Flame-retardant polyurethane coating as well as preparation method and application thereof in field of unmanned aerial vehicles

Abstract

The invention discloses a flame-retardant polyurethane coating as well as a preparation method and application thereof in the field of unmanned aerial vehicles, and relates to the technical field of flame-retardant coatings. The flame-retardant polyurethane coating is prepared from the following raw materials in parts by weight: 25 to 30 parts of polyether polyol, 15 to 20 parts of isocyanate, 0.1 to 0.2 part of a catalyst, 4 to 6 parts of 2, 2-dimethylolpropionic acid, 1 to 1.5 parts of a cross-linking agent, 4 to 6 parts of a flame-retardant anti-aging agent, 0.3 to 0.5 part of a de-foaming agent, 0.5 to 0.8 part of a dispersing agent, 5 to 8 parts of silicon dioxide and 70 to 80 parts of de-ionized water. The flame-retardant polyurethane coating prepared by the invention has excellent tensile strength, ageing resistance and flame retardance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of flame-retardant coatings, in particular to a flame-retardant polyurethane coating, a preparation method thereof and application thereof in the field of unmanned aerial vehicles. BACKGROUND

[0002] With the rapid development of unmanned aerial vehicles in the fields of military reconnaissance, emergency rescue, logistics transportation, agricultural and forestry plant protection and industrial inspection, the service environment of unmanned aerial vehicles is becoming increasingly complex, and higher requirements are put forward for the surface protective coating of the unmanned aerial vehicles. Unmanned aerial vehicles are exposed to environmental conditions such as ultraviolet radiation, alternating temperature and humidity, rain erosion, sand abrasion and chemical medium erosion during flight, and also need to withstand take-off and landing impact, vibration load and local stress concentration, so the mechanical strength, weather resistance and flame-retardant properties of the coating material are strictly required. In recent years, polyurethane coatings have been rapidly developed due to their low volatile organic compound emissions, excellent physical properties and environmental protection characteristics. However, the traditional polyurethane resin still has problems such as insufficient mechanical properties, weather resistance and flame-retardant properties.

[0003] A Chinese patent application with the publication number CN112646475A discloses a preparation and application method of a flame-retardant wear-resistant low-VOC polyurethane coating. In a reaction container, polyester diol, isocyanate and dibutyl tin dilaurate are added and stirred at 75-90℃ to obtain a polyurethane prepolymer. Hydrophilic chain extender dimethylol propionic acid, nitrogen-phosphorus intumescent flame retardant and acetone solvent are added to the polyurethane prepolymer and stirred at 70-90℃ for 1-2h. Triethylamine and water are further added and emulsified for 20-60min. Subsequently, substance A and epoxy-terminated polyether silicone oil are added, the pH is adjusted to 6.5, and the mixture is stirred at 70-80℃ to obtain the flame-retardant wear-resistant low-VOC polyurethane coating. The obtained polyurethane film has a low flaming combustion time and good melt dripping resistance, but the mechanical properties are still insufficient. SUMMARY

[0004] In view of the deficiencies in the prior art, the purpose of the present application is to provide a flame-retardant polyurethane coating.

[0005] To achieve the above-mentioned purpose, the present application realizes the following technical solutions: A flame-retardant polyurethane coating comprises the following raw materials in parts by weight: polyether polyol 25-30 parts, isocyanate 15-20 parts, catalyst 0.1-0.2 parts, 2,2-dimethylol propionic acid 4-6 parts, crosslinking agent 1-1.5 parts, triethylamine 6-8 parts, flame-retardant antioxidant 4-6 parts, defoaming agent 0.3-0.5 parts, dispersing agent 0.5-0.8 parts, silicon dioxide 5-8 parts, deionized water 70-80 parts; The flame-retardant antioxidant is prepared by the following method: S1: reaction of oleylamine with 2-hydroxy-4-(methacryloyloxy)benzophenone to obtain intermediate 1, whose reaction structural formula is as follows:

[0006] S2: reaction of intermediate 1 with L-cysteine to obtain intermediate 2, whose reaction structural formula is as follows:

[0007] S3: reaction of intermediate 2 with 3,9-dichloro-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane 3,9-dioxide to obtain the flame-retardant antioxidant. Its reaction structural formula is as follows:

[0008] In step S1, the molar ratio of oleylamine to 2-hydroxy-4-(methacryloyloxy)benzophenone is (1.05-1.1):1.

[0009] In step S2, the molar ratio of intermediate 1 to L-cysteine is 1:(1.1-1.15).

