In situ synthesis of nanoclay filled polyethylene using polymer supported metallocene catalyst system

ARTIGO

In situ synthesis of nanoclay filled polyethylene using polymer supported metallocene catalyst system

Z. V. P. Murthy^I,*; Parimal A. Parikh^I; Smitha Rajesh^II

^IChemical Engineering Department, Sardar Vallabhbhai National Institute of Technology, Surat 395007, Gujarat, India

^IIChemical Engineering Department, Sardar Vallabhbhai National Institute of Technology, Surat 395007, Gujarat / Reliance Technology Group, Hazira Manufacturing Division, Reliance Industries Limited, Surat 394510, Gujarat, India

ABSTRACT

In situ ethylene polymerizations were performed using bis(cyclopentadiene)titanium dichloride supported on polyethersulfone as catalyst. The bis(cyclopentadiene)titanium dichloride supported on polyethersulfone catalyst activity estimated by ethylene polymerization was 360 kgPE/molTi/h. During polymerization the fillers used were montmorillionite nanoclays having surface modifications with 35-45 wt% dimethyl dialkyl(14-18)amine (FA) and 25-30 wt% trimethyl stearyl ammonium (FB). These fillers were pretreated with methylaluminoxine (MAO; cocatalyst) for better dispersion onto the polymer matrix. The formation of polyethylene within the whole matrix was confirmed by FTIR studies. It was found that the nature of nanofiller did not have any remarkable effect on the melting characteristics of the polymer. TGA study indicates that nanoclay FB filled polyethylene has higher thermal stability than nanoclay FA filled polyethylene. The melting temperature of the obtained polyethylenes was 142 ºC, which corresponds to that synthesized by the polyether sulfone supported catalyst.

Keywords: nanoclay; polymeric support; bis(cyclopentadiene)titanium dichloride.

INTRODUCTION

Polymer nanocomposites have attracted great interest, both in industry and in academy, because they exhibit remarkable improvement in material properties compared to virgin polymer or conventional micro- and macro-composites.^1-3 Preparation of desired products from polymer nanocomposites is a challenging area that requires considerable efforts. Polyethylene (PE) is one of the most widely used polyolefin but the use is restricted in certain applications because of its low melting point, non-degradability, and the tendency to crack when stressed. Research on reactions like grafting, cross-linking and blending of PE with inorganic fillers to mitigate these disadvantages has been extensively investigated for many years.^4-6 Introduction of nanofillers onto polyethylene matrix have a strong impact on the macroscopic properties of the polymer even though the filler content is hardly few weight percent. The properties such as heat distortion temperatures, flame resistance, modulus, barrier properties and electronic and optical properties are dramatically enhanced.^7-9

Various methodologies of nanofiller filled PE synthesis are available globally which includes foremost ones like sol-gel reactions, intercalative polymerization, melt processing and polymerization filling technique (PFT). Originated by Howard et al.,¹⁰ PFT in which onto the surface of filler, Ziegler-Natta-type catalyst is immobilized where further on olefin monomer is polymerized, has been further explored by Jerome et. al.^11-13 and Das et. al..^14,15 This methodology provides with such nanocomposites which have higher filler content but without sacrificing their enhanced mechanical properties.

The commodity polymers i.e. polyethylenes are synthesized using organometallic coordination catalysts such as heterogeneous Ziegler-Natta catalysts and homogeneous metallocene catalysts.^16,17 Metallocene catalysts have created a new area in the polymerization of olefins by providing new polymer structures and resulting properties in comparison to the traditional Ziegler catalysts. These complexes exhibit high catalyst activities and only one type of active site (single site catalyst). Control of molar mass, narrow molecular weight distributions and structural uniformity related to comonomer content are achieved through precise selection of reaction conditions.^18-22

In recent years, special attention has been devoted to the immobilization methodology of these catalyst systems either on organic or inorganic materials. Apart from using inorganic supports like magnesium chloride, silica, alumina, TiO₂ etc, polymeric support like pentaerythritol trimethacrylate, divinyl benzene, 2-hydroxyethyl methacrylate, styrene etc have been utilized for immobilization of metallocenes.^23-29 Beside performing the basic role of the support like exposure of active sites, stabilization of active sites, high activity of catalyst and precise control over polymer properties, polymeric support also eliminates the side effects of inorganic support like reduction in ash content etc.

