A nitrogen-phosphorus flame retardant, a composition and a composite material containing the flame retardant, and a preparation method and application thereof

Abstract

This invention discloses a nitrogen-phosphorus flame retardant, a composition and composite material containing the flame retardant, its preparation method, and its application. The flame retardant has a structure as shown in formula (I), where n is an integer from 100 to 150. The preparation method of the flame retardant includes: synthesizing bis(4-formylphenyl) phenylphosphonate and bis(4-aminophenyl)phenylphosphonate, reacting the two to generate a Schiff base, and then reacting it with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide to obtain the nitrogen-phosphorus flame retardant PCNPO. This flame retardant, when compounded with melamine cyanurate (MCA), can be used to prepare flame-retardant nylon 6 composite materials. This invention, through the synergistic effect of PCNPO and MCA, enables nylon 6 to achieve the UL94 V-0 flame retardant standard at a low addition level, while maintaining excellent mechanical properties and processing stability, and exhibiting strong charring ability, simple process, and environmental friendliness. (I)

Description

Technical Field

[0001] This invention relates to the field of flame-retardant polymer materials technology, and more specifically, to a novel nitrogen-phosphorus flame retardant, a flame retardant composition containing the flame retardant, a flame-retardant nylon composite material containing the composition, and their preparation methods and applications. Background Technology

[0002] Nylon 6 (PA6), a general-purpose engineering plastic with excellent comprehensive properties, possesses superior mechanical properties, heat resistance, chemical corrosion resistance, and good processing flowability, making it widely used in electronics, automobiles, and machinery. However, Nylon 6 has a limiting oxygen index (LOI) of only about 21%, classifying it as a flammable material. Furthermore, it produces a large number of molten droplets during combustion, easily igniting secondary fires. This severely limits its application in some high-end fields (such as automotive electronics, rail transportation, and aerospace). Therefore, flame-retardant modification of Nylon 6 to improve its flame-retardant properties has become crucial for expanding its application areas.

[0003] Currently, flame retardant modification of nylon 6 mainly employs additive flame retardants, which can be categorized based on the type of flame-retardant element, including halogen-based, phosphorus-based, nitrogen-based, and inorganic flame retardants. While halogen-based flame retardants offer high flame retardant efficiency, they release toxic and harmful gases (such as hydrogen chloride and hydrogen bromide) and corrosive fumes during combustion, posing serious threats to the environment and human health, and their use has been gradually restricted. Inorganic flame retardants (such as magnesium hydroxide and aluminum hydroxide) require high addition amounts to achieve ideal flame retardant effects, which significantly reduces the mechanical properties and processing fluidity of nylon 6, making it difficult to meet engineering application requirements. Nitrogen-phosphorus flame retardants, as an environmentally friendly flame retardant, possess advantages such as low toxicity, low smoke, and halogen-free properties. Their flame retardant mechanism primarily involves the synergistic effect of gas-phase flame retardancy (releasing inert gases such as nitrogen and ammonia to dilute oxygen and combustible gases) and condensed-phase flame retardancy (forming a dense carbon layer to block heat and mass transfer), making them a research hotspot in the modification of flame-retardant nylon 6. Melamine cyanurate (MCA) is a commonly used nitrogen-based flame retardant with high flame retardant efficiency, good thermal stability, and excellent compatibility. However, when used alone, it requires a high addition amount to enable Nylon 6 to meet the UL94 V-0 flame retardant standard, which can also lead to a decrease in the mechanical properties of the material.

[0004] To address these issues (such as high single flame retardant addition levels and difficulty in balancing flame retardancy and mechanical properties), researchers often employ flame retardant compounding techniques to improve flame retardant efficiency. For example, Gu Lei et al. (Gu Lei et al. Preparation of aromatic Schiff base flame retardants and their application in polyamide 6 [J]. Plastics Technology, 2023, 51(02):36-42, hereinafter referred to as prior art 1) disclosed an aromatic Schiff base phosphorus-nitrogen synergistic flame retardant TMA-DOPO synthesized from terephthalaldehyde, p-aminobenzoic acid, and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), and compounded it with MCA for flame retardant modification of PA6. Although this scheme enabled nylon 6 to reach the UL94 V-0 flame retardant standard, the resulting composite material exhibited a significant decrease in mechanical properties, with a tensile strength of only about 26.5 MPa, a decrease of about 63% compared to pure nylon 6, which may be insufficient to meet the requirements of engineering applications.

[0005] To address the issue of decreased mechanical properties during flame-retardant modification, another prior art discloses a flame-retardant chain-extending modification process for polyamide plastics (see CN117736574A, hereinafter referred to as Prior Art 2). This document describes the synthesis of an intermediate product, D-DDS, using diaminodiphenyl sulfone, p-hydroxybenzaldehyde, and DOPO as raw materials. This intermediate product is then reacted with epichlorohydrin to prepare a DOPO derivative, DEP, containing biepoxide groups. DEP is then compounded with MCA for flame-retardant modification of nylon 6. The epoxy groups in DEP can undergo chain-extending reactions with the end groups of nylon 6, thereby increasing the molecular weight and tensile strength of the material. While this method increases the tensile strength to 77.5 MPa, it reduces the impact strength by approximately 45%, requires precise control of the chain-extending reaction, and involves a complex process. Furthermore, the introduction of epoxy groups may affect the long-term thermo-oxidative stability of the material.

