Introduction

Plastics are versatile materials that are used in many products of daily life due to their outstanding and multifunctional properties. In particular, the packaging sector is dominated by plastics, because these materials are flexible, stable, versatile in processing, and cheap to produce. Therefore, plastic production was steadily increasing with peak production in 2019 of 368 million annual tons and is anticipated to grow to 589 million annual tons in 2050 [1]. In 2020, the top production share was polypropylene (PP, 18%), followed by low-density polyethylene (LD-PE 18%) and high-density polyethylene (HD-PE, 12%) [2]. By application, plastics are mainly used for packaging (36%) and have therefore a very short product lifecycle [3]. From an environmental point of view, the persistence of plastics [4,5,6,7,8] causes a pollution problem when they are handled carelessly and end up uncontrolled in the environment [9]. Plastic items can weather and form microplastics [10, 11]. The extent of this pollution is unknown but estimated as concerningly high as, that out of 8.3 billion tons of plastics that were produced from the 1950ies to now, 56% were discarded. This discarded fraction is expected to have been landfilled (more or less controlled) or, more concerningly, that it was littered [3]. Therefore, strategies for the reduction of the shelf-life of plastics were sought-after. Besides, the development of biodegradable plastics, which are composed of biodegradable polymers [12, 13], also additives were developed to accelerate the degradation of conventional plastics (e.g., polyolefins) under certain conditions. The advantage of such additives is that production processes need to be changed only marginally and the basic material properties of the original materials can be retained. Plastics made with prodegradant additives are classified as oxo-degradable plastics (ODP) [14] or pre-oxidant additive-containing plastics (PAC) [15]. The characteristics of such plastics are that the additives catalyze, in the presence of UV radiation, oxygen, and/or heat the oxidation of the persistent alkyl bonds, that leads to hydrophilization of the plastics and shortening of the long polymer chains [16, 17]. The combination of these processes fosters the enzymatic breakdown of the oxidized polymers and the formation of a biofilm on the plastics by microorganisms, which is reported to lead to accelerated degradation [18]. Therefore, such plastics were merchandised as a solution to reduce (macro-)plastic pollution from littering or plastics that were unintentionally released to the environment [15]. However, concerns were raised that prevailing environmental conditions differ strongly from the ideal conditions for the initial oxidation, and therefore, ODPs could not be degraded entirely under realistic conditions, leading to partial degradation and fragmentation into microplastics [15]. This, together with the concerns of the plastic industry that these types of plastics will lower the quality of recycled materials, led to a ban on ODP in the EU [19]. However, due to the different options of available catalysts, which are referred to as prodegradants, it is difficult to assess whether a plastic product contains such additives and if it falls under the ban or not. Inorganic and organic compounds like polyunsaturated compounds, transition metal ions, and metal complexes (e.g., dithiocarbamates) can be used as such prodegradants [14, 20]. In addition, the composition of these additives remains undisclosed by the manufacturers and only a few formulations can be tracked by publications in patents, which, however, does not guarantee that the disclosed formulation is the actual marketed one [14].

Recently, new additives were developed and merchandized that induce the biodegradability of polyolefins and avoid the formation of microplastics [21], which was one of the main arguments for banning prodegradant additives under the “single use plastics directive” [19]. Additionally, one of the products is claimed to be food safe (FDA approved) and compliant with several test standards (ASTM D6954, ASTM D5988, and OECD 202, 207, 222) [18, 22]. Another type of additive technology is based on enzymes, and it is promoted to enhance the ability of microorganisms to decompose persistent plastics by, e.g., fostering microorganisms to secrete acids [18, 23], when it is added.

In the present study, four different plastic packaging materials were investigated, to which two different additives were added. Two PP specimen types were prepared with a prodegradant additive based on a biotransformation technology, and one PP and one PET specimen type was prepared with an enzyme-based additive. The specimens were investigated for their degradability under aerobic and anaerobic conditions and inspected for the formation of microplastics of sizes > 1 mm. In addition, a more in-depth investigation was performed using ATR-FTIR investigation to elucidate molecular changes in the materials.

