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The use of photodegradable polyethylene film containing radiation-modified atactic polypropylene for mulching
PolymerPhoux:l'u~misrry1 (1981) 15-23
THE
USE OF PHOTODEGRADABLE POLYETHYLENE FILM CONTAINING RADIATION-MODIFIED ATACTIC POLYPROPYLENE FOR MULCHING
H. OMICm and M. HAGtWARA Japan Atomic Energy Research Institute, Takasaki Radiation Chemistry Research Establishment, Takasaki, Gunma-ken, Japan
ABSTRACT
Polyethylene film containing radiation-modified atactic polypropylene as photosensitiser was used indoors (inside a greenhouse) and outdoors as a protective cover (plastic mulch) for seedbeds of strawberry. Both the yield strength and elongation of polyethylene (PE) film used indoors for hall-a-year were maintained at 70-80% of the initial values, while those used outdoors were continuously decreased to about 50 and 30%, respectively. It was found that the increase in IR absorption at 1720 cm -1 of PE film was closely related with the decrease in elongation. The weight loss and IR absorption showed that the oxidised PE film was biodegradable when it was contacted with bacteria which were found in soil around petroleum wells.
INTRODUCTION
Recently, it has become necessary for the plastics used as packaging materials, containers, mulching films for agriculture, etc. to be photodegradable in order to make their disposal easier. 1~ In spite of the intensive studies about photodegradable polymers, many problems still remain unsolved. One is the controllability of photodegradation. For example, the plastics must maintain their initial strength during use and then rapidly decompose at the end of their useful lifetime. No such plastics with a high reliability have been developed. It would be convenient, therefore, if the means to predict their lifetime could be developed. Another problem is the development of an effective treatment of these plastics after use. Most of them are photo-oxidatively decomposed into small 15 Polymer Photochemistry
0144-2880/81/0001-0015/$2.50©Applied
England, 1981 Printed in Northern Ireland
Science Publishers Ltd,
16
H. OMICI-II,M. HAGIWARI
pieces and then become consumed by biodegradation in the soil or in sludge. 7-11 However, the biodegradability of oxidised plastics has not been thoroughly investigated. In this study, polyethylene (PE) film blended with radiation-modified atactic polypropylene (APP) was used as mulching film for strawberry #ants and the controllability of photodegradation and the subsequent biodegradation with bacteria were investigated.
EXPERIMENTAL A crude APP (Chisso Co.) was irradiated with 6°Co gamma rays of dose rate 4 . 0 x l 0 4 r a d / h in CC14 solution at 60°C. The sensitiser M-1 was made by irradiating the APP for 20 h under the mixed gas flow of oxygen and chlorine (1 : 1, volume ratio), while M-2 was made by irradiating the APP for 70 h under the flow of oxygen. The irradiated APP was purified by reprecipitation with methanol and obtained in the form of faintly yellowish granules (ca. 1-2 mm in diameter). The amounts of oxygen and chlorine introduced into M-1 during the irradiation were estimated by elemental analysis6 as 0.32 wt% and 0"23 wt%, respectively; and those in M-2 were 0-34wt% and 0-17wt%, respectively. Polyethylene pellets (20 kg) (UBE, F-222) were blended with I kg of each APP sensitiser (M-1 or M-2) followed by moulding into films (27 ~m thick) by extrusion at 170°C. An antioxidant, 2,6-di-t-butyl-4-methylphenol (0-2%) was mixed with the PE pellets and the control PE film was made in a similar manner. The films (5 m x 1 m) were spread over the seedbeds of strawberry plants indoors (inside a greenhouse) and outdoors as shown in Fig. 1. The test site was located at longitude 139° and latitude 36 ° north. The weathering test was started in November, 1977 and ended in July, 1978.
(a)
(b)
Fig. 1. Seedbedscoverof strawberry:(a) in a greenhouse,(b) outdoor.
