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Article

Morphology, Mechanical Properties, and Biodegradability of Modified Thermoplastic Starch/PETG Blends with In Situ Generated Graft Copolymers

by
Apoorva Kulkarni
1,2,* and
Ramani Narayan
1
1
Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI 48824, USA
2
Saint-Gobain NRDC, Northborough, MA 01352, USA
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(3), 2227; https://doi.org/10.3390/su15032227
Submission received: 27 November 2022 / Revised: 2 January 2023 / Accepted: 21 January 2023 / Published: 25 January 2023
(This article belongs to the Section Sustainable Engineering and Science)
Browse Figures
Versions Notes

Abstract

:
This paper reports on synthesis of modified thermoplastic starch (MTPS) and glycol-modified polyethylene terephthalate (PETG) blends in a twin-screw extruder. Scanning electron microscopy (SEM) images showed uniform, microdispersion of MTPS in PETG matrix, confirming compatibilization of the blend by graft copolymers generated in situ during the reactive extrusion process. Incorporating 30% by wt. MTPS in the blend gives a biobased carbon content of 22.8%, resulting in reduced carbon footprint by removal of 0.5 kg CO2 from the environment/kg resin relative to unmodified PETG. MTPS with 80% glycerol grafted onto starch was prepared by reactive extrusion in the twin-screw extruder. A total of 33% of added PETG was grafted on MTPS backbone as determined by soxhlet extraction with dichloromethane (DCM). The grafting was confirmed by presence of PETG peak in the TGA analysis of residue and appearance of carbonyl peak in FTIR spectra of the residue after Soxhlet extraction. The synthesized MTPS–PETG reactive blend had lower but acceptable mechanical properties. Even after a 15% reduction in the tensile stress and 40% reduction in the strain and impact strength obtained after adding 30% MTPS, this blend still had good mechanical properties and can be used in many applications requiring a balance of cost, mechanical properties, and biobased content. Aqueous biodegradability studies using ISO 14852 showed that the 30% starch component in the blend biodegraded rapidly within 80 days, whereas PETG remained as it was even after 150 days. Thus, this study categorically proves that addition of starch does not improve the biodegradability of nonbiodegradable polymers.

