Preparation of phosphorus-containing flame retardant with different oxidation states by click chemistry and its application

Abstract

The application belongs to the technical field of flame retardants, and discloses a click chemistry preparation of a flame retardant containing a core-shell structure with different oxidation states of phosphorus and application thereof, and specifically includes a core and a polymer shell coated on the surface of the core; the core is a halogen-free flame retardant; the polymer shell is a crosslinked polymer containing oxidation state phosphorus, which is formed by crosslinking and curing of polythiol and phosphorus-containing acrylate monomers through thiol-ene click chemistry, and the phosphorus-containing acrylate monomer is an acrylate monomer containing oxidation state phosphorus of -1 valence, +1 valence, +3 valence or +5 valence. The preparation process is mild and efficient, green and controllable, and easy to industrialize, and the obtained flame retardant has excellent hydrophobicity, polyolefin interfacial compatibility and gas phase-condensed phase space-time synergistic flame retardant effect, can significantly improve the flame retardant grade, mechanical properties and long-term heat aging resistance of polyolefin materials, is suitable for flame retardant modification of polyolefin materials, and is easy to realize large-scale industrial production.

Description

Technical Field

[0001] This invention belongs to the field of flame retardant technology, and particularly relates to the preparation of flame retardants with phosphorus core-shell structures of different oxidation states by click chemistry and their applications. Background Technology

[0002] Polyolefins (PO) possess excellent chemical resistance, mechanical properties, ease of processing, light weight, and superior insulation, making them widely used in construction, communications, nuclear power plants, and new energy vehicles. However, PO is composed of pure carbon, hydrogen, and oxygen elements, with a linear carbon-carbon single-bond structure in its molecular backbone, making it highly flammable. Maintaining flame retardancy while simultaneously ensuring excellent physical properties and durability has become crucial for its further development. Therefore, researching high-performance polyolefin materials with high flame retardancy, long-lasting durability, and excellent overall properties is of great significance.

[0003] Currently, additive flame retardants are widely used in polyolefins due to their low cost and ease of processing. Traditional halogenated flame retardants used in polyolefin materials produce large amounts of toxic fumes and corrosive hydrogen halide gases during combustion, causing secondary hazards. Currently, there are two types of halogen-free flame retardant solutions for polyolefins: one is a high-addition flame retardant system using metal hydroxides; the other is a flame retardant system primarily based on phosphorus and nitrogen compounds. Although metal hydroxides are inexpensive, their large-scale addition leads to a decrease in the physical and mechanical properties of polyolefin materials. Phosphorus and nitrogen flame retardant systems have high flame retardant efficiency and good flame retardant effects. Their flame retardant mechanisms include condensed-phase and gas-phase mechanisms: low-valence phosphorus (e.g., +1 valence) primarily uses a gas-phase flame retardant mechanism with a secondary condensed-phase mechanism; +3 valence phosphorus exhibits both gas-phase and condensed-phase flame retardant mechanisms; and +5 valence phosphorus primarily uses a condensed-phase flame retardant mechanism with a secondary gas-phase mechanism.

[0004] However, existing phosphorus-nitrogen flame retardants have significant technical drawbacks. The polarity of phosphorus-nitrogen flame retardants and hydroxides is not compatible with the polyolefin matrix. The high hydroxide content and the "acidic" characteristics of phosphorus-nitrogen flame retardants adversely affect the overall performance and durability of flame-retardant polyolefins, thus limiting their long-term application in polyolefin materials. Existing technologies utilize microencapsulation to incorporate flame retardants containing phosphorus of different valence states into a core-shell structure, leveraging the interaction between the gas and condensed phases to significantly improve the overall performance of flame-retardant polyolefins. However, traditional methods for preparing microencapsulated flame retardants suffer from drawbacks such as long processing time, high energy consumption, high cost, and low production efficiency, making it difficult to achieve stable industrial-scale production.

