Biological Enzymatic Hydrolysis—Single Screw Co-Extrusion Treatment to Improve the Mechanical Properties of Biodegradable Straw Fiber Mulching Films

Abstract

Biodegradable agricultural films manufactured with straw serve as a viable substitute for plastic films, effectively addressing the issue of white pollution. However, existing biodegradable straw fiber films exhibit insufficient mechanical properties, primarily characterized by their susceptibility to fracture damage. To address this issue, a novel method for the preparation of film raw materials was proposed, which employs the synergistic treatment of bioenzymes and a single screw extruder, with the aim of enhancing the mechanical properties of the film. The method begins with the application of microbial agents to pretreat the straw, for improving its fiber morphology and inducing beneficial physicochemical structural changes. Subsequently, single screw extrusion technology is employed to further enhance the quality of the straw fibers and the mechanical performance of the film. The bio-mechanical pulp produced with this method demonstrated an increase in the crystallinity index (CrI) from 50.33% to 60.78%, while the degree of polymerization (DP) decreased from 866.51 to 749.60. Furthermore, the tensile strength, tear strength, and burst strength of the fiber covering film increased by 35.74%, 16.22%, and 11.65%, respectively, which meet the mechanical durability requirements for farmland mulching. This research effectively mitigates agricultural white pollution by converting agricultural waste straw into biodegradable mulch film, which promotes the recycling of straw resources. This study presents a novel method with significant potential application value for the production of bio-pulping in the paper industry.

1. Introduction

Plastic mulch film presents considerable challenges regarding its natural degradation after use. Residual film obstructs water infiltration, diminishes the soil water retention capacity, and contributes to soil compaction and reduced porosity. This disruption adversely affects the stability of the bacteria–fungi interaction network, reduces the soil’s ability to decompose organic matter, and leads to a temporary decrease in soil bulk density while exacerbating soil structure fragmentation in the long term [1]. Additionally, residual film adheres to crop roots, reducing root surface area and impairing the efficiency of water and nutrient absorption, which hinders root development and inhibits crop growth. Furthermore, aged mulch film releases small molecules such as lactic acid, which serve as an additional carbon source for microorganisms, resulting in increased CO₂ emissions in the short term. However, this does not contribute to an increase in the soil carbon pool over the long term; rather, it accelerates the mineralization of organic carbon and exacerbates the greenhouse effect [2]. Lastly, fragments of mulch film can be dispersed by wind to adjacent farmland, leading to ‘visual pollution’ and posing a risk to livestock that may mistakenly ingest them, potentially resulting in digestive diseases or even death.

In recent years, numerous experts and scholars have researched environmentally friendly biodegradable mulching films. Films made from natural polymers, such as cellulose and protein, have emerged as viable alternatives to traditional plastic mulching films [3]. Their performance is comparable to that of conventional mulching films; however, biodegradable products can decompose in the soil, thereby enhancing soil quality [4] and minimizing environmental harm. Agricultural waste biomass, particularly rice straw, represents a significant source of cellulose. Utilizing rice straw to produce biodegradable materials offers an effective solution for resource utilization and addresses environmental pollution issues. Bilo et al. utilized rice straw as a raw material to produce biodegradable composite materials whose mechanical properties are comparable to those of plastics [5]. Beniwal et al. successfully enhanced the tensile properties and water resistance of polylactic acid (PLA) films containing straw fibers [6]. Senthil et al. extracted fibers from elephant grass to create all-cellulose-based composite materials suitable for crop mulching. Common fiber extraction methods include physical, chemical, and biological techniques, with enzymatic hydrolysis being an environmentally friendly treatment that effectively improves fiber characteristics and enhances the mechanical properties of the resulting products. Pretreatment using lignin-degrading fungi and enzymes can reduce the need for chemicals, decrease cooking time, and lower energy consumption while improving pulp quality [7]. Research indicates that MA formulations effectively facilitate the dissociation and degradation of cellulose fibers, enhance the performance of rice straw (RS) bundles, and significantly reduce pulping energy consumption [8]. Gabriel Banvillet et al. produced cellulose nanofibrils (CNFs) through in situ enzymatic hydrolysis combined with twin-screw extrusion technology. The incorporation of in situ enzymes during the twin-screw extrusion process effectively increased the proportion of fine fibers and the average fiber length, thereby positively influencing the formation of the fiber network. Furthermore, the addition of the in situ enzyme solution resulted in an enhanced crystallinity index and a reduced degree of polymerization, ultimately leading to improved quality indicators of CNFs [9]. Despite these significant advancements, physical and chemical methods for fiber extraction pose challenges such as environmental pollution, high energy consumption, and elevated production costs, while the mechanical properties of biodegradable mulch films remain suboptimal.

