Preparation method and application of mold-proof and flame-retardant composite coating for bamboo material

Abstract

This invention discloses a method for preparing and applying a mildew-proof and flame-retardant composite coating for bamboo. In the preparation method, the quaternary composite core material solution uses phytic acid–Fe²⁺. + The quaternary synergistic core material system composed of tannic acid–PG-CS organically integrates natural flame retardant components, metal ion synergists, plant polyphenol antifungal agents, and polymeric antibacterial carriers, not only through Fe² + Complexing with phytic acid promotes high-temperature char formation and improves flame retardant efficiency; Fe²⁺ is also utilized. + The oxidative stress effect and the antibacterial effect of tannic acid / PG-CS achieve long-lasting broad-spectrum antifungal properties; at the same time, by precisely controlling the reaction pH of the core material, Fe²⁺ is effectively inhibited. + Oxidative deactivation ensures functional stability. The prepared anti-mildew and flame-retardant microcapsules have a tight core wall and good dispersibility. When combined with waterborne polyurethane, they can improve the flame-retardant properties of the composite material while maintaining good mechanical properties.

Description

Technical Field

[0001] This invention relates to the field of anti-mildew and flame-retardant materials technology, and in particular to a method for preparing and applying an anti-mildew and flame-retardant composite coating for bamboo. Background Technology

[0002] my country boasts the world's richest bamboo forest resources, with 7.01 million hectares of bamboo forests. Of these, over 60% are suitable for industrial processing, making bamboo a widely used and highly sought-after material. However, bamboo is primarily composed of cellulose, hemicellulose, and lignin, all flammable organic compounds, making it highly flammable. Furthermore, bamboo is rich in nutrients and its fibers are relatively porous, making it highly susceptible to mold growth. Therefore, developing a coating that combines flame retardant and mold-resistant properties is essential.

[0003] With increasing public awareness of safety and increasingly stringent environmental regulations, traditional flame-retardant materials cannot meet practical application needs due to issues such as the release of toxic and harmful substances and environmental pollution. Waterborne polyurethane (WPU), as a green and environmentally friendly polymer material, possesses excellent flexibility, adhesion, and weather resistance, and is widely used in coatings, adhesives, and other fields. However, WPU itself is flammable, and combustion produces a large amount of smoke and toxic gases, limiting its application in construction, electronics, transportation, and other fields. To improve the flame-retardant properties of WPU, the commonly used method is to add flame retardants, such as inorganic flame retardants, organophosphorus flame retardants, and nitrogen-based flame retardants. However, inorganic flame retardants require large amounts to achieve the ideal flame-retardant effect, which can easily lead to a decrease in the mechanical properties of WPU composite materials; organic flame retardants, although added in small amounts, suffer from poor compatibility, easy migration, and unstable long-term flame-retardant effects.

[0004] Microencapsulation technology offers a novel approach to addressing these issues: encapsulating antifungal agents and flame retardants within microcapsules reduces direct contact between the flame retardant and the WPU matrix, minimizing its impact on the matrix's mechanical properties, while simultaneously inhibiting the migration and volatilization of the flame retardant. However, some existing flame-retardant microcapsule preparation processes utilize aldehyde-containing monomers as crosslinking agents, posing environmental risks. Furthermore, the bonding stability and dispersibility between the microcapsule wall material and core material are poor, making it impossible to simultaneously achieve antifungal, flame-retardant, and mechanical properties when combined with WPU.

[0005] Therefore, developing a green, environmentally friendly, and highly dispersible anti-mildew and flame-retardant microcapsule is of great significance for promoting the application of WPU materials in the field of anti-mildew and flame retardant bamboo materials. Summary of the Invention

[0006] The technical problem this invention aims to solve is to overcome the shortcomings of existing technologies and provide a method for preparing anti-mildew and flame-retardant microcapsules for bamboo and their composite with waterborne polyurethane. This method does not require the use of aldehydes and is environmentally friendly. The prepared anti-mildew and flame-retardant microcapsules have a tightly bonded core wall and good dispersibility. When combined with WPU, they can improve the flame-retardant properties of the composite material while maintaining good mechanical properties.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: A method for preparing a mildew-resistant and flame-retardant composite coating for bamboo includes the following steps: A1. The silanized modified tannic acid solution, phytic acid and propyl gallate-chitosan graft copolymer were stirred and mixed, the pH value was adjusted to 5.5-6.0, ferrous sulfate heptahydrate was added and stirred to obtain a quaternary composite core material solution. A2. Using a formaldehyde-free silane-modified polyurethane prepolymer with an isocyanate group content of 8-10% as the wall material, the wall material is added to a quaternary composite core material solution for a coating reaction. After solid-liquid separation, a precipitate is obtained. The precipitate is washed and dried to obtain microcapsule powder. A3. Using microcapsule powder as a crosslinking agent, the crosslinking agent and water-based polyurethane emulsion are mixed and reacted, and after emulsification, a mildew-proof and flame-retardant composite coating for bamboo is obtained.

[0008] The stirring reaction in this application uses ferrous sulfate heptahydrate; ferrous chloride cannot be added because it introduces chloride ions, affecting the purity and safety of the final product. Ferrous sulfate heptahydrate has good compatibility with the system, is easily hydrolyzed and oxidized, and sulfate ions can be removed by washing, resulting in high safety.

[0009] As a further improvement to the above technical solution: The silanized modified tannic acid solution was prepared using the following steps: The silane coupling agent KH-560 was dissolved in an aqueous ethanol solution, and the pH was adjusted to 4.0–4.5 to obtain an aqueous solution of KH-560. KH-560 aqueous solution was added dropwise to tannic acid aqueous solution, and the pH value was adjusted to 4.0-4.5 to obtain silanized modified tannic acid solution.

[0010] The propyl gallate-chitosan graft copolymer was prepared using the following steps: Chitosan was dissolved in an aqueous acetic acid solution, and the pH was adjusted to 8-9 to obtain a chitosan aqueous solution. Epichlorohydrin and chitosan aqueous solution were mixed and stirred at 50-60°C to generate an intermediate solution. Propyl gallate and intermediate solution were mixed and stirred, purified, and dried to obtain propyl gallate-chitosan graft copolymer.

