High-strength flame-retardant plastic hollow plate and preparation method thereof

Abstract

The application discloses a kind of high-strength flame-retardant plastic hollow plate and preparation method thereof.The hollow plate includes core honeycomb layer, interface reinforcing layer and outer flame-retardant surface layer.Core honeycomb layer is made of 70wt%-85wt% polypropylene and 15wt%-30wt% carbon fiber reinforced material, extrusion molding honeycomb aperture, provide high strength and light weight.Interface reinforcing layer uses nanocellulose whisker-ZIF-8 composite system, and the interfacial adhesion of reinforcing layer is enhanced.Preparation method includes extrusion molding core layer, ultrasonic dispersion preparation interface reinforcing layer solution, melt blending outer flame-retardant surface layer, hot-pressing composite and annealing post-processing.Product total thickness is 10-20 mm, bending strength is 52.3-58.4 MPa, LOI is 29.5-31.8%, reaches UL-94 V-0 level, and is suitable for high requirement scene such as building, packaging, etc.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of high-strength flame-retardant plastic hollow plates, in particular to a high-strength flame-retardant plastic hollow plate and a preparation method thereof. BACKGROUND

[0002] As a kind of light-weight, high-strength and good thermal insulation layered composite material, plastic hollow plate is widely used in the fields of construction, packaging, logistics and automobile industry. In recent years, with the increasing requirements on material performance and safety, the research and development of high-strength flame-retardant plastic hollow plate has become a hot topic in the industry. The existing technology mainly adds flame retardants (such as halogen, phosphorus or inorganic flame retardants) and reinforcing materials (such as glass fiber, nano filler) in polypropylene (PP) or polyethylene (PE) matrix to improve the flame retardant performance and mechanical properties. In addition, multi-layer co-extrusion technology and interface modification technology (such as maleic anhydride grafted PP) have made significant progress in improving the interlaminar bonding strength of the hollow plate. These technological advances have promoted the application of plastic hollow plate in fireproof building templates, electrical equipment packaging and other high-demand scenarios. However, there is still room for improvement in process optimization and functional integration of the existing technology, especially in balancing high strength and flame retardant performance, improving interfacial stability and other key technical challenges.

[0003] Although the above-mentioned technology lays a foundation for the development of high-strength flame-retardant plastic hollow plate, there are still significant deficiencies. The layered structure (panel-core panel) of the existing hollow plate has deficiencies in interfacial bonding. For example, in the traditional co-extrusion process, the interfacial bonding strength between the panel and the core layer is low, which is prone to delamination due to the difference in thermal expansion coefficient or external force, resulting in a decrease in overall mechanical properties and limited flame retardant effect. Although the existing interface modification technology (such as silane coupling agent or graft modification) has improved, it is not suitable for complex multi-layer structures, and it is difficult to achieve the synergistic optimization of high-strength interfacial bonding and flame retardant function. Our invention aims to solve the strength-flame retardant contradiction and interfacial bonding problem through innovative material formulation and structure design, aiming to significantly improve the comprehensive performance and application reliability of plastic hollow plate. SUMMARY

[0004] The application provides a high-strength flame-retardant plastic hollow plate, which comprises a core honeycomb layer, an interface enhancement layer and an outer flame-retardant surface layer. The core honeycomb layer is made of a polypropylene matrix and a carbon fiber reinforced composite material. The interface enhancement layer adopts a nano-cellulose whisker-metal organic framework (MOF) composite system, which forms a molecular-level interlocking structure through in-situ self-assembly. The outer flame-retardant surface layer is composed of a graphene chemically modified polyethylene matrix, a composite flame retardant and an intumescent carbonization promoter. The graphene forms a covalent bond with the polyethylene matrix through chemical grafting. The multi-layer structure of the hollow plate is integrally formed by a hot pressing composite process.

[0005] It should be noted that the core honeycomb layer is composed of a polypropylene matrix and a carbon fiber reinforced composite material, and the honeycomb pores are formed by extrusion molding to provide high mechanical strength and lightweight characteristics. The interface reinforcing layer adopts a nanocellulose whisker-metal organic framework (MOF) composite system, in which the surface of the nanocellulose whisker (CNC) is anchored with ZIF-8 (zinc-based MOF) through coordination chemical reaction. In the hot-pressing process, the hydroxyl groups of CNC form hydrogen bonds and van der Waals forces with the polypropylene matrix of the core honeycomb layer and the graphene-modified polyethylene matrix of the outer flame-retardant surface layer, and the zinc ion coordination center of ZIF-8 forms coordination bonds with the polyamide resin and the outer layer functional groups. Through in-situ self-assembly, a molecular-level interlocking structure is constructed to enhance the interlayer bonding force and flame-retardant performance. The outer flame-retardant surface layer is composed of a graphene chemically modified polyethylene matrix, a phosphate ester and melamine cyanurate composite flame retardant, and expanded graphite. Graphene forms C-C covalent bonds with polyethylene through free radical grafting, improving thermal stability, and synergizing with flame retardants and expanded graphite to form a dense carbon layer during combustion, blocking oxygen and heat transfer. The hot-pressing process promotes molecular chain diffusion and chemical bonding through high temperature and high pressure to form an integrated multi-layer structure.

[0006] As a preferred technical solution of the high-strength flame-retardant plastic hollow plate, the mass percentage of the polypropylene matrix in the core honeycomb layer is 70wt%-85wt%, and the mass percentage of the carbon fiber reinforced material is 15wt%-30wt%. The honeycomb pores are formed by extrusion molding, and the pore diameter is 5-10 mm.

[0007] It should be noted that the 5-10 mm honeycomb pores are formed by extrusion molding to optimize strength and lightweight, polypropylene provides toughness, carbon fiber enhances rigidity, the honeycomb structure disperses stress, reduces density, maintains high compression and impact resistance, and provides lightweight, high-strength support for the multi-layer structure of the hollow plate.

