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Biodegradability of some food packaging materials in soil
Soil Eiol. Biochem. Vol. 25, No. 11, pp. 146S1475, Printed in Great Britain. All rights reserved
1993 Copyright
ACCELERATED
0
0038-0717/93 $6.00 + 0.00 1993 Pergamon Press Ltd
PAPER
BIODEGRADABILITY OF SOME FOOD PACKAGING MATERIALS IN SOIL Department of Biochemistry and Microbiology, NJAES Rutgers University, New Brunswick, NJ 08903-0231, U.S.A. (Accepied 2 May 1993) Summary-The imminent shift of urban solid waste disposal from landfilling to recycling-composting programs directs attention to the biodegradability of certain food packaging items that are not suitable for recycling. Novel plastic wrap formulations claiming to be more degradable than their predecessors are now being marketed. Because biodegradability claims often appear in poorly documented promotional literature only, a comparative biodegradability survey of some novel highly plasticized polyvinyl chloride (PVC), traditional polyethylene (PE) and polypropylene (PP) wraps as well as coated or impregnated paper products was made. All materials were exposed in soil under conditions generally favorable for biodegradation. The measurement of CO, evolution in Biometer flasks, decline in tensile strengths, in combination with newly developed residual weight determinations, and other residue analysis techniques were utilized in assessing biodegradation. During 3 months of exposure, traditional PE and PP films did not undergo measurable deterioration, but a group of newly formulated and heavily plasticized PVC films underwent extensive biodeterioration and up to 27.3% of their carbon was converted to CO,. However, gas chromatography, residual weight determination, chloride release and viscosity measurements indicated that only the plasticizer but not the PVC resin was mineralized. The wax-impregnated paper products were rapidly and completely mineralized. The PE-coating did not prevent the rapid mineralization of the cardboard, even when the material was left unshredded.
INTRODUCTION
In 1988, packaging materials made up 31.6% of municipal solid waste in the U.S.A. (USEPA, 1990). A large portion of this waste consisted of food containers, and the effect of this waste on the environment in general and on landfills in particular was the subject of the Institute of Food Technology (IFT) workshop ‘Food Packaging, Food Protection and the Environment’ (IFT, 1990). Municipal solid waste handling is currently in a state of profound transition, moving from the traditional landfill disposal towards some combination of recycling, composting and/or incineration. As a first phase, numerous communities have initiated recycling programs and are studying future composting or incineration alternatives for garbage and non-recyclable organics, including various food wraps and containers. This situation has renewed societal and regulatory interest in biodegradable packaging materials and manufacturers are responding with ‘environmentally friendly’ products, some of them controversial (Donnelly, 1990; Thayer, 1990). Earlier work on plastics typically focused on biodeterioration rather than on biodegradability (Aminabhavi et al., 1990). Even when biodegradation was the focus of the work, the older test approaches that measured growth yields in pure cultures (Stahl *Author for correspondence.
and Pessen, 1953) are now obsolete. More recent 14C02 liberation studies (Albertsson et al., 1978; Albertsson and Banhidi, 1980) preferentially used pure cultures but also soil. The availability of labeled material restricted these studies to polyethylene only. Against the described background, we compared the biodegradability of 12 polymeric film samples and 4 traditional or composite packaging materials in soil, under conditions generally favorable for biodegradation. To ensure complete objectivity for the study, we received from our sponsor all polymeric films with letter codes only. The manufacturer and the composition of the test materials was not revealed or discussed with us until the study was completed and our final report submitted to the sponsor. To avoid excessive length, detailed time-course data are presented only for four polymeric films. The similar fate of the other test compounds are shown in a summary form only. EXPERIMENTAL Characterization
of the test materials
IR-spectra of the polymeric films were compared with published spectra (Haslam and Willis, 1965; Sadtler Research Laboratories, 1980). In addition, the Beilstein test for organochlorine (Hollernan and Richter, 1957) and the solubility characteristics of the films aided qualitative characterization. Plasticizers
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ASHA YABANNAVAR
and
were qualitatively and quantitatively dete~ined by GC measurements on methanol extracts. Retention times and peak areas were compared to an authentic dioctyl adipate (DOA) and dioctyl phthalate (DOP) standard (Aldrich, Milwaukee, Wis.). Epoxidized soybean oil (ESO), an additive to some PVC films, was not sufficiently volatile to be determined by gas chromatography. Its amount was calculated by subtracting from the total plastic film weight the weight of precipitable PVC plus the weight of DOA, as determined by GC measurements. The weight difference was assumed to be ESO. The hydrocarbon and fiber content of the two impregnated paper products was determined by Soxhlet-extracting 1 g material, using hexane. The solvent was evaporated in a tared flask at 60°C under vacuum in a Rotavap apparatus. The hydrocarbon residue and the fibre remaining in the Soxhlet thimble were weighed and the sum of these weights was compared to the starting weight of the product. The composition of the PE-coated cardboard products was dete~ined using a similar approach, extracting 1 g of the cardboard samples with hot 1,2,4-trichlorobenzene (TCB) and weighing the residual cardboard. The PE resin was precipitated from the TCB solution by diluting with ethanol and cooling the solution. The precipitate was filtered, dried and weighed. Again, the sum of the cardboard and PE resin weights were compared to the starting weight of the product. Total C, H and ash of all samples was determined by combustion followed by sequential absorptive removal of the combustion products, while measuring changes in the~oconductivity. The latter analyses were made by Midwest Microlab, Indianapolis, Ind. Exposure conditions and CO, evolution measurements Nixon sandy loam (fine, loamy, mixed, me&g, typic hapludolt), freshly collected from College Farm, New Brunswick, N.J. was used in all biodegradation studies. In terms of texture, this soil is composed of 50% sand, 21% silt and 29% clay. Its organic matter content is 5% and its pH is 5.5-6.5 (Bartha and Bordeleau, 1969). Its moisture content and water holding capacity were determined and used in adjusting the water content of the soil for the biodegradation studies to 50% of capacity. This moisture content is considered ideal for aerobic biodegradation processes in soil (Pramer and Bartha, 1972) and was maintained throughout the incubation by weighing the samples at time 0 and adjusting with distilled water for any weight loss during incubation. The pH of the soil was adjusted by the addition of CaCO, (liming) to 7.5. This pH was found to be optimal for hydrocarbon biodegradation (Dibble and Bartha, 1979) and it was assumed that it would also favor the biodegradation of plastic materials. The soil used in CO2 evolution tests were limed 5 days prior to the start of the experiment to avoid measuring any
RICHARD BARTHA
CO2 released by neutralization as part of the biodegradation process. Per gram test material, 38 mg of (NH.,)2HP03 was added as fertilizer (approx. C:N ratio 1OO:l) in order to provide sufficient N and P for the enrichment of a polymerdegrading microbial population. For easier distribution and incorporation in soil, for Biometer and residual weight measurements the film materials were shredded in a Waring blender. The other materials were cut into ca 3 x 2 mm pieces using scissors. From all materials, 250mg portions were mixed with 25 g dry wt of soil (1% w/w). From the food carton (FC), single pieces of exactly 250 mg were also incorporated into soil to test the effect of cutting up on degradation rates. Water and dissolved fertilizer were added. Biometer flasks for CO, evolution measurement (Bartha and Pramer, 1965) and foil-covered 250 ml beakers for residual weight measurements were kept at 27°C. Both types of samples were aerated every 2-3 days. Poisoned soil controls contained 1% (w/w) HgCl,. Tensile strength &terminations For tensile strength measurements, 15 strips each (2.5 x 20 cm) of the polymeric films were incubated in a bin containing 25 kg of soil. All conditions (liming, fertilization, aeration, moisture and temperature) were identical to the ones described earlier. After 3 months, the strips were retrieved, washed with water and mild detergent, dried and subjected to tensile strength tests as specified by the American Society for Testing Materials (ASTM, 1983). Force and work needed to break the strips were compared to five non-exposed control strips of each material. Residual weight analysis Retrieval and weighing of thin plastic films after soil exposure is inherently inaccurate, since brittleness may cause incomplete recovery; encrustation and oxidation add weight. Therefore, we attempted to develop analytical techniques for plastic films similar to those used in pesticide and hydrocarbon residue analysis. At the end of an exposure period, the soil was air-dried at room temperature and subsequently solvent-extracted. After considerable experimentation with solvents and extraction techniques, the following two procedures were adopted. Residues of the two PVC films were Soxhlet-extracted for 6 h (at least 100 cycles) using 100 ml of methyl-ethyl ketone (MEK). The extract was split 9O:lO for residual weight measurement and gas chromatography, respectively. For residual weight, MEK was evaporated at 50°C and under vacuum in a tared extraction flask using a Rotavap evaporator. After 24 h in a vacuum desiccator with Drierite and paraffin chips, the flasks were re-weighed on an analytical balance. For reuse, the flasks needed to be cleaned using alkaline permanganate. PE and PP residues were extracted from soil contained in a Soxhlet thimble using six aliquots of
Biodegradability of some food packaging materials in soil boiling TCB. The thimble itself was kept at boiling temperature in a beaker placed upon a hot plate. A total volume of 125 ml TCB was used. Since gas chromatography detected no plasticizer in these samples, the extract was not split but mixed with an equal volume of methanol and cooled. The precipitated plastic material was filtered through a tared Whatman No. 114 analytical filter. The filter was dried for 10 min at 50°C and re-weighed. No satisfactory solvent was found for the irradiated polyolefin (PO) sample and, therefore, no residual weight was determined for this material.
