21st IAPRI World Conference on Packaging ISBN: 978-1-60595-046-4

Mechanical Properties and Anaerobic Biodegradation of Thermoplastic Starch/Polycaprolactone Blends

Ryan Nunziato1, Swati Hedge2, Elizabeth Dell3, Thomas Trabold2, Christopher Lewis3 and Carlos Diaz*1 1Packaging Science, Rochester Institute of Technology, Rochester, New York United States 2Golisano Institute for Sustainability, Rochester Institute of Technology, Rochester, New York United States 3Manufacturing & Mechanical Engineering Technology, Rochester Institute of Technology, Rochester, New York United States

Abstract: Thermoplastic starch (TPS) has emerged as a readily biodegradable and inexpensive biomaterial that can replace traditional plastics in applications such as food service and packaging. TPS is an abundant, renewable resource with a low processing temperature when compared with other biodegradable polymers. However, TPS has very low strength and it is brittle. Therefore, there is a need to modify it or blend it with other biodegradable polymers to achieve the desired performance. Polycaprolactone (PCL) is a rubbery biodegradable plastic that can be used to improve the ductile behavior of TPS. The goal of this research was to determine the viability of TPS as an inexpensive majority component in polymer blends with PCL. A ductile bioplastic that is cost-competitive can promote the use of compostable packaging. This study also evaluated biodegradation through anaerobic digestion under mesophilic (37±2°C) and thermophilic (52±2°C) conditions. Preliminary studies on the anaerobic degradation of PCL revealed that thermophilic degradation conditions were favorable compared to mesophilic conditions. While mesophilic digestion can work when a high percentage of TPS is blended with PCL, thermophilic conditions offer an advantage even at a lower TPS percentage. Addition of TPS improved the biodegradability of PCL significantly, possibly due to increased surface area of PCL exposed after TPS degrades from the blend.

Keywords: Polycaprolactone, Thermoplastic starch, Anaerobic digestion

1 Introduction

Thermoplastic starch (TPS) has emerged as a readily biodegradable and cheap biomaterial which can replace traditional plastics in applications such as food service and packaging. TPS is an abundant, renewable resource with a low processing temperature when compared with other biodegradable polymers. However, TPS has very low strength and it is brittle. Therefore, there is a need to modify it or blend it with other biodegradable polymers to achieve the desired performance. Polycaprolactone (PCL) is a rubbery biodegradable plastic that can be used to improve the ductile behavior of TPS. The goal of this research was to determine the viability of TPS as a cheap majority component in polymer blends with PCL. A ductile bioplastic that is cost-competitive can promote the use of compostable packaging.

* Correspondence to: Carlos Diaz Packaging Science, Rochester Institute of Technology, Rochester, New York United States. E-mail: [email protected]

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Degradation of PCL has been studied using various methods such as anaerobic digestion [3], enzymatic degradation [4], [5], aerobic composting [6] and landfill simulations [7]. As opposed to organic wastes, bioplastics require specific process requirements to be degraded completely. For example, the microbial degradation of bioplastics either by composting or anaerobic degradation requires an aqueous environment. Hydrolytic degradation of PCL is slow due to its hydrophobic nature and high flexibility that allows the polymer to crystallize easily, leaving a small amorphous fraction which slows down the rate of degradation [8]. In addition to physical and chemical properties, the morphology of bioplastics has been found to affect microbial activity, with the amorphous regions preceding crystalline regions during degradation [9], [10]. Some European organizations recommend sequential anaerobic digestion and composting processes for complete degradation of bioplastic materials [11]. Blending synthetic polymers like PCL with highly degradable starch and cellulose were previously reported as an option to improve biodegradation [2], [3]. While published studies recommended several methods for bioplastics degradation, anaerobic digestion is an attractive alternative as it is the most practical option and also yields energy in the form of methane gas. This preliminary study evaluated biodegradation of PCL and PCL with blended thermoplastic starch (PCL/TPS) through anaerobic digestion under mesophilic (37 ±2°C) and thermophilic (52 ±2°C) conditions.

2 Methodology

2.1 Sample preparation

Corn starch was obtained from MP Biomedicals LLC. Thermoplastic starch was made using an internal shear mixer (CWB Brabender Intelli-torque Plasticorder torque rheometer with a 60cc 3-piece mixing head) with a mix of 7:3 starch to glycerol ratio and an equal amount of water added. The mixer was run open to allow venting of water vapor. TPS starch was blended at 30, 40, 50, 60, and 70 wt. % with PCL in the mixer at 100ºC for 8 minutes and 50 rpm. Compression molded samples were made at 2 tons pressure with a heated press Carver 4391.

