Effect of chain extending cross-linkers on the disintegration behavior of composted PBAT/PLA blown films

Citation: Azevedo, J.V.C.; Hausnerova, B.; Möginger, B.; Sopik, T. Effect of Chain Extending Cross-Linkers on the Disintegration Behavior of Composted PBAT/PLA Blown Films. Int. J. Mol. Sci. 2023,24, 4525. https://doi.org/10.3390/ ijms24054525 Academic Editors: Terence L. Marsh and Rafael Auras Received: 10 October 2022 Revised: 21 February 2023 Accepted: 22 February 2023 Published: 24 February 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). International Journal of Molecular Sciences Article Effect of Chain Extending Cross-Linkers on the Disintegration Behavior of Composted PBAT/PLA Blown Films Juliana V. C. Azevedo 1,2,3 , Berenika Hausnerova 1,4,* , Bernhard Möginger 2and Tomas Sopik 4 1Faculty of Technology, Tomas Bata University in Zlín, Vavreckova 275, 76001 Zlín, Czech Republic 2Department of Natural Sciences, University of Applied Sciences Bonn-Rhein-Sieg, von Liebig Str. 20, 53359 Rheinbach, Germany 3 BIO-FED, Branch of AKRO-PLASTIC GmbH, BioCampus Cologne, Nattermannallee 1, 50829 Köln, Germany 4Centre of Polymer Systems, University Institute, Tomas Bata University in Zlín, Nam. T.G. Masaryka 5555, 76001 Zlín, Czech Republic *Correspondence: hausner[email protected] Abstract: A biodegradable blend of PBAT—poly(butylene adipate-co-terephthalate)—and PLA— poly(lactic acid)—for blown film extrusion was modified with four multi-functional chain extending cross-linkers (CECL). The anisotropic morphology introduced during film blowing affects the degradation processes. Given that two CECL increased the melt flow rate (MFR) of tris(2,4-ditert-butylphenyl)phosphite (V1) and 1,3-phenylenebisoxazoline (V2) and the other two reduced it (aromatic polycarbodiimide (V3) and poly(4,4-dicyclohexylmethanecarbodiimide) (V4)), their compost (bio-)disintegration behavior was investigated. It was significantly altered with respect to the unmodified reference blend (REF). The disintegration behavior at 30 and 60 ◦ C was investigated by determining changes in mass, Young’s moduli, tensile strengths, elongations at break and thermal properties. In order to quantify the disintegration behavior, the hole areas of blown films were evaluated after compost storage at 60 ◦ C to calculate the kinetics of the time dependent degrees of disintegration. The kinetic model of disintegration provides two parameters: initiation time and disintegration time. They quantify the effects of the CECL on the disintegration behavior of the PBAT/PLA compound. Differential scanning calorimetry (DSC) revealed a pronounced annealing effect during storage in compost at 30 ◦ C, as well as the occurrence of an additional step-like increase in the heat flow at 75 ◦ C after storage at 60 ◦ C. The disintegration consists of processes which affect amorphous and crystalline phase of PBAT in different manner that cannot be understood by a hydrolytic chain degradation only. Furthermore, gel permeation chromatography (GPC) revealed molecular degradation only at 60 ◦ C for the REF and V1 after 7 days of compost storage. The observed losses of mass and cross-sectional area seem to be attributed more to mechanical decay than to molecular degradation for the given compost storage times. Keywords: poly(butylene adipate terephthalate); poly(lactic acid); blown film; chain extending cross-linker; degree of disintegration; disintegration kinetics; molecular mass degradation 1. Introduction The advances in materials sciences and technology allow for developing environmentally suitable packaging products [ 1 – 3 ]. As a response to increasing plastic use and the related environmental pollution, bioplastics have become highly interesting in the last decade [ 4 – 6 ]. The development of biodegradable polymers seems to be an effective way to partly solve the plastics waste problem. In this respect, commercially available PBAT— poly(butylene adipate-co-terephthalate)—and PLA—poly(lactic acid)—are predominantly selected by bioplastics manufacturers [ 7 – 11 ] and used for the production of blown films mainly for shoppers bags, fruit and vegetable bags, and waste bags (e.g., Ecovio ® by BASF). PBAT is a random copolymer of butylene adipate and terephthalate that owes its biodegradability to the butylene adipate groups and its stability and mechanical properties Int. J. Mol. Sci. 2023,24, 4525. https://doi.org/10.3390/ijms24054525 https://www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2023,24, 4525 2 of 21 to the terephthalate groups [ 11 – 14 ]. It is a flexible polymer with high elongation at break and good processing properties. PLA is entirely renewable if originating from a