were the tensile strength determinations made at the end of the experiments. CC& evolution measurement by the Biometer method is a highly consistent technique. Sharabi and Bartha (1993) have shown that the cumulative CO, evolution of replicate samples, even in long-term experiments, deviate ~5% from the mean. In the experiments on packaging materials, we interpreted net CO* evolution from additives as significant if it consistently exceeded the cumulative CO, evolution of the unamended soil control by >5%. RESULTS
Gas chromatographic analysis
Plasticizer in the PVC samples was measured after adding an equal volume of cold methanol to the MEK extract. The precipitated plastic fiber was removed by filtration and in some cases its weight was determined after drying. Of the clarified filtrate, 1~1 vol were injected into a Hewlett-Packard model 5890 instrument fitted with a 10 m x 0.53 mm macrobore capillary column (RSL 150, 1.2pm thick poly dimethyl-siloxane bonded liquid phase). Carrier flow was 10 ml N, min-‘. Temperatures: oven 200°C isothermal, injector 250°C detector 300°C. DOA standards were used for the identification and quantification of the plasticizer peaks. Viscosimetric and chloride determinations
Viscosity was measured on the solvent extracts of the plastic-amended soil samples as follows. The original extracting solvent (MEK) was evaporated under reduced pressure and the residue was dissolved in cyclohexane (half of the original volume). The viscosity of this cyclohexane solution was measured according to the ASTM (1972) and was expressed as ‘kinematic viscosity’. Such measurements were made on extracts of plastic amended soil samples at 0 time and after 5 weeks. Chloride and viscosity were measured on PVC-l and -2, since CO* evolution and residual weight measurements indicated substantial degradation of these two materials. Chloride was determined according to Bergman and Sanik (1957), on soil blanks (25 g dry wt) and on 25 g soil + 250 mg plastic foil at time 0 and after 5 weeks exposure. Quality control
Because of the volume of samples, it was not feasible to replicate all data points. As the extraction and residual weight measurement of plastic residues from soil were techniques newly developed for our study, the time 0 recoveries were evaluated on triplicate samples for every material. These recoveries and their standard deviation were used as confidence limits for the subsequent time-course determinations. Even without replication, in time-course experiments aberrant readings are easily spotted when they deviate from the regression curve, and in such cases the determinations were repeated. Statistically controlled
1471
Composition of the test materials
IR spectra and solubility characteristics identified one of the polymeric films as polyethylene (PE) and two as polypropylene (PP), respectively. Irradiation treatment of polyolefins (PO) induces cross-linking and decreases solubility, preventing the extraction of this polymer from soil. None of these films contained plasticizer and they were negative for organochlorine. All other films gave a positive Beilstein test for organochlorine; their IR spectra and solubility characteristics identified them as polyvinyl chloride (PVC) polymers. Gas chromatography of their methanol extracts revealed the presence of DOA in most of the plasticized PVC samples; PVC-8 contained DOP. DOA or DOP did not account for all methanol-extractable material. A standard for the missing ingredient, epoxidized soybean oil (ESO) was provided by the manufacturer but, because of its low volatility, we determined this ingredient in the films negatively, i.e. as the film weight that was neither PVC resin nor DOA. The compositions of the tested polymeric films are summarized in Table 1. Our analyses agreed within 1% with the manufacturer’s specifications, made available to us after the conclusion of our study. The separately determined wax plus fiber and PE plus cardboard weights of the WP, BW and OC samples added up to within 99-100% of the starting weights of the products. Only a 96% weight recovery for the FC material was caused by coloring inks and dyes that stayed in the solvent solution when PE was precipitated. The good weight agreements served as quality control for our data on these materials, since manufacturer’s specifications for them were not available to us. CO, evolution The PE and PP-2 films, added to soil at 1% (w/w), did not increase CO, production as compared to unamended control soil during a 3 month exposure (Fig. 1). In the same period, the two PVC films released 2.74 and 1.8 mmol of CO2 above soil background for PVC-l and -2, respectively. The COZ release corresponded to 27.3 and 19.4% of the total organic C of the materials. The impregnated paper products and the food carton released between 76
ASHAYABANNAVARand RICHARLI BARTHA
1412
Table I. Codes, descriptions and compositions of test materials and the summary of degradative changes they underwent during 3 months of aerobic exposure in soil*
Code
Sample composition (%I
Same description
PO PP- I PP-2 PE PVC-I PVC-2 PVC-3 PVC-4 PVC-5 PVC-6 PVC-7 PVC-8 WP BW FC oc
Packaging filmt Packaging filmt Packaging fibnt Packaging film? Produce/mushroom filmf Bakery tilmt Meat-poultry fihnt Meat-poultry filmf Meat-poultry filmi Shrink tilmf: Shrink filmf DOP-plasticized film Wax papert Bread wrapt Food cartont Frozen juice container?
