A phosphogypsum-based fireproof and mildewproof interior coating and a preparation method thereof

Abstract

This invention discloses a phosphogypsum-based fire-retardant and mildew-resistant indoor coating and its preparation method. The coating is made from unwashed phosphogypsum powder, polyvinyl alcohol aqueous solution, low-activity zinc oxide, melamine, boric acid-glycerol pre-complex, dispersant, defoamer, and deionized water. This invention utilizes the combination of unwashed phosphogypsum powder and low-activity zinc oxide to undergo an in-situ reaction during the coating drying stage. Water evaporation promotes the dissolution of the passivation layer of zinc oxide, which neutralizes with free phosphoric acid to form a slightly soluble zinc phosphate precipitate. This reaction directly consumes free acid and generates mildew-resistant substances in situ within the coating. The boric acid-glycerol pre-complex cross-links with polyvinyl alcohol, improving the water resistance of the coating. Melamine and phosphates synergistically catalyze char formation to achieve flame retardancy. The preparation method uses jacket cooling to control the low temperature, suppressing the acid-base reaction rate during the slurry preparation stage and ensuring the system's storage stability. This invention eliminates the need for the water washing and impurity removal process of unwashed phosphogypsum, reducing production costs.

Description

Technical Field

[0001] This invention relates to the field of architectural coatings technology, specifically to a phosphogypsum-based fireproof and mildew-proof indoor coating and its preparation method. Background Technology

[0002] Phosphogypsum is an industrial solid waste discharged during the production of phosphoric acid. With the advancement of solid waste resource utilization, using phosphogypsum in the preparation of building materials has become a common development trend. In the field of interior coatings, unwashed phosphogypsum contains impurities such as free phosphoric acid and soluble phosphates. Existing comprehensive utilization technologies for phosphogypsum typically require multiple water washings to remove these acidic substances. This pretreatment process consumes water resources and increases the production cost of coatings as well as the burden of subsequent wastewater treatment.

[0003] If unwashed and untreated phosphogypsum is directly applied to water-based coating systems, the free acid released will disrupt the acid-base balance of the liquid coating system. This acidic environment causes the film-forming binder in the coating to degrade or prematurely cross-link and flocculate, resulting in loss of storage stability and flowability of the liquid coating. Furthermore, existing anti-mold interior coatings often achieve their anti-mold function by directly adding anti-mold powder. However, directly added anti-mold powder has poor compatibility with water-based coating binders, easily agglomerating during slurry preparation and failing to disperse evenly in the system. Uneven powder dispersion creates physical defects within the coating, affecting the mechanical strength of the film, and the anti-mold substance cannot form a uniform anti-mold network within the film, resulting in poor long-term anti-mold performance after film formation. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a phosphogypsum-based fire-retardant and mildew-proof indoor coating and its preparation method. This solves the problems of existing phosphogypsum comprehensive utilization technologies, which typically require washing the virgin phosphogypsum with water to remove free phosphoric acid and soluble phosphates, and adding mildew-proof powder. This results in uneven particle dispersion, easy agglomeration and sedimentation, and poor compatibility with the base material in the finished mildew-proof coating. Furthermore, if unwashed phosphogypsum is directly used in water-based coatings, the free acid within it will disrupt the acid-base balance of the coating system, leading to degradation or flocculation of the film-forming base material.

[0005] In a first aspect, the present invention provides a phosphogypsum-based fire-retardant and mildew-resistant indoor coating, comprising the following technical solution: a phosphogypsum-based fire-retardant and mildew-resistant indoor coating, comprising the following raw materials in the indicated mass percentages: 40.0% to 50.0% unwashed phosphogypsum powder; 20.0% to 25.0% polyvinyl alcohol aqueous solution with a solid content of 10.0% to 15.0%; 3.0% to 5.0% low-activity zinc oxide; 5.0% to 8.0% melamine; 4.0% to 6.0% boric acid-glycerol pre-complex; 0.3% to 0.6% sodium polycarboxylate dispersant; 0.2% to 0.4% modified polyether siloxane defoamer; and 10.0% to 27.5% deionized water; the sum of the mass percentages of the above raw materials is 100%.

