Article Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX
pubs.acs.org/est
Biodegradable Plastic Blends Create New Possibilities for End-of-Life Management of Plastics but They Are Not a Panacea for Plastic Pollution Tanja Narancic,† Steven Verstichel,‡ Srinivasa Reddy Chaganti,§ Laura Morales-Gamez,∥ Shane T. Kenny,∥ Bruno De Wilde,‡ Ramesh Babu Padamati,*,§,∥ and Kevin E. O’Connor*,†,⊥ †
UCD Earth Institute and School of Biomolecular and Biomedical Science, University College Dublin, Belfield, Dublin 4, Ireland OWS nv, Dok Noord 5, 9000 Gent, Belgium § AMBER Centre, CRANN Institute, School of Physics, Trinity College Dublin, Dublin 2, Ireland ∥ Bioplastech Limited, Nova UCD, Belfield Innovation Park, University College Dublin, Belfield, Dublin 4, Ireland ⊥ BEACON - Bioeconomy Research Centre, University College Dublin, Belfield, Dublin 4, Ireland
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‡
S Supporting Information *
ABSTRACT: Plastic waste pollution is a global environmental problem which could be addressed by biodegradable plastics. The latter are blended together to achieve commercially functional properties, but the environmental fate of these blends is unknown. We have tested neat polymers, polylactic acid (PLA), polyhydroxybutyrate, polyhydroxyoctanoate, poly(butylene succinate), thermoplastic starch, polycaprolactone (PCL), and blends thereof for biodegradation across seven managed and unmanaged environments. PLA is one of the world’s best-selling biodegradable plastics, but it is not home compostable. We show here that PLA when blended with PCL becomes home compostable. We also demonstrate that the majority of the tested bioplastics and their blends degrade by thermophilic anaerobic digestion with high biogas output, but degradation times are 3−6 times longer than the retention times in commercial plants. While some polymers and their blends showed good biodegradation in soil and water, the majority of polymers and their blends tested in this study failed to achieve ISO and ASTM biodegradation standards, and some failed to show any biodegradation. Thus, biodegradable plastic blends need careful postconsumer management, and further design to allow more rapid biodegradation in multiple environments is needed as their release into the environment can cause plastic pollution.
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INTRODUCTION Biodegradable polymers are promoted as an environmental solution to plastic pollution. However, their degradation in managed and unmanaged environments is not well understood. Thus, humankind does not understand the end-of-life management possibilities nor the environmental impact of these polymers if they are released into the environment. The fact that biodegradable polymers are blended together to achieve functionality introduces further uncertainty for biodegradable plastic waste management and environmental impact. The aim of this study is to understand the biodegradation of individual polymers and their blends in managed and unmanaged environments. Polymers are ideal materials used in thousands of industrial and consumer products. A major application area for polymers is plastics. However, the recalcitrant nature of oil-based plastic coupled with a single-use throw away culture and release into the environment results in negative environmental consequences; e.g., 4.6−12.6 million tonnes of plastic waste generated in 2010 by 192 costal countries have ended up in the ocean.1 © XXXX American Chemical Society
While extreme weather events can result in plastic from seaside properties entering into the marine environment, single-use plastics are the major single contributor to plastic waste in terrestrial and marine environments.2 For example, single use plastic items make up 50% of marine and beach litter in Europe.3 Plastic debris kills hundreds of thousands of sea turtles, seals, whales, and seabirds due to ingestion or entanglement.2,4,5 Some countries are singled out as major contributors to plastic pollution, while others have a landfill ban in place.6,7 Nearly 30% of plastic waste is still landfilled in Europe, and the current EU plan is to reduce landfilling of plastic to no more than 10% by 2030.7 In the USA, 53% of total municipal solid waste (MSW) is landfilled, with plastic waste representing 13% of MSW.8 The recycling rate of nondegradable plastics remains Received: June 1, 2018 Revised: July 27, 2018 Accepted: July 27, 2018
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DOI: 10.1021/acs.est.8b02963 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology Table 1. Biodegradable Polymers Used in This Study, Their Characteristics and Application polymer
origin
characteristics
application
ref
polylactic acid (PLA) polyhydroxybutyrate (PHB) polyhydroxyoctanoate (PHO) poly(butylene succinate) (PBS) thermoplastic starch (TPS) polycaprolactone (PCL)
biobased biobased biobased biobased, petrochemical biobased petrochemical
