Thermal Analysis of Aliphatic Polyester Blends with Natural Antioxidants

The aim of this research was to enhance thermal stability of aliphatic polyester blends via incorporation of selected natural antioxidants of plant origin. Thermal methods of analysis, including differential scanning calorimetry (DSC) and thermogravimetry (TGA), are significant tools for estimating the stabilization effect of polyphenols in a polymer matrix. Thermal stability was determined by analyzing thermogravimetric curves. Polymers with selected antioxidants degraded more slowly with rising temperature in comparison to reference samples without additives. This property was also confirmed by results obtained from differential scanning calorimetry (DSC), where the difference between the oxidation temperatures of pure material and polymer with natural stabilizers was observed. According to the results, the materials with selected antioxidants, including trans-chalcone, flavone and lignin have higher oxidation temperature than the pure ones, which confirms that chosen phytochemicals protect polymers from oxidation. Moreover, based on the colour change results or FT-IR spectra analysis, some of the selected antioxidants, including lignin and trans-chalcone, can be utilized as colorants or aging indicators. Taking into account the data obtained, naturally occurring antioxidants, including polyphenols, can be applied as versatile pro-ecological additives for biodegradable and bio-based aliphatic polyesters to obtain fully environmentally friendly materials dedicated for packaging industry.


Introduction
Nowadays, many scientists are focused on creating non-toxic, bio-based and biodegradable materials [1][2][3][4]. In such composites, additives play an important role and should also have positive impact on natural environmental and human health. The application of selected phytochemicals as additions to polymers with biodegradability and derived from renewable resources is an innovative way to obtain pro-ecological polymeric materials [5][6][7][8].
Phytochemicals are substances with a variety of physical, chemical and biological characteristics that play important roles in plant growth and development [9][10][11]. The majority of them have antioxidant properties and their role relies on protecting against the harmful impact of free radicals. These substances, called antioxidants, are able to delay or inhibit oxidation reactions which are responsible for the aging processes [11]. Many of the phytochemicals are, because of their beneficial properties, used as food supplements, bioactive pharmaceutical components and additives for cosmetics and perfumes [12,13]. A diet enriched with vegetables and fruit, which are full of natural of science. According to the present studies, other different natural antioxidants, including flavone, trans-chalcone and lignin can become pro-ecological additives for polymers.

Sample Preparation
In the first stage, samples were prepared by melt blending in a DSM-5 twin-screw compounder (Xplore ® , Sittard, The Netherlands) with mixing time of 2.5 min and speed of 100 rpm. The mixing temperature was 170 • C in the case of polyhydroxybutyrate (P(3.4 HB)) reference material and 190 • C for PLA reference material and (PLA/P(3.4 HB)) blends with selected antioxidants. Samples of about 8-9 g were then hot pressed at 170 • C (P(3.4 HB)) referential sample) and 190 • C (PLA and PLA/P(3.4 HB) specimens) with pressing time of 2.5 min. The thin square plates obtained had dimensions of 80 × 80 × 1 mm. The compositions of prepared blends are presented in Table 1.

Differential Scanning Calorimetry (DSC)
Differential scanning calorimetry (DSC) was used for determining the temperature ranges of the characteristic phase changes, glass transition temperature (T g ), cold crystallization temperature (T cc ) and melting temperature (T m ), as well as oxidation temperature (T o ). Enthalpy changes (∆H) were also analysed. The tests were carried out using a Mettler Toledo ® DSC1 instrument (TA 2920; TA Instruments, Greifensee, Switzerland) equipped with a STARe System and Gas Controller GC10. The analyser was calibrated on the basis of n-octane and indium standards. The samples of about 8-9 mg, placed in open, aluminum crucibles, were heated from 0 • C to 200 • C at a rate of 20 • C/min under an argon atmosphere. After 10 min at 200 • C, the samples were cooled to 0 • C and the gas was switched from argon to air at a flow rate of 50 mL/min. Finally, samples were heated to 350 • C and the curves obtained were analysed.

