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Thermal oxidative degradation of polyppropylene—II. The kinetics of the initial oxidation stage
THERMAL OXIDATIVE DEGRADATION OF POLYPROPYLENE--II. THE KINETICS OF THE INITIAL OXIDATION STAGE* V. B. MILLER, M. B. NEIMAN and IU. A. SHLLAPNLKOV Institute of Physical Chemistry, U.S.S.R. Academy ot" Medical Scien(-es
(Received 12 August 1959) THE study of the oxidative degradation of polypropylene at temperatures close to the melting point has a considerable practical bearing on the formulation of convenient methods to process this polymer. Furthermore, extrapolation of the findings to lower temperatures should yield information on the ageing process of polypropylene products about which little is known. In common with the greater proportion of organic compounds polypropyleue oxidizes by an autocatalytic mechanism [1, 2] at an initially very low rate. All extremely convenient rate characteristic for this process, therefore, is the induction period ~ conventionally taken as the time from initiation of the reaction to a given point very soon after. In the initial reaction stage while the rate is low,the admission of oxygen to the site of the reaction exceeds its consumption owing to diffusion and the rate of the process is governed solely t)y chemical factors. At the end of the induction period the rate of oxidation very rapidly becomes constant, evidently depending only on the rate of diffusion of the oxygen into the interior of the substance, i.e. on the geometrical dimensions of the sample [3 ]. Consequently the problem of polypropylene stabilization is t a n t a m o u n t to extending the induction period of the oxidation process. We studied the kinetics of the initial polypropylene oxidation stage at 124~ 150°C, using powdered polypropylene, with a melting point of 152°C, a mean molecular weight of about 200,000 and an ash content of 0.07 per cent. Technicalgrade oxygen was washed with water; water and other impurities were frozen out by means of liquid air. Oxidation was studied on a static apparatus. A cylindrical reaction vessel was charged with a weighed portion of the polypropylene, the air pumped out and the temperature raised to 140°C. The polypropylene was kept under these conditions for about 1 hr with continuous pumping to remove the volatile iml)urities, then the thermostat was adjusted to the required temperature and oxygen introduced into the reaction vessel. The reaction was followed by a decrease * \rV,~l)kO|l|(lL !) I){)l~ilt~'i"
Irk2
soc,:lin. 1: No. 11. 1703-170.5, 1959. ]2,(,)
130
V.B. MILLERet al.
in oxygen pressure. Inasmuch as gaseous products form at an advanced stage polypropylene oxidation our method was reliable for just the initial stage. Figure 1 gives specimen kinetic curves for the oxidation of polypropylene at 140°C, and they have a clearly marked autocatalytic character. We took the induction period conventionally as the time for w h i c h - - A P = l . 0 mm Hg. A slight error in determining--AP little affects the accuracy of determining the induction period. As an instance, if the induction period for curve 2 is taken as the time corresponding t o - - A P values of 0.5 or 2.0 rather than 1.0 mm, the induction period is altered only 15 per cent. Figure 2 plots the induction period against temperature and pressure. The Figure shows that at low pressures (below 150-200 mm Hg) the induction period grows strongly inversely with the pressure. For these pressures the reciprocal of the induction period l/z, proportional to the mean rate of the initial reaction stage, is directly proportional to the pressure (1/rPo=const.). At higher pressures t h e induction period alters little with pressure. The effective activation energy, comr(min
- ~ f f , Mr,
$o 8
0
,
2.0
40
Sgt.(min)
FIG. 1. Kinetic curves ibr the oxidation of polypropylene, T= 140°C; (1) P=588 ram, (2) 146 mm, (3) 88 mm.
