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Effect of polydimethylsiloxane on oxidation of isotactic polypropylene
Polymer Degradation and Stability 66 (1999) 149±151
Short communication
Eect of polydimethylsiloxane on oxidation of isotactic polypropylene T.V. Monakhova, T.A. Bogaevskaya, Yu.A. Shlyapnikov* United Institute of Chemical Physics, 117334 Moscow, Russia Received 4 December 1998; received in revised form 12 March 1999; accepted 18 March 1999
Abstract Addition of liquid polydimethylsiloxane to isotactic polypropylene retards its non-inhibited and inhibited oxidation by molecular oxygen. We propose that the reasons for this retardation are the migration of mobile free radicals and the reversible transition of antioxidant into inclusions of liquid siloxane in the polymer matrix. # 1999 Elsevier Science Ltd. All rights reserved.
As shown earlier [1±3], in the induction period of oxidation, strong antioxidants in many polymers are consumed according to a ®rst order law, i.e. their concentration [IH] changes as: dIH keff IH dt
1
where keff is the so called apparent rate constant which depends on the nature of the oxidizing compound and the oxygen concentration. When the antioxidant concentration changing according to (1) becomes equal to its critical concentration IHcr , the law (1) becomes violated and after a relatively short time, cr , the induction period ends and the reaction rate increases. The dependence of induction period on initial antioxidant concentration following from this assumption will be: cr
1 IHo ln keff IHcr
2
If the main reason for ®rst order kinetics of antioxidant consumption is direct oxidation of the anti oxidant by molecular oxygen (IH O2 ! I HO2 ) followed by various processes in which the oxidizing compound takes part, we may try to prolong the
* Corresponding author. Fax:+7-095-137-4101. E-mail address: [email protected] (Y.A. Shlyapnikov)
antioxidant action, i.e. the induction period, by protecting the antioxidant from oxidation. In this study we wanted to increase the eectiveness of a strong antioxidant 2,20 -methylene-bis(4-methyl-6-tert.butylphenol) (MBP) by adding to the polymer another more stable compound, polydimethylsiloxane (PDMS) (O± Si(CH3)2ÿ)n. Denote the partition coecient of MBP between isotactic polypropylene and PDMS IHipp =IHpdms Kd , and the apparent rate constants of its consumption in these media kipp and kpdms . PDMS is more stable towards oxidation than IPP, and we may assume that kpdms is less than kipp . Extracting a part of the antioxidant from IPP into siloxane decreases the average rate of its consumption, at the same time decreasing the critical antioxidant concentration calculated for all the mixture. We may write the average antioxidant concentration as: mpdms IHpdms mipp IHipp mpdms mipp kd mpdms nipp IHipp mpdms mipp
IHa
3
where IHpdms and IHipp are antioxidant concentrations in PDMS and IPP, and mpdms and mipp are correspondingly the masses of PDMS and IPP in the oxidizing sample. It follows from (3) that antioxidant concentration in the polymer is:
0141-3910/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(99)00057-9
150
T.V. Monakhova et al. / Polymer Degradation and Stability 66 (1999) 149±151
Fig. 1. Oxygen consumption in the course of oxidation of IPP containing PDMS: 1, 0%; 2, 2.25%; 3, 4.5%; 4, 10%; 5, 20% (130 C, oxygen pressure 300 mm Hg).
IHipp
mpdms nipp kd mpdms mipp
4
Similarly the average rate constant of antioxidant consumption will be: ka
kpdms mpdms IHpdms kipp mipp IHipp mpdms IHpdms mipp IHipp
5
Without cumbersome calculations we may see that in adding PDMS to IPP we decrease the rate of antioxidant consumption. At the same time decreasing the antioxidant concentration in IPP increases its critical concentration calculated for the whole mixture because only part of the antioxidant is present in the IPP. However, in the formula of induction period (2), IH appears as the logarithm and ka as the denominator, thus the eect of increasing IHcr will be much less than that of decreasing ka , and we may predict that PDMS, being insoluble in IPP, will prolong the antioxidant action in it. In this study we investigated the eect a liquid siliconeÐolydimethylsiloxaneÐproduces on the antioxidant eect of 2,20 -methylene-bis(4-methyl-6-tert. butylphenol) (BMP). 1. Experimental Isotactic polypropylene with molecular mass Mw =92000 was reprecipitated from m-xylene, several times washed with alcohol and dried in vacuum at gradually increasing temperature. Liquid PDMS was heated in vacuum at 200 C for removal of volatile admixtures. Antioxidant BMP was vacuum-distilled, m.p.=135 C. The powder of IPP was mixed with a heptane solution
Fig. 2. Induction period of oxidation of IPP as function of antioxidant concentration. 1, Without PDMS; 2, with PDMS; 15% (200 C, oxygen, 300 mm Hg, antioxidant MBP).