[0010] In step S3, the molar ratio of intermediate 2 to 3,9-dichloro-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane 3,9-dioxide is (2.08-2.1):1.

[0011] The crosslinking agent is prepared by the following method: N1: reaction of N,N,N',N'-tetra(p-aminophenyl)-p-phenylenediamine with 6-maleimidyl hexanoic acid to obtain a tetraamide compound, whose reaction structural formula is as follows:

[0012] N2: reaction of the tetraamide compound with furfuryl alcohol to generate the crosslinking agent. Its reaction structural formula is as follows:

[0013] In step N1, the molar ratio of N,N,N',N'-tetra(p-aminophenyl)-p-phenylenediamine to 6-maleimidyl hexanoic acid is 1:4.1; in step N2, the molar ratio of the tetraamide compound to furfuryl alcohol is 1:4.2.

[0014] The isocyanate is one of diphenylmethane-4,4'-diisocyanate and 2,4-toluene diisocyanate; the catalyst is one of dibutyltin dilaurate and stannous octoate.

[0015] The defoaming agent is BYK-011; the dispersing agent is BYK-190.

[0016] A preparation method of a flame-retardant polyurethane coating, comprising the following steps: (1) take by weight parts: polyether polyol 25-30 parts, isocyanate 15-20 parts, catalyst 0.1-0.2 parts, 2,2-dimethylol propionic acid 4-6 parts, crosslinking agent 1-1.5 parts, triethylamine 6-8 parts, flame-retardant antioxidant 4-6 parts, defoaming agent 0.3-0.5 parts, dispersing agent 0.5-0.8 parts, silicon dioxide 5-8 parts, deionized water 70-80 parts; (2) stir and mix the polyether polyol, isocyanate and catalyst uniformly, heat and react, add 2,2-dimethylol propionic acid and react, then add the crosslinking agent and continue to react to obtain a polyurethane prepolymer; (3) stir and mix the deionized water, polyurethane prepolymer and triethylamine uniformly, and then add the flame-retardant antioxidant, defoaming agent, dispersing agent and silicon dioxide in sequence and stir and mix uniformly to obtain the flame-retardant polyurethane coating.

[0017] The application of a flame-retardant polyurethane coating in the field of unmanned aerial vehicles.

[0018] Due to the adoption of the above technical scheme, the application has the following beneficial effects: The flame-retardant polyurethane coating provided by the application has excellent tensile strength, anti-aging performance and flame-retardant performance. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 Nuclear magnetic resonance hydrogen spectrum of the flame-retardant antioxidant prepared in step S3 of Example 1; Figure 2 High-resolution mass spectrum of the flame-retardant antioxidant prepared in step S3 of Example 1; Figure 3 Nuclear magnetic resonance hydrogen spectrum of the crosslinking agent prepared in step N2 of Example 4; Figure 4 High-resolution mass spectrum of the crosslinking agent prepared in step N2 of Example 4. DETAILED DESCRIPTION

[0020] The application will be further described below in conjunction with examples, but the application is not limited to these examples.

