High-transmittance flame-retardant PMMA (polymethyl methacrylate) material and preparation method thereof

Abstract

The invention provides a high-transparency flame-retardant PMMA (polymethyl methacrylate) material, and belongs to the technical field of high polymer materials. A preparation method of the high-transparency flame-retardant PMMA material comprises the following steps: firstly, synthesizing a nitrogen-phosphorus synergistic flame-retardant monomer; the nitrogen and phosphorus synergistic flame-retardant monomer is a micromolecular derivative which is bifunctionalized by a phosphite group and a methacrylate group and is rich in nitrogen and phosphorus flame-retardant elements; then, carrying out copolymerization reaction on the nitrogen-phosphorus synergistic flame-retardant monomer and a methyl methacrylate monomer to obtain a polymer solution or slurry; and finally, carrying out devolatilization, molding and other subsequent treatments to obtain the high-transparency flame-retardant PMMA material. A flame-retardant structure containing phosphorus and nitrogen elements is introduced into a PMMA molecular chain through a covalent bond, so that the flame-retardant property of PMMA is effectively improved, meanwhile, high transparency of the material is kept, and the flame-retardant effect has a long-term effect.

Description

Technical Field

[0001] The present invention belongs to the technical field of polymer materials, and particularly relates to a flame-retardant modified polymethyl methacrylate (PMMA) material, and more particularly to a PMMA copolymer having both high transparency and flame retardancy, its preparation method, and an intermediate flame-retardant monomer thereof. Background Art

[0002] Polymethyl methacrylate (PMMA), commonly known as plexiglass, has excellent optical transparency, good weather resistance, easy processing and molding, and relatively high mechanical strength and surface hardness. Therefore, it is widely used in many fields such as building lighting, automotive lamp housings, aviation transparent parts, optical instruments, display panels, advertising signs, and daily necessities.

[0003] However, PMMA is a flammable material with a limiting oxygen index of only about 17%. When burning, it is easy to melt and drip, and releases a large amount of heat and smoke, posing serious safety hazards. With the increasingly strict safety regulations and the expansion of application fields (such as crowded public transportation, electronic and electrical equipment casings, building materials, etc.), higher and higher requirements are put forward for the flame-retardant performance of PMMA, usually requiring it to reach the UL94 V-0 or V-1 level.

[0004] Traditional methods for flame-retardant modification of PMMA mainly involve physically blending with flame retardants. Commonly used flame retardants include halogenated flame retardants (such as brominated flame retardants) and non-halogenated flame retardants (such as phosphorus-based flame retardants, nitrogen-based flame retardants, silicon-based flame retardants, inorganic flame retardants, etc.). Although halogenated flame retardants are highly efficient, they release toxic and harmful gases and smoke during combustion, which are harmful to the environment and human health, and their use has been gradually restricted. Inorganic flame retardants (such as aluminum hydroxide and magnesium hydroxide) need to be added in large amounts to achieve effective flame retardancy, which will cause a sharp decrease in the transparency of PMMA and deterioration of mechanical properties. Organic phosphorus-based flame retardants (such as phosphates) are commonly used non-halogenated flame retardants for PMMA, but when used as additives, they have problems such as easy migration and volatilization, resulting in non-persistent flame-retardant effects, and they will also significantly reduce the glass transition temperature Tg, mechanical strength, and transparency of PMMA at high addition amounts.

[0005] To solve the problems of migration and compatibility of additive flame retardants, the development of reactive flame retardants has become an important research direction. Reactive flame retardants are bonded to the polymer molecular chain through chemical bonds, which can achieve long-term flame-retardant effects and reduce the adverse effects on material properties. For example, Patent CN113698527B discloses a phosphorus-containing reactive flame-retardant monomer and a method for copolymerizing it with MMA to prepare transparent flame-retardant PMMA. By introducing a phosphorus-containing group into the side chain, a V-0 level of flame retardancy and a light transmittance of about 92% are achieved. However, the flame-retardant structural unit used in this patent is different from that of the present invention.

