Assessing the Biodegradation Characteristics of Poly(Butylene Succinate) and Poly(Lactic Acid) Formulations Under Controlled Composting Conditions

Abstract

Biopolymers and bio-based plastics, such as polylactic acid (PLA) and polybutylene succinate (PBS), are recognized as environmentally friendly materials and are widely used, especially in the packaging industry. The purpose of this study was to assess the degradation of PLA- and PBS-based formulations in the forms of granules and films under controlled composting conditions at a laboratory scale. Biodegradation tests of bio-based materials were conducted under controlled aerobic conditions, following the standard EVS-EN ISO 14855-1:2012. Scanning electron microscopy (SEM) was performed using a high-resolution Zeiss Ultra 55 scanning electron microscope to analyze the samples. After the six-month laboratory-scale composting experiment, it was observed that the PLA-based materials degraded by 47.46–98.34%, while the PBS-based materials exhibited a final degradation degree of 34.15–80.36%. Additionally, the PLA-based compounds displayed a variable total organic carbon (TOC) content ranging from 38% to 56%. In contrast, the PBS-based compounds exhibited a more consistent TOC content, with a narrow range from 53% to 54%. These findings demonstrate that bioplastics can contribute to reducing plastic waste through controlled composting, but their degradation efficiency depends on the material composition and environmental conditions. Future efforts should optimize bioplastic formulations and composting systems while developing supportive policies for wider adoption.

1. Introduction

Plastic pollution has become a major global concern as the accumulation of plastic waste threatens environmental sustainability [1]. The increasing use and disposal of plastic materials have inevitably increased improperly managed plastic waste, which most likely remains intact in the environment due to its non-biodegradable characteristics [2,3]. Plastic litter and particles pollute the ocean, rivers, urban environments, and remote habitats [4]. A growing body of evidence suggests that plastics in agricultural soils and aquatic biota pose potential risks to human health through the ingestion of food [3].

The abovementioned issues have spurred substantial interest in the area of degradation of plastics [5]. Biodegradation is a complex physicochemical and biological process driven by microorganisms such as bacteria, fungi, algae, and other biologically derived agents [6]. It presents a promising alternative for mitigating plastic waste by enabling the recycling and treatment of environmental pollutants [7,8]. The microbial degradation of plastics typically progresses through several stages: deterioration, fragmentation, assimilation, and mineralization [9]. Biodegradation processes can be studied under various environmental conditions, including in compost, soil, seawater, and municipal solid waste, as well as during anaerobic digestion. The extent and rate of degradation can be evaluated using several analytical methods, including CO₂ evolution tests, oxygen demand measurements, and mass loss analysis [5,10,11]. The emergence of plastic biodegradation as a sustainable and environmentally friendly approach to managing plastic waste accumulation is prompted by its milder and less energy-intensive conditions [1].

While some bio-based plastics are biodegradable, it is important to note that not all bio-based plastics share this characteristic [5,12]. The term “bio-based” refers exclusively to a material’s manufacturing process and does not indicate its behavior at the end of its lifespan [12]. Therefore, the end-of-life options for bio-based plastics may differ considerably, as shown in Figure 1 [13,14,15,16,17,18].

Plastic degradation is influenced by various factors, including UV radiation, oxygen, temperature, water absorption, and molecular size. Therefore, the rate at which any plastic degrades is largely determined by environmental conditions [5,19,20].

Biopolymers and bio-based plastics, such as polylactic acid (PLA) and polybutylene succinate (PBS), are considered environmentally friendly materials and are widely used, especially in the packaging industry [21]. They are produced from renewable sources such as biomass [21,22]. These types of plastics are biodegradable; however, their biodegradation properties can differ and depend on the degradation environment, conditions, and type of material [21].

PLA, which is synthesized from renewable materials such as wheat, potato starch, rice, corn, and biomass, has attracted significant attention in the last two decades, owing to its characteristics that are comparable with, or even superior to, those of conventional petroleum-based polymers, such as polypropylene (PP), polyethylene terephthalate (PET), and polyethylene (PE) [15].