[0005] Click chemistry, due to its high selectivity, mild reaction conditions, high efficiency, and ease of separation and purification, aligns with the concepts of atom economy and green chemistry. In recent years, thiol-olefin click chemistry has been widely researched and applied in fields such as varnishes, inks, adhesives, photoresists, optical materials, and biomaterials. However, currently, there are no reports on applying thiol-olefin click chemistry to the surface coating modification of halogen-free flame retardants, constructing core-shell structured flame retardants containing phosphorus of different oxidation states, and achieving an integrated design that combines gas-phase and condensed-phase spatiotemporal synergistic flame retardancy, long-term antioxidant effects, and improved interfacial compatibility. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention proposes a click chemistry method for preparing flame retardants with core-shell structures containing phosphorus of different oxidation states and their applications. Through a thiol-olefin click chemistry reaction, polymer shells containing phosphorus of different oxidation states are precisely constructed on the surface of a halogen-free flame retardant, achieving controllable synthesis of the core-shell structure of the flame retardant. This process also endows the flame retardant with highly efficient synergistic flame retardant properties, excellent interfacial compatibility, long-lasting resistance to thermo-oxidative aging, and hydrophobic stability, thus meeting the application requirements of polyolefin materials.

[0007] To achieve the above objectives, the present invention provides a flame retardant containing phosphorus core-shell structure with different oxidation states, comprising a core and a polymer shell layer covering the surface of the core;

[0008] The core is a halogen-free flame retardant, and the surface of the core is grafted with carbon-carbon double bonds that can participate in the thiol-alkene click reaction through a silane coupling agent.

[0009] The polymer shell is a cross-linked polymer containing phosphorus in a specific oxidation state. It is formed by cross-linking and curing polythiols and phosphorus-containing acrylate monomers through a thiols-olefin click chemical reaction. The phosphorus-containing acrylate monomers are acrylate monomers containing phosphorus in oxidation states of -1, +1, +3, or +5.

[0010] Preferably, the halogen-free flame retardant is any one or a combination of ammonium polyphosphate, triazine charring agent, magnesium hydroxide, piperazine pyrophosphate, and melamine polyphosphate.

[0011] Preferably, the polythiol is any one or a combination of trimethylolpropane tris(3-mercaptopropionic acid), pentaerythritol tetra(mercaptoacetic acid), and 4,4'-dimercaptodiphenyl ether;

[0012] The phosphorus-containing acrylate monomer is any one of the following: trimethylolpropane triacrylate phosphine oxide (TAMPO) containing -1 valent phosphorus, pentaerythritol triacrylate diphenylphosphine oxide (PETAO) containing +1 valent phosphorus, hydroxyethyl diacrylate phenylphosphine oxide (DABP) containing +3 valent phosphorus, and hydroxyethyl triacrylate phosphine oxide (TAEP) containing +5 valent phosphorus.

[0013] Preferably, the mass ratio of the core to the polymer shell is 80-90:10-20.

[0014] A method for preparing flame retardants containing phosphorus core-shell structures with different oxidation states is also provided, comprising the following steps:

[0015] S1. Core surface modification: Disperse the halogen-free flame retardant in an organic solvent, add a silane coupling agent, heat and stir to react, and after the reaction is completed, wash and dry to obtain the modified halogen-free flame retardant;

[0016] S2. Preparation of precursor solution: Mix polythiols and phosphorus-containing acrylate monomers at a molar ratio of thiol to double bond of 1:1, add photoinitiator and polymerization inhibitor, dissolve in organic solvent, stir evenly to obtain precursor solution.

[0017] S3, Thiol-olefin click reaction coating: The modified halogen-free flame retardant obtained in S1 is mixed with the precursor solution obtained in S2, and the mixture is stirred under ultraviolet light to carry out the thiol-olefin click polymerization reaction. After the reaction is completed, the product is filtered, washed and dried to obtain the flame retardant containing phosphorus core-shell structure with different oxidation states.

[0018] Preferably, the organic solvent in step S1 is anhydrous ethanol; the amount of silane coupling agent added is 5% of the mass of the halogen-free flame retardant, the reaction temperature is 80°C, and the reaction time is 4 hours; the solvent used for washing is deionized water.

[0019] Preferably, the photoinitiator in step S2 is trimethylbenzoyl-diphenylphosphine oxide, and the amount added is 1%-3% of the total mass of polythiol and phosphorus-containing acrylate monomer; the polymerization inhibitor is dibutylhydroxytoluene, and the amount added is 0.03%-0.08% of the total mass of polythiol and phosphorus-containing acrylate monomer; the organic solvent is tetrahydrofuran.

[0020] Preferably, in step S3, the modified halogen-free flame retardant and the precursor solution are mixed at a mass ratio of 80:20; the wavelength of the ultraviolet light is 365nm, and the time for the click polymerization reaction is 20-40min; the filtration, washing, and drying process specifically involves washing the filter repeatedly with ethanol three times and then drying it overnight in a 100℃ oven.

