Introduction

Plastics are the most omnipresent group of materials today and are used practically everywhere. Their low cost, lightweight, durability, and ease of production allow them to be widely used almost in every industry, from packaging, and toys, to medical products [1]. Industrial use plastics are synthetic polymers obtained via chemical reactions from basic chemicals that are mainly derived from crude oil. Worldwide production of plastics reached over 360 Million tons in 2020 [2] and only 10.2 million tonnes of post-consumer plastic waste were collected and sent to recycling facilities inside and outside Europe [2]. Generally, it is a packaging market, in particular food and beverages packaging, which represents the largest end-use market (ca. 40%) [2], where plastic-based packaging is non-degradable and has a short lifetime (once used they are discarded) [3]. Fortunately the increasing social awareness of the environmental crisis subject and all related matters, for instance shrinking landfill capacity, the accumulation of non-degradable waste, and secondary nano- and microplastics pollution in the ocean, etc., makes people appear to be ready to accept new solutions and alternatives to petroleum-based plastics [3]. Recycling procedures are currently used, including the popular pyrolysis, which leads to lower greenhouse gas (GHG) net emissions. Bioplastics might represent a better alternative in light of the shortage of fossil resources and global warming. since they promote the advancement of circular economy and environmental sustainability [3]. However, interest in bioplastics has only recently been renewed, due to the huge amount of plastic produced. the ban on the export of plastic waste and contamination with nano- and microplastics. In the present study, we have focused on the comparison of a so-called “double green” (due to the sustainable nature of both raw materials and products [3]) thermoplastic polyester, poly(lactide) (PLA), with biobased thermoplastic polyesters (poly(ethylene 2.5-furandicarboxylate) (PEF), poly(trimethylene 2.5-furandicarboxylate) (PTF) and poly(hexamethylene 2.5-furandicarboxylate) (PHF)), and their petrochemical counterparts based on glycols with the same amount of methylene groups. by mean of poly(ethylene terephthalate) (PET), poly(trimethylene terephthalate) (PTT) and poly(hexamethylene terephthalate) (PHT).

Thermoplastic polyesters can be used in many areas of everyday life, for example, implants, packaging, fibers for the production of clothes, or as components for the automotive and aviation industries. The main representatives of thermoplastic polyesters used in the packaging industry are PET, PEF, and PLA. PLA has been of special research interest due to it’s the so-called “double green” feature resulted from its biocompatibility and biodegradability, along with high mechanical properties, thermal plasticity, and fabric ability that lead to applications in medical science and biotechnology [4, 5], but also in the packaging industry [6, 7]. The first commercially available and fully biodegradable PLA material is called NatureWorks®. However, despite the undoubted advantages of PLA, PET is the most widely used material for the production of carbonated and non-carbonated beverage bottles. Although various forms of PET recycling are practiced all over the world, this does not match the real demand. Hence, it is extremely important to replace PET with other materials, which firstly contribute to reducing the amount of waste, but also reduce the consumption of the so-called primary raw materials: PET is obtained directly from the crude oil fraction [8]. Therefore, PEF is an important example of "green" thermoplastic polyester used in industry and constitutes an eco-alternative to PET. This bio-based polyester has achieved great attention, not only because of its properties but also due to the fact that it leads to a significant reduction in energy use and GHG emissions. Its characteristics, by means of barrier performance, mechanical (almost two times higher Young’s modulus), and thermal are superior to those of PET. In particular, the gas barrier properties of PEF are extremely good. It exhibits a 10 times lower O2 permeability and a 20 times lower CO2 permeability than those of PET [1, 3, 9,10,11]. In comparison to PET, PTT, can be characterized by a lower melting point, ease of processing or dyeing, and greater flexibility [12], make it suitable for many industries, including in the textile markets or packaging [12, 13]. It is also worth mentioning that its production is based on a process, in which raw materials are used for its production that consuming 40% less energy and reduces GHG emissions by 20% compared to raw materials based on crude oil. Another semicrystalline aromatic thermoplastic polyester of increasing interest is poly(hexamethylene terephthalate) (PHT). Due to the production of one of the monomers for its synthesis (hexanediol) based on renewable raw materials, and its good mechanical properties, very good chemical resistance, and a relatively low melting point (which makes it easy to process), it has gained great interest in both industrial and academic environments [14, 15].

However, as mentioned above, the decreasing resources of fossil fuels, fluctuating crude oil prices, significant GHG, and the above-mentioned limited biodegradability of plastics obtained from petrochemical raw materials necessitate the search for renewable raw materials for their production [16, 17]. It is from plant biomass that, utilizing biochemical and/or chemical methods, it is possible to obtain compounds that are an alternative to petrochemical raw materials, which include 2,5-furanedicarboxylic acid (FDCA), on the basis of which the furan-based polyesters can be obtained. Not only PEF, but also polyesters with a longer sequence of methylene groups in the main chain [18], i.e. PTF [19], PBF [20], or PHF [21], are of great interest [22, 23]. PTF exhibits exceptional barrier properties, especially with respect to CO2 [24], and good mechanical performance (Young’s modulus of 1.6–2.7 MPa, σb of 79 MPa, and εb of 3% [25]) which allows its use in rigid packaging. These improved barrier properties of furan polyesters are due to the limited mobility of the furan ring and its polarization [24]. Most of the current research on PTF concerns the production of thin films, possibility of spinning fibers [24] or obtaining nanocomposites [26]. Another thermoplastic polyester based on FDCA or its diester is PHF, can be characterized by lower values of the characteristic temperatures of phase transitions (if compared to PEF and PTF), i.e. Tm and Tg, 145 °C and 7 °C, respectively [27]. It has good mechanical and thermal properties, however, due to its high flammability, it is not widely used in industrial applications [28]. For this reason, copolyester-ethers based on PHF are gaining importance, e.g. PHF-b-PTMO [28], PHF-b-PEO [29] or copolyesters, e.g. poly(ethylene-co-hexamethylene furanoate) (PEHF) [30].