[0021] Example 1: Preparation of a flame-retardant antioxidant S1: 300ml of anhydrous acetonitrile, 0.105mol of oleylamine and 0.1mol of 2-hydroxy-4-(methacryloyloxy)benzophenone were added into a reaction kettle, stirred and mixed uniformly, heated to 40℃, reacted for 6h, distilled at 40℃ under reduced pressure for 1h, recrystallized using a mixed solution of 180ml of acetone and n-hexane (the volume ratio of acetone to n-hexane was 1:9), filtered, and vacuum dried at 50℃ for 10h to obtain an intermediate 1; the nuclear magnetic hydrogen spectrum data are as follows:1 H NMR (400 MHz, Chloroform- d ) δ 11.74 (s, 1H), 7.79 -7.71 (m, 2H), 7.67 (d, J = 8.5 Hz, 1H), 7.56 - 7.45 (m, 3H), 7.05 (dd, J =8.5, 1.9 Hz, 1H), 6.84 (d, J = 1.9 Hz, 1H), 5.33 (t, J = 5.5 Hz, 2H), 4.60 (tt, J = 6.0, 4.8 Hz, 1H), 2.89 - 2.61 (m, 5H), 2.06 - 1.96 (m, 4H), 1.54 -1.43 (m, 2H), 1.38 - 1.22 (m, 22H), 1.08 (d, J = 7.3 Hz, 3H), 0.92 - 0.84 (m,3H); HRMS (m / z): 550.3824[M+H] + ; S2: Under nitrogen protection, 400 ml of anhydrous ethanol, 0.1 mol of intermediate 1, 0.11 mol of L-cysteine, and 3 g of photoinitiator 184 were added to a reaction vessel and stirred for 30 min. The mixture was then irradiated under 300 W UV light for 10 min at 25 °C with stirring, followed by vacuum distillation at 45 °C for 1 h. 300 ml of toluene was added and stirred until homogeneous. The mixture was washed with deionized water (3 × 100 ml), dried with 30 g of anhydrous magnesium sulfate, filtered, and then vacuum distilled at 70 °C for 2 h to obtain intermediate 2. Its 1H NMR spectrum data are as follows: 1 H NMR (400 MHz, Chloroform- d) δ 11.74 (s, 1H), 11.39 (s, 1H), 7.79 - 7.71 (m, 2H), 7.67 (d, J = 8.5 Hz, 1H), 7.58 - 7.45 (m, 3H), 7.05 (dd, J = 8.5, 1.9 Hz, 1H), 6.84 (d, J = 1.9 Hz, 1H), 4.60 (tt, J = 6.0, 4.8 Hz, 1H), 3.61 (q, J = 1.5 Hz, 3H), 3.03 - 2.61 (m, 8H), 1.61 - 1.43 (m, 6H), 1.40 - 1.22 (m, 24H), 1.08 (d, J = 7.3 Hz, 3H), 0.93 - 0.84 (m, 3H); HRMS (m / z): 671.4025 [M+H] + ; S3: 1000 ml of anhydrous acetonitrile, 0.208 mol of intermediate 2, 0.22 mol of triethylamine were added into the reaction kettle, stirred and mixed, 0.1 mol of 3,9-dichloro-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane 3,9-dioxide was added in batches (divided into 5 batches, each batch interval 10 min), heated to 70°C, reacted for 7h, cooled to room temperature, filtered, 45°C reduced pressure distillation for 1h, purified by column chromatography, eluent was a mixed solution of ethyl acetate and petroleum ether (volume ratio of ethyl acetate to petroleum ether was 2:3), 50°C reduced pressure distillation for 2h, to obtain the flame retardant antioxidant. Its nuclear magnetic resonance spectrum is shown in Figure 1 . 1 H NMR (400MHz, Chloroform- d) δ 11.74 (s, 2H), 11.45 (s, 2H), 7.78 - 7.71 (m, 4H), 7.67 (d, J = 8.5 Hz, 2H), 7.58 - 7.44 (m, 6H), 7.05 (dd, J = 8.5, 1.9 Hz, 2H), 6.84 (d, J = 1.9 Hz, 2H), 5.45 (d, J = 9.3 Hz, 2H), 4.60 (tt, J = 6.0, 4.8 Hz, 2H), 4.05 (s, 10H), 2.99 - 2.60 (m, 16H), 1.60 - 1.43 (m, 12H), 1.40 - 1.23 (m, 48H), 1.08 (d, J = 7.3 Hz, 6H), 0.93 - 0.84 (m, 6H); its high resolution mass spectrum is shown in Figure 2 Figure 1, HRMS (m / z): 1565.7669 [M+H] + .

[0022] Example 2: Preparation of flame-retardant antioxidant S1: 300 ml of anhydrous acetonitrile, 0.108 mol of oleylamine, 0.1 mol of 2-hydroxy-4-(methacryloyloxy)benzophenone were added into a reaction kettle, stirred and mixed, heated to 45°C, reacted for 5.5 h, distilled at 40°C under reduced pressure for 1 h, recrystallized using a mixed solution of 180 ml of acetone and n-hexane (volume ratio of acetone to n-hexane is 1:9), filtered, and dried at 50°C under vacuum for 10 h to obtain intermediate 1; S2: Under nitrogen protection, 400 ml of anhydrous ethanol, 0.1 mol of intermediate 1, 0.112 mol of L-cysteine and 3 g of photoinitiator 184 were added into a reaction kettle, stirred for 30 min, irradiated under 300 W ultraviolet light under stirring at 25°C for 15 min, distilled at 45°C under reduced pressure for 1 h, stirred and mixed with 300 ml of toluene, washed with deionized water (3 x 100 ml), dried with 30 g of anhydrous magnesium sulfate, filtered, distilled at 70°C under reduced pressure for 2 h to obtain intermediate 2; S3: 1000 ml of anhydrous acetonitrile, 0.209 mol of intermediate 2, 0.22 mol of triethylamine were added into the reaction kettle, stirred and mixed, 0.1 mol of 3,9-dichloro-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane 3,9-dioxide was added in batches (divided into 5 batches, each batch interval 10 min), heated to 75°C, reacted for 6h, cooled to room temperature, filtered, 45°C reduced pressure distillation for 1h, purified by column chromatography, eluent was a mixed solution of ethyl acetate and petroleum ether (volume ratio of ethyl acetate to petroleum ether was 2:3), 50°C reduced pressure distillation for 2h, to obtain the flame-retardant antioxidant.

