Flame-retardant low-smoke acrylic plate and preparation process thereof

Abstract

The invention relates to the technical field of plates, in particular to a flame-retardant low-smoke acrylic plate and a preparation process thereof, and the plate is provided with a core layer and symmetrical functional surface structures on two sides. Each of the functional surface layer and the core layer comprises a reactive flame-retardant comonomer, a crosslinking monomer containing a dynamic disulfide bond and surface grafting modified silicon dioxide micro-nano particles which are bonded through copolymerization. And the prepared plate has the characteristics of efficient flame retardance, low smoke density, high light transmittance and low haze through a multi-time casting molding and segmented curing process. Especially, by means of the dynamic exchange effect of disulfide bonds, the material can achieve self-repairing of surface microcracks in the using process, the long-term weather resistance and durability are remarkably improved, and the defect that safety and durability of a traditional acrylic plate are difficult to consider at the same time is overcome.

Description

Technical Field

[0001] This invention relates to the field of sheet technology, and in particular to a flame-retardant, low-smoke acrylic sheet and its preparation process. Background Technology

[0002] Polymethyl methacrylate (PMMA) acrylic sheets are widely used in architectural lighting, advertising signage, and transportation due to their excellent optical transparency, weather resistance, and processing performance. However, PMMA itself is a flammable material, burning rapidly and releasing a large amount of heat and dense smoke when exposed to fire, which severely limits its application in environments with high safety standards. To improve its flame retardant properties, a common technique is to add flame retardants, such as halogenated, phosphorus-based, or inorganic flame retardants.

[0003] While early halogenated flame retardants, though highly efficient, released toxic and corrosive gases during combustion, posing environmental and health risks. Therefore, halogen-free flame retardant systems have become a research focus, with phosphorus-nitrogen intumescent flame retardants attracting significant attention due to their char-forming barrier properties. However, simple physical blending of these flame retardants results in poor compatibility with the PMMA matrix, leading to migration and precipitation during processing or long-term use. This not only degrades flame retardant performance but also causes surface blooming, increased haze, and significantly accelerates yellowing under outdoor UV radiation, rapidly deteriorating the optical quality and aesthetics of the sheet material.

[0004] To overcome the shortcomings of physical blending, existing technologies propose copolymerizing reactive phosphorus- or nitrogen-containing monomers with methyl methacrylate (PMMA). The aim is to chemically bond flame-retardant elements to the polymer backbone, thereby achieving a more durable and stable flame-retardant effect. However, this method still faces challenges: on the one hand, the introduction of flame-retardant monomers may alter the polymerization kinetics and final network structure of PMMA, negatively impacting the material's mechanical strength, toughness, and light transmittance; on the other hand, inorganic nanofillers (such as silica) introduced to achieve low-smoke properties, if not surface-modified, are prone to agglomeration in the polymer matrix, forming stress defect points that become the origin of microcracks in the material under thermo-oxidative aging or mechanical stress.

[0005] Crucially, for outdoor acrylic sheets, the formation and propagation of surface microcracks are almost inevitable after prolonged exposure to sunlight, rain, and thermal cycles. These microcracks not only scatter light, leading to increased haze and decreased light transmittance, but also become channels for flame and smoke penetration, accelerating material damage in a fire. Existing technologies primarily focus on optimizing initial performance, lacking the design capability for self-repair of damage (such as microcracks) throughout the material's lifespan, resulting in insufficient long-term durability and reliability. Therefore, developing an acrylic sheet manufacturing technology that can fundamentally and synergistically solve the problems of flame retardancy, low smoke, weather resistance, and long-term durability has become a critical breakthrough urgently needed in this field. Summary of the Invention

[0006] In view of this, the purpose of this invention is to propose a flame-retardant and low-smoke acrylic sheet and its preparation process, so as to overcome the shortcomings of existing PMMA sheets in that it is difficult to achieve both flame retardancy and long-term weather resistance, and to endow it with the ability to self-repair surface micro-cracks during use, thereby preparing an acrylic sheet with high flame retardancy, low smoke, high transparency and long-term durability.

[0007] To achieve the above objectives, the present invention provides a flame-retardant low-smoke acrylic sheet, comprising a core layer, and a first functional surface layer and a second functional surface layer respectively disposed on both sides of the core layer, wherein the first functional surface layer and the second functional surface layer have the same composition.

[0008] By weight, the first functional surface layer and the second functional surface layer are prepared from the following raw materials: 175-185 parts methyl methacrylate, 15-25 parts reactive flame retardant comonomer, 3-5 parts bis(2-methacryloyloxyethyl) disulfide, 7-12 parts surface-grafted modified silica micro / nano particles, 0.2 parts dodecyl mercaptan, and 1.3-1.7 parts azobisisobutyronitrile;

[0009] The core layer is prepared from the following raw materials in parts by weight: 1180-1220 parts methyl methacrylate, 65-80 parts reactive flame retardant comonomer, 4-8 parts bis(2-methacryloyloxyethyl) disulfide, 4-8 parts surface-grafted modified silica micro / nanoparticles, 0.35-0.45 parts dodecyl mercaptan, and 2.3-2.7 parts azobisisobutyronitrile.

[0010] Preferably, the thickness of each of the functional surfaces is 280-320 μm.

[0011] Preferably, the total thickness of the flame-retardant, low-smoke acrylic sheet is 5mm.

[0012] Preferably, the reactive flame-retardant comonomer is prepared by the following method: phenyl dichlorophosphate is dissolved in anhydrous tetrahydrofuran and cooled to 5°C under ice-water bath conditions. Diethylamine is added dropwise, followed by triethylamine to form a phosphorus-nitrogen intermediate. Then, 4-maleimide-based phenol is added, and the mixture is stirred at 5°C and then heated to 25°C to continue the reaction. After the reaction is completed, the mixture is filtered to remove salt, concentrated under reduced pressure, and then recrystallized and dried under vacuum to obtain the reactive flame-retardant comonomer. The weight ratio of phenyl dichlorophosphate, anhydrous tetrahydrofuran, diethylamine, triethylamine, and 4-maleimide-based phenol is 120:600:73:101:100.