The aforementioned numerous examples of thermoplastic polyesters used in packaging prove the interest of entrepreneurs in new solutions in the field of polymeric materials, and the importance of the subject matter in this paper. Here, we have compared the susceptibility of degradation in the compost heap of three types of packaging polyesters, by means of PET and biobased PEF and PLA, with other thermoplastic polyesters with more methylene groups (three and six) bio-based (PTF and PHF, respectively) and petrochemically-based (PTT and PHT, respectively. Another important issue discussed in this publication was the assessment of the impact of the consecutive cycle of processing (injection moulding) on the utilitarian properties of these materials, which were analogies to the subsequent recycling cycles. The susceptibility to degradation was assessed in the context of changes in the structure (analyzed by FTIR and DSC), intrinsic viscosity, and mechanical performance, and additionally, chromatographic analysis of the solutions of the analyzed samples in methanol was carried out in order to determine whether and what low-molecular compounds were released from the analyzed polyesters.

Materials and Methods

Polyesters’ Characterization

In the present study two series of polyesters, bio-based and petrochemically-based, were subjected to multiple injection moulding procedures (simulating the return cycles for recycling) and compared to the reference material, which was chosen PLA. PLA and PET were purchased in the form of granules, directly from the manufacturers to use the polyesters generally used in the packaging industry. PLA, Ingeo 4042 D grade (95.8% LLA) with Mw = 113000 g/mol was purchased from NatureWorks (Minnetonka, USA). PET bottle-grade (ELPET®) was purchased from Elana S.A., Torun, Poland, currently Boryszew S.A., Elana Department in Torun, Poland) exhibited limited viscosity number (LVN) of 0.78 dl/g, Mn = 23 500 g/mol, density in the crystalline form 1.40 g/cm3, density in the amorphous form 1.33 g/cm3, and melting temperature of 257 °C. The remaining polyesters, i.e. PEF, PTF, PTT, PHF, and PHT, were obtained using a two-stage melt polycondensation process in a 1 dm3 steel reactor equipped with the condenser, stirrer, gas inlet, and a vacuum pump. The following chemicals have been used in this study: dimethyl furan-2,5-dicarboxylate (DMFDC, 99%, Henan Coreychem Co. Ltd, China), dimethyl terephthalate (DMT, Sigma-Aldrich, Germany), 1,2-ethylene glycol [ED, BioUltra, ≥ 99.5% (GC), Sigma–Aldrich], 1.3-propylene glycol (bio-derived PD, DuPont Tate & Lyle BioProducts, Loudon, TN), 1.6-hexylene glycol (HD, 99%, Rennovia Inc., Santa Clara, CA, USA), catalyst [tetrabuthyl orthotitanate, Ti(OBu)4, Fluka], and antioxidant (Irganox 1010,Ciba–Geigy, Switzerland). A 1:2 molar ratio of diester to diol was used in all syntheses of polyesters. In the first stage of the reaction DMFDC (in the case of PEF, PTF, and PHF) or DMT (in the case of PTT and PHT), suitable glycol, the first portion of the catalyst, and the thermal stabilizer were inserted into the reactor and heated up to 170–185 °C, and the transesterification between DMFDC or DMT and glycol took place, wherein the by-product (methanol) was dripping. When the amount of the distilled methanol achieved 90% of the theoretical value (stoichiometrically calculated), the second stage, by means of melt polycondensation, began. The temperature in the reactor was gradually raised to 230–255 °C (depending on the polyester type, as summarized in Table 1), while the pressure was reduced to ca. 25–30 Pa. Progress of this stage was monitored by an increase of the stirrer torque. Finally, the resulting polyesters were extruded from the reactor under nitrogen pressure, cooled to room temperature in a water bath, and then granulated.

Table 1 Details on the preparation procedure and the monomers used for the polycondensation synthesis of the materials (bio-based chemicals and final materials coded in green. petrochemical ones coded in black) (Color figure online)
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Testing Samples` Preparation

The testing samples (dumbbell shape sample, type A3) were injection moulded using Boy 15 (Dr Boy GmbH&Co., Neustadt, Germany). Then, as mentioned above, to simulate the recycling procedure, materials were pelletized using a grinder after the first injection moulding procedure (coded as “I”), injection moulded again (coded as “II), pelletized again, and, finally, injection moulded for the third time (coded as “III”). Prior to each injection moulding pellets were dried for 24 h under a vacuum at the temperature of 60 °C. The parameters of injection moulding can be found in Table 2.