[0011] In the above steps, the reaction is carried out with stirring at a temperature of 50-60℃. Excessive temperature may cause over-hydrolysis of KH-560, reducing grafting efficiency. Insufficient temperature results in a slow reaction rate and incomplete reaction.

[0012] The mass ratio of phytic acid to ferrous sulfate is 5-10:0.5-1, which can significantly enhance the flame retardant-char formation synergistic effect of the system and promote the formation of a dense and continuous insulating char layer when the material is heated.

[0013] The formaldehyde-free silane-modified polyurethane prepolymer was prepared using the following steps: Using isocyanate and polypropylene glycol as reactive monomers, the reactive monomers, crosslinking agents and coupling agents are mixed and stirred at 75-80°C to obtain a prepolymer by controlling the isocyanate group content to be 8-10%. The prepolymer was diluted with ethyl acetate to obtain an aldehyde-free silane-modified polyurethane prepolymer.

[0014] In the above steps, isocyanate is used as a reactant monomer to construct the main chain structure; polypropylene glycol is used as a reactant monomer to improve the flexibility and elasticity of the prepolymer; trimethylolpropane is used as a crosslinking agent to increase the crosslinking density; and γ-aminopropyltriethoxysilane is used as a coupling agent to achieve silane modification. The stirring reaction temperature is 75–80°C. If the temperature is too low, the prepolymer will not cure completely, and γ-... Aminopropyltriethoxysilane will not undergo premature hydrolysis and condensation at 75°C, thus avoiding thickening or gelation of the system. At temperatures above 80°C, isocyanates may self-polymerize, affecting the reaction results.

[0015] The crosslinking agent is trimethylolpropane, and the coupling agent is γ-aminopropyltriethoxysilane. The mass ratio of isocyanate, polypropylene glycol, trimethylolpropane, and γ-aminopropyltriethoxysilane is 10–13:18–22:0.4–1.0:0.5–1.5. Trimethylolpropane, as a crosslinking agent, is present in a small amount, while polypropylene glycol, as a reactant monomer, constitutes the largest proportion.

[0016] Step A2 includes the following steps: Add Tween 80 to the quaternary composite core material solution, stir to dissolve, and form an emulsion; Formaldehyde-free silane-modified polyurethane prepolymer was dropped into the emulsion, and the mixture was stirred to carry out the coating reaction to obtain a suspension. The suspension is subjected to solid-liquid separation to obtain a precipitate; The precipitate was washed with an organic solvent and then with deionized water. After drying, microcapsule powder was obtained.

[0017] Tween 80 can promote the uniform dispersion of fillers in solvents, prevent agglomeration and clumping, avoid stratification and sedimentation, and at the same time improve the compatibility between fillers and solvents, ensuring the uniformity and stability of the system.

[0018] Step A3 includes the following steps: Using polypropylene glycol and toluene diisocyanate as the basic reactants, a hydrophilic chain extender is added, and a catalyst is added dropwise. The reaction is carried out at 73–77°C under a protective atmosphere to form an isocyanate-terminated prepolymer. The temperature is then lowered to 55–60°C, and a chain extender is added again to carry out a chain extension reaction. A solvent is added to reduce the viscosity of the system to 600–800 mPa. s, to obtain a mixed system; Using microcapsule powder and trimethylolpropane as crosslinking agents, the crosslinking agent was added to the mixture to carry out the crosslinking reaction. The temperature was lowered to 30-40℃, and an alkaline catalyst was added to carry out the neutralization reaction to obtain the mixture. The mixture was emulsified with deionized water at 1500±100 rpm, acetone was removed by rotary evaporation, and defoamer was added to obtain a mildew-proof and flame-retardant composite coating for bamboo.

[0019] The hydrophilic chain extender is dimethylolpropionic acid, which enables the polymer to be dispersed in water and thus achieve chain growth. The catalyst is dibutyltin dilaurate, whose main function is to catalyze the polycondensation reaction between isocyanate and hydroxyl groups in the synthesis of polyurethane, thereby accelerating the reaction rate, lowering the activation energy, and promoting polymer chain growth.

[0020] A catalytic reaction temperature between 73 and 77°C is conducive to a stable and rapid reaction and suppresses side reactions.

[0021] The chain extender is 1,4-butanediol. The reaction temperature for the chain extension reaction is between 55 and 60°C. If the temperature is too high, side reactions will increase, the color will darken, and the system will become unstable.

[0022] Acetone is used as the solvent to disperse the raw materials, reduce the viscosity of the system, and dilute the reaction system to ensure more uniform mixing. The viscosity is controlled between 600 and 800 mPa. s, the entire system is easy to flow and can be coated normally.

[0023] During the cross-linking reaction, trimethylolpropane is the main cross-linking agent, which chemically cross-links with the film-forming substance, improving the system's water resistance and other properties. The microcapsule powder is encapsulated at room temperature and released after the system is heated to stabilize. Using trimethylolpropane and microcapsule powder together is beneficial to both the stability of the system and the degree of cross-linking, thus improving water resistance and other properties.

[0024] The alkaline catalyst is triethylamine, which neutralizes the small molecule acid generated in the reaction, preventing the system from becoming too acidic and causing side reactions and decomposition of raw materials, while stabilizing the system and improving its purity.

[0025] The mass ratio of microcapsule powder, trimethylolpropane, and the mixed system is 1.5–4.5:10:32–35.

