Mechanical Properties of Metallocene Linear Low-Density Polyethylene Mulch Films Correlate with Ultraviolet Irradiation and Film Thickness

Abstract

Mulching technology has created a major problem of agricultural plastic pollution. This is because the mulch is severely degraded by UV (ultraviolet) irradiation and the mechanical properties deteriorate, which makes mechanical recycling or manual recycling difficult. This study was conducted on m-LLDPE (metallocene linear low-density polyethylene) mulch films. The difference in mechanical properties of specimens in the TD (transverse direction) and PD (parallel direction) was investigated, and the changes in the mechanical properties after UV irradiation were compared. Finally, an initial mulch mechanical property evaluation model was developed to adapt to different recovery machines and crop agronomic requirements. The results suggest that the mechanical properties of m-LLDPE mulch films were strongly influenced by the film thickness, and also showed directional differences in tensile and tear properties. After UV irradiation, the mechanical properties of the specimens were enhanced after a short period of time, but the overall trend was a non-linear decay which gradually slowed down with time.

1. Introduction

Plastic has important and wide applications in production. In agriculture, the reasonable use of plastic helps to increase crop yields [1]. As one of the plastic applications, mulching has become an essential technology in arid and cold regions of the world, increasing the water conservation in soils. The proper use of mulch has the effect of improving crop yields [2].

Aging of the mulch films inevitably occurs during use due to UV (ultraviolet) and other environmental influences. Structural and chemical changes occur in the polymer chain of the mulch films; methyl and isopropyl groups appear to increase and alcohol groups, ester groups, carbonyl groups, carboxyl groups, unsaturated content, and cross-linked structures increase [3]. Although the reactions start with cross-linking as the dominant association, fracture reactions soon occur, eventually reducing the polymer molecular weight [4]. These irreversible phenomena cause changes in the mechanical properties of the mulch, mainly in the form of decreases in tensile, tear, and puncture resistance, resulting in the mulch remaining in the soil during recycling.

The area covered by mulch in China reached 1.73868 × 10⁷ hm² in 2020, exceeding the total land area under micro-irrigation of the world [5]. However, as mulch is not recycled, the problem of residual mulch pollution in China has become increasingly prominent. Cheap mulch is popular in the Chinese market, and it is easy to break and difficult to recycle after aging. The consumption of cheap mulch has increased exponentially, while the recycling rate is generally less than 2/3, resulting in a higher soil microplastic (MP) content. The extent of microplastic residues is more severe in deep tillage layers below 10 cm soil depth, significantly reducing soil pH [6,7]. These microplastics clog the soil tillage layer and surface layer, seriously affecting water infiltration into the soil, hindering water movement in the soil, and weakening the drought resistance of the soil, which is extremely harmful to crop cultivation in arid areas and poses a serious threat to human health and environmental safety [8,9]. Therefore, high-quality mulch recycling is an effective way to solve these problems.

Mulch recycling effectiveness depends on its mechanical properties and weather resistance. Thickness is one of the factors affecting the mechanical properties of mulch, and the thicker the mulch, the better its mechanical properties. Chen [10] showed that thicker mulch films have a higher recycling rate in mechanical recycling. Wen [11] and Szlachetka [12] found that the specimen thickness has a strong effect on the strength, the strain, and the tensile modulus of polyethylene. Li Chen [13] and Li Bingru [14] showed that UV irradiation is the main factor that causes aging of films. Zhang [15] found that the direction of film placement has an impact on the extent of mechanical recycling; the maximum tensile force of the mulch films in the parallel direction is generally about 1/3 higher than that in the transverse direction, suggesting that there are directional differences in the mechanical properties of mulch films.

Previous studies on the tensile properties of mulch films before aging are more common, but less research has been conducted on the tensile properties, tear resistance, and puncture resistance of mulch films after aging. In this paper, our research object is a m-LLDPE (metallocene linear low-density polyethylene) mulch film, which is popular in the Chinese market. After sampling, the films were aged for 0 h, 45 h, 90 h, 135 h, and 180 h by using a UV aging tester, which was similar to the irradiation times of 20, 40, 60, and 80 days used by Xinjiang Urumqi [16]. Mechanical tests were conducted after the aging treatment had been completed, and the effects of thickness and UV irradiation time on the mechanical properties of the specimens were analyzed. Since mechanical recycling depends on the mechanical properties of the mulch films after aging, a mulch film mechanical property evaluation model was initially developed based on this test. The mulch scores were used to precisely select the appropriate mulch and to better adapt the recycling machinery. From the aspect of recycling, selecting the appropriate type and thickness of mulch films is important regarding the recycling machinery; the properties of the selected film need to be appropriate for the recycling machinery. From the aspect of agronomy, different crops have different uncovering times and planting methods, and the selected mulch needs to meet the agronomic requirements of crop cultivation. An evaluation model was developed to provide an important theoretical basis for future soil environment improvement and soil microplastic pollution reduction.