crystalline, brittle crystalline, brittle elastomeric crystalline brittle elastomeric
packaging packaging medical packaging packaging medical
13, 14 15 15, 20 16 19 18, 21
stubbornly low globally.9 While recycling technologies are available, the high price and relatively low quality of the recycled plastics limits market applications. Biodegradable plastics are especially interesting for nondurable applications such as packaging and agricultural films where biodegradability offers new end-of-life management options not open to nondegradable plastics, e.g., composting and anaerobic degradation. Biodegradability is an advantage when inadvertent environmental release occurs to aquatic and terrestrial environments. Human behavior is a critical factor in preventing this uncontrolled release. However, biodegradable plastics that degrade across a wide range of managed and unmanaged environmental conditions offer greater economic and environmental rewards compared to those that are limited to biodegradation in a narrower set of environments. While biodegradability is an attractive postconsumer characteristic, these plastics must demonstrate performance in the market prior to disposal. Nonetheless, many biodegradable plastics often lack characteristics such as flexibility, strength, and toughness. Thus, they do not perform as well as their established nondegradable counterparts, and they must be blended together to combine characteristics and offer functionality for applications such as packaging. We have blended polymers to achieve characteristics suitable for packaging as the latter represents one of the largest uses of plastic material and carries the highest risk for environmental pollution. The polymers and blends used in this study (Table 1) were selected based on their application as packaging material10 and mechanical properties such as tensile strength 11 and elongation.12 Polylactic acid (PLA) is one of the major bioplastics on the market used mainly as packaging material.13 PLA was previously blended with other materials to improve its material modulus.14 Polyhydroxybutyrate (PHB) and polyhydroxyoctanoate (PHO) belong to the family of bacterial polyesters, with the same backbone but different side-chain lengths.15 Because of this structural difference, PHB is highly crystalline and brittle, while PHO is a rubber-like material of low crystallinity. With the emergence of biobased succinic acid and 1,4-butanediol, poly(butylene succinate) (PBS) can now be a biobased plastic16 with a melting temperature similar to those of polyethylene and polypropylene.17 While polycaprolactone (PCL) is of petrochemical origin, it is biodegradable and exhibits good elongation and good compatibility with many types of polymers.18 Thermoplastic starch (TPS) is completely biobased and biodegradable but is fragile compared to petrochemical-based polymers such as polyethylene.19 The diversity of biodegradable materials and environments makes it difficult to make simple and generic assessments of their end-of-life fate. We have therefore investigated the fate of a range of selected biodegradable plastics and their blends simulating controlled vs uncontrolled environments to gain a greater understanding of their potential environmental fate and
possible future end-of-life management options. We identified a surprising synergy but also antagonism between polymers in plastic blends that affects both their biodegradability and biodegradation rates which opens up new end-of-life management options but also raises concerns about the release of some of these biodegradable plastics in uncontrolled environments, e.g., soil, oceans, and rivers.
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MATERIALS AND METHODS Biodegradable polymers polylactic acid (PLA), polyhydroxybutyrate (PHB), polyhydroxyoctanoate (PHO), poly(butylene succinate) (PBS), thermoplastic starch (TPS), polycaprolactone (PCL), and their blends were tested for biodegradation in managed (industrial composting, anaerobic digestion, home composting) and unmanaged (marine, fresh water, aquatic anaerobic digestion, soil) environments. The polymers were blended at ratios that gave plastic characteristics suitable for packaging applications. The mechanical properties and morphology of the neat polymers and blends were investigated. Biodegradable Plastics Compounding. Neat polymers and their plastic blends (Table 2) were processed using a Table 2. Matrix of Polymers Used in This Study polymer
trade name
supplier
PLA PHB PHO PCL PBS TPS
Biopolymer-4043D ENMATY1000 Bioplastech R CAPA 6500 PBE 003 Bioplast TPS
Nature Works, USA TiTAN Bioplastech Perstorp NaturePlast BIOTEC
