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Polystyrene Produced by a Multifunctional Initiator
10th International Symposium on Process Systems Engineering - PSE2009 Rita Maria de Brito Alves, Claudio Augusto Oller do Nascimento and Evaristo Chalbaud Biscaia Jr. (Editors) © 2009 Elsevier B.V. All rights reserved.
1077
Polystyrene Produced by a Multifunctional Initiator Paula F. de M. P. B. Machadoa, Liliane M. F. Lonab a
LASSPQ/DPQ- School of Chemical Engineering - University of Campinas Cidade Universitária “Zeferino Vaz”, s/n, CEP 13083-970, Campinas - SP, Brazil. E-mail address: [email protected] b LASSPQ/DPQ- School of Chemical Engineering - University of Campinas Cidade Universitária “Zeferino Vaz”, s/n, CEP 13083-970, Campinas - SP, Brazil. E-mail address: [email protected]
Abstract Free-radical polymerization of styrene in the presence of trifunctional initiator was investigated to understand the behavior of initiators with functionality greater than two. The trifunctional initiator selected was the TRIGONOX 301 from Akzo Nobel. This study proposes a kinetic mechanism and a mathematical model to free radical polymerization of styrene using a trifunctional initiator. The complexity of the kinetic mechanism increases as the functionality of the initiator increases. The mathematical model was built and predicts results as conversion, molecular weight, radical concentration and polymer concentration. An experimental investigation was also explored to validate the simulations results. The experimental part followed the method of polymerization in ampoules. Experiments were also carried out to verify the effect of temperature and initiator concentration on the polymerization kinetics and molecular weights. Keywords: multifunctional initiator, polystyrene, polymerization.
1. Introduction The study of chemical initiators that have functionality superior than two has been explored in the scientific and industrial field. Free-radical polymerization in the presence of multifunctional initiator has been also investigated to understand the behavior of these initiators. The multifunctional initiator is able to increase the reaction rate in a free radical polymerization without decrease the molecular weight of the formed polymer and it can also changes the polymer structure. Literature shows experimental and simulation studies related to free radical polymerization using bifunctional initiators (Villalobos et al., 1991; Dhib et al., 2000; Machado and Lona 2005 and others). Cerna et al. (2002) and Sheng et al. (2004) present detailed and important experimental studies with trifunctional initiators. There are very few studies presenting models of free radical polymerization using multifunctional initiators (Scorah, 2005 and Scorah et al., 2007). To understand the behavior of initiators with functionality greater than two, trifunctional initiators were chosen. The trifunctional initiator selected was the TRIGONOX 301 from Akzo Nobel. It is a cyclic triperoxide and very unstable at temperatures greater than 40ºC. It must be kept at room temperature. This study proposes a kinetic mechanism and a mathematical model to free radical polymerization of styrene using
1078
P.F. de M.P.B. Machado and L.M.F. Lona
this trifunctional initiator since there are very few studies presenting models of free radical polymerization using multifunctional initiators. Styrene is chosen because it is a very well known monomer and there are a lot of data about it. The mathematical model was built in Fortran 90, based on the Method of Moments Equations for all species, and predicts results as conversion, molecular weight (Number Average, Mn, and Weight Average, Mw), radical concentration and polymer concentration. An experimental investigation was also explored to validate the simulations results. The experimental part followed the method of polymerization in ampoules. Full-conversion-range experiments were carried out to validate the model and also to verify the effects of the temperature and initiator concentration on the polymerization kinetics and molecular weights. Conversion was calculated based on differences of masses (gravimetry). Gel Permeation Chromatography (GPC) was used to measure the molecular weights of polystyrene. The validation was made to conversion and molecular weights.
