science 183 (2019) 105362
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Research Paper
Ziegler-Natta catalyst produced from MgCl2/organically modified Mt/DI/ TiCl4 for in situ synthesis of polypropylene nanocomposites
T
Renata da Silva Cardoso, Jaqueline da Silva Oliveira, Maria de Fátima Vieira Marques
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Instituto de Macromoleculas, Universidade Federal do Rio de Janeiro, IMA-UFRJ, Cidade Universitária. Av. Horacio Macedo, 2030, Centro de Tecnologia, Bloco J, Brazil
ARTICLE INFO
ABSTRACT
Keywords: Ziegler-Natta catalyst Nanocomposite in situ polymerization Polypropylene Clay mineral Montmorillonite
Spherical Ziegler-Natta catalysts containing clay mineral are a more favorable way for the synthesis of nanocomposites and masterbatches of polyolefins. With the application of in situ polymerization technique, it was possible to overcome challenges such as intercalation/exfoliation of clay mineral, avoiding lamellae reaggregation in order to obtain materials with superior properties than the currently available ones, such as gas barrier and thermal resistance. In this study, support precursors and catalysts were prepared by chemical route with sodium clay mineral (Mt, montmorillonite) modified with different amounts of ammonium quaternary salt. The preparation of the catalyst precursor was performed using the mass ratios of MgCl2 to modified sodium Mt of 1:1 and 1:2 and it was possible to observe different thermal decomposition profiles compared to the standard catalytic support precursor (adduct MgCl2.EtOH) prepared as a reference. The catalysts obtained therefrom maintained a spherical morphology and X-ray diffractions (XRD) exhibit peak shift showing an increase of interlayer space of the Mt In order to obtain nanocomposites and masterbatches of polypropylene/modified Mt, the polymerization reaction was conducted in different reaction times. Polypropylene nanocomposites presented the high thermal degradation temperature (459 °C) and isotacticity (98%). XRD and transmission electron microscopy (TEM) analyses revealed exfoliated and intercalated morphologies even with high Mt contents in the polypropylene matrix.
1. Introduction Great attention has been paid to the preparation of polypropylene nanocomposites using nanoparticles, especially clay minerals, carbon nanotubes, graphene, silica and metal oxides due to the resulting dramatic modification in the physical and mechanical properties. However, the great challenge remains to disperse nanomaterials into polymers matrices, especially into hydrophobic polyolefin as polypropylene (PP). Since nanomaterials tend to agglomerate and/or may exhibit incompatibility with the organic polymer matrix, many strategies have been studied. To profit from the properties of the filler, a strong interface between filler and the polymeric matrix, and also a homogeneous distribution of the individual particles (or layered silicates or fibers) are necessary (Kaminsky, 2018). Among the techniques used to produce nanocomposites (melt processing, solution, and direct synthesis), in situ polymerization is more attractive because the dispersion obtained is the most efficient, particularly in the fully exfoliated polyolefin/filler (load) nanocomposite formation (Abedi and Abdouss, 2014). In the last one, Ziegler-Natta (ZN) catalysts are extensively used, as widely reported in the literature
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(Ma et al., 2001; Rong et al., 2001; Yang et al., 2003; Haddadi et al., 2018; Dong and Qin, 2019a, 2019b). Through the last technique, silicate sheets are swollen and dispersed in the liquid monomer (or a monomer solution) and the polymer synthesis can occur into the interlayer space. Another more interesting way is to introduce transition metal catalyst through cationic exchange inside the sheets of the clay mineral or by mixing it with support powders. In the latter case, it is possible to maintain the morphology of the support. The shape of catalysts and resultant nanocomposites is less investigated, although morphology is one of the most influential considerations in the preparation of industrial Ziegler-Natta catalysts (Zhang et al., 2016a, 2016b). The shape and size of the catalyst particle make it possible to perform reactions in flow reactors with the spherical shape of granules of the polymer product being preserved (Nifant'ev et al., 2016). A wide range of systems was investigated in order to promote polymer intercalation into the clay mineral interlayer space, with variables being molecular weight of the PP, maleic anhydride content in the compatibilizer, clay mineral modification and content, etc. This last approach can be done by ion-exchange reactions of the interlayer
Corresponding author at: Av. Horacio Macedo, 2030, CEP 21941-598 Rio de Janeiro, RJ, Brazil. E-mail address: [email protected] (M.d.F. Vieira Marques).
https://doi.org/10.1016/j.clay.2019.105362 Received 5 July 2019; Received in revised form 4 November 2019; Accepted 5 November 2019 0169-1317/ © 2019 Elsevier B.V. All rights reserved.
Applied Clay Science 183 (2019) 105362
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Fig. 1. Scheme to represent the synthesis of catalyst in the space of ammonium salt modified Mt
inorganic cations with organic cationic surfactants, including primary, secondary, tertiary and quaternary alkylammonium compounds (Gordana, 2014). Therefore, it is necessary to change its surface to increase hydrophobic character, which increases compatibility with the polymer and at the same time increases the interlayer space of the clay mineral, which facilitates exfoliation. The advantages of using clay mineral compared to other nanomaterials are abundance in nature, low cost, facility for chemical modification and good chemical and thermal stability. Among them, montmorillonite (Mt) is the most widely chosen filler, mainly due to its swelling property leading to exfoliation and high interlayer ion exchange capacity (Tong et al., 2014). Additionally, layered nanofillers, such as Mt, usually improve the thermal stability of the polymer because of a physical barrier effect that retards the diffusion of degradation products, gases, and heat (Dittrich et al., 2013). It is now well-known that the properties of the nanocomposites depend significantly on the characteristics of the polymer matrices, the nature and chemical surface of the clay mineral, as well as the preparation methods (Liu et al., 2018). In our previous study (Cardoso et al., 2018), commercial organophilic Mt was used to prepare spherical support based on the adduct of MgCl2.EtOH/claytone HY. The Ziegler-Natta catalyst produced was effective for the synthesis of nanocomposites and masterbatches as revealed by TEM and XRD. Therefore, in this work, spherical support based on the adduct MgCl2.EtOH/Mt was prepared in order to produce Ziegler-Natta catalyst with different Mt contents. In order to organophilize the sodium Mt, different amounts of hexadecyltrimethylammonium bromide (CTAB) were used. This study proposed to achieve a point that the amount of salt was sufficient to increase interlayer space of Mt without compromising the activity of the inorganic catalyst. Ziegler-Natta catalyst based on MgCl2/Mt/internal donor (ID)/TiCl4 with the addition of external donor (ED) and alkyl aluminum was evaluated in propylene polymerizations and different reaction times to produce Mt polymer nanocomposites (CPN) and masterbatches with high amounts of Mt Thus, we studied the effect of using organophilized Mt in the structure of support precursor and catalyst, on activity catalytic and properties of CPN by in situ polymerization.
triethylaluminium (TEA) (10 mass% solution in hexane, obtained from Akzo Nobel, USA), anhydrous MgCl2 (Toho Catalyst Co. LTD, Japan), inorganic montmorillonite Argel 40 (Bentonit União Nordeste S.A), the ammonium salt was hexadecyltrimethylammonium bromide (CTAB) (Pharma Special, Brazil). 2.2. Chemical treatment of montmorillonite Montmorillonite (A40) (cation exchange capacity: 1000 cmol/kg) was modified with the quaternary ammonium salt, hexadecyltrimethylammonium bromide (CTAB). The treatments were carried out using increasing amounts of ammonium salt (90, 180 and 360 cmol/kg of A40) in order to obtain A40 with different degrees of organophilization (A40M): A40M-9, A40M-18 and A40M-36, respectively. The chemical treatment was carried out in a Beaker using 450 mL of distilled water, 15 g of A40 and the ammonium salt. The suspension was kept under magnetic stirring, at room temperature, for 48 h. Then, the solid was filtered and washed with distilled water for an additional 3 times, maintaining between each wash a period of 48 h. In the end, the filtrate was tested with 1% silver nitrate solution to ensure that the unreacted ammonium salt was discarded. The objective of using different amounts of the ammonium salt was to obtain A40M with different degrees of organophilization and to evaluate its efficiency in increasing the interlayer spaces, as depicted in Fig. 1. On the other hand, the ammonium salt could hinder the fixation of catalyst components inside the A40M spaces. 2.3. Support and catalyst preparation Catalyst precursors were prepared in a Kettle reactor using 31.5 mmol of anhydrous magnesium chloride, 189 mmol of anhydrous alcohol and 80 mL of mineral oil. The system was heated for 4 h until it reached 110 °C. Then, the organophilized A40M was added, previously kept in mineral oil for 20 h at 60 °C and posteriorly at 110 °C for 4 h. The resulting emulsion was transferred through a ¼-inch stainless steel tubing to a flask containing isoparaffin at −40 °C under stirring. Thus, the support precursors obtained were maintained under stirring for 4 h in order to crystallize, being subsequently washed with hexane. The support catalyst precursor based on MgCl2.nEtOH/A40M was prepared with different mass ratios (1:1 and 1:2) of MgCl2/A40M (Table 1). One support was prepared without Mt, as standard. To prepare the catalysts, the supports were treated with TiCl4 (0.5 TiCl4:1EtOH molar ratio), at −10 °C for 10 min in isoparaffin. After the system reached room temperature, the material was washed with dry hexane at 60 °C. Then, the support was suspended again in isoparaffin (20 mL/g support), internal
2. Material and methods 2.1. Materials All reagents were manipulated under a nitrogen atmosphere using the Schlenk technique. Propylene and nitrogen were purified by sequential passage through columns containing 4 Å molecular sieves and copper catalyst to remove oxygen, carbon dioxide, and moisture. Hexane (Polymerization grade, purchased from Petroflex, Brazil), was used as a solvent for the polymerization medium, and also for catalyst preparation. It was treated with molecular sieve and bubbled with N2 before use. Anhydrous ethanol (Vetec Quimica Fina Ltda, Brazil, purity ≥99.5%), butyl phthalate (Aldrich, Brazil, purity ≥98%) as an internal donor, dimethoxy-diphenyl silane (ToKyo Kasei Kogyo Co., Japan, purity: 99%) as an external donor, were dried with 4 Å molecular sieve. TiCl4 was distilled under nitrogen atmosphere prior to use. Isoparaffin (Unipar Comercial, Brazil) was also treated with molecular sieve. Mineral oil (EMCA 350, Ipiranga, Brazil) was employed after heating to eliminate humidity. Other materials were used as received:
Table 1 Modification of sodium Mt and mass ratios used for support catalyst preparation. Content of salta (cmol/kg of Mt)
Samples
MgCl2:Mt in support (mass ratio)
Catalyst
90 180 360 360
A40M-9 A40M-18 A40M-36 A40M-36
1:2 1:2 1:1 1:2
C19 C20 C13 C18
a
2
hexadecyltrimethylammonium bromide (CTAB).