PBS is typically produced via the polycondensation of 1,4-butanediol with succinic acid obtained from petrochemical sources. PBS can also be produced from renewable sources, such as sugarcane, cassava, and corn [15].

These biopolymers are compostable and can be broken down by microorganisms into carbon dioxide and water within 6–12 months under controlled composting conditions. According to Awasthi et al. (2022) [23], the degradation rate of PLA in compost at a laboratory scale over 30 days was 60%, while in a synthetic material at a reactor scale, its degradation rate was 63% over 90 days. PBS exhibited even better results, with a 90% degradation rate in compost at a laboratory scale within 160 days [24].

Some of the earliest studies on the biodegradability of bio-based plastics date back to 2006. For example, early studies examined the biodegradability of polybutylene succinate-co-terephthalate (PBST) (thermal degradation in nitrogen and air atmospheres) [25], PBS bio-composites in natural and compost soil [26], and starch blends with polyethylene in soil [27], among others. In recent years, there has been a significant amount of research in this area. Recent studies have investigated the biodegradation of PLA-based plastics in salt marsh environments and soil, comparing their degradation to that of conventional plastics [10,19], showing that the degree of degradation varies widely depending on the type of product, temperature, and other conditions.

For instance, research has concluded that PLA (polylactic acid) in different forms (plastic film, packaging, bottles, and biodegradable medical devices) is biodegradable in various conditions (both biotically or abiotically) [28]. Although PBS (polybutylene succinate) is degradable, its degree of degradation may vary. Compost, containing microorganisms and producing natural enzymes, is the most favorable environment for its biodegradation [29,30].

Although these biodegradable plastics offer the potential to reduce plastic waste and minimize environmental impact, it is important to note that they require specific conditions to biodegrade effectively. Proper disposal is necessary for these plastics to break down, and they may not degrade in landfills or in natural environments. Nevertheless, the development and use of biodegradable plastics such as PLA and PBS represent a promising step towards more sustainable and environmentally friendly packaging options.

Even though numerous studies have focused on the biodegradation of PLA and PBS materials, several key differences distinguish this work. Previous research has focused on blends such as PBST/PLA foams without addressing their biodegradation under controlled composting conditions [31], or evaluated PLA/PBS in soil environments with emphasis on disintegration [32]. Other studies assessed biodegradation indirectly via oxygen consumption [33], used PBS or PLA as minor additives rather than the primary components [34], or investigated alternative blend compositions using simpler standards than ISO 14855-1 [35,36]. Some studies focused on marine conditions [37], examined enzymatic degradation [38], or tested pure PBS in compost [39].

In contrast to previous studies, the present study investigates PLA- and PBS-based formulations in the forms of granules and films as the primary materials under ISO 14855-1-compliant controlled composting conditions to gain a deeper understanding of their degradation pathways. Biodegradation was assessed over a six-month period at the laboratory scale based on titration-based CO₂ evolution to directly quantify mineralization, providing a more accurate and standardized evaluation of the biodegradation performance. Additionally, scanning electron microscopy (SEM) was employed to examine structural changes before and after the composting experiment, offering new insights into long-term surface degradation.

2. Materials and Methods

2.1. Materials

The biodegradation tests were performed on PBS- and PLA-based materials supplied by Arctic Biomaterials (ABM, Arvo Ylpön katu 41, 33520,Tampere, Finland) and NaturePlast (NPL, Rue Ada Lovelace 6, 14120 Mondeville, Rue François Arago, France). All bio-based polymers were initially prepared to be less than 2 by 2 cm in size. Subsequently, the required assessments were conducted in accordance with the specifications outlined in EVS-EN ISO 14855-1:2012, entitled “Determination of the ultimate aerobic biodegradability of plastic materials under controlled composting conditions—Method by analysis of evolved carbon dioxide—Part 1: General method” [40].

Table 1. The materials tested for compostability and their properties.

