Biomacromolecules 2001, 2, 880-885
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UV-Irradiated Biodegradability of Ethylene-Propylene Copolymers, LDPE, and I-PP in Composting and Culture Environments Jitendra K. Pandey and R. P. Singh*,† Polymer Chemistry Division, National Chemical Laboratory, Pune-411008, India Received March 5, 2001
The biodegradability of UV-irradiated films of ethylene-propylene copolymers (E-P copolymer), isotactic polypropylene (i-PP), and low-density polyethylene (LDPE), was measured in composting and culture environments by monitoring the variations in intrinsic viscosity [η], weight loss per surface area, surface changes by SEM, colonization of fungus, chain scission, and evolution of hydroxyl and carbonyl groups by FT-IR spectroscopy. Photooxidation was used as a pretreatment for biodegradation of polymers. A systematic decrease in intrinsic viscosity [η] and increase in carbonyl/hydroxyl regions in FT-IR spectra was found from 0 to 100-h irradiated samples. The degradation rate was strongly dependent on the composition of copolymers and markedly increased with decrease in ethylene content. Important surface erosion was detected after composting by SEM for longer UV-irradiated samples. It was estimated that chain-scission was directly related to photoirradiation. Introduction
Experimental Section
Synthetic carbon-based polymers are mostly inert toward microorganism in the initially produced form. The long-term properties in the synthetic and natural polymers have attracted more interest during the past decades as environmental concerns have been increased, due to the accumulation of municipal solid waste, generated by the commodity polymers. It is well-known that some traditional polymers such as polyethylene and polystyrene undergo very slow biodegradation,1,2 and it is also known that extremely useful polyolefins also can be degraded through the introduction of keto/ carbonyl species in both the stabilized3 and unstabilized samples.4 Although several kinds of formulations filled with starch as biodegradable natural fillers5-7 and starch with prooxidants8-10 have been well documented in order to achieve biodegradability in synthetic polymers, there has been no real significant discussion on short term UV-irradiated biodegradability of “additive-free” E-P copolymers whereas E-P copolymers have a range of useful properties from thermoplastics to soft elastomers, depending upon the relative composition of the two monomers and manner of their entanglement. Photooxidation of these copolymers has already been well reported.11,12 Scott et al.13,14 have concluded that microbial action on the polymers is a secondary process and bioassimilation is related to oxygenated products. Albertsson et al.15,16 investigated that biodegradation can be initiated by photooxidation where carboxylic acid parts, generated through Norrish type-I and II mechanisms during the oxidation process, can be consumed by microbial attack. The present investigation is intended to study the extent and effect of short-term photoirradiation on the biodegradability of i-PP, LDPE, and E-P copolymers.
Materials. Commercial samples of isotactic polypropylene (i-PP, Koylene S 30330) and low-density polyethylene (LDPE, Indothene 16 MA 400) were obtained from M/s Indian Petrochemicals Corp., Baroda, India, and heterophasic ethylene propylene copolymers (EPF 30R, EPQ 30R) were obtained from M/s Himont Italia. The molar percentages of ethylene content in copolymer samples were 7.7 and 15.1, for EPF-30R and EPQ-30R, respectively. These polymer pallets were purified by dissolving in refluxing xylene under nitrogen atmosphere. The solution was precipitated with methanol, filtered, and dried at 50 °C in a vacuum oven. These samples were assumed to be additive free and were designated as purified samples. Preparation of Films and UV Irradiation. The method of sample preparation (∼100 µm thickness) has already been reported.17 The photoirradiation of the films was carried out in a accelerated weathering chamber (SEPAP 12/24) at 60 °C. The chamber consists of (4 × 400 W) medium-pressure mercury vapor lamps supplying radiation longer than 290 nm. The details of the equipment are described elsewhere.18 Viscosity Measurements. The method of viscosity measurement has already been described19 where the intrinsic viscosity [η], dL/g, was measured by using successive dilution of only one solution (concentration 0.2 wt %) at 135 ( 5 °C in Decaline. The error due to expansion of flask is negligible as preheated flask and pipet (140 °C) were used to mix the solvent into an Ubbelohde viscometer. Incubation in Compost. The biodegradability tests were performed in a laboratory scale composter, and the size of films was 5 × 5 cm. The constitution20 of solid waste mixture (compost) used for biodegradability testing of photooxidized samples was as follows (dry weight): 40.8% shredded leaves, 11.4% cow manure/dung, 15.8% newspaper and computer
†
E-mail:
[email protected].