[0006] By adopting the above technical solution, the following beneficial effects are achieved: This invention employs a reaction mechanism that generates a slightly soluble antifungal substance in situ during the coating drying and curing stage. This not only directly consumes the free acid in the original unwashed phosphogypsum but also avoids compatibility problems caused by directly adding finished powder. The reaction process and mechanism are specifically divided into the following steps: Step 1: Preparation and Storage Stage. During the preparation and storage stage of the liquid coating system, low-activity zinc oxide, calcined at high temperature, is selected. Its surface activity is low and a passivation layer exists. Furthermore, the system temperature is controlled, and the zinc oxide and the free phosphoric acid released from the original unwashed phosphogypsum are in a state of physical coexistence and low reactivity, maintaining the stability of the liquid coating system. Step 2: Moisture Evaporation and Activation Stage. When the coating is applied to the substrate surface and begins to dry, the liquid system gradually concentrates as moisture evaporates from the coating. Moisture loss disrupts the original physicochemical equilibrium, causing the passivation layer on the surface of the low-activity zinc oxide to dissolve, exposing reaction sites. Step 3: In-situ Salt Formation Stage. The activated zinc oxide then undergoes a neutralization reaction with the free phosphoric acid and soluble phosphates enriched in the system, generating a slightly soluble zinc phosphate precipitate in situ within the coating network. This reaction consumes free hydrogen ions, changing the pH of the system from slightly acidic to slightly alkaline. The in-situ generated zinc phosphate is uniformly dispersed in the cross-linked network, providing anti-mildew properties. Step Four: Cross-linking and Flame Retardant Stage. During the moisture evaporation stage, the polyhydroxy structure in the boric acid-glycerol pre-complex undergoes acetalization and hydrogen bonding cross-linking reactions with the hydroxyl groups on the polyvinyl alcohol molecular chain, constructing a cross-linked network and improving the density and water resistance of the coating film. When exposed to fire, melamine decomposes upon heating, releasing non-flammable gases to dilute the oxygen concentration. Simultaneously, phosphogypsum and the in-situ generated phosphates synergistically catalyze the formation of char, forming a heat-insulating and oxygen-free charred layer, achieving synergistic physical and chemical flame retardancy.

[0007] Preferably, the moisture content of the unwashed phosphogypsum powder is not higher than 15%, the internal free phosphoric acid and soluble phosphate are 0.5% to 1.5% by mass fraction of phosphorus pentoxide, and the fineness is 100 to 200 mesh. The polyvinyl alcohol in the aqueous solution is a partially alcoholyzed homopolymer with a degree of alcoholysis of 87.0% to 89.0% and a degree of polymerization of 1700 to 1800; the low-activity zinc oxide is low-activity zinc oxide that has undergone constant-temperature calcination at a temperature above 800℃; the melamine has a purity of not less than 99.0% and a fineness of not less than 300 mesh. The boric acid-glycerol pre-complex is prepared by reacting sodium tetraborate, glycerol, and deionized water; the sodium tetraborate is sodium tetraborate decahydrate; the glycerol is an anhydrous reagent with a purity of not less than 98.0%; the mass ratio of sodium tetraborate, glycerol, and deionized water is (1~3):(1~2):(1~2). The preparation method of boric acid-glycerol pre-complex includes: mixing glycerol with deionized water, heating to 50°C-60°C, and slowly adding solid sodium tetraborate in portions under mechanical stirring; after the addition is complete, raising the system temperature to 70°C-80°C and stirring at a constant temperature for 1.0h-2.5h, stopping heating and allowing the system to cool naturally to 25°C to obtain the boric acid-glycerol pre-complex. By adopting the above technical solution, the physicochemical parameters and microstructure of the core materials are clearly defined, ensuring the occurrence of in-situ salt formation reaction. The specific parameters of polyvinyl alcohol and the boric acid-glycerol pre-complex work synergistically to improve the crosslinking density after film formation while ensuring the leveling properties of the coating during application. The pre-complex obtained by limiting the proportions and preparation temperature is uniform and transparent, avoiding premature flocculation of the base material caused by free borate ions.

[0008] Secondly, the present invention provides a method for preparing a phosphogypsum-based fireproof and mildew-proof indoor coating, which adopts the following technical solution: A method for preparing a phosphogypsum-based fireproof and mildew-proof indoor coating includes the following steps: (1) Polyvinyl alcohol particles are added to the corresponding deionized water according to the set solid content, and the mixture is soaked and dispersed under mechanical stirring. Then, heating is turned on, and the mixture is kept warm and stirred until completely dissolved. The temperature is then lowered to below 30°C to obtain a polyvinyl alcohol aqueous solution for later use; (2) The remaining deionized water in the formula is mixed with the polyvinyl alcohol aqueous solution obtained in step (1) and transferred to a high-speed dispersion tank. Under low-speed stirring, sodium polycarboxylate dispersant and modified polyether silicone are added in sequence. Mix the alkyl defoamer evenly; gradually add low-activity zinc oxide and melamine powder; adjust the speed of the high-speed disperser to continuously disperse, and control the temperature inside the cylinder to not exceed 40°C to obtain a suspension slurry; (3) reduce the speed of the disperser, turn on the jacket cooling of the dispersion cylinder to control the system temperature to below 25°C, and add the original unwashed phosphogypsum powder evenly and slowly to the suspension slurry obtained in step (2). After the addition is completed, maintain stirring to obtain a mixed slurry; (4) maintain low-speed stirring, slowly add boric acid-glycerol pre-complex to the mixed slurry obtained in step (3), and continue low-speed stirring after the addition is completed. Filter under pressure through a filter screen, fill and seal to obtain a single-component liquid coating.

