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The mechanism of biodegradation of polyethylene
Polymer Degradation and Stability 18 (1987) 73-87
The Mechanism of Biodegradation of Polyethylene
Ann-Christine Albertsson, Sven Ove Andersson & Sigbritt Karlsson Department of Polymer Technology, The Royal Institute of Technology, S-100 44 Stockholm, Sweden (Received 24 November 1986; accepted 8 December 1986)
ABSTRACT LDPE films have been exposed to abiotic and biotic environments. The films were UV irradiated for periods of O, 7, 14, 26 and 42 days before being mixed with water and soil. Degraded LDPE films were examined by infra-red spectroscopy. The carbonyl peak increased with time in the abiotic environment and the oxidative degradation reported in our earlier works was confirmed. In the presence of a biotic atmosphere, however, this peak decreased. A t the same time there was an increase in double bonds which was related to weight loss. An explanation of this behavior is presented as a proposedmechanism for the biodegradation of polyethylene. This mechanism is compared, on the one hand, with abiotic photooxidation, Norrish type I and H degradation, and, on the other, with the biotic paraffin degradation. Abiotic, as well as biotic, ester formation mechanisms are also presented. An ESR spectrum confirms the presence of radicals on the polyethylene samples. At the beginning of the degradation the main agents seem to be UV light and/or oxidizing agents. When carbonyl groups have been produced, these are attacked by microorganisms which degrade the shorter segments of polyethylene chains and form carbon dioxide and water as end products. There is a synergistic effect between photooxidative degradation and biodegradation. The biodegradation of polyethylene can be compared with the biodegradation of paraffin. 73 Polymer Degradation and Stability 0141-3910/87/$03"50 © Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain
74
Ann-Christine Albertsson, Sven Ore Andersson, Sigbritt Karlsson
INTRODUCTION In almost every survey of the biological degradation of synthetic polymers, it is fairly clearly stated that polyethylene is an inert polymer with good resistance to microorganisms. Several reports mention, however, that fungal growth can occur on the surface of polyethylene.1'2 Connolly 3 and Dolezel 1 have also reported a change in tensile strength for polyethylene exposed to a biotic environment, and Kestelman et al. 4 have demonstrated a higher water uptake in polyethylene overseeded with growing molds. The increase in water uptake was accompanied by an increase in the degree of crystallinity. Jen-hao and Schwartz 5 claimed that the number of bacteria that PolYethylene was able to support was dependent on its molecular weight. Later, Potts and his collaborators 6 investigated the relationship between molecular weight and fungal growth. They found that polyethylene was not biodegradable, but the linear paraffin molecules below a molecular weight of about 500 were utilized by several microorganisms. Tsuchii et aL 7 suggested, however, that biodegradation was dependent on the availability of a substrate and was not affected by the molecular weight. Hueck a emphasized the role of criteria for the inertness of polyethylene; its hydrophobic nature and its large molecular dimensions. Scott 9 concluded that attack by microorganisms is a secondary process. The step which determines the rate at which degradable polyethylene is returned to the biological cycle appears to be the rate of the oxidation process which reduces the molecular weight of the molecule to the value required for biodegradation to occur. Even in the absence of any biodegradative attack, the carboxylic acids produced ultimately oxidize to carbon dioxide and water. Griffin 10 also questioned whether polyethylene is biodegradable. He emphasized the complex nature of the process proposed, in view of the interacting synergistic autooxidation and biodegradation effects to which polyethylene is exposed in the great variety of natural environments available. From a chemical point of view, it can be anticipated that microorganisms may be able to degrade polyethylene since the chemical structure of polyethylene is similar to that of linear alkanes, which are known to be biodegradable. However, since it is not possible to study the biodegradation of polyethylene under single environment conditions, we have compared the changes induced by a biotic and an abiotic environment. The differences observed are interpreted as being the result of biodegradation alone. We have shown that the biodegradation of polyethylene is affected by preliminary irradiation from a UV source, 11'12 by the presence of photodegradative enhancers, 11,12 by its morphology and surface area, ~3,~4 by additives, ~5.16 by antioxidants, 17 and by its molecular weight) 7
The mechanism of biodegradation of polyethylene
75
We have earlier studied the biodegradation and ageing of polyethylenes in carbon-14 marked films and powder and have measured 14CO2 generation as an indication of the degradation. The degradation curves--the evolution of CO2 as a function of time--were characterized by a straight line progression in the first hundred days of observation before a parabolic decline. The trend was occasionally reversed to a progressive increase in 14CO2 liberation in some of the long-term runs, especially after 2-3 years. These tests have been going on since 1973 and, to complement these earlier results, we have now looked at the same films mainly with the help of IR analysis. We interpret the results by suggesting a mechanism of degradation and we compare this mechanism with the mechanism of degradation of paraffin to carboxylic acids and the subsequent well known fl-oxidation and citric acid cycle.