Irradiated polyolefin, 100 PP. 100 PP, 100 PE, 100 PVC, 69.5; WA 23.6; ESO, 6.9 PVC, 76.5; DOA, 16.0; ESO, 7.5 PVC, 70.0; DOA, 19.2; ESO, 10.8 PVC, 72.0; DOA, 22.0; ESO, 6.0 PVC, 70.0; DOA, 13.6; ESO, 16.0 PVC, 84.0; DOA, 2.0; ESO, 14.0 PVC, 70.5; WA, 8.6; ESO, 16.0 PVC, 68.5; DOP, 24.8; ESO, 7.0 Fiber, 75; wax, 25 Fiber and filler, 68.0; wax, 32.0 Cardboard, 96.0; PE, 4.0 Cardboard, 95; PE, 5.0
Sample carbon (mg)
Conversion to co, (“/)
Weight loss (%I
Loss of elongation w
211.2 215.0 214.5 214.7 120.5 116.5 122.7 121.4 123.4 108.3 109.9 123.1 126.0 126.2 104.7 110.2
1.5 ND ND
NA ND ND ND 26.8 23.2 38.4 27.2 18.4 14.8 9.6 20.8 NA NA NA NA
-33.0 ND ND ND -94.5 -92.6 -94.0 -94.0 -92.0 -67.0 ND -15.0 NA NA NA NA
K 19.4 25.4 25.7 18.8 7.3 5.9 9.7 78.6 76.2 81.9 64.2
*All samples were applied at 250 mg in 25 g soil. Abbreviations: NA: not analyzed; ND: not de&table; PO: polyolefin; PP: polypropylene; PE: polyethylene; PVC: polyvinyl chloride; DOA: dioctyl adipate; DOP: dioctyl phthalate. TManufacturer not known to us. SManufactured by Borden Resinite Co., Andover, Mass., U.S.A.
and 82% of their carbon as CO, (Fig. 2). The thicker-walled orange juice container released 64% of its carbon as COZ. Net CO, production rates were initially high and gradually declined as the biodegradable and physically accessible carbon neared exhaustion. Since for convenient and reproducible applications the test materials were cut up or shredded, the question arose how this mechanical procedure affected the rate and extent of biodegradation. As shown for the food carton in Fig. 3, cutting up increased the rate of CO* evolution only slightly and the extent of degradation was not influenced significantly.
(O-time) recoveries were performed in triplicate. The average recoveries and standard deviations are shown in Table 2. We assumed similar confidence limits for the time-course experiment (Table 3) that involved the analysis of single samples per time point. The standard deviations ranged from +2.7 to + 11 mg. Conservatively, we judged weight losses as significant if recovery declined more than 11 mg below the initial recovery. By this measure, the PE and PP-2 films did not decline significantly in residual weight, but both PVC samples did. The fact that the latter films did not loose significant weight in metabolically inactive (poisoned) soil pointed to a biodegradative mechanism of the weight losses.
Residual weight analysis
Plasticizer
These measurements were made on the polymeric films only. To establish confidence limits, the initial
The GC measurements of DOA plasticizer recovery with time are shown in Table 4. 6n a-TErage, > 50%
7 15 2329
’ ’
5.5
’ ’
37
’
50
64
76
loss
92
PVC-I
#
2.5-
f
1.5-
PVC-2
G lj
0.5-1
RP-2 PF ._ TIME (DAYS)
Fig. 1. Net CO, evolution with time from 250 mg amounts of polymeric film samples in 25 g soil, measured in Biometer flasks. PVC-l and -2: plasticized polyvinyl chloride films. PE and PP-2 designate polyethylene and polypropylene films, respectively. The numbers above the curves designate the days the readings were taken.
Fig. 2. Net CO, evolution with time from 250 mg amounts of impregnated paper and plasticized cardboard food packaging materials in 25 g soil. BW, bread wrap; WP, wax paper; FC, food carton; OC, orange juice (frozen concentrate) container. The numbers above the curves designate the days the readings were taken.
1473
Biodegradability of some food packaging materials in soil 7
15 23
37 ”
8,“’
49
63
77
91 I
Table 4. Gas chromatographic analysis of dioctyl adipate plasticizer loss during the exposure of PVC films in soil
F.C.cutUP
Recovery (mg) with incubation time (months)
F.C.uncut
Material PVC-1 PVC-2
Plasticizer
0
I
2
3
DOA WA
59.2 40.1
22.9 12.7
20.5 16.0
21.5 13.5
occurred in case of the films PVC-I and -2. The PP-2 that showed no CO* evolution and weight also failed to decline in tensile strength. Film elongated without breaking under the standard conditions. 0
so TIMEE*YS~
30
I20
Fig. 3. Net CO, evolution with time from 250 mg amounts of food carton (FC) material in 25 g soil. FC was either cut into ca 3 x 2 mm pieces or was left uncut as a single piece of plasticized cardboard. The numbers above the curves designate the days the readings were taken.
of the DOA plasticizer was lost during the first month of exposure to soil with little additional change thereafter. Even after 3 months, a substantial amount of the plasticizer remained undegraded. Since free DOA spikes degraded in the same soil rapidly (results not shown), we conclude that the remaining plasticizer was physically inaccessible for biodegradation, being trapped within the polymer network. DOP of PVC-8 was degraded at a somewhat slower rate than the DOA plasticizer but the extents of their biodegradation were similar (data not shown). ES0 was not measured directly, but was calculated, in some cases, from the total weight minus plasticizer and polymer precipitate. We assume that ES0 biodegradation contributed to weight loss and CO, evolution and most likely behaved in a similar manner as the plasticizer in the extent of its biodegradation. Tensile strength tests
Changes in tensile strength are summarized in Table 5. A strong decrease in elongation at break Table 2. Time 0 recoveries of 250 mg amounts of polymeric film from 25 g triplicate soil samples Description
Average recovery Code mg*lSD
Packaging film Packaging film Produce-mushroom Bakerv film
PE PP-2 PVC-I PVC-2
film
218 f 226 * 246+ 237 +
Recovery (%)
2.7 9.0 1.2 10.9
87.2 90.0 98.4 94.8
Chloride determinations ments on polymericJilms
measure-
Table 5. Changes in tensile strength of polymeric films during 3 months burial in soil Eloneation at break (%j* Material
Table 3. Decrease in residual weight of 250 mg amounts of polymeric films during incubation in soil Recovery (mg) with incubation time (months) PE PP-2 PVC-1 PVC-2
and viscosimetric PVC-l and -2
The observed CO2 evolution, residual weight and plasticizer losses of PVC-l and -2 were at rates that left some question whether they originated from their DOA and ES0 components alone, or whether it represented also some breakdown of the PVC resin itself. To clarify whether and to what extent the PVC polymer itself was affected, we determined the release of chloride. If PVC was mineralized, inorganic chloride would be released. PVC polymer biodegradation would be evident also by a decrease in viscosity of the solvent extract as large PVC molecules were converted to smaller ones. Chloride readings were consistently higher in PVCspiked soil after 5 weeks of exposure than at time 0, but the differences were extremely small (data not shown). Less than 0.1 mg Cl was released per flask. This would account for the ultimate biodegradation of 0.2 mg PVC only, or < 0.1% of the PVC resin in each flask. Therefore, Cl determinations did not indicate a significant breakdown of PVC resin. As a positive control for our measurement technique, we treated similar soil samples with 25 mg chloracetic acid (Na-salt), and determined Cl immediately after treatment and after 1 week of exposure. Subtracting the 0 time determination from the 1 week determination, it was calculated that 9.07 mg (36.3%) of the 25 mg chloroacetic acid was mineralized during 1 week. We would have clearly detected any significant chloride release from the PVC resin that was added to the soil samples at a IO-fold higher concentration than the chloroacetic acid.
SD: standard deviation.
Material
0
1
2
3
3PC
218 226 246 237
215 236 230 225
212 256 194 187
213 225 179 205
209 229 261 263
PC: poisoned control (1% HgCI,).
film loss PE test
PE PP-2 PVC-1 PVC-2
Controlt +
I SD
No break 49.2 6.4 267.3 20.3 233.1 39.8
Exoosedf + 1 SD No Break 46.9 7.3 15.0 O.O§ 15.0 0.05
*ASTM (1983). TAverage of five test strips. SAverage of 15 test strips. @Alltest strips broke before the lowest elongation (15%) the instrument was able to record. SD: standard deviation.
