A flame-retardant PMMA sheet and its preparation method

Abstract

The present invention discloses a flame-retardant PMMA sheet and a preparation method thereof, belonging to the technical field of acrylic sheets. The raw materials include the following parts by weight: 70-80 parts of methyl methacrylate, 8-12 parts of methyl acrylate, 5-15 parts of a synergistic flame retardant, 10-18 parts of modified nano-silica, 0.2-0.4 part of an initiator, and 3-5 parts of a demolding agent. Among them, the modified nano-silica has strong surface hydrophobicity and better compatibility with the PMMA matrix, greatly enhancing the wear resistance and mechanical properties of the PMMA sheet; among them, the synergistic flame retardant can enhance the mechanical properties of the PMMA sheet. In addition, the molecule contains three elements of silicon, nitrogen, and phosphorus, and the three can play a synergistic role, significantly improving the flame retardancy of the sheet and having good stability; therefore, the PMMA sheet prepared by the present invention has good wear resistance, excellent mechanical properties, and also has stable and efficient flame retardancy, and has important application value in the technical field of acrylic sheets.

Description

Technical Field

[0001] The present invention belongs to the technical field of acrylic plates, and specifically, relates to a flame-retardant PMMA plate and a preparation method thereof. Background Art

[0002] Polymethyl methacrylate (PMMA), commonly known as plexiglass or acrylic, is an amorphous plastic copolymerized from methyl methacrylate (MMA) monomers and a small amount of acrylate esters. It has high transparency, a light transmittance of up to 92%, good optical properties, strong corrosion resistance, and good resistance to ultraviolet rays and weather, making it suitable for outdoor use and capable of maintaining transparency for a long time. It is often used as a substitute for glass and has the reputation of "plastic glass". Due to its excellent properties, it is widely used in various fields. Especially in construction, PMMA plates are used for making doors, windows, partitions, lenses, etc. because of their high light transmittance and impact strength, which meet the indoor lighting and safety requirements.

[0003] With the expansion of the field of material applications and the continuous improvement of people's requirements, PMMA materials also face the requirement of flame retardancy. However, due to the poor thermal stability of PMMA, it is extremely easy to burn, and the limiting oxygen index of pure PMMA is only 17, which greatly restricts its application in more fields. By adding flame retardants, the flame retardant performance of PMMA materials can be effectively improved. However, most traditional organic flame retardants are small molecules and are prone to migration and exudation in PMMA materials, affecting the flame retardant performance of the materials; while inorganic flame retardants have poor compatibility with PMMA materials, and adding a large amount will affect the mechanical properties of the materials; moreover, the surface wear resistance of PMMA plates is generally average, and scratches are generated during installation and use, affecting the beauty and light transmittance, so it cannot meet the existing market demand. Therefore, it is urgent to solve the above problems to meet the higher requirements in the technical field of acrylic plates. Summary of the Invention

[0004] The purpose of the present invention is to overcome the defects of the prior art and provide a flame-retardant PMMA plate and a preparation method thereof.

[0005] The purpose of the present invention can be achieved by the following technical solutions:

[0006] A preparation method of a flame-retardant PMMA plate, comprising the following steps:

[0007] First, methyl methacrylate, methyl acrylate, a synergistic flame retardant, and modified nano-silica are added to a reaction kettle, then an initiator and a demolding agent are added. Stirring is started and the temperature is raised. When the raw materials in the reaction kettle are fully mixed and boiling, keep the temperature for 20 - 40 min, stop heating, let it stand and cool to obtain a slurry; inject the slurry into a sealed glass mold, remove the gas, heat to carry out a polymerization reaction. After the reaction is completed, perform annealing treatment in an oven. After cooling to room temperature, open the mold, and finally polish to obtain a flame-retardant PMMA sheet.

[0008] Further, the amounts of each raw material are as follows by weight: 70 - 80 parts of methyl methacrylate, 8 - 12 parts of methyl acrylate, 5 - 15 parts of synergistic flame retardant, 10 - 18 parts of modified nano-silica, 0.2 - 0.4 parts of initiator, and 3 - 5 parts of demolding agent.

[0009] Further, the initiator is one of benzoyl peroxide and lauroyl peroxide.

[0010] Further, the demolding agent is one of zinc stearate, magnesium stearate, and montan wax.

