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Autoxidation of polypropylene glycol and its catalysis with ferric acetylacetonate
European Polymer Journal 36 (2000) 1001±1010
Autoxidation of polypropylene glycol and its catalysis with ferric acetylacetonate Mohammad A. Semsarzadeh a,*, Hamed Salehi b a
Polymer Group, Chemical Engineering Department, Tarbiat Modarres University, P.O. Box 14155-4838, Tehran, Iran Engineering Research Center, Jahad Sazandegi and Chemistry Department, School of Science, Tarbiat Modarres University, P.O. Box 14155-4838, Tehran, Iran
b
Received 2 November 1998; received in revised form 15 March 1999; accepted 14 May 1999
Abstract The autoxidation of polypropylene glycol (PPG) alone and with ferric acetylacetonate (FeAA) was studied in air at 75, 85 and 938C. The overall time-conversion follows an exponential equation. These results were con®rmed by GPC, FTIR and NMR spectroscopy. The atuoxidation of polymer shows that ester, ketone, alcohol and formate compounds are also formed. Calculated activation energy of autoxidation of PPG is 84 kJ molÿ1 and the average activation energy for PPG-FeAA autoxidation is 64 kJ molÿ1. The induction period of autoxidation of PPG with catalyst is very short, while the rate of autoxidation is faster. # 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Today the production of rigid and ¯exible polyurethane foams, polyurethane coatings, elastomers and adhesives is based on polyether polyols [1±3]. The stability of polyurethane (PU) have been a major area of research during last decade [4±6]. It is now clear that there are number of factors aecting the stability and degradation of polyurethanes. These factors have shown signi®cant eect on the kinetics of the reaction. The stability of polyurethanes are related to the stability of their components. Since the poor oxidative stability of polyethers like polypropylene glycol (PPG) in¯uences the thermal stability and properties of the polymer, kinetic studies of this monomer and the eect of ferric acetylacetonate (FeAA) catalyst on the stability of PPG used in polyurethane, specially during the ageing of the product was studied [7]. For this pur-
* Corresponding author. Fax: +98-800-6544.
pose, we have studied the kinetics of autoxidation of PPG and explored further the possibility of stabilization of PU at lower temperature range of 75±958C, where the degradation is believed to be more signi®cant in service life time storage.
2. Experimental 2.1. Materials and methods Ferric acetylacetonate (FeAA), carbon tetrachloride and toluene (chromatography grade), were supplied from Merck (Darmstadt, Germany). Polypropylene glycol (PPG) was supplied from BDH, the nominal molecular weight of polymer is 2025. The polymer was dissolved in carbon tetrachloride in a laboratory glass dish. The solvent was allowed to evaporate at room temperature for 1 h and then dried for 30 min in vacuum (120 mbar) to form a layer of polymer with thickness of about 50±80 mm. The container with its layer
0014-3057/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 9 9 ) 0 0 1 4 7 - 0
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M.A. Semsarzadeh, H. Salehi / European Polymer Journal 36 (2000) 1001±1010
Fig. 1. Infrared spectra of carbonyl domain at 938C for dierent oxidation times.
of polymer was then placed in the oven. The eect of catalyst was studied using a known amounts of FeAA in PPG. Prior to FTIR and GPC studies, the isothermal oxidation experiments were performed at dierent time intervals in an air-ventilated ovens at 75, 85 and 938C. GPC experiments were carried out with GPC-LC4A, equipped with data processing facilities of CR4AX (Shimadzu, Japan). The polymer samples were dissolved in toluene and ®ltered through Milex ®lter. Calibration curves were plotted using PPG standards supplied from Waters. The tests were carried out using refractive index detector and a 103 AÊ column with toluene as mobile phase and 1 ml/min ¯ow rate. The mobile phase was degassed by ultrasonic vibration.
FTIR spectra were used to study the absorption of carbonyl band at 1728 cmÿ1. They were recorded with FTIR IFS 88, Brucker, Germany. The samples were prepared in KBr in form of pellets. Carbonyl index (CI) was measured using baseline method, normalized with respect to methyl vibration at 2975 cmÿ1. H-NMR spectra were measured with a JEOL-EX90A spectrometer and CDCl3 was used as a solvent.