[0008] As a preferred technical solution of the high-strength flame-retardant plastic hollow plate, the nanocellulose whisker-metal organic framework composite system in the interface reinforcing layer is prepared by anchoring zinc-based MOF on the surface of nanocellulose whiskers through coordination chemical reaction.

[0009] It should be noted that in the hot-pressing process, the hydroxyl groups of CNC form hydrogen bonds and van der Waals forces with the polypropylene matrix of the core honeycomb layer and the graphene-modified polyethylene matrix of the outer flame-retardant surface layer, and the zinc ion coordination center of ZIF-8 forms coordination bonds with the polyamide resin and the outer layer functional groups. Combined with the heat-induced molecular chain diffusion of the polyamide resin, a molecular-level interlocking network is constructed by in-situ self-assembly to enhance the interlayer bonding force and flame-retardant auxiliary performance.

[0010] As a preferred technical scheme of the high-strength flame-retardant plastic hollow plate, the outer flame-retardant surface layer graphene chemically modified polyethylene matrix is prepared by radical grafting reaction of graphene oxide and polyethylene molecular chain, the content of graphene is 0.5wt%-3wt%, the mass percentage of polyethylene matrix is 60wt%-75wt%, the composite flame retardant is composed of phosphate and melamine cyanurate, the mass percentage is 25wt%-40wt%, and 1wt%-3wt% of expanded graphite is added as a carbonization promoter.

[0011] It should be noted that the graphene chemically modified polyethylene matrix in the outer flame-retardant surface layer is prepared by radical grafting reaction of graphene oxide (GO) and polyethylene molecular chain at 170-190℃ induced by peroxide initiator, the content of GO is 0.5wt%-3wt%, C-C covalent bond is formed, and the thermal stability and carbonization tendency of polyethylene are improved. The polyethylene matrix (60wt%-75wt%) provides flexibility and processability, the composite flame retardant (phosphate and melamine cyanurate, 25wt%-40wt%) releases non-combustible gas and promotes the formation of carbon layer by phosphorus-nitrogen synergistic effect during combustion, and 1wt%-3wt% of expanded graphite is added as a carbonization promoter, which expands at high temperature to form a dense carbon layer, blocks oxygen and heat transfer, and significantly enhances the flame retardant performance.

[0012] As a preferred technical scheme of the high-strength flame-retardant plastic hollow plate, the total thickness of the multi-layer structure hollow plate is 10-20 mm, wherein the core honeycomb layer accounts for 60-70%, the interface reinforcing layer accounts for 5-10%, and the outer flame-retardant surface layer accounts for 20-30%.

[0013] It should be noted that the core honeycomb layer provides high strength and light weight, the interface reinforcing layer enhances the interfacial bonding force, and the outer flame-retardant surface layer improves the flame retardant performance. The proportion optimization ensures the structural stability and functional synergy, and is suitable for various application scenarios.

[0014] As a preferred technical scheme of the high-strength flame-retardant plastic hollow plate, the interface reinforcing layer further comprises a hindered phenolic antioxidant, and the content is 0.5-2 wt%.

[0015] It should be noted that the 0.5-2 wt% hindered phenolic antioxidant in the interface reinforcing layer effectively prevents the oxidation and aging of the interface material, prolongs the service life and performance stability of the multi-layer structure hollow plate.

[0016] In addition, the present application also provides a preparation method of a high-strength flame-retardant plastic hollow plate, comprising the following steps:

[0017] Step S1. Preparation of the core honeycomb layer: 70wt%-85wt% of polypropylene is mixed with 15wt%-30wt% of carbon fiber reinforcement material, extruded through an extruder at 180-220℃ to form a honeycomb structure with a pore diameter of 5-10 mm, and cooled for standby;

[0018] Step S2. Preparation of the interface reinforcement layer solution: 3wt%-8wt% of nanocellulose whiskers is ultrasonically dispersed with zinc salt and 2-methylimidazole in a molar ratio of 1:2:4 in a water-ethanol mixed solvent, followed by stirring at 50-70℃ under nitrogen protection for 4-6 hours to form ZIF-8 anchored nanocellulose whiskers; which is mixed with 86wt%-95wt% of polyamide resin, 0.5wt%-2wt% of hindered phenolic antioxidant and 2wt%-5wt% of hydroxypropyl cellulose, ultrasonically dispersed at 80-100℃ for 20-40 minutes to prepare a uniform composite system solution;

[0019] Step S3. Preparation of the outer flame-retardant surface layer: 0.5wt%-3wt% of graphene oxide is free radical grafted with 60wt%-75wt% of polyethylene matrix at 170-190℃ by peroxide initiator to prepare a graphene chemically modified polyethylene matrix, which is then melt blended with 25wt%-40wt% of phosphate ester and melamine cyanurate composite flame retardant and 1wt%-3wt% of intumescent graphite in a twin-screw extruder at 160-190℃ to extrude an outer flame-retardant surface layer with a thickness of 2-6 mm, and cooled for standby; wherein benzoyl peroxide is used as the initiator, the addition amount is 0.1wt%-0.5wt%, and the reaction time is 1-2 hours;

[0020] Step S4. Hot pressing composite molding: the core honeycomb layer is preheated to 150-180℃, uniformly coated with the interface reinforcement layer solution, and stacked with the outer flame-retardant surface layer, and hot pressed at a pressure of 5-10 MPa and a temperature of 160-200℃ for 10-20 minutes to avoid decomposition of the flame retardant by gradient temperature control;

[0021] Step S5. Post-processing: the molded multilayer hollow plate is annealed at 50-60℃ for 1-2 hours and cut to the desired size.

