A polylactic acid foamed material and a method for preparing the same

Abstract

The application discloses a kind of polylactic acid foamed materials and preparation method thereof, belong to foamed material technical field.The method includes polylactic acid and second resin dry, foaming agent, chain extender, auxiliary crosslinking agent, antioxidant, nucleating agent are mixed evenly, and foaming sheet base sheet is obtained by single screw extruder extrusion;Foaming sheet base sheet is irradiated and crosslinked;Finally, the irradiated material is free foaming through vertical foaming oven, and polylactic acid foamed material is obtained.Compared with prior art, the application solves the problems of low melt strength, uneven cell structure and other problems of traditional polylactic acid foamed material by unique formula design and process optimization, significantly improves the mechanical properties and cell structure stability of foamed material, expands the application range of polylactic acid foamed material, and has important market application value and environmental friendliness.

Description

Technical Field

[0001] This invention relates to the field of foaming materials technology, and in particular to a polylactic acid foaming material and its preparation method. Background Technology

[0002] Traditional polymer materials such as polyethylene, polypropylene, and polystyrene are all petroleum-based, meaning their raw materials come from petroleum. This not only consumes large amounts of non-renewable resources but also makes them difficult to completely degrade under natural conditions, leading to serious "white pollution" and posing an increasingly severe threat to the environment. To address this challenge, biodegradable materials have emerged as an important approach to solving the plastic pollution problem.

[0003] Among numerous biodegradable materials, polylactic acid (PLA) stands out due to its excellent physical and mechanical properties. Its strength and stiffness are comparable to polypropylene, demonstrating broad application prospects. However, the large-scale application of PLA faces two major bottlenecks: firstly, its cost is high, more than twice that of polypropylene (PP); secondly, PLA has low melt strength and poor melt viscoelasticity, making it difficult to directly perform foaming processes, which significantly limits its application in the field of foamed materials. To overcome this limitation, existing technologies attempt to improve the foaming performance of PLA through formulation optimization.

[0004] For example, Chinese patent application CN119463439A discloses a biodegradable expandable polylactic acid polymer foam material and its preparation method. The method involves melting and mixing polylactic acid, acylated starch, and polyamide in a certain proportion to obtain a polylactic acid-starch-polyamide blended modified mixture. This mixture is then mixed with waste scraps of polylactic acid foam material, foaming agent, nucleating agent, and activator. The mixture is extruded and granulated to obtain foaming masterbatch, which is then foamed and molded. This method improves the expandability of polylactic acid material, and the resulting foam material has a uniform cell size distribution, small cell size, and excellent mechanical properties. However, while this invention improves expandability, it suffers from insufficient cell uniformity, and the hydrophilicity of starch leads to a decrease in the material's moisture resistance. Chinese patent application CN115746524A discloses a biodegradable microfoamed polylactic acid material, whose components include polylactic acid resin, biopolyester resin, polybenzidine-pyridine-benzimidazole, initiator, antioxidant, multifunctional monomer, plant fiber powder, starch, inorganic powder, coupling agent, chain extender, dispersant, organic nucleating agent, and foaming agent. Through innovative formulation design and preparation process, the polylactic acid foam material possesses a stable branched molecular chain structure, ensuring excellent mechanical properties and processing performance, making it suitable for large-scale factory production. While this invention enhances the branched structure, the process is cumbersome, further increasing costs, and the cell refinement effect is limited.

[0005] None of the above solutions have fundamentally solved the core contradiction between low melt strength and poor nucleation efficiency. Traditional nucleating agents (talc, calcium carbonate, etc.) have poor dispersibility, insufficient heterogeneous nucleation sites, and difficulty in breaking through the cell density. Chain extenders alone cannot simultaneously improve the viscoelasticity of the melt, and gas escapes quickly during foaming, making it easy for cells to merge and rupture.

[0006] Therefore, developing a PLA foaming material that can simultaneously achieve high melt strength, high-density cell structure, and simplified process has become a technical challenge that the industry urgently needs to overcome. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention aims to provide a polylactic acid foam material and its preparation method, which significantly improves the mechanical properties and cell structure stability of the foam material through unique formulation design and process optimization.