[0013] Preferably, the 4-maleiminophenol is obtained by reacting p-aminophenol with maleic anhydride to produce 4-hydroxyphenylmaleamic acid, which is then reacted in the presence of acetic anhydride and anhydrous sodium acetate under nitrogen protection at a temperature of 80°C, followed by water precipitation, filtration, washing, and vacuum drying; the weight ratio of p-aminophenol to maleic anhydride is 109:98; and the weight ratio of 4-hydroxyphenylmaleamic acid, acetic anhydride, and anhydrous sodium acetate is 180:500:50.

[0014] Preferably, the surface-grafted modified silica micro / nanoparticles are obtained by grafting trimethoxysilane graft groups onto fumed silica to form disulfide acrylate monomers; the mass ratio of fumed silica to disulfide acrylate monomers with trimethoxysilane graft groups is 50:8-12.

[0015] Preferably, the specific surface area of ​​the fumed silica is 180-220 m². 2 / g.

[0016] Preferably, the disulfide bond acrylate monomer of the trimethoxysilane graft group is obtained by a mercapto-olefin addition reaction of bis(2-methacryloyloxyethyl) disulfide and 3-mercaptopropyltrimethoxysilane; the weight ratio of bis(2-methacryloyloxyethyl) disulfide to 3-mercaptopropyltrimethoxysilane is 60:35.

[0017] Furthermore, the present invention provides a method for preparing flame-retardant, low-smoke acrylic sheets, comprising the following steps:

[0018] S1, methyl methacrylate, reactive flame retardant comonomer, bis(2-methacryloyloxyethyl) disulfide, surface-grafted modified silica micro-nano particles, dodecyl mercaptan and azobisisobutyronitrile are mixed, deoxygenated and prepolymerized according to the raw material ratio of the first functional surface layer and the second functional surface layer, respectively, to obtain the first functional surface layer prepolymer slurry and the second functional surface layer prepolymer slurry.

[0019] S2, methyl methacrylate is deoxygenated and prepolymerized with reactive flame retardant comonomer, and then bis(2-methacryloyloxyethyl) disulfide, surface-grafted modified silica micro-nano particles and dodecyl mercaptan are added, dispersed and degassed to obtain core layer prepolymer slurry.

[0020] S3, the first functional surface prepolymer slurry, the core prepolymer slurry and the second functional surface prepolymer slurry are poured and molded in sequence using a staged pouring method, followed by degassing and curing, demolding and post-heat treatment to obtain the flame-retardant low-smoke acrylic sheet.

[0021] Preferably, in step S1, the prepolymerization is carried out by stirring at 70°C for 35-45 minutes.

[0022] Preferably, in step S2, the prepolymerization is carried out at 70°C for 1.3-1.7 hours.

[0023] Preferably, in step S3, the weight ratio of the first functional surface layer prepolymer slurry, the core layer prepolymer slurry, and the second functional surface layer prepolymer slurry is 95-105:1280-1295:100-110.

[0024] Preferably, in step S3, the curing process involves sequentially holding the product at 50°C for 2 hours, at 65°C for 4 hours, at 80°C for 4 hours, at 100°C for 2 hours, and at 120°C for 1 hour.

[0025] Preferably, in step S3, the post-heat treatment involves first holding at 105°C for 2 hours, and then holding at 85°C for 2 hours.

[0026] The beneficial effects of this invention are:

[0027] Superior Flame Retardancy, Smoke Suppression, and Durability: By incorporating specific reactive flame-retardant comonomers into the copolymerization process, making them integral components of the polymer molecular chain, the flame-retardant effect is sustained and highly efficient. This structure promotes the formation of a stable, dense, expanded char layer on the material surface at high temperatures. This char layer not only effectively insulates against heat and oxygen, inhibiting further decomposition of the internal substrate, but also significantly reduces the generation of flammable volatiles and the release of smoke, thus achieving a synergistic effect of flame retardancy and low smoke at the source. Simultaneously, the chemically bonded flame-retardant units prevent the migration and precipitation of small molecules, allowing the panels to maintain excellent flame-retardant efficiency and optical stability even after long-term outdoor aging.

[0028] Significant self-healing and mechanical recovery capabilities: An innovative approach introduces disulfide-bonded acrylate monomers into the polymer network. These disulfide bonds undergo dynamic exchange reactions during post-heat treatment, allowing polymer molecular chains to rearrange and flow. When microcracks form on the material surface due to external forces, this dynamic characteristic drives the crack interface to re-bond and close, effectively repairing the damage. This not only restores the material's appearance integrity but also significantly restores its mechanical properties, substantially delaying performance degradation caused by microcrack propagation and greatly improving the service life and reliability of the sheet material.

[0029] Excellent optical performance and interfacial compatibility: Utilizing surface grafting modification technology, polymerizable organic functional groups are introduced onto the surface of silica micro / nano particles, significantly improving their interfacial compatibility with the organic PMMA matrix. The modified particles can be uniformly dispersed within the matrix, avoiding visible light scattering caused by agglomeration. This provides both reinforcement and auxiliary barrier effects while ensuring the substrate exhibits excellent optical qualities of high transmittance and low haze, meeting the stringent requirements of high-end applications for transparent materials.

[0030] Optimized structural and performance gradient distribution: Through a unique double-layer prepolymer slurry design and a staged casting process, a symmetrical gradient structure with a functional surface layer, a core layer, and another functional surface layer is constructed. This structure concentrates the functional layers, rich in flame-retardant, smoke-suppressing, and self-healing components, on both surfaces of the board, rapidly forming an effective protective barrier when exposed to fire. The core layer primarily ensures the overall mechanical strength and dimensional stability of the material. This spatial functional allocation optimizes resource utilization, enabling the board to achieve superior overall protective performance with the same amount of raw materials.

[0031] Stable processing technology and product uniformity: A segmented prepolymer feeding sequence control strategy is adopted, first forming a prepolymer of a certain viscosity, and then introducing functional monomers and modified particles. This facilitates the uniform dispersion and stable existence of each component in subsequent processing, avoiding sedimentation or separation caused by density differences. This process design ensures the uniformity of the chemical structure and the consistency of the physical properties of the final cured product, providing a reliable guarantee for large-scale industrial production of high-quality products. Detailed Implementation

[0032] 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.

[0033] Example 1:

[0034] Step 1: Weigh 109g of p-aminophenol and add it to 1000g of acetone. Cool the mixture to 5°C in an ice-water bath and stir for 30min. Then add 98g of maleic anhydride and maintain the reaction at 5°C-10°C for 30min. Remove the ice bath and continue stirring at 25°C for 2h. Filter the reaction solution and wash the filter cake twice with 200g of acetone. Then dry it under vacuum at 50°C for 12h to obtain 4-hydroxyphenylmaleamic acid.

