Degradation of starch modified polyethylene bags in a compost field study

Polymer Degradation and Stability 39 (1993) 251-259

Degradation of starch modified polyethylene bags in a compost field study Hebe B. Greizerstein, Joseph A. Syracuse & Paul J. Kostyniak Toxicology Research Center, State University of New York, Buffalo, NY, USA (Received 16 August 1991; revised version received and accepted 27 January 1992)

The degradation of commercially available leaf bags made of polyethylene compounded with ECOSTAR PLUS®, a starch based additive, was determined after exposure to a passive composting environment during the summer of 1990. The bags were filled with partially composted leaves and yard waste. Half of the bags were buried inside and half on the surface of the pile. One bag from each group plus a control was removed at 14, 28, 42 and 49 days of exposure. The tensile properties, FTIR and UV-vis spectra, differential scanning calorimetry and light microscopy determinations were performed on these samples. The surface bags showed oxidation, loss of starch, lower melting point and embrittleness after 14 days. The buried bags had lower melting points at 14 days and showed oxidation at 28 and 49 days. These results indicate that exposure to daylight had a marked effect in the degradation of the surface bags while the degradation of buried bags may involve other processes.

as starch to facilitate the decomposition of polyethylene was introduced by Griffin in 1973.11 The present report evaluates the performance of degradable compost or leaf bags made of polyethylene compounded with E C O S T A R PLUS ®, a starch based additive, exposed in a passive composting operation for a period of two months. A secondary aim of this study is to standardize the methodology for future research into the characterization of the type and mechanisms of degradation of plastic under controlled environmental conditions. The measured parameters include changes in the polymer such as oxidation and unsaturation of hydrocarbon chains; in the composite structure such as appearance, mechanical and thermal properties; and in the additives such as amount of starch remaining in the plastic.

INTRODUCTION Degradable plastics are materials designed to be broken into smaller components or to disintegrate and eventually be converted to non harmful substances after predetermined periods of time and under average environmental conditions. Polyethylene is resistant to attack by microorganisms or by chemical means other than photo-oxidation in most naturally occurring environments. 1"2 In order to increase its degradation a number of different approaches are being used such as copolymerization with ketone containing materials, compounding with metal salts, starch, and other additives. 3,4 Long term studies on the biodegradation of 14C-polyethylene films show less than 0-2% (by weight) evolution of CO2. ~-s The effect of additives such as UV-sensitizer 9 and Ndotriacontane 1° have shown a five-fold increase and an initial increase followed by a slow down, respectively, in the conversion to CO2. The addition of a biodegradable additive such

METHODS This study was carried out on a compost pile maintained by the township of Amherst, New York. The pile was initiated during the month of October 1989. Physical indications, i.e. unifor-

Polymer Degradation and Stability 0141-3910/92/$05.00 © 1992 Elsevier Science Publishers Ltd. 251

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Hebe B. Greizerstein, Joseph A. Syracuse, Paul J. Kostyniak

mity of color, temperature changes, odor, indicates that this was a mature compost pile. Our access to the compost pile was restricted to provide minimal disruption to the operations of the township personnel. The leaf bags made of polyethylene compounded with E C O S T A R PLUS ® additive were translucent and cream colored. The thickness of the plastic was 2-1 mil (53-3/~m) and the size of the starch grains was 6-4 + 3.0/~m. The bags were filled with material from the compost pile and placed in the pile during the months of July-August of 1990. Half of the bags were placed approximately 2 - 3 f t below the surface (buried group) and the other half on top of the pile (surface group). One bag from each group was removed at 14 days, 28 days, 42 days and 49 days of exposure and tested for chemical and physical characteristics. The temperature of the compost pile measured at a depth of 3 ft ranged between 130°F and 135°F. Each sample bag was split in two, one half was kept for future reference, the other half was washed with distilled water, dried in a hood and cut in small pieces for testing. The washed samples were tested for tensile strength and elongation at break. The changes in chemical composition were assessed by infrared (F-FIR) and UV-vis spectroscopy and light microscopy. The thermal characteristics of the films were determined by differential scanning calorimetry (DSC).

equipped with a MCT detector. The spectral region between 4000 and 400 cm -1 was scanned at 2 wavenumber resolution and the average spectrum of 100 scans was used for the quantitative measurements. The area of the absorbance bands at 1894 c m -1, 1745 cm -1, 1715 cm -1, and 988cm -1 were used to monitor relevant chemical changes in the plastic.

