Spherical-like pine cone-like laminated structure aluminum phosphinate flame retardant, and preparation method and application thereof

Abstract

This invention belongs to the field of flame retardant materials technology, specifically relating to a spherical, pinecone-like layered structure aluminum hypophosphite flame retardant, its preparation method, and its application. The flame retardant is an inorganic phosphorus-based halogen-free flame retardant. The aluminum hypophosphite flame retardant exhibits a spherical, pinecone-like layered structure, with each particle composed of multiple thin lamellar layers stacked alternately to form a regular spherical outer contour. The thermal decomposition temperature of the aluminum hypophosphite flame retardant is >330℃, and its particle size distribution meets the following requirements: D10 particle size 8~15μm, D50 particle size 25~40μm, D90 particle size 60~80μm, D97 particle size 80~100μm, and a specific surface area of ​​1.2~3.5m² / g. This invention employs a continuous preparation process involving preliminary crystallization in a crystallization kettle, dissolution in a dissolving kettle, and crystal growth in a precipitation kettle. The preparation process parameters are controllable, requires no additives, and is suitable for industrial mass production. This flame retardant can be applied to engineering plastics, achieving both high-efficiency flame retardancy and good mechanical properties.

Description

Technical Field

[0001] This invention relates to the field of flame retardant materials technology, and in particular to a spherical, pinecone-like layered aluminum hypophosphite flame retardant, its continuous preparation method, and its application. Background Technology

[0002] Aluminum hypophosphite, as a halogen-free, low-smoke, and environmentally friendly phosphorus-based inorganic flame retardant, possesses advantages such as moderate thermal stability, excellent flame retardant efficiency, and good compatibility with polymer matrices, making it a promising core product to replace traditional halogen-based flame retardants. Currently, most commercially available aluminum hypophosphite flame retardants are prepared using a batch-step crystallization process, resulting in products with generally small particle sizes and irregular morphologies. The D50 particle size is mostly concentrated in the 5-10 μm range, and the particles are mostly fine crystals with a large specific surface area. When applied to engineering plastics, this process presents several insurmountable drawbacks: First, the small particle size itself has an excessively large specific surface area, resulting in strong van der Waals forces and electrostatic interactions between particles. During the high-temperature melt blending process of engineering plastics, it easily agglomerates, forming agglomerates that cannot be uniformly dispersed in the resin matrix, significantly deteriorating tensile and impact mechanical properties. Agglomeration also shields flame-retardant active sites, leading to reduced flame retardant efficiency. Second, the thermal decomposition temperature is relatively low, making it difficult to adapt to the high-temperature processing technology of engineering plastics, and prone to premature decomposition.

[0003] Existing technologies address the aforementioned problems primarily through physical modification methods such as post-processing surface coating and the addition of dispersants. These methods only alleviate agglomeration issues and cannot fundamentally resolve the defects from the perspective of the intrinsic particle structure and preparation process. Furthermore, coating modification introduces impurities, reduces the thermal stability of the flame retardant, and dilutes the effective flame-retardant components. Some processes attempt to control particle size and morphology, but lack continuous and precisely controllable reaction processes, making it impossible to produce aluminum hypophosphite products with large particle size and high thermal stability. Moreover, these processes suffer from poor continuity and low industrial applicability. Therefore, developing an aluminum hypophosphite flame retardant with regular morphology, suitable particle size, excellent thermal stability, and continuous production capability is crucial to overcoming the current technological bottlenecks. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a spherical, pinecone-like, layered aluminum hypophosphite flame retardant, its continuous preparation method, and its application.

[0005] This invention is achieved through the following technical solution:

[0006] In a first aspect, the present invention provides a spherical pinecone-like layered structure aluminum hypophosphite flame retardant, which exhibits a spherical pinecone-like layered structure in its microstructure. Each particle is composed of multiple thin layers stacked alternately, forming a regular spherical outer contour. The aluminum hypophosphite flame retardant has a thermal decomposition temperature >330℃ and a particle size distribution that meets the following requirements: D10 particle size 8~15μm, D50 particle size 25~40μm, D90 particle size 60~80μm, D97 particle size 80~100μm, and a specific surface area of ​​1.2~3.5m² / g.

[0007] The flame retardant of this invention features a spherical, pinecone-like layered structure, large particle size, and low specific surface area. When applied to engineering plastics, it disperses uniformly in the resin matrix without agglomeration, achieving excellent flame retardant effects without requiring high addition amounts. Simultaneously, it maximizes the preservation of the original mechanical properties of engineering plastics, overcoming the shortcomings of traditional flame retardants that cannot simultaneously achieve both flame retardancy and mechanical properties. The product's thermal decomposition temperature is >330℃, higher than conventional aluminum hypophosphite flame retardants, making it suitable for high-temperature processing of engineering plastics and meeting the flame retardant modification requirements of engineering plastics.

[0008] As a preferred embodiment, the aluminum hypophosphite flame retardant has a thermal decomposition temperature >335℃ and a particle size distribution that meets the following requirements: D10 particle size 10~15μm, D50 particle size 30~40μm, D90 particle size 65~80μm, D97 particle size 80~90μm, and a specific surface area of ​​2~3m² / g.