of PDMS with subsequent drying. For investigation of noninhibited oxidation, ®lms 0.07 cm thick were pressed in vacuum; the ®lms were oxidized in standard vacuum apparatus described in [3]. All concentrations were are shown in mols per kg of the mixture. Microphotographs of ®lms show that PDMS is noncompatible with IPP and present as a multitude of separate inclusions. As shown in Fig. 1, PDMS added to IPP (2 to 20) increases induction period of its oxidation at 130 C and decreases the oxidation rate at later stages of oxidation.
T.V. Monakhova et al. / Polymer Degradation and Stability 66 (1999) 149±151
This eect may be explained by transition of active free radicals from IPP to the less oxidizable medium. PDMS also increases the induction period of inhibited oxidation of IPP at 200 C (Fig. 2). Comparing the curves with and without PDMS (15%) we may see that this increase is equivalent to 40% increase of antioxidant concentration. If we suppose that PDMS does not dissolve MBP and behaves as an inert ®ller the increase would be equivalent to only 15% increase of its concentration. It is interesting that the presence of PDMS also slightly decreases critical concentration of antioxidant, which may be explained by trapping of a part
151
of the active radicals by inclusions of PDMS competing with low, near-critical antioxidant concentrations [4]. References [1] Shlyapnikov YuA, Tyuleneva NK. Polym Degrad Stab 1997;56:311. [2] Shlyapnikov YuA, Kiryushkin SG, Tyuleneva NK, Iring M, Fodor Zs. Polym Bull 1988;19:449. [3] Shlyapnikov YuA, Kiryushkin SG, Mar'in AP. Antioxidative polymer stabilization. London: Taylor and Francis, 1996. [4] Shlyapnikov YuA, Bogaevskaya TA, Monakhova TV. Khimicheskaya Fizika (Russian Chemical Physics) 1984;3:997.
Short communication
Eect of polydimethylsiloxane on oxidation of isotactic polypropylene T.V. Monakhova, T.A. Bogaevskaya, Yu.A. Shlyapnikov* United Institute of Chemical Physics, 117334 Moscow, Russia Received 4 December 1998; received in revised form 12 March 1999; accepted 18 March 1999
Abstract Addition of liquid polydimethylsiloxane to isotactic polypropylene retards its non-inhibited and inhibited oxidation by molecular oxygen. We propose that the reasons for this retardation are the migration of mobile free radicals and the reversible transition of antioxidant into inclusions of liquid siloxane in the polymer matrix. # 1999 Elsevier Science Ltd. All rights reserved.
As shown earlier [1±3], in the induction period of oxidation, strong antioxidants in many polymers are consumed according to a ®rst order law, i.e. their concentration [IH] changes as: dIH keff IH dt
1
where keff is the so called apparent rate constant which depends on the nature of the oxidizing compound and the oxygen concentration. When the antioxidant concentration changing according to (1) becomes equal to its critical concentration IHcr , the law (1) becomes violated and after a relatively short time, cr , the induction period ends and the reaction rate increases. The dependence of induction period on initial antioxidant concentration following from this assumption will be: cr
1 IHo ln keff IHcr
2
If the main reason for ®rst order kinetics of antioxidant consumption is direct oxidation of the anti oxidant by molecular oxygen (IH O2 ! I HO2 ) followed by various processes in which the oxidizing compound takes part, we may try to prolong the
* Corresponding author. Fax:+7-095-137-4101. E-mail address: [email protected] (Y.A. Shlyapnikov)
antioxidant action, i.e. the induction period, by protecting the antioxidant from oxidation. In this study we wanted to increase the eectiveness of a strong antioxidant 2,20 -methylene-bis(4-methyl-6-tert.butylphenol) (MBP) by adding to the polymer another more stable compound, polydimethylsiloxane (PDMS) (O± Si(CH3)2ÿ)n. Denote the partition coecient of MBP between isotactic polypropylene and PDMS IHipp =IHpdms Kd , and the apparent rate constants of its consumption in these media kipp and kpdms . PDMS is more stable towards oxidation than IPP, and we may assume that kpdms is less than kipp . Extracting a part of the antioxidant from IPP into siloxane decreases the average rate of its consumption, at the same time decreasing the critical antioxidant concentration calculated for all the mixture. We may write the average antioxidant concentration as: mpdms IHpdms mipp IHipp mpdms mipp kd mpdms nipp IHipp mpdms mipp
IHa
3
where IHpdms and IHipp are antioxidant concentrations in PDMS and IPP, and mpdms and mipp are correspondingly the masses of PDMS and IPP in the oxidizing sample. It follows from (3) that antioxidant concentration in the polymer is:
0141-3910/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(99)00057-9
150
T.V. Monakhova et al. / Polymer Degradation and Stability 66 (1999) 149±151
Fig. 1. Oxygen consumption in the course of oxidation of IPP containing PDMS: 1, 0%; 2, 2.25%; 3, 4.5%; 4, 10%; 5, 20% (130 C, oxygen pressure 300 mm Hg).
