A reinforced, toughened, and easily degradable PBAT / modified nanocellulose composite film and its preparation method

Abstract

This invention relates to the field of biodegradable polymer materials, specifically to a reinforced, toughened, and easily degradable PBAT / modified nanocellulose composite film and its preparation method. The method involves extracting cellulose nanocrystals from plant straw, sequentially modifying them with maleic anhydride and aminosilane coupling agents in a dual chemical process. This constructs a diverse chemical bonding network containing ester-carboxyl-amide bonds-siloxanes on the filler surface, yielding dual-modified nanocellulose. Subsequently, this modified nanocellulose is blended with PBAT resin and cast into a film. The composite film obtained by this invention achieves stable interfacial adhesion between the nanoskeleton and the polymer matrix, resulting in a simultaneous increase in tensile strength and elongation at break. Furthermore, it possesses excellent degradation characteristics that accelerate hydrolysis and metabolism in natural soil, making it highly effective for promoting plant growth and increasing yield when used as an agricultural mulch film.

Description

Technical Field

[0001] This invention relates to the field of biodegradable polymer materials technology, specifically to a reinforced, toughened, and easily degradable PBAT / modified nanocellulose composite film and its preparation method. Background Technology

[0002] Polybutylene adipate-butylene terephthalate (PBAT) belongs to a class of aliphatic-aromatic copolyester polymers with fully biodegradable properties. Its molecular chains combine the excellent flexibility of aliphatic chains with the good machinability provided by aromatic rings. This material exhibits high biocompatibility and is widely regarded as an ideal substrate to replace traditional non-degradable plastics, possessing broad market potential in fields with high ecological and environmental protection requirements, such as disposable packaging materials and modern agricultural mulch films. To enhance the overall performance of this substrate, cellulose nanocrystals (CNCs) extracted from natural plant resources, with their high modulus, high crystallinity, and completely renewable natural properties, are often used as green reinforcing fillers in composite material systems.

[0003] For applications combining CNC machining with PBAT matrices, various surface modification schemes have emerged within the existing technological system. Patent document CN202110203802.2 discloses a reinforced PBAT mulch film using tung oil anhydride and octadecane-modified ultrafine rice husk powder; this system shows a slight increase in tensile strength when a higher proportion of filler is added. Another document, CN201610813912.X, uses butyl methacrylate to modify bagasse, resulting in a certain degree of change in the tensile strength and elongation at break of the PBAT composite material. Furthermore, to improve interfacial compatibility, some studies have attempted multi-step modification strategies, such as pre-constructing carboxyethyl nanocellulose followed by modification with advanced alkylamines, or using acetic anhydride and butyric acid to perform an acylation reaction on hemicellulose before blending it with the resin matrix.

