E ﬀ ects of Tensile Stress and Soil Burial on Mechanical and Chemical Degradation Potential of Agricultural Plastic Films

: Plastic ﬁlm mulching is widely practiced in arid and semiarid farming systems, but the accumulation of plastic residues in soils can negatively a ﬀ ect soil properties. Therefore, e ﬃ cient means of plastic ﬁlm degradation are urgently needed to mitigate its unfriendly environmental impacts for sustainable land use. Here, we characterized the e ﬀ ects of tensile stress (TS) and soil burial (SB) on potential degradation properties of three ﬁlm types: Polyethylene ﬁlm (PEF), oxo-biodegradable ﬁlm (OBDF), and biodegradable ﬁlm (BDF). Weight loss, mechanical properties, hydrophilicity, functional groups, and crystallinity were recorded after TS and SB treatments. The results indicated that: (1) Weight loss of plastic ﬁlms was associated with SB, although the extent of weight loss depended on ﬁlm type and was highest in BDF, (2) application of TS before SB weakened the mechanical properties of the ﬁlms and increased their hydrophilicity, creating favorable conditions for the settlement of microorganisms on the ﬁlm surface, (3) PEF treated with TS and SB had higher functional group indices and lower crystallinity. Our results highlighted that the combination of TS and SB has the potential to accelerate plastic ﬁlm degradation.


Introduction
Plastic film mulching (PFM) is an important agricultural practice in arid and semiarid farming systems [1]. It increases food production and security by providing multiple benefits, including weed control [2][3][4], increased soil temperature [5], and reduced soil water evaporation [6]. However, long-term use of plastic film is not conducive to the sustainable development of land.
The most commonly used plastic film is primarily composed of polyethylene (PE), which is highly chemically inert [7] and difficult to degrade in soil. Attempts to effectively remove or degrade plastic residues in soil have been hampered by practical difficulties and high costs [8]. Accumulation of plastic residue in soil may have negative effects on soil quality and crop production. Plastics can act as vectors for the transport of other soil contaminants and plastic film residues may release phthalate acid esters, which are potentially mutagenic and carcinogenic. Consequently, plastic residue is now considered to be a serious environmental concern. In recent decades, biodegradable mulch films have been proposed  3 9.61 black PBAT Qingdao Hongda BDF 4 12.98 transparent PBAT Lanzhou Jintudi PEF, OBDF, and BDF are polyethylene film, oxo-biodegradable film, and biodegradable film, respectively. FM (functional masterbatch). PBAT (poly (butyleneadipate-co-terephthalate)). a measured with a scanning electron microscope (SEM) (S-3400N, Hitachi, Japan), b the material produced and named by the manufacturer.
TS was applied to plastic films using a computer tensile tester (HH-LL 300, Huahan, Hangzhou, China). The stress was imposed by pre-tensioning the plastic films up to 50% of the measured ε tb (nominal tensile strain at breaking) value. The 50% ε tb value (50% reduction in ε tb ) is considered to show film degradation [27]. To reach this intensity, plastic film samples were exposed to 100 mm/min TS speeds. Samples exposed to no tensile stress (nTS) were used as controls.
The plastic films were manually cut into a rectangular shape (30 × 20 cm 2 ) and weighed, placed into tagged nylon-mesh (aperture 2 mm) bags to allow the access of microorganisms and moisture and the easy retrieval of the degraded samples, and the nylon bags with plastic film samples were completely covered with a layer of soil (about 10 cm) in 10 replicates for each plastic film. At the end of each month (3,6,12,15,18,24,27, and 30 months), one nylon bags with plastic film was retrieved, and the corresponding film samples were washed with distilled water and soaked in 2% sodium dodecyl sulfate (SDS) for 4 h on a rotary shaker at 120 r/min. Finally, they were cleaned with deionized water and dried at 40 • C for 12 h.

Film Weight Loss
Film weight loss [28] was calculated using the following formula: Weight loss = [(Initial weight − Final weight)/Initial weight] × 100%.

