Self-suspension modified magnesium hydroxide flame retardant, preparation method and application thereof

Abstract

The present application relates to self-suspension modified magnesium hydroxide flame retardant, preparation method and application thereof, and belongs to the field of preparation of flame retardant materials, and the preparation method comprises the following steps: dispersing magnesium hydroxide filter cake A into magnesium hydroxide slurry, adding KH560 and titanate coupling agent to modify the slurry to obtain slurry B; dispersing attapulgite, adding CTAB solution to modify to obtain colloid C; adding slurry B into colloid C and stirring; adding a crosslinking agent into the stirring solution, and performing suction filtration and drying to obtain coated modified magnesium hydroxide M; dispersing magnesium hydroxide M into magnesium hydroxide slurry D; taking sodium-based bentonite, shearing after modification by PAAS to obtain dispersion liquid F; adding slurry D into dispersion liquid F and mixing, adding PVP K30 and stirring; and performing spray drying to obtain the magnesium hydroxide flame retardant. The self-suspension modified magnesium hydroxide flame retardant provided by the present application can realize self-dispersion and suspension in water-based polyurethane coating, can solve the problem of affecting the stability of water-based polyurethane, and can improve the flame retardance and smoke suppression of water-based polyurethane.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of flame-retardant material preparation, in particular to a self-suspension modified magnesium hydroxide flame retardant, a preparation method and application thereof. BACKGROUND

[0002] Waterborne polyurethane coating is a kind of coating with waterborne polyurethane resin as base material and water as dispersion medium, which has good storage stability, film mechanical properties, water resistance, solvent resistance and aging resistance, and has been widely used in the field of waterborne coatings. However, the flame-retardant effect of waterborne polyurethane coating itself is poor, so the research on improving the flame-retardant effect of waterborne polyurethane coating has become a research hotspot in recent years.

[0003] The current technical implementation mainly achieves the flame-retardant requirement by directly adding flame retardants in the coating. The commonly used high-efficiency flame-retardant coating contains halogen. When the halogen-containing material burns, a large amount of toxic smoke and carbon monoxide, hydrogen halide and other corrosive gases are released, thereby hindering the evacuation and fire extinguishing work of people in the fire, causing "secondary" pollution after the fire, and causing great loss of life and property. The world attaches great importance to the environmental protection problem of flame-retardant coating, so halogen-free flame-retardant coating has become the development trend of coating.

[0004] The flame-retardant additive used in the commonly used halogen-free flame-retardant coating is the conventional aluminum hydroxide, magnesium hydroxide, silicate and expanded graphite system. Magnesium hydroxide stands out due to its cost and resource advantages. When the conventional magnesium hydroxide is used as an additive, its flame-retardant rate is low, the compatibility with organic components is poor, the required filling amount is large, and the flame-retardant effect can be achieved only when the filling amount is more than 50% of the total weight of the coating. Moreover, hard or soft settlement easily occurs, which brings great difficulty to the use after long-term transportation. Moreover, the medium-strong alkaline nature of magnesium hydroxide easily leads to the imbalance of the stability of waterborne polyurethane coating, and a plurality of chemical additives need to be added to improve the use performance of the coating, which greatly reduces the environmental protection of the waterborne coating and limits its development and application. SUMMARY

[0005] The present application aims to provide a self-suspension modified magnesium hydroxide flame retardant, a preparation method and application thereof, so as to solve the technical problems of poor flame-retardant effect and easy settlement of the conventional magnesium hydroxide as an additive mentioned in the background.

[0006] To achieve the above-mentioned purpose, the present application provides the following technical solutions:

[0007] The present application provides a preparation method of a self-suspension modified magnesium hydroxide flame retardant, which comprises the following steps:

[0008] (1) obtaining magnesium hydroxide filter cake A;

[0009] (2) dispersing the magnesium hydroxide filter cake A into a magnesium hydroxide slurry with a solid content of 10-15%, and adding KH560 and a titanate coupling agent to modify the slurry to obtain slurry B;

[0010] (3) dispersing the attapulgite, and adding a CTAB solution to modify the colloid to obtain colloid C;

[0011] (4) adding the slurry B to the colloid C and stirring;

[0012] (5) adding a crosslinking agent to the stirring solution of step (4), and performing suction filtration and drying to obtain modified magnesium hydroxide M coated with a layer;

[0013] (6) dispersing the magnesium hydroxide M into a magnesium hydroxide slurry D;

[0014] (7) shearing sodium-based bentonite modified by PAAS to obtain dispersion F;

[0015] (8) adding the magnesium hydroxide slurry D to the dispersion F and mixing, and adding PVP K30 and stirring;

[0016] (9) spray drying to obtain a magnesium hydroxide flame retardant.