[0026] The application of a mildew-resistant and flame-retardant composite coating for bamboo in the application of a mildew-resistant and flame-retardant composite coating for bamboo, wherein the mildew-resistant and flame-retardant composite coating for bamboo is prepared by the aforementioned preparation method. Compared with the prior art, the beneficial effects of the present invention are as follows: The method for preparing the anti-mildew and flame-retardant composite coating for bamboo of the present invention uses a quaternary composite core material solution with phytic acid–Fe²⁺. + The quaternary synergistic core material system composed of tannic acid–PG-CS organically integrates natural flame retardant components, metal ion synergists, plant polyphenol antifungal agents, and polymeric antibacterial carriers, not only through Fe² + Complexing with phytic acid promotes high-temperature char formation and improves flame retardant efficiency; Fe²⁺ is also utilized. + The oxidative stress effect and the antibacterial effect of tannic acid / PG-CS achieve long-lasting broad-spectrum antifungal properties; at the same time, by precisely controlling the core material reaction pH (5.5–6.0), Fe²⁺ is effectively inhibited. + Oxidative deactivation ensures functional stability. The quaternary composite core material solution constructs a thermally stable three-dimensional network structure through coordination complexation and multiple hydrogen bonding between ferrous ions, phytic acid, and tannic acid. The prepared anti-mildew and flame-retardant microcapsule core wall is tightly bonded and has good dispersibility. When combined with waterborne polyurethane, it can improve the flame-retardant performance of the composite material while maintaining good mechanical properties. It does not require the use of aldehydes, and the process is green and environmentally friendly. It solves the problems of easy migration, single function, poor compatibility, and environmental pollution of traditional flame-retardant and anti-mildew agents. It can be widely used in the functional treatment of bamboo surface and expand its application in high fire-resistant and mildew-resistant scenarios such as construction and home furnishing. Detailed Implementation

[0027] The present invention will be further described in detail below. Unless otherwise specified, the instruments or materials used in the present invention are commercially available.

[0028] Example 1: The preparation method of the anti-mildew and flame-retardant composite coating for bamboo in this embodiment includes the following steps: S1. Weigh 3.0g of KH-560 (γ-glycidoxypropyltrimethoxysilane, CAS: 2530-83-8) and measure 10ml of ethanol and 5ml of deionized water. Add all three to a beaker and place it on a magnetic stirrer. Adjust the pH of the system to the range of 4.0-4.5 by slowly adding glacial acetic acid to obtain KH-560 hydrolysate.

[0029] S2. Transfer the beaker containing 10g of tannic acid and 60ml of deionized water to the center of a magnetic stirrer and stir until the tannic acid is completely dissolved. Add all of the KH-560 hydrolysate dropwise. After the addition is complete, adjust the pH of the solution to the range of 4.0-4.5 with glacial acetic acid. Place the beaker back in the water bath and let it stand to allow the temperature to equalize, thus obtaining the modified tannic acid solution.

[0030] In steps S1 and S2, tannic acid is silanized and modified by hydrolysis and activation with silane coupling agent KH-560 to construct a precursor containing active sites.

[0031] In this invention, the silane coupling agent selected is KH-560 (γ-glycidoxypropyltrimethoxysilane, CAS: 2530-83-8). The main purposes are: 1) to introduce organosilicon into the system, increasing the flame retardant properties of the material; 2) to introduce active hydroxyl groups, forming microcapsule wall materials through surface polymerization; 3) to allow the epoxy groups on KH-560 to react with tannic acid, increasing the water solubility of tannic acid; and 4) to improve the crosslinking degree of the core material, effectively preventing the migration and leakage of flame-retardant / mildew-resistant components in the core material. After hydrolysis, KH-560 is grafted onto tannic acid, introducing silanol active sites into the wall material precursor, significantly enhancing the core-wall interface bonding strength and laying a good foundation for the subsequent preparation of microcapsule powder. The main purpose of silane modification is to introduce more silicon to further enhance the flame retardancy and surface hydrophobicity of the material.

[0032] S3. Dissolve 2.0 g of chitosan in 50 mL of 1% (v / v, volume fraction) acetic acid aqueous solution. After complete dissolution, slowly adjust the pH of the system to 8-9 with dilute NaOH solution to deprotonate the amino group of chitosan and activate the nucleophilicity of the phenolic hydroxyl group. Then add 0.3 g of epichlorohydrin (ECH) and stir at 55 °C for 1-2 h to react with chitosan to generate an epoxy-containing intermediate. Next, add 0.5 g of propyl gallate and continue to react at the same temperature for 2-4 h to promote nucleophilic ring-opening of the phenolic hydroxyl group of PG to the epoxy group, achieving effective grafting. After the reaction is complete, dialysis is performed to purify the product to remove unreacted small molecules. Finally, freeze-dry to obtain propyl gallate-chitosan graft copolymer (PG-grafted chitosan, PG-CS) powder. PG-CS provides broad-spectrum antibacterial and bacteriostatic effects and solves compatibility issues. S4. Place the modified tannic acid solution in a water bath, add 6g of phytic acid and 0.6g of PG-CS powder, and stir until completely dissolved. Add K2CO3 powder in small amounts several times using a dropper until the pH of the system reaches 5.5-6.0. Weigh 0.8g of FeSO4·7H2O, add it to the solution, and react for 1 hour to obtain the quaternary composite core material solution.

[0033] Steps S3 and S4, using phytic acid and Fe²⁺ +Modified tannic acid solution and propyl gallate-chitosan graft copolymer (PG-CS) are assembled to form a quaternary synergistic core material, in which Fe²⁺ + It has the dual function of promoting char formation and flame retardancy, and catalyzing the generation of active oxygen to inhibit mold growth.

[0034] Ferrous ions in FeSO4·7H2O participate in both flame retardancy and mildew prevention. 1) They react with phytic acid and tannic acid to form a stable three-dimensional network structure, stabilizing the two charring agents; 2) They catalyze the generation of active oxygen to inhibit mold growth, thus achieving anti-mold and antibacterial effects. In this step, controlling the pH value between 5.5 and 6.0 effectively inhibits Fe²⁺ ions. + Oxidized into ineffective Fe³ + This ensures the stability of its flame-retardant and mildew-proof functions. Therefore, ferrous ions (Fe²⁺) + (Derived from ferrous sulfate heptahydrate), it not only acts as a complexation center for phytic acid, promoting char formation and flame retardancy, but also possesses significant antibacterial and antifungal properties. Fe²⁺ + It can effectively inhibit common bamboo decay fungi such as Aspergillus niger by disrupting the permeability of mold cell membranes, interfering with electron transport in their respiratory chain, and catalyzing the production of reactive oxygen species (ROS). Aspergillus niger ), Penicillium ( Penicillium spp. Compared to traditional organic mildew inhibitors, Fe²⁺ promotes the growth of fungi such as Fiber 25 and Fiber 26. + It is safe to source, inexpensive, and forms a stable complex structure with phytic acid and tannic acid, which significantly prolongs the anti-mildew effect.