P H O T O D E G R A D A B L E P O L Y E T H Y L E N E FILM
17
T h e temperature, rainfall, and solar radiation energy were measured during the test period. T h e temperature and humidity in the greenhouse, which was made of poly(vinylchloride) (PVC) film (Mitsubishi-Monsanto, Cleanace, 0.1 mm thick), were automatically controlled. The solar radiation energy was measured with an Iio Electric Sunshine Integrator, Model SPI-531. Once a month a portion of the films (ca. 50 cm x 50 cm) was recovered and the yield strength and elongation were measured with an Instron Tensile Tester, Model 1130. The IR absorption spectra were measured with an Hitachi IR Spectrophotometer, Model EPI-G2. In order to investigate the biodegradability, the P E film which was previously exposed for 500 h in a xenon weathermeter (Suga Test Instrument, Model W E 6H-X) was immersed in the culture media composed of NH4NO3 (3.0 g), K2HPO4 (3.0g), MgSO4.7H20 (1-0g), NaC1 (0.5g), CaC12 (0.2g), F e S O 4 . 7 H 2 0 (0.01 g), and H 2 0 (1.0 litre). Ninety-two kinds of bacteria recovered from the soil around petroleum wells were added to the culture media. One millilitre of kerosene was added to the media in order to accelerate the metabolisation. After three weeks' cultivation at 30°C, the weight loss and change in IR absorption spectra of the film were measured.
RESULTS A N D DISCUSSION
Y i e l d strength a n d elongation
When plastic films are degraded outdoors, the primary cause of the degradation is the absorption of solar energy. 12 However, other conditions concerning temperature and humidity are also important for the photodegradation. Table 1 shows the meteorological data (temperature, rainfall and solar radiation energy) for the present test. The indoor temperature in winter (15-17°C) was about 10°C higher than the outdoor temperature. In summer, on the other hand, the difference was reduced to ca. 3°C. The indoor relative humidity was TABLE 1 METEOROLOGICAL CONDITIONS
Temperature °C Month
indoors
outdoors
Rainfall mm
x 103 cal/ cm 2
Solar energy
1977 12 1978 1 2 3 4 5 6 7
17.0 15.7 15.3 16.9 21.2 20.8 23.6 27.8
4.3 2-1 3.3 6-3 13-1 16.9 20.6 24.3
25.3 15-2 30"1 38-9 68.5 90.4 195.0 142.8
6.3 12.3 18.9 29-0 39.6 52.9 62.3 73.1
18
I-I. OMICHI, M. HAGIWARI
c a . 60-80%. The rainfall was increased from winter to summer. The increase in monthly solar radiation energy was broken off in June (Japanese rainy season). The last column in this table shows the accumulated solar energy from the start of the test. Based on these meteorological data, it is evident that the present weathering test proceeds with an increase in temperature, rainfall, and solar radiation energy. The films for measuring mechanical properties and IR absorption spectra were recovered from the parts which were not shadowed by strawberry leaves. As shown in Fig. 2, the yield strength of the sensitiser-containing PE films used indoors was gradually decreased to c a . 70% of the initial value in seven months, while that of the control PE was almost kept constant at the initial value. The elongation of the sensitiser-containing PE films was decreased by c a . 20% in the first two months and was kept nearly constant for the following four months. A marked decrease was again observed in the final month. The decrease in elongation of the control PE film was not observed until seven months had passed. When these films were used outdoors, the yield strength of the sensitisercontaining PE films decreased continuously and reached one-half the initial value after seven months, while that of the control PE film was kept almost constant for six months, followed by a rapid decrease in the final month as shown in Fig. 3. The elongation of the sensitiser-containing PE film was also continuously decreased and finally reached less than 10% of the initial value. The samples after seven months became very brittle and were easily torn into small pieces. The elongation of the control PE film, on the other hand, was increased by about 15% from the initial value for six months, followed by a decrease to 85% of the initial value in the next month. The elongation was plotted against the accumulated solar radiation energy
"~
1.2
1.2 "~.
0 1.0
~'
1.0
_
, 0.8 W
a6 u-I e 4
Q4
-~0.2
0.2
0
i
i
i
i
Z
4
6
8
TIME (month)
Fig. 2.
i 0
2
4
6
8
TIME (month)
Tensile strength and elongation of PE film cover used indoors: (0) PE film containing the photosensitiser M-I, (~) PE film containing M-2, (O) control PE film.
19
P H O T O D E G R A D A B L E P O L Y E T H Y L E N E FILM i
0 "I"
1.2
t.2
.