3. Results and discussion 3.1. PPG autoxidation kinetics The autoxidation kinetics of the polyether was stu-
Fig. 2. Carbonyl build-up in the autoxidation of PPG at dierent temperatures.
M.A. Semsarzadeh, H. Salehi / European Polymer Journal 36 (2000) 1001±1010
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Table 1 Pre-exponential factors (A0), activation energies Ea and correlation factors (R ) for induction time and stationary state rates for autoxidation of PPG FTIR
A0 Ea (kJ molÿ1) R
GPC
Induction time
Stationary state rate
Induction time
Stationary state rate
29 108 87 0.999
6.8 108 82 0.994
29 108 86 0.998
13 108 82 0.994
died with FTIR and GPC; and this process was used to study the activation energy of reaction.The FTIR studies were based on the increase in concentration of carbonyl group as shown in Fig. 1; and the time-conversion curves were calculated during the autoxidation of polypropylene glycol. Carbonyl index was calculated from the intensity ratio of carbonyl absorption at 1728 cmÿ1 to the absorption intensity of methyl group at 2975 cmÿ1. The time-conversion curves are shown in Fig. 2. These curves show three important stages: the ®rst stage is the induction period where the carbonyl growth takes place. The second stage is the steady state stage, where the rate of reaction is constant, and the third stage is the decrease in rate, where the mass losses in reaction takes place. The induction period and maximum steady rate have been used in analysis of the ®rst and second stages of the reaction, where the eects are more important in the studies of stability and ageing of polymer. Fig. 2 shows the end of the induction period or ti and the maximum steady rate or rs. FTIR and GPC results were used to calculate the activation energy of the autoxidation of the ®rst and second stages of the process. Table 1 shows the calculated pre-exponential and activation energy of the autoxida-
tion of PPG. The Arrhenius plots of rs and ti is also shown in Fig. 3. The calculated Ea of the steady state rate (82 kJ molÿ1) is in good agreement with the reported one from oxygen uptake method (84 kJ molÿ1) [4]. The calculated activation energy for the induction time is 87 kJ molÿ1, and this is some how lower than the reported activation energy by Griths (115 kJ molÿ1) [4]. It is noteworthy that, the reported results on the autoxidation of PPG is at higher temperature range (95±1408C) than those reported here. The autoxidation reaction of PPG and the eect of time and temperature on the molecular weight of polymer was also studied with GPC and the results are shown in Figs. 4 and 5. In this case the autoxidation reaction was studied on the basis of scission index (SI) or the number of broken bonds per initial macromolecule [8,9]. The scission index is de®ned according to Eq. (1)
n t ÿ n 0 Mn 0 SI ÿ1 n 0 Mn t
1
In this equation, n(t ) represents the number of polymer chains at time t and n(0) at initial condition or t = 0. The scission index is expected to show the autoxi-
Fig. 3. Arrhenius plot of induction time and stationary state rate (comparing GPC and FTIR analysis).
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M.A. Semsarzadeh, H. Salehi / European Polymer Journal 36 (2000) 1001±1010
Fig. 4. GPC chromatograms for dierent times of PPG autoxidation at 858C.
dation of PPG at dierent temperatures. Fig. 6 shows the overall changes in the SI during oxidation. This process is similar to the oxidation reaction presented earlier in Fig. 2, where carbonyl build-up of the polymer was considered as the major factor in autoxidation. This indicates that the chain scission are in principal due to the formation of carbonyl group during the reaction. Since the ®rst and second stages of the reaction, measured from both GPC and FTIR methods show a
very good ®tting with experimental data (Fig. 7), Eq. (2) was used to express the existing statistical relation of CI or SI with time. exp y kyt c
2
In this equation y is the carbonyl index (CI) from FTIR or the scission index (SI) from GPC and t is the exposure time. Table 2 shows the results and the calculated k and c constants of the equation.
Fig. 5. Changes of Mn in the autoxidation of PPG at dierent temperatures.
M.A. Semsarzadeh, H. Salehi / European Polymer Journal 36 (2000) 1001±1010
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Fig. 6. Changes of scission index in the autoxidation of PPG at dierent temperatures.