2.2 Mechanical property characterization

Tensile testing of the blend was carried out using an Instron Universal Testing Machine model 5567 at a crosshead speed of 12.5 mm/min. At least five specimens of each sample were tested according to ASTM D638. Samples were conditioned at room temperature for at least 24 hours prior to mechanical testing.

2.3 Anaerobic degradation

Sample preparation: PCL, TPS, 80PCL/20TPS (i.e., 80% PCL blended with 20% TPS), 60PCL/40TPS and 40PCL/60TPS films were cut into approximately 1 cm square pieces to obtain uniform particle size for degradation. The total solids (TS) and volatile solids (VS) content of the samples were determined according to a standard protocol [13]. Inoculum and AD conditions were set up to test biochemical methane potential (BMP) of PCL and blends[14]. The inoculum originated from the effluent of a commercial digester that co-digests industrial food waste with cow manure. The effluent was separately pre-incubated in

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a BOD incubator for 7-9 days at 37°C and 52°C to develop the inoculum for mesophilic and thermophilic digestion experiments, respectively. The samples for degradation studies were prepared to obtain an inoculum to substrate ratio of 2 (g VS inoculum: g VS substrate added). An Automated Methane Potential Testing System (AMPTS II, Bioprocess Control, Lund, Sweden) was used to perform BMP assays. The AMPTS II system continuously records the biomethane production at regular time intervals (Figure 1). The reactors were and incubated at 52±2°C for thermophilic and 35±2°C for mesophilic digestion studies, with mixing at 160 rpm, with an ‘ON’ cycle of 10 seconds and ‘OFF’ cycle of 50 seconds. Commercial cellulose powder (Sigma-Aldrich) was used as control samples. Before starting the data acquisition, the reactors were purged with nitrogen gas to create an anaerobic environment by displacing residual oxygen. Methane production was recorded on a daily basis using the online flow cell detector array shown in Figure 1.

a

d b

c

Figure 1. AMPTSII system used in studying degradation of bioplastic materials under anaerobic conditions. A water bath incubator (b) set at the desired temperature helps to maintain the optimum temperature of reactors (a). A carbon dioxide fixing unit (c) connects to the biogas outlet tubing of the reactors. Biogas mainly contains methane and CO2; the fixing unit absorbs CO2 from the biogas and methane passes through a flow cell detector (d). The flow cell detector continuously records methane production at regular intervals. Calculation of BMP: BMP is defined as the volume of methane produced per unit mass of the volatile solids added in a defined period. Daily methane production volumes were converted into BMP using Equation 1. Volatile solids represent the biodegradable fraction of a material and VS fractions vary for different materials. Therefore, BMP is used to normalize methane production for comparison purposes. mLCH4 mL methane produced in specified time BMP ( ) = gVS total gram volatile solids added Equation 1 Calculation of theoretical BMP and degradability: The theoretical maximum BMP for different materials was estimated using Buswell’s reaction stoichiometry for anaerobic digestion suggested by various literature [7], [14]. The general reaction stoichiometry for anaerobic degradation is given in Equation 2 that assumes only methane and carbon dioxide as the end products of degradation [7].

724 3 Results and Discussion

Figure 2 and 3 show the tensile properties of the blends. The stiffness shows a synergistic effect because some of the blends have higher modulus of elasticity than that of TPS or PCL alone. The blend with 30 wt.% TPS showed the highest modulus of elasticity at around 160 MPa. The tensile strength showed an increasing trend as the amount of PCL increases.

Figure 2. Elastic Modulus and Tensile Strength of TPS/PCL Blends and PCL and TPS Reference Materials.

Figure 3. Elongation at Break of TPS/PCL Blends and break types from tensile test.

