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Photodegradation of polypropylene and polypropylene containing pyrene
Polymer Photochemistry 3 (1983) 379-390
Photodegradation of Polypropylene and Polypropylene Containing Pyrene A y a k o Torikai, K a k u z o Suzuki a n d K e n j i F u e k i Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Chikusa-ku, Nagoya 464, Japan (Received: 9 July, 1982)
ABSTRACT The photodegradation of polypropylene (PP) and polypropylene conruining pyrene (Py) as an additive has been studied by electron spin resonance (ESR), infra-red (1R) and ultra-violet (UV) absorption measurements. When PP film is irradiated with U V light at -196 °C, alkyl-type radicals are produced. The radicals convert to the polyenyltype on warming to room temperature. Oxygenated products are detected by IR spectroscopy. On photo-irradiating PP film containing Py, the same radicals and products as those in pure PP film are produced. Their amounts, however, are suppressed by the addition of Py. The photodegradation mechanism of PP and the protective effect of Py on the photodegradation of PP film are discussed on the basis of the experimental results.
INTRODUCTION Photodegradation and photostabilization of commercial polymers have been investigated by many authors. Previously we have studied the photodegradation and photostabilization mechanism of poly(vinyl chloride) 1 and poly(methyl methacrylate).2 We have also reported on the photo-sensitized degradation of polystyrene as a basis for improving the stability of the polymer. 3 The photodegradation of polypropylene has been investigated by many authors, 4"5 and the 379 Polymer Photochemistry 0144-2880/83/$3.00(~ Applied Science Publishers Ltd, England, 1983. Printed in Northern Ireland
380
A yako Torikai, Kakuzo Suzuki, Kenji Fueki
light-absorbing species which initiate the photo-induced reaction have been a topic of some controversy in the last decade or so. Polynuclear aromatic compounds are known to sensitize the degradation of polyolefins on irradiating with light of ~ >--300 nm. 6 However, it has been reported that the addition of polynuclear aromatic compounds to polyethylene, one possible source of photoinitiation, 4 suppresses the photo-induced radical formation in polyethylene7 when irradiation is carried out with a xenon arc lamp without an optical filter. Among several polynuclear aromatic compounds, Py suppresses most efficiently radical formation in the polymer. In this paper, we intend to elucidate the mechanism of photodegradation of PP film and the effect of Py on the photodegradation of the polymer by analysis with ESR, IR and UV absorption spectra.
EXPERIMENTAL Isotactic polypropylene (PP) powder was supplied from Aldrich Chemicals Co. Ltd without any additives. Pyrene was obtained from Tokyo Kasei Ind. Co. and used as received. PP films were prepared by hot-pressing with a pressure of 150 kg/cm2 for 10 min at 180 °C. Polypropylene films containing Py were made by blending the polymer with Py and then hot-pressing under the same conditions as those for pure PP film. The thickness of the films was c. 0.1 mm for ESR measurements, c. 0.05 mm for IR absorption measurements and c. 0.3 mm for UV absorption measurements. A Toshiba H-400P medium-pressure mercury lamp was used for the photo-irradiation ( ) t - 250 nm). The distance between the sample and the light source was 10 cm and the light intensity at this position was 1500 J i m 2 s. Temperature variations were performed by blowing the cold nitrogen gas from liquid nitrogen to a sample cell, controlling the rate of nitrogen flow. The standing time at each temperature was 10 min to attain thermal equilibrium. The ESR and optical absorption spectra of the photo-irradiated samples were measured at room temperature or - 1 9 6 °C on a JEOL-3BX spectrometer and an Hitachi 323 type spectrophotometer, respectively. Infra-red spectra of the photoirradiated PP films were taken on a Nippon Bunko DS-402 infra-red spectrophotometer.
