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Scanning electron microscopy observation of nascent polypropylene particles
Eur. Polym. J. Vol. 29, No. 10, pp. 1319--1322,1993
0014-3057/93 $6.00+ 0.00 Copyright © 1993PergamonPress Ltd
Printed in Great Britain.All rights reserved
SCANNING ELECTRON MICROSCOPY OBSERVATION OF NASCENT POLYPROPYLENE PARTICLES L. C. SANTAMARIA, 1 F. M. B. COUTINHO 2. and J. C. BRUNO 3 tDepartmento de Fisica e Quimica, Instituto de Cirncias, Escola Federal de Engenharia de Itajub~i~ Rio de Janeiro, Brazil qnstituto de Macromolrculas, Universidade Federal do Rio de Janeiro, P.O. Box 68525, Rio de Janeiro, 21945-RJ, Brazil 3Instituto Militar de Engenharia, Rio de Janeiro, Brazil (Received 26 November 1992)
Abstraet--A study to achieve the control of polymer particles obtained by propylene polymerization with Ziegler-Natta catalyst based on TiCI3 was carried out. Two different catalysts were employed: Cat.A was made through TiCI4 reduction with diethylaluminium chloride (DEAC) in the presence of di-n-butyl ether (DBE) as first internal base and Cat.B was prepared through TiCI4 reduction with DEAC in the presence of DBE as first internal base and ethyl benzoate (EB) as second internal base, added after the reduction and before the thermal treatment at 70°. The progress of propylene polymerization was evaluated. Fragmentation of catalyst particles and a "replica" phenomenon were observed during the polymerization. The rupture of polypropylene particles was attributed to their friability and to the uncontrolled kinetics of the polymerization.
INTRODUCTION Propylene polymerization employing heterogeneous Ziegler-Natta-type catalysts is of both commercial and academic interest. Technologies based on these catalysts offer significant advantages for olefin polymerization when compared with high pressure processes. Great developments in this field have been achieved and Himont and Union Carbide have recently produced polymers with controlled morphology. There are many kinds of catalysts used for propylene polymerization, though some features are common to most of them. The main catalyst constituent is the active component, titanium trichloride or tetrachloride, the latter is normally deposited on a support such as MgC12 which, in most cases, improves catalyst performance. A co-catalyst [generally diethylaluminium chloride (DEAC) or triethylaluminium (TEA)] is used for activating the catalytic centres. Additional modifiers are often employed to produce polymers with selected properties. For instance, modifiers such as electron-donors [i.e. ethyl benzoate EB) and di-n-butyi ether (DBE)] will normally produce a higher stereospecificity and activity in the polymerizations [1-6]. The catalysts prepared with electron-donors are more porous and consequently more breakable. A characteristic of the olefin polymerization is the rupture of the initial catalyst structure into small fragments, with dimensions thousands of times smaller than the initial particles. This process, usually referred to as fragmentation, has been studied. The microparticles of catalyst formed by fragmentation become independent particles in the reactor causing the formation of small particles at the end of the polymerization. *To whom all correspondence should be addressed.
The understanding and control of the polymer growth mechanism are the key points which have made it possible to obtain polymeric materials with suitable properties. The catalyst-to-polymer replication phenomenon has been known for a long time and has been exploited industrially. This phenomenon occurs when the polymer chains begin to grow, not only on the surface of the external crystals, but also within the catalyst granules, causing rupture of the particles. This process is particularly critical in the first stages of ~t-olefin polymerization, because a too-fast growth could cause catalyst "explosion", consequently preventing regular replication. To achieve a suitable performance, it is necessary not only to modify the nature of active centres (polymerization kinetics) but also to act on the morphology and structure of catalyst particle in order to give it a porous structure with properly-sized crystals with primary particles homogeneously dispersed. These characteristics would allow the monomer equal access to the active centres. Pre-polymerization is normally used in order to maintain the mechanical stability of catalyst granules. Polymer growth on or inside the catalyst particles becomes more controlled in this process. The polymer covers the internal structures of catalyst particles, maintaining them as a whole, thus preventing or making difficult the fragmentation of the catalyst granules. A proper balance of catalyst architecture, the nature, number and distribution of active sites and polymerization conditions can result in a progressive expansion of different catalyst layers and consequently a controlled replication process by monomer diffusion. It is also possible that a more random expansion of the catalyst crystallites (polymerization nuclei) occurs and consequently a controlled replication process.