Materials and methods

Test specimen description

Four different specimens were investigated that were produced in four different packaging forms (Table 1). Out of these plastic specimens, three were PP-based and one was a PET-based packaging item. The PP plastic packaging had the form of a light cup (‘yogurt cup’), a coffee capsule (‘coffee capsule’), and a ketchup-bottle (‘bottle’). The PET specimen was a transparent container (‘jar’). The specimens were made by a large plastic producer by adding masterbatch materials from ‘Biosphere Plastics’, that is described to make polymers biodegradable by a biotransformation technology (referred to as additive based on biotransformation technology), or from ‘Polymateria’, that is based on an enzyme technology (referred to as enzyme technology) during the production process. The dimensions of the coffee capsule specimen were 2.46 cm in diameter and 2.7 cm in height, the yogurt cup specimen had a diameter of 7.5 cm and a height of 12.2 cm, the jar specimen had a diameter of 8.8 cm and a height of 15 cm, and the bottle specimen had a long diameter of 4.5 cm, a short diameter of 4 cm, and a height of 17 cm (images of the specimen are shown in the supplementary material Fig. S 3).

Table 1 Specimen description by polymers, form, and additive technology used
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Composting trial

Pre-treated input material from a technical composting plant in Vienna (Austria) was used as rotting material for this study. Before putting into the rotting reactors in the laboratory, the material was shredded to < 20 mm and moistened to optimum water content (typically 50–65%).

The laboratory composting experiment was conducted in an adapted 240-l residual waste container situated in a climate chamber to minimize temperature losses. A defined cavity was created in the lower area by a metal mesh, through which air was evenly supplied to the above lying rotting material. The applied air volume was determined by a gas meter and recorded twice a working day. A permanently installed temperature probe was used to monitor the rotting temperatures, which were recorded two to four times per working day. The exhaust air was diverted through an opening in the lid. The exhaust air was analyzed via the biogas measuring unit (IR detector for CO2 and CH4, and chemical O2 cell, portable landfill gas analyser LFG 20 ADC, CEA Geotechnical instruments, UK) that was connected by a T-piece to the exhaust gas, for its composition (CO2, O2, and CH4 content in Vol%). The gas composition of the exhaust gas was measured and recorded twice a working day.

The temperature of the climate chamber was always kept 1 °C below the actual rotting temperature and was readjusted at least twice a working day. On the one hand, this procedure minimizes temperature losses, and on the other hand, it avoids an increase of the “natural” rotting temperature by the external energy supply.

The supplied air quantities were controlled and adjusted at least twice per working day, based on the CO2 concentration in the exhaust air. The air volume was adjusted in such a way that the CO2 concentration in the exhaust air remained in the range of 10–15 vol%, which guaranteed aerobic milieu conditions similar to technical composting plants working according to Austrian state-of-the-art of composting [24].

For the aerobic degradation test, five plastic bodies of each type were cut into halves, individually marked, examined by ATR-FTIR, weighed, and photographed, before mixing them with the biowaste. These ten halves of each type of specimen were used in the composting experiment. The specimens were placed individually and separately in the rotting container before they were covered with rotting material in layers (Fig. S1 A in the supplementary materials). The smaller coffee capsule specimens were mixed with biowaste and placed in pollen bags for better retrieval from the input material (Fig. S1 B–D in the supplementary materials). Care was taken not to stack specimens. During the turning events, the test specimens were sorted out of the rotting material, washed with tap water, and dried with paper towels before they were photographed and weighted. The mass changes of all test specimens were recorded during all turning processes. Afterwards the specimens were placed back into the rotting reactor. Statistical significance of the mass changes was determined by a one-way ANOVA on ranks with pairwise post hoc Tukey test calculated in Sigma Plot 14.0 for each type of specimen.

The composting experiment was continuously monitored and controlled. The rotting material was turned seven times, after 10, 16, 30, 35, 42, 47, and 62 days. The total rotting process took 84 days.