A
d 1.01
LOI
Z 0
LU 0.8 n¢ 0.6 IM _..4. (/) O.4 Z tl.I I-- O.2
a8
~: o.e
S W
0.4
0.2 I 2
I 4
I 6
TIME
i 8
2
4
TIME
(month)
6
8
(month)
Fig. 3. Tensile strength and elongation of PE film cover used outdoors: (&) PE film containing the photosensitiser M-l, (~x)PE film containing M-2, (A) control PE film. because the elongation at breaking point was a good measure of the brittleness of the plastic film (Fig. 4). It is clear that the elongation of PE films containing the sensitisers APP ( M - l , M-2) is decreased continuously with increasing solar energy, while that of the control PE film is not greatly affected by the solar energy within the test period. This result indicates that the radiation-modified APP is sufficiently effective for promoting the photodegradation of PE films. 6 As shown in Figs 2 and 3, a marked difference in sensitising effect was observed between the indoor and outdoor exposure. When considered with the results in Fig. 4, this might be explained as follows: the solar radiation energy was reduced by the PVC roof of the greenhouse especially in the range 2 8 0 - 4 0 0 nm, which was effective for the photodegradation of PE, The absorption of U V light in this range may be due to the U V absorber contained in the PVC roof material. 1
.
2
~
LO Z
a4 o,2 I i i I I, 0 2 4 6 8 SOLAR RADIATION ENERGY (IO"cal/cnf)
Fig. 4.
T h e relation of e l o n g a t i o n with solar radiation energy. ( S y m b o l s are the s a m e as in Fig. 3.)
20
I-I. OMICI-I/, M. HAGIWARI
In this study, two kinds of radiation-modified APP (M-1, M-2) were used as sensitisers. However, no marked difference in their effectiveness was observed. This may be due to the fact that there is only a small difference in their oxygen and chlorine contents as reported in the section headed 'Experimental'.
IR absorption and degradation Figure 5 shows the IR absorption spectra of sensitiser-containing PE films which were exposed outdoors for: (a) not exposed (i.e. control), (b) two months, (c) four months and (d) six months. A remarkable change was observed in the range 1600-1800 cm -1. The peak at 1720 cm -1 which increases with exposure time is due to ketonic carbonyl groups (1725 cm -1) and carboxylic acids (1715 cm-~)) x14 These changes indicate that the main functional groups introduced into PE film with sunlight exposure have been oxidised. The absorption log Io/I at 1720 cm -~ was plotted against exposure time in order to measure the degree of photo-oxidation of PE film, because this absorption changed most remarkably with time (Fig. 6). As for the sensitisercontaining PE film, a rapid increase in IR absorption is not observed until the end of the induction period. This could be due to the fact that the PE contains an antioxidant 2,6-di-t-butyl-4-methylphenol. Its concentration may be reduced by reaction with peroxy radicals of PE which are introduced during photo-oxidation with the sensitiser APP. 6 Therefore, the rapid increase is considered to be due to the depletion of the antioxidant. That no appreciable acceleration of oxidation is observed in the control PE may indicate that the antioxidant in PE still remains after seven months' exposure. The comparison of the results in Fig. 3 with those in Fig. 6 shows that the elongation of the sensitiser-containing PE film is little decreased until the IR
I00
g o0
~
40
O00
2000
I?00
1500
1200
I000
700
500
WAVE NUMBER (cm")
Fig. 5.
I R absorption spectra of P E films exposed outdoors for (a) 0 month, (b) two months, (c)
four monthsand (d) six months.
21
PHOTODEGRADABLE POLYETHYI~NE FILM
.
Q3
0.1
_e I 2
0
4
6
0
(mon~)
Fig. 6.
Effect of exposure time on increase in IR absorption l o g / o / / a t 1720cm-1: (A) PE film containing the photosensitiser, (O) control PE film.
absorption at 1720 cm -1 has attained a certain value. In order to represent this result quantitatively, elongation was plotted against the degree of oxidation (Fig. 7). Obviously, the relation can be expressed by a single curve. This result indicates that a certain amount of oxidised group is necessary for producing appreciable changes in elongation. That is, the elongation is rapidly decreased from 80% of the initial value to 20%, with the increase in log Io/I at 1720 cm -1 from 1.5 x 10 -2 to 6-0 x 10 -2. A further increase in this value has little effect on the elongation change. The relation in Fig. 7 gives a convenient measure for predicting the lifetime of plastic films. For example, it is possible to regulate the lifetime of PE mulch films according to the kind of plants which are planted in various seasons for various periods, as this relation may hold independently of the season and period. LO
d
oo
W 02
0 IJ~O0"j
,
!
log
Fig. 7.