Themogravimetric Analysis
Thermogravimetric Analysis (TGA) was utilized for detecting the thermal degradation process which is related to the mass loss of the specimen as a function of rising temperature. The analysis was performed by means of a TGA/DSC1 device provided by Mettler Toledo ® (TA Instruments, Greifensee, Switzerand), with prior calibration using indium and zinc as standards. The test was carried out in a temperature range of 25-600 • C with heating rate of 15 • C/min under an argon atmosphere at a flow rate of 45 mL/min. The crucibles used in this analysis with volume of 70 µL were made of ceramic (polycrystal aluminium oxide).

Dynamic Mechanical Analysis
The rheological measurements in the solid state were carried out by means of an ARES G2 rotational rheometer (TA Instruments, New Castle, DE, USA) using a small amplitude oscillatory frequency and a temperature ramp within the range of −100 until 150 • C. The heating rate was set at 5 K/min. A frequency of 1 Hz was used and the strain amplitude was 0.05%. All measurements were carried out using nitrogen as the heating gas. Both storage and loss modulus, were measured as a function of the temperature respectively.

Solar Aging
Samples were aging using Atlas SC340 MHG Solar Simulator climate chamber (AMETEK, Inc., Berwyn, IL, USA) with a 2500 W MHG lamp. The solar radiation intensity is reported to be 1200 W/m 2 at 100% lamp power intensity. Accelerated ageing of the samples was carried out in an Atlas SC340 MHG Solar Simulator climate chamber with a 2500 W MHG lamp. The solar radiation intensity is reported to be 1200 W/m 2 at 100% lamp power intensity. The specimens were tilted a bit from the horizontal (8 • ± 2 • ) in order to let the water run off the surface. During the ageing period the UVA and UVB radiation intensity was measured at various times to be lying within the interval 60-80 W/m 2 (a 6% UVA fraction (like in sunlight) of the total solar intensity (1200 W/m 2 ) yields 72 W/m 2 UVA radiation) and 3-6 W/m 2 , respectively. The UV measurements were performed with a radiometer/photometer with an UVA sensor and an UVB sensor. The exposure duration consisted of 30 whole cycles of 24 h, each cycle divided into 20 h with a solar radiation intensity of 1200 W/m 2 and 4 h with no solar radiation exposure. The solar aging lasted 265 h, with the temperature of T = 60 • C and the humidity of 70%.

Mechanical Tests
Static mechanical properties, tensile strength (TS) and elongation at break (E b ), were determined with a Zwick/Roell Z2.5 test machine provided by ZwickRoell GmbH & Co. KG (Ulm, Germany) before and after solar aging. Tests were carried out on "dumbbell" shaped samples type 5B, approximately 1 mm thick and 1.7 mm wide centre portion, according to DIN EN ISO 527-2/5B/5. The samples were cut from prepared plates using a stamp. The speed of test was 5 mm/min. The pre-stress was about 0.05 N. The number of individual samples used in this research was 6 for every tested material. Basing on the obtained results, the aging coefficient (A f ) was calculated from the presented equation [30]: where A f is the aging coefficient, TS is tensile strength (MPa), and E B is elongation at break (%).

Fourier Transform Infrared Spectroscopy (FT-IR) Absorbance Spectra Analysis
Fourier transform infrared spectroscopy (FT-IR) absorbance spectra was obtained in the range of 4000-400 cm −1 taking advantage of Thermo Scientific Nicolet 6700 FT-IR spectrometer with diamond Smart Orbit ATR sampling equipment. The number of used scans amounted to 64 at a resolution of Polymers 2020, 12, 74 5 of 16 4 cm −1 . In this research the structural changes of tested materials which occurred as a result of ageing processes were investigated by the difference of the peak intensity at 1748 cm −1 and the reference peak at 1451 cm −1 [31]. The comparison of -C-O (1187cm −1 ) and -C=O (1748 cm −1 ) absorbance was also determined [31,32].

Colour Measurement
The optical investigation of samples was performed before and after solar aging by means of colour measurement. The colour of PLA/P(3.4 HB) blends containing selected antioxidants was measured on the basis of PN-EN ISO 105-J01 using a Spectrophotometer UV-VIS CM-36001 (Konica Minolta Sensing, Inc., Osaka, Japan). This device measures the signal reflected from the surface of the sample and converts it into the impression of colour that is perceived by the human eye. The results were described with the CIE-Lab system (L: lightness, a: red-green, b: yellow-blue). Moreover, colour difference (∆E), whiteness index (W i ), chroma (C ab ) and hue angle (h ab ) values were calculated according to Equations (2)- (5). The values ∆a, ∆b and ∆L used in these equations were calculated as the difference of a, b and L parameters between samples with and without natural stabilizer. These parameters are useful for estimating polymer colour change after incorporating natural antioxidants to polyester blend matrix [33].

