A low-cost, long-lasting flame-retardant, and highly transparent co-extruded wood-plastic composite material and its preparation method.

Abstract

A low-cost, long-lasting flame-retardant, and highly transparent co-extruded wood-plastic composite material is disclosed. This co-extruded wood-plastic composite material consists of a surface layer and a core layer. The core layer is mainly made from recycled polyester bottle flakes, wood flour, coupling agents, compatibilizers, etc., extruded and granulated, and finally co-extruded with the surface layer. Because the surface layer uses a special bulk flame-retardant polyester resin component without the addition of additional flame retardants, its transparency is excellent. Furthermore, the recycled bottle flake polyester resins of the surface layer and the core material belong to the same type of plastic resin, exhibiting good compatibility. Therefore, the bonding ability between the surface layer and the core material is extremely strong, resulting in excellent resistance to prolonged boiling in water without cracking. The flame retardancy and transparency properties are also well retained after boiling. This invention extensively utilizes recycled polyester bottle flakes, significantly reducing the cost of the wood-plastic composite material and providing a high-value-added comprehensive utilization method for recycled polyester bottle flakes.

Description

Technical Field

[0001] This invention relates to the field of wood-plastic composite materials technology, specifically to a low-cost, durable, flame-retardant, and highly transparent co-extruded wood-plastic composite material and its preparation method. Background Technology

[0002] PET material boasts advantages such as high transparency, high temperature resistance, and high toughness, making it widely used in daily life. Especially common are mineral water bottles and beverage bottles, which, after being sorted and recycled, are used as low-end plastics with low costs. However, due to limited application areas, their added value is low. But as people's living standards improve, disposable polyester plastic bottles are becoming increasingly popular for convenience. Therefore, it is crucial to increase the utilization value of these polyester plastic bottles and develop new high-value-added applications. Wood-plastic composites (WPC) are a new type of composite material that has seen rapid development both domestically and internationally in recent years. As a new type of building and decorative material, it has advantages over wood, such as better water resistance and corrosion resistance. With the expansion of WPC applications, and given the large amount of plastic particles used, reducing the cost of plastic particles is key to lowering the overall cost of WPC. Simultaneously, with increasing demands for decorative appeal and safety, achieving durable flame retardancy and high transparency in the co-extruded surface layer is also a goal pursued by the industry. Current flame retardant methods mostly rely on the addition of external flame retardants, which reduces the transparency of the surface layer. Furthermore, the added small-molecule flame retardants are prone to leakage due to insufficient water resistance, leading to decreased long-lasting flame retardant performance and compromised safety. The industry's development direction is to achieve a long-lasting flame retardant and highly transparent surface layer while using low-cost plastics for processing and molding high-performance wood-plastic composites. Summary of the Invention

[0003] To address the aforementioned problems, this invention provides a low-cost, long-lasting flame-retardant, high-transparency co-extruded wood-plastic composite material and its preparation method. This co-extruded wood-plastic composite material, due to the use of a flame-retardant and highly transparent surface layer, exhibits excellent long-term flame retardancy and water resistance, along with high transparency and low cost. The co-extruded wood-plastic composite material consists of a surface layer and a core material. The co-extruded surface layer primarily uses a specially prepared flame-retardant, high-transparency polyester resin. The core material mainly uses recycled polyester flakes, wood flour, coupling agents, compatibilizers, etc., as raw materials for extrusion granulation, and is finally co-extruded with the surface layer.

[0004] A low-cost, long-lasting flame-retardant, and highly transparent co-extruded wood-plastic composite material, comprising a surface layer and a core material, wherein the core material comprises the following raw materials in parts by weight:

[0005]

[0006] Preferably, the recycled polyester bottle flakes are made by cleaning and drying the recycled mineral water bottles, and then cutting them into flakes (preferably with both length and width within 3cm); the wood flour has a size of 100-120 mesh, and can be purchased from Shijiazhuang Zhuzhong Technology Co., Ltd.; the compatibilizer is maleic anhydride modified high-density polyethylene, and can be purchased from Huangshan Benuo Technology Co., Ltd.; the lubricant is stearic acid; and the coupling agent is silane coupling agent KH-550.