I
SOXlO4 o~OxH) 4
~l~Om~"t
I% L
at
IT20
0.OxlO"*
~ 0 • IO"°
cm"
The relation of elongation of PE film cover with IR absorption log
Io/I at 1720 cn'1-1.
22
H. OMICHI, M. HAGIWARI
Biodegradation The sensitiser-containing PE film used outdoors for more than half-a-year is broken down into fine pieces and can easily be ploughed into the soil. There will be no harmful effect on the plant if these pieces are completely decomposed into carbon dioxide and water. In this work the possibility of biodegradation of the oxidised PE film was investigated using various kinds of bacteria in the soil. The PE which was previously oxidised in a weathermeter was immersed for three weeks in the culture media with bacteria, which had been collected from the soil around petroleum wells. A remarkable weight loss (18.7%) was observed when a bacterium named Acinetobacter calcoaceticus was used. Further, six samples exposed to other bacteria also showed the weight loss by more than 8%. These results indicate that the oxidised PE film can be metabolised by the bacteria. Figure 8 shows the IR absorption spectra of the films which exhibited a weight loss. The absorption of oxidised groups in the range 1600-1800 cm -1 decreased and a new absorption in the range 1600-1700 cm -1 appeared. When compared with the weight loss, the decrease in IR absorption of oxidised groups indicated that the PE film is attacked preferentially at these groups by bacteria. The resultant PE film loses its weight and gains unsaturated groups. The concentration of bacteria in the natural soil is much smaller than that used in the present study and may differ from place to place. However, it could be said that the oxidised PE film is metabolised in the soil by these bacteria in the long term.
I
~
i
I
I
I
l
|
l
200O
l
l
l
*
l
l
l
I~ WAVE
NUMBER
l
l
l
~ (cm "~)
Fig. 8. IR absorption spectra of PE film degraded by bacteria: (a) control PE film, (b) PE film of weight loss 6.9%, (c) PE film of weight loss 8.6% and (d) PE film of weight loss 18.7%.
PHOTODEGRADABLEPOLYETHYI..ENEF1LM
23
ACKNOWLEDGEMENT
The authors are indebted to Mr K. Kuribara of Gunma Horticultural Experimental Station for his assistance in mulching and to Professor H. Iizuka and Dr Y. Nishimura of the Science University of Tokyo for assisting with the biodegradation experiment.
REFERENCES 1. 2. 3. 4. 5. 6. 7.
8. 9. 10. 11. 12. 13. 14.
Gunfa~r, J. E., Polymers and ecological problems, Plenum, New York, 1973. GU-~ERT, R. D., US 3,950,282 (1976). UNIONCAPam~E CoPa~., US 3,901,838 (1975). UNXONCARBIDE COPa'., US 3,929,937 (1975). COLOROt~La~, US 4,021,388 (1977). O~crH, H., ASANO,M., I-IAGrWARA,M. and tM~,rd, K., J. Appl. Polym. Sci., 2,4 (1979) 2311. JONES, P. H., P a ~ s ~ , D., I-/~KrNS, M., MORGAN, M. H. and G u z ~ , E., Environmental Sci. and Technol., 8 (1974) 919. NYKWSr, N. B., Plastics and Polymers, Oct. (1974) 195. DENN'~NBERG,R. J., BOTI-tAST,R. J. and ABBOTt, T. P., J. Appl. Polym. Sci., 22 (1978) 459. G~, G. I. L., In: Fillers and reinforcement for plastics. Deanin, R. D. and Scott, N. R. (eds.), American Chemical Society, Washington, 1974. OTEY, F. H., MARK, A. H.,/vl~r~'ra~a'rE~, C. L. and RUSSEL,C. R., Ind. Eng. Chem., Prod. Res. Devel., 13 (1974) 90. 1L~rBY, B. and tL~3EK, J. F., Photodegradaaon, photo-oxidaaon, and photostabilization o[ polymers. Wiley, New York, 1975. ADAMS, J. H., J. Polym. Sci., A - l , 8 (1970) 1279. RUGG, F. M., SMrm, J. J. and BACON, R. C., J. Polym. Sci., 13 (1954) 535.