1. Introduction

With increasing efforts for a sustainable environment, several studies have suggested replacing conventional petroleum-based plastics partly or fully with biobased counterparts. Using biobased carbon in place of petro-fossil carbon in products offers a reduced carbon footprint, empowers rural agrarian economy, and reduces dependence on fossil resources [1,2,3]. Biobased plastics are “plastics in which the (organic) carbon (of the polymer molecule) in part or whole comes from plant-biomass such as agricultural crops and residues, marine and forestry materials, algae, and fungi living in a natural environment in equilibrium with the atmosphere” [4]. They remove CO2 from the environment and incorporate it into plastic in a short time frame of 1–10 years, thus supporting the goals of “circular plastic economy” [5]. Substituting petro-fossil carbon with biobased carbon has received immense interest in recent years. Production of biobased polyethylene terephthalate (PET), producing blends of biobased polymers such as polylactide (PLA), starch, and polyhydroxybutyrate-co-hydroxyvalerate (PHBV), with various nonbiodegradable polymers are efforts in such direction [6].
PETG is a glycol-modified version of PET. It is synthesized by replacing a portion of glycol component of PET with a cyclic diol called cyclohexanedimethanol (CHDM) [7]. Addition of the cyclic diol makes PETG an amorphous copolyester [8]. It is a highly hydrophobic polymer with excellent tensile strength and elongation properties. PETG is used in a wide range of industrial applications, including manufacture of implants, medical device packaging, and graphic displays, and is one of the most important polymers in 3D printing [8]. More recently, the shape memory effect (SME) of PETG has attracted a lot of interest, making it an important polymer for fused deposition modeling (FDM) in 3D and 4D printing [8,9,10]. Although efforts have been made to increase the biobased carbon content of PET, the work on biobased PETG has been limited. One of the main shortcomings of PETG is its end of life—it is not biodegradable in aqueous or composting environments.
Starch has generated increasing interest as an additive or blend component in plastic formulations, being 100% biobased and readily biodegradable in almost all the environments. Starch is also abundantly available and inexpensive. To achieve high biocarbon content, provide high oxygen barrier properties and crystallinity, and to reduce the cost, a large number of studies have been reported by blending different polymers such as low-density polyethylene (LDPE) [11], high-density polyethylene (HDPE) [12], polystyrene (PS) [13], and polypropylene (PP) [14] with starch. In this study, we made use of modified thermoplastic starch as a biobased component in the MTPS/PETG blend. A maximum of 30% MTPS by wt. was added to the blend. According to ISO 22526, introducing 30% by wt. MTPS gives a biobased carbon content of 22%. Carbon footprint is defined as the amount of CO2 removed from the air and incorporated into the plastic [15].
Using granular starch as it is in the blends has some processing and compatibility limitations [16,17,18,19,20,21,22]. Starch cannot be processed as it is using extrusion or injection molding because of its decomposition before melting [23,24]. Adding plasticizers such as glycerol and sorbitol solves the problem of processability, but these thermoplastic starches (TPSs) still have the shortcoming of leaching of plasticizers and poor mechanical properties over time due to less compatibility with other polymers [21,22,25,26]. Chemical modification of starch using malic anhydride to covalently bond glycerol to the starch backbone has been extensively studied in our group [21,22,27]. This modification eliminates the possibility of glycerol leaching over time, thus giving good mechanical properties in blends. In addition, MTPS is shown to have better compatibility with polyesters such as PLA and poly(butylene adipate-co-terephthalate) PBAT, which was shown by the SEM studies of Hwang et al. (2013), Detyothin et al. (2015), and Kulkarni et al. (2021) [22,28,29]. It was observed that MTPS showed much better distribution and smaller particle sizes of 4–9 um in the blends with PLA which were smaller as compared to the 30 um size observed by Clasen et al. (2015) in the PLA–TPS blends without any compatibilizer [22,30].
MTPS was also found to react with PBAT via transesterification reaction to form in situ graft copolymer, which improved the compatibility of the blends [27]. Similar studies of grafting other polymers such as polycaprolactone (PCL) and poly-L-lactide (PLLA) on starch backbone by reactions of their hydroxyl groups or through ring-opening graft polymerization were carried out by Chen et al. (2005), Najemi et al. (2010), and Zerroukhi et al. (2012) [31,32,33]. Grafting reaction in these studies showed over 60% grafting efficiency; mechanical properties were improved, and a completely biodegradable copolymer was obtained. However, these reactions were carried out in solvents and on a very small scale. Reactive extrusion (REX) was not used. REX offers several advantages over traditional batch and flow reactors (CSTR, PFR), such as fast reaction time, enhanced heat and mass transfer, and better mixing, and it does not require any solvents [34]. To the best of our knowledge, there are no reports of using this grafting reaction for PETG. In this study, the synthesis of in situ MTPS–g–PETG copolymers using reactive extrusion was reported as a novel, one-step procedure. Chemically modified thermoplastic starch (MTPS) was prepared first using malic anhydride and glycerol via reactive extrusion, using the method described in Kulkarni et al. (2021) [22]. This MTPS was then melt-blended with PETG in a twin-screw extruder to give compatibilized blends of MTPS–PETG. Graft copolymers generated in situ via transesterification reactions function as compatibilizers between the starch and PETG phases. Soxhlet extraction, TGA, and FTIR were used for analyzing and quantifying the percent grafting. Further, mechanical and biodegradation properties of the blend were also studied.
A number of studies claim that addition of starch to nonbiodegradable polymers effectively render nonbiodegradable polymers biodegradable. Bulatovic et al. (2022) claimed that addition of TPS to low-density polyethylene (LDPE) made the blend biodegradable [11]. Chandra et al. (1998) suggested that if microorganisms are able to utilize the biodegradable component in the plastic film, then the remaining inert components should easily disintegrate and disappear. Their claim was that this increased the overall biodegradability of the plastic [35]. Datta et al. (2018) also claimed that dispersing starch into low-density polyethylene (LDPE) made the film more biodegradable. They observed that biodegradation of starch from the film increased the surface area and helped in facilitating the number of microbes initially to act as a catalyst in enhancing the degradation process of LDPE [36]. A quick search on Springer with the search terms involving “starch”, “LDPE”, and “biodegrada*” gives 1300 such articles claiming enhanced biodegradation of the blends by addition of starch. Similar claims have been made in several articles for other polymers such as PS, and HDPE as well [13]. However, the majority of these articles do not use polymer carbon conversion to CO2 by microbial metabolism for reporting degree of biodegradation. Other methods, such as reduction in mechanical properties, weight loss, or disintegration, have been used to indicate that the blend or the nonbiodegradable polymer in the blend undergoes biodegradation.
However, true and complete biodegradation is a process when all the polymer carbon is assimilated by microorganisms as food and energy source, as measured by evolved carbon dioxide (aerobic) and methane (anaerobic) [2,37]. Some studies from S. Selke, E.F. Gomez, and F.C. Michael showed that addition of biodegradation-promoting additives to polyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP), etc., did not show any significant degradation when tested in similar environments [38]. Thus, there remains a considerable controversy regarding the biodegradation of these polymers.
This study evaluates the validity of these biodegradability claims using our synthesized MTPS/PETG blend containing 30% starch. A blend containing well-dispersed starch with increased compatibility was prepared using reactive extrusion. Aqueous biodegradation of the synthesized blend was tested according to ISO 14852. The goal of this test was to determine the effect of grafting and addition of starch on the biodegradability of this compatibilized blend. The hypothesis was that if MTPS is evenly dispersed throughout the PETG matrix, then both PETG and MTPS will be equally accessible to the microorganisms. The consumption of MTPS should increase the surface area of PETG available for the microbes, resulting in enhanced biodegradability. However, aqueous biodegradation studies showed that only the MTPS portion of the blend biodegraded, leaving the PETG intact. Addition of starch did not improve the biodegradability of nonbiodegradable polymers. Claims of accelerated biodegradation of nonbiodegradable polymers by the addition of organic constituents or additives needs to be re-examined and carefully evaluated using accepted international standards [3,37].

