A hydrogel composite flame retardant as well as a preparation method and application thereof

Abstract

The present application relates to a kind of hydrogel composite flame retardant and its preparation method and application, belong to flame retardant technical field.The flame retardant includes the following weight parts of raw materials: oxidized starch 5~10 parts, crosslinking agent 1~2 parts, surfactant 0.4~0.6 parts, antioxidant 2~4 parts, polyvinyl alcohol 10~20 parts and initiator 0.1~0.2 parts;Wherein, the antioxidant includes at least one of erythorbic acid, phytic acid;The surfactant includes at least one of alkyl naphthalene sulfonate sodium salt, sodium lignosulfonate.The flame retardant covers oxygen by water retention, water absorption cooling, antioxidant can absorb the free radical generated in the process of coal oxidation, block the chain lock reaction of free radical, delay low temperature oxidation reaction, so as to achieve the purpose of inhibiting spontaneous combustion.The flame retardant uses environment-friendly material, easy to degrade, no pollution to environment;No corrosive, no corrosion to surrounding equipment and facilities.Low cost, simple spraying operation, suitable for large coal yard.

Description

Technical Field

[0001] This invention relates to the technical field of coal flame retardants, specifically to a hydrogel composite flame retardant, its preparation method, and its application. Background Technology

[0002] Spontaneous combustion of coal is one of the major hazards in thermal power plants and coal mines, seriously impacting the safety of operations and causing enormous economic losses and environmental pollution. Spontaneous combustion is the result of low-temperature oxidation and accumulation in coal; therefore, prevention and control must begin with inhibiting this low-temperature oxidation.

[0003] The following are ways to inhibit low-temperature oxidation of coal: 1) Isolate oxygen from contact with coal. The most common method is to form a covering layer on the surface of the coal pile. This can be done by spraying non-combustible substances on the surface of the coal pile to seal the pores of the coal particles and cut off the air passage. Another method is to fill with inert gas, but this can easily cause health hazards to workers in the enclosed space; 2) Use halogen salt inhibitors. The principle is that the hygroscopic and water-retaining properties of halogen salts can reduce the temperature of the coal pile and form a water film layer to isolate oxygen. However, halogens have a more serious corrosive effect on the steel of the boiler. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a hydrogel composite flame retardant with environmentally friendly and high efficiency, as well as its preparation method and application.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: In a first aspect, the present invention provides a hydrogel composite flame retardant, comprising the following raw materials in parts by weight: 5-10 parts oxidized starch, 1-2 parts crosslinking agent, 0.4-0.6 parts surfactant, 2-4 parts antioxidant, 10-20 parts polyvinyl alcohol, and 0.1-0.2 parts initiator. The antioxidant comprises at least one of isoascorbic acid and phytic acid; the surfactant comprises at least one of sodium alkylnaphthalene sulfonate condensate and sodium lignin sulfonate.

[0006] The flame retardant of this invention uses non-toxic, biodegradable natural materials to synthesize a hydrogel. It employs an initiator to graft polyvinyl alcohol and oxidized starch, and adds a small amount of crosslinking agent to improve the degree of crosslinking of the hydrogel, making the hydrogel into a network structure, so that it can form a gel coating layer on the surface of the coal pile. Then, an inhibitor and a surfactant with wetting function are added. On the one hand, the gel blocks oxygen from entering the coal pile, and on the other hand, the gel absorbs moisture and retains water to reduce the temperature of the coal pile. The antioxidant can absorb free radicals generated during the coal oxidation process, block the chain reaction of free radicals, and prevent low-temperature oxidation reaction.

[0007] Polyvinyl alcohol (PVA) is used to graft with oxidized starch to form a hydrogel. PVA, acrylic acid, and other covalently bonded substances can be grafted with starch. PVA is a water-soluble polymer, and its aqueous solution has similar properties to starch, being viscous. PVA contains hydroxyl groups, and the crosslinking agent borax hydrolyzes to produce some boric acid. Boric acid forms a complex with polar groups (such as hydroxyl or carboxyl groups) in starch, leading to a series of hydroxyl chain reactions and forming a network structure. This can improve the coagulation of coal particles and form a relatively hard shell with the coal particles, playing a role in oxygen isolation.