[0035] Step 2: Weigh 180g of 4-hydroxyphenylmaleamic acid and add it to 500g of acetic anhydride. Add 50g of anhydrous sodium acetate as a catalyst and acid scavenger. Stir and heat to 80℃ for 2h under nitrogen protection. Pour the hot reaction solution into 5000g of deionized water and stir for 10min. Filter the precipitated solid and wash it with 1000g of deionized water until the washing liquid has no obvious acetic anhydride odor. Dry the filter cake under vacuum at 60℃ for 12h to obtain 4-maleimide phenol.

[0036] Step 3: Weigh 120g of phenyl dichlorophosphate and dissolve it in 600g of anhydrous tetrahydrofuran. Cool the solution to 5°C in an ice-water bath. Add 73g of diethylamine and 101g of triethylamine dropwise over 30 minutes. After the addition is complete, maintain the temperature at 5°C and stir for 30 minutes to form a phosphorus-nitrogen intermediate. Then add 100g of 4-maleimide-phenol and stir at 5°C for 60 minutes. Increase the temperature to 25°C and continue the reaction for 6 hours. After the reaction is complete, filter to remove the triethylamine salt. Concentrate the filtrate under reduced pressure to remove the tetrahydrofuran. Redissolve the filtrate in 400g of ethyl acetate and recrystallize it in 600g of n-hexane. Filter the solution and dry it under vacuum at 50°C for 12 hours to obtain the reactive flame-retardant comonomer.

[0037] Step 4: Weigh 60g of bis(2-methacryloyloxyethyl) disulfide and dissolve it in 600g of anhydrous toluene. After deoxygenation by purging with nitrogen for 30min, add 1g of azobisisobutyronitrile and heat to 65℃. Add 35g of 3-mercaptopropyltrimethoxysilane dropwise over 30min and stir the reaction at 65℃ for 6h. After the reaction, cool to 25℃ and distill under reduced pressure to obtain a disulfide bonded acrylate monomer containing a trimethoxysilane graft group.

[0038] Step 5: Weigh 50g of fumed silica (Evonik, model AEROSIL 200, specific surface area approximately 200m² / g) and add it to a mixed solvent of 900g anhydrous ethanol and 100g deionized water, and disperse for 30min; add 8g of disulfide bond acrylate monomer containing trimethoxysilane grafted groups, and then add 2g of glacial acetic acid, heat to 70℃ and stir for 240min; after the reaction is completed, cool to 25℃ and filter, wash the filter cake twice with 500g anhydrous ethanol, and then vacuum dry at 80℃ for 12h to obtain surface-grafted modified silica micro / nanoparticles;

[0039] Step 6: Weigh 175g of methyl methacrylate, 15g of reactive flame-retardant comonomer, 3g of bis(2-methacryloyloxyethyl) disulfide, 7g of surface-grafted modified silica micro / nanoparticles, and 200mg of dodecyl mercaptan and add them to the reactor. Disperse at 25°C for 30min. After deoxygenation by purging with nitrogen for 20min, add 1300mg of azobisisobutyronitrile. Heat to 70°C and stir for 35min. Then cool to 25°C and degas at an absolute pressure of 20kPa for 10min to obtain the functional surface layer prepolymer slurry.

[0040] Step 7: Weigh 1220g of methyl methacrylate and 65g of reactive flame-retardant comonomer and add them to the reactor. Stir at 25°C and purge with nitrogen for 30 minutes to remove oxygen. Then add 2300mg of azobisisobutyronitrile and heat to 70°C to react for 1.3 hours to obtain the first-stage prepolymer slurry. Then add 4g of bis(2-methacryloyloxyethyl) disulfide, 4g of surface-grafted modified silica micro-nano particles, and 350mg of dodecyl mercaptan. Disperse at 700rpm for 15 minutes at 25°C, and then degas at 20kPa for 10 minutes to obtain the core layer prepolymer slurry.

[0041] Step 8: Weigh 2g of silicone oil and evenly coat it onto the inner surfaces of two 500mm×500mm tempered glass pieces, allowing it to evaporate and stand for 10 minutes. Assemble and clamp the casting mold using a 5mm thick stainless steel frame shim. First, evenly pour 95g of functional surface layer prepolymer slurry to the bottom of the mold and scrape it to a thickness of 280μm. Place it in a 40℃ constant temperature oven and let it stand for 15 minutes. Then, slowly pour 1295g of core layer prepolymer slurry into the middle of the mold and let it stand for 8 minutes to self-level. Finally, pour 100g of functional surface layer prepolymer slurry to the top and scrape it to a thickness of 280μm. The entire mold was placed in a vacuum chamber and evacuated to an absolute pressure of 20 kPa for 7 minutes to complete the overall degassing. After returning to normal pressure, the mold was sealed and placed in a programmed temperature oven for curing at 50℃ for 2 hours, 65℃ for 4 hours, 80℃ for 4 hours, 100℃ for 2 hours, and 120℃ for 1 hour. Then, the heating was turned off and the temperature was lowered to 60℃ in the oven before demolding. The mold was then placed in a hot air circulation box for 105℃ for 2 hours, followed by 85℃ for 2 hours, and then cooled to 25℃ to obtain a 5mm thick flame-retardant low-smoke acrylic sheet.

[0042] Example 2:

[0043] Step 1: Weigh 109g of p-aminophenol and add it to 1000g of acetone. Cool the mixture to 5°C in an ice-water bath and stir for 30min. Then add 98g of maleic anhydride and maintain the reaction at 5°C-10°C for 30min. Remove the ice bath and continue stirring at 25°C for 2h. Filter the reaction solution and wash the filter cake twice with 200g of acetone. Then dry it under vacuum at 50°C for 12h to obtain 4-hydroxyphenylmaleamic acid.

[0044] Step 2: Weigh 180g of 4-hydroxyphenylmaleamic acid and add it to 500g of acetic anhydride. Add 50g of anhydrous sodium acetate as a catalyst and acid scavenger. Stir and heat to 80℃ for 2h under nitrogen protection. Pour the hot reaction solution into 5000g of deionized water and stir for 10min. Filter the precipitated solid and wash it with 1000g of deionized water until the washing liquid has no obvious acetic anhydride odor. Dry the filter cake under vacuum at 60℃ for 12h to obtain 4-maleimide phenol.

