PC / PMMA co-extrusion scratch-resistant mobile phone back cover sheet and preparation method thereof

Abstract

The application relates to the technical field of composite materials, in particular to a PC / PMMA co-extrusion scratch-resistant mobile phone back cover plate and a preparation method thereof. The plate has a three-layer structure, including a polycarbonate core layer and upper and lower poly (methyl methacrylate) surface layers. The surface layer is made by mixing a special functional masterbatch and PMMA resin, the functional masterbatch contains PMMA copolymer resin containing an epoxy side group, ZIF-8 powder which is double-grafted by an amino group and a fluorine-containing alkyl group, a siloxane precursor and a catalyst. Through a co-extrusion forming process, the surface layer forms an organic-inorganic hybrid network structure. The plate has excellent scratch resistance, wear resistance, high light transmittance, low haze and firm interlayer bonding force, and effectively solves the problem that traditional single materials or coating technologies are difficult to balance toughness, hardness and optical performance.

Description

Technical Field

[0001] This invention relates to the field of composite material technology, and in particular to a PC / PMMA co-extruded scratch-resistant mobile phone back cover material and its preparation method. Background Technology

[0002] As smartphones rapidly evolve towards thinner, lighter designs and more aesthetically pleasing aesthetics, the overall performance requirements for phone back covers, as crucial exterior and structural components, are becoming increasingly stringent. Currently, polycarbonate (PC) and polymethyl methacrylate (PMMA) are two widely used transparent polymer materials for phone back covers. PC excels in its excellent toughness, impact resistance, and dimensional stability, providing reliable protection for internal phone components; while PMMA is known for its high surface hardness, excellent light transmittance, and gloss, effectively resisting daily friction and maintaining a clean appearance. However, a single material often cannot simultaneously meet the dual requirements of high hardness and high toughness. While PC ensures drop resistance, its relatively low surface hardness makes it prone to fine scratches from daily contact with hard objects such as keys and dust, leading to decreased light transmittance, increased haze, and a "fogging" phenomenon, severely impacting aesthetics and user experience. PMMA, on the other hand, has higher surface hardness and improved scratch resistance, but the material itself is brittle and prone to brittle fracture upon impact, resulting in insufficient reliability.

[0003] To address the performance limitations of single materials, the industry has explored surface coating technologies, such as applying organic-inorganic hybrid hardening coatings to PC or PMMA substrates. While these methods can improve surface hardness to some extent, they also introduce new challenges. First, the difference in thermal expansion coefficients between the coating and the plastic substrate, as well as interfacial adhesion issues, are significant, especially after long-term use or temperature changes, leading to risks of edge cracking, peeling, or even complete detachment. Second, coating processes typically involve solvents and curing, potentially increasing energy consumption and environmental impact, and complex curved surfaces can result in uneven coating thickness, affecting the consistency of optical performance. Furthermore, the coating itself increases the overall thickness of the product, which contradicts the trend towards thinner and lighter end products.

[0004] To further enhance integration and structural reliability, co-extrusion technology has been introduced, aiming to directly produce sheets with a gradient structure of a hard surface layer and a tough core layer through a one-step molding process. This approach attempts to solve the interfacial bonding problem from the perspective of the material's bulk structure. However, in the co-extrusion system of PC and PMMA, achieving a strong interfacial bond is itself a major challenge due to the significant differences in the polarity and melt viscoelasticity of the two polymers. Simple physical blending or lamination is insufficient to form a stable interpenetrating or chemically bonded interface, and insufficient interlayer bonding can easily lead to delamination during use. More importantly, even if the bonding problem is solved, how to substantially and stably introduce and maintain highly efficient scratch-resistant and abrasion-resistant components in the PMMA surface layer, so that it not only has high initial hardness but also effectively inhibits plastic deformation, microcrack propagation, and optical degradation under external mechanical stress (such as scratching and sliding friction), while not sacrificing the material's basic optical properties such as high light transmittance and low haze, remains a bottleneck that current technologies have not yet been able to overcome well. Simply adding inorganic fillers often leads to a decrease in light transmittance and a significant increase in haze, while organic modification may sacrifice heat resistance or long-term stability. Summary of the Invention

[0005] In view of this, the purpose of this invention is to propose a PC / PMMA co-extruded scratch-resistant mobile phone back cover material and its preparation method, so as to provide a mobile phone back cover material that can substantially achieve the organic combination of high scratch resistance and wear resistance of the surface layer and high toughness of the core layer through the material's own gradient structure design without relying on external coatings, and ensure strong interlayer bonding, excellent optical performance and long-term stability.

[0006] To achieve the above objectives, the present invention provides a PC / PMMA co-extruded scratch-resistant mobile phone back cover material, comprising: a polycarbonate core layer; and polymethyl methacrylate (PMMA) surface layers respectively disposed on the upper and lower surfaces of the polycarbonate core layer; wherein the PMMA surface layer is formed by mixing PMMA surface layer functional masterbatch and PMMA resin in a mass ratio of 1600-2400:2400-1600.

[0007] The polymethyl methacrylate (PMMA) surface functional masterbatch is prepared from a PMMA copolymer resin containing epoxy side groups, a PMMA resin, a tetraethoxysilane, a 3-glycidoxypropyltrimethoxysilane, a zirconium acetylacetonate, and modified ZIF-8 powder. The mass ratio of the PMMA copolymer resin containing epoxy side groups, the PMMA resin, the tetraethoxysilane, the 3-glycidoxypropyltrimethoxysilane, the zirconium acetylacetonate, and the modified ZIF-8 powder is 450-500:4200-4400:70-90:50-70:8-12:40-60.

[0008] The modified ZIF-8 powder has a silicon-oxygen coating layer on its surface, and aminosilyl groups and fluorinated alkylsilyl groups are grafted onto the silicon-oxygen coating layer; the aminosilyl groups are derived from 3-aminopropyltriethoxysilane grafting, and the fluorinated alkylsilyl groups are derived from 1H,1H,2H,2H-perfluorodecyltriethoxysilane grafting.

[0009] Preferably, the specific preparation steps of the modified ZIF-8 powder are as follows:

[0010] (a) Preparation of ZIF-8 nanocrystal powder;

[0011] (b) The ZIF-8 nanocrystal powder is coated with a sol-gel under alkaline conditions using tetraethoxysilane to obtain ZIF-8 powder with a surface silicon oxide coating; the mass ratio of the tetraethoxysilane to the ZIF-8 nanocrystal powder is 45-55:100.

[0012] (c) The ZIF-8 powder with surface silicon oxide coating is reacted with 3-aminopropyltriethoxysilane and 1H,1H,2H,2H-perfluorodecyltriethoxysilane to simultaneously graft aminosilyl groups and fluorinated alkylsilyl groups onto the silicon oxide coating layer to obtain the modified ZIF-8 powder. The mass ratio of the surface silicon oxide coated ZIF-8 powder, 3-aminopropyltriethoxysilane and 1H,1H,2H,2H-perfluorodecyltriethoxysilane is 100:8-12:8-12.

