Green Development of Natural Fibre-Based Paper Mulch from Recyclable Cow Dung and Flax Straw Waste

Abstract

Livestock dung, discarded crop straws, and residual plastic film are the primary agricultural non-point sources of pollution. For livestock dung and discarded crop straw, the general treatment focuses on compost, animal fodder, industrial raw material, and new energy. The development of degradable mulch film is the main way to solve pollution from residual plastic film. However, an effective way to solve the above three types of pollution simultaneously and use them for ecological circular agriculture has been less studied. In this study, using cow dung and flax straw wastes as raw materials, we prepared natural, fibre-based paper mulch using the rapid-Kothen method and analysed the film-forming mechanism. Based on the Van Soest method, the cow dung and flax straw waste contain abundant cellulose fibres: 36.75% and 54.69%, respectively. The tensile strength and tear strength of fibre paper mulch are 1.87 kN/m and 19.91 N/mm, respectively. To enhance the adaptability of the fibre paper mulch in humid environments, the surface of the mulch was treated with alkyl ketene dimer (AKD). The AKD-coated fibre paper mulch displays hydrophobic properties, indicated by a contact angle of 128° ± 2°. It has a wet tensile strength of 0.64 kN/m and a wet tear strength of 8.23 N/mm. Additionally, it exhibits a dry tensile strength and a tear strength of 2.13 kN/m and 16.43 N/mm, respectively. Notably, the dry tensile strength is increased by 16.31%. In this way, the livestock dung and discarded crop straw can be reused, reducing dung pollution and straw burning in livestock farms, and the final products can alleviate the residual film pollution simultaneously.

1. Introduction

Film mulching is an essential agricultural production technology [1] that improves crop yields and changes the agricultural production mode in water-deficient areas. Due to its significant warming, moisture retention, and yield-increasing effects [2,3], mulch film has been widely used worldwide. China is the country that uses the most mulch film and covers the largest area in the world. Northwest China is marked by arid and high-altitude climate characteristics, and a large amount of plastic film is particularly required for the production of economic crops in this area, such as vegetables, tobacco, medicinal herbs, and various bulk grain crops such as corn [4]. Traditional plastic mulch film is prepared using polyethene (PE) as the primary raw material, which is difficult to degrade and has caused severe environmental problems due to long-term application accumulation. For example, it seriously damages soil structure, affects crop water and fertilizer absorption, hinders crop root growth, and causes soil compaction [5,6]. The global polyethylene production was about 104.4 million tons in 2020 and is expected to reach 121.4 million tons in 2026 [7]. Plastics comprise various long-chain polymers from different sources, including natural gas, petroleum, and coal [8]. The incineration of plastic garbage also causes air pollution [7]. To address these environmental pollution issues, it is imperative to urgently develop alternative products that can substitute for traditional plastic films.

In the study of the material composition of degradable mulch film, it has been found that cellulose, as a green natural polymer material, can be thoroughly degraded, has good biocompatibility, and is low cost. Therefore, plant fibre paper mulch based on cellulose has become a research hotspot. Plant fibres mainly come from wood, crop straw (rice, corn, soybeans, flax straw, etc.), energy crops (sorghum, sugarcane, etc.), etc. [9,10]. Among them, wood and crop straw are also widely used in construction [11], papermaking [10], composite materials [12], fertilizers [13], feed, energy, and other fields. If plant fibre paper mulch takes a vital role in future, it will mean a significant consumption of plant fibres, so further expansion and research are needed regarding the source of plant fibres.

A sharp increase in cow breeding bases with the rapid development of the animal husbandry industry has generated a large amount of cow dung. The accumulation of cow dung generates greenhouse and foul gases, seriously affecting air quality [14], and it is also a source of groundwater pollution, disrupting the ecological balance [15,16,17]. Among them, methane, as one of the main sources of greenhouse gas, is produced through the open-air fermentation of cow dung. This portion of greenhouse gas emissions accounts for 19% [15] of all emissions. Therefore, livestock dung has become the primary pollutant in the rural environment, and there is an urgent need for harmless treatment and resource utilization. Cow dung is rich in phosphorus content [18]. Currently, cow dung is still used for fertilization [19] and fermentation [20], and its applications in other fields include clean energy [21,22], sewage treatment [21,22], etc. Recent research has shown that livestock dung consists of plenty of fibres, especially cow dung [14]. This is because the digestion process of cows is exceptional. The forage eaten by cows is treated with microorganisms in the rumen, equivalent to the pre-treatment of forage fibres. It makes the fibre content in cow dung different from that in grass feed. The fibres in the cow dung are typical straw fibre, which can be used as a new source of plant fibres. The extraction of cellulose fibres from cow dung, in turn, provides the possibility for the resource utilization of cow dung [23,24].

