Degradation and Stabilization of Poly(Butylene Adipate-Co- Terephthalate)/Polyhydroxyalkanoate Biodegradable Mulch Films Under Different Aging Tests

The degradation and stability of biodegradable lms determine the service length of mulch lms in actual use. Most biodegradable polymers degrade too fast to meet the required durability of mulch lms. The objective of this work is to investigate the degradation behaviors of poly(butylene adipate-co-terephthalate) (PBAT) /polyhydroxyalkanoate (PHA) blend mulch lms. Several different types of stabilizers were incorporated in the biodegradable blends to provide protection for the PHA/PBAT lms during thermal processing and aging on agricultural elds. The degradation process of the lms was systematically studied under an Accelerated Aging Test (AAT) and a Soil Aging Test (SAT). Adding a light stabilizer, UV stabilizer, or antioxidant to the mulch lms led to signicant improvement in the retention of mechanical properties of the lms under both AAT and SAT. Morphological evolution of the lms with or without a UV stabilizer as a function of aging times was studied by Scanning Electron Microscopy (SEM). The results of thermal properties and crystallinity revealed damage of crystal structure of the lms during AAT. Spectrocopic results indicated that the lms underwent both hydrolysis and photodegradative chain scissions (Norrish Type I/II reactions and photo-oxidation). The degradation mechanisms of the PHA/PBAT biodegradable mulch lms were proposed.


Introduction
Plastic mulch lms have contributed to improving crop yields by increasing soil temperature, reducing water loss, and limiting weed growth, etc. [1], use of mulch lms is especially important for arid regions around the world. However, the polymers used in commercial mulch lms are not biodegradable, polyole ns (PO) [2], ethylene-vinyl acetate (EVA), and polyvinyl chloride (PVC) [3] are the most common polymers used in manufacturing mulch lms. As these polymer lms remain intact after crop harvesting, and due to the high cost of lm collection, these mulch lms are typically plowed into soil as plastic fragments.
Over time, plastic lm residues in soil get accumulated to a level which starts to affect seed germination and crop growth, causing environmental problems for sustainable use of farmland [4]. As a result, there is a growing need to develop biodegradable mulch lms which can provide the mulch functions during the crop growth but get biodegraded within a crop planting cycle. This has led to signi cant interest from both industrial and academic scientists to investigate material options to create biodegradable mulch lms.
Polyhydroxyalkanoates (PHA) are emerging biodegradable materials after research and development of several decades [5][6][7], they are accumulated intracellularly in cells by various microorganisms via microbial fermentation [8][9][10], using renewable carbon resources as feedstocks such as sugars, vegetable oils, etc. [11][12][13]. One of the advantages of PHA is their ease to biodegradation in a variety of environments including compost, soil or aquatic media, etc. Another advantage of PHA is that there are many PHA copolymers available to tailor their properties for different applications.
However, several challenges still remain for using PHA for wide range of commercial applications. PHA have narrow processing window and they are prone to thermal degradation during lm extrusion [11].
Besides lm performance, a key challenge for biodegradable mulch lms is to improve their durability in  [37], however, the effect of stabilizers on improving UV stability was not addressed in their work.
In this work, there are two main objectives: (1) to understand the photodegradation process of PBAT/PHA mulch lms under an accelerated aging test, and to explore the feasibility of using this accelerated test to predict degradation in soil, (2) to determine which stabilizer is the most effective in extending the use life of PBAT/PHA lms in mulch lm applications. Speci cally, three different kinds of stabilizers: an antioxidant, a UV stabilizer, and a hydrolysis resistance additive were evaluated. The PBAT/PHA mulch lms were prepared from melt blends made by a twin screw extrusion process. The degradation process of PBAT/PHA lms was systematically studied by different characterization and test methods (DSC, XRD, ATR-FTIR, XPS, SEM, and tensile testing).