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donor (n-butyl phthalate) was added (molar ratio MgCl2/DI = 12) at 60 °C and kept under stirring for 2 h. Then, TiCl4 (10 mL/g MgCl2) was added on the support and the temperature was raised to 120 °C for 2 h. After a period of 2 h stirring the supernatant was removed and the catalyst obtained was washed with hexane until the HCl residue was no longer observed. Then, the solid was dried under N2 flow to constant mass. Propylene polymerization was carried out in a Büchi reactor (Brooks – 5850D) equipped with a mechanical stirrer, and attached to a thermostatic system. Hexane (100 mL) was used as a polymerization medium. The amounts of triethylaluminium (TEA) and MgCl2 used in all the polymerizations were 10 mmol and 3.15 mmol, respectively. The concentration of external electron donor (ED) in the system was according to the molar ratio TEA/ED = 50. The reactor temperature and pressure were maintained constant at 70 °C and 4 bar for 30 or 60 min (evaluated in polymerizations). The polymerization was quenched with 5% HCl in ethanol solution. The polypropylene nanocomposites obtained was filtered, washed, and dried under reduced pressure at 50 °C to constant mass. The amount of MgCl2 added into the reactor (which is the support that contains the Ti active species for polymerization) was kept constant.
Fig. 2. XRD patterns of sodium Mt and organically modified Mt (A40M) with different salt contents (CTAB): A40M-9 (90 cmol/kg Mt), A40M-18 (180 cmol/ kg Mt), A40M-36 (360 cmol/kg Mt).
exhibit in all organically modified A40M. There was an increase in basal spacing d001 in relation to the original sodium Mt The interlayer space increased from 1.10 (Mt) to 1.37 (A40M-9) and 1.68 (A40M-36) nm. Montmorillonite A40M-18, modified with 180 cmol/kg of CTAB, displayed an X-ray diffractogram with a very similar profile to that of the sodium Mt A40M-36, but with a reduced interlayer space. The higher salt content used led to a higher value of d001, indicating that increasing amounts of salt were exchanged in the A40M, as well as successful intercalation of CTAB. The effective intercalation of organic molecules is important since it allows the introduction of catalyst components and favors a complete exfoliation of the Mt in propylene polymerization. The quantity of 90 cmol/kg of CTAB added for cation exchange was sufficient to promote a structural change in Mt As a result of the introduction of CTAB molecules into the Mt interlayer space, an increase in the temperature of degradation can also be observed (Table 2), especially when treated with 90 and 180 cmol/kg Mt The increase in the thermal stability of materials is a very positive factor since the processing temperature of the PP is high and the degradation of the ammonium salt can cause stacking of Mt sheets and promote degradation of the PP matrix. The thermogravimetric analysis also confirms an effective organophilization of the materials. Montmorillonite presents diffraction peaks corresponding to the mass loss at 112 °C, probably resulting from water in its structure, and 651 °C, relative to the dehydroxylation of Mt structure. However, the modified inorganic montmorillonites showed a mass loss corresponding to the organic fraction at temperatures above 350 °C. This fact can be better observed when higher salt content was employed. Sample A40M-36 displayed mass loss at 3 different temperatures and content of volatile was 24%, while in the original sodium Mt it was 14%. In the most recent works in the literature, the significant advances in the performance of the Ziegler-Natta catalysts were achieved through modifications carried out in the catalytic support. However, there are
2.4. Characterization of materials Wide-angle X-ray diffraction (XRD) analysis was performed on a Rigaku Miniflex diffractometer (15 mA, 30 kV, Cukα λ = 1.5418 Ǻ), the scanned 2θ range was between 2 and 30 °C and the scanning rate was 0.05 °C/min. The basal space (001) was calculated using Bragg's eq. (2d sin θ = nλ). The catalyst was characterized by XRD under the same conditions of A40M analysis. Titanium, magnesium and chloride contents in the catalyst were measured by energy dispersive X-ray spectrometry in a Shimadzu model 720 EDX apparatus. Previously, dried samples of Ziegler-Natta catalysts were examined under vacuum for 320 s in powdered form. The absorbance reading was performed in a Turner SP 870 UV–Vis spectrophotometer using a wavelength of 370–410 nm. A scanning electronic microscope (SEM), JEOL model JFC, was used to evaluate the morphologies of the support precursors and catalysts. The polymers were characterized by optical microscopy (OM) (Olympus, model 5210) coupled to a camera. X-ray patterns (XRD), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) were also employed. TGA analyses were carried out using TA Instruments equipment (Series Q model Q500). Samples were heated from room temperature to 700 °C, under an inert atmosphere, using a heating rate of 10 °C/min and the initial degradation temperature (Tonset) and the temperature of maximum degradation rate (Tmax) were determined. Melting (Tm) and crystallization (Tc) temperatures were determined from the second heating using DSC equipment (TA Instruments, Q1000) under a nitrogen atmosphere. Samples (~3 mg) in aluminum pans were first melted from 30 to 180 °C, then cooled down to 30 °C, and melted again using the same procedure. The heating and cooling rate were 10 °C/min. The isotactic index (I·I.) test was carried out in boiling heptane for 6 h using a Soxhlet extractor (ASTM D 297). Transmission electron microscopy (TEM) was carried out on a Philips equipment model EM208S, using tungsten filament. Samples for TEM analysis were prepared as films. The films were prepared by melt pressing at 180 °C for one minute using a 2 mm spacer.
Table 2 Characterization by TGA of inorganic and organically modified Mt (A40M).
3. Results 3.1. Characterization of organically modified Mt and support precursors MgCl2.EtOH/Mt produced therefrom In this work, we started from sodium Mt, a 2:1 layered silicate. Diffractograms of A40M organophilized with different amounts of ammonium salt (CTAB) were compared (Fig. 2). Intense reflection was 3
Mt
Tonset (°C)
Tmax (°C)
Volatile content (%)
Estimated ammonium salt content (%)
Sodium Mt A40M-9 A40M-18 A40M-36
112/604 371/547 381/564 277/396/ 566
112/651 416/605 421/616 313/427/ 592
14 19 20 24
0 5 6 10
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Fig. 3. TGA patterns of support precursors: AD03 (standard); AD19 (1:2) based on MgCl2.nEtOH/A40M–9; AD20 (1:2) based on MgCl2.nEtOH/A40M-18; AD18 (1:1) based on MgCl2.nEtOH/40 M-36; AD13 (1:2) based on MgCl2.nEtOH/40 M-36.
no papers in the literature that report TGAs of adducts based on MgCl2.nEtOH/Mt The main reference in obtaining Ziegler-Natta catalysts with controlled morphology and synthesized with support based on MgCl2.nEtOH/Mt is found in WO 2009080568 (Collina et al., 2009). The supports prepared with 0.1 to 1% of Mt in MgCl2 were analyzed by XRD and DSC and results demonstrated different physicochemical properties when compared to conventional adducts but the information is limited. To the best of our knowledge, supports were evaluated with high contents of Mt and might benefit the expansion of interlayer spacing. Thermal stability of the catalyst precursors (AD) produced and the influence of the Mt in its structure were investigated by TGA measurements (Fig. 3) The thermograms show the derivative curve peaks (DTG) that provide information about the structure and mechanism of thermal decomposition of the adducts produced with different contents of Mt compared to standard MgCl2.nEtOH. In the standard adduct (AD03) there are peaks at 188 °C, 219 °C and 254 °C for MgCl2 molecules with low alcohol content. According to the literature (Jiang et al., 2008), up to 300 °C, the mass loss may correspond to ethanol and HCl molecules generated by the adduct decomposition. Above 300 °C, peaks at 435 °C and 434 °C probably correspond to the decomposition of compound Mg(OH)Cl. As demonstrated (Bart and Roovers, 1995), peaks near 450 °C are caused by the decomposition of this compound. Support precursors based on MgCl2.EtOH, as AD03, exhibited no mass loss in the temperature range of 300 and 400 °C. Instead, Mt treated with different CTAB contents presented thermal decomposition peaks in the range of 300 to 500 °C corresponding to the organic fraction in its structure. Comparing thermograms of organically modified montmorillonites with produced precursors (AD) therefrom, it was possible to observe the
mass loss of organic fraction at temperatures above 300 °C for all substrates. DTG profile of precursor AD19, prepared with A40M-9, allows one to observe well distributed mass loss peaks in the temperature range between 50 °C and 200 °C indicating good incorporation of the EtOH molecules. Peaks are also verified at 392 °C and 557 °C, similar to Mt present in its composition. Similar to adduct AD19, support precursor AD20 exhibited DTG peaks corresponding to those of Mt in the same temperature range, as well as peaks corresponding to the loss of EtOH molecules. The increase of the CTAB content to 180 cmol/kg in the treatment of A40M resulted in a slight increase in the organic fraction in its composition, and this profile is very similar to the A40M treated with 90 cmol/kg. Adduct AD18 revealed several peaks in the temperature range of 75 °C to 200 °C, corresponding to the mass loss of MgCl2 molecules with different numbers of moles of alcohol. At 321 °C, there is a slight enlargement of the peak, corresponding to the decomposition of organic fraction present in its structure. It was revealed that the mass loss up to 300 °C was 50%. Similarly, support precursor AD13, prepared with the same Mt, but with MgCl2:Mt ratio of 1:2, showed DTG peaks in the same temperature range. Above 300 °C, the profile of the TG derivative is very similar to that of the Mt used in its formulation. For precursors AD19 and AD20, a lower number of peaks and mass loss values were found to be much lower than those for AD18 and AD13. At a temperature of up to 300 °C support precursor AD20 lost only 15% of the mass. This can be explained by the fact that organophilic A40M with 90 and 180 cmol/kg of CTAB has the largest inorganic fraction in its structure.