[0007] The surface layer of the aforementioned low-cost, long-lasting flame-retardant, and highly transparent co-extruded wood-plastic composite material comprises the following raw materials in parts by weight:

[0008]

[0009] The raw materials also include catalysts and antioxidants.

[0010] Preferably, the catalyst is tetrabutyl titanate, and the amount used is 0.8-1.2% of the mass of 1,4-dibromo-2,3-butanediol; the antioxidant is antioxidant 1010, and the amount used is 1-1.5% of the mass of recycled polyester bottle flakes.

[0011] The preparation method of the low-cost, long-lasting flame-retardant, and highly transparent wood-plastic composite material as described above includes the following steps:

[0012] A. Add the prescribed amount of 1,4-dibromo-2,3-butanediol and catalyst to the reactor, start stirring and heat to 220-225℃, then add the prescribed amount of recycled polyester bottle flakes in three batches. Only after the recycled polyester bottle flakes added in the previous batch have completely melted and dispersed into a homogeneous phase can the next batch of recycled polyester bottle flakes be added. After all the recycled polyester bottle flakes in the formula have been added and melted into a homogeneous phase, continue to keep warm to carry out the transesterification alcoholysis reaction.

[0013] B. Sampling and testing: When the amount of 1,4-dibromo-2,3-butanediol participating in the transesterification reaction in the reactor reaches 60-65% of the added amount, add the formulated amount of dibromoneopentyl glycol and tetrabromoterephthalic acid, and heat to 230-235℃ to carry out the chain extension polymerization reaction.

[0014] C. Take samples to test the acid value of the polymer. When the acid value of the polymer reaches 40-50 mg KOH / g, add the amount of the tetrafunctional chain extender bicondensate (1,1,1-trimethylolpropane) as specified in the formula, and at the same time raise the temperature to 240-245℃ to continue the chain extension reaction.

[0015] D. Take samples to test the acid value of the polymer. When the acid value of the polymer reaches 20-28 mg KOH / g, add antioxidants and start the vacuum system to carry out high vacuum polycondensation.

[0016] E. Take samples to test the acid value of the polymer. When the acid value of the polymer reaches below 2mgKOH / g, release the vacuum system, discharge the material at high temperature, and crush it after cooling to obtain highly transparent flame-retardant polyester resin surface particles for later use.

[0017] F. The raw materials for preparing the core material are first thoroughly vacuum dried at 70-80℃ according to the formula, then thoroughly mixed in a high-speed mixer, and discharged into a cold roller to cool to room temperature before being discharged to prepare a core material premix.

[0018] G. Add the core material premix to a parallel twin-screw extruder for extrusion granulation;

[0019] H. The core material particles obtained in step G are melted and fed into the core material cavity of the co-extrusion mold; the highly transparent flame-retardant polyester resin surface layer particles are melted and fed into the flame-retardant surface layer cavity of the co-extrusion mold. After shaping and cooling, the co-extruded wood-plastic composite material is obtained.

[0020] Preferably, in step D, the vacuum degree is controlled between -0.098 MPa and -0.099 MPa; in step E, the material is crushed after cooling with a steel strip. In step G, the barrel temperature of the parallel twin-screw extruder is 270–280°C, and the screw speed is 250–350 rpm. In step H, the melting process is carried out entirely through an extruder, and the melt extrusion temperature of both the core material particles and the surface layer is 270–280°C.

[0021] For example, a method for preparing a low-cost, durable, flame-retardant, and highly transparent wood-plastic composite material includes the following steps:

[0022] A. Add the prescribed amount of 1,4-dibromo-2,3-butanediol and catalyst to the reactor, start stirring and heat to 220-225℃, then add the prescribed amount of recycled polyester bottle flakes in three batches. Only after the recycled polyester bottle flakes added in the previous batch have completely melted and dispersed into a homogeneous phase can the next batch of recycled polyester bottle flakes be added. After all the recycled polyester bottle flakes in the formula have been added and melted into a homogeneous phase, continue to keep warm to carry out the transesterification alcoholysis reaction.

[0023] B. Sampling and testing: When the amount of 1,4-dibromo-2,3-butanediol participating in the transesterification reaction in the reactor reaches 60-65% of the added amount, it indicates that the transesterification reaction has met the application requirements. At this time, add the formulated amount of dibromoneopentyl glycol and tetrabromoterephthalic acid, and heat to 230-235℃ to carry out the chain extension polymerization reaction.