2. Materials and Methods

2.1. Materials

High-amylose corn starch was obtained from National Starch (Bridgewater, NJ, USA). The equilibrium moisture content of starch was about 12% (w/w). Glycerol was obtained from J.T. Baker (Phillipsburg, NJ, USA) and was used as received. Maleic anhydride (MA) and 2,5-bis (tert-butyl-2,5-dimethylhexane, tech. 90% (Luperox 101) were obtained from Sigma-Aldrich (Milwaukee, WI, USA). Glycol-modified PET (PETG) was purchased from the Eastman Chemical Company (Kingsport, TN, USA) and was used as is. Figure 1 shows the structure of PETG.

2.2. Preparation of MTPS–g–PETG Graft Copolymers

MTPS was prepared in a twin-screw corotating CENTURY ZSK-30 extruder as explained in our previous article [22]. MTPS with more than 80% grafting was dried for at least 24 h to remove excess moisture. Similarly, PETG was also dried to remove moisture. The dried MTPS and PETG were mixed in a 30:70 w/w ratio and fed through the single-screw feeder. The feed rate was maintained at 100 g/min or 6 kg/h for the mixture by calibrating it. The temperature profile was set at 90/110/120/130/140/150/160/160/160/150. The screw speed was set at 100 rpm, and melt temperature was about 140 °C. The resulting MTPS/PETG blend was pelletized in line after cooling in water bath using a Scheer Bay pelletizer. The overall process is as shown in Figure 2.
The pellets were dried overnight in an oven at 65 °C before storing. Further details for the apparatus and setup can be found in our reference [22].

2.3. Characterization and Analysis

After preparing the blend in the extruder, the extent of reaction or grafting was measured using Soxhlet extraction and was further confirmed using thermogravimetric analysis (TGA). FT-IR spectroscopy was used to provide further validation of transesterification reaction. The mechanical properties of the MTPS/PETG blend were determined and compared with neat PETG properties. Scanning electron microscopy (SEM) was used to study the dispersion of MTPS in PETG phase. Aqueous biodegradation of the reactive blend was also studied using ISO 14852.

2.3.1. Soxhlet Extractions

Selective solubility of glycerol in acetone was used to establish and determine percent covalent grafting of glycerol on starch, as explained in our previous studies [21,22,27,39]. The percentage of covalently grafted glycerol translated in a weight gain in the residue. It was calculated from the mass balance as shown in the following equation:
%   g r a f t i n g = W 1 − W 2 W 1 × 100
where W1 is the weight of glycerol present in the sample originally, and W2 is the glycerol in the extract after 72 h, i.e., free glycerol. Similar Soxhlet analysis was used to determine the % grafting of PETG on MTPS backbone. In this, the solvent used was dichloromethane (DCM), which selectively dissolves PETG. Hence, it was expected that any covalently bonded PETG will stay in the residue with MTPS and only the free, ungrafted PETG will be extracted with DCM. The percent grafting for PETG was also calculated using Equation (1). The residues and extracts of all the samples were analyzed using TGA in order to confirm the results from Soxhlet analysis.

