Preparation method of biodegradable polylactic acid plastic

Abstract

The invention relates to the technical field of high polymer material processing and biodegradable plastic, in particular to a preparation method of biodegradable polylactic acid plastic. The invention discloses a preparation method of biodegradable polylactic acid plastic. The polylactic acid plastic is composed of a polylactic acid matrix, an elastomer toughening phase, a nanocellulose enhancer, a reactive compatibilizer and a three-layer core-shell structure microencapsulated enzyme preparation. The preparation method comprises the steps of raw material pretreatment and master batch preparation, reactive melt blending, multi-stage hot stretching orientation and cooling forming, the microencapsulated enzyme preparation sequentially comprises a pH response layer, a temperature response layer and a polymer protection outer layer from inside to outside, and intelligent control over the material degradation behavior is achieved through a synergistic triggering mechanism. The product prepared by the invention has the characteristics of high tensile strength, high elongation at break, high impact strength, high thermal deformation temperature and controllable biodegradability, and effectively solves the technical problem that the toughness, heat resistance and controllable degradability of the polylactic acid material are difficult to consider at the same time.

Description

[0001] This invention relates to the field of polymer material processing and biodegradable plastics technology, specifically to a method for preparing biodegradable polylactic acid plastic. Background Technology

[0002] Polylactic acid (PLA) is a biodegradable polymer derived from renewable resources. It possesses excellent mechanical properties and biocompatibility, and has been widely used in packaging, fibers, 3D printing, and medical materials. However, PLA materials also have significant drawbacks, such as inherent brittleness, low elongation at break, and a slow crystallization rate leading to lower heat resistance temperatures, limiting its application in high-performance fields. To improve the toughness of PLA, blending is often employed. However, due to the poor polarity and compatibility between PLA and polybutylene terephthalate / polybutylene succinate (PBAT / PBS), simple physical blending often results in significant phase separation, weak interfacial bonding, and poor toughening efficiency. Without compatibilization, the mechanical properties of PLA / PBAT blends show limited improvement, and the material strength may even decrease.

[0003] To address the aforementioned issues, existing research has explored several technical methods to improve the interfacial compatibility between PLA and elastomers. For example, CN120737570A discloses a method for preparing a high-toughness transparent plastic cup. This method uses isosorbide-based plasticizer, polydimethylsiloxane, citric acid, and the remainder polycaprolactone to prepare a modified elastomer toughening agent. Cinnamaldehyde bamboo pulp antibacterial reinforcing agent is prepared using cinnamon, bamboo pulp, bio-based 1,3-propanediol (PO3G), and the remainder water. High-toughness polylactic acid is then prepared using the cinnamaldehyde bamboo pulp antibacterial reinforcing agent, the modified elastomer toughening agent, and polylactic acid. The resulting high-toughness transparent plastic cup exhibits high strength, excellent toughness, good Staphylococcus aureus inhibition, and meets biodegradability requirements. Advantages: CN120737568A discloses a strong and tough fully bio-based polylactic acid / polyamide composite material and its preparation method. This material is based on a core-shell amide modified material formed from an amino nanoparticle reactive-filled functional elastomer copolymer. This modified material possesses an in-situ formed microscopic core-shell structure and a large number of amide bonds, which simultaneously increases the number of hydrogen bonds between molecular chains in the blend system. This improves the entanglement between microscopic molecular chains and the intermolecular forces, thus improving the system's compatibility and enhancing the material's absorption of external stress and energy. Under stress and deformation, the material exhibits more significant plastic deformation, giving it better mechanical properties. However, existing technologies focus more on improving the material's strength, toughness, and flowability to meet the requirements of injection molding, extrusion, and other molding processes, producing robust and durable products, while neglecting issues such as material degradability and performance balance.

[0004] In summary, how to comprehensively utilize material formulation and processing technology to innovate a novel method for preparing PLA-based composite materials, and achieve breakthroughs in the performance of PLA composite materials in terms of high toughness, high heat resistance, and controllable biodegradability, has become an urgent technical problem to be solved in this field. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for preparing biodegradable polylactic acid plastics, so as to solve the technical problems in the prior art such as poor compatibility between PLA and most elastomer toughening agents, difficulty in balancing performance, narrow processing technology window, and poor controllability of degradation behavior.

[0006] The specific technical solution is as follows: A method for preparing biodegradable polylactic acid (PLA) plastic, wherein the preparation method involves constructing a PLA + elastomer + nanocellulose ternary composite system in a PLA matrix, and employing a technical route combining reaction compatibilization, multi-level orientation stretching, and enzymatic degradation triggering to prepare biodegradable PLA plastic.

[0007] Furthermore, the ternary composite system comprises 100 parts of PLA matrix resin, 15-30 parts of elastomer toughening phase, 3-8 parts of nanocellulose reinforcing agent, 0.3-1.5 parts of maleic anhydride monomer, 0.03-0.15 parts of peroxide initiator, 1-5 parts of microencapsulated enzyme preparation, 0.5-1.5 parts of composite nucleating agent, 0.1-0.8 parts of antioxidant, and 0.2-1.5 parts of lubricant.