Thermal analysis by DSC A Perkin-Elmer DSC 4 with nitrogen as the purge gas and calibrated using the melting transition of pure indium at 156.6°C was used for all the determinations. Small pieces of the films, approximately 8-10 mg, were heated at 10°C/min from 25°C to 210°C (heat 1), left 5 min at 210°C and then cooled at 10°C/min to 25°C (cool run). After 5 min the samples were heated again (heat 2) under the same conditions as heat 1. The heat of fusion, onset and peak temperatures of the first order transitions were recorded.

UV-vis absorbance The UV-visible spectra were obtained using a Varian DMS-90 spectrophotometer scanning in the 200-400nm range at 100nm/min. The samples consisted of pieces of film at least 25 mm x 9 mm in size.

Tensile strength Secondary emission electron microscopy Representative samples of control and experimental bags, approximately 2 mm x 5 mm in size, were mounted on the sampling discs and grounded with a graphite paste. These discs were coated with gold using an ISI Sputter Coater. Static was reduced within the chamber prior to coating by flushing several times with Argon gas. The plastic samples were scanned with a Hitachi model S-450 Electron Microscope using secondary electron emission detection. The fields were photographed using a Polaroid camera and Polaroid 550 high contrast film. The angle of observation was 70 degrees with a magnification of 200x and lO00x.

Infrared spectroscopy The transmission infrared spectra of the plastic samples were obtained in a 20DX Nicolet FTIR

An Instron model 1125 Universal Testing Instrument was used to determine tensile strength and elongation at break. The samples consisted of strips 110 mm long and 30 mm wide. The initial grip separation was 75 mm and the strain rate 1 in/min.

Starch granule determinations Square pieces of the plastic (1 cm x 1 cm) were cut from several random locations and treated with Gram's iodine solution for 2 days. Half of the sample was mounted on a microscope slide and the coverslip sealed with clear nail polish. The purple granules within the counting reticle of twenty five fields were counted for each of three sample slides using a Reichert Microstar IV Compound Microscope and 400x magnification. The Grams's iodine solution was prepared by dissolving 2 g of potassium iodide in 300 ml of

Degradation of starch modified polyethylene bags in compost

253

distilled water followed by the addition of I g of iodine crystals. The well mixed solution was stored in a dark sealed bottle until added to the plastic. Ten controls were counted to establish the variability of the starch content measurements. Samples from buried and surface bags exposed for 14 days and 49 days were analyzed by this technique.

RESULTS The plastic bags exposed on the surface of the compost pile broke down into small pieces while the buried bags remained intact after 49 days of exposure in the compost pile. Deterioration of the surface of the plastic was revealed by the appearance of holes observed by electron microscopy after day 14. The extent of damage was similar in surface and buried plastic bags throughout the 49 days of observation. Figure 1 shows the photograph of a plastic control and Fig. 2 that of a buried bag at 42 days of exposure. The last one represents the characteristic effects observed in the exposed plastic independent of the site of exposure. Bags removed from the pile, unwashed, were examined visually and by

Fig. 2. Surface of plastic from buried bags at 42 days of exposure.

electron microscopy. Although surface particulate contamination was observed, no microbial culture measurements were performed to determine whether there was microbial growth.

Infrared spectroscopy

Fig. 1. Surface of plastic from control bags.

The infrared absorbance spectra of the fingerprint region (1800-450cm -1) of control, buried and surface bags exposed for 49 days are shown in Fig. 3. The 1745 band present in the control is assigned to an additive (ester carbonyl) and the 900-1100 region to the starch. The ratio of bands 1368/909 cm-1 have been used in infrared studies of polyethylene to control for variations in crystallinity and thickness of the samples. 12 These bands could not be used in our studies because of interferences. The 909 band was overshadowed by the 988 band due to the COC band of the starch additive and the area of the 1368 cm -~ band (assigned to CH2 wag in the chain) showed random variations among all groups. Therefore the band at 1894cm -1, which has been assigned to the crystalline phase on polyethylene films, ~3 was used instead to normalize against differences in film thickness.