Fig. 4. SEM micrographs of (a) support AD19 (1:2) and catalysts: (b) C13 (1:2) and (c) C18 (1:2) (MgCl2:A40M mass ratios). 4
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samples are presented in Fig. 5. Comparing characterizations results of organically modified Mt (Fig. 2) with the catalysts produced therefrom, it was possible to observe the presence of characteristic diffraction peak of Mt displaced to small angles (catalyst C19) and strong reduction in tactoids size (catalysts C13 and C18) accompanied by the decrease of the interlayer space of the Mt The catalyst C19 presented a very wide diffraction peak and more displaced to smaller angles (2θ = 5.8°, corresponding to the basal space of d = 1,52 nm), which reveals a wider distribution of interlayer spaces, with lamellae presenting different basal sizes. Comparing the results of the XRD and TG of montmorillonites A40M-18 and A40M-9, it was not possible to observe significant differences between them. However, the XRD analysis revealed that each amount of CTAB used to treat Mt resulted in a catalyst with different structural properties. Catalyst C19 showed to be more exfoliated and/or intercalated when smaller salt content was used. This fact is important because it favors good fixation of the internal donor and Ti species on the MgCl2 crystal. On the other hand, catalyst C18 and C13 presented diffraction peak at 2θ = 6.8° and 2θ = 6.5°, corresponding to the basal distance of d = 1,25 nm and d = 1,36 nm, respectively, while A40M-36 presented diffraction peak at 2θ = 5.2°, corresponding to the basal distance of d = 1,36 nm. The increase of the CTAB content in the organophilized sodium Mt to 360 cmol/kg of salt possibly resulted in the greater difficulty of access of the MgCl2 to the interior of the interlayer space. In addition, the reaction of TiCl4 directly in the OH groups on the surface of the Mt lamellae, including into the space between sheets, may have caused the lamella to approach, reducing the basal spacing. The catalyst prepared with A40M-18 (catalyst C20) presented a diffraction peak at 2θ = 6,5° (corresponding to the basal space of 1.36 nm), which is discretely more shifted, wider and less pronounced compared to Mt This result clearly confirms that there was intercalation of the catalytic components in the interlayer spacing of the Mt, as catalyst C19. At higher angles, above 20°, there is also a greater structural disorder for all catalysts.
Table 3 Characterization by EDX of the catalysts based on MgCl2/Mt/DI/TiCl4. Catalyst
MgCl2:Mt (mass ratios)
Mt
Ti (%)
Mg (%)
Cl (%)
C19 C20 C18 C13
1:2 1:2 1:1 1:2
A40M-9 A40M-18 A40M-36 A40M-36
5.8 4.5 3.0 2.0
1.8 3.9 3.7 4.0
16.0 17.9 17.7 17.2
Fig. 5. XRD patterns of the catalysts containing organically modified Mt with different amount of CTAB.
3.2. Characterization of catalysts produced from MgCl2.EtOH/Mt
3.3. Polypropylene/Mt nanocomposites produced by in situ polymerization with catalysts based on MgCl2/Mt/DI/TiCl4
SEM micrographs of the catalyst support precursors and the final catalysts produced with different mass ratios of MgCl2/A40M Mt (Fig. 4) reveals the presence of Mt at different amounts in the preparation of the catalytic support and the catalyst did not compromise the spherical morphology of the same. EDX technique was also used to quantify chemical composition (Table 3) and identify variations in the samples. Ti contents were close to those of the literature (Yank et al., 2007; Nikolaeva et al., 2018; Sukulova et al., 2019), the mass average was 3.8%. However, the percentages of magnesium found were low. It can be justified by the presence of the inorganic filler in excess compared to MgCl2. There is no Mg species in the Mt structure. Since XRD gives information on the crystalline and well-ordered part of the catalysts, the incorporation of the Mt and its degree of dispersion can be well evaluated by this technique. XRD patterns of the
In order to evaluate the influence of reaction time on the catalytic activity (Table 4) and properties of CPN produced (Table 5), polymerization reactions were performed in 30 and 60 min. The catalyst C19, prepared A40M with the lowest CTAB content (90 cmol/kg Mt), showed superior activity. The increase of the salt content to 180 and 360 cmol/kg Mt in catalyst C20, C13 and C18, respectively, resulted in a decrease in the catalytic activity for polymerizations in 30 and 60 min. These results corroborate XRD diffraction analysis of the catalysts, where it was possible to verify that catalyst C19 exhibits greater interlayer space, followed by the catalysts C20 and C13. The amount of CTAB incorporated in the silicate sheets and used to make catalyst support may exert influence on the activity of the catalyst. The
Table 4 Results of nanocomposites and masterbatches of propylene polymerization catalyzed by MgCl2/Mt/DIBP/TiCl4. Catalyst
MgCl2/Mt ratio (wt%)
Content of salt (cmol/kg of Mt)
Time reaction (min.)
Sample
Polymer produced (g)
a C.A (gPP/gMgCl2*h)
C19
1:2
90
C20
1:2
180
C13
1:2
360
C18
1:1
360
30 60 30 60 30 60 30 60
NPP19-2 NPP19 NPP20-2 NPP20 NPP13-2 NPP13 NPP18-2 NPP18
2.7 7.0 2.0 2.8 1.5 2.4 0.1 2.0
18.0 23.3 9.3 13.3 8.0 10.0 0.7 6.3
Reactions conditions: Hexane as solvent (100 mL); Pressure of 4 bar; TEA as cocatalyst ([TEA] = 0,15 mmol/g MgCl2; molar ratio TEA/ ED = 50 a C.A: catalytic activity.
5
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Table 5 Characterization of nanocomposites and masterbatches of PP/A40M. DSC
TGA
Catalyst
Polymer
Mt (wt%)
Tm (°C)
Tc (°C)
∆Hm (J/g)
Tonset (°C)
Tmax (°C)
Residue (°C)
I.I. (%)
C19–1:2
NPP19–2 NPP19 NPP20–2 NPP20 NPP13–2 NPP13 NPP18–2 NPP18
22.0 8.5 30.0 21.0 40.0 25.0 ~100.0 16.0
155 158 154 154 155 153 145 157
118 121 119 119 121 119 115 119
7.4 86.3 42.5 80.8 58.6 69.0 48.7 69.1
nd 440 260 404 248/421 287/431 236 428
nd 455 291 430 287/431 247/421 276/416 450
nd nd nd nd 6.4 6.6 nd nd
90 98 nd 53 nd nd nd 84
C-1:2 C-1:2 C-1:1
I.I.: isotacticity index by heptane soluble; nd: not determined.
ammonium salt used in the treatment of A40 has a long chain of carbon atoms (C16), which may hinder the fixation of the Ti atoms and internal donor over MgCl2 crystal in the interlayer space. Furthermore, the Mt lamellae can also hinder the access of the monomer to the active centers. In the literature (Zhang et al., 2016), the decrease of catalyst activity in ethylene polymerization was attributed mainly to the layered shape of the graphene (present in the catalyst) acting as a physical barrier, thus retarding the diffusion of the monomer into the catalyst core. Propylene polymerization is even more complex, due to the presence of methyl in the monomer and internal and external electron donors (present in the structure of the catalyst and polymerization reaction, respectively) that plays an important role in the catalyst activity, as well as in PP stereoregularity. However, one cannot rule out the possibility that the Ti species supported on MgCl2 are bound to other OH-Si groups from nearby lamellae, thus inactivating some sites of polymerization. In our previous study (Cardoso et al., 2018), reduction of catalyst activity when commercial organic Mt content was increased in support catalyst composition was reported. Other similar and important aspect in previous study was catalyst response to the kinetic profile. It has reduced the polymerization rate in the beginning of reaction and increased with the polymerization time. This behavior is opposite to that of conventional ZN catalyst that showed high initial polymerization rates in the first minutes of reaction and a decay with time, as reported in the literature (Wang et al., 2005; McKenna et al., 2010). A particularly important structural effect, usually reported when Mt is well dispersed in the matrix of PP, is an increase of crystallization temperature Tc, normally in nanocomposites where montmorillonites
act as nucleating agents (Shi and Dou, 2013). In this work, no relevant changes were observed for Tc of NPC obtained in different time reaction, as well as, in the melting temperature (Tm). Meanwhile, melting enthalpies (ΔHm) were much lower for all samples synthesized in 30 min polymerization and sharply increased with increase in time of polymerization, indicating enhancement of crystallinity of nanocomposites and masterbatches. These results were expected due to the high A40M content. The presence of Mt sheets forms a barrier making it difficult to pack polymer chain and resulting in many imperfections. It may be a cause of low isotacticity of the samples NPP19–2 and NPP18. Nevertheless, the results reveal good crystallinity content for the samples, since they present a percentage of A40M higher than 8%. The melting endotherms of the samples also were obtained (Fig. 6). It revealed an enlargement of the melt endotherms, especially in the samples obtained in 30 min, and the displacement of the Tm to lower temperatures with the increase of the content of A40M. This fact reveals the formation of imperfect crystals of PP in the presence of the Mt as reported previously (Wang and He, 2015; Cardoso et al., 2018). It is interesting to note that the stereoregularity of CPN synthesized with the catalyst C19 was high, isotacticity index (I.I.) reached 98% in the polymerization of 60 min, despite the high A40M content present (8.5%). Wang and He (2011) used 1-hexadecane-3-methylimidazolium bromine to prepare organically modified Mt to produce MgCl2/Mt/ TiCl4 catalyst and I.I of PP/Mt reached 99%, but the amount of Mt obtained in nanocomposites was 0.9%, a value much lower than that described in this paper. This result is typical for titanium‑magnesium Ziegler-Natta catalyst with high stereoselectivity (Zhu et al., 2017) and suggests that although clay minerals were present in the composition of
1,5
Polymerization time: 30 min. NPP13-2
2,0
NPP19
1,0
NPP20 1,5
NPP20-2
Heat Flow (W/g)
Heat Flow (W/g)
NPP18-2
0,5
NPP19-2
NPP13 1,0
0,5
NPP18 0,0 40
60
80
100
120
140
160
180
Temperature (°C)
0,0 40
60
80
100
120
Temperature (°C) Fig. 6. DSC thermogram of heating curves of polypropylene/A40M synthetized in (a) 30 min and (b) 60 min. 6
140
160
180
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Fig. 7. XRD patterns of the nanocomposites and masterbatches produced in different reaction times: (a) NPP19 (22% A40M-9, 60 min of polymerization); b) NPP20–2 (30% A40M-18, 30 min of polymerization) and NPP20 (21% A40M-18, 60 min of polymerization); (c) NPP18–2 (~100% A40M-36, 30 min of polymerization) and NPP18 (16% A40M-36, 60 min of polymerization); (d) NPP13–2 (40% A40M-36, 30 min of polymerization) and NPP13 (25% A40M-36, 60 min of polymerization.