Digestion trials

For the digestion experiments, six apparatuses (1 l glass bottle, Schott, Germany) were assembled according to the setup, as shown in Fig. S 2 (supplementary material). The reaction bottle (right bottle) contained approx. 500 g digestate with a water content (WC) of approx. 80% wet mass (WM) and either a 10 g dry mass (DM) plastic specimen (cut to approx. 5 cm2) or microcrystalline cellulose (control material). Microcrystalline cellulose was used as the reference material as it is suggested in ISO EN 15985 [25], by which the experimental setup for anaerobic testing was inspired. The digestate was obtained from a mesophilic digestor in Vienna that is operated at 37 °C and processes 22,000 t a−1 of kitchen waste. The bottles were incubated in a water bath at 52 °C for about 50 days. The digestion gases, which were generated during the process, increase the pressure in the gas volume between the reaction bottle and the barrier liquid bottle (bottle in the middle in Fig. S 2), which were connected by a tube. The rising pressure (gray-dashed arrow) displaces the barrier liquid from the second bottle via a glass tube into a measuring bottle (left bottle), which was also equipped with a glass tube that was immersed into the liquid and hindered atmospheric air to enter the system. A graduation allowed to determine the displaced volume as the difference between the initial value and the value at the time of readout. Calibration of the graduation was made beforehand with barrier liquid and a graduated cylinder. After readout, the valve on the first bottle was opened, which resulted in a pressure equalization (with ambient air pressure) inducing the backflow of the barrier liquid from the third bottle into the second one. As soon as the valve was closed again, a new measuring cycle started. The readouts were taken at least twice a day on working days and sporadically on weekends. With long measuring intervals, it happened in some cases, during the most active phase of digestion, that the entire barrier liquid was displaced into the measuring bottle and the maximum measuring level was overgone. In these cases, an estimated value was linearly interpolated based on 5 values before and after the missing values. As barrier liquid, saturated NaCl solution was used, which reduces the gas solubility in the solution. The displaced volume was corrected for atmospheric pressure and room temperature, which were also recorded during each readout.

The digestion trials were carried out consecutively for each plastic specimen in triplicates, with triplicate controls (microcrystalline cellulose). Therefore, the reactivity of the digestate was different between the trials, but the same in comparison to the control.

FTIR analysis

Molecular changes were determined by an augmented total reflection-Fourier transform infrared (ATR-FTIR) spectrometer (Alpha Bruker, Germany) using a diamond ATR unit and a Deuterated Triglycine Sulfate (DTGS) detector. Spectra were recorded in a wavelength range of 500–4000 cm−1 at a resolution of 4 cm−1. Treated specimens were washed with water and dried with paper towels before analyzing them. The spectra were corrected by the atmospheric compensation algorithm (OPUS Software, Bruker, Germany) and normalized by dividing the absorbances at all wavelengths by the maximum absorbance. The normalization and representation of the spectra were performed in Microsoft Excel 365 after export from OPUS.

Microplastic analysis and loss on ignition

The analyses for microplastics were conducted in different ways. For aerobically treated samples, the rotting material close to the specimen was inspected by eye for fragments in sizes of > 1 mm. In case of anaerobic treatment, the whole digestate was air-dried for at least 2 days and inspected by eye during the separation of the specimens. As no smaller fragments with the same appearance (colour) as the specimens were found in the samples from both treatment methods by eye, it was assumed that no microplastics were formed by fragmentation.

Loss on ignition of each sample was determined before and after aerobic and anaerobic treatment. Ca. 2.5 g of each plastic specimen type was placed in porcelain pots, weighted, and dried at 105 °C in duplicates. After putting the hot pots into a desiccator for 15 min, the pots were weighted again (to determine the residual water content) and placed in a muffle furnace (Nabertherm L9/R). The temperature was increased to 550 °C within a period of ca. 7 h followed by maintaining 550 °C for further 5 h. After cooling down, the still hot samples were placed for 15 min in a desiccator before the pots were weighted again.