[0035] PG-CS not only acts as an anti-mildew component, but its amino and hydroxyl groups can also react with phytic acid / Fe² in the core material. + The complex and the silanized polyurethane wall material form hydrogen bonds and electrostatic adsorption, achieving a dual fixation mechanism of "core loading – wall anchoring". Using only propyl gallate results in poor antibacterial effect and lacks the large molecular chain structure necessary for char formation. Using only chitosan offers limited antibacterial effect and has poor compatibility with the system. However, the PG-CS formed by graft copolymerization has amino and hydroxyl groups that can respectively react with phytic acid / Fe²⁺ in the core material. + The complex and the silanized polyurethane wall material form hydrogen bonds and electrostatic adsorption, which enhance the flame retardant-char formation synergy and realize the dual mechanism of core loading and wall fixation.

[0036] S5. Mix 10 g IPDI (isophorone diisocyanate), 20.0 g PPG (polypropylene glycol), 0.5 g TMP (trimethylolpropane), and 1.2 g KH-550 (γ-aminopropyltriethoxysilane) and react at 75 °C for 2 h, controlling the NCO content to be 8%, to obtain a prepolymer. Dilute the prepolymer with ethyl acetate to prepare an ethyl acetate solution with a prepolymer solid content of 40%.

[0037] S6. Place the quaternary composite core material solution into a beaker, add 0.5 g of Tween 80, and stir manually until completely dissolved. Run at 5000-8000 rpm for 3 minutes to form an emulsion. Slowly add 30 g of the ethyl acetate solution of the prepolymer dropwise into the quaternary composite core material emulsion to obtain a mixed solution.

[0038] The -NCO content of the prepolymer is controlled at 8%~10%. The amino and hydroxyl groups in the core material emulsion will react with the -NCO groups in the prepolymer to form hydrogen bonds or covalent bonds. In this embodiment, the total mass of the core material solution is about 95.4g, and the mass ratio of the core material emulsion to the prepolymer is 1.2~1.5, so that the prepolymer can completely coat the core material and the core-wall interface can fully react.

[0039] S7. Aliquot the above mixed solution into centrifuge tubes, centrifuge at 8000 rpm for 10 minutes, discard the supernatant, and collect the white microcapsule precipitate at the bottom. Add 5 volumes of anhydrous acetone to the precipitate, vortex thoroughly to resuspend the precipitate, centrifuge, and discard the supernatant. Repeat 3 times to remove organic impurities. Add 5 volumes of anhydrous ethanol to the precipitate, resuspend thoroughly, centrifuge, and discard the supernatant. Repeat 2 times to remove ethyl acetate and inorganic salts. Freeze-dry the washed, purified microcapsule precipitate under vacuum to obtain dried microcapsule powder.

[0040] In steps S4 and S5, formaldehyde-free silane-modified polyurethane prepolymer is used as the wall material, and the core material is coated by interfacial polymerization to obtain mildew-proof and flame-retardant microcapsules with uniform particle size, stable dispersion, and tight core-wall bonding.

[0041] The wall material primarily uses water as a medium, and ethyl acetate can be removed through washing, reducing pollution and safety hazards, thus meeting the requirements of green chemistry. Combining these two methods maximizes the preservation of bamboo's "ecological attributes," aligning with current environmental regulations and consumer demand for green materials.

[0042] The microcapsules prepared by the present invention through emulsion interfacial polymerization have a spherical core-shell structure with an average particle size of 100–300 nm (measured by DLS), a particle size distribution index (PDI) <0.2, and an absolute value of Zeta potential >30 mV, indicating that they have good colloidal stability in the aqueous phase.

[0043] S8. Polypropylene glycol (PPG-2000, 20 g) and toluene diisocyanate (TDI, 12.54 g) were used as the basic reactive monomers. Hydrophilic properties were imparted to the system by introducing dimethylolpropionic acid (DMPA, 1.34 g), and dibutyltin dilaurate (DBTDL, 2 drops) were added dropwise. The reaction was carried out at 75°C for 2 hours under a nitrogen atmosphere to form an isocyanate-terminated prepolymer. The temperature was then lowered to 55°C, and 1,4-butanediol (BDO, 0.97 g) was added to the above reaction system for a chain extension reaction for 1 hour. Acetone was then added to reduce the viscosity of the system. Next, the microcapsules obtained in S7 (2.79 g) (microcapsule powder was added to the waterborne polyurethane (WPU) matrix at 10 wt%) and trimethylolpropane (TMP, 10 g) were added and reacted for 30 min. Subsequently, triethylamine (TEA, 0.9 g) was added at 35°C to neutralize the carboxyl groups in the system, and the reaction was carried out for 30 min. The mixture was then emulsified with deionized water (DI water) at 1500 rpm for 60 min. Finally, acetone was removed by rotary evaporation at 40°C for 30 min to obtain the Sch-WPU2 emulsion. The final solid content of the resulting emulsion was approximately 53 wt%. Ten minutes before the end of the reaction, 1-2 drops of defoamer (polydimethylsiloxane (PDMS), C2H6OSi)n) were added to obtain the composite material, which was used as a mildew-resistant and flame-retardant composite coating for bamboo.

[0044] In step S8, after adding the microcapsule powder to the waterborne polyurethane (WPU) matrix at 10 wt%, a strong chemical bond is established between the microcapsules and the WPU matrix, which greatly improves the interfacial bonding strength between the two and prevents the microcapsules from falling off during processing or use.

[0045] Example 2: The method for preparing a mildew-proof and flame-retardant composite coating for bamboo in this embodiment, compared to reducing the iron ion content in step S4 of Example 1, includes the following steps: S1. Weigh 3.0g of KH-560 and measure 10ml of ethanol and 5ml of deionized water. Add all three to a beaker and place it on a magnetic stirrer. Adjust the pH of the system to the range of 4.0-4.5 by slowly adding glacial acetic acid to obtain KH-560 hydrolysate.

