Full-biodegradable material based on straw waste and preparation method thereof

Abstract

The invention provides a full-biodegradable material based on straw waste and a preparation method thereof.The method comprises the following steps that S1, the straw waste is smashed and then treated with alkali liquor, and pretreated straw is obtained; s2, the pretreated straw and epsilon-caprolactone are subjected to a reaction under the condition of an organic tin catalyst, and polycaprolactone grafted straw is obtained; and S3, mixing polylactic acid, poly (adipic acid) / butylene terephthalate, the polycaprolactone grafted straw, the isocyanate-terminated polyurethane prepolymer and an antioxidant, and performing melt extrusion to obtain the full-biodegradable material. According to the method, a flexible polycaprolactone chain segment is constructed on the surface of the straw, and the flexible polycaprolactone chain segment, a polyurethane prepolymer and thermoplastic degradable polyester synergistically construct a multi-component compatible network, so that the high-doping-amount straw is uniformly dispersed in the composite material, the interface is continuous, the elongation at break and the flexibility are remarkably improved, and meanwhile, the biodegradability is reserved; and high-value utilization of waste straw resources is realized.

Description

Technical Field

[0001] The present application relates to the technical field of waste utilization, and in particular to a fully biodegradable material based on straw waste and a preparation method thereof. Background Art

[0002] Typical fully biodegradable polyester materials, such as polylactic acid (PLA) and polybutylene adipate / terephthalate (PBAT), have been widely used in packaging, agricultural films, and disposable products due to their mechanical properties, biodegradability, and industrial suitability. However, their relatively high raw material costs have limited their widespread application in major consumer sectors.

[0003] Agricultural waste (such as corn straw and wheat straw) is one of the most abundant biomass resources on Earth. Its main components are cellulose, hemicellulose, and lignin, and it has good biodegradability and reusability potential. Therefore, how to incorporate straw waste into biodegradable plastic systems to effectively reduce material costs while improving resource utilization and environmental benefits has become an important research direction.

[0004] However, existing technologies for preparing biodegradable materials from straw waste still face numerous challenges. For one thing, straw contains a large amount of wax, fat-soluble impurities, and some lignin. Its surface inertness and strong polarity result in poor compatibility with polyester-based biodegradable resins (such as PLA and PBAT), making interfacial delamination prone to occur, significantly reducing the mechanical properties and processing stability of the composite material. Furthermore, as a natural fiber, straw exhibits high rigidity and poor flexibility. High addition levels can easily lead to increased brittleness and a significant decrease in elongation at break, severely impacting the material's performance and product quality.

[0005] To improve the above problems, some studies have attempted to improve the interfacial compatibility between straw and polyester matrix through surface grafting modification, chemical bonding, compatibilizer compounding, etc. However, the relevant technologies still have problems such as low reaction efficiency, lack of synergy of modified structures or poor adaptability. In particular, there is still a lack of practical and effective solutions to achieve stable utilization of high-doped straw while ensuring the flexibility, ductility and degradability of the composite materials.

[0006] Therefore, there is an urgent need to develop a method that can achieve efficient utilization of straw waste while ensuring the mechanical properties of biodegradable materials, so as to promote the high-value utilization of renewable resources and the industrial development of green plastics. Summary of the Invention

[0007] The present application provides a fully biodegradable material based on straw waste and a preparation method thereof, aiming to solve the problems in the prior art of poor compatibility between straw waste and biodegradable resin and the reduced elongation at break of the material caused by high addition amount.

[0008] In a first aspect, the present application provides a method for preparing a fully biodegradable material based on straw waste, comprising the following steps: S1: crushing the straw waste and treating it with alkali solution to remove wax, fat-soluble substances and a small amount of lignin in the straw waste, exposing surface hydroxyl groups to obtain pretreated straw; S2: reacting the pretreated straw with ε-caprolactone in the presence of an organotin catalyst, so that the ε-caprolactone is ring-opened and reacts with hydroxyl groups on the surface of the pretreated straw to form polycaprolactone-grafted straw; S3: mixing polylactic acid, polybutylene adipate / terephthalate, the polycaprolactone grafted straw, isocyanate terminated polyurethane prepolymer and antioxidant, and then melt-extruding to obtain a fully biodegradable material.

[0009] According to this application, by constructing flexible polycaprolactone chain segments on the surface of straw and collaboratively constructing a multi-component compatible network with polyurethane prepolymer and thermoplastic degradable polyester, the high-dosage straw is evenly dispersed in the composite material and the interface is continuous, which significantly improves the elongation at break and flexibility while retaining biodegradability, thereby realizing high-value utilization of waste straw resources.