[0006] Cyclotriphosphazene (such as hexachlorocyclotriphosphazene, HCCP) is an inorganic cyclic compound containing phosphorus and nitrogen elements, with high thermal stability, inherent flame retardancy, and easy functional modification characteristics. By introducing different organic functional groups on its P atoms, a variety of cyclotriphosphazene derivatives can be prepared and used as high-performance flame retardants in various polymers. Introducing the cyclotriphosphazene structure into PMMA is expected to achieve efficient flame retardancy by utilizing the P-N synergistic effect.

[0007] However, how to design and synthesize a new type of nitrogen-phosphorus synergistic reactive flame retardant monomer that can effectively copolymerize with MMA and maximize the high transparency and mechanical properties of PMMA while achieving efficient flame retardancy remains a challenge in this field. In particular, introducing multiple functional groups (such as reactive groups and other groups to improve flame retardancy efficiency) into the cyclotriphosphazene ring simultaneously and studying their effects on the final properties of PMMA have important research and application values. Summary of the Invention

[0008] The present invention aims to overcome the deficiencies of the prior art and provides a new type of nitrogen-phosphorus synergistic flame retardant monomer, as well as a PMMA material with both high transparency and long-term flame retardant performance prepared using this monomer and its preparation method. The goal is to significantly improve its flame retardancy grade on the basis of maintaining the original excellent optical and processing properties of PMMA to meet broader application requirements.

[0009] To achieve the above object, the present invention provides the following technical solutions:

[0010] A nitrogen-phosphorus synergistic flame retardant monomer, the general formula of which can be expressed as N3P3R1(R2) x Cl y , where N3P3 represents a matrix rich in nitrogen and phosphorus flame retardant elements, R1 is the -O-CH2CH2-O-C(O)C(CH3)=CH2 group, providing a copolymerization reaction site; R2 is a phosphite group, which mainly contributes to the flame retardant performance, x is an integer from 1 to 5, representing the number of phosphite (or phosphonate) groups; y is an integer from 0 to 4, representing the number of remaining chlorine atoms; and x + y = 5, that is, there is a total of 1 R1 group, x R2 groups and y chlorine atoms on the ring. Preferably, x is from 2 to 4 and y is from 1 to 3, which means that in addition to a methacrylate reaction group, there are multiple phosphonate flame retardant groups.

[0011] A preparation method of a nitrogen-phosphorus synergistic flame retardant monomer. This method utilizes the difference in the reactivity of hexachlorocyclotriphosphazene (HCCP), and by controlling the reaction conditions, the R1 and R2 groups are introduced stepwise or selectively. For example, HCCP can first react with stoichiometric HEMA in the presence of a base (such as NaH, K2CO3 or triethylamine) at low temperature to preferentially form a mono-HEMA substituted product N3P3(R1)(Cl)5; then, this intermediate is reacted with x equivalents of dialkyl phosphite (such as P(O)(OMe)2H or P(O)(OEt)2H) under the catalytic conditions of a suitable catalyst (such as a palladium catalyst, referring to the P-P bond coupling or base in the feasibility report), so that the phosphite group is connected to the phosphazene ring through a P-P bond or a similar manner, replacing some or all of the remaining chlorine atoms to obtain the target product. The reaction conditions such as solvent, temperature, time, etc. need to be optimized according to the specific substituents and the target degree of substitution.

[0012] A highly transparent flame retardant PMMA material is prepared by free radical copolymerization of methyl methacrylate (MMA) monomer and the above-mentioned nitrogen-phosphorus synergistic flame retardant monomer. The flame retardant monomer units are randomly distributed in the PMMA molecular chain in the form of covalent bonds. The content of the flame retardant monomer can be adjusted according to the required flame retardant level and transparency requirements, usually between 1% and 30% by mass, preferably 5% to 20%.

[0013] A preparation method of a highly transparent flame retardant PMMA material. This method adopts the free radical polymerization mechanism. MMA monomer, the above-mentioned nitrogen-phosphorus synergistic flame retardant monomer, an initiator (such as AIBN, BPO or tert-butyl peroxide) and an optional chain transfer agent (such as n-octyl mercaptan, dodecyl mercaptan) are mixed and then polymerized; the polymerization method can be bulk polymerization (such as casting method or continuous bulk polymerization combined with devolatilization, extrusion granulation), solution polymerization or suspension polymerization. For bulk polymerization, a two-step method of prepolymerization-curing polymerization can be adopted, or polymerization can be carried out in a continuous reactor. The reaction temperature, the dosages of the initiator and the chain transfer agent need to be selected and optimized according to the selected polymerization method and the target molecular weight.

