1-butene copolymers in liquid pool and gas-phase processes: A comparative analysis

Available online at www.sciencedirect.com

European Polymer Journal 44 (2008) 1102–1113

EUROPEAN POLYMER JOURNAL www.elsevier.com/locate/europolj

Synthesis of propylene/1-butene copolymers in liquid pool and gas-phase processes: A comparative analysis Fabricio Machado a,b,*, Enrique Luis Lima b, Jose´ Carlos Pinto b, Timothy F. McKenna a,1 a b

LCPP-CNRS/ESCPE-Lyon, 43 Blvd du 11 Novembre 1918, Baˆt 308F, BP 2077, 69616 Villeurbanne Cedex, France Programa de Engenharia Quı´mica/COPPE, Universidade Federal do Rio de Janeiro, Cidade Universita´ria, CP 68502, Rio de Janeiro 21945-970, RJ, Brazil Received 14 November 2007; received in revised form 22 January 2008; accepted 24 January 2008 Available online 9 February 2008

Abstract Batch liquid pool and semibatch gas-phase polymerizations were performed with high-activity Ziegler–Natta catalysts to evaluate the effect of 1-butene on the crystallinity, the melt temperature and the average molecular weights of the final 1butene/propylene copolymers and alloys. According to the obtained results, 1-butene can be significantly incorporated into the polymer chain over the whole range of copolymer compositions in both gas and liquid-phases, leading to the decrease of the melting temperature of the copolymer resins. On the other hand, the properties of the polymer alloys seem to be less sensitive to 1-butene incorporation, indicating the development of a distinct 1-butene phase. The average molecular weights, the polydispersities and the reactivity ratios are quite different in the liquid pool and gas-phase processes, indicating that sorption/ diffusion effects may exert an important role during the copolymerization. The obtained reactivity ratios in the gas-phase are close to 1, while the reactivity ratios of propylene are systematically higher than the reactivity ratios of 1-butene in the liquid pool process. Polymer materials with large molecular weights and good particle morphology can be obtained in all analyzed cases, indicating that development of propylene/1-butene copolymer grades is indeed possible in both liquid pool and gas-phase processes. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Propylene/1-butene copolymers; Liquid pool, gas-phase and sequential polymerizations; Ziegler–Natta catalysts; Deconvolution of molecular weight distributions

1. Introduction *

Corresponding author. Present address: Nova Petroquı´mica, Rua Hidrogeˆnio, 1404, Po´lo Petroquı´mico, CEP: 42810-000 Camacßari, BA, Brazil. Tel.: +55 71 3797 3709; fax: +55 71 3632 2206. E-mail address: [email protected] (F. Machado). 1 Present address: Department of Chemical Engineering, Queen’s University, 19 Division Street, Kingston, ON, Canada K7L 3N6.

The appropriate choice of comonomers can allow for drastic modification of the properties of homopolymer resins, leading to considerable improvement of product performance. In propylene polymerizations, comonomers can be used for modification of a number of important end-use properties, such as hardness, tensile strength, stiffness,

0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.01.040

F. Machado et al. / European Polymer Journal 44 (2008) 1102–1113

density, melt point, impact strength and transparency of the final polymer resin. It is well known that the melting point and the crystallinity of isotactic polypropylene-based polymers are lowered by incorporation of comonomer units. According to Arnold et al. [1], propene/ 1-butene copolymers (CPP1B) obtained with heterogeneous Ziegler–Natta catalysts may present very interesting properties for a large number of distinct applications. For instance, propene/1-butene random copolymers can be used for development of special film applications, which require printing and metallization for improvement of the aesthetics of the final products. In this particular case, CPP1B may be regarded as very promising materials, because of the lower sensibility to the surface treatment [2]. In addition, the incorporation of 1-butene into the copolymer chains leads to decrease of the melting and the sealing initiation temperatures of the polymer film, which is advantageous when lower processing temperatures are required. Liquid pool polymerizations are extensively used in industrial practice for production of polypropylene resins because of the higher polymerization rates and easier separation and purification of the final polymer material. Gas-phase polymerizations also constitute very competitive industrial processes, because process constraints related to liquid viscosity and solubility in the liquid-phase can be eliminated. The absence of solvent treatment also reduces plant operation costs very significantly [3]. For these reasons, both liquid pool and gas-phase polymerizations based on a high-activity Ziegler– Natta catalyst system can be used successfully to produce propylene resins with different properties and copolymer compositions [4,5]. Compared to propylene and ethylene homopolymerizations and to ethylene/a-olefin copolymerizations, there are relatively few studies in the open literature about the synthesis of propylene/1-butene copolymers. Among available published studies, most investigated polymerizations in suspensions of heptane, hexane or toluene [6–11], although some few studies regard polymerizations performed in bulk [12,13] and in gas-phase [14]. These studies show that, depending on the features of the Ziegler–Natta catalyst system (for example, type of external and internal donors, catalyst carriers, isospecificity of the catalyst sites, etc.), both random and block copolymers can be obtained. There are also few studies in the open literature concerning the synthesis of propylene alloys via