[0009] By adopting the above technical solution, the following beneficial effects are achieved: This preparation method ensures the realization of the in-situ salt formation reaction described in the first aspect by controlling the order of feeding and the reaction temperature. Before adding the unwashed phosphogypsum powder, the low-activity zinc oxide is first dispersed at high speed with the base material and additives to ensure that it is uniformly suspended in the system. Subsequently, when adding the phosphogypsum powder containing free acid, the jacket cooling is turned on to control the temperature below 25°C. The low-temperature environment further inhibits the acid-base neutralization reaction rate between zinc oxide and phosphoric acid, preventing gelation or precipitation during the pulping stage. Finally, the boric acid-glycerol pre-complex is added dropwise at a low speed to avoid high shear force destroying the pre-formed primary coordination bonds, thus ensuring the physicochemical stability of the coating during sealed storage.

[0010] Preferably, in step (1), the soaking and dispersion temperature is 20°C to 25°C, the mechanical stirring speed is 150 rpm to 200 rpm, and the soaking and dispersion time is 15 min to 20 min; the temperature inside the vessel is raised to 90°C to 95°C, and the holding and stirring time is 1.5 h to 2.0 h. In step (2), the low-speed stirring speed is 300 rpm to 400 rpm; the high-speed disperser speed is 1200 rpm to 1500 rpm, and the continuous dispersion time is 25 min to 30 min. In step (3), the disperser speed is reduced to 300 rpm to 500 rpm, the time for uniform and slow feeding is 10 min to 15 min, the stirring speed is maintained at 400 rpm to 500 rpm after feeding, and the stirring time is maintained at 15 min to 20 min. In step (4), the stirring speed is maintained at 200 rpm to 300 rpm, and the slow dripping time is 5 min to 8 min, with the continued low-speed stirring time being 10 min to 15 min; the filter screen is an 80-100 mesh stainless steel filter screen. By adopting the above technical solution, the flow and dispersion state of the material are standardized by quantifying the temperature range, shear rate, and processing time of each step. Specific low-speed stirring parameters and constant temperature duration can effectively eliminate air bubbles entrained during the mixing process, ensuring the uniformity of the liquid coating and the smoothness and mechanical continuity of the cured film.

[0011] This invention provides a phosphogypsum-based fire-retardant and mildew-resistant indoor coating and its preparation method. It has the following beneficial effects: 1. This invention achieves in-situ salt formation during the coating drying stage by combining unwashed phosphogypsum powder with high-temperature calcined low-activity zinc oxide. The evaporation of moisture after coating causes the passivation layer on the surface of the low-activity zinc oxide to dissolve, allowing it to neutralize the free phosphoric acid enriched in the system, generating slightly soluble zinc phosphate. This reaction process directly consumes free acid that easily degrades the base material, maintaining the stability of the system. Simultaneously, it uniformly generates anti-mold substances in the cross-linked network, avoiding the problems of easy agglomeration and poor compatibility of traditional externally added anti-mold powders.

[0012] 2. This invention employs a polyvinyl alcohol aqueous solution with specific parameters and a boric acid-glycerol pre-complex to construct a film-forming system, significantly improving the mechanical properties and water resistance of the coating film. During the sealed storage of the coating, the specific structure of the pre-complex prevents premature flocculation of the base material caused by free borate ions. During the water evaporation stage after coating, the polyhydroxy structure in the boric acid-glycerol pre-complex undergoes hydrogen bonding cross-linking with the hydroxyl groups on the polyvinyl alcohol molecular chain, constructing a dense cross-linked network within the coating, thereby improving the density and water resistance of the final film.

[0013] 3. This invention utilizes melamine, phosphogypsum, and in-situ generated phosphate substances to construct a synergistic physical and chemical flame-retardant system within the coating. Under high-temperature conditions when exposed to fire, the melamine in the system decomposes upon heating and releases non-flammable gases, effectively diluting the oxygen concentration on the coating surface. Simultaneously, the original phosphogypsum and salts such as zinc phosphate generated by the neutralization reaction synergistically catalyze the dehydration and charring reaction of the polymer matrix, forming a heat-insulating and oxygen-free charred layer on the substrate surface, thereby effectively blocking the combustion process. Attached Figure Description

[0014] Figure 1 This is a flowchart of the preparation method of the present invention. Detailed Implementation

[0015] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0016] raw material: The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Reagents not specifically mentioned are all commercially available analytical grade or higher grade products.

[0017] The main chemical component of unwashed phosphogypsum powder is calcium sulfate dihydrate, with a moisture content not exceeding 15%. The internal free phosphoric acid and soluble phosphates are mainly composed of phosphorus pentoxide (…). The mass fraction is 0.5% to 1.5%, and the fineness is 100 mesh to 200 mesh.

[0018] Polyvinyl alcohol is a partially alcoholyzed homopolymer with a degree of alcoholysis of 87.0% to 89.0% and a degree of polymerization of 1700 to 1800.

[0019] Zinc oxide, specifically low-activity zinc oxide calcined at a constant temperature above 800℃, has a specific surface area of ​​no more than [missing value]. .

[0020] Melamine, also known as melamine, has a purity of not less than 99.0% and a fineness of not less than 300 mesh.

[0021] Sodium tetraborate is sodium tetraborate decahydrate, industrial grade.

[0022] Glycerol, also known as glycerin, is an anhydrous reagent with a purity of not less than 98.0%.