Since the autoxidation of PPG does not follow a single reaction or a simple mechanism, one may expect
that the changes in the shape of the curves actually re¯ect the combination of the activation energies re-
Table 2 Curvilinear equation of kinetic curves of PPG atuoxidation by GPC and FTIR methods at dierent temperatures T (8C)
FTIR ( y = Cl)
GPC ( y = SI)
75 85 93
ey = 2.92 10ÿ4( yt )+1.00 (R = 0.9999) ey = 6.27 10ÿ4( yt )+1.01 (R = 0.9997) ey = 12.0 10ÿ4( yt )+1.01 (R = 0.9999)
ey = 3.03 10ÿ4( yt )+1.00 (R = 0.9999) ey = 6.74 10ÿ4( yt )+1.01 (R = 0.9998) ey = 13.0 10ÿ4( yt )+1.01 (R = 0.9996)
Fig. 7. Carbonyl build-up in the autoxidation of PPG comparing experimental and theoretical data (FIT).
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M.A. Semsarzadeh, H. Salehi / European Polymer Journal 36 (2000) 1001±1010
Fig. 8. Proton NMR spectra of PPG and PPG-catalyst (4 10ÿ5 mol/g) before and after autoxidation at 858C. (a) PPG; (b) PPG after 1780 min; (c) PPG-catalyst after 200 min.
lated to each stage of the reaction. Therefore, FTIR and GPC results are used to calculate the total activation energies of each process. The calculated activation energies are shown in Table 1. The fact that the Arrhenius plot of the induction time and stationary state rate is linear with respect to 1=T; indicated that the mechanism involved is the same.
3.2. Autoxidation reaction of PPG In spite of the fact that samples were measured with a precision weight of 10ÿ5 g, there were no observable changes in the weight of the material during the ®rst and second stages of the autoxidation. The degradation products of the polypropylene glycol with H-
M.A. Semsarzadeh, H. Salehi / European Polymer Journal 36 (2000) 1001±1010
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Fig. 9. IR spectra of PPG and PPG-catalyst (4 10ÿ5 mol/g) before and after autoxidation at 858C. (a) PPG; (b) PPG after 1780 min; (c) PPG-catalyst after 200 min.
NMR and FTIR show that polymer chain apparently breaks during the degradation and form carbonyl products. The H-NMR spectra of the PPG before and after autoxidation at 858C after 1780 min are shown in Fig. 8. The appearance of new peaks at d 2:03, 2.10,
and 8.05 ppm are related to the carbonyl products formed after autoxidation. The model compounds show these peaks are related to CH3COO± (acetate), CH3CO± (ketone) and HCOO± (formate), respectively [4]. The other resonances at d 3:8±4:4 ppm are
Fig. 10. Relationship between carbonyl index and scission index.
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M.A. Semsarzadeh, H. Salehi / European Polymer Journal 36 (2000) 1001±1010
Fig. 11. Mn % vs volatile % in the PPG and PPG-catalyst (0.6 10ÿ5 mol/g) autoxidation at 858C.
attributed to the formation of ester or ketone groups. Deutration result shows that the resonance at d 3:1 ppm in the original polymer is due to OH and its disappearance after autoxidation is related to interaction with the (C.O) groups and subsequently a shift to a higher value. The FTIR spectra of the polymer before and after autoxidation is shown in Fig. 9. The carbonyl stretch indicates a strong absorption at 1728 cmÿ1. The additional peaks at 1190 and 1240 cmÿ1 are related to the formate and acetate groups, respectively [10,11]. More signi®cantly, degraded PPG shows an increase in the absorption of hydroxyl group at 3500 cmÿ1, which can be attributed to alcohol formation. Linear relations between carbonyl index and scission index of polymer at dierent temperatures are shown
in Fig. 10. It is therefore clear that, the molecular weight decreases upon degradation at 75, 85 and 938C. Fig. 11 shows the exponential decay of the molecular weight of the polymer with respect to the loss of materials. Furthermore, the H-NMR results also indicate the autoxidation products are mostly formates, ketones and acetates with integrated intensity ratio of 3/2 for the formate with respect to ketone and ester. The formation of these materials most probably follows a free radical reaction of hydroperoxide formation followed by decomposition to carbonyl compounds as reported earlier by others [4,10,12]. 3.3. Iron-salt catalyzed autoxidation The proton NMR spectra of PPG and PPG with
Fig. 12. Carbonyl build-up in the autoxidation of PPG at dierent concentration of FeAA (mol/g) at 858C.