[0045] Step 3: Weigh 120g of phenyl dichlorophosphate and dissolve it in 600g of anhydrous tetrahydrofuran. Cool the solution to 5°C in an ice-water bath. Add 73g of diethylamine and 101g of triethylamine dropwise over 30 minutes. After the addition is complete, maintain the temperature at 5°C and stir for 30 minutes to form a phosphorus-nitrogen intermediate. Then add 100g of 4-maleimide-phenol and stir at 5°C for 60 minutes. Increase the temperature to 25°C and continue the reaction for 6 hours. After the reaction is complete, filter to remove the triethylamine salt. Concentrate the filtrate under reduced pressure to remove the tetrahydrofuran. Redissolve the filtrate in 400g of ethyl acetate and recrystallize it in 600g of n-hexane. Filter the solution and dry it under vacuum at 50°C for 12 hours to obtain the reactive flame-retardant comonomer.

[0046] Step 4: Weigh 60g of bis(2-methacryloyloxyethyl) disulfide and dissolve it in 600g of anhydrous toluene. After deoxygenation by purging with nitrogen for 30min, add 1g of azobisisobutyronitrile and heat to 65℃. Add 35g of 3-mercaptopropyltrimethoxysilane dropwise over 30min and stir the reaction at 65℃ for 6h. After the reaction, cool to 25℃ and distill under reduced pressure to obtain a disulfide bonded acrylate monomer containing a trimethoxysilane graft group.

[0047] Step 5: Weigh 50g of fumed silica (Evonik, model AEROSIL 200, specific surface area approximately 200m²). 2 / g) was added to a mixed solvent of 900g anhydrous ethanol and 100g deionized water and dispersed for 30min; 10g of disulfide bond acrylate monomer containing trimethoxysilane grafted groups was added, followed by 2g of glacial acetic acid, and the mixture was heated to 70℃ and stirred for 240min; after the reaction was completed, the mixture was cooled to 25℃ and filtered, the filter cake was washed twice with 500g anhydrous ethanol, and then vacuum dried at 80℃ for 12h to obtain surface-grafted modified silica micro-nano particles;

[0048] Step 6: Weigh 170g of methyl methacrylate, 20g of reactive flame-retardant comonomer, 4g of bis(2-methacryloyloxyethyl) disulfide, 10g of surface-grafted modified silica micro / nanoparticles, and 200mg of dodecyl mercaptan and add them to the reactor. Disperse at 25°C for 30min. After deoxygenation by purging with nitrogen for 20min, add 1500mg of azobisisobutyronitrile. Heat to 70°C and stir for 40min. Then cool to 25°C and degas at 20kPa absolute pressure for 10min to obtain the functional surface layer prepolymer slurry.

[0049] Step 7: Weigh 1200g of methyl methacrylate and 70g of reactive flame-retardant comonomer and add them to the reactor. Stir at 25°C and purge with nitrogen for 30 minutes to remove oxygen. Then add 2500mg of azobisisobutyronitrile and heat to 70°C to react for 1.5 hours to obtain the first-stage prepolymer slurry. Then add 6g of bis(2-methacryloyloxyethyl) disulfide, 6g of surface-grafted modified silica micro-nano particles, and 400mg of dodecyl mercaptan. Disperse at 800rpm for 20 minutes at 25°C, and then degas at 20kPa absolute pressure for 10 minutes to obtain the core layer prepolymer slurry.

[0050] Step 8: Weigh 2g of silicone oil and evenly coat it on the inner surface of two 500mm×500mm tempered glass pieces, then let it evaporate and stand for 10 minutes. Assemble the casting mold using a 5mm thick stainless steel frame shim and clamp it. First, evenly pour 100g of functional surface layer prepolymer slurry to the bottom of the mold and scrape it to a thickness of 300μm. Place it in a 40℃ constant temperature oven and let it stand for 20 minutes. Then, slowly pour 1290g of core layer prepolymer slurry to the middle of the mold and let it stand for 10 minutes to self-level. Finally, pour 105g of functional surface layer prepolymer slurry to the top and scrape it to a thickness of 300μm. The entire mold was placed in a vacuum chamber and evacuated to an absolute pressure of 20 kPa for 8 minutes to complete the overall degassing. After returning to normal pressure, the mold was sealed and placed in a programmed temperature oven for curing at 50℃ for 2 hours, 65℃ for 4 hours, 80℃ for 4 hours, 100℃ for 2 hours, and 120℃ for 1 hour. Then, the heating was turned off and the temperature was lowered to 60℃ in the oven before demolding. The mold was then placed in a hot air circulation box for 105℃ for 2 hours, followed by 85℃ for 2 hours, and then cooled to 25℃ to obtain a 5mm thick flame-retardant low-smoke acrylic sheet.

[0051] Example 3:

[0052] Step 1: Weigh 109g of p-aminophenol and add it to 1000g of acetone. Cool the mixture to 5°C in an ice-water bath and stir for 30min. Then add 98g of maleic anhydride and maintain the reaction at 5°C-10°C for 30min. Remove the ice bath and continue stirring at 25°C for 2h. Filter the reaction solution and wash the filter cake twice with 200g of acetone. Then dry it under vacuum at 50°C for 12h to obtain 4-hydroxyphenylmaleamic acid.

[0053] Step 2: Weigh 180g of 4-hydroxyphenylmaleamic acid and add it to 500g of acetic anhydride. Add 50g of anhydrous sodium acetate as a catalyst and acid scavenger. Stir and heat to 80℃ for 2h under nitrogen protection. Pour the hot reaction solution into 5000g of deionized water and stir for 10min. Filter the precipitated solid and wash it with 1000g of deionized water until the washing liquid has no obvious acetic anhydride odor. Dry the filter cake under vacuum at 60℃ for 12h to obtain 4-maleimide phenol.

