Water-based intumescent fire-retardant coatings, their preparation methods and applications

Abstract

This invention relates to the field of fire-retardant coating production technology, specifically to a water-based intumescent fire-retardant coating, its preparation method, and its application. The coating comprises, by weight, 25-35 parts of a water-based acrylic emulsion, 40-60 parts of a flame retardant, 15-25 parts of water, 5-10 parts of titanium dioxide, and 1-5 parts of a film-forming aid. Its core lies in the use of N-methylpiperazine-modified ammonium polyphosphate as a single-component flame retardant. This modified flame retardant integrates an acid source, a carbon source, and a gas source at the molecular scale. Compared to traditional physical compounding systems and sterically hindered substituent modification schemes, this invention utilizes the low steric hindrance and excellent hydrophilic-philic balance of N-methylpiperazine to significantly improve the uniformity and controllability of the grafting reaction, and improve the dispersion stability and compatibility of the flame retardant in the water-based emulsion.

Description

Technical Field

[0001] This invention relates to the field of fire-retardant coating production technology, specifically to water-based intumescent fire-retardant coatings, their preparation methods, and applications. Background Technology

[0002] With the rapid development of my country's building industrialization and steel structure building industry, the fire resistance requirements for load-bearing steel structure components are continuously being upgraded. Fire-retardant coatings, as special functional coatings that can be directly applied to the surface of substrates and have both structural protection and decorative functions, are core protective materials for delaying the spread of fire, preventing high-temperature instability and collapse of steel structures, and protecting the structural integrity of substrates. They play a crucial role in ensuring the fire safety of building structures and reducing casualties and property losses caused by fires. According to their fire-retardant mechanism, fire-retardant coatings can be divided into two main categories: non-intumescent fire-retardant coatings and intumescent fire-retardant coatings.

[0003] Among them, intumescent fire-retardant coatings, with their advantages of low additive dosage, high fire resistance efficiency, thin coating, and strong adaptability to substrate appearance and construction scenarios, have become the mainstream technical route in the field of fire protection for steel structure substrates in industrial and civil buildings. Its core flame-retardant system achieves fire protection based on the synergistic flame-retardant effect of three components: acid source, carbon source, and gas source. When exposed to fire and heat, the acid source decomposes and releases acidic dehydrating substances, catalyzing the carbon source to undergo a dehydration and carbonization reaction to generate a dense carbonaceous skeleton. Simultaneously, the gas source decomposes upon heating, releasing a large amount of inert gas, which promotes the uniform foaming and expansion of the carbonaceous skeleton, forming a continuous, porous, honeycomb-shaped heat-insulating intumescent carbon layer. This carbon layer can significantly block heat conduction from the heat source to the substrate and isolate external oxygen from contact with the substrate, greatly slowing down the temperature rise rate of the substrate through a physical barrier effect, thereby achieving stable and long-term fire protection for the substrate.

[0004] In the current field of water-based intumescent fire-retardant coatings, the mainstream flame-retardant systems mostly adopt a multi-component physical compounding mode using acid source, carbon source, and gas source. Among them, the ternary compounding system using ammonium polyphosphate (APP) as the acid source, melamine (MEL) as the gas source, and pentaerythritol (PER) as the carbon source is the most widely used. Usually, the above three components are dispersed together with water-based film-forming emulsions, pigments, fillers, functional additives, etc. to prepare coatings. However, this physical compounding system has inherent technical shortcomings: the density, polarity, and surface properties of the multiple components are significantly different, which easily leads to uneven dispersion, sedimentation and agglomeration, and poor storage stability in water-based systems; at the same time, the thermal decomposition process of the three components is difficult to match precisely, resulting in a significant reduction in synergistic flame-retardant efficiency, and significant batch-to-batch fluctuations in char layer expansion ratio, density, and thermal stability, making it difficult to consistently achieve the optimal flame-retardant protection effect.

[0005] Addressing the inherent limitations of the aforementioned physical compound systems, monomolecular intumescent flame retardants integrating acid, carbon, and gas sources have become a key research focus in the industry. These flame retardants achieve uniform distribution of the three components at the molecular scale through molecular structure design, fundamentally solving the synergy and dispersion challenges of multi-component compound systems. While some research findings and patented technologies for this type of monomolecular integrated intumescent flame retardant have been published, existing solutions still suffer from varying degrees of technical deficiencies and fail to completely resolve the core application pain points of water-based fire-retardant coatings. Chinese patent application CN112961528A, published on February 8, 2021, discloses an environmentally friendly fire-retardant coating and its preparation method. The solution uses an NP cage-type macromolecular intumescent flame retardant as the core flame-retardant component. This flame retardant has a single-molecule structure that integrates carbon source, acid source, and gas source. During combustion, it can form a closed porous carbon layer to achieve heat insulation and oxygen isolation, and also has the effects of smoke suppression and anti-dripping. Moreover, the amount of flame retardant added is only 1 / 2 to 1 / 3 of that of the classic ternary system, which can reduce the raw material cost. However, the synthesis route of the NP cage-type macromolecular flame retardant used in this scheme is complicated and the reaction conditions are harsh, making it difficult to achieve large-scale mass production. At the same time, the cage-type molecular structure is rigid and has poor compatibility with aqueous film-forming emulsions, which can easily lead to insufficient continuity of coating film formation, reduced adhesion, and phase separation and sedimentation problems during long-term storage. In addition, the char layer formed by this flame retardant is brittle and is prone to cracking and peeling under high temperature when exposed to fire, making it unable to continuously and stably perform the thermal insulation and protection effect, and difficult to meet the long-term fire protection requirements of steel structure substrates.