[0017] In a preferred embodiment, in step (1), a magnesium hydroxide slurry is filtered and washed with water to obtain a magnesium hydroxide filter cake A, and the particle size of the magnesium hydroxide is 0.8-1.2 μm.

[0018] In a preferred embodiment, in step (3), the length of the attapulgite is 0.96-2.4 μm, and the particle size is 40-120 nm.

[0019] In a preferred embodiment, in step (4), the pH of the slurry B and the colloid C is first adjusted to 9-10, and then the slurry B is added to the colloid C and stirred.

[0020] In a preferred embodiment, in step (4), the mass ratio of the slurry B to the colloid C is 4-5:1; and in step (8), the mass ratio of the magnesium hydroxide slurry D to the dispersion F is 2-4:1.

[0021] In a preferred embodiment, in step (6), a polydopamine coating layer is coated on the surface of the magnesium hydroxide M.

[0022] In a preferred embodiment, in step (7), the sodium-based bentonite has a small-size lamellar structure with a diameter of <100 nm.

[0023] In a preferred embodiment, in step (9), silica aerogel is added during the spray drying process.

[0024] The second aspect of the present application provides the self-suspension modified magnesium hydroxide flame retardant prepared by the preparation method.

[0025] The third aspect of the present application provides a halogen-free flame-retardant water-based polyurethane coating, comprising: a water-based polyurethane resin, the modified magnesium hydroxide flame retardant, a dispersing agent, a defoaming agent and a mildew-proof agent, and the mass ratio of the water-based polyurethane resin, the modified magnesium hydroxide flame retardant, the dispersing agent, the defoaming agent and the mildew-proof agent is (50-70):(32-36):(1-2):(0-1):(0-1).

[0026] Compared with the prior art, the present application has the following advantages: the self-suspension modified magnesium hydroxide flame retardant provided by the present application has thixotropy, and when applied in water-based polyurethane coating, it can realize self-dispersion and suspension without adding other anti-settling agents, can achieve 6 months without water separation, and the outer coating layer is a silicate-based weak alkaline compound, which solves the problem of the influence of the high pH value of magnesium hydroxide on the stability of water-based polyurethane, and further improves the flame retardance and smoke suppression of water-based polyurethane.

[0027] (1) The self-suspension modified magnesium hydroxide flame retardant provided by the present application realizes the stable shell-core structure of double-layer coated magnesium hydroxide by using layer-by-layer self-assembly technology and effective bridging aids such as polyetheramine and polyvinylpyrrolidone, and when used in water-based polyurethane coating, the outermost sodium-based bentonite layer can thicken the emulsion through the hydration swelling effect of the sheet layer, and has the effect of long-term stable suspension and non-settling.

[0028] (2) Under the support of the two thixotropic shell layers, the modified magnesium hydroxide also has thixotropy, which can realize good dispersion and stable suspension and non-settling in water-based polyurethane coating and other water-based resins, thereby breaking the application limitation of magnesium hydroxide.

[0029] (3) Under the action of the two silicate shell layers, the flame retardance and smoke suppression of magnesium hydroxide are greatly improved, thereby reducing the addition amount of magnesium hydroxide.

[0030] (4) The double-layer coated magnesium hydroxide can also effectively avoid the destruction of the stability of water-based polyurethane coating caused by the medium-strong alkalinity of magnesium hydroxide. The pH value of unmodified magnesium hydroxide is high, and a large amount of addition can easily lead to imbalance of the stability of water-based polyurethane, resulting in gel demulsification. The modified magnesium hydroxide of the present application has a sodium-based bentonite outer coating layer, and after being dispersed in the water-based system, its pH value is close to that of the water-based polyurethane, so it does not affect the system stability. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1The sedimentation state of the waterborne polyurethane coating prepared for the present application example 3 after being placed for 6 months;

[0032] Figure 2 The sedimentation state of the waterborne polyurethane coating prepared for the present application comparative example 2 after being placed for 2 days. DETAILED DESCRIPTION

[0033] The technical solutions in the embodiments of the present application will be apparently and completely described below with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all the other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the present application.