Results and discussion

Composting trial

The temperature course (Fig. 1A) showed the characteristic rapid initial self-heating of rotting materials with a peak temperature of 70.4 °C after about 3 days and a following slow decrease of the temperature to an almost constant temperature of about 39 °C, starting at day 29, which was continuing until the end of the composting experiment. Sharp negative temperature peaks marked the turning events, during which the material was removed from the reactor, when also the plastic test specimens were retrieved and characterized. Before the rotting material and the specimens were placed back into the rotting reactor, water was added if necessary. The turning resulted in heat loss, which was usually compensated within 1 or 2 days after continuing the rotting process. Similar peaks were also observed in the pore gas composition (Fig. 1B), with negative peaks for CO2 and positive peaks for O2 in each case, as fresh air (nearly 21 vol% O2 and approx. 0.03 vol% CO2) entered the pores during the turning events. The cumulative amount of air supplied (Fig. 1A) shows that, during the intensive rotting (until approx. day 26), higher amounts of air were required for sufficient oxygen supply. During the curing period (day 26–45), the oxygen demand decreased sharply, which led to significantly lower amounts of air supply. From day 45, the air supply was increased again to dry the rotting material. CH4 levels (Fig. 1B) were below the detection limit of the measuring device, except for a few short phases, in which values < 1 vol% were reached in the exhaust air. This shows that the rotting process was exclusively aerobic. Although no comparative rotting was carried out, it can be assumed, based on the rotting process itself, that the plastics, which accounted for a total of 1.1% of the wet mass (WM) of the rotting material (35 kg WM) at the beginning, had no negative effects on the composting process.

Fig. 1
figure 1

Monitoring parameters evaluated during the composting process. A Rotting temperature and B CO2 (black solid), O2 (gray), and CH4 (gray, dotted) content in the exhaust air of the rotting process. The horizontal black dotted line shows 21 vol% (rounded up O2 content of supplied air and maximum sum of O2 and CO2 in the experiment)

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The mass changes of each specimen were recorded during all turning events and reported as the mean mass change of ten individual specimens, each compared to the weight before adding the specimens to the rotting material (Fig. 2). Compared to the mass of the materials before rotting, the mass after 15 days increased slightly for all plastic types, with the largest increase observed for the yogurt cups (8%) and the smallest for coffee capsules (1.2%). The changes were only statistically significant for the yogurt specimen after 42 and 62 days of treatment versus the control. Jar and bottle specimens showed a remarkably similar increase of about 3%, but this remained constant during the continuous rotting process with a maximum of 4% after 61 days for the bottles. The other two plastic specimens still showed an increase to a maximum of 10.3% for yogurt cups and 7.8% for coffee capsules, both on day 61. This evaluation indicated a general mass uptake, presumably of water, during the rotting experiment. This mass uptake could not be removed by washing, absorption of moisture using paper towels, and air drying. It was clearly shown that the materials did not lose weight, suggesting that no significant fragmentation or aerobic degradation occurred at the macroscopic level. In addition, no fragments of the materials were found in the rotting material close to the specimen that could be identified by eye. The difference between the yogurt specimens and the other specimens cannot be explained as a property of the polymer, since yogurt specimens were made of PP, as were bottle and coffee capsules specimens. The differences between specimens consisted in their thickness, which was the smallest in the yogurt cups. The morphological examinations showed no differences in PP materials, except for deformations (Fig. S3). These deformations could have taken place due to the sustained, elevated temperatures and pressure (loading) by the rotting material in the reactor. Already after 10 days, some jar specimens showed opaqueness appearing at the thread and at the bottom of the plastic specimens (Fig. S4 A and B) and a few yogurt cups and bottles showed some local yellow coloring (Fig. S4 C and D). However, these changes remained after this period and did not spread over the whole material. In addition, the loss on ignition (LOI) and water content (WC) was determined for the specimen (before and after the treatment), which showed that the organic content changed only marginally for the bottle and yogurt cup specimens (− 0.1%), and that yogurt cups exhibited the largest WC (+ 0.8%). These results indicated that the mass increase was mostly caused by the adsorption of water (Tab. 2 in the supplementary materials).

Fig. 2
figure 2

Mean mass changes of the plastic specimen jars (black solid line and circles), yogurt cups (dashed line and squares), bottles (gray solid line and circles), and coffee capsules (gray-dashed line and squares) during rotting for 84 days. The error bars correspond to the standard deviation of the mass change of 10 individual specimens of each plastic type. mt corresponds to the actual mass and m0 to the mass at beginning of experiment. Tests on statistical significance were made by one-way ANOVA on ranks with pairwise post hoc Tukey test on the raw data. Significance is highlighted by * for p < 0.05 versus the initial mass of each specimen (0 days)