This study effectively integrates biological and physical methods by employing a composite microbial EM bacterial solution to treat rice straw. It investigates the effects of biological pretreatment and screw extrusion on the physicochemical properties of rice straw and compares the impacts of various process parameters on the performance of fiber raw materials. Through optical microscopy observation and measurements of crystallinity, degree of polymerization, and other parameters, the influence of the proposed method on rice straw fiber raw materials is analyzed. This approach facilitates the separation and degradation of lignin, hemicellulose, and cellulose in rice straw. By deconstructing rice straw cellulose fibers and enhancing the properties of rice straw (RS) bundles, in conjunction with the application of screw extrusion technology, the mechanical properties of the film can be significantly improved.

2. Materials and Methods

2.1. Materials

The rice straw (RS) utilized in this study was harvested in 2022 from the Sui Geng 18 rice variety in Suihua City, Heilongjiang Province, China. The straw was subsequently processed into segments measuring 10–15 cm using a straw cutting machine for further applications. The composition of the rice straw includes 50.18% cellulose, 14.6% lignin, and 27.46% hemicellulose. The biological pretreatment microbial agent employed is an EM composite microbial liquid, which was self-prepared by the College of Life Sciences at Northeast Agricultural University, exhibiting an effective microbial count of 3.5 × 10¹¹ cfu/g.

2.2. Instrumentation and Equipment

D200 straw fiber preparation machine (self-made by the Biomass Material Testing and Analysis Laboratory for Main Crops in Arid Regions, Northeast Agricultural University) ZT4-00 Valley beater (Zhong Tong Experimental Equipment Co., Ltd., Xingping, China). ATV312HU75N4 frequency converter (Schneider Electric, frequency range 0–50 Hz) ZJG-100 Schopper–Riegler freeness tester (accuracy 1°SR, Changchun Yue Ming Experimental Device Co., Ltd., Changchun, China). ZCX-A sheet former (Changchun Yue Ming Small Experimental Machine Co., Ltd., Changchun, China). DCP-DGG-9070AD electric thermostatic blast drying oven (Shanghai Sen Xin Experimental Instrument Co., Ltd., Shanghai, China). JA5003N electronic balance (accuracy 0.001 g, Shanghai Jing Hai Instrument Co., Ltd., Shanghai, China). Straw kneading machine (Harbin Long Mu Machinery Co., Ltd., Harbin, China). ANKOM200i semi-automatic fiber analyzer (Shanghai He Guan Instrument Co., Ltd., Shanghai, China). DCP-SLY1000 computer-controlled paper tearing tester (Sichuan Yangtze Paper Instrument Co., Ltd., Yibin, China). Burst strength tester, TM4000 tabletop scanning electron microscope (Hitachi, Chiyoda, Japan), capillary viscometer (Yancheng Hai Kuo Experimental Equipment Co., Ltd., Yancheng, China). X-ray diffractometer (Bruker AXS GmbH, Karlsruhe, Germany).