EXPERIMENTAL LDPE films with a thickness of 20/~m were used. Some samples contained an additive (ND) while others were used without additive (0). The additive ND is a substance which increases the photochemical degradation rate. The LDPE was radioactively marked with carbon-14. The samples all came from Imperial Chemical Industries (ICI), London. The films were exposed to UV irradiation from an Osram Ultra Vitalux 300W lamp for periods of 0, 7, 14, 27 and 42 days. The samples were then mixed with natural soil and water. The samples were kept, for 10 years, at 25°C in darkness with an airflow (purified to remove CO2). Abiotic samples were prepared by including silver nitrate in the medium. Biotic samples were kept in non-sterile soil. For every abiotic sample, an identical biotic sample was prepared and handled in the same way. The degradation of the films was monitored with an IR instrument, Perkin-Elmer 580B. The films were cold drawn to make it possible to obtain transmission spectra. The reflectance IR analyses were performed with a M I R crystal (Multiple Internal Reflectance) KRS-5 prism consisting of a thallium halogen compound. Samples (2 × 5 mm) to be analyzed by IR were cut out from the films and washed with distilled water to remove traces of soil, etc. In the IR spectra, special interest was focused on the following absorption peaks: 1740cm- 1, ester carbonyl ( - - C O 0 - - ) ; 1715 c m - 1, ketone carbonyl (--CO--); 1640cm -1, double bonds ( - - C ~ C - - ) ; 915-905cm -1, double bonds (H2C~---C--). The carbonyl index (=A1715/1465, where A1465 corresponds to - - C H 2 - - ) and the double bond index ( = A1640/1465) were calculated from the IR spectra. The carbonyl index is a measure of the concentration of
76
Ann-Christine Albertsson, Sven Ore Andersson, Sigbritt Karlsson
carbonyl groups and the double bond index a measure of the double bond concentration. ESR spectra were recorded on a JES-ME-IX instrument from JEOLCO, Tokyo, Japan, using 100kHz field modulation and a cylindrical cavity (TE 011 model).
RESULTS A N D DISCUSSION In any discussion of the degradation of polymers many factors must be taken into consideration. Polymers can be degraded by thermal, oxidative, chemical, radiative and mechanical means as well as by biological agents. An initial step in the degradation of polyethylene (in the absence of additive to prevent degradation) is known to involve photooxidation. 18 Photooxidation increases the amount of low molecular weight material by breaking bonds and increasing the surface area through embrittlement. It also increases the hydrophilicity by the introduction ofcarbonyl groups. All these effects of photooxidation promote the degradation of polymers, as we have shown earlier. 16 The evolution of CO 2 increases with increasing exposure to UV light. The effect of UV radiation can be modified by using protective or sensitizing additives. The evolution of CO 2 increases greatly if the irradiated polyethylene contains a photosensitizer. 11'x2 Figure 1 shows the traditional mechanism for the photooxidation of PE. Initially, UV radiation is absorbed which leads to radical formation. Eventually, oxygen is absorbed and hydroperoxides are formed, the end products being carbonyl groups. Additional exposure to UV radiation causes the carbonyl groups to undergo the Norrish type I and/or II degradation (see Fig. 2). 18 The photooxidation can be initiated by impurities. UV degradation can also begin at locations of trace hydroperoxide or ketone groups, introduced during the manufacturing processing or fabrication. The oxidative degradation of polyolefins can be followed by measuring the level of carbonyl group adsorption by infra-red spectroscopy (IR). The measured carbonyl groups are usually expressed as a carbonyl index, which is the ratio ofcarbonyl absorbance to an invariant absorbance characteristic of the polymer, both in the IR. The use of this index compensates for differences in the thickness of polymer films. The formation of carbonyl groups is increased by photooxidation, but also by increasing stress even after storage in an abiotic environment. 16'19 If Norrish type I or II degradation (or both) occur, additional peaks are observed in the IR spectrum of the polymer. For example, a terminal double
The mechanism of biodegradation of polyethylene /XACH2-CH~.-CH2A,'k ~
77
.V'CH~CH-CH2,,v'x
J
½
0-6 I •,'vxC H2- C H--C H2 ,,~
O'-O-H
,~C H2- I~-C H2,v~ H
+
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+
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-t"
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1
O n
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Fig. 1.
fIR 1715 cm'l" I
Photooxidation of polyethylene.
bond appears at 905-915cm -1 and it is also possible to trace ester formation. Figure 3 gives a proposed mechanism for ester formation in abiotically-treated LDPE films. Norrish type I cleavage yields a carbonyl radical which can react with an alkoxy radical on the PE chain shown in Fig. 1. A peak appears at 1740cm-1 in the IR spectrum if this ester formation occurs. Radical formation in the samples can be evaluated by the use of ESR.20, 21 O O Ii NI ~l "w'CH~'CH~'C-CH2-CH2 "~'v ~ ""~CHECH'2"C"
+
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/\CH.v~v ~
~
+
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1 oII
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Fig. 2. Norrish type I and II degradations.
78
Ann-Christine Albertsson, Sven Ore Andersson, Sigbritt Karlsson o
II
,~CH2--C-C H2,w~
0 II ,~wCH2-C.
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~ , CH2"(~-I-CH2,~O-(~-CH2 "~'~' O IR'1740 cm"1 i Fig. 3.
Abiotic ester formation.
Reference is frequently made to paraffin degradation by microorganisms when PE and its instability towards environmental factors are discussed. This is fairly obvious since a paraffin can be classified as a very short-chain PE, the length of typical paraffins being 10-20 carbons. 22 Microorganisms preferentially use linear n-paraffins, whereas the corresponding branched isomers are almost completely inert to biodegradation. R-C H2-- Cl-i~CH 3
~
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~
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' 4
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Fig. 4.