Polymer Type	Product	Composition	Sample Name	Size (mm)	TDS (%)	TVS (%)	TOC (%)
PBS-based	Soft packaging	>85% PBS + <15% additives (mostly mineral fillers)	SP-PBS-1-G	3.5 × 2	99.76	89.97	53
	Soft packaging	85–90% PBS + 8–15% mineral + <3% processing additives + <2% anti-hydrolysis additives	SP-PBS-2-G	3.2 × 2.7 × 1.9	99.83	90.35	54
			SP-PBS-2-F	0.1	99.78	89.76	54
			SP-PBS-2-TF	0.01	99.68	89.88	54
PLA-based	Mulch film	50–70% PLA + 10–15% PBAT + 5–15% plasticizer + <5% mineral + <5% processing additives + <15% compatibilized plasticized starch	MF-PLA-1-F	0.15	99.19	98.03	56
	Cutlery	80% PLA-based compound + 20% ArcBiox X5 degradable glass fiber	C-PLA-1-G	3 × 2.5	99.77	80.20	44
	Cutlery	80% PLA-based compound + 20% ArcBiox X5 degradable glass fiber + small amount of processing additives	C-PLA-2-G	1–2	99.86	82.32	45
			C-PLA-2-G-D	1–2	99.86	79.80	44
	Rigid packaging	60% PLA-based compound + 10% ArcBiox X5 degradable glass fiber + 30% filler + small amount of processing additives	RP-PLA-1-G	1–2	99.94	61.02	38

specifies the materials tested for compostability and their properties. These materials included soft packaging, mulch film, cutlery, and rigid packaging, The samples were also presented in granulated form and as thin film. The tested polymer materials included bio-based samples with varying compositions. The PBS-based soft packaging material consists of >85% PBS with <15% additives (primarily mineral fillers), while another variant contains 85–90% PBS, 8–15% mineral additives, <3% processing aids, and <2% anti-hydrolysis agents. The PLA-based materials include mulch film (50–70% PLA, 10–15% PBAT, 5–15% plasticizer, <5% mineral fillers, <5% processing additives, and <15% compatibilized plasticized starch), cutlery (80% PLA compound with 20% degradable glass fiber, sometimes with a trace mount of processing additives), and rigid packaging (60% PLA compound, 10% degradable glass fiber, 30% filler, and a trace amount of processing additives).

Typical additives include mineral fillers (e.g., calcium carbonate) to improve stiffness and reduce cost, processing aids to enhance flow and mixing, and plasticizers to increase flexibility in films. Anti-hydrolysis agents help protect polyester chains from moisture, while compatibilizers improve the performance of polymer blends.

In this study, a shorthand notation is utilized to represent the materials and conditions under investigation. The following abbreviations are employed: SF for soft packaging, MF for mulch film, C for cutlery, RP for rigid packaging, G for granules form, F for film form, T for thin, and D for after 100 dishwashing cycles. For instance, the abbreviation ‘C-PLA-2-G-D’ stands for ‘Cutlery application with PLA-based polymer of version 2, in granules form, after 100 dishwashing cycles’.

2.2. Methods

2.2.1. Composting at a Laboratory Scale

According to the specified standard, each plastic test material was combined with the inoculum (activated vermiculite) at a ratio of 1:4 (dry mass to dry mass). The mixture was then placed in a stationary composting vessel in darkness, with active aeration using carbon dioxide-free air (preliminarily treated with a sodium hydroxide (NaOH) 0.5 M solution) under the optimal conditions of moisture content (50 ± 5%), oxygen concentration (>6%), temperature (58 ± 2 °C), and pH (ranging from 7.0 to 9.0) for 6 months. Multiple composting vessels were used for the experiment, with three designated for the inoculum (blank), three for the control sample (TLC (Thin-Layer Chromatography)-grade cellulose with particles smaller than 20 μm (micrometers), TDS (total dry solids)—90.78%, TVS (total volatile solids)—90.52%, and TOC (total organic carbon)—44%), and three for every test material. A schematic representation of the test system for degradation testing is presented in Figure 2, while the actual installation is shown in Figure 3.

Continuous monitoring and measurement of CO₂ production were performed at regular intervals for all composting vessels. The quantity of CO₂ generated from the combination of the test material and inoculum in a composting vessel was compared with the CO₂ produced solely from the inoculum in a composting vessel (blank sample). Additionally, weekly measurements involved adjusting humidity, shaking the composting vessels, and conducting pH measurements to ensure a stable composting process.