The Process of Making Straw Using Single Screw Extrusion Puffing

The D200-type straw fiber preparation machine operates by generating power from the main motor, which is transmitted to the spindle via a gear mechanism, resulting in spindle rotation as shown in Figure 1. The straw material, pre-soaked in water at room temperature to achieve its saturated moisture content, is introduced into the main unit through a forced feeding device. The spindle’s spiral movement facilitates the forward transport of the straw along its axis. In the feeding and extrusion sections, a laminar flow field is formed. The straw material continuously rubs against the screw, leading to an increase in the internal temperature of the machine. As the straw undergoes compression, shearing, and kneading during these stages, its structure transforms into a coarse, fibrous form, thereby completing the preliminary processing of the raw material. Upon entering the explosion section, the straw material experiences compression via the screw conveyor, achieving a specific compression ratio. Within the high-temperature and high-pressure steam environment of the explosion section, the coarse straw fibers are extruded from the die head. The abrupt pressure drop of the high-pressure steam induces an explosive expansion phenomenon, resulting in the rapid enlargement of the internal bundle-like structure. This explosion effect fragments the plant fibers into smaller fibers, yielding a fiber slurry with enhanced brooming characteristics and fine branching.

2.3. Pretreatment of Straw Using Biological Fermentation

To investigate whether biological pretreatment has a significant effect on the physicochemical properties of straw, the pretreatment time was selected as the experimental factor to explore its impact. Harvested rice straw was cleaned to remove soil, gravel, and other impurities, then processed into small sections of 10–15 cm using a straw kneading machine. Clean water was added to the kneaded straw to achieve a moisture content of 50–55%. A biological agent solution was then applied to the soaked straw at a rate of 0.5% based on the dry weight of the straw, with a mass ratio of 1:1 for the microbial agent and the fermentation solution in the biological solution. The straw was continuously mixed during the application to ensure thorough integration with the microbial solution. The treated straw was placed in a fermentation chamber for closed fermentation at room temperature for different durations.

To determine whether the biological pretreatment causes significant changes in the rice straw and to evaluate the depolymerization effect of the exogenous microbial agent on the straw, the physicochemical properties and weight loss rate of the rice straw at different pretreatment times were tested. These evaluations aimed to assess the depolymerization effect of the microbial agent on the rice straw.

2.3.1. Macroscopic Quality Loss During Fermentation

During the fermentation process of rice straw, both the control and experimental groups exhibit a decline in material quality. To evaluate the depolymerization effect of exogenous microbial agents on rice straw fibers, the weight loss rate is monitored. The rate of weight loss change is utilized to assess the enhancing effect of exogenous microbial agents on the fermentation process of rice straw.

2.3.2. Physical Performance Testing

Rice straw subjected to varying fermentation durations was tested for tensile strength using a universal tensile testing machine. This investigation aims to explore the impact of biological pretreatment on the physical properties of the straw. Each group undergoes 30 tests, and after excluding significant outliers, the average tensile strength value is calculated. These values are recorded to assess the changes in tensile strength.

2.3.3. Test Methods for Changes in Chemical Properties

The cellulose, hemicellulose, and lignin content of rice straw fibers at various pretreatment durations were measured using the ANKOM200i semi-automatic fiber analyzer, in accordance with the modified Van Soest method [10]. The results were analyzed to summarize the changes in the proportions of these components.

2.4. Preparation of Straw Fiber Mulch Film

2.4.1. Fiber Preparation Method for Mulch Film

After pretreatment, the rice straw is soaked in clean water to achieve a saturated moisture content. The straw, now saturated, is subsequently fed into the D200 rice straw fiber preparation machine. To prevent starch gelatinization, the processing temperature is maintained at 90 °C, with the main shaft speed set at 100 rpm and the explosion pressure regulated at 2.1 MPa [11].

2.4.2. Mechanical Pulping and Preparation Straw Fiber Mulch Film

The raw materials were pulped using a pulping machine. The fiber raw materials were soaked for 4 h, and the pulp consistency was maintained at 2 wt%. Specifically, 200 g of raw materials were added to 10 L of water, and the pulping degree was set to 50 ± 3 °SR. When the pulping degree reached the predetermined value, the time used was recorded [12]. The energy consumption was then evaluated. The basis weight was established at 75 g/m², and the temperature was maintained at 100 °C. Straw fiber mulch film was produced by following the papermaking process using a paper forming machine, which was set at 100 °C for 7 min as shown in Figure 2. The physical properties of the straw fiber mulch film were tested in accordance with ISO 5270 [13].