Film Mechanical Properties
The mechanical deterioration of plastic film is typically assessed by measuring ε tb and tensile strength [29]. The more stable the film, the higher the ε tb and tensile strength. Measurements of film mechanical properties were performed using a computer tensile tester following China National Standard GB/T 1040-2006 (plastics-determination of tensile properties). ε tb and tensile strength were measured with a crosshead speed of 100 mm/min. Each of the plastic films was measured five times, and the average value was used for statistical analysis.

Film Hydrophobicity
Hydrophobicity is an important surface property for polymer degradation because it determines the extent of microbial colonization of polymer substrates. Film hydrophobicity is inversely related to the water contact angle (WCA) [30]: The more hydrophilic the surface, the smaller the WCA. Changes in film hydrophobicity were determined by measuring WCA [31] using a contact angle measurement device (JC2000P, Zhongchen, Shanghai, China) connected to a computer equipped with image recording software, and contact angle measurement tools. The WCA was measured at five different regions of each film sample.

Statistical Analysis
Statistical analyses were performed using GenStat 18th edition (VSN International Ltd., Rothamsted, UK), and graphs were created in Origin 9.2 (OriginLab OriginPro 2015, Northampton, MA, USA). Repeated measures of ANOVAs test were adopted, using burial time (BT) as the repeated factor and TS and plastic film (PF) as two fixed factors, was conducted to assess treatment effects on ε tb , WCA, weight loss and tensile strength. One-way analysis of variance was used to evaluate the differences in KCBI, ECBI, VBI, IDBI, and crystallinity among PF followed by Fisher's protected least significant difference test at the level p < 0.05. All reported measurements are the means and standard errors of three independent replicates.

Changes in Mechanical Properties
Change in ε tb and tensile strength for the three types of plastic films over 30 months of SB are shown in Figure 1 and Figure S1. Regardless of TS treatment, the ε tb of all film types decreased with time (p < 0.001 at two sites) ( Table S1). The ε tb of PEF and OBDF decreased relatively slowly throughout the experiment, whereas that of BDF dropped rapidly to undetectable levels by the end of 15 months for BDF 1 and BDF 2 and by the end of 24 months for BDF 3 and BDF 4 .
Application of TS lowered the initial ε tb of the films by 9-66%. Initial ε tb differed markedly between TS-treated and untreated films, and at the end of the experiment, final ε tb of the TS and SB-treated films had declined to a greater extent than that of the nTS and SB films (p < 0.001 at two sites) (Table S1). Specifically, ε tb of TS and SB-treated PEF and OBDF was reduced by 57-81% relative to the initial values, whereas that of nTS and SB films was reduced by only 15-52%. The 50% ε tb value (50% reduction in ε tb ) is a standard measure of mechanical degradation [29]. The ε tb of TS and SB-treated PEF and OBDF was near or below 50% ε tb , whereas that of nTS and SB films (except OBDF 1 ) was well above this level ( Figure 1). In BDF, ε tb decreased rapidly to levels far below 50% ε tb , and its final values were not substantially altered by TS. (except OBDF1) was well above this level ( Figure 1). In BDF, εtb decreased rapidly to levels far below 50% εtb, and its final values were not substantially altered by TS.
The tensile strength of all plastic films decreased over time (p < 0.001 at two sites) ( Figure S1, Table S1), similar to εtb. However, unlike εtb, application of TS greatly increased the tensile strength of PEF and OBDF (except OBDF2). The tensile strength of BDF (except BDF4) was also enhanced by TS treatment, but to a much lesser extent than in the other two film types. Nominal tensile strain at break (εtb %) of plastic films exposed to tensile stress or no tensile stress, measured at intervals up to 30 months after soil burial. BDF dropped rapidly to an undetectable level by the end of 15 (for BDF1 and BDF2) or 24 (for BDF3 and BDF4) months. The dotted line indicates 50% εtb, the mechanical degradation threshold of no tensile stress and no soil burial film. PEF (polyethylene film). OBDF (oxo-biodegradable film). BDF (biodegradable film). nTS + SB (no tensile stress and soil burial). TS + SB (tensile stress and soil burial).