[0008] Ordinary starch granules contain approximately 80% amylose and 20% amylopectin. Amylose dissolves in hot water, while amylopectin remains insoluble. Adding an oxidizing agent causes the hydroxyl groups in the amylopectin to gradually separate. During oxidation, glycoside chains inevitably break down, reducing the molecular weight; this reaction is directly proportional to the degree of oxidation. Therefore, by appropriately controlling the oxidation depth, oxidized starch with a certain viscosity can be obtained. As the temperature increases, the starch swells and gelatinizes, the starch molecules extend, increasing the number of hydrophilic groups in contact with the external environment, thereby increasing its binding force.

[0009] The antioxidants selected in this invention include at least one of isoascorbic acid and phytic acid, more preferably isoascorbic acid; their function is to inert the activity of the active groups and interrupt the chain reaction of the active groups. Moreover, they are biological reagents, have no corrosive effect on austenitic steel, and are non-polluting to humans and the environment. As an antioxidant inhibitor, it has significant advantages over common inorganic inhibitors. Specifically, calcium chloride, aluminum chloride, and calcium hydroxide are all chlorine-containing inhibitors that achieve flame retardancy through moisture absorption and cooling, but they are easily lost after spraying, have limited moisture absorption, and chlorine has a corrosive effect on boiler austenitic steel.

[0010] The surfactant used in this invention includes at least one of sodium alkyl naphthalene sulfonate condensate and sodium lignosulfonate, which acts as a wetting agent in the system. Preferably, the surfactant is a mixture of sodium alkyl naphthalene sulfonate condensate and sodium lignosulfonate in a mass ratio of (1.8~2.1):1; more preferably, the mass ratio is 2:1. The surfactant enhances the diffusion of the flame retardant, enabling it to form a coating layer of a certain thickness on the surface of the coal pile. The combination of sodium alkyl naphthalene sulfonate condensate and sodium lignosulfonate provides even better wetting.

[0011] Preferably, the oxidized starch includes at least one of oxidized corn starch and oxidized cassava starch.

[0012] Preferably, the method for preparing oxidized starch includes the following steps: mixing starch with water to form a starch solution, then heating the solution to 45-55°C with stirring, then adding acid and potassium permanganate, maintaining a constant temperature until the potassium permanganate solution changes from red to colorless, filtering the generated oxidized starch emulsion, drying it, and pulverizing it to obtain oxidized starch.

[0013] Specifically, ordinary starch granules contain approximately 80% amylose and 20% amylopectin. Amylose dissolves in hot water, while amylopectin remains insoluble. Adding an oxidizing agent causes the hydroxyl groups in the amylopectin to gradually separate. During oxidation, glycoside chains inevitably break down, reducing the molecular weight; this reaction is directly proportional to the degree of oxidation. Therefore, by appropriately controlling the oxidation depth, oxidized starch with a certain viscosity can be obtained. As the temperature increases, the starch swells and gelatinizes, extending the starch molecules and increasing the number of hydrophilic groups in contact with the external environment, thereby increasing its binding force. The oxidizing agent used is potassium permanganate.

[0014] Preferably, the crosslinking agent is borax.

[0015] Preferably, the initiator is potassium persulfate.

[0016] Secondly, the present invention provides a method for preparing a hydrogel composite flame retardant, comprising the following steps: S1 disperses oxidized starch with water to prepare an oxidized starch dispersion; S2 is used to gelatinize the oxidized starch dispersion to obtain an oxidized starch gelatinized solution; Polyvinyl alcohol was mixed with water, heated and stirred, an initiator was added and stirring continued, then oxidized starch gelatinized liquid was added to carry out graft copolymerization reaction, and then a crosslinking agent was added to carry out complexation reaction to obtain polyvinyl alcohol graft modified starch gel. S4 involves adding antioxidants and surfactants to polyvinyl alcohol-grafted modified starch gel to obtain the hydrogel composite flame retardant.

[0017] Preferably, in step S2, the gelatinization process involves adding alkali to the oxidized starch dispersion at a temperature of 20-30°C, stirring until homogeneous, and reacting for 0.2-1 h; wherein the mass ratio of the oxidized starch to the alkali is (9-11):1.

[0018] Preferably, in step S3, polyvinyl alcohol is mixed with water and then heated and stirred; wherein the heating and stirring parameters are as follows: first, heating and stirring at 70-100 r / min and 90-100℃ to completely dissolve the polyvinyl alcohol; then cooling to 50-60℃, stirring at 200-400 r / min, and then adding the initiator and stirring for 20-40 min; wherein the graft copolymerization reaction temperature is 20-35℃ and the time is 1-2 h; the complexation reaction temperature is 30-40℃ and the time is 20-40 min.