Linum usitatissimum, or flax, is an annual or perennial herb. It is one of the crucial characteristic oil crops in the northwest and northern regions of China [25], among which the Gansu Province is one of the main producing areas in China. Flax straw fibres are mainly located in the phloem of the stems [26]. They are extractable, renewable, biodegradable, and have excellent mechanical properties (friction resistance, high strength, etc.), and UV protection, which can be used as reinforcement for composite materials [27] and also in the textile industry [28]. In recent years, some researchers have used flax straw fibres as a raw material for the preparation of degradable mulch film [29], which has good film-forming properties. Still, flax straw fibre products have high production costs and low popularity due to their complex preparation process (needle-punched nonwoven technology). Although flax straw fibres have many applications, in rural areas of China, most flax straw is burned, polluting the environment and wasting resources. Straw burning has therefore become another primary pollution source in rural areas.

In the present work, we used cow dung and flax straw as raw materials. We adopted a simplified papermaking process to prepare natural, fibre-based paper mulch. We characterized the physical and chemical structures of the pulps and fibre paper mulches. Furthermore, we explained the film-forming mechanism of the fibre paper mulch based on its mechanical properties. For better environmental adaptability, we also carried out surface modification on the fibre paper mulch. No harmful chemical additives were added during the whole preparation process, and the product can be directly returned to the field after being used. In addition, this mixed utilization of cow dung and flax straw wastes as paper mulch has opened up a new direction for the source and application of plant fibres and compensated for the high-cost problem of flax straw as a single raw material. This resource utilization of cow dung and flax straw can solve the pollution problems caused by livestock dung, crop straw burning, and the use of PE films. After the degradation of the fibre paper mulch, the final product should be a high-quality organic fertilizer, which may avoid excessive use of fertilizers and pesticides. At the same time, the addition of cellulose fibres from cow dung can compensate for the high cost of flax straw fibre products, so the obtained fibre paper mulch has more potential to replace agricultural plastic mulch film and promote the sustainable development of agriculture.

2. Materials and Methods

2.1. Materials

Cow dung, after it was cleaned to remove impurities (sand, soil, etc.), was purchased from the Huarui Agricultural Company in the Minle County Ecological Industrial Park, Zhangye City, Gansu Province, China. Flax straw (Variety: Dingya No. 23, with a flax content of 40 ± 2%) was purchased from the Xizhai Oil Testing Station in Dingxi City, Gansu Province, China.

The reagents used in the experiment are as follows: Grass ash (weakly alkaline), disodium ethylenediaminetetraacetic acid (C₁₀H₁₄O₈Na₂·2H₂O, also known as EDTA-2Na, AR), sodium borate (Na₂B₄O₇·10H₂O, AR), sodium dodecyl sulfate (C₁₂H₂₅NaO₄S, AR), ethylene glycol ether (C₄H₁₀O₂, AR), Na₂HPO₄(AR), hexadecane trimethylammonium bromide (C₁₉H₄2BrN, AR), H₂SO₄ (72%), H₂SO₄(98%), decalin (C₁₀H₁₈, AR), Na₂SO₄, acetone (CH₃COCH₃, AR), petroleum ether (C₆H₁₄, AR), alkyl ketene dimer (AKD, MX-103, solid content 15 ± 0.5%, Mingxiang Chemical Technology Group Co., Ltd., Qingzhou, China).

2.2. Test Methods

2.2.1. Composition Determination

The contents of crude fibre (CF), neutral washing fibre (NDF), acidic washing fibre (ADF), hemicellulose (HCEL), cellulose (CEL), acidic washing lignin (ADL), and acid-insoluble ash (AIA) in cow dung and flax straw were determined using the polyester fibre filter bag method combined with the Van Soest cellulose analysis method [30]. The measurement process is shown in Figure 1.

A total of 0.5 g of each crushed sample to be tested was weighed and placed in a polyester mesh bag. Each sample was divided into three parallel groups, and a blank control group was set up. After weighing, we used nylon ropes to tie the filter bags tightly. The samples were treated by boiling with a neutral detergent, and the insoluble residue was NDF, which includes hemicellulose, cellulose, acid-insoluble lignin, and silicate. The samples were treated with acid detergent, and the remaining residue was ADF, which includes cellulose, acid-insoluble lignin, and silicate. The difference between NDF and ADF is the HCEL content. The residue of acid-washed fibres after 72% sulfuric acid hydrolysis was acid-insoluble lignin and silicate. The residue was dried and ashed in a muffle furnace at 550 °C for 3 h. The component that escaped during the ashing process was the ADL. The content of each element was calculated using the following formula:

NDF (%):

$$\mathrm{N}\mathrm{D}\mathrm{F}\left(\mathrm{\%}\right)=\frac{m_2-m_1}{m}\times 100\%$$

(1)

ADF (%):

$$\mathrm{A}\mathrm{D}\mathrm{F}\left(\mathrm{\%}\right)=\frac{m_3-m_1}{m}\times 100\%$$

(2)

HCEL (%):

$$\mathrm{HCEL}\left(\%\right)=\mathrm{NDF}\left(\%\right) - \mathrm{ADF}\left(\%\right)$$

(3)

ADL (%):

$$\mathrm{A}\mathrm{D}\mathrm{L}\left(\mathrm{\%}\right)=\frac{m_4-m_5}{m}\times 100\%$$

(4)

In the formula, m is the mass of the sample, m₁ is the mass of the filter bag, m₂ is the total mass of neutral washing fibre residue and filter bag, m₃ is the total mass of acidic washing fibre and filter bag, m₄ is the filter residue after acid hydrolysis, and m₅ is the mass of acid insoluble ash.