[0013] Preferably, the polymethyl methacrylate copolymer resin containing epoxy side groups is a copolymer resin obtained by copolymerizing methyl methacrylate and glycidyl methacrylate.

[0014] Preferably, the mass ratio of methyl methacrylate to glycidyl methacrylate is 870-930:70-130.

[0015] Preferably, the thickness of the upper and lower polymethyl methacrylate surface layers is 80μm-120μm each, the thickness of the polycarbonate core layer is 760μm-840μm, and the total thickness of the board is 920μm-1080μm.

[0016] Furthermore, the present invention also provides a method for preparing a PC / PMMA co-extruded scratch-resistant mobile phone back cover material, comprising the following steps:

[0017] (1) Preparation of polymethyl methacrylate surface functional masterbatch: polymethyl methacrylate resin, polymethyl methacrylate copolymer resin containing epoxy side groups, zirconium acetylacetonate and modified ZIF-8 powder are melt-blended, and a pre-hydrolyzed siloxane precursor solution is introduced during the melt-blending process to form a siloxane network by condensation of the tetraethoxysilane and 3-glycidoxypropyltrimethoxysilane, while removing the low-boiling components carried by the siloxane precursor solution to obtain the polymethyl methacrylate surface functional masterbatch; wherein, the siloxane precursor solution contains tetraethoxysilane, 3-glycidoxypropyltrimethoxysilane, water and alcohol;

[0018] (2) The polymethyl methacrylate surface functional masterbatch obtained in step (1) is mixed with polymethyl methacrylate resin to obtain surface material, and polycarbonate resin is provided as core material.

[0019] (3) Co-extrusion molding is used to form a polycarbonate core layer from the core material and a polymethyl methacrylate surface layer from both sides of the polycarbonate core material to obtain a PC / PMMA co-extruded scratch-resistant mobile phone back cover material.

[0020] Preferably, in step (1), the melt mixing is carried out in a twin-screw extruder with a barrel temperature of 195℃-245℃ and a screw speed of 180-220r / min. After melting and plasticizing for 1-2 minutes, the pre-hydrolyzed silica precursor solution is introduced, and after mixing for another 1-3 minutes, the mixture is extruded and pelletized to obtain the polymethyl methacrylate surface functional masterbatch.

[0021] Preferably, in step (1), a vacuum of -0.06 MPa to -0.09 MPa is maintained at the vacuum exhaust port of the twin-screw extruder to remove the low-boiling component.

[0022] Preferably, in step (3), co-extrusion is carried out on a parallel three-layer co-extrusion flat production line. The temperature of each zone of the surface extruder is 205℃-245℃ and the die head temperature is 235℃-245℃. The temperature of each zone of the core extruder is 235℃-260℃ and the die head temperature is 250℃-260℃. The dwell time in the co-extrusion die area is 1min-3min.

[0023] The beneficial effects of this invention are:

[0024] This invention, through specific material combinations and process design, endows the final sheet surface with excellent scratch and abrasion resistance. The three-dimensional silicon-oxygen network constructed in the surface system provides a robust inorganic framework, significantly enhancing the material's resistance to scratches from sharp objects and repeated friction. Simultaneously, the introduction of specific functionalized nanocrystalline materials, uniformly dispersed in the system, further enhances the surface's microscopic hardness and load-bearing capacity, effectively inhibiting the formation and deepening of furrows under external forces, reducing wear debris generation, and thus maintaining the surface's smoothness and integrity for a long time.

[0025] The sheet material of this invention can maintain its initial optical properties to the maximum extent after being scratched or worn. The silicon-oxygen network formed inside the surface layer has good compatibility with the organic polymer matrix, reducing light scattering centers caused by phase separation. Specific modifications to the surface of the functionalized nanocrystalline material not only promote its dispersion stability in the matrix but also help reduce light scattering. This allows the sheet material to maintain high light transmittance and low haze even after a certain degree of surface damage, significantly reducing visible scratches and maintaining a new appearance for a long time.

[0026] A strong interfacial bond between the PMMA surface layer and the PC core layer was achieved through a co-extrusion molding process. The epoxy reactive groups in the surface layer synergistically interact with specific catalytic components during processing, promoting the formation of chemical bonds in the interfacial region and thus creating a strong and robust transition zone between the two layers. This strong interfacial bond effectively avoids problems such as interlayer separation and warping that may occur during use, ensuring the overall mechanical integrity and reliability of the gradient structure.

[0027] The sheet material prepared by this invention exhibits excellent thermal and structural stability on its surface. The presence of the silicon-oxygen network increases the polymer's resistance to thermal decomposition, while the functionalized nanocrystalline materials and their surface coatings also contribute to good thermal stability. This allows the sheet material to better resist thermal stress during subsequent processing (such as hot bending) or high-temperature use, reducing deformation and performance degradation, and ensuring the product's dimensional stability and long service life.

[0028] The surface system achieves effective surface lubrication by introducing low surface energy components. Fluoroalkyl segments grafted onto the surface of the functionalized nanocrystalline material significantly reduce the coefficient of friction of the plate surface. This not only reduces sliding resistance with contacting objects and improves tactile feel, but more importantly, it reduces tangential stress during frictional wear, thereby mitigating shear damage to the material surface. This, combined with enhanced hardness and load-bearing capacity, contributes to excellent wear resistance. Overall, this invention achieves an excellent balance between hardness, toughness, interfacial strength, optical properties, and surface lubricity. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0030] Example 1:

[0031] Step 1: Preparation of polymethyl methacrylate copolymer resin containing epoxy side groups

[0032] 1500g of deionized water and 12g of polyvinyl alcohol (polyvinyl alcohol type 1788) were added to a glass reactor equipped with stirring and temperature control. After stirring was started, 930g of methyl methacrylate and 70g of glycidyl methacrylate were added at room temperature, followed by 4g of azobisisobutyronitrile initiator and 1g of dodecyl mercaptan chain transfer agent. The temperature was raised to 68°C under nitrogen protection and maintained at this temperature for 3 hours. During the reaction, the mixture was stirred at a medium speed to obtain a uniform suspended particle system. After the reaction was completed, the mixture was naturally cooled to room temperature, filtered, and then dried in a 75°C hot air oven for 6 hours to obtain a polymethyl methacrylate copolymer resin containing epoxy side groups.