[0019] Preferably, in step S4, the mixing temperature of adding antioxidants and surfactants to the polyvinyl alcohol grafted modified starch gel is 45-55°C, and the mixing time is 20-40 min.

[0020] Thirdly, the present invention provides the application of gel composite flame retardants in the preparation of coal flame retardant products.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The flame retardant of the present invention simultaneously possesses the functions of isolating air, absorbing moisture and cooling, and resisting oxidation. Its main raw materials are polyvinyl alcohol, antioxidant, initiator, crosslinking agent, surfactant, and oxidized starch, which enables the flame retardant to be biodegradable and environmentally friendly. Moreover, its raw materials are all non-toxic and harmless substances. Compared with ammonium salt antioxidants, it does not produce NH3, which is a toxic gas. Compared with inert gases, it is less likely to cause asphyxiation for coal yard workers. The antioxidant in this flame retardant acts to inertize the activity of the active groups and interrupt the chain reaction of the active groups. It is a biological reagent that has a corrosive effect on austenitic steel and does not pollute the human body or the environment. Because the raw materials are all organic substances, they do not affect the combustion performance of coal. Therefore, the flame retardant of the present invention can achieve the purpose of inhibiting spontaneous combustion by using hydrogel to retain water, absorb water and cool, cover and isolate oxygen, and the antioxidant can absorb free radicals generated during the coal oxidation process, block the chain reaction of free radicals, delay the low-temperature oxidation reaction, and use environmentally friendly materials that are easy to degrade and do not pollute the environment. The formula is non-corrosive and does not corrode equipment and facilities around the coal yard. This flame retardant is low in cost and easy to apply, making it suitable for large coal yards.

[0022] (2) The preparation method of the flame retardant of the present invention includes the following steps: mixing oxidized starch gelatinized liquid and polyvinyl alcohol hydrogel, performing a graft copolymerization reaction, then adding a crosslinking agent to perform a complexation reaction, obtaining a polyvinyl alcohol graft-modified starch gel with a network structure, and then adding an antioxidant and a surfactant to mix, thereby obtaining the flame retardant. This preparation method overcomes the technical problem that oxidized starch is difficult to dissolve in water. First, the oxidized starch is gelatinized, and the gelatinization promotes the dissolution of 80% of the amylose in the starch, accelerates the graft copolymerization reaction, and completely dissolves the starch through gelatinization and oxidation. This preparation method has simple steps, generates no toxic substances during production, and is suitable for large-scale preparation. Detailed Implementation

[0023] To better illustrate the purpose, technical solution, and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0024] The raw materials used in the following examples are as follows: Sodium alkyl naphthalene sulfonic acid condensate: manufactured by AkzoNobel, trade name Morwet EFW.

[0025] Unless otherwise specified, all other materials and reagents used in the examples are commercially available.

[0026] Example 1 A hydrogel composite coal flame retardant comprises the following raw materials: 20g oxidized starch, 4g borax, 0.66g Morwet EFW, 0.33g sodium lignosulfonate, 6g isoascorbic acid, 30g polyvinyl alcohol, 140g deionized water, and 0.5g potassium persulfate.

[0027] The preparation method of oxidized starch is as follows: Weigh 30g of corn starch and add it to a four-necked flask equipped with a heating and reflux device. Add 70ml of deionized water and stir evenly. Heat the mixture to 50℃ while stirring. Add 3.5ml of 30% H2SO4 solution and 20mL of 2% KMnO4 solution. Maintain a constant temperature reaction until the KMnO4 solution changes from red to colorless. Transfer the generated oxidized starch emulsion to a Buchner funnel and filter it three times. Dry it in a drying oven at 50℃ until the moisture is completely evaporated. Crush the dried starch to obtain oxidized starch.

[0028] The preparation method of the above-mentioned hydrogel composite coal flame retardant includes the following steps: (1) Weigh 20g of dried oxidized starch and add it to a four-necked flask equipped with a heating and reflux device. Add water to prepare a 20% oxidized starch emulsion. Then add 2g of NaOH and keep the temperature at 20℃~30℃. Stir evenly and react for 0.5h to obtain oxidized starch gelatinized liquid.