2. Experimental Procedure Styrene was first washed with NaOH solution and deionized water. Then it is dried with CaCl2. The washed styrene is distillate using a vertical rotative evaporator equipped with vacuum pump and a hot bath. After that determined quantities of the solution initiator-monomer are transferred to ampoules. They are all sealed after the elimination of oxygen and put into a bath of oil at a desirable temperature. At certain intervals of time, each ampoule is removed from the bath, cooled and weighted. Then the ampoule is broken and the solution polymer + monomer + initiator is dissolved in tetrahydrofuran (THF) and then the polymer is precipitated with ethanol. After ethanol evaporation, the sample of polymer goes to a dryer to guarantee the total evaporation of the solvent. After this, the dried samples are weighted, and conversion is calculated based on differences of masses (gravimetry). Finally, the polymer is dissolved in THF and characterized by GPC analysis, providing molecular weights data. These results bring elements to understand the behavior of trifunctional initiator in the production of polystyrene, since they provide data to validate the mathematical model and to estimate parameters.
3. Results The simulation results and experimental data are shown in Figures 1, 2, 3 e 4. The experimental data are used to validate the model. In Figures 1 and 2 it was considered temperature equal to 130°C and initiator concentration of 0.005M.
1079
Polystyrene Produced by a Multifunctional Initiator T=130°C, [I]=0,005M
1,0
T=130°C e [I]=0,005M 450000 400000
0,8
300000
Mn and Mw
Conversion
350000
0,6
0,4
250000 200000 150000
0,2
Mn Exp Data Mw Exp Data Mn Model Mw Model
100000
Exp Data Model
50000
0,0
0
0
40
80
120
160
200
240
0,0
0,2
0,4
0,6
0,8
Conversion
Time (min)
Figure 1. Conversion x Time.
Figure 2. Weight and Number Average Molecular Weights x Conversion.
Figures 4 and 5 show results from a different operating condition; temperature of 125°C and initiator concentration of 0.0029M.
T=125°C, [I]=0,0029M Model Exp
Mn Exp Mw Exp Mn Model Mw Model
600000
0,6
500000
Mn and Mw
Conversion
T=125°C, [I]=0,0029M
700000
0,8
0,4
400000
300000
200000 0,2 100000
0
0,0 0
30
60
90
120
Time (min)
Figure 3. Conversion x Time.
150
180
0,0
0,1
0,2
0,3
0,4
0,5
Conversion
Figure 4. Weight and Number Average Molecular Weights x Conversion.
It is possible to verify from Figures 1 to 4 that the model presents agreement with experimental data. It was possible to explore the model using different operating conditions providing a study of effects of temperature and initiator concentration on the reaction rate and proprieties of the polymer. Figures 5 and 6 show the effect of temperature in conversion and number average molecular weight.
1080
P.F. de M.P.B. Machado and L.M.F. Lona Effect of Temperature, [I]=0,005M
1,0
0,8
200000
0,6
Mn
Conversion
Effect of Temperature, [I]=0,005M
135°C 130°C 125°C
0,4
150000
135°C 130°C 125°C
0,2
0,0
100000
0
40
80
120
160
200
240
0
40
80
Time (min)
120
160
200
240
Time (min)
Figure 5. Conversion x Time.
Figure 6. Number Average Molecular Weight x Time.
It is possible to verify in Figure 5 that conversion increases as the temperature increases. That is expected since an increasing in temperature increases the reaction rate generating more radicals to react with the molecules of monomer. Figure 6 shows that Mn decreases as the temperature increases, what was also expected, since a large quantity of radicals implies in a large quantity of chains, but short ones, making the molecular weight decreases. Figures 7 and 8 show the effect of initiator concentration in conversion and number average molecular weight. Effect of Initiator Concentration, T=130°C
1,0
0,8
200000
0,6
Mn
Conversion
Effect of Initiator Concentration, T=130°C
[I]=0,01M [I]=0,005M [I]=0,0025M
0,4
150000
[I]=0,01M [I]=0,005M [I]=0,0025M
0,2
0,0
100000 0
40
80
120
160
Time (min)
Figure 7. Conversion x Time.
200
240
0
40
80
120
160
200
240
Time (min)
Figure 8. Number Average Molecular Weight x Time.