Preparation of PBAT/PHA lms
PBAT/PHA lms were prepared by adding the PBAT/PHA blends (in pellet form) to the feeding zone of a Collin single-screw extruder (screw diameter: 19 mm, L/D: 25/1) tted with a blown lm die having a diegap of 0.8 mm. The temperatures of the extruder were set to 50 o C, 175 o C, 175 o C, 180 o C, and 170 o C from its feed zone to die, screw speed was 30 rpm, lm was drawn at speed of 6 m/s. The thickness of lms ranged from 13 to 20 μm, detailed process conditions are listed in Table S2. The average lm thickness was calculated from data measured at 5 different locations on each lm sample.

Degradation test
In the accelerated aging test (AAT), PBAT control and different PBAT/PHA lms were exposed to UV light in a Xenon Test Chamber (Model: Q-SUN Xe-3, Q-Lab Corporation, Cleveland, Ohio, USA). Its irradiance was 0.51 W/m 2 at a wavelength of 340 nm. The temperature inside of the chamber was 38 o C, and the humidity was kept at 50%. In a typical test, four samples were placed on xed aluminum holders in the chamber. The inside surface of the chamber was made from highly re ective aluminum. To simulate mulching conditions, the lms were exposed to repeating cycles of UV light for 108 minutes, and then followed by spraying water for 18 minutes to mimic the effects of hydrolysis resulting from rain fall. At each additional 24 hours, a piece of lm was cut from each lm sample for characterizations of samples after AAT at the time period.

Soil degradation test
The soil degradation tests of mulch lms were performed by placing mulch lms on soil (leached noncalcareous soils, Shanghai) in ower pots from October to November of 2019, placed in an unshaded area, in Pudong New District, Shanghai, China. The mulch lms tested were 50 × 50 cm 2 squares. A piece of lm was cut from the lm samples after every 7 days for property tests. In order to keep the soil moist, an adequate amount of water was added to each pot every 3 days. Typically, at least 3 different lm samples were tested for each lm to achieve a good reproducibility.

Characterization
Differential scanning calorimetry (DSC) was conducted on a DISCOVERY DSC FC100 with a refrigerated cooling system Model 90, supplied by Thermal Analysis, New Castle, USA. A sample of about 7 mg was rst kept at 40°C and then heated to 220°C at a heating rate of 10°C/min under a ow of nitrogen, it was then cooled down to -50 o C at a rate of 10°C/min during the rst cooling cycle, followed by heating to 220 o C at a heating rate of 10°C/min. Attenuated total re ection FTIR (ATR-FTIR) spectra were recorded on a Frontier FTIR spectrometer with a universal ATR sampling accessory, supplied by PerkinElmer, Waltham, MA, USA.
X-ray photoelectron spectroscopy (XPS) spectra were collected using a Kratos Axis Ultra DLD with a monochomatic aluminum Kα light source at room temperature.
Wide angle X-Ray diffraction (WAXRD) analysis was performed using Scintag XDS-2000 with Ni-ltered Cu Ka radiation (1.5418 Å) at room temperature in the range of 2θ = 1.5-40 o with a scanning rate of 2 o /min. Scanning electron microscopy (SEM) was conducted on a ZEISS Merlin electron microscope with an acceleration voltage of 2 kV and the working distance of 5 mm.
Tensile tests were performed on an Instron 3344 testing system, supplied by Instron-USA, Norwood, MA, USA, according to ISO 527-3-2018. Tensile tests of the lms were performed at a grip distance of 50 mm, a testing rate of 100 mm/min, and a specimen width of 15 mm. All the lm samples were stored in a constant humidity chamber at a temperature 23 o C and a relative humidity of 60% for 24 hours before tests.