Photodegradation of polypropylene and polypropylene containing pyrene
381
RESULTS AND DISCUSSION Photodegradation
of PP films
W h e n a P P film is photo-irradiated in a v a c u u m at 40 °C, it develops a yellow colour and gives an E S R spectrum shown in Fig. 1. The line width (21G) and the shape of this spectrum exactly coincide with those of the polyenyl-type radical (I) which is f o r m e d by the photo-irradiation of poly(vinyl chloride) film in a v a c u u m at - 5 0 °C.1 CH2 - t~(R) - ( C H ~ C H ) , - C H 2 -
(I)
w h e r e R = H or CH3. As has b e e n extensively discussed, PP has no functional group in the molecule which absorbs the irradiated light. T h e P P film used in this experiment shows a small absorption band below 330 nm which may b e responsible for the light absorption. C o m m e n t s on the light-absorbing species will be m a d e in a later section. T h e relative E S R peak height of radical (I) is plotted against the irradiation time in Fig. 2. T h e intensity increases with increasing irradiation time and then appears to reach a plateau value. T h e P P films were photo-irradiated at - 1 9 6 ° C to clarify the precursor of radical (I). T h e E S R spectrum of photo-irradiated P P in v a c u u m at - 196 °C is shown in Fig. 3. The spectrum is similar to that obtained for U V - i r r a d i a t e d (predominant wavelengths, 2 5 3 7 / ~ and 1 8 4 9 A ) s t e r e o b l o c k pp8 and consists of a sharp quartet with a hyperfine splitting of 2 3 G (shown by circles in Fig. 3 A and a b r o a d quartet. The spectral change with heat treatment of photo-irradiated P P at - 1 9 6 °C is shown in Fig. 3 B - 3 E . The sharp quartet decays at
l~° 1. ESR spectrum of photo-irradiated PP film at 40 °C in a vacuum and 10 cm
from the light source. Irradiation time, 13 h.
A yako Torikai, Kakuzo Suzuki, Kenji Fueki
382
=
10.0
c-
-r _
5.0
0
o
2b Irradiation
Fig. 2.
3o
Time(h)
Changes of relative E S R peak heights of photo-irradiated PP film at 40 °C in a vacuum with the irradiation time.
C
D
,
H
Fig. 3. E S R spectra of photo-irradiated PP films in a vacuum. A, after irradiation at - 1 9 6 ° C ; B, on warming to - 1 5 0 °C; C, on warming to 0°C; D and E, on warming to 25°C. The E S R spectra were taken at - 1 9 6 ° C for A - D . Spectrum E was measured at 25°C. Arrows and circles indicate the signal of Mn 2+ and "CH3, respectively,
Photodegradation of polypropylene and polypropylene containing pyrene
383
a r o u n d - 1 6 0 °C and can be assigned to the methyl radical, " C H 3 . The r e m a i n d e r of the spectrum obtained at - 1 9 6 ° C is attributed to the following type of radical by several authors. 8"9'1° CH2 - CH((2H2) - C H 2 -
(II)
Since the methyl radical is detected at - 1 9 6 ° C , the initial reaction which takes place by photo-irradiation may be the d e t a c h m e n t of a •CH3 or hydrogen atom, resulting in the formation of the alkyl-type radicals shown below. -CH2 - CH - CH2-
(III)
CH2
(IV)
- C(CH3)
--
CH2-
The spectrum of radical (III) may overlap that of radical (II). The radical conversion reaction from (IV) to (II) by photo-irradiation has also been reported previously, 8'9"1° i.e. hv
-CHz-C(CH3)-CH2-~
' -CH2-CH(CH2)-CH2A
(IV)
(II)
Only radical (II) can be detected at - 1 9 6 ° C because the radical conversion from (IV) to (II) occurs by the successive photoirradiation u n d e r our experimental conditions. T h e radicals ('CH3 and (II)) f o r m e d at - 1 9 6 ° C change their spectral shape and convert into another type of radical by heat treatment. T h e y change gradually to a broad singlet spectrum having AHmsl = 21G (Fig. 3D and 3E), which is attributed to radical (I). The small peaks near the signal of Mn 2+ may arise from radical (IV). 8 Although the broad singlet was detected by H a m a et al. 8 for U V irradiated PP, they did not identify this spectrum with radical (I). Changes in relative peak heights of E S R spectra with increasing t e m p e r a t u r e are shown in Fig. 4A. T h e glass transition point, T~, of isotactic PP is reported to be a r o u n d - 1 0 ° C . The radical decay below Tg is due to a small molecular motion. The faster decay begins near T~, and about onehalf of the radicals p r o d u c e d at - 1 9 6 ~ C decay in the course of warming to r o o m t e m p e r a t u r e during this experiment. The remaining radicals are relatively stable for standing at r o o m temperature. Infra-red absorption spectra of photo-irradiated PP in air at 40 °C
384
A yako Torikai, Kakuzo Suzuki, Kenfi Fueki
10.0
cT
-~ 5.0 0_
cr
° oo
i
- oo
6
Temp. ( °C ) Fig. 4. Change of relative ESR peak heights for the highest peak of the radical produced after photo-irradiation at - 1 9 6 °C with temperature. Curve A, PP; Curve B, PP containing 0.1 wt% of Py. Irradiation time, 2 h.