As expected, Figure 7 shows that an increasing in initiator concentration is also very important to achieve faster conversions, since a large quantity of initiator implies in a large quantity of consumed monomer. Figure 8 shows that, first, Mn decreases as the initiator concentration increases and then this behavior changes and Mn starts to increase with the increasing in the initiator concentration. The first part was expected, since a large quantity of radicals implies in a large quantity of chains, but short ones, making the molecular weight decrease, but the behavior in the second part is really unexpected. Maybe, this can be explained by the fact that with the increase on the initiator concentration, more radicals are formed resulting in the increasing of the number of chains and before 120 minutes, Mn (which depends on the number of chains)
Polystyrene Produced by a Multifunctional Initiator
1081
decreases. However, after 120 minutes, as the initiator concentration is smaller (there is less initiator in the reaction), when all initiator is consumed, the polymeric chains that have undecomposed peroxides start to break, making Mn decrease. At this point, the inversion in the behavior begins because the highest initiator concentration has not all initiator consumed, so its chains have not yet their undecomposed peroxide broken, so molecular weights of the highest initiator concentrations keep still growing.
4. Conclusion Results from model simulation of styrene polymerization using trifunctional initiator TRIGONOX 301 showed agreement with experimental data. These results from simulation are new, since there is no research published about this subject. The existent models are very few and they are not based in the Method of Moments. So far it was possible to verify some aspects of the behavior of a trifunctional initiator through the study of the effects of temperature and initiator concentration and it was possible to see how interest is the behavior of a multifunctional initiator in a polymerization reaction and to verify the benefits of its use. References J. R. Cerna, G. Morales, G. N. Eyler, A. I. Canizo, 2002, J. Applied polymer sci., 83, 111. R. Dhib, J. Gao, A. Penlidis, 2000, Polymer Reaction Engineering, v.8, n.4, p. 299-464. P.F.M.P.B. Machado, L.M.F. Lona, 2005, E. Symp. on Comp. Aided Proc. Eng.-15, 2005, 20 A, 445-450. M. J. Scorah, 2005, PhD thesis, Dept. of Chem. Eng., University of Waterloo, Waterloo, Ontario, Canada. M. J. Scorah, R. Dhib, A. Penlidis, 2007, Macrom. React. Eng., 1, 209-221. W. C., Sheng, J. Y. Wu, G. R. Shan, Z. M. Huang, Z. X. Weng, 2004, J. Appl. Polymer Sci. , 94 , 1035-1042. M. A. Villalobos, A. E. Hamielec, P. E. Wood, 1991, Journal of Applied Polymer Science, 42, 629-641.
1077
Polystyrene Produced by a Multifunctional Initiator Paula F. de M. P. B. Machadoa, Liliane M. F. Lonab a
LASSPQ/DPQ- School of Chemical Engineering - University of Campinas Cidade Universitária “Zeferino Vaz”, s/n, CEP 13083-970, Campinas - SP, Brazil. E-mail address: [email protected] b LASSPQ/DPQ- School of Chemical Engineering - University of Campinas Cidade Universitária “Zeferino Vaz”, s/n, CEP 13083-970, Campinas - SP, Brazil. E-mail address: [email protected]
Abstract Free-radical polymerization of styrene in the presence of trifunctional initiator was investigated to understand the behavior of initiators with functionality greater than two. The trifunctional initiator selected was the TRIGONOX 301 from Akzo Nobel. This study proposes a kinetic mechanism and a mathematical model to free radical polymerization of styrene using a trifunctional initiator. The complexity of the kinetic mechanism increases as the functionality of the initiator increases. The mathematical model was built and predicts results as conversion, molecular weight, radical concentration and polymer concentration. An experimental investigation was also explored to validate the simulations results. The experimental part followed the method of polymerization in ampoules. Experiments were also carried out to verify the effect of temperature and initiator concentration on the polymerization kinetics and molecular weights. Keywords: multifunctional initiator, polystyrene, polymerization.