IHipp
mpdms nipp kd mpdms mipp
4
Similarly the average rate constant of antioxidant consumption will be: ka
kpdms mpdms IHpdms kipp mipp IHipp mpdms IHpdms mipp IHipp
5
Without cumbersome calculations we may see that in adding PDMS to IPP we decrease the rate of antioxidant consumption. At the same time decreasing the antioxidant concentration in IPP increases its critical concentration calculated for the whole mixture because only part of the antioxidant is present in the IPP. However, in the formula of induction period (2), IH appears as the logarithm and ka as the denominator, thus the eect of increasing IHcr will be much less than that of decreasing ka , and we may predict that PDMS, being insoluble in IPP, will prolong the antioxidant action in it. In this study we investigated the eect a liquid siliconeÐolydimethylsiloxaneÐproduces on the antioxidant eect of 2,20 -methylene-bis(4-methyl-6-tert. butylphenol) (BMP). 1. Experimental Isotactic polypropylene with molecular mass Mw =92000 was reprecipitated from m-xylene, several times washed with alcohol and dried in vacuum at gradually increasing temperature. Liquid PDMS was heated in vacuum at 200 C for removal of volatile admixtures. Antioxidant BMP was vacuum-distilled, m.p.=135 C. The powder of IPP was mixed with a heptane solution
Fig. 2. Induction period of oxidation of IPP as function of antioxidant concentration. 1, Without PDMS; 2, with PDMS; 15% (200 C, oxygen, 300 mm Hg, antioxidant MBP).
of PDMS with subsequent drying. For investigation of noninhibited oxidation, ®lms 0.07 cm thick were pressed in vacuum; the ®lms were oxidized in standard vacuum apparatus described in [3]. All concentrations were are shown in mols per kg of the mixture. Microphotographs of ®lms show that PDMS is noncompatible with IPP and present as a multitude of separate inclusions. As shown in Fig. 1, PDMS added to IPP (2 to 20) increases induction period of its oxidation at 130 C and decreases the oxidation rate at later stages of oxidation.
T.V. Monakhova et al. / Polymer Degradation and Stability 66 (1999) 149±151
This eect may be explained by transition of active free radicals from IPP to the less oxidizable medium. PDMS also increases the induction period of inhibited oxidation of IPP at 200 C (Fig. 2). Comparing the curves with and without PDMS (15%) we may see that this increase is equivalent to 40% increase of antioxidant concentration. If we suppose that PDMS does not dissolve MBP and behaves as an inert ®ller the increase would be equivalent to only 15% increase of its concentration. It is interesting that the presence of PDMS also slightly decreases critical concentration of antioxidant, which may be explained by trapping of a part
151
of the active radicals by inclusions of PDMS competing with low, near-critical antioxidant concentrations [4]. References [1] Shlyapnikov YuA, Tyuleneva NK. Polym Degrad Stab 1997;56:311. [2] Shlyapnikov YuA, Kiryushkin SG, Tyuleneva NK, Iring M, Fodor Zs. Polym Bull 1988;19:449. [3] Shlyapnikov YuA, Kiryushkin SG, Mar'in AP. Antioxidative polymer stabilization. London: Taylor and Francis, 1996. [4] Shlyapnikov YuA, Bogaevskaya TA, Monakhova TV. Khimicheskaya Fizika (Russian Chemical Physics) 1984;3:997.