Biotic paraffin degradation according to H. G. Schlegel (1976). 23
The mechanism of biodegradation of polyethylene
79
Figure 4 gives the mechanism for degradation of paraffins by microorganisms. 23 The microorganisms responsible for the attack on paraffins can be, for example, Mycobacterium, Nocardia, Corynebacterium, Candida or Pseudomonas. The alkane chain is oxidized to a carboxylic acid and the resultant acid undergoes fl-oxidation which, by reaction with coenzyme A, removes two carbon fragments from the carboxylic molecule. The two carbon fragments, acetyl-SCoA, enter the citric acid cycle, from which carbon dioxide and water are released. From a paraffin molecule with n carbons, a total of n molecules of carbon dioxide are evolved. 24 Figure 5 presents an example of ester formation due to microbial action on alkanes. F r o m hexadecane (C~6H34), cetylpalmitate is formed by the action of Acinetobacter and Micrococcus. Each step is catalyzed by sets of specific enzymes. 23 2
2 c~-(c~-c~.
~'
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Biotic ester formation according to H. G. Schlegel (1976). 23
When polyethylene films were studied after 10 years' storage in a mixture of soils, we could find no connection between time of UV irradiation and carbonyl index. As shown in Fig. 6, the carbonyl index increased with UV irradiation up to a certain limit, after which it decreased. A parabolic relationship was also obtained when the carbonyl index was plotted as a function of the percentage by weight of polyethylene converted to carbon dioxide (Fig. 7). The carbonyl index increased up to 0.3 and then decreased
Ann-Christine Albertsson, Sven Ore Andersson, Sigbritt Karlsson
80
02[
•
O-PE
0.1
Days
Fig. 6.
Variation in carbonyl index after exposure to UV light. Samples with and without additive to improve the sensitivity to UV radiation.
with increasing degradation. The samples stored in air increased their carbonyl index with time, but all samples in contact with soil showed a decrease of carbonyl index with time. We have earlier presented similar results for high density polyethylene 16'19 and our interpretation was that biodegradation decreased the number of carbonyl groups, in contrast to most other environmental factors. A proposed mechanism for the biodegradation of PE is given in Fig. 8. It is thought that the alteration of the PE molecule is initially the same in both ~.1715 ~,14~5
I 0,1
Fig. 7.
i
t 0,3
i
I 0,5
t
0~.7
'
weight~ converted to CD2
Carbonyl index as a function of percentage by weight of polyethylene converted to carbon dioxide.
The mechanism of hiodegradation of polyethylene
81
/vvlCH2-- ~ --CN2~vv~
9H
,,~C~C,-~-~ c~,
?H
[ IR,1700-25 crn-lJ -'v~O¢2"-C ~O -4-
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]
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Fig. 8. Proposed mechanism for the biodegradation of polyethylene. the abiotic and the biotic samples. When carbonyl groups had been formed, the abiotic sample evidently did not undergo Norrish type II degradation, as no double bond peak could be found in the IR spectra from abiotically degraded PE (Fig. 9). According to the Figure, the carbonyl index increased with prolonged incubation in an abiotic atmosphere.l 6 Ester formation had, however, taken place, as was confirmed by the presence of a peak at 1740 cm- t. This peak was nevertheless rather small when compared with the peak obtained in the biotically degraded samples (Fig. 9).
Ann-Christine Albertsson, Sven Ore Andersson, Sigbritt Karlsson
82
f tO
o
"6
I dT~ ..J
0
z E 0
The mechanism of biodegradation of polyethylene
83
For a study of the IR spectra of biotically degraded PE samples (Fig. 9) it was observed that the carbonyl index decreased with prolonged incubation time. ~6 The explanation for this is given by the mechanism in Fig. 8. In some IR spectra of biotically degraded PE, a peak was noted corresponding to double bond groups (IR 905-915 cm-1). This peak was missing in abiotic samples (see above). In Fig. 10, the double bond index is plotted against weight % PE converted to CO 2. There is an increase in A~64o A14~
0.~
0.1
A
0 t.1
Fig. 10.
0.3
45
weight% PE converted to CO2
Double bond index as a function of percentage by weight of polyethylene converted to CO2. • corresponds to ND-PE and • to O-PE.
double bonds with increasing C O 2 evolution and a possible mechanism for the double bond formation is shown in Fig. 8b. This step is not necessarily a biotic one. It may be a result of the splitting of the hydroperoxide and the formation of the carboxylic acid. The formation of double bonds in biotic samples but not in abiotic ones needs further analysis before a plausible explanation can be given. Figure 11 shows an ESR spectrum for biotically degraded PE. (The sample was HDPE powder. It is included here as an introduction to a forthcoming paper.) A characteristic peak attributable to peroxy radicals 2° is evident. The ESR spectral intensity increases with temperature (25, 45 and 65°C) and illumination. No difference in peroxide content could be found between samples of different ~4C concentration. In earlier studies, the rate of x4CO2 evolution from degraded PE was measured. In Fig. 12, the % PE converted to CO 2 is plotted against time. One can distinguish between two separate mechanisms, a and b. 12 b symbolizes the always present autooxidative degradation in an abiotic environment. When exposure to UV light is prolonged, this results in a steeper curve due to more portions of the PE chain being degraded to shorter pieces with
84
Ann-Christine Albertsson, Sven Ore Andersson, Sigbritt Karlsson
? ESR
SPECTRA
BIOTIC DEGRADED
Fig. 11. ESR spectra of a biotic degraded polyethylene sample.
carboxylic groups which can undergo fl-oxidation. 12a symbolizes biotic degradation. The autooxidative degradation has been subtracted, so that 12a only shows the evolution of 14CO2 from biotically degraded PE. Evolution of carbon dioxide is fast at the beginning, but thereafter the curve flattens out towards an asymptotic value. Polymers containing ester groups have been found to be much more prone to undergo biodegradation. This is to be expected, since it is well known that esterase (enzymes which hydrolyze ester links) exist in nature. Degradation% by weight
~
~ a
j
~
j
~
b
L time
Fig. 12. Percentage of polyethylene converted to carbon dioxide plotted against time.