10.1021/bm010047s CCC: $20.00 © 2001 American Chemical Society Published on Web 06/26/2001
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Figure 1. Compost temperature changes during study.
paper, 2% white bread, 7.8% sawdust, 19.2% food waste (dry milk, potato, carrot, banana, and other vegetables), and 3.0% urea. Total dry weight was ∼5 kg. The moisture content was maintained by periodic addition of water, and the temperature profile of the compost during testing is shown in Figure 1. It is clear here that composting temperature varied with the temperature of the surrounding atmosphere. The biodegradability was determined by measuring the gravitational weight loss (per surface area in gm/cm2) on a digital balance, Prescisa 205 A SCS, Switzerland. Incubation in culture. The test fungi (Aspergillus niger) was collected from the Biochemistry Division, National Chemical Laboratory, Pune, India, and nutrient salt agar was prepared by dissolving potassium dihydrogen phosphate (0.700 g), magnesium sulfate (0.700 g), ammonium nitrate (1.0 g), sodium chloride (0.005 g), ferrous sulfate (0.002 g), manganese sulfate (0.001 g), and agar (15.00 g) in 1 L of distilled water. After the medium was sterilized at 120 ( 5 °C for 25 min, the pH was adjusted between 6.5 and 7.0 by the addition of a 0.1 N solution of NaOH. For providing the solidified agar layer (depth 4-7 mm) nutrient salt was poured into sterilized Petri dish. The surface of test specimen (3.0 × 3.0 cm) was inoculated by spraying the spore suspension. The Petri dishes were incubated at 28-30 °C after being sealed by wax to avoid contamination. FT-IR Spectroscopy. FT-IR (Fourier transform infrared 16 PC spectrometer) was used to characterized the photooxidation in the polymer films, and interest was focused mainly on the changes in hydroxyl (3700-3100 cm-1) and carbonyl region (1600-1800 cm-1). Scanning Electron Microscopy. The UV-exposed films were placed in stoppered bottles containing osmium tetroxide (2% aqueous) and allowed to stand for 48 h. The films were washed with water and dry ethanol. The stained samples dried under vacuum for 24 h at 50 °C. The gold-coated samples were examined under electron microscope (Leica Cambridge Stereoscan 440 model) for morphological changes. Results and Discussion Incubation in Compost. Weight loss is one of the most valuable data indicating the actual biodegradation of polymeric material after composting whenever validated by parallel monitoring of the neat respirometric microbial
Figure 2. (a) Weight loss of controlled (unirradiated) samples during compost incubation. (b) Weight loss of 50-h treated samples during compost incubation. (c) Weight loss of 100-h UV-treated samples during compost incubation.
activity bound to the carbon content of the sample under testing. The weight loss of 0-, 50-, and 100-h irradiated, compost-buried, polymer samples (∼100 µm thickness and 5 × 5 cm size) was measured periodically 1, 2, 3, 4, 5, and 6 months from the compost, after washing the samples with distilled water and drying in a vacuum oven at 60-65 °C until constant weight. Figure 2, parts a-c, illustrates the comparative weight loss, per surface area in g/cm2, of irradiated and unirradiated polymer samples related to the incubation time in compost. It is quite obvious that degradation rate (gravitational weight loss) increased rapidly with the increasing in irradiation period. Among all the samples, 100-h irradiated films showed much more weight loss (more than 75%) after 5 months and were unrecoverable after 6
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Table 1. Variations in the Intrinsic Viscosity [η] of Irradiated and Incubated (Buried in Compost) Samples
i-PP
LDPE
EPF
EPQ
month
0
50
100
0
50
100
0
50
100
0
50
100
0 2 4 6
3.41 3.40 3.31 3.10
1.98 1.53 0.909 0.654
0.785 0.554 0.402 0.098
1.96 1.80 1.51 1.18
0.416 0.310 0.200 0.101
0.095 0.065 0.043
2.56 2.40 2.40 2.30
0.634 0.600 0.501 0.369
0.092 0.062 0.041
2.89 2.87 2.86 2.80
0.701 0.659 0.600 0.401
0.096 0.076 0.049