In the above experimental process, the yields of NDF, ADF, and ADL in cow dung were 86.1 ± 0.5%, 54.5 ± 0.5%, and 17.7 ± 0.5%, respectively. The yields of NDF, ADF, and ADL in flax straw were 81.9 ± 0.5%, 65.8 ± 0.5%, and 11.1 ± 0.5%, respectively.

2.2.2. Preparation of Paper Mulch Samples

The sample preparation process is shown in Figure 2. The cow dung was based on the digestion treatment of the cow’s stomach, so we only steamed the flax straw (under a pressure of 0.6 MPa and a temperature of 160 °C for 30 min). The cow dung and flax straw, which had been treated beforehand, were pulped using a Wali beater (model TD6-23, manufactured by Xianyang Tongda Light Industry Equipment Co., Ltd., located in the Xianyang, Shaanxi Province, China) at concentrations of 30.0 g/L and 17.0 g/L, respectively. During the beating process, the load was gradually increased (1, 1.5, 2.5, 4, and 4.5 kg). The beating degree of the pulp was measured every 15 min using a beating degree tester (TD9-M, Xianyang Tongda Light Industry Equipment Co., Ltd., Xianyang, Shaanxi Province, China). The degree of beating was determined using the method outlined in GB/T 3332-2004 [31]. We conducted experiments using pulp at various beating degrees (cow dung: 33, 37, 41, 45, and 49 °SR; flax straw fibre: 73, 77, 81, 85, and 89 °SR), with paper weights of 50, 60, 70, 80, and 90 g/m², and different amounts of flax straw fibres were added (35%, 45%, 55%, 65%, 75%). The paper-forming machine (TD10-200 Xianyang Tongda Light Industry Equipment Co., Ltd., Xianyang, Shaanxi Province, China) was used to prepare the samples at a drying temperature of 80 °C. In our previous work, we used the four-factors and three-levels centre combination test method to confirm and optimise the parameters of the preparation process. The optimal process parameters were determined: the beating degree of cow dung fibres was 37 °SR, the beating degree of flax straw fibres was 85 °SR, the paper basis weight was 80 g/m², and the addition of flax straw fibres was 65% [32]. We prepared samples using the rapid-Kothen method [33] under the same experimental conditions and let the prepared paper mulch stand for 24 h at a room temperature of 25 °C and a relative humidity of 30~40% before conducting performance measurements.

The purchased AKD lotion was diluted with distilled water to about 3.5%, and then was evenly brushed on the surface of the prepared paper mulch; the paper mulches were placed in the drying oven and dried at 65 °C, and the samples in the dryer were used in subsequent tests.

2.2.3. Measurement and Characterization of the Pulps

(1)

Fourier transform infrared (FTIR) spectroscopy analyses:

The functional group content of the mixed pulp of the cow dung and flax straw was scanned and analysed using an FTIR (iS50, Thermo Fisher Scientific, Waltham, MA, USA), with a resolution of 2 cm⁻¹ and was recorded in the range of 4000 to 640 cm⁻¹.

(2)

X-ray diffraction (XRD) analyses:

The phase and crystallinity of the mixed pulp were determined by XRD (D8 ADVANCE, Bruker, Germany) under experimental conditions of 40 kV, 30 mA, and 2θ = 5°–90°, monochromatic Cu-Kα radiation. The scanning speed was 2°/min, and the sampling interval within the scale area was 0.01°. Based on the empirical Formula (5) [14], the crystallinity index C_rI of the sample was determined.

$$C_{r}I=\frac{I_{200}-I_{\mathrm{am}}}{I_{200}}\times 100\%$$

(5)

Among them, I₂₀₀ was the maximum diffraction intensity on the (200) plane, and I_am was the intensity diffraction at approximately 2θ = 18°.

(3)

Scanning electron microscopy (SEM) analyses of the pulps:

The microstructure of cow dung pulp, the flax straw pulp, and the mixed-fibre pulp were observed by using an SEM (S-3400N Suzhou Sainz Instrument Co., Ltd., Suzhou, China). All samples were treated with gold spray before detection (we first placed the sample in the gold spray chamber. When the pointer on the vacuum gauge dial was observed to be below 10, the sample was subjected to gold spraying treatment, which takes approximately 10 s).

2.2.4. Measurement and Characterization of the Fibre Paper Mulch

(1)

Thermogravimetric analyses (TGA)

The thermal stability of the mixed-paper mulch of the cow dung and flax straw was analysed using a TGA (DSC 214, NETZSCH) in a nitrogen environment (flow rate of 2525 mL/min) at an initial temperature of 30 °C, a maximum temperature of 800 °C, and a heating rate of 5 °C/min.