[0023] Example 3 Preparation of flame-retardant antioxidant: S1: 300 ml of anhydrous acetonitrile, 0.11 mol of oleylamine, 0.1 mol of 2-hydroxy-4-(methacryloyloxy)benzophenone were added into the reaction kettle, stirred and mixed, heated to 50°C, reacted for 5h, 40°C reduced pressure distillation for 1h, recrystallized using a mixed solution of 180 ml of acetone and n-hexane (volume ratio of acetone to n-hexane was 1:9), filtered, 50°C vacuum dried for 10h, to obtain intermediate 1; S2: 400 ml of anhydrous ethanol, 0.1 mol of intermediate 1, 0.115 mol of L-cysteine and 3 g of photoinitiator 184 were added into the reaction kettle under nitrogen protection, stirred for 30 min, 25°C stirring condition, 300W ultraviolet light was used for light irradiation for 20 min, 45°C reduced pressure distillation for 1h, 300 ml of toluene was added and stirred and mixed, washed with deionized water (3x100 ml), dried with 30 g of anhydrous magnesium sulfate, filtered, 70°C reduced pressure distillation for 2h, to obtain intermediate 2; S3: 1000 ml of anhydrous acetonitrile, 0.21 mol of intermediate 2, 0.22 mol of triethylamine were added into the reaction kettle, stirred and mixed, 0.1 mol of 3,9-dichloro-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane 3,9-dioxide was added in batches (divided into 5 batches, each batch interval 10 min), heated to 80°C, reacted for 5h, cooled to room temperature, filtered, 45°C reduced pressure distillation for 1h, purified by column chromatography, eluent was a mixed solution of ethyl acetate and petroleum ether (volume ratio of ethyl acetate to petroleum ether was 2:3), 50°C reduced pressure distillation for 2h, to obtain the flame-retardant antioxidant.

[0024] Example 4 Preparation of crosslinking agent: N1: 200 ml of anhydrous acetonitrile, 0.041 mol of 6-maleimide hexanoic acid, 0.041 mol of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide, 0.004 mol of 1-hydroxybenzotriazole, 0.041 mol of 4-dimethylaminopyridine were added into a reaction kettle, stirred for 20 min, 0.01 mol of N,N,N',N'-tetra(p-aminophenyl) p-phenylenediamine was added, reacted at 25°C for 12 h, filtered, 250 ml of deionized water was added and stirred to precipitate, filtered, washed with a mixed solution of tetrahydrofuran and deionized water (volume ratio of tetrahydrofuran to deionized water was 1:9) (2 x 25 ml), vacuum dried at 60°C for 12 h, to obtain a tetraamide compound; its nuclear magnetic hydrogen spectrum data are as follows: 1 H NMR (400 MHz, DMSO- d 6) δ 9.72 (s, 4H), 7.57 - 7.49 (m, 8H),7.24 (s, 4H), 7.20 - 7.12 (m, 8H), 6.71 (s, 8H), 3.81 - 3.63 (m, 8H), 2.33(t, J = 8.3 Hz, 8H), 1.76 - 1.56 (m, 16H), 1.46 - 1.35 (m, 8H);HRMS (m / z):1245.5343[M+H] + ; N2: 150 ml of dichloromethane, 0.01 mol of tetraamide compound and 0.042 mol of furfuryl alcohol were added into a reaction kettle, stirred and mixed under nitrogen protection, refluxed for 24 h, cooled to room temperature, 300 ml of anhydrous ether was added and stirred to precipitate, filtered, washed with anhydrous ether (2 x 20 ml), vacuum dried at 40°C for 8 h, to obtain a crosslinking agent; its nuclear magnetic hydrogen spectrum is shown in Figure 3 , and the data are as follows: 1 H NMR (400 MHz, DMSO- d6) δ 9.72 (s, 4H), 7.58 - 7.49 (m, 8H), 7.24 (s,4H), 7.20 - 7.12 (m, 8H), 6.39 (ddt, J = 8.5, 1.7, 0.8 Hz, 4H), 6.28 (ddd, J= 8.4, 5.9, 1.7 Hz, 4H), 5.09 (tt, J = 5.8, 0.9 Hz, 4H), 4.44 (d, J = 12.3Hz, 4H), 4.02 (dd, J = 11.0, 6.2 Hz, 4H), 3.92 (dd, J = 11.0, 6.2 Hz, 4H),3.62 (dt, J = 12.4, 6.9 Hz, 4H), 3.52 (dt, J = 12.3, 6.9 Hz, 4H), 3.13 (td, J= 6.0, 1.7 Hz, 4H), 3.00 (dd, J = 6.3, 1.7 Hz, 4H), 2.33 (t, J = 8.3 Hz, 8H),1.73 - 1.53 (m, 16H), 1.49 - 1.33 (m, 8H); its high resolution mass spectrum is shown in Figure 4 HRMS (m / z): 1637.6815 [M+H] + .