[0021] A flame-retardant polyolefin composite material is also provided, comprising a polyolefin matrix, a flame retardant containing phosphorus core-shell structures of different oxidation states, and a crosslinking aid.

[0022] It also provides the application of flame retardants with phosphorus core-shell structures of different oxidation states in the flame retardant modification of polyolefin materials.

[0023] Compared with the prior art, the present invention has the following advantages and technical effects:

[0024] 1) This invention is the first to apply thiol-olefin click chemistry to the preparation of flame retardants with phosphorus core-shell structures of different oxidation states. By introducing reactive carbon-carbon double bonds onto the surface of halogen-free flame retardants using silane coupling agents, precise grafting sites are provided for the click reaction. By controlling the functionality, feed ratio, and reaction conditions of phosphorus-containing acrylate monomers and polythiols, a polymer shell with uniform and controllable crosslinking density, chemical composition, and shell thickness can be formed on the flame retardant surface, overcoming the shortcomings of traditional microencapsulation processes such as uneven shell formation, low encapsulation rate, and uncontrollable structure. The preparation process of this invention can be rapidly completed at room temperature and under ultraviolet light irradiation, without the need for high-temperature and high-pressure environments, complex catalysts, or large amounts of organic solvents. The reaction conditions are mild, with high atom utilization and no harmful byproducts, conforming to the concept of green chemistry. Furthermore, it requires low raw material and equipment inputs, has high production efficiency, and is easy to scale up for industrial production.

[0025] 2) This invention integrates +5 valent phosphorus (condensed-phase flame retardant) in the core and low-valent phosphorus (gas-phase flame retardant and free radical capture) in the shell structure. During combustion, the core-shell structure achieves a spatiotemporal synergistic effect of "gas-phase physical flame retardancy and free radical quenching in the shell, and char formation and heat insulation in the core condensed-phase structure," significantly improving flame retardant efficiency. Simultaneously, by using click chemistry to macromolecularize and fix the antioxidant units (low-valent phosphorus) in the shell, the problem of migration and precipitation of small-molecule antioxidants is fundamentally solved, endowing the material with excellent long-term resistance to thermo-oxidative aging.

[0026] 3) The organic polymer shell constructed by the thiol-olefin click chemistry reaction of this invention can effectively reduce the surface energy of halogen-free flame retardants, improve their hydrophobicity (e.g., water contact angle > 100°) and interfacial compatibility with the polyolefin matrix. This not only reduces the negative impact of adding flame retardants on the mechanical properties of materials (e.g., tensile strength, elongation at break) and volume resistivity, but sometimes even has a reinforcing effect, while ensuring the stability of the material's performance under long-term use or harsh environments.

[0027] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0028] Figure 1 A schematic diagram of the synthesis of acrylates with different phosphorus valence states, where A is TAMPO, B is PETAO, C is DABP, and D is TAEP;

[0029] Figure 2 The 1H NMR spectrum of TAMPO prepared in Example 1 of this invention;

[0030] Figure 3 The 1H NMR spectrum of PETAO prepared in Example 1 of this invention;

[0031] Figure 4The 1H NMR spectrum of the DABP prepared in Example 1 of this invention;

[0032] Figure 5 The proton nuclear magnetic resonance spectrum of the TAEP prepared in Example 1 of this invention;

[0033] Figure 6 The figure shows the phosphorus nuclear magnetic resonance spectra of TAMPO, PETAO, DABP and TAEP prepared in Example 1 of the present invention. In the figure, A is TAMPO, B is PETAO, C is DABP and D is TAEP.

[0034] Figure 7 This is a schematic diagram of the synthesis route of the flame retardants with different phosphorus valence states and core-shell structures of the present invention;

[0035] Figure 8 The SEM image and contact angle test image of the APP prepared in Example 2 of the present invention;

[0036] Figure 9 The images show the SEM image and contact angle test image of APP@Si prepared in Example 2 of this invention.

[0037] Figure 10 SEM image and contact angle test image of APP@Si@PS(-1P) prepared in Example 2 of the present invention;

[0038] Figure 11 SEM image and contact angle test image of APP@Si@PS(+1P) prepared in Example 2 of the present invention;

[0039] Figure 12 SEM image and contact angle test image of APP@Si@PS(+3P) prepared in Example 2 of this invention;

[0040] Figure 13 SEM image and contact angle test image of APP@Si@PS(+5P) prepared in Example 2 of this invention;

[0041] Figure 14 The image shows the X-ray photoelectron spectroscopy (XPS) spectra of APP, CFA and their corresponding core-shell flame retardants prepared in Example 2 of this invention. In the figure, A represents APP and its corresponding core-shell flame retardant, and B represents CFA and its corresponding core-shell flame retardant.