starch [ 15 ]. Due to the brittle behavior of PLA, it is inappropriate for applications requiring high deformation strains [ 16 , 17 ]. Therefore, it is often modified by plasticizers and chain extenders [18–20]. The flexibility and toughness of PBAT may compensate for the brittleness of PLA in their blends. The studies conducted on the processing of PBAT and PLA blends focus on balancing the properties required for specific applications [21–23]. An important factor involves denoting the experimental conditions under which biodegradation takes place in order to investigate its effects on the disintegration of PLA/PBAT blends. Weng et al. [ 24 ] investigated a 40/60 PBAT/PLA blend cast to films after granulation, which were then composted in a soil environment. DSC analysis of the pure polymers showed that the melting temperature of the PBAT was slightly decreased after degradation, while the melting temperature of the PLA was increased. The changes in the melting temperatures of the blend before and after degradation were identical to those of the corresponding polymers. However, combined thermogravimetric and elemental analyses revealed that the degradation rates of the components in the PBAT/PLA blend differed from those of the individual polymers. Nevertheless, all samples of PBAT, PLA and PBAT/PLA were fragmented after four months of degradation in soil. Scanning Electron Microscope (SEM) revealed an increased roughness on the surfaces, but the blend did not show voids. Given that the introduction of filler and additives also affects the biodegradation behavior and kinetics of PBAT/PLA blends, due to increased diffusion paths and the possible abiotic properties of the filler systems, Tolga et al. [ 25 ] investigated the disintegration process of a PLA containing mineral fillers blended with other biodegradable polymers. For 2 mm thick plates of the 30/70 PBAT/PLA blend, they found that the disintegration process started after an initiation time of 4 weeks in a compost at 60 ◦ C and 70% relative humidity. After 12 weeks, the degree of disintegration exceeded 25%. Further, from the behavior of PLA/PBAT blends, one can conclude that the disintegration rates decrease with increasing filler contents during the first stage of disintegration [ 25 ]. Freitas et al. [ 26 ] studied the effect of montmorillonite clay (MMT) filler on PLA/PBAT blends that were modified with chain extenders. For unfilled PLA/PBAT blends, they found the highest emission rate of CO 2 , which indicated that the degradation rate depended on the available polymer surface. Filler particles may also hinder microorganisms from penetrating the polymer or have partly antibiotic effects. In each case, MMT reduced the available surface, which led to lower emission rates of CO 2 , and, thus, lower biodegradation rates. Touchaleaume et al. [ 27 ] investigated 40 µ m thick films of a 70/30 PBAT/PLA blend in vineyard soil for 24 months at ambient temperatures with respect to the amount of degraded surface area. They observed no degradation over 24 months, which indicated that the PBAT matrix does not allow significant degradation at ambient temperatures, although an increased surface roughness was found. This result is supported by degradation tests of a 55/45 PBAT/PLA blend for 180 days at 30 ◦ C, in which Lamparelli et al. [ 28 ] found weight gains of 0.7% in soil and 1.4% in an aqueous medium. On the other hand, compost storage for 70 days at 60 ◦ C [ 27 ] revealed initiation times of the degradation process of 4 to 5 days for an 18 month aged blend and of 6 days for the fresh blend. They successfully fitted their measurements (R 2 > 0.99) using the Hill model [ 29 ], but they did not provide any fit parameters of degradation kinetics. From the above-mentioned investigations, one can conclude that biodegradation can occur only after certain initiation times in which the polymer chains are fractured by hydrolysis, photo-oxidation, and thermal oxidation into pieces that can be metabolized by micro-organisms [25,27,30,31]. Deeper insights into degradation modes of PBAT films are given in a review article by Liu et al. [32]. At present, an effect of multi-functional chain extending cross-linkers (CECL) on disintegration, as well as mechanical and thermal properties, have been investigated for the Int. J. Mol. Sci. 2023,24, 4525 3 of 21 extrusion of blown films of PBAT/PLA blends very scarcely. Most studies were carried out on samples manufactured by other processing techniques, e.g., injection or compression molding that did not take into account the pronounced anisotropic morphology introduced by blown film extrusion. Our previous study [ 33 ] showed that the chemical reactions caused by CECL in PBAT/PLA blends were incomplete after compounding, and that the stretching during blown film extrusion brought the appropriate molecular groups into reach by promoting further cross-linking, chain scission, or other reactions. The objective of this study is to investigate in detail the effects of four CECLs on the disintegration behavior and kinetics of a PBAT/PLA blend under compost storage. 