2.3.2. Thermal Analysis

The degradation temperature of samples was obtained by thermogravimetric analyzer, TGA Q50 (TA Instruments, New Castle, DE, USA). The general sample weight used was 5–7 mg. The sample was heated in an aluminum pan from room temperature to 600 °C at the rate of 10 °C/min under inert nitrogen atmosphere. Weight loss (%) of the sample as a function of temperature (°C) was obtained using TA universal analysis 2000 software. Since PETG is an amorphous polymer, further analysis of melting point and crystallinity was not performed.

2.3.3. Tensile Testing

A tabletop DSM 15 cc mini extruder (DSM Research B. V., Sittard-Geleen, The Netherlands) and 3.5 cc mini-injection-molder (DACA Instruments, Santa Barbara, CA, USA) were used for making the injection-molded test bars of PETG and the blend. Neat PETG, as well as the MTPS/PETG blend, was measured in the required quantity and fed to the corotating twin-screw microcompounder at a temperature of 210 °C and screw speed of 100 rpm. After sufficient mixing and melting, the mixture was transferred to the DACA microinjector set at 210 °C. The mixture was then injected into the mold maintained at 65 °C with an injection pressure of 140 psi. Then, the sample was held in the mold for about 15 s to cool down before being removed. The samples were preconditioned for 2 days at 25 °C in a humidity chamber with RH of 50% before any analysis. Tensile testing was performed using an Instron model 5565-P6021 (Instron, Norwood, MA, USA) with a 5 kN load cell and grip separation speed of 12.5 mm/min, as per ASTM D882. A minimum of 5 replicates were used to ensure repeatability of the data.

2.3.4. Impact Testing

Injection-molded test bars required for notched Izod impact testing were also prepared by a DSM 15 cc mini-extruder (DSM Research B.V., Sittard-Geleen, The Netherlands) and 3.5 cc mini-injection molder (DACA Instruments, Santa Barbara, CA, USA) using similar conditions to those explained in Section 2.3.3. The samples with a dimension of 64 × 12.7 × 4 mm were notched using a Tinius Olsen Model 22–05-03 Motorized Specimen Notcher (West Conshohocken, PA, USA). The notch marked was 2.54 mm deep. The testing was carried out in accordance with the ASTM D256 standard test method for determining the Izod pendulum impact resistance of plastics [40]. Six replicates were used per sample to ensure repeatability of the data.

2.3.5. Fourier Transform Infrared Spectroscopy

FT-IR spectra of pure PETG, MTPS, and of MTPS–g–PETG residue after Soxhlet extraction were recorded on Shimadzu IRAffinity-1 spectrometer (Columbia, Portland, OH, USA) equipped with MIRacle ATR attachment. The spectra were recorded between the wavelengths of 500–4000 cm−1 in absorption mode.

2.3.6. Scanning Electron Microscopy

A JOEL 6610 LV scanning electron microscope (JEOL Ltd., Tokyo, Japan) was used to study the dispersion of MTPS in PETG in the pellets using a similar procedure to that explained in Kulkarni (2021) [22]. MTPS phase in the pellets was dissolved using 6 N HCl and air-dried for 12 h in a fume hood. Then, they were mounted on aluminum stubs using high-vacuum carbon tabs and coated with gold using a sputter coater. The dispersion of MTPS was indicated by cavities formed in the pellet and was examined using JOEL at 3000× magnification at 10 kV. A set of fractured tensile bars was also examined to look at the fracture morphology of the samples. The bars were immersed in liquid nitrogen for ~2 min and then fractured. Fracture surfaces were mounted on aluminum stubs and examined using JOEL at 500× magnification at 10 kV.

2.3.7. Aqueous Biodegradation

The biodegradability of MTPS/PETG samples was tested in an aqueous environment. The testing was carried out in an aerobic environment, maintaining the temperature at 30 °C. International Standard ISO 14852 was followed for the test procedure and setup [41]. Two blank tests (without any carbon source addition) and two positive controls (cellulose) were also tested, along with the test material (MTPS/PETG). Further details for the setup and testing procedure can be found in our reference [22].