Decreases in Hydrophobicity
The three film types differed somewhat in initial WCA, and WCA of all films decreased with BT (p < 0.001 at two sites) ( Figure 2, Table S1). The WCA of BDF decreased more rapidly than that of PEF and OBDF, TS and SB-treated plastic film samples had much lower WCA than nTS and SB samples, especially between 3 and 15 months. After that, the BDF samples had completely fragmented and could not be subjected to further WCA testing. For PEF and OBDF, the differences in WCA caused by the TS treatment were no longer apparent at the end of 27 months. Nominal tensile strain at break (ε tb %) of plastic films exposed to tensile stress or no tensile stress, measured at intervals up to 30 months after soil burial. BDF dropped rapidly to an undetectable level by the end of 15 (for BDF 1 and BDF 2 ) or 24 (for BDF 3 and BDF 4 ) months. The dotted line indicates 50% ε tb , the mechanical degradation threshold of no tensile stress and no soil burial film. PEF (polyethylene film). OBDF (oxo-biodegradable film). BDF (biodegradable film). nTS + SB (no tensile stress and soil burial). TS + SB (tensile stress and soil burial).
The tensile strength of all plastic films decreased over time (p < 0.001 at two sites) ( Figure S1, Table S1), similar to ε tb . However, unlike ε tb , application of TS greatly increased the tensile strength of PEF and OBDF (except OBDF 2 ). The tensile strength of BDF (except BDF 4 ) was also enhanced by TS treatment, but to a much lesser extent than in the other two film types.

Decreases in Hydrophobicity
The three film types differed somewhat in initial WCA, and WCA of all films decreased with BT (p < 0.001 at two sites) (Figure 2, Table S1). The WCA of BDF decreased more rapidly than that of PEF and OBDF, TS and SB-treated plastic film samples had much lower WCA than nTS and SB samples, especially between 3 and 15 months. After that, the BDF samples had completely fragmented and could not be subjected to further WCA testing. For PEF and OBDF, the differences in WCA caused by the TS treatment were no longer apparent at the end of 27 months.

The Formation of Functional Groups
FTIR spectra after 27 months of SB revealed that the transformations of all PEF functional groups (KCBI, ECBI, VBI, and IDBI) were broadly similar between the two SB sites, Yuzhong and Dingxi (Table 2). Functional group indices were not substantially affected by the combined TS and SB treatment in PEF 2 , but they were significantly increased in PEF 1 . PEF 1 is much thinner than PEF 2 (Table 1).  Figure 2. Film water contact angle (°) of plastic films exposed to tensile stress or no tensile stress, measured at intervals up to 27 months after soil burial. BDF dropped rapidly to an undetectable level at 27 months. PEF (polyethylene film). OBDF (oxo-biodegradable film). BDF (biodegradable film). nTS + SB (no tensile stress and soil burial). TS + SB (tensile stress and soil burial).

The Formation of Functional Groups
FTIR spectra after 27 months of SB revealed that the transformations of all PEF functional groups (KCBI, ECBI, VBI, and IDBI) were broadly similar between the two SB sites, Yuzhong and Dingxi (Table 2). Functional group indices were not substantially affected by the combined TS and SB treatment in PEF2, but they were significantly increased in PEF1. PEF1 is much thinner than PEF2 (Table 1).

Decreases in Crystallinity
As shown in Figure 3, the combination of TS and SB significantly decreased the crystallinity of PEF (p ≤ 0.001 at two sites). In Yuzhong, compared with controls (nTS and nSB), the crystallinity values of nTS and SB-treated PEF 1 and PEF 2 was reduced by 8.9% and 5.7%, respectively, whereas that of PEF 1 and PEF 2 treated with both TS and SB were significantly reduced by 16.1% and 18.1%, respectively ( Figure 3A). In Dingxi, the combination of TS and SB significantly reduced the crystallinity values of PEF 1 and PEF 2 by 22.7% and 11.2%, respectively, but the crystallinity of PEF treated with nTS and SB did not differ significantly from that of the controls (nTS and nSB) ( Figure 3B). values of nTS and SB-treated PEF1 and PEF2 was reduced by 8.9% and 5.7%, respectively, whereas that of PEF1 and PEF2 treated with both TS and SB were significantly reduced by 16.1% and 18.1%, respectively ( Figure 3A). In Dingxi, the combination of TS and SB significantly reduced the crystallinity values of PEF1 and PEF2 by 22.7% and 11.2%, respectively, but the crystallinity of PEF treated with nTS and SB did not differ significantly from that of the controls (nTS and nSB) ( Figure  3B).