(2)

SEM analyses of the paper mulch samples:

The microstructure of the surface of the pure cow dung fibre paper mulch (CDPM), the pure flax straw fibre paper mulch (FSPM), and the mixed-fibre paper mulch (MPM) were observed by an SEM under 5 kV working conditions. All samples were treated with gold spray before detection (The spraying process was the same as (3) in Section 2.2.3).

(3)

Mechanical properties testing:

Based on the standards of GB/T 35795-2017 [34], GB/T 12914-2018 [35], GB/T 1040.1-2018 [36], and QB/T 1130-1991 [37], an E43.104 electromechanical universal testing machine (accuracy level: 0.5 level; Shenzhen MTS Systems Co., Ltd., Shenzhen, Guangdong Province, China) was used to conduct tensile and tear tests on the samples at a speed of 20 mm/min and a room temperature of 25 °C. To ensure repeatability, eight experiments were conducted in both vertical and horizontal directions, and the data were taken with a precision of 0.01 N.

Tensile strength:

$$S=\frac{F}{{\omega }_i}$$

(6)

where S is tensile strength, kN/m; F is the tensile resistance, N; and ${\omega }_i$ is the initial width of the sample, mm.

$$T_s=\frac{F}{d}$$

(7)

where $T_s$ is the tear strength, N/mm; F is the tear load, N; and d is sample thickness, mm.

2.2.5. Measurement and Characterization of the Physical Properties of AKD-Coated Paper Mulch

(1)

Contact angle testing:

The contact angles of the MPM before and after treatment were tested using a contact angle measuring instrument (SDC-200S, Dongguan Shengding Precision Instrument Co., Ltd., Dongguan, China), and the droplet size of the contact angle was 4 μL. The droplets were made using laboratory-made deionized water. To reduce experimental errors, 10 points on the sample’s surface were randomly selected for the contact angle during testing, and the test results were averaged.

(2)

Three-dimensional roughness analyses:

A three-dimensional, non-contact surface profiler (OLS5000, Olympus, Tokyo, Japan) was used to observe the surface roughness of the MPM. We made the paper mulch sample into an observation sample of 5 mm × 5 mm.

(3)

X-ray photoelectron spectroscopy (XPS) analyses:

The types of O, C, N, and Cl elements on the surface of the fibre paper mulch were analysed using an XPS (Nexsa G2, Thermo Fisher Scientific, Waltham, MA, USA).

(4)

Mechanical properties testing:

The method steps were the same as (3) in Section 2.2.4.

(5)

TGA analyses:

The method steps were the same as (1) in Section 2.2.4.

3. Results and Analyses

3.1. Component Analyses

Table 1. The fibre composition analyses of the raw materials.

Name of Material	Indicators of Components
	Hemicellulose (%)	Cellulose (%)	Acid Detergent Lignin (%)	Acid Insoluble Ash (%)	Bibliography
Cow dung	31.6	36.8	16.4	0.9	-
Flax straw	16.1	54.7	12.1	0.7	-
Wheat straw	26.0	51.0	16.0	-	[41]
Sorghum straw	24.43	41.50	15.04	-	[42]
Oat straw	-	31.0–48.0	16.0–19.0	-	[41]
Alfalfa	38.5	45.4	14.9	-	[38]
Rice straw	19.5	41.2	21.9	-	[10]

shows the fibre composition analyses of the cow dung pulp, the flax straw pulp, and other crop straws (wheat, sorghum, oats, etc.). The results show that the cellulose content of the cow dung and flax straw pulp is 36.8% and 54.7%, respectively; the hemicellulose content is 31.6% and 16.1%, respectively; the lignin content is 16.4% and 12.1%, respectively; and the ash content is only 0.9% and 0.7%, respectively. The cellulose content in the flax straw pulp is higher than that of other crop straws, and the hemicellulose content is lower than that of other crop straws but is close to the hemicellulose content of rice straw (19.5%). The ADL is slightly lower than that of other crop straws. The cellulose content of cow dung pulp is similar to that of rice straw (41.2%), oat straw (31.0–48.0%), and sorghum straw (41.5%), while the ADL (16.34%) is lower than that of rice straw (21.9%) and is similar to that of wheat straw (16.0%), sorghum straw (15.04%), oat straw (16.0–19.0%), and alfalfa (14.9%). The content of various components in the material directly affects the film-forming properties of the paper mulch. The higher the cellulose content, the better the mechanical properties of the mulch are [38]. Hemicellulose increases the strength of the paper mulch, especially its tensile strength [38]. The lower the lignin and ash content, the more favourable film-forming properties its pulp shows [39,40]. Therefore, cow dung and flax straw can be used as raw materials for mulch preparation.