M.A. Semsarzadeh, H. Salehi / European Polymer Journal 36 (2000) 1001±1010
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Table 3 rs and ti at dierent concentration of catalyst (T = 858C) [FeAA] 105 (mol/g)
rs 104 (minÿ1)
ti (min)
Linear equation in second stage ( y = Cl)
0(uncatalyzed) 0.6 1.8 4 12
6.97 11.3 16.7 26.3 57.5
1274 98.3 34.9 16 5.4
y y y y y
= = = = =
6.97 10ÿ4 11.3 10ÿ4 16.7 10ÿ4 26.3 10ÿ4 57.5 10ÿ4
t t t t t
ÿ ÿ ÿ ÿ ÿ
0.888 0.106 0.059 0.042 0.031
(R (R (R (R (R
Fig. 13. Relationship between induction time and stationary state rate with concentration of FeAA at 858C.
Fig. 14. Arrhenius plots of induction time and stationary state rate for catalyzed and uncatalyzed system.
= = = = =
0.9996) 0.9999) 0.9999) 0.9999) 0.9998)
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M.A. Semsarzadeh, H. Salehi / European Polymer Journal 36 (2000) 1001±1010
Table 4 rs, ti and Ea in catalyzed and uncatalyzed systems Ea (ti)a
Ea (rs)
Ea (av)
T (8C) Uncatalyzed Catalyzedb a b
87 62
82 66.5
84.5 64.2
rs 104 (minÿ1)
ti (min)
75
85
93
75
85
93
3.6 27.5
7 57.4
14.4 83.5
3055 9.4
1274 5.4
713 3.3
Ea (kJ molÿ1). [FeAA] = 12 10ÿ5 (mol/g).
catalyst (FeAA = 4 10ÿ5 mol/g PPG) before and after autoxidation at 858C are shown in Fig. 8. The result shows that inspite of the fact that the peak intensities of the oxidative products (at d 2:03, 2.10 and 8.05 ppm) are almost the same, but the rate of formation of these products are at least 8.2 times faster for the catalyzed autoxidation reaction. The FTIR spectra of PPG and PPG-Catalyst before and after autoxidation are also shown in Fig. 9, and quantitative analysis for the rate of carbonyl build up in both catalyzed and uncatalyzed system is similar to H-NMR result. The eect of catalyst on the degradation of PPG is also shown in Fig. 11. The carbonyl build up during the autoxidation of PPG at dierent concentration of FeAA (0.6 10ÿ5 to 12 10ÿ5 mol/g PPG) are shown in Fig. 12. These results show that the induction time decreases with the concentration of catalyst and reaches a higher maximum steady rate. Presumably the catalyst can accelerate the reaction of oxygen with the polymer by a fast decomposition of the hydroperoxides [13±15]. The hydroperoxides subsequently form acetates, ketones and formates. The calculated rs and ti for dierent concentration of catalyst are shown in Table 3. The linear relation of rs and 1=ti with concentration of catalyst is shown in Fig. 13. The Arrhenius relation of induction time and stationary state rate for the catalyzed and uncatalyzed systems are also shown in Fig. 14. Table 4 shows rs and ti at dierent temperatures, the calculated energy of activation for the catalyzed system is 20.3 kJ molÿ1 less than uncatalyzed system.
Acknowledgements The support of this work by Engineering Research
Center of Jihad-e-Sazandegi is gratefully acknowledged.