[0023] Sodium polycarboxylate dispersant and modified polyether siloxane defoamer are both commercially available coating additives commonly used in this field.

[0024] Preparation example: Preparation Example 1: This preparation example provides a borate-glycerol pre-complex, comprising the following steps: Sodium tetraborate, glycerol, and deionized water are accurately weighed in a mass ratio of 1:2:1. Glycerol and deionized water are mixed and added to a three-necked flask equipped with a reflux condenser. The water bath is turned on and the temperature is raised to 50°C. Solid sodium tetraborate is slowly added in three portions at a mechanical stirring speed of 200 rpm. After the addition is complete, the system temperature is raised to 70°C and stirred at this constant temperature for 2.5 hours. During this period, sodium tetraborate completely dissolves and undergoes coordination complexation with glycerol. Heating is stopped, and the system is allowed to cool naturally to 25°C to obtain a homogeneous, transparent, and fluid liquid borate-glycerol pre-complex.

[0025] Preparation Example 2: This preparation example provides a borate-glycerol pre-complex, comprising the following steps: Sodium tetraborate, glycerol, and deionized water are accurately weighed at a mass ratio of 2:1.5:1.5. Glycerol and deionized water are mixed and added to a three-necked flask equipped with a reflux condenser. The water bath is turned on and the temperature is raised to 55°C. Solid sodium tetraborate is slowly added in three portions at a mechanical stirring speed of 250 rpm. After the addition is complete, the system temperature is raised to 75°C and stirred at this constant temperature for 1.5 hours. During this period, sodium tetraborate completely dissolves and undergoes coordination complexation with glycerol. Heating is stopped, and the system is allowed to cool naturally to 25°C to obtain a homogeneous, transparent, and fluid liquid borate-glycerol pre-complex.

[0026] Preparation Example 3: This preparation example provides a boric acid-glycerol pre-complex, comprising the following steps: Sodium tetraborate, glycerol, and deionized water are accurately weighed in a mass ratio of 3:1:2. Glycerol and deionized water are mixed and added to a three-necked flask equipped with a reflux condenser. The water bath is turned on and the temperature is raised to 60°C. Solid sodium tetraborate is slowly added in four portions at a mechanical stirring speed of 300 rpm. After the addition is complete, the system temperature is raised to 80°C and stirred at a constant temperature for 1.0 h. During this period, sodium tetraborate completely dissolves and undergoes coordination complexation with glycerol. Heating is stopped and the system is allowed to cool naturally to 25°C to obtain a homogeneous, transparent, and slightly yellow high-viscosity liquid boric acid-glycerol pre-complex.

[0027] Example: Example 1: This example provides a phosphogypsum-based fireproof and mildew-proof indoor coating and its preparation method, including the following steps: (1) According to the mass percentage of this example, prepare the following raw materials: 40.0% unwashed phosphogypsum powder, 20.0% polyvinyl alcohol aqueous solution (solid content 10.0%), 3.0% low-activity zinc oxide, 5.0% melamine, 0.3% sodium polycarboxylate dispersant, 0.2% modified polyether siloxane defoamer, 27.5% deionized water, and 4.0% boric acid-glycerol precomplex prepared by Example 1 (this amount of addition is equivalent to 1.0% sodium tetraborate, 2.0% glycerol, and 1.0% water in the pure system). (2) Preparation of the main film-forming base material: Polyvinyl alcohol particles were added to the corresponding deionized water according to the set solid content, and soaked and dispersed for 15 minutes at 20°C and mechanical stirring speed of 150 rpm. Then, the heating was turned on to raise the temperature inside the vessel to 90°C, and the mixture was kept warm and stirred for 1.5 hours until completely dissolved. The circulating cooling water was turned on to cool down to below 30°C to obtain a polyvinyl alcohol aqueous solution for use. (3) High-speed dispersion and premixing of functional powder: 27.5% deionized water and 20.0% polyvinyl alcohol aqueous solution were mixed and transferred to a high-speed dispersion tank. At a low stirring speed of 300 rpm, 0.3% sodium polycarboxylate dispersant and 0.2% modified polyether siloxane defoamer were added in sequence and stirred for 5 minutes to mix evenly. 3.0% low-activity zinc oxide and 5.0% melamine powder were added gradually. The speed of the high-speed disperser was adjusted to 1200 rpm and continuously dispersed for 25 minutes. The temperature inside the tank was controlled not to exceed 40°C to obtain a uniform white suspension slurry. (4) Low-temperature composite of in-situ acid source initiator: Reduce the speed of the disperser to 300 rpm, turn on the jacket cooling of the dispersion tank to control the system temperature below 25℃, and slowly add 40.0% of the original unwashed phosphogypsum powder to the suspension slurry at a uniform speed within 10 min. After the addition is completed, maintain stirring at 400 rpm for 15 min. (5) Construction of competitive crosslinking network: Maintain a low stirring speed of 200 rpm, and slowly add 4.0% of boric acid-glycerol pre-complex to the mixed slurry dropwise within 5 min. After the addition is completed, continue stirring at low speed for 10 min. Filter under pressure through an 80-mesh stainless steel filter screen, fill and seal to obtain a single-component liquid coating.