THE
USE OF PHOTODEGRADABLE POLYETHYLENE FILM CONTAINING RADIATION-MODIFIED ATACTIC POLYPROPYLENE FOR MULCHING
H. OMICm and M. HAGtWARA Japan Atomic Energy Research Institute, Takasaki Radiation Chemistry Research Establishment, Takasaki, Gunma-ken, Japan
ABSTRACT
Polyethylene film containing radiation-modified atactic polypropylene as photosensitiser was used indoors (inside a greenhouse) and outdoors as a protective cover (plastic mulch) for seedbeds of strawberry. Both the yield strength and elongation of polyethylene (PE) film used indoors for hall-a-year were maintained at 70-80% of the initial values, while those used outdoors were continuously decreased to about 50 and 30%, respectively. It was found that the increase in IR absorption at 1720 cm -1 of PE film was closely related with the decrease in elongation. The weight loss and IR absorption showed that the oxidised PE film was biodegradable when it was contacted with bacteria which were found in soil around petroleum wells.
INTRODUCTION
Recently, it has become necessary for the plastics used as packaging materials, containers, mulching films for agriculture, etc. to be photodegradable in order to make their disposal easier. 1~ In spite of the intensive studies about photodegradable polymers, many problems still remain unsolved. One is the controllability of photodegradation. For example, the plastics must maintain their initial strength during use and then rapidly decompose at the end of their useful lifetime. No such plastics with a high reliability have been developed. It would be convenient, therefore, if the means to predict their lifetime could be developed. Another problem is the development of an effective treatment of these plastics after use. Most of them are photo-oxidatively decomposed into small 15 Polymer Photochemistry
0144-2880/81/0001-0015/$2.50©Applied
England, 1981 Printed in Northern Ireland
Science Publishers Ltd,
16
H. OMICI-II,M. HAGIWARI
pieces and then become consumed by biodegradation in the soil or in sludge. 7-11 However, the biodegradability of oxidised plastics has not been thoroughly investigated. In this study, polyethylene (PE) film blended with radiation-modified atactic polypropylene (APP) was used as mulching film for strawberry #ants and the controllability of photodegradation and the subsequent biodegradation with bacteria were investigated.
EXPERIMENTAL A crude APP (Chisso Co.) was irradiated with 6°Co gamma rays of dose rate 4 . 0 x l 0 4 r a d / h in CC14 solution at 60°C. The sensitiser M-1 was made by irradiating the APP for 20 h under the mixed gas flow of oxygen and chlorine (1 : 1, volume ratio), while M-2 was made by irradiating the APP for 70 h under the flow of oxygen. The irradiated APP was purified by reprecipitation with methanol and obtained in the form of faintly yellowish granules (ca. 1-2 mm in diameter). The amounts of oxygen and chlorine introduced into M-1 during the irradiation were estimated by elemental analysis6 as 0.32 wt% and 0"23 wt%, respectively; and those in M-2 were 0-34wt% and 0-17wt%, respectively. Polyethylene pellets (20 kg) (UBE, F-222) were blended with I kg of each APP sensitiser (M-1 or M-2) followed by moulding into films (27 ~m thick) by extrusion at 170°C. An antioxidant, 2,6-di-t-butyl-4-methylphenol (0-2%) was mixed with the PE pellets and the control PE film was made in a similar manner. The films (5 m x 1 m) were spread over the seedbeds of strawberry plants indoors (inside a greenhouse) and outdoors as shown in Fig. 1. The test site was located at longitude 139° and latitude 36 ° north. The weathering test was started in November, 1977 and ended in July, 1978.
(a)
(b)
Fig. 1. Seedbedscoverof strawberry:(a) in a greenhouse,(b) outdoor.