Table 2 Conditions of the injection moulding process (Color figure online)
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Composting Procedure

The prepared dumb-bell shape samples were subjected to composting process in a compost heap at a burial depth of about 5 cm. The composting process was carried out using a ready-made substrate available on the market (garden biocompost substrate, COMPO), which was mixed with a small amount of garden waste, grass clippings, leaves, egg shells, and coffee and tea grounds. The composting process was carried out in a laboratory dryer (POL-EKO) at 30 °C and 50% RH. The aim was to create an environment that is as close to natural as possible so that the results of the decomposition simulation are as reliable as possible. Moreover, the samples were watered every 3 days to keep the soil moist.

The fittings of each material are divided into 5 groups, respectively:

  • • Not subjected to the composting process marked with index "0".

  • • Subjected to composting process for 1 month—index "1".

  • • Subjected to composting process for 2 months—index "2".

  • • Subjected to composting process for 3 months—index "3".

  • • Subjected to composting process for 6 months—index "6".

The dumb-bell shape samples were placed in separate compost containers and appropriately marked to identify the samples correctly. In each of the containers, there are six samples from the 1st, 2nd, and 3rd injection, each (Fig. 1). There were also selected six samples that had not been composted. In addition, photos of the dumb-bell shape samples not subjected to the composting process and subjected to the 6-month composting process are shown in Fig. 2. The coloration of all furan-based polyesters is due to the use of a thermal stabilizer (Irganox 1010) which causes slight staining of materials.

Fig. 1
figure 1

Photos of all materials (after the first, second and third injection moulding cycle) inserted in the pots filled with compost soil

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Fig. 2
figure 2

Photos of all polyesters (after I injection moulding) before and after 6 months of compositing

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Characterization methods

The infrared spectra were acquired at room temperature with a Nicolet 380 FT-IR spectrometer (Thermo Scientific. USA). Sixteen scans were averaged for each sample in the range of 4000–400 cm−1.

Control over the composting process was also carried out by gas chromatography coupled with mass spectrometry. Methanol samples obtained as a result of the 24-h extraction process of a given polyester (weight 50 ± 5 mg) in methanol solvent (analytical grade, Chempur, Poland) were analyzed. Qualitative and quantitative analysis was performed with the GC–MS ThermoQuest apparatus, equipped with a Voyager detector and a DB-5 column (filled with phenylmethylsiloxanes, 30 m 9 0.25 mm 9 0.5 um). The analysis parameters were as follows: helium flow 1 ml/min, inlet temperature 150 °C, furnace temperature isothermally for 2.5 min at 50 °C, then an increase at a rate of 10 °C/min to 300 °C. Distillate compositions were calculated using the internal normalization method taking into account response factors.

Differential Scanning Calorimetry (DSC) measurements were performed using F1 Phoenix (Netzsch, Selb, Germany). The measurements were carried out in the heating/cooling/heating cycles in different temperature ranges depending on the material, i.e. PLA at 0–200 °C, PEF and PHT at – 50–300 °C, PET at 25–300 °C, PTF and PHF at – 50–250 °C, and PTT at 25–250 °C. All measurements were performed at the heating/cooling rate of 10 °C/min. The first heating was used to unify the structure of the samples after preparation and processing. The melting and crystallization temperatures (Tm, Tc) were measured at the maximum of endo- and exothermic peaks, respectively. The glass transition temperature (Tg) was taken as the midpoint of the change of the heat capacity (ΔCp). The degree of crystallinity was calculated from Eq. 1:

$$X_{c} = \frac{{\Delta H_{m} - \Delta H_{cc} }}{{\Delta H_{m}^{0} }} \cdot 100\%$$
(1)

where ΔHm and ΔHcc is the enthalpy of melting and enthalpy of cold crystallization (if observed), respectively, and \(\Delta {H}_{m}^{0}\) is the melting enthalpy of 100% crystalline sample depending on the material, i.e. for PLA equals to 105.97 J/g [31], for PEF equals to 137 J/g [32], for PET equals to 140.1 J/g [24], for PTF equals to 141.7 J/g [33], for PTT equals to 146 J/g [12], for PHF equals to 143 J/g [27] and for PHT equals to 146 J/g [34].

The limiting viscosity number (LVN) was estimated in order to observe the influence of both, the “recycling” and compositing procedure on the changes in polymers’ chain lengths. The measurement was taken at 30 °C in the mixture of phenol/1.1.2.2- tetrachloroethane (60/40 by weight). The concentration of polymer solution was 0.5 g/dl. The intrinsic viscosity was characterized using a capillary Ubbehlode viscometer (type Ic, K = 0.03294).

The static mechanical properties were evaluated using Autograph AG–X plus (Shimadzu, Kyoto, Japan), equipped with an optical extensometer, 1 kN Shimadzu load cell, and the TRAPEZIUM X software. In the beginning, the materials were extended to 1% with a crosshead speed of 1 mm/min. Subsequently, the stress–strain curves for polyesters were obtained at a rate of 5 mm/min. Each reported value is an average of five test specimens, The measurements were performed according to PN-EN ISO 527.

Results and Discussion

Fourier-Transformed Infrared Spectroscopy

In order to determine changes in the structure of materials resulting from their triple injection processing and composting in a compost heap, FTIR analysis was performed. The analysis concerned samples that were not composted and composted for 6 months in a compost heap. The differences in the spectra resulting from the multiple processing cycle as well as the degradation process in the heap were analysed (Fig. 3).