References [1] Hepburn C. In: Polyurethane elastomers. London: Elsevier Applied Science, 1992. p. 19±26. [2] Woods G. The ICI polyurethane book. New York: Wiley, 1987. [3] Newton RD. In: Kirk-Othmer, Encyclopedia of chemical technology, vol. 18. New York: John Wiley, 1982. p. 633. [4] Griths PGF, Hughes JG, Park GS. Eur Polym J 1993;29(2/3):437. [5] Gahde J, Fischer TH, Falkenhagen J, Dittmar A, Gnauck R. J Appl Polym Sci 1997;64:2449. [6] Jayabalan M, Lizymol PP. Polym Degrad Stab 1997;58:251. [7] Wirpsza Z. In: Polyurethanes, chemistry, technology and applications. Chichester, UK: Ellis Horwood, 1993. p. 86±8. [8] Bamford CH, Tipper CFH. In: Degradation of polymers, vol. 472. Oxford: Elsevier Science, 1975. p. 476±85. [9] Nguyen TQ. Polym Degrad Stab 1994;46:99. [10] Costa L, Camino G, Luda MP, Cameron GG, Qureshi MY. Polym Degrad Stab 1996;53:301. [11] Bellamy LJ. In: The infra-red spectra of complex molecules. New York: Wiley, 1962. p. 179, 189. [12] Hahner V, Habicher WD, Schwetlick K. Polym Degrad Stab 1991;34:111. [13] Gavin P, Lmaire J, Sallet D. Makromol Chem 1987;188:1815. [14] Achimsky L, Audouin L, Verdu J. Polym Degrad Stab 1997;57:231. [15] Dexter M. In: Mark HF, Bikales NM, Overberger CG, Menges G, editors. Antioxidant, Encyclopedia of polymer science and engineering, vol. 2. New York: Wiley, 1985. p. 75.
Autoxidation of polypropylene glycol and its catalysis with ferric acetylacetonate Mohammad A. Semsarzadeh a,*, Hamed Salehi b a
Polymer Group, Chemical Engineering Department, Tarbiat Modarres University, P.O. Box 14155-4838, Tehran, Iran Engineering Research Center, Jahad Sazandegi and Chemistry Department, School of Science, Tarbiat Modarres University, P.O. Box 14155-4838, Tehran, Iran
b
Received 2 November 1998; received in revised form 15 March 1999; accepted 14 May 1999
Abstract The autoxidation of polypropylene glycol (PPG) alone and with ferric acetylacetonate (FeAA) was studied in air at 75, 85 and 938C. The overall time-conversion follows an exponential equation. These results were con®rmed by GPC, FTIR and NMR spectroscopy. The atuoxidation of polymer shows that ester, ketone, alcohol and formate compounds are also formed. Calculated activation energy of autoxidation of PPG is 84 kJ molÿ1 and the average activation energy for PPG-FeAA autoxidation is 64 kJ molÿ1. The induction period of autoxidation of PPG with catalyst is very short, while the rate of autoxidation is faster. # 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Today the production of rigid and ¯exible polyurethane foams, polyurethane coatings, elastomers and adhesives is based on polyether polyols [1±3]. The stability of polyurethane (PU) have been a major area of research during last decade [4±6]. It is now clear that there are number of factors aecting the stability and degradation of polyurethanes. These factors have shown signi®cant eect on the kinetics of the reaction. The stability of polyurethanes are related to the stability of their components. Since the poor oxidative stability of polyethers like polypropylene glycol (PPG) in¯uences the thermal stability and properties of the polymer, kinetic studies of this monomer and the eect of ferric acetylacetonate (FeAA) catalyst on the stability of PPG used in polyurethane, specially during the ageing of the product was studied [7]. For this pur-
* Corresponding author. Fax: +98-800-6544.
pose, we have studied the kinetics of autoxidation of PPG and explored further the possibility of stabilization of PU at lower temperature range of 75±958C, where the degradation is believed to be more signi®cant in service life time storage.
2. Experimental 2.1. Materials and methods Ferric acetylacetonate (FeAA), carbon tetrachloride and toluene (chromatography grade), were supplied from Merck (Darmstadt, Germany). Polypropylene glycol (PPG) was supplied from BDH, the nominal molecular weight of polymer is 2025. The polymer was dissolved in carbon tetrachloride in a laboratory glass dish. The solvent was allowed to evaporate at room temperature for 1 h and then dried for 30 min in vacuum (120 mbar) to form a layer of polymer with thickness of about 50±80 mm. The container with its layer
0014-3057/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 9 9 ) 0 0 1 4 7 - 0
1002
M.A. Semsarzadeh, H. Salehi / European Polymer Journal 36 (2000) 1001±1010
Fig. 1. Infrared spectra of carbonyl domain at 938C for dierent oxidation times.
of polymer was then placed in the oven. The eect of catalyst was studied using a known amounts of FeAA in PPG. Prior to FTIR and GPC studies, the isothermal oxidation experiments were performed at dierent time intervals in an air-ventilated ovens at 75, 85 and 938C. GPC experiments were carried out with GPC-LC4A, equipped with data processing facilities of CR4AX (Shimadzu, Japan). The polymer samples were dissolved in toluene and ®ltered through Milex ®lter. Calibration curves were plotted using PPG standards supplied from Waters. The tests were carried out using refractive index detector and a 103 AÊ column with toluene as mobile phase and 1 ml/min ¯ow rate. The mobile phase was degassed by ultrasonic vibration.