[0011] Further, the modified nano-silica is prepared by the following steps:

[0012] Disperse nano-silica in a three-necked flask containing an ethanol aqueous solution. After ultrasonic treatment for 10 min, add γ-methacryloxypropyltrimethoxysilane (silane coupling agent KH-570), gradually raise the temperature to 60 °C, and react under mechanical stirring for 5 h. After the reaction is completed, cool to room temperature, filter by suction, take the filter residue, wash it with absolute ethanol 3 - 4 times, and place it in a vacuum oven for drying to obtain modified nano-silica.

[0013] Further, the dosage ratio of the nano-silica, ethanol aqueous solution, and γ-methacryloxypropyltrimethoxysilane is 1 g:100 mL:3.4 g.

[0014] There are many -OH groups on the surface of nano-silica. Reacting with the silane coupling agent KH-570 can introduce unsaturated carbon-carbon double bonds: Through this reaction, the hydrophobicity of the surface of nano-silica can be improved, the compatibility between nano-silica and the PMMA matrix can be enhanced, so that nano-silica is fully dispersed in the PMMA matrix, enabling the performance of nano-silica to be fully exerted, and greatly enhancing the wear resistance and mechanical properties of the matrix.

[0015] Further, the synergistic flame retardant is prepared by the following steps:

[0016] S1. In a four-necked flask equipped with a constant-pressure dropping funnel, a stirrer, a thermometer, nitrogen inlets and outlets, a reflux condenser and a drying tube (connected to a hydrogen chloride absorption device), add pentaerythritol and tetrahydrofuran. Introduce nitrogen as a protective gas, turn on the stirrer, slowly heat up to 60 °C, and dropwise add phosphorus oxychloride. The reaction starts, and a large amount of gas is released. The reaction system gradually changes from a white turbid liquid to transparent, and then gradually becomes turbid again until the addition of phosphorus oxychloride is completed. Then heat up to 70 °C and reflux for 4 h. After the reaction is completed, remove the solvent by vacuum distillation, wash successively with absolute ethanol and chloroform, and dry to obtain intermediate 1. The dosage ratio of pentaerythritol, tetrahydrofuran, and phosphorus oxychloride is 15.1 g: 100 mL: 14.3 g;

[0017] Pentaerythritol reacts with phosphorus oxychloride. By controlling the molar ratio of the two to be close to 1:1 and with pentaerythritol in excess, intermediate 1 is obtained;

[0018] ;

[0019] S2. At room temperature, introduce nitrogen into a four-necked flask equipped with a stirrer, a thermometer and a fractionation device to remove the air in the device. Then add intermediate 1 and 3-aminopropyltrimethoxysilane. Raise the temperature of the reaction device to 140 °C and carry out fractionation reaction for 2 h. Then raise the temperature to 160 °C and carry out fractionation reaction for 6 h. Keep a slight vacuum in the flask during the reaction to separate methanol. After the reaction is completed, cool to room temperature, add absolute ethanol and stir to disperse, then filter and dry to obtain intermediate 2. The dosage ratio of intermediate 1 and 3-aminopropyltrimethoxysilane is 17.9 g: 53.9 g;

[0020] Using the solvent-free method, intermediate 1 reacts with 3-aminopropyltrimethoxysilane. By controlling the molar ratio of the two to be close to 3:1, intermediate 2 is obtained;

[0021] ;

[0022] S3. In a three-necked flask equipped with a thermometer and a stirring device, mix intermediate 2 and N,N-dimethylformamide (DMF) and turn on the stirrer. Add potassium carbonate (acid-binding agent) and 1-chlorononane in sequence, control the reaction temperature at 65 °C, and keep the reaction for 4 h. After the reaction is completed, filter, remove the solvent by vacuum distillation, and dry under vacuum to obtain intermediate 3. The dosage ratio of intermediate 2, N,N-dimethylformamide, potassium carbonate, and 1-chlorononane is 62.3 g: 150 mL: 13.8 g: 16.2 g;

[0023] Potassium carbonate removes the hydrogen chloride generated in the reaction and accelerates the reaction. Intermediate 2 and 1-chlorononane undergo nucleophilic substitution to obtain intermediate 3. The specific reaction is as follows:

[0024] ;

