Foaming polylactic acid bead capable of being quickly formed by microwaves and molded part of foaming polylactic acid bead

Abstract

The invention discloses a foamed polylactic acid bead capable of being quickly formed by microwaves, the foamed polylactic acid bead has a core-shell structure, a core layer material comprises polylactic acid resin with a melting point of more than or equal to 145 DEG C, a shell layer material comprises polyester resin with a melting point of less than or equal to 130 DEG C and a certain amount of microwave heating aid, and the interior or surface of the shell layer of the foamed bead also contains a surfactant. The microwave heating aid can be rapidly heated under relatively low microwave heating power, so that the low-melting-point polyester material on the surface layer is rapidly melted and is fused and sintered with other surrounding foamed polyester shell layers, and the heating efficiency can be further improved by the surfactant; the interior of the foamed polylactic acid bead does not contain a microwave heating aid and a surfactant, low-power microwaves cannot enable the internal temperature of the foamed bead to rise too much, an original independent closed-pore structure can be kept, an obtained molded part is high in curing degree and good in surface sintering, the whole molding process is short in time and low in energy consumption, a large amount of cost is saved, and the method is suitable for large-scale production. The environment is protected.

Description

Technical Field

[0001] The invention relates to a foamed polylactic acid bead capable of rapid microwave molding and a molded product thereof, belonging to the field of foaming materials. Background Art

[0002] Foam materials are widely used in many fields due to their light weight, good heat insulation, cushioning and sound insulation effects, high strength and low production energy consumption. The mainstream foam materials on the market include foamed polyethylene, foamed polypropylene, foamed polystyrene and some other blended or copolymerized foamed resins.

[0003] As a biodegradable material with relatively mature preparation technology and green environmental protection, polylactic acid can effectively solve environmental pollution problems and achieve the goal of energy conservation and emission reduction. At present, the preparation of foamed polylactic acid molded parts by foaming polylactic acid beads usually adopts steam compression molding, that is, the foamed polylactic acid beads are first filled into the mold, and then the mold is closed, and water vapor is injected into the cavity through the pores inside the mold to heat the beads. After the foamed beads are sintered, they are cooled and demolded to obtain foamed parts. In this steam molding process, in order to make the foamed beads heat and sinter more evenly and fully, it is often necessary to preheat the mold with water vapor; moreover, during the molding process, the mold also needs to be kept at a certain temperature by steam. These two factors lead to a large amount of steam energy consumption actually being consumed in the heating and insulation process of the mold, and the heating and sintering of the foamed polylactic acid beads only account for less than 10% of the total steam energy consumption.

[0004] Microwave molding of foamed polylactic acid materials can alleviate this problem well. Compared with steam compression molding, microwave molding does not require heating of the mold. The heat is generated by the electromagnetic waves acting on the foamed polylactic acid material. The foamed polylactic acid material can absorb most of the heat, with high energy efficiency and low energy consumption. On the other hand, in order to ensure that the foamed bead material is not overheated and burned during the microwave molding process, especially polylactic acid, a material that is more sensitive to heat, a relatively low microwave heating power is often used. However, lower microwave power will result in longer heating time, which has a greater impact on the molding time cycle and lower work efficiency. In addition, long-term microwave heating may potentially bring about some degradation risks of polylactic acid and loss of some mechanical properties. This also limits the further promotion and application of microwave molding in actual production.

[0005] Therefore, it is necessary to develop a foamed polylactic acid bead material and foamed parts that can be quickly microwaved. Summary of the invention

[0006] In order to solve the problem that the molding time of microwave-molded foamed polylactic acid bead materials is long and the molding efficiency is low, the present invention discloses a foamed polylactic acid bead and a molded product thereof that can be quickly microwave-molded. The surface layer of the foamed polylactic acid bead contains a certain amount of microwave heating aid and surfactant, which can help the foamed bead material to heat up rapidly under a low microwave heating power, so that the low-melting-point polyester material of the surface layer melts rapidly and melts and sinters with other surrounding polyester surfaces; while the interior of the foamed polylactic acid bead does not contain microwave heating aid and surfactant, and the polylactic acid material of the core layer has a high melting point, and the low-power microwave will not cause the internal temperature of the foamed bead to rise too much, and the original independent pore structure can be maintained. After the microwave heating is finished and cooled, a foamed polylactic acid product with high degree of maturation, good surface sintering, and the original closed-cell structure inside the foamed bead can be obtained; more importantly, the entire molding process is short in time, low in energy consumption, high in energy utilization, saving a lot of costs, and being green and environmentally friendly. At the same time, the foamed polylactic acid product has good mechanical properties.