Differential Scanning Calorimetry (DSC)
Differential Scanning Calorimetry was useful for oxidation stability detection of prepared materials. Moreover, some thermal parameters, such as glass transition temperature (T g ), cold crystallization temperature (T cc ) and melting point (T m ), were determined. As can be seen in the curve (Figure 1), the blend based on polylactide (PLA) and polyhydroxybutyrate (P(3.4 HB)) has one glass transition temperature, which is higher than for P(3.4 HB) material and lower than for polylactide (PLA). The glass transition temperature (T g ) of prepared blends was about 54 • C and this result corresponds with the data obtained by Zhang et. al. [34]. Adding antioxidants, including flavone and trans-chalcone, did not change this parameter significantly. Only lignin turned out to be a type of plasticizer and caused a decrease of T g to 52 • C. Based on the results obtained by Gordobil et. al., this phenomenon was not observed with pure PLA, where the addition of different types of lignin did not change this parameter [35]. Moreover, the phenomenon of cold crystallization, which is characteristic of materials crystallizing very slowly, such as PLA or P(3.4 HB), has been detected. Prepared blends of PLA and P(3.4 HB) crystallize quicker and have the highest enthalpy of this process, amounting to 24 J/g, which is presented in Table 2. This is much more than for the referential samples of PLA or P(3.4 HB). This enthalpy might be a result of two types of crystal structure creation, where dispersed P(3.4 HB) crystals act as nucleating agents in PLA matrix [36]. The crystallization temperatures of PLA/P(3.4 HB) blends are between that of pure PLA and P(3.4 HB), furthermore, the addition of flavonoids did not affect this parameter substantially. In the DSC run, the melting point of PLA/P(3.4 HB) materials can also be detected. For the polylactide/polyhydroxybutyrate blend, a bimodal melting peak is observed. According to the data obtained from DSC measurements, two types of crystal are present Polymers 2020, 12, 74 6 of 16 in this blend. The first stage of melting is related to one type of crystals melting, which probably belongs to PLA. The second one belongs to the P(3.4 HB) crystals, which also corresponds with data given in literature [34,36]. As can be seen, the addition of polyphenols did not change this parameter significantly but it caused an increase in the temperature of the oxidation process. This parameter is related to improvement of material resistance to oxidation. Thanks to flavone, the temperature of the start of the oxidation process in PLA/P(3.4 HB) blend was raised from 233 • C to 241 • C, hence improving the material's stability. Adding trans-chalcone to PLA/P(3.4 HB) blend caused an increase in the temperature of oxidation from about 232 • C to 269 • C, which also made the blends more thermally stable. A similar phenomenon was observed in the case of other antioxidants, including quercetin [23] and lawsone [25].
Polymers 2020, 12, x FOR PEER REVIEW 6 of 17 types of crystal are present in this blend. The first stage of melting is related to one type of crystals melting, which probably belongs to PLA. The second one belongs to the P(3.4 HB) crystals, which also corresponds with data given in literature [34,36]. As can be seen, the addition of polyphenols did not change this parameter significantly but it caused an increase in the temperature of the oxidation process. This parameter is related to improvement of material resistance to oxidation. Thanks to flavone, the temperature of the start of the oxidation process in PLA/P(3.4 HB) blend was raised from 233 °C to 241 °C, hence improving the material's stability. Adding trans-chalcone to PLA/P(3.4 HB) blend caused an increase in the temperature of oxidation from about 232 °C to 269 °C, which also made the blends more thermally stable. A similar phenomenon was observed in the case of other antioxidants, including quercetin [23] and lawsone [25].