[0033] Step 2: Preparation of ZIF-8 nanocrystals

[0034] Metal salt solution and organic ligand solution were prepared in two beakers, each equipped with a magnetic stirrer. First, 950 g of deionized water and 27 g of zinc nitrate hexahydrate were added to the first beaker and stirred until completely dissolved to obtain the metal salt solution. Then, 950 g of deionized water and 75 g of 2-methylimidazole were added to the second beaker and stirred until dissolved to obtain the organic ligand solution. The metal salt solution was poured into the organic ligand solution at room temperature and stirred for 4 min. After standing for 20 min, a milky white suspension was formed. The suspension was centrifuged, washed three times with deionized water and twice with anhydrous ethanol, and finally dried in a vacuum oven at 58 °C for 10 h to obtain ZIF-8 nanocrystalline powder.

[0035] Step 3: Introduce a silicon-oxygen coating layer on the ZIF-8 surface

[0036] 100g of ZIF-8 nanocrystalline powder, 950g of anhydrous ethanol and 180g of deionized water were added to a reaction flask equipped with a heating and reflux condenser. The mixture was ultrasonically dispersed for 25min to obtain a suspension. The pH was adjusted to 9.0 with 28wt% ammonia. After stirring at 48℃ for 25min, 45g of tetraethoxysilane was added dropwise. The mixture was stirred at 48℃ for 2h. After the reaction, the mixture was cooled to room temperature, and the solid was separated by centrifugation. The solid was washed twice with ethanol and twice with water, and finally dried in a vacuum oven at 58℃ for 8h to obtain ZIF-8 powder with a silicon-oxygen coating.

[0037] Step 4: Preparation of amino and fluorinated alkyl double-grafted ZIF-8

[0038] 100g of ZIF-8 powder with a silica coating, 750g of anhydrous ethanol, and 180g of deionized water were added to a round-bottom flask equipped with a stirrer and temperature control. The mixture was mechanically stirred and ultrasonically dispersed at room temperature for 25 min, then heated to 48℃. 8g of 3-aminopropyltriethoxysilane and 8g of 1H,1H,2H,2H-perfluorodecyltriethoxysilane were added sequentially while stirring. The pH was adjusted to 8.0 with 28wt% ammonia water, and the reaction was continued at 48℃ for 2 h. After the reaction was completed, the mixture was cooled to room temperature, the solid was separated by centrifugation, washed three times with anhydrous ethanol, and finally dried under vacuum at 58℃ for 10 h to obtain ZIF-8 powder with both amino and fluorinated alkyl groups on its surface.

[0039] Step 5: Preparation of polymethyl methacrylate surface functional masterbatch

[0040] Add 4400g of polymethyl methacrylate resin granules (ROHM PLEXIGLAS 8N) to the upstream metering and mixing unit of the twin-screw extruder, then add 450g of polymethyl methacrylate copolymer resin containing epoxy side groups, 8g of zirconium acetylacetonate, and 40g of ZIF-8 powder containing both amino and fluorinated alkyl groups on its surface. After premixing evenly, feed the mixture into the main feed port of the twin-screw extruder. Simultaneously, prepare a silica precursor solution in a pressure-resistant stainless steel container with a stirrer: add 70g of tetraethoxysilane, 50g... 3-Glycidoxypropyltrimethoxysilane was added, along with 70g of deionized water and 35g of anhydrous ethanol. The pH was adjusted to 8.0 with 28wt% ammonia water. The mixture was stirred and hydrolyzed at room temperature for 20 minutes and kept sealed for later use. The extruder barrel temperature was set sequentially from the feeding section to the die head as 195℃, 205℃, 215℃, 225℃, and 235℃, and the screw speed was set to 180r / min. The mixed resin was melted and plasticized in the extruder for 1 minute. After the material was completely melted, the above pre-hydrolyzed silica precursor solution was added to the liquid injection port in the middle through a metering pump. The mixture was continued to be mixed in the machine for 1 minute, and a vacuum of -0.06MPa was maintained at the vacuum exhaust port in the rear section. The material was extruded from the die head, cooled, stretched, water-cooled, and pelletized to obtain surface functional masterbatch granules.

[0041] Step Six: Preparation of Special Material for Polycarbonate Core Layer

[0042] Add 8000g of polycarbonate resin granules (Covestro Makrolon 2407) to a hot air dryer and dry at 115°C for 3 hours. After drying, cool to no more than 75°C and seal for later use. This is the special material for polycarbonate core layer.

[0043] Step 7: Co-extrusion molding of polycarbonate / polymethyl methacrylate to form gradient structure sheets

[0044] In a parallel three-layer co-extrusion flat plate production line, 1600g of the polymethyl methacrylate surface functional masterbatch prepared in step five is mixed with polymethyl methacrylate resin (ROHM PLEXIGLAS). 2400g of 8N) mixture is fed into two surface extruders (each processing 2000g of the mixture), and 8000g of polycarbonate core material is fed into the intermediate core extruder. The temperatures of each zone of the surface extruder are set to 205℃, 215℃, 225℃ and 235℃, and the die head temperature is set to 235℃. The temperatures of each zone of the core extruder are set to 235℃, 245℃, 250℃ and 250℃, and the die head temperature is set to 250℃. The extruded materials from the three extruders are stacked in the co-extrusion channel and then enter the co-extrusion die, where they remain in the die area for 1 minute. By adjusting the screw speed and flow rate of each extruder, the thickness of the upper and lower surface layers in the final sheet is controlled to be about 80μm, the core layer thickness is 840μm, and the total thickness is 1.0mm. The extruded sheet is cooled by a 55℃ cooling roller and traction-setting before being wound up to obtain the co-extruded sheet.

[0045] Step 8: Annealing to eliminate residual stress and stabilize the hardened structure

[0046] The co-extruded sheet was cut into samples with a size of 200mm×300mm, placed in a hot air circulating oven, kept at 95℃ for 45min, and then slowly cooled to room temperature at a rate not exceeding 6℃ per minute to obtain PC / PMMA co-extruded scratch-resistant mobile phone back cover sheet.

[0047] Example 2:

[0048] Step 1: Preparation of polymethyl methacrylate copolymer resin containing epoxy side groups

[0049] 1600g of deionized water and 15g of polyvinyl alcohol (polyvinyl alcohol type 1788) were added to a glass reactor equipped with stirring and temperature control. After stirring was started, 900g of methyl methacrylate and 100g of glycidyl methacrylate were added at room temperature, followed by 5g of azobisisobutyronitrile initiator and 2g of dodecyl mercaptan chain transfer agent. The temperature was raised to 70°C under nitrogen protection and maintained at this temperature for 4 hours. During the reaction, the mixture was stirred at a medium speed to obtain a uniform suspended particle system. After the reaction was completed, the mixture was naturally cooled to room temperature, filtered, and then dried in an 80°C hot air oven for 8 hours to obtain a polymethyl methacrylate copolymer resin containing epoxy side groups.