[0007] In order to achieve the above object, the present invention adopts the following technical solution: A foamed polylactic acid bead capable of rapid microwave molding has a core-shell structure, wherein the core layer material comprises 80-99.96wt%, preferably 90-99.96wt% of polylactic acid resin, the shell layer material comprises 70-99.94wt%, preferably 80-99.94wt% of polyester resin and 0.01-5wt% of microwave auxiliary heating agent, the polylactic acid resin is one or more of L-lactic acid homopolymer, D-lactic acid homopolymer and L-lactic acid and D-lactic acid copolymer, with a melting point of ≥145°C, the polyester resin is a biodegradable polyester with a melting point of ≤130°C, the microwave auxiliary heating agent is one or more of metal powder, metal oxide, nitride, glass fiber, bamboo fiber, carbon fiber, graphene, ferrite and ceramic, the size of the microwave auxiliary heating agent is 10-1000nm, and the mass ratio of the core layer material to the shell layer material is 80:20-99:1, more preferably 90:10-99:1.

[0008] Furthermore, the shell material of the foamed polylactic acid beads further includes 0.01 to 5 wt% of a surfactant, and the surfactant is at least one of an anionic surfactant, a cationic surfactant, a zwitterionic surfactant, a nonionic surfactant, etc. Under the action of the microwave electromagnetic field, the ionic surfactant is ionized, which can promote the absorption capacity of the foamed beads to microwaves; the nonionic surfactant contains a hydrophilic group, which promotes the absorption of water wetted on the surface of the beads. The polarity of water is high and can be quickly heated under the action of microwaves. These factors can make the shell material quickly heat up and melt, and quickly increase the degree of sintering between the beads.

[0009] Furthermore, the anionic surfactant is at least one of stearic acid, oleic acid, lauric acid, sulfates, sulfonates, etc., the cationic surfactant is at least one of ammonium salts, quaternary ammonium salts, heterocyclic surfactants, etc., the zwitterionic surfactant is at least one of lecithin, amino acid, betaine surfactants, etc., and the nonionic surfactant is at least one of fatty acid glycerides, polyols, fatty acid sorbitan, polysorbate, polyoxyethylene, polyoxyethylene-polyoxypropylene copolymers, etc.

[0010] Furthermore, the cell structure of the core layer is an independent closed-cell cell structure with a pore size of 50-200 microns, and more preferably 50-120 microns.

[0011] Furthermore, the melt index of the polylactic acid resin is 3-5g / 10min (190°C, 2.16kg), the tensile modulus is ≥3000MPa, and the tensile strength is ≥40MPa. The lower melt index can provide the necessary viscosity and melt strength to ensure the formation of a closed-cell structure of polylactic acid during the foaming process and reduce the porosity.

[0012] Furthermore, the polyester resin has a melt index of 3-5 g / 10 min (190° C., 2.16 kg), a Vicat softening point of ≥90° C., a tensile strength of ≥25 MPa, and an elongation at break of ≥500%.

[0013] Furthermore, the biodegradable polyester is preferably one or more of polyethylene succinate, polybutylene succinate, polybutylene succinate, polybutylene adipate, and polybutylene adipate / butylene terephthalate copolymer, preferably polybutylene adipate / butylene terephthalate copolymer.

[0014] Furthermore, the metal powder is iron powder, copper powder, copper-tin alloy powder, gold powder and / or silver powder, etc., the metal oxide is magnesium oxide, iron oxide, aluminum oxide and / or copper oxide, etc., the nitride is silicon nitride and / or boron nitride, etc., the ferrite is a sintered composite of ferric oxide and nickel oxide, zinc oxide, manganese oxide, etc. The microwave heating aid has a high dielectric constant and dielectric loss, which is conducive to absorbing microwaves and quickly converting them into heat energy, heating the skin layer of the foamed polylactic acid beads and fully sintering them.