[0034] The present application provides a preparation method of self-suspension modified magnesium hydroxide flame retardant, comprising the following steps:

[0035] First step: magnesium hydroxide slurry is prepared by using hydrothermal reaction, and the magnesium hydroxide filter cake A is obtained by filtering and washing with water for multiple times; in this step, the particle size of the magnesium hydroxide is 0.8-1.2 μm, and the crystal morphology is regular hexagonal flake, and the pH after washing is 9-10;

[0036] Second step: the magnesium hydroxide filter cake A is dispersed at high speed with deionized water to prepare the magnesium hydroxide slurry with the mass solid content of 10-15%, and the modification agent is continuously dispersed at high speed, wherein the modification agent is the mixture of KH560 (silane coupling agent) and titanate coupling agent, and the total addition amount of the modification agent is 1%-2% of the mass of the magnesium hydroxide slurry;

[0037] The modification is carried out for 1 h to obtain the modified slurry B, and the mass ratio of KH560 to titanate coupling agent is 3:1;

[0038] Third step: the attapulgite is dispersed at high speed in deionized water to form the stable colloid C, and the mass solid content is 10%, and 1% of the mass of the attapulgite is CTAB (cetyltrimethylammonium bromide), and the modification is carried out for 2 h under the condition of 60°C; in this step, the length of the attapulgite is 0.96-2.4 μm, and the particle size range is 40-120 nm; meanwhile, 0.1% of the mass of the CTAB in 1% of the CTAB contains sodium polyacrylate;

[0039] Fourth step: the pH of the slurry B and the colloid C is adjusted to 9-10 by adding appropriate amount of auxiliary agent, then the slurry B is slowly added into the colloid C, and the constant temperature stirring is carried out for 2 h under the condition of 60°C-70°C;

[0040] The auxiliary agent is one of NaOH, tetramethylammonium hydroxide and sodium carbonate, and the mass ratio of the slurry B to the colloid C is 4-5:1;

[0041] Fifth step: adding crosslinking agent (0.8% of aluminum chloride or polyetheramine D230 in the solution) to the solution formed in the fourth step, filtering, drying after multiple washing, and obtaining modified magnesium hydroxide M;

[0042] Sixth step: dispersing the magnesium hydroxide M after the surface is oxidized to form a polydopamine coating, and preparing a magnesium hydroxide slurry D with a solid content of 10%;

[0043] Seventh step: preparing a dispersion liquid of sodium bentonite with a solid content of 10% by adding water, modifying the sodium bentonite with 2% of polyacrylic acid sodium (PAAS, industrial grade, brand: Karamar), and preparing a nanolayer dispersion liquid F by high-speed shearing (10000 rpm, 20 min);

[0044] Among them, the sodium bentonite is obtained by centrifugal classification and has a small size lamellar structure with a diameter of less than 100 nm;

[0045] Eighth step: slowly adding the magnesium hydroxide slurry D to the dispersion liquid F, adjusting the mass ratio of the magnesium hydroxide slurry D to the dispersion liquid F to 2-4:1, adjusting the pH of the mixed liquid to 7.0, adding 0.1% of polyvinylpyrrolidone (PVP K30) as a hydrogen bond mediator, and stirring at a low speed (200 rpm) at 40°C for 4h;

[0046] Ninth step: spray drying the liquid formed in the eighth step, adding 3% of silica aerogel to prevent caking, and obtaining a double-layer modified magnesium hydroxide flame retardant with a stable shell-core structure after drying.

[0047] The present application solves the problem of preventing magnesium hydroxide from settling by comprehensively considering viscosity, particle size, electrostatic effect and thixotropic mechanism.

[0048] 1. The pH value of the magnesium hydroxide treated by the modifier and the attapulgite treated by CATB is about 8, and they are positively charged, which is not conducive to interlayer adsorption due to electrostatic repulsion. The present application adjusts the pH value of the attapulgite dispersion liquid to be negatively charged by a pH adjusting aid, realizes electrostatic coating, and adds an interfacial bridging agent polyetheramine to form a Si-O-C-NH- covalent network between the epoxy group of KH560 and the -SiOH in the attapulgite, realize a stable shell structure, and the addition of titanate coupling agent can more completely and densely coat the surface of the magnesium hydroxide in cooperation with KH560. At the same time, a layer of polydopamine coating is coated on the outer shell to enhance the stability of the shell-core, ensure that the water in the outermost layer does not further infiltrate under swelling, and ensure the stability of the double-layer shell-core structure.