[0008] Furthermore, the elastomer toughening phase is a mixture of polybutylene adipate-terephthalate (PBAT) and polybutylene succinate (PBS) in a weight ratio of 7:3; the nanocellulose reinforcing agent is cellulose nanocrystals (CNC); the reactive compatibilizer is maleic anhydride-grafted polylactic acid (PLA-g-MAH); the wall material of the microencapsulated enzyme preparation is a pH-temperature dual-responsive copolymer, and the core material is thermophilic lipase; the composite nucleating agent is a mixture of talc and zinc phenyl phosphate in a weight ratio of 3:1.

[0009] Furthermore, the compatibilization reaction first involves pre-reacting a portion of maleic anhydride (MAH) with a peroxide and an elastomer (PBAT / PBS) to graft MAH functional groups onto its molecular chains. Subsequently, it is master-blended with polylactic acid (PLA), the remaining MAH, and an initiator. During this stage, MAH grafting also occurs on the PLA molecular chains, leading to in-situ reactions between the grafted products to form a polylactic acid-co-polybutylene adipate terephthalate / polybutylene succinate (PLA-co-PBAT / PBS) block copolymer at the two-phase interface. This process achieves synergistic compatibilization from within the elastomer phase to the two-phase interface, significantly improving the compatibility of the multi-component system.

[0010] Furthermore, the multi-level orientation stretching is based on the reaction compatibilization and uses a precisely controlled multi-level thermal stretching process to make PLA molecular chains and nanocellulose whiskers highly oriented along the stretching direction, forming a nanofiber-crystal interpenetrating network structure that runs through the material.

[0011] Furthermore, the enzymatic degradation triggering is achieved by uniformly dispersing a microencapsulated enzyme preparation with a three-layer core-shell structure within the material system, realizing intelligent synergy between material performance and degradation behavior. During material processing and use, the outermost high glass transition temperature (Tg) polymer protective shell maintains structural integrity, effectively isolating the internal enzyme preparation and thus ensuring the material's mechanical properties and long-term stability. When the material is disposed of and placed in a specific composting environment, the middle temperature-sensitive polymer undergoes phase transition shrinkage, while the inner pH-responsive gel dissolves. The synergistic mechanical stress generated by these two processes is sufficient to rupture the protective shell, precisely releasing thermophilic lipase, which efficiently catalyzes the hydrolysis of the polylactic acid matrix. This design successfully realizes the intelligent transformation of the material from high-performance long-term use to rapid degradation after disposal.

[0012] A method for preparing biodegradable polylactic acid plastic includes the following steps: S1: Thermophilic lipase and Tween 80 were dissolved in deionized water and thoroughly mixed to form the aqueous phase. Dichloromethane and Span 80 were mixed to form the oil phase. At room temperature, the aqueous phase was slowly poured into the oil phase, and high-speed shear emulsification was performed to form a stable primary emulsion. A sodium alginate solution was prepared and slowly added dropwise to the primary emulsion while maintaining stirring. After the addition was complete, a calcium chloride solution was slowly added, followed by centrifugation. The microcapsules were collected and washed with deionized water to obtain microcapsules with an inner layer. The microcapsules with the inner layer were redispersed in liquid paraffin, and N-isopropylacrylamide and methacrylic acid monomers were added. The mixture was stirred at low speed to ensure that the monomers were uniformly dispersed on the surface of the microcapsules. The system was heated to 60°C, and ammonium persulfate was added under nitrogen protection. A temperature-pH dual-responsive polymer middle layer was formed on the surface of the microcapsules through free radical copolymerization. After the reaction was completed, the mixture was centrifuged and washed with cyclohexane to obtain microcapsules with inner and middle layers. Poly(N-isopropylacrylamide-co-methacrylic acid) was dissolved in a mixed solvent of ethanol and water, and 0.15 parts of methylenebisacrylamide were added as a crosslinking agent to form a homogeneous solution. The prepared inner and middle bilayer microcapsules were dispersed in this solution and then coated using spray drying technology. At the high temperature of spray drying, the crosslinking agent was activated, forming a crosslinked network during film formation. The powder was collected to obtain a microencapsulated enzyme preparation with a complete three-layer shell structure.

[0013] S2: Place PLA granules in a vacuum drying oven and dry them at 80°C and 0.1MPa for later use. After drying CNC at 105°C for 4 hours, mix it with PLA powder and silane coupling agent KH-550 to prepare CNC masterbatch. Mix microencapsulated enzyme preparation with PLA powder at low temperature to prepare enzyme preparation masterbatch.

[0014] S3: A twin-screw extruder is used, with temperature settings from the feed port to the die head as follows: 170℃, 175℃, 180℃, 185℃, 180℃, 175℃. Pre-dried PLA granules, PBAT, PBS, and a portion of PLA-g-MAH are first added through the main feed port. After the material enters the melting zone, another portion of PLA-g-MAH is added through the side feed port. During this stage, an in-situ reaction occurs in the molten state, generating a graft copolymer that acts as a compatibilizer. Subsequently, CNC masterbatch is added through the side feed port. At the low-temperature zone at the end of the extruder, a composite nucleating agent, antioxidant, lubricant, and enzyme preparation masterbatch are added. The melt residence time is 2.5 minutes. The extruded strip is cooled in a water bath and then pelletized.

