Halogen-free intrinsic flame-retardant nylon 66 with phosphorus-nitrogen-silicon multi-synergism and preparation method thereof

Abstract

The application discloses a phosphorus-nitrogen-silicon multi-coordinated halogen-free intrinsic flame-retardant nylon 66 and a preparation method thereof. The halogen-free intrinsic flame-retardant nylon 66 is prepared from a kind of phosphorus, nitrogen and silicon element-containing amino-terminated polysiloxane-based reactive flame-retardant monomer and nylon 66 salt through in-situ copolymerization. The flame-retardant monomer is prepared by carrying out a silicon-hydrogen addition reaction on an amino-terminated polysiloxane precursor with active Si-H bonds and a phosphorus / nitrogen compound containing unsaturated double bonds. Through the copolymerization reaction, the flame-retardant unit is introduced into the nylon 66 main chain at the molecular level, and the compatibility problem and the migration problem of the traditional additive flame retardant are solved. The application utilizes the synergistic flame-retardant effect of phosphorus, nitrogen and silicon elements, so that the obtained copolymerized nylon 66 can achieve excellent flame-retardant performance under the condition of a low flame-retardant monomer addition amount, and meanwhile, the inherent high strength, high toughness and other excellent mechanical properties of the nylon 66 are maximally maintained, the technical problem that the flame retardance and the mechanical properties are difficult to balance is solved, and the application has a wide industrial application prospect.

Description

Technical Field

[0001] This invention relates to the field of flame-retardant polymer materials technology, specifically to a phosphorus-nitrogen-silicon multi-synergistic halogen-free intrinsic flame-retardant nylon 66 and its preparation method. Background Technology

[0002] Nylon 66 (PA66) is a high-performance thermoplastic engineering plastic formed by the condensation polymerization of hexamethylenediamine and adipic acid. Due to its comprehensive advantages such as high strength, high heat resistance, abrasion resistance, and excellent chemical corrosion resistance, it is widely used in the automotive, machinery, electronics, and chemical industries. However, as a flammable material, Nylon 66 typically has a limiting oxygen index (LOI) of only 21-24%, and its UL-94 vertical flammability rating is mostly HB. This inherent flammability severely limits its application in situations requiring high fire safety standards. Therefore, effective flame-retardant modification of Nylon 66 to improve its fire safety has become a key research focus and industrialization need in this field.

[0003] Currently, flame-retardant modification of nylon 66 mainly employs two technical approaches: additive and reactive. Additive flame retardancy involves physical blending to composite the flame retardant with the nylon 66 matrix; this method is simple and low-cost. However, to achieve the ideal flame retardant rating (such as UL-94V-0), a large amount (typically as high as 20-30 wt%) of flame retardant is often required, which inevitably and severely impairs the material's mechanical properties (such as strength and toughness) and processing flowability. Furthermore, migration and precipitation problems caused by poor compatibility between the flame retardant and the matrix also challenge the durability of the flame-retardant effect. In contrast, reactive flame retardant technology introduces unit structures containing flame-retardant elements into the nylon 66 molecular backbone through chemical reactions, forming intrinsically flame-retardant copolymers. This method fundamentally solves the compatibility problem, endowing the material with durable and highly efficient flame-retardant properties. Among them, in-situ copolymerization, which uses flame-retardant monomers with amino or carboxyl groups to participate in the polycondensation reaction with nylon 66 salt, is recognized as the most promising way to prepare high-performance intrinsic flame-retardant nylon.

[0004] In the selection of flame retardant systems, given that traditional halogenated flame retardants produce toxic and corrosive gases during combustion, posing a threat to the environment and health, halogen-free flame retardancy has become an inevitable trend. In halogen-free systems, phosphorus, nitrogen, and silicon, the three flame-retardant elements, each have their own characteristics: phosphorus-based flame retardants mainly function in the condensed phase by promoting char formation and forming a thermal and material insulating barrier; nitrogen-based flame retardants mainly function in the gas phase by decomposing and releasing non-flammable gases to dilute the concentration of combustibles; while silicon-based flame retardants tend to form a stable silicate protective layer on the material surface and can effectively synergize with phosphorus-based flame retardants to enhance the density and stability of the char layer. Based on this, integrating phosphorus, nitrogen, and silicon through rational molecular design to construct a multi-element synergistic flame-retardant system is expected to achieve a "1+1+1>3" high-efficiency flame retardancy with lower addition amounts, becoming a key breakthrough in existing technological bottlenecks.

[0005] Despite some exploration in existing technologies, both achievements and challenges remain. For example, existing technology "CN202211086418.X" discloses a series of phosphorus-containing copolymer flame-retardant monomers, which exhibit good flame retardancy at low addition levels, but their flame-retardant mechanism is relatively simple, and the synthesis route is complex and costly. Existing technology "CN202311433885.X" discloses a phosphorus-nitrogen synergistic copolymer flame-retardant monomer based on DOPO. Its examples show that when the material's flame retardancy rating reaches UL-94V-0, its tensile strength drops significantly, even falling below 50 MPa, severely weakening the value of Nylon 66 as a structural material. This clearly reveals a common and thorny dilemma in the current technology field: while the introduction of reactive flame-retardant monomers improves flame retardant performance, it often inevitably causes significant degradation of the material's key mechanical properties. Especially when pursuing high flame retardancy ratings, the sacrifice of mechanical properties is even more prominent, which has become a technical bottleneck widely recognized by those skilled in the art.

[0006] Therefore, to address the aforementioned problems, this invention provides a halogen-free intrinsically flame-retardant nylon 66 with phosphorus-nitrogen-silicon synergistic properties and its preparation method. By in-situ melt copolymerizing a terminal amino-terminated polysiloxane flame-retardant monomer with a specific molecular structure with a nylon 66 salt, phosphorus, nitrogen, and silicon flame-retardant elements are successfully introduced into the nylon 66 backbone through chemical bonding, achieving uniform distribution and efficient synergy of flame-retardant elements at the molecular level. This method not only endows the material with excellent intrinsic flame-retardant properties (high LOI value and UL-94V-0 rating), but also effectively overcomes the problem of deterioration in the mechanical properties of traditional flame-retardant nylons through the toughening effect of the flexible siloxane segments and the synergistic and stable char layer structure. Thus, while achieving highly efficient flame retardancy, it maintains and even optimizes the comprehensive mechanical properties of nylon 66. Summary of the Invention

[0007] The purpose of this invention is to provide a halogen-free intrinsic flame-retardant nylon 66 with phosphorus-nitrogen-silicon multi-synergistic properties and its preparation method, so as to achieve uniform distribution and efficient synergy of flame-retardant elements at the molecular level; and to maintain or even optimize the comprehensive mechanical properties of nylon 66 while achieving high flame retardancy.