2. Results The visual inspection of the films shows that temperature had a significant effect on the disintegration behavior of the PBAT/PLA compounds modified by four CECLs (for details see Section 4.1). Figure 1shows that, at 30 ◦ C, the samples of REF (unmodified blend), V1 (CECL = tris(2,4-di-tert-butylphenyl) phosphite), and V2 (CECL = 1,3phenylenebisoxazoline) exhibited small stains on the surface, which indicated a starting disintegration after 2 months. At 30 ◦ C, PLA is in the glassy state (glass temperature of 60 ◦C), which prevents its degradation. Figure 1. Compost storage for 2 months at 30 ◦ C ( a ) and 7 days at 60 ◦ C ( b ) on 25 µ m-thick films of PBAT/PLA compounds; scaling in mm. At 60 ◦ C, the REF exhibited a mean disintegration after 7 days, and V1 and V2 exhibited severe disintegrations, whereas V3 (CECL = aromatic polycarbodiimide) showed a starting disintegration, and V4 (CECL = poly(4,4-dicyclohexylmethane carbodiimide) still seemed to be unaffected. The states of disintegration were in line with the increased melt flow ratios of V1 and V2 when compared to REF, as well as with the decreased melt flow ratios of V3 and V4 [ 33 ]. This showed that disintegration was eased in the case of shorter and more mobile polymer chains. Int. J. Mol. Sci. 2023,24, 4525 4 of 21 2.1. Effects of Compost Storage on Mass Change At 30 ◦ C, the relative masses of the samples of REF and V1 to V4 remained more or less unchanged within the range of the scatter of the mass determination after compost storage for 8 weeks. Figure 2indicates that the disintegration processes did not lead to mass decreases yet. The mass variations can be attributed to processes which occur within the initiation time, e.g., mass increase due to chemical reactions, humidity uptake, and remaining compost particles, or mass decrease due to releases of degradation reactions or loss of micro-plastic particles during cleaning. Figure 2. Relative mass change of 25 µ m-thick films of REF and V1 to V4 over 8 weeks at 30 ◦ C ( a ) and 7 days at 60 ◦ C ( b ) of disintegration in compost; standard deviations (STD) are between 0.03 and 0.07, the lines serve for better visualization. At 60 ◦ C, the relative masses remained unchanged only for 4 days. Figure 2shows that, for V1 and V2, a pronounced mass decrease was consequently observed. The REF exhibited a less pronounced mass decrease between day 5 and 6. The V3 showed a tendency of decreasing mass, whereas the V4 seemed to not be disintegrated, even after 7 days. This shows that the initiation times of disintegration are significantly affected by the chosen CECL and the induced chemical changes. Furthermore, significant disintegration was observed for the REF, V1, and V2 after 7 days at 60 ◦ C. This supports the interpretation that the stains on the REF, V1, and V2 in Figure 1represented a beginning disintegration. Because of the scatter of the mass measurements, the disintegration was confirmed only if the masses had decreased to less than 90% of the initial value. Thus, the time t 80% was the determined time at which the masses of the films reached 80% of their initial masses, as is shown in Figure 2, and it represents the upper limit of the initiation time. The data indicated that there was a mass increase at the beginning of the compost storage due to the initial disintegration processes. To estimate the mass scatter due to adsorbed and biodegradation-generated low molecular weight components, the initial masses of the DSC experiments were compared to the masses after the second heating runs. The mass losses were between 2 and 12%, which showed that there was a remarkable content of low molecular components in the films, as is shown in Table 1. Int. J. Mol. Sci. 2023,24, 4525 5 of 21 Table 1. Effects of conditions of compost storage on mass loss and relative mass loss after second run, and on enthalpies from 70 to 90 ◦C of first and second run. Storage 8 Weeks at 30 ◦C 7 Days at 60 ◦C ∆H between 70 to 90 ◦C Mass Loss Relative Mass Loss ∆H between 70 to 90 ◦C Mass Loss Relative Mass Loss Unit mJ mg - mJ mg - REF 15 0.25 0.04 34 0.43 0.07 V1 21 0.44 0.07 46 0.67 0.11 V2 23 0.45 0.08 45 0.74 0.12 V3 26 0.24 0.04 28 0.25 0.04 V4 27 0.20 0.03 25 0.13 0.02 After 8 weeks at 30 ◦ C, the REF had lost 4% of its mass, and 