[0022] It should be noted that 70wt%-85wt% polypropylene is mixed with 15wt%-30wt% carbon fiber reinforced material to form a honeycomb-shaped hole (diameter 5-10 mm) through an extruder at 180-220℃, the polypropylene provides toughness and processability, the carbon fiber enhances rigidity and strength, the honeycomb structure disperses stress, realizes the balance of lightweight and high mechanical strength, and forms a stable core layer after cooling. 3wt%-8wt% nanocellulose whiskers (CNC) are ultrasonically dispersed with zinc salt and 2-methyl imidazole (molar ratio 1:2:4) in a water-ethanol mixed solvent, and reacted at 50-70℃ under nitrogen protection for 4-6 hours, so that the hydroxyl groups on the surface of CNC are anchored with ZIF-8 through coordination bonds (O-Zn) to form a CNC-ZIF-8 composite structure; then 86wt%-95wt% polyamide resin, 0.5wt%-2wt% hindered phenolic antioxidant and 2wt%-5wt% hydroxypropyl cellulose are ultrasonically dispersed at 80-100℃ for 20-40 minutes to prepare a uniform composite system solution, CNC provides high specific surface area, ZIF-8 enhances interfacial bonding, and antioxidants and hydroxypropyl cellulose improve stability and compatibility. 0.5wt%-3wt% graphene oxide (GO) is subjected to free radical grafting reaction (1-2 hours) with 60wt%-75wt% polyethylene matrix at 170-190℃ under the initiation of 0.1wt%-0.5wt% benzoyl peroxide, to form C-C covalent bonds, and a graphene chemically modified polyethylene matrix is prepared to improve thermal stability; then 25wt%-40wt% phosphate ester and melamine cyanurate composite flame retardant and 1wt%-3wt% intumescent graphite are melt blended in a twin-screw extruder at 160-190℃, and a 2-6 mm thick surface layer is extruded, the phosphorus-nitrogen flame retardant releases non-combustible gas to promote charring, and the intumescent graphite forms a dense carbon layer to block oxygen and heat. The core honeycomb layer is preheated to 150-180℃, the interfacial reinforcement layer solution is coated, and the outer flame-retardant surface layer is superimposed, and hot pressing is carried out at 160-200℃ and 5-10 MPa for 10-20 minutes, the hydroxyl groups and coordination bonds of CNC-ZIF-8 form hydrogen bonds and coordination networks with polypropylene and modified polyethylene, and gradient temperature control (initial 180℃ to 120℃) is used to promote molecular chain diffusion and in-situ self-assembly, avoid decomposition of the flame retardant, and form a molecular-level interlocking structure. Annealing at 50-60℃ for 1-2 hours eliminates internal stress, and the desired size is cut to ensure the integration of the multilayer structure and the stability of the performance, and high strength and flame retardant performance are achieved synergistically.

[0023] As a preferred technical scheme of the preparation method of the high-strength flame-retardant plastic hollow plate, in step S2, the power of ultrasonic dispersion is 300-500 W, the frequency is 20-40 kHz, and the time is 30-60 minutes.

[0024] It is necessary to ensure that the nanocellulose whiskers and ZIF-8 are uniformly dispersed, and the stability of the interface reinforced layer composite system and the formation efficiency of the molecular level interlocking structure are enhanced.

[0025] As a preferred technical solution of the preparation method of the high-strength flame-retardant plastic hollow plate, in step S3, the screw rotation speed of the double-screw extruder is 100-200 rpm, and the feeding rate is controlled at 5-10 kg / h.

[0026] It is necessary to ensure that the graphene modified polyethylene matrix is uniformly blended with the flame retardant and expanded graphite, and the dispersibility of the outer flame-retardant surface layer and the carbon layer formation efficiency are improved.

[0027] As a preferred technical solution of the preparation method of the high-strength flame-retardant plastic hollow plate, in step S4, the hot pressing composite process adopts multi-stage pressure control, the initial pressure is 5 MPa, and gradually increases to 10 MPa, and maintains constant pressure for 5 minutes.

[0028] It is necessary to promote the penetration and chemical bonding of the molecular chains of the interface reinforced layer, and to enhance the formation efficiency of the molecular level interlocking structure and the interlayer bonding force.

[0029] The high-strength flame-retardant plastic hollow plate of the application significantly improves the comprehensive performance through innovative material formula and structure design. The core honeycomb layer combines polypropylene and carbon fiber reinforced material to provide high mechanical strength and lightweight characteristics. The interface reinforced layer adopts a nanocellulose whisker-ZIF-8 composite system, which significantly enhances the interlayer bonding force through a molecular level interlocking structure, effectively preventing delamination. The outer flame-retardant surface layer uses a graphene chemically modified polyethylene matrix in combination with phosphorus-nitrogen flame retardant and expanded graphite to form a dense carbon layer, significantly improving the flame retardant performance. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 The infrared spectrum of the ZIF-8 anchored CNC formed in step S2 in Example 1. DETAILED DESCRIPTION

[0031] In order to make the above-mentioned purposes, features and advantages of the application more obvious and easy to understand, the specific embodiments of the application will be described in detail below with reference to the description examples.

[0032] In the following description, many specific details are set forth in order to provide a thorough understanding of the application, but the application can also be implemented in other ways different from those described herein, and those skilled in the art can make similar generalizations without departing from the scope of the application, therefore the application is not limited to the specific examples disclosed below.

[0033] Second, the term "one embodiment" or "an embodiment" as may appear in various places in the specification are not necessarily all referring to the same embodiment, nor are they necessarily all alternative or mutually exclusive embodiments. Example 1

[0034] Example 1 provides a method for preparing a high-strength flame-retardant plastic hollow board, comprising the following steps:

[0035] Step S1. Preparation of core honeycomb layer: 70wt% polypropylene (PP, melt index 10 g / 10min) and 30wt% chopped carbon fiber (length 3 mm, modulus 230 GPa) were mixed in a twin-screw extruder at 180°C, screw speed 150 rpm, feed rate 5 kg / h, and extruded into a honeycomb-shaped pore (pore diameter 5 mm), and cooled to room temperature for standby.