Weight Loss of Plastic Film
The weight loss of the three types of plastic film varied greatly after TS and SB in different months ( Figure 4, Table S1). There were no significant differences in the weight loss of PEF and OBDF. However, for the four BDFs, weight loss increased with months of soil burial.

Weight Loss of Plastic Film
The weight loss of the three types of plastic film varied greatly after TS and SB in different months ( Figure 4, Table S1). There were no significant differences in the weight loss of PEF and OBDF. However, for the four BDFs, weight loss increased with months of soil burial.  . Weight loss (%) of plastic films exposed to tensile stress or no tensile stress, measured at intervals up to 30 months after soil burial. PEF (polyethylene film). OBDF (oxo-biodegradable film). BDF (biodegradable film). nTS + SB (no tensile stress and soil burial). TS + SB (tensile stress and soil burial).

Discussion
PE is the most common type of plastic film used in agriculture due to its low cost, outstanding mechanical properties, and excellent chemical resistance properties. The degradation of plastic . Weight loss (%) of plastic films exposed to tensile stress or no tensile stress, measured at intervals up to 30 months after soil burial. PEF (polyethylene film). OBDF (oxo-biodegradable film). BDF (biodegradable film). nTS + SB (no tensile stress and soil burial). TS + SB (tensile stress and soil burial).