[0049] 2. The selected attapulgite and bentonite in the application are all nanoscale, which are easy to agglomerate and affect the coating effect. In the application, high-speed shearing is used for dispersion, and polyvinylpyrrolidone is added as a hydrogen bond mediator during the reaction process to effectively bridge between attapulgite and bentonite. In the later drying process, silica aerogel is synchronously added as a nanometer support to maintain the pore structure, prevent the collapse of the core-shell structure during the drying process, and prevent the agglomeration of the material.

[0050] The application further provides a halogen-free flame-retardant water-based polyurethane coating, which comprises a flame-retardant filler, a water-based polyurethane resin, a dispersing agent, a defoaming agent and a mildew-proof agent, wherein the flame-retardant filler is the self-suspension modified magnesium hydroxide flame retardant prepared by the preparation method, and the mass ratio of the water-based polyurethane resin, the modified magnesium hydroxide flame retardant, the dispersing agent, the defoaming agent and the mildew-proof agent is (50-70):(32-36):(1-2):(0-1):(0-1).

[0051] The application will be further described in detail in combination with the following examples. It should be understood that the following examples are only used to further illustrate the application, and cannot be understood as a limitation on the protection scope of the application. Some non-essential improvements and adjustments made by those skilled in the art according to the above content of the application all belong to the protection scope of the application. The specific process parameters in the following examples are only one example in the appropriate range, that is, those skilled in the art can select within the appropriate range through the description herein, and are not limited to the specific values in the following examples.

[0052] In the following examples, the raw materials used are commercially available, and the processes used are conventional processes.

[0053] Example 1

[0054] The preparation method of the self-suspension modified magnesium hydroxide flame retardant provided in this embodiment comprises the following steps:

[0055] First step: using hydrothermal reaction to prepare hexagonal flake-shaped magnesium hydroxide slurry with a particle size of 0.8-0.9 μm, filtering and washing with water for multiple times, measuring the pH value to be 10, and obtaining magnesium hydroxide filter cake A;

[0056] Second step: dispersing the magnesium hydroxide filter cake A with deionized water at high speed to prepare magnesium hydroxide slurry with a solid content of about 10%, continuing to disperse at high speed, adding 1% of a mixture of KH560 and titanate coupling agent at 60°C, modifying for 1 h, and obtaining modified slurry B;

[0057] Third step: dispersing attapulgite (length 0.96 μm, diameter 40 nm) in deionized water at high speed, with a solid content of 10%, adding 1% of CTAB (cetyltrimethylammonium bromide), and modifying for 2 h at 60°C to form stable colloid C;

[0058] Fourth step: add 1% of tetramethylammonium hydroxide with a concentration of 1.5 mL / g to adjust the pH value of slurry B and colloid C to 9, then slowly add slurry B into colloid C, the mass ratio of slurry B to colloid C is 4:1, constant temperature stirring at 60-70℃ for 2h;

[0059] Fifth step: add crosslinking agent (0.8% of polyetheramine D230 in the solution mass) to the solution of the fourth step, filter, wash and dry to obtain the coated modified magnesium hydroxide M;

[0060] Sixth step: after the surface of magnesium hydroxide M is oxidized to self-polymerize a layer of polydopamine coating, disperse with water to prepare magnesium hydroxide slurry D with a solid content of 10%;

[0061] Seventh step: prepare a dispersion liquid of sodium-based bentonite (diameter <100nm) with a solid content of 10% by adding water, modify with 2% of polyacrylic acid sodium (PAAS), and high-speed shear (10000rpm, 20min) to prepare nanolayer dispersion liquid F;

[0062] Eighth step: slowly add magnesium hydroxide slurry D into dispersion liquid F, the mass ratio of slurry D to dispersion liquid F is 3:1, adjust pH=7.0, add 0.1% of polyvinylpyrrolidone (PVP K30) as a hydrogen bond mediator, and stir at low speed (200rpm) at 40℃ for 4h;

[0063] Ninth step: spray drying, add 3% of silica aerogel to prevent caking, and obtain the double-layer modified magnesium hydroxide flame retardant with stable shell-core structure after drying.

[0064] The embodiment also provides a halogen-free flame-retardant water-based polyurethane coating, which comprises water-based polyurethane resin, modified magnesium hydroxide flame retardant, dispersant, defoaming agent, and mildew-proof agent in a mass ratio of 60:35:2:1:1, and is uniformly stirred with a high-speed dispersant to obtain the halogen-free flame-retardant water-based polyurethane coating.