2.5. Fiber Morphology Analysis

Fiber morphology analysis was performed using the FQA-360 Fiber Quality Analyzer image acquisition system. A 300 mg sample of pulp was dispersed in 7 L of water and continuously circulated within the device. The fiber length was restricted to 200 μm, with a resolution of approximately 5 μm. The average fiber length, fine fiber content, and other morphological parameters of the sample were measured.

2.6. SEM Ultrastructural Characterization

The microscopic structure of the rice straw at various pretreatment durations was examined using the TM4000 desktop scanning electron microscope (Hitachi, Chiyoda, Japan). The magnification was set to 500× and 1000×, and the corresponding images were captured for subsequent analysis.

2.7. Determination of Fiber Polymerization

Cellulose was extracted from the samples [14]. The characteristic viscosity was determined according to the British Standard [15], and the degree of polymerization was calculated using Formula (1) [16]. The degree of polymerization reflects the length of cellulose chains, which in turn influences the strength of the fibers. Therefore, it is crucial to consider not only the disassembly of fibers but also the impact of microorganisms on fiber strength during the biological pretreatment process.

$$P^{0.9}=1.65[\eta ]/\mathrm{mL}{\mathrm{g}}^{-1}$$

(1)

2.8. Fiber Crystallinity Determination

X-ray diffractograms of the samples were recorded using an X-ray diffractometer (XRD Bruker D8ADVANCE) with a scanning range of 5–40°. Cu Kα rays, tube current 40 mA, tube voltage fiber 40 kV, Ni sheet filtering, scanning rate 6°/min, and the crystallinity of the samples [7] was calculated according to Equation (2).

$$Crl(\%)=[(I_{002}-I_{am})]/I_{002}\ast 100$$

(2)

3. Results

3.1. Results of Biological Fermentation Pretreatment Test

3.1.1. Mass Loss Rate

As illustrated in Figure 3, the degradation rates for the BPF and control group (Ck) reached 28.34% and 24.86%, respectively, at 22 days. The degradation rate of straw in the BPF group was higher than that in the Ck group, indicating that the involvement of microorganisms accelerated the fermentation process of the rice straw. The mass loss of the straw in the BPF group was relatively rapid due to the external microbial agents utilizing the nutrients provided by the straw substrate for extensive reproduction. This activation of microorganisms facilitated the fermentation process, resulting in a significant decline in straw mass. The decomposition of straw commenced with the early-stage depolymerization of macromolecular chains into smaller molecular chains [17], leading to substantial mass loss.

3.1.2. Changes in Mechanical Properties During Fermentation

To investigate the changes in the physical structure of straw caused by microbial agents, the tensile strength of straw during the bio-pre-treatment process was measured. As shown in

Table 1. The straw’s tensile test results.

Pre-Treatment Time/d	Tensile Strength/Mpa
0	2.87 ± 0.18
3	2.68 ± 0.21
6	2.5 ± 0.2
9	2.2 ± 0.15
12	1.89 ± 0.37

, the tensile strength of the straw decreased with the extension of the pre-treatment time. After 12 days of fermentation, the tensile strength of the straw was 1.89 MPa, a 22.9% decrease compared to untreated straw. The trend showed an initial slow decline followed by a more rapid decrease.

3.1.3. Changes in Chemical Composition

To investigate the changes in the chemical composition of straw caused by microbial agents, the chemical composition of straw during the bio-pre-treatment process was measured. As illustrated in Figure 4, with the extension of pre-treatment time, the cellulose content in the straw showed a downward trend, the lignin content showed an upward trend, and the hemicellulose content showed an overall decline. After 12 days of treatment, the contents of lignin, cellulose, and hemicellulose were 19.41%, 48.56%, and 23.72%, changing by 5.31%, 2.11%, and 3.74%, respectively.