[0009] Secondly, the present invention provides a method for preparing a spherical, pinecone-like layered aluminum hypophosphite flame retardant, employing a three-stage continuous reaction process of crystallization-dissolution-precipitation, specifically including the following steps:

[0010] (1) Preparation of raw materials and continuous reaction equipment: Prepare sodium hypophosphite monohydrate and aluminum sulfate octadecade aqueous solution at a constant temperature of 90-95℃ in advance; set up a continuous reaction equipment, including a crystallization vessel, a dissolving vessel and a precipitation vessel connected in sequence, add deionized water accounting for 50%-70% of the effective volume of the three vessels to the three vessels respectively, start stirring and control the temperature of the three vessels at 90-95℃, and the difference in the effective volume of the three vessels should not exceed 20%;

[0011] (2) Start and control the continuous reaction: Turn on the continuous reaction device and control the simultaneous and continuous injection of the two salt solutions into the crystallization vessel. The feed weight ratio of sodium hypophosphite monohydrate and aluminum sulfate octadecahydrate per unit time is 1:(1.02~1.08), and the total feed rate of the two salt solutions is the effective volume of the crystallization vessel ÷ (3~8h). Control the rate at which the slurry is continuously introduced from the crystallization vessel into the dissolving vessel to be equal to the total feed rate of the two salt solutions into the crystallization vessel. At the same time, continuously introduce sulfuric acid solution with a mass concentration of 15%~35% into the dissolving vessel to maintain the pH value of the slurry in the dissolving vessel stable at a certain level. The pH value is controlled between 2.9 and 3.1; the rate at which the slurry is continuously introduced from the dissolving vessel to the precipitation vessel is the sum of the total feed rate of the two salt solutions to the crystallization vessel and the feed rate of the sulfuric acid solution to the dissolving vessel, while a sodium hydroxide aqueous solution with a mass concentration of 10% to 30% is continuously introduced into the precipitation vessel to maintain the pH value of the slurry in the precipitation vessel stable between 7.1 and 7.5; the rate at which the finished slurry is continuously discharged from the precipitation vessel is the sum of the total feed rate of the two salt solutions to the crystallization vessel, the feed rate of the sulfuric acid solution to the dissolving vessel, and the feed rate of the sodium hydroxide aqueous solution to the precipitation vessel;

[0012] (3) Post-processing: After the particle size of the finished slurry discharged from the precipitation vessel is found to be up to standard, the slurry is collected and then subjected to filtration, deionized water rinsing and drying in sequence to obtain the aluminum hypophosphite flame retardant.

[0013] This invention employs a three-reactor continuous reaction process of crystallization-dissolution-precipitation to achieve continuous production throughout the entire process. Before the reaction begins, deionized water is pre-added to each of the three sequentially connected reactors. During the continuous reaction, as the two salt solutions are continuously and synchronously injected, the raw material concentration in the crystallization reactor gradually increases and approaches the initial feed concentration. Once the system reaches steady state, the material concentration in each reactor maintains a dynamic equilibrium. In the crystallization reactor, sodium hypophosphite monohydrate and aluminum sulfate octadecade react to generate aluminum hypophosphite, which then crystallizes. In the dissolution reactor, fine crystals are dissolved while the core crystal nuclei are retained. In the precipitation reactor, the dissolved aluminum hypophosphite recrystallizes and grows on the surface of the core crystal nuclei. This solution, through a three-reactor continuous reaction process and precise control of the raw material ratio, reaction temperature, feed and discharge rates of each reactor, sulfuric acid and sodium hydroxide concentrations, and pH values ​​in the dissolution and precipitation reactors, achieves the generation of crystal nuclei, dissolution of fine crystals, and recrystallization to produce a flame retardant product with larger particles and a spherical, pinecone-like layered structure. The entire process is controllable and requires no additional crystal form regulators or dispersants.

[0014] In step (1), the temperature of the raw material solution and the preheating temperature of the three reactors are both set at 90-95℃. This temperature range ensures that the raw materials are completely dissolved without crystallization and matches the appropriate reaction rate. If the temperature is below 90℃, the raw materials will not dissolve sufficiently, the reaction rate will be too slow, the crystal particle size will be too small and the morphology will be messy. If the temperature is above 95℃, it will easily cause excessive evaporation of water, imbalance of system concentration, and uncontrolled growth of crystal nuclei, making it impossible to form a regular layered structure. The amount of water added to the three reactors is 50%-70% of the effective volume, which can ensure that the materials are fully mixed and leave enough reaction space to avoid overflow. Insufficient water will lead to uneven mixing and excessive local reaction, while excessive water will reduce reaction efficiency and prolong the production cycle. The effective volume difference of the three reactors is ≤20%, which is the core premise for a constant liquid level in continuous reaction. If the volume difference is too large, it will lead to poor material flow, imbalance of liquid levels in the front and rear reactors, failure to achieve continuous and stable discharge, and damage to the continuity of the process.