Fig. 8. TEM micrographs of polypropylene/A40M: a) e b) NPP19 (22% A40M-9, 60 min of polymerization); c) NPP13–2 (40% A40M-36, 30 min of polymerization) and d) NPP13 (25% A40M-36, 60 min of polymerization).
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adduct MgCl2.EtOH, as previously mentioned by TGA curves, they did not prevent the formation of active centers with high specificity. The role that internal and external donors exert on the surface of magnesium chloride to control the stereoselectivity of the catalyst does not appear to be compromised by the presence of high Mt contents. The thermogravimetric analysis revealed high Tmax for the materials synthesized in 60 min polymerizations, especially NPP19 (455°). It can be verified that in the polymerizations of 30 min the thermal degradation temperature assumes lower values. TGA profile obtained from NCP and masterbatch NPP13, NPP13-2, and NPP18 were bimodal according to the profile of Mt present in their composition (A40M-36). The effect of A40M in the crystalline structure of CPN and masterbatches were also studied by XRD (Fig. 7). For sample NPP19, it is possible to verify that no diffraction peaks are related to A40M-9, indicating good dispersion of the Mt in the matrix of polypropylene. For sample NPP20-2 produced with 30 min of polymerization, an enlarged diffraction peak is exhibited at 2θ = 6.25° (1.46 nm). There was the displacement of the diffraction peak to a lower angle compared to the peak of the Mt present in the catalyst. In 60 min of reaction (sample NPP20), the diffraction peak was shifted towards the smallest angles (2θ = 5.7°), which indicates higher polymer intercalation with increasing polymerization time. Samples NPP13-2 and NPP13 exhibited intense diffraction peak at 6.35° and 6.2°, respectively, corresponding to an interlayer distance of 1.39 nm and 1.43 nm, respectively) close to that of the Mt in the catalyst. This case suggests there was little intercalation. Moreover, it is possible to observe the presence of diffraction peaks corresponding to the γ phase of polypropylene. We believe that the γ-crystallites are formed by the action of the Mt as nucleating, as reported by (Nam et al., 2001; Maiti et al., 2002; Cardoso et al., 2018). The NCP diffractograms obtained in 30 and 60 min with the catalyst C18 show that the increase in reaction time led to the complete exfoliation of the Mt (NPP18). The catalyst C18 was prepared with the same Mt of catalyst C13, but with less proportion of MgCl2 in its formulation (MgCl2: Mt = 1:1). In general, it is possible to observe major structural disorder in the crystalline phases of polypropylene/Mt compared to pure polypropylene as previously reported (Kim et al., 2004). The morphology of CPN was evaluated by TEM (Fig. 8). Analysis of sample NPP19 confirmed good dispersion of the Mt in the polypropylene matrix, despite the high Mt content (22%). Part of Mt's sheets is fully exfoliated, while other part contains polymer chains interspersed between Mt's lamellae, forming stacks of various thicknesses. Comparing with TEM result of sample NPP13, which contains a similar amount of Mt (25%), a lower degree of exfoliation was reached. It is reasonably believed that it is a consequence of poor fixation of the catalytic components inside the interlayer space of the A40M-36 as demonstrated for the catalyst C13 by XRD. This fact resulted in the lower production of polymer in this space and, consequently, in the less intercalation/exfoliation of the Mt when compared to sample NPP19.
interlayer space and even the exfoliation of the Mt Nanocomposites obtained presented good thermal properties, achieving a high degree of isotacticity even with 8.5% of Mt As observed in TEM images, the resultant PP/Mt nanocomposites and masterbatches exhibited Mt sheet homogeneously distributed in the polymeric matrix, with intercalated and exfoliated morphology. Acknowledgements This work was supported by the Ministerio da Ciencia e Tecnologia, Inovações e Comunicações, CNPq; The Coordenação de Aperfeiçoamento de Pessoal de Nivel Superior (CAPES); and the Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro – FAPERJ (Grant Nos. 41472045). References Abedi, S., Abdouss, M., 2014. Review of clay-supported Ziegler–Natta catalysts for production of polyolefin/clay nanocomposites through in situ polymerization. Appl. Catalysis A Gen. 475, 386–409. Bart, J.C.J., Roovers, W., 1995. Magnesium chloride - ethanol adducts. J. Mater. Sci. 30, 2809–2820. Cardoso, R.S., Oliveira, J.S., Ramis, L.B., Marques, M.F.V., 2018. Ziegler-Natta catalyst based on MgCl2/Clay/ID/TiCl4 for the synthesis of spherical particles of polypropylene nanocomposites. J. Nanosci. Nanotechnol. 18, 5124–5132. Collina, G., Evangelist, D., Giampiero, M., Ferrara, G., 2009. Magnesium Dichloride-Alcohol Adducts and Catalyst obtained Therefrom. WO 2009080568. Dittrich, B., Wartig, K.A., Hofmann, D., Mülhaupt, R., Schartel, B., 2013. Carbon black, multiwall carbon nanotubes, expanded graphite and functionalized graphene flame retarded polypropylene nanocomposites. Polym. Adv. Technol. 24, 916–926. Dong, J.-Y., Qin, Y., 2019a. Polypropylene nanocomposites containing multi-walled carbon nanotubes and graphene oxide by Ziegler-Natta Catalysis. In: Aliofkhazraei, M. (Ed.), Advances in Nanostructured Composites. CRC Press 1. Taylor and Francis group, Boca Raton, pp. 424–441 (Chapter 19). Dong, J.-Y., Qin, Y., 2019b. Polypropylene nanocomposites containing multi-walled carbon nanotubes and graphene oxide by Ziegler-Natta Catalysis. In: Aliofkhazraei, M. (Ed.), Advances in Nanostructured Composites. CRC Press 1. Taylor and Francis group, Boca Raton, pp. 424–441 (Chapter 19). Gordana, B.G., 2014. Advances in polypropylene based materials, Contributions. Sec. Nat. Math. Biotech. Sci. 35, 121–138. Haddadi, S.A., Amini, M., Kheradmand, A., 2018. In-situ preparation and characterization of ultra-high molecular weight polyethylene/diamond nanocomposites using Bi-supported Ziegler-Natta catalyst: Effect of nanodiamond silanization. Mater. Today Commun. 14, 53–64. Jiang, X., Tian, X., Fan, Z., 2008. Crystal structure of ball-milled mixture of sodium chloride and magnesium chloride-ethanol adduct. Mater. Res. Bull. 43, 343–352. Kaminsky, W., 2018. Polyolefin-nanocomposites with special properties by in-situ polymerization. Front. Chem. Sci. Eng. 12, 555–563. Kim, B., Lee, S., Lee, D., Ha, B., Park, J., Char, K., 2004. Crystallization kinetics of maleated polypropylene/clay hybrids. Ind. Eng. Chem. Res. 43, 6082–6089. Ma, J., Qi, Z., Hu, Y., 2001. Synthesis and characterization of polypropylene/clay nanocomposites. J. Appl. Polym. Sci. 82, 3611–3617. Maiti, P., Nam, P.H., Okamoto, M., Hasegawa, N., Usuki, A., 2002. Influence of crystallization on intercalation, morphology, and mechanical properties of polypropylene/clay nanocomposites. Macromolecules 35, 2042–2049. McKenna, T.F.L., Martino, A.D., Weickert, G., Soares, J.B.P., 2010. Particle growth during the polymerisation of olefins on supported catalysts, 1 – nascent polymer structures. Macromol. React. Eng. 4, 40–64. Nam, P.H., Maiti, P., Okamoto, M., Kotaka, T., Hasegawa, N., Usuki, A., 2001. A hierarchical structure and properties of intercalated polypropylene/clay nanocomposites. Polym. 42, 9633–9640. Nifant’ev, I.E., Smetannikov, O.V., Tavtorkin, A.N., Chinova, M.S., Ivchenko, P.V., 2016. Titanium–magnesium nanocatalysts of polymerization (Review). Pet. Chem. 56, 480–490. Nikolaeva, M., Matsko, M., Zakharov, V., 2018. Propylene polymerization over supported Ziegler–Natta catalysts: effect of internal and external donors on distribution of active sites according to stereospecificity. J. Appl. Polym. Sci. 135, 46291–46301. Rong, J., Li, H., Jing, Z., Hong, X., Sheng, M., 2001. Novel organic/inorganic nanocomposite of polyethylene. I. Preparation via in situ polymerization approach. J. Appl. Polym. Sci. 82, 1829–1837. Shi, Y., Dou, Q., 2013. Effect of β-nucleating agent on the crystallization, Mechanical properties, and heat resistance of injection-molded isotactic polypropylene. J. J. Macromol. Sci. B. 52, 476–488. Sukulova, V.V., Barabanov, A.A., Matsko, M.A., Zakharov, V.A., Mikenas, T.B., 2019. Kinetic features of ethylene copolymerization with 1-hexene over titanium-magnesium Ziegler–Natta catalysts: effect of comonomer on the number of active centers and the propagation rate constant. J. Catal. 369, 276–282. Tong, Z.-Z., Zhou, B., Huang, J., Xu, J.-T., Fan, Z.-Q., 2014. Hierarchical structures of olefinic blocky copolymer/montmorillonite nanocomposites with collapsed and intercalated clay layers. RSC Adv. 4, 15678–15688. Wang, L., He, A., 2011. Preparation of polypropylene/clay nanocomposites by in situ polymerization with TiCl4/MgCl2/clay compound catalyst. Chinese J. Polym. Sci. 29, 597–601. Wang, L., He, A., 2015. Microstructure and thermal properties of polypropylene/Clay nanocomposites with TiCl4/MgCl2/Clay compound catalyst. J. Nanomater. 5, 1–5. Wang, Q., Murayama, N., Liu, B., Terano, M., 2005. Effects of electron donors on active sites
4. Conclusions The catalyst precursors prepared with high amounts of Mt (MgCl2.EtOH/A40M) resulted in significant differences in thermal decomposition profile compared to conventional adduct based on MgCl2.nEtOH. The TG measurements showed thermal decomposition peaks corresponding to the organic fraction of the Mt in its structure. It was verified that among the CTAB concentrations evaluated, the use of lower content (90 cmol/kg Mt) to organophilize inorganic Mt was sufficient to produce a catalyst with a good distribution in its interlayer space. The catalyst produced therefrom exhibited higher catalytic activity in the synthesis of nanocomposite with the highest properties. It was possible to notice that there was good incorporation of the Mt in the catalyst composition without compromising the spherical morphological character. XRD analysis confirmed that catalyst components were found in the interlayer space of Mt and the growth of polypropylene chains during polymerization led to the increase of the 8
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R. da Silva Cardoso, et al. distribution of MgCl2-supported Ziegler-Natta catalysts investigated by multiple active sites model. Macromol. Chem. Phys. 206, 961–966. Yang, F., Zhang, X., Zhao, H., Chen, B., Huang, B., Feng, Z., 2003. Preparation and properties of polyethylene/montmorillonite nanocomposites by in situ polymerization. J. Appl. Polym. Sci. 89, 3680–3684. Yank, k., Huang, Y., Dong, J., 2007. Preparation of polypropylene/montmorillonite nanocomposites by intercalative polymerization: effect of in situ polymer matrix functionalization on the stability of the nanocomposite structure. Polym. 52, 181–187. Zhang, H.-X., Ko, E.-B., Park, J.-H., Moon, Y.-K., Zhang, X.-Q., Yoon, K.-B., 2016a. Fabrication
of polyethylene/graphene nanocomposites through in situ polymerization with a spherical graphene/MgCl2-supported Ziegler-Natta catalyst. Compos. Sci. Technol. 136, 61–66. Zhang, H.X., Ko, E.B., Park, J.H., Moon, Y.K., Zhang, X.Q., Yoon, K.B., 2016b. Fabrication of polyethylene/graphene nanocomposites through in situ polymerization with a spherical graphene/MgCl2-supported Ziegler-Natta catalyst. Compos. Sci. Technol. 136, 61–66. Zhu, W., Tian, Z., Cheng, R., He, X., Liu, Z., Zhao, N., Liu, B., 2017. Exploring Si/Mg composite supported Ziegler-Natta Ti-based catalysts for propylene polymerization. Chinese J. Polym. Sci. 35, 1474–1487.