P H O T O D E G R A D A B L E P O L Y E T H Y L E N E FILM
17
T h e temperature, rainfall, and solar radiation energy were measured during the test period. T h e temperature and humidity in the greenhouse, which was made of poly(vinylchloride) (PVC) film (Mitsubishi-Monsanto, Cleanace, 0.1 mm thick), were automatically controlled. The solar radiation energy was measured with an Iio Electric Sunshine Integrator, Model SPI-531. Once a month a portion of the films (ca. 50 cm x 50 cm) was recovered and the yield strength and elongation were measured with an Instron Tensile Tester, Model 1130. The IR absorption spectra were measured with an Hitachi IR Spectrophotometer, Model EPI-G2. In order to investigate the biodegradability, the P E film which was previously exposed for 500 h in a xenon weathermeter (Suga Test Instrument, Model W E 6H-X) was immersed in the culture media composed of NH4NO3 (3.0 g), K2HPO4 (3.0g), MgSO4.7H20 (1-0g), NaC1 (0.5g), CaC12 (0.2g), F e S O 4 . 7 H 2 0 (0.01 g), and H 2 0 (1.0 litre). Ninety-two kinds of bacteria recovered from the soil around petroleum wells were added to the culture media. One millilitre of kerosene was added to the media in order to accelerate the metabolisation. After three weeks' cultivation at 30°C, the weight loss and change in IR absorption spectra of the film were measured.
RESULTS A N D DISCUSSION
Y i e l d strength a n d elongation
When plastic films are degraded outdoors, the primary cause of the degradation is the absorption of solar energy. 12 However, other conditions concerning temperature and humidity are also important for the photodegradation. Table 1 shows the meteorological data (temperature, rainfall and solar radiation energy) for the present test. The indoor temperature in winter (15-17°C) was about 10°C higher than the outdoor temperature. In summer, on the other hand, the difference was reduced to ca. 3°C. The indoor relative humidity was TABLE 1 METEOROLOGICAL CONDITIONS
Temperature °C Month
indoors
outdoors
Rainfall mm
x 103 cal/ cm 2
Solar energy
1977 12 1978 1 2 3 4 5 6 7
17.0 15.7 15.3 16.9 21.2 20.8 23.6 27.8
4.3 2-1 3.3 6-3 13-1 16.9 20.6 24.3
25.3 15-2 30"1 38-9 68.5 90.4 195.0 142.8
6.3 12.3 18.9 29-0 39.6 52.9 62.3 73.1
18
I-I. OMICHI, M. HAGIWARI
c a . 60-80%. The rainfall was increased from winter to summer. The increase in monthly solar radiation energy was broken off in June (Japanese rainy season). The last column in this table shows the accumulated solar energy from the start of the test. Based on these meteorological data, it is evident that the present weathering test proceeds with an increase in temperature, rainfall, and solar radiation energy. The films for measuring mechanical properties and IR absorption spectra were recovered from the parts which were not shadowed by strawberry leaves. As shown in Fig. 2, the yield strength of the sensitiser-containing PE films used indoors was gradually decreased to c a . 70% of the initial value in seven months, while that of the control PE was almost kept constant at the initial value. The elongation of the sensitiser-containing PE films was decreased by c a . 20% in the first two months and was kept nearly constant for the following four months. A marked decrease was again observed in the final month. The decrease in elongation of the control PE film was not observed until seven months had passed. When these films were used outdoors, the yield strength of the sensitisercontaining PE films decreased continuously and reached one-half the initial value after seven months, while that of the control PE film was kept almost constant for six months, followed by a rapid decrease in the final month as shown in Fig. 3. The elongation of the sensitiser-containing PE film was also continuously decreased and finally reached less than 10% of the initial value. The samples after seven months became very brittle and were easily torn into small pieces. The elongation of the control PE film, on the other hand, was increased by about 15% from the initial value for six months, followed by a decrease to 85% of the initial value in the next month. The elongation was plotted against the accumulated solar radiation energy
"~
1.2
1.2 "~.
0 1.0
~'
1.0
_
, 0.8 W
a6 u-I e 4
Q4
-~0.2
0.2
0
i
i
i
i
Z
4
6
8
TIME (month)
Fig. 2.
i 0
2
4
6
8
TIME (month)
Tensile strength and elongation of PE film cover used indoors: (0) PE film containing the photosensitiser M-I, (~) PE film containing M-2, (O) control PE film.
19
P H O T O D E G R A D A B L E P O L Y E T H Y L E N E FILM i
0 "I"
1.2
t.2
.