[0015] Compared with traditional steam heating molding, most of the heat is lost in the mold. Microwave heating can increase the temperature of the foamed beads themselves, and very little heat is consumed in the mold. Under the action of the external alternating electromagnetic field generated by microwaves, the polar molecules in the foamed polylactic acid beads are polarized to form dipoles or the existing dipoles are rearranged, and swing at a speed of hundreds of millions of times per second with the high-frequency alternating electromagnetic field. In order for the molecules to rearrange with the changing direction of the high-frequency magnetic field, it is necessary to overcome the obstacles and interference of the original molecular thermal motion and intermolecular interactions, generate "frictional heat", and directly convert the electromagnetic field energy into thermal energy, which greatly increases the temperature of the foamed polylactic acid beads. Compared with polylactic acid materials, microwave-assisted heating agent materials with higher dielectric constants and dielectric losses have higher and faster heat generation capabilities during the "friction" process, which results in the foamed polylactic acid shell containing microwave-assisted heating agents having a higher and faster temperature speed under the same microwave power; in addition, the shell layer of the foamed polylactic acid itself uses polyester with a lower melting point as the base material, which allows the shell layer material to melt quickly under lower microwave power, and the beads are quickly sintered, but the core layer of the foamed beads is kept at a relatively low temperature to ensure that the independent bubble structure in the core layer is not destroyed by melting.

[0016] Furthermore, the core layer material further comprises 0.01-5wt% of a chain extender and / or 0.01-1wt% of a nucleating agent and / or 0.01-1wt% of an antioxidant and / or 0.01-1wt% of a lubricant.

[0017] Furthermore, the chain extender is a glycidyl methacrylate containing epoxy functional groups or a glycidyl acrylate containing epoxy functional groups. The chain extender can increase the molecular weight and branching degree of polylactic acid, thereby increasing the melt strength and reducing the porosity of foamed polylactic acid.

[0018] Further, the nucleating agent is one or more of talcum powder, zinc borate, polytetrafluoroethylene powder, barium sulfate, calcium carbonate, glass fiber, preferably glass fiber. The nucleating agent needs to have a certain incompatibility with the polylactic acid substrate, promotes the growth of pores at the junction of the nucleating agent and the polylactic acid, and plays a role in heterogeneous nucleation. At the same time, the nucleating agent also has the effect of reducing foaming pressure and uniform pores. In addition, relative to several other nucleating agents, the water absorption of glass fiber is relatively low, and it has good hydrophobicity during the foaming process, which can effectively reduce the degradation of polylactic acid.

[0019] Furthermore, the antioxidant is one or more of hindered phenols, hindered amines, phosphites, etc., preferably 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene.

[0020] Furthermore, the lubricant is one or a mixture of oleamide, erucamide, stearic acid monoglyceride, and vinyl bis stearamide, preferably erucamide. The lubricant can increase the fluidity of polylactic acid during extrusion, making production more continuous and stable.

[0021] Furthermore, the shell material further comprises 0.01-10wt% of an anti-hydrolysis agent and / or 0.01-5wt% of a compatibilizer and / or 0.01-1wt% of an antioxidant and / or 0.01-1wt% of a lubricant.

[0022] Furthermore, the anti-hydrolysis agent is preferably one or more of glycidyl ether, triglycidyl isocyanate, and carbodiamine, and more preferably carbodiamine. The anti-hydrolysis agent usually reacts with the terminal carboxyl group of the polyester to effectively block the end and inhibit hydrolysis.

[0023] Further, the compatibilizer is a silane coupling agent, a higher fatty acid, an unsaturated organic acid, etc. Preferably, the silane coupling agent can be vinyl trimethoxy silane, vinyl triethoxy silane, phenyl trimethoxy silane, phenyl triethoxy silane, methyl triethoxy silane, etc. The alkoxy hydrolysis at one end of the silane coupling agent can react with the inorganic microwave-assisted heating agent to form a chemical bond, while the lipophilic group at the other end can be entangled with the molecular chain of the polyester to improve the interface ability between the organic and inorganic. Under the action of a high-shear high-speed twin-screw, the microwave-assisted heating agent can be more evenly dispersed in the shell polyester substrate.

[0024] Furthermore, the foamed polylactic acid beads may be added with masterbatches, anti-ultraviolet agents, etc. as required.