1319
1320
L.C. SANTAMARIA et
During the last three years, we have been studying and developing a new high active catalyst for propylene polymerization. Such a catalyst, under proper operating conditions, makes it possible to obtain a highly stereoregular polymer with a low content of catalyst residues due to the high catalyst activity. It is now our aim to control the morphology of the catalyst granules. The present paper refers to the effect of electron-donors, employed in the catalyst preparation, on propylene polymerization. EXPERIMENTAL PROCEDURES
All manipulations of catalyst components were carried out under dry N 2 using standard inert atmosphere techniques. The sources of materials and their purification and the polymerization procedure have been reported [2].
al.
Two catalysts (Cat.A and Cat.B) were synthesized for this study. Cat.A was prepared by TiC14 reduction with AIC1Et2 in the presence of DBE as a first internal base and Cat.B was also prepared by TiCI4 reduction with AIC1Et2 in the presence of DBE as a first internal base but was added as a second internal base before thermal treatment at 70°. The details of these syntheses have been described [2]. RESULTS AND DISCUSSION The aim of this work was to observe the growth of polymer particles during propylene polymerization employing Ziegler-Natta-type catalyst based on TiC13 modified by DBE and/or EB. It is well known that the catalyst obtained in the presence of electron-donors has a porous structure and the catalyst crystals show defects (6-TiCI3) that cause higher activity and higher stereospecificity than other TiC13 forms (fl-, ct- and y-TiCI 3).
Fig. 1. (a) Electron micrography of polypropylene particles obtained from Cat.A (synthesized with di-n-butyl ether as first internal base)~magnification: 1000x and polymerization time: 10min. (b) Electron micrography of polypropylene particles obtained from Cat.B (synthesized with di-n-butyl ether as first internal base and ethyl benzoate as second internal base)--magnification: 250 x and polymerization time: 15 min. (c) Electron micrography of polypropylene particles obtained from Cat.B--magnification: 1500 x and polymerization time: 15 min.
SEM observation of nascent polypropylene particles The formation of a porous structure enhances the catalyst activity, because more active centres areexposed, but the particles become more friable. The fragility of catalyst granules produces uncontrolled fragmentation of catalyst particles, consequently small particles and particles without uniformity are formed. This problem can be controlled by carrying out a pre-polymerization with the catalyst granules. Figures l(a), (b) and (c) show that the polymer covered the TiCI 3 layers which form the initial catalyst structure. As can be seen in these figures, the formation of "scales" is a result of the covering of TiC13 layers. Thus one can conclude that the polymer particles underwent a replication phenomenon, although the rupture process has been predominant for both catalysts (Cat.A and Cat.B). As can be seen from Figs 2(a), (b) and (c), the progress of propylene polymerization caused the rupture of nascent polymer particles and many particles of different sizes were formed. This rupture could be
1321
attributed to the fragmentation phenomenon and to the uncontrolled growth of the catalyst granules that caused the "explosion" of the particles. That effect is normally due to the uncontrolled growth of polymer chains, because the insertion reaction of monomer to the polymer chain is very fast. Uniform polymer growth throughout the catalyst crystallites (polymerization nuclei) would maintain the substructures of the catalyst solid connected to each other, avoiding the fragmentation and the "explosion" processes. This step is the pre-polymerization and it is carried out under mild reaction conditions. The polymer growth is then controlled and the initial particles can be preserved. As can be also observed in Figs 2(a), (b) and (c), the polymer particles obtained from Cat.B had more fragments than those obtained from Cat.A. It is possible that the ethyl benzoate employed during its synthesis made the catalyst granules more porous and consequently more fragile.