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The elongation at break shows a clear brittle-ductile transition starting at around 40 wt.% PCL. A material that is able to stretch is a key characteristic for manufacturing films for packaging applications. The commercial biodegradable bag used as a benchmark had a modulus of elasticity, tensile strength and elongation at break of 115 MPa, 20MPa, and 620 % respectively. Comparable properties can be achieved with the right combination of TPS/PCL. Preliminary studies on degradation behavior observed for 35 days under anaerobic mesophilic conditions resulted in a methane yield of 25.5 mL/gVS of PCL that corresponded to only 4% degradability. However, blending PCL with 40% (60PCL/40TPS) and 60% TPS (40PCL/60TPS) resulted in 18.5% and 32.3% degradation respectively. Degradation of 70-81% for various types of starch, 2% for PCL and 21-24.3% for PCL/starch blends containing 30% of various starches have been previously reported under mesophilic conditions [3]. These authors also reported that the degradation improved to 26.7-30.7% when starch was plasticized using glycerol. For PCL under mesophilic conditions, 16-17% of the theoretical biogas production was observed during a 42-day period [2]. The daily biomethane potential for PCL, 60PCL/40TPS, and 40PCL/60TPS over a 30-day period are shown in Figure 4a. The replicates from mesophilic digestion displayed a high degree of inconsistency even though control experiments (food waste and cellulose) had acceptable consistency as shown in Figure 4b. The inconsistency could be because the microorganisms were not completely acclimated to degrade petroleum-based bioplastics like PCL. Also, mesophilic temperatures offer a smaller diversity of PCL degrading organisms [1]. It is also possible that the hydrophobicity of PCL and degraded oligomers contributed to inconsistent microbial degradation due to a random dispersal of exposed and unexposed surfaces.

Figure 4. (a) Daily biomethane potential measured during mesophilic anaerobic degradation of bioplastics; (b) Daily biomethane potential from control experiments; the error bars correspond to standard deviation among three replicates.

The degradation of PCL is predominantly reported to be favorable under thermophilic conditions, either by thermotolerant bacteria and fungi or by the action of fungal enzymes that break polyester bonds under high-temperature conditions [15], [16]. While TPS significantly improved biodegradability under mesophilic conditions, a higher TPS fraction is necessary to achieve better degradability. However, increasing the TPS fraction in blends compromises

726 mechanical properties of PCL based materials. Therefore, thermophilic conditions would provide more favorable conditions for PCL degradation. Several authors have hypothesized that the higher temperature conditions could favor degradation of PCL by enhancing the mobility of PCL chains (e.g.,[3]), thereby improving the mass transfer of PCL molecules into the microbial cell wall. Thermophilic conditions used in anaerobic digestion range between 50 and 600C [17] which is also close to PCL melting temperature of approximately 600C [3]. Under thermophilic conditions, 80PCL/20TPS produced 93.6 mL methane/gVS, which corresponded to 26.2% degradation in 30 days; this is higher than the degradation observed for 60PCL/40TPS under mesophilic condition. While under thermophilic conditions PCL on its own showed an improved 11.3% degradation relative to mesophilic digestion, the degradation rate of TPS was comparable to food waste and cellulose powder with 77.1% degradation in 30 days. A 2.3 fold improvement in the biodegradation was observed in 80PCL/20TPS relative to PCL with no added TPS. The daily biomethane potential for PCL and 80PCL/20PCL are shown in Figure 5a; Figure 5b shows BMP from respective control experiments.

Figure 5 (a) Daily biomethane potential during measured during thermophilic anaerobic degradation of bioplastics; (b) Daily biomethane potential from control experiments; the error bars correspond to standard deviation among three replicates; 0PCL/100TPS is daily average from duplicate runs.

Irrespective of the conditions, TPS degrades at a faster rate than PCL under anaerobic conditions as starch is a more naturally occurring polymer. Degradation of TPS from PCL/TPS blends possibly increased the surface area of PCL available for microbial action, thus improving PCL degradation. The PCL/TPS blends, in general, showed an improved and consistent degradation pattern compared to PCL by itself. Thermophilic conditions were more favorable than mesophilic conditions, possibly attributed to the diversity of microorganisms capable of degrading PCL at higher temperature. The preliminary results from anaerobic degradation studies are summarized in Table 1.

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Table 1. Summary of anaerobic degradation studies.

$averaged from duplicate runs

4 Acknowledgements

Funding provided by the NYS Pollution Prevention Institute through a grant from the NYS Department of Environmental Conservation. Any opinions, findings, conclusions or recommendations expressed are those of the authors and do not necessarily reflect the views of the Department of Environmental Conservation.

5 Conclusion

This study evaluated the modification of mechanical properties and biodegradation behavior of thermoplastic starch blended with polycaprolactone. The addition of PCL in concentrations greater than 40% rendered a ductile material, showing a brittle-ductile transition. A synergistic effect was observed in the modulus of elasticity. Preliminary studies on the anaerobic degradation of PCL revealed that thermophilic degradation conditions were favorable compared to mesophilic conditions. While mesophilic digestion can work when a high percentage of TPS is blended with PCL, thermophilic conditions offer an advantage even at a lower TPS percentage. Addition of TPS improved the biodegradability of PCL significantly, possibly due to increased surface area of PCL exposed after TPS degrades from the blend. This research can contribute to developing affordable biodegradable plastics in an effort to reduce solid waste buildup on promote organic waste streams.

6 References

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