[0010] Specifically, in step S1, the alkali solution treatment preferentially destroys the cuticle and wax structure on the surface of the straw, while partially breaking the ester bonds between hemicellulose and lignin, exposing more hydroxyl functional groups on the surface, thereby providing sufficient reaction sites for the subsequent grafting reaction; In step S2, ε-caprolactone undergoes cationic ring-opening polymerization under organotin catalysis, starting at the hydroxyl sites on the straw surface. This results in a covalently grafted polycaprolactone segment (PCL) forming a stable, flexible coating on the straw surface. These flexible segments exhibit excellent biodegradability and plasticity, providing stress buffering and stretch-guiding channels within the composite system, preventing microcrack propagation at the interface and effectively improving the composite's elongation at break. In step S3, when the polycaprolactone grafted straw is blended with the isocyanate-terminated polyurethane prepolymer, the isocyanate end in the polyurethane prepolymer can further react with the hydroxyl residue of the grafted straw or the blended polyester to form a carbamate bond. At the same time, the flexible chain segment in the isocyanate-terminated polyurethane prepolymer is also easy to entangle with other polar segments in the system, which is conducive to the construction of a multiphase synergistic entanglement structure; at the same time, PLA gives the material basic skeleton strength, PBAT provides a flexible phase, and the three and the modified straw synergistically construct an elastic energy dissipation system during the melt extrusion process, thereby effectively relieving stress concentration without introducing irreversible crosslinking, and significantly improving the elongation at break and flexibility of the material.

[0011] In the final composite material, the hard straw skeleton, after being coated with flexible PCL chain segments, exhibits good interface transition performance in the composite matrix. The flexible segments and polyurethane construct a local flexible buffer zone that can inhibit crack initiation and expansion, and can improve the overall flexibility of the material. All components are biodegradable structures and do not produce cross-linked irreversible structures, ensuring the overall degradable performance of the material.

[0012] In some embodiments, step S1 includes: The straw waste is crushed and passed through a 60-120 mesh sieve, and then soaked in a 1wt%-3wt% sodium hydroxide aqueous solution at 60-80°C for 2-4 hours to obtain pretreated straw.

[0013] In some of the above-mentioned embodiments, the use of a sieve particle size of 60-120 mesh is beneficial for increasing the reaction specific surface area while maintaining the structural integrity of the straw, making the subsequent alkali solution treatment and grafting reaction more uniform. The alkali solution concentration is controlled between 1wt% and 3wt%, which can effectively destroy the cuticle and wax structure on the surface of the straw, while moderately breaking the ester bond between hemicellulose and lignin, releasing more hydroxyl functional groups, and providing active sites for the subsequent ε-caprolactone grafting reaction. The soaking temperature is set at 60-80°C and the soaking time is controlled at 2-4 hours, which helps to increase the reaction rate and wax dissolution efficiency, while avoiding excessive hydrolysis that leads to degradation of the cellulose backbone, thereby ensuring the structural strength of the straw and subsequent processing performance.

[0014] Compared with the original straw powder that has not been sieved or coarsely sieved, the pretreated straw obtained under this embodiment has a larger surface area and a higher surface active hydroxyl density, which can form a more continuous interface transition zone in the subsequent polycaprolactone grafting and melt blending, thereby improving the elongation at break and flexibility of the composite material, and enhancing the interface compatibility and stress conduction efficiency between the straw filler and the polymer matrix.

[0015] In some embodiments, the straw waste material is derived from at least one of wheat straw, corn straw, rice straw, and soybean straw.

[0016] In some of the above embodiments, these types of crop straw generally have the characteristics of high cellulose content, loose structure, wide distribution, and easy collection. They are naturally rich in a certain amount of hemicellulose and lignin. After appropriate alkali treatment, a large number of surface hydroxyl functional groups can be exposed, providing reaction sites for subsequent grafting modification. In addition, these straw raw materials usually come from the field harvest residues of major crops and are agricultural wastes that are large in quantity and difficult to process. Using them as the main raw material for the fully biodegradable materials of this application can not only significantly reduce the cost of raw materials, but also conform to the development direction of resource recycling and green manufacturing.

[0017] In some embodiments, step S2 includes: 100 parts of pretreated straw, 10-20 parts of ε-caprolactone, 0.1-0.3 parts of stannous octoate and 50-100 parts of toluene were mixed and reacted at 90-110°C for 2-4 hours to obtain a prepolymer grafted straw; 80-120 parts of ε-caprolactone and 0.3-0.6 parts of stannous octoate are added to the prepolymer grafted straw, and the mixture is reacted at 120-140° C. for 3-4 hours under a nitrogen atmosphere to obtain caprolactone grafted straw.