[0015] S4: Remelt the extruded granules and press them into sheets. Perform three-stage stretching on the sheets with a total stretch ratio of 7.5:1 and heat set them at this temperature for 10 seconds.

[0016] S5: The stretched sheet is subjected to gradient cooling through a 60°C hot air zone and a 25°C cooling water roller, and finally wound up to obtain the final biodegradable PLA composite film.

[0017] Furthermore, the nanocellulose CNC in S2 has a mass of 3 to 8 parts; the microencapsulated enzyme preparation has a mass of 1 to 5 parts.

[0018] Furthermore, the PLA-g-MAH in S3 is a mixture of maleic anhydride monomer and peroxide initiator, accounting for 50% of the total PLA-g-MAH.

[0019] Furthermore, the total mass of PBAT and PBS added in S3 is 15-30 parts.

[0020] Furthermore, in the three-stage tensile test described in S4, the first stage tensile test is completed at 75°C with a segmented tensile ratio of 1.0~2.0:1; the second stage tensile test is completed at 80°C with a segmented tensile ratio of 2.0~3.5:1; and the third stage tensile test is completed at 85°C with a segmented tensile ratio of 1.5~2.5:1.

[0021] Furthermore, the residence time of the microencapsulated enzyme preparation described in S4 is controlled to be between 30 and 90 seconds. Compared with the prior art, the present invention has the following beneficial effects: (1) Balancing high toughness and high strength: By reactive compatibilizing and blending PLA with the elastomer toughening phase, the two phases are uniformly dispersed at the nanoscale and firmly bonded at the interface, effectively exerting the toughening effect of the elastomer while maintaining the strength and modulus of the material.

[0022] (2) Significantly improved thermal stability and heat resistance: On the one hand, the efficient nucleating component zinc phenyl phosphate in the composite nucleating agent promotes the crystallization of the PLA matrix, shortens the crystallization time and improves the crystallinity; on the other hand, multi-stage stretching induces a highly oriented microcrystalline structure, which is stabilized by heat setting, giving the material a higher degree of crystallinity.

[0023] (3) Comprehensive improvement of mechanical properties: The introduction of nanocellulose reinforcing agent and the fiber mesh structure formed by stretching and orientation have a reinforcing effect on PLA matrix, significantly improving the rigidity and creep resistance of the material.

[0024] (4) Controllable and environmentally friendly degradation behavior: The composite material of the present invention has stable performance under normal use conditions and will not age or decrease in strength due to the presence of degradation agents. At the same time, after being disposed of and placed in a composting environment, the material can rapidly biodegrade under triggered conditions. This characteristic of stable use period and rapid decomposition after disposal enables the material to meet its functional requirements while significantly reducing the risk of environmental pollution, demonstrating excellent environmental friendliness.

[0025] (5) Good process adaptability: This invention combines reactive extrusion compatibilization technology with multi-stage stretching and orientation process, but all the equipment used are conventional plastic processing equipment. The process route is feasible and easy to scale up industrially. By selecting appropriate process parameters, PLA-based green materials can be prepared without significantly increasing costs, which has good application prospects. Attached Figure Description

[0026] Figure 1 This is a flowchart of a method for preparing biodegradable polylactic acid plastic according to the present invention.

[0027] Figure 2 This is a scanning electron microscope image of the cross-section of the polylactic acid plastic film in Example 1 of the present invention.

[0028] Figure 3 This is a comparison chart of the experimental results of tensile strength, elongation at break, impact strength, heat distortion temperature, and compost degradation rate in Experiment Example 1 of the present invention. Detailed Implementation

[0029] The following embodiments further explain and illustrate the technical solutions of the present invention. It should be specifically noted that each specific embodiment is a concretization and explanation of the technical solution and should not be considered as a limitation on the scope of protection of the present invention. Those skilled in the art still have the right to modify the technical solutions of these embodiments and make equivalent substitutions for some or all of the technical features, and these modifications or substitutions do not change the essence of the corresponding technical solutions, nor do they cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions described in the present invention.

[0030] The technical solution designed by this invention to solve the existing problems includes the following key points: 1. Constructing a reaction-compressed ternary composite matrix Unlike existing technologies that involve simple physical blending or direct addition of pre-synthesized compatibilizers, this invention employs a ternary composite system comprising a PLA matrix, a biodegradable elastomer, and nanocellulose whiskers, and innovatively introduces an in-situ reaction compatibilization strategy based on maleic anhydride monomers. This strategy does not involve direct addition of pure monomers, but rather the controlled introduction of maleic anhydride and peroxide initiators into the melt blend system to ensure precise metering and uniform dispersion. Under peroxide initiation, maleic anhydride functional groups are first grafted onto the PLA molecular chain, allowing the in-situ grafted material to chemically react with the active groups at the ends of the elastomer phase, generating a polylactic acid-co-polybutylene adipate terephthalate / polybutylene succinate (PLA-co-PBAT / PBS) block copolymer at the two-phase interface. This chemical bonding, which is directly constructed during processing, fundamentally improves the compatibility and adhesion of multiphase interfaces. Its interface strength and efficiency are significantly better than traditional physical blending or pre-compensation methods, effectively preventing phase separation and creating a uniformly dispersed and firmly interfacial multiphase structure foundation for subsequent stretching processes.