[0050] Step 2: Preparation of ZIF-8 nanocrystals

[0051] Metal salt solution and organic ligand solution were prepared in two beakers, each equipped with a magnetic stirrer. First, 1000g of deionized water and 30g of zinc nitrate hexahydrate were added to the first beaker and stirred until completely dissolved to obtain the metal salt solution. Then, 1000g of deionized water and 82g of 2-methylimidazole were added to the second beaker and stirred until dissolved to obtain the organic ligand solution. The metal salt solution was poured into the organic ligand solution at room temperature and stirred for 5 minutes. After standing for 25 minutes, a milky white suspension was formed. The suspension was centrifuged, washed three times with deionized water and twice with anhydrous ethanol, and finally dried in a vacuum oven at 60℃ for 12 hours to obtain ZIF-8 nanocrystalline powder.

[0052] Step 3: Introduce a silicon-oxygen coating layer on the ZIF-8 surface

[0053] 100g of ZIF-8 nanocrystalline powder, 1000g of anhydrous ethanol and 200g of deionized water were added to a reaction flask equipped with a heating and reflux condenser. The mixture was ultrasonically dispersed for 30 min to obtain a suspension. The pH was adjusted to 9.1 with 28wt% ammonia water. After stirring at 50℃ for 30 min, 50g of tetraethoxysilane was added dropwise. The mixture was stirred at 50℃ for 3 h. After the reaction, the mixture was cooled to room temperature, and the solid was separated by centrifugation. The solid was washed twice with ethanol and twice with water, and finally dried in a vacuum oven at 60℃ for 10 h to obtain ZIF-8 powder with a silicon-oxygen coating.

[0054] Step 4: Preparation of amino and fluorinated alkyl double-grafted ZIF-8

[0055] 100g of ZIF-8 powder coated with silica, 800g of anhydrous ethanol, and 200g of deionized water were added to a round-bottom flask equipped with a stirrer and temperature control. The mixture was mechanically stirred and ultrasonically dispersed at room temperature for 30 min, then heated to 50℃. While stirring, 10g of 3-aminopropyltriethoxysilane and 10g of 1H,1H,2H,2H-perfluorodecyltriethoxysilane were added sequentially. The pH was adjusted to 8.1 with 28wt% ammonia water, and the reaction was continued at 50℃ for 3 h. After the reaction was completed, the mixture was cooled to room temperature, the solid was separated by centrifugation, washed three times with anhydrous ethanol, and finally dried under vacuum at 60℃ for 12 h to obtain ZIF-8 powder with both amino and fluorinated alkyl groups on its surface.

[0056] Step 5: Preparation of polymethyl methacrylate surface functional masterbatch

[0057] Add 4300g of polymethyl methacrylate resin granules (ROHM PLEXIGLAS 8N) to the upstream metering and mixing unit of the twin-screw extruder, then add 500g of polymethyl methacrylate copolymer resin containing epoxy side groups, 10g of zirconium acetylacetonate, and 50g of ZIF-8 powder containing both amino and fluorinated alkyl groups on its surface. After premixing evenly, feed the mixture into the main feed port of the twin-screw extruder. Simultaneously, prepare a silica precursor solution in a pressure-resistant stainless steel container with a stirrer: add 80g of tetraethoxysilane, 60g... 3-Glycidoxypropyltrimethoxysilane was added, along with 80g of deionized water and 40g of anhydrous ethanol. The pH was adjusted to 8.1 with 28wt% ammonia water. The mixture was stirred and hydrolyzed at room temperature for 30 minutes and kept sealed for later use. The extruder barrel temperature was set sequentially from the feeding section to the die head to 200℃, 210℃, 220℃, 230℃, and 240℃. The screw speed was set to 200r / min. The mixed resin was melted and plasticized in the extruder for 1 minute. After the material was completely melted, the above pre-hydrolyzed silica precursor solution was added to the liquid injection port in the middle through a metering pump. The mixture was continued to be mixed in the machine for 2 minutes, and a vacuum of -0.08MPa was maintained at the vacuum exhaust port in the rear section. The material was extruded from the die head, cooled, stretched, water-cooled, and pelletized to obtain surface functional masterbatch granules.

[0058] Step Six: Preparation of Special Material for Polycarbonate Core Layer

[0059] Add 8000g of polycarbonate resin granules (Covestro Makrolon 2407) to a hot air dryer and dry at 120°C for 4 hours. After drying, cool to no more than 80°C and seal for later use. This is the special material for polycarbonate core layer.

[0060] Step 7: Co-extrusion molding of polycarbonate / polymethyl methacrylate to form gradient structure sheets

[0061] In a parallel three-layer co-extrusion flat plate production line, 2000g of the polymethyl methacrylate surface functional masterbatch prepared in step five is mixed with polymethyl methacrylate resin (ROHM PLEXIGLAS). 2000g of 8N) mixture is fed into two surface extruders (each processing 2000g of the mixture), and 8000g of polycarbonate core material is fed into the intermediate core extruder. The temperatures of each zone of the surface extruder are set to 210℃, 220℃, 230℃, and 240℃, and the die head temperature is set to 240℃. The temperatures of each zone of the core extruder are set to 240℃, 250℃, 255℃, and 255℃, and the die head temperature is set to 255℃. The extruded materials from the three extruders are stacked in the co-extrusion channel and then enter the co-extrusion die, where they remain in the die area for 2 minutes. By adjusting the screw speed and flow rate of each extruder, the thickness of the upper and lower surface layers in the final sheet is controlled to be approximately 100μm, the core layer thickness is 800μm, and the total thickness is 1.0mm. The extruded sheet is cooled by a 60℃ cooling roller and then wound up after traction shaping to obtain the co-extruded sheet.

[0062] Step 8: Annealing to eliminate residual stress and stabilize the hardened structure

[0063] The co-extruded sheet was cut into samples with a size of 200mm×300mm, placed in a hot air circulating oven, kept at 100℃ for 60min, and then slowly cooled to room temperature at a rate not exceeding 5℃ per minute to obtain PC / PMMA co-extruded scratch-resistant mobile phone back cover sheet.

[0064] Example 3:

[0065] Step 1: Preparation of polymethyl methacrylate copolymer resin containing epoxy side groups

[0066] 1700g of deionized water and 18g of polyvinyl alcohol (polyvinyl alcohol type 1788) were added to a glass reactor equipped with stirring and temperature control. After stirring was started, 870g of methyl methacrylate and 130g of glycidyl methacrylate were added at room temperature, followed by 6g of azobisisobutyronitrile initiator and 3g of dodecyl mercaptan chain transfer agent. The temperature was raised to 72°C under nitrogen protection and maintained at this temperature for 5 hours. During the reaction, the mixture was stirred at a medium speed to obtain a uniform suspended particle system. After the reaction was completed, the mixture was naturally cooled to room temperature, filtered, and then dried in an 85°C hot air oven for 10 hours to obtain a polymethyl methacrylate copolymer resin containing epoxy side groups.

