Chapter 12
Evaluation of Polymer Degradation in Controlled Microbial Chemostats 1
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Gary L. Loomis , James A. Romesser , and William J. Jewell 1
Warner-Lambert Company, 201 Tabor Road, Morris Plains, NJ 07950 Celgene Corporation, 7 Powderhorn Drive, Warren, NJ 07060 Microgen Corporation, Ithaca, NY 14853 2
3
We have developed protocols for the assessment of polymer degradation utilizing stable, well-controlled microbial chemostats (digesters) with easily measurable and quantifiable populations of microorganisms. This paper describes the use of three different chemostats [thermophilic (60°C) anaerobic, thermophilic (60°C) aerobic, and mesophilic (35°C) aerobic] to assess the degradation of a variety of commercially available polymer films. Environmental concerns have recently triggered a flurry of new research into the environmental fate of polymers. 1>2,3,4 Considering the importance of the subject, surprisingly little is understood about the molecular-level interactions of many common polymers with microorganisms. The varying claims regarding the degradation of these materials may be due in part to inconsistencies in the use of terminology describing degradation and to the lack of accepted standard tests for assessing the biodegradability of plastics.5 Most of the existing methodology for the study of polymer degradation suffers in the utilization of uncontrolled and poorly defined biological milieu - e.g. soil^, compost^, sewage sludge**, sea water, etc. can have highly variable compositions in terms of both chemistry and microbial populations. Commonly used evaluative methods based solely on physical properties (i.e. tensile strength, % elongation, etc.) may often be unsuitable, since observed property changes can be effected by changes in polymer morphology and, therefore, are not necessarily a measure of degradation. Conclusions about polymer degradation based solely on respiration and biogas production data is often inconsistent^ 10, possibly because mechanisms other than digestion of the polymer component of a system can alter the respiration of microorganisms. Finally, assessment of material biodegradability by observation of the degree of microbial growth* * has been often inconclusive, since common polymer additives may serve as a nutrient source while the molecular structure of the polymer remains intact.
0097-6156/92/0513-0163$06.00/0 © 1992 American Chemical Society
In Emerging Technologies in Plastics Recycling; Andrews, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
164
EMERGING TECHNOLOGIES IN PLASTICS RECYCLING
EXPERIMENTAL: The standard feed for all three reactors was composed of cellulose and sorghum 12 i sorghum being the main source of nutrients, and a supplemental solution of trace nutrients. Reactors were semi-continuously fed twice per week and a relatively low organic loading rate (OLR) of 1.42 g of V S per 1.0 kg of reactor mass per day was maintained in order to insure equal exposure of the sample materials to the microbes while providing a significant microbial population. Volatile solids (VS) is a standard, surrogate, operational measure of the organic fraction of a complex organic material or mixture and is based on the fact that most organic compounds volatilize at 5 5 0 ° C in air* 3. The chemostats were run in a slurry mode maintained at 4.5 wt.% dry solids at near neutral pH. The biological activity of these system was monitored via analysis of quantity and composition of biogas and volatile fatty acids (VFA) produced for the anaerobic system and C 0 2 evolution for the aerobic systems.
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Polymer samples were in the form of 2.0 cm X 10.0 cm strips of film. These film samples had a thickness of between 0.5 and 3.0 mils with most materials being in the range of 1.5 to 2.0 mils. Films were immersed in the reactors for up to 28 days and retrieved materials were evaluated by a variety of analytical techniques with particular attention paid to changes in molecular weight and molecular weight distribution. Obviously, since the microorganisms in this digester were fed with sorghum as the major carbon source and the polymer samples represent only a minor portion of the total available carbon, it is not possible to accurately assess the degradation of the samples by monitoring biogas (methane in the anaerobic chemostat and carbon dioxide in the aerobic chemostats) production. Simultaneously, two sets of control samples were maintained; one set in sterile, phosphate buffer at pH=7.5 at the appropriate temperature, and a second set in a dry environment at 2 0 ° C .
Molecular weight data was obtained using Waters model 150°C gel permeation chromatographs (GPC). Polyolefin materials were run in 1,2,4trichlorobenzene at 1 3 5 ° C using two Shodex A T - 8 0 M / S columns with calibration as polyethylene via the universal calibration technique using polystyrene as primary standard. Polyester materials were run at 3 5 ° C with a mobile phase of 0.01 M sodium trifluoroacetate in hexafluoroisopropanol and two Shodex KF80M/HFIP columns with an in-house laboratory standard of poly(ethylene terephthalate) used for calibration.
The stress-strain data presented in the accompanying bar graphs were generated from Instron testing using the average of five specimens of each sample, and the error bars (only the top half of the bar is shown) represent the standard error as (S/Nl/2).
In Emerging Technologies in Plastics Recycling; Andrews, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
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LOOMIS ET AL.