[0015] In step (2), sodium hypophosphite monohydrate and aluminum sulfate octadecade water react in the crystallization vessel to generate aluminum hypophosphite and crystallize. Since the weight ratio of sodium hypophosphite monohydrate to aluminum sulfate octadecade water per unit time is 1:(1.02~1.08), this weight ratio allows sodium hypophosphite monohydrate and aluminum sulfate octadecade water to react completely, reducing the impurity content in the product and improving the product purity and thermal stability. The total feed rate of the two salt solutions is set to the effective volume of the crystallization vessel ÷ (3~8h), which can control the primary crystal nucleus generation rate to be slow and uniform, avoiding rapid feeding that leads to excessive reaction and instantaneous precipitation of a large number of fine particles. By controlling the total rate of the two raw material solutions injected into the crystallization vessel, the reaction is ensured to proceed fully, and a primary crystallization slurry is obtained in the crystallization vessel. The primary crystal nuclei in the slurry lay the foundation for subsequent morphology reconstruction and crystal growth. After the slurry enters the dissolution vessel, the fine crystal nuclei are selectively dissolved under acidic conditions, while the core crystal nuclei are retained. Because the solubility of aluminum hypophosphite crystals is significantly particle size-dependent, fine crystal nuclei have a large specific surface area, high solubility, and fast dissolution rate, while dense core crystal nuclei have a small specific surface area, low solubility, and high stability. By controlling the slurry feed rate from the crystallization vessel, the mass concentration of the sulfuric acid solution, and maintaining the pH value between 2.9 and 3.1, a weakly acidic environment is used to selectively dissolve fine and irregular particles in the primary crystal nuclei, purifying and standardizing the core crystal nuclei, removing unqualified fine crystals, and avoiding agglomeration caused by residual small particles in subsequent products. If the pH value is >3.1, the system is not acidic enough to effectively dissolve fine and irregular particles, resulting in poor purification and residual small-diameter particles in the final product, and the agglomeration problem cannot be solved. If the pH value is <2.9, the system is too acidic, which will dissolve the standard core crystal nuclei as well, leading to excessive crystal nucleus loss, insufficient seed crystals for subsequent recrystallization, inability to form large-particle layered structures, and increased costs for subsequent alkali neutralization. In the precipitation vessel, a 10-30% sodium hydroxide solution is used to achieve precise acid-base neutralization. After the system reaches steady state, the precipitation vessel becomes slightly alkaline, allowing the dissolved aluminum hypophosphite to redefine and grow. Under pH conditions of 7.1-7.5, aluminum hypophosphite crystals exhibit anisotropic growth, with crystal faces preferentially growing along two-dimensional directions to form thin lamellar layers. Multiple lamellar layers are stacked in an orderly fashion on the surface of the core crystal nucleus, ultimately forming a spherical pinecone-like layered structure. If the pH value is >7.5, it will lead to over-neutralization, causing a large amount of dissolved aluminum hypophosphite to precipitate instantaneously, forming fine agglomerated particles that cannot form a layered structure. If the pH value is <7.1, neutralization is not timely, the system remains acidic for a long time, the recrystallization rate is too slow, particle growth is insufficient, and the particle size is too small. By adjusting the pH, the dissolved aluminum hypophosphite component can be redefined and grow on the surface of the core crystal nucleus, gradually forming a spherical pinecone-like layered structure with a larger particle size than conventional aluminum hypophosphite flame retardants.

[0016] In step (3), the particle size of the finished slurry continuously discharged from the precipitation vessel is detected in real time. Once the particle size meets the target requirements, it indicates that the reaction system has stabilized. Collecting the slurry then ensures that the particle size and morphology of the continuously discharged product meet the requirements, thus avoiding batch non-compliance caused by the discharge when the reaction is not stable in the early stage.

[0017] In this invention, the feed and discharge rates of the crystallization vessel, dissolution vessel, and precipitation vessel are strictly controlled, meaning that the feed and discharge rates of each vessel are consistent. This facilitates the rapid attainment of a steady state of the slurry in each vessel within the continuous reaction apparatus, and allows the reaction crystallization, fine crystal dissolution, and recrystallization to proceed stably and controllably in the three vessels, respectively. If the feed and discharge rates of the crystallization vessel are too fast (too short a residence time), a large number of irregular fine crystals will form, making it impossible to purify and regularize the crystal nuclei after adding acid in the dissolution vessel. If the feed and discharge rates are too slow (too long a residence time), the production efficiency will be extremely low, failing to meet the requirements of industrial mass production. This invention employs a continuous three-vessel preparation process, eliminating the need for complex post-modification treatments and introducing no impurities. It enables continuous industrial production, and the resulting aluminum hypophosphite flame retardant product exhibits a controllable large particle size, low specific surface area, and a spherical, pinecone-like layered structure, with good batch stability.

[0018] Furthermore, in the sodium hypophosphite monohydrate aqueous solution, the weight ratio of sodium hypophosphite monohydrate to deionized water is 1:(1-1.5); in the aluminum sulfate octadecade aqueous solution, the weight ratio of aluminum sulfate octadecade to deionized water is 1:(1-1.3). As the continuous reaction proceeds, the concentrations of each raw material in the crystallization vessel gradually reach a stable state. Limiting the weight ratio of raw materials to water within a specific range ensures that the concentration of the raw material solution is appropriate. Too high a concentration will lead to an excessively rapid reaction and severe particle agglomeration, while too low a concentration will result in low reaction efficiency and insufficient yield, making it unsuitable for industrial production.