Fig. 3
figure 3

FTIR spectra for all polyesters after I, II, and III injection moulding cycle and subjected to the composting process for 0 and 6 months

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All FTIR spectra of PLA (Fig. 3a) exhibit the characteristic bands related to the CH stretching overtones at 2998–2850 cm−1, C = O stretching of ester bonds at 1750 cm−1, CH3 bending at 1452 cm−1, CH3 asymmetric stretching at 1348–1388 cm−1, O–C = O stretching ∼1190–1080 cm−1, OH bending at 1041 cm−1, C–C backbone vibrations at 901 cm−1, C–C stretching of C–COO at 870 cm−1, and C = O bending at 754 cm−1 [35]. The spectra of all analysed PLA samples did not differ significantly from each other, the only differences were observed at 2998–2850 cm−1 which corresponds to CH stretching overtones. These differences in intensity between samples that weren't subjected to the composting process and composted for 6 months are probably due to the greater number of shorter PLA chains resulting from the degradation of PLA_X_6 materials (where X = I, II, or III) in the compost. On the other hand, between the samples subjected to 6-month composting but subjected to I, II, and III injection times, no visible differences were observed. It has thus been confirmed that the material can be recycled within the processing cycle (without structural changes), and at the same time it is degraded in a compost heap, which is very promising from the point of view of packaging applications.

Similar observations on the effect of composting time and the processing cycle on the chemical structure were made for all analysed furan-based polyesters. The differences in FTIR spectra in individual materials (PEF, PTF, and PHF) (Figs. 3b–d) resulted only from the influence of composting time, but not from subsequent processing cycles, i.e. differences were observed at ca. 2900–2800 cm−1 that corresponds to CH stretching overtones and resulted from a greater number of shorter polymer chains. As for other differences between the spectra for individual furan-based polyester, no differences were observed, and the materials after 0 and 6 months of composting showed only bands characteristic for a given material, i.e.:

  • PEF_0 and PEF_6 (Fig. 3b) exhibit absorption bands at 3645 cm−1 due to the RCH2OH hydroxyl group vibration, at 3563 cm−1 due to the O–H bending of the carboxylic hydroxyls, at 3435 cm−1 due to the O–H bending of the hydroxyl end groups of the polyester, at 3123 cm−1 due to the C-H bending of the furan ring, at 2960–2850 cm−1 of the C-H bending vibrations, at 1714 cm−1 of the carbonyl group’s bending, at 1580 cm−1 due to the bending vibrations of the = C-H of the furan ring, at 1388 cm − 1 and 1348 cm − 1 due to the Csp2-O and Csp3-O bonds of the furan ring, and at 1190 cm.−1 due to the C-O bending vibration of the ester group [36, 37]

  • PTF_0 and PTF_6 (Fig. 3c) exhibit strong absorption bands at 3125 cm−1 and 1575 cm−1 attributed to C≡C stretching and = C–H bending vibrations, respectively. Moreover, absorption bands assigned to = C–O–C = antisymmetric stretching and = C–O–C = ring vibrations appear at 1261 cm−1 and 1218 cm−1, respectively. The strong absorption peak at 1710 cm−1 is attributed to C = O stretching vibration. The furan ring bending peaks are featured in three locations, 962, 822, 760 cm.−1 [38, 39]

  • PHF_0 and PHF_6 (Fig. 3d) exhibit strong absorption peaks originating from the C = O and C(= O)–O stretching mode of ester groups at 1718 cm−1 and 1222 cm−1, respectively, absorption peaks at 1576 cm−1 corresponding to aromatic C = C stretching vibrations, and at 1222 cm−1 attributed to = C–O–C = ring vibrations. Moreover, some weak signals ascribed to furan ring out-of-plane deformation appear at 967 cm−1, 820 cm−1, and ~ 766 cm.−1 [28, 38]

In turn, for the series of polyesters based on dimethyl terephthalate (PET, PTT, and PHT) (Figs. 3e–g), no changes between the FTIR spectra of the materials subjected to composting and not subjected to composting, were observed. For each material only characteristic bands were detected, i.e.:

  • PET_0 and PET_6 (Fig. 3e) exhibit absorption bands at 3412 cm−1 corresponded to O–H stretching vibrations, at 3000–2800 cm−1 bands corresponded to the asymmetric and symmetric aliphatic C-H stretching vibrations [40], at 1730 cm−1 corresponded to the C = O of ester groups, at 1590 cm−1 corresponded to stretching vibration of O–H, two peaks at 1453 and 1180 cm−1 are ascribed to –CH2– deformation band and C(O)–O stretching of ester groups, respectively [40, 41], and at 725 cm−1 the C-H out-of-plane deformation of two carbonyl substituents on the aromatic ring [40,41,42];

  • PTT_0 and PTT_6 (Fig. 3f) exhibit absorption bands at 2958 cm−1 corresponded to the asymmetric and symmetric aliphatic C-H stretching vibrations, at 1710 cm−1 corresponded to C = O stretching vibration of the terephthalate units, at 1245 and 1095 cm−1 corresponded, respectively, to C–O–C asymmetric and symmetric stretching vibration, two peaks at 1018 and 871 cm−1 correspond, respectively, to the C–H out-plane and in-plane deformation vibration of paradisplacement benzene ring, and at 723 cm−1 corresponded the C-H out-of-plane deformation of two carbonyl substituents on the aromatic ring [43];

  • PHT_0 and PHT_6 (Fig. 3g) exhibit absorption bands at 2919 cm−1 corresponding to the asymmetric and symmetric aliphatic C-H stretching vibrations, at 1710 cm−1 corresponded to C = O stretching vibration of the terephthalate units, 1474 cm−1 corresponded to –CH2– deformation, at 1267 and 1101 cm−1 corresponded, respectively, to C–O–C asymmetric and symmetric stretching vibration, two peaks at 1018 and 875 cm−1 correspond, respectively, to the C–H out-plane and in-plane deformation vibration of paradisplacement benzene ring, and at 724 cm−1 corresponded the C-H out-of-plane deformation of two carbonyl substituents on the aromatic ring [44].