3.2. Analyses of the Pulps

3.2.1. FTIR Analyses of the Mixed Pulp

Figure 3a shows the FTIR spectrum of the mixed pulp of the cow dung and flax straw. It is evident from the spectrum that the sample mainly absorbs infrared spectra in the regions of 800~1750 cm⁻¹ and 2800~3600 cm⁻¹, which indicates that the sample contains cellulose, hemicellulose, and lignin [14]. The FTIR spectrum of the samples shows that the absorption band at 3320 cm⁻¹ is attributed to the hydroxyl (-OH)-stretching vibrations of cellulose and lignin [43]. The aliphatic-saturated C-H group produces stretching vibrations on cellulose, hemicellulose, and lignin fractions near 2915 cm⁻¹ [44]. The absorption peak at 1731 cm⁻¹ is due to the stretching vibration of the C=O bond of the unconjugated ketones, which is characteristic of hemicellulose [45]. The acetyl-stretching band of hemicellulose is at 1124 cm⁻¹. At 1512 cm⁻¹, there is a stretching vibration of the lignin benzene skeleton [46]. In addition, the peak at 1031 cm⁻¹ is related to the stretching vibration of the cellulose pyran ring [46], and the peak at 896 cm⁻¹ is related to the b-glycosidic bond between monosaccharides and the β-glycosides in cellulose [14]. The presence of these characteristic peaks indicates that there are typical cellulose, hemicellulose, lignin, and amorphous structures in the cow dung pulp and flax straw pulp, which is consistent with the results determined by the Van Soest method.

3.2.2. XRD Analyses of Mixed Pulp

The XRD pattern of the mixed-pulp sample is shown in Figure 3b. The XRD pattern of the mixed-pulp sample exhibits broad diffraction peaks. The four diffraction peaks of Bragg’s angle (2θ) around 15.0°, 16.0°, 22.4°, and 34.0° can be assigned to the 101, 10Î, 002, and 004 reflections, respectively [47,48]. The diffraction peak of the sample at 2θ = 15.0°, 22.4° attributed to the (101) and (002) crystal planes, correspond to the diffraction patterns for cellulose I according to French and Azubike et al. [49,50]. The diffraction peak of the sample at 2θ = 16.0°, 34.0° attributed to the (10Î) and (004) crystal planes, correspond to the diffraction patterns for cellulose II [48,49]. Calculations based on the Formula (5) show that the degree of crystallinity of the mixed pulp is about 57.25%. To some degree, the crystallinity of the cellulose reflects the physical and chemical properties of the fibres. In general, the tensile strength and stiffness increase with an increase in the crystallinity of the cellulose [51]. Meanwhile, the broad diffraction peaks and the relatively high-back bottom may come from the amorphous structure of the cow dung and flax straw fibres, which are consistent with the literature that reported that cellulose includes both crystalline and amorphous regions [48].

3.2.3. The SEM Analyses of the Three Kinds of Pulps

Figure 3c–e shows the SEM images of the cow dung pulp, the flax straw pulp, and the mixed pulps. It can be seen from Figure 3c that the fibres in the cow dung pulp as a whole have a beehive structure, presenting a thin-walled and porous morphology, and the surface is relatively smooth and free of burrs. As shown in Figure 3d, the fibres in the flax straw pulp are filamentous, with long, thin single fibres, and the fibres are entangled and curled to form flocculent groups or fibre bundles [52]. Figure 3e shows the SEM image of the mixed pulps. The holes in the cow dung fibres disappear, and the interpenetration and superposition between the flax straw and the cow dung fibres make the voids smaller and form a dense, sheet-like structure.

From the SEM images of the pulps (Figure 3c–e), it can be seen that the fibres in the cow dung pulp do not appear to have obvious filamentation, the fibres are relatively dispersed and the combination of the fibres is weak. After the pulping of the flax straw, the filamentation of the broom occurs in the fibres. The fibres in the flax straw pulp are intertwined to form a complex network structure, which provides a possibility for the fibre combination of the cow dung pulp. The mutual filling between the two pulps forms a dense, sheet-like structure.

3.3. The Characterization and Performance of the Paper Mulch

3.3.1. The TGA Analyses of the MPM

The results of the tests performed on the thermal stability and the FTIR spectrum of the MPM are shown in Figure 4. The TG curves in Figure 4 indicate that the first weight loss phase of the MPM samples arises from 10 °C to 110 °C, corresponding to the loss or evaporation of water adsorbed by low-molecular-weight compounds in the samples during the heating process [47]. The mass loss of this region is about 7%. At 110 °C, the dissociation and desorption of water molecules and other adsorbents are completed. The MPM sample shows a stable trend at 120–220 °C, indicating no thermal decomposition and weight loss. Due to the different chemical structures of hemicellulose, cellulose, and lignin, they decomposed at different temperatures [53]. The second thermal degradation process of the MPM sample takes place with an initial degradation temperature of about 210 °C, corresponding to the decomposition of hemicellulose [44] and a total mass loss of about 71%. The third stage of the MPM sample occurs at approximately 390 °C, corresponding to the decomposition of the cellulose [53] and a mass loss of roughly 7%. A broad peak is observed in the MPM between 220 °C and 390 °C, which is the pyrolysis of lignin [53,54].