[0006] Chinese patent application CN112322167A, published on November 13, 2021, discloses an intumescent fire-retardant coating for steel structures. This scheme uses a modified ammonium polyphosphate, triazine charring agent, and expandable graphite as an intumescent flame-retardant system, combined with water-based resin emulsion and inorganic fillers to prepare the coating. While retaining the flame-retardant effect, it improves the coating's water resistance and heat resistance, and the preparation process is simple, enabling large-scale industrial production. However, this scheme still adopts a multi-component physical compound flame-retardant mode, failing to fundamentally solve the inherent defects of uneven dispersion and poor matching of thermal decomposition processes caused by differences in density and polarity among the acid, carbon, and gas sources. There is still considerable room for improvement in synergistic flame-retardant efficiency. Furthermore, the large density difference of the expandable graphite added to the system easily leads to sedimentation and stratification during coating storage, resulting in high surface roughness and poor application compatibility. In addition, the multi-component compound system is prone to component migration problems, leading to rapid degradation of the flame-retardant performance and insufficient durability during long-term service.

[0007] Chinese patent application CN119823621A, published on April 15, 2025, discloses a water-based intumescent fire-retardant coating, its preparation method, and its application. This scheme uses 1-cyclohexylpiperazine-modified ammonium polyphosphate as a single-component flame retardant, combined with water-based film-forming emulsion, titanium dioxide, and other components to prepare the coating, achieving a single-molecule integrated design of acid source, carbon source, and gas source. Compared with the traditional ternary physical compound system of ammonium polyphosphate, pentaerythritol, and melamine, this scheme significantly improves the dispersibility of the flame retardant in the water-based system, the carbon expansion ratio of the coating, char formation, and thermal stability. The coating preparation process is simple and environmentally friendly. However, the 1-cyclohexylpiperazine modification system used in this scheme still has room for optimization: First, the substituent of 1-cyclohexylpiperazine is a six-membered alicyclic structure with large steric hindrance, which easily forms a steric shield on the reactive sites of the piperazine ring. This causes the grafting reaction to be concentrated on the outer surface of the ammonium polyphosphate particles, making it difficult to achieve uniform grafting of active sites inside and outside the particles. The controllability of the grafting rate and the batch stability of the product are insufficient. Second, the strong hydrophobic structure of cyclohexyl makes it difficult to balance the compatibility between modified ammonium polyphosphate and the aqueous emulsion system. This can easily lead to slight sedimentation and flocculation during long-term storage of the coating, affecting the continuity of the coating film. Third, 1-cyclohexylpiperazine is prone to crystallization at room temperature. An additional heating and dissolution process is required in the synthesis of flame retardants, which increases the complexity of the synthesis process and energy consumption. At the same time, the market procurement cost of this raw material is relatively high, and there is still room for optimization in its suitability for large-scale mass production. Summary of the Invention

[0008] In view of the shortcomings of the prior art, the purpose of this invention is to provide a water-based intumescent fire-retardant coating. This invention uses N-methylpiperazine, which has low steric hindrance and excellent hydrophilic-hydrophobic balance properties, to chemically modify ammonium polyphosphate, constructing a novel "three-in-one" high-efficiency intumescent flame retardant. This is then applied to a water-based fire-retardant coating system to obtain a water-based intumescent fire-retardant coating with better storage stability, superior fire resistance, and better overall coating performance.

[0009] Another object of the present invention is to provide a method for preparing a water-based intumescent fire-retardant coating, which is suitable for industrial production.

[0010] Another object of the present invention is to provide an application of a water-based intumescent fire-retardant coating for use in fire-retardant coatings for fire protection of steel structure substrates.

[0011] This invention is achieved using the following technical solution: The water-based intumescent fire retardant coating comprises the following components by weight: Aqueous acrylic emulsion: 25-35 parts; Flame retardant: 40-60 parts; Water: 15-25 parts; Titanium dioxide: 5-10 parts; Film-forming aid: 1-5 parts; Anti-settling agent: 0.1–0.5 parts; Dispersant: 0.1–0.5 parts; Defoamer: 0.1–0.5 parts; Anti-flash rust agent: 0.1-0.5 parts; The flame retardant is N-methylpiperazine-modified ammonium polyphosphate.

[0012] The structural formula of ammonium polyphosphate is: .

[0013] The raw materials for preparing the N-methylpiperazine-modified ammonium polyphosphate include ammonium polyphosphate and N-methylpiperazine, wherein the molar ratio of the ammonium polyphosphate to the N-methylpiperazine, based on its structural units, is 1:(1-2).

[0014] The structural formula of N-methylpiperazine is: .

[0015] The structural formula of N-methylpiperazine-modified ammonium polyphosphate is: .

[0016] The N-methylpiperazine-modified ammonium polyphosphate is prepared by the following steps: under an inert gas atmosphere, ammonium polyphosphate and N-methylpiperazine are refluxed in an alcohol-water mixed solvent, and after the reaction is completed, the mixture is cooled, washed and dried.

[0017] The alcohol-water mixed solvent is a mixture of ethanol and water, and the volume ratio of ethanol to water is 100:(1-10); Preferably, the reaction temperature is 85–95°C, and the reaction time is 3–6 hours.

[0018] The aqueous acrylic emulsion is an aqueous vinyl acetate-acrylate copolymer emulsion.

[0019] The water-based intumescent fire retardant coating also includes 0.1 to 0.5 parts by weight of an anti-settling agent; Preferably, the anti-settling agent is organic bentonite.

[0020] The water-based intumescent fire-retardant coating also includes 0.1 to 0.5 parts by weight of dispersant and 0.1 to 0.5 parts by weight of defoamer; Preferably, the dispersant is a polycarboxylate dispersant, and the defoamer is an organosilicone defoamer.

[0021] The preparation method of the water-based intumescent fire-retardant coating includes the following steps: mixing water-based acrylic emulsion, N-methylpiperazine-modified ammonium polyphosphate, water, titanium dioxide and film-forming aid evenly.