[0004] Most of the aforementioned conventional processes suffer from the limitation of introducing only a single type of functional group, resulting in the interfacial interaction between the filler and the polymer matrix remaining at the level of physical entanglement or weak intermolecular hydrogen bonding for a long time. This weak interface, lacking directional chemical bonding, is prone to debonding failure under long-term physical stretching or in complex service environments such as high soil moisture and microbial metabolic acid production, leading to the dilemma of extremely high reinforcement amounts but low actual reinforcement efficiency. Existing research often focuses excessively on simply improving the mechanical properties of composite materials at the factory, seriously neglecting the systematic intervention and evaluation of the final natural degradation performance of the product. As agricultural mulch film materials that directly enter the soil environment, how to effectively regulate and accelerate the degradation rate of film materials in natural or alkaline soils while simultaneously inhibiting interfacial delamination and significantly improving mechanical strength has become a long-standing and unresolved common technical challenge in this field. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a reinforced, toughened, and easily degradable PBAT / modified nanocellulose composite film and its preparation method, which solves the problems of poor dispersibility of reinforcing fillers, weak interfacial bonding, and difficulty in balancing mechanical properties and degradation performance in existing PBAT composite film systems.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing a reinforced, toughened, and easily degradable PBAT / modified nanocellulose composite film, comprising the following steps: Step 1: Extraction and preparation of cellulose nanocrystals Plant straw raw materials are obtained and then subjected to impurity removal, alkali treatment and bleaching operations in sequence to remove non-cellulose components such as hemicellulose and lignin. The purified cellulose is then subjected to acid hydrolysis and mechanical dissociation to obtain cellulose nanocrystals with a diameter of less than 50 nm and an aspect ratio of greater than 100. Step 2, First Stage Surface Chemical Modification The cellulose nanocrystals obtained in step one are uniformly dispersed in an organic solvent medium to form a suspension. Maleic anhydride and an initiator are introduced into the system. The pH value of the reaction system is adjusted to the range of 3 to 5. The reaction is carried out under constant temperature stirring for 2 to 6 hours under heating conditions. After the reaction system is cooled, it is separated by centrifugation, washed until neutral and dried to prepare modified cellulose nanocrystals with maleic anhydride grafted on the surface. Step 3, Second Stage Coupled Connection and Branch Modification Prepare an aqueous solution of aminosilane coupling agent in an acidic environment of pH 3-4 and allow it to pre-hydrolyze at room temperature for 3-8 hours. Disperse the maleic anhydride modified cellulose nanocrystals obtained in step 2 in a mixed solvent of ethanol and water. Slowly add the pre-hydrolyzed silane coupling agent solution, control the reaction system temperature between 50-70℃ and continuously stir the reaction for 4-7 hours. After filtration, washing and freeze-drying, obtain the double-modified nanocellulose. Step 4: Casting and Shaping of Composite Film According to a mass ratio of 1:99 to 10:90, the double-modified nanocellulose obtained in step three and polybutylene adipate-butylene terephthalate resin are added together into an organic solvent. The mixture is continuously stirred until the solid phase is completely dissolved and a uniform and stable casting liquid is formed. The casting liquid is uniformly coated on the surface of the substrate and subjected to a drying and curing process at a specified temperature. After peeling off the film, the target composite film is obtained.

[0007] Preferably, the raw material pretreatment process in step one is as follows: The impurity removal process is as follows: extract plant straw powder, add it to an alkaline aqueous solution containing 1% to 3% surfactant, and continuously stir it at room temperature; The alkali treatment process is as follows: the impurity-removed cellulose raw material is immersed in a sodium hydroxide solution with a mass concentration of 10% to 20% at a solid-liquid mass ratio of 1:10 to 20, the system temperature is maintained at 50 to 60°C and the reaction is carried out for 1 to 3 hours. The bleaching process involves contacting the alkali-treated cellulose with a mixed aqueous solution of sodium chlorite and hydrochloric acid, reacting at a relatively high temperature range of 80-95°C for 0.5-2 hours, and then using sodium metabisulfite solution to remove residues.

[0008] Preferably, the plant straw raw material in step one is selected from any one or more combinations of wheat straw, oat straw, or corn straw; the mechanical dissociation treatment is selected from one or more combined applications of high-pressure homogenization, micro-jet, ultrasonic treatment, ball milling, and bead milling technologies.

[0009] Preferably, the acid hydrolysis process in step one is as follows: the purified fiber that has completed the bleaching operation is placed in a sulfuric acid solution with a concentration of 50% to 65% according to a solid-liquid ratio of 1:20 to 40, and stirred and reacted in an environment of room temperature to 60°C for 30 to 120 minutes. At the end of the reaction, cold water is immediately injected to terminate the reaction process.

[0010] Preferably, the initiator in step two is ammonium persulfate or azobisisobutyronitrile; the mass ratio of maleic anhydride to cellulose nanocrystals is 1:4~6; the reaction temperature for the first stage of surface chemical modification is set at 70~130℃; after this stage of treatment, the carboxyl content on the surface of the obtained maleic anhydride-modified cellulose nanocrystals reaches 1.0~3.0 mmol·g. -1 .

[0011] Preferably, the aminosilane coupling agent used in step three is 3-aminopropyltriethoxysilane or N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane; the mass ratio of the aminosilane coupling agent to the maleic anhydride-modified cellulose nanocrystals is 0.05~0.2:1; in the mixed solvent of ethanol and water, the volume ratio of ethanol to water is 3~5:1; and N,N'-dicyclohexylcarbodiimide is added as a reaction catalyst in the reaction system of this step.