1103

sequential polymerization [15–26]. More important, these works are essentially focused upon the synthesis of polypropylene-based materials, where polyethylene and/or poly(ethylene-co-polypropylene) constitute the second phase. To the best of our knowledge, the work developed by Cecchin et al. [27] is the only one that presents experimental data for propylene/1-butene alloys produced in sequential polymerizations. However, 1-butene was used in this case only to provide information about the mechanism of polypropylene growth over MgCl2/ TiCl4 catalyst system, so that no information was provided about the quality of the final polymer products. Recently, Machado et al. [28–30] developed a family of random polypropylene/1-butene copolymer grades for gas-phase and bulk processes intended for packaging and film applications. It was shown for the first time that it is possible to produce propylene/1-butene random copolymers in semibatch gas-phase polymerizations using highactivity Ziegler–Natta catalysts [28]. It was also shown that 1-butene can be incorporated into the polymer chain over the whole range of copolymer compositions during liquid pool polymerizations [29]. It was observed in both cases that 1-butene is incorporated at random into the polymer chains at high polymerization rates, resulting in polymer materials with lower melting temperatures. It was also shown that the microstructure of the resulting polymer chains can be controlled through adjustment of the propylene and hydrogen partial pressures. Machado et al. [30] also synthesized a new family of polypropylene/1-butene alloys through in situ sequential two-stage polymerizations, using a high-activity MgCl2/Ziegler–Natta catalyst. In the first stage, liquid pool propylene polymerizations were carried out in batch mode, while 1-butene was polymerized inside the polypropylene matrix in gas-phase in semibatch mode during the second polymerization stage. It was shown that it is possible to incorporate 1-butene in the polypropylene matrix in polymerizations performed at low pressures, indicating that polypropylene/1-butene alloys can also be prepared in situ for future applications as high-performance structured materials. In this paper, the results obtained by Machado and coworkers are reviewed and compared. The main objective here is presenting a critical evaluation of the main differences observed among the distinct analyzed polymerization processes and obtained polymer products. In addition, new analytical and

1104

F. Machado et al. / European Polymer Journal 44 (2008) 1102–1113

characterization results are also presented for the propylene/1-butene copolymer products obtained here. 2. Chemicals, experimental procedure and analyses

the cocatalyst and catalyst concentrations were kept constant and equal, respectively, to 1 mg of TEA/ 5 g of monomer and 1 mg of catalyst/25 g of monomer. Detailed description of reactor apparatus is presented by Machado et al. [29].

2.1. Chemicals 3.2. Gas-phase polymerizations Propylene with minimum purity of 99.5%, 1butene with minimum of purity 99.0% and hydrogen with minimum purity of 99.9% were purchased from AGA S/A (Rio de Janeiro, Brazil). Heptane (VETEC, Rio de Janeiro, Brazil) was used for preparation of cocatalyst solution and catalyst system ˚ molecular sieves slurry after pre-treatment on 3 A (purchased from Spectrum Chemical, USA). The triethylaluminum (TEA) cocatalyst was provided by Akzo Nobel, Sa˜o Paulo, Brazil. Polymerization grade cyclohexyl-methyl-dimethoxysilane (DMMCHS), kindly supplied by Suzano Petroquı´mica, was used as external electron donor. Nitrogen purchased from AGA S/A (Rio de Janeiro, Brazil), with minimum purity of 99.0%, was used to keep the reaction environment free of oxygen. The gases used in the reaction were used after purification in succes˚ molecular sive beds of copper catalysts and 3 A sieves. Unless otherwise stated, chemicals were used as received, without additional purification. A commercial MgCl2-supported TiCl4 catalyst, with catalyst titanium content of 3.0 wt.% containing diisobutyl phthalate (DIBP) as internal donor, was used to perform the polymerizations. Suzano Petroquı´mica kindly provided the catalyst samples. The reader is encouraged to read the original references for additional information. 3. Experimental procedure 3.1. Liquid pool polymerizations Bulk polymerizations were carried out in a 450mL mini bench top PARR 4562 reactor at 60 °C. The system was kept under isothermal conditions and constant agitation of 500 rpm. Gas feed lines were equipped with Brooks mass flow meters (model 5860 i). The reaction temperature and the gas feed flow rates were monitored in line with a microcomputer equipped with an AD/DA data acquisition system PCI-1710 (Advantech Brazil, Sa˜o Paulo – SP). The software ADPol 2.0 was used for data acquisition [31]. The 1-butene content of the feed was varied in the range of 0–100%, while

Gas-phase polymerizations were carried out in semibatch mode. The system was kept under isothermal conditions and constant agitation of 270 rpm. The reactor used was a 2.5 L thermostatted turbosphere stainless steel reactor, equipped with injection valves for the catalyst and comonomer feeds. Polymerizations were performed with 20 mg of commercial MgCl2-supported TiCl4 catalyst and 10 mg of DMMCHS. The 1-butene content was varied in the range of 0–15 mol%, while the TEA/DMMCHS mole ratio was kept equal to 40. The experimental setup used to carry out the polymerization reactions was similar to the one described by Machado et al. [28] and Kittilsen and McKenna [42], and the reader is referred to these publications for a more detailed description of the protocol.

3.3. Sequential polymerizations Sequential polymerizations were carried out in a 1000 mL moveable PARR 4531 reactor equipped with a PARR 4842 temperature controller. The system was kept under isothermal conditions and constant agitation of 500 rpm. Gas feed lines were equipped with Kobold MAS-4010 mass flow meters, which provided real time flowrate data. The reaction temperature and the gas feed flowrates were monitored in line with a microcomputer equipped with an AD/DA data acquisition system Advantech PCI-1710. The software ADPol 2.0 was used for data acquisition [31]. In the experimental runs, the catalyst/DMMCHS weight ratio ðRCAT=ED Þ was varied within the range of 1.8–2.3. In the first stage, liquid pool propylene polymerizations were carried out in batch mode. In the second stage, 1-butene was polymerized into the polypropylene matrix in semibatch mode in gas-phase. The experimental setup used to carry out the sequential polymerizations was similar to the one described by Machado et al. [29,30] and the reader is referred to this publication for detailed information.