[0008] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0009] A method for preparing a polylactic acid foam material is as follows:

[0010] Step 1: After drying polylactic acid and the second resin, add foaming agent, chain extender, crosslinking agent, antioxidant and nucleating agent and mix evenly. Extrude through a single screw extruder to obtain a foamed sheet substrate.

[0011] Step 2: Irradiate and crosslink the foamed sheet substrate;

[0012] Step 3: The irradiated photographic material is freely foamed in a vertical foaming furnace to obtain polylactic acid foam material.

[0013] Preferably, the polylactic acid foam material is prepared by the following method, in parts by weight:

[0014] Step 1: Dry 60-80 parts of polylactic acid and 20-40 parts of the second resin in a 60-90℃ forced-air oven for 3-10 hours, controlling the moisture content to ≤200-400ppm. Then add 5-10 parts of foaming agent, 0.5-4 parts of chain extender, 1-5 parts of crosslinking agent, 0.5-1.5 parts of antioxidant, and 0-4.5 parts of nucleating agent to a high-speed mixer at a temperature of 50-70℃ and a speed of 500-1000rpm for 5-15 minutes until uniform. Then extrude the mixture through a single-screw extruder. The temperature of the feeding section of the single-screw extruder is 150-160℃, the melting section is 170-180℃, the homogenization section is 185-195℃, the die is 180-190℃, the screw speed is 150-250rpm, and the extruded sheet thickness is 1-3mm. After water cooling and shaping, the sheet is wound up to obtain a foamed sheet substrate.

[0015] Step 2: Irradiate the foamed sheet substrate prepared in Step 1 to crosslink it. The irradiation dose is 5-20 kGy.

[0016] Step 3: The irradiated material is freely foamed in a vertical foaming furnace at a foaming temperature of 160-200℃ to obtain polylactic acid foam material.

[0017] The polylactic acid is at least one of L-polylactic acid and D-polylactic acid.

[0018] Preferably, the polylactic acid is composed of L-polylactic acid and D-polylactic acid in a weight ratio of 1-3:1.

[0019] The second resin is at least one of PBAT and polycaprolactone.

[0020] Preferably, the second resin is composed of PBAT and polycaprolactone in a weight ratio of 1-3:1.

[0021] The foaming agent is at least one of azodicarbonamide, azodicarbonyl esters, and acylhydrazides.

[0022] The chain extender is at least one of the following: multifunctional epoxy compounds, bisoxazolium compounds, isocyanate compounds, acid anhydride compounds, and acyl halide compounds.

[0023] The crosslinking agent is at least one of trimethylolpropane trimethacrylate, triallyl isocyanurate, and triallyl cyanurate.

[0024] The antioxidant is at least one of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], 2,6-di-tert-butyl-4-ethylphenol, diethyl 3,5-di-tert-butyl-4-hydroxybenzyl phosphate, and tributyl phosphite.

[0025] The nucleating agent is at least one of talc, calcium carbonate, silica powder, and modified nucleating agents.

[0026] The preparation method of the modified nucleating agent is as follows:

[0027] A metal ion compound was added to an aqueous ethanol solution and stirred until homogeneous to obtain a metal salt solution. A ligand compound was added to the aqueous ethanol solution and mixed until homogeneous to obtain a ligand solution. The ligand solution was added dropwise to the metal salt solution and stirred until homogeneous. The mixture was then heated under reflux and stirred until naturally cooled. After natural cooling, the mixture was filtered to obtain a wet filter cake. Carboxylated multi-walled carbon nanotubes were added to anhydrous ethanol and sonicated. During sonication, the wet filter cake was added and stirred to form a homogeneous slurry. After filtration, the slurry was dried in a vacuum oven to obtain the modified nucleating agent.