The X-Ray diffraction (XRD) test results (Figure 3b) demonstrate the presence of cellulose I, cellulose II, and an amorphous structure within the mixed pulps. Additionally, both cellulose I and cellulose II exhibit hydrophilic groups. Hence, the water absorption/release shows rapid weight loss during the first stage of the heating process 47. The main thermal degradation temperature range of hemicellulose is 210–315 °C, and the initial decomposition temperature of cellulose (in the temperature range of 310–400 °C) is higher than that of the hemicellulose 48. This is because cellulose has a tight and ordered molecular structure, which is composed of a long chain of unbranched glucose polymers [55]. The broad peak between 220 °C and 390 °C is due to the complex aromatic ring and wide coverage of the chemical bond activity of lignin. Therefore, its temperature degradation range covers the entire thermal degradation process, which is also why it is difficult to find the thermal decomposition peak of lignin in DTG spectra [53,56]. The components in the paper mulch are relatively stable. According to the TG and DTG analysis results, the initial pyrolysis temperature of the MPM sample is 298 °C, as compared to the microcrystalline cellulose composite material (initial pyrolysis temperature of 240 °C) [57] and the garlic peel fibres (initial pyrolysis temperature of 235 °C) [58]. After comparing and analyzing the FTIR spectrum of the mixed pulp and MPM, it was found that there was no significant change, indicating that there was no fibre damage or reaction during the fibre paper mulch-forming process. The MPM sample has better thermal stability and environmental adaptability.

3.3.2. SEM Analyses of the Fibre Paper Mulch

Figure 5a–d shows the SEM images of the surface and cross-section of the base fibre paper mulch. As shown in Figure 5a, the morphology of CDPM has uneven fibre distribution on the surface of the paper mulch, with coarse fibres, significant fibre gaps, and numerous pores, which indicate a weak bonding effect. As shown in Figure 5b, it is the SEM image of the FSPM. It can be observed that the fibre structure on the surface of the paper mulch is complex and tight, indicating a well-bonded effect. However, there are still some tiny voids in local areas, and the surface is relatively rough and uneven. As shown in Figure 5c, the SEM image of the MPM sample shows that the pores on the surface of the paper mulch have disappeared, the surface is relatively flat and dense, and no coarse fibres exposed on the surface can be observed in the cow dung and flax straw pulp. The cross-sectional SEM image of the MPM in Figure 5d shows that the internal structure of the paper mulch prepared by mixing the two types of pulp is dense, and there are no apparent gaps.

The CDPM (Figure 5a) was made by pressing stacked cow dung fibres under a vacuum. However, the fibre structure is single, and there is no branching or brooming. Therefore, the paper mulch structure is not dense, reflecting the poor film-forming performance of pure cow dung fibres. In the FSPM (Figure 5b), the fibres in the flax straw pulp exhibit significant fibre splitting and brooming (Figure 3d), so the binding effect among the fibres is better. However, its surface is rough and uneven, and there are small voids; this is because the flax straw fibres are easy to entangle into a group and uneven dispersion. The surface of the MPM is flat, the fibres are evenly dispersed, and the structure is dense (Figure 5c). This is because the cow dung pulp fills the network gaps among the flax straw fibres (as shown in Figure 3e). The high film-forming effect of the flax straw fibres effectively promotes the bonding effect of the cow dung pulp, and a dense layered structure is formed during the vacuum-pressing process of the rapid-Kothen method. The simultaneous mixing of the two types of pulp provides the possibility for cow dung fibres to be used as a kind of raw material for paper mulch, which offers a new way of utilizing cow dung. Moreover, it reduces the use of flax straw, which can effectively reduce the preparation cost of paper mulch.

3.3.3. Mechanical Properties of the Three Kinds of Base Fibre Paper Mulch Samples

(1)

Tensile strength analyses of three kinds of samples:

The paper basis weight is 80 g/m², and the tensile strength of the CDPM, PSPM, and MPM are 0.43 kN/m, 2.11 kN/m, and 1.85 kN/m, respectively. Through thermogravimetric analysis of the MPM, it was found that the drying temperature of the paper mulch (80 °C) is not reached at the decomposition temperature of fibre polymers (298 °C). Therefore, the molecular structure of these three mulches is not damaged during the paper filming process, indicating that no chemical reaction occurs during the process, but only a physical combination under external force. However, the MPM shows higher mechanical properties than those of the CDPM. According to the above SEM analyses, the bonding effect among the cow dung fibres is unsatisfactory, indicating that the mechanical properties of cow dung fibre mulch are limited. However, the separation and brooming of the flax straw fibres are apparent, and the bonding effect among the fibres is significant. Therefore, after mixing the two types of pulp, a dense fibre paper mulch is formed, indicating the key role of the addition of the flax straw fibres (rich in cellulose) and the interface adhesion with the cow dung fibres [59], which significantly improves the strength of the paper mulch. Therefore, compared with other lignified plant fibres such as jute or kenaf, flax has better performance.