[0022] Specifically, the following steps are included: S1: Mix water, dispersant and defoamer, and stir once to obtain mixture one; S2: Add N-methylpiperazine-modified ammonium polyphosphate, titanium dioxide and anti-settling agent to the mixture one, and stir twice to obtain mixture two; S3: Add water-based acrylic emulsion, film-forming aid and flash rust inhibitor to the mixture 2, and stir three times to obtain the water-based intumescent fireproof coating.

[0023] The application of the water-based intumescent fire-retardant coating is in the preparation of fire-retardant coatings for fire protection of steel structure substrates.

[0024] Specifically, the present invention adopts the following technical solution: In a first aspect, the present invention provides a water-based intumescent fire-retardant coating, comprising the following components by weight: 25-35 parts of water-based acrylic emulsion; 40-60 parts of flame retardant; 15-25 parts of water; 5-10 parts of titanium dioxide; and 1-5 parts of film-forming aid.

[0025] The core of this invention lies in the fact that the flame retardant is N-methylpiperazine-modified ammonium polyphosphate. This flame retardant integrates the "acid source," "carbon source," and "gas source" required for fire prevention in a single molecular structure, fundamentally solving the inherent defects of uneven dispersion of components and difficulty in matching thermal decomposition processes in traditional physical compound systems.

[0026] Compared with the 1-cyclohexylpiperazine-modified ammonium polyphosphate used in existing technologies, the N-methylpiperazine selected in this invention has significant technical advantages. The steric hindrance of the "methyl" substituent in N-methylpiperazine is minimal, making it less likely to create a steric shielding effect on the reactive sites (secondary amine groups) on the piperazine ring. This allows the grafting reaction to extend beyond the outer surface of the ammonium polyphosphate particles, penetrating deeper into their microporous structure, achieving uniform and complete reaction of the active sites both inside and outside the particles. This significantly improves the controllability of the grafting reaction, the grafting rate, and the batch stability of the final product.

[0027] In this invention, the aqueous acrylic emulsion is preferably an aqueous vinyl acetate-acrylate copolymer emulsion (referred to as vinyl acetate-acrylic acid emulsion). Vinyl acetate-acrylic acid emulsions possess excellent film-forming properties, weather resistance, and coating ability for pigments and fillers. More importantly, the introduction of N-methylpiperazine results in a higher polarity match between the surface polarity of the modified ammonium polyphosphate and the polarity of the vinyl acetate-acrylic acid emulsion system. The moderate hydrophobicity of the methyl group and the hydrophilicity of the piperazine ring form a good hydrophilic-hydrophobic balance, improving the poor compatibility between strongly hydrophobic sterically hindered substituents (such as cyclohexyl groups) and aqueous emulsions. This effectively reduces the tendency of flame retardant particles to agglomerate in the emulsion, making the coating system less prone to sedimentation, stratification, or flocculation during long-term storage, ensuring excellent storage stability. Simultaneously, good compatibility ensures that the emulsion can effectively wet and bond the flame retardant particles, minimizing interference during the film-forming process. The resulting coating is continuous, dense, and possesses excellent adhesion, flexibility, and water resistance.

[0028] In this invention, the raw materials for preparing the N-methylpiperazine-modified ammonium polyphosphate include ammonium polyphosphate (APP) and N-methylpiperazine. To ensure efficient synergistic effects of the three sources, the preferred molar ratio of ammonium polyphosphate to N-methylpiperazine, based on its structural unit (NH4PO3), is 1:(1-2). If the proportion of N-methylpiperazine is less than 1, the grafted carbon and gas source groups will be insufficient, affecting the expansion and charring effect; if the proportion is greater than 2, the excessive N-methylpiperazine will not only waste raw materials but may also lead to an increase in free small molecules in the product, affecting coating performance and further increasing costs.

[0029] Furthermore, the N-methylpiperazine-modified ammonium polyphosphate can be prepared by a specific chemical synthesis method. A preferred preparation method includes the following steps: under an inert gas atmosphere (such as nitrogen or argon), ammonium polyphosphate and N-methylpiperazine are refluxed in an alcohol-water mixed solvent, and after the reaction is completed, the mixture is cooled, washed, and dried to obtain the final product.

[0030] The inert gas atmosphere is set up to remove oxygen and moisture from the air, prevent unnecessary oxidation or other side reactions during the heating reaction, and ensure the purity of the target product.

[0031] An alcohol-water mixed solvent, preferably a mixture of ethanol and water, with a volume ratio preferably of 100:(1-10), is used. This mixed solvent system can effectively disperse the solid phase of ammonium polyphosphate and effectively dissolve the liquid phase of N-methylpiperazine, providing an excellent reaction medium for the two-phase reaction. The appropriate addition of water can adjust the solvent polarity, promoting partial dissolution and activation of the reactants.

[0032] The reaction temperature is preferably controlled between 85 and 95°C, and the reaction time is 3 to 6 hours. This temperature range is the highly efficient region for the grafting reaction, ensuring a sufficient reaction rate while avoiding significant thermal decomposition of ammonium polyphosphate. The reaction time ensures the complete progress of the grafting reaction.

[0033] The washing step after the reaction is preferably performed with anhydrous ethanol to effectively remove unreacted N-methylpiperazine and other small molecule impurities, thus purifying the product. Drying is usually performed using vacuum drying to remove the solvent from the product at a lower temperature, avoiding damage to the product structure caused by high temperatures.

[0034] To further improve the overall performance and application properties of the coating, the water-based intumescent fire-retardant coating of this invention may selectively include one or more additives. For example: Anti-settling agent: 0.1 to 0.5 parts by weight. Organic bentonite is preferred, as it forms a three-dimensional network structure in the coating system, effectively increasing the static viscosity of the coating and imparting excellent thixotropic properties, thereby preventing the settling of high-density fillers such as flame retardants and titanium dioxide during storage.