The mechanism of biodegradation of polyethylene
85
Therefore, Bailey and Gapud 25 synthesized a new degradable polyethylene with ester carbonyl groups. Hydrolytic degradation takes place when polymers containing hydrolyzable groups are exposed to moisture. If hydrolysis is achieved enzymatically, then the process is usually considered to be biodegradation. 26,27 Guillet et al. 28 produced a copolymer with ketone groups in the main polymer chain, and these carbonyl groups absorb UV light in the first step of degradation. Scott, 9 0 m i s h i and Hagiwara 29 and Griffin 3° have used different additives to increase the degradation of polyethylene. The question is, however, whether ordinary polyethylene is biodegradable. Colin et a/. 31'32 did not interpret their results as showing that polyolefins were biodegradable even though their embrittlement data could be evidence for that. Dolezel, 1 on the other hand, has shown that the tensile strength of polyethylene changes in a biotic environment. Later, he also showed that polyethylene was changed chemically in a way similar to that observed by us, i.e. the amount of carbonyl groups decreased with prolonged exposure to a biotic environment. 33 The mechanism for paraffin biodegradation is comparable with that of polyethylene. There are many steps which are the same, but in the degradation of polyethylene abiotic steps will also contribute to the total degradation. At the beginning, the main agents seem to be UV light and/or oxidizing agents, but when carbonyl groups have been produced these are attacked by microorganisms which degrade the shorter fragments of polyethylene chains to the end products, carbon dioxide and water. There is a strong synergism between biodegradation and environmental factors, and biodegradation can, in practice, never be entirely separated from the purely physical and chemical--mainly autooxidative--progressive ageing, always present as an unavoidable slow but cumulative background effect. Biodegradation is seldom due to a single cause, but a combined effect including heat, UV light, stress and water. The presence of water is a necessity for biodegradation. Studies concerning the relationship between surface and biodegradation of PE will be presented in the future, as well as the results of an investigation on the presence of radicals on the surfaces and their effects on the fate of PE in nature. ACKNOWLEDGEMENT This work was supported by the National Swedish Board for Technical Development (STU) and by the Swedish Council for Building Research (BFR).
86
Ann-Christine Albertsson, Sven Ore Andersson, Sigbritt Karlsson REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.
B. Dolezel, Brit. Plast., 49, 105 (1967). E Demmer, Mater. Organismen, 3, 19 (1968). R. A. Connolly, Bell Syst. Tech., 51, 1 (1972). V. N. Kestelman, V. L. Yaravenko and E. I. Melnikova, Int. Biodetn. Bull., 8, 15 (1972). L. Jen-hao and A. Schwartz, Kunststoffe, 51, 317 (1961). J. E. Potts, R. A. Clendinning, W. B. Ackart and W. D. Niegischi, Polymer Preprints, 13, 629 (1972). A. Tsuchii, T. Suzuki and S. Fukuoka, Biseibutsu Kogyo Gijutsu Kenkya Hokoku, 55, 35 (1980). H, J. Hueck, Int. Biodetn. Bull., 10, 87 (1974). G. Scott, Polymer Age, 6, 54 (1975). G.J.L. Griffin, Conference on Polyethylenes 1933-1983, The Plastic and Rubber Institute, London, C.4.8.1 (1983). A.-C. Albertsson and B. Rfinby, Proceedings of the 3rd International Biodegradation Symposium (J. M. Sharpley and K. M. Kaplan (Eds)), Applied Science, London, 743 (1976). A.-C. Albertsson, Eur. Polym. J., 16, 623 (1980). A.-C. Albertsson, J. Appl. Polym. Sci., 22, 3419 (1978). A.-C. Albertsson, Z. G. Banhidi and L.-L. Beyer-Ericsson, J. Appl. Polym. Sci., 22, 3434 (1978). A.-C. Aibertsson and B. Rfinby, AppL Polym. Symp., 35, 4243 (1979). A.-C. Albertsson, Eighth Annual Conference on Advances in the Stabilization and Controlled Degradation of Polymers, Lucerne, May (1986). A.-C. Albertsson and Z. G. Banhidi, J. Appl. Polym. Sci., 25, 1655 (1980). P. Vink, Degradation and stabilisation ofpolyolefins (N. S. Allen (Ed.)), Applied Science Publishers, London and New York, 213 (1983). A.-C. Albertsson, Thesis, The Royal Institute of Technology, Stockholm, Sweden (1977). B. Rfinby and J. Rabek, ESR studies in polymer research, Springer-Verlag, Heidelberg (1977). T. Fujimura, N. Hayakawa and N. Tamura, J. Macromol. Sci.-Phys., 16, 511 (1979). A. Tsuchii, T. Suzuki and S. Fukuoka, Report of the Fermentation Research Institute, 55, 10 (1980). H. G. Schlegel, Allgemeine Mikrobiologie, 4 Auflage, Georg Thieme Verlag, Stuttgart, 356 (1976). R. C. Bohinski, Modern concepts of biochemistry (3rd edn), Allyn and Bacon Inc., 505 (1979). W. J. Bailey and B. Gapud, Macromolecules as drugs and as carriers for biologically active materials (D. A. Tirell, L. G. Donaruma and A. B. Tusek (Eds)), New York Academy of Science, New York, 42 (1985). A.-C. Albertsson and O. Ljungquist, J. Macromol. ScL-Chem., A23, 393 (1986). A.-C. Albertsson and O. Ljungquist, J.. Macromol. Sci.-Chem., A23, 411 (1986). G. E. Guillet, T. W. Regulski and T. B. McAneney, Environ. Sci. Techn., 8, 923 (1974). H. Omishi and M. Hagiwara, Polym. Photochem., 1, 15 (1981).