[0008] The objective of this invention is achieved through the following technical solution:

[0009] A phosphorus-nitrogen-silicon multi-synergistic halogen-free intrinsic flame-retardant nylon 66 is a copolymer obtained by melt polycondensation reaction of nylon 66 salt, amino-terminated polysiloxane flame-retardant monomer and adipic acid; the amino-terminated polysiloxane flame-retardant monomer has the structure shown in Formula I.

[0010] , Formula I;

[0011] Where m is an integer from 0 to 50, and n is an integer from 0 to 50; the -R group is an organic group containing phosphorus, nitrogen, or a combination of phosphorus and nitrogen.

[0012] Preferably, when -R is a phosphorus-containing group, the flame-retardant monomer is a phosphorus-silicon synergistic system, -R is selected from the structure shown in A or B of Formula II, and -X is selected from the structure shown in C, D, E or F of Formula III;

[0013] Formula II;

[0014] Formula III.

[0015] More preferably, when -R is a phosphorus-containing group, the flame-retardant monomer is a phosphorus-silicon synergistic system, -R is selected from structure B in formula II, and -X is selected from one of structures C, D and F in formula III.

[0016] Preferably, when -R is a nitrogen-containing group, the flame-retardant monomer is a nitrogen-silicon synergistic system, -R has the structure shown in Formula IV, wherein -X is selected from A, B, C or D in Formula V, and -Y is selected from A, B or C in Formula VI:

[0017] Formula IV;

[0018] , formula V;

[0019] , style VI.

[0020] More preferably, when -R is a nitrogen-containing group, the flame-retardant monomer is a nitrogen-silicon synergistic system, -X is selected from one of structures A, C and D in formula V, and -Y is selected from one of structures A and C in formula VI.

[0021] Preferably, when -R is a phosphorus-nitrogen coexisting group, the flame retardant monomer is a phosphorus-nitrogen-silicon synergistic system, and -R has the structure shown in Formula VII, wherein -X is selected from the structure shown in Formula III as C, D, E or F;

[0022] Equation VII;

[0023] More preferably, when -R is a phosphorus-nitrogen coexisting group, the flame retardant monomer is a phosphorus-nitrogen-silicon synergistic system, and -X is selected from one of structures C, D and F in structural formula III.

[0024] Preferably, the terminal amino polysiloxane flame retardant monomer is prepared by grafting a phosphorus / nitrogen compound containing unsaturated double bonds onto a terminal amino methylhydrosiloxane-dimethylsiloxane copolymer via a hydrosilylation reaction; the phosphorus / nitrogen compound containing unsaturated double bonds is selected from one of dimethyl acryloyl phosphate (monomer 1), DOPO-AA (monomer 2), 2-acryloyloxyethyl-benzylphenylphosphonate (monomer 3), N-allyl-1,1,1-trimethyl-N-(trimethylsilyl)silamide (monomer 4), dibenzylallyl phosphoramide (monomer 5), dimethylallyl phosphoramide (monomer 6), and 6-((allylamino)methyl)dibenzo[c,e][1,2]oxaphosphacyclohexane 6-oxide (monomer 7).

[0025] Furthermore, the phosphorus / nitrogen compounds containing unsaturated double bonds (i.e., the precursors of the -R groups of the flame-retardant monomers) described in this invention can be prepared by organic synthesis methods known in the art. For example, the phosphorus-containing compounds (such as monomers 1, 2, 3, 5, 6, and 7) can be obtained by esterification or substitution reactions of compounds containing phosphorus hydroxyl (-OH) or amino (-NH2) groups with acyl halides or halogenated derivatives containing unsaturated bonds, such as (meth)acryloyl chloride and allyl chloride, in the presence of an alkaline acid-binding agent (such as triethylamine) in an organic solvent (such as dichloromethane or toluene). The nitrogen-containing compounds (such as monomer 4) can be obtained by amination reactions of nitrogen-containing olefins with silane reagents, or can be directly purchased commercially. Preferred synthetic routes for each specific compound are described in the "Detailed Description" section of this specification.

[0026] Furthermore, the phosphorus-nitrogen-silicon multi-element synergistic system described in this invention includes, but is not limited to, specific implementations such as P-Si synergy, PN-Si synergy, and N-Si synergy.

[0027] This application also claims a method for preparing the above-mentioned phosphorus-nitrogen-silicon multi-synergistic halogen-free intrinsic flame-retardant nylon 66, comprising the following steps:

[0028] S1. Synthesis of terminal aminomethylhydrosiloxane-dimethylsiloxane copolymer: Tetramethylcyclotetrasiloxane D4H and octamethylcyclotetrasiloxane D4 were used as comonomers, 1,3-diamino-1,1,3,3-tetramethyldisiloxane was used as a capping agent / molecular weight regulator, and tetramethylammonium hydroxide TMAH was used as a catalyst. Anionic ring-opening copolymerization was carried out under nitrogen protection. After the reaction was completed, the temperature was raised to deactivate the catalyst, and unreacted monomers and byproducts were removed by vacuum distillation to obtain terminal aminomethylhydrosiloxane-dimethylsiloxane copolymer.

[0029] S2. Preparation of flame retardant monomer: The amino-terminated methylhydrosiloxane-dimethylsiloxane copolymer obtained in step S1 is reacted with a phosphorus / nitrogen compound containing unsaturated double bonds in an organic solvent under the catalysis of Karstedt catalyst. After the reaction is complete, the solvent is removed to obtain an amino-terminated polysiloxane flame retardant monomer.

[0030] S3: Copolymerization reaction: Preparation of Nylon 66 salt: Adipic acid and hexamethylenediamine are reacted in water to form a salt, the pH is adjusted to 7.6±0.2, and the mixture is cooled to crystallize, filtered, and dried to obtain Nylon 66 salt;

[0031] Preparation of flame retardant salt: The flame retardant monomer obtained in step S2 is heated and refluxed in ethanol to carry out a salt formation reaction. After cooling and precipitation, the product is filtered and dried to obtain the hexamethylenediamine salt of the flame retardant.