3. Results and Discussion

The pellets obtained after reactive extrusion of MTPS and PETG were dried for 12 h before using for any further characterization. Figure 2 shows the appearance of the pellets as light brown solids.

3.1. Biobased Value Proposition

Biobased carbon content and carbon footprint reduction was calculated according to ISO 22526 parts 1 and 2 [15]. Carbon percent of the polymers was calculated using molecular formula of PETG and MTPS. The percent carbon content of PETG was found to be 65%, whereas for MTPS, %C was 46.8. Adding 30% biobased MTPS by wt. corresponded to a biobased content of 22.8%. Using experimentally determined biobased carbon content and applying fundamental stoichiometric calculations, the amount of CO2 removed from the environment by 1 kg of material was calculated. A total of 100% petro-fossil carbon-based PETG would result in zero CO2 removal from the environment. It was found that if 30% of biobased MTPS is added to make a blend with petro-fossil PETG, it would result in 0.52 kg of CO2 removal from the environment. Recently, some companies such as Anellotech announced efforts towards making 100% biobased PET [42]. If such partially biobased PETG was used in the blend (biobased terephthalic acid and ethylene glycol) in which about 85% of carbon is replaced by biobased carbon, the CO2 removal from the environment was calculated to be 1.92 kg/kg of polymer. These results are graphically shown in Figure 3.
Eventually, this carbon will be released back to the environment as CO2 at the end of its life through incineration, composting, or anaerobic digestion etc., and it is then captured by the next season crops/plants, resulting in a net zero carbon footprint [4]. Thus, replacement of petro-fossil carbon in PETG by biobased MTPS carbon offers a value proposition of reduced carbon footprint and moving towards a “circular economy”.

3.2. Soxhlet Extraction

Previous studies from our group regarding synthesis of in situ graft copolyesters with MTPS and PBAT by Raquez et al. (2008) and Nigam (2018) showed that the percent grafting was maximum when the ratio of polyester to MTPS was taken as 70:30 (wt./wt) [27,39]. When the amount of MTPS was increased further, the grafting was found to be reduced. Assuming that the same would be true for MTPS/PETG blends, a 70:30 ratio of PETG to MTPS was selected for this experiment. MTPS/PETG (30/70) blends prepared by reactive extrusion were analyzed using Soxhlet extraction and TGA. Figure 4 shows the comparative TGA graphs for MTPS, PETG, and MTPS/PETG reactive blend. It was found that the degradation temperature for the reactive blend was in between that of pure MTPS and PETG. It showed two degradation stages. The first one corresponds with MTPS, and the second with PETG.
These PETG pellets were analyzed for % grafting using Soxhlet extraction, as explained in Section 2.3.1. PETG is soluble in DCM and MTPS is not; hence, DCM was used for Soxhlet extraction to find the % grafting of PETG on MTPS. To confirm that the grafting reaction had occurred, the residues and extracts of the resin after Soxhlet extraction were compared with a control. The control here was a physical mixture of PETG and MTPS. PETG and MTPS pellets were mixed in the same ratio of 70:30. The mixture was then extracted with DCM for 72 h. It was found that the entirety of PETG was extracted with DCM in 72 h and none of the MTPS was dissolved in DCM. Thus, the residue consisted of pure MTPS, and the extract was pure PETG. The results were confirmed by TGAs of residue and extract. When MTPS/PETG extruded resins were extracted with DCM, it was found that some PETG remained in the residue even after 72 h. Since DCM dissolves only PETG, it was expected that after extraction the extract TGA should show a peak only for PETG and the residue should only show MTPS if no grafting reaction had occurred. However, it was observed that the residue showed a distinct peak for PETG, accounting for 25% of total weight as shown in Figure 5. Considering that 70% of PETG was introduced while making the blend, it was concluded from the mass balance that around 33% of added PETG was grafted on the MTPS backbone.