2. Materials and Methods

2.1. Materials

The m-LLDPE mulch films used in the tests were produced in Shuyang, Jiangsu Province, China, with densities of 0.914–0.928 g/cm³ and melting points of 94–121 °C. The specimen thicknesses were selected as 0.005 mm, 0.010 mm, 0.015 mm, and 0.020 mm.

The more popular type of mulch film in the Chinese market is the m-LLDPE film, which is processed with metallocene catalysts. The strong polymerization ability of the catalysts leads to m-LLDPE films with a narrow molecular weight distribution and a uniform composition distribution [17]. The mechanical properties of polymers are affected by the amount of Tie molecules, and the Tie molecular weight in m-LLDPE films is three times that of LLDPE [18], which results in a greater improvement in the elongation at break, as well as better performance in tensile resistance, tear resistance, and puncture resistance. [19,20].

2.2. Test Equipment

As shown in Figure 1, a UV aging tester was used to age the mulch, and the equipment selected was the LUV-II UV aging machine (a) produced by Shanghai Pusun Company, Shanghai, China, which uses UVB-313 lamps with an irradiance of 0.48 W/m² @ 310nm. The punching equipment and cutter need to be used in combination. The specimen pad was held below the tool and the square head of the punch was held against the cutter and then punched to take the sample. The equipment used was a HongYun JM-16 square head punching machine (b) and two sizes of specimen cutter (d), a 6 mm × 115 mm dumbbell-type cutter and a 50 mm × 150 mm trouser-type cutter manufactured by Five Stars Knife and Mold Factory. An electronic universal testing machine was sued for tensile, tearing, and puncture tests, and the equipment selected was a Sanshi UTM6000 electronic universal testing machine (c). A puncture needle with a cross-sectional diameter of 1 mm (e) and a holding ring with an inner diameter of 10 mm (f) was used for puncture tests.

2.3. Experimental Program

2.3.1. UV Irradiation

According to ISO 4892, the UVB-313 test program was selected and the experimental temperature was chosen to be 45 °C. After the experimental setup was completed, the mulch films were sampled in two directions, marked as PD (parallel direction) and TD (transverse direction) [21]; the PD was parallel to the rolled direction of the film and the TD was transverse to the rolled direction of the film. The sampling size was 150 mm × 80 mm for the tensile test and 170 mm × 80 mm for the tear test to make the test operation easier. In Figure 2, the outer outline is the mulch film and the inner rectangles are the sampling illustration. As shown in

Table 1. Number of specimens of each size.

Size	Direction	Quantity
150 mm × 80 mm	TD	100
	PD	100
170 mm × 80 mm	TD	100
	PD	100
25 mm × 25 mm	-	200

, sampling was performed five times for four thicknesses and five aging times along the PD and TD of the film for a total of 100 times in each direction. In the puncture test, the direction did not need to be distinguished, the puncture test film sampling size was 25 mm × 25 mm, and the total number of specimens was 200. After sampling was completed, the rectangular specimens were securely installed on the specimen rotating shelf of the UV aging tester, and the irradiation times were 0 h (0d), 45 h (20d), 90 h (40d), 135 h (60d), and 180 h (80d).

2.3.2. Tensile Properties

Following the requirements of ISO 1184, rectangular films were cut into dumbbell-type (6 mm × 115 mm) specimens, as shown in Figure 3. According to the requirements, the edge of the specimens should not be notched. However, the films were soft and their thicknesses were less than 0.05 mm, so the edges of the dumbbell specimens cut by normal knives were inevitably notched. Thus, the test results could not reflect the tensile properties of the mulch films due to the breakage from the notches when they were elongated. Therefore, we used a special dumbbell-type cutter, and the punch press was used cooperatively with it. The paper was padded under the specimens before punching, and the specimens adhered to the paper after punching, This method ensured that the edges of the specimens were free from notches, while also preventing the specimens from being compressed during storage.

The test room temperature was 25 ± 1 °C, the dumbbell-type specimens were held on the universal testing machine, the holding distance was 80 mm, and the test speed was 200 mm/min. The maximum tensile force and elongation at break were recorded, then the standard deviation was calculated, and the number of standard deviation samples was the number of samples of the whole test.

The elongation at break, $\delta$, is calculated as follows:

$$\delta =\frac{\Delta L}{L}\times 100\%$$

(1)

where $\Delta L$ is the change in the distance between the clamps in mm and $L$ is the initial distance between the clamps in mm.

2.3.3. Tearing Properties

According to ISO 6383, the specimens were cut into trouser-type (50 mm × 150 mm) samples by using a special trouser-type cutter. The cutting direction was similar to that of the dumbbell-type specimens. The specimens were held between the clamps, the central axis of the specimens coincided with the central line of the clamps, the holding distance was (75 ± 1) mm, and the testing speed was 250 mm/min. The maximum tearing force and tear resistance of the whole length were recorded, then the standard deviation was calculated, and the number of standard deviation samples was the number of samples of the whole test.