[0032] Melt polycondensation: Nylon 66 salt and flame retardant salt are mixed, deionized water is added, and pre-polymerization is carried out under nitrogen protection. Then, the pressure is slowly released to atmospheric pressure and the temperature is raised. Vacuum is drawn to continue the polycondensation reaction. After the reaction is completed, nitrogen is introduced to break the vacuum. The product is then extruded, cooled, pelletized, and dried to obtain intrinsic flame retardant nylon 66.

[0033] Preferably, in step S1, the conditions for the anionic ring-opening copolymerization reaction are: reaction temperature 100℃, reaction time 4~8 hours; the amount of catalyst TMAH is 0.05%~0.1% of the total monomer mass; and the conditions for catalyst deactivation are heating to 130~160℃ and holding for 0.5~5 hours.

[0034] Preferably, in step S2, the conditions for the hydrosilylation reaction are: reaction temperature 80°C, reaction time 2-6 hours; the amount of Karstedt catalyst is 5-25 ppm; and the amount of the phosphorus / nitrogen compound containing unsaturated double bonds is 1.1 molar amounts relative to the Si-H bonds in the terminal aminomethylhydrosiloxane-dimethylsiloxane copolymer.

[0035] Preferably, during the melt polycondensation process in step S3, the pre-polycondensation temperature is 200~270℃ and the time is 2 hours; during vacuum polycondensation, the system pressure is below 100Pa and the time is 1 hour; the amount of flame retardant salt added accounts for 1%~10% of the total mass of nylon 66 salt and flame retardant salt.

[0036] This application also claims the use of the above-mentioned phosphorus-nitrogen-silicon multi-synergistic halogen-free intrinsic flame-retardant nylon 66 in the fields of electronics, transportation, and building materials.

[0037] Due to the application of the above technical solution, the present invention has the following beneficial effects compared with the prior art:

[0038] 1. This invention achieves synergistic optimization of flame retardancy and mechanical properties, breaking through the bottleneck of traditional technology. This invention integrates a highly efficient phosphorus / nitrogen flame retardant unit with a flexible polysiloxane segment into a reactive monomer and successfully introduces it into the main chain of nylon 66 molecules. This structure fundamentally solves the contradiction between "improved flame retardant performance" and "deterioration of mechanical properties" in traditional flame retardant technology. Experimental results show that the intrinsically flame-retardant nylon 66 prepared by this invention achieves excellent flame retardant performance with a limiting oxygen index (LOI) of over 33% and UL-94V-0 rating, while its tensile strength can still be stably maintained at a high level of over 60MPa. Compared with pure nylon 66, there is almost no loss, which is significantly better than the level in the prior art where the tensile strength drops significantly to below 50MPa in order to achieve the same flame retardant rating. Thus, while ensuring fire safety, the structural application value of nylon 66 as an engineering plastic is fully preserved.

[0039] 2. This invention exhibits superior synergistic flame retardant efficiency, achieving high-efficiency flame retardancy with low addition amounts. This invention constructs a phosphorus-nitrogen-silicon multi-element synergistic flame retardant system, cleverly combining the stepwise flame retardant effects of phosphorus and nitrogen in the gas and condensed phases, as well as the stabilizing and reinforcing effects of silicon on the char layer. This multi-layered synergistic mechanism greatly improves the flame retardant efficiency. Therefore, only about 5 wt% of the flame retardant monomer of this invention is needed to impart excellent flame retardant properties to Nylon 66, and the required addition amount is far lower than the 20-30 wt% typically required by traditional additive flame retardants. This low addition amount not only effectively reduces raw material costs, but more importantly, it avoids problems such as a sharp increase in melt viscosity and deterioration of processing fluidity caused by the introduction of a large amount of filler, significantly improving the processing performance of the material.

[0040] 3. Because the flame-retardant elements of this invention are incorporated into the polymer molecular chain through strong chemical bonding rather than physical blending, this invention fundamentally eliminates the risk of flame retardant migration, precipitation, or failure after washing or prolonged use. This intrinsic flame-retardant property endows the material with a permanent and stable flame-retardant effect, greatly improving the fire safety reliability and durability of the final product throughout its entire service life.

[0041] 4. The flame-retardant system used in this invention is completely halogen-free and does not produce toxic or corrosive dioxins or other hydrogen halide gases during combustion, reducing potential harm to the environment and human health. Furthermore, the high flame-retardant efficiency and low additive content also mean less resource consumption and lower potential hazards from waste during production and use, meeting current requirements for the green and sustainable development of polymer materials. Attached Figure Description

[0042] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, some of the drawings in the following description are some embodiments of the present invention. For those skilled in the art, other drawings can be made based on these drawings without creative effort.

[0043] Figure 1 This is a Fourier transform infrared spectrum of the Si-H bond transformation process in the flame retardant monomer M-1 prepared in Example 1 of this invention. Detailed Implementation

[0044] To provide a clearer understanding of the technical features, objectives, and effects of this invention, specific implementation schemes are now described in detail.

[0045] The present invention will be further described below with reference to embodiments, but the present invention is not limited to the following embodiments. The implementation conditions used in the embodiments can be further adjusted according to different requirements of specific use, and the implementation conditions not specified are conventional conditions in the industry. The technical features involved in the various embodiments of the present invention can be combined with each other as long as they do not conflict with each other.

[0046] Examples of the synthesis of compounds containing unsaturated double-bonded phosphorus / nitrogen compounds are provided below, with specific preparation methods for each example.

[0047] Synthesis Example 1: Synthesis of dimethyl acryloyl phosphate (monomer 1):

[0048] A 500 ml three-necked flask equipped with a constant pressure dropping funnel and a magnetic stirrer was placed in a 0°C cryogenic bath. 34.8 g of dimethyl phosphate, 38.36 ml of triethylamine, and 150 ml of dichloromethane were added. Under nitrogen protection, the mixture was stirred thoroughly until completely dissolved. Over two hours, 50 ml of dichloromethane containing 25 g of acryloyl chloride was added dropwise to the flask. After the addition was complete, the mixture was stirred at room temperature for 12 hours to ensure complete reaction. The triethylamine hydrochloride in the reaction product was removed by filtration. The filtrate was washed with 200 ml of deionized water and dried with MgSO4. After rotary evaporation, a pale yellow transparent liquid product, dimethyl acryloyl phosphate, was obtained. The synthetic route of dimethyl acryloyl phosphate (monomer 1) is shown in formula (1):

[0049] ; Equation (1).