Fig. 2. (a) Electron micrography of polypropylene particles obtained from Cat.A--magnification: 1000 x and polymerization time: 45 min. (b) Electron micrography of polypropylene particles obtained from Cat.B--magnification: 3000 x and polymerization time: 4 min. (c) Electron micrography of polypropylene particles obtained from Cat.B--magnification: 100 x and polymerization time: 20 min.
L.C. SANTAMARIA et al.
1322 CONCLUSION
The use of EB as a second internal base (Cat.B) produced a very porous structure. The granules of polypropylene obtained with the catalyst based on TiCl 3 modified by D B E alone (Cat.A) and those obtained with Cat.B, synthesized with both DBE and EB, display also the "replica" phenomenon but they underwent fragmentation during propylene polymerization. Probably the effect was due to the fragility of the porous catalyst particles.
Acknowledgements--The authors thank the Conselho National de Desenvolvimento Cientifico e Tecno16gico (CNPq), Coordena~5.o de Aperfeit;oamento de Pessoal de Nivel Superior (CAPES), Conselho de Ensino
para Graduados e Pesquisa (CEPG/UFRJ) and Polibrasil S.A. REFERENCES
1. F. M. B. Coutinho and L. C. Santa Maria. Eur. Polym. J. 27, 987 0991). 2. F. M. B. Coutinho and L. C. Santa Maria. Polym. Bull. 26, 535 (1991). 3. F. M. B. Coutinho, C. Alencar and L. C. Santa Maria. Polym. Bull. (in press). 4. F. M. B. Coutinho, M. A. S. Costa and L. C. Santa Maria. Polym. Bull. (in press). 5. N. S. Bukharkina, V. P. Konovalov, L. L. Yezhenkova, I. A. Valoshin, O. M, Zvyagin, A. A. Baulin, M. V. Mal'giva, N. Y. Ivanova and R. Kh. Denilov. Polym. Sci. USSR 30, 621 (1988). 6. K. Soga, J. R. Park, T. Shiono and N. Kashiwa. Makromolek. Chem., Rapid Commun. 11, l i t (1990).
0014-3057/93 $6.00+ 0.00 Copyright © 1993PergamonPress Ltd
Printed in Great Britain.All rights reserved
SCANNING ELECTRON MICROSCOPY OBSERVATION OF NASCENT POLYPROPYLENE PARTICLES L. C. SANTAMARIA, 1 F. M. B. COUTINHO 2. and J. C. BRUNO 3 tDepartmento de Fisica e Quimica, Instituto de Cirncias, Escola Federal de Engenharia de Itajub~i~ Rio de Janeiro, Brazil qnstituto de Macromolrculas, Universidade Federal do Rio de Janeiro, P.O. Box 68525, Rio de Janeiro, 21945-RJ, Brazil 3Instituto Militar de Engenharia, Rio de Janeiro, Brazil (Received 26 November 1992)
Abstraet--A study to achieve the control of polymer particles obtained by propylene polymerization with Ziegler-Natta catalyst based on TiCI3 was carried out. Two different catalysts were employed: Cat.A was made through TiCI4 reduction with diethylaluminium chloride (DEAC) in the presence of di-n-butyl ether (DBE) as first internal base and Cat.B was prepared through TiCI4 reduction with DEAC in the presence of DBE as first internal base and ethyl benzoate (EB) as second internal base, added after the reduction and before the thermal treatment at 70°. The progress of propylene polymerization was evaluated. Fragmentation of catalyst particles and a "replica" phenomenon were observed during the polymerization. The rupture of polypropylene particles was attributed to their friability and to the uncontrolled kinetics of the polymerization.