ASHAYABANNAVAR and RICHARD BARTHA
1474
Table 6. Viscosity of solvent extracts of soil amended with polymeric film samples, at time 0 and after 5 weeks of incubation Kinematic viscosity* Material
Control
Deviationt
Exposed
Deviation?
PVC- 1 PVC-2
2.881 2.891
0.091 0.074
2.947 2.948
0.138 0.073
*ASTM (1972). tDeviation ( f ) from the mean of duplicate measurements.
Viscosity was measured after two separate exposures. The results were averaged in Table 6 and the deviation from the means are indicated. No significant changes occurred in viscosity during soil exposure, and these measurements did not indicate polymer breakdown. DOA and ESO, when added to solvent, influenced viscosity marginally (results not shown). Therefore, we consider it unlikely that DOA and ES0 disappearance influenced our measurements. Both the Cl release and the viscosimetric tests indicated that there had been little, if any, degradation of the PVC resin. DISCUSSION
Test method development and definition of biodegradation for polymeric jilms
In tests on the environmental fate of polymeric materials, until very recently, the emphasis has been on the prevention of biodeterioration (Bessems, 1988). The methodology developed for this purpose was less than satisfactory when questions shifted to the rate and extent of biodegradation of packing materials that are typically used in the form of thin sheets, may consist of several component and are exposed to the complex physicochemical and biochemical actions of the soil environment. For this reason, our aim in this study, beyond a comparison of biodegradability of several packing materials, was the development of a test methodology package that would answer questions about biodegradability of novel polymeric products in a more satisfactory manner. From the classic polymer biodeterioration tests, the most common ones are loss of tensile strength and loss of residual weight (Wendt et al., 1970). For comparison purposes, we used the tensile strength test in its classic form (ASTM, 1983), but we modified residual weight measurement procedures to make them more accurate and reproducible. We applied solvent extraction techniques common to pesticide and hydrocarbon residue analysis and, for the materials tested, found these extraction techniques very satisfactory. We found CO, evolution measurements in Biometer flasks (Bartha and Pramer, 1965), used as a standard FDA procedure for biodegradability testing (USFDA, 1987) applicable and highly predictive of packaging material biodegradability in soil. When applied to composite materials both residual weight and CO2 evolution procedures have the common limitation that in case of partial biodegradation, they
do not indicate which component of the composite was biodegraded and which persisted. To answer such questions, auxiliary techniques such as G.C., viscosimetry and Cl release need to be applied. A generally accepted terminology pertaining to the environmental degradation of polymeric films has yet to be developed. For lack of standards, much controversy surrounded some ‘environmentally friendly’ products that were advertised as biodegradable (Donnelly, 1990; Thayer, 1990). In these instances, the generally-accepted criteria of biodeterioration, such as loss of tensile strength and microbial encrustation of surfaces were equated with biodegradation. An ASTM study group currently seeks to define ‘biodegradation’ as the biochemical breakdown of the polymer resin molecules to smaller size, as documented by gel permeation chromatography measurements (Ramini Narayan, pers. commun.). This restrictive definition would effectively rule out misleading claims that equate additive degradation with degradation of the whole polymeric product. On the other hand, it is definitely awkward to call a material like PVC-l ‘biodeteriorable’, when close to 30% of its total C was converted to CO, and the material was formulated to undergo rapid albeit only partial biodegradation. Comparative persistence of polymeric films, impregnated paper and plasticized cardboard containers in soil.
Our findings are generally consistent with earlier reports on synthetic polymer behavior in soil (Aminabhavi et al., 1990). By both conventional and newly-developed test methods, polyolefin (PE and PP) polymers were much more resistant to biodeterioration than plasticized PVC. Not surprisingly, impregnated paper and plasticized cardboard was, in turn, more biodegradable than plasticized PVC. However, compared to earlier studies, our measurements provide a more quantitative and detailed understanding of the test material behavior in soil. Table 1 compares the C content of each sample to the net CO2 evolved during the 3 month test period. Percentage weight losses are also shown in this table. A lack of net CO2 evolution or of significant weight loss by the PE and PP samples was consistent with their tensile strength behavior. The plasticized PVC films underwent substantial conversion to CO, and showed losses in tensile strength and in residual weight, but the losses did not exceed the additive contents of these compounds. Moreover, viscosimetric and Cl release measurements ruled out a significant degradation of the PVC resin. Percentage weight loss and percentage C conversion to CO, showed definite correlations. Weight losses ranged from equal to twice as high when compared with C conversion to CO2. Some substrate C can be expected to be sequestered temporarily in microbial biomass and soil organic matter, but this is compensated for by a priming of soil organic matter
Biode~adability
1475
of some food packaging materials in soil
degradation (Sharabi and Bartha, 1993). As a consequence, Xl-100% of the degraded substrate carbon registers as net CO;, evolution in Biometer tests a
month or longer in duration. Although the solventextraction based residual weights are more reliable than manual retrieval and weighing, residual-weight m~s~ements showed a higher variability than CO, evolution measurements in Biometer flasks. Carbon conversions to CO1 between 18.8 and 27.3% (PVC-l-5) were accompanied by over 90% reduction in elongation at break but PVC-8 changed little and PVC-7 atypically increased its elongation at break. Thus tensile strength measurements alone do not reliably predict biodegradative changes. Not surprisingly, impregnated paper products were completely biodegraded as shown by the extensive conversion of their C to CO*. Net CO2 evolution also indicated extensive biodegradation of the plasticized cardboard materials. When adjustment is made for the 45% nonbiodegradable PE component, 90.2 and 70.7% of the cardboard C was converted to CO, in case of the FC and OC samples, respectively, instead of the somewhat lower total C-based conversion percentages shown in Table 1. Apparently, PE coating does not prevent the extensive biodegradation of cardboard containers. The use of plasticized composites as food containers, where mechanical strength is provided by biodegradable cardboard and the synthetic polymer is used only in low (45%) proportion to provide a hydrophobic barrier, may not be the ‘ultimate’ en~ronmentai solution. Nevertheless, as compared to all-plastic containers, in a composting situation such composites would reduce the non-biodegradable container material by a factor of 20. Acknowledgements-The work described in this New Jersey Agricultural Experiment Station Publication No. D-0150801-92 was performed with the support of Borden Resinite Inc. We are indebted to S. Gilbert, Professor Emeritus, Rutgers University, for introducing us to the biodegradation problems of food packaging materials. The work was also supported by New Jersey State funds.
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__
Stahl W. and Pessen H. (1953) The microbiological degradation of plasticizers. Applied Microbiology 1, 3&35. Thayer A. (1990) Degradable plastics generate controversy in solid waste issues. Chemical and Engineering News 69, 7-14.
REFERENCES Albertsson A. C. and Banhidi Z. G. (1980) ~icrobiai and oxidative effects in degradation of polyethylene. Journai of Applied Polymer Science 25, 1655-1671. Albertsson A. C., Banhidi Z. G. and Beyer-Ericson L. L. (1978) Biodegradation of synthetic polymers. III. The
USEPA (U.S. Environmental
Protection Agency) (1990)
Characterization of Municipal Solid Waste in the United States: 1990 Update. USEPA, Washington, D.C.
Wendt T. M., Kaplan A. M. and Greenberger M. (1970) Weight loss as a method for estimating deterioration of PVC film in soil burial. International Biodeterioration Bulletin 6, 139-143.