[0067] Step 2: Preparation of ZIF-8 nanocrystals

[0068] Metal salt solution and organic ligand solution were prepared in two beakers, each equipped with a magnetic stirrer. First, 1050 g of deionized water and 33 g of zinc nitrate hexahydrate were added to the first beaker and stirred until completely dissolved to obtain the metal salt solution. Then, 1050 g of deionized water and 90 g of 2-methylimidazole were added to the second beaker and stirred until dissolved to obtain the organic ligand solution. The metal salt solution was poured into the organic ligand solution at room temperature and stirred for 6 min. After standing for 30 min, a milky white suspension was formed. The suspension was centrifuged, washed three times with deionized water and twice with anhydrous ethanol, and finally dried in a vacuum oven at 62 °C for 14 h to obtain ZIF-8 nanocrystalline powder.

[0069] Step 3: Introduce a silicon-oxygen coating layer on the ZIF-8 surface

[0070] 100g of ZIF-8 nanocrystalline powder, 1050g of anhydrous ethanol and 220g of deionized water were added to a reaction flask equipped with a heating and reflux condenser. The mixture was ultrasonically dispersed for 35min to obtain a suspension. The pH was adjusted to 9.2 with 28wt% ammonia. After stirring at 52℃ for 35min, 55g of tetraethoxysilane was added dropwise. The mixture was stirred at 52℃ for 4h. After the reaction, the mixture was cooled to room temperature, and the solid was separated by centrifugation. The solid was washed twice with ethanol and twice with water, and finally dried in a vacuum oven at 62℃ for 12h to obtain ZIF-8 powder with a silicon-oxygen coating.

[0071] Step 4: Preparation of amino and fluorinated alkyl double-grafted ZIF-8

[0072] 100g of ZIF-8 powder coated with silica, 850g of anhydrous ethanol, and 220g of deionized water were added to a round-bottom flask equipped with a stirrer and temperature control. The mixture was mechanically stirred and ultrasonically dispersed at room temperature for 35 min, then heated to 52℃. While stirring, 12g of 3-aminopropyltriethoxysilane and 12g of 1H,1H,2H,2H-perfluorodecyltriethoxysilane were added sequentially. The pH was adjusted to 8.2 with 28wt% ammonia water, and the reaction was continued at 52℃ for 4 h. After the reaction was completed, the mixture was cooled to room temperature, the solid was separated by centrifugation, washed three times with anhydrous ethanol, and finally dried under vacuum at 62℃ for 14 h to obtain ZIF-8 powder with both amino and fluorinated alkyl groups on its surface.

[0073] Step 5: Preparation of polymethyl methacrylate surface functional masterbatch

[0074] Add 4200g of polymethyl methacrylate resin granules (ROHM PLEXIGLAS 8N) to the upstream metering and mixing unit of the twin-screw extruder, then add 550g of polymethyl methacrylate copolymer resin containing epoxy side groups, 12g of zirconium acetylacetonate, and 60g of ZIF-8 powder containing both amino and fluorinated alkyl groups on its surface. After premixing evenly, feed the mixture into the main feed port of the twin-screw extruder. Simultaneously, prepare a silica precursor solution in a pressure-resistant stainless steel container with a stirrer: add 90g of tetraethoxysilane, 70g... 3-Glycidoxypropyltrimethoxysilane was added, along with 90g of deionized water and 45g of anhydrous ethanol. The pH was adjusted to 8.2 with 28wt% ammonia water. The mixture was stirred and hydrolyzed at room temperature for 40 minutes and kept sealed for later use. The extruder barrel temperature was set sequentially from the feeding section to the die head as 205℃, 215℃, 225℃, 235℃, and 245℃, and the screw speed was set to 220r / min. The mixed resin was melted and plasticized in the extruder for 2 minutes. After the material was completely melted, the above pre-hydrolyzed silica precursor solution was added to the liquid injection port in the middle through a metering pump. The mixture was continued to be mixed in the machine for 3 minutes, and a vacuum of -0.09MPa was maintained at the vacuum exhaust port in the rear section. The material was extruded from the die head, cooled, stretched, water-cooled, and pelletized to obtain surface functional masterbatch granules.

[0075] Step Six: Preparation of Special Material for Polycarbonate Core Layer

[0076] Add 8000g of polycarbonate resin granules (Covestro Makrolon 2407) to a hot air dryer and dry at 125°C for 5 hours. After drying, cool to no more than 85°C and seal for later use. This is the special material for polycarbonate core layer.

[0077] Step 7: Co-extrusion molding of polycarbonate / polymethyl methacrylate to form gradient structure sheets

[0078] In a parallel three-layer co-extrusion flat plate production line, 2400g of the polymethyl methacrylate surface functional masterbatch prepared in step five is mixed with polymethyl methacrylate resin (ROHM PLEXIGLAS). 1600g of 8N) mixture is fed into two surface extruders (each processing 2000g of the mixture), and 8000g of polycarbonate core material is fed into the intermediate core extruder. The temperatures of each zone of the surface extruder are set to 215℃, 225℃, 235℃ and 245℃, and the die head temperature is set to 245℃. The temperatures of each zone of the core extruder are set to 245℃, 255℃, 260℃ and 260℃, and the die head temperature is set to 260℃. The material from the three extruders is stacked in the co-extrusion channel and then enters the co-extrusion die, where it stays for 3 minutes. By adjusting the screw speed and flow rate of each extruder, the thickness of the upper and lower surface layers of the final sheet is controlled to be about 120μm, the core layer thickness is 760μm, and the total thickness is 1.0mm. The extruded sheet is cooled by a 65℃ cooling roller and then wound up after traction shaping to obtain the co-extruded sheet.

[0079] Step 8: Annealing to eliminate residual stress and stabilize the hardened structure

[0080] The co-extruded sheet was cut into 200mm×300mm samples, placed in a hot air circulating oven, and kept at 105℃ for 75 minutes. Then it was slowly cooled to room temperature at a rate not exceeding 4℃ per minute to obtain PC / PMMA co-extruded scratch-resistant mobile phone back cover sheet.

[0081] Comparative Example 1:

[0082] The difference between Comparative Example 1 and Example 2 is that, in step five, when preparing the polymethyl methacrylate surface functional masterbatch, 500g of polymethyl methacrylate copolymer resin containing epoxy side groups is not added, but is replaced by an equal mass of polymethyl methacrylate resin particles, so that the total amount of solid resin remains unchanged; the other conditions are the same as in Example 2.

[0083] Comparative Example 2:

[0084] The difference between Comparative Example 2 and Example 2 is that in step five, when preparing the polymethyl methacrylate surface functional masterbatch, 10g of zirconium acetylacetonate is not added, but is replaced by an equal mass of polymethyl methacrylate resin particles to keep the total solid mass unchanged; the other conditions are the same as in Example 2.