1. Introduction The study of chemical initiators that have functionality superior than two has been explored in the scientific and industrial field. Free-radical polymerization in the presence of multifunctional initiator has been also investigated to understand the behavior of these initiators. The multifunctional initiator is able to increase the reaction rate in a free radical polymerization without decrease the molecular weight of the formed polymer and it can also changes the polymer structure. Literature shows experimental and simulation studies related to free radical polymerization using bifunctional initiators (Villalobos et al., 1991; Dhib et al., 2000; Machado and Lona 2005 and others). Cerna et al. (2002) and Sheng et al. (2004) present detailed and important experimental studies with trifunctional initiators. There are very few studies presenting models of free radical polymerization using multifunctional initiators (Scorah, 2005 and Scorah et al., 2007). To understand the behavior of initiators with functionality greater than two, trifunctional initiators were chosen. The trifunctional initiator selected was the TRIGONOX 301 from Akzo Nobel. It is a cyclic triperoxide and very unstable at temperatures greater than 40ºC. It must be kept at room temperature. This study proposes a kinetic mechanism and a mathematical model to free radical polymerization of styrene using
1078
P.F. de M.P.B. Machado and L.M.F. Lona
this trifunctional initiator since there are very few studies presenting models of free radical polymerization using multifunctional initiators. Styrene is chosen because it is a very well known monomer and there are a lot of data about it. The mathematical model was built in Fortran 90, based on the Method of Moments Equations for all species, and predicts results as conversion, molecular weight (Number Average, Mn, and Weight Average, Mw), radical concentration and polymer concentration. An experimental investigation was also explored to validate the simulations results. The experimental part followed the method of polymerization in ampoules. Full-conversion-range experiments were carried out to validate the model and also to verify the effects of the temperature and initiator concentration on the polymerization kinetics and molecular weights. Conversion was calculated based on differences of masses (gravimetry). Gel Permeation Chromatography (GPC) was used to measure the molecular weights of polystyrene. The validation was made to conversion and molecular weights.
2. Experimental Procedure Styrene was first washed with NaOH solution and deionized water. Then it is dried with CaCl2. The washed styrene is distillate using a vertical rotative evaporator equipped with vacuum pump and a hot bath. After that determined quantities of the solution initiator-monomer are transferred to ampoules. They are all sealed after the elimination of oxygen and put into a bath of oil at a desirable temperature. At certain intervals of time, each ampoule is removed from the bath, cooled and weighted. Then the ampoule is broken and the solution polymer + monomer + initiator is dissolved in tetrahydrofuran (THF) and then the polymer is precipitated with ethanol. After ethanol evaporation, the sample of polymer goes to a dryer to guarantee the total evaporation of the solvent. After this, the dried samples are weighted, and conversion is calculated based on differences of masses (gravimetry). Finally, the polymer is dissolved in THF and characterized by GPC analysis, providing molecular weights data. These results bring elements to understand the behavior of trifunctional initiator in the production of polystyrene, since they provide data to validate the mathematical model and to estimate parameters.
3. Results The simulation results and experimental data are shown in Figures 1, 2, 3 e 4. The experimental data are used to validate the model. In Figures 1 and 2 it was considered temperature equal to 130°C and initiator concentration of 0.005M.
1079
Polystyrene Produced by a Multifunctional Initiator T=130°C, [I]=0,005M
1,0
T=130°C e [I]=0,005M 450000 400000
0,8
300000
Mn and Mw
Conversion
350000
0,6
0,4
250000 200000 150000
0,2
Mn Exp Data Mw Exp Data Mn Model Mw Model
100000
Exp Data Model
50000
0,0
0
0
40
80
120
160
200
240
0,0
0,2
0,4
0,6
0,8
Conversion
Time (min)
Figure 1. Conversion x Time.
Figure 2. Weight and Number Average Molecular Weights x Conversion.
Figures 4 and 5 show results from a different operating condition; temperature of 125°C and initiator concentration of 0.0029M.
T=125°C, [I]=0,0029M Model Exp
Mn Exp Mw Exp Mn Model Mw Model
600000
0,6
500000
Mn and Mw
Conversion
T=125°C, [I]=0,0029M
700000
0,8
0,4
400000
300000
200000 0,2 100000
0
0,0 0
30
60
90
120
Time (min)
Figure 3. Conversion x Time.
150
180
0,0
0,1
0,2
0,3
0,4
0,5
Conversion
Figure 4. Weight and Number Average Molecular Weights x Conversion.
It is possible to verify from Figures 1 to 4 that the model presents agreement with experimental data. It was possible to explore the model using different operating conditions providing a study of effects of temperature and initiator concentration on the reaction rate and proprieties of the polymer. Figures 5 and 6 show the effect of temperature in conversion and number average molecular weight.