[0046] S2. Transfer the beaker containing 10 g of tannic acid and 60 ml of deionized water to the center of a magnetic stirrer and stir until the tannic acid is completely dissolved. Add all of the KH-560 hydrolysate dropwise. After the addition is complete, adjust the pH of the solution to the range of 4.0-4.5 with glacial acetic acid. Place the beaker back in the water bath and let it stand to allow the temperature to equalize, thus obtaining the modified tannic acid solution.

[0047] S3. Dissolve 2.0 g of chitosan in 50 mL of 1% (v / v) acetic acid aqueous solution. After complete dissolution, slowly adjust the pH of the system to 8-9 with dilute NaOH solution to deprotonate the amino group of chitosan and activate the nucleophilicity of the phenolic hydroxyl group. Then add 0.3 g of epichlorohydrin (ECH) and stir at 55 °C for 1-2 h to react with chitosan to generate an epoxy-containing intermediate. Next, add 0.5 g of propyl gallate and continue to react at the same temperature for 2-4 h to promote nucleophilic ring-opening of the epoxy group by the phenolic hydroxyl group of PG, achieving effective grafting. After the reaction is completed, the product is purified by dialysis to remove unreacted small molecules, and finally freeze-dried to obtain propyl gallate-chitosan graft copolymer (PG-grafted chitosan, PG-CS) powder.

[0048] S4. Place the modified tannic acid solution in a water bath, add 6g of phytic acid and 0.6g of PG-CS powder, and stir until completely dissolved. Add K2CO3 powder in small amounts several times using a dropper until the pH of the system reaches 5.5-6.0. Weigh 0.5g of FeSO4·7H2O, add it to the solution, and react for 1 hour to obtain the quaternary composite core material solution.

[0049] S5. Mix 10.0g IPDI, 20.0g PPG, 0.5g TMP and 1.2g KH-550, and react at 75℃ for 2h, controlling the NCO content to be 8%, to obtain a prepolymer. Dilute the prepolymer with ethyl acetate to prepare an ethyl acetate solution with a prepolymer solid content of 40%.

[0050] S6. Place the quaternary composite core material solution into a beaker, add 0.5g of Tween 80, and stir manually until completely dissolved. Run at 5000-8000 rpm for 3 minutes to form an emulsion. Slowly add 30g of the ethyl acetate solution of the prepolymer to the core material emulsion to obtain a mixed solution. In this example, the total mass of the core material solution is approximately 95.1g, which is 0.3g less than that of ferrous sulfate heptahydrate in Example 1.

[0051] S7. Aliquot the above mixed solution into centrifuge tubes, centrifuge at 8000 rpm for 10 minutes, discard the supernatant, and collect the white microcapsule precipitate at the bottom. Add 5 volumes of anhydrous ethanol to the precipitate, vortex thoroughly to resuspend the precipitate, centrifuge, and discard the supernatant. Repeat 3 times to remove organic impurities. Add 5 volumes of deionized water to the precipitate again, resuspend thoroughly, centrifuge, and discard the supernatant. Repeat 2 times to remove inorganic salts and ethyl acetate. Freeze-dry the washed, purified microcapsule precipitate under vacuum to obtain dried microcapsule powder.

[0052] S8. Polypropylene glycol (PPG-2000, 20 g) and toluene diisocyanate (TDI, 12.54 g) were used as the basic reactive monomers. Hydrophilic properties were imparted to the system by introducing dimethylolpropionic acid (DMPA, 1.34 g), and dibutyltin dilaurate (DBTDL, 2 drops) were added dropwise. The reaction was carried out at 75°C for 2 hours under a nitrogen atmosphere to form an isocyanate-terminated prepolymer. The temperature was then lowered to 55°C, and 1,4-butanediol (BDO, 0.97 g) was added to the above reaction system for a chain extension reaction for 1 hour. Acetone was then added to reduce the viscosity of the system. Next, the microcapsules obtained in S7 (2.79 g) (microcapsule powder was added to the waterborne polyurethane (WPU) matrix at 10 wt%) and trimethylolpropane (TMP, 10 g) were added and reacted for 30 min. Subsequently, triethylamine (TEA, 0.9 g) was added at 35°C to neutralize the carboxyl groups in the system, and the reaction was carried out for 30 min. The mixture was then emulsified with deionized water (DI water) at 1500 rpm for 60 min. Finally, acetone was removed by rotary evaporation at 40°C for 30 min to obtain the Sch-WPU2 emulsion. The final solids content of the resulting emulsion was approximately 53 wt%. Ten minutes before the end of the reaction, 1-2 drops of defoamer (same as in Example 1) were added to obtain the composite material, which was used as a mildew-resistant and flame-retardant composite coating for bamboo.

[0053] Example 3: The preparation method of the anti-mildew and flame-retardant composite coating for bamboo in this embodiment is largely the same as that in Example 2, except that the amount of FeSO4·7H2O in step S4 is increased to 1.0 g.

[0054] Example 4: The preparation method of the anti-mildew and flame-retardant composite coating for bamboo in this embodiment is largely the same as that in Example 2, except that the amount of phytic acid in step S4 is increased to 10g and the amount of FeSO4·7H2O is increased to 0.8g.

[0055] Example 5: The preparation method of the anti-mildew and flame-retardant composite coating for bamboo in this embodiment is largely the same as that in Example 2, except that the amount of phytic acid used in step S4 is reduced to 5g.

[0056] Example 6: The preparation method of the anti-mildew and flame-retardant composite coating for bamboo in this embodiment is largely the same as that in Example 2, except that the amount of PG-CS powder in step S4 is increased to 1.0g.

[0057] Example 7: The preparation method of the anti-mildew and flame-retardant composite coating for bamboo in this embodiment is largely the same as that in Example 5, except that the amount of PG-CS powder used in step S4 is reduced to 0.8 g.

[0058] Example 8: The preparation method of the anti-mildew and flame-retardant composite coating for bamboo in this embodiment is largely the same as that in Example 2, except that the amount of IPDI used in step S5 is 9.5g and the NCO content is controlled at 7.5%.