[0054] Step 3: Weigh 120g of phenyl dichlorophosphate and dissolve it in 600g of anhydrous tetrahydrofuran. Cool the solution to 5°C in an ice-water bath. Add 73g of diethylamine and 101g of triethylamine dropwise over 30 minutes. After the addition is complete, maintain the temperature at 5°C and stir for 30 minutes to form a phosphorus-nitrogen intermediate. Then add 100g of 4-maleimide-phenol and stir at 5°C for 60 minutes. Increase the temperature to 25°C and continue the reaction for 6 hours. After the reaction is complete, filter to remove the triethylamine salt. Concentrate the filtrate under reduced pressure to remove the tetrahydrofuran. Redissolve the filtrate in 400g of ethyl acetate and recrystallize it in 600g of n-hexane. Filter the solution and dry it under vacuum at 50°C for 12 hours to obtain the reactive flame-retardant comonomer.

[0055] Step 4: Weigh 60g of bis(2-methacryloyloxyethyl) disulfide and dissolve it in 600g of anhydrous toluene. After deoxygenation by purging with nitrogen for 30min, add 1g of azobisisobutyronitrile and heat to 65℃. Add 35g of 3-mercaptopropyltrimethoxysilane dropwise over 30min and stir the reaction at 65℃ for 6h. After the reaction, cool to 25℃ and distill under reduced pressure to obtain a disulfide bonded acrylate monomer containing a trimethoxysilane graft group.

[0056] Step 5: Weigh 50g of fumed silica (Evonik, model AEROSIL 200, specific surface area approximately 200m² / g) and add it to a mixed solvent of 900g anhydrous ethanol and 100g deionized water, and disperse for 30min; add 12g of disulfide bond acrylate monomer containing trimethoxysilane grafted groups, and then add 2g of glacial acetic acid, heat to 70℃ and stir for 240min; after the reaction is completed, cool to 25℃ and filter, wash the filter cake twice with 500g anhydrous ethanol, and then vacuum dry at 80℃ for 12h to obtain surface-grafted modified silica micro / nanoparticles;

[0057] Step 6: Weigh 185g of methyl methacrylate, 25g of reactive flame-retardant comonomer, 5g of bis(2-methacryloyloxyethyl) disulfide, 12g of surface-grafted modified silica micro / nano particles, and 200mg of dodecyl mercaptan and add them to the reactor. Disperse at 25°C for 30min. After deoxygenation by purging with nitrogen for 20min, add 1700mg of azobisisobutyronitrile. Heat to 70°C and stir for 45min. Then cool to 25°C and degas at an absolute pressure of 20kPa for 10min to obtain the functional surface layer prepolymer slurry.

[0058] Step 7: Weigh 1180g of methyl methacrylate and 80g of reactive flame-retardant comonomer and add them to the reactor. Stir at 25°C and purge with nitrogen for 30 minutes to remove oxygen. Then add 2700mg of azobisisobutyronitrile and heat to 70°C to react for 1.7h to obtain the first-stage prepolymer slurry. Then add 8g of bis(2-methacryloyloxyethyl) disulfide, 8g of surface-grafted modified silica micro-nano particles, and 450mg of dodecyl mercaptan. Disperse at 25°C with strong shear at 900rpm for 25 minutes, and then degas at an absolute pressure of 20kPa for 10 minutes to obtain the core layer prepolymer slurry.

[0059] Step 8: Weigh 2g of silicone oil and evenly coat it on the inner surface of two 500mm×500mm tempered glass pieces, then let it evaporate and stand for 10 minutes. Assemble the casting mold using a 5mm thick stainless steel frame shim and clamp it. First, evenly pour 105g of functional surface layer prepolymer slurry to the bottom of the mold and scrape it to a thickness of 320μm. Place it in a 40℃ constant temperature oven and let it stand for 25 minutes. Then, slowly pour 1280g of core layer prepolymer slurry to the middle of the mold and let it stand for 12 minutes to self-level. Finally, pour 110g of functional surface layer prepolymer slurry to the top and scrape it to a thickness of 320μm. The entire mold was placed in a vacuum chamber and evacuated to an absolute pressure of 20 kPa for 9 minutes to complete the overall degassing. After returning to normal pressure, the mold was sealed and placed in a programmed temperature oven for curing at 50℃ for 2 hours, 65℃ for 4 hours, 80℃ for 4 hours, 100℃ for 2 hours, and 120℃ for 1 hour. Then, the heating was turned off and the temperature was lowered to 60℃ in the oven before demolding. The mold was then placed in a hot air circulation box for 105℃ for 2 hours, followed by 85℃ for 2 hours, and then cooled to 25℃ to obtain a 5mm thick flame-retardant low-smoke acrylic sheet.

[0060] The difference between Comparative Example 1 and Example 2 is as follows: In step 6, the reactive flame-retardant comonomer is not added; instead, 190g of methyl methacrylate, 0g of reactive flame-retardant comonomer, 4g of bis(2-methacryloyloxyethyl) disulfide, 10g of surface-grafted modified silica micro / nano particles, and 200mg of dodecyl mercaptan are weighed and added to the reactor. In step 7, the reactive flame-retardant comonomer is not added; instead, 1270g of methyl methacrylate and 0g of reactive flame-retardant comonomer are weighed and added to the reactor for the first stage of prepolymerization. The remaining conditions are the same as in Example 2.

[0061] The difference between Comparative Example 2 and Example 2 is as follows: In step 6, bis(2-methacryloyloxyethyl) disulfide is not added; instead, 174g of methyl methacrylate, 20g of reactive flame-retardant comonomer, 0g of bis(2-methacryloyloxyethyl) disulfide, 10g of surface-grafted modified silica micro / nanoparticles, and 200mg of dodecyl mercaptan are weighed and added to the reactor. In step 7, bis(2-methacryloyloxyethyl) disulfide is not added; instead, 0g of bis(2-methacryloyloxyethyl) disulfide is added after the first-stage prepolymer slurry is prepared, and 6g of methyl methacrylate is added simultaneously. The remaining conditions are the same as in Example 2.

[0062] The difference between Comparative Example 3 and Example 2 is that, in steps 6 and 7, surface-grafted modified silica micro / nanoparticles are not used; instead, fumed silica (Evonik, model AEROSIL 200, with a specific surface area of ​​approximately 200 m²) is used. 2 / g) were replaced in equal amounts: in step 6, 10g of fumed silica was added to replace 10g of surface-grafted modified silica micro-nano particles, and in step 7, 6g of fumed silica was added to replace 6g of surface-grafted modified silica micro-nano particles; the remaining conditions were the same as in Example 2.