The mechanism of biodegradation of polyethylene
87
30. G. J. L. Griffin, Pure Appl. Chem., 52, 399 (1980). 31. G. Colin, J. D. Cooney, D. J. Carlsson and D. M. Wiles, J. Appl. Polym. Sci., 26, 509 (1981). 32. G. Colin, J. D. Cooney and D. M. Wiles, Int. Biodetn. Bull., 12, 67 (1976). 33. B. Dolezel, Die Bestiindigkeit yon Kunststoffen und Gummi, Carl Hanser Verlag, Miinchen, Wien, 655 (1978).
The Mechanism of Biodegradation of Polyethylene
Ann-Christine Albertsson, Sven Ove Andersson & Sigbritt Karlsson Department of Polymer Technology, The Royal Institute of Technology, S-100 44 Stockholm, Sweden (Received 24 November 1986; accepted 8 December 1986)
ABSTRACT LDPE films have been exposed to abiotic and biotic environments. The films were UV irradiated for periods of O, 7, 14, 26 and 42 days before being mixed with water and soil. Degraded LDPE films were examined by infra-red spectroscopy. The carbonyl peak increased with time in the abiotic environment and the oxidative degradation reported in our earlier works was confirmed. In the presence of a biotic atmosphere, however, this peak decreased. A t the same time there was an increase in double bonds which was related to weight loss. An explanation of this behavior is presented as a proposedmechanism for the biodegradation of polyethylene. This mechanism is compared, on the one hand, with abiotic photooxidation, Norrish type I and H degradation, and, on the other, with the biotic paraffin degradation. Abiotic, as well as biotic, ester formation mechanisms are also presented. An ESR spectrum confirms the presence of radicals on the polyethylene samples. At the beginning of the degradation the main agents seem to be UV light and/or oxidizing agents. When carbonyl groups have been produced, these are attacked by microorganisms which degrade the shorter segments of polyethylene chains and form carbon dioxide and water as end products. There is a synergistic effect between photooxidative degradation and biodegradation. The biodegradation of polyethylene can be compared with the biodegradation of paraffin. 73 Polymer Degradation and Stability 0141-3910/87/$03"50 © Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain
74
Ann-Christine Albertsson, Sven Ore Andersson, Sigbritt Karlsson
INTRODUCTION In almost every survey of the biological degradation of synthetic polymers, it is fairly clearly stated that polyethylene is an inert polymer with good resistance to microorganisms. Several reports mention, however, that fungal growth can occur on the surface of polyethylene.1'2 Connolly 3 and Dolezel 1 have also reported a change in tensile strength for polyethylene exposed to a biotic environment, and Kestelman et al. 4 have demonstrated a higher water uptake in polyethylene overseeded with growing molds. The increase in water uptake was accompanied by an increase in the degree of crystallinity. Jen-hao and Schwartz 5 claimed that the number of bacteria that PolYethylene was able to support was dependent on its molecular weight. Later, Potts and his collaborators 6 investigated the relationship between molecular weight and fungal growth. They found that polyethylene was not biodegradable, but the linear paraffin molecules below a molecular weight of about 500 were utilized by several microorganisms. Tsuchii et aL 7 suggested, however, that biodegradation was dependent on the availability of a substrate and was not affected by the molecular weight. Hueck a emphasized the role of criteria for the inertness of polyethylene; its hydrophobic nature and its large molecular dimensions. Scott 9 concluded that attack by microorganisms is a secondary process. The step which determines the rate at which degradable polyethylene is returned to the biological cycle appears to be the rate of the oxidation process which reduces the molecular weight of the molecule to the value required for biodegradation to occur. Even in the absence of any biodegradative attack, the carboxylic acids produced ultimately oxidize to carbon dioxide and water. Griffin 10 also questioned whether polyethylene is biodegradable. He emphasized the complex nature of the process proposed, in view of the interacting synergistic autooxidation and biodegradation effects to which polyethylene is exposed in the great variety of natural environments available. From a chemical point of view, it can be anticipated that microorganisms may be able to degrade polyethylene since the chemical structure of polyethylene is similar to that of linear alkanes, which are known to be biodegradable. However, since it is not possible to study the biodegradation of polyethylene under single environment conditions, we have compared the changes induced by a biotic and an abiotic environment. The differences observed are interpreted as being the result of biodegradation alone. We have shown that the biodegradation of polyethylene is affected by preliminary irradiation from a UV source, 11'12 by the presence of photodegradative enhancers, 11,12 by its morphology and surface area, ~3,~4 by additives, ~5.16 by antioxidants, 17 and by its molecular weight) 7
The mechanism of biodegradation of polyethylene
75
We have earlier studied the biodegradation and ageing of polyethylenes in carbon-14 marked films and powder and have measured 14CO2 generation as an indication of the degradation. The degradation curves--the evolution of CO2 as a function of time--were characterized by a straight line progression in the first hundred days of observation before a parabolic decline. The trend was occasionally reversed to a progressive increase in 14CO2 liberation in some of the long-term runs, especially after 2-3 years. These tests have been going on since 1973 and, to complement these earlier results, we have now looked at the same films mainly with the help of IR analysis. We interpret the results by suggesting a mechanism of degradation and we compare this mechanism with the mechanism of degradation of paraffin to carboxylic acids and the subsequent well known fl-oxidation and citric acid cycle.