The tear resistance, $\sigma$, is calculated as follows:

$$\sigma =\frac{F_t}{d}\times 100\%$$

(2)

where $F_t$ is the average tearing force in N and $d$ is the thickness of the specimens in mm.

2.3.4. Puncture Properties

According to GB/T 37841–2019, 25 mm × 25 mm rectangular specimens were gripped using the holding ring and it was ensured that the specimens were kept flat and unbroken in the ring. The steel needle was fixed on the universal testing machine, and the holding ring was placed under the steel needle directly. The puncture speed was (50 ± 5) mm/min, and the maximum puncture force and puncture elongation were recorded. Finally, the standard deviation was calculated, and the number of standard deviation samples was the number of samples of the whole test.

3. Results

3.1. Mechanical Properties of Initial m-LLDPE Mulch Films

This part was designed to study the mechanical properties (tensile, tearing, and puncture resistance) of the initial m-LLDPE mulch films. The effect of thickness on the mechanical curves of the specimen were analyzed, and the mechanical properties in different directions were compared. The effects of thickness and direction on the mechanical properties were analyzed.

3.1.1. Tensile Properties

The tensile properties of the initial mulch films shown in Figure 4 suggest that the maximum tensile force and elongation at break are proportional to the thickness of the mulch film. In the TD, the elongation at break is higher and the ductility is better, while in the PD, the strain hardening is more obvious, the tangential modulus is larger, the elongation at break is lower, and the ductility is lower.

The yielding stage of the TD curve exhibits large fluctuations because local stress concentration occurs during the tensile process of the specimen, and these points of stress concentration produce a yield deformation band (this deformation band is called Lüders band in the field of metals), which expands continuously to both sides during the tensile process. This will be accompanied by a sudden drop in tensile force and then an immediate rise; this phenomenon is expressed in the tensile curve as each fluctuation in the yielding stage. The above phenomenon is not obvious in the PD; there is no obvious deformation band throughout its tensile process and the curve of the yielding stage is relatively smooth.

The maximum tensile force and elongation at break of the specimens are shown in Figure 5 and

Table 2. Maximum tensile force and elongation at break results (mean values) in the TD and PD.

Thickness (mm)	TD		PD
	Maximum Tensile Load (N)	Elongation at Break (%)	Maximum Tensile Load (N)	Elongation at Break (%)
0.005	0.64	915.27	1.05	770.10
0.010	1.26	1051.64	1.49	823.65
0.015	2.06	1129.33	2.53	1008.86
0.020	3.14	1267.69	3.59	1063.05

; the tensile properties of m-LLDPE films in the TD and PD are significantly different. Based on the overall analysis, the maximum tensile force of the specimens in the TD is lower than that in the PD for all four tested thicknesses, but the elongation at break in the TD is higher than that in the PD due to the tangential modulus in the TD being less than that in PD. Based on the analysis of maximum tensile force, the TD maximum tensile forces of specimens with thicknesses of 0.005 mm, 0.01 mm, 0.015 mm, and 0.02 mm are lower than the those in the PD by 39.05%, 15.44%, 18.58%, and 12.53%, respectively. Based on the analysis of elongation at break, the elongations at break in the TD are 18.85%, 27.68%, 11.94%, and 19.25% higher than those of the PD for the four thicknesses of specimens.

3.1.2. Tearing Properties

The tearing properties of the initial m-LLDPE films shown in Figure 6 indicate that the tearing force is proportional to the thickness of the film (note: this does not mean that the thickness of the material has a linear relationship with the tearing force). The specimens were elongated and produced yield deformation bands during the tearing process in the TD, which exhibited a flatter curve in the figure. While this phenomenon did not occur in the PD due to the larger tangential modulus and shorter yield deformation, the curve in the figure is steeper. As a result of the existence of elongation in the tearing, the tearing displacement is longer than the initial length (Figure 7). The tearing displacements in the PD and TD are different due to the difference in mechanical properties; it is longer in the TD with a higher ductility and it is shorter in the PD with a lower ductility.

The maximum tearing force and tear resistance of the specimens are shown in Figure 8 and

Table 3. Maximum tearing force and tear resistance results (mean values) in the TD and PD.

Thickness (mm)	TD		PD
	Maximum Tear Load (N)	Tear Resistance (N/mm)	Maximum Tear Load (N)	Tear Resistance (N/mm)
0.005	1.38	273.20	1.14	222.50
0.010	2.72	267.94	1.98	196.33
0.015	4.00	264.92	3.62	239.61
0.020	5.20	257.91	4.50	223.31

. Based on the analysis of the maximum tearing force, the maximum tearing forces of the TD are 21.05%, 37.37%, 10.50%, and 15.56% higher than those of the PD for the four thicknesses of specimens. Based on the analysis of the tear resistance, there is little variation in the tear resistance in the same direction, while the difference between the TD and PD is more pronounced; the tear resistances of the TD are 22.79%, 36.47%, 10.56%, and 15.49% higher than those of the PD for the four thicknesses of specimens, indicating that the TD of the specimens has a stronger tear resistance than the PD.