[0035] Dispersant: 0.1 to 0.5 parts by weight may be added. Polycarboxylate dispersants (such as BYK-190) are preferred, which can efficiently wet and disperse solid particles through electrostatic repulsion and steric hindrance, reduce the viscosity of the system, and improve the fluidity and storage stability of the coating.

[0036] Defoamer: 0.1 to 0.5 parts by weight. Preferably, an organosilicon defoamer (such as BYK-024) is used, which can quickly eliminate bubbles generated during production mixing and application, ensuring a smooth coating surface free of pinholes.

[0037] Anti-flash rust agent: 0.1 to 0.5 parts by weight. Preferably, a 30 wt% sodium nitrite aqueous solution is used to temporarily inhibit flash rust formation on the surface of the substrate during the evaporation of water-based coatings applied to steel structure substrates.

[0038] Secondly, the present invention also provides a method for preparing the above-mentioned water-based intumescent fire-retardant coating, which involves uniformly mixing water-based acrylic emulsion, N-methylpiperazine-modified ammonium polyphosphate, water, titanium dioxide, and film-forming aids.

[0039] A more preferred preparation method, which achieves better dispersion, specifically includes the following steps: S1 (Pre-dispersion): Add water, dispersant and defoamer to a mixing tank and stir once at a low speed (e.g. 700-900 r / min) to make the additives dissolve or disperse evenly in the water to obtain mixture one.

[0040] S2 (Grinding and Dispersion): Add N-methylpiperazine-modified ammonium polyphosphate, titanium dioxide, and anti-settling agent to mixture one in sequence, increase the rotation speed to a higher level (e.g., 1400-1600 r / min) for secondary stirring or grinding, so that all powders are fully wetted, dispersed, and uniformly dispersed under the action of the dispersant to form a fine color paste, thus obtaining mixture two.

[0041] S3 (Paint Mixing): Reduce the rotation speed to a lower level (e.g., 500-700 r / min), slowly add water-based acrylic emulsion, film-forming aid, and other liquid additives (e.g., anti-flash rust agent) to mixture two, and stir three times until the system is evenly mixed to obtain the final water-based intumescent fire retardant coating.

[0042] This step-by-step feeding and variable-speed stirring process conforms to the conventional logic of coating production, maximizes the function of each additive, ensures the optimal dispersion of powder, and thus ensures the stable performance of the final coating product.

[0043] Thirdly, the present invention also provides the application of the above-mentioned water-based intumescent fire-retardant coating in the preparation of a fire-retardant coating for fire protection of steel structure substrates. This coating can be directly applied to the surface of appropriately treated load-bearing steel structural components in industrial and civil buildings. In the event of a fire, the coating rapidly expands and foams upon heating, forming a dense, robust honeycomb-like insulating char layer, effectively delaying the transfer of heat to the steel structure substrate and improving the fire resistance limit of the steel structure.

[0044] This invention addresses the common technical problems in existing monomolecular intumescent flame retardants, such as the difficulty in simultaneously achieving steric hindrance of substituents and grafting reaction efficiency, compatibility with aqueous systems, and synergistic flame retardant performance from three sources. It employs N-methylpiperazine to chemically graft ammonium polyphosphate, constructing a monomolecular intumescent flame retardant system that integrates acid, carbon, and gas sources.

[0045] N-methylpiperazine, with its short-chain methyl group as a substituent, exhibits low steric hindrance and is less likely to form significant steric shielding on the reactive sites on the piperazine ring. This allows it to penetrate into the secondary pores of ammonium polyphosphate particles, achieving uniform grafting of active sites both inside and outside the particles. This overcomes the shortcomings of sterically hindered substituents, which tend to graft only onto the surface of ammonium polyphosphate and result in uneven grafting. Under the same reaction conditions, this structural design makes the grafting reaction more stable and controllable, improving the sufficiency of the grafting reaction, reducing unreacted monomer residues, and ensuring batch-to-batch consistency of product performance.

[0046] This invention uses the ammonium polyphosphate backbone as the acid source, the piperazine ring's dinitrogen structure as the gas source, and the carbon structure introduced by the methyl group as the carbon source. This achieves a uniform distribution of the acid source, carbon source, and gas source within a single molecule, resulting in a higher degree of matching in their thermal decomposition processes. This improves upon the problems of asynchronous thermal decomposition and insufficient synergy among the three components in traditional physical compound systems. It also alleviates the drawbacks of insufficient carbon source supply from small sterically hindered substituents and delayed thermal decomposition from large sterically hindered substituents. When heated, the acidic substances released from the decomposition of the acid source simultaneously catalyze the dehydration of the carbon source into char, while the inert gas released from the decomposition of the gas source simultaneously promotes the foaming of the char layer. This optimizes the char layer's formation rate, density, and closed-porosity, improving the coating's fire resistance limit and reducing flue gas release from incomplete combustion.

[0047] The methyl structure of N-methylpiperazine and the piperazine ring structure can form a hydrophilic-hydrophobic balance, which has a higher polarity matching degree with the aqueous vinyl acetate-acrylic emulsion film-forming system selected in this invention, and can improve the defect of poor compatibility between strongly hydrophobic sterically hindered substituents and aqueous emulsions. This modified structure can reduce the aggregation tendency of ammonium polyphosphate particles, further optimize its dispersion effect in aqueous vinyl acetate-acrylic emulsions, reduce the dispersed particle size, and the prepared coating is less prone to significant sedimentation, stratification, and flocculation during room temperature storage, with small viscosity changes and improved storage stability. At the same time, the aqueous vinyl acetate-acrylic emulsion has good wetting and adhesion to the modified ammonium polyphosphate, and this modified structure has little interference with the film-forming process of the emulsion, which can ensure the continuity of coating film formation, improve the adhesion, flexibility and water resistance of the coating, and broaden the adaptability to the construction environment.