Retention of mechanical properties under accelerated aging tests (AAT)
The PBAT/PHA lms were made from polymer blends prepared by a twin screw extrusion process, the lms contain 80 wt.% PBAT and 20 wt.% PHA with or without stabilizing additives. The mechanical properties of these lms before Accelerate Aging Test (AAT) are listed in Table 1. The data showed that adding a UV stabilizer (UVA) or a hydrolysis resistance additive (HA) to PBAT/PHA lms slightly decreased both tensile strength (σ) and elongation at break (ε b ) of the biodegradable lms, while adding an antioxidant (AO) led to a signi cant increase in tensile strength of the lm (from 18 MPa to 35 MPa), which could be attributed to decreased thermal degradation of the lm in the presence of the antioxidant during the melt extrusion blending process and subsequent lm extrusion.
The retention ratios of mechanical properties of PBAT, PBAT/PHA, and PBAT/PHA with different additives under AAT are summarized in Table 1. The effects of adding PHA to PBAT on lm properties are shown as Figs. 1a and 1b, the presence of only 20% PHA caused a sharp decrease in both the elongation at break and the tensile strength of PBAT/PHA lm, as compared to pure PBAT lm. A 77% reduction in elongation and 39% reduction in tensile strength were found respectively at 72 h. However, the difference became smaller at 100 h. The results suggest that PHA is more easily degradable than PBAT under the repeated UV exposure and water spray cycles under AAT conditions, this is in agreement with a previous study which found PHA degraded faster than other polymers in accelerated soil degradation [38]. This difference could be due to the fully aliphatic backbone of PHA versus aliphatic and aromatic copolymer backbone of PBAT as shown in Figure 2. Nevertheless, this also emphasizes the importance to stabilize PHA-containing PBAT blends in order to meet durability requirements for mulch lm applications.
The retention behaviors of PBAT/PHA lm as a control and thee PBAT/PHA lms with stabilizing additives during AAT are shown as Figs. 1c and 1d. Adding an antioxidant (AO) and a hydrolysis resistance additive (HA) to the lms apparently did not contributed to lms' stability at a short test time (e.g. 24 h.), however, these additives did provide stabilizing effects for elongation retention at extended exposure times (>48 h.), which showed 16%-24% improvement over the lm without these additives. This could be attributed to the fact that an antioxidant only reacts with peroxyl radical generated during oxidation [39][40][41], and hydrolysis resistance additive only reacts with carboxyl group generated during hydrolysis of the biodegradable polymers [42]. In the early stage of UV-induced photodegradation process, it is reasonable to assume that neither peroxyl radical nor carboxyl group are generated in su cient amount for them to react with an antioxidant or a hydrolysis resistant additive. As a result, both AO and HA are only effective in improving the lms' stability after a relatively longer aging period, when more peroxyl radical and carboxyl group are produced in the lms.
Among all the additives tested, the most effective stabilizer is the UV stabilizer, i.e. UVA. The ε b decrease of the PBAT/PHA-UVA lm was signi cantly less than the other thee lms, and it retained the highest ε b (46%) at 100 h. The retention of tensile strength of the PBAT/PHA-UVA lm was also the best among all the lms as well. The UV stabilizer in PBAT/PHA lms can absorb UV radiation energy and undergo structural transformation [43] to provide a stabilizing effect. The above results demonstrated the importance of incorporating a UV stabilizer to provide protection against UV irradiation for the lms.