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Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research Paper
Ziegler-Natta catalyst produced from MgCl2/organically modified Mt/DI/ TiCl4 for in situ synthesis of polypropylene nanocomposites
T
Renata da Silva Cardoso, Jaqueline da Silva Oliveira, Maria de Fátima Vieira Marques
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Instituto de Macromoleculas, Universidade Federal do Rio de Janeiro, IMA-UFRJ, Cidade Universitária. Av. Horacio Macedo, 2030, Centro de Tecnologia, Bloco J, Brazil
ARTICLE INFO
ABSTRACT
Keywords: Ziegler-Natta catalyst Nanocomposite in situ polymerization Polypropylene Clay mineral Montmorillonite
Spherical Ziegler-Natta catalysts containing clay mineral are a more favorable way for the synthesis of nanocomposites and masterbatches of polyolefins. With the application of in situ polymerization technique, it was possible to overcome challenges such as intercalation/exfoliation of clay mineral, avoiding lamellae reaggregation in order to obtain materials with superior properties than the currently available ones, such as gas barrier and thermal resistance. In this study, support precursors and catalysts were prepared by chemical route with sodium clay mineral (Mt, montmorillonite) modified with different amounts of ammonium quaternary salt. The preparation of the catalyst precursor was performed using the mass ratios of MgCl2 to modified sodium Mt of 1:1 and 1:2 and it was possible to observe different thermal decomposition profiles compared to the standard catalytic support precursor (adduct MgCl2.EtOH) prepared as a reference. The catalysts obtained therefrom maintained a spherical morphology and X-ray diffractions (XRD) exhibit peak shift showing an increase of interlayer space of the Mt In order to obtain nanocomposites and masterbatches of polypropylene/modified Mt, the polymerization reaction was conducted in different reaction times. Polypropylene nanocomposites presented the high thermal degradation temperature (459 °C) and isotacticity (98%). XRD and transmission electron microscopy (TEM) analyses revealed exfoliated and intercalated morphologies even with high Mt contents in the polypropylene matrix.
1. Introduction Great attention has been paid to the preparation of polypropylene nanocomposites using nanoparticles, especially clay minerals, carbon nanotubes, graphene, silica and metal oxides due to the resulting dramatic modification in the physical and mechanical properties. However, the great challenge remains to disperse nanomaterials into polymers matrices, especially into hydrophobic polyolefin as polypropylene (PP). Since nanomaterials tend to agglomerate and/or may exhibit incompatibility with the organic polymer matrix, many strategies have been studied. To profit from the properties of the filler, a strong interface between filler and the polymeric matrix, and also a homogeneous distribution of the individual particles (or layered silicates or fibers) are necessary (Kaminsky, 2018). Among the techniques used to produce nanocomposites (melt processing, solution, and direct synthesis), in situ polymerization is more attractive because the dispersion obtained is the most efficient, particularly in the fully exfoliated polyolefin/filler (load) nanocomposite formation (Abedi and Abdouss, 2014). In the last one, Ziegler-Natta (ZN) catalysts are extensively used, as widely reported in the literature
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(Ma et al., 2001; Rong et al., 2001; Yang et al., 2003; Haddadi et al., 2018; Dong and Qin, 2019a, 2019b). Through the last technique, silicate sheets are swollen and dispersed in the liquid monomer (or a monomer solution) and the polymer synthesis can occur into the interlayer space. Another more interesting way is to introduce transition metal catalyst through cationic exchange inside the sheets of the clay mineral or by mixing it with support powders. In the latter case, it is possible to maintain the morphology of the support. The shape of catalysts and resultant nanocomposites is less investigated, although morphology is one of the most influential considerations in the preparation of industrial Ziegler-Natta catalysts (Zhang et al., 2016a, 2016b). The shape and size of the catalyst particle make it possible to perform reactions in flow reactors with the spherical shape of granules of the polymer product being preserved (Nifant'ev et al., 2016). A wide range of systems was investigated in order to promote polymer intercalation into the clay mineral interlayer space, with variables being molecular weight of the PP, maleic anhydride content in the compatibilizer, clay mineral modification and content, etc. This last approach can be done by ion-exchange reactions of the interlayer
Corresponding author at: Av. Horacio Macedo, 2030, CEP 21941-598 Rio de Janeiro, RJ, Brazil. E-mail address: [email protected] (M.d.F. Vieira Marques).
https://doi.org/10.1016/j.clay.2019.105362 Received 5 July 2019; Received in revised form 4 November 2019; Accepted 5 November 2019 0169-1317/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Scheme to represent the synthesis of catalyst in the space of ammonium salt modified Mt
inorganic cations with organic cationic surfactants, including primary, secondary, tertiary and quaternary alkylammonium compounds (Gordana, 2014). Therefore, it is necessary to change its surface to increase hydrophobic character, which increases compatibility with the polymer and at the same time increases the interlayer space of the clay mineral, which facilitates exfoliation. The advantages of using clay mineral compared to other nanomaterials are abundance in nature, low cost, facility for chemical modification and good chemical and thermal stability. Among them, montmorillonite (Mt) is the most widely chosen filler, mainly due to its swelling property leading to exfoliation and high interlayer ion exchange capacity (Tong et al., 2014). Additionally, layered nanofillers, such as Mt, usually improve the thermal stability of the polymer because of a physical barrier effect that retards the diffusion of degradation products, gases, and heat (Dittrich et al., 2013). It is now well-known that the properties of the nanocomposites depend significantly on the characteristics of the polymer matrices, the nature and chemical surface of the clay mineral, as well as the preparation methods (Liu et al., 2018). In our previous study (Cardoso et al., 2018), commercial organophilic Mt was used to prepare spherical support based on the adduct of MgCl2.EtOH/claytone HY. The Ziegler-Natta catalyst produced was effective for the synthesis of nanocomposites and masterbatches as revealed by TEM and XRD. Therefore, in this work, spherical support based on the adduct MgCl2.EtOH/Mt was prepared in order to produce Ziegler-Natta catalyst with different Mt contents. In order to organophilize the sodium Mt, different amounts of hexadecyltrimethylammonium bromide (CTAB) were used. This study proposed to achieve a point that the amount of salt was sufficient to increase interlayer space of Mt without compromising the activity of the inorganic catalyst. Ziegler-Natta catalyst based on MgCl2/Mt/internal donor (ID)/TiCl4 with the addition of external donor (ED) and alkyl aluminum was evaluated in propylene polymerizations and different reaction times to produce Mt polymer nanocomposites (CPN) and masterbatches with high amounts of Mt Thus, we studied the effect of using organophilized Mt in the structure of support precursor and catalyst, on activity catalytic and properties of CPN by in situ polymerization.
triethylaluminium (TEA) (10 mass% solution in hexane, obtained from Akzo Nobel, USA), anhydrous MgCl2 (Toho Catalyst Co. LTD, Japan), inorganic montmorillonite Argel 40 (Bentonit União Nordeste S.A), the ammonium salt was hexadecyltrimethylammonium bromide (CTAB) (Pharma Special, Brazil). 2.2. Chemical treatment of montmorillonite Montmorillonite (A40) (cation exchange capacity: 1000 cmol/kg) was modified with the quaternary ammonium salt, hexadecyltrimethylammonium bromide (CTAB). The treatments were carried out using increasing amounts of ammonium salt (90, 180 and 360 cmol/kg of A40) in order to obtain A40 with different degrees of organophilization (A40M): A40M-9, A40M-18 and A40M-36, respectively. The chemical treatment was carried out in a Beaker using 450 mL of distilled water, 15 g of A40 and the ammonium salt. The suspension was kept under magnetic stirring, at room temperature, for 48 h. Then, the solid was filtered and washed with distilled water for an additional 3 times, maintaining between each wash a period of 48 h. In the end, the filtrate was tested with 1% silver nitrate solution to ensure that the unreacted ammonium salt was discarded. The objective of using different amounts of the ammonium salt was to obtain A40M with different degrees of organophilization and to evaluate its efficiency in increasing the interlayer spaces, as depicted in Fig. 1. On the other hand, the ammonium salt could hinder the fixation of catalyst components inside the A40M spaces. 2.3. Support and catalyst preparation Catalyst precursors were prepared in a Kettle reactor using 31.5 mmol of anhydrous magnesium chloride, 189 mmol of anhydrous alcohol and 80 mL of mineral oil. The system was heated for 4 h until it reached 110 °C. Then, the organophilized A40M was added, previously kept in mineral oil for 20 h at 60 °C and posteriorly at 110 °C for 4 h. The resulting emulsion was transferred through a ¼-inch stainless steel tubing to a flask containing isoparaffin at −40 °C under stirring. Thus, the support precursors obtained were maintained under stirring for 4 h in order to crystallize, being subsequently washed with hexane. The support catalyst precursor based on MgCl2.nEtOH/A40M was prepared with different mass ratios (1:1 and 1:2) of MgCl2/A40M (Table 1). One support was prepared without Mt, as standard. To prepare the catalysts, the supports were treated with TiCl4 (0.5 TiCl4:1EtOH molar ratio), at −10 °C for 10 min in isoparaffin. After the system reached room temperature, the material was washed with dry hexane at 60 °C. Then, the support was suspended again in isoparaffin (20 mL/g support), internal
2. Material and methods 2.1. Materials All reagents were manipulated under a nitrogen atmosphere using the Schlenk technique. Propylene and nitrogen were purified by sequential passage through columns containing 4 Å molecular sieves and copper catalyst to remove oxygen, carbon dioxide, and moisture. Hexane (Polymerization grade, purchased from Petroflex, Brazil), was used as a solvent for the polymerization medium, and also for catalyst preparation. It was treated with molecular sieve and bubbled with N2 before use. Anhydrous ethanol (Vetec Quimica Fina Ltda, Brazil, purity ≥99.5%), butyl phthalate (Aldrich, Brazil, purity ≥98%) as an internal donor, dimethoxy-diphenyl silane (ToKyo Kasei Kogyo Co., Japan, purity: 99%) as an external donor, were dried with 4 Å molecular sieve. TiCl4 was distilled under nitrogen atmosphere prior to use. Isoparaffin (Unipar Comercial, Brazil) was also treated with molecular sieve. Mineral oil (EMCA 350, Ipiranga, Brazil) was employed after heating to eliminate humidity. Other materials were used as received:
Table 1 Modification of sodium Mt and mass ratios used for support catalyst preparation. Content of salta (cmol/kg of Mt)
Samples
MgCl2:Mt in support (mass ratio)
Catalyst
90 180 360 360
A40M-9 A40M-18 A40M-36 A40M-36
1:2 1:2 1:1 1:2
C19 C20 C13 C18
a
2
hexadecyltrimethylammonium bromide (CTAB).