[0048] Compared with the prior art, the beneficial effects of the present invention are: (1) This invention uses N-methylpiperazine to chemically modify ammonium polyphosphate. Its core advantage lies in the extremely small steric hindrance of the methyl substituent. Compared with the large-volume groups such as cyclohexyl groups in the prior art, the methyl group is less likely to shield the reaction sites of the piperazine ring, allowing the grafting reaction to proceed uniformly on the inner and outer surfaces of the ammonium polyphosphate particles, rather than being limited to the surface. This results in a more stable and controllable reaction process, higher grafting efficiency, and less unreacted monomer residue, ensuring the batch consistency and stability of the "three-in-one" flame retardant product performance from the source.

[0049] (2) N-methylpiperazine-modified ammonium polyphosphate has optimized hydrophilicity-hydrophobicity balance characteristics and a high polarity match with the preferred aqueous vinyl acetate-acrylic emulsion system of this invention. This greatly improves the dispersibility and suspension stability of the flame retardant in aqueous media, and the prepared coating maintains stable viscosity during long-term storage, making it less prone to hard sedimentation, stratification, or flocculation. At the same time, good compatibility ensures that the emulsion film-forming process is not disturbed, resulting in a continuous and dense coating that effectively guarantees key physical properties such as adhesion, flexibility, and water resistance to the substrate.

[0050] (3) In the single-molecule flame-retardant system of the present invention, the acid source (ammonium polyphosphate backbone), carbon source (methyl and piperazine ring skeleton), and gas source (piperazine ring nitrogen atom) are well integrated at the molecular scale. When exposed to fire and heat, the thermal decomposition processes of the three are highly synchronized and matched: the acidic substances produced by the decomposition of polyphosphate can catalyze the dehydration of carbon source into char in an immediate and nearby manner, while the inert gas released by the decomposition of gas source simultaneously promotes the uniform foaming of carbon skeleton. This efficient synergistic effect results in a faster formation rate, higher expansion ratio, more uniform and fine pore size, and more robust structure of the expanded char layer. Detailed Implementation

[0051] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be described in detail below. It should be understood that the specific embodiments described herein are only for explaining this invention and are not intended to limit this invention.

[0052] Sources of raw materials used in the examples and comparative examples: Ammonium polyphosphate (APP): Quzhou Weikai Chemical Co., Ltd., industrial grade, degree of polymerization > 1000; N-Methylpiperazine: Shandong Guobang Pharmaceutical Co., Ltd., industrial grade; Anhydrous ethanol: Shandong Xuran Biotechnology Co., Ltd., industrial grade; Water-based vinyl acetate-acrylic emulsion: Ausnutria Chemical (Beijing) Co., Ltd., 707; Titanium dioxide: Shandong Dongjia Group Co., Ltd., rutile titanium dioxide; Film-forming aid, alcohol ester twelve: Badifu Group Co., Ltd., industrial grade; Anti-flash rust agent, 30% sodium nitrite aqueous solution: Shandong Jinshengrun Chemical Co., Ltd., industrial grade; 1-Cyclohexylpiperazine: Suzhou Haofan Biotechnology Co., Ltd., industrial grade; Melamine: Quzhou Weikai Chemical Co., Ltd., industrial grade; Pentaerythritol: Quzhou Weikai Chemical Co., Ltd., industrial grade; Anhydrous piperazine: Shanghai Aladdin Biochemical Technology Co., Ltd., ≥99%; Anti-settling agent, organobentonite: Zhejiang Fenghong New Material Co., Ltd., CLAYMINTON LB; Dispersant, polycarboxylate dispersant, BYK (Germany), BYK-190; Defoamer, silicone defoamer, BYK (Germany), BYK-024.

[0053] Test method: Storage stability: The viscosity change rate of the coatings in the examples and comparative examples was measured after 30 days of storage at room temperature. The smaller the change rate, the better the stability.

[0054] Bond strength: The bond strength test was conducted in accordance with GB 14907-2018 standard 6.4.4, and the maximum tensile load was measured.

[0055] Flexibility: Perform a shaft bending test according to GB / T 1731-2020 standard and give the minimum shaft diameter at which the coating does not crack.

[0056] This invention provides a high-performance water-based intumescent fire retardant coating, the core of which lies in the use of a novel "three-in-one" intumescent flame retardant, which is optimized and compounded with a specific water-based film-forming system and additives.

[0057] By weight, the water-based intumescent fire-retardant coating of the present invention may include the following components: 25-35 parts of water-based acrylic emulsion, 40-60 parts of flame retardant, 15-25 parts of water, 5-10 parts of titanium dioxide, and 1-5 parts of film-forming aid.

[0058] The key innovation of this invention lies in the fact that the flame retardant is a specially chemically modified ammonium polyphosphate, specifically N-methylpiperazine-modified ammonium polyphosphate. This flame retardant ingeniously integrates the "acid source," "carbon source," and "gas source" required for an intumescent fire-retardant system within a single molecular structure. Specifically, the long-chain skeleton of the ammonium polyphosphate acts as a highly efficient "acid source," decomposing upon heating to release phosphoric acid or polyphosphoric acid, catalyzing dehydration and char formation. The grafted piperazine ring structure, rich in nitrogen, decomposes at high temperatures to release non-flammable gases such as nitrogen, serving as a "gas source" and providing the impetus for the foaming and expansion of the char layer. The carbon skeleton of the piperazine ring, together with the introduced methyl group, acts as a "carbon source," providing the material basis for the formation of a dense char layer. This "three-in-one" design at the molecular level fundamentally overcomes the problems of uneven dispersion and thermal decomposition mismatch caused by differences in component density and polarity in traditional physical blending systems, ensuring highly efficient synergistic effects during the fire-retardant process.