[0085] Comparative Example 3:

[0086] The difference between Comparative Example 3 and Example 2 is that the solution injected into the liquid injection port in step five does not contain tetraethoxysilane and 3-glycidoxypropyltrimethoxysilane, but consists only of 80g deionized water and 40g anhydrous ethanol. The pH value is adjusted to 8.1 with 28% ammonia water, stirred at room temperature for 30 minutes and sealed for later use. Then it is injected in the same way as in Example 2; the other conditions are the same as in Example 2.

[0087] Comparative Example 4:

[0088] The difference between Comparative Example 4 and Example 2 is that the vacuum exhaust port in the latter part of step five does not maintain a vacuum of -0.08 MPa, but instead does not evacuate the exhaust; the other conditions are the same as in Example 2.

[0089] Comparative Example 5:

[0090] The difference between Comparative Example 5 and Example 2 is that in step five, when preparing the polymethyl methacrylate surface functional masterbatch, the added powder is not ZIF-8 powder containing both amino and fluorinated alkyl groups on the surface, but an equal mass of ZIF-8 powder with surface silicon oxide coating is added; the other conditions are the same as in Example 2.

[0091] Comparative Example 6:

[0092] The difference between Comparative Example 6 and Example 2 is that in step four, when preparing ZIF-8 powder, only 10g of 3-aminopropyltriethoxysilane was added instead of 1H,1H,2H,2H-perfluorodecyltriethoxysilane, resulting in ZIF-8 powder with amino groups on the surface but without fluorinated alkyl groups. In step five, this powder was used to replace the double-grafted ZIF-8 powder of Example 2 (the amount added was still 50g); the other conditions were the same as in Example 2.

[0093] Performance testing:

[0094] Thermogravimetric analysis: Material was scraped from the surface of the polymethyl methacrylate (PMMA) plates of each sample in the examples and comparative examples and ground evenly. 8 mg of the sample was weighed and placed in an alumina crucible. Under a nitrogen protective atmosphere (flow rate 50 mL / min), the temperature was increased from 30 °C to 800 °C at a rate of 10 °C / min. After reaching 800 °C, the temperature was held for 10 min. Each sample was tested 3 times and the average was taken. The 5% weight loss temperature, the maximum weight loss rate temperature, and the residual mass fraction at 800 °C were recorded. The results are shown in Table 1.

[0095] Interlayer peel strength (polycarbonate / polymethyl methacrylate surface interface bonding): 180° peel strength test was conducted according to GB / T 2790-1995: A strip specimen with a length of 200 mm and a width of 25 mm was cut from each sample board. A 50 mm long starting section was pre-peeled from one end of the specimen along the interface direction using a blade (forming two free ends that can be clamped separately for the polycarbonate layer and the polymethyl methacrylate surface layer). Both ends were clamped in the upper and lower clamps of a universal testing machine. The peel angle was maintained at 180°, and the beam speed was set to 100 mm / min. The average peel force within the stable peel range (peel displacement 50-150 mm) was recorded, and the interlayer peel strength was expressed as N / 25 mm. Five strips were tested for each sample, and the average value was taken. The results are shown in Table 1.

[0096] Scratch performance: Scratch performance was tested according to GB / T 41878-2022. Using polymethyl methacrylate (PMMA) functional surface as the test surface, 100mm × 100mm specimens were cut from each sample sheet. A scratch testing machine with a spherical diamond scraper (scraper radius 1.0mm) was used. The scratch length was set to 60mm, the scratching speed to 100mm / min, and the load was increased linearly from 1N to 20N (uniformly increasing within a 60mm stroke). Three parallel scratches were made on each specimen with a 5mm spacing between adjacent scratches. A 10x optical microscope was used to observe and record the critical load at which continuous whitening / continuous grooves first appeared along the scratches. Simultaneously, the occurrence of surface cracking, particle pull-out, or interlayer warping was recorded. At least five specimens were tested for each sample, and the average critical load was calculated. The results are shown in Table 1.

[0097] Quantitative assessment of scratch visibility: Scratch damage and visibility were quantitatively assessed according to GB / T 44303-2024. Along the scratch trajectory obtained from the scratch performance test, unscratched areas 1 mm away from the center line on both sides of the scratch center line were taken as control areas. The color coordinates L*, a*, and b* of the scratch area and the control area were measured using a spectrophotometer under D65 light source and 10° field of view, and the color difference ΔE* was calculated. Five equidistant measurement points (10, 20, 30, 40, and 50 mm away from the starting point) were selected along the length of each scratch to calculate ΔE, and the maximum value ΔEmax was taken as the visibility index of the scratch. At least 15 scratches were counted for each sample and the average value was taken. The results are shown in Table 1.

[0098] Transmittance and haze: Transmittance and haze were tested according to GB / T 2410-2008: Five 50mm×50mm specimens were cut from each sample board. The visible light transmittance and haze were measured using a haze meter under standard optical geometry conditions. The initial transmittance T0 and haze H0 of each specimen were recorded. Then, three scratches (60mm in length, with an increasing load of 1-20N) were prepared in the center area of ​​the same batch of specimens according to the scratch conditions of test item five. The transmittance T1 and haze H1 after scratching were measured again, and the haze change ΔH = H1 - H0 and the transmittance change ΔT = T1 - T0 were calculated. The results are shown in Table 1.

[0099] Sliding friction and wear: The sliding friction and wear test was conducted according to GB / T 3960-2016. A 30mm×30mm×1.0mm sample was cut from each sample plate as a flat plate sample. A ball-disc sliding friction and wear device was used, with an alumina ball of 6mm diameter as the pair. The normal load was 5N, the sliding speed was 0.1m / s, the sliding distance was 1000m, and the test environment was (23±2)℃ and (50±5)% relative humidity. The friction coefficient was recorded in real time as it changed over time, and the average value of the stable stage was taken. After the test, the cross-sectional area of ​​the wear track was measured by a three-dimensional profilometer and the wear volume was obtained by integration. The results are shown in Table 1.