[0012] Preferably, the preparation parameters for the composite film in step four are as follows: The mass ratio of the double-modified nanocellulose to polybutylene adipate-butylene terephthalate is (5~7):(93~95). The organic solvent used to dissolve the polymer components is selected from chloroform, dichloromethane, or N,N-dimethylformamide alone; The casting solution is applied by vacuum doctor blade coating, spin coating, or casting film formation process; The drying temperature during the film-forming stage is maintained in the range of 40~80℃, and the drying time is 2~12 hours.

[0013] Preferably, the composite film contains a multi-faceted chemical bonding network constructed on the surface of nanocellulose due to sequential dual chemical modification. This network achieves a strong interfacial bond with the PBAT matrix through the synergistic effect of ester groups, carboxyl groups, amide bonds, and siloxanes.

[0014] Preferably, when the mass fraction of the double-modified nanocellulose inside the composite film is in the range of 5% to 7%, it exhibits significant mechanical strengthening characteristics. At this time, the tensile strength of the film is increased by more than 35% compared with the pure PBAT film without the addition of the modified material, and its elongation at break is not less than 390%.

[0015] Preferably, the composite film has excellent natural environmental degradation characteristics and growth-promoting value. When buried in a high-humidity alkaline soil environment with pH ≥ 8.5, after a 90-day natural metabolic cycle, the mass degradation rate of the material itself is not less than 50%. When it is laid out as an agricultural mulch film, relying on its specific water vapor barrier and oxygen permeability balance system, it can effectively induce the root length of crops to increase to more than 87% and promote the chlorophyll content of plant leaves to increase by about 62%.

[0016] This invention provides a reinforced, toughened, and easily degradable PBAT / modified nanocellulose composite film and its preparation method. It possesses the following beneficial effects: 1. This invention employs a two-step sequential chemical modification process, using covalent bonding to sequentially introduce maleic anhydride and aminosilane coupling agents onto the surface of cellulose nanocrystals. This successfully constructs a diverse chemical bonding network containing ester groups, carboxyl groups, amide bonds, and siloxane functional groups on the outer layer of the filler structure. This transforms the original highly polar hydrophilic CNC surface into an active structure with steric hindrance and high hydrophobic compatibility, breaking the physical limitation of easy aggregation of nanocellulose in organic matrices and achieving a stable and dense interfacial adhesion between the nano-reinforced framework and PBAT macromolecular chain segments.

[0017] 2. This invention, through the uniform dispersion of the modified filler in the PBAT matrix and the strong interfacial stress transfer mechanism, controls the dosage of the modified reinforcing agent within the range of 5% to 7%, which can maximally constrain the initiation and propagation of microcracks in the polymer matrix. This results in the prepared composite film achieving a tensile strength increase of over 35% when subjected to external traction, while maintaining an excellent elongation at break of no less than 390%. This completely avoids the performance inversion phenomenon that polymer composite materials inevitably sacrifice material flexibility while improving strength.

[0018] 3. This invention utilizes a labyrinthine physical barrier constructed within the composite membrane space by dual-modified nanocellulose with high aspect ratio, which greatly extends the free path for external water vapor and oxygen molecules to penetrate into the material, effectively compressing the free volume ratio of the resin matrix. This endows the composite film with excellent water vapor barrier capability and moderate oxygen permeability, laying a solid physical foundation for the subsequent production of agricultural mulch film and the long-term maintenance of root zone microclimate stability in complex field environments.

[0019] 4. This invention retains a large number of active sites such as carboxyl and hydroxyl groups on the surface of the double-modified nanocellulose, enabling the material to quickly adsorb surrounding free water and soil microbial communities with degradation effects when in natural contact with moist soil. This establishes a micro-hydrolysis catalytic zone inside the material, which greatly accelerates the spontaneous hydrolysis and bio-enzymatic chain breaking rate of ester bonds on the PBAT main chain. As a result, the weight loss rate of the membrane material after being buried in alkaline soil with a pH value of 8.5 or higher for three months soars to about 50%, achieving an ecological closed-loop design that increases strength and doubles degradation. Attached Figure Description

[0020] Figure 1 These are microscopic morphology characterization images of the unmodified CNC, the MCNC obtained from the first step of modification, and the final product AMCNC of this invention. Figure 2 These are FT-IR characterization images of the CNC, MCNC, and AMCNC of this invention; Figure 3 This is a comparison diagram of the mechanical properties of the composite film of the present invention; Figure 4 This is a comparison chart of the light transmittance of the composite film of the present invention. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] Example 1: This invention provides a process for extracting CNC from plant fiber sources, as detailed below: Weigh 8.0 g of dried and mechanically pulverized wheat straw powder as the initial reaction material, and completely disperse and immerse it in 300 mL of a 2% (w / w) conventional detergent aqueous solution. Run a magnetic stirrer continuously for 1 hour at room temperature. After discarding the waste liquid, rinse the fiber residue repeatedly with plenty of deionized water 6 to 8 times, and then place it in a 90°C forced-air drying oven for continuous dehydration and drying for 6.5 hours, yielding 4.85 g of preliminarily purified and peeled plant fiber.