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ATR-FTIR examination of the specimens did not reveal any major spectral abnormalities prior to composting that could be attributed to the additives (Fig. 3). It should be noted, however, that the ATR method allows only the examination of a few µm of the specimens, and therefore, only changes near the surface can be detected. All PP specimens (Fig. 3A, C, and D) exhibited characteristic bands at 2914 cm−1 (C–H-stretching vibration of alkanes) whose changes could not be investigated due to the normalization procedure. In the case of yogurt cups (Fig. 3C), a broad band < 1000 cm−1 was found after composting. Since this band was not found in specimens from bottles (Fig. 3A), it seemed that the band was specific for the material mixture of the yogurt cup specimens and not related to the additive, which was used in both specimens. After composting, changes in the relative band heights at 1450 cm−1 (methyl C–H-bending vibration) and 1380 cm−1 (alkyl C–H-bending vibration) of all three PP specimens could be detected (Fig. 3A, C, and D). It was also noticeable that a sharp band at 1250 cm−1 (C–O stretching) and a broad band in the range 950–1180 cm−1 appeared, consisting of at least three superimposed bands at 1150 cm−1, 1190 cm−1, and 1013 cm−1, indicating the formation of C–O bands. In addition, a band at 800 cm−1 (multi-substituted C–H-bending vibrations) increased. Interestingly, these bands were independent of the additive, which was used in yogurt cups and bottles, while coffee capsules were made with the other additive (enzyme-based technology). These appearing bands were typically also observed in PET bottles before composting, together with the most significant band at 1709 cm−1 (aliphatic ketone), which was also used for normalization. However, in PET jars, only slight changes in the band intensities at 1240 cm−1 and 800 cm−1 were observable, which suggested that PET jars did not degrade at all, or that the degradation took directly place without the formation of intermediate products (Fig. 3B). The comparison of the PP specimens with PET indicated that the chemical structure of both types of additives was quite similar and resembled features of PET, most probably C–O, but no C=O functionalities. The emergence of these bands, only after composting, indicated that these functionalities were intermediate products of the additives. Changes in the infrared pattern were also observed by other authors for conventional plastics (LD-PE) after composting, which were mainly in the aliphatic region [26].

Fig. 3
figure 3

Normalized ATR-FTIR spectra of plastic specimens before (grey) and after the composting experiment (black) of A PP bottles, B PET jars, C PP yogurt cups, and D PP coffee capsules. In B, the opaque region of jars was also examined (light grey)

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Also, opaque PET specimens were investigated by ATR-FTIR analysis (Fig. 3B light grey spectrum). These specimens exhibited only weak signals, indicating a weak absorption of infrared radiation. It is likely that the turbidity was related to a phase transition (crystalline-to-amorphous) in PET, which could be induced by the elevated temperatures in the reactors, which may have also caused an expansion of the material. It must be noticed that only the parts that were thicker and appeared more brittle (thread and bottom) became opaque.

Digestion trials

Figure 4 shows the results as the cumulative (normalized) gas volumes in the experimental variants with plastic specimens (P) shown in black (circles) and controls (C) in light grey (squares). The quotient of the two gas quantity values (subtracted by 1), which is displayed in grey, illustrates the different dynamics of gas generation, where positive values indicate more gas generation in experiments with plastics than in the controls and vice versa. The quotients were calculated based on the cumulative gas volumes.

Fig. 4
figure 4

Mean gas generation during the fermentation process at 52 °C of 500 g WM digestate and 10 g specimen material of A jars (PET), B bottles (PP), C yogurt cups (PP), and D coffee capsules (PP). The black lines and circles show the mean gas generations (left y-axes) of three reactors that contained 10 g of plastic and light grey lines and squares indicate the mean gas generation of three control reactors, that each contained 10 g of microcrystalline cellulose instead of plastic. The normalized quotient of the volume in the specimen with plastic specimen V(P) and the control V(C) is shown in gray lines and triangles (V(P) − V(C))/V(C) × 100 [%] on the right y-axes