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donor (n-butyl phthalate) was added (molar ratio MgCl2/DI = 12) at 60 °C and kept under stirring for 2 h. Then, TiCl4 (10 mL/g MgCl2) was added on the support and the temperature was raised to 120 °C for 2 h. After a period of 2 h stirring the supernatant was removed and the catalyst obtained was washed with hexane until the HCl residue was no longer observed. Then, the solid was dried under N2 flow to constant mass. Propylene polymerization was carried out in a Büchi reactor (Brooks – 5850D) equipped with a mechanical stirrer, and attached to a thermostatic system. Hexane (100 mL) was used as a polymerization medium. The amounts of triethylaluminium (TEA) and MgCl2 used in all the polymerizations were 10 mmol and 3.15 mmol, respectively. The concentration of external electron donor (ED) in the system was according to the molar ratio TEA/ED = 50. The reactor temperature and pressure were maintained constant at 70 °C and 4 bar for 30 or 60 min (evaluated in polymerizations). The polymerization was quenched with 5% HCl in ethanol solution. The polypropylene nanocomposites obtained was filtered, washed, and dried under reduced pressure at 50 °C to constant mass. The amount of MgCl2 added into the reactor (which is the support that contains the Ti active species for polymerization) was kept constant.
Fig. 2. XRD patterns of sodium Mt and organically modified Mt (A40M) with different salt contents (CTAB): A40M-9 (90 cmol/kg Mt), A40M-18 (180 cmol/ kg Mt), A40M-36 (360 cmol/kg Mt).
exhibit in all organically modified A40M. There was an increase in basal spacing d001 in relation to the original sodium Mt The interlayer space increased from 1.10 (Mt) to 1.37 (A40M-9) and 1.68 (A40M-36) nm. Montmorillonite A40M-18, modified with 180 cmol/kg of CTAB, displayed an X-ray diffractogram with a very similar profile to that of the sodium Mt A40M-36, but with a reduced interlayer space. The higher salt content used led to a higher value of d001, indicating that increasing amounts of salt were exchanged in the A40M, as well as successful intercalation of CTAB. The effective intercalation of organic molecules is important since it allows the introduction of catalyst components and favors a complete exfoliation of the Mt in propylene polymerization. The quantity of 90 cmol/kg of CTAB added for cation exchange was sufficient to promote a structural change in Mt As a result of the introduction of CTAB molecules into the Mt interlayer space, an increase in the temperature of degradation can also be observed (Table 2), especially when treated with 90 and 180 cmol/kg Mt The increase in the thermal stability of materials is a very positive factor since the processing temperature of the PP is high and the degradation of the ammonium salt can cause stacking of Mt sheets and promote degradation of the PP matrix. The thermogravimetric analysis also confirms an effective organophilization of the materials. Montmorillonite presents diffraction peaks corresponding to the mass loss at 112 °C, probably resulting from water in its structure, and 651 °C, relative to the dehydroxylation of Mt structure. However, the modified inorganic montmorillonites showed a mass loss corresponding to the organic fraction at temperatures above 350 °C. This fact can be better observed when higher salt content was employed. Sample A40M-36 displayed mass loss at 3 different temperatures and content of volatile was 24%, while in the original sodium Mt it was 14%. In the most recent works in the literature, the significant advances in the performance of the Ziegler-Natta catalysts were achieved through modifications carried out in the catalytic support. However, there are
2.4. Characterization of materials Wide-angle X-ray diffraction (XRD) analysis was performed on a Rigaku Miniflex diffractometer (15 mA, 30 kV, Cukα λ = 1.5418 Ǻ), the scanned 2θ range was between 2 and 30 °C and the scanning rate was 0.05 °C/min. The basal space (001) was calculated using Bragg's eq. (2d sin θ = nλ). The catalyst was characterized by XRD under the same conditions of A40M analysis. Titanium, magnesium and chloride contents in the catalyst were measured by energy dispersive X-ray spectrometry in a Shimadzu model 720 EDX apparatus. Previously, dried samples of Ziegler-Natta catalysts were examined under vacuum for 320 s in powdered form. The absorbance reading was performed in a Turner SP 870 UV–Vis spectrophotometer using a wavelength of 370–410 nm. A scanning electronic microscope (SEM), JEOL model JFC, was used to evaluate the morphologies of the support precursors and catalysts. The polymers were characterized by optical microscopy (OM) (Olympus, model 5210) coupled to a camera. X-ray patterns (XRD), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) were also employed. TGA analyses were carried out using TA Instruments equipment (Series Q model Q500). Samples were heated from room temperature to 700 °C, under an inert atmosphere, using a heating rate of 10 °C/min and the initial degradation temperature (Tonset) and the temperature of maximum degradation rate (Tmax) were determined. Melting (Tm) and crystallization (Tc) temperatures were determined from the second heating using DSC equipment (TA Instruments, Q1000) under a nitrogen atmosphere. Samples (~3 mg) in aluminum pans were first melted from 30 to 180 °C, then cooled down to 30 °C, and melted again using the same procedure. The heating and cooling rate were 10 °C/min. The isotactic index (I·I.) test was carried out in boiling heptane for 6 h using a Soxhlet extractor (ASTM D 297). Transmission electron microscopy (TEM) was carried out on a Philips equipment model EM208S, using tungsten filament. Samples for TEM analysis were prepared as films. The films were prepared by melt pressing at 180 °C for one minute using a 2 mm spacer.
Table 2 Characterization by TGA of inorganic and organically modified Mt (A40M).
3. Results 3.1. Characterization of organically modified Mt and support precursors MgCl2.EtOH/Mt produced therefrom In this work, we started from sodium Mt, a 2:1 layered silicate. Diffractograms of A40M organophilized with different amounts of ammonium salt (CTAB) were compared (Fig. 2). Intense reflection was 3
Mt
Tonset (°C)
Tmax (°C)
Volatile content (%)
Estimated ammonium salt content (%)
Sodium Mt A40M-9 A40M-18 A40M-36
112/604 371/547 381/564 277/396/ 566
112/651 416/605 421/616 313/427/ 592
14 19 20 24
0 5 6 10
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Fig. 3. TGA patterns of support precursors: AD03 (standard); AD19 (1:2) based on MgCl2.nEtOH/A40M–9; AD20 (1:2) based on MgCl2.nEtOH/A40M-18; AD18 (1:1) based on MgCl2.nEtOH/40 M-36; AD13 (1:2) based on MgCl2.nEtOH/40 M-36.
no papers in the literature that report TGAs of adducts based on MgCl2.nEtOH/Mt The main reference in obtaining Ziegler-Natta catalysts with controlled morphology and synthesized with support based on MgCl2.nEtOH/Mt is found in WO 2009080568 (Collina et al., 2009). The supports prepared with 0.1 to 1% of Mt in MgCl2 were analyzed by XRD and DSC and results demonstrated different physicochemical properties when compared to conventional adducts but the information is limited. To the best of our knowledge, supports were evaluated with high contents of Mt and might benefit the expansion of interlayer spacing. Thermal stability of the catalyst precursors (AD) produced and the influence of the Mt in its structure were investigated by TGA measurements (Fig. 3) The thermograms show the derivative curve peaks (DTG) that provide information about the structure and mechanism of thermal decomposition of the adducts produced with different contents of Mt compared to standard MgCl2.nEtOH. In the standard adduct (AD03) there are peaks at 188 °C, 219 °C and 254 °C for MgCl2 molecules with low alcohol content. According to the literature (Jiang et al., 2008), up to 300 °C, the mass loss may correspond to ethanol and HCl molecules generated by the adduct decomposition. Above 300 °C, peaks at 435 °C and 434 °C probably correspond to the decomposition of compound Mg(OH)Cl. As demonstrated (Bart and Roovers, 1995), peaks near 450 °C are caused by the decomposition of this compound. Support precursors based on MgCl2.EtOH, as AD03, exhibited no mass loss in the temperature range of 300 and 400 °C. Instead, Mt treated with different CTAB contents presented thermal decomposition peaks in the range of 300 to 500 °C corresponding to the organic fraction in its structure. Comparing thermograms of organically modified montmorillonites with produced precursors (AD) therefrom, it was possible to observe the
mass loss of organic fraction at temperatures above 300 °C for all substrates. DTG profile of precursor AD19, prepared with A40M-9, allows one to observe well distributed mass loss peaks in the temperature range between 50 °C and 200 °C indicating good incorporation of the EtOH molecules. Peaks are also verified at 392 °C and 557 °C, similar to Mt present in its composition. Similar to adduct AD19, support precursor AD20 exhibited DTG peaks corresponding to those of Mt in the same temperature range, as well as peaks corresponding to the loss of EtOH molecules. The increase of the CTAB content to 180 cmol/kg in the treatment of A40M resulted in a slight increase in the organic fraction in its composition, and this profile is very similar to the A40M treated with 90 cmol/kg. Adduct AD18 revealed several peaks in the temperature range of 75 °C to 200 °C, corresponding to the mass loss of MgCl2 molecules with different numbers of moles of alcohol. At 321 °C, there is a slight enlargement of the peak, corresponding to the decomposition of organic fraction present in its structure. It was revealed that the mass loss up to 300 °C was 50%. Similarly, support precursor AD13, prepared with the same Mt, but with MgCl2:Mt ratio of 1:2, showed DTG peaks in the same temperature range. Above 300 °C, the profile of the TG derivative is very similar to that of the Mt used in its formulation. For precursors AD19 and AD20, a lower number of peaks and mass loss values were found to be much lower than those for AD18 and AD13. At a temperature of up to 300 °C support precursor AD20 lost only 15% of the mass. This can be explained by the fact that organophilic A40M with 90 and 180 cmol/kg of CTAB has the largest inorganic fraction in its structure.