[0029] (2) In a four-necked flask equipped with a stirrer, thermometer, reflux device, and heating element, add polyvinyl alcohol and water, and stir at a speed of 80 r / min; heat to 95°C to completely dissolve, cool to 55°C, adjust the stirring speed to 300 r / min, add potassium persulfate and continue stirring. After about 30 min, add the gelatinized oxidized starch solution in proportion and perform a constant temperature graft copolymerization reaction for 1.5 h, stir and cool to 35°C, add 2% (w / w) borax solution, complex for 0.5 h, stir and cool to room temperature to obtain polyvinyl alcohol grafted modified starch gel.

[0030] (3) Morwet EFW, sodium lignosulfonate and isoascorbic acid were added to polyvinyl alcohol grafted modified starch and stirred and heated at 50°C for 30 min until all components were completely dissolved to obtain hydrogel composite coal flame retardant.

[0031] Example 2 The difference between Example 2 and Example 1 is that the hydrogel composite coal flame retardant of Example 2 includes the following raw materials: 10g oxidized starch, 2g borax, 0.53g Morwet EFW, 0.26g sodium lignosulfonate, 4g isoascorbic acid, 15g polyvinyl alcohol, 163g deionized water, and 0.25g potassium persulfate.

[0032] Example 3 The difference between Example 3 and Example 1 is that the hydrogel composite coal flame retardant of Example 3 includes the following raw materials: 20g oxidized starch, 4g borax, 1g Morwet EFW, 4g phytic acid, 40g polyvinyl alcohol, 130g demineralized water, and 0.5g potassium persulfate.

[0033] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that oxidized starch in Comparative Example 1 is replaced with unoxidized corn starch.

[0034] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that the flame retardant in Comparative Example 2 comprises the following raw materials in parts by weight: 20g oxidized starch, 4g borax, 1g sodium dodecyl sulfonate, 6g isoascorbic acid, 30g polyvinyl alcohol, and 140g deionized water. Specifically, Morwet EFW and sodium lignosulfonate in Example 1 are replaced with sodium dodecyl sulfonate.

[0035] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that isoascorbic acid is replaced with calcium chloride in Comparative Example 3.

[0036] Comparative Example 4 The difference between Comparative Example 4 and Example 1 is that the preparation method of the flame retardant in Comparative Example 4 does not involve the gelatinization treatment of oxidized starch in step (1).

[0037] Performance testing I. Activation Energy of Coal The activation energy of coal refers to the energy required for organic molecules to transform into activated molecules and participate in the reaction during coal combustion. The activation energy of the coal-oxygen reaction is the sum of the activation energies of multiple elementary reactions, also known as the apparent activation energy. A higher apparent activation energy indicates that the coal sample is less likely to spontaneously combust. The apparent activation energy of easily spontaneously combustible coal was compared before and after using the gel-type flame retardant of this invention. The implementation steps are as follows: Sample selection and preparation Coal sample preparation: Indonesian lignite, weighing 3 kg, was selected as the self-igniting coal and prepared into a sample with a particle size of 50 mesh.

[0038] The above Indonesian lignite was pulverized into 8 parts, each weighing 0.5 kg. One part was not added with flame retardant, and the other 7 parts were added with flame retardants from Examples 1-3 and Comparative Examples 1-4, respectively. The amount of flame retardant added was 10% of the coal sample.

[0039] The apparent activation energy of the two samples was analyzed by thermogravimetric analysis. The experimental equipment was a STA449F3 simultaneous thermal analyzer manufactured by Netzsch Instruments GmbH, Germany. The experimental conditions were as follows: ① The heating rate was controlled at 5 ℃ / min, the initial temperature was 25 ℃, and the final temperature was 600 ℃; ② The nitrogen flow rate was controlled at 40 mL / min, and the oxygen flow rate was controlled at 10 mL / min; ③ The sample mass was 10~13 mg. The experimental results are shown in Table 1.

[0040] Table 1 Activation energy of Indonesian lignite before and after flame retardancy. As shown in Table 1, compared with the sample without flame retardant, the activation energy of the coal sample increased in all three stages after adding flame retardant, indicating that the flame retardant has a flame-retardant effect. In Example 1, the activation energy of the coal sample increased by 6.4, 26.2, and 16.9 kJ / mol in the three stages, respectively, with increases of 12.3%, 41.91%, and 18.86%, indicating the best flame-retardant effect. In Example 2, the mass of oxidized starch and polyvinyl alcohol was lower than in Example 1, resulting in a lower increase in coal activation energy in the three stages. This is because the amount of polyvinyl alcohol-grafted modified starch was less, reducing the oxygen-barrier effect of covering the coal pile. In Example 3, the mass of polyvinyl alcohol was greater than in Example 1, and the increase in coal activation energy in the three stages was similar to that of Example 1, but the increased reagents raised the cost.