[0006] In recent years, existing technologies (see Fan S et al., A novel phosphorus-nitrogen-based hyperbranched polysiloxane for improving the fire safety of PA6 with suppressed melt droplets and good mechanical properties. Heliyon, 2023, 9(12), hereinafter referred to as Existing Technology 3) have also explored integrating DOPO and Schiff base structures into the same polymer molecule in order to achieve a balance between flame retardancy and mechanical properties through molecular structure optimization. Specifically, this existing technology discloses a method for preparing hyperbranched polysiloxane PBDSi containing DOPO and Schiff base. It uses 3-aminopropyltriethoxysilane, diphenylsilanediol, DOPO and benzaldehyde as raw materials to synthesize hyperbranched polysiloxane PBDSi, and uses it for flame retardant modification of nylon 6. This scheme demonstrates the feasibility of integrating DOPO and Schiff base into the polymer backbone. However, the backbone is a siloxane structure, and the flame retardant effect mainly depends on the phosphorus element provided by the side chain DOPO. The backbone itself does not participate in char formation catalysis, so there is still room for improvement in flame retardant efficiency. Furthermore, it does not involve the compounding with MCA.

[0007] In summary, although existing technologies have proposed integrating DOPO derivatives with Schiff base structures for flame retardant modification of nylon 6, the following technical problems still need to be addressed: (1) High amounts of flame retardant still significantly affect mechanical properties; (2) It is difficult to balance flame retardant efficiency and mechanical properties, often resulting in a trade-off; (3) Some solutions rely on the chemical reaction of active functional groups to compensate for the loss of mechanical properties, which is complex and may introduce new stability issues; (4) The flame retardant mechanism is mainly gas-phase flame retardant, with limited charring ability, making it difficult to form a dense protective char layer. Therefore, developing a flame-retardant nylon 6 composite material with high flame retardant efficiency, excellent mechanical property retention, good charring ability, and simple process, as well as its preparation method, has important practical significance and application value. Summary of the Invention

[0008] To address the shortcomings of existing technologies, this invention provides a novel nitrogen-phosphorus flame retardant, a composition and composite material containing the flame retardant, a preparation method thereof, and its applications. This flame retardant possesses a unique molecular structure, with a phosphonate ester backbone (containing phosphorus) and DOPO side links forming a dual phosphorus source polymer. When combined with MCA, it produces a significant synergistic flame retardant effect, achieving a high flame retardant rating for nylon 6 material while reducing the total amount of flame retardant used, and exhibiting good mechanical properties and processing stability.

[0009] This invention is mainly achieved through the following technical solutions:

[0010] In a first aspect, the present invention provides a nitrogen-phosphorus flame retardant having a structure as shown in formula (I):

[0011]

[0012] (I)

[0013] In equation (I), n is an integer between 100 and 150.

[0014] In some implementations, n in equation (I) is an integer from 110 to 140, and can be an integer from 119 to 134.

[0015] Secondly, the present invention provides a method for preparing the above-mentioned nitrogen-phosphorus flame retardant, which may include the following steps:

[0016] (1) Reaction of p-hydroxybenzaldehyde and phenylphosphonic dichloro in an organic solvent in the presence of an acid-binding agent yields the intermediate bis(4-formylphenyl) phenylphosphonate (named P-CHO);

[0017] (2) Reaction of p-aminophenol and phenylphosphonodichloro in an organic solvent in the presence of a strong base yields the intermediate bis(4-aminophenyl)phenylphosphonate (named P-NH2);

[0018] (3) Dissolve the P-NH2 obtained in step (2) and the P-CHO obtained in step (1) in an organic solvent to react and generate Schiff base polymer. Then add 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) to continue the reaction and obtain the nitrogen-phosphorus flame retardant (named PCNPO).

[0019] In some embodiments, in the above method, in step (1), the molar ratio of phenylphosphonodichloro to p-hydroxybenzaldehyde is 1:(2.0-2.2); the molar ratio of p-hydroxybenzaldehyde to the acid-binding agent is 1:(2.0-2.2); optionally, the acid-binding agent is selected from one or more of triethylamine, pyridine, N,N-diisopropylethylamine and 4-dimethylaminopyridine; the organic solvent may be anhydrous dichloromethane.

[0020] In some embodiments, in the above method, in step (2), the molar ratio of phenylphosphonodichloro to p-aminophenol is 1:(2.0-2.2); the molar ratio of p-aminophenol to the strong base is 1:(1.0-1.1); optionally, the strong base may be one or more selected from sodium hydroxide, potassium hydroxide, potassium tert-butoxide, sodium hydride, triethylamine, N,N-diisopropylethylamine and 1,8-diazabicyclo[5.4.0]undec-7-ene; the organic solvent may be anhydrous dichloromethane or anhydrous tetrahydrofuran.

[0021] In some embodiments, in the above method, in step (3), the molar ratio of the bis(4-formylphenyl) phenylphosphonate to the bis(4-aminophenyl)phenylphosphonate is 1:(1.3-1.5); the molar ratio of the bis(4-aminophenyl)phenylphosphonate to the 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide is 1:(0.8-1.0); optionally, the organic solvent may be anhydrous ethanol.