months except LDPE. Sample of i-PP are showing a comparatively high linear increase in degradation than that observed for LDPE (22% weight loss) after 6 months, indicating the greater susceptibility of i-PP to microorganisms of compost during degradation process furthermore this fact was supported by more weight loss in i-PP (in both forms irradiated and unirradiated) in comparison of other samples. In the case of E-P copolymers, the degradation rate was increased rapidly after a 4-month incubation, and both samples (EPF 30R and EPQ 30R) were not recoverable after 6 months, suggesting that initially the degradation rate was slow due to comparatively less degradable ethylene component. The 50-h irradiated samples showed significant weight loss in both the copolymers but less than the 100-h irradiated samples. During the observation of unirradiated samples, weight loss of i-PP and EPF 30R in compost was 22% and 10% respectively, after 6 months whereas EPQ 30R and LDPE underwent less than 15% weight loss. We did not study the kind of enzymes used by microorganism during the consumption of polymer samples. Variation in Viscosity. The variation in intrinsic viscosity [η] of i-PP, EPF 30R, EPQ 30R, and LDPE films after photooxidation and burial in the composting environment are tabulated in Table 1. Ethylene content has an effect on molecular weight of pure PP matrix; therefore, [η] of EPQ 30R is higher than EPF 30R of un-irradiated samples. The [η] of irradiated samples was decreasing as a function of irradiation time and negligible changes were found for unirradiated samples. A gradual decrease in [η] was observed after one month for all the 100-h irradiated samples in the composting environment with the incubation time. The presence of any appreciable cross-linking was not observed in this system as films remained completely soluble in Decaline. The 50 h irradiated samples of i-PP and EPF 30R showed a significant decrease in [η] after the burial in the compost but in the case of LDPE and EPQ 30R no such type marked decrease was observed. Chain Scission. Chain scission, which indicates average number of dissociated bonds related from photo cleavage in the main chain scission, is simply expressed by eq 1,21 chain scission (S) ) M0/Mt - 1
(1)
where, M0 and Mt are the viscosity average molecular weight before and after degradation, respectively. M0 values are as follows (×10-5): LDPE, 2.21; i-PP, 2.07; EPF30R, 2.68; and EPQ30R, 2.87. Viscosity average molecular weight (M h v) was measured by the Mark-Houwink-Sakurada equation, eq 2, where K and a are the constants and values were taken from the literature.22
Figure 3. (a) Chain scission in the samples with UV irradiation time. (b) Changes in degradation rate with the chain scission.
[η] ) K M h va
(2)
Figure 3, parts a and b, illustrates the rate of chain scission in different polymer samples with the irradiation time and percentage gravitational weight loss after composting, respectively. Chain scission was increasing more rapidly with UV irradiation than composting suggesting a slow action of microorganism on the polymer substrate, responsible for scission, than UV irradiation whereas significant and high chain scission was seen for 100-h irradiated and compostincubated samples. The slow increase in the chain scission of 0- and 50-h irradiated copolymers after composting may be due to the heterophasic nature of copolymers, but the increase in weight loss indicates that the degradation process occurs initially and dominantly on the surface and after that entered into the polymer matrix by the action of microbes. FT-IR Spectroscopy. Figure 4, parts a-d, shows the carbonyl and hydroxyl region changes and their rate of formation upon irradiation. A very broad hydroxyl absorption region (3700-3200 cm-1) with a maximum centered at 3400
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Figure 4. (a) Increase in hydroxyl region during UV irradiation of samples. (b) Rate of hydroxyl group formation with the time of UV treatment. (c) Increase of carbonyl region during UV irradiation of samples. (d) Carbonyl group formation rate with time of UV treatment. (e) EPF30R before composting after UV irradiation. (f) EPF30R UV-irradiated sample after 6 months composting.