[0059] Ammonium polyphosphate (APP): Degree of polymerization n>1000, type II, D50 approximately 10~25μm.

[0060] Waterborne acrylic emulsions (waterborne vinyl acetate emulsions): solid content 45%~48%, minimum film-forming temperature (MFFT) 5℃, glass transition temperature (Tg) 15℃.

[0061] In preparing this N-methylpiperazine-modified ammonium polyphosphate, the main raw materials include ammonium polyphosphate and N-methylpiperazine. To achieve the best synergistic flame retardant effect, when calculating the molar ratio of ammonium polyphosphate with N-methylpiperazine using its basic structural unit (e.g., NH4PO3), the molar ratio can be set to 1:(1-2). When the proportion of N-methylpiperazine is less than 1, the number of carbon and gas source groups grafted onto the ammonium polyphosphate molecular chain is insufficient, which may lead to poor expansion ratio and density of the final carbon layer; while when the proportion is greater than 2, excessive N-methylpiperazine will increase costs and may introduce unreacted free small molecules, affecting product purity and the performance of the final coating.

[0062] In a preferred preparation method, the N-methylpiperazine-modified ammonium polyphosphate can be synthesized by the following method: In a reaction vessel equipped with stirring, reflux condensation, and gas protection devices, an inert gas (such as nitrogen or argon) is first introduced to fully replace the air in the vessel, providing an anaerobic environment for the reaction and preventing side reactions. Subsequently, under the protection of the inert gas, an alcohol-water mixed solvent is added, and the ammonium polyphosphate and N-methylpiperazine are then added to it. Preferably, the mixed solvent is a mixture of ethanol and water with a volume ratio of 100:(1-10). This solvent system can effectively disperse the solid ammonium polyphosphate and dissolve the liquid N-methylpiperazine, providing a good medium for the reaction. Next, the reaction system is heated to a temperature range of 85-95°C for reflux reaction, and the reaction time is 3-6 hours to ensure that the grafting reaction proceeds fully. After the reaction, the product is post-treated, including natural cooling, filtration, washing, and drying. Anhydrous ethanol can be used as the washing agent to effectively remove residual unreacted raw materials and small molecule impurities from the product. Finally, the product is dried under vacuum at 60–70°C to remove the solvent under mild conditions, yielding a powdered N-methylpiperazine-modified ammonium polyphosphate product.

[0063] In the coating formulation of this invention, the aqueous acrylic emulsion serves as the primary film-forming substance, providing the coating with fundamental adhesion, durability, and mechanical properties. In a preferred embodiment, the emulsion is an aqueous vinyl acetate-acrylate copolymer emulsion (vinyl acetate-acrylic acid emulsion). The advantage of choosing a vinyl acetate-acrylic acid emulsion is that its polarity is highly compatible with the surface characteristics of N-methylpiperazine-modified ammonium polyphosphate. The introduction of N-methylpiperazine imparts excellent hydrophilic-hydrophobic balance properties to the flame retardant, giving it excellent dispersibility and compatibility in the vinyl acetate-acrylic acid emulsion system, significantly improving the storage stability of the coating, and preventing pigment and filler sedimentation.

[0064] To further optimize the overall performance of the coating, the formulation may also include one or more functional additives. For example: It may contain 0.1 to 0.5 parts by weight of an anti-settling agent, preferably organobentonite. It can form a three-dimensional network structure when the coating is left to stand, giving the coating excellent thixotropic properties and effectively preventing the settling of high-density flame retardants and titanium dioxide particles.

[0065] It may contain 0.1 to 0.5 parts by weight of dispersant, preferably a polycarboxylate dispersant. It can efficiently wet and stabilize the dispersion of powder particles through electrostatic repulsion and steric hindrance effects, reduce the viscosity of the system, and improve the flowability and uniformity of the coating.

[0066] It may contain 0.1 to 0.5 parts by weight of defoamer, preferably an organosilicon defoamer. It can quickly eliminate air bubbles during the preparation and application of the coating, ensuring a smooth and defect-free final coating film.

[0067] It may contain 0.1 to 0.5 parts by weight of an anti-flash rust agent, such as a 30 wt% sodium nitrite aqueous solution. It can effectively inhibit flash rust formation on steel surfaces during the drying process when water-based coatings are applied.

[0068] This invention also provides a method for preparing the above-mentioned water-based intumescent fire-retardant coating. A preferred preparation process that ensures uniform dispersion of each component is as follows: The first step is to pre-disperse the additives. Add the prescribed amount of water, dispersant, and defoamer to a mixing container and stir at a low to medium speed (e.g., 700–900 rpm) for 10–30 minutes to ensure that the additives are fully dissolved or dispersed in the water.

[0069] The second step is to grind and disperse the powder. While maintaining stirring, add N-methylpiperazine-modified ammonium polyphosphate, titanium dioxide, and an anti-settling agent sequentially to the above mixture. Then, increase the stirring speed to a high speed (e.g., 1400–1600 r / min) and continue stirring or grinding for 20–40 minutes. The high-speed shear force can break up the powder agglomerates, allowing them to be uniformly dispersed into fine primary particles with the help of the dispersant, forming a stable color paste.

[0070] The third step is paint mixing. Reduce the stirring speed to medium-low (e.g., 500-700 rpm), slowly add the water-based acrylic emulsion, film-forming aid, and other liquid additives, and continue stirring for 10-20 minutes until the entire system is uniformly mixed, free of lumps, and of consistent color. This yields the final water-based intumescent fire-retardant coating product. Using a reduced stirring speed is to avoid high-speed shearing that could destabilize the emulsion particles and cause emulsion breakage.