1993 Copyright
ACCELERATED
0
0038-0717/93 $6.00 + 0.00 1993 Pergamon Press Ltd
PAPER
BIODEGRADABILITY OF SOME FOOD PACKAGING MATERIALS IN SOIL Department of Biochemistry and Microbiology, NJAES Rutgers University, New Brunswick, NJ 08903-0231, U.S.A. (Accepied 2 May 1993) Summary-The imminent shift of urban solid waste disposal from landfilling to recycling-composting programs directs attention to the biodegradability of certain food packaging items that are not suitable for recycling. Novel plastic wrap formulations claiming to be more degradable than their predecessors are now being marketed. Because biodegradability claims often appear in poorly documented promotional literature only, a comparative biodegradability survey of some novel highly plasticized polyvinyl chloride (PVC), traditional polyethylene (PE) and polypropylene (PP) wraps as well as coated or impregnated paper products was made. All materials were exposed in soil under conditions generally favorable for biodegradation. The measurement of CO, evolution in Biometer flasks, decline in tensile strengths, in combination with newly developed residual weight determinations, and other residue analysis techniques were utilized in assessing biodegradation. During 3 months of exposure, traditional PE and PP films did not undergo measurable deterioration, but a group of newly formulated and heavily plasticized PVC films underwent extensive biodeterioration and up to 27.3% of their carbon was converted to CO,. However, gas chromatography, residual weight determination, chloride release and viscosity measurements indicated that only the plasticizer but not the PVC resin was mineralized. The wax-impregnated paper products were rapidly and completely mineralized. The PE-coating did not prevent the rapid mineralization of the cardboard, even when the material was left unshredded.
INTRODUCTION
In 1988, packaging materials made up 31.6% of municipal solid waste in the U.S.A. (USEPA, 1990). A large portion of this waste consisted of food containers, and the effect of this waste on the environment in general and on landfills in particular was the subject of the Institute of Food Technology (IFT) workshop ‘Food Packaging, Food Protection and the Environment’ (IFT, 1990). Municipal solid waste handling is currently in a state of profound transition, moving from the traditional landfill disposal towards some combination of recycling, composting and/or incineration. As a first phase, numerous communities have initiated recycling programs and are studying future composting or incineration alternatives for garbage and non-recyclable organics, including various food wraps and containers. This situation has renewed societal and regulatory interest in biodegradable packaging materials and manufacturers are responding with ‘environmentally friendly’ products, some of them controversial (Donnelly, 1990; Thayer, 1990). Earlier work on plastics typically focused on biodeterioration rather than on biodegradability (Aminabhavi et al., 1990). Even when biodegradation was the focus of the work, the older test approaches that measured growth yields in pure cultures (Stahl *Author for correspondence.
and Pessen, 1953) are now obsolete. More recent 14C02 liberation studies (Albertsson et al., 1978; Albertsson and Banhidi, 1980) preferentially used pure cultures but also soil. The availability of labeled material restricted these studies to polyethylene only. Against the described background, we compared the biodegradability of 12 polymeric film samples and 4 traditional or composite packaging materials in soil, under conditions generally favorable for biodegradation. To ensure complete objectivity for the study, we received from our sponsor all polymeric films with letter codes only. The manufacturer and the composition of the test materials was not revealed or discussed with us until the study was completed and our final report submitted to the sponsor. To avoid excessive length, detailed time-course data are presented only for four polymeric films. The similar fate of the other test compounds are shown in a summary form only. EXPERIMENTAL Characterization
of the test materials
IR-spectra of the polymeric films were compared with published spectra (Haslam and Willis, 1965; Sadtler Research Laboratories, 1980). In addition, the Beilstein test for organochlorine (Hollernan and Richter, 1957) and the solubility characteristics of the films aided qualitative characterization. Plasticizers
1469
1470
ASHA YABANNAVAR
and
were qualitatively and quantitatively dete~ined by GC measurements on methanol extracts. Retention times and peak areas were compared to an authentic dioctyl adipate (DOA) and dioctyl phthalate (DOP) standard (Aldrich, Milwaukee, Wis.). Epoxidized soybean oil (ESO), an additive to some PVC films, was not sufficiently volatile to be determined by gas chromatography. Its amount was calculated by subtracting from the total plastic film weight the weight of precipitable PVC plus the weight of DOA, as determined by GC measurements. The weight difference was assumed to be ESO. The hydrocarbon and fiber content of the two impregnated paper products was determined by Soxhlet-extracting 1 g material, using hexane. The solvent was evaporated in a tared flask at 60°C under vacuum in a Rotavap apparatus. The hydrocarbon residue and the fibre remaining in the Soxhlet thimble were weighed and the sum of these weights was compared to the starting weight of the product. The composition of the PE-coated cardboard products was dete~ined using a similar approach, extracting 1 g of the cardboard samples with hot 1,2,4-trichlorobenzene (TCB) and weighing the residual cardboard. The PE resin was precipitated from the TCB solution by diluting with ethanol and cooling the solution. The precipitate was filtered, dried and weighed. Again, the sum of the cardboard and PE resin weights were compared to the starting weight of the product. Total C, H and ash of all samples was determined by combustion followed by sequential absorptive removal of the combustion products, while measuring changes in the~oconductivity. The latter analyses were made by Midwest Microlab, Indianapolis, Ind. Exposure conditions and CO, evolution measurements Nixon sandy loam (fine, loamy, mixed, me&g, typic hapludolt), freshly collected from College Farm, New Brunswick, N.J. was used in all biodegradation studies. In terms of texture, this soil is composed of 50% sand, 21% silt and 29% clay. Its organic matter content is 5% and its pH is 5.5-6.5 (Bartha and Bordeleau, 1969). Its moisture content and water holding capacity were determined and used in adjusting the water content of the soil for the biodegradation studies to 50% of capacity. This moisture content is considered ideal for aerobic biodegradation processes in soil (Pramer and Bartha, 1972) and was maintained throughout the incubation by weighing the samples at time 0 and adjusting with distilled water for any weight loss during incubation. The pH of the soil was adjusted by the addition of CaCO, (liming) to 7.5. This pH was found to be optimal for hydrocarbon biodegradation (Dibble and Bartha, 1979) and it was assumed that it would also favor the biodegradation of plastic materials. The soil used in CO2 evolution tests were limed 5 days prior to the start of the experiment to avoid measuring any
RICHARD BARTHA
CO2 released by neutralization as part of the biodegradation process. Per gram test material, 38 mg of (NH.,)2HP03 was added as fertilizer (approx. C:N ratio 1OO:l) in order to provide sufficient N and P for the enrichment of a polymerdegrading microbial population. For easier distribution and incorporation in soil, for Biometer and residual weight measurements the film materials were shredded in a Waring blender. The other materials were cut into ca 3 x 2 mm pieces using scissors. From all materials, 250mg portions were mixed with 25 g dry wt of soil (1% w/w). From the food carton (FC), single pieces of exactly 250 mg were also incorporated into soil to test the effect of cutting up on degradation rates. Water and dissolved fertilizer were added. Biometer flasks for CO, evolution measurement (Bartha and Pramer, 1965) and foil-covered 250 ml beakers for residual weight measurements were kept at 27°C. Both types of samples were aerated every 2-3 days. Poisoned soil controls contained 1% (w/w) HgCl,. Tensile strength &terminations For tensile strength measurements, 15 strips each (2.5 x 20 cm) of the polymeric films were incubated in a bin containing 25 kg of soil. All conditions (liming, fertilization, aeration, moisture and temperature) were identical to the ones described earlier. After 3 months, the strips were retrieved, washed with water and mild detergent, dried and subjected to tensile strength tests as specified by the American Society for Testing Materials (ASTM, 1983). Force and work needed to break the strips were compared to five non-exposed control strips of each material. Residual weight analysis Retrieval and weighing of thin plastic films after soil exposure is inherently inaccurate, since brittleness may cause incomplete recovery; encrustation and oxidation add weight. Therefore, we attempted to develop analytical techniques for plastic films similar to those used in pesticide and hydrocarbon residue analysis. At the end of an exposure period, the soil was air-dried at room temperature and subsequently solvent-extracted. After considerable experimentation with solvents and extraction techniques, the following two procedures were adopted. Residues of the two PVC films were Soxhlet-extracted for 6 h (at least 100 cycles) using 100 ml of methyl-ethyl ketone (MEK). The extract was split 9O:lO for residual weight measurement and gas chromatography, respectively. For residual weight, MEK was evaporated at 50°C and under vacuum in a tared extraction flask using a Rotavap evaporator. After 24 h in a vacuum desiccator with Drierite and paraffin chips, the flasks were re-weighed on an analytical balance. For reuse, the flasks needed to be cleaned using alkaline permanganate. PE and PP residues were extracted from soil contained in a Soxhlet thimble using six aliquots of
Biodegradability of some food packaging materials in soil boiling TCB. The thimble itself was kept at boiling temperature in a beaker placed upon a hot plate. A total volume of 125 ml TCB was used. Since gas chromatography detected no plasticizer in these samples, the extract was not split but mixed with an equal volume of methanol and cooled. The precipitated plastic material was filtered through a tared Whatman No. 114 analytical filter. The filter was dried for 10 min at 50°C and re-weighed. No satisfactory solvent was found for the irradiated polyolefin (PO) sample and, therefore, no residual weight was determined for this material.