Sample
Tg

Thermogravimetric Analysis (TGA)
Thermogravimetric analysis allowed a determination of material thermal stability. According to the TGA thermogram ( Figure 2) and Table 3, blending PLA with P(3.4 HB) reduces the resistance of PLA to high temperatures. The blend lost 5% of its weight at a temperature of 284.5 • C, while the temperature of 5% PLA decomposition amounted to 328.7 • C. Adding polyphenols to PLA/P(3.4 HB) blend caused the improvement in thermal stability At first glance, the stability is not noticeable and PLA/P(3.4 HB) blend with antioxidants loses 5% of its weight at a similar temperature as the pure blend. Nevertheless, the added natural stabilizers subsequently slowed down the blend decomposition. This phenomenon starts to occur in case of 15% material weight loss and is the most visible for 50% of sample weight loss. According to the gathered data, PLA/P(3.4 HB) with trans-chalcone is the most thermally stable and loses 50% of its mass at 346.0 • C, while the pure blend loses the same amount of mass at 328.0 • C. Other selected antioxidants, including flavone and lignin, also improve the thermal stability of polylactide and polyhydroxybutyrate blend. In the case of PLA/P(3.4 HB) blend with flavone, the material loss of 50% takes place at the temperature of 335 • C, and in the case of the blend with lignin, this parameter amounted to 338.5 • C.

Thermogravimetric Analysis (TGA)
Thermogravimetric analysis allowed a determination of material thermal stability. According to the TGA thermogram ( Figure 2) and Table 3, blending PLA with P(3.4 HB) reduces the resistance of PLA to high temperatures. The blend lost 5% of its weight at a temperature of 284.5 °C, while the temperature of 5% PLA decomposition amounted to 328.7 °C. Adding polyphenols to PLA/P(3.4 HB) blend caused the improvement in thermal stability At first glance, the stability is not noticeable and PLA/P(3.4 HB) blend with antioxidants loses 5% of its weight at a similar temperature as the pure blend. Nevertheless, the added natural stabilizers subsequently slowed down the blend decomposition. This phenomenon starts to occur in case of 15% material weight loss and is the most visible for 50% of sample weight loss. According to the gathered data, PLA/P(3.4 HB) with transchalcone is the most thermally stable and loses 50% of its mass at 346.0 °C, while the pure blend loses the same amount of mass at 328.0 °C. Other selected antioxidants, including flavone and lignin, also improve the thermal stability of polylactide and polyhydroxybutyrate blend. In the case of PLA/P(3.4 HB) blend with flavone, the material loss of 50% takes place at the temperature of 335 °C, and in the case of the blend with lignin, this parameter amounted to 338.5 °C.  The thermal stability phenomenon can be seen more noticeably by using differential thermogravimetry (DTG) (Figure 3 and Table 4). According to DTG curves (Figure 3), it can also be  Table 3. Temperatures of the tested materials weight loss. T x% is temperature at which the weight loss is x%. The thermal stability phenomenon can be seen more noticeably by using differential thermogravimetry (DTG) (Figure 3 and Table 4). According to DTG curves (Figure 3), it can also be observed that the thermal degradation of blends has two stages. The reason is that the first stage is related to P(3.4 HB) decomposing, and the second one is associated with the PLA degradation. The decomposing of P(3.4 HB) in a blend occurs at a higher temperature than in the case of pure material. This means that the presence of PLA in a blend delays the degradation of polyhydroxybutyrate by acting as a type of barrier, which was also verified by Arrieta et al. [37]. However, polylactide is more thermally stable than the P(3.4 HB) and PLA/P(3.4 HB) blend. Nevertheless, adding selected natural antioxidants, including flavone, trans-chalcone and lignin to the blend, can improve the thermal stability of the PLA/P(3.4 HB) material. According to the data obtained, the beginning of the pure blend's main decomposing occurred at lower temperature compared to the blend with antioxidants. The most thermally stable blend contains trans-chalcone. This antioxidant changed blend's beginning temperature of main decomposing (T onset ) from 284.8 • C to 327.0 • C. The addition of other antioxidants, flavone and lignin caused an increase in this parameter to 311.4 • C and 318.7 • C respectively. Furthermore, the addition of flavone, trans-chalcone and lignin to PLA/P(3.4 HB) blend caused an increase in the temperature of the most intensive weight loss (T d ) from 353.3 • C to 360.9 • C, 371.6 • C and 363.4 • C, respectively. observed that the thermal degradation of blends has two stages. The reason is that the first stage is related to P(3.4 HB) decomposing, and the second one is associated with the PLA degradation. The decomposing of P(3.4 HB) in a blend occurs at a higher temperature than in the case of pure material. This means that the presence of PLA in a blend delays the degradation of polyhydroxybutyrate by acting as a type of barrier, which was also verified by Arrieta et al. [37]. However, polylactide is more thermally stable than the P(3.4 HB) and PLA/P(3.4 HB) blend. Nevertheless, adding selected natural antioxidants, including flavone, trans-chalcone and lignin to the blend, can improve the thermal stability of the PLA/P(3.4 HB) material. According to the data obtained, the beginning of the pure blend's main decomposing occurred at lower temperature compared to the blend with antioxidants. The most thermally stable blend contains trans-chalcone. This antioxidant changed blend's beginning temperature of main decomposing (Tonset) from 284.8 °C to 327.0 °C. The addition of other antioxidants, flavone and lignin caused an increase in this parameter to 311.4 °C and 318.7° C respectively. Furthermore, the addition of flavone, trans-chalcone and lignin to PLA/P(3.4 HB) blend caused an increase in the temperature of the most intensive weight loss (Td) from 353.3 °C to 360.9 °C, 371.6 °C and 363.4 °C, respectively.