[0031] 2. Perform multi-stage thermal stretching to construct nanofiber network structures. This invention chooses a multi-stage uniaxial stretching process instead of the more common biaxial stretching in the field, based on its unique performance requirements. Firstly, it aims to achieve ultimate uniaxial performance: the two-dimensional network structure formed by biaxial stretching disperses the orientation of molecular chains, sacrificing strength limits in any direction while balancing performance. This invention aims to obtain ultra-high axial strength, high modulus, and optimal barrier properties, which necessitates achieving a high degree of uniaxial molecular orientation through uniaxial stretching. Secondly, it aims to construct directional degradation channels: the dense mesh structure formed by biaxial stretching hinders the penetration of water and enzymes. The highly oriented nanofiber-crystal network formed by uniaxial stretching creates regular, continuous channels in the orientation direction, providing the physical conditions for the efficient and orderly hydrolysis reaction along a specific direction after microencapsulation and enzyme triggering, a key design element for achieving controllable and complete degradation. Finally, it aims to achieve precise structural control.

[0032] 3. Introducing a smart-triggered enzymatic degradation mechanism This invention designs a microencapsulated enzyme preparation with a three-layer core-shell structure and introduces it as a smart degradation trigger into a PLA composite material system. The microcapsule is constructed with three functionally defined layers: the innermost layer is a pH-responsive calcium alginate gel layer, the middle layer is a temperature-responsive poly(N-isopropylacrylamide-co-methacrylic acid) copolymer layer, and the outermost layer is a protective layer of poly(N-isopropylacrylamide-co-methacrylic acid) (PNIPAM-co-MAA) copolymer with a high glass transition temperature (Tg), the thickness of which is controlled between 0.5 μm and 2.0 μm. This thickness serves both as a thermal barrier, effectively preventing a large amount of heat from entering during the short processing time, and as a guarantee that it can reliably rupture due to internal synergistic stress after disposal.

[0033] The specific implementation process of the intelligent degradation mechanism is as follows: During material processing and use, the outermost layer remains rigid and solid due to its high properties, rather than melting, effectively resisting high-temperature shearing and maintaining the integrity of the microcapsule structure. This ensures complete isolation of the enzyme preparation and that the material's mechanical properties remain unaffected. When the material is disposed of and enters an industrial composting environment, the middle layer of the thermosensitive polymer in the microcapsule undergoes phase transition shrinkage at 55-60°C, generating significant inward tensile stress. Simultaneously, the inner layer of calcium alginate gradually dissolves in a weakly acidic environment, disintegrating the internal supporting structure. These two synergistic cell-wall breaking effects generate significant mechanical stress concentrated on the outermost shell. At this point, although the outermost layer is hard at room temperature, it is essentially a polymer material that can be broken down. Under the combined action of high temperature, humidity, and continuous internal stress, the microcapsule structure eventually ruptures, precisely releasing the encapsulated thermophilic lipase. The released thermophilic lipase can immediately act on the ester bonds in the PLA molecular chain, achieving efficient degradation of the material through a specific hydrolysis reaction. This mechanism successfully enables the intelligent transformation of materials from high-performance, long-term use to rapid degradation after disposal, providing a new technological path for the development of controllable biodegradable plastics.

[0034] Meanwhile, the outermost vitrified polymer of the microencapsulated enzyme preparation, through surface design and particle size control, can form good compatibility with the PLA matrix. When introduced at low dosages and uniformly dispersed, it acts as a rigid filler, having a positive or neutral impact on the material's mechanical properties without substantially damaging the PLA's tensile strength, elongation at break, and impact strength. This dispersion process is completed at the end of the extruder, with the process time controlled between 30 and 90 seconds. This time window is set to ensure that the microcapsules achieve sufficiently uniform dispersion while minimizing their exposure time to the high-temperature melt; this is a key process parameter to ensure that the activity of the internal enzyme preparation is not compromised. Figure 2The image shown is a scanning electron microscope (SEM) image of the polylactic acid (PLA) film obtained in Example 1. It reveals that the microencapsulated enzyme preparation is uniformly dispersed in the PLA film matrix, maintaining a complete spherical shape without cracking or collapse. This indicates that it successfully withstood the high temperature and shear stress of melt blending and was well embedded in the matrix.