Hebe B. Greizerstein, Joseph A. Syracuse, Paul J. Kostyniak

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Ratios against the 1894 cm -~ band were given for the bands assigned to carbonyl ester at 1745, to carbonyl formation by oxidation at 1715, and the COC stretch assigned to ether linkages in the starch additive. The appearance of bands in the 18001700cm -1 region, the characteristic area for carbonyl absorptions, is an indication that oxidation in the polyethylene chain is taking place. A band at 1715 cm -~ was observed in the spectra of surface bags after 14 days of exposure. The intensity of this band increased at 28 days and kept constant for the duration of the study. In the buried bags this band was small and appeared only at 28 days and 49 days of exposure (Fig. 4). The carbonyl band at 1745 cm -~ assigned to an

ester additive, decreases with time in both buried and surface bags (Fig. 5). The strong C---O C stretch band at 988 cm -1, assigned to the starch, decreases with time in both buried and surface bags (Fig. 6). The variability of the data was great and no significant differences were detected among the three groups by analyses of variance. The loss of starch was also measured by light microscopy. The number of starch granules per measured field in the buried and surface samples after 49 days of exposure was significantly lower (p < 0.05) than in controls (Fig. 7). This decrease was more pronounced in the buried group. To insure that the iodine aqueous solution reacts with starch granules in the film and not with only the surface granules, a series of duplicate samples were also stained by using iodine crystals. The results were not different

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between the two methods. Examination of the stained samples by light microscopy and polarized light microscopy showed that all starch granules were stained regardless of their position in the film or the form of the reagent used. Thermal analysis Three thermal curves were obtained during the thermal analysis by differential scanning calorimetry. The results of the first determination, or heat 1, reflect the thermal history or characteristics of the film resulting from its exposure to different temperatures prior to testing. In the case of controls the melting curve was the result of the conditions used during the manufacturing process, while in the surface and buried bags go-

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there is the additional effect of the elevated temperatures reached in the compost pile. The second determination, after controlled annealing conditions, compares the characteristics of the films themselves. The thermal curves of control, buried and surface exposed films at 42 days are illustrated in Fig. 8. Two first order thermal transitions were observed, a major peak at 125°C and a smaller at 111°C. These two peaks relate to crystals of different thermodynamic stability causing different melting points. ~4 The general shape of the melting curve of the surface bags changed with time of exposure. The melting peak at 111°C decreased in intensity and after 49 days it was barely discernible. The melting point and the heat of fusion of the major peak were not significantly different among the three groups. There is a tendency to lower melting points in the experimental bags as time of exposure increases (Fig. 9). Tensile strength

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At 28 days of exposure the bags placed on the top of the compost pile had disintegrated into small pieces. These fragments were too small to allow testing of tensile properties, at 28 days only one piece of plastic was found of the appropriate size to be tested and none was found after 42 days and 49 days. The percent elongation at

256

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DISCUSSION break at day 14 was significantly lower than controls. The bags buried inside the compost pile remained in one piece throughout the experiment. The percent elongation at break and the ultimate tensile strength of buried samples exposed for up to 49 days were not significantly different than controls (Fig. 10). UV-vis spectrometry The UV-vis spectrum of a control bag (Fig. 11) shows the presence of an absorbance band associated with a diene structure at 245 nm. This band was not present in any of the treated bags from day 14 to day 49. No other significant changes were detected.

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Polyethylene leaf bags compounded with ECOSTAR PLUS ® additive and exposed to a natural environment degraded at different rates according to their location. The plastic film of bags placed on the surface of the compost pile showed oxidation, loss of starch, lower melting point, and embrittleness after 14 days of exposure. These bags disintegrated into small pieces by day 28. The same plastic bags buried inside the compost pile experienced small changes, namely loss of starch, oxidation and a decrease in the melting point but they remained intact for the 49 days of duration of the study. The data of this report is in agreement with previously reported effects of natural weathering on different grades of polyethylene with and without photoinitiators. 15'16 The observed differences in the degradation between surface and buried bags must be related to the environment surrounding the samples since all have the same chemical composition. The factors involved in both types of exposure, surface and buried, were different. Surface bags were exposed to daylight, temperatures ranging from 40°F to 90°F, and oxygen. The environment around the buried bags had more moisture, less oxygen, no sunlight and reached higher temperatures (130-135°F). The progress of oxidation of polyethylene measured at the molecular level by the formation of carbonyl groups as indicated in the infrared spectrum indicate that oxidation proceeded faster and reached a maximum between 2 and 4 weeks of exposure in the surface bags. These results