[0025] S4. In a three-necked flask equipped with a thermometer and a stirring device, mix intermediate 3, allyl chloride, potassium carbonate, and N,N-dimethylformamide evenly and stir. Control the reaction temperature at 70 °C and keep the reaction for 6 h. After the reaction is completed, filter, and then purify by column chromatography (the eluent uses a mixed solvent of petroleum ether / ethyl acetate, and the volume ratio of the two is 5:1). Rotate and evaporate to remove the eluent to obtain the synergistic flame retardant; the dosage ratio of intermediate 3, allyl chloride, potassium carbonate, and N,N-dimethylformamide is 74.9 g:7.6 g:13.8 g:150 mL;

[0026] Under the catalysis of potassium carbonate, intermediate 3 and allyl chloride undergo nucleophilic substitution to obtain the synergistic flame retardant; the structure of the synergistic flame retardant is shown as follows:

[0027] ;

[0028] The prepared synergistic flame retardant molecule contains silicon element, nitrogen element, cage-like phosphonate, long carbon chain, and carbon-carbon double bond structure. Among them, the silicon element will form -Si-O- or -Si-C- structure at high temperature. This structure can play a role in heat insulation and flame retardancy on the material surface, prevent the escape of combustion decomposition products, and inhibit the thermal decomposition of polymer materials, promoting charring. The introduced nitrogen element will decompose when heated during combustion, releasing non-combustible gases such as nitrogen, ammonia, and water vapor. These non-combustible gases can not only dilute the concentration of oxygen and combustible gases in the air but also react with oxygen in the air to generate nitrogen, water, and deep oxides, thereby consuming the oxygen on the material surface to achieve a good flame retardant effect. Moreover, the introduced cage-like phosphonate structure is a phosphorus-based flame retardant with strong stability. It will promote the formation of a carbon layer and inhibit smoke release during combustion, and can cooperate with silicon and nitrogen elements to significantly improve the flame retardant performance of the matrix. In addition, the introduced long carbon chain belongs to methylene chain segments, which are flexible chain segments, and can also improve the mechanical properties of the matrix, and can penetrate into the macromolecular chains of the matrix to enhance stability. Finally, the introduced carbon-carbon double bond can crosslink with the PMMA matrix under the action of an initiator, making the small molecule synergistic flame retardant not easy to migrate and exude in the matrix, further improving the stability of the synergistic flame retardant.

[0029] Advantages of the present invention:

[0030] 1. The PMMA sheet prepared by the present invention adds modified nano-silica to the raw materials. Compared with ordinary nano-silica, it has stronger surface hydrophobicity and better compatibility with the PMMA matrix, greatly enhancing the wear resistance and mechanical properties of the PMMA sheet;

[0031] 2. The prepared synergistic flame retardant molecule contains three elements: silicon, nitrogen, and phosphorus. The three elements can play a synergistic role, significantly improving the flame retardancy of PMMA plates. In addition, it can also enhance the mechanical properties of PMMA plates and has good stability.

[0032] Therefore, the PMMA plates prepared by the present invention have good wear resistance, excellent mechanical properties, and also have stable and efficient flame retardancy, and have important application value in the field of acrylic plate technology. Specific Embodiments

[0033] The following will combine the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

[0034] Example 1

[0035] Preparation of synergistic flame retardant:

[0036] S1. In a four-necked flask equipped with a constant pressure dropping funnel, a stirrer, a thermometer, a nitrogen inlet and outlet, a reflux condenser and a drying tube (connected to a hydrogen chloride absorption device), add 15.1 g of pentaerythritol and 100 mL of tetrahydrofuran, introduce nitrogen as a protective gas, and start stirring. Slowly heat up to 60 °C, and dropwise add 14.3 g of phosphorus oxychloride. The reaction starts, and a large amount of gas is released. The reaction system gradually changes from a white turbid liquid to transparent, and then gradually becomes turbid until the addition of phosphorus oxychloride is completed. Heat up to 70 °C and reflux for 4 h. After the reaction is completed, remove the solvent by vacuum distillation, wash successively with anhydrous ethanol and chloroform, and dry to obtain Intermediate 1.