are given in Fig. 5. A peak centred at 1720 cm -1 and a small shoulder about 1780 cm -1 are observed in the photo-irradiated PP films. The bands a r e a t t r i b u t e d to the ~ O stretching vibrations of aliphatic ketone, uc_-o. H The absorption band at about 1620 cm -a (seen in Fig. 5D) is assigned to the C~--C stretching vibration, uc_-c. The intensity of Vc__--o increases and appears to b e c o m e broad with increasing irradiation time. The broadening of the uc__--oband is attributed to the coexistence of various 1r-conjugated D O and ~ C bonds, as in the case of the photo-irradiation of poly(vinyl chloride), which has been reported on previouslyJ T h e following reaction mechanism can reasonably explain the photodegradation of PP film• Although peroxy-type radicals were not directly detected by E S R spectroscopy, the formation of the radicals is assured by the observation of the carbonyl group. hv
PP(I) ,PP(I)* where (I) indicates impurities in PP.
(1)
Photodegradation of polypropylene and polypropylene containing pyrene
385
"C'.
r"
E c"
I--
2000
1800
1600
Waven umber (c m-t )
Fig. 5. Infra-red spectra of photo-irradiated PP films in air at 40 °C and 10 cm from the light source. Irradiation time: A, unirradiated; B, 2h; C, 4h; D, 5h. Film thickness, 0-05 ram. PP(I)*
by, - C H 2 - - C H - - C H 2 - -
+-CH3
CH2--(~(CH3)--CH2-
+. H
(2) (3)
( a l k y l - t y p e radical)
~CH2--CH--CH2
~
- CHa--CH(I~H2)--CH2~
(4)
-CH2--C;H--CH2~ ~ CH2--C(CH3)--CH2~
1 ~ CH2--CR--CH~--~CHCH2~
(5)
386
A yako Torikai, Kakuzo Suzuki, Ken]i Fueki
w h e r e R = H o r CH3 (allyl-type radical)
l - CH2--CR--(eH=eH).--CH2(polyenyl-type radical)
(6)
In the presence of air: ~ CHz--CR--(CH~CH).--CH2[+02 OwO
L
•
(7) ! R
\
o/ H -CH2--C
-C--(CH=CH).--CHz-
I
II
O
R
(9)
(8)
Reaction (8) is known as the /3-scission of PP. Since various ~-conjugated C - - O and C - - C bonds are found by I R absorption, reaction (9) can not be excluded in this case. The I R spectrum of unirradiated PP film shows an absorption band at 1720 cm-'- which is characteristic for the aliphatic ketone (vc_-o). The k e t o n e is possibly a light-absorbing species responsible for the photodegradation of PP film in o u r experiments. Another possible mechanism for the formation of oxygenated products may be written as follows: P" + 02 PO~ + PO;
, PO~
' "~"~" + ~ + Oz II 0 OH
where P" represents the polymer radical.
(10) (11)
Photodegradation of polypropylene and polypropylene containing pyrene
387
Photodegradation of P P film in the presence ot pyrene Various aromatic hydrocarbons, especially polynuclear aromatic hydrocarbons (PNA), have been known to affect the p h o t o d e g r a d a t i o n of olefins in a different manner. W h e n polyethylene containing a small a m o u n t of P N A was p h o t o - i r r a d i a t e d with light of A-->300 nm, the a m o u n t of p o l y m e r radicals increased. 6 On the contrary, the yield of radicals decreased on photo-irradiation of the same sample with the light from a xenon arc lamp. 7 P N A from exhaust gases of automobile fuel are believed to be adsorbed on the surface of PP films to a 10-3-10 -5 M concentration level? 2 W e will study the effect of P N A such as Py by E S R and I R absorption spectroscopy to analyze the reaction intermediates and final products, since Py has been reported to suppress the radical formation efficiently.7 On irradiating PP films containing low concentrations of Py with U V light in v a c u u m at 40 °C, E S R spectra similar to those of p h o t o - i r r a d i a t e d pure PP film were obtained. The spectra can also be attributed to radical (I). The spectral intensities, however, are lower than that in pure PP film at any irradiation time. The results are shown in Fig. 6. T h e relative E S R peak heights of the PP films containing Py decrease with increasing Py concentration. T h e formation of radical
D
6 10.0
0 I
~ 5.0
Zx
Q_
[]
rl
0 Irradiation Time(h) Fig. 6. Relative ESR peak heights in photo-irradiated PP film (O) and PP films containing 0.1wt% (A), 0.25wt% (Q), 0.51wt% (I-q)and 1-0wt% (&)of Py at 40 °C, in vacuum, against the irradiation time.