Polymer Degradation in Microbial Chemostats 165
DISCUSSION A N D R E S U L T S :
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Cellophane: Uncoated cellophane (regenerated cellulose) film was used as a positive control material and since, due to the high cellulose content of the feed, the digesters are rich in microorganisms which secrete cellulases (cellulose degrading enzymes), it was expected that the cellophane would degrade readily within the 28 day duration of the experiments. This was indeed the case, with no trace of the cellophane recoverable from any of the three reactors. Commercial cellophanes, which are generally coated with a thin, moisturebarrier coating of either nitrocellulose or poly(vinylidene chloride) were not evaluated in this study. Polvolefins: Figures 1 and 2 show stress-strain data for films of high density polyethylene (Alathon A7030), linear low density polyethylene (Dowlex 2045), two polyolefins before and after exposure to the anaerobic system. These property changes, particularly the H D P E , (given the inconsistencies inherent in testing physical properties of films) are significantly beyond experimental error and are therefore probably real. However, since the G P C traces shown in Figures 3 and 4 indicate no change in molecular weight distribution, we conclude that no significant molecular level degradation (i.e. no chain scission) has occurred. The physical property changes are likely due to changes in orientation or crystal morphology of the polymer caused by the 28 day 6 0 ° C "thermal exposure". Various commercial "plastic" bags were evaluated with similar results. Polyesters: Figures 1 and 2 (stress-strain data) and Figures 5 and 6 (molecular weight data) show a comparison between the polyesters P E T (Mylar™, from DuPont) and poly (hydroxybutyrate/valerate) copolymer (Biopol™ from ICI). It can be easily seen that, in the anaerobic system, a film of the commercial Biopol™ shows no significant physical property change while showing substantial loss of molecular weight. Notice that the Mylar™, on the other hand, shows significant changes in physical properties under the same conditions with no accompanying change in molecular weight (therefore no molecular level degradation). Like the polyethylene examples above, the changes in physical properties of the Mylar™ are likely due to changes in morphology, in this case possibly loss of orientation. The apparent degradation of the Biopol™ without measurable changes in stress-strain properties is likely due to the high initial molecular weight ( M >400K) of this microbially produced polyester and further exposure would no doubt cause the material to exhibit property deterioration. It must be pointed out that we have no direct evidence that the molecular weight loss of the poly(hydroxy-butyrate/valerate) in our systems is due to the action of microorganisms - we may well be seeing simple hydrolysis. It is also interesting, but not yet explainable, that the hydrolysis of the Biopol™ film appears to be faster in p H 7.5 phosphate buffer than in the anaerobic chemostat (see Figure 6). w
In Emerging Technologies in Plastics Recycling; Andrews, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
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EMERGING TECHNOLOGIES IN PLASTICS RECYCLING
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ANAEROBIC DIGESTION 60°C/28DAYS
M CONTROL • EXPOSED
HIGH DENSITY PE LINEAR LOW DENSITY PE
MYLAR (PET)
BIOPOL
Figure 1. Tensile strength before and after anaerobic digestion.
ANAEROBIC DIGESTION 60°C/28DAYS
HIGH DENSITY PE LINEAR LOW DENSITY PE
1 CONTROL g EXPOSED
MYLAR (PET)
BIOPOL
Figure 2. Elongation before and after anaerobic digestion.
In Emerging Technologies in Plastics Recycling; Andrews, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
Polymer Degradation in Microbial Chemostats
LOOMIS ET AL.
Molecular Weight Distribution Downloaded by KTH ROYAL INST OF TECHNOLOGY on November 26, 2015 | http://pubs.acs.org Publication Date: November 13, 1992 | doi: 10.1021/bk-1992-0513.ch012
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LLDPE 1.0 mil film 28 Day Exposure
control
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aerobic (60°C) anaerobic (60°C)
2.5
3.0
3.5
/
4.0
4.5
5.0
5.5
6.0
6.5
Log MW Figure 3. Molecular weight distribution of L L D P E before and after digestion.
Molecular Weight Distribution HDPE 1.0 mil film 28 Day Exposure
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2.5
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5.5
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Figure 4. Molecular weight distribution of HDPE before and after digestion.
In Emerging Technologies in Plastics Recycling; Andrews, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
168
EMERGING TECHNOLOGIES IN PLASTICS RECYCLING
Molecular Weight Distribution
Downloaded by KTH ROYAL INST OF TECHNOLOGY on November 26, 2015 | http://pubs.acs.org Publication Date: November 13, 1992 | doi: 10.1021/bk-1992-0513.ch012
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Figure 5. Molecular weight distribution of Biopol
6.5
7.0
before and after digestion.
Molecular Weight Distribution
control
/
60°C/pH7.5
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3.0
3.5
Mvlar™ 28 days
^
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5.0
Figure 6. Molecular weight distribution of Mylar
5.5
6.0
before and after digestion.
In Emerging Technologies in Plastics Recycling; Andrews, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
12.
LOOMIS
ET AL.
Polymer Degradation in Microbial Chemostats
Downloaded by KTH ROYAL INST OF TECHNOLOGY on November 26, 2015 | http://pubs.acs.org Publication Date: November 13, 1992 | doi: 10.1021/bk-1992-0513.ch012
References
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In Emerging Technologies in Plastics Recycling; Andrews, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
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