[0028] Example 2: This example provides a phosphogypsum-based fireproof and mildew-proof indoor coating and its preparation method, including the following steps: (1) According to the mass percentage of this example, prepare the following raw materials: 45.0% unwashed phosphogypsum powder, 25.0% polyvinyl alcohol aqueous solution (solid content 12.0%), 4.0% low-activity zinc oxide, 6.5% melamine, 0.5% sodium polycarboxylate dispersant, 0.3% modified polyether siloxane defoamer, 13.7% deionized water, and 5.0% boric acid-glycerol precomplex prepared by Example 2 (this amount of addition is equivalent to 2.0% sodium tetraborate, 1.5% glycerol, and 1.5% water in the pure system). (2) Preparation of the main film-forming base material: Polyvinyl alcohol particles were added to the corresponding deionized water according to the set solid content, and soaked and dispersed for 18 minutes at 22°C and mechanical stirring speed of 180 rpm. Then, the heating was turned on to raise the temperature inside the vessel to 92°C, and the mixture was kept warm and stirred for 1.8 hours until completely dissolved. The circulating cooling water was turned on to cool down to below 30°C to obtain a polyvinyl alcohol aqueous solution for use. (3) High-speed dispersion and premixing of functional powder: 13.7% deionized water and 25.0% polyvinyl alcohol aqueous solution were mixed and transferred to a high-speed dispersion tank. At a low stirring speed of 350 rpm, 0.5% sodium polycarboxylate dispersant and 0.3% modified polyether siloxane defoamer were added in sequence and stirred for 8 minutes to mix evenly. 4.0% low-activity zinc oxide and 6.5% melamine powder were added gradually. The speed of the high-speed disperser was adjusted to 1350 rpm and continuously dispersed for 28 minutes. The temperature inside the tank was controlled not to exceed 40°C to obtain a uniform white suspension slurry. (4) Low-temperature composite of in-situ acid source initiator: Reduce the speed of the disperser to 400 rpm, turn on the jacket cooling of the dispersion tank to control the system temperature below 25℃, and slowly add 45.0% of the original unwashed phosphogypsum powder to the suspension slurry at a uniform speed within 12 min. After the addition is completed, maintain stirring at 450 rpm for 18 min. (5) Construction of competitive crosslinking network: Maintain a low stirring speed of 250 rpm, and slowly add 5.0% of boric acid-glycerol pre-complex to the mixed slurry dropwise within 6 min. After the addition is completed, continue stirring at low speed for 12 min. Filter under pressure through a 90-mesh stainless steel filter, fill and seal to obtain a single-component liquid coating.

[0029] Example 3: This example provides a phosphogypsum-based fireproof and mildew-proof indoor coating and its preparation method, including the following steps: (1) According to the mass percentage of this example, prepare the following raw materials: 50.0% unwashed phosphogypsum powder, 20.0% polyvinyl alcohol aqueous solution (solid content 15.0%), 5.0% low-activity zinc oxide, 8.0% melamine, 0.6% sodium polycarboxylate dispersant, 0.4% modified polyether siloxane defoamer, 10.0% deionized water, and 6.0% boric acid-glycerol precomplex prepared by Example 3 (this amount of addition is equivalent to 3.0% sodium tetraborate, 1.0% glycerol, and 2.0% water in the pure system). (2) Preparation of the main film-forming base material: Polyvinyl alcohol particles were added to the corresponding deionized water according to the set solid content, and soaked and dispersed for 20 minutes at 25°C and mechanical stirring speed of 200 rpm. Then, the heating was turned on to raise the temperature inside the vessel to 95°C, and the mixture was kept warm and stirred for 2.0 h until completely dissolved. The circulating cooling water was turned on to cool down to below 30°C to obtain a polyvinyl alcohol aqueous solution for use. (3) High-speed dispersion and premixing of functional powder: 10.0% deionized water and 20.0% polyvinyl alcohol aqueous solution were mixed and transferred to a high-speed dispersion tank. At a low stirring speed of 400 rpm, 0.6% sodium polycarboxylate dispersant and 0.4% modified polyether siloxane defoamer were added in sequence and stirred for 10 minutes to mix evenly. 5.0% low-activity zinc oxide and 8.0% melamine powder were added gradually. The speed of the high-speed disperser was adjusted to 1500 rpm and continuously dispersed for 30 minutes. The temperature inside the tank was controlled not to exceed 40°C to obtain a uniform white suspension slurry. (4) Low-temperature composite of in-situ acid source initiator: Reduce the speed of the disperser to 500 rpm, turn on the jacket cooling of the dispersion tank to control the system temperature below 25℃, and slowly add 50.0% of the original unwashed phosphogypsum powder to the suspension slurry at a uniform speed within 15 min. After the addition is completed, maintain stirring at 500 rpm for 20 min. (5) Construction of competitive crosslinking network: Maintain a low stirring speed of 300 rpm, and slowly add 6.0% of boric acid-glycerol pre-complex to the mixed slurry dropwise within 8 min. After the addition is completed, continue stirring at low speed for 15 min. Filter under pressure through a 100-mesh stainless steel filter screen, fill and seal to obtain a single-component liquid coating.