1080
P.F. de M.P.B. Machado and L.M.F. Lona Effect of Temperature, [I]=0,005M
1,0
0,8
200000
0,6
Mn
Conversion
Effect of Temperature, [I]=0,005M
135°C 130°C 125°C
0,4
150000
135°C 130°C 125°C
0,2
0,0
100000
0
40
80
120
160
200
240
0
40
80
Time (min)
120
160
200
240
Time (min)
Figure 5. Conversion x Time.
Figure 6. Number Average Molecular Weight x Time.
It is possible to verify in Figure 5 that conversion increases as the temperature increases. That is expected since an increasing in temperature increases the reaction rate generating more radicals to react with the molecules of monomer. Figure 6 shows that Mn decreases as the temperature increases, what was also expected, since a large quantity of radicals implies in a large quantity of chains, but short ones, making the molecular weight decreases. Figures 7 and 8 show the effect of initiator concentration in conversion and number average molecular weight. Effect of Initiator Concentration, T=130°C
1,0
0,8
200000
0,6
Mn
Conversion
Effect of Initiator Concentration, T=130°C
[I]=0,01M [I]=0,005M [I]=0,0025M
0,4
150000
[I]=0,01M [I]=0,005M [I]=0,0025M
0,2
0,0
100000 0
40
80
120
160
Time (min)
Figure 7. Conversion x Time.
200
240
0
40
80
120
160
200
240
Time (min)
Figure 8. Number Average Molecular Weight x Time.
As expected, Figure 7 shows that an increasing in initiator concentration is also very important to achieve faster conversions, since a large quantity of initiator implies in a large quantity of consumed monomer. Figure 8 shows that, first, Mn decreases as the initiator concentration increases and then this behavior changes and Mn starts to increase with the increasing in the initiator concentration. The first part was expected, since a large quantity of radicals implies in a large quantity of chains, but short ones, making the molecular weight decrease, but the behavior in the second part is really unexpected. Maybe, this can be explained by the fact that with the increase on the initiator concentration, more radicals are formed resulting in the increasing of the number of chains and before 120 minutes, Mn (which depends on the number of chains)
Polystyrene Produced by a Multifunctional Initiator
1081
decreases. However, after 120 minutes, as the initiator concentration is smaller (there is less initiator in the reaction), when all initiator is consumed, the polymeric chains that have undecomposed peroxides start to break, making Mn decrease. At this point, the inversion in the behavior begins because the highest initiator concentration has not all initiator consumed, so its chains have not yet their undecomposed peroxide broken, so molecular weights of the highest initiator concentrations keep still growing.
4. Conclusion Results from model simulation of styrene polymerization using trifunctional initiator TRIGONOX 301 showed agreement with experimental data. These results from simulation are new, since there is no research published about this subject. The existent models are very few and they are not based in the Method of Moments. So far it was possible to verify some aspects of the behavior of a trifunctional initiator through the study of the effects of temperature and initiator concentration and it was possible to see how interest is the behavior of a multifunctional initiator in a polymerization reaction and to verify the benefits of its use. References J. R. Cerna, G. Morales, G. N. Eyler, A. I. Canizo, 2002, J. Applied polymer sci., 83, 111. R. Dhib, J. Gao, A. Penlidis, 2000, Polymer Reaction Engineering, v.8, n.4, p. 299-464. P.F.M.P.B. Machado, L.M.F. Lona, 2005, E. Symp. on Comp. Aided Proc. Eng.-15, 2005, 20 A, 445-450. M. J. Scorah, 2005, PhD thesis, Dept. of Chem. Eng., University of Waterloo, Waterloo, Ontario, Canada. M. J. Scorah, R. Dhib, A. Penlidis, 2007, Macrom. React. Eng., 1, 209-221. W. C., Sheng, J. Y. Wu, G. R. Shan, Z. M. Huang, Z. X. Weng, 2004, J. Appl. Polymer Sci. , 94 , 1035-1042. M. A. Villalobos, A. E. Hamielec, P. E. Wood, 1991, Journal of Applied Polymer Science, 42, 629-641.