3.1.3. Puncture Properties

The puncture properties of the initial mulch films shown in Figure 9 indicate that the maximum puncture force and puncture elongation are proportional to the thickness of the specimens. As the puncture needle gradually advanced, the specimen gradually entered the yielding stage around the force point and the yield radius of the specimen gradually expanded. With the circular holding ring and uniform force on the puncture needle, the force–displacement graph showed an almost linear trend from the loading to the breaking stage. The thickness of the specimens is an important factor in determining their puncture properties, with thicker films typically exhibiting a higher maximum puncture force and puncture elongation.

As shown in Figure 10 and

Table 4. Maximum tearing force and tear resistance results (mean values) in the TD and PD.

Thickness (mm)	Maximum Puncture Force (N)	Puncture Elongation (mm)
0.005	0.95	15.82
0.010	1.89	16.57
0.015	2.79	18.53
0.020	4.73	21.47

, the data indicate that the rate of increase in puncture elongation is not as significant as that of the maximum anti-puncture force. Specifically, in the thickness intervals of 0.005 mm to 0.01 mm, 0.01 mm to 0.015 mm, and 0.015 mm to 0.02 mm, the increase rates of the maximum puncture force are 98.95%, 47.62%, and 70.61%, respectively, while those of the puncture elongation are 4.97%, 12.11%, and 15.77%, respectively. In addition, over the entire range of thicknesses from 0.005 mm to 0.02 mm, the increase rates of the maximum puncture force and the puncture elongation are 398.95% and 35.46%, respectively.

3.2. Mechanical Properties of m-LLDPE Mulch Films after UV Irradiation

The above experiments analyzed the effect of the film thickness and direction on the mechanical properties of the mulch. This part of the test added UV irradiation treatment to the specimens to analyze the effect of three factors (UV irradiation time, direction, and thickness) on the mechanical properties.

3.2.1. Tensile Properties after UV Irradiation

As shown in Figure 11a,b, there are two aging trends of m-LLDPE mulch films in the TD. The first is that for the specimens with thicknesses of 0.005 mm and 0.01 mm, the maximum tensile force decreases slightly before 90 h of UV irradiation and starts to recover after 135 h of irradiation. It continues to increase at 180 h but is always lower than the 0 h control group. The other is that for specimens with thicknesses of 0.015 mm and 0.02 mm, the maximum tensile force maintains an increasing trend until 45 h of irradiation, then decreases rapidly, has a certain recovery trend until 135 h, and then finally continues to decrease in the 135 h to 180 h stage. While in the PD, none of the above fluctuations are obvious and show a decreasing trend. Both trends indicate that the maximum tensile force of the specimens decreases nonlinearly with the UV irradiation time. In terms of aging resistance, the most serious loss in properties in the TD is the specimen with a thickness of 0.02 mm, which exhibits a decrease in the maximum tensile force of 20.99% after UV irradiation for 180 h. The most serious loss in property in the PD is the specimen with a thickness of 0.005 mm, which exhibits a decrease of 45.51%. Such a phenomenon was caused by the residual non-crystalline regions and incomplete crystalline parts in the molecular chain, which caused partial secondary crystallization of the film during processing, resulting in a slight increase in the crystallinity of the film and a corresponding slight increase in the macroscopic mechanical properties.

The elongation at break in the TD of the specimens shows similar trends to the maximum tensile force and also appears to fluctuate. The specimens of 0.02 mm thickness have the most obvious decrease in the elongation at break at 90 h, from 1267.687% before UV irradiation to 1006.882% after, which then recovers to 1203.570%. The trend in the elongation at break in the PD was relatively flat compared to the TD, and the elongation at break of the 0.005 mm thickness specimen decreased the most significantly after UV irradiation, with a decrease of 20.57% compared to 0 h.

In order to better understand the variation in mechanical properties with thickness and UV irradiation time, multivariate equations were constructed based on each mechanical property test. Since there were fluctuations in the mechanical properties of the specimens, a polynomial function model was used to fit the surfaces. The surfaces in Figure 11a–d were fitted with multivariate Equations (3)–(6), respectively.

$$\begin{array}{cc}\hfill Z_{1\mathrm{a}}=& 1.23892-255.07275X+34725.92279X^2-949913.60946X^3-718265.77329X^4+2.16853{\times 10}^8{X}^5\hfill \\ & +0.00725Y-7.29853\times {10}^{-5}{Y}^2-2.76266{\times 10}^{-6}{Y}^3+3.594\times {10}^{-8}{Y}^4-1.10225{\times 10}^{-10}{Y}^5\hfill \end{array}$$