FTIR spectra were used to study the absorption of carbonyl band at 1728 cmÿ1. They were recorded with FTIR IFS 88, Brucker, Germany. The samples were prepared in KBr in form of pellets. Carbonyl index (CI) was measured using baseline method, normalized with respect to methyl vibration at 2975 cmÿ1. H-NMR spectra were measured with a JEOL-EX90A spectrometer and CDCl3 was used as a solvent.
3. Results and discussion 3.1. PPG autoxidation kinetics The autoxidation kinetics of the polyether was stu-
Fig. 2. Carbonyl build-up in the autoxidation of PPG at dierent temperatures.
M.A. Semsarzadeh, H. Salehi / European Polymer Journal 36 (2000) 1001±1010
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Table 1 Pre-exponential factors (A0), activation energies Ea and correlation factors (R ) for induction time and stationary state rates for autoxidation of PPG FTIR
A0 Ea (kJ molÿ1) R
GPC
Induction time
Stationary state rate
Induction time
Stationary state rate
29 108 87 0.999
6.8 108 82 0.994
29 108 86 0.998
13 108 82 0.994
died with FTIR and GPC; and this process was used to study the activation energy of reaction.The FTIR studies were based on the increase in concentration of carbonyl group as shown in Fig. 1; and the time-conversion curves were calculated during the autoxidation of polypropylene glycol. Carbonyl index was calculated from the intensity ratio of carbonyl absorption at 1728 cmÿ1 to the absorption intensity of methyl group at 2975 cmÿ1. The time-conversion curves are shown in Fig. 2. These curves show three important stages: the ®rst stage is the induction period where the carbonyl growth takes place. The second stage is the steady state stage, where the rate of reaction is constant, and the third stage is the decrease in rate, where the mass losses in reaction takes place. The induction period and maximum steady rate have been used in analysis of the ®rst and second stages of the reaction, where the eects are more important in the studies of stability and ageing of polymer. Fig. 2 shows the end of the induction period or ti and the maximum steady rate or rs. FTIR and GPC results were used to calculate the activation energy of the autoxidation of the ®rst and second stages of the process. Table 1 shows the calculated pre-exponential and activation energy of the autoxida-
tion of PPG. The Arrhenius plots of rs and ti is also shown in Fig. 3. The calculated Ea of the steady state rate (82 kJ molÿ1) is in good agreement with the reported one from oxygen uptake method (84 kJ molÿ1) [4]. The calculated activation energy for the induction time is 87 kJ molÿ1, and this is some how lower than the reported activation energy by Griths (115 kJ molÿ1) [4]. It is noteworthy that, the reported results on the autoxidation of PPG is at higher temperature range (95±1408C) than those reported here. The autoxidation reaction of PPG and the eect of time and temperature on the molecular weight of polymer was also studied with GPC and the results are shown in Figs. 4 and 5. In this case the autoxidation reaction was studied on the basis of scission index (SI) or the number of broken bonds per initial macromolecule [8,9]. The scission index is de®ned according to Eq. (1)
n t ÿ n 0 Mn 0 SI ÿ1 n 0 Mn t
1
In this equation, n(t ) represents the number of polymer chains at time t and n(0) at initial condition or t = 0. The scission index is expected to show the autoxi-
Fig. 3. Arrhenius plot of induction time and stationary state rate (comparing GPC and FTIR analysis).
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M.A. Semsarzadeh, H. Salehi / European Polymer Journal 36 (2000) 1001±1010
Fig. 4. GPC chromatograms for dierent times of PPG autoxidation at 858C.
dation of PPG at dierent temperatures. Fig. 6 shows the overall changes in the SI during oxidation. This process is similar to the oxidation reaction presented earlier in Fig. 2, where carbonyl build-up of the polymer was considered as the major factor in autoxidation. This indicates that the chain scission are in principal due to the formation of carbonyl group during the reaction. Since the ®rst and second stages of the reaction, measured from both GPC and FTIR methods show a
very good ®tting with experimental data (Fig. 7), Eq. (2) was used to express the existing statistical relation of CI or SI with time. exp y kyt c
2
In this equation y is the carbonyl index (CI) from FTIR or the scission index (SI) from GPC and t is the exposure time. Table 2 shows the results and the calculated k and c constants of the equation.