[0036] Step S2. Preparation of interface reinforcement layer solution: 3wt% nanocellulose whiskers (CNC, diameter 10-20 nm, length 100-200 nm) were dispersed in a water-ethanol mixed solvent (volume ratio 1:1) with zinc nitrate and 2-methylimidazole (molar ratio 1:2:4) for 30 minutes at 300 W power and 20 kHz frequency, followed by stirring at 50°C under nitrogen protection for 4 hours to form ZIF-8 anchored CNC; then mixed with 95wt% polyamide resin (PA6, molecular weight 20,000), 0.5wt% hindered phenol antioxidant (Irganox 1010), and 2wt% hydroxypropyl cellulose, and ultrasonically dispersed at 80°C for 20 minutes at 300 W power and 20 kHz frequency to prepare a uniform composite system solution, with a total ZIF-8 content of 2wt%.

[0037] Step S3. Preparation of outer flame-retardant surface layer: 0.5wt% graphene oxide (GO, flake diameter 0.5-5 μm) was mixed with 75wt% polyethylene (PE, melt index 2 g / 10min) at 170°C, and 0.1wt% benzoyl peroxide (BPO) was added to initiate a free radical grafting reaction, and the reaction was carried out for 1 hour to prepare a graphene chemically modified polyethylene matrix; then mixed with 24.5wt% phosphate ester (tris(2-chloroethyl) phosphate) and melamine cyanurate (mass ratio 1:1), and 1wt% expanded graphite (expansion ratio 200 mL / g) in a twin-screw extruder at 160°C, screw speed 100 rpm, and feed rate 5 kg / h, and melt blended to form a surface layer with a thickness of 2 mm, and cooled for standby.

[0038] Step S4. Hot-pressing compounding molding: Preheat the core honeycomb layer to 150 °C, uniformly coat the interface reinforcement layer solution (coating thickness 0.5 mm), superimpose the outer flame-retardant surface layer, hot-press at 160 °C, initial pressure 5 MPa for 5 minutes, gradually increase to 10 MPa, maintain constant pressure for 5 minutes, avoid decomposition of the flame retardant by gradient temperature control (decrease to 120 °C), form a multilayer structure (core honeycomb layer 6 mm, interface reinforcement layer 0.5 mm, outer flame-retardant surface layer 3.5 mm) with a total thickness of 10 mm.

[0039] Step S5. Post-processing: Anneal the hollow plate of the molded multilayer structure at 50 °C for 1 hour, cut to the required size, and obtain a high-strength flame-retardant plastic hollow plate. Example 2

[0040] Example 2 provides a method for preparing a high-strength flame-retardant plastic hollow plate, comprising the following steps:

[0041] Step S1. Preparation of core honeycomb layer: Take 77.5wt% polypropylene (PP, melt index 12 g / 10min) and 22.5wt% chopped carbon fibers (length 4 mm, modulus 240 GPa), mix in a double-screw extruder at 200 °C, screw speed 175 rpm, feed rate 7.5 kg / h, and extrude to form a honeycomb-shaped pore (pore diameter 7.5 mm), and cool to room temperature for standby.

[0042] Step S2. Preparation of interface reinforcement layer solution: Take 5.5wt% nanocellulose whiskers (CNC, diameter 10-20 nm, length 100-200 nm) and zinc nitrate and 2-methylimidazole (molar ratio 1:2:4) in a water-ethanol mixed solvent (volume ratio 1:1), ultrasonic dispersion for 45 minutes at 400 W power and 30 kHz frequency, then stir at 60 °C under nitrogen protection for 5 hours to form ZIF-8 anchored CNC; mix it with 90.5wt% polyamide resin (PA66, molecular weight 25,000), 1.25wt% hindered phenol antioxidant (Irganox 1098), and 3.5wt% hydroxypropyl cellulose, ultrasonic dispersion for 30 minutes at 90 °C, 400 W power and 30 kHz frequency, to prepare a uniform composite system solution, with a total ZIF-8 content of 4wt%.

[0043] Step S3. Preparation of outer flame-retardant face layer: 1.75 wt% graphene oxide (GO, flake diameter 0.5-5 pm) and 67.5 wt% polyethylene (PE, melt index 3 g / 10 min) were taken and a free radical grafting reaction was initiated by adding 0.3 wt% benzoyl peroxide (BPO) at 180 °C for 1.5 hours to prepare a graphene chemically modified polyethylene matrix; then 29 wt% phosphonate (tris(2-chloroethyl) phosphate) and melamine cyanurate (mass ratio 1:1) and 2 wt% intumescent graphite (expansion ratio 250 mL / g) were melt blended in a twin-screw extruder at 175 °C, screw speed 150 rpm, feeding rate 7.5 kg / h, and extruded into a face layer with a thickness of 4 mm, which was cooled for later use.

[0044] Step S4. Hot-pressing composite molding: the core honeycomb layer was preheated to 165 °C, uniformly coated with an interfacial reinforcement layer solution (coating thickness 1 mm), and overlaid with an outer flame-retardant face layer, and then hot-pressed at 180 °C, initial pressure 5 MPa, for 7.5 minutes, gradually increased to 10 MPa, and kept constant for 5 minutes, and a gradient temperature control (reduced to 120 °C) was used to avoid decomposition of the flame retardant, to form a multilayer structure (core honeycomb layer 10 mm, interfacial reinforcement layer 1 mm, outer flame-retardant face layer 4 mm) with a total thickness of 15 mm.