[0022] In some implementations, the above method may include the following specific steps:

[0023] (1) Synthesis of intermediate P-CHO: p-hydroxybenzaldehyde was dissolved in an organic solvent, an acid-binding agent was added, and phenylphosphonodichloro soluble in the organic solvent was added dropwise under a nitrogen atmosphere at 0-5℃. The reaction was carried out for 8-10 hours. The solution was filtered, and the filtrate was washed three times successively with water, 2M NaOH solution, and water. The solvent was removed by vacuum distillation of the washed organic phase to obtain the final product bis(4-formylphenyl) phenylphosphonate (P-CHO). The reaction process is shown below.

[0024]

[0025] (2) Synthesis of intermediate P-NH2: p-Aminophenol was dissolved in an organic solvent with a strong base. Phenylphosphonyl dichloride dissolved in the organic solvent was added dropwise to the reaction solution under a nitrogen atmosphere at 0-5°C for 8-10 hours. The solution was filtered, and the obtained organic phase was distilled under reduced pressure to remove the solvent. The resulting solid was then dissolved in another organic solvent. The solution was washed three times successively with water, 2M NaOH solution, and water. The washed organic phase was distilled under reduced pressure to remove the solvent, yielding the final product bis(4-aminophenyl)phenylphosphonate (P-NH2). The reaction process is shown below.

[0026]

[0027] (3) Synthesis of the target product PCNPO: P-NH2 and P-CHO were added to an organic solvent and reacted at 50°C for 6 hours under a nitrogen atmosphere to generate Schiff base polymer PCNH; then DOPO was added and the mixture was heated to 80°C and refluxed for 12 hours. The solution was filtered, and the filter cake was slurried with an organic solvent for 30 minutes and then filtered. The solid product was dried in a vacuum drying oven at 60°C to obtain the final molecular PCNPO. The reaction process is shown below.

[0028]

[0029] Thirdly, the present invention provides a flame retardant composition which may include the above-mentioned nitrogen-phosphorus flame retardant PCNPO and melamine cyanurate MCA.

[0030]

[0031] In some embodiments, the flame retardant composition comprises PCNPO and MCA in a weight ratio of 1:(1.0-3.0), preferably 1:(1.1-2.25).

[0032] Fourthly, the present invention provides a flame-retardant nylon 6 composite material, which, by weight, may include or be composed of the following components:

[0033] 100 parts of Nylon 6 resin;

[0034] 4-7 parts of nitrogen-phosphorus flame retardant PCNPO;

[0035] Melamine cyanurate (MCA) 8-11 parts;

[0036] Antioxidant 0.5-1 part; and

[0037] 0.3-0.5 parts of lubricant.

[0038] In some embodiments, the antioxidant may be selected from hindered phenolic antioxidant 1076, phosphite antioxidant 168, or a combination thereof; optionally, the antioxidant may be a mixture of hindered phenolic antioxidant 1076 and phosphite antioxidant 168 in a weight ratio of 1:1, which can effectively inhibit the oxidative degradation of nylon 6 during processing and use, and improve the thermo-oxidative stability of the material.

[0039] In some embodiments, the lubricant may be ethylene bis-stearamide (EBS), which has a melting point of 140-145°C. It can reduce the melt viscosity during material processing, reduce friction between materials and equipment, improve processing fluidity, and improve the surface finish of materials.

[0040] Fifthly, the present invention provides a method for preparing the above-mentioned flame-retardant nylon 6 composite material, which may include the following steps:

[0041] Nylon 6 resin, nitrogen-phosphorus flame retardant, melamine cyanurate, antioxidant and lubricant are mixed evenly to obtain a mixture.

[0042] The mixture was added to a twin-screw extruder for melt blending, extrusion granulation, and the resulting flame-retardant nylon 6 granules were obtained.

[0043] After drying, the flame-retardant nylon 6 particles are added to an injection molding machine for injection molding to obtain a flame-retardant nylon 6 composite material.

[0044] In some embodiments, in the above method, the processing temperature of the twin-screw extruder can be 220-260℃, and the rotation speed can be 70-80 rpm; the processing temperature of the injection molding machine can be 220-235℃, and the injection pressure can be 50-60 MPa.

[0045] In a sixth aspect, the present invention provides the application of the above-mentioned nitrogen-phosphorus flame retardant or flame retardant composition in the preparation of flame-retardant polymer materials.

[0046] In some embodiments, the polymer material may be one or more selected from nylon 6, nylon 66, polyester, and epoxy resin.

[0047] Compared with the prior art, the present invention has the following beneficial effects:

[0048] 1. Synergistic Flame Retardancy from Dual Phosphorus Sources: The PCNPO molecule designed in this invention possesses both skeletal phosphorus (from phenylphosphonodichloro) and side-chain phosphorus (from DOPO), forming a dual phosphorus source. During thermal decomposition, the skeletal phosphorus preferentially releases phosphoric acid, catalyzing the char formation of nylon 6 and forming a dense char layer. The side-chain phosphorus decomposes to generate PO· free radicals, quenching the combustion chain reaction and achieving a synergistic flame retardant effect from the dual phosphorus sources. Furthermore, the PCNPO provided by this invention combines the condensed-phase flame retardant effect of phosphorus with the gas-phase flame retardant effect of nitrogen. When combined with MCA, it forms a "phosphorus-nitrogen" synergistic flame retardant system. The phosphoric acid produced by the decomposition of PCNPO catalyzes the dehydration and char formation of nylon 6, resulting in a dense char layer. This technical solution reduces the amount of MCA added to nylon 6 and enables nylon 6 materials to meet the UL94 V-0 flame retardant standard (1.6mm sample).