(3)

$$\begin{array}{cc}\hfill Z_{1\mathrm{b}}=& 0.4371+135.35205X-1302.82645X^2-28387.2591X^3-526298.99419X^4+2.16226{\times 10}^8{X}^5\hfill \\ & +0.01094Y-5.64433{\times 10}^{-4}{Y}^2+8.31143{\times 10}^{-6}{Y}^3-5.19605\times {10}^{-8}{Y}^4+1.15713\times {10}^{-10}{Y}^5\hfill \end{array}$$

(4)

$$\begin{array}{cc}\hfill Z_{1\mathrm{c}}=& 793.58407+26306.68707X+198880.03774X^2-3.63175{\times 10}^7{X}^3-2.98874{{\times 10}^{8}X}^4+6.03527{\times 10}^{10}{X}^5\hfill \\ & +0.68367Y-0.03786Y^2-5.26478\times {10}^{-4}{Y}^3+1.0039{\times 10}^{-5}{Y}^4-3.44439\times {10}^{-8}{Y}^5\hfill \end{array}$$

(5)

$$\begin{array}{cc}\hfill Z_{1\mathrm{d}}=& 911.6265-61573.88823X+8379345.95186X^2-2.66481{\times 10}^8{X}^3-3.80142\times {10}^8{X}^4+5.62376{\times 10}^{10}{X}^5\hfill \\ & +1.167Y-0.01863Y^2-3.74643{\times 10}^{-4}{Y}^3+5.16605{\times 10}^{-6}{Y}^4-1.55923\times {10}^{-8}{Y}^5\hfill \end{array}$$

(6)

where $Z_{\mathrm{i}\mathrm{d}}$ (i = a, b, c, d) is the value of the corresponding mechanical parameter for each graph, $X$ is the thickness of the specimen and $Y$ is the UV irradiation time. The fitted R² values of the four surfaces are 0.97658, 0.97731, 0.9529, and 0.92256, respectively, and the equations are well fitted.

3.2.2. Tearing Properties after UV Irradiation

As shown in Figure 12a,b, the maximum tearing force is the maximum force that the film can resist tearing during the tearing process. The maximum tearing force in the TD of the specimens tends to decrease without obvious fluctuations, and only the 0.02 mm thickness specimen exhibits slight fluctuations at the 90 h point, while the 0.01 mm thickness specimen exhibits the most obvious decrease after 180 h of UV irradiation, with a decrease of 14.26%. In the PD, the 0.015 mm thickness specimen also exhibits fluctuations at 90 h.

The tearing process of m-LLDPE mulch film is a complex process in which multiple forces participate, and the spreading force of the tearing process is not the maximum tearing force, but the average tearing force, which is smaller than the maximum tearing force. The maximum tearing force cannot be used to represent the tear resistance of the overall tearing process, so the average tearing force should be used to calculate the tearing resistance. The variations in the tear resistance of the specimens are shown in Figure 12c,d, where the tear resistance in the TD and PD fluctuates with the UV irradiation time, and the fluctuation time point is different for each thickness. From a comparison of Figure 12a–d, it can be found that the maximum tearing force does not show an obvious fluctuation phenomenon, but the fluctuation in the tearing force during the tearing process affects the value of the average tearing force, which has a huge impact on the stability of the tear resistance.

Similar to the tensile test, a multivariate equation was also fitted using the polynomial model, and the multivariate equation was constructed as follows:

$$\begin{array}{cc}\hfill Z_{2\mathrm{a}}=& 0.69601+67.73976X+19957.43091X^2-677533.67147X^3-1410013.89865X^4+2.34474\times {10}^8{X}^5\hfill \\ & +0.00941Y-4.1813{\times 10}^{-4}{Y}^2+5.01979{\times 10}^{-6}{Y}^3-2.60284{\times 10}^{-8}{Y}^4+5.02438{\times 10}^{-11}{Y}^5\hfill \end{array}$$

(7)

$$\begin{array}{cc}\hfill Z_{2\mathrm{b}}=& 1.28935-178.66606X+35578.26816X^2-907490.27834X^3+727613.08892X^4-1.18784{\times 10}^8{X}^5\hfill \\ & +0.01495Y-6.71645\times {10}^{-4}{Y}^2+8.1994e^{-6}{Y}^3-3.87636{\times 10}^{-8}{Y}^4+6.2531{\times 10}^{-11}{Y}^5\hfill \end{array}$$

(8)

$$\begin{array}{cc}\hfill Z_{2\mathrm{c}}=& 276.51061-6117.49266X+729193.93636X^2-2.29843e^7{X}^3-1.90509{{\times 10}^{7}X}^4+2.07493{\times 10}^9{X}^5\hfill \\ & -0.09024Y-0.00458Y^2+6.74712{\times 10}^{-5}{Y}^3-6.15629{\times 10}^{-7}{Y}^4+2.03292\times {10}^{-9}{Y}^5\hfill \end{array}$$