[0065] Example 2

[0066] The embodiment is different from example 1 in that the hexagonal flaky magnesium hydroxide slurry with a particle size of 1.0-1.1 μm is prepared in the first step, and the length of attapulgite in the third step is 1.2 μm and the diameter is 80 nm.

[0067] The halogen-free flame-retardant water-based polyurethane coating provided by the embodiment comprises water-based polyurethane resin, modified magnesium hydroxide flame retardant, dispersant, defoaming agent, and mildew-proof agent in a mass ratio of 70:32:1:1:1, and is uniformly stirred with a high-speed dispersant to obtain the halogen-free flame-retardant water-based polyurethane coating.

[0068] Example 3

[0069] The embodiment is different from embodiment 1 in that the hexagonal flaky magnesium hydroxide slurry with a particle size of 1.1-1.2 μm is prepared in the first step; and the attapulgite has a length of 2.4 μm and a diameter of 120 nm in the third step.

[0070] The halogen-free flame-retardant water-based polyurethane coating provided by the embodiment comprises water-based polyurethane resin, modified magnesium hydroxide flame retardant, dispersant, defoaming agent and mildew-proof agent in a mass ratio of 60:35:2, and is uniformly stirred with a high-speed dispersant to obtain the halogen-free flame-retardant water-based polyurethane coating.

[0071] Embodiment 4

[0072] The embodiment is different from embodiment 1 in that the pH adjusting agent in the fourth step is NaOH with a concentration of 0.6 mL / g.

[0073] The halogen-free flame-retardant water-based polyurethane coating provided by the embodiment comprises water-based polyurethane resin, modified magnesium hydroxide flame retardant, dispersant, defoaming agent and mildew-proof agent in a mass ratio of 60:35:2, and is uniformly stirred with a high-speed dispersant to obtain the halogen-free flame-retardant water-based polyurethane coating.

[0074] Embodiment 5

[0075] The embodiment is different from embodiment 1 in that the pH adjusting agent in the fourth step is Na2CO3 with a concentration of 2.0 mL / g.

[0076] The halogen-free flame-retardant water-based polyurethane coating provided by the embodiment comprises water-based polyurethane resin, modified magnesium hydroxide flame retardant, dispersant, defoaming agent and mildew-proof agent in a mass ratio of 60:35:2, and is uniformly stirred with a high-speed dispersant to obtain the halogen-free flame-retardant water-based polyurethane coating.

[0077] Embodiment 6

[0078] The embodiment is different from embodiment 1 in that the mass ratio of the slurry B to the colloid C in the fourth step is 5:1.

[0079] Comparative example 1

[0080] The comparative example is different from embodiment 1 in that the hexagonal flaky magnesium hydroxide slurry with a particle size of 5 μm is prepared in the first step.

[0081] Comparative example 2

[0082] The comparative example is different from embodiment 1 in that the attapulgite has a length of 10 μm and a diameter of 500 nm in the third step.

[0083] Comparative example 3

[0084] The comparative example is different from embodiment 1 in that no pH adjusting agent is added in the fourth step.

[0085] Comparative example 4

[0086] The difference between the present comparative example and Example 1 is that no crosslinking agent polyetheramine is added in the fifth step.

[0087] Comparative Example 5

[0088] The difference between the present comparative example and Example 1 is that no polydopamine coating is coated on the surface of the magnesium hydroxide M product in the sixth step.

[0089] Comparative Example 6

[0090] The difference between the present comparative example and Example 1 is that in the fourth step, the mass ratio of slurry B to colloid C is 8:1, and in the eighth step, the mass ratio of slurry D to dispersion F is 6:1.

[0091] A portion of the halogen-free flame-retardant waterborne polyurethane coating prepared in Examples 1-6 and Comparative Examples 1-6 is placed in a 60°C oven for 7 days, and whether there is sedimentation or water separation is observed; a portion is poured into an oxygen index and vertical combustion sample mold, and after curing, it is taken out for testing, with the oxygen index detection standard being GB / T 2406.2-2009; the vertical combustion grade detection standard being the UL94 flame-retardant grade standard, and the observation and test results being shown in Table 1.

[0092] Table 1 Test results of the halogen-free flame-retardant waterborne polyurethane coating obtained in Examples 1-6 and Comparative Examples 1-6

[0093]

[0094] Comparative analysis of the test data results in Table 1 shows that the halogen-free flame-retardant waterborne polyurethane coating obtained by the present application has good suspension stability, and the good core-shell structure is conducive to improving the flame retardancy and smoke suppression.