0
1/70 COg 30g ~00 500~n~
FI(L 2. Induction period of polypropylene oxidation as a function of temperature and pressure. (1) 120°C; (2) 130°C; (3) I40°C; (4)150°C.
puted from the temperature dependence of the induction period, (more precisely, from 1 / v P o, Fig. 3), was 26.0 ± 0 . 5 kcal/mol for pressures up to 100 mm Hg inclusive, and 22.5 ~ 0 . 5 kcal/mol for a pressure of 500 mm Hg. The first oxidation product of saturated hydrocarbons is hydrogen peroxide [2]. Our experiments disclosed that slightly oxidized polypropylene liberates iodine from a solution of potassium iodide acidified with acetic acid, viz. it contains peroxide compounds. The decomposition of hydrogen peroxide to form free radicals initiates further oxidation of the polypropylene. If a reaction that has already started is terminated by pumping out the oxygen rather than by
131
The thermal degradation of polypropylene
decreasing the t e m p e r a t u r e , a considerable q u a n t i t y is given off of gaseous products t h a t h a v e o b v i o u s l y b e e n f o r m e d on d e c o m p o s i t i o n of h y d r o g e n peroxide. T h e rise in pressure a t t a i n s 10-30 per cent of the c o m p l e t e d r o p in pressure occurring on o x i d a t i o n a n d continues for 30-60 m i n a t a r a p i d l y decreasing rate. On r e p e a t e d a d m i s s i o n of o x y g e n t h e i n d u c t i o n period arises again, increasing as a function of the t i m e for which the r e a c t i o n is i n t e r r u p t e d (Fig. 4). r I (min)
t9
•
•
3
2
~0 B
0
Zas
I
I
2.4e
z.4s
10
20
t (rain)
t
Z}0 ~.i0 ~
FIG. 3. lg TP0 as a function of the reciprocal of the temperature (l) for Po--100 ram: (2) f o r t ' o 500 ram.
0
~
6'L?
~0
120t (rain)
Fit~. 4. Induction period on repeated admission of oxygen us a function of the period of interruption of the reaction, T== 14@'C; p--300 ram. inset--specimen kinetic curves: (l) before interruption, (2) interruption 120 rain, (3) 60 rain, (4) I rain.
i n a special series of e x p e r i m e n t s (at 140°C) the reaction was initiated a t a pressure of 97.5 m m H g , i n v o l v i n g an induction period r~ = 4 2 min. D u r i n g the i n d u c t i o n period t h e pressure j u m p e d to 300 m m H g , e q u i v a l e n t to a n induction period of r,,--1S min. Besides t i m e t 2 was linearly proi)ortional to t~ f r o m the ta(,nin)
Fig. 5. Experiments on increasing pressure during induction period; (tl) time fl'om a(hnission of oxygen to pressure rise, (t.,.) time from pressure rise to end of induction period. s t a r t of tile pressure rise to the end of the i n d u c t i o n period, a n d from tile s t a r t of the e x p e r i m e n t to t h e s t a r t of t h e pressure rise (Fig. 5), i.e. fldfilling tile condition 9*
132
V.B. MILLERet al. tl
ts
~--=1.
Consequently the same relationship of rate to pressure is preserved for any extension in the induction period. Exposure of the polypropylene to direct sunlight beforehand (for about 5 hr) did not curtail the induction period. Ill the presence of water vapour (P~,o = h n m Hg, Po,----300 m m Hg) the induction period decreased some 20 per cent. Our experiments also showed t h a t different polypropylene samples oxidize at dissimilar rates. The reason for this behaviour has not yet been elucidated. CONCLUSIONS
(1) The initial oxidation stage of polypropylene is studied. The character of the relationship between the induction period and the pressure is elucidated. (2) I t is shown t h a t on oxidation polypropylene forms unstable peroxide compounds. (3) A series of experiments is conducted to study the induction period with varying oxygen pressure. Translated by G. CAMERON REFERENCES
1. M. B. NEIMAN, V. B. MILLER, V. S. PUDOV and L. I. LAFER, Vy.~okomol.soedin. 1: 1696, 1959 2. D. G. KNOPPE, Z. K. MAIZUS, L. K. OBUKHOVA and N. M. EMANUEL', Usp. khim. 26: 416, 1957 3. A. G. POPOV, S. E. BRESLER, E. N. KASBEKOV, A. G. OS'MINSKAIIA and E. M.