[0037] S2. At room temperature, introduce nitrogen into a four-necked flask equipped with a stirrer, a thermometer and a fractionation device to remove the air in the device. Then add 17.9 g of Intermediate 1 and 53.9 g of 3-aminopropyltrimethoxysilane. Raise the temperature of the reaction device to 140 °C and carry out a fractionation reaction for 2 h, then raise the temperature to 160 °C and carry out a fractionation reaction for 6 h. Keep the inside of the bottle slightly vacuum during the reaction to separate methanol. After the reaction is completed, cool to room temperature, add anhydrous ethanol and stir to disperse, and then filter and dry to obtain Intermediate 2.

[0038] S3. In a three-necked flask equipped with a thermometer and a stirring device, mix 62.3 g of Intermediate 2 and 150 mL of N,N-dimethylformamide and start the stirrer. Add 13.8 g of potassium carbonate and 16.2 g of 1-chlorononane in sequence, control the reaction temperature at 65 °C, and keep the reaction for 4 h. After the reaction is completed, filter, remove the solvent by vacuum distillation, and dry under vacuum to obtain Intermediate 3.

[0039] S4. In a three-necked flask equipped with a thermometer and a stirring device, 74.9 g of Intermediate 3, 7.6 g of allyl chloride, 13.8 g of potassium carbonate and 150 mL of N,N-dimethylformamide were mixed and stirred evenly. The reaction temperature was controlled at 70 °C and the reaction was carried out for 6 h with heat preservation. After the reaction was completed, filtration was carried out, and then purification was carried out by column chromatography (the eluent was a mixed solvent of petroleum ether / ethyl acetate, and the volume ratio of the two was 5:1). The eluent was removed by rotary evaporation to obtain the synergistic flame retardant.

[0040] Example 2

[0041] Preparation of synergistic flame retardant:

[0042] S1. In a four-necked flask equipped with a constant-pressure dropping funnel, a stirrer, a thermometer, a nitrogen inlet and outlet, a reflux condenser and a drying tube (connected to a hydrogen chloride absorption device), 30.2 g of pentaerythritol and 200 mL of tetrahydrofuran were added. Nitrogen was introduced as a protective gas, and stirring was started. The temperature was slowly raised to 60 °C, and 28.6 g of phosphorus oxychloride was added dropwise. The reaction started, and a large amount of gas was released. The reaction system gradually changed from a white turbid liquid to transparent and then gradually became turbid again until the addition of phosphorus oxychloride was completed. The temperature was raised to 70 °C and the reflux reaction was carried out for 4 h. After the reaction was completed, the solvent was removed by vacuum distillation, and it was washed successively with absolute ethanol and chloroform and dried to obtain Intermediate 1;

[0043] S2. At room temperature, nitrogen was introduced into a four-necked flask equipped with a stirrer, a thermometer and a fractionating device to remove the air in the device. Then 35.8 g of Intermediate 1 and 107.8 g of 3-aminopropyltrimethoxysilane were added. The temperature of the reaction device was raised to 140 °C and the fractionation reaction was carried out for 2 h, and then the temperature was raised to 160 °C and the fractionation reaction was carried out for 6 h. During the reaction, a slight vacuum was maintained in the flask to separate methanol. After the reaction was completed, the temperature was lowered to room temperature, absolute ethanol was added and stirred to disperse, and then filtration and drying were carried out to obtain Intermediate 2;

[0044] S3. In a three-necked flask equipped with a thermometer and a stirring device, 124.6 g of Intermediate 2 and 300 mL of N,N-dimethylformamide were mixed and the stirrer was started. 27.6 g of potassium carbonate and 32.4 g of 1-chlorononane were added successively. The reaction temperature was controlled at 65 °C and the reaction was carried out for 4 h with heat preservation. After the reaction was completed, filtration was carried out, the solvent was removed by vacuum distillation, and vacuum drying was carried out to obtain Intermediate 3;

[0045] S4. In a three-necked flask equipped with a thermometer and a stirring device, 149.8 g of Intermediate 3, 15.2 g of allyl chloride, 27.6 g of potassium carbonate and 300 mL of N,N-dimethylformamide were mixed and stirred evenly. The reaction temperature was controlled at 70 °C and the reaction was carried out for 6 h with heat preservation. After the reaction was completed, filtration was carried out, and then purification was carried out by column chromatography (the eluent was a mixed solvent of petroleum ether / ethyl acetate, and the volume ratio of the two was 5:1). The eluent was removed by rotary evaporation to obtain the synergistic flame retardant.