[0042] Figure 15 The figure shows the test results of the flame-retardant EVA composite material prepared in Example 3 of the present invention. In the figure, A is the stress-strain curve of each group of flame-retardant EVA composite materials, and B is the curve showing the relationship between the elongation at break of each group of flame-retardant EVA composite materials and the aging time under the heat aging condition of 150℃. Detailed Implementation

[0043] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0044] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0045] Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this invention. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards. Experimental instruments, equipment, and reagents in the following embodiments that do not specify their sources are all commercially available materials.

[0046] Unless otherwise defined or stated, all technical and scientific terms used in this invention have the same meaning as those skilled in the art. Furthermore, any methods and materials similar to or equivalent to those described herein can be applied to the methods of this invention. It should be noted that, unless otherwise specified, the embodiments and features described in this invention can be combined with each other.

[0047] Example 1

[0048] Preparation of acrylate monomers with different phosphorus valence states (e.g.) Figure 1 (As shown).

[0049] 1) Preparation of TAMPO containing -1 valent phosphorus.

[0050] 0.1 mol of tris(hydroxymethyl)phosphine oxide (THPO), 0.35 mol of triethylamine (TEA), and 0.0648 g of 2,4-dimethyl-6-tert-butylphenol were added to a 500 mL round-bottom flask, followed by 300 mL of dichloromethane and stirred until homogeneous. The flask was then immersed in an ice bath, and 0.35 mol of acryloyl chloride (AC) was added dropwise. After the addition was complete, the reaction was allowed to proceed at room temperature for 12 h. After the reaction was complete, the mixture was washed three times with 1 M hydrochloric acid solution and once with saturated sodium bicarbonate solution. The organic phase was dried over anhydrous sodium sulfate, filtered, and concentrated to 70 mL. The solution was then passed through a 2 cm diameter, 6 cm long neutral alumina column, and the solvent was evaporated under vacuum at room temperature to obtain a slightly viscous, deep yellow liquid, namely TAMPO containing -1 valent phosphorus.

[0051] 2) Preparation of PETAO containing +1 valent phosphorus.

[0052] 0.105 mol of pentaerythritol triacrylate (PETA) was added to 100 mL of dichloromethane and stirred to disperse. Then, 0.105 mol of triethylamine was added, and the mixture was stirred for 15 min under ice bath conditions to ensure uniform dispersion. 0.1 mol of diphenylphosphine chloride (DC) was slowly added dropwise using a separatory funnel, ensuring the addition was completed within 1 h. The reaction was maintained for 1 h, followed by continued reaction at room temperature for 12 h. After the reaction was completed, the mixture was filtered, and the lower layer was washed three times with deionized water. The lower layer was extracted, and the mixture was rotary evaporated under vacuum at 30 °C for 30 min to obtain PETAO containing +1 valent phosphorus.

[0053] 3) Preparation of DABP containing +3 valent phosphorus.

[0054] 0.2 mol of 2-hydroxyethyl acrylate (HEA) was added to 100 mL of chloroform and stirred to disperse. Then, 0.3 mol of triethylamine was added, and the mixture was stirred for 15 min under ice bath conditions to ensure uniform dispersion. 0.12 mol of phenylphosphonodichlorophenyl BPOD was slowly added dropwise using a separatory funnel, completing the addition within 1 h. The reaction was maintained for 1 h, and then transferred to a 30 °C oil bath for 24 h. After the reaction was completed, the mixture was filtered, and the lower layer was washed three times with deionized water. The lower layer was extracted, and the mixture was rotary evaporated under vacuum at 45 °C for 30 min to obtain DABP containing +3 valent phosphorus.

[0055] 4) Preparation of TAEP containing +5 valent phosphorus.

[0056] A solution of 0.305 mol hydroxyethyl acrylate (HEA) and 0.355 mol triethylamine in 100 mL of diethyl ether was added to a 250 mL round-bottom flask equipped with a calcium chloride drying tube and stirred until homogeneous. Under ice bath conditions, a mixture of 0.102 mol phosphorus oxychloride (POCl3) and 20 mL of diethyl ether was slowly added dropwise to the flask. After the addition was complete, the reaction was allowed to proceed at room temperature for 12 h. After the reaction was completed, the triethylcarbamate byproduct was removed by filtration. The resulting filtrate was extracted twice, successively with 1 M hydrochloric acid, 10% sodium bicarbonate solution, and saturated sodium chloride aqueous solution. The organic phase was dried over anhydrous sodium sulfate and then the solvent was removed under vacuum to obtain a colorless liquid product, namely TAEP containing +5 valent phosphorus.