[0024] C. Take samples to test the acid value of the polymer. When the acid value of the polymer reaches 40-50 mg KOH / g, add the amount of the tetrafunctional chain extender bicondensate (1,1,1-trimethylolpropane) as specified in the formula, and at the same time raise the temperature to 240-245℃ to continue the chain extension reaction.

[0025] D. Take samples to test the acid value of the polymer. When the acid value of the polymer reaches 20-28 mg KOH / g, add antioxidants and start the vacuum system to carry out high vacuum polycondensation. The vacuum degree is controlled between -0.098 MPa and -0.099 MPa.

[0026] E. Take samples to test the acid value of the polymer. When the acid value of the polymer reaches below 2mgKOH / g, it indicates that the polymerization reaction has been completed. Release the vacuum system, discharge the material at high temperature, and crush it after cooling to obtain highly transparent flame-retardant polyester resin surface particles for later use.

[0027] F. The raw materials for preparing the core material are first thoroughly vacuum dried at 70-80℃ according to the formula, then thoroughly mixed in a high-speed mixer, and discharged into a cold roller to cool to room temperature before being discharged to prepare a core material premix.

[0028] G. Add the core material premix to a parallel twin-screw extruder for extrusion granulation. The barrel temperature is 270-280℃ and the screw speed is 250-350rpm.

[0029] H. The core material particles obtained in step G are melted and fed into the core material cavity of the co-extrusion mold; the high-transparency flame-retardant polyester resin surface layer particles are melted and fed into the flame-retardant surface layer cavity of the co-extrusion mold, and after shaping and cooling, the co-extruded wood-plastic composite material is obtained.

[0030] The melting process was carried out by an extruder, and the melting and extrusion temperature of the core material particles and the surface layer was 270-280℃.

[0031] Beneficial effects:

[0032] This invention relates to a low-cost, long-lasting flame-retardant, and highly transparent co-extruded wood-plastic composite material. The co-extruded wood-plastic composite material consists of a surface layer and a core layer. The surface layer is primarily made from recycled polyester bottle flakes through a special process to obtain a flame-retardant, highly transparent polyester resin surface layer. The core layer is mainly made from recycled polyester bottle flakes, wood flour, coupling agents, compatibilizers, etc., extruded and granulated, and finally co-extruded with the surface layer. This co-extruded wood-plastic composite material, due to the use of a self-flammable and highly transparent surface layer, exhibits excellent long-term flame retardancy and water resistance, as well as high transparency and low cost. Because the surface layer uses a self-flammable polyester resin component without the addition of additional flame retardants, its transparency is excellent. Moreover, the polyester resin in the surface layer and the recycled bottle flake polyester resin in the core material belong to the same resin class, exhibiting good compatibility. Therefore, the bonding ability between the surface layer and the core material is extremely strong, resulting in excellent long-term (e.g., 50 hours) water boiling resistance without cracking, and the flame retardant performance and surface transparency show almost no decrease after boiling. Because both the surface layer and the core material of this invention use a large amount of recycled polyester bottle flakes, it not only greatly reduces the cost of wood-plastic composite materials and facilitates their market application and promotion, but also provides a high-value-added comprehensive utilization method for recycled polyester bottle flakes. Detailed Implementation

[0033] The present invention will be further described below with reference to embodiments. It should be noted that, unless otherwise specified, the embodiments and technical features described in this application can be combined with each other. It should also be pointed out that, unless otherwise specified, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. The terms "comprising" or "including" and similar words used in this invention refer to elements or objects preceding the word that encompass the elements or objects listed following the word and their equivalents, without excluding other elements or objects.

[0034] All raw materials described in this invention are commercially available.

[0035] Example 1

[0036] A low-cost, long-lasting flame-retardant, and highly transparent co-extruded wood-plastic composite material, comprising a surface layer and a core material, wherein the core material comprises the following raw materials in parts by weight:

[0037]

[0038] The recycled polyester flakes are produced by cleaning and drying recycled mineral water bottles, then cutting them into flakes with a length and width of less than 3 cm. The wood flour is 100 mesh and was purchased from Shijiazhuang Zhuzhong Technology Co., Ltd. The compatibilizer is maleic anhydride modified high-density polyethylene and was purchased from Huangshan Benuo Technology Co., Ltd. The lubricant is stearic acid. The coupling agent is silane coupling agent KH-550.