[0025] Example 5 Preparation of flame-retardant polyurethane coating (1) Take: polyether polyol 25g, isocyanate (diphenylmethane-4,4'- diisocyanate) 15g, catalyst (dibutyltin dilaurate) 0.1g, 2,2-dimethylol propionic acid 4g, crosslinking agent (prepared in example 4) 1g, triethylamine 6g, flame-retardant antioxidant (prepared in example 1) 4g, defoaming agent (BYK-011) 0.3g, dispersant (BYK-190) 0.5g, silicon dioxide 5g, deionized water 70g; (2) Stir the polyether polyol, isocyanate and catalyst for 20 min, heat to 70°C and react for 3h, add 2,2-dimethylol propionic acid and react for 1h, add crosslinking agent and continue to react for 2h, cool to room temperature to obtain polyurethane prepolymer; (3) Stir the deionized water, polyurethane prepolymer and triethylamine at 500r / min for 20 min, add the flame-retardant antioxidant, defoaming agent, dispersant and silicon dioxide in turn, and stir at 800r / min for 30 min to obtain the flame-retardant polyurethane coating.

[0026] Example 6 Preparation of flame-retardant polyurethane coating (1) Take: polyether polyol 28 g, isocyanate (2,4-toluene diisocyanate) 19 g, catalyst (stannous octoate) 0.16 g, 2,2-dimethylol propionic acid 5 g, crosslinking agent (prepared in example 4) 1.2 g, triethylamine 7 g, flame-retardant antioxidant (prepared in example 2) 5 g, defoaming agent (BYK-011) 0.4 g, dispersing agent (BYK-190) 0.6 g, silicon dioxide 7 g, deionized water 75 g; (2) Stir the polyether polyol, isocyanate, and catalyst for 20 min, heat to 75°C and react for 2.5 h, add 2,2-dimethylol propionic acid and react for 1 h, add the crosslinking agent and continue to react for 2 h, and cool to room temperature to obtain a polyurethane prepolymer; (3) Stir the deionized water, polyurethane prepolymer, and triethylamine at 500 r / min for 20 min, add the flame-retardant antioxidant, defoaming agent, dispersing agent, and silicon dioxide in sequence, and stir at 800 r / min for 30 min to obtain a flame-retardant polyurethane coating.

[0027] Example 7 Preparation of a flame-retardant polyurethane coating (1) Take: polyether polyol 30 g, isocyanate (2,4-toluene diisocyanate) 20 g, catalyst (stannous octoate) 0.2 g, 2,2-dimethylol propionic acid 6 g, crosslinking agent (prepared in example 4) 1.5 g, triethylamine 8 g, flame-retardant antioxidant (prepared in example 3) 6 g, defoaming agent (BYK-011) 0.5 g, dispersing agent (BYK-190) 0.8 g, silicon dioxide 8 g, deionized water 80 g; (2) Stir the polyether polyol, isocyanate, and catalyst for 20 min, heat to 80°C and react for 2 h, add 2,2-dimethylol propionic acid and react for 1 h, add the crosslinking agent and continue to react for 2 h, and cool to room temperature to obtain a polyurethane prepolymer; (3) Stir the deionized water, polyurethane prepolymer, and triethylamine at 500 r / min for 20 min, add the flame-retardant antioxidant, defoaming agent, dispersing agent, and silicon dioxide in sequence, and stir at 800 r / min for 30 min to obtain a flame-retardant polyurethane coating.

[0028] Comparative Example 1 The raw material composition and preparation method of the flame-retardant polyurethane coating are basically the same as in example 6, except that the flame-retardant antioxidant is replaced with an equal weight of a flame-retardant antioxidant prepared by the following method: The preparation method of the flame-retardant antioxidant is basically the same as in example 2, except that 2-hydroxy-4-(methacryloyloxy) benzophenone in step S1 is replaced with an equal molar amount of 4-methacryloyloxy benzophenone.