Degradation of starch modified polyethylene bags in compost indicate that exposure to daylight resulting in photodegradation was a major contributor to the effects on the surface bags. Photodegradation of polyethylene has been shown to occur by formation of oxidation products followed by chain scission and crosslinking pathways? 7,18 Gugumus 19'2° postulated that photo-oxidation proceeds via two different pathways, oxidation on the bulk of the film and oxidation on the surface of the film. The latter is characterized by formation of hydroxyls, peroxides, ketones and carboxylic acids, it follows radical chain oxidation and is not observed during natural weathering exposure. Our results concur with this hypothesis since we have not observed formation of peroxides, ketones or similar groups. Mechanical changes monitor bulk and localized changes in polymer structure. Regardless of the way in which oxidation spreads throughout the polymeric material, the oxidized sites enhance changes in mechanical properties. The oxidation site is more sensitive to thermal catalytic reaction propagation and mechanical effects. The observed embrittlement on the surface bags correlates well with the formation of oxidation products. The same explanation applies to the effects of the buried bags, i.e. less oxidation and consequently no loss of mechanical properties. The degradation of a polymer is usually assessed macroscopicaUy by determining the stress-strain curve and the elongation at break and tensile strength parameters. The differences in the degree of degradation between plastic exposed to the surface and inside the compost pile are best shown by the observed physical and mechanical property measurements. The buried bags remain intact throughout the study. The ultimate tensile strength, the elongation at break and accordingly the area under the load extension curve changed very little in these samples. The surface bags have disintegrated and at 14 days, when these measurements were possible, the area under the curve and the elongation at break were already significantly lower. These changes are in agreement with previous reports on degradation of polyethylene exposed to a chemically controlled compost under laboratory conditions.2~ The thermal analyses by DSC of untreated bags shows two endotherms very similar to those observed in degradation studies of LDPE and LLDPE blends.22 The overall crystallinity, determined by the heat of melting, does not

257

change significantly with time in the weathered bags. However, there are trends to lower heat capacity values in the buried bags and higher heat capacity values in the surface bags as exposure time increases. The observed shift to lower melting values in the surface and buried bags after 42 days and 49 days, respectively, may be due to chain branching or chain scission or their combination. Random chain scission without branching reduces molecular weight and also lowers the melting point. The infrared data was not conclusive, since the ratios of the absorbance bands due to terminal-CH3 and -CH2 groups did not show significant changes. Molecular weight distribution determinations by gel permeation techniques are being carried out to elucidate this point. The lower melting point component (shoulder at lll°C) in the surface bags disappeared as time of exposure increased. As a result of a reorganization of the less ordered fraction of the polymer, the shoulder component becomes part of the higher melting fraction resulting in higher enthalpy of melting of the second peak. Indeed, the heat of melt increased slightly and the shoulder peak disappeared in the surface bags as time increased. Another plausible explanation for the changes in the endotherm curve would be that the degradation is taking place in materials with lower molecular weight. An increase in the amount of lower molecular weight compounds and in the crystallinity of film has been reported after abiotic degradation of LDPE." The degradation of LDPE polyethylene is manifested by different characteristics according to the type of exposure. 23 Photo-irradiation of polyethylene increases gel fraction formation. 24 Oxidation damages specific sites namely the boundary region between crystalline and amorphous phases. The photostability of polyethylene is dependent among other factors; on its crystallinity and crystal size) 7,25 Our attempts to measure gel formation by extraction with organic solvents were not successful. The rate of degree of carbonyl formation on the surface bags was significantly higher than in the buried samples. Because the surface bags were exposed to daylight, photo-oxidation played the most important part. Buried bags, on the other hand, were not exposed to daylight and the contribution of photo-oxidation must be