[0025] The preparation method of the above-mentioned foamed polylactic acid beads capable of rapid microwave molding is as follows: (1) The core layer material of the foamed polylactic acid beads is mixed evenly and then put into a twin-screw extruder A, and the shell layer material of the foamed polylactic acid beads is mixed evenly and then put into a twin-screw extruder B; (2) Co-extrusion is achieved through a double-layer die, with the material in extruder A being the core layer and the material in extruder B being the shell layer; (3) The extruded filaments are water-cooled and pelletized using a pelletizer to prepare expandable polylactic acid microparticles that can be quickly microwave-molded; (4) The expandable polylactic acid microparticles are placed in a high-pressure foaming kettle for foaming to obtain foamed polylactic acid beads.

[0026] When the shell layer of the foamed beads does not contain a surfactant, when preparing a foamed polylactic acid molded product, it is necessary to spray a surfactant solution on the surface of the foamed polylactic acid beads before microwave molding. The mass ratio of the surfactant to water in the surfactant solution is 1:1 to 1:10, and the mass ratio of the spraying amount of the surfactant solution to the foamed polylactic acid beads is 1:2 to 1:20. The microwave heating power is 1 to 100 kW, more preferably 10 to 60 kW, and further preferably 10 to 30 kW. The time is 1 to 500 s, more preferably 10 to 100 s, and further preferably 15 to 60 s.

[0027] When the shell layer of the foamed beads contains a surfactant, a small amount of water needs to be sprayed onto the surface of the foamed beads before microwave molding. The mass ratio of water to foamed polylactic acid beads is 1:10 to 1:50. The foamed polylactic acid beads are then subjected to microwave molding to prepare foamed polylactic acid molded parts. The microwave heating power is 1 to 100 kW, more preferably 10 to 60 kW, and further preferably 10 to 30 kW. The time is 1 to 500 s, more preferably 10 to 100 s, and further preferably 15 to 60 s.

[0028] The present invention has the following beneficial effects: 1. The shell layer of the above-mentioned expanded polylactic acid beads contains a microwave heating aid, which has a high dielectric constant and dielectric loss. It can quickly generate high heat through polarization and oscillation of polar molecules at a lower microwave power, so that the temperature of the shell layer of the expanded polylactic acid beads rises rapidly. On the other hand, since the core layer of the expanded polylactic acid beads does not contain a microwave heating aid with a high dielectric constant and dielectric loss, less heat is generated at the same low microwave power, resulting in a lower temperature of the core layer of the expanded polylactic acid beads. The low-melting-point polyester substrate used in the shell layer of the expanded polylactic acid itself can make the shell layer material melt quickly under the action of a lower microwave power, and the beads are quickly sintered, but the independent pore structure in the core layer is kept from being destroyed by melting.

[0029] 2. The surfactant in the shell layer of the expanded polylactic acid beads or on the surface of the beads is ionized under the action of the microwave electromagnetic field, or contains hydrophilic groups to promote the absorption of highly polar water, thereby improving the heating efficiency and heat conduction efficiency, further causing the shell material to heat up and melt rapidly, and improving the degree of sintering between the beads.

[0030] 3. Ordinary microwave molding, in order to ensure that the foamed bead material is not overheated and burned during the microwave molding process, often adopts a relatively low microwave heating power. However, a lower microwave power will lead to a longer heating time, which has a greater impact on the molding time cycle, has a lower working efficiency, and will potentially bring about the risk of partial loss of mechanical properties of the foamed polylactic acid parts. The foamed polylactic acid bead material described in the present invention can be quickly molded at a lower microwave power to obtain a foamed molded part with a good sintering degree. The entire molding production process has low energy consumption, a short cycle, is green and energy-saving, and is environmentally friendly, and the obtained foamed polylactic acid parts have good mechanical properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is the polylactic acid foam molding prepared in Example 1. DETAILED DESCRIPTION

[0032] The present invention will now be further described in detail with reference to embodiments.