[0019] Furthermore, in the post-processing stage, a rake dryer is used for drying, and the drying temperature is controlled at 60~120℃.

[0020] Furthermore, the temperature of the deionized water used for rinsing the filter cake in the post-treatment stage is 40~70℃, and the rinsing endpoint is controlled at a filtrate conductivity of <3.0ms / cm. A washing water temperature of 40~70℃ can quickly remove water-soluble impurities such as residual sodium sulfate and free ions from the filter cake without damaging the already formed layered structure. Controlling the filtrate conductivity to <3.0ms / cm at the washing endpoint allows for precise determination of complete impurity removal. Excessive conductivity indicates excessive residual soluble salts, which will significantly lower the product's thermal decomposition temperature, leading to yellowing and decomposition problems during engineering plastic processing.

[0021] Thirdly, this invention provides a spherical, pinecone-like, layered aluminum hypophosphite flame retardant as a halogen-free flame retardant, and its application in the field of flame retardant modification of engineering plastics.

[0022] Furthermore, the flame retardant is compounded with melamine polyphosphate and applied to PA6, PA66 or PBT systems.

[0023] This invention provides a spherical, pinecone-like, layered aluminum hypophosphite flame retardant, specifically designed for flame-retardant modification of engineering plastics. It can be widely applied to various engineering plastics such as PA6, PA66, PBT, PET, ABS, and PC / ABS alloys. Application utilizes a conventional melt-blending process, mixing the flame retardant with engineering plastic resins, antioxidants, lubricants, and other additives in a specific ratio. This mixture is then melt-blended and granulated using a twin-screw extruder, followed by injection molding into various products. When applied to engineering plastics, this flame retardant, with its spherical, pinecone-like layered structure, large particle size distribution, and low specific surface area, disperses uniformly in the resin matrix without agglomeration. It achieves excellent flame-retardant effects without requiring high addition amounts, while maximizing the preservation of the original mechanical properties of the engineering plastics. This overcomes the shortcomings of traditional flame retardants that cannot simultaneously achieve both flame retardancy and mechanical properties. The resulting products have a smooth appearance and good processing flowability, making them widely applicable in high-end fields such as electronics, automotive parts, and rail transportation.

[0024] The present invention has the following beneficial effects:

[0025] First, the aluminum hypophosphite flame retardant of this invention has a spherical, pinecone-like layered morphology and a large particle size distribution, with a relatively low specific surface area, which can significantly reduce the surface energy of particles and weaken the interparticle forces, fundamentally solving the agglomeration problem of traditional small-particle-size products, and achieving a balance between high-efficiency flame retardancy and mechanical properties. Second, the product's thermal decomposition temperature is >330℃, which is higher than that of conventional aluminum hypophosphite products, making it suitable for high-temperature processing of engineering plastics such as PA66 and PBT, and meeting the flame retardant modification requirements of engineering plastics. Third, it adopts a three-stage continuous reaction process of crystallization-dissolution-precipitation, and by precisely controlling the raw material ratio, reaction temperature, feed rate, and system pH value, the entire process of crystal nucleation, fine crystal dissolution, and recrystallization of the layered structure can be controlled, without the need for additional crystal form regulators or dispersants, complex post-modification treatment, or the introduction of impurities, enabling continuous industrial production. Attached Figure Description

[0026] Figure 1 The image shows a SEM image of the spherical sheet-like stacked aluminum hypophosphite structure prepared in Example 1.

[0027] Figure 2 The TGA diagrams are of the aluminum hypophosphite flame retardants prepared in Example 1 and Comparative Example 1.

[0028] Figure 3 This is a process flow diagram for preparing the aluminum hypophosphite flame retardant according to the present invention.

[0029] Figure 4 The image shows the SEM image of aluminum hypophosphite obtained in Comparative Example 1. Detailed Implementation

[0030] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. 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. These all fall within the scope of protection of the present invention.

[0031] The raw materials used in this invention are sourced from the following sources:

[0032] Aluminum sulfate octahydrate: purity > 98%, commercially available;

[0033] Sodium hypophosphite monohydrate: purity > 98%, commercially available;

[0034] Sodium hydroxide: Commercially available;

[0035] Sulfuric acid: Commercially available.

[0036] Example 1: Preparation of aluminum hypophosphite using a three-stage continuous reaction process of crystallization-dissolution-precipitation.

[0037] (1) Preparation of raw materials and continuous reaction apparatus: Prepare an aqueous solution of sodium hypophosphite monohydrate with a weight ratio of sodium hypophosphite monohydrate to deionized water of 1:1.2 and preheat it to 92°C; prepare an aqueous solution of aluminum sulfate octadecade with a weight ratio of aluminum sulfate octadecade to deionized water of 1:1.1 and preheat it to 92°C. Set up a continuous reaction apparatus, including a crystallization vessel, a dissolving vessel, and a precipitation vessel connected in sequence. Add deionized water to each of the three vessels, accounting for 60% of the effective volume of the vessel, start stirring, and adjust the temperature of the three vessels to 92°C. The difference in effective volume between the three vessels should be ≤15%.