[0039] The surface layer comprises the following raw materials in parts by weight:

[0040]

[0041] The catalyst is tetrabutyl titanate, used at 1% of the mass of 1,4-dibromo-2,3-butanediol; the antioxidant is antioxidant 1010, used at 1.5% of the mass of recycled polyester bottle flakes.

[0042] The preparation method of the low-cost, long-lasting flame-retardant, and highly transparent wood-plastic composite material includes the following steps:

[0043] A. Add the prescribed amount of 1,4-dibromo-2,3-butanediol and catalyst to the reactor, start stirring and heat to 220°C, then add the prescribed amount of recycled polyester bottle flakes in three batches. Only after the previous batch of recycled polyester bottle flakes has completely melted and dispersed into a homogeneous phase can the next batch of recycled polyester bottle flakes be added. After all the recycled polyester bottle flakes in the formula have been added and melted into a homogeneous phase, continue to keep warm to carry out the transesterification alcoholysis reaction.

[0044] B. Sampling and testing: When the amount of 1,4-dibromo-2,3-butanediol participating in the transesterification reaction in the reactor reaches 65% of the added amount, it indicates that the transesterification reaction has met the application requirements. At this time, add the formulated amount of dibromoneopentyl glycol and tetrabromoterephthalic acid, and heat to 230℃ to carry out the chain extension polymerization reaction.

[0045] C. Take samples to test the acid value of the polymer. When the acid value of the polymer reaches 50 mg KOH / g, add the prescribed amount of the tetrafunctional chain extender bicondensate (1,1,1-trimethylolpropane) and simultaneously raise the temperature to 240℃ to continue the chain extension reaction.

[0046] D. Take samples to test the acid value of the polymer. When the acid value of the polymer reaches 22 mg KOH / g, add an antioxidant and start the vacuum system to carry out high vacuum polycondensation. The vacuum degree is controlled at -0.099 MPa.

[0047] E. Take samples to test the acid value of the polymer. When the acid value of the polymer reaches below 2mgKOH / g, it indicates that the polymerization reaction has been completed. Release the vacuum system, discharge the material at high temperature, and crush it after cooling to obtain highly transparent flame-retardant polyester resin surface particles for later use.

[0048] F. The raw materials for preparing the core material are first thoroughly vacuum dried at 70-80℃ according to the formula, then thoroughly mixed in a high-speed mixer, and discharged into a cold roller to cool to room temperature before being discharged to prepare a core material premix.

[0049] G. Add the core material premix to a parallel twin-screw extruder for extrusion granulation. The barrel temperature is 270℃ and the screw speed is 350rpm.

[0050] H. The core material granules obtained in step G are melted and fed into the core material cavity of the co-extrusion mold; the high-transparency flame-retardant polyester resin surface layer granules are melted and fed into the flame-retardant surface layer cavity of the co-extrusion mold. After shaping and cooling, the co-extruded wood-plastic composite material is obtained. The melting process is carried out by an extruder, and the melting and extrusion temperature of the core material granules and the surface layer is 280℃.

[0051] Example 2

[0052] A low-cost, long-lasting flame-retardant, and highly transparent co-extruded wood-plastic composite material, comprising a surface layer and a core material, wherein the core material comprises the following raw materials in parts by weight:

[0053]

[0054] The recycled polyester bottle flakes are made by cleaning and drying the recycled mineral water bottles, and then cutting them into flakes with a length and width of less than 3cm; the wood flour is 120 mesh; the compatibilizer is maleic anhydride modified high-density polyethylene, purchased from Huangshan Benuo Technology Co., Ltd.; the lubricant is stearic acid; and the coupling agent is silane coupling agent KH-550.

[0055] The surface layer comprises the following raw materials in parts by weight:

[0056]

[0057] The catalyst is tetrabutyl titanate, used at 1.2% of the mass of 1,4-dibromo-2,3-butanediol; the antioxidant is antioxidant 1010, used at 1.5% of the mass of recycled polyester bottle flakes.

[0058] The preparation method is the same as in Example 1.