Fig. 5. Changes of Mn in the autoxidation of PPG at dierent temperatures.
M.A. Semsarzadeh, H. Salehi / European Polymer Journal 36 (2000) 1001±1010
1005
Fig. 6. Changes of scission index in the autoxidation of PPG at dierent temperatures.
Since the autoxidation of PPG does not follow a single reaction or a simple mechanism, one may expect
that the changes in the shape of the curves actually re¯ect the combination of the activation energies re-
Table 2 Curvilinear equation of kinetic curves of PPG atuoxidation by GPC and FTIR methods at dierent temperatures T (8C)
FTIR ( y = Cl)
GPC ( y = SI)
75 85 93
ey = 2.92 10ÿ4( yt )+1.00 (R = 0.9999) ey = 6.27 10ÿ4( yt )+1.01 (R = 0.9997) ey = 12.0 10ÿ4( yt )+1.01 (R = 0.9999)
ey = 3.03 10ÿ4( yt )+1.00 (R = 0.9999) ey = 6.74 10ÿ4( yt )+1.01 (R = 0.9998) ey = 13.0 10ÿ4( yt )+1.01 (R = 0.9996)
Fig. 7. Carbonyl build-up in the autoxidation of PPG comparing experimental and theoretical data (FIT).
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M.A. Semsarzadeh, H. Salehi / European Polymer Journal 36 (2000) 1001±1010
Fig. 8. Proton NMR spectra of PPG and PPG-catalyst (4 10ÿ5 mol/g) before and after autoxidation at 858C. (a) PPG; (b) PPG after 1780 min; (c) PPG-catalyst after 200 min.
lated to each stage of the reaction. Therefore, FTIR and GPC results are used to calculate the total activation energies of each process. The calculated activation energies are shown in Table 1. The fact that the Arrhenius plot of the induction time and stationary state rate is linear with respect to 1=T; indicated that the mechanism involved is the same.
3.2. Autoxidation reaction of PPG In spite of the fact that samples were measured with a precision weight of 10ÿ5 g, there were no observable changes in the weight of the material during the ®rst and second stages of the autoxidation. The degradation products of the polypropylene glycol with H-
M.A. Semsarzadeh, H. Salehi / European Polymer Journal 36 (2000) 1001±1010
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Fig. 9. IR spectra of PPG and PPG-catalyst (4 10ÿ5 mol/g) before and after autoxidation at 858C. (a) PPG; (b) PPG after 1780 min; (c) PPG-catalyst after 200 min.
NMR and FTIR show that polymer chain apparently breaks during the degradation and form carbonyl products. The H-NMR spectra of the PPG before and after autoxidation at 858C after 1780 min are shown in Fig. 8. The appearance of new peaks at d 2:03, 2.10,
and 8.05 ppm are related to the carbonyl products formed after autoxidation. The model compounds show these peaks are related to CH3COO± (acetate), CH3CO± (ketone) and HCOO± (formate), respectively [4]. The other resonances at d 3:8±4:4 ppm are
Fig. 10. Relationship between carbonyl index and scission index.
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M.A. Semsarzadeh, H. Salehi / European Polymer Journal 36 (2000) 1001±1010
Fig. 11. Mn % vs volatile % in the PPG and PPG-catalyst (0.6 10ÿ5 mol/g) autoxidation at 858C.