cm-1 during photooxidation appeared. This band is due to the neighboring intramolecular hydrogen bonded hydroperoxide and alcohols. Hydrogen-bonded hydroperoxide (3420 cm-1) and associated alcohols (3380 cm-1) were also present. The absorption in hydroxyl region is more intense in i-PP and EPF 30R. The carbonyl region (1850-1550 cm-1) is broad in i-PP and EPF 30R copolymer but sharp and narrow in LDPE and EPQ 30R. The absorption at 1712, 1722, 1740, and 1785 cm-1 has been assigned to carboxylic acid, ketone, ester and lactones, respectively. Figure 4, parts e and f, shows
the peak of the carbonyl group before and after composting of the EPF 30R copolymer, and it is evident that after incubation in compost, there was a decrease in the carbonyl region that may be due to the release of short chain carboxylic acids in the form of degradation products during the biotic step such as in polyethylene, where the carboxyl functionalized short pieces can undergo β-oxidation by coenzymatic action and the reaction mechanism can be compared with paraffin degradation.15,23,24 The absence of primary alcohol after treating with NaOH in LDPE suggests
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Figure 5. (a) i-PP 100 h after composting (magnification 400 ×). (b) EPF 100 h after composting (magnification 1.00 × 103×). (c) EPQ 100 h after composting (magnification 500×). (d) LDPE 100 h after composting (magnification 500×). (e) i-PP 50 h after composting (magnification 500×). (f) EPF 50 h after composting (magnification 200×). (g) EPQ 50 h after composting (magnification 200×).
the microbial oxidation of the terminal methyl group of short and long chains. Morphological Aspects. Morphological changes upon UV irradiation of copolymers have been studied in our earlier
work.25 In Figure 5, parts a-d are showing the scanning electron micrographs of 100-h irradiated and composted samples of i-PP, EPF 30R, EPQ 30R, and LDPE, respectively. It is evident from these micrographs that there is much
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UV-Irradiated Biodegradability Table 2. Visual Growth Rating of A. niger on Polymer Films
i-PP
LDPE
EPF
4. Conclusions
EPQ
weeks
0
50
100
0
50
100
0
50
100
0
50
100
1 2 3 4 5 6
0 0 0 0 0 0
0 0 0 0 1 1
0 0 0 1 2 2
0 0 0 1 2 2
0 0 1 2 2 3
0 0 1 2 3 4
0 0 0 0 1 1
0 0 1 1 2 2
0 0 1 2 2 4
0 0 0 0 0 0
0 0 0 1 1 2
0 0 1 2 2 3
more surface deformation in 100-h irradiated i-PP and EPF 30R. In 50-h irradiated samples (Figure 5e-g) such types of deformation or deepness of erosion was much less, which is due to the presence of a lesser amount of oxidation products in comparison with 100-h irradiated samples, generated during photoirradiation of polymer, and their greater consumption by microbes, resulting in a good surface erosion in polymer films. A network of crack formation was observed with an increase in irradiation time. Formation of microcracks on the polymer surface is due to chain scission of polymer after UV irradiation. The cavities on the surface were observed after composting of irradiated samples, suggesting that microorganisms penetrate the polymer matrix during the degradation process. The presence of small and large cavities on the surface may be due to the absence of a uniform distribution of short branches or photodegradable products in the polymer matrix, which are preferable “foods” for microorganisms. Incubation in Culture. The visual growth rating26 test is valuable in assessing the performance of polymer during its use under such conditions. The samples were used as the sole carbon source for the fungus, e.g., A. niger. Table 2 represents the fungal colonization data (visual growth) on polymer films surface after 6 weeks of culture incubation but the initiation of fungal growth is seen by microscope within 10-15 days. The absence of any colonization in a controlled Petri dish (Petri dish without film sample), clearly suggests that fungus is using polymer films as a source of survival, as there was complete absence of carbon in the nutrient agar. Among unirradiated samples, microscopic growth was observed only on i-PP and EPF 30R surfaces after 6 weeks. In the case of 100-h irradiated samples, the coverage of films surface by fungus was much more than others, and further, it may be due to the easy consumption or use of short chains as energy source by fungus, generated during the photooxidation. The EPF 30R films are showing higher colonization than EPQ 30R, which is attributed to the greater ethylene content in EPQ 30R. The intrinsic viscosity [η] of culture-incubated samples was not changing significantly for 0- and 50-h irradiated samples, whereas an increase in [η] in 100-h irradiated samples may be due to the greater number of long chains remaining after elimination of low molecular weight short chains by fungal attack, as this fact is supported from unchanged readings of chain scission. In FT-IR spectra, peaks at 1715-1720 cm-1 for ketones and acids was observed. The SEM of cultured samples were showing eroded surfaces with some residue of mycelium.