[0029] Comparative Example 2 The raw material composition and preparation method of the flame-retardant polyurethane coating are basically the same as those in Example 6, except that the flame-retardant antioxidant is replaced with an equal weight of the flame-retardant antioxidant prepared by the following method: The preparation method of the flame retardant and antioxidant is basically the same as that in Example 2, except that 2-hydroxy-4-(methacryloyloxy)benzophenone in step S1 is replaced with 0.21 mol, the reaction temperature is 60℃, and the reaction time is 15 h.

[0030] The structural formula of the flame retardant and antioxidant is as follows:

[0031] Comparative Example 3 The raw material composition and preparation method of the flame-retardant polyurethane coating are basically the same as those in Example 6, except that the flame-retardant antioxidant is replaced with an equal weight of the flame-retardant antioxidant prepared by the following method: The preparation method of the flame retardant and anti-aging agent is basically the same as that in Example 2. The difference is that the 3,9-dichloro-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane 3,9-dioxide in step S3 is replaced with 0.2 mol of 5,5-dimethyl-2-chloro-1,3,2-dioxaphosphacaprolactone phosphate.

[0032] Comparative Example 4 The raw material composition and preparation method of the flame-retardant polyurethane coating are basically the same as those in Example 6, except that the crosslinking agent is replaced with an equal weight of a crosslinking agent prepared by the following method: The preparation method of the crosslinking agent is basically the same as that in Example 4, except that N,N,N',N'-tetra(p-aminophenyl)p-phenylenediamine in step N1 is replaced with 0.02 mol of bis(4-aminophenyl)aniline (CAS: 4117-90-2); and furfuryl alcohol in step N2 is replaced with 0.021 mol.

[0033] Comparative Example 5 The raw material composition and preparation method of the flame-retardant polyurethane coating are basically the same as those in Example 6, except that the crosslinking agent is replaced with an equal weight of a crosslinking agent prepared by the following method: The preparation method of the crosslinking agent is basically the same as that in Example 4, except that 6-maleimide hexanoic acid in step N1 is replaced with an equimolar amount of 11-maleamido undecanoic acid.

[0034] Comparative Example 6 The raw material composition and preparation method of the flame-retardant polyurethane coating are basically the same as those in Example 6, except that the crosslinking agent is replaced with an equal weight of a crosslinking agent prepared by the following method: N1: 200 ml of anhydrous acetonitrile, 0.041 mol of 6-maleimide hexanoic acid, 0.041 mol of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide, 0.004 mol of 1-hydroxybenzotriazole, and 0.041 mol of 4-dimethylaminopyridine were added to a reaction vessel and stirred for 20 min. Then, 0.01 mol of N,N,N',N'-tetra(p-aminophenyl)p-phenylenediamine was added, and the mixture was reacted at 25 °C for 12 h. After filtration, 250 ml of deionized water was added and stirred to precipitate the precipitate. The precipitate was then filtered and washed with a mixed solution of tetrahydrofuran and deionized water (volume ratio of tetrahydrofuran to deionized water was 1:9) (2 × 25 ml). The mixture was then dried under vacuum at 60 °C for 12 h to obtain the tetraamide compound. N2: Add 150 ml of anhydrous acetonitrile, 0.01 mol of tetraamide compound and 0.042 mol of 3-aminopropanol to a reaction vessel, stir and mix well, react at 45 °C for 6 h, cool to room temperature, add 300 ml of anhydrous diethyl ether and stir to precipitate, filter, wash with anhydrous diethyl ether (2 × 20 ml), and vacuum dry at 40 °C for 8 h to obtain the crosslinking agent.

[0035] The polyether polyol used in the embodiments and comparative examples of this application is CHE-303, produced by Changhua Chemical Technology Co., Ltd.; the silica is HDS100, produced by Shandong Hongruitong New Material Technology Co., Ltd.

[0036] The flame-retardant polyurethane coatings prepared in Examples 5-7 and Comparative Examples 1-6 of this application were subjected to anti-aging and flame-retardant performance tests.

[0037] Sample preparation: The flame-retardant polyurethane coating was poured into a polytetrafluoroethylene mold (120mm×30mm×1.5mm) and cured for 7 days under standard curing conditions (temperature 23℃, humidity 50%). The coated sample was then demolded.

[0038] Anti-aging performance test: The samples were placed in a QUV accelerated aging test chamber for aging tests, with an ultraviolet wavelength of 340nm and an irradiance of 0.76W / m. 2 The sample was aged at 60℃ for 1200 hours, and the tensile properties of the sample before and after aging were tested.