Fig. 4. SEM micrographs of (a) support AD19 (1:2) and catalysts: (b) C13 (1:2) and (c) C18 (1:2) (MgCl2:A40M mass ratios). 4
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samples are presented in Fig. 5. Comparing characterizations results of organically modified Mt (Fig. 2) with the catalysts produced therefrom, it was possible to observe the presence of characteristic diffraction peak of Mt displaced to small angles (catalyst C19) and strong reduction in tactoids size (catalysts C13 and C18) accompanied by the decrease of the interlayer space of the Mt The catalyst C19 presented a very wide diffraction peak and more displaced to smaller angles (2θ = 5.8°, corresponding to the basal space of d = 1,52 nm), which reveals a wider distribution of interlayer spaces, with lamellae presenting different basal sizes. Comparing the results of the XRD and TG of montmorillonites A40M-18 and A40M-9, it was not possible to observe significant differences between them. However, the XRD analysis revealed that each amount of CTAB used to treat Mt resulted in a catalyst with different structural properties. Catalyst C19 showed to be more exfoliated and/or intercalated when smaller salt content was used. This fact is important because it favors good fixation of the internal donor and Ti species on the MgCl2 crystal. On the other hand, catalyst C18 and C13 presented diffraction peak at 2θ = 6.8° and 2θ = 6.5°, corresponding to the basal distance of d = 1,25 nm and d = 1,36 nm, respectively, while A40M-36 presented diffraction peak at 2θ = 5.2°, corresponding to the basal distance of d = 1,36 nm. The increase of the CTAB content in the organophilized sodium Mt to 360 cmol/kg of salt possibly resulted in the greater difficulty of access of the MgCl2 to the interior of the interlayer space. In addition, the reaction of TiCl4 directly in the OH groups on the surface of the Mt lamellae, including into the space between sheets, may have caused the lamella to approach, reducing the basal spacing. The catalyst prepared with A40M-18 (catalyst C20) presented a diffraction peak at 2θ = 6,5° (corresponding to the basal space of 1.36 nm), which is discretely more shifted, wider and less pronounced compared to Mt This result clearly confirms that there was intercalation of the catalytic components in the interlayer spacing of the Mt, as catalyst C19. At higher angles, above 20°, there is also a greater structural disorder for all catalysts.
Table 3 Characterization by EDX of the catalysts based on MgCl2/Mt/DI/TiCl4. Catalyst
MgCl2:Mt (mass ratios)
Mt
Ti (%)
Mg (%)
Cl (%)
C19 C20 C18 C13
1:2 1:2 1:1 1:2
A40M-9 A40M-18 A40M-36 A40M-36
5.8 4.5 3.0 2.0
1.8 3.9 3.7 4.0
16.0 17.9 17.7 17.2
Fig. 5. XRD patterns of the catalysts containing organically modified Mt with different amount of CTAB.
3.2. Characterization of catalysts produced from MgCl2.EtOH/Mt
3.3. Polypropylene/Mt nanocomposites produced by in situ polymerization with catalysts based on MgCl2/Mt/DI/TiCl4
SEM micrographs of the catalyst support precursors and the final catalysts produced with different mass ratios of MgCl2/A40M Mt (Fig. 4) reveals the presence of Mt at different amounts in the preparation of the catalytic support and the catalyst did not compromise the spherical morphology of the same. EDX technique was also used to quantify chemical composition (Table 3) and identify variations in the samples. Ti contents were close to those of the literature (Yank et al., 2007; Nikolaeva et al., 2018; Sukulova et al., 2019), the mass average was 3.8%. However, the percentages of magnesium found were low. It can be justified by the presence of the inorganic filler in excess compared to MgCl2. There is no Mg species in the Mt structure. Since XRD gives information on the crystalline and well-ordered part of the catalysts, the incorporation of the Mt and its degree of dispersion can be well evaluated by this technique. XRD patterns of the
In order to evaluate the influence of reaction time on the catalytic activity (Table 4) and properties of CPN produced (Table 5), polymerization reactions were performed in 30 and 60 min. The catalyst C19, prepared A40M with the lowest CTAB content (90 cmol/kg Mt), showed superior activity. The increase of the salt content to 180 and 360 cmol/kg Mt in catalyst C20, C13 and C18, respectively, resulted in a decrease in the catalytic activity for polymerizations in 30 and 60 min. These results corroborate XRD diffraction analysis of the catalysts, where it was possible to verify that catalyst C19 exhibits greater interlayer space, followed by the catalysts C20 and C13. The amount of CTAB incorporated in the silicate sheets and used to make catalyst support may exert influence on the activity of the catalyst. The
Table 4 Results of nanocomposites and masterbatches of propylene polymerization catalyzed by MgCl2/Mt/DIBP/TiCl4. Catalyst
MgCl2/Mt ratio (wt%)
Content of salt (cmol/kg of Mt)
Time reaction (min.)
Sample
Polymer produced (g)
a C.A (gPP/gMgCl2*h)
C19
1:2
90
C20
1:2
180
C13
1:2
360
C18
1:1
360
30 60 30 60 30 60 30 60
NPP19-2 NPP19 NPP20-2 NPP20 NPP13-2 NPP13 NPP18-2 NPP18
2.7 7.0 2.0 2.8 1.5 2.4 0.1 2.0
18.0 23.3 9.3 13.3 8.0 10.0 0.7 6.3
Reactions conditions: Hexane as solvent (100 mL); Pressure of 4 bar; TEA as cocatalyst ([TEA] = 0,15 mmol/g MgCl2; molar ratio TEA/ ED = 50 a C.A: catalytic activity.
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Table 5 Characterization of nanocomposites and masterbatches of PP/A40M. DSC
TGA
Catalyst
Polymer
Mt (wt%)
Tm (°C)
Tc (°C)
∆Hm (J/g)
Tonset (°C)
Tmax (°C)
Residue (°C)
I.I. (%)
C19–1:2
NPP19–2 NPP19 NPP20–2 NPP20 NPP13–2 NPP13 NPP18–2 NPP18
22.0 8.5 30.0 21.0 40.0 25.0 ~100.0 16.0
155 158 154 154 155 153 145 157
118 121 119 119 121 119 115 119
7.4 86.3 42.5 80.8 58.6 69.0 48.7 69.1
nd 440 260 404 248/421 287/431 236 428
nd 455 291 430 287/431 247/421 276/416 450
nd nd nd nd 6.4 6.6 nd nd
90 98 nd 53 nd nd nd 84
C-1:2 C-1:2 C-1:1
I.I.: isotacticity index by heptane soluble; nd: not determined.
ammonium salt used in the treatment of A40 has a long chain of carbon atoms (C16), which may hinder the fixation of the Ti atoms and internal donor over MgCl2 crystal in the interlayer space. Furthermore, the Mt lamellae can also hinder the access of the monomer to the active centers. In the literature (Zhang et al., 2016), the decrease of catalyst activity in ethylene polymerization was attributed mainly to the layered shape of the graphene (present in the catalyst) acting as a physical barrier, thus retarding the diffusion of the monomer into the catalyst core. Propylene polymerization is even more complex, due to the presence of methyl in the monomer and internal and external electron donors (present in the structure of the catalyst and polymerization reaction, respectively) that plays an important role in the catalyst activity, as well as in PP stereoregularity. However, one cannot rule out the possibility that the Ti species supported on MgCl2 are bound to other OH-Si groups from nearby lamellae, thus inactivating some sites of polymerization. In our previous study (Cardoso et al., 2018), reduction of catalyst activity when commercial organic Mt content was increased in support catalyst composition was reported. Other similar and important aspect in previous study was catalyst response to the kinetic profile. It has reduced the polymerization rate in the beginning of reaction and increased with the polymerization time. This behavior is opposite to that of conventional ZN catalyst that showed high initial polymerization rates in the first minutes of reaction and a decay with time, as reported in the literature (Wang et al., 2005; McKenna et al., 2010). A particularly important structural effect, usually reported when Mt is well dispersed in the matrix of PP, is an increase of crystallization temperature Tc, normally in nanocomposites where montmorillonites
act as nucleating agents (Shi and Dou, 2013). In this work, no relevant changes were observed for Tc of NPC obtained in different time reaction, as well as, in the melting temperature (Tm). Meanwhile, melting enthalpies (ΔHm) were much lower for all samples synthesized in 30 min polymerization and sharply increased with increase in time of polymerization, indicating enhancement of crystallinity of nanocomposites and masterbatches. These results were expected due to the high A40M content. The presence of Mt sheets forms a barrier making it difficult to pack polymer chain and resulting in many imperfections. It may be a cause of low isotacticity of the samples NPP19–2 and NPP18. Nevertheless, the results reveal good crystallinity content for the samples, since they present a percentage of A40M higher than 8%. The melting endotherms of the samples also were obtained (Fig. 6). It revealed an enlargement of the melt endotherms, especially in the samples obtained in 30 min, and the displacement of the Tm to lower temperatures with the increase of the content of A40M. This fact reveals the formation of imperfect crystals of PP in the presence of the Mt as reported previously (Wang and He, 2015; Cardoso et al., 2018). It is interesting to note that the stereoregularity of CPN synthesized with the catalyst C19 was high, isotacticity index (I.I.) reached 98% in the polymerization of 60 min, despite the high A40M content present (8.5%). Wang and He (2011) used 1-hexadecane-3-methylimidazolium bromine to prepare organically modified Mt to produce MgCl2/Mt/ TiCl4 catalyst and I.I of PP/Mt reached 99%, but the amount of Mt obtained in nanocomposites was 0.9%, a value much lower than that described in this paper. This result is typical for titanium‑magnesium Ziegler-Natta catalyst with high stereoselectivity (Zhu et al., 2017) and suggests that although clay minerals were present in the composition of
1,5
Polymerization time: 30 min. NPP13-2
2,0
NPP19
1,0
NPP20 1,5
NPP20-2
Heat Flow (W/g)
Heat Flow (W/g)
NPP18-2
0,5
NPP19-2
NPP13 1,0
0,5
NPP18 0,0 40
60
80
100
120
140
160
180
Temperature (°C)
0,0 40
60
80
100
120
Temperature (°C) Fig. 6. DSC thermogram of heating curves of polypropylene/A40M synthetized in (a) 30 min and (b) 60 min. 6
140
160
180
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Fig. 7. XRD patterns of the nanocomposites and masterbatches produced in different reaction times: (a) NPP19 (22% A40M-9, 60 min of polymerization); b) NPP20–2 (30% A40M-18, 30 min of polymerization) and NPP20 (21% A40M-18, 60 min of polymerization); (c) NPP18–2 (~100% A40M-36, 30 min of polymerization) and NPP18 (16% A40M-36, 60 min of polymerization); (d) NPP13–2 (40% A40M-36, 30 min of polymerization) and NPP13 (25% A40M-36, 60 min of polymerization.