Gas Chromatography

Low molecular weight compounds may be released during the degradation of polymers. Some of them are present in polymeric materials from the very beginning of the polymer synthesis, such as residual unreacted monomer, catalyst, stabilizer, and other additives. They can migrate and diffuse in the polymer during its application and be released from the surface of the material into the environment. The amount of polymer degradation products ranged from one or two in the case of, for example, PLA, to even several hundred in the case of engineering plastics, such as, for example, PET [45]. Therefore, chromatographic analysis of the solutions of the analyzed samples in methanol was carried out in order to determine whether and what low-molecular compounds are released from the analyzed polyesters.

As a result of the analysis, it was shown that despite the 6-month composting process and the observed changes in the chemical structure confirmed by FTIR analysis (for biopolyesters: PLA, PEF, PTF, and PHF) and changes in the limited viscosity number values (for all analyzed materials), no low molecular weight compounds were extracted from the materials. The analyzed chromatographs showed only the peak corresponding to the retention time for methanol. This is an important observation that suggests the possibility of composting samples based on furan compounds without negative environmental impact. The study did not show the presence of low molecular weight compounds, which means that materials based on furan diester can be a worthy replacement for plastics based on non-renewable raw materials, as they are not invasive to the environment. It is worth carrying out more specific research in this direction so that these materials are more often used in various branches of the economy.

Limiting Viscosity Number

Figure 4 shows changes in the limited viscosity number values depending on the composting time (for 0, 1, 2, 3, and 6 months) and the number of processing cycles (I, II, III, respectively). Moreover, all data are summarized in Table S1 (in Supplementary materials). For all analysed polyesters, a clear influence of both the composting time and subsequent injection processing cycles was observed. However, when comparing the samples subjected to the same composting time (indexed with 0, 1, 2, 3, 6), a strong influence of the number of injection moulding cycle was clearly visible, suggesting that the polyesters are more vulnerable to the subsequent processing cycle (simulating the recycling procedure) rather than composting. At mechanical recycling materials are subjected to thermal, mechanical, and oxidative degradation, which leads to chain scission. This results in a decrease in molecular weight, as previously noted by others for most commonly used polyesters, by means of PET [46, 47] and PLA [48]. In addition, during the composting process, materials are affected by hydrolysis and biodegradation. The prime properties which are influenced by these two processes are the polymer’s molecular weight, intermolecular forces, and crystallinity.

Fig. 4
figure 4

Changes in limiting viscosity number as a function of composting time for all polyesters after I, II and III injection moulding cycle and subjected to the composting process for 0, 1, 2, 3 and 6 months

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PLA injected once (PLA_I) did not show such significant differences in the LVN values, since, between the samples not subjected to the composting process (index “0”) and those placed in it for six months (index “6”), a decrease of 22% was recorded (from 0.843 to 0.654 dl/g). In the case of samples injected twice (PLA_II) and three times (PLA_III), the reduction of the LVN value was significant and on a similar level, approx. 40% (a reduction from 0.348 to 0.194 dl/g, and 0.162 to 0.093 dl/g, respectively).

In the case of PEF, the greatest reduction in the value of LVN was noticed between the samples subjected to two- (“II”) and three-times (“III”) injection processing cycle. For samples with the index "I", the reduction between the samples that were not subjected to and those subjected to the six-month composting process was as high as 51% (reduction of the value from 0.533 to 0.263 dl/g). The reduction in the value for samples with the index "II" was not that significant—it was only 27% for samples that were not subjected to and composted for 6 months (reduction from 0.373 to 0.274 dl/g). The samples with the index "III" showed the greatest reduction in the LVN value, as it amounted to as much as 66% (0.341 dl/g for samples that were not subjected to the composting process, up to 0.117 dl/g for samples that were subjected to the composting process for 6 months). On the other hand, when comparing PEF_I_0 with PEF subjected to a triple processing cycle and at the same time a 6-months composting process (PEF_III_6), the decrease in the LVN value was over 4 times (0.533 to 0.117 dl/g, respectively).

PET showed a similar decrease in the LVN value between samples subjected and not subjected to the composting process. This reduction was in the range of 37–48%. The samples with the index "I" showed the greatest reduction—the LVN value decreased from 0.784 to 0.408 dl/g. For the samples with the index "II", the value of the LVN decreased from 0.521 to 0.310 dl/g, while for the samples with the index "III" from 0.335 to 0.211 dl/g. Comparing PET injected once (“I”) and three times (“III”) and subject and not subjected to the composting process, i.e. PET_I_0 with PET_III_6, the reduction in LVN was more than 3.5 times.