Brabender melt processor. Melt mixing of the polymers was carried out for 10 min at a rotor speed of 50 rpm. Before blending, all polymers pellets were dried in vacuum at 80 °C overnight and 60 °C for PCL, PHA, and TPS. The meltprocessed samples were compression molded to 20 × 20 × 0.2 cm rectangular specimens at 180 °C with an applied pressure of 200 MPa using a Servitech Polystat 200 T compression press. Mechanical Testing. Biodegradable plastic sheets were punched with a cutter to dumbbell-shaped samples with dimensions of 25 mm × 4 mm × 1 mm and were used for stress−strain measurements. Tensile measurements were carried out using a Zwick twin column tensile tester with a 100 N load cell and calibrated with a 2 kg standard. The tensile tests were carried out at room temperature and a cross head speed of 25 mm/min. Young’s modulus, ultimate tensile strength, breaking strength, elongation at break and toughness values were calculated by integrating the stress−strain data obtained from the samples. B
DOI: 10.1021/acs.est.8b02963 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology Scanning Electron Microscopy (SEM). The surface morphology of polymers and blends was examined using SEM. The samples were fixed on the SEM stubs using carbon tape and sputtered with gold/palladium (80/20 ratio) for 15 s. The cross-section images of the composites were obtained by a high-resolution field emission Zeiss Ultra Plus-SEM (Carl Zesis AG, Oberkochen, Germany) with an accelerating voltage of 5 kV under high vacuum. The internal morphology of the biodegradable plastics was studied using the fractured surface of the systems. Differential Scanning Calorimetry (DSC). The glass transition temperature (Tg) and melting temperature (Tm) of polymers and biodegradable plastics were analyzed using a PerkinElmer Pyris Diamond calorimeter calibrated to indium standards. The samples were sealed in aluminum pans and heated from −90 to +200 °C at a rate of 10 °C min−1. The samples were run through a first cycle of heating and cooling to remove the thermal history. Biodegradation Tests. All tests were done according to available international standards, where a minimum incubation time was applied. In some cases, we prolonged the incubation until we observed a plateau, and these cases are described for each biodegradation test. During the aerobic biodegradation of organic materials, oxygen is consumed and carbon is converted to gaseous, mineral carbon as carbon dioxide, CO2. Part of the organic material is assimilated for cell growth. Under anaerobic conditions, the organic carbon of the material is converted into biogas CO2 and CH4. Biodegradation is calculated as the percentage of organic carbon content of the tested sample which has been converted to gaseous, mineral C in the form of CO2 (aerobic conditions) or CO2 and CH4 (anaerobic conditions). The organic carbon content of cellulose as the reference material used in these tests and bioplastics was determined by subtracting the total inorganic carbon content from the total carbon content using a Primacs SNC-100 analyzer. The carbon content (g) at the start of incubation is designated as Ci. A cumulative CO2 production (g) was then measured for each reactor in all aerobic tests and this value was divided by the molar mass of CO2 (44 g/mol) and multiplied by the molar mass of carbon (12 g/mol) to obtain the amount of gaseous carbon produced (Cg; g). For the tests performed under anaerobic conditions, 1 mol of solid carbon of the test item is converted into 1 mol of gaseous CO2 or CH4 (C + 2 H2→ CH4 and C + O2→ CO2). One mole of gaseous carbon occupies 22.414 Nl (Nl is a liter at 0 °C and 1013.25 hPa), and this is converted to g of C using the molar mass of carbon (12 g/mol). The biodegradation (%) is calculated by dividing the cumulative average net gaseous carbon production of the test compound by the original cumulative average amount of solid carbon of the test compound and multiplying by 100:
relative biodegradation of the reference material cellulose, were always reached. Standard deviation among three independent replicates was calculated. A short description for each test is given below. Controlled Industrial Composting Conditions (ISO 14855). The controlled composting biodegradation test is an optimized simulation of an intensive aerobic composting process where the biodegradability of a test item under dry, aerobic conditions is determined. The test is performed according to ISO 14855-1 (2012). The maximum test period is 180 days. The inoculum is derived from the organic fraction of municipal solid waste, which is stabilized and matured in a composting bin at the laboratory under controlled aeration for more than 20 weeks. Before use, the mature compost is sieved through a 5 mm sieve and the fine fraction is used as the inoculum, rich in bacteria and fungi. Total solids content of the compost inoculum is 50−55%, while volatile solids concentration is >30% of dry solids. Typically, 80 g of test material is added to 1200 g of inoculum. The reactors are incubated at 58 ± 2 °C. The flow rate of pressurized dry air is controlled by a gas flow controller and directed into the