A yako Torikai, Kakuzo Suzuki, Ken]i Fueki
388
(I) is suppressed by the addition of Py. This suggests that Py acts as a protective agent against photodegradation. When PP film containing 0"1 wt% of Py was photo-irradiated at - 1 9 6 ° C and then on warming up to room temperature, a spectral change similar to that of pure PP film was found. The change of relative ESR peak heights with temperature is given in Fig. 4B. The radical concentrations at each temperature were always lower than those of pure PP film. The radicals begin to decay at lower temperatures in PP containing Py than in pure PP film. This fact may be interpreted as being due to a lowering of Tg with the inclusion of Py in an amorphous region of PP. The protective effect of Py on photodegradation of PP was also confirmed by IR spectroscopy. The formation of C-~-O groups (vc_--o) in PP film were found to be suppressed in the presence of Py. The change in intensity of the ~ O group is given as a carbonyl index, Drr2o/D138o, to remove the effect of film thickness, where D1~20 and D1380 a r e the absorbances at 1720 cm -1 and 1380 cm -1, respectively. The results are shown in Fig. 7. Carbonyl group formation is greatly suppressed by the addition of only 0-1 wt% of Py and a similar degree of ~ O suppression is attained in the case of the PP film containing 1 wt% of Py. However, the radical concentration differs depending on the Py concentration as shown in Fig. 6. This result suggests that all the polyenyl radicals
0.2 c5
0.1
0
0
7
Irradiation Time(h) Fig. 7. Carbonyl group formation in photo-irradiated PP film ((3) and PP films containing 0-1 wt% (Zk) and 0-25 wt% (0) of Py against the irradiation time.
Photodegradation of polypropylene and polypropylene containing pyrene
389
produced by photo-irradiation do not always contribute to the formation of carbonyl groups. The radical combination reactions other than carbonyl formation may participate in overall reactions in this system. On the behaviour of added pyrene The optical absorption spectrum of unirradiated PP film has only a small absorption at A < 3 3 0 nm, which may be responsible for the initiation of photodegradation of PP(I). This is shown in Fig. 8A.
5 4 d (5
-*B, C
360
~..0.2 ~
,
o
2 4 ]qme ( h )
0
460 Wavelength ( n m )
Fig. 8. Optical absorption spectra of PP film (A) and PP films containingPy (B, C). A and B, unirradiated; C, photo-irradiated at -196°C in vacuum for 5 h. Insert shows the dependence of OD at 11107nm on the irradiation time. When a PP film contains 0.1 wt% of Py as an additive (PP-Py), the spectrum displays the absorption bands arising from Py in the UV region (Fig. 8B). On photo-irradiation of PP-Py film in vacuum at - 1 9 6 ° C , a new absorption band appears at Am~x=407nm. This absorption band has been assigned to the cyclohexadienyl-type radical of Py, and has the following structure. 13
390
A yako Torikai, Kakuzo Suzuki, Ken]i Fueki
This result indicates that the reaction, P y + H - ~ PyH; takes place in this system. An additional mechanism of protection by Py may be the absorption of a photon by Py, the molecule which would otherwise be absorbed by PP(I), i.e. screening.
REFERENCES 1. Torikai, A., Tsuruta, H. and Fueki, K., Polym. Photochem., 2 (1982) 227. 2. Torikai, A. and Fueki, K., Polym. Photochem., 2 (1982) 297. 3. Torikai, A., Takeuchi, T. and Fueki, K., Polym. Photochem., 3 (1983) 307. 4. McKellar, J. F. and Allen, N. S., In: Photochemistry of man-made polymers, Applied Science Publishers Ltd, London, 1979. 5. R~nby, B. and Rabek, J. F., In: ESR spectroscopy in polymer research, Springer-Verlag, New York, 1977, p. 184. 6. Takeshita, T., Tsuji, K. and Seiki, T., J. Polym. Sci., A - l , 10 (1972) 2315. 7. Tsuji, K., Seiki, T. and Takeshita, T., J. Polym. Sci., Polym. Chem. Ed., 10 (1972) 3119. 8. Hama, Y., Ooi, T., Shiotsubo, M. and Shinohara, K., Polymer, 15 (1974) 787. 9. Loy, B. R., J. Polym. Sci., (A), 1 (1963) 2251. 10. Iwasaki, M., Ichikawa, T. and Toriyama, K., J. Polym. Sci., (B), 5 (1967) 423. 11. Colthup, N. B., J. Opt. Soc. Am., 40 (1950) 397. 12. Tsunooka, M. and Tanaka, M., Plastics Age, 27 (1981) 112. 13. Torikai, A., Adachi, T. and Fueki, K., J. Polym. Sci., Polym. Chem. Ed., 19 (1981) 2929.