To summarize the data for ethylene glycol-based polyesters: green PEF with the petrochemical PET, one can observe a similar trend, i.e. the decrease in LVN values along with an increase in the number of injection moulding cycle and the length of composting time.

For PTF, the greatest reduction in the LVN value was noticed between the samples subjected to the injection moulding process once and twice. For the samples with the index “I”, the reduction in LVN value between the samples that were not subjected to and those subjected to the six-month composting process was 16% (reduction from 0.554 to 0.466 dl/g). Samples injected twice (index “II”) showed a decrease in the value of LVN by 34% (from 0.423 to 0.281 dl/g for PTF_II_0 and PTF_II_6, respectively). In turn, the reduction of the LVN value for samples with the index "III" was as much as 46% (reduction from 0.322 to 0.173 dl/g for PTF_III_0 and PTF_III_6, respectively). When comparing the outliers samples, i.e. PTF_I_0 with PTF_III_6, a decrease in the LVN value by almost 70% was observed. Similarly, for PTT one also observed a decrease in the LVN values along with an increase in the number of injection moulding cycles and length of composting time. For samples with the index "I", the reduction between the samples that were not subjected to and those subjected to the 6-month composting process was 26% (reduction from 0.631 to 0.470 dl/g). For PTT samples injected twice (index “II”) a decrease in LVN value was as much as 42% (from 0.519 to 0.303 dl/g for PTT_II_0 and PTT_II_6, respectively). For the PTT samples with index “III”, the reduction of the LVN value equals 47% (from 0.476 to 0.254 dl/g for PTT_III_0 and PTT_III-6, respectively). While, when comparing the extreme values for PTT-based series, i.e. PTT-I_0 and PTT_III_6, the difference was 2.5 times. Thus, one can draw a conclusion that furan-based polyester (PTF) was more prone to the influence of the subsequent processing cycle and composting time than petrochemically-based PTT.

When analyzing polyesters based on bio-1.6-hexylene glycol, i.e. PHF and PHT, one can also observe a strong influence on the number of injection moulding cycle and the composting time. Similarly, as for the series based on 1,3-propylene glycol, also here, furan-based polyester (PHF) was more prone to the influence of simulated recycling and composting procedures than PHT, the decrease in the extreme values of LVN for PHF was 2.1 times (from 0.741 to 0.359 dl/g for PHF_I_0 and PHF_III_6, respectively) while for PHT equals to 1.9 times (from 0.837 to 0.437 dl/g for PHF_I_0 and PHF_III_6, respectively).

Differential Scanning Calorimetry

The influence of the injection moulding cycle and composting time on the supramolecular structure and phase transition temperatures was investigated with DSC analysis (Fig. 5). All determined data, by means of phase transition temperatures, the corresponding heat capacity (ΔCp), enthalpies of melting and (cold)crystalization (ΔHm and ΔHcc/ΔHc, respectively), along with the degree of crystallinity (Xc) are summarized in Table S2 (in Supplementary Materials), while partial results are additionally presented in Table 3.

Fig. 5
figure 5figure 5

DSC curves recorded during cooling and 2nd heating for all polyesters after I. II and III injection moulding cycle and subjected to the composting process for 0, 3, and 6 months

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Table 3 Characteristic phase transition temperatures from the second heating cycle for all materials I. II and III injection moulding cycle and subjected to the composting process for 0, 3, and 6 months (Color figure online)
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For PLA (Fig. 5a, Tables S2 and 3) one observed that the values of Tg and corresponding ΔCp decrease along with the subsequent injection moulding cycle and composting time. All samples indexed as “PLA_I” did not crystallize from the melt, while the subsequent processing cycles caused the decrease in the molecular mass (observed via LVN) and thus the samples crystallized. Moreover, all samples indexed as “PLA_I” exhibited cold crystallization (Tcc) but no changes in these values were observed along with the composting time. For all PLA_based samples, the values of Tm decreased, while the values of degree of crystallinity (Xc) increased along with subsequent injection moulding cycle and composting time. When comparing the extreme samples, i.e. PLA_I_0 and PLA_III_6, the difference in Xc was 20%. In turn, the values of crystallization temperatures (Tc) did not differ significantly from one another, regardless of the number of injection moulding cycle and composting time.

For all furan-based polyesters (PEF, PTF, PHF) one observed a decrease in the value of Tg (and the corresponding ΔCp) along with the subsequent injection moulding cycle and composting time, whereas the greatest decline was noticed for PEF, almost 50 °C when comparing extreme samples, i.e. PEF_I_0 and PEF_III_6. In addition, PEF_I was found to be amorphous (no Tm and Tc were detected), but subjecting it to subsequent processing cycles (PEF_II and PEF_III) caused the changes in crystallization behavior, wherein PEF_II started to crystallize after subjecting it to 2-months composting and PEF_III started to crystallize right away (Fig. 5b). For both series, i.e. PEF_II and PEF_III, the values of Tm did not change significantly, Tcc decreased, while the Xc increased along with the composting time. In turn, PTF_I and PTF_II didn’t crystallize from the melt, while PTF_III crystallized after being subjected to 3 months of composting (PTF_III_3) (blue panel in Fig. 5c), but the values of Tc increased slightly along with composting time. In addition, PTF_I initially was amorphous, but after subjecting it to composting for 2 months one found a melting peak at ca. 170°, but it didn’t change much along with increasing composting time. In turn, for PTF_II the melting point was observed from the very beginning, and its value didn’t change along with composting time, while the cold crystallization appeared after six months of composting. For PTF_III one observed the Tm and Tcc peaks at once, wherein the value of Tm wasn’t changed much with composting time, while the value of Tcc decreased from 132.6 °C (PTF_III_0) to 117.6 °C (PTF_III_6). For all PTF-based samples, where the melting was observed, the values of Xc increased along with the subsequent injection moulding cycle and composting time. Finally, for the series based on PHF (Fig. 5d) no significant changes were observed in the values of phase transition temperatures, i.e. the Tm and Tc values were comparable for all samples, while the values of Xc increased slightly along with a subsequent number of processing cycle and the length of composting process.