258

Hebe B. Greizerstein, Joseph A. Syracuse, Paul J. Kostyniak

minimal. However, temperature and humidity were higher in the buried bag environment and made a stronger contribution to thermal oxidation in this case. The light microscopy determinations measured the nature and distribution of the starch granules in controls and in degraded films. There was a pronounced loss of starch granules in the buried bags after 49 days of exposure. The removal of starch granules creates voids in the films hence increasing the surface area. Others have speculated that this, together with the formation of oxidative products, can lead to an enhanced potential for biological attack. Besides starch, the principal additives in the plastic film were transition metal salts and an unsaturated oil. UV-vis and FTIR spectroscopy showed the disappearance of diene and carbonyl bands associated with the oil additive suggesting loss of this material after a short time of exposure (less than 14 days). The significance of this change is not yet clear. It has been shown that oils are involved in the degradation of polyethylene composites.21 In this report we have referred to the breakdown of the material as photodegradation or degradation. Degradation can proceed by thermal, oxidative, chemical, radiative, mechanical and biological mechanisms. Biodegradation and ageing of polyethylene occurs at very low rates; linear for the first 3 months, parabolic decline for the next 2-3 years and sporadic increases in rate after 3 years. Biodegradation is affected by preliminary irradiation from a UV source, additives, morphology, surface area and molecular weight.~28 In paraffins, oxidation or chain scission of the alkane chain seem to be requisites to sensitize or increase the susceptibility of the polymer to further attack by microorganisms. The oligomer fraction of this oxidation being the entity capable of sustaining microbial growth. 29 Our data do not allow us to ascertain whether biodegradation has taken place. Longer laboratory studies are required to demonstrate possible biodegradation. In conclusion, various factors affect the degradation of polyethylene-starch composites: molecular weight distribution, processing techniques (quenching and annealing), temperatures during processing, density, extent and distribution of crystallinity, presence of unsaturation, antioxidants and metal catalysts. In our study we have shown that exposure of polyethylene-starch

composite films to a compost environment undergo degradation. The observed degree of deterioration of the film is dependent on the immediate surroundings as exemplified by the surface bags degrading faster than bags buried inside the mature compost pile. Further studies are underway to determine the relation between polymer structure and degradation during the entire compost cycle in field and laboratory studies.

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Biodegradability and Utilization, A CS Symposium Series 433, ed. J. E. Glass & G. Swift. ACS, Washington 1990, p. 2. Albertsson, A. C., J. Appl. Polym. Sci., 22 (1978) 3419. Albertsson, A. C. & Banhidi, Z. G., J. Appl. Polym. Sci., 2,5 (1980) 1655. Albertsson, A.-C., Andersson, S. O. & Karlsson, S., Polym. Degr. and Stab., 18 (1987) 73. Albertsson, A.-C. & Karlsson, S., In Agricultural and Synthetic Polymers Biodegradability and Utilization, ACS Symposium Series 433, ed. J. E. Glass & G. Swift. ACS, Washington, 1990, p. 60. Albertsson, A.-C. & Karlsson, S., J. Appl. Polym. Sci., 35 (1988) 1289. Albertsson, A. C. & Ranby, B., J. Appl. Polym. Sci., Applied Polymer Symposium, 35 (1979) 423. Griffin, G. J. L., British Patent (1973) 1 489 050. Gonzalez-Orosco, J. A., Rego, J. A. & Katime, I., J. Appl. Polym. Sci., 40 (1990) 2219. Glenz, W. & Peterlin, A., J. Macromol. Sci.-Phys., B4(3) (1970) 473. Hosoda, S., Kojima, K. & Furuta, M., Makromol. Chem., 187 (1986) 1501. Daro, A., Trojan, M., Jacobs, R. & David, C., Eur. Polym. J., 26 (1990) 47. Gonzalez, A. & DeSaja, J. A., J. Appl. Polym. Sci., 41 (1990) 1961. Torikai, A., Shirakawa, H., Nagaya, S. & Fueki, K., J. Appl. Polym. Sci., 40 (1990) 1637. La Mantia, F. P., Polym. Degr. and Stab., 13 (1985) 297. Gugumus, F., Angewandte Makromoleculare Chemie, 182 (1990) 85. Gugumus, F., Angewandte Makromolekulare Chemie, 182 (1990) 111. Griffin, G. J. L., J. Polym. Sci., 57 (1976) 281. Trojan, M., Daro, A., Jacobs, R. & David, C., Polym. Degr. and Stab., 25 (1990) 275. Henry, J. L. & Garton, A., J. Polym. Sci., Part A, Polym. Chem., 25 (1990) 945. Geetha, R., Torikai, A., Nagaya, S. & Fueki, K., Polym. Degr. and Stab., 19 (1987) 279. Torikai, A., Geetha, R., Nagaya, S. & Fueki, K., J. Polym. Sci., Part A, Polym. Chem., 25 (1990) 3639. Albertsson, A. C., Eur. Polym. J., 16 (1980) 623.

Degradation of starch modified polyethylene bags in compost 27. Albertsson, A. C., Banhidi, Z. G. & Beyer-Ericsson, L.-L., J. Appl. Polym. Sci., 22 (1978) 3435. 28. Johannessen, S. M., J. Appl. Polym. Sci. Appl. Polym.

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Symp., 35 (1978) 415. 29. Cornell, J. H., Kaplan, A. M., & Rogers, M. R., J. Appl. Polym. Sci., 29 (1984) 2581.