composting vessel at the bottom through a porous plate. CO2 produced during the test as well as O2 are analyzed continuously as part of the gas leaving each individual reactor using a gas chromatograph (PerkinElmer Clarus 500), while the flow rate is measured with a Brooks 5860S mass flow meter. When the microbial activity is reduced the test is converted to static conditions in which the oxygen in the headspace is consumed by the microorganisms. The evolved CO2 is absorbed in a beaker using KOH and determined by titration with a Metrohm 888 Titrando. Controlled Composting Conditions at Ambient Temperature (ISO 14855 at 28 °C). The test procedure is the same as defined in ISO 14855 and described in the paragraph above, but the test temperature is decreased to 28 °C ± 2 °C to simulate home composting conditions. Maximum test duration is 1 year. High-Solids Anaerobic Biodegradation Test: Anaerobic Digestion (AD; ISO 15985). The biodegradability of samples in a solid-state anaerobic digestion system or in a sanitary landfill is determined through high-rate dry anaerobic batch fermentation. This method simulates and accelerates the biodegradation process that takes place in a landfill because it is a stationary (no mixing) and dry fermentation under optimal conditions. The incubation temperature is 52 ± 2 °C, and the duration of the test is at least 15 days. Test item (15 g) is added to a large amount of highly active inoculum (1000 g) derived from a digester that has been operated during several months on the organic fraction of household waste and has been stabilized during a short postfermentation of several days to reduce biogas production rate to a level below 1.5 Nl biogas/kg/day. The generated biogas is collected in an inverted graduated cylinder in water. The water in contact with the gas has pH < 2 during the whole period of the test to avoid CO2 loss through absorption in the water. A gas chromatograph (PerkinElmer Clarus 500) and TotalChem software are used for the determination of the concentration of CH4 and CO2 in the biogas. Soil Biodegradation Test (ISO 17556). The biodegradation in soil is evaluated according to ISO 17556 (2012) using standard soil. Maximum testing period is 2 years. The standard soil consists of a mixture of 70% industrial quartz
%biodegradation =
[mean C test(g ) − mean Ccontrol(g )] × 100 Ci
In all biodegradation tests, control reactors without test item are taken along to be able to measure the net CO2 or biogas production derived from the degradation of the test material. Cellulose was used as a reference material to evaluate the validity of the test. The validity criteria, which were 90% C
DOI: 10.1021/acs.est.8b02963 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology
Figure 1. Mechanical properties of biobased and biodegradable plastics (purple circles) and fossil-based biodegradable polycaprolactone (green square) and their blends (blue triangles). By blending these biodegradable polymers (purple circles) with each other they are moving into new design space (blue triangle) and thus opening up new opportunities for applications such as plastics.
other organic carbon sources, and spiked with microorganisms. The source of microorganisms is a mixture of activated sludge, obtained from different aerobic wastewater treatment plants (Destelbergen, Gent and Lokeren, all located in Belgium), that treat wastewater from domestic and/or industrial origin. At start-up, each reactor is filled with 245 mL mineral medium and 5 mL of sludge inoculum. The reference and test item (approximately 25 mg) are added directly to the reactors. The reactors are incubated at a constant temperature (21 °C ± 1 °C) in the dark for a period of minimum 28 days. The evolved CO2 is absorbed in KOH and determined by titration with a Metrohm 888 Titrando. Aqueous Anaerobic Biodegradation Test (ISO 14853). The test is performed according to ISO 14853 (2005). A 90 g portion of anaerobic sludge (corresponding to about 10% of the sludge concentration in a real digester), suspended in an oxygen-free medium, is placed in a suitable vessel leaving headspace into which any gases produced may be evolved. The anaerobic sludge is derived from the wastewater treatment plant Ossemeersen, Gent, Belgium. Prior to sealing, 25 mg of test compound is added. The vessels are incubated at a constant temperature of 35 °C ± 2 °C. The headspace pressure resulting from the production of gas is measured with a pressure transducer (UMS, INFIELD7c with T1 Stitch-Tensiometer), and the DIC (dissolved inorganic carbon, this is the inorganic carbon (mainly CO2) that is dissolved in the liquid phase) content of the digesting liquid is determined by addition of hydrochloric acid to the reactors and measuring the subsequent pressure increase. The biodegradation is calculated from the sum of the measured biogas in the headspace and DIC in the liquid medium. The total test duration is 56 days.