EXPERIMENTAL LDPE films with a thickness of 20/~m were used. Some samples contained an additive (ND) while others were used without additive (0). The additive ND is a substance which increases the photochemical degradation rate. The LDPE was radioactively marked with carbon-14. The samples all came from Imperial Chemical Industries (ICI), London. The films were exposed to UV irradiation from an Osram Ultra Vitalux 300W lamp for periods of 0, 7, 14, 27 and 42 days. The samples were then mixed with natural soil and water. The samples were kept, for 10 years, at 25°C in darkness with an airflow (purified to remove CO2). Abiotic samples were prepared by including silver nitrate in the medium. Biotic samples were kept in non-sterile soil. For every abiotic sample, an identical biotic sample was prepared and handled in the same way. The degradation of the films was monitored with an IR instrument, Perkin-Elmer 580B. The films were cold drawn to make it possible to obtain transmission spectra. The reflectance IR analyses were performed with a M I R crystal (Multiple Internal Reflectance) KRS-5 prism consisting of a thallium halogen compound. Samples (2 × 5 mm) to be analyzed by IR were cut out from the films and washed with distilled water to remove traces of soil, etc. In the IR spectra, special interest was focused on the following absorption peaks: 1740cm- 1, ester carbonyl ( - - C O 0 - - ) ; 1715 c m - 1, ketone carbonyl (--CO--); 1640cm -1, double bonds ( - - C ~ C - - ) ; 915-905cm -1, double bonds (H2C~---C--). The carbonyl index (=A1715/1465, where A1465 corresponds to - - C H 2 - - ) and the double bond index ( = A1640/1465) were calculated from the IR spectra. The carbonyl index is a measure of the concentration of
76
Ann-Christine Albertsson, Sven Ore Andersson, Sigbritt Karlsson
carbonyl groups and the double bond index a measure of the double bond concentration. ESR spectra were recorded on a JES-ME-IX instrument from JEOLCO, Tokyo, Japan, using 100kHz field modulation and a cylindrical cavity (TE 011 model).
RESULTS A N D DISCUSSION In any discussion of the degradation of polymers many factors must be taken into consideration. Polymers can be degraded by thermal, oxidative, chemical, radiative and mechanical means as well as by biological agents. An initial step in the degradation of polyethylene (in the absence of additive to prevent degradation) is known to involve photooxidation. 18 Photooxidation increases the amount of low molecular weight material by breaking bonds and increasing the surface area through embrittlement. It also increases the hydrophilicity by the introduction ofcarbonyl groups. All these effects of photooxidation promote the degradation of polymers, as we have shown earlier. 16 The evolution of CO 2 increases with increasing exposure to UV light. The effect of UV radiation can be modified by using protective or sensitizing additives. The evolution of CO 2 increases greatly if the irradiated polyethylene contains a photosensitizer. 11'x2 Figure 1 shows the traditional mechanism for the photooxidation of PE. Initially, UV radiation is absorbed which leads to radical formation. Eventually, oxygen is absorbed and hydroperoxides are formed, the end products being carbonyl groups. Additional exposure to UV radiation causes the carbonyl groups to undergo the Norrish type I and/or II degradation (see Fig. 2). 18 The photooxidation can be initiated by impurities. UV degradation can also begin at locations of trace hydroperoxide or ketone groups, introduced during the manufacturing processing or fabrication. The oxidative degradation of polyolefins can be followed by measuring the level of carbonyl group adsorption by infra-red spectroscopy (IR). The measured carbonyl groups are usually expressed as a carbonyl index, which is the ratio ofcarbonyl absorbance to an invariant absorbance characteristic of the polymer, both in the IR. The use of this index compensates for differences in the thickness of polymer films. The formation of carbonyl groups is increased by photooxidation, but also by increasing stress even after storage in an abiotic environment. 16'19 If Norrish type I or II degradation (or both) occur, additional peaks are observed in the IR spectrum of the polymer. For example, a terminal double
The mechanism of biodegradation of polyethylene /XACH2-CH~.-CH2A,'k ~
77
.V'CH~CH-CH2,,v'x
J
½
0-6 I •,'vxC H2- C H--C H2 ,,~
O'-O-H
,~C H2- I~-C H2,v~ H
+
R°
1 .,~v~.C H~ C- CH 2
+
6H
-t"
H20
1
O n
N ~ CHff C - CH2 ~'~
Fig. 1.
fIR 1715 cm'l" I
Photooxidation of polyethylene.
bond appears at 905-915cm -1 and it is also possible to trace ester formation. Figure 3 gives a proposed mechanism for ester formation in abiotically-treated LDPE films. Norrish type I cleavage yields a carbonyl radical which can react with an alkoxy radical on the PE chain shown in Fig. 1. A peak appears at 1740cm-1 in the IR spectrum if this ester formation occurs. Radical formation in the samples can be evaluated by the use of ESR.20, 21 O O Ii NI ~l "w'CH~'CH~'C-CH2-CH2 "~'v ~ ""~CHECH'2"C"
+
"CH~'CH2 Nv
O"-H .,,vC~\
/\CH.v~v ~
~
+
CH~CH,'V~
1 oII
. v w C - CHa [ 'R 1715cm-1l
Fig. 2. Norrish type I and II degradations.