[0038] (2) Start and control the continuous reaction: Turn on the continuous reaction device and control the simultaneous and continuous injection of the two salt solutions into the crystallization vessel. The feed weight ratio of sodium hypophosphite monohydrate and aluminum sulfate octadecahydrate per unit time is 1:1.05, and the total feed rate of the two salt solutions is the effective volume of the crystallization vessel ÷ 5h. Control the rate at which the slurry is continuously introduced from the crystallization vessel to the dissolving vessel to be equal to the total feed rate of the two salt solutions into the crystallization vessel. At the same time, continuously introduce a 25% sulfuric acid solution into the dissolving vessel to maintain the pH value of the slurry in the dissolving vessel at a stable level. 3.0; The rate at which the slurry is continuously introduced from the dissolving vessel to the precipitation vessel is controlled to be the sum of the total feed rate of the two salt solutions to the crystallization vessel and the feed rate of the sulfuric acid solution to the dissolving vessel. At the same time, a 20% sodium hydroxide aqueous solution is continuously introduced into the precipitation vessel to maintain the pH value of the slurry in the precipitation vessel at a stable level of 7.3. The rate at which the finished slurry is continuously discharged from the precipitation vessel is controlled to be the sum of the total feed rate of the two salt solutions to the crystallization vessel, the feed rate of the sulfuric acid solution to the dissolving vessel, and the feed rate of the sodium hydroxide aqueous solution to the precipitation vessel.

[0039] (3) Post-processing: After the particle size of the finished slurry discharged from the precipitation vessel is measured by a laser particle size analyzer, the qualified slurry is collected, filtered, and the filter cake is repeatedly washed with 55℃ deionized water until the conductivity of the filtrate is <3.0ms / cm; the washed filter cake is placed in a rake dryer and dried at a constant temperature of 90℃ to obtain the finished aluminum hypophosphite flame retardant.

[0040] Example 2: Preparation of aluminum hypophosphite using a three-stage continuous reaction process of crystallization-dissolution-precipitation.

[0041] (1) Preparation of raw materials and continuous reaction apparatus: Prepare an aqueous solution of sodium hypophosphite monohydrate with a weight ratio of sodium hypophosphite monohydrate to deionized water of 1:1, and preheat it to 90°C; prepare an aqueous solution of aluminum sulfate octadecade with a weight ratio of aluminum sulfate octadecade to deionized water of 1:1, and preheat it to 90°C. Set up a continuous reaction apparatus, including a crystallization vessel, a dissolving vessel, and a precipitation vessel connected in sequence. Add deionized water to the three vessels, accounting for 50% of the effective volume of the vessels, start stirring, and adjust the temperature of the three vessels to 90°C. The difference in effective volume between the three vessels should be ≤20%.

[0042] (2) Start and control the continuous reaction: Turn on the continuous reaction device and control the simultaneous and continuous injection of the two salt solutions into the crystallization vessel, wherein the feed weight ratio of sodium hypophosphite monohydrate and aluminum sulfate octadecahydrate per unit time is 1:1.02, and the total feed rate of the two salt solutions is the effective volume of the crystallization vessel ÷ 3h; control the rate at which the slurry is continuously introduced from the crystallization vessel into the dissolving vessel to be equal to the total feed rate of the two salt solutions into the crystallization vessel, and at the same time continuously introduce a 15% sulfuric acid solution into the dissolving vessel to maintain the pH value of the slurry in the dissolving vessel at a stable 2.9; control the rate at which the precipitate is introduced from the dissolving vessel into the dissolving vessel. The rate at which the slurry is continuously introduced into the crystallization vessel is the sum of the total feed rate of the two salt solutions into the crystallization vessel and the feed rate of the sulfuric acid solution into the dissolving vessel. Simultaneously, a 10% sodium hydroxide aqueous solution is continuously introduced into the precipitation vessel to maintain the pH value of the slurry in the precipitation vessel at a stable 7.1. The rate at which the finished slurry is continuously discharged from the precipitation vessel is controlled to be the sum of the total feed rate of the two salt solutions into the crystallization vessel, the feed rate of the sulfuric acid solution into the dissolving vessel, and the feed rate of the sodium hydroxide aqueous solution into the precipitation vessel. Reaction crystallization, fine crystal dissolution, and recrystallization occur in the crystallization vessel, dissolving vessel, and precipitation vessel, respectively.

[0043] (3) Post-processing: The finished slurry discharged from the precipitation vessel is collected after the particle size is tested by a laser particle size analyzer and the slurry meets the standard. After filtration, the filter cake is repeatedly washed with 40℃ deionized water until the conductivity of the filtrate is <3.0ms / cm. The washed filter cake is placed in a rake dryer and dried at a constant temperature of 60℃ to obtain the finished aluminum hypophosphite flame retardant.

[0044] Example 3: Preparation of aluminum hypophosphite using a three-stage continuous reaction process of crystallization-dissolution-precipitation.