Photodegradation of Polypropylene and Polypropylene Containing Pyrene A y a k o Torikai, K a k u z o Suzuki a n d K e n j i F u e k i Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Chikusa-ku, Nagoya 464, Japan (Received: 9 July, 1982)
ABSTRACT The photodegradation of polypropylene (PP) and polypropylene conruining pyrene (Py) as an additive has been studied by electron spin resonance (ESR), infra-red (1R) and ultra-violet (UV) absorption measurements. When PP film is irradiated with U V light at -196 °C, alkyl-type radicals are produced. The radicals convert to the polyenyltype on warming to room temperature. Oxygenated products are detected by IR spectroscopy. On photo-irradiating PP film containing Py, the same radicals and products as those in pure PP film are produced. Their amounts, however, are suppressed by the addition of Py. The photodegradation mechanism of PP and the protective effect of Py on the photodegradation of PP film are discussed on the basis of the experimental results.
INTRODUCTION Photodegradation and photostabilization of commercial polymers have been investigated by many authors. Previously we have studied the photodegradation and photostabilization mechanism of poly(vinyl chloride) 1 and poly(methyl methacrylate).2 We have also reported on the photo-sensitized degradation of polystyrene as a basis for improving the stability of the polymer. 3 The photodegradation of polypropylene has been investigated by many authors, 4"5 and the 379 Polymer Photochemistry 0144-2880/83/$3.00(~ Applied Science Publishers Ltd, England, 1983. Printed in Northern Ireland
380
A yako Torikai, Kakuzo Suzuki, Kenji Fueki
light-absorbing species which initiate the photo-induced reaction have been a topic of some controversy in the last decade or so. Polynuclear aromatic compounds are known to sensitize the degradation of polyolefins on irradiating with light of ~ >--300 nm. 6 However, it has been reported that the addition of polynuclear aromatic compounds to polyethylene, one possible source of photoinitiation, 4 suppresses the photo-induced radical formation in polyethylene7 when irradiation is carried out with a xenon arc lamp without an optical filter. Among several polynuclear aromatic compounds, Py suppresses most efficiently radical formation in the polymer. In this paper, we intend to elucidate the mechanism of photodegradation of PP film and the effect of Py on the photodegradation of the polymer by analysis with ESR, IR and UV absorption spectra.
EXPERIMENTAL Isotactic polypropylene (PP) powder was supplied from Aldrich Chemicals Co. Ltd without any additives. Pyrene was obtained from Tokyo Kasei Ind. Co. and used as received. PP films were prepared by hot-pressing with a pressure of 150 kg/cm2 for 10 min at 180 °C. Polypropylene films containing Py were made by blending the polymer with Py and then hot-pressing under the same conditions as those for pure PP film. The thickness of the films was c. 0.1 mm for ESR measurements, c. 0.05 mm for IR absorption measurements and c. 0.3 mm for UV absorption measurements. A Toshiba H-400P medium-pressure mercury lamp was used for the photo-irradiation ( ) t - 250 nm). The distance between the sample and the light source was 10 cm and the light intensity at this position was 1500 J i m 2 s. Temperature variations were performed by blowing the cold nitrogen gas from liquid nitrogen to a sample cell, controlling the rate of nitrogen flow. The standing time at each temperature was 10 min to attain thermal equilibrium. The ESR and optical absorption spectra of the photo-irradiated samples were measured at room temperature or - 1 9 6 °C on a JEOL-3BX spectrometer and an Hitachi 323 type spectrophotometer, respectively. Infra-red spectra of the photoirradiated PP films were taken on a Nippon Bunko DS-402 infra-red spectrophotometer.