78
Ann-Christine Albertsson, Sven Ore Andersson, Sigbritt Karlsson o
II
,~CH2--C-C H2,w~
0 II ,~wCH2-C.
~'~CI-12-?H--C O H2AA~
1 +
• C H2,,,'~.
"t
~ , CH2"(~-I-CH2,~O-(~-CH2 "~'~' O IR'1740 cm"1 i Fig. 3.
Abiotic ester formation.
Reference is frequently made to paraffin degradation by microorganisms when PE and its instability towards environmental factors are discussed. This is fairly obvious since a paraffin can be classified as a very short-chain PE, the length of typical paraffins being 10-20 carbons. 22 Microorganisms preferentially use linear n-paraffins, whereas the corresponding branched isomers are almost completely inert to biodegradation. R-C H2-- Cl-i~CH 3
~
R- CH2-CPI2-CH2OH
1
CoASH
'~
#
R-CI-12CH~C,,, SCoA
"
,o,
R-'CH~-CI-~C-OH
/
~
R-CH=CH-C"'SC oA
/
r~,~
,,
0 If R-C-SCoA
\
' 4
. _
o
R-CH OH- CH~r~;"'SCoA
o
CH-&SCo
oI
oII
R4-cH-c ~, sc
CoASH "#r--~.nCO 2 4- nH20
Fig. 4.
Biotic paraffin degradation according to H. G. Schlegel (1976). 23
The mechanism of biodegradation of polyethylene
79
Figure 4 gives the mechanism for degradation of paraffins by microorganisms. 23 The microorganisms responsible for the attack on paraffins can be, for example, Mycobacterium, Nocardia, Corynebacterium, Candida or Pseudomonas. The alkane chain is oxidized to a carboxylic acid and the resultant acid undergoes fl-oxidation which, by reaction with coenzyme A, removes two carbon fragments from the carboxylic molecule. The two carbon fragments, acetyl-SCoA, enter the citric acid cycle, from which carbon dioxide and water are released. From a paraffin molecule with n carbons, a total of n molecules of carbon dioxide are evolved. 24 Figure 5 presents an example of ester formation due to microbial action on alkanes. F r o m hexadecane (C~6H34), cetylpalmitate is formed by the action of Acinetobacter and Micrococcus. Each step is catalyzed by sets of specific enzymes. 23 2
2 c~-(c~-c~.
~'
/
\
"
2 CH3-(CH2)I~"
CH2OH
2NADH 2 2NAD
o
II CH.j-(CH2~) C -H
"2O--L2[.J
/ NADH 2
\ NAD
F IP /2 CH~- (CH2")3~" C-OH
,.
Fig.
5.
CH~(CH2~I~ C - O - C H 2- (CH2)I2F-CH 3
Biotic ester formation according to H. G. Schlegel (1976). 23
When polyethylene films were studied after 10 years' storage in a mixture of soils, we could find no connection between time of UV irradiation and carbonyl index. As shown in Fig. 6, the carbonyl index increased with UV irradiation up to a certain limit, after which it decreased. A parabolic relationship was also obtained when the carbonyl index was plotted as a function of the percentage by weight of polyethylene converted to carbon dioxide (Fig. 7). The carbonyl index increased up to 0.3 and then decreased
Ann-Christine Albertsson, Sven Ore Andersson, Sigbritt Karlsson
80
02[
•
O-PE
0.1
Days
Fig. 6.
Variation in carbonyl index after exposure to UV light. Samples with and without additive to improve the sensitivity to UV radiation.
with increasing degradation. The samples stored in air increased their carbonyl index with time, but all samples in contact with soil showed a decrease of carbonyl index with time. We have earlier presented similar results for high density polyethylene 16'19 and our interpretation was that biodegradation decreased the number of carbonyl groups, in contrast to most other environmental factors. A proposed mechanism for the biodegradation of PE is given in Fig. 8. It is thought that the alteration of the PE molecule is initially the same in both ~.1715 ~,14~5
I 0,1
Fig. 7.
i
t 0,3
i
I 0,5
t
0~.7
'
weight~ converted to CD2
Carbonyl index as a function of percentage by weight of polyethylene converted to carbon dioxide.
The mechanism of hiodegradation of polyethylene
81
/vvlCH2-- ~ --CN2~vv~
9H
,,~C~C,-~-~ c~,
?H
[ IR,1700-25 crn-lJ -'v~O¢2"-C ~O -4-
~CH2JW~
• CH=CH2NVV
]
CoASH"~I O ~CH~" C*~SCoA IS
1o
•v~ CH~-CH=CH-(~:,,,SCoA
I 1
oii
CH,J-CHOH-CH~C'VSCoA
, ' ~ - C9H f - ~,SCoA
t
/ CoASH
\ CH$~'~SCoA
I
Citric acid cycle
Fig. 8. Proposed mechanism for the biodegradation of polyethylene. the abiotic and the biotic samples. When carbonyl groups had been formed, the abiotic sample evidently did not undergo Norrish type II degradation, as no double bond peak could be found in the IR spectra from abiotically degraded PE (Fig. 9). According to the Figure, the carbonyl index increased with prolonged incubation in an abiotic atmosphere.l 6 Ester formation had, however, taken place, as was confirmed by the presence of a peak at 1740 cm- t. This peak was nevertheless rather small when compared with the peak obtained in the biotically degraded samples (Fig. 9).