[0045] Step S5. Post-processing: the multilayer structure after molding was annealed at 55 °C for 1.5 hours, cut to the required size, and a high-strength flame-retardant plastic hollow plate was obtained. Example 3

[0046] Example 3 provides a method for preparing a high-strength flame-retardant plastic hollow plate, comprising the following steps:

[0047] Step S1. Preparation of core honeycomb layer: 85 wt% polypropylene (PP, melt index 15 g / 10 min) and 15 wt% chopped carbon fibers (length 5 mm, modulus 250 GPa) were mixed in a twin-screw extruder at 220 °C, screw speed 200 rpm, and feeding rate 10 kg / h, and extruded into a honeycomb-shaped pore (pore diameter 10 mm), which was cooled to room temperature for later use.

[0048] Step S2. Preparation of interface reinforced layer solution: 8 wt% nanocellulose whiskers (CNC, diameter 10-20 nm, length 100-200 nm) were dispersed with zinc nitrate and 2-methylimidazole (molar ratio 1:2:4) in water-ethanol mixed solvent (volume ratio 1:1) by ultrasonic dispersion at 500 W power and 40 kHz frequency for 60 minutes, followed by stirring reaction at 70°C under nitrogen protection for 6 hours to form ZIF-8 anchored CNC; which was mixed with 86 wt% polyamide resin (PA6, molecular weight 30,000), 2 wt% hindered phenol antioxidant (Irganox 1010) and 4 wt% hydroxypropyl cellulose, and a uniform composite system solution was prepared by ultrasonic dispersion at 100°C, 500 W power and 40 kHz frequency for 40 minutes, and the total content of ZIF-8 was 6 wt%.

[0049] Step S3. Preparation of outer flame-retardant surface layer: 3 wt% graphene oxide (GO, flake diameter 0.5-5 μm) was mixed with 60 wt% polyethylene (PE, melt index 4 g / 10 min) at 190°C, and 0.5 wt% benzoyl peroxide (BPO) was added to initiate free radical grafting reaction, and the reaction was carried out for 2 hours to prepare a graphene chemically modified polyethylene matrix; which was then melt blended with 34 wt% phosphate (tris(2-chloroethyl) phosphate) and melamine cyanurate (mass ratio 1:1) and 3 wt% intumescent graphite (expansion ratio 300 mL / g) in a twin-screw extruder at 190°C, screw speed 200 rpm, and feeding rate 10 kg / h, and extruded into a surface layer with a thickness of 6 mm, and cooled for use.

[0050] Step S4. Hot pressing composite molding: the core honeycomb layer was preheated to 180°C, uniformly coated with the interface reinforced layer solution (coating thickness 2 mm), and the outer flame-retardant surface layer was stacked, and hot pressed at 200°C, initial pressure 5 MPa for 10 minutes, gradually increased to 10 MPa, and kept constant pressure for 5 minutes, and the gradient temperature control (reduced to 120°C) was used to avoid decomposition of the flame retardant, and a multilayer structure (core honeycomb layer 14 mm, interface reinforced layer 2 mm, outer flame-retardant surface layer 4 mm) with a total thickness of 20 mm was formed.

[0051] Step S5. Post-processing: the hollow plate of the molded multilayer structure was annealed at 60°C for 2 hours, cut to the required size, and a high-strength flame-retardant plastic hollow plate was obtained. Example 4

[0052] Example 4 provides a method for preparing a high-strength flame-retardant plastic hollow plate, comprising the following steps:

[0053] Step S1. Preparation of core honeycomb layer: 75 wt% polypropylene (PP, melt index 11 g / 10 min) and 25 wt% short carbon fibers (length 3.5 mm, modulus 235 GPa) were mixed in a twin-screw extruder at 190 °C, screw rotation speed 160 rpm, feeding rate 6 kg / h, and extruded into a honeycomb structure with a pore diameter of 6 mm. After cooling to room temperature, the honeycomb layer was stored for further use.

[0054] Step S2. Preparation of interface reinforcement layer solution: 4 wt% nanocellulose whiskers (CNC, diameter 10-20 nm, length 100-200 nm) were dispersed in a mixture of water and ethanol (volume ratio 1:1) with zinc nitrate and 2-methylimidazole (molar ratio 1:2:4) by ultrasonic dispersion at a power of 350 W and a frequency of 25 kHz for 40 min, followed by stirring at 55 °C under nitrogen protection for 4.5 h to form ZIF-8 anchored CNC. The ZIF-8 anchored CNC was mixed with 92 wt% polyamide resin (PA66, molecular weight 22,000), 1 wt% hindered phenol antioxidant (Irganox 1098), and 3 wt% hydroxypropyl cellulose, and a homogeneous composite system solution was prepared by ultrasonic dispersion at a power of 350 W and a frequency of 25 kHz for 25 min at 85 °C. The total content of ZIF-8 was 3 wt%.

[0055] Step S3. Preparation of outer flame-retardant surface layer: 1 wt% graphene oxide (GO, flake diameter 0.5-5 μm) was mixed with 70 wt% polyethylene (PE, melt index 2.5 g / 10 min) at 175 °C, and 0.2 wt% benzoyl peroxide (BPO) was added to initiate a free radical grafting reaction. The reaction was carried out for 1.2 h to prepare a graphene chemically modified polyethylene matrix. The graphene chemically modified polyethylene matrix was then melt blended with 27 wt% phosphoric acid ester (tris(2-chloroethyl) phosphate) and melamine cyanurate (mass ratio 1:1) and 2 wt% intumescent graphite (expansion ratio 220 mL / g) in a twin-screw extruder at 170 °C, screw rotation speed 120 rpm, and feeding rate 6 kg / h. The extruded surface layer with a thickness of 3 mm was cooled and stored for further use.