[0030] Comparative example: Comparative Example 1: Compared with Example 2, the difference is that the "original unwashed phosphogypsum powder" in the raw materials was replaced with an equal amount of "high-purity phosphogypsum powder that has been washed multiple times to neutrality" (i.e., free phosphoric acid and soluble phosphate impurities were removed), and all other aspects were the same.

[0031] Comparative Example 2: Compared with Example 2, the difference is that the "boronic acid-glycerol pre-complex" was not synthesized and used in advance. Instead, in step (5), an equivalent amount of sodium tetraborate decahydrate solid powder and glycerol were directly added to the mixed slurry in a physical mixing manner. All other aspects are the same.

[0032] Comparative Example 3: Compared with Example 2, the difference is that the "low-activity zinc oxide" component in the formulation was removed and replaced with an equal mass of a conventional commercially available isothiazolinone organic fungicide, while the rest are the same.

[0033] Comparative Example 4: Compared with Example 2, the difference is that the "melamine" component in the formula was removed and replaced with an equal mass of conventional heavy calcium carbonate powder, while the rest are the same.

[0034] Comparative Example 5: Compared with Example 2, the difference is that in step (4), the jacket cooling of the dispersion cylinder was not turned on, and the system was not temperature controlled below 25°C. Instead, the unwashed phosphogypsum powder was added directly under natural heating (or above 50°C) for compounding. All other aspects are the same.

[0035] Test example: Test Example 1: pH Dynamic Response Test During Film Formation and Curing This test case aims to verify the reaction process of in-situ generation of slightly soluble zinc phosphate inside the coating during the natural drying and curing stage through continuous monitoring of macroscopic physicochemical parameters.

[0036] I. Experimental Description and Test Procedure The coating samples prepared in Examples 1 to 3 and Comparative Examples 1 to 5 were poured onto polytetrafluoroethylene (PTFE) test plates and coated with an initial thickness of 2.0 mm using a wet film preparer. The operating environment was maintained at 25°C and 50% relative humidity. Before testing, a surface pH meter equipped with a planar composite electrode was calibrated using standard buffer solutions of pH 4.00 and pH 6.86. Starting from hour 0 when the coating preparation was completed, the planar composite electrode was attached to the coating surface to take readings. Subsequently, at time points of 4, 8, 12, 24, 36, 48, 60, and 72 hours, three locations were randomly selected on each sample plate for repeated measurements, and the arithmetic mean was recorded. During the testing period, the samples were allowed to air dry under the same conditions.

[0037] II. Experimental Data The following are the surface pH monitoring results of each group of samples during the 72-hour film formation cycle.

[0038] Table 1. Surface pH value test data of each coating sample during the natural drying film formation process.

[0039] III. Data Analysis and Conclusions Based on the experimental data in Table 1 and the chemical reaction mechanism of this technical solution, the following analytical conclusions are drawn: Analysis of the data in Table 1 shows that Comparative Example 1 used water-washed phosphogypsum, resulting in a system without free acid, and the coating pH remained neutral throughout the testing period. Examples 1 to 3 used unwashed phosphogypsum, and the initial pH decreased gradually with increasing phosphogypsum addition, indicating a slightly acidic state. During the evaporation period from 4 to 48 hours, the pH of the examples increased over time, tending towards a slightly alkaline range of 7.3 to 7.5 after 60 hours. This indicates that coating concentration caused the zinc oxide surface layer to dissolve and neutralize with free phosphoric acid to form zinc phosphate, consuming hydrogen ions in the system.

[0040] Comparative Example 2 did not use a pre-complexing compound, and its pH change trend was similar to that of Example 2, indicating that the complexation process mainly affects the crosslinking rheological properties and has no substantial impact on the acid-base neutralization process. Comparative Example 3 removed zinc oxide, and the system lacked substances that consumed free acid; the pH remained consistently in the acidic range of around 4.5, and no in-situ neutralization occurred. Comparative Example 4 used calcium carbonate instead of melamine. Because calcium carbonate reacted rapidly with free acid during the slurry preparation stage, the initial pH of the coating was too high, and a gradual neutralization and release process was not formed. Comparative Example 5 did not control the low temperature during preparation, causing zinc oxide to react prematurely with free acid in the dispersion tank. The initial pH of the coating at film formation was close to neutral, losing the reaction conditions for in-situ generation of zinc phosphate during the curing stage.

[0041] Test Example 2: Long-lasting mold resistance test This test case refers to GB / T 1741-2007 "Determination of Resistance to Mold in Coatings" to evaluate the long-term resistance of coating systems to mold erosion under high temperature and high humidity conditions.