7% of its mass after 7 days at 60 ◦ C. The decrease in the V1 and V2 exceeded that of the REF, which showed that the biodegradation of the V1 and V2 was more pronounced. For the V3 and V4, the decreases were less than for the REF, which showed that cross-linking hindered biodegradation and decelerated disintegration. However, the decreases were still within the scatter of the relative masses during the initiation times. This is a further indication for a mass increase due to the uptake and generation of low molecular weight components during the initiation times of compost storage, in addition to the contributions of adherent compost particles. For all compounds, the DSC traces of the first run differed significantly from the second run, as is shown in Figure 3, and the enthalpies also differed between 20 and 170 ◦C . These differences can be attributed to the processing history (which was similar for all films) and the reached state of biodegradation. As polyamides are typically dried at 80 ◦ C (a temperature which is also above the T g of PLA), the enthalpies between 70 and 90 ◦ C were determined, as is shown in Table 1. It can be seen that these enthalpies correlated qualitatively with the mass loss for the REF, V1, and V2 and that they were almost equal for the V3 and V4. Given that the content of CECLs was low, one can assume that the water contents of all films were identical. Thus, the differences in the mass losses in Table 1 have to have been generated by volatile low molecular linked to the initial processes of biodegradation. Figure 3. First and second heating run of REF after compost storage for 7 days at 60 ◦ C showing the enthalpies between 20 and 170 ◦ C as hatched areas. The dashed lines mark the range 70 to 90 ◦ C in which most of the water desorption is expected to have occurred. 2.2. Effects of Compost Storage on Disintegration Kinetics After compost storage at 60 ◦ C, the films were further investigated with respect to their disintegration kinetics by determining the cross-sectional areas of holes to elucidate the kinetic parameters of initiation time t init and disintegration time τdisint for the REF and for V1 to V4. The experimental data can satisfactorily be fitted by Equation (8) with a reasonable R 2 , as are shown in Figure 4and Table 2. In the model, a single dominating disintegration process was assumed using an approach that was used to describe the Int. J. Mol. Sci. 2023,24, 4525 6 of 21 hydrolytic degradation of neat PLA, PLA modified with a carbodiimide, and PLA wood flour composites [ 34 , 35 ]. The smaller measured time dependent degrees of disintegration x disint of the REF, V1, and V2 when compared to the predicted values can be explained by the fact that the disintegration had already happened prior to the first visible holes. Figure 4. Time dependent degree of disintegration at 60 ◦ C of REF and V1 to V4; comparison of experimental data (symbols) to fits according to Equation (8); STD from 0.03 to 0.05. Table 2. Kinetic parameters of REF and V1 to V3 compounds over 7 days at 60 ◦ C of disintegration in compost. Kinetic Parameters Compound Unit REF V1 V2 V3 Time t80% to 80% of initial mass h 132.7 105.6 105.8 174.1 Initiation time tinit h 32.8 34.1 23.7 45.8 Disintegration time τdisint h 385.1 147.3 94.8 1179.5 R2of fits 0.96 0.95 0.97 0.83 The times t 80% (to the mass decrease of 80%) exceeded the initiation times t init by three to four factors, as is shown in Table 2. This shows that the initiation of disintegration depends on the considered property—mass loss or hole generation. Obviously, some degree of disintegration has to happen prior to the appearance of the first holes. Thus, the determined initiation times t init have to depend on the film thickness. If one assumes that the first appearance of holes depends linearly on the film thickness, the initiation time of V1 (30 µ m instead of 25 µ m) would be roughly 20% too long—leading to a corrected t init ≈ 27 h. The V4 did not show any holes within 7 days, which indicated that the Int. J. Mol. Sci. 2023,24, 4525 7 of 21 molecular structure generated by the CECL poly(4,4-dicyclohexylmethane carbodiimide) prevented significant disintegration. The disintegration times τdisint show that the CECLs significantly affected the disintegration processes. The disintegration rates of the V1 and V2 exceeded that of the REF by 2.5 times and 4 times, respectively, whereas it was decreased to a factor 0.25 for the V3. Given that the V4 did not show any holes within 7 days, it was not evaluated. 