were the tensile strength determinations made at the end of the experiments. CC& evolution measurement by the Biometer method is a highly consistent technique. Sharabi and Bartha (1993) have shown that the cumulative CO, evolution of replicate samples, even in long-term experiments, deviate ~5% from the mean. In the experiments on packaging materials, we interpreted net CO* evolution from additives as significant if it consistently exceeded the cumulative CO, evolution of the unamended soil control by >5%. RESULTS
Gas chromatographic analysis
Plasticizer in the PVC samples was measured after adding an equal volume of cold methanol to the MEK extract. The precipitated plastic fiber was removed by filtration and in some cases its weight was determined after drying. Of the clarified filtrate, 1~1 vol were injected into a Hewlett-Packard model 5890 instrument fitted with a 10 m x 0.53 mm macrobore capillary column (RSL 150, 1.2pm thick poly dimethyl-siloxane bonded liquid phase). Carrier flow was 10 ml N, min-‘. Temperatures: oven 200°C isothermal, injector 250°C detector 300°C. DOA standards were used for the identification and quantification of the plasticizer peaks. Viscosimetric and chloride determinations
Viscosity was measured on the solvent extracts of the plastic-amended soil samples as follows. The original extracting solvent (MEK) was evaporated under reduced pressure and the residue was dissolved in cyclohexane (half of the original volume). The viscosity of this cyclohexane solution was measured according to the ASTM (1972) and was expressed as ‘kinematic viscosity’. Such measurements were made on extracts of plastic amended soil samples at 0 time and after 5 weeks. Chloride and viscosity were measured on PVC-l and -2, since CO* evolution and residual weight measurements indicated substantial degradation of these two materials. Chloride was determined according to Bergman and Sanik (1957), on soil blanks (25 g dry wt) and on 25 g soil + 250 mg plastic foil at time 0 and after 5 weeks exposure. Quality control
Because of the volume of samples, it was not feasible to replicate all data points. As the extraction and residual weight measurement of plastic residues from soil were techniques newly developed for our study, the time 0 recoveries were evaluated on triplicate samples for every material. These recoveries and their standard deviation were used as confidence limits for the subsequent time-course determinations. Even without replication, in time-course experiments aberrant readings are easily spotted when they deviate from the regression curve, and in such cases the determinations were repeated. Statistically controlled
1471
Composition of the test materials
IR spectra and solubility characteristics identified one of the polymeric films as polyethylene (PE) and two as polypropylene (PP), respectively. Irradiation treatment of polyolefins (PO) induces cross-linking and decreases solubility, preventing the extraction of this polymer from soil. None of these films contained plasticizer and they were negative for organochlorine. All other films gave a positive Beilstein test for organochlorine; their IR spectra and solubility characteristics identified them as polyvinyl chloride (PVC) polymers. Gas chromatography of their methanol extracts revealed the presence of DOA in most of the plasticized PVC samples; PVC-8 contained DOP. DOA or DOP did not account for all methanol-extractable material. A standard for the missing ingredient, epoxidized soybean oil (ESO) was provided by the manufacturer but, because of its low volatility, we determined this ingredient in the films negatively, i.e. as the film weight that was neither PVC resin nor DOA. The compositions of the tested polymeric films are summarized in Table 1. Our analyses agreed within 1% with the manufacturer’s specifications, made available to us after the conclusion of our study. The separately determined wax plus fiber and PE plus cardboard weights of the WP, BW and OC samples added up to within 99-100% of the starting weights of the products. Only a 96% weight recovery for the FC material was caused by coloring inks and dyes that stayed in the solvent solution when PE was precipitated. The good weight agreements served as quality control for our data on these materials, since manufacturer’s specifications for them were not available to us. CO, evolution The PE and PP-2 films, added to soil at 1% (w/w), did not increase CO, production as compared to unamended control soil during a 3 month exposure (Fig. 1). In the same period, the two PVC films released 2.74 and 1.8 mmol of CO2 above soil background for PVC-l and -2, respectively. The COZ release corresponded to 27.3 and 19.4% of the total organic C of the materials. The impregnated paper products and the food carton released between 76
ASHAYABANNAVARand RICHARLI BARTHA
1412
Table I. Codes, descriptions and compositions of test materials and the summary of degradative changes they underwent during 3 months of aerobic exposure in soil*
Code
Sample composition (%I
Same description
PO PP- I PP-2 PE PVC-I PVC-2 PVC-3 PVC-4 PVC-5 PVC-6 PVC-7 PVC-8 WP BW FC oc
Packaging filmt Packaging filmt Packaging fibnt Packaging film? Produce/mushroom filmf Bakery tilmt Meat-poultry fihnt Meat-poultry filmf Meat-poultry filmi Shrink tilmf: Shrink filmf DOP-plasticized film Wax papert Bread wrapt Food cartont Frozen juice container?
Irradiated polyolefin, 100 PP. 100 PP, 100 PE, 100 PVC, 69.5; WA 23.6; ESO, 6.9 PVC, 76.5; DOA, 16.0; ESO, 7.5 PVC, 70.0; DOA, 19.2; ESO, 10.8 PVC, 72.0; DOA, 22.0; ESO, 6.0 PVC, 70.0; DOA, 13.6; ESO, 16.0 PVC, 84.0; DOA, 2.0; ESO, 14.0 PVC, 70.5; WA, 8.6; ESO, 16.0 PVC, 68.5; DOP, 24.8; ESO, 7.0 Fiber, 75; wax, 25 Fiber and filler, 68.0; wax, 32.0 Cardboard, 96.0; PE, 4.0 Cardboard, 95; PE, 5.0
Sample carbon (mg)
Conversion to co, (“/)
Weight loss (%I
Loss of elongation w
211.2 215.0 214.5 214.7 120.5 116.5 122.7 121.4 123.4 108.3 109.9 123.1 126.0 126.2 104.7 110.2
1.5 ND ND
NA ND ND ND 26.8 23.2 38.4 27.2 18.4 14.8 9.6 20.8 NA NA NA NA
-33.0 ND ND ND -94.5 -92.6 -94.0 -94.0 -92.0 -67.0 ND -15.0 NA NA NA NA
K 19.4 25.4 25.7 18.8 7.3 5.9 9.7 78.6 76.2 81.9 64.2
*All samples were applied at 250 mg in 25 g soil. Abbreviations: NA: not analyzed; ND: not de&table; PO: polyolefin; PP: polypropylene; PE: polyethylene; PVC: polyvinyl chloride; DOA: dioctyl adipate; DOP: dioctyl phthalate. TManufacturer not known to us. SManufactured by Borden Resinite Co., Andover, Mass., U.S.A.