[0056] Step S4. Hot-pressing composite molding: The core honeycomb layer was preheated to 160 °C, uniformly coated with the interface reinforcement layer solution (coating thickness 0.8 mm), and overlaid with the outer flame-retardant surface layer. Hot-pressing was carried out at 170 °C, initial pressure 5 MPa, for 6 min, gradually increased to 10 MPa, and maintained at a constant pressure for 5 min. Gradient temperature control (reduced to 120 °C) was used to avoid decomposition of the flame retardant. A multilayer structure with a total thickness of 12 mm was formed (core honeycomb layer 7.5 mm, interface reinforcement layer 0.8 mm, outer flame-retardant surface layer 3.7 mm).

[0057] Step S5. Post-processing: annealing the hollow plate of the multi-layer structure after molding at 52℃ for 1.2 hours, cutting to the required size, to obtain a high-strength flame-retardant plastic hollow plate.

[0058] Comparative Example 1 provides a method for preparing a plastic hollow plate, compared with Example 1, only omitting the ZIF-8 anchoring process in the interface reinforcing layer (i.e. not adding zinc salt and 2-methyl imidazole, not forming a nanocellulose whisker-metal organic framework composite system), and using only nanocellulose whiskers mixed with polyamide resin as the interface reinforcing layer, and other steps and parameters remain the same, including the core honeycomb layer, the outer flame-retardant surface layer, and the hot-pressing composite process. The specific steps are as follows:

[0059] Step S1. Preparation of the core honeycomb layer: take 70wt% polypropylene (PP, melt index 10 g / 10min) and 30wt% chopped carbon fiber (length 3 mm, modulus 230 GPa) and mix them in a double screw extruder at 180℃, screw speed 150 rpm, feeding rate 5 kg / h, to form a honeycomb-shaped pore (pore diameter 5 mm) and cool to room temperature for standby.

[0060] Step S2. Preparation of the interface reinforcing layer solution: take 3wt% nanocellulose whiskers (CNC, diameter 10-20 nm, length 100-200 nm) in water-ethanol mixed solvent (volume ratio 1:1) and ultrasonically disperse for 30 minutes at 300 W power and 20 kHz frequency; mix it with 95wt% polyamide resin (PA6, molecular weight 20,000), 0.5wt% hindered phenol antioxidant (Irganox 1010), and 2wt% hydroxypropyl cellulose, and ultrasonically disperse for 20 minutes at 80℃, 300 W power and 20 kHz frequency, to obtain a uniform composite system solution.

[0061] Step S3. Preparation of the outer flame-retardant surface layer: take 0.5wt% graphene oxide (GO, flake diameter 0.5-5 μm) and 75wt% polyethylene (PE, melt index 2 g / 10min) and add 0.1wt% benzoyl peroxide (BPO) to initiate free radical grafting reaction at 170℃, and react for 1 hour to prepare a graphene chemically modified polyethylene matrix; then melt blend with 24.5wt% phosphate ester (tris(2-chloroethyl) phosphate) and melamine cyanurate (mass ratio 1:1) and 1wt% expanded graphite (expansion ratio 200 mL / g) in a double screw extruder at 160℃, screw speed 100 rpm, and feeding rate 5 kg / h, to extrude a 2 mm thick surface layer and cool for standby.

[0062] Step S4. Hot-pressing compounding molding: Preheat the core honeycomb layer to 150 °C, uniformly coat the interface reinforcement layer solution (coating thickness 0.5 mm), superimpose the outer flame-retardant surface layer, hot-press at 160 °C, initial pressure 5 MPa for 5 minutes, gradually increase to 10 MPa, maintain constant pressure for 5 minutes, avoid decomposition of the flame retardant by gradient temperature control (decrease to 120 °C), form a multilayer structure (core honeycomb layer 6 mm, interface reinforcement layer 0.5 mm, outer flame-retardant surface layer 3.5 mm) with a total thickness of 10 mm.

[0063] Step S5. Post-processing: Anneal the hollow plate of the molded multilayer structure at 50 °C for 1 hour, cut to the required size, and obtain the plastic hollow plate.

[0064] Comparative Example 2

[0065] Comparative Example 2 provides a method for preparing a plastic hollow plate, compared with Example 1, only no graphene chemical modification (i.e. no graphene oxide and free radical grafting reaction, using 75wt% ordinary polyethylene matrix) in the outer flame-retardant surface layer, other steps and parameters remain the same, including the core honeycomb layer, the interface reinforcement layer and the hot-pressing compounding process. The specific steps are as follows:

[0066] Step S1. Preparation of core honeycomb layer: Take 70wt% polypropylene (PP, melt index 10 g / 10min) and 30wt% chopped carbon fiber (length 3 mm, modulus 230 GPa), mix in a twin-screw extruder at 180 °C, screw speed 150 rpm, feeding rate 5 kg / h, extrude to form a honeycomb-like pore (pore diameter 5 mm), and cool to room temperature for standby.

[0067] Step S2. Preparation of interface reinforcement layer solution: Take 3wt% nanocellulose whiskers (CNC, diameter 10-20 nm, length 100-200 nm) and zinc nitrate and 2-methylimidazole (molar ratio 1:2:4) in a water-ethanol mixed solvent (volume ratio 1:1), ultrasonic dispersion for 30 minutes at 300 W power and 20 kHz frequency, then stir at 50 °C under nitrogen protection for 4 hours to form ZIF-8 anchored CNC; mix it with 95wt% polyamide resin (PA6, molecular weight 20,000), 0.5wt% hindered phenol antioxidant (Irganox 1010) and 2wt% hydroxypropyl cellulose, ultrasonic dispersion for 20 minutes at 80 °C with 300 W power and 20 kHz frequency, to prepare a uniform composite system solution, with a total ZIF-8 content of 2wt%.

[0068] Step S3. Preparation of outer flame-retardant skin layer: 75 wt% polyethylene (PE, melt index 2 g / 10 min) was melt-blended with 24.5 wt% phosphate ester (tris(2-chloroethyl) phosphate) and melamine cyanurate (mass ratio 1:1) and 1 wt% intumescent graphite (expansion ratio 200 mL / g) in a twin-screw extruder at 160 °C, screw rotation speed 100 rpm, feeding rate 5 kg / h, and extruded into a skin layer with a thickness of 2 mm, and cooled for standby.