SAMINSKII, Tezy dokladov VIII Mendeleevskogo s"ezda 6, 97, 1959
(Received 12 August 1959) THE study of the oxidative degradation of polypropylene at temperatures close to the melting point has a considerable practical bearing on the formulation of convenient methods to process this polymer. Furthermore, extrapolation of the findings to lower temperatures should yield information on the ageing process of polypropylene products about which little is known. In common with the greater proportion of organic compounds polypropyleue oxidizes by an autocatalytic mechanism [1, 2] at an initially very low rate. All extremely convenient rate characteristic for this process, therefore, is the induction period ~ conventionally taken as the time from initiation of the reaction to a given point very soon after. In the initial reaction stage while the rate is low,the admission of oxygen to the site of the reaction exceeds its consumption owing to diffusion and the rate of the process is governed solely t)y chemical factors. At the end of the induction period the rate of oxidation very rapidly becomes constant, evidently depending only on the rate of diffusion of the oxygen into the interior of the substance, i.e. on the geometrical dimensions of the sample [3 ]. Consequently the problem of polypropylene stabilization is t a n t a m o u n t to extending the induction period of the oxidation process. We studied the kinetics of the initial polypropylene oxidation stage at 124~ 150°C, using powdered polypropylene, with a melting point of 152°C, a mean molecular weight of about 200,000 and an ash content of 0.07 per cent. Technicalgrade oxygen was washed with water; water and other impurities were frozen out by means of liquid air. Oxidation was studied on a static apparatus. A cylindrical reaction vessel was charged with a weighed portion of the polypropylene, the air pumped out and the temperature raised to 140°C. The polypropylene was kept under these conditions for about 1 hr with continuous pumping to remove the volatile iml)urities, then the thermostat was adjusted to the required temperature and oxygen introduced into the reaction vessel. The reaction was followed by a decrease * \rV,~l)kO|l|(lL !) I){)l~ilt~'i"
Irk2
soc,:lin. 1: No. 11. 1703-170.5, 1959. ]2,(,)
130
V.B. MILLERet al.
in oxygen pressure. Inasmuch as gaseous products form at an advanced stage polypropylene oxidation our method was reliable for just the initial stage. Figure 1 gives specimen kinetic curves for the oxidation of polypropylene at 140°C, and they have a clearly marked autocatalytic character. We took the induction period conventionally as the time for w h i c h - - A P = l . 0 mm Hg. A slight error in determining--AP little affects the accuracy of determining the induction period. As an instance, if the induction period for curve 2 is taken as the time corresponding t o - - A P values of 0.5 or 2.0 rather than 1.0 mm, the induction period is altered only 15 per cent. Figure 2 plots the induction period against temperature and pressure. The Figure shows that at low pressures (below 150-200 mm Hg) the induction period grows strongly inversely with the pressure. For these pressures the reciprocal of the induction period l/z, proportional to the mean rate of the initial reaction stage, is directly proportional to the pressure (1/rPo=const.). At higher pressures t h e induction period alters little with pressure. The effective activation energy, comr(min
- ~ f f , Mr,
$o 8
0
,
2.0
40
Sgt.(min)
FIG. 1. Kinetic curves ibr the oxidation of polypropylene, T= 140°C; (1) P=588 ram, (2) 146 mm, (3) 88 mm.