attributed to the formation of ester or ketone groups. Deutration result shows that the resonance at d 3:1 ppm in the original polymer is due to OH and its disappearance after autoxidation is related to interaction with the (C.O) groups and subsequently a shift to a higher value. The FTIR spectra of the polymer before and after autoxidation is shown in Fig. 9. The carbonyl stretch indicates a strong absorption at 1728 cmÿ1. The additional peaks at 1190 and 1240 cmÿ1 are related to the formate and acetate groups, respectively [10,11]. More signi®cantly, degraded PPG shows an increase in the absorption of hydroxyl group at 3500 cmÿ1, which can be attributed to alcohol formation. Linear relations between carbonyl index and scission index of polymer at dierent temperatures are shown
in Fig. 10. It is therefore clear that, the molecular weight decreases upon degradation at 75, 85 and 938C. Fig. 11 shows the exponential decay of the molecular weight of the polymer with respect to the loss of materials. Furthermore, the H-NMR results also indicate the autoxidation products are mostly formates, ketones and acetates with integrated intensity ratio of 3/2 for the formate with respect to ketone and ester. The formation of these materials most probably follows a free radical reaction of hydroperoxide formation followed by decomposition to carbonyl compounds as reported earlier by others [4,10,12]. 3.3. Iron-salt catalyzed autoxidation The proton NMR spectra of PPG and PPG with
Fig. 12. Carbonyl build-up in the autoxidation of PPG at dierent concentration of FeAA (mol/g) at 858C.
M.A. Semsarzadeh, H. Salehi / European Polymer Journal 36 (2000) 1001±1010
1009
Table 3 rs and ti at dierent concentration of catalyst (T = 858C) [FeAA] 105 (mol/g)
rs 104 (minÿ1)
ti (min)
Linear equation in second stage ( y = Cl)
0(uncatalyzed) 0.6 1.8 4 12
6.97 11.3 16.7 26.3 57.5
1274 98.3 34.9 16 5.4
y y y y y
= = = = =
6.97 10ÿ4 11.3 10ÿ4 16.7 10ÿ4 26.3 10ÿ4 57.5 10ÿ4
t t t t t
ÿ ÿ ÿ ÿ ÿ
0.888 0.106 0.059 0.042 0.031
(R (R (R (R (R
Fig. 13. Relationship between induction time and stationary state rate with concentration of FeAA at 858C.
Fig. 14. Arrhenius plots of induction time and stationary state rate for catalyzed and uncatalyzed system.
= = = = =
0.9996) 0.9999) 0.9999) 0.9999) 0.9998)
1010
M.A. Semsarzadeh, H. Salehi / European Polymer Journal 36 (2000) 1001±1010
Table 4 rs, ti and Ea in catalyzed and uncatalyzed systems Ea (ti)a
Ea (rs)
Ea (av)
T (8C) Uncatalyzed Catalyzedb a b
87 62
82 66.5
84.5 64.2
rs 104 (minÿ1)
ti (min)
75
85
93
75
85
93
3.6 27.5
7 57.4
14.4 83.5
3055 9.4
1274 5.4
713 3.3
Ea (kJ molÿ1). [FeAA] = 12 10ÿ5 (mol/g).
catalyst (FeAA = 4 10ÿ5 mol/g PPG) before and after autoxidation at 858C are shown in Fig. 8. The result shows that inspite of the fact that the peak intensities of the oxidative products (at d 2:03, 2.10 and 8.05 ppm) are almost the same, but the rate of formation of these products are at least 8.2 times faster for the catalyzed autoxidation reaction. The FTIR spectra of PPG and PPG-Catalyst before and after autoxidation are also shown in Fig. 9, and quantitative analysis for the rate of carbonyl build up in both catalyzed and uncatalyzed system is similar to H-NMR result. The eect of catalyst on the degradation of PPG is also shown in Fig. 11. The carbonyl build up during the autoxidation of PPG at dierent concentration of FeAA (0.6 10ÿ5 to 12 10ÿ5 mol/g PPG) are shown in Fig. 12. These results show that the induction time decreases with the concentration of catalyst and reaches a higher maximum steady rate. Presumably the catalyst can accelerate the reaction of oxygen with the polymer by a fast decomposition of the hydroperoxides [13±15]. The hydroperoxides subsequently form acetates, ketones and formates. The calculated rs and ti for dierent concentration of catalyst are shown in Table 3. The linear relation of rs and 1=ti with concentration of catalyst is shown in Fig. 13. The Arrhenius relation of induction time and stationary state rate for the catalyzed and uncatalyzed systems are also shown in Fig. 14. Table 4 shows rs and ti at dierent temperatures, the calculated energy of activation for the catalyzed system is 20.3 kJ molÿ1 less than uncatalyzed system.
Acknowledgements The support of this work by Engineering Research
Center of Jihad-e-Sazandegi is gratefully acknowledged.
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