Dynamic Mechanical Analysis
Basing on the dynamic mechanical analysis results (Figure 4), it can be noticed that blending PLA with P(3.4 HB) at a proportion of 60/40 caused an interruption in PLA crystallization which is observed in the storage modulus decreasing. This phenomenon was confirmed also by Jung Seop Lim et al. in a polylactide blended with poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) at different mass ratios [38]. Moreover, loss modulus of PLA also diminished significantly after mixing with P(3.4 HB) at the temperature of about 75 • C. Furthermore, pure PLA sample revealed an intensive peak of tanδ in a comparison to the blend. The reduced height of this peak in case of PLA/P(3.4 HB) blend was a result of the addition of polyhydroxybutyrate to polylactide, which caused restraint of the PLA chain mobility. The similar occurrence was proved also by Jung Seop Lim et al. [38]. Adding Polymers 2020, 12, 74 9 of 16 natural stabilizers to the blend did not cause changes in the dynamic mechanical analysis results as expected, because such stabilizers should not affect these properties. observed in the storage modulus decreasing. This phenomenon was confirmed also by Jung Seop Lim et al. in a polylactide blended with poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) at different mass ratios [38]. Moreover, loss modulus of PLA also diminished significantly after mixing with P(3.4 HB) at the temperature of about 75 °C. Furthermore, pure PLA sample revealed an intensive peak of tanδ in a comparison to the blend. The reduced height of this peak in case of PLA/P(3.4 HB) blend was a result of the addition of polyhydroxybutyrate to polylactide, which caused restraint of the PLA chain mobility. The similar occurrence was proved also by Jung Seop Lim et al. [38]. Adding natural stabilizers to the blend did not cause changes in the dynamic mechanical analysis results as expected, because such stabilizers should not affect these properties.

Mechanical Tests
According to the mechanical test results illustrated in Figure 5, the blend of polylactide and polyhydroxybutyrate in proportion 60 and 40 is characterised by lower maximum stress results in comparison to reference material of PLA. This parameter decreased from 57.79 MPa to 37.12 MPa after adding polyhydroxybutyrate to polylactide. Also, strain at maximum stress (ε M ) of PLA, which was 6.79%, was halved after adding polyhydroxybutyrate. As can be seen, the addition of P(3.4 HB) to polylactide reduced not only thermal parameters, but also mechanical ones. The reason is that too large an amount of P(3.4 HB) crystals could not be finely dispersed in PLA matrix in order to behave as a filler, as was proven by Arrieta et. al. [37] in the case of 75/25 PLA/PHB blend. Nevertheless, addition of natural antioxidants did not change σ M and ε M significantly.