0
1/70 COg 30g ~00 500~n~
FI(L 2. Induction period of polypropylene oxidation as a function of temperature and pressure. (1) 120°C; (2) 130°C; (3) I40°C; (4)150°C.
puted from the temperature dependence of the induction period, (more precisely, from 1 / v P o, Fig. 3), was 26.0 ± 0 . 5 kcal/mol for pressures up to 100 mm Hg inclusive, and 22.5 ~ 0 . 5 kcal/mol for a pressure of 500 mm Hg. The first oxidation product of saturated hydrocarbons is hydrogen peroxide [2]. Our experiments disclosed that slightly oxidized polypropylene liberates iodine from a solution of potassium iodide acidified with acetic acid, viz. it contains peroxide compounds. The decomposition of hydrogen peroxide to form free radicals initiates further oxidation of the polypropylene. If a reaction that has already started is terminated by pumping out the oxygen rather than by
131
The thermal degradation of polypropylene
decreasing the t e m p e r a t u r e , a considerable q u a n t i t y is given off of gaseous products t h a t h a v e o b v i o u s l y b e e n f o r m e d on d e c o m p o s i t i o n of h y d r o g e n peroxide. T h e rise in pressure a t t a i n s 10-30 per cent of the c o m p l e t e d r o p in pressure occurring on o x i d a t i o n a n d continues for 30-60 m i n a t a r a p i d l y decreasing rate. On r e p e a t e d a d m i s s i o n of o x y g e n t h e i n d u c t i o n period arises again, increasing as a function of the t i m e for which the r e a c t i o n is i n t e r r u p t e d (Fig. 4). r I (min)
t9
•
•
3
2
~0 B
0
Zas
I
I
2.4e
z.4s
10
20
t (rain)
t
Z}0 ~.i0 ~
FIG. 3. lg TP0 as a function of the reciprocal of the temperature (l) for Po--100 ram: (2) f o r t ' o 500 ram.
0
~
6'L?
~0
120t (rain)
Fit~. 4. Induction period on repeated admission of oxygen us a function of the period of interruption of the reaction, T== 14@'C; p--300 ram. inset--specimen kinetic curves: (l) before interruption, (2) interruption 120 rain, (3) 60 rain, (4) I rain.
i n a special series of e x p e r i m e n t s (at 140°C) the reaction was initiated a t a pressure of 97.5 m m H g , i n v o l v i n g an induction period r~ = 4 2 min. D u r i n g the i n d u c t i o n period t h e pressure j u m p e d to 300 m m H g , e q u i v a l e n t to a n induction period of r,,--1S min. Besides t i m e t 2 was linearly proi)ortional to t~ f r o m the ta(,nin)
Fig. 5. Experiments on increasing pressure during induction period; (tl) time fl'om a(hnission of oxygen to pressure rise, (t.,.) time from pressure rise to end of induction period. s t a r t of tile pressure rise to the end of the i n d u c t i o n period, a n d from tile s t a r t of the e x p e r i m e n t to t h e s t a r t of t h e pressure rise (Fig. 5), i.e. fldfilling tile condition 9*
132
V.B. MILLERet al. tl
ts
~--=1.
Consequently the same relationship of rate to pressure is preserved for any extension in the induction period. Exposure of the polypropylene to direct sunlight beforehand (for about 5 hr) did not curtail the induction period. Ill the presence of water vapour (P~,o = h n m Hg, Po,----300 m m Hg) the induction period decreased some 20 per cent. Our experiments also showed t h a t different polypropylene samples oxidize at dissimilar rates. The reason for this behaviour has not yet been elucidated. CONCLUSIONS
(1) The initial oxidation stage of polypropylene is studied. The character of the relationship between the induction period and the pressure is elucidated. (2) I t is shown t h a t on oxidation polypropylene forms unstable peroxide compounds. (3) A series of experiments is conducted to study the induction period with varying oxygen pressure. Translated by G. CAMERON REFERENCES
1. M. B. NEIMAN, V. B. MILLER, V. S. PUDOV and L. I. LAFER, Vy.~okomol.soedin. 1: 1696, 1959 2. D. G. KNOPPE, Z. K. MAIZUS, L. K. OBUKHOVA and N. M. EMANUEL', Usp. khim. 26: 416, 1957 3. A. G. POPOV, S. E. BRESLER, E. N. KASBEKOV, A. G. OS'MINSKAIIA and E. M.
SAMINSKII, Tezy dokladov VIII Mendeleevskogo s"ezda 6, 97, 1959