For the series based on dimethyl terephthalate (PET, PTT, PHT) one also observed the decrease in the Tg values with subsequent injection moulding cycle and composting time, but this change wasn’t as pronounced as for the series of polyesters based on dimethyl furan-2,5-dicarboxylate. For semicrystalline PET (Fig. 5e), the melting peak slightly moved towards higher temperatures along with the subsequent processing cycle, but no influence of the composting time was visible. Moreover, the values of Tc increased along with the composting time and injection moulding cycle, and when comparing the values for the extreme samples (PET_I_0 and PET_III_6) this change equaled 17 °C. In addition, the values of Xc were affected by the number of injection moulding cycle (increase) and weren’t dependent on composting time. In the case of PTT (Fig. 5f), no significant changes were observed between samples subjected to subsequent processing cycles and composting time, with only a slight decrease in the value of Tm and Tc, and a slight increase in the values of Xc (ca. 3%) was visible. Finally, for PHT (Fig. 5g) one could have observed a double melting peak resulting from the existence of two crystal structures, α and β [19, 44], at 134–138 °C and 141–147 °C, respectively, depending on the sample. Nevertheless, the values of Tm haven’t changed much between samples. In turn, one could have observed the influence of the number f injection moulding cycle on the values of Tc, especially for the samples subjected to composting for 6 months. When comparing the extreme samples, i.e. PHT_I_0 and PHT_III_6 this change was over 10 °C. Additionally, the degree of crystallinity of the samples wasn’t affected by the number of the injection moulding cycle, but it increased along with the composting time by over 15%.

One can observe an increase in Xc value along with subsequent injection moulding cycle and longer composting time for most samples. This is associated with a decrease in molecular weight as described in the section above, which causes an increase in the mobility of macromolecules that facilitates crystallization. In addition, the decrease in molecular weight is also reflected in Tg values, with longer composting time and a higher number of injection moulding cycles the value of Tg decreased. The presented relationship between Tg and molecular weight is consistent with Fox-Flory predictions [49].

Tensile properties

The mechanical properties of the analyzed polyesters were determined for the samples subjected to composting process for 0 to 6 months. Values such as Young’s modulus (E), stress at yield (σy), elongation at yield (εy) stress at break (σb), and elongation at break (εb) for samples that were not subjected and subjected to composting process for 3 and 6 months are summarized in Table 4. All data can be found in Table S3 (Supplementary materials). Figure 6 shows the representative stress–strain curves for the tested materials not subjected and subjected to the composting process for 3 and 6 months.

Table 4 Mechanical properties for all polyesters after I, II, and III injection moulding cycle and subjected to the composting process for 0, 3, and 6 months (Color figure online)
Full size table
Fig. 6
figure 6figure 6

The representative stress–strain curves all polyesters after I, II and III injection moulding cycle and subjected to the composting process for 0, 3, and 6 months

Full size image

For PLA one observed a strong influence of the subsequent processing cycles and the composting time on the stress–strain characteristics (Fig. 6). The increase in Young’s modulus (Table 4, Table S3) resulted from the stiffening effect ensuing from crystallization behavior, i.e. shorter polymer chains were more prone to crystallize, and thus higher value of the degree of crystallinity was observed (Table 3, Table S1). The highest values of Young’s modulus were observed for the samples subjected to the composting process for up to 3 months. The further decrease in the value of E (observed for the samples PLA_I, II, III_6), resulted from the decrease in the molecular masses (values of LVN). Moreover, only samples not subjected to (PLA_I, II, III_0) and subjected to composting for up to three months but only after single injection moulding (PLA_I_1-3) exhibit yield point (σy and εy). In addition for all analysed PLA-based samples one observed a decrease in the values of σb and εb resulting from both the subsequent processing cycles and the composting time.

For the series of furan-based polyesters (PEF, PTF, and PHF) one observed no linear dependence in the values of Young’s modulus (E) from the number of the injection moulding cycle. However, the pronounced influence of the composting time on the values of E was visible, i.e. an increase in the value of E along with the length of the composting process resulting from the stiffening effect (higher crystallinity) was seen. Moreover, only PHF exhibited yield point, in comparison to PEF and PTF, and the values of σy increased, while the values of εy decreased along with the number of processing cycles and the composting time. For PEF and PTF one observed the decrease in the values of σb and εb along with the subsequent processing cycle and composting time, but only in the case of the samples subjected to the composting time for 1 month (PEF_I, II, III_1 and PTF_I, II, III_1), an inconsiderable increase in the values of σb and εb was seen. This resulted probably from the stiffening effect, without a significant reduction in the molecular mass. Therefore, further composting, along with the concomitant mass loss of the samples, resulted in a decrease in the values of σb and εb. In turn, for PHF, an increase in the value of σb along with an increase in the length of the composting process. Moreover, when comparing the samples after three subsequent injection moulding cycles but at the same composting time, one observed a decrease in the value of σb. Additionally, when comparing the extreme values obtained for the series of furan-based polyester, one observed the following: (i) for PEF_I_0 and PEF_III_6 the difference between σb and εb were 2.5 and 2.0 times, respectively; (ii) for PTF_I_0 and PTF_III_6 the difference between σb and εb were 1.8 and 1.3 times, respectively; and (iii) for PHF_I_0 and PHF_III_6 the difference between σb and εb were ca. 20% and ca. 10%, respectively.