3.3. FT-IR Analysis

To confirm the grafting of PETG on MTPS, FT-IR spectra of PETG, MTPS, and residue of MTPS–g–PETG copolymer were recorded, as shown in Figure 6. The MTPS spectra clearly showed presence of a broad OH peak between 3100–3600 cm−1. The PETG spectrum shows presence of C=O and C–C–O bonds at 1740 cm−1 and 1240 cm−1. The MTPS–g–PETG residue showed presence of both MTPS and PETG characteristic peaks. The presence of PETG in the residue was confirmed from the characteristic peaks of the ester linkage at 1741 and 1240 cm−1.
These observations from Soxhlet extraction and FT-IR attested to the formation of in situ PETG-g-MTPS graft copolymer derived from PETG and MTPS, covalently linked to each other through acid-promoted transesterification reactions between the ester functionalities from the PETG backbone and the hydroxyl groups from MTPS. Figure 7 presents a possible scheme for the possible transesterification reaction between the two polymers. As observed in our previous studies, it can be noted that these transesterification reactions are promoted by the acidic maleic anhydride moieties grafted on starch backbone which were formed during synthesis of MTPS [21,22,27]. The detailed mechanism and structure of MTPS are not included here; they can be found in our previous references. Figure 6 shows the three most possible routes for the transesterification reaction. MTPS has three primary carbons at which the transesterification reaction can occur. PETG, being a copolyester, can attach to the starch -OH groups from either direction; from the ethylene glycol side or cyclic diol side, as shown in Figure 7 (1). PETG can also be grafted on any one of the primary carbons of glycerol covalently bonded to the starch backbone, as shown in Figure 7 (2 and 3). The efficiency of each reaction and the exact ratio of each of these byproducts was not determined here. A previous study carried out in our group for transesterification reactions between PBAT and MTPS showed that the third pathway was the most probable pathway for that blend [16]. The same could be true for MTPS/PETG as well; however, a detailed study and understanding of this reaction mechanism was not the scope of the current study. For the purpose of this study, it was assumed that the resulting graft copolymer will be a mixture of these products.

3.4. Mechanical Properties

Tensile properties of PETG and MTPS/PETG (30:70) blend were analyzed on a universal testing machine (UTS). These data were compared to the tensile properties of PETG. A minimum of five samples were used to ensure the repeatability of the data. The results are shown in Table 1. Figure 8 shows the average stress–strain curves for the data. It was observed that inclusion of 30% MTPS in PETG caused only a 15% reduction in the tensile stress of the sample, whereas the elongation properties decreased by 40%. However, the resulting elongation is still significant when looking at the brittle nature of MTPS. The resulting blend still has acceptable mechanical properties for many applications.
Impact properties of MTPS/PETG (30:70) blend were also analyzed and compared with neat PETG, as shown in Figure 9. Around 40% reduction in notched Izod impact strength was observed for the reactive blend. Impact strength testing for neat MTPS could not be performed because of its extremely brittle nature.
This MTPS–g–PETG graft copolymer can be further used as a compatibilizer for making the MTPS/PETG blends. Several studies have shown synthesis of such graft copolymers and their use as compatibilizers for making the blends of the two original polymers. Lopez et al. (2019) showed that the use of St–g–PCL as a compatibilizer in starch/PCL blends improved the interfacial adhesion between the two polymers, and the maximum tensile strength and tensile modulus were significantly increased compared to the noncompatibilized blends [43]. Similar results were also obtained by Giri (2018), where melt blending of PLA–g–PDMS copolymer with PLA showed significant improvement in tensile properties and toughness of PLA [34]. It would be very interesting to see the impact of MTPS–g–PETG on the properties of MTPS/PETG blends. This was not included in the current study.

3.5. Phase Morphology

Figure 10a shows the morphology of the MTPS/PETG blends produced by extrusion after selective removal of the MTPS phase. MTPS from the blend was dissolved using 6N HCl, resulting in the cavities shown in Figure 10a. Analysis by ImageJ showed that the proportion of cavities was close to 30%, which correlated with the percentage of MTPS added in the blend. Figure 10a shows micron-sized distribution of MTPS particles evenly distributed in the PETG matrix. It was observed that the particle size for MTPS was between 1–5 μm, which indicated good compatibilization between MTPS and PETG. The reduction in the mechanical properties of the blend, as observed in Section 3.3, might be due to the high amount of MTPS added and the inherent brittle nature of MTPS.
The morphological behavior of neat PETG and the blend was analyzed using SEM images of tensile fracture surfaces. The representative images are as shown in Figure 10b,c. The SEM images of tensile fractured surfaces of PETG showed stretch marks, indicating a ductile fracture, whereas for the reactive blend, rougher surface was observed, indicating comparatively brittle fracture.