[0045] (1) Preparation of raw materials and continuous reaction apparatus: Prepare an aqueous solution of sodium hypophosphite monohydrate with a weight ratio of sodium hypophosphite monohydrate to deionized water of 1:1.5 and preheat it to 95°C; prepare an aqueous solution of aluminum sulfate octadecade with a weight ratio of aluminum sulfate octadecade to deionized water of 1:3 and preheat it to 95°C. Set up a continuous reaction apparatus, including a crystallization vessel, a dissolving vessel, and a precipitation vessel connected in sequence. Add deionized water to the three vessels, accounting for 70% of the effective volume of the vessels, start stirring, and adjust the temperature of the three vessels to 95°C. The difference in effective volume between the three vessels should be ≤20%.

[0046] (2) Start and control the continuous reaction: Turn on the continuous reaction device and control the simultaneous and continuous injection of the two salt solutions into the crystallization vessel. The feed weight ratio of sodium hypophosphite monohydrate and aluminum sulfate octadecade per unit time is 1:1.08, and the total feed rate of the two salt solutions is the effective volume of the crystallization vessel ÷ 8h. Control the rate at which the slurry is continuously introduced from the crystallization vessel into the dissolving vessel to be equal to the total feed rate of the two salt solutions into the crystallization vessel. At the same time, continuously introduce a 35% sulfuric acid solution into the dissolving vessel to maintain the pH value of the slurry in the dissolving vessel at a stable level. 3.1; The rate at which the slurry is continuously introduced from the dissolving vessel to the precipitation vessel is controlled to be the sum of the total feed rate of the two salt solutions to the crystallization vessel and the feed rate of the sulfuric acid solution to the dissolving vessel. At the same time, a 30% sodium hydroxide aqueous solution is continuously introduced into the precipitation vessel to maintain the pH value of the slurry in the precipitation vessel at a stable level of 7.5. The rate at which the finished slurry is continuously discharged from the precipitation vessel is controlled to be the sum of the total feed rate of the two salt solutions to the crystallization vessel, the feed rate of the sulfuric acid solution to the dissolving vessel, and the feed rate of the sodium hydroxide aqueous solution to the precipitation vessel.

[0047] (3) Post-processing: The finished slurry discharged from the precipitation vessel is collected after the particle size is measured by a laser particle size analyzer and the slurry meets the standard. After filtration, the filter cake is repeatedly washed with 70℃ deionized water until the conductivity of the filtrate is <3.0ms / cm. The washed filter cake is placed in a rake dryer and dried at a constant temperature of 120℃ to obtain the finished aluminum hypophosphite flame retardant.

[0048] Comparative Example 1: Preparation of aluminum hypophosphite using a conventional batch one-step crystallization process

[0049] (1) Prepare an aqueous solution of sodium hypophosphite monohydrate with a weight ratio of sodium hypophosphite monohydrate to deionized water of 1:1.2 and preheat to 92°C; prepare an aqueous solution of aluminum sulfate octadecade with a weight ratio of aluminum sulfate octadecade to deionized water of 1:1.1 and preheat to 92°C; add deionized water to the crystallization vessel to account for 60% of the effective volume of the vessel, start stirring, and adjust the vessel temperature to 92°C.

[0050] (2) Sodium hypophosphite monohydrate and aluminum sulfate octadecade in a weight ratio of 1:1.05 were injected into the crystallization vessel and the neutralization and crystallization reaction was carried out for 5 hours to generate a crystallization slurry.

[0051] (3) Drain the slurry from the crystallization vessel, filter it, and repeatedly wash the filter cake with 55°C deionized water until the conductivity of the filtrate is <3.0ms / cm; put the washed filter cake into a rake dryer and dry it at a constant temperature of 90°C to obtain the finished aluminum hypophosphite flame retardant.

[0052] Comparative Example 2: Preparation of aluminum hypophosphite using a three-stage continuous reaction process of crystallization-dissolution-precipitation.

[0053] Compared with Example 1, only the total feed rate of the two salt solutions in step (2) is adjusted to the effective volume of the crystallization vessel ÷ 1h, and the feed and discharge rates of the other vessels are adjusted synchronously according to the reaction conditions of Example 1. Specifically, the discharge rate of each vessel is still equal to the sum of its feed rates, maintaining the dynamic balance of the system's feed and discharge. Therefore, the feed rates of the dissolving vessel and the precipitation vessel change accordingly based on the adjustment of the total feed rate of the two salt solutions in the crystallization vessel. The other process parameters are exactly the same as in Example 1.

[0054] Comparative Example 3: Preparation of aluminum hypophosphite using a three-stage continuous reaction process of crystallization-dissolution-precipitation.

[0055] Compared with Example 1, only the pH of the slurry in the dissolving vessel in step (2) was adjusted to 2.5, and the other process parameters were exactly the same as in Example 1.

[0056] Comparative Example 4: Preparation of aluminum hypophosphite using a three-stage continuous reaction process of crystallization-dissolution-precipitation.

[0057] Compared with Example 1, only the pH of the slurry in the precipitated vessel in step (2) was adjusted to 8.5, and the other process parameters were exactly the same as in Example 1.