[0050] Synthesis Example 2: Synthesis of DOPO-AA (monomer 2):

[0051] 108 g of DOPO and 200 ml of xylene were added to a 500 ml four-necked flask equipped with a reflux condenser. The reflux condenser was connected to a drying tube (anhydrous calcium chloride). The mixture was stirred and heated to 90 °C under nitrogen protection. After complete dissolution, paraformaldehyde was added, and the mixture was heated to 100 °C and refluxed for 8 hours. The reaction product was filtered through xylene, washed to remove the precipitate, and then dried under vacuum at 70 °C for 10 hours to obtain a white solid product, DOPO-OH.

[0052] A 250 ml four-necked flask equipped with a constant-pressure dropping funnel and a magnetic stirrer was placed in a -5°C cryogenic bath. 12.3 g of DOPO-OH, 80 ml of dichloromethane, and 6.08 g of triethylamine were added. After thorough stirring until completely dissolved, 50 ml of dichloromethane containing 5.45 g of acryloyl chloride was added dropwise to the flask over 3.5 hours. The reaction was continued at -5°C for 2 hours to ensure complete reaction, followed by stirring at room temperature for 6 hours. The reaction product was repeatedly washed with 5 wt% sodium hydroxide solution and deionized water until the organic phase was neutral. The dichloromethane was then evaporated to obtain a pale yellow liquid product, DOPO-AA. The synthetic route of DOPO-AA (monomer 2) is shown in formulas (2-1) and (2-2).

[0053] Equation (2-1);

[0054] Equation (2-2).

[0055] Synthetic Example 3: Synthesis of 2-acryloyloxyethyl-benzylphenylphosphonate (monomer 3):

[0056] A 250 ml four-necked flask equipped with a constant pressure dropping funnel and a magnetic stirrer was placed in an ice-water bath at 0 °C. 14.6 g of benzylphenyl hydroxyethyl phosphonate, 80 ml of dichloromethane, and 6.08 g of triethylamine were added. After thorough stirring until completely dissolved, 50 ml of dichloromethane solution containing 5.45 g of acryloyl chloride was added dropwise to the flask over 2 hours. The reaction was continued at 0 °C for 2 hours to ensure the initial reaction was complete. Then, stirring was continued at room temperature for 6 hours. After the reaction was completed, the triethylamine hydrochloride precipitate was first removed by filtration. The filtrate was repeatedly washed with 5 wt% sodium hydroxide solution and deionized water until the organic phase was neutral. The dichloromethane was then evaporated to obtain a pale yellow oily liquid product, 2-acryloyloxyethyl-benzylphenyl phosphonate. The synthetic route of 2-acryloyloxyethyl-benzylphenyl phosphonate (monomer 3) is shown in formula (3):

[0057] Equation (3);

[0058] Synthesis Example 4: Synthesis of N-allyl-1,1,1-trimethyl-N-(trimethylsilyl)silane (monomer 4): Monomer 4 is readily available. N-allyl-1,1,1-trimethyl-N-(trimethylsilyl)silane (monomer 4) is shown in formula (4):

[0059] Equation (4).

[0060] Synthesis Example 5: Synthesis of Dibenzyl Allyl Phosphamide (Monomer 5):

[0061] A 1000 ml four-necked flask equipped with a constant-pressure dropping funnel and a magnetic stirrer was placed in a -5°C cryogenic bath. 76.7 g of phosphorus oxychloride and 200 ml of anhydrous dichloromethane were added. Over two hours, a 100 ml solution of dichloromethane containing 108.1 g of benzyl alcohol and 101.2 g of triethylamine was added dropwise to the flask, and the reaction was continued at -5°C for another 2 hours. Then, the mixture was stirred at room temperature for 10 hours to generate the intermediate dibenzyl chlorophosphate. Subsequently, the mixture was... The reaction system was cooled to -5°C again, and 100 ml of dichloromethane solution containing 31.4 g allylamine and 55.7 g triethylamine was added dropwise over 1 hour. The reaction was continued at -5°C for 2 hours, followed by stirring at room temperature for 6 hours. The reaction product was filtered under reduced pressure to remove the hydrochloric acid precipitate. The filtrate was repeatedly washed with 5 wt% dilute hydrochloric acid and deionized water until the organic phase was neutral. The dichloromethane was then evaporated to obtain a pale yellow oily liquid product, dibenzylallyl phosphoramide. The synthetic route of dibenzylallyl phosphoramide (monomer 5) is shown in formula (5):

[0062] Equation (5).

[0063] Synthesis Example 6: Synthesis of dimethylallylphosphamide (monomer 6):

[0064] A 500 ml three-necked flask equipped with a constant pressure dropping funnel and a magnetic stirrer was placed in a 0°C cryogenic bath. Dimethyl chlorophosphate (38.66 g, 0.276 mol), triethylamine (38.36 ml, 0.276 mol), and dichloromethane (150 ml) were added separately. Under nitrogen protection, the mixture was stirred thoroughly until completely dissolved. Over 2 hours, 50 ml of dichloromethane containing acrylamine (15.76 g, 0.276 mol) was added dropwise to the flask. After the addition was complete, the mixture was stirred at room temperature for 12 hours to ensure complete reaction. The triethylamine hydrochloride in the reaction product was removed by filtration. The filtrate was washed with 200 ml of deionized water and dried with MgSO4. After rotary evaporation, a pale yellow transparent liquid product, dimethylallylphosphamide, was obtained. The synthetic route of dimethylallylphosphamide (monomer 6) is shown in formula (6):

[0065] Equation (6).