[0071] The film-forming aid is 12-ol ester; the organic bentonite is CLAYMINTON LB; the flash rust inhibitor is a 30% sodium nitrite aqueous solution; and the titanium dioxide is rutile titanium dioxide.

[0072] The water-based intumescent fire-retardant coating prepared by this invention is particularly suitable for fire protection of steel structural load-bearing components in industrial and civil buildings. Under fire conditions, it can form a highly efficient heat-insulating intumescent carbon layer, significantly improving the fire resistance limit of the steel structure.

[0073] To further illustrate the present invention, specific embodiments will be described below. However, it should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0074] Example 1 Water-based intumescent fire-retardant coating comprises the following raw materials in parts by weight: 30 parts water-based vinyl acetate-acrylic emulsion, 19.2 parts water, 5 parts titanium dioxide, 50 parts flame retardant (N-methylpiperazine modified ammonium polyphosphate), 0.2 parts organic bentonite, 2 parts film-forming aid, 0.4 parts BYK-190, 0.1 parts BYK-024, and 0.2 parts flash rust inhibitor (30% sodium nitrite aqueous solution).

[0075] Example 2 Water-based intumescent fire-retardant coating comprises the following raw materials by weight: 31 parts water-based vinyl acetate-acrylic emulsion, 15 parts water, 6 parts titanium dioxide, 45 parts flame retardant (N-methylpiperazine modified ammonium polyphosphate), 0.2 parts organic bentonite, 2 parts film-forming aid, 0.4 parts BYK-190, 0.2 parts BYK-024, and 0.2 parts flash rust inhibitor (30% sodium nitrite aqueous solution).

[0076] Example 3 Water-based intumescent fire-retardant coating comprises the following raw materials in parts by weight: 35 parts water-based vinyl acetate-acrylic emulsion, 15 parts water, 6 parts titanium dioxide, 41 parts flame retardant (N-methylpiperazine modified ammonium polyphosphate), 0.2 parts organic bentonite, 2 parts film-forming aid, 0.4 parts BYK-190, 0.2 parts BYK-024, and 0.2 parts flash rust inhibitor (30% sodium nitrite aqueous solution).

[0077] Comparative Example 1 Water-based intumescent fire-retardant coating comprises the following raw materials in parts by weight: 30 parts water-based vinyl acetate-acrylic emulsion, 15 parts water, 7 parts titanium dioxide, 45 parts unmodified ammonium polyphosphate, 0.2 parts organic bentonite, 2 parts film-forming aid, 0.4 parts BYK-190, 0.2 parts BYK-024, and 0.2 parts flash rust inhibitor.

[0078] Comparative Example 2 Water-based intumescent fire-retardant coating comprises the following raw materials in parts by weight: 30 parts water-based vinyl acetate-acrylic emulsion, 15 parts water, 7 parts titanium dioxide, 45 parts 1-cyclohexylpiperazine modified ammonium polyphosphate, 0.2 parts organic bentonite, 2 parts film-forming aid, 0.4 parts BYK-190, 0.2 parts BYK-024, and 0.2 parts flash rust inhibitor.

[0079] Comparative Example 3 Water-based intumescent fire-retardant coating comprises the following raw materials in parts by weight: 30 parts water-based vinyl acetate-acrylic emulsion, 19.2 parts water, 5 parts titanium dioxide, a mixture of unmodified ammonium polyphosphate (48.2 parts) and N-methylpiperazine (1.8 parts), 0.2 parts organobentonite, 2 parts film-forming aid, 0.4 parts BYK-190, 0.2 parts BYK-024, and 0.2 parts flash rust inhibitor.

[0080] Comparative Example 4 Water-based intumescent fire-retardant coating comprises the following raw materials by weight: 30 parts water-based vinyl acetate-acrylic emulsion, 19.2 parts water, 5 parts titanium dioxide, 50 parts traditional ternary flame-retardant compound (26 parts unmodified ammonium polyphosphate, 12 parts melamine, 12 parts pentaerythritol), 0.2 parts organic bentonite, 2 parts film-forming aid, 0.4 parts BYK-190, 0.2 parts BYK-024, and 0.2 parts flash rust inhibitor.

[0081] Comparative Example 5 A water-based intumescent fire-retardant coating comprises the following raw materials in parts by weight: 30 parts water-based vinyl acetate-acrylic emulsion, 19.2 parts water, 5 parts titanium dioxide, 50 parts unsubstituted piperazine-modified ammonium polyphosphate (using the same synthesis process as the N-methylpiperazine-modified ammonium polyphosphate of this invention, except that N-methylpiperazine is replaced with an equimolar amount of anhydrous piperazine), 0.2 parts organobentonite, 2 parts film-forming aid, 0.4 parts BYK-190, 0.2 parts BYK-024, and 0.2 parts flash rust inhibitor.

[0082] The performance parameters of the water-based intumescent fire retardant coatings obtained in Examples 1-3 and Comparative Examples 1-7 were tested according to the relevant fire retardant coating standard (GB14907-2018), and the results are shown in Table 1.

[0083] Table 1: Test data of Examples 1-3 and Comparative Examples 1-5

[0084] As shown in Table 1 of the test results, the surface drying time of the water-based intumescent fire-retardant coating of this invention is ≤12h, the bonding strength is ≥0.15MPa, and there are no cracks in the initial drying crack resistance, meeting the basic application requirements of fire-retardant coatings for steel structures. The temperature on the back of the steel plate is below 225℃, the expansion ratio can reach up to 28 times, and the char layer structure formed after fire is solid and dense, possessing excellent fireproof and heat-insulating performance. This invention constructs a single-molecule flame-retardant system integrating acid source, carbon source, and gas source by grafting ammonium polyphosphate with N-methylpiperazine, achieving uniform distribution of the three components. When exposed to fire and heat, the flame retardant can simultaneously complete the entire process of acid source catalytic dehydration, carbon source char formation, and gas source foaming, forming a continuous and complete honeycomb-shaped heat-insulating char layer under the synergy of the film-forming system, effectively blocking the transfer of heat to the substrate and achieving a stable fire protection effect.