[0039] Tensile property test: The specimen was cut into dumbbell shape 1 and tested according to GB / T 19250-2013 standard.

[0040] Flame retardant performance test: The coated sample is vertically clamped in a glass jar containing nitrogen and oxygen in a certain ratio. The upper end of the sample is ignited with a propane igniter. The sample burns steadily from top to bottom for 3 minutes. The minimum volume fraction of oxygen required to support the sample burning to 50 mm is determined (if the burning length is less than 50 mm within 30 minutes, the oxygen concentration is increased; if the burning length exceeds 50 mm, the oxygen concentration is decreased). This is the limiting oxygen index of the sample.

[0041] Table 1 Performance Test Table

[0042] As can be seen from the data in Examples 5-7 in Table 1, the flame-retardant polyurethane coating prepared by the present invention has excellent tensile strength, anti-aging properties and flame-retardant properties.

[0043] The flame-retardant and antioxidant agent added to the flame-retardant polyurethane coating prepared in this invention is based on a symmetrical phosphoramide structure, and incorporates benzophenone, ortho- and ortho-phenolic hydroxyl groups, thioethers, carboxyl groups, and secondary amines. The carboxyl groups and secondary amines in the flame-retardant and antioxidant agent can strongly interact with isocyanate or hydrogen bonding sites in the polyurethane system, allowing the flame-retardant and antioxidant agent to embed into the polyurethane network structure, enhancing intermolecular cohesion, effectively reducing the migration and volatilization of small-molecule flame-retardant and antioxidant agents, and improving the long-term flame-retardant and aging-resistant stability of the coating. Regarding flame retardancy: the phosphoramide structure in the flame-retardant and antioxidant agent molecule decomposes upon heating, releasing inert gases such as NH3 to dilute oxygen, while simultaneously generating phosphoric acid or polyphosphoric acid. This catalyzes the dehydration and carbonization of the material surface to form a dense, heat-insulating carbon layer, and interrupts the combustion reaction in the gas phase by capturing combustion free radicals, thereby improving the flame-retardant performance of the coating. In terms of anti-aging: the intramolecular hydrogen bond formed by the carbonyl group and the ortho-phenolic hydroxyl group in the flame retardant and anti-aging agent molecule undergoes excited-state intramolecular proton transfer after absorbing ultraviolet radiation, rapidly dissipating ultraviolet energy as heat, thereby effectively inhibiting the generation of polymer photo-oxidative free radicals; the thioether can effectively delay the thermo-oxidative aging of the material by decomposing the peroxides generated during polymer aging and blocking the propagation stage of the free radical chain oxidation reaction. The symmetrical phosphoramide core enables the carbonyl group, phenolic hydroxyl group, thioether, carboxyl group and secondary amine to present an ordered spatial distribution, forming a multiple synergistic effect of flame retardancy, anti-aging and interface enhancement, improving the tensile properties, anti-aging properties and flame retardant properties of the coating.

[0044] The flame retardant and antioxidant added in Comparative Example 2 has a larger molecular weight and more complex structure, making it more prone to phase separation in the polyurethane system. This reduces its effective volume fraction in the material, hindering the full realization of the synergistic effect of anti-aging and flame retardancy. The flame retardant and antioxidant used in Comparative Example 3 has a smaller molecular weight and poorer migration resistance, affecting its anti-aging performance.

[0045] The crosslinking agent added to the flame-retardant polyurethane coating prepared by this invention has a four-arm structure, and simultaneously introduces a rigid benzene ring, alkyl chain, and a dynamic six-membered ring and hydroxyl group generated by the Diels-Alder reaction. The hydroxyl group in the crosslinking agent molecule reacts with the unreacted isocyanate group in the polyurethane prepolymer. Its four-arm structure can increase the crosslinking density, construct a dense three-dimensional crosslinking network structure in the system, enhance the overall cohesion of the coating, and thus improve the mechanical properties of the coating. The benzene ring provides a rigid skeleton, restricts the free rotation of chain segments, increases local hard domains, and effectively improves impact resistance. The flexible alkyl chain increases the degree of freedom of chain segments, alleviates the stress concentration of the crosslinking network, and improves the mechanical properties of the coating. The Diels-Alder six-membered ring, as a dynamic reversible bond, can partially break and recombine under stress or heat conditions, absorb and alleviate stress concentration, and thus endow the coating with good mechanical properties. The four-arm topology of the crosslinking agent provides efficient crosslinking density, the benzene ring provides rigid support, the alkyl chain provides flexible transition, and the Diels-Alder dynamic six-membered ring endows the network with reversible adjustment capability, forming a composite crosslinking network of "static strong crosslinking + dynamic adjustable crosslinking" with the polyurethane matrix, thereby improving the mechanical properties of the coating. In Comparative Example 5, the alkyl chain in the flame retardant was too long, which led to a reduction in intermolecular forces, thus reducing the tensile strength of the coating.