[0018] In some of the above embodiments, the "two-step grafting method" is adopted instead of the conventional one-pot blending method, which can more effectively control the grafting density and segment configuration of ε-caprolactone on the straw surface, thereby constructing a more uniform, flexible and anchoring interface transition structure.

[0019] In the first step, a limited amount of ε-caprolactone is added under relatively mild conditions (90-110°C). Using toluene as a diluent significantly reduces the concentration of free monomers in the system, aiding in uniform dispersion of the reactants, reducing the system viscosity, and inhibiting local side reactions. This allows the ε-caprolactone to preferentially react with the hydroxyl groups on the straw surface, forming a primary graft layer primarily composed of short-chain polycaprolactone. This primary graft layer not only activates the surface but, due to its short chain segments and high flexibility, effectively alleviates the incompatibility between the polarity of the straw fiber surface and the matrix polymer, providing a regular grafting platform for subsequent epitaxial grafting. The second step, carried out at a higher temperature (120-140°C) and with an ample supply of ε-caprolactone monomer, significantly enhances the system's reactivity, allowing unsaturated hydroxyl groups or the terminal hydroxyl groups of the grafted segments to further participate in ring-opening polymerization, generating medium- and long-chain polycaprolactone, which forms a coating structure with a certain thickness and flexible ductility. This structure can undergo segmental nesting and entanglement with the polycaprolactone or polyester matrix in the molten polymer matrix, inducing the formation of a continuous interfacial transition zone during melt extrusion.

[0020] Compared with the one-pot method that is prone to over-polymerization, uneven grafting distribution or side reactions, the two-step grafting method significantly improves the directionality and uniformity of grafting through step-by-step temperature and quantity control operations. The resulting grafted straw has a higher grafting density and a more uniform distribution, enhancing the physical intercalation and interface anchoring with the polymer matrix; at the same time, the length of the polycaprolactone chain segment is controlled, and the flexible chain layer can effectively buffer the stress concentration area, thereby improving the elongation at break and impact resistance; the obtained caprolactone grafted straw presents a "rigid core-flexible shell" structure, which is easier to disperse during the blending process and has a better viscoelastic match with the matrix, thereby obtaining a highly flexible and fully biodegradable composite material.

[0021] In some embodiments, in step S3, the isocyanate-terminated polyurethane prepolymer is prepared by the following method: Mix 50 parts of polyester diol and 10-30 parts of isocyanate, and react at 70-100°C for 1-3 hours to obtain an isocyanate-terminated polyurethane prepolymer.

[0022] In some of the above embodiments, the isocyanate-terminated polyurethane prepolymer uses polyester polyol as the flexible main chain. During the synthesis process, the polyester diol reacts with the isocyanate to generate a polyurethane prepolymer containing an -NCO end group. During the subsequent blending process, the prepolymer can react in situ or undergo chain entanglement with caprolactone-grafted straw and polyester matrices such as polylactic acid and PBAT to form an interfacial transition network with synergistic physical entanglement and chemical anchoring.

[0023] Compared to prepolymers prepared from conventional polyether polyols, these polyester prepolymers, due to their structure closer to the polyester matrix and higher polarity compatibility, exhibit better dispersibility and interfacial fusion during melt blending, effectively avoiding microscopic phase separation caused by poor compatibility. Furthermore, the moderate rigidity and flexibility of their chain segments enhance the overall viscoelastic coordination of the material, creating a flexible and continuous stress-conducting structure and contributing to increased flexibility.

[0024] Therefore, the introduction of this polyester isocyanate-terminated polyurethane prepolymer not only enhances the interfacial adhesion strength between the grafted straw filler and the polyester matrix, but also synergistically improves the flexibility, processing fluidity and structural stability of the composite material during degradation, showing comprehensive performance that is better than the polyether system.

[0025] In some embodiments, the polyester diol includes polycaprolactone diol, and the isocyanate includes hexamethylene diisocyanate.

[0026] In some of the aforementioned embodiments, the polycaprolactone diol serves as the flexible backbone of the isocyanate-terminated polyurethane prepolymer. Its structure is highly consistent with the polycaprolactone segments grafted onto the straw surface, enabling synergistic nesting and segment reconstruction between homogeneous segments during the blending process. This segment structural consistency significantly enhances the interfacial adhesion strength between the prepolymer and the grafted straw, promoting the formation of a continuous, stable, and flexible physical-chemical composite anchoring zone at the interface, thereby improving the composite's flexibility.