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The gas generation in reactors with jars (PET) and bottles (PP) specimens showed that both reactors containing plastics and controls generated similar amounts of gas (Fig. 4A and B). Although both experiments generated a sum of about 28 l of gas after about 50 days, the gas amounts increased in a staggered manner. From the normalized gas volumes (V(P) − V(C))/V(C) (× 100 in %), it can be seen that the reactors containing PP had a 28% higher increase in gas evolution within the first 2 days, which was equalized by the control at day 5. A second peak was observed after 10 days, indicating that there was a 42% higher gas generation in jar specimens, compared to the control. The gas generation decreased thereafter and after a period in which the control generated more gas (from about day 15), the gas generation leveled off toward the end of the experiment. The results show that although the degradation processes occurred after different durations in the experiments, in total, they resulted in equal amounts. The digestion tests of bottle specimens showed almost the same behaviour as those of the jar specimens, although the polymers were different, as the jar specimens consisted of PET, and different additives were used in both plastic types (Fig. 4B). In total, 29 l of gas was generated in the digestion experiments of bottle specimens and control. Also, in this case, two characteristic peaks in the (V(P)−V(C))/V(C) curves were observed after 2 and 9 days.

The experiments with yogurt cups and coffee capsule specimens (both PP with different additives, Fig. 4C and D) showed a lower gas generation compared to the experiments with jar and bottle specimen. The gas volume that was generated in the controls was 4 l (yogurt cups trial) and 3 l (coffee capsules trial) after 30 days. After that period, almost no gas was generated anymore. Samples containing plastics, on the other hand, produced a total of 8 l (yogurt cups) and 7 l (coffee capsules) in the same period, with the most active phases of 250% (V(P)−V(C))/V(C), in the case of yogurt cup specimens and around 750% (V(P) − V(C))/V(C), in the case of coffee capsules until day 20.

The results show that the digestate used for the last two series of experiments did not reach the biological activity as in the first two experiments. However, the presence of plastics led to an increase in biological activity compared to the control in the less-reactive digestate. Although it is likely to assume that the increased generation of biogas is caused by the prodegradant additives (independent of its type), also other materials (additives or fillers) used in the plastics could be the cause for the observation. Since the control substance was microcrystalline cellulose, which is easily degradable, and the control always generated less gas in the initial phases, than the experiments containing plastics, it can be concluded that the gas generation in the starting phase (first peak of the (V(P) − V(C))/V(C) in Fig. 4A and B) was not caused by the biological reaction, but more due to direct chemical degradation of the additives. Such a degradation of the additives could have led to a raising pH and thus fostering the biological activity. This hypothesis is supported by the result that digestates with low activity in control experiments (Fig. 4C and D) showed increased activities in the presence of the plastic specimens. This hypothesis cannot be conclusively clarified by this series of tests, since the chemical composition of the additives was unknown, and several tests would have to be undertaken with digestates with different inhibiting effects. In addition, each molecule of cellulose (C12H20O10) can be digested theoretically to 6 molecules CH4 and 6 molecules CO2, when hydrolyzed by 2 H2O molecules. This amount would lead to the production of ca. 10 l of gas (for 10 g of cellulose), considering the simple calculation via ideal gas law (pV = nRT) and assuming atmospheric pressure in the reactors and a temperature of 52 °C (325.15 K). As a higher gas generation would have been expected in reactors with microcrystalline cellulose and because the plastics did not degrade, the additional gas quantity in the reactors containing plastics must have originated from the digestion of the biomass of the digestate itself, which was obviously promoted by components in the plastics. Due to the described properties of the prodegradant additives, it appears reasonable to assume that these additives were responsible for the higher biogas generation. However, a final conclusion on the reason of the increased biogas generation cannot be drawn from these experiments. For clarification, control experiments with the same formulation of plastics and lacking the prodegradant additives would be necessary.