[0023] The obtained 4.85 g of purified fiber was transferred to a reaction vessel, and a 14% sodium hydroxide strong alkaline solution (prepared by dissolving 11.6 g of solid NaOH in 72.75 mL of deionized water) was added. The temperature of the constant temperature bath was adjusted to 50°C and the stirring speed was set to 839 rpm to promote vigorous reaction for 1 hour. After the reaction was terminated, the black liquor was drained and the fiber was washed with clean water until the waste liquid was neutral. It was then dried again at 90°C for 6 hours to obtain 2.8 g of deeply alkaline washed fiber. Subsequently, decolorization was completed using a sodium chlorite hydrochloric acid system according to conventional bleaching standards. Finally, controlled acid hydrolysis was carried out with a 60% sulfuric acid solution at 45°C for 45 minutes. After quenching the reaction with ice water, the suspension was collected. After centrifugation, dialysis, and high-frequency ultrasonic mechanical dissociation, uniform rod-shaped cellulose nanocrystals (CNC) were finally obtained.

[0024] Example 2: This invention provides a two-step chemical modification method for preparing AMCNC, as detailed below: 1.0 g of the CNC powder prepared in Example 1 was weighed and immersed in 100 mL of a selected organic solvent, followed by ultrasonic vibration for 30 minutes to ensure particle dispersion. 0.2 g of maleic anhydride reagent was weighed and added to the system (maintaining a fixed CNC to MAH mass ratio of 5:1), and a trace amount of sulfuric acid solution was added dropwise to precisely adjust the pH to 4.0. 0.01 g of ammonium persulfate was added as a free radical initiator, and the entire reaction system was transferred to an oil bath heating device with a stable temperature of 120–130 °C. Mechanical stirring was started for a grafting reaction lasting 4 hours. After the reaction solution cooled naturally, the precipitate was collected by centrifugation and washed multiple times with deionized water to remove unreacted monomers until the supernatant was neutral. After a freeze-drying process, modified cellulose nanocrystals (MCNC), an intermediate product with a large number of reactive carboxyl groups, was obtained. High-precision conductivity titration showed that the surface carboxyl group content of the MCNC's outer structure reached 1.85 mmol·g. -1 .

[0025] 0.5 g of dried MCNC powder was dispersed in 50 mL of anhydrous ethanol and sonicated for half an hour. In another clean container, 0.05 g of APTES (3-aminopropyltriethoxysilane) was weighed and dissolved in 10 mL of an ethanol-water co-solvent with a volume ratio of 4:1. Acetic acid was added dropwise to force the pH of the system to drop to 3.5, and the mixture was allowed to stand at room temperature (25°C) for 4 hours for hydrolysis. The pre-hydrolyzed APTES solution was then added extremely slowly dropwise to the continuously stirred MCNC ethanol dispersion using a constant-pressure dropping funnel. After mixing, the reactor was heated to 60°C for a isothermal silanization coupling reaction for 5 hours. The mixture was cooled, discharged, and washed several times by centrifugation with anhydrous ethanol. The purified wet material was then thoroughly freeze-dried under vacuum to obtain a double-modified nanocellulose (AMCNC) with an ester-carboxyl-amide bond-siloxane crosslinked network framework. Figure 1 As can be seen from the morphological characteristics, the surface of the material after grafting with APTES exhibits a distinctive layered stacking feature, which is relatively smooth compared to the original CNC.