[0069] Step S4. Hot-pressing composite molding: the core honeycomb layer was preheated to 150 °C, uniformly coated with the interfacial reinforcement layer solution (coating thickness 0.5 mm), and superimposed with the outer flame-retardant skin layer, and hot-pressed at 160 °C, initial pressure 5 MPa for 5 minutes, gradually increased to 10 MPa, and kept constant pressure for 5 minutes, and the gradient temperature control (reduced to 120 °C) was used to avoid decomposition of the flame retardant, to form a multilayer structure (core honeycomb layer 6 mm, interfacial reinforcement layer 0.5 mm, outer flame-retardant skin layer 3.5 mm) with a total thickness of 10 mm.

[0070] Step S5. Post-processing: the hollow plate of the molded multilayer structure was annealed at 50 °C for 1 hour, and cut to the required size to obtain a plastic hollow plate.

[0071] Performance test methods

[0072] The test methods are based on Chinese national standards (GB / T) or international standards (ASTM / UL) and are applicable to plastic composites. Sample preparation: standard size samples (e.g., 80 mm x 10 mm x 4 mm strips for bending tests) were cut from the prepared hollow plates, and the test environment was 23 ± 2 °C, relative humidity 50 ± 5%.

[0073] Bending strength: determined according to GB / T 9341-2008 (equivalent to ASTM D790), using a three-point bending method, span 64 mm, loading speed 2 mm / min, and calculating the strength at the maximum load (unit: MPa).

[0074] Compressive strength: determined according to GB / T 1041-2008 (equivalent to ASTM D695), using parallel plate compression, loading speed 1 mm / min, until the sample is destroyed, and calculating the peak stress (unit: MPa).

[0075] Impact strength: determined according to GB / T 1043.1-2008 (equivalent to ASTM D256), using a simply supported beam impact testing machine, notch type A, and calculating the fracture energy absorption (unit: kJ / m2).

[0076] Peel strength: determined according to GB / T 2790-1995 (equivalent to ASTM D903) using 180° peel test, peel rate 300 mm / min, average peel force calculated (unit: N / cm).

[0077] Limiting oxygen index (LOI): determined according to GB / T 2406.2-2009 (equivalent to ASTM D2863) by igniting samples in a nitrogen-oxygen mixed gas stream, the lowest oxygen concentration (unit: %) to maintain combustion was recorded.

[0078] UL-94 vertical burning rating: determined according to UL-94 standard, samples were vertically hung, and a flame was applied twice for 10 s, the self-extinguishing time, dripping and burning length were observed, rated as V-0, V-1, V-2 or HB.

[0079] Table 1 is the experimental data of Examples 1 to 4 and Comparative Examples 1 to 2

[0080]

[0081] In combination with Example 1 and Figure 1 It can be seen from the provided FT-IR spectrum that the high-intensity broad peak at 3330 cm -1 corresponds to the O-H stretching vibration of nanocellulose whiskers, indicating the presence of abundant hydroxyl groups, which is a typical feature of CNC; the peak at 2890 cm -1 corresponds to C-H stretching vibration, the peaks at 1160 cm -1 and 1055 cm -1 correspond to C-O-C and C-O stretching vibrations, respectively, further confirming the presence of the CNC skeleton. At the same time, the peak at 1584 cm -1 corresponds to the C=N / C=C vibration of the imidazole ring in ZIF-8, the peak at 995 cm -1 indicates the plane vibration of the imidazole ring, while the peaks at 756 cm -1 and 421 cm -1 correspond to the out-of-plane vibration and the characteristic absorption of Zn-N coordination bond, respectively, which are all the hallmark signals of ZIF-8 metal-organic framework. The coexistence of these CNC and ZIF-8 characteristic peaks, especially the appearance of the Zn-N coordination peak at 421 cm -1 in the low wave number region, indicates that ZIF-8 has been bound to the functional groups (such as hydroxyl groups) on the surface of CNC through coordination bonds, thus successfully anchoring on the nanocellulose whiskers and forming a stable CNC-ZIF-8 composite material. In addition, the relative intensity and distribution of the peaks in the spectrum also suggest the effective compounding of the two materials, and no obvious interference peaks are seen, supporting the formation of this structure.

[0082] It can be seen from the combination of embodiments 1 to 4 and table 1 that the bending strength ranges from 52.3-58.4 MPa, the compressive strength is 45.1-50.2 MPa, the impact strength is 12.5-14.1 kJ / m2, the peeling strength is 21.2-24.5 N / cm, and the limiting oxygen index (LOI) is 29.5-31.8%, all reaching the UL-94 V-0 flame retardant level. The present application significantly enhances the interlayer bonding force and flame retardant performance through the nano-cellulose whisker-ZIF-8 composite system and the chemical modification of graphene, while maintaining high mechanical strength.

[0083] It can be seen from the combination of embodiments 1, comparative example 1 and table 1 that the high-strength flame-retardant plastic hollow plate of embodiment 1 is significantly superior in performance to comparative example 1. The bending strength of embodiment 1 is 52.3 MPa, the compressive strength is 45.1 MPa, the impact strength is 12.5 kJ / m 2 , the peeling strength is 21.2 N / cm, the limiting oxygen index (LOI) is 29.5%, and the UL-94 level is V-0; the bending strength of comparative example 1 is 42.6 MPa, the compressive strength is 38.9 MPa, the impact strength is 10.3 kJ / m 2 , the peeling strength is only 11.4 N / cm, the LOI is 29.0%, and the UL-94 level is also V-0. The interface reinforcing layer of embodiment 1 uses a nano-cellulose whisker-ZIF-8 composite system to form a molecular-level interlocking structure through coordination chemical reaction, significantly enhancing the interlayer bonding force, while comparative example 1 only uses pure nano-cellulose whiskers, lacking the coordination bond and interlocking network of ZIF-8, resulting in insufficient interface adhesion strength and easy delamination, thereby reducing the overall mechanical properties.