[0049] 2. The nitrogen-phosphorus flame retardant PCNPO provided by this invention exhibits a different flame retardant mechanism in the synergistic flame retardant process compared to when MCA is used alone: ​​when MCA is used alone, it mainly uses a gas-phase flame retardant mechanism, resulting in a loose char structure and insufficient long-term stability; after compounding, it transforms into a condensation phase mechanism, where the nitrogen-phosphorus flame retardant PCNPO can catalyze the formation of a continuous and dense char layer as an excellent protective barrier covering the material surface, effectively blocking the transfer of combustible gases, heat, and oxygen.

[0050] 3. Excellent mechanical property retention: The PCNPO provided by this invention has a polymer structure and good compatibility with the nylon 6 matrix. When the total flame retardant (PCNPO and MCA) is added in an amount of 12-15 parts, the tensile strength of the composite material is maintained above 55 MPa (retention rate >98%), and the impact strength is maintained above 4.07 kJ / m² (retention rate >83%).

[0051] 4. Excellent charring ability: The phosphonate structure in the PCNPO skeleton catalyzes the charring of nylon 6 during combustion, and the char residue rate of the composite material reaches 4.23-6.02%. The char layer is dense and continuous, which can effectively block the transfer of heat and oxygen.

[0052] 5. Good thermal stability and processing performance: The initial decomposition temperature of PCNPO is higher than 290℃, and the initial decomposition temperature of MCA is higher than 350℃, both of which are much higher than the processing temperature of Nylon 6 (220-260℃). Therefore, no smoke or odor is generated during processing, making it suitable for industrial production.

[0053] 6. Environmentally friendly: The flame retardants used in this invention are all halogen-free flame retardants, which do not release toxic or harmful gases during combustion, comply with current environmental regulations, and can be widely used in fields with high environmental protection requirements such as electronics, electrical appliances, and automotive parts. Attached Figure Description

[0054] Figure 1 This is the 1H NMR spectrum of the intermediate P-CHO molecule synthesized in this invention.

[0055] Figure 2 This is the 1H NMR spectrum of the intermediate P-NH2 molecule during the synthesis process of this invention.

[0056] Figure 3 The images show the phosphorus NMR spectra of DOPO, P-NH2, the intermediate Schiff base PCNH, and the flame retardant molecule PCNPO of this invention.

[0057] Figure 4 Fourier transform infrared spectra of DOPO, the intermediate Schiff base PCNH, and the flame retardant molecule PCNPO of this invention.

[0058] Figure 5 This is a thermogravimetric curve of the flame retardant molecule PCNPO of the present invention in an air atmosphere. Detailed Implementation

[0059] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. However, it should be understood that the specific embodiments of this invention are only for explaining the invention and are not intended to limit the scope of protection of this invention.

[0060] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0061] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0062] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0063] In the description of this article, it should be noted that, unless otherwise stated, "one or more" means two or more.

[0064] Raw materials used in this invention and their sources:

[0065] Nylon 6 resin was purchased from Jiangsu Ruimeifu Co., Ltd.

[0066] p-Hydroxybenzaldehyde, phenylphosphonodichloro, triethylamine, p-aminophenol, sodium hydroxide, and DOPO: all were analytical grade and purchased from Aladdin Technology Co., Ltd.

[0067] MCA, hindered phenolic antioxidant 1076, phosphite antioxidant 168, and EBS: all industrial products, purchased from Qingdao Yousuo Chemical Technology Co., Ltd.

[0068] Performance testing methods:

[0069] 1. Limiting Oxygen Index (LOI): Tested using an oxygen indexer according to ASTM D2863 standard, with a sample size of 127.0mm × 12.7mm × 3.2mm.

[0070] 2. Vertical Burning Test (UL-94): Tested using a vertical burning chamber according to ASTM D3801 standard, with a sample size of 127.0mm × 12.7mm × 1.6mm.

[0071] 3. Residual mass: Thermogravimetric analysis (TGA) was used under nitrogen atmosphere, with a heating rate of 10℃ / min, from 30℃ to 800℃.

[0072] 4. Tensile strength: The test was conducted using a universal testing machine according to ISO 527-1 standard. The tensile speed was 20 mm / min, and each sample was tested 5 times. The average value was taken as the final test result.

[0073] 5. Impact strength: Tested using a cantilever beam impact testing machine according to ISO 180 standard.

[0074] 6. Thermal decomposition behavior: The thermal decomposition behavior of the pretreated flame retardant PCNPO was tested using a thermogravimetric analyzer (TGA) at a heating rate of 10℃ / min from 30℃ to 800℃ in air.

[0075] 7. GPC determination of degree of polymerization: Standard polystyrene was used as the standard sample. The chromatographic column was a PSt microgel column (HR1, HR4). The parameters were set as follows: 40℃, DMF mobile phase flow rate 1.0 mL / min.