INTRODUCTION Propylene polymerization employing heterogeneous Ziegler-Natta-type catalysts is of both commercial and academic interest. Technologies based on these catalysts offer significant advantages for olefin polymerization when compared with high pressure processes. Great developments in this field have been achieved and Himont and Union Carbide have recently produced polymers with controlled morphology. There are many kinds of catalysts used for propylene polymerization, though some features are common to most of them. The main catalyst constituent is the active component, titanium trichloride or tetrachloride, the latter is normally deposited on a support such as MgC12 which, in most cases, improves catalyst performance. A co-catalyst [generally diethylaluminium chloride (DEAC) or triethylaluminium (TEA)] is used for activating the catalytic centres. Additional modifiers are often employed to produce polymers with selected properties. For instance, modifiers such as electron-donors [i.e. ethyl benzoate EB) and di-n-butyi ether (DBE)] will normally produce a higher stereospecificity and activity in the polymerizations [1-6]. The catalysts prepared with electron-donors are more porous and consequently more breakable. A characteristic of the olefin polymerization is the rupture of the initial catalyst structure into small fragments, with dimensions thousands of times smaller than the initial particles. This process, usually referred to as fragmentation, has been studied. The microparticles of catalyst formed by fragmentation become independent particles in the reactor causing the formation of small particles at the end of the polymerization. *To whom all correspondence should be addressed.
The understanding and control of the polymer growth mechanism are the key points which have made it possible to obtain polymeric materials with suitable properties. The catalyst-to-polymer replication phenomenon has been known for a long time and has been exploited industrially. This phenomenon occurs when the polymer chains begin to grow, not only on the surface of the external crystals, but also within the catalyst granules, causing rupture of the particles. This process is particularly critical in the first stages of ~t-olefin polymerization, because a too-fast growth could cause catalyst "explosion", consequently preventing regular replication. To achieve a suitable performance, it is necessary not only to modify the nature of active centres (polymerization kinetics) but also to act on the morphology and structure of catalyst particle in order to give it a porous structure with properly-sized crystals with primary particles homogeneously dispersed. These characteristics would allow the monomer equal access to the active centres. Pre-polymerization is normally used in order to maintain the mechanical stability of catalyst granules. Polymer growth on or inside the catalyst particles becomes more controlled in this process. The polymer covers the internal structures of catalyst particles, maintaining them as a whole, thus preventing or making difficult the fragmentation of the catalyst granules. A proper balance of catalyst architecture, the nature, number and distribution of active sites and polymerization conditions can result in a progressive expansion of different catalyst layers and consequently a controlled replication process by monomer diffusion. It is also possible that a more random expansion of the catalyst crystallites (polymerization nuclei) occurs and consequently a controlled replication process.
1319
1320
L.C. SANTAMARIA et
During the last three years, we have been studying and developing a new high active catalyst for propylene polymerization. Such a catalyst, under proper operating conditions, makes it possible to obtain a highly stereoregular polymer with a low content of catalyst residues due to the high catalyst activity. It is now our aim to control the morphology of the catalyst granules. The present paper refers to the effect of electron-donors, employed in the catalyst preparation, on propylene polymerization. EXPERIMENTAL PROCEDURES
All manipulations of catalyst components were carried out under dry N 2 using standard inert atmosphere techniques. The sources of materials and their purification and the polymerization procedure have been reported [2].
al.
Two catalysts (Cat.A and Cat.B) were synthesized for this study. Cat.A was prepared by TiC14 reduction with AIC1Et2 in the presence of DBE as a first internal base and Cat.B was also prepared by TiCI4 reduction with AIC1Et2 in the presence of DBE as a first internal base but was added as a second internal base before thermal treatment at 70°. The details of these syntheses have been described [2]. RESULTS AND DISCUSSION The aim of this work was to observe the growth of polymer particles during propylene polymerization employing Ziegler-Natta-type catalyst based on TiC13 modified by DBE and/or EB. It is well known that the catalyst obtained in the presence of electron-donors has a porous structure and the catalyst crystals show defects (6-TiCI3) that cause higher activity and higher stereospecificity than other TiC13 forms (fl-, ct- and y-TiCI 3).