[0066] Synthesis Example 7: Synthesis of 6-((allylamino)methyl)dibenzo[c,e][1,2]oxaphosphacyclohexane 6-oxide (monomer 7):

[0067] In a 500 ml four-necked flask equipped with a reflux condenser and magnetic stirrer, DOPO (21.6 g, 0.1 mol), paraformaldehyde (3.3 g, 0.11 mol), allylamine (6.3 g, 0.11 mol), and toluene (250 ml) were added. Under nitrogen protection, the mixture was heated under reflux for 6 hours. After the reaction, the system was cooled to room temperature, and a large amount of white solid precipitated. The product was collected by filtration, washed with a small amount of cold toluene, and then dried under vacuum at 70 °C for 12 hours to obtain a white powdery product, 6-((allylamino)methyl)dibenzo[c,e][1,2]oxaphosphacyclohexane 6-oxide. The synthetic route of 6-((allylamino)methyl)dibenzo[c,e][1,2]oxaphosphacyclohexane 6-oxide (monomer 7) is shown in formula (7):

[0068] Equation (7).

[0069] Examples of flame retardant monomer preparation are provided below, with specific preparation methods for each example as follows.

[0070] Preparation Example 1: Preparation of Phosphorus-Silicon Synergistic Flame Retardant Monomer M-1:

[0071] Synthesis of S1, terminal aminomethylhydrosiloxane-dimethylsiloxane copolymer: In a four-necked flask equipped with a mechanical stirrer, thermometer, reflux condenser, and nitrogen protection device, metered amounts of D4H, D4, and 1,3-diamino-1,1,3,3-tetramethyldisiloxane were added according to a preset molar ratio (n / m determines the ratio of Si-H to Si-CH3); under a nitrogen atmosphere, stirring was started, and the mixture was heated to 100°C; after the reaction system temperature stabilized, a catalyst amount of TMAH (usually its aqueous or methanolic solution, at a concentration of 0.05%~0.1% of the total monomer mass) was added; after the catalyst was added, the viscosity of the system gradually increased; the reaction was maintained at this temperature for 6 hours until the system reached the desired viscosity. The desired viscosity or conversion rate is reached; after the polymerization reaction reaches its endpoint, the temperature of the reaction system is raised to 150°C and kept at this temperature for 3 hours; the high temperature will cause TMAH to decompose into volatile trimethylamine and methanol, thereby deactivating the catalyst, terminating the polymerization reaction, and preventing the product from depolymerizing in subsequent processing; after the catalyst is deactivated, vacuum distillation is carried out under vacuum conditions (<1kPa) to remove unreacted cyclic monomers and low molecular weight linear or cyclic byproducts generated during polymerization; after no distillate is obtained, heating is stopped and the product is cooled to room temperature under nitrogen protection; the obtained product is a colorless and transparent liquid with a certain viscosity, which is the target product - terminal aminomethylhydrosiloxane-dimethylsiloxane copolymer. The synthetic route of terminal aminomethylhydrosiloxane-dimethylsiloxane copolymer is shown in formula (8):

[0072] Equation (8).

[0073] S2. Preparation of flame-retardant monomers (via hydrosilylation reaction): In a four-necked flask equipped with a mechanical stirrer, thermometer, constant-pressure dropping funnel, and nitrogen protection device, a measured amount of terminal aminomethylhydrosiloxane-dimethylsiloxane copolymer precursor was added, along with an appropriate amount of toluene to adjust the viscosity. Under a nitrogen atmosphere, stirring was started, and the system was heated to the reaction temperature of 80°C. A toluene solution of 20 ppm Karstedt catalyst was added to the reaction system, and the mixture was stirred and activated at this temperature for 40 minutes. Over 2 hours, a measured amount of the selected monomer, such as monomer 1 (whose molar amount relative to the Si-H bond is 1.1 times, slightly excess), was slowly added dropwise to the reaction flask through a constant-pressure dropping funnel. Since the hydrosilylation reaction is exothermic, the dropping rate must be controlled to maintain a stable reaction temperature. After the addition was complete, the mixture was kept at the reaction temperature and stirred for another 5 hours. The reaction progress was monitored using Fourier transform infrared spectroscopy. When the temperature reached 2160 cm⁻¹... -1 When the characteristic absorption peaks representing Si-H bonds in the vicinity completely disappear, it indicates that the reaction has been completed (see attached). Figure 1(As shown in the figure); after the reaction is complete, the solvent is removed by rotary evaporation under reduced pressure to obtain the product for the next step of nylon polycondensation reaction. The synthetic route of flame retardant monomer M-1 is shown in formula (9):

[0074] Equation (9).

[0075] Preparation Example 2: Preparation of Phosphorus-Silicon Synergistic Flame Retardant Monomer M-2: This preparation example is based on Preparation Example 1 above. The similarities with Preparation Example 1 above will not be repeated. In this preparation example, 0.1 mol of the terminal aminomethylhydrosiloxane-dimethylsiloxane copolymer precursor and 0.11 mol of monomer 2 synthesized in the synthesis example are reacted according to the method of Preparation Example 1 above to obtain flame retardant monomer M-2.

[0076] Preparation Example 3: Preparation of Phosphorus-Silicon Synergistic Flame Retardant Monomer M-3: This preparation example is based on Preparation Example 1 above. The similarities with Preparation Example 1 above will not be repeated. In this preparation example, 0.1 mol of the terminal aminomethylhydrosiloxane-dimethylsiloxane copolymer precursor and 0.11 mol of monomer 3 synthesized in the synthesis example are reacted according to the method of Preparation Example 1 above to obtain flame retardant monomer M-3.

[0077] Preparation Example 4: Preparation of Nitrogen-Silicon Synergistic Flame Retardant Monomer M-4: This preparation example is based on Preparation Example 1 above. The similarities with Preparation Example 1 above will not be repeated. In this preparation example, 0.1 mol of the terminal aminomethylhydrosiloxane-dimethylsiloxane copolymer precursor and 0.11 mol of monomer 4 synthesized in the synthesis example are reacted according to the method of Preparation Example 1 above to obtain flame retardant monomer M-4.

[0078] Preparation Example 5: Preparation of Nitrogen-Silicon Synergistic Flame Retardant Monomer M-5: This preparation example is based on Preparation Example 1 above. The similarities with Preparation Example 1 above will not be repeated. In this preparation example, 0.1 mol of the obtained terminal aminomethylhydrosiloxane-dimethylsiloxane copolymer precursor and 0.11 mol of monomer 5 synthesized in the synthesis example are reacted according to the method of Preparation Example 1 above to obtain flame retardant monomer M-5.