[0057] The four monomers prepared above were subjected to proton and phosphorus NMR spectra, respectively, and the results are as follows: Figure 2-6 As shown, the characteristic peaks in the spectrum perfectly match the structure of the target monomer, proving that four acrylate monomers containing phosphorus of different oxidation states were successfully synthesized.

[0058] Example 2

[0059] Preparation of flame retardants with different phosphorus valence states and core-shell structures (e.g.) Figure 7 (As shown).

[0060] Preparation of S1 and APP@Si:

[0061] 100g of ammonium polyphosphate (APP) was dispersed in anhydrous ethanol, and 5g of silane coupling agent (KH570) was added. The mixture was stirred at 80℃ for 4h. After the reaction was completed, the product was washed three times with deionized water and dried under vacuum to obtain silane coupling agent modified APP@Si.

[0062] S2. Preparation of precursor solution:

[0063] Trimethylolpropane tris(3-mercaptopropionic acid) ester (TMPMP) and the phosphorus-containing acrylate (such as PETAO) prepared above were mixed at a 1:1 molar ratio of mercapto to double bond. 2 wt% of trimethylbenzoyl-diphenylphosphine oxide (TPO) and 0.05 wt% dibutylhydroxytoluene (BHT) were added and dissolved in tetrahydrofuran (THF). The mixture was stirred until homogeneous to obtain a precursor solution.

[0064] S3, preparation of core-shell structured flame retardants by click chemistry of thiols-olefins:

[0065] APP@Si was mixed with the precursor solution at a mass ratio of 80:20 and stirred under 365nm UV light for 30 min to complete the click polymerization reaction. After the reaction, the product was filtered, washed three times with ethanol, and dried overnight in a 100℃ oven to obtain the APP@Si@PS(+1P) core-shell flame retardant.

[0066] S4. Preparation of microencapsulated ammonium polyphosphate (APP) with shell materials of different phosphorus valence states:

[0067] Following the same method described above, APP@Si@PS(-1P), APP@Si@PS(+3P), and APP@Si@PS(+5P) core-shell flame retardants were prepared by replacing the phosphorus-containing acrylates with TAMPO, DABP, and TAEP, respectively.

[0068] By replacing the core halogen-free flame retardant with triazine charring agent (CFA), CFA@Si, CFA@Si@PS(-1P), CFA@Si@PS(+1P), CFA@Si@PS(+3P), and CFA@Si@PS(+5P) were prepared.

[0069] SEM was used to test the water contact angle of APP, APP@Si, and phosphorus core-shell structure flame retardants with different oxidation states. The results are as follows: Figure 8-13As shown, the water contact angle of unmodified APP is only 44.2°, exhibiting strong hydrophilicity; after silane modification and click chemical coating, the water contact angle of all core-shell structure flame retardants is greater than 100°, reaching a maximum of 116.2°, with significantly improved hydrophobicity; SEM images show that a uniform and dense polymer shell is formed on the surface of the coated flame retardant particles, with no obvious agglomeration, proving that the core-shell structure was successfully constructed.

[0070] XPS tests were performed on APP, CFA and their corresponding core-shell flame retardants, and the results are as follows: Figure 14 As shown, the spectrum shows obvious characteristic peaks of phosphorus, proving that the phosphorus-containing polymer shell was successfully grafted onto the flame retardant surface.

[0071] Example 3

[0072] Flame-retardant EVA composite materials with phosphorus core-shell structure of different oxidation states.

[0073] This embodiment prepares EVA composite materials modified with phosphorus core-shell structure flame retardants of different oxidation states. The specific steps are as follows:

[0074] According to the formulation shown in Table 1, 73.6 parts of ethylene vinyl acetate copolymer (EVA), 18.75 parts of APP@Si@PS(-1P), 6.25 parts of CFA@Si@PS(-1P), and 1.4 parts of triallyl isocyanurate (TAIC) were added to a Banbury mixer and mixed evenly at 80°C. The mixture was then pressed into sheets of different thicknesses in a flat vulcanizing machine and crosslinked by 200 kGy electron beam irradiation to prepare irradiated crosslinked flame-retardant EVA-3 composite material. Other material formulations are shown in Table 1. Irradiated crosslinked polyolefin and flame-retardant polyolefin materials were prepared using the same method as the flame-retardant EVA-3 composite material.