[0041] In Comparative Example 1, corn starch was not oxidized, so the amylopectin in the starch could not be effectively dissolved, resulting in a smaller amount of polyvinyl alcohol-grafted modified starch. The degree of improvement in the activation energy of the coal sample in the three stages was lower than that in Example 1.

[0042] In Comparative Example 2, sodium dodecyl sulfonate was used as the surfactant. Due to the poor wetting effect of sodium dodecyl sulfonate, the thickness of the coating layer formed was thinner than that in Example 1, and the oxygen barrier and flame retardant effect decreased. Therefore, the degree of increase in coal sample activation energy in the three stages was lower than that in Example 1.

[0043] In Comparative Example 3, calcium chloride was used as the flame retardant, which achieved the flame retardant effect by absorbing moisture and lowering temperature. In contrast, the antioxidant used in Example 1 activated the active group by inertizing it and interrupting the chain reaction of the active group. Therefore, the flame retardant effect of calcium chloride was lower than that of the mixture of sodium alkyl naphthalene sulfonate condensate and sodium lignosulfonate. As a result, the degree of increase in the activation energy of the coal sample in the three stages was lower than that in Example 1.

[0044] In Comparative Example 4, the oxidized starch was not gelatinized, resulting in a decrease in the solubility of the starch and a smaller amount of polyvinyl alcohol-grafted modified starch. The degree of increase in the activation energy of the coal sample in the three stages was lower than that in Example 1.

[0045] II. Influence of Temperature at Crossroads Cross point temperature (CPT) is also an indicator of the ease of spontaneous combustion. Cross point temperature refers to the temperature at which the coal sample is heated by the furnace. When the coal sample is heated to a certain extent, its temperature may exceed the furnace temperature due to self-heating. The higher the cross point temperature, the more difficult it is to spontaneously combust.

[0046] Indonesian bituminous coal and Indonesian lignite were used. The cross-point temperatures of the raw coal samples and the samples with flame retardants added in Examples 1-3 and Comparative Examples 1-4, as well as the GE crusting agent, were measured. The addition amount of the two flame retardants was 10%. The results are shown in Table 2.

[0047] Table 2 Cross-point temperatures of coal samples before and after flame retardancy. As shown in Table 2, the cross-point temperature of Indonesian bituminous coal increased by 8.4°C with the addition of this patented flame retardant, while it increased by 5.6°C with the addition of GE crusting agent. This indicates that both flame retardants increased the cross-point temperature and had a flame-retardant effect, but the increase from this patented flame retardant was greater, indicating that its flame-retardant effect was superior to that of GE crusting agent. The cross-point temperature of Indonesian lignite increased by 17.6°C with the addition of this patented flame retardant, while it increased by 14.2°C with the addition of GE crusting agent. This indicates that the addition of both flame retardants had a better inhibitory effect on easily self-igniting coal, further proving that the flame retardant from this patented flame retardant had a better flame-retardant effect. In Example 2, the mass of oxidized starch and polyvinyl alcohol added was lower than in Example 1, resulting in a smaller amount of polyvinyl alcohol-grafted modified starch, a decreased oxygen-barrier effect on the coal pile, and a lower increase in cross-point temperature compared to Example 1. In Example 3, the mass of polyvinyl alcohol was greater than in Example 1, and the increase in cross-point temperature was similar to that of Example 1, but the increased amount of reagents increased the cost.

[0048] In Comparative Example 1, corn starch was not oxidized, so the amylopectin in the starch could not be effectively dissolved, resulting in a smaller amount of polyvinyl alcohol-grafted modified starch and a lower increase in cross-point temperature compared to Example 1.

[0049] In Comparative Example 2, sodium dodecyl sulfonate was used as the surfactant. Due to the poor wetting effect of sodium dodecyl sulfonate, the resulting coating thickness was thinner than in Example 1, leading to a decrease in oxygen barrier and flame retardant effect. Consequently, the increase in cross-point temperature was less significant than in Example 1. This demonstrates that the mixture of sodium alkyl naphthalene sulfonate condensate and sodium lignin sulfonate used in Example 1 effectively improves the wetting properties of the flame retardant, resulting in a significantly improved coating layer and thus enhanced flame retardant effect.