[0063] The difference between Comparative Example 4 and Example 2 is that, instead of pouring in stages to form the upper and lower surface layers and the middle core layer in step 8, the functional surface layer prepolymer slurry obtained in step 6 and the core layer prepolymer slurry obtained in step 7 are mixed evenly at 25°C for 5 minutes at a mass ratio of 100:1290:105, and then poured into the same casting mold in one go after degassing at an absolute pressure of 20 kPa for 8 minutes and scraped to a thickness of 5 mm; the other conditions are the same as in Example 2.

[0064] The difference between Comparative Example 5 and Example 2 is that in step 7, instead of adding bis(2-methacryloyloxyethyl) disulfide and surface-grafted modified silica micro-nanoparticles after the first-stage prepolymerization, 1200g of methyl methacrylate, 70g of reactive flame-retardant comonomer, 6g of bis(2-methacryloyloxyethyl) disulfide, 6g of surface-grafted modified silica micro-nanoparticles, and 400mg of dodecyl mercaptan are added to the reactor at the beginning of step 7. The mixture is stirred at 25°C and dispersed under strong shear at 800rpm for 20min, then nitrogen is introduced for 30min for deoxygenation. 2500mg of azobisisobutyronitrile is added and the temperature is raised to 70°C for 1.5h. After the reaction, the temperature is lowered to 25°C and degassed at an absolute pressure of 20kPa for 10min to obtain the core layer prepolymer slurry. The remaining conditions are the same as in Example 2.

[0065] Performance testing:

[0066] Sample preparation: 5 mm thick plate samples were prepared according to the examples and comparative examples. After demolding, the plates were placed in a standard environment of (23±2)℃ and (50±5)% relative humidity for 48 h for conditioning before sampling and processing. All samples were taken from the central area of ​​the same plate. Water-cooled diamond saws were used for cutting, and the edges were lightly sanded with 600 grit sandpaper in one direction to remove burrs and wiped with a lint-free cloth. When the samples needed to be thinned to 3.0 mm or 4.0 mm, double-sided equal-volume grinding was used to ensure that the surface layers on both sides were intact and of consistent thickness. The fire-exposed side or the exposed side was marked on each sample.

[0067] Thermogravimetric analysis: Thermogravimetric analysis was performed using a thermogravimetric analyzer. (8.0 ± 0.5) mg of fragments from each sample were placed in a platinum crucible. High-purity nitrogen was used as the carrier gas at a flow rate of 50 mL / min. The heating program was from 30℃ to 800℃ at a heating rate of 10℃ / min. The temperature at which 5% mass loss occurred, T, was recorded. 5% The temperature T corresponding to the maximum rate of weight loss max and the residual mass fraction R at 700℃ 700 ;

[0068] Oxygen index flame retardant performance: The oxygen index was determined according to GB / T 2406.2-2009. The sample size was fixed at 80mm×10mm×3.0mm. The sample thickness was obtained by grinding the plate on both sides with equal amount of material. The edges were chamfered by 0.5mm and deburred. The room temperature test was carried out at (23±2)℃. The oxygen-nitrogen mixed gas was used to flow from bottom to top. The oxygen concentration was adjusted in steps of 0.2% (volume fraction). The lowest oxygen concentration that could maintain the sample burning continuously for 180s or the burning length reached 50mm was measured as the oxygen index.

[0069] Vertical combustion performance: The vertical combustion test was conducted according to GB / T 2408-2021. The sample size was fixed at 125mm×13mm×3.0mm, and the surface exposed to the flame was uniformly the surface of the board. The ignition condition was a 50W flame. Five samples were tested for each sample. The flaming time t1 and t2 after the first and second flame exposures were recorded, as well as whether dripping occurred and ignited the degreased cotton. The flammability rating was determined according to the standard.

[0070] Smoke density: Smoke density was determined using the single-chamber method according to GB / T 8323.2-2008. The sample size was fixed at 75mm×75mm×5.0mm, and the exposed surface was uniformly the surface of the board. The test was conducted under conditions with an ignition flame, and the radiant heat flux was fixed at 25kW / m². 2 Record the curve of smoke density change over time and read the maximum specific optical density Dm as the evaluation index;

[0071] Transmittance and haze: Transmittance and haze were determined according to GB / T 2410-2008. The sample size was fixed at 50mm×50mm×5.0mm. The haze meter method was used for testing. After instrument calibration, the total transmittance and haze were measured separately. Five samples were tested for each sample and the average was taken.

[0072] Tensile properties: Tensile properties were determined according to GB / T 1040.2-2022. Type 1A dumbbell specimens were used, with a gauge length of 50 mm and a thickness of 4.0 mm. The surface structure was preserved by grinding both sides with equal amount of material. The tensile speed was fixed at 50 mm / min. The tensile strength and elongation at break were recorded.

[0073] Weather resistance, yellowing, color difference, and optical integrity: Accelerated aging was performed using a xenon arc lamp according to GB / T 16422.2-2022, with a fixed irradiance of 0.55 W / m². 2 (340nm), blackboard temperature 63℃, chamber temperature 38℃, relative humidity 50%, the cycle program was fixed as 102min light exposure and 18min light exposure with water spraying, the cumulative aging time was 500h, and the exposed surface of the sample was uniformly the surface of the board; before aging and after 500h aging, the yellow index YI was determined according to GB / T 39822-2021 and the change value of yellow index ΔYI was calculated; according to the standard illuminant D65 and geometric conditions specified in GB / T 3978-2008 and the transmission mode chromaticity coordinates were determined according to GB / T 3979-2008 and the color difference ΔE was calculated.

[0074] Surface microcrack self-healing capability: Three strip specimens of 100mm×20mm×5.0mm were cut from each sample. Displacement was applied under a span of 80mm according to the three-point bending loading method of GB / T9341-2008, so that the strain of the outer surface fiber reached 2.0% and was held for 60s before unloading, thereby inducing observable surface microcracks. Immediately after unloading, 10 cracks were selected on each specimen using a 200x digital microscope to measure the initial crack width w0 and take the average. Then, the specimens were placed in a hot air circulation chamber and subjected to the heat treatment conditions of Example 2, which were 105℃ for 2h and 85℃ for 2h, and then naturally cooled to 25℃. The width of the repaired crack w1 was measured again at the same location and the crack closure rate η was calculated. c = (w0-w1) / w0×100%; Simultaneously, type 1A tensile specimens were machined along the crack zone on the same batch of specimens, and the tensile strength was retested at 50 mm / min according to GB / T 1040.2-2022, with the strength retention rate η as the metric. s The mechanical recovery after injury was evaluated; the test results are shown in Table 1.