F. Machado et al. / European Polymer Journal 44 (2008) 1102–1113

3.4. Analyses

r1 ¼ 2

The weight-average molecular weight and the MWD of the polymers were measured on a Waters Alliance GPCV 2000. The system was equipped with a refractometer, a viscometer and Waters Styragel HT2 and HT6E gel columns. Analyses were performed at 150 °C using trichlorobenzene as solvent. Copolymer composition was determined by liquid 13C NMR in a Bruker DRX 400 spectrometer, operating at 100.6 MHz and equipped with probes of 5 mm. The 13C NMR copolymer spectra were obtained at 90 °C. Typical accumulations included 70° flip angle and 4.44 s recycle time. Samples were dissolved in tetrachloroethylene and benzene-d6 (2/1 v/v). The melting temperature was determined by DSC measurements in a Pyris 1 calorimeter at heating rates of 5 °C/min. Surface morphology of polymer particles was determined using scanning electron microscopy (SEM). The images were recorded with a S800 Hitachi microscope operating at accelerating voltages of 15 keV. 4. Results and discussion Polymer composition was determined by 13C NMR through quantitative analysis of the characteristic peaks of the group CH2. Fig. 1 shows the characteristic CH2 peak placed at 46–47.1 ppm for polypropylene, at 40–40.1 ppm for poly(1-butene) and 43–43.6 for polypropylene/1-butene copolymer. The 1-butene content of the copolymer, the number average sequence lengths and the reactivity ratio were determined from the dyad distributions by using standard relationships [32,33]. Number average sequence lengths could also be determined from the dyad distributions and used to characterize the molecular microstructure of polymer materials through the following relationships [32]:  nB ¼

½BB þ 0:5½BP ½PP þ 0:5½BP and  nP ¼ 0:5½BP 0:5½BP

ð1Þ

The dyad sequence distribution was also used for the calculation of the reactivity ratio products, expressed in accordance with the following equation [33]: r1  r2 ¼ 4

½PP½BB ½BP2

ð2Þ

The reactivity ratios of propylene (r1) and of 1-butene (r2) can be expressed as [33]

½PPW ½BB and r2 ¼ 2 ½BP  ½BPW

1105

ð3Þ

where W is the ratio between 1-butene and propylene concentrations in the feed. P denotes propylene and B denotes 1-butene monomer units. Table 1 and Fig. 2 show how the incorporation of 1-butene (I¼ C4 Þ depends on the initial monomer feed composition (X ¼ C4 Þ. It can be observed that 1butene is significantly incorporated into the polymer chains over the entire range of feed compositions employed in the liquid pool and the gas-phase polymerization processes. According to the results presented in Table 1, the kinetics of the polymerization are not affected by the increasing 1-butene concentration, when the 1-butene concentration in the medium is relatively small (up to 10 wt.%). The reactivity ratios of both monomers change slightly and polymerizations rate are not significantly different when the 1-butene content increases. In the particular case of the gasphase propylene/1-butene polymerizations, both reactivity ratios are very close to 1. This was very surprising, because olefins of larger molecular weights are believed to react at lower rates. The distinct reactivity ratios obtained during the liquid pool and the gas-phase polymerizations performed with the same catalyst indicate that mass transfer effects (sorption and/or diffusion) should play an important role during the reaction. As a matter of fact, the solubility of 1-butene into the copolymer matrix is favored in the gas-phase with respect to propylene because of its larger molecular weight and lower vapor pressure. These effects are expected to be minimized in the liquid-phase because of the much more significant swelling of the polymer phase and the much larger density of the bulk monomer phase. This seems to indicate that the higher solubility of 1-butene in the polymer matrix in the gas-phase compensates its intrinsically lower reactivity, leading to reactivity ratios that are close to 1. This behavior changes when the concentration of 1-butene in the reaction medium is higher than 10 wt.% in the liquid-phase. As shown in Table 1 for liquid pool polymerizations, the propylene reactivity ratios increase linearly, while the 1-butene reactivity ratios tend to go through a maximum and decrease slowly to a value around 0.50. Again, this is probably related to the preferential swelling and higher diffusivity of propylene into the copolymer matrix. This also reflects the higher

1106

F. Machado et al. / European Polymer Journal 44 (2008) 1102–1113

8 4

2

7 5

3

1

6 50

45

47

40

35

46

45

30

25

44

43

20

15

42

10

41

5

40

ppm

Fig. 1. Typical

13

C NMR spectrum of propylene/1-butene copolymers.