[0026] Figure 2 The three curves correspond to CNC, MCNC, and AMCNC, respectively. MCNC: at approximately 1728cm. -1 A new strong absorption peak appears at approximately 3350 cm⁻¹, which is attributed to the carbonyl (C=O) stretching vibration of carboxylic acids and esters; CNC is at approximately 3350 cm⁻¹. -1 The broad and strong hydroxyl (OH) stretching vibration peak intensity significantly decreased, indicating its involvement in the esterification reaction. AMCNC: at approximately 1635 cm⁻¹ -1 A distinct shoulder peak or independent peak appears at approximately 1100 cm⁻¹, which is attributed to the carbonyl (C=O) stretching vibration peak of the amide bond; at approximately 1100 cm⁻¹... -1 A new broad absorption band appears nearby, attributed to the stretching vibrations of Si-OC and Si-O-Si, at 660 cm⁻¹. -1 The nearby absorption peaks are attributed to the bending vibrations of Si-O-Si, and the appearance of these characteristic peaks proves the success of the modification.

[0027] Example 3: This embodiment provides a method for preparing a PBAT / AMCNC composite film, specifically as follows: Polybutylene adipate-butylene terephthalate (PBAT) particles are vacuum dried at 80°C for 6 hours, and AMCNC is vacuum dried at 60°C for 10 hours for later use. 0.05 g of AMCNC and 4.95 g of dried PBAT are accurately weighed. PBAT is dissolved in 100 mL of chloroform and stirred at 50°C until completely dissolved. AMCNC is dispersed in 20 mL of chloroform and sonicated for 30 minutes. Then, it is slowly added to the PBAT chloroform solution, and the mixture is stirred and mixed at 50°C for 4 hours to obtain a uniform casting solution. The casting solution is poured onto a polytetrafluoroethylene release film, and the thickness is controlled with a doctor blade. The film is then transferred to a 50°C forced-air oven and dried for 12 hours until constant weight. The film is carefully peeled off to obtain a PBAT composite film with an AMCNC content of approximately 1 wt%, labeled as PBAT-AMCNC1.

[0028] Example 4: This embodiment provides a method for preparing a PBAT / AMCNC composite film. The preparation method is the same as in Example 3, except that the amounts of AMCNC and PBAT are adjusted to 0.15 g and 4.85 g, respectively, to obtain a composite film with an AMCNC content of about 3 wt%, which is labeled PBAT-AMCNC3.

[0029] Example 5: This embodiment provides a method for preparing a PBAT / AMCNC composite film. The preparation method is the same as in Example 3, except that the amounts of AMCNC and PBAT are adjusted to 0.25 g and 4.75 g, respectively, to obtain a composite film with an AMCNC content of approximately 5 wt%, which is labeled as PBAT-AMCNC5.

[0030] Example 6: This embodiment provides a method for preparing a PBAT / AMCNC composite film. The preparation method is the same as in Example 3, except that the amounts of AMCNC and PBAT are adjusted to 0.35 g and 4.65 g, respectively, to obtain a composite film with an AMCNC content of approximately 7 wt%, which is labeled PBAT-AMCNC7.

[0031] Example 7: This embodiment provides a method for preparing a PBAT / AMCNC composite film. The preparation method is the same as in Example 3, except that the amounts of AMCNC and PBAT are adjusted to 0.5 g and 4.5 g, respectively, to obtain a composite film with an AMCNC content of approximately 3 wt%, which is labeled as PBAT-AMCNC10.

[0032] Comparative Example 1: The present invention provides a comparative example of a method for preparing a pure PBAT film. Except for the absence of AMCNC, the other preparation steps are exactly the same as in Example 3, and a pure PBAT film is obtained.

[0033] Comparative Example 2: Commercially available PBAT films were purchased directly from commercial channels without any modification or processing. They served as blank control samples in this study to compare the performance differences of modified PBAT films.

[0034] 1. Performance Testing and Result Analysis Referring to GB / T 1040.3-2006 standard, each film sample was cut into dumbbell-shaped strips (gauge length 10 mm, width 4 mm). Tensile strength and elongation at break were tested using a universal testing machine at a tensile rate of 5 mm / min. Five parallel samples were tested for each sample, and the average value was taken. The results are shown in Table 1. Table 1 Mechanical properties of PBAT and its composite films Results analysis: Mechanical properties comparison Figure 3 As shown, the tensile strength of pure PBAT is approximately 17.5 MPa, and the elongation at break is approximately 407%, while the tensile strength of commercially available PBAT is approximately 17.7 MPa, and the elongation at break is approximately 215%. After introducing AMCNC, both the tensile strength and elongation at break of the composite film are simultaneously improved. Among them, PBAT-AMCNC7 exhibits the best overall mechanical properties, with a tensile strength of 24.1 MPa, an improvement of approximately 37.7% compared to pure PBAT; and an elongation at break of 439%, achieving a synergistic effect of reinforcement and toughening. When the AMCNC addition increases to 10%, the mechanical properties decrease, indicating that excessive filler may agglomerate and form stress concentration points.