[0035] Example 1 Table 1 Raw Material Information Table

[0036] A method for preparing biodegradable polylactic acid plastic includes the following steps: S1: Dissolve 1.0 part of thermophilic lipase and 0.2 parts of Tween 80 in 20 parts of deionized water and mix thoroughly to form the aqueous phase; mix 100 parts of dichloromethane and 1.0 part of Span 80 to form the oil phase; at room temperature, slowly pour the aqueous phase into the oil phase and emulsify at 1000 rpm for 5 minutes to form a stable primary emulsion. Dissolve 1.0 part of sodium alginate in 10 parts of deionized water to prepare a sodium alginate solution. Slowly add the sodium alginate solution dropwise to the primary emulsion while maintaining stirring at 500 rpm. After the addition is complete, slowly add 0.5 parts of calcium chloride solution and react for 30 minutes. Then centrifuge to collect the microcapsules and wash them twice with deionized water to obtain the inner-layer-coated microcapsules. The inner-layer-coated microcapsules were redispersed in 150 parts of liquid paraffin, and 3.0 parts of N-isopropylacrylamide and 0.5 parts of methacrylic acid monomer were added. The mixture was stirred at low speed to ensure uniform dispersion of the monomers on the microcapsule surface. The system was heated to 60°C, and under nitrogen protection, 0.05 parts of ammonium persulfate were added. The reaction was carried out for 4 hours, forming a temperature-pH dual-responsive polymer middle layer on the microcapsule surface through free radical copolymerization. After the reaction, the mixture was centrifuged and washed with cyclohexane to obtain microcapsules with inner and middle layers. 1.5 parts of poly(N-isopropylacrylamide-co-methacrylic acid) were dissolved in a 1:1 mixture of 50 parts of ethanol and water, and 0.15 parts of methylenebisacrylamide were added as a crosslinking agent to form a homogeneous solution. The prepared inner and middle-layer-coated microcapsules were dispersed in this solution and then coated using spray drying technology. At the high temperature of spray drying, the crosslinking agent was activated, forming a crosslinked network during film formation. The powder was collected to obtain a microencapsulated enzyme preparation with a complete three-layer shell structure.

[0037] S2: Place 100 parts of PLA granules in a vacuum drying oven and dry for 6 hours at 80℃ and 0.1MPa. Dry 5.5 parts of CNC at 105℃ for 4 hours, then mix with 5 parts of PLA powder and 0.1 parts of silane coupling agent KH-550 using a high-speed mixer to prepare CNC masterbatch. Mix 3 parts of microencapsulated enzyme preparation with 5 parts of PLA powder at a low temperature of 55℃ to prepare enzyme preparation masterbatch.

[0038] S3: Use a co-rotating twin-screw extruder with temperature settings from the feed port to the die head of 170℃, 175℃, 180℃, 185℃, 180℃, and 175℃ respectively. First, add 100 parts of pre-dried PLA granules, 15.75 parts of PBAT, 6.75 parts of PBS, 0.36 parts of maleic anhydride, and 0.036 parts of dicumyl peroxide through the main feed port. After the material enters the melting section, add 0.44 parts of maleic anhydride and 0.044 parts of dicumyl peroxide through the side feed port. In this stage, an in-situ reaction occurs in the molten state to generate a graft copolymer that acts as a compatibilizer. CNC masterbatch is then added through the side feed port. At the low-temperature zone at the end of the extruder, 1.0 part of a composite nucleating agent (a mixture of talc and zinc phenyl phosphate in a 3:1 weight ratio), 0.3 parts of hindered phenolic antioxidant 1010, 0.5 parts of the lubricant calcium stearate, and 8.57 parts of enzyme masterbatch are added. The screw speed is 200 rpm, the melt residence time is 2.5 minutes, and the residence time of the microencapsulated enzyme is controlled at 60 seconds. The extruded strip is cooled in a water bath and then pelletized.

[0039] S4: The extruded granules are remelted and pressed into sheets with a thickness of 1 mm using a single-screw extruder. The sheets are then subjected to a first-stage stretching process at 75°C with a segment stretching ratio of 1.5:1 and a total stretching ratio of 1.5:1. The sheets are then subjected to a second-stage stretching process at 85°C with a segment stretching ratio of 2.5:1 and a total stretching ratio of 3.75:1. Finally, the sheets are subjected to a third-stage stretching process at 95°C with a segment stretching ratio of 2.0:1 and a total stretching ratio of 7.5:1. The sheets are then held at this temperature for 10 seconds for heat setting.

[0040] S5: The stretched sheet was subjected to gradient cooling via a 60°C hot air zone and a 25°C cooling water roller, and finally wound up to obtain the final biodegradable PLA composite film. Subsequently, film cross-sectional samples were prepared using a liquid nitrogen quenching method, and the microstructure of the film cross-sectional samples was characterized using electron microscopy. The test conditions were as follows: after vacuum sputtering, the samples were subjected to secondary electron imaging mode with an accelerating voltage of 5.0 kV and a working distance of 10 mm. The microstructure was observed for subsequent analysis of the dispersion state and structural integrity of the microcapsules in the matrix.