Table 1 Copolymer composition and average sequence lengths of propylene-based polymers Run

T (°C)

P (Bar)

X¼ C4 (wt.%)

I¼ C4 (wt.%)

nB

nP

r1

r2

r1  r2

25.4 24.5 23.6 26.6 16.5 9.38 8.49 7.60

0 5 10 20 50 90 95 100

0.00 4.28 6.25 13.16 33.73 82.68 89.82 100

– 1.01 1.02 1.20 1.53 5.12 8.29 –

– 30.12 20.01 10.56 4.00 1.43 1.25 –

– 1.15 1.58 2.39 2.25 2.91 3.62 –

– 0.24 0.24 0.80 0.70 0.61 0.51 –

– 0.28 0.38 1.91 1.58 1.78 1.85 –

0 5 10 15

0.00 4.30 8.20 11.90

– 1.1 1.1 1.2

– 23.5 12.4 8.6

– 1.18 1.27 1.34

– 1.13 0.94 0.94

– 1.34 1.19 1.26

a

Liquid pool polymerization LP00 60 LP01 60 LP02 60 LP03 70 LP04 60 LP05 60 LP06 60 LP07 60 Gas-phase polymerizationb GF00 60 GF01 60 GF02 60 GF03 40 a b

4 4 4 4

The TEA/catalyst weight ratio was kept equal to 5. The polymerizations were performed without hydrogen. The TEA/catalyst weight ratio was kept equal to 15. The hydrogen concentration was kept equal to 3 mol%.

intrinsic reactivity of propylene molecules, when compared to the 1-butene monomer. Therefore, the comparative analysis of reactivity ratios obtained for the different reaction systems shows

that the intrinsic kinetic behavior of propylene/ 1-butene copolymerizations cannot be investigated without the support of good thermodynamic analysis.

F. Machado et al. / European Polymer Journal 44 (2008) 1102–1113

4 3

r1 : Liquid Pool

r1 : Gas-Phase

r2 : Liquid Pool

r2 : Gas-Phase

r1 .r2 : Liquid Pool

r1 .r2 : Gas-Phase

2 1 0 32

Average Sequence Lengths

n1 : Liquid Pool n1 : Gas-Phase

24

n2 : Liquid Pool n2 : Gas-Phase

16 8 0

1-Butene Composition in the Copolymer (wt-%)

100

Liquid Pool Gas-Phase

80 60 40 20 0 0

10

20

30

40

50

60

70

80

90

100

Feed Composition of 1-Butene (wt-%)

Fig. 2. Copolymer composition, average sequence lengths and reactivity ratios of propylene-based polymers.

According to Table 1, the product of the reactivity ratios (r1  r2) for samples obtained from liquid pool experiments increase with the 1-butene content of the copolymer chain. This indicates that deviation from ideal random copolymerizations and relative increase of 1-butene incorporation can be observed when the 1-butene content increases. The scenario is very different in the gas-phase, where conditions resemble the ideal copolymerization, where all reactivity ratios are very similar to 1. This means that liquid pool polymerizations give birth to longer monomer blocks in the polymer chain that the corresponding gas-phase polymerizations. This also means that liquid pool polymerizations are more sensitive to changes of the feed conditions than gas-phase polymerizations, because feed changes can also cause significant changes of reactivity ratios because of sorption/diffusion effects. The reactivity ratios and the number average sequence lengths of the polymeric material obtained from sequential polymerizations cannot be evaluated from the dyad distributions by using standard

relationships [32,33]. Polymer samples obtained from this process were polymer alloys, not real copolymers, because of the experimental procedure adopted to perform the polymerizations [30]. As the reactions were started in the absence of propylene in the second stage of the polymerization, BP dyad sequences were not observed in the final polymer chains. This confirmed the production of 1butene homopolymer chains in the gas-phase during the second polymerization stage. Therefore, poly(1butene) can be produced both during liquid pool and gas-phase polymerizations, which encourages the development of future poly(1-butene) grades. Fig. 3 illustrates the dyad sequences obtained through 13C NMR of the final polymer samples. It can be observed that PP dyad sequences are present in higher concentrations than BB dyad sequences, given the lower amounts of produced poly(1butene). Fig. 4 illustrates the triads distributions (PBP, PBB and BBB) [34–37], as determined from the CH signal peaks of 1-butene placed at 35.05– 35.10 ppm. As can be observed, the triad concentrations are proportional to the content of 1-butene in the copolymer chain, independent of the used polymerization process. The triads/I¼ C4 ratio is obtained in the range of 0.18–0.30 for the liquid pool polymerization, of 0.25–0.66 for the sequential polymerization and of 1.05–1.21 for the semibatch gas-phase process, indicating that the formation of triads is favored in the gas-phase polymerization process, as illustrated in Fig. 4. Therefore, it seems clear that the incorporation of 1-butene is favored in the gasphase, when compared to the results obtained for the liquid pool polymerizations. As 1-butene vapor pressures are not very low at typical reaction PP Dyad Distribution BB Dyad Distribution (mol-%) (mol-%)

Reactivity Ratios

5

1107

0.5 0.4 0.3 0.2 0.1 0.0 38 37 36 35 34 33 32 31 30

SP01

SP02

SP01

SP02

SP03

SP04

SP05

SP03

SP04

SP05

Experiment Code

Fig. 3. Dyad distribution for propylene/1-butene copolymers.