[0028] Preferably, the modified nucleating agent is prepared by the following method, in parts by weight:

[0029] Weigh 8-12 parts of the metal ion compound and add them to 400-600 parts of a 90-98 wt% ethanol aqueous solution. Stir at room temperature until homogeneous to obtain a metal salt solution. Weigh 10-20 parts of the ligand compound and add them to 120-280 parts of a 90-98 wt% ethanol aqueous solution. Mix until homogeneous to obtain a ligand solution. Add the ligand solution dropwise to the metal salt solution at a rate of 1-3 mL / min while stirring. Then heat the mixture to 75-85℃ and reflux for 3-8 hours. After natural cooling, the mixture is filtered to obtain a wet filter cake. 0.1-0.3 parts of carboxylated multi-walled carbon nanotubes are added to 200-400 parts of anhydrous ethanol and sonicated at a power of 200-400W, a time of 20-40min, and a frequency of 20-60kHz. During the sonication process, the wet filter cake is added and stirred at 300-800rpm for 0.5-2 hours to form a uniform slurry. The slurry is then filtered and dried overnight in a vacuum oven at 25-35℃ to obtain the modified nucleating agent.

[0030] The metal ion compound is at least one of manganese acetate and ferrous fumarate.

[0031] The ligand compound is at least one of L-phenyllactic acid, malic acid, and tartaric acid.

[0032] The functions of each component in this invention are as follows:

[0033] Polylactic acid (PLA) serves as a biodegradable matrix, providing the main framework for materials. A proportional mixture of L-PLA and D-PLA forms a stereocomposite crystal, significantly improving melt strength and crystallization temperature, thus solving the foaming collapse problem caused by the low melt strength of pure PLA.

[0034] The second resin (PBAT / polycaprolactone) uses PBAT as a flexible toughening agent, introducing an aliphatic-aromatic copolymer structure to improve elongation at break through elastic segments and reduce PLA brittleness. Polycaprolactone (PCL) lowers the glass transition temperature (Tg), enhances low-temperature toughness, and synergistically inhibits cell wall rupture with PBAT.

[0035] The foaming agent (azodicarbonamide) decomposes upon heating, releasing gases such as N2 and CO, forming cell nuclei. Matching its decomposition temperature to the foaming process achieves a balance between the gas release rate and the viscoelasticity of the melt, preventing cell collapse.

[0036] Chain extenders (hexamethylene diisocyanate) react with the terminal carboxyl or hydroxyl groups of PLA / PBAT through isocyanate groups, extending the molecular chain and increasing the degree of branching, thereby improving melt strength and preventing pore wall rupture during bubble expansion.

[0037] The crosslinking agent (trimethylolpropane trimethacrylate) generates free radicals under irradiation (12 kGy), which initiates crosslinking of PLA molecular chains to form a three-dimensional network structure, enhances melt elasticity, and inhibits cell coalescence.

[0038] Antioxidants capture free radicals generated during high-temperature processing, inhibit the oxidative breakage of PLA molecular chains, maintain stable melt strength, and ensure uniform foaming.

[0039] Metal ion compounds (manganese acetate) serve as heterogeneous nucleation cores. Transition metal ions (such as Mn²⁺) coordinate with the oxygen atoms of PLA ester groups through d orbital electrons, lowering the nucleation energy barrier and increasing the pore density. At the same time, variable valence states (such as Mn²⁺ / Mn³⁺) consume free radicals and inhibit PLA chain breakage.

[0040] The ligand compound (malic acid) forms a stable complex by chelating with metal ions through its carboxyl group. The small, flexible chain of malic acid has a high degree of lattice matching with PLA, inducing regular crystallization.

[0041] Carboxylated multi-walled carbon nanotubes serve as a dispersion carrier, enhancing the uniformity of metal-ligand complex dispersion in the PLA matrix; simultaneously, they act as thermally conductive channels, promoting heat transfer during foaming and preventing localized overheating that could lead to cell collapse.

[0042] Compared with existing technologies, it has the following advantages:

[0043] 1) This invention constructs a branched and three-dimensional network structure of PLA molecular chains through the triple synergy of chain extender, crosslinking agent and irradiation crosslinking, which significantly improves the viscoelasticity of the melt, solves the problem of foam collapse caused by the low melt strength of traditional PLA, achieves uniform cell growth, reduces the scrap rate in the production process, improves production efficiency, and is conducive to the large-scale production of polylactic acid foam materials.

[0044] 2) Through unique formula design and process optimization, the polylactic acid foam material prepared by this invention exhibits excellent mechanical properties such as tensile strength and elongation at break, which can better meet the requirements of material strength and toughness in practical applications and effectively expand the application range of polylactic acid foam material.