3.2. Results of SEM Ultrastructural Characterization

The microstructure of straw subjected to varying pretreatment durations was examined using scanning electron microscopy (SEM) as shown in Figure 5. The images illustrate the surface microstructure of straw at different pretreatment stages. It is evident that untreated straw maintains a smooth and flat surface structure, signifying the preservation of its original tissue and microstructure. As the pretreatment duration increases, noticeable alterations in the straw’s surface structure occur. After six days of pretreatment, the dense surface structure of the straw is compromised; the surface tissue begins to flake off in chunks, and microbial agents penetrate the internal fiber structure. These microbial agents initiate the depolymerization of the fiber structure, potentially breaking down the lignin that encases the cellulose, which may account for the observed decrease in lignin content. With further extension of the pretreatment duration, the surface of the straw fibers becomes increasingly rough and uneven. Numerous cracks develop on the surface, disrupting the originally dense structure. The microbial agents, which utilize the nutrients from the straw substrate for rapid reproduction, secrete cellulase that infiltrates the straw fiber structure. This process leads to further degradation of the fiber structure, resulting in visible grooves on the surface and damage to the cellulose structure.

After 12 days of pretreatment, it is clear that the dense outer layer of the straw fiber is severely compromised, exposing the internal fiber structure. Additionally, fiber separation begins to occur, diminishing the bonding force between fibers. Post-treatment, the straw exhibits a significantly increased quantity of fine fibers, and its tactile properties become notably softer.

3.3. Results of Fiber Morphology Analysis

Using the FQA-360 fiber quality analyzer, a quantitative analysis of the changes in fiber morphology was conducted. As shown in

Table 2. Fiber quality analyzer results.

Time (d)	Fine Content (%)	Average Fiber Length (μm)
0	59.6	0.489 ± 0.007
10	64.8	0.437 ± 0.009
15	69.8	0.431 ± 0.005

, the addition of microbial agents significantly increased the proportion of fine fibers. Furthermore, with an increase in the enzymatic treatment duration, the proportion of fine fibers also rose.

3.4. Results of Cellulose Degree of Polymerization (DP) Analysis

Cellulose with a high degree of polymerization (DP) and crystallinity (Crl) exhibits greater resistance to hydrolysis, as well as increased density and tensile strength [18]. Consequently, the degree of polymerization (DP) of cellulose was measured during the pretreatment process to investigate the degradation of cellulose and the role of microbial agents in cellulose depolymerization. The results of these measurements are presented in

Table 3. Polymerization test results.

Samples	[$\mathit{\eta }$] (mL/g)	DP
Ck	267	866.51
BPF-10	234.35	749.60
BPF-15	203.17	639.64

. As the pretreatment duration increased, both the characteristic viscosity and the degree of polymerization of straw cellulose consistently decreased as shown in Figure 6. Compared to untreated straw, after 12 days of biological pretreatment, the degree of polymerization and characteristic viscosity decreased by 23.91% and 26.18%, respectively. The primary reason for this decrease in the degree of polymerization is attributed to the cellulose enzymes secreted by the microbial agents in the exogenous microbial inoculum. These enzymes cleave the β-1,4-glycosidic bonds between cellulose chains, resulting in the shortening of cellulose chains and the disruption of chemical bonds among the straw components, ultimately leading to the depolymerization of cellulose fibers.

3.5. Results of the Crystallinity Test

Crystallinity is a crucial factor influencing the mechanical properties of rice straw fibers, and an increase in crystallinity has been shown to enhance fiber strength [19]. Figure 7 presents the diffraction patterns of the films. Notably, diffraction peaks are observed at scanning angles of 18° and 22.5°, with the optimized group exhibiting higher peak intensities compared to the control (Ck). The positions of the XRD diffraction peaks for the samples have not changed significantly. The crystallinity data presented in

Table 4. Crystallinity index test results.

Sample	Crystallinity Index (wt.%)
Ck	53.33
BPF-10	60.78
BPF-15	52.14

reveal a 13.97% increase in crystallinity for the optimized group relative to the untreated group, whereas the crystallinity of the control (Ck) has decreased by 4.89% compared to the untreated biological group.

3.6. Results of Mechanical Property Determination

To evaluate the impact of biological pretreatment fermentation on the performance of straw fiber mulch film, we tested the mechanical properties of mulch films produced through various processes. The results are presented in

Table 5. Mechanical properties test results.