[0079] Preparation Example 6: Preparation of Phosphorus-Nitrogen-Silicon Synergistic Flame Retardant Monomer M-6: This preparation example is based on Preparation Example 1 above. The similarities with Preparation Example 1 above will not be repeated. In this preparation example, 0.1 mol of the obtained terminal aminomethylhydrosiloxane-dimethylsiloxane copolymer precursor and 0.11 mol of monomer 6 synthesized in the synthesis example are reacted according to the method of Preparation Example 1 above to obtain flame retardant monomer M-6.

[0080] Preparation Example 7: Preparation of Phosphorus-Nitrogen-Silicon Synergistic Flame Retardant Monomer M-7: This preparation example is based on Preparation Example 1 above. The similarities with Preparation Example 1 above will not be repeated. In this preparation example, 0.1 mol of the obtained terminal aminomethylhydrosiloxane-dimethylsiloxane copolymer precursor and 0.11 mol of monomer 7 synthesized in the synthesis example are reacted according to the method of Preparation Example 1 above to obtain flame retardant monomer M-7.

[0081] Example 1

[0082] See appendix Figure 1 This embodiment provides a method for preparing intrinsically flame-retardant nylon 66, which specifically includes the following steps:

[0083] Preparation of Nylon 66 salt: In a 1000 mL reaction vessel equipped with a mechanical stirrer, a dropping funnel, and a pH meter, adipic acid (146.1 g, 1.0 mol) and 400 mL of deionized water were added, heated to 70 °C, and stirred until completely dissolved. Hexamethylenediamine (116.2 g, 1.0 mol) was accurately weighed, dissolved in 100 mL of deionized water, and slowly added to the adipic acid solution through a dropping funnel under vigorous stirring. After the addition was complete, the pH of the solution was adjusted to 7.6 ± 0.2 with a small amount of adipic acid or hexamethylenediamine. The resulting nylon 66 salt solution was cooled to crystallize, filtered, and vacuum dried to obtain white powdered nylon 66 salt. The synthetic route of nylon 66 salt is shown in formula (10):

[0084] Formula (10).

[0085] Preparation of flame-retardant nylon 6X salt: 0.05 mol of flame-retardant monomer M-1 obtained in Preparation Example 1 was mixed with 0.05 mol of hexamethylenediamine in 200 mL of ethanol and heated under reflux for 2 hours to carry out the salt formation reaction; after the reaction was completed, the solid was cooled and precipitated, filtered and vacuum dried to obtain the hexamethylenediamine salt of the flame retardant. The synthetic route of the hexamethylenediamine salt of the flame retardant is shown in formula (11):

[0086] ; Equation (11).

[0087] Melt polycondensation: 950g of nylon 66 salt and 50g of hexamethylenediamine salt of flame retardant were mixed evenly in a high-pressure reactor, 30wt% of deionized water was added, and the air in the reactor was replaced three times with high-purity nitrogen. The reactor was then sealed. The oil bath was preheated to raise the temperature inside the reactor to 270℃. Under these conditions, the pre-polymerization was carried out for 2 hours. Then, the pressure was slowly reduced to atmospheric pressure over 1 hour. During this process, a large amount of water vapor was discharged. After the pressure was reduced to atmospheric pressure, the pressure inside the reactor was evacuated to make it lower than 100Pa. The polycondensation reaction was continued for 1 hour at the corresponding temperature to further increase the molecular weight of the polymer. After the reaction was completed, the evacuation was stopped and high-purity nitrogen was introduced to atmospheric pressure. The molten polymer was extruded into strips through the valve at the bottom of the reactor. After cooling in a water bath, the strips were granulated into granules by a pelletizer. The strips were then vacuum dried at 60℃ for 24 hours to obtain intrinsic flame-retardant nylon 66 chips (denoted as FR-PA66-1). The synthesis route of intrinsic flame-retardant nylon 66 is shown in formula (12):

[0088] Equation (12).

[0089] Example 2

[0090] This embodiment is based on the above embodiment 1. The similarities with the above embodiment 1 will not be repeated. In this embodiment, flame retardant monomer M-2 replaces M-1 in embodiment 1; thus, intrinsic flame retardant nylon 66 chips (denoted as FR-PA66-2) are obtained.

[0091] Example 3

[0092] This embodiment is based on the above embodiment 1. The similarities with the above embodiment 1 will not be repeated. In this embodiment, the flame retardant monomer M-3 replaces M-1 in embodiment 1; and intrinsic flame retardant nylon 66 chips (denoted as FR-PA66-3) are obtained.

[0093] Example 4

[0094] This embodiment is based on the above embodiment 1. The similarities with the above embodiment 1 will not be repeated. In this embodiment, the flame retardant monomer M-4 replaces M-1 in embodiment 1; and intrinsic flame retardant nylon 66 chips (denoted as FR-PA66-4) are obtained.

[0095] Example 5

[0096] This embodiment is based on the above embodiment 1. The similarities with the above embodiment 1 will not be repeated. In this embodiment, the flame retardant monomer M-5 replaces M-1 in embodiment 1; and intrinsic flame retardant nylon 66 chips (denoted as FR-PA66-5) are obtained.

[0097] Example 6

[0098] This embodiment is based on the above embodiment 1. The similarities with the above embodiment 1 will not be repeated. In this embodiment, the flame retardant monomer M-6 replaces M-1 in embodiment 1; and intrinsic flame retardant nylon 66 chips (denoted as FR-PA66-6) are obtained.

[0099] Example 7

[0100] This embodiment is based on the above embodiment 1. The similarities with the above embodiment 1 will not be repeated. In this embodiment, the flame retardant monomer M-7 replaces M-1 in embodiment 1; and intrinsic flame retardant nylon 66 chips (denoted as FR-PA66-7) are obtained.

[0101] Comparative Example 1

[0102] This comparative example uses pure nylon 66. 1000g of nylon 66 salt was polycondensed according to the polymerization process in Example 1 to obtain pure nylon 66 chips.

[0103] Comparative Example 2

[0104] This comparative example is additive flame-retardant nylon 66. 1000g of pure nylon 66 chips obtained from comparative example 1 were melt-blended with 50g (5wt%) of commercial phosphorus-nitrogen flame retardant (melamine polyphosphate, MPP) through a twin-screw extruder and pelletized to obtain additive flame-retardant nylon 66.