[0035] As shown in Table 1, the application test results indicate that adding modified nanocellulose filaments can improve the tensile strength and elongation at break of PBAT films. When the addition amount is 7%, the tensile strength reaches a maximum of 24.1 MPa, an increase of approximately 37.7% compared to pure PBAT film (17.5 MPa), and the elongation at break increases from 407% to 439%. This improvement indicates a strong interfacial bond between AMCNC and the PBAT matrix (through interactions such as amide bonds and hydrogen bonds), allowing stress to be effectively transferred from the matrix to AMCNC, thereby improving tensile strength and elongation at break. When the addition amount increases to 10%, the tensile strength decreases, indicating that excessive AMCNC may agglomerate, forming stress concentration points.

[0036] 2. Barrier performance and transmittance testing and result analysis Barrier performance testing: The water vapor transmission rate (WVT) of the composite film was tested according to GB / T1037-2021, and the O2 transmission rate (OT) of the composite film was tested using the differential pressure method according to GB / T1038-2000. Transmittance testing was conducted according to GB / T 2410-2008 standard, using a UV-Vis spectrophotometer to scan the transmittance within the incident light wavelength range of 200~800nm ​​at room temperature. The sample film thickness was 0.02±0.002mm, and the surface was cleaned before testing. Each sample was tested three times, and the average value was taken. The results are shown in Table 2. Table 2. Barrier performance and transmittance test results From Table 2 and Figure 4 The application test results show that both water vapor transmission rate and oxygen transmission rate exhibit a trend of first decreasing and then increasing. Specifically, compared with pure PBAT (WVT≈416g / (m... 2 ·d) OT≈435cm 3 / (m 2 ·d·MPa -1 Compared to when the AMCNC content was 7%, WVP decreased to approximately 291 g / (m²). 2 ·d) (approximately 30% lower than pure PBAT), OT is simultaneously reduced to approximately 281cm 3 / (m 2 ·d·MPa -1 The improved barrier properties are attributed to: the uniform dispersion of AMCNC in the PBAT matrix increases the tortuosity of the gas diffusion path; at the same time, the strong interfacial bonding between AMCNC and PBAT reduces interfacial defects, hindering the penetration of small molecules.

[0037] The amount of AMCNC added is negatively correlated with light transmittance. When the AMCNC content is 7%, it drops to approximately 39%, and further decreases to 31% when the content is 10%. The decrease in light transmittance is related to the scattering and absorption of light by AMCNC. The PBAT-AMCNC7 composite film achieves a good balance between light transmittance (approximately 39%) and barrier properties, meeting the need for moderate shading in agricultural mulch films while enhancing mechanical and barrier properties through nanofillers.

[0038] 3. Soil degradation performance test of composite film Soil degradation tests were conducted in alkaline soil with a pH of 8.5, following GB / T19275-2003. Soil moisture content was maintained at 60% of maximum water holding capacity, and the temperature was 28±1℃. Samples were cut into 2cm×2cm square slices, weighed, and buried in the soil. They were periodically removed, cleaned, dried, and weighed again. The weight loss rate was calculated, and the results are shown in Table 3. Table 3 Soil degradation performance of PBAT-AMCNCx composite film Results analysis: As shown in Table 3, after 90 days of burial in alkaline soil, the weight loss rate of the PBAT-AMCNC7 composite film reached 49.46%, an increase of approximately 80% compared to pure PBAT (27.47%). The accelerated degradation mechanism lies in the strong water absorption of the carboxyl and hydroxyl groups on the surface of AMCNC, which promotes the hydrolysis of PBAT ester bonds; simultaneously, AMCNC enriches soil microorganisms, accelerating the enzymatic hydrolysis process. This invention achieves a synergistic effect of mechanical enhancement and accelerated degradation, solving the dual problems of insufficient strength and slow degradation in traditional PBAT mulch films.