Sample	Tensile Strength /N·m·g−1	Tear Strength /mN·m2·g−1	Burst Strength /kPa·m2·g−1	Initial Pulping Degree /°SR	Pulping Time /min
Ck	6.15 ± 0.21 b	0.74 ± 0.07 b	1.03 ± 0.05 b	16 ± 2 b	101 ± 3 a
BPF-10	9.57 ± 0.16 a	0.86 ± 0.03 a	1.15 ± 0.06 a	21 ± 1 a	81 ± 2 b
BPF-15	5.32 ± 0.17 c	0.68 ± 0.05 b	0.97 ± 0.07 b	23 ± 1 a	70 ± 3 c

Note: Means followed by different letters are significantly different at p < 0.05 (Duncan’s test). Means followed by the same letter are not significantly different.

.

4. Discussion

4.1. Discussion on Results of Biological Fermentation Pretreatment Experiments

4.1.1. Mass Loss Rate

In the Ck group, the microorganisms that accumulated naturally during the fermentation process were sourced from the surrounding air and the straw itself. The slower mass loss rate observed can be attributed to the fact that the primary microorganisms present in the naturally fermented straw originated from those that were naturally occurring in the straw and moisture, which were initially in limited quantities. Consequently, the fermentation accumulation time required was extended, leading to a slower degradation rate throughout the fermentation process. Therefore, when compared to the results of pre-treatment utilizing only the naturally occurring microorganisms present on the straw, the bio-pretreated results displayed a delayed degradation and a relatively lower degradation rate. Notably, at 14 days of bio-pre-treatment, the mass loss of the straw began to show a significant increase. Taking into account both the macroscopic mass loss of the straw and the economic factors associated with production, alongside the team’s previous research [11], the optimal pre-treatment duration for the straw was determined to be 12 days.

4.1.2. Changes in Mechanical Properties During Fermentation

Tensile strength shows a trend of slow decline in the initial stage, followed by a faster decline. The variation in this trend can be attributed to the complex chemical composition and dense physical structure of the outer cell wall of straw, which restricts enzymatic hydrolysis by microbial agents. However, as the pre-treatment time increases, microbial agents begin to proliferate by utilizing the nutrients within the fermentation substrate. This proliferation leads to the degradation of the dense physical structure of the straw’s outer layer, resulting in a decline in its mechanical properties. Once the microbial agents penetrate the interior of the straw, they enzymatically hydrolyze the straw fibers, further accelerating the reduction in tensile strength. Enzymatic hydrolysis over a certain duration renders the straw fibers more susceptible to breakage, which is advantageous for the fibrillation and pulping processes. However, excessively prolonged pre-treatment times can significantly compromise the mechanical properties of the straw fibers, adversely affecting the overall mechanical characteristics of the straw fiber mulch film.

4.1.3. Changes in Chemical Composition

Microbial agents utilize straw for growth and metabolism, leading to the secretion of cellulase, which reduces the content of cellulose and hemicellulose. In contrast, lignin, due to its stable three-dimensional structure, is resistant to degradation and continues to accumulate [16]. The alterations in the chemical composition of straw resulting from bio-pre-treatment indicate that this process significantly affects the chemical properties of straw, which may enhance the fibrillation and pulping processes. To further investigate the effects of bio-pre-treatment, the pre-treated straw was subjected to steam explosion to assess its impact on the performance differences in the film.

4.2. Discussion on SEM Ultrastructural Characterization

It has been observed that an appropriate pretreatment duration effectively opens the fiber structure, facilitating a more thorough high-temperature, high-pressure steam explosion treatment in the straw fiber extraction machine. This process results in easier splitting and fibrillation of the fibers, while also improving inter-fiber bonding, which may enhance their mechanical properties. In comparison to untreated pulp fibers, which exhibit greater stiffness, depolymerized straw fibers are notably softer. Furthermore, the roughened surface structure of these fibers enhances their water absorption and swelling properties, which are advantageous for the direct shear forces exerted by the cutter teeth and for the rubbing and loosening of fibers, ultimately leading to a reduction in pulping time.