[0075] The prepared EVA composite material was subjected to vertical burning and loss on ignition (LOI) tests, and the results are shown in Table 1; cone calorimetry tests were performed, and the results are shown in Table 2; mechanical properties and thermal aging tests were performed, and the results are shown in Table 2. Figure 15 As shown.

[0076] Among them, EVA-1 is a pure EVA blank sample, EVA-2 is an unmodified flame retardant control sample, and EVA-3 to EVA-6 are core-shell structure flame retardant modified samples of the present invention.

[0077] Table 1 Final Formulation and Combustion Performance of EVA Composite Materials

[0078]

[0079] Where X represents the -1, +1, +3 or +5 oxidation states of phosphorus in different oxidation states.

[0080] Table 2 Summary of main parameters of EVA and its composites tested by cone calorimeter

[0081]

[0082] The results in Tables 1 and 2 show that pure EVA is highly flammable, has no flame retardant rating, and has an LOI of only 20%. The unmodified flame retardant system EVA-2 can only reach the UL-94V-1 rating, while all EVA composite materials modified with core-shell flame retardants in this invention can reach the UL-94V-0 rating, with the LOI increasing to a maximum of 29%. Compared with EVA-2, the modified samples show a significant decrease in PHRR and THR, and a significant reduction in CO and CO2 emissions. This indicates that their flame retardant efficiency and smoke suppression and toxicity reduction performance are greatly improved.

[0083] from Figure 15 The results show that the tensile strength and elongation at break of the modified EVA composite material of this invention are better than those of the unmodified control sample. Furthermore, during the thermal aging process at 150℃, the elongation at break retention rate is much higher than that of the control sample, proving that the material has excellent mechanical properties and long-term resistance to thermo-oxidative aging.

[0084] Example 4

[0085] Preparation of flame retardants with a core-shell structure containing +1 phosphorus valence and their flame-retardant polyethylene composites.

[0086] This embodiment prepares a +1 valent phosphorus core-shell structure flame retardant with magnesium hydroxide as the core, and the corresponding flame-retardant polyethylene composite material. The specific steps are as follows:

[0087] 1) Preparation of MH@Si:

[0088] 100g of magnesium hydroxide (MH) was dispersed in anhydrous ethanol, and 5g of A151 was added. The mixture was stirred at 40°C for 6 hours. After the reaction was completed, the product was washed three times with anhydrous ethanol and dried under vacuum to obtain MH@Si modified with silane coupling agent.

[0089] 2) Preparation of precursor solution:

[0090] Pentaerythritol tetra(thioglycolic acid) ester was mixed with PETAO prepared in Example 1 at a thiol to double bond molar ratio of 1:1. 2 wt% of TPO and 0.05 wt% of BHT were added to the mixture, dissolved in THF, and stirred until homogeneous to obtain the precursor solution.

[0091] 3) Preparation of core-shell structured flame retardants by thiol-olefin click chemistry:

[0092] MH@Si was mixed with the precursor solution at a mass ratio of 90:10 and stirred under 365 nm UV light for 30 min to complete the click polymerization reaction. After the reaction, the product was filtered, washed three times with anhydrous ethanol, and dried to obtain MH@Si@PS (+1P), a microencapsulated magnesium hydroxide flame retardant with a core-shell structure containing a +1 phosphorus valence state.

[0093] 4) Preparation of flame-retardant polyethylene composite materials:

[0094] According to the formulations shown in Table 3, polyethylene (PE), MH@Si@PS (+1P), triallyl isocyanurate (TAIC), and dicumyl peroxide (DCP) were added to a Banbury mixer and mixed evenly at 100°C. Then, the mixture was vulcanized at 165°C for 15 minutes in a flat vulcanizing machine to prepare a thermally vulcanized crosslinked flame-retardant polyethylene composite material. Other material formulations are shown in Table 3, and thermally crosslinked polyolefin and flame-retardant polyolefin materials were prepared using the same method.

[0095] The prepared polyethylene composite material was subjected to flame retardancy, mechanical properties, and thermal aging tests, and the results are shown in Table 3; a cone calorimeter test was also performed, and the results are shown in Table 4.

[0096] Among them, PE-1 is a pure PE blank sample, PE-2 is an unmodified MH control sample, and PE-3 is a modified sample of the core-shell structure flame retardant of this invention.