[0033] Preparation of rapidly microwaveable expanded polylactic acid beads: (1) According to the mass ratio of Example 1 in Table 1, the core layer of polylactic acid (brand name 4032D, purchased from Nature Works, USA), epoxy functional group-containing glycidyl methacrylate chain extender (purchased from BASF AG, Germany), nucleating agent glass fiber (purchased from Hangzhou Sansi Chemical Technology Co., Ltd.), 1,3,5-trimethyl-2,4,6-tris (3,5-di-tert-butyl-4-hydroxybenzyl) benzene antioxidant (purchased from BASF AG, Germany), erucic acid amide lubricant (purchased from Croda Group Co., Ltd., UK) were mixed uniformly and put into a twin-screw extruder A, and the shell layer of polyester resin (brand name HF101, purchased from Zhejiang Huafeng Group Co., Ltd.), carbodiamine hydrolysis resistance agent (purchased from Yitailong (Tianjin) Synthetic Materials Co., Ltd.) were mixed uniformly and put into a twin-screw extruder A. ), compatibilizer vinyl trimethoxy silane coupling agent KH171 (purchased from Hangzhou Jessica Chemical Co., Ltd.), microwave heating aid (boron nitride, CW-BN-002, average particle size 600nm, purchased from Shanghai Chaowei Nano Technology Co., Ltd.), surfactant (tridecanol polyether-10 phosphate, i.e. PEG-10 ester, purchased from Nantong Chenrun Chemical Co., Ltd.), 1,3,5-trimethyl-2,4,6-tris (3,5-di-tert-butyl-4-hydroxybenzyl) benzene antioxidant (purchased from BASF AG, Germany), erucamide lubricant (purchased from Croda Group Co., Ltd., UK) were mixed evenly and put into twin-screw extruder B; (2) Co-extrusion is achieved through a double-layer die, with the material in extruder A being the core layer and the material in extruder B being the shell layer, with the mass ratio of the core layer to the shell layer being 95:5; (3) Cooling the extruded filaments with water and pelletizing them to prepare expandable polylactic acid microparticles with a length of 1.2 to 2.5 mm and a unit weight of 0.5 to 1.8 mg; (4) Add the above-mentioned expandable polylactic acid microparticles and water into a foaming kettle, add dispersant butter and surfactant sodium dodecyl sulfate at the same time, heat the foaming kettle and introduce carbon dioxide physical foaming agent to make the foaming agent penetrate into the interior of the polylactic acid microparticles to form a homogeneous system. When the set foaming temperature of 130°C and the foaming pressure of 3.3 MPa are reached in the reactor, they are maintained for 2 minutes, and then the expandable polylactic acid microparticles are instantly released into a room temperature foaming pipe with an internal air pressure of less than 0.1 MPa for foaming and expansion, thereby finally obtaining foamed polylactic acid beads that can be quickly microwaved.

[0034] Preparation of foamed polylactic acid molded parts: (1) The expanded polylactic acid beads were subjected to an air pressure of 0.15 MPa for 10 hours.

[0035] (2) The surface of the foamed polylactic acid beads after air pressure was sprayed with water for wetting treatment, the mass ratio of water to foamed polylactic acid beads was 1:20, and then the wet foamed polylactic acid beads were subjected to microwave molding, the microwave frequency was 2450MHz, and the microwave power was 15kw. The molded parts were baked and shaped in a drying room at 80℃ to finally obtain the finished foamed parts. The process parameters during the molding process and the degree of maturation of the foamed parts are shown in Table 1.

[0036] Except that the microwave auxiliary heating agent in the shell layer was replaced by alumina (CW-AL203-003, average particle size 200 nm, purchased from Shanghai Chaowei Nanotechnology Co., Ltd.), other components and preparation conditions were consistent with those in Example 1.

[0037] Except that the microwave heating aid in the shell layer was replaced by nano iron powder (CW-Fe-002, average particle size 800 nm, purchased from Shanghai Chaowei Nanotechnology Co., Ltd.), other components and preparation conditions were consistent with those in Example 1.

[0038] The difference from Example 1 is that when preparing the foamed polylactic acid beads that can be quickly microwaved, no surfactant is added to the extruder B; when preparing the foamed polylactic acid molded parts, the water spraying wetting treatment in step (2) is replaced by: a surfactant (tridecanol polyether-10 phosphate, i.e., PEG-10 ester, purchased from Nantong Chenrun Chemical Co., Ltd.) and water are mixed in a mass ratio of 1:3 and stirred to prepare a surfactant solution, and the above-mentioned surfactant solution is evenly sprayed on the surface of the foamed polylactic acid beads after pressure loading, and the ratio of the surfactant solution spraying amount to the foamed beads mass is 1:9, to obtain a foamed polylactic acid bead material attached with the surfactant solution and microwave molding is performed on it. The other components and preparation conditions are consistent with those of Example 1.

[0039] Except that the sprayed surfactant was fatty amine polyoxyethylene ether, AC-1812 (purchased from Nantong Chenrun Chemical Co., Ltd.), other components and preparation conditions were the same as those in Example 4.