[0027] Furthermore, hexamethylene diisocyanate (HDI) is a typical aliphatic linear diisocyanate. Its molecular structure contains no rigid cyclic or sterically hindered side groups, resulting in moderate end group reactivity and a uniform and controllable reaction process. This makes it particularly suitable for the end-capping control required to prevent crosslinking in this application. Compared to other aliphatic isocyanates (such as isophorone diisocyanate), HDI possesses a more flexible chain segment structure and polarity matching properties. This not only makes it easier to form a continuous and flexible interfacial network with the polycaprolactone segments, but also significantly improves the viscoelastic compatibility and stress dispersion capabilities of the blend.

[0028] Therefore, the combination of polycaprolactone diol and hexamethylene diisocyanate helps to construct a polyurethane prepolymer structure with greater flexibility, continuity and interfacial fusion capability, synergistically improving the flexibility and molding processing performance of straw-based composite materials, and maintaining excellent mechanical properties and degradability under high straw doping conditions.

[0029] As an example, in one embodiment of the present application, polycaprolactone diol-1000 (PCL-1000) is used as the polyester diol.

[0030] In some embodiments, step S3 includes: 50 parts of polylactic acid, 20-30 parts of polybutylene adipate / terephthalate, 30-40 parts of the polycaprolactone grafted straw, 5-15 parts of isocyanate terminated polyurethane prepolymer and 0.1-1 part of antioxidant are mixed and melt-extruded to obtain a fully biodegradable material.

[0031] In some of the aforementioned embodiments, PLA and PBAT are used as the matrix resins in the composite system, with a reasonable proportion of synergy to balance the material's rigidity and flexibility. PLA has a high modulus and biodegradability, while PBAT has excellent flexibility and impact strength. The blend of the two achieves a balance between performance and degradation rate. The addition of PBAT significantly improves PLA's high brittleness and poor toughness, making the composite material more suitable for practical processing applications such as injection molding or extrusion.

[0032] Straw filler grafted with polycaprolactone serves as a highly doped natural reinforcing phase, providing excellent degradability and mechanical support. Its flexible polycaprolactone segments can nest or entangle with the PBAT and prepolymer in the matrix, significantly enhancing interfacial bonding. Simultaneously, the isocyanate-terminated polyurethane prepolymer, acting as a reactive compatibilizer, reacts in situ with residual hydroxyl groups, polyester segments, and functional groups in the straw surface during the melt extrusion process, forming a cross-phase chemical connection network. This effectively enhances the interfacial adhesion strength between the filler and the matrix, suppressing the risk of interfacial debonding and performance degradation caused by high doping.

[0033] In addition, the addition of antioxidants can inhibit the thermal oxidative degradation of polylactic acid and PBAT during high-temperature melt processing, avoid the deterioration of melt fluidity and mechanical properties caused by the decrease in molecular weight, and further ensure the processing stability and service life of the material.

[0034] Therefore, through the above-mentioned coordinated design of component ratios and structures, the present invention achieves comprehensive improvements in toughness, interface bonding strength and processing performance of the fully biodegradable composite material while maintaining a high straw content.

[0035] In some embodiments, in step S3, the melt index of the polylactic acid at 210°C and 2.16 kg is 6-15 g / 10 min; the melt index of the polybutylene adipate / terephthalate at 190°C and 2.16 kg is 2-6 g / 10 min; and the antioxidant includes antioxidant 1010.

[0036] In some of the above embodiments, the melt index of polylactic acid is controlled at 6-15 g / 10 min, which can ensure that it has suitable fluidity and dispersibility during the melt extrusion process, so that it can still effectively coat the filler in a highly filled straw environment and form a continuous phase with PBAT and polyurethane prepolymer, which helps to improve the formability and interfacial adhesion efficiency of the composite material; PBAT, as a flexible modified phase, has a melt index set to 2-6 g / 10 min, which is conducive to matching with PLA with medium fluidity, constructing a blending network structure with viscoelastic coordination, which can not only fully wrap the flexible chain segments, but also provide lubrication and toughening effects on rigid PLA and grafted straw, and synergistically achieve uniform dispersion and stable extrusion of the composite material; antioxidant 1010 has good thermal stability and processing stability, can effectively capture free radicals during melt processing, inhibit thermal oxidative aging and molecular chain breakage of PLA and PBAT, especially in systems containing natural components (such as grafted straw) that are easily affected by heat, playing an important protective role, significantly improving the processing stability and long-term performance of the material.