2.3. Effects of Compost Storage on Mechanical Properties The Young’s moduli in the extrusion direction (ED) were roughly double of those in transverse direction (TD), which reflected the film anisotropy due to processing. Compost storage of the REF and V1 to V4 for 8 weeks at 30 ◦ C initially led to an increase in the Young’s moduli in the order of 20% before they were slowly decreased again, as is shown in Figure 5. This increase can be explained by the annealing effects in the amorphous phases of PLA [ 36 ] and by the post-crystallization of PBAT. The PLA in its disperse phase was protected by the PBAT matrix against any kind of degradation and disintegration at the beginning of the compost storage at 30 ◦ C, and annealing could happen undisturbed. Jian et al. [8] reported that PBAT starts to crystallize at 60 ◦ C if heated for 10 ◦ C/min during DSC. Therefore, a slow post-crystallization of amorphous PBAT can already be expected at 30 ◦ C if humidity provides more mobility to the polymer chains. Both processes may increase stiffness. Interestingly, the maximum Young’s moduli were determined after 2 weeks in the ED, while 4 weeks were needed in the TD, which indicated that mechanical degradation happened differently in the ED and TD during initiation time. This can be understood by the fact that the films exhibit significantly different morphologies on their fracture surfaces in the ED and TD [ 37 ]. Thus, during the initiation phase, one direction can be more affected by biodegradation processes than the other. Figure 5. Time dependent effects of disintegration on mechanical properties (Young’s modulus E, tensile strength σmax , elongation at break εbreak ) in extrusion direction (ED) and transverse direction (TD) of films after storage in compost at 30 ◦C. Int. J. Mol. Sci. 2023,24, 4525 8 of 21 Compost storage for 8 weeks at 30 ◦ C led to a continuous decrease in the tensile strengths of the REF, V1, and V2 in the order of 50 to 70% in the ED and TD. The V3 and V4 exhibited a plateau of tensile strength for 4 weeks in the ED before a decrease of 60 to 70% occurred. In the TD, the V3 exhibited a plateau of tensile strength for 4 weeks, and the V4 exhibited a plateau of tensile strength for 2 weeks. The elongations at break exhibited a plateau for 2 weeks for the REF, V1, and V2 in the ED, with subsequent decreases of 35% (REF) and almost 100% (V1 and V2). For the V3, the plateau lasted 4 weeks, followed by a decrease of 20%, whereas no decrease was observed for the V4, even after 8 weeks. In the TD, the elongations at break of the REF and V2 exhibited a plateau for 2 weeks before they decreased, whereas the V1 showed a continuous decrease. Their elongations at break after 8 weeks dropped to 5 to 30% of the initial values. The V3 and V4 exhibited plateaus of elongations at break in the TD that lasted 4 weeks and 2 weeks, respectively, with subsequent decreases of 30 and 70%, respectively. Compost storage at 60 ◦ C, as is depicted in Figure 6, had pronounced impact on the mechanical properties of the films. Annealing effects in the amorphous phase of the PLA did not continue to occur, as the PLA was now above the glass transition temperature. Therefore, no increases in Young’s moduli were further observed. After one week, it was hardly possible to perform tensile tests with films of the REF, V1, and V2, due to severe mechanical disintegration. In the ED and TD, the Young’s moduli of the REF remained on a plateau for the first 3 days. Then, they continuously decreased close to zero within a week. The Young’s moduli of the V1 and V2 continuously decreased after 7 days to 70% and 50%, respectively, in the ED and TD. For the V3, its Young’s moduli in the ED and TD remained on a plateau for 14 days before they decreased by at least 50%. The Young’s moduli of the V4 remained on a plateau for almost 28 days. Only the modulus in the ED was 15% lower after 28 days. Figure 6. Time dependent effects of disintegration on mechanical properties (Young’s modulus E, tensile strength σmax , elongation at break εbreak ) in extrusion direction (ED) and transverse direction (TD) of films after storage in compost at 60 ◦C. Int. J. Mol. Sci. 2023,24, 4525 9 of 21 In the ED and TD, the tensile strengths of the REF, V1, and V2 decreased continuously to approximately 20% of their initial values after 7 days. For the V3, its Young’s moduli in the ED decreased continuously to less than 20% after 28 days, whereas in the TD, they decreased to 30% during the first 2 days and remained on that plateau for 2 weeks before they further decreased to less than 20%. For the V4, the tensile strength in the ED decreased continuously to approximately 40% after 28 days, whereas in the TD, it remained on a plateau for 3 days before it decreased to 30%. The elongations at break in the ED and TD of the REF remained on a plateau for 1 