[0075] Table 1 Summary of Performance Test Results

[0076] project Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 <![CDATA[T 5% (℃)]]> 271.6 268.4 262.7 279.3 273.1 267.5 268.2 266.8 <![CDATA[T max (℃)]]> 369.2 368.0 366.1 372.4 369.0 368.6 367.7 367.0 <![CDATA[R 700 (%)]]> 5.8 8.9 11.8 1.3 8.4 9.6 8.7 8.8 Oxygen index (%) 26.8 28.4 28.6 18.0 28.8 27.8 27.2 27.0 Vertical flammability rating V-1 V-0 V-0 HB V-0 V-1 V-1 V-2 Maximum specific optical density 305 220 265 720 255 345 415 455 Light transmittance (%) 91.7 91.9 90.8 92.4 91.6 90.9 90.5 90.2 Haze (%) 1.2 1.0 1.8 0.9 1.1 2.6 2.1 2.8 Tensile strength (MPa) 68.5 69.8 66.1 71.2 70.5 64.9 69.1 66.8 Elongation at break (%) 5.8 6.6 5.1 6.3 4.2 4.7 5.0 4.5 ΔYI@500h 1.35 0.98 1.62 1.08 1.80 1.55 1.42 1.66 ΔE@500h 1.82 1.30 2.10 1.65 2.61 2.35 2.05 2.48 <![CDATA[η c (%)]]> 74.0 88.5 83.0 90.2 12.5 60.5 70.2 65.0 <![CDATA[η s (%)]]> 82.5 92.0 87.8 93.5 56.1 74.8 80.2 77.2

[0077] Data Analysis:

[0078] As can be seen from the data in Examples 1-3 of Table 1, the flame-retardant and low-smoke acrylic sheets prepared by this invention show a synergistic improvement in flame retardancy, smoke suppression, and thermal decomposition residue. Simultaneously, the light transmittance and haze remain within the acceptable engineering range for transparent sheets, and good appearance stability is maintained even after weathering. This may be because: the reactive flame-retardant comonomer introduces phosphorus- and nitrogen-containing structures into the polymer backbone through copolymerization, promoting char formation and the release of inert components upon heating, thus weakening the continuous supply of combustible volatiles at the source; surface-grafted modified silica micro-nano particles further construct an inorganic barrier network, reducing the migration rate of thermal and decomposition products. Meanwhile, the bis(2-methacryloyloxyethyl) disulfide, under the dynamic exchange triggered by post-heat treatment, facilitates chain rearrangement and interface re-bonding, inhibiting the propagation of surface microcracks, thereby achieving both optical retention and mechanical stability, demonstrating the comprehensive performance improvement brought about by the coupling of materials and processes.

[0079] As can be seen from the data in Table 1 for Example 2 and Comparative Example 1, when no reactive flame-retardant comonomer was added in steps 6 and 7, the flame retardancy rating and smoke suppression performance significantly deteriorated, while the optical, mechanical, and self-healing capabilities did not deteriorate simultaneously. The main reason is that Comparative Example 1 lacked the introduction of a phosphorus- and nitrogen-containing copolymer, making it difficult to form a stable and dense char layer when heated. The combustion process relied more on the pyrolysis and volatilization of the polymer bulk, leading to sustained combustion and increased smoke production. However, because bis(2-methacryloyloxyethyl) disulfide and surface-grafted modified silica micro / nano particles were still present, the dynamic exchange of chain segments and interfacial compatibility remained, so self-healing and transparency did not decrease by the same extent. Therefore, it is evident that the flame-retardant comonomer and inorganic barrier / self-healing are not simply additive, but rather the key factors determining flame retardancy and low smoke levels.

[0080] As can be seen from the data in Table 1 for Example 2 and Comparative Example 2, when bis(2-methacryloyloxyethyl) disulfide is not added in steps 6 and 7, the closure of surface microcracks and mechanical recovery are significantly limited, and weather yellowing and color difference are more easily amplified. Even if the flame retardant related indicators are still at a high level, it is difficult to obtain the comprehensive optimal effect as in Example 2. The main reason is that without the dynamic exchange points of disulfide bonds, the post-heat treatment can only cause limited thermal relaxation. The stress concentration at the crack tip is difficult to be effectively passivated by chain segment rearrangement. Microcracks are more likely to expand under photothermal coupling and induce light scattering and appearance deterioration. At the same time, the re-bonding of the interface after repair is insufficient, resulting in limited strength recovery.

[0081] As can be seen from the data in Example 2 and Comparative Example 3 in Table 1, after replacing the surface-grafted modified silica micro-nano particles with an equal amount of unmodified fumed silica, the haze increased, the mechanical properties and weather resistance decreased, and the smoke suppression and combustion rating did not improve synchronously with the increase in residue, resulting in an anomaly of high residue but worse overall performance. The main reason is that the unmodified fumed silica has insufficient interfacial compatibility with the polymer matrix, and is prone to agglomeration during prepolymerization and casting curing, forming light scattering centers and stress concentration points. Agglomeration defects are more likely to form discontinuous pore channels during combustion, allowing heat and combustible decomposition products to escape more quickly, thereby weakening the barrier effect and increasing smoke production.

[0082] As can be seen from the data in Table 1 for Example 2 and Comparative Example 4, after canceling the sandwich spatial gradient structure formed by the staged casting in step 8 and instead mixing and casting the surface prepolymer slurry and core prepolymer slurry at one time, the flame retardant and smoke suppression performance decreased, and the optical haze and weather resistance deteriorated simultaneously. The main reason is that the gradient structure enables the functional phase containing flame retardant and inorganic particles to form a more continuous barrier and char-forming interface on the surface, preferentially establishing a thermal barrier on the surface and inhibiting dripping when exposed to fire; while one-time mixing dilutes the surface functional phase and spreads the particles throughout the thickness direction, which reduces the surface barrier efficiency and increases bulk scattering and accumulation of micro-defects after aging.