A
d 1.01
LOI
Z 0
LU 0.8 n¢ 0.6 IM _..4. (/) O.4 Z tl.I I-- O.2
a8
~: o.e
S W
0.4
0.2 I 2
I 4
I 6
TIME
i 8
2
4
TIME
(month)
6
8
(month)
Fig. 3. Tensile strength and elongation of PE film cover used outdoors: (&) PE film containing the photosensitiser M-l, (~x)PE film containing M-2, (A) control PE film. because the elongation at breaking point was a good measure of the brittleness of the plastic film (Fig. 4). It is clear that the elongation of PE films containing the sensitisers APP ( M - l , M-2) is decreased continuously with increasing solar energy, while that of the control PE film is not greatly affected by the solar energy within the test period. This result indicates that the radiation-modified APP is sufficiently effective for promoting the photodegradation of PE films. 6 As shown in Figs 2 and 3, a marked difference in sensitising effect was observed between the indoor and outdoor exposure. When considered with the results in Fig. 4, this might be explained as follows: the solar radiation energy was reduced by the PVC roof of the greenhouse especially in the range 2 8 0 - 4 0 0 nm, which was effective for the photodegradation of PE, The absorption of U V light in this range may be due to the U V absorber contained in the PVC roof material. 1
.
2
~
LO Z
a4 o,2 I i i I I, 0 2 4 6 8 SOLAR RADIATION ENERGY (IO"cal/cnf)
Fig. 4.
T h e relation of e l o n g a t i o n with solar radiation energy. ( S y m b o l s are the s a m e as in Fig. 3.)
20
I-I. OMICI-I/, M. HAGIWARI
In this study, two kinds of radiation-modified APP (M-1, M-2) were used as sensitisers. However, no marked difference in their effectiveness was observed. This may be due to the fact that there is only a small difference in their oxygen and chlorine contents as reported in the section headed 'Experimental'.
IR absorption and degradation Figure 5 shows the IR absorption spectra of sensitiser-containing PE films which were exposed outdoors for: (a) not exposed (i.e. control), (b) two months, (c) four months and (d) six months. A remarkable change was observed in the range 1600-1800 cm -1. The peak at 1720 cm -1 which increases with exposure time is due to ketonic carbonyl groups (1725 cm -1) and carboxylic acids (1715 cm-~)) x14 These changes indicate that the main functional groups introduced into PE film with sunlight exposure have been oxidised. The absorption log Io/I at 1720 cm -~ was plotted against exposure time in order to measure the degree of photo-oxidation of PE film, because this absorption changed most remarkably with time (Fig. 6). As for the sensitisercontaining PE film, a rapid increase in IR absorption is not observed until the end of the induction period. This could be due to the fact that the PE contains an antioxidant 2,6-di-t-butyl-4-methylphenol. Its concentration may be reduced by reaction with peroxy radicals of PE which are introduced during photo-oxidation with the sensitiser APP. 6 Therefore, the rapid increase is considered to be due to the depletion of the antioxidant. That no appreciable acceleration of oxidation is observed in the control PE may indicate that the antioxidant in PE still remains after seven months' exposure. The comparison of the results in Fig. 3 with those in Fig. 6 shows that the elongation of the sensitiser-containing PE film is little decreased until the IR
I00
g o0
~
40
O00
2000
I?00
1500
1200
I000
700
500
WAVE NUMBER (cm")
Fig. 5.
I R absorption spectra of P E films exposed outdoors for (a) 0 month, (b) two months, (c)
four monthsand (d) six months.
21
PHOTODEGRADABLE POLYETHYI~NE FILM
.
Q3
0.1
_e I 2
0
4
6
0
(mon~)
Fig. 6.
Effect of exposure time on increase in IR absorption l o g / o / / a t 1720cm-1: (A) PE film containing the photosensitiser, (O) control PE film.
absorption at 1720 cm -1 has attained a certain value. In order to represent this result quantitatively, elongation was plotted against the degree of oxidation (Fig. 7). Obviously, the relation can be expressed by a single curve. This result indicates that a certain amount of oxidised group is necessary for producing appreciable changes in elongation. That is, the elongation is rapidly decreased from 80% of the initial value to 20%, with the increase in log Io/I at 1720 cm -1 from 1.5 x 10 -2 to 6-0 x 10 -2. A further increase in this value has little effect on the elongation change. The relation in Fig. 7 gives a convenient measure for predicting the lifetime of plastic films. For example, it is possible to regulate the lifetime of PE mulch films according to the kind of plants which are planted in various seasons for various periods, as this relation may hold independently of the season and period. LO
d
oo
W 02
0 IJ~O0"j
,
!
log
Fig. 7.