[0059] Example 9: The preparation method of the anti-mildew and flame-retardant composite coating for bamboo in this embodiment is largely the same as that in Example 2, except that: in step S5, the amount of IPDI used is 12.3 g and the NCO content is controlled at 10%; in step S6, the amount of ethyl acetate solution of the prepolymer used is 40 g.

[0060] With a high NCO content, sufficient wall material is needed to completely cover the core material, so the amount of ethyl acetate used is 40g.

[0061] Example 10: The preparation method of the anti-mildew and flame-retardant composite coating for bamboo in this embodiment is largely the same as that in Example 2, except that the amount of ethyl acetate solution containing prepolymer in step S6 is 25g.

[0062] Example 11: The preparation method of the anti-mildew and flame-retardant composite coating for bamboo in this embodiment is largely the same as that in Example 2, except that the amount of microcapsule powder used in step S8 is 1.82 g.

[0063] Example 12: The preparation method of the anti-mildew and flame-retardant composite coating for bamboo in this embodiment is largely the same as that in Example 2, except that the amount of microcapsule powder used in step S8 is 4.05 g.

[0064] Comparative Example 1: The preparation method of the anti-mildew and flame-retardant composite coating for bamboo in this comparative example is roughly the same as that in Example 2, except that KH-560 is replaced with KH-550 in step S1.

[0065] Comparative Example 2: The preparation method of the anti-mildew and flame-retardant composite coating for bamboo in this comparative example is roughly the same as that in Example 2, except that tannic acid is replaced with phytic acid in step S2.

[0066] Comparative Example 3: The preparation method of the anti-mildew and flame-retardant composite coating for bamboo in this comparative example is roughly the same as that in Example 2, except that phytic acid is replaced with tannic acid in step S4.

[0067] Comparative Example 4: The preparation method of the anti-mildew and flame-retardant composite coating for bamboo in this comparative example is roughly the same as that in Example 2, except that KH-550 is replaced with KH-560 in step S4.

[0068] Comparative Example 5: The preparation method of the anti-mildew and flame-retardant composite coating for bamboo in this comparative example is roughly the same as that in Example 2, except that FeSO4·7H2O is replaced with ZnSO4 in step S4.

[0069] Comparative Example 6: The preparation method of the anti-mildew and flame-retardant composite coating for bamboo in this comparative example is roughly the same as that in Example 2, except that tannic acid is replaced with catechin in step S2 and phytic acid is replaced with citric acid in step S4.

[0070] Comparative Example 7: The preparation method of the anti-mildew and flame-retardant composite coating for bamboo in this comparative example is roughly the same as that in Example 2, except that step S2 is missing and propyl gallate is used directly to replace PG-CS powder in the same amount in step S3.

[0071] Comparative Example 8: The preparation method of the anti-mildew and flame-retardant composite coating for bamboo in this comparative example is roughly the same as that in Example 2, except that step S2 is omitted and in step S3, the same amount of chitosan is used directly to replace PG-CS powder.

[0072] As shown in Table 1, the polydispersity index (PDI) of the examples is less than 0.3 (0.128~0.293), indicating narrow particle size distribution and good uniformity. In contrast, the PDI of Comparative Examples 1-6 is relatively large (0.56~0.881) and wide, while the PDI of Comparative Examples 7-8 is relatively small. The encapsulation efficiency of the products of Examples 1-12 is all above 90% (up to 97.1%), while the encapsulation efficiency of Comparative Examples 1-6 is generally low (29.8%~45.5%). The encapsulation efficiency of the examples remains at a high level (88.7%~92.4%) after 30 days of storage, with a small decrease. However, the 30-day encapsulation efficiency of Comparative Examples 1-6 drops sharply to 2.5%~32.2%, and that of Comparative Examples 7-8 also drops to around 80%, indicating significantly insufficient stability. The overall performance of Comparative Examples 7-8 is not as good as that of this application. This demonstrates that the reagents used in this invention have a synergistic effect. The comparative examples show that the grafted structure in PG-CS (propyl gallate grafted with chitosan) is crucial for maintaining the high encapsulation efficiency and long-term storage stability of the microcapsules. Without grafting, using only chitosan as the core material makes migration easier, illustrating the technical advantages of using PG-CS in this invention.

[0073] Table 1. Basic properties of the microcapsules prepared in the various embodiments and comparative examples of the present invention.

[0074] As shown in Table 2, the embodiments of the present invention exhibit excellent performance in four aspects: high Zeta potential (high stability), high preparation success rate (no gel), excellent anti-precipitation properties, and long-term storage stability, with all indicators showing balanced excellence. In contrast, the comparative examples generally suffer from significant defects such as excessively low potential (easy aggregation), preparation failure (gelation), or precipitation. Specifically, all embodiments successfully prepared a normal-looking "milky white" liquid with a good product appearance. Comparative Examples 4 and 6 showed "gels," indicating preparation failure and the inability to form a normal microcapsule suspension. The absolute values ​​of the Zeta potentials in all embodiments were generally high, ranging from 42.3 mV to 59.2 mV. The higher the absolute value of the Zeta potential (generally considered stable above 30 mV), the lower the probability of precipitation. The Zeta potentials of Comparative Examples 1 (18.8 mV) and 5 (16.7 mV) were extremely low, indicating a higher probability of flocculation or aggregation. The comparative examples demonstrate that the grafting structure in PG-CS can improve the stability of microcapsules in waterborne polyurethane and prevent aggregation, illustrating the technical advantages of using PG-CS in maintaining emulsion stability in this invention.

[0075] Table 2. Basic properties of the composite coatings prepared in the various embodiments and comparative examples of the present invention.

[0076] The coatings of the various embodiments and comparative examples of the present invention were applied to bamboo surfaces of the same size using conventional coating methods. The bamboo surfaces were first dried and sanded. The coatings obtained in step S8 were then applied by roller coating machine to evenly coat the bamboo surfaces. The coating thickness was controlled by smoothing and leveling the coating. This process was repeated 2-3 times (each time ensuring that the previous coating was completely dry before continuing to coat) to ensure that each sample surface was evenly coated. The samples were then randomly placed in a dryer to dry and obtain the final composite coating. The performance of the composite coating was then tested.