[0105] The dried nylon 66 slices prepared in the above examples and comparative examples were molded into standard samples by injection molding and tested. The test results are shown in Table 1.

[0106] Table 1

[0107]

[0108] As can be clearly seen from the test results in Table 1, all Examples 1 to 7 of this invention successfully prepared high-performance intrinsically flame-retardant nylon 66. Regarding flame retardant performance, all examples achieved the highest UL-94 rating, V-0, and their LOI values ​​were significantly higher than those of Comparative Example 1 (pure PA66) and Comparative Example 2 (additive type). In particular, Examples 6 and 7 used a phosphorus-nitrogen-silicon synergistic flame-retardant monomer, achieving an LOI value of over 33%, demonstrating optimal flame retardant effect. This proves the correctness and superiority of the multi-element synergistic design concept of this invention.

[0109] In terms of mechanical properties, the tensile strength of all embodiments of the present invention remained above 56 MPa, showing minimal performance loss compared to 66.5 MPa of Comparative Example 1 (pure PA66). In stark contrast, while Comparative Example 2 (traditional additive type) also achieved a V-0 rating, its tensile strength plummeted to 49.3 MPa, resulting in a severe loss of mechanical properties. This comparison powerfully demonstrates that the technical solution of the present invention, by introducing a silicon-grafted flame-retardant system, successfully overcomes the technical challenge of simultaneously achieving high flame retardancy and high mechanical properties.

[0110] The test data of the above embodiments and comparative examples were tested according to the following method.

[0111] 1. Flame retardant performance test:

[0112] 1.1 Limiting Oxygen Index (LOI):

[0113] Test standard: Based on GB / T2406.2-2009 "Determination of flammability of plastics by oxygen index method - Part 2: Test at room temperature" (equivalent to ISO4589-2:1996);

[0114] Testing instrument: JF-3 limiting oxygen index tester;

[0115] Sample size: The injection-molded sample was cut into strips of 80mm × 10mm × 4mm.

[0116] Test procedure: The sample is vertically clamped in the combustion chamber, the top of the sample is ignited, and the mixing ratio of oxygen and nitrogen is adjusted at the same time. The lowest oxygen concentration percentage that can maintain the sample burning stably for 180 seconds or the burning length reaches 50 mm is recorded, which is the limiting oxygen index (LOI) of the material.

[0117] 1.2 Vertical flammability rating (UL-94):

[0118] Test standard: Based on the vertical burning test method in ANSI / UL94 "Tests on flammability of plastic materials for use in equipment and appliance parts";

[0119] Testing instrument: CZF-5 vertical combustion test apparatus;

[0120] Sample size: Standard strip specimen, 125mm × 13mm × 1.6mm;

[0121] Test procedure: The sample is suspended vertically, and degreased cotton is placed 10 mm below it; the lower end of the sample is ignited with a standard flame (20 mm high) for 10 seconds and then removed, and the first flaming time (t1) is recorded; after the flame is extinguished, the flame is immediately applied again for 10 seconds and then removed, and the second flaming time (t2) and the glow time (t3) are recorded; at the same time, it is observed whether any molten drips ignite the degreased cotton below; based on the times t1, t2, t3 and the dripping situation, the flame retardancy rating of the material is evaluated (V-0, V-1, or V-2).

[0122] 2. Thermal performance test:

[0123] 2.1 Melting point (Tm):

[0124] Test standard: Based on GB / T19466.3-2004 "Plastics - Differential scanning calorimetry (DSC) - Part 3: Determination of melting and crystallization temperature and enthalpy" (equivalent to ISO11357-3:2018);

[0125] Test instrument: Mettler Toledo DSC1 differential scanning calorimeter;

[0126] Test procedure: Accurately weigh 5-10 mg of dry sample and place it in an aluminum crucible; under the protection of high-purity nitrogen (flow rate 50 mL / min), first heat from 30℃ to 300℃ at a heating rate of 10℃ / min, hold for 5 minutes to eliminate thermal history; then cool down to 30℃ at a rate of 10℃ / min; finally heat up to 300℃ again at a rate of 10℃ / min; take the peak temperature of the endothermic peak in the second heating scan curve as the melting point (Tm) of the material.

[0127] 3. Mechanical property testing:

[0128] 3.1 Tensile strength and elongation at break:

[0129] Test standard: Based on GB / T1040.2-2022 "Determination of tensile properties of plastics - Part 2: Test conditions for molded and extruded plastics" (equivalent to ISO527-2:2012);

[0130] Testing instrument: Instron 5567 universal testing machine;

[0131] Sample size: Standard type 1A dumbbell-shaped spline;

[0132] Test procedure: The dumbbell-shaped specimen, after being conditioned, is mounted on the fixture of the testing machine and subjected to a tensile test at a tensile rate of 50 mm / min until the specimen breaks; the maximum stress (i.e., tensile strength) and the percentage elongation of the specimen at the time of fracture (i.e., elongation at break) are automatically recorded and calculated by the software built into the instrument; 5 valid data points are tested for each group of samples, and the average value of the results is taken.

[0133] In summary, this invention successfully constructs a phosphorus-nitrogen-silicon multi-synergistic flame-retardant system by integrating phosphorus / nitrogen flame-retardant units with flexible polysiloxane segments into a single reactive monomer and introducing them into the nylon 66 molecular backbone. This fundamentally resolves the contradiction between improved flame-retardant performance and deteriorated mechanical properties. With only about 5 wt% of flame-retardant monomer added, this material achieves excellent flame retardancy with a limiting oxygen index exceeding 33% and a UL-94 V-0 rating, while maintaining a tensile strength consistently above 60 MPa, superior to most existing technologies. Furthermore, the intrinsic flame-retardant structure ensures the durability of the flame-retardant effect, avoids migration and precipitation problems, and its halogen-free nature meets green environmental protection requirements. It achieves a highly efficient, durable, and environmentally friendly flame-retardant effect while maintaining the inherent engineering properties of nylon 66.