Morphology change under AAT
The surface morphology images of PBAT/PHA lms at different aging times were studied by SEM. In Fig.  3a, the surface of PBAT/PHA lm before aging was smooth with clear orientation marks along the machine direction formed during blown lm process. The orientation helps to improve mechanical properties of lms [44,45]. After 24 hours of aging, as shown in Fig. 3b, the surface began to show some roughness, some small protrusions appeared on surface, which may be formed after partial surface degradation of the lm and degraded material was washed away by sprayed water. After 72 hours of accelerated aging (Fig. 3c), several small cavities can be clearly observed on the surface. Surprisingly, a large hole with a diameter of approximately 10 μm (in Fig. 3d), was formed at 100 h.
As a result, it seemed that due to UV irradiation, the surface of PBAT/PHA lm underwent partial erosion of the outer layer at rst, followed by formation of small cavities, and nally large cavities were formed.
Also, it is noticed that surface orientation of PBAT/PHA became less distinct after aging, which may be due to the gradual destruction and wash away of the surface layer after extended UV exposure.
For comparison, the SEM images of PBAT/PHA-UVA at different aging times are shown in Fig. 4. After 24 hours of aging, there were no signi cant defects or holes observed on the surface (Fig. 4b). After 72 hours, the surface started to appear coarse (Fig. 4c). Moreover, surface orientation became less distinct for PBAT/PHA-UVA after 100 h aging, but still no formation of cavities or large holes were observed.
As a result, it can be concluded that adding UVA can effectively slow down the photodegradation process for PBAT/PHA lms. As a result, the PBAT/PHA-UVA had a much higher retention of ε b after 100 hours of accelerated aging as compared to all other additives.

Thermal properties and crystalline structures
The thermal properties of PBAT/PHA lms before and after AAT were studied by DSC. In Figs. 5A and 5B, as aging time increases from 0 to 50 h and 100 h, both the crystallization temperature (T c ) and melting temperature (T m ) of PBAT component in PBAT/PHA lms shifted to lower temperatures, indicating a decrease of crystal size [34], the peaks also lost sharpness and became broader than the peak at 0 h. This could be resulted from chain scission taking place during the photodegradation process in AAT, PBAT tends to partially transform from ordered crystalline structure to amorphous form.
Moreover, after 100 hours of aging, the most signi cant broadening of T c peak was observed, and this also provides support for chain scission could have happened during degradation. This is in agreement with a study by Souza et. al. [35], they reported that chain scission in the photodegradation process was predominant in PBAT while studying accelerated aging of PBAT lms with added carbon black and light stabilizers.
For the PBAT/PHA lm with 0.5% UV stabilizer (Fig. 5c), the T c peak location and shape only had slight changes. This shows the UV stabilizer provided protection for PBAT/PHA against chain scission under UV radiation.
The quantitative changes of crystallization peak temperatures (T c ) of PBAT, PBAT/PHA, and PBAT/PHA lms with different stabilizers as a function of aging time are summarized in Fig. 6. Fig. 6a shows that adding 20% PHA to PBAT caused pronounced decreases of T c as compared to pure PBAT, i.e. making PBAT more prone to photodegradation. This could be resulted from the PHA degradation products having carboxyl groups at chain ends, the acidic carboxyl groups would have a catalytic or accelerating on the hydrolytic degradation of PBAT.  Fig. S1) [47]. These were attributed to the Phase II crystals as reported by Chen [48]. For aging from 0 to 48 hours, the diffraction peaks of PBAT/PHA lms remained relatively stable. However, after 100 h of aging, the intensities all thee speci c diffraction peaks were drastically reduced. This indicates that under accelerated aging test, a large fraction of crystalline region of PBAT in PBAT/PHA lms was transformed into amorphous region.
In comparison, the intensities of diffraction peaks of PBAT/PHA lm with the UVA additive remained almost unchanged after 24, 48 and 100 h of aging (Fig. 7b), however, the peak location of the peaks were shifted slightly to the right by 0.19 o after 100 h of aging.
The crystallinity study provides a structural interpretation for the trend of mechanical properties previously discussed. The results strongly support that the deterioration of the mechanical properties of PBAT/PHA lms after accelerated aging is closely related to the change of crystallinity of PBAT in the lms. Nevertheless, it should be noted that the change of crystalline structure during degradation is highly dependent on the backbone structure of polymer involved. For example, crystallinity of PLA lm actually increased after degradation [42,49].