[0042] I. Experimental Description and Test Procedure Liquid coatings from Examples 1 to 3 and Comparative Examples 1 to 5 were applied to standard asbestos cement samples using a coating preparation device. These samples were then cured for 14 days at 25°C and 50% relative humidity to produce test samples with a dry film thickness of approximately 150 micrometers. A mixed mold spore suspension containing *Aspergillus niger*, *Aspergillus flavus*, *Chaetomium globosum*, and *Brachystomium buddingense* was uniformly sprayed onto the sample surface for inoculation. The inoculated samples were then placed flat in a constant temperature and humidity incubator, with the incubation temperature set at 28°C and the relative humidity controlled between 90% and 95%. Samples were removed on the 28th and 56th days of continuous incubation, and the surface mold growth was observed using a magnifying glass with a graduated grid. The percentage of colony coverage was recorded, and the corresponding mold growth level was assessed. The criteria for judging the degree of mold growth are as follows: Level 0 (no obvious mold growth), Level 1 (mold growth area less than 10%), Level 2 (mold growth area 10% to 30%), Level 3 (mold growth area 30% to 60%), and Level 4 (mold growth area greater than 60%).

[0043] II. Experimental Data Table 2. Results of mold resistance tests for each coating sample at different incubation periods.

[0044] III. Data Analysis and Conclusions Regarding antifungal properties, the systems in the examples exhibited a relatively long-lasting ability to resist fungal colonization. Comparative Example 3, using a commercially available organic antifungal agent, inhibited mold growth within a 28-day period, but the area percentage increased to 24.7% by day 56, indicating that the free migration of the added antifungal agent in the matrix resulted in a certain time-limited effect. Examples 1 to 3 maintained a low mold growth level within a 56-day period, corresponding to the in-situ salt formation reaction process. The free acid released from phosphogypsum and the low-activity zinc oxide underwent a neutralization reaction during the liquid phase concentration stage of the coating, and the resulting slightly soluble zinc phosphate precipitate was dispersed in the cross-linked network formed by polyvinyl alcohol. This in-situ generated inorganic salt forms a long-term stable slightly soluble release system in the local microenvironment, avoiding the precipitation of the product. In Comparative Example 1, the free acid source was removed by water washing, so the system could not induce a neutralization reaction to generate zinc phosphate; in Comparative Example 5, no low-temperature control was used, and the neutralization reaction occurred prematurely during the liquid phase preparation stage, causing the generated antifungal substance to agglomerate and settle. Both examples reached level 3 in the 56-day mold growth test.

[0045] Test Example 3: Flame Retardant and Combustion Heat Release Performance Test This test case uses the limiting oxygen index and cone calorimetry to obtain the heat release parameters and char formation behavior of the coating system during heating and combustion, and evaluates its fire-retardant performance.

[0046] I. Experimental Description and Test Procedure Each group of liquid coatings was poured into a polytetrafluoroethylene mold pre-coated with a release agent, and demolded after surface curing at room temperature. The resulting samples were then transferred to a 60℃ forced-air drying oven and dried to constant weight, and processed into strips and templates conforming to the corresponding test standard dimensions. The limiting oxygen index of the strips was tested using a digital oxygen index meter according to GB / T 2406.2 standard. A cone calorimeter was used to test a 100mm×100mm×3mm template, with the radiometer power set to 50 kW / m². The peak heat release rate and total heat release from ignition to the end of combustion were recorded, and the residue was collected after the test to calculate the char rate. The char rate was expressed as a percentage of the mass of the residue after the test to the initial mass of the template before the test.

[0047] II. Experimental Data Table 3 Limiting oxygen index and cone calorimetry test parameters for each coating sample

[0048] III. Data Analysis and Conclusions Regarding flame retardant heat release, the data confirmed the synergistic charring and flame retardant effect of melamine and in-situ generated salts. Comparative Example 4, which removed melamine, saw its peak heat release rate increase to 294.5 kW / m², and its limiting oxygen index decrease to 22.8%. This indicates the absence of the non-flammable gas phase dilution effect generated by the high-temperature decomposition of melamine. The char residue of Examples 1 to 3 increased with increasing raw material proportions, reaching a maximum of 59.2%. Compared to Comparative Example 1, which removed free phosphoric acid, and Comparative Example 5, which lost the in-situ salt formation conditions, the total heat release of the example systems was lower, and the char residue was higher. This is because the in-situ generated zinc phosphate and the remaining soluble phosphates altered the thermal degradation pathway of the polyvinyl alcohol matrix during the high-temperature heating stage. Phosphate substances promoted the early dehydration and carbonization of the polymer macromolecular chains, and the increased dense carbonaceous layer covering the coating surface hindered the escape of the underlying combustible volatiles and the heat transfer from external oxygen. Comparative Example 2 lacks the acetal crosslinking structure constructed from the boric acid-glycerol pre-complex, resulting in lower coating continuity and a slightly decreased char density, leading to a lower char residue rate compared to the corresponding Example 2. The data relationships between the groups are consistent with the design mechanism of in-situ acid source initiator synergistic physical vapor phase flame retardancy.

[0049] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A phosphogypsum-based fireproof and moldproof interior coating material, characterized in that, It is made from the following raw materials in the following mass percentages: 40.0%~50.0% unwashed phosphogypsum powder; 20.0%~25.0% polyvinyl alcohol aqueous solution with a solid content of 10.0%~15.0%; 3.0%~5.0% low-activity zinc oxide; 5.0%~8.0% melamine; 4.0%~6.0% boric acid-glycerol pre-complex; 0.3%~0.6% sodium polycarboxylate dispersant; 0.2%~0.4% modified polyether siloxane defoamer; 10.0%~27.5% deionized water; the sum of the mass percentages of the above raw materials is 100%.