[0134] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A phosphorus-nitrogen-silicon multi-synergistic halogen-free intrinsic flame-retardant nylon 66, characterized in that, It is a copolymer obtained by melt polycondensation reaction of nylon salt obtained by copolymerizing nylon 66 salt with amino-terminated polysiloxane flame retardant monomer and adipic acid; the amino-terminated polysiloxane flame retardant monomer has the structure shown in Formula I. Equation I; Where m is an integer from 0 to 50, and n is an integer from 0 to 50; the -R group is an organic group containing phosphorus, nitrogen, or a combination of phosphorus and nitrogen; and m and n are not 0. The terminal amino polysiloxane flame retardant monomer is prepared by grafting a phosphorus / nitrogen compound containing unsaturated double bonds onto a terminal amino methylhydrosiloxane-dimethylsiloxane copolymer via a hydrosilylation reaction.

2. The phosphorus-nitrogen-silicon multi-synergistic halogen-free intrinsic flame-retardant nylon 66 according to claim 1, characterized in that, When -R is a phosphorus-containing group, the flame-retardant monomer is a phosphorus-silicon synergistic system, -R is selected from the structure shown in A or B of Formula II, and -X is selected from the structure shown in C, D, E or F of Formula III; Formula II; Formula III.

3. The phosphorus-nitrogen-silicon multi-synergistic halogen-free intrinsic flame-retardant nylon 66 according to claim 1, characterized in that, When -R is a nitrogen-containing group, the flame-retardant monomer is a nitrogen-silicon synergistic system, and -R has the structure shown in Formula IV, wherein -X is selected from A, B, C or D in Formula V, and -Y is selected from A, B or C in Formula VI: Formula IV; , formula V; , Formula VI.

4. The phosphorus-nitrogen-silicon multi-synergistic halogen-free intrinsic flame-retardant nylon 66 according to claim 1, characterized in that, When -R is a phosphorus-nitrogen coexisting group, the flame retardant monomer is a phosphorus-nitrogen-silicon synergistic system, and -R has the structure shown in Formula VII, wherein -X is selected from the structure shown in C, D, E or F in Formula III; Equation VII.

5. The phosphorus-nitrogen-silicon multi-synergistic halogen-free intrinsic flame-retardant nylon 66 according to claim 1, characterized in that, The phosphorus / nitrogen compound containing unsaturated double bonds is selected from one of dimethyl acryloyl phosphate, DOPO-AA, 2-acryloyloxyethyl-benzylphenylphosphonate, N-allyl-1,1,1-trimethyl-N-(trimethylsilyl)silamide, dibenzylallyl phosphoramide, dimethylallyl phosphoramide, and 6-((allylamino)methyl)dibenzo[c,e][1,2]oxaphosphacyclohexane 6-oxide.

6. A method for preparing phosphorus-nitrogen-silicon multi-synergistic halogen-free intrinsic flame-retardant nylon 66 as described in any one of claims 1 to 5, characterized in that, Includes the following steps: S1. Synthesis of terminal aminomethylhydrosiloxane-dimethylsiloxane copolymer: Tetramethylcyclotetrasiloxane D4H and octamethylcyclotetrasiloxane D4 were used as comonomers, 1,3-diamino-1,1,3,3-tetramethyldisiloxane was used as a capping agent / molecular weight regulator, and tetramethylammonium hydroxide TMAH was used as a catalyst. Anionic ring-opening copolymerization was carried out under nitrogen protection. After the reaction was completed, the temperature was raised to deactivate the catalyst, and unreacted monomers and byproducts were removed by vacuum distillation to obtain terminal aminomethylhydrosiloxane-dimethylsiloxane copolymer. S2. Preparation of flame retardant monomer: The amino-terminated methylhydrosiloxane-dimethylsiloxane copolymer obtained in step S1 is reacted with a phosphorus / nitrogen compound containing unsaturated double bonds in an organic solvent under the catalysis of Karstedt catalyst. After the reaction is complete, the solvent is removed to obtain an amino-terminated polysiloxane flame retardant monomer. S3: Copolymerization reaction: Preparation of Nylon 66 salt: Adipic acid and hexamethylenediamine are reacted in water to form a salt, the pH is adjusted to 7.6±0.2, and the mixture is cooled to crystallize, filtered, and dried to obtain Nylon 66 salt; Preparation of flame retardant salt: The flame retardant monomer obtained in step S2 is heated with adipic acid in ethanol under reflux to carry out a salt formation reaction, cooled to precipitate, filtered, and dried to obtain flame retardant salt; Melt polycondensation: Nylon 66 salt and flame retardant salt are mixed, deionized water is added, and pre-polymerization is carried out under nitrogen protection. Then, the pressure is released to normal pressure and the temperature is raised. Vacuum is drawn to continue the polycondensation reaction. After the reaction is completed, nitrogen is introduced to break the vacuum. The product is then extruded, cooled, pelletized, and dried to obtain intrinsic flame retardant nylon 66.

7. The method for preparing phosphorus-nitrogen-silicon multi-synergistic halogen-free intrinsic flame-retardant nylon 66 according to claim 6, characterized in that, In step S1, the conditions for the anionic ring-opening copolymerization reaction are: reaction temperature 100℃, reaction time 4~8 hours; the amount of catalyst TMAH is 0.05%~0.1% of the total monomer mass; the conditions for catalyst deactivation are heating to 130~160℃ and holding for 0.5~5 hours.

8. The method for preparing phosphorus-nitrogen-silicon multi-synergistic halogen-free intrinsic flame-retardant nylon 66 according to claim 6, characterized in that, In step S2, the conditions for the hydrosilylation reaction are: reaction temperature 80°C, reaction time 2-6 hours; the amount of Karstedt catalyst is 5-25 ppm; and the amount of the phosphorus / nitrogen compound containing unsaturated double bonds is 1.1 molar amounts relative to the Si-H bonds in the terminal aminomethylhydrosiloxane-dimethylsiloxane copolymer.

9. The method for preparing phosphorus-nitrogen-silicon multi-synergistic halogen-free intrinsic flame-retardant nylon 66 according to claim 6, characterized in that, In the melt polycondensation process of step S3, the pre-polycondensation temperature is 200~270℃ and the time is 2 hours; the system pressure is lower than 100Pa during vacuum polycondensation and the time is 1 hour; the amount of flame retardant salt added accounts for 1%~10% of the total mass of nylon 66 salt and flame retardant salt.

10. The application of phosphorus-nitrogen-silicon multi-synergistic halogen-free intrinsic flame-retardant nylon 66 prepared by any one of claims 1 to 5 or any one of claims 6 to 9 in the fields of electronics, transportation, and building materials.

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