[0046] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. However, any modifications, alterations, and variations made by those skilled in the art without departing from the scope of the present invention based on the disclosed technical content are equivalent embodiments of the present invention. Furthermore, any modifications, alterations, and variations made to the above embodiments based on the essential technology of the present invention are still within the protection scope of the present invention.

Claims

1. A flame-retardant polyurethane coating, characterized in that, The ingredients include the following parts by weight: 25-30 parts polyether polyol, 15-20 parts isocyanate, 0.1-0.2 parts catalyst, 4-6 parts 2,2-dimethylolpropionic acid, 1-1.5 parts crosslinking agent, 6-8 parts triethylamine, 4-6 parts flame retardant and antioxidant, 0.3-0.5 parts defoamer, 0.5-0.8 parts dispersant, 5-8 parts silica, and 70-80 parts deionized water; The flame retardant and antioxidant is prepared by the following method: S1: The reaction of oleylamine with 2-hydroxy-4-(methacryloyloxy)benzophenone yields intermediate 1. S2: Intermediate 1 reacts with L-cysteine ​​to give intermediate 2. S3: Intermediate 2 reacts with 3,9-dichloro-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane 3,9-dioxide to obtain a flame retardant and antioxidant.

2. The flame-retardant polyurethane coating according to claim 1, characterized in that, In step S1, the molar ratio of oleylamine to 2-hydroxy-4-(methacryloyloxy)benzophenone is (1.05-1.1):

1.

3. The flame-retardant polyurethane coating according to claim 1, characterized in that, In step S2, the molar ratio of intermediate 1 to L-cysteine ​​is 1:(1.1-1.15).

4. The flame-retardant polyurethane coating according to claim 1, characterized in that, In step S3, the molar ratio of intermediate 2 to 3,9-dichloro-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5.5]undecane 3,9-dioxide is (2.08-2.1):

1.

5. The flame-retardant polyurethane coating according to claim 1, characterized in that, The crosslinking agent is prepared by the following method: N1: N,N,N',N'-Tetra(p-aminophenyl)p-phenylenediamine reacts with 6-maleimide hexanoic acid to give a tetraamide compound. N2: Tetraamide compounds react with furfuryl alcohol to form crosslinking agents.

6. The flame-retardant polyurethane coating according to claim 5, characterized in that, In step N1, the molar ratio of N,N,N',N'-tetra(p-aminophenyl)p-phenylenediamine to 6-maleimide hexanoic acid is 1:4.1; in step N2, the molar ratio of the tetraamide compound to furfuryl alcohol is 1:4.

2.

7. The flame-retardant polyurethane coating according to claim 1, characterized in that, The isocyanate is one of diphenylmethane-4,4'-diisocyanate and 2,4-toluene diisocyanate; the catalyst is one of dibutyltin dilaurate and stannous octoate.

8. The flame-retardant polyurethane coating according to claim 1, characterized in that, The defoamer is BYK-011; the dispersant is BYK-190.

9. A method for preparing a flame-retardant polyurethane coating according to any one of claims 1-8, characterized in that, Includes the following steps: (1) Weigh out the following by weight: 25-30 parts of polyether polyol, 15-20 parts of isocyanate, 0.1-0.2 parts of catalyst, 4-6 parts of 2,2-dimethylolpropionic acid, 1-1.5 parts of crosslinking agent, 6-8 parts of triethylamine, 4-6 parts of flame retardant and antioxidant, 0.3-0.5 parts of defoamer, 0.5-0.8 parts of dispersant, 5-8 parts of silica, and 70-80 parts of deionized water; (2) Stir and mix the polyether polyol, isocyanate and catalyst, heat and react, add 2,2-dimethylolpropionic acid and react, then add crosslinking agent and continue the reaction to obtain polyurethane prepolymer; (3) Stir deionized water, polyurethane prepolymer and triethylamine together, then add flame retardant and anti-aging agent, defoamer, dispersant and silica in sequence and stir to obtain flame retardant polyurethane coating.

10. The application of the flame-retardant polyurethane coating according to any one of claims 1-8 in the field of unmanned aerial vehicles (UAVs).

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