[0041] Example 2 The preparation method is the same as in Example 1, except that: S2: 5.5 parts of nanocellulose CNC are replaced with 3 parts of nanocellulose CNC; 3 parts of microencapsulated enzyme preparation are replaced with 1 part of microencapsulated enzyme preparation; S3: 15.75 parts PBAT were replaced with 10.5 parts PBAT; 6.75 parts PBS were replaced with 4.5 parts PBS; the reaction mixture consisting of 0.36 parts maleic anhydride and 0.036 parts dicumyl peroxide was replaced with the reaction mixture consisting of 0.15 parts maleic anhydride and 0.015 parts dicumyl peroxide; 8.57 parts enzyme masterbatch were replaced with 2.86 parts enzyme masterbatch. S4: The total stretching ratio of 7.5:1 is replaced with a total stretching ratio of 3.75:1; the residence time of the microencapsulated enzyme preparation is controlled at 60 seconds, which is replaced with a residence time of 30 seconds. All other steps are the same.

[0042] Example 3 The preparation method is the same as in Example 1, except that: S2: 5.5 parts of nanocellulose CNC are replaced with 8 parts of nanocellulose CNC; 3 parts of microencapsulated enzyme preparation are replaced with 5 parts of microencapsulated enzyme preparation; S3: 15.75 parts PBAT were replaced with 21.0 parts PBAT; 6.75 parts PBS were replaced with 9.0 parts PBS; the reaction mixture consisting of 0.36 parts maleic anhydride and 0.036 parts dicumyl peroxide was replaced with the reaction mixture consisting of 0.75 parts maleic anhydride and 0.075 parts dicumyl peroxide; 8.57 parts enzyme masterbatch were replaced with 14.29 parts enzyme masterbatch. S4: The total stretching ratio of 7.5:1 is replaced with a total stretching ratio of 11.5:1; the residence time of the microencapsulated enzyme preparation is controlled at 60 seconds is replaced with a residence time of 90 seconds for the microencapsulated enzyme preparation. All other steps are the same.

[0043] Comparative Example 1 Biodegradable polylactic acid (PLA) plastics were prepared using traditional methods: PLA, PBS, and PBAT were dried at 80°C for 4 hours and then poured into a high-speed mixer and mixed for 3-5 minutes. The mixed material was then melt-blended, extruded, cooled, and pelletized using a twin-screw extruder. The pellets were dried at 85°C for 4 hours and then injection-molded into standard test strips.

[0044] Comparative Example 2 The preparation method is the same as in Example 1, except that: S1: The step of preparing microencapsulated enzyme masterbatch is omitted; S3: No microencapsulated enzyme preparations added; All other steps are the same.

[0045] Comparative Example 3 The preparation method is the same as in Example 1, except that: S3: The steps of adding maleic anhydride and dicumyl peroxide are omitted; All other steps are the same.

[0046] Comparative Example 4 The preparation method is the same as in Example 1, except that: S4: The multi-stage stretching step is omitted and replaced with biaxial stretching. The extruded granules are added to a casting machine to form a cast sheet, which is then biaxially stretched in a biaxial stretching machine. First, longitudinal stretching is performed with a stretch ratio of 2~5:1 and a stretching temperature of 90~100℃. Then, transverse stretching is performed with a stretch ratio of 2~6:1 and a stretching temperature of 95~105℃. All other steps are the same.

[0047] Experimental Example 1 The films obtained in Examples 1-3 and Comparative Examples 1-4 were measured: (1) Tensile strength and elongation at break: Referring to GB / T 1040.3 "Determination of tensile properties of plastics - Part 3: Test conditions for films and sheets", after the film has been conditioned in a standard environment of 23±2℃ and 50±10% relative humidity for at least 40 hours, it is cut into dumbbell-shaped or strip-shaped specimens with a width of 15mm and a length of 150mm using a standard cutter. The standard-cut film specimens are tested using a universal testing machine. The clamp spacing is 100mm, the tensile speed is 50mm / min, and the testing machine automatically records the stress-strain curve. The tensile strength and elongation at break are calculated from the curve. The formulas are: Tensile strength = maximum load / original cross-sectional area; Elongation at break = [(gauge length at break - original gauge length) / original gauge length] × 100%. Three valid specimens are taken for the experiment, and the average value of the results is taken.

[0048] (2) Impact strength: Refer to ASTM D1709 "Standard Test Method for Resistance of Plastic Films to Free Falling Darts" and use a free falling dart impact tester to test. The result is the impact energy (J) required to puncture 50% of the specimen under the general method. Three valid specimens were tested and the average value was taken.

[0049] (3) Heat distortion temperature: Referring to GB / T 1634.2 "Determination of load deformation temperature of plastics - Part 2: Plastics and hard rubber", the material was first prepared into a standard strip with a length of 120 mm, a width of 10 mm, and a thickness of 4 mm by injection molding or compression molding. The heat distortion temperature tester was used to test the strip. The strip was placed on a support with a span of 100 mm and subjected to a three-point bending load. The result is the temperature at which the strip bends by 0.34 mm, which is the heat distortion temperature. Three valid strips were tested, and the average value of the results was taken.

[0050] (4) Composting degradation rate: Referring to ISO 14855 "Determination of the final aerobic biodegradability of plastic materials under controlled composting conditions—Method by determination of carbon dioxide released", the film sample was cut into fragments smaller than 10 mm × 10 mm, uniformly mixed with mature compost inoculum, and the mixture was placed in a reactor at 58℃ ± 2℃ with a constant flow of humid air. The amount of carbon dioxide generated by the sample in the reactor outlet gas was continuously monitored and calculated. The biodegradation rate was calculated using the formula: actual CO2 generated / theoretical CO2 amount after complete mineralization of the sample × 100%. The result was the biodegradation rate after 90 days. Three valid samples were used for the experiment, and the average value was taken.