F. Machado et al. / European Polymer Journal 44 (2008) 1102–1113 25.0

Triad Distribution (mol-%)

22.5 20.0 17.5 15.0 12.5 10.0 7.5 5.0 2.5

LP 01 LP 02 LP 03 LP 04 LP 05 LP 06 G F0 1 G F0 2 G F0 3 SP 01 SP 02 SP 03 SP 04 SP 05

0.0

Experiment Code

Fig. 4. Triad distribution for propylene/1-butene copolymers.

temperatures, the use of 1-butene in monomer feed lines for production of new copolymer grades in gas-phase polypropylene facilities seems to be encouraging. The fraction of atactic polymer, soluble in hot xylene (XS) was determined through extractions via Soxhlet technique with boiling xylene, stabilized with BHT to avoid oxidative degradation, for 3 h and vacuum dried at 100 °C. Table 2 shows the results obtained to the XS. It is observed that the XS value is directly correlated with 1-butene content in the final copolymer. As a consequence, the polymer solubility in xylene increases with the incorporation of 1-butene in the polymeric chain. According to Table 1 (Run 06), the r1/r2 ratio is about 7, which indicates that the produced copolymers do not present a homogeneous chemical composition in reactions performed in batch mode because of the preferential polymerization of propylene, leading to the formation of copolymer rich in propylene in the beginning of the polymerization and very rich in 1-butene at the end of the polymerization. The isotactic index obtained for pure polypropylene is higher than the one obtained for pure poly(1-butene), as shown in Table 2. In the particular case of copolymer material with a high content of 1-butene, it is expected that the contribution of

Table 2 Xylene soluble in propylene-based polymers in liquid pool polymerizations

1-butene for the formation of copolymeric chain in the beginning of the reaction be more important for determination of the isotactic index than the propylene incorporation at the end of the polymerization. Fig. 5 illustrates the influence of the 1-butene content on melting temperature and crystallinity of propylene-based polymers. It can be observed that a small increase of the 1-butene content may lead to very significant reduction of the melting temperature of the copolymer when compared to the melting temperature of the propylene homopolymer. The crystallinity of propylene-based polymers is also influenced by the 1-butene incorporation very significantly, probably because of the regular perturbations of the geometrical structure of the polymer chains. (The incorporation of comonomer units into a homopolymer chain and reduction of crystallinity can also contribute to an increase of polymer transparency.) Depending on the 1-butene content, the melting temperature of the random copolymer can be 30 °C lower than the melting temperature of the polypropylene homopolymer, which is very significant for development of new film applications. Surprisingly, the melting temperatures of the polymer samples obtained from the two-stage polymerization processes do not change much and are even slightly higher than the ones obtained for polypropylene. This seems to confirm once more that poly(1-butene) is produced in gas-phase during the second stage of the polymerization. In addition, the formation of different crystalline structure of the propylene-based polymer in both liquid pool and gas-phase processes should be taken into consideration. 170 Gas-Phase Liquid Pool Sequential Process

165

Melting Temperature (°C)

1108

160 155 150 145 140 135 130

Run I¼ C4

(wt.%) XS (wt.%)

LP00 0.00 2.32

LP01 4.28 3.16

LP02 6.25 5.46

LP06 89.82 12.41

LP07 100.00 4.17

0

2

4

6

8

10

12

14

1-Butene Content (mol-%)

Fig. 5. Effect of 1-butene content on the melting temperature.

F. Machado et al. / European Polymer Journal 44 (2008) 1102–1113

It is well known that supported Ziegler–Natta catalysts present multiple catalyst sites, which may lead to development of multimodal distributions of final molecular properties of polymer materials, such as chain size and chain composition. Fig. 6 presents typical DSC thermograms obtained for polymer samples produced in the distinct analyzed processes. It is well known that the incorporation of comonomer units into a homopolymer chain leads to small structural imperfections, allowing for decreasing of the melting point and the crystallinity of isotactic polypropylene-based polymers. Depending on the extent of the structural changes in the polymer chain, different patterns can be observed for the shape of DSC curve. As one can observe, bimodal curves can be obtained in many of the samples. This clearly indicates that the polymer material is not homogeneous and that polymer chains with distinct composition are produced both in the gas-phase and in the liquid-phase polymerizations. In general, the bimodal behavior seems to be more pronounced for the copolymer materials obtained in the liquid pool polymerization process, which can also be related to the more pronounced variation of reactivity ratios observed for increasing 1-butene feed compositions. Therefore, detailed kinetic understanding of the analyzed copolymerizations can only be performed if the multi-site nature of the catalyst is taken into consideration. This is beyond the scope of this text and will not be performed here because the comonomer composition distribution depends on several factors, such as the nature of the catalyst system, the polymerization conditions, the polymerization process, the monomer reactivities, etc. 31 30

Liquid Pool: Run LP03 Gas-Phase: Run GF03 Sequential Process: Run SP02

χ (%) = 45.69

Heat Flow (mW)

29 28 27 χ (%) = 34.34

26

χ (%) = 31.39

25 24 23 100

110

120

130

140

150

160

170

180

Temperature (°C)

Fig. 6. Typical DSC curves of the propylene/1-butene samples.

1109

Deconvolution of molecular weight distributions (MWDs) has been extensively employed for obtaining information about the polymerization kinetics and interpretation of the main features of catalyst systems. The MWDs can generally be described as the summation of the Schulz–Flory distributions [38]. Wi ¼

NS X

aj wi;j

ð4Þ

j¼1

where NS is the number of active sites, aj is the mass fraction of polymer produced by the individual catalyst site j an wi,j is the weight Schulz–Flory [39] distributions given as wi;j ¼ ið1  qj Þ2 qji1