The hydroxyl and carbonyl groups were observed to be increasing rapidly during 0-100 h UV irradiation. In general a decrease in intrinsic viscosity and increase in chain scission was also observed with the irradiation and incubation time in compost. Polypropylene was more susceptible than lowdensity polyethylene to microbial attack in neat and irradiated samples. The higher weight losses in low viscosity samples and longer irradiated samples suggested that chain scission and oxidized functional groups are important biodegradable units in the bio-/photodegradation of polymers. The copolymer composition has an effect on biodegradability as EPF30R (7.7% ethylene) degrades faster than EPQ-30R (15.1% ethylene). Increasing viscosity of 100-h irradiated samples after culture incubation as well as a decrease in the carbonyl region in FT-IR spectra after composting must be due to the elimination of those short chains, which are responsible for increases in the carbonyl region after absorbing the IR light. In general, it can be concluded that photooxidation is a precursor of bioassimilation13,14 in the copolymers, and it can follow the same pattern during microbial attack as has been well investigated for polyethylene.15,24 Acknowledgment. The authors are grateful to Dr. S. Sivaram, Deputy Director and Head, Polymer Chemistry Division, National Chemical Laboratory, Pune, India, for fruitful discussions and encouragement. References and Notes (1) Albertsson, A. C.; Karlsson, S. J. Appl. Polym. Sci.1988, 35, 1289. (2) Albertsson, A. C. J. Appl. Polym. Sci. 1978, 22, 3419. (3) Scott, G. Atmospheric Oxidation and Antioxidants, Elsevier: Amsterdam, 1965; p 276. (4) Grassie, N.; Scott, G. Degradation and Stablization of Polymers; Cambridge University Press: Cambridge, U.K., 1985; p 91. (5) Griffin, G. J. L. U.S. Patent 4 021 338, 1977. (6) Griffin, G. J. L. U.S. Patent 4 021 388, 1977. (7) Griffin, G. J. L. U.K. Patent 1485833, 1978. (8) Griffin, G. J. L. International Patent PCT/GB 88/00386, 1988. (9) Chiquet, A. U.S. Patent 4 931 488, 1990. (10) Griffin, G. J. L. U.S. Patent 4 983 651, 1991. (11) Singh, R. P.; Singh, A. J. Macromol. Sci.sChem. 1991, A28, 487. (12) Sivaram, S.; Singh, R. P. AdV. Polym. Sci. 1991 101, 169. (13) Scott, G. In Degradable Polymers Principles and Applications; Scott, G., Gillead, D., Eds.; Chapman and Hall: London, 1995; Chapter 1. (14) Arnaud, R.; Davin, P.; Lemaire, J.; Al-Malaika, S.; Chohan, S.; Coker, M.; Scott, G.; Fauve, Maaroufi, A. Polym. Degrd. Stab. 1994, 46, 211. (15) Albertsson. A. C.; Andersson S. O.; Karlsson, S. Polym. Degrd. Stab. 1987, 18, 73. (16) Albertsson, A. C.; Karlsson, S. Acta Polym. 1995, 46, 114. (17) Singh, R. P.; Mani, R.; Sivaram, S.; Lacoste, J.; Vaillant, D.; Lemaire, J. J. Appl. Polym. Sci. 1993, 50, 1872. (18) Tang, L.; Sallet, D.; Lemaire, J. Macromolecules 1981, 15, 1437. (19) Mani, R.; Singh, R. P.; Sivaram, S.; Lacoste, J.; Lemaire, J. J. Elastomers Plast. 1996, 18, 183. (20) Eldsater, C.; Karlsson, S.; Albertsson, A. C. Polym. Degrd. Stab. 1999, 64, 177-183. (21) Eiji Ikada, J. Photopolym. Sci. Technol. 1997, 10, 265. (22) Mani, R. A Study of Thermal and Photodegradation of Polyolefins. Ph.D. Thesis, 1994, India. (23) Albertsson, A. C.; Karlsson, S. In Degradable Polymers Principles and Applications; Scott, G., Gillead, D., Eds.; Chapman and Hall: London, 1995; p 32. (24) Albertsson, A. C.; Barenstedt, C.; Karlsson, S.; Lindberg, T. Polymer 1995 36, 3075. (25) Sarwade, B. D.; Singh, R. P. J. Appl. Polym Sci. 1995, 72, 225. (26) ASTM G-21 70 (reapproved 1985).
BM010047S