Mechanical Tests
According to the mechanical test results illustrated in Figure 5, the blend of polylactide and polyhydroxybutyrate in proportion 60 and 40 is characterised by lower maximum stress results in comparison to reference material of PLA. This parameter decreased from 57.79 MPa to 37.12 MPa after adding polyhydroxybutyrate to polylactide. Also, strain at maximum stress (εM) of PLA, which was 6.79%, was halved after adding polyhydroxybutyrate. As can be seen, the addition of P(3.4 HB) to polylactide reduced not only thermal parameters, but also mechanical ones. The reason is that too large an amount of P(3.4 HB) crystals could not be finely dispersed in PLA matrix in order to behave as a filler, as was proven by Arrieta et. al. [37] in the case of 75/25 PLA/PHB blend. Nevertheless, addition of natural antioxidants did not change σM and εM significantly. For the tested samples, mechanical parameters were measured also after solar aging and the aging factor [30] was calculated based on the Equation (6): where Af is the aging coefficient, TS is tensile strength (MPa), and EB is the elongation at break (%). According to Table 5, solar aging caused severe deterioration of tested materials mechanical properties. Based on the aging factor results, pure PLA proved to be the most stable in extreme conditions prevailing in solar simulator climate chamber. P(3.4 HB) and PLA/P(3.4 HB) are much more sensitive and even addition of natural antioxidants did not prevent materials from degradation. Selected antioxidants are good thermal stabilizers, but they are not sufficient in terms of other conditions, including higher humidity and solar radiation intensity. For the tested samples, mechanical parameters were measured also after solar aging and the aging factor [30] was calculated based on the Equation (6): A f = (TS·E B ) a f ter aging /(TS·E B ) a f ter aging (6) where A f is the aging coefficient, TS is tensile strength (MPa), and E B is the elongation at break (%). According to Table 5, solar aging caused severe deterioration of tested materials mechanical properties. Based on the aging factor results, pure PLA proved to be the most stable in extreme conditions prevailing in solar simulator climate chamber. P(3.4 HB) and PLA/P(3.4 HB) are much more sensitive and even addition of natural antioxidants did not prevent materials from degradation. Selected antioxidants are good thermal stabilizers, but they are not sufficient in terms of other conditions, including higher humidity and solar radiation intensity.

Fourier Transform Infrared Spectroscopy (FT-IR) Absorbance Spectra Analysis
The -C-O (1187 cm −1 ) stretching band in PLA/P(3.4 HB) [31] is sensitive to solar aging process. which is observable in the Figure 6 During the solar aging. the absorbance of this band decreased in the case of pure P(3.4 HB) sample and its blend with polylactide and flavone. Only PLA/P(3.4 HB) with trans-chalcone blend turned out to be an exception and for this sample. where absorbance of the -C-O stretching band increased after solar aging. In the case of pure polylactide as well as PLA/P(3.4 HB) with lignin. changes in the absorbance did not occur significantly. A similar phenomenon is observed in the case of -C=O (1748 cm −1 ) and -CH 3 (1451 cm −1 ) [31,32]. This means that only lignin reveals such stabilizing effect on surface PLA/P(3.4 HB) blend.

Colour Measurement
Some additives are able to change the colour of the polymer matrix, which can be sometimes considered as an advantage. Such natural stabilizers with colouring properties can be utilized as colorants for polymers. which was also proposed in literature [39]. On the other hand, in certain cases, the colour changing of polymer is not desirable, especially for packaging materials. where the transparency of the composite is infinitely preferable. Nevertheless, it is important to investigate the impact of selected stabilizers on material appearance. Colour measurement results presented in a bar