[0100] Table 1 Performance Test Results

[0101] project Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 Comparative Example 6 5% weight loss temperature (°C) 312 317 318 315 318 302 309 314 316 Temperature at maximum rate of weight loss (°C) 378 383 384 382 379 366 375 381 382 Residual mass fraction at 800℃ / % 2.1 2.3 2.6 2.2 2.1 1.1 2.1 2.4 2.3 Interlayer peel strength (N / 25mm) 24.6 28.8 27.5 11.8 18.6 30.1 26.4 23.9 27.9 Scratch critical load (N) 12.3 15.8 14.6 13.2 13.7 8.4 12.8 11.9 14.3 Scratch visibility ΔE*max 2.7 1.85 2.1 3.2 2.6 7.8 4.5 5.6 2.9 Light transmittance (%) 90.8 90.4 89.6 90.3 89.9 91.2 88.1 89.1 90.0 Haze (%) 0.8 0.9 1.3 0.9 1.1 0.6 2.1 1.7 1.0 Change in transmittance ΔT (%) after scratching -0.8 -0.6 -1.1 -1.3 -1.0 -3.6 -2.2 -2.8 -1.1 Change in haze ΔH (%) after scratching 1.2 0.9 1.5 2.3 1.9 7.5 4.8 5.6 1.8 coefficient of sliding friction 0.265 0.238 0.292 0.276 0.262 0.228 0.310 0.345 0.318 <![CDATA[Sliding wear volume (mm 3 )]]> 0.518 0.412 0.603 0.556 0.502 0.984 0.792 1.132 0.646

[0102] Data Analysis:

[0103] As can be seen from the data in Examples 1-3 in Table 1, the scratch-resistant mobile phone back cover material prepared by this invention maintains high light transmittance and low haze while exhibiting minimal optical degradation after scratching. Furthermore, it demonstrates low friction and small wear volume in the friction and wear test, indicating that the surface layer can effectively inhibit furrow deepening and debris accumulation under tangential load. Combined with thermogravimetric results, it is evident that the surface layer retains some residue during thermal decomposition, indicating that the silicon-oxygen network formed by tetraethoxysilane and 3-glycidoxypropyltrimethoxysilane provides a stable inorganic framework within the polymethyl methacrylate matrix. Simultaneously, the zeolite imidazole framework material ZIF-8, after being coated with silica, further enhances its microscopic load-bearing capacity and cutting resistance. On the other hand, polymethyl methacrylate copolymers containing epoxy side groups are more likely to undergo epoxy ring opening and form an interfacial chemical anchor with 3-aminopropyltriethoxysilane under the promotion of zirconium acetylacetonate, so as to form a continuous force path between the hard framework and the organic phase. In addition, the 1H,1H,2H,2H-perfluorodecyltriethoxysilane reduces the surface energy and tangential friction, thus improving scratch resistance, wear resistance and optical stability without significantly sacrificing transparency.

[0104] As can be seen from the data in Example 2 and Comparative Example 1 in Table 1, when the epoxy-containing polymethyl methacrylate copolymer was removed, the interlayer peel strength decreased significantly, the critical scratch load decreased, and the scratch visibility increased. The changes in haze and light transmittance after scratching were also amplified. The main reason is that without the epoxy side groups, the surface system struggles to form effective reactive bridging points at the polycarbonate / polymethyl methacrylate interface. This makes the silicon-oxygen network and inorganic filler more prone to interfacial slippage and micro-delamination under shear, leading to enhanced whitening scattering and amplified color difference. Therefore, the reactive epoxy side groups do not simply improve adhesion, but rather provide a load-transferring interfacial basis for subsequent zirconium acetylacetonate-catalyzed network growth.

[0105] As can be seen from the data in Example 2 and Comparative Example 2 in Table 1, when zirconium acetylacetonate was removed, both the critical scratch load and the interlayer peel strength decreased, while the visibility of scratches and the change in haze after scratching tended to increase, resulting in a weakening of overall scratch resistance and abrasion resistance. This is because zirconium acetylacetonate in the hybrid system can both promote the condensation and densification of the silicon-oxygen network and catalyze the ring-opening process of epoxy, thereby simultaneously improving the strength of the inorganic framework and the continuity of the organic network. Without this catalytic effect, the network is more prone to exhibiting structural defects of localized density and overall discontinuity, leading to earlier microcracks and spalling under the tangential load of the scratching needle. It should be noted that the fluctuations observed in the comparative example at the initial stage of thermal decomposition do not necessarily indicate a superior structure; rather, they suggest insufficient load-bearing and damage resistance at high temperatures after the network construction path is altered.

[0106] As can be seen from the data in Example 2 and Comparative Example 3 in Table 1, when the injection of tetraethoxysilane and 3-glycidoxypropyltrimethoxysilane is omitted, preventing the formation of a silicon-oxygen network in the system, the initial optical properties may be superior. While the apparent peel strength and coefficient of friction do not deteriorate in some cases, the critical scratch load is significantly reduced, scratch visibility is enhanced, and the changes in haze and light transmittance after scratching are significantly amplified, along with an increase in wear volume. The main reason for this is that without a silicon-oxygen network, the surface layer, dominated by polymethyl methacrylate, undergoes more complete plastic deformation, resulting in reduced initial scattering and higher light transmittance. However, under actual scratching / sliding conditions, due to the lack of an inorganic framework for support and the locking of the zeolite imidazole framework material ZIF-8, the tangential load is transformed into deeper furrows and larger debris accumulation, thus rapidly inducing whitening and optical degradation. Therefore, low friction or high transparency alone cannot achieve comprehensive scratch resistance.

[0107] As can be seen from the data in Example 2 and Comparative Example 4 in Table 1, when the vacuuming and degassing step is omitted, the initial haze increases, the transmittance decreases, and the optical changes and visibility after scratching deteriorate significantly, while the wear volume increases. This is because ethanol and water in the system act as a sol-gel medium. If not fully removed, they will form micropores and bubbles during extrusion and annealing. These defects directly cause light scattering and, as stress concentration sources, promote microcrack propagation during scratching, preventing the hardening contribution of the silicon-oxygen network from being uniformly utilized. Ultimately, this results in more pronounced scratch whitening and easier accumulation of wear debris. This result demonstrates that the vacuuming step in this invention is not a process detail, but a necessary condition for ensuring network density and optical uniformity.

[0108] As can be seen from the data in Example 2 and Comparative Example 5 in Table 1, when only the silica-coated zeolite imidazole framework material ZIF-8 is used, and the surface grafting of 3-aminopropyltriethoxysilane and 1H,1H,2H,2H-perfluorodecyltriethoxysilane is omitted, the coefficient of friction increases, the wear volume increases, the visibility of scratches is enhanced, and the haze change after scratching is amplified. At the same time, the interlayer peel strength also shows a decreasing trend. The main reason is that without aminosilane, the reaction anchoring between the filler and the polymethyl methacrylate copolymer containing epoxy side groups is significantly weakened, making it easier for particles to be pulled out and for interfacial micro-deadhesion to occur during sliding, forming a third-body abrasive amplification wear; at the same time, without fluorinated alkylsilane, the surface energy and tangential force are difficult to reduce, and the scriber / dual ball produces stronger plowing on the surface, resulting in increased whitening and color difference.

[0109] As can be seen from the data in Example 2 and Comparative Example 6 in Table 1, when 3-aminopropyltriethoxysilane is retained while 1H,1H,2H,2H-perfluorodecyltriethoxysilane is removed, the interlayer peel strength can still be maintained at a high level, but the coefficient of friction increases, the wear volume increases, and the visibility of scratches and the change in haze after scratching also deteriorate. The reason is that aminosilane can undergo a ring-opening reaction with epoxy side groups and effectively anchor the zeolite imidazole skeleton material ZIF-8 and the silicon-oxygen network, thereby ensuring the continuity of interfacial forces; however, without fluorinated alkyl groups, the surface is not sufficiently de-energized, and the increased tangential friction makes the surface layer more prone to ploughing and microcrack propagation. Even if it can adhere, it is difficult to maintain appearance stability under sliding conditions. This comparative example further demonstrates that the present invention achieves a simultaneous match between mechanical load-bearing and surface lubrication through amino chemical anchoring and fluorinated friction reduction, demonstrating a significant synergistic effect.