[0076] Example 1: Synthesis of PCNPO and Preparation of Flame-Retardant Nylon 6 Composite Material

[0077] (1) Preparation of nitrogen-phosphorus flame retardant PCNPO

[0078] (a) Synthesis of intermediate P-CHO: 36.6 g (0.3 mol) of p-hydroxybenzaldehyde was dissolved in 350 mL of anhydrous dichloromethane, and 83.2 mL (0.6 mol) of triethylamine was added. Under nitrogen protection at 0–5 °C, 34 mL (0.15 mol) of phenylphosphonodichlorodichloride dissolved in dichloromethane was slowly added dropwise to the reaction system, and the reaction was carried out for 8–10 hours. The mixture was filtered, and the filtrate was washed successively with water, 2 M NaOH solution, and water. The organic phase after washing was distilled under reduced pressure to remove the solvent, yielding the final product bis(4-formylphenyl) phenylphosphonate (P-CHO). Figure 1 The hydrogen NMR spectrum of the P-CHO molecule is shown.

[0079] (b) Synthesis of intermediate P-NH2: 32.7 g (0.3 mol) of p-aminophenol and 12 g (0.3 mol) of sodium hydroxide were dissolved in 350 mL of tetrahydrofuran. Under nitrogen protection at 0–5 °C, 34 mL (0.15 mol) of phenylphosphonodichlorophenol was dissolved in 100 mL of tetrahydrofuran and then slowly added dropwise, reacting for 8–10 hours. After filtration, the solvent was removed by vacuum distillation of the obtained organic phase. The solid was dissolved in dichloromethane and washed successively with water, 2 M NaOH solution, and water. The solvent was removed by vacuum distillation of the organic phase to obtain the final product bis(4-aminophenyl)phenylphosphonate (P-NH2). Figure 2 The hydrogen nuclear magnetic resonance spectrum of the P-NH2 molecule is shown.

[0080] (c) Synthesis of the target product PCNPO: 51.0 g (0.15 mol) P-NH2 and 36.6 g (0.1 mol) P-CHO were added to anhydrous ethanol and reacted at 50 °C for 6 hours under a nitrogen atmosphere. Then, 25.92 g (0.12 mol) DOPO was added, and the mixture was heated to 80 °C and refluxed for 12 hours. After the reaction was completed, the mixture was filtered, and the filter cake was washed with anhydrous ethanol for 30 minutes and filtered again. The solid product was dried under vacuum at 60 °C to constant weight to obtain PCNPO (the degree of polymerization was determined to be 119 by GPC (gel permeation chromatography)). Figure 3 The NMR phosphorus spectra of DOPO, P-NH2, the intermediate Schiff base PCNH, and the flame retardant molecule PCNPO are shown. Figure 4 Fourier transform infrared (FTIR) spectra of DOPO, the intermediate Schiff base PCNH, and the flame retardant molecule PCNPO are shown. Figure 3 and Figure 4 It can be seen that PCNPO, in the presence of the intermediate Schiff base PCNH, adds a doublet similar to that on the DOPO molecule, proving that DOPO is successfully attached to the molecule and that there is no residue of unreacted DOPO molecules. (1238 cm⁻¹) -1 Corresponding to the P=O absorption band, 1477 cm⁻¹ -1 Corresponding to PC tensile vibration, 1045cm -1 This corresponds to the POC bond stretching vibration. For the molecule PCNPO, the absorption peak of the PH bond disappears, and the POC stretching vibration, the P=O absorption band, and the PC stretching vibration appear, proving that DOPO was successfully attached to the molecule. In summary, both NMR phosphorus spectroscopy and Fourier transform infrared spectroscopy jointly demonstrate the successful synthesis of the flame-retardant molecule PCNPO.

[0081] The thermal decomposition behavior of the prepared flame retardant PCNPO in air atmosphere was tested, and the results are as follows: Figure 5As shown, the decomposition of PCNPO exhibits multi-stage characteristics: at 300-400℃, the phosphonate structure in the skeleton preferentially decomposes, releasing phosphoric acid substances (catalyzing carbonization); at 400-600℃, the side chain DOPO decomposes, releasing PO· free radicals (vapor-phase quenching); above 600℃, the residual carbon layer further cross-links, forming a stable protective layer. This staged decomposition may be a direct manifestation of the "skeleton phosphorus + side chain phosphorus" dual phosphorus source design. Furthermore, it can be seen that PCNPO has almost no mass loss below 300℃ (maintaining 98%), and reaches a 5% loss at around 350℃ (indicating that its initial decomposition temperature is about 350℃), which fully meets the processing requirements of Nylon 6 at 220-260℃ (PCNPO will not decompose prematurely during the melt blending process of Nylon 6, ensuring processing stability and the integrity of the flame retardant). Moreover, PCNPO can still maintain a char rate of about 15% at a high temperature of 800℃, indicating that PCNPO itself has excellent char-forming ability. The char formed after its decomposition can serve as a char layer precursor, catalyzing the char formation of Nylon 6.