[0014] Beneficial effects:

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] (1) Highly effective flame retardancy and high transparency: By introducing flame retardant groups containing phosphorus (from phosphazene ring and phosphite / phosphonate groups) and nitrogen (from phosphazene ring) elements into the PMMA molecular chain through covalent bonds, a highly effective PN (and possibly Cl, if y>0) synergistic flame retardant effect is achieved; since the flame retardant units are copolymerized onto the molecular chain, the serious decrease in transparency caused by compatibility issues when adding flame retardants at high addition levels is avoided, so that the material can achieve a higher flame retardant level (UL94V-0 or V-1) while still maintaining excellent light transmittance (visible light transmittance ≥ 90%).

[0017] (2) Long-lasting flame retardancy: The flame retardant groups are permanently fixed to the polymer chain through chemical bonds, overcoming the problem of added flame retardants being easy to migrate and volatilize, causing the flame retardant properties to decay over time, and giving the material long-lasting flame retardant protection.

[0018] (3) Good comprehensive performance: The copolymerization method helps to maintain or improve the mechanical and thermal properties of PMMA. Compared with the sharp drop in Tg and strength caused by the addition of a large amount of small molecule flame retardants, the copolymer of the present invention is expected to maintain a higher Tg and mechanical strength; at the same time, the designed monomer structure is expected to have good compatibility with the PMMA matrix.

[0019] (4) Design flexibility: By adjusting the type (selection of R3) and quantity (value of x) of the phosphite / phosphonate group (R2) in the cyclotriphosphazene monomer, as well as its content in the copolymer, the flame retardant, thermal, mechanical and optical properties of the final PMMA material can be regulated to meet the needs of different application scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a schematic diagram of the structure of a nitrogen-phosphorus synergistic flame retardant monomer in an embodiment of the present invention;

[0021] Figure 2 Schematic diagram of the synthetic route for preparing nitrogen-phosphorus synergistic flame retardant monomers in an embodiment of the present invention;

[0022] Figure 3 The flame retardant monomer prepared in Example 1 of the present invention 1 HNMR spectrum. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. Unless otherwise specified, the technical means adopted in the embodiments of the present invention are conventional means well known to those skilled in the art, and the reagents and materials used can be obtained through commercial channels.

[0024] Synthesis example 1

[0025] A method for preparing a nitrogen-phosphorus synergistic flame retardant monomer [N3P3(R1)(R2)2(Cl)3, R1=HEMA derivative, R2=-P(O)(OMe)2] comprises the following steps:

[0026] (a) In a 500mL four-necked flask equipped with a stirrer, a thermometer, a dropping funnel and a nitrogen protection device, 34.8g (0.1mol) of hexachlorocyclotriphosphazene (N3P3Cl6) and 200mL of anhydrous tetrahydrofuran (THF) were added and cooled to 0°C. Under stirring, 50mL of a THF solution in which 13.0g (0.1mol) of hydroxyethyl methacrylate (HEMA) and 10.1g (0.1mol) of triethylamine (TEA) were pre-dissolved was slowly added dropwise. After the addition was completed, the reaction was continued at 0-5°C for 4 hours, the reaction mixture was filtered to remove the generated triethylamine hydrochloride precipitate, and the filtrate was decompressed on a rotary evaporator to remove most of the THF to obtain a crude product N3P3(R1)(Cl)5;