Ann-Christine Albertsson, Sven Ore Andersson, Sigbritt Karlsson
82
f tO
o
"6
I dT~ ..J
0
z E 0
The mechanism of biodegradation of polyethylene
83
For a study of the IR spectra of biotically degraded PE samples (Fig. 9) it was observed that the carbonyl index decreased with prolonged incubation time. ~6 The explanation for this is given by the mechanism in Fig. 8. In some IR spectra of biotically degraded PE, a peak was noted corresponding to double bond groups (IR 905-915 cm-1). This peak was missing in abiotic samples (see above). In Fig. 10, the double bond index is plotted against weight % PE converted to CO 2. There is an increase in A~64o A14~
0.~
0.1
A
0 t.1
Fig. 10.
0.3
45
weight% PE converted to CO2
Double bond index as a function of percentage by weight of polyethylene converted to CO2. • corresponds to ND-PE and • to O-PE.
double bonds with increasing C O 2 evolution and a possible mechanism for the double bond formation is shown in Fig. 8b. This step is not necessarily a biotic one. It may be a result of the splitting of the hydroperoxide and the formation of the carboxylic acid. The formation of double bonds in biotic samples but not in abiotic ones needs further analysis before a plausible explanation can be given. Figure 11 shows an ESR spectrum for biotically degraded PE. (The sample was HDPE powder. It is included here as an introduction to a forthcoming paper.) A characteristic peak attributable to peroxy radicals 2° is evident. The ESR spectral intensity increases with temperature (25, 45 and 65°C) and illumination. No difference in peroxide content could be found between samples of different ~4C concentration. In earlier studies, the rate of x4CO2 evolution from degraded PE was measured. In Fig. 12, the % PE converted to CO 2 is plotted against time. One can distinguish between two separate mechanisms, a and b. 12 b symbolizes the always present autooxidative degradation in an abiotic environment. When exposure to UV light is prolonged, this results in a steeper curve due to more portions of the PE chain being degraded to shorter pieces with
84
Ann-Christine Albertsson, Sven Ore Andersson, Sigbritt Karlsson
? ESR
SPECTRA
BIOTIC DEGRADED
Fig. 11. ESR spectra of a biotic degraded polyethylene sample.
carboxylic groups which can undergo fl-oxidation. 12a symbolizes biotic degradation. The autooxidative degradation has been subtracted, so that 12a only shows the evolution of 14CO2 from biotically degraded PE. Evolution of carbon dioxide is fast at the beginning, but thereafter the curve flattens out towards an asymptotic value. Polymers containing ester groups have been found to be much more prone to undergo biodegradation. This is to be expected, since it is well known that esterase (enzymes which hydrolyze ester links) exist in nature. Degradation% by weight
~
~ a
j
~
j
~
b
L time
Fig. 12. Percentage of polyethylene converted to carbon dioxide plotted against time.
The mechanism of biodegradation of polyethylene
85
Therefore, Bailey and Gapud 25 synthesized a new degradable polyethylene with ester carbonyl groups. Hydrolytic degradation takes place when polymers containing hydrolyzable groups are exposed to moisture. If hydrolysis is achieved enzymatically, then the process is usually considered to be biodegradation. 26,27 Guillet et al. 28 produced a copolymer with ketone groups in the main polymer chain, and these carbonyl groups absorb UV light in the first step of degradation. Scott, 9 0 m i s h i and Hagiwara 29 and Griffin 3° have used different additives to increase the degradation of polyethylene. The question is, however, whether ordinary polyethylene is biodegradable. Colin et a/. 31'32 did not interpret their results as showing that polyolefins were biodegradable even though their embrittlement data could be evidence for that. Dolezel, 1 on the other hand, has shown that the tensile strength of polyethylene changes in a biotic environment. Later, he also showed that polyethylene was changed chemically in a way similar to that observed by us, i.e. the amount of carbonyl groups decreased with prolonged exposure to a biotic environment. 33 The mechanism for paraffin biodegradation is comparable with that of polyethylene. There are many steps which are the same, but in the degradation of polyethylene abiotic steps will also contribute to the total degradation. At the beginning, the main agents seem to be UV light and/or oxidizing agents, but when carbonyl groups have been produced these are attacked by microorganisms which degrade the shorter fragments of polyethylene chains to the end products, carbon dioxide and water. There is a strong synergism between biodegradation and environmental factors, and biodegradation can, in practice, never be entirely separated from the purely physical and chemical--mainly autooxidative--progressive ageing, always present as an unavoidable slow but cumulative background effect. Biodegradation is seldom due to a single cause, but a combined effect including heat, UV light, stress and water. The presence of water is a necessity for biodegradation. Studies concerning the relationship between surface and biodegradation of PE will be presented in the future, as well as the results of an investigation on the presence of radicals on the surfaces and their effects on the fate of PE in nature. ACKNOWLEDGEMENT This work was supported by the National Swedish Board for Technical Development (STU) and by the Swedish Council for Building Research (BFR).
86
Ann-Christine Albertsson, Sven Ore Andersson, Sigbritt Karlsson REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.
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The mechanism of biodegradation of polyethylene
87
30. G. J. L. Griffin, Pure Appl. Chem., 52, 399 (1980). 31. G. Colin, J. D. Cooney, D. J. Carlsson and D. M. Wiles, J. Appl. Polym. Sci., 26, 509 (1981). 32. G. Colin, J. D. Cooney and D. M. Wiles, Int. Biodetn. Bull., 12, 67 (1976). 33. B. Dolezel, Die Bestiindigkeit yon Kunststoffen und Gummi, Carl Hanser Verlag, Miinchen, Wien, 655 (1978).