(2)

Tear strength analyses of three kinds of samples:

Figure 5f shows the tear force-displacement curves of the CDPM, the FSPM, and the MPM. These three kinds of samples were subjected to tear tests on an electromechanical universal testing machine (E43.104), and the torn paper mulch samples broke at the maximum tear stress, with maximum tear strengths of 4.08 N/mm, 21.84 N/mm, and 19.34 N/mm, respectively. From the tear force-displacement curve, it can be seen that the CDPM, FSPM, and MPM sample tears at relative displacements of 0.25 mm, 0.51 mm, and 0.47 mm, respectively. Therefore, the strength of the CDPM is relatively low and tends to tear apart more quickly as compared to the other two samples. The MPM sample has a tear strength close to that of the FSPM. This is because the tear strength is greatly affected by the strength of the fibres themselves and the comprehensive force among the fibres. Because of the weaker bonding effect and lower fibre strength of the cow dung fibres compared to the flax straw fibres, the tear strength of the CDPM is inevitably lower than that of the MPM. The addition of flax straw fibres changed the internal fibre structure of the paper mulch sample and promoted the film-forming of the cow dung fibres.

3.4. The Performance and Characterization of the Fibre Paper Mulch before and after Coating with AKD

3.4.1. Contact Angle of the Fibre Paper Mulch before and after Coating with AKD

Testing the static contact angle on the surface of the paper mulch is a critical indicator for measuring its hydrophobicity. Figure 6 depicts the behaviour of water contact and separation on the paper mulch’s surface. As shown in Figure 6a, the contact angle between the base fibre paper mulch sample and the droplet is 50 ± 2°. During the experiment, it was observed that the droplet easily permeates when it comes into contact with the surface of the paper mulch. In contrast, Figure 6b illustrates that the AKD-coated paper mulch has a contact angle of 128 ± 2°. The experiment revealed that droplets are not easily adhered to the surface when the AKD-coated paper mulch comes into contact with water.

The results of the contact angle test showed that the base fibre paper mulch does not have hydrophobic properties. The FTIR spectrum shown in Figure 3a indicates that the outermost surface of the base fibre paper mulch is enriched in hydrophilic groups (-OH at 3320 cm⁻¹ in the infrared spectrum), which also show significant hydrophilicity [60]. The contact angle of the AKD-coated paper mulch is 128 ± 2°, which is increased by 78 ± 2° compared to the base fibre paper mulch. Therefore, the AKD-coated paper mulch has hydrophobic properties. We observe in Figure 6d that the contact of liquid droplets on the surface of the AKD-coated paper mulch conformed to Cassie’s model [61]. The results show that the AKD-coated paper mulch possesses water resistance and can be more suitable to the climatic conditions and production environment.

3.4.2. The Roughness Analyses of the Fibre Paper Mulch before and after Coating with AKD

Figure 7a,b shows the three-dimensional morphology of the paper before and after coating with AKD. The arithmetic mean height Sa of the surface of the base fibre paper mulch measured by the surface roughness measuring instrument is 8.962 μm (Figure 7a), the surface arithmetic average height Sa of the fibre paper mulch sample coated with AKD is 10.515 μm (Figure 7b), and the surface roughness of the fibre paper mulch increased by 1.553 μm after the coating. The roughness value of a paper mulch is related to its Sa value, meaning that the higher the Sa value, the greater its roughness will be. AKD adheres to the fibres’ mulch surface, preventing direct contact between the water droplets and the surface of the paper mulch. A rough structure is constructed on the surface of the paper mulch, which forms a Cassie model after contact with water droplets, thereby increasing the contact angle between the surface of the paper mulch and water droplets and improving the hydrophobicity of the base fibre paper mulch. This is consistent with the test results of the contact angle on the surface of the paper mulch (Figure 6).

3.4.3. The XPS Analyses of the Fibre Paper Mulch before and after Coating with AKD

To further determine the surface characteristics of the fibre paper mulch, the XPS analyses were conducted on the fibre paper mulch before and after coating with AKD. Figure 7c shows the XPS full spectrum of the surface of the paper mulch sample. The paper mulch without the AKD coating shows O1s, N1s, and C1s peaks, and the AKD-coated paper mulch shows O1s, N1s, C1s, and Cl2p peaks. Figure 7d displays the XPS spectra of the base fibre paper mulch surface, revealing the C1, C2, and C3 peaks at 284.7 eV, 286.2 eV, and 287.7 eV, respectively. The C1 peak corresponds to C-C/C-H [62,63], C2 corresponds to C-O, and C3 corresponds to O-C-O/C=O [44,57]. The XPS spectrum of the AKD-coated paper mulch is shown in Figure 7e. The fitting results show that C1 and C2 have two peaks, and C1 is significantly enhanced.