2.2.2. CO₂ Measurement Using Titration Method

After the preparation of the samples at the start of the experiment, titration was performed at regular intervals throughout the 6 months of the experiment to determine the amount of CO₂ generated from the biodegradation of a plastic sample. This approach is described in the ISO 19679:2020 standard, entitled “Plastics—Determination of aerobic biodegradation of non-floating plastic materials in a seawater/sediment interface—Method by analysis of evolved carbon dioxide” [41], and is a common technique of CO₂ determination [42,43]. Hydrochloric acid (0.05 M HCL) was used as the titrant.

During this process, if CO₂ from the degradation process enters the bottle, it transforms into K₂CO₃ [44]. Then, two-stage titration is used.

First, phenolphthalein is added to a 10 mL sample that is basic, so it appears pink in color. As titration with HCl proceeds, KOH is neutralized, all the CO₂ absorbed by the KOH solution transforms from K₂CO₃ to KHCO₃, and the color disappears. The amount of HCL used is recorded (VHCl(I)).

Then, methyl orange is added, which has a yellow hue at this pH. Further adding of the titrant transforms all the KHCO₃ to H₂CO₃, and the color changes to red. The readings from the burette are recorded (VHCl(II)) [43].

The difference between two recordings represents the H⁺ equivalents needed to react with KHCO₃, i.e., the moles of CO₂ produced, and its amount can be calculated as per Equation (1):

mmol CO₂ = (V_HCl(II) − V_HCl(I)) × [HCl] = (V_HCl) × [HCl]

(1)

The amount of CO₂, expressed in mg, is calculated according to Equation (2):

mgCO₂ = mmol CO₂ × 44

(2)

2.2.3. Biodegradation Evaluation

The degree of biodegradation is determined by the amount of carbon dioxide produced [40]. Through continuous monitoring and regular measurement of CO₂ produced by every sample and blank vessel, the cumulative CO₂ production was obtained [45]. The theoretical amount (ThCO₂) of carbon dioxide produced by every sample is defined based on the relation of total dry solids (M_TOT), total organic carbon (C_TOT), the molecular mass of carbon dioxide (44 g/g), and the atomic mass of carbon (g/g) (Equation (3)).

$${ThCO}_2=M_{TOT}·C_{TOT}·{\displaystyle \frac{44}{12}}$$

(3)

The percentage of material biodegradation (D_t) is calculated using Equation (4):

$$D_t={\displaystyle \frac{{\left({CO}_2\right)}_T-{\left({CO}_2\right)}_B}{{ThCO}_2}}·100\mathrm{\%}$$

(4)

where ${\left({CO}_2\right)}_T$ is the cumulative CO₂ production per composting vessel (g); ${\left({CO}_2\right)}_B$ is the average CO₂ production per blank vessel (g); and ${ThCO}_2$ is the theoretical amount of CO₂ produced by the test material (g).

In accordance with the guidelines provided in EVS-EN 13432:2000, entitled “Packaging—Requirements for packaging recoverable through composting and biodegradation—Test scheme and evaluation criteria for the final acceptance of packaging”, the degree of biodegradation of the bio-based materials was normalized and expressed as the maximum degradation of the reference material, after the plateau phase had been attained, for both the test samples and TLC-grade cellulose, which should be at least 90% [46].

2.2.4. Scanning Electron Microscopy

Scanning electron microscopy (SEM) was performed using a high-resolution Zeiss Ultra 55 scanning electron microscope to analyze the test samples. SEM images were obtained using the Secondary Electron (SE2) and Back-Scattered Electron (AsB) techniques at a voltage of 15 kV.

2.2.5. Total Organic Carbon

Total organic carbon (TOC) in the polymers was determined using the Vario TOC, Solids Module; 950 °C apparatus from Elementar GmbH, Germany, following the standard methodology outlined in EVS-EN ISO 14855-1:2012.