[0110] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A PC / PMMA co-extruded scratch-resistant mobile phone back cover material, characterized in that, include: Polycarbonate core layer; And polymethyl methacrylate (PMMA) surface layers respectively disposed on the upper and lower surfaces of the polycarbonate core layer; the PMMA surface layer is formed by mixing PMMA surface layer functional masterbatch and PMMA resin at a mass ratio of 1600-2400:2400-1600; The polymethyl methacrylate (PMMA) surface functional masterbatch is prepared from a PMMA copolymer resin containing epoxy side groups, PMMA resin, tetraethoxysilane, 3-glycidoxypropyltrimethoxysilane, zirconium acetylacetonate, and modified ZIF-8 powder; the mass ratio of the PMMA copolymer resin containing epoxy side groups, PMMA resin, tetraethoxysilane, 3-glycidoxypropyltrimethoxysilane, zirconium acetylacetonate, and modified ZIF-8 powder is 450-500:4200-4400:70-90:50-70:8-12:40-60. The preparation method of the polymethyl methacrylate surface functional masterbatch is as follows: polymethyl methacrylate resin, polymethyl methacrylate copolymer resin containing epoxy side groups, zirconium acetylacetonate, and modified ZIF-8 powder are melt-blended in a twin-screw extruder. During the melt-blending process, a pre-hydrolyzed siloxane precursor solution is introduced to allow the tetraethoxysilane and 3-glycidoxypropyltrimethoxysilane to condense and form a siloxane network. At the same time, the twin-screw extruder maintains a vacuum of -0.06 MPa to -0.09 MPa to remove the low-boiling components carried by the siloxane precursor solution, thereby obtaining the polymethyl methacrylate surface functional masterbatch. The modified ZIF-8 powder has a silicon-oxygen coating layer on its surface, and aminosilyl groups and fluorinated alkylsilyl groups are grafted onto the silicon-oxygen coating layer; the aminosilyl groups are derived from 3-aminopropyltriethoxysilane grafting, and the fluorinated alkylsilyl groups are derived from 1H,1H,2H,2H-perfluorodecyltriethoxysilane grafting.

2. The PC / PMMA co-extruded scratch-resistant mobile phone back cover material according to claim 1, characterized in that, The specific preparation steps of the modified ZIF-8 powder are as follows: (a) Preparation of ZIF-8 nanocrystal powder; (b) The ZIF-8 nanocrystal powder was coated with a sol-gel under alkaline conditions using tetraethoxysilane to obtain ZIF-8 powder with a surface silicon oxide coating; the mass ratio of the tetraethoxysilane to the ZIF-8 nanocrystal powder was 45-55:

100. (c) The ZIF-8 powder with surface silicon oxide coating is reacted with 3-aminopropyltriethoxysilane and 1H,1H,2H,2H-perfluorodecyltriethoxysilane to simultaneously graft aminosilyl groups and fluorinated alkylsilyl groups onto the silicon oxide coating layer to obtain the modified ZIF-8 powder. The mass ratio of the surface silicon oxide coated ZIF-8 powder, 3-aminopropyltriethoxysilane and 1H,1H,2H,2H-perfluorodecyltriethoxysilane is 100:8-12:8-12.

3. The PC / PMMA co-extruded scratch-resistant mobile phone back cover material according to claim 1, characterized in that, The polymethyl methacrylate copolymer resin containing epoxy side groups is a copolymer resin obtained by copolymerizing methyl methacrylate and glycidyl methacrylate.

4. The PC / PMMA co-extruded scratch-resistant mobile phone back cover material according to claim 3, characterized in that, The mass ratio of methyl methacrylate to glycidyl methacrylate is 870-930:70-130.

5. The PC / PMMA co-extruded scratch-resistant mobile phone back cover material according to claim 1, characterized in that, The thickness of the upper and lower polymethyl methacrylate surface layers is 80μm-120μm each, the thickness of the polycarbonate core layer is 760μm-840μm, and the total thickness of the sheet is 920μm-1080μm.

6. A method for preparing a PC / PMMA co-extruded scratch-resistant mobile phone back cover material according to any one of claims 1-5, characterized in that, Includes the following steps: (1) Preparation of polymethyl methacrylate surface functional masterbatch: Polymethyl methacrylate resin, polymethyl methacrylate copolymer resin containing epoxy side groups, zirconium acetylacetonate and modified ZIF-8 powder are melt-blended in a twin-screw extruder, and a pre-hydrolyzed siloxane precursor solution is introduced during the melt-blending process to form a siloxane network by polycondensation of tetraethoxysilane and 3-glycidoxypropyltrimethoxysilane. At the same time, the twin-screw extruder maintains a vacuum of -0.06MPa to -0.09MPa to remove the low-boiling components carried by the siloxane precursor solution, thereby obtaining the polymethyl methacrylate surface functional masterbatch; wherein, the siloxane precursor solution contains tetraethoxysilane, 3-glycidoxypropyltrimethoxysilane, water and alcohol; (2) The polymethyl methacrylate surface functional masterbatch obtained in step (1) is mixed with polymethyl methacrylate resin to obtain surface material, and polycarbonate resin is provided as core material. (3) Co-extrusion molding is used to form a polycarbonate core layer from the core material and a polymethyl methacrylate surface layer from both sides of the polycarbonate core material to obtain a PC / PMMA co-extruded scratch-resistant mobile phone back cover material.

7. The method for preparing PC / PMMA co-extruded scratch-resistant mobile phone back cover material according to claim 6, characterized in that, In step (1), melt mixing is carried out in a twin-screw extruder with a barrel temperature of 195℃-245℃ and a screw speed of 180-220r / min. After melt plasticizing for 1-2 minutes, the pre-hydrolyzed silica precursor solution is introduced, and after mixing for 1-3 minutes, the mixture is extruded and pelletized to obtain the polymethyl methacrylate surface functional masterbatch.

8. The method for preparing PC / PMMA co-extruded scratch-resistant mobile phone back cover material according to claim 6, characterized in that, In step (3), co-extrusion is carried out on a parallel three-layer co-extrusion flat production line. The temperature of each zone of the surface extruder is 205℃-245℃ and the die head temperature is 235℃-245℃. The temperature of each zone of the core extruder is 235℃-260℃ and the die head temperature is 250℃-260℃. The dwell time in the co-extrusion die area is 1min-3min.