I
SOXlO4 o~OxH) 4
~l~Om~"t
I% L
at
IT20
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~ 0 • IO"°
cm"
The relation of elongation of PE film cover with IR absorption log
Io/I at 1720 cn'1-1.
22
H. OMICHI, M. HAGIWARI
Biodegradation The sensitiser-containing PE film used outdoors for more than half-a-year is broken down into fine pieces and can easily be ploughed into the soil. There will be no harmful effect on the plant if these pieces are completely decomposed into carbon dioxide and water. In this work the possibility of biodegradation of the oxidised PE film was investigated using various kinds of bacteria in the soil. The PE which was previously oxidised in a weathermeter was immersed for three weeks in the culture media with bacteria, which had been collected from the soil around petroleum wells. A remarkable weight loss (18.7%) was observed when a bacterium named Acinetobacter calcoaceticus was used. Further, six samples exposed to other bacteria also showed the weight loss by more than 8%. These results indicate that the oxidised PE film can be metabolised by the bacteria. Figure 8 shows the IR absorption spectra of the films which exhibited a weight loss. The absorption of oxidised groups in the range 1600-1800 cm -1 decreased and a new absorption in the range 1600-1700 cm -1 appeared. When compared with the weight loss, the decrease in IR absorption of oxidised groups indicated that the PE film is attacked preferentially at these groups by bacteria. The resultant PE film loses its weight and gains unsaturated groups. The concentration of bacteria in the natural soil is much smaller than that used in the present study and may differ from place to place. However, it could be said that the oxidised PE film is metabolised in the soil by these bacteria in the long term.
I
~
i
I
I
I
l
|
l
200O
l
l
l
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l
l
l
I~ WAVE
NUMBER
l
l
l
~ (cm "~)
Fig. 8. IR absorption spectra of PE film degraded by bacteria: (a) control PE film, (b) PE film of weight loss 6.9%, (c) PE film of weight loss 8.6% and (d) PE film of weight loss 18.7%.
PHOTODEGRADABLEPOLYETHYI..ENEF1LM
23
ACKNOWLEDGEMENT
The authors are indebted to Mr K. Kuribara of Gunma Horticultural Experimental Station for his assistance in mulching and to Professor H. Iizuka and Dr Y. Nishimura of the Science University of Tokyo for assisting with the biodegradation experiment.
REFERENCES 1. 2. 3. 4. 5. 6. 7.
8. 9. 10. 11. 12. 13. 14.
Gunfa~r, J. E., Polymers and ecological problems, Plenum, New York, 1973. GU-~ERT, R. D., US 3,950,282 (1976). UNIONCAPam~E CoPa~., US 3,901,838 (1975). UNXONCARBIDE COPa'., US 3,929,937 (1975). COLOROt~La~, US 4,021,388 (1977). O~crH, H., ASANO,M., I-IAGrWARA,M. and tM~,rd, K., J. Appl. Polym. Sci., 2,4 (1979) 2311. JONES, P. H., P a ~ s ~ , D., I-/~KrNS, M., MORGAN, M. H. and G u z ~ , E., Environmental Sci. and Technol., 8 (1974) 919. NYKWSr, N. B., Plastics and Polymers, Oct. (1974) 195. DENN'~NBERG,R. J., BOTI-tAST,R. J. and ABBOTt, T. P., J. Appl. Polym. Sci., 22 (1978) 459. G~, G. I. L., In: Fillers and reinforcement for plastics. Deanin, R. D. and Scott, N. R. (eds.), American Chemical Society, Washington, 1974. OTEY, F. H., MARK, A. H.,/vl~r~'ra~a'rE~, C. L. and RUSSEL,C. R., Ind. Eng. Chem., Prod. Res. Devel., 13 (1974) 90. 1L~rBY, B. and tL~3EK, J. F., Photodegradaaon, photo-oxidaaon, and photostabilization o[ polymers. Wiley, New York, 1975. ADAMS, J. H., J. Polym. Sci., A - l , 8 (1970) 1279. RUGG, F. M., SMrm, J. J. and BACON, R. C., J. Polym. Sci., 13 (1954) 535.