and 82% of their carbon as CO, (Fig. 2). The thicker-walled orange juice container released 64% of its carbon as COZ. Net CO, production rates were initially high and gradually declined as the biodegradable and physically accessible carbon neared exhaustion. Since for convenient and reproducible applications the test materials were cut up or shredded, the question arose how this mechanical procedure affected the rate and extent of biodegradation. As shown for the food carton in Fig. 3, cutting up increased the rate of CO* evolution only slightly and the extent of degradation was not influenced significantly.
(O-time) recoveries were performed in triplicate. The average recoveries and standard deviations are shown in Table 2. We assumed similar confidence limits for the time-course experiment (Table 3) that involved the analysis of single samples per time point. The standard deviations ranged from +2.7 to + 11 mg. Conservatively, we judged weight losses as significant if recovery declined more than 11 mg below the initial recovery. By this measure, the PE and PP-2 films did not decline significantly in residual weight, but both PVC samples did. The fact that the latter films did not loose significant weight in metabolically inactive (poisoned) soil pointed to a biodegradative mechanism of the weight losses.
Residual weight analysis
Plasticizer
These measurements were made on the polymeric films only. To establish confidence limits, the initial
The GC measurements of DOA plasticizer recovery with time are shown in Table 4. 6n a-TErage, > 50%
7 15 2329
’ ’
5.5
’ ’
37
’
50
64
76
loss
92
PVC-I
#
2.5-
f
1.5-
PVC-2
G lj
0.5-1
RP-2 PF ._ TIME (DAYS)
Fig. 1. Net CO, evolution with time from 250 mg amounts of polymeric film samples in 25 g soil, measured in Biometer flasks. PVC-l and -2: plasticized polyvinyl chloride films. PE and PP-2 designate polyethylene and polypropylene films, respectively. The numbers above the curves designate the days the readings were taken.
Fig. 2. Net CO, evolution with time from 250 mg amounts of impregnated paper and plasticized cardboard food packaging materials in 25 g soil. BW, bread wrap; WP, wax paper; FC, food carton; OC, orange juice (frozen concentrate) container. The numbers above the curves designate the days the readings were taken.
1473
Biodegradability of some food packaging materials in soil 7
15 23
37 ”
8,“’
49
63
77
91 I
Table 4. Gas chromatographic analysis of dioctyl adipate plasticizer loss during the exposure of PVC films in soil
F.C.cutUP
Recovery (mg) with incubation time (months)
F.C.uncut
Material PVC-1 PVC-2
Plasticizer
0
I
2
3
DOA WA
59.2 40.1
22.9 12.7
20.5 16.0
21.5 13.5
occurred in case of the films PVC-I and -2. The PP-2 that showed no CO* evolution and weight also failed to decline in tensile strength. Film elongated without breaking under the standard conditions. 0
so TIMEE*YS~
30
I20
Fig. 3. Net CO, evolution with time from 250 mg amounts of food carton (FC) material in 25 g soil. FC was either cut into ca 3 x 2 mm pieces or was left uncut as a single piece of plasticized cardboard. The numbers above the curves designate the days the readings were taken.
of the DOA plasticizer was lost during the first month of exposure to soil with little additional change thereafter. Even after 3 months, a substantial amount of the plasticizer remained undegraded. Since free DOA spikes degraded in the same soil rapidly (results not shown), we conclude that the remaining plasticizer was physically inaccessible for biodegradation, being trapped within the polymer network. DOP of PVC-8 was degraded at a somewhat slower rate than the DOA plasticizer but the extents of their biodegradation were similar (data not shown). ES0 was not measured directly, but was calculated, in some cases, from the total weight minus plasticizer and polymer precipitate. We assume that ES0 biodegradation contributed to weight loss and CO, evolution and most likely behaved in a similar manner as the plasticizer in the extent of its biodegradation. Tensile strength tests
Changes in tensile strength are summarized in Table 5. A strong decrease in elongation at break Table 2. Time 0 recoveries of 250 mg amounts of polymeric film from 25 g triplicate soil samples Description
Average recovery Code mg*lSD
Packaging film Packaging film Produce-mushroom Bakerv film
PE PP-2 PVC-I PVC-2
film
218 f 226 * 246+ 237 +
Recovery (%)
2.7 9.0 1.2 10.9
87.2 90.0 98.4 94.8
Chloride determinations ments on polymericJilms
measure-
Table 5. Changes in tensile strength of polymeric films during 3 months burial in soil Eloneation at break (%j* Material
Table 3. Decrease in residual weight of 250 mg amounts of polymeric films during incubation in soil Recovery (mg) with incubation time (months) PE PP-2 PVC-1 PVC-2
and viscosimetric PVC-l and -2
The observed CO2 evolution, residual weight and plasticizer losses of PVC-l and -2 were at rates that left some question whether they originated from their DOA and ES0 components alone, or whether it represented also some breakdown of the PVC resin itself. To clarify whether and to what extent the PVC polymer itself was affected, we determined the release of chloride. If PVC was mineralized, inorganic chloride would be released. PVC polymer biodegradation would be evident also by a decrease in viscosity of the solvent extract as large PVC molecules were converted to smaller ones. Chloride readings were consistently higher in PVCspiked soil after 5 weeks of exposure than at time 0, but the differences were extremely small (data not shown). Less than 0.1 mg Cl was released per flask. This would account for the ultimate biodegradation of 0.2 mg PVC only, or < 0.1% of the PVC resin in each flask. Therefore, Cl determinations did not indicate a significant breakdown of PVC resin. As a positive control for our measurement technique, we treated similar soil samples with 25 mg chloracetic acid (Na-salt), and determined Cl immediately after treatment and after 1 week of exposure. Subtracting the 0 time determination from the 1 week determination, it was calculated that 9.07 mg (36.3%) of the 25 mg chloroacetic acid was mineralized during 1 week. We would have clearly detected any significant chloride release from the PVC resin that was added to the soil samples at a IO-fold higher concentration than the chloroacetic acid.
SD: standard deviation.
Material
0
1
2
3
3PC
218 226 246 237
215 236 230 225
212 256 194 187
213 225 179 205
209 229 261 263
PC: poisoned control (1% HgCI,).
film loss PE test
PE PP-2 PVC-1 PVC-2
Controlt +
I SD
No break 49.2 6.4 267.3 20.3 233.1 39.8
Exoosedf + 1 SD No Break 46.9 7.3 15.0 O.O§ 15.0 0.05
*ASTM (1983). TAverage of five test strips. SAverage of 15 test strips. @Alltest strips broke before the lowest elongation (15%) the instrument was able to record. SD: standard deviation.
ASHAYABANNAVAR and RICHARD BARTHA
1474
Table 6. Viscosity of solvent extracts of soil amended with polymeric film samples, at time 0 and after 5 weeks of incubation Kinematic viscosity* Material
Control
Deviationt
Exposed
Deviation?