[0082] (2) Raw material pretreatment

[0083] Nylon 6 resin was placed in a vacuum drying oven and dried at 100℃ for 4 hours to remove moisture and avoid defects such as bubbles and silver streaks caused by moisture during processing. PCNPO, MCA, antioxidant (1076:168=1:1) and lubricant EBS were dried at 60-80℃ for 4-6 hours to remove surface adsorbed water and improve flame retardant efficiency and dispersion uniformity.

[0084] (3) Preparation of flame-retardant nylon 6 composite material

[0085] Weigh out the following components by weight: 100 parts Nylon 6 resin, 4 parts PCNPO, 11 parts MCA, 1 part antioxidant, and 0.5 parts EBS. Mix all components evenly in a high-speed mixer and add them to a twin-screw extruder. The extruder temperatures are as follows: Zone 1 226℃, Zone 2 228℃, Zone 3 235℃, Zone 4 240℃, Zone 5 243℃, Zone 6 240℃, and the main extruder speed is 76 rpm. Extrusion granulation yields flame-retardant Nylon 6 granules. After vacuum drying the granules at 100℃ for 4 hours, they are added to an injection molding machine for injection molding. The injection molding machine temperatures are as follows: Zone 1 230℃, Zone 2 225℃, Zone 3 225℃, Zone 4 220℃, Zone 5 220℃, and the injection pressures are as follows: Zone 1 55MPa, Zone 2 55MPa, Zone 3 55MPa.

[0086] The properties of the obtained flame-retardant nylon 6 composite material are shown in Table 1.

[0087] Example 2

[0088] The preparation method of Example 2 is basically the same as that of Example 1, except that the degree of polymerization of PCNPO is different (n is 134 in this example). The specific ratio is shown in Table 1.

[0089] Examples 3-5

[0090] The preparation methods of Examples 3-5 are basically the same as those of Example 1, except that the ratio of PCNPO and MCA is different. The specific ratio is shown in Table 1.

[0091] Comparative Example 1: Pure PA6

[0092] The preparation method is the same as in Example 1, but without adding any flame retardant. The ratio is: 100 parts PA6, 1 part antioxidant, and 0.5 parts EBS.

[0093] Comparative Example 2: Adding only MCA

[0094] The preparation method is the same as in Example 1, but without the addition of PCNPO. The amount of MCA added is 15 parts, and the ratio is: 100 parts PA6, 15 parts MCA, 1 part antioxidant, and 0.5 parts EBS.

[0095] Comparative Example 3: TMA-DOPO+MCA

[0096] TMA-DOPO was synthesized according to the method described in Existing Technology 1. TMA-DOPO was compounded with MCA, and the addition amounts were as described in Existing Technology 1 to achieve a V-0 rating (PA6 100 parts, TMA-DOPO 5 parts, MCA 3 parts). The remaining steps were the same as in Example 1. Performance test results are shown in Table 1.

[0097] Comparative Example 4: PCNPO used alone (without MCA)

[0098] PCNPO was synthesized according to Example 1 and then blended separately with PA6 at a dosage of 15 parts. The remaining steps were the same as in Example 1. The performance test results are shown in Table 1.

[0099] Comparative Example 5: Synthesis of low-polymerization-degree PCNPO (n<100) and its flame-retardant nylon 6 composite material compounded with MCA

[0100] (1) Synthesis of low degree of polymerization PCNPO

[0101] Following the synthesis method of Example 1, only the molar ratio of P-NH2 to P-CHO in step (3) was changed. Specifically, 42.5 g (0.125 mol) of P-NH2 and 36.6 g (0.1 mol) of P-CHO were added to anhydrous ethanol (i.e., the molar ratio of P-NH2:P-CHO was 1.25:1). The reaction was carried out at 50°C for 6 hours under a nitrogen atmosphere, and then 21.6 g (0.1 mol) of DOPO (the molar ratio of DOPO to P-NH2 was 0.8:1) was added, and the mixture was heated to 80°C and refluxed for 12 hours. The post-treatment was the same as in Example 1, and a pale yellow powder product was obtained. According to GPC determination, its degree of polymerization n was approximately 90, and the product was named PCNPO-L (low degree of polymerization PCNPO (n<100)).

[0102] (2) Preparation of flame-retardant nylon 6 composite material

[0103] Flame-retardant nylon 6 composite material was prepared according to the formulation of Example 4: 100 parts nylon 6 resin, 7 parts PCNPO-L, 8 parts MCA, 1 part antioxidant, and 0.5 parts EBS. The preparation method was the same as in Example 1. The performance test results are shown in Table 1.

[0104] Comparative Example 6: Synthesis of high degree of polymerization PCNPO (n>150) and its flame-retardant nylon 6 composite material compounded with MCA

[0105] (1) Synthesis of high degree of polymerization PCNPO

[0106] Following the synthesis method of Example 1, only the molar ratio of P-NH2 to P-CHO in step (3) was changed. Specifically, 61.2 g (0.18 mol) of P-NH2 and 36.6 g (0.1 mol) of P-CHO were added to anhydrous ethanol (i.e., the molar ratio of P-NH2 to P-CHO was 1.8:1). The reaction was carried out at 50°C for 6 hours under a nitrogen atmosphere, and then 31.1 g (0.144 mol) of DOPO (the molar ratio of DOPO to P-NH2 was 0.8:1) was added, and the mixture was heated to 80°C and refluxed for 12 hours. The post-treatment was the same as in Example 1, and a brown powder was obtained. According to GPC determination, its degree of polymerization n was approximately 153, and the product was named PCNPO-H (high degree of polymerization PCNPO (n>150)).