According to ISO 10694:1995, entitled “Soil quality—Determination of organic and total carbon after dry combustion (elementary analysis)”, the carbon present in the plastic samples would undergo complete decomposition and oxidation to carbon dioxide (CO₂) when heating it to 950 °C in an oxygen-containing, carbon dioxide-free gas [47]. The released carbon dioxide was subsequently measured using the apparatus mentioned above.

3. Results and Discussion

3.1. Degree of Biodegradation

As specified in ISO 14855-1, TLC-grade cellulose (<20 μm) was used as a positive control sample. During the six-month compostability test, the degradation rate of cellulose, its plateau, and the pH of the soil were continuously monitored. Cellulose reached its degradation plateau on day 110, achieving a maximum average degradation degree of 97.19%. The test was considered valid, as the biodegradation degree of the reference material exceeded 70% within the first 45 days, as required by the standard. Throughout the experiment, the pH of the activated vermiculite remained stable, ranging between 7.0 and 9.0.

3.1.1. Biodegradation of PBS-Based Compounds

The tested PBS-based compounds exhibited relatively similar amounts of TOC. In the case of SP-PBS-1, the amount was 53%, while for SP-PBS-2, it slightly increased to 54% (see Table 1). The cumulative CO₂ evolution (g/g) of the PBS-based compounds during the six-month composting period is shown in Figure 4. The biodegradation degree of the PBS-based compounds during the six-month composting period is presented in Figure 5.

SP-PBS-1-G (granule size of 3.5 × 2.0 mm) demonstrated an average CO₂ generation of 0.52 g/g and reached a 34.15% degradation rate, showing low potential for composting. This material is composed of more than 85% bio-based components and less than 15% processing additives.

SP-PBS-2 (granule size of 3.2 × 2 mm) exhibited significantly higher CO₂ generation per gram compared to SP-PBS-1. The carbon dioxide production of SP-PBS-2-G reached 1.42 g/g, with a biodegradation rate of 74.09%, while SP-PBS-2-F (film thickness of 0.1 mm) achieved a CO₂ generation rate of 1.26 g/g and a biodegradation rate of 66.79%. Meanwhile, SP-PBS-2-TF (film thickness of 0.01 mm) reached an average CO₂ production of 1.54 g/g and an 80.36% biodegradation rate, showcasing high potential for degradation under composting conditions. The latter compound is composed of more than 90% bio-based materials and less than 10% non-degradable components such as minerals, processing additives, and anti-hydrolysis agents, which may enhance the biodegradation rate. Recent studies have shown that adding specific additives, such as pro-oxidants, metal iron, and non-degradable plastics, in small amounts can significantly enhance biodegradation rates [48,49,50].

Comparing SP-PBS-1 and SP-PBS-2, the biodegradation of SP-PBS-2 increased under aerobic composting conditions. This can be explained by the presence of non-degradable components in the first blend, which hindered degradation, and the higher bio-based content in the second blend, which enhanced degradation [51,52]. However, the degradation of SP-PBS-2-G showed better results compared with SP-PBS-2-F. This difference can be attributed to the film having poor contact with the inoculum due to its significantly higher volume. The degree of biodegradation of SP-PBS-2-TF increased compared to that of SP-PBS-1 and SP-PBS-2, which can be explained by the material’s thickness of 0.01 mm and its larger surface area [53].

Kunioka et al. (2009) reported that the biodegradation degree of poly(butylene succinate) powder after 74 days was 56.2% under controlled composting conditions at 58 °C [36]. These results correlate with those reported by Zhao et al. (2005), who investigated the composting of three PBS samples under identical conditions. These authors observed that as the surface area of the PBS sample increased, the biodegradation rate also increased. After 90 days, the powder achieved approximately 72% biodegradation, while the films (with a thickness of 0.04 mm) reached a degradation rate of over 60%, but the degradation of granules (with a size of 3 mm) was only 14% [39].

The degradation results of the PBS-based materials tested in this study are in line with previous findings and demonstrate that the addition of additives (mostly mineral fillers) did not significantly impact the degree of biodegradation. Moreover, the results suggest that maintaining the thickness of PBS-based materials at a minimum level is crucial to achieving the highest degree of biodegradation under controlled composting conditions [54].