[0037] Therefore, by controlling the melt index matching of PLA and PBAT and introducing the highly effective thermal stabilizer antioxidant 1010, while ensuring the uniform dispersion of the highly doped biofiller, the fully biodegradable material achieves controllable rheological behavior and structural stability during the extrusion molding process, further enhancing the material's processability. For example, the melt index of polylactic acid used in one embodiment of this application at 210°C and 2.16 kg is 10 g / 10 min; the melt index of polybutylene adipate / terephthalate at 190°C and 2.16 kg is 4 g / 10 min.

[0038] In some embodiments, the melt extrusion conditions include: A twin-screw extruder is used for melt extrusion, and the temperatures of each zone of the twin-screw extruder are 140~150℃, 150~160℃, 165~175℃, 175~180℃, and 175~180℃, respectively, and the screw speed is 60~100r / min.

[0039] In some of the aforementioned embodiments, the use of a twin-screw extruder for melt extrusion offers strong shear, mixing, and material dispersion capabilities, making it particularly suitable for composite systems containing high amounts of natural fillers (e.g., polycaprolactone grafted onto straw). Zoned temperature control facilitates the sequential softening, mixing, and uniform dispersion of the different components during the melting process, preventing degradation of PLA or PBAT or thermal cracking of the straw filler due to rapid temperature increases. Screw speed is controlled between 60 and 100 r / min, ensuring adequate mixing while avoiding degradation of components such as PLA due to excessive shear. By setting these melt extrusion process parameters, the components are fully melted, dispersed, and reacted during processing, enabling synergistic nesting and continuous structural construction between multiphase systems, significantly improving the flexibility of the composite material.

[0040] In a second aspect, the present application provides a fully biodegradable material based on straw waste, which is prepared according to the method described in any embodiment of the first aspect.

[0041] According to the present application, the fully biodegradable material uses polycaprolactone grafted modified straw waste as the main bio-based reinforcing component, and is synergistically blended with polylactic acid, poly(butylene adipate / terephthalate) and isocyanate-terminated polyurethane prepolymers. Through step-by-step chemical grafting, introduction of structure-matching prepolymers and optimization of extrusion processing conditions, the interface adhesion enhancement and stress conduction continuity between natural straw waste and thermoplastic polyester are achieved.

[0042] In the material structure, the straw particles grafted with polycaprolactone provide good polar compatibility and a flexible interface, while the isocyanate-terminated polyurethane prepolymer forms a synergistically nested transition network between the polyester matrix and the grafted straw, effectively alleviating the problems of interfacial debonding and concentrated fracture caused by high straw doping. At the same time, the introduction of the polyester prepolymer improves the fluidity and dispersibility of the blending system, and enhances the processing stability and controllable formability of the highly doped system.

[0043] Therefore, the fully biodegradable material provided in this application maintains a high bio-based content while still having good flexibility, ductility and thermal processing adaptability. It is particularly suitable for packaging, agricultural film, disposable products and other thermoplastic processing fields that require green environmental protection, degradability and structural strength.

[0044] Compared with the prior art, the present invention has the following advantages: 1. This application uses polycaprolactone to graft-modify straw waste, significantly improving the interfacial compatibility and flexible synergy between natural straw and the polyester matrix, solving the problem of high brittleness and easy fracture of the material under high straw doping conditions, and improving the elongation at break and flexibility of the material; 2. By introducing a structurally matching polyester-type isocyanate-terminated polyurethane prepolymer, a physical-chemical composite transition network connecting the grafted straw and the polyester matrix was constructed, achieving a synergistic enhancement of interfacial adhesion and chain segment continuity without inducing cross-linking, effectively avoiding interfacial debonding and phase separation.

[0045] 3. The overall material system is a thermoplastic structure, which can be continuously processed and formed through a twin-screw melt extrusion process. It has good dispersibility and fluidity and is suitable for industrial-scale preparation. DETAILED DESCRIPTION

[0046] The various embodiments or implementation schemes in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments.

[0047] Throughout this specification, reference to terms such as "one embodiment," "some embodiments," "illustrative embodiments," "examples," "specific examples," or "some examples" means that a specific feature, structure, material, or characteristic described in conjunction with an embodiment or example is included in at least one embodiment or example of the present application. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0048] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features being referred to. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of such features. Throughout the description of this application, "plurality" means at least two, for example, two, three, etc., unless otherwise specifically defined.

[0049] In the description of this specification, unless otherwise specified, "parts" refer to "parts by mass".