day and then decreased to approximately 10% of their initial values after 3 days. For the V1, a plateau was observed in the ED for 1 day with a subsequent decrease that was almost close to zero after 7 days. In the TD, the decrease close to zero was already reached after 3 days. The V2 exhibited a continuous decrease and reached elongations at break of a few percentage points after 7 days in the ED and after 3 days in the TD. The V3 exhibited a plateau for 1 day and a slow decrease to a few percentage points after 14 days in both the ED and TD. For the V4, a plateau was observed for 3 days, followed by a decrease to 50% in the ED and 10% in the TD. Tensile strengths and elongations at break indicate a high sensitivity with respect to structural changes of the films, and they showed that the films of the REF and V1 to V4 had also been mechanically degraded during compost storage at 30 ◦ C. Obviously, these processes were still in the state of initiation with negligible effects on film masses. At 60 ◦ C, the degradation processes were significantly accelerated, as the REF, V1, and V2 were severely disintegrated after 7 days (see Figure 1), and both their tensile strengths and elongations at break dropped to low values. 2.4. Effects of Compost Storage on Thermal Properties The biodegradation caused by compost storage affected the thermal properties in a significant manner, as the DSC curves of the first heating run after 8 weeks at 30 ◦ C differed significantly from those after 7 days at 60 ◦ C (see Figure 7), which also occurred for the corresponding transition temperatures and heats of fusions, as is shown in Table 3. Figure 7. DSC traces of the first heating run of REF and V1 to V4 after compost storage for 8 weeks at 30 ◦C (a) and 7 days at 60 ◦C (b). Int. J. Mol. Sci. 2023,24, 4525 16 of 21 It is obvious that the disintegration processes occurring in the PBAT/PLA films are complex and act on different time scales with respect to properties such as mass loss, tensile strength, elongation at break, transition temperatures, and heats of fusion. If the blown films are subjected to compost storage, the first processes are mainly water diffusion in the amorphous phases of the PBAT and PLA and the settling of micro-organisms on the film surfaces. As a compost consists of a manifold of micro-organisms, other substances of low molecular weights—being yet unknown—can diffuse in the available free volume of water-softened amorphous phases, where they undergo further chemical reactions and initiate stress cracking. This propagates mechanical disintegration by micro-void formation and fracturing. As fracturing is a defect-controlled process, elongations at break and tensile strengths can already decrease if there are small void concentrations close to the surface at the early stages of biodegradation. Internal stress fields in the films (what is probable due to the anisotropic structure generated by the film blowing process), propagate these voids and ease the fracturing of the films to micro-plastic particles. With respect to complete degradation, this fracturing towards micro-plastic particles is beneficial. Firstly, the diffusion paths for low molecular weight components (such as water) decrease, and hydrolytic molecular chain degradation requires shorter initiation times. Secondly, the fracturing generates a new surface area where micro-organisms can settle and metabolize the polymer. 4. Materials and Methods 4.1. Materials and Sample Preparation Four chain extending cross-linkers (1 wt.%) were compounded to the reference PBAT/PLA blend (REF) M · VERA ® B5029 [ 40 ] from BIO-FED, a branch of AKRO-PLASTIC GmbH, Cologne, Germany. M · VERA ® B5029 consists of 65% PBAT (matrix), and 11% PLA (disperse phase) and 24% of CaCO 3 filler particles. It is mainly used for packaging and agricultural applications. The following CECLs were employed: • V1—tris(2,4-di-tert-butylphenyl)phosphite, Songnox 1680 (Songwon Industrial Co, Ulsan, South Korea) [41], •V2—1,3-phenylenebisoxazoline, 1,3-PBO powder (Evonik, Essen, Germany) [42], • V3—aromatic polycarbodiimide, Stabaxol P110 (Lanxess, Cologne, Germany) [43], and • V4—poly(4,4-dicyclohexylmethane carbodiimide), Carbodilite HMV-15CA (Nisshinbo, Tokyo, Japan) [44]. All ingredients were evenly mixed using a Mixaco CM 150-D (Mixaco Maschinenbau, Neuenrade, Germany) and compounded by a twin-screw extruder (FEL 26 MTS, Feddem GmbH, Sinzig, Germany) with 26 L/D, screw speed of 260 rpm, and output rate of 20 kg h−1 . The films were manufactured using the blow molding machine LF-400, (Labtech Engineering Company Co., Ltd., Samut Prakan, Thailand) with an extrusion temperature of 165 ◦ C and a blow-up ratio (BUR) of 