sand, 10% kaolinite clay, 16% natural soil, and 4% mature compost. The soil was collected from a sandy field in Lokeren and 2 types of forest in Moerbeke (all located in Belgium). The mixture consists of 1/3 field soil and 2/3 forest soils. The soil is sieved over a 2 mm sieve to remove stones and other inert materials, roots, and other plant debris and thoroughly mixed. The mature compost is derived from the organic fraction of municipal solid waste. The waste is stabilized and aerated in a composting bin at the laboratory under controlled conditions for at least 20 weeks. Before use, the compost is sieved through a 5 mm sieve. Finally, salts are added to the standard soil by means of nutrients solution (per l: KH2PO4 9.6 g, MgSO4 4.8 g, NaNO3 19.2 g, urea 9.6 g and NH4Cl 19.2 g/L) to obtain the final inoculum. At start-up, 2.0 g of reference or test item is mixed with 500 g of soil inoculum, while the control reactors contain only 500 g soil inoculum. The reactors are closed airtight and placed in the dark at 25 °C ± 2 °C. The evolved CO2 is absorbed in a beaker with KOH and determined by titration with a Metrohm 888 Titrando. Marine Biodegradation (ASTM D6691). Testing is performed according to ASTM D6691 (2009) and determines the biodegradation of a test item under laboratory conditions by incubation in seawater. The test material is brought into natural seawater enriched with inorganic nutrients (0.05 g/L of NH4Cl and 0.1 g/L of KH2PO4) and containing an indigenous population of micro-organisms. The natural seawater was collected from open sea via a 1 km pipeline from SEA LIFE, Blankenberge, Belgium. At start-up, each reactor is filled with 250 mL of enriched seawater. The reference and test item (approximately 25 mg) are added directly to the reactors. The reactors are incubated at a constant temperature (30 °C ± 1 °C) in the dark for a period of minimum 28 days. The evolved CO2 is absorbed in KOH and determined by titration with a Metrohm 888 Titrando. Biodegradation in Aerobic, Aquatic Conditions (Fresh Water; ISO 14851). The test is performed according to ISO 14851 (2005). The test material is brought into a chemically defined (mineral) liquid medium, essentially free of
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RESULTS Mechanical Properties of Biobased and Biodegradable Plastics. To address shortcomings in performance of individual plastics (i.e., PLA is hard but brittle and PHO is flexible but soft), blends were created where characteristics of D
DOI: 10.1021/acs.est.8b02963 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology
Table 3. Compatibility of the Blends Used in This Study Was Determined by Differential Scanning Chromatography (DSC) blend
compatibility
PLA−PCL(80/20) PLA−PBS(80/20) PLA−PHB(80/20) PLA−PHO(85/15) PHB−PCL(60/40) PHB−PBS(50/50) PHB−PHO(85/15) PCL−TPS(70/30) PBS−TPS(60/40)
compatible inconclusive compatible inconclusive incompatible compatible incompatible incompatible compatible
comment mechanical mechanical mechanical mechanical mechanical mechanical mechanical mechanical mechanical
properties properties properties properties properties properties properties properties properties
of of of of of of of of of
the the the the the the the the the
blend blend blend blend blend blend blend blend blend
are better than neat polymer; single Tg peak observed are better than neat polymer; multiple melting peaks observed are better than neat polymers are better than neat polymers; multiple Tg peaks observed in DSC inferior than neat polymers are better than neat polymer inferior than neat polymers inferior than neat polymers are better than neat polymer
Figure 2. Biodegradation of individual polymers and their plastic blends in multiple managed environments: home composting (ISO 14855, 28 °C); anaerobic digestion (ISO 15985, 52 °C); industrial composting (ISO 14855, 58 °C). Biodegradation is expressed as a percentage of biodegradation of cellulose (green dotted line). The cutoff point for a material to be considered biodegradable under tested conditions is presented with a red dashed line (90% of the reference material). *PHB, PHB−PCL(60/40), and PCL−TPS (70/30) were not tested in this study for home composting as PHB is certified for home composting,28 and PCL and TPS are both home compostable.
properties (Figure 1, Table 3, Figure S1). This is in keeping with previous literature reports where immiscible blends of polymers still showed improved properties.24 While PCL in general shows good miscibility with other polymers,26 DSC analysis showed that PCL and PHB are incompatible, resulting in a blend with poorer mechanical properties compared to neat polymers (Table 3). Despite some polymer blends showing decreased mechanical properties, they were brought forward to allow a broad examination of their end-of-life fate in simulated managed and unmanaged environments. Biodegradation of Bioplastic in Managed Environments. Surprisingly, the PLA−PCL(80/20) plastic blend
individual polymers were combined with additive and synergistic effects to give properties suitable for packaging applications. The ratio of polymers in the blends was chosen based on the scientific literature and our experience with these materials.22−25 The blending of these polymers moved the created blends into a plastic designer space previously unoccupied by biodegradable polymers (Figure 1). By adding PCL or PHO to PLA at a relatively low loading (