4.3. Discussion on Fiber Morphology Analysis

The increase in fine fiber content significantly enhances the strength of agricultural films. Research by RS Reth indicates that incorporating 8.2% fine fibers into untreated raw materials improved the elasticity modulus of paper by 48% and increased its folding endurance by nearly 3.5 times [20]. This enhancement is primarily due to the fine fibers facilitating closer contact between the fibers, which positively influences their bonding strength. The increase in fine fibers effectively enlarges the bonding area and the number of interwoven fiber connections, thereby improving the tensile strength of the agricultural film. Conversely, fine fibers exhibit a smaller volume and a larger specific surface area, resulting in a higher surface charge compared to long fibers and untreated pulp. These surface characteristics enable fine fibers to adsorb fine components and additives more readily during the agricultural film preparation process [21]. Furthermore, the small size of the fibers allows them to be effectively adsorbed onto long fibers, filling the pores in the agricultural film and enhancing its mechanical properties. Regarding fiber length, the microbial agent effectively depolymerizes the fibers, leading to a reduction in fiber length. Generally, fibers with a larger aspect ratio provide more bonding points and exhibit higher intrinsic strength. Consequently, the aspect ratio of fibers is positively correlated with the strength of agricultural films. The influence of fiber morphology on film strength necessitates a comprehensive consideration of fine fiber content and fiber aspect ratio, which can inform the adjustment of processing techniques.

In the context of fiber morphology, the curl index and knot index serve as critical parameters for characterizing fiber structure. The data indicate that enzymatic treatment enhances both the curl and knotting of fibers. Agricultural film produced from straw fibers, which comprises fibers, additives, and moisture, forms a multi-phase, network-like porous structure. The increased curling and knotting of fibers not only improves the interweaving degree among them but also contributes to a more complex network structure. This complexity facilitates the adhesion of fine fibers and functional additives, thereby augmenting the mechanical properties of the agricultural film. Additionally, the enhanced curling and knotting increase the contact area between fibers. Enzymatic treatment disrupts the smooth structure of the fibers, rendering their surfaces rougher. Under stress, these rough, curled, and knotted fibers provide greater friction and contact points, thereby improving fiber interactions and enhancing the tensile properties of the film [22]. However, it is crucial to note that excessively depolymerized fibers may lead to significantly reduced mechanical performance, potentially compromising the film’s properties.

As demonstrated in Table 2, the fiber branching index exhibited a significant increase at 10 and 15 days of treatment. This enhancement can be attributed to the secretion of cellulases by microorganisms within the fermentation system, which effectively cleave theβ-1,4-glycosidic bonds between cellulose chains, leading to substantial alterations in the fiber structure. The enzymatic process facilitates the breakdown of both the primary and secondary walls of the fiber, resulting in a softer and rougher outer layer that is more susceptible to water absorption and swelling. During the refining process, the fibers become more prone to fuzzing, tearing, or splitting due to the action of blades and water flow, thereby increasing their surface area and promoting better inter-fiber bonding. The observed increase in the fiber branching index during the subsequent agricultural film-forming process creates additional bonding points, thereby enhancing the mechanical performance of the film. However, the data indicate no significant difference in the branching index between the 10-day and 15-day treatments. This may be attributed to the fact that after 10 days of treatment, the depolymerization of the fiber surface is largely complete, with further depolymerization primarily occurring within the fiber, consequently leading to a notable decline in fiber mechanical performance. Therefore, extended pretreatment durations can adversely affect the effectiveness of enzymatic treatment.

In conclusion, appropriate enzymatic pretreatment significantly modifies fiber morphology, enhancing fine fiber content and improving the mechanical properties of agricultural film. However, excessive treatment duration may negatively impact fiber strength, making the optimization of pretreatment duration essential for achieving optimal performance.

4.4. Analysis of Cellulose Degree of Polymerization

The degradation rate of cellulose polymerization degree was observed to be more rapid in the early stages compared to the later stages. This phenomenon can be attributed to the fact that, as pretreatment progresses, exogenous microorganisms proliferate rapidly by utilizing nutrients from the straw substrate, thereby activating microbial activity during the fermentation process. This activation increases the likelihood of cellulose enzymes cleaving the β-1,4-glycosidic bonds [23]. Furthermore, the enzymes secreted by the microorganisms primarily target the amorphous regions of cellulose, resulting in a swift decline in the degree of polymerization. In contrast, the crystalline regions of cellulose possess a more stable structure and exhibit greater resistance to degradation, leading to a deceleration in the decrease in polymerization degree during the later stages of treatment. In conclusion, biological pretreatment significantly reduces the degree of polymerization of cellulose, indicating that microbial agents play a pivotal role in the depolymerization of cellulose chains. This depolymerization process, facilitated by cellulase enzymes, is essential for enhancing the accessibility of cellulose fibers and improving subsequent fiber processing, such as pulping or film formation. However, it is crucial to recognize that excessive degradation over prolonged pretreatment periods may result in undesirable changes to the mechanical properties of the fibers.