Discussion
PE is the most common type of plastic film used in agriculture due to its low cost, outstanding mechanical properties, and excellent chemical resistance properties. The degradation of plastic mulch film is mainly governed by external factors, such as ultraviolet radiation (photo-oxidation), thermal and mechanical stress during outdoor application. During photo-or thermo-oxidation, chain scission, cross-linking, and branching can occur simultaneously, which is accompanied by the formation of various degradation products (such as carbonyl groups). However, due to the practical stability of PE below 100 • C, thermal degradation is not important for application on PE degradation in the agricultural field, meanwhile, photo-oxidation can only take place on the surface layers of the film, while the ubiquitous presence of mechanical stress can cause fracture and chemical changes in the film [17]. The degradation of plastic mulch films can be initiated when exposed to TS, which further leads to the increased susceptibility of polymers to biodegradation. Therefore, we determined the influence of TS initially applied to the tested films on their further behavior during their incubation in soil. In this study, the degradation of three types of plastic mulch films was analyzed by assessing their weight loss, mechanical properties, hydrophilicity, functional groups, and crystallinity.
The degradability of plastic films depends mainly on the nature of the chemical bonds and the properties of additives in their polymers. The structural characteristics of the strong C-C and C-H bonds in PEF backbones make it particularly resistant to degradation [13]. It has been reported that PEF buried in soil exhibits a weight loss of approximately 0.2% in 10 years [35], with the appearance of whitening points on the surface after 32 years [26].
This study showed that the mechanical stability of PEF and OBDF was higher than that of BDF during biodegradation, as reflected in their smaller weight losses ( Figure 4) and stronger mechanical properties (Figure 1 and Figure S1). Biodegradation of BDF is likely by the cleavage of chemical bonds susceptible to enzymes (esters), promoting easier breakdown and more rapid degradation of its mechanical properties. However, SB after TS led to greater decreases in ε tb of PEF and OBDF, suggesting that the addition of TS could accelerate the degradation process. It has also been reported that TS shortens the time required for mechanical degradation of films under UV radiation [36].
Hydrophobicity is an important property of the plastic film surface that determines the extent of microbial colonization of the polymer substrates. If a polymer has a hydrophilic surface, microorganisms can attach to it. In this study, the hydrophilicity of TS and SB treated BDF samples increased more than those of PEF and OBDF samples. However, the increase of hydrophilicity of the BDF samples is rather related to the microorganism actions (cleavage of chemical bonds susceptible to enzymes) than the TS pre-treatment. Research has shown that the hydrophobicity/hydrophilicity of the plastic film surface depends on the nature of its functional groups [30]. In this study, SB after TS decreased plastic film hydrophobicity ( Figure 2, Table S1). This is partially supported by the results of FTIR analysis because TS-treated PEF 1 showed significantly greater formation of relevant functional groups (Table 2).
FTIR spectra show the effects of microbial activity on the functional groups of the film surface: The appearance and disappearance of IR spectral bands provide evidence of biodegradation of buried films. There were no substantial changes in the functional groups of nTS and SB treated PEF or in TS and SB treated PEF 2 (Table 2). It has been reported that masterbatch can make films resistant to oxidation under certain conditions [37]. In this study, it was clear that the increased presence of functional groups in PEF 1 was caused by the combination of TS and SB (Table 2), indicating that together these two factors promoted the oxidation of PE structures and promoted PEF biodegradation. PEF 1 is much thinner than PEF 2 (Table 1), and thin films have been reported to degrade at a faster rate due to their higher relative surface area [38]. Furthermore, it is likely that the application of TS and SB to PEF 1 caused more cracks to appear on the plastic film, it was easier for oxygen to penetrate inside via the cracks, plastic film may undergo more oxidation reaction to produce more molecular chain breaks and more free radicals, particularly peroxy radicals, which cause the formation of low molecular byproducts in the presence of oxygen [19,39]. In addition, the combination of TS and SB lowered PE film hydrophilicity, presumably aiding microorganism attachment to the film surface, once microorganisms have settled on the film surface, they begin to grow, using the polymer as a source of carbon, breaking down polymer chains, and converting them to low molecular weight fragments, thereby promoting effective biodegradation [23,37].
Polyethylene is a semi-crystalline polymer comprised of crystalline microstructures surrounded by amorphous regions. It has been demonstrated experimentally that the amorphous regions of plastic films are degraded before the crystalline microstructures [30]. The SB after TS treatment was also found to decrease the crystallinity of PEF (Figure 3). According to Bonhomme [40], the presence of radicals in the absence of oxygen leads to the formation of PE cross-links, which reduce crystallinity [41]. As previously reported [29], the reduction in crystallinity occurs because of cross-linking that takes place in the amorphous phase, with a concomitant decrease in film ε tb . The reduction in PEF crystallinity measured here is consistent with the results of film ε tb (Figures 1 and 3). To date, most studies have shown increases in plastic film crystallinity due to the fact that the amorphous regions are more accessible to microorganisms than the crystalline regions. This study shows a decrease in plastic film crystallinity, indicating that degradation had reached a very advanced stage in which both the amorphous and crystalline phase had degraded, as reported previously [42]. Hence, the application of TS to plastic film before SB is of crucial importance in promoting its degradation.
From the results above, we drew two key conclusions on the degradation process of the three types of films: First, the type of plastic mulch film had a strong effect on degradation, with BDF exhibiting faster degradation than that of PEF and OBDF. Second, the combination of TS and SB had a significant effect on the degradation of the three types of plastic films. To discuss these observations, it is important to understand the degradation mechanism and the influencing factors of the degradation process. It is known that degradation mechanisms are morphology-dependent. Stress causes morphological changes, which affect the degradation rates [43] as well as the ability of a geminate radical pair to recombine or diffuse apart. Morphological changes can also affect the rate of radical reactions following radical formation [18]. For example, previous studies have shown that the breaking time of molecular chains under high stresses (10 MPa) was decreased with the increasing degree of cross-linking, and the breaking of specimens was mainly due to the mechanical properties under high stress. Specifically, as the degree of cross-linking increased, the density of cross-linking was increased, the molecular chains in network structure were shortened, the orientation distance of the molecular chains before fracture was shortened, and the breaking time was ultimately decreased [44]. Huang et al., [39] studied the stress and photo-oxidative on high-density polyethylene (HDPE), the results showed that the carbonyl index and cracks increased with aging time. TS primarily acts on plastic film, and the surface of TS plastic film may display variations in topography and roughness, in comparison with those of nTS plastic film. Hence, TS was initially applied to the tested films, which can provide more sites for microbial colonization on the polymer surface, compared with nTS film. Then, the degradation of plastic films might be initiated, which further increased the susceptibility of polymers to biodegradation with SB. Several studies have reported that microorganisms can decrease plastic film hydrophobicity [31], which can explain the much lower WCA of TS-and SB-treated plastic film samples than that of nTS and SB samples, especially between 3 and 15 months. Furthermore, the degradation process in SB could be defined as 'a biochemical transformation of compounds by microorganisms and the propensity of a material to break down into its constituent molecules by natural processes' [37]. It has been reported that the extent of polymer biodegradation depends on the microbial population, the prevailing environment (temperature, pH, humidity, and nutrition), and the properties of the polymer [45]. In this study, the soils of the two sites were relatively similar, suggesting that biotic and abiotic conditions at the two sites might be similar as well, and differences among the degradation of the three types of films primarily resulted from the differences in their material properties. Moreover, we found that BDF was degraded to a greater extent when buried in the soil, which is at least partially attributed to the physicochemical properties and biotic and abiotic processes. BDF contains additives such as degradable masterbatch, split agent, and poly (butyleneadipate-co-terephthalate) (PBAT). These additives are hydrophilic and can promote water absorption, which provides microbial colonization sites on the polymer surface, which causes weakened bonding forces between the additives and the PE, and further weakens the mechanical properties of BDF. As degradation progressed, the dimensions of the BDF declined, the films became fragmented and eventually collapsed, which increased the surface area for microbial growth and, consequently, elevated the rate of degradation [37,46].
In this study, the combination of TS and SB had significant effects on the degradation of plastic film, and we propose a series of mechanisms that may explain this finding. First, the application of TS probably increased surface roughness and caused more molecular chain breaks, which increased the surface area of the film and weakened its mechanical properties. Burial of films in the soil initiated chemical hydrolysis of their surface due to moisture uptake, creating a microenvironment that was conducive to microbial colonization. The degradation of the films was reflected in a further weakening of mechanical properties. Next, more molecular chains were broken and more peroxy radicals were produced, promoting the oxidation of the plastic film chains and ultimately resulting in the formation of low molecular weight byproducts of microbial activity. At the same time, these radicals also caused the PE to cross-link, decreasing its crystallinity.