The presence of C-C/C-H is attributed to AKD, C-O is associated with cellulose, and C=O is indicative of hemicellulose, which is consistent with the analysis results of FTIR of the mixed pulp (infrared patterns at 1731 cm⁻¹ and 2915 cm⁻¹, etc.). The N1s exist on the fibres’ surface, probably due to elemental N in the cow dung and the flax straw [64]. After coating with AKD, the XPS spectrum of the fibre paper mulch sample undergoes significant changes, and the intensity of the C1 peak is significantly enhanced. The weakening of the C2 and C3 peaks is because after coating the surface of the paper mulch with AKD, the cellulose on the surface of the paper mulch is covered. Meanwhile, there is a Cl2p peak on the surface of the AKD-coated paper mulch, and the Cl element mainly comes from AKD [65]. All XPS data here have also been corrected for C1s spectra.

3.4.4. Mechanical Properties of the AKD-Coated Paper Mulch

Figure 8 shows the dry and wet tensile strength and tear strength of the AKD-coated paper mulch. The comparison of the fibre paper mulch strength before and after applying AKD is shown in

Table 2. Comparison of the fibre paper mulch strength before and after AKD coating.

Sample		The Base MPM	The AKD-Coated Fibre Paper Mulch
Dry sample	The tensile strength (kN/m)	1.85	2.13
	The tear strength (N/mm)	19.34	16.43
Wet sample	The tensile strength (kN/m)	-	0.64
	The tear strength (N/mm)	-	8.23

. The dry tensile strength of the AKD-coated paper mulch is 2.13 kN/m, and the dry tear strength is 16.43 N/mm. Compared with the base MPM (the tensile and tear strength are 1.85 kN/m and 19.34 N/mm, respectively), the dry tensile strength is increased by 15.14%, and the tear strength is decreased by 15.05%. The wet tensile strength is increased to 0.64 kN/m, and the wet tear strength is increased to 8.23 N/mm (the wet strength of the base fibre paper mulch is almost undetectable). According to data analysis, coating with AKD can improve the dry tensile strength of the paper mulch to a certain extent, but the dry tear strength slightly decreases. At the same time, the wet mechanical strength is significantly enhanced. This is because the AKD molecules contain hydrophobic and reactive groups, which react with the carbonyl groups of fibres to form covalent bonds [66,67,68], creating a stable film on the fibres’ surfaces [69]. It further binds the fibres on the surface of the paper mulch, enhancing the comprehensive force among the fibres and increasing the tensile strength. However, the resulting film is brittle in a dry state, resulting in a slight decrease in the tear strength. On the other hand, the hydrophobic groups of AKD exist on the fibres’ surfaces, forming a thin film that blocks direct contact between the fibres and the water [69]. In this experiment, it was found that the film formed by the AKD coating’s wet state has a certain degree of toughness. Therefore, under the binding effect between the AKD film and the paper mulch fibres, the wet strength of the paper mulch is improved. This indicates that the coating of AKD can improve the mechanical strength of fibre paper mulch as a whole, and its dry tensile strength is comparable to the strength of the mulch studied by Coconut Husk [40], Koray-Gulsoy [70], etc. It can effectively enhance the mulch’s adaptability in agricultural production environments, meet production requirements, and be used in the agricultural field.

4. Conclusions

Based on the fibre composition analyses, cow dung and flax straw contain abundant cellulose fibres. However, as a raw material for preparing paper mulch, the cow dung fibres exhibit unsatisfactory film-forming properties. When combined with flax straw fibres, the characteristic results clearly show that the mixed pulps have a more significant bonding effect among fibres than single pulp fibre, and the mechanical properties of MPM are also higher than CDPM’s properties. The strength of MPM is close to that of FSPM, with a tensile strength of 1.87 kN/m and a tear strength of 19.91 N/mm. However, the base MPM exhibits significant hydrophilicity, and the contact angle is only 50.1° and is easily destroyed in humid environments. To better adapt to the climatic conditions and production environment, the AKD was coated on the sample’s surface, and the contact angle was increased to 127.6 ° and shows good hydrophobicity. The dry tensile strength and the dry tear strength are 2.13 kN/m and 16.43 N/mm, respectively. For the AKD-coated paper mulch, except for the increase of dry strength, the wet tensile strength and tear strength were also increased to 0.64 kN/m and 8.23 N/mm, respectively. In addition, no harmful chemical additives were added during the whole preparation process, and the product can be directly returned to the field after being used. This mixed utilization of cow dung and flax straw wastes as paper mulch film has opened up a new direction for the source and application of plant fibres and helps to compensate for the high cost problem faced by flax straw as a single raw material. The final paper mulch film has great potential for simultaneously solving the three main types of pollution (livestock dung, discarded crop straws, and residual plastic film) in rural areas and being used for ecological circular agriculture. Afterward, we will further modify the fibre paper mulch and conduct field experiments to determine its degradation performance and its impact on the soil after degradation.