ð5Þ

The propagation probability (q) is represented as q¼

K PM P K P M þ k K Tk X k

ð6Þ

where M and X are monomer and chain transfer agent concentrations, KP and KT are the kinetic constants for propagation and transfer to the chain transfer agents. Fig. 7 illustrates MWD obtained from GPC analyses that were deconvoluted into different Flory– Schulz distributions. Depending on the sample, it can be observed that up to five Flory-distributions may be necessary to describe the GPC data of the polymer samples. In the particular case of Fig. 7B (sample obtained from liquid pool polymerization), only four sites are required to describe the MWD. The estimated model parameters are presented in Table 3. Weight-average molecular weights (M w Þ predicted by the deconvoluted Schulz–Flory distributions agree very well with the M w obtained from the GPC measurements (see Fig. 7 and Table 3). Shapes of the MWDs are significantly different because the different active sites respond distinctly to the reaction conditions (the reaction temperature and the 1-butene content). For this reason, the Mws and the as predicted by the individual Schulz–Flory distributions are significantly different, as shown in Table 3. The analysis of molecular weight distributions indicates that the average molecular weights and the polydispersities can be quite different for polymer samples obtained in the different processes and different conditions, as shown in Tables 2 and 3. The reader is encouraged to refer to the original references [28–30] for detailed analysis of molecular

1110

F. Machado et al. / European Polymer Journal 44 (2008) 1102–1113 0.9 GPC Data All Sites Individual Sites

0.8

dwt/d[Log(M)]

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

Log(Mw) 0.6

0.8 GPC Data All Sites Individual Sites

0.7

GPC Data All Sites Individual Sites

0.5

dwt/d[Log(M)]

dwt/d[Log(M)]

0.6 0.5 0.4 0.3

0.4 0.3 0.2

0.2

0.1

0.1 0.0

0.0 3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

Log(Mw)

Log(Mw)

¼ Fig. 7. Deconvolution of MWD into Schulz–Flory distributions: (A) gas-phase: GF03 – I¼ C4 (wt.%) = 11.90; (B) liquid pool: LP03 – IC4 ¼ (wt.%) = 13.16; (C) sequential polymerization, SP02 – IC4 (wt.%) = 1.59.

Table 3 Deconvolution of MWD into Schulz–Flory distributions Process

Mw GPC

Schulz–Flory

Gas-phase (Run GF03)

105,923

Liquid pool (Run LP03)

Sequential (Run SP02)

Error (%)

Site

a

q

103,986

1.83

I II III IV V

0.051 0.081 0.174 0.324 0.370

0.9996 0.9737 0.9989 0.9938 0.9972

594,479 7923 196,760 33,639 75,379

237,871

226,193

4.91

I II III IV

0.113 0.144 0.322 0.421

0.9998 0.9941 0.9992 0.9976

876,910 35,344 264,155 87,779

549,454

577,772

5.15

I II III IV V

0.109 0.130 0.248 0.254 0.260

0.9942 0.9999 0.9997 0.9976 0.9992

35,685 2,379,187 713,942 87,945 250,741

Mw

F. Machado et al. / European Polymer Journal 44 (2008) 1102–1113

weight distributions of all polymer samples. However, it is important to say that, despite the multisite nature of the catalyst, obtained MWD’s do not present multimodal behavior. Nevertheless, molecular weight distributions are usually very broad and present large polydispersities in all cases (larger than 4). It is also interesting to observe that molecular weights of commercial interest are obtained in all cases, although molecular weight distributions are shifted towards larger molecular weights when polymerizations are performed in the liquid-phase. This is to be expected, given the much larger monomer concentrations in the liquid-phase. Table 4 shows, however, that hydrogen exerts a pronounced effect on the molecular weight distribution and can be used for control purposes, as usual. Surface morphology of polymer particles was determined using SEM. Images were recorded at accelerating voltages of 15 keV. Fig. 8 shows the

1111

surface morphology of typical polymer particles. Polymer particles with good morphology can be obtained in gas-phase, liquid pool and sequential processes. The morphological control of polymer particles is desired because of problems associated to process viability and reduction of plant operation costs. Generally, it is desired that polymer particles present regular shape because the absence of fines prevents reactor fouling problems and undesirable fluidization effects. It can be observed that polymer particles grow uniformly in all cases, leading to formation of polymer material with good morphological features in the whole range of analyzed experimental conditions. This probably means that heat and temperature effects associated with overheating and melting of polymer particles did not occur at the analyzed process conditions. As the employed catalyst is provided as a solid suspension in a mineral oil, it is possible that the inert material contributes with the reduction of local reaction rates

Table 4 Polymer composition and average molecular weight in sequential polymerizations Run

Temperature (°C) a

SP00 SP01 SP02 SP03 SP04 SP05 a b

RCAT=ED (w/w)

H2 (mol%)

I¼ C4 (wt.%)

M n (g/mol)

M w (g/mol)

PDI

1.87 1.80 1.83 1.84 1.84 1.75

0.38 0.34 0.00 0.00 0.38 0.38

0.00 1.06 1.59 0.67 0.80 0.27

73,781 141,440 77,707 77,926 80,395 76,936

538,293 566,295 549,454 538,471 570,449 463,605

7.30 4.00 7.07 6.91 7.10 6.03

b

Stage I

Stage II

70 70 70 70 70 70

– 70 70 60 60 60

First stage pressure equal to 31 bars. The TEA/catalyst weight ratio was kept equal to 5. Second stage pressure equal to 1 bar g.

Fig. 8. Morphology of polymer particles: (A) gas-phase; (B) liquid pool; (C) sequential polymerization.

1112

F. Machado et al. / European Polymer Journal 44 (2008) 1102–1113

during particle breakup, allowing for more uniform catalyst fragmentation and production of particles with good morphology [40,41].