[0107] (2) Preparation of flame-retardant nylon 6 composite material

[0108] According to the proportions in Example 4: 100 parts of nylon 6 resin, 4 parts of PCNPO-H, 11 parts of MCA, 1 part of antioxidant, and 0.5 parts of EBS, flame-retardant nylon 6 composite material was prepared.

[0109] However, due to the excessively high molecular weight of PCNPO-H and its extremely low melt flow rate, excessive torque in the extruder causes overload protection of the equipment, and the material undergoes severe degradation in the screw, making normal extrusion granulation impossible. Even if extrusion granulation is successfully achieved, the extremely high melt viscosity and poor flowability of the material prevent it from filling the mold cavity during injection molding to prepare test specimens, making it impossible to obtain defect-free specimens that meet the testing standards.

[0110] Table 1

[0111]

[0112] The measurement results in Table 1 show that:

[0113] Comparative Example 1 (pure PA6) provides a performance benchmark: UL94 NR, LOI 21.7%, char residue (i.e., residual mass) 3.89%, tensile strength 57.08 MPa, and impact strength 4.90 kJ / m². Comparative Example 2 (15 parts MCA only) shows the limitations of using MCA alone: ​​although it increases LOI to 27.2% and achieves UL94 V-2 rating, the char residue only increases to 4.28%, tensile strength decreases by 13.6% to 49.31 MPa, and impact strength decreases by 24.5% to 3.70 kJ / m². This demonstrates that a balance between "high flame retardancy + high performance" cannot be achieved with MCA alone. Although Comparative Example 3 (5 parts TMA-DOPO, 3 parts MCA) exhibits highly efficient gas-phase flame retardant properties with a low total flame retardant addition, it severely sacrifices the material's thermal stability and mechanical properties. Data shows that its LOI value reached 30.7%, the highest among all comparative examples, and its UL94 rating reached V-0. However, its char residue was only 2.00%, even lower than the 3.89% of pure PA6, indicating that it hardly promoted char formation and relied entirely on gas-phase mechanisms. More importantly, the introduction of TMA-DOPO caused the tensile strength to plummet to 26.5 MPa, a decrease of 53.6% compared to pure PA6, indicating that the introduction of the flame retardant TMA-DOPO severely damaged the integrity of the molecular chain. According to the general rules of polymer composites, the higher the amount of small molecule flame retardant added, the more severe its tendency to agglomerate in the matrix, and the negative impact on mechanical properties usually tends to intensify. Therefore, it is reasonable to expect that if the total amount of TMA-DOPO and MCA is increased to 15 parts (e.g., 7 parts TMA-DOPO + 8 parts MCA), its mechanical properties will further deteriorate. In contrast, the PCNPO flame retardant of this invention exhibits better compatibility and less loss of mechanical properties when compounded with the matrix at the same addition amount. Comparative Example 4 (PCNPO only, 15 parts) demonstrated the significant advantages of condensed-phase flame retardancy, but its use alone was insufficient to achieve a UL94 V-0 rating. This formulation achieved a char residue of 9.10%, the highest among all comparative examples, proving that PCNPO effectively promotes char formation in PA6, creating a dense char layer to insulate against heat and oxygen. However, its LOI value was only 25.9%, and it only achieved a UL94 V-2 rating, indicating that the condensed-phase flame retardancy alone is insufficient for rapid self-extinguishing in vertical burning tests. In terms of mechanical properties, tensile strength and impact strength decreased to 51.22 MPa and 3.90 kJ / m², respectively, which, while better than Comparative Example 2 (MCA only), still showed a significant loss compared to pure PA6. Comparative Example 5 (using PCNPO-L+MCA) revealed the crucial influence of the flame retardant's molecular weight on its performance.With the same addition amount as in Example 5 (8 parts MCA + 7 parts flame retardant), PCNPO-L's performance was inferior to PCNPO in all aspects: its UL94 rating was only V-2 (Example 5 was V-0), its LOI value of 28.1% was lower than 30.2% in Example 5, and its char residue of 5.08% was also lower than 6.02% in Example 5. This indicates that PCNPO-L with a lower degree of polymerization is significantly less effective than PCNPO with a higher degree of polymerization in promoting char formation and overall flame retardancy. This may be due to its shorter molecular chains, which are more prone to migration or decomposition during processing or combustion, making it difficult to form a stable and effective cross-linked network. In terms of mechanical properties, the tensile strength of the PCNPO-L formulation (56.02 MPa) and the impact strength (4.26 kJ / m²) were also lower than those of Example 5.

[0114] Examples 1-5 (using PCNPO + MCA of this invention, with a total flame retardant addition of 12-18 parts) all achieved UL94 V-0 rating, LOI values ​​of 29.2-30.2%, tensile strength maintained above 55.89 MPa (close to 57.08 MPa of pure PA6, retention rate >98%), char residue of 4.23-6.02%, and impact strength maintained above 4.07 kJ / m² (impact strength retention rate >83%). This demonstrates that the present invention achieves an excellent balance between flame retardant performance and mechanical properties. Combined with the results of Comparative Examples 1 and 2, it can be seen that the addition of PCNPO not only reduces the amount of MCA used but also improves flame retardant performance and restores mechanical properties through the synergistic effect of dual phosphorus sources, reflecting the substantial progress of the present invention.