[0058] The aluminum hypophosphite prepared in Examples 1-3 and Comparative Examples 1-4 was characterized, and the results are shown in Table 1. The microstructure was measured using a field emission scanning electron microscope (HITACHI SU1000), the particle size distribution was measured using a laser particle size analyzer (Malvern Mastersizer 3000), the thermal decomposition temperature was measured using a thermogravimetric analyzer (NETZSCH TG 209 F3), and the specific surface area was measured using a specific surface area and pore size analyzer (BSD~660M) via static volumetric method. The SEM image of the near-spherical, lamellar stacked structure aluminum hypophosphite prepared in Example 1 is shown below. Figure 1 As shown, the SEM image of aluminum hypophosphite obtained in Comparative Example 1 is as follows. Figure 4 As shown, the TGA diagrams of the aluminum hypophosphite flame retardants prepared in Example 1 and Comparative Example 1 are as follows. Figure 2 As shown.

[0059] Table 1. Morphology and parameters of aluminum hypophosphate products from Examples 1-3 and Comparative Examples 1-4

[0060]

[0061] Application performance testing:

[0062] PA6 composite materials were prepared using aluminum hypophosphite prepared in Examples 1-3 and Comparative Examples 1-4 as flame retardants. The composite material formulation was as follows: 55 parts PA6 resin, 25 parts glass fiber, 15 parts aluminum hypophosphite, 5 parts melamine polyphosphate, and 0.2 parts antioxidant. Except for the aluminum hypophosphite, which was prepared using the product from Examples 1-3 or Comparative Examples 1-4, all other components were commercially available products in the art. Each component was weighed according to its weight, and the components were added to a mixer and mixed until homogeneous to obtain a premix. The premix was then fed into a twin-screw extruder for melt mixing and extrusion granulation to obtain the composite material. The mechanical and flame-retardant properties of the composite material were tested. The test parameters were: vertical flammability rating (GB / T2408 2008, 1.6 mm specimen), limiting oxygen index (LOI) (GB / T2406.2 2009), tensile strength (GB / T1040.2 2006), flexural strength (GB / T9341 2008), and notched impact strength of cantilever beam (GB / T1843 2008). The results are shown in Table 2.

[0063] Table 2 Application test results of Examples 1-3 and Comparative Examples 1-4

[0064]

[0065] Based on the product characterization results in Table 1 and the application test results in Table 2, it can be concluded that Examples 1-3 adopted a three-stage continuous process of crystallization-dissolution-precipitation, and the preparation process is as follows: Figure 3 As shown, by controlling various reaction parameters, aluminum hypophosphite flame retardants with a spherical, pinecone-like layered structure can be prepared. The D50 particle size is above 25 μm, the thermal decomposition temperature is above 330℃, and the specific surface area is between 1.2 and 3.5 m² / g. The aluminum hypophosphite flame retardants of Examples 1-3 all achieved excellent mechanical properties and high flame retardancy in PA6 and glass fiber systems. This is mainly because... Figure 1 and Figure 2 As shown, the aluminum hypophosphite flame retardants in Examples 1-3 exhibit a specific layered microstructure with large particle size and small specific surface area. Their thermal decomposition temperatures are all greater than 330℃. They achieve uniform dispersion without agglomeration in the resin matrix and can form a synergistic flame retardant effect when compounded with melamine polyphosphate. They rapidly form char to block heat and flammable gases, achieving excellent flame retardant effects without requiring high addition amounts, while preserving the original mechanical properties of engineering plastics to the greatest extent. Comparative Example 1 uses a traditional intermittent one-step method, such as... Figure 2 and Figure 4As shown, the product has a low thermal decomposition temperature and an irregular blocky morphology with a high specific surface area of ​​6.8 m² / g. The particles are severely agglomerated, which easily forms a large number of defect points inside the composite material. This significantly damages the continuity of the resin matrix and the bonding between the resin matrix and the glass fiber interface, resulting in a significant decrease in tensile, flexural, and impact strength. The impact strength is only 5.7 kJ / m², which is about 60% of that in Example 1. At the same time, the agglomeration shields the flame-retardant active sites, causing a sharp drop in flame-retardant efficiency. The oxygen index is only 26.3%, and the vertical burning level is only V-2, which cannot meet the requirements for the use of PA6 materials. Comparative Example 2, due to its excessively fast feeding rate in the crystallization reactor, easily generates a large number of fine crystal nuclei instantly, making it impossible to purify and form a regular structure. The particles exhibit poor dispersion and significant agglomeration, resulting in mechanical properties that are only slightly better than Comparative Example 1, and a flame retardant rating of only V-1. Comparative Example 3, with its excessively acidic dissolution reactor, easily leads to over-dissolution of the core crystal nuclei, resulting in fine and loose particles with low thermal stability. The composite material exhibits moderate to low mechanical and flame retardant properties. Comparative Example 4, with its excessively alkaline precipitation reactor, causes instantaneous particle precipitation and localized agglomeration. Although its performance is better than the first two comparative examples, it still suffers from dispersion defects. Its oxygen index and mechanical properties are far lower than those of the example group, and it cannot achieve a V-0 rating during vertical combustion.