[0050] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0051] Soybean straw: collected from surrounding rural areas, washed, dried, and initially crushed for later use; Polylactic acid: melt index at 210°C and 2.16 kg is 10 g / 10 min; Polybutylene adipate / terephthalate: The melt index at 190°C and 2.16 kg is 4 g / 10 min. Example

[0052] Preparation of fully biodegradable materials based on straw waste: Take dried soybean straw, crush it, and pass it through an 80-mesh sieve. Remove the sieve residue. Add the straw powder to a 2 wt% sodium hydroxide aqueous solution and soak it in a 70°C water bath for 3 hours. After the reaction is complete, wash it thoroughly with deionized water to a pH of approximately 7. Dry it and use it as pretreated straw. Preparation of polycaprolactone grafted straw: 100 parts of pretreated straw were added to a reactor, and then 15 parts of ε-caprolactone, 0.2 parts of stannous octoate and 80 parts of toluene were added and mixed. The mixture was stirred and refluxed in an oil bath at 95°C for 3 hours under continuous nitrogen protection according to a condensation reflux device. The stirring speed was maintained at about 300 r / min during the reaction. After the reaction was completed, the mixture was cooled to room temperature and the toluene was recovered by rotary evaporation to obtain polymer grafted straw. Then, 100 parts of ε-caprolactone and 0.5 parts of stannous octoate were added to the prepolymer grafted straw, and the reaction was continued at 130°C for 3.5 hours under a nitrogen atmosphere. After the reaction was completed, the system was cooled to room temperature, poured into precooled methanol for precipitation, stirred thoroughly, and ultrasonically cleaned for 20 minutes to remove unreacted ε-caprolactone and free polycaprolactone. The precipitate was collected by filtration and washed with methanol three times. The final product was dried in a vacuum drying oven at 60°C for 12 hours to obtain polycaprolactone grafted straw. Preparation of isocyanate-terminated polyurethane prepolymer: 50 parts of PCL-1000 and 20 parts of hexamethylene diisocyanate were added to a reactor, heated to 90°C, stirred and reacted for 2 hours to obtain an isocyanate-terminated polyurethane prepolymer, and cooled for later use; 50 parts PLA, 25 parts PBAT, 35 parts polycaprolactone-grafted straw, 10 parts isocyanate-terminated polyurethane prepolymer, and 0.5 parts antioxidant 1010 were mixed in a mixer and then melt-extruded in a twin-screw extruder. The extrusion temperature zones were set as follows: Zone 1: 145°C; Zone 2: 155°C; Zone 3: 170°C; Zone 4: 178°C; and Zone 5: 178°C. The screw speed was set at 80 r / min. After cooling, the extruded strands were pelletized to produce a fully biodegradable material. Example

[0053] Preparation of fully biodegradable materials based on straw waste: The method is similar to Example 1, except that the preparation method of polycaprolactone grafted straw is different, specifically: 100 parts of pretreated straw were added to the reactor, followed by 115 parts of ε-caprolactone and 0.7 parts of stannous octoate. The reaction was carried out at 130°C under a nitrogen atmosphere for 3.5 hours. After the reaction, the system was cooled to room temperature and poured into pre-cooled methanol for precipitation. The mixture was stirred thoroughly and ultrasonically cleaned for 20 minutes to remove unreacted ε-caprolactone and free polycaprolactone. The precipitate was collected by filtration and washed three times with methanol. The final product was dried in a vacuum drying oven at 60°C for 12 hours to obtain polycaprolactone-grafted straw. Example

[0054] Preparation of fully biodegradable materials based on straw waste: The method is similar to Example 1, except that the preparation method of the isocyanate-terminated polyurethane prepolymer is different, specifically: 50 parts of polytetrahydrofuran-1000 and 20 parts of hexamethylene diisocyanate were added to a reactor, heated to 90° C., stirred and reacted for 2 hours to obtain an isocyanate-terminated polyurethane prepolymer, which was then cooled for later use. Example

[0055] Preparation of fully biodegradable materials based on straw waste: The method is similar to Example 1, except that the preparation method of the isocyanate-terminated polyurethane prepolymer is different, specifically: 50 parts of polybutylene adipate diol-1000 and 20 parts of hexamethylene diisocyanate were added to a reactor, heated to 90° C., stirred and reacted for 2 hours to obtain an isocyanate-terminated polyurethane prepolymer, which was then cooled for later use. Example