(9)

$$\begin{array}{cc}\hfill Z_{2\mathrm{d}}=& 294.56533-25864.4284X+2271838.52516X^2-5.22012{\times 10}^7{X}^3+1.28276{\times 10}^8{X}^4-2.04389{\times 10}^{10}{X}^5\hfill \\ & +0.41723Y-0.02709Y^2+3.633{\times 10}^{-4}{Y}^3-1.84571\times {10}^{-6}{Y}^4+3.24132{\times 10}^{-9}{Y}^5\hfill \end{array}$$

(10)

where $Z_{\mathrm{i}\mathrm{d}}$ (i = a, b, c, d) is the value of the corresponding mechanical parameter for each graph, $X$ is the thickness of the specimen and $Y$ is the UV irradiation time. The R² values after fitting the four surfaces were 0.99606, 0.98945, 0.7825, and 0.52523, respectively, with a better fit for the maximum tearing force but a poorer fit for the tearing strength, due to the fact that the tearing strength was calculated using the average tearing force. The values observed in the calculation range fluctuated widely, similar to that observed by Bao Han et al. [22], which would lead to deviations in the fit.

3.2.3. Maximum Puncture Force after UV Irradiation

As shown in Figure 13, the values of the maximum puncture force of the specimens are affected by the thickness more obviously, and a similar trend of slight fluctuations is observed. Within a specific UV irradiation time, the puncture force also increased slightly due to UV irradiation, with specimens of 0.005 mm, 0.01 mm, 0.015 mm, and 0.02 mm thicknesses exhibiting enhanced performance periods of 0–135 h, 90–135 h, 0–45 h, and 90–135 h, respectively.

The puncture elongation of the specimens clearly varies with thickness and fluctuates with regard to the UV irradiation time. Specimens with thicknesses of 0.005 mm and 0.01 mm exhibit enhancements at 135 h, while 0.015 mm and 0.02 mm thickness specimens are enhanced after 45 h and 135 h. Since the m-LLDPE mulch films are soft plastic materials, when puncture damage occurs, the puncture needles have different friction relations with the specimens and the UV aging is uneven, resulting in different puncture results.

The puncture performance surface was also fitted using the polynomial model, and the multivariate equations were constructed as follows:

$$\begin{array}{cc}\hfill Z_{3\mathrm{a}}=& 0.29859+169.32412X-3292.90795X^2+156115.14873X^3-1527266.55169X^4+3.83431\times {10}^8{X}^5\hfill \\ & +0.00919Y-2.97822\times {10}^{-4}{Y}^2+2.12674\times {10}^{-6}{Y}^3+3.31584{\times 10}^{-10}{Y}^4-2.67988{\times 10}^{-11}{Y}^5\hfill \end{array}$$

(11)

$$\begin{array}{cc}\hfill Z_{3\mathrm{b}}=& 20.22507-1784.24046X+163627.94483X^2-3386933.20061X^3+6898051.68897X^4-1.43767\times {10}^9{X}^5\hfill \\ & +0.04134Y+2.10858{\times 10}^{-4}{Y}^2-2.6979\times {10}^{-5}{Y}^3+2.87578{\times 10}^{-7}{Y}^4-8.44007{\times 10}^{-10}{Y}^5\hfill \end{array}$$

(12)

where $Z_{\mathrm{i}{\mathrm{a}}^{\mathrm{i}}}$ (i = a, b) is the value of the corresponding mechanical parameter for each graph, $X$ is the thickness of the specimen and $Y$ is the UV irradiation time. The R² values after fitting the two surfaces are 0.99413 and 0.8272, respectively, and the equation has a good fit.

4. Initial Development of an Evaluation Model for the Mechanical Properties of Mulch Films

4.1. Evaluation Model Building

In this experiment, it was found that the mechanical properties of the mulch are related to thickness, direction, and UV irradiation time. However, the experimental results are only a description of the mechanical properties and cannot provide effective support for the actual recycling work. To achieve the goal of improving the recovery rate of mulch films, it is necessary to consider the working principle of recycling machinery and crop cultivation agronomy to select the most suitable mulch films. Therefore, an initial model was developed to evaluate the mechanical properties of mulch films. The scores resulting from the evaluation model can directly reflect the suitability of the mulch in the current environment, providing more efficient solutions in the field of mulch recycling. This evaluation model has important practical values in helping us better analyze the properties and characteristics of mulch and guiding future research on mulch recycling. We hope that this research will contribute to solving environmental pollution problems and sustainable development.