[0077] As can be seen from Table 3: 1. The tensile strength of the examples is generally high (71.3 MPa ~ 91.5 MPa), while the elongation at break is excellent (612% ~ 891%). This indicates that the materials of the examples are both strong and flexible, and not easily broken. The tensile strength of comparative examples 1, 2, and 3 is extremely low (27.5 MPa ~ 43.1 MPa), and the elongation at break is very poor (98% ~ 132%). Although comparative example 5 has a high tensile strength (71.2 MPa), its elongation at break (235%) is much lower than that of the examples, indicating insufficient toughness. The mechanical properties of comparative examples 7 and 8 are close to those of the examples, but there are differences in subsequent indicators (such as water absorption).

[0078] 2. The hardness of all examples reached 2H, indicating that the surface has strong scratch resistance after curing. The hardness of comparative examples 1, 2 and 3 was only HB (hardness lower than 2H), and the surface protection performance was poor. Comparative examples 4 and 6 failed to be prepared and there is no data.

[0079] 3. The adhesion of the embodiments ranges from 2.85 MPa to 4.31 MPa, showing a relatively high and stable overall performance, indicating strong bonding with the substrate. The adhesion of Comparative Examples 1 (1.36 MPa), 2 (0.87 MPa), and 3 (1.93 MPa) is significantly lower, and the coatings are prone to peeling off. While the values ​​of Comparative Examples 5 (4.28 MPa) and 7 (3.58 MPa) are acceptable, considering other properties (such as poor toughness in Comparative Example 5 and high water absorption in Comparative Example 7), their overall performance is inferior to the embodiments.

[0080] 4. The water absorption rates of the examples were low and well controlled, ranging from 10.5% to 17.0%. Low water absorption means that the material is less prone to swelling and hydrolysis in humid environments, exhibiting good long-term stability. Comparative Example 1 had an extremely high water absorption rate (54.2%) and very poor water resistance. The water absorption rates of Comparative Examples 2, 3, 5, 7, and 8 (17.5% to 28.6%) were generally higher than those of the examples. In particular, Comparative Examples 7 (20.5%) and 8 (21.3%), although their mechanical properties were acceptable, had significantly high water absorption rates, which would affect their service life in humid environments.

[0081] Therefore, it can be seen that the embodiments of the present invention achieve an excellent balance between mechanical properties (strength, toughness, hardness) and application properties (adhesion, water resistance). In contrast, the comparative examples show obvious shortcomings. The mechanical properties of comparative examples 1-3 are generally poor (low strength, high brittleness, low hardness) and water resistance is also poor. Comparative example 5 has insufficient toughness (low elongation at break), and the water resistance of comparative examples 7-8 is significantly worse than that of the embodiments (excessively high water absorption).

[0082] Table 3 Physical properties of the composite coatings in the embodiments and comparative examples of the present invention

[0083] As shown in Table 4, the limiting oxygen index (LOI) of the embodiments is generally high, ranging from 27.8% to 37.4%. According to the classification of flame retardant grades, an LOI greater than 27% is considered a flame-retardant material, and the embodiments are generally at the flame-retardant to non-combustible level. The ignition time is generally long, ranging from 16 s to 21 s, indicating that the material requires a longer period of heat accumulation before ignition, thus exhibiting better fire safety. The burning time is short, ranging from 25 s to 34 s, indicating that once ignited, the flame spreads slowly or has good self-extinguishing properties. The burning area is small, ranging from 26% to 41%, indicating that the flame does not easily spread on the material surface, resulting in a significant flame-retardant effect. The embodiments of the present invention achieve an excellent balance in terms of flame retardant performance: high LOI (flame-retardant), difficult to ignite (long ignition time), strong self-extinguishing properties (short burning time), and small flame spread (small burning area). The comparative examples show a clear polarization. Comparative examples 2, 3, and 5 have poor flame retardant performance (extremely low LOI, fast ignition, long burning time, and large burning area) and are flammable. Comparative example 1 has flame retardant performance that is close to but slightly lower than the examples. Comparative examples 7 and 8 have good flame retardant performance, but their overall performance is not as balanced as the examples when combined with the preceding data (stability, water resistance, etc.).

[0084] Table 4 Flame retardant properties of the composite coatings in the embodiments and comparative examples of the present invention

[0085] As shown in Table 5, Comparative Examples 1-3 barely approached the lower limit of the embodiments in some individual data regarding their mold prevention efficacy. However, considering the previous data, Comparative Examples 1-3 exhibited extremely poor mechanical properties (low tensile strength, high brittleness), poor flame retardancy (flammable), and poor stability (low Zeta potential). Even with slight performance in mold prevention, they could not be used as reliable functional materials. Although Comparative Examples 5, 7, and 8 had acceptable mechanical or flame retardant properties in some aspects, their mold prevention efficacy was a fatal weakness (all below 50%), indicating poor antibacterial ability.

[0086] Table 5. Mold prevention efficacy of the composite coatings in the embodiments and comparative examples of the present invention.

[0087] Thermogravimetric analysis (TGA) showed that the microcapsule powder had a char residue of ≥35% at 800℃ under a nitrogen atmosphere. When the microcapsule powder was added at 10 wt% to a waterborne polyurethane (WPU) matrix, the resulting composite coatings in each embodiment exhibited a limiting oxygen index (LOI) of over 28.5%, achieved a vertical burning test (UL-94) rating of V-1, reduced total heat release (THR) by over 40%, and significantly decreased smoke generation rate (SPR). The overall flame retardant performance (LOI ≥ 28.5%, UL-94 V-1 rating) met the flame-retardant standard for building materials (GB 8624 B1 level), demonstrating excellent flame retardant properties. After aging at 70℃ and 90% relative humidity for 168 h, the WPU composite coating maintained a mildew resistance rating of 0, with no significant core material seepage or frosting. The mechanical properties (tensile strength ≥15 MPa, elongation at break ≥200%) retained over 90%, fully demonstrating its long-term stability and practicality.