All the PBS-based materials exhibited a fragile property after the six-month compostability test. This shared attribute implies that these materials tend to become brittle or break more easily over time, and their overall strength diminishes under the moisture content (50 ± 5%), oxygen concentration (>6%), temperature (58 ± 2 °C), and pH (ranging from 7.0 to 9.0) conditions tested in this study. Additionally, these properties indicate that the disintegration process occurs within the material. More detailed information about the changes happening at the surface of the PLA-based polymers throughout the biodegradation process is presented in Section 3.2.1.

3.1.2. Biodegradation of PLA-Based Compounds

The PLA-based compounds displayed variations in the amount of TOC within them. For instance, in the case of MF-PLA-1-F, the TOC content was 56%, while for C-PLA-1-G, it slightly decreased to 44%. C-PLA-2-G exhibited a TOC content of 45%. However, after a series of dishwashing cycles, the TOC content of C-PLA-2-G-D decreased to 44%. Lastly, RP-PLA-1-G displayed a TOC content of 38% (see Table 1). The cumulative CO₂ evolution (g/g) of the PLA-based compounds during the six-month composting period is shown in Figure 6. The biodegradation degree of the PLA-based compounds during the six-month composting period is presented in Figure 7.

MF-PLA-1-F (film thickness of 0.15 mm) displayed an average CO₂ production rate of 1.15 g/g and a biodegradation rate of 58.39%, showcasing low potential for degradation under composting conditions. This sample is composed of more than 80% bio-based materials and less than 20% non-degradable components such as plasticizers and processing additives, which may hinder the biodegradation rate by increasing hydrophobicity and/or limiting microbial accessibility [55,56].

C-PLA-1-G (granule size of 3 × 2.5 mm) exhibited a CO₂ production rate of 0.6 g/g and reached a 47.46% biodegradation rate under the controlled composting conditions. Meanwhile, C-PLA-2-G (granule size of 1–2 mm) displayed an average CO₂ production rate of 1.33 g/g and an 83.27% biodegradation rate, indicating high potential biodegradability under controlled composting conditions. The degradation of this material was slow at the beginning of the experiment, which aligns with the findings by Yang et al., who reported that PLA degradation is influenced by the additional time required for hydrolysis [57]. Additionally, C-PLA-2-G-D (granule size of 1–2 mm) achieved an average CO₂ production rate of 1.54 g/g and a biodegradation rate of 98.34% on average, suggesting a significant degree of biodegradability under controlled composting conditions. These results indicate that after 130 days of composting, the biodegradation degree of C-PLA-2-G-D exceeded 83%. This is consistent with the findings by Kalita et al., who reported a 94.2% biodegradation rate for neat PLA in 136 days [58].

Comparing C-PLA-1-G and C-PLA-2, the biodegradation of C-PLA-2 under aerobic conditions increased due to the smaller size of the granules [57]. Additionally, the processing additive used for C-PLA-2 could potentially enhance the biodegradation rate by increasing matrix flexibility and reducing crystallinity, thereby accelerating degradation [48,59]. Furthermore, dishwashing influenced the degree of biodegradation. According to ISO EN 13432, at least 90% of a plastic material should be converted to CO₂ within 6 months. However, C-PLA-1-G and C-PLA-2-G are not fully degradable under the controlled composting conditions, while C-PLA-2-G-D, after 100 dishwashing cycles, is degradable under these conditions.

RP-PLA-1-G (granule size of 1–2 mm) exhibited an average carbon dioxide (CO₂) production rate of 1.13 g/g. Comprising 60% bio-based materials, it displayed an impressive 83.62% biodegradation rate, suggesting a significant level of biodegradability under the controlled composting conditions. These results underscore the potential of RP-PLA-1-G as an environmentally friendly material suitable for composting applications.