[0097] Table 3 Formulation, flame retardant properties, mechanical properties and mechanical properties after thermal aging of flame-retardant polyethylene composite materials

[0098]

[0099] Table 4. Summary of main parameters of flame-retardant polyethylene and its composites tested by cone calorimeter.

[0100]

[0101] As can be seen from the results in Tables 3 and 4, the unmodified MH system PE-2 has no flame retardant rating, while the modified sample PE-3 of this invention can reach UL-94V-0 rating, with LOI increased to 35%. Compared with PE-1, PE-2, and PE-3, the PHRR and THR are significantly reduced, the tensile strength is significantly improved, and the elongation at break is significantly improved. After heat aging at 150℃ for 168h, the mechanical properties are well retained, achieving a simultaneous improvement in flame retardant performance, mechanical properties, and long-term aging resistance.

[0102] Example 5

[0103] Preparation of +3 phosphorus valence core-shell structure flame retardant and its flame retardant EPDM rubber composite material.

[0104] This embodiment prepares a +3 valent phosphorus core-shell structure flame retardant with piperazine pyrophosphate and melamine polyphosphate as the core, and the corresponding flame-retardant ethylene propylene diene monomer (EPDM) rubber composite material. The specific steps are as follows:

[0105] 1) Preparation of PAPP@Si:

[0106] 100g of piperazine pyrophosphate (PAPP) was dispersed in anhydrous ethanol, and 5g of A171 was added. The mixture was stirred at 60℃ for 5h. After the reaction was completed, the product was washed three times with anhydrous ethanol and dried under vacuum to obtain silane coupling agent modified PAPP@Si.

[0107] 2) Preparation of precursor solution: Mix 4,4'-dimercaptodiphenyl ether and DABP at a 1:1 molar ratio of thiol to double bond, add 2wt% TPO and 0.05wt% BHT of total monomer mass, dissolve in THF, and stir until homogeneous to obtain precursor solution.

[0108] 3) Preparation of core-shell flame retardants via thiol-olefin click chemistry: PAPP@Si was mixed with the precursor solution at a mass ratio of 85:15 and stirred under 365nm UV light for 30 min to complete the click polymerization reaction. After the reaction, the product was filtered, washed three times with anhydrous ethanol, and dried to obtain a core-shell microencapsulated piperazine pyrophosphate flame retardant with a +3 phosphorus valence state (PAPP@Si@PS(+3P)). A core-shell microencapsulated melamine polyphosphate flame retardant with a +3 phosphorus valence state (MPP@Si@PS(+3P)) was also prepared using the same method.

[0109] 4) Preparation of flame-retardant EPDM rubber composite material:

[0110] According to the formulations shown in Table 5, EPDM rubber, flame retardant, TAIC, and DCP were added to a Banbury mixer and mixed evenly at 100°C. The mixture was then vulcanized at 165°C for 15 minutes in a flat vulcanizing machine to prepare a heat-vulcanized crosslinked flame-retardant EPDM composite material. Other material formulations are shown in Table 5, and heat-vulcanized EPDM rubber and flame-retardant EPDM rubber materials were prepared using the same method.

[0111] The prepared EPDM composite material was subjected to flame retardancy, mechanical properties, and thermal aging tests, and the results are shown in Table 5; a cone calorimeter test was also performed, and the results are shown in Table 6.

[0112] Among them, EPDM-1 is a blank sample of pure EPDM, EPDM-2 is a control sample of unmodified flame retardant, and EPDM-3 is a modified sample of the core-shell structure flame retardant of this invention.

[0113] Table 5 Formulation and Flame Retardant Properties of Flame Retardant EPDM Rubber Composites

[0114]

[0115] Table 6 Summary of main parameters of flame-retardant EPDM rubber and its composites tested by cone calorimeter

[0116]

[0117] As can be seen from the results in Tables 5 and 6, the unmodified flame retardant system EPDM-2 has no flame retardant rating, while the modified sample EPDM-3 of this invention can reach UL-94V-0 rating, with LOI increased to 26%. Compared with EPDM-2, EPDM-3 has significantly reduced PHRR, THR, total smoke release, and toxic gas release, and its mechanical properties and performance retention rate after thermal aging are greatly improved. This proves that the core-shell structure flame retardant of this invention also has excellent flame retardant effect, interfacial compatibility, and long-term aging resistance in the EPDM system.