FT-IR and XPS spectroscopy
ATR-FTIR helps to provide more structural information for studying the photodegradation mechanism of PBAT/PHA lms. As shown in Fig. 8a, the intensities of multiple bands of ester groups decreased after degradation. In particular, C=O stretch band is found at 1710 cm -1 [26], and C-O stretch bands are located at 1269 cm -1 and 1019 cm -1 [36,40]. The observed weakening of these thee bands indicates breaking of ester groups during photodegradation. Meanwhile, as shown in Fig. 8d, the intensity of bands at 1685 cm -1 and 3435 cm -1 slightly increased, which belong to stretch absorption of carboxyl and hydroxyl groups of PHA respectively [39]. This provides evidence that hydrolysis happened during the photodegradation. The band at 667 cm -1 assigned to stretch vibration of C=C-H [46] increased after degradation (Fig. 8b).
Moreover, the intensity of bands from 1578 to 1654 cm -1 due to C=C stretching [50] also increased. The increase of these bands suggests formation of C=C structures, which is typical of Norrish II photodegradation mechanism [51][52][53].
As a comparison, the FTIR spectra of PBAT/PHA with UV stabilizer before and after aging are shown as Fig. 8e. The respective bands did exhibit signi cant changes after AAT, indicating that the presence of the UV stabilizer in PBAT/PHA lm has signi cantly improved the stability of the lm during photodegradation process.
The C 1s XPS spectra of different PBAT/PHA lms before and after AAT are shown in Fig. 9 to provide an in-depth understanding of the carbon binding states on the lms' surface. Detailed information on the deconvoluted peaks of C 1s is summarized in Table S3. For the PBAT/PHA lm without additive (Fig. 9a and 9b), the sp 2 carbon calculated from tted curve increased from 26.7% before AAT to 36.3% after AAT, while the sp 3 carbon decreased from 53.1% (before AAT) to 41.1% (after AAT). This suggests that about 10% sp 3 carbon transformed to sp 2 carbon during the photodegradation process, providing evidence supporting the Norrish II [50] photodegradation mechanism for PBAT/PHA lm without additive.
The amount of C=O increased from 6.5% before AAT to 10% after AAT. This could be resulted from a photo-oxidation process which involves a classical hydrogen abstraction typically at a tertiary carbon in α-position of an ester group on a polymeric backbone, leading to formation of macroradicals [49,54,55] (see scheme 1).
For the PBAT/PHA lm with the UVA additive (Figs. 9C and 9D), amazingly, the sp 3 carbon only decreased slightly from 47.4% before AAT to 46.8% after AAT, while C=O increased moderately from 9.4% to 12.4% before and after AAT, respectively. This indicates that adding UVA has effectively inhibited photodegradation and provided protection to PBAT and PHA during AAT.

Degradation mechanism of PBAT/PHA under AAT
There were basically two degradation mechanisms in the literature proposed for biodegradable polyesters under a UV accelerated test: 1) photo-oxidative reactions, and 2) Norrish I/II type photodegradative reactions. The photo-oxidation mechanism was widely used to explain PLA (polylactic acid) degradation under UV exposure [54][55][56]. J. Rychlý [55] discussed that the tertiary carbon in PLA is the predominant site for oxidation, which is the most unstable carbon as compared to primary and secondary carbons when reacting with a hydroperoxy radical.
Since the PHA used in this work is a poly(3-hydroxybutyrate-4-hydroxybutyrate) (P3HB4HB) and it is composed of 90% of 3HB and 10% of 4HB units (Fig. 1), the tertiary carbon in 3HB is most likely to undergo photo-oxidative reactions as illustrated in Scheme 1. This may also provide a mechanistic basis for the observed much worse deterioration of mechanical properties of PBAT/PHA lm than PBAT lm under UV exposure during AAT, the reactive tertiary carbon in 4HB of PHA led to extensive photo-oxidation of PBAT/PHA and this causes the mechanical properties to signi cantly decrease.
As shown in Scheme 1, the hydrogen at the tertiary carbon is abstracted by an alkyl radical, which further reacts with oxygen to form a peroxy radical, followed by coupling with hydrogen abstraction forming a hydroperoxide and then cleaving the O-C bond, and this breaks a PHA chain into two separate chains, one chain has a carboxylic acid chain end and the other chain has a methyl-keto chain end. This process generates one more carbonyl group which accounts for the increased C=O level as observed on the XPS of PBAT/PHA lm after subjecting to AAT test.
Beside the photo-oxidation for 3HB of PHA, the Norrish I and II type degradative reactions can also occur on polymer chains under an accelerated UV test [35,49,57]. As illustrated in Scheme 2, for the butylene terephthalate structure of PBAT, the Norrish type II reaction can produce C=C bond as a terminal vinyl group after a chain is cleaved, this accounts for the increased Sp 2 carbon level after degradation from the XPS results. The Norris I type reactions occur around the aromatic carboxyl ester group in three possible chain scission reactions, forming different chain end structures and even CO 2 [52].