Photodegradation of polypropylene and polypropylene containing pyrene
381
RESULTS AND DISCUSSION Photodegradation
of PP films
W h e n a P P film is photo-irradiated in a v a c u u m at 40 °C, it develops a yellow colour and gives an E S R spectrum shown in Fig. 1. The line width (21G) and the shape of this spectrum exactly coincide with those of the polyenyl-type radical (I) which is f o r m e d by the photo-irradiation of poly(vinyl chloride) film in a v a c u u m at - 5 0 °C.1 CH2 - t~(R) - ( C H ~ C H ) , - C H 2 -
(I)
w h e r e R = H or CH3. As has b e e n extensively discussed, PP has no functional group in the molecule which absorbs the irradiated light. T h e P P film used in this experiment shows a small absorption band below 330 nm which may b e responsible for the light absorption. C o m m e n t s on the light-absorbing species will be m a d e in a later section. T h e relative E S R peak height of radical (I) is plotted against the irradiation time in Fig. 2. T h e intensity increases with increasing irradiation time and then appears to reach a plateau value. T h e P P films were photo-irradiated at - 1 9 6 ° C to clarify the precursor of radical (I). T h e E S R spectrum of photo-irradiated P P in v a c u u m at - 196 °C is shown in Fig. 3. The spectrum is similar to that obtained for U V - i r r a d i a t e d (predominant wavelengths, 2 5 3 7 / ~ and 1 8 4 9 A ) s t e r e o b l o c k pp8 and consists of a sharp quartet with a hyperfine splitting of 2 3 G (shown by circles in Fig. 3 A and a b r o a d quartet. The spectral change with heat treatment of photo-irradiated P P at - 1 9 6 °C is shown in Fig. 3 B - 3 E . The sharp quartet decays at
l~° 1. ESR spectrum of photo-irradiated PP film at 40 °C in a vacuum and 10 cm
from the light source. Irradiation time, 13 h.
A yako Torikai, Kakuzo Suzuki, Kenji Fueki
382
=
10.0
c-
-r _
5.0
0
o
2b Irradiation
Fig. 2.
3o
Time(h)
Changes of relative E S R peak heights of photo-irradiated PP film at 40 °C in a vacuum with the irradiation time.
C
D
,
H
Fig. 3. E S R spectra of photo-irradiated PP films in a vacuum. A, after irradiation at - 1 9 6 ° C ; B, on warming to - 1 5 0 °C; C, on warming to 0°C; D and E, on warming to 25°C. The E S R spectra were taken at - 1 9 6 ° C for A - D . Spectrum E was measured at 25°C. Arrows and circles indicate the signal of Mn 2+ and "CH3, respectively,
Photodegradation of polypropylene and polypropylene containing pyrene
383
a r o u n d - 1 6 0 °C and can be assigned to the methyl radical, " C H 3 . The r e m a i n d e r of the spectrum obtained at - 1 9 6 ° C is attributed to the following type of radical by several authors. 8"9'1° CH2 - CH((2H2) - C H 2 -
(II)
Since the methyl radical is detected at - 1 9 6 ° C , the initial reaction which takes place by photo-irradiation may be the d e t a c h m e n t of a •CH3 or hydrogen atom, resulting in the formation of the alkyl-type radicals shown below. -CH2 - CH - CH2-
(III)
CH2
(IV)
- C(CH3)
--
CH2-
The spectrum of radical (III) may overlap that of radical (II). The radical conversion reaction from (IV) to (II) by photo-irradiation has also been reported previously, 8'9"1° i.e. hv
-CHz-C(CH3)-CH2-~
' -CH2-CH(CH2)-CH2A
(IV)
(II)
Only radical (II) can be detected at - 1 9 6 ° C because the radical conversion from (IV) to (II) occurs by the successive photoirradiation u n d e r our experimental conditions. T h e radicals ('CH3 and (II)) f o r m e d at - 1 9 6 ° C change their spectral shape and convert into another type of radical by heat treatment. T h e y change gradually to a broad singlet spectrum having AHmsl = 21G (Fig. 3D and 3E), which is attributed to radical (I). The small peaks near the signal of Mn 2+ may arise from radical (IV). 8 Although the broad singlet was detected by H a m a et al. 8 for U V irradiated PP, they did not identify this spectrum with radical (I). Changes in relative peak heights of E S R spectra with increasing t e m p e r a t u r e are shown in Fig. 4A. T h e glass transition point, T~, of isotactic PP is reported to be a r o u n d - 1 0 ° C . The radical decay below Tg is due to a small molecular motion. The faster decay begins near T~, and about onehalf of the radicals p r o d u c e d at - 1 9 6 ~ C decay in the course of warming to r o o m t e m p e r a t u r e during this experiment. The remaining radicals are relatively stable for standing at r o o m temperature. Infra-red absorption spectra of photo-irradiated PP in air at 40 °C
384
A yako Torikai, Kakuzo Suzuki, Kenfi Fueki
10.0
cT
-~ 5.0 0_
cr
° oo
i
- oo
6
Temp. ( °C ) Fig. 4. Change of relative ESR peak heights for the highest peak of the radical produced after photo-irradiation at - 1 9 6 °C with temperature. Curve A, PP; Curve B, PP containing 0.1 wt% of Py. Irradiation time, 2 h.