[0051] Table 2 Comparison of experimental results of Examples 1-3 and Comparative Examples 1-4

[0052] The experimental results of Examples 1-3 and Comparative Examples 1-4 are shown in Table 2. Figure 3 As shown, the four indicators of Example 1 are at the highest level overall, and the heat distortion temperature is much higher than that of the conventional injection-molded sample in Comparative Example 1. This indicates that the formulation and process achieve the best balance between strength, toughness, impact resistance, and heat resistance, successfully constructing a high-performance material that combines rigidity and toughness. Therefore, this invention represents the optimal implementation point for biodegradable polylactic acid plastics.

[0053] Data from Example 2 shows that all data points are at the lower limit of the examples. This is because insufficient CNC cannot form an effective nano-network to bear and transfer stress. Furthermore, the low elastomer and compatibilizer content results in limited toughening effect and potential weak points at the interface. The low stretching ratio leads to insufficient orientation of PLA molecular chains and CNC, failing to fully unleash its reinforcing potential. The material structure is not dense, which is conducive to early degradation, but lacks sufficient enzyme content and efficient degradation channels, resulting in insufficient degradation momentum in the later stages and not as thorough as in Example 1. In contrast, Example 3, by significantly increasing the content of reinforcing phase and compatibilizer and employing an extreme stretching process, achieves the highest tensile strength and heat distortion temperature among all examples. However, its excessively high CNC content increases the risk of stress concentration, easily becoming a crack initiation point. Excessive stretching restricts molecular chain slippage, causing the material to become brittle, and the composting degradation rate is lower than that of Example 1. This means that while the dense microstructure brings extremely high mechanical properties, it also hinders the penetration and diffusion of moisture and enzymes within the material, delaying the later degradation rate and resulting in incomplete degradation.

[0054] Comparative Examples 1-4, lacking key technologies, showed varying degrees of reduction in overall performance compared to the Examples. Comparative Example 1, following a conventional approach, demonstrated that existing technologies focus on manufacturing durable plastic products, resulting in the lowest mechanical and thermal properties. This is because this approach relies solely on simple physical blending, lacking nanoscale reinforcement, robust interfacial bonding, and the core process of enhancing performance through stretching and orientation. Comparative Example 2, omitting the microencapsulated enzyme preparation, maintained excellent mechanical and thermal properties despite the absence of enzymes, but its degradation rate was significantly lower than Example 1. This indicates that the microencapsulated enzyme preparation is the engine that triggers and accelerates the degradation process, achieving rapid and complete degradation. Comparative Example 3, lacking a reactive compatibilizer, exhibited reduced mechanical properties, including tensile strength and fracture strength. The elongation at break and impact strength decreased sharply, and the material reverted to brittleness. Due to severe phase separation, a uniform and stable overall structure could not be formed, resulting in a decrease in heat resistance. This proves that reactive compatibilization is the skeleton and binder for successfully constructing a high-performance ternary composite system. Comparative Example 4 lacked a multi-stage stretching and orientation process, and its data were between those of Comparative Example 3 and Example 1. The material had certain toughness and strength, which was due to the improved interface of reactive compatibilization. However, without stretching, the PLA molecular chains and nanocellulose were in a disordered state and could not form a highly oriented nanofiber network to bear the load and restrict chain segment movement. Therefore, its mechanical properties and heat distortion temperature were much lower than those of Example 1 after stretching. The material's reinforcing potential, rigidity, and heat resistance were not stimulated.

[0055] In summary, this invention solves the interfacial compatibility problem of multi-component systems through PLA-g-MAH reaction compatibilization, and constructs a highly oriented reinforcing network through multi-stage thermal stretching, thereby achieving a leap in the material's mechanical properties and heat resistance. Simultaneously, it innovatively introduces a pH-temperature dual-responsive microencapsulation enzyme, endowing the material with intelligent characteristics of stability during its service life and rapid degradation after disposal. These three aspects work synergistically and are indispensable, successfully resolving the long-standing technical contradiction of balancing high strength, high toughness, high heat resistance, and controllable rapid degradation in PLA materials.

Claims

1. A method for preparing biodegradable polylactic acid plastic, characterized in that, The preparation method involves a reactive melt blending process combined with a multi-stage stretching process to form a highly unidirectional nanofiber-crystal network structure within the material. The reactive melt blending process involves simultaneously introducing graft monomers and peroxide initiators during the melt blending of polylactic acid (PLA) and biodegradable elastomer, generating block copolymers in situ at the interface between the PLA and the biodegradable elastomer, thus improving interfacial compatibility and adhesion. At the end of the reactive melt blending process, microencapsulated enzyme preparations are added as degradation triggers to achieve controllable adjustment of the material degradation rate. The multi-stage stretching process involves unidirectional, staged, multiple stretching operations, with differentiated stretching conditions at different stages to achieve a high degree of unidirectional orientation and ultimate performance of the material in a specific direction.