This model combines different mechanical properties of mulch films (e.g., maximum tensile force, elongation at break, maximum tear force, tear resistance, puncture force, and puncture elongation) and gives them a weighted score, where the mulch film with the highest score is the most suitable one. The specific evaluation model is as follows:

$$S_i=A_i{C+B}_{i}D$$

(13)

where $S_i$ (i = 1, 2, 3, 4) is the weighted score of maximum tensile force, elongation at break, tearing force, and tear resistance, respectively, $A_i$ (i = 1, 2, 3, 4) is the weighted factor of maximum tensile force, elongation at break, tearing force, and tear resistance in the TD, respectively, $B_i$ (i = 1, 2, 3, 4) is the weighted factor of maximum tensile force, elongation at break, tearing force, and tear resistance in the PD, respectively, and C and D are the normalized values in the TD and PD, respectively.

$$S=\sum _{i=1}^4{\mu }_i{S}_i+\alpha P+\beta E$$

(14)

where $S$ is the total evaluation score of the mulch, ${\mu }_i$(i = 1, 2, 3, 4) are the weighting factors of each property of $S_i$ in Equation 13, $P$ and $E$ are the normalized values of puncture force and puncture elongation, respectively, and α and β are the weighting factors of puncture force and puncture elongation, respectively.

4.2. Evaluation Model Application

The weighting factors in S_i represent the importance of the TD and PD in recycling, where the selection of parameters A_i, B_i (A_i + B_i = 1) needs to be considered in the working principle of current recycling machinery. If the working direction of the recycling machinery is parallel to the PD, the weighting factor of B_i needs to be reasonably increased, and the weighting factor of A_i needs to be decreased accordingly. If the working direction of recycling machinery is horizontal, A_i needs to be increased and B_i needs to be decreased.

The parameter selection of ${\mu }_i,\alpha \mathrm{a}\mathrm{n}\mathrm{d} \beta$ (${\mu }_i+\alpha +\beta =1$) in S need to take into account the relationship between the properties of the mulch films and the practical situation. In terms of recycling, different recycling machinery has different emphases on the mechanical properties of the mulch. For the winding-type and air-suction-type machinery, which have higher requirements for the tensile force, ${\mu }_1$ needs to be reasonably increased, while for the needle-roller-type and other recycling machinery with tearing and impact effects, ${\mu }_3{,\mu }_4,\alpha , \mathrm{a}\mathrm{n}\mathrm{d} \beta$ need to be reasonably increased. In terms of agronomic requirements, the thickness of the mulch affects the crop seedling rate, and some crops can break the mulch independently, such as wheat, corn, pumpkin, etc. These crops have different film breaking abilities. Therefore, the α and β of the mulch need to be adjusted appropriately after comprehensive consideration of the practical situation, finally resulting in an overall score of S. The higher the value of S, the more practical the mulch is.

The evaluation of the suitability of m-LLDPE mulch was used as an example, corn and potatoes were selected as crops, and transverse and parallel mulch rollers were selected as the recovery machines to evaluate mulch suitability. As shown in Figure 14a, the transverse roll pick-up direction is orthogonal to the mulching direction, while the mulch roll pick-up direction in Figure 14b is parallel to the mulching direction. When using transverse roll pick-up recycling machine for recovery, the tensile properties of the films in the PD are more important than in the TD, and accordingly B₁ and B₂ are given a larger weighting factor of 0.6, while A₁ and A₂ are given a smaller weighting factor of 0.4. The films are easily torn in the TD, resulting in an interruption to the film pick-up process. Thus, the films should have a higher tear resistance in the PD, and A₃ and A₄ are given a larger weighting factor of 0.7, while B₃ and B₄ are given a smaller weighting factor of 0.3. When using parallel roll pick-up recycling machine for recovery, the tensile properties of the film in the PD are more important than in the TD, and accordingly B₁ and B₂ are given a greater weighting factor of 0.8, while A₁ and A₂ are given a smaller weighting factor of 0.2. The film is easily torn along the mulching direction, so the film should have a higher tear resistance in the TD, so B₃ and B₄ are given a greater weighting factor of 0.7, while A₃ and A₄ are given smaller weights of 0.3. The puncture resistance weighting factors α and β need to be defined based on the growth characteristics of corn and potato. They both have the ability to break the film growth; corn has a stronger ability to break the film and thus is given a smaller value of α and β, taking the value of 0.18, and potato is given a larger value of α and β, taking the value of 0.22 [23]. In the recovery process, the tensile and tear resistance are of equal importance, so after α and β are determined, the average distribution of μ₁, μ₂, μ₃, and μ₄ and the specific weighting factors are shown in

Table 5. Evaluation weighting factors.

	T 1 + Corn	T + Potato	P 2 + Corn	P + Potato
A1	0.4	0.4	0.2	0.2
A2	0.4	0.4	0.2	0.2
A3	0.7	0.7	0.3	0.3
A4	0.7	0.7	0.3	0.3
B1	0.6	0.6	0.8	0.8
B2	0.6	0.6	0.8	0.8
B3	0.3	0.3	0.7	0.7
B4	0.3	0.3	0.7	0.7
μ1	0.16	0.14	0.16	0.14
μ2	0.16	0.14	0.16	0.14
μ3	0.16	0.14	0.16	0.14
μ4	0.16	0.14	0.16	0.14
α	0.18	0.22	0.18	0.22
β	0.18	0.22	0.18	0.22

¹ Transverse roll pick-up recycling machine. ² Parallel roll pick-up recycling machine.

