*2.1. Basic Information of the Test Field*

Maigaiti County is located in the southwestern part of Xinjiang Uygur Autonomous Region, which includes the western part of the Tarim Basin, the eastern part of the Kashgar region, the southwestern edge of the Taklimakan Desert, the northern foot of the Karakoram Mountains, the lower reaches of the Yarkant River, and the lower reaches of the Tiznafu River (77◦28 –79◦05 east longitude, 38◦25 –39◦22 north latitude). This county has a temperate continental dry climate with sufficient sunshine, a large temperature difference between day and night, very little precipitation, hot summers and cold winters, and a windy and sandy spring. The average annual sunshine is 2836.5 h, the annual average temperature is 11.8 ◦C, and the annual average precipitation is 56.5 mm.

#### *2.2. Test Materials and Field Management*

Considering local production conditions, the high-performance film and ordinary polyethylene film with thicknesses of 0.008 mm and 0.01 mm were laid on the cotton test field in Maigaiti county on 30 April 2021. The film-laying site is shown in Figure 1. The planting mode of one film, which covered three pipes and six rows with 660 mm + 100 mm of machine-harvested cotton was adopted in the test field. The plant spacing was 12.5 cm, and routine management of the field was adopted for water–fertilizer management. The high-performance film was manufactured by Guangdong Siico Technology Co., Ltd., (Guangdong, China); the ordinary polyethylene film is manufactured by Xingnong Industry and Trade Co., Ltd. in Bayingolin Mongol Autonomous Prefecture, in Xinjiang province, China. The film-laying situation in the test field is shown in Figure 1.

**Figure 1.** Diagram of plastic film laying in test field.

#### *2.3. Test Design*

#### 2.3.1. Test Factors and Levels

Both high-performance film and ordinary polyethylene film are made from highmolecular compounds [17]. Therefore, at the same sampling spot, their tensile performance is affected mainly by natural erosion, material aging, material thickness, and material anisotropy [18]. Thus, the film-laying period, sampling position, film thickness, and sampling direction were chosen as the test factors in the test on the film tensile property.

Material anisotropy determines that different tensile properties are obtained by testing the film from different directions. Hence, the direction along the film-laying direction was defined as the horizontal direction, while the perpendicular direction of the film-laying direction was defined as the vertical direction. The degree of sunniness and the natural erosion effect on the different positions of the film (near and far away from the plants) may vary due to degree of shading of the cotton plants on the film, thus, the sample-taking positions on the film were divided into near-end positions and far-end positions.

#### 2.3.2. Test Indexes

According to the requirements of GB/T 1040.3-2006 Plastics—Determination of Tensile Properties, the elongation at break of the film and the tensile yield stress were taken as the test indexes, and the calculation method is as follows:

$$
\varepsilon\_t = \frac{L - L\_0}{L\_0} \times 100\% \tag{1}
$$

where *L* is the distance between the marked lines when the sample is torn off, mm; *L*<sup>0</sup> is the distance between the original graticule lines, mm.

$$
\sigma\_t = \frac{F\_b}{bd} \tag{2}
$$

where *Fb* is the breaking load of the sample, N; *b* is the sample width, mm; and *d* is the sample thickness, mm.

### 2.3.3. Determination of Test Parameters

The strain data sample frequency is obtained based on test speed, the ratio of the distance between the original graticule lines of the standard sample and the original clamp distance, and the minimum resolution of the obtained strain signal of the accurate data, and its calculation method is as follows:

$$f\_{\min} = \frac{vL\_0}{60L\_c r} \tag{3}$$

where *f* min is the sampling frequency of minimum strain data, Hz; *v* is the test speed, mm/min; *Lc* is original clamp distance, mm; and *r* is the minimum resolution of the obtained strain signal of the accurate data, mm.

According to the recommended test speed and the original clamp distance of the standard samples in GB/T 1040.1-2018, *v* = 10 mm/min, *Lc* = 115 mm, the CMT-6103 electronic universal testing machine, which is controlled by a microcomputer, obtained the minimum resolution of the obtained strain signal of the accurate data, which was 0.008 mm. After calculation, the sampling frequency of the minimum strain data was obtained, and *f* min = 9.06 Hz.

The load data sampling frequency is based on the test speed, strain range, minimum resolution of the obtained strain signal of accurate data, and the initial clamp distance, in which the elastic modulus, test speed, and clamp distance determine the load growth rate. The ratio between the load growth rate and the minimum resolution of the obtained strain

signal of accurate data determines the load data sampling frequency of the test machine. The calculation method is as follows:

$$f\_{force} = \frac{\stackrel{\bullet}{F}}{r} = \frac{v}{\Delta \varepsilon \times 60 \times L\_c \times 5 \times 10^{-3}}\tag{4}$$

where • *<sup>F</sup>* is the load growth rate, %, and <sup>Δ</sup>*<sup>ε</sup>* is the strain range of the samples. <sup>Δ</sup>*<sup>ε</sup>* = <sup>3</sup> × <sup>10</sup>−<sup>2</sup> was selected according to standard requirements, and the sampling frequency of the load data was calculated to be 9.66 Hz.

In this test, an extensometer is used as the strain indicating device, and it should be a Level 1 extensometer as required by GB/T 12160-2019, that is, the relative error of the gauge length is ±1%, the percent of reading is 0.5%, the absolute value is 1 μm, the relative error is ±1%, and the absolute error is ±3 μm.