In the present work, we have immobilized bis(cyclopentadiene)titanium dichloride onto polyethersulphone as the support. The resultant light red colored solid catalyst was added to the MAO pre-treated nanaoclays having surface modification of 35-45 wt% dimethyl dialkyl(14-18)amine (FA) and 25-30 wt% trimethyl stearyl ammonium (FB). This mixture of nanofiller, cocatalyst and supported catalyst was used for atmospheric ethylene polymerization. The resultant in situ nanoclay filled polyethylenes were characterized by FTIR, DSC for melting temperature and crystallinity impact and TGA for filler weight percent. The ratio of catalyst to MAO treated nanoclay were also varied to understand the impact on the polymerization performance, melting properties, crystallinity and inclusion of filler onto PE matrix.

EXPERIMENTAL

Materials

All the chemicals were procured from Sigma-Aldrich and used as such or otherwise stated. Poly(oxy-1,4-phenylene-sulfonyl-1,4-phenylene; PES), n-hexane (99.9%), toluene (99.9%), nanoclay based on montmorillonite, whose surface has been modified with 35-45 wt% dimethyl amine (FA) and nanoclay modified with 25-30 wt% trimethyl stearyl ammonium (FB) were used as received. Bis(cyclopentadiene)titanium dichloride (Cp₂TiCl₂; Strem) was used as such. High purity polymerization grade ethylene gas of 99.9% purity was taken from commercial plant and used with out further purification. Methylaluminoxane (MAO; 10% v/v in toluene) was used as obtained from Crompton, GmbH.

Synthesis of PES supported Cp₂TiCl₂ catalyst

For synthesizing the polymeric supported metallocene catalyst, commercially available PES (having M_n = 38000 g mol^-1; melt flow index =16 g/10 min) was taken as the support. A 5.0 g of PES was dissolved in the dry dichloromethane by stirring for 45 min. To this solution, 0.2 g of bis(cyclopentadiene)titanium dichloride was added to keep the ratio 0.04 with continuous stirring. The reaction mixture was allowed to stir at ambient temperature for 24 h, washed with dry toluene (100 mL × 5) to remove the excess metallocene and dried under vacuum. The dried light red catalyst was subjected to titanium estimation, which was 0.80 wt% by calorimetric method using UV-Vis spectroscopy.

Polymerization of ethylene using PES supported Cp₂TiCl₂ catalyst

All the polymerizations were carried out in 1 L double jacketed glass reactor equipped with a top mounted mechanical stirrer under nitrogen atmosphere. After the addition of toluene (100 mL), it was saturated with ethylene followed by the addition of MAO (3 mL; 10% v/v; Al/Ti mole ratio = 2500). Finally the catalyst (0.25 g) was added to start the polymerization at ambient conditions. Addition of ethylene was continued during the 60 min polymerization. Polymerization was stopped by the addition of sufficient amount of acidified methanol and the precipitated polymer was filtered, washed with methanol and dried under vacuum.

Synthesis of in situ nanoclay filled polyethylene using PES supported catalyst

In 100 mL toluene, MAO (2.6 mL; 10%, v/v; Al/Ti mole ratio = 2500) was added followed by the nanoclay (0.12, 0.25, 0.50 g) and stirred for 30 min. Then 200 mL toluene was added followed by MAO (3mL, 10% v/v; Al/Ti mole ratio = 2500) and finally the catalyst (0.25 g) was added to start the polymerization, then ethylene addition was continued during the 60 min polymerization. The polymerization was stopped by the addition of sufficient amount of acidified methanol. The precipitated polymer was filtered, washed with methanol and dried under vacuum.