Suzano Petroquı´mica support.

for

providing

technical

References 5. Conclusions A family of propylene/1-butene copolymer grades can be successfully developed for processes intended for packaging and film applications. Polypropylene/1-butene in-reactor alloy can also be successfully developed for applications as highperformance structured materials. It was observed that 1-butene can be successfully incorporated into polypropylene chains at high polymerization rates and low 1-butene partial pressures, resulting in polymer materials with lower melting and sealing initiation temperatures. It is possible to incorporate 1-butene upon the polypropylene matrix at random in liquid pool and gas-phase polymerizations and sequentially through combination of liquid pool and gas-phase polymerization processes. The average molecular weights, the polydispersity index and the reactivity ratios are quite different in the analyzed processes. The reactivity ratios are close to 1 and essentially constant in the gas-phase, while it depends significantly on the 1-butene composition in the liquid-phase. The obtained results indicate that 1-butene solubilization in the polymer matrix seems to be favored in the gas-phase process, which partially compensates for its lower reactivities, as observed in the liquid-phase. The multi-site nature of the Ziegler–Natta catalysts leads to production of materials with heterogeneous compositions and broad molecular weights. Gas-phase materials are more homogeneous and present lower molecular weights. However, all analyzed processes can produce molecular weights of commercial interest in the whole range of 1-butene compositions. Polymer particles with good morphology can be obtained in gas-phase, liquid pool and sequential processes, without significant formation of fines and particle sticking. Acknowledgements The authors thank Coordenacßa˜o de Aperfeicßoamento de Pessoal de Nı´vel Superior (CAPES, Brazilian Agency, Project No. BEX 2813/03-3), Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico (CNPq) and UCBL-1 for providing scholarships and research funds. The authors thank

[1] Arnold M, Henschke O, Knorr J. Copolymerization of propene and higher a-olefins with the metallocene catalyst Et[Ind]2HfCl2/methylaluminoxane. Macromol Chem Phys 1996;197:563. [2] Marega C, Marigo A, Saini R, Ferrari P. The influence of thermal treatment and processing on the structure and morphology of poly(propylene-ran-1-butene) copolymers. Polym Int 2001;50(4):442–8. [3] Xie T, McAuley KB, Hsu JCC, Bacon DW. Modeling molecular weight development of gas-phase a-olefin copolymerization. AIChE J 1995;41(5):1251. [4] Dotson NA, Galva´n R, Laurence RL, Tirrel M. Polymerization process modeling. New York: VCH Publishers.; 1996. [5] Lieberman RB, Lenoir RT. Manufacturing. In: Moore EP, editor. Polypropylene Handbook Polymerization Characterization Properties Processing Applications. New York: Hanser Publishers; 1996. p. 287–302. [6] Kissin YV, Beach DL. A kinetic method of reactivity ratio measurement in olefin co-polymerization with Ziegler–Natta catalysts. J Polym Sci Part A Polym Chem 1983;21(4): 1065–74. [7] Locatelli P, Sacchi MC, Tritto I, Zannoni G. Propene–1butene copolymerization with a heterogeneous Ziegler– Natta catalyst – inhomogeneity of isotactic active-sites. Makromol Chem Rapid Commun 1988;9(8):575–80. [8] Kakugo M, Miyatake T, Mizunuma K, Kawai Y. Characteristics of ethylene–propylene and propylene–1-butene copolymerization over TiCl31/3AlCl3–Al(C2H5)2Cl. Macromolecules 1988;21(8):2309–13. [9] Sacchi MC, Shan CJ, Forlini F, Tritto I, Locatelli P. Effect of internal and external Lewis-bases on propene/1-butene copolymerization with MgCl2-supported Ziegler–Natta catalysts. Makromol Chem Rapid Commun 1993;14(4):231–8. [10] Xu J, Feng L, Yang S, Yang Y, Kong X. Influence of electron donors on the tacticity and the composition distribution of propylene–butene copolymers produced by supported Ziegler–Natta catalysts. Macromolecules 1997;30(25):7655–60. [11] Abiru T, Mizuno A, Weigand F. Microstructural characterization of propylene–butene-1 copolymer using temperature rising elution fractionation. J Appl Polym Sci 1998;68(9):1493–501. [12] Collina G, Noristi L, Stewart CA. Propene-co-butene random copolymers synthesized with superactive Ziegler– Natta catalyst. J Mol Catal A Chem 1995;99(3):161–5. [13] Machado F, Santos RTP, Nele M, Crossetti GL, Melo PA, Lima EL, et al. Bulk copolymerization of propylene/1butene using a high-activity Ziegler–Natta catalyst. Dechema Monographs 2004;138:267–73. [14] Oliva L, Serio MD, Peduto N, Santacesaria E. Chain propagation rate constants for gas-phase polymerization of propene and 1-butene with Ziegler–Natta catalysts. Macromol Chem Phys 1994;195(1):211–6. [15] Fu Z, Xu J, Zhang Y, Fan Z. Chain structure and mechanical properties of polyethylene/polypropylene/

F. Machado et al. / European Polymer Journal 44 (2008) 1102–1113

[16]

[17]

[18]

[19]

[20]

[21]

[22]

[23]

[24]

[25] [26]

[27]