[0050] The inhibitor used in Comparative Example 3 is calcium chloride, which achieves flame retardancy by absorbing moisture and lowering temperature. However, the isoascorbic acid used in Example 1 inactivates the active group and interrupts the chain reaction of the active group. Therefore, the inhibitory effect of the inorganic inhibitor used in Comparative Example 3 is lower than that in Example 1. Consequently, the increase in the cross-point temperature in Example 5 is lower than that in Example 1.

[0051] The oxidized starch in Comparative Example 4 was not gelatinized, resulting in a decrease in the solubility of the starch, a smaller amount of polyvinyl alcohol grafted modified starch was synthesized, and a lower increase in the cross-point temperature compared to Example 1.

[0052] III. Security Testing The flame retardant of this patent and the GE crusting agent were tested for toxicity. The tests included mercury, cadmium, lead, chromium, arsenic, formaldehyde, and oral toxicity (LD50). The results are as follows: Table 3 Hazardous Factors Indicators of Flame Retardants and GE Carburizing Agents As shown in Table 3, all harmful factors of the flame retardant in Example 1 are within the standard requirements, while the oral toxicity of the crusting agent exceeds the standard requirements. At the same time, all harmful factor indicators of the flame retardant are lower than those of the GE crusting agent, demonstrating excellent environmental performance.

[0053] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A hydrogel composite flame retardant, characterized in that, The raw materials include the following parts by weight: 5-10 parts oxidized starch, 1-2 parts crosslinking agent, 0.4-0.6 parts surfactant, 2-4 parts antioxidant, 10-20 parts polyvinyl alcohol, and 0.1-0.2 parts initiator; wherein the antioxidant includes at least one of isoascorbic acid and phytic acid; The crosslinking agent is borax, and the surfactant is a mixture of sodium alkyl naphthalene sulfonate condensate and sodium lignin sulfonate in a mass ratio of (1.8~2.1):

1.

2. The hydrogel composite flame retardant as described in claim 1, characterized in that, The oxidized starch includes at least one of oxidized corn starch and oxidized cassava starch.

3. The hydrogel composite flame retardant as described in claim 1, characterized in that, The initiator is potassium persulfate.

4. The method for preparing the hydrogel composite flame retardant according to any one of claims 1-3, characterized in that, Includes the following steps: S1. Disperse oxidized starch with water to prepare an oxidized starch dispersion; S2. The oxidized starch dispersion is gelatinized to obtain an oxidized starch gelatinized solution; S3. Mix polyvinyl alcohol with water, heat and stir, add an initiator and continue stirring, then add oxidized starch gelatinized liquid to carry out graft copolymerization reaction, and then add a crosslinking agent to carry out complexation reaction to obtain polyvinyl alcohol graft modified starch gel. S4. Add antioxidants and surfactants to polyvinyl alcohol grafted modified starch gel and mix to obtain the hydrogel composite flame retardant.

5. The preparation method of the hydrogel composite flame retardant as described in claim 4, characterized in that, In step S2, the gelatinization process is as follows: alkali is added to the oxidized starch dispersion at a temperature of 20-30℃, and the mixture is stirred until homogeneous and reacted for 0.2-1 h; wherein the mass ratio of the oxidized starch to the alkali is (9-11):

1.

6. The preparation method of the hydrogel composite flame retardant as described in claim 4, characterized in that, In step S3, polyvinyl alcohol is mixed with water and then heated and stirred. The heating and stirring parameters are as follows: first, heating and stirring at 70-100 r / min and 90-100℃ to completely dissolve the polyvinyl alcohol; then cooling to 50-60℃, stirring at 200-400 r / min, adding the initiator and stirring for 20-40 min. The graft copolymerization reaction temperature is 20-35℃ and the time is 1-2 h; the complexation reaction temperature is 30-40℃ and the time is 20-40 min.

7. The preparation method of the hydrogel composite flame retardant as described in claim 4, characterized in that, In step S4, the mixing temperature of the antioxidant and surfactant added to the polyvinyl alcohol grafted modified starch gel is 45-55℃, and the mixing time is 20-40 min.

8. The application of the hydrogel composite flame retardant according to any one of claims 1 to 3 in the preparation of coal flame retardant products.

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