[0045] 3) This invention produces polylactic acid foam material with smaller foam pore size and higher pore density, which helps to improve the material's heat insulation, sound insulation and other properties, while making the material's appearance more delicate and uniform, thus improving the product's quality and market competitiveness. Detailed Implementation

[0046] Main source of materials:

[0047] Polylactic acid (PLLA), product number: PLLA granules, Dongguan Nabaichuan Plastics Co., Ltd.

[0048] Dextrorotatory polylactic acid, specification: PDLA D070, brand: Total (Thailand).

[0049] PBAT, brand name: C1200, manufacturer (place of origin): BASF, Germany.

[0050] Polycaprolactone, grade: 6800, manufacturer (origin): Solvay, USA.

[0051] Carboxylated multi-walled carbon nanotubes, fineness: 10-20nm, item number: 008, Shenzhen Guosen Leading Technology Co., Ltd.

[0052] All other raw materials used in the embodiments and comparative examples of this invention are commercially available products.

[0053] Example 1

[0054] A method for preparing a polylactic acid foam material is as follows, in parts by weight:

[0055] Step 1: Dry 70 parts of polylactic acid and 30 parts of the second resin in an 80℃ forced-air oven for 6 hours, controlling the moisture content to ≤300ppm, then add them to a high-speed mixer; then add 8 parts of azodicarbonamide, 0.5 parts of hexamethylene diisocyanate, 2 parts of trimethylolpropane trimethacrylate, 1 part of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], and 3 parts of modified nucleating agent to the high-speed mixer at 60℃ and 800 rpm. Mix at rpm for 10 minutes until homogeneous, then extrude through a single-screw extruder. The single-screw extruder has a feeding section temperature of 155°C, a melting section temperature of 175°C, a homogenization section temperature of 188°C, and a die temperature of 182°C. The screw speed is 190 rpm, and the extruded sheet thickness is 2 mm. After water cooling and shaping, it is wound up to obtain a foamed sheet substrate. The polylactic acid is composed of L-polylactic acid and D-polylactic acid in a weight ratio of 2:1. The second resin is composed of PBAT and polycaprolactone in a weight ratio of 2:1.

[0056] Step 2: Irradiate the foamed sheet substrate prepared in Step 1 to crosslink it. The irradiation dose is 12 kGy.

[0057] Step 3: The irradiated material is freely foamed in a vertical foaming furnace at a foaming temperature of 180℃ to obtain polylactic acid foam material.

[0058] The modified nucleating agent is prepared as follows, in parts by weight:

[0059] Weigh 10 parts of manganese acetate and add it to 500 parts of 95wt% ethanol aqueous solution. Stir at room temperature until homogeneous to obtain a metal salt solution. Weigh 15 parts of malic acid and add it to 150 parts of 95wt% ethanol aqueous solution. Mix homogeneously to obtain a ligand solution. Add the ligand solution dropwise to the metal salt solution at a rate of 2 mL / min. Stir and mix. Heat the mixture to 80°C and reflux for 5 hours. After natural cooling, filter to obtain a wet filter cake. Add 0.2 parts of carboxylated multi-walled carbon nanotubes to 300 parts of anhydrous ethanol and sonicate at a power of 300 W for 30 min and a frequency of 40 kHz. Add the wet filter cake during sonication and stir at 500 rpm for 1 hour to form a homogeneous slurry. Filter to separate the slurry and dry in a vacuum oven at 30°C overnight to obtain the modified nucleating agent.

[0060] Example 2

[0061] The preparation method of a polylactic acid foam material is basically the same as that in Example 1, except that the preparation method of the modified nucleating agent is different.