[0040] Except that the sprayed surfactant was methoxy polyethylene glycol, MPEG-600 (purchased from Nantong Chenrun Chemical Co., Ltd.), other components and preparation conditions were the same as those in Example 4.

[0041] When preparing the foamed polylactic acid molded parts, the water spraying wetting treatment in step (2) is replaced by: a surfactant (PEG-10 ester, purchased from Nantong Chenrun Chemical Co., Ltd.) and water are mixed in a mass ratio of 1:3 and stirred to prepare a surfactant solution, and the surfactant solution is evenly sprayed onto the surface of the foamed polylactic acid beads after pressure loading, and the ratio of the surfactant solution spraying amount to the foamed beads mass is 1:9, to obtain a foamed polylactic acid bead material attached with the surfactant solution. Other components and preparation conditions are the same as those in Example 1.

[0042] Comparative Example 1: Except that the microwave auxiliary heating agent boron nitride is not added to the shell layer of the foamed polylactic acid beads, other components and preparation conditions are consistent with those of Example 1.

[0043] Comparative Example 2: Except that the surfactant PEG-10 is not added to the shell layer of the foamed polylactic acid beads, other components and preparation conditions are the same as those in Example 1.

[0044] Comparative Example 3: Except that the microwave auxiliary heating agent boron nitride is not added to the shell layer of the foamed polylactic acid beads, other components and preparation conditions are consistent with those of Example 4.

[0045] Comparative Example 4: Except that the microwave heating aid boron nitride and the surfactant PEG-10 are not added to the shell layer of the foamed polylactic acid beads, the rest is the same as Example 1.

[0046] Table 1 The ripening degree of the foamed parts in Table 1 is divided from good to bad, and is divided into 5 very good, 4 good, 3 average, 2 poor, and 1 very poor. 5: When the number of beads with destroyed cells on the broken surface of the foamed parts accounts for 90% or more of the total number of beads; 4: The number of beads with destroyed cells on the broken surface of the foamed parts accounts for 70%-90% of the total number of beads; 3: The number of beads with destroyed cells on the broken surface of the foamed parts accounts for 50%-70% of the total number of beads; 2: The number of beads with destroyed cells on the broken surface of the foamed parts accounts for 30%-50% of the total number of beads; 1: The number of beads with destroyed cells on the broken surface of the foamed parts accounts for less than 30% of the total number of beads.

[0047] Compression strength test: According to ISO 844:2014, the polylactic acid foamed parts were cut into 100*100*50mm specimens and the compression strength was tested at 10% compression deformation.

[0048] Combining Examples 1-3 and Comparative Example 1, as well as Example 4 and Comparative Example 3, it can be seen that the addition of microwave heating aids significantly reduces the microwave heating time during molding, and the obtained foamed parts have a good degree of maturation. Different microwave heating aids have different efficiencies in converting heat energy in a high-frequency alternating electric field. For foamed polylactic acid materials, boron nitride and nano iron powder have better effects on reducing microwave heating time.

[0049] Combining Examples 1 and 4 with Comparative Example 2, it can be seen that the surfactant blended in the shell layer or sprayed on the surface of the foamed beads can also shorten the microwave heating time during molding and achieve a better degree of maturation. Moreover, spraying the surfactant on the surface of the foamed beads has the characteristics of simple processing technology and more significant shortening of microwave heating time.

[0050] It can be seen from Examples 4-6 that spraying different surfactants on the surface of the foamed beads can reduce the microwave heating time during molding to varying degrees.

[0051] From Examples 1, 4 and 7, it can be seen that, compared with only spraying the surfactant on the surface of the foamed beads or blending it in the shell of the foamed beads, the effect of simultaneously blending and coating the surfactant on shortening the microwave heating time is not obvious. Therefore, it is sufficient to choose one of the surfactant application methods.

[0052] Combining Examples 1 and 4 with Comparative Examples 1-4, it can be seen that the simultaneous addition of microwave heating aid and surfactant (blended or coated) has a better synergistic effect than the addition of microwave heating aid or surfactant (blended or coated) alone, and can significantly reduce the microwave heating time. In Comparative Example 4, when neither microwave heating aid nor surfactant is added, the microwave heating time of the foamed product is long, the efficiency is low, and the degree of maturation of the product is also poor. In addition, due to the long microwave heating time, the compression performance of the foamed polylactic acid product is also lost to a certain extent.