are given in Fig. 5. A peak centred at 1720 cm -1 and a small shoulder about 1780 cm -1 are observed in the photo-irradiated PP films. The bands a r e a t t r i b u t e d to the ~ O stretching vibrations of aliphatic ketone, uc_-o. H The absorption band at about 1620 cm -a (seen in Fig. 5D) is assigned to the C~--C stretching vibration, uc_-c. The intensity of Vc__--o increases and appears to b e c o m e broad with increasing irradiation time. The broadening of the uc__--oband is attributed to the coexistence of various 1r-conjugated D O and ~ C bonds, as in the case of the photo-irradiation of poly(vinyl chloride), which has been reported on previouslyJ T h e following reaction mechanism can reasonably explain the photodegradation of PP film• Although peroxy-type radicals were not directly detected by E S R spectroscopy, the formation of the radicals is assured by the observation of the carbonyl group. hv
PP(I) ,PP(I)* where (I) indicates impurities in PP.
(1)
Photodegradation of polypropylene and polypropylene containing pyrene
385
"C'.
r"
E c"
I--
2000
1800
1600
Waven umber (c m-t )
Fig. 5. Infra-red spectra of photo-irradiated PP films in air at 40 °C and 10 cm from the light source. Irradiation time: A, unirradiated; B, 2h; C, 4h; D, 5h. Film thickness, 0-05 ram. PP(I)*
by, - C H 2 - - C H - - C H 2 - -
+-CH3
CH2--(~(CH3)--CH2-
+. H
(2) (3)
( a l k y l - t y p e radical)
~CH2--CH--CH2
~
- CHa--CH(I~H2)--CH2~
(4)
-CH2--C;H--CH2~ ~ CH2--C(CH3)--CH2~
1 ~ CH2--CR--CH~--~CHCH2~
(5)
386
A yako Torikai, Kakuzo Suzuki, Ken]i Fueki
w h e r e R = H o r CH3 (allyl-type radical)
l - CH2--CR--(eH=eH).--CH2(polyenyl-type radical)
(6)
In the presence of air: ~ CHz--CR--(CH~CH).--CH2[+02 OwO
L
•
(7) ! R
\
o/ H -CH2--C
-C--(CH=CH).--CHz-
I
II
O
R
(9)
(8)
Reaction (8) is known as the /3-scission of PP. Since various ~-conjugated C - - O and C - - C bonds are found by I R absorption, reaction (9) can not be excluded in this case. The I R spectrum of unirradiated PP film shows an absorption band at 1720 cm-'- which is characteristic for the aliphatic ketone (vc_-o). The k e t o n e is possibly a light-absorbing species responsible for the photodegradation of PP film in o u r experiments. Another possible mechanism for the formation of oxygenated products may be written as follows: P" + 02 PO~ + PO;
, PO~
' "~"~" + ~ + Oz II 0 OH
where P" represents the polymer radical.
(10) (11)
Photodegradation of polypropylene and polypropylene containing pyrene
387
Photodegradation of P P film in the presence ot pyrene Various aromatic hydrocarbons, especially polynuclear aromatic hydrocarbons (PNA), have been known to affect the p h o t o d e g r a d a t i o n of olefins in a different manner. W h e n polyethylene containing a small a m o u n t of P N A was p h o t o - i r r a d i a t e d with light of A-->300 nm, the a m o u n t of p o l y m e r radicals increased. 6 On the contrary, the yield of radicals decreased on photo-irradiation of the same sample with the light from a xenon arc lamp. 7 P N A from exhaust gases of automobile fuel are believed to be adsorbed on the surface of PP films to a 10-3-10 -5 M concentration level? 2 W e will study the effect of P N A such as Py by E S R and I R absorption spectroscopy to analyze the reaction intermediates and final products, since Py has been reported to suppress the radical formation efficiently.7 On irradiating PP films containing low concentrations of Py with U V light in v a c u u m at 40 °C, E S R spectra similar to those of p h o t o - i r r a d i a t e d pure PP film were obtained. The spectra can also be attributed to radical (I). The spectral intensities, however, are lower than that in pure PP film at any irradiation time. The results are shown in Fig. 6. T h e relative E S R peak heights of the PP films containing Py decrease with increasing Py concentration. T h e formation of radical
D
6 10.0
0 I
~ 5.0
Zx
Q_
[]
rl
0 Irradiation Time(h) Fig. 6. Relative ESR peak heights in photo-irradiated PP film (O) and PP films containing 0.1wt% (A), 0.25wt% (Q), 0.51wt% (I-q)and 1-0wt% (&)of Py at 40 °C, in vacuum, against the irradiation time.