[0046] Example III

[0047] Prepare modified nano-silica:

[0048] Disperse 1 g of nano-silica in a three-necked flask containing 100 mL of ethanol aqueous solution. After ultrasonic treatment for 10 min, add 3.4 g of γ-methacryloxypropyltrimethoxysilane, gradually heat up to 60 °C, and react under mechanical stirring for 5 h. After the reaction is completed, cool to room temperature, filter by suction, take the filter residue, wash it 3 times with absolute ethanol, place it in a vacuum oven for drying to obtain modified nano-silica.

[0049] Example IV

[0050] First, add 70 g of methyl methacrylate, 8 g of methyl acrylate, 5 g of the synergistic flame retardant prepared in Example I and 10 g of the modified nano-silica prepared in Example III to the reaction kettle, then add 0.2 g of dibenzoyl peroxide and 3 g of zinc stearate, start stirring and heating. When the raw materials in the reaction kettle are fully mixed and boiling, keep warm for 20 min, stop heating, let it stand and cool to obtain a slurry; inject the slurry into a sealed glass mold, remove the gas, heat to 65 °C, carry out polymerization reaction for 4 h, after the reaction is completed, carry out annealing treatment in an oven at 80 °C for 2 h, cool to room temperature and then open the mold, and finally polish to obtain a flame-retardant PMMA sheet.

[0051] Example V

[0052] First, add 75 g of methyl methacrylate, 10 g of methyl acrylate, 10 g of the synergistic flame retardant prepared in Example II and 14 g of the modified nano-silica prepared in Example III to the reaction kettle, then add 0.3 g of lauroyl peroxide and 4 g of magnesium stearate, start stirring and heating. When the raw materials in the reaction kettle are fully mixed and boiling, keep warm for 30 min, stop heating, let it stand and cool to obtain a slurry; inject the slurry into a sealed glass mold, remove the gas, heat to 65 °C, carry out polymerization reaction for 4 h, after the reaction is completed, carry out annealing treatment in an oven at 80 °C for 3 h, cool to room temperature and then open the mold, and finally polish to obtain a flame-retardant PMMA sheet.

[0053] Example VI

[0054] First, add 80 g of methyl methacrylate, 12 g of methyl acrylate, 15 g of the synergistic flame retardant prepared in Example 2, and 18 g of the modified nano-silica prepared in Example 3 into the reaction kettle. Then add 0.4 g of lauroyl peroxide and 5 g of montan wax. Start stirring and heating. When the raw materials in the reaction kettle are fully mixed and boiling, keep the temperature for 40 min, stop heating, let it stand and cool to obtain a slurry. Inject the slurry into a sealed glass mold, remove the gas, heat to 65 °C, carry out a polymerization reaction for 4 h. After the reaction is completed, carry out an annealing treatment in an oven at 80 °C for 3 h. After cooling to room temperature, open the mold, and finally polish to obtain a flame-retardant PMMA sheet.

[0055] Comparative Example 1

[0056] Replace the synergistic flame retardant in Example 6 with a commercially available triphenyl phosphate flame retardant, and the remaining steps are the same as in Example 6.

[0057] Comparative Example 2

[0058] Use commercially available PMMA.

[0059] The light transmittance was measured according to the national standard GB / T 2410-2008 "Determination of Light Transmittance and Haze of Transparent Plastics".

[0060] The wear resistance was determined according to the national standard GB 3960 "Test Method for Sliding Friction and Wear of Plastics".

[0061] The impact strength was determined according to the national standard GB / T 1043.1-2008 "Determination of Izod Impact Strength of Plastics - Part 1: Non-Instrumented Impact Test".

[0062] The limiting oxygen index of the specimens was determined according to the national standard GB / T 2406-2008 "Test Method for Flammability of Plastics". After standing at room temperature for 200 days for Examples 4, 5, 6 and Comparative Example 1, the limiting oxygen index was measured again.

[0063] The measured results are shown in the following table:

[0064]

[0065] As can be seen from the above table, the PMMA sheet prepared in the examples of the present invention has excellent wear resistance. And because the synergistic flame retardant is added in the examples, its mechanical properties and flame retardancy are higher than those of the comparative examples, and the performance is stable, which has important application value in the technical field of acrylic sheets.