2. The phosphogypsum-based fire-retardant and mildew-proof indoor coating according to claim 1, characterized in that, The original unwashed phosphogypsum powder has a moisture content of no more than 15%, and the internal free phosphoric acid and soluble phosphates are 0.5%~1.5% by mass fraction of phosphorus pentoxide, with a fineness of 100 mesh~200 mesh.

3. The phosphogypsum-based fire-retardant and mildew-proof indoor coating according to claim 1, characterized in that, The polyvinyl alcohol in the aqueous solution is a partially hydrolyzed homopolymer with a degree of hydrolysis of 87.0% to 89.0% and a degree of polymerization of 1700 to 1800; the low-activity zinc oxide is low-activity zinc oxide that has been calcined at a constant temperature above 800°C; the melamine has a purity of not less than 99.0% and a fineness of not less than 300 mesh.

4. The phosphogypsum-based fire-retardant and mildew-proof indoor coating according to claim 1, characterized in that, The boric acid-glycerol precomplex is prepared by reacting sodium tetraborate, glycerol and deionized water; the sodium tetraborate is sodium tetraborate decahydrate; the glycerol is an anhydrous reagent with a purity of not less than 98.0%; the mass ratio of sodium tetraborate, glycerol and deionized water is (1~3):(1~2):(1~2).

5. The phosphogypsum-based fire-retardant and mildew-proof indoor coating according to claim 4, characterized in that, The method for preparing the boric acid-glycerol precomplex includes: mixing glycerol with deionized water, heating to 50°C~60°C, and slowly adding solid sodium tetraborate in portions under mechanical stirring; after the addition is complete, raising the system temperature to 70°C~80°C and stirring at a constant temperature for 1.0h~2.5h, stopping the heating and allowing the system to cool naturally to 25°C to obtain the boric acid-glycerol precomplex.

6. A method for preparing a phosphogypsum-based fire-retardant and mildew-resistant indoor coating according to any one of claims 1-5, characterized in that, Includes the following steps: (1) Add polyvinyl alcohol particles to the corresponding deionized water according to the set solid content, soak and disperse under mechanical stirring, then turn on the heating, keep warm and stir until completely dissolved, cool down to below 30°C to obtain a polyvinyl alcohol aqueous solution for later use; (2) Mix the remaining deionized water in the formula with the polyvinyl alcohol aqueous solution obtained in step (1) and transfer it to a high-speed dispersion tank. Under low-speed stirring, add sodium polycarboxylate dispersant and modified polyether siloxane defoamer in sequence and mix well; gradually add low-activity zinc oxide and melamine powder; adjust the speed of the high-speed disperser. (2) Disperse continuously at high speed, control the temperature inside the cylinder to not exceed 40°C, and obtain a suspension slurry; (3) Reduce the speed of the disperser, turn on the jacket cooling of the dispersion cylinder to control the system temperature to below 25°C, and add the original unwashed phosphogypsum powder to the suspension slurry obtained in step (2) at a uniform and slow speed. After the addition is completed, maintain stirring to obtain a mixed slurry; (4) Maintain low-speed stirring, slowly add boric acid-glycerol pre-complex to the mixed slurry obtained in step (3), continue low-speed stirring after the addition is completed, filter under pressure through a filter screen, fill and seal to obtain a single-component liquid coating.

7. The method for preparing a phosphogypsum-based fire-retardant and mildew-proof indoor coating according to claim 6, characterized in that, In step (1), the soaking and dispersion temperature is 20℃~25℃, the mechanical stirring speed is 150rpm~200rpm, the soaking and dispersion time is 15min~20min; the heating temperature inside the vessel is 90℃~95℃, and the heat preservation and stirring time is 1.5h~2.0h.

8. The method for preparing a phosphogypsum-based fire-retardant and mildew-proof indoor coating according to claim 6, characterized in that, In step (2), the low-speed stirring speed is 300 rpm to 400 rpm; the high-speed disperser speed is 1200 rpm to 1500 rpm; and the continuous dispersion time is 25 min to 30 min.

9. The method for preparing a phosphogypsum-based fire-retardant and mildew-proof indoor coating according to claim 6, characterized in that, In step (3), the speed of the disperser is reduced to 300 rpm to 500 rpm, the time for feeding the material slowly and evenly is 10 min to 15 min, the speed of stirring is maintained at 400 rpm to 500 rpm after the material is fed, and the stirring time is maintained at 15 min to 20 min.

10. The method for preparing a phosphogypsum-based fire-retardant and mildew-proof indoor coating according to claim 6, characterized in that, In step (4), the stirring speed is maintained at 200 rpm to 300 rpm, the slow dripping time is 5 min to 8 min, and the stirring time is 10 min to 15 min; the filter screen is an 80 mesh to 100 mesh stainless steel filter screen.

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