[0039] 4. Plant growth promotion effect test Referring to *Experimental Techniques in Plant Physiology* (3rd Edition) and the agricultural industry standard NY / T3042-2016 *Technical Specifications for Monitoring Residue of Agricultural Mulch Film*, Chinese cabbage was used as the experimental crop. Mulch film samples were grouped according to the design and covered in experimental plots. The uncovered group served as a blank control (Control 1), pure PBAT mulch film as a control (Comparative Example 1), and commercially available PBAT mulch film as a control (Comparative Example 2). PBAT-AMCNCx series modified mulch film was used for Examples 3-7. Root scanning, leaf area metering, weighing, acetone extraction spectrophotometry, and ruler measurement were used to test root length, leaf area, fresh weight, dry weight, chlorophyll content, and plant height under different treatments to evaluate the promoting effect of modified mulch film on crop growth. The results are shown in Table 4. Table 4. The effect of composite film on plant growth In summary, this invention provides a green and efficient preparation method for high-performance biodegradable films from agricultural straw. Through an innovative sequential dual-modification process, a multi-faceted chemical bonding interface of ester-carboxyl-amide bonds-siloxane was successfully constructed, solving the problems of dispersion and interfacial adhesion of nanocellulose in PBAT. The prepared composite film, with an AMCNC addition of 7%, exhibits a tensile strength increase of over 37.7%, a simultaneous increase in elongation at break, and a soil degradation rate of approximately 50% after 90 days, while also significantly promoting crop growth. This material combines excellent mechanical properties, accelerated degradation characteristics, and agricultural application value, providing key technical support for the development of environmentally friendly biodegradable mulch films.

[0040] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a reinforced, toughened, and easily degradable PBAT / modified nanocellulose composite film, characterized in that, Includes the following steps: Step 1: Extraction and preparation of cellulose nanocrystals Plant straw raw materials are obtained and then subjected to impurity removal, alkali treatment and bleaching operations in sequence to remove non-cellulose components such as hemicellulose and lignin. The purified cellulose is then subjected to acid hydrolysis and mechanical dissociation to obtain cellulose nanocrystals with a diameter of less than 50 nm and an aspect ratio of greater than 100. Step 2, First Stage Surface Chemical Modification The cellulose nanocrystals obtained in step one are uniformly dispersed in an organic solvent medium to form a suspension. Maleic anhydride and an initiator are introduced into the system. The pH value of the reaction system is adjusted to the range of 3 to 5. The reaction is carried out under constant temperature stirring for 2 to 6 hours under heating conditions. After the reaction system is cooled, it is separated by centrifugation, washed until neutral and dried to prepare modified cellulose nanocrystals with maleic anhydride grafted on the surface. Step 3, Second Stage Coupled Connection and Branch Modification Prepare an aqueous solution of aminosilane coupling agent in an acidic environment of pH 3-4 and allow it to pre-hydrolyze at room temperature for 3-8 hours. Disperse the maleic anhydride modified cellulose nanocrystals obtained in step 2 in a mixed solvent of ethanol and water. Slowly add the pre-hydrolyzed silane coupling agent solution, control the reaction system temperature between 50-70℃ and continuously stir the reaction for 4-7 hours. After filtration, washing and freeze-drying, obtain the double-modified nanocellulose. Step 4: Casting and Shaping of Composite Film According to a mass ratio of 1:99 to 10:90, the double-modified nanocellulose obtained in step three and polybutylene adipate-butylene terephthalate resin are added together into an organic solvent. The mixture is continuously stirred until the solid phase is completely dissolved and a uniform and stable casting liquid is formed. The casting liquid is uniformly coated on the surface of the substrate and subjected to a drying and curing process at a specified temperature. After peeling off the film, the target composite film is obtained.

2. The method for preparing a reinforced, toughened, and easily degradable PBAT / modified nanocellulose composite film according to claim 1, characterized in that, The specific process of raw material pretreatment in step one is as follows: The impurity removal process is as follows: extract plant straw powder, add it to an alkaline aqueous solution containing 1% to 3% surfactant, and continuously stir at room temperature; The alkali treatment process is as follows: the impurity-removed cellulose raw material is immersed in a sodium hydroxide solution with a mass concentration of 10% to 20% at a solid-liquid mass ratio of 1:10 to 20, the system temperature is maintained at 50 to 60°C and the reaction is carried out for 1 to 3 hours. The bleaching process involves contacting the alkali-treated cellulose with a mixed aqueous solution of sodium chlorite and hydrochloric acid, reacting at a relatively high temperature range of 80-95°C for 0.5-2 hours, and then using sodium metabisulfite solution to remove residues.