Mass changes between the plastic specimens before the anaerobic treatment and after removal from the reactors after approximately 45–50 days are shown in Fig. 5A. The mass of bottle and coffee capsule specimens (PP) hardly changed, in contrast to yogurt cups that increased in mass by 2.3% (Fig. 5A, black striped bars). These slight positive masses indicated an uptake of compounds from the digestate. Visually, only changes in colouration (Fig. 5B “bottle”) of these specimens, but no surface wrinkling at the edges were observed, which would have at least indicated an incipient degradation. Similar observations were already made in the composting test materials. Jar specimens (PET) had an average mass reduction of − 6.5%, which indicated an indicative low degradation of the material. This loss of mass was not previously observed in the composting experiment. As also found in the aerobic trial, all specimens showed only negligible changes in the LOI (Tab. 2 in the supplementary materials). The degradation of the jar specimens could not be confirmed by morphological observations. As already observed in the composting test, parts of the jars (threads and bottom pieces) became opaque also under anaerobic conditions (Fig. 5B). In addition, no microplastic fragments (> 1 mm) of the same appearance as the specimen could be found in the digestate. It is important to note that the digestate had different activities. The digestates that were used to investigate jar and bottle specimens evolved more biogas than the digestates used for investigating yogurt cup and coffee capsule specimens. Therefore, it cannot be excluded that if the coffee capsule and the yogurt cup specimen would have been incubated with more reactive digestates (such as in Fig. 4A and B) a degradation might have been observed.

Fig. 5
figure 5

A Change of mass of plastic specimen jars (black), bottles (grey), yogurt cups (black striped), and coffee capsules (white) after anaerobic treatment mend compared to initial mass m0 and B photographs of plastic specimens jars, opaque jars, bottle, yogurt cups, and coffee capsule after anaerobic treatment. S1, S2, and S3 are the masses of the specimen in the reactors and ‘mean’ represent the mean mass of S1–S3 (colour figure online)

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The normalized ATR-FTIR spectra show molecular changes after anaerobic treatment (Fig. 6), which were evident in bottles (Fig. 6A) as a reduction in absorbance for bands at 1452 cm−1 (C–H methyl group bending vibration) and 1376 cm−1 (C–H-bending vibration). In contrast to the changes found after aerobic treatment (Fig. 3), fewer band changes were observed under anaerobic conditions, which were also found in the PP specimen. In case of coffee capsule specimen, which were also made of PP, two new bands of equal height at 1570 cm−1 and 1535 cm−1 were found, indicating N–O bonds that did not originate from the polymer, and can be hypothesized to originate from degradation products of the additive (Fig. 6D). In the third PP specimen (yogurt cups), higher intensities of most of the characteristic bands were found (Fig. 6C). In addition to the two N–O bands, a characteristic band was also found at 1727 cm−1 (C=O-stretching vibration) indicating degradation processes.

Fig. 6
figure 6

Normalized ATR-FTIR spectra of plastic specimens before (gray) and after (black) the anaerobic treatment. A Spectra of the specimen bottle (PP), B jar (PET), C the yogurt cup (PP), and D coffee capsule (PP). All spectra represent mean spectra from four individual spectra

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In the transparent jar specimens, only small reductions of the characteristic infrared bands were found, which were observed in the region around the main band at 1711 cm−1, 1060 cm−1, 1020 cm−1, and 840 cm−1 (Fig. 6B). The latter vibrations involve heterocompounds typical for PET and indicated the degradation of the polymer material itself. In addition, bands appeared in the range of 2800–2990 cm−1 (C–H-stretching vibrations), which were not typical for pristine PET, but may indicate degradation products of the polymer.

Discussion of the behavior of the investigated plastics in different end-of-life scenarios