[0062] The modified nucleating agent is prepared as follows, in parts by weight:

[0063] 10 parts of ferrous fumarate were weighed and added to 500 parts of 95wt% ethanol aqueous solution. The mixture was stirred at room temperature until homogeneous to obtain a metal salt solution. 15 parts of malic acid were weighed and added to 150 parts of 95wt% ethanol aqueous solution. The mixture was stirred until homogeneous to obtain a ligand solution. The ligand solution was added dropwise to the metal salt solution at a rate of 2 mL / min. The mixture was stirred and mixed. The mixture was then heated to 80°C and refluxed for 5 hours. After natural cooling, it was filtered to obtain a wet filter cake. 0.2 parts of carboxylated multi-walled carbon nanotubes were added to 300 parts of anhydrous ethanol and sonicated at a power of 300 W for 30 min and a frequency of 40 kHz. During the sonication process, the wet filter cake was added and stirred at 500 rpm for 1 hour to form a homogeneous slurry. The slurry was filtered and dried overnight in a vacuum oven at 30°C to obtain the modified nucleating agent.

[0064] Example 3

[0065] The preparation method of a polylactic acid foam material is basically the same as that in Example 1, except that the preparation method of the modified nucleating agent is different.

[0066] The modified nucleating agent is prepared as follows, in parts by weight:

[0067] Weigh 10 parts of manganese acetate and add it to 500 parts of 95wt% ethanol aqueous solution. Stir at room temperature until homogeneous to obtain a metal salt solution. Weigh 15 parts of tartaric acid and add it to 150 parts of 95wt% ethanol aqueous solution. Mix homogeneously to obtain a ligand solution. Add the ligand solution dropwise to the metal salt solution at a rate of 2 mL / min. Stir and mix. Heat the mixture to 80°C and reflux for 5 hours. After natural cooling, filter to obtain a wet filter cake. Add 0.2 parts of carboxylated multi-walled carbon nanotubes to 300 parts of anhydrous ethanol and sonicate at a power of 300 W for 30 min and a frequency of 40 kHz. Add the wet filter cake during sonication and stir at 500 rpm for 1 hour to form a homogeneous slurry. Filter to separate the slurry and dry in a vacuum oven at 30°C overnight to obtain the modified nucleating agent.

[0068] Example 4

[0069] The preparation method of a polylactic acid foam material is basically the same as that in Example 1, except that the preparation method of the modified nucleating agent is different.

[0070] The modified nucleating agent is prepared as follows, in parts by weight:

[0071] 10 parts of manganese acetate were weighed and added to 500 parts of 95wt% ethanol aqueous solution. The mixture was stirred at room temperature until homogeneous to obtain a metal salt solution. 15 parts of L-phenyllactic acid were weighed and added to 150 parts of 95wt% ethanol aqueous solution. The mixture was stirred until homogeneous to obtain a ligand solution. The ligand solution was added dropwise to the metal salt solution at a rate of 2 mL / min. The mixture was stirred and mixed. The mixture was then heated to 80°C and refluxed for 5 hours. After natural cooling, it was filtered to obtain a wet filter cake. 0.2 parts of carboxylated multi-walled carbon nanotubes were added to 300 parts of anhydrous ethanol and sonicated at a power of 300 W for 30 min and a frequency of 40 kHz. During the sonication process, the wet filter cake was added and stirred at 500 rpm for 1 hour to form a homogeneous slurry. The slurry was filtered and dried overnight in a vacuum oven at 30°C to obtain the modified nucleating agent.

[0072] Comparative Example 1

[0073] The preparation method of a polylactic acid foam material is basically the same as that in Example 1, except that the preparation method of the modified nucleating agent is different.

[0074] The modified nucleating agent is prepared as follows, in parts by weight:

[0075] Weigh 10 parts of magnesium acetate tetrahydrate and add it to 500 parts of 95wt% ethanol aqueous solution. Stir at room temperature until homogeneous to obtain a metal salt solution. Weigh 15 parts of malic acid and add it to 150 parts of 95wt% ethanol aqueous solution. Mix homogeneously to obtain a ligand solution. Add the ligand solution dropwise to the metal salt solution at a rate of 2 mL / min. Stir and mix. Heat the mixture to 80°C and reflux for 5 hours. After natural cooling, filter to obtain a wet filter cake. Add 0.2 parts of carboxylated multi-walled carbon nanotubes to 300 parts of anhydrous ethanol and sonicate at a power of 300 W for 30 min and a frequency of 40 kHz. Add the wet filter cake during sonication and stir at 500 rpm for 1 hour to form a homogeneous slurry. Filter to separate the slurry and dry in a vacuum oven at 30°C overnight to obtain the modified nucleating agent.