3. The method for preparing a reinforced, toughened, and easily degradable PBAT / modified nanocellulose composite film according to claim 1, characterized in that, The plant straw raw material mentioned in step one is selected from any one or more combinations of wheat straw, oat straw, or corn straw; the mechanical dissociation treatment is selected from one or more superimposed applications of high pressure homogenization, micro-jet, ultrasonic treatment, ball milling, and bead milling technologies.

4. The method for preparing a reinforced, toughened, and easily degradable PBAT / modified nanocellulose composite film according to claim 1, characterized in that, The acid hydrolysis process in step one is as follows: According to the solid-liquid ratio of 1:20~40, the purified fiber that has completed the bleaching operation is placed in a sulfuric acid solution with a concentration of 50%~65%, and stirred and reacted in an environment of room temperature to 60℃ for 30~120 minutes. At the end of the reaction, cold water is immediately injected to terminate the reaction process.

5. The method for preparing a reinforced, toughened, and easily degradable PBAT / modified nanocellulose composite film according to claim 1, characterized in that, In step two, the initiator used is ammonium persulfate or azobisisobutyronitrile; the mass ratio of maleic anhydride to cellulose nanocrystals added is 1:4~6; the reaction temperature for the first stage of surface chemical modification is set at 70~130℃; after this stage of treatment, the carboxyl group content on the surface of the obtained maleic anhydride-modified cellulose nanocrystals reaches 1.0~3.0 mmol·g. -1 .

6. The method for preparing a reinforced, toughened, and easily degradable PBAT / modified nanocellulose composite film according to claim 1, characterized in that, The aminosilane coupling agent used in step three is 3-aminopropyltriethoxysilane or N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane; the mass ratio of the aminosilane coupling agent to the maleic anhydride-modified cellulose nanocrystals is 0.05~0.2:1; in the mixed solvent of ethanol and water, the volume ratio of ethanol to water is 3~5:1; N,N'-dicyclohexylcarbodiimide is added as a reaction catalyst in the reaction system of this step.

7. The method for preparing a reinforced, toughened, and easily degradable PBAT / modified nanocellulose composite film according to claim 1, characterized in that, The preparation parameters for the composite film in step four are as follows: The mass ratio of the double-modified nanocellulose to polybutylene adipate-butylene terephthalate is 5~7:93~95; The organic solvent used to dissolve the polymer components is selected from chloroform, dichloromethane, or N,N-dimethylformamide alone; The casting solution is applied by vacuum doctor blade coating, spin coating, or casting film formation process; The drying temperature during the film-forming stage is maintained in the range of 40~80℃, and the drying time is 2~12 hours.

8. A reinforced, toughened, and easily degradable PBAT / modified nanocellulose composite film, characterized in that, The thin film product is obtained by the preparation method according to any one of claims 1 to 7; the composite film contains a diversified chemical bonding network constructed on the surface of nanocellulose due to sequential dual chemical modification, and the network achieves a strong interfacial bond with the PBAT matrix through the synergistic effect of ester groups, carboxyl groups, amide bonds and siloxanes.

9. The composite film according to claim 8, characterized in that, When the mass fraction of the double-modified nanocellulose inside the composite film is in the range of 5% to 7%, it exhibits significant mechanical strengthening characteristics. At this time, the tensile strength of the film is increased by more than 35% compared with the pure PBAT film without the addition of the modified material, and its elongation at break is not less than 390%.

10. The composite film according to claim 8, characterized in that, This composite film possesses excellent natural environmental degradation characteristics and growth-promoting value. When buried in a high-humidity alkaline soil environment with pH ≥ 8.5, after a 90-day natural metabolic cycle, the mass degradation rate of the material itself is no less than 50%. When it is laid out as an agricultural mulch film, relying on its specific water vapor barrier and oxygen permeability balance system, it can effectively induce crop root length to increase to more than 87% and promote the chlorophyll content of plant leaves to increase by about 62%.

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