[0083] As can be seen from the data in Table 1 for Example 2 and Comparative Example 5, when the feeding sequence of adding bis(2-methacryloyloxyethyl) disulfide and surface-grafted modified silica micro-nano particles in step 7 is changed from the first-stage prepolymerization to a one-time addition at the beginning stage, the haze, smoke suppression, combustion rating, and self-healing performance all deteriorate to varying degrees. The main reason is that the segmented prepolymerization strategy increases the viscosity of the system in advance, which is beneficial to suppressing particle sedimentation and improving dispersion stability. At the same time, it is also beneficial to form a more uniform dynamic exchange point distribution of disulfide bond comonomers in the system. Conversely, one-time addition is more likely to cause particle agglomeration and uneven dynamic point distribution, resulting in local hard spots and stress concentration. After weathering, it is more likely to generate irreversible microcracks and amplify smoke production and appearance loss.

[0084] 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 flame-retardant, low-smoke acrylic sheet, comprising a core layer, and a first functional surface layer and a second functional surface layer respectively disposed on both sides of the core layer, characterized in that, The first functional table layer and the second functional table layer have the same composition; By weight, the first functional surface layer and the second functional surface layer are respectively prepared from the following raw materials: 175-185 parts methyl methacrylate, 15-25 parts reactive flame retardant comonomer, 3-5 parts bis(2-methacryloyloxyethyl) disulfide, 7-12 parts surface-grafted modified silica micro / nano particles, 0.2 parts dodecyl mercaptan, and 1.3-1.7 parts azobisisobutyronitrile; The core layer, by weight, is prepared from the following raw materials: 1180-1220 parts methyl methacrylate, 65-80 parts reactive flame-retardant comonomer, 4-8 parts bis(2-methacryloyloxyethyl) disulfide, 4-8 parts surface-grafted modified silica micro / nano particles, 0.35-0.45 parts dodecyl mercaptan, and 2.3-2.7 parts azobisisobutyronitrile; The reactive flame-retardant comonomer was prepared as follows: phenyl dichlorophosphate was dissolved in anhydrous tetrahydrofuran and cooled to 5°C under ice-water bath conditions. Diethylamine was added dropwise, followed by triethylamine to form a phosphorus-nitrogen intermediate. Then, 4-maleimide-based phenol was added, and the mixture was stirred at 5°C before being heated to 25°C to continue the reaction. After the reaction was completed, the mixture was filtered to remove salt, concentrated under reduced pressure, and then recrystallized and dried under vacuum to obtain the reactive flame-retardant comonomer. The weight ratio of phenyl dichlorophosphate, anhydrous tetrahydrofuran, diethylamine, triethylamine, and 4-maleimide-based phenol was 120:600:73:101:

100. The surface-grafted modified silica micro / nano particles are obtained by grafting trimethoxysilane graft groups onto fumed silica and then onto disulfide bonded acrylate monomers; the mass ratio of fumed silica to disulfide bonded acrylate monomers with trimethoxysilane graft groups is 50:8-12.

2. The flame-retardant, low-smoke acrylic sheet according to claim 1, characterized in that, The thickness of each functional surface layer is 280-320 μm; the total thickness of the flame-retardant low-smoke acrylic sheet is 5 mm.

3. The flame-retardant, low-smoke acrylic sheet according to claim 1, characterized in that, The 4-maleimide-based phenol is obtained by reacting p-aminophenol with maleic anhydride to produce 4-hydroxyphenylmaleamic acid, followed by reaction in the presence of acetic anhydride and anhydrous sodium acetate at a nitrogen protection temperature of 80°C, and then by water precipitation, filtration, washing and vacuum drying; the weight ratio of p-aminophenol to maleic anhydride is 109:98; the weight ratio of 4-hydroxyphenylmaleamic acid, acetic anhydride and anhydrous sodium acetate is 180:500:

50.

4. The flame-retardant, low-smoke acrylic sheet according to claim 1, characterized in that, The specific surface area of ​​the fumed silica is 180-220 m². 2 / g.

5. The flame-retardant, low-smoke acrylic sheet according to claim 1, characterized in that, The disulfide bond acrylate monomer of the trimethoxysilane graft group is obtained by a mercapto-olefin addition reaction of bis(2-methacryloyloxyethyl) disulfide and 3-mercaptopropyltrimethoxysilane; the weight ratio of bis(2-methacryloyloxyethyl) disulfide to 3-mercaptopropyltrimethoxysilane is 60:

35.

6. A method for preparing a flame-retardant, low-smoke acrylic sheet according to any one of claims 1-5, characterized in that, Includes the following steps: S1, methyl methacrylate, reactive flame retardant comonomer, bis(2-methacryloyloxyethyl) disulfide, surface-grafted modified silica micro-nano particles, dodecyl mercaptan and azobisisobutyronitrile are mixed, deoxygenated and prepolymerized according to the raw material ratio of the first functional surface layer and the second functional surface layer, respectively, to obtain the first functional surface layer prepolymer slurry and the second functional surface layer prepolymer slurry. S2, methyl methacrylate is deoxygenated and prepolymerized with reactive flame retardant comonomer, and then bis(2-methacryloyloxyethyl) disulfide, surface-grafted modified silica micro-nano particles and dodecyl mercaptan are added, dispersed and degassed to obtain core layer prepolymer slurry. S3, the first functional surface prepolymer slurry, the core prepolymer slurry and the second functional surface prepolymer slurry are poured and molded in sequence using a staged pouring method, followed by degassing and curing, demolding and post-heat treatment to obtain the flame-retardant low-smoke acrylic sheet.

7. The method for preparing flame-retardant low-smoke acrylic sheet according to claim 6, characterized in that, In step S1, the prepolymerization is carried out at 70°C with stirring for 35-45 minutes; in step S2, the prepolymerization is carried out at 70°C for 1.3-1.7 hours.

8. The method for preparing flame-retardant low-smoke acrylic sheet according to claim 6, characterized in that, In step S3, the weight ratio of the first functional surface layer prepolymer slurry, the core layer prepolymer slurry, and the second functional surface layer prepolymer slurry is 95-105:1280-1295:100-110.

9. The method for preparing flame-retardant low-smoke acrylic sheet according to claim 6, characterized in that, In step S3, the curing process involves sequentially holding the product at 50°C for 2 hours, at 65°C for 4 hours, at 80°C for 4 hours, at 100°C for 2 hours, and at 120°C for 1 hour.

10. The method for preparing flame-retardant, low-smoke acrylic sheet according to claim 6, characterized in that, In step S3, the post-heat treatment involves first holding at 105°C for 2 hours, and then holding at 85°C for 2 hours.