Out of the four packaging materials, which had additives designed to increase the biodegradability of conventional polymers, only the PET specimens degraded to a minor degree under anaerobic conditions. Independent of the method of treatment, the polymer type, and the additive, no morphological features that indicated a degradation could be found with the exception of some colouring on PP specimens and some parts of transparent PET becoming opaque. However, on the molecular level, FTIR measurements of PP specimens that were composted revealed the appearance of bands that could be assigned to C–O bands, which indicated the oxidation of the polyolefins. This mechanism is known to be induced by prodegradant additives, particularly those, that can be expected in ODPs after abiotic oxidation [17, 27]. Interestingly, both additives showed the same FTIR band pattern after composting, which suggests that the degradation processes of PP were independent of the claimed mechanism of oxidation, which was abiotic for specimens with additives based on the biotransformation technology and enzymatic for the other additive. This suggests that although the definition of the ODPs covers only the abiotic degradation of plastics, plastic formulation that are not covered by the definition may cause the same problems that led to the restriction of ODPs in the EU and should also be considered in the future. The characteristic FTIR band pattern observed for PP could not be observed in PET specimens, as these characteristic bands were the same as in pristine PET. Consistent with the claims of both additive producing companies, also in these experiments, no evidence for fragmentation of the plastics (> 1 mm) was found, which means that microplastic formation is very unlikely to occur during composting or digestion processes by abiotic (e.g., oxidation, UV degradation) or biotic processes. However, microplastic formation can still be caused by mechanical processes, in the same way as for conventional plastics, e.g., in composts [28]. Regarding the effects of the materials on the composting processes, no negative impacts on the process were found, as high temperatures could be reached and the conditions for compost sanitisation, according to the Austrian state-of-the-art of composting, were fulfilled [29]. In the anaerobic process, plastic trials in active digestate showed the same gas generation compared to microcrystalline cellulose as control, while in less-active digestate, the presence of plastic specimens led to a superior gas evolution. FTIR patterns showed only negligible changes after anaerobic treatment. This is counterintuitive as it is reported that plastics in digestate lead to a reduction of biogas formation rather than an increase [30]. However, the reason for the stimulation of the biogas production by the plastic specimen made with these additives in less-active digestate cannot be answered by the experiments presented here, but could be uncovered in further comparative experiments made with additional controls, containing plastics without such additives, and analyzing the biogas on its CH4 and CO2 content during the process.

Regarding the impact of plastic products made with such additives on different end-of-life scenarios, some conclusions can be deduced from the results of this study. Although no negative effects on the composting process could be found (based on the temperature course), it cannot be excluded that the material can have a negative impact on the compost quality itself, other than representing an impurity. However, since the plastics were not degraded, the material that ends up in the environment will be slowly degraded, possibly forming microplastics and emitting methane under anaerobic conditions.

Regarding the potential of these materials under anaerobic conditions to generate more biogas, compared to controls, a problematic situation can arise. Since the material was also not significantly degraded during the digestion process, the material still retained its intrinsic degradability. If prevailing conditions become advantageous for anaerobic degradation, as, for example, if the material is landfilled and the landfill evolves into the methanogenic phase, the material could start or continue to degrade and add to the unwanted and greenhouse active methane emission of a landfill. The same considerations apply to materials that are littered or unintentionally released into the environment and end up in locations where anaerobic conditions may occur (e.g., anaerobic sediments of surface waters). On the other hand, if littering occurs in aerobic environments (different from composting, like on soils or grassland), effects on the biodegradability cannot be deduced from this study, due to different prevailing conditions mainly regarding the temperature, radiation, and moisture, which are known to affect biodegradation [31]. Risks can occur from the fragmentation of the materials under such conditions, which would release microplastics in the same way as oxo-degradable plastic.

If the material is incinerated, no benefit can be expected from the additive, while the plastics made with prodegradant additives are regarded as troublesome in mechanical recycling [15].

However, the degradation of materials could depend on their dimension and, in particular, their thickness, which means that other product forms, which are thinner, such as films, may be degraded during the usual timeframe of composting or fermentation. Nevertheless, such thinner forms are only used for certain packaging applications and cannot be broadly used for food packaging, such as in jars, cups, bottles, etc.

Conclusion

Packaging plastics made from PP or PET that were produced with additives to foster a degradation by enzymatic or oxidative mechanisms show features of degradation on the molecular level but not on the macroscopic one. Therefore, no additional generation of microplastic other than from conventional plastics can be expected. However, the active generation of large amounts of biogas under anaerobic conditions can lead to unfavorable conditions of such plastics in landfills and the environment and to larger emissions of methane, compared to conventional plastics. Considering the anticipated problems for mechanical recycling and no expectable benefit for thermal treatment leaves no recognizable positive effects for the material’s end-of-life. From an environmental perspective, the uncertainties, their behaviour, and consequently their risks are even higher, since more complex processes are involved. Materials may mineralize earlier than conventional plastics, when littered, which strongly depends on the prevailing conditions and would need further research under realistic conditions to prove this assumption. Although these materials are not classified as already banned (in the EU) oxo-degradable materials, it seems that these new materials behave in a similar way. Therefore, analytical techniques to recognize such plastic compositions and to safely treat them, at best by incineration, will have to be developed in the future.