PVC- 1 PVC-2
2.881 2.891
0.091 0.074
2.947 2.948
0.138 0.073
*ASTM (1972). tDeviation ( f ) from the mean of duplicate measurements.
Viscosity was measured after two separate exposures. The results were averaged in Table 6 and the deviation from the means are indicated. No significant changes occurred in viscosity during soil exposure, and these measurements did not indicate polymer breakdown. DOA and ESO, when added to solvent, influenced viscosity marginally (results not shown). Therefore, we consider it unlikely that DOA and ES0 disappearance influenced our measurements. Both the Cl release and the viscosimetric tests indicated that there had been little, if any, degradation of the PVC resin. DISCUSSION
Test method development and definition of biodegradation for polymeric jilms
In tests on the environmental fate of polymeric materials, until very recently, the emphasis has been on the prevention of biodeterioration (Bessems, 1988). The methodology developed for this purpose was less than satisfactory when questions shifted to the rate and extent of biodegradation of packing materials that are typically used in the form of thin sheets, may consist of several component and are exposed to the complex physicochemical and biochemical actions of the soil environment. For this reason, our aim in this study, beyond a comparison of biodegradability of several packing materials, was the development of a test methodology package that would answer questions about biodegradability of novel polymeric products in a more satisfactory manner. From the classic polymer biodeterioration tests, the most common ones are loss of tensile strength and loss of residual weight (Wendt et al., 1970). For comparison purposes, we used the tensile strength test in its classic form (ASTM, 1983), but we modified residual weight measurement procedures to make them more accurate and reproducible. We applied solvent extraction techniques common to pesticide and hydrocarbon residue analysis and, for the materials tested, found these extraction techniques very satisfactory. We found CO, evolution measurements in Biometer flasks (Bartha and Pramer, 1965), used as a standard FDA procedure for biodegradability testing (USFDA, 1987) applicable and highly predictive of packaging material biodegradability in soil. When applied to composite materials both residual weight and CO2 evolution procedures have the common limitation that in case of partial biodegradation, they
do not indicate which component of the composite was biodegraded and which persisted. To answer such questions, auxiliary techniques such as G.C., viscosimetry and Cl release need to be applied. A generally accepted terminology pertaining to the environmental degradation of polymeric films has yet to be developed. For lack of standards, much controversy surrounded some ‘environmentally friendly’ products that were advertised as biodegradable (Donnelly, 1990; Thayer, 1990). In these instances, the generally-accepted criteria of biodeterioration, such as loss of tensile strength and microbial encrustation of surfaces were equated with biodegradation. An ASTM study group currently seeks to define ‘biodegradation’ as the biochemical breakdown of the polymer resin molecules to smaller size, as documented by gel permeation chromatography measurements (Ramini Narayan, pers. commun.). This restrictive definition would effectively rule out misleading claims that equate additive degradation with degradation of the whole polymeric product. On the other hand, it is definitely awkward to call a material like PVC-l ‘biodeteriorable’, when close to 30% of its total C was converted to CO, and the material was formulated to undergo rapid albeit only partial biodegradation. Comparative persistence of polymeric films, impregnated paper and plasticized cardboard containers in soil.
Our findings are generally consistent with earlier reports on synthetic polymer behavior in soil (Aminabhavi et al., 1990). By both conventional and newly-developed test methods, polyolefin (PE and PP) polymers were much more resistant to biodeterioration than plasticized PVC. Not surprisingly, impregnated paper and plasticized cardboard was, in turn, more biodegradable than plasticized PVC. However, compared to earlier studies, our measurements provide a more quantitative and detailed understanding of the test material behavior in soil. Table 1 compares the C content of each sample to the net CO2 evolved during the 3 month test period. Percentage weight losses are also shown in this table. A lack of net CO2 evolution or of significant weight loss by the PE and PP samples was consistent with their tensile strength behavior. The plasticized PVC films underwent substantial conversion to CO, and showed losses in tensile strength and in residual weight, but the losses did not exceed the additive contents of these compounds. Moreover, viscosimetric and Cl release measurements ruled out a significant degradation of the PVC resin. Percentage weight loss and percentage C conversion to CO, showed definite correlations. Weight losses ranged from equal to twice as high when compared with C conversion to CO2. Some substrate C can be expected to be sequestered temporarily in microbial biomass and soil organic matter, but this is compensated for by a priming of soil organic matter
Biode~adability
1475
of some food packaging materials in soil
degradation (Sharabi and Bartha, 1993). As a consequence, Xl-100% of the degraded substrate carbon registers as net CO;, evolution in Biometer tests a
month or longer in duration. Although the solventextraction based residual weights are more reliable than manual retrieval and weighing, residual-weight m~s~ements showed a higher variability than CO, evolution measurements in Biometer flasks. Carbon conversions to CO1 between 18.8 and 27.3% (PVC-l-5) were accompanied by over 90% reduction in elongation at break but PVC-8 changed little and PVC-7 atypically increased its elongation at break. Thus tensile strength measurements alone do not reliably predict biodegradative changes. Not surprisingly, impregnated paper products were completely biodegraded as shown by the extensive conversion of their C to CO*. Net CO2 evolution also indicated extensive biodegradation of the plasticized cardboard materials. When adjustment is made for the 45% nonbiodegradable PE component, 90.2 and 70.7% of the cardboard C was converted to CO, in case of the FC and OC samples, respectively, instead of the somewhat lower total C-based conversion percentages shown in Table 1. Apparently, PE coating does not prevent the extensive biodegradation of cardboard containers. The use of plasticized composites as food containers, where mechanical strength is provided by biodegradable cardboard and the synthetic polymer is used only in low (45%) proportion to provide a hydrophobic barrier, may not be the ‘ultimate’ en~ronmentai solution. Nevertheless, as compared to all-plastic containers, in a composting situation such composites would reduce the non-biodegradable container material by a factor of 20. Acknowledgements-The work described in this New Jersey Agricultural Experiment Station Publication No. D-0150801-92 was performed with the support of Borden Resinite Inc. We are indebted to S. Gilbert, Professor Emeritus, Rutgers University, for introducing us to the biodegradation problems of food packaging materials. The work was also supported by New Jersey State funds.
liberation of 14C0, by molds like Fusarium redolens from “C-laheled pulverized high-density polyethylene. Journal of Appiied Polymer Science 22, 3435-3447.
Aminabha~ T. M., Bahmdgi R. H. and Cassidy P. E. (1990) A review on biodegradable plastics. Polymer-Plastic Technology and Engineering 29, 235-262.
ASTM (American Society for Testing Materials) (1972) Standard method of test for dilute solution viscosity of vinyl chloride polymers. D 1243-66. In Annual Book of ASTM Star&r& Vol. 08 01. ASTM, Pa. ASTM (American Society for Testing Materials) (1983) Standard test methods for tensile properties of thin plastic sheeting. D 882-83. In Annual Book of ASTM Standards, Vol. 09 01. ASTM, Pa. Bartha R. and Bordeleau L. M. (1969) Cell-free peroxidases in soil. Soil Biology & Biochemistry 1, 1399143. Bartha R. and Pramer D. (1965) Features of a flask and method for measuring the persistence and biological effects of pesticides in soil. Soil Science 100, 68-70. Bergman J. C. and Sanik J. Jr (1957) Determination of trace amounts of chlorine in naphtha. Analytical Chemistry 29, 241-243.
Bessems E. J. (1988) The biodeterioration of plasticized PVC and its prevention. JournaI of Vinyl Tec~ology 10, 3-6.
Dibble J. T. and Bartha R. (1979) The effect of environmental parameters on the biodegradation of oil sludge. Applied and Environmental
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