[0085] As can be seen from the comparison between the examples and the comparative examples, Comparative Example 1, prepared using unmodified ammonium polyphosphate, achieved a steel plate back temperature of 380°C and an expansion ratio of only 7 times, failing to form an effective thickness of expandable heat-insulating char layer. Its fire resistance was significantly weaker than the scheme of this invention, verifying the core role of the three-in-one structure of the modified flame retardant in improving fire resistance. Comparative Example 3, prepared using a physical mixing system of unmodified ammonium polyphosphate and N-methylpiperazine, achieved a steel plate back temperature of 325°C and an expansion ratio of only 10 times, with a bond strength of only 0.1 MPa. Furthermore, the coating showed obvious filamentous cracks during initial drying, and the char layer was loose and porous after exposure to fire, easily detaching. All performance characteristics were significantly inferior to Example 1 of this invention. These results indicate that the uniform distribution of the three components cannot be achieved solely through physical mixing of raw materials, and a stable and synergistic flame-retardant system cannot be formed.

[0086] Comparative Example 4, prepared using the traditional ternary physical compound system of ammonium polyphosphate-pentaerythritol-melamine, under the same total flame retardant addition, had a steel back surface temperature of 254°C, an expansion ratio of only 12 times, uneven char layer thickness, and average density. Its fire resistance was significantly weaker than that of the present invention, demonstrating the performance advantages of the single-molecule integrated flame retardant system of the present invention over the mainstream traditional compound system in the industry.

[0087] Comparative Example 2, prepared using 1-cyclohexylpiperazine-modified ammonium polyphosphate, exhibited a steel backside temperature of 226°C and an expansion ratio of 22 times under the same flame retardant addition. In contrast, the steel backside temperature of the same addition group in this invention was reduced to a minimum of 210°C, while the expansion ratio reached a maximum of 28 times, demonstrating further optimization of fire resistance. This indicates that the structural design of N-methylpiperazine better matches the char formation and foaming process of the flame retardant system, improving thermal insulation efficiency. Comparative Example 5, prepared using unsubstituted piperazine-modified ammonium polyphosphate, showed a steel backside temperature of 232°C and an expansion ratio of only 15 times under the same conditions, with a bond strength of 0.14 MPa and moderate char layer density. Its overall performance was inferior to the same addition group in this invention. These results suggest that the introduction of N-methyl substituents helps regulate the hydrophilic-hydrophobic balance of the modified ammonium polyphosphate, optimizes its compatibility with aqueous vinyl acetate-acrylic emulsion, and supplements the system with a matching carbon source structure, further enhancing synergistic flame retardant efficiency.

Claims

1. A water-based intumescent fire-retardant coating, characterized in that, The water-based intumescent fire-retardant coating comprises the following components by weight: Aqueous acrylic emulsion: 25-35 parts; Flame retardant: 40-60 parts; Water: 15-25 parts; Titanium dioxide: 5-10 parts; Film-forming aid: 1-5 parts; Anti-settling agent: 0.1–0.5 parts; Dispersant: 0.1–0.5 parts; Defoamer: 0.1–0.5 parts; Anti-flash rust agent: 0.1-0.5 parts; The flame retardant is N-methylpiperazine-modified ammonium polyphosphate.

2. The water-based intumescent fire-retardant coating according to claim 1, characterized in that, The raw materials for preparing the N-methylpiperazine-modified ammonium polyphosphate include ammonium polyphosphate and N-methylpiperazine, wherein the molar ratio of the ammonium polyphosphate to the N-methylpiperazine, based on its structural units, is 1:(1-2).

3. The water-based intumescent fire-retardant coating according to claim 2, characterized in that, The N-methylpiperazine-modified ammonium polyphosphate is prepared by the following steps: under an inert gas atmosphere, ammonium polyphosphate and N-methylpiperazine are refluxed in an alcohol-water mixed solvent, and after the reaction is completed, the mixture is cooled, washed and dried.

4. The water-based intumescent fire-retardant coating according to claim 3, characterized in that, The alcohol-water mixed solvent is a mixture of ethanol and water, with a volume ratio of ethanol to water of 100:(1-10); the reaction temperature is 85-95℃, and the reaction time is 3-6h.

5. The water-based intumescent fire-retardant coating according to claim 1, characterized in that, The aqueous acrylic emulsion is an aqueous vinyl acetate-acrylate copolymer emulsion.

6. The water-based intumescent fire-retardant coating according to claim 1, characterized in that, The anti-settling agent is organic bentonite.

7. The water-based intumescent fire-retardant coating according to claim 1, characterized in that, The dispersant is a polycarboxylate dispersant, and the defoamer is an organosilicone defoamer.

8. The water-based intumescent fire-retardant coating according to claim 1, characterized in that, The flash rust inhibitor is a 30wt% sodium nitrite aqueous solution.

9. The method for preparing the water-based intumescent fire-retardant coating according to any one of claims 1-8, characterized in that, Includes the following steps: S1: Mix water, dispersant and defoamer, and stir once to obtain mixture one; S2: Add N-methylpiperazine-modified ammonium polyphosphate, titanium dioxide and anti-settling agent to the mixture one, and stir twice to obtain mixture two; S3: Add water-based acrylic emulsion, film-forming aid and flash rust inhibitor to the mixture 2, and stir three times to obtain the water-based intumescent fireproof coating.

10. The application of a water-based intumescent fire-retardant coating according to any one of claims 1-8, characterized in that, Application in the preparation of fire-retardant coatings for fire protection of steel structure substrates.

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