[0118] In summary, this invention integrates high-valence phosphorus (condensed-phase flame retardancy) in the core and low-valence phosphorus (gas-phase flame retardancy and free radical capture) in the shell structure, significantly improving the flame retardant efficiency of polyolefin composites. Furthermore, the flame-retardant, antioxidant, and cross-linked polyolefin composite material and its preparation method provided by this invention not only endow polyolefin materials with excellent flame retardant properties but also improve the long-term heat and oxygen aging resistance of polyolefin composites, making them suitable for cable sheathing materials.

[0119] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A flame retardant comprising a core-shell structure of phosphorus in different oxidation states, characterized in that, It includes a core and a polymer shell covering the surface of the core; The core is a halogen-free flame retardant; The polymer shell is a cross-linked polymer containing oxidized phosphorus, which is formed by cross-linking and curing polythiols and phosphorus-containing acrylate monomers through a thiol-olefin click chemical reaction. The phosphorus-containing acrylate monomers are acrylate monomers containing oxidized phosphorus in the form of -1, +1, +3, or +5 valence. The polythiol is any one or a combination of trimethylolpropane tris(3-mercaptopropionic acid), pentaerythritol tetra(mercaptoacetic acid), and 4,4'-dimercaptodiphenyl ether; The phosphorus-containing acrylate monomer is any one of the following: trimethylolpropane triacrylate phosphine oxide (TAMPO) containing -1 valent phosphorus, pentaerythritol triacrylate diphenylphosphine oxide (PETAO) containing +1 valent phosphorus, hydroxyethyl diacrylate phenylphosphine oxide (DABP) containing +3 valent phosphorus, and hydroxyethyl triacrylate phosphine oxide (TAEP) containing +5 valent phosphorus.

2. The flame retardant containing different oxidation state phosphorus core-shell structure according to claim 1, characterized in that, The halogen-free flame retardant is any one or a combination of ammonium polyphosphate, triazine charring agent, magnesium hydroxide, piperazine pyrophosphate, and melamine polyphosphate.

3. The flame retardant containing different oxidation state phosphorus core-shell structure according to claim 1, characterized in that, The mass ratio of the core to the polymer shell is 80-90:10-20.

4. A process for the preparation of the flame retardant comprising a core-shell structure of phosphorus in different oxidation states according to any one of claims 1 to 3, characterized in that, Includes the following steps: S1. Core surface modification: Disperse the halogen-free flame retardant in an organic solvent, add a silane coupling agent, heat and stir to react, and after the reaction is completed, wash and dry to obtain the modified halogen-free flame retardant; S2. Preparation of precursor solution: Mix polythiols and phosphorus-containing acrylate monomers at a molar ratio of thiol to double bond of 1:1, add photoinitiator and polymerization inhibitor, dissolve in organic solvent, stir evenly to obtain precursor solution. S3, Thiol-olefin click reaction coating: The modified halogen-free flame retardant obtained in S1 is mixed with the precursor solution obtained in S2, and the mixture is stirred under ultraviolet light to carry out the thiol-olefin click polymerization reaction. After the reaction is completed, the product is filtered, washed and dried to obtain the flame retardant containing phosphorus core-shell structure with different oxidation states.

5. The method of claim 4, wherein, The organic solvent in step S1 is anhydrous ethanol; the amount of silane coupling agent added is 5% of the mass of the halogen-free flame retardant, the reaction temperature is 80℃, and the reaction time is 4h; the solvent used for washing is deionized water.

6. The method of claim 4, wherein, The photoinitiator in step S2 is trimethylbenzoyl-diphenylphosphine oxide, and the amount added is 1%-3% of the total mass of polythiol and phosphorus-containing acrylate monomer; the polymerization inhibitor is dibutylhydroxytoluene, and the amount added is 0.03%-0.08% of the total mass of polythiol and phosphorus-containing acrylate monomer; the organic solvent is tetrahydrofuran.

7. The method of claim 4, wherein, In step S3, the modified halogen-free flame retardant and the precursor solution are mixed at a mass ratio of 80:20; the wavelength of the ultraviolet light is 365nm, and the time for the click polymerization reaction is 20-40min; the filtration, washing, and drying process specifically involves washing the filter repeatedly with ethanol three times and then drying it overnight in a 100℃ oven.

8. A flame-retardant polyolefin composite material, characterized in that, It includes a polyolefin matrix, flame retardants with phosphorus core-shell structures of different oxidation states as described in any one of claims 1-4, and crosslinking aids.

9. The application of a flame retardant containing phosphorus core-shell structure with different oxidation states as described in any one of claims 1-3 in the flame retardant modification of polyolefin materials.

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