.

The evaluation model was applied to calculate the mechanical properties of the film at the time of uncovering and seedling emergence. The best time to uncover the film for both corn yield and film recyclability is at the trumpet stage [24], about 70 days after sowing, while potatoes have the highest uncovering to production ratio at the tuber formation stage, about 80 days after sowing [25]. The emergence periods for corn and potato are 8 days and 33 days [26], respectively. By using the multivariate equation for the mechanical properties of the mulch (Equations (3)–(12)), the mechanical properties of the mulch corresponding to the above time points were calculated, and the evaluation model was applied to derive the final mulch score, S. The higher the score, the better the applicability (

Table 6. m-LLDPE mulch film evaluation score.

Thickness (mm)	T + Corn	T + Potato	P + Corn	P + Potato
0.005	0.220	0.050	0.145	0.079
0.010	0.382	0.087	0.190	0.108
0.015	0.538	0.109	0.237	0.127
0.020	0.692	0.086	0.228	0.091

).

The values of all weighting factors are relative and depend on the importance of the evaluation of the respective mechanical property parameters. Therefore, the values should be adjusted according to the specific situation. An evaluation model with suitable values of the weighting factors can meet the mechanical recovery rates and consider the crop growth, to achieve accurate film selection, accurate film spreading, and high-quality recovery.

5. Discussion

The tensile curves of the m-LLDPE films showed directional differences in the maximum tensile force and elongation at break, exhibiting similar trends to the LDPE construction films tested by Olga Szlachetka [12]. The test results showed that the average elongation at break of m-LLDPE films was 1267.69% compared to 549.5% for LDPE films, indicating a significant increase in the film elongation after metallocene catalysis.

m-LLDPE mulch films were found to exhibit property fluctuations after UV irradiation. In a study by D. Briassoulis [27], plastic films of different materials showed varying property fluctuations, especially for LDPE films during UV aging. Such a phenomenon is due to the residual non-crystalline regions and incomplete crystalline parts in the molecular chain, which cause partial secondary crystallization of the film during the aging process, resulting in a slight increase in the crystallinity of the film and a corresponding slight increase in the macroscopic mechanical properties.

By studying the mechanical properties of the mulch in depth, a better understanding of the damage and contamination caused by mulch during use can be achieved. New solutions and recommendations are necessary to reduce and solve the problem of microplastic pollution, for example, the use of evaluation models to evaluate the suitability and recycling value of mulch, which is important for the mulch industry and environmental protection to guide the production and use of mulch and is expected to have a positive impact on reducing plastic pollution and promoting a circular economy. Therefore, the application of a mulch film mechanical property evaluation model is meaningful in the relevant academic research and practical applications.

The initial evaluation model is innovative but also has some shortcomings. The aging process of the mulch can be scored in steps, instead of a simple score for the aged mulch, and the evaluation model also takes into account the directional difference; weighting scores are assigned to both the TD and PD. The advantage of such a method is that it can accurately be adapted to the agronomic requirements of planting and the recycling machinery types to improve the mechanical recycling rate and prevent soil microplastic pollution. However, the evaluation model is not necessarily based on a complete set of mulch properties. In practical applications, there may be other important properties of mulch films that need to be considered, and the specific weighting factor values are not given for these. Thus, it is necessary to supplement the evaluation model with a large number of experiments to improve the evaluation model based on the requirements of farm machinery and agronomy.

6. Conclusions

All of the m-LLDPE mulch films in the test showed lower maximum tensile force in the TD than in the PD, averaging at 21.40% lower, while the elongation at break was higher in the TD than in the PD, averaging at 19.43% higher. The tear resistance in the TD was higher than in the PD, averaging at 21.33% higher, and the TD showed longer tear displacement due to a higher ductility, while the PD showed a shorter tear displacement. The maximum puncture force was influenced by the thickness, and the puncture elongation slightly increased with thickness.

After UV irradiation, the mechanical properties of the m-LLDPE mulch films exhibited mostly non-linear decays, with a gradual slowing of the decay rate. The curved surfaces of maximum tensile force, elongation at break, tear resistance, and puncture elongation changed unevenly and clearly fluctuated, while the curved surfaces of maximum tearing force and maximum puncture force changed smoothly and fluctuated little.

An evaluation model of the mechanical properties was initially designed, which could combine the conditions necessary for application in agricultural machinery and agronomy to select the most suitable mulch. The reasonable use of the evaluation model will make the accurate selection and spreading of mulch with high-quality recycling a possibility, and it will contribute to the prevention of the further increase in soil microplastic content and the reduction in soil burden.