4.5. Analysis of Crystallinity Test Results

Biological pre-treatment primarily influences the amorphous region of cellulose, where hydrolysis and damage occur, while the crystalline region maintains its structural stability without significant decomposition. This process initially leads to an increase in the fiber index. However, as the duration of pre-treatment extends, cellulase begins to act on the crystalline region, disrupting its structure and consequently decreasing the fiber crystallinity index. This suggests that an optimal pre-treatment duration enhances the crystallinity index, while excessively prolonged pre-treatment can diminish fiber crystallinity, adversely affecting fiber performance.

4.6. Determination of Mechanical Properties

According to Table 5, biological fermentation pretreatment significantly enhanced the initial freeness of the fiber slurry, reduced pulping time, and demonstrated a trend where prolonged pretreatment duration led to an increase in the initial freeness of the fiber slurry and a decrease in pulping time. This trend can be attributed to the disruption of the surface structure of RS fibers, which increased their water absorption and swelling capacity, rendering the straw fibers softer and more pliable. Furthermore, the cellulase secreted by the microbial agent weakened the bonding forces between fibers, facilitating their separation and breakage during mechanical pulping [24]. Consequently, the initial freeness of the straw fiber slurry improved markedly, and the pulping time was significantly reduced.

In comparison to the mulch film produced without the addition of microbial agents (Ck), the mulch film created with biological pretreatment for 10 days (BPF-10) exhibited a notable enhancement in mechanical properties. The tensile strength increased from 6.15 N·mg⁻¹ to 9.49 N·mg⁻¹, reflecting a 35.74% increase. This improvement in mechanical properties is primarily due to the biological fermentation pretreatment, which breaks down macromolecular cellulose into smaller molecular chains, thus facilitating the utilization and processing of straw fibers. Additionally, the tear index and burst index increased by 16.22% and 11.65%, respectively. This enhancement is likely due to the microbial agent disrupting the dense surface structure of the straw, thereby exposing the fiber structure and improving the fiber’s bursting performance [25]. The enhanced structural integration among fibers created additional bonding points, thereby improving the mechanical properties of the product.

However, excessive pretreatment duration resulted in significant damage to the straw fiber structure, leading to over-depolymerization of cellulose fibers and a subsequent decline in the mechanical performance of the fiber mulch film.

5. Conclusions

This study confirms the feasibility of producing straw fiber films using bio-enzymatic hydrolysis combined with single screw extrusion puffing technology. The bio-enzymatic pretreatment promotes changes in fiber properties, effectively enhancing the mechanical properties of the fiber films. The results indicate that an appropriate pretreatment time can effectively depolymerize straw fibers and improve the properties of fiber bundles, making the straw looser and softer, which positively impacts the refining process. Additionally, the fermentation treatment significantly enhances the tensile strength, tear strength, and burst strength of the fiber films. Meanwhile, the increase in crystallinity and the decrease in polymerization degree confirm the depolymerization effect of bio-enzymes, positively influencing the production process and enhancing the mechanical properties of the final film products. Therefore, biological pretreatment is an effective method for depolymerizing straw fibers; however, prolonged pretreatment time can negatively affect the fibers.

Biodegradable mulch films demonstrate significant advantages in mitigating white pollution and restoring soil health; however, their production costs remain higher than conventional plastic films. Our self-developed biodegradable mulch has been successfully implemented in field applications with functional performance comparable to plastic counterparts. Future research will prioritize cost reduction strategies and develop crop-specific, customized formulations to advance widespread agricultural adoption.