Claims

1. A foamed polylactic acid bead that can be quickly microwaved, characterized in that: The invention has a core-shell structure, wherein the core layer material comprises 80-99.96wt% of a polylactic acid resin, the shell layer material comprises 80-99.94wt% of a polyester resin and 0.01-5wt% of a microwave auxiliary heating agent, wherein the polylactic acid resin is one or more of an L-lactic acid homopolymer, a D-lactic acid homopolymer and an L-lactic acid and D-lactic acid copolymer, and the melting point is ≥145°C, the polyester resin is a biodegradable polyester, and the melting point is ≤130°C, the microwave auxiliary heating agent is one or more of metal powder, metal oxide, nitride, glass fiber, bamboo fiber, carbon fiber, graphene, ferrite and ceramic, the size of the microwave auxiliary heating agent is 10-1000nm, and the mass ratio of the core layer material to the shell layer material is 80:20-99:

1.

2. The rapidly microwaveable expanded polylactic acid beads according to claim 1, characterized in that: The shell material further comprises 0.01 to 5 wt % of a surfactant, wherein the surfactant is at least one of an anionic surfactant, a cationic surfactant, a zwitterionic surfactant, and a nonionic surfactant.

3. The rapidly microwaveable foamed polylactic acid beads according to claim 2, characterized in that: The anionic surfactant is at least one of stearic acid, oleic acid, lauric acid, sulfates, and sulfonates; the cationic surfactant is at least one of ammonium salts, quaternary ammonium salts, and heterocyclic surfactants; the zwitterionic surfactant is at least one of lecithin, amino acid, and betaine surfactants; and the nonionic surfactant is at least one of fatty acid glycerides, polyols, fatty acid sorbitan, polysorbate, polyoxyethylene, and polyoxyethylene-polyoxypropylene copolymers.

4. The rapidly microwaveable foamed polylactic acid beads according to claim 1 or 2, characterized in that: The cell structure of the core layer is an independent closed-cell cell structure with a pore diameter of 50-200 microns.

5. The rapidly microwaveable foamed polylactic acid beads according to claim 1 or 2, characterized in that: The polylactic acid resin has a melt index of 3-5 g / 10 min, a tensile modulus of ≥3000 MPa, and a tensile strength of ≥40 MPa.

6. The rapidly microwaveable expanded polylactic acid beads according to claim 1 or 2, characterized in that: The polyester resin has a melting index of 3-5 g / 10 min, a Vicat softening point of ≥90° C., a tensile strength of ≥25 MPa, and an elongation at break of ≥500%.

7. The rapidly microwaveable foamed polylactic acid beads according to claim 1 or 2, characterized in that: The metal powder is iron powder, copper powder, copper-tin alloy powder, gold powder and / or silver powder, the metal oxide is magnesium oxide, iron oxide, aluminum oxide and / or copper oxide, the nitride is silicon nitride and / or boron nitride, and the ferrite is a sintered composite of ferric oxide and nickel oxide, zinc oxide and manganese oxide.

8. The rapidly microwaveable foamed polylactic acid beads according to claim 1 or 2, characterized in that: The core layer material further includes 0.01-5wt% of a chain extender and / or 0.01-1wt% of a nucleating agent and / or 0.01-1wt% of an antioxidant and / or 0.01-1wt% of a lubricant, and the shell layer material further includes 0.01-10wt% of an anti-hydrolysis agent and / or 0.01-5wt% of a compatibilizer and / or 0.01-1wt% of an antioxidant and / or 0.01-1wt% of a lubricant.

9. A foamed polylactic acid molded part, characterized in that: The foamed polylactic acid beads according to claim 1 are prepared by a microwave heating molding process, wherein a surfactant solution is sprayed on the surface of the foamed polylactic acid beads before microwave molding, the mass ratio of the surfactant to water in the surfactant solution is 1:1 to 1:10, the mass ratio of the spraying amount of the surfactant solution to the foamed polylactic acid beads is 1:2 to 1:20, the microwave heating power is 1 to 100 kW, and the time is 1 to 500 s.

10. A foamed polylactic acid molded part, characterized in that: The foamed polylactic acid beads described in any one of claims 2-8 are prepared by a microwave heating molding process, water is sprayed on the surface of the foamed polylactic acid beads before microwave molding, the mass ratio of water to the foamed polylactic acid beads is 1:10-1:50, the microwave heating power is 1-100kw, and the time is 1-500s.

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