Tolga et al. (2020) conducted disintegration tests on blends containing poly(lactic acid) (PLA) under industrial composting conditions and demonstrated a relationship between sample thickness and weight loss. Samples with a thickness of 1.0 mm exhibited a 40% greater weight loss compared to samples with a thickness of 2.0 mm. Additionally, it is worth noting that the blends with relatively small disintegration rates at 1.0 mm displayed results comparable to the 2.0 mm samples [60].

Kalita et al. (2020) investigated the biodegradation behavior of pure poly(lactic acid) (PLA), polycaprolactone (PCL), and their blends with microcrystalline cellulose (MCC) under composting conditions for 140 days. Their findings indicate that the blend composition and filler concentration play a crucial role in the biodegradation behavior of the composite materials. They concluded that by adjusting the blend composition and filler concentration, the biodegradability of a biopolymer blend can be effectively tuned [61].

This conclusion aligns with the results observed for the PLA-based samples, where samples with a higher concentration of biodegradable additives and smaller granules exhibited better degradation rates. These findings underscore the significant influence of blend composition and filler concentration on degradation rates. Additionally, maintaining an optimal polymer thickness is critical to maximizing the contact surface area for biodegradation.

Following the six-month compostability test, it is evident that all the materials display a fragile characteristic. This common trait suggests a tendency for these materials to become brittle or more prone to breaking, resulting in an overall reduction in strength under specific conditions [62,63,64]. Additionally, these properties indicate that the disintegration process occurs within the material. More detailed information about the changes happening at the surface of the PLA-based polymers throughout the biodegradation process is presented in Section 3.2.2.

3.2. Surface Morphological Features

SEM images were taken of the samples before and after 6 months in the compost medium to observe the macroscopic changes happening at the surface of both polymers throughout the biodegradation process. The samples’ SEM images show different structural changes, which will be described further below.

3.2.1. Surface Morphology of PBS-Based Compounds

The SEM images in Figure 8 illustrate the evolution of surface erosion of the PBS-based samples after the six-month composting experiment.

The first SP-PBS-1-G sample shows extensive damage on the granules’ surface. However, when recalling Figure 5, which displays the degree of biodegradation, the extent of surface damage for the SP-PBS-1-G sample is approximately half of that for the other samples.

At the same time, the SEM image of the second SP-PBS-2-G sample shows a mildly damaged surface of its granules after 6 months of composting, although there is a higher degree of internal transformation, as indicated by Figure 5. This can be explained by the differences in the samples’ composition. It can be assumed that the additives in the SP-PBS-2-G sample promote material breakdown in the depths of the sample. Compared with SP-PBS-1-G, the SP-PBS-2-G, SP-PBS-2-F, and SP-PBS-2-TF samples have processing and anti-hydrolysis additives in their composition, aside from mineral fillers.

The third SP-PBS-2-F sample, which is in the form of a film and has the same composition as the SP-PBS-2-G sample, shows a similar level of surface alteration. Figure 5 illustrates that these two samples also have a similar pattern in terms of material conversion over time.

For the fourth SP-PBS-2-TF sample, which has the same composition as the SP-PBS-2-G and SP-PBS-2-F samples, a more pronounced structural change to the sample surface after the 6-month composting experiment can be observed, which is consistent with the results of Figure 5—this sample displays the highest extent of material transformation. This result can be explained by the form in which the sample is presented (thin film) [65,66], which provides a larger area of contact with the compost medium, thus accelerating the transformation rate.

It is worth mentioning that PBS breaks down quite easily, up to 90%, within 160 days; thus, it can be concluded that the additives slow down the breakdown process, helping to preserve the integrity of the materials.

3.2.2. Surface Morphology of PLA-Based Compounds

The SEM images in Figure 9 depict the evolution of surface erosion in PLA-based samples after the six-month composting experiment. Due to the C-PLA-2-G sample’s heavy disintegration, it was impossible to visually distinguish it from the soil particles and retrieve it from the compost medium. Therefore, an SEM image of this sample after the 6-month composting experiment is not available.

The first MF-PLA-1-F sample, presented in the form of a film, shows moderate structural alteration, which affects the entire film layer. However, its degree of biodegradation reaches only around 50%, which can be attributed to the presence of additives such as plasticizers in this sample.