[0095] In Comparative Example 1, the magnesium hydroxide used has a large particle size and a small specific surface area, resulting in poor anchoring of the coupling agent and insufficient coverage of the coating material, and the defects in the core-shell structure lead to sedimentation and low flame-retardant efficiency.

[0096] In Comparative Example 2, the size of the attapulgite is too large, on the one hand, the specific surface area is too small, and the magnesium hydroxide core cannot be completely wrapped; on the other hand, the large size of the rod material has stronger rigidity, and it is difficult to tightly wrap around the smaller magnesium hydroxide particles, resulting in holes and unevenness in the coating layer, and a loose structure.

[0097] In Comparative Example 3, the pH of slurry B and colloid C in the fourth step is not adjusted, so that both of them have positive charges, repel each other, are not conducive to interlayer adsorption, and further lead to unstable inner core-shell structure, hard sedimentation, and poor flame-retardant and smoke suppression effect.

[0098] In Comparative Example 4, no crosslinking agent polyetheramine is added in the fifth step, the shell structure does not form a covalent network, the coating layer is easy to fall off, and the stability is poor.

[0099] In Comparative Example 5, the magnesium hydroxide M product surface is not coated with a polydopamine coating in the sixth step, and under the swelling of the outermost layer, water infiltrates, damages the stable core-shell structure, and causes soft settling.

[0100] In Comparative Example 6, the relative content of the magnesium hydroxide slurry in the fourth step and the eighth step is set to be too high, the ratio is unbalanced, and then the coating layer structure is incomplete, which affects the suspension and the flame retardant effect.

[0101] The above shows and describes the basic principles, main features and advantages of the present application. Those skilled in the art should understand that the present application is not limited to the above examples, and the above examples and descriptions in the specification are only preferred examples of the present application and are not intended to limit the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the present application. The scope of protection of the present application is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a self-suspended modified magnesium hydroxide flame retardant, characterized in that, Includes the following steps: (1) Obtain magnesium hydroxide filter cake A, wherein the particle size of the magnesium hydroxide is 0.8-1.2 μm; (2) Disperse magnesium hydroxide filter cake A into magnesium hydroxide slurry with a solid content of 10-15%, and add KH560 and titanate coupling agent to modify it to obtain slurry B; (3) Disperse the attapulgite and modify it with CTAB solution to obtain colloid C. The length of the attapulgite is 0.96-2.4μm and the particle size is 40-120nm. (4) Adjust the pH of slurry B and colloid C to 9-10, add slurry B to colloid C and stir, the mass ratio of slurry B to colloid C is 4-5:1; (5) Add a crosslinking agent to the stirred solution in step (4), filter and dry to obtain coated modified magnesium hydroxide M, wherein the crosslinking agent is aluminum chloride or polyetheramine; (6) Coating the surface of the magnesium hydroxide M with a polydopamine coating, and dispersing the magnesium hydroxide M into a magnesium hydroxide slurry D; (7) Sodium-based bentonite was modified with PAAS and then sheared to obtain dispersion F; (8) Add magnesium hydroxide slurry D to dispersion F and mix, add PVP K30 and stir. The mass ratio of magnesium hydroxide slurry D to dispersion F is 2-4:

1. (9) Spray drying to obtain magnesium hydroxide flame retardant.

2. The preparation method of the self-suspended modified magnesium hydroxide flame retardant according to claim 1, characterized in that, In step (1), magnesium hydroxide slurry is filtered and washed with water to obtain magnesium hydroxide filter cake A.

3. The preparation method of the self-suspended modified magnesium hydroxide flame retardant according to claim 1, characterized in that, In step (7), the sodium-based bentonite has a small-sized lamellar structure with a diameter of <100nm.

4. The preparation method of the self-suspended modified magnesium hydroxide flame retardant according to claim 1, characterized in that, In step (9), silica aerogel is added during the spray drying process.

5. A self-suspending modified magnesium hydroxide flame retardant prepared by any one of the preparation methods of claims 1-4.

6. A halogen-free flame-retardant waterborne polyurethane coating, characterized in that, include: The waterborne polyurethane resin, the modified magnesium hydroxide flame retardant as described in claim 5, the dispersant, the defoamer, and the mildew inhibitor, wherein the mass ratio of the waterborne polyurethane resin, the modified magnesium hydroxide flame retardant, the dispersant, the defoamer, and the mildew inhibitor is (50-70):(32-36):(1-2):(0-1):(0-1).

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