[0056] Preparation of fully biodegradable materials based on straw waste: The method is similar to Example 1, except that the preparation method of the isocyanate-terminated polyurethane prepolymer is different, specifically: 50 parts of PCL-1000 and 20 parts of isophorone diisocyanate were added to the reactor, the temperature was raised to 90°C, and the mixture was stirred and reacted for 2 hours to obtain an isocyanate-terminated polyurethane prepolymer, which was then cooled for later use. Comparative Example 1 Preparation of fully biodegradable materials based on straw waste: Take dried soybean straw, crush it, and pass it through an 80-mesh sieve. Remove the sieve residue. Add the straw powder to a 2 wt% sodium hydroxide aqueous solution and soak it in a 70°C water bath for 3 hours. After the reaction is complete, wash it thoroughly with deionized water to a pH of approximately 7. Dry it and use it as pretreated straw. Preparation of polycaprolactone grafted straw: 100 parts of pretreated straw were added to a reactor, and then 15 parts of ε-caprolactone, 0.2 parts of stannous octoate and 80 parts of toluene were added and mixed. The mixture was stirred and refluxed in an oil bath at 95°C for 3 hours under continuous nitrogen protection according to a condensation reflux device. The stirring speed was maintained at about 300 r / min during the reaction. After the reaction was completed, the mixture was cooled to room temperature and the toluene was recovered by rotary evaporation to obtain polymer grafted straw. Then, 100 parts of ε-caprolactone and 0.5 parts of stannous octoate were added to the prepolymer grafted straw, and the reaction was continued at 130°C for 3.5 hours under a nitrogen atmosphere. After the reaction was completed, the system was cooled to room temperature, poured into precooled methanol for precipitation, stirred thoroughly, and ultrasonically cleaned for 20 minutes to remove unreacted ε-caprolactone and free polycaprolactone. The precipitate was collected by filtration and washed with methanol three times. The final product was dried in a vacuum drying oven at 60°C for 12 hours to obtain polycaprolactone grafted straw. 55 parts PLA, 30 parts PBAT, 35 parts polycaprolactone-grafted straw, and 0.5 parts antioxidant 1010 were mixed in a mixer and then melt-extruded in a twin-screw extruder. The extrusion temperature zones were set as follows: Zone 1: 145°C; Zone 2: 155°C; Zone 3: 170°C; Zone 4: 178°C; and Zone 5: 178°C. The screw speed was set at 80 rpm. After cooling, the extruded strands were pelletized to produce a fully biodegradable material. Test section The elongation at break (%) of the fully biodegradable material was obtained by testing each embodiment and comparative example in accordance with GB / T 1040.3-2006 "Plastics - Determination of Tensile Properties - Part 3: Test Conditions for Film and Sheeting," which includes the following steps: molding the samples using an injection molding process and then cutting them into strips with a width of 10 mm, a length of 200 mm, and a thickness of 1 mm; storing them in a standard environment (23±2°C, relative humidity of 50±5%) for at least 48 hours, and testing them using an electronic universal tensile testing machine with a clamping length of 50 mm and a tensile speed of 50 mm / min at a temperature of 23±2°C and a humidity of 50±5%. Five samples were tested in each group, and the average value was taken. The results are shown in Table 1. Table 1 Elongation at break (%) Example 1 309 Example 2 248 Example 3 226 Example 4 282 Example 5 275 Comparative Example 1 156 According to Table 1, the elongation at break of each example is better than that of Comparative Example 1, indicating that the fully biodegradable material solution provided by this application, based on the synergistic modification of polycaprolactone-grafted straw and isocyanate-terminated polyurethane prepolymer, can significantly improve the elongation at break of the composite material, thereby enhancing its flexibility and deformation resistance. It is particularly suitable for the application of biodegradable products with high toughness requirements in high-straw filling scenarios. Specifically, in Comparative Example 1, no isocyanate-terminated polyurethane prepolymer was added, and the lack of flexible chain segments to assist in the construction of the interfacial transition layer resulted in poor interfacial adhesion between the straw filler and the polylactic acid matrix, and stress concentration, resulting in poor ductility of the overall material and the lowest elongation at break of only 156%.

[0057] According to Examples 1 and 2, the preparation path of polycaprolactone grafted straw has a significant impact on the flexibility of the final composite material. Example 1 uses a "two-step grafting method" to prepare polycaprolactone grafted straw with a higher grafting degree and more flexible segments. It can form a dense and continuous flexible layer on the straw surface, enhancing the interface fusion ability with the polymer matrix and prepolymer, so the elongation at break is as high as 309%; while Example 2 uses a "one-step grafting method", the resulting grafted segments are shorter and unevenly distributed, and the interface buffering capacity is insufficient, resulting in a drop in elongation at break to 248%. This shows that optimizing the grafting path to improve segment coverage and configuration control can help significantly improve the flexibility of the composite material.