[0076] Comparative Example 2

[0077] The preparation method of a polylactic acid foam material is basically the same as that in Example 1, except that the preparation method of the modified nucleating agent is different.

[0078] The modified nucleating agent is prepared as follows, in parts by weight:

[0079] Weigh 10 parts of manganese acetate and add it to 500 parts of 95wt% ethanol aqueous solution. Stir at room temperature until homogeneous to obtain a metal salt solution. Weigh 15 parts of L-mandelic acid and add it to 150 parts of 95wt% ethanol aqueous solution. Mix homogeneously to obtain a ligand solution. Add the ligand solution dropwise to the metal salt solution at a rate of 2 mL / min. Stir and mix. Then heat the mixture to 80°C and reflux for 5 hours. After natural cooling, filter to obtain a wet filter cake. Add 0.2 parts of carboxylated multi-walled carbon nanotubes to 300 parts of anhydrous ethanol and sonicate at a power of 300 W for 30 min and a frequency of 40 kHz. Add the wet filter cake during sonication and stir at 500 rpm for 1 hour to form a homogeneous slurry. Filter to separate the slurry and dry it overnight in a vacuum oven at 30°C to obtain the modified nucleating agent.

[0080] Comparative Example 3

[0081] The preparation method of a polylactic acid foam material is basically the same as that in Example 1, except that the modified nucleating agent is replaced with an equal amount of calcium carbonate.

[0082] Test Example 1

[0083] Mechanical property testing:

[0084] Tensile strength and elongation at break were tested in accordance with GB / T6344-2008 "Determination of tensile strength and elongation at break of flexible foam polymer materials".

[0085] The test results are shown in Table 1.

[0086] Table 1

[0087]

[0088] Test Example 2

[0089] Testing of foam pore size and pore density:

[0090] Scanning electron microscopy (SEM) was used to examine the pore appearance of the foamed samples. The procedure involved immersing the samples in liquid nitrogen for 2 hours, then quickly breaking them in half. The broken cross-section was then sputter-coated with gold and observed. Image analysis software was then used to analyze the SEM images to determine the pore size (average diameter) and pore density.

[0091] The test results are shown in Table 2.

[0092] Table 2

[0093]

[0094] Under the condition of malic acid as a ligand, the mechanical properties and cell structure of Example 1 (manganese acetate) are superior to those of Example 2 (ferrous fumarate) and Comparative Example 1 (magnesium acetate tetrahydrate). This is because Mn²⁺ has strong coordination ability and redox activity, which can increase the heterogeneous nucleation density, consume free radicals and enhance melt strength; its complex with malic acid can induce PLA to form a high-toughness β-crystal form, increasing the elongation at break; it can also catalyze the regular arrangement of PLA molecular chains and combine with radiation crosslinking to form a three-dimensional network, enhancing melt strength and stabilizing the cell structure. On the other hand, Fe²⁺ is easily oxidized, leading to a decrease in the stability of the complex, and Mg²⁺ has low charge density and weak coordination ability, both of which reduce nucleation efficiency and mechanical properties.

[0095] Under conditions where manganese acetate is used as the metal source, Example 1 (malic acid) exhibits the best mechanical properties and cell structure. The mechanism may lie in the fact that the dicarboxyl structure of malic acid has low steric hindrance, highly matching the lattice parameters of PLA, forming a regular chelate that provides numerous uniform heterogeneous nucleation sites, increasing crystal density and rate. Malic acid promotes the formation of a high proportion of α-crystals in PLA, enhancing cell wall strength and supporting high tensile strength. The short-chain flexible structure of malic acid enhances molecular chain mobility, and in conjunction with irradiation crosslinking, forms a three-dimensional network, increasing melt strength and suppressing cell coalescence and gas escape during foaming. Other ligands, due to structural defects (such as hydroxyl interference and benzene ring steric hindrance), reduce nucleation efficiency and cell stability.