The second C-PLA-1-G granule sample has a distinguished initial structure with visible glass fibers, which remain seemingly unchanged in the sample after 6 months of composting, while the rest of the sample body displays mild degradation. This sample also has the lowest degree of biodegradation, at about 40%, according to Figure 7.

As mentioned above, it is impossible to describe the third C-PLA-2-G sample due to the inability to take an SEM image after six months of composting.

It was possible to capture SEM images of the fourth C-PLA-2-G-D sample, which has the same composition as the third one, after the six-month period, and the SEM image shows substantial material breakdown. Even the glass fibers cannot be distinguished from the sample body. This result is in accordance with Figure 7, which shows that the sample’s degree of biodegradation is more than 90%, a higher rate than that for the third C-PLA-2-G sample (around 80%). This can be explained by the 100 dishwashing cycles the fourth C-PLA-2-G-D sample was subjected to.

The fifth RP-PLA-1-G sample has a slightly different initial structure than samples 2–4 due to the presence of material fillers. The SEM image after the 6-month composting experiment looks similar to the one for the fourth C-PLA-2-G-D sample, with the glass fibers not visible. The degree of biodegradation is around 80%, which is the same as that for the third C-PLA-2-G sample.

4. Conclusions

The application and usage of plastics are unavoidable to meet daily needs. Their demand is steadily rising and contributing to the continuous accumulation of plastic waste, posing a significant environmental threat. Biodegradation offers a sustainable solution to this issue; however, it is influenced by various factors, including environmental conditions and plastic characteristics. Consequently, the composting process plays a crucial role in the environmental impact and sustainability of developed materials.

This study focuses on the biodegradation of bio-based granules and films, measured by cumulative CO₂ production via titration. In this context, bio-based polymers underwent thorough examination to assess their potential environmental impact. The composites, primarily composed of PLA and PBS, demonstrated promising biodegradation rates during composting under controlled temperature, humidity, oxygen concentration, and pH conditions.

During the six-month laboratory-scale composting experiment, the PLA-based materials exhibited biodegradation rates ranging from 47.46% to 98.34%, while the PBS-based materials showed rates between 34.15% and 80.36%. The biodegradation performance of the PLA-based compounds varied significantly, depending on the material composition, granule size, and the presence of additives.

C-PLA-1-G demonstrated the lowest biodegradation rate among the PLA-based materials (47.46%), while MF-PLA-1-F achieved a slightly higher rate (58.39%). In contrast, C-PLA-2-G reached 83.27%, and C-PLA-2-G-D exhibited the highest rate (98.34%), notably after undergoing dishwashing. RP-PLA-1-G also showed high biodegradability (83.62%) despite a lower bio-based content.

For the PBS-based materials, SP-PBS-1-G had the lowest biodegradation rate (34.15%). In contrast, the SP-PBS-2 samples showed significantly improved results: 74.09% for SP-PBS-2-G and 66.79% for SP-PBS-2-F, both benefiting from a higher bio-based content and fewer non-degradable additives. SP-PBS-2-TF achieved the highest PBS-based biodegradation rate (80.36%).

Notably, among the PLA-based materials, variations in total organic carbon (TOC) content ranged from 38% to 56%. Conversely, the PBS-based materials exhibited a more consistent TOC content, with a narrow range from 53% to 54%.

While both material types demonstrated degradation under controlled composting conditions, the rate and extent of degradation differed significantly. The PBS-based materials showed lower biodegradation rates compared to the PLA-based materials. Moreover, it was observed that materials with a higher bio-based content demonstrated better degradation results. Additionally, there was a correlation between the size of the material and the degree of biodegradation: materials with smaller granules degraded faster than those with larger granules, while thin films achieved the highest biodegradation rates. However, very thin films exhibited a slower degradation rate due to their small contact surface, requiring extra time to degrade.

Regarding further actions, it is crucial to implement adequate measures for better monitoring of the behavior of PLA- and PBS-based materials under controlled composting conditions; further studies are needed to investigate how these materials interact in different environments to identify appropriate measures for optimizing their use and end-of-life.