A yako Torikai, Kakuzo Suzuki, Ken]i Fueki
388
(I) is suppressed by the addition of Py. This suggests that Py acts as a protective agent against photodegradation. When PP film containing 0"1 wt% of Py was photo-irradiated at - 1 9 6 ° C and then on warming up to room temperature, a spectral change similar to that of pure PP film was found. The change of relative ESR peak heights with temperature is given in Fig. 4B. The radical concentrations at each temperature were always lower than those of pure PP film. The radicals begin to decay at lower temperatures in PP containing Py than in pure PP film. This fact may be interpreted as being due to a lowering of Tg with the inclusion of Py in an amorphous region of PP. The protective effect of Py on photodegradation of PP was also confirmed by IR spectroscopy. The formation of C-~-O groups (vc_--o) in PP film were found to be suppressed in the presence of Py. The change in intensity of the ~ O group is given as a carbonyl index, Drr2o/D138o, to remove the effect of film thickness, where D1~20 and D1380 a r e the absorbances at 1720 cm -1 and 1380 cm -1, respectively. The results are shown in Fig. 7. Carbonyl group formation is greatly suppressed by the addition of only 0-1 wt% of Py and a similar degree of ~ O suppression is attained in the case of the PP film containing 1 wt% of Py. However, the radical concentration differs depending on the Py concentration as shown in Fig. 6. This result suggests that all the polyenyl radicals
0.2 c5
0.1
0
0
7
Irradiation Time(h) Fig. 7. Carbonyl group formation in photo-irradiated PP film ((3) and PP films containing 0-1 wt% (Zk) and 0-25 wt% (0) of Py against the irradiation time.
Photodegradation of polypropylene and polypropylene containing pyrene
389
produced by photo-irradiation do not always contribute to the formation of carbonyl groups. The radical combination reactions other than carbonyl formation may participate in overall reactions in this system. On the behaviour of added pyrene The optical absorption spectrum of unirradiated PP film has only a small absorption at A < 3 3 0 nm, which may be responsible for the initiation of photodegradation of PP(I). This is shown in Fig. 8A.
5 4 d (5
-*B, C
360
~..0.2 ~
,
o
2 4 ]qme ( h )
0
460 Wavelength ( n m )
Fig. 8. Optical absorption spectra of PP film (A) and PP films containingPy (B, C). A and B, unirradiated; C, photo-irradiated at -196°C in vacuum for 5 h. Insert shows the dependence of OD at 11107nm on the irradiation time. When a PP film contains 0.1 wt% of Py as an additive (PP-Py), the spectrum displays the absorption bands arising from Py in the UV region (Fig. 8B). On photo-irradiation of PP-Py film in vacuum at - 1 9 6 ° C , a new absorption band appears at Am~x=407nm. This absorption band has been assigned to the cyclohexadienyl-type radical of Py, and has the following structure. 13
390
A yako Torikai, Kakuzo Suzuki, Ken]i Fueki
This result indicates that the reaction, P y + H - ~ PyH; takes place in this system. An additional mechanism of protection by Py may be the absorption of a photon by Py, the molecule which would otherwise be absorbed by PP(I), i.e. screening.
REFERENCES 1. Torikai, A., Tsuruta, H. and Fueki, K., Polym. Photochem., 2 (1982) 227. 2. Torikai, A. and Fueki, K., Polym. Photochem., 2 (1982) 297. 3. Torikai, A., Takeuchi, T. and Fueki, K., Polym. Photochem., 3 (1983) 307. 4. McKellar, J. F. and Allen, N. S., In: Photochemistry of man-made polymers, Applied Science Publishers Ltd, London, 1979. 5. R~nby, B. and Rabek, J. F., In: ESR spectroscopy in polymer research, Springer-Verlag, New York, 1977, p. 184. 6. Takeshita, T., Tsuji, K. and Seiki, T., J. Polym. Sci., A - l , 10 (1972) 2315. 7. Tsuji, K., Seiki, T. and Takeshita, T., J. Polym. Sci., Polym. Chem. Ed., 10 (1972) 3119. 8. Hama, Y., Ooi, T., Shiotsubo, M. and Shinohara, K., Polymer, 15 (1974) 787. 9. Loy, B. R., J. Polym. Sci., (A), 1 (1963) 2251. 10. Iwasaki, M., Ichikawa, T. and Toriyama, K., J. Polym. Sci., (B), 5 (1967) 423. 11. Colthup, N. B., J. Opt. Soc. Am., 40 (1950) 397. 12. Tsunooka, M. and Tanaka, M., Plastics Age, 27 (1981) 112. 13. Torikai, A., Adachi, T. and Fueki, K., J. Polym. Sci., Polym. Chem. Ed., 19 (1981) 2929.