3.6. Biodegradability Testing

The test was set up for aqueous biodegradation according to ISO 14852. The bioreactors were closed with rubber septa and then incubated in a dark, temperature-controlled room maintained at 30 °C. The bioreactors were agitated throughout the run with the help of magnetic stirrers. The CO2 that evolved from the bioreactors was collected in NaOH solution and titrated with HCl to determine the CO2 that evolved from the samples and % biodegradation, as described our previous paper [22]. Figure 11 shows the setup used for the testing [22].
The initial curve obtained for aqueous biodegradation of MTPS/PETG till day 80 is as shown in Figure 12a. During the first 80 days, rapid CO2 production was observed. The slope for this biodegradation curve was linear and increasing. It continued till the biodegradation reached about 30%. However, once it reached a biodegradation of 30%, the biodegradation curve reached a plateau, and no further biodegradation was observed till day 150, as shown in Figure 12b. This might be because this blend contained only 30% of MTPS, which is a component that is readily biodegradable. PETG is not biodegradable in aqueous or composting environments, as observed in our other study (data not shown here). Hence, once the MTPS present in the blend was consumed by the microbes, no further biodegradation was observed. From the SEM images, it was confirmed that the dispersion of MTPS in PETG was uniform, which indicated that PETG was as accessible to the microorganisms as MTPS.
Even then, only MTPS was selectively consumed, leaving nonbiodegradable PETG. This showed that formation of in situ graft copolymer of PETG does not improve the biodegradability of the PETG. An increase in surface area due to MTPS biodegradation had no impact on the biodegradation of PETG. This contradicts the results observed by many studies that claim that addition of biodegradable polymers to nonbiodegradable polymers such as LDPE accelerates the biodegradation of those polymers [11,35,36,44]. The increase in biodegradation claimed by Bulatović et al. (2022) was by monitoring the weight loss of the blend [11]. In the study by Datta et al. (2018), biodegradability of the sample was assessed by determining the changes in the elongation at break and tensile strength at different time intervals for different-starch-content biodegradable films [36]. These are not indications of true biodegradation. The increased biodegradation that is observed in these blends is only due to the consumption of the biodegradable polymer. The nonbiodegradable part of the blend is still expected to last in the environment for a long time and form microplastics. Another study carried out by Selke et al. (2015) actually used CO2 evolution as the method for measuring the biodegradation did not find any improvement in biodegradation of polyethylene (PE), PET, and PP when tested in similar environments. These findings coincide with our work, so there is no reason or evidence to expect that addition of starch will increase the biodegradability of the nonbiodegradable polymer. Thus, we should be aware of the invalid claims made for accelerated biodegradability of such plastics.

4. Conclusions and Future Study

This article reports synthesis of MTPS/PETG blends using a twin-screw extruder through reactive extrusion processing. Maleated thermoplastic starch (MTPS) containing 80% grafted glycerol was melt blended with PETG to form compatibilized MTPS–PETG blends. In situ MTPS–g–PETG graft copolymers generated by transesterification functioned as compatibilizer. The addition of starch reduces the strength and percent extension of the blend; however, it is acceptable for many applications requiring a balance of cost, mechanical properties, and biobased content. The grafting of PETG on MTPS was 33%, which was quite lower. This grafting was obtained without any external catalyst. Therefore, efforts can be made to improve the grafting by adding external transesterification catalyst(s). Studies have shown that polymers such as PLA, PCL, and PBAT were grafted on starch backbone using transesterification catalysts such as imidazole, Sn(Oc)2, and N-methylimidazole [31,32,33]. These reactions showed more than 60% grafting, and the resulting copolymers showed improved mechanical properties. Similar results can be expected from a high grafting content of current MTPS–g–PETG copolymer. Increased grafting might affect the properties of the blend, which needs to be studied further. SEM analysis of the pellets showed that MTPS was evenly distributed in the PETG matrix. Aqueous biodegradation studies showed that addition of starch did not enhance the biodegradability of PETG. The observed biodegradation of the blend was solely due to the biodegradation of MTPS. After consumption of 30% MTPS, the biodegradation did not increase further. We underline the importance of using polymer carbon conversion to CO2 by microbial metabolism as the primary requirement for reporting biodegradability, and reiterate caution to avoid false conclusions that addition of biodegradable polymers will render nonbiodegradable plastics biodegradable.