[0059] Example 3

[0060] A low-cost, long-lasting flame-retardant, and highly transparent co-extruded wood-plastic composite material, comprising a surface layer and a core material, wherein the core material comprises the following raw materials in parts by weight:

[0061]

[0062] The recycled polyester flakes are made by cleaning and drying the recycled mineral water bottles, and then cutting them into flakes with a length and width of less than 3cm; the wood flour is 110 mesh; the compatibilizer is maleic anhydride modified high-density polyethylene, purchased from Huangshan Benuo Technology Co., Ltd.; the lubricant is stearic acid; and the coupling agent is silane coupling agent KH-550.

[0063] The surface layer comprises the following raw materials in parts by weight:

[0064]

[0065] The catalyst is tetrabutyl titanate, used at 1% of the mass of 1,4-dibromo-2,3-butanediol; the antioxidant is antioxidant 1010, used at 1% of the mass of recycled polyester bottle flakes.

[0066] The preparation method is the same as in Example 1.

[0067] Example 4

[0068] A low-cost, long-lasting flame-retardant, and highly transparent co-extruded wood-plastic composite material, comprising a surface layer and a core material, wherein the core material comprises the following raw materials in parts by weight:

[0069]

[0070] The recycled polyester bottle flakes are made by cleaning and drying the recycled mineral water bottles, and then cutting them into flakes with a length and width of less than 3cm; the wood flour is 100 mesh; the compatibilizer is maleic anhydride modified high-density polyethylene, purchased from Huangshan Benuo Technology Co., Ltd.; the lubricant is stearic acid; and the coupling agent is silane coupling agent KH-550.

[0071] The surface layer comprises the following raw materials in parts by weight:

[0072]

[0073]

[0074] The catalyst is tetrabutyl titanate, used at 0.8% of the mass of 1,4-dibromo-2,3-butanediol; the antioxidant is antioxidant 1010, used at 1% of the mass of recycled polyester bottle flakes.

[0075] The preparation method is the same as in Example 1.

[0076] Performance testing: The technical indicators of this invention are tested in accordance with the standard GB17657-2013 "Test Methods for Physical and Chemical Properties of Wood-based Panels and Decorative Wood-based Panels", wherein the static bending strength is tested according to the three-point bending test method; the flame retardant performance of the surface layer is tested according to GBT 2406.2-2009 "Determination of Combustion Behavior by Oxygen Index Method for Plastics"; the transparency of the surface layer is judged by visually observing the clarity of the surface of the underlying core material.

[0077] Table 1. Performance of co-extruded wood-plastic composites

[0078]

[0079]

[0080] As shown in the table above, this invention mainly uses recycled polyester bottle flakes to prepare a durable, flame-retardant, and highly transparent surface layer through a special formula and process. The recycled polyester bottle flakes are also used in the core material to replace high-density polyethylene or polypropylene plastics, significantly reducing the cost of wood-plastic composites. The final result also achieves a static bending strength of over 43 MPa in the co-extruded wood-plastic composite material. Furthermore, the surface layer obtained through co-extrusion using the special surface layer exhibits good transparency and excellent water resistance. After 50 hours of boiling, the transparency of the surface layer remains unchanged, and there is no cracking between the surface layer and the new material. This indicates that the polyester surface layer of this invention has excellent compatibility and bonding strength with the recycled bottle flake polyester in the core material. Simultaneously, the co-extruded surface layer demonstrates excellent flame-retardant and water-resistant properties; after 50 hours of boiling, its oxygen index shows almost no significant decrease, exhibiting excellent durable flame-retardant characteristics. This invention comprehensively utilizes recycled bottle flake polyester, not only reducing the cost of wood-plastic composites but also providing a new way to comprehensively utilize recycled bottle flake polyester for high added value.

[0081] The foregoing has only described certain exemplary embodiments of the present invention by way of illustration. Undoubtedly, those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the above description is illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.