[0088] The composite coating prepared by this invention has high strength (70-95 MPa) and high elongation at break (600%-900%), while also having high flame retardant properties, a limiting oxygen index (LOI) between 28% and 37%, an ignition time of 16-21 seconds, and a burning area of ​​26%-41%. Compared with the comparative example, the mechanical properties and flame retardant properties are poor (LOI<18%), indicating that this invention achieves a synergistic improvement in mechanical properties and flame retardant properties, which is a technical effect that the comparative example cannot achieve.

[0089] While the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the invention. Any person skilled in the art can make many possible variations and modifications to the technical solutions of the present invention, or modify them into equivalent embodiments, without departing from the scope of the present invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention, without departing from the scope of the present invention, should fall within the protection scope of the present invention.

Claims

1. A method for preparing a mildew-resistant and flame-retardant composite coating for bamboo, characterized in that: Includes the following steps: A1. The silanized modified tannic acid solution, phytic acid and propyl gallate-chitosan graft copolymer are stirred and mixed, the pH value is adjusted to 5.5-6.0, ferrous sulfate is added and stirred to react, and a quaternary composite core material solution is obtained. A2. Using a formaldehyde-free silane-modified polyurethane prepolymer with an isocyanate group content of 8-10% as the wall material, the wall material is added to a quaternary composite core material solution for a coating reaction. After solid-liquid separation, a precipitate is obtained. The precipitate is washed and dried to obtain microcapsule powder. A3. Using microcapsule powder as a crosslinking agent, the crosslinking agent and water-based polyurethane emulsion are mixed and reacted, and after emulsification, a mildew-proof and flame-retardant composite coating for bamboo is obtained.

2. The method for preparing the anti-mildew and flame-retardant composite coating for bamboo according to claim 1, characterized in that: The silanized modified tannic acid solution was prepared using the following steps: The silane coupling agent KH-560 was dissolved in an aqueous ethanol solution, and the pH was adjusted to 4.0–4.5 to obtain an aqueous solution of KH-560. KH-560 aqueous solution was added dropwise to tannic acid aqueous solution, and the pH value was adjusted to 4.0-4.5 to obtain silanized modified tannic acid solution.

3. The method for preparing the anti-mildew and flame-retardant composite coating for bamboo according to claim 1, characterized in that: The propyl gallate-chitosan graft copolymer was prepared using the following steps: Chitosan was dissolved in an aqueous acetic acid solution, and the pH was adjusted to 8-9 to obtain a chitosan aqueous solution. Epichlorohydrin and chitosan aqueous solution were mixed and stirred at 50-60°C to generate an intermediate solution. Propyl gallate and intermediate solution were mixed and stirred, purified, and dried to obtain propyl gallate-chitosan graft copolymer.

4. The method for preparing the anti-mildew and flame-retardant composite coating for bamboo according to any one of claims 1 to 3, characterized in that: The mass ratio of phytic acid to ferrous sulfate is 5-10:0.5-1.

5. The method for preparing a mildew-proof and flame-retardant composite coating for bamboo according to any one of claims 1 to 3, characterized in that: The formaldehyde-free silane-modified polyurethane prepolymer was prepared using the following steps: Using isocyanate and polypropylene glycol as reactive monomers, the reactive monomers, crosslinking agents and coupling agents are mixed and stirred at 75-80°C to obtain a prepolymer by controlling the isocyanate group content to be 8-10%. The prepolymer was diluted with ethyl acetate to obtain an aldehyde-free silane-modified polyurethane prepolymer.

6. The method for preparing the anti-mildew and flame-retardant composite coating for bamboo according to claim 5, characterized in that: The crosslinking agent is trimethylolpropane, the coupling agent is γ-aminopropyltriethoxysilane, and the mass ratio of isocyanate, polypropylene glycol, trimethylolpropane and γ-aminopropyltriethoxysilane is 10-13:18-22:0.4-1.0:0.5-1.

5.

7. The method for preparing a mildew-proof and flame-retardant composite coating for bamboo according to any one of claims 1 to 3, characterized in that: Step A2 includes the following steps: Add Tween 80 to the quaternary composite core material solution, stir to dissolve, and form an emulsion; Formaldehyde-free silane-modified polyurethane prepolymer was dropped into the emulsion, and the mixture was stirred to carry out the coating reaction to obtain a suspension. The suspension is subjected to solid-liquid separation to obtain a precipitate; The precipitate was washed with an organic solvent and then with deionized water. After drying, microcapsule powder was obtained.

8. The method for preparing a mildew-proof and flame-retardant composite coating for bamboo according to any one of claims 1 to 3, characterized in that: Step A3 includes the following steps: Using polypropylene glycol and toluene diisocyanate as the basic reactants, a hydrophilic chain extender is added, and a catalyst is added dropwise. The reaction is carried out at 73–77°C under a protective atmosphere to form an isocyanate-terminated prepolymer. The temperature is then lowered to 55–60°C, and a chain extender is added again to carry out a chain extension reaction. A solvent is added to reduce the viscosity of the system to 600–800 mPa. s, to obtain a mixed system; Using microcapsule powder and trimethylolpropane as crosslinking agents, the crosslinking agent was added to the mixture to carry out the crosslinking reaction. The temperature was lowered to 30-40℃, and an alkaline catalyst was added to carry out the neutralization reaction to obtain the mixture. The mixture was emulsified with deionized water at 1500±100 rpm, acetone was removed by rotary evaporation, and defoamer was added to obtain a mildew-proof and flame-retardant composite coating for bamboo.

9. The method for preparing the anti-mildew and flame-retardant composite coating for bamboo according to claim 8, characterized in that: The mass ratio of microcapsule powder, trimethylolpropane, and the mixed system is 1.5–4.5:10:32–35.

10. The application of a mildew-proof and flame-retardant composite coating for bamboo in the mildew-proof and flame-retardant composite coating of bamboo, characterized in that: The mildew-proof and flame-retardant composite coating for bamboo is prepared by the preparation method according to any one of claims 1 to 9.

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