Author Contributions

Conceptualization, A.K. and R.N.; methodology, A.K.; investigation, data curation, A.K.; writing—original draft preparation, A.K.; writing—review and editing, R.N. and A.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. Structure of PETG.
Figure 1. Structure of PETG.
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Figure 2. Synthesis of MTPS/PETG blend.
Figure 2. Synthesis of MTPS/PETG blend.
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Figure 3. Material carbon footprint—amount of CO2 removed from the environment per kg of resin.
Figure 3. Material carbon footprint—amount of CO2 removed from the environment per kg of resin.
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Figure 4. TGA graphs for MTPS, PETG, and MTPS–g–PETG.
Figure 4. TGA graphs for MTPS, PETG, and MTPS–g–PETG.
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Figure 5. DTG curve of residue remaining after Soxhlet extraction.
Figure 5. DTG curve of residue remaining after Soxhlet extraction.
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Figure 6. Comparative FT-IR spectra of PETG, MTPS, and MTPS-PETG residue.
Figure 6. Comparative FT-IR spectra of PETG, MTPS, and MTPS-PETG residue.
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Figure 7. Proposed mechanism for transesterification reactions between PETG and MTPS promoted by MA-derived acidic moieties grafted on starch backbone.
Figure 7. Proposed mechanism for transesterification reactions between PETG and MTPS promoted by MA-derived acidic moieties grafted on starch backbone.
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Figure 8. The average stress–strain curves of PETG and MTPS/PETG.
Figure 8. The average stress–strain curves of PETG and MTPS/PETG.
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Figure 9. Notched Izod impact strength of PETG and MTPS/PETG reactive blend.
Figure 9. Notched Izod impact strength of PETG and MTPS/PETG reactive blend.
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Figure 10. SEM Images of (a) MTPS/PETG blend after leaching of MTPS phase with HCL; (b) tensile fracture surface of PETG; (c) tensile fracture surface of MTPS/PETG.
Figure 10. SEM Images of (a) MTPS/PETG blend after leaching of MTPS phase with HCL; (b) tensile fracture surface of PETG; (c) tensile fracture surface of MTPS/PETG.
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Figure 11. Experimental setup for aqueous biodegradation [22].
Figure 11. Experimental setup for aqueous biodegradation [22].
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Figure 12. Biodegradation curve for MTPS–PETG for (a) initial 80 days; (b) total 150 days.
Figure 12. Biodegradation curve for MTPS–PETG for (a) initial 80 days; (b) total 150 days.
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Table
Table 1. Tensile properties of PETG and MTPS/PETG blend.
Table 1. Tensile properties of PETG and MTPS/PETG blend.
Modulus (MPa)Tensile Stress at Yield (MPa)Tensile Strain at Yield (mm/mm)Tensile Stress at Break (MPa)Elongation at Break (%)
PETG632.7 ± 45.753.8 ± 0.82.2 ± 0.141.9 ± 0.8215 ± 10.6
MTPS/PETG483.68 ± 29.344.5 ± 1.21.5 ± 0.122.9 ± 1.5120.7 ± 15.5
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Kulkarni, A.; Narayan, R. Morphology, Mechanical Properties, and Biodegradability of Modified Thermoplastic Starch/PETG Blends with In Situ Generated Graft Copolymers. Sustainability 2023, 15, 2227. https://doi.org/10.3390/su15032227

AMA Style

Kulkarni A, Narayan R. Morphology, Mechanical Properties, and Biodegradability of Modified Thermoplastic Starch/PETG Blends with In Situ Generated Graft Copolymers. Sustainability. 2023; 15(3):2227. https://doi.org/10.3390/su15032227

Chicago/Turabian Style

Kulkarni, Apoorva, and Ramani Narayan. 2023. "Morphology, Mechanical Properties, and Biodegradability of Modified Thermoplastic Starch/PETG Blends with In Situ Generated Graft Copolymers" Sustainability 15, no. 3: 2227. https://doi.org/10.3390/su15032227

APA Style

Kulkarni, A., & Narayan, R. (2023). Morphology, Mechanical Properties, and Biodegradability of Modified Thermoplastic Starch/PETG Blends with In Situ Generated Graft Copolymers. Sustainability, 15(3), 2227. https://doi.org/10.3390/su15032227

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