2. The method for preparing a biodegradable polylactic acid plastic according to claim 1, characterized in that, Includes the following steps: S1: Using calcium ions to crosslink sodium alginate, a pH-responsive inner gel shell is formed around the enzyme-containing primary emulsion droplets; subsequently, the inner microcapsules are dispersed in liquid paraffin containing N-isopropylacrylamide and methacrylic acid monomers, and ammonium persulfate is added for polymerization reaction, generating a temperature-pH dual-responsive copolymer middle layer in situ on the surface of the microcapsules; finally, spray drying technology is used to uniformly coat the outermost layer of the microcapsules with the copolymer solution, forming a dense and high-temperature resistant polymer protective outer layer, resulting in a three-layer core-shell microencapsulated enzyme preparation; S2: Polylactic acid granules are vacuum dried and set aside for use; nanocellulose is dried and mixed with polylactic acid powder and silane coupling agent to prepare nanocellulose masterbatch; microencapsulated enzyme preparation is mixed with polylactic acid powder at low temperature to prepare enzyme preparation masterbatch; S3: Pre-dried polylactic acid granules, elastomer, and a portion of reactive compatibilizer are blended and melted. In the melting section, another portion of reactive compatibilizer is added through the side feed port, followed by nanocellulose masterbatch. Finally, composite nucleating agent, antioxidant, lubricant, and enzyme preparation masterbatch are added to form polylactic acid strips. The melt residence time is 2.5 minutes. The strips are cooled in a water tank and then cut into pellets. S4: Remelt the granules and press them into sheets. Perform three-stage stretching on the sheets with a total stretch ratio of 7.5:1 and heat set them at a high temperature for 10 seconds. S5: The stretched sheet is subjected to gradient cooling and then wound up to obtain the final biodegradable polylactic acid composite film.

3. The method for preparing a biodegradable polylactic acid plastic as described in claim 2, characterized in that, The biodegradable polylactic acid plastic has the following formulation: 100 parts polylactic acid matrix resin, 15-30 parts elastomer toughening phase, 3-8 parts nanocellulose reinforcing agent, 0.3-1.5 parts maleic anhydride monomer, 0.03-0.15 parts peroxide initiator, 1-5 parts microencapsulated enzyme preparation, 0.5-1.5 parts composite nucleating agent, 0.1-0.8 parts antioxidant, and 0.2-1.5 parts lubricant.

4. The method for preparing a biodegradable polylactic acid plastic as described in claim 2, characterized in that, The enzyme-containing primary emulsion described in S1 contains thermophilic lipase. The microencapsulated enzyme preparation has a three-layer core-shell structure, consisting of, from the inside out: a pH-responsive calcium alginate gel inner layer, a temperature-pH dual-responsive polymer middle layer, and a polymer protective outer layer with a high glass transition temperature. The thickness of the polymer protective outer layer is controlled between 0.5 μm and 2.0 μm. Under neutral conditions at room temperature, the microcapsules maintain structural integrity to isolate the enzyme. When the material is disposed of in a composting environment at 55-60°C and weakly acidic conditions, the inner layer dissolves, and the middle layer undergoes a phase transition and shrinkage simultaneously. The resulting synergistic cell disruption effect causes the outer layer to rupture, releasing the thermophilic lipase.

5. The method for preparing a biodegradable polylactic acid plastic as described in claim 2, characterized in that, The blending and melting process described in S3 includes a one-step in-situ reaction compatibilization step, which is achieved by introducing maleic anhydride monomer and free radical initiator into the polylactic acid and elastomer melt blending system.

6. The method for preparing a biodegradable polylactic acid plastic as described in claim 2, characterized in that, The elastomer described in S3 is a mixture of polybutylene adipate terephthalate and polybutylene succinate in a weight ratio of 7:

3.

7. The method for preparing a biodegradable polylactic acid plastic as described in claim 2, characterized in that, The partially reactive compatibilizer described in S3 is a mixture of maleic anhydride monomer and peroxide initiator, accounting for 50% of the total reactive compatibilizer.

8. The method for preparing a biodegradable polylactic acid plastic as described in claim 2, characterized in that, The melt residence time described in S3 is 2.5 minutes, and the residence time of the enzyme preparation masterbatch described in S3 is controlled within a short range of 30 to 90 seconds.

9. The method for preparing a biodegradable polylactic acid plastic as described in claim 2, characterized in that, The three-stage stretching described in S4 specifically includes: a first-stage stretching at 75±5℃ with a segment stretching ratio of 1.0~2.0:1; a second-stage stretching at 85±5℃ with a segment stretching ratio of 2.0~3.5:1; and a third-stage stretching and heat setting at 95±5℃ with a segment stretching ratio of 1.5~2.5:

1.

10. The method for preparing a biodegradable polylactic acid plastic as described in claim 2, characterized in that, The gradient cooling described in S5 requires passing through a 60°C hot air zone and a 25°C cooling water roller for gradient cooling.

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