In order to avoid the toe at the initial stage in the stress–strain curve, in measuring the related stress, the prestress on the sample before the test should satisfy Equation (5) as follows:

$$0 < \sigma\_0 \le \sigma^\* / 100 \tag{5}$$

where *σ*<sup>0</sup> is the prestress at the beginning of the test, MPa; *σ*∗ is the tensile yield stress of the material, MPa. In order to make the prestress at the beginning of the test adapt to the two types of film, *σ*∗ should be less than the lower value of the tensile yield stress of the two types of film; thus, *σ*<sup>0</sup> = 0.09 Mpa was selected [19].

#### 2.3.4. Sample Collection

The service period of the film laid on the cotton field of south Xinjiang in China is about 180 d. In order to reflect the tensile property variation process of the two types of film during their service periods, film samples were collected every 30 d from the filmlaying date to carry out the tensile property test; the samples were collected seven times. Each time, the sampling objects included two sets of high-performance film and ordinary polyethylene film of 0.008 mm and 0.01 mm in thickness, with a width of slightly more than 300 mm and a length of slightly more than 660 mm. After sample collection, the film samples were rinsed to remove the impurities for airing. On each selected sample film, eight standard tensile pieces were cut down by a cutter and used as test material, as shown in Figure 2. The size of the standard tensile film pieces is shown in Figure 3. During each instance of sample collection, the intact film sample pieces were obtained on dry, hard, flat land, and the sampling positions were marked on the film.

**Figure 2.** Schematic diagram of sampling location: 1—cotton plant, 2—vertical film sampling, 3—horizontal film sampling, 4—film sample piece, I—near-end position, II—far-end position.

**Figure 3.** Standard tensile sample of film.

#### 2.3.5. Test Scheme

Before the test, a low-power magnifying glass was used to check the test samples; the sample pieces with unsmooth and frayed edges or damages were eliminated to avoid test errors caused by stress concentration on the damaged parts of the sample pieces in the test. The CMT-6103 electronic universal testing machine controlled by a microcomputer was used to carry out a test on the film tensile property. According to Equations (1) and (2), the elongation at break and tensile yield stress of the film were calculated. The test was repeated four times, and test results were averaged. The test process is shown in Figure 4. Figure 4a shows the state of the sample after prestressing, and Figure 4b–d show the tensile process of the sample after loading.

**Figure 4.** Process of the tensile test of film. (**a**) shows the state of the sample after prestressing, (**b**–**d**) show the tensile process of the sample after loading.

#### **3. Test on Curl-Up Force in Film Collecting**

The curl-up residual plastic film collector is generally composed of the film pickup mechanism, film-guiding mechanism, film-curling mechanism, impurity separation mechanism, and film-unloading mechanism [20]. During operation, the film pick-up mechanism loosens the soil on the film surface on both sides of the film and separates the film from the soil [21]. Then, the film-guiding mechanism transmits the film to the impurity separation mechanism to the film-curling mechanism. The impurity separation mechanism separates the soil, roots, and stems from the film through vibration or sweeping. The film-curling mechanism curls up the film to a suitable size, and, finally, the film-unloading device unloads the residue film package after curling up.

In the test on the curl-up force during film collecting, by simulating the process of overcoming the force from the soil to the film during curl-up collecting of the residue film, the tensile stresses on the film while the curl-up film collector pulls up the film under different test factors were obtained. In collecting film, the film pick-up mechanism separates the film from the soil and forms a film pick-up angle *α*; the curl-up force *F* is formed in curl-up collecting film. The force between the film and soil under the effect of the curl-up force is shown in Figure 5. Since the soil on the film's surface at the slope has the tendency to move downwards, there is a friction *f* <sup>2</sup> from the film against the soil on the film at the slope. At the same time, the film is uncovered by the film pick-up mechanism along the film pick-up angle *α*. The cohesion force between the film and soil prevents the film from moving and forms a downward force *Fa* along the film pick-up angle *α*.

**Figure 5.** Diagram of force between plastic film and soil under the action of curl-up force: 1—soil under the film, 2—soil on the film, 3—film, 4—film-curling mechanism.

In Figure 5, *N*<sup>1</sup> is the support force from the soil and film on the flat ground to the soil on the film; *G*<sup>1</sup> is the gravity of the soil on the film; *N*<sup>2</sup> is the support force from the film at the slope to the soil on the film; and *G*<sup>2</sup> is the gravity of soil on the film at the slope. Then, the mechanics equilibrium equation during operation of the curl-up residual plastic film collector is established as follows:

$$\begin{cases} \quad F = F\_a + G\_2 \sin \alpha - f\_2\\ \quad N\_2 - G\_2 \cos \alpha = 0\\ \quad N\_1 - G\_1 = 0 \end{cases} \tag{6}$$

In order to prevent the film from being torn down due to the speed difference between the linear velocity of the film-curling mechanism and the advancing speed of the machine, the linear velocity of the curling speed should be equal to the advancing speed of the machine, and the speed should be uniform, so as to avoid tearing down the film with the rigid impact from an abrupt change in the film collecting speed. The test on the curl-up force in film collecting was carried out. By measuring the curl-up force *F*, the tensile stresses on film during the curl-up collecting process under different factor levels were obtained.