Characterization of PES supported Cp₂TiCl₂ catalyst

For the determination of titanium content in the catalyst, the catalyst was dissolved in an acidic media. The titanium contained in the catalysts was converted in to Ti⁴⁺ by the addition of H₂O₂. UV-vis spectra of the resultant solution of peroxotitanium complexes were recorded on a Perkin-Elmer UV Lambda12 spectrometer.³⁰ The intensity of a peak at 410 nm (ε = 782.7 L^-1 mol^-1 cm^-1) was used to quantify the titanium content. FTIR studies were performed on Perkin-Elmer spectrum GX instrument with 4 cm^-1 resolution and 32 scans with KBr pellet for determining the characteristic vibrational modes of the catalyst.³¹

Characterization of in situ nanoclay filled polyethylene

FTIR studies were performed on Perkin-Elmer spectrum GX instrument with 4 cm^-1 resolution and 32 scans with KBr pellet in transmittance mode. The melting points of PEs were measured by differential scanning calorimetry (DSC) with a Perkin-Elmer DSC-7 at a heating rate of 10 ºC/min from 50 to 300 ºC (with held for 3 min at 300 ºC), cooling rate of 10 ºC/min from 300 to 50 ºC and again heating from 50 to 300 ºC. Thermal stability studies were performed using thermogravimetric analysis (TGA) with a TGA V 50 1A Perkin Elmer thermo gravimetric analyzer in the presence of nitrogen in the temperature range of 50-650 ºC and at a heating rate of 10 ºC/min.³¹

RESULTS AND DISCUSSION

Synthesis and characterization of PES supported Cp₂TiCl₂

Bis(cyclopentadiene)titanium dichloride (Cp₂TiCl₂) was immobilized on PES, since PES has potential Lewis base sites in the form of oxygen of sulfonyl group where the coordination of titanium can be expected.^32,33 The resulting PES supported Cp₂TiCl₂ catalyst had 0.8 wt% of titanium content. The FTIR spectra of pure PES, PES supported Cp₂TiCl₂ and Cp₂TiCl₂ is shown in Figure 1. The pure PES shows a strong absorption band at 1486 cm^-1 attributed to the stretching vibration of the S=O groups belonging to the sulfone groups.³¹ After the incorporation of titanium onto the polymer a weak vibration centered at 820 cm^-1 corresponding to the complexation of titanium with sulfur group of the polymer was observed.^15,33 The room temperature atmospheric polymerization of ethylene was carried out using PES supported Cp₂TiCl₂ which showed activity of 670 kg PE/mol Ti/h indicating its potential for polymerization.

Synthesis of in situ nanoclay filled polyethylene

After ensuring the reproducibility of titanium loading on to the support (PES), this supported Cp₂TiCl₂ was further utilized for preparing in situ nanoclay filled PE. The two fillers used for this study are nanoclay based on montmorillonite, whose surface has been modified with 35-45 wt% dimethyl dialkyl(14-18)amine (FA) and with 25- 30 wt% trimethyl stearyl ammonium (FB). The whole methodology of synthesizing in situ nanoclay filled PE has been depicted in Scheme 1.

For homogenous dispersion of fillers in the polyethylene matrix, the fillers were activated in advance with MAO. This pretreatment of fillers with MAO minimizes the deactivation effects of their surfaces on catalyst activity and generates active centres for ethylene polymerization.¹⁴

For in situ nanoclay filled PE synthesis, catalyst to MAO treated nanoclay ratio was varied in order to see how the activity of the PES supported Cp₂TiCl₂ was affected. Table 1 shows the experimental data and the results of the variations carried out. For system where MAO treated FB filler was used, with the introduction of the FB filler, the activity (950 kg PE/mol Ti/h) of PES supported Cp₂TiCl₂ increased. As the PES supported Cp₂TiCl₂ catalyst to MAO treated FB filler ratio was increased to 2, activity showed decreasing trend, whereas, when the ratio was changed to 0.5, marked increase in activity was observed. The observed increase in activity with increase in MAO treated filler content may be due to the generation and stabilization of active centers which are formed when enough cocatalyst is present to reduce titanium as depicted in Scheme 2a. At lower supported catalyst to FB ratio, there is not enough cocatalyst to activate the sites for polymerizing ethylene but once the amount of cocatalyst is enough, all the sites are activated to get the polymer.

For system where MAO treated FA filler was used, with introduction of filler, activity was found to be unaffected. This may be due to the bulkiness in the chain length of FB filler. As the PES supported catalyst to MAO treated FA filler ratio was changed to 2, activity got decreased. Even when the ratio was changed to 0.5, one observed decrease in activity. Again this might be assumed that bulkiness of FA filler is decreasing the activity by resisting the chain growth of ethylene monomer as shown in Scheme 2b.