Finally, for the series of dimethyl terephthalate (PET, PTT, and PHT) one observed different effects on the changes in the mechanical performance depending on methylene unit sequence length. In the case of PET, one observed an almost linear increase in the value of E for the samples subjected to the composting for up to 3 months, and afterward, after 6 months of composting a distinct decrease was seen. In turn, for PTT, the increase in the value of E was observed along with the subsequent processing cycles and the composting time. For PHT, generally one observed an increase in the value of E, however, the highest values of E were observed for the samples subject to the composting time of 2 and 3 months. But, as mentioned above, the further decrease in the molecular mass, despite the stiffening effect caused the value of E to decrease (however the value of E observed for the PHT_I, II, III_6 is still higher than the one observed for PHT_I_0). Only PET and PHT exhibit yield point, and the values of σy and εy initially decrease (after 1 month of composting) and then increase. Additionally, one can observe that the value of σb for PET decreases along with the subsequent processing cycle, at the same composting time. In turn, the increase in the composting time for the samples after the first and second processing cycles are comparable to one another, and just in the case of the samples injected three times, the values of σb initially increase up to 2 months of composting and then decrease. Moreover, for PET one couldn’t observe an unequivocal trend towards the impact of injection number or composting time: the difference between the extreme samples, i.e. PET_I_0 and PET_III_6 equals 10% (statistically insignificant). While, for PTT one observed the decrease in the value of σb and εb, both with the subsequent injection moulding cycle and composting time, e.g. the difference in the values of σb between PTT_I_0 and PTT_III_6 was over 2 times. In turn, for PHT one couldn’t observe an unequivocal trend along with the subsequent injection moulding cycle. The value of σb for PHT-based samples initially decrease, but only for the samples subjected to composting process for 3 months, a slight increase was observed. Similar observations were made on the changes in the values of εb for PHT-based materials, but herein the increase was observed for the samples subjected to composting process for 2 months.

Summarizing the above observations, it can be concluded that there was a clear influence of subsequent processing cycles and composting time on the mechanical properties of the obtained materials, which results from changes in the supramolecular structure and the decreasing molecular mass.

Conclusions and Future Perspectives

In this study, the properties of thermoplastic polyesters based on petrochemical dimethyl terephthalate (PET, PTT, PHT) and biobased dimethyl furandicarboxylate (PEF, PTF, PHF) were compared. The reference material was the so-called "double green" PLA. Except for PLA and PET, which were commercially available, the rest of the analysed materials were obtained by the melt polycondensation, and then all polyesters were processed three times. The triple injection moulding cycle was applied to simulate the recycling process of the tested polyesters. Each of the materials was tested in terms of chemical structure as well as physicochemical and mechanical properties.

It was found that the values of LVN for all tested polyesters decreased along with the length of composting time and the number of processing cycles. FTIR analysis also showed differences in the structure of polymers, but only in the case of materials based on renewable raw materials (PLA, PEF, PTF, and PHF). Moreover, the significant influence of the composting time and the number of processing cycles on the values of phase transition temperatures along with the changes in supramolecular structure and mechanical properties of each of the materials was confirmed. In general, furan-based polyesters were more prone to composting procedure and subsequent injection moulding cycle, and thus one could have observed a stronger influence on the changes in phase transition temperatures. For instance, the greatest change (decrease) in the values of glass transition temperature was visible for PEF between the samples injected twice and three times (50 °C). In addition, the degree of crystallinity increased with increasing composting time and subsequent injection cycle. Moreover, along with an increase in the number of processing cycles and the composting time, the mechanical properties of each of the samples deteriorated significantly. This results are in accordance with two important review papers from the group coordinated by Andreia Sousa [10, 18] on a perspectives on the synthesis, properties and end-life targeting of PEF [10] and replacement of PET, which represents 7.7% market-share in the global plastic demand (Plastics–the Facts 2019), by renewable alternatives in packaging, fiber, and film materials [18]. The HPGC analysis confirmed that despite 24 h of extraction in the methanol, in the analyzed solvent (methanol) no low molecular weight compounds were visible, which means that materials based on furan diester can be a worthy replacement for plastics based on non-renewable raw materials, as they are not invasive to the environment.

Fossil fuels are a non-renewable resource and make a significant contribution to GHG emissions. Due to the current climate change, raw materials based on plant biomass are becoming more and more desirable and their production reflects today's economy. The conducted research has shown that materials based on dimethyl furan-2,5-dicarboxylate are promising substitutes for petroleum-based materials, since they exhibit many desirable properties, e.g. excellent barrier properties, making them suitable for packaging applications.