[0084] It can be seen from the combination of embodiments 1, comparative example 2 and table 1 that the high-strength flame-retardant plastic hollow plate of embodiment 1 is significantly superior in performance to comparative example 2. The bending strength of embodiment 1 is 52.3 MPa, the compressive strength is 45.1 MPa, the impact strength is 12.5 kJ / m 2 , the peeling strength is 21.2 N / cm, the limiting oxygen index (LOI) is 29.5%, and the UL-94 level is V-0; the bending strength of comparative example 2 is 50.8 MPa, the compressive strength is 44.2 MPa, the impact strength is 12.1 kJ / m 2, the peel strength is 20.5 N / cm, the LOI is only 23.7%, and the UL-94 rating is V-2. The LOI of Example 1 is about 24% higher, the UL-94 rating is improved from V-2 to V-0, and the mechanical properties are slightly improved (about 2%-3%). This is mainly due to the use of graphene chemically modified polyethylene matrix in the outer flame-retardant layer of Example 1, which forms C-C covalent bonds through free radical grafting, enhances thermal stability, and cooperates with phosphorus-nitrogen flame retardant and intumescent graphite to form a dense carbon layer to effectively block oxygen and heat; while the ordinary polyethylene matrix used in Comparative Example 2 lacks the thermal stability and carbonization promotion of graphene, resulting in a significant decrease in flame retardant performance.

[0085] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent replacements to some technical features. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A high-strength flame-retardant hollow plastic board, characterized in that, It includes a core honeycomb layer, an interface reinforcement layer, and an outer flame-retardant surface layer, wherein: The core honeycomb layer is made of a polypropylene matrix and a carbon fiber reinforced composite material; The interface enhancement layer adopts a nanocellulose whisker-metal-organic framework composite system, which forms a molecular-level interlocking structure through in-situ self-assembly. The outer flame-retardant surface layer is composed of a graphene-modified polyethylene matrix, a composite flame retardant, and an intumescent carbonization accelerator, wherein the graphene is chemically grafted to form a covalent bond with the polyethylene matrix. Hollow panels with multi-layered structures are integrally formed using a hot-pressing composite process. The method for preparing the high-strength flame-retardant plastic hollow board includes the following steps: Step S1. Preparation of core honeycomb layer: Mix 70wt%-85wt% polypropylene with 15wt%-30wt% carbon fiber reinforcement material, and extrude the mixture at 180-220℃ using an extruder to form a honeycomb structure with a pore diameter of 5-10 mm. After cooling, set aside for later use. Step S2. Preparation of the interface reinforcement layer solution: 3wt%-8wt% of nanocellulose whiskers, zinc salt and 2-methylimidazolium are ultrasonically dispersed in a water-ethanol mixed solvent at a molar ratio of 1:2:4, and then stirred and reacted at 50-70℃ under nitrogen protection for 4-6 hours to form ZIF-8 anchored nanocellulose whiskers; these whiskers are then mixed with 86wt%-95wt% of polyamide resin, 0.5wt%-2wt% of hindered phenolic antioxidant and 2wt%-5wt% of hydroxypropyl cellulose, and ultrasonically dispersed at 80-100℃ for 20-40 minutes to obtain a uniform composite system solution, wherein the sum of the percentages of each component in the interface reinforcement layer solution is 100%. Step S3. Preparation of the outer flame-retardant surface layer: 0.5wt%-3wt% of graphene oxide and 60wt%-75wt% of polyethylene matrix are subjected to a free radical grafting reaction at 170-190℃ using a peroxide initiator to prepare a graphene-modified polyethylene matrix. Then, it is melt-blended with 25wt%-40wt% of phosphate ester and melamine cyanurate composite flame retardant and 1wt%-3wt% of expanded graphite in a twin-screw extruder at 160-190℃, and extruded to form an outer flame-retardant surface layer with a thickness of 2-6 mm. After cooling, the sum of the percentages of each component in the outer flame-retardant surface layer is 100%. Benzoyl peroxide is used as the initiator, with an addition amount of 0.1wt%-0.5wt%, and the reaction time is 1-2 hours. Step S4. Hot pressing composite molding: Preheat the core honeycomb layer to 150-180℃, uniformly apply the interface reinforcement layer solution, stack the outer flame retardant surface layer, and hot press for 10-20 minutes at a pressure of 5-10 MPa and a temperature of 160-200℃. Avoid decomposition of flame retardant by gradient temperature control. Step S5. Post-processing: Anneal the formed multi-layer plastic hollow board at 50-60℃ for 1-2 hours, and cut it to the required size.

2. The high-strength flame-retardant hollow plastic board according to claim 1, characterized in that, The total thickness of the multi-layer plastic hollow board is 10-20 mm, of which the core honeycomb layer accounts for 60-70%, the interface reinforcement layer accounts for 5-10%, and the outer flame-retardant surface layer accounts for 20-30%.

3. The high-strength flame-retardant hollow plastic board according to claim 2, characterized in that, In step S2, the ultrasonic dispersion power is 300-500 W, the frequency is 20-40 kHz, and the time is 30-60 minutes.

4. The high-strength flame-retardant hollow plastic board according to claim 3, characterized in that, In step S3, the screw speed of the twin-screw extruder is 100-200 rpm, and the feeding rate is controlled at 5-10 kg / h.

5. The high-strength flame-retardant hollow plastic board according to claim 4, characterized in that, In step S4, the hot-pressing composite process uses multi-stage pressure control, with an initial pressure of 5 MPa, gradually increasing to 10 MPa, and maintaining constant pressure for 5 minutes.

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