[0115] In summary, this invention, by designing a dual phosphorus source polymer PCNPO with a phosphorus-containing skeleton and DOPO-containing side chains, and controlling the degree of polymerization within the range of 100-150 (preferably 110-140, more preferably 119-134), and compounding it with MCA, can achieve UL94 V-0 flame retardancy at a relatively low addition amount, while maintaining excellent mechanical properties (tensile strength retention rate >98%, impact strength retention rate >83%), overcoming the difficulty in balancing flame retardancy efficiency and mechanical properties in the prior art.

[0116] The above detailed description further illustrates the purpose, technical solution, and beneficial effects of the invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A nitrogen-phosphorus flame retardant having a structure as shown in formula (I): （I） In equation (I), n is an integer between 100 and 150.

2. The nitrogen-phosphorus flame retardant according to claim 1, wherein n is an integer from 110 to 140, preferably an integer from 119 to 134.

3. A method for preparing the nitrogen-phosphorus flame retardant as described in claim 1, comprising the following steps: (1) Reaction of p-hydroxybenzaldehyde with phenylphosphonic dichloro in the presence of an acid-binding agent yields bis(4-formylphenyl) phenylphosphonate; (2) Reaction of p-aminophenol with phenylphosphonodichloro in the presence of a strong base yields bis(4-aminophenyl)phenylphosphonate; and (3) The bis(4-aminophenyl)phenylphosphonate obtained in step (2) and the phenylphosphonic acid bis(4-formylphenyl) ester obtained in step (1) are reacted to generate a Schiff base polymer, and then 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide is added to continue the reaction to obtain the nitrogen-phosphorus flame retardant.

4. The method according to claim 3, wherein: In step (1), the molar ratio of phenylphosphonic dichloro to p-hydroxybenzaldehyde is 1:(2.0-2.2); the molar ratio of p-hydroxybenzaldehyde to the acid-binding agent is 1:(2.0-2.2); optionally, the acid-binding agent is one or more selected from triethylamine, pyridine, N,N-diisopropylethylamine and 4-dimethylaminopyridine; and / or In step (2), the molar ratio of phenylphosphonodichloro to p-aminophenol is 1:(2.0-2.2); the molar ratio of p-aminophenol to the strong base is 1:(1.0-1.1); optionally, the strong base is one or more selected from sodium hydroxide, potassium hydroxide, potassium tert-butoxide, sodium hydride, triethylamine, N,N-diisopropylethylamine and 1,8-diazabicyclo[5.4.0]undec-7-ene; and / or In step (3), the molar ratio of bis(4-formylphenyl) phenylphosphonate to bis(4-aminophenyl)phenylphosphonate is 1:(1.3-1.5); the molar ratio of bis(4-aminophenyl)phenylphosphonate to 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide is 1:(0.8-1.0).

5. A flame retardant composition comprising the nitrogen-phosphorus flame retardant as described in claim 1 or 2 and melamine cyanurate.

6. The flame retardant composition according to claim 5, wherein the weight ratio of the nitrogen-phosphorus flame retardant to melamine cyanurate is 1:(1.0-3.0), optionally 1:(1.1-2.25).

7. A flame-retardant nylon 6 composite material, comprising or composed of the following components in parts by weight: 100 parts of Nylon 6 resin; 4-7 parts of the nitrogen-phosphorus flame retardant as described in claim 1 or 2; 8-11 parts of melamine cyanurate; Antioxidant 0.5-1 part; and 0.3-0.5 parts of lubricant.

8. The flame-retardant nylon 6 composite material according to claim 7, wherein the antioxidant is selected from one or more of the following: hindered phenolic antioxidant 1076 and phosphite antioxidant 168; optionally, the antioxidant is a mixture of hindered phenolic antioxidant 1076 and phosphite antioxidant 168 in a weight ratio of 1:1; and / or The lubricant is ethylene bis-stearamide.

9. A method for preparing the flame-retardant nylon 6 composite material as described in claim 7 or 8, comprising the following steps: Nylon 6 resin, nitrogen-phosphorus flame retardant, melamine cyanurate, antioxidant and lubricant are mixed evenly to obtain a mixture. The mixture was added to a twin-screw extruder for melt blending and extrusion granulation to obtain flame-retardant nylon 6 particles. and After drying, the flame-retardant nylon 6 particles are added to an injection molding machine for injection molding to obtain a flame-retardant nylon 6 composite material.

10. The method according to claim 9, wherein the twin-screw extruder has a processing temperature of 220-260°C and a rotation speed of 70-80 rpm; and the injection molding machine has a processing temperature of 220-235°C and an injection pressure of 50-60 MPa.

11. The use of the nitrogen-phosphorus flame retardant as described in claim 1 or 2, or the flame retardant composition as described in claim 5 or 6, in the preparation of flame-retardant polymer materials.

12. The application according to claim 11, wherein the polymeric material is one or more selected from nylon 6, nylon 66, polyester and epoxy resin.

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