Fig. 1. (a) Electron micrography of polypropylene particles obtained from Cat.A (synthesized with di-n-butyl ether as first internal base)~magnification: 1000x and polymerization time: 10min. (b) Electron micrography of polypropylene particles obtained from Cat.B (synthesized with di-n-butyl ether as first internal base and ethyl benzoate as second internal base)--magnification: 250 x and polymerization time: 15 min. (c) Electron micrography of polypropylene particles obtained from Cat.B--magnification: 1500 x and polymerization time: 15 min.
SEM observation of nascent polypropylene particles The formation of a porous structure enhances the catalyst activity, because more active centres areexposed, but the particles become more friable. The fragility of catalyst granules produces uncontrolled fragmentation of catalyst particles, consequently small particles and particles without uniformity are formed. This problem can be controlled by carrying out a pre-polymerization with the catalyst granules. Figures l(a), (b) and (c) show that the polymer covered the TiCI 3 layers which form the initial catalyst structure. As can be seen in these figures, the formation of "scales" is a result of the covering of TiC13 layers. Thus one can conclude that the polymer particles underwent a replication phenomenon, although the rupture process has been predominant for both catalysts (Cat.A and Cat.B). As can be seen from Figs 2(a), (b) and (c), the progress of propylene polymerization caused the rupture of nascent polymer particles and many particles of different sizes were formed. This rupture could be
1321
attributed to the fragmentation phenomenon and to the uncontrolled growth of the catalyst granules that caused the "explosion" of the particles. That effect is normally due to the uncontrolled growth of polymer chains, because the insertion reaction of monomer to the polymer chain is very fast. Uniform polymer growth throughout the catalyst crystallites (polymerization nuclei) would maintain the substructures of the catalyst solid connected to each other, avoiding the fragmentation and the "explosion" processes. This step is the pre-polymerization and it is carried out under mild reaction conditions. The polymer growth is then controlled and the initial particles can be preserved. As can be also observed in Figs 2(a), (b) and (c), the polymer particles obtained from Cat.B had more fragments than those obtained from Cat.A. It is possible that the ethyl benzoate employed during its synthesis made the catalyst granules more porous and consequently more fragile.
Fig. 2. (a) Electron micrography of polypropylene particles obtained from Cat.A--magnification: 1000 x and polymerization time: 45 min. (b) Electron micrography of polypropylene particles obtained from Cat.B--magnification: 3000 x and polymerization time: 4 min. (c) Electron micrography of polypropylene particles obtained from Cat.B--magnification: 100 x and polymerization time: 20 min.
L.C. SANTAMARIA et al.
1322 CONCLUSION
The use of EB as a second internal base (Cat.B) produced a very porous structure. The granules of polypropylene obtained with the catalyst based on TiCl 3 modified by D B E alone (Cat.A) and those obtained with Cat.B, synthesized with both DBE and EB, display also the "replica" phenomenon but they underwent fragmentation during propylene polymerization. Probably the effect was due to the fragility of the porous catalyst particles.
Acknowledgements--The authors thank the Conselho National de Desenvolvimento Cientifico e Tecno16gico (CNPq), Coordena~5.o de Aperfeit;oamento de Pessoal de Nivel Superior (CAPES), Conselho de Ensino
para Graduados e Pesquisa (CEPG/UFRJ) and Polibrasil S.A. REFERENCES
1. F. M. B. Coutinho and L. C. Santa Maria. Eur. Polym. J. 27, 987 0991). 2. F. M. B. Coutinho and L. C. Santa Maria. Polym. Bull. 26, 535 (1991). 3. F. M. B. Coutinho, C. Alencar and L. C. Santa Maria. Polym. Bull. (in press). 4. F. M. B. Coutinho, M. A. S. Costa and L. C. Santa Maria. Polym. Bull. (in press). 5. N. S. Bukharkina, V. P. Konovalov, L. L. Yezhenkova, I. A. Valoshin, O. M, Zvyagin, A. A. Baulin, M. V. Mal'giva, N. Y. Ivanova and R. Kh. Denilov. Polym. Sci. USSR 30, 621 (1988). 6. K. Soga, J. R. Park, T. Shiono and N. Kashiwa. Makromolek. Chem., Rapid Commun. 11, l i t (1990).