Soil degradation test
In the AAT test results discussed above, adding a UV stabilizer to PBAT/PHA lm was shown to effectively reduce degradation under UV accelerated aging test conditions. A soil degradation test was carried out to test the effectiveness of different stabilizers for up to 30 days. The elongation at break (ε b ) retention results and tensile strength at break (σ b ) are presented in Fig. 10.
It clearly showed that the UV stabilizer is effective to protect the PBAT/PHA lms by achieving the best retention of ε b and σ b , this nding is consistent with the best performance of UVA during AAT (Figure 2).
Moreover, the retention of σ b of PBAT/PHA-UVA lm after 100 h of AAT is almost the same as that after 30 days of degradation in soil. This indicates the potential to use AAT to predict soil degradation results under certain soil test conditions, and our test was conducted in Pudong New district of Shanghai, China, from October to November.
However, the hydrolysis resistance additive (HA) did not exhibit consistent retention of both properties, and the lm with AO additive had poorer retention than the PBAT/PHA control lm without additive in soil test. This may result from biodegradation during the degradation test in soil, the cleavage of ester bonds by extracellular enzymes of microorganisms may have actually accelerated abiotic hydrolysis [42] which lead to more extensive degradation.

Conclusions
Poly(butylene adipate-co-terephthalate) (PBAT)/polyhydroxyalkanoate (PHA) biodegradable mulch lms were prepared from a series of PBAT/PHA 20/80 wt./wt. blends with or without stabilizers via a twin screw extrusion process. The degradation behaviors of the lms were systematically studied under an Accelerated Aging Test (AAT). It was found that adding a low level (20 wt.%) of PHA to PBAT has substantially decreased the retention of mechanical properties of PBAT.
By studying the effects of a UVA stabilizer, an antioxidant, and a hydrolysis resistance additive, it was found that the UV stabilizer under investigation was the most effective in protecting the PBAT/PHA lms.
Adding 0.3% UV stabilizer achieved more than 12 times improvement in mechanical properties. The surface morphology of the PBAT/PHA lm without additives had an evolution of staged degradation eventually forming large defects on the lm surface, while adding a UV stabilizer effectively inhibited or delayed such damages caused by photodegradation.
Thermal properties, XRD, FTIR, and XPS all provided evidences consistently supporting the trends of mechanical properties. A photodegradation mechanism was proposed for the PBAT/PHA lms under accelerated aging conditions, suggesting several types of chain session pathways involving photooxidation and Norrish Type I/II reactions.
Soil degradation test showed that the UV stabilizer was the most effective stabilizer which is consistent with its performance in AAT, this has indicated potential to use AAT to predict the degradation behaviors in soil under certain conditions. Overall, this work has provided systematic basis for understanding the photo-degradation behaviors of PBAT/PHA biodegradable mulch lms.    The peak position of Tc of neat PHA-PBAT lms (a) and PHA-PBAT with different stabilizers (b) after accelerated aging.