Conclusions
The use of plastic film mulch has numerous benefits for agriculture in arid and semiarid regions, but the accumulation of plastic mulch residues in soil is unfriendly to the environment. It is important to examine the process by which plastic films breakdown in the soil so that we can develop efficient means of promoting their degradation. Here, we show that a combination of TS and SB promotes the breakdown of plastic films by altering their mechanical properties, hydrophobicity, functional groups, and crystallinity. Weakened mechanical properties and increased hydrophilicity created conditions that were favorable for the attachment of microbes to the film surfaces. Taken together, our results demonstrate that TS and SB have the potential to accelerate plastic film degradation.

•
We tested the effects of soil burial (SB) after tensile stress (TS) on film degradation. • SB after TS significantly weakened the mechanical properties of plastic film. • SB after TS increased the presence of functional groups on thin polyethylene film (PEF). • SB after TS increased the hydrophilicity of plastic film but decreased PEF crystallinity.
Supplementary Materials: The following are available online at http://www.mdpi.com/2071-1050/12/19/7985/s1, Figure S1: tensile strength (MPa) of plastic films exposed to tensile stress or no tensile stress, measured at intervals up to 30 months after soil burial, Table S1: Results of repeated measure analysis of variance on nominal tensile strain at break, water contact angle, weight loss and tensile strength using burial time (BT) as the repeated factor and tensile stress (TS) and plastic film (PF) as two fixed factors within each of the two sites.