Claims

1. A low-cost, long-lasting flame-retardant, and highly transparent co-extruded wood-plastic composite material, characterized in that, Co-extruded wood-plastic composite material consists of a surface layer and a core material. The core material comprises raw materials in the following proportions by weight: 55-70 portions of recycled polyester bottle flakes; 23-27 parts wood flour; 8-11 parts calcium carbonate; 1-2 parts compatibilizer; 0.5-1 part coupling agent; Lubricant 0.5-1 part; Among them, the compatibilizer is maleic anhydride-modified high-density polyethylene; the lubricant is stearic acid; and the coupling agent is silane coupling agent KH-550. The surface layer comprises the following raw materials in parts by weight: 62-75 portions of recycled polyester bottle flakes; 26-32 parts of 1,4-dibromo-2,3-butanediol; 7-10 parts of dibromonepentylene glycol; 65-80 parts of tetrabromo-terephthalic acid; 5-6 parts of dimethylolpropane (1,1,1-trimethylolpropane); The raw materials also include catalysts and antioxidants; The catalyst is tetrabutyl titanate, used at 0.8-1.2% of the mass of 1,4-dibromo-2,3-butanediol; the antioxidant is antioxidant 1010, used at 1-1.5% of the mass of recycled polyester bottle flakes. The preparation method of the co-extruded wood-plastic composite material includes the following steps: A. Add the prescribed amount of 1,4-dibromo-2,3-butanediol and catalyst to the reactor, start stirring and heat to 220-225℃, then add the prescribed amount of recycled polyester bottle flakes in three batches. After the recycled polyester bottle flakes added in the previous batch are completely melted and dispersed into a homogeneous phase, add the next batch of recycled polyester bottle flakes. After all the recycled polyester bottle flakes in the formula have been added and melted into a homogeneous phase, continue to keep warm to carry out the transesterification alcoholysis reaction. B. Sampling and testing: When the amount of 1,4-dibromo-2,3-butanediol participating in the transesterification reaction in the reactor reaches 60-65% of the added amount, add the formulated amount of dibromoneopentyl glycol and tetrabromoterephthalic acid, and heat to 230-235℃ to carry out the chain extension polymerization reaction. C. Take samples to test the acid value of the polymer. When the acid value of the polymer reaches 40-50 mg KOH / g, add the amount of the tetrafunctional chain extender bicondensate (1,1,1-trimethylolpropane) as specified in the formula, and at the same time raise the temperature to 240-245℃ to continue the chain extension reaction. D. Take samples to test the acid value of the polymer. When the acid value of the polymer reaches 20-28 mg KOH / g, add antioxidants and start the vacuum system to carry out high vacuum polycondensation. E. Take samples to test the acid value of the polymer. When the acid value of the polymer reaches below 2mgKOH / g, release the vacuum system, discharge the material at high temperature, and crush it after cooling to obtain highly transparent flame-retardant polyester resin surface particles for later use. F. The raw materials for preparing the core material are first thoroughly vacuum dried at 70-80℃ according to the formula, then thoroughly mixed in a high-speed mixer, and discharged into a cold roller to cool to room temperature before being discharged to prepare a core material premix. G. Add the core material premix to a parallel twin-screw extruder for extrusion granulation; H. The core material particles obtained in step G are melted and fed into the core material cavity of the co-extrusion mold; the highly transparent flame-retardant polyester resin surface layer particles are melted and fed into the flame-retardant surface layer cavity of the co-extrusion mold. After shaping and cooling, the co-extruded wood-plastic composite material is obtained.

2. The low-cost, long-lasting flame-retardant, and highly transparent co-extruded wood-plastic composite material as described in claim 1, characterized in that, Recycling polyester bottle flakes involves cleaning and drying recycled mineral water bottles, then cutting them into flakes.

3. The low-cost, long-lasting flame-retardant, and highly transparent co-extruded wood-plastic composite material as described in claim 1, characterized in that, The wood flour is ordinary commercially available wood flour, with a mesh size of 100-120.

4. The low-cost, long-lasting flame-retardant, and highly transparent co-extruded wood-plastic composite material as described in claim 1, characterized in that, In step D, the vacuum level is controlled between 0.098 MPa and 0.099 MPa; in step E, the steel strip is cooled and then crushed.

5. The low-cost, long-lasting flame-retardant, and highly transparent co-extruded wood-plastic composite material as described in claim 1, characterized in that, In step G, the barrel temperature of the parallel twin-screw extruder is 270–280°C, and the screw speed is 250–350 rpm.

6. The low-cost, long-lasting flame-retardant, and highly transparent co-extruded wood-plastic composite material as described in claim 1, characterized in that, In step H, the melting process is carried out by an extruder, and the melt extrusion temperature of the core material particles and the surface layer is 270-280℃.

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