Fig. 8. TEM micrographs of polypropylene/A40M: a) e b) NPP19 (22% A40M-9, 60 min of polymerization); c) NPP13–2 (40% A40M-36, 30 min of polymerization) and d) NPP13 (25% A40M-36, 60 min of polymerization).
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adduct MgCl2.EtOH, as previously mentioned by TGA curves, they did not prevent the formation of active centers with high specificity. The role that internal and external donors exert on the surface of magnesium chloride to control the stereoselectivity of the catalyst does not appear to be compromised by the presence of high Mt contents. The thermogravimetric analysis revealed high Tmax for the materials synthesized in 60 min polymerizations, especially NPP19 (455°). It can be verified that in the polymerizations of 30 min the thermal degradation temperature assumes lower values. TGA profile obtained from NCP and masterbatch NPP13, NPP13-2, and NPP18 were bimodal according to the profile of Mt present in their composition (A40M-36). The effect of A40M in the crystalline structure of CPN and masterbatches were also studied by XRD (Fig. 7). For sample NPP19, it is possible to verify that no diffraction peaks are related to A40M-9, indicating good dispersion of the Mt in the matrix of polypropylene. For sample NPP20-2 produced with 30 min of polymerization, an enlarged diffraction peak is exhibited at 2θ = 6.25° (1.46 nm). There was the displacement of the diffraction peak to a lower angle compared to the peak of the Mt present in the catalyst. In 60 min of reaction (sample NPP20), the diffraction peak was shifted towards the smallest angles (2θ = 5.7°), which indicates higher polymer intercalation with increasing polymerization time. Samples NPP13-2 and NPP13 exhibited intense diffraction peak at 6.35° and 6.2°, respectively, corresponding to an interlayer distance of 1.39 nm and 1.43 nm, respectively) close to that of the Mt in the catalyst. This case suggests there was little intercalation. Moreover, it is possible to observe the presence of diffraction peaks corresponding to the γ phase of polypropylene. We believe that the γ-crystallites are formed by the action of the Mt as nucleating, as reported by (Nam et al., 2001; Maiti et al., 2002; Cardoso et al., 2018). The NCP diffractograms obtained in 30 and 60 min with the catalyst C18 show that the increase in reaction time led to the complete exfoliation of the Mt (NPP18). The catalyst C18 was prepared with the same Mt of catalyst C13, but with less proportion of MgCl2 in its formulation (MgCl2: Mt = 1:1). In general, it is possible to observe major structural disorder in the crystalline phases of polypropylene/Mt compared to pure polypropylene as previously reported (Kim et al., 2004). The morphology of CPN was evaluated by TEM (Fig. 8). Analysis of sample NPP19 confirmed good dispersion of the Mt in the polypropylene matrix, despite the high Mt content (22%). Part of Mt's sheets is fully exfoliated, while other part contains polymer chains interspersed between Mt's lamellae, forming stacks of various thicknesses. Comparing with TEM result of sample NPP13, which contains a similar amount of Mt (25%), a lower degree of exfoliation was reached. It is reasonably believed that it is a consequence of poor fixation of the catalytic components inside the interlayer space of the A40M-36 as demonstrated for the catalyst C13 by XRD. This fact resulted in the lower production of polymer in this space and, consequently, in the less intercalation/exfoliation of the Mt when compared to sample NPP19.
interlayer space and even the exfoliation of the Mt Nanocomposites obtained presented good thermal properties, achieving a high degree of isotacticity even with 8.5% of Mt As observed in TEM images, the resultant PP/Mt nanocomposites and masterbatches exhibited Mt sheet homogeneously distributed in the polymeric matrix, with intercalated and exfoliated morphology. Acknowledgements This work was supported by the Ministerio da Ciencia e Tecnologia, Inovações e Comunicações, CNPq; The Coordenação de Aperfeiçoamento de Pessoal de Nivel Superior (CAPES); and the Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro – FAPERJ (Grant Nos. 41472045). References Abedi, S., Abdouss, M., 2014. Review of clay-supported Ziegler–Natta catalysts for production of polyolefin/clay nanocomposites through in situ polymerization. Appl. Catalysis A Gen. 475, 386–409. Bart, J.C.J., Roovers, W., 1995. Magnesium chloride - ethanol adducts. J. Mater. Sci. 30, 2809–2820. Cardoso, R.S., Oliveira, J.S., Ramis, L.B., Marques, M.F.V., 2018. Ziegler-Natta catalyst based on MgCl2/Clay/ID/TiCl4 for the synthesis of spherical particles of polypropylene nanocomposites. J. Nanosci. Nanotechnol. 18, 5124–5132. Collina, G., Evangelist, D., Giampiero, M., Ferrara, G., 2009. Magnesium Dichloride-Alcohol Adducts and Catalyst obtained Therefrom. WO 2009080568. Dittrich, B., Wartig, K.A., Hofmann, D., Mülhaupt, R., Schartel, B., 2013. Carbon black, multiwall carbon nanotubes, expanded graphite and functionalized graphene flame retarded polypropylene nanocomposites. Polym. Adv. Technol. 24, 916–926. Dong, J.-Y., Qin, Y., 2019a. Polypropylene nanocomposites containing multi-walled carbon nanotubes and graphene oxide by Ziegler-Natta Catalysis. In: Aliofkhazraei, M. (Ed.), Advances in Nanostructured Composites. CRC Press 1. Taylor and Francis group, Boca Raton, pp. 424–441 (Chapter 19). Dong, J.-Y., Qin, Y., 2019b. Polypropylene nanocomposites containing multi-walled carbon nanotubes and graphene oxide by Ziegler-Natta Catalysis. In: Aliofkhazraei, M. (Ed.), Advances in Nanostructured Composites. CRC Press 1. Taylor and Francis group, Boca Raton, pp. 424–441 (Chapter 19). Gordana, B.G., 2014. Advances in polypropylene based materials, Contributions. Sec. Nat. Math. Biotech. Sci. 35, 121–138. Haddadi, S.A., Amini, M., Kheradmand, A., 2018. In-situ preparation and characterization of ultra-high molecular weight polyethylene/diamond nanocomposites using Bi-supported Ziegler-Natta catalyst: Effect of nanodiamond silanization. Mater. Today Commun. 14, 53–64. Jiang, X., Tian, X., Fan, Z., 2008. Crystal structure of ball-milled mixture of sodium chloride and magnesium chloride-ethanol adduct. Mater. Res. Bull. 43, 343–352. Kaminsky, W., 2018. Polyolefin-nanocomposites with special properties by in-situ polymerization. Front. Chem. Sci. Eng. 12, 555–563. 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4. Conclusions The catalyst precursors prepared with high amounts of Mt (MgCl2.EtOH/A40M) resulted in significant differences in thermal decomposition profile compared to conventional adduct based on MgCl2.nEtOH. The TG measurements showed thermal decomposition peaks corresponding to the organic fraction of the Mt in its structure. It was verified that among the CTAB concentrations evaluated, the use of lower content (90 cmol/kg Mt) to organophilize inorganic Mt was sufficient to produce a catalyst with a good distribution in its interlayer space. The catalyst produced therefrom exhibited higher catalytic activity in the synthesis of nanocomposite with the highest properties. It was possible to notice that there was good incorporation of the Mt in the catalyst composition without compromising the spherical morphological character. XRD analysis confirmed that catalyst components were found in the interlayer space of Mt and the growth of polypropylene chains during polymerization led to the increase of the 8
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R. da Silva Cardoso, et al. distribution of MgCl2-supported Ziegler-Natta catalysts investigated by multiple active sites model. Macromol. Chem. Phys. 206, 961–966. Yang, F., Zhang, X., Zhao, H., Chen, B., Huang, B., Feng, Z., 2003. Preparation and properties of polyethylene/montmorillonite nanocomposites by in situ polymerization. J. Appl. Polym. Sci. 89, 3680–3684. Yank, k., Huang, Y., Dong, J., 2007. Preparation of polypropylene/montmorillonite nanocomposites by intercalative polymerization: effect of in situ polymer matrix functionalization on the stability of the nanocomposite structure. Polym. 52, 181–187. Zhang, H.-X., Ko, E.-B., Park, J.-H., Moon, Y.-K., Zhang, X.-Q., Yoon, K.-B., 2016a. Fabrication
of polyethylene/graphene nanocomposites through in situ polymerization with a spherical graphene/MgCl2-supported Ziegler-Natta catalyst. Compos. Sci. Technol. 136, 61–66. Zhang, H.X., Ko, E.B., Park, J.H., Moon, Y.K., Zhang, X.Q., Yoon, K.B., 2016b. Fabrication of polyethylene/graphene nanocomposites through in situ polymerization with a spherical graphene/MgCl2-supported Ziegler-Natta catalyst. Compos. Sci. Technol. 136, 61–66. Zhu, W., Tian, Z., Cheng, R., He, X., Liu, Z., Zhao, N., Liu, B., 2017. Exploring Si/Mg composite supported Ziegler-Natta Ti-based catalysts for propylene polymerization. Chinese J. Polym. Sci. 35, 1474–1487.
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