[0066] In summary, the spherical pinecone-like layered aluminum hypophosphite flame retardant prepared by this invention through a three-reactor continuous process and optimized reaction parameters, when compounded with melamine polyphosphate in a glass fiber reinforced PA6 system, can achieve both excellent mechanical properties and highly efficient halogen-free flame retardant properties. This solves the problems of poor dispersion, severe agglomeration, and the inability to achieve both flame retardancy and mechanical properties in traditional flame retardants. In contrast, conventional intermittent processes or deviations from core process parameters can lead to the deterioration of the flame retardant's morphology, particle size, and thermal stability, directly causing a significant decrease in the performance of the composite material.

[0067] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. For those skilled in the art, the present invention can have various modifications and variations. 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 spherical, pinecone-shaped, layered aluminum hypophosphite flame retardant, characterized in that: The aluminum hypophosphite flame retardant exhibits a spherical, pinecone-like layered structure in its microstructure, with each particle consisting of multiple thin layers stacked alternately, forming a regular spherical outer contour. The aluminum hypophosphite flame retardant has a thermal decomposition temperature >330℃ and a particle size distribution that meets the following requirements: D10 particle size 8~15μm, D50 particle size 25~40μm, D90 particle size 60~80μm, D97 particle size 80~100μm, and a specific surface area of ​​1.2~3.5m² / g.

2. The spherical, pinecone-like, layered aluminum hypophosphite flame retardant according to claim 1, characterized in that, The aluminum hypophosphite flame retardant has a thermal decomposition temperature >335℃ and a particle size distribution that meets the following requirements: D10 particle size 10~15μm, D50 particle size 30~40μm, D90 particle size 65~80μm, D97 particle size 80~90μm, and a specific surface area of ​​2~3m² / g.

3. A method for preparing the spherical, pinecone-like, layered aluminum hypophosphite flame retardant according to claim 1 or 2, characterized in that: The process employs a three-stage continuous reaction process of crystallization-dissolution-precipitation, specifically including the following steps: (1) Preparation of raw materials and continuous reaction equipment: Prepare sodium hypophosphite monohydrate and aluminum sulfate octadecade aqueous solution at a constant temperature of 90-95℃ in advance; set up a continuous reaction equipment, including a crystallization vessel, a dissolving vessel and a precipitation vessel connected in sequence, add deionized water accounting for 50%-70% of the effective volume of the three vessels to the three vessels respectively, start stirring and control the temperature of the three vessels at 90-95℃, and the difference in the effective volume of the three vessels should not exceed 20%; (2) Start and control the continuous reaction: Turn on the continuous reaction device and control the simultaneous and continuous injection of the two salt solutions into the crystallization vessel. The feed weight ratio of sodium hypophosphite monohydrate and aluminum sulfate octadecahydrate per unit time is 1:(1.02~1.08), and the total feed rate of the two salt solutions is the effective volume of the crystallization vessel ÷ (3~8h). Control the rate at which the slurry is continuously introduced from the crystallization vessel into the dissolving vessel to be equal to the total feed rate of the two salt solutions into the crystallization vessel. At the same time, continuously introduce sulfuric acid solution with a mass concentration of 15%~35% into the dissolving vessel to maintain the pH value of the slurry in the dissolving vessel stable at a certain level. The pH value is controlled between 2.9 and 3.1; the rate at which the slurry is continuously introduced from the dissolving vessel to the precipitation vessel is the sum of the total feed rate of the two salt solutions to the crystallization vessel and the feed rate of the sulfuric acid solution to the dissolving vessel, while a sodium hydroxide aqueous solution with a mass concentration of 10% to 30% is continuously introduced into the precipitation vessel to maintain the pH value of the slurry in the precipitation vessel stable between 7.1 and 7.5; the rate at which the finished slurry is continuously discharged from the precipitation vessel is the sum of the total feed rate of the two salt solutions to the crystallization vessel, the feed rate of the sulfuric acid solution to the dissolving vessel, and the feed rate of the sodium hydroxide aqueous solution to the precipitation vessel; (3) Post-processing: After the particle size of the finished slurry discharged from the precipitation vessel is found to be up to standard, the slurry is collected and then filtered, rinsed with deionized water and dried in sequence to obtain the aluminum hypophosphite flame retardant.

4. The preparation method according to claim 3, characterized in that: In the sodium hypophosphite monohydrate aqueous solution, the weight ratio of sodium hypophosphite monohydrate to deionized water is 1:(1-1.5); in the aluminum sulfate octadecade aqueous solution, the weight ratio of aluminum sulfate octadecade to deionized water is 1:(1-1.3).

5. The preparation method according to claim 3, characterized in that: The post-processing stage uses a rake dryer for drying, with the drying temperature controlled at 60~120℃.

6. The preparation method according to claim 3, characterized in that: The temperature of the deionized water used for rinsing the filter cake in the post-treatment stage is 40~70℃, and the rinsing endpoint is controlled when the conductivity of the filtrate is <3.0ms / cm.

7. The application of the spherical pinecone-like layered aluminum hypophosphite flame retardant according to claim 1 or 2, characterized in that: This flame retardant, as a halogen-free flame retardant, is used in the field of flame retardant modification of engineering plastics.

8. The application of the spherical pinecone-like layered aluminum hypophosphite flame retardant according to claim 7, characterized in that: This flame retardant is compounded with melamine polyphosphate and applied to PA6, PA66 or PBT systems.

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