[0066] In the description of the specification, the descriptions referring to terms such as "one embodiment", "example", "specific example", etc. mean that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner.

[0067] The above content is only an illustration and description of the present invention. Those skilled in the art of this technology can make various modifications or supplements to the described specific embodiments or use similar ways to replace them. As long as they do not deviate from the invention or exceed the scope defined by the claims of the present invention, they should fall within the protection scope of the present invention.

Claims

1. A method for preparing a flame retardant PMMA sheet, characterized in that: The following steps are involved: First, methyl methacrylate, methyl acrylate, synergistic flame retardant and modified nano-silica are added to a reactor, and then an initiator and a release agent are added, stirring is started and the temperature is increased. When the raw materials in the reactor are fully mixed and boiled, the temperature is kept for 20-40 minutes, and the heating is stopped, and the mixture is allowed to stand and cool to obtain a slurry. The slurry is injected into a sealed glass mold, gas is removed, and a polymerization reaction is generated by heating. After the reaction is completed, an annealing treatment is performed in an oven, and the mold is opened after cooling to room temperature. Finally, the slurry is ground and polished to obtain a flame-retardant PMMA sheet. Wherein, the synergistic flame retardant is prepared by the following steps: S1. Add pentaerythritol and tetrahydrofuran into a four-necked flask, introduce nitrogen and start stirring, heat to 60°C, add phosphorus oxychloride dropwise, start the reaction, and heat to 70°C until the addition of phosphorus oxychloride is complete. Reflux for 4 hours. After the reaction is complete, distill under reduced pressure, wash, and dry to obtain intermediate 1; the ratio of pentaerythritol, tetrahydrofuran, and phosphorus oxychloride is 15.1 g:100 mL:14.3 g; S2. At room temperature, nitrogen was introduced into a four-necked flask, and then intermediate 1 and 3-aminopropyltrimethoxysilane were added. After fractionation reaction at 140°C for 2 hours, the temperature was raised to 160°C and fractionation reaction was performed for 6 hours. During the reaction, a slight vacuum was maintained in the flask. After the reaction was completed, the temperature was lowered to room temperature, anhydrous ethanol was added, stirred and dispersed, and then filtered and dried to obtain intermediate 2. The ratio of intermediate 1 to 3-aminopropyltrimethoxysilane was 17.9 g:53.9 g. S3, intermediate 2 and N, N-dimethylformamide were mixed and a stirrer was turned on, potassium carbonate and 1-chlorononane were added in sequence, and the mixture was kept warm for 4 hours at 65°C. After the reaction was completed, the mixture was filtered, distilled under reduced pressure, and dried in vacuo to obtain intermediate 3; the ratio of intermediate 2, N, N-dimethylformamide, potassium carbonate, and 1-chlorononane was 62.3 g: 150 mL: 13.8 g: 16.2 g; S4. Mix and stir the intermediate 3, allyl chloride, potassium carbonate and N,N-dimethylformamide evenly, and keep the mixture at 70°C for 6 hours. After the reaction, filter, purify by column chromatography, and rotary evaporate to obtain a synergistic flame retardant. The ratio of the intermediate 3, allyl chloride, potassium carbonate and N,N-dimethylformamide is 74.9 g:7.6 g:13.8 g:150 mL.

2. The method for preparing a flame retardant PMMA sheet according to claim 1, characterized in that: The raw materials are calculated in parts by weight as follows: 70-80 parts of methyl methacrylate, 8-12 parts of methyl acrylate, 5-15 parts of synergistic flame retardant, 10-18 parts of modified nano silicon dioxide, 0.2-0.4 parts of initiator, and 3-5 parts of release agent.

3. The method for preparing a flame-retardant PMMA sheet according to claim 1, characterized in that: The modified nano silicon dioxide is prepared by the following steps: Nano-silica was dispersed in a three-necked flask filled with ethanol aqueous solution. After ultrasonic treatment for 10 minutes, γ-methacryloxypropyltrimethoxysilane was added, and the temperature was gradually raised to 60°C. The mixture was reacted for 5 hours under mechanical stirring. After the reaction was completed, the mixture was cooled to room temperature, filtered, and the residue was washed and dried to obtain modified nano-silica.

4. A flame retardant PMMA sheet, characterized in that: Prepared according to the method according to any one of claims 1 to 3.

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