1:2.5 for 25 µ m-thick films. This was to affirm almost similar morphology developments in all investigated compounds. CECLs influence the melt flow ratio and, thus, affect the melt viscosities of the compounds [ 33 ]. For the given blowing condition, this may affect film thicknesses and draw ratios (DR), as is shown in Table 6. The film thicknesses (Table 6) exhibited relatively small standard deviations, because they were determined on stripes before compost storage from a relatively short section of blown films. A device for inline-monitoring of the thickness was not available. The film thicknesses during blown film extrusion typically may vary by up to 15% [45,46] for both the ED and TD due to process fluctuations such as changes in melt viscosity due to variations in die temperature or die gap control. Since the overall mean thickness in Table 6 was (28 ±2) µm, the variation range was within the limit. The samples were stabilized at 23 ◦ C for 24 h after extrusion due to customers’ requirements enabling further packaging processing. Films were cut to tensile test stripes with the dimensions 170 × 15 mm and weighed. Trays with at least 3 L of volume were filled with a compost (Plantiflor Pro Natur torffrei, Dortmund, Germany) with 3 subsequent layers: a bottom layer of compost (thickness 5 cm), a layer of individualized test specimens, Int. J. Mol. Sci. 2023,24, 4525 17 of 21 and a top layer of compost (thickness > 5 cm). To prevent cross-contamination, differently colored trays were used for the REF and V1 to V4 and stored in an oven at 30 ◦ C or 60 ◦ C, respectively, with 60% relative humidity to be close to industrial composting conditions described by ISO 16926:2018. The storage times at 30 ◦ C were 168, 336, 672 and 1344 h; at 60 ◦C they were 24, 48, 72, 96, 120, 144 and 168 h. Since the V3 and V4 hardly showed visual disintegration, they were also subjected to compost storage at 60 ◦ C for 336 and 672 h. To follow ISO 16926:2018, pH values were determined when the samples were removed and cleaned. Table 6. Mean thicknesses of films of the reference (REF) and CECL-modified (V1 to V4) PBAT/PLA blends and corresponding draw ratio. Thickness in µm Draw Ratio REF 25.8 ±0.7 12.4 V1 30.5 ±0.5 10.5 V2 26.3 ±0.5 12.2 V3 27.3 ±0.6 11.7 V4 29.9 ±0.8 10.7 Note: Thicknesses were determined on stripes before compost storage. 4.2. Methods 4.2.1. Determination of Mass Change after Disintegration in Compost The film masses after compost exposure were measured using the Sartorius BP221S balance (Sartorius AG, Göttingen, Germany) with a measuring accuracy of 0.1 mg. The determined masses showed scatters in the range of 3 to 7% due to sticking compost particles, absorbed disintegration substances, and loss of micro-plastic particles during cleaning in cases of severe film disintegration. For better comparison of mass changes of the REF and V1 to V4, the data were converted to relative masses: mrel(t)=m(t) m0 (1) with mass m(t) after the given storage time in compost and initial mass m0. 4.2.2. Kinetics of the Process of Disintegration in Compost Due to the Decrease in Film Cross-Section After the given storage times, the samples were taken out of the compost, cleaned with a soft paint brush to remove compost particles, and subsequently stored at 23 ◦C/50% r.h. Then pictures of the PBAT/PLA films were taken with a Sony Cyber-shot DSCHX60V camera. The sample films appeared in greyish colors, whereas holes appeared in black. Then, the pictures were discriminated to black and white pixels, and the corresponding pixel areas were calculated using the public domain image analysis software Fiji [ 47 ]. The degree of disintegration xdisint was determined by xdisint(t)=Ablack(t) Awhite(t)+Ablack(t)(2) with the black pixel area represented as Ablack and white pixel area represented as Awhite. The quantitative kinetic approach assumes that the micro-organisms from the compost attack the film at the contact areas and start disintegration processes from the surface. Since the compost has a coarse and crumbly structure, the disintegration does not happent uniformly on the surface but locally at certain points. After an initiation time t init , first, holes appear in the films, which grow in size and number, and, thus, decrease the remaining Int. J. Mol. Sci. 2023,24, 4525 18 of 21 film cross-section. If one assumes that the time dependent change of the film cross-section dAf ilm dt depends on the available film cross-section Af ilm, one gets dAf ilm dt ∼Af ilm(t)=A0−Ahole(t)(3) with hole area Ahole. After dividing the initial cross-section A 0 , one gets the normalized rate equation with the temperature dependent disintegration constant kdisint: dAf ilm rel