[0027] (b) The crude product N3P3(R1)(Cl)5 obtained in step (a) (assuming a yield of 100%, about 0.1 mol) was dissolved in 150 mL of anhydrous toluene, 22.0 g (0.2 mol) of dimethyl phosphite (DMHP, P(O)(OMe)2H) and 20.2 g (0.2 mol) of triethylamine (TEA) were added, and 1.16 g (1 mol%) of tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) was added as a catalyst, and the mixture was heated under nitrogen protection. Reflux reaction for 24 hours. After the reaction is completed, cool to room temperature, filter to remove solids, wash the filtrate with saturated sodium bicarbonate solution, then wash with water, dry over anhydrous sodium sulfate, evaporate the solvent under reduced pressure, and purify the residue by column chromatography (e.g., silica gel, petroleum ether / ethyl acetate gradient elution) to obtain the target product N3P3(R1)(R2)2(Cl)3, wherein R1 is -O-CH2CH2-OC(O)C(CH3)=CH2, R2 is -P(O)(OMe)2, and the product structure is as follows 1 HNMR, 31 PNMR,MS confirmed.

[0028] Synthesis example 2

[0029] A preparation method of a nitrogen-phosphorus synergistic flame retardant monomer [N3P3(R1)(R2)4(Cl)1, R1 = HEMA-derived group, R2 = -P(O)(OEt)2] is as follows: Repeat step a of Synthesis Example 1. Dissolve the obtained crude product N3P3(R1)(Cl)5 (about 0.1 mol) in 150 mL of anhydrous toluene, add 55.2 g (0.4 mol) of diethyl phosphite (DEHP, P(O)(OEt)2H) and 55.3 g (0.4 mol) of potassium carbonate (K2CO3). Under nitrogen protection, heat and reflux for 48 hours. The post-treatment and purification methods after the reaction are similar to those in Synthesis Example 1 to obtain the target product N3P3(R1)(R2)4(Cl)1, where R1 is -O-CH2CH2-O-C(O)C(CH3)=CH2 and R2 is -P(O)(OEt)2.

[0030] Example 1

[0031] A preparation method of a highly transparent flame retardant PMMA material (flame retardant monomer content 5 wt%). Add 95 g of methyl methacrylate (MMA, the polymerization inhibitor needs to be removed in advance), 5 g of the flame retardant monomer [N3P3(R1)(R2)2(Cl)3] prepared in Synthesis Example 1, 0.1 g of azobisisobutyronitrile (AIBN) as an initiator, and 0.2 g of n-dodecyl mercaptan (n-DDM) as a chain transfer agent into a reaction vessel. After stirring and dissolving evenly, inject the mixed solution into a mold composed of two parallel glass plates (the mold thickness is 3 mm, sealed with a flexible gasket). Place the mold in a water bath and carry out pre-polymerization at 60 °C for 4 hours, then transfer it to an oven, gradually raise the temperature to 110 °C, keep warm for 2 hours for curing, and demold after cooling to room temperature to obtain a transparent PMMA sheet.

[0032] Example 2

[0033] A preparation method of a highly transparent flame retardant PMMA material (flame retardant monomer content 10 wt%). Except for using 90 g of MMA and 10 g of the flame retardant monomer prepared in Synthesis Example 1, the other steps are the same as those in Example 1.

[0034] Example 3

[0035] A preparation method of a highly transparent flame retardant PMMA material (flame retardant monomer content 15 wt%). Except for using 85 g of MMA and 15 g of the flame retardant monomer prepared in Synthesis Example 2, the other steps are the same as those in Example 1.

[0036] Example 4

[0037] Preparation method of a highly transparent flame-retardant PMMA material (continuous bulk polymerization - extrusion granulation), using a continuous bulk polymerization process, continuously pumping MMA, the flame-retardant monomer prepared in Synthesis Example 1 (the flame-retardant monomer accounts for 10% of the total monomer mass), an initiator (tert-butyl peroxy-3,5,5-trimethylhexanoate, added at 0.01% of the total monomer mass), and a chain transfer agent (n-octyl mercaptan, added at 0.3% of the total monomer mass) into a prepolymerization reactor, controlling the reaction temperature at 130°C, after reaching a certain conversion rate (50%), continuously feeding the formed polymer slurry into a devolatilizer (temperature 220°C) to remove unreacted monomers and volatiles, obtaining a polymer melt, then feeding the melt into a twin-screw extruder, extruding, cooling, and pelletizing at a set temperature range and rotation speed (200 rpm) to obtain PMMA copolymer particles.