Characterization of in situ nanoclay filled polyethylene

A comparison of FTIR spectra of PE synthesized by Cp₂TiCl₂; PES supported Cp₂TiCl₂; in situ FB filled PE and in situ FA filled PE is shown in Figure 2. The strong absorption band at 2856 cm^-1 corresponding to υ(C-H) stretching of PE is being observed in Figures 2a,^b. This band is also present in nanoclay filled PE (Figures 2c,d). The occurrence of these IR bands indicates that PE has been formed in the presence of organic support and nanoclay during the polymerization. A strong band at 1474 cm^-1 corresponding to υ(C-H) deformation of tertiary carbon is also present. The ¹³C{¹H} NMR spectrum of PE synthesized by PES supported Cp₂TiCl₄ catalyst where only one strong resonance peak was observed at 29.41 ppm, which is attributed to the main chain methylene carbon.¹⁴ Non-existence of other resonance peaks indicates the absence of branching in the homopolymer synthesized using the polymeric supported catalyst.

The DSC analysis (Figure 3) shows the melting and crystallization temperatures of nanoclay filled PE. The melting as well as crystallization temperatures show no remarkable effect by the addition of nanoclays. There is insignificant change in the crystallization onset temperature indicating that the inclusions induce heterogeneous nucleation. The melt enthalpy observed for FB case increases as the supported catalyst to FB ratio decreases which can be attributed to the difference in molecular weight of PE chain as the productivity is different. The presence of nanoclay shows major influence on the degree of crystallinity of PE as shown in Table 2. The degree of crystallinity^{14, 31} of nanoclay filled PEs were measured using the following formula

where X = percent crystallinity of the composites; ΔH_m= melting enthalpy (J g^-1) of the nanoclay filled PE; ΔH_o= melting enthalpy (293 J g^-1) of 100% crystalline PE;¹⁴ m_p= mass (g) of PE in the nanoclay filled PE; and m_c= mass (g) of the nanoclay filled PE. The high melting point of the polymers (139 ºC) indicates the branchless structure of the homopolymer.

To understand the weight percentage of the nanoclay into the polymer, thermo gravimetric analyses were conducted (Figure 4). The results obtained clearly indicate that when PES supported catalyst with FB at 1:0.5 ratio based PE was heated from 40 to 400 ºC (Figures 4a-c); there was a gradual reduction in weight percentage. The weight loss of ~68% was observed from 400 to 500 ºC, which remains constant after 500 ºC temperature. At 600 ºC, 12 wt% of material was obtained. Similar was the case when the catalyst-to-clay ratio was maintained as 1:1 and 1:2 and 1:3 indicating there is no much change in the weight reduction during heating due to the incorporation of clay to the PE.

 

For FA filled PE, the weight loss was observed to be ~20% on reaching 400 ºC (Figures 4d-f), followed by reduction in weight (~64%) from 400 to 500 ºC. At temperature above 550 ºC no significant weight loss was observed, indicating that ~19 wt% clay has been associated with PE. The gradual weight loss was observed in all the cases where nanoclay-FA was incorporated with PE.

The weight percentages of the fillers, as elucidated by ash content, are also in close agreement as shown in Table 3. These data indicate that highest residual weight percent is for FB filled at a ratio of 1:2 PE which is also supported by the high activity observed for this system for polymerizing ethylene.

CONCLUSIONS

In situ polymerization of ethylene in the presence of MAO treated nanoclay using polyethersulfone supported Cp₂TiCl₂ catalyst has been successfully carried out to formulate nanoclay filled polyethylene with a polymer supported catalyst system. FTIR studies confirmed the formation of polyethylene. DSC study shows that both nanoclay FA and FB filled PE have high melting enthalpies with high crystallinity. The high melting point of the polymers indicates the branchless structure of the homopolymer. TGA study indicates that nanoclay FB filled polyethylene has higher thermal stability than nanoclay FA filled polyethylene. Use of Cp₂TiCl₂ for the anchoring of titanium species on to the surface of polymeric support can be used for catalyst preparation, which may be effectively and efficiently used for in situ polymerization.

Recebido em 30/8/10; aceito em 4/3/11; publicado na web em 12/5/11

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Publication Dates

• Publication in this collection
  12 Sept 2011
• Date of issue
  2011

History

• Received
  30 Aug 2010
• Accepted
  04 Mar 2011