dt =−kdisint(T)Af ilm rel (t)(4) Solving by separation of variables and subsequent integration over equivalent limits gives: ln Af ilm rel (t)=−kdisint(T)t+C(5) with the integration constant C= 0. Solving for Af ilm rel (t)and substitution by xdisint(t)yields: xdisint(t)=1−e−kdisint(T)t(6) To take into account the initiation time t init of the disintegration process, one has to modify Equation (6) into: xdisint(t)=1−e−kdisint(T) (t−tinit)(7) or xdisint(t)=1−e−(t−tinit) τdisint (8) with the disintegration time τdisint being a measure of how fast the disintegration process proceeds. A least square fitting procedure for Equation (8) with the excel solver was used to determine tinit and τdisint to quantify the disintegration kinetics. 4.2.3. Tensile Tests and Mechanical Properties Young’s modulus, tensile strength, and fracture strain were determined according to ISO 527-3 using a tensile testing machine (2.5 kN Zwicki, Zwick Roell, Ulm, Germany) at 23 ◦ C/50% r.h. and a crosshead speed of 200 mm min −1 . Tensile tests were performed in the extrusion direction (ED) and transversal direction (TD) with n = 5 as long as the sample disintegration allowed that. All samples having initial dimensions of 170 × 15 mm were conditioned at 23 ◦ C and 50% r.h. for 24 h before testing to adjust the same testing conditions under which the initial properties were determined in [33]. 4.2.4. Differential Scanning Calorimetry (DSC) and Thermal Properties DSC experiments were performed using a DSC Diamond (Mettler Toledo, Greifensee, Switzerland) for granules and a DSC 214 Polyma (Netzsch Gerätebau GmbH, Selb, Germany) for films in standard Al pans with pinholed lids in three steps—1st heating, cooling, 2nd heating. The measuring conditions were: •Sample weight ms: (6 ±1) mg •Starting temperature Tstart: 0 ◦C •End temperature Tend: 200 ◦C •Heating/cooling rate: 10 K min−1 •Repetition: n ≥2 Int. J. Mol. Sci. 2023,24, 4525 19 of 21 The DSC traces according to ISO 11357-3:2018 were evaluated with respect to glass transition temperatures of the hard segment T g,hs , melting temperatures of PBAT T m1 and PLA T m2 and the corresponding heats of fusion ∆ H m1 , ∆ H m2 (1st run). Crystallization temperatures T cr and heats of crystallization ∆ H cr were determined from the cooling run. All samples were also conditioned at 23 ◦C and 50% r.h. for 24 h before testing. To estimate the content of humidity, the enthalpies between 70 and 90 ◦ C were determined from the 1st and 2nd run. Finally, the film masses after the 2nd run were measured to determine the mass loss due to all low molecular weight components. 4.2.5. Gel Permeation Chromatography (GPC) The granules of pure PBAT and PLA as well as film samples of the REF and V1 to V4 were dissolved in THF and filtered using PTFE filters with a mesh of 0.45 µ m into vials of 2 mL. All samples showed a turbidity due to a filler content of 24%. The sample concentration in each vial was 2.5 g/L. The GPC measurements were conducted by a Waters HPLC system equipped with a Waters model e2695 and Waters model 2414 differential refractometer (Waters Corporation, Massachusetts United States of America). The GPC system was calibrated with polystyrene standards having molecular masses of 580; 10,440; 38,640; 132,900; 492,500; and 990,500 g/mol. The measuring conditions were: • Series of gel-mixed bed columns: PL gel MIXED-A (300 × 7.5 mm, 20 µ m) + PL gel MIXED-B (300 ×7.5 mm, 10 µm) + PL gel MIXED-D (300 ×75 mm, 5 µm); •Mobile phase: tetrahydrofuran (THF) stabilized with butylated hydroxytoluene; •Temperature: 40 ◦C; •Injection volume: 100 µL; •Flow rate: 1 mL/min; •Detector: refractive index detector (RI). All data processing was carried out using Empower 3 software (Version FR 3). Author Contributions: Conceptualization, B.M.; methodology, B.M.; software, J.V.C.A.; validation, J.V.C.A.; formal analysis, J.V.C.A.; investigation, J.V.C.A. and T.S.; resources, J.V.C.A.; data curation, J.V.C.A., T.S.; writing—original draft preparation, J.V.C.A. and B.M.; writing—review and editing, B.H.; visualization, J.V.C.A.; supervision, B.H. and B.M.; funding acquisition, B.H. All authors have read and agreed to the published version of the manuscript. Funding: The authors B.H. and T.S. acknowledge the Ministry of Education, Youth, and Sports of the Czech Republic—DKRVO (RP/CPS/2022/003) and (RP/CPS/2022/002), respectively. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Datasets analyzed and generated during the study will be provided upon request. Acknowledgments: BIO-FED, a branch of AKRO-PLASTIC GmbH, is acknowledged for sponsoring the samples and working place of the presented work. 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