Colour Measurement
Some additives are able to change the colour of the polymer matrix, which can be sometimes considered as an advantage. Such natural stabilizers with colouring properties can be utilized as colorants for polymers. which was also proposed in literature [39]. On the other hand, in certain cases, the colour changing of polymer is not desirable, especially for packaging materials. where the transparency of the composite is infinitely preferable. Nevertheless, it is important to investigate the impact of selected stabilizers on material appearance. Colour measurement results presented in a bar chart (Figure 7) indicate that lignin changed the colour of the reference samples most significantly. The colour change index amounted to 34, which means that even an inexperienced observer is able to detect the difference between the test and reference samples. The PLA/P(3.4 HB) specimen containing lignin got the lowest whiteness index and was the darkest of all the materials. On the other hand, according to the data obtained, trans-chalcone also changed PLA/P(3.4 HB) blend, but in a different way. The addition of this antioxidant lightened the blend and lowered the chroma from about nine to three. This means that either lignin or trans-chalcone can be utilized as colorant. In contrast to these natural stabilizers, the third one, flavone, did not change the appearance of the blend. This compound seems to be the most appropriate additive with antioxidant and thermal stability properties dedicated for colourless packaging. contrast to these natural stabilizers, the third one, flavone, did not change the appearance of the blend. This compound seems to be the most appropriate additive with antioxidant and thermal stability properties dedicated for colourless packaging. According to the results revealed in Figure 8, it can be observed that PLA/P(3.4 HB) blend with flavone and trans-chalcone changed their colour most significantly during solar aging. Their chroma (Cab) results got significantly higher and the whiteness index (Wi) noticeably decreased. Depending on the point of view. This phenomenon can be treated as a drawaback or an asset. The colour changing of the tested sample during the aging process can be used as information about the degradation degree of the material and the flavone and trans-chalcone additives can be utilized as aging indicators, which is also noticeable in Figure 9. This idea has already been revealed in the case of β-carothene [6]. On the other hand, when the colour changing of a material during aging process is not needed. The choice of PLA/P(3.4 HB) blends with lignin would be the most appropriate because the colour of this composite is the most stable. According to the results revealed in Figure 8, it can be observed that PLA/P(3.4 HB) blend with flavone and trans-chalcone changed their colour most significantly during solar aging. Their chroma (C ab ) results got significantly higher and the whiteness index (W i ) noticeably decreased. Depending on the point of view. This phenomenon can be treated as a drawaback or an asset. The colour changing of the tested sample during the aging process can be used as information about the degradation degree of the material and the flavone and trans-chalcone additives can be utilized as aging indicators, which is also noticeable in Figure 9. This idea has already been revealed in the case of β-carothene [6]. On the other hand, when the colour changing of a material during aging process is not needed. The choice of PLA/P(3.4 HB) blends with lignin would be the most appropriate because the colour of this composite is the most stable.

Conclusions
Nowadays, it is of considerable importance to create non-toxic, biobased, and biodegradable materials with environmentally friendly additives. This research focused on blends based on the main pro-ecological polymers, namely polylactide and poly(hydroxy butyrate) (P(3.4 HB)), with the addition of selected natural antioxidants, flavone, trans-chalcone, and lignin. According to the results obtained, blending polylactide (PLA) with polyhydroxybutyrate (P(3.4 HB)) caused a reduction of polylactide material's mechanical and thermal properties. The summary of the revealed properties and application of the tested antioxidant are presented in Table 6. On the basis of the data obtained.

Conclusions
Nowadays, it is of considerable importance to create non-toxic, biobased, and biodegradable materials with environmentally friendly additives. This research focused on blends based on the main pro-ecological polymers, namely polylactide and poly(hydroxy butyrate) (P(3.4 HB)), with the addition of selected natural antioxidants, flavone, trans-chalcone, and lignin. According to the results obtained, blending polylactide (PLA) with polyhydroxybutyrate (P(3.4 HB)) caused a reduction of polylactide material's mechanical and thermal properties. The summary of the revealed properties and application of the tested antioxidant are presented in Table 6. On the basis of the data obtained.

Conclusions
Nowadays, it is of considerable importance to create non-toxic, biobased, and biodegradable materials with environmentally friendly additives. This research focused on blends based on the main pro-ecological polymers, namely polylactide and poly(hydroxy butyrate) (P(3.4 HB)), with the addition of selected natural antioxidants, flavone, trans-chalcone, and lignin. According to the results obtained, blending polylactide (PLA) with polyhydroxybutyrate (P(3.4 HB)) caused a reduction of polylactide material's mechanical and thermal properties. The summary of the revealed properties and application of the tested antioxidant are presented in Table 6. On the basis of the data obtained. selected antioxidants are able to improve the thermal properties of polymeric material by inhibiting its oxidation and sever decomposition at increased temperatures. This was confirmed by thermogravimetry and differential scanning calorimetry. Moreover, lignin as well as trans-chalcone are able to change the colour of blend significantly and may also be applied as colorants, in contrast to flavone which can be used to keep a material colorless. Therefore, selected natural antioxidants can be used as multifunctional and environmentally friendly additives for polymer blends of polylactide and polyhydroxybutyrate (P(3.4 HB)).

Conflicts of Interest:
The authors declare no conflict of interest.