[0038] Comparative Example 1

[0039] Preparation method of pure PMMA, except for not adding any flame-retardant monomers, the remaining steps are the same as in Example 1.

[0040] Experimental Example

[0041] The plates prepared in Examples 1 - 3 and Comparative Example 1 were processed into test specimens according to the standard for performance testing. The specific test results are as follows in the table:

[0042]

[0043]

[0044] It can be seen from the results in the above table that with the introduction of the nitrogen-phosphorus synergistic flame-retardant monomer described in the present invention, the flame-retardant performance of the PMMA copolymer is significantly improved, its transparency is comparable to that of pure PMMA resin, and the heat distortion temperature (HDT) is also close to that of pure PMMA resin, and the tensile strength is also significantly improved. This indicates that the technical solution provided by the present invention can effectively prepare PMMA materials with both high transparency and excellent flame-retardant performance.

[0045] The embodiments of the present invention are given for purposes of illustration and description, and are not exhaustive or limit the present invention to the disclosed form. Many modifications and variations are obvious to those of ordinary skill in the art. The embodiments are selected and described to better illustrate the principles and practical applications of the present invention, and enable those of ordinary skill in the art to understand the present invention and design various embodiments with various modifications suitable for specific purposes.

Claims

1. A highly transparent flame-retardant PMMA material, characterized in that, It contains a structural unit formed by polymerization of methyl methacrylate monomer and a structural unit formed by polymerization of a nitrogen-phosphorus synergistic flame retardant monomer, and the nitrogen-phosphorus synergistic flame retardant monomer has a structure shown in the following general formula I: N3P3(R1)(R2) x (Cl) y (I), wherein, R1 is a -O-CH2CH2-O-C(O)C(CH3)=CH2 group; R2 is a phosphite group; x is an integer from 1 to 5; y is an integer from 0 to 4; and x + y = 5.

2. The high-transparency flame-retardant PMMA material according to claim 1, characterized in that, The mass percentage content of the nitrogen-phosphorus synergistic flame retardant monomer in the copolymer is 1-30%.

3. The high-transparency flame-retardant PMMA material according to claim 1, characterized in that, R2 is a phosphonate group having the structure -P(O)(OR3)2, where R3 is selected from C1-C4 alkyl or phenyl.

4. The high-transparency flame-retardant PMMA material according to claim 3, characterized in that, R3 is methyl or ethyl.

5. The high-transparency flame-retardant PMMA material according to claim 1, wherein, The preparation method of the nitrogen-phosphorus synergistic flame retardant monomer comprises the following steps: (a) Reacting hexachlorocyclotriphosphazene with 1 equivalent of 2-hydroxyethyl methacrylate in the presence of an inert atmosphere and a base to obtain a monosubstituted product N3P3(R1)(Cl)5; (b) Reacting the monosubstituted product N3P3(R1)(Cl)5 obtained in step (a) with x equivalents of a phosphite precursor under the action of a catalyst to substitute x of the remaining chlorine atoms to obtain a product represented by general formula I.

6. The high-transparency flame-retardant PMMA material according to claim 5, characterized in that, In step (a), the base is an organic base or an inorganic base, and the reaction temperature is -10°C to 50°C; in step (b), the catalyst is a palladium catalyst or a base.

7. A preparation method of a high-transparency flame-retardant PMMA material, characterized in that, Methyl methacrylate monomer and the nitrogen-phosphorus synergistic flame retardant monomer are subjected to free radical copolymerization in the presence of an initiator and an optional chain transfer agent.

8. The preparation method of the high-transparency flame-retardant PMMA material according to claim 7, characterized in that, The copolymerization reaction is carried out by bulk polymerization, solution polymerization or suspension polymerization.

9. The preparation method of the high-transparency flame-retardant PMMA material according to claim 8, characterized in that, Using bulk polymerization, it includes a prepolymerization stage and a curing polymerization stage.

10. The preparation method of the high-transparency flame-retardant PMMA material according to claim 7, wherein, The initiator is an azo initiator or a peroxide initiator; the chain transfer agent is a mercaptan chain transfer agent.

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