[28]

poly(ethylene-co-propylene) in-reactor alloys synthesized with a spherical Ziegler–Natta catalyst by gas-phase polymerization. J Appl Polym Sci 2005;97(2):640–7. Xu J, Feng L, Yang S, Wu Y, Yang Y, Kong X. Separation and identification of ethylene–propylene block copolymer. Polymer 1997;38(17):4381–5. Xu J, Fu Z, Fan Z, Feng L. Temperature rising elution fractionation of PP/PE alloy and thermal behavior of the fractions. Eur Polym J 2002;38:1739–43. Zhang X, Olley RH, Huang B, Bassett DC. Characterization of propylene/ethylene copolymers sequentially polymerized with catalyst system d-TiCl3/Et2AlCl. Polym Int 1997;43:45–54. Hongjun C, Xiaolie L, Xiangxu C, Dezhu M, Jianmin W, Hongsheng T. Structure and properties of impact copolymer polypropylene II. Phase structure and crystalline morphology. J Appl Polym Sci 1999;71:103–13. Hongjun C, Xiaolie L, Dezhu M, Jianmin W, Hongsheng T. Structure and properties of impact copolymer polypropylene I. Chain structure. J Appl Polym Sci 1999;71:93–101. Fu Z, Fan Z, Zhang Y, Xu J. Chain structure of polyethylene/polypropylene in-reactor alloy synthesized in gas phase with spherical Ziegler–Natta catalyst. Polym Int 2004;53(8):1169–75. Xu J, Jin W, Fu Z, Fan Z. Composition distributions of different particles of a polypropylene/poly(ethylene-co-propylene) in situ alloy analyzed by temperature-rising elution fractionation. J Appl Polym Sci 2005;98(1):243–6. Mckenna TF, Bouzid D, Matsunami S, Sugano T. Evolution of particle morphology during polymerisation of high impact polypropylene. Polym React Eng 2003;11(2):177–97. Bouzid D, McKenna TF. Improving impact poly(propylene) morphology and production: selective poisoning of catalyst surface sites and the use of antistatic agents. Macromol Chem Phys 2006;207:13–9. Xu J, Linxian Feng. Characterization of microstructure of polypropylene alloys. Polym Int 1998;47(4):433–8. Fan ZQ, Zhang YQ, Xu JT, Wang HT, Feng LX. Structure and properties of polypropylene/poly(ethylene-co-propylene) in-situ blends synthesized by spherical Ziegler–Natta catalyst. Polymer 2001;42:5559–66. Cecchin G, Marchetti E, Baruzzi G. On the mechanism of polypropene growth over MgCl2/TiCl4 catalyst system. Macromol Chem Phys 2001;202(10):1987–94. Machado F, Lima EL, Pinto JC, McKenna TF. Synthesis of propylene/1-butene copolymers with Ziegler–Natta catalyst

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39] [40]

[41]

[42]

1113

in gas-phase copolymerizations, 1. Kinetics and macromolecular properties. Macromol Chem Phys 2005;206:2333–41. Machado F, Melo PA, Nele M, Lima EL, Pinto JC. Liquid pool copolymerization of propylene/1-butene with a MgCl2supported Ziegler–Natta catalyst. Macromol Mater Eng 2006;291:540–51. Machado F, Lima EL, Pinto JC, McKenna TF. In-situ preparation of polypropylene/1-butene alloys using a MgCl2-supported Ziegler–Natta catalyst. Eur Polym J 2008;44(4):1130–9. Machado F, Lenzi MK, Software para Aquisicßa˜o de Dados de Reacßo˜es de Polimerizacßa˜o. ADPol versa˜o 2.0: Manual do Usua´rio, PEQ/COPPE/UFRJ, Rio de Janeiro; 2003. Randall JC. A 13C NMR determination of the comonomer sequence distributions in propylene–butene-1 copolymers. Macromolecules 1978;11(3):592–7. Uozumi T, Soga K. Copolymerization of olefins with Kaminsky-Sinn-type catalysts. Die Makromol Chem 1992;193(4):823–31. Fisch MH, Dannenberg JJ. Propylene incorporation in 1butene copolymers by carbon-13 nuclear magnetic resonance spectrometry. Anal Chem 1977;49(9):1405–8. Zhang Y-D, Gou Q-Q, Wang J, Wu C-J, Qiao J-L. C-13 NMR, GPC, and DSC study on a propylene–ethylene–1butene terpolymer fractionated by temperature rising elution fractionation. Polym J 2003;35(7):551–9. Zhang Y-D. Monomer sequence distributions in propylene– 1-butene random copolymers. Macromolecules 2004;37:2471–7. Zhang Y. Monomer sequence characterization of propylene–1-olefin copolymers by carbon-13 NMR spectroscopy. Polymer 2004;45:2651–6. Nele M, Pinto JC. Molecular-weight multimodality of multiple flory distributions. Macromol Theory Simul 2002;11:293–307. Flory PJ. Molecular size distribution in linear condensation polymers. J Am Chem Soc 1936;58(10):1877–85. Merquior DM, Lima EL, Pinto JC. Modeling of particle fragmentation in heterogeneous olefin polymerization reactions, 2. A Two Phase Model. Macromol Mater Eng 2005;290(6):511–24. Merquior DM, Lima EL, Pinto JC. Modeling of particle fragmentation in heterogeneous olefin polymerization reactions. Polym React Eng 2003;11(2):133–54. Kittilsen P, McKenna TF. Study of the kinetics, mass transfer, and particle morphology in the production of highimpact polypropylene. J Appl Polym Sci 2001;82(5):1047–60.