[0058] According to Examples 1, 3 and 4, the thermal stability and polarity matching of the polyurethane prepolymer segment structure are the key factors determining material properties. In Example 3, polyether diol is used to synthesize prepolymer. The polyether segment has poor thermal stability during high-temperature extrusion and is mismatched with the polarity of polylactic acid and polycaprolactone segments, resulting in poor interface bonding and insufficient mobility. Elongation at break is only 226%; Example 4 uses polybutylene adipate diol to prepare prepolymer. Its polyester segment is close to the polarity of the matrix and straw surface structure, and has better compatibility and thermal stability. Elongation at break is increased to 282%; and in Example 1, the prepolymer prepared using PCL-1000 is highly consistent with the straw grafting segment, and the interface reconstruction ability is stronger, reaching a maximum elongation at break of 309%. It is shown that the main chain structure of the prepolymer should be highly polar matched with the grafting segment and the matrix structure to collaboratively construct a flexible through network.

[0059] Examples 1 and 5 demonstrate that the type of isocyanate significantly influences the flexible network-building ability of polyurethane prepolymers. In Example 1, hexamethylene diisocyanate, with its linear, flexible structure, is polarly compatible with the polycaprolactone segments and offers minimal steric hindrance, facilitating the formation of continuous, mobile, flexible segments, resulting in a high elongation at break (309%). In contrast, in Example 5, isophorone diisocyanate, with its rigid ring structure, reduces segment flexibility and interfacial stress transfer, resulting in a drop in elongation at break of only 275%. This suggests that the use of hexamethylene diisocyanate in end-capped prepolymer design can further improve the flexibility of the resulting isocyanate-terminated polyurethane prepolymer. Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some or all of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for preparing a fully biodegradable material based on straw waste, characterized in that: The following steps are involved: S1: crushing the straw waste and treating it with alkali solution to remove wax, fat-soluble substances and a small amount of lignin in the straw waste, exposing surface hydroxyl groups to obtain pretreated straw; S2: reacting the pretreated straw with ε-caprolactone in the presence of an organotin catalyst, so that the ε-caprolactone is ring-opened and reacts with hydroxyl groups on the surface of the pretreated straw to form polycaprolactone-grafted straw; S3: mixing polylactic acid, polybutylene adipate / terephthalate, the polycaprolactone grafted straw, isocyanate terminated polyurethane prepolymer and antioxidant, and then melt-extruding to obtain a fully biodegradable material.

2. The method according to claim 1, characterized in that The step S1 comprises: The straw waste is crushed and passed through a 60-120 mesh sieve, and then soaked in a 1wt%-3wt% sodium hydroxide aqueous solution at 60-80°C for 2-4 hours to obtain pretreated straw.

3. The method according to claim 1, characterized in that The straw waste material comes from at least one of wheat straw, corn straw, rice straw and soybean straw.

4. The method according to claim 1, wherein The step S2 comprises: 100 parts of pretreated straw, 10-20 parts of ε-caprolactone, 0.1-0.3 parts of stannous octoate and 50-100 parts of toluene were mixed and reacted at 90-110°C for 2-4 hours to obtain a prepolymer grafted straw; 80-120 parts of ε-caprolactone and 0.3-0.6 parts of stannous octoate are added to the prepolymer grafted straw, and the mixture is reacted at 120-140° C. for 3-4 hours under a nitrogen atmosphere to obtain caprolactone grafted straw.

5. The method according to claim 1, wherein In step S3, the isocyanate-terminated polyurethane prepolymer is prepared by the following method: Mix 50 parts of polyester diol and 10-30 parts of isocyanate, and react at 70-100°C for 1-3 hours to obtain an isocyanate-terminated polyurethane prepolymer.

6. The method according to claim 5, characterized in that The polyester diol includes polycaprolactone diol, and the isocyanate includes hexamethylene diisocyanate.

7. The method according to any one of claims 1 to 6, characterized in that The step S3 comprises: 50 parts of polylactic acid, 20-30 parts of polybutylene adipate / terephthalate, 30-40 parts of the polycaprolactone grafted straw, 5-15 parts of isocyanate terminated polyurethane prepolymer and 0.1-1 part of antioxidant are mixed and melt-extruded to obtain a fully biodegradable material.

8. The method according to claim 7, characterized in that In the step S3: The melt index of the polylactic acid at 210° C. and 2.16 kg is 6 to 15 g / 10 min; The melt index of the polybutylene adipate / terephthalate at 190° C. and 2.16 kg is 2 to 6 g / 10 min; The antioxidant includes antioxidant 1010 .

9. The method according to claim 7, characterized in that The conditions for the melt extrusion include: A twin-screw extruder is used for melt extrusion, and the temperatures of each zone of the twin-screw extruder are 140~150℃, 150~160℃, 165~175℃, 175~180℃, and 175~180℃, respectively, and the screw speed is 60~100r / min.

10. A fully biodegradable material based on straw waste, characterized in that: Prepared according to the method according to any one of claims 1 to 9.