Claims

1. A method for preparing a polylactic acid foam material, characterized in that, The method is as follows: Step 1: After drying polylactic acid and the second resin, add foaming agent, chain extender, crosslinking agent, antioxidant and nucleating agent and mix evenly. Extrude through a single screw extruder to obtain a foamed sheet substrate. Step 2: Irradiate and crosslink the foamed sheet substrate; Step 3: The irradiated material is freely foamed in a vertical foaming furnace to obtain polylactic acid foam material; The second resin is at least one of PBAT and polycaprolactone; The nucleating agent is a modified nucleating agent; The modified nucleating agent is prepared as follows, in parts by weight: Weigh 8-12 parts of the metal ion compound and add them to 400-600 parts of 90-98wt% ethanol aqueous solution. Stir at room temperature until the mixture is homogeneous to obtain a metal salt solution. Weigh 10-20 parts of the ligand compound and add them to 120-280 parts of 90-98wt% ethanol aqueous solution. Mix the mixture homogeneously to obtain a ligand solution. The ligand solution was added dropwise to the metal salt solution at a rate of 1-3 mL / min, and the mixture was stirred. The mixture was then heated to 75-85°C and refluxed for 3-8 hours. After natural cooling, it was filtered to obtain a wet filter cake. 0.1-0.3 parts of carboxylated multi-walled carbon nanotubes were added to 200-400 parts of anhydrous ethanol and sonicated at a power of 200-400 W for 20-40 min and a frequency of 20-60 kHz. During sonication, the wet filter cake was added and stirred at 300-800 rpm for 0.5-2 hours to form a homogeneous slurry. The slurry was filtered and dried overnight in a vacuum oven at 25-35°C to obtain the modified nucleating agent. The metal ion compound is at least one of manganese acetate and ferrous fumarate; the ligand compound is at least one of L-phenyllactic acid, malic acid, and tartaric acid.

2. The method for preparing polylactic acid foamed material as described in claim 1, characterized in that, The method is as follows, by weight: Step 1: Dry 60-80 parts of polylactic acid and 20-40 parts of the second resin in a 60-90℃ forced-air oven for 3-10 hours, controlling the moisture content to ≤300ppm, and then add them to a high-speed mixer; then add 5-10 parts of foaming agent, 0.5-4 parts of chain extender, 1-5 parts of crosslinking agent, 0.5-1.5 parts of antioxidant, and 3 parts of nucleating agent to the high-speed mixer, at a temperature of 50-70℃ and a speed of 500-1000rpm, mix for 5-15 minutes until uniform, and then extrude through a single-screw extruder. The single-screw extruder has a feeding section temperature of 150-160℃, a melting section temperature of 170-180℃, a homogenization section temperature of 185-195℃, a die temperature of 180-190℃, a screw speed of 150-250rpm, and an extruded sheet thickness of 1-3mm. After water cooling and shaping, the sheet is wound up to obtain a foamed sheet substrate. Step 2: Irradiate the foamed sheet substrate prepared in Step 1 to crosslink it. The irradiation dose is 5-20 kGy. Step 3: The irradiated material is freely foamed in a vertical foaming furnace at a foaming temperature of 160-200℃ to obtain polylactic acid foam material.

3. The method for preparing polylactic acid foamed material as described in claim 1 or 2, characterized in that, The polylactic acid is at least one of L-polylactic acid and D-polylactic acid.

4. The method for preparing polylactic acid foamed material as described in claim 1 or 2, characterized in that, The foaming agent is at least one of azodicarbonamide, azodicarbonyl esters, and acylhydrazides.

5. The method for preparing polylactic acid foamed material as described in claim 1 or 2, characterized in that, The chain extender is at least one of the following: multifunctional epoxy compounds, bisoxazolium compounds, isocyanate compounds, acid anhydride compounds, and acyl halide compounds.

6. The method for preparing polylactic acid foamed material as described in claim 1 or 2, characterized in that, The crosslinking agent is at least one of trimethylolpropane trimethacrylate, triallyl isocyanurate, and triallyl cyanurate.

7. The method for preparing polylactic acid foamed material as described in claim 1 or 2, characterized in that, The antioxidant is at least one of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], 2,6-di-tert-butyl-4-ethylphenol, diethyl 3,5-di-tert-butyl-4-hydroxybenzyl phosphate, and